Optical waveguide second harmonic generating element and method of making the same

An optical waveguide second harmonic generating element comprises a KTP single-crystal substrate sliced at a z-plane, a plurality of polarization reversal areas having spontaneous polarization, whose direction is reversed, and formed periodically along a predetermined direction on the z-plane of the substrate, and a channel waveguide, extending along the predetermined direction across polarization reversal areas, for propagating a light.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an optical waveguide second harmonic 
generating element (SHG element), and more particularly to a second 
harmonic generating element suitable for use in a compact laser light 
source of a short wavelength to be used in fields of optical information 
processing, optical measurement and medical treatment, and a method of 
making the same. 
2. Description of the Related Art 
An oscillation wavelength of a currently practical semiconductor laser is 
in a range of infrared to red, and a semiconductor laser which oscillates 
a light of a shorter wavelength, for example, a green or blue wavelength 
has not yet been put into practice. Accordingly, it has been proposed to 
generate a second harmonic from an output light of a semiconductor laser, 
which oscillates an infrared light, thereby to produce a laser beam of a 
short wavelength. 
In order to efficiently convert a wavelength of a light to a second 
harmonic, it is necessary to meet a phase matching condition so that 
phases of a number of second harmonics generated at various points in the 
SHG element match each other. To this end, methods of using angular phase 
matching, temperature phase matching or a waveguide have been known. 
Recently, a quasi-phase matching method using a periodic structure shown, 
for example, in Phys. Rev. Vol. 127, Sep. 15, 1962, J. A. Armstrong et al. 
pages 1918-1939 is attracting an attention in the field. In this method, a 
polarization direction in a crystal is periodically reversed to compensate 
for phase mismatching between a fundamental wave and a harmonic. 
The polarization reversal means the reversal of the polarization direction 
of a single crystalline dielectric material polarized in a predetermined 
direction. When applied to an SHG element, it is called a polarization 
reversal type SHG element. 
Various methods have been practiced to attain the polarization reversal. 
For example, in a lithium niobate (LiNbO.sub.3) or (LN) single-crystal, 
thermal diffusion of a Ti metal is used. In a lithium tantalate (LT) 
single-crystal, a method of rapid heating after proton exchange is used. 
FIG. 6 shows an SHG element in which polarization reversal areas and 
polarization non-reversal areas are formed alternately in a comb shape in 
a substrate of LN or LT crystal, and a channel waveguide 20 is formed 
orthogonally thereto. 
In generating a second harmonic, the following relationship exists between 
power of a fundamental wave (frequency .omega.) and a power of a second 
harmonic (frequency 2.omega.) 
EQU P(2.omega.).infin.{S(n,n,m)}.sup.2 .times.{P(.omega.)}.sup.2 /W(1) 
where P(.omega.) and P(2.omega.) are powers of the fundamental wave and the 
second harmonic, respectively, W is a width of the waveguide, S(n,n,m) is 
a spatial coupling constant representing overlap of electromagnetic field 
distributions of the fundamental wave and the second harmonic, and n and m 
are orders of mode of the second harmonic and the fundamental wave, 
respectively. 
EQU {S(n,n,m)}=.intg.f.sup.2 (n,.omega.).multidot.f(m,2.omega.)dS(2) 
where f(n,.omega.) and f(m, 2.omega.) are electric field distributions of 
the fundamental wave and the harmonic, respectively, and S is a 
cross-section. 
In an SHG element which uses the quasi-phase matching by using the lithium 
niobate (LiNbO.sub.3) single-crystal which is one of the proposed SHG 
element; as disclosed in Electron Lett. Vol. 25, No. 3, Feb. 2, 1989, 
pages 174-175, by E. J. Lim and M. M. Fejer, a refractive index of an 
optical waveguide is changed by the generated P(2.omega.) because the 
lithium niobate is weak to light damage so that P(2.omega.) does not 
increase in proportion to the increase of a term {P(.omega.)}.sup.2 in the 
formula (1) but it tends to saturate. Further, since the refractive index 
of the optical waveguide changes with time, it is difficult to stabilize 
P(2.omega.) over a long time periods. 
