Nanosecond AgGaS2 type-I singly resonant optical parametric oscillator pumped by a Q-switched 1.064 µm Nd:YAG laser is demonstrated experimentally. Continuously tunable 2.6–5.3 µm radiation and output pulse energy up to 0.6 mJ at 4 µm are achieved in a single-stage conversion process. The analysis of pump threshold is investigated both theoretically and experimentally.
© 2006 Optical Society of America
Solid state lasers operated in the range of atmospheric transparency (3–5 µm, 8–12 µm) are of great interest for many applications such as eye-safety lidars, target designations, obstacle avoidance, and infrared countermeasures. Optical parametric oscillator (OPO) is an effective tool to cover the range. As a pumping source, Nd:YAG lasers, which have high output energy and good beam quality, are widely distributed and used in mobile systems, and in combination with well designed UV to visible range frequency converters permits to design extra-wide band 0.2–14.0 µm source of coherent radiation. In turn, frequency conversion of most efficient wide-band 0.7 to 1.1 µm Ti:sapphire laser is the simplest way in design of middle IR tunable femtosecond sources. Unfortunately, most efficient middle IR nonlinear crystals such as CdGeAs2, ZnGeP2, Tl3AsSe3 and AgGaSe2 are not transparent or have big loss at near infrared and visible, and that is why they cannot be used as laser frequency converters in these cases.
Negative silver thiogallate (AgGaS2 or AGS) crystal with high nonlinear optical coefficient and high optical transmission from 0.5–12 µm makes it realistic to generate infrared parametric radiation. Its most characteristic is that it is one of the few crystals which can be pumped by commercially available 1.064 µm Nd:YAG laser to achieve phase-matched down-conversion into the λ>5µm region. That is why numerous experiments with one and multistage OPO/OPG pumped by ns and ps IR dye, Nd:YAG and another near IR solid state lasers, so as by fs Ti:sapphire and Cr:forsterite lasers, are carried out during last decade [1–9]. Significant attention is paid also to harmonic, sum and difference frequency generators, and up-converters. As high as from 0.1 to 30% efficiencies  and OPG at 1.2–10 µm wide range pumped by 20 ps Nd:YAG laser are demonstrated . But usual tuning range of OPO pumped by ns Nd:YAG laser does not cover all 3–5 µm range [2, 3, 5] because surface damage of the crystal. The only wide-range 3.9–11.3 µm singly resonant non-selective cavity OPO pumped by ns Nd:YAG laser is designed. It is type-II OPO to maximize the effective second-order nonlinearity and minimize spectral bandwidth, which yields limited energy of 372 µJ output pulses .
In this paper, considering the low damage threshold, the methods to reduce the oscillating threshold are discussed. AgGaS2 type-I singly resonant optical parametric oscillator (SRO OPO) pumped by a ns 1.064 µm Nd:YAG laser is demonstrated experimentally. We put our emphasis on the interested parametric wavelength range with 3–5 µm. A comparison on the threshold energy density is made between the single pump pass SRO OPO and the double pump pass both theoretically and experimentally.
2. Analysis of Oscillation threshold
Due to the low damage threshold in AGS crystal, how to lower the pump threshold is considered firstly. In order to describe the threshold pump energy density (oscillation threshold) of a pulsed single- and double pump pass single resonant oscillator, simplified theoretical model including the effects of Poynting vector walk-off is used . Simplified expression for the threshold pump energy density (threshold fluency) is given by
Where gs, leff and k are the signal spatial mode coupling coefficient and the effective parametric gain length and the interaction coefficient listed below, respectively.
The walk-off length lw is given by
Note: ωs, ωi are frequencies for the signal and idle light; deff is effective nonlinear coefficient; ns, ni, np are refractive indexes for signal, idle and pumping light, respectively; C is the light velocity; τ is the pulse width of pumping light; L,l are the cavity length and the crystal physical length, respectively; γ is the ratio of backward to forward pump field amplitude inside the crystal; α is the coefficient of field absorption in the crystal; ln(Pn/P0) is the ratio of the threshold power to noise power; R is the effective cavity loss (the product of mirror reflectivities). The relations between the Gaussian mode electric field radii wp, ws and the resonant spot radii w̄s are
If the ratio of backward to forward pump field amplitude γ is fixed to zero and SRO is considered, the threshold pump intensity of a pulsed singly resonant oscillator including the effects of Poynting vector walk-off is concluded as follows:
Double pump pass means that pump wave passes through the nonlinear crystal twice. It is an effective method to lower the pump threshold. Figure 1(a) shows a comparison between single- and double pump pass (DSRO, double-pump singly resonant, SSRO, single-pump singly resonant). The different ratio of backward to forward pump field amplitude inside the crystal follows with the different threshold. With the increasing of γ, threshold declines obviously as shown in Fig. 1(b).
Threshold pump energy density is mainly determined as a function of cavity length, crystal physical length, pump spot size, pump pulse width. The influence of the parameters on the threshold is investigated as shown in Fig. 2. It is clear: (1) the longer the cavity length is, the higher the threshold is in Fig. 2(a); (2) when the crystal length is less than 1 cm, the threshold is abruptly increased, when the length is more than 2 cm, it nearly keeps flat in Fig. 2(b); (3) the increase in threshold as well as the decrease in gain is due to the small pump spot size as a result of Poynting vector walk-off in Fig. 2(c); (4) the relatively shorter pulse width is benefit to the lower threshold in Fig. 2(d). It is also evident that the oscillation threshold increase with the output wavelength (threshold at 5µm >that at 4 µm >one at 3µm) from the Fig. 2. It can be interpreted as: 1) the oscillation threshold is inversely proportional to the product of the idler and signal angular frequency as shown in Eq. (1), so as to the product of the two wavelength; 2) the effective nonlinear coefficient deff is decreasing at the range of 3–5µm as well as the gain in the AGS crystal, the oscillation threshold is inversely proportional to deff, too. Thus a larger threshold is predicted at longer output wavelength certainly.
