High-pulse-energy mid-infrared laser source based on optical parametric amplification in ZnGeP_2

High-pulse-energy mid-infrared lasersource based on optical parametric

amplification in ZnGeP2

Magnus W. Haakestad, Gunnar Arisholm, Espen Lippert,Stephane Nicolas, Gunnar Rustad, and Knut Stenersen

Norwegian Defence Research Establishment (FFI)P O Box 25, NO-2027 Kjeller, Norway

[email protected]

Abstract: Nonlinear optical conversion of 1.064 μm pulses from aQ-switched Nd:YAG laser to the mid-infrared is demonstrated. The ex-perimental setup is based on a two-stage master-oscillator/power-amplifier(MOPA) design with a KTiOPO4 based MOPA in the first stage and aKTiOAsO4/ZnGeP2 based MOPA in the second stage. The setup can betuned to provide output at 8 μm or in the 3–5 μm wavelength region. Weobtain more than 8 mJ at 8 μm, and up to 33 mJ at 3–5 μm. The measuredbeam quality factors are in the range M 2 =2–4 for both wavelength regions.

© 2008 Optical Society of America

OCIS codes: (140.3070) Infrared and far-infrared lasers; (140.3580) Lasers, solid state;(190.2620) Frequency conversion; (190.4410) Nonlinear optics, parametric processes;(190.4970) Parametric oscillators and amplifiers.

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optical parametric oscillator using a Cr,Er:YSGG laser to pump CdSe and ZnGeP2 crystals,” Opt. Lett. 22,597–599 (1997).

2. K. L. Vodopyanov, F. Ganikhanov, J. P. Maffetone, I. Zwieback, and W. Ruderman, “ZnGeP2 optical parametricoscillator with 3.8–12.4-μm tunability,” Opt. Lett. 25, 841–843 (2000).

3. P. A. Budni, C. R. Ibach, S. D. Setzler, L. A. Pomeranz, M. L. Lemons, P. A. Ketteridge, E. J. Gustafson, Y. E.Young, P. G. Schunemann, T. M. Pollak, R. T. Castro, and E. P. Chicklis, “20 mJ, 3-5 micron ZnGeP2 OpticalParametric Oscillator Pumped by a 2.09 micron Ho:YAG Laser,” in Advanced Solid-State Photonics. Paper PD12.

4. H. R. Lee, J. Yu, N. P. Barnes, and Y. Bai, “High pulse energy ZnGeP2 singly resonant OPO,” in AdvancedSolid-State Photonics, (2004) Paper TuC3.

5. P. B. Phua, K. S. Lai, R. F. Wu, and T. C. Chong, “Coupled tandem optical parametric oscillator (OPO): an OPOwithin an OPO,” Opt. Lett. 23, 1262–1264 (1998).

6. F. Ganikhanov, T. Caughey, and K. L. Vodopyanov, “Narrow-linewidth middle-infrared ZnGeP2 optical paramet-ric oscillator,” J. Opt. Soc. Am. B 18, 818–822 (2001).

7. S. Haidar, K. Miyamoto, and H. Ito, “Generation of tunable mid-IR (5.5–9.3 μm) from a 2-μm pumped ZnGeP2optical parametric oscillator,” Opt. Commun. 241, 173–178 (2004).

8. M. Henriksson, M. Tiihonen, V. Pasiskevicius, and F. Laurell, “ZGP Mid-Infrared Laser Source Pumped byNearly-Degenerate PPKTP Parametric Oscillator,” in Advanced Solid-State Photonics (2007), Paper TuB22.

9. A. Godard, “Infrared (2–12 μm) solid-state laser sources: a review,” C. R. Physique 8, 1100–1128 (2007).10. B. C. Johnson, V. J. Newell, J. B. Clark, and E. S. McPhee, “Narrow-bandwidth low-divergence optical paramet-

ric oscillator for nonlinear frequency-conversion applications,” J. Opt. Soc. Am. B 12, 2122–2127 (1995).11. Y. Ehrlich, S. Pearl, and S. Fastig, “High brightness tunable tandem optical parametric oscillator at 8-12 μm,” in

Advanced Solid-State Photonics (2004), Paper TuB15.12. A. Dergachev, D. Armstrong, A. Smith, T. Drake, and M. Dubois, “3.4-μm ZGP RISTRA nanosecond optical

parametric oscillator pumped by a 2.05-μm Ho:YLF MOPA system,” Opt. Express 15, 404–14,413 (2007).

