JPH01107214A - Optical waveguide element - Google Patents

Abstract

PURPOSE:To converge a beam spot sufficiently by varying the aspect ratio of a diffraction grating so that the intensity distribution of projection light has a nearly Gaussian distribution in the traveling direction of waveguide light. CONSTITUTION:The aspect ratio [ratio of grating-array directional thickness d1 of grating to grating pitch (d)] of a Linear Grating Coupler (LGC) 21 for light projection is increased gradually and then decreased gradually in the traveling direction of the waveguide light 13', namely, in the grating array direction of the LGC 21. The light intensity distribution of the light beam 13'' is therefore the nearly Gaussian distribution. Consequently, the light beam projected from the optical waveguide 11 is converged into the small spot W and the accuracy of image recording and reading is improved.

Description

Detailed Description of the Invention (Industrial Field of Application) The present invention provides a guiding waveguide element, in particular, a guiding waveguide device, in which a diffraction grating is provided on the surface of an optical waveguide, and guided light is emitted out of the optical waveguide by the diffraction grating, or This invention relates to a guiding waveguide element in which external light is made to enter the guiding wavepath 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 a guiding waveguide are attracting attention. This optical deflection device consists of a slab-shaped leading wavepath 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 light beam guided within the leading wavepath. It has means for generating an elastic wave in the leading wavepath (for example, composed of a pair of crossed comb-shaped electrodes and a driver that applies an alternating voltage whose frequency changes continuously to this pair of electrodes). In this optical deflection device, the light beam guided in the guide 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 guide wavepath. The light beam deflected in this manner can be emitted out of the leading waveguide by, for example, a diffraction grating (grating coupler) or a prism coupler formed on the surface of the leading 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 It is required to narrow down the beam to a spot and to make the light intensity distribution take a Gaussian distribution at least in the beam sub-scanning direction.

However, when the guided wave is emitted from the guide 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 leading waveguide, the emitted light is The light intensity of the beam 43'' gradually decreases exponentially along the traveling direction of the guided wave 43, as shown by curve g in the figure.As shown in FIG. When the guided light is deflected by a surface acoustic wave, the direction of deflection (main scanning direction) is a direction that intersects the plane of the paper in this figure, and the sub-scanning direction is the direction of arrow V in the figure. Since the beam 43' has a light intensity distribution that gradually decreases or gradually increases along this sub-scanning direction, it is necessary to create a beam spot where the light intensity distribution becomes a Gaussian distribution along this direction. It becomes very difficult to obtain.

Above, we have discussed the problems when guiding light is emitted out of the optical waveguide using a diffraction grating, but it has also been widely used to make external light enter the guided 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 Problems) As described above, the guiding waveguide element of the present invention allows the surface of the guiding waveguide to emit the guided light traveling within the guiding waveguide to the outside, or to emit external light. In an optical waveguide element in which a diffraction grating is formed, which allows guided light to enter the guiding waveguide, the aspect ratio of the diffraction grating (the ratio of the thickness of the grating in the direction in which the gratings are arranged to the grating pitch) is set to It is characterized in that the intensity distribution of the emitted light is changed along the traveling direction of the guided light so that it becomes a substantially Gaussian distribution when emitted.

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

1 and 2 are a side view and a perspective view, respectively, showing an optical waveguide device 10 according to an embodiment of the present invention.

For example, this guiding waveguide element 10 constitutes an optical deflector for image recording, and includes a slab-shaped guiding waveguide 11 formed on a transparent substrate 16 and a side end of this optical waveguide 11. A pair of crossed comb-shaped electrodes (Inter D
1g1tal transducer, hereinafter ■
DT) 15 and a linear diffraction grating for light incidence (L
It has an inner grating coupler (hereinafter referred to as LGC) 20 and a light emitting LGC 21. Further, on the surface lea of the substrate 16 on the side opposite to the optical waveguide 11, a light input prism 30 and a light output prism 31 are attached. Light incidence prism 30
has a triangular cross section, and has a first light passing surface Boa and a second light passing surface Boa.
The first light passing surface 30a is tightly fixed to the substrate surface 16a by being strongly pressed against the substrate surface 16a or by using an adhesive having a high refractive index. There is. The light emitting prism 31 also has the same shape as the light entering prism 30, and the first light passing surface 3
1a1 has 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 leading 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 the leading wavepath, for example, T, Tal1i
r) ed. “Ingrated Obtics (Int.
egrated Optics)J()Pix in Applied Physics(Topics in
Applied Physics Volume 7) 'S pringer-Verl
ag) (1975); “Optical Integrated Circuits” co-authored by Nishihara, Onna, and Narahara, published by Ohmsha (1985). can also be used. However, this leading wave path 11
is formed of a material such as the above-mentioned Ti diffusion film through which a surface acoustic wave, which will be described later, can propagate. Further, the guiding 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 (laser beam) 13.

