Nonlinear optical conversion of 1.064 µm pulses from a Q-switched Nd:YAG laser to the mid-infrared is demonstrated. The experimental setup is based on a two-stage master-oscillator/power-amplifier (MOPA) design with a KTiOPO4 based MOPA in the first stage and a KTiOAsO4/ZnGeP2 based MOPA in the second stage. The setup can be tuned to provide output at 8 µm or in the 3–5 µm wavelength region. We obtain more than 8 mJ at 8 µm, and up to 33 mJ at 3–5 µm. The measured beam quality factors are in the range M 2=2–4 for both wavelength regions.
©2008 Optical Society of America
High energy mid-infrared laser sources have several applications including spectroscopy and remote sensing. In order to cover a wide wavelength range, nonlinear optical conversion is commonly employed. Zinc germanium phosphide (ZGP) is an attractive material for this purpose due to its high nonlinear coefficient, good transmission in the mid-infrared range, and high thermal conductivity.
ZGP must be pumped at wavelengths above 2 µm due to its high absorption below 2 µm. Such pump beams can either be generated directly by a laser [1–4], or by nonlinear conversion from a shorter wavelength laser such as Nd:YAG [5–8].
Nonlinear optical conversion to the mid-infrared range has also been demonstrated using other nonlinear crystals, such as CdSe, AgGaS2, AgGaSe2, and orientation patterned GaAs. A recent review of results obtained using these materials, in addition to ZGP, is given in Ref. .
Optical parametric oscillators (OPOs) are the most common nonlinear conversion devices. A problem with pulsed OPOs is that conversion is not efficient until the signal power has grown comparable to that of the pump, so the energy in the leading part of the pump pulses is wasted. Secondly, a problem with high energy OPOs is that the beam diameter must be large to avoid exceeding the damage threshold of the nonlinear crystal. This may lead to a resonator with a high Fresnel-number, resulting in poor beam quality due to a large number of transverse modes. This problem can be addressed using confocal unstable resonators [10,11], or by image-rotating resonators . An alternative solution is the master-oscillator/power-amplifier (MOPA) approach [13–18]. Here, a master OPO is pumped with a low-energy, narrow, pump beam, to suppress higher order transverse modes. The signal or idler from the master OPO is then used as a seed in an optical parametric amplifier (OPA), where a large pump diameter does not necessarily cause a poor beam quality. In addition, the seed pulse for the OPA can be delayed to overlap optimally with the pump pulse for maximum nonlinear conversion. The MOPA approach is used in the present work, which aims at demonstrating high pulse energies combined with good beam quality in the mid-infrared.
Previously, Ehrlich et al. obtained up to 6.8 mJ at 8.5 µm with M 2=5.5 in an AgGaSe2 OPO with an unstable confocal resonator . Mennerat and Kupecek obtained as much as 10 mJ at 10.4 µm using difference-frequency generation (DFG) in a CdSe crystal , but the beam quality was not reported. Rustad et al. have obtained up to 28 mJ in the 3–5 µm range using a ZGP OPO, but the beam quality was poor (M 2 > 100) . However, they also obtained 21 mJ in the 3–5 µm range with M 2≈15 using a ZGP-based MOPA. Similar energy levels have also been reported by Budni et al. and by Lee et al. using a ZGP OPO [3,4], but they do not state the beam quality. Recently, Dergachev et al. obtained as much as 30 mJ at 3.4 µm in a ZGP OPA, but the beam quality was not reported .
In this paper, we demonstrate a two-stage MOPA setup where pulses from a Nd:YAG laser are converted to 2.08 µm in the first stage and to the mid-IR in the second stage. The first stage consists of an OPO and an OPA, both based on KTP. The second stage consist of a KTA-based OPO, which is also pumped at 1.06 µm, and a ZGP-based OPA, which is pumped by the 2 µm beam from stage 1. The output wavelengths from the two-stage MOPA can be tuned to provide output in the important 3-5 µm and 8-12 µm atmospheric transmission bands. It is shown that back-conversion affects the quantum efficiency and beam quality of the ZGP OPA, and that both the length of the ZGP crystal and the seed energy must be adjusted to achieve optimum performance.
