We report on high energy optical parametric oscillation of 118 mJ output with ~70% slope efficiency in 10 ns duration of 30 Hz operation by using Mg-doped congruent composition LiTaO3 (MgLT). The periodically poled MgLT device with ~30 µm period for quasi-phase matching (QPM) in 5-mm-thick crystal are prepared. MgLT crystal could become a candidate for high-energy and higher durability material of QPM device, compared to conventional Mg-doped congruent composition LiNbO3.
©2010 Optical Society of America
Quasi-phase matching (QPM) technique [1,2] have realized an efficient and various types of nonlinear wavelength conversion using a desired nonlinear coefficient in arbitrary wavelength by specially designed crystal structure, which is different with conventional birefringent phase matching technique. Various types of crystals have been used for materials of QPM device. Ferroelectric crystals such as LiNbO3 (LN) [3–7], LiTaO3 (LT) [8–12], and KTiOPO4 [13,14], and semiconductor crystals such as GaAs  and GaP , and crystal quartz  are typical materials for the QPM device with periodically inverted structure. Although semiconductor materials have large nonlinear coefficients and wide transparent range up to mid-IR range, multi-photon absorption prevents to use high-power 1 µm laser for pumping. Crystal quartz is hard material with short absorption edge, though its nonlinearity is low. Ferroelectrics with large nonlinearity and wide transparent range from UV to mid-IR are suitable for effective QPM device in the range from visible to mid-IR region. Also, ferroelectrics with spontaneous polarization can be poled to the specially designed structure by the high electric-field poling technique after crystal growth , although it is difficult to apply the field poling technique to the semiconductors.
Both nonlinear optical coefficients and transparent range (absorption coefficient) are important parameters for an effective nonlinear wavelength conversion using QPM devices. LN has relatively high nonlinear coefficient (d 33 ~25.2 pm/V @ 1.064 µm ), although transparent range is narrow (0.35 ~5 µm ) compared to LT. Because Mg-doped congruent LN (MgLN) shows an improved photorefractive-damage resistance and a decreased coercive field to invert crystal polarization compared to undoped congruent LN, many researchers have reported efficient wavelength conversion, such as second-harmonic generation (SHG) [6,7], difference-frequency generation (DFG), and optical-parametric oscillation (OPO) [4,5], by using periodically poled MgLN (PPMgLN).
Last several years, we have reported highly efficient and high-energy QPM-OPO using 5-mm-thick PPMgLN devices [4,5]. As increasing of both conversion efficiency and handling power/energy in our QPM-OPO experiments using large-aperture PPMgLN devices, crystal damage inside of the PPMgLN device have become severe problem. Figure 1 shows an example of a broken PPMgLN device in high-energy QPM-OPO experiment at maximum input pump energy of ~270 mJ (pumped by Q-sw Nd:YAG laser of 1.064 µm, Rep. rate = 30 Hz, Pulse duration ~10 ns).
We assume that the damage of the broken PPMgLN was caused by unnecessarily and imperfectly phase-matched high-order harmonics between pump, signal and idler waves in OPO. Actually, we can see various colors such as green, red, orange, and blue lights from OPO cavity in our experiments as shown in Fig. 2(a) . Operation wavelength of signal and idler waves in OPO are 1.736 µm and 2.749 µm, respectively. Even in low pumping region of ~60 mJ using 5-mm-thick PPMgLN device, variously cascaded high-order harmonic waves can be measured as Fig. 2(b). As a typical example in our high-energy PPMgLN-OPO, green light can be seen below oscillation threshold, red~orange light starts to generate simultaneously with OPO start, and blue light is produced in higher pumping region. Further pumping results in the damage of PPMgLN as shown in Fig. 1.