This light beam 13, which is a diverging beam, is made into a parallel beam by the collimator lens 25, and then enters the light incidence prism 30 from the second light passing surface 30b, and passes through the first light passing surface 30a. The light passes through the substrate 16, passes through the optical waveguide 11, and enters the LGC 20 formed on the surface thereof. Thereby the light beam 13
is diffracted by this LGC 20 and enters the leading waveguide 11,
The wave propagates in the direction of arrow A in the guided 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 leading 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' propagates across the surface acoustic wave 12, and in this case, the guided light 13' becomes a Bragg (Bragg) beam due to acousto-optic interaction with the surface acoustic wave 12.
g) Diffraction. As is well known, guided light 13 due to this diffraction
The deflection angle ' is approximately proportional to the frequency of the surface acoustic wave 12. As described above, the drive circuit 17 applies an alternating voltage whose frequency changes continuously to the IDT 15, so that the frequency of the surface acoustic wave 12 changes continuously and the deflection angle changes continuously. Therefore, as shown by arrow C, this guided light 13' is diffracted and deflected so that the diffraction angle changes continuously. The waveguide light 13° deflected in this way is diffracted by the LGC 21 and is sent from the guide waveguide 11 to the substrate 1B.
Emits to the side.

In this way, the light beam 13'' that is emitted from the leading 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 second light passing surface. The light passes through the atb 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 main scanning direction, to perform sub-scanning, so that the photoreceptor 23 is two-dimensionally scanned by the light beam 13''. As mentioned above, this light beam 13'' is the image signal S.
Since the image signal S is modulated based on the image signal S, an image carried by this image signal S is recorded on the photoreceptor 23.

° - 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 and sent to the IDT 15. 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 a characteristic part of the present invention, as schematically shown in FIG. 3(A), the light emitting L The traveling direction of the wave light 13°, that is, LGC21
The light intensity distribution of the light beam 13'' in this direction is approximately as shown by the curve f in FIG. 3(B). This is a Gaussian distribution.Hereinafter, the grating height distribution for obtaining such a light intensity distribution will be explained in detail.

As shown in FIG. 3(B), the position in the traveling direction of the guided light on the LGC 21 is y (0≦y≦L), the radiation loss coefficient of the LGC 21 is α, and the aspect ratio is β(y)=dt (y
)/d, and the length is L, then a (y) mB-s i
n2(π*β(y)) -----.-(1) [B is a coefficient determined by the grating height, etc.]. Since the amount of guided light in the optical waveguide 11 in the part where the LGC 21 is installed is a function of y, this is set as P (y). The amount of attenuation per unit length in the y direction of this guided light ff1P(y) is
When the light intensity distribution of the light beam 13'' emitted from the LGC 21 has a Gaussian distribution in the y-axis direction as shown by the curve f in FIG. This is the radiation loss coefficient at each position.

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 formula and the above (?5 formula), the above (
4) The sum of the first and second terms on the right side of the equation is the amount P of the guided light 13° guided to the LGC 21 installation part. By knowing HPo, the value of P (y) can be found from equation (4), and the value of α(y) can then be found from equation (5).

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,
An aspect ratio β(y) that realizes such a radiation loss coefficient α(y) is determined.

Two examples of the distribution of the aspect ratio β obtained in this manner are shown in FIG. Curve a in FIG. 4 shows the aspect ratio distribution for obtaining an output beam 13'' with a beam diameter of 2W-2fffffl when the length of the diffraction grating is 3 mm, and curve a° shows the corresponding radiation loss coefficient ' α The force curve shows the aspect ratio distribution to obtain an output beam 13'' with a beam diameter of 2 w -4 and 7 mm when the length of the diffraction grating is 7 mm.

shows the distribution of the corresponding radiation loss coefficient α.

