DIPLOMA THESIS

Femtosecond Laser Induced Damage ThresholdMeasurements with Compressed Laser Pulses

Benedek Nagy

Supervisor: Dr. Peter Dombi

senior research fellow

Wigner Research Centre for Physics

Consultant: Dr. Zsolt Papp

assistant professor

BME Institute of Physics

Department of Physics

BME

2014

Nagy Benedek, the undersigned, Master of Physics student at the Budapest Uni-

versity of Technology and Economics declare that this Diploma Thesis was prepared

on my own, with the leading of the supervisor without any aid not allowed, I used

only the referred sources.

Every part I took from other sources word by word or in the same meaning, but

rephrased is cited with the source.

Budapest, January 2, 2014

signature

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3

Contents

1 Introduction 5

1.1 Laser Damage and its Basic Phenomena . . . . . . . . . . . . . . . . 6

1.2 Models for Damage Mechanisms . . . . . . . . . . . . . . . . . . . . . 6

1.2.1 Theoretical Models . . . . . . . . . . . . . . . . . . . . . . . . 6

1.2.2 Dependence of Damage on Various Laser Parameters . . . . . 10

2 Methods and Instruments Used 13

2.1 ISO Standard Based and Other Measurement Procedures . . . . . . . 13

2.2 Lasers Used . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

2.2.1 Amplified Ti:S Laser System . . . . . . . . . . . . . . . . . . . 15

2.2.2 Long Cavity Ti:S Oscillators . . . . . . . . . . . . . . . . . . . 18

2.3 Measurement Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

2.4 Sample Optics Tested . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

3 Results 25

3.1 Damage Thresholds of Different Optical Elements . . . . . . . . . . . 26

3.1.1 Damage Thresholds of Femtosecond Beam Steering Mirrors . . 26

3.1.2 Damage Threshold of Gratings . . . . . . . . . . . . . . . . . 27

3.1.3 Damage Threshold of Broadband Mirrors . . . . . . . . . . . . 29

3.2 Focal Spot Size Dependence of Damage . . . . . . . . . . . . . . . . . 29

3.3 Repetition Rate Dependence of Damage . . . . . . . . . . . . . . . . 31

3.4 Measurement Errors . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

3.5 Alternative Real Time and Offline Diagnostic Techniques . . . . . . . 34

3.5.1 Thermal Camera . . . . . . . . . . . . . . . . . . . . . . . . . 34

3.5.2 ZyGO Interferometer . . . . . . . . . . . . . . . . . . . . . . . 35

4 Conclusion and Future Plans 38

4.1 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

4.2 Future Plans . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

References 40

4

1 Introduction

At leading high intensity femtosecond laser systems in the world, the optical damage

threshold of various optical elements used in the laser chain is a critical parameter.

Because of damage limitations, the laser beam has to be substantially expanded

at the duty amplifier stages of the system, resulting in the usage of large-aperture,

expensive optical elements (with up to 1 meter diameter in some cases). This rep-

resents a significant cost factor during the building of such a laser system.

The above technology is (or will be) used e.g. at the Rutherford Appleton Lab-

oratory (RAL), whose Astra Gemeni project is able to produce laser pulses with

focused intensity higher than 1021 W/cm2[1], at the National Ignition Laboratory

(NIF), where a 500 TW (1 TW= 1012 W) laser is planned to apply for fusion

ignition[2] and at the Petawatt-Field-Synthesizer (PFS) project at the Max-Planck-

Institut fur Quantenoptik (Garching), which aims to combine few-cycle pulse dura-

tions with petawatt scale peak powers[3] as well as at the Extreme Light Infrastruc-

ture (ELI), where the aim is to achieve femtosecond multi-PW (1 PW= 1015 W)

powered pulses[4]. It is a challenge to apply smaller and thus more cost-efficient

optical elements with higher damage threshold in these high energy ultrafast lasers.

Therefore, Laser Induced Damage Threshold (LIDT) investigations with femtosec-

ond pulses are necessary for developing and designing mirrors, gratings and optical

coatings[5].

In this Thesis I give an overview of my femtosecond LIDT measurements results.

LIDT is an important specification of optical elements such as mirrors, gratings,

etc. In high energy pulsed laser physics LIDT must be taken into account during

construction of a laser system. Therefore, LIDT measurements are inevitable for

various optical elements. The LIDT depends on the pulse energy, pulse duration,

pulse repetition rate and other parameters of the beam. There are various theories

describing the damage mechanisms that explain the measured effects. LIDT is better

known in the pico- and nanosecond region so experiments made by femtosecond

laser pulses are important. In this Thesis, I report on the results of my femtosecond

damage threshold measurements that I performed on various samples. Due to the

fact that there are only few comparable LIDT measurements with different repetition

rate lasers I report on LIDT caused by a 1 kHz and a 3.6 MHz repetition rate lasers

as well.

The structure of my Thesis is as follows. I briefly summarize the most important

terminology in the next subsection. In Subsection 1.2 I present models for damage

5

mechanisms and introduce earlier results on LIDT dependencies from other studies.

I will introduce basic measurement procedures, the lasers available in the laboratory

at the Wigner Research Centre for Physics, the applied measurement setup and the

detailed properties of the samples in subsections of Section 2. In Section 3 I present

my results on LIDT measurements. I close my Thesis with a short summary of the

results, a conclusion of the work performed and future plans in Section 4.

1.1 Laser Damage and its Basic Phenomena

Physical damage mechanisms of materials with large band gap (>3 eV) and in

picosecond pulse length region are ablation of atomic layers, conventional melt-

ing, boiling, formation of color centres, shallow strap and lattice defects[6]. In the

femtosecond region, Coulomb explosion due to various optical ionization processes,

plasma formation and thermoelastic breaking may occur as well[5].

An interaction is called optical damage if there is an irreversible alteration, prac-

tically visible on the surface of the material with a modern high resolution optical

microscope or some other suitable method[6]. Damage threshold is the minimum

power density which causes damage on the surface. The ablation parameter is a

quantitatively measurable parameter of the damaged surface, e.g. the depth or the

diameter of the crater. The ablation threshold is the optical power density which

causes ablation[6]. Below the damage threshold so called incubation is visible. These

are temporary defects not followed by ablation[7].

The common unit for LIDT is the peak fluence in J/cm2. This is given by[5]:

F = 2P

frep

cos θ

w20π

, (1)

where P is the average power, frep is the repetition frequency, θ is the beam angle

of incidence, w0 is the beam radius (1/e2 radius, assuming a Gaussian beam).

1.2 Models for Damage Mechanisms

1.2.1 Theoretical Models

In general, damage is caused at a critical electron density in the conduction band

(1016 − 1018 1/cm3), which corresponds to a critical power density where damage

occurs to the material. This critical electron density, and the corresponding critical

power density, is dependent on different parameters of the laser pulse.

6

In dielectrics with a band gap higher than 3 eV the few tens of picoseconds and

longer pulsed laser beam excites the conduction-band electrons and increases their

kinetic energy. This is transferred to the lattice and when the heat is sufficient,

damage occurs. This model predicts a τ12 dependence of the damage threshold on

the pulse length[6, 8] for longer than 20 ps pulses due to the time that the kinetic

energy of the electron transfers to the lattice. At the beginning the heat increases

at lattice defects, surface injuries where the absorption is higher than in the rest of

the material. For damage the temperature at the local absorption center needs to

increase near the melting point or the temperature where material cracking occurs.

