KEYWORDS: Optical parametric oscillators, Second harmonic generation, Nd:YAG lasers, Sum frequency generation, Thermal optics, Gas lasers, Ferroelectric materials, Solids, Crystals, Refractive index
We report the experimental results on the temperature-dependent phase-matching properties of BaGa2GeS6 for second-harmonic generation (SHG) and sum-frequency generation (SFG) of Nd:YAG laser-pumped KTiOPO4 (KTP) and HgGa2S4 (HGS) optical parametric oscillators (OPOs) and a CO2 laser in the 0.7674-10.5910 μm range. The experimental values for type-2 90° phase-matching SHG wavelengths in the short and long wavelength branches were measured to be λ1 = 2.0376 and 7.6740 μm, respectively. By using a BaGa2GeS6 crystal cut at θ = 48.1˚ and Φ = 0˚, we measured the type-1 and type-2 phase-matching angles for SHG and SFG of a Nd:YAG laser-pumped KTP OPO at λ = 3.1092 μm and HGS OPO at λ = 7.6740 μm as well as those for SHG and SFG of a waveguide CO2 laser. In addition, we derived the Sellmeier equations that provide a good reproduction of the experimental results for the phase-matching data. The temperature phase-matching bandwidths (FWHM) were also obtained in eight different phase-matching conditions by using the measured temperature variation of the pump wavelengths, the measured temperature dependence of the phase-matching angles (Δθext/ΔT) from 20℃ to 120℃ and the angular acceptance (Δθext‧l) calculated with our Sellmeier equations. Moreover, the temperature phase-matching bandwidth (FWHM) for type-2 90˚ phase-matching SHG was determined to be ΔT‧l = 12.2 ℃‧cm from the measured value of dλ1/dT = +0.3 nm/℃ and the calculated spectral bandwidth (Δλ1‧l). From these experimental results and the dn/dT measured at five wavelengths, we derived a thermo-optic dispersion formula valid in the 0.7674 − 10.5910 μm spectral range.
We performed direct measurements of phase-matching conditions of Second-Harmonic Generation (SHG) and Difference-Frequency Generation using the sphere method to determine reliable Sellmeier’s equations valid in the 1-12 microns transparency range of the new BGSe monoclinic nonlinear crystal. We also recorded SHG conversion efficiencies under and out-of phase-matching conditions to determine without the magnitudes and relative signs of the associated non zero quadratic nonlinear coefficients of BGSe. By combining all these data, we were able to calculate pump wavelengths enabling the generation of a widely tunable light in the infrared range from phase-matched Optical Parametric Generation.
We employed an HgGa2S4 nonlinear crystal to achieve the highest average power from a high-repetition rate mid-infrared femtosecond optical parametric oscillator (OPO), synchronously pumped near 1 μm. The continuous tuning of the nonresonated idler extended from below 4.4 to beyond 12 μm. The OPO was pumped by a 6.5-W, 1.035-μm, 81-MHz, 220-fs Yb-fiber laser. The idler output reached an average power of over 355 mW at 5.2 μm (at a signal power of > 2 W at 1.25 μm). The peak quantum efficiency of the OPO approached 40%. The signal output was diffraction limited with M2 < 1.1. No crystal damage was observed at average pump intensities as high as 60 kW/cm2.
We employed mercury thiogallate (HgGa2S4 or HGS) to achieve the highest average output power from a 1 µm synchronously pumped optical parametric oscillator (SPOPO) generating femtosecond idler pulses continuously tunable in the mid-IR. HGS is one of the most promising wide band-gap non-oxide nonlinear crystals without two-photon absorption (TPA) losses at such pump wavelengths. The SPOPO was pumped by an 80 MHz mode-locked Yb-fiber laser with 6.5 W of average power at 1.035 µm and a pulse duration of 220 fs. Continuous tuning of the non-resonated idler was achieved from 4.4 to beyond 12 µm with an average output power of over 350 mW at 5.2 µm (>2 W at 1.25 µm for the signal) and tens of mW at the edges of the idler tuning range (hundreds of mW for the signal). The quantum efficiency of the device exceeded 20 % up to 8 µm (> 30% in the maximum). The output of the OPO was diffraction limited with a beam quality factor (M²) below 1.1. No crystal damage was observed at average power densities as high as 60 kW / cm² although using lower power densities was necessary to achieve the best possible beam quality. The only other crystals that were successfully employed in lower power SPOPOs in the past, CdSiP2 (CSP) and OPGaP, lack either the tuning range due to limited mid-IR transparency or the capability for continuous tuning.
