Photonic integration offers the potential to bring complex high-performance optical systems to the form factor of a compact semiconductor chip. However, the range of system functions accessible critically depends on the extent to which free-space and fiber components can be made integrable. The ultralow-expansion cavity-stabilized laser$-$often used in precision metrology, high-resolution sensors, and advanced systems in atomic physics$-$is one component that currently has no direct parallel on chip. Lasers stabilized to photonically-integrated resonators exist, but exhibit considerably higher frequency noise and are accompanied by large levels of frequency drift. We demonstrate here a new architecture for an ultranarrow linewidth integrated laser based on stabilization to a sinusoidal fringe of an interferometer having a long 25-m unbalanced delay line. Our interferometric laser not only advances the state-of-the-art for on-chip lasers, but we in addition introduce an amplitude locking scheme that greatly suppresses the laser's long-term frequency wander. We achieve a record on-chip fractional frequency noise of $5.6 \times 10^{-14}$, corresponding to a linewidth of 12 Hz centered at 1348 nm. To showcase the utility of this laser, we divide the optical carrier to microwave frequencies, demonstrating the ability to outperform state-of-the-art quartz crystal oscillators by 15 dB or more.
Bulk reference cavities constructed from ultralow-expansion (ULE) glass [1][2][3][4][5] or cryogenic crystalline silicon [6,7] form the basis for the most stable lasers in existence today.
Such lasers are currently the backbone for numerous applications in basic and applied science, including those of trapped-ion quantum computers [8], optical-atomic clocks based on a narrow-linewidth transition [9][10][11][12][13][14][15], ultralow-noise optical-to-microwave frequency synthesizers [16][17][18][19][20], Hertz-level precision spectrometers [21], and high-resolution sensors [22,23].
As a condition for their extraordinary performance, these reference cavities require both active vacuum pumping and a considerable degree of temperature stabilization and isolation from the environment. The extensive stabilization combined with an already sizable optical cavity make cavity-stabilized lasers physically large and too unwieldy for use outside of the laboratory. Yet, in the context of future applications that demand scalability, portability, mass-manufacturability, and robustness, a laser capable of matching the performance of bulk cavity-stabilized lasers while maintaining integrability with other system components on chip remains a highly desirable goal.
Over the last decade, substantial effort has been devoted to developing compact lownoise lasers based upon either miniaturizing the configuration of existing ULE cavities [24][25][26][27] or directly establishing optical cavities on chip for laser stabilization [28][29][30][31]. On-chip cavities, while attractive due to their amenability for system integration, face an increased set of challenges due to (1) thermal noise and drift in the waveguiding material, (2) lower quality factors (Q) resulting from higher propagation loss, and (3) large levels of intensityinduced frequency noise. To date, integrated lasers have showcased exceptionally low levels of intrinsic/fundamental linewidth derived from offset frequencies far removed from the carrier. Yet for lower offset frequencies most relevant to real-world applications, the noise often increases sharply, which considerably degrades the laser’s actual linewidth. The current record performance across all integrated laser technologies was achieved by a 6.1-meter spiral cavity, demonstrating a fractional frequency noise of 7.5 × 10 -14 and a linewidth of 16.7 Hz at 1348 nm wavelength [32]. The long spiral waveguide [33][34][35][36] effectively averages down the laser’s thermorefractive noise, while the ultralow optical losses greatly narrow the width of the cavity’s resonances. However, as a consequence of this optimized geometry, the spiral cavity architecture also reaches a limit where further improvements are possible only if the physical spiral length achievable on chip, the quality factor of the resonances, and the frequency drift at longer time scales can be improved simultaneously.
on using an unbalanced Mach-Zehnder (MZ) interferometer in lieu of a traditional resonant cavity to serve as the master reference. The structure of our interferometer consists of two integrated waveguide couplers with 25 meters of excess spiral waveguide delay on one interferometer arm. Such delay-line interferometers have also been previously implemented in fiber platforms [37,38] and have demonstrated levels of performance rivaling that of cavity-stabilized lasers, but with the caveat of requiring 1 km or more of fiber delay length. A key question remains as to whether chip-integrated interferometers, with their substantially shorter delay lengths, would be viable for laser stabilization. Our work here not only affirms this to be true, but demonstrates that this scheme surpasses the best on-chip stabilization methods of today, thereby enabling continued future scaling of integrated lasers to higher levels of performance. In comparison to prior endeavors in utilizing on-chip interferometers for laser stabilization [39,40], our approach solves two important limitations. First, we avoid the use of any resonant structures within the interferometer (ring resonators, spiral resonators, etc), whose added noise would inadvertently limit the interferometer stability to that of the cavity. Second, owing to our ultralow optical losses, we are able to maximize the spiral mode volume that can be laid out on chip and achieve delay lengths of 25 m, 2800× larger than that previously reported. Beyond these improvements, we introduce an amplitude locking scheme specific to the interferometer architecture that stabilizes its longterm frequency drift by an order of magnitude, down to the level of 24 Hz/s. Altogether, we set a new record for the stability achievable by an integrated laser of 5.6 × 10 -14 , which we then use to perform optical frequency division down to 10 GHz and showcase phase noise 15 dB or more better than state-of-the-art quartz crystal oscillators.
An interferometer offers many important advantages in comparison to a resonator f
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