JPS6353428A - Wave front detection element - Google Patents

Abstract

PURPOSE:To achieve a higher accuracy, by arranging a plurality of gratings in a path of incident light. CONSTITUTION:This element is so designed that an outgoing convergent light 22a diffracted with a grating 20a converges at a specified position most efficiently when a gradient angle phi of a wave front gives a desired value in the perimeter of an incident divergent light 21a. An outgoing convergent light 21b is diffracted with a grating 20b. Here, the cycle and the directivity of the grating 20b are so set that an outgoing light 22b converges at a specified position most efficiently when the gradient angle phi of a wave front gives a desired value in the perimeter of the incident light 21b. An equiphase plane is determined by mutual interference between the incident light 21a of 21b and the outgoing light 22a or 22b to allow the setting of a grating distribution. Thus, a plurality of gratings different in selected combination are used to detect a wave front selectively and the results are integrated thereby achieving highly accurate detection of the state of wave fronts.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to an element for detecting the wavefront state of a light wave, and in particular, a device that uses a plurality of gratings to selectively diffract the light wave with different degrees of coupling. The present invention relates to an element that detects a wavefront by detecting light.

[Prior Art] In an optical information recording/reproducing device such as an optical disk device or a magneto-optical disk device, a light beam is used to record information on a recording medium (for example, by irradiating a spot of light for 1f raw time and further recording the information). Detection of reflected light from the medium is performed.

In such optical information recording and reproduction, 8 yellow means that the irradiation light is focused on the recording V body with a predetermined accuracy. Therefore, the optical head is spot on! I+ detects the reflected light from the recording medium, detects the size and direction of the out-of-focus from the shape of the reflected light, and uses this detection result to
; By appropriately displacing the light-focusing objective lens of the optical head optical system in the optical axis direction, the optimal optical focus/focus on the recording medium is always achieved.
A so-called autofocusing control is performed so that the diameter of the lens is maintained and an accurate focused state 5S is achieved.

FIG. 6 is a diagram showing a wavefront detection optical system for autofocusing control in a conventional optical head.

In FIG. 6, l is a semiconductor laser, 2 is a collimator lens, 3 is a beam splitter, 4 is a light focusing objective lens, 6 is a light focusing lens, and 7 is a cylindrical lens. and 8 is 4 minutes-1,l
It is a photodetector. Further, 5 is an information recording medium.

The diverging light beam emitted from the semiconductor laser 1 is made into a parallel light beam by the collimator lens 2, passes straight through the beam splitter 3, and is focused by the objective lens 4 to form a beam spot on the information recording medium 54-. The reflected light from the recording medium passes through the objective lens 4 again, and at this time is focused into a substantially upward beam, reflected by the beam splitter 3, and focused by the lens 6. The focused light beam is focused in only one direction by the cylindrical lens 7 and is made incident on the photodetector 8. By appropriately setting the directions of the four divisions of the photodetector, the focusing error of the light beam incident on the recording medium 5 can be detected from calculation of the output of each division of the photodetector. This is nothing but detecting the wavefront shape jB of the light flux incident on the photodetector 8 by utilizing the fact that the cylindrical lens has a unique aberration pattern.

Based on this detection result, an actuator (not shown) is driven,
The objective lens 4 is moved in the optical axis direction to bring it closer to an accurate focusing state. In this way, autofocusing control is realized.

In addition to the above-mentioned methods, methods conventionally used for wavefront detection in optical heads include the Knifezi method and the like.

[Problems to be Solved by the Invention] However, the wavefront detection elements that have been proposed so far require a light-focusing lens and have a long optical path, and therefore are difficult to miniaturize. There are limitations, and furthermore, there are problems such as a small degree of freedom in design.

Therefore, in view of the above-mentioned conventional techniques, the present invention aims to provide a wavefront detection element that does not require a light focusing lens, can be miniaturized, and has a greater degree of freedom in design. .

[Means for Solving the Problems] According to the invention, a plurality of sets of gratings and photodetectors are arranged in parallel, and the gratings are Provided is a wavefront detection element disposed in the path of the detection incident light, wherein each grating has a different selectivity of coupling by diffraction between the incident light wave ll111 and the output light wavefront reaching the photodetector. .

