Second harmonic generator for obtaining an aberration free plane wave and information processing system using the same

A second harmonic generator of the present invention utilizes Cerenkov radiation as a useful coherent short-wavelength light source. The second harmonic generator includes an aberration correction means to collimate the second harmonic to form a parallel plane wave having a high flatness. The aberration correction means is provided on an exit end face of a nonlinear waveguide which generates the second harmonic, and a glued cone prism, a cone prism having a conic exit end face changed in accordance with birefringence of the nonlinear waveguide, or a diffraction grating is used as the aberration correction means.

BACKGROUND OF THE INVENTION 
The present invention relates to a second harmonic generator which utilizes 
Cerenkov radiation useful as a source of coherent short-wave light. 
Such a second harmonic generator is useful as a light source for recording 
and/or reproducing in optical information processing systems such as 
optical disks, laser printers or color printers. 
There has heretofore been known technology for generating second harmonics 
of Cerenkov radiation by forming organic materials having nonlinear 
characteristics into a device of fiber form and inputting coherent 
fundamental light thereinto, as described, for example, in Nonlinear 
Optical Materials-Extended Abstract, 1985, pp. 97-99. 
The Cerenkov radiation is composed of second harmonics radiated by a 
polarization wave having a phase velocity greater than the phase velocity 
of a medium. However, since the beam shape of the Cerenkov radiation has a 
cone or an arc shape, it cannot be focused into a good spot. 
Furthermore, the generation of second harmonics by Cerenkov radiation from 
waveguides fabricated on the surface of a substrate of nonlinear crystals 
such as lithium niobate (LiNbO.sub.3) has been reported in CLEO '87, 
Technical Digest, pp. 198-199. This method has many advantages in that the 
secondary light having a wavelength one-half that of the fundamental light 
can be generated with a high conversion efficiency. In addition, the phase 
matching of the two light waves having different wavelengths can be 
achieved relatively easily. 
With this method, however, as shown in FIG. 8, since a radiation mode is 
established from a narrow line-shaped waveguide into the substrate, the 
light diverges in the direction of radiation while the light is collimated 
in parallel in the direction at right angles thereto. Therefore, the light 
cannot be focused to a good spot. Specifically, in the prior art shown in 
FIG. 8, the substrate of the waveguides consists simply of a block having 
a flat surface without being devised in particular. Thus, the Cerenkov 
beam which has passed through the substrate develops a large aberration, 
and it cannot be focused into a good spot unless a correction measure is 
employed. 
As described above, in the prior art technologies, the countermeasure for 
the aberration of the Cerenkov beam is insufficient, and thus, it is 
impossible to focus the Cerenkov beam into a diffraction limited spot. As 
a result, the Cerenkov beam cannot be used as a light beam for recording 
and/or reproducing in optical information processing system such as 
optical disks and the like. 
SUMMARY OF THE INVENTION 
It is a first object of the present invention to provide a second harmonic 
generator which is capable of collimating a second harmonic such as the 
Cerenkov beam to a flat parallel plane wave. 
It is a second object of the present invention to provide a second harmonic 
generator which is capable of converting a second harmonic such as the 
Cerenkov beam to an aberration free plane wave without being influenced by 
a variation in wavelength of incident light (fundamental light) thereby 
making it possible to focus the beam a diffraction limited spot. 
It is a third object of the present invention to provide a compact second 
harmonic generator which is capable of generating a coherent second 
harmonic which is collimated to a high flatness without being influenced 
by birefringence of a nonlinear crystal. 
It is a fourth object of the present invention to provide a second harmonic 
generator which is capable of generating a second harmonic of an 
aberration free plane wave which can be focused into a diffraction limited 
spot stably regardless of fluctuations of temperature and mechanical 
fluctuations. 
It is a fifth object of the present invention to provide an information 
precessing system capable of recording and/or reproducing information with 
high recording density by using the second harmonic generator as mentioned 
above as a source of coherent short-wavelength light. 
