We fabricated large-aperture axis-slant quasi-phase-matching (AS-QPM) device with 8 mm x 11 mm acceptable aperture size in 2-mm-thick Mg-doped LiNbO3 crystal at 65° slant angle with 75-µm surface period. The AS-QPM has a possibility of wafer-scale-aperture device, suitable for handling high power/energy lasers.
©2011 Optical Society of America
Quasi-phase matching (QPM) device by using ferroelectric materials , such as LiNbO3 (LN), LiTaO3 (LT), and KTiOPO4 (KTP), can be realized by periodic inversion of spontaneous polarization by applying high electric field . The LN crystal presents highest nonlinearlity among ferroelectric materials for QPM device, and efficient nonlinear-optic wavelength conversion, such as second-harmonic generation (SHG) and optical-parametric oscillation (OPO), has been reported [3–5]. For handling high-power/energy lasers by QPM device, increase of device aperture is simple and practical method to avoid damages of both end faces and device inside. In conventional right-angled QPM (RA-QPM) structure, as shown in Fig. 1(a) , beam propagation axis and crystallographic z-axis cross vertically. In this structure, the device aperture is limited by the crystal thickness, and the thickness is limited by various factors, such as QPM period, quality of periodic poling, and coercive field to invert the crystal polarization. Diffusion-bonded structure by periodically poled LN and single-domain LN was reported to increasing the device aperture , because undoped congruent LN has a high coercive field of ~21 kV/mm at room temperature, and thick QPM device by undoped congruent LN was hard to be realized. Mg-doped congruent LN (MgLN), which has a decreased coercive field of ~4.5 kV/mm  and an improved resistance to photo-refractive damage, is widely used for current QPM applications. The decreased coercive field of MgLN enabled to increase the thickness of QPM device, and we have reported fabrication, demonstration, and application of large-aperture periodically poled MgLN (PPMgLN) device with up to 5 mm x 5 mm aperture [8–10]. Large-aperture PPMgLN could be also used for optical-parametric amplification in ultra short pulse generation .
Although large-aperture PPMgLN enables us to handle high-power/energy lasers, fundamental limitation of the device aperture still remains in the RA-QPM structure. To overthrow the limitation of aperture size, another arrangement of QPM structure has been needed. Here, we comment about conventional slant structure to distinguish difference of each QPM structures. In the slant structure as Fig. 1(b), called as slant QPM or tilted QPM in previous reports, crystallographic z-axis works as a rotation axis of the slant structure, and beam propagation axis and crystallographic z-axis cross vertically. In most case, QPM grating k-vector is slant against beam propagation axis. In special case of slant angle (or tilting angle) equals zero, this slant structure as Fig. 1(b) is same as the RA-QPM structure as Fig. 1(a). This slant structure has been already used for various applications, such as THz generation [12,13], and tunable QPM device . We have also reported this slant structure as tilted QPM device, to realize continuously tunable high-energy OPO by using 5-mm-thick PPMgLN device . Although this slant structure is useful and can be fabricated easily, problem of device-aperture limitation is same as the RA-QPM structure.
To overthrow the limitation of aperture size, innovative change of QPM-structure arrangement as shown in Fig. 1(c) was proposed and demonstrated by using near-stoichiometric LT (SLT) [16,17], which has a low coercive field, though nonlinear coefficient is low compared to LN. Here, we call this arrangement axis-slant QPM (AS-QPM) structure. In the AS-QPM structure, crystallographic z-axis is slant against input/output plane of the device, and the beam-propagation axis outside of the device does not cross vertically with the z-axis. The AS-QPM structure has a possibility of scalable device aperture up to wafer size, and is suitable for large-aperture QPM device for ultra-short pulse application to handle high power/energy lasers. Also, realization of the AS-QPM structure can increase a freedom of rotation in QPM device, based on the concept of angular QPM . In this work, we demonstrate fabrication and evaluation of the AS-QPM device by using MgLN, which shows larger nonlinearity compared to SLT.
2. Axis-slant QPM
Figure 2(a) & (b) present schematic views of the AS-QPM structure. In this structure, input and output beams pass through large surface of substrate crystal, which enable to realize an extremely large aperture as a QPM device.
