1. Introduction

Various applications of green light sources in biomedicine, optical storage and laser machining require a reliable and compact nonlinear optical device for building an efficient source. One of the streams is single-pass continuous wave (CW) second harmonic generation (SHG) by quasi-phase matching (QPM) using periodically poled ferroelectric crystal family [1] among which stoichiometric lithium tantalate (SLT) can handle multi-watt visible power [1]. The key issues of such devices are a high effective nonlinear coefficient deff of 10 pm/V [2], a high thermal conductivity of 8.8 W/mK [3], and a high photorefractive damage threshold [3]. Previously laser-linewidth and beam-quality effects in QPM SHG were presented with a bulk PPMgSLT [1]. In SLT at room temperature a continuous wave green power of 7 W [1] and 10.5 W [4] has been reported with 2-cm and 4-cm long devices. In our present work we investigated thermal behavior of PPMgSLT in high power region to understand the limiting factors, thermal dephasing and thermal lensing. The results demonstrate that green light absorption is a dominant factor of heat generation in multi-watt CW SHG in Mg: SLT, and the upper limit of the SH power strongly depends on confocal parameter b. At room temperature, 16.1 W green light at 532 nm was achieved with 17.6% efficiency by using single-pass SHG of a CW 1064 nm single-mode and single-frequency Nd:YAG laser.

2. Experimental setup

Here we used a stoichiometric LiTaO₃ (SLT) grown by the double-crucible Czochralski method. An electric-field poling technique was used for the fabrication of periodic domain structures to satisfy QPM conditions. Polarization reversal (PR) was controlled by integrating PR current to meet the 50% duty ratio of the reversed domains to the period. A metal electrode was evaporated on a patterned insulator on a SLT wafer with 2 inch diameter and 0.5 mm thickness. A Mg content of 1mol% was doped during crystal growth to increase photorefractive damage threshold. We applied low-electric-field poling for precise control of domain wall movement [3]. The applied electric field was 1.3 kV/mm, which is lower than a typical SLT’s coercive field of 1.7 kV/mm.

We investigated the SHG characteristics of PPMgSLT with periods of 8.4 and 8.3 μm (20 mm length) using a CW Yb-doped fiber laser (1084 nm) with a single transverse mode and a maximum output power of 47 W. Another PPMgSLT with periods of 8.0 μm (10 mm length) was utilized for SHG of a CW single-mode and single-frequency Nd:YAG laser (1064nm) with maximum output power of 100 W, which was originally developed for a light source of an interferometric gravitational wave detector [5, 6]. The fiber lasers supported several oscillation lines near 1084 nm with different linewidths, δλ_laser, from 0.03 to 0.17 nm. The Yb-doped fiber laser with M² = 1.0 yields the linewidth δλ_laser = 0.17 nm at a maximum output infrared (IR) power of 47 W. The Nd:YAG laser with M² = 1.2 presents the linewidth δλ_laser ∼ 3.8*10^-9 nm (1 kHz) at a maximum output IR power of 100 W. Several lenses with different focusing lengths were used to focus the 1084 nm radiation (Yb-fiber laser) to spot sizes of 44 μm, 32 μm and 21 μm radius (focusing parameter ξ ranging from 0.8 to 3.7 for the 20 mm long device) and to focus the 1064 nm radiation (Nd:YAG laser) to spot sizes of 22 μm, 15 μm and 11 μm radius (focusing parameter ξ ranging from 2 to 7.9 for the 10 mm long device). We used a combination of a half-wave plate and a polarizer for attenuating the input power of the IR lasers sources and high power harmonic separator to separate IR and SHG power. The device temperature was controlled by a thermoelectric cooler (TEC) with an accuracy of 0.1°C.

3. Thermal effects

There are two main thermal effects in high power region which are limiting factors for CW SHG: (i) longitudinal thermal effect or thermal de-phasing and (ii) in-plane thermal effect or thermal lensing. In high power region the distribution of temperature along device length is nonuniform [7, 8]. Due to thermal de-phasing the SHG power and the efficiency saturate. To overcome this problem we placed our samples into metal chip bed, which provide effective heat removal from four sides (Fig. 1).

 

Fig. 1. Positional dependence of normalized conversion efficiency in periodically poled Mg-doped stoichiometric lithium tantalate with non parallel domains.

