Tunable Optical Parametric Oscillators Pumped by Ti:sapphire Lasers

H.H. Zenzie and P.F. Moulton

Schwartz Electro-Optics (now Q-Peak, Inc.)

Research Division

45 Winthrop Street

Concord, MA 01742

We report on the use of tunable, pulsed Ti:sapphire lasers as efficient pump sources for optical parametric oscillators. Using a 90-degree-phase-matched KTP crystal, we have generated output from 1030-1280 nm (signal) and 2180-3030 nm (idler) by tuning the pump laser from 700-900 nm, with 49 mJ of combined signal and idler outputs at a pump level of 110 mJ. In addition, we have demonstrated degenerate OPO operation with a KNbO3 crystal over the pump range 720-818 nm, and observed 44% conversion efficiency.

Ti:sapphire lasers pumped by frequency-doubled, Q-switched Nd:YAG lasers can be configured to produce a single, high-peak-power pulse. With the use of unstable resonators, the pulsed energy can be generated in a nearly diffraction-limited mode, suitable for driving a variety of nonlinear processes. In this letter, we report on the use of a tunable, nanosecond-pulse Ti:sapphire laser as a pump source for an optical parametric oscillator (OPO) [1] In contrast to other common OPO pump sources, such as fundamental and harmonic wavelengths of Q-switched neodymium-doped lasers, the Ti:sapphire pump source is broadly tunable, which allows additional flexibilty in the operation of the OPO. In this paper we describe OPOs based on two nonlinear crystals, K(TiO)PO4 (KTP) and KNbO3.

The first tunable-laser pumping of an OPO was demonstrated by Wallace [2], who used a rhodamine 6G, pulsed dye laser combined with the nonlinear material LiNbO3. One attractive feature demonstrated in the initial work was the ability to operate the OPO with non-critical phasematching (NCPM) over a wide span of signal and idler wavelengths. Recently, Kato [3] used a Nd:YAG-pumped dye laser to drive a NCPM KTP OPO, and was able to obtain signal and idler outputs over the range 1040-1380 and 2150-3090 nm, respectively, by using 5 different styryl dyes and tuning the pump laser from 700-950 nm. In the work of Kato a maximum of 23% of the pump energy was converted to the combined signal and idler outputs, for a total OPO output energy of 15 mJ.

The Ti:sapphire pump lasers we used in our experiments employed graded-reflectivity-mirror (GRM) unstable resonators and were in turn pumped by multi-mode, frequency-doubled, Q-switched Nd:YAG oscillator-amplifier systems operating at 10 Hz. In addition to the GRM and a high reflector, the unstable resonators contained two Brewster-cut, Ti:sapphire crystals for dispersion compensation and four prisms for line-narrowing (to ~1 nm) and tuning. We have previously reported on design details of these resonators [4]. The laser we used with the KTP OPO was capable of emitting up to 500 mJ at 800 nm in a gain-switched pulse of approximately 20-nsec duration. For the KNbO3 OPO we employed a smaller, commercial system (SEO Titan-P) capable of 100 mJ of output. The near-field spatial profile of the high-energy system output beam was Gaussian with a diameter of 3-mm (1/e2) and was estimated to be 1.5x the diffraction limit. The lower-energy system had a smaller Gaussian-profile beam, with a 1/e2 full-width of 2.5 mm.

The NCPM KTP OPO consisted of a 0.5x0.5x1.5 cm3, anti-reflection (AR)-coated (1050-1250-nm) flux-grown KTP crystal placed between two plane parallel mirrors spaced 3.5-cm apart. The input mirror of the OPO cavity, placed 25 cm from the pump-laser output mirror with no focusing optics, was coated for high transmission (92% T at 760 nm) at the pump wavelength (700-900 nm) and high reflection at the signal wavelength (1030-1280 nm). The output mirror, which was partially transmitting at the signal wavelength, was coated to highly reflect the pump wavelength in order to reduce the OPO threshold by double-passing the pump beam through the crystal. In this Type-I configuration, the beams propagated down the x axis, the pump and signal were polarized along the y axis and the idler was polarized parallel to the z axis, where nz > ny > nx for the index ellipsoid.

We generated the curve shown in Figure 1 by tuning the Ti:sapphire laser from 700 to 900 nm and measuring the second harmonic of the signal wavelength on an optical multi-channel analyzer (OMA); two sets of OPO cavity mirrors were required to cover the range. A 1-mm-thick KD*P doubler converted the near-IR signal output to the visible, so that the silicon-detector OMA could respond to the signal. We measured the pump wavelength by directing <1 mJ from the Ti:sapphire pump laser into a Burleigh pulsed wavemeter. The signal wavelengths were 7-10 nm shorter than predicted by Kato [3], a difference that may be the result of slight index differences in the KTP crystals. (The discrepancy is not the result of an error in crystal orientation away from 90 degrees, as that would lead to longer, rather than shorter signal wavelengths.) The signal wavelength range of 1030-1280 nm corresponds to an idler tuning range of 2180-3030 nm.

