Design and Fabrication of Wide Wavelength Range 25.8-Gb/S, 1.3-μM, Push-Pull-Driven DMLs

We design and fabricate wide wavelength range directly modulated lasers (DMLs) on the same InP wafer to realize a large-capacity monolithically integrated light source array up to 1 Tb/s. We demonstrate a 25.8-Gb/s push-pull operation over a 30-nm wavelength range, which is conventionally about 14 nm for 100 GbE applications. To extend the operating wavelength range, we design the wavelength dependence of the differential gain (∂G/∂n) for an InGaAlAs multiple quantum well structure, and realize high frequency relaxation oscillation resulting from the high differential gain over a wide range. Next, we also design the wavelength detuning (Δλ) under an operating injection current condition by taking account of the thermal effect of the chip, because Δλ determines ∂G/∂n, and the threshold current of the laser diode (LD). In addition, to achieve a 25.8-Gb/s push-pull operation, we fabricate a ridge waveguide structure buried in benzocyclobutene (BCB) with a low parasitic capacitance, and electrically isolate the DML from the neighboring chip by etching off n-InP. By using this design and structure, we achieve a 3-dB-down frequency bandwidth of over 20 GHz from 1290 to 1320 nm. We also achieve a mean output power of 8.0 dBm, and a dynamic extinction ratio of 5 dB. We measure the 25.8-Gb/s transmission characteristics, and obtain clear eye openings for a back-to-back configuration. We also measure the bit-error-rate performance, and obtain error-free operation for a 30-nm operating wavelength range.