Compared to LN-type crystals, LT-type crystals have relatively wide transparent range (0.28 µm ~5 µm ) and small absorption especially in visible~UV range, although their nonlinear coefficients are relatively low (d 33 ~13.8 pm/V @ 1.064 µm ), which are supposed to be a suitable candidate of material for more efficient and high power/energy QPM device. In last several years, LT crystals with Mg-doped stoichiometric composition have been used to handle higher power/energy QPM application compared to MgLN, especially for high power second-harmonic green generation [10,11].
In this study, we focus on the recently developed Mg-doped congruent LT (MgLT) for high-energy optical-parametric systems (OPS) such as OPO, optical-parametric amplification, and optical-parametric generation. The MgLT with congruent composition have possibility of stable and mass production by using a conventional Czochralski method, which is different from stoichiometric-composition crystals prepared by using DCCZ  or VTE [20,21] method. We previously reported various properties of MgLT for material of QPM device, such as optical properties, thermal conductivity, and coercive field . Also, we found that characteristics on the field poling process of MgLT is similar to that of MgLN, which is favorable for us to use both MgLT and MgLN. In this paper, MgLT with 7 mol% Mg doping (the doping level of Mg means the amount in the melt state at the Czochralski process) is evaluated for material of highly efficient and high-energy QPM device. We present preparation of periodically poled MgLT (PPMgLT) device with 5-mm thickness, and demonstrate high-energy QPM-OPO using the PPMgLT device. Finally, results by PPMgLT are compared with that by PPMgLN.
2. Periodical poling of MgLT
The coercive field of MgLT (7 mol% Mg), measured by REFVR (ramping electric field with various rates) method with the ramping rate S = 1 kV/mm-s, was ~3.4 kV/mm at 23°C, and can be decreased to ~2.0 kV/mm by increasing crystal temperature to 150°C . Condition of periodical poling depends mainly on QPM period and crystal thickness. In this paper, period of QPM structure (Λ QPM) for PPMgLT was chosen to ~30 µm for OPO experiments by 1.064 µm laser pumping, and 5-mm-thick MgLT was used to compare with MgLN. An aluminum electrode of 0.1 µm thickness with periodical structure was fabricated on the MgLT by a conventional vacuum evaporation method. The temperature-elevated field poling in an insulation-oil bath was done by the same method as that for MgLN . Finally, penetrating periodic structure with ~30 µm period in 5-mm-thick MgLT could be demonstrated by applying ~9 kV voltage pulses at crystal temperature of 150°C. The applied voltage of ~9 kV for periodical poling in 5-mm-thick MgLT is considerably low compared to that of 5-mm-thick MgLN (~16 kV at 120°C), which means that MgLT has a possibility of larger device-aperture than MgLN. Figure 3 show photographs of obtained periodical structure in 5-mm-thick MgLT. Although the obtained periodical structures needs more improvement for realizing uniform structure, penetrating structures with ~30 µm period from + z surface to -z surface in 5-mm-thick MgLT can be confirmed. In general, uniformity of the periodical structure can be improved by using higher voltage pulses less than damage threshold of electric field, if adequate high-voltage and high-speed power supply can be prepared. In case of MgLT crystal, we can expect to realize fine periodical structure with suitable QPM period and crystal thickness for each application by further improvement of field poling condition.
3. OPO using 5-mm-thick PPMgLT
PPMgLT device with Λ QPM = 32.5 µm and QPM-pattern size of 5 mm x 37 mm (W x L, for y and x-axis) was prepared for OPO experiments. The device includes two QPM patterns in device size of 5 mm x 16 mm x 39 mm (H x W x L, for z, y, and x-axis). Both input and output faces are uncoated. A Q-switched Nd:YAG laser (Spectra Physics, LAB-170-30) with 1.064 µm wavelength (Rep. rate = 30 Hz, Pulse duration ~10 ns) was used as a pump source. Polarization of the pump source was set parallel to the z axis of PPMgLT device. The input coupler was a plane mirror with high reflectivity for both signal (~1.8 µm) and idler (~2.6 µm) waves and with high transmission for pump wave. The 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. The PPMgLT device was placed in a temperature controlled oven between above mirrors with cavity length of 10 cm. Diameter of pump beam was set to ~4 mm.
