Optical head using birefringent diffraction grating

A light radiated from a light source is focused on an optical disc by a focusing lens. A light reflected from the optical disc is introduced to a signal detection system. In the signal detection system, the introduced light is diffracted by a birefringent diffraction grating element utilizing an optical crystal. The diffracted light is received by a photodetector. Consequently, focus and track errors are detected, and information stored in the optical disc is re-produced.

FIELD OF THE INVENTION 
This invention relates to an optical head, and more particularly to, an 
optical head used for erasable magnetooptic disc and phase-change optical 
disc, a write-once optical disc, a read-only optical disc, etc. 
BACKGROUND OF THE INVENTION 
An optical head which is applied to an optical recording apparatus 
utilizing an optical disc is required to have functions of detecting a 
focus error to form a minute light spot on the optical disc and a track 
error to precisely trace a predetermined track on the optical disc. 
Further, the optical head is required to have a function of detecting an 
intensity of a light reflected from the optical disc to reproduce 
information stored in the optical disc, where the optical disc is a 
read-only optical disc, a write-once optical disc, or a phase-change 
optical disc. 
In one type of a conventional optical head, a light reflected from an 
optical disc is partly introduced to a signal detecting system by a 
polarizing beam splitter, and is divided into a focus error detecting 
light and a track error detecting light in the signal detecting system by 
a beam splitter. Then, a focus error is detected in the knife edge method 
by receiving the focus error detecting light, and a track error is 
detected in the push-pull method by receiving the track error detecting 
light. 
In a further type of a conventional optical head, a light reflected from a 
magnetooptic disc is partly introduced to first and second signal 
detecting systems by first and second polarizing beam splitters arranged 
in tandem on a light path. In the first signal detecting system, the 
introduced light is divided into first and second lights by a beam 
splitters, so that information stored in the optical disc is re-produced 
by a difference of light intensity between the first and second divided 
lights. In the second signal detecting system, the introduced light is 
also divided into third and fourth lights by a beam splitters, so that a 
focus error is detected in the knife edge method by receiving the third 
divided light, and a track error is detected in the push-pull method by 
receiving the fourth divided light. 
In a still further type of a conventional optical head, a light reflected 
from a magnetooptic disc is divided into first and second diffraction 
lights by first and second birefringent diffraction grating elements 
having specified light axes and predetermined filling materials and 
arranged in tandem on a light path, so that information stored in the 
magnetooptic disc is re-produced by a difference of light intensity 
between the first and second diffraction lights. 
According to the conventional optical heads, however, there is a 
disadvantage in that it is difficult that the optical heads become light 
and compact, because a number of parts such as a polarizing beam splitter, 
a half wave plate, a quarter wave plate, etc. are arranged in a 
predetermined designed pattern as explained in detail later. There is a 
further disadvantage in that it is difficult that the optical heads are 
decreased in cost and price, because the number of parts is large, the 
polarizing beam splitter is high in price, and the number of assembling 
steps is large as also explained later. There is a still further 
disadvantage in that a light utilizing factor is decreased especially in 
the optical head having the magnetooptic disc, because the first and 
second diffraction lights exist partly on a single light path, through 
which a light radiated from a light source is propagated in a direction to 
the magnetooptic disc. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of this invention to provide an optical head 
which becomes light and compact. 
It is a further object of this invention to provide an optical head which 
becomes low in cost and price. 
It is a still further object of this invention to provide an optical head 
having a high utilizing factor of light. 
According to this invention, an optical head comprises: 
means for focusing a light radiated from a light source on an optical disc; 
an optical diffraction element for diffracting a light reflected from the 
optical disc; and 
a photodetector device for detecting a light diffracted by the optical 
diffraction element; 
wherein the optical diffraction element is a birefringent diffraction 
grating element utilizing an optical crystal.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Before explaining an optical head in the preferred embodiments according to 
the invention, the aforementioned conventional optical heads will be 
explained in more detail. 
FIG. 1 shows the first type of the conventional optical head comprising a 
semiconductor laser 1, a polarizing beam splitter 4, a quarter wave plate 
5, an optical disc 7, a beam splitter 9, etc. 
