I5 April 1997

OPTICS COMMUNICATIONS

ELSEVIER Optics Communications 137 (1997) 83-88

Improvement of the pulse and spectrum characteristics of a mode-locked argon laser with a phase-conjugating external cavity

David Gay, Nathalie McCarthy &pipe Lmer ct Opriyue Guidbe. Centre d’optique. Photonique et Laser 1COPLI. D&artement de Physique. Uniuersite’ La&,

QuChec, Canada GIK 7P4

Received 4 October 1996; accepted 20 November 19%

Abstract

We have investigated the effects of a small feedback from a phase-conjugate mirror on the properties of a mode-locked argon ion laser. These properties are the noise in the low frequency range (from 0 to 20 GHz), the width and the shape of the pulse and the optical spectrum. Depending on the laser cavity detuning with respect to the mode-locker period, an external cavity shorter than the laser cavity can control the pulse shape, the temporal fluctuations, the intensity noise in a wide range of frequencies (from kHz to GHz).

Keywords: Phase conjugation; Noise reduction; Mode-locked argon laser; External cavity

1. Introduction

Argon ion laser has found many applications in the industry, medical treatment, and in the pumping of other lasers such as color center lasers and dye lasers because of its ability to produce a high output power in the blue-green wavelength region. Especially for optical pumping, the output beam of the argon laser must exhibit the best transverse intensity profile and the lowest noise level as possible. Over the last decade, theoretical and experimen- tal work has been done on argon lasers to characterize the optical noise due to the gain medium and to improve the behavior of the output beam [l-4]. It has been demon- strated with a CPM (colliding-pulse mode-locked) dye laser optically pumped with a continuous wave (cw) argon laser that the noise of the CPM mainly originates from the longitudinal mode beating of the pump laser [5]. The

insertion of an &Ion inside the argon laser cavity has allowed to run in the single longitudinal mode regime then reducing by up to 50 dB the amplitude noise of the CPM

’ Corresponding author. E-mail: nmccat@phy.ulaval.ca.

emission. However, this operation typically reduces the pump power by a factor of 2.

The reinjection of a small feedback provided by an external cavity terminated by a phase-conjugate mirror (PCM) has been successfully exploited to reduce the noise of a cw argon laser with no loss of power [6]. In mode- locked regime, the gain medium dynamic and the noise behave quite differently. The mode-locking of an argon laser is usually achieved with an acousto-optic mode- locker. When carefully optimized, the laser can produce pulses as short as - 50 ps of duration. The noise arises from the fluctuations of the pulse amplitude, the repetition rate (jitter), the pulse width and from the phase modula- tion. The technique called CPS (coherent photon seeding) has been studied experimentally and theoretically with synchronously-pumped mode-locked lasers and has given great results on the noise reduction [7-l I].

In this paper, we compare the pulse and noise charac- teristics of an argon ion laser mode-locked by an acousto- optic modulator operated without and with a small optical feedback provided by an external cavity terminated by a PCM. The experimental set-up and apparatus are presented in the next section. This is followed by the experimental results. Without optical feedback, there is a compromise to

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84 D. Gay. N. McCarthy/Optics Communications I37 (1997) 83-88

be made between short pulse duration and low-noise am- plitude in different frequency range. The use of the exter- nal cavity allows the operation in shortest pulse with a lowest noise level in different spectral range simultane- ously.

2. Experimental set-up

The set-up of the experiment is shown in Fig. 1. The argon ion laser (Spectra-Physics model 2020-05) is oper- ated at A = 5 I4 nm in the mode-locked regime with an acousto-optic modulator (Spectra-Physics model 342A). In order to synchronize the pulse round-trip duration in the laser cavity L with the resonant frequency of the mode- locker (40.9765 MHz), the output coupler (transmission of 4.6%) is mounted outside the laser at 1.832 m from the laser end mirror. When the mode-locker is turned off, the average output power is 2.2 W with a 50 A plasma current.

The external cavity of length L,,, is terminated by a phase conjugate mirror formed in a single domain BaTiO, crystal (5 mm X 5 mm X 5 mm) used in the 2-m (two interaction regions) configuration [ 121. With the PCM, the external cavity is self-aligned and the reinjected field is best adapted to the intracavity field. The PCM reflectivity is estimated to be - 35%. It is not necessary to optimize the reflectivity since only a small fraction of the conjugate beam is needed to feed back the laser. The amount of feedback is controlled with a variable attenuator. A half- wave plate is placed inside the external cavity in order to obtain the linear polarization of the beam parallel to the plane of Fig. I containing the c-axis of the crystal.