A singly-resonant OPO (SRO) based on AgGaSe2 (AGSe) intracavity pumped at ~1.85 μm by the signal pulses of a Rb:PPKTP doubly-resonant OPO (DRO) provided extremely broad tuning (5.8 to ~18 μm) for the non-resonated idler. In a similar set-up with the same nonlinear crystals, we studied intracavity difference-frequency generation (DFG). Both AGSe and the new monoclinic crystal BaGa4Se7 (BGSe) generated single pulse energies of ~0.7 mJ near 7 μm at an overall conversion efficiency from the 1.064 μm pump of 1.2%. The main advantage of BGSe is its damage resistivity up to the maximum pump levels applied at 100 Hz.
On the basis of our previous Dy3+:PbGa2S4 laser study, laser output wavelength temporal evolution as well as tuning possibilities in the range 4.3–4.7 μm were investigated. Active crystal was pumped by a fiber-coupled Brightlase Ultra- 50 diode laser (1.7 μm, max. power 7.5 W). Laser resonator was formed by flat dichroic pumping mirror (T = 70%@1.7 μm, R~100% @ 3.5 - 5 μm) and a concave (r = 200 mm) output coupler with R~99% @ 3.5 – 5 μm. The laser output wavelength dependence on the pump pulse duration and its evolution during the pulse was investigated first without any spectrally-selective element in the cavity. At pump pulse duration of 1 ms, generation just near Dy3+ fluorescence maximum of 4.35 μm has been observed. Prolongation of the pulse up to 5 ms led to similar lasing at 4.35 μm in the first millisecond, followed by simultaneous generation at 4.35 and 4.38 μm in the next millisecond, and further lasing at 4.6 μm till the end of the pump pulse. Increase of pump pulse duration up to 10 ms led to similar oscillation pulse development followed by generation at 4.6 μm only. Furthermore, output wavelength tuning using MgF2 birefringent filter as a cavity spectral selective element was investigated under 10 ms pumping. Almost continuous tuning without any significant dip has been observed within spectral range from 4.3 up to 4.7 μm. Due to practically closed cavity mean output power in the maximum of tuning curve was in the order of 400 μW.
We present an efficient coherent source widely tunable in the mid-infrared (mid-IR) spectral range consisting of a novel femtosecond Yb-fiber laser operating at ~50 MHz repetition rate, a synchronously-pumped OPO (SPOPO) and difference-frequency generation (DFG) in AgGaSe2. With an average input power of 5 W for ~260 fs pump pulses at 1034 nm, the SPOPO outputs are tunable from ~1710 to 1950 nm (signal) and from 2200 to 2600 nm (idler) with pulse durations between 200 and 250 fs over the entire tuning range. After temporally overlapping signal and idler through a delay line, the two beams are spatially recombined with a dichroic mirror and focused to a beam diameter of ~75 μm. For DFG we employ an uncoated 2-mm-thick AgGaSe2 nonlinear crystal cut for type-I interaction at θ=57°. The generated femtosecond mid-IR pulses are continuously tunable between 5 and 17 μm with average power of up to 69 mW at 6 μm and more than 1 mW at 17 μm. Their spectra and autocorrelation traces are measured up to 12 μm and 8 μm, respectively, and indicate that the input spectral bandwidth and pulse duration are maintained to a great extent in the nonlinear frequency conversion processes. The DFG pulse width measured at 7.2 μm amounts to ~300 fs (FWHM). The measured spectral bandwidth supports ~150 fs Gaussian pulse durations across the entire DFG tuning range. For the first time mid-IR pulses with energy exceeding 1 nJ are generated at such high repetition rates.
We investigated optical damage (surface and bulk) in one of the most promising wide bandgap nonoxide nonlinear crystals, HgGa 2 S 4 , that can be used in ∼1 -μm pumped optical parametric oscillators (OPOs) and synchronously pumped OPOs (SPOPOs) for generation of idler pulses above 4 μm without two-photon absorption losses at the pump wavelength. The optical damage has been characterized at the pump wavelength for different repetition rates using uncoated and antireflection-coated (mainly with a single layer for pump and signal wavelengths) samples. HgGa 2 S 4 is the most successful nonlinear crystal (both in terms of output energy and average power) for such OPOs, but optical damage inside the OPO has a lower threshold and represents at present the principal limitation for the achievable output. It is related to peak pulse and not to average intensity, and bulk damage in the form of scattering centers occurs before surface damage. Such bulk damage formation is faster at higher repetition rates. Lower repetition rates increase the lifetime of the crystal but do not solve the problem. The safe pump fluence in extracavity measurements is <1 J/cm 2 , which corresponds to ∼100 MW/cm 2 for the 8-ns pulse duration (both values peak on-axis). In the OPO, however, peak on-axis fluence should not exceed 0.3 J/cm 2 limited by the formation of bulk scattering centers in orange-phase HgGa 2 S 4 . In the nanosecond OPO regime, the damage resistivity of Cd-doped HgGa 2 S 4 is higher and that of the almost colorless CdGa 2 S 4 is roughly two times higher, but the latter has no sufficient birefringence for phase-matching. In SPOPOs operating in the ∼100 MHz regime, the damage limitations are related both to the peak pulse and the average intensities, but here HgGa 2 S 4 seems the best nonoxide candidate to obtain first steady-state operation with Yb-based mode-locked laser pump sources.
