An Integrated Ultralow Noise Spiral Interferometric Laser

Abstract

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.

Figures (4)

• Figure 1: Spiral MZ Interferometer Configurationa, Layout of a 25-m spiral MZ interferometer on a 2.6 cm $\times$ 2.6 cm semiconductor chip. The zoomed-in sections show the input ports (lower left), output ports (lower right), and the individual waveguide traces of the interferometer. b, Photographs of the spiral MZ interferometer. The top photograph shows the scatter out of the spiral delay line when red light sent is through the input port. The bottom photograph depicts the spiral MZ interferometer with fiber arrays bonded to the input and output ports. c, Spiral waveguide cross section depicting a Si$_3$N$_4$ core region bounded above and below by SiO$_2$ cladding. The optical mode is illustratively shown to be loosely confined by the Si$_3$N$_4$, with most of the power guided in the SiO$_2$. d, Plot of the curvature along the path of the spiral delay line. Over the 25-m length of the spiral, the minimum bending radius is 3.4 mm.
• Figure 2: Spiral MZ Interferometer Characterizationa, Sinusoidal interference pattern measured by two independent photodetectors (PD) tracking the outputs of the spiral MZ interferometer. Balanced detection doubles the slope of the interference fringe. b, Measured RIN of the interrogating laser prior to laser stabilization. Balanced photodetection suppresses the measured RIN by 50 dB. c, Broadband scan of the interferometer response from 1290 nm -- 1365 nm and 1454 nm -- 1530 nm. The zoomed-in sections near 1350 nm and 1460 nm show the sinusoidal interference pattern to be preserved across a broad wavelength range.
• Figure 3: Laser Stabilization to Spiral MZ Interferometera, Schematic of a spiral interferometric laser consisting of an amplified seed laser that probes the interferometer chip. Various components support the locking of the seed laser to the interferometer and also the subsequent measurement of the stabilized laser performance. PD, photodetector; Bal. PD, balanced photodetector; Freq. Comb, frequency comb. b, Measured frequency noise of the spiral interferometric laser. Also shown for comparison are the laser operating without the amplitude modulation (AM) lock, the thermorefractive (TR) noise limit for a 25-meter spiral, the performance limit derived from the laser SNR, and the value of the integrated phase noise (PN) from higher to lower offset frequencies. c, Fractional frequency noise of the spiral interferometric laser with and without the use of AM locking. The AM lock primarily affects the laser's long-term frequency drift. d, Measurement of the laser frequency for three different laser configurations across a one minute time span. The spiral interferometric laser is compared with and without the use of the AM lock. The current best-performing chip-integrated laser is also graphed, whose operation utilizes a 6.1-m spiral cavity for stabilization.
• Figure 4: Optical Frequency Division of Spiral Interferometric Lasera, Spectrum of the laser lineshape before and after optical frequency division to 10 GHz. The resolution bandwidth is 5 Hz and 1 Hz for the optical spectrum and the divided-down microwave spectrum, respectively. b, Measured phase noise of the spiral interferometric laser after optical frequency division to 10 GHz. Also shown are the measured phase noise at optical frequencies and projected to 10 GHz, as well as the phase noise of an ultralow noise oven-controlled crystal oscillator reference (Rohde $\&$ Schwarz SMB100B-B711).