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Here we provide a new original method to separate infrared (IR), green (SHG) and green induced-infrared absorptions (GRIIRA). In this fabrication batch, we intentionally fabricated non parallel domains from top to bottom allowing continuous change of the normalized conversion efficiency $\left({\eta }_{\mathrm{norm}}=\frac{P_{\mathrm{SHG}}}{P_{\mathrm{input}}^2}\right)$ by shifting the focusing position. The Fig. 1 illustrates the device, where upper focusing point 1 exhibits η_norm = 0.66 %/W whereas middle focusing point 2 - η_norm = 0.36 %/W. This structure enables us various combinations of fundamental and SH powers in a unique device. By using the unique device it is possible to fix thermal conductance between TEC and the device. We then measured the temperature increase of the device by lowering the thermoelectric cooler temperature to obtain the maximum efficiency in SHG. Consequently we could define a dominant factor of heat generation in high power region. If the temperature increase is a function of the product of IR- and green power, the thermal de-phasing could be attributed to green-induced IR absorption (GRIIRA). Figure 2 shows the dependence of SHG power (left scale) and TEC temperature (right scale) on input IR power for two cases of 0.66 and 0.36%/W. In the temperature data we observed different behaviors of the TEC temperature for different η_norm, where nonlinear behavior appears in the region of input power higher than 10-15 W. Recently it was suggested that linear increase of the device temperature in low power region up to 20 W IR power, is assumed to be connected with IR absorption [1, 4] and further nonlinear increase of the temperature is thought to be connected with GRIIRA. We investigated heat generation in the 20-mm-long devises with 8.4 and 8.3 μm period, with various η_norm and come to the conclusion that the temperature increase observed at high power is attributed to neither IR absorption nor GRIIRA. The dependence of the TEC temperature for the 8.4 and 8.3 μm periodically poled structures on the SHG green power is shown in Fig. 3, where the TEC data was on the same line with a linear slope of 0.38°C/W for different η_norm. The inset shows the TEC temperature dependence on SHG green power with different η_norm for the Nd:YAG laser where TEC data was also on the same line with a linear slope of 0.1°C/W. All these data (Fig. 2 and Fig. 3) demonstrates that the green light absorption becomes a dominant factor to induce heat generation in high power region in Mg:SLT-based CW SHG.

 

Fig. 2. CW 542 nm green output power (black circle and square) and TEC temperature (open circle and square) via 1084 nm input power for δλ_laser = 0.17 nm. Solid and dash lines: quadratic dependences of SHG power.

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Fig. 3. TEC temperature versus SHG green power for 8.3 and 8.4 μm periodically poled structures for Yb-fiber laser with δλ_laser = 0.17 nm at different η_norm. Inset TEC temperature versus SHG green power for 8.0 μm periodically poled structures for the Nd:YAG laser at different η_norm.

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Another problem in SHG is in-plane thermal effect or thermal lensing: changing of the refractive index in high power region lead to divergence of SHG beam and to cause catastrophic damage of the device. We observed beam divergence as a sign of damage and determine an upper limit of SHG power (P^Limit _SHG) by the value of the green power at that point. We investigated P^Limit _SHG by changing the confocal parameter b, defined by

where n – refractive index, w ₀ - beam waist radius, λ – fundamental wavelength in vacuum. By shifting the focusing position we kept η_norm for different confocal parameter b in the unique device. The normalized conversion efficiency was about 0.5 %/W in case 20 mm long device and 0.2 %/W in case 10 mm long device. The Figure 4 (a) show the SHG upper limit for 20 mm long device using Yb-fiber laser and Figure 4 (b) - for 10 mm device using Nd:YAG laser. In both cases the SHG upper limit increases with the decrease of confocal parameter b. We assume that P^Limit _SHG becomes higher than that in the case of short b, because interaction length is shorter than that in long confacal parameter. To achieve high SHG power we used 10 mm long device with the Nd:YAG laser at short confocal parameter b = 1.3 mm. The dependences of the SHG power (left-hand scale) and the efficiency (right-hand scale) on input IR power are presented in Figure 5. A maximum 16.1 W green 532 nm radiation and 17.6% efficiency were achieved at a fundamental power of 91.5 W. Signs of saturation in SHG efficiency and power were observed in the high power region due to thermal de-phasing. Normalized conversion efficiency was 0.23 %/W for input power < 70 W.

 

Fig. 4. (a) Upper limit of SHG power via confocal parameter b for 20 mm long device using Yb-fiber laser; (b) upper limit of SHG power via confocal parameter b for 10 mm device using the Nd:YAG laser.

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Fig.5. CW 532 nm green output power and efficiency for 10 mm long device via 1064 nm input power for the Nd:YAG laser. Solid and dash lines: quadratic and linear dependences of SHG power and efficiency.

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4. Conclusion

We investigated thermal behavior of high-quality periodically poled Mg:SLT with 8.4, 8.3 and 8.0 μm periods in high power region. From the dependence of the device temperature on IR and green power at different normalized conversion efficiencies we determined direct relationship between green power and generated heat. Green light absorption becomes a dominant factor of heat generation in multi-watt CW SHG in Mg:SLT, thus the Mg:SLT can be used without any damage in intracavity doubling. We define that short confocal parameter gives the higher upper limit of green SHG power. Single-pass second-harmonic generation of 16.1 W was demonstrated in the green region at an efficiency of 17.6% with the Nd:YAG laser at 1064 nm.