JPH01149006A - Light guide element - Google Patents

Abstract

PURPOSE:To remove aberrations by a simple aberration correcting optical system and to reduce the size, weight and cost of a device by forming a focusing diffraction grating to the shape at which the exit angle of the light beam emitting from central position of the arraying direction of the grid attains nearly 0 deg.. CONSTITUTION:The focusing diffraction grating to be formed on the surface of a light guide is formed to the shape at which the exit angle of the light beam emitting from the central position in the arraying direction of the grating attains nearly 0 deg.. The aberration nearly the same as the aberration in the case of condensing the light beam by an ordinary single spherical lens is generated in the case of emitting the guided light 13 to the outside of the focusing diffraction 11 from the focusing diffraction grating at such angle. The aberration is, therefore, removed by passing the light beam emitted from the light guide element 10 through the correcting optical system consisting of the spherical lens. This correcting optical system is easily constituted from, for example, a concave lens 30 and a convex lens 31.

Description

Detailed Description of the Invention (Industrial Field of Application) The present invention relates to an optical waveguide element, in particular, a light beam that is provided with a condensing diffraction grating on the surface of the optical waveguide, and that allows a light beam guided through the optical waveguide to be The present invention relates to an optical waveguide element that is configured to diffract light in a light-concentrating diffraction grating and emit light out of the optical waveguide while converging the light.

(Prior Art) For example, mechanical optical deflectors such as galvanometer mirrors and polygon mirrors and EOD (electro-optic optical deflectors) have been used as optical deflection devices for deflecting light beams in optical scanning recording devices, optical scanning reading devices, etc. AOD (acousto-optic optical deflector) is widely used. However, mechanical optical deflectors have problems such as poor durability and easy increase in size.On the other hand, EOD and AOp cannot obtain a large optical deflection angle, so the beam optical path becomes long, and optical scanning recording devices etc. There is a problem in that it leads to an increase in size.

Recently, as an optical deflection device that can solve the above-mentioned problems,
Optical deflection devices using optical waveguides are attracting attention. This optical deflection device consists of a slab-shaped optical waveguide made of a material that allows surface acoustic waves to propagate, and a surface whose frequency changes continuously as it travels in the direction intersecting the optical beam guided inside the optical waveguide. It has a means for generating an elastic wave in the optical waveguide (for example, composed of a pair of crossed comb-shaped electrodes and a driver that applies an alternating voltage whose frequency changes continuously to the pair of interdigitated electrodes). In this optical deflection device, the light beam guided in the optical waveguide undergoes Bragg diffraction due to acousto-optic interaction with the surface acoustic wave, and this diffraction angle changes depending on the surface acoustic wave frequency, so the surface acoustic wave By varying the frequency as described above, the light beam can be continuously deflected within the optical waveguide. Regarding such a light deflection device, for example, Japanese Patent Laid-Open No. 6
A detailed description is given in Japanese Patent No. 2-77761.

In the optical deflection device using the optical waveguide described above, a focusing diffraction grating (Focus grating) is provided on the surface of the optical waveguide in order to condense the deflected light beam and emit it out of the optical waveguide.
sing Grating Coupler: FG
C) is often used. Such a condensing diffraction grating is extremely thin and compact compared to a normal spherical lens disposed outside the optical waveguide, and is therefore advantageous in reducing the size and weight of the optical deflection device.

(Problem to be solved by the invention) However, on the other hand, when guided light enters this condensing diffraction grating obliquely, that is, at an angle to the direction in which the diffraction gratings are arranged, A problem arises in that the focused light beam has a special aberration that is completely different from that when the light beam is focused by a spherical lens, and it is difficult to correct this aberration with a correction optical system made of a spherical lens. When guided light is deflected by surface acoustic waves as described above, the direction of incidence of the guided light on the condensing diffraction grating changes moment by moment, so most of the time during deflection the guided light is incident obliquely as described above. I decided to do it,
The occurrence of the above-mentioned special aberration cannot be avoided.

When performing precise image recording or image reading by scanning a flat scanned surface with a deflected light beam, it is necessary to remove the aberrations mentioned above. A complex correction optical system must be provided.

However, if such a complicated correction optical system is provided, the optical scanning recording device or the optical scanning reading device will naturally become larger.

The problems with optical deflection devices made of optical waveguide elements have been described above, but optical waveguide elements that propagate surface acoustic waves in an optical waveguide and diffract the guided light by the surface acoustic waves can also be applied to, for example, high-frequency spectrum analyzers. Even in such a case, if the guided light is emitted out of the optical waveguide using a condensing diffraction grating as described above, the same problem will occur.

Therefore, an object of the present invention is to provide an optical waveguide element that can solve the above-mentioned problems.

