Difference Frequency Generation Source Research Articles

6.7-8.1 µm tunable single-frequency difference frequency generation in ZGP for high-resolution spectroscopy.

Difference frequency generation (DFG) based tunable single-frequency mid-infrared (MIR) light sources are desirable for high-resolution spectroscopy, sensing, and imaging. In this work, we demonstrate a continuous-wave (CW) single-frequency DFG in a ZnGeP2 (ZGP) crystal driven by all-fiber near-infrared (NIR) fiber lasers, for the first time to our knowledge. The all-fiber NIR laser sources consist of a 1.5 µm erbium-doped fiber amplifier seeded by a CW tunable fast scanning single-frequency laser and a 1.9 µm CW tunable single-frequency thulium-doped fiber laser. Taking advantage of the high nonlinear coefficient and large birefringence of the ZGP crystal, single-frequency DFG in ZGP achieves a broad spectral tuning range from 6.7 to 8.1 µm, with an output power at the 10 µW level. Precise detection of C2H2 gas by continuously scanning the DFG source across a spectral range of 1.1 THz (∼34 cm-1) is also presented, highlighting the potential of the tunable DFG source for high-resolution optical spectroscopy applications. We anticipate this work will provide what we believe to be a new platform for spectroscopy in the molecular fingerprint spectral region.

High-Power, Continuous-Wave, Fiber-Pumped Difference-Frequency-Generation at 2.26 μm

We report a high-power source of continuous-wave (cw) mid-infrared (mid-IR) radiation at 2.26 μm based on difference-frequency-generation (DFG) of a cw Yb-fiber laser at 1.064 μm and a cw Tm-fiber laser at 2.010 μm in MgO:PPLN. Using a 50-mm-long crystal with a single grating period of 32.16 μm and a simple single-pass setup, we generate up to 3.84 W of cw output power at 2.26 μm at a power conversion efficiency of ~0.47%/W. The DFG output beam is characterized by a Gaussian TEM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">00</sub> spatial profile with M <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> <; 1.2, and exhibits excellent passive power stability of 0.6% rms and spectral stability better than 0.01% over 1 hour. We find close agreement between the measured conversion efficiencies and theoretical predictions, confirming optimum performance of the DFG source. Our approach represents a viable and practical alternative to the existing methods for generating this useful wavelength. To the best of our knowledge, the generated output represents the highest cw power obtained from any single-pass nonlinear frequency conversion source in the mid-IR.

Frequency measurements of the 2ν3A1−ν3 band transitions of methane in comb-referenced infrared-infrared double-resonance spectroscopy

We carry out infrared-infrared double-resonance spectroscopy of the $2{\ensuremath{\nu}}_{3}{A}_{1}\text{\ensuremath{-}}{\ensuremath{\nu}}_{3}$ band of methane. Ten transitions of Q(1) to Q(4) are observed with an 88.5-THz difference-frequency-generation (DFG) source, while the lower levels are pumped through the ${\ensuremath{\nu}}_{3}$ band transition by another 90.5-THz DFG source phase-locked to an optical frequency comb (OFC). The transition frequencies are determined using the OFC with an uncertainty of 10 kHz, which is an improvement of 4 orders of magnitude compared with the previous studies.

Intensity noise optimization of a mid-infrared frequency comb difference-frequency generation source

We experimentally demonstrate in a difference-frequency generation mid-infrared frequency comb source the effect of temporal overlap between pump and signal pulses on the relative intensity noise (RIN) of the idler pulse. When scanning the temporal delay between our 130 fs long signal and pump pulses, we observe a RIN minimum with a 3 dB width of 20 fs delay and a RIN increase of 20 dB in 40 fs delay at the edges of this minimum. We also demonstrate active long-term stabilization of the mid-infrared frequency comb source to the temporal overlap setting corresponding to the lowest RIN operation point by an online RIN detector and active feedback control of the pump-signal pulse delay. This active stabilization setup allows us to dramatically increase the signal-to-noise ratio of mid-infrared absorption spectra.

