Trace gas detection and high-precision spectroscopy in the

Trace gas detection and high-precisionspectroscopy in the mid-infrared and visible

wavelength regions

Jari Peltola

University of HelsinkiFaculty of Science

Department of ChemistryLaboratory of Physical Chemistry

A.I. Virtasen aukio 1 (P.O. BOX 55)FI-00014 University of Helsinki, Finland

Academic Dissertation

To be presented, with the permission of the Faculty of Science of theUniversity of Helsinki, for public discussion in auditorium A129,Department of Chemistry (A. I. Virtasen aukio 1, Helsinki),

on the 2nd of December, 2015, at 12 o’clock.

Helsinki 2015

Supervisor: Professor Lauri HalonenDepartment of ChemistryUniversity of HelsinkiHelsinki, Finland

Instructors: Dr. Markku VainioDepartment of ChemistryUniversity of HelsinkiHelsinki, Finland

Dr. Mikael SiltanenPicodeon Ltd OyOulu, Finland

Reviewers: Professor emeritus Jouko Korppi-TommolaDepartment of ChemistryUniversity of Jyvaskyla

Dr. Florian SchmidtDepartment of Applied Physics and ElectronicsUmea University

Opponent: Dr. Paolo De NataleNational Institute of OpticsFirenze, Italy

ISBN 978-951-51-1663-5 (paperback)ISBN 978-951-51-1664-2 (PDF)

http://ethesis.helsinki.fiUnigrafia, Helsinki 2015

Abstract

This thesis is based on four experimental spectroscopic studies where novel highlysensitive laser absorption spectroscopy spectrometers are developed and used for tracegas detection and precision spectroscopy. Most of the studies are carried out in the mid-infrared region between 3 and 4 µm, where a homebuilt continuous-wave singly resonatingoptical parametric oscillator is used as a light source. In addition, one study has beenperformed in the visible region using a commercial green laser at 532 nm.

Two of the developed spectroscopic applications are based on cavity ring-down spec-troscopy. In this thesis, the first off-axis re-entrant cavity ring-down spectrometer in themid-infrared is demonstrated and utilized for highly sensitive detection of formaldehyde.The second study presents an optical frequency comb referenced mid-infrared continuous-wave singly resonating optical parametric oscillator, which is applied to high-precisioncavity ring-down spectroscopy of nitrous oxide and methane. Furthermore, this studypresents a new method for referencing a mid-infrared optical parametric oscillator to anear-infrared optical frequency comb. This new method allows large mode-hop-free fre-quency tuning ranges in the mid-infrared region.

The other two experiments are based on cantilever-enhanced photoacoustic spec-troscopy, presenting the first reported studies of cantilever-enhanced-based trace gas de-tection in the mid-infrared and visible region. These studies show the great potential ofcantilever-enhanced photoacoustic detection for substantial enhancement of the sensitiv-ity of trace gas detection. For instance, the best nitrogen dioxide detection limit everreported using photoacoustic spectroscopy is presented in this thesis.

Preface

The present work has been carried out at the Laboratory of Physical Chemistry ofthe University of Helsinki and at the Centre for Metrology (MIKES) during the yearsbetween 2010 and 2015.

I owe my gratitude to a great many people who have contributed to this work. Firstof all, I would like to express my gratitude to my supervisor, Professor Lauri Halonen forgiving me the opportunity to work in his group, and the interesting field of science. Thiswork began essentially as early as 2008 when I started as an undergraduate to workingwith the development of mid-infrared laser sources. Ever since, Lauri has been supportiveand has encouraged me throughout my studies and research.

I am very grateful to my instructors, Dr. Markku Vainio and Dr. Mikael Siltanen.Your assistance, knowledge of science, and endless new ideas has helped me through thisprocess. My very special thanks go to Markku who has taught and guided me since 2008,and without your impact this thesis would not have been anywhere near as good as itbecame.

I also want to thank all the colleagues, present and former, at the University and at theMIKES for a nice working environment and all the cheer we have had in- and out-officeactivities during the years. Especially I want to thank Dr. Tuomas Hieta who towardsthe end became like a third instructor to me.

I thank all the co-authors of the publications for their contribution to the thesis. I alsowant to thank the reviewers, Professor emeritus Jouko Korppi-Tommola and Dr. FlorianSchmidt, for careful reading of the manuscript. In addition, I want to thank Dr. CraigRichmond for checking the language of my thesis.

The financial support of the University of Helsinki, the Academy of the Finland, theEmil Aaltonen Foundation, the Magnus Ehrnrooth Foundation, the Doctoral Programmein Chemistry and Molecular Sciences, and the European Metrology Research Programmeis gratefully acknowledged. The support of Gasera Ltd. is also gratefully acknowledged.

I want to express my sincere gratitude to my family for their support throughout mystudies. Especially to Kalervo and Marja-Leena, the most loving and supportive parents.I also want to thank my uncle Oiva who, like my father, represents a true working classhero to me.

I also thank my non-scientist friends in Helsinki and Lappajarvi by offering me nu-merous joyful moments in the fields of non-science: sports, music, fishing, get-together,etc. I especially would like to thank Antti and Vesa, and my bandmates Ari-Matti andJani, friends that have been close to me more than a half of my life.

Finally, I want to thank Sari, the love of my life. You were always there cheering meup and stood by me through the good times and the bad ones within this journey.

Jari PeltolaHelsinki, October 2015

List of publications

This thesis contains the following publications, which are referred to by the Romannumerals I-IV:

I J. Peltola, M. Vainio, V. Ulvila, M. Siltanen, M. Metsala and L. Halonen. Off-axis re-entrant cavity ring-down spectroscopy with a mid-infrared continuous-waveoptical parametric oscillator. Appl. Phys. B, 107(3):839–847, 2012.

II J. Peltola, M. Vainio, T. Hieta, J. Uotila, S. Sinisalo, M. Metsala, M. Siltanen and L.Halonen. High sensitivity trace gas detection by cantilever-enhanced photoacousticspectroscopy using a mid-infrared continuous-wave optical parametric oscillator.Opt. Express, 21(8):10240–10250, 2013.

III J. Peltola, T. Hieta and M. Vainio. Parts-per-trillion level detection of nitrogen diox-ide by cantilever-enhanced photoacoustic spectroscopy. Opt. Lett., 40(13):2933–2936, 2015.

IV J. Peltola, M. Vainio, T. Fordell, T. Hieta, M. Merimaa and L. Halonen. Frequency-comb-referenced mid-infrared source for high-precision spectroscopy. Opt. Express,22(26):32429–32439, 2014.

The author has prepared the manuscripts for Publications I, II, and III. He has alsowritten the final version of the manuscript for Publication IV. In Publication I, the authorhas been responsible for most of the experimental work and data analysis. The authorhas been responsible for all the experimental work in Publication II. He has also analyzedmost of the data presented in this publication. In Publications III and IV, the authorhas carried out all the experimental work and analysed all the measurements presentedin these publications.

Related publications

i M. Vainio, J. Peltola, S. Persijn, F. J. M. Harren and L. Halonen. Singly resonantcw OPO with simple wavelength tuning. Opt. Express, 16(15):11141–11146, 2008.

ii M. Vainio, J. Peltola, S. Persijn, F. J. M. Harren and L. Halonen. Thermal effectsin singly resonant continuous-wave optical parametric oscillators. Appl. Phys. B,94(3):411–427, 2009.

Contents

1 Introduction 1

2 Laser sources 4

2.1 Continuous-wave optical parametric oscillator . . . . . . . . . . . . . . . . 4

2.1.1 Nonlinear frequency conversion . . . . . . . . . . . . . . . . . . . . 4

2.1.2 Optical parametric generation . . . . . . . . . . . . . . . . . . . . . 5

2.1.3 Quasi-phase-matched lithium niobate crystal . . . . . . . . . . . . . 6

2.1.4 Wavelength tuning . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.2 Optical frequency comb . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

2.2.1 Principle of optical frequency comb . . . . . . . . . . . . . . . . . . 10

3 Laser absorption spectroscopy 13

3.1 Absorption of light . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

3.2 Continuous-wave cavity ring-down spectroscopy . . . . . . . . . . . . . . . 15

3.2.1 Off-axis injection with re-entrant configuration . . . . . . . . . . . . 16

3.2.2 Formaldehyde detection with off-axis re-entrant CRDS . . . . . . . 19

3.3 Cantilever-enhanced photoacoustic spectroscopy . . . . . . . . . . . . . . . 21

3.3.1 Optical cantilever microphone . . . . . . . . . . . . . . . . . . . . . 22

3.3.2 Detection of HCN and CH4 in the mid-infrared . . . . . . . . . . . 24

3.3.3 Detection of NO2 using a high power green laser . . . . . . . . . . . 27

3.4 Optical frequency comb assisted mid-infrared spectroscopy . . . . . . . . . 28

3.4.1 Frequency-comb-referenced CW-SRO for precision spectroscopy . . 30

3.4.2 Precision spectroscopy of N2O and CH4 . . . . . . . . . . . . . . . . 31

4 Conclusions 35

Chapter 1

Introduction

Trace gases are gases that exist in small concentrations in a gas mixture. A good exampleof such a gas mixture is the atmosphere around us, which is mainly composed of nitro-gen, oxygen, and argon. Together these compounds account for about 99.9% of the totalatmosphere. The remaining ∼0.1% consists of trace gases including, among others, car-bon dioxide (CO2), ozone (O3), methane (CH4), hydrogen cyanide (HCN), nitrous oxide(N2O), and oxides of nitrogen (NOx). Of these, e.g., hydrogen cyanide is a highly toxiccompound, which is used and released in many industrial processes and is formed duringthe combustion of nitrogen-containing compounds. In general, trace gases are involvedin global warming, reduction of ozone layer, formation of photochemical smog and acidrain, and are the cause of health problems. Many of them are also absorbed (inhaled)and emitted (exhaled) by living organisms. Hence, the detection and quantification of thecomposition of trace gases for various scientific, industrial, medical, and environmentalpurposes is essential. Concentrations of trace gases vary typically from hundreds of parts-per-million (ppm) to a few of parts-per-trillion (ppt) by volume. Low concentrations andchallenging measurement conditions create strict requirements for trace gas detection.

