JPH0431805A - Method for inputting light to optical waveguide and method for outputting light therefrom - Google Patents

Abstract

PURPOSE:To make the width of guided light sufficiently narrower so as to realize a high light inputting efficiency by making the cross section of an optical beam used for irradiating a diffraction grating elliptic, with the diameter in the width direction being shorter than that in the direction perpendicular to the width direction. CONSTITUTION:A parallel optical beam 16 emitted from a laser light source 17 is made incident to a linear grating coupler (LGC) 14 for light incidence through the obliquely-cut end face 11a of a substrate 11 and optical waveguide 12. At the time of the incidence, the optical beam 16 is only condensed in the direction parallel to the grating width of the LGC 14 through a cylindrical lens 7 and becomes to have an elliptic cross section with a minor diameter 2ws in the width direction of the LGC 14 and major diameter 2wL in the direction perpendicular to the width direction at the location where the beam 16 is made incident to the LGC 14. Therefore, the width of the guided light can be reduced sufficiently and, at the same time, the light inputting efficiency to the optical waveguide can be improved.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to a method of inputting light into an optical waveguide, and more particularly, the present invention relates to a method of inputting light into an optical waveguide, and in particular, irradiating a light beam onto a diffraction grating formed on the surface of an optical waveguide, The invention relates to a method of diffracting a beam therein and inputting the beam into an optical waveguide.

The present invention also relates to a method of diffracting guided light using a diffraction grating as described above and outputting the diffraction light to the outside of an optical waveguide.

(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, the modulation speed is limited by the time required for the surface acoustic wave to cross the guided light.

Therefore, in this optical modulator, there is a demand for setting the width of the guided light as small as possible in order to increase the modulation speed.

On the other hand, one method for inputting a light beam into the optical waveguide as described above is to form a diffraction grating on the surface of the optical waveguide.
A known method is to irradiate a light beam thereon and cause it to diffract. It is also widely practiced to diffract a light beam guided in an optical waveguide by a diffraction grating as described above and output it outside the optical waveguide (for example, see Japanese Patent Laid-Open No. 1-107214).

When implementing such a light input method or a light output method, the coupling length can be shortened by increasing the grating height, but there is a limit to this. In other words, even if the height of the grating is made higher than the percolation of guided light power into the diffraction grating, strong coupling will not occur and the coupling length cannot be shortened.

Therefore, a situation arises in which the coupling length is long compared to the guided light width required for high-speed modulation.

(Problem to be Solved by the Invention) However, in the case of optical input, it is known that the optical input efficiency can be maximized by making the input beam diameter and the coupling length approximately equal. For this purpose, the diameter of the light beam irradiated onto the diffraction grating must also be set large. This is contrary to the above-mentioned requirement that the width of guided light be made smaller for high-speed modulation.

On the other hand, for optical output, if the width of the guided light is made as small as possible for high-speed modulation, the diameter of the light beam emitted outside the optical waveguide in the width direction of the diffraction grating is shorter than the diameter in the direction perpendicular to it. , resulting in a flat cross-sectional shape.

Although a modulated light beam is often required to be focused into a perfect circular spot, it is impossible to focus a light beam having a flat cross-sectional shape as described above.

The present invention has been made in view of the above-mentioned circumstances, and provides a method for inputting light to an optical waveguide, which can sufficiently reduce the width of guided light and achieve high optical input efficiency. The purpose is to

Furthermore, the present invention provides a method for efficiently outputting guided light having an extremely narrow beam width out of the optical waveguide, and furthermore, making it possible to focus the light beam into a circular spot. The purpose is to provide an output method.

(Means and Effects for Solving the Problems) As described above, the method of inputting light into an optical waveguide according to the present invention is to irradiate a light beam onto a diffraction grating on the surface of a slab-shaped optical waveguide and diffract it there. This method is characterized in that the light beam irradiated onto the diffraction grating has a flat cross-sectional shape in which the diameter in the width direction of the diffraction grating is shorter than the diameter in the direction perpendicular thereto.

