We present a next generation of large-aperture periodically poled Mg-doped LiNbO3 (PPMgLN) device with 10-mm thickness. Efficient optical parametric oscillation with 540 mJ output energy at 709 mJ pumping by 1.064 µm laser in 10 nanoseconds operation could be demonstrated using the 10-mm-thick PPMgLN with an inversion period of 32.2 µm at total conversion efficiency > 76%. We also confirmed that degradation effect of conversion-efficiency distribution by wedged-inversion structures, which is inevitable in current poling condition of the large-aperture PPMgLN, can be ignored in high-intensity operation.
©2012 Optical Society of America
Optical parametric oscillation (OPO) pumped by conventional 1 µm laser source can realize efficient and practical mid-infrared light sources , which are of interest for various applications, such as molecular spectroscopy and remote sensing . Efficient 2.1-µm generation by optical parametric master oscillator and power amplifier by cascading of bulk KTiOPO4 (KTP) crystals was reported .
Quasi-phase matching (QPM) device with periodically inverted structure of nonlinear coefficient can enable an efficient and various wavelength conversion at arbitrary wavelength in the transparent range of the QPM material [4, 5]. Ferroelectric materials, such as LiNbO3 (LN) [6–8], LiTaO3 (LT) [9–11], and KTP [12, 13], and semiconductor materials, as GaAs [14, 15], GaP , and GaN , have been used as major materials for the QPM device. Recently, crystal quartz has been also investigated as a material for the QPM device, especially in ultra-violet region . Among these materials, the crystal polarization of ferroelectrics with spontaneous polarization can be inverted by applying an intense electric field, which enabled us to realize a QPM ferroelectric device by periodic field poling technique after crystal growth .
Mg-doped congruent LN (MgLN) shows relatively high nonlinearity (d33 ~25.2 pm/V @ 1.064 µm ) with wide transparency range (0.35 ~ 5 µm) among ferroelectrics, and has been used widely for the QPM device, such as second-harmonic blue/green generation [7, 8], and optical-parametric mid-infrared light generation . Although the conversion efficiency of a QPM device generally increases with increasing pumping intensity and QPM-device length, maximum pumping intensity is limited by the endurance of input/output face or inside of the QPM device . Therefore, a simple way to realize both high-energy and highly efficient QPM wavelength conversion is to use large-aperture QPM device pumped by adequate-intensity pumping source. Last several years, we have reported a high-energy QPM-OPO using a large-aperture periodically poled MgLN (PPMgLN) up to 5-mm thickness . QPM-OPO with 124 mJ output by 193 mJ pumping in 10-ns pulse region using a 5-mm-thick PPMgLN device could be demonstrated . We have also reported 5-mm-thick periodically poled device using Mg-doped congruent LT (MgLT), as another candidate of the high-energy QPM device . Zukauskas et al. have reported high-quality periodically poled Rb-doped KTP with 5-mm thickness for high-energy OPO . Imura et al. have reported room-temperature-bonding of GaAs plates with 4 mm x 5 mm aperture for THz-wave generation .
In this paper, we present a next generation of large-aperture PPMgLN device with 10-mm thickness for higher-energy QPM-OPO. Temperature-elevated field poling of 10-mm-thick MgLN with around 30 µm poling period for OPO pumped by 1.064 µm laser source was realized. Aperture-area size of the 10-mm-thick PPMgLN device simply increases 4 times larger compared to conventional 5-mm-thick device in case of a circular beam, which enabled to demonstrate an improved QPM-OPO with half-joule class output energy by joule-class pump source in 10-ns pulse region. Also, relations between beam shape of OPO-output wave and periodically inverted wedge-like structure of the large-aperture PPMgLN device were characterized.