Laser-based absorption spectroscopy has provided versatile techniques for trace gasanalysis [1–8], atmospheric monitoring [9–11], medical diagnostics [12–15], as well asfor optical frequency metrology [16–19]. Spectroscopic methods such as cavity ring-down spectroscopy (CRDS) and photoacoustic spectroscopy (PAS) allow fast detectionwith high sensitivity and good spectral resolution, and need little or no pre-treatmentof the sample. Tunable narrow linewidth lasers improve the selectivity and help toidentify a molecule or its isotope from interfering compounds and background signals.This is the case especially in the mid-infrared (MIR), in the so-called fingerprint region,where many molecules have their characteristic strong fundamental vibrations and re-lated rotational-vibrational structure. The MIR region covers wavelengths between 2.5and 25 µm (400− 4000 cm−1). The strong absorption and the large number of detectablemolecules make the mid-infrared region ideal for trace gas analysis. This is the case, inparticular, in the wavelength region between 3.0 and 4.0 µm, which contains, for example,CH, NH, and OH stretching modes (see Fig. 1.1). In addition, the region has a relativelysmall absorption due to water vapor and carbon dioxide. However, laser manufacturinghas proven to be very challenging for this region.

In this particular spectral region, 1064 nm pumped continuous wave (CW) opticalparametric oscillators (OPOs), based on periodically poled lithium niobate (PPLN) non-

1

CHAPTER 1. INTRODUCTION 2

Figure 1.1: Some molecular stretching motions and their characteristic absorbing wavelength

regions in the mid-infrared region.

linear crystals, have been widely used as light sources for high-resolution MIR spec-troscopy. These CW OPOs possess broad spectral coverage (2.5 − 4.0 µm), narrowlinewidth (∼ 1 MHz), wide mode-hop-free tuning (> 10 GHz), and high output power(> 0.5 W) when configured as singly resonant OPOs (SROs).

In laser absorption spectroscopy, a measured spectrum presents the absorption of asample as a function of light frequency. In particular, the frequency of light has previ-ously been difficult to measure with high precision and accuracy. Consequently, one ofthe most important technological advances in the field of laser spectroscopy has been theinvention of the optical frequency comb (OFC). The OFC enables one to synthesize andmeasure optical frequencies with extremely high accuracy, typically with relative uncer-tainties from 10−13 to 10−15. This has led to remarkable improvements in high-precisionlaser spectroscopy.

This thesis consists of four refereed publications, where novel highly sensitive laserabsorption spectroscopy spectrometers are described and used for trace gas detection andprecision spectroscopy in the mid-infrared and visible regions. A brief summary of eachpublication is given below.

Publication I describes the first re-entrant off-axis cavity ring-down spectroscopy spec-trometer for the mid-infrared region, which provides high spectral resolution while main-taining high measurement speed. The spectrometer was applied to the spectroscopy offormaldehyde at 3.4 µm using a mid-infrared CW-SRO as a light source.

Publication II describes a highly sensitive CW-SRO-based spectrometer, which uti-lizes photoacoustic spectroscopy for the detection of hydrogen cyanide and methane in themid-infrared region. Very high sensitivity was achieved using a novel cantilever-enhancedphotoacoustic detection combined with the high output power of the CW-SRO.

Publication III reports a simple cantilever-enhanced photoacoustic spectrometer forsensitive detection of nitrogen dioxide in the visible region reaching the lowest NO2 detec-tion limit (50 ppt in 1 s) ever reported using PAS. Unlike in the previous publications, thelight source used in this work was a high-power continuous-wave intra-cavity frequency

CHAPTER 1. INTRODUCTION 3

doubled Nd:YVO4 laser at 532 nm.Publication IV presents a tunable mid-infrared CW-SRO, which is referenced to a

fully stabilized near-infrared optical frequency comb using a new referencing method. Themethod is based on frequency doubling of the mid-infrared beam. The comb-referencedCW-SRO was applied to high-precision cavity-ring-down spectroscopy of nitrous oxide(N2O) and water (H2O) at 2.85 µm and of methane (CH4) at 3.2 µm.

This thesis is organized as follows. Chapter 2 explains the general theory of the lasersources related to this thesis. In Chapter 3, I review the theoretical and experimentalbackground of the spectroscopic techniques used and developed in this thesis. Chap-ter 3 also summarizes the results of the work presented in Publications I-IV. Chapter 4summarizes and concludes the thesis.

Chapter 2

Laser sources

2.1 Continuous-wave optical parametric oscillator

All the CW-OPOs used in this thesis are self-developed and homebuilt. The more detaileddescription of their operation and design is described in publications [20,21]. The followingsections explain the main theory and principle of optical parametric oscillator that arerelevant to this thesis.

2.1.1 Nonlinear frequency conversion

In a nonlinear optical interaction, the frequency of light can change. This effect enablescreation of coherent mid-infrared radiation starting from visible or near-infrared laserradiation. When an electromagnetic field (EMF) E interacts with a medium, it causesbound electrons (dipoles) to move inducing a changing dipole moment. The macroscopicpolarization of the oscillating dipoles can be expressed as P = ε0χE, where ε0 is the freespace dielectric constant and χ is the susceptibility. The interaction with the magneticfield is small compared to the electric field and hence the magnetic part can generallybe neglected. The susceptibility describes the response of a matter to an applied electro-magnetic field. The induced polarization P can be represented as a power series of theelectric field

P = ε0χ(1)E + ε0(χ(2)E2 + χ(3)E3 + · · · ), (2.1)

where the first term is the polarization induced by the linear susceptibility χ(1) and thenext two terms represent the polarization induced by the nonlinear susceptibilities χ(2)

and χ(3). Normally, the intensity of electric field is small and the susceptibility will haveonly a linear dependence on the electric field. The effects of nonlinear susceptibilitiesbecome significant only when the intensity of the electric field is strong enough becausetheir values are many orders of magnitude smaller than that of the linear one. In practice,nonlinear optical phenomena are only observed by using laser light.

When two electromagnetic fields with frequencies ω1 and ω2 interact with a second-order nonlinear medium, four additional frequencies, 2ω1, 2ω2, ω1 + ω2, and ω1 − ω2 are

4

CHAPTER 2. LASER SOURCES 5

created [22]. The first two frequencies are products of an effect called second harmonicgeneration (SHG) and the third and fourth effects are called sum frequency generation(SFG) and difference frequency generation (DFG), respectively. Optical parametric gen-eration, which will be discussed in the next section, is based on the polarization inducedby the second-order susceptibility χ(2).

2.1.2 Optical parametric generation

Optical parametric generation (OPG) is a coherent three-wave mixing process (see Fig.2.1a), where a pump field at frequency ωp, creates two new fields at frequencies ωs andωi, that obey the law of conservation of energy

ωp = ωs + ωi. (2.2)

For historical reasons, the two output waves are called signal ωs and idler ωi, where theoutput wave with a lower frequency (in the MIR) is called the idler. Unlike in the othersecond-order nonlinear processes, the new fields arise from optical parametric fluorescence,and the process does not have to involve input fields at ωs and ωi. After that, DFG betweenthe initial pump and a signal photon creates a new idler photon, which, in turn, createsa new signal photon by DFG with the pump. This process repeats itself throughout themedium amplifying the signal and idler fields. The amplification is possible only if themomentum is conserved in the process

Figure 2.1: a) A three-wave mixing process. b) A singly resonant optical parametric oscillator

configuration. The symbol HRM stands for a highly reflective mirror.

kp = ks + ki, (2.3)

where kx, (x = p, s, i) are the wave vectors of the pump, signal, and idler fields. The wavevector depends on the refractive index (nx) as kx = (nxωx/c)k, where c is the speed oflight, and k is a unit vector. Since all materials are in reality dispersive, the three wavespropagate at different phase velocities corresponding to different refractive indices. Thisinduces a phase-mismatch between the fields, which can be describe as

∆k = kp − (ks + ki), (2.4)


and it is illustrated in Fig. 2.2. The symbol ∆k is called the phase-mismatch parameter.

Figure 2.2: Phase-mismatch in parametric generation.

The single pass parametric conversion efficiency of a nonlinear medium is typicallyvery small. Because of this, a parametric device usually needs a positive feedback to am-plify the generated fields. The feedback can be achieved by placing the nonlinear mediuminside an optical cavity (see Fig. 2.1b). This configuration is called an optical paramet-ric oscillator. The particular configuration, where only one of the wavelengths (in thiscase the signal) is in resonance inside the cavity, is called a singly resonant OPO. Allthe CW-OPOs used in this thesis are CW-SROs [20, 21]. In order for an OPO to reachthe oscillation threshold, the single-pass parametric gain has to overcome the round-triplosses of the cavity. A typical pump power at threshold is a few watts for a CW-SRO [20].

The CW-SROs are considered as one of the most useful configurations for moleculardetection in the mid-infrared region. Due to the CW operation, they possess narrowlinewidth which is needed for selective high-resolution spectroscopy. In Publication I, weestimated that the 1-µs short-term and 1-s long-term linewidths of our SRO are approx-imately 200 kHz and 1 MHz, respectively. The CW-SROs have also a high idler outputpower of up to several watts [20, 23], which is useful, especially, for photoacoustic spec-troscopy. But above all, they possess broad spectral coverage, which enables detectionof numerous molecules in the mid-infrared. The wavelength tuning range of all period-ically poled lithium niobate (PPLN)-based SROs used in Publications I, II, and IV isapproximately 2.7 − 3.5 µm, which covers, among other, the fundamental ro-vibrationaltransitions of formaldehyde (CH2O), hydrogen cyanide (HCN), methane (CH4), and ni-trous oxide (N2O).

