Zitong Feng

In a hollow-core fiber (HCF), light propagates through an air/vacuum core rather than a solid material, resulting in a low thermo-optic coefficient and ability to handle high powers. Here, we demonstrate a laser locked to a hollow-core fiber reference, which thanks to the low HCF thermal sensitivity, shows long-term stability an order of magnitude better than compact commercially available low-noise lasers. The laser frequency variation within ±600 kHz was measured over 50 h. The stability of our proof-of-concept laser is ensured via a strong self-injection ratio of −15 dB, enabled by the high-power handling and low loss of the hollow-core fiber’s resonator. Moreover, our results show appealing performance parameters, including a fractional frequency stability of 4×10⁻¹³ at 1 s averaging time and a Lorentzian component of the linewidth of 0.2 Hz.

Unfortunately, we have made a mistake on the name of co-author and missed the DOI of our dataset. It would have no influence on any of our results, and the correct name and DOI are given here.

SummaryQ factor over 90 billion is demonstrated for the first time in hollow core fibre based Fabry-Perot resonators. This is achieved thanks to low-loss of the latest hollow core fibers. Such high Q factor together with other beneficial properties of hollow core fibers such as low thermal sensitivity and high power handling is of interest for a range of applications including laser stabilization in metrology.

We demonstrate the simultaneous distribution of stable frequency and data signals over hollow-core fibers (HCF). We show that the use of HCF reduces frequency stability degradation observed in links made of standard single-mode fibers (SMFs) due to nonlinear interaction between the co-propagating signals which is orders of magnitude weaker in HCF.

Light traversing an optical fiber is subject to various local phase perturbations driven by temperature. This thermal phase sensitivity is undesirable in fiber interferometers and their applications which require that a fixed, stable phase be received after propagation. The use of hollow core fiber (HCF) has been shown to reduce this thermal phase sensitivity over solid core fibers and here we propose and demonstrate how coiling HCF to a prescribed geometry can further significantly reduce this sensitivity. Our proof-of-concept experiment shows reduction by a factor of ∼90 with respect to the uncoiled HCF, and over three orders of magnitude with respect to uncoiled solid core optical fiber. Our strategy exploits a nuance of the elastic properties of fiber coils whereby the constrained thermal expansion of the composite material (fiber + coating) can result in a coil having compressed inner layers and expanded outer layers. Thermal expansion is the dominant effect responsible for thermal phase sensitivity in HCFs, and in this scheme the compressed inner coil layers compensate the thermal expansion of the outer layers. In this study we design the coil parameters using finite element simulations, studying the relationship between coil performance and its key parameters. The proof-of-principle coil has 160 mm diameter and incorporates a 548 m length of HCF out of which a 230 m section shows almost zero (slightly negative) thermal phase sensitivity. Though the coil shows low thermal phase sensitivity over tens of hours, the long-time constant viscoelastic properties of the coating materials used in the HCF under study are shown to limit these benefits. To make this strategy practical for systems with fast temperature dynamics, a coating having more stable mechanical properties could be used. For precision timing systems in which long thermal time constants are already the norm, this scheme represents a low-cost and provides a significant reduction to thermal sensitivity which is immediately practicable.

Optical fibers with a low thermal coefficient of delay (TCD) have been developed for frequency and timing transmission/distribution. However, their temperature sensitivity changes as a function of temperature and, to date, no study of such fibers has demonstrated improved performance over extended temperature ranges, especially at sub-zero temperatures. Here, we show that a hollow core fiber (HCF) with a thin acrylate coating can have a TCD within ±2.0 ps/km/°C over a broad temperature range from −150°C to +60°C. In addition, this thinly coated HCF can be fully insensitive to temperature around −134°C, making it of interest, e.g., for laser stabilization close to cryogenic temperatures.

There are a host of applications in communications, sensing, and science, in which analogue signal transmission is preferred over today’s dominant digital transmission. In some of these applications, the advantage is in lower cost, while in others, it lies in superior performance. However, especially for longer analogue photonics links (up to 10 s of km), the performance is strongly limited by the impairments arising from using standard single-mode fibres (SSMF). Firstly, the three key metrics of analogue links (loss, noise figure, and dynamic range) tend to improve with received power, but this is limited by stimulated Brillouin scattering in SSMF. Further degradation is due to the chromatic dispersion of SSMF, which induces radio-frequency (RF) signal fading, increases even-order distortions, and causes phase-to-intensity-noise conversion. Further distortions still, are caused by the Kerr nonlinearity of SSMF. We propose to address all of these shortcomings by replacing SSMFs with hollow-core optical fibres, which have simultaneously six times lower chromatic dispersion and several orders of magnitude lower nonlinearity (Brillouin, Kerr). We demonstrate the advantages in this application using a 7.7 km long hollow-core fibre sample, significantly surpassing the performance of an SSMF link in virtually every metric, including 15 dB higher link gain and 6 dB lower noise figure.

Environmental temperature fluctuations cause phase changes of the light propagating through optical fibers due to their thermal sensitivity. This limits the stability of fiber delay-line interferometers. Here, we present a thermally-insensitive fiber Mach-Zehnder interferometer. It consists of a hollow core fiber (HCF), which has a low thermal sensitivity coefficient, in one branch and a standard single-mode fiber with a thermal sensitivity coefficient about 25 times larger in the other. By setting their associated length ratio to approximately 25:1, the optical phase of the light in both arms changes by the same amount with temperature, making the difference in their optical paths insensitive to temperature and thus producing thermally-insensitive interference. As the thermal sensitivity coefficient of the optical fibers is itself slightly temperature dependent, exactly-zero sensitivity is achieved at a specific temperature only. We show how this zero-sensitivity temperature can be controlled via control of the relative fiber lengths in the two interferometer arms and set it to room temperature. Further, we show that our interferometer is over 100 times less sensitive to temperature than a single-mode fiber-based interferometer over temperature range as large as 25–50 °C. When considering a simple temperature control that keeps the interferometer within ±1 °C, the demonstrated interferometer achieves over 2000 times lower thermal sensitivity than a single-mode fiber-based interferometer and over 100 times lower sensitivity than an HCF-only based interferometer. Finally, we discuss how the thermal sensitivity of such interferometer depends on the light source wavelength.

We demonstrate that optical frequency distribution using hollow core fiber links allows for the same backward and forward propagating signal carrier wavelength (due to low backscattering), potentially reducing non-reciprocal link noise.