Evidence of gain modulation in km-long lasers DRAFT

Gain modulation in km-long lasers is experimentally demonstrated and explained. 1. Introduction A new kind of laser raises much attention for a few years. It consists in a Reflective SOA, a km-long fibre, an AWG and a mirror. Thus, the round-trip time of photons is in the s scale, and yet output power can be modulated at Gbit/s scale frequencies [1, 2, 3]., that is to say with modulation periods much faster than the photon lifetime. Up to now, this unusual property has only been explained by self-gain modulation effects in the RSOA. In other words, the circulating data coming back from the remote mirror would be erased by gain compression of the RSOA, whose output power could therefore only depend on the injected current. However, it is usually assumed that gain of the active section of a laser should be clamped, and consequently the physical phenomena leading to a clean modulation of the self-seeded cavity are not fully understood. In this paper, we will first measure the gain compression dynamics of a standard RSOA [4], and then measure the static gain compression of a RSOA when placed in a cavity. Then, we will explain why gain compression is possible in this kind of lasers. 2. Self-gain modulation of a RSOA An optical signal is launched into a standard RSOA, and the reflected signal is analyzed on a sampling oscilloscope. Both the input signal and the RSOA current can be either continuous (CW) or modulated at 2.5 Gbit/s with an extinction ratio of 6 dB and 3 dB respectively.