mid-IR Optical Parametric Oscillator Research Articles

Experimental Investigation of Laser Damage Limit for ZPG Infrared Single Crystal Using Deep Magnetorheological Polishing of Working Surfaces

Zinc germanium phosphide (ZGP) crystals have garnered significant attention for their nonlinear properties, making them good candidates for powerful mid-IR optical parametric oscillators and second-harmonic generators. A ZnGeP2 single crystal was treated by deep magnetorheological processing (MRP) until an Angstrom level of roughness. The studies presented in this article are devoted to the experimental evaluation of the influence of deep removal (up to 150 μm) from the surface of a ZnGeP2 single crystal by magnetorheological polishing on the parameters of optical breakdown. It was shown that the dependence of the ZnGeP2 laser-induced damage threshold on MRP depth is a smooth monotonically decreasing logarithmic function. The obtained logarithmic dependence indicates the thermal nature of optical breakdown and the dependence of the ZnGeP2 laser-induced damage threshold on the concentration of surface absorbing defects.

Physical and Technological Aspects of Laser-Induced Damage of ZGP Single Crystals under Periodically Pulsed Laser Irradiation at 2.1 μm

The nonlinear properties of zinc germanium diphosphide (ZGP) crystals enable their applications in powerful mid-IR optical parametric oscillators and second-harmonic generators. This paper summarizes the mechanisms of the laser-induced damage (LID) of high-purity ZGP crystals under periodically pulsed nanosecond irradiation by a Ho3+:YAG laser at 2.1 μm. The ZGP samples were manufactured by “LOC” Ent., Tomsk, Russia, or the Harbin Institute of Technology, China. The impact of processing techniques and the post-growing methods for polishing and anti-reflective coatings on the LID threshold are discussed. The importance of the defect structure of the crystal lattice and the parameters of transparent coatings for increasing the LID threshold are also discussed. The impact of the test laser parameters on the LID threshold and the transient area near the LID threshold obtained using digital holography are analyzed. The influence of the pre-damage processes on the optical parametric oscillations is reported. Lastly, the prospects for improving ZGP crystals to further increase the LID threshold are discussed.

High power widely tunable mid-IR (5–7.2 μm) ZnGeP2 optical parametric oscillator pumped by a 2.09 μm laser

A high-power widely tunable nanosecond (ns) mid-infrared (mid-IR) optical parametric oscillator (OPO) in a ZnGeP2 crystal has been demonstrated. The OPO is pumped by a Q-switched 2.09 μm Ho: YAG laser with 10 kHz repetition frequency. Based on optimizing the parameters of ZGP-OPO system, a maximum idler power as high as 5.47 W is achieved at 5.05 μm with photon conversion efficiency of up to 46.4 %. By rotating the angle of the ZGP crystal, the idler tuning region across from 5 to 7.2 μm is realized, with >1 W of idler power over almost the entire tuning region. The idler operation displays high pulse stability with peak to peak amplitude fluctuation less than ±2.8 % in near-Gaussian spatial mode.

High efficiency in-band pumped Tm- and Ho-doped 2-μm solid-state lasers

The expansion of laser sources operating at 2-3 μm is motivated in large by strong absorption in atmospheric gases, liquid water, and plastics, critical for applications in remote sensing, soft and hard tissue surgery, and materials processing. 2-μm novel solid-state lasers based on Tm3+-doped ceramics (Lu2O3 or Y2O3) or YAP crystals in-band pumped by a fiber laser at 1670 nm are presented in this report. CW, actively or passively Q-switch operations of the Tm3+-doped lasers are described. The Tm3+ laser characteristics are compared with parameters of an in-band fiber-laser-pumped Ho3+:YAG laser. The 2-μm output of the lasers are used to pump the Cr2+-doped ZnSe or CdSe crystals. The lasers based on the Tm3+:Lu2O3 ceramics and the Tm3+:YAP crystal are also used as pump sources for mid-IR optical parametric oscillators (OPOs) or second-harmonic generators (SHGs). The high-efficiency oscillations are obtained at 3.8-4.1 μm in the OPOs based on a fun-out PPMg:LN; the SHGs provided the repetitively-pulsed radiations at 970-983 nm.

