JPH0786625B2 - Light deflection device - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an optical deflecting device, and more particularly to a surface acoustic wave in which a light beam is guided in an optical waveguide and the light beam is propagated in the optical waveguide. The present invention relates to an optical deflecting device which is deflected by and is extracted from an optical waveguide.

(Prior Art) As an optical deflecting device for deflecting a light beam in an optical scanning recording device or an optical scanning reading device, a mechanical optical deflector such as a galvanometer mirror or a polygon mirror or an EOD has been conventionally used.
(Electro-optical light deflector) and AOD (acousto-optical light deflector) are widely used. However, in the mechanical light deflector,
Durability is difficult and there is a problem that it tends to increase in size. On the other hand, in EOD and AOD, there is a problem that the beam optical path becomes long because the optical deflection angle cannot be made large, which leads to the size increase of optical scanning recording devices. .

Recently, as an optical deflector capable of solving the above problems,
Attention has been paid to an optical deflector using an optical waveguide. This optical deflector comprises a thin film optical waveguide formed of a material capable of propagating a surface acoustic wave, and a surface acoustic wave whose frequency continuously changes in a direction intersecting with a light beam guided in the optical waveguide. Means for generating in the optical waveguide (for example, composed of a pair of cross-shaped electrodes and a driver for applying an alternating voltage whose frequency continuously changes to the pair of electrodes)
And have. In this optical deflector, the light beam guided in the optical waveguide is black-diffracted by the acousto-optic interaction with the surface acoustic wave, and this diffraction angle changes according to the surface acoustic wave frequency. 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 outside the optical waveguide by, for example, a diffraction grating (grating coupler) or a prism coupler formed on the surface of the optical waveguide.

An example of the light deflector as described above is disclosed in
The detailed description is given in JP 62-77761 A.

(Problems to be Solved by the Invention) By the way, the thin film optical waveguide constituting the above-mentioned optical deflecting device is generally formed from a uniaxial anisotropic crystal substrate such as LiNbO ₃ or LiTaO ₃ , and the deflected light beam is It has been conventionally recognized that the guided light propagates in the waveguide in the TM guided mode, the TE guided mode, or a mode in which they are converted to each other, but the guided light is easily radiated to the substrate side and lost.
The reason why the radiation loss of the guided light is likely to occur will be described below.

The uniaxial anisotropic thin film optical waveguide described above is formed so that the optical axis is included in the waveguide surface (so-called X-cut or Y-cut). In that case, the radiation loss coefficient α of the guided light changes in accordance with the angle θ formed by the guided light propagation direction with respect to the optical axis, and its characteristics are generally as shown in FIG. In order to minimize the radiation loss, the guided light may be propagated at an angle of 90 ° or close to 0 ° with respect to the optical axis. However, in the above-mentioned optical deflector, since the guided light is diffracted and deflected by the surface acoustic wave on the way, the propagation direction naturally changes before and after this diffraction, and therefore the guided light before and after the diffraction propagates at the angle as described above. It is impossible to let them do it. In this way, when the traveling direction of the guided light before or after diffraction deviates from the value close to 90 ° or 0 °, the guided light causes a large radiation loss. The radiation loss will be specifically described below. For example X
In the optical waveguide composed of −cut LiNbO ₃ substrate, the wavelength λ
A light beam of 633 nm is guided, and the light beam is diffracted by a surface acoustic wave having a frequency f = 1 GHz and a propagation velocity v = 3500 m / s, and after diffraction, in the y-axis direction (90 ° relative to the optical axis Direction), if the Bragg angle is θ _B ,
When the effective refractive index Neff of the optical waveguide is 2.2, Becomes The angle θ formed by the light beam with the optical axis before entering the surface acoustic wave is θ = 90−2θ _B 85 °, and the radiation loss coefficient α = 1.5 dB / cm at this angle θ. Assuming that the guided distance L from the light beam entering the optical waveguide to the surface acoustic wave is L = 5 mm, the guided light propagation efficiency η is Becomes That is, the radiation loss theoretically reaches about 16% only after the light beam enters the optical waveguide and reaches the surface acoustic wave.

As described above, if the radiation loss of the guided light is large, the light utilization efficiency naturally lowers, and a high-power and expensive light source such as a semiconductor laser is required, and the light using this optical waveguide type optical deflector is required. The power consumption of the scanning recording device and the like also increases.

Therefore, an object of the present invention is to provide an optical waveguide type optical deflector capable of suppressing the radiation loss of guided light.

