Midinfrared frequency combs from coherent supercontinuum in chalcogenide and optical parametric oscillation

>10 GHz femtosecond fiber laser system at 2.0 μm

We demonstrate a high-power 2.0-μm fiber laser system delivering femtosecond pulses with a fundamental repetition rate of >10 GHz, the highest value so far, to the best of our knowledge. The seed is a self-started fundamentally mode-locked Tm-doped fiber laser that has excellent power and spectral stabilities. The laser system can provide an average power of >600 mW, and the use of soliton-effect-based pulse compression allows the achievement of a pulse duration of 163 fs, leading to a compression factor of ∼ 13. It is anticipated that this new high-power femtosecond fiber laser with a 10-GHz-level fundamental repetition rate can serve as a promising light source for various applications, including laser surgery, micromachining, frequency comb spectroscopy, and nonlinear frequency conversion.

Chalcogenide Glass Microfibers for Mid-Infrared Optics

With diameters close to the wavelength of the guided light, optical microfibers (MFs) can guide light with tight optical confinement, strong evanescent fields and manageable waveguide dispersion and have been widely investigated in the past decades for a variety of applications. Compared to silica MFs, which are ideal for working in visible and near-infrared regions, chalcogenide glass (ChG) MFs are promising for mid-infrared (mid-IR) optics, owing to their easy fabrication, broad-band transparency and high nonlinearity, and have been attracting increasing attention in applications ranging from near-field coupling and molecular sensing to nonlinear optics. Here, we review this emerging field, mainly based on its progress in the last decade. Starting from the high-temperature taper drawing technique for MF fabrication, we introduce basic mid-IR waveguiding properties of typical ChG MFs made of As2S3 and As2Se3. Then, we focus on ChG-MF-based passive optical devices, including optical couplers, resonators and gratings and active and nonlinear applications of ChG MFs for mid-IR Raman lasers, frequency combs and supercontinuum (SC) generation. MF-based spectroscopy and chemical/biological sensors are also introduced. Finally, we conclude the review with a brief summary and an outlook on future challenges and opportunities of ChG MFs.

Two-octave-wide (3-12 µm) subharmonic produced in a minimally dispersive optical parametric oscillator cavity

We report a subharmonic (frequency-divide-by-2) optical parametric oscillator (OPO) with a continuous wavelength span of 3 to 12 µm (-37dB level) that covers most of the molecular rovibrational "signature" region. The key to obtaining such a wide spectral span is the use of an OPO with a minimal dispersion-through the choice of intracavity elements, the use of all gold-coated mirrors, and a special "injector" mirror. The system delivers up to 245 mW of the average power with the conversion efficiency exceeding 20% from a 2.35 µm Kerr-lens mode-locked pump laser.

Broadband high-resolution molecular spectroscopy with interleaved mid-infrared frequency combs

Traditionally, there has been a trade-off in spectroscopic measurements between high resolution, broadband coverage, and acquisition time. Originally envisioned for precision spectroscopy of the hydrogen atom in the ultraviolet, optical frequency combs are now commonly used for probing molecular ro-vibrational transitions throughout broad spectral bands in the mid-infrared providing superior resolution, speed, and the capability of referencing to the primary frequency standards. Here we demonstrate the acquisition of 2.5 million spectral data points over the continuous wavelength range of 3.17-5.13 µm (frequency span 1200 cm-1, sampling point spacing 13-21 MHz), via interleaving comb-tooth-resolved spectra acquired with a highly-coherent broadband dual-frequency-comb system based on optical subharmonic generation. With the original comb-line spacing of 115 MHz, overlaying eight spectra with gradually shifted comb lines we fully resolve the amplitude and phase spectra of molecules with narrow Doppler lines, such as carbon disulfide (CS2) and its three isotopologues.