In one feature of the present invention, a nonlinear waveguide means 
including a waveguide of a fiber form or a channel type formed of 
materials having nonlinear optics effects comprises an aberration 
correction means which collimates a second harmonic to a flat parallel 
plane wave. The nonlinear waveguide means is composed of a fiber form or a 
channel type waveguide (a core layer of a fiber, or a channel layer which 
is formed by diffusing Ti in a surface of a nonlinear optical crystal and 
the Ti-diffused layer is subjected to proton exchange to enhance a 
refractive index), and a holding member (a cladding layer of the fiber, or 
the nonlinear optical crystal such as LiNbO.sub.3) for retaining the 
waveguide. When the waveguide having nonlinear optical characteristics is 
irradiated by coherent light, a Cerenkov beam is radiated from the 
waveguide towards the holding member in a cone shape, and the Cerenkov 
beam is output from the exit end face of the holding member. The holding 
member has the aberration correction means provided on the exit end face. 
A technique devised to convert the wavefront of the cone shape due to the 
Cerenkov radiation into a parallel plane wave by means of a conic prism is 
n U.S. Ser. No. 408,537 filed Sept. 18, 1989, now U.S. Pat. No. 4,972,422. 
By using this conic prism, it is possible to convert the Cerenkov beam 
into a parallel plane wave. However, the direction of travel of the 
Cerenkov beam is varied sensitively with respect to a change in wavelength 
of the fundamental light due to a temperature change or the like, and as a 
result, chromatic aberration is caused. Therefore, a diffraction limited 
spot cannot be obtained stably. 
On the other hand, in channel waveguides having a substrate formed of a 
nonlinear crystal (for example, a uniaxial crystal or a biaxial crystal) 
such as lithium niobate or the like having optical anisotropy, another 
problem arises in which an aberration of a wavefront is caused due to 
birefringence or double refraction of the nonlinear crystal. 
In another feature of the present invention, as the aberration correction 
means provided on the exit and face of the nonlinear waveguide means, a 
conic prism of a glued type formed by bonding two or more materials having 
different refractive indices is used. Specifically, with respect to a 
variation of a Cerenkov radiation angle caused by a wavelength change of 
the fundamental light due to fluctuations of temperature, a so-called 
achromatic cone lens formed by the conic prism of the glued type is used 
as a chromatic aberration correction means, thereby to convert a variation 
of an angle of incidence at which the Cerenkov beam enters the conic prism 
into a parallel displacement of the Cerenkov beams while maintaining 
parallel beams. 
The Cerenkov angle .alpha. is given by the following formula in accordance 
with the phase matching condition. 
##EQU1## 
Here, n(2.omega.) is the refractive index with respect to the second 
harmonic (Cerenkov beam), and N(.omega.) is the refractive index with 
respect to the fundamental wave. The refractive index N(.omega.) can be 
considered as an effective refractive index of the nonlinear waveguide 
means. In the case of the fiber form waveguide, the effective refractive 
index is determined by respective refractive indices of the core layer and 
the cladding layer, and in the case of the channel type waveguide, the 
effective refractive index is determined by respective refractive indices 
of the proton-exchanged waveguide and the nonlinear crystal substrate. 
Because these effective refractive indices are changed sensitively 
following a wavelength change of the fundamental wave, the Cerenkov angle 
a is also varied sensitively according thereto. 
When a parallel plane wave is to be obtained by focusing such a Cerenkov 
radiation beam by using the conic prism, if the Cerenkov angle is changed 
by a variation of wavelength of the fundamental wave, the angle of 
incidence of the Cerenkov radiation beam entering the conic prism is 
varied and the parallel plane wave cannot be obtained. 
Accordingly, a conic prism formed of two or more materials having different 
refractive indices is used, and the variation of incident angle with 
respect to the conic prism is converted into a translation or a parallel 
displacement while maintaining parallel beams. As a result, even when the 
radiation angle of Cerenkov radiation emitted from the nonlinear waveguide 
is varied due to fluctuations in ambient temperature, after the Cerenkov 
radiation beam is focused by the conic prism of the glued type, the 
parallel plane wave is maintained and a beam shift is merely caused. Thus, 
it is possible to obtain an aberration free plane wave without being 
affected by wavelength fluctuations of the incident light (fundamental 
wave), and to ensure the precision of a spot of a diffraction limit by 
focusing the plane wave through a lens. 
In another feature of the present invention, as the aberration correction 
means, the shape of the exit end face of the nonlinear waveguide means is 
changed in accordance with birefringent characteristics of the nonlinear 
waveguide means. Specifically, when the birefringence is present in the 
nonlinear waveguide means, in particular, in the case of the channel type 
waveguide having a substrate formed of a nonlinear optical crystal, the 
refractive index n(2.omega.) is changed depending on an angle .phi. 
between an optical axis and a Cerenkov beam. Thus, the Cerenkov angle 
.alpha. is changed depending on the angle .phi. in accordance with the 
following phase matching condition, that is, 
##EQU2## 
which must be satisfied at the time of Cerenkov radiation. In other words, 
the Cerenkov angle .alpha. is given by the following formula; 
##EQU3## 
For this reason, when a mere conic prism is used as the aberration 
correction means which is provided on the exit end face of the nonlinear 
waveguide means, it is impossible to make all Cerenkov radiation beams 
into a parallel plane wave. Hence, the vertex angle .theta. of the conic 
prism is modulated in accordance with the angle a between the optical axis 
and the Cerenkov beam to satisfy the following relationship, 
##EQU4## 
so that the shape of the conic exit end face of the conic prism is 
changed. In this manner, any Cerenkov beam radiated from the nonlinear 
waveguide can be converted into the parallel plane wave. Therefore, the 
aberration free plane wave can be obtained without being affected by 
birefringent characteristics of the nonlinear waveguide, and the coherent 
second harmonic which is collimated to a high flatness can be obtained. 
In still another feature of the present invention, as the aberration 
correction means, a conic prism of the glued type formed by bonding two or 
more materials having different refractive indices is employed, and 
furthermore, the shape of a conic exit end face of the glued type conic 
prism is changed in accordance with birefringent characteristics of the 
nonlinear waveguide means. Specifically, by combining the above-mentioned 
correction of chromatic aberration with the correction of wave front 
aberration due to birefringence, even when the nonlinear waveguide means 
having the birefringence is used, an aberration free plane wave can be 
obtained without being affected by wavelength fluctuations of the 
fundamental wave and by the birefringent characteristics of the nonlinear 
waveguide means. As a result, it is possible to always obtain a stable 
flat parallel plane wave, and thus, to focus the flat parallel plane wave 
into a spot of a diffraction limit. 
In another feature of the present invention, as the aberration correction 
means, a diffraction grating is provided on the exit end face of the 
non-linear waveguide means. More specifically, a conic Cerenkov beam 
emitted from a fiber form or a channel type nonlinear optical medium is 
converted into a parallel plane wave by diffracting by the diffraction 
grating formed in the exit end face of the nonlinear guide wave in which 
grooves of the diffraction grating are concentric circles centered at a 
core of the fiber or a channel waveguide. 
In still another feature of the present invention, the exit end face of the 
nonlinear waveguide has a spherical surface, and the diffraction grating 
is formed in the spherical surface. Specifically, in the course of 
manufacturing of the diffraction grating, the occurrence of a 
manufacturing error is unavoidable, and it is difficult to make the center 
of the diffraction grating of concentric circle form perfectly coincide 
with the position of the fiber form or channel type waveguide. For this 
reason, the sine condition which is a characteristic to be possessed by a 
microscope objective lens, that is, the condition for removing a coma 
aberration is introduced. In other words, an exit end face of a fiber 
cladding, or an exit end face of a waveguide substrate which is the exit 
end face of the nonlinear waveguide means is formed in a spherical 
surface, and the concentric circle shaped diffraction grating is formed in 
the spherical surface, and the center of the spherical surface is made in 
coincidence with the center of the fiber form or channel type waveguide. 
As a result, even when a deviation is present more or less between the 
center of the diffraction grating and the position of the fiber form or 
channel type waveguide, the aberration never becomes large, and it is 
possible to collimate to a plane wave having a high flatness, and 
subsequently to focus into a spot of a diffraction limit through a lens. 