Using a ZGP OPA optimised for high conversion and good beam quality, we obtain more than 8mJ at 8 µm with a beam quality factor M 2≈3.6. By increasing the pump energy on the ZGP crystal, we obtained as much as 10 mJ at 8 µm, but then the input surface of the crystal was slightly damaged. Up to 33 mJ is obtained in the 3–5 µm region, with M 2=2–4. However, we observe that there are problems with parasitic OPO effects in this wavelength region due to imperfect AR-coatings for the ZGP crystals.
2. Experimental setup
The experimental setup is schematically shown in Fig. 1. The Q-switched Nd:YAG laser provides linearly polarised, 6 ns pulses at 1.064 µm. It is injection seeded to operate in a single longitudinal mode. An optical isolator is used to prevent feedback into the laser, and the pulse energy after the isolator is ~500 mJ. The repetition rate is 10 Hz, and the beam quality is M 2≈2.
The 1.064 µm beam is split into three parts. One low-energy beam at 8mJ is used to pump the KTP master OPO, and another low-energy beam at 8-10 mJ is used to pump the KTA master OPO. Relay imaging telescopes are utilised to reduce the pump diameter (1/e 2) to 1.0 mm for both OPOs while preserving the transverse distribution of the laser beam. The remaining available pulse energy at 1.064 µm is used to pump the KTP OPA, where the pump diameter (1/e 2) is set to 5.3 mm using a relay imaging telescope. The KTP MOPA is described in more detail elsewhere . It provides a ~140 mJ signal beam at 2.08 µm with M 2≈2 and aFWHM bandwidth of 2.3 nm. This beam is used as a pump in the second MOPA stage. In addition, the KTP MOPA provides about 80 mJ idler at 2.18 µm. However, a beam quality factor M 2≈2.7 for the 2.18 µm beam makes it less suited as a pump in the second MOPA stage.
2.1. KTA OPO
The role of the KTA OPO is to provide seed pulses for the ZGP OPA. KTA is an attractive nonlinear optical crystal due to its high damage threshold, and reasonably high nonlinear coefficient. In addition, the absorption of KTA is lower than that of KTP in the 3–4 µm range, allowing for tuning to longer idler wavelengths.
The OPO consists of two 15 mm long KTA crystals, in a walk-off compensating configuration, and two flat mirrors (see Fig. 3). The KTA crystals are cut at θ=42.5° for type II phase-matching, and can be angle-tuned to generate idler wavelengths of 2.8 µm or 3.7 µm. These wavelengths are chosen to obtain a DFG beam at 8 µm or at 4.7 µm in the ZGP OPA.
The OPO resonator is designed for singly resonant oscillation with double-pass pumping. The reflectivity of the input mirror (M1 in Fig. 3) is <1.5% (pump), >99% (signal), and <5% (idler), and the reflectivity of the output mirror (M2 in Fig. 3) is >99% (pump), 62%–69% (signal), and <10% (idler).
Measured in/out curves for the KTA OPO are shown in Fig. 2 for the two idler wavelengths used in the experiments. During operation of the two-stage MOPA, we keep the pump energy of the KTA OPO at 8 mJ or 10 mJ, for an idler wavelength of 2.8 µm or 3.7 µm, respectively, and use filters after the OPO to attenuate the idler in order to vary the seed energy to the ZGP OPA.
The measured beam quality of the idler is M 2=2.3–2.4 for both idler wavelengths, and the measured FWHM idler bandwidth is 2.0 nm for an idler wavelength of 2.8 µm and 21.6 nm for an idler wavelength of 3.7 µm.
2.2. ZGP OPA
The setup for the second MOPA stage is shown in Fig. 3. The idler from the KTA OPO is expanded using lenses L1 and L2 to make its diameter somewhat larger than that of the 2.08 µm beam. A long-pass filter after the KTA OPO removes the signal at 1.5 µm/1.7 µm. The idler at 2.8 µm/3.7 µm from the KTA OPO is made to overlap spatially and temporally with the 2.08 µm beam from the KTP MOPA. The phase-matching conditions of the ZGP OPA are type I.
