We report the first realization of widely tunable continuous-wave (cw) optical parametric oscillator (OPO) based on periodically-poled KTiOPO4 (PPKTP) at room temperature. By exploiting fan-out grating design in a 30-mm PPKTP crystal, and configured in an output-coupled singly-resonant oscillator (OC-SRO) configuration pumped at 532 nm in the green, the OPO provides finely tunable radiation across 741-922 nm in the signal and 1258-1884 nm in the idler, at a fixed temperature of 25 °C. The use of output coupling for the signal wave enables enhancement of OPO extraction efficiency to 30%, providing a maximum total output power of 1.65 W (450 mW of signal at 901 nm and 1.2 W of idler at 1299 nm) for 5.5 W of pump power. The output idler exhibits passive power stability better than 3.2% rms over >2 mins, and the extracted signal exhibits frequency stability of 194 MHz over more than 35 seconds, in excellent beam quality. The OPO performance in pure SRO configuration has also been investigated.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Continuous-wave (cw) optical parametric oscillators (OPOs) capable of providing widely tunable radiation in the near-infrared (near-IR) in single-frequency output with good beam quality are of great importance for many applications including spectroscopy and trace gas sensing . Such OPOs, typically pumped in the green, are now recognized as viable sources for such radiation. Given the small parametric gain available under low pump intensities in the cw regime, to exploit the highest nonlinear coefficient in the nonlinear optical crystal, practical cw OPOs are exclusively based on quasi-phase-matched (QPM) nonlinear materials with long interaction lengths under noncritical phase-matching. The QPM materials which have been explored in the past in green-pumped cw OPOs include periodically-poled LiNbO3 (PPLN), LiTaO3 (PPLT), KTiOPO4 (PPKTP), as well as MgO-doped PPLN (MgO:PPLN) and stoichiometric PPLT (MgO:sPPLT) . To date, the vast majority of cw OPOs pumped at 1 μm have been based on PPLN, which are capable of providing spectral output up to ~4 μm . However, given the photorefractive damage induced by the visible pump or signal radiation below 1 μm, the development of green-pumped OPO based on PPLN is challenging. As such, there has been an ongoing quest for the exploitation of alternative nonlinear materials for the realization of viable cw OPOs in the visible and near-IR pumped in the green. By using a frequency-doubled Nd:YAG laser at 532 nm, a pump-enhanced PPLT cw OPO was previously reported , but attainment of high output powers was hampered by photorefractive damage in the crystal under green pumping. In PPLN and PPLT, doping with MgO has reduced the photorefractive susceptibility to visible radiation, thus enabling high power generation with wide wavelength tunability using green-pumped cw OPOs [5,6]. With advances in QPM fabrication technology, cw OPO based on MgO:sPPLT in fan-out grating design have been developed, generating high powers across 734-1929 nm in the near-IR, while operating at room temperature . On the other hand, the nonlinear crystal, PPKTP, having effective nonlinearity, deff ~10 pm/V, high damage threshold , and transparency across ~0.35-4 μm, has higher photorefractive damage threshold and negligible green-induced infrared absorption compared to both PPLN and PPLT. As such, it also does not require high temperature operation as in the case of PPLN and PPLT. Based on PPKTP, various cw OPOs have been previously demonstrated. Table 1 summarizes the performance characteristics of the tunable cw OPOs reported to date using PPKTP. The first demonstration was based on doubly-resonant oscillator (DRO) configuration and pumped at 532 nm by frequency-doubled Nd:YAG laser, providing spectral output across 1037–1093 nm . A PPKTP-based cw DRO was further demonstrated for spectroscopic applications . Also, a cw singly-resonant oscillator (SRO) based on PPKTP was reported under intracavity pumping scheme . Later, a pump-enhanced cw OPO in signal-resonant configuration was reported , and a cw SRO using a pair of multiple-grating PPKTP crystals was demonstrated . Further, a cw output-coupled DRO with high conversion efficiency at 805 nm and 1522 nm, and wavelength tunability across 791-807 nm in signal and 1512-1573 nm in idler was reported . In all such PPKTP-based cw OPOs reported to date, wavelength tuning has been achieved by using uniform QPM grating structure in combination with temperature tuning. Given that the change in crystal temperature is a slow process, as the crystal needs to be thermally stable at each defined temperature, wavelength tuning under this approach is not rapid. Moreover, damage in PPKTP has been observed at high temperatures when pumped at high green powers , thus limiting the tunable spectral range attainable at high powers when using uniform grating periods.