In this example, the wavelength λ of the guided light 13' is 633n
II, and the refractive index n of LGC21 is 2.4.

If the light intensity distribution of the light beam 13'' emitted from the LGC 21 is as explained 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.

In the embodiment described above, the light beam 13'' is emitted from the LGC 21 to the substrate 16 side, but the light beam 13'' is emitted to the air side opposite to the substrate 16, that is, to the upper side in FIG. The present invention is also applicable to the case where the above-described effect is obtained. Furthermore, when the light beam 13'' is emitted from the substrate 1B side, it is not necessarily necessary to use the prism 31 described above.
As has been widely practiced in the past, the end face of the substrate 16 may be cut obliquely, and the light beam 13'' may be emitted therefrom.

Further, in the above embodiment, the aspect ratio of the LGC 21 is changed so that it gradually increases and then gradually decreases along the traveling direction of the guided light 13', but this is because the aspect ratio d1/d is 0≦d1/d. It is effective in the range of ≦0゜5, and otherwise this aspect ratio d1/d is 0.5
In the range of ≦dt/d≦1, as shown in FIG. 6(A), it may gradually decrease and then increase along the traveling direction of the guided light 13, or in the range of 0≦di/d≦1. Now, even if the increase is made gradually along the above direction as shown in FIG. 7(A), 0≦d1/d≦1
In the range of , even if it gradually decreases along the above direction as shown in FIG. 8(A), the curves f in FIG. 6(B), FIG. As shown in , it is possible to obtain the same output light intensity distribution as in the above embodiment.

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.

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

Furthermore, although the guiding waveguide element of the embodiment described above constitutes an optical deflector, it is not limited to such an optical deflector, and the optical beam emitted from the guiding waveguide is used to adjust the light intensity distribution in the direction of propagation of the guided light. 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 nearly Gaussian light intensity distribution incident into a leading waveguide with high incident 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 in detail above, in the guiding waveguide element of the present invention, the aspect ratio of the diffraction grating formed on the surface of the optical waveguide is set to change along the traveling direction of the guided light. The light intensity distribution of the light beam emitted from the diffraction grating can be made into a substantially Gaussian distribution. Therefore, this guiding waveguide element makes it 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. When applied to a high frequency spectrum analyzer etc., the resolution of frequency analysis can be improved.

In addition, in the guiding waveguide element of the present invention, by changing the aspect ratio of the diffraction grating as described above, external light such as a general laser beam with a Gaussian light intensity distribution can be efficiently transmitted. It becomes possible to introduce it into an optical waveguide. Therefore, according to the leading waveguide element of the lever, the efficiency of light utilization in the above-mentioned optical deflector, high frequency spectrum analyzer, etc. can be sufficiently increased by using the numerical laser beam as it is without shaping it. be able to.

[Brief explanation of the drawing]

1 and 2 are a side view and a perspective view, respectively, showing a guiding waveguide element according to an embodiment of the present invention, and FIG. 3(A) is an enlarged view of the diffraction grating portion of the guiding waveguide element of the above embodiment. FIG. 3(B) is a graph showing the intensity distribution of the emitted light along the direction in which the diffraction gratings are arranged, and FIG. A graph showing an example of the distribution of the radiation loss coefficient corresponding to this.
B), Figures 7(A) and (B), and Figures 8(A) and (B)
) are a side view showing another example of the shape of the diffraction grating in the guiding waveguide element of the present invention, and a graph showing the intensity distribution of the emitted light along the direction in which the diffraction gratings are arranged. lO... Optical waveguide element 11... Guide waveguide 12
...Surface acoustic wave 13...Light beam 13°・
... Guided light 13"... Light beam 21 emitted from the guiding waveguide...
Diffraction grating for light emission Fig. 3 Fig. 5 Opening ■ (T) y (mm) Fig. 6, T, -7 Fig. 8 November 24, 1988

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 aspect ratio of the diffraction grating is such that the aspect ratio of the diffraction grating couples the guided light traveling in the optical waveguide with external light. The light guide is characterized in that the intensity distribution of the emitted light along the traveling direction of the guided light is changed along the direction so that when the light is emitted out of the optical waveguide by the diffraction grating, the intensity distribution becomes a substantially Gaussian distribution. Wave path element.