#96734 - $15.00 USD Received 28 May 2008; accepted 11 Aug 2008; published 28 Aug 2008

(C) 2008 OSA 1 September 2008 / Vol. 16, No. 18 / OPTICS EXPRESS 14263

13. W. R. Bosenberg and D. R. Guyer, “Broadly tunable, single-frequency optical parametric frequency-conversionsystem,” J. Opt. Soc. Am. B 10, 1716–1722 (1993).

14. J. C. McCarthy, R. C. Day, and E. P. Chicklis, “Novel, Efficient, High Brightness KTP Optical Parametric Oscil-lator - Amplifier in single beamline,” in Advanced Solid State Lasers 2001 Technical Digest, pp. 656–659.

15. J. Saikawa, M. Fujii, H. Ishizuki, and T. Taira, “52 mJ narrow-bandwidth degenerated optical parametric systemwith a large-aperture periodically poled MgO:LiNbO3 device,” Opt. Lett. 31, 3149–3151 (2006).

16. J. Saikawa, M. Miyazaki, M. Fujii, H. Ishizuki, and T. Taira, “Tunable, narrow-bandwidth Mid-IR generation inZnGeP2 crystals pumped by a large aperture periodically poled Mg doped LiNbO3 optical parametric system,”in Advanced Solid-State Photonics (2008), Paper MC46.

17. G. Arisholm, Ø. Nordseth, and G. Rustad, “Optical parametric master oscillator and power amplifier for efficientconversion of high-energy pulses with high beam quality,” Opt. Express 12, 4189–4197 (2004).

18. G. Rustad, S. Nicolas, Ø. Nordseth, and G. Arisholm, “High pulse energy mid-infrared laser source,” Proc. ofSPIE 5989 (2005). Article no. 598904.

19. G. Mennerat and P. Kupecek, “High-energy narrow-linewidth tunable source in the mid infrared,” in AdvancedSolid State Lasers 1998 Technical Digest, pp. 269–272.

20. A. Dergachev, D. Armstrong, A. Smith, T. Drake, and M. Dubois, “High-power, high-energy ZGP OPA pumpedby a 2.05-μm Ho:YLF MOPA system,” Proc. of SPIE 6875, 2008. Article no. 687507.

21. G. Arisholm, “Quantum noise initiation and macroscopic fluctuations in optical parametric oscillators,” J. Opt.Soc. Am. B 16, 117–127 (1999).

22. G. Anstett, M. Nittmann, A. Borsutzky, and R. Wallenstein, “Experimental investigation and numerical simula-tion of the spatio-temporal dynamics of nanosecond pulses in Q-switched Nd:YAG lasers,” Appl. Phys. B 76,833–838 (2003).

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of single-crystal ZnGeP2,” J. Opt. Soc. Am. B 23, 2310–2316 (2006).

1. Introduction

High energy mid-infrared laser sources have several applications including spectroscopy andremote sensing. In order to cover a wide wavelength range, nonlinear optical conversion is com-monly employed. Zinc germanium phosphide (ZGP) is an attractive material for this purposedue to its high nonlinear coefficient, good transmission in the mid-infrared range, and highthermal 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 conversionfrom a shorter wavelength laser such as Nd:YAG [5–8].

Nonlinear optical conversion to the mid-infrared range has also been demonstrated usingother nonlinear crystals, such as CdSe, AgGaS2, AgGaSe2, and orientation patterned GaAs. Arecent review of results obtained using these materials, in addition to ZGP, is given in Ref. [9].