By setting the cross-sectional shape of the light beam as described above, the coupling length between the light beam and the diffraction grating can be made sufficiently long, and regardless of this, the width of the guided light must be made sufficiently small. I can do it.

On the other hand, the method of outputting light from an optical waveguide according to the present invention is a method in which a light beam guided in a slab-shaped optical waveguide is diffracted by a diffraction grating formed on the surface of the optical waveguide and outputted to the outside of the optical waveguide. After setting the coupling length of the diffraction grating to be longer than the width of the guided light, the optical beam with a flat cross section emitted from the optical waveguide is passed through a beam shaping optical system to be shaped into an approximately circular cross section. This is a characteristic feature.

By setting the coupling length of the diffraction grating as described above, it is possible to satisfy the requirement of reducing the width of guided light and also meet the requirement of weak coupling. Then, by shaping the light beam with a flat cross section emitted from the optical waveguide as described above,
The light beam can be focused into a perfect circular spot.

(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 of an optical waveguide device 10 in which light input and light output are performed according to the method of the present invention. This optical waveguide element 10 constitutes an optical modulator for a recording device, as an example, and includes a slab-shaped optical waveguide 12 formed on a transparent substrate 11, and this optical waveguide 1.
2, and a linear diffraction grating for light incidence (L 1near grating couple) provided at a distance from each other on the surface of this optical waveguide 120.
r: hereinafter referred to as LGC) 14 and light emitting LG
C15. Note that the IDT 13 has an inclined finger chirved interdigitated electrode pair (T i
Ited-Finger ChirpedIDT
).

In this embodiment, as an example, the substrate 11 has LiNbO
The optical waveguide 12 is formed by using three wafers and providing a T1 diffusion film on the surface of the wafer. Note that substrate 1
1 may also be a crystalline substrate made of sapphire, Si, or the like. Furthermore, the optical waveguide 12 is not limited to the above-mentioned Ti diffusion, but can also be formed by sputtering or vapor depositing other materials on the substrate 11. Regarding optical waveguides, for example, Taa+ir (T, Taa+ir)
) edition [Ingrated Optics (I nte
Grated Optics) J (Topics in
Applied Physics (Topics in A)
pplied Physics) Volume 7) Springer-Verlag
) (1975); "Optical Integrated Circuits" co-authored by Nishihara, Haruna, and Suhara, published by Ohmsha (1985), etc., have detailed descriptions, and in the present invention, these known optical waveguides can be widely used as the optical waveguide 12. It is.

However, this optical waveguide 12 is formed from 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.

Incidentally, the above-mentioned inclined finger chirve IDT 13 is manufactured by, for example, applying a positive electron beam resist to the surface of the optical waveguide 12, further depositing an Au conductive thin film thereon, drawing an electrode pattern with an electron beam, and developing after peeling off the Au thin film. After depositing a Cr thin film and an AI thin film, the film can be formed by performing lift-off in an organic solvent.

A laser light source 17, such as a He-Ne laser, emits a modulated light beam 16, which emits a modulated light beam 16.
6 is arranged so as to pass through the obliquely cut end surface 11a of the substrate 11, pass through the optical waveguide 12, and enter the LGC14 portion. Thereby, the light beam 1
6 is diffracted by this L G C 14, enters the optical waveguide 12, and travels in the direction of arrow A within the optical waveguide 12 in a waveguide mode.

Note that the light beam 16 is passed through a cylindrical lens 7 and subjected to beam shaping, which will be described later. Furthermore, in this embodiment, the LiNbO3 substrate 11 is x-cut or y-cut.
t is used, and the waveguide direction (arrow direction) is the Z-axis direction.

