Wavelength extension beyond 3 µm in a Ho3+/Pr3+ co-doped AlF3-based fiber laser

In this Letter, we report continuous-wave (CW) lasers with wavelengths beyond 3 µm in homemade Ho3+/Pr3+ co-doped AlF3-based glass fibers. The laser cavity was established through the integration of a dichroic mirror (DM, HR@3-3.1 µm) positioned at the pump end and a partial reflectivity (PR) fiber Bragg grating (FBG) situated at the laser emission end. The FBGs in AlF3-based glass fibers were fabricated by a fs laser direct-writing method, and the resonant wavelengths were 3.009, 3.036, and 3.064 µm, respectively. Under the pump of 1.15 µm laser, a maximum unsaturated output power of 1.014 W was obtained at 3.009 µm with an overall laser efficiency of 11.8% and FWHM bandwidth of 0.88 nm. Furthermore, in order to enhance the optical-thermal stability, the FBG was heat-treated at 200°C for 30 min, and a higher output power of 1081 mW (348 mW without heat treatment) at 3.036 µm was achieved. To the best of our knowledge, this is the first demonstration of 3-3.1 µm lasers by using FBGs in Ho3+/Pr3+ co-doped AlF3-based fibers.

Mid-infrared optical frequency comb spectroscopy using an all-silica antiresonant hollow-core fiber

We present the first mid-infrared optical frequency comb spectrometer employing an absorption cell based on self-fabricated, all-silica antiresonant hollow-core fiber (ARHCF). The spectrometer is capable of measuring sub-mL sample volumes with 26 m interaction length and noise equivalent absorption sensitivity of 8.3 × 10-8 cm-1 Hz-1/2 per spectral element in the range of 2900 cm-1 to 3100 cm-1. Compared to a commercially available multipass cell, the ARHCF offers a similar interaction length in a 1000 times lower gas sample volume and a 2.8 dB lower transmission loss, resulting in better absorption sensitivity. The broad transmission windows of ARHCFs, in combination with a tunable optical frequency comb, make them ideal for multispecies detection, while the prospect of measuring samples in small volumes makes them a competitive technique to photoacoustic spectroscopy along with the robustness and prospect of coiling the ARHCFs open doors for miniaturization and out-of-laboratory applications.

Highly Responsive Mid-Infrared Metamaterial Enhanced Heterostructure Photodetector Formed out of Sintered PbSe/PbS Colloidal Quantum Dots

Efficient and simple-to-fabricate light detectors in the mid infrared (MIR) spectral range are of great importance for various applications in existing and emerging technologies. Here, we demonstrate compact and efficient photodetectors operating at room temperature in a wavelength range of 2710-4250 nm with responsivities as high as 375 and 4 A/W. Key to the high performance is the combination of a sintered colloidal quantum dot (CQD) lead selenide (PbSe) and lead sulfide (PbS) heterojunction photoconductor with a metallic metasurface perfect absorber. The combination of this photoconductor stack with the metallic metasurface perfect absorber provides an overall ∼20-fold increase of the responsivity compared against reference sintered PbSe photoconductors. More precisely, the introduction of a PbSe/PbS heterojunction increases the responsivity by a factor of ∼2 and the metallic metasurface enhances the responsivity by an order of magnitude. The metasurface not only enhances the light-matter interaction but also acts as an electrode to the detector. Furthermore, fabrication of our devices relies on simple and inexpensive methods. This is in contrast to most of the currently available (state-of-the-art) MIR photodetectors that rely on rather expensive as well as nontrivial fabrication technologies that often require cooling for efficient operation.

Pump tuning of a mid-infrared whispering gallery optical parametric oscillator

Optical parametric oscillators (OPOs) constitute an important coherent, narrow-linewidth and widely tunable light source with applications in spectroscopy and many other fields. Their realizations based on whispering gallery resonators (WGRs) provide a small footprint and ultra-low thresholds, with demonstrations of tunability typically done via temperature variation. In this work, we show the pump tuning capabilities of a mm-sized WGR mid-infrared OPO made of CdSiP₂. By tuning a telecom wavelength diode laser by 16 nm, we generate tunable light from 2708 to 3575 nm. Furthermore, we show controlled tuning in steps of 1 free spectral range (FSR) and the possibility of 12 GHz of continuous tuning. All these features are in good agreement with the theoretical predictions. We conclude that tuning from 2.4 to 4.9 µm is even possible, while still using commercially available near-infrared diode lasers. This work highlights the advantages of pump tuning of WGR OPOs and provides valuable insights for their precise control.

