Enhancing the sensitivity of atom-interferometric inertial sensors using robust control.

Atom-interferometric quantum sensors could revolutionize navigation, civil engineering, and Earth observation. However, operation in real-world environments is challenging due to external interference, platform noise, and constraints on size, weight, and power. Here we experimentally demonstrate that tailored light pulses designed using robust control techniques mitigate significant error sources in an atom-interferometric accelerometer. To mimic the effect of unpredictable lateral platform motion, we apply laser-intensity noise that varies up to 20% from pulse-to-pulse. Our robust control solution maintains performant sensing, while the utility of conventional pulses collapses. By measuring local gravity, we show that our robust pulses preserve interferometer scale factor and improve measurement precision by 10× in the presence of this noise. We further validate these enhancements by measuring applied accelerations over a 200 µg range up to 21× more precisely at the highest applied noise level. Our demonstration provides a pathway to improved atom-interferometric inertial sensing in real-world settings.

Using an injection-locked VCSEL to produce Fourier-transform-limited optical pulses.

In this Letter, we present Fourier-transform-limited, nanosecond scale optical pulses from a vertical cavity surface emitting laser (VCSEL) using injection locking with a narrow-band seed laser. We examine two different injection-locking architectures and show that we can achieve an effective injection-locking range of over 8 GHz with an extinction ratio of 20,000:1. These results indicate that injection-locked VCSELs could become a key component of large-scale photonic quantum networks.

Hertz-level frequency comparisons between diverse color lasers without a frequency comb.

We present a simple yet powerful technique to measure and stabilize the relative frequency noise between two lasers emitting at vastly different wavelengths. The noise of each laser is extracted simultaneously by a frequency discriminator built around an unstabilized Mach-Zehnder fiber interferometer. Our protocol ensures that the instability of the interferometer is canceled and yields a direct measure of the relative noise between the lasers. As a demonstration, we measure the noise of a 895 nm diode laser against a reference laser located hundreds of nm away at 1561 nm. We also demonstrate the ability to stabilize the two lasers with a control bandwidth of 100 kHz using a Red Pitaya and reach a sensitivity of 1Hz2/Hz limited by detector noise. We independently verify the performance using a commercial frequency comb. This approach stands as a simple and cheap alternative to frequency combs to transport frequency stability across large spectral intervals or to characterize the noise of arbitrary color sources.

Ultra-sensitive lithium niobate thermometer based on a dual-resonant whispering-gallery-mode cavity.

We exploit the strong polarization dependence of the thermooptic coefficients in a lithium niobate whispering-gallery-mode resonator to create a self-referenced thermometer. An unprecedented temperature sensitivity of 3.0 GHz/K in the frequency difference between modes of orthogonal polarizations is demonstrated. In order to lock the lasers to the mode resonances, we use a simple intracavity phase modulation approach that provides for superbly low frequency instability. We demonstrate a record room-temperature thermometer detectivity of 40 nK with 1 s of averaging time. Simulations based on the fluctuation-dissipation theorem are performed to calculate the fundamental thermorefractive noise, showing that the detectivity could be improved with reduced laser-locking instabilities.

Bidirectional microwave and optical signal dissemination.

We describe a technique to disseminate highly stable microwave and optical signals from physically separated frequency standards to multiple locations. We demonstrate our technique by transferring the frequency stability performance of a microwave frequency reference to the repetition-rate stability of an optical frequency comb in a different location. The stabilized optical frequency comb becomes available in both locations for measurements of both optical and microwave signals. We show a microwave frequency stability of 4×10(-15) in both locations for integration times beyond 100 s. The control system uses only a standard Ethernet connection.

Coherent radio-frequency detection for narrowband direct comb spectroscopy.

We demonstrate a scheme for coherent narrowband direct optical frequency comb spectroscopy. An extended cavity diode laser is injection locked to a single mode of an optical frequency comb, frequency shifted, and used as a local oscillator to optically down-mix the interrogating comb on a fast photodetector. The high spectral coherence of the injection lock generates a microwave frequency comb at the output of the photodiode with very narrow features, enabling spectral information to be further down-mixed to RF frequencies, allowing optical transmittance and phase to be obtained using electronics commonly found in the lab. We demonstrate two methods for achieving this step: a serial mode-by-mode approach and a parallel dual-comb approach, with the Cs D1 transition at 894 nm as a test case.