2. Effective coercive field of MgLN
Mg doping to LN crystal drastically improved various properties of MgLN as a material for the QPM device, such as a reduced coercive field EC to invert the crystal polarization and a suppressed photo-refractive damage. Although periodic poling of MgLN is still difficult compared to undoped LN crystal, temperature elevation to decrease EC and multi short-pulse application to suppress sidewise motion of the inverted region can help to improve the quality of periodic poling in MgLN. In a previous report, we presented the dependence of EC on crystal temperature in static condition (continuous-wave electric-field application) , which is not suitable as the reference for the multi short-pulse application method, because the response characteristics of EC in short-pulse application condition is totally different from that in the static condition. We call the EC in short-pulse application as effective EC in this study. Recently, we have proposed a REFVR method to characterize the effective EC, which is applicable as the reference in short-pulse application condition .
The effective EC of MgLN was measured by using a 1-mm-thick crystal with a circular electrode of 1-mm diameter in an insulation-oil bath. Ramping electric field with ramping ratio S was applied to the crystal at various temperature T. Further information is given in our previous report . Measured effective EC on various S and T is shown in Fig. 1 . Various conditions of S from 10−2 kV/mm-s to 104 kV/mm-s and T from 25°C to 200°C were evaluated, and large dependence of the effective EC on both S and T could be measured, which revealed that shorter-pulse (higher S) poling needs higher-voltage application to invert crystal polarization. In a practical situation at periodic poling of thick MgLN, we have to find applicable poling condition within the various limitations, such as maximum process temperature of poling set up, maximum voltage and ramping rate of high-voltage power supply.
3. Periodic poling in 10-mm-thick MgLN
Commercially available MgLN crystal (Yamaju Ceramics) with 10-mm thickness in z-axis was prepared for periodic poling. The period Λ of QPM structure was set to around 30 µm for realizing OPO pumped by 1.064 µm laser around room temperature. Temperature-elevated field poling was demonstrated in an insulation-oil bath at T ~120°C [6, 10, 11]. High-voltage continuous triangle-pulse train of 33-kV peak voltage with 2-ms pulse-repetition time as shown in Fig. 2 was applied to the MgLN crystal through periodic aluminum electrode by using a high-voltage and high-speed power amplifier (Trek, model 40/15). The applied voltage of 33 kV to the 10-mm-thick crystal was almost twice compared to that of conventional 5-mm-thick crystal at the same process temperature.
Figure 3 shows y-cut photographs of typical periodic structure in 10-mm-thick MgLN from + z to -z surface. Penetrating periodic structure with Λ around 30 µm could be realized in 10-mm-thick MgLN, though the obtained structure has a wedge-like shape that the periodic structure near + z surface is almost merged with neighboring patterns and that penetration of inversion near -z surface is insufficient. Because leakage current in the periodic poling of MgLN becomes severe compared to undoped LN, exact penetration-depth control of inverted structure in MgLN becomes difficult, and periodic structure obtained by current poling condition shows a slightly insufficient penetration, as Fig. 3. Although poling condition for the 10-mm-thick MgLN should be improved to reduce the wedged structure and to control the penetration depth, we can use the 10-mm-thick PPMgLN device with Λ around 30 µm for high-power/energy laser source with large beam size up to 10-mm diameter.
4. Effect of wedged structure in large-aperture PPMgLN
Here, we discuss the effect of the wedged structure in the large-aperture PPMgLN device at high-power/energy operation by using a duty ratio D, defined as a ratio of inverted-region width to the QPM period Λ. Theoretical investigations in case of second-harmonic generation (SHG) using a QPM device are already reported [4, 5].
In 1st-order QPM, dependence of effective nonlinear coefficient deff on D is given asFig. 4(a) . In this case, deff becomes uniform to 2d / π at any point along z-axis, and conversion efficiency η becomes also uniform along z-axis, at any pumping intensity from continuous wave to ultra-short high-peak pulse operation, and at any QPM-area length.