Ultra-broadband mid-infrared generation in dispersion-engineered thin-film lithium niobate

Thin-film lithium niobate (TFLN) is an emerging platform for compact, low-power nonlinear-optical devices, and has been used extensively for near-infrared frequency conversion. Recent work has extended these devices to mid-infrared wavelengths, where broadly tunable sources may be used for chemical sensing. To this end, we demonstrate efficient and broadband difference frequency generation between a fixed 1-µm pump and a tunable telecom source in uniformly-poled TFLN-on-sapphire by harnessing the dispersion-engineering available in tightly-confining waveguides. We show a simultaneous 1-2 order-of-magnitude improvement in conversion efficiency and ∼5-fold enhancement of operating bandwidth for mid-infrared generation when compared to equal-length conventional lithium niobate waveguides. We also examine the effects of mid-infrared loss from surface-adsorbed water on the performance of these devices.

Versatile optical frequency combs based on multi-seeded femtosecond optical parametric generation

This study proposes and demonstrates a versatile method for near- and mid-infrared optical frequency comb generation using multi-seeded femtosecond optical parametric generation. The method allows one to divide the repetition rate by an arbitrarily large integer factor, freely tune the offset frequency, and adjust the common phase offset of the comb modes. Since all possible degrees of freedom are adjustable, the proposed method manifests itself as versatile optical frequency synthesis.

Spectral Interferometry with Frequency Combs

In this review paper, we provide an overview of the state of the art in linear interferometric techniques using laser frequency comb sources. Diverse techniques including Fourier transform spectroscopy, linear spectral interferometry and swept-wavelength interferometry are covered in detail. The unique features brought by laser frequency comb sources are shown, and specific applications highlighted in molecular spectroscopy, optical coherence tomography and the characterization of photonic integrated devices and components. Finally, the possibilities enabled by advances in chip scale swept sources and frequency combs are discussed.

Recent developments in lanthanide-doped mid-infrared fluoride fiber lasers [Invited]

Mid-infrared fiber sources, emitting between 2.5 µm and 5.0 µm, are interesting for their great potential in several application fields such as material processing, biomedicine, remote sensing and infrared countermeasures due to their high-power, their diffraction-limited beam quality as well as their robust monolithic architecture. In this review, we will focus on the recent progress in continuous wave and pulsed mid-infrared fiber lasers and the components that bring these laser sources closer to a field deployment as well as in industrial systems. Accordingly, we will briefly illustrate the potential of such mid-infrared fiber lasers through a few selected applications.

Advances in mid-infrared spectroscopy enabled by supercontinuum laser sources

Supercontinuum sources are all-fiber pulsed laser-driven systems that provide high power spectral densities within ultra-broadband spectral ranges. The tailored process of generating broadband, bright, and spectrally flat supercontinua-through a complex interplay of linear and non-linear processes-has been recently pushed further towards longer wavelengths and has evolved enough to enter the field of mid-infrared (mid-IR) spectroscopy. In this work, we review the current state and perspectives of this technology that offers laser-like emission properties and instantaneous broadband spectral coverage comparable to thermal emitters. We aim to go beyond a literature review. Thus, we first discuss the basic principles of supercontinuum sources and then provide an experimental part focusing on the quantification and analysis of intrinsic emission properties such as typical power spectral densities, brightness levels, spectral stability, and beam quality (to the best of the authors' knowledge, the M² factor for a mid-IR supercontinuum source is characterized for the first time). On this basis, we identify key competitive advantages of these alternative emitters for mid-IR spectroscopy over state-of-the-art technologies such as thermal sources or quantum cascade lasers. The specific features of supercontinuum radiation open up prospects of improving well-established techniques in mid-IR spectroscopy and trigger developments of novel analytical methods and instrumentation. The review concludes with a structured summary of recent advances and applications in various routine mid-IR spectroscopy scenarios that have benefited from the use of supercontinuum sources.

A comparative study of an Yb-doped fiber gain-managed nonlinear amplifier seeded by femtosecond fiber lasers

In this work, we show that the nonlinear evolution of femtosecond seed pulses with different parameters (temporal and spectral shapes, repetition rate, pulse energy) in an Yb-fiber amplifier leads to gain-managed nonlinear amplification, enabling robust generation of high-peak-power and nearly transform-limited pulses after external compression. We demonstrate a compressed pulse duration of 33 fs with an energy of 80.5 nJ and a peak power of 2.29 MW for a source with a repetition rate of 30 MHz. For a second seed source with a repetition rate of 125 MHz, we obtained a pulse duration of 51 fs with an energy of 22.8 nJ and a peak power of 420 kW. Numerical simulations incorporating rate equations and nonlinear propagation in the amplifier provide evolutions that agree well with the experimental results. The discrepancies in the amplifier’s absorption edge appearing at low repetition rates and higher pump powers are attributed to the temperature dependence of the amplifier’s gain cross-sections. Here, we experimentally verify this attribution and thus underline the importance of accounting for the fiber core temperature for precise modelling of the short-wavelength spectral edge of the output pulses in nonlinear Yb-fiber amplifiers. We also measure, for the first time, the relative intensity noise of an amplifier operating in the gain-managed nonlinear regime. The measurements reveal a significant contribution of the amplification process to the overall output noise of the system.

