1. Introduction

Quasi-phase-matching optical parametric oscillators (OPOs) which use PPLN as nonlinear medium are a promising method to obtain 3-5 µm mid-infrared radiation to meet specific requirements of spectroscopic application, remote sensing and so on [1–4]. SRO has advantages in spectral and power stability and tuning ability over doubly resonant OPO (DRO), but the much higher oscillation threshold it requires compared to DRO often hinders its efficient operation continuous-wave (CW) regime. In recent years, driven by progress in high quality pump source and nonlinear crystal fabrication, many CW extra-cavity SROs pumped by well-refined solid-state lasers, fiber lasers or even diode lasers have been demonstrated, usually equipped with four-mirror ring cavity and etalon [5–8]. For this extra-cavity structure, CW output power could reach multi watts and pump depletion could exceed 90% when pumped well above threshold. In the intra-cavity pumping scheme, the nonlinear medium is located within the cavity of parent laser. By exploiting high circulating pump power, the efficient SRO operation can be realized with much smaller primary pump power [9–11]. ICSROs exhibit extremely low pump threshold, high down-conversion efficiency and output power, which, meanwhile, have a less complicated and more compact structure than the extra-cavity pumping scheme. However, unlike the pump power stability of extra-cavity pumped SRO can be optimized first, pump power fluctuation induced by thermal effects and mechanical disturbance can completely pass on to SRO power instability when the SRO is intra-cavity pumped. The OPO pump depletion also has influence on the stable operation of pump laser. The circulating pump field of CW ICSROs which use Nd-doped vanadate lasers as parent pump lasers often exhibit spontaneous relaxation oscillations that harm seriously on the transient stability of SRO output power [11]. As a result, ICSROs suffer from poor power stability, especially in CW regime which is more sensitive to external disturbance, thus limits their application.

In the past few years, in-band pumping scheme has received increasing attention. Exciting Nd³⁺ ions from the ground-state (⁴I_9/2) directly to the upper lasing level (⁴F_3/2) without the relaxation process of ⁴F_5/2→⁴F_3/2 could diminish the quantum defect and heat generation effectively. Nd:YVO₄ lasers in-band pumped at 880 nm showed great superiority in power scaling, conversion efficiency, threshold and power stability compared to lasers traditionally pumped at 808 nm [12–15]. Therefore, better results can be expected if in-band pumped Nd:YVO₄ laser is used as parent pump laser of the intra-cavity OPO. In this work, we report the first intra-cavity PPLN-OPO utilizing in-band pumped Nd:YVO₄ laser as pump source. The mid-infrared idler wavelength tuning range is 3.66-4.22 µm through grating period tuning. A maximum output power of 1.54 W is obtained; the optical efficiency with respect to 21.9 W LD pump power is 7.0%. Compared to ICSRO under traditional 808 nm pumping, in-band pumped ICSRO exhibits significant improvement in the aspects mentioned above, especially in power stability.

2. Experimental setup

Our experimental setup is sketched in Fig. 1 . The LD is an 880-nm fiber-coupled diode laser array with a fiber core diameter of 400 µm. The multi-lens coupler re-imaged the diode laser into a Nd:YVO₄ crystal with a ratio of 1:1. The 3×3×10 mm³ Nd:YVO₄ crystal is 0.5 at.%-doped and α-cut. Its entrance face (S₀) is coated for antireflective (AR, T>98%) at 880 nm and highly reflective (HR, R>99.5%) at 1064 nm. The other face is AR (R<0.2%) coated at 1064 nm and 880 nm to minimize cavity loss and make the measurement of pump absorption more accurate. A crystal length of 10 mm ensures ~85% absorption of non-polarized 880 nm LD pump. It can be higher if a shorter fiber is used so that the LD pump is kept π-polarized. M₁ is a concave mirror with 100 mm radius of concave (ROC) which is HR coated at 1064 nm (R>99%) and 1.4-1.55 µm signal range (R>99%). It makes the 1064 nm laser cavity along with S₀. Because M₁ is also idler output coupler, it is a CaF₂ mirror with transmittance of ~95% at 3.6-4.5 µm. The SRO resonator consists of M₁, a flat-flat beam splitter (BS) which is coated AR at 1064 nm on both sides and HR at 1.4-1.55 µm on one side, and a concave mirror M₂ (ROC = 90 mm) which is coated HR at 1.4-1.55 µm signal wavelength range (R>99%). A PPLN crystal with dimensions of 24 × 8 × 1 mm³ is chosen as the nonlinear medium. It contains 7 different gratings, equally spaced in periods from 26 µm to 29 µm. Both faces of the crystal are AR coated at 1064 nm pump, 1.4-1.55 µm signal and 3.6-4.5 µm idler wavelength ranges. A focus lens L with a 100 mm focal length, which is AR coated (R<0.2%) at 1064 nm on both sides, is used to narrow the beam waist of 1064 nm laser for the purpose of sufficient pump intensity and mode matching between the pump and the resonant signal of the SRO. It can also make the size and location of 1064 nm laser waist insensitive to the variation of Nd:YVO₄ crystal’s thermal focal length as pump power changes. In order to facilitate temperature tuning and to avoid photorefractive damage, the PPLN crystal is placed in a servo-controlled oven with a precision of 0.1°C. The Nd:YVO₄ crystal is wrapped in indium foil and clamped in an aluminium holder which is cooled by refrigerant water at the temperature of 10°C. To make a comparison between in-band pumped and traditionally pumped ICSROs, another fiber coupled LD array emitting at 808 nm is used to pump a 3×3×10 mm³, 0.3 at.%-doped Nd:YVO₄ crystal with the same pump spot diameter of 400 µm. The entrance face of the crystal is coated for AR at 808 nm and HR at 1064nm. The other face is coated for AR at 808 nm and 1064nm. It can absorb ~97% of the incident 808 nm pump.