In order to solve a drawback of the SHG element which uses lithium niobate, 
a structure shown in FIG. 5 has been proposed as an SHG element which uses 
KTP (KTi OPO.sub.4) which has been known to be stronger to the light 
damage by the order of two figures than lithium niobate (Appl. Phys. Lett. 
Vol. 57, No. 20, Nov. 12, 1990, pages 2074-2076, by C. J. Van der Poel, J. 
D. Bierlein and J. B. Brown). In FIG. 5, numeral 51 denotes a substrate 
sliced at a z-plane of a KTP single-crystal, and numeral 52 denotes a Rb 
ion exchange waveguide made by exchanging a part of K ions by Rb ions. 
When Ba ions are added during the manufacture of the Rb ion exchanqe 
waveguide, the direction of spontaneous polarization of the waveguide 
portion is reversed relative to the bulk or substrate so that the SHG is 
attained by the quasi-phase matching. However, since the optical waveguide 
is discontinuous in the structure, a light passes through a bulk crystal 
portion in which no waveguide is formed when the light propagates from a 
waveguide portion to a next waveguide portion. Since a light confinement 
effect disappears in the bulk crystal portion, both the incident beam and 
the generated second harmonic beam spread. Namely, P(.omega.) in the 
formula (1) attenuates as the light travels. As a result, a power of 
P(2.omega.) as calculated is not attained. 
The optical waveguide which uses KTP is also disclosed in Appl. Phys. Lett. 
Vol. 50, No. 18, May 4, 1987, pages 1216-1218, by J. D. Bierlein et al. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an optical waveguide 
second harmonic generating element which efficiently generates a second 
harmonic while keeping a high power density in an optical waveguide by 
providing a continuous channel optical waveguide on a KTP substrate which 
is resistive to light damage. 
It is another object of the present invention to provide a method of 
manufacturing the above optical waveguide second harmonic generating 
element. 
In order to achieve the above object, the optical waveguide second harmonic 
generating element of the present invention comprises a KTP single-crystal 
substrate sliced at a z-plane, a plurality of polarization reversal areas 
periodically formed along a predetermined direction on the z-plane of the 
substrate and having direction of spontaneous polarization reversed, and a 
channel waveguide for propagating a light, extending across the plurality 
of polarization reversal areas along the predetermined direction. 
The method of manufacturing the optical waveguide second harmonic 
generating element of the present invention comprises the steps of forming 
a pattern mask of a selected metal material on a minus z-plane of a KTP 
single-crystal substrate such that a plurality of areas exposing a 
substrate surface are periodically arranged along a predetermined 
direction, applying ion exchange process to the plurality of exposed areas 
on the substrate to form a plurality of polarization reversal areas 
periodically arranged along the predetermined direction, and forming a 
channel waveguide on the substrate extending across the plurality of areas 
along the predetermined direction. 
In the optical waveguide second harmonic generating element of the present 
invention, a second harmonic is generated in a phase matched condition by 
passing a light through areas where directions of spontaneous polarization 
are alternately reversed. A reversal period .LAMBDA. is given by 
EQU .LAMBDA.=(2m-1).times.2.pi./{.beta.(2.omega.)-(.beta.(.omega.)}(3) 
where .beta.(.omega.) is a propagation constant of a fundamental wave in 
the channel optical waveguide, .beta.(2.omega.) is a propagation constant 
of an SH wave, and m is a natural number. 
The light is tightly confined in the waveguide by generating the second 
harmonic in the continuous channel optical waveguide. As a result, 
P(.omega.) and S(n,n,m) in the formula (1) are made larger than those in 
the prior art non-continuous optical waveguide structure, and the SHG is 
attained efficiently.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A second harmonic generating element in accordance with a first embodiment 
of the present invention is explained with reference to FIG. 1. 
In FIG. 1, numeral 1 denotes a z-plate of a KTP single-crystal. Spontaneous 
polarization reversal areas 2 and non-reversal areas 7 are alternately 
formed on the z-plane of the single-crystal. An output light of a 
semiconductor laser 3 is coupled to the KTP single-crystal so that it is 
directed to an optical waveguide 4. The output light of the semiconductor 
laser is coupled to the optical waveguide formed on the KTP substrate such 
that an electric field component which is parallel to the z-direction is 
formed so that a maximum non-linear optical constant d.sub.33 
=14.times.10.sup.-12 m/v of the KTP is utilized to generate the second 
harmonic. Assuming that a light having a wavelength of 850 nm is directed, 
as a fundamental wave, to the optical waveguide 4 from the semiconductor 
laser 3, a reversal period .LAMBDA. is 3-5 .mu.m when m=1 in the formula 
(2). The second harmonic generated in the waveguide is emitted from an end 
plane of the crystal opposite to the laser incident plane, and may be 
condensed by a lens as required for other utilization. 
