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Keywords:

  • Optical parametric oscillators;
  • mid-infrared;
  • mercury thiogallate

Abstract

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. HGS OPO: experimental results
  5. 3. Conclusion
  6. Acknowledgements
  7. References

The defect chalcopyrite crystal HgGa2S4 has been employed in a 1064-nm pumped optical parametric oscillator operating at 100 Hz, to generate ∼5 ns long idler pulses near 4 µm with energies as high as 6.1 mJ and average power of 610 mW. At crystal dimensions comparable to those available for the commercial AgGaS2 crystal, operation of the 1064-nm pumped HgGa2S4 OPO is characterized by much lower pump threshold and higher conversion efficiency, with the most important consequence that such a device might become practical at pump levels sufficiently lower than the optical damage threshold.

1. Introduction

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. HGS OPO: experimental results
  5. 3. Conclusion
  6. Acknowledgements
  7. References

The so-called defect chalcopyrite nonlinear optical crystals with chemical formula AIIBIII2CVI4 and point group inline image symmetry possess substantially higher second order nonlinear susceptibility in comparison to their chalcopyrite analogues, especially having in mind the close band-gap values. However, in fact only one such ternary crystal, mercury thiogallate HgGa2S4 (HGS), and the quaternary CdxHg1−xGa2S4 (a solid solution of mercury and cadmium thiogallates) have been used so far for phase-matched nonlinear frequency conversion [1]. All the properties, frequency conversion experiments and the existing literature on these nonlinear crystals have been reviewed in [1]. The most important advantage of HGS over the commercially available chalcopyrite AgGaS2 (AGS, the standard non-oxide mid-IR material when it comes to pumping down-conversion schemes near 1 µm) is the ∼1.8 times higher nonlinear coefficient d36 of HGS and this at slightly increased band-gap value (2.79 eV for HGS against 2.7 eV for AGS), i.e. at somewhat improved damage resistivity [2]. In fact, the effective nonlinearity of HGS can be even better but at present, the unknown relative sign of the d36 and d31 tensor components does not allow one to utilize both of them by optimizing the azimuthal angle φ [1].

Due to its wide band-gap, HGS, similar to AGS, can be pumped near 1 µm by short or ultrashort pulse laser sources (e.g. Nd:YAG at 1064 nm) without two-photon absorption. Only few other non-oxide crystals possess this property but most of them (the biaxial LiGaS2, LiInS2, LiGaSe2, LiInSe2, BaGa4S7, and BaGa4Se7) exhibit nonlinear susceptibility lower than AGS. The recently developed chalcopyrite CdSiP2 (CSP) is highly nonlinear and non-critically phase-matchable crystal but its transparency extends only up to ∼6.5 µm, with an optical damage threshold not better than that of AGS [2].

Recently, an optical parametric oscillator (OPO) based on HGS, with tunability from ∼3.7 to ∼5.7 µm, showed the highest idler output energy level of any non-oxide nonlinear material pumped near 1 µm [3], in fact the only OPO of this kind generating millijoule idler energy [2]. In the present work we examine the potential of HGS for high-power OPO application with 1064 nm pumping achieving almost 10-fold increase in the idler average power reaching 610 mW (6.1 mJ at 100 Hz) in damage free operation.

2. HGS OPO: experimental results

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. HGS OPO: experimental results
  5. 3. Conclusion
  6. Acknowledgements
  7. References

The HGS element used in the present study, see Fig. 1, was 13.4 mm-long with an aperture of ∼10 × 13.6 mm2. It was cut at θ = 52.7° and φ = 45° for type-I (oo-e) interaction utilizing only the d36 component of the nonlinear tensor. The crystal faces were antireflection-coated for the signal wavelength range resulting in low Fresnel losses also at the pump wavelength of 1064 nm.

image

Figure 1. Photograph of the antireflection-coated HGS element.

