36  Hz integral linewidth laser based on a photonic integrated 4.0  m coil resonator

Abstract

Laser stabilization sits at the heart of many precision scientific experiments and applications, including quantum information science, metrology, and atomic timekeeping. Many of these systems narrow the laser linewidth and stabilize the carrier by use of Pound–Drever–Hall (PDH) locking to a table-scale, ultrahigh quality factor (Q), vacuum spaced Fabry–Perot reference cavity. Integrating these cavities to bring characteristics of PDH stabilization to the chip scale is critical to reducing their size, cost, and weight, and enabling a wide range of portable and system-on-chip applications. We report a significant advance in integrated laser linewidth narrowing, stabilization, and noise reduction by use of a photonic integrated 4.0 m long coil resonator to stabilize a semiconductor laser. We achieve a 36 Hz 1 / π -integral linewidth, Allan deviation of 1.8 × 10 − 13 at 10 ms measurement time, and a 2.3 kHz/s drift—to the best of our knowledge, the lowest integral linewidth and highest stability demonstrated for an integrated waveguide reference cavity. This performance represents over an order of magnitude improvement in integral linewidth and frequency noise over previous integrated waveguide PDH stabilized reference cavities and bulk-optic and integrated injection locked approaches, and over two orders of magnitude improvement in frequency and phase noise than integrated injection locked approaches. Two different wavelength coil designs are demonstrated, stabilizing lasers at 1550 nm and 1319 nm. The resonator is bus-coupled to a 4.0 m long coil, with a 49 MHz free spectral range, mode volume of 1.0 × 10 10 µ m 3 , and 142 million intrinsic Q , fabricated in a CMOS compatible, ultralow loss silicon nitride waveguide platform. Our measurements and simulations show that the thermorefractive noise floor for this particular cavity is reached for frequencies down to 20 Hz in an ambient environment with simple passive vibration isolation and without vacuum or thermal isolation. The thermorefractive noise limited performance is estimated to yield an 8 Hz 1 / π -integral linewidth and Allan deviation of 5 × 10 − 14 at 10 ms, opening a stability regime that heretofore has been available only in fundamentally non-integrated systems. These results demonstrate the potential to bring the characteristics of laboratory-scale stabilized lasers to the integrated, wafer-scale compatible chip scale, and are of interest for a number of applications in quantum technologies and atomic, molecular, and optical physics, and with further developments below 10 Hz linewidth, can be highly relevant to ultralow noise microwave generation.

Chip-based laser with 1-hertz integrated linewidth

“…The result is an order of magnitude lower than what has been reported for the best integrated planar waveguide, fiber, and compact solid dielectric cavities (25,31,32,41,50,51), illustrating the clear advantage of our miniaturized vacuum-gap approach (29). With a 1-s fractional frequency instability of 10 −14 and 1.1-Hz linewidth, our cavity-stabilized laser is capable of probing comparable 1-Hz atomic lines on the 1-s time scale and faster (1,2).…”

“…Using a cutoff of 1/π rad 2 , we obtain linewidths of 8.9 kHz (free-running), 620 Hz (SIL), and 1.3 Hz (PDH). The disagreement between the β-separation line and phase integration methods in the free-running and SIL laser cases highlights the estimation oversimplification for nontrivial frequency noise PSD ( 25 , 49 ). However, in the case of our cavity-locked laser, these estimations agree on hertz-level linewidth.…”

“…Silicon nitride (Si 3 N 4 ) photonics adds even more functionality, taking advantage of complementary metal-oxide semiconductor (CMOS) compatibility, wide bandgap, and low-loss integrated waveguides (21,22). Si 3 N 4 -based lasers have especially leveraged low loss (10,(23)(24)(25) and have demonstrated coherence on par with commercial fiber lasers (26). However, they are ultimately limited by thermo-refractive noise (TRN), which has kept the fractional frequency instability of planar waveguide and solid dielectric resonators above the 10 −13 level typical of quartz oscillators (27).…”

“…Similarly, compact reference cavities have demonstrated excellent frequency stability but lack a path to scalability and integration (31)(32)(33)(34)(35)(36). On the other hand, integrated planar waveguide devices have demonstrated a clear path to efficient scalability but suffer from higher frequency noise inherent to the waveguide medium (25). By using lithographically fabricated micromirrors in our cavity, we capitalize on the noise performance of vacuum-gap systems while introducing parallel manufacturability previously reserved for planar waveguide systems.…”

Compact, Portable, Thermal-Noise-Limited Optical Cavity with Low Acceleration Sensitivity

“…A laser locked to the cavity operates at the cavity thermal noise limit for noise offset frequencies ranging from ∼ 1 Hz to ∼ 10 kHz. To our knowledge, the phase noise level at 10 kHz, at approximately −108 dBc/Hz on the optical carrier, is one of the lowest reported for any vacuum-gap FP [18,28], or dielectric resonator [29,30]. If paired with an optical frequency comb, the laser system can support state-of-the-art microwave phase noise that is comparable to the lowest phase noise achieved to date for offset frequencies above ∼ 100 Hz.…”