The spontaneous polarization reversal areas 2 are formed in the following 
manner. First, a minus z-plane of a z-plate of a KTP single-crystal having 
a length of 10 mm, a width of 2 mm and a thickness of 1 mm is covered by a 
Ti film at areas other than the areas where the direction of the 
spontaneous polarization is to be reversed by a conventional 
photolithography technique. Then, the KTP crystal is subjected to thermal 
treatment at approximately 350.degree. C. for approximately 10 minutes in 
molten liquid of a mixture of Ba(NO.sub.3).sub.2 and RbNO.sub.3 at a ratio 
of 20/80. As a result, exchange of Rb.sup.+ ions by K.sup.+ ions is 
effected in the areas not covered by the Ti film so that the spontaneous 
polarization reversal areas are formed. After the ion exchanqe, the Ti 
film is removed by etching. Each reversal area may have a length d1 of 
approximately 2 .mu.m, in a light propagation direction, an orthogonal 
width d2 of approximately 4 .mu.m to 1 mm and a depth of approximately 3 
.mu.m and a spacing between the reversal areas is about 2 .mu.m. 
The optical waveguide 4 is also formed by the ion exchange in the same 
manner as the spontaneous polarization reversal areas. First, areas other 
than an area on which the waveguide is to be formed are covered by a Ti 
film. Then, it is subjected to thermal treatment at approximately 
380.degree. C. for approximately 20 minutes in molten liquid of a mixture 
of RbNO.sub.3 and TlNO.sub.3 at a ratio of 50/50. Since a refractive index 
increases in the area in which the ion exchange has been effected by the 
thermal treatment, a continuous channel optical waveguide is formed. A 
width and a depth of the optical waveguide may be about 4.0 .mu.m and 4.0 
.mu.m, respectively. 
In accordance with the element of the present embodiment, a high power 
laser beam having a stable short wavelength is attained by generating the 
second harmonic from the light source such as a semiconductor laser which 
oscillates a light at wavelength of infrared to red. 
In the present embodiment, the molten salt of nitrate of barium and 
rubidium is used in the manufacture of the spontaneous polarization 
reversal areas 2 and the optical waveguide 4. Reference is made to Appl. 
Phys. Lett. Vol. 50, No. 18, page 1216, 1987, by J. D. Bieriein, A. 
Ferretti, L. H. Brizner and W. Y. Hsu. 
Where the nitrate is used, a melting point is as high as 
300.degree.-450.degree. C. that the crystal may be cracked or the surface 
of the crystal is roughened after the manufacture of the optical 
waveguide, resulting in the necessity of repolishing after the treatment. 
The toughened surface or the crack of the crystal may be presented and the 
propagation loss may be reduced by manufacturing the optical wavelength at 
a lower temperature by using the molten salt of acetate such as rubidium, 
cesium, talium or barium which melts at a relatively low temperature (its 
meting point is 194.degree.-246.degree. C., and the melting point of a 
mixture thereof is approximately 150.degree. C.). An embodiment therefor 
is described below. The embodiment is applicable to the manufacture of not 
only the element of FIG. 1 which includes the reversal areas 2 and the 
optical waveguide 4 but also an element having only the optical waveguide 
without the reversal areas 2. 
Rubidium acetate was put in a pot and melt at 250.degree.-320.degree. C. A 
z-plate of a KTP single-crystal cut in a z-plane was treated in the molten 
salt for 10 minutes to 4 hours to form a planar waveguide on the surface. 
The treated crystal exhibited a smooth surface. 
An equivalent refractive index difference was measured by a prism coupling 
method by a He-Ne laser (.LAMBDA.=633 nm). It was 0.0019 for a TM mode on 
a +Z-plane. 
Results of treatments by various salts are shown in Table 1. 