Optical parametric oscillators (OPOs) are the most common nonlinear conversion devices. Aproblem with pulsed OPOs is that conversion is not efficient until the signal power has growncomparable 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 avoidexceeding the damage threshold of the nonlinear crystal. This may lead to a resonator with ahigh 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-rotatingresonators [12]. An alternative solution is the master-oscillator/power-amplifier (MOPA) ap-proach [13–18]. Here, a master OPO is pumped with a low-energy, narrow, pump beam, tosuppress higher order transverse modes. The signal or idler from the master OPO is then usedas a seed in an optical parametric amplifier (OPA), where a large pump diameter does not nec-essarily 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 ap-proach is used in the present work, which aims at demonstrating high pulse energies combinedwith 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 OPOwith an unstable confocal resonator [11]. Mennerat and Kupecek obtained as much as 10 mJat 10.4 μm using difference-frequency generation (DFG) in a CdSe crystal [19], but the beamquality was not reported. Rustad et al. have obtained up to 28 mJ in the 3–5 μm range using aZGP OPO, but the beam quality was poor (M 2 > 100) [18]. However, they also obtained 21 mJin the 3–5 μm range with M2 ≈ 15 using a ZGP-based MOPA. Similar energy levels have alsobeen reported by Budni et al. and by Lee et al. using a ZGP OPO [3,4], but they do not state thebeam 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 [20].

In this paper, we demonstrate a two-stage MOPA setup where pulses from a Nd:YAG laserare converted to 2.08 μm in the first stage and to the mid-IR in the second stage. The first stageconsists of an OPO and an OPA, both based on KTP. The second stage consist of a KTA-basedOPO, which is also pumped at 1.06 μm, and a ZGP-based OPA, which is pumped by the 2 μmbeam from stage 1. The output wavelengths from the two-stage MOPA can be tuned to provideoutput in the important 3-5 μm and 8-12 μm atmospheric transmission bands. It is shown thatback-conversion affects the quantum efficiency and beam quality of the ZGP OPA, and thatboth the length of the ZGP crystal and the seed energy must be adjusted to achieve optimumperformance.

Using a ZGP OPA optimised for high conversion and good beam quality, we obtain morethan 8 mJ at 8 μm with a beam quality factor M 2 ≈ 3.6. By increasing the pump energy on theZGP crystal, we obtained as much as 10 mJ at 8 μm, but then the input surface of the crystal wasslightly 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 toimperfect AR-coatings for the ZGP crystals.

2. Experimental setup

The experimental setup is schematically shown in Fig. 1. The Q-switched Nd:YAG laser pro-vides linearly polarised, 6 ns pulses at 1.064 μm. It is injection seeded to operate in a singlelongitudinal mode. An optical isolator is used to prevent feedback into the laser, and the pulseenergy after the isolator is ∼ 500 mJ. The repetition rate is 10 Hz, and the beam quality isM2 ≈ 2.

Nd:YAG KTP OPO

KTA OPO

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Fig. 1. Schematic overview of the two-stage MOPA.

The 1.064 μm beam is split into three parts. One low-energy beam at 8 mJ is used to pump theKTP master OPO, and another low-energy beam at 8-10 mJ is used to pump the KTA masterOPO. Relay imaging telescopes are utilised to reduce the pump diameter (1/e 2) to 1.0 mmfor both OPOs while preserving the transverse distribution of the laser beam. The remainingavailable pulse energy at 1.064 μm is used to pump the KTP OPA, where the pump diameter



(1/e2) is set to 5.3 mm using a relay imaging telescope. The KTP MOPA is described in moredetail elsewhere [17]. It provides a∼ 140 mJ signal beam at 2.08 μm with M 2 ≈ 2 and a FWHMbandwidth of 2.3 nm. This beam is used as a pump in the second MOPA stage. In addition, theKTP MOPA provides about 80 mJ idler at 2.18 μm. However, a beam quality factor M 2 ≈ 2.7for 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 attractivenonlinear optical crystal due to its high damage threshold, and reasonably high nonlinear co-efficient. 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 config-uration, and two flat mirrors (see Fig. 3). The KTA crystals are cut at θ = 42.5 ◦ for type IIphase-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).

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Fig. 2. Measured output energy in the forward direction from the KTA OPO for (a) an idlerwavelength of 2.8 μm, and (b) an idler wavelength of 3.7 μm.