Laser-Induced Damage Threshold of Single Crystal ZnGeP2 at 2.1 µm: The Effect of Crystal Lattice Quality at Various Pulse Widths and Repetition Rates

The ZnGeP2 crystal is a material of choice for powerful mid-IR optical parametric oscillators and amplifiers. In this paper, we present the experimental analysis of the optical damage threshold of ZnGeP2 nonlinear crystals induced by a repetitively-pulsed Ho3+:YAG laser at 2091 nm. Two types of ZnGeP2 crystals grown under different conditions were examined using the laser and holographic techniques. The laser-induced damage threshold (LIDT) determined by the pulse fluence or peak intensity was studied as a function of the pulse repetition rate (PRR) and laser exposure duration. The main crystal structure factor for a higher LIDT was found to be a reduced dislocation density of crystal lattice. The ZnGeP2 nonlinear crystals characterized by the high structural perfection with low density of dislocations and free from twinning and stacking faults were measured to have a 3.5 J/cm2 pulse fluence damage threshold and 10.5 MW/cm2 peak intensity damage threshold at 12 kHz PRR; at 40 kHz PRR the pulse fluence damage threshold increased to over 6 J/cm2, but the peak intensity damage threshold dropped to 5.5 MW/cm2.

Mid-IR Optical Parametric Oscillator Based on Periodically Polled LiNbO3 Pumped by Tm3+:Lu2O3 Ceramic Laser

Optical parametric oscillator based on a periodically poled MgO:LiNbO3 crystal pumped by a repetitevely pulsed Tm3+:Lu2O3 ceramics laser at 1966 nm was studied. The OPO average output power up to 530 mW at wavelength of 3.7–4.2 μm was achieved at the ceramics laser power of 7.9 W. The optical-to-optical efficiency of OPO energy conversion was up to 6.7%, and the slope efficiency was up to 8%.

Investigation on four-wave mixing toward mid-infrared waveband in tellurite photonic crystal fiber

The evolution characteristics of the degenerate four-wave mixing (DFWM) effect in the proposed tellurite photonic crystal fiber (PCF) are investigated systematically. The influence of the pitch distance and diameter-to-pitch ratio on the phase-matching condition is analyzed numerically. The effects of fiber length, signal wavelength, idler wavelength, initial pump power and initial signal power on the signal gain and idler conversion efficiency are investigated. Based on the DFWM occurring in the proposed tellurite PCF, the signal (anti-Stokes) wave is efficiently amplified and the idler (Stokes) wave is efficiently generated at the mid-infrared (mid-IR) waveband with the pump wavelength of 1.553 μm. The signal gain can be increased by increasing the initial pump power. The idler conversion efficiency can reach ~ 26%. The simulated results will be instructive for the experimental FWM process to generate mid-IR optical parametric oscillators.

Compact and efficient mid-IR OPO source pumped by a passively Q-switched Tm:YAP laser.

We describe a compact and efficient mid-infrared (mid-IR) source based on zinc germanium phosphide (ZGP) and cadmium silicon phosphide (CSP) optical parametric oscillators (OPOs), operating in near degenerate condition, directly pumped by a 1.94μm thulium (Tm)-doped yttrium-aluminum-perovskite (YAP) laser. The Tm:YAP laser is passively Q-switched by a chromium-doped zinc sulfide saturable absorber, and is operated to 4W average power with a peak power of 29kW. The laser emission was used to pump CSP and ZGP doubly resonant linear OPO cavities, generating a maximum 3.5-4.2μm mid-IR emission of 2.5W for CSP and 2.3W for ZGP, with maximum optical conversion efficiencies of 65% and 58%, respectively, achieved for the two OPO crystals.

Widely tunable and narrow-bandwidth pulsed mid-IR PPMgLN-OPO by self-seeding dual etalon-coupled cavities.

Integrating narrow-bandwidth and wavelength tunability together is crucially important in upgrading the applications of optical parametric oscillators (OPO). Here, we have demonstrated a widely tunable, narrow-bandwidth and efficient mid-IR OPO pumped by a single-longitudinal-mode pulsed Yb-fiber laser. By restricting the bandwidth of the oscillated signal via self-seeding dual etalon-coupled cavities, the bandwidth of the idler can be suppressed to about 0.35 nm, with a wide tunable range of 2.85-3.05 μm, which can be achieved by synergistically adjusting the temperatures of PPMgLN crystal and one of the etalons. The maximum idler power at 3.031 μm is 2.67 W with an optical-to-optical conversion efficiency of 17.4%.

