JPH01178918A - Waveguide type optical modulator - Google Patents

Abstract

PURPOSE:To suppress the attenuation of surface acoustic waves low within a range wherein it crosses waveguide light by arranging an IDT (Inter Digial Transducer) which generates surface acoustic waves at the nearly breadthwise center position of the waveguide light. CONSTITUTION:The IDT 15 which generates the surface acoustic waves 12a and 12b is arranged in an optical waveguide 11 at the nearly breadthwise center position of a light beam 13 propagated in the optical waveguide 11. The surface acoustic waves 12a and 12b which are generated by the IDT 15 are propagated in two directions so as to travel from the nearly breadthwise center part of the light beam 13 to right and left end parts. Thus, the distances where the surface acoustic waves 12a and 12b which are propagated in the mutually opposite directions cross the light beam 13 are almost a half as long as the distance where one surface acoustic wave crosses the light beam from one end to the other like a conventional device. Consequently, the extent of the attenuation of the surface acoustic waves 12a and 12b by absorption by the waveguide 11 while the waves cross the light beam is suppressed lower than the conventional device.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to an optical modulator, in particular, a light beam guided in an optical waveguide is diffracted by a surface acoustic wave propagated through the optical waveguide, The present invention also relates to a waveguide type optical modulator that modulates the light beam by changing the diffraction efficiency by controlling the intensity of the surface acoustic wave.

(Prior Art) Recently, an optical modulator using an optical waveguide has been attracting attention as an optical modulator that modulates a light beam in, for example, an optical scanning recording device or the like. This optical modulator includes a slab-shaped optical waveguide formed from a material that allows surface acoustic waves to propagate, and a surface acoustic wave that generates in the optical waveguide a surface acoustic wave that travels in a direction that intersects the optical beam guided within the optical waveguide. (for example, a pair of crossed comb-shaped electrodes and a driver for applying an alternating voltage to the pair of electrodes). In this waveguide type optical modulator, a light beam guided in an optical waveguide undergoes Bragg diffraction due to acousto-optic interaction with a surface acoustic wave, and this diffraction efficiency changes depending on the intensity of the surface acoustic wave. , the light beam can be modulated by controlling the intensity of the surface acoustic waves.

In this waveguide type optical modulator, as mentioned above, a pair of interdigitated electrodes (Inte
r D 1g1tal Transducer
%IDT) is used. In this case, the value of the alternating voltage applied to the IDT may be continuously controlled,
Or by controlling voltage application ON-OFF,
The intensity of surface acoustic waves can be controlled.

Conventionally, the IDT has been placed in an optical waveguide at a position laterally away from the light beam guided therein so as not to directly interfere with the light beam.

By the way, in the waveguide type optical modulator having the above configuration, there is a demand for setting the width of the light beam guided in the optical waveguide as large as possible.

In other words, when recording an image by scanning a light beam emitted from an optical waveguide onto a recording material, for example, in order to increase the number of resolution points, it is necessary to increase the width of the light beam and then scan it as described above. This is because it is necessary to narrow down the selection sufficiently.

(Problem to be Solved by the Invention) However, when the width of the light beam guided through the optical waveguide is increased, the ID disposed at the above-mentioned position
Naturally, the distance that the surface acoustic wave generated from T traverses the optical beam becomes large, and during that time, the surface acoustic wave is absorbed by the optical waveguide and is greatly attenuated. The intensity ■ of the surface acoustic wave propagating through the optical waveguide is given by ImI (, e...(1).In other words, the surface acoustic wave intensity I significantly attenuates exponentially as the distance X increases (see Figure 3 (1)).-Blade surface elasticity The diffraction efficiency η of a light beam by a wave is 4m1IIsin2AIv/T'', where the intensity of the surface acoustic wave is ■, and A is a coefficient, and decreases as the surface acoustic wave intensity ■ decreases.Therefore, the surface acoustic wave The diffraction efficiency η of the light beam due to the surface acoustic waves decreases significantly as the distance from the IDT increases.Therefore, the intensity distribution of the light beam before being diffracted by the surface acoustic wave is shown by the solid line in Figure 3 (2). If the light beam has a Gaussian distribution as shown in FIG.

In other words, the surface acoustic waves are greatly attenuated by the time they reach the center position in the width direction of the light beam where the light intensity P is maximum, so the light intensity of the light beam after diffraction becomes quite low overall. . Moreover, the intensity distribution of the light beam after diffraction becomes far from a Gaussian distribution, which poses a problem in sufficiently narrowing down the light beam.