We investigated optical damage (surface and bulk) in wide band-gap (absorption edge below 532 nm) sulphide and
selenide nonlinear crystals that can be used in 1064-nm pumped optical parametric oscillators (OPOs) for generation of
idler pulses above 4 μm without two-photon absorption losses at the pump wavelength. The optical damage has been
characterized at the pump wavelength for different repetition rates. Surface damage has been studied for uncoated and
antireflection-coated (mainly with a single layer for pump and signal wavelengths) samples. Optical damage inside the
OPO has a lower threshold and represents at present the principal limitation for the achievable output. It is related to
peak and not to average intensities and in many of the studied crystals bulk damage in the form of scattering centers
occurs before surface damage. Such bulk damage formation is faster at higher repetition rate. Lower repetition rates
increase the lifetime of the crystal but do not solve the problem. In the most successful nonlinear crystal (both in terms of
output energy and average power), orange-phase HgGa2S4, the safe pump intensity in extracavity measurements is below
100 MW/cm2 which corresponds to less than 1 J/cm2 for the 8 ns pulse duration (both values peak on-axis). In the OPO,
however, peak on-axis fluence should not exceed 0.3 J/cm2 limited by the formation of bulk scattering centers. The
damage resistivity of yellow-phase HgGa2S4 or Cd-doped HgGa2S4 is higher and of the almost colorless CdGa2S4 it is
roughly two times higher but the latter has no sufficient birefringence for phase-matching.
This paper reports the 90° phase-matched fourth-harmonic generation of the CO2 laser wavelength at 10.5910 μm in BaGa4S7. The 1.4 cm long BaGa4S7 crystal used in the present experiment was cut at θ=9.2° in the xz(=bc) plane. The pump source was the second harmonic of the wave-guide CO2 laser (10.5910 μm) operating with an average output power of 18.3 W at 130 kHz. By focusing the 7.6 W, 5.2955 μm pulses (peak power is 6kW with 10 ns pulse duration) generated from the 2 cm long AgGa0.65In0.35Se2 crystal into the BaGa4S7 crystal (θPM=9.9°) with the 6 cm focal length ZnSe lens, the maximum average output power of 0.3 W was obtained at 2.6478 μm without any damage to the crystal. In addition, the new Sellmeier equations of BaGa4S7 are presented.
One of the promising active ions giving the possibility of laser radiation generation in 4 - 5 μm region is trivalent
dysprosium in lead thiogallate crystal. On the basis of our previous Dy3+:PbGa2S4 laser study, this work is showing a
several wavelengths generation possibility. The laser was working at the room temperature and it was in-band pumped
by the Er:YLF laser radiation with the wavelength 1.73 μm. The investigated crystal Dy3+:PbGa2S4 was synthesized
using Bridgman technique from the melt. Dimension of the sample was 16 mm long and 19 mm in diameter and the
nominal Dy3+ ion concentration was ~ 0.7 at. %. The laser operation was tested with three resonator configurations. The
first was the non-selective resonator with the length 41 mm. To perform the wavelength tuning, in the second case the
MgF2 Lyot filter was inserted under Brewster angle inside the resonator which length was 100 mm. In both these cases
the laser resonator was formed by the incoupling flat dichroic mirror with low reflectivity at pumping wavelength (T =
90%@1.73 μm) and high reflectivity (R~100%) within the 3.5 - 4.5 μm spectral range, and by an out-coupling concave
(r = 500 mm) mirror with reflectivity of 95 % or 98 % in the same region of wavelengths. For the third case the selective
mirrors supporting the 3.0 - 4.0 μm spectral region were chosen. The generation possibility on three lines 4 μm, 4.3 μm,
and 4.6 μm were found without continuous tuning between them.