Generation of a dual optical frequency comb by large signal modulation of a semiconductor laser

In this paper, we present and theoretically investigate a simple and power efficient scheme for dual optical frequency comb generation residing on a single directly modulated semiconductor laser driven by two superimposed current waveforms. Our detailed model estimates dual combs comprising teeth pairs within 20 dB margin spanning up to 116 GHz with power per teeth pair up to 8.8 µW. In addition, we report dual combs with 40 GHz span comprising ultraflat teeth pairs, with flatness of the order of 1 dB.

Mid-infrared frequency comb with 25 pJ threshold via CW-seeded optical parametric generation in nonlinear waveguide

We demonstrate efficient generation of mid-infrared frequency combs based on continuous-wave-seeded femtosecond optical parametric generation in nonlinear waveguides. Conversion of the near-infrared pump to signal and idler light takes place with very high efficiency (74%), and the threshold (25 pJ for 100 fs pulses) is over 300 times lower than in bulk analogs. Relative intensity noise of the mid-infrared comb is exceptionally low, below 5×10^-5 (integrated from 10 Hz to 2 MHz). Furthermore, the mid-infrared bandwidth can be increased by driving the process with a broadband pump obtained via supercontinuum generation.

Laser Beam Atmospheric Propagation Modelling for Aerospace LIDAR Applications

Atmospheric effects have a significant impact on the performance of airborne and space laser systems. Traditional models used to predict propagation effects rely heavily on simplified assumptions of the atmospheric properties and their interactions with laser systems. In the engineering domain, these models need to be continually improved in order to develop tools that can predict laser beam propagation with high accuracy and for a wide range of practical applications such as LIDAR (light detection and ranging), free-space optical communications, remote sensing, etc. The underlying causes of laser beam attenuation in the atmosphere are examined in this paper, with a focus on the dominant linear effects: absorption, scattering, turbulence, and non-linear thermal effects such as blooming, kinetic cooling, and bleaching. These phenomena are quantitatively analyzed, highlighting the implications of the various assumptions made in current modeling approaches. Absorption and scattering, as the dominant causes of attenuation, are generally well captured in existing models and tools, but the impacts of non-linear phenomena are typically not well described as they tend to be application specific. Atmospheric radiative transfer codes, such as MODTRAN, ARTS, etc., and the associated spectral databases, such as HITRAN, are the existing tools that implement state-of-the-art models to quantify the total propagative effects on laser systems. These tools are widely used to analyze system performance, both for design and test/evaluation purposes. However, present day atmospheric radiative transfer codes make several assumptions that reduce accuracy in favor of faster processing. In this paper, the atmospheric radiative transfer models are reviewed highlighting the associated methodologies, assumptions, and limitations. Empirical models are found to offer a robust analysis of atmospheric propagation, which is particularly well-suited for design, development, test and evaluation (DDT&E) purposes. As such, empirical, semi-empirical, and ensemble methodologies are recommended to complement and augment the existing atmospheric radiative transfer codes. There is scope to evolve the numerical codes and empirical approaches to better suit aerospace applications, where fast analysis is required over a range of slant paths, incidence angles, altitudes, and atmospheric conditions, which are not exhaustively captured in current performance assessment methods.

Miniaturized integrated spectrometer using a silicon ring-grating design

We introduce and experimentally demonstrate a miniaturized integrated spectrometer operating over a broad bandwidth in the short-wavelength infrared (SWIR) spectrum that combines an add-drop ring resonator narrow band filter with a distributed Bragg reflector (DBR) based broadband filter realized in a silicon photonic platform. The contra-directional coupling DBR filter in this design consists of a pair of waveguide sidewall gratings that act as a broadband filter (i.e., 3.9 nm). The re-directed beam is then fed into the ring resonator which functions as a narrowband filter (i.e., 0.121 nm). In this scheme the free spectral range (FSR) limitation of the ring resonator is overcome by using the DBR as a filter to isolate a single ring resonance line. The overall design of the spectrometer is further simplified by simultaneously tuning both components through the thermo-optic effect. Moreover, several ring-grating spectrometer cells with different central wavelengths can be stacked in cascade in order to cover a broader spectrum bandwidth. This can be done by centering each unit cell on a different center wavelength such that the maximum range of one-unit cell corresponds to the minimum range of the next unit cell. This configuration enables high spectral resolution over a large spectral bandwidth and high extinction ratio (ER), making it suitable for a wide variety of applications.

