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Heterogeneous Integration for Reduced Phase Noise and Improved Reliability of Semiconductor Lasers


Significant savings in cost, power and space are possible in existing optical data transmission networks, sensors and metrology equipment through photonic integration. Photonic integration can be broadly classified into two categories, hybrid and monolithic integration. The former involves assembling multiple single functionality optical devices together into a single package including any optical coupling and/or electronic connections. On the other hand monolithic integration assembles many devices or optical functionalities on a single chip so that all the optical connections are on chip and require no external alignment. This provides a substantial improvement in reliability and simplifies testing. Monolithic integration has been demonstrated on both indium phosphide (InP) and silicon (Si) substrates. Integration on larger 300mm Si substrates can further bring down the cost and has been a major area of research in recent years. Furthermore, with increasing interest from industry, the hybrid silicon platform is emerging as a new technology for integrating various active and passive optical elements on a single chip. This is both in the interest of bringing down manufacturing cost through scaling along with continued improvement in performance and to produce multi-functional photonic integrated circuits (PIC).

The goal of this work is twofold. First, we show four laser demonstrations that use the hybrid silicon platform to lower phase noise due to spontaneous emission, based on the following two techniques, viz. confinement factor reduction and negative optical feedback. The first two demonstrations are of mode-locked lasers and the next two are of tunable lasers. Some of the key results include; (a) 14dB white frequency noise reduction of a 20GHz radio-frequency (RF) signal from a harmonically mode-locked long cavity laser with greater than 55dB supermode noise suppression, (b) 8dB white frequency noise reduction from a colliding pulse mode-locked laser by reducing the number of quantum wells and a further 6dB noise reduction using coherent photon seeding from long on-chip coupled cavity, (c) linewidth reduction of a tunable laser down to 160kHz using negative optical feedback from coupled ring resonator mirrors, and (d) linewidth reduction of a widely tunable laser down to 50kHz using on-chip coupled cavity feedback effect.

Second, we present the results of a reliability study conducted to investigate the influence of molecular wafer bonding between Si and InP on the lifetime of distributed feedback lasers, a common laser source used in optical communication. No degradation in lasing threshold or slope efficiency was observed after aging the lasers for 5000hrs at 70°C and 2500hrs at 85°C. However, among the three chosen bonding interface layer options, the devices with an interface superlattice layer showed a higher yield for lasers and lower dark current values in the on-chip monitor photodiodes after aging.

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