Here we define surface-normal ZS-axis and surface-parallel XS-axis, which slant by slant angle θ0 against crystallographic z-axis and x-axis in x-z plane of the crystal structure. In this arrangement, crystallographic y-axis works as a rotation axis of the axis-slant structure. Surface QPM period Λ1 produces minimum QPM period Λ2 inside the crystal. When pump laser is inputted to the device at angle of θ1 against ZS axis, the laser passes into the device at angle of θ2 against ZS-axis and θ3 against x-axis. Original crystal thickness is denoted by d1, and effective crystal thickness along z-axis for periodic inversion of spontaneous polarization is presented by d2. Effective QPM device length and effective QPM period in the device are also denoted by d3 and Λ3. Refractive index of QPM material is presented by n2. At special case of θ1 = θ0 = θB (Brewster angle), θ3 becomes zero, and Λ3 equals to Λ2. In general, difficulty of periodic poling increase with increasing θ0 and d1, and decreasing Λ1. Also, the value of coercive field affects to maximum value of d1, and finally limit θ0.
3. Axis-slant QPM vs. Right-angled QPM
Here we discuss about fundamental difference of conversion-efficiency distribution between the AS-QPM and the RA-QPM. The distribution characteristics of the slant structure as Fig. 1(b) is same as that of RA-QPM. Although the field poling technique is simple and effective method to realize QPM devices by various ferroelectrics, resulting periodic structure becomes essentially wedged shape, which means that periodically poled (PP) structure in one side becomes wide, and the other side becomes narrow. This fundamental characteristics is inevitable in all QPM devices realized by the field poling technique, and becomes serious problems in thick crystal such as our previous report , and in short-QPM-period structure. Because the ratio of poled region to QPM period largely affect to the conversion efficiency of QPM, the wedged structure decide the conversion-efficiency distribution of the QPM device.
In case of the RA-QPM with optimum structure, that means perfectly vertical structure from + z to -z face, the efficiency distribution along z-axis should becomes uniform, as shown in Fig. 3(a) . In actual case of the RA-QPM with wedged structure, the efficiency distribution along z-axis becomes non-uniform, and depends on the wedged shape as Fig. 3(b) and (c).
In case of the AS-QPM, the effect of wedged structure is different from the RA-QPM. In optimum structure as Fig. 4(a) , the efficiency distribution along XS-axis becomes uniform. In wedged structure as Fig. 4(b), the efficiency distribution along XS-axis becomes also uniform because the beam passes through different point of wedged structure, though peak conversion efficiency is low compared to optimum structure because of the averaging effect of the wedged structure along the beam propagation axis.
Therefore, when we compare the AS-QPM device and the RA-QPM device with the same aperture, the AS-QPM device should have an improved conversion-efficiency distribution compared to the RA-QPM device, although peak conversion efficiency is degraded with depending on the wedged structure. It is needless to say, the AS-QPM device has a fundamental limitation in QPM device length, and the RA-QPM device has a fundamental limitation in device thickness (connects to aperture size).
4. Measurement of inversion field in axis-slant MgLN
We have proposed a REFVR method to characterize coercive field of various ferroelectrics for QPM device  at the conventional RA-QPM structure. In the AS-QPM structure, inversion field to invert the polarization of axis-slant crystal increase by factor of 1/cosθ0 compared to the coercive field of the RA-QPM structure. Figure 5(a) shows a measured inversion field of 1-mm-thick axis-slant crystal with various θ0 by the REFVR method at temperature T = 120°C and slope of ramping electric field S = 10 kV/mm-s. The measured inversion field can be well fitted by theoretical curve. As increasing of θ0, effective crystal thickness along z-axis also increase, which results in increasing of the inversion field, and the inversion field at θ0 = 65° increase more than two times compared to the coercive field.
Figure 5(b) presents a measured inversion field dependence on crystal temperature of 1-mm-thick axis-slant crystal with θ0 = 65° at S = 10 kV/mm-s. As noted in our previous report [7,19], the coercive field of MgLN drastically decreases with increasing crystal temperature. Therefore, increased inversion field by increasing the slant angle such as θ0 = 65° could be suppressed by temperature elevation at the field poling process.
5. Fabrication of axis-slant QPM device
For practical use of high-power/energy nonlinear device, AS-QPM device with large θ0 and thick d1 are suitable, although difficulty in device fabrication increase. We tried to fabricate a AS-QPM device by MgLN with θ0 = 65°, d1 = 2 mm, and Λ1 ~75 µm, which results in d3 ~2.2 mm and Λ3 = Λ2 ~32 µm at the condition of θ1 = θ0 ~θB. Temperature elevated field poling for AS-QPM was done in an insulation-oil bath at T = 120°C with high voltage pulses of ~13 kV, as same method as our previous large-aperture QPM device. Periodic electrode on + ZS face and uniform electrode on -ZS face were prepared for applying the high voltage pulses. Electrode size on + ZS face was 9 mm for y-axis and 15 mm for XS-axis. After finished the periodic inversion, the electrode was removed by hydrofluoric-acid etching.
