JP2839250B2 - Concentrator - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an improvement in a light collecting device used for an optical pickup or the like. 2. Description of the Related Art For example, as one type of optical pickup used for an optical disk, there is a type in which various optical elements are formed on a substrate having a two-dimensional optical waveguide by using a planar technique. In such an optical pickup, light propagating in a thin-film optical waveguide is condensed at one point outside the optical waveguide by using a diffraction effect of a diffraction grating, and reflected light reflected at the one point is again reflected. Condensing grating couplers of the type that lead into a thin film optical waveguide are known. Problems to be Solved by the Invention By the way, in the above-mentioned conventional condensing grating coupler, a diffraction grating formed by photolithography, a relief (relief) type diffraction grating formed by thin film deposition or etching on the surface of the thin film optical waveguide, A diffraction grating or the like in which a refractive index difference is periodically formed by electron beam scanning is provided. For this reason, in a condensing device such as the condensing grating coupler, since the diffraction grating once formed has a constant grating constant, it is necessary to move the condensing point on the optical disc for tracking or focusing, for example. In some cases, other converging point changing means such as an optical element and its driving device and substrate moving device must be provided outside. Means for Solving the Problems The present invention has been made in view of the above circumstances, and its purpose is to provide a light collecting point without preparing another light collecting point changing means outside. It is an object of the present invention to provide a light-collecting device that can be moved for tracking or focusing. In order to achieve the above object, the gist of the present invention is to focus light propagating in a two-dimensional optical waveguide at a point outside a plane including the two-dimensional optical waveguide by using a diffraction effect of a diffraction grating. A condensing device of the following type, comprising: (a) a substrate on which the two-dimensional optical waveguide is formed; and (b) an arc-shaped comb-shaped electrode curved concentrically. A surface acoustic wave generating element for generating a concentric surface acoustic wave on the two-dimensional optical waveguide and forming a diffraction grating in the two-dimensional optical waveguide by an acousto-optic effect based on periodic coarse and dense; (c) the two-dimensional optical waveguide; A drive signal generator for supplying a drive signal whose frequency has been swept to the surface acoustic wave generator in order to set a condensing position of light propagating in the optical waveguide to a desired position. In this manner, surface acoustic waves are generated from concentric arc-shaped comb-shaped electrodes on the two-dimensional optical waveguide of the substrate, and the acousto-optic effect based on the periodic density is achieved. And a drive signal whose frequency is periodically swept in order to set a condensing position of light propagating in the two-dimensional optical waveguide to a desired position. Since a drive signal generator for supplying to the surface acoustic wave generator is provided, by changing the drive signal output from the drive signal generator, the light condensing position of light propagating in the two-dimensional optical waveguide can be changed. It is moved to a desired position. Therefore, the focal point can be moved for tracking or focusing without preparing any other focal point changing means outside. Further, the drive signal generating device is preferably such that light propagating in the optical waveguide changes a spot diameter at the one point, or such that a light condensing position follows a track at the one point. Output drive signal. Embodiment Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. In FIG. 1, a substrate 10 is made of a transparent and piezoelectric material such as LiNbO ₃ single crystal,
It has a thickness of about 0.5mm. On one surface of the substrate 10, a two-dimensional optical waveguide 12 for transmitting light while confining light in the thickness direction is provided.
Is provided. This two-dimensional optical waveguide 12 is, for example,
It is formed by depositing a metal such as Ti to a thickness of about 30 nm and then thermally diffusing it into the substrate 10. And the substrate 10
Absorbers 14 for absorbing surface acoustic waves,
At the same time, a laser diode 18 functioning as a light source is mounted on one end of the substrate 10. A pair of comb electrodes 20 for generating surface acoustic waves is provided on one surface of the substrate 10 on the side where the two-dimensional optical waveguide 12 is provided. The pair of comb-shaped electrodes 20 are each concentrically curved, and are opposed to each other at an interval corresponding to a half wavelength of the surface acoustic wave. For example, if the substrate 10 is LiNbO ₃ , its sound velocity
Since 6.57 × 10 ⁵ cm / sec, when applying a high-frequency drive signal for generating an ultrasonic wave having a center frequency of 100 MHz, the distance between the pair of comb electrodes 20 in the ultrasonic wave propagation direction is
An interval of about 33 μm is formed. The driving signal generator 22 is provided on the pair of comb electrodes 20.