2.1.3 Quasi-phase-matched lithium niobate crystal

The effect of phase dependence of the pump, signal, and idler fields inside the nonlinearmedium on the parametric process has already been mentioned briefly in the previoussection. Due to dispersion, a phase difference (∆k, see Equation 2.4) is induced betweenthe pump, signal, and idler while they travel along the medium as seen in Fig 2.2. Phasematching is a method for achieving a constant phase relationship between the gener-ated and propagating fields in order to obtain an effective parametric conversion. In thecase of a phase-mismatched situation ∆k 6= 0, the interference between the pump andother waves becomes destructive after the coherence length (Lc = π/∆k) after whichthe signal and idler waves are converted back to the pump frequency. This will createan oscillating power behaviour in the medium, which is illustrated by the bottom (red)line in Fig. 2.3. The top (purple) line presents the situation for perfect (birefringent)phase-matching (∆k = 0). The middle (green) line describes the principle of so called


Figure 2.3: The principle of quasi-phase-matching is presented by the middle line (green).

The topmost (purple) line presents perfect phase-matching. The bottom line (red) presents a

phase-mismatched situation.

Table 2.1: The properties of some commercially available periodically poled crystals. The

symbol dlm is the nonlinear coefficient for a specific crystal orientation [25]

.

Crystal dlm(pm/V) Transparency (µm)PPLN(LiNbO3) d33 = 27 0.33− 5.5

PPKTP(KTiOPO4) d33 = 10.7 0.35− 4.5PPRTA(RbTiOAsO4) d33 = 12.1 0.35− 5.8

PPLT(LiTaO3) d33 = 13.8 0.28− 5.5

quasi-phase-matching (QPM). In QPM, phase matching is achieved by reversing the crys-tal structure of the nonlinear material after every coherence length [24]. The inversionintroduces an additional phase difference of π, which makes the waves travel in phaseagain.

Lithium niobate is one the most used nonlinear materials, since it has a high nonlin-ear coefficient (see Table 2.1). In addition, it is transparent up to 5.5µm (see Table 2.1).However, its use becomes difficult above 4µm where the absorption becomes stronger(see Fig. 2.4). The quasi-phase-matching of lithium niobate is done usually by periodicpoling. In the periodic poling, the dipoles of lithium niobate are permanently re-arrangedunder a high electric field. A typical poling period of PPLN crystal for MIR (∼ 3 µm)applications is around 30 µm. Using periodical poling, QPM can be done for the crys-tal orientation which has the strongest nonlinearity. The most common commerciallyavailable periodically poled nonlinear materials and their properties are listed in Table2.1.


Figure 2.4: Transparency of lithium niobate between ∼2 and 4µm [21]. The absorption at

∼ 2.8µm is caused by OH. The measurement is carried out with the crystal used in this thesis

using a Fourier transform infrared spectrometer. The crystal is 50 mm long and its both ends

are antireflection coated for the pump, signal, and idler wavelengths.

2.1.4 Wavelength tuning

The wavelength of an SRO is determined by the energy conservation (Eq. 2.2) andthe phase-matching condition (Eq. 2.3) within the parametric gain. While the signalwavelength is kept constant inside the SRO cavity, the idler wavelength can be tunedwithin the wavelength region supported by the gain curve by scanning the pump laser.Due to energy conservation, the pump frequency tuning is transferred entirely into theidler frequency. The width of the parametric gain is typically a few hundred gigahertz andusing widely tunable pump lasers, it is possible to achieve wide continuous mode-hop freeidler tuning ranges. For example, continuous mode-hop-free frequency-comb referencedtuning of the idler frequency over 40 GHz by pump tuning is demonstrated in PublicationIV. In certain circumstances, a tuning range of up to a few terahertz can be achieved [26].

A tuning range larger than the width of the parametric gain curve requires a changein the phase-matching condition. This is typically done by changing the temperature ofthe nonlinear crystal. The temperature adjustment will change the length of the polingperiod by the thermal expansion. The effective poling period will also depend on thetemperature dependence of refractive indices. Hence, a wide gapless tuning range ispossible to achieve by combining the coarse temperature tuning of the crystal and thepump laser fine tuning [20]. A typical PPLN crystal has several poling periods in a singleor fan-out configuration for different wavelength regions (see Fig. 2.5).

In addition, the signal frequency of an SRO depends on the cavity length (Lcav). TheSRO tends to oscillate at the cavity mode that experiences the largest net gain, which istypically close to the maximum of the parametric gain curve. The parametric gain curveis usually wide compared to the spacing of the cavity modes. This is illustrated in Fig.


Figure 2.5: The signal and idler wavelengths of the SRO as a function of temperature of

periodically poled lithium niobate crystal for periods 29.0− 31.5µm [32].

2.6. The spacing between two adjacent modes is called free spectral range (FSR) and itis determined for a standing wave cavity as FSR = c/2Lcav. A typical FSR of an SRO is<1 GHz. Ambient perturbations (such as temperature variations and air flows) can causefrequency fluctuations of the cavity modes and the parametric gain, leading to mode-hops.These mode-hops can be prevented or reduced by stabilizing the crystal temperature andby covering the SRO cavity [20,27]. Often, an etalon is also placed inside the SRO cavityfor improved single-mode operation and/or for frequency tuning [28–31].

Figure 2.6: A schematic picture of cavity modes within the gain profile of the nonlinear medium.


2.2 Optical frequency comb

The frequency of electromagnetic waves in the optical region is typically hundreds of THz.Historically, absolute phase-coherent measurements of these frequencies have been chal-lenging [33]. Electronically it is possible to measure and create directly radio frequencies(RFs) up to a few hundreds of GHz. The most stable and accurate frequency standardstoday are carried out in the RF region based on the 9.2-GHz cesium and the 1.42-GHzhydrogen hyperfine transitions of which the cesium transition defines the second in the in-ternational system of units (SI). An absolute measurement of frequency must be based onthe time unit second, so the measurements in the optical region require a phase-coherentlink to the microwave region.

In the past, there were only a few optical frequencies that were known relative to theprimary standard and could be used as references for accurate frequency measurementsin the optical region. These measurements were produced using frequency chains, whichwere complicated and their use required a lot of effort. The first phase-coherent measure-ment of visible radiation was performed as late as in 1996 [34].

A direct phase-coherent link between the radio frequencies and optical frequencies be-came possible at the end of the 1990s by the demonstration of the optical frequency comb(OFC) generator [35, 36]. The spectrum of a fully stabilized OFC consists of numerousequidistant laser lines whose frequencies are known extremely accurately. The OFC allowsdirect absolute measurement of any frequency within the comb range by optical hetero-dyne detection. The optical frequency comb has revolutionized the field of metrology sinceits discovery. It has enabled measurements of fundamental constants [37,38], frequency oflasers stabilized on transitions of atoms or molecules [17], or trapped and cooled ions [18]or atoms [19]. In particular, it has provided a versatile tool for spectroscopy to synthe-size and measure optical frequencies with extremely low relative uncertainties [39–43],typically ranging from 10−13 to 10−15.

2.2.1 Principle of optical frequency comb

Consider a femtosecond mode-locked pulsed laser emitting a train of pulses with a rep-etition rate of frep. The pulsed laser has a wide spectrum due to the extremely shortpulse duration. The electric field of the pulses circulates inside the laser cavity with acarrier frequency fc and can be represented as E(t) = A(t)e−i(2πfc)t. The pulse envelopefunction A(t) is periodic in time having a period of 1/frep, where the repetition rate ofthe pulses depends on the length Lcav of the laser cavity and the group velocity vg of thepulse as frep = vg/2Lcav. In frequency space, this periodicity of pulses creates regularlyspaced frequency modes around the carrier frequency where the distance between adjacentmodes is the repetition rate of the pulses (see Fig. 2.7). However, the pulses can have aconstant pulse-to-pulse phase shift between the electric field and the pulse envelope. Thisphenomena shifts the spectrum of the comb by a factor of fCEO from zero and is referredto as carrier-envelope offset (CEO).

Taking the CEO into account the electric field can be now described as Fourier ex-pansion [44]


E(t) =∞∑

k=−∞

Ake−i2π(fCEO+nfrep)t + c.c. (2.5)

where c.c. is the complex conjugate. The Fourier transformation of this equation corre-sponds to a comb spectrum in the frequency space. The frequency of the nth mode ofthe comb can be simply described as

fn = nfrep + fCEO. (2.6)

Figure 2.7: The principle of an optical frequency comb and f − 2f self-referencing technique.

In the optical region, n is usually a large number of the order of 106. Typically the pulserepetition rate and the CEO frequency are from tens of MHz to a few GHz and can bestabilized to a radio frequency standard, such as an atomic clock. The frep can be easilymeasured with a fast photodiode, whereas the measurement of the fCEO is more difficult.It can be measured using an interferometric f − 2f self-referencing technique. The prin-ciple of this technique is schematically shown in Fig. 2.7. The comb frequency fn fromthe red part of the spectrum is frequency doubled to a frequency 2fn = 2(nfrep + fCEO).If the comb extends over an optical octave the doubled frequency 2fn can be comparedin a heterodyne measurement with the comb component f2n = 2nfrep + fCEO in the bluepart of the spectrum. The beat note of the heterodyne measurement between the fre-quency doubled mode and the mode 2n directly gives the value of the offset frequencysince 2(nfrep + fCEO) − 2nfrep + fCEO = fCEO. When the frep and fCEO are phase sta-bilized to a radio frequency standard, its accuracy is shifted to the optical region of theelectromagnetic spectrum providing the phase-coherent link between radio and opticalfrequencies. In Publication IV, the repetition rate of 100 MHz and the CEO frequency of


20 MHz of a commercial OFC were stabilized to a hydrogen maser, which has a relativefrequency stability of about 10−13 in 1 s.

The absolute frequency flaser of a laser within the comb spectrum can be determinedfrom the beat note fbeat between the laser and the closest comb line as

flaser = nfrep + fCEO + fbeat. (2.7)

The multiple integer (mode number) n can be determined from a coarse measurement witha wavemeter or by measuring the beat note with at least two different mode numbers (nand n + 1), which requires a change of the repetition frequency of the comb. However,normally this is not necessary because many laser frequencies are already known at a levelmuch better than the comb mode spacing.

