JPH01107213A - Optical waveguide element - Google Patents

Abstract

PURPOSE:To converge a light beam which is projected from an optical waveguide into a small spot by setting the grating height of a diffraction grating formed on the surface of the optical waveguide so as to have a mountain-shaped distribution in the traveling direction of waveguide light. CONSTITUTION:The grating height of the diffraction grating 21 is varies in the mountain shape in the traveling direction of the waveguide light 13' so that when waveguide light 13' is projected from the optical waveguide 11 by the diffraction grating 21, the intensity distribution of the projection light 13'' is nearly a Gaussian distribution in the traveling direction. Therefore, the light beam 13'' projected from the optical waveguide element 10 is converted into the small beam spot Q through a scanning lens 26 to scan (main scan) on a photosensitive body 23 as shown by an arrow (u). Simultaneously, the photosensitive body 23 moves by a conveying means 22 almost at right angles to the main scanning direction as shown by an arrow (v) to make a subscan, so the photosensitive body 23 is scanned with the light beam 13'' in two dimensions. Consequently, the beam spot Q is converted sufficiently and a fine image can be recorded.

Description

Detailed Description of the Invention (Industrial Field of Application) The present invention relates to an optical waveguide element, and more particularly, to an optical waveguide that includes a diffraction grating on the surface of the optical waveguide, and uses the diffraction grating to emit guided light to the outside of the optical waveguide. This invention relates to an optical waveguide element in which external light is made to enter an optical waveguide through this diffraction grating.

(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 tend to be bulky.On the other hand, EOD and AOD do not have 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. The light beam thus deflected can be emitted out of the optical waveguide by, for example, a diffraction grating (grating coupler) or a prism coupler formed on the surface of the optical waveguide. Further, such a light deflecting device is described in detail in, for example, Japanese Patent Laid-Open No. 62-77761.

(Problem to be Solved by the Invention) By the way, when recording an image by scanning a light beam on a recording medium using an optical deflection device, in order to record a single high-definition image, it is necessary to direct the scanning beam to a small spot. It is required that the light intensity distribution has a Gaussian distribution over at least the beam sub-scanning direction.

However, when guided light is emitted from an optical waveguide to the outside using a diffraction grating as described above, it is extremely difficult to obtain a beam spot with the above-mentioned light intensity distribution. That is, as shown in FIG. 5, when a linear diffraction grating 42 with a constant grating height and pitch is formed on the surface of an optical waveguide 41 on a substrate 40 and the guided light 43 is emitted to the outside of the optical waveguide, the output The light intensity of the beam 43° gradually decreases exponentially along the traveling direction of the guided light 43, as shown by the curve g in the figure. As shown in Fig. 5, when a guided light is made to travel through an optical waveguide and is deflected by a surface acoustic wave, the direction of deflection (main scanning direction) is a direction that intersects the plane of the paper of this figure, and The scanning direction is the direction of arrow V in the figure. In other words, the output beam 43° has a light intensity distribution that gradually decreases or gradually increases along this sub-scanning direction, so it becomes a beam whose light intensity distribution becomes a Gaussian distribution along this direction. It becomes very difficult to get a spot.

Above, we have discussed the problems when guiding light is emitted out of an optical waveguide using a diffraction grating, but it has also been widely used to make external light enter an optical waveguide using such a diffraction grating. , a problem arises in that the incident coupling efficiency decreases. In other words, as derived from the reciprocity theorem for light output and light input, in the case of light input, unless the incident light beam has a light intensity distribution as shown by curve g in Figure 5, the entire Therefore, the light cannot enter the optical waveguide efficiently. Light beams emitted from light sources such as various lasers generally have a Gaussian light intensity distribution in the beam radial direction, and such light beams are gradually decreased (gradually increased) exponentially as described above. It is extremely difficult to shape a beam into a beam with a light intensity distribution that

SUMMARY OF THE INVENTION Therefore, the present invention provides an optical guide that does not cause the above-mentioned problems when guided light is outputted from an optical waveguide using a diffraction grating formed on the surface of an optical waveguide, or when external light is input into an optical waveguide. The object is to provide a wave path element.

