  Stable narrow-linewidth light sources play a significant role in many
precision optical systems. Electro-optic laser frequency stabilization systems,
such as the well-known Pound-Drever-Hall (PDH) technique, have been key
components of stable laser systems for decades. These control loops utilize an
optical frequency noise discriminator (OFND) to measure frequency noise and
convert it to an electronic servo signal. Despite their excellent performance,
there has been a trade-off between complexity, scalability, power consumption,
and noise measurement sensitivity. Here, we propose and experimentally
demonstrate a modulation-free laser stabilization technique using an integrated
cavity-coupled Mach-Zehnder interferometer (MZI) as an OFND. The proposed
architecture maintains the sensitivity and performance of the PDH architecture
without the need for any modulation. This significantly improves overall power
consumption, simplifies the architecture, and makes it easier to miniaturize
into an integrated photonic platform. An on-chip microring resonator with a
loaded quality factor of 2.5 million is used as the frequency reference. The
implemented chip suppresses the frequency noise of a semiconductor laser by 4
orders of magnitude. The integral linewidth of the free-running laser is
suppressed from 6.1 MHz to 695 KHz. The passive implemented photonic integrated
circuit occupies an area of 0.456 mm$^2$ and is integrated on AIM Photonics 180
nm silicon-on-insulator process.