FIG. 3 shows the surrounding portion of the IDT 13 and the electric circuit. As shown in the figure, the tilted finger chirp IDT 13 is connected to an RF amplifier 70, an attenuator 20, a multiplexer 21, and four high-frequency oscillators 31, 32, 3, for example.
3.34 are connected in parallel. A switching circuit 41.42.43.44 is interposed between each of these high frequency oscillators 31.32.33.34 and the multiplexer 21. High frequency oscillator 31.32.83 above
．． 34 are respectively different frequencies f1, T2,
High frequency voltages RF1, RF2, RF3, RF4 of T3, '4 (fl<fz<T3<T4) are generated. These high frequency voltages RF1 s RF 2,
RF3, RF are switching circuits 41, 42, respectively.
If 43.44 is in the closed state, the inclined finger chirve I
Applied to DT13.

The inclined finger chirp IDT13 receives the above-mentioned high frequency voltage RF1.
, RF2, RF8, and RF4 are applied to generate surface acoustic waves 1 with frequencies f□, T2, T3, and T4, respectively.
Generates 8. Note that if a plurality of high frequency voltages RF, RFz, RF3, and RF4 are simultaneously applied to the inclined finger chirve IDT 13, the surface acoustic waves 18 become frequency-superimposed waves. For example, high frequency voltage RFz, RF2%
RF3, RF.

are all simultaneously applied to the tilting chart IDTl3, the surface acoustic waves 18 will have frequencies f1, f2, f3 and f
This results in a superposition of l.

The tilting chirve IDT 13 has the above-mentioned surface acoustic wave 18,
It is arranged so as to travel in a direction intersecting the optical path of the light beam (guided light) 1B. The light beam 1B thus travels across the surface acoustic wave 1B, while the light beam 16 undergoes Bragg diffraction due to acousto-optic interaction with the surface acoustic wave I8.

Bragg diffraction of guided light by surface acoustic waves has been known for some time, but will be briefly explained here. If the angle between the traveling direction of the surface acoustic wave 18 propagating through the optical waveguide 12 and the traveling direction of the optical beam IB (B ragg angle) is θ, then the rotation of the optical beam 16 due to the acousto-optic interaction with the surface acoustic wave 18 is The angle δ becomes δ−2θ. If the wavelength and effective refractive index of the forward beam 16 are λ and Ne, and the wavelength, frequency, and velocity of the surface acoustic wave 18 are A, f, and v, respectively, then 2θ−2sin' (λ/(2Ne ・Δ)) Yu λ/
(Ne・Δ) -λ・f/(Ne −v), which is 20
In other words, δ is approximately proportional to the frequency f of the surface acoustic wave 18.

Therefore, a high frequency voltage R is applied to the tilting chart IDT13.
By applying FI, RFz, RF3, and RF4, lea, 16b, 16c, and l are shown in FIG. 2, respectively.
As shown by ad, diffracted lights traveling in mutually different directions are generated.

As shown in FIG.
The light is diffracted at C15 and exits from the obliquely cut end face flb of the substrate 11.

The switching circuits 41, 42, 43, and 44 described above open and close based on the input modulation signals S1, S2, S3, and S4, respectively. Therefore, the high frequency voltage RFL, R
F2, RF9, RF, each tilt technique chart IDT
13 are these modulated signals S1, S2, S3,
Ite 0N-OFF control is performed based on S4. Thereby,
Each frequency f1, f2, f3, f of the surface acoustic wave 16
4 component is turned 0N-OFF, and as a result, the light beams 16a, 1
6b, L6c, led respectively signal S1, S2, S3, S
0N-OFF modulation is performed based on 4.

Next, an example of a recording device using the optical waveguide element 10 will be described with reference to FIG. 4. This recording device 50
consists of a light modulation device including the optical waveguide element 10 described above, a laser light source 17 as a recording light source that makes the light beam 16 enter the light modulation device, and a rotating polygon mirror 5I as an optical deflector. , a cylindrical platen 53 that holds a photosensitive material 52, which is an example of a recording medium, and a sub-scanning motor 5 that rotates this platen 53 in the direction of arrow Y.
4. Although not shown in FIG. 4, the optical modulation device is driven by the electric circuit shown in FIG. 3. That is, the modulation control circuit 29 shown in FIG.
A recording signal SP is input to the modulation control circuit 29, and the modulation control circuit 29 outputs modulation signals 51 to S4 based on the recording signal SP in parallel.

