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

Tunable and mode-locked Tm,Ho:GdScO3 laser

A novel, to the best of our knowledge, Tm,Ho:GdScO3 crystal grown using the Czochralski method was investigated for its polarized spectroscopic properties and laser performance in both tunable continuous-wave (CW) and mode-locked regimes. The crystal's multisite structure (Gd3+/Sc3+ site) and Tm3+/Ho3+ dopants contributed to spectral broadening, enabling a tunable laser operation from 1914 to 2125 nm (with a broad range of 215 nm). Additionally, a pulse duration of 72 fs was achieved for E || b polarization. These results demonstrate the potential of the Tm,Ho:GdScO3 perovskite crystal as a promising gain material for ultrafast lasers operating around 2 µm.

Mode-locking of a Tm,Ho:CALYGO laser delivering 50 fs pulses at 2.08 µm

We study the polarization-dependent laser performance of a novel, to the best of our knowledge, "mixed" Tm,Ho:CaYGdAlO4 crystal in the continuous-wave (CW) and mode-locked regimes. Both in terms of the CW tunability range (261 nm) and the minimum pulse duration (50 fs at 2078 nm, spectral width of 95 nm) in the mode-locked regime, σ-polarization is superior. With extended inhomogeneous spectral broadening due to structural and compositional disorder, Tm,Ho:CaYGdAlO4 is promising for few-optical-cycle pulse generation around 2 µm.

SESAM mode-locked Tm:Y2O3 ceramic laser

We demonstrate a widely tunable and passively mode-locked Tm:Y₂O₃ ceramic laser in-band pumped by a 1627-nm Raman fiber laser. A tuning range of 318 nm, from 1833 to 2151 nm, is obtained in the continuous-wave regime. The SESAM mode-locked laser produces Fourier-transform-limited pulses as short as 75 fs at ∼ 2.06 µm with an average output power of 0.26 W at 86.3 MHz. For longer pulse durations of 178 fs, an average power of 0.59 W is achieved with a laser efficiency of 29%. This is, to the best of our knowledge, the first mode-locked Tm:Y₂O₃ laser in the femtosecond regime. The spectroscopic properties and laser performance confirm that Tm:Y₂O₃ transparent ceramics are a promising gain material for ultrafast lasers at 2 µm.

Watt-level femtosecond Tm-doped "mixed" sesquioxide ceramic laser in-band pumped by a Raman fiber laser at 1627 nm

We report on a semiconductor saturable absorber mirror mode-locked Tm:(Lu,Sc)₂O₃ ceramic laser in-band pumped by a Raman fiber laser at 1627 nm. The nonlinear refractive index (n₂) of the Tm:(Lu,Sc)₂O₃ ceramic has been measured to be 4.66 × 10^-20 m²/W at 2000 nm. An average output power up to 1.02 W at 2060 nm is achieved for transform-limited 280-fs pulses at a repetition rate of 86.5 MHz, giving an optical efficiency with respect to the absorbed pump power of 36.4%. Pulses as short as 66 fs at 2076 nm are produced at the expense of output power (0.3 W), corresponding to a spectral bandwidth of 69 nm. The present work reveals the potential of Tm³⁺-doped sesquioxide transparent ceramics for power scaling of femtosecond mode-locked bulk lasers emitting in the 2-µm spectral range.

Tunable thulium-doped mode-locked fiber laser with watt-level average power

We demonstrate a wavelength-tunable, sub-200 fs, and watt-level thulium-doped ultrafast fiber oscillator with a fundamental frequency repetition rate of 509.7 MHz. The wavelength can be tuned between 1918.5 nm and 2031 nm by adjusting the intra-cavity waveplates. When the wavelength is tuned to below 2000 nm, the average output power exceeds 1 W. The oscillator provides a maximum average power of 1.314 W (corresponding to a pulse energy of 2.58 nJ) and a highest peak power of 12.5 kW at 1940 nm. Such a high-power, tunable 2-µm mode-locked fiber laser is an ideal light source candidate for a variety of applications, such as frequency metrology, molecular spectroscopy, and ultrafast pump-probe spectroscopy.