10-W mid-IR optical parametric oscillators based on ZnGeP2 elements pumped by a fibre-laser-pumped Ho: YAG Laser. Experimental and numerical study

The possibility of increasing the power and efficiency of 3.5 – 5.0 μm optical parametric oscillators (OPOs) based on one or two ZnGeP2 crystals pumped by a repetitively pulsed Ho: YAG laser (λ = 2097 nm), which, in turn, is pumped by a thulium fibre laser (λ = 1908 nm), is studied. The OPO based on a tandem of ZnGeP2 elements is demonstrated to generate high-quality beams with an average power up to 10 W in pulses with durations of 15 – 25 ns at a repetition rate of ∼17 kHz. The total conversion efficiency of the fibre laser power to the mid-IR radiation reached 25%. A theoretical OPO model explaining a cluster spectrum and higher output power in the case of the ZnGeP2 tandem than in the case of a single element is developed.

10 W mid-IR optical parametric oscillator pumped by a passively Q-switched Ho:YAG ceramic laser

We report on a ZnGeP2 (ZGP) optical parametric oscillator (OPO) pumped by a Cr2+:ZnS saturable absorbed passively Q-switched (PQS) Ho:YAG ceramic laser. The maximum output power of the ZGP OPO was 10 W at a 27.4 W incident PQS Ho:YAG ceramic laser, corresponding to a slope efficiency of 41.3% with respect to the incident pump power. The minimum mid-IR pulse width of 21 ns in 3.8 and 4.6 μm was obtained from the ZGP OPO. The beam quality was measured to be M2 < 3.22.

A Mid-IR Optical Parametric Oscillator Pumped by an Actively Q-Switched Ho:YAG Ceramic Laser**Supported by the National Natural Science Foundation of China under Grant Nos 61308009, 61405047 and 50990301, the Fundamental Research Funds for the Central Universities under Grant Nos HIT.NSRIF.2014044 and HIT.NSRIF.2015042, and the Science Fund for Outstanding Youths of Heilongjiang Province under Grant No JQ201310.

We demonstrate a mid-IR ZnGeP2 (ZGP) optical parametric oscillator (OPO) pumped by a dual-end-pumped actively acousto-optic Q-switched Ho:YAG ceramic laser. The maximum average output power of 35 W is obtained at a pulse repetition frequency of 20kHz from the Ho:YAG ceramic laser. Under the maximum incident pump power of Ho:YAG ceramic laser, the maximum output power of 14 W is obtained from the ZGP OPO, corresponding to the slope efficiency of 49.6% with respect to the incident pump power. The wavelength can be tuned from 3.5 μm to 4.2 μm (signal), corresponding to 5.2–4.1 μm (idler). The beam quality M2 is less than 2.3 from the ZGP OPO.

Progress in 1-μm Pumped Mid-IR Optical Parametric Oscillators Based on Non-Oxide Nonlinear Crystals

Pumping mid-IR optical parametric oscillators near 1 μm is attractive for its simplicity and power scaling capability. To this aim, wide bandgap non-oxide nonlinear crystals are needed. This paper reviews the recent developments in crystal growth and singly resonant optical parametric oscillators for the generation of high single pulse energy and average powers in the mid-IR from 3 to 10 μm.

Measurements of N2-broadening and pressure-shift coefficients in the ν3-band of 12CH4 using a cw-OPO

We have measured the nitrogen-collision-induced line broadening and line shift coefficients of three singlet lines – P(1), R(0) and R(1) – belonging to the ν3 band of methane (12CH4) at 3.29–3.32μm using direct laser absorption spectroscopy by means of a continuous-wave mid-IR optical parametric oscillator. Owing to the well-isolated features of the singlet lines, the lineshapes were modeled with a simple Voigt profile in order to retrieve the line parameter coefficients. The values of the N2-broadening and pressure-shift coefficients agree fairly well with the previously published ones that use either Fourier-transform or mid-infrared difference-frequency spectrometers along with more complex analytic spectroscopic models.

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.

Few-cycle, broadband, mid-infrared optical parametric oscillator pumped by a 20-fs Ti:sapphire laser.