On the other hand, wavelength tuning with continuous variation of QPM grating period can be achieved by using a fan-out structure at fixed temperature, which is a relatively fast process, and is potentially capable of providing broad spectral coverage . With progress in QPM fabrication technology, the development of PPKTP in fan-out grating design has now become possible, motivating the exploitation of such structures for the development of cw OPOs capable of wide and rapid wavelength coverage without resort to temperature tuning. Here we report, for the first time to our knowledge, a green-pumped cw OPO based on a fan-out grating design in PPKTP, enabling rapid and continuous wavelength tuning with lateral translation of the nonlinear crystal while operating at room temperature. Moreover, by deploying signal output coupling in a PPKTP-based cw SRO, also for the first time, we investigate the performance of the device and demonstrate effective reduction in thermal loading of the crystal, resulting in an enhancement of output efficiency.
2. Experimental setup
A schematic of the experimental setup is shown in Fig. 1. The OPO is pumped by a cw frequency-doubled Nd:YVO4 solid-state laser delivering up to 10 W of output power at 532 nm in a single-frequency linearly-polarized beam with M2<1.1. The PPKTP crystal used has dimensions of 30x11x1 mm3, in a fan-out grating design with periods varying over Λ = 9.00-10.85 μm across its lateral dimension. A laboratory photograph of the crystal is also shown in Fig. 1. The crystal is housed in an oven with a stability of ± 0.1°C, adjustable from room temperature to 200 °C, and mounted on a linear translation stage (resolution of 10 μm) to enable smooth grating tuning across its lateral dimension in ± y direction with fine and continuous translation of the crystal. The end faces of the crystal are antireflection-coated at 532 nm (R<0.5%), 720–990 nm (R<0.5%), and 1130–2040 nm (R<5%). The OPO is configured in a compact ring cavity comprising two concave mirrors, M1,2 (r = −100 mm), one plane mirror, M3, and one plane output coupler, M4. Mirrors, M1-3, are highly reflecting (R>99.8%) for the signal (620–1030 nm) and highly transmitting (T>97%) for the idler (1078–3550 nm) and pump, while M4 has an output coupling of ~1% across 635-1100 nm, ensuring singly-resonant oscillation in the signal and single-pass pump. A dichroic mirror, M’, is used to separate the output idler from the transmitted pump.
3. Crystal characterization
Given the first demonstration of PPKTP in fan-out grating design, and in particular at high green powers, it is important to first characterize the crystal under single-pass pumping condition. To this end, we measured the transmission of the crystal as a function of pump intensity at various temperatures, with the pump beam in phase-matched polarization and orthogonal non-phase-matched polarization. The results are shown in Figs. 2(a) and 2(b), respectively. The measurements were performed at room temperature (T~25 °C), moderate temperature (T = 50 °C), and high temperature (T = 170 °C), while keeping the crystal at a fixed position. Transmission of ~89.6%, ~89.6% and ~87% for phase-matched polarization, and ~93.2%, ~93.2% and ~91.7% for orthogonal non-phase-matched polarization were obtained at T~25 °C, 50 °C and 170 °C, respectively. As evident from Fig. 2(a), the variation of crystal transmission with pump intensity at all temperatures remains negligible. We also observe no significant drop in transmission at high pump intensities, thus implying the absence of two-photon absorption. At high temperature, however, a drop in transmission by ~2.5% is observed for all pump intensities as compared to low temperatures. The behavior of the crystal under orthogonal non-phase-matched polarization across the pump intensities, as seen in Fig. 2(b), is also similar, although at all temperatures the transmission is ~4% higher than that under phase-matched polarization. The lower transmission under phase-matched polarization as compared to non-phase-matched polarization has been previously observed in uniform grating QPM structure KTP .