3-6 µm dispersive mirrors compensating for dispersion introduced by the GaAs crystal

A new broadband dispersive mirror (DM), which consists of Si and SiO₂ layer materials operating in the spectral range of 3-6 µm, is developed for the first time, to our knowledge. The DM is able to compensate for the dispersion of a 0.5 mm thick GaAs crystal per bounce. Pulse analysis proves that the compensation effect of the DM is much better than a CaF₂ plate, which is the commonly used dispersion compensation element in mid-infrared optical parametric oscillators (MIR OPOs), and the new MIR DM could improve the MIR OPO spectrum with better pulse quality and short pulse duration.

Supercontinuum generation from a quasi-stationary doubly resonant optical parametric oscillator

We report a novel, to the best of our knowledge, quasi-stationary operation-state in a femtosecond doubly resonant optical parametric oscillator (OPO). At high pump rates, a doubly resonant OPO (DRO) could evolve into a nonstationary state, and we demonstrate that through an averaging procedure, most of the power fluctuations could be suppressed, resulting in a quasi-stationary output. Moreover, the quasi-stationary DRO could be configured to generate broadband supercontinuum, with a bandwidth only limited by the parametric gain bandwidth of the nonlinear crystal. Experimentally, from a DRO based on a 3-mm-long MgO: PPLN crystal, we generated a supercontinuum with a -20dB bandwidth of 86.9 THz, covering 1.54-2.78 µm. This indeed opens up new opportunities for generating light emission with an ultra-wide bandwidth at various wavelength regions.

Generation of few-cycle pulses from a mode-locked Tm-doped fiber laser

We report a compact, self-starting dispersion-managed mode-locked thulium-doped fiber oscillator that delivers 2.6 nJ pulses at 2 µm with a repetition rate of 250 MHz. The average output power and spectral bandwidth of the pulses reach impressive values of 648 mW and 103 nm, respectively. The generated pulses are near linearly chirped, capable of linearly compressing to 74 fs in a normal dispersion fiber after power attenuation. Using a nonlinear fiber compression scheme can even compress the pulses to 29 fs (4.3-cycle). The remaining pulse energy is 1.15 nJ, and the corresponding peak power is estimated as 39.4 kW. To the best of our knowledge, this is the first demonstration of nonlinearly compressing the pulse of a 2 µm fiber oscillator to the sub-5 cycle regime. Such a few-cycle fiber laser could be an ideal candidate source for short-wavelength mid-infrared frequency metrology and molecular spectroscopy applications.

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.

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.

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.

Sub-200 fs, 344 MHz mode-locked Tm-doped fiber laser

We demonstrate a compact, self-starting mode-locked thulium-doped fiber laser based on nonlinear polarization evolution (NPE), with a fundamental repetition rate of ∼344MHz and a pulse duration of ∼160fs. The generated pulses centered at ∼1975nm have a maximum output power of ∼560mW, corresponding to a pulse energy of ∼1.63nJ. To the best of our knowledge, the achieved repetition rate represents the highest value of fundamentally NPE mode-locked fiber lasers at ∼2µm, while the average output power is also higher than the previously reported 2 µm ultrafast single-mode fiber oscillators. The timing jitter in the integrated range [5 kHz, 10 MHz] and the integrated relative intensity noise in the range [10 Hz, 10 MHz] reach ∼35fs and ∼0.009%, respectively. Our high-performance laser is an ideal candidate for various applications, including mid-infrared frequency metrology and high-speed optical sampling.