Is supplied.
As described above, since the substrate 10 has the piezoelectric effect, when a high-frequency drive signal is applied between the pair of comb electrodes 20, local distortion is repeatedly generated between the comb electrodes 20. Thereby, a surface acoustic wave is generated. This surface acoustic wave propagates in the normal direction of the comb-shaped electrode 20 at a wavefront having a curvature corresponding to the curvature of the comb-shaped electrode 20, and its wavefront interval (pitch)
Is changed according to the cycle of the high-frequency drive signal. The wavefront 24 of the surface acoustic wave shown by the solid line in FIGS. 1 and 2 is schematically shown to facilitate understanding of the phase front. Thus, when a surface acoustic wave of a predetermined frequency exists, the substrate
On the surface of the two-dimensional optical waveguide 12 provided on one surface of 10, the density of the material is periodically formed, so that the refractive index periodically changes in the propagation direction of the surface acoustic wave as shown by the wavefront 24. The diffraction grating 26 is formed by the acousto-optic effect that increases. As shown in FIG. 3, the diffraction grating 26 based on the acousto-optic effect converts the laser light emitted from the laser diode 18 and propagating in the two-dimensional optical waveguide 12 out of the plane including the two-dimensional optical waveguide 12 by the diffraction effect. While focusing on one point A,
The reflected light from the point A is guided again into the two-dimensional optical waveguide 12. That is, as shown in FIG. 4, the drive signal supplied from the drive signal generator 22 to the comb-shaped electrode 20 has a center frequency f _o.
Those which are swept in a sawtooth shape in period t ₁ in a range to a maximum frequency f _max from the minimum frequency f _min around the. Therefore, assuming that on the substrate 10 the surface acoustic wave requires the same t ₁ hour as the above-mentioned sweeping time from the point B to the point C, the frequency of the surface acoustic wave at the point B every t ₁ hour f _B is the minimum frequency f _min, the frequency f _c of the surface acoustic wave at the point C becomes a maximum frequency f _max, since the f _c> f _B, the diffraction angle theta _B at the point B the diffraction angle at the point C theta _c
Becomes smaller than, if you look at the firing or every sampling period t ₁ hour is the detection timing of the reflected light from the laser diode 18, the laser beam emitted from the laser diode 18 is in a state of focusing on the point A. On the contrary, the spherical wave reflected from the point A is incident on the two-dimensional optical waveguide 12 through the same optical path as above in the opposite direction. Therefore, the diffraction grating 26 based on the acousto-optic effect formed by the surface acoustic waves functions as a so-called condensing grating coupler. The reflected light reflected from the point A and made incident on the two-dimensional optical waveguide 12 is usually separated by a not-shown beam splitter provided between the laser diode 18 and the comb-shaped electrode 20 and is not shown in the drawing. It is detected by a sensor. As this beam splitter, for example, a twin-grating focusing beam splitter which is a waveguide-type beam splitter is used.
Spuritter: TGFBS) is preferably used. Normally, when applied to an optical pickup such as an optical disk, the focal point A is moved in the tracking direction and the focus direction in order to perform constant feedback control of the focal spot size and to control the focal spot to follow the track. Need to be done. In the present embodiment, the drive signal generator 22 changes the frequency of the drive signal based on the focus error signal and the track error signal included in the light reflected from the surface of the optical disk or the like, thereby moving the focal point a. It is supposed to be. That is,
For example, as shown in FIG. 5, when the focal point a is moved to the point A 'in the tracking direction (the direction perpendicular to the track), the drive signal is changed to B as shown by the broken line in FIG. the frequency f _B of the surface acoustic wave 'into the frequency f _c of the surface acoustic wave at point C f _c' f _B at the point by changing to, is generally low shift frequency. This allows
The constant (interval) of the diffraction grating 26 due to the acousto-optic effect is increased as a whole, and the diffraction angles θ _{B at the} points B and C are increased.