[Embodiments] Hereinafter, specific embodiments of Ul will be described without reference to the drawings.

FIG. 1 is a schematic plan view showing a first embodiment of the wavefront detection using the unreleased 11 and the detector.

In FIG. 1, 20a and 20b are gratings with different characteristics arranged parallel to each other, and 23a and 23
Reference numerals b denote photodetectors such as photodiodes arranged in pairs with respect to the gratings 20a and 20b, respectively.

In this embodiment, the incident light to be detected first enters the grating 20a from the right side, the light diffracted by the grating is focused on the photodetector 23a, and the light not diffracted by the grating 20a is The light that travels straight and enters the grating 20b and is diffracted by the grating jig is focused and incident on the photodetector W23b. The light that is not diffracted by the grating 20b travels straight.

The operation of the device of this example will be explained below.

FIGS. 2(a) and 2(b) are schematic plan views for explaining the characteristics of the grating of this embodiment.

In this embodiment, the grating 20a is for detecting a diverging light wavefront, and the grating 20b is for detecting a convergent light wavefront.

In FIG. 2(a), 21a is the incident diverging light, and 2
2a is an emitted focused light diffracted by the grating 20a. The grating period Δ and directionality of the grating are determined by the inclination angle Φ of the peripheral wavefront of the incident light 21a.
is a desired value, the emitted light 22a is designed to be most efficiently focused at a predetermined position (the position of the photodetector 23a in FIG. 1). Similarly, in FIG. 2(b), 21b is the incident focused light, and 22b is the outgoing focused light diffracted by the grating 20b. The grating period and directionality of the grating are such that when the inclination angle φ of the peripheral wavefront of the incident light 21b is a desired value, the output light 22b is most efficiently directed to a predetermined position (the photodetector 23b in FIG. 1). It is designed to be focused at the position of

As described above, the period and inclination of the grating can be easily designed based on the principle of holography. That is, the grating distribution can be set by finding an equal phase plane due to mutual interference between the corresponding incident light 21a or 21b and output light 22a or 22b.

The angular selectivity Δ0 of the gratings 20a and 20b generally varies depending on the location, and as shown in the figure, if the grating period is Δ and the grating thickness is d, from coupled wave theory, it is approximately given by Δ0=A/d. . Therefore, by increasing the thickness d of the grating, sharp angle selectivity can be achieved, and a sharp output can be obtained only for a specific incident light wavefront.

FIG. 3 is a graph showing the wavefront selection characteristics of the grating of the device of this example.

In Fig. 3, the horizontal axis shows the inclination angle φ of the wavefront at the periphery of the incident light (in the case of a diverging light wavefront, Φ〉O, and in the case of a converging light wavefront, φ〉0), and the vertical axis shows the inclination angle φ of the wavefront in the peripheral part of the incident light. Detector 23
a and 23b show the output light intensity detected by 23b. As can be seen from this graph, the ratio of the output of the photodetector 23a and the output of the photodetector 23b differs depending on the inclination angle φ of the wavefront of the fM side of the incident light. The inclination angle φ of the surface can be determined. In addition, in this detection, there is an overlamp region in the peripheral wavefront inclination angle φ of the incident light diffracted by both gratings, and the incident light to be detected is made to enter the gratings sequentially, so this point should be taken into account. Correct it.

FIG. 4 is a schematic plan view showing a second embodiment of the wavefront detection element according to the present invention.

In FIG. 4, 33 is a slab optical waveguide, and a part of the optical waveguide is provided with two mutually parallel channel optical waveguides 34a and 34b. Gratings 30a and 30b are formed in a portion of the channel optical waveguide, respectively. In addition, the channel optical waveguides 34a, 3
Photodetectors 23a and 23b are connected to one end of 4b, respectively.

