Indirect Amplitude Stabilization of a Tunable Laser Through Control of the Intensity of a Pump Laser by an Electro-Optic Modulator

C H I E U D. TRAN* and RICARDO J . F U R L A N Department of Chemistry, Marquette University, Milwaukee, Wisconsin 53233

A novel method has been developed to stabilize the intensity of a tunable laser. In this method, the tunable laser is amplitude-stabilized indirectly by controlling the intensity of the pump laser through an electro-optic modulator placed between the pump and the tunable lasers. A small portion of the tunable laser beam was split into a reference photodiode to provide a reference signal for the feedback driver to drive the Pockels cell. Any fluctuation in the intensity of the tunable laser is compensated for by varying the intensity of the pump laser through the feedback driven Pockels cell. Results obtained on the Ti-sapphire laser pumped by an ion laser demonstrate that up to 100 × reduction in the laser noise level can be accomplished by use of this method. Furthermore, with this method, it is possible to adjust the intensity of the laser to be exactly equal for different wavelengths, and to maintain this level for as long as the stabilization is activated. Applications of this method for different types of tunable lasers, including dye and F-center lasers, are discussed.

Index Headings: Infrared; Lasers, tunable; Near-IR; Spectroscopic tech- niques.

INTRODUCTION

Applications of the laser to spectrochemical analysis techniques have increased significantly in recent years. In order to be used as a light source for such techniques, a laser needs to be able to tune to particular atomic or molecular transitions. This requirement is due to the fact that most chemical analysis techniques are based on the interactions between the light and atoms or molecules. The requirement limits the applications of ion lasers (e.g., argon, krypton lasers), because these lasers have only a number of discrete lines. Truly tunable lasers, such as dye, Ti-sapphire, and color center (F-center) lasers, are particularly useful for such techniques since they can be continuously tuned over a large wavelength range. However, in spite of their advantages, the use of these lasers is still limited. A variety of reasons may account for this underutilization, but the most likely one is the instability in the intensity of these lasers. In fact, the fluctuations in the output intensity of these tunable lasers are relatively higher than those of gas lasers. This is because the tunable lasers are generally pumped by ion lasers. Therefore, their instability increases because, in addition to their own instability, there is also the instability inherited from the pump lasers.

Considerable effort has been made in the last few years to stabilize laser power. Some success has been achieved, but mainly in the field of ion laser power stabilization. 1-3 This is because of the wide availability and ease of im- plementation of various techniques to these lasers. Tech-

Received 28 September 1992. * Author to whom correspondence should be sent.

niques which are currently available rely on the use of a feedback loop to control either the current or the piezo- electric transducer (which is attached to a high reflector), or an electro-optic modulator (Pockels cell). 1 3 While the former two techniques are effective in stabilizing the power of ion lasers, they also produce an unwanted ef- fec t -namely , beam-pointing or beam-walking instabil- ity. 1-3 As a consequence of this beam-pointing problem, more noise will be introduced into the output of a tunable laser (dye or Ti-sapphire or F-center laser) if the pump ion laser is stabilized by these two techniques.

The method which is based on the use of an electro- optic modulator is powerful in reducing the noise in the laser beam and is not limited to only ion lasers. In this method a laser beam passes through a Pockels cell placed between two polarizers whose axes are parallel to each other. A beamsplitter is used to split part of the laser beam into a reference photodiode whose output provides the reference signal for the feedback loop, which controls the high-voltage supply to the electro-optic modulator. Any fluctuation in the laser beam intensity is detected by the reference photodiode and compensated for by varying the applied voltage via the feedback loop. 1-3

The use of a Pockels cell to stabilize the laser power is, in principle, applicable to all types of lasers. However, in practice it cannot be used to stabilize tunable lasers in the near-IR and IR region (i.e., Ti-sapphire and F-cen- ter lasers). This problem is due not to the principle of the method but rather to the lack of a polarizer and/or a material which has the required electro-optical prop- erties and is transparent in these wavelength regions. 4,5 In fact, it is particularly surprising to discover that, in spite of the significant advances in the field of material science, ADP (NH4H2P04), KDP (KH2P04), and their corresponding deuterated forms (AD*P and KD*P)-- which were discovered more than 40 years ago--still re- main the most widely used crystals for electro-optic mod- ulators. 4,5 These materials cannot be used in the near-IR and IR regions because of the fundamental and overtone absorption in these wavelength regions by their O-H

P

Ion Laser H Pockels + - Ti-Sapphire j F e e d b a c k D r i v e r , , I P D I

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FiG. 1. Schematic diagram of the system to stabilize the intensity of the Ti-sapphire laser by controlling the intensity of the pump laser through the electro-optic modulator. P, polarizer; PD, photodiode.

