Narrow linewidth lasers have many applications, such as higher order coherent communications, optical sensing, and metrology. While semiconductor lasers are typically unsuitable for such applications due to relatively low coherence, recent advances in heterogeneous integration of III-V with silicon have shown that this is no longer true. In this tutorial, we discuss in-depth techniques that are used to drastically reduce the linewidth of a laser. The heterogeneous silicon-III/V platform can fully utilize these techniques, and fully integrated lasers with Lorentzian linewidth on the order of 100 Hz and tuning range of 120 nm are shown.

We demonstrate a fully integrated extended distributed Bragg reflector (DBR) laser with ∼1 kHz linewidth and over 37 mW output power, as well as a ring-assisted DBR laser with less than 500 Hz linewidth. The extended DBR lasers are fabricated by heterogeneously integrating III-V material on Si as a gain section plus a 15 mm long, low-kappa Bragg grating reflector in an ultralow-loss silicon waveguide. The low waveguide loss (0.16 dB/cm) and long Bragg grating with narrow bandwidth (2.9 GHz) are essential to reducing the laser linewidth while maintaining high output power and single-mode operation. The combination of narrow linewidth and high power enable its use in coherent communications, RF photonics, and optical sensing.

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This paper presents recent results on widely-tunable narrow-linewidth semiconductor lasers using a ring-resonator based mirror as the extended cavity. Two generations of lasers on the heterogeneous Si/InP photonic platform are presented. The first-generation lasers, with a total footprint smaller than 0.81 mm 2 , showed an intrinsic linewidth of ∼2 kHz over a 40 nm wavelength tuning range across C+L bands. The second-generation lasers using ultra-low loss silicon waveguides and a novel cavity design achieved an intrinsic linewidth below 220 Hz. The lasers also possess an ultrawide wavelength tuning range of 110 nm across three optical communication bands (S+C+L). These are records among all fully integrated semiconductor lasers reported in the literature.

Acquisition of laser frequency with high resolution under continuous and abrupt tuning conditions is important for sensing, spectroscopy and communications. Here, a single microresonator provides rapid and broad-band measurement of frequencies across the optical C-band with a relative frequency precision comparable to conventional dual frequency comb systems. Dual-locked counter-propagating solitons having slightly different repetition rates are used to implement a Vernier spectrometer. Laser tuning rates as high as 10 THz/s, broadly step-tuned lasers, multiline laser spectra and also molecular absorption lines are characterized using the device. Besides providing a considerable technical simplification through the dual-locked solitons and enhanced capability for measurement of arbitrarily tuned sources, this work reveals possibilities for chipscale spectrometers that greatly exceed the performance of table-top grating and interferometerbased devices.Frequency-agile lasers are ubiquitous in sensing, spectroscopy and optical communications 1-3 and measurement of their optical frequency for tuning and control is traditionally performed by grating and interferometerbased spectrometers, but more recently these measurements can make use of optical frequency combs 4-6 . Frequency combs provide a remarkably stable measurement grid against which optical signal frequencies can be determined subject to the ambiguity introduced by their equally spaced comb lines. The ambiguity can be resolved for continuously frequency swept signals by counting comb teeth 7 relative to a known comb tooth; and this method has enabled measurement of remarkably high chirp rates 8 . However, signal sources can operate with abrupt frequency jumps so as to quickly access a new spectral region or for switching purposes, and this requires a different approach. In this case, a second frequency comb with a different comb line spacing can provide a Vernier scale 9 for comparison with the first comb to resolve the ambiguity under quite general tuning conditions 9-11 . This Vernier concept is also used in dual comb spectroscopy 12,13 , but in measuring active signals the method can be significantly enhanced to quickly identify signal frequencies through a signal correlation technique 9 . The power of the Vernier-based method relies upon mapping of optical comb frequencies into a radio-frequency grid of frequencies, the precision of which is set by the relative line-by-line frequency stability of the two frequency combs. This stability can be guaranteed by self-referencing each comb using a common high-stability radio-frequency source or through optical locking of each comb to reference lasers whose relative stability is ensured by mutual locking to a common optical cavity.Here, a broad-band, high-resolution Vernier soliton microcomb spectrometer is demonstrated using a single miniature comb device that generates two mutuallyphase-locked combs. The principle of operation relies upon an optical phase locking effect observed in the generation of counter-pr...