The first fully stabilized optical frequency combs were based on Kerr-lens mode-lockedTitanium-sapphire (Ti:sapphire) lasers [33,35] whose spectra were broadened in a highlynonlinear microstructured fiber to cover a full octave between 500 and 1000 nm. Nowa-days, fiber laser based solutions have become more popular because they provide higherstability and are more user-friendly. The most common realizations are based on erbium-[45] and ytterbium-doped [46] fiber lasers emitting around 1550 nm and 1030 nm, re-spectively. The spectrum of such femtosecond laser can also be broadened in a highlynonlinear microstructured fiber to cover a full octave [45, 46]. The erbium-based fre-quency comb produces a spectrum in the near-infrared region between 1100-2100 nm andcan also be exploited as a frequency ruler for high spectral resolution spectroscopy inthe mid-infrared region, as described in Publication IV. Optical frequency comb assistedmid-infrared spectroscopy is discussed in more detail in section 3.4.

Chapter 3

Laser absorption spectroscopy

3.1 Absorption of light

The familiar Beer-Lambert law expresses the transmitted intensity I of the simplest directabsorption experiment (see Fig. 3.1) for a gas at frequency ν, temperature T , and atpressure p in the linear (weak field) regime according to

I(ν, LOPL) = I0exp(−α(ν, T, p)LOPL), (3.1)

where α(ν, T, p) is the absorption coefficient, and LOPL is the optical path length (OPL).The absorption coefficient α(ν, T, p) can be expressed as [47]

α(ν, T, p) = Sηη′(T )f(ν, νηη′ , T, p)N, (3.2)

where Sηη′ is the line intensity for a non-degenerate transition at frequency νηη′ betweenlower and upper states η and η′, f is the area-normalized line profile function, and N is thenumber density of absorbing molecules. The spectral line intensity for a single molecule atHITRAN reference temperature Tref = 296K for the two states of a vibrational-rotationalsystem is [47]

Sηη′(Tref ) =hνηη′

c

nηN

(1− gηnη′

gη′nη)Bηη′ (3.3)

Figure 3.1: A direct laser absorption experiment.

13

CHAPTER 3. LASER ABSORPTION SPECTROSCOPY 14

where Bηη′ is the Einstein coefficient for induced absorption, nη and nη′ are the populationsof the lower and upper states, respectively, gη and gη′ are the state statistical weights,and h is the Planck constant. The temperature dependent line intensity Sηη′(T ) can becalculated from the line intensity at reference temperature Sηη′(Tref) according to [47]

Sηη′(T ) = Sηη′(Tref )Q(Tref )

Q(T )exp(−hcEη

k(

1

T− 1

Tref))(

1− exp(−hcνηη′kT

)

1− exp(−hcνηη′kTref

)), (3.4)

where Q(T ) is the total internal partition sum, Eη is the lower state energy, and k isthe Boltzmann constant. The third term in Eq. 3.4 accounts for the ratio of Boltzmannpopulations between temperature T and the reference temperature Tref, and the last termfor the effects of stimulated emission.

At pressures below 0.1 atm (<100 mbar, <100 Torr), the frequency dependence (broad-ening) of the absorption line profile is mostly determined by the Doppler effect causedby the thermal motion of the absorbing molecules relative to the light source. At agiven temperature, different velocities determined by the Maxwell-Boltzmann distribu-tion cause different Doppler shifts. The resulting line profile is called the Doppler profile.The Doppler profile, the function f in Eq. 3.2, is defined as

f(ν, νηη′ , T ) =1

γD(T )√π

exp(−(ν − νηη′)γD(T )

)2 (3.5)

where γD(T ) is the Doppler profile’s full width at half maximum (FWHM) in frequencyunits (Hz) and it can be calculated as [48]

γD(T ) = 7.16 · 10−7νηη′

√T

M(3.6)

where M is the molar mass.An uncertainty in the energy of the states involved in the transition causes natural

broadening of the line profile, and is characterized by a Lorentzian profile. Furthermore,the collisions between molecules interrupt the emission process decreasing the lifetime(broadening the line). This pressure dependent effect can also be characterized by aLorentzian profile as

f(ν, νηη′ , T, p) =1

π

γ(p, T )

γ(p, T )2 + (ν − νηη′)2(3.7)

where γ(p, T ) is the FWHM of the Lorentzian profile. In many circumstances, the mea-sured line profile is affected by both broadening mechanisms, and can be described as aconvolution of the Doppler and the Lorentzian profiles, which results in a Voigt profile [48].The normalized (area = 1) Lorentzian, Gaussian and Voigt line profiles of equal halfwidthsare shown in Figure 3.2. For example, in the 3 µm region, at a typical pressure of ∼0.1atm used in this thesis, the Lorentzian (pressure broadening) and Gaussian FWHMs are


Figure 3.2: The area-normalized Gaussian (red), Lorentzian (blue) and Voigt (black) line

profiles of equal halfwidths.

around 300 MHz for methane. The natural linewidth of methane ro-vibrational transitionsin the MIR region is very narrow, even less than 100 Hz. Normally, the pressure broad-ened linewidth of a molecular transition in the MIR region is a few GHz at atmosphericpressure. The high resolution spectra fittings in Publication IV have been performedusing an approximate solution [49] for Voigt line profile to speed up the fitting routine.

3.2 Continuous-wave cavity ring-down spectroscopy

The sensitivity of direct absorption spectroscopy is limited by the relatively short opticalpath length. Cavity ring-down spectroscopy (CRDS) is a technique where the OPL isenhanced by using an optical cavity [8, 50–53]. In CRDS, the laser beam is coupled intoan external high-quality optical cavity, which is typically formed by two highly reflectingmirrors. The quality factor of the cavity is related to the reflectivities (R) of the mir-rors through so-called finesse F = π

√R/(1 − R) [48]. The effective optical path length

(Leff = 2FLcav/π) of the cavity can reach up to several kilometers already with a moder-ate finesse of ∼5000 and a typical cavity length (Lcav) of 0.5 m.

Originally, the cavity ring-down technique was invented for measuring the reflectivityof mirrors in the early 1980s [54], but later in the same decade O’Keefe et. al. [55] noticedits usefulness for spectroscopy. Continuous-wave CRDS was introduced by Romanini et.al. [56] in 1997. A typical CW-CRDS setup is shown in Fig. 3.3. It consists of a tun-able laser, whose output beam is passed through an acousto-optical modulator (AOM).The AOM is used as an optical switch to interrupt continuous light injection into thering-down cavity (RDC). The light leaking out of the cavity is focused by a lens on thephotodetector (PD).

The principle of CRDS is to measure the rate of absorption instead of the light inten-sity. When the light injection is interrupted, the intensity of light inside the cavity decaysexponentially with a time constant τ [54, 55], which is called the ring-down time of thecavity. The ring-down time of the empty cavity (τ0) depends only on the reflectivity of


Figure 3.3: A typical continuous-wave cavity ring-down spectroscopy setup.

the mirrors (R) and the length of the cavity (Lcav). In the presence of an absorber, thering-down time (τ) is shorter due to the molecular absorption losses (α) inside the cavity

τ =Lcav

c(1−R + αLcav). (3.8)

The spectrum is obtained from the variation of the cavity ring-down time as a functionof the laser frequency. The ring-down time does not depend on the initial power, whichmakes the CRDS independent of laser power fluctuations. The absorption coefficient (incm−1), at frequency ν, can be calculated from the ring-down time variation as

α(ν) =1

c

( 1

τ(ν)− 1

τ0

). (3.9)

The relative concentration (Crel) of a molecular species in the sample can be calculatedfrom the integrated absorption coefficient αint =

∫α(ν) dν as

Crel =αint

Sηη′(T )N. (3.10)

It is worth mentioning that Eq. 3.10 is only valid for small absorption (α < 0.05), whenthe Beer-Lambert law can be linearized. One of the benefits of CRDS is that, if theline intensity Sηη′(T ) of the probed molecular transition is known, the measurement iscalibration-free. It is clear that the sensitivity of CRDS depends on the molecular transi-tion being probed, which favours the use of strong fundamental ro-vibrational transitionsin the mid-infrared region for trace gas detection.

3.2.1 Off-axis injection with re-entrant configuration

The frequency resolution of a passive CRDS cavity is limited by the transmission combdetermined by the cavity FSR (see Fig. 3.4). Normally, the injected laser beam is mode


Figure 3.4: Schematic picture of transmission patterns of the conventional ”on-axis” (left)

and off-axis re-entrant (right) CRDS cavity within a molecular absorption profile.

matched to the lowest transverse electro-magnetic mode (TEM00) of the cavity. Theformula of eigenfrequencies νq,mn of a spherical cavity formed by two mirrors is givenby [57]

νsphericalq,mn =c

2Lcav

[q + (m+ n+ 1)θ

2π], (3.11)

where q is the longitudinal mode index, m and n are transverse mode indices θ = 2 arccos gis the Gouy phase shift of the TEM00 mode, and g = 1− Lcav/R is the geometric cavityparameter. Generally, high spectral resolution is achieved by measuring the RD-signalwith different cavity lengths [56, 58]. The cavity length is typically controlled with apiezoelectric transducer (PZT) attached to one of the cavity mirrors [56]. A small changein the cavity length shifts the eigenfrequencies according to Eq. 3.11, where already a λ/2change in length corresponds to one FSR shift in frequency. It is also clear that the spectralresolution improves if the cavity length is increased, but in many cases long cavities areimpractical to use and more sensitive to ambient noise and vibrations. A typical 0.5 mcavity length corresponds to a FSR of 300 MHz, which would give only a few measurabledata points within a molecular absorption profile at the typical measurement conditionsused in this thesis (see section 3.1 or 3.2.2).

