JPH03170902A - Diffraction element unit - Google Patents

Abstract

PURPOSE:To securely photodetect diffracted light by a photodetector by providing a diffraction angle variation suppressing means formed of a material, which receives diffracted light from a diffraction element and varies in the quantity of variation in projection angle with the quantity of variation in incidence angle, behind the diffraction element. CONSTITUTION:The diffraction element unit is provided with the diffraction element 2a where a diffraction grating is formed on a transparent substrate 2 made of glass, etc., at a grating interval nearly equal to the wavelength of object laser light, etc., and the diffraction angle variation suppressing means 3 which is arranged behind the diffraction element 2a so that the diffracted light from the diffraction element 2a is made incident and made of the transparent material so that the quantity of variation in projection angle decreases with the quantity of variation in incidence angle. Then the diffracted light from the diffraction element 2a is made incident on the diffraction angle variation suppressing means 3a and projected so that the quantity of variation in projection angle is smaller than the quantity of variation in the angle of diffraction by the diffraction element 2a. Consequently, the variation in the angle of diffraction by the diffraction element 2a is suppressed and the diffracted light which is diffracted by the diffraction element 2a can securely be photodetected by the photodetector 37 without increasing the photodetection area of the photodetector 37 specially.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a diffraction element unit incorporated in an optical head that performs recording, reproduction, or erasing on various optical memory elements, and particularly relates to a diffraction element unit that is incorporated in an optical head that performs recording, reproduction, or erasing in various optical memory elements, and in particular, The present invention relates to a diffraction element unit that can suppress fluctuations in diffraction angle caused by.

[Conventional technology]

In recent years, the development of magneto-optical memory elements that are capable of high density and large capacity, and that can be repeatedly recorded and erased, have been actively developed. This magneto-optical memory element is, for example, as shown by reference numeral 21 in FIG. 6, in which a magnetic thin film 2lb having an axis of easy magnetization in a direction perpendicular to the film surface is formed on a transparent substrate 21a. The magnetic thin film 2lb is magnetized in either direction by initialization prior to recording.

During recording, an external magnetic field (not shown) is applied to the magnetic thin film 2lb in a direction opposite to the direction of the external magnetic field during initialization, and a relatively large magnetic field is emitted from a laser light source such as the semiconductor laser 22. Laser light L1 is emitted with high intensity, and transmitted light L2 is transmitted through polarizing beam splitter 23. By irradiating the magnetic thin film 2lb, the coercive force is lowered in the irradiated portion due to a temperature rise, and the magnetization is reversed in the direction of the external magnetic field. Note that the polarizing beam splitter 23
The reflected light LSI in is not used.

On the other hand, during reproduction, the laser beam L. is emitted from the semiconductor laser 22 with relatively low intensity. is emitted, and the transmitted light Lgo from the polarizing beam splitter 23 is irradiated onto the magnetic thin film 2lb. The plane of polarization of the reflected light L3 from the magnetic thin film 2lb is rotated according to the direction of magnetization due to the magneto-optic effect. A part of the reflected light L3 is reflected at a right angle by the polarizing beam splitter 23, and this reflected light L3 is reflected at right angles. The information is reproduced by detecting the inclination of the plane of polarization at the photodetector 25 via the analyzer 24. Note that the transmitted light L4 in the polarizing beam splitter 23. is not used.

Furthermore, in order to further improve the accuracy of information reproduction, it is conceivable to configure the optical head as shown in FIG. 7 to perform differential detection. That is, the magneto-optical memory element 3l, the semiconductor laser 32 and the polarizing beam splitter 33 in FIG. After the polarization plane of the reflected light L41 is rotated by 45 degrees by the (1/2) wavelength plate 34, the reflected light L41 is rotated by 45 degrees by the (1/2) wavelength plate 34, and then the reflected light L41 is transferred to the substrate 3 made of glass or the like.
The light is incident on the diffraction element 35a formed at 5.

Then, the incident light is converted into 0th order diffracted light L,s by the diffraction element 35a.
o and the 1st-order diffracted light L and I, and the 0th-order diffracted light L
,. is received by the photodetector 36, while the first-order diffracted light L
The light s+ is received by a photodetector 37, and the outputs of the photodetectors 36 and 37 are differentiated by a differential amplifier 38, so that information on the magnetic thin film 3lb is detected. As mentioned above, to separate the reflected light L41 into polarized light,
Using the diffraction element 35a has the advantage that the optical head can be made lighter than when using a polarizing beam splitter or the like. Note that, as shown in FIG. 8, condenser lenses 40 and 41 are arranged between the diffraction element 35a and each of the photodetectors 36 and 37, respectively, as necessary.