Mid-infrared dual-comb spectroscopy with absolute frequency calibration using a passive optical reference

We demonstrate an absolute-frequency-calibrated mid-infrared dual-comb spectrometer by using a reference absorption cell. The source is based on a singly-resonant OPO containing two MgO:PPLN crystals in a common ring cavity, synchronously pumped by two mode-locked Yb-fiber lasers. The repetition-rate of the two pumps are stabilized while their offset frequencies and the OPO cavity length are not actively controlled. The reference spectrum is used to correct the frequency fluctuations in the sample spectrum providing a high-quality averaged spectrum with spectral resolution of 6 GHz and calibration precision of 120 MHz, without adding any complexity to the experimental setup or signal processing.

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.

67-fs pulse generation from a mode-locked Tm,Ho:CLNGG laser at 2083 nm

We report on a mode-locked Tm,Ho:CLNGG laser emitting in the 2 µm spectral range using single-walled carbon nanotubes (SWCNTs) as a saturable absorber (SA). Pulses with duration of 98 fs are generated at 99.28 MHz repetition rate with an average output power of 123 mW, yielding a pulse energy of 1.24 nJ. Using a 0.5% output coupling, pulses as short as 67 fs, i.e., 10 optical cycles, are produced after extracavity compression with a 3-mm-thick ZnS plate.

Mid-Infrared Tunable Laser-Based Broadband Fingerprint Absorption Spectroscopy for Trace Gas Sensing: A Review

The vast majority of gaseous chemical substances exhibit fundamental rovibrational absorption bands in the mid-infrared spectral region (2.5–25 μm), and the absorption of light by these fundamental bands provides a nearly universal means for their detection. A main feature of optical techniques is the non-intrusive in situ detection of trace gases. We reviewed primarily mid-infrared tunable laser-based broadband absorption spectroscopy for trace gas detection, focusing on 2008–2018. The scope of this paper is to discuss recent developments of system configuration, tunable lasers, detectors, broadband spectroscopic techniques, and their applications for sensitive, selective, and quantitative trace gas detection.

Thulium-doped nonlinear fiber amplifier delivering 50 fs pulses at 20 W of average power

In this Letter, we present an optimized nonlinear amplification scheme in the 2 μm wavelength region. This laser source delivers 50 fs pulses at an 80 MHz repetition rate with exceptional temporal pulse quality and 20 W of average output power. According to predictions from numerical simulations, it is experimentally confirmed that dispersion management is crucial to prevent the growth of side pulses and an increase of the energy content in a temporal pedestal surrounding the self-compressed pulse. Based on these results, we discuss guidelines to ensure high temporal pulse quality from nonlinear femtosecond fiber amplifiers in the anomalous dispersion regime.

Phase-stabilized 100 mW frequency comb near 10 μm

Long-wavelength mid-infrared (MIR) frequency combs with high power and flexible tunability are highly desired for molecular spectroscopy, including investigation of large molecules such as C60. We present a high power, phase-stabilized frequency comb near 10 μm, generated by a synchronously pumped, singly resonant optical parametric oscillator (OPO) based on AgGaSe2. The OPO can be continuously tuned from 8.4 to 9.5 μm, with a maximum average idler power of 100 mW at the center wavelength of 8.5 μm. Both the repetition rate (f rep) and the carrier-envelope offset frequency (f ceo) of the idler wave are phase-locked to microwave signals referenced to a Cs clock. We describe the detailed design and construction of the frequency comb, and discuss potential applications for precise and sensitive direct frequency comb spectroscopy.

Flexible pulse-stretching for a swept source at 2.0 μm using free-space angular-chirp-enhanced delay

Dispersive pulse-stretching at 2.0 μm has long been hindered by the high intrinsic optical loss from conventional dispersive media. Here a flexible pulse-stretching technique at 2.0 μm is demonstrated over a broad bandwidth with large-scale dispersion and low intrinsic optical loss. The technique employs the newly proposed pulse-stretching scheme, namely, free-space angular-chirp-enhanced delay. Both normal and anomalous temporal dispersion (up to ±500  ps/nm) with low intrinsic loss (<6  dB) over a spectral bandwidth of ∼84  nm at 2.0 μm is obtained with low nonlinear effects. Based on this method, an optical wavelength-swept source at 2.0 μm is realized and applied to spectrally encoded imaging at a line scan rate of ∼19  MHz, proving the potential of this pulse-stretching technique for continuous single-shot measurements at the 2.0 μm wavelength regime, particularly for optical microscopy and spectroscopy.

