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The semiconductor laser (SCL) is the principal light source powering the worldwide optical fiber network. The ever-increasing demand for data is causing the network to migrate to phase-coherent modulation formats, which place strict requirements on the temporal coherence of the light source that no longer can be met by current SCLs. This failure can be traced directly to the canonical laser design, in which photons are both generated and stored in the same, optically lossy, III-V material. This leads to an excessive and large amount of noisy spontaneous emission commingling with the laser mode, thereby degrading its coherence. High losses also decrease the amount of stored optical energy in the laser cavity, magnifying the effect of each individual spontaneous emission event on the phase of the laser field. Here, we propose a new design paradigm for the SCL. The keys to this paradigm are the deliberate removal of stored optical energy from the lossy III-V material by concentrating it in a passive, low-loss material and the incorporation of a very high-Q resonator as an integral (i.e., not externally coupled) part of the laser cavity. We demonstrate an SCL with a spectral linewidth of 18 kHz in the telecom band around 1.55 μm, achieved using a single-mode silicon resonator with Q of 10 6 . narrow linewidth | silicon photonics | phase noise | coherent optical communications A lmost from the inception of the semiconductor distributed feedback (DFB) laser, there has been a continuous effort to improve its coherence. The methods used to this end include long cavities (1), longitudinal mode engineering via multiple phaseshifts (2, 3), optimization of the active medium [e.g., strained quantum well (QW)] (4), and wavelength detuning (5, 6). Progress has been hindered by the inevitable penalty paid for the coherencelimiting optical absorption, the result of spatially colocalizing both photons and electrons in a highly absorbing active medium.The finite coherence of laser light is of fundamentally quantum-mechanical origin, the result of spontaneously generated photons entering the lasing mode from the active region of the laser medium. Under the effect of many independent spontaneous emission events, the laser field phasor performs a random walk in the complex plane, which results in a phase excursion given by (7)where N th is the number of excited carriers at threshold, W sp is the spontaneous emission rate ðs −1 Þ into the lasing mode, n is the average number of coherent photons in the lasing mode, α is the linewidth enhancement factor due to coupling of amplitude and phase fluctuations, and τ is the symbol duration (s). The numerator and denominator of Eq. 1 conceptually represent spontaneous photon generation and photon storage, respectively. Increasing the quality factor, Q, of the laser cavity provides a double benefit to phase noise by reducing the number of excited carriers needed to reach threshold, thus decreasing spontaneous photon generation while increasing photon storage.The quality factor of conventional I...

SignificanceThe semiconductor laser, arguably the most versatile member of the family of lasers, has become a technological staple of a massively interconnected, data-driven world, with its spectral purity (i.e., temporal coherence) an increasingly important figure of merit. The present work describes a conceptually fundamental “recipe” for the enhancement of coherence, predicated on direct control of the coherence-limiting process itself, the field–matter interaction. As such, it is inherently adaptable and technologically scalable. As photonic materials and fabrication techniques continue to improve, the described approach has the potential of serving as a roadmap for major and sustained improvements in coherence. With experimentally demonstrated coherence limited at 1 kHz in this work, we envision “deep” sub-kilohertz-level coherence to be soon within reach.

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