(Means and effects for solving the problem) As described above, the optical waveguide element of the present invention has a surface of the optical waveguide that emits guided light traveling inside the optical waveguide to the outside, or emits external light to the surface of the optical waveguide. In an optical waveguide element in which a diffraction grating is formed, which allows the light to enter the optical waveguide, the grating height of the diffraction grating is such that when the guided light is emitted out of the optical waveguide by the diffraction grating, the intensity distribution of the emitted light is similar to that of the guided light. It is characterized by changing in the shape of a mountain along the traveling direction so as to have a substantially Gaussian distribution along the direction.

Note that changing the height of a diffraction grating provided on an optical waveguide is shown, for example, in Japanese Patent Application Laid-open No. 111220/1982, but the optical waveguide element shown in this publication does not change the height of the grating for guided light. The object of the present invention cannot be achieved with such a configuration.

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

FIG. 1 and FIG. 2 are a side view and a perspective view, respectively, showing an optical waveguide element IO according to an embodiment of the present invention. As an example, this optical waveguide element IO constitutes an optical deflector for image recording, and includes a slab-shaped optical waveguide 11 formed on a transparent substrate 16 and a side end of this optical waveguide 11. interdigitated electrode pair (Inter D 1
g1tal Transdueer, hereinafter ■DT
) 15 and a linear diffraction grating for light incidence (L 1
It has a near grating coupler (hereinafter referred to as LGC) 20 and a light emitting LGC 21. Further, on the surface 11sa of the substrate 1G on the opposite side to the optical waveguide 11, a light input prism 30 and a light output prism 31 are attached. The light entrance prism 30 has a triangular cross section and has a first light passing surface 30a and a second light passing surface 30b, and the first light passing surface 30a is strongly pressed against the substrate surface lea. It is closely fixed to the surface lea by using a high refractive index adhesive or by using an adhesive with a high refractive index. The light emitting prism 31 has the same shape as the light entering prism 30, has a first light passing surface 31a1 and a second light passing surface 31b, and is fixed to the substrate lea in the same manner as described above. .

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 B, 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. Tamir (T, Tam1r)
Edited by “Ingrated Optics”
rated Opiics)J()Pix in Applied Physics(Topics in Ap
pHed Physics) Volume 7) Sbringer
Springer-Verlag
Published (1975); Co-authored by Nishihara, Onna, and Suhara, "Optical Integrated Circuits"
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.

The semiconductor laser 18 that emits recording light emits a light beam (
It is arranged so as to emit a laser beam (13). This light beam 13, which is a diverging beam, is made into a parallel beam by a collimator lens 25, and then enters the light entrance prism 30 from the second light passing surface 30b,
The light passes through the first light passing surface 30a and enters the substrate 16, passes through the optical waveguide 11, and enters the LGC 20 formed on the surface thereof. Thereby light beam 1
3 is diffracted by this L G C 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 a laser beam 13, and at the same time, an alternating voltage whose frequency changes continuously is applied from a 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
Guided light (parallel beam) 2 is arranged so as to travel in a direction intersecting the 13° optical path.

Therefore, the guided light 13° travels across the surface acoustic wave 12, and in this case, the guided light 13' is caused by Bragg (Bragg) interaction with the surface acoustic wave 12.
) to diffract. As is well known, the guided light of 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, this guided light 13' is
As shown in , it is diffracted and deflected so that the diffraction angle changes continuously. The guided light 13' deflected in this way is L
The light is diffracted by the GC 21 and output from the optical waveguide 11 to the substrate 16 side.

In this way, the light beam 13'' that is emitted from the optical waveguide 11 and becomes external light passes through the first light passing surface 31a of the light emitting prism 31, enters the prism 31, and enters the light passing surface 31b of i2. passes vertically and exits the prism.

The light beam 13'' emitted to the outside of the optical waveguide element 1O as described above is transmitted through a scanning lens 2B consisting of, for example, an fθ lens.
The beam passes through the beam and is narrowed down to a small beam spot Q, and the photoreceptor 2
3. Scan the top in the direction of arrow U (main scan). At the same time, the photoreceptor 23 is transferred by the transfer means 22 in the direction of arrow V, which is substantially perpendicular to the top-fertilization 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 this image signal S is used as a trigger signal.
The timing of voltage application to the IDT 15 may be controlled.