Volume 47, Number 2, 1993 ooo3-7o2s/93/47o2-o23552.o0/o APPLIED SPECTROSCOPY 235 © 1993 Society for Applied Spectroscopy

• PD

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FIG. 2. C i r c u i t r y d i a g r a m o f t h e f e e d b a c k d r i v e r fo r t h e P o c k e l s cell . R1 = 10 k0 ; R 2 = 3.3 k•; R 3 = R 4 = 12 k~; R 5 = 25 .64 k~; R 6 = 100 fZ; R 7 = 27 ~; R 8 = 1 MO; R 9 = 10 kQ; C1 = 1 n F ; C 2 = C 4 = 10 n F ; C 3 = C 5 = 10 p F ; C 6 = 100 n F ; C I 1 = C I 2 = C I 3 = C I 4 = C I 6 = 1 /4 L F 3 4 7 ( + 1 2 V d c , - 1 2 Vdc) ; C I 5 = P A 8 5 ( + 4 0 0 V d c , - 1 5 V d c ) ; a n d P I = 10 k ~ p o t e n t i o m e t e r .

groups (ADP and KDP) and O-D groups (AD*P and KD*P).4.5

Theoretically, it is possible to amplitude-stabilize tun- able lasers (dye, Ti-sapphire, and F-center) by use of the aforementioned electro-optic modulators. This is be- cause these lasers are pumped by visible radiation from ion lasers, and the modulators are usable in the visible region. Therefore, the modulator is used to control the intensity not of the tunable laser directly but of the pump ion laser. The reference signal for the feedback system is, however, derived from the tunable laser. Because the intensity of the tunable laser is a monotonic function of the intensity of the pump laser, any fluctuation in the intensity of the tunable laser, detected by the reference diode, is compensated for by varying the intensity of the pump laser through the electro-optic modulator. This possibility was investigated for the first time in this study, and the results obtained demonstrate that stabilization of the Ti-sapphire laser can be, in fact, accomplished by

controlling the intensity of the pumped laser through the electro-optic modulator.

RESULTS AND DISCUSSION

A standing wave Ti-sapphire laser purchased from Co- herent (Model 890) was used in this study. It was pumped by a 6-W multi-line argon-ion laser (Coherent Innova 70-5). Since the intensity of the Ti-sapphire laser is a monotonic function of the pump laser power, the inten- sity of the former was controlled indirectly by controlling the intensity of the pump laser. This was accomplished with the use of a transverse type electro-optic modulator fabricated from KD*P crystal (Model 370, Conoptics, Danbury, CT). As illustrated in Fig. 1, the Pockels cell was placed between the ion laser and a Glan-Thompson prism polarizer. The axis of the polarizer was aligned to be parallel to the plane of the polarization of the ion laser. The intensity of the ion laser beam emerging from

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236 Volume 47, Number 2, 1993

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the polarizer is a function of the voltage applied to the Pockels cell. If I 0 is the intensity of the laser incident to the Pockels cell, the intensity of the laser beam emerging from the polarizer,/, in this case, can be written as:

I /Io oc [1 - sin2(k~V + k2)]

where V is the voltage applied to the Pockels cell, and k~ and k2 are characteristic constants of the electro-optic modulator. It is thus evident from this equation that the intensity of the laser beam transmitted from the Pockels cell is inversely proportional to the applied voltage; i.e., increasing the applied voltage results in a decrease in the intensity of the laser beam.