The increase of the temperature is controlled by the thermal diffusion of the lattice

which leads to the τ12 scaling of LIDT. The typical time scale for the lattice to

transfer energy is 0.1-1 ps. This was observed for narrow band gap materials as

femtosecond region laser induced surface modification was monitored by von der

Linde et al. with a femtosecond time resolve microscope[9].

In the femtosecond region the electrons will be excited into the conduction band

from the valence band by photoionization, but they cannot transfer their energy

to the lattice due to the short time scale. Getting more energy from the radiation

of the laser, collisional avalanche ionization starts. Below a certain pulse length

the avalanche ionization effect is enough to cause damage. Decreasing the pulse

length more the LIDT decrease slows down hence the thermal effects have less

importance because the electron density in the conduction band is nearly permanent,

a saturation can be observed. However under 100 fs a drop can may be predicted in

the damage threshold[6] due to the predominance of multiphoton ionization. This

was measured by Mero et al. on Al2O3 [10] and by Stuart et al. on SiO2[6].

In dielectrics the band gap is quite wide, the conduction band is empty and the

material is often transparent. At infrared (IR) and near infrared (NIR) regions single

photon ionization is nearly impossible in wide band gap dielectrics with femtosecond

lasers. Measurements down to 7 fs were made[7]. At ultrashort pulse length non-

linear effects (multiphoton ionization, tunnel ionization) are able to transfer the

electrons to the conduction band. The kinetic energy of the charge carrier electrons

increase due to the additional laser radiation and above a critical kinetic energy

they transfer more electrons to the conduction band by collision, this is the collision

ionization. The newly freed electrons may get more energy from the laser radiation

and excite even more electrons to the conduction band and so on. This is the so

called avalanche ionization. Below 350 fs pulse length, tunnel ionization starts to

play role[7], but in certain processes only tunnel ionization can be dominant[11].

7

The ionized dielectric material reacts as a metal with a time dependent electron

density in the conduction band and thus kinetic energy of the electrons transfers to

the lattice by collision.

The description of the ionization procedure can be evaluated from the Fokker-

Planck equation[6] and leads to an electron density rate equation[7]:

∂ne

∂t=

nv − ne

nv

(wPI + newAI)−ne

τr, (2)

where ne and nv denote the electron density in the conduction and valence band

respectively, wPI the photoionization rate in 1/cm3s, wAI the avalanche ionization

rate in 1/s and τr to the electron relaxation time from the conduction band. At

low energies collision of two electrons is dominant which is accompanied by photon

radiation. At high energies three body collision may occur. In simpler models the

energy transferred to the photons or electrons is transferred simply to the electrons

in the conduction band. Although it is possible that the relaxation of the free

charge carriers leads to metastable self-trapped excitons (STE)[12]. STE increases

the ablation threshold for pulse length greater than 50 fs, thus decreases the maximal

kinetic energy of the electrons in the conduction band. Moreover, when the pulse

duration is higher than the recombination time, STE can be retransferred to the

conduction band by photoionization or avalanche ionization.

The photoionization process depends on the electromagnetic field as introduced

by Keldysh in his theory[13] which is often used nowadays as well. The Keldysh

parameter is

γ = ω

√m∗U

eEl

, (3)

where El is the electric field applied to the laser radiation, e is the elementary

charge, U is the band gap, m∗ is the electron effective mass and ω is the laser central

frequency. This parameter tells which type of ionization process is dominant for a

certain intensity, wavelength and ionization potential[14]. If the Keldysh parameter

is significantly below 1.5 tunnel ionization occurs, if it is much above 1.5 multiphoton

ionization plays role in the process and if it is approximately around 1.5 intermediate

of the tunnel and multiphoton ionization dominates. Whether the Keldysh theory is

valid for pulsed laser fields was questioned as measurement showed other results as

predicted by the theory[15], however it has been widely used and validated for other

cases in atomic and molecular physics as well as for various metallic photoemission

processes, therefore, I will also use this scale quantity.

According to the Keldysh theory multiphoton ionization dominates at low in-

tensity and tunnel ionization may be neglected[7]. However the effective band gap

8

decreases with the intensity of the laser in the Keldysh formula, the multiphoton

ionization is less significant and the real photoionization will be less than estimated

by the multiphoton ionization. At higher intensity the probability of tunnel ioniza-

tion increases and the process becomes dominant. Collision ionization occurs when

the energy of a free electron is high enough to excite another one from the valence

band to the conduction band. This energy level is above the ionization potential and

depends on the intensity of the laser and on the electron density in the conduction

band.

Once the electron is excited to the conduction band (with multiphoton or tunnel

ionization) it can decrease its energy by Joule-heat during the time of the laser pulse.

The time dependence of the thermal energy (Wth) can be written in the

∂neWth

∂t=

1

2σRE

2 − PimpneJeff (4)

form[8], where E is the electric field of the laser, σR is the conductivity, therefore12σRE

2 is the energy decrease of the electrons due to the Joule-heat, PimpneJeff is

the energy loss due to collision ionization as Pimp is the probability of ionization by

electron impact and Jeff is the band gap including the quiver energy of the electron

which means that the band gap J decreases due to the electron quiver and therefore

the electron needs less transferred energy.

It is an important issue what to define as LIDT in the models. A simple approach

based on the Drude-Lorentz model leads to critical electron density in the conduction

band[5]:

ncr =ϵ0ϵelm

∗

e2ω2, (5)

where m∗ denotes the effective mass of the electron, ϵel the susceptibility of the

bound electrons, ω the laser frequency, ϵ0 the vacuum permittivity and e to the

elementary charge. Various groups found that the typical critical electron density

is about 1021 cm−3[6, 10, 15, 16]. That is, near the critical density, the absorption

length is on the order of a wavelength. This implies that for a pulse just above the

damage threshold, a thin layer of dense plasma will be formed at the tail end of

the pulse. This layer will be even thinner as the material fully ionizes at intensities

above threshold[6]. The damage and ablation thresholds are often related to the

electron density in the conduction band, and particularly to the critical electron

density. When the electron density exceeds the critical electron density (ncr =

1.7 × 1021 cm−3 in fused silica for a laser wavelength of 800 nm[16]), the material

turns highly absorbing. The criterion ne = ncr thus appears as a necessary condition

9

for dramatic modification (ablation and/or damage) of a material, but in general

does not describe with the highest accuracy the exact numerical and experimental

conditions yielding such profound changes of the material[7].

It is important to mention that single shot and multiple shot LIDT may dif-

fer significantly as shown by the theories as well. As described above, there are

mechanisms that require multiple shots or in the case of one shot having insufficient

energy, multiple shots can transfer enough energy to start the mechanism which

leads to damage in the material. Therefore it is obvious that the LIDT decreases

as the number of pulses increase. Although above a certain number of pulses the

threshold fluence does not decrease significantly[17, 18], as the number of electrons

being trapped is proportional to the concentration of conduction band electrons

and pre-existing defect sites. This process will introduce additional energy levels

and excitation routes for the following laser shot. The relative strength of this pro-

cess to additionally initiate free and heated electrons by combined multiphoton and

avalanche ionization seems to decrease with increasing the number of pulses until

finally reaching a constant level[18].

1.2.2 Dependence of Damage on Various Laser Parameters

Dependence on pulse length As described above the LIDT peak fluence is

scaling with the square root of the pulse length (F ∝ τ12 ) in the picosecond region.

This dependence does not apply in sub picosecond region as there is not enough

time to transfer the energy to the lattice. Under 100 fs a break down in the a LIDT

peak fluence was predicted and proved by measurements[6, 10].

