Meas. Sci. Technol. 1 (1990) 1254-1256. Printed in the UK

DESIGN NOTE

Measuring the frequency jitter andspectral width of nanosecond, single-mode laser pulses

J Guéna, Ph Jacquier and L PottierLaboratoire de Spectroscopie Hertziennet, Département de Physique de l'ENS,24, rue Lhomond,F-75231Paris Cédex 05, France

Received 25 June 1990, accepted for publication 24 July 1990

Abstract.A pulse-by-pulseanalysis system provides a simple and direct method formeasuring the pulse-to-pulse frequency jitter and the spectral width of nanosecondpulses from single-mode lasers, up to repetition rates of a few kHz. To test themethod, we measure the jitter (;(; 1 MHz) and bandwidth (~80 MHz) of our pulsedlaser system at À.= 540nm.

1. Introduction

Pulsed lasers are preferred to cw lasers whenever veryhigh instantaneous power is required. For certain appli-cations, however, a narrow bandwidth and a stable cen-tral frequency are important as well. For example, theaim of the present work is to achieve a selective excitationof one HFS component of a highly forbidden atomictransition (Cs 6S1/2-7S1/2), and subsequent Doppler-freedetection by a cw probe laser. (The final purpose is aprecise measurement of parity violation effects in thistransition [1].) This raises the problem of accuratelymeasuring the spectral profile and the frequency jitter ofa pulsed laser system.

The method uses the transmission of a high-finesseconfocal Fabry- Pérot etalon. The novelty is the sub-sequent pulse-by-pulse analysis system, one advantage ofwhich is that the data are flot distorted by any transferfunction of the detection electronics, so that the par-ameters of interest (frequency jitter, spectral width, ...)are readily obtained. This method cao be applied to anysingle-mode laser with a repetition rate up to a few kHz.It is more versatile and easier to set up than spectroscopiemeasurements. We apply the method to the nanosecondpulses of our laser system.

2. Apparatus and method

Figure 1 shows a schematic diagram of the experimentalset-up. The cw laser is a home-made ring dye laser tun-

t Unité de recherche de l'Ecole Normale Supérieure et del'Université Paris VI, associée au CNRS (URA 18).

0957-0233/90/111254+03 $03.50 @ 1990 IOP Publishing Ltd

able around 540 nm (using Rhodamine 110). The fre-quency is locked to the side of the peak of a scannableFabry-Pérot cavity (FPC;FSR1.5 GHz; finesse 110)placedin a temperature-stabilized vacuum chamber. The servo-loop reduces the jitter to less than 1 MHz. The cw beamis fed iota a pulsed iwo-stage amplifier chain. Each stageconsists of a dye cell (Coumarin 540 at 4 x 10- 3 moll-1

in methanol) pumped transversely by a frequency-tripledNd:YAG laser beam (FWHMpulse duration ",15 os; rep-etition rate 12.5 Hz). The amplified pulse has FWHMdur-ation '" 10 os.

The Fabry- Pérot detection cavity is a Tropel Model240 (FSR 1.5 GHz; finesse", 120; instrumental width

Figure 1. Schematic diagram of the laser system (Ieft) andthe spectral analysis system (right).

'" 12MHz). The photodetector is a fast photodiode(0 1 mm, rise lime in the ns range).

According to methods common in nuclear spec-trometry [2], the photocurrent pulse is integrated in acharge amplifier (figure 2) thaï delivers a voltage pulse(short rise lime; 50 J1Sfall lime) whose height is pro-portion al to the light pulse energy. This pulse is fed intoaRC-CR shaping filter, followed by a pulse-stretcher topro vide sufficient lime for subsequent analogue-to-digitalconversion of the pulse height. The data are finally trans-ferred to the computer via a MC 6821 PIAinterface card.A computer-controlled digital-to-analogue converter isused to step the FPCpiezo voltage by small increments(correspondingtypically to '" 1MHz).