 

Fig. 1 Schematic illustration of the in-band pumped intra-cavity SRO.

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When absorbed 880 nm LD pump power reaches its allowed maximum value of 21.9 W (25.7 W incident power), thermal focal length of the Nd:YVO₄ crystal is measured to be 150 mm through criterion of cavity stability [16]. It is larger than theoretical value calculated from the equation given by Innocenzi which has been cited widely [17]. Compared with the ~80 mm thermal focal length measured under 17.1 W 808 nm pump, 880 nm in-band pumping can make use of higher pump power before the onset of cavity stability or crystal damage induced by thermal effects. The laser cavity length (S₀ -M₁) is set to 170 mm and the lens L is located close to the Nd:YVO₄ crystal. The distance from PPLN crystal to M₁ is 85 mm. As a result, TEM₀₀ mode spot radius of 1064 nm laser in the Nd:YVO₄ crystal is 225 µm to matched 200 µm pump spot radius. Beam waist of 1064 nm laser in the PPLN crystal is 90 µm. The SRO signal cavity length (M₁ -BS -M₂) is set to 190 mm. Beam waist of 1.5 µm signal wave (when idler wavelength is 3.66 µm) in the PPLN crystal is narrowed to 110 µm. Focusing parameters of the pump and the signal beams are ξ_p=L/b_p= 0.22 and ξ_s=L/b_s=0.21, respectively. Where L is crystal length and confocal parameter b = 2πnω²/λ. The focusing parameters are relatively small because the pump power available is relatively high, so the device does not need to be optimized for lowest threshold. Well mode matching is achieved in both processes of lasing and parametric oscillation. The folding angle of SRO cavity (2θ) is 40°. When using the 808 nm pumping, the thermal focal length is much smaller. Therefore the cavity arrangement is adjusted to keep the mode sizes the same as those under 880 nm pumping thus reasonable comparison between the in-band and traditional pumping can be carried out. The laser cavity length is shortened to 165 mm and lens L is 15 mm apart from Nd:YVO₄ crystal. The distance between PPLN and M₁ is still 85 mm.

3. Results and discussion

Since idler wavelengths are out of the response range of the optical spectrum analyzer (Agilent 86142B) in use, they are calculated from signal wavelengths measured behind M₂. Figure 2 shows experimental idler wavelengths and the theoretical curve calculated from Sellmeier equation [18]. When grating period varies from 27.5 µm to 29 µm at 140°C, idler tuning range of 3.66-4.22 µm is obtained. Limited by coating range of mirrors, other grating periods are not used. The signal spectral width is ~70 GHz and the frequency variation is less than 40 GHz when signal wavelength is 1.5 µm (idler wavelength of 3.66 µm). The maximum output power is 1.54 W at 3.66 µm when absorbed pump power is 21.9 W (measured by power meter, Molectron EPM1000). Because of lower photon energy and transmittance in the PPLN crystal, idler output power drops with increasing wavelength under fixed pump power.

 

Fig. 2 Idler wavelength and output power as functions of grating period. (T = 140°C & P_abs = 21.9 W).