Enhancement of third-order nonlinearity of thermally evaporated GeSbSe waveguides through annealing

Chalcogenides are a promising platform for infrared nonlinear optics but are susceptible to structural changes during fabrication that affect their linear and nonlinear optical properties. We analyze the structure and optical properties of thermally evaporated and annealed chalcogenide films. Thermally evaporated Ge₂₈Sb₁₂Se₆₀ has an increased selenium content, bandgap, and concentration of heteropolar bonds. The concentration of heteropolar bonds can be reduced by annealing above the glass transition temperature, resulting in improved optical nonlinearity. We demonstrate a 4-fold enhancement of third-order nonlinearity in Ge₂₈Sb₁₂Se₆₀ chalcogenide waveguides by thermal annealing and a decrease in propagation loss from 2.5 dB/cm to 1 dB/cm as an added benefit.

Impact of deuteration on the ultrafast nonlinear optical response of toluene and nitrobenzene

Nonlinear pulse propagation inside highly nonlinear media requires accurate knowledge on the temporal response function of the materials used particular in the case of liquids. Here we study the impact of deuteration on the ultrafast dynamics of toluene and nitrobenzene via all optical Kerr gating, showing substantially different electronic and molecular contributions, which was quantified by fitting a multichannel decay model to the data points. Specifically we found that deuteration imposes the time-integrated nonlinearities to reduce particular for toluene which could be caused by both reduced electronic hyperpolarizabilities as well as weaker intermolecular interactions. The results achieved reveal that deuterated organic solvents represent promising materials for infrared photonics since they offer extended infrared transmission compared to their non-deuterated counterparts while maintained strong nonlinear responses.

Long-term stable supercontinuum generation and watt-level transmission in liquid-core optical fibers

Due to their unique properties such as transparency, tunability, nonlinearity, and dispersion flexibility, liquid-core fibers represent an important approach for future coherent mid-infrared light sources. However, the damage thresholds of these fibers are largely unexplored. Here we report on the generation of soliton-based supercontinua in carbon disulfide (CS₂) liquid-core fibers at average power levels as high as 0.5 W operating stably for a long term (>70  h) without any kind of degradation or damage. Additionally, we also show stable high-power pulse transmission through liquid-core fibers exceeding 1 W of output average power for both CS₂ and tetrachloroethylene as core materials.

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.

Guiding light in a water core all-solid cladding photonic band gap fiber - an innovative platform for fiber-based optofluidics

We present a single-channel photonic band gap fiber design allowing for guiding light inside a water core, which is surrounded by solid microstructured cladding, consisting of an array of high refractive index strands in silica. We address all relevant properties and show that the microstructure substantially reduces loss. We also introduce a ray reflection model, matching numerical modelling and allowing for time-effective large-scale parameter sweeps. Our single channel fiber concept is particularly valuable for applications demanding fast and reliable injection of liquids into the core, with potential impact in fields such as optofluidics, spectroscopy or bioanalytics.

Hybrid soliton dynamics in liquid-core fibres

The discovery of optical solitons being understood as temporally and spectrally stationary optical states has enabled numerous innovations among which, most notably, supercontinuum light sources have become widely used in both fundamental and applied sciences. Here, we report on experimental evidence for dynamics of hybrid solitons-a new type of solitary wave, which emerges as a result of a strong non-instantaneous nonlinear response in CS2-filled liquid-core optical fibres. Octave-spanning supercontinua in the mid-infrared region are observed when pumping the hybrid waveguide with a 460 fs laser (1.95 μm) in the anomalous dispersion regime at nanojoule-level pulse energies. A detailed numerical analysis well correlated with the experiment uncovers clear indicators of emerging hybrid solitons, revealing their impact on the bandwidth, onset energy and noise characteristics of the supercontinua. Our study highlights liquid-core fibres as a promising platform for fundamental optics and applications towards novel coherent and reconfigurable light sources.Here, Chemnitz et al. report experimental evidence for hybrid solitons - a type of solitary wave, which emerges as a result of a strong non-instantaneous nonlinear response in CS2-filled liquid-core optical fibres, demonstrating efficient soliton-driven supercontinuum generation.

