Tunable and ultra-narrow linewidth lasers that are fully integrated remain a missing component and challenge for the thin-film lithium niobate platform, while being useful for applications ranging from data communication to signal processing. Here, we present, for the first time, the demonstration of fully integrated, extended cavity diode lasers combining C-band semiconductor gain chips with TFLN using photonic wire bonding. By leveraging the scalability of photonic wirebonding the laser, with two intra-cavity RSOAs, produces a high on-chip output power of 35 mW and shows single frequency operation with more than 61 dB side mode suppression. By adjusting on-chip heaters the laser can be tuned over >40 nm across the entire gain bandwidth. Using delayed self-heterodyne detection an ultra-narrow, intrinsic linewidth of 1.4 kHz is measured.
We present a framework for designing high-performance Time-Stepped Optical Frequency Comb (TSOFC) lasers comprising: (i) Thin Film Lithium Niobate (TFLN) Integrated Circuit (IC) for high-bandwidth photonic devices; (ii) Semiconductor Optical Amplifier for optical gain; (iii) silicon CMOS IC for drive circuits; and (iv) three-dimensional integration for dense connectivity between CMOS and TFLN. We target high-resolution Optical Coherence Tomography (OCT) as an example application; our TSOFC design switches between 128 optical wavelengths at 3 GHz switching rate (to meet lateral, axial, and temporal resolution requirements) with <2 Watts total system power consumption (using a 180 nm technology node for CMOS IC).
Time-stepped optical frequency comb (TSOFC) sources have extended the imaging capabilities of optical coherence tomography (OCT). However, existing TSOFC laser architectures have significant limitations in cost, complexity, and performance. A recently developed active integrated photonic circuit platform – lithium niobate-on-insulator (LNOI) – has the potential to address these limitations. We designed and fabricated custom LNOI integrated photonic circuits and demonstrated LNOI-based TSOFC sources for OCT. The fabricated LNOI photonic chips support optical frequency switching at the 1 ns time-scale, although the current laser prototypes are limited by the driving electronics to switching times of several hundred nanoseconds.
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