Longwave infrared (6.6-11.4†µm) dual-comb spectroscopy with 240,000 comb-mode-resolved data points at video rate.

Using sub-3-cycle pulses from mode-locked Cr:ZnS lasers at λ ≈ 2.4 µm as a driving source, we performed high-resolution dual-frequency-comb spectroscopy in the longwave infrared (LWIR) range. A duo of highly coherent broadband (6.6-11.4 µm) frequency combs were produced via intrapulse difference frequency generation in zinc germanium phosphide (ZGP) crystals. Fast (up to 0.1 s per spectrum) acquisition of 240,000 comb-mode-resolved data points, spaced by 80 MHz and referenced to a Rb clock, was demonstrated, resulting in metrology grade molecular spectra of N2O (nitrous oxide) and CH3OH (methane). The key to high-speed massive spectral data acquisition was low intensity and phase noise of the LWIR combs and high (7.5%) downconversion efficiency, resulting in a LWIR power of 300 mW for each comb.

Post-compression of long-wave infrared 2 picosecond sub-terawatt pulses in bulk materials.

We have experimentally demonstrated the post-compression of a long-wave infrared (9.2 µm) 150 GW peak power pulse from 2 ps to less than 500 fs using a sequence of two bulk materials with negative group velocity dispersion (GVD). The compression resulted in up to 1.6-fold increase of the peak power and up to 2.8-fold increase of the intensity in the center of a quasi-Gaussian beam. The partial decoupling of the self-phase modulation and chirp compensation stages by using two materials with significantly different ratios of nonlinear refractive index to GVD provides accurate optimization of the compression mechanism and promises a viable path to scaling peak powers to supra-terawatt levels. During the preparatory study, we measured, for the first time to our knowledge, the nonlinear refractive indices of NaCl, KCl, and BaF2 for picosecond pulses in the long-wave infrared region.

Single-shot measurement of the nonlinear refractive index of air at 9.2 µm with a picosecond terawatt CO2 laser.

We developed a simple, accurate single-shot method to determine the nonlinear refractive index of air by measuring the evolution of the spatial shape of a laser beam propagating through the atmosphere. A distinctive feature of this new method, which relies on a modified Fresnel propagation model for data analysis, is the use of a hard aperture for producing a well-defined, high-quality beam from a comparatively non-uniform quasi-flat-top beam, which is typical for high-peak-power lasers. The nonlinear refractive index of air for a very short (2 ps) long-wave infrared (LWIR) laser pulse was measured for the first time, to the best of our knowledge, yielding n2=3.0×10-23m2/W at 9.2 µm. This result is 40% lower than a corresponding measurement with longer (200 ps) LWIR pulses at a similar wavelength.

Multi-octave visible to long-wave IR femtosecond continuum generated in Cr:ZnS-GaSe tandem.

We report a technique for generation of broad and coherent femtosecond (fs) continua that span several octaves from visible to long-wave IR parts of the spectrum (0.4-18 µm). The approach is based on simultaneous amplification of few-cycle pulses at 2.5 µm central wavelength at 80 MHz repetition rate, and augmentation of their spectrum via three-wave mixing in a tandem arrangement of polycrystalline Cr:ZnS and single crystal GaSe. The obtained average power levels include several mW in the 0.4-0.8 µm visible, 0.23 W in the 0.8-2 µm near-IR, up to 4 W in the 2-3 µm IR, and about 17 mW in the 3-18 µm long-wave IR bands, respectively. High brightness and mutual coherence of all parts of the continuum was confirmed by direct detections of the carrier envelope offset frequency of the master oscillator.

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

This erratum reports a correction to the labeling of Figs. 2(b) and 3(b) in the original manuscript, Opt. Lett.42, 4756 (2017)OPLEDP0146-959210.1364/OL.42.004756.

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.

Mid-wave infrared beam steering based on high-efficiency liquid crystal diffractive waveplates.

We demonstrated two liquid crystal diffractive waveplates: one optimized for near-infrared (1.06 µm), and another for mid-wave infrared (MWIR, 3~5 µm). By employing a low loss liquid crystal mixture UCF-M3, whose absorption loss is below 2% in the 4~5 µm spectral region, the grating achieves over 98% diffraction efficiency in a broad MWIR range. To switch the grating, both active and passive driving methods can be considered. In our experiment, we used a polymer-stabilized twisted nematic cell as the polarization rotator for passive driving. The obtained rise time is 0.2 ms and decay time is 10 ms.

Performance estimation of dual-comb spectroscopy in different frequency-control schemes.

Dual-comb spectroscopy (DCS) has shown unparalleled advantages but at the cost of highly mutual coherence between comb lasers. Here, we investigate spectral degradation induced by laser frequency instabilities and improvement benefited from active laser stabilization. Mathematical models of DCS in the cases of direct radio-frequency (RF) locking and optical phase stabilization were separately established first. Numerical simulations are utilized to study the impact of laser intrinsic stability and the improvement by different locking strategies on spectral performance in the following. Finally, both simulations are proven by corresponding experiments. It shows that an optically phase-stabilized system owns a better immunity of laser frequency fluctuations than a direct RF-stabilized one. Furthermore, the performance improvement by the feedback servos is also more effective in the optically phase-stabilized system. In addition, the simulations could instruct optimal design and system improvement.

Spectral narrowing and stabilization of interband cascade laser by volume Bragg grating.

A volume Bragg grating recorded in photo-thermo-refractive glass was used to spectrally lock the emission from an 18-µm-wide interband cascade laser ridge to a wavelength of 3.12 µm. The spectral width of emission into the resonant mode is narrowed by more than 300 times, and the thermal wavelength shift is reduced by 60 times. While the power loss penalty is about 30%, the spectral brightness increases by 200 times.

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

We observe the coherence of the supercontinuum generated in a nanospike chalcogenide-silica hybrid waveguide pumped at 2 µm. The supercontinuum is shown to be coherent with the pump by interfering it with a doubly resonant optical parametric oscillator (OPO) that is itself coherent with the shared pump laser. This enables coherent locking of the OPO to the optically referenced pump frequency comb, resulting in a composite frequency comb with wavelengths from 1 to 6 µm.

Fractional-length sync-pumped degenerate optical parametric oscillator for 500-MHz 3-µm mid-infrared frequency comb generation.

We demonstrate a mid-IR frequency comb centered at 3120 nm with 650-nm (20-THz) bandwidth at a comb-teeth spacing of 500 MHz. The generated comb is based on a compact ring-type synchronously pumped optical parametric oscillator (SPOPO) operating at degeneracy and pumped by a mode-locked Er-doped 1560 nm fiber laser at a repetition rate of 100 MHz. We achieve high-repetition rate by using a fractional-length cavity with a roundtrip length of 60 cm, which is one-fifth of the length dictated by conventional synchronous pumping.

Octave-spanning supercontinuum generation in in situ tapered As2S3 fiber pumped by a thulium-doped fiber laser.

We report a supercontinuum spanning well over an octave of measurable bandwidth from about 1 to 3.7 µm in a 2.1 mm long As2S3 fiber taper using the in situ tapering method. A sub-100-fs mode-locked thulium-doped fiber laser system with ~300 pJ of pulse energy was used as the pump source. Third-harmonic generation was observed and currently limits the pump pulse energy and achievable spectral bandwidth.

In-situ tapering of chalcogenide fiber for mid-infrared supercontinuum generation.

Supercontinuum generation (SCG) in a tapered chalcogenide fiber is desirable for broadening mid-infrared (or mid-IR, roughly the 2-20 µm wavelength range) frequency combs(1, 2) for applications such as molecular fingerprinting, (3) trace gas detection, (4) laser-driven particle acceleration, (5) and x-ray production via high harmonic generation. (6) Achieving efficient SCG in a tapered optical fiber requires precise control of the group velocity dispersion (GVD) and the temporal properties of the optical pulses at the beginning of the fiber, (7) which depend strongly on the geometry of the taper. (8) Due to variations in the tapering setup and procedure for successive SCG experiments-such as fiber length, tapering environment temperature, or power coupled into the fiber, in-situ spectral monitoring of the SCG is necessary to optimize the output spectrum for a single experiment. In-situ fiber tapering for SCG consists of coupling the pump source through the fiber to be tapered to a spectral measurement device. The fiber is then tapered while the spectral measurement signal is observed in real-time. When the signal reaches its peak, the tapering is stopped. The in-situ tapering procedure allows for generation of a stable, octave-spanning, mid-IR frequency comb from the sub harmonic of a commercially available near-IR frequency comb. (9) This method lowers cost due to the reduction in time and materials required to fabricate an optimal taper with a waist length of only 2 mm. The in-situ tapering technique can be extended to optimizing microstructured optical fiber (MOF) for SCG(10) or tuning of the passband of MOFs, (11) optimizing tapered fiber pairs for fused fiber couplers(12) and wavelength division multiplexers (WDMs), (13) or modifying dispersion compensation for compression or stretching of optical pulses.(14-16.)

Carrier envelope offset frequency of a doubly resonant, nondegenerate, mid-infrared GaAs optical parametric oscillator.

