Volume 44, number 5 OPTICS COMMUNICATIONS I February 1983

A NEW SYNCHROSCAN STREAK-CAMERA READ-OUT SYSTEM

FOR USE WITH CW MODE-LOCKED LASERS

DJ. BRADLEY, J. McINERNEY and W.M. DENNISLaser Group, Physics Department, Trinity College, Dublin 2, Ireland

and

J.R. TAYLOR

Optics Section, Blackett Laboratory, Imperial College, London SW7 2AZ, England

Received 15 November 1982

The synchroscan streak-camera system has been further developed to obtain both picosecond temporal resolution anddirect photomultiplier read-out from a single image tube. Advantages incJilde reduction in cost, potentially improved SIN

ratio, greater dynamic range and "free temporal range", and ultimately improved time-resolution in the subpicosecond re­gime. An application to signal-to-noise improvement in Raman spectroscopy is described.

1. Introduction

Mode-locked CW lasers are being increasingly em­ployed []] in in vestigations in the fields of chemistry,biology, physics and optoelectronics. In all of these ap­plications it is necessary to be able to measure with

adequate time-resolution the picosecond and sub-pico­second light pulses generated in the lasers themselvesor luminous phenomena arising from the interactionof the laser pulses with the materials and devices un­der study. During the] 970's the range of ultra-fast ­electron-optical photochronoscopy [2] was extendedto sub-picosecond time-resolution [3] and with theequally rapid progress in developing mode-locked CWlasers during the decade, (initially dye lasers [4] andlater semicond uctor diode lasers [5]) comparable time­resolution has been obtained with the Synchroscanmode of operation of pi cosecond streak-cameras [6].In this mode of operation the image tube deflection

plates are driven inexact synchronization wiJh thepulse repetition rate of the CW laser [7] as originallysuggested by one of us [8]. Space charge effects [9,] 0]in the streak tube are avoided because of the low val­

ues of instantaneous photo-electron currents involved.If an optical multichannel analyser (OMA) is used toreplace -photographic mm as the camera recording

003 -4018/83/0000-0000/$ 03.00 © 1983 North-Holland

medium a completely linear recording,is obtained.With an improved streak-tube (photochron II A) atime-resolution of ~ 1 ps has recently been obtained[] I]. The production ofpu]ses of duration < ]00 fsfrom CW dye lasers [12,13] makes it desirable to im­prove performance of the synchronously driven streak­camera to achieve a comparable time-resolution. Anew streak image tube (the Ph otochron HI) has beendesigned [14] to achieve a time-resolution of 200 fsor less with high spatial resolution over an extended

fi.eld for single-shot operation (streak or frame), or ­when used in the Synchroscan mode. In either case,the use of an OMA detector is needed to maintain

linear photoelectric recording tJUOUgl10ut the camerasystem. Apart from being costly and difficult to alignwith the streak-camera image, the OMA adds noise tothe signal from the Vidi con preamplifier stage [] 0] .Both cost and noise can be greatly reduced by em­ploying a direct photomultiplier read-out. In this newmode of operation the time-resolving deflection platesof the streak-tube are simultaneously used to providea read-out signal by relatively slowly scanning the re­petitive time-dispersed image across a slit (placed per­pendicular to the direction of scan) in front of a photo­multiplier. In this way the deflection system of thestreak-tube takes over the read-out role of the OMA.

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2. Experimental

An important additional>advantage of this system forsubpicosecond temporal resolution is that the time­resolved element read out can always be arranged on­

axis in the image plane of the streak-tube so that de­flection defocussing [15] of the slit image should beeliminated. In addition, the spatial resolution in thetemporal scan direction, and hence the time resolution,will always be optimum on axis. We report the first ex­perimen tal demonstration of this new system and dis­cuss alternative read-out arrangements and an applica­tion to noise reduction in Raman spectroscopy wherethe temporal resolution obtained is only incidental.