In this optical head, a linearly polarized light radiated from the 
semiconductor laser 1 is converted to a parallel light 3 to be supplied to 
the polarizing beam splitter 4 by a collimating lens 2. A polarized 
direction of the parallel light 3 is set to transmit through the 
polarizing beam splitter 4. The light transmitted through the polarizing 
beam splitter 4 is transmitted through the quarter wave plate 5 to be 
focused on the optical disc 7 by a focusing lens 6. A light reflected from 
the optical disc 7 is transmitted through the quarter wave plate 5 back to 
the polarizing beam splitter 4 as a linearly polarized light orthogonal in 
polarization to the radiated light of the semiconductor laser 1. The light 
incident to the polarizing beam splitter 4 is bent in its light path by 90 
degrees, so that it is introduced to a signal detecting system. This 
introduced light is divided into a light 10 for detecting a focus error 
and a light 11 for detecting a track error by the beam splitter 9. Then, 
the focus error is detected in the knife edge method using a knife edge 
10B by a photodetector 10A receiving the light 10, while the track error 
is detected in the push-pull method by a photodetector 11A receiving the 
light 11. In addition, information stored in the optical disc 7 is 
re-produced by a sum of outputs of the optical detectors 10A and 11A. 
In an erasable magnetooptic disc, on the other hand, information is 
re-produced by detecting a rotating direction of a linearly polarized 
light incident to the disc. Therefore, it is required that a rotation of a 
polarizing direction is precisely detected, even if it is a minute 
rotation. 
FIG. 2 shows the second type of the conventional optical head comprising a 
semiconductor laser 12, first and second polarizing beam splitters 15 and 
16, a magnetooptic disc 18, a half wave plate 19, a third polarizing beam 
splitter 20, a beam splitter 23, etc. 
In this optical head, a light radiated from the semiconductor laser 12 is 
converted to a parallel light 14 by a collimating lens 13, and the 
parallel light 14 is transmitted through the first and second polarizing 
beam splitters 15 and 16 to be focused on the magnetooptic disc 18 by a 
focusing lens 17. A light reflected from the magnetooptic disc 18 is 
introduced to first and second signal detecting systems by the first and 
second polarizing beam splitters 15 and 16. The introduced light of the 
second polarizing beam splitter 16 is rotated in its main polarized 
direction by 45 degrees, when it is transmitted through the half wave 
plate 19. Then, the light is supplied to the third polarizing beam 
splitter 20, so that the light is divided into a first light to be 
supplied to a first photodetector 21 and a second light to be supplied to 
a second photodetector 22. Then, information stored in the magnetooptic 
disc 18 is re-produced by a subtraction of outputs of the first and second 
photodetectors 22 and 24. The reason why the first and second lights 
divided by the polarizing beam splitter 20 are received by the first and 
second photodetectors 22 and 24, after the 45 degree rotation of the 
polarized light is carried out by the half wave plate 19, is that a 
magnetooptic signal is improved in its quality by removing in-phase signal 
components in light intensity change of the reflected light, etc. The 
introduced light of the first polarizing beam splitters 15 is transmitted 
through a focusing lens 15A to be supplied to the beam splitter 23, so 
that the light is divided into a third light to be supplied to a third 
photodetector 24 and a fourth light partly to be interrupted by a knife 
edge 25 and supplied to a fourth photodetector 26. Thus, a track error is 
detected in the push-pull method by the third photodetector 24, and a 
focus error is detected in the knife edge method by the fourth 
photodetector 26. 
FIG. 3 shows the third type of the conventional optical head utilizing 
birefringent diffraction grating-elements, for instance, as explained in 
the Japanese Patent Application No. 62-81571, and comprising a 
semiconductor laser 27, birefringent diffraction grating elements 28a and 
28b, a magnetooptic disc 18, etc. The birefringent diffraction grating 
elements 28a and 28b have a function of diffracting a light having a 
particular polarized component, and a structure as shown in FIG. 4. That 
is, each of the birefringent diffraction grating elements 28a and 28b is 
composed of an anisotropic plate 32 having two different refractive 
indexes in directions orthogonal to an optic axis 33, and a material 
having a refractive index equal to one of the two refractive indexes and 
contained in grating grooves 32A provided on a surface of the anisotropic 
plate 32, and one of the birefringent diffraction grating elements 28a and 
28b has grating grooves different in interval period from the other. 