In the above configuration, as shown in FIG. 9, the diffraction element 35a has diffraction gratings 35b, 35b, . It is formed by Further, the reflected light from the magneto-optical memory element 3l is reflected by the diffraction gratings 35b, 35b, . . . at a so-called Bragg angle θ. (θ.=sin-'
The inclination of the substrate 35 is set so that the light is incident at (λ/2D)). However, λ is the wavelength of the laser light used.

Here, when the grating spacing D of the diffraction gratings 35b, 35b... is 0.59λ, and the refractive index of the substrate 35 is 1.45, the 0th order diffracted light L by the diffraction gratings 35b, 35b...
,. and the intensity ratio of two mutually orthogonal polarization components in each of the first-order diffraction light Ls+ and the above-mentioned diffraction grating 35b
・The relationship between 35b and the depth d is shown in FIG. However, L5. (TE) is the 0th order diffracted light L, A direction parallel to the direction of the diffraction gratings 35b, 35b, etc. (direction perpendicular to the paper surface of FIG. 9)? Indicates the polarization component, LS, (TE)
are the diffraction gratings 35b and 35b in the first-order diffraction light LSI
... indicates a certain amount of polarization in a direction parallel to... Also, L5. (T
M) is the 0th order diffracted light L,. Diffraction grating 35b.3 in
5b... indicates the polarization command in the direction perpendicular to L, . (T
M) is the diffraction grating 35b.3 in the first-order diffracted light Lst.
5b... shows a certain amount of polarization in a direction perpendicular to .

As is clear from the figure, Lso (TM): L
The intensity ratio of sI(TM) is approximately 100:O, while
L.S. (TE): The intensity ratio of L, , (TE) changes depending on the depth d of the diffraction gratings 35b, 35b, . . . that constitute the diffraction element 35a. In the above diffraction grating 35b,
L. (TE): LSI (TE) L=. O
: 100, the depth is set to about 1.2λ (see Fig. 10). As a result, the diffraction element 35a
In, the 0th order diffracted light L,. almost all become TM waves, and almost all of the first-order diffracted light LSI becomes TE waves. In addition, since the polarization plane of the reflected light L from the polarizing beam splitter 33 is rotated by 45 degrees by the (l/2) wavelength plate 34 on the front side of the diffraction element 35a, the distribution of the polarized light by the diffraction element 35a is The reference axial direction has an angle of 45° with respect to the linearly polarized light emitted from the semiconductor laser 31.

[Problem to be solved by the invention]

However, in the above configuration, when the first-order diffracted light LSI from the diffraction element 35a is used for detecting information, if the wavelength of the laser light L1 emitted from the semiconductor laser 32 changes due to temperature change, etc. Diffraction element 3 accordingly
The diffraction angle θ at 5a varies. In that case, if the amount of variation in the diffraction angle θ1 is large, the first-order diffracted light 1-s+ in the diffraction element 35a cannot be received by the photodetector 37, and as a result, there is a possibility that information cannot be detected. Note that if the light-receiving area of the photodetector 37 is made sufficiently large so that the first-order diffracted light LSI can be reliably received by the photodetector 37, a problem arises in that the response speed of the photodetector 37 decreases.

[Means to solve the problem]

In order to solve the above-mentioned problems, a diffraction element unit according to the present invention has a diffraction grating formed on a transparent substrate made of glass or the like with a grating interval approximately equal to the wavelength of light such as a target laser beam. a diffraction angle formed by a transparent material so that the amount of variation in the output angle is smaller than the amount of variation in the angle of incidence; The basic feature is that it is equipped with a fluctuation suppressing means.

Note that the diffraction angle variation suppressing means may be constituted by, for example, a beam shaping prism arranged so as to expand the beam diameter of the diffracted light in the direction in which the diffraction angle in the diffraction element changes.