Furthermore, the diffraction grating can be used also for correcting the 
above-mentioned chromatic aberration and for correcting the wave front 
aberration due to birefringence. For example, in correcting the chromatic 
aberration, a plurality of diffraction gratings are used, and the 
plurality of diffraction gratings may be disposed so that the fluctuations 
of Cerenkov radiation angle caused by wavelength fluctuations of the 
fundamental wave are converted into a translation or parallel displacement 
while maintaining parallel beams. Further, in correcting the wavefront 
aberration due to birefringence, the groove intervals of the diffraction 
grating may be changed in accordance with the birefringence 
characteristics of the nonlinear waveguide means. 
Still further advantages of the present invention will become apparent to 
those of ordinary skill in the art upon reading and understanding the 
following detailed description of the preferred and alternate embodiments.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
An embodiment of the present invention will be described with reference to 
FIGS. 1A and 1B. In this embodiment, a semiconductor laser 1 is used as a 
light source of fundamental light, and a fiber form waveguide 3 is used as 
a nonlinear waveguide means. The light, for example, having a single 
wavelength in a range between 600 nm and 1.5 .mu.m, from the semiconductor 
laser 1 is focused through a lens 2, and introduced into a fiber form 
waveguide (core) 31 formed of a nonlinear optical material, thereby to 
generate a second harmonic (a wavelength of 300 to 750 nm) 4 of Cerenkov 
radiation type with a radiation angle .alpha.. The fiber form waveguide 
(core) 31 retained in a cladding 33, and the Cerenkov beam 4 from the core 
layer 31 is radiated within the cladding 33. A parallel plane wave 5 is 
obtained from the Cerenkov beam 4 by a conic prism 35 provided on an exit 
end face of the cladding 33, and a spot 7 is produced by focusing the 
parallel plane wave 5 through a focusing lens 6. 
However, when an oscillation wavelength of the semiconductor laser 1 which 
is the light source is changed due to fluctuations of ambient temperature 
or the like, the Cerenkov radiation angle .alpha. is changed, for example, 
to a radiation angle .alpha.'. The ray of light radiated at the angle 
.alpha.' cannot any longer become the parallel plane wave by the conic 
prism of the same shape, and the ray of light becomes a diverging or a 
converging beam and involves an aberration. 
For this reason, in the present invention, as shown in FIGS. 1A and 1B, the 
conic prism 35 is formed by bonding materials having different refractive 
indices n1 and n2 to form a glued conic prism. For example, prism elements 
having angles .theta..sub.1 and .theta..sub.2 are bonded to each other. 
Such a glued conic prism 35 may be formed by integrally forming one prism 
element integral with the cladding 33 as a unitary member as shown in FIG. 
1A, or the glued prism 35 may be bonded to the cladding 33 as shown in 
FIG. 1B. 
Two rays of light Cerenkov-radiated in different directions with the 
radiation angles .alpha. and .alpha.' pass through a refracting surface A 
(a bonding surface of the glued conic prism) and a refracting surface B 
(the exit surface of the glued conic prism). Thus, either of the two rays 
of light can be converted into the parallel plane wave 5 by suitably 
selecting the refractive indices n1 and n2, and the angles .theta..sub.1 
and .theta..sub.2 of individual prism elements constituting the glued 
conic prism 35. 
Accordingly, even when the Cerenkov radiation angle is changed by 
fluctuations of the wavelength of the semiconductor laser 1 due to a 
change in ambient temperature, the parallel plane wave can be obtained 
without being affected by the change in the Cerenkov radiation angle, and 
the parallel plane wave involving no aberration can be obtained. As a 
result, it becomes possible to obtain the focused spot 7 of a diffraction 
limit by focusing the parallel plane wave through a focusing optical 
system such as the lens 6 or the like. 