Dependence on pulse repetition rate There are only very few measurements

which investigated the LIDT dependence on various repetition rates and these mea-

surements are not matching either. Mero et al. performed measurements on a Ta2O5

single layer at 1, 10, 100 and 1000 Hz and did not observe significant changes or any

trends in the LIDT[17]. Hertwig et al. achieved similar results on ion-doped glass

sample[19]. In spite of these results Bonse et al. demonstrated a decreasing trend

in LIDT values, with measurements taken at 10, 100 and 1000 Hz on a Ta2O5/SiO2

high reflector multilayer[20]. One order of magnitude increase in the repetition rate

leads to approximately 25% decrease in the LIDT. Due to the sparse literature, it is

an important task to investigate the LIDT dependence on the pulse repetition rate

especially for higher repetition rates than 1 kHz.

10

Dependence on beam diameter Martin and Hertwig et al. studied the spot

size dependence of the LIDT[19, 21]. An earlier study was used to predict the

dependence published by DeShazer et al. in 1973[22]. The results matched well

with the theory therefore I briefly summarize it here. The model assumes Poisson

distributed defects and lower LIDT at a defect site. The probability of a defect site

got hit by a Gaussian laser beam is

P (w0) = 1− exp

[−π

8ln 2

(w0

d0

)2], (6)

where w0 is the Gaussian spot size and d0 is the average distance between defects.

For the next step, the model assumes the coating defects on the surface to have a

LIDT Fd much less than the intrinsic LIDT Fi. Therefore the total LIDT can be

given using the probability in Eq. (6):

F (w0) = FdP (w0) + Fi [1− P (w0)] . (7)

This leads to an increasing LIDT with the decrease of the spot size. It has to

be noted that the LIDT predicted by that model reaches a boundary Fd quite

quickly depending on the w0

d0ratio. Decreasing the beam diameter leads to Fi hence

F (w0 → 0) → Fi. Therefore increase or decrease of the spot size above and below a

certain one respectively does not influence the LIDT significantly.

Dependence on central wavelength There are many results available at 800 nm

due to the widespread use of Titanium-Sapphire (Ti:S) lasers. Therefore wavelength

dependent measurements are in the vicinity of 800 nm. The LIDT dependence on

the central wavelength can be summarized by using a study published by Jupe et

al.[16]. The avalanche ionization (WAI) rate is scaling linearly with the absorption

cross section (σ), and from the Drude model

σ =e2

cϵ0n0m

τc1 + ω2τ 2c

, (8)

where τc is the resulting collision time, ω is the laser frequency, m∗ is effective

electron mass, e the elementary charge, n0 is the photon number and ϵ0 is the vac-

uum permittivity. Multiphoton ionization provides a substantial contribution to the

damage mechanism for ultrashort laser pulses. Considering the large differences in

cross-sections of n and n+1 photon absorption, a quantized behavior of the damage

threshold is expected. The presented theoretical approach indicates clear steps in

the ionization characteristic behavior of the material with photon energies varying

11

over orders of the multiphoton process. Therefore, the LIDT should also show a

respective change at the transition point between the n and n+1 photon orders. A

step of between two and three photon ionization is approximately at 670 nm which

could be observed experimentally as well. Thus corresponding to this model the

calculated LIDT has a non-monotonous dependence on the wavelength: at all n and

n+1 photon orders it has a sharp step and then a slow relaxation until the next n+1

and n+2 photon order. This effect was measured by Ristau et al. as well[23].

In my experiments, I will focus on the following aspects of LIDT: I will compare

different sample optics, discuss the results on focal spot size and repetition rate

dependence focusing on a comparison between kHz and MHz repetition rates for

the first time to my knowledge.

12

2 Methods and Instruments Used

2.1 ISO Standard Based and Other Measurement Proce-

dures

The LIDT measurement procedure is mostly based on the ISO 112541 standards[24].

There are two main principles in the ISO standard. One is the 1-on-1 and other is

the S-on-1 measurement routine. The basics are that LIDT is measured by single

shot or S shot induced damage respectively. Therefore the procedure is different for

the two cases. The standard defines them as follows[24]:

1-on-1 A number of test sites are positioned into the beam and irradiated at differ-

ent energy densities or power densities. From this data, the damage threshold

can be determined. Test a minimum of 10 sites for each energy-density or

power-density increment. The range of pulse energies or beam powers em-

ployed shall be sufficiently broad to include points of zero damage frequency,

as well as points of 100% damage probability.

S-on-1 An unexposed test site is positioned into the beam and irradiated by a

series of Np pulses with a selected energy, Qtp, of the typical pulse. If damage

is observed by the on-line damage detection system before the series of Np

pulses is completed, stop the irradiation of the site and record the minimum

number of pulses Nmin. Repeat this procedure for different energy densities

of the typical pulse. The number of pulses, Np, shall be constant for the

entire test procedure and shall be selected such that the specific laser-induced

damage behavior of the specimen is registered by the S-on-1 test.

At both procedures the threshold is defined as the highest quantity of incident

laser radiation to the optical surface for which the extrapolated probability of dam-

age is zero. The probability corresponding to a certain energy differs with the two

methods. The probability is simply the ratio of the damaged sites and total number

of exposed sites on that certain energy for the 1-on-1 procedure. The probability

calculation for the S-on-1 process is more difficult and has to be done as described

here. The pulse energy scale is divided into a series of intervals (Q−∆Q,Q+∆Q)

covering the energy range accessible by the experimental set-up. For the calculation

of damage probability for a certain energy value, Q, and for a selected number, N ,

1There is a revision of the standard published in 2011 (ISO 21254) but the main principles are

the same.

13

of pulses, data points with Qtp(Q−∆Q,Q+∆Q) are selected from the file of data

points. Data points with Nmin ≤ N correspond to sites damaged, meanwhile data

points with Nmin > N or Np ≥ N correspond to sites not damaged during the test.

The damage probability for the energy, Q, is calculated by the ratio of the number

of data points corresponding to damaged sites with respect to the total number of

data points considered for the evaluation[24].

It must be mentioned that the papers published in this theme do not use the ISO

rule in general. The reason for this are different optics specifications which do not

match the ISO standard and makes another procedure acceptable. I also did not

apply the ISO standards strictly due to practical reasons. I performed a mixture of

the 1-on-1 and the S-on-1 procedures. I was able to measure the illumination time

but I was unable to stop the beam suddenly at a damage occasion. Therefore my

test was using the same number of pulses as in S-on-1 but the probability was only

the ratio of the damaged and all sites as in 1-on-1. The damage threshold fluence

was defined as the intersection of the linear fit of the data points and the y = 0

line as shown in Fig. 2 The test in practice used during my measurements works as

follows.

1. Direct a low power laser beam on the optical element at 10 different points

one after the other as shown in Fig. 1(a).

2. Record the histogram of the exposure sites as in Fig. 1(b).

3. If the surface is damaged, decrease the power, if it is not, increase it. Than

return to step 1.

(a) (b)

Figure 1: LIDT measurement [25]. Exposure sites shown on a mirror (a) and the

exposure histogram (b).

14

Figure 2: Damage probabilities of borosilicate glass and the determination of damage

threshold value from the measurement.

The exposure histogram can be recorded in a more obvious way (see Fig. 2.)

This way the LIDT may be read easily from the histogram. If the dependence on

different pulse attributes needs to be measured the procedure has to be repeated at

the desired pulse parameters.

