May 15, 2001 - Received October 20, 2000. Narrow-linewidth optical pulses at wavelengths near 630 nm with 2.2-mJ energy were generated with 61%.
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High-efficiency parametric oscillation and spectral control in the red spectral region with periodically poled KTiOPO4 V. Pasiskevicius, H. Karlsson, and F. Laurell Department of Physics, Royal Institute of Technology, Stockholm, Sweden
R. Butkus, V. Smilgevicius, and A. Piskarskas Department of Quantum Electronics, Vilnius University, Vilnius, Lithuania Received October 20, 2000 Narrow-linewidth optical pulses at wavelengths near 630 nm with 2.2-mJ energy were generated with 61% efficiency in a periodically poled KTiOPO4 parametric oscillator pumped by a frequency-doubled Q-switched Nd:YAG laser. The tuning range was extended to 30 nm by a noncollinear elliptical pumping geometry. We demonstrate that by angular dispersion a noncollinear optical parametric oscillator can be used to control the spectral and spatial characteristics of the output signal beam. © 2001 Optical Society of America OCIS codes: 190.4410, 190.4970, 160.4330.
Laser radiation in the red spectral region is important in such f ields as laser displays, biology, and medicine. Many applications in these fields require high-energy and, preferentially, tunable red laser radiation. A potentially eff icient strategy for the generation of red light is the use of parametric downconversion of frequency-doubled radiation from widely available Nd31 -doped lasers. Typically, b-barium borate or lithium triborate crystals were used in optical parametric oscillators (OPOs) and optical parametric generators (OPGs) pumped by the third harmonic of a Nd31 :YAG laser to generate visible signal light with a conversion eff iciency exceeding 30%.1,2 However, because of an additional sum-frequency mixing step that is required, the overall conversion efficiency from the fundamental infrared radiation is less than 10%, and, typically, demonstrating this efficiency requires fundamental pulses with an energy of hundreds of millijoules. Periodically poled (PP) crystals, e.g., PP lithium niobate and PP potassium titanyl phosphate (KTP), because of their high optical nonlinearity and the possibility of quasi-phase matching over an entire transparency region, can be used in the red OPO and OPG devices pumped directly by the second harmonic of a Nd31 :YAG laser. Because of the high nonlinearity of these crystals, eff icient oscillation can be obtained at relatively low pump pulse energies. For instance, an OPG with an efficiency of 23% at a pump energy of 33 mJ with a 55-mm-long PP lithium niobate crystal has been demonstrated.3 Here we investigate the performance of PP KTP in a visible OPO pumped at 532 nm and generating tunable radiation in the red spectral region. PP KTP has good optical power-handling capabilities at room temperature when it is used in infrared OPOs4 as well as for frequency doubling of Nd:YAG, where millijoule-level pulses have been generated at 532 nm.5 Moreover, PP KTP has a relatively high effective nonlinearity, deff 艐 10 pm兾V,6 facilitating high-efficiency parametric conversion in short crystal lengths. Here we also investigate the possibility of extending the tuning range of the red OPO by using noncollinear interaction with an elliptical pumping beam. 0146-9592/01/100710-03$15.00/0
The PP KTP crystal that we used was fabricated from a f lux-grown KTP wafer by a standard electric field poling technique.7 A domain inversion period of 12.77 mm was chosen to generate signal wavelengths near 650 nm in the OPO pumped at 532 nm. The crystal was 8 mm long and 1 mm thick, and the width of the domain inversion region was 5 mm. The crystal was polished and antiref lection coated at the pump and signal wavelengths. The coating had a residual 3% ref lectivity in the idler region. The PP KTP crystal was mounted upon a temperature-controlled holder and could be heated to 170 ±C. An actively Q-switched and frequency-doubled Nd:YAG laser (Microlase, New Wave Research) generating 5-ns pulses at a 20-Hz repetition rate was used as the pump source. A value of M 2 艐 8 was measured for the pump beam. The pump beam was focused with an f 苷 500-mm lens to a beam waist radius of 270 mm with the intensity prof ile shown in the inset of Fig. 1(a). The 15-mm-long OPO cavity was completed by two f lat mirrors: an input mirror with ref lectivity Rs 苷 98% and an experimentally selected output mirror with Rs 苷 18% for the signal and high transmission for the idler. The pump ref lectivity of the output mirror was 90%, which resulted in better utilization of the pump in two passes through the PP KTP crystal. Although the pump ref lectivity of the input mirror was 50%, the pump buildup in the OPO cavity was not observed