Volume 54, number 6 OPTICS COMMUNICATIONS 15 July 1985 AN EXPERIMENTAL STUDY OF THE PRIMARY PARAMETERS THAT DETERMINE THE TEMPORAL COMPRESSION OF CW Nd:YAG LASER PULSES A.S.L. GOMES, U. OSTERBERG \ W. SIBBETT and J.R. TAYLOR Photonics Group, Optics Section, Blackett Laboratory, Imperial College, London, SW72BZ, UK Received 20 March 1985 An experimental study of temporally compressed CW Nd:Y AG laser pulses using an optical fibre and diffraction grating pair technique has been carried out. The relevant experimental parameters have been varied so that a better insight into the physical processes could be gained and to enable some comparisons to be made with theory. Under optimum conditions an overall 47 X pulsewidth compression and 10 X enhancement in peak power have been achieved. Changes in fibre length, peak power and grating separation gave rise to outputs which showed good qualitative agreement with theoretical predictions. 1. Introduction The temporal compression of ultrashort pulses by means of self-phase-modulation (SPM) induced chirp during propagation in a monomode optical fibre in the regime of positive group velocity dispersion (GVD) and subsequent de-chirp using a suitable dispersive de- lay line has proved to be a powerful technique in short- ening picosecond and femtosecond laser pulses [1-7]. The use of a grating pair as the delay line has been demonstrated to be more effective than other schemes [1] . Several authors have reported successful attempts to compress laser pulses in the picosecond regime [1,2] as well as in the femtosecond regime [3-5] and more recently, relatively "long" (30-100 ps) pulses from a CW mode-locked Nd:Y AG laser have been compressed both at the fundamental frequency [6] and when fre- quency doubled [7]. Theoretical treatments for opti- cal pulse compression have also been reported [8] and an extensive article containing normalised expressions has been published recently [9]. In this letter, we report a study relating the depen- dence of the compression characteristics of the pulses from a CW mode-locked Nd:YAG laser on several of the primary parameters involved; notably fibre length, input peak power, input pulsewidth and grating separa- 1 Permanent address: Institute of Optical Research, KTH, SI0044, Stockholm, Sweden. 0030-4018/85/$03.30 © Elsevier Science Publishers B.Y. (North-Holland Physics Publishing Division) tion. Nonlinear (second-order autocorrelation) and linear (Synchroscan streak camera) measurement tech- niques were employed in order to provide complemen- tary diagnostics. An overal147X pulse shortening and 10X enhancement in peak power have been achieved, in good agreement with other reported results [7]. 2. Experimental The experimental arrangement is shown schemat- ically in fig. 1. The CW mode-locked Nd:Y AG laser (Quantronix model 116) has already been described [10] and it suffices to mention here that it produces ~80 ps pulses at a repetition rate of 100 MHz with an average power of7 W. The non-polarisation main- taining monomode optical fibre had a 7 J1Ill core diam- eter,l dB/km loss at 1.06p.ffi and 35 ps/km nm group velocity dispersion. To establish optimum conditions for compression, various lengths of fibre from 200 to 125 m in steps of 25 m were used. Part of the laser output was directed via beam splitter BSI to a Photocron 11streak camera with an extended SI pho- tocathode [11] which, as well as being used "in-line" to measure the input pulsewidth to the fibre, also dis- played any feedback features caused by reflection at the input face of the fibre. The remainder of the laser beam was focused into the fibre by a X20 uncoated microscope objective. At the output end of the fibre, 377