2.2 Lasers Used

I used two different laser sources for damage measurements at Wigner Research

Centre for Physics. In this section these systems are introduced.

2.2.1 Amplified Ti:S Laser System

A mode-locked Ti:S laser with a Coherent LEGEND regenerative amplifier is used

at the laboratory of the Ultrafast and Attosecond Physics Group. The spectrum of

the pulse before and after the amplifier is shown in Fig. 3(a). The pulse length is

measured by an APE Mini autocorrelator. A typical measurement window is shown

in Fig. 3(b). The parameters of the laser beam after the amplifier are shown in

Table 1.

Laser Parameters at the Damage Workstation The laser described above

is used in other measurements in the laboratory as well. Therefore the parameters

at the damage workstation differs from the above, the maximum average power is

600 mW. The beam diameter depends on the lens built into the setup (see Fig. 10.),

the beam diameter in focus is 20 µm and 160 µm with a 200 mm and 750 mm

focusing lens respectively. I will call these focusing geometries medium and loose

focusing below.

15

(a) (b)

Figure 3: Spectra of the pulse before (black) and after (red) the LEGEND amplifier

(a). Pulse length measured by autocorrelator (b).

Parameter Value

Central wavelength (λ) 800 nm

Average power (P ) 4.1 W

Beam diameter at 1/e2, Gaussian-fit 9 mm ± 0.5 mm

Polarization Linear, horizontal

Repetition rate 1 kHz

Pulse length, FWHM (τ) 35 fs

Pulse energy stability rms 0.25 %

Pulse bandwidth, FWHM 30 nm

Table 1: Laser beam parameters after the amplifier.

In LIDT measurement it is very important to calculate the peak fluence. This

depends on the shape of the beam. I measured the beam profile with a Thorlabs

BC106-VIS CCD camera. The beam can be well fitted with a Gaussian (Fig. 4(a)

and 4(b)) hence the peak fluence may be calculated as already mentioned in Eq. (1)

(including the correction factor for Gaussian beams). The measurement results with

the 750 mm focusing lens are shown in Fig. 4 and 5.

I also applied a lens with short, 35 mm focal length to achieve tight focusing.

Due to the 6 µm pixel pitch of the camera I had to enlarge the focal spot with a

simple microscope objective and calibrate the magnification of the system. Thus

the size of the focal spot is 8.95 ± 0.23 µm and 9.04 ± 0.21 µm for the x and y

16

(a) (b)

Figure 4: Beam profile at the workstation measured with Thorlabs BC106-VIS CCD

with f=750 mm. Beam profile and Gaussian fit on the x axis (a) and y axis (b).

(a) (b)

Figure 5: Beam profile at the workstation measured with Thorlabs BC106-VIS CCD

with f=750 mm. 2D (a) and 3D (b) view of the profile.

Name Lens focal length (mm) Focal spot size (µm)

Tight 35 9

Medium 200 20

Loose 750 160

Table 2: A summery of the used focusing geometries and their notation.

directions, respectively. The diameter is measured at 1/e2 width. For the damage

measurements to be discussed below, these three focusing geometries are to be used

as summarized in Table 2.

17

(a) (b)

Figure 6: Scheme of a long cavity oscillator (a) and the circular pattern formed on

the M5 mirror in the so-called Herriott-cell (b). M1-M10: chirped and high reflector

mirrors, P1, P2: Brewster prisms, OC: output coupler, CP: compensation plate, L:

lens with 60 mm focal length, SESAM: semiconductor saturable absorber mirror.

2.2.2 Long Cavity Ti:S Oscillators

7.36 MHz Repetition Rate Oscillator Another light source to investigate dam-

age phenomena with is a long-cavity Ti:S oscillator of the same group. This provides

pulses with much higher energy than standard oscillators with a high (MHz) repeti-

tion rate[26]. Thus, we expect damage processes to behave substantially differently

than in the case of a beam with kHz repetition rate. A former student built this

laser[27]. My task was to put this laser to work with a new Ti:S crystal.

Therefore, I installed a new Ti:S crystal with an α = 4.95 1/cm absorption and

4.6 mm optical path length which was put into the laser head. Firstly, a short cavity

laser was set up with the new crystal where 1.5 W output power was achieved with

6 W pump power. Then, I set up the long cavity oscillator the scheme of which is

shown in Fig. 6(a). By extending the cavity, the repetition rate decreases and high

pulse energies can be achieved. The so-called Herriott-cell extends the cavity length

as the beam bounces between e.g. two mirrors, a spherical and a flat mirror in our

case[28]. The Herriott-cell is formed by the M5 and M6 mirrors together with the

M4 incoupling and M7 outcoupling mirrors. The Herriott-cell is not depicted fully

realistically, as the subsequent reflections form a circular pattern on the mirrors as

shown in Fig. 6(b). A certain combination of radii of curvatures, number of bounces

and mirror distances results in a net ABCD matrix of unity for the Herriott-cell.

Thus, introducing this into the cavity, oscillator stability is not compromised. In

18

(a) (b)

Figure 7: The spectrum of the chirped pulse oscillator (red line) together with the

reflectivity of the SESAM used (blue line) and the intracavity GDD of the laser (black

line) as a function of wavelength (a). The autocorrelation of 7.36 MHz repetition rate

oscillator measured by the APEMini autocorrelator (b). The FWHM is approximately

147 fs which corresponds to a 97 fs pulse length at FWHM.

the 7.36 MHz laser that I put into operation there are 7 such bounces from each

large-aperture mirror.

Thus, the laser cavity is 40 m long and the available continuous wave mode

(CW) power was 650 mW. The mode locking can be started by a semiconductor

saturable absorber mirror (SESAM). The best pulse was achieved by 680 mW mode

locked power (The highest power was 820 mW), and its spectra can be seen in

Fig. 7(a) (red line). The Figure also shows the reflectivity of the SESAM (blue line)

and the intracavity Group Delay Dispersion (GDD) of the laser (black line). The

repetition rate of the laser is 7.36 MHz, so the pulse energy is 92 nJ. I also built

the pulse compressor with which the achieved best pulse length was 82 fs, but the

average achievable is 93-99 fs, measured by an APE Mini autocorrelator as shown in

Fig. 7(b). The FWHM of the autocorrelation shown in the figure is approximately

147 fs which corresponds to a 97 fs pulse length at FWHM. The compressor is shown

in Fig. 8.

Since I was not able to cause damage at the quick tests with this system the

setup was moved to another long-cavity laser with even lower repetition rate and

approximately two times higher pulse energy as described in the next section.

3.6 MHz Repetition Rate Oscillator The third laser of the group is a Ti:S

oscillator with 80 m long cavity. This laser has been working for years so my task

19

Figure 8: The compressor after the laser oscillator (a) and the compressor gratings

enlarged (b).

Figure 9: The spectrum of the 3.6 MHz oscillator.

was only to perform the LIDT measurements. First I made a quick test whether

the peak fluence available is high enough to cause damage. The lower repetition

rate leads to a higher peak fluence. The output power with short cavity, used for

proper setting of the laser head, is more then 1.2 W, with long cavity the CW

power is 520 mW, and the mode locked power is 680 mW. This is all achieved with

10.2 W pump power. The oscillator setup was shown in Fig. 6 at above paragraph

of this section. M1, M2, M6 and M9 mirrors are curved with radii of curvature

RM1 = 10 cm, RM2 = 15 cm, RM6 = 16 m, RM9 = 80 cm respectively, the others

are flat. P1 and P2 Brewster prisms are for fine-tuning intracavity dispersion.