We measure the carrier envelope offset (CEO) frequency of the mid-infrared frequency comb (wavelength tunable between 3 and 6 µm) from a doubly resonant nondegenerate synchronously pumped optical parametric oscillator (SPOPO) as a function of the CEO frequency of the Tm-fiber pump laser. We show that the CEO frequency of the SPOPO signal wave is a linear function of the CEO frequency of the pump laser, with a slope determined by the signal to pump center-frequency ratio.

Sub-50 fs pulses around 2070 nm from a synchronously-pumped, degenerate OPO.

We report generation of 48 fs pulses at a center wavelength of 2070 nm using a degenerate optical parametric oscillator (OPO) synchronously-pumped with a commercially available 36-MHz, femtosecond, mode-locked, Yb-doped fiber laser. The spectral bandwidth of the output is ~137 nm, corresponding to a theoretical, transform-limited pulse width of 33 fs. The threshold of the OPO is less than 10 mW of average pump power. By tuning the cavity length, the output spectrum covers a spectral width of more than 400 nm, limited only by the bandwidth of the cavity mirrors.

Mid-infrared supercontinuum generation in tapered chalcogenide fiber for producing octave-spanning frequency comb around 3 µm.

We demonstrate mid-infrared (mid-IR) supercontinuum generation (SCG) with instantaneous bandwidth from 2.2 to 5 µm at 40 dB below the peak, covering the wavelength range desirable for molecular spectroscopy and numerous other applications. The SCG occurs in a tapered As(2)S(3) fiber prepared by in-situ tapering and is pumped by femtosecond pulses from the subharmonic of a mode-locked Er-doped fiber laser. Interference with a narrow linewidth c.w. laser verifies that the coherence properties of the near-IR frequency comb have been preserved through these cascaded nonlinear processes. With this approach stable broad mid-IR frequency combs can be derived from commercially available near-IR frequency combs without an extra stabilization mechanism.

Degenerate 1 GHz repetition rate femtosecond optical parametric oscillator.

We report a degenerate femtosecond optical parametric oscillator (OPO) that is synchronously pumped by a mode-locked Ti:sapphire laser at 1 GHz repetition rate. The OPO produces an 85 nm (10 THz) wide frequency comb centered at 1.6 µm. Stable long-term operation with >100 mW of average output power has been achieved.

All-optical quantum random bit generation from intrinsically binary phase of parametric oscillators.

We demonstrate a novel all-optical quantum random number generator (RNG) based on above-threshold binary phase state selection in a degenerate optical parametric oscillator (OPO). Photodetection is not a part of the random process, and no post processing is required for the generated bit sequence. We show that the outcome is statistically random with 99% confidence, and verify that the randomness is due to the phase of initiating photons generated through spontaneous parametric down conversion of the pump, with negligible contribution of classical noise sources. With the use of micro- and nanoscale OPO resonators, this technique offers a promise for simple, robust, and high-speed on-chip all-optical quantum RNGs.

Octave-spanning ultrafast OPO with 2.6-6.1 µm instantaneous bandwidth pumped by femtosecond Tm-fiber laser.

We report the extension of broadband degenerate OPO operation further into mid-infrared. A femtosecond thulium fiber laser with output centered at 2050 nm synchronously pumps a 500-µm-long crystal of orientation patterned GaAs providing broadband gain centered at 4.1 µm. We observe a pump threshold of 17 mW and output bandwidth extending from 2.6 to 6.1 µm at the -30 dB level. Average output power was 37 mW. Appropriate resonator group dispersion is a key factor for achieving degenerate operation with instantaneously broad bandwidth. The output spectrum is very sensitive to absorption and dispersion introduced by molecular species inside the OPO cavity.

Coherence properties of a broadband femtosecond mid-IR optical parametric oscillator operating at degeneracy.

We study coherence properties of a χ(²) optical parametric oscillator (OPO), which produces 2/3-octave-wide spectrum centered at the subharmonic (3120 nm) of the femtosecond pump laser. Our method consists of interfering the outputs of two identical, but independent OPOs pumped by the same laser. We demonstrate that the two OPOs show stable spatial and temporal interference and are mutually locked in frequency and in phase. By observing a collective heterodyne beat signal between the two OPOs we show that one can deterministically choose, by cavity length adjustment, between the two frequency states corresponding to the two sets of modes shifted with respect to each other by half of the laser pulse repetition rate. Moreover, we observe that the existence of two opposite phase states, a known common feature of a parametrically driven n = 2 subharmonic oscillator, reveals itself in our experiment as a common phase, 0 or π, being established through the whole set of some 300 thousand longitudinal modes.