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

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.