Optical PPM generator by direct-frequency shifting

Abstract

Summary form only given. Modulation formats for optical communication systems [e.g., return to zero (RZ) versus non-return to zero (NRZ)I is at present a subject of intense discussion. Recently new format options for RZ systems, such as pulse position modulation (PPM), quasi-soliton, and nearly return to zero have been proposed in order to increase the transmission capacity. PPM is an interesting option because the fact that we always have pulses (for mark and space) makes the system more independent of the bit sequence, thus facilitating clock recovery and alleviating problems arising from average power variations in the bit stream. Also, in the case of solitons, the PPM format allows for the exploration of stabilization techniques based on soliton trapping effects. One of the major problems for PPM implementation in high-bit-rate systems is the lack of simple and reliable schemes for the pulse position control and detection. Presently, this requires special modulators and precise synchronization. We present a modulator-less scheme of PPM based on a simple configuration, which converts a digital FM signal applied directly on the laser source to a PPM signal with a natural synchronization pulse.