Super-flat coherent supercontinuum source in As38.8Se61.2 chalcogenide photonic crystal fiber with all-normal dispersion engineering at a very low input energy

We numerically report super-flat coherent mid-infrared supercontinuum (MIR-SC) generation in a chalcogenide As_38.8Se_61.2 photonic crystal fiber (PCF). The dispersion and nonlinear parameters of As_38.8Se_61.2 chalcogenide PCFs by varying the diameter of the air holes are engineered to obtain all-normal dispersion (ANDi) with high nonlinearities. We show that launching low-energy 50 fs optical pulses with 0.88 kW peak power (corresponding to pulse energy of 0.05 nJ) at a central wavelength of 3.7 μm into a 5 cm long ANDi-PCF generates a flat-top coherent MIR-SC spanning from 2900 to 4575 nm with a high spectral flatness of 3 dB. This ultra-wide and flattened spectrum has excellent stability and coherence properties that can be used for MIR applications such as medical diagnosis of diseases, atmospheric pollution monitoring, and drug detection.

Mid-infrared nonlinear absorption in As2S3 chalcogenide glass

We report mid-infrared (MIR) nonlinear absorption in As₂S₃ glasses which results from two-photon excitation of valence electron to the Urbach extension followed by strong linear absorption of excited states. The measured MIR nonlinear absorption can be 3 to 4 orders of magnitude stronger than the two-photon absorption in the near-infrared for similar laser intensities and does not result from contaminants, but it is intrinsic to As₂S₃ glasses. As₂S₃ fibers are widely used to generate supercontinuum by pumping them with high peak power laser pulses. For a 100 kilowatt peak power MIR soliton propagating in single mode As₂S₃ fiber, the nonlinear absorption can be of similar magnitude than the fiber background loss. Finally, for laser peak power around 1 MW, the MIR nonlinear absorption can be ~2 orders of magnitude larger than the fiber background loss in single mode As₂S₃ fiber.

Analytic model for the complex effective index dispersion of metamaterial-cladding large-area hollow core fibers

We present a mathematical model that allows interpreting the dispersion and attenuation of modes in hollow-core fibers (HCFs) on the basis of single interface reflection, giving rise to analytic and semi-analytic expressions for the complex effective indices in the case where the core diameter is large and the guiding is based on the reflection by a thin layer. Our model includes two core-size independent reflection parameters and shows the universal inverse-cubed core diameter dependence of the modal attenuation of HCFs. It substantially reduces simulation complexity and enables large scale parameter sweeps, which we demonstrate on the example of a HCF with a highly anisotropic metallic nanowire cladding, resembling an indefinite metamaterial at high metal filling fractions. We reveal design rules that allow engineering modal discrimination and show that metamaterial HCFs can principally have low losses at mid-IR wavelengths (< 1 dB/m at 10.6 µm). Our model can be applied to a great variety of HCFs with large core diameters and can be used for advanced HCF design and performance optimization, in particular with regard to dispersion engineering and modal discrimination.

Visible to near-infrared supercontinuum generation in yttrium orthosilicate bulk crystal and ion implanted planar waveguide

This paper reports on the supercontinuum generation in yttrium orthosilicate bulk crystal and 6-mm-long ion implanted planar waveguide. The waveguide is fabricated by 6 MeV oxygen ions implantation with fluence of 5 × 10¹⁴ ions/cm² at room temperature. The yttrium orthosilicate bulk crystal and waveguide are pumped using a mode-locked Ti:Sapphire laser with a center wavelength of 800 nm. The generated broadest supercontinuum spans 720 nm (at −30 dB points) from 380 to 1100 nm in bulk crystal and 510 nm (at −30 dB points) from 490 to 1000 nm in ion implanted waveguide, respectively. Compared to the bulk crystal, the ion implanted waveguide requires almost three orders of magnitude lower pump power to achieve a similar level of broadening. The supercontinuum is generated in the normal dispersion regime and exhibits a relatively smooth spectral shape. Our research findings indicate that ion implantation is an efficient method to produce waveguide in yttrium orthosilicate crystal for low-threshold supercontinuum generation.