On the other hand, in case of the wedged structure, D depends on the position along z-axis, and distributions of deff and η along z-axis depends on various factors, such as the inversion depth of the wedged structure, the intensity of pumping source, and QPM-area length. In case of moderate penetration, which has an inverted structure with triangular shape of slightly merged region (D = 1) near + z surface, optimum inversion (D = 0.5) at middle, and slight penetration (D = 0) near -z surface as Fig. 4(b), an area with highest deff exists around middle region along z-axis, and areas around + z and -z surface has low deff. In case of insufficient penetration as Fig. 4(c), highest deff area moves toward + z surface side, and area without penetration of inversion around -z surface has no effective nonlinearity. Also in case of over penetration as Fig. 4(d), highest deff stays near -z surface side, and area around + z surface has no effective nonlinearity because of merged inversion. As a result, position of maximum η by highest deff along z-axis depends on the inversion depth of the wedged structure.
Though η depends on D, η also depends on the set up of device operation, such as pumping intensity and QPM-area length. Because η becomes less sensitive to D-distribution (shape of the wedged structure) by saturation of η in high η condition, such as high-intensity pumping and long device length, distribution of η along z-axis becomes smooth except for the area near + z and -z surface. For example, conversion efficiency ηSHG on D in case of QPM-SHG can be presented as4, 5]. These three parameters relate to the set up of device operation. Figure 5 shows calculated distribution shapes of ηSHG normalized by ηSHG(D = 0.5) on z in case of moderate penetration as Fig. 4(b) at various conditions of α. As α increases, ηSHG also increases toward saturation, and the distribution of ηSHG along z becomes smooth except for the area near + z and -z surface. Similar characteristics can be estimated in case of other wavelength conversion as OPO. As a result, distribution of η along z-axis becomes less sensitive to the wedged structure in high-efficiency operation.
As shown in Fig. 3, the periodic structure of the 10-mm-thick PPMgLN obtained in current poling conditions has a wedged structure with slightly insufficient penetration, similar to Fig. 4(c). Therefore, it can be estimated that conversion efficiency of the 10-mm-thick PPMgLN becomes higher in + z side at low pump-intensity condition and that smooth conversion-efficiency distribution can be obtained by increasing pump intensity. The large-aperture PPMgLN device is prepared especially for high-power/energy laser operation. Therefore, if the conversion-efficiency distribution on the aperture face of the large-aperture PPMgLN becomes a problem, it means that pumping condition should be changed to realize a smooth conversion-efficiency distribution, by increasing pump the intensity, device length, and shrinking the pump-beam diameter, under the endurance limitation of the PPMgLN device. It is needless to say that the wedged structure can be suppressed in fabrication of small-aperture PPMgLN device.
5. Optical parametric oscillation by 10-mm-thick PPMgLN
PPMgLN device with 10-mm thickness was prepared for high-energy OPO as shown in Fig. 6 . Size of the PPMgLN device and area of the QPM poling were 40 mm x 15 mm x 10 mm (x, y, and z axis), and 38 mm x 12 mm (x, y), respectively. The QPM period Λ was 32.2 µm, which can generate a signal wave, λs, around 1.9 µm and an idler wave, λi, around 2.4 µm by 1.064 µm pumping at 25°C operation. Both input and output faces are anti-reflection coated for pump, signal and idler waves. Input mirror was a plane mirror with high reflectivity for both signal and idler waves, and high transmission for pump wave. Output coupler was also a plane mirror with a reflectivity of ~40% for signal wave, high transmission for idler wave, and high reflection for pump wave. PPMgLN device was placed in a temperature controlled oven stabilized at 25°C between the above mirrors with a cavity length of 10 cm.
To evaluate the effect of the wedged structure, point dependence of OPO output in the 10-mm-thick PPMgLN was characterized using a middle-energy Q-switched Nd:YAG laser (λp = 1.064 µm, frep = 30 Hz, τpulse = 10 ns) with a 5-mm diameter . Wavelength of signal and idler waves at 25°C operation were measured to 1.90 µm and 2.42 µm, respectively. The OPO characteristics of four different points on the aperture face could be measured as presented in Fig. 7 , which showed that threshold energy for the upper points (Point 1 and 2) was low compared to that for the lower points (Point 3 and 4), and that the slope efficiency in high-energy pumping region (> 40 mJ) was almost the same at all points. There was no significant difference between the OPO characteristics of the left points (Point 1 and 3) and the right points (Point 2 and 4). These results show that the wedged structure largely affects the low-energy OPO characteristics such as the threshold energy, and that the wedged effect can be suppressed in the high-energy region because of the saturation of conversion efficiency η, as noted in Section 4.