Measured in/out curves for the KTA OPO are shown in Fig. 2 for the two idler wavelengthsused in the experiments. During operation of the two-stage MOPA, we keep the pump energyof 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 ZGPOPA.

The measured beam quality of the idler is M 2 =2.3–2.4 for both idler wavelengths, and themeasured FWHM idler bandwidth is 2.0 nm for an idler wavelength of 2.8 μm and 21.6 nm foran 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 isexpanded using lenses L1 and L2 to make its diameter somewhat larger than that of the 2.08μm beam. A long-pass filter after the KTA OPO removes the signal at 1.5 μm/1.7 μm. Theidler at 2.8 μm/3.7 μm from the KTA OPO is made to overlap spatially and temporally withthe 2.08 μm beam from the KTP MOPA. The phase-matching conditions of the ZGP OPA aretype I.



2.08 µm (from KTP MOPA)

1.064 µm

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LP

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8 µm or 4.7 µmM3

Fig. 3. Schematic overview of the second MOPA stage. M1 and M2 form the resonator ofthe master oscillator (KTA OPO), L1 and L2 expand the seed beam for the ZGP OPA, along-pass filter (LP) removes the 1.7 μm or 1.5 μm beam, and M3 combines the seed andthe pump for the ZGP OPA.

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Fig. 4. Measured pump fluence at the ZGP crystal. The data is normalised to a unity peakvalue.

We limit the maximum pump energy in most of the experiments to approximately 75 mJ toavoid damaging the ZGP crystals. The 1.06 μm pump beam has a somewhat irregular fluencedistribution, which is transferred to the 2.08 μm beam. Therefore, a pump energy of 75 mJcorresponds to a peak fluence of approximately 0.6 J/cm 2. We are thus not able to use the fullavailable pump energy of ∼140 mJ at 2.08 μm. Figure 4 shows the measured pump fluencedistribution at the ZGP crystal.

Due to loss in optical components, the seed energy at the position of the ZGP crystal isapproximately 0.3 mJ. However, since the seed is somewhat larger than the pump, the amountof seed overlapping with the pump on the ZGP crystal is approximately 0.1 mJ.

3. Results

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 fortwo different ZGP crystals. The first ZGP crystal is 10 mm long and is cut at θ ≈ 53 ◦, and thesecond ZGP crystal is 14 mm long and is cut at θ ≈ 55◦. Both crystals have a 6 mm × 8 mmaperture.

We simulate the ZGP OPA using a model that can include all relevant effects, such as diffrac-tion, temporal and spatial walk-off, and absorption [21]. The model can also handle the complexspatio-temporal variation of the pump, but unfortunately we do not have such information forour pump laser. Detailed characterisation of a similarly designed laser (unstable resonator with



variable-reflectivity output coupler) has been reported [22], and the spatial field distributionwas found to be strongly time-dependent: It was approximately Gaussian in the beginning ofthe pulse and ring-shaped in the end, similar to a hollow cone [22]. Since we lack detailed in-formation on our laser we have used the measured pulse and fluence distribution and assumedthat the pump is separable in space and time. Although this cannot be expected to give accurateresults, the simulations can be useful as design guidelines. The relevant nonlinear coefficient inZGP is taken to be d36 = 75 pm/V.

The simulated quantum efficiency of the ZGP OPA is shown in Fig. 5(a) and (b) for a ZGPlength of 10 mm and 14 mm, respectively, and for different seed energies. We observe thata maximum quantum efficiency of 50% can be reached for the highest seed energy. We alsoobserve that there is back-conversion, i.e. that sum-frequency generation of the signal and idlerwaves transfers energy back to the pump. For a given seed energy, the back-conversion resultsin a maximum in the quantum efficiency at a certain pump energy. The peak conversion is seento decrease with decreasing seed energy. This means that the crystal length should be chosensuch 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 ZGPcrystal. We observe in Fig. 5(a) that a 10 mm long ZGP crystal should be optimal for themaximum 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 weplot the pulse energy at 8 μm instead of the quantum efficiency. In addition, we have taken intoaccount the measured coating reflectance at 8 μm of 2.3% and 13.2% for the 10 mm and 14 mmlong ZGP crystal, respectively. This is done to simplify comparison of the simulated results withthe 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 andsimulated in/out curves, despite the fact that the simulations were performed with a simplifiedmodel 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. Inan attempt to obtain an output of 10 mJ at 8 μm we increased the pump energy, thus increasingthe risk of damaging the crystal. The seed energy was ∼0.1 mJ in this case. We obtained 9.6mJ at 8 μm for a pump energy of about 90 mJ and 10 mJ at 108 mJ pump energy, as shownin Fig. 5(e). These results indicate a roll-off, as expected from Fig. 5(c). However, even thoughthe ZGP crystal was pumped at 108 mJ for only a couple of seconds, the input surface of thecrystal was slightly damaged.