In this embodiment, the test light 31 enters from the right in the form of slab waveguide light and first reaches the grating 30a of the channel optical waveguide 34a, whereupon a portion is Bragg diffracted and becomes the channel waveguide light 32a. The remaining light passes through the channel optical waveguide 34a. The passing light then passes through the grating 3 of the channel optical waveguide 34b.
0b, a part thereof undergoes Bragg diffraction and is coupled to the channel optical waveguide 32b, and the rest passes through the channel optical waveguide 34b. Channel guided light 3
2a and 32b reach photodetectors 23a and 23b, respectively, and are detected.

In this embodiment, the grating 30a.

30b are the gratings 20 of the first embodiment, respectively.
It is designed to have the same binding selectivity as a and 20b. Also in this embodiment, since the ratio between the output of the photodetector 23a and the output of the photodetector 23b differs depending on the inclination angle φ of the wavefront in the peripheral part of the incident light, the ratio between the output of the photodetector 23a and the output of the photodetector 23b is detected. The tilt angle φ can be determined.

In the above embodiment, the channel optical waveguide 34a
, 34b are formed on the gratings 30a, 30.
By using b, the slab waveguide light to be tested is coupled to the channel waveguide light of a predetermined width without being diffused, and efficiently introduced into the photodetector. Furthermore, this configuration has advantages such as being able to shorten the optical path, making it possible to downsize the device, and having a large degree of freedom in design. The grating structure for this purpose is detailed in Japanese Patent Application No. 60-262655].

In the wavefront detection element having a waveguide structure as in this embodiment, a photodetector etc. can be integrated on one chip by using a conductor such as 31 as the waveguide substrate.

FIG. 5 is a schematic perspective view showing an embodiment of an optical head of an optical information recording/reproducing apparatus having a wavefront detection element according to the present invention as a focusing control f stage.

In FIG. 5, 41 is a glass substrate, and 40 is a slab optical waveguide having a relatively high refractive index formed on the surface thereof. Three channel optical waveguides 43° 44a and 44b parallel to each other are provided in a part of the slab optical waveguide. Gratings 47, 48a and 48b are formed in a portion of the channel optical waveguide, respectively. The gratings 48a and 48b are set to have the same bonding selectivity as the gratings 20a and 20b shown in FIG. 1 above, respectively. Furthermore, a grating coupler 46 having a light focusing function is formed on the surface of the slab optical waveguide.

One end of the channel optical waveguide 43 extends to the end face of the slab optical waveguide 40, and a semiconductor laser 42 serving as a light source is connected thereto. Further, a photodetector 'J is connected to one end of the channel optical waveguides 44a and 44b.
A45a and 45b are formed.

In this embodiment as described above, the light beam from the conductor laser 42 is coupled to channel guided light propagating in the direction of the arrow within the channel optical waveguide 43. The channel guided light reaches the grating 47, undergoes Bragg diffraction there, and is coupled to the slab guided light that propagates to the right in the direction of the arrow within the slab optical waveguide 40. The slab guided light propagates to the optical focusing grating coupler 46 with a width corresponding to the length of the grating 47 in the longitudinal direction of the channel optical waveguide 43. The grating coupler has a width that approximately corresponds to the width of the step guided light. 1. The slab waveguide light that has reached the grating cuff 46 is coupled by the coupler into concentrated light that propagates between the slabs. The emitted concentrated light is made to be smaller in the figure; and is focused at a predetermined position between -AS and f. Although it is not shown in this position, there is a tlli-ki S! ,
A medium is placed, and an optical substrate is formed to the north of the recording medium.

On the other hand, the reflected light from the information recording medium travels backwards along the path described in
It is coupled to the step waveguide light propagating leftward in the direction of the arrow in the four slab light wave paths 40 through the light focusing grating coupler 46. The slab waveguide light reaches the channel optical waveguide 44a and further reaches the channel optical waveguide 44b. As described above with respect to the embodiment shown in FIG. 4, the slab guided light is subjected to Bragg diffraction by the gratings 48a and 48b at a ratio corresponding to the wavefront state,
They are coupled to channel guided light propagating in the direction of the arrow in channel optical waveguides 44a and 44b, respectively. The channel guided lights are respectively incident on photodetectors 45a and 45b and detected.

