We report herein the enhancement in both power and efficiency performance of a continuous-wave intra-cavity singly resonant optical parametric oscillator (ICSRO) by introducing finite resonant wave output coupling. While coupling out the resonant wave to useful output, the output coupling increases the SRO threshold properly thus suppresses the back-conversion under high pump power. Therefore, the down-conversion efficiency is maintained under high pump without having to raise the threshold by defocusing. With a T = 9.6% signal wave output coupler used, the SRO threshold is 2.46 W and the down-conversion efficiency is 72.9% under the maximum pump power of 21.4 W. 1.43 W idler power at 3.66 μm and 5.03 W signal power at 1.5 μm are obtained, corresponding to a total extraction efficiency of 30.2%. The resonant wave out coupling significantly levels up the upper limit for the power range where the ICSRO exhibits high efficiency, without impeding its advantage of low threshold.
© 2012 OSA
Singly resonant optical parametric oscillators (SROs) have been established as efficient coherent radiation sources in spectral regions inaccessible to lasers, from visible to mid-infrared region, as a result of its superior wavelength tuning capability and stabilities of both power and spectrum, compared to doubly resonant optical parametric oscillators (DROs) [1–3]. In early stage, the SROs usually operate in pulsed regime because of the high threshold. Even though periodically poled crystals with large effective nonlinearities and long interaction lengths like periodically poled LiNbO3 (PPLN) and periodically poled LiTaO3 (PPLT) have been utilized together with high quality pump sources since the late 1990s, the continuous-wave (CW) oscillation thresholds still remain at a level of several watts [3,4]. Considering the fact that these extra-cavity SROs only exhibit good down-conversion efficiency when pumped sufficiently far above threshold, such devices often require high power primary pump sources and are very inefficient when only low output powers are needed [3,5]. To circumvent this problem, the intra-cavity pumping scheme was introduced to SROs by Colville and Ebrahim-Zadeh et al. By locating the nonlinear medium within the cavity of a CW laser to exploit high circulating power, the efficient CW operation of SROs can be realized with relatively moderate primary pump power. The first examples of such CW intra-cavity SROs (ICSROs) are Ti:Sapphire-KTP/KTA devices pumped by Argon-ion laser, which show good power and efficiency characteristics [6,7]. Then a Nd:YVO4-PPLN device pumped by laser diode (LD) with extremely low threshold of 310 mW LD power was reported, which also exhibited high conversion efficiency .
In recent years, the further development in pump sources and nonlinear mediums allows the CW extra-cavity SRO threshold to be reduced to 2-3 W . With low threshold and high pump power, the back-conversion process, which only occurred in pulsed regime before, becomes an influential factor on the CW SROs [8,9]. The pump depletion reaches its maximum value when pumped ca. 2.5 times above the threshold and then gradually tails off as pump power continue to rise because the OPO over-couples the laser field [10,11]. By using a resonant wave output coupler instead of the highly reflective mirror, the resonant wave can be extracted to useful output as well as the non-resonant wave, and meanwhile, it increases the SRO threshold properly for the purpose of higher pump depletion. In 2008, Samanta and Ebrahim-Zadeh carried out a detailed comparison between the output-coupled SRO (OC-SRO) and common SRO under extra-cavity pumping scheme . Significant results were obtained with an OC-SRO, including the similar non-resonant idler output power and much higher total extraction efficiency compared with those of a common SRO. In the ICSROs, the available primary pump power could be 10 times higher than the SRO threshold even with relatively small focusing parameters . This would lead to an optimal SRO threshold for down-conversion efficiency  much higher than the lowest threshold in the experiment. Therefore, the influence of back-conversion could be serious. Sometimes it is even necessary to introduce defocusing in the nonlinear medium by adjusting the cavity mirror to artificially raise the SRO threshold with the aim of enhancing down-conversion efficiency .
Obviously, it is more efficient to optimize the SRO threshold by appropriate resonant wave output coupling instead of defocusing. Here we focus on the influence of resonant signal wave output coupling on the oscillation threshold, non-resonant idler output power, down-conversion efficiency and total extraction efficiency of the CW ICSRO. By using three mirrors with different signal transmittances in a LD end-pumped Nd:YVO4-PPLN ICSRO, the performance indicators (vide supra) are investigated in detail. 1.43 W idler and 5.03 W signal power could be extracted under 21.4 W absorbed LD power, corresponding to an overall extraction efficiency of 30.2%. It also demonstrated that increasing the SRO threshold towards its optimal value by proper resonant wave output coupling is an effective and efficient method to suppress the back-conversion and keep high down-conversion efficiency in CW ICSRO.