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The OPO was pumped by a diode-pumped Q-switched Nd:YAG laser/amplifier system (Innolas GmbH) delivering up to 250 mJ per pulse at 100 Hz, as shown in Fig. 2. The pump pulse duration was typically ∼8 ns and the M2 factor was measured to be ∼1.4. A half-wave plate (λ/2) and a polarizer (P) were used to attenuate the pump beam and a telescope (T) was applied to expand it to a diameter of 9.6 and 8.45 mm in the horizontal and vertical directions, respectively. The HGS crystal rotation axis for tuning was vertical and parallel to the 13.6 mm edge of the active element. The pump beam reached the HGS crystal after reflection at the ZnSe bending mirror (BM) and passing through the output coupler (OC) which transmitted 75%. The wedged ZnSe OC had a transmission of 29% for the signal wave and 79% for the idler while BM was highly transmitting for both (66% and 64%, respectively). A plane Ag-mirror was used as a total reflector (TR) for all three waves in a double pump pass singly resonant OPO configuration. The diaphragm (D) served to align the OPO and the filters (F) were used for suppression of the residual pump and signal pulses.

image

Figure 2. HGS OPO experimental set-up.

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Figure 3 shows the input-output characteristics obtained at normal incidence for a cavity length of 17 mm. Only the idler is effectively out-coupled and measured, and the energy values in the figure are corrected for the transmission of F and BM. The threshold of 2.6 mJ corresponds to an axial pump fluence of 8 mJ/cm2 or a peak intensity of ∼1 MW/cm2.

image

Figure 3. Input-output characteristics of the HGS OPO at normal incidence for a cavity length of 17 mm and 100 Hz.

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The maximum pump level of < 40 MW/cm2 axial intensity applied in Fig. 3 is considered to be just the safe limit above which optical surface damage in HGS may occur [1]. The threshold pump intensity is lower, by roughly a factor of ∼15, compared to [3] where the HGS crystal parameters were similar but single-pass pumping was applied and the number of cavity round trips was almost 3 times smaller. The slope efficiency (idler only) in the initial stage of Fig. 3 is ∼8.5% but decreases at higher pump levels. The absolute efficiency of conversion from pump to idler energy at maximum output power was around 6%, higher, by a factor of ∼1.5 in comparison to [3]. The highest idler energy obtained, 6.1 mJ, corresponds to an average power of 610 mW which is an improvement by a factor of more than 9 compared to [3] and represents the highest idler power ever achieved with a 1-µm pumped OPO based on a non-oxide material. Damage-free operation far above threshold (>10 times) was possible with the present HGS OPO, however, it is known that highest conversion efficiencies are achieved at lower ratios and in Fig. 3, the maximum conversion to the idler of ∼7.5% (reached at a pump energy ∼40 mJ) indicates that yet higher output idler energies and average power could be possible using larger apertures for the active element or a flat top shaped pump beam profile.

Close to the OPO threshold we measured at normal incidence a signal wavelength of 1446 nm, accordingly the idler wavelength was 4.03 µm. Figure 4 shows a comparison of the experimental OPO tuning capability (symbols) measured at an incident pump energy of 15 mJ and at 100 Hz with a slightly lengthened (21 mm) cavity, with calculations based on the Sellmeier equations for HGS that gave the best agreement [4]. The idler tuning range for the given crystal cut extends from below 3 to above 8 µm. However, the energy dependence is not symmetric, with stronger decrease at longer wavelengths (Fig. 5), as could be expected from the effective nonlinearity and parametric gain wavelength dependence. The “anomalous” peak in Fig. 5 at normal incidence is due to enhanced feedback, especially for the idler wave. The energy decrease in the left limit of Fig. 5 is related to increasing OC transmission at the signal wavelength.

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Figure 4. Experimental signal (triangles) and idler (circles) OPO wavelengths vs. internal phase-matching angle θ compared with calculations (line) based on Sellmeier equations [4].

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Figure 5. Relative idler energy of the HGS OPO vs. idler wavelength.

