Linear dual-comb interferometry at high power levels.

Detector non-linearity is an important factor limiting the maximal power and hence the signal-to-noise ratio (SNR) in dual-comb interferometry. To increase the SNR without overwhelming averaging time, photodetector non-linearity must be properly handled for high input power. Detectors exhibiting nonlinear behavior can produce linear dual-comb interferograms if the area of the detector's impulse response does not saturate and if the overlap between successive time-varying impulse responses is properly managed. Here, a high bandwidth non-amplified balanced photodetector is characterized in terms of its impulse response to high intensity short pulses to exemplify the conditions. With a 23.5 mW average power on each detector in a balanced pair, nonlinear spectral artifacts are at least 40 dB below the spectral baseline. Absorption lines of carbon dioxide are measured to reveal lines discrepancies smaller than 0.1% with HITRAN. A spectral shape independent formulation for the dual-comb figure of merit is proposed, reaching here 7.2 × 107 Hz1/2 limited by laser relative intensity noise, but corresponding to an ideal, shot-noise limited, figure of merit for an equivalent 0.85 mW average power per comb.

Correcting spectral baseline fluctuations in dual-comb interferometry.

A method to measure and correct for spectral baseline fluctuations in dual-comb interferometry is presented. Fluctuations can be measured from the amplitude of beat notes between combs and a continuous wave laser or from a separate measurement of the combs' repetition rates, filtered around the spectral region of interest. Amplitude-dependent spectral variations are characterized using low-resolution Fourier transforms around the centerburst of several interferograms, and a nonstationary filter is applied to properly account for the combs' variations during the measurement. This allows removal of this source of statistical, as well as systematic, errors.

Correcting photodetector nonlinearity in dual-comb interferometry.

Photodetector nonlinearity, the main limiting factor in terms of optical power in the detection chain, is corrected to improve the signal-to-noise ratio of a short-time measurement in dual-comb spectroscopy. An iterative correction algorithm minimizing out-of-band spectral artifacts based on nonlinearity correction methods used in classical Fourier-transform spectrometers is presented. The exactitude of the nonlinearity correction is validated using a low power linear measurement. Spectroscopic lines of H12CN are provided and the increase in absorption depth of 24% caused by the saturation of the detector is corrected yielding residuals limited by the measurement noise.

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

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.

Toward free-running operation of dual-comb fiber lasers for methane sensing.

The phase information provided by the beat note between frequency combs and two continuous-wave lasers is used to extrapolate the phase evolution of comb modes found in a spectral region obtained via nonlinear broadening. This thereafter enables using interferogram self-correction to fully retrieve the coherence of a dual-comb beat note between two independent fiber lasers. This approach allows the $ f - 2f $f-2f self-referencing of both combs, which is a significant simplification. Broadband near-infrared methane spectroscopy has been conducted to demonstrate the simplified system's preserved performance.

An open and flexible digital phase-locked loop for optical metrology.

This paper presents an open and flexible digital phase-locked 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 PC-based 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 canceller 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.