A thick beamsplitter BS, is used to monitor different characteristics of the laser beam. The optical output power, the temporal fluctuations (AC signal) of the optical power and the low-frequency noise spectrum (from 0 to 5 MHz) are measured by photodiode PD, (response time of IO ns). The low-frequency noise measurements of the optical sig- nal are achieved with a Tektronix spectrum analyzer (plug- in 7L5) in a 150 MHz analog Tektronix oscilloscope. A second photodiode PD, is used to measure the feedback level relatively to the output power. The high-frequency noise components (100 MHz-2.5 GHz), covering several

acousto-optic mode-locker

Fig. 1. Experimental set-up of the mode-locked argon ion laser with the phase-conjugating external cavity.

g 400

g 300

x 3 200 P

2 100

0

200

“, 150 n^

L

100

1.5 -1 -0.5 0 0.5 1

50

L - LPmax (mm)

Fig. 2. Variations of the optical output power (left scale, open

circles) and of the pulse width (right scale, full circles) as a

function of the difference AL between the laser cavity length L

and the length L Pmax yielding the maximum output power.

harmonics of longitudinal and transverse mode beating frequencies, are measured with a fast photodetector (Opto-Electronics, model PD-15, risetime of _ 50 ps) and processed by a spectrum analyzer Hewlett-Packard 8563A. Finally, the pulse duration is monitored by another fast photodiode PD, (New Focus, mode1 1404, response time of - 20 ps) connected to a HP-54120B sampling oscillo- scope (bandwidth of 34 GHz).

3. Experimental results

3.1. Without external cavity

The emission characteristics of the laser without any external cavity have been investigated first. With the acousto-optic modulator active, the maximum output power is obtained when the laser cavity length L is equal to L *_ (- c/4f, where f is the resonant frequency of the modulator). In this case, the laser cavity is said to be tuned and the output power is 455 mW with 50 A plasma current. The pulse width is then of the order of 95 ps. Fig. 2 shows the variations of the average output power and of the pulse width as a function of AL (== L - Lpmax), the laser cavity detuning with respect to the modulator reso- nant frequency. We can see that the shortest pulse (dura- tion of 60 ps, as shown in Fig. 3a) is obtained when AL = - 270 pm (here called the length # 1). The pulse is then nicely shaped, without any satellite but the output power is approximately 250 mW.

Another interesting operation point corresponds to A.L = - 300 pm (called the length #2). In this latter case, the low-frequency components of the noise (from 0 to I MHz) are minimum. The upper trace of Fig. 4 shows the noise components of the beam from 0 to 2 MHz for the cavity length #I while the lower trace holds for the length #2. We see that noise reduction reaching 25 dB can be achieved with length #2. The effects of the reduction of these noise components are directly observable by comparing the tem-

D. Gay, N. McCarthy/Optics Communications 137 (1997) 83-88 85

, I ‘..”

--.-,. 1 h N4...+_,_...... -n ,-.._ ,.oQz

“--h”.--.~.r_-_._“: __.“,_ __A

I,., ,, ,,,/,,,/, U// /,i/ ,/,, L,( ‘11.11:

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0:6 0.9 1

TIME (ns)

b

- .

,,e .:j I : . . ‘*

“./ “.‘x .: I_.._ ,.... “... . ...’ ‘L. .,_.,...,.

-L -..%_.\..-”

8. L L ,, ,.A : , . /,,, L _.l ,,. I. i_L, .,.. 1 .L.LS

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.6 0.9 1

TIME (ns)

Fig. 3. Pulse shape (a) with length #I (optical power of 250 mW> and (b) with length #2 (optical power of 280 mW), for the same plasma current.

pot-al fluctuations of the optical output power. Fig. 5a shows the AC-signal amplitude from the photodiode PD, for the length #l, which corresponds to the maximum temporal fluctuations for a given plasma current. Fig. 5b shows that the AC-signal amplitude for the length #2 is reduced by a factor greater than 2.