The emission spectrum depends on various parameters, such as on the beam

position on the SESAM and on the fine tuning of the laser head, especially on the

stability zone. The pulse was compressed by a double prism pair compressor[29].

With best spectrum, shown in Fig. 9, 87 fs pulse length could be reached. This was

the best achievable pulse length fluctuating between 82-93 fs depending on the laser

condition. The measurements were performed at these pulse length.

20

The laser parameters at the damage workstation do not differ significantly from

the above presented. To achieve tight focusing a lens with 11 mm focal length was

built into the setup. Medium and loose focusing geometries are not possible in this

case as they do not deliver high enough focused intensity for damage. The mea-

surement of the focal spot diameter was the same method as described in Sec. 2.2.1

above. The measured values are 8.42 ± 0.22 µm and 7.92 ± 0.19 µm for the x and

y directions, respectively.

2.3 Measurement Setup

In Fig. 10(a) and 10(b) a scheme and a photograph is shown respectively of the

medium and loose focusing measurement setup. The laser beam power is 0.5 W at

the work station powered by the LEGEND amplifier. The power is decreased by

a wheel absorption filter (WF). A shutter (SH) is built in the way of the beam to

enable single shot and repetition rate dependent measurements. The power reaching

the sample may be monitored with a photodiode (PD1). The beam is focused by

a f=200 mm (L) or a f=750 mm (L’) lens on the sample which is fixed on an x-y-z

translation stage (TS).

There are two real-time monitoring systems shown in Fig. 10(a). One shown in

the center is a stereo microscope built into the setup. A CCD camera is attached

to one of the oculars and the illumination of the sample is performed through the

(a) (b)

Figure 10: The schematic draw (a) and photograph (b) of the workstation. SH:

shutter, WF: wheel density filter, PD1: photo diode, L’: f=750 mm lens, L: f=200 mm

lens, BS: beam splitter (R=20%, T=80%), TS: translation stage, IR-CCD: thermal

camera.

21

other ocular. For measuring dielectric elements, a back illumination of the sample

proved to be more usable. Filtering the IR scatter of the sample the damage is

visible certainly on the CCD’s picture. The other on the left center is a thermal

camera (IR-CCD) with which the heating of the sample can also be visualized.

The setup used for the tight focusing measurements differs slightly from the one

used at loose focus. Due to the shortness of the focal length the lens had to be

put between the sample and the stereo microscope. So the microscope and the light

source had to be shifted and the beam could be perpendicular to the sample. This

setup is better than the previous one because setup could be kept unchanged after

changing the focusing lens. This setup was built on a breadboard, so it is portable

and all of the necessary elements can be handled easily.

2.4 Sample Optics Tested

Mirrors The most commonly and widely used optical elements in laser construc-

tion are mirrors. They play role in beam steering and focusing after amplifiers.

Chirped mirrors may be used in pulse compression systems as well with continu-

ously increasing pulse energy. There are many materials to deal with these tasks

and it is important to know the damage threshold for applying the proper mirror

in each case. Therefore I tested metallic and dielectric mirrors as well. A silver

and a high reflector mirror centered at 800 nm was manufactured by Optilab Ltd.

I will refer to these mirrors as Ag and HR800 mirrors respectively. The high reflec-

tor mirror is coated with a dielectric layer structure as described further on. The

dielectric layer stack is a TiO2-SiO2 layer pair repeated 24 times which is a (LH)24

stack, where L denotes the SiO2 (n=1.44) low index and H the TiO2 (n=2.3) high

index layer respectively. The whole stack is 5421 nm thick since every layer is a λ/4

layer at λ = 800 nm. Commercial mirrors supplied by Thorlabs were tested as well,

the samples used are summarized in Table 3. They can provide a comparison for

800 nm high reflectors even though they are not optimized for femtosecond appli-

cation as their group delay dispersion is not controlled, only their reflectivity. The

Thorlabs protected metal mirrors all have a protecting overcoat which may be, in the

most common case, an approximately 100 nm thick SiO2 layer. Unfortunately the

manufacturer does not provide with precise information on the overcoat thickness.

Gratings Since gratings are very important parts of amplified laser systems, espe-

cially in pulse compressors and amplifiers, it is important to investigate the damage

22

Mirror Material Protection Coating Design wavelength

HR800 mirror TiO2-SiO2 - 800 nm

Ag mirror Ag ∼100 nm SiO2 400 nm - 2 µm

BB05-E02 Unknown dielectric - 400 - 750 nm

BB05-E03 Unknown dielectric - 750 - 1100 nm

ME1-G01 Protected Al 450 nm - 20 µm

PF05-03-P01 Protected Ag ∼100 nm SiO2 450 nm - 2 µm

PF05-03-M01 Protected Au ∼100 nm SiO2 800 nm - 20 µm

Table 3: The used mirrors and their main properties.

threshold for these type of optical elements. As a first rough proof-of-principle test

I measured three commercial gratings with different groove densities, two with loose

focusing and one with tight focusing.

The gratings are not optimized for femtosecond pulse compression. The goal

of the study on these test pieces was to see whether a dielectric λ/4 overcoating

structure can increase their damage threshold significantly without compromising

diffraction efficiency. All of them are ruled gratings with Al reflective overcoat

provided by the manufacturer. Further on each one had a dielectric layer overcoated

half with three pairs of λ/4 layers of Si02 and TiO2. This leads to a (LH)3 layer,

what corresponds to a 677 nm overcoating thickness. The overcoating (performed

by Optilab Ltd.) of the gratings is shown in Fig. 11. The diffraction efficiency is not

influenced significantly at 800 nm what can be observed in Table 4. It can be seen

that at the GR25-0610 and GR25-1210 gratings the diffraction efficiency loss due to

the overlayers is approximately 10%, and at the third an efficiency increase can be

observed at approximately 20% due to the complexity of the hybrid structure not

particularly optimized for efficiency increase. Therefore, the hybrid metal-dielectric

grating structure can be efficiently compared to the pure metal grating structure in

LIDT measurements.

23

(a) GR25-0310 (b) GR25-0610 (c) GR25-1210

Figure 11: The three Thorlabs gratings with metal coated (M) and dielectric coated

(D) halves.

Grating d(µm) αblaze ηmetal(%) ηhybrid(%)

Thorlabs GR25-0310 3.3 8◦36’ 52.5 65.0

Thorlabs GR25-0610 1.6 17◦27’ 66.2 61.5

Thorlabs GR25-1210 0.83 36◦52’ 23.0 20.5

Table 4: The diffraction efficiency of the gratings used at 800 nm, where d, αblaze,

ηmetal and ηhybrid is the grating constant, the blaze angle, the first order diffraction

efficiency without and with overlayer respectively.

