Fred N. Baynes

Dual-microcavity narrow-linewidth Brillouin laser

Ultralow-noise yet tunable lasers are a revolutionary tool in precision spectroscopy, displacement measurements at the standard quantum limit, and the development of advanced optical atomic clocks. Further applications include lidar, coherent communications, frequency synthesis, and precision sensors of strain, motion, and temperature. While all applications benefit from lower frequency noise, many also require a laser that is robust and compact. Here, we introduce a dual-microcavity laser that leverages one chip-integrable silica microresonator to generate tunable 1550 nm laser light via stimulated Brillouin scattering (SBS) and a second microresonator for frequency stabilization of the SBS light. This configuration reduces the fractional frequency noise to 7.8×10^(−14)  1/√Hz at 10 Hz offset, which is a new regime of noise performance for a microresonator-based laser. Our system also features terahertz tunability and the potential for chip-level integration. We demonstrate the utility of our dual-microcavity laser by performing spectral linewidth measurements with hertz-level resolution.

Nano-Kelvin thermometry and temperature control: beyond the thermal noise limit

We demonstrate thermometry with a resolution of 80  nK/Hz using an isotropic crystalline whispering-gallery mode resonator based on a dichroic dual-mode technique. We simultaneously excite two modes that have a mode frequency ratio that is very close to two (±0.3  ppm). The wavelength and temperature dependence of the refractive index means that the frequency difference between these modes is an ultrasensitive proxy of the resonator temperature. This approach to temperature sensing automatically suppresses sensitivity to thermal expansion and vibrationally induced changes of the resonator. We also demonstrate active suppression of temperature fluctuations in the resonator by controlling the intensity of the driving laser. The residual temperature fluctuations are shown to be below the limits set by fundamental thermodynamic fluctuations of the resonator material.

High-Power and High-Linearity Photodetector Modules for Microwave Photonic Applications

We demonstrate hermetically packaged InGaAs/InP photodetector modules for high performance microwave photonic applications. The devices employ an advanced photodiode epitaxial layer known as the modified uni-traveling carrier photodiode (MUTC-PD) with superior performance in terms of output power and saturation. To further improve the thermal limitations, the MUTC-PDs were flip-chip bonded on high thermal conductivity substrates such as Aluminum Nitride (AlN) and Diamond. Modules using chips with active area diameters of 40, 28, and 20 μm were developed. The modules demonstrated a 3-dB bandwidth ranging from 17 GHz up to 30 GHz. In continuous wave mode of operation, very high RF output power was achieved with 25 dBm at 10 GHz, 22 dBm at 20 GHz, and 17 dBm at 30 GHz. In addition, the linearity of the modules was characterized by using the third order intercept point (OIP3) as a figure of merit. Very high values of OIP3 were obtained with 30 dBm at 10 GHz, 25 dBm at 20 GHz and more than 20 dBm at 30 GHz. Under short pulse illumination conditions and by selectively filtering the 10 GHz frequency component only, a saturated power of >21 dBm was also measured. A very low AM-to-PM conversion coefficient was measured, making the modules highly suitable for integration in photonic systems for ultralow phase noise RF signal generation.

Test of special relativity using a fiber network of optical clocks

Phase compensated optical fiber links enable high accuracy atomic clocks separated by thousands of kilometers to be compared with unprecedented statistical resolution. By searching for a daily variation of the frequency difference between four strontium optical lattice clocks in different locations throughout Europe connected by such links, we improve upon previous tests of time dilation predicted by special relativity. We obtain a constraint on the Robertson-Mansouri-Sexl parameter |α|≲1.1×10^{-8}, quantifying a violation of time dilation, thus improving by a factor of around 2 the best known constraint obtained with Ives-Stilwell type experiments, and by 2 orders of magnitude the best constraint obtained by comparing atomic clocks. This work is the first of a new generation of tests of fundamental physics using optical clocks and fiber links. As clocks improve, and as fiber links are routinely operated, we expect that the tests initiated in this Letter will improve by orders of magnitude in the near future.

Geodesy and metrology with a transportable optical clock

Optical atomic clocks, due to their unprecedented stability1–3 and uncertainty3–6, are already being used to test physical theories7,8 and herald a revision of the International System of Units9,10. However, to unlock their potential for cross-disciplinary applications such as relativistic geodesy11, a major challenge remains: their transformation from highly specialized instruments restricted to national metrology laboratories into flexible devices deployable in different locations12–14. Here, we report the first field measurement campaign with a transportable 87Sr optical lattice clock12. We use it to determine the gravity potential difference between the middle of a mountain and a location 90 km away, exploiting both local and remote clock comparisons to eliminate potential clock errors. A local comparison with a 171Yb lattice clock15 also serves as an important check on the international consistency of independently developed optical clocks. This campaign demonstrates the exciting prospects for transportable optical clocks.An atomic clock has been deployed on a field measurement campaign to determine the height of a mountain location 1,000 m above sea level, returning a value that is in good agreement with state-of-the-art geodesy.