We limit the maximum pump energy in most of the experiments to approximately 75 mJ to avoid damaging the ZGP crystals. The 1.06 µm pump beam has a somewhat irregular fluence distribution, which is transferred to the 2.08 µm beam. Therefore, a pump energy of 75 mJ corresponds to a peak fluence of approximately 0.6 J/cm 2. We are thus not able to use the full available pump energy of ~140 mJ at 2.08 µm. Figure 4 shows the measured pump fluence distribution at the ZGP crystal.
Due to loss in optical components, the seed energy at the position of the ZGP crystal is approximately 0.3 mJ. However, since the seed is somewhat larger than the pump, the amount of seed overlapping with the pump on the ZGP crystal is approximately 0.1 mJ.
3.1. ZGP OPA tuned to 8 µm
The performance of the ZGP OPA, tuned to provide a DFG beam at 8 µm, is investigated for two different ZGP crystals. The first ZGP crystal is 10 mm long and is cut at θ≈53°, and the second ZGP crystal is 14 mm long and is cut at θ≈55°. Both crystals have a 6 mm×8 mm aperture.
We simulate the ZGP OPA using a model that can include all relevant effects, such as diffraction, temporal and spatial walk-off, and absorption . The model can also handle the complex spatio-temporal variation of the pump, but unfortunately we do not have such information for our pump laser. Detailed characterisation of a similarly designed laser (unstable resonator with variable-reflectivity output coupler) has been reported , and the spatial field distribution was found to be strongly time-dependent: It was approximately Gaussian in the beginning of the pulse and ring-shaped in the end, similar to a hollow cone . Since we lack detailed information on our laser we have used the measured pulse and fluence distribution and assumed that the pump is separable in space and time. Although this cannot be expected to give accurate results, the simulations can be useful as design guidelines. The relevant nonlinear coefficient in ZGP is taken to be d 36=75 pm/V.
The simulated quantum efficiency of the ZGP OPA is shown in Fig. 5(a) and (b) for a ZGP length of 10 mm and 14 mm, respectively, and for different seed energies. We observe that a maximum quantum efficiency of 50% can be reached for the highest seed energy. We also observe that there is back-conversion, i.e. that sum-frequency generation of the signal and idler waves transfers energy back to the pump. For a given seed energy, the back-conversion results in a maximum in the quantum efficiency at a certain pump energy. The peak conversion is seen to decrease with decreasing seed energy. This means that the crystal length should be chosen such that the peak conversion at full seed energy occurs at the maximum available pump energy. The maximum usable pump energy is in our case limited by the damage threshold of the ZGP crystal. We observe in Fig. 5(a) that a 10 mm long ZGP crystal should be optimal for the maximum available seed energy of 0.1 mJ and a pump energy of about 75 mJ.
The same data as in Fig. 5(a) and (b) is shown in Fig. 5(c) and (d), respectively, where we plot the pulse energy at 8 µm instead of the quantum efficiency. In addition, we have taken into account the measured coating reflectance at 8 µm of 2.3% and 13.2% for the 10 mm and 14mm long ZGP crystal, respectively. This is done to simplify comparison of the simulated results with the measurements. The measurements corresponding to Fig. 5(c) and (d) are shown in Fig. 5(e) and (f), respectively. We observe that there is a reasonable agreement between measured and simulated in/out curves, despite the fact that the simulations were performed with a simplified model of the pump beam. We consider 75 mJ as the maximum safe pump energy (0.6 J/cm 2 peak fluence) and obtained 8 mJ output at that pump energy level with the 10 mm crystal. In an attempt to obtain an output of 10 mJ at 8 µm we increased the pump energy, thus increasing the risk of damaging the crystal. The seed energy was ~0.1 mJ in this case. We obtained 9.6 mJ at 8 µm for a pump energy of about 90 mJ and 10 mJ at 108 mJ pump energy, as shown in Fig. 5(e). These results indicate a roll-off, as expected from Fig. 5(c). However, even though the ZGP crystal was pumped at 108 mJ for only a couple of seconds, the input surface of the crystal was slightly damaged.
