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This is the accepted version of a paper published in Optical Materials Express. This paper has been peer-reviewed but does not include the final publisher proof-corrections or journal pagination.
Citation for the original published paper (version of record):
Coetzee, R S., Thilmann, N., Zukauskas, A., Canalias, C., Pasiskevicius, V. (2015)
Nanosecond laser induced damage thresholds in KTiOPO4 and Rb:KTiOPO4 at 1 µm and 2 µm.
Optical Materials Express, 5(9): 2090-2095 http://dx.doi.org/10.1364/OME.5.002090
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High-energy optical parametric amplifiers in the mid-infrared with large- aperture periodically poled Rb:KTiOPO
4Riaan Coetzee, Andrius Zukauskas, and Valdas Pasiskevicius
Department of Applied Physics, Royal Institute of Technology, Roslagstullsbacken 21, 106 91 Stockholm, Sweden.
High-energy nanosecond pulses in the mid-infrared and tunable over the spectral range of 7 µm – 10 µm are of great interest for remote detection of air pollution by potentially hazardous chemical species. Differential absorption LIDARs would typically require narrowband tunable nanosecond pulses with pulse energies of several millijoule in that spectral range. One possible approach is to employ two tandem frequency down-conversion stages using a high-energy Q-switched laser operating at 1.064 µm. The first down-conversion stage would operate close to degeneracy at 2.128 µm which would increase utilization of the pump and circumvent issues of remnant absorption in the second stage, often built using birefringence phase-matched ZGP crystals. Indeed, generation of about 6 ns-long pulses with energy of about 140 mJ at 2.1 µm with efficiency of 28% have been demonstrated in arrangement consisting of an optical parametric oscillator (OPO) followed by parametric amplifier (OPA) employing critically phase matched KTiOPO4 (KTP) [1]. We recently demonstrated a tandem setup, where the second ZGP OPO stage was pumped by degenerate OPA, employing periodically poled Rb:KTP (PPRb:KTP) and generating pulse energies of 25 mJ with efficiency of 43% [2]. Here we investigate nondegenerate OPA using large aperture PPRb:KTP crystals and seeded at 2.5 µm idler produced by a PPRb:KTP OPO. The temperature-tunable signal and idler generated by the OPA can then be used in the second difference frequency mixing (DFM) stage for tunable long-wavelength mid-infrared generation.
In both the OPO and OPA we used PPRb:KTP crystals with poling period of 38.52 µm which, for 1.064 µm pumping produces 1.84 µm signal and 2.52 µm idler radiation at room temperature. This would correspond to the DFM wavelength of 6.8 µm. By changing crystal temperature to 90C the signal and idler could be tuned to 1.94 µm and 2.35 µm, respectively (DFM wavelength of 11 µm). The crystals had homogeneous poling over optical aperture of 5 mm 7 mm along the crystal z- and y-axes, respectively. The length of the crystals along x- axis was 7 mm. The crystals were antireflection-coated for all interacting wavelengths. One such crystal was used in a singly resonant OPO to generate seed pulses at 2.5 µm, while the OPA could include up to three such crystals bringing the total interaction length in the OPA to 21 mm. The setup was pumped with 10 ns pulses at repetition rate of 100 Hz and the total energy budget of 150 mJ, of which, 20mJ was used for the OPO pumping.
The maximum pump fluence in the OPA was 3 J/cm2. The OPO generated up to 2.8 mJ of idler which was separated from the pump and signal and used for seeding the OPA. The measured FWHM bandwidth of the OPO idler was about 1.55 THz, narrower than the FWHM bandwidth of 2.5 THz in 7 mm-long OPA crystal, while somewhat larger than 1.44 THz bandwidth in 21-mm OPA. The measured OPA idler energy as a function of the OPA pump is shown in Fig.1 (a) where the 2.5 µm seed energy was fixed at 1.8 mJ. The overall extraction efficiency for the OPA with three PPRb:KTP crystals (signal and idler) slightly exceeded 40% delivering more than 52 mJ (5.2 W of average power) in the output for further use in the DFM stage. For the surface optical damage threshold of 10 J/cm2 in uncoated Rb:KTP crystals measured with 10 ns pulses at 1 µm [3], the pump energy could be increased to 300 mJ (provided that the beam profile is smooth enough) and adjusting pump beam radius to 2 mm. This would bring the pump fluence to 4.8 J/cm2 and for the same efficiency one could reduce the OPA length.
This brings a concomitant advantage of a larger OPA bandwidth which facilitates fast-tuning arrangements when a narrowband seed source is used.
References
[1] G. Arisholm, Ø. Nordsteth, G. Rustad, “Optical parametric master oscillator and power amplifier for efficient conversion of high-energy pulses with high beam quality,” Opt. Express, 12, 4189 (2004).
[2] G. Stoeppler, N. Thilmann, V. Pasiskevicius, A. Zukauskas, C. Canalias, and M. Eichhorn, “Tunable Mid-infrared ZnGeP2 RISTRA OPO pumped by periodically-poled Rb:KTP optical parametric master-oscillator power amplifier,” Opt. Express, 20, 4509 (2012).
[3]R. S. Coetzee, N. Thilmann, A. Zukauskas, C. Canalias, V. Pasiskevicius, “Nanosecond laser induced damage thresholds in KTiOPO4
and Rb:KTiOPO4 at 1 µm and 2 µm,” Opt. Materials Express, 5, 2090 (2015).
Fig.1. (a) The OPA idler energy as a function of pump for different amplifier crystal lengths. The maximum pump fluence is 3 J/cm2. (b) The total efficiency of the OPA seeded with 1.8 mJ of idler at 2.5 µm.