Alternatively, the spectral resolution of a CRD-measurement can be enhanced byusing an off-axis (OA) injection technique [50, 51, 59–63]. Using the OA injection, itis possible to produce a dense or almost continuous cavity transmission comb, withoutactive cavity modulation (see Fig. 3.4). Originally, the off-axis injection was used tocreate long optical path lengths in multipass cells [64, 65]. The dense transmission combowes to the excitation of high order transverse electromagnetic modes of the cavity. Itsuse in cavity-enhanced absorption spectroscopy (CEAS) was first reported by Meijer et.al. [66] in 1994. Typically OA injection is used for integrated cavity output spectroscopy(ICOS) [51,60], but it has been utilized for cavity ring-down spectroscopy as well [51,52].A real advantage of this approach is that the dense mode pattern enables rapid trace gasdetection with high spectral resolution using fast tunable lasers [52,60].


The theory of off-axis technique can be derived by geometrical optics [64,65,67] or waveoptics analysis [68]. The geometrical approach is presented in more detail in PublicationI. In normal CRDS, the beam is injected into the cavity along the optical (on)-axis andthe beam repeats itself every round trip. This is not the case in off-axis injection. Thepropagation of the beam during one round trip of the off-axis cavity can be described bya transfer matrix T, where the position xN and direction (angle ϕN) on the rear mirrorafter N round trip can be expressed as

(ϕNxN

) = TN(ϕ0

x0

), (3.12)

where x0 and ϕ0 are the beam initial position on the rear mirror and the angle betweenthe ray and the optical axis, respectively. The off-axis injected beam repeats itself onthe rear mirror after N round trips if the transfer matrix TN is identity matrix I. Thenumber N is called the re-entrant order of the chosen cavity configuration. This happensif the cavity length is chosen as

Lre-entrant = R[1− cos(Kπ

N)], (3.13)

where K represents how many times the ray trajectory cycles around the optical axis.With the re-entrant length, the eigenfrequency formula becomes

νsphericalq,mn =c

2LN[Nq + (m+ n+ 1)K]. (3.14)

If the re-entrant condition is fulfilled, the cavity length provides degeneracy of transversemodes where every mode family is separated by K/N . This divides the FSR to N groupsof degenerated modes which are equally frequency-spaced [67]. Due to the exact re-entrantlength (Lre-entrant) of the RD-cavity, the off-axis technique provides a calibrated frequencyaxis with a better spectral resolution than the conventional on-axis injection technique.

In the MIR region, the off-axis injection has previously been used only for the in-tegrated cavity output spectroscopy (ICOS) [59–63]. In this thesis, off-axis re-entrantCRDS has been utilized for the first time in the MIR. A detailed description of the de-veloped measurement setup (Fig. 3.5) can be found in Publication I. In short, a simpletwo-mirror CW-SRO was used as the light source [20, 21]. The OPO pump laser systemhas a fast (up to 750 GHz s−1) mode-hop-free tuning range of ∼100 GHz at 1064 nm.The SRO provides a high output power (>0.5 W) in the wavelength region 2.75 − 3.45µm. The high SRO power is useful for reaching a good signal to noise ratio (SNR) in themeasurement, since the optical power is divided equally between the generated modes,each group of modes having 1/N2 of the initial power [67].

An acousto-optic modulator (AOM) was used as an optical switch to interrupt lightinjection into the cavity so that a ring-down signal could be recorded. The ring-downcavity was formed by two concave mirrors which were connected to each other with atube of flexible stainless steel. The re-entrant length of the cavity is 0.293 m (K = 1 andN = 4). The purpose of the ”mode-matching” optics (M1 and M2) was to reshape thebeam profile so that it would excite as many high order TEMnm (n,m > 0) modes of thering-down cavity as possible [69]. The MIR beam leaking out of the cavity was focused on


Figure 3.5: Schematic picture of the off-axis re-entrant CRDS experimental setup. Abbrevi-

ations: FI - Faraday isolator, HWP - half-wave plate, PBS - polarizing beam splitter, L1-L7 -

lenses, HR - OPO mirror, MgO:PPLN - MgO-doped periodically poled lithium niobate, DM -

dichroic mirror, GM - gold mirror, WBS - wedge beam splitter, AOM - acousto-optic modulator,

PH - pinhole, M1-M4 - mirrors, RDC - ring-down cavity, PD -photodetector [Publication I].

a fast photodetector (Vigo system, PVI-2TE-5/VPDC-20I) and the detected ring-downsignal was fitted by the least-squares method to a single exponential function using theLevenberg Marquadt algorithm in a LabVIEW program.

3.2.2 Formaldehyde detection with off-axis re-entrant CRDS

The instrument described in the previous section can be used to record a spectrum withgood resolution and high speed by scanning the fast tunable laser over several FSRs ofthe cavity. Since the cavity length is fixed, the system is simple compared to, for example,laser-locked CRDS instruments [70, 71], where the cavity length is actively locked to thelaser frequency. In Publication I, the performance of the off-axis spectrometer was studiedby measuring a mid-infrared spectrum of formaldehyde at 2943.08 - 2943.24 cm−1 (3397.62- 3397.80 nm). For this, we used a mixture of 10 ppmv (parts-per-million by volume) ofH2CO in nitrogen. Because of strong adsorption of H2CO on the stainless steel surfacesof the ring-down cavity, the measurements were carried out in a continuous gas flow. Themeasurements were recorded at room temperature and at a sample pressure of about0.1 atm. The FWHM of a pressure-broadened peak of formaldehyde at this pressure andwavelength is about 1 GHz which would results in only two measurable data points withinthe absorption profile using a passive 0.293 m long on-axis CRDS system.

Experimental results for the performance of the off-axis re-entrant CRDS spectrometerand formaldehyde detection are given in Publication I. The most important conclusionsof the study can be summarized as follows:

• The off-axis injection divides the original FSR of 500 MHz into four groups ofdegenerated modes having the effective free spectral range (FSReff = c/2LN) of 125MHz, which gives the needed spectral resolution for fast trace gas detection.


• The re-entrant condition of the cavity was found to be sensitive to deviations of thecavity length from the optimum re-entrant length. Already a deviation of ∆L ∼250 µm caused splitting of the degenerate transverse cavity modes. The splittingdecreases the sensitivity of the spectrometer due to the non-uniform excitation ofthe cavity modes.

• The optimum trigger level for AOM switching depends on the splitting of transversemode degeneracy caused by mirror astigmatism and other imperfections. With lowtrigger levels, the ring-down distributions depend on the direction of the frequencyscan. Due to the imperfect cavity mode degeneracy, the timing of the excitation ofthe different modes is not uniform.

• A detection limit of ∼75 ppb for formaldehyde at 2943.176 cm−1 with a signalaveraging time of 1 s was achieved. This results noise equivalent absorption (NEA)coefficient of ∼2.5 × 10−8 cm−1 Hz−1/2.

• With the fast frequency tuning of the CW-SRO, we were able to measure theformaldehyde spectrum during a fast single 5 ms long sweep of the idler frequencyover the wavenumber region 2943.12− 2943.22 cm−1. This corresponds to a repeti-tion rate of 4.6 kHz for the ring-down events. An example of a measured formalde-hyde spectrum at 2943.176 cm−1 is shown in Fig. 3.6.

Figure 3.6: A fast 5 ms scan of 10 ppm H2CO spectrum at a pressure of 0.1 atm [Publication

I]. The red line is the least-square Voigt line shape fit to the measured data points.

Moreover, with the developed spectrometer, there is no significant difference betweenthe sensitivity whether the injection is on-axis or off-axis. The achieved NEA coefficient


is of the same order of magnitude as to what has previously been reported for other mid-infrared cavity-enhanced absorption spectroscopy based spectrometers [72, 73] and otherfast-scanning off-axis CRDS setups [52]. The achieved detection limit for formaldehydeis about two times better than the previously reported detection limit of 150 ppb in 3 susing an off-axis integrated cavity output spectroscopy with an interband cascade laserat 2832.485 cm−1 [63]. Also note that the line strength of the formaldehyde transitionat 2832.485 cm−1 is approximately two times stronger than that the one used by us at2943.176 cm−1. Thus, the detection limit of our spectrometer can be increased by shiftingthe detection to the longer wavelengths, which was not possible with the ring-down-mirrors used in this study.

Although the sensitivity of our spectrometer is comparable to other simple off-axisCRDS and CEAS setups, a lower data-rate normalized minimum detectable absorptioncan be reached with other simple CW-CRDS techniques in the near-infrared. For example,using rapidly swept cavity [74] or laser [75] designs. In conclusion, the achieved detectionlimit of 75 ppb for formaldehyde is about 10 times higher than that required for breathand indoor air studies, since the ambient concentration of formaldehyde is ∼10 ppb. Inaddition to the purchase of better mirrors, the required enhancement in the sensitivitycould be achieved by improving the mechanical stability of the re-entrant cavity and byincreasing the scanning speed of the OPO.

3.3 Cantilever-enhanced photoacoustic spectroscopy

Photoacoustic (PA) phenomenon is based on the conversion of optical energy into heat.It was discovered by Alexander Graham Bell in 1880 [76]. The PA signal originates fromthe absorption of light, which excites the ground state molecules to higher energy levelsin the sample matrix. The relaxation of these excited states can happen either radiativelyby emitting photons or nonradiatively trough collisions with other molecules or atoms.In the gas phase, the time between collisions is typically shorter than the radiative lifetime of the excited transition. Therefore the optical energy is converted into translationalenergy in the sample, which leads to thermal expansion and produces a pressure wavethat propagates away from its origin. This pressure wave can be detected by a pressuresensor, such as a microphone. When the amplitude of light is modulated, an acousticsignal is generated at a frequency equal to that of the modulation. For small absorptions,the amplitude of the photoacoustic signal S is given by [77]

S = P ·R · α · pa, (3.15)

where P is the optical power, α is the absorption coefficient, pa is the partial pressure ofthe sample, and R describes the responses of the PA cell and the microphone.