The beam quality at 8 µm is measured by focusing the beam and measuring the second moments at waist and in the far field using a pyroelectric camera. It is well-known that background noise may dominate the second moment integrals, since the second moments heavily weighs the outer wings of the beam . A correction is therefore done on the near field and far field fluence profiles to make sure that the average fluence of the background is zero. In addition, we use circular apertures enclosing 99% of the energy in the near field and far field when determining the second moments. Both corrections are done numerically on the raw data from the camera.
For the 14 mm long ZGP crystal we estimate the beam quality to be M 2≈4.3 at a pump energy of 30 mJ and M 2≈6.1 at 73 mJ, at full seed power. For the 10 mm long ZGP crystal, we obtain a beam quality of M 2≈3.6 at a pump energy of 75 mJ, with full seed power. The corresponding simulated beam qualities are 1.5, 3.1, and 1.6, respectively. Figure 6 shows measured fluence at 8 µm for the 10 mm long ZGP crystal with 75 mJ pump energy and full seed power.
We observe that the measured M 2-values are higher than the simulated ones. This may be ascribed to the time-dependence of the pump beam profile, which was not included in the simulations. However, it is clear from both the simulations and experiments that back-conversion reduces the beam quality. Physically, the nonuniform pump intensity means that the intense part of the pump is depleted before the low intensity parts are effectively converted. Increasing the gain to improve conversion in the low intensity parts means that one obtains back-conversion in the high intensity parts . This distorts the intensity distribution of the 8 µm beam, leading to a poorer beam quality.
Figure 7 shows the measured spectrum at 8 µm.
3.2. ZGP OPA tuned to the 3–5 µm range
The two-stage MOPA can be tuned to provide output in the 3–5 µm wavelength region by angle-tuning the KTA OPO and the ZGP OPA. In order to optimise the performance at 3–5 µm, we use a ZGP crystal with a coating reflectance of less than 2% at 3.7–4.7 µm. The cut angle of the crystal is θ≈53°, the length is 10 mm, and the aperture is 5×7 mm. This aperture is marginal for the present pump size, but we estimate that the clipping of the pump beam is less than 3%. In addition, the mirror M3 in Fig. 3 is changed to maximise its reflection at 3.7 µm.
An advantage when tuning to this wavelength region is that both the amplified seed at 3.7 µm and the DFG beam at 4.7 µm are within the 3–5 µm atmospheric transmission band. However, since the output from the KTA OPO is lower for an idler wavelength of 3.7 µm than for 2.8 µm, as seen in Fig. 2, the KTA OPO must be pumped at higher energy to produce the same idler energy as before. We therefore pump the KTA OPO at 10 mJ in this case.
There are two major differences when tuning the ZGP MOPA to the 3–5 µm wavelength region, compared to the 8 µmcase. The first difference is that the coupling constant for the DFG process in the ZGP OPA is somewhat higher than at 8 µm, mainly due to the (λ seed λ DFG)-1/2 factor in the nonlinear coupling constant .
Secondly, we have observed nonlinear conversion in the ZGP crystal, even if the seed beam is blocked. It turns out that the coatings on the front and back surface of the ZGP crystal act as OPO resonator mirrors for a type I, non-collinearly phase-matched parasitic OPO. This conclusion is based on the observation that when the crystal is rotated, the parasitic signal remains normal to the crystal face, and its wavelength changes in fair agreement with phase-matching calculations based on the Sellmeier equations in Ref. . Such calculations also show that our crystal does not support parasitic OPO effects when it is tuned for 8 µm operation.