(Means for Solving the Problem) As described above, the optical deflecting device of the present invention is a uniaxial anisotropic device capable of propagating a surface acoustic wave, formed so that the optical axis is included in the waveguide surface. Thin-film optical waveguide, and means for generating in the optical waveguide a surface acoustic wave whose frequency continuously changes by advancing in a direction intersecting with a light beam which is incident on the optical waveguide and is guided in the optical waveguide. In an optical waveguide type optical deflecting device including an optical path conversion element for converting an optical path of a light beam before entering the surface acoustic wave, the optical path of the light beam before the optical path conversion and the surface acoustic wave The respective angles formed by the optical path of the light beam after being diffracted and deflected and the optical axis are compared with the angle formed by the optical path of the light beam until it enters the surface elastic liquid after the optical path conversion and the optical axis. Corners with less radiation loss of wave light The surface acoustic wave generating portion of the surface acoustic wave generating means and the optical path changing element are arranged so that the optical path changing element has the same degree, and the above-mentioned optical path changing element is arranged close to the surface acoustic wave.

(Operation) When the optical path conversion element as described above is provided, the optical path of the light beam from entering the optical waveguide to entering the element,
Both the optical path of the light beam guided in the optical waveguide after being diffracted by the surface acoustic wave is 90 ° or 0 with respect to the optical axis.
It is possible to set the angle close to °. These two optical paths are
For example, a diffraction grating (grating coupler) or prism coupler formed on the surface of the optical waveguide tends to make the length longer in order to allow external light to enter the optical waveguide or to emit the guided light outside the optical waveguide. If the angle of the optical path is as described above, it is possible to suppress the radiation loss of the guided light there to be extremely low.

Further, the optical path of the light beam guided in the optical waveguide from the optical path conversion in the optical path conversion element until it is incident on the surface acoustic wave has an angle at which radiation loss tends to increase,
If the optical path conversion element is arranged close to the surface acoustic wave, the optical path becomes short and the radiation loss of the guided light there can be suppressed low.

(Examples) Hereinafter, the present invention will be described in detail based on examples shown in the drawings.

1, 2 and 3 show an optical deflecting device according to an embodiment of the present invention. This optical deflecting device 10 constitutes an optical scanning recording device as an example, and a transparent substrate
The thin-film optical waveguide 11 formed on the 16 and a pair of tilted finger chirped interdigital transducers (Tilted-Finger Chirped Inter Digital Transduce) provided at the side end of the optical waveguide 11.
r, hereinafter referred to as IDT) 15, and an electro-optic grating (Electrooptic G) as an optical path changing element that also functions as an optical modulator.
rating, hereinafter referred to as EOG) 14, a light-incident linear diffraction grating (hereinafter referred to as LGCT) 20 and a light-exiting LGC 21 provided on the surface of the optical waveguide 11 so as to be separated from each other. ing. Further, on the surface 16a of the substrate 16 opposite to the optical waveguide 11, the light incidence prism 30 is provided.
And a light emitting prism 31 is attached. The light incident prism 30 has a triangular cross section, and has a first light passage surface.
30a and a second light passage surface 30b, the first light passage surface 30
The a is tightly fixed to the surface 16a by being strongly pressed against the surface 16a of the substrate or by using an adhesive having a high refractive index. The light emitting prism 31 also has the same shape as the light incident prism 30, has a first light passage surface 31a and a second light passage surface 31b, and is fixed to the substrate 16a in the same manner as described above. .

In this example, as an example, a LiNbO ₃ wafer, which is a negative uniaxial anisotropic crystal, is used as the substrate 16, and the surface of this wafer is
The optical waveguide 11 is formed by providing a Ti diffusion film. Here, the substrate 16 is X-cut, and the optical axis (Z axis)
Are formed so as to extend in the vertical direction in FIG. The optical waveguide 11 is not limited to the Ti diffusion described above, but may be formed on the substrate 16 by sputtering, vapor deposition, or the like. Further, in the present invention, the optical waveguide 11 can be formed using not only the above-mentioned LiNbO ₃ but also other known substrates such as LitaO ₃ which is a positive uniaxial anisotropic crystal. The optical waveguide 11 is formed such that the optical axis is included in the waveguide surface. The optical waveguide 11 is made of a material such as the Ti diffusion film described above that can propagate surface acoustic waves, which will be described later.

