We present an original instrument designed to accomplish high-speed spectroscopy of individual optical lines based on a frequency comb generated by pseudo-random phase modulation of a continuous-wave (CW) laser. This approach delivers efficient usage of the laser power as well as independent control over the spectral point spacing, bandwidth and central wavelength of the comb. The comb is mixed with a local oscillator generated from the same CW laser frequency-shifted by an acousto-optic modulator, enabling a self-heterodyne detection scheme. The current configuration offers a calibrated spectrum every 1.12 µs. We demonstrate the capabilities of the spectrometer by producing averaged, as well as time-resolved, spectra of the D1 transition of cesium with a 9.8-MHz point spacing, a 50-kHz resolution and a span of more than 3 GHz. The spectra obtained after 1 ms of averaging are fitted with complex Voigt profiles that return parameters in good agreement with expected values.
This paper presents an open and flexible digital phase lock loop optimized for laser stabilization systems. It is implemented on a cheap and easily accessible FPGA-based digital electronics platform (Red Pitaya) running a customizable open-source firmware. A PCbased software interface allows controlling the platform and optimizing the loop parameters remotely. Several tools are included to allow measurement of quantities of interest smoothly and rapidly. To demonstrate the platform's capabilities, we built a fiber noise canceler over a 400 m fiber link. Noise cancellation was achieved over a 30 kHz bandwidth, a value limited mainly by the delays introduced by the actuator and by the round-trip propagation over the fiber link. We measured a total latency of 565 ns for the platform itself, limiting the theoretically achievable control bandwidth to approximately 225 kHz.
The elastic backscattering of light in optical fiber is a fundamental phenomenon that sets the ultimate performance of several fiber systems such as gyroscopes and bidirectional transfer links. Until now, efforts to reduce the backscattering coefficient have yielded limited results, with the lowest value sitting at around − 76 d B / m in Ge-free silica-core fiber at 1.55 µm. Here, we present what we believe to be the first measurement of backscattering from a low-loss antiresonant hollow-core fiber and show that it is more than 40 dB below reported values in silica-core fiber and hollow-core photonic bandgap fiber. The record-low level of − 118 d B / m measured with our custom-built optical frequency-domain reflectometer is in good agreement with simulations in which we assume the scattering to originate from the intrinsic surface roughness. Our demonstration also shows that a tailored instrument can localize and quantify weak faults within a hollow-core fiber, enabling its detailed characterization.
We present a theoretical analysis into the fundamental physical mechanisms contributing to backscattering in hollow-core antiresonant fibers. We consider Rayleigh scattering originating from the hollow regions of the fiber, which may be filled with gases, Rayleigh scattering from within the glass membranes, as well as the contribution from scattering at the glass surfaces. We derive expressions for the backscattering coefficient into any specified guided mode for an arbitrary excitation. These lead to general scaling relations with the core size and wavelength, which are found to be the key parameters for backscattering, regardless of the exact antiresonant geometry. For a nested antiresonant hollow-core fiber with a core diameter of 35 µm, the only antiresonant fiber geometry for which the experimental data are available in the literature, we find that the surface-scattering limited backscattering coefficient of the fundamental mode into itself is nearly 40 dB below that of a single mode fiber, in good agreement with recently published measurements.
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