Furthermore, as mentioned above, if the width of the light beam (guided light) is increased, the time required for the surface acoustic wave to traverse it will naturally become longer, which also causes the problem that it becomes impossible to sufficiently increase the modulation speed. arise.

Therefore, the present invention utilizes the above-mentioned ID as a surface acoustic wave generating means.
It is an object of the present invention to provide a waveguide type optical modulator that can solve the above-mentioned problems while using T.

(Means and effects for solving the problems) As described above, the waveguide type optical deflector of the present invention uses an IDT that generates a surface acoustic wave in an optical waveguide to direct a light beam guided through the optical waveguide. It is characterized by being disposed at approximately the center position in the width direction.

If the IDT is placed in such a position, the IDT
The surface acoustic waves generated by
It propagates in two directions from approximately the center in the width direction to the left and right ends, respectively. In this way, the distance that each surface acoustic wave propagating in opposite directions traverses the light beam is approximately 1/2 of the distance that one surface acoustic wave traverses the light beam from end to end as in conventional devices. becomes.

Therefore, the degree to which each surface acoustic wave is absorbed and attenuated by the optical waveguide while traversing the optical beam is suppressed to a lower level than in conventional devices.

Furthermore, if the surface acoustic waves propagate in two directions from the center position in the width direction of the light beam as described above, the time required for the surface acoustic waves to cross the light beam is approximately halved compared to the conventional device.
becomes.

(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 showing a waveguide type optical modulator IO according to an embodiment of the present invention.

For example, this optical modulator 10 also has the function of deflecting a light beam and constitutes an optical scanning recording device.
A slab-shaped optical waveguide 11 formed on a transparent substrate 16, an IDT 15 provided at the center of this optical waveguide 11, and a linear diffraction grating for light incidence provided at a distance from each other on the surface of this optical waveguide 11. (L 1near Gra
tlng Coupler (hereinafter referred to as LGC)
20 and a light emitting LGC21.

Further, a light input prism 30 and a light output prism 31 are attached to the surface lea of the substrate 1B on the side opposite to the optical waveguide 11. 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, with the first light passing surface 30a facing the substrate surface I.
It is tightly fixed to the surface 16a by being strongly pressed by Ela or by using an adhesive with a high refractive index. The light emitting prism 31 has the same shape as the light inlet prism 30, has a first light passing surface 31a1 and a second light passing surface 31b, and is fixed to the substrate surface lea in the same manner as described above. .

In this embodiment, as an example, LiNbO is used on the substrate IB.
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 1B. Regarding optical waveguides, for example, T. Tamir (T, Tam1r)
“Integrated Obtics” (ed.)
rated Optics)J()Pix in Applied Physics(Topics in Ap
plied Physlcs) Volume 7) Springer-Verlag
Published (1975); Co-authored by Nishihara, Haruna, 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 of a material such as the above-mentioned Ti diffusion film that allows surface acoustic waves to propagate, 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 1B, passes through the optical waveguide 11, and enters the L G C 20 formed on the surface thereof. Thereby, the light beam 13 is diffracted by this LGC 20, enters the optical waveguide 11, and travels in the direction of the arrow in the waveguide mode within the optical waveguide 11.

As mentioned earlier, in order to improve the number of resolution points of the recorded image, the width d of the waveguide light ■3° (see Figure 5) is set to 10 to 15.
It is set to be sufficiently wide, about mm.

In addition, the IDT 15 is made up of a large number of electrode fingers whose line widths are changed little by little and are arranged in parallel in the vertical direction in FIG. The parallel length of the fingers (vertical length in FIG. 5) is, for example, about 600 μm. And this IDT
15 is arranged at approximately the center position in the width direction of the guided light 13'.

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

Then, the semiconductor laser ■B is driven by the 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. Note that this drive circuit 1
7 is controlled by a modulation circuit 24, which changes the level of the alternating voltage according to the image signal S or turns the alternating voltage ON-OFF.

By applying the voltage as described above to the IDT 15, the surface acoustic waves 12a112b propagate on the surface of the optical waveguide 11 in the directions of arrows A and B in FIGS. 2 and 5, respectively. The IDT 15 is arranged so that these surface acoustic waves 12aS12b travel in a direction intersecting the optical path of the guided light (parallel beam) 13'. Therefore, the guided light is 13°
propagates across the surface acoustic waves 12a, 12b, and at this time, the guided light 13' passes through the surface acoustic waves 12a, 1
Bragg (B rag
g) Diffraction.