The beam quality at 8 μm is measured by focusing the beam and measuring the second mo-ments at waist and in the far field using a pyroelectric camera. It is well-known that backgroundnoise may dominate the second moment integrals, since the second moments heavily weighsthe outer wings of the beam [23]. A correction is therefore done on the near field and far fieldfluence profiles to make sure that the average fluence of the background is zero. In addition, weuse circular apertures enclosing 99% of the energy in the near field and far field when deter-mining the second moments. Both corrections are done numerically on the raw data from thecamera.

For the 14 mm long ZGP crystal we estimate the beam quality to be M 2 ≈ 4.3 at a pumpenergy of 30 mJ and M2 ≈ 6.1 at 73 mJ, at full seed power. For the 10 mm long ZGP crys-tal, we obtain a beam quality of M2 ≈ 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 showsmeasured fluence at 8 μm for the 10 mm long ZGP crystal with 75 mJ pump energy and fullseed power.

We observe that the measured M2-values are higher than the simulated ones. This may beascribed to the time-dependence of the pump beam profile, which was not included in the sim-



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Fig. 5. Simulated and measured output from the ZGP OPA, for a DFG wavelength of 8μm. The data is shown for a 10 mm long ZGP crystal (left column) and for a 14 mm longZGP crystal (right column), for different seed energies. (a) Simulated quantum efficiency,10 mm ZGP. (b) Simulated quantum efficiency, 14 mm ZGP. (c) Simulated output at 8 μm,10 mm ZGP. (d) Simulated output at 8 μm, 14 mm ZGP. (e) Measured output at 8 μm, 10mm ZGP. (f) Measured output at 8 μm, 14 mm ZGP. A perfect AR coating for the ZGPcrystal is assumed in (a) and (b), while a measured coating reflection at 8 μm of 2.3% and13.2% is taken into account in (c) and (d), respectively.

ulations. However, it is clear from both the simulations and experiments that back-conversionreduces the beam quality. Physically, the nonuniform pump intensity means that the intense partof the pump is depleted before the low intensity parts are effectively converted. Increasing thegain to improve conversion in the low intensity parts means that one obtains back-conversion inthe high intensity parts [17]. This distorts the intensity distribution of the 8 μm beam, leadingto a poorer beam quality.



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Fig. 6. Measured beam fluence at 8 μm after an f = 244 mm lens. (a) Waist. (b) Corre-sponding far field. The data is normalised to a unity peak value.

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Fig. 7. Measured spectrum at 8 μm.

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 byangle-tuning the KTA OPO and the ZGP OPA. In order to optimise the performance at 3–5μm, we use a ZGP crystal with a coating reflectance of less than 2% at 3.7–4.7 μm. The cutangle of the crystal is θ ≈ 53◦, the length is 10 mm, and the aperture is 5 × 7 mm. This apertureis marginal for the present pump size, but we estimate that the clipping of the pump beam isless than 3%. In addition, the mirror M3 in Fig. 3 is changed to maximise its reflection at 3.7μm.

An advantage when tuning to this wavelength region is that both the amplified seed at 3.7 μmand 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 2.8μm, as seen in Fig. 2, the KTA OPO must be pumped at higher energy to produce the sameidler 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 wavelengthregion, compared to the 8 μm case. The first difference is that the coupling constant for the DFGprocess in the ZGP OPA is somewhat higher than at 8 μm, mainly due to the (λ seedλDFG)−1/2

factor in the nonlinear coupling constant [24].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 crystalact 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 signalremains normal to the crystal face, and its wavelength changes in fair agreement with phase-matching calculations based on the Sellmeier equations in Ref. [25]. Such calculations alsoshow that our crystal does not support parasitic OPO effects when it is tuned for 8 μm operation.