Fig. 1. Measured KTP OPO signal wavelength as a function of Ti:sapphire pump wavelength (data points with solid line) compared to theory (dotted line).

The energy input-output relation at a pump wavelength of 760 nm is shown in Figure 2, where the total (signal and idler) output energy is plotted as a function of pump energy input to the cavity. At the 760-nm pump wavelength, the output mirror reflected 31% at the signal wavelength of 1089 nm. The slope efficiency was 55% (60% with input-mirror losses accounted for) with a maximum energy of 49 mJ (signal and idler) produced at a pump input of 110 mJ. The maximum pump energy was self-imposed rather than damage-limited, and operation at higher outputs may be possible for the same beam size.

Fig. 2. Total KTP and KNbO3 OPO output energies as a function of pump energy. The pump, signal and idler wavelengths were 760, 1089 and 2515 nm, respectively for the KTP device, while the KNbO3 OPO was set for degenerate operation at a pump wavelength of 787 nm.

We documented the temporal behavior of the KTP OPO operating at 1089 nm by using a Tektronix 7912 digitizer and an Electro-Optics Technology ET2000 fast Si photodetector. Figure 3 shows the depleted pump (solid line) and the OPO signal output (dashed line) at a pump-energy input of 90 mJ. The OPO threshold was exceeded 5 nsec after the start of the pump pulse, as indicated by the rapid rise of the signal pulse. The pulsewidth of the signal was 18 nsec.

Fig. 3. KTP OPO pump (solid) and signal waveforms (dashed) at 90 mJ of pump energy. The pump and signal wavelengths were 760 and 1089 nm, respectively.

The KNbO3 OPO utilized a 1-cm-long crystal, with transverse dimensions 0.3x0.5 cm. In the notation for the index ellipsoid in which nz > ny > nx with x = c, y = a and z = b [5], the crystal was cut for propagation in the xz plane; Type I phasematching occurs with the signal and idler polarizations parallel to the y axis and the pump polarization contained in the xz plane. Guyer et al. [6] have shown that phasematching in this orientation is at least an order-of-magnitude less temperature sensitive than orientations used for frequency doubling into the blue wavelength region. In contrast to the case for KTP, NCPM cannot be obtained for Ti:sapphire pump wavelengths with this crystal geometry. Previous Type-I KNbO3 OPOs have been operated with 532-nm pumping and propagation along the z direction, where NCPM is possible at elevated temperatures [7, 8].

We chose to operate the KNbO3 OPO at or close to the degenerate point. Based on the Sellmeir coefficients of Zysset et al. [9], we calculated that the phase-matching angle for degenerate operation at a pump wavelength of 770 nm should be at 51.6 degrees to the x(c) axis. We had the crystal fabricated for this orientation and AR-coated for the 1400-1800-nm wavelength region. The OPO cavity, located 50 cm from the pump-laser output mirror, consisted of two flat mirrors spaced 1.8 cm apart, with the input mirror coated for high transmission (87% at 790 nm) at the pump wavelengths and high reflectivity at the signal and idler wavelengths. The output coupler had moderate reflectivity for the signal and idler beams, 40% R at 1540 nm, and only low reflectivity for the pump, 15% R at 790 nm. As with the KTP OPO, we used the direct output of the Ti:sapphire laser for pumping.

In Figure 2 we show the total KNbO3 OPO energy output as a function of pump energy presented to the input mirror, for a 787-nm pump wavelength. Using a constant pump energy of 40 mJ and adjusting the crystal angle for degenerate operation, we tuned the pump wavelength from 764 to 818 nm, and the total output energy varied from 12.5 to 10.5 mJ. At the 720-nm pump wavelength OPO operation at 40 mJ of pump was observed, but was terminated after a short period by damage to the pump-input, coated face of the crystal, possibly related to a resonance of the signal and idler with a peak in water absorption at 1440 nm. We examined the spectral behavior of the device pumped at 787 nm by frequency doubling the output, as described above for the KTP OPO, and observing the resultant spectrum on the OMA. Slightly away from degeneracy the signal had a spectral width of 16 nm, but the width increased to at least 80 nm for degenerate operation, where the measurement was likely limited by the spectral acceptance of the doubling crystal.

Correcting for the transmission loss of the KNbO3 OPO input mirror, we compute a slope efficiency at 787 nm of 44% for conversion of pump to combined signal and idler energies centered at 1574 nm. The calculated phase-matching angle monotonically increases from 53.6 to 50.2 degrees as the pump wavelength increases from 720 to 818 nm, in accord with the experimental result that OPO operation over the pump tuning range was observed at or close to normal incidence on the crystal.