When the information recording medium is in the normal position S and the light irradiated to the information recording medium is accurately focused, the light reflected by the medium is coupled by the grating coupler 46 into parallel slab waveguide light.

On the other hand, if the information recording medium is deviated from the normal position S in the optical axis direction and the irradiation light to the information recording medium is not accurately focused, the light reflected by the medium is transmitted to the grating coupler 46. It is coupled into non-parallel slab waveguide light (1 (i.e., diverging four-wave light or convergent waveguide light)).

Therefore, it is possible to detect the wavefront of the guided light incident on the gratings 48a and 48b in the same manner as in the first embodiment, and to determine the distance to which side the information recording medium is separated from the position S. I can see if there are any. Therefore, the grating 48a.

Autofocusing control is realized by moving the optical head using an actuator (not shown) so that parallel slab waveguide light is incident on 48b.

In this embodiment, information can be recorded on the information recording medium by modulating the light beam from the semiconductor laser 42 in accordance with the recording signal when recording information.
Furthermore, when reproducing information, by keeping the intensity of the light beam from the semiconductor laser 42 constant, recorded information on the information recording medium is detected and reproduced by the sum of the output of the photodetector 45a and the output of the photodetector 45b. can do.

In the above embodiment, there are two gratings, and one of the gratings has relatively high coupling to the diverging light wavefront, and the other grating has relatively high coupling to the converging light wavefront. Although an example is shown, the present invention is of course not limited to this, and also includes those having cratings with three or more different binding selectivities.

[Effects of the Invention] According to the present invention as described above, a wavefront state is detected with high accuracy by selectively detecting a wavefront using a plurality of gratings having different coupling selectivities, and integrating the results. It becomes possible to detect. Further, according to the present invention, it is possible to significantly reduce the size of the device compared to wavefront detection using a lens, and in particular, it becomes possible to apply the device as an optical integrated circuit device. Furthermore, according to the present invention, desired characteristics can be obtained by varying the modulation degree, thickness, and other characteristics of each grating over a considerable range, providing a large degree of freedom in design.

[Brief explanation of the drawing]

FIG. 1 is a schematic plan view of a wavefront detection element according to the present invention. Figure 2 (a) and (b) explain the characteristics of the grating 1
FIG. FIG. 3 is a graph showing the wavefront selection characteristics of the grating. FIG. 4 is a schematic plan view showing the wavefront detection element according to the present invention. FIG. 5 is a schematic perspective view showing an optical head of an optical information recording/reproducing device having a wavefront detection element as a focusing control means according to the present invention. FIG. 6 is a diagram showing a wavefront detection optical system for autofocusing control in a conventional optical head. 20a, 20b nigerating, 21a, 2Lb: incident light, 22a, 22b: output light, 23a, 23b: photodetector, 30a, 30b nigerating, 33 Nislab optical waveguide, 34a, 34b: channel optical waveguide, 40 Nislab optical waveguide Wavepath, 41: Substrate, 42: Semiconductor laser, 43.44a, 44b: Channel optical waveguide, 45
a, 45b: photodetector, 46: grating coupler, 47. 48a, 48b nigerating. Agent Patent Attorney Minoru Yamashita Mo Figure 1 23b 23゜Figure 2 (G) Figure 2 (b) Figure 3 Pressure 77e 5 False Figure 4 23b 23a Figure 5 Figure 6

Claims (3)

[Claims] (1) A plurality of pairs of gratings and photodetectors are arranged in parallel, and the gratings are arranged in the path of the incident light to be detected, and each grating has a wavefront of the incident light and a wavefront of the output light that reaches the photodetector. A wavefront detection element characterized in that the selectivity of coupling by diffraction with the element differs. (2) Two sets of gratings and photodetectors are provided, one grating having relatively high coupling to the diverging light wavefront and the other grating having relatively high coupling to the converging light wavefront. A wavefront detection element according to claim 1, having: (3) The light flux to be tested is slab waveguide light, the grating is formed in a channel optical waveguide provided in a part of the slab optical waveguide, and a photodetector is coupled to the end of the channel optical waveguide. , a wavefront detection element according to claim 1.