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Optics Letters

External serrodyne modulation for the suppression of low-frequency noise in quadrature interferometry

Vincent Michaud-Belleau, Mark Charlet, Alex Tourigny-Plante, Jean-Daniel Deschênes, Jérôme Genest

Published Dec 9, 2019• 4 citations
Publisher

Abstract

In interferometry, reaching a high signal-to-noise ratio at low frequencies can be challenging when the additive noise is nonstationary. Although this problem is typically solved by inserting a frequency shifter into one of the arms, in some cases, the interferometer cannot or should not be modified in this way. This Letter presents an alternative solution, based on external serrodyne frequency modulation, which is comparable to the typical approach in terms of complexity and performance yet does not require the modification of a passive interferometer. We demonstrate a prototype that achieves frequency shifting at 500 kHz with 89% power efficiency, leading to the wideband suppression of low-frequency additive noise by more than 19 dB. This enables a fully passive measurement of the thermoconductive noise of a 100 m single-mode fiber.

DOI
10.1364/ol.376542

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In interferometry, reaching a high signal-to-noise ratio at low frequencies can be challenging when the additive noise is nonstationary. Although this problem is typically solved by inserting a frequency shifter into one of the arms, in some cases, the interferometer cannot or should not be modified in this way. This Letter presents an alternative solution, based on external serrodyne frequency modulation, which is comparable to the typical approach in terms of complexity and performance yet does not require the modification of a passive interferometer. We demonstrate a prototype that achieves frequency shifting at 500 kHz with 89% power efficiency, leading to the wideband suppression of low-frequency additive noise by more than 19 dB. This enables a fully passive measurement of the thermoconductive noise of a 100 m single-mode fiber.
Atomic state manipulation by laser radiation requires a stringent control of the light phase noise for precise spectroscopy and quantum computation . In this work we describe a novel interferometer recently employed to estimate the phase noise induced by an optical frequency shifter based on serrodyne modulation for optical frequency standards (Barbiero et al., 2023). The interferometer is generated by an acousto-optic actuator working in the Raman-Nath regime. Using the heterodyne detection between the unshifted optical mode with the first higher order optical components , we are able to detect the differential phase between the two optical path arms with a reduction of the phase noise induced by the actuator. We provide a theoretical treatment for the actuator phase suppression and we estimate the detectable phase noise limit of this interferometer. Finally, we discuss the possible applications in the field of precision measurements.
Fiber interferometers are widely used in precision measurements, yet phase-noise suppression over the mHz-kHz band remains a key bottleneck. Here, a specialty hollow-core photonic bandgap fiber (HC-PBGF) is introduced as the sensing arm to reduce temperature-induced phase noise. Comparative measurements show that, relative to a solid-core polarization-maintaining fiber (PMF) interferometer, the HC-PBGF interferometer achieves an ∼18 dB reduction in phase-noise power spectral density (PSD) in the mHz-Hz band while maintaining an ∼7 dB advantage in the Hz-k Hz band, which is qualitatively consistent with the simplified theoretical prediction. These results support the development of HC-PBGF-based broadband low-noise fiber interferometry.
Quantum noise limited interferometry for subhertz phase measurement is of essence in precision metrology applications, such as gravitational wave detection. However, suppression of subhertz classical noises below the shot-noise level is very challenging in practice. Two-frequency interferometry has been previously proposed to avoid low-frequency noises and may be useful for squeezing-enhanced high-precision phase measurement. Here we experimentally demonstrate a subhertz interferometer at the standard quantum noise (shot-noise) limit, by use of dual-frequency coherent probe light at the input and phase-sensitive heterodyne detection to measure the output interfered light. When the interferometer is locked at a dark fringe, the noise floor of the phase variance in the interferometer reaches the shot-noise level down to Fourier frequencies below 1 Hz. This work should pave a way for a realization of subhertz interferometry beyond the shot-noise limit.
Phase estimation has attracted much attention in the field of sensing, where weak measurement has become an effective solution for the study of small physical quantities because of its anomalous amplification property. However, it is difficult for weak measurement to cope with large low-frequency noise signals in the environment due to its extremely narrow linear range, which makes the extension of weak measurement to practical applications a challenging task. In order to solve this issue, we propose and experimentally verify a phase demodulation scheme based on weak measurement, which solves the problem of working point offset caused by the noise through artificially introducing a high-frequency carrier and performing demodulation operations on the output light. The experimental results show that the scheme can improve the signal-to-noise ratio by 17 dB compared to standard weak measurement and ensures low distortion in phase demodulation. Moreover, the scheme is not limited to a specific modulation depth, which greatly improves the robustness of the scheme.
Deterministic frequency manipulation of single photons is an essential tool for quantum communications and quantum networks. We demonstrate a 15.65 GHz frequency shift for classical and nonclassical light using a commercially available quadrature phase-shift keying modulator. The measured spectrum of frequency-shifted single photons indicates a high carrier-to-sideband ratio of 30 dB. We illustrate our frequency shifter's utility in quantum photonics by performing Hong-Ou-Mandel quantum interference between two photons whose initial frequency spectra overlap only partially, and showing visibility improvement from 62.7 to 89.1% after one of the photons undergoes a corrective frequency shift.
Suppressing high-frequency laser phase noise, particularly at frequencies near and beyond typical feedback bandwidths of a few MHz, is a critical yet challenging task in many advanced applications. Feedforward-based methods generally outperform feedback in high-frequency range, but their performances are more susceptible to perturbations. In this work, we focus on the Pound-Drever-Hall (PDH)-feedforward method we demonstrated recently [Yu-Xin Chao et al., Optica 11(7), 945-950 (2024)] and analyze the factors that affect its long-term stability. By constructing a simple circuit allowing for adaptive control of the feedforward gain in response to power fluctuations of cavity transmission, we demonstrate a robust ≥40\geq 40≥40 dB suppression of laser phase noise around 2 MHz and a noise suppression bandwidth up to 50 MHz. In comparison, when using normal PDH feedback, robust noise suppression of over 40 dB can only occur for frequencies below tens of kHz in most setups. Our findings may pave the way for general usage of PDH feedforward and allow for simple construction of low-noise lasers for precise quantum controls and precision metrology.