4.1. Spectral coverage
Wavelength tuning in the present OPO can be achieved by lateral translation of the PPKTP crystal to vary the QPM grating period or by changing the crystal temperature, while keeping either parameter fixed. With the OPO in output-coupled SRO (OC-SRO) configuration, we initially varied the crystal position laterally while keeping the crystal at room temperature, and recorded the signal wavelength at optimum pump powers generating maximum signal powers. Figure 3(a) shows the generated signal and corresponding idler wavelengths as function of crystal position at room temperature. As evident, the OPO is rapidly and continuously tunable across 741-922 nm in signal and 1258-1884 nm in idler. We were able to use 8.08 mm of the 11 mm crystal width, with the OPO ceasing to operate towards the edges of the crystal, accompanied by distortion of the pump beam. The initial operating position of the crystal was considered as the starting point (0 mm).
Wavelength tuning was also obtained by varying the crystal temperature at the two possible extreme crystal positions. At position of 0.492 mm, as seen in Fig. 3(b), signal tuning across 865-901 nm and idler tuning across 1299-1381 nm was obtained by heating the crystal from room temperature to 112 °C. The wavelength tuning range in signal and idler was further expanded to 739 nm and 1901 nm, respectively, by temperature tuning the crystal at position of 8.08 mm, as seen in Fig. 3(c). However, as evident, in both cases the variation of wavelength with temperature is small. In particular, at crystal position of 8.08 mm, shown in Fig. 3(c), as the OPO is tuned further away from degeneracy, the variation in the signal and idler wavelength with temperature is only 3 and 20 nm, respectively, due to the dispersion properties of PPKTP, similar to that in MgO:sPPLT . We further observed that at higher temperatures the OPO would cease to operate, and the maximum temperature at which oscillation could be maintained decreases as we move away from degeneracy along the crystal lateral dimension. The OPO performance at high temperature and variation of wavelength with temperature show similar behavior at other crystal positions. This behavior is consistent with earlier reports, where operation of OPO has also been observed to be challenging above 150 °C, with damage also observed to the PPKTP crystal . These limitations, thus, further support the merits of the fan-out grating design in PPKTP as a robust and efficient approach for the attainment of wide wavelength tuning at room temperature, as evident from Fig. 3(a), a more practical device architecture without the need for an oven, as well as higher output stability by avoiding increased thermal fluctuations at higher operating temperatures.
In order to verify our wavelength tuning measurements, we also performed theoretical calculations based on the Sellmeier equations and thermo-optic dispersion relations for PPKTP, which have been studied extensively in many earlier reports [16–20]. To determine the grating period at a corresponding crystal position, we calculated the grating period variation across the width of the crystal, using relevant Sellmeier equations, with the results shown in Fig. 4(a). The calculations were performed within the generated spectral range in the OC-SRO configuration under room temperature operation, using three Sellmeier equations [16,18,19]. As evident, the calculated variation in grating period with crystal position is different for the three Sellmeier equations. Thus, accordingly, signal tuning across 741-922 nm is achieved for calculated grating period variation across Λ = 9.14-10.38 μm, Λ = 9.12-10.47 μm, and Λ = 9.20-10.58 μm, as shown in Fig. 4(a), obtained from ,  and , respectively.
At a fixed crystal position of 0.492 mm, we also simulated temperature tuning curves using four different sets of refractive-index temperature derivatives and Sellmeier equations, with the results shown in Fig. 4(b). The first and second set include the refractive-index temperature derivatives from , and Sellmeier equations from  and , respectively. The third set includes the refractive-index temperature derivatives and Sellmeier equations from , while the fourth set includes the refractive-index temperature derivatives from  and Sellmeier equations from . We further compared the theoretical calculations in Fig. 4(b) with the experimental temperature tuning data in Fig. 3(b), and observed that the theoretical curves are in good agreement with experimental data at higher temperatures, with the closest agreement between theory and measurement data obtained using the third set.
