JPS59192232A - Optical deflector of waveguide type - Google Patents

Abstract

PURPOSE:To extend a frequency band and to eliminate the dependence of delay time upon frequency by forming a curved reed screen type electrode so that intervals of intersections between the normal passing an optional point of a center electrode finger and the other electrode fingers traversed by this normal are constant. CONSTITUTION:An optical beam 5 propagated in a plane waveguide 2 is subjected to Bragg diffraction with a surface acoustic wave 4 radiated by a curved reed screen type electrode 3, where electrode fingers are curved in the lengthwise direction, to obtain a deflected light 6. In this case, a reed screen type electrode 11 is so formed that intervals of electrode fingers of this electrode 11 are decreased or increased gradually and are approximately constant in the propagation direction of the surface acoustic wave. Consequently, the time when the surface acoustic wave radiated from the reed screen type electrode 11 reaches an optical beam 12 is hardly different between a case 13 of a low frequency f1 and a case 14 of a high frequency f2 and is not dispersed. Thus, the frequency band is extended, and the dependence of delay time upon frequency is eliminated.

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. Currently, in such optical deflectors, improving deflection efficiency and widening the frequency band are extremely important themes.

In order to widen the band, a type using chirped electrodes in which the spacing between electrode fingers is gradually changed along the SAW propagation direction has been proposed. In the chirp electrode, the electrode finger spacing gradually changes, and each electrode finger is tilted to each other so that the plug condition with the light beam is satisfied at each frequency.
The frequency characteristics of the deflector become almost flat, making it easy to achieve a wide band. However, since the time from when the SAW is excited until it intersects with the original beam differs depending on the frequency, it cannot be applied to signal processing devices that require uniform delay (linear phase) frequency characteristics, such as correlators. is not suitable. Therefore, an optical deflector is desired that has a wide band and has no frequency dependence in the time taken to intersect with the optical beam.

Another example of broadband expansion is a type that uses curved interdigital electrodes in which the electrode fingers are curved in the longitudinal direction.
This was proposed by Mr. Tsai (C0S), and was published by Transaction Action on Circuits and Systems (Tr) published by IBEB.
Answer on C1rcuitsand
Systems) magazine, December 1979 issue, page 1072.
Acoust Optic Plug Module Lakes for Wide Band Inch Gradient Optics Communication and Fist Signal Process In'y'' (Gu 1ded -WaveA
coustooptic Bragg Modulat
ors for Wide-Band Integr
ated 0ptic Comrllunicati
ons and Signal Processing
) shows a diagram of a curved interdigital electrode as an example of a broadband optical deflector.

This figure shows that the electrode finger spacing of the curved interdigital electrode changes gradually in the length direction and is almost constant in the SAW propagation direction, resulting in so-called non-dispersive deflection with a wide band and delay time independent of frequency. It looks like it would be easy to implement the device. However, in principle, it is necessary to satisfy the plug condition for SAWs of all frequencies in the band and to keep the electrode finger spacing constant in the SAW propagation direction so that broadband and non-dispersion can be realized. Therefore, some kind of approximate construction method is necessary, but none have been proposed, including the cited document by Mr. Soai mentioned above.

Therefore, it would be extremely useful if a method for configuring curved interdigital electrodes with a wide frequency band and a delay time independent of frequency could be obtained.

The object of the present invention is to provide such a structure of curved interdigital electrodes, and the waveguide type optical deflector of the present invention has electrode fingers curved in the longitudinal direction on a thin film optical waveguide and has a finite frequency band. In the waveguide type optical deflector, a curved interdigital electrode is provided, and a light beam propagating in the thin film optical waveguide is deflected by a surface acoustic wave emitted from the curved interdigital electrode, wherein the center of the curved interdigital electrode is The surface acoustic wave emitted from the electrode finger and the original beam always satisfy the Plaque diffraction condition within the frequency band, and furthermore, the normal passing through any point on the central electrode finger sequentially crosses other electrode fingers. The present invention is characterized in that the curved blind electricity collector 1 is configured such that the distance between each intersection of this normal line and each electrode finger is constant.

