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.

Methane spectroscopy using a free-running chip-based dual-comb laser.

Absorption lines of methane in the 2ν3 band centered at 1650 nm were measured with a free-running mode-locked dual-comb laser based on a single erbium-doped glass chip. The laser's spectra were broadened up to 1670 nm using amplifiers and highly nonlinear fiber. A comb was used to interrogate the complex transmission spectrum of a methane-filled gas cell with an optical point spacing of 968 MHz and an interferogram (IGM) rate of 27 kHz to yield absorption lines of the R and Q branches. A 1.28 s sequence of IGMs was measured and phase-corrected using a self-sufficient correction algorithm seeded only by the IGMs. The associated transmission spectrum was then compared to HITRAN yielding residuals limited by photodetector nonlinearity.

Dual electro-optic frequency comb spectroscopy using pseudo-random modulation.

An approach for dual-comb spectroscopy using electro-optic (EO) phase modulation is reported. Maximum-length pseudo-random binary sequences allow for energy-efficient and flexible comb generation. Self-correction of interferograms is shown to remove relative comb drifts and improve mutual coherence, even for EO combs derived from the same laser source. Methane spectroscopy is reported over a ∼10 GHz spectral range, limited by the modulators' bandwidth. The potential of a simple EO comb instrument is demonstrated to rapidly quantify atmospheric methane emissions with ppb precision over 1 km.

Highly coherent free-running dual-comb chip platform.

We characterize the frequency noise performance of a free-running dual-comb source based on an erbium-doped glass chip running two adjacent mode-locked waveguide lasers. This compact laser platform, contained only in a 1.2 L volume, rejects common-mode environmental noise by 20 dB thanks to the proximity of the two laser cavities. Furthermore, it displays a remarkably low mutual frequency noise floor around 10 Hz2/Hz, which is enabled by its large-mode-area waveguides and low Kerr nonlinearity. As a result, it reaches a free-running mutual coherence time of 1 s since mode-resolved dual-comb spectra are generated even on this time scale. This design greatly simplifies dual-comb interferometers by enabling mode-resolved measurements without any phase lock.

Self-corrected chip-based dual-comb spectrometer.

We present a dual-comb spectrometer based on two passively mode-locked waveguide lasers integrated in a single Er-doped ZBLAN chip. This original design yields two free-running frequency combs having a high level of mutual stability. We developed in parallel a self-correction algorithm that compensates residual relative fluctuations and yields mode-resolved spectra without the help of any reference laser or control system. Fluctuations are extracted directly from the interferograms using the concept of ambiguity function, which leads to a significant simplification of the instrument that will greatly ease its widespread adoption and commercial deployment. Comparison with a correction algorithm relying on a single-frequency laser indicates discrepancies of only 50 attoseconds on optical timings. The capacities of this instrument are finally demonstrated with the acquisition of a high-resolution molecular spectrum covering 20 nm. This new chip-based multi-laser platform is ideal for the development of high-repetition-rate, compact and fieldable comb spectrometers in the near- and mid-infrared.

Ultrafast pulse generation in a mode-locked Erbium chip waveguide laser.

We report mode-locked ~1550 nm output of transform-limited ~180 fs pulses from a large mode-area (diameter ~50 µm) guided-wave erbium fluorozirconate glass laser. The passively mode-locked oscillator generates pulses with 25 nm bandwidth at 156 MHz repetition rate and peak-power of 260 W. Scalability to higher repetition rate is demonstrated by transform-limited 410 fs pulse output at 1.3 GHz. To understand the origins of the broad spectral output, the laser cavity is simulated by using a numerical solution to the Ginzburg-Landau equation. This paper reports the widest bandwidth and shortest pulses achieved from an ultra-fast laser inscribed waveguide laser.

Dual-comb spectroscopy with a phase-modulated probe comb for sub-MHz spectral sampling.

We present a straightforward and efficient method to reduce the mode spacing of a frequency comb based on binary pseudo-random phase modulation of its pulse train. As a proof of concept, we use such a densified comb to perform dual-comb spectroscopy of a long-delay Mach-Zehnder interferometer and a high-quality-factor microresonator with sub-MHz spectral sampling. Since this approach is based on binary phase modulation, it combines all the advantages of other densification techniques: simplicity, single-step implementation, and conservation of the initial comb's power.

Complex direct comb spectroscopy with a virtually imaged phased array.

We demonstrate a simple interferometric technique to directly measure the complex optical transmittance over a large spectral range using a frequency-comb spectrometer based on a virtually imaged phased array. A Michelson interferometer encodes the phase deviations induced by a sample contained in one of its arms into an interferogram image. When combined with an additional image taken from each arm separately, along with a frequency-calibration image, this allows full reconstruction of the sample's optical transfer function. We demonstrate the technique with a vapor cell containing H13C14N, producing transmittance and phase spectra spanning 2.9 THz (∼23 nm) with ∼1 GHz resolution.

Self-heterodyne interference spectroscopy using a comb generated by pseudo-random modulation.

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.

A quantitative mode-resolved frequency comb spectrometer.

We have developed a frequency-comb spectrometer that records 35-nm (4 THz) spectra with 2-pm (250 MHz) spectral sampling and an absolute frequency accuracy of 2 kHz. We achieve a signal-to-noise ratio of ~400 in a measurement time of 8.2 s. The spectrometer is based on a commercial frequency comb decimated by a variable-length, low-finesse Fabry Pérot filter cavity to fully resolve the comb modes as imaged by a virtually imaged phased array (VIPA), diffraction grating and near-IR camera. By tuning the cavity length, spectra derived from all unique decimated combs are acquired and then interleaved to achieve frequency sampling at the comb repetition rate of 250 MHz. We have validated the performance of the spectrometer by comparison with a previous high-precision absorption measurement of H13C14N near 1543 nm. We find excellent agreement, with deviations from the expected line centers and widths of, at most, 1 pm (125 MHz) and 3 pm (360 MHz), respectively.

Coherent dual-comb interferometry with quasi-integer-ratio repetition rates.

We demonstrate a generalized method for dual-comb interferometry that involves the use of two frequency combs with quasi-integer-ratio repetition rates. We use a 16.67 MHz comb to probe an 80-cm-long ring cavity and a 100 MHz comb to asynchronously sample its impulse response. The resulting signal can be seen as six time-multiplexed independent interferograms. We perform a deconvolution of the photodetector's impulse response to prevent any crosstalk between these multiplexed data sets. The measurement is then demultiplexed and corrected with referencing signals. We obtain a measurement with a spectral point spacing of 16.67 MHz and a spectral SNR of 55 dB by averaging 15,000 interferograms, corresponding to a measurement time of 500 s. Compared to conventional dual-comb spectroscopy, this generalized technique allows to either reduce the spectral point spacing or the acquisition time by changing the repetition rate of only one of the combs.