4.2. Output power
We characterized the OPO with regard to output power by recording the extracted signal and idler power across the tuning range. We performed the measurements by grating tuning at room temperature, as well as by temperature tuning at fixed grating period for the shorter and longer periods of Λ = 9.28 μm and Λ = 10.58 μm, corresponding to crystal positions of 0.492 mm and 8.08 mm, respectively. We were not able to use the shortest grating period of Λ = 9.00 μm and longest period of Λ = 10.85 μm, because the OPO ceased to operate towards the crystal edges, and the pump beam quality was observed to deteriorate after passing through the crystal at these extreme positions. Figure 5(a) shows the maximum generated power over the signal (741-922 nm) and idler (1258-1884 nm) tuning range at room temperature under optimum pumping, achieved with continuous grating tuning by lateral translation of the crystal across its width. As evident, the OPO can provide >150 mW of signal power over the entire tuning range with up to 450 mW at 901 nm, and > 400 mW of idler power over 66% of the tuning range with up to 1.2 W at 1299 nm, corresponding to total extraction efficiency of 30% for 5.5 W of pump power. We also recorded the maximum generated power at the shortest and longest grating periods of Λ = 9.28 μm and Λ = 10.58 μm, respectively, by temperature tuning, with the results shown in Figs. 5(b) and 5(c). As seen, at the shortest grating period, signal and idler powers of >200 mW over 72% of the tuning range were generated, while at the longest grating period, signal and idler powers of >40 mW over 88% and 86% of the tuning range were obtained, respectively. The drop in power at shorter signal and longer idler wavelengths with the crystal at lower temperatures, as seen in Figs. 5(a) and 5(c), is attributed to the reduction in parametric gain away from degeneracy. However, the decline in power has also been observed at shorter idler wavelengths when the crystal temperatures are increased, as seen in Fig. 5(b), which is due to the degradation of the pump beam quality observed at higher temperatures. We note that in earlier work it has been reported that the operation of the PPKTP-based OPO ceases at higher temperatures , and this could be due to the degradation of the beam quality observed here.
4.3. Power scaling
To investigate the power scaling capability of the OPO, we recorded the variation of output power in the signal and the corresponding idler with pump power, with the results shown in Fig. 6(a). The measurements were performed at room temperature for a fixed grating period of Λ = 9.21 μm. As evident, the output power increases with pump power, displaying small sharp transitions in signal and idler power as the pump power is increased, generating maximum idler power of 640 mW at 4.1 W of pump power and a maximum signal power of 367 mW at 4.5 W of pump power. The output powers recorded are the maximum powers observed at the corresponding pump power, with the cavity length kept unchanged throughout the power scaling measurements. As such, we believe the sharp transitions in the output powers observed with the increase in pump power could be due to the temperature fluctuations in the crystal at a given pump power, resulting in mode-hopping. Given that we have not observed any beam quality distortion after long-term operation at these pump powers, we believe there is negligible thermal lensing, but a stable cavity length is required at each pumping level to minimize output power fluctuations. With the increase in pump power beyond 5 W, under long-term operation, we observed distortion in the output beam profile. As such, to avoid thermal loading in the PPKTP crystal, we limited the maximum pump power to 4.5 W. The threshold pump power was recorded to be 3.6 W. We further investigated the OPO signal wavelength as a function of pump power for the same grating period of Λ = 9.21 μm at room temperature, with the results shown in Fig. 6(b). As evident, the signal wavelength is observed to undergo sudden transitions with small variations in pump power, also decreasing from 921 nm to 917 nm with the increase in pump power from 3.6 W to 4.5 W. This decrease in signal wavelength is attributed to the increase in crystal temperature with the increase in pump power due to residual absorption of ~10% at 532 nm (see section 3). Using the relevant Sellmeier and thermo-optic dispersion relations , we have theoretically calculated that for the period of Λ = 9.21 μm at room temperature, an increase in the crystal temperature by 7.6 °C results in a decrease in signal wavelength by ~4 nm.
