JPS59192233A - Optical deflector of waveguide type - Google Patents

Abstract

PURPOSE:To extend a frequency band and eliminate the dependence of delay time upon frequency by constituting an optical deflector so that the decrease or increase rate of intervals of reflective elements for the propagation distance of a surface acoustic wave is equal to that of intervals of electrode fingers of a chirp reed screen type electrode. CONSTITUTION:The surface wave reflected from a chirp reed screen type electrode 11 is reflected by a reflective grating 12 and reaches an optical beam 13. In this case, the decrease or increase rate of intervals of electrode fingers is equal to that of reflective elements. Therefore, a surface wave 14 having a low frequency which is radiated in a position before electrode fingers is turned back before the reflective grating 12 and reaches the optical beam 13. A surface wave 15 having a high frequency which is radiated from the deepest position of electrode fingers is reflected by the depth of the reflective grating and reaches the optical beam 13. Consequently, surface waves radiated from electrode fingers reach the optical beam approximately simultaneously in the frequency band to perform Bragg diffraction, and the optical deflector whose delay time is not dependent upon the frequency is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical deflector using surface acoustic waves used in a photoacoustic signal processing device.

Waveguide optical deflectors using surface acoustic wave (SAW) converters are being studied in various ways as a means of integrating, miniaturizing, and improving the performance of optical/acoustic signal processing devices such as real-time spectrum analyzers and correlators. is being done. In such optical deflectors, improving deflection efficiency and widening the frequency band are currently extremely important themes.

To date, a type using chirped interdigital electrodes in which the spacing between electrode fingers is gradually changed along the SAW propagation direction has been proposed to widen the band. In the chirped interdigital electrode, the spacing between the electrode fingers gradually changes, and each electrode finger is tilted relative to each other so that the plug condition with the light beam is satisfied at each frequency for use as a deflector. Therefore, the frequency characteristics of the deflector become almost flat, and a wide band can be easily realized. However, since the time from when the SAW is excited until it intersects with the optical beam varies depending on the frequency, signal processing devices that require -like delay (linear phase) frequency characteristics,
For example, it is unsuitable for application to devices such as collators.

Therefore, it would be extremely useful if a waveguide optical deflector with a wide frequency band and a delay time independent of frequency could be obtained.

An object of the present invention is to provide a waveguide optical deflector that has a wide frequency band and whose delay time does not depend on frequency as described above.

The waveguide type optical deflector of the present invention includes a chirped interdigital electrode on a planar optical waveguide in which the spacing between the electrode fingers gradually decreases or increases along the electrode finger width direction, and a chirped interdigital electrode that emits light from the chirped interdigital electrode. a reflection grating that reflects the generated surface acoustic waves so as to intersect with the optical beam propagating in the planar optical waveguide at a Bragg angle; The surface acoustic wave emitted from the chirped sag electrode gradually decreases or increases along the propagation direction, and the rate of decrease or increase of the reflective element interval with respect to the surface acoustic wave propagation distance becomes the chirped sagging structure. There is.

Next, the present invention will be explained with reference to the drawings.

FIG. 1 is a three-dimensional diagram showing an embodiment of the waveguide type optical deflector according to the present invention, in which 1 is a substrate, 2 is a planar optical waveguide provided on the substrate, and for example, T1 is diffused onto a LiNbO3 substrate. It can be obtained by 3 is a chirped interdigital electrode in which the spacing between electrode fingers gradually decreases or increases along the electrode finger width direction; 4 is a reflection grating that reflects the surface waves emitted from electrode 3; for example, a groove is dug in the substrate; It is formed by metal strips made of metal vapor-deposited films. The spacing between the reflective elements constituting the reflective grating gradually decreases or increases along the direction of incidence of the surface waves. The rate of decrease or increase of this reflective element spacing with respect to the propagation distance of surface waves is:
This is the same as the chirped interdigital electrode 3. Interdigital electrode 3
The surface acoustic wave 5 radiated from the surface and reflected by the reflection grating 4 is
The light beam 6 propagating within the planar optical waveguide is subjected to plug diffraction, and as a result, polarized light 7 is obtained. There are various means for propagating the incident light 8 into a waveguide and for taking out the polarized light 7, and for example, prisms such as 9 and 10 are used.

Figure 2 is an enlarged view of the chirped interdigital electrode and reflection grating in Figure 1. Reference numeral 11 is a chirp interdigital electrode in which the electrode finger spacing gradually decreases or increases along the electrode finger width direction. The radiated surface acoustic waves are reflected by the reflection grating 12
and reaches the light beam 13. In this case, the rate of decrease or increase in the electrode finger spacing is the same as that of the reflective element. That is, the center frequency of the electrode is f.