Part of Fig. 2 is the electronic circuitry of the driver which provides the voltage to the Pockels cell. The figure also shows the circuitry of the negative feedback loop which was used to stabilize the power of the Ti-sapphire laser. As shown in Fig. 1, the reference signal for the feedback loop was obtained by splitting part of the Ti- sapphire laser beam with a beamsplitter and was de- tected with a PIN photodiode (PD, United Detector Technology Model 10DP). Since the photodiode provid- ed small output current signal, it was amplified and con- verted to the voltage signal (e.g., Vl), by means of CI1, R1, and C1 (Fig. 2). This amplifier has a current-to- voltage conversion factor of 10 V/milliamperes and a cutoff frequency of about 15.9 kHz. This amplified volt- age signal, Vl, is proportional to the intensity of the Ti- sapphire laser. An integrator (i.e., CI2, R2, and C2) was used to compare the Yl signal with the Y2 reference signal, which was provided by P1. This P1 potentiometer can provide a reference voltage signal of any value be- tween 0 and 12 V. Error signal will be generated when there is a difference between Vl and V2. The integrator was utilized because it provides the error correction volt-

age when there is no error in the power level (i.e., old memory error). The integrated error signal was then am- plified by CI3, CI4, and CI5. The amplified signal was then inverted by CI3 to provide the negative feedback. The feedback loop is closed when the signal, after being amplified by CIh, is connected to the Pockels cell. The last amplification was needed because, in order to vary the power of the transmitted laser beam (from the po- larizer) from 9 to 70% of the incident laser beam, a voltage between 400 and 0 V was required to drive the Pockels cell. The CIh, which can provide up to 40x amplification, brings the signal to the voltage range re- quired to drive the modulator.

For data acquisition, the power of the amplified signal from CI1 was amplified and filtered by an amplifier/low- pass filter (CI6, 10, and C6). The output signal was then connected to a microcomputer [IBM AT compatible with a 386 microprocessor (Northgate Computer Systems, Eden Prairie, MN)] through an AD board of the 12-bit DAS 16 board (Metra-Byte, Taunton, MA).

Shown in Figs. 3 and 4 are the intensities of the Ti- sapphire laser as a function of time without (3a and 4a) and with (3b and 4b) the electro-optic stabilization. As illustrated, the Ti-sapphire laser suffers from short-term and long-term instability. The short-term fluctuation, as shown in Fig. 3a for a 3-s measurement, is probably due to the plasma relaxation oscillations and microphonics induced by the flow of cooling water. 1 The long-term instability, shown in Fig. 4a for the measurement per- formed in 3000 s, is due to the 1/[ noise, the mechanical and optical drifts. 1 As depicted in Figs. 3b and 4b, the stabilization supplied by the electro-optic modulator provides substantial reduction in the low-frequency as well as the high-frequency noises. However, it seems that this stabilization method is more effective in improving

APPLIED SPECTROSCOPY 237

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the stability of the laser in the long term than the short term. Specifically, the ratio of the rms of the laser power with the stabilization and without the stabilization was found to be 111 for the long term (i.e., 3000 s) and 28 for the short term.

The present stabilization method can also be used to maintain the intensity of the Ti-sapphire laser to be constant for different laser wavelengths. For the specific output coupler used in this work, the Ti-sapphire laser can be tuned from 650 to 850 nm. As shown in Fig. 5a, the output laser intensity is different for different wave- lengths. This inequality in the laser intensity can be removed when the stabilization is activated. As shown in Fig. 5b, the electro-optic stabilization not only pro- vides equal intensity of the laser at different wavelengths but also maintains it at this level as long as it is activated.

In conclusion, a novel method to stabilize the intensity of a tunable laser has been developed. The present study, performed with a Ti-sapphire laser pumped by an ion

laser, shows that up to 100 × in the laser noise level can be accomplished by indirectly controlling the intensity of the pump laser by an electro-optic modulator. Because the tunable laser intensity is stabilized by controlling the intensity of the pump laser, the method is applicable to other types of tunable lasers including dye and F-center lasers.

ACKNOWLEDGMENT

The authors are grateful to Mr. Mark Bartelt for his competent technical assistance.

1. P. J. Miller, Photonics Spectra 25, 183 (1991). 2. P. Miller and C. Hoyt, Photonics Spectra 20, 129 (1986). 3. R. F. Enscoe and R. J. Kocka, Lasers & Applications 3, 91 (1984). 4. L. J. Pinson, Electro-Optics (John Wiley, New York, 1985). 5. W. G. Driscoll and W. Vaughan, Handbook of Optics (McGraw-Hill,

New York, 1978), Chaps. 7 and 17.

238 Volume 47, Number 2, 1993