Near and mid-infrared optical vortex parametric oscillator based on KTA

We investigated high energy, near and mid-infrared optical vortex lasers formed by a 1 μm optical vortex-pumped KTiOAsO₄ (KTA) optical parametric oscillator. The orbital angular momentum (OAM) of the pump beam can be selectively transferred to the signal or idler output by changing the reflectivity of the output coupler. With this system, 1.535 µm vortex signal output with an energy of 2.04 mJ and 3.468 µm vortex idler output with an energy of 1.75 mJ were obtained with a maximum pump energy of 21 mJ, corresponding to slope efficiencies of 14% and 10%, respectively. The spectral bandwidth (full width at half maximum, FWHM) of the signal and idler vortex outputs were measured to be Δλ_s ~ 1.3 nm (~ 5.5 cm⁻¹) and Δλ_i ~ 1.7 nm (~ 1.4 cm⁻¹), respectively.

Electro-Optic Control of Lithium Niobate Bulk Whispering Gallery Resonators: Analysis of the Distribution of Externally Applied Electric Fields

Whispering gallery resonators made out of lithium niobate allow for optical parametric oscillation and frequency comb generation employing the outstanding second-order nonlinear-optical properties of this material. An important knob to tune and control these processes is, e.g., the linear electro-optic effect, the Pockels effect via externally applied electric fields. Due to the shape of the resonators a precise prediction of the electric field strength that affects the optical mode is non-trivial. Here, we study the average strength of the electric field in z-direction in the region of the optical mode for different configurations and geometries of lithium niobate whispering gallery resonators with the help of the finite element method. We find that in some configurations almost 100% is present in the cavity compared to the ideal case of a cylindrical resonator. Even in the case of a few-mode resonator with a very thin rim we find a strength of 90%. Our results give useful design considerations for future arrangements that may benefit from the strong electro-optic effect in bulk whispering gallery resonators made out of lithium niobate.

High-resolution, broadly-tunable mid-IR spectroscopy using a continuous wave optical parametric oscillator

We report on the design and automation of a mid-infrared, continuous wave, singly-resonant optical parametric oscillator. Hands-free controls and the implementation of a tuning algorithm allowed for hundreds of nanometers of continuous, effective-mode-hop-free tuning over the range of 2190-4000 nm. To demonstrate the applicability of this light source and algorithm to mid-IR spectroscopy, we performed a sample spectroscopy measurement in a C₂H₂ gas cell and compared the experimentally-measured absorption spectrum to HITRAN 2016 simulations. We found excellent agreement with simulation in both peak heights and peak centers; we also report a reduced uncertainty in peak centers compared to simulation.

Efficient carrier-envelope phase tunable mid-infrared frequency combs based on CW-seeded optical parametric generation

We present an efficient method for generating frequency combs in the mid-infrared (MIR) spectral range in bulk lithium niobate crystals as well as in waveguides. Our approach is simple and robust, since it is based on single-pass configuration of femtosecond Optical Parametric Generation (OPG) seeded by a telecom continuous-wave laser. Precise and fast tuning of the seed laser allows to rapidly change the offset frequency of the generated MIR comb independent of its repetition rate. The MIR comb offset frequency is inherently known, so its direct detection is not required. An additional degree of freedom that further complements the versatility of our light source is a unique way of repetition rate or mode spacing adjustments. This is achieved by our new concept of coherent multi-seeding that depending on the configuration allows for pulse train modulation and mode spacing, or repetition rate division by an integer number.

Infrared Comb Spectroscopy of Buffer-Gas-Cooled Molecules: Toward Absolute Frequency Metrology of Cold Acetylene

We review the recent developments in precision ro-vibrational spectroscopy of buffer-gas-cooled neutral molecules, obtained using infrared frequency combs either as direct probe sources or as ultra-accurate optical rulers. In particular, we show how coherent broadband spectroscopy of complex molecules especially benefits from drastic simplification of the spectra brought about by cooling of internal temperatures. Moreover, cooling the translational motion allows longer light-molecule interaction times and hence reduced transit-time broadening effects, crucial for high-precision spectroscopy on simple molecules. In this respect, we report on the progress of absolute frequency metrology experiments with buffer-gas-cooled molecules, focusing on the advanced technologies that led to record measurements with acetylene. Finally, we briefly discuss the prospects for further improving the ultimate accuracy of the spectroscopic frequency measurement.