Bragg diffraction of guided light by surface acoustic waves has been known for some time, but will be briefly explained here. An angle between the traveling direction of the surface acoustic waves 12a and 12b propagating in the optical waveguide 11 and the traveling direction of the guided light 13° (Bragg angle)
When θ is the deflection angle (diffraction angle) δ of the guided light 13′ due to the acousto-optic interaction with the surface acoustic waves 12a and L2b,
It becomes δ−2θ. The wavelength of the guided light 13°, the effective refractive index λ and Ne, the wavelengths of the surface acoustic waves 12a and L2b,
If the frequency and speed are Δ, f, and v, respectively, 2θ−2sin' (λ/(2Ne−A))Sλ/(
Ne-A)-λ·f/(Ne-v), and 2θ, that is, δ, is approximately proportional to the frequency f of the surface acoustic waves 12a and 12b. As mentioned above, the drive circuit 17
Since an alternating voltage whose frequency changes continuously is applied to the DT 15, the frequency of the surface acoustic waves 12a, 12b changes continuously, and the deflection angle changes continuously. Therefore, as shown by arrow C, this guided light 13'
It is diffracted and deflected so that the diffraction angle changes continuously. The guided light 13' thus deflected is diffracted by the LGC 2L and exits from the optical waveguide 11 to the substrate 16 side. In this way, the light beam 13'' that is emitted from the optical waveguide 1 and becomes external light is transmitted to the first light passing surface 3 of the light emitting prism 31.
1a, the light enters the prism 31, passes vertically through the second light passing surface 31b, and exits the prism.

The light beam 1 emitted to the outside of the optical waveguide 11 as described above
3'' passes through a scanning lens 26 made of, for example, an fθ lens, is focused into a small beam spot Q, and scans (main scan) over the photoreceptor 23 in the direction of arrow U.

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 front beam I3''. As mentioned above, the level or ON-OFF of the alternating voltage applied to the IDT 15 is controlled based on the image signal S, so the surface acoustic wave 12a
, 12b changes (including ON-OFF).

Therefore, the diffraction efficiency of the guided light 13' by the surface acoustic waves 12a and 12b changes, and the light beam 13'' is modulated based on the image signal S.
3, an image carried by this image signal S is recorded.

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.
What is necessary is to control the timing of voltage application to IDTL5.

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 a characteristic part of the present invention, as mentioned above, the IDT
15 is arranged at approximately the center position in the width direction of the guided light 13'. Therefore, the distance that the surface acoustic waves 12a and 12b propagating in opposite directions from the IDT 15 cross the guided light 13' is the same as the distance that one surface acoustic wave crosses the guided light 13' from the end, as in the conventional device. This is approximately 1/2 of the distance to cross to the end. Therefore, the attenuation characteristics of the surface acoustic waves 12a and 12b in this case are (1) in FIG.
). In other words, compared to the case where the surface acoustic waves are propagated toward the guided light from outside the guided light as in the conventional device, each surface acoustic wave 12a, 12
The absorption of the surface acoustic waves 12a and 12b in the optical waveguide 11 is suppressed by the short distance that b crosses the guided light 13', and the surface acoustic waves 1 in the range where they intersect with the guided light 13' are suppressed.
2a.

12b becomes less attenuated. Also, the surface acoustic wave 12a
, 12b is maximum at the center of the guided light 13°, so the light intensity distribution of the guided light 13° before diffraction becomes a Gaussian distribution as shown by the solid line in Figure 4 (2). In this case, the diffraction efficiency η is maximum at the portion where the light intensity is highest. Therefore, in this case, the light intensity distribution after the diffraction of the guided light 13'' is such that the high light intensity in the central portion is not attenuated so much, as shown by the broken line in FIG. 4(2). ′ becomes sufficiently high.

Moreover, since it is as described above, the surface acoustic waves 12a and 12b
The time required for the surface acoustic wave to completely cross the guided light 13' is also approximately 1/2 of the time required for the surface acoustic wave to propagate toward the guided light from outside the guided light. therefore,
When the level of the alternating voltage applied to the IDT 15 is changed or this alternating voltage is turned off, the time required for the intensity of the diffracted waveguide light 13' to change accordingly becomes shorter. The modulation speed of the wave light 13' can be made sufficiently high. In other words, if there are no other limiting conditions, this modulation speed can be approximately twice that of the case where the surface acoustic wave is propagated from outside the guided light toward the guided light. .