Furthermore, by controlling the driving timing of the transfer means 22 using this blanking signal sb, the main scanning and sub-scanning can be synchronized.

Here, as an i feature of the present invention, as schematically shown in FIG. 3(A), the light emitting LGC 21 has a grating height of 13° in the traveling direction of the guided light (that is, the grating direction of the L G C 21). The light intensity distribution of the light beam 13'' in this direction becomes a substantially Gaussian distribution as shown by the curve f in FIG. 3CB). The grating height distribution for obtaining such a light intensity distribution will be explained in detail below.

Assuming that the radiation loss coefficient of LGC21 is α, the length is L, and the grating height h does not exceed the evanescent wave seepage depth hc, α-B h2 [13 is a coefficient] ...
- (1). As shown in FIG. 3(B), L G
If the position in the traveling direction of the guided light on C21 is y (O5y≦L), the amount of guided light in the optical waveguide 11 where the LGC 21 is installed is a function of y, so this is set as P (y). The amount of attenuation per unit length in the y direction of this guided light JIP(y) is the light beam 1 emitted from the LGC 21.
When the light intensity distribution of 3" becomes a Gaussian distribution in the y-axis direction as shown by the curve f in Figure 3(B), the following equation is obtained. α(y) is the radiation loss coefficient at each position in the y-axis direction. It is.

On the other hand, the above curve f is generally expressed by the following formula. Note that W is the half value of the grating length in the range where the light intensity is 1/e2 or more of the maximum value. Using the above equation (3), the amount of guided light P (y) is expressed by the following equation.

+δ Let the above equation be equation (4). The first term on the right side of this equation (4) is L
The total amount of light extracted by GC21, the second term is LGC
The amount of light that is constantly guided in the range 0≦y≦L regardless of the effect of 21, the third term is LGC2 up to a certain position y.
It is the amount of light emitted from 1, and from this formula, with A as a coefficient, it becomes. Therefore, from this equation and the above equation (2), the above (
4) Since the sum of the first and second terms on the right side of the equation is equal to the light amount Po of the guided light 13' guided to the LGC 21 installation section, by knowing this light ffi Po, P can be calculated from equation (4).
The value of (y) is calculated, and α
The value of (y) is found.

As described above, if the radiation loss coefficient α(y) for obtaining the light beam 13'' with the light intensity distribution as shown by the curve f in FIG. 3(B) is determined, based on the above equation (1), hand,
The grating height h that realizes such a radiation loss coefficient α(y)
(y) is found. Two examples of the distribution of the grating height h obtained in this manner are shown in FIG. Curve a in FIG. 4 shows the grating height distribution for obtaining an output beam 13'' with a beam diameter of 2w-211III when the length of the diffraction grating is 3fflIIl, and curve a' shows the corresponding radiation loss coefficient. The -force curve shows the distribution of α, and the beam diameter 2w = 4.7mg with the length L of the diffraction grating being 7111Im.
The curve b° shows the distribution of the corresponding radiation loss coefficient α.The wavelength length of the guided light 13' in this example is 63".
3 nm, and the refractive index n of LGC21 is 2.4.

As mentioned above, the grating height h is the evanescent wave seepage depth. It is necessary to change it within a range lower than that. This evanescent wave seepage depth h2 is hc-λ/, where N is the effective refractive index of the optical waveguide 11, and nge is the average refractive index of the LGC 21 and the medium outside it (usually air). 2π buff; 7'' Note that nte is given by nte, where n is the refractive index of the LGC 21, n is the refractive index of the medium, d1 is the grating pitch, and dl is the thickness of each grating in the lattice alignment direction.