Hermetic optical-fiber iodine frequency standard.

We have built an optical-frequency standard based on interrogating iodine vapor that has been trapped within the hollow core of a hermetically sealed kagome-lattice photonic crystal fiber. A frequency-doubled Nd:YAG laser locked to a hyperfine component of the P(142)37-0 I2127 transition using modulation transfer spectroscopy shows a frequency stability of 3×10(-11) at 100 s. We discuss the impediments in integrating this all-fiber standard into a fully optical-fiber-based system, and suggest approaches that could improve performance of the frequency standard substantially.

Saturation spectroscopy of iodine in hollow-core optical fiber.

We present high-resolution spectroscopy of I(2) vapor that is loaded and trapped within the core of a hollow-core photonic crystal fiber (HC-PCF). We compare the observed spectroscopic features to those observed in a conventional iodine cell and show that the saturation characteristics differ significantly. Despite the confined geometry it was still possible to obtain sub-Doppler features with a spectral width of ~6 MHz with very high contrast. We provide a simple theory which closely reproduces all the key observations of the experiment.

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.

Large-core acetylene-filled photonic microcells made by tapering a hollow-core photonic crystal fiber.

We report on kagomé-lattice photonic microcells with low losses, large outer diameters, and large cores. The large (40-70microm) cores are accommodated by tapering the fibers and splicing the reduced ends to a single-mode fiber. We demonstrate the repeatability of this process and obtain splice losses of 0.6dB by optimizing the taper transition length. Narrow electromagnetically induced transparencies and saturable absorption are demonstrated in an acetylene-filled photonic microcell.

10 kHz accuracy of an optical frequency reference based on (12)C2H2-filled large-core kagome photonic crystal fibers.

Saturated absorption spectroscopy reveals the narrowest features so far in molecular gas-filled hollow-core photonic crystal fiber. The 48-68 mum core diameter of the kagome-structured fiber used here allows for 8 MHz full-width half-maximum sub-Doppler features, and its wavelength-insensitive transmission is suitable for high-accuracy frequency measurements. A fiber laser is locked to the (12)C2H2 nu(1); + nu(3) P(13) transition inside kagome fiber, and compared with frequency combs based on both a carbon nanotube fiber laser and a Cr:forsterite laser, each of which are referenced to a GPS-disciplined Rb oscillator. The absolute frequency of the measured line center agrees with those measured in power build-up cavities to within 9.3 kHz (1 sigma error), and the fractional frequency instability is less than 1.2 x 10(-11) at 1 s averaging time.

Double photonic bandgap hollow-core photonic crystal fiber.

We report on the design, fabrication and characterization of hollow-core photonic crystal fiber with two robust bandgaps that bridge the benchmark laser wavelengths of 1064 nm and 1550 nm. The higher-order bandgap arises due to the extremely thin struts of the silica cladding and their fine-tuning relative to the apex size. The optimum strut thickness was found to be approximately one hundredth of the cladding pitch.

A phase-stabilized carbon nanotube fiber laser frequency comb.

A frequency comb generated by a 167 MHz repetition frequency erbium-doped fiber ring laser using a carbon nanotube saturable absorber is phase-stabilized for the first time. Measurements of the in-loop phase noise show an integrated phase error on the carrier envelope offset frequency of 0.35 radians. The carbon nanotube fiber laser comb is compared with a CW laser near 1533 nm stabilized to the nu(1) + nu(3) overtone transition in an acetylene-filled kagome photonic crystal fiber reference, while the CW laser is simultaneously compared to another frequency comb based on a Cr:Forsterite laser. These measurements demonstrate that the stability of a GPS-disciplined Rb clock is transferred to the comb, resulting in an upper limit on the locked comb's frequency instability of 1.2 x 10(-11) in 1 s, and a relative instability of <3 x 10(-12) in 1 s. The carbon nanotube laser frequency comb offers much promise as a robust and inexpensive all-fiber frequency comb with potential for scaling to higher repetition frequencies.