We applied the pulsed OPO threshold theory of Brosnan and Byer [10] to the KTP and KNbO3 devices to compare observed and calculated thresholds. The case of pulsed degenerate (or near-degenerate) operation of the experimental KNbO3 system is more complex than the singly resonant OPO considered in [10], but we were interested in the predicted singly resonant threshold to estimate the level of enhancement afforded by double resonance. Our experimental thresholds were derived by least-squares fitting of the data in Figure 2 to a straight line and then determining the intercepts. Table I lists the key parameters we used in the calculations, following the notation used in Equations 17 and 27 in [10]. In order to calculate the spatial mode coupling coefficient, gs, we solved Equation A-5 in [10] to determine the gain-guided mode radius for the signal, ws, which we show in the Table. We derived our value for the effective nonlinearity, deff by the application of Miller's delta rule [11], where the base value of d24 for KTP is 3.92 pm/V for second-harmonic generation at 880 nm [12] and the base value for |d31| of KNbO3 is 15.8 pm/V for the same process at 1064 nm [8]. In the angle-matched KNbO3 OPO, the value for deff is d31sinq , where q is the angle from the x axis. The walk-off angle in the KNbO3 crystal is 19 mrad, and for the pump beam size we used the effective interaction length, L, is the essentially the same as the crystal length. For both crystals we set absorption and scattering losses to zero. We corrected the theoretical threshold values to account for partial pump transmission through the OPO cavity mirror. The agreement between data and theory for KTP indicates that the latest values for d24 [11] are more valid than those from earlier estimations. If we assume the published values of d31 are appropriate, then the reduction in threshold afforded by doubly resonant operation of the KNbO3 system is not significant.

The high OPO conversion efficiencies, damage-free operation and good agreement between observed and predicted thresholds confirm the good beam quality of the unstable-resonator Ti:sapphire pump laser. These results are consistent with previous experiments [12] that demonstrated high conversion efficiencies (>60%) for second-harmonic generation in b -BaB2O4 (BBO) crystals, which have limited angular acceptance. For the KTP system we maintained damage-free operation up to 5.5x threshold and expect that even higher energies could be obtained by an increase in the pump-beam size. The 1280 to 2180-nm "gap" in the Ti:sapphire-pumped KTP tuning range can be filled in by resorting to critical phase matching. For example, operation at an angle of 55 degrees results in wavelength coverage from 1320-2000 nm for pump tuning from 700-900 nm. In addition, degenerate operation over the same pump region is obtained by the use of crystal angles ranging from 57 to 51 degrees. A major limit to KTP is the onset of crystal absorption at wavelengths beyond 3000 nm [14]. The KNbO3 OPO performance was limited by coating failure, and may be improved in the future with advances in coating techniques. A major advantage of KNbO3 is high transparency out to beyond 4000 nm, and with the appropriate choice of angle one could use a Ti:sapphire-pumped KNbO3 OPO to generate energy out in this region.

We gratefully acknowledge the support of Gregory Mizell at Virgo Optics (Port Richey, FL), who supplied the KNbO3 crystal used in our experiments.

1. We use the phrase "optical" parametric oscillator by tradition, even though none of the wavelengths involved are visible.

2. R.W. Wallace, IEEE J. Quantum Electron. QE-8, 819 (1972).

3. K. Kato and M. Masutani, Opt. Lett. 17, 178 (1992).

4. G. Rines and P.F. Moulton, Opt. Lett. 15, 434 (1990).

5. The nomenclature we use is consistent with refs. 6- 9 but is not the standard proposed in D.A. Roberts, IEEE J. Quantum Electron. 28, 2057 (1992). For a discussion of the standard with relation to KNbO3, see W.R. Bosenberg and R.H. Jarman, Opt. Lett. 18, 1323 (1993).

6. D.R. Guyer, W.R. Bosenberg and F.D. Braun, Proc. Soc. Photo-Opt. Instrum. Eng. 1409, 14 (1991).

7. K. Kato, IEEE J. Quantum Electron. QE-18, 451 (1982).

8. I. Biaggio, P. Kerkoc, L.-S. Wu, P. Günter and B. Zysset, J. Opt. Soc. Am. B9, 507 (1992).

9. B. Zysset, I. Biaggio and P. Günter, J. Opt. Soc. Am. B9, 380 (1992).

10. S.J. Brosnan and R.L. Byer, IEEE J. Quantum Electron. QE-15, 415 (1979).

11. R.C. Miller, Appl. Phys. Lett., 5, 17 (1964).

12. H. Vanherzeele and J.D. Bierlein, Opt. Lett. 17, 982 (1992).

13. G.A. Rines, H.H. Zenzie and P.F. Moulton, "Recent advances in Ti:Al2O3 unstable-resonator lasers," in Advanced Solid State Lasers, 1991, Technical Digest Series (Optical Society of America, Washington, DC 1991), pp. 64-65.

14. K. Kato, IEEE J. Quantum Electron. 27, 1137 (1991).

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