24

3 Results

There where four types of measurements performed during my work. First loose

and medium focus measurements were taken with the 1 kHz repetition rate laser

described in Sec. 2.2.1 using a 750 mm and a 200 mm focal length. These will be

called loose and medium focusing, respecitvely. The second series was measured

with the 3.6 MHz repetition rate laser described in Sec. 2.2.2 with tight focus using

a 11 mm focal length lens. The third series was taken again at the 1 kHz repetition

rate laser with tight focus with 35 mm focal length to get comparable results with

measurements of the 3.6 MHz repetition rate laser since in this case focal spot sized

were exactly the same as with the 3.6 MHz measurements. The loose, medium

and tight focusing corresponds to 160 µm, 20 µm and 9 µm focal spot diameter

Sample

Measurement by focusing

MHz kHz

tight tight medium loose

Borosilicate glass ✓

HR800 mirror, Optilab Ltd. ✓ ✓ ✓

Ag mirror, Optilab Ltd. ✓

BB05-E02 Broadband diel. mirror ✓

BB05-E03 Broadband diel. mirror ✓

ME1-G01 Prot. Al mirror ✓ ✓ ✓ ✓✓

PF05-03-P01 Prot. Ag mirror ✓ ✓

PF05-03-M01 Prot. Au mirror ✓ ✓

Evap. GR25-0310 ✓

Not evap. GR25-0310 ✓

Evap. GR25-0610 ✓

Not evap. GR25-0610 ✓

Evap. GR25-1210 ✓ ✓

Not evap. GR25-1210 ✓ ✓

Table 5: Summary of the samples tested in various focusing geometries.

25

respectively as shown in Table 2 of Sec. 2.2.1. Not all of the samples presented in

Sec. 2.4 were measured in all cases and in addition a borosilicate glass was measured

with medium focus. This is summarized in Table 5.

All measurements were taken with the method described in Sec. 2.1 and a damage

occasion was detected using a stereo microscope with a CCD as presented on the

setup in Sec. 2.3, since this proved to be the best solution for real time damage

detection. The microscope is also needed for positioning the beam on the sample.

Also the scattered light can be observed while damage occurs and after shuttering

the beam the damaged spot can be seen on the CCD. Therefore I used the microscope

with the CCD for all measurements. In each case the average power was measured

and rendered to the damage probabilities. The peak fluence was calculated according

to Eq. (1). For the peak fluence calculation at the different measurements the spot

size given in Sec. 2.2.1 and Sec. 2.2.2 was used for the 1 kHz and 3.6 MHz lasers

respectively. Results are presented further on sorted by importance.

3.1 Damage Thresholds of Different Optical Elements

3.1.1 Damage Thresholds of Femtosecond Beam Steering Mirrors

The four samples measured at the kHz laser with medium focus as shown in Table 5

are a borosilicate glass, a dielectric (HR800) and Ag mirror by Optilab Ltd. and a

Thorlabs Prot. Al mirror. The damaged exposure sites are shown in Fig. 12. The

highest LIDT was demonstrated by the borosilicate glass with 3.72 J/cm2 (shown

in Fig. 2 in Sec. 2.1). This is followed by the Ag mirror with 1.29 J/cm2. The third

is the HR800 mirror with 0.67 J/cm2 and the last is the Al mirror with 0.20 J/cm2.

With the knowledge of the band structure difference between metals and di-

electrics it is surprising that the LIDT of the Ag mirror is higher than that of the

HR800 mirror. Although this is consistent with earlier studies[5]. The band struc-

ture of metals allow easy excitation which should lead to a low LIDT. As all mirrors

are coated with a protective coating with unknown thickness (and material) I do not

measure the correct LIDT of the pure metal mirror only of the metal-dielectric struc-

ture. This is a very probable reason of the results. Due to the difference between

the methods and the applied laser parameters the results can only be compared

with earlier studies qualitatively. I could not find other publications comparing the

same or similar samples except the earlier cited Bachelor Thesis of von Conta[5].

Martin et al. measured Al as well and presented result of LIDT in the same order of

26

Figure 12: Damage probabilities of Al, Ag and HR800 mirrors with f=200 mm

focusing lens.

magnitude at various pulse numbers during S-on-12 tests[30]. Moreover, comparable

measurements can only be done between samples of the same manufacturer as the

coatings can be different and LIDT of the same mirror types but made by different

manufacturers may differ. A possible reason of the lower LIDT of the Al mirror

compared to the Ag mirror can be sought in the electron configuration difference of

the two materials. The Al has three electrons on the 3s and 3p orbitals in contrary

with the only one electron on the 5s orbital of Ag. Therefore the ionization energy

of Al is lower than the ionization energy of Ag and so the electrons can be excited

easier in Al and the energy transferred from the radiation gives greater part to the

electrons the kinetic energy of which leads to the avalanche ionization effect with

higher possibility.

3.1.2 Damage Threshold of Gratings

There were three gratings tested. All of them has a metal (Al) reflective coating on

the whole surface. One half of each gratings is evaporated with a dielectric overcoat

as described in detail in Sec. 2.4. The evaporated coating is a (LH)3 coating on top

of the metal coating as described above. The exposure sites of the gratings can be

seen in Fig. 13 for the two gratings participating in the ”loose kHz” measurement

and in Fig. 15(c) for the grating participating in the ”tight MHz” measurement. It

can be seen in these figures that the LIDT of the evaporated surfaces are higher as

the metal coated surfaces, as expected. To highlight this I summarized the LIDT

2For details see Sec.2.1.

27

Figure 13: Damage probabilities of different Thorlabs gratings with and without

dielectric layer with f=750 mm focusing lens (”loose kHz” measurements).

MeasurementDamage threshold (J/cm2)

Impr. factor

Evap. Not Evap.

Thorlabs GR25-0310 ”loose kHz” 0.084 0.032 2.63

Thorlabs GR25-0610 ”loose kHz” 0.16 0.06 2.67

Thorlabs GR25-1210 ”tight kHz” 0.56 0.28 2.00

Table 6: LIDT peak fluence values for the measured gratings with the metal- and

dielectric coated half and the improvement factor due to the dielectric overcoat.

of the gratings in Table 6. In most cases an improvement of more than a factor

of 2 was measured in the LIDT of the evaporated part over the metal coated part.

The diffraction efficiency is not influenced significantly by the evaporation of the

protective coating as shown in Sec. 2.4.

The significant improvement in LIDT for the hybrid gratings is clearly demon-

strated by these measurements. Since the high reflectivity (or more precisely, high

diffraction efficiency) effect is distributed over a number of layers in this case, such

high field strengths can not be created as in the case of the pure metal gratings.

This explains the improved damage thresholds. The samples were not produced for

800 nm pulse compression purposes. For this purpose it is rather gold coated holo-

graphic gratings that are used. However, my samples give a good approximation

on how much LIDT improvement could be achieved with a hybrid gold-dielectric

28

Figure 14: Damage probabilities of Thorlabs broadband mirrors with f=750 mm

focusing lens.

holographic grating. It is an interesting challenge for the future to find an optimum

in the layer number to increase LIDT and diffraction efficiency at the same time.

3.1.3 Damage Threshold of Broadband Mirrors

In Sec. 2.4 the Thorlabs broadband mirrors are presented. Even though these sam-

ples are not suitable for femtosecond pulse steering, they can provide a good com-

parison with standard 800 nm high reflectors as their reflectivity at the target wave-

length is high, therefore light is not absorbed or transmitted. Fig. 14 shows that

especially broadband mirrors LIDT is higher than the other designed for visible-NIR

light. The measured LIDT values are 0.148 J/cm2 and 0.074 J/cm2 for the BB05-

E03 IR mirror (high reflector for 750-1100 nm) and for the BB05-E02 visible-NIR

mirror (high reflector for 400-750 nm) respectively. This is also expected due to the

specifications of the mirrors.

High reflectors for the target (800 nm) wavelength will allow the damaging beam

to penetrate into the multilayer stack only up to a certain depth. This is not the case

for the BB05-E02 mirror any more as some spectral components of the damaging

beam can even be transmitted. This means a more complex electric field distribution

within the mirror structure and thus, damage has a higher probability.

3.2 Focal Spot Size Dependence of Damage

The aim of these measurement was to compare LIDT at different focal spot sizes of

the damaging beam. As described in Sec. 1.2.2 there is a theoretical model for the

spot size dependence of the damage threshold.

29

As shown in Table 5 the Optilab HR800 and the Thorlabs Al mirrors were

measured with the 1 kHz laser at two different focusing geometries. As at the mea-

surements of the tight focus the pulse length differs, one can only give a qualitative

prediction spot size dependence of the damage threshold. At the tight focus mea-

surement the pulse length was 82 fs in contrast with the medium or loose focus where

it was 35 fs. As described in Sec. 1.2.2 decreasing the spot size leads to an increase

in the LIDT. Therefore the predicted LIDT at loose focusing has to be higher as at

medium focusing.

As also presented in Sec. 1.2.2 there is a drop in LIDT below 100 fs pulse length.

This indicates an increasing LIDT at the transition from 35 fs to 82 fs which cor-

responds to the loose/medium and tight focus measurements respectively. As both

of pulse length increase and spot size decrease lead to increase in LIDT the LIDT

measured at tight focus is predicted to be the highest.

In Table 7 the LIDT peak fluence values are summarized for the loose, medium

and tight focus measurements on the kHz repetition rate laser. It can be seen that

the Al mirror behaves as predicted as the LIDT increases in strict monotonicity

from loose to tight focus. Looking at the total improvement factor of the Al it

has to be considered that the pulse length at the loose focusing geometry was 35 fs

(τl = 35 fs) in contrast to the tight focusing geometry, where it was 82 fs (τt = 82 fs).

A total improvement factor of six seems high at first sight compared to the former

published results where a factor of two[19] and a factor of three[22] can be found.

However, if calculating with the τ12 scaling law of LIDT and divide and multiply the

tight focus result with√τt and

√τl respectively we get 0.33 J/cm2 which leads to a

scaling factor of four. It has to be noted that the τ12 scaling law underestimates the

LIDT in the subpicosecond region[6] and there is a breakdown in the LIDT under

100 fs[10]. Therefore the difference between the pulse length plays a significant

SampleDamage threshold (J/cm2)

loose medium tight

ME1-G01 Prot. Al mirror 0.083; 0.075 0.20 0.5

HR800 mirror, Optilab Ltd. - 0.67 0.4

Table 7: LIDT peak fluence values for the measured samples at different spot size

on the Thorlabs Al and the HR800 mirrors.

30

role in the improvement factor and my results can be considered as satisfactory.

On the other hand, the HR800 mirror does not correspond to the predictions. A

possible reason is that the two samples used in the measurement were different in

the optimal angle of incidence. The one measured at medium focus was optimized

for 45 degrees incidence and the other used at tight focus was optimized for normal

incidence. It is known that mirrors from different coating charges can have different

damage thresholds due to varying chamber conditions, cleanliness etc. In addition

(and probably this is the largest effect), the values given carry a significant error

which I estimate to be 15% discussed in detail in Sec. 3.4. These effects together

can explain the observed differences in the damage thresholds, therefore the latter

measurements on the high reflectors can not be considered as conclusive and will

have to be repeated.

3.3 Repetition Rate Dependence of Damage

My aim of the tight focusing measurements was to compare LIDT induced by 1 kHz

and 3.6 MHz repetition rate lasers. Since the pulse energy of the high repetition rate

oscillator only allows for damaging at very tight focusing conditions with a sub-10-

micron focal spot, I had to perform similar measurements with the kHz amplifier too.

There were many effects measured and published as shown in Sec. 1.2 but there was

no information about LIDT measured by kHz-MHz repetition rates with the same

parameters. The prediction was that at higher repetition rate the LIDT decreases

since the sample has less time to ”relax”, the possible heating effects’ relaxation

time may be longer than the time between two pulses in the MHz region but shorter

than or much comparable to the time between two pulses in the kHz region. In the

literature for Hz-kHz comparison we can find decreasing[20] and comparable (almost

equal)[17, 19] results as well.

Because one of the laser was an amplified Ti:S laser system (Sec. 2.2.1) and the

other was a long cavity Ti:S laser (Sec. 2.2.2) it was a quite difficult task to achieve

the same parameters. The most important parameters to match were the pulse

length, the focal spot size and the beam quality. Both lasers are operating at 800 nm

wavelength. Due to the more exact, precise and reproducible adjustment of the

amplified laser system I tried to achieve the best pulse on the long cavity oscillator

and after that I adjusted the amplified laser system to match the conditions. The

amplified laser system’s pulse length can be easily adjusted with the amplifier’s

compressor grating to match the typical 80-90 fs pulse length of the MHz oscillator

31

SamplePeak Fluence (J/cm2)

PfkHz

PfMHz

1 kHz 3.6 MHz

Not evaporated grating GR25-1210 0.28 0.031 9.03

Evaporated grating GR25-1210 0.56 0.043 13.02

Thor. Prot. Ag mirror: PF05-03-P01 1.26 0.133 9.47

Thor. Prot. Au mirror: PF05-03-M01 1.20 0.095 12.63

Thor. Prot. Al mirror: ME1-G01 0.52 0.024 21.67

HR800 mirror, Optilab Ltd. 0.42 0.102 4.12

Table 8: LIDT peak fluence the values for the measured samples at the kHz and

MHz lasers, and its ratio.

so this seemed to be a proper solution. I chose a 11 mm lens, with which I could

cause damage during the quick tests, which led to a spot size of 8.42± 0.22 µm and

7.92± 0.19 µm for the x and y direction respectively as described in Sec. 2.2.2. Due

to the different beam diameter of the two lasers I had to try lenses with different

focal length to ensure the same spot size at the amplified laser system. A 35 mm lens

was the best solution even in spot size and in focusing properties. So the achieved

focal spot size at the amplified laser system was 8.95± 0.23 µm and 9.04± 0.21 µm

for the x and y direction respectively as described in Sec. 2.2.1. This approximately

1 µm difference does not influence the LIDT significantly[19, 21].

I measured five samples as shown by Table 5. One grating, with both pure

metal and dielectric overcoated sides, one protected gold, protected aluminum and

protected silver mirror as described in Sec. 2.4. Further on I measured a 800 nm

high reflector mirror made by Optilab Ltd. also presented in Sec. 2.4. The damage

probability results on the mirrors with the 1 kHz laser and the 3.6 MHz laser and

the grating with both lasers are shown in Fig. 15(a), 15(b) and 15(c) respectively.

The LIDT values are summarized in Table 8. In the last column on the ratio

of the kHz and MHz damage thresholds can be seen. Except for the HR800 mirror

all ratios are between 9 and 22 with remarkable stablity irrespective of the sample

type. The approximately x10 scaling factor is noticeable comparing Fig. 15(a) and

15(b) as well. One possible reason for the smaller scaling factor of the HR800 mirror

may be the lower heat conduction ability thus heating effects have longer relaxation

time than time between two pulses at both leasers. The damaging probability of

32

(a) (b)

(c)

Figure 15: Damage probabilities of different samples. Prot. Au, prot. Ag, prot. Al

and HR800 mirrors with the kHz (a) and the MHz laser system (b). Grating with

both kHz and MHz laser system (c).

the grating is plotted with a logarithmic scale on the x axis. This highlights the

difference between the two measurement as the MHz results are in the 0.02-0.1 and

the kHz results are in the 0.2-1 region as observable in Fig. 15(c). The origin of

such a huge difference between kHz and MHz damage thresholds need to be inves-

tigated further on to identify potentially different damage mechanisms (including,

for example, thermally assisted femtosecond damage). However these measurements

represent a first step to investigate an important new phenomenon. The low damage

threshold at high repetition rate has to be taken into account in the construction

33

of some high intensity fiber lasers delivering femtosecond pulses at MHz repetition

rates.