4.4. Performance with respect to time
In order to further investigate the temporal behavior of OPO output power, we recorded the variation of signal and idler power with corresponding variation of pump power, while keeping the crystal under continuous exposure to pump in time. The results are shown in Fig. 7. As can be seen, while the pump power was kept constant for initial 5 seconds at 5.35 W, the idler power was observed to decrease from 1.191 W to 702 mW, and the signal, after a sudden initial rise to 449 mW in 1 second drops to 388 mW in 5 seconds. The abrupt increase in signal power at exposure to pump power and subsequent drop in total OPO output power at a constant input power may be attributed to thermal lensing in the PPKTP crystal at high pump powers beyond 5 W. It is to be noted that while measuring the output power at sudden exposure to high pump power of 5.35 W, we observed thermal lag in the idler power detector, which could be due to sudden increase in the power onto the highly-sensitive thermal power sensor. This resulted in no idler power data for the first 2.8 seconds. After 5 seconds, by decreasing the pump power the OPO output power also begins to decrease to 193 mW in the signal and 386 mW in the idler at 4.6 W of pump power. With further decrease in the pump power, the thermal loading in the crystal becomes negligible and the signal and idler powers start to increase again, reaching more stable values of 330 mW and 537 mW, respectively, at 4.4 W of pump power. The signal and idler power is then observed to decrease gradually to 273 mW and 471 mW, respectively, with further decrease in pump power from 4.4 W to 4 W, beyond which, at low pump powers, the output power curves have similar behaviour as that of pump power. This behaviour indicates that thermal loading in the PPKTP crystal under exposure to green pump can be a limiting factor to OPO operation at high input powers above 4.5 W. However, improvements in the transmission loss of PPKTP (currently ~10% at 532 nm) will enable stable and practical operation of the OPO at increased powers.
Also shown in Fig. 7 is the variation of signal wavelength with time under continuous exposure of the crystal to the varying pump power. As can be seen, the signal wavelength decreases over 5 nm, when the pump is kept constant at high power of 5.35 W for the initial 5 seconds. This is due to the heating of the crystal at high pump power, which results in change in phase-matching temperature, as also noted in section 4.3, and observed in Fig. 6(b). The decrease in wavelength continues even when the pump power is gradually reduced to 4.6 W over next 10.8 seconds. By further decreasing the pump power below 4.6 W, the signal wavelength begins to increase again, due to the reduction in the temperature of the crystal at low pump powers. Similar behaviour was observed when the crystal was kept at a temperature of 50 °C.
4.5. Power stability
We also performed the power stability measurements of the output idler at 1517 nm and corresponding signal at 820 nm, under free-running conditions for a pump power of 5.43 W at the input to the OPO, with the crystal kept at room temperature, and after allowing a few minutes to reach steady-state operation. The results are shown in Figs. 8(a) and 8(b).
As can be seen, the idler and signal powers exhibit a passive stability better than 3.2% rms and 5.5% rms, respectively, over 2.6 minutes. We also recorded long-term power stability for the maximum power of 8.9 W at the output of the pump laser, to be better than 0.08% rms over 1.62 hours, as shown in Fig. 8(c). The fluctuation in OPO output power could be attributed to mechanical vibrations and in large part to the temperature fluctuations in the PPKTP crystal due to thermal lensing combined with heating of the crystal at high pump powers >4.5 W. We recorded similar output power stability at a pump power of 4.5 W, which we attribute to temperature fluctuations in the crystal even at pump powers below 5.43 W, as also observed in the power scaling measurements up to 4.5 W of pump power. Further, we also monitored the OPO output power stability while keeping the OPO operating over a few hours, and we observed that the OPO remains stable for about ~3 minutes, beyond which a sudden sharp fluctuation in powers is observed. However, after a few minutes, the output power becomes stable again and shows low fluctuations. The sudden fluctuation in output power under long-term operation could be attributed to mechanical vibrations or air current changes in the laboratory.
