CdSiP2 (CSP) is a very promising nonlinear crystal for the mid-infrared spectral range with a nonlinear coefficient slightly larger than that of ZnGeP2 (ZGP). In contrast to ZGP, CSP is phase-matchable and can be employed in 1.064-μm pumped optical parametric oscillators (OPOs) without two-photon absorption. Although low damage resistivity has been reported in such initial OPO tests of CSP, no reliable data on the damage threshold of uncoated CSP exists. In this work, we compare the damage resistivity of uncoated CSP with ZGP at two wavelengths, 2.09 μm (1 kHz, 21 ns) and 1.064 μm (100 Hz, 8 ns).
CdSiP2 (CSP) is a very promising nonlinear crystal for the mid-IR spectral range with a nonlinear coefficient slightly larger than that of ZnGeP2 (ZGP). In contrast to ZGP, CSP is phase-matchable and can be employed in 1.064-μm pumped optical parametric oscillators (OPOs) without two-photon absorption. Although low damage resistivity has been reported in such initial OPO tests of CSP, no reliable data on the damage-threshold of uncoated CSP exists. We compare in this work the damage resistivity of uncoated CSP with ZGP at two wavelengths, 2.09 μm (1 kHz, 21 ns) and 1.064 μm (100 Hz, 8 ns).
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.
We employed a 9-mm long periodically-poled KTiOPO4 (PPKTP) crystal with a domain inversion period of 37.8 μm in
an optical parametric oscillator (OPO) to generate sub-nanosecond pulses around 2.8 μm. With a 1-cm long OPO cavity
in a singly resonant configuration with double pass pumping the OPO threshold was 110 μJ at 1064 nm (1-ns pump
pulses at 1064 nm). The maximum idler output energy reached 110 μJ (quantum conversion efficiency of 32.5%). The
signal pulse duration (FWHM) was 0.72 ns and the estimated idler pulse duration was 0.76 ns. At room temperature the
signal and idler wavelengths were at 1722 and 2786 nm.
The beam quality of the idler output of a 1064 nm pumped OPO based on CdSiP2 is compared for linear and Rotated
Image Singly-Resonant Twisted RectAngle (RISTRA) cavities. For similar mirrors and cavity round trip times the
RISTRA cavity yielded 64 μJ of idler energy (6.4 mW of average power at 100 Hz) compared to 34 μJ with the linear
cavity, at a pump level of 21.5 mJ, roughly two times above threshold. The RISTRA cavity generated a somewhat
smoother idler beam spatial profile (characterized by moving a knife-edge) and the intensity in the focus of a 10-cm lens
was about 50% higher.
Passive Q-switching of a diode-pumped (Tm,Yb):KLu(WO4)2 laser, operating between 1910 and 1950 nm, has been
achieved using polycrystalline Cr2+:ZnS as a saturable absorber in a hemispherical L-shaped cavity. The dependence on
the doping levels of Tm and Yb and the low-signal absorption of the saturable absorber has been studied. The highest
average output power reached 272 mW at a repetition rate of 2.04 kHz corresponding to a pulse energy of 133 μJ. This
was achieved with (8.8 at. % Tm, 2.3 at. % Yb):KLu(WO4)2 active element and a saturable absorber with 85% lowsignal
transmission. The highest pulse energy, 148 μJ, was achieved with the same combination at lower (0.94 kHz)
repetition rate. The obtained pulse energy with the different samples is compared with theoretical analysis and the pulse
duration and peak power are estimated from the calculations.
LiGaSe2 and LiInSe2 are promising nonlinear crystals for conversion of laser radiation to the mid-IR spectral range
which are transparent down to the visible and UV. We successfully grew a new mixed crystal as a solid solution in the
system LiGaSe2 - LiInSe2, with a composition of LiGa0.5In0.5Se2 which has the same orthorhombic structure (mm2) as
the parent compounds (LiGaSe2 and LiInSe2). The new crystal is more technological with regard to the growth process
in comparison with LiGaSe2 and LiInSe2 since its homogeneity range is broader in the phase diagram. We established
that about 10% of the Li ions are found in octahedral position with coordination number of 3. The band-gap of
LiGa0.5In0.5Se2 is estimated to be 2.94 eV at room temperature. The transparency at the 0-level extends from 0.47 to
13 μm. The dispersion of the principal refractive indices was measured and Sellmeier equations were constructed. The
fundamental wavelength range for the SHG process extends from 1.75 to 11.8 μm. The nonlinear coefficients of
LiGa0.5In0.5Se2 have values between those of LiGaSe2 and LiInSe2.
Two essential advantages can be expected from adding S to the well known nonlinear crystal GaSe: increase of the bandgap
value or the short wave cut-off limit and improved hardness. Recently, we confirmed that the non-centrosymmetric
structure of GaSe is preserved up to a GaS content of 40 mol. % while the nonlinear coefficient d22 is reduced by only
24%. The increased band-gap results also in higher surface damage threshold. Our preliminary Sellmeier equations for
GaS0.4e0.6 were based on refractive index measurements. These equations are refined in the present work by fitting
second-harmonic generation and optical parametric amplification phase-matching angle data in the mid-infrared as well
as birefringence data in the visible and near-infrared obtained with thin phase retardation plates. The two-photon
absorption effect was studied for GaS0.4e0.6 and GaSe using amplified picosecond pulses at 1064 nm, at a repetition rate
of 10 Hz. For intensities in the GW/cm2 range, the two-photon absorption coefficient of GaS0.4e0.6 for the o-polarization
is 3.5 times smaller than the corresponding coefficient of GaSe. This means that GaS0.4e0.6 could be safely used in
Nd:YAG laser pumped nanosecond optical parametric oscillators or picosecond optical parametric amplifiers, without
nonlinear absorption losses. The dynamic indentation measurements with Berkovich type indenter of c-cut GaS0.4e0.6and GaSe plates indicate about 30% higher indentation modulus and microhardness of GaS0.4e0.6 in comparison to
GaSe.
LiInSe2 is one of the few (in the meanwhile 6) non-oxide nonlinear crystals whose band-gap (2.86 eV) and transparency
enabled in the past nanosecond optical parametric oscillation in the mid-IR without two-photon absorption for a pump
wavelength of 1064 nm. However, the first such demonstration was limited to the 3.34-3.82 μm spectral range with a
maximum idler energy of 92 μJ at 3.457 μm for a repetition rate of 10 Hz. Now we achieved broadly tunable operation,
from 4.7 to 8.7 μm, reaching maximum idler pulse energy of 282 μJ at 6.514 μm, at a repetition rate of 100 Hz
(~28 mW of average power).
LiInSe2 is one of the few (in the meanwhile 6) non-oxide nonlinear crystals whose band-gap (2.86 eV) and transparency
enabled in the past nanosecond optical parametric oscillation in the mid-IR without two-photon absorption for a pump
wavelength of 1064 nm. However, the first such demonstration was limited to the 3.34-3.82 μm spectral range with a
maximum idler energy of 92 μJ at 3.457 μm for a repetition rate of 10 Hz. Now we achieved broadly tunable operation,
from 4.65 to 7.5 μm, with a single crystal, reaching maximum idler pulse energy of 282 μJ at 6.514 μm, at a repetition
rate of 100 Hz (~28 mW of average power).
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