[Function] According to the above structure, the diffracted light from the diffraction element is made incident on the diffraction angle variation suppressing means formed of a translucent material, and the amount of variation in the incident angle, in other words, the amount of variation in the diffraction angle in the diffraction element is By emitting light so that the amount of variation in the output angle is smaller than the amount of variation in the diffraction angle, it becomes possible to suppress variation in the diffraction angle in the diffraction element. For example, if the above diffraction element unit is incorporated into an optical head that uses a diffraction element, fluctuations in the diffraction angle in the diffraction element can be suppressed, and the diffraction element can be used without particularly increasing the light-receiving area of the photodetector. The diffracted light is reliably received by the photodetector.

[Embodiment 1] An embodiment of the present invention will be described below based on FIGS. 1 to 4.

The diffraction element unit 1 according to this embodiment is used by being incorporated into an optical head as shown in FIG. 2, for example. This optical head is for recording and reproducing information in the magneto-optical memory element 31, and its main parts are constructed in the same way as the optical head shown in FIG. Here, members having the same structure as the optical head in FIG. 7 are given the same reference numerals, and their explanations will be omitted.

In this embodiment, a diffraction element unit 1 is provided in place of the diffraction element 35a in FIG. As shown in FIG. 1, this diffraction element unit 1 includes a diffraction element 2a formed on a transparent substrate 2 made of glass or the like, and a first-order diffracted light L 51 from this diffraction element 2a in the optical path. A beam shaping prism 3 is provided as a diffraction angle fluctuation suppressing means. Further, condenser lenses 40 and 41 are arranged between the diffraction element 2a and the photodetector 36 and between the beam shaping prism 3 and the photodetector 37, respectively.

Diffraction grating that constitutes the diffraction element 2a (not specifically shown)
Similar to the diffraction grating in the diffraction element 35a in FIG. 7, the grating interval is about the wavelength of the laser beam used, for example,
The thickness is set to about 0.5 μm. Further, the direction of the diffraction grating constituting the diffraction element 2a is perpendicular to the paper plane of FIGS. 1 and 2, similar to the diffraction grating in the diffraction element 35a of FIG. 7. Furthermore, the depth of the diffraction grating that covers the diffraction element 2a is such that almost all of the TM waves in the light beam from the (1/2) wavelength plate 34 are Oth-order diffracted light L. On the other hand, the value is set so that almost all of the TE waves in the light beam from the (1/2) wavelength plate 34 are diffracted as first-order diffracted light LSI.

The beam shaping prism 3 changes the traveling direction of the first-order diffracted light LSI from the diffraction element 2a to the left in the figure and emits it. As shown in FIG. 3, this beam shaping prism 3 allows the output light from the beam shaping prism 3 to The optical characteristics are set so that the beam diameter r2 of L, in the direction parallel to the plane of the paper in FIG. 3, is expanded by about three times, for example. On the other hand, the beam diameters in the direction perpendicular to the plane of FIG. 3 between the incident light and the outgoing light to the beam shaping prism 3 are 1:1. Note that only the center line of the light beam is shown in FIGS. 1 and 2.

In the above structure, when reproducing information, a part of the linearly polarized laser light L1 from the semiconductor laser 32 is transmitted through the polarization beam splitter 33, and this transmitted light L2 is transmitted. is irradiated onto the magneto-optical memory element 31.

A portion of the reflected light L3 from the magneto-optical memory element 31 is reflected at right angles by the polarizing beam splitter 33, and after this reflected light L41 has its polarization plane rotated by 45 degrees by the (1/2) wavelength plate 34, The light is incident on the diffraction element 2a.

Here, due to the polarization characteristics mentioned above, the TM wave is approximately lOO
%Oth order diffracted light L,. The light is incident on the photodetector 36 via the condensing lens 40. On the other hand, the (l/2) wavelength plate 34
The TE wave in the light flux from LS is almost 100% {th order diffracted light LS
It is diffracted as I, and is incident on the condenser lens 4l via the beam shaping prism 3 and the condenser lens 41. and,
Information on the magneto-optical memory element 31 is detected by differentially outputting the output signals of the photodetectors 36 and 37 by a differential amplifier 38.

Note that when the wavelength of the laser beam L emitted from the semiconductor laser 32 changes, the diffraction angle θ1 in the diffraction element 2a changes accordingly. For example, laser light L. When the wavelength of the laser beam LSI is a standard value, the center line of the first-order diffracted light LSI is at the position indicated by A in FIG. 1, and the diffraction angle is θ1. When the wavelength of the first-order diffracted light LSI becomes shorter than the standard value, the center line of the first-order diffracted light LSI becomes, for example, the position indicated by B, and the diffraction angle becomes smaller than θ1.