Regarding the optical waveguide, for example, Titamir (T.
Tamir) “Integrated Optics (Integ
rated Optics))) (Topics in Applied
Physics (Topics in Applied Physics Volume 7)
Detailed descriptions are given in publications such as "Springer-Verlag"(1975); Nishihara, Haruna, and Suhara "Optical Integrated Circuits" published by Ohmsha (1985).

The semiconductor laser 18 that emits recording light is a prism 30 for light incidence.
Is arranged so as to emit a light beam (laser beam) 13 having a wavelength λ = 780 nm perpendicularly to the second light passage surface 30b. This light beam 13, which is a divergent beam, is made into a parallel beam by the collimator lens 27, and then the second beam
From the light passing surface 30b into the light entrance prism 30, passes through the first light passing surface 30a, enters into the substrate 16, passes through the optical waveguide 11, and is formed on the surface thereof. LGC20
Is incident on the part of. As a result, the light beam 13 is
The light is diffracted by and is incident on the inside of the optical waveguide 11.
It travels in the direction of arrow A in the guided mode.

When recording an image, the photoconductor 23 is set on the transfer means 22 such as an endless belt. Then, the semiconductor laser 18 is driven by the laser drive circuit 19 so as to emit the laser beam 13, and at the same time, an alternating voltage whose frequency continuously changes between 1 and 2 GHz is applied to the IDT 15 from the drive circuit 17. It On the other hand, the voltage V is applied from the drive circuit 24 to the EOG 14 arranged in the optical path of the guided light (parallel beam) 13. The value of the voltage V is ON-OFF modulated by the modulation circuit 25 that receives the image signal S so that it changes according to the signal S (that is, it continuously changes when the light beam 13 is intensity-modulated). If it does, one of the two values is controlled selectively).

The above EOG14 forms a diffraction grating.
The guided light 13 is diffracted by 14 and the diffracted light 13 'travels in the direction shown by the solid line in FIG. 1 and the 0th order light 13 "travels in the direction shown by the broken line. When this is done, the refractive index of the optical waveguide 11 changes due to the electro-optic effect, and the diffraction efficiency changes. Since the change rate of the diffraction efficiency changes according to the value of the voltage V applied to the EOG 14, Eventually, the diffracted light 13 'is modulated according to the image signal S.

The EOG14 in the above embodiment has an electrode finger line width of 3.75.
μm, electrode finger cycle is 15 μm, effective length of electrode finger is 1.3 mm,
The number of electrode finger pairs is 100, and the maximum diffraction efficiency η _M = 9
We were able to achieve 3% and a modulation frequency f _M = 25 MHz. EO like this
G14 can be formed by a known photolithography method or the like. On the other hand, when the above-mentioned voltage is applied to the IDT 15, the surface acoustic wave 12 propagates on the surface of the optical waveguide 11 in the direction of arrow B in FIG. The IDT 15 is arranged so that the surface acoustic wave 12 travels in a direction intersecting the optical path of the guided light (the diffracted light) 13 '. Therefore, the guided light 13 ′ travels across the surface acoustic wave 12 while the guided light 1 ′ is being transmitted.
3'is Bragg-diffracted by the acousto-optic interaction with the surface acoustic wave 12. As is well known, the deflection angle of the guided light 13 'due to this diffraction is substantially proportional to the frequency of the surface acoustic wave 12. As described above, the drive circuit 17 applies to the IDT 15 an alternating voltage whose frequency changes continuously, so that the frequency of the surface acoustic wave 12 changes continuously, or the surface acoustic wave 12 is adjusted so that the Bragg condition is always satisfied. The direction of propagation of is continuously changed, whereby the deflection angle is continuously changed. Therefore, this guided light 13 'is, as shown by the arrow C,
Diffract and deflect so that the diffraction angle changes continuously. The guided light 13 ′ thus deflected is diffracted by the LGC 21 and emitted from the optical waveguide 11 to the back plate 16 side. In this way, the light beam 13 'which is emitted from the optical waveguide 11 and becomes external light passes through the first light passage surface 31a of the light emission prism 31 and enters the prism 31 to form the second light passage surface. It passes through 31b vertically and goes out of the prism.

The light beam 1 emitted outside the optical deflector 10 as described above.
3'is passed through the scanning lens 26 formed of, for example, an f.theta. Photoconductor 23
However, since the sub-scanning is carried out by being carried by the carrying means 22 in the direction of arrow v substantially perpendicular to the main scanning direction, the photoconductor 23 is two-dimensionally scanned by the light beam 13 '. Since the light beam 13 'is modulated based on the image signal S as described above, the image carried by the image signal S is recorded on the photoconductor 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 to generate the IDT1
The timing of voltage application to 5 may be controlled. Further, by controlling the drive timing of the transfer means 22 by the blanking signal Sb, the main scanning and the sub scanning can be synchronized.