Octave-spanning supercontinuum generation in hybrid silver metaphosphate/silica step-index fibers

We reveal the potential of step-index fibers consisting of a metaphosphate glass core and a silica cladding as an ultrafast octave-spanning supercontinuum source. The hybrid waveguide was fabricated by pressure-assisted melt filling and possesses a sophisticated dispersion behavior with two zero-dispersion points in the proximity of the Erbium laser bands. The fiber generates an octave-spanning supercontinuum from 0.7 to 2.4 μm if pumped at 1.56 μm with 30 fs pulses and energies as low as 300 pJ. Numerical simulations reveal soliton fission and double dispersive wave generation as the dominant broadening effect. This study highlights phosphate glasses as a promising new candidate for the next generation of broadband photonic devices, as they allow for high rare earth-doping levels and dispersion posttuning via plasmonic nanoparticle growth.

Single mode criterion - a benchmark figure to optimize the performance of nonlinear fibers

Optical fibers with sub-wavelength cores are promising systems for efficient nonlinear light generation. Here we reveal that the single-mode criterion represents a convenient design tool to optimize the performance of nonlinear fibers circumventing intense numerical calculations. We introduce a quasi-analytic expression for the nonlinear coefficient allowing us to investigate its behavior over a large parameter range. The study is independent of the actual value of the material nonlinearity and shows the fundamental dependencies of the nonlinear coefficient on wavelength, refractive index and core diameter, elucidated by detailed case studies of fused silica and chalcogenide tapers and hybrid fibers.

Dual optical frequency comb architecture with capabilities from visible to mid-infrared

In this paper, a new approach to dual comb generation based on well-known optical techniques (Gain-Switching and Optical Injection Locking) is presented. The architecture can be implemented using virtually every kind of continuous-wave semiconductor laser source (DFB, VCSEL, QCL) and without the necessity of electro-optic modulators. This way, a frequency-agile and adaptive dual-comb architecture is provided with potential implementation capabilities from mid-infrared to near ultraviolet. With a RF comb comprising around 70 teeth, the system is validated in the 1.5 μm region measuring the absorption feature of H¹³CN at 1538.523 nm with a minimum integration time of 10 μs.

Coherent octave-spanning mid-infrared supercontinuum generated in As2S3-silica double-nanospike waveguide pumped by femtosecond Cr:ZnS laser

A more than 1.5 octave-spanning mid-infrared supercontinuum (1.2 to 3.6 μm) is generated by pumping a As₂S₃-silica "double-nanospike" waveguide via a femtosecond Cr:ZnS laser at 2.35 μm. The combination of the optimized group velocity dispersion and extremely high nonlinearity provided by the As₂S₃-silica hybrid waveguide enables a ~100 pJ level pump pulse energy threshold for octave-spanning spectral broadening at a repetition rate of 90 MHz. Numerical simulations show that the generated supercontinuum is highly coherent over the entire spanning wavelength range. The results are important for realization of a high repetition rate octave-spanning frequency comb in the mid-infrared spectral region.

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.

Silver metaphosphate glass wires inside silica fibers--a new approach for hybrid optical fibers

Phosphate glasses represent promising candidates for next-generation photonic devices due to their unique characteristics, such as vastly tunable optical properties, and high rare earth solubility. Here we show that silver metaphosphate wires with bulk optical properties and diameters as small as 2 µm can be integrated into silica fibers using pressure-assisted melt filling. By analyzing two types of hybrid metaphosphate-silica fibers, we show that the filled metaphosphate glass has only negligible higher attenuation and a refractive index that is identical to the bulk material. The presented results pave the way towards new fiber-type optical devices relying on metaphosphate glasses, which are promising materials for applications in nonlinear optics, sensing and spectral filtering.

Mid-infrared optical parametric oscillators and frequency combs for molecular spectroscopy

Review of mid-infrared optical parametric oscillators and frequency combs for high-resolution spectroscopy, including applications in trace gas detection and fundamental research.

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

We pump a degenerate frequency-divide-by-two optical parametric oscillator (OPO) based on orientation-patterned GaAs with a stable Tm frequency comb at 2 micrometer wavelength and measure the OPO comb offset frequency and linewidth. We show frequency division by two with sub-Hz relative linewidth of the comb teeth. The OPO thermally self-stabilizes and oscillates for nearly an hour without any active control.