FIG. 2 shows a second embodiment of the present invention. In this 
embodiment, a channel type waveguide 8 is used as the nonlinear waveguide 
means. Similar to the first embodiment, the beam from a semiconductor 
laser 1 which is a light source for fundamental light is focused through a 
lens 2 and is introduced into an optical waveguide 81 of the channel type 
waveguide 8, thereby to radiate a Cerenkov beam 4 towards a substrate 83 
at an angle .alpha.(.phi.). In this case, for example, when a nonlinear 
optical crystal of lithium niobate (LiNbO.sub.3) is used as the substrate 
83 of the channel type waveguide 8, since a maximum nonlinear constant 
d.sub.33 is used, its optical axis (C-axis) is directed in a perpendicular 
direction with respect to the optical waveguide 81 as shown in FIG. 2. 
Accordingly, each Cerenkov angle .alpha.(.phi.) is associated with a 
different refractive index n.sub.e (.phi.) depending upon the angle .phi. 
with respect to the C-axis. This means that, when the direction of 
polarization of the semiconductor laser 1 is selected to be in parallel to 
the C-axis, the direction of polarization of the second harmonic will be 
perpendicular to the C-axis, and a so-called extraordinary ray will be 
emitted. The refractive index n.sub.e (.phi.) of the extraordinary ray has 
a different value depending on the angle .phi.. 
For this reason, in this embodiment, the shape of a conic exit surface of a 
conic prism 11 provided on an exit end face of the channel type waveguide 
8 is modulated, for example, a vertex angle .theta.(.phi.) of the conic 
prism 11 is slightly changed in accordance with the angle .phi. so that 
the vertex angle .phi.(.theta.) satisfies the following relationship, 
##EQU5## 
In this manner, the Cerenkov beam after passing through the conic prism 11 
becomes a parallel plane wave without being affected by the birefringence 
characteristics of the channel type waveguide 8, and the aberration free 
parallel wave can be obtained. Accordingly, it becomes possible to always 
obtain a focused spot 7 by focusing the parallel plane wave through a 
focusing optical system such as a lens 6 or the like. 
The conic prism 11 as described above may be a unitary type in which the 
conic prism 11 integrally is formed with the substrate 83 of the channel 
type waveguide 8, or a separate type in which the conic prism 11 is a 
separate member from the substrate 83. However, when taking position 
matching into consideration, the separate type is advantageous. 
Furthermore, also in the second embodiment, when the conic prism 11 is 
constituted by the glued conic prism shown in the first embodiment of 
FIGS. 1A and 1B, the correction of chromatic aberration is combined with 
the correction for birefringence, and the aberration free parallel plane 
wave can be obtained without being affected by the fluctuations of the 
Cerenkov radiation angle due to the wavelength fluctuations of the 
fundamental light and the birefringent characteristics of the nonlinear 
waveguide means. 
FIG. 3 shows a third embodiment of the present invention. In this 
embodiment, a channel type waveguide 8 is used as the nonlinear waveguide 
means, and a diffraction grating 15 is used as the aberration correction 
means. Similar to the embodiments described above, a light beam from an 
exciting light source (not shown) such as a semiconductor laser or the 
like is focused through a lens 2, and the focused light beam enters a 
waveguide 81 for generating a second harmonic having nonlinear optical 
characteristics, thereby to radiate a second harmonic 4 of the Cerenkov 
type towards a substrate 83 as a cone shaped beam. 
The Cerenkov radiation beam 4 is collimated by the diffraction grating 15 
carved or ruled in an exit end face of the substrate 83, and travels as a 
parallel plane wave 16. The diffraction grating 15 consists of a group of 
grooves of concentric circles centered at the waveguide 81. Such a 
diffraction grating, can be formed, for example, by mechanically ruling 
lines using a ruling engine, an etching process using an electron beam, or 
the like. Since the beams collimated by the diffraction grating 15 have a 
flat wavefront, similar to the embodiments described in the foregoing, the 
beams are focused into a spot of a diffraction limit by a focusing optical 
system such as a lens 6 or the like. 