Mid-infrared saturated absorption spectroscopy inside a hollow glass waveguide

We report the saturation spectroscopy of carbon dioxide inside a hollow glass waveguide (GW) using a tunable mid-infrared difference frequency generation (DFG) source. This DFG source was generated in a periodically poled lithium niobate crystal with a tuning range from 3.97 to 4.71μm. Saturated spectroscopy was performed on the R(60) transition of CO20001 ← 0000 band at 4.2μm inside a hollow GW of diameter 300μm and length 1 m. The saturation dip signal was found with a pump power as low as 0.18 mW inside the hollow GW. The linewidth of the dip was determined to be 4.40 MHz.

Kagome Hollow Core Fiber-Based Mid-Infrared Dispersion Spectroscopy of Methane at Sub-ppm Levels.

In this paper, we demonstrate the laser-based gas sensing of methane near 3.3 µm inside hollow-core photonic crystal fibers. We exploit a novel anti-resonant Kagome-type hollow-core fiber with a large core diameter (more than 100 µm) which results in gas filling times of less than 10 s for 1.3-m-long fibers. Using a difference frequency generation source and chirped laser dispersion spectroscopy technique, methane sensing with sub-parts-per-million by volume detection limit is performed. The detection of ambient methane is also demonstrated. The presented results indicate the feasibility of using a hollow-core fiber for increasing the path-length and improving the sensitivity of the mid-infrared gas sensors.

Energy scalable, offset-free ultrafast mid-infrared source harnessing self-phase-modulation-enabled spectral selection.

We demonstrate a high-power offset-free ultrafast mid-infrared (IR) laser source based on difference-frequency generation (DFG). Powerful signal pulses are obtained by filtering the rightmost spectral lobe of the optical spectra broadened by fiber-optic nonlinearities dominated by self-phase modulation. The resulting mid-IR pulses are tunable from 7 to 18μm with up to 5.4mW average power. We experimentally and numerically investigate power scaling of this DFG source and demonstrate that increasing the signal power is an efficient approach for generating high-power mid-IR pulses.

Continuous-Wave 3.1–3.6 μm Difference-Frequency Generation of Dual Wavelength-Tunable Fiber Sources in PPMgLN-Based Rapid-Tuning Design

We report on a single-frequency continuous wave (CW) difference-frequency generation (DFG) source based on single-frequency wavelength-tunable polarization-maintaining ytterbium- and erbium-doped fiber master oscillator–power amplifiers (MOPAs), acting as the pump and signal source, respectively, and a 40-mm long periodically poled MgO-doped LiNbO3 (PPMgLN) crystal. Owing to the dual wavelength-tuning of the MOPAs, the generated idler light reaches a wavelength-tuning range of close to 500 nm, from ∼3117.2 to ∼3598.8 nm, only by tuning the launched pump and signal wavelengths from 1040 to 1084.6 nm and from 1545.2 to 1561.4 nm, respectively, without any change of temperature or grating period of the PPMgLN. Compared to temperature-based idler-wavelength-tuning, this method is potentially faster in speed. The maximum idler power exceeds 60 mW, which is the highest reported power for a wavelength-tunable single-frequency CW DFG source. A rapidly wideband-tunable DFG source with tens of milliwatts of output power in a narrow line can be a practical tool for mid-infrared molecular spectroscopy, detection, and sensing at high measurement rates.

All-fiber mid-infrared source tunable from 6 to 9 μm based on difference frequency generation in OP-GaP crystal

We report the first fully fiberized difference frequency generation (DFG) source, delivering a broadly tunable idler in the 6 to 9 μm spectral range, using an orientation-patterned gallium phosphide (OP-GaP) crystals with different quasi-phase matching periods (QPM). The mid-infrared radiation (MIR) is obtained via mixing of the output of a graphene-based Er-doped fiber laser at 1.55 μm with coherent frequency-shifted solitons at 1.9 μm generated in a highly nonlinear fiber using the same seed. The presented setup is the first truly all-fiber, all-polarization maintaining, alignment-free DFG source reported so far. Its application to laser spectroscopy was demonstrated by the absorption spectrum measurement of ν4 band of methane in 7.5 - 8.3 µm spectral range. The system simplicity and compactness paves the way for applications in field-deployable optical frequency comb spectroscopy systems for gas sensing.

