Optical wavelength conversion element and method of manufacturing the same

An optical wavelength conversion element is formed of a ferroelectric material which has a nonlinear optical effect, and is provided with periodic domain reversals arranged in one direction and converts the wavelength of a fundamental wave impinging thereupon in the direction in which the periodic domain reversals are arranged. The ferroelectric material is LiNb.sub.x Ta.sub.1-x O.sub.3 (0.ltoreq..times..ltoreq.1) doped with at least one of Zn, Sc and In.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to an optical wavelength conversion element which 
converts a fundamental wave to a second harmonic, and more particularly to 
an optical wavelength conversion element in which periodic domain 
reversals are formed on a ferroelectric material having a nonlinear 
optical effect. This invention further relates to a method of 
manufacturing such an optical wavelength conversion element having domain 
reversals. 
2. Description of the Related Art 
There has been proposed by Bleombergen and et al. a method of converting a 
fundamental wave to a second harmonic by use of an optical wavelength 
conversion element formed with a region where the spontaneous polarization 
(domain) of a ferroelectric material having a nonlinear optical effect is 
periodically reversed. (See Phys. Rev., vol. 127, No. 6, 1918 (1962)) In 
this method, by setting pitches .LAMBDA. of the domain reversals to an 
integer multiple of the coherence length .LAMBDA.c given by formula 
EQU .LAMBDA.c=2.pi./{.beta.(2.omega.)-2.beta.(.omega.)} (1) 
wherein .beta.(2.omega.) represents the propagation constant of the second 
harmonic and .beta.(.omega.) represents the propagation constant of the 
fundamental wave, phase matching (artificial phase matching) between the 
fundamental wave and the second harmonic can be obtained. When the 
wavelength is converted by use a bulk crystal of a nonlinear optical 
material, phase matching can be achieved only at a particular wavelength 
inherent to the crystal. However in accordance with the method described 
above, phase matching can be efficiently achieved for any wavelength by 
selecting pitches .LAMBDA. of the domain reversals to satisfy the formula 
(1). 
As a ferroelectric material suitable for forming such periodic domain 
reversals, there has been known LiNbO.sub.3 doped with Mg as disclosed, 
for instance, in U.S. Pat. No. 5,568,308. Mg-doped LiNbO.sub.3 is higher 
than non-doped LiNbO.sub.3 by more than two digits in the optical damage 
threshold value. Accordingly when periodic domain reversals are formed on 
Mg-doped LiNbO.sub.3, an optical wavelength conversion element which can 
generate a high power wavelength-converted wave with a high wavelength 
conversion efficiency can be obtained. 
As another ferroelectric material suitable for forming periodic domain 
reversals, there has been known LiTaO.sub.3 doped with Mg. There have been 
made various attempts making optical waveguide type or bulk crystal type 
optical wavelength conversion elements by use of such ferroelectric 
materials. 
As a method of forming periodic domain reversals on a ferroelectric 
material, there has been known a method in which periodical electrodes 
each having a predetermined width are formed on a ferroelectric substrate 
at predetermined pitches and electric fields are imparted to the 
ferroelectric substrate through the periodical electrodes as disclosed in 
U.S. Pat. No. 5,568,308. 
However the conventional optical wavelength conversion elements comprising 
a substrate of Mg-doped LiNbO.sub.3 or Mg-doped LiTaO.sub.3 formed with 
periodic domain reversals are disadvantageous in that the pitches of the 
periodic domain reversals are apt to fluctuate and it is difficult to 
achieve a high wavelength conversion efficiency. 
Further in the conventional optical wavelength conversion elements, each of 
the periodic domain reversals is apt to be formed wider than the width of 
each electrode though it should be equal to the width of each electrode. 
Thus there has been a problem that it is difficult to form each of the 
periodic domain reversals precisely in a desired width. 
SUMMARY OF THE INVENTION 
In view of the foregoing observations and description, the primary object 
of the present invention is to provide an optical wavelength conversion 
element in which the pitches of the periodic domain reversals and the 
width of each periodic domain reversal are formed precisely in desired 
values, whereby a high wavelength conversion efficiency can be obtained. 
Another object of the present invention is to provide a method of 
manufacturing such an optical wavelength conversion element. 
