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Register a new userUltra-Narrow Linewidth Brillouin Lasers with Nanokelvin Thermometry
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Ultrastable lasers serve as the backbone for some of the most advanced scientific experiments today and enable the ability to perform atomic spectroscopy and laser interferometry at the highest levels of precision possible. With the recent and increasing interest in applying these systems outside of the laboratory, it remains an open question as how to realize a laser source that can reach the extraordinary levels of narrow linewidth required and yet still remain sufficiently compact and portable for field use. Critical to the development of this ideal laser source is the necessity for the laser to be insensitive to both short- and long-term fluctuations in temperature, which ultimately broaden the laser linewidth and cause drift in the lasers center frequency. We show here that the use of a large mode-volume optical resonator, which acts to suppress the resonators fast thermal fluctuations, together with the stimulated Brillouin scattering (SBS) optical nonlinearity presents a powerful combination that enables the ability to lase with an ultra-narrow linewidth of 20 Hz. To address the lasers long-term temperature drift, we apply the narrow Brillouin line as a metrological tool that precisely senses a minute change in the resonators temperature at the level of 85 nK. The precision afforded by this temperature measurement enables new possibilities for the stabilization of resonators against environmental perturbation.
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We demonstrate a hybrid integrated and widely tunable diode laser with an intrinsic linewidth as narrow as 40 Hz, achieved with a single roundtrip through a low-loss feedback circuit that extends the cavity length to 0.5 meter on a chip. Employing solely dielectrics for single-roundtrip, single-mode resolved feedback filtering enables linewidth narrowing with increasing laser power, without limitations through nonlinear loss. We achieve single-frequency oscillation with up to 23 mW fiber coupled output power, 70-nm wide spectral coverage in the 1.55 $mu$m wavelength range with 3 mW output, and obtain more than 60 dB side mode suppression. Such properties and options for further linewidth narrowing render the approach of high interest for direct integration in photonic circuits serving microwave photonics, coherent communications, sensing and metrology with highest resolution.
Optical frequency stabilization is a critical component for precision scientific systems including quantum sensing, precision metrology, and atomic timekeeping. Ultra-high quality factor photonic integrated optical resonators are a prime candidate for reducing their size, weight and cost as well as moving these systems on chip. However, integrated resonators suffer from temperature-dependent resonance drift due to the large thermal response as well as sensitivity to external environmental perturbations. Suppression of the cavity resonance drift can be achieved using precision interrogation of the cavity temperature through the dual-mode optical thermometry. This approach enables measurement of the cavity temperature change by detecting the resonance difference shift between two polarization or optical frequency modes. Yet this approach has to date only been demonstrated in bulk-optic whispering gallery mode and fiber resonators. In this paper, we implement dual-mode optical thermometry using dual polarization modes in a silicon nitride waveguide resonator for the first time, to the best of our knowledge. The temperature responsivity and sensitivity of the dual-mode TE/TM resonance difference is 180.7$pm$2.5 MHz/K and 82.56 $mu$K, respectively, in a silicon nitride resonator with a 179.9E6 intrinsic TM mode Q factor and a 26.6E6 intrinsic TE mode Q factor. Frequency stabilization is demonstrated by locking a laser to the TM mode cavity resonance and applying the dual-mode resonance difference to a feedforward laser frequency drift correction circuit with a drift rate improvement to 0.31 kHz/s over the uncompensated 10.03 kHz/s drift rate. Allan deviation measurements with dual-mode feedforward-correction engaged shows that a fractional frequency instability of 9.6E-11 over 77 s can be achieved.
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