A new method for electrical frequency locking of optical FDM transmitters

A novel method for locking the frequency spacing between semiconductor lasers electrically is presented. The method is a modification of the offset frequency master-slave principle. The method allows the frequency spacing between optical channels to be chosen independently, with an accuracy determined by the selectivity of the electronics, and it improves the frequency stability of the locked lasers. A rigorous analytical model of the locking configuration is presented. This model is used to investigate how laser frequency noise and electrical noise from the locking circuit affect the frequency stability of the locked lasers. It is found that the improvements in frequency stability can be achieved for up to 350 lasers, each having a linewidth of 1 MHz. Consequently, the method is suited to lock the transmitters in a coherent optical FDM system.>