A picosecond (ps) mid-infrared (MIR) optical parametric amplifier (OPA) with LiInSe2 crystal was demonstrated for the first time. The MIR OPA was pumped by a 30 ps 1064 nm Nd:YAG laser and injected by a barium boron oxide (BBO)-based widely tunable near-infrared seed. A maximum idler pulse energy of 433 μJ at 4 μm has been obtained under a pump energy of 17 mJ, and the corresponding pulse duration was estimated to be ~13 ps. To our knowledge, this is the highest single pulse energy generated by LiInSe2 crystal. Furthermore, an idler spectrum tuning from 3.6 to 4.8 μm was investigated at fixed pump energy of 15 mJ.
© 2017 Optical Society of America
Tunable, efficient mid-Infrared (MIR) coherent sources in the 3-5 μm atmospheric windows are always been intensively desired for a variety of applications, such as spectrum analysis, laser radar, remote sensing, and medicine [1–3]. The parametric frequency down-conversion based on nonlinear optical (NLO) crystals, such as optical parametric generator (OPG), optical parametric oscillator (OPO), optical parametric amplifier (OPA) and difference frequency generation (DFG) can generate widely tunable radiation in the MIR. The parametric frequency down-conversion pumped by a commonly available ∼1 μm laser has been proved to be an effective way to obtain MIR radiation. Oxide-based NLO crystals, such as KTiOAsO4, LiNbO3 and MgO:LiNbO3, can be pumped by widely-spread high-power diode-pumped laser systems and perform well up to 4 μm [4–6]. Unfortunately, these oxide crystals transmit only to ∼5 μm and their performance above 4 μm is dramatically affected by the onset of multi-phonon MIR absorption. For wavelengths generation exceeding 4 μm, there are only few non-oxide NLO crystals for efficient down-conversion in the MIR. ZnGeP2 has got great achievements in recent years due to its wide transparency region, high effective nonlinearity and thermal conductivity [7–9]. But it must be pumped by the long-wavelength pump sources above 2 μm due to two-photon absorption (TPA) at near-IR wavelengths. AgGaS2/Se2 possess large NLO coefficients and wide transparent regions in the MIR region. However, AgGaS2/Se2 have poor thermal conductivities and low laser damage thresholds . GaS0.4Se0.6 has large nonlinearity (d22≈40 pm/V) and birefringence, but its problems related to cutting, polishing, and coating are still need to be solved. Based on GaS0.4Se0.6-OPA, a tuning range from 5 to 11 μm was achieved, but the maximum energy at 6.45 μm was only∼10 μJ at 10 Hz, which corresponds to an energy conversion efficiency of ~0.33% from the pump to the idler . CdSiP2 exhibits exceptionally high nonlinearity and can be non-critically phase-matched but transparent only up to ∼6.5 μm [12, 13]. BaGa4S7 has higher surface damage threshold and the transparency extends up to 12 μm, but its NLO coefficients are roughly two times lower in comparison to the commercially available AgGaS2 [14, 15]. Recently, a newly developed crystal BaGa4Se7 has broader transparency covering from 0.47 to 18 μm, and its second-harmonic generation (SHG) effect is about 2-3 times that of the AgGaS2 . But BaGa4Se7 has extremely poor thermal conductivities, inferior to that of AgGaS2/Se2, and quite modest surface laser damage threshold with ∼0.09 J/cm2 under the conditions of 1064 nm, 30 ps, and 10 Hz [16, 17]. Pumped by a 30 ps 1064 nm Nd:YAG laser, a tuning performance at 3-5 μm in BGSe-OPA was demonstrated in 2013 . The maximum idler pulse energy at 3.9 μm was 830 μJ, corresponding to an energy conversion efficiency of ∼9%. Furthermore, a wide wavelength ranging from 6.4 to 11 μm based on BGSe-OPA was also achieved in 2015, and the highest idler pulse energy at 7.8 μm was ~125 μJ with an energy conversion efficiency of ~1.4% .
As a promising MIR NLO crystal, LiInSe2 (LISe) is an orthorhombic crystal with large bandgap (2.86 eV) and wide transparency extending across 0.47-13.7 μm without TPA for a pump wavelength of 1064 nm . The crystal exhibits effective nonlinearity (d33 = 16 pm/V) comparable to that of AgGaS2. The most relevant advantages of LISe are its excellent thermo-mechanical properties [16, 20]: isotropic expansion, thermal conductivity ~5 W·m−1·K−1 (3 to 5 times higher than in AgGaS2/Se2) as well as high damage threshold, which are suitable for high-power laser applications. In addition, LISe can be successfully grown in large sizes and with good optical quality .