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, where 1 is a substrate and 2 is a planar optical waveguide provided on the substrate. For example, TI is diffused on an L 1NbOs substrate. It can be obtained by 3 is a curved interdigital electrode whose electrode fingers are curved in the length direction, and a light beam 5 propagating in the planar waveguide is plug-diffracted by the surface acoustic wave 4ζ emitted from this electrode to obtain polarized light 6. . There are various means for propagating the incident light 7 into the waveguide and extracting the output light 6, but in this embodiment, prisms such as 8 and 9 are used. Figure 2 shows the curved interdigital electrode 3 of Figure 1.
In this enlarged view, the interdigital electrode 11 has a structure in which the electrode finger spacing gradually decreases or increases along the length direction of the electrode fingers, and further remains almost constant in the propagation direction of the surface acoustic wave. Therefore, the time it takes for the surface acoustic wave emitted from the interdigital electrode 11 to reach the light beam 12 is almost the same whether the frequency is low like f8 (13) or high like f2 (14), and the dispersion is do not have. FIG. 3 is a diagram showing a specific method of constructing the curved intersecting electrode shown in FIG. 2.

That is, if the number of pairs of curved interdigital electrodes is N, then N/2
In order for the surface waves emitted from the th central electrode finger 21 to satisfy the plug diffraction conditions at all frequencies within the band of the electrode, the shape of the electrode finger 21 must first be adjusted along the y-axis as shown in FIG.
Once the y-axis is determined, it is necessary and sufficient that it is expressed by the following equation.

At this time, the frequency excited by each electrode finger changes linearly depending on the value of X, and is expressed as Xl f=fo+()Δf(2) 2 . Here, W is the length of the electrode in the X-axis direction and is approximately the intersecting width of the electrode fingers, fo is the center frequency of the electrode, and Δf is the frequency bandwidth of the electrode, λ. , represents the wavelength of the light beam, ne represents the refractive index of the optical waveguide, and vo represents the propagation speed of the surface wave.

Also, the propagation direction of the light beam is parallel to the straight line.

Next, for the electrode fingers 22 to 25 other than the central electrode finger, the normal line 26 is drawn on the electrode finger 21 determined by formula (1), and the electrode finger spacing is set at the frequency (calculated from formula (2)) )
The wavelength is set to be half of the wavelength λ determined by

In other words, the normal line 26 set at point A on the electrode finger 21 in FIG.
In the above, the electrode finger spacing d is a function of and X.

can be realized.

FIG. 4 shows frequency characteristics obtained as a result of an optical deflection experiment using the waveguide optical deflector according to the present invention. Center frequency 200 MH2, frequency band 100■, crossover width 2
-6mvt, curved interdigital electrodes with 14 pairs of electrode fingers were provided on a Ti-diffused optical waveguide formed on a Y-type LiNbO3 substrate, and the frequency characteristics of the deflector were measured using a He-Ne laser. As a result, the bandwidth (-3dB
) A deflector with a wide band of 80 MJ and a fractional band of 40% was realized, demonstrating the usefulness of the present invention.

[Brief explanation of the drawing]

Figure 1 of /p1 is an elevational view showing one implementation of the waveguide type optical deflector according to the present invention, 1: substrate, 2: optical waveguide, 3: curved interdigital electrode, 4:
surface acoustic wave, 5: light beam, 6: polarized light, 7: incident light,
Shows 8.9 nibrism. Fig. 2 is an enlarged view of the curved interdigital electrode, showing 11: curved interdigital electrode, 12: light beam, 13.14: surface acoustic wave, and Fig. 3 shows the construction method of the curved interdigital electrode in Fig. 2. In the figure, 21 to 25: electrode fingers, 26 two normal lines. FIG. 4 shows the experimental results of the frequency characteristics of the waveguide type optical deflector according to the present invention. ? rc Founder's Attorney ±1 Mohara - 175- Funeral Figure 1, ff / Otsu Figure 2 Ge 3 Figure A

Claims (1)

[Claims] A thin film optical waveguide is provided with a curved interdigital electrode in which electrode fingers are curved in the length direction and has a finite frequency band, and a light beam propagating within the thin film optical waveguide is radiated from the curved interdigital electrode. In a waveguide type optical deflector that deflects using a surface wave, the surface acoustic wave emitted from the center electrode finger of the curved interdigital electrode and the light beam always satisfy the plug diffraction condition within the frequency band, and The curved interdigital electrode is configured so that when a normal line passing through an arbitrary point on the central electrode finger successively crosses other electrode fingers, the distance between each intersection of this normal line and each electrode finger is constant. A waveguide type optical deflector characterized by: