Chirp Compensation in Active Mode-Locked Semiconductor Diode Laser Using a DFB

This thesis investigates the possibility of canceling chirp in an actively modelocked semiconductor laser using a DFB structure. The chirp is caused by the nonlinearity of the diode, namely, the line-width enhancement factor, a. The mode-locking equations are formed, and the postulated solution is a gaussian pulse shape. The diode is modeled primarily using the rate equations for carrier and photon densities, and the DFB is modeled by transfer matrix theory. Using these models, a program was written to simulate a one-way ring structure. A mathematical analysis of the carrier density, given current and pulse shape, enabled predictions on gain. From these results, the modelocking equations could be solved, thus defining the pulse uniquely and determining the amount of dispersion necessary for no chirp. Once dispersion was known, the DFB structure could be designed. First, numerical simulations were run with no dispersion and a=O and also a=5. These simulations demonstrated that the program works, and that a does produce a significant amount of chirp. Following these results, simulations were run with the DFB designed for three different amounts of dispersion: Dfb=1.2662x10 2 5s 2 , 2.5332x10-2 6 s2, and 7.1061x10-2 7s 2 . At Dfb=1.2662x10-2 5s2, where no chirp was calculated, there was very little -chirp, but double pulsing was a major problem. At Dfb=2.5332x10-26s2, there was less double pulsing, and the chirp seemed acceptably low. At Dfb=7.1061x102 7s2, there was quite a bit of chirp, however the pulse was clean. It is suggested that the best among the three is Dfb=2.5332x1026s2 with K=250cm 1 and a=5. Thesis Supervisor: Professor H. A. Haus Title: Institute Professor