<title>Generation Of Sub-Picosecond Pulses From Mode Locked Algaas Semiconductor Lasers</title>

Abstract

Generation of Sub -picosecond Pulses from Mode Locked AlGaAs Semiconductor LasersJ. P. van der Ziel600 Mountain Avenue, Murray Hill, New Jersey 07974AbstractThe pulse widths obtained from A1GaAs lasers have been reduced to the subpicosecondregime by passive and active mode locking of A1GaAs lasers containing a region of saturableabsorption in an external cavity. Bursts of 4 pulses each of 0.65 psec long and separatedby the 11 psec round trip time through the laser have been obtained by passive mode lockingusing a linear external cavity. By inserting a bandwidth limiting element in the cavity,the pulse width increased to ti 20 psec. The widths obtained are in reasonable agreementwith a theoretical model and correspond to dispersion and bandwidth limited propagation,respectively. Pulses as short as 0.56 psec have been obtained by the colliding pulsetechnique using a ring geometry external cavity.IntroductionRecently we reported the first observation of bursts of sub -picosecond pulses generatedusing a semiconductor laser mode locked in an external cavity.1'2 Crucial to the attainmentof such short pulses is the presence of a region of optically saturable absorption in theactive medium. The saturable absorption in our case was introduced by proton bombardmentof the exit facet.3 Mode locking of these lasers yielded pulses as short as 0.65 psecl ina linear external cavity, and 0.56 psec2 in a ring cavity geometry. Mode lockin of laserscontaining degraded regions had previously resulted in pulses as short as 5 psec4'5 and1.3 psec6, and recently as short as 0.58 psec.7 In the absence of saturable absorption,mode locking typically results in pulses of 20 -40 psec duration.4The saturable absorption also significantly modifies the shape of the pulse. Thetemporal behavior of the pulses is obtained from the two beam autocorrelation techniaueusing phase matched second harmonic generation from a LiIO3 crystal. For active mode lock-ing using lasers without saturable absorption, the autocorrelation is highly Gaussian,indicating the pulses have a Gaussian shape. The introduction of saturable absorption foractive mode locking results in a symmetric exponential autocorrelation. This suggests thatthe actual pulse shape is an asymmetric exponential. In the limit of a infinitely steepleading edge the pulse shape is a single sided exponential. This is the shape assumed foractive mode locking. For passive mode locking a sech2 pulse shape is observed; this resultsin an exponential autocorrelation well away from the zero delay of the two beams, but aGaussian like autocorrelation near zero delay.Mode Locking TheoryThe duration and the shape of the pulse are determined by the time dependence of the gain,the saturable loss, and the dispersive phase shift experienced by the pulse as it makes around trip transit through the laser and external cavity. We show that the pulse widthsfrom the theory are in good agreement with the experimental results described in thefollowing section. In the steady state the pulse must replicate itself after each transit.This results in the operator equation for the mode amplitude v(t) of a single pulse propa-gating in an external cavity having for simplicity, the ring geometry.8,9