Midinfrared frequency comb from self-stable degenerate GaAs optical parametric oscillator

Sub-Hz relative linewidths from an interferometrically stabilized mid-infrared frequency comb

Frequency combs present a unique tool for high-precision and rapid molecular spectroscopy. Difference frequency generation (DFG) of near-infrared sources is a common approach to generate passively stabilized mid-infrared combs. However, only little attention has been paid so far to precisely measure the coherence properties of such sources. Here, we investigate these using a Raman-soliton based DFG source driven by an Yb:fiber frequency comb. A heterodyne beat between the second harmonic of the phase-locked DFG comb near 4 µm and a 2 µm Tm:fiber frequency comb locked to the same optical reference is performed. Using this method, we measure the relative phase noise power spectral density of both combs. This results in a sub-Hz relative linewidth between the DFG comb and the Tm:fiber comb. We also introduce a new pump/seed delay locking mechanism based on interferometry for long-term stable intensity noise suppression.

Continuous wavelength tuning of nondegenerate femtosecond doubly resonant optical parametric oscillators

We report an approach for continuous tuning of the central wavelengths of a femtosecond nondegenerate doubly resonant optical parametric oscillator (DRO). Key in this scheme is the insertion of a wavelength-selective element into the DRO cavity-length-locking system, which allows selected narrow-band emission from a DRO to be detected and enables the DRO cavity length to be locked away from the peak of its cavity resonances. In a preliminary experiment, we demonstrated a femtosecond nondegenerate DRO with its central-wavelength continuously-tunable over each of its cavity resonances and achieved a wavelength-tuning range of 1910-2070 nm for the signal wave and 2140-2340 nm for the idler wave. This is, to the best of our knowledge, the first demonstration of the continuous-wavelength-tuning capability of DRO systems.

Mid infrared gas spectroscopy using efficient fiber laser driven photonic chip-based supercontinuum

Directly accessing the middle infrared, the molecular functional group spectral region, via supercontinuum generation processes based on turn-key fiber lasers offers the undeniable advantage of simplicity and robustness. Recently, the assessment of the coherence of the mid-IR dispersive wave in silicon nitride (Si3N4) waveguides, pumped at telecom wavelength, established an important first step towards mid-IR frequency comb generation based on such compact systems. Yet, the spectral reach and efficiency still fall short for practical implementation. Here, we experimentally demonstrate that large cross-section Si3N4 waveguides pumped with 2 μm fs-fiber laser can reach the important spectroscopic spectral region in the 3-4 μm range, with up to 35% power conversion and milliwatt-level output powers. As a proof of principle, we use this source for detection of C2H2 by absorption spectroscopy. Such result makes these sources suitable candidate for compact, chip-integrated spectroscopic and sensing applications.

Dual-channel operation in a synchronously pumped optical parametric oscillator for the generation of broadband mid-infrared coherent light sources

We report, to the best of our knowledge, a novel approach for generating broadband mid-infrared (mid-IR) light by implementing a dual-channel scheme in a synchronously pumped optical parametric oscillator (SPOPO). Two-channel operation was achieved by inserting a prism pair and two reflection mirrors inside an optical parametric oscillator (OPO) cavity. Pumped by a Yb-fiber laser, the OPO generated an idler wave at ∼3150  nm with a -10  dB bandwidth of ∼13.2  THz, which was twice as much as that of the pump source. This scheme represents a promising technical route to transform conventional SPOPOs into a device capable of generating mid-IR light with very broad instantaneous bandwidth.

Efficient half-harmonic generation of three-optical-cycle mid-IR frequency comb around 4 µm using OP-GaP

We report a broadband mid-infrared frequency comb with three-optical-cycle pulse duration centered around 4.2 µm, via half-harmonic generation using orientation-patterned GaP (OP-GaP) with ~43% conversion efficiency. We experimentally compare performance of GaP with GaAs and lithium niobate as the nonlinear element, and show how properties of GaP at this wavelength lead to generation of the shortest pulses and the highest conversion efficiency. These results shed new light on half-harmonic generation of frequency combs, and pave the way for generation of short-pulse intrinsically-locked frequency combs at longer wavelengths in the mid-infrared with high conversion efficiencies.

Coherent frequency division with a degenerate synchronously pumped optical parametric oscillator

Synchronously pumped optical parametric oscillators (OPOs) are important tools for frequency comb (FC) generation in the mid-IR spectral range, where few suitable laser gain materials exist. For degenerate OPOs, self-phase-locking to the pump FC has been demonstrated. Here, we present a phase noise study of the carrier envelope offset frequency, revealing a -6  dB reduction compared to the pump FC over a wide Fourier frequency range. These results demonstrate that a degenerate OPO can be an ideal coherent frequency divider without any excess noise.

Stabilization of femtosecond optical parametric oscillators for infrared frequency comb generation

A synchronously pumped optical parametric oscillator (SP-OPO) is one of the most common techniques to generate femtosecond frequency combs in the mid-infrared region. Stable long-term operation of an SP-OPO requires active locking of the OPO resonator round-trip time to the pump pulse interval. A simple modulation-free locking method based on stabilization of narrow-band frequency-doubled power of the SP-OPO output comb is demonstrated in this Letter. The method relies on the strong dependency of frequency-doubled power on spectral shape of the comb, leading to better stability of the comb envelope spectrum than the commonly used dither-and-lock method.

Fully stabilized mid-infrared frequency comb for high-precision molecular spectroscopy

A fully stabilized mid-infrared optical frequency comb spanning from 2.9 to 3.4 µm is described in this article. The comb is based on half-harmonic generation in a femtosecond optical parametric oscillator, which transfers the high phase coherence of a fully stabilized near-infrared Er-doped fiber laser comb to the mid-infrared region. The method is simple, as no phase-locked loops or reference lasers are needed. Precise locking of optical frequencies of the mid-infrared comb to the pump comb is experimentally verified at sub-20 mHz level, which corresponds to a fractional statistical uncertainty of 2 × 10^-16 at the center frequency of the mid-infrared comb. The fully stabilized mid-infrared comb is an ideal tool for high-precision molecular spectroscopy, as well as for optical frequency metrology in the mid-infrared region, which is difficult to access with other stabilized frequency comb techniques.

Coherence properties of a 2.6-7.5 μm frequency comb produced as a subharmonic of a Tm-fiber laser

We study the temporal coherence of an ultrabroadband frequency comb produced in a degenerate GaAs optical parametric oscillator (OPO) pumped by a stabilized Tm-fiber comb, by observing multiheterodyne beats in the RF domain. We infer that in such a regime the OPO automatically produces a stable frequency comb that is phase and frequency locked to the pump. By varying intracavity dispersion, we achieve a comb spanning 2.6-7.5 μm at a -20  dB level. Low pump threshold (down to 7 mW), high average power (up to 73 mW), broad spectral coverage, flat spectrum, and high coherence make this comb a source suitable for various applications, foremost dual-comb molecular spectroscopy.

Dual-comb spectroscopy

Dual-comb spectroscopy is an emerging new spectroscopic tool that exploits the frequency resolution, frequency accuracy, broad bandwidth, and brightness of frequency combs for ultrahigh-resolution, high-sensitivity broadband spectroscopy. By using two coherent frequency combs, dual-comb spectroscopy allows a sample's spectral response to be measured on a comb tooth-by-tooth basis rapidly and without the size constraints or instrument response limitations of conventional spectrometers. This review describes dual-comb spectroscopy and summarizes the current state of the art. As frequency comb technology progresses, dual-comb spectroscopy will continue to mature and could surpass conventional broadband spectroscopy for a wide range of laboratory and field applications.