Suppression of relaxation-oscillations in intra-cavity optical parametric oscillators by two-photon absorption

The applications of continuous-wave (cw), intra-cavity optical parametric oscillators (ICOPO) in molecular sensing and spectroscopy have been hampered by their relaxation-oscillation and power-stability problems. To solve these problems, we propose a two-photon-absorption (TPA) mechanism into ICOPOs. In a proof-of-principle experiment, we inserted a CdTe plate into an ICOPO as a TPA medium and demonstrated efficient suppression of relaxation-oscillations, obtaining an intensity-noise reduction of over 70 dB at the relaxation-oscillation frequency. To the best of our knowledge, this is the first demonstration of relaxation-oscillation suppression in ICOPOs based on TPA.

Mid-infrared frequency combs at 10 GHz

We demonstrate mid-infrared (MIR) frequency combs at 10 GHz repetition rate via intra-pulse difference-frequency generation (DFG) in quasi-phase-matched nonlinear media. Few-cycle pump pulses (≲15fs, 100 pJ) from a near-infrared electro-optic frequency comb are provided via nonlinear soliton-like compression in photonic-chip silicon-nitride waveguides. Subsequent intra-pulse DFG in periodically poled lithium niobate waveguides yields MIR frequency combs in the 3.1-4.8 µm region, while orientation-patterned gallium phosphide provides coverage across 7-11 µm. Cascaded second-order nonlinearities simultaneously provide access to the carrier-envelope-offset frequency of the pump source via in-line f-2f nonlinear interferometry. The high-repetition rate MIR frequency combs introduced here can be used for condensed phase spectroscopy and applications such as laser heterodyne radiometry.

All-fiber frequency comb at 2 µm providing 1.4-cycle pulses

We report an all-fiber approach to generating sub-2-cycle pulses at 2 µm and a corresponding octave-spanning optical frequency comb. Our configuration leverages mature erbium:fiber laser technology at 1.5 µm to provide a seed pulse for a thulium-doped fiber amplifier that outputs 330 mW average power at a 100 MHz repetition rate. Following amplification, nonlinear self-compression in fiber decreases the pulse duration to 9.5 fs, or 1.4 optical cycles. The spectrum of the ultrashort pulse spans from 1 to beyond 2.4 µm and enables direct measurement of the carrier-envelope offset frequency. Our approach employs only commercially available fiber components, resulting in a design that is easy to reproduce in the larger community. As such, this system should be useful as a robust frequency comb source in the near-infrared or as a pump source to generate mid-infrared frequency combs.

Parallel gas spectroscopy using mid-infrared supercontinuum from a single Si3N4 waveguide

Efficient third-order nonlinear optical processes have been successfully integrated on silicon nitride (Si₃N₄) waveguides. In particular, owing to Si₃N₄ wide transparency window spanning from the visible to the middle-infrared (mid-IR), efficient mid-IR dispersive-wave (DW) generation from a fiber laser has been recently demonstrated, and its potential as a source for absorption spectroscopy of a single gas has been established. Here we show that the system can be further engineered to broaden the coverage of a single DW without losing efficiency, as to enable simultaneous and discrete detection of several gas-phase molecules within the 2900 and 3380cm^-1 functional group region. We demonstrate quantitative detection of acetylene, methane, and ethane using a simple direct-absorption spectroscopy scheme, achieving a several hundreds of parts-per-million noise-equivalent detection limit with a 5 cm long gas cell.

Coherent mid-infrared supercontinuum generation in tapered suspended-core As39Se61 fibers pumped by a few-optical-cycle Cr:ZnSe laser

We report on efficient supercontinuum generation in tapered suspended-core $ {{\rm As}_{39}}{{\rm Se}_{61}} $As₃₉Se₆₁ fibers pumped by a femtosecond mode-locked Cr:ZnSe laser. The supercontinuum spectrum spans the mid-infrared spectral region from 1.4 to 4.2 µm, and its spectral coherence is proved by heterodyning with a single-frequency narrow-linewidth Er-fiber laser at 1.55 µm, measuring a beat note with 27-dB signal-to-noise ratio in a resolution bandwidth of 100 kHz. The intensity stability of the supercontinuum radiation is also characterized by relative intensity noise measurements.