The amplitude of the PA signal depends linearly on all of the variables in Eq. (3.15).Therefore, high-power sources like CW-SROs combined with PAS permit high sensitivityin the mid-infrared region where the absorption is strong [1, 30, 78–81]. Such measure-ment systems enable trace gas detection in sub-ppb concentration levels. However, thesensitivity of commonly used condenser microphones has already been pushed close tothe fundamental limit. In recent years, the development in PAS research has led to noveldetection schemes, where the microphone is replaced with new type pressure sensors,


such as a quartz tuning fork [82], an optical fiber based microphone [83], and an opticalcantilever microphone [84]. In this thesis, a cantilever based photoacoustic detection iscombined, for the first time, with a CW-SRO operating in the mid-infrared region, aswell as with a commercial high power continuous-wave laser at 532 nm. The review ofthe cantilever principles and the results of the trace gas measurements accomplished withcantilever based photoacoustic spectrometers are presented in the following sections.

3.3.1 Optical cantilever microphone

The cantilever-enhanced photoacoustic spectroscopy (CEPAS) is based on an extremelysensitive miniature silicon cantilever microphone introduced by Kauppinen et al. [84,85].A schematic picture of a microfabrication manufactured cantilever structure is presentedin Fig. 3.7. The typical length (lc) and width (w) of the cantilever are ∼5 mm and ∼ 1−2mm, respectively. The cantilever has a thickness h of 5− 10µm, and it is separated of itsthicker frame with small gap (∆ = 3 − 5µm) from three sides. A pressure wave makesthe cantilever move like a pendulum. The movement is about two orders of magnitudelarger than that of a conventional membrane microphone. The response of the cantileveris also more linear, because the cantilever is connected to its frame just on one side, andthus only the bending motion is excited.

The movement of the cantilever is measured using a Michelson type laser interfer-ometer, which is schematically shown in Fig. 3.8. A phase detection technique thatutilizes multiple photodetectors enables measurement of over 2π phase differences [86],which increases the dynamical range of the optical microphone. The phase change of2π corresponds to a displacement of λ/2 of the cantilever, where λ is the wavelength ofthe interferometer laser. A typical cantilever has a resonant frequency around 600 Hz,depending on its effective mass and string constant, and the measurement conditions [87].The frequency response of the cantilever is virtually constant below the resonant fre-quency (see Fig. 3.9). This means that the modulation frequency can be chosen freelybelow the resonance frequency. In many cases, the best SNR is achieved with modulationfrequencies between 10 and 100 Hz [86].

Even though CEPAS is a rather new technique, it has already been used with manydifferent light sources in the near- and mid-infrared region, such as black body radia-tors [85, 88], tunable diode lasers [89–92], MIR light-emitting diodes (LEDs) [93], andQCLs [94, 95]. In the previous studies, high sensitivities and excellent trace gas de-tection limits have been achieved for many molecules. For example, a carbon dioxidedetection limit of 300 ppb has been reported using a tunable diode laser in the near-infrared [91]. In the MIR, sub-ppb detection of formaldehyde has been reached with aQCL [95]. The best reported normalized noise equivalent absorption (NNEA) coefficient(3σ) with CEPAS spectroscopy is 3.4 ·10−10 W cm−1 Hz−1/2, which was achieved by mea-suring carbon dioxide at 1568.78 nm using a tunable diode laser [92]. Nevertheless, inorder to reach extremely high detection sensitivities, a high power laser source is needed.Hence, in Publication II, we have combined cantilever-based detection with a CW-SRO.We show that this combination is capable of highly sensitive trace gas detection in themid-infrared owing to the high watt-level output power of the CW-SRO. This study andits results are summarized in the following section.


Figure 3.7: Schematic picture of a cantilever.

Figure 3.8: A Michelson type laser interferometer.


Figure 3.9: Experimental frequency response of a silicon cantilever microphone without mod-

ulation (black) and with 90 Hz modulation of the optical power (red). The wide peak at around

600 Hz is the resonant peak of the cantilever. In addition to the sharp peak at the modulation

frequency of 90 Hz, peaks at the harmonics of the modulation frequency are also observed.

3.3.2 Detection of HCN and CH4 in the mid-infrared

The SRO-CEPAS system was used to measure HCN and CH4 in the mid-infrared. Themeasurement setup presented in Publication II is shown in Fig. 3.10. The CW-SRO wassimilar to the one described in section 3.2.1 [20,21] except that the SRO cavity was a fourmirror bow-tie ring cavity. The wavelength of the CW-SRO was tunable between 3.0 and3.4 µm (3330 to 2950 cm−1), where the single-mode idler output power was over 0.5 W.The long-term linewidth of the idler beam was of the order of 1 MHz. The idler wave-length was monitored with a wavemeter. Most of the idler power was focused through acommercial cantilever-enhanced photoacoustic analyser (PA201) manufactured by GaseraLtd. The PA cell was 95 mm long and 4 mm in diameter with a total volume of about7 ml. The transmitted SRO power was monitored after the analyser by a power meter(PM). The SRO wavelength was modulated at the frequency f = 70 Hz. The PA signalwas detected at the harmonic frequency 2f . The measurements were recorded at a sampletemperature of 50◦C and at a sample pressure of ∼400 mbar (∼0.35 atm).

Photoacoustic spectroscopy is not an absolute absorption technique and needs to becalibrated with a reference gas. The calibration measurements were carried out by record-ing the photoacoustic signal with five different HCN concentrations, ranging from 8 ppb to320 ppb. The calibration curve in Publication II shows a linear behaviour of the CEPASsignal as a function of HCN concentration measured at the center of the HCN transitionat 3331.58 cm−1. With the CW-SRO power of 0.5 W, a detection limit (1σ) of 190 ppt in1 s was reached, which is, to our knowledge, the lowest HCN detection limit ever reportedusing PAS. This yields an NNEA of 1.8 × 10−9 W cm−1 Hz −1/2. Hydrogen cyanidehas also been measured with an OPO based PAS system [96]. In this study, a detection


Figure 3.10: A schematic picture of the SRO-CEPAS setup.

limit of 400 ppt in 10 s at 3287.25 cm−1 was reached. In addition, hydrogen cyanide hasbeen measured with another novel PAS system. A detection limit (1σ) of 155 ppb in 1s yielding an NNEA of 4.3 × 10−9 W cm−1 Hz−1/2 has previously been reported using aPAS method where a quartz tuning fork is used as a pressure sensing element [97]. Inthis quartz enhanced photoacoustic spectroscopy (QEPAS) study, a 50 mW telecom diodelaser was used to measure an HCN line at 6539.11 cm−1.

The results reported in this thesis and in Publication II show the potential of our CW-SRO based CEPAS for significant enhancement in the sensitivity of PAS gas detection.However, it is worth noting that the CW-SRO power fluctuations become the dominantnoise source in the measurement system when measuring high concentrations. This isillustrated in Fig. 3.11, where the behavior of the signal to noise ratio is shown as afunction of PA signal level. The effect of SRO intensity noise is small or comparable withother noise sources up to 50 ppb and the SNR increases linearly. After that, the slope ofthe SNR starts to decrease and saturates at around 200 with high HCN concentrations.Consequently, this behavior is not critical for the performance of the measurement systemwhen measuring low concentrations.

Figure 3.12 shows the result of a stability measurement of the SRO-CEPAS system.We used Allan deviation [98] to investigate of the optimum averaging time of the SRO-CEPAS system. Because of strong adsorption of HCN on the gold coated surfaces ofthe photoacoustic cell, we used methane for this measurement. The measurements weredone by measuring the CEPAS signal at the center of the methane absorption line at3057.68 cm−1. A detection limit of 65 ppt with an averaging time of 30 s was reachedwhen the CW-SRO power fluctuations were compensated. This is ∼4 times better thanwhat can be obtained without the compensation (Fig. 3.12). In comparison with ourresults, a detection limit of ∼200 ppt with an averaging time of 100 s has been previouslyreported for methane using a CEPAS setup with a black body radiation source [85]. Tothe best of our knowledge, our detection limit is approximately by a factor of three betterthan the best previously reported photoacoustic spectroscopy measurement done with amid-infrared CW-OPO and a resonant cell [78].


Figure 3.11: Signal to noise ratios (SNRs) for different PA signal levels at the center of the

absorption peak at 3331.584 cm−1 with an OPO power of 0.6 W. The inset shows the SNR over

a wider PA signal level range. The red slope is a linear least-square fit to the first three points

of the SNR vs. the PA signal level plot [Publication II].

Figure 3.12: Allan deviation plots of the CEPAS system with compensation of OPO power

fluctuations (red triangles) and without the compensation (black squares). The lower panel shows

the measured PA signal without the power compensation [Publication II].


3.3.3 Detection of NO2 using a high power green laser

Usually the best detection limits of trace gases are achieved by measuring their fundamen-tal ro-vibrational transitions in the MIR region. However, usually, electronic transitionsin the visible region are much stronger and, therefore, provide better sensitivity. One suchexample is nitrogen dioxide (NO2), which has almost two orders of magnitude strongerabsorption at around 400 nm than in the MIR region at 3.4 µm. The fundamental ro-vibrational transitions at around 6.1 µm are even almost five times weaker than the visibletransitions. Publication III reports a CEPAS system, which is applied to NO2 detectionin the visible region. Nitrogen dioxide is a highly reactive gas and also an air pollutantthat is released to the atmosphere mainly by combustion processes. The troposphere NO2

concentration varies depending on the geographical location. It is typically a few tens ofppb, but can be in the sub-ppb level in clean areas and rise up to the ppm range in highlypolluted areas. Since nitrogen dioxide has a wide and strong absorption spectrum in thevisible region between 250 and 650 nm [99], a commercial high power continuous-waveintra-cavity frequency doubled Nd:YVO4 laser at 532 nm was used as a light source. Itis also worth mentioning that nowadays there are commercially available compact andaffordable green lasers with relatively high output powers. Such a laser combined withthe simple and sensitive measurement system described in this thesis provides an NO2

sensor, which is capable of in situ measurements with high sensitivity.The experimental setup (Fig. 3.13) of the CEPAS system used Publication III is simi-

lar to the one described in Publication II. The same CEPAS unit model (Gasera, PA201)was used. The laser has a maximum optical output power of 10.5 W. The amplitude ofthe laser output was modulated at a frequency of 90 Hz using a chopper with a duty cycleof 50%. The measurements were recorded at room temperature (23◦C) and at a samplepressure of 360 mbar (∼0.35 atm).