On the other hand, when the wavelength of the laser beam L1 becomes longer than the standard value, the center line of the first-order diffracted light LSI is at a position indicated by C, for example, and the diffraction angle becomes larger than θ.

By the way, in this embodiment, when the first-order diffracted light LSI passes through the beam shaping prism 3, the amount of variation in the output angle of the output light L6 from the beam shaping prism 3 is equal to It is smaller than the amount of variation in the diffraction angle. For example, the beam shaping prism 3 is set to expand the beam diameter of the output light to about three times the beam diameter of the incident light in the paper plane direction of FIG. 1, that is, in the direction of the arrow X in which the diffraction angle changes. In this case, the amount of variation in the output angle of the emitted light L6 is approximately 1/33 of the amount of variation in the diffraction angle of the first-order diffraction lights L and l.

That is, assuming that the wavelength of the laser light emitted from the semiconductor laser 32 is 780 nm, and that it varies by a maximum of Onm, the maximum variation of the diffraction angle in the diffraction element 2a is about 1.8°. In that case, the amount of variation in the output angle of the output light L from the beam shaping prism 3 is at most 0.
．． It is about 58°, that is, about (1/3) of the amount of variation in the diffraction angle.

Note that if the beam shaping prism 3 is made of flint-based glass or the like with large wavelength dispersion, it is advantageous in suppressing fluctuations in the exit angle from the beam shaping prism 3 due to the effect of wavelength dispersion in which the refractive index changes with wavelength fluctuations. Become.

As described above, in this embodiment, by arranging the beam shaping prism 3 after the diffraction element 2a, the diffraction element 2a
Since the amount of variation in the diffraction angle at point a can be suppressed to, for example, about (1/3), the light flux that should be incident on the photodetector 37 can be reduced even without particularly increasing the light-receiving area of the photodetector 37. There is no possibility of it coming off the detector 37. This allows information to be detected reliably.

FIG. 4 shows a modified example.

In this modification, the beam shaping prism 3 is arranged in the opposite direction to that of the above embodiment, and the first-order diffracted light L% from the diffraction element 2a is
The configuration is such that the traveling direction of the + beam is changed to the right in the figure and the beam is emitted. In this case, if the maximum variation amount of the diffraction angle in the diffraction element 2a is about 1.86, the variation amount of the exit angle from the beam shaping prism 3 is about 0.61'.

[Example 2] Next, a second embodiment will be described based on FIG.

The diffraction element unit 11 according to the second embodiment can be incorporated, for example, in the optical head shown in FIG. 2 instead of the diffraction element unit 1. This diffraction element unit 11 includes a transparent substrate 12 made of glass or the like, and a substrate 12.
A diffraction element 12a is formed on the surface portion of the side facing the (1/2) wavelength plate 34. The diffraction element 12a has a structure substantially similar to the diffraction element 2a of the first embodiment, with a diffraction grating (not specifically shown) extending in a direction perpendicular to the plane of the paper.

On the other hand, on the surface portion of the substrate 12 opposite to the surface portion on which the diffraction element 12a is formed, a diffraction grating for compensating for diffraction angle fluctuation (specifically A diffraction element 12b for compensating for diffraction angle fluctuations is provided (not shown). This diffraction element 12b for compensating for diffraction angle variation has a role as a diffraction angle variation suppressing means.

Condensing lenses 40 and 41 are arranged in parallel behind the substrate 12, and a photodetector 3 is arranged behind each condensing lens 40 and 41.
6 and 37 are arranged in parallel.

In the above structure, the light beam that has passed through the (1/2) wavelength plate 34 is incident on the diffraction element 12a, where almost 100% of the TM wave is transmitted as O-th order diffracted light LSG, while the TM wave is transmitted as O-th order diffracted light LSG.
Almost 100% of the waves are diffracted as l-th order diffracted light Ls+.

The above O-th order diffracted light L,.

is incident on the diffraction element 12b for diffraction angle variation compensation, and approximately 10
0% Oth order diffracted light L6. After passing through the light beam, it enters the photodetector 36 via the condensing lens 40.