The investigation of the spectrum in the frequency range of the longitudinal mode beatings with the HP-8563A

Fig. 4. Noise components from 0 to 2 MHz for the cavity length #I (upper trace) and for length #2 (lower trace), recorded with a Tektronix 7L5 plug-in. Vertical scale is IO dB/div.

B

Fig. 5. Temporal fluctuations (AC-signal) of the ouput beam (a) for the length #l and (b) the length #2. Horizontal scale is 50 Fs/div and vertical scale is 200 mV/div.

spectrum analyzer has revealed that cavity lengths # 1 and #2 yield the same amplitude for the beating of adjacent longitudinal modes as shown by the central peak (near 82 MHz) of Fig. 6a and Fig. 6b, respectively. However, the side peaks in these figures reveal that the transverse mode beating amplitudes of length #2 were 24 dB lower than those obtained for length # 1. Despite the noise reduction obtained with length #2, this cavity length is not optimal since it produces a parasitic pulse that follows the main pulse, as shown in Fig. 3b. These pulses are separated by 430 ps while two consecutive main pulses are separated by 12.2 ns. The appearance of this parasitic pulse is accompa- nied by a sudden increase of the output power, passing from 250 to 280 mW (see Fig. 2). This double pulse region has been studied by other authors [2-41. The main pulse with length #2 is slightly longer (67 ps instead of 60 ps) but presents less jitter than the pulse obtained with length #l.

3.2. Wirh the external cavity

The reinjection of a very small fraction of the output laser beam into the laser cavity gives the possibility of

D. Guy, N. McCarthy/ Optics Communications 137 (1997) 83-88

I

a I

1 ! ! !I.! !!! !i

I I I I I I IIll I TIME (ns)

Fig. 6. Output beam spectra at frequencies in the range of the first longitudinal mode beating recorded with a HP-8563A spectrum analyzer (a) for the length #l and (b) the length #2. Horizontal scale is IO MHz/div and the display is centered at 82 MHz. Vertical scale is 10 dB/div.

combining the advantages obtained with lengths #1 and #2 in the same configuration, then improving the overall characteristics of the emission in the shortest pulse. The feedback is provided by an external cavity terminated by a PCM. The feedback level was 2.3 X 10d6, measured out- side the laser cavity. If we take into account the output coupler transmittivity, the feedback measured inside the cavity will be 2 x 10e3 weaker. Such a small feedback corresponds to the energy of a few photons. This corre- sponds to equivalent feedback levels provided in the CPS technique used with other laser types. It consists essen- tially of injecting a coherent seeding pulse ahead of the main pulse in the cavity; the coherent photons forming the injected pulse are amplified and migrate towards the laser pulse. Therefore, the amplified pulse contains relatively less photons due to spontaneous emission than in the case without feedback. The external cavity length I.,,, is 1.77 1 m long corresponding to 6.1 cm shorter than the laser cavity length (set to the value of length #1 for all the measurements done with the PCM external cavity). The value of L,,, is not critical as we will see below.

With a feedback of 2.3 X 10-6, the pulse jitter is reduced. Fig. 7 compares the pulse shape with and without feedback, for the same optical output power. The pulse obtained with the feedback is shown on the left of the

Fig. 7. Pulse shapes with length #l, without any feedback (right pulse) and with the feedback (left pulse).

figure. Its duration is 65 ps and we can see that the pulse shape is smoother than the one obtained without external cavity. The low-frequency noise components for the length #1 without (upper trace) and with the external cavity (lower trace) are shown in Fig. ga. The temporal fluctua- tions of the optical output power (Fig. 8b) occurring with

A

B

Fig. 8. (a) Noise components from 0 to 2 MHz for the cavity length #I without any feedback (upper trace) and with the external feedback (lower trace). Vertical scale is 10 dB/div. (b) Temporal fluctuations (AC-signal) of the ouput beam with the external feedback. Horizontal scale is 50 ps/div and vertical scale is 200 mV /div.

D. Gay. N. McCurthy/Optics Communications 137 0997) 83-88 87

the external cavity can be compared to those shown in Fig. 5. We can see that the noise components, as well as the AC-signal, have been reduced at a level comparable to the one obtained with the length #2.