(Means for Solving the Problems) The optical waveguide element of the present invention has a condensing diffraction grating formed on the surface of the optical waveguide as described above, and an emission angle of a light beam emitted from the central position in the direction in which the gratings are arranged. It is characterized by having a shape in which the angle is approximately 0°.

(Function) According to the research conducted by the present inventors, when the guided light is emitted from the condensing diffraction grating to the outside of the optical waveguide at the above-mentioned angle, the light beam is condensed by an ordinary spherical single lens. It was found that almost the same aberrations as those in the case were generated. Therefore, in this case, the aberration can be removed simply by passing the light beam emitted from the optical waveguide element through a correction optical system consisting of a spherical lens, which has been widely used in the past. The above correction optical system can be easily constructed from, for example, one concave lens and one convex lens.

(Example) Hereinafter, the present invention will be described in detail based on an example shown in the drawings.

FIG. 1 shows the side profile of an optical waveguide element IO according to an embodiment of the present invention, and FIG.
This figure shows an optical deflector for image recording constructed using O. The optical waveguide element 10 includes a slab-shaped optical waveguide 11 formed on a transparent substrate 16 and a pair of interdigitated electrodes (Inter D
ID below 1g1tal transducer s
15 (referred to as T) and a linear diffraction grating for light incidence (Li
near cratting coupler: hereafter L
GC) 20 and a light-emitting condensing diffraction grating (hereinafter referred to as FCC) 21. Further, below the FGC 21 of this optical waveguide element IO, there is a concave lens 30.
A convex lens 31 is arranged, and a collimator lens 32 is arranged below the LGC 20.

In this embodiment, as an example, the substrate 16 is made of LiNbO.
The optical waveguide 11 is formed by using three wafers and providing a Ti diffusion film on the surface of the wafer. Note that substrate 1
As the material 6, a crystalline substrate made of sapphire, Si, etc. may also be used. Furthermore, the optical waveguide 11 is not limited to the above-mentioned Ti diffusion, but can also be formed by sputtering, vapor depositing, or the like other materials on the substrate 16. Regarding optical waveguides, for example, T-9Tamir
“Integrated Obtics” (ed.)
rated 0ptics)J()Pix in Applied Physics(Topics In Ap
plied Physics) Volume 7) Springer-Verlag
Published (1975): “Optical Integrated Circuits” by Nishihara, Onna, and Suhara.
There are detailed descriptions in published works such as Ohmsha (1985).
In the present invention, any of these known optical waveguides can be used as the optical waveguide 11. However, this optical waveguide 11 is formed from a material such as the above-mentioned Ti diffusion film that allows surface acoustic waves to be propagated, which will be described later. Further, the optical waveguide may have a laminated structure of two or more layers.

A semiconductor laser 18 that emits recording light is arranged so as to emit a light beam (laser beam) 13 from below the collimator lens 32 toward the light input LGC 20 . The light beam 13, which is a diverging beam, is made into a parallel beam by the collimator lens 32, passes through the transparent substrate IB and the optical waveguide 11, and enters the L G C 20 formed on the surface thereof. do.

Thereby, the light beam 13 is diffracted by this LGC 20, enters the optical waveguide 11, and travels in the direction of arrow A in the optical waveguide 11 in a waveguide mode.

When recording an image, the photoreceptor 23 is set on a transport means 22 such as an endless belt.

The semiconductor laser 18 is driven by a laser drive circuit 19.
The IDT 15 is driven to emit the laser beam 3, and at the same time, an alternating voltage whose frequency changes continuously is applied from the drive circuit 17 to the IDT 15. The laser drive circuit 19 is controlled by a modulation circuit 24 so as to change the optical output according to the image signal S (i.e., the intensity of the light beam 13, the number of pulses and the pulse width when emitting the light beam 13 in a pulsed manner). ) the semiconductor laser 18 is driven so as to change the

By applying the voltage as described above to the IDT 15, the surface acoustic wave 12 travels on the surface of the optical waveguide 11 in the direction of arrow B in FIG. The IDT 15 uses this surface acoustic wave 1
2 is arranged so as to travel in a direction intersecting the optical path of the guided light (parallel beam) 13'.

Therefore, the guided light 13° travels across the surface acoustic wave 12;
Bragg by acousto-optic interaction with 2
diffract. As is well known, the guided light 13' due to this diffraction
The deflection angle of is approximately proportional to the frequency of the surface acoustic wave 12.

As mentioned above, since the drive circuit 17 applies an alternating voltage whose frequency changes continuously to the IDT 15, the surface acoustic wave 12
The frequency changes continuously, and the deflection angle changes continuously. Therefore, as shown by arrow C, this guided light 13' is diffracted so that the diffraction angle changes continuously.
deflect. The guided light 13° deflected in this way is F
The light is diffracted by the CC 21 and output from the optical waveguide 11 to the substrate 16 side.