, frequency band Δf1 delay dispersion ΔT1 surface wave propagation velocity v
If fz, then the distance X of the i-th electrode finger from the leftmost electrode finger is as follows, for example, when the electrode finger interval decreases sequentially. However, f, =: fo - Δf/2.

Therefore, the position of each reflective element forming the reflective grating is also described by the above-mentioned parameters, and the distance yI from the leftmost reflective element to the jth reflective element is as follows.

Furthermore, in order for the surface acoustic wave and light beam reflected in the perpendicular direction by the reflection grating to satisfy the plug diffraction condition within the frequency band, the angle θ3 of the j-th reflection element with respect to the propagation direction of the incident surface acoustic wave is as follows. It becomes like the expression.

Here, also, λ0. is the wavelength of the optical beam, ne is the refractive index of the optical waveguide, and v3! , vsx are the velocities of the surface waves propagating in the reflection grating in the incident direction and reflection direction, respectively.

When the electrode fingers and reflection elements are determined as described above, the low frequency surface wave 15 emitted at a location in front of the electrode finger is folded back in front of the reflection grating and reaches the light beam J3.

Furthermore, the high-frequency surface wave 15 emitted from the innermost part of the electrode finger is also reflected at the innermost part of the reflection grating and reaches the light beam 13. Therefore, the surface waves emitted from the electrode fingers reach the optical beam almost simultaneously within the frequency band, and an optical deflector whose delay time does not depend on frequency can be obtained. Furthermore, if the slope of the reflective element is set as shown in equation (3),
Plug diffraction conditions are always satisfied between the reflected surface wave and the optical beam within the frequency band, and a broadband deflector can be easily realized.

Note that the angle of the reflective element with respect to the incident surface wave differs for each reflective element as shown in equation (3), but especially the fractional band (Δf/f
When o) is large and ΔT is small, the angular difference between adjacent elements also becomes large. In such a case, the spacing between the reflective elements is not constant along the length of the elements. Therefore, if the width of the element is constant, the frequency of the reflected surface wave will vary along the length of the element.

FIG. 3 is a diagram showing a method for determining the width of the reflective element so that the frequency of the reflected surface wave does not change along the length of the reflective element in such a case. In other words, if the angle between the center line O1-〇□ parallel to the propagation direction of the incident surface wave and the ji-th reflecting element 21 is θj-1, and the angle with the first reflecting element 22 is θJ, then the fractional band Δf When /fo is large and ΔT is small, the difference between θ, -1 and θ becomes large, and therefore the spacing between both reflective elements varies significantly in the length direction of the element.

However, as shown in the figure, when the distance between the reflective elements 21 and 22 is d1 and the element width is ω, if the following formula is satisfied, then the length direction of the reflective element is always (
This surface wave of frequency f is reflected. Here, vm z is the velocity of a surface wave propagating on a reflective element, for example, on a metal film in the case of a metal slab IJ tube. When formula (6) is satisfied, the width of the reflective element is not constant in the length direction. but,
In this case, even when the fractional bandwidth Δf/fo is large and ΔT is small, the frequency of the reflected surface wave does not change, and therefore a waveguide optical deflector with better frequency characteristics can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a three-dimensional diagram showing an embodiment of a waveguide type optical deflector according to the present invention, which includes 1 substrate, 2. Planar optical waveguide, 3-chave interdigital electrode,
4. surface acoustic wave reflection grating, 51 surface acoustic wave, 6. light beam, 7. Polarized light, 8. Incident light, 9.10. The prism is shown, and Figure 2 is an enlarged view of the chirped interdigital electrode and reflection grating. 11. Chirped interdigital electrode, 129 reflection grating.
light beam, 14. Low frequency surface acoustic waves, 15. High-frequency surface acoustic waves are shown. Figure 3 is an enlarged view of the reflection grating, 2], , , 22. Reflective element is shown. A patent attorney in the hole Internal pressure π Shino 1 Figure'f 2 (i) 13 Spear 3 Mouth 2

Claims (1)

[Claims] A chirped interdigital electrode is provided on the planar optical waveguide, and the spacing between the electrode fingers gradually decreases or increases by 1 along the electrode finger width direction, and the surface acoustic waves emitted from the chirped interdigital electrode are transferred to the planar optical waveguide. A reflection grating is provided so as to intersect (reflect) the light beam propagating in the wave path at a Bragg angle, and further, the interval between each reflection element constituting the reflection grating is
The surface acoustic wave emitted from the chirped interdigital electrode gradually decreases or increases along the propagation direction, and the rate of decrease or increase of the reflective element spacing relative to the surface acoustic wave propagation distance is the electrode finger spacing of the chirped interdigital electrode. A waveguide type optical deflector characterized in that the rate of decrease or increase is the same as that of .