So far, LISe has been widely used in the OPO. A broadly tunable operation from 4.7 to 8.7 μm of a 1064 nm pumped nanosecond (ns) OPO was demonstrated in 2009, and the maximum idler pulse energy at ~6.5 μm was 282 μJ at a repetition rate of 100 Hz, which corresponds to an energy conversion efficiency of 1.7% from the pump to the idler . In 2014, the difference-frequency generation of picosecond (ps) or femtosecond (fs) Yb-fiber laser synchronously pumped optical parametric oscillators (SPOPOs) at 53 and 80 MHz were reported . Single pulse energies in excess of 1 nJ in fs mode of operation were generated, and the continuous tuning extends from 5 to 12 μm. However, there is no report on LISe-OPA up to now.
In this letter, we demonstrate, for the first time to our knowledge, a ps MIR OPA based on LISe pumped by a ps Nd:YAG laser at 1064 nm. A maximum idler pulse energy of 433 μJ at 4 μm was obtained under pump energy of 17 mJ, corresponding to an energy conversion efficiency of ∼2.55% from the 1064 nm pump beam to the 4 μm idler beam. The idler pulse duration at 4 μm was estimated to be ~13 ps. Moreover, a tuning performance in the range of 3.6-4.8 μm was also demonstrated.
2. Experimental details
The experimental setup is shown in Fig. 1. The ps MIR OPA system consists of two branches: one is the widely tunable near-infrared seed based on a BBO-OPG/OPA configuration pumped by the second-harmonic (SH) of a 1064 nm laser and the other is the seed-injected MIR OPA based on LISe crystal pumped by the same 1064 nm laser. The pump source at 1064 nm is a mode-locked Nd:YAG laser system (EKSPLAPL2250) with pulse duration of 30 ps (FWHM) at a repetition rate of 10 Hz. As shown in Fig. 1, the pump beam at 1064 nm was reshaped first by a spherical telescope (TS) to reduce the beam diameter from 8 mm to ∼3 mm. Then, a combination of a half-wave plate (HWP) and a polarizing beam splitter (PBS) were used as an adjustable energy divider, where the total available pump energy was split into two arms. The second HWP in the LBO arm was used to direct the polarization of the 1064 nm laser along the non-critical phase-matching (PM) direction of LBO crystal to produce the SH output at 532 nm with the pulse duration of ~21 ps. The LBO crystal is cut along θ = 90°, φ = 0° with an aperture of 4 × 4 mm2 and a length of 20 mm. The generated SH laser was separated from the residual fundamental 1064 nm laser by two dichroic mirrors M7 and M8, then the 532 nm beam was reflected by mirror M9 entering the Type I (e-oo) phase-matched BBO crystal (cut at θ = 22.3°) with an aperture of 8 × 8 mm2 and a length of 12 mm. The 532 nm pumped BBO-OPG/OPA stage was double-pass geometry. The idler beam and the residual 532 nm pump beam were separated by a dichroic mirror M10 after the first pass. The residual 532 nm pump beam was also reflected back to the BBO crystal by M11 after a suitable time delay (DL2) and overlapped with the idler beam reflected by M12. The residual 532 nm beam after the second pass was reflected by mirror M9 and separated from the incident 532 nm beam with a small angle in the direction perpendicular to the PM direction of BBO crystal. The output in the second pass after the dichroic mirror M9 consists two portions: the signal beam and the idler beam. The signal beam was reflected by the dichroic mirror M13 into a spectrum meter (SP1: Ocean Optics, HR4000) for wavelength monitoring, and the amplified idler beam as the seed was injected into the LISe-OPA stage. In the experiment, the idler beam from BBO-OPG/OPA covers a spectral range of 1510-1367 nm with an adjustable energy up to ~200 μJ.
The other 1064 nm beam transmitted through the PBS was used to pump the MIR LISe-OPA stage after a suitable time delay (DL1) for temporal overlap with the injected seed beam. The pump beam at 1064 nm was introduced and collinearly mixed in LISe crystal with the collimated seed beam through a dichroic mirror M14. The LISe crystal employed was grown by the Bridgman-Stockbarger method. The LISe crystal is with an aperture of 5.1 mm (along z-axis) × 6.2 mm and a length of 6.76 mm, and optically polished in both end-faces without coatings. The optical transmission of this crystal was measured as shown in Fig. 2. It shows that the LISe crystal has relatively good transmission before 8 μm, and extends to roughly 13.7 μm at the “zero”-level with the characteristic dip near 10 μm. It was cut for propagation in the x-y plane, e-oe type-II PM, which is characterized by maximum effective nonlinearity. The LISe crystal was cut at ϕ = 60.2° for the idler wavelength of 4 μm at normal incidence. Figure 3 shows the calculated Type II PM curve pumped by 1064 nm laser for the idler tuning from 3.6 to 4.8 μm using the Sellmeier equations of LISe given in . It indicates that the tuning of idler beam from 3.6 to 4.8 μm, which corresponds to a signal range of 1510-1367 nm, can be expected within the PM angle range of 68°-51.7°. Figure 3 also shows the measured PM angles in experiment. It can be seen that the deviation between calculated values and experimental data is less than ± 0.55°.