TABLE 1 
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Results of treatments by various salts 
(+Z plane, .lambda. = 633 nm) 
Equivalent 
refractive 
index Surface 
difference 
Condition Crack 
______________________________________ 
Example 
Rubidium acetate 
0.0019 good .smallcircle. 
Cesium acetate 0.0020 good .smallcircle. 
Thallium acetate 
0.0017 good .smallcircle. 
Mixture of rubidium 
0.0018 good .smallcircle. 
acetate and barium 
acetate (Rb 95 mol % + 
Ba 5 mol %) 
Comparative example 
Rubidium nitrate 
0.0019 no good, x 
occasionally 
broke 
Cesium nitrate 0.0019 no good x 
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A second embodiment of the present invention is now explained with 
reference to FIGS. 2A and 2B. 
In the optical waveguide second harmonic generating element of the second 
embodiment, a ridge optical waveguide 5 having a refractive index which is 
smaller than that of an optical waveguide area for guiding a second 
harmonic and larger than that of a KTiOPO.sub.4 substrate 1 which is 
resistive to light damage is formed, as shown FIG. 2A, on a z-plane of the 
substrate orthogonally to the spontaneous polarization reversal areas 2 as 
shown in FIG. 2A, or the periodic spontaneous polarization reversal areas 
2. Alternatively, as shown in FIG. 2B, a first continuous channel 
waveguide 4 is formed on the areas 2 for propagating a light in the 
direction of periodicity and a ridge optical wavelength 5 having a 
refractive index which is smaller than that of an optical guide area for 
guiding the second harmonic and larger than that of the substrate is 
formed to extend in parallel to the first continuous channel waveguide 4. 
In the present embodiment, an efficiency of transforming an incident light 
to a secondary harmonic is improved by increasing an integrated value 
{S(n,n,m)} of the formula (2) by providing the ridge portion. As shown in 
FIGS. 4A and 4B, the refractive index is changed at a portion including 
the ridge layer and the channel waveguide by providing the ridge layer. As 
a result, as shown in FIG. 3, the electric field distribution of the 
fundamental wave f(.omega.) is almost the same as that of the second 
harmonic f(2.omega.) at the channel waveguide 82, but the former is 
different from the latter at the ridge portion 81 so that the cancellation 
of the electric field is reduced thereby increasing the value of S(n,n,m) 
and improving the transform efficiency. 
The spontaneous polarization areas and the first channel waveguide in the 
second embodiment are identical to the spontaneous polarization reversal 
areas and the optical waveguide of the first embodiment, and they are 
manufactured in a similar process. The ridge optical waveguide 5 is 
manufactured after the formation of the spontaneous polarization reversal 
areas 2 in the embodiment of FIG. 2A, and manufactured after the formation 
of the spontaneous polarization reversal areas 2 and the first channel 
waveguide 4 in the embodiment of FIG. 2B. A thickness of the ridge optical 
wavelength is approximately 0.1-2 .mu.m and a width thereof is 
approximately 2-10 .mu.m (substantially equal to a width of the first 
channel waveguide). 
In manufacturing the ridge optical waveguide, the ridge layer is formed by 
making a hole in a resist perpendicularly to the spontaneous polarization 
line by a photo-lithography technique, applying a thin film, whose 
refractive index is adjusted by SiO.sub.2 mixed with Ta.sub.2 O.sub.5 at a 
ratio of 0-100 weight %, by an RF sputtering method, and removing the 
resist to form the structure shown in FIG. 2A. 
In the present embodiment, the molten salt of nitrate or acetate of 
rubidium and/or barium is used to locally reverse the polarization of the 
KTP single-crystal. However, when this method is used to reverse the 
polarization, a difference appeared in the refractive index between the 
ion-exchanged area and the non-exchanged area. Thus, in designing and 
manufacturing the SHG element, it is necessary to calculate the width of 
the polarization reversal area by taking the small difference between the 
refractive indices into consideration. 
In a third embodiment of the present invention, the ion exchange is 
effected by using a molten salt of a mixture of potassium and barium so 
that the polarization reversal is achieved without presenting a difference 
between the refractive indices. 