On the other hand, the above-mentioned first-order diffracted light L5l is almost 100% first-order diffracted light L. The light is diffracted as 1 and enters the photodetector 37 via the condenser lens 4I.

By the way, since the diffraction grating constituting the diffraction element 12a and the diffraction grating constituting the diffraction element 12b for compensating for diffraction angle fluctuations have the same grating interval and direction, the diffraction angle θ in the diffraction element 12a and the diffraction grating for compensating for diffraction angle fluctuations are the same. The diffraction angles θ2 in the element 12b are equal to each other, and therefore, the first-order diffracted light L 7+ from the diffraction angle variation compensation diffraction element 12b is the 0th
It is emitted in parallel with the next diffracted light L 60.

When the diffraction angle at the diffraction element 12a decreases to, for example, θ1'' due to the wavelength fluctuation of the laser beam, the diffraction angle at the diffraction angle variation compensation diffraction element 12b decreases to θ2'', but at this time as well, θ1 and θ2' are equal, the first-order diffracted light L from the diffraction element 12') for compensating for diffraction angle fluctuations
'l+ is the 0th order diffracted light L6. It emits parallel to. In addition,
The position of the center line of the first-order diffracted light L8 shifts slightly as the diffraction angle changes, but if the thickness of the substrate 12 is set to be sufficiently small, the shift of the center line of the first-order diffracted light L71 is slight. Yes, therefore, the lth order diffracted light L? + comes to be incident on the photodetector 37 regardless of the fluctuation of the diffraction angle.

[Effects of the Invention] As described above, the diffraction element unit according to the present invention includes a diffraction element in which a diffraction grating is formed on a transparent substrate with a grating interval approximately equal to the wavelength of target light, and this diffraction element. and a diffraction angle fluctuation suppressing means formed of a translucent material so that the amount of change in the output angle is smaller than the amount of change in the angle of incidence. It is planned that

According to such a structure, the diffracted light from the diffraction element is made incident on the diffraction angle variation suppressing means, and the amount of variation in the incident angle, in other words, the amount of variation in the output angle is smaller than the variation in the diffraction angle at the diffraction element. By emitting light in such a way that the angle of diffraction becomes small, fluctuations in the diffraction angle can be suppressed. Note that, for example, by incorporating the above-mentioned diffraction element unit into an optical head using a diffraction element, fluctuations in the diffraction angle in the diffraction element can be suppressed, and the diffracted light diffracted by the diffraction element can be reliably received by the photodetector. You will be able to do this.

[Brief explanation of the drawing]

1 to 4 show one embodiment of the present invention. FIG. 1 is an explanatory diagram showing a diffraction element unit. FIG. 2 is an explanatory diagram showing the optical head. FIG. 3 is an explanatory diagram showing how the beam diameter is expanded by the beam shaping prism. FIG. 4 is an explanatory diagram showing a diffraction element unit in a modified example. FIG. 5 is an explanatory diagram showing a diffraction element unit in the second embodiment. 6 to 10 show conventional examples. FIG. 6 is an explanatory diagram showing an optical head using a polarizing beam splitter. FIG. 7 is an explanatory diagram showing an optical head using a diffraction element. FIG. 8 is an explanatory diagram showing a part of the optical head shown in FIG. 7 in detail. Figure 9 is a schematic vertical cross-sectional view showing the diffraction grating. FIG. 10 is a graph showing the relationship between the depth of the diffraction grating and the intensity ratio of TE waves and TM waves. 1 and 11 are diffraction element units, 2 and 12 are substrates, and 2a.
12a is a diffraction element, and 3 is a beam shaping prism (diffraction angle fluctuation suppressing means). 1 Figure 1 Figure 2 Figure 6 Figure 8 Figure 1q

Claims (1)

[Claims] 1. A diffraction element in which a diffraction grating is formed on a transparent substrate with a grating interval approximately equal to the wavelength of the target light, and diffracted light by the diffraction element is incident on the back of the diffraction element. and a diffraction angle fluctuation suppressing means formed of a translucent material so that the amount of fluctuation in the output angle is smaller than the amount of fluctuation in the incident angle. 2. The diffraction angle fluctuation suppressing means comprises a beam shaping prism arranged so as to enlarge the beam diameter of the diffracted light in the direction in which the diffraction angle in the diffraction element changes. Diffraction element unit.