Self-referenced octave-wide subharmonic GaP optical parametric oscillator centered at 3 μm and pumped by an Er-fiber laser

We report an octave-wide mid-IR spectrum (2.3-4.8 μm) obtained from a subharmonic optical parametric oscillator (OPO) based on a newly developed nonlinear crystal, orientation-patterned gallium phosphide (OP-GaP), which was synchronously pumped by a femtosecond 1560 nm fiber laser. We proved that the octave-wide output is in the form of a single frequency comb. The observed f-to-2f frequency beats, originating directly from the OPO, can be used for self-referencing and phase locking of the pump laser comb with no need for supercontinuum generation. With an average output power of ∼30  mW, this setup might be beneficial for a variety of spectroscopic applications in the mid-IR.

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.

High-efficiency femtosecond Raman soliton generation with a tunable wavelength beyond 2 μm

We report an efficient Raman soliton laser system by optimizing the chirp of input pulses. The highest efficiency of energy transferred to a Raman soliton is up to 97%, and the tunable range is from 1.98 to 2.31 μm. The pulse width of these tunable Raman solitons remains below 200 fs. The efficiency and the wavelength range are mainly limited by the background loss of the silica fiber.

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.

Gas-phase broadband spectroscopy using active sources: progress, status, and applications

Broadband spectroscopy is an invaluable tool for measuring multiple gas-phase species simultaneously. In this work we review basic techniques, implementations, and current applications for broadband spectroscopy. We discuss components of broad-band spectroscopy including light sources, absorption cells, and detection methods and then discuss specific combinations of these components in commonly-used techniques. We finish this review by discussing potential future advances in techniques and applications of broad-band spectroscopy.

Three-photon absorption in optical parametric oscillators based on OP-GaAs

We report on, to the best of our knowledge, the first singly resonant (SR), synchronously pumped optical parametric oscillator (OPO) based on orientation-patterned gallium arsenide (OP-GaAs). Together with a doubly resonant (DR) degenerate OPO based on the same OP-GaAs material, the output spectra cover 3 to 6 μm within ∼3  dB of relative power. The DR-OPO has the highest output power reported to date from a femtosecond, synchronously pumped OPO based on OP-GaAs. We observed strong three-photon absorption with a coefficient of 0.35±0.08  cm³/GW² for our OP-GaAs sample, which limits the output power of these OPOs as mid-IR light sources. We present a detailed study of the three-photon loss on the performance of both the SR- and DR-OPOs, and compare them to those without this loss mechanism.

Stabilizing carrier-envelope offset frequency of a femtosecond laser using heterodyne interferometry

We propose a time-domain f_ceo stabilization method of a femtosecond laser using heterodyne interferometry. A femtosecond pulse train that is delayed by a spatial delay line interferes with the original pulse train. The phase difference between heterodyne interference signals extracted from different spectral regions is used to stabilize the relative position of the two pulse trains; then the heterodyne interference phase is used to stabilize the carrier-envelope offset frequency f_ceo. The experimental results show that, after being stabilized, the relative Allan deviations of f_ceo are 1.0×10^-9 at 0.5 s and 4.6×10^-10 at 50 s.

Dispersion-compensation-free femtosecond Tm-doped all-fiber laser with a 248 MHz repetition rate

In this Letter, we report a dispersion-compensation-free ultrafast thulium-doped all-fiber laser based on nonlinear polarization evolution (NPE) mode locking, delivering 330 fs soliton pulses at 1950 nm. A multifunctional hybrid fiberized device was applied in the oscillator to minimize the physical cavity length to ∼80  cm with a total dispersion of -0.045  ps², enabling a state-of-the-art fundamental mode-locking repetition rate of 248 MHz in an NPE-based oscillator at ∼2  μm.