Light beam (diffraction light) emitted from the optical waveguide element 101.
6a, 16b, 18c, and 16d pass through a cylindrical lens 8.9.19, which will be described later, and then turn into a rotating polygon mirror 51.
The light beam is reflected and deflected by , passes through a scanning lens 55 made of an fθ lens, etc., and main-scans over the photosensitive material 52 in the direction of arrow X. Note that the rotating polygon mirror 51 is disposed so that the light beams 1ea to 16d main scan the photosensitive material 52 in a direction substantially perpendicular to the direction in which they are lined up.

In this way, the main scanning of the light beams lea to led is performed, and the rotation of the platen 53 moves the photosensitive material 52 in the Y direction substantially perpendicular to the main scanning direction X, so that the light beams 1.6a to A sub-scan of 16d is performed. In this way, the photosensitive material 52 is two-dimensionally scanned by the light beams IBa to 16d, and these light beams 16
By ON-OFF modulating signals a to 16d as described above, the content carried by the recording signal SP (see FIG. 3) is recorded on the photosensitive material 52.

Here, as shown in FIG. 1.2, the light beam 16 is
Passed through the aforementioned cylindrical lens 7, L G C
The light is focused only in the direction parallel to the grating width direction of 14. As a result, the light beam 16 at the position where it enters the L G C14
is an elliptical cross-sectional shape with the width direction of L G C14 being the minor axis 2WS and the direction perpendicular to it being the major axis 2WL (9 in Figure 1).
The direction is changed by 0°, and it has a diagonally shaded portion D1). In this embodiment, the diameter of the light beam 16 is 800 mm.
It is assumed to be μm. Therefore, the major axis 2WL is 800 μm and the minor axis 2WS is 130 μm at the position of incidence on the LGC14. The above valley diameter is
The position where the value is 1/e2 of the maximum light intensity is defined as the beam periphery.

The width of the guided light 16 in the optical waveguide element 10 is equal to the short axis 2WS, which is 130 μm. When the guided light 16 is modulated by the surface acoustic wave 18, if the response time is τ, then
Since its minimum value is limited to τ-2w5/1.5 v, by making the width of the guided light 1B extremely narrow as described above, response time can be shortened, that is, the modulation speed can be increased. .

L G C14 grid height is 650 people, grid pitch is 3
μm, and when the incident angle of the light beam 16 to the LGC 14 is 65°, the relationship between the beam diameter in the direction perpendicular to the LGC width direction and the overlap integral is as shown in FIG. As shown in the figure, when the beam diameter in the above direction is 800 μm, the value of the overlap integral is 0. &0.

This overlap integral is explained in detail in the aforementioned work by Nishihara et al.

Furthermore, when the wavelength of the light beam 16 is around 830 nm, L
The power distribution ratio of GC14 is approximately 0.74. Since the light input efficiency in the diffraction grating is the value obtained by multiplying the overlap integral by the power distribution ratio, the light input efficiency in the L GC 14 of this embodiment is approximately 59%.

On the other hand, as has been done in the past, L G
The light beam irradiated to C14 has a perfect circular cross section,
In order to enable high-speed modulation, the beam diameter was set to 130μ.
When it is set to m, the overlap integral becomes 0.28, as shown in FIG. Therefore, the light input efficiency in that case is about 20%, which is significantly lower than about 59% in this embodiment.

The light output LG C15 is the pre-input LG described above.
It is formed to the same specifications as C14. Therefore, its coupling length is sufficiently longer than the width 2Ws = 130 μm of the guided light 1θ, and the light beam L[
1a to 1d have an elliptical cross-sectional shape (turned by 90 degrees in FIG. 1 and shown as a hatched portion D2). These light beams 16a-d are focused by cylindrical lenses 8 and 9 only within the plane shown in FIG. Perfect circular shape (first
In the figure, the direction is changed by 90°, and the result is shown by a diagonally shaded area D3).