3. Discussion

Our preliminary results clearly demonstrate the sim­plicity and ease of operation of this new Synchroscanstreak-camera system. With a passively mode-lockeddye laser source ~ I ps time resolution should be at-

operated in the normal Synchroscan mode. When a

steady image is obtained on the streak-tube phosphorthe 50 Hz read-out deflection voltage is applied to theplates and the resulting doubly time-dispersed streak isimaged on to a 100 Mm slit by a,F/l.3, 50 mm focallength, lens. Care was taken to ensure that the camerainput slit and the read-out slit were matched in widthsafter allowing for magnification through the imagetube. The output signal from the photo multiplier(Hamamatsu TypeR 647101) is displayed on an oscil­loscope, triggered by the 50 Hz supply. Fig. 2 showsthe resulting oscillogram obtained, demonstrating alaser pulse duration of 100 ± 5 ps. The temporal pulseshape with a rise time of ~ 80 ps and a slower tail canbe seen. The recorded pulse halfwidth is clearly limit­ed by the duration of the laser pulses. Since this is theonly laser source currently available to us it is not pos­

sible to demonstrate the full time-resolution capabilityof the Photochron II tube. However, the high qualityof this photomultiplier read-out system can be seenfrom fig. 2. With weaker light sources phase sensitivedetection could be employed.

Fig. 2. Oscilloscope trace of 100 ps recorded pulsewidths fromthe laser. Calibrated pulse separation 800 ps.

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Fig. 1. Experimental arrangement.

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The experimental arrangement is shown schematical­ly in fig. 1. The deflection plates of the Photochron IIstreak-tube are synchronously driven at 100 MHz by aRF power amplifier fed from the oscillator circuit ofthe acousto-optic modulator used to mode-lock theCW Nd-Y AG laser (Quantronix Model 116 R). Afterharmonic generation, the 0.53 Mm train of pulses ispassed in to a Michelson in terferometer delay line toprovide temporal calibration of the streak-tube read­out. The resulting pairs of pulses, separated by a knowninterval are then focussed on the input slit of thestreak-camera. The read-out deflection signal is gener­ated from a 0-500 V, 50 Hz mains supply across a lowinductance chip capacitor in series with the RF ,reso­nance inductance, and both are in parallel with thestreak-tube deflection plates. The system is initially

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Volume 44, number 5 OPTICS COMMUNICATIONS 1 February 1983

tainable with a Photochron II streak-tube. Using thenew Photochron III tube design [14] (with three cy­lindrical focussing electrodes which permit a consider­able reduction in temporal dispersion by maximisingthe mean axial potential in the tube) and an extrac­tion mesh field of 50 kY cm-I a resolution of < 200

fs would be possible since neither intensity dependentspace charge effects [9,1 0] nor deflection defocussingwould reduce the resolution set by time dispersion in­side the tube [16]. With sufficient internal gain in thestreak-tube and efficient optical coupling of the tubephosphor to the photomultiplier, the signal-to-noiseratio and the dynamic range of the overall system areessentially limited by photo-electron statistics. Forvery weak light sources photon-counting could be em­ployed. Multiplexing could be obtained by employingan array of parallel read-out slits coupled to separatephotomultipliers by fibre-optics.

The limit on the speed of read-out is set by the 100J.lS decay time of the P20 phosphor employed. A 100J.lm effective slit width at the phosphor would thenlimit the read-out (slow) scanning speed to 100 cm s-Icorresponding to scanning 5 mm of the time-dispersedstreak at 50 Hz. A greater extent of the streak couldbe read out by operating at a correspondingly lowerscan. A linear slow ramp read-out scan would be suit­able for use with a pen-recorder.

The limiting effect of the phosphor lifetime couldbe removed by employing an internal slit directly fol­lowed by an electron-multiplier. In this case a verysimple fast read-out system could be obtained with asingle voltage drive in the tube deflection plates. If theCW laser train had a repetition frequency of (say) 100MHz and the tube deflection system was driven at 101MHz then the time-dispersed signal would be read-outat a 1 MHz rate, the beat frequency. This mode ofoperation which we have demonstrated at low frequen­cies (50-100 Hz) would be most clearJy described as

. "Stroboscopic Synchroscan". Again, there could bean array of internal slits and electron multipliers togive multiplexing. One particular advantage of thestroboscopic synchroscan technique is that the time­resolved event would extend over the whole of the

periodic time of the scanning voltage (~ IOns) insteadof being limited to the linear part of the voltage si­nusoidal waveform (~2.5 ns) as in the presently usedsynchroscan system. Thus, the "free-temporal range"of the instrument is considerably extended, of impor-

tance in the study of relatively long lived fluorescencelifetimes. The rate of read-out in stroboscopic synchro­scan is lirnited on the low frequency side by the stabil­ity of the laser and the RF voltage sources and on thehigh frequency side by the res.ponse time of the photo­electron detector. It is clear that various combinations

of the stroboscopic and "ancillary scan" read-out sys­tems are possible with a tube manufactured with acentral internal slit followed by an electron-multiplier.The rest of the scan could then be recorded on a

phosphor screen on either side of the photoelectronslit.