Mid-infrared heterodyne phase-sensitive dispersion spectroscopy using difference frequency generation

Difference frequency generation is a flexible method for the generation of mid-infrared light. It allows downshifting a near-infrared optical signal to the mid-infrared region by means of a nonlinear process, enabling to take full advantage of the availability of optical communications components for the generation of the multi-tone signals that are characteristic of molecular dispersion spectroscopic methods. In this way, it is possible to avoid several issues associated with optical instruments in which directly modulated mid-infrared quantum cascade lasers are employed. In this paper, we take full advantage of this benefit to implement the first heterodyne phase sensitive dispersion spectroscopy gas sensor based on a difference frequency generation source. The performance of the instrument is validated by detecting with high sensitivity methane in the 3.4 µm mid-infrared band.

All-fiber mid-infrared difference frequency generation source and its application to molecular dispersion spectroscopy

A widely tunable, fully monolithic, mid-infrared difference frequency generation source and its application in the dispersion-spectroscopy-based laser trace gas detection of methane and ethane, near 2938 and 2998 cm−1, is presented. Utilizing a fiber pigtailed nonlinear crystal module radically simplified the optical setup, while maintaining a superb conversion efficiency of 20% W−1. Seeded directly from two laser diodes, the source delivered ~0.5 mW of tunable radiation, which was used in a chirped laser dispersion spectroscopy setup, enabling the highly sensitive detection of hydrocarbons.

Picosecond difference-frequency-generation in orientation-patterned gallium phosphide.

We report the generation of tunable high-repetition-rate picosecond radiation in the mid-infrared using the new quasi-phase-matched nonlinear material of orientation-patterned gallium phosphide (OP-GaP). The source is realized by single-pass difference-frequency-generation (DFG) between the output signal of a picosecond optical parametric oscillator (OPO) tunable across 1609-1637 nm with input pump pulses at 1064 nm in OP-GaP, resulting in tunable radiation across 3040-3132 nm. Using a 40-mm-long crystal, we have generated up to 57 mW of DFG average power at ~80 MHz repetition rate for a pump power of 5 W and signal power of 0.9 W, with >30 mW over >50% of the tuning range. The DFG source exhibits a passive power stability better than 3.2% rms over 1 hour in good spatial beam quality. To the best of our knowledge, this is the first picosecond frequency conversion source based on OP-GaP.

Nanosecond difference-frequency generation in orientation-patterned gallium phosphide.

We report a tunable, single-pass, pulsed nanosecond difference-frequency generation (DFG) source based on the new semiconductor nonlinear material, orientation-patterned gallium phosphide (OP-GaP). The DFG source is realized by mixing the output signal of a nanosecond OPO tunable over 1723-1827nm with the input pump pulses of the same OPO at 1064nm in an OP-GaP crystal, resulting in tunable generation over 233nm in the mid-infrared from 2548 to 2781nm. Using a 40-mm-long crystal, we have produced ∼14 mW of average DFG output power at 2719nm for a pump power of 5W and signal power of 1W at 80kHz repetition rate. To the best of our knowledge, this is the first single-pass nanosecond DFG source based on OP-GaP. The DFG output beam has a TEM00 spatial mode profile and exhibits passive power stability better than 1.7%rms over 1.4h at 2774nm, compared to 1.6% and 0.1%rms for the signal and pump, respectively. The OP-GaP crystal is recorded to have a temperature acceptance bandwidth of 17.7°C.

High-power frequency comb source tunable from 2.7 to 4.2 μm based on difference frequency generation pumped by an Yb-doped fiber laser.

We demonstrate a broadband mid-infrared (MIR) frequency comb source based on difference frequency generation (DFG) in periodically poled lithium niobate crystal. MIR radiation is obtained via mixing of the output of a 125 MHz repetition rate Yb-doped fiber laser with Raman-shifted solitons generated from the same source in a highly nonlinear fiber. The resulting idler is tunable in the range of 2.7-4.2 μm, with average output power reaching 237 mW and pulses as short as 115 fs. The coherence of the MIR comb is confirmed by spectral interferometry and heterodyne beat measurements. Applicability of the developed DFG source for laser spectroscopy is demonstrated by measuring absorption spectrum of acetylene at 3.0-3.1 μm.

