Rare-Earth-Doped Lasers on Silicon Photonics Platforms

Abstract

Laser sources are of great interest due to their wide applications in detection, sensing, communication, material processing and medicine. Silicon photonics is a promising technology which enables these laser devices to be fabricated in a standard complementary-metal-oxide-semiconductor (CMOS) foundry, with the advantages of reliability, compactness, low cost and large-scale production. In addition to the common method of bonding III-V based lasers on silicon photonics platforms, deposition of rare-earth-doped Al2O3 glass as gain medium for laser integration has proven to be effective for several key reasons. In this thesis, we present integrated lasers based on rare-earth-doped Al2O3 glass, which is deposited on a wafer through a single-step back-end-of-line process. By changing the rare-earth dopant, erbium, thulium, and holmium doped integrated lasers are demonstrated, which provide emission around 1.5 µm, 1.8 µm, and beyond 2.0 µm, respectively. For the erbium doped laser, a narrow linewidth design and a reliable curved cavity design are included, showing the linewidth of 5 kHz and > 6 times lower lasing threshold compared to straight cavity design, respectively. Additionally, the tunable laser designs utilizing both external fiber gain and integrated Al2O3:Er3+ gain are demonstrated, showing tuning over the entire C-band. For the thulium doped laser, a microring cavity design and straight grating based cavity design are included, showing low lasing threshold of 226 μW and high output power of 387 mW, respectively. For the holmium doped laser, a straight grating based cavity design is demonstrated, showing the lasing wavelength beyond 2 μm on a CMOS compatible silicon photonics platform for the first time. In addition, the lasing wavelength shift is demonstrated by changing the Al2O3:Ho3+ gain film thickness. Lastly, applications of the lasers in optical communication, beam steering, and optical frequency synthesis are demonstrated. For optical communication, two system level integrations are demonstrated, including cascading grating based lasers to form a WDM light source, and link up a grating-based erbium doped laser with a WDM filter to achieve athermal synchronization. For beam steering, a CMOS-compatible optical phased array powered by a monolithically-integrated erbium laser is demonstrated, achieving one-dimentional beam steering with a 0.85°×0.20° full-width at half-maximum. For optical signal synthesis, the first silicon photonics based frequency synthesizer with a relative frequency instability < 1×10-15 at 100s is demonstrated. The optical frequency synthesizer utilizes a monolithically integrated erbium doped tunable laser locked to the silicon-photonic-based octave spanning optical frequency combs. These results demonstrate integrated rare-earth-doped lasers to be of interest as efficient monolithic light sources for emerging silicon-based photonic microsystems.