As a result, the light beam 16a incident on the scanning lens 55
-d can be narrowed down to a sufficiently small beam spot by this simple lens 55.

The beam shaping optical system used in the light input method and light output method of the present invention is the cylindrical lens 7 or the cylindrical lens 8.9.1 of the above embodiment.
9, other examples such as anamorphic prism bears can also be used. moreover,
Such a cylindrical lens or anamorphic prism bear may be used in combination with a spherical lens for beam diameter adjustment.

Furthermore, in the above embodiments, an LGC is used as a diffraction grating, but the method of the present invention uses a focusing diffraction grating (Focus grating) as a diffraction grating for light input or light output.
The present invention is also applicable in the same way even when a grading coupler is used.

(Effects of the Invention) As explained in detail above, the method of inputting light into an optical waveguide according to the present invention is such that the diameter of the diffraction grating in the width direction is shorter than the diameter in the direction perpendicular to it. By having a flat cross-sectional shape, the coupling length between the light beam and the diffraction grating can be made sufficiently long, while the width of the guided light can be made sufficiently small.

Therefore, according to this method, the efficiency of light input to the optical waveguide is increased, and when the optical waveguide constitutes an optical modulator, for example, it is possible to increase the modulation speed.

On the other hand, in the method of outputting light from an optical waveguide according to the present invention, the coupling length of the diffraction grating is set longer than the width of the guided light, and then
Since the light beam with a flat cross section emitted from the optical waveguide is passed through a beam shaping optical system and shaped into a nearly perfect circular cross section, it is possible to meet the requirement of reducing the width of the guided light and to pass it through a beam shaping optical system. The emitted light beam can be made to have a circular cross-section or one close to it.

Therefore, according to this method, for example, when an optical waveguide constitutes an optical modulator, the modulation speed can be increased, and the optical beam emitted outside the optical waveguide can be reduced to a sufficiently small amount using a simple optical system. It is also possible to narrow down your search to specific spots.

[Brief explanation of the drawing]

1 and 2 are a side view and a perspective view, respectively, showing an example of an optical waveguide device for carrying out the method of the present invention, and FIG. 3 is a schematic diagram showing the main parts and electric circuit of the optical waveguide device, FIG. 4 is a perspective view showing an example of an optical scanning recording device using the optical waveguide element, and FIG. 5 is a graph showing the relationship between the overlap integral and the incident beam diameter in the diffraction grating of the optical waveguide element. be. 7.8.9.19...Cylindrical lens 10...
- Optical waveguide element 11...Substrate I2...Slab-shaped optical waveguide 13...IDT14...LGC15 for light input...LGC1B for light output, 18axd...
・Light beam 18...Surface acoustic wave Figure 1 Figure 3 Figure 2 Figure 2 Figure input beam

Claims (2)

[Claims] (1) In a method in which a light beam is irradiated onto a diffraction grating formed on the surface of a slab-shaped optical waveguide, and the light beam is diffracted and input into the optical waveguide, the light beam irradiated onto the diffraction grating is beam-shaped. A method for inputting light to an optical waveguide, characterized in that the diameter of the diffraction grating in the width direction is shorter than the diameter in the direction perpendicular to the diffraction grating. (2) In a method in which a light beam guided in a slab-shaped optical waveguide is diffracted by a diffraction grating formed on the surface of the optical waveguide and outputted from the optical waveguide, the coupling length of the diffraction grating is A method of outputting light from an optical waveguide, characterized in that the width of the optical waveguide is set to be longer than the width of the optical waveguide, and then the optical beam with a flat cross section emitted from the optical waveguide is passed through a beam shaping optical system to be shaped into a substantially circular cross section. .