Hereinafter, the attenuation of the guided light 13' will be explained using specific numerical examples. When the ultrasonic absorption coefficient α in equation (1) mentioned above is 3.0d when the surface acoustic wave frequency f=1.0GHz
B/cm, and assuming that the width d of the guided light 13' is IQmm, the surface acoustic waves 12a and 12b of the intensity IO generated from the IDT 15 will be 0 at the side end of the guided light 13'.
．． 7d. The strength will be . On the other hand, under similar conditions, a surface acoustic wave is applied from the outside of the guided light 13' to the guided light 13'.
When the surface acoustic wave propagates toward the guided light 13'
When the surface acoustic wave is emitted from a position touching one side end of the waveguide, the surface acoustic wave intensity I at the position where the surface acoustic wave completely crosses the guided light 13' is attenuated to 0.51o.

- The speed V of the blade surface acoustic wave 12aS12b is 3463 m/
see, the width d of the guided light 13' is 10 mm as above.
(=10-2m), the surface acoustic wave 12a,
The time required for light 12b to completely pass through guided light 13' is 10'/(3463X2)-1,45xlO-6=1
．． 45. czse( becomes.

Also, as shown in the figure, the light intensity distribution of the guided light 13゛ after diffraction is as follows:
As with the one before diffraction, the distribution becomes approximately Gaussian, and the light beam 13'' emitted from the optical waveguide 11 can be narrowed down to a sufficiently small beam spot Q by the scanning lens 26.

Note that the optical modulator IO of the embodiment described above is formed so as to also have an optical deflection function, but the IDT1
If the frequency of the alternating voltage applied to the waveguide 5 is not swept, the diffraction angle of the guided light 13' by the surface acoustic waves 12a and L2b will be constant, and an optical modulator that only performs optical modulation can be obtained.

(Effects of the Invention) As explained in detail above, in the waveguide type optical modulator of the present invention, the IDT that generates a surface acoustic wave is disposed at approximately the center position in the width direction of the guided light. Attenuation of surface acoustic waves in the intersecting range can be suppressed to a low level. Therefore, with this optical modulator, the light utilization efficiency can be sufficiently increased, and the width of the guided light can be increased, so that the number of resolution points in optical scanning recording can be increased.

In the optical modulator of the present invention, the ID is located at the above position.
By arranging the T, the time required for the surface acoustic wave that diffracts the guided light to cross the guided light becomes shorter, so that the modulation speed can be sufficiently increased.

Furthermore, in the optical modulator of the present invention, by arranging the IDT at the above-mentioned position, if the light intensity of the guided light before diffraction has a Gaussian distribution, the light intensity distribution of the guided light after diffraction also has a Gaussian distribution. Since it is possible to form a Gaussian distribution, it is possible to perform precise scanning by concentrating the beam emitted from the optical waveguide into a sufficiently small spot.

[Brief explanation of the drawing]

1 and 2 are a side view and a schematic perspective view, respectively, showing a waveguide type optical modulator according to an embodiment of the present invention, and FIG. 3 (1) and FIG. Figure 4 is a graph showing the attenuation characteristics of surface acoustic waves in a waveguide type optical modulator and the light intensity distribution characteristics before and after diffraction of guided light.
1) and FIG. 4(2) are graphs respectively showing the attenuation characteristics of the surface acoustic wave in the optical modulator of the present invention and the light intensity distribution characteristics before and after diffraction of the guided light, and FIG. 5 is the graph showing the above-mentioned example. FIG. 3 is a plan view showing an enlarged portion of the IDT installed in the optical modulator of FIG.

Claims (1)

[Claims] It has a pair of intersecting comb-shaped electrodes that generates in the optical waveguide a surface acoustic wave that propagates in a direction intersecting the optical path of the optical beam guided in the optical waveguide and diffracts the optical beam, and an alternating voltage is applied to the electrode pair. In the waveguide type optical modulator that modulates the light beam by changing the diffraction efficiency of the light beam by controlling the application and changing the intensity of the surface acoustic wave, the intersecting comb-shaped electrode pair A waveguide-type optical modulator, characterized in that it is disposed at approximately the center position in the width direction of a light beam guided through an optical waveguide.