Next, we demonstrated a high-energy OPO pumped by a high-energy Q-switched Nd:YAG laser (λp = 1.064 µm, frep = 10 Hz, τpulse = 10 ns) of top-hat shape, 9-mm diameter beam with 710-mJ maximum energy, as shown in Fig. 8 . Oscillation threshold energy was about 10 mJ, and maximum output energy of 540 mJ (average power of 5.4W) for total of both signal and idler waves could be obtained without device damage at a pumping energy of 709 mJ, which is 4-times improvement compared to previous 5-mm-thick PPMgLN . Pulse-energy fluctuations of the high-energy Nd:YAG laser was measured to 3%, and pulse-energy fluctuations of the OPO at the maximum output condition was 3%. Although we could not evaluate the individual energy of signal and idler waves because of the damage in optical filters, a total conversion efficiency > 76% at the maximum output energy of 540 mJ and a high slope efficiency ~82% at the low pump-energy range could be realized. Though the maximum output energy in this demonstration was limited by an available pump energy, final limitation of the maximum output energy is a damage problem of 10-mm-thick PPMgLN device, which is the same as that of 5-mm-thick device.
Figure 9 presents a beam-shape evaluation of the OPO-signal wave (λs ~1.9 µm) at various pumping energy from 29 mJ to 130 mJ, measured by a pyroelectric camera (Spiricon, Pyrocam NIR) after pump- and idler-cut filters. As commented previously, the 10-mm-thick PPMgLN, realized by the current poling conditions, has a wedged structure with insufficient penetration, which results in both the point-dependence of conversion efficiency along the z-axis in low pump-intensity condition and the smoothing of conversion-efficiency distribution by increasing the pump intensity. The measured results shown in Fig. 9 well present these characteristics, and agree with the point dependence of OPO by small-diameter pump beam presented in Fig. 7. In the low pump region as 29 mJ pumping, efficient OPO occurs around the + z side inside of the 9-mm-diameter circular beam of the pump laser. As the pump energy increases, OPO beam extended toward -z side in whole area of the pump beam. Roughly, the point-dependence of conversion efficiency could be ignored at a pump energy > 100 mJ in the current OPO set up, which is the pump-energy region of the original purpose for the 10-mm-thick PPMgLN. Beam-size transition of the OPO output after the output coupler was evaluated at the 130-mJ pumping condition. Initial output beam with 9-mm diameter in FWHM around the output coupler expanded its diameter to 25 mm by 900-mm propagation with a full divergence angle of 18 mrad. In addition to both the flat-flat cavity-mirror configuration and the top-hat-shape pumping source, inhomogeneous nonlinearlity by the wedged structure may degraded the output beam quality, which should be improved at practical application of the 10-mm-thick PPMgLN device.
We fabricated a 10-mm-thick periodically poled Mg-doped congruent LiNbO3 device for handling higher-energy laser pulses than the previous 5-mm-thick device. Also, we demonstrated high-energy optical-parametric oscillation with half-joule output energy in 10-nanoseconds pulse region using the 10-mm-thick device, which is a 4-times improvement compared to the previous 5-mm-thick device. Beam-shape evaluation revealed that point-dependence of conversion efficiency, which is due to the wedged structure of polarization inversion, can be ignored by increasing the pump intensity. We hope that our large-aperture PPMgLN device can handle joule-class energy in the nanoseconds region in the near future.
This research was partially supported by Grant-in-Aid for Scientific Research (C) 22560046 by JSPS, and Photon-Frontier-Consortium Project by MEXT of Japan.
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