Stabilized all-fiber source for generation of tunable broadband fCEO-free mid-IR frequency comb in the 7 - 9 µm range

A compact and robust all-fiber difference frequency generation-based source of broadband mid-infrared radiation is presented. The source emits tunable radiation in the range between 6.5 µm and 9 µm with an average output power up to 5 mW at 125 MHz repetition frequency. The all-in-fiber construction of the source along with active stabilization techniques results in long-term repetition rate stability of 3 Hz per 10 h and a standard deviation of the output power better than 0.8% per 1 h. The applicability of the presented source to laser spectroscopy is demonstrated by measuring the absorption spectrum of nitrous oxide (N₂O) around 7.8 µm. The robustness and good long- and short-term stability of the source opens up for applications outside the laboratory.

Infrared electric field sampled frequency comb spectroscopy

Probing matter with light in the mid-infrared provides unique insight into molecular composition, structure, and function with high sensitivity. However, laser spectroscopy in this spectral region lacks the broadband or tunable light sources and efficient detectors available in the visible or near-infrared. We overcome these challenges with an approach that unites a compact source of phase-stable, single-cycle, mid-infrared pulses with room temperature electric field-resolved detection at video rates. The ultrashort pulses correspond to laser frequency combs that span 3 to 27 μm (370 to 3333 cm-1), and are measured with dynamic range of >106 and spectral resolution as high as 0.003 cm-1. We highlight the brightness and coherence of our apparatus with gas-, liquid-, and solid-phase spectroscopy that extends over spectral bandwidths comparable to thermal or infrared synchrotron sources. This unique combination enables powerful avenues for rapid detection of biological, chemical, and physical properties of matter with molecular specificity.

A Broadband Mid-Infrared Trace Gas Sensor Using Supercontinuum Light Source: Applications for Real-Time Quality Control for Fruit Storage

We present a fully integrated and transportable multi-species trace gas sensor based on a mid-infrared (MIR) supercontinuum light source. The high brightness (surpassing synchrotron) and ultra-broad spectral bandwidth (2-4 μm) of this light source allows simultaneous detection of multiple broadband absorbing gas species. High sensitivity in the sub-ppmv level has been achieved by utilizing an astigmatic multipass cell. A grating-based spectrometer at a scanning rate of 20 Hz is developed employing a balanced detection scheme. A multi-component global fitting algorithm is implemented into a central LabVIEW program to perform real-time data analysis. The obtained concentration values are validated by the standard gas chromatography mass spectrometry (GC-MS) method. Field application of the sensor for quality control of stored fruits at a small scale is demonstrated, involving the detection of ethylene, ethanol, ethyl acetate, acetaldehyde, methanol, acetone, and water simultaneously. The sensor also shows promising potentials for other applications, such as environmental monitoring and biomedical research.

Observation of spectral mode splitting in a pump-enhanced ring cavity for mid-infrared generation

We report on experimental and theoretical investigation of mode-splitting dynamics in a ring cavity under the perturbation of fractional Bragg reflection from a periodically-poled nonlinear crystal. Counterintuitively, pronounced mode splitting in the spectral domain could have been observed even with a tiny intensity reflection of 0.0003. The breaking of running-wave operation in the ring-cavity configuration resulted in comparable circulating fields in forward- and counter-propagation directions, which thus dramatically reduced the enhancing factor for the resonating field. In contrast, a linear cavity with intrinsically bidirectional operation was immune to the small intra-cavity reflection. Therefore, the linear-cavity layout could provide an expedient solution for a given internal reflection to obtain more stable and higher enhancement, which was confirmed by comparative studies of mid-infrared generation based on pump-enhanced difference frequency conversion. The underlying mechanism was further modeled by numerical simulations, which agreed well with experimental results. These findings could not only shed light on the understanding of the exotic feature of concatenated optical cavities, but also provide a useful guide to practical design of enhancement cavities for cavity-based frequency conversion with periodically-poled nonlinear crystals.

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.

Quantitative Evaluation of Broadband Photoacoustic Spectroscopy in the Infrared with an Optical Parametric Oscillator