2. Experimental arrangement
The experiment arrangement is depicted in Fig. 1 . The primary pump source of the ICSRO is an 880-nm fiber-coupled laser diode array with a fiber core diameter of 400 µm and a NA of 0.22. A multi-lens coupler re-imaged the pump into a Nd:YVO4 crystal with a ratio of 1:1. Pumping the Nd:YVO4 with the wavelength of 880 nm instead of 808 nm helps to reduce the thermal load, hence the maximum pump power allowed is higher. The 3 × 3 × 10 mm3 Nd:YVO4 crystal is 0.5-at.%-doped and α-cut, which could absorb ~85% of the incident non-polarized LD pump. It is coated for anti-reflection (AR) at 880 nm and highly reflective (HR) at 1064 nm on the entrance face (S0) and AR at 1064 nm on the other face. The crystal is wrapped with indium foil and mounted in an aluminum holder cooled at 10°C by refrigerant water. M1 is a CaF2 concave mirror (ROC = 100 mm) which is coated for HR at 1064 nm (R>99%) and 1.4-1.55 µm signal range (R>99%) and AR at 3.6-4.5 µm. It constitutes the 1064 nm parent laser resonator along with S0.
A focusing lens L with a focal length of 100 mm is used to control the 1064 nm laser beam waist in the nonlinear medium, as well as to circumvent the influence of thermal focal length variation on the parent laser. The 24-mm-long PPLN crystal is AR coated at pump, signal and the idler wavelengths. It contains 7 gratings ranging from 26 μm to 29 μm at the interval of 0.5 μm. In this work, we are focusing on the influence of resonant wave output-coupling on the down-conversion efficiency of CW ICSRO but not the tuning characteristics and other properties. Therefore, the experiment is carried out with the grating period of 29 μm and the crystal temperature of 140°C only (corresponding signal and idler wavelengths are 1.5 µm and 3.66 µm, respectively). The SRO resonator is defined by M1, a flat-flat beam splitter (BS), and another concave mirror M2 (ROC = 100 mm). To investigate the influence of resonant wave output-coupling, mirrors with different transmittances at 1.5 µm signal wavelength (T = 0.1%, 5.3% and 9.6%) are used as M2, separately. The thermal focal length in the Nd:YVO4 crystal is measured to be ca. 150 mm under the maximum pump power used of 25.2 W (21.4 W absorbed). Take the thermal lensing into consideration, the 165 mm Nd:YVO4 laser cavity (S0 -M1) results in theoretical beam radii of 1064 nm laser in the Nd:YVO4 crystal and the PPLN crystal of 210 µm and 80 µm, respectively, while the 195 mm signal cavity lead to a 94 mm signal beam waist in the PPLN crystal. The focusing parameters of both the pump and signal beams are 0.28. Two laser powermeters, a Molectron EPM1000 and a Newport 842-PE, are used to record the idler and the signal output power simultaneously.
3. Results and discussion
The T = 0.1% mirror is used first. The thresholds of the 1064 nm parent laser and the SRO are 0.34 W and 0.95 W absorbed LD power, respectively. As shown in Fig. 2 , the idler output power reached 1.13 W under 21.4 W absorbed LD pump before the onset of cavity instability induced by thermal effects (cycle). Take the laser threshold and the maximum pump power into the optimal threshold condition for maximum down-conversion efficiency of ICSRO 
Then the output couplers with 5.3% and 9.6% transmittances at 1.5 µm are used to replace the HR mirror as M2. When the cavity is optimized for low threshold with each output coupler, the SRO threshold increases from the 0.95 W with HR M2 mirror to 1.55 W and 2.46 W absorbed LD power, respectively. Figure 3 gives the idler output power and down-converted power as functions of pump power with the three different M2 mirrors. It can be seen that accompanied with the increasing of SRO threshold induced by higher signal wave output coupling, the output power growing become faster. When using the T = 5.3% and T = 9.6% output couplers, the idler power under the maximum LD pump power of 21.4 W are 1.37 W and 1.43 W, respectively. The M2 factors of idler output at 1.43 W are 1.62 and 1.71 in x and y directions, respectively, better than those of 1.74 and 1.87 measured at the 1.13 W with the HR M2 mirror used. This is because the circulating signal power, which is the primary reason for PPLN crystal heating, is reduced significantly by the output coupling. The long-term idler power stability at 1.43 W is better than 1.3% (rms) over 1 hour. However, the transient behavior is harmed seriously by the relaxation oscillation because of the long upper-laser-level lifetime of Nd:YVO4 . The down-converted power is calculated through the relation PDC = [2Pi/ηi]/[λp/λi], where the factor of 2 accounts for the two-way idler propagation and ηi = 0.95 is the idler output coupling efficiency. The solid curve represents the maximum 1064 nm laser power could be coupled out of the parent laser under optimized output coupling.