We simulate the ZGP OPA for a seed wavelength of 3.7 µm, corresponding to a DFG-wavelength at 4.7 µm. The parasitic OPO process is not taken into account in the simulations. It is found that the results are qualitatively similar to Fig. 5(a). Figure 8(a) shows the simulated quantum efficiency for a seed energy of 0.1 mJ. The maximum in quantum efficiency in Fig. 8(a) occurs at a somewhat lower pump energy than in Fig. 5(a). This is mainly due to the slightly higher nonlinear coupling constant in this case. The corresponding output in Fig. 8(b) is in reasonable agreement with the measurement in Fig. 8(c), despite the problems with the parasitic OPO process, which is also shown in the figure. Note that the blue curve in Fig. 8(c) is the sum of the signal and idler energy from the OPA process in the ZGP crystal, and does not include the beams from the parasitic process. The maximum 3–5 µm energy is approximately 33 mJ. The measured parasitic idler energy is shown for blocked seed, and at full seed power. The parasitic idler wavelength is 3.0 µm (the term “parasitic idler” here refers to the non-resonant parasitic beam), and the coating reflection for the parasitic signal at 6.8 µm is approximately 10% from each surface.
We observe that the energy of the parasitic idler is significantly reduced at full seed energy. The reason for this is that at full seed, some of the pump energy is consumed in the OPA process, leaving less energy for the parasitic OPO. On the other hand, when the parasitic OPO process takes place, the available pump energy for the OPA is reduced. It is clear from Fig. 8(c) that the effect of the parasitic OPO process on the OPA output is small at full seed power. We do however observe experimentally that the parasitic OPO process significantly reduces the OPA output at low seed energies, and for a 14 mm long ZGP crystal.
The beam qualities at 3–5 µm are measured in the same way as for the DFG beam at 8 µm. Figure 9 shows the measured beam fluence at 3.7 µm and 4.7 µm in the near field and the far field. The pump energy is 75 mJ, and the seed energy is 0.1 mJ. The resulting beam qualities are M 2=3.8 at 3.7 µm and M 2=2.0 at 4.7 µm. The corresponding simulated beam qualities are M 2=1.9 at both 3.7 µm and 4.7 µm.
The MOPA setup shows good conversion and reasonably good beam quality at both 3–5µmand 8 µm for a 10 mm long ZGP crystal and full seed power. However, the experiments show that there are parasitic OPO effects at 3–5 µm, in agreement with calculations based on Sellmeier equations for ZGP. It is difficult to design crystal coatings with low reflectance over a wide wavelength range, but a wedge-shaped crystal could be used to suppress the parasitic OPO.
Because the pump energy is limited by the damage threshold of the ZGP crystal, we are not able to exploit the full potential of the current setup. This could be solved by using ZGP crystals with larger aperture, or by using ZGP crystals with higher damage thresholds . An alternative solution is to use the remaining pump at 2.08 µm to pump a second ZGP OPA, where the DFG beam from the first ZGP OPA is used as seed.
The results for the ZGP MOPA are presented for DFG wavelengths of 4.7 µm and 8 µm. The ZGP MOPA could in principle also be tuned to shorter wavelengths than 4.7 µm, but then material absorption in KTA at the corresponding seed wavelength would decrease the performance of the KTA OPO. Similarly, material absorption in ZGP, which increases rapidly above 8.3 µm would reduce the output from the ZGP OPA above 8.3 µm.
The present setup has a number of advantages, compared to the setup in Ref. . Firstly, we here only use one ZGP crystal, compared to 3 for the MOPA setup presented in Ref. . Secondly, the DFG bandwidth is smaller than for the setup in Ref. . However, by using PPLN instead of KTA, we could in principle reduce the bandwidth at 4.7 µm in the present setup.
A source of high energy mid-infrared pulses, with reasonably good beam quality, is demonstrated. More than 8 mJ is obtained at 8 µm, and up to 33 mJ is obtained in the 3–5 µm region by using an appropriate length of the ZGP crystal.
We observe parasitic OPO operation in the 3–5 µm region due to an imperfect AR coating of the ZGP crystals, but these problems are reduced by maximizing the seed energy for the ZGP OPA. In future systems, such problems can be eliminated by use of wedged crystals.
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