In accordance with a first aspect of the present invention, there is 
provided an optical wavelength conversion element comprising a 
ferroelectric material which has a nonlinear optical effect, which is 
provided with periodic domain reversals arranged in one direction and 
which converts the wavelength of a fundamental wave impinging thereupon in 
the direction in which the periodic domain reversals are arranged, wherein 
the improvement comprises that 
said ferroelectric material is LiNb.sub.x Ta.sub.1-x O.sub.3 
(0.ltoreq..times..ltoreq.1) doped with at least one of Zn, Sc and In. 
In accordance with a second aspect of the present invention, there is 
provided a method of manufacturing the optical wavelength conversion 
element comprising the step of 
applying electric fields to a single domain ferroelectric material of 
LiNb.sub.x Ta.sub.1-x O.sub.3 (0.ltoreq..times..ltoreq.1) doped with at 
least one of Zn, Sc and In through periodic electrodes formed in a 
predetermined pattern, thereby forming periodic domain reversals on the 
ferroelectric material. 
When LiNb.sub.x Ta.sub.1-x O.sub.3 (0.ltoreq..times..ltoreq.1) doped with 
at least one of Zn, Sc and In is used as the ferroelectric material, the 
pitches of the periodic domain reversals and the width of each periodic 
domain reversal can be controlled more precisely to desired values as 
compared with the case where LiNb.sub.x Ta.sub.1-x O.sub.3 doped with Mg 
is used as the ferroelectric material, whereby a higher wavelength 
conversion efficiency can be obtained.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
An optical wavelength conversion element in accordance with a first 
embodiment of the present invention was formed. In FIG. 1, reference 
numeral 1 denotes a substrate of LiNbO.sub.3 doped with7.5 mol % Zn, which 
is a ferroelectric material having a nonlinear optical effect. This 
ferroelectric material will be referred to as "LN:Zn", hereinbelow. The 
LN:Zn substrate 1 was made to have a single domain and was cut into 0.5 mm 
in thickness. Further the LN:Zn substrate 1 was polished in a Z-face so 
that a nonlinear optical constant d.sub.33, which is the largest, can be 
efficiently used. 
Ta film was formed on +Z-face 1a (FIG. 2) of the Ln:Zn substrate 1 by 
sputtering of metal Ta and periodic electrodes 2 of Ta were formed by 
photolithography at pitches .LAMBDA. as shown in FIG. 1. Each of the 
periodic electrodes 2 was 4 .mu.m in width and the pitches .LAMBDA. of the 
electrodes 2 was set to 12.9 .mu.m taking into account the 
wavelength-dependent dispersion of the refractive index of LN:Zn so that 
the pitches become linear along x-direction of the substrate near 1313 nm. 
Then while maintaining the substrate 1 at 90.degree. C. and evacuating the 
space which +Z-face 1a faced to 10.sup.-4 Ps, an electric field was 
applied to the substrate 1 by corona charging through a corona wire 4 
disposed on the side of Z-face -Z-face 1b of the substrate 1 with the 
periodic electrodes 2 grounded by a ground wires. In this embodiment, a 
voltage of -20 kV/cm was applied for 4.5 seconds by a high voltage source 
5 through the corona wire 4. 
Then the periodic electrodes 2 were removed and the substrate 2 thus formed 
with periodic domain reversals 9 was cut along a Y-face. The cut surface 
(Y-face) was polished and was subjected to selective etching by use of 
etching liquid of a 1:2 mixture of HF and HNO.sub.3. When the cut surface 
(Y-face) was visually inspected, it was found that the domain was 
periodically reversed through the substrate 1 from the -Z-face 1b to the 
+Z-face 1a at portions opposed to the periodic electrodes 2 as denoted by 
reference numeral 9 in FIG. 2. Arrows 10 in FIG. 2 indicate the directions 
of the domains. 
Thereafter the -X face and the +X face of the LN:Zn substrate 1 were 
subjected to optical polishing to make the faces light transmission faces 
20a and 20b, whereby a bulk crystal type optical wavelength conversion 
element 20 shown in FIG. 3 was obtained. Then the optical wavelength 
conversion element 20 was disposed in a resonator of a laser diode-pumped 
YLF laser as shown in FIG. 3 and second harmonic was generated. 