If the light intensity distribution of the light beam 13'' emitted from the LGC 21 is as described above, the beam spot Q scanning the photoreceptor 23 has a Gaussian light intensity distribution in the beam sub-scanning direction. Therefore, the beam spot Q can be sufficiently focused in this direction, making it possible to record extremely fine images.
Although the light beam 13'' is emitted from the substrate 16 to the side of the substrate 1B, the present invention can also be applied to a case where the light beam 13'' is emitted to the air side opposite to the substrate 16, that is, to the upper side in FIG. In that case as well, the same effect as described above can be obtained. In addition, when emitting the light beam 13'' to the substrate I6 side, it is not necessarily necessary to use the prism 3 described above, but instead of cutting the end face of the substrate 1B obliquely and then The light beam 13' may also be emitted.

Further, in the above embodiments, the present invention is applied to the light output LGC 21, but it is also possible to apply the present invention to the light input LGC 20. In that case,
Effects that are contradictory to those in the above embodiments can be obtained.

In other words, in this case, if a general laser beam or the like whose light intensity distribution is approximately Gaussian is used as the light beam 13 to be input into the optical waveguide ll, the beam 1
3 has substantially the maximum incident coupling efficiency and is taken into the optical waveguide 11.

Furthermore, although the optical waveguide element of the embodiment described above constitutes an optical deflector, it is not limited to such an optical deflector, and is used to control the light beam emitted from the optical waveguide so that the light intensity distribution in the guided light propagation direction is There is a widespread demand to shape a light beam so that it has a Gaussian distribution, and furthermore, to make a general light beam with a substantially Gaussian light intensity distribution enter an optical waveguide with high coupling efficiency. Therefore, the present invention is applicable and effective to all optical waveguide devices that meet such requirements.

(Effects of the Invention) As explained above in detail, in the optical waveguide element of the present invention, the grating height of the diffraction grating formed on the optical waveguide surface is set so as to have a mountain-shaped distribution along the traveling direction of the guided light. With this setting, the light intensity distribution of the light beam emitted from the diffraction grating can be made into a substantially Gaussian distribution. Therefore, according to this optical waveguide element, it is possible to focus the light beam emitted from the optical waveguide into an extremely small spot, and when applied to an optical deflector for image recording or image reading, it is possible to sufficiently improve the accuracy of image recording or reading. Increased to
Furthermore, when applied to a high frequency spectrum analyzer etc., the resolution of frequency analysis can be improved.

Furthermore, in the optical waveguide device of the present invention, by forming the diffraction grating in the above-described shape, external light such as a general laser beam having a Gaussian light intensity distribution can be
It becomes possible to efficiently introduce the light into the optical waveguide. Therefore, according to this optical waveguide element, it is possible to sufficiently increase the light utilization efficiency in the above-mentioned optical deflector, high frequency spectrum analyzer, etc. by using the numerical laser beam as it is without shaping it. Can be done.

[Brief explanation of the drawing]

1 and 2 are a side view and a perspective view, respectively, showing an optical waveguide device according to an embodiment of the present invention, and FIG. 3(A) is an enlarged view of the diffraction grating portion of the optical waveguide device of the above embodiment. FIG. 3B is a graph showing the intensity distribution of the emitted light along the direction in which the diffraction gratings are arranged, and FIG. 4 is a graph showing the distribution of the height of the diffraction grating in the optical waveguide element of the present invention. A graph showing an example of the distribution of the radiation loss coefficient corresponding thereto. FIG. 5 is an explanatory diagram illustrating the intensity distribution of the light beam emitted from the diffraction grating of the conventional optical waveguide element. 10... Optical waveguide element 11... Optical waveguide 12
...Surface acoustic wave 13...Light beam 13゛
... Waveguide light 13" ... Light beam 21 emitted from the optical waveguide...
Diffraction grating for light emission Fig. 3 Fig. 5 Fig. 4

Claims (1)

[Scope of Claims] An optical waveguide element in which a diffraction grating is formed on the surface of an optical waveguide for coupling guided light traveling in the optical waveguide with external light, wherein the grating height of the diffraction grating is set to a height of When emitted out of the optical waveguide by the diffraction grating, the intensity distribution of the emitted light along the traveling direction of the guided light is changed into a mountain shape along the direction so that it becomes a substantially Gaussian distribution. Characteristic optical waveguide device.