Unisolated 2.5 Gbit/s uncooled gain-coupled 1300 nm DFB lasers for low cost applications

We investigated 1300 nm gain-coupled DFB lasers for low cost isolator free 2.488 Gbit/s applications. The package used was a coaxial laser package, incorporating ball lens coupling optics for reduced cost. The CD was a standard header with no electrical matching resistor. The laser was based on our standard buried heterostructure design, incorporating an n-type doped blocking grating to induce the gain-coupling which was separated from the active region by an InP spacer layer. The real part of the grating coupling strength was approximately 90 cm^-1. The coaxial packages were mounted on a microwave test fixture and tested in an oven based system. A calibrated optical return loss was injected into the package using a polarisation controller and variable optical attenuator. The polarisation state was altered to produce the maximum perturbation. To assess the system performance, the devices were modulated at 2.488 Gbit/s using a NRZ 2 ²³-1 PRBS pattern with a 10:1 extinction ratio. The filtered and unfiltered eye diagrams were measured at -20°C, 25°C and 85°C for 0 dBm mean power. The bit error rate curves were measured back-to-back at the three temperatures and no saturation of the BER curves was found even under -14 dB return loss, down to an error rate of 8e-13. The maximum penalty measured between -40 dB and -14 dB back reflection was 0.75 dB at -20°C for a bit error rate of 1e-10