The mid-infrared radiation generated by bulk Dy:PbGa2S4 laser working at room temperature was characterized and for
its delivery the special type of COP/Ag hollow waveguide was used. The optical pumping of Dy:PbGa2S4 laser was
performed by flashlamp pumped Er:YLF laser at 1.73 μm wavelength. The compact 60 mm long Dy:PbGa2S4 laser
oscillator worked in free-running mode with the repetition rate 1.5 Hz. The output energy was 5.1 mJ in 80 μs long pulse
at 4.3 μm wavelength. The spatial beam structure was close to the Gaussian shape.
The goal of the presented study was the preliminary investigation of the mid-infrared Dy:PbGa2S4 radiation delivery
possibility by the cyclic olefin polymer and silver coated hollow glass waveguide. The length of the waveguide was
103 cm and the inner diameter was 700 μm. The thickness of the polymer inner layer was calculated for the optimal
4 μm radiation transmission. Mid-infrared laser radiation was coupled into the waveguide by the CaF2 lens with the focal
length 55 mm. The characterization of delivered 4.3 μm radiation was provided. It was observed that the spatial structure
is changing essentially, which follows from the transmission principle of the hollow waveguide. As conclude the
delivery system for 4.3 μm mid-infrared Dy:PbGa2S4 laser radiation was investigated for the first time.
The aim of the presented project was comparison of two Fe:ZnSe lasers based on Fe:ZnSe bulk active crystals grown by
two different methods - Bridgman and floating zone. For pumping the Q-switched Er:YAG laser generating 15 mJ and
300 ns giant pulses was used. The highest Fe:ZnSe laser generated output energy was 1.2 - 1.3 mJ for both investigated
crystals, the pulse duration was 150 - 200 ns. The Fe:ZnSe laser threshold was reached at absorbed pumping energy of
~ 1 mJ. Tuning properties using intracavity CaF2 prism were also investigated and tuning range ~ 4 - 5 μm was observed
for both crystals.
The goal of this work was to design and investigate a gain switched, at room temperature lasing Fe:ZnSe laser. The active medium was a bulk, by Bridgman-technique grown Fe:ZnSe sample with the thickness 3.4 mm. The pumping was provided by electro-optically Q-switched Er:YAG laser with the oscillation wavelength 2.937 μm matching the local maximum of the Fe:ZnSe absorption. The Er:YAG Q-switched operation was obtained by the Brewster angle cut LiNbO3 Pockels cell placed between the rear mirror and the laser active medium. No additional intracavity polarizers were used. The maximum pumping pulse energy and length was 15 mJ, and ~300 ns, respectively. This pulse-length is close to room-temperature measured lifetime of Fe2+ ions in Fe:ZnSe crystal.
The pump radiation was directed into the Fe:ZnSe crystal which was placed inside the cavity formed by dichroic
pumping mirror (THR=92% at 2.94 μm and RHR~100% for 3.5-5.2 μm) and optimal output coupler with the reflectance ROC=90% at 4.5 μm, radius of curvature r = -200 mm. The maximum obtained output Fe:ZnSe laser energy was 1.2 mJ, the generated output pulse duration on the wavelength 4.5 μm was 65 ns (FWHM). The output pulse profile was approximately Gaussian. The crystal showed rather high uniformity of oscillation properties throughout its volume. For the case of tuning the CaF2 prism was implemented into the resonator. The tuning curve of generated Fe:ZnSe laser radiation covered the spectral range 3.9 - 4.7 μm.
Cr:ZnSe laser coherently longitudinally pumped with Tm:YAP microchip laser was realised.
The pumping laser consisted of Tm:YAP crystal (3x3 mm) with resonator mirrors deposited
directly on its faces (on rear face the dielectric layer with high reflectance for 1998 nm
wavelength and high transmittance for 790 nm pumping radiation wavelength; on output face
the dielectric layer with reflectance 97% at 1998 nm wavelength). The maximal output power
was 5.5 W and the generated radiation wavelength was 1998 nm. The main advantage of this
pumping was stable and still output without relaxation spikes (non-spiking).
The Tm:YAP laser radiation was collimated and focused by the set of two CaF2 lenses. The
pumping beam spot diameter inside the Cr:ZnSe crystal was 300 μm. The Cr:ZnSe laser
resonator consisted of flat rear mirror (HT at 1998 nm and HR at 2100 - 2900 nm) and curved
output coupler (r = -150 mm, R = 95% at 2100 - 2700 nm). The maximal output energy of
stable radiation was 4 mJ (pulse duration 10 ms, repetition rate 10 Hz). For wavelength tuning
the Lyott filter (quartz plate under Brewster angle) was placed between the Cr:ZnSe crystal
and output coupler. The generated radiation wavelength was continuously tunable from 2246
- 2650 nm.