Highly efficient mid-infrared difference-frequency generation using synchronously pulsed fiber lasers.

We report the development of a high average power, picosecond-pulse, mid-infrared source based on difference-frequency generation (DFG) of two synchronous master oscillator power fiber amplifier systems. The generated idler can be tuned over the range 3.28-3.45 μm delivering greater than 3.4 W of average power, with a maximum pump to total DFG power conversion efficiency of 78%. The benefits of a synchronously pumped scheme, compared to CW seeding of DFG sources, are discussed.

Rapid spectrum measurement at 3 μm over 100 nm wavelength range using mid-infrared difference frequency generation source.

We demonstrate a broadband rapid scanning light source in the 3-μm region by using difference frequency generation (DFG). The DFG source consists of a module with quasi-phase-matched LiNbO3 ridge waveguides, a 1-μm-band wide swept range laser for the pump source, and a 1.5-μm continuous wave laser for the signal source. The sweep rate and the tuning bandwidth of this source are 20 kHz and 100 nm, respectively. This source enables us to evaluate the temperature dependence of absorbance of methane gas.

High-power femtosecond mid-IR sources for s-SNOM applications

We demonstrate two high-power femtosecond mid-infrared (mid-IR) sources that can be combined with a scattering-type scanning near-field optical microscope (s-SNOM). The first one is based on difference frequency generation (DFG) between the two signal wavelengths of a high-power dual-signal-wavelength periodically poled lithium niobate (PPLN) optical parametric oscillator (OPO) and covers the spectral range from 10.5 μm to 16.5 μm. The second one is an AgGaSe2 OPO pumped by the PPLN OPO. With this mid-IR OPO we obtained up to 113 mW average idler power at 4857 nm with more than 40 cm−1 FWHM spectral width. We demonstrate mid-IR near-field spectra and near-field images that we obtained by combining the broadband femtosecond mid-IR DFG source with an s-SNOM.

Ultrabroadband XFROG of few-cycle mid-infrared pulses by four-wave mixing in a gas

Cross-correlation frequency-resolved optical gating (XFROG) based on four-wave mixing (FWM) in a gas medium is shown to enable dispersion-free characterization of few-cycle mid-infrared (mid-IR) pulses tunable within a spectral range of more than two octaves. The FWM XFROG technique is used to measure spectra and pulse shapes, as well as to retrieve the phase of a few-cycle output of difference-frequency generation (DFG) tunable from 3 to 11&#xA0;&#x3BC;m. With Ti:sapphire laser pulses used as a reference, this FWM process maps the entire tunability range of the DFG source, spanning over more than two octaves, onto a wavelength region of only 50&#xA0;nm in the visible, allowing convenient XFROG measurements and revealing the reshaping of few-cycle mid-IR field waveforms by molecular rovibrational modes.

Widely tunable difference frequency generation source for high-precision mid-infrared spectroscopy

We have developed a widely tunable mid-infrared difference frequency generation (DFG) source by mixing ~ 1 W Ti:sapphire laser and 6 W Nd:YAG laser beams in a 50-mm MgO-doped long periodically poled lithium niobate (MgO:PPLN). The power of the DFG source is > 2 mW over the tuning range of 2.66-4.77 μm and its free-running linewidth is about 100 kHz. Combining various frequency stabilisation schemes for the Nd:YAG laser and the Ti:sapphire laser, the DFG frequency can be precisely controlled. Besides, its frequency can be determined better than 12 kHz by measuring the Ti:sapphire laser frequency using an optical frequency comb. Two high resolution spectroscopic studies on (12)C(16)O(2) molecule are demonstrated using this DFG source. The saturation spectra of R(18) and R(60) transitions of 00(0)1 ← 00(0)0 fundamental band at 4.2 μm and P(20) transition of [10(0)1, 02(0)1](I) ← 00(0)0 band at 2.7 μm have been observed and their absolute transition frequencies are measured with an accuracy better than 30 kHz.

Erratum: Hyperfine-resolved 3.4-μm spectroscopy of CH3I with a widely tunable difference frequency generation source and a cavity-enhanced cell: A case study of a local Coriolis interaction between thev1=1and(v2,v6l)=(1,22)states [Phys. Rev. A83, 012505 (2011)

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