Converging point is the point A 'and θ since _c are both reduced. Conversely, if the drive signal is shifted higher overall,
The focal point A is moved in a direction opposite to the above. In addition,
In FIG. 5, reference numeral 28 denotes the surface of the optical disk. Also, for example, as shown in FIG. 7, the focus point A is set in the focus direction (a direction perpendicular to the surface of the optical disc).
In the case of moving to the point A ″ in
As shown by the broken line in the figure, the frequency of the surface acoustic wave at point B
The f _B f _B "to the frequency f _c of the surface acoustic wave at point C f _c"
By changing to, the width of frequency change within one scanning cycle is increased. In other words, the line indicating the frequency change in FIG. 8 is made steeper. As a result, the diffraction angle θ B at the point _B decreases and the diffraction angle θ _{c at the} point C increases, so that the light condensing point becomes the point A ″. The light-collecting point A is moved in the opposite direction to the above-described one.As described above, according to the present embodiment, light is condensed at a desired position by adjusting the frequency of the drive signal supplied to the comb-shaped electrode 20. Therefore, the focal point can be moved for tracking, focusing, etc., without providing any other focal point changing means outside the apparatus. will be described. in the case of moving the focus direction is a focusing point a as FIG. 7, as indicated by the broken line in Fig. 9, the solid line a frequency shift of the drive signal in one scanning period t ₁ Change and run By appropriately setting the is allowed and the period t ₂ perform the sampling at a short period t ₂ than the period t _1, the desired light collecting device capable of focusing control and tracking control at the same time can be obtained. Further, the aforementioned In the embodiment of the present invention, a condensing grating coupler for an optical pickup for reproducing information from an optical disc is described, but the present invention can also be applied to a condensing device for other uses. By adjusting the grating interval of the diffraction grating 26 according to the driving signal, it is possible to condense light at a desired position.The above is merely an example of the present invention, and the spirit of the present invention is Various changes can be made without departing from the scope of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view showing one embodiment of the present invention. Second
FIG. 3 is a plan view of a main part of the embodiment of FIG. 1, FIG. 3 is a sectional view of a main part of the embodiment of FIG. 1, and FIG. 4 is a diagram for explaining drive signals of the embodiment of FIG. FIG. 5 is a diagram for explaining tracking control in the embodiment of FIG. FIG. 6 is a diagram for explaining a drive signal in the tracking control of FIG. FIG. 7 is a diagram for explaining the focusing control in the embodiment of FIG. FIG. 8 is a diagram for explaining drive signals in the embodiment of FIG. FIG. 9 is a diagram corresponding to FIG. 8 in another embodiment of the present invention. 10: substrate 20: comb-shaped electrode (surface acoustic wave generation element) 22: drive signal generator 26: diffraction grating by acousto-optic effect

Continuation of front page    (72) Inventor Aki Suzuki               9-35 Horitadori, Mizuho-ku, Nagoya-shi, Aichi               Within Brother Industries, Ltd.                    Panel     Referee Takehiko Katayose     Judge Akira Watanuki     Referee Masao Kashiwazaki                (56) References JP-A-61-232429 (JP, A)

Claims (1)

(57) [Claims] A light condensing device of a type in which light propagating in a two-dimensional optical waveguide is condensed at a point outside a plane including the two-dimensional optical waveguide by using a diffraction effect by a diffraction grating, wherein the two-dimensional optical waveguide is The formed substrate has an arc-shaped comb-shaped electrode curved concentrically, and generates concentric surface acoustic waves on the two-dimensional optical waveguide from the comb-shaped electrode, and periodically and densely. A surface acoustic wave generating element that forms a diffraction grating based on an acousto-optic effect based on the two-dimensional optical waveguide, and a frequency is swept to set a condensing position of light propagating in the two-dimensional optical waveguide to a desired position. A driving signal generator for supplying a driving signal to the surface acoustic wave generating element. 2. The drive signal generating device outputs the drive signal such that light propagating in the two-dimensional optical waveguide changes a spot diameter at the one point, or such that a condensing position of the light follows a track at the one point. The light condensing device according to claim 1, wherein