The results of the experimental study for the performance of the CEPAS spectrom-eter are given in Publication III. The most important conclusions of the study can besummarized as follows:

• With the maximum laser power of 4.7 W, an NO2 detection limit (1σ) of 50 ppt in1 s was achieved.

• A linear increase of the CEPAS signal was observed within the entire optical powerrange without any indication of photochemical dissociation of NO2.

• A normalized noise equivalent absorption coefficient of 2.6 × 10−10 W cm−1 Hz−1/2

was obtained.

To our knowledge, the achieved NO2 detection limit of 50 ppt (1 s) is the best that has beenreported using photoacoustic spectroscopy [100]. Our excellent detection limit is mainlydue to large electronic transition moment and the stable high power laser operating inthe region where the photochemical dissociation does not occur [101]. Moreover, the highstability of the system makes it possible to improve the detection limit to as low as a fewparts-per-trillion by using longer averaging times [Publication III]. Altogether, the NO2

detection limit demonstrated here is among the best ever reported, even when comparingto more complex laser spectroscopy methods. For example, a detection limit (2σ) of22 ppt in 1 s has previously been reported using cavity ring-down spectroscopy in the


Figure 3.13: A schematic picture of the SRO-CEPAS setup [Publication III].

blue region [11]. With a more conventional NO2 detection setup based on laser inducedfluorescence, a detection limit (1σ) of 15 ppt in 10 s has been reported [102].

3.4 Optical frequency comb assisted mid-infrared spec-

troscopy

It is difficult to exaggerate the impact of the optical frequency comb on absorption spec-troscopy. The OFC is a versatile light source that provides excellent frequency accuracyand high spectral purity within a broad spectral region. As mentioned in section 2.2.1,optical frequency combs are commonly based on mode-locked Ti:sapphire lasers (0.5-1.1µm) or mode-locked Er-doped fiber lasers (1-2 µm). They can be directly applied tospectroscopy, enabling measurements of broad spectral spans at once [39, 103, 104]. Anindividual comb mode can also be applied to probe narrow (< frep) one- or two-photonexcited atomic transitions [105, 106]. High finesse optical cavities have been used to in-crease the sensitivity of direct OFC spectroscopy [40, 107–110]. Dual comb spectroscopyutilizes the combination of two frequency combs with slightly different repetition rates asan interferometric setup [111,112], which allows fast multiheterodyne spectroscopy over abroad spectral range. In turn, a tunable single-frequency laser referenced to an OFC canbe used to measure gas absorption line parameters, such as line strength, center frequency,width, and line broadening coefficients with high resolution and accuracy [41,43,113,114].

The extension of mode-locked laser based frequency combs to the MIR region hasproven difficult [115]. Although a mid-infrared comb can be realized using nonlinearfrequency conversion starting from a near-infrared frequency comb [116–129], these tech-nologies are still rather complex. Thereby, MIR high precision spectroscopy is often doneby referencing the MIR laser source to an NIR OFC via nonlinear frequency conversion.Figure 3.14 presents three often used approaches based on the frequency conversion in asecond order nonlinear crystal.

Difference frequency generation between two CW NIR laser sources produces MIRradiation. Since the energy is conserved in the DFG process (see Eq. 2.2), the absoluteMIR (idler) frequency can be determined by measuring the pump and signal frequencieswith an OFC [130,131]. An advantage of DFG-based methods is that the carrier-envelope


Figure 3.14: Schemes for referencing a mid-infrared continuous-wave laser source to an OFC,

based on nonlinear optical frequency conversion. The abbreviations DFG, SFG, and OPO stand

for difference frequency generation, sum frequency generation, and optical parametric oscillator,

respectively.

offset frequency of the comb is cancelled and the frequency of the idler can be determinedas fDFG = (np − ns)frep + fp

beat − f sbeat. The np and ns are the numbers of the closest

comb modes of the pump and signal frequencies, respectively, and fpbeat and f s

beat are thecorresponding beat frequencies of the pump and signal between the closest comb modes,respectively. Hence, only the repetition frequency of the comb needs to be stabilized in or-der to measure the absolute frequency of the MIR beam. The DFG-based frequency-combreferenced spectrometers have been used, for example, to measure absolute frequenciesof CO2 transitions near 4.2 µm [130] and CH4 around 3.4 µm [131–133]. These high-precision spectroscopy setups are typically based on PPLN crystals, whose transparencystarts to decrease above 4 µm (see Fig. 2.4). In addition, the optical power produced byCW-DFG in the mid-infrared is typically in the sub-milliwatt range.

In longer wavelengths, including most of the molecular fingerprint region, a differentreferencing scheme, such as SFG [134–143], is typically used. Commonly, the referencingprocess is implemented by taking a narrow part of the comb spectrum or a comb-referencedCW-NIR laser, which is then mixed, for instance, with the light of a QCL [134–140] or aCO2 laser [141–143]. The SFG process produces a new frequency within the spectral regionof the near-infrared comb. The SFG method is also independent of the carrier-envelope-offset frequency. Most spectroscopic studies with comb-referenced QCLs have been carriedout around the 4.5 µm region [136–140]. However, the SFG referencing scheme has alsobeen applied to phase-lock a QCL [135] at 9 µm and a CO2 laser [141, 142] at 10 µm toan OFC.

The absolute frequency measurement of the idler beam of an OPO is usually doneby measuring the frequencies of the near-infrared pump and signal beams with an OFC[144–147]. Often, the signal and pump frequencies are phase-locked to the comb in whichcase the wide scanning range of the idler is difficult to reach. The scanning of the rep-etition rate of the OFC results in scanning of both pump and signal frequencies, andsince the signal is resonating inside the OPO cavity with a typical FSR of < 1 GHz, themode-hop free tuning range is limited approximately to a FSR/2. This problem can beavoided by using an electro-optic modulator for scanning only one of the offset-lockedfrequencies, but even then the continuous scanning range of the MIR frequency is small,typically a few tens of MHz [144]. A much wider tuning range has been demonstrated only


Figure 3.15: A novel method based on frequency doubling (second harmonic generation, SHG)

of the MIR beam for referencing an OPO to an OFC.

recently, using a novel method that is not limited by the tuning range of the electro-opticmodulator [148].

In Publication IV, a novel scheme for referencing the idler frequency of a CW-SROto a fully-stabilized NIR OFC is presented. The principle of the method is shown inFig. 3.15. The idler frequency is frequency doubled to the near-infrared in an MgO-doped PPLN crystal, after which its frequency can be directly referenced or locked to theNIR OFC. The benefit of the method is that now only one beam needs to be locked tothe comb. However, this method requires a fully stabilized OFC where also the carrier-envelope-offset frequency is known. While the doubled frequency is locked to the OFC,the frequency of the idler beam can be simply tuned by scanning the repetition rate ofthe comb allowing for wide continuous tuning ranges.

3.4.1 Frequency-comb-referenced CW-SRO for precision spec-troscopy

The experimental setup presented in Publication IV is shown in Fig. 3.16. The design ofthe CW-SRO was similar to that described in section 3.3.2. Coarse tuning of the MIRidler wavelength between 2.7 and 3.4 µm is done by changing the poling period and tem-perature of the crystal. Mode-hop-free fine tuning of the MIR frequency over 100 GHzcan be done by tuning the frequency of the pump laser system. The single-mode idleroutput power is about 0.5 W with a 1-s linewidth of ∼1 MHz. The frequency-doublingsystem used in the Publication IV allows the doubling of the idler frequency within thewhole wavelength tuning range of the CW-SRO. The frequency-doubled idler beam wasguided through an optical fiber link to a beat frequency measurement with a commercial,fully stabilized OFC (Menlo Systems FC1500-100). The scanning of the MIR frequencyof the SRO was done by adjusting the repetition rate of the OFC. The repetition rateand the offset frequency of the OFC were referenced to an active hydrogen maser, whichwas traceable to the SI-second. The OPO MIR frequency was locked to the OFC bystabilizing the beat note between the frequency-doubled idler beam and the OFC to apre-set value (typically 20 MHz) by applying a correction from a proportional-integral(PI) controller to the OPO pump laser frequency. The bandwidth of the PI control wasa few tens of hertz, which was limited by the shortest available gate time (4 ms) of the


frequency counter that was used to measure the beat frequency.

Figure 3.16: The experimental setup of frequency-comb-linked SRO based cavity ring-down

spectrometer

3.4.2 Precision spectroscopy of N2O and CH4

The frequency-comb referenced CW-SRO was used as a light source for high-precisioncavity-ring down spectroscopy of N2O and CH4. The CRD spectrometer is briefly de-scribed below, and more details can be found in Publication IV. A small fraction of theidler beam was on-axis coupled into a 0.5 m long ring-down cavity formed by two highlyreflecting concave mirrors (see Fig. 3.16). The ring-down signal was recorded by a pho-todetector, after switching off the MIR beam incident on the ring-down cavity using anAOM. The driving signal of the AOM caused a 50 MHz frequency shift to the MIR beam,which was directed to the CRD measurement. The 50 MHz driving signal was referencedto the same time base (hydrogen maser) as the OFC. For high frequency resolution, thering-down cavity frequency was matched at every measuring point to the OFC referencedidler frequency by modulating the length of the cavity using piezoelectric actuators. Themodulation frequency was typically 10 to 30 Hz and the recorded ring-downs were syn-chronized with the OFC-measurement of the MIR frequency.