Next, the radiation loss of the guided lights 13 and 13 'will be described. As shown in Fig. 3, the length of LGC20 and LGC21 are respectively
5mm, 14mm, EOG14 from the center of the LGC20 length direction
The distance from the center of the EOG 14 to the center of is 3.5 mm, and the distance from the center of the EOG 14 to the diffraction point by the surface acoustic wave 12 is 1.5 m.
The distance from the diffraction point to the center of the LGC21 in the length direction is 8 mm in the Y-axis direction. Then, the guided light 13 is emitted from the LGC 20.
In order to travel between EOG14 at an angle of 87 ° with respect to the optical axis (Z-axis) and guided light 13 'at an angle of 81 ° with respect to the optical axis between EOG14 and surface acoustic wave 15,
The EOG 14 and the IDT 15 are arranged so that the guided light 13 'diffracted by the surface acoustic wave 12 travels at an angle of 90 ° with respect to the optical axis at the center of the deflection angle. In this example, the angular range of deflection by the surface acoustic wave 12 is set to 6 °, and therefore the diffracted guided light 13 'is at least 87 ° (= 90-6 / 2) with respect to the optical axis. Will make an angle.

As described above, the angle formed by the optical path of the guided light before the EOG 14 and the optical axis and the angle formed by the optical path of the guided light after being diffracted by the surface acoustic wave 12 with the optical axis are 87 ° and 87-90 °, respectively. Therefore, as is clear from FIG. 4, the radiation loss coefficient α of the guided light in this portion is extremely small.
As described above, the lengths of the two optical paths tend to be relatively long, but since the radiation loss coefficient a is small as described above, it is possible to suppress the radiation loss of guided light in these optical path portions to be low. it can. On the other hand, surface acoustic wave 1 from EOG14
The angle between the guided light path up to 2 and the optical axis is 81 °,
The radiation loss coefficient α in this optical path portion is larger than that in the above two optical paths (see FIG. 4). However, since this EOG 14 is arranged sufficiently close to the surface acoustic wave 12, the length of the optical path where this radiation loss coefficient α becomes relatively large is extremely short, and therefore the radiation loss of the guided light in this optical path. Can be kept low. In this example, when the radiation loss of the guided light between LGC20 and LGC21 was measured, it was a minimum of 21% when the frequency of the surface acoustic wave 12 was f = 1.5 GHz, and when f = 1.0 GHz and 2.0 GHz, that is, The maximum deflection was 28% at the minimum and maximum deflection angles. In this example, the effective refractive index neff of the optical waveguide 11 is 2.
2. The propagation velocity of the surface acoustic wave 12 is v = 3500 m / s.

As a comparative example, an optical deflecting device as shown in FIG. 5 was prepared and the radiation loss of guided light therein was examined. In this comparative example, the optical waveguide 11, LGC20, 21, IDT15, light beam 13
And the frequency range of the alternating voltage applied to the IDT 15 (that is, the deflection angle range of the guided light 13 ') are the same as those in the above embodiment, while the EOG 14 described above is not provided and the guided light 13' is Therefore, the light is guided directly at an angle of 81 ° with respect to the optical axis. In this comparative example LGC20
The measured radiation loss of the guided light from the to the LGC21 is 47 when the frequency of the surface acoustic wave 12 is f = 1.5GHz.
%, While at f = 1.0 GHz and 2.0 GHz, that is, at the maximum and minimum deflection angles of 52%. It can be seen that the radiation loss of the guided light in the above example is extremely small as compared with that in this comparative example.