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OPA output, 3−5 μmParasite, seed blockedParasite, full seed

Fig. 8. Simulated and measured output from the ZGP OPA in the 3–5 μm region. The data isshown for a 10 mm long ZGP crystal. (a) Simulated quantum efficiency. (b) Correspondingoutput at 3-5 μm. (c) Measured output at 3-5 μm at full seed power, parasitic idler whenthe seed is blocked, and parasitic idler at full seed power. A perfect AR coating for the ZGPcrystal is assumed in (a) and (b).

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 simu-lated quantum efficiency for a seed energy of 0.1 mJ. The maximum in quantum efficiency inFig. 8(a) occurs at a somewhat lower pump energy than in Fig. 5(a). This is mainly due to theslightly 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 theparasitic OPO process, which is also shown in the figure. Note that the blue curve in Fig. 8(c) isthe sum of the signal and idler energy from the OPA process in the ZGP crystal, and does not in-clude the beams from the parasitic process. The maximum 3–5 μm energy is approximately 33mJ. The measured parasitic idler energy is shown for blocked seed, and at full seed power. Theparasitic idler wavelength is 3.0 μm (the term “parasitic idler” here refers to the non-resonantparasitic beam), and the coating reflection for the parasitic signal at 6.8 μm is approximately10% 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 OPOprocess 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 theOPA output at low seed energies, and for a 14 mm long ZGP crystal.

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Fig. 9. Measured beam fluence at 3.7 μm and 4.7 μm after an f = 518 mm and f = 408mm lens, respectively. (a) Waist, 3.7 μm beam. (b) Corresponding far field. (c) Waist, 4.7μm beam. (d) Corresponding far field. The data is normalised to a unity peak value.

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 farfield. The pump energy is 75 mJ, and the seed energy is 0.1 mJ. The resulting beam qualitiesare M2 = 3.8 at 3.7 μm and M2=2.0 at 4.7 μm. The corresponding simulated beam qualitiesare M2 = 1.9 at both 3.7 μm and 4.7 μm.

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Fig. 10. Measured spectrum at (a) 3.7 μm and (b) 4.7 μm.



The measured spectrum at 3–5 μm from the ZGP OPA is shown in Fig. 10. The spectrum inFig. 10(a) corresponds well to the measured seed spectrum from the KTA OPO.

4. Discussion

The MOPA setup shows good conversion and reasonably good beam quality at both 3–5 μm and8 μm for a 10 mm long ZGP crystal and full seed power. However, the experiments show thatthere are parasitic OPO effects at 3–5 μm, in agreement with calculations based on Sellmeierequations for ZGP. It is difficult to design crystal coatings with low reflectance over a widewavelength 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 arenot able to exploit the full potential of the current setup. This could be solved by using ZGPcrystals with larger aperture, or by using ZGP crystals with higher damage thresholds [26].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, butthen material absorption in KTA at the corresponding seed wavelength would decrease theperformance of the KTA OPO. Similarly, material absorption in ZGP, which increases rapidlyabove 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. [18]. Firstly,we here only use one ZGP crystal, compared to 3 for the MOPA setup presented in Ref. [18].Secondly, the DFG bandwidth is smaller than for the setup in Ref. [18]. However, by usingPPLN instead of KTA, we could in principle reduce the bandwidth at 4.7 μm in the presentsetup.

5. Conclusion

A source of high energy mid-infrared pulses, with reasonably good beam quality, is demon-strated. More than 8 mJ is obtained at 8 μm, and up to 33 mJ is obtained in the 3–5 μm regionby 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 ofthe ZGP crystals, but these problems are reduced by maximizing the seed energy for the ZGPOPA. In future systems, such problems can be eliminated by use of wedged crystals.



High-pulse-energy mid-infrared laser source based on optical parametric amplification in ZnGeP_2

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