High-performance iodine fiber frequency standard

We have constructed a compact and robust optical frequency standard based around iodine vapor loaded into the core of a hollow-core photonic crystal fiber (HC-PCF). A 532 nm laser was frequency locked to one hyperfine component of the R(56) 32-0 (127)I(2) transition using modulation transfer spectroscopy. The stabilized laser demonstrated a frequency stability of 2.3×10(-12) at 1 s, almost an order of magnitude better than previously reported for a laser stabilized to a gas-filled HC-PCF. This limit is set by the shot noise in the detection system. We present a discussion of the current limitations to the performance and a route to improve the performance by more than an order of magnitude.

Testing Lorentz invariance using an odd-parity asymmetric optical resonator

We present the first experimental test of Lorentz invariance using the frequency difference between counter-propagating modes in an asymmetric odd-parity optical resonator. This type of test is {approx}10{sup 4} more sensitive to odd-parity and isotropic (scalar) violations of Lorentz invariance than equivalent conventional even-parity experiments due to the asymmetry of the optical resonator. The disadvantages of odd-parity resonators have been negated by the use of counter-propagating modes, delivering a high level of immunity to environmental fluctuations. With a nonrotating experiment our result limits the isotropic Lorentz violating parameter {kappa}-tilde{sub tr} to 3.4{+-}6.2x10{sup -9}, the best reported constraint from direct measurements. Using this technique the bounds on odd-parity and scalar violations of Lorentz invariance can be improved by many orders of magnitude.

Compact, thermal-noise-limited reference cavity for ultra-low-noise microwave generation

We demonstrate an easy-to-manufacture 25-mm-long ultra-stable optical reference cavity for transportable photonic microwave generation systems. Employing a rigid holding geometry that is first-order insensitive to the squeezing force and a cavity geometry that improves the thermal noise limit at room temperature, we observe a laser phase noise that is nearly thermal noise limited for three frequency decades (1 Hz to 1 kHz offset) and supports 10 GHz generation with phase noise near -100  dBc/Hz at 1 Hz offset and <-173  dBc/Hz for all offsets >600  Hz. The fractional frequency stability reaches 2×10-15 at 0.1 s of averaging.

Low mechanical loss coatings for LIGO optics: progress report

A significant limiting factor on the sensitivity of interferometric gravitational wave detectors has been identified as thermal noise generated by mechanical loss in the high reflectivity dielectric mirror coatings on the test masses. The development of coatings which maintain high optical performance and minimize mechanical loss is therefore vital if the current designs of interferometers are to achieve adequate sensitivity. While the origins of the mechanical loss are yet to be fully elucidated, some progress has been made toward minimizing it, although there is still some way to go before specifications can be met. The work reported here is progress made toward achieving low mechanical loss coatings on behalf of the LIGO consortium. The current directions include attempts to reduce the loss in the coating materials by control of the coating stoichiometry and intrinsic stress. This includes such methods as ion bombardment of the growing films and optimization of post-deposition thermal treatments.

A microrod-resonator Brillouin laser with 240 Hz absolute linewidth

We demonstrate an ultralow-noise microrod-resonator based laser that oscillates on the gain supplied by the stimulated Brillouin scattering optical nonlinearity. Microresonator Brillouin lasers are known to offer an outstanding frequency noise floor, which is limited by fundamental thermal fluctuations. Here, we show experimental evidence that thermal effects also dominate the close-to-carrier frequency fluctuations. The 6 mm diameter microrod resonator used in our experiments has a large optical mode area of ~100 μm2, and hence its 10 ms thermal time constant filters the close-to-carrier optical frequency noise. The result is an absolute laser linewidth of 240 Hz with a corresponding white-frequency noise floor of 0.1 Hz2 Hz−1. We explain the steady-state performance of this laser by measurements of its operation state and of its mode detuning and lineshape. Our results highlight a mechanism for noise that is common to many microresonator devices due to the inherent coupling between intracavity power and mode frequency. We demonstrate the ability to reduce this noise through a feedback loop that stabilizes the intracavity power.