3.4 Measurement Errors

All measurements presented above have errors due to measurement inaccuracies,

laser instabilities and statistical error.

In all cases measurement inaccuracies of the power, the pulse length, the focal

spot size (given by the magnification and/or the beam profiler camera evaluation

software) are estimated to be less than 1%. The laser instability, mostly appearing in

the power instability, caused error higher than the measurement inaccuracies. These

errors were evaluated to account for a total 4-7% error in the measured damage

threshold values.

Above all errors the highest corresponds to the statistical error observed in the

measurement procedure. Due to the properties of statistical errors it is very hard

to evaluate precisely. This type of error originates from the finite numbers and size

of the damage exposure sites and the statistical distribution of surface and bulk

defects on the mirror. Since the damage fluence depends strong on whether the

beam hits a crystal defect or not the results have great statistical fluctuation. This

can be decreased by measuring many samples with the same method or increase the

measurement points of the sample. I estimate the statistical error to be around 15%

according to experiences after a large number of tests.

3.5 Alternative Real Time and Offline Diagnostic Techniques

3.5.1 Thermal Camera

An alternative, useful and not typically used real time diagnostic technique is a

thermal camera with which the thermally assisted femtosecond damage processes

can be investigated. In spite of the fact that heating effects are not expected in

the femtosecond region with kHz repetition rate, upon measuring dielectric samples

the camera gives a nice image of the heating of the sample. In Fig. 16(a). the

evaluation software can be seen with an exposure of a HR800 mirror. The maximum

temperature was nearly 70 degrees. The camera does not work properly with metal

mirrors due to the fact that metal has better heat conductivity and reflects thermal

rays as well. So the heat of the environment is seen by the camera with a very little

heating point as shown in Fig. 16(b).

34

(a) (b)

Figure 16: Thermal camera exposure evaluation on HR800 (a) and Al mirror (b).

The heat camera diagnostics will enable testing damage mechanisms and differ-

ences between kHz and MHz repetition rates in the future. It will help to investigate

the assumption of thermally assisted femtosecond damage at MHz repetition rate.

3.5.2 ZyGO Interferometer

Offline diagnostics can be made by a ZyGO interferometer available in the laboratory

which is a high resolution microscope with white-light interferometric objectives

resulting in a depth resolution of down to 0.1 nm. The sample can be scanned

in z direction giving proper information about the characteristic of the damage.

Two dimensional, a lineout and a three dimensional image made by the ZyGO

interferometer is shown in Fig. 17(a), 17(b) and 17(c) respectively.

It is known that the different pulse length leads to different shaped damage

spots[6, 31] due to different damage mechanisms[32]. Different peak fluence at the

same pulse length gives different sites as well[23]. Therefore it seemed to be interest-

ing whether there are differences between the kHz and MHz laser caused exposure

sites. I took the protected gold mirror, hence I could determine precisely the dam-

age sites on the surface, and scanned the sample with the ZyGO interferometer. I

found that there are similar and different damage sites as well for the two lasers.

The pictures taken by the ZyGO are shown in Fig. 18. Both the kHz and the MHz

laser caused damage sites with bumps to their edges and fey hundreds nm deep

craters sites. Only the kHz laser caused small and very shallow sites with a depth

of approximately 15 nm and only the MHz laser caused sharp and very deep sites

35

(a) (b)

(c)

Figure 17: ZyGO based diagnostic technique shown on Al mirror’s crater: ZyGO

2D image (a), lineout of the 2D image (b), 3D image (c).

with a depth of approximately 2 µm. The mechanisms which correspond to deep

and shallow crater sites of the same laser needs further investigations.

36

(a)kHzbumps3D

profile

(b)kHzbumps1D

profile

(c)MHzbumps3D

profile

(d)MHzbumps1D

profile

(e)kHzshallow

2D

profile

(f)kHzshallow

1Dprofile

(g)MHzshallow

2Dprofile

(h)MHzshallow

1Dprofile

(i)kHzveryshallow

2Dprofile

(j)kHzveryshallow

1Dprofile

(k)MHzverydeep2D

profile

(l)MHzverydeep1D

profile

Figure

18:Sim

ilar

(a)-(h)an

ddifferentdamag

essites(i)-(l)of1kHz(left)

and3.6

MHz(right)

laseron

aThor.

Prot.

Aumirror:

PF05-03-M01

sample.

37

4 Conclusion and Future Plans

4.1 Conclusion

The optical damage threshold of mirrors and gratings play an important role in

laser system manufacturing and development, therefore it is essential to know the

LIDT of the elements used. However, in spite of the abundance of nanosecond and

picosecond data, less information is available for femtosecond damage threshold.

For this reason I made several femtosecond LIDT measurements on typically used

ultrafast optical elements as my Master Thesis work and took conclusions to be

summarized here.

During my master Thesis work, first I acquired useful skills in Ti:S laser sys-

tems, especially in adjusting the laser head and the oscillator, in building and ad-

justing pulse compressors (containing grating pairs and double prism pairs) and in

using femtosecond diagnostic devices such as spectrometers, autocorrelators, beam

profiler, photodiodes, etc. I built a portable LIDT measurement setup and took

measurements on different samples with two different laser systems based on ISO

standards and other consistent methods. I measured and discussed the LIDT of some

gratings and mirrors as well. The results, where comparable, were well matching

the earlier ones found in literature. I showed that even a 3-pair dielectric multi-

layer overcoating on a metal grating can significantly increase its damage threshold

without affecting its diffraction efficiency. For the first time to my knowledge, I also

compared femtosecond damage thresholds with kHz and MHz repetition rates and

found a remarkable order-of-magnitude difference the origin of which requires further

investigation. I investigated practical real time and offline diagnostic methods which

can be applied during LIDT measurements. In addition, I also compared damage

thresholds with different focal spot sizes in tight and loose focusing geometries.

38

4.2 Future Plans

We plan to write an article on the results of these measurements mainly focusing

on the striking differences between the kHz and MHz damage threshold values.

A further task is to devise a semi-automatic detection system with a quantitatively

measurable threshold indicating the onset of damage (e.g. using a simple photodiode

to detect scattered light from the damage sites) and to build a Schlieren detection

setup using automatic image processing and evaluation. With these tools at hand, a

large number of optical components can be tested for ultrashort-pulse femtosecond

damage and even online damage monitoring methods can be devised to enable safe

laser operation at large-scale facilities.

Acknowledgments

I would like to acknowledge my supervisor Peter Dombi, who made possible all

these measurements, gave me a great confidence and guidelines, reacted quickly to

my questions, even on holidays in the last days and encouraged me with his manner.

I have to thank also Lenard Vamos’ and Peter Racz’s practical advices given

me during the LIDT measurements and laser adjustment. I also thank to Daniel

Oszetzky who has helped me taking the ZyGO images.

A very special acknowledgment goes to my parents Tibor Nagy and Marianna

Nagyne Ficza, who believed in me all the time and accepted that I wrote the main

of my Thesis during the feast days of the birth our Lord, Jesus Christ.

39

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