The laser diode-pumped YLF laser comprised a laser diode 22 which emitted a 
pumping laser beam 21 of a wavelength of 795 nm, a condenser lens 23 which 
converged the diverging laser beam 21, a YLF crystal 24 which was a laser 
medium doped with Nd and on which the laser beam 21 was converged, and a 
resonator mirror 25 disposed forward (rightward in FIG. 3) of the YLF 
crystal 24. The optical wavelength conversion element 20 was disposed 
between the YLF crystal 24 and the resonator mirror 25. 
Pumped with the laser beam 21 of 795 nm, the YLF crystal 24 emits light of 
1313 nm. The light resonates between an end face 24a of the YLF crystal 24 
provided with a predetermined coating and the mirror surface 25a of the 
resonator mirror 25, whereby a solid laser beam 26 is generated. The solid 
laser beam 26 enters the optical wavelength conversion element 20 and is 
converted to a second harmonic 27 whose wavelength is 657 nm, one half of 
that of the laser beam 26. Substantially only the second harmonic 27 
emanates from the resonator mirror 25. Phase matching (so-called 
artificial phase matching) is achieved in the reversed domain regions of 
the optical wavelength conversion element 20. The intensity of the second 
harmonic 27 and the like will be described later with reference to FIG. 4. 
Optical wavelength conversion elements in accordance with second and third 
embodiment of the present invention and a control optical wavelength 
conversion element will be described, hereinbelow. These optical 
wavelength conversion elements differ from the optical wavelength 
conversion element of the first embodiment in the doping material, the 
amount of the doping material and the time for which the electric voltage 
is applied by corona charging as follows. 
1st embodiment! LN:Zn (7.5 mol %), -20 kV/cm.times.4.5 sec. 
2nd embodiment! LN:Sc (1.5 mol %), -20 kV/cm.times.3.5 sec. 
3rd embodiment! LN:In (1.8 mol %), -20 kV/cm.times.3.0 sec. 
control! LN:Mg (5.0 mol %), -20 kV/cm.times.9.0 sec. 
The optical wavelength conversion elements of the second and third 
embodiments and the control were disposed in the laser diode-pumped YLF 
laser shown in FIG. 3 in place of the optical wavelength conversion 
element of the first embodiment and the laser was operated to generate a 
second harmonic. The intensities of the second harmonics for the 
respective cases are shown in FIG. 4 in relative values together with that 
for the case where the optical wavelength conversion element of the first 
embodiment was employed. The results of the cases where the optical 
wavelength conversion elements of the second and third embodiments were 
employed were the substantially the same. 
As can be understood from FIG. 4, when the optical wavelength conversion 
elements of the first to third embodiments of the present invention were 
employed, a higher intensity of second harmonic was obtained in a narrower 
temperature range as compared with when the conventional optical 
wavelength conversion element (the control), that is, a higher wavelength 
conversion efficiency was obtained, which proved that periodicity of the 
periodic domain reversals was improved in the optical wavelength 
conversion elements of the present invention. 
The domain reversal threshold voltage, that is, the voltage above which 
domain reversal is caused, was investigated for each of the doping 
materials and the result is shown in FIG. 5. As shown in FIG. 5, when LN 
(LiNbO.sub.3) is doped with Sc or In, the domain reversal threshold 
voltage is lower than when LN is doped with Mg irrespective of the doping 
amount. When LN is doped with Zn, the domain reversal threshold voltage is 
lower than when LN is doped with Mg so long as the doping amount is not 
smaller than about 6.5 mol %. Thus by doping LN with Zn, Sc or In, domain 
reversal can be facilitated. 
FIGS. 6A to 6D are 400.times. microphotographs respectively showing the 
Y-faces of the substrates of the optical wavelength conversion elements of 
the control and the first to third embodiments. 
As can be seen from the microphotographs, though the width of the domain 
reversals is partly increased and periodicity of the periodic domain 
reversals is bad in the control, such a defect is hardly seen in any one 
of the first to third embodiments. 
Though a LiNbO.sub.3 substrate doped with Zn, Sc or In is employed in the 
embodiments described above, substantially similar results can be obtained 
even if a substrate of LiTaO.sub.3 or LiNbTaO.sub.3 doped with Zn, Sc or 
In or a substrate of LiNb.sub.x Ta.sub.1-x O.sub.3 
(0.ltoreq..times..ltoreq.1) doped with two or three of Zn, Sc and In is 
employed.