The direct modulation bandwidth of widely tunable DBR laser diodes

Widely tunable laser diodes are currently regarded as promising light sources for WDM communication systems in which they can economically be used as spare transmitters. Their functionality could however also be useful in a number of new system concepts that are currently under investigation. For some of these new system concepts, it is relevant to know up to which bit rate the widely tunable laser diodes could be directly modulated. We have studied the direct modulation properties of 3 types of widely tunable laser diodes: the Altitun GCSR laser, the NTT SSG-DBR laser and the Marconi SGDBR laser. Different types of measurements, from parasitic-free RIN measurements to eye-diagram measurements and link experiments, have been carried out. None of the investigated laser designs however was particularly optimised for high-speed operation. From the RIN measurements, the K-factor and the theoretical maximum modulation frequency under small-signal modulation have been derived. For all investigated lasers and for typically 40 to 50 channels with a spacing of 100 GHz, the theoretical maximum modulation frequency was always found to be close to and generally over 10 GHz.