JPS61213823A - Optical modulator - Google Patents

Abstract

PURPOSE:To make an optical modulation small in size and high-frequency without requiring an external oscillator by providing a surface acoustic wave device whose one end is inserted into a liquid so that two reed screen type electrodes arranged apart from each other on a piezoelectric substrate are exposed to air, an amplifier, a phase shifter, and a light source and obtaining a modulated light as a diffracted light from an ultrasonic wave in the liquid. CONSTITUTION:The oscillator oscillates when prescribed conditions are satisfied, and a surface acoustic wave 16 propagated in the direction of a liquid 30 is subjected to mode conversion to become an ultrasonic wave 24 propagated in the liquid 30. When a light 22 emitted from a laser 34 is made incident on the inside of the liquid, the light 22 is diffracted by the acoustic field of the liquid, and a diffracted pattern 44 is obtained from a window 28. Then frequency spectrum of each diffracted light of this diffracted pattern 44 is changed in accordance with the variance of the oscillator, and frequency modulation is possible. Demodulation is performed by detecting the frequency in a frequency measuring means through an amplifier 40 and an output terminal 42 after receiving and converting, for example, the diffracted light of the 0th order to an electric signal in a photodetector 36.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an ultrasonic light modulator using a surface acoustic wave device having interdigitated electrodes on a piezoelectric substrate.

(Prior Art) In recent years, with the progress of optical communications, research on optical modulators, which are essential for optical communications, has been attracting attention. Among these, an ultrasonic optical modulator that uses photo-acoustic interaction using ultrasonic waves has an excellent extinction ratio, and is used as a small and inexpensive optical modulation element in laser printers and the like. An ultrasonic optical modulator using this optical-acoustic interaction is capable of modulating the intensity of light.

On the other hand, a surface acoustic wave interdigital transducer efficiently operates as a device for emitting ultrasonic waves in liquid at the liquid-solid interface, so it can be applied to ultrasonic microscopes and the like.

(Problems to be Solved by the Invention) However, since the transducer used in the conventional ultrasonic optical modulator uses bulk waves, the excitation frequency of the ultrasonic wave is determined by the thickness of the piezoelectric element of the transducer. It is difficult to handle high frequencies of 200 MHz or higher. Furthermore, since it is necessary to separately provide an oscillator for driving the transducer, there is a problem that the device becomes large and expensive.

Therefore, the present invention aims to solve the above problems.

(Means for Solving the Problems) The present invention provides a surface acoustic wave device in which two sets of interdigital electrodes spaced apart on a piezoelectric substrate are inserted into a liquid at one end so as to be exposed in the air; It has an amplifier and a phase shifter that are connected in series between two sets of interdigital electrodes, and a light source that is also positioned transversely to the direction of propagation of the ultrasonic waves in the liquid. The light is obtained as diffracted light from ultrasonic waves in the liquid.

(Operation) A loop consisting of a surface acoustic wave device, an amplifier, and a phase shifter oscillates when predetermined amplitude and phase conditions are satisfied.

At this time, surface acoustic waves propagate through the piezoelectric substrate. On the one hand, this surface acoustic wave propagates in the direction of the liquid, and when it reaches the liquid surface, its mode is converted into a longitudinal sound wave that propagates in the liquid.

This ultrasonic wave in the liquid acts as a diffraction grating for the light from the light source. Therefore, light from the light source is diffracted. At this time, modulation is possible because the frequency spread of the diffracted light changes as the oscillation frequency changes. Note that this frequency change is detected by converting the diffracted light into an electrical signal and then measuring it with a squitrem analyzer or the like.

(Example) Hereinafter, the present invention will be described in detail based on an example with reference to the drawings.

First, the principle of this invention will be explained with reference to FIG. In FIG. 2, 10 is a surface acoustic wave device, and 12 is a piezoelectric substrate. 14a and 14b are interdigital electrodes, which are arranged to be spaced apart from each other. As shown in the figure, the surface acoustic wave device 10 includes interdigital electrodes 14a,
14b is exposed to the air, in other words, the end on the side where the interdigital electrodes 14 a p 14 b are not provided is inserted into the liquid. With this configuration, when an electric signal is applied to the interdigital electrode 14a on the input side, a surface acoustic wave 16 propagating in the direction of the liquid and a surface acoustic wave propagating in the direction of the interdigital electrode 14b are generated.

The interdigital electrode 14b on the output side outputs an electrical signal corresponding to this surface acoustic wave. An oscillator is formed by feeding back this electric signal to the interdigital electrode 14m according to predetermined amplitude and phase conditions. In order to obtain the above two conditions, an amplifier and a phase shifter (not shown in FIG. 2) are connected in series between the input-side interdigital electrode and the output-side interdigital electrode. In order for the oscillator formed in this way to oscillate continuously, 8〉Ls
It is necessary to satisfy the following amplitude condition (1). Here, Ls is the insertion loss of the somatic surface wave device, and GA is the gain of the amplifier. Also, the phase difference of the surface acoustic wave between the input side and the output side is φ8, and the phase delay of the electrical system including the amplifier and phase shifter is φ. Then, the phase condition is φ
, +φ. =2πfot/vR+φ8=2nπ −(2)
becomes. Here, n is an integer related to the oscillation mode, fo is the oscillation frequency, t is the propagation path length of the surface acoustic wave, and vR is the velocity of the surface acoustic wave.