We evaluate the spectral resolution and the detection thresholds achievable for a photoacoustic spectroscopy (PAS) system in the broadband infrared wavelength region 3270 n m ≲ λ ≲ 3530 n m driven by a continuous wave optical parametric oscillator (OPO) with P ¯ ≈ 1.26 W . The absorption spectra, I PAS ( λ i ) , for diluted propane, ethane and methane test gases at low concentrations ( c ∼ 100 ppm ) were measured for ∼1350 discrete wavelengths λ i . The I PAS ( λ i ) spectra were then compared to the high resolution cross section data, σ FTIR , obtained by Fourier Transform Infrared Spectroscopy published in the HITRAN database. Deviations of 7.1(6)% for propane, 8.7(11)% for ethane and 15.0(14)% for methane with regard to the average uncertainty between I PAS ( λ i ) and the expected reference values based on σ FTIR were recorded. The characteristic absorption wavelengths λ res can be resolved with an average resolution of δ λ res ∼ 0.08 nm . Detection limits range between 7.1 ppb (ethane) to 13.6 ppb (methane). In an additional step, EUREQA, an artificial intelligence (AI) program, was successfully applied to deconvolute simulated PAS spectra of mixed gas samples at low limits of detection. The results justify a further development of PAS technology to support e.g., biomedical research.

Widely tunable, continuous-wave, intra-cavity optical parametric oscillator based on an Yb-doped fiber laser

We report an optical parametric oscillator (OPO) intra-cavity pumped by an Yb-doped fiber laser. In comparison to an intra-cavity OPO based on a solid-state laser gain medium, the benefits of using a fiber-based scheme include a superior beam quality of the generated mid-infrared idler light at high power levels, a more efficient process of nonlinear frequency conversion, and the prospect of scaling to higher power. In a preliminary experiment, we obtained a slope efficiency of the down-converted power of as high as 66% with respect to the absorbed laser diode power. To the best of our knowledge, this is the first demonstration of an intra-cavity OPO based on a fiber laser.

Chirped-pulse optical parametric oscillators

We demonstrated that by introducing an extra optical material with a high nonlinear refractive index into a conventional optical parametric oscillator (OPO) cavity and managing the intra-cavity dispersion, a chirped-pulse OPO (CPOPO) can be constructed, generating optical pulses with an instantaneous bandwidth much wider than the pump bandwidth. Experimentally, from a CPOPO pumped by a Yb-fiber laser system centered at 1058 nm with a pulse duration of 230 fs and a -20  dB bandwidth of 8.7 THz, we obtained a signal wave with a -20  dB bandwidth of 28.4 THz and an idler wave covering 2900-3960 nm with a -20  dB bandwidth of 27.7 THz.

Continuous-wave, singly-resonant, intracavity optical parametric oscillator based on a single-mode-laser-diode-pumped Yb:KYW laser

We report a continuous-wave (cw), intracavity, singly resonant optical parametric oscillator (ICSRO) based on an Yb:KYW laser pumped by a single-mode laser diode (LD). Pumping the ICSRO by a low-noise single-mode LD, combined with the reduced heat generation due to the lower quantum defect of the Yb:KYW laser system, effectively eliminated the onset of relaxation oscillations, which have been a long-standing problem in previous multimode-LD-pumped ICSROs, and resulted in a cw ICSRO being operated free of relaxation oscillations. At an LD power of 515 mW, the generated idler power was 21 mW at ∼3500  nm. To the best of our knowledge, this is the first demonstration of a single-mode-LD-pumped ICSRO.

Mid-infrared frequency comb generation via cascaded quadratic nonlinearities in quasi-phase-matched waveguides

We experimentally demonstrate a simple configuration for mid-infrared (MIR) frequency comb generation in quasi-phase-matched lithium niobate waveguides using the cascaded-χ⁽²⁾ nonlinearity. With nanojoule-scale pulses from an Er:fiber laser, we observe octave-spanning supercontinuum in the near-infrared with dispersive wave generation in the 2.5-3 μm region and intrapulse difference frequency generation in the 4-5 μm region. By engineering the quasi-phase-matched grating profiles, tunable, narrowband MIR and broadband MIR spectra are both observed in this geometry. Finally, we perform numerical modeling using a nonlinear envelope equation, which shows good quantitative agreement with the experiment-and can be used to inform waveguide designs to tailor the MIR frequency combs. Our results identify a path to a simple single-branch approach to mid-infrared frequency comb generation in a compact platform using commercial Er:fiber technology.

Multiplexed direct-frequency-comb Vernier spectroscopy of carbon dioxide 2ν1 + ν3 ro-vibrational combination band

We investigated a set of nineteen ¹²C¹⁶O₂ transitions of the 2ν₁ + ν₃ ro-vibrational band in the spectral region from 5064 to 5126 cm^-1 at different pressures, using frequency-comb Vernier spectroscopy. Our spectrometer enabled the systematic acquisition of molecular absorption profiles with high precision. Spectroscopic parameters, namely, transition frequency, linestrength, and self-pressure broadening coefficient, have been accurately determined by using a global fit procedure. These data are in agreement with theoretical values contained in HITRAN2016 database [I. E. Gordon et al., J. Quant. Spectrosc. Radiat. Transfer 203, 3-69 (2017)] at the same precision level. A moderate improvement of the line intensity determinations, by a factor 1.5 in the best case [P(10) transition at 5091.6 cm^-1], should be noticed, projecting direct-comb-Vernier-spectroscopy as an adequate tool for spectral intensity calibration.