This FGC 21 is a diffraction grating having a curve and a chirve period, or a curve, and when the light beam 13'' is diffracted here and emitted to the outside of the optical waveguide 11, the light beam 13'' is focused by the focusing action of the FGC 21. As a characteristic part of the present invention, as clearly shown in FIG. The grating pitch is set so that the light is emitted vertically).

The light beam 13'' emitted from the optical waveguide element 1O at such an angle passes through the concave lens 30 and the convex lens 31, is focused into a small beam spot P, and scans the photoreceptor 23 in the direction of the arrow X (main scanning). At the same time, photoreceptor 2
3 is transferred by the transfer means 22 in the direction of the arrow y, which is substantially perpendicular to the main scanning direction, to perform sub-scanning, so that the photoreceptor 23 is two-dimensionally scanned by the light beam 13''.As described above, Since this light beam 13' is modulated based on the image signal S, an image carried by this image signal S is recorded on the photoreceptor 23.

Note that in order to synchronize the image signal S for one main scanning line with the main scanning of the light beam 13'', the blanking signal sb included in the image signal S is used as a trigger signal to control the voltage to the IDT 15. It is only necessary to control the application timing. Furthermore, by controlling the drive timing of the transfer means 22 using this blanking signal sb, the main scanning and sub-scanning can be synchronized.

A light beam 13″ from the FCC21 at the angle mentioned above
When emitted, the concave lens 30 and the convex lens 31
When the light beam 13'' is scanned on the flat photoreceptor 23 without passing through it, astigmatism and field curvature occur.At this time, the radius of curvature of the field curvature of the beam spot P in the main scanning direction X is: It is about 1/3 of that in the sub-scanning direction y.Such aberration is almost equal to that when a parallel beam is focused by a spherical single lens.Therefore, as the concave lens 30 and the convex lens 31, conventionally a spherical single lens is used. By using what is used to correct astigmatism and field curvature, almost all of the aberrations caused by the F G C21 can be removed.

FIG. 3 shows a concave lens 8 that removes aberrations as described above.
An example of an aberration correction optical system including a convex lens 31 and a convex lens 31 will be shown in detail. In this example, the rear surface lea of the substrate 1B from which the light beam 13'' is emitted is also formed as a surface having negative refractive power, and constitutes a part of the correction optical system. It shows the distance between surfaces, and the unit is mm.In addition, in this example, the refractive index of the substrate 1B, concave lens 30, and convex lens 31 is 2.202.1.5151.1.7786, respectively.In addition, in the figure, rl %rs The table below shows the radius of curvature of each surface.

Naomen r! is the FGC 21 installation surface, and in this example, its focal length is 2206.63 mm, so this value is shown as the radius of curvature of the surface r1.

An example of the relationship between the amount of field curvature in the main scanning direction and sub-scanning direction after correction by the aberration correction optical system shown in Fig. 3, and the angle θ that the optical axis makes with the scanned surface in Fig. 1. is shown in Figure 4.

Of course, the light beam 13 emitted from the FGC 21
The optical system for correcting the aberrations mentioned above is not limited to those with the above specifications, but can be formed in various ways, similar to the optical systems conventionally used to correct the aberrations of spherical single lenses. be.

(Effects of the Invention) As explained in detail above, in the optical waveguide element of the present invention, the light beam emitting condensing diffraction grating formed on the surface of the optical waveguide for emitting the light beam is By creating a shape where the exit angle is approximately 0°,
The aberration of this emitted light beam can be made almost equal to that when it passes through a single spherical lens. Therefore, according to this optical waveguide element, the above-mentioned aberrations can be removed using a relatively simple aberration correcting optical system consisting of one concave lens, one convex lens, etc., which has been used in the past. Therefore, if the optical waveguide device of the present invention is applied to an optical scanning recording device or the like, the device can be made smaller, lighter, and less expensive.

[Brief explanation of the drawing]

1 and 2 are a side view and a perspective view, respectively, showing an optical waveguide element according to an embodiment of the present invention, FIG. 3 is a side view showing details of the aberration correction optical system in the above embodiment, and FIG. 4 is a graph showing characteristics of field curvature in the above embodiment having the aberration correction optical system shown in FIG. 3. FIG.

Claims (1)

[Scope of Claims] An optical waveguide element in which a light-concentrating diffraction grating is formed on the surface, and guided light is diffracted by the light-concentrating diffraction grating to be focused and emitted to the outside, comprising: An optical waveguide element characterized in that the diffraction grating has a shape such that the emission angle of the light beam emitted from the central position in the direction in which the gratings are arranged is approximately 0°.