3. Experimental setup
The schematic setup is shown in Fig. 3. AgGaS2 crystal used with 10×7 mm2 in cross section, 20 mm in length, θ=47° and ψ=45° cut for type-I phase-matching is supplied by MolTech GMBH, Germany. In order to reduce the loss so as to the oscillation threshold, both cross sections are well antireflection AR coated: high transparent HT1.06>99% at pump wavelength 1.06 µm, and also at signal and idler wavelengths HT1.3–1.7~97.5%–99%, HT3–5~97%–98.5%, respectively. The two identical flat mirrors M1, M2, which are used as the cavity mirrors with coatings of HT1.06>95%, HR1.3–1.7>99–99.4% and HT3–5>88–98%, are designed for SRO OPO. A homemade electro-optically Q-switched Nd:YAG laser and amplifier pumped by flashlamp with pulse width 10–30ns depending on the input energy is used as OPO pump source. A diaphragm is inserted in the cavity of pump laser to restrain the higher-order mode oscillation. The laser can operate at the frequency of 20 Hz.
As shown in Fig. 3, D1, detector of fast Si PIN photodiode, is used to monitor pump laser and also as the trigger signal for D2, detector of LN-cooled 1×1 mm2 sensitive area MCT P775-10 photoresistor, which is used to detect the idle signal. Mirror, m1, is glass mirror, which can reflect a little part of pump laser at 1.064 µm. Glan prism G combined with mirrors m2 and m3 is used to control the polarization direction of pump beam. Filter F and mirror m5 with the identical coating have high reflectivity R>99% at 1.064 µm and transmission T~65%–90% at 3–5 µm, T~88% at 4 µm. A He-Ne laser and mirror m4 are used to indicate the idler radiation propagation. The output idler is measured by a step-motor-driven computer-controlled UV-FIR monochromator SBP300, Zolix Instruments Co., Ltd, China. Step-motor-driven computer-controlled rotational positioner with positioning accuracy 9″ is used for precision determination of the phase-matching angles.
4. Experimental results and analysis
Considering the theoretical analysis on the threshold, the cavity length for oscillation is about 2.8 mm, just at the minimal distance allowed to rotate by the 2 cm length of AGS crystal. The wavelength tuning range is from 2.6 to 5.3 µm as shown in Fig. 4, which is pumped by a 1.064 µm Nd:YAG laser with pulse width 15 ns and spot diameter 1.5 mm. To our knowledge, it is the biggest range for nanosecond AgGaS2 type-I SRO OPO. The solid line in Fig. 4 is simulated based on the Sellmeier’s equation in Ref. , which gives the best agreement with our experimental data. The longer wavelength is limited by the damage of cavity mirror.
In order to lower the oscillation threshold, the double-pump scheme is considered. As shown in Fig. 3, the pump laser is not fed back into the OPO cavity when pump laser is not vertical to 1.064 µm reflective filter, F, but the pump laser can be fed back into the cavity for a double pump pass by adjusting the filter, F vertical to pump laser. Figure 5(a) shows the pump threshold energy density as a function of various idle wavelength outputs for single and double pump pass AGS SRO OPO. 1.4 times higher oscillation threshold of SSRO than DSRO at 4 µm is concluded, which is well consistent with 1.6 times of the theoretical prediction in Fig. 1(a). The predicted variabilities of threshold on the idler are confirmed by two schemes. The longer the output wavelength is, the higher the oscillation threshold is, which can be interpreted in section two.
Output energy of the idler is our main interest. Firstly, the output energy of idle light is monitored with single pump pass. The pump pulse width (FWHM) is 15 ns and its beam diameter is 1.3 mm with frequency of 1 Hz. The relation between pump energy and output energy is shown in Fig. 5(b). Square points are at fixed output wavelength 4 µm, 2.8 cm in cavity length. 270 µJ is recorded with the maximum laser-to-idler conversion efficiency 3.5% without any damage appears. The optical damage of cavity mirrors inside occurs at the input pump power density up to 34 MW/cm2, while no damage appears on the crystal surface in OPO cavity, which is due to the good growth, polishing and coating technique on the crystal. In fact, we also observe the backward energy output after mirror m5 as shown in Fig. 3.
In order to improve the output energy, a telescope with two lens of f=5 cm, f=10 cm is inserted between Glan prism G and mirror m5 to enlarge the pump spot size. The output energy with double pump pass is shown in Fig. 5(b) by the circular points. The maximum energy 560 µJ is recorded. Considering the filter’s loss at the idler, the maximum one of 620 µJ has been generated. Farther improvement is limited by the damage of coating on cavity mirrors.
Nanosecond singly resonant type-I AgGaS2 optical parametric oscillator pumped by a Q-switched 1.064 µm Nd:YAG laser is demonstrated experimentally. Methods to reduce the oscillation threshold are investigated both theoretically and experimentally. The effect of parameters such as OPO cavity length, AGS crystal physical length, pump spot size and pulse width on the oscillation threshold is investigated theoretically. A comparison on the threshold pump energy density is made between the single pump pass SRO OPO and the double pump pass one both theoretically and experimentally. Continuously tunable 2.6 to 5.3 µm radiation and maximum output energy 0.6 mJ per pulse are obtained.
The authors would like to acknowledge the support from the National Natural Science Foundation under grant No. 10334010 and the international cooperation program by the Ministry of Education of China.
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