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Figure 3 illustrates the 3.66 µm idler output power and circulating 1064 nm laser power (P_L) versus LD pump power (grating period of 29 µm and temperature of 140°C). The P_L is estimated by measuring its leakage behind the mirror M₁. P_L reaches the SRO threshold of 7 W (two ways) when the LD power is 1.22 W, corresponding power intensity is ~32kW/cm². Then idler power begins to increase but P_L is clamped at this level because of pump depletion. When the LD pump power goes higher than 5 W, P_L starts increasing along with the idler power. The circulating power growing occurs in such early stage because thermal effects make the mode matching in the PPLN crystal depart from its ideal situation. Then the back-conversion from SRO signal and idler to pump can play a role in the rapid growing of P_L as pump power continue to rise [19]. The increase of P_L slows down after the LD power exceeds 16.5 W because of the strong thermal effects in the crystals. However, its influence on the increase of idler power is not obvious. Both the circulating laser power and the idler output power roll over after the LD pump power goes beyond 21.9 W. Serious thermal lens effect takes the pump laser cavity out of the stable region. By virtue of smaller absorption coefficient and lower heat generation with 880 nm in-band pumping, the crystal is not damaged under this pump power.

 

Fig. 3 3.66 µm idler output power and circulating 1064 nm laser power versus absorbed LD pump power. (PPLN grating period of 29 µm and temperature of 140°C)

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Figure 4 plots the 3.66 µm idler output power and corresponding conversion efficiency versus absorbed LD power at 880 nm and 808 nm, respectively. When using the 808 nm traditional pump source under the same focusing condition, the SRO threshold is 1.39 W, higher than that of 1.22 W under 880 nm in-band pumping. A maximum idler power of 1.54 W is obtained when absorbed 880 nm LD power is 21.9 W, corresponding to an optical efficiency of 7.0% and a slope efficiency of 7.6%. Taking backward-propagating idler wave not collected into account, the total down-converted power is ~10.1 W. When using an output coupler with 11% transmittance at 1064 nm and 100 mm ROC instead of M1, 13.9 W 1064 nm laser output is obtained with 21.9 W absorbed 880 nm pump power. The down-conversion efficiency from extractable pump to SRO field is 72.6%. It is worth pointing out that, with primary pump power and 1064 nm laser power at this level, an efficient extra-cavity SRO pumped at least twice above threshold with similar non-resonant idler output power, and meanwhile less susceptible to thermal effects, could also be realized. The intra-cavity scheme show advantages under moderate primary pump power but the extra-cavity scheme may be better when pump power is high enough. Therefore, the device is operating at a point perhaps the upper ceiling of the power regime where the intra-cavity scheme is the correct one to adopt. When the pump wavelength is 808 nm, the maximum usable pump power is limited to 17.1 W by heavy thermal load and 1.15 W idler output power is obtained, with the optical efficiency of 6.7% and the slope efficiency of 7.3%. In-band pumped intra-cavity SRO certainly benefits as well as in-band pumped lasers.

 

Fig. 4 3.66 µm idler output power and conversion efficiency versus absorbed LD pump power under 880 nm and 808 nm pump.

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Last, but most importantly, because the reduction of thermal load leads to better power stability of the 1064 nm pump laser, the RMS fluctuation of SRO idler output power over 1 hour is only 1.1% (Fig. 5a ). It is much better than the 2.9% fluctuation measured under the 808 nm traditional pumping and is comparable with single-frequency extra-cavity SRO reported. However, its transient stability is still compromised by the onset of relaxation oscillations, which have been a long standing problem with ICSROs in CW regime [11]. Figure 5b shows the transient behavior of the circulating pump field over 1 ms. The long-lived relaxation oscillations occur spontaneously without perturbation. The beam quality of idler output is evaluated by measuring the beam radii with the knife-edge method at different locations and hyperbolic fitting the measured data. When the output power is 1.5 W, the M² factors are 1.92 and 2.80 in parallel and perpendicular directions, respectively. The photorefractive damage is not observed during the experiment.

 

Fig. 5 Comparison of idler power stability between ICSROs under 880 nm in-band pumping and 808 nm traditional pumping over an hour (a) and the transient behavior of circulating pump field under 880 nm pumping (b).

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4. Conclusions

In conclusion, in-band pumping is introduced to ICSRO for the first time, by utilizing 1064 nm Nd:YVO₄ laser in-band pumped at 880 nm as the parent pump laser and PPLN crystal as the nonlinear medium. 3.66-4.22 µm idler output is obtained through grating period tuning. The SRO threshold is 1.22 W LD power and the maximum output power is 1.54 W at 3.66 µm with 21.9 W LD power, corresponding to an optical efficiency of 7.0%. In-band pumped ICSRO shows lower threshold, higher output power and conversion efficiency than the SRO traditionally pumped at 808 nm. It also improves the CW ICSRO power stability significantly, what makes the robust, compact intra-cavity SRO more practicable.