Contents 1. Introduction
There is increasing demand for high-bit-rate fiber transmission systems that operate at over 100 Gbit/s. One possible solution is a 160-Gbit/s-transmission system employing optical time-division multiplexing (OTDM). In this scheme, demultiplexing in the receiver requires optical clock recovery (OCR) at a subharmonic frequency corresponding to the base data rate. To recover fast subharmonic optical clock signal effectively, we have proposed an optical clock recovery device that uses a regeneratively mode-locked laser diode (MLLD-OCR), and demonstrated successful recovery of 40-GHz [2] and 80-GHz [3] optical clock signals from a 160-Gbit/s data stream. On the other hand, recovery of an optical clock signal at 160 GHz is also indispensable for retiming or wavelength-conversion. So far, several efforts have been made to realize the recovery of an ultrafast optical clock using semiconductor lasers [4], [5], and, very recently, 160-GHz clock extraction has been achieved using a colliding pulse mode-locked laser diode (CPM-LD) [6]. For practical use, a reasonable tolerance of the repetition frequency is also important. In this paper, we report the recovery of an optical clock signal at 160 GHz from a 160-Gbit/s optical data stream using a mode-locked laser diode (MLLD) integrated with a chirped distributed Bragg reflector (DBR). A wide locking range has been achieved at around 162 Gbit/s.
