Experimental Demonstration of Wavelength-tunable In-Series DFB Laser Array with 100-GHz Spacing

We have proposed and experimentally demonstrated a four-channel wavelength-tunable in-series DFB laser array with 100-GHz channel spacing. Compared to the in-parallel laser array, the optical combiner is not utilized and the extra power loss induced by the combiner is avoided. A linearly chirped Bragg grating with multiple phase shifts implemented in the laser cavity is utilized to reduce the grating interference among different lasers. Besides, the reflector sections are inserted at two ends to provide grating feedback for two side channels. With the feedback, all the channels work with low threshold currents. The linearly chirped Bragg grating requires precise control of grating phase variation, which is quite difficult for traditional e-beam-lithography-based method. Here we used the reconstruction-equivalent-chirp technique to fabricate the grating, and precise control of the grating phase is achieved. From the experimental results, the laser arrays work with high output power (>10 mW), relative intensity noise near the relaxation oscillation frequency of below -130 dB/Hz, stable single-mode operation, and high-uniform channel spacing. Fast channel switchings (<600 ns) are obtained by on/off laser switching. Such laser performance enables the proposed laser array to be applied in the dense wavelength division multiplexing technology, especially when fast channel switching is needed, such as the NG-PON2 system.