Figure 6 presents y-cut face photograph of obtained axis-slant periodic structure. The axis-slant periodic structure could be fabricated in 2-mm-thick MgLN with θ0 = 65°. The wedged shape of periodic structure become serious in thick MgLN, and situation is same as in the axis-slant periodic poling in MgLN, which means that periodic structures near + ZS surface is almost merged with neighboring structures, and that periodic poling near -ZS surface is insufficient. In current poling condition, periodic structures near + ZS surface is almost merged with neighboring around ~200 µm depth from + ZS surface. As noted before, the effect of the wedged structure to the conversion-efficiency distribution could be averaged along propagation direction in the AS-QPM device. Also, the effect of the merged region by the wedged structure around ± ZS surface area could be partially removed, because these region should be polished for realizing input/output faces.
6. Evaluation of axis-slant QPM device by SHG experiment
To evaluate aperture size of the fabricated AS-QPM device, preliminary experiment by picosecond SHG was demonstrated. A Q-switched Nd:YAG laser of 1064 nm wavelength with 35 ps duration at 10Hz repetition rate (Quantel, YG901C10) was used for pump source. QPM-SHG for 1st-order phase matching at 1064 nm pumping by d33 of MgLN needs QPM period ~6.9 µm . The fabricated AS-QPM device with θ0 = 65° and Λ1 ~75 µm produces minimum QPM period Λ2 ~32 µm inside the device, which is close to the QPM-period for 5th-order phase matching of ~34.5 µm at conventional collinear set up with pump and SH waves. Therefore, we can realize 5th-order QPM-SHG by large-aperture axis-slant QPM device at slightly non-collinear set up, as shown in Fig. 7(a) .
Both ± ZS surface of the device with original 2-mm thickness were polished to 1.67 mm thickness for making input/output faces. QPM size of the device along y-axis was 9 mm. Also, QPM electrode size along XS-axis at the field poling was 15 mm, which resulted in full ( + ZS to -ZS) PP-region of ~11 mm, with sandwiched by partial PP-region of ~4 mm and ~4 mm at both side, as shown in Fig. 7(a). Pump beam diameter was set to ~2 mm at intensity FWHM. Maximum input energy was 22 mJ in current set up.
SH wave of 532nm wavelength could be easily obtained because of high intensity pumping of the picosecond laser. At the incident angle θ1 = 65°, walk off angle between pump and SH wave outside of the device was measured to 4°. Figure 7(b) shows typical input/output characteristics of full PP-region at room temperature. Maximum SH energy of 1.2 mJ was obtained at pumping energy of 22 mJ, with conversion efficiency of 5.5%. The low conversion efficiency, which is resulted by the short QPM device length and the high-order QPM, enables a sensitive measurement of periodic structure.
Figure 8 shows measured SH distribution results along (a) y-axis and (b) XS-axis of the device, operated at 17 mJ pumping. Measured result along y-axis as Fig. 8(a) shows wide aperture size > 8 mm (y-axis from 1 mm to 9 mm). One of possible reason of periodic SH-intensity fluctuation around center region (2 mm < y < 8 mm) in y-axis scanning may be discrepancy between crystallographic y-axis and scanning y-axis. Result along XS-axis as Fig. 8(b) well present both full PP-region and partial PP-region. Although the SH intensity at center region ( = full PP region) is high, the intensity at side area ( = partial PP region) decrease slowly as going to the edge of partial PP-region. Most of SH-intensity fluctuation at center region (3 mm < XS < 14 mm) in XS-axis scanning may attribute to the quality of periodic poling. As presented in Fig. 6, penetrations of each inverted structures to -ZS surface are insufficient and non-uniform in current poling condition, which may cause the fluctuation of SH-intensity in XS-axis scanning. Although it is needed further improvement of the poling quality for large-aperture, uniform conversion-efficiency-distribution QPM device, we could confirm the expanded aperture size of the AS-QPM device of > 8 mm for y-axis and > 11 mm for XS-axis, which is 3-times larger device-aperture size than our previous RA-QPM device with 5mm x 5mm aperture.
We demonstrated a fabrication of axis-slant QPM structure in 2-mm-thick MgLN crystal at 65° slant angle with 75-µm surface QPM period. Also we presented a preliminary evaluation of acceptable aperture size by picosecond SHG experiment, and confirmed an expanded device aperture of the fabricated axis-slant QPM device, compared to the right-angled QPM and the conventional slant-structure QPM. We can expect that the axis-slant QPM can realize a wafer-scale-aperture device, which can easily handle high-power/energy lasers for giant micro-photonics.
The authors would like to thank to Prof. M. M. Fejer of Stanford University, for fruitful discussion about axis-slant QPM. This research was partially supported by Grant-in-Aid for Scientific Research 22560046 by JSPS, and Photon-Frontier-Consortium Project by MEXT of Japan.
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