FIG. 4 illustrates a variant example of the third embodiment, and a fiber 
form waveguide 3 is used as the nonlinear waveguide. Specifically, similar 
to the first embodiment in FIGS. 1A and 1B, the fundamental light is 
directed to enter a nonlinear optical waveguide 31 of fiber form to 
generate a Cerenkov beam 4. In an exit end face of the fiber form 
waveguide 3, similar to the third embodiment, there is formed a 
diffraction grating 15 of a concentric circle shape centered at the 
waveguide (core) 31, and the Cerenkov beam 4 having a cone shape is 
diffracted by the diffraction grating 15 to become a parallel plane wave 
16. 
FIG. 5 illustrates a fourth embodiment of the present invention. In this 
embodiment, similar to the third embodiment in FIG. 3, a channel type 
waveguide 8 is used as the nonlinear waveguide, and a diffraction grating 
15 is used as the aberration correction means. In this case, however, an 
exit end face 89 of the channel type waveguide 8 is formed into a 
spherical surface centered with respect to an incident end face 87 of a 
waveguide 81 in advance, and the diffraction grating 15 is formed in the 
exit end face 89. In this manner, even when the center of the diffraction 
grating 15 and the position of the waveguide 81 are deviated, or inclined 
more or less relative to each other, beams 16 which have been collimated 
can be made a parallel plane wave maintaining a high flatness. 
FIG. 6 illustrates a variant example of the fourth embodiment, and a fiber 
form waveguide 3 is used as the nonlinear waveguide means. Specifically, 
similar to the embodiment in FIG. 4, an exit end face 39 of the fiber form 
waveguide 3 is formed into a spherical surface centered with respect to an 
end face 37 of a waveguide (core) 31. As a result of this, the sine 
condition is satisfied, and even when the center of the fiber form 
waveguide 3 and the center of concentric circle grooves of a diffraction 
grating 15 are deviated, or inclined relative to each other, it is 
possible to obtain a parallel plane wave 16 having a high flatness. 
Furthermore, the diffraction grating 15 described in the embodiments in 
FIGS. 3 to 6 can be used for correcting the chromatic aberration and for 
correcting the wavefront aberration due to birefringence described in the 
embodiment in FIGS. 1A and 1B or FIG. 2. For example, in correcting the 
chromatic aberration, two or more diffraction gratings are employed, and 
the plurality of diffraction gratings may be disposed so that with respect 
to the fluctuations of Cerenkov radiation angle caused by wavelength 
fluctuations of the fundamental wave, the Cerenkov beams are converted 
into a parallel displacement while maintaining parallel beams. 
In addition, in order to correct the wavefront aberration, the groove 
intervals of the diffraction grating may be changed in accordance with the 
birefringent characteristics of the nonlinear waveguide means. 
Further, when the second harmonic (wavelength .lambda.) formed in a flat 
parallel wave by the second harmonic generator described in the 
embodiments and the variant examples in FIGS. 1A and 1B and 2 to 6 is 
focused by an objective lens having a relatively large numerical aperture 
(NA), it is possible to obtain a spot which is determined by a so-called 
.lambda./NA. Accordingly, a short-wavelength light source suitable for 
information processing systems such as optical disks, laser printers, and 
the like can be obtained. 
FIG. 7 illustrates an embodiment of the information processing system. The 
parallel plane wave from a short-wavelength light source 20 constituted by 
the second harmonic generator (the arrangement with the exception of the 
objective lens 6) described in the embodiments and their variant examples 
in FIGS. 1A and 1B and 2 to 6 is permitted to pass through a focusing 
optical system composed of various lenses, beam splitter and a mirror, and 
is focused into a diffraction limited spot to irradiate an optical disk 
30. The reflected light or diffracted light from pits recorded in the 
surface of the disk 30 is received by photodetectors 40, and an 
auto-focusing signal, a tracking signal, a data signal, and the like are 
detected. 
The invention has been described with reference to the preferred and 
alternate embodiments. Obviously, modifications and alternations will 
occur to those of ordinary skill in the art upon reading and understanding 
the present invention. It is intended that the invention be construed as 
including all such modifications and alternations in so far they come with 
the scope of the appended claims or the equivalent thereof.