Modulated Quasi-Continuous-Wave Laser Module for Free-Space Optics

A quasi-continuous-wave (QCW) laser module based on a half-bridge structure is proposed for the low-voltage silicon photonics application, which forms a continuous-wave (CW) laser output when it equally distributes the heat dissipation into all lasers. Such a QCW laser module is modulated into a CW laser source for the chip-to-chip or board-to-board communication. The source current is alternatively diverted to the high-side and the low-side lasers by turning the corresponding gallium nitride high-electron-mobility transistor (GaN HEMT) on and off. The current redirection modulates multiple QCW laser outputs into a CW laser output; however, an undesirable laser downtime is produced during the transition time of the current redirection. Although for the 10 Gbps data rate transmission, a short laser downtime period may be scheduled for the time to perform either the laser steering task of the free-space optics (FSO) operation or the data pause for the fan-out delay, which is still preferred to be minimized for higher data rate transmission. The power efficiency and the laser downtime are functions of the parameters of the laser diodes, switch parasitic capacitances, input voltage, and the inductor. According to the mathematical derivation of the circuit response, the circuit design rules and the switching control strategy are provided to achieve high efficiency and low laser downtime. In the experiment, we implemented a laser module to achieve an FSO specification with a laser downtime of less than 3 ns, total harmonic distortion (THD) less than 10%, power efficiency greater than 60% and laser power higher than 1 W.