Using the down-converted power and the maximum laser output power above, the down-conversion efficiency of the ICSRO can be obtained, as shown in Fig. 4 . To take a comparison, the theoretical down-conversion efficiencies are also calculated using the measured laser threshold of 0.34 W and SRO thresholds of 0.95 W, 1.55 W and 2.46 W, respectively. The theoretical 100% down-conversion efficiency with the three signal output couplers (SRO thresholds) appear at the 2.5 W, 6.7 W and 16.8 W and would fall to 78.3%, 94.0% and 99.7%, respectively, under the maximum absorbed LD power of 21.4 W. When using the mirror HR at signal in the experiment, the down-conversion efficiency rises rapidly after exceeding the SRO threshold and reaches its maximum value of 90 ± 0.5% at 2.8-3.7 W pump power. Then the back-conversion starts to cause a decay in efficiency together with other factors such as thermal effects and finally an efficiency of 57.4% is obtained. When it turns to the 5.3% and 9.6% signal output coupler, the down-conversion efficiencies under the maximum pump power of 21.4 W are 69.7% and 72.9%, respectively. Just as theory predicted, the higher SRO thresholds, which are closer to the optimal value, help to suppress the back-conversion, which lead to less efficiency decline under high pump power. The differences between the down-conversion efficiencies obtained with the three mirrors also roughly agree with the theoretical values. However, the measured down-conversion efficiencies under maximum pump power with all the three mirrors are 20-30% lower than their theoretical values. We attribute this to the degeneration of beam quality induced by serious thermal effects in the two crystals of Nd:YVO4 and PPLN under high pump power. Another observation reflects the influence of thermal effects is that the highest down-conversion efficiency obtained with the 9.6% output coupler is only 84.2% under 11.5 W pump power, lower than the ca. 90% reached with the HR and 5.3% mirrors. Meanwhile, it comes before its theoretical 100% point, just opposite to those with the other two mirrors. This is because the back-conversion occurs later with the relatively high threshold of 2.46 W and the thermal effects begin to influence on the efficiency significantly before that.
The useful signal output is another important aspect of the resonant wave output coupled SRO. The signal powers, total output powers and corresponding extraction efficiencies with the two output couplers are plotted in Fig. 5 . The 1.5 µm signal output power is 5.03 W under 21.4 W pump power when using the 9.6% output coupler and 3.79 W is obtained with the 5.3% output coupler, corresponding to extraction efficiencies with respect to absorbed LD power of 30.2% and 24.1%, respectively. Of course, the SRO threshold can be also controlled by adjusting the focusing parameters or cavity alignment, thereby suppressing the back-conversion. However, resonant wave output coupling is an efficient way that simultaneously makes full use of the high pump power to generate idler and signal output. Compared to the extra-cavity SRO, the ICSRO is more suitable for resonant wave output coupling because of its low threshold. In reference , the threshold of CW extra-cavity SRO was 10.5 W with 3.8% signal wave output coupling and could exceed 15 W when the output coupling increased to 6%. For this ICSRO, its threshold is only 2.46 W with 9.6% signal output coupling and the down-conversion efficiency is higher than 70% under 7 W primary pump power (Fig. 4). This means, even with relatively high output coupling, the ICSRO can still keep efficient under moderate pump power. Considering a typical SRO cavity round-trip loss of ca. 2-3%, the contribution of relatively high output coupling of ca. 10% (compared to the less than 5% with the extra-cavity SRO) on extracting more signal power generated to useful output is non-negligible. After optimizing the SRO cavity under the 21.4 W pump power, the 3.66 µm idler output power with the T = 5.3% and T = 9.6% mirrors increases from 1.37 W and 1.43 W to 1.48 W and 1.49 W, respectively. This indicates that the 9.6% output coupling has eliminated the influence of back-conversion substantially.
In conclusion, we have demonstrated that by deploying resonant wave output coupling in a CW ICSRO thus increase its threshold properly instead of introducing defocusing in the nonlinear medium, the back-conversion process can be suppressed and the down-conversion efficiency can be enhanced as well. With a T = 9.6% signal output coupler, 1.43 W idler and 5.03 W signal are coupled as useful output under 21.4 W absorbed LD pump power, corresponding to an overall extraction efficiency of 30.2%. Meanwhile, the CW SRO threshold still remains at a low level of 2.46 W and its down-conversion efficiency is higher than 70% in a large primary pump power range of 7 W to 21.4 W. The resonant wave out coupling significantly levels up the upper limit for the pump power range where the ICSRO exhibits high efficiency, without impeding its advantage of low threshold.
This work is supported by the National Natural Science Foundation of China (Grant Nos. 60978021 & 61178028), Program for New Century Excellent Talents in University (NCET-10-0610) and National High Technology Research and Development Program (863 Program) of China (No. 2011AA03020).
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