because of the short coherence time (10 ps) of the pump and passive losses. Thus, here for the pump powers we take into account the incoupling losses. The OPO reached oscillation threshold at the pump energy of 120 mJ, and the signal-generation efficiency at room temperature saturated at 50% at a pump pulse energy of 0.5 mJ, as shown in Fig. 1(a). The signal’s dependence on the pump energy was linear, and at the pump energy of 3.5 mJ the OPO produced 1.79 mJ of signal radiation at 652-nm wavelength. By using a coated Ge filter we found that the idler generated at 2.89 mm constituted less than 3% of the maximum OPO output in the forward direction. Most of the idler power that was generated was absorbed in the output coupler made from BK7 glass. © 2001 Optical Society of America
May 15, 2001 / Vol. 26, No. 10 / OPTICS LETTERS
Fig. 1. (a) OPO signal energy (open circles) and signal eff iciency (f illed squares) at room temperature. Inset, the CCD image of the pump beam at focus. (b) OPO signal generation eff iciency as a function of PP KTP temperature.
Previously reported experimental data8 suggest that high-peak-power visible-light generation at room temperature induces absorbing color centers in KTP by means of two-photon absorption of the pump and signal light. Absorption by the color centers should decrease the efficiency of the OPO. Indeed, we observed a weak gray tracking of the PP KTP sample. To assess the inf luence of the color centers of the OPO performance we measured the temperature dependence of the signal-generation efficiency at the pump pulse energy of 3.7 mJ. As shown in Fig. 1(b), the efficiency remained approximately unchanged up to 70 ±C, but it then started to increase in a linear fashion. At the PP KTP temperature of 160 ±C the signal-generation eff iciency reached 61% and the OPO produced 2.2 mJ of red light. This thresholdlike behavior suggests that at temperatures above 70 ±C the creation of color centers and thermal annealing compete to produce a new steady state with a lower concentration of absorbing centers. We could tune the OPO signal in the usual way by changing the temperature of the PP KTP crystal. At a PP KTP temperature increase from 15 to 170 ±C, the signal wavelength was tuned only 10 nm, from 652 to 642 nm, as shown in Fig. 2 by open circles. The inset of Fig. 2 shows signal spectra taken at three different temperatures. The FWHM width of the signal spectra was approximately 0.3 nm 共7 cm21 兲. The width of the spectrum is very close to the expected theoretical value of 0.34 nm.9 To increase the signal-tuning range we investigated a noncollinear OPO conf iguration.10 Here we used an R 苷 50% output coupler
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because of increased oscillation thresholds. We simultaneously tilted the OPO cavity mirrors to achieve oscillation at a specific angle with respect to the pump beam while the position of the PP KTP crystal and the direction of the pump were kept constant. Initially, we used a circularly symmetric pump beam with a beam waist of 270 mm. We could tune the OPO signal at room temperature to 631 nm by tilting the OPO cavity axis by 40 mrad, as shown in Fig. 2 (filled squares). The tuning range in the noncollinear configuration is limited by the lateral separation of the interacting beams inside the nonlinear crystal, and, as a consequence, by decreasing effective interaction length, which increases threshold power. By appropriately shaping the pump beam one should have access to larger noncollinear angles and be able to widen the OPO signal-tuning range further. We employed a cylindrical telescope to form a 2-mm-wide collimated beam in the plane perpendicular to the PP KTP z axis; a cylindrical lens was used to focus the beam to a waist radius of approximately 300 mm in the plane perpendicular to the crystal y axis. As shown in Fig. 3, the increase in OPO threshold with noncollinear angle was much weaker in the case of an elliptical beam, which enables us to extend the OPO signal tuning range to 30 nm; i.e., the range was three times larger than what could be realized with collinear temperature tuning. The measured noncollinear angle tuning characteristics correspond fairly well to the dependence calculated with the Sellmeir expansion from Ref. 11 (solid curve in Fig. 2). Eventually, the tuning range was limited by the idler beam walk-off from the parametric gain channel, whose phase-matching angle, according to our calculations, was 174 mrad at the maximum signal noncollinear angle of 48 mrad. This extended tuning range can be achieved at the price of lower signal-generation efficiency owing to decreased overlap between the interacting beams. However, the noncollinear OPO with elliptical pumping still generated 0.78-mJ pulses with an eff iciency of 19.5%.