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In view of the high average powers, it is very important to evaluate the effect of residual absorption leading to crystal heating. We measured the dependence of the HGS crystal temperature on the incident pump energy at 10 Hz (employing a shutter in front of the attenuator in Fig. 2) and at 100 Hz (Fig. 6). It is known that the thermo-optic coefficients of HGS are relatively low (more than two times compared to AGS) and the temperature tuning potential is modest [5,6]. The temperature rise relative to the ambient temperature of 22°C was only 8°C for an incident pump power of 4 W. The measured OPO signal wavelength shift corresponding to this temperature rise was ∼ −2.2 nm in excellent agreement (< 0.1 nm) with calculations based on [4-6]. Thus, radial dependence of the crystal temperature is expected to contribute on the first place to the spectral bandwidth (seen experimentally) and hence top-hat pump profiles will be preferable in narrow-band OPO set-ups, e.g. such based on seeding, which are beyond the scope of the present work.

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Figure 6. Measured HGS crystal temperature vs. incident pump energy at 10 and 100 Hz, for a cavity length of 17 mm and normal incidence.

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We measured the HGS OPO linewidth at the signal wavelength using a 1-mm-thick Ag-coated CaF2 Fabry-Perot etalon and obtained ∼57 GHz (1.9 cm−1 or 0.4 nm at 1446 nm).

The temporal characteristics of the HGS OPO were measured at 5-times threshold, 100 Hz, 17 mm cavity length, and normal incidence using fast photodiodes and 0.5 GHz oscilloscope, Fig. 7. The (HgCdZn)Te detector used for the idler (Vigo systems model PCI-9) had a specified time constant of < 2 ns at 9 µm but its temporal response near 4 µm seems slower and from the second-harmonic (SH, generated in a 3-mm thick type-I GaSe crystal) pulse profile one can conclude that the actual idler pulse duration is ∼5 ns, shorter than the pump pulse.

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Figure 7. Simultaneously measured temporal pulse profiles of the pump, signal, idler and SH of the idler.

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The M2 beam quality factor of the HGS OPO was far above the value corresponding to diffraction limited beams: for the non-resonated idler we fitted data measured by the knife-edge method with M2 ∼ 180–190 in the two planes. The very high M2 values can be attributed to a combination of factors including the large pump diameter, short pump pulse duration and cavity length and the operation far above threshold [7]. It is expected that this parameter could be substantially improved implementing the RISTRA cavity OPO concept [8].

3. Conclusion

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. HGS OPO: experimental results
  5. 3. Conclusion
  6. Acknowledgements
  7. References

We conclude that HGS has a great potential compared to all known non-oxide nonlinear crystals that can be pumped near 1-µm (powerful Nd- or Yb-based pulsed laser systems) for coverage of the mid-IR wavelength range up to ∼12 µm. Different crystal cut will be required to study such extended tuning capabilities in the future. However, one essential possible improvement is obvious: HGS, similar to AGS, exhibits for such three-wave processes substantially higher (roughly by 50%) effective nonlinearity if type-II (eo-e) interaction is applied with a crystal cut at φ = 0°. Ideally this should be combined with increased crystal aperture because the OPO threshold will be reduced. It is expected in this way to reach output energies on the 10 mJ level (e.g. > 1 W) and more that 5 mJ at idler wavelengths > 5 µm using commercially available pump sources at 1064 nm. Judging from our extensive experience with other non-oxide nonlinear crystals pumped at 1064 nm, one of the most important aspects is long term damage-free operation with such OPOs, especially at increased repetition rates (in the past most experiments were performed at 10 Hz to avoid cumulative damage). Consequently, further reduction of the HGS OPO threshold by using longer crystal samples and type-II interaction will dramatically improve the chances to reach the high reliability and stability required for commercialization. The same can be achieved using flat top pump beam shaping and/or RISTRA cavity design, which has proved to be the best solution for reaching highest idler pulse energies (e.g. near 6 µm) from similar OPOs pumped in the 2-µm spectral range [8].

Acknowledgements

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. HGS OPO: experimental results
  5. 3. Conclusion
  6. Acknowledgements
  7. References

The research leading to these results has received funding from the European Community's Seventh Framework Programme FP7/2007-2011 under grant agreement n°224042 and DLR (Germany) under RUS 11/019 (bilateral cooperation with Russia). A. E.-M. acknowledges support from the Catalan Agència de Gestió d'Ajuts Universitaris I de Recerca (AGAUR) through grant (BE-DGR 2011, BE100777).

References

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. HGS OPO: experimental results
  5. 3. Conclusion
  6. Acknowledgements
  7. References