Pulse Narrowing in a Nonlinear Directional Coupler Consisting of Two Parallel Semiconductor Optical Amplifiers

Abstract

The pulse break-up phenomenon during nonlinear coupling in directional couplers made of Kerr media, such as optical fiber, can lead to pulse narrowing and has been used for passive mode-locking of Er-doped fiber lasers [1,2]. In this paper, we demonstrate the similar mechanism in directional couplers made of two parallel semiconductor optical amplifiers. In such a directional coupler with an appropriate length, the leading edge of an input pulse can couple into the cross amplifier; however, the high power of the central part saturates the bar amplifier and breaks the phase-matching condition for linear coupling. Because of the long recovering time of gain saturation, the trailing edge of the input pulse is effectively depressed, although it stays in the bar amplifier. Therefore, the pulse is effectively narrowed. By placing such a directional coupler in a ring cavity, we have numerically shown that periodical short pulses evolved from a long pulse inside the cavity under appropriate conditions, similar to the passively mode-locking results using a saturable absorber. To extract laser energy, we used the cross amplifier as the output coupler and found that the peak power (pulse width) of the mode-locked output could be higher (shorter) than the intracavity peak power (pulse width). A detailed investigation showed that the relative strength of the output pulse and the intracavity pulse depended on the length of the directional coupler. Also, we found that mode-locking results could be adjusted by the relative gain of the two amplifiers. There exists quite a large parameter space for the mode-locking phenomenon to occur. This implies that the implementation of such a passively mode-locked semiconductor laser should be feasible.