The measurements of the spectrum of the first longitu- dinal mode beating (originating from the beating of adja- cent longitudinal modes) have shown that the optical feed- back significantly narrows the peak (Fig. 9a). The longitu- dinal mode beating width passed from approximately 1.8

to 0.5 MHz, as measured at the bottom of the peaks. A similar behavior has been observed for the other notes of longitudinal mode beating, at least up to 8 GHz. Moreover, we can see in Fig. 9b that the feedback level we used allows the reduction of the transverse mode beating ampli- tude by up to 20 dB; higher feedback would further

decrease the transverse mode beating but at the expense of a pulse shape deterioration.

Finally, Fig. 10 shows the pulse width and the feedback

level needed to reduce the transverse mode beating ampli- tude (side Peaks of Fig. 9b) by 10 dB as a function of the external cavity length. This reduction of 10 dB corre- sponds to a reduction of the low-frequency components of approximately 15 dB. We can see that the exact value of L,,, is not critical; we observed the same pulse length and a constant behavior of the transverse mode beatings over

Fig. 9. Output beam spectra near the frequency of the first longitudinal mode beating for the length #I without external feedback (upper trace) and with the feedback (lower trace). Hori- zontal scale is (a) 0.5 MHz/div and (b) 5 MHz/div. Vertical scale is 10 dB/div and the display is centered at 82 MHz for both.

ot ’ I I -_j,(p -500 -400 -300 -200 -100 0

Lex,- L (PS)

Fig. IO. pulse width (left scale, open circles) and feedback level needed to reduce the transverse mode beating amplitude (side peaks of Fig. 9b) by 10 dB (right scale, full circles) as a function of the difference between L,,, and the laser length L.

an interval of 10 cm (corresponding to an interval of 300

ps in the advance of the injected pulse with respect to the amplified laser pulse).

4. Conclusion

Our experiments have shown that the reinjection of a very small pulse in the laser cavity ahead of the amplified laser pulse allows operation at shortest pulse duration with a lowest noise level in different spectral bands simultane- ously. Without optical feedback, there was a compromise to make between short pulse duration and low-noise ampli- tude in different frequency bands. With the external cavity, we have reduced the pulse jitter, the low-frequency noise components by up to 25 dB, the longitudinal mode beating width (it passed from 1.8 to 0.5 MHz, as measured at the bottom of the peaks) and the transverse mode beating components by 20 dB. The use of the external cavity was facilitated by the use of a phase conjugate mirror since the beam feedback into the laser was automatically aligned. Moreover, the PCM makes possible the exact evaluation of the beam really feedback into the laser since its size and wavefront were adapted to the intracavity beam.

Acknowledgements

We would like to acknowledge M. Marc D’Auteuil for his technical support and Pr RCal Tremblay for the use of his laser. This work was supported by grants and scholar- ships from Natural Science and Engineering Research Council (Ottawa) and Fonds pour la Formation de

Chercheurs et 1’Aide 3 la Recherche (Qutbec).

References

[I] D. von der Linde, Appl. Phys. B 39 (1986) 201. [2] H.D. Harvey, N. Christensen, J.M. Dudley and D.B. Hirst,

Optics Comm. 97 (1993) 219.

88 D. Gay, N. McCarthy/ Optics Communications 137 (1997) 83-88

[3] A.A. Apolonsky, A.E. Kuklin, SD. Shchebetov and E.I.

Zinin, Optics Comm. 98 (1993) 291.

[4] D.L. MacFarIane and L.W. Casperson, J. Opt. Sot. Am. I? 8

(1987) 1777.

[5] M.C. Nuus, U. Keller, G.T. Harvey, M.S. Heutmaker and

P.R. Smith, Optics Lett. 15 (1990) 1026.

[6] N. McCarthy and D. Gay, Optics Lett. 16 (1991) 1004.

[7] P. Beaud, J.Q. Bi, W. Hodel and H.P. Weber. Optics Comm.

80 (1990) 31.

[8] G.H.C. New, Optics Lett. 15 (1990) 1306.

191 C.J. Hooker, J.M.D. Lister and I.N. Ross, Optics Comm. 80

(1991) 375.

[IO] J.Q. Bi, W. Hodel and H.P. Weber, Optics Comm. 81 (1991)

408.

[I 11 N. McCarthy, S. Mailhot and J.-F. Cormier, Optics Comm.

88 (1992) 403.

[I21 J. Feinberg, Optics Lett. 7 (1982) 486.