4.6. Single-frequency operation and spatial profile
We further performed spectral characterization of OPO output using a confocal Fabry-Perot interferometer (FSR = 1 GHz, finesse = 400) at an input pump power of 5.2 W. The signaltransmission spectrum at 831 nm is shown in Fig. 9(a), where an instantaneous linewidth of 7.5 MHz was measured, confirming single-frequency operation at room temperature. Similar behaviour was observed across the signal tuning range. Figure 9(b) shows the signal spectrum across the tuning range at room temperature, measured using a spectrometer (Ocean Optics HR4000) at stable output powers. Given the detector sensitivity of the spectrometer across 190-1100 nm, we were not able to record the spectrum across the idler tuning range.We also investigated the frequency stability of the output signal using a wavemeter (HighFinesse, WS-U 30). The measurements were performed under free-running conditions, and in the absence of any thermal isolation. The results are shown in Fig. 9(c), where it can be seen that the signal exhibits a peak-to-peak frequency deviation of Δν~194 MHz over 37 seconds, measured at a central wavelength of 831.17005 nm. The far-field energy distribution together with orthogonal intensity profile of the signal beam at 831 nm obtained at maximum power is shown in Fig. 9(d). As can be seen, a Gaussian profile with circularity >95% was recorded, confirming high spatial beam quality.
5. Performance under SRO configuration
We finally investigated the performance of the fan-out grating PPKTP cw OPO in pure SRO configuration by replacing the output coupling mirror, M4 in Fig. 1, with a plane high reflector (R>99.8% over 620–1030 nm) for the signal. We characterized the OPO to understand its performance at high intracavity powers. We found that the SRO could provide spectral tuning across 742-864 nm in signal and 1386-1879 nm in idler at room temperature, with maximum recorded idler power of 210 mW at 1588 nm for 5.2 W of pump power, corresponding to an idler extraction efficiency of 4%. The idler output was measured to exhibit a passive power stability better than 2.6% rms over 1.5 minutes at a pump power of 3 W and at an idler wavelength of 1461 nm. The far-field energy distribution of the signal beam at 838 nm was also recorded, where a Gaussian profile with circularity >93% was obtained. These measurements, which resulted in reduced wavelength coverage and lower output power, confirm inferior performance of the OPO in pure SRO configuration compared to that in OC-SRO scheme. This degradation in performance could be attributed to the higher intracavity signal power resulting in additional thermal loading of the PPKTP crystal at lower critical pump powers beyond 3 W.
We have demonstrated a cw OPO based on fan-out grating design PPKTP crystal at room temperature, providing wide and rapid wavelength tunability across 741-922 nm in signal and 1258-1884 nm in idler providing a maximum total power of 1.65 W, with >150 mW of extracted signal power over the entire tuning range and >400 mW of idler power over 66% of the tuning range. The idler and signal powers exhibit passive stability better than 3.2% rms and 5.5% rms, respectively, over 2.6 minutes. The OPO has single-frequency operation with good spatial beam quality. With improvements in the transmission loss of the PPKTP in the green, proper thermal management and crystal temperature control, the performance of the OPO in providing high and stable output powers can be further enhanced. In addition, the superior performance of the OPO in OC-SRO configuration compared to the pure SRO scheme suggests that further improvements in output power, stability, and power scaling can be obtained by optimization of output coupling and intracavity signal power.
The European Union (Mid-TECH, H2020-MSCA-ITN-2014, 642661); Ministerio de Ciencia, Innovación y Universidades (MICINN) (nuOPO, TEC2015-68234-R); Generalitat de Catalunya (CERCA Programme); Severo Ochoa Programme for Centres of Excellence in R&D (SEV-2015-0522-16-1); Fundació Privada Cellex.