A few-cycle, broadband, singly-resonant optical parametric oscillator (OPO) for the mid-infrared based on MgO-doped periodically-poled LiNbO3 (MgO:PPLN), synchronously pumped by a 20-fs Ti:sapphire laser is reported. By using crystal interaction lengths as short as 250 µm, and careful dispersion management of input pump pulses and the OPO resonator, near-transform-limited, few-cycle idler pulses tunable across the mid-infrared have been generated, with as few as 3.7 optical cycles at 2682 nm. The OPO can be continuously tuned over 2179-3732 nm (4589-2680 cm-1) by cavity delay tuning, providing up to 33 mW of output power at 3723 nm. The idler spectra exhibit stable broadband profiles with bandwidths spanning over 422 nm (FWHM) recorded at 3732 nm. The effect of crystal length on spectral bandwidth and pulse duration is investigated at a fixed wavelength, confirming near-transform-limited idler pulses for all grating interaction lengths. By locking the repetition frequency of the pump laser to a radio-frequency reference, and without active stabilization of the OPO cavity length, an idler power stability better than 1.6% rms over >2.75 hours is obtained when operating at maximum output power, in excellent spatial beam quality with TEM00 mode profile. Photograph shows a multigrating MgO:PPLN crystal used as a nonlinear gain medium in the few-cycle femtosecond mid-IR OPO. The visible light is the result of non-phase-matched sum-frequency mixing between the interacting beams.

Performance of a high-resolution mid-IR optical-parametric-oscillator transient absorption spectrometer.

We report on a mid-IR optical parametric oscillator (OPO)-based high resolution transient absorption spectrometer for state-resolved collisional energy transfer. Transient Doppler-broadened line profiles at λ = 3.3 μm are reported for HCl R7 transitions following gas-phase collisions with vibrationally excited pyrazine. The instrument noise, analyzed as a function of IR wavelength across the absorption line, is as much as 10 times smaller than in diode laser-based measurements. The reduced noise is attributed to larger intensity IR light that has greater intensity stability, which in turn leads to reduced detector noise and better frequency locking for the OPO.

Wildly Tunable, High-Efficiency MgO:PPLN Mid-IR Optical Parametric Oscillator Pumped by a Yb-Fiber Laser

We demonstrate a wildly tunable, high-efficiency mid-infrared (mid-IR) output-coupled single resonant optical parametric oscillator (OC-SRO) pumped by a Yb-fiber laser. The compact mid-infrared source employs a 50-mm-long, multi-grating MgO-doped periodically poled lithium niobate (MgO-PPLN) crystal, providing as much as 1.73 W idler power at 3.012 μm, and 1.27 W signal power at 1645 nm, corresponding to an overall conversion efficiency of 41.7% and a slope efficiency of 77.9%. In particular, the mid-infrared output power of 1.03 W and 0.67 W are obtained at 3.7 μm and 3.9 μm, respectively, with an optical-to-optical conversion efficiency of 14.3% and 9.3%. Furthermore, the idler light is tunable from 3 μm to 3.9 μm by changing the periods from 31 to 28.5 μm, with the output power >1 W over 78% of the tuning range. Our experimental results are pump power limited and further mid-IR power and conversion efficiency could be obtained with a suitable high-power pump source. The total OC-SRO output power rms at 2.6 W is about 0.6% during 2 h measurement.

High-speed multi-THz-range mode-hop-free tunable mid-IR laser spectrometer

We report on a widely (2.25 THz or 75 cm(-1)) and rapidly (4.5 THz/s) mode-hop-free (MHF) tunable mid-IR laser source at ~3.3 μm, consisting of a 5%-MgO:LiNbO(3) singly resonant optical parametric oscillator (SRO) pumped by an automated broadly MHF tunable extended-cavity diode laser (ECDL). The broad and rapid MHF tuning capability of the ECDL is readily transferred to the SRO idler wave owing to the quasi-noncritical pump spectral acceptance bandwidth of the quasi-phase-matching. Fast and broadband high-resolution Doppler spectroscopy measurements of the ν(3) band of CH(4) are presented to illustrate the performance of the mid-IR optical parametric oscillator spectrometer.

Chalcogenide optical parametric oscillator

We demonstrate the first optical parametric oscillator (OPO) based on chalcogenide glass. The parametric gain medium is an As(2)Se(3) chalcogenide microwire coated with a layer of polymer. The doubly-resonant OPO oscillates simultaneously at a Stokes and an anti Stokes wavelength shift of >50 nm from the pump wavelength that lies at λ(P) = 1,552 nm. The oscillator has a peak power threshold of 21.6 dBm and a conversion efficiency of >19%. This OPO experiment provides an additional application of the chalcogenide microwire technology; and considering the transparency of As(2)Se(3) glass extending far in the mid-infrared (mid-IR) wavelengths, the device holds promise for realizing mid-IR OPOs utilizing existing optical sources in the telecommunications wavelength region.