In the third embodiment, the polarization reversal areas are formed with 
the same refractive index as that of the non-reversal areas so that the 
design of element including a transfer error of a mask pattern in the 
manufacture of the element is simplified and more accurate element can be 
manufactured. By forming the channel waveguide normally to the stripe-type 
polarization reversal areas, the SHG element which is free from a scatter 
loss and has a high transform efficiency is manufactured. 
The SHG element of the third embodiment has a structure similar to that of 
the element of FIG. 1. Resist is spin-coated on a minus z-plane of a KTP 
single-crystal substrate 1, and the resist is formed in comb-like or 
stripe pattern by the photo-lithography technique. The areas appear every 
4 .mu.m. The resist is removed after Ti sputtering to form a Ti pattern. 
It is immersed in a molten salt (300.degree.-450.degree. C.) of a mixture 
of potassium nitrate and barium nitrate mixed at a ratio of 80/20 (other 
salt may be used) for 10 minutes to 4 hours thereby to form alternately 
polarization non-reversal areas and polarization reversal areas having the 
same refractive index as that of the substrate. The Ti pattern is then 
removed and a channel waveguide is formed orthogonally to the stripe 
pattern in the same manner as that described above by using a Ti mask and 
immersing it in a molten salt of rubidium, cesium or thallium. In this 
manner, the second harmonic generating element is manufactured. 
The substrate may be immersed in a molten salt (300.degree.-450.degree. C.) 
of mixture of rubidium nitrate and barium nitrate mixed at a ratio of 
80/20 (or other salt may be used) for 10 minutes to 4 hours, instead of 
the molten salt of the mixture of potassium nitrate and barium nitrate, 
thereby to form alternately the polarization non-reversal areas and the 
polarization reversal areas. Then, it is immersed in a molten salt 
(300.degree.-450.degree. C.) of potassium nitrate for 10 minutes to 4 
hours to exchange the rubidium ions with potassium ions. Then, the Ti 
pattern is removed and a channel waveguide is formed orthogonally to the 
stripe pattern in the same manner as that described above by using a Ti 
mask and immersing it in a molten salt of rubidium, cesium or thallium. In 
this manner, a light wavelength transform element is manufactured. 
In the present embodiment, nitrate is used although other salt may be used. 
As the ratio of barium salt in the mol ratio of potassium salt and the 
barium salt increases, the time of polarization reversal is shortened. 
Magnesium salt calcium salt or strontium salt may be used instead of 
barium salt. While the Ti mask is used, a mask pattern forming metal such 
as Al, Ta, Ni, Cr or an alloy thereof may be used depending on the type of 
salt used. 
TABLE 2 
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Example 
Fundamental wave wavelength 
850 nm 
Fundamental wave power 10 mW 
Second harmonic power 1 .mu.W 
Comparative Example (Segment type: FIG. 2) 
RbNO.sub.3 :Ba(NO.sub.3).sub.2 = 80/20 
Fundamental wave wavelength 
857 nm 
Fundamental wave power 10 mW 
Second harmonic power 0.9 .mu.W 
______________________________________ 
As seen from Table 2, essentially identical characteristic is attained. 
Both element lengths are 5 mm. An equivalent refractive index attained 
when it was immersed in a molten salt (350.degree. C.) of a mixture of 
potassium nitrate and barium nitrate at a ratio of 80/20 for 10 minutes 
is shown in Table 3. 
TABLE 3 
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Change of Refractive Index 
______________________________________ 
Before ion-exchange 1.861 
After ion-exchange 1.861 
______________________________________ 
Magnesium salt, calcium salt or strontium salt may be used instead of 
barium salt. While Ti mask is used, a metallic material such as Al, Ta, 
Ni, Cr or an alloy thereof may be used for making the mask pattern 
depending on the type of salt used. 
In accordance with the present embodiment, the comb-like mask pattern is 
formed by the Ti metal on the minus z-plane of the KTP crystal, and the 
ion exchange is effected by immersing it in the molten salt of the mixture 
of potassium and barium to form alternately the polarization non-reversal 
areas and the polarization reversal areas which have the same refractive 
index as that of the substrate. The consideration to the difference of 
refractive index between the polarization non-reversal areas and the 
polarization reversal areas in the manufacture of the element and the 
design of the element including the transfer error of the mask pattern are 
simplified, and more accurate second harmonic generating element can be 
manufactured.