Mid-infrared optical frequency combs based on difference frequency generation for molecular spectroscopy

Mid-infrared femtosecond optical frequency combs were produced by difference frequency generation of the spectral components of a near-infrared comb in a 3-mm-long MgO:PPLN crystal. We observe strong pump depletion and 9.3 dB parametric gain in the 1.5 μm signal, which yields powers above 500 mW (3 μW/mode) in the idler with spectra covering 2.8 μm to 3.5 μm. Potential for broadband, high-resolution molecular spectroscopy is demonstrated by absorption spectra and interferograms obtained by heterodyning two combs.

Broadband and transparent wavelength conversion based on dispersion-flattened double-slot waveguide

The flat and low second-order dispersion of 0∼-13  ps²/km in a double-slot waveguide is optimized for broadband wavelength conversion. Subsequently, a 3 dB conversion bandwidth of about 1200 nm is presented by four-wave mixing based on the above structure in simulation. Attributed to the small phase mismatch caused by low second- and fourth-order dispersion parameter in a large waveband, the 3 dB conversion bandwidth is almost constant at about 1200 nm with the pump wavelength varying from 2200 to 2800 nm. The results indicate that this dispersion-flattened double-slot waveguide is not only an ideal device for broadband wavelength conversion but also can realize the transparency of pump wavelength, which has potential applications in wavelength conversion.

Frequency comb offset detection using supercontinuum generation in silicon nitride waveguides

We present the first direct carrier-envelope-offset (CEO) frequency detection of a modelocked laser based on supercontinuum generation (SCG) in a CMOS-compatible silicon nitride (Si(3)N(4)) waveguide. With a coherent supercontinuum spanning more than 1.5 octaves from visible to beyond telecommunication wavelengths, we achieve self-referencing of SESAM modelocked diode-pumped Yb:CALGO lasers using standard f-to-2f interferometry. We directly obtain without amplification strong CEO beat signals for both a 100-MHz and 1-GHz pulse repetition rate laser. High signal-to-noise ratios (SNR) of > 25 dB and even > 30 dB have been generated with only 30 pJ and 36 pJ of coupled pulse energy from the megahertz and gigahertz laser respectively. We compare these results to self-referencing using a commercial photonic crystal fiber and find that the required peak power for CEO beat detection with a comparable SNR is lowered by more than an order of magnitude when using a Si(3)N(4) waveguide.

An octave-spanning mid-infrared frequency comb generated in a silicon nanophotonic wire waveguide

Laser frequency combs, sources with a spectrum consisting of hundred thousands evenly spaced narrow lines, have an exhilarating potential for new approaches to molecular spectroscopy and sensing in the mid-infrared region. The generation of such broadband coherent sources is presently under active exploration. Technical challenges have slowed down such developments. Identifying a versatile highly nonlinear medium for significantly broadening a mid-infrared comb spectrum remains challenging. Here we take a different approach to spectral broadening of mid-infrared frequency combs and investigate CMOS-compatible highly nonlinear dispersion-engineered silicon nanophotonic waveguides on a silicon-on-insulator chip. We record octave-spanning (1,500-3,300 nm) spectra with a coupled input pulse energy as low as 16 pJ. We demonstrate phase-coherent comb spectra broadened on a room-temperature-operating CMOS-compatible chip.

As₂S₃-silica double-nanospike waveguide for mid-infrared supercontinuum generation

A double-nanospike As2S3-silica hybrid waveguide structure is reported. The structure comprises nanotapers at input and output ends of a step-index waveguide with a subwavelength core (1 μm in diameter), with the aim of increasing the in-coupling and out-coupling efficiency. The design of the input nanospike is numerically optimized to match both the diameter and divergence of the input beam, resulting in efficient excitation of the fundamental mode of the waveguide. The output nanospike is introduced to reduce the output beam divergence and the strong endface Fresnel reflection. The insertion loss of the waveguide is measured to be ∼2  dB at 1550 nm in the case of free-space in-coupling, which is ∼7  dB lower than the previously reported single-nanospike waveguide. By pumping a 3-mm-long waveguide at 1550 nm using a 60-fs fiber laser, an octave-spanning supercontinuum (from 0.8 to beyond 2.5 μm) is generated at 38 pJ input energy.