Figure 4 shows dependence of OPO output wavelength on device temperature measured at pumping energy of ~30 mJ. As increasing of the device temperature, OPO approaches to degeneracy point of 2.128 µm, and spectral bandwidth of output waves are slightly broadened. For realizing a high-energy and narrow-bandwidth OPO using PPMgLT, spectral narrowing optics such as volume Bragg grating are effective as shown in previous reports [5,14].
Figure 5 presents typical OPO output dependence on input pump energy by 5-mm-thick PPMgLT at 25°C operation, and wavelength for signal and idler waves were 1.84 µm and 2.52 µm, respectively. The oscillation threshold energy was ~19 mJ, and maximum total output energy of signal and idler waves reached 118 mJ at input energy of 196 mJ with initial slope efficiency of ~70%. High energy QPM-OPO by PPMgLT can be successfully demonstrated with 5-mm-thick device.
We also demonstrated QPM-OPO by 5-mm-thick PPMgLN using the same set up (mirrors, cavity, pump source, etc) for comparison. Wavelength of signal and idler waves by PPMgLN was 1.89 µm and 2.43 µm with Λ QPM = 32.2 µm at temperature of 25°C. Obtained total output was measured to 124 mJ at 193 mJ pumping, and threshold energy was ~6 mJ. Although threshold energy differs more than three times, which is because of the difference of nonlinear coefficient between two materials, total output energy by PPMgLT was almost comparable with that by PPMgLN. We suppose that nonlinear coefficients of material cannot become a dominant parameter in highly efficient and high energy region, and that MgLT, as same as MgLN, can become a candidate for material of high energy QPM.
For practical use of PPMgLN and PPMgLT, end-face coating is important for both anti reflection and surface protection. Because current PPMgLT devices equip no end-face coating, maximum pumping energy is limited by surface damage of end face, although PPMgLN devices already equipped . Therefore, we can expect higher energy operation at higher pumping energy by using PPMgLT device with appropriate end-face coating.
4. Comparison of PPMgLT with PPMgLN
As presented in previous report, MgLT has higher thermal conductivity and shorter absorption edge in UV region compared to MgLN , although nonlinear coefficients are small. Figure 6 shows typical example of high-energy PPMgLT-OPO experiment, for comparison with that of PPMgLN-OPO as shown in Fig. 2. Wavelength of signal and idler waves are 1.740µm and 2.739 µm, respectively, pumped by 1.064 µm laser. The pump energy for PPMgLT-OPO in Fig. 6 was ~120 mJ, which is almost twice higher than that for PPMgLN-OPO in Fig. 2. Although cascaded high-order harmonic waves between pump, signal, and idler waves could be measured as Fig. 6(b), efficiency of the cascaded high-order harmonic generation in PPMgLT-OPO is much low compared to that in PPMgLN-OPO, and undesired lights except for green (second harmonics of pump) decreased as shown in Fig. 6(a), compared to Fig. 2(a). These results mean that we can expect higher durability of MgLT compared to MgLN in high energy region.
We presented periodical poling in 5-mm-thick MgLT with ~30 µm period. Subsequently, we demonstrated high energy OPO up to 118 mJ output at 196 mJ pumping with ~70% slope efficiency using 5-mm-thick PPMgLT (pulse operation of 10 ns duration) in device temperature of 25°C, which reached almost comparable value with that by PPMgLN.
In spite of relatively small nonlinear coefficient of MgLT compared to MgLN, PPMgLT could be used for efficient QPM-OPO. Also, we suppose that decreasing of undesired cascaded harmonic generation connect to suppress a device damage especially in high power/energy operation. We can expect higher energy operation using PPMgLT device with end-face coating, which is a next work of us.
The authors would like to acknowledge Yamaju Ceramics Co., Ltd. for their help to investe MgLT crystals.
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