Fe:ZnSe is one of the most promising materials capable of generating broadly tunable laser radiation in the wavelength
range from 3.5 to 5 μm. The aim of the work was to test laser properties of the Bridgman-method-grown Fe2+:ZnSe
crystal activated through the synthesis process as an active medium coherently pumped with the Q-switched Er:YAG
laser whose oscillation wavelength (2937 nm) corresponds to the maximum of the Fe2+:ZnSe absorption spectrum. The
Er:YAG laser generated giant pulses with the duration 160 - 200 ns and energy 20 - 30 mJ. The repetition-rate was set
to be 1 Hz. The oscillation properties, such as the pulse length, energy, and generated beam spatial structure, of the
Bridgman-method-grown Fe2+:ZnSe crystal used as an active medium of Fe2+:ZnSe laser operated at room temperature
were investigated. The maximal obtained output energy of room temperature Fe2+:ZnSe laser was 580 μJ for the
absorbed energy of 5.3 mJ which corresponds to slope efficiency of 38%. The generated pulse waveform was found to
follow that of the pump one.
Cr:ZnSe laser active material is one of the favourite possibility how to generate broadly tunable mid-infrared
laser radiation at room-temperature. The aim of this study was to demonstrate and analyze pulsed as well as
continuous-wave laser action in bulk Cr:ZnSe crystals grown by the floating-zone method or by the Bridgman
method. The absorption spectra of Cr:ZnSe were measured to be from 1500 to 2000 nm, therefore various lasers
were utilized for coherent longitudinal pumping of Cr:ZnSe laser, namely flashlamp-pumped Er:YAP laser
(generated wavelength 1658 nm), diode-pumped Tm:YLF laser (generated wavelength 1912 nm), and diodepumped
Tm:YAP laser (generated wavelength 1980 nm).
In the first case, the Cr:ZnSe crystal grown by the Bridgman method was investigated. In the second case, the
Cr:ZnSe crystal grown by the floating zone method was studied. In both cases, the homogeneity of the active
Cr:ZnSe crystals was found reasonable good. The emission spectrum was from 2000 up to 2800 nm. The
Cr:ZnSe laser generated radiation was broadly continuously tunable in the range from 2050 nm up to 2750 nm.
The generated radiation beam spatial structure was close to TEM00.
Stimulated Raman scattering (SRS) in BaWO4 crystal under 1.56 μm pumping was investigated. Several (up to
fourth) Stokes components were observed. Mid IR spectral range 2.75 µm and 3.7 μm radiation was obtained using
BaWO4 crystalline Raman shifter. Under 1.318 μm pumping 4.3 μm and 4.7 μm oscillations in low phonon
PbGa2S4:Dy3+ laser were obtained.
Broadly tunable mid-infrared laser sources operated at room-temperature are desired in many technological and
medical applications. The aim of the project was to design and construct broadly tunable powerful Cr:ZnSe laser.
The investigated Cr:ZnSe various shaped bulk crystals were grown by the Bridgman method or by the floating zone method. The absorption spectrum was measured to be from 1500 to 2000 nm and the emission spectrum was from 2100 to 2800 nm. Three different lasers were utilized for coherent longitudinal pumping of Cr:ZnSe laser, namely flashlamp-pumped Er:YAP laser (generated wavelength 1660 nm), diode-pumped Tm:YLF laser (generated wavelength 1912 nm) and diode-pumped Tm:YAP laser (generated wavelength 1980 nm). The constructed Cr:ZnSe laser operated in pulsed as well as in continuous-wave regime. In the first case the Cr:ZnSe crystal grown by the floating zone method was studied. The maximal output power in continuous-wave regime was 310 mW with the slope-efficiency 73% for the Tm:YAP laser pumping. In the second case the Cr:ZnSe prism grown by the Bridgman method which served simultaneously as laser active medium and intracavity dispersive element was investigated. For the Er:YAP laser pumping the maximal output energy was 20 mJ with the slope-efficiency 36%. The output radiation was tunable in the range from 2050 nm up to 2750 nm. For the Tm:YAP laser pumping the maximal output power in continuous-wave regime was 175 mW with the slope-efficiency 24%. The output radiation was tunable in the interval from 2220 nm up to 2680 nm. The generated radiation beam spatial structure was close to TEM00.
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