Fig. 2. OPO signal tuning: open circles, temperature tuning of a collinear OPO; f illed squares, room-temperature noncollinear angle tuning for the OPO pumped with a circular beam; open squares, room-temperature noncollinear angle tuning for the OPO pumped with an elliptical beam. Solid curve, calculated tuning characteristic. Inset, collinear OPO signal spectra at different temperatures.
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Fig. 3. Pump energy density at threshold for a noncollinear OPO pumped with a circular beam (open circles) and an elliptical beam (f illed squares).
signal beam divergence in the scanning direction was ⬃2.1 mrad, and the beam had an almost linear spatial chirp of dfPP KTP 兾dl 苷 1.1 mrad nm21 . The linear spatial chirp in the far field can be compensated for by a diffraction grating. We employed a 1200-line mm21 diffraction grating whose spatial dispersion for the first diffraction order at the diffraction angle of 35± is dfg 兾dl 苷21.46 mrad nm21 . An angular scan with the same slit aperture (filled circles in Fig. 4) revealed that the spatial chirp had been virtually completely compensated for and that, at the same time, the signal beam divergence was reduced to ⬃0.75 mrad, effectively reducing the M 2 value in this plane by three times. In conclusion, a visible OPO in the red spectral region with 61% eff iciency and as much as 2.2-mJ signal pulse energies has been demonstrated, with 8-mm-long PP KTP pumped at 532 nm and operated at 160 ±C. Because of absorption by induced color centers, the OPO signal eff iciency decreased to 50% at room temperature, but otherwise the operation remained stable. We used the noncollinear OPO with elliptical pumping to achieve a tunability range three times broader than could be realized with temperature tuning. This noncollinear pumping geometry also produces almost linear spatial chirp across the signal beam, which can be compensated for by a diffraction grating and at the same time substantially increase brightness of the OPO signal.
Fig. 4. Far-f ield scan of the noncollinear OPO signal beam: open squares, without spatial compression; filled circles, compressed with a diffraction grating. Error bars show the FWHM spectral bandwidth.
V. M. Pasiskevicius’s optics.kth.se.
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One can also use the angular dispersion that enhances the tunability of the OPO signal to manipulate the spectral bandwidth. The bandwidth of noncollinear OPO is determined by both the noncollinear angle and the divergence of the OPO cavity mode. For instance, from Fig. 2 it is evident that the signal wavelength of 643 nm can be reached either by temperature tuning (140 ±C) or at the noncollinear angle of 25 mrad. In the first case, the OPO bandwidth is 0.3 nm; in the noncollinear case it will increase to ⬃2 nm, considering that the experimentally measured OPO mode divergence is 2.1 mrad. By using an elliptical pumping beam we could reach the largest noncollinear angles and thus obtain the broadest signal bandwidths of 6 nm at the noncollinear angle of 50 mrad. This marks a 203 increase in the signal bandwidth over the collinear OPO. Another consequence of the angular dispersion is the spatial chirp of the generated noncollinear OPO beam. To measure the spatial chirp, we collimated the elliptical OPO signal beam along the fast axis (parallel to the crystal z axis) while the beam was scanned in the direction parallel to the PP KTP y axis by a 0.2-mm slit aperture in the far f ield, 3.4 m away from the OPO cavity. The measured signal wavelength as a function of the noncollinear angle near a 35-mrad median is shown in Fig. 4 by open squares. The error bars indicate the FWHM bandwidth at each measurement point. The
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