Design consideration on a DBR-laser transmitter for fast frequency-switching in an optical FDM cross-connect system

The design of a laser transmitter for fast frequency switching in an optical frequency-division-multiplexing (FDM) cross-connect system that uses tunable laser diodes and passive frequency-channel selection filters is considered. Small-signal analysis of the laser rate equation is used to estimate laser switching time. The results show that the frequency switching time depends on the allowable frequency detuning as well as the frequency spacing and the carrier lifetime. Frequency switching time is obtained from the calculation of power penalty due to the frequency detuning at the receiver section with ASK and FSK direct detection schemes. To achieve frequency switching without bit loss in the cross-connect module, switching times of 4-200 ns are required to change frequencies of 500 GHz. This suggests that a duplex transmitter configuration using two laser diodes must be employed to perform fast frequency switching and bit-error-free operation in a cross-connect system. The thermally induced frequency drift is experimentally evaluated. The feasibility of the duplex transmitter configuration is experimentally verified with a 156 Mb/s cross-connect unit