Next, a second embodiment of the present invention will be described with reference to FIG. 6 showing its planar shape. In the optical deflecting device 40 of the second embodiment, the guided light 13 'diffracted by the first surface acoustic wave 12 generated from the first IDT 15 is converted into the second guided light 13'.
It is again diffracted by the second surface acoustic wave 42 emitted from the IDT 45, so that a wide deflection angle range can be obtained. In this embodiment, the angle formed by the guided light 13 'diffracted and deflected by the first surface acoustic wave 12 with the optical axis (Z axis) is 78 to 84 ° (that is, the deflection angle range is 6 °), and the second surface The angle formed by the guided light 13 ′ diffracted and deflected by the elastic wave 42 with the optical axis is 84 to 90 ° (the deflection angle range is (90−84) × 2 = 12.
)), And the LGC20 is incident on the optical waveguide 11 to cause EOG14
The propagating direction of the guided light 13 traveling up to 84 ° is 84 ° with respect to the optical axis, while the propagating direction of the guided light 13 ′ between the EOG 14 and the first surface acoustic wave 12 and the optical axis are. The angle formed is 72 °. So again in this case EOG1
If the 4 is placed close to the surface acoustic wave 12, the radiation loss coefficient α tends to be large.
It is possible to suppress the radiation loss of the guided light in the optical path portion up to 2. The optical paths of the guided light before and after this optical path portion tend to be relatively long, but since the angles of these optical paths are as described above, the radiation loss coefficient α in the optical path portion becomes small, and as a whole, It is possible to suppress the radiation loss of wave light to a low level.

Although the EOG14 described above also serves as a modulator, the optical path conversion element used in the present invention is not limited to such, and a simple diffraction grating or a mirror provided in the optical waveguide may be used. May be. In addition, when the optical path of the guided light is changed using the above EOG,
Therefore, when the diffracted light is deflected by the surface elastic liquid, it is desirable to set the diffraction angle by EOG so that the 0th-order light becomes stray light and does not enter the scanning range of the light beam.

Further, in the two embodiments described above, the optical path of the guided light before entering the optical path changing element and the optical path of the guided light after being diffracted by the surface acoustic wave are the optical path changing element and the surface acoustic wave. Compared with the optical path between them, the angle formed with the optical axis of the optical waveguide is made closer to 90 ° to reduce the guided light radiation loss. As is clear from FIG. The angle formed by each of the two optical paths and the optical axis of the optical waveguide is set close to 0 °, while the optical path between the optical path conversion element and the surface acoustic wave (the angle formed by the two optical paths is It is also possible to reduce the radiation loss of the guided light in the same manner as described above by shortening the length of each angle (which is larger than each angle formed with the optical axis).

(Effects of the Invention) As described in detail above, in the optical deflecting device of the present invention, an optical path conversion element that converts the optical path of guided light before entering the surface elastic liquid is provided, and in front of the element and on the surface. The optical path of the guided light after the elastic wave is set to an angle with a small radiation loss, and the optical path conversion element is placed close to the surface elastic liquid to shorten the length of the optical path between them, which tends to increase the radiation loss coefficient. By doing so, the radiation loss of the guided light can be suppressed to a very low level as a whole. Therefore, in this device, it is possible to use an inexpensive light source with a low output, and it is possible to reduce the cost, and it is also possible to obtain the effect of reducing the power consumption.

[Brief description of drawings]

1, 2 and 3 are a perspective view, a side view and a partial plan view showing an optical deflecting device according to a first embodiment of the present invention, respectively, and FIG. 4 is a guided light propagation angle with respect to an optical waveguide optical axis according to the present invention. FIG. 5 is a plan view showing an example of a conventional optical deflecting device, and FIG. 6 is a plan view showing an optical deflecting device according to a second embodiment of the present invention. is there. 10, 40 ...... Optical deflector, 11 ...... Optical waveguide 12, 42 ...... Surface acoustic wave, 13 ...... Light beam 13 '...... Guided light after optical path conversion 14 ...... Optical path conversion EOG, 15, 45 ... … Light deflection IDT 16 …… Substrate, 17 …… IDT driving circuit 18 …… Semiconductor laser, 20 …… Light incident diffraction grating 20 …… Light emitting diffraction grating, 24 …… EOG driving circuit

Claims (1)

[Claims] 1. A uniaxial anisotropic thin film optical waveguide capable of propagating a surface acoustic wave, formed so that its optical axis is included in a waveguide surface, and an inside of the optical waveguide which is made incident upon the optical waveguide. And a means for generating in the optical waveguide a surface acoustic wave having a frequency that continuously advances in a direction intersecting with the guided light beam, and an optical path of the light beam before entering the surface acoustic wave. An optical path converting element for converting the optical path of the optical beam before the optical path conversion and the optical path of the optical beam after being diffracted and deflected by the surface acoustic wave and the respective optical axes are formed by the optical path conversion surface. The surface acoustic wave generating portion and the optical path of the surface acoustic wave generating means are such that the radiation loss of the guided light is smaller than the angle formed by the optical path of the light beam and the optical axis until it enters the elastic wave. The conversion element It is, and the optical path conversion element, an optical deflection device, characterized in that it is arranged adjacent to the surface acoustic wave.