To separate the generated idler beam from the residual pump and amplified seed beams, mirrors M15 and M16 were adopted after the LISe crystal. M15 was made of CaF2 to avoid MIR absorption and coated with high reflection (HR) at 1064 nm and antireflection (AR) at idler beam at 0°-5° incident angle. The mirror M16 was an uncoated germanium (Ge) filter plate with transmittance of about 48% in the spectral range of 3-5 μm and used as a filter to cutoff the residual pump and signal beams. The energy of the generated idler beam was measured by a pyroelectric sensor (OPHIR, PE10-V2), and the wavelength of amplified seed was monitored by a spectrum meter (SP2: Ocean Optics, NIR Quest), simultaneously.
3. Results and discussion
In the BBO-OPG/OPA stage, the idler seed of ~200 µJ was achieved at pump energy of ~3.5 mJ at 532 nm, which corresponds to an energy conversion efficiency of ∼5.7%. Figure 4 shows the generated MIR idler energy at a wavelength of 4 μm versus the pump energy at 1064 nm under ∼200 μJ seed energy injection. It can be seen from Fig. 4 that the output energy of the idler increases monotonously with the pump energy. At the pump energy of ∼17 mJ, the highest idler pulse energy was 433 μJ, which corresponds to an energy conversion efficiency of ∼2.55% and a photon conversion efficiency of 9.63% from the pump beam at 1064 nm to the idler beam at 4 μm. Surface damage was observed under the maximum pump energy of 17 mJ within seconds. The damage threshold is calculated to be ∼0.24 J∕cm2, which corresponds to a pump intensity of ∼8 GW∕cm2 under the conditions of 1064 nm, 30 ps, and 10 Hz. The damage threshold of LISe crystal is about 2.7 times higher than that of BGSe crystal . Furthermore, as shown in Fig. 4, the MIR idler output energy does not show any roll-over effect up to the maximum pump energy, which seems to indicate that higher idler output energy may be achieved with higher pump energy. Figure 5 shows the typical spectrum characterization of the amplified signal beam measured by spectrum meterSP2. The central wavelength of the signal was located at 1450 nm with spectral width of ~33 nm (FWHM), corresponding to the idler wavelength of 4 μm. The calculated idler spectrum at 4 μm with spectral width of ~254 nm was plotted in Fig. 5 as an inset. Due to the limitation of the autocorrelator in NIR-MIR regions, the pulse duration of the signal and idler beams could not be measured. By assuming approximately the signal pulse duration of 21 ps, the idler pulse duration at 4 μm under the maximum pump energy of ∼17 mJ was estimated to be ~13 ps by using the SNLO software.
Under an injected seeding energy of ~200 μJ, the tuning performances of LISe-OPA ranging from 3.6 to 4.8 μm were also measured as shown in Fig. 6. The tuning curve was measured at fixed pump energy of ∼15 mJ, and the corresponding seed beam with tunability extending from 1510 to 1367 nm. The generated idler output energies were ∼90 μJ at 4.8 μm and up to ∼250 μJ at 3.6 μm. We expect that the tuning range can be extended by the use of a high quality crystal with larger aperture.
In summary, we have demonstrated a ps MIR OPA based on LISe crystal pumped by a Nd:YAG laser at 1064 nm for the first time. The maximum idler output energy of 433 μJ at 4 μm with an energy conversion efficiency of ∼2.55% was obtained under pump energy of 17 mJ. The single pulse energies exceeding any previous results obtained with LISe crystal. Further, tuning from 3.6 to 4.8 μm was achieved under the pump energy of 15 mJ. The results show that LISe crystal can be an excellent candidate for the MIR laser output with the advantages of excellent thermo-mechanical properties and high damage threshold. The MIR output energy and the wavelength tunable region can be increased by using a large size crystal. Future experiments will be devoted to the investigation of its performance in the long-mid-IR range of 7-12 μm.
National Natural Science Foundation of China (11504389,51572155, and 51321091); National Key Research and Development Program of China (2016YFB1102201); Shandong Provincial Natural Science Foundation, China (ZR2014EMM015).
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