Non-linear Phase Chirp Retrieval of a 1.55μm Gain Switched Pulse Laser using Two Photon Absorption Autocorrelation

Abstract

We suggest a new scheme to retrieve phase chirp of a gain-switched short pulse using a simple interferometric TPA autocorrelation trace and have confirmed the accuracy of the retrieved phase chirp with linear pulse compression.Complete analysis of fast optical pulses down to subpicosecond requires information about both the amplitude and the phase of the pulses. The phase information of a pulse has received a great deal of special interest because the minimized chirp is one of the conditions to generate the shortest pulses [1,2]. Several methods have been developed to obtain the intensity and the phase chirp information of a pulse [3,4]. Of all, frequency resolved optical gating (FROG) has been used mostly to characterize optical pulses, but it is very sensitive to phase matching conditions in a nonlinear crystal and requires relatively expensive components and complicated algorithm to retrieve pulses [5]. In this paper, an interferometric TPA autocorrelator using a Si-APD was utilized to diagnose a gain-switched pulse laser. In general, a gain-switched pulse includes an inherent problem of a large frequency chirp across the pulse due to variations in carrier density in the gain region of an LD during the buildup time of a pulse [6][7][8].A new scheme based on fitting analysis of an autocorrelation trace to reconstruct the phase chirp of a pulse was introduced. Comparing the intensity autocorrelation curve and the envelope of the second-harmonic (SH) interference fringe in an autocorrelation trace with some fitting analysis, we were able to retrieve the phase chirp of a gain-switched pulse. For practical applications of gain switched laser pulses, chirp compensation is usually employed to make pulses temporally narrower. After fitting analysis we did a linear pulse compression experiment using a dispersion compensating fiber (DCF) in order to verify the retrieved phase chirp of the pulse. We compared measured pulsewidths with those of numerical simulation for linearly compressed output pulses after propagating different lengths of a DCF. For the calculated initial phase chirp of a gain-switched pulse the linearly compressed pulse widths after propagating various lengths of a DCF agree quite well with the simulated results. We have shown that this complete pulse characterization method with a simple fitting process of an autocorrelation trace allows us to predict a condition for the optimized pulse compression for a gain-switched pulse.The schematic setup of interferometric TPA autocorrelator is sketched in figure 1. The various [ Fig.1 Schematic diagram of autocorrelator] semiconductor devices such as photodiodes, laser diodes and light emitting diode can be used as two photon absorbers [9][10][11] and in this experiment we used a Silicon Avalanche photodiode whose internal multiplication factor is 100. A step motor was used to scan an optical delay line of a Mach-Zehnder interferometer. The sweeping speed of the motor was 3.1 ps/sec, which gives the center frequency of corresponding fringe resolved auto...