Vibrational Energy Redistribution in a Gas-Surface Encounter: State-to-State Scattering of CH_{4} from Ni(111)

The fate of vibrational energy in the collision of methane (CH_{4}) in its antisymmetric C-H stretch vibration (ν_{3}) with a Ni(111) surface has been studied in a state-to-state scattering experiment. Laser excitation in the incident molecular beam prepared the J=1 rotational state of ν_{3}, and a bolometer in combination with selective laser excitation detected the scattered methane. The rovibrationally resolved scattering distributions reveal very efficient vibrational energy redistribution from ν_{3} to the symmetric C-H stretch vibration (ν_{1}). The branching ratio ν_{1}/ν_{3} is near 0.4 and insensitive to changes in incident kinetic energy in the range from 100 to 370 meV. State-resolved angular distributions and measurements of the residual Doppler linewidths prove that the scattering is direct. The observed vibrationally inelastic scattering provides direct experimental evidence for surface-induced vibrational energy redistribution.

Microresonator-based high-resolution gas spectroscopy

We report the first demonstration of a microresonator-based tunable mode-locked frequency comb source. We achieve a mode-hop-free tuning range of 16 GHz by simultaneously tuning both the pump laser and the cavity resonance while keeping the system in a multi-soliton mode-locked state. The optical spectrum spans 2520-4125  cm^-1 (2.425-3.970 μm) pumping at 3508  cm^-1 (2.850 μm) in a silicon microresonator with a comb line spacing of 4.23  cm^-1 (127 GHz). Our scanning technique can be used to increase the effective resolution of the microresonator-based comb spectroscopy. As a proof-of-principle demonstration, we record the absorption spectrum of the rovibrational transitions of the υ₃ and υ₂+(υ₄+υ₅)+0 bands of acetylene. We measure absorption features as narrow as 0.21  cm^-1 (6.4 GHz) full width at half-maximum at a frequency sampling step of 80 MHz.

Probing methane in air with a midinfrared frequency comb source

We employed a midinfrared frequency comb source for methane detection in ambient air. The transmitted spectra over a bandwidth of about 500 nm were recorded with an optical spectrum analyzer under various experimental conditions of different path lengths. The normalized absorption spectra were compared and fitted with simulations, yielding quantitative values of concentrations of methane and water vapor in the ambient air. The 3σ detection limit was ∼6.6×10^-7  cm^-1 in ambient air for a broad spectral range, achieved with a path length of ∼590  m. This approach provides a broad spectral range, a large dynamic range, high sensitivity, and accurate calibration. The performed analysis of the residuals shows that an excellent agreement between the measured and calculated spectral profiles was obtained.

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.

Comb-locked cavity ring-down saturation spectroscopy

We present a new method of comb-locked cavity ring-down spectroscopy for the Lamb-dip measurement of molecular ro-vibrational transitions. By locking both the probe laser frequency and a temperature-stabilized high-finesse cavity to an optical frequency comb, we realize saturation spectroscopy of molecules with kilohertz accuracy. The technique is demonstrated by recording the R(9) line in the υ = 3 - 0 overtone band of CO near 1567 nm. The Lamb-dip spectrum of such a weak line (transition rate 0.0075 s^-1) is obtained using an input laser power of only 3 mW, and the position is determined to be 191 360 212 770 kHz with an uncertainty of 7 kHz (δν/ν∼3.5×10^-11), which is currently limited by our rubidium clock.

Frequency comb assisted two-photon vibrational spectroscopy

We report a setup for high-resolution two-photon spectroscopy using a mid-infrared continuous wave optical parametric oscillator (CW-OPO) and a near-infrared diode laser as the excitation sources, both of which are locked to fully stabilized optical frequency combs. The diode laser is directly locked to a commercial near-infrared optical frequency comb using an optical phase-locked loop. The near-infrared frequency comb is also used to synchronously pump a degenerate femtosecond optical parametric oscillator to produce a fully stabilized mid-infrared frequency comb. The beat frequency between the mid-infrared comb and the CW-OPO is then stabilized through frequency locking. We used the setup to measure a double resonant two-photon transition to a symmetric vibrational state of acetylene with a sub-Doppler resolution and high signal-to-noise ratio.

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.