The spectrometer is capable of large, mode-hop-free scans while maintaining lockingof the idler frequency of the SRO to the OFC. As an example, a 40 GHz wide spectrumof an N2O sample at 2.85 µm in the presence of water was measured (Fig. 3.17a). Thelarge tuning range of the spectrometer makes frequency calibrated measurements of sev-eral transitions possible in a single scan in the wavelength region below 4.0 µm whereQCLs are not yet available. For comparison, a relatively wide phase-locked QCL tuning


Figure 3.17: a) A continuous scan of 40 GHz measured with a frequency comb locked OPO.

(b) The R44e transition of the ν1 + ν3 band of N2O at a pressure of 100 mbar [Publication IV].

range of ∼ 20 GHz has previously been demonstrated at 4.3 µm [140]. True phase- orfrequency-locking to an OFC, in many cases, increases the measurement time of the spec-trum considerably. For example, the measurement presented in Fig. 3.17a took aboutone hour. The same measurement could be done much faster by reducing the number ofacquired data points, or by performing the measurement without locking the SRO to thecomb. These kind of OFC-referenced (but not locked) spectroscopic studies have beendemonstrated with QCLs [136, 139]. Knabe et al. [139] achieved a tuning rate of ∼ 0.9GHz/s by referencing an external-cavity QCL at 4.5 µm to an NIR OFC using a SFG-based referencing method.

As has already been mentioned, accurate calibration of the frequency axis is neededfor the precision measurements of absorption line parameters, such as the line intensityor center frequency. While the frequency axis is calibrated with the OFC, the absorbanceaxis can be accurately measured by cavity ring-down spectroscopy (see Section 3.2). Inother words, the line intensity of, e.g., a molecular transition can be measured usingCRDS by measuring the spectrum of a known concentration sample in a known mea-suring condition. The line intensity at the measurement temperature can be calculatedfrom the determined integrated absorption coefficient of the measured spectrum using Eq.3.10 while the line intensity at HITRAN reference temperature Tref = 296 K can then becalculated using Eq. 3.4.

Figure 3.17b shows a measurement of the line intensity of the R44e transition of theν1 + ν3 band of N2O. In the spectrum, each frequency point (black marker) is an averageof 50 ring-down events recorded during 5 s and the solid (red) line is a least-square Voigtfit to them. The line intensity deduced from the integrated absorption coefficient of thefitted spectrum was Sηη′(296 K) = 2.7472 × 10−21 cm−1/(molecule/cm−2). The observedvalue deviated only 0.25 % from the HITRAN database value of Sηη′(296 K) = 2.7540× 10−21 cm−1/(molecule/cm−2 [99]. The uncertainty of the HITRAN value is between


2 and 5%. The estimated total uncertainty of our line intensity measurement was 3.4%.The biggest contribution to the uncertainty comes from the fitting procedure (3.0%). Wealso tried to fit with the Galatry line profile [149–151], which takes into account the effectof collisional, so-called Dicke narrowing, but the residual of the fitted profile showed nosignificant difference compared to the Voigt fit (see Fig. 3.17b). The uncertainty budgetincludes the statistical uncertainty of the ring-down fitting of 0.3%, mole fraction of 1.5%,temperature of 0.3%, and the pressure gauge of 0.2%.

The line intensity measurement was also part of an intercomparison of mole frac-tion measurements of N2O within the European Metrology Research Programme project”EUMETRISPEC”. Four national metrology institutes (NMIs) from Germany (PTB),Netherlands (VSL), Denmark (DMF), and Finland (MIKES) were represented. The pur-pose of the intercomparison was to show that reliable molecular line data is important foraccurate determination of mole fractions of molecules in the gas phase. In the comparison,each NMI measured the line intensity with their own spectrometer after which it was usedfor the determination of the mole fraction of a gravimetrically prepared reference sample.Our measurement was in good agreement with the reference value within the reporteduncertainty.

In Publication IV, we also demonstrated the capability of the OFC-locked CRD spec-trometer for saturated spectroscopy by measuring R8 transition of the ν3 band of methaneat 3.2 µm. Owing to the high-finesse ring-down cavity, the intracavity power easily reachedthe watt-level even with modest input powers. High intracavity power is sufficient tosaturate the strong fundamental rovibrational transitions of many molecules within thetuning range of the spectrometer. As an example, Fig. 3.18 shows the Lamb dip (satu-

rated absorption feature) of the F(2)1 R8 component of CH4 transition. The high spectral

resolution at the center of the transition was achieved by scanning the repetition rate ofthe OFC in steps of ∼ 0.5 − 1.0 Hz (∼ 0.5 − 1.0 MHz in the mid-IR). In the wings ofthe transition, the frequency step was larger. The saturation of the absorption allowed usto carry out measurements with sub-Doppler resolution, which significantly improved theprecision in determining the absorption line center. The line center determined from theLorentzian profile fit to the sub-Doppler spectrum was 93 064 092 684 662.80 Hz, whichis in good agreement with the previously reported values for the same transition. Themeasured value is about 530 kHz smaller than an OFC-referenced value reported by Abeet. al [133] and 2.7 MHz smaller than the value reported in HITRAN [99].

Experimental results of the performance of the frequency-comb referenced CRD-spec-trometer are given in more detail in Publication IV. The most important conclusions ofthe study can be summarized as follows:

• The frequency-doubling method for referencing an MIR CW-SRO to an OFC iscapable of large mode-hop-free scans while maintaining locking of the MIR frequencyto the OFC.

• The maximum mode-hop-free scanning range of the MIR frequency is 410 GHz,which is limited by the repetition rate of the comb. In practise, it was limited to100 GHz due to the SRO pump laser tunability.

• The Allan deviation of the OFC-locked idler frequency showed the achieved fre-quency stability of about 50 kHz in 1 s, which corresponded to a relative frequencystability of ∼4 × 10−10.


Figure 3.18: A spectrum of a Doppler broadened F(2)1 R8 component of CH4 with a Lamb dip

at the center of the transition at the pressure of 1 mbar [Publication IV].

• The noise-equivalent absorption coefficient of the CRDS spectrometer was 9.8 ×10−8 cm−1 Hz−1/2, which is of the same order of magnitude as the sensitivity of theoff-axis CRDS spectrometer presented in Publication I.

• In addition to the accurate frequency axis calibration, the spectrometer also pro-vides an accurate absorbance axis owing to CRDS, and is suitable for saturatedspectroscopy.

In addition, the required beat-note SNR for reliable frequency counting could be ob-tained with an MIR power of as low as 100 mW, which is already in the achievable powerregion of current QCLs and interband cascade lasers between the 3 and 4 µm region [152].

Chapter 4

Conclusions

Four applications of two different spectroscopic methods have been demonstrated andstudied in the laboratory for high sensitivity molecular gas-phase detection and high-precision spectroscopy. Most of the studies were carried out in the mid-infrared regionwith the exception of nitrogen dioxide detection in the visible. The mid-infrared experi-ments utilized continuous-wave singly resonating optical parametric oscillator (CW-SRO),a coherent high power light source in the mid-infrared wavelength region between 3 and4 µm, which is one of the most important spectral regions for molecular spectroscopy.The demonstrated spectroscopic applications are based on cavity ring-down spectroscopyand photoacoustic spectroscopy, including, the first off-axis re-entrant CRDS experimentin the mid-infrared, two cantilever enhanced photoacoustic spectroscopy setups, and ahigh-precision CRDS spectrometer.

In this thesis, measurement of numerous trace gases with extremely high sensitivitywas achieved by combining a versatile CW-SRO with state-of-the-art spectroscopic meth-ods. As an example, a detection limit of 75 parts-per-billion in 1 s for formaldehydewas demonstrated by using off-axis re-entrant CRDS in the MIR. The off-axis re-entrantconfiguration provides a simple system for rapid trace gas detection with high spectralresolution without modulation of the ring-down cavity. Moreover, the high output powerof the CW-SRO in the MIR shows great potential for significant enhancement of the sen-sitivity of trace gas detection by cantilever-enhanced photoacoustic spectroscopy. Usingthe CW-SRO based CEPAS setup developed in this thesis, noise equivalent detectionlimits of 190 ppt in 1 s and 65 ppt in 30 s were achieved for HCN and CH4, respectively.

In this thesis, a new method for referencing a MIR CW-SRO to an optical frequencycomb was demonstrated. The method is based on frequency doubling of the MIR beamused for spectroscopy, after which it can be referenced to a commercially available, fullystabilized Er-fiber-laser OFC in the near-infrared region. In this method, only one beamneeds to be locked to the OFC, which significantly simplifies the scanning of the MIR fre-quency. Another benefit of this method is that it is not restricted to parametric sources,but can also be used with any other CW laser sources that have enough power (≥100mW) for the frequency doubling and are tunable in the MIR region, between 2 and 4 µm,so that the doubled frequency overlaps with the spectrum of the Er-fiber-laser OFC. Inpractise, the method can be combined with any standard absorption spectroscopy tech-nique, such as direct absorption spectroscopy, wavelength modulation absorption spec-troscopy, or cavity-enhanced spectroscopy. In this thesis, high-precision cavity ring-down

35

CHAPTER 4. CONCLUSIONS 36

spectroscopy of nitrous oxide and methane was demonstrated using an OFC-referencedCW-SRO. In general, the OFC-referenced CRD spectrometer is a versatile tool for pre-cision spectroscopy, allowing measurements of many absorption line parameters, such asline center frequencies, line shapes, broadening, as well as line intensities.

In addition to mid-infrared spectroscopy, this thesis shows the potential of cantilever-enhanced photoacoustic spectroscopy for extremely sensitive trace gas detection in thevisible region. A noise equivalent detection limit (1σ) of 50 ppt in 1 s was achievedfor nitrogen dioxide. This is the best NO2 detection limit that has been reported us-ing photoacoustic spectroscopy. Commercial availability of compact and affordable greenhigh-power lasers makes our simple measurement setup potential for substantial enhance-ment in the sensitivity of NO2 and also the other compound measurements, which havestrong absorption in the green region.

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