Squeezed dual-comb spectroscopy.

Optical frequency combs have enabled unique advantages in broadband, high-resolution spectroscopy and precision interferometry. However, quantum mechanics ultimately limits the metrological precision achievable with laser frequency combs. Quantum squeezing has led to significant measurement improvements with continuous wave lasers, but experiments demonstrating metrological advantage with squeezed combs are less developed.

Mode-resolved optical frequency comb fixed point localization via dual-comb interferometry.

We describe improved methods for locating the fixed point of an optical frequency comb. Two continuous-wave lasers are locked to a reference frequency comb and track the optical phase of a second comb-under-test (CUT) at two points separated by approximately 1.6 THz.

Graphics card-based real-time processing for dual comb interferometry.

The technique of performing interferometry with two optical frequency combs is used by an increasing number of research groups and even for field deployed commercial applications. Real-time interferogram acquisition, correction, and averaging are, however, still not broadly accessible. This limits the deployment and wider adoption of this high resolution, high sensitivity technique.

Pulse interaction induced systematic errors in dual comb spectroscopy.

Systematic errors are observed in dual comb spectroscopy when pulses from the two sources travel in a common fiber before interrogating the sample of interest. When sounding a molecular gas, these errors distort both the line shapes and retrieved concentrations. Simulations of dual comb interferograms based on a generalized nonlinear Schrodinger equation highlight two processes for these systematic errors.

Fast rate dual-comb spectrometer in the water-transparent 7.5-11.5 µm region.

We introduce a dual-comb spectrometer based on erbium fiber oscillators at 250 MHz that operates in the 7.5-11.5 µm spectral range over optical bandwidths up to 9 THz with a multi-kHz acquisition rate.

Removing biases in dual frequency comb spectroscopy due to digitizer nonlinearity.

Operation of any dual-comb spectrometer requires digitization of the interference signal before further processing. Nonlinearities in the analog-to-digital conversion can alter the apparent gas concentration by multiple percent, limiting both precision and accuracy of this technique. This work describes both the measurement of digitizer nonlinearity and the development of a model that quantitatively describes observed concentration bias over a range of conditions.

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.

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.

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.

Balanced photodetector nonlinearity for the short-pulse regime.

Short-pulse lasers are used to characterize the nonlinear response of amplified photodetectors. Two widely used balanced detectors are characterized in terms of amplitude, area, broadening, and balancing the mismatch of their impulse response. The dynamic impact of pulses on the detector is also discussed.

Widely tunable silicon-fiber laser at 2 µm.

Laser sources operating in the 2 µm spectral region play an important role for sensing and spectroscopy, and potentially for optical communication systems. In this work, we demonstrate a widely tunable hybrid silicon-fiber laser operating in the 2 µm band. By introducing a silicon-integrated Vernier filter in a fiber laser, we achieved continuous wavelength tuning over a range of 100 nm, from 1970 to 2070 nm.

Etchless chalcogenide microresonators monolithically coupled to silicon photonic waveguides.

Integration of chalcogenide waveguides in silicon photonics can mitigate the prohibitive nonlinear losses of silicon while leveraging the mature complementary metal-oxide-semiconductor (CMOS)-compatible nanophotonic fabrication process. In this work, we demonstrate, for the first time, to the best of our knowledge, a method of integrating high- chalcogenides microring resonators onto the silicon photonics platform without post-process etching. The method uses micro-trench filling and a novel thermal dewetting technique to form low-loss chalcogenide strip waveguides.

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.

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.

Single-frequency mid-infrared waveguide laser.

A guided-wave chip laser operating in a single longitudinal mode at 2860 nm is presented. The cavity was set in the Littman-Metcalf configuration to achieve single-frequency operation with a side-mode suppression ratio above 33 dB. The chip laser's 2 MHz linewidth on a 10 ms scale was found to be limited by mechanical fluctuations, but its Lorentzian contribution was estimated to be lower than 1 Hz using a heterodyne technique.

Amplified noise nonstationarity in a mode-locked laser based on nonlinear polarization rotation.

Beat note measurements between a mode-locked (ML) and a continuous-wave laser, as well as between two ML sources, were used to demonstrate that the sub-threshold, cavity filtered, amplified spontaneous emission is not stationary, even when a fast mode-locking mechanism, such as nonlinear polarization rotation, is used to generate short pulses. A relatively small gain modulation of a few percent created by high-intensity pulses can produce a significant modulation of the amplified noise once synchronously accumulated over several cavity round-trips, even if the repetition rate is faster than the gain dynamics.

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

Absorption lines of methane in the 2ν 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.

Measuring the nonlinear phase shift in a mode-locked laser.

The spectrum of a mode-locked laser (MLL) is down-mixed to electrical frequencies using a tunable continuous-wave laser. By characterizing the sub-threshold low-intensity emission relative to the laser mode positions, one can measure the nonlinear phase shift of the MLL while in operation.

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.

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.

Precision spectroscopy of HCN using a free-running, all-fiber dual electro-optic frequency comb system.

We demonstrate the precision molecular spectroscopy of HCN using a free-running, all-fiber dual electro-optic frequency comb system. Successive interferograms, acquired at a rate of Δf=1  MHz, were phase-corrected in post-processing, averaged, and normalized to yield the complex transmission spectrum of several transitions within the 2νHCN band centered near λ=1545  nm. With spectral signal-to-noise ratios as high as 326:1 achieved in 2 ms of integration time, we report accurate measurements of HCN transition intensities which will aid in the study of extreme astrophysical environments.

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.

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.

Improving the signal-to-noise ratio of the beat note between a frequency comb and a tunable laser using a dynamically tracking optical filter.

An acousto-optic filter is locked to a tunable continuous wave (CW) laser so that a frequency comb can be dynamically filtered around the wavelength of the CW source. The signal-to-noise ratio (SNR) of the heterodyne beat note between the comb and the CW laser is improved by a factor of up to 19 dB. Furthermore, a SNR of more than 56 dB in 100 kHz is obtained over an 85 nm wavelength span.