In this optical head, a light radiated from the semiconductor laser 27 is 
transmitted through the birefringent diffraction grating elements 28a and 
28b to be focused on the magnetooptic disc 18 by a focusing lens 31. A 
light reflected from the magnetooptic disc 18 is diffracted to be supplied 
to a two divided photodetector 29 by the birefringent diffraction grating 
element 28a, so that a track error is detected in the push-pull method, 
while the reflected light is also diffracted to be supplied to a four 
divided photodetector 30 by the birefringent diffraction grating element 
28b, so that a focus error is detected in the knife edge method. The 
birefringent diffraction grating elements 28a and 28b are designed, such 
that the optic axes 33 and the filling materials 34 are adequately 
selected to divide the reflected light into the two diffraction lights 
each having a different polarization from the other. Information stored in 
the magnetooptic disc 18 is re-produced by an intensity difference between 
the two diffraction lights, that is, a subtraction between a first sum of 
outputs of the two divided photodetector 29 and a second sum of outputs of 
the four divided photodetector 30. 
Next, an optical head in the first preferred embodiment according to the 
invention will be explained in FIG. 5. The optical head comprises a 
semiconductor laser 48 for radiating a linearly polarized light 49, a 
birefringent diffraction grating element 50 (to be explained later in 
detail), a collimating lens 51, a quarter wave plate 52, a focusing lens 
53, an optical disc 54, and first and second photodetectors 56 and 58. 
In this optical head, the linearly polarized light 49 radiated from the 
semiconductor laser 48 is incident to the birefringent diffraction grating 
element 50 consisting of a lithium niobate crystal having a Z-axis 
orthogonal to the polarization of the light 49. The light 49 is not 
affected by the birefringent diffraction grating element 50, because the 
incident light 49 is an ordinary light relative to the birefringent 
diffraction grating element 50. A light transmitted through the 
birefringent diffraction grating element 50 is converted to a parallel 
light by the collimating lens 51, and is transmitted through the quarter 
wave plate 52 to be focused on the optical disc 54 by the focusing lens 
53. A light reflected from the optical disc 54 is converted to a parallel 
light to be supplied to the quarter wave plate 52, in which a linearly 
polarized light orthogonal in polarization to the linearly polarized light 
49 is obtained to be incident to the birefringent diffraction grating 
element 50, so that the incident light is diffracted due to the function 
of a grating pattern of the birefringent diffraction grating element 50, 
because the reflected light is an extraordinary light relative to the 
lithium niobate crystal. Then, a plus primary diffraction light 55 is 
received by the first photodetector 56, and a minus primary diffraction 
light 57 is received by the second photodetector 58. Thus, focus and track 
error signals are obtained by outputs of the first photodetector 56, and 
information stored in the optical disc 54 is re-produced by a sum of 
outputs of the first and second photodetectors 56 and 58. 
In this first preferred embodiment, the birefringent diffraction grating 
element 50 and the first photodetector 56 are arranged to have a relation 
shown in FIG. 6. As clearly illustrated therein, the birefringent 
diffraction grating element 50 has first to fourth grating pattern 
sections 59 to 62, and the first photodetector 56 has first to sixth 
segments 63 to 68, on which four light spot points A to D are indicated. 
On the first grating pattern section 59, a grating pattern is defined to 
provide a light spot focused on the point A by a light incident to the 
section 59. In the same manner, grating patterns are defined on the second 
to fourth grating pattern sections 60 to 62 to provide light spots focused 
on the points B to D by lights incident to the sections 60 to 62, 
respectively. In this arrangement, a focus error signal is obtained by a 
subtraction between two sums of output signals of diagonal segments among 
the central segments 63 to 66 of the first photodetector 56, while a track 
error signal is obtained by a subtraction between output signals of the 
fifth and sixth segments 67 and 68 thereof. In this first preferred 
embodiment, although the grating patterns of the birefringent diffraction 
grating element 50 are arranged to provide the plus and minus primary 
diffraction lights 55 and 57 to the left and right of the semiconductor 
laser 48, the plus and minus primary diffraction lights 55 and 57 may be 
directed on the upper and lower sides of the semiconductor laser 48, 
because a relation between grating patterns of a birefringent diffraction 
grating element and a crystal axis of a lithium niobate crystal is 
arbitrary. 
Here, the birefringent diffraction grating element 50 will be explained in 
more detail. 
If proton-exchange is applied to X-plate or Y-plate of lithium niobate by 
benzoic acid, refractive indexes are increased relative to an 
extraordinary light by approximately 0.13, and decreased relative to an 
ordinary light by approximately 0.04. Then, it is possible that an 
refractive index change occurs relatively only to an extraordinary light 
due to proton-exchange, in a state that a phase difference occurring 
between ordinary lights transmitting through a proton-exchange section and 
a proton-non-exchange section is cancelled by means of a phase 
compensation film, etc. Therefore, when the proton-exchange and 
non-exchange sections are provided periodically, and a phase difference 
compensation measure such as the aforementioned film is adopted, a 
diffraction grating operating to diffract only an extraordinary light is 
obtained. In this circumstance, if a phase difference of the extraordinary 
light is set to be ".pi." between the proton-exchange section and the 
proton-non-exchange section, a transmission factor of the extraordinary 
light is 0%, while that of the ordinary light is 100%. A polarizer has 
been discussed to be applied to such a birefringent diffraction grating, 
as described on pages 168 and 169 of "a technical digest" in "the second 
optoelectronics conference (OEC '88)". 
This grating element has a function of a polarizer, and can be applied to 
an optical head of a read only type, a write-once type, or a phase-change 
type, as understood from a following explanation. 
In FIG. 7, it is arranged that Z-axis of the lithium niobate crystal 36 is 
orthogonal to a linear polarization 35 of the light radiated from the 
semiconductor laser 48 in the first preferred embodiment. Therefore, the 
radiated light is not affected by the grating pattern of the lithium 
niobate crystal 36. As shown in FIG. 8, on the other hand, a linear 
polarization 39 of the light reflected from the optical disc 54 is rotated 
to be supplied to the lithium niobate crystal 36 relative to the linear 
polarization 35 of the radiated light by 90 degrees, because the radiated 
light is transmitted through the quarter wave plate 52 two times. As a 
result, the supplied light is diffracted in its almost all light amount to 
be diffracted lights 40 and 41 by the lithium niobate crystal 36. 
Therefore, almost all the radiated light is not returned to the 
semiconductor laser 48. Consequently, the semiconductor laser 48 can 
continue a stable lasing oscillation in a state that it does not have 
returning light noise, so that a quality of reproduced information is not 
deteriorated. Even more, at least two diffraction lights are received to 
improve the quality by the photodetectors which can compensate the 
decrease of light amount. 
FIG. 9 shows an optical head in the second preferred embodiment according 
to the invention. The optical head comprises a semiconductor laser 69, 
photodetectors 70, and a birefringent diffraction grating element 71 and a 
quarter wave plate 72 which are included integrally in an optical module 
73. The optical head further comprises a focusing lens 53, and an optical 
disc 54. This optical head can be compact, because the optical module 73 
is utilized. Operation of this optical head is almost identical to that of 
the optical head in the first preferred embodiment. Therefore, the 
explanation thereof is not made here. 
FIG. 10 shows an optical head in the third preferred embodiment according 
to the invention. The optical head comprises a semiconductor laser 74, a 
collimating lens 75, a polarizing beam splitter 77, a focusing lens 78, a 
magnetooptic disc 79, a focusing lens 80, a birefringent diffraction 
grating element 82, and first to third photodetectors 84, 86 and 88. 
In this optical head, a light radiated from the semiconductor laser 74 is 
converted to a parallel light 76 by the collimating lens 75, and the 
parallel light 76 is transmitted through the polarizing beam splitter 77 
to be focused on the magnetooptic disc 79 by the focusing lens 78. A light 
reflected from the magnetooptic disc 79 is converted to a parallel light 
by the focusing lens 78, and the parallel light is supplied to the 
polarizing beam splitter 77, and is thereby introduced to a signal 
detecting system, in which a light supplied from the polarizing beam 
splitter 77 is focused to be supplied to the birefringent diffraction 
grating element 82 composed of a lithium niobate crystal by the focusing 
lens 80. The Z-axis of this crystal has 45 degrees in its arrangement 
relative to a main polarization direction of the reflected light. A plus 
primary diffraction light 83, a transmitted light 85, and a minus primary 
light 87 are supplied from the birefringent diffraction grating element 82 
to the first to third photodetectors 84, 86 and 88, respectively, as 
illustrated in FIG. 10. Consequently, information stored in the 
magnetooptic disc 79 is re-produced by a difference signal between an 
output signal of the second photodetector 86 and a sum signal of output 
signals of the first and second photodetectors 84 and 88. 