Finally, this new arrangement could be used to im­prove considerably the signal-to-noise ratio in sponta­

neous Raman spectroscopy in the presence of a (spec­trally) nearby fluorescence line of nanosecond orlonger lifetime. If the Raman spectrum is excited bya CW mod~ocked laser then the phasing of a singlehigh-frequency scan voltage could be arranged so thatthe temporally dispersed ultra-short duration Ramanemission was emitted through the output slit. Thelonger lived fluorescence emission would be spreadover the whole of the streak-tune image plane. An im­provement in SIN ratio by X 103 could thus be obtain­ed for a 5 ps Raman pulse and a 5 ns fluorescence sig­nal using this "temporal filtering" technique.

4. Conclusion

We have demonstrated a new method for real-time

direct linear measurement of ultra-short light pulsesgenerated by CW mode-locked lasers. Apart from con­siderably reducing the cost of synchroscan streak-cam­era systems by employing a single deflection systemto produce both time-resolution and photoelectricread-out. the system provides opportunities for im­proved signal-to-noise ratios, increased dynamic rangeand "free temporal range" and, ultimately, improvedtime-resolution in the sub-picosecond regime.

Acknowledgement

We wish to thank Professor Y. Kiuchi and Dr. W.E.

Sleat for helpful discussions on electronic techniquesand to acknowledge financial assistance from the IrishNational Board for Science and Technology.

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Volume 44, number 5 OPTICS COMMUNICATIONS I February 1983

References

[I] See e.g. Ultra-short laser pulses, eds. DJ. Bradley, G.Porter and M.H. Key (The Royal Society, London 1980);and Picosecond phenomena n, eds. R.M. Hochstrasser,W. Kaiser and C.V. Shank (Springer Verlag, New York1980).

[2] DJ. Bradley, B. Liddy and W.E. Sleat, Optics Comm. 2(1971) 391.

(3] DJ. Bradley and W. Sibbett, Appl. Lett. 27 (1975) 382.[4] DJ. Bradley, in: Topics in Appl. Physics, Vol. 18. Ultra­

short light pulses, ed. S.L. Shapiro (Springer-Verlag,New York 1977). pp. 17-81 and references therein.

[5] DJ. Bradley, M.H. Holbrook and W.E. Sleat, Proc.1EEE1. Q.E. QE-17 (1981) 658, and references therein.

[6] M.C. Adams, W. Sibbett and DJ. Bradley, Advances inelectronics and electron physics, V01. 52 (AcademicPress London 1979) p. 265.

(7] DJ. Bradley, J. Phys. Chem. 82 (1978) 2259.

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[8] DJ. Bradley, B. Liddy, A.G. Roddie, W. Sibbett andW.E. Sleat, Advances in electronics and electron physics,Vol. 33b (Academic Press Londen 1972) p. 1145.

(91 DJ. Bradley, S.F. Bryant, J.R. Taylor and W. Sibbett,Rev. Sci. Instrum. 49 (1978) 15.

(10] DJ. Bradley, S.F. Bryant and W. Sibbett, Rev. Sci.Instrum. 51 (1980) 824.

[11] W. Sibbett, W.E. Sleat, J.R. Taylor and J.P. Wilson,paper 348-68, 15th Int. Congo on High Speed Photog. andPhotonics, San Diego, August 1982 (to be published bySPIE).

(12] R.L. Fork, B.!. Greene and C.V. Shank, Appl. Phys. Lett.38 (1981) 671.

[13] G. Mourou and T. Sizer, Optics Comm. 41 (1982) 47.[14] DJ. Bradley and K.W. J ones, paper 348-72, 15th Int.

Congo on High Speed Photog. and Photonics, San Diego,August 1982 (to be published by SPIE).

(15] DJ. Bradley, K.W. Jones and W. Sibbett, Phil. Trans.Roy. Soc. London A 298 (1980) 281.

[16] DJ. Bradley and G.H.C. New, Proc. IEEE 62 (1974) 313.