The voltage is incremented typically every five lasershots. A complete profile is swept in less th an 1 min.Then the voltage increment is reversed and the profile isimmediately swept backwards. The two curves generallydo flot coïncide, because of thermal drift of the FPC(es-sentially constant over lime periods far longer th an oneforward-backward sweep). While the width of a singlecurve is flot exactly reproducible, the average of the for-ward and backward widths is.

The width of the average curve is the spectral widthof the laser (as far as the instrumental width of the modeanalyser, about 12 MHz, can be neglected). For a givenpiezo voltage, the vertical dispersion of the shots dependson both the frequency jitter and the energy fluctuationsof the laser pulses. ln the half-height regions the sensi-tivity to frequenty jitter is high, near the apex it is low.This provides a method for disentangling the frequencyjitter and energy fluctuations. If necessary, a second seriesof shots can be recorded with the FPC removed, whichyields directly the energy fluctuations. Alternatively onecan implement a second detection channel, similar to thefirst one except thaï it con tains no FPC.The energy andfrequency of every individu al shot are then recorded sim-ultaneously.

Compared with analogue averaging and pen recorderoutput, our method has the advantage thaï the transferfunction of the processing electronics has no effect on theshape of the recorded profile (it concerns only the vertical

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Figure 2. Schematic diagram of the photocurrent pulseprocessing chain. Photodiode load resistor RL= 1 Mf.!;coupling capacitor C,= 100 nF; charge integrator: Ri =10 Mf.!, Ci= 5 pF (mica); CR-RC filter: RC= 3 J-ts;stretcher:Tennelec TC309;AOC: Analog Deviee HAS-1002.

Design note

scale). The shot-by-shot results are directly available, sothaï averages, standard deviations, frequency jitter andeven possible correlations can be computed in a straight-forward, reliable way.

3. Results

Figure 3 shows two typical records. Each cross corre-sponds to a single shot. ln figure 3(a) the cw dye laserfrequency was locked. The vertical dispersion, larger atthe apex of the curve than on the sides, originales mainlyin pulse-to-pulse energy fluctuations (",6% RMS).Oncethese are subtracted from the noise observed at half-maximum, the remaining noise is interpreted in terms oflaser frequency jitter. We ob tain an upper limit of:t 1 MHz. This demonstrates thaï the low jitter of the cwdye laser is practically flOt spoiled in the pulsed amplifi-

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Figure 3. Typical records of the transmitted light pulseenergy plotted against Fabry-Pérot scanning voltage: cwdye laser frequency servo-Iocked (a) or unlocked (b).Voltage Increment ~ 1.6 MHz. One cross per laser shot;five shots at each voltage step; total. record in 40 s (500shots).

1255

Design note

cation process. ln the opposite case where the cw laserfrequency is unlocked (figure 3(b)), the noise in the recordis much larger. It essentially reflects the frequency jitter-:t 10 MHz of the free-running cw laser.

The width of the profile, averaged over severalforward-backward records, is -80 MHz. Since thedetection chain does not introduce any kind of broaden-ing, except for a very small contribution from the instru-mental width of the FPC (-12 MHz), we interpret theobserved width as the spectral width of the laser pulses.It is noticeably above the Fourier Iimit (45 MHz for 10 nspulses).

4. Conclusion

We have presented a direct and simple method for meas-uring the spectral characteristics of nanosecond pulses

up to repetition rates of a few kHz. Its main interest isits ability to give the pulse-to-pulse frequency jitter of .

single-mode pulsed lasers, an important parameter inhigh-resolution spectroscopie measurements performedon short time scales.

References

[lJ Bouchiat MA, Guena J, Jacquier Ph, Lintz M andPottier L 1990 Experimental progress using nonlinearoptics for precision measurements of the nuclear weakcharge in the 6S- 7S Cs transition Opt. Commun.77 374-80

[2J Nicholson P W 1974 Nuc/ear Electronics (New York:WiIey)