In a modified arrangement of the third preferred embodiment according to 
the invention, the Z-axis of the lithium niobate crystal has 46.5 degrees 
relative to the main polarization direction of the reflected light from 
the magnetooptic disc 79. A grating pattern of the birefringent 
diffraction grating element 82 is obtained by the proton-exchange, as 
explained before, and has a cross-section of a rectangle. When a grating 
pattern is provided to have a depth of grooves, such that a transmission 
factor of an extraordinary light is 0%, a diffraction efficiency of plus 
and minus primary diffraction lights is approximately 4.0%. As a result, a 
light receiving diffraction efficiency is approximately 80%. According to 
a definition to be explained later, a light intensity ratio .gamma. of a 
receiving diffraction light is 0.8. Consequently, a crystal angle .alpha. 
of the lithium niobate crystal relative to an incident linearly polarized 
light can be set to be an angle between 45 and 48 degrees, as shown in 
FIG. 3 to be explained later. In the modified preferred embodiment, the 
angles is set to be 46.5 degrees. 
FIG. 11 shows a relation between grating pattern sections of the 
birefringent diffraction grating element 82 and divided segments of the 
first photodetector 84. The birefringent diffraction grating element 82 is 
provided with first to fourth grating pattern sections 91 to 94, and the 
first photodetector is provided with first to fourth segments of 95, fifth 
segment 97 and sixth segment 98, on which light spots A to D are formed. 
When a light spot is adequately formed on the magnetooptic disc 79, a 
light incident to the first grating pattern section 91 of the birefringent 
diffraction grating element 82 is focused to be the light spot A on the 
first photodetector 84. In the same manner, grating patterns are defined 
on the second to fourth grating pattern sections 92 to 94, such that 
lights incident to those sections 92 to 94 are focused to be the light 
spots B to D on the first photodetector 84. In this arrangement, a focus 
error signal is obtained by a difference signal between a first output sum 
of first diagonal segments and a second output sum of second diagonal 
segments, respectively, of the four segments 95 in the first photodetector 
84, and a track error signal is obtained by a difference signal between 
output signals of the fifth and sixth segments 97 and 98 in the first 
photodetector 84. 
Here, a function of the birefringent diffraction grating element 82 
relative to the magnetooptic disc 79 will be explained in more detail in 
FIGS. 12 to 14. 
FIG. 12 shows a relation in which a main polarization direction 110 of a 
light reflected from the magnetooptic disc 79 is approximately 45 degrees 
relative to the Z-axis 109 of the lithium niobate crystal for the 
birefringent diffraction grating element 82 fabricated as explained 
before. Consequently, extraordinary light components 42 and 43 of the 
reflected light are divided to be diffraction lights 44 and 45, while an 
ordinary component 41 thereof is transmitted through the birefringent 
diffraction grating element 82 to be a transmitted light 47. As explained 
before, information stored in the magnetooptic disc 79 is re-produced by a 
difference signal between a summed light intensity of the diffraction 
lights 44 and 45 and a light intensity of the transmitted light 47. 
Otherwise, the main polarization direction 110 of the reflected light from 
the magnetooptic disc 79 may be changed at any angle other than 0 and 90 
degrees relative to the Z-axis of the lithium niobate crystal 108 for the 
birefringent diffraction grating element 107. Even in such an arrangement, 
the same result that the extraordinary light components 42 and 43, and the 
ordinary light component 46 are divided in the form of the diffraction 
lights 44 and 45, and the transmitted light 47 is obtained. In this 
circumstance, a light intensity ratio is determined by a crystal 
angle.sup..alpha. of the crystal relative to a polarization direction of 
the incident linearly polarized light, a diffraction efficiency .eta., and 
an order number n of receiving diffraction lights, where the order number 
n is an integer including "one". That is, it may includes plus and minus 
diffraction lights, and, even more, secondary diffraction light. For the 
purpose of removing same phase noise components in a signal detection, it 
is desired that direct current components of the diffraction and 
transmission lights are equal in light amount to each other. In this 
point, when the diffraction efficiency .eta., and the order number n of 
the receiving diffraction lights are specified, the aforementioned 
angle.sup..alpha. is determined to make the direct current components 
equal to each other. 