1. C. W. Freudiger, W. Min, B. G. Saar, S. Lu, G. R. Holtom, C. He, J. C. Tsai, J. X. Kang, and X. S. Xie, “Label-Free Biomedical Imaging with High Sensitivity by Stimulated Raman Scattering Microscopy,” Science 322(5909), 1857–1861 (2008). [CrossRef] [PubMed]
2. M. Ebrahim-Zadeh, “Continuous-wave optical parametric oscillators,” in Handbook of Optics (OSA/McGraw-Hill, 2010), pp. 1–33.
3. M. Ebrahim-Zadeh, S. C. Kumar, and K. Devi, “Yb-Fiber-Laser-Pumped Continuous-Wave Frequency Conversion Sources from the Mid-Infrared to the Ultraviolet,” IEEE J. Sel. Top. Quantum Electron. 20(5), 0902823 (2014). [CrossRef]
4. U. Strössner, A. Peters, J. Mlynek, S. Schiller, J.-P. Meyn, and R. Wallenstein, “Single-frequency continuous-wave radiation from 0.77 to 1.73 microm generated by a green-pumped optical parametric oscillator with periodically poled LiTaO3,” Opt. Lett. 24(22), 1602–1604 (1999). [CrossRef] [PubMed]
6. S. Zaske, D.-H. Lee, and C. Becher, “Green-pumped cw singly resonant optical parametric oscillator based on MgO:PPLN with frequency stabilization to an atomic resonance,” Appl. Phys. B 98(4), 729–735 (2010). [CrossRef]
8. J. D. Bierlein and H. Vanherzeele, “Potassium titanyl phosphate: properties and new applications,” J. Opt. Soc. Am. B 6(4), 622–633 (1989). [CrossRef]
10. G. M. Gibson, M. Ebrahimzadeh, M. J. Padgett, and M. H. Dunn, “Continuous-wave optical parametric oscillator based on periodically poled KTiOPO4 and its application to spectroscopy,” Opt. Lett. 24(6), 397–399 (1999). [CrossRef] [PubMed]
11. M. Ebrahim-Zadeh, G. A. Turnbull, T. J. Edwards, D. J. M. Stothard, I. D. Lindsay, and M. H. Dunn, “Intracavity continuous-wave singly resonant optical parametric oscillators,” J. Opt. Soc. Am. B 16(9), 1499–1511 (1999). [CrossRef]
12. D. R. Weise, U. Strößner, A. Peters, J. Mlynek, S. Schiller, A. Arie, A. Skliar, and G. Rosenman, “Continuous-wave 532-nm-pumped singly resonant optical parametric oscillator with periodically poled KTiOPO4,” Opt. Commun. 18, 329–333 (2000). [CrossRef]
13. U. Strößner, J.-P. Meyn, R. Wallenstein, P. Urenski, A. Arie, G. Rosenman, J. Mlynek, S. Schiller, and A. Peters, “Single-frequency continuous-wave optical parametric oscillator system with an ultrawide tuning range of 550 to 2830 nm,” J. Opt. Soc. Am. B 19(6), 1419–1424 (2002). [CrossRef]
14. C. Liu, X. Guo, Z. Bai, X. Wang, and Y. Li, “High-efficiency continuously tunable single-frequency doubly resonant optical parametric oscillator,” Appl. Opt. 50(10), 1477–1481 (2011). [CrossRef] [PubMed]
16. T. Y. Fan, C. E. Huang, B. Q. Hu, R. C. Eckardt, Y. X. Fan, R. L. Byer, and R. S. Feigelson, “Second harmonic generation and accurate index of refraction measurements in flux-grown KTiOPO4,” Appl. Opt. 26(12), 2390–2394 (1987). [CrossRef] [PubMed]
18. K. Fradkin, A. Arie, A. Skliar, and G. Rosenman, “Tunable midinfrared source by difference frequency generation in bulk periodically poled KTiOPO4,” Appl. Phys. Lett. 74(7), 914–916 (1999). [CrossRef]