In a paper at OFC 2004, to be held in Los Angeles later this month, Bookham Technology will describe the performance of a full band monolithic tunable laser that it claims allows it to challenge the cost/performance characteristics of the current distributed feedback (DFB) technology.
Test modules with the new laser chips have fibre-coupled powers of +16dBm on all C-band channels, and Bookham claims that these are the highest performance monolithic tunable devices yet demonstrated.
Operating over the C-band, the new laser module design uses a digital supermode distributed Bragg-reflector DBR (DS-DBR) laser, monolithically integrated with a semiconductor amplifier (SOA).
Bookham claims that the combination of the base high-power DS-DBR laser coupled to the SOA operating at low gain leads to the breakthrough performance in sidemode suppression and spectral linewidth – better than 45dB and 2MHz on all channels.
The integrated DS-DBR/SOA laser is a surface-ridge device, fabricated by conventional InP processes, and has five main sections. The gain section provides the necessary optical gain within the laser cavity, and the phase section enables continuous tuning via adjustment of the optical cavity length.
The front tuning section is a linearly chirped holographic Bragg grating, and the rear section has an e-beam-written phase grating reflector that provides a sharp and flat comb reflectance response.
The continuous linear chirp of the front grating over the bandwidth of 70nm confers excellent modal discrimination. Output power is controlled by the SOA current; when free running (no tuning current), powers greater than 100mW are possible. Typically, 200mA in the gain section and 150mA in the SOA provides 70mW ex-facet power.The front grating section is the key to the tuning mechanism and is unique to the Bookham design. It is a chirped grating structure that, when activated electrically, selects one of the supermode reflection peaks created by the rear phase grating reflector acting as a comb filter, thereby tuning the laser output.
The front section is a linearly chirped grating with sub-sections addressed by individual contacts. The supermode selected depends on which contact receives current, rather than on the size of the current. This fact is responsible for the laser’s excellent power uniformity, as there is very little current induced absorption in the device.