FIG. 13 shows a dependency of a residual direct current component R on a 
relative angle.sup..alpha. of the crystal axis to a polarization direction 
of an incident linearly polarized light by use of a parameter of the 
diffraction light intensity ratio .gamma. after a signal detection 
normalized by a light intensity of the transmission light. As apparent 
from the explanation of FIG. 12, when the angle .alpha. is 0.degree., the 
polarization direction of the incident linearly polarized light is 
coincided with an ordinary light axis of the crystal, while, when the 
angle .alpha. is 90.degree., the polarization direction is coincided with 
an extraordinary light axis thereof. Although the residual direct current 
component R is zero in a curve having a diffraction light intensity ratio 
.gamma. of "1", when the angle .alpha. is 45 degrees, the value of the 
diffraction light intensity ratio .gamma. is not practical. The 
diffraction light intensity ratio .gamma. is practically less than "1". As 
the diffraction light intensity ratio .gamma. is decreased, the angle 
.alpha., at which the residual direct current component R becomes zero, is 
increased. 
On the other hand, FIG. 14 shows a calculation result how a magnetooptic 
signal is dependent on the relative angle .alpha.. As clearly seen 
therein, the magnetooptic signal is maximum, when the relative angle is 45 
degrees, regardless of a value of the diffraction light intensity ratio 
.gamma.. 
Therefore, if it is designed that the relative angle.sub..alpha. is set to 
be 45 degrees, at which direct current components of the diffraction and 
transmission lights are equal in light amount to each other, a 
signal/noise ratio can be maximum. In addition, if a grating pattern 
provided on the lithium niobate crystal becomes in the form of a hologram, 
focus and track errors can be detected by receiving the diffraction 
lights. 
FIG. 15 shows an optical head in the fourth preferred embodiment according 
to the invention. The optical head comprises a semiconductor laser 74, a 
collimating lens 75, a polarizing beam splitter 77, a focusing lens 78, a 
magnetooptic disc 79, first to third photodetectors 84, 86 and 88, a total 
reflection prism 100, a birefringent diffraction grating element 101, and 
a focusing lens 102. In this arrangement, the total reflection prism 100, 
the birefringent diffraction grating element 101, and the focusing lens 
102 are integrally combined to provide a single optical package. As a 
result, the optical head becomes small and compact. In this optical 
package, the birefringent diffraction grating element 101 is bonded to the 
total reflection prism 100 by use of adhesives, while the focusing lens 
102 is combined to the total reflection prism 100 by polishing both 
contacting surfaces thereof or by use of adhesives. 
In operation, plus and minus primary diffraction lights 83 and 87 are 
received by the first and third photodetectors 84 and 88, and transmission 
light 85 is received by the second photodetectors 86, as explained in the 
preferred embodiment of FIG. 10. 
FIG. 16 shows an optical head in the fifth preferred embodiment according 
to the invention. The optical head comprises a semiconductor laser 74, a 
collimating lens 75, a polarizing beam splitter 77, a focusing lens 78, a 
magnetooptic disc 79, a focusing lens 80, and a signal detecting package 
105 including a birefringent diffraction grating element 104 and 
photodetectors 103. The signal detecting package 105 may include the 
focusing lens 80. By the adoption of the signal detecting package 105, the 
optical head can be compact. 
The aforementioned birefringent grating elements can be fabricated by a 
planar batch process. For instance, Ti-diffused layer is formed on a 
Y-plate lithium niobate substrate, and proton-exchange is carried out in 
the Ti-diffused layer by a predetermined grating pattern. 
Although the invention has been described with respect to specific 
embodiment for complete and clear disclosure, the appended claims are not 
to be thus limited but are to be construed as embodying all modification 
and alternative constructions that may occur to one skilled in the art 
which fairly fall within the basic teaching herein set forth.