On the other hand, the surface acoustic wave 16 propagating in the liquid direction travels in the perpendicular direction 18 of the surface acoustic wave device 10 at the interface with the liquid.
For, C1= m−' (VJ”R) −(
3) The mode is converted into a longitudinal ultrasonic wave that travels in the direction of arrow 20 in the liquid at an angle of . Here, Vt is the ultrasonic velocity in the liquid, and vR is the velocity of the surface acoustic wave. As can be seen from equation (1), the traveling direction of the ultrasonic waves in the liquid does not depend on the frequency of the applied electric signal. When the light 22 passes through a portion where ultrasonic waves are present, the wavefront 24 of the sound wave acts as a diffraction grating, and the light is diffracted. This is because the refractive index of the liquid changes periodically due to ultrasonic waves in the liquid. The phase grating e parameter Q is Q=2πLλo /n/f2≦0.3 ・
In the case of (4), Raman Nurse diffraction occurs in which the diffracted light reaches a higher order. Here, A and L are the wavelength and width of the ultrasonic wave, n is the refractive index of the liquid, and λ0 is the wavelength of light in the air. At this time, the intensity factor and the diffraction angle θ of the N-order diffracted light 22N are expressed by the following equation.

IN=J, 7(v)...(5
)θ,=gln(Nλo/A) −(
6) Here, υ is the amplitude of the phase change of light, - If the change in liquid refractive index due to ultrasound is δn, then τ = 2πLδ
n/λ (7).

Next, one embodiment of the present invention will be described.

In FIG. 1, 26 is a liquid tank containing a liquid 3o, and a window 28 for light is formed on the side wall of the tank.

In this liquid tank 26, the surface acoustic wave device 10 described above is positioned as shown. The interdigital electrode 14b is connected to an input terminal of an amplifier 46. The output terminal of amplifier 4a is connected to the input terminal of phase shifter 480. Phase shifter 48 changes the phase of the input signal according to the electrical signal applied via terminal 32. The output of the phase shifter 48 is connected to the interdigital electrode 14a. In other words, the surface acoustic wave device 10. Amplifier 46 and phase shifter 4
8 forms an oscillator.

In the transverse direction with respect to the advancing direction of the in-liquid sound wave 24 propagating in the liquid 30, a radar (for example, He-Ne) constituting the light source is arranged.
34 is positioned. 36 is a light receiving element constituting a demodulator, and is installed so as to be able to slide in the direction of arrow 38 so that the diffraction pattern 44 can be detected. Reference numeral 40 denotes an amplifier that amplifies the electrical signal output from the light receiving element 36 and supplies it to the output terminal 42 .

Next, the operation will be explained.

As explained earlier using Fig. 2, the oscillator is expressed by formula (1)
And when the predetermined conditions shown in (2) are satisfied, oscillation occurs. The surface acoustic wave 16 propagating in the direction of the liquid 30 undergoes mode conversion and becomes an ultrasonic wave 24 propagating in the liquid 30 . laser 3
When the light 22 emitted from 4 enters the liquid, the light 22
becomes a sound field in the liquid and is diffracted. As a result, a diffractive turn 44 is obtained from the window 28. The frequency spectrum of each diffracted light of this diffraction/IP turn 44 changes according to the change in the oscillation frequency of the oscillator. Therefore, frequency modulation becomes possible. Note that demodulation is performed by, for example, receiving the O-th order strong light with the light receiving element 36 and converting it into an electrical signal, and then detecting the frequency with a frequency measuring means (not shown) via the amplifier 40 and the output terminal 42.

Next, an experimental example of the apparatus shown in FIG. 1 will be explained. As the piezoelectric substrate 12 of the surface acoustic wave device 10, TDK
Piezoelectric ceramic 91 manufactured by Co., Ltd., length 50cm, width 2
0tlI+! , 5 m thick was used.

The logarithm of each of the two pairs of interdigital electrodes 14a and 14b is 20, the period length P is 800 μm, and the distance between them is 28.
．． At 8 m, the velocity of surface acoustic waves is 2100%/8. As the light source 34, He-N with a wavelength of 0.6328 μm is used.
An e-laser was used. Further, as the light receiving element 36, a photodiode S-1223 manufactured by Hamamatsu Co., Ltd. was used. Furthermore,
Water was used as the liquid.

FIG. 3 is a diagram showing the results of measuring the frequency spectrum of the O-order strong light measured using the above device. From the same figure, the 0th order strong light is a single mode ultrasonic wave (2,575MHz
) can be seen to be modulated.

FIG. 4 is a diagram showing the results of measuring the frequency spectrum of the O-th order strong light when a 10 kHz modulation electrical signal is applied to the phase shifter 48. From the same figure, the O-th strong light is 1
It can be seen that the signal is subjected to frequency modulation of 0 kHz.

FIG. 5 is a diagram showing the results of measuring the relationship between the voltage of the modulation electric signal applied to the phase shifter 48 and the frequency shift of the 0th order strong light.

FIG. 6 is a diagram showing the relationship with the output level of diffracted light when the voltage of the modulation electric signal is held constant and the modulation frequency is varied. From the figure, it can be seen that the frequency is flat up to about 100 kHz.

(Effects of the Invention) As described above, according to the present invention, it is possible to provide an ultrasonic optical modulator that does not require an external oscillator, is small and inexpensive, and can easily be used at high frequencies.

[Brief explanation of drawings]

FIG. 1 is a diagram showing an embodiment of the present invention, FIG. 2 is a diagram for explaining the invention in detail, FIGS. 3 and 4 are diagrams showing the frequency spectrum of O-order strong light, respectively, FIG. 5 is a diagram showing the relationship between the modulation electric signal voltage and the frequency shift of the O-th order strong light, and FIG. 6 is a diagram showing the relationship between the modulation frequency and the leveling of the diffracted light output. 10... Surface acoustic wave device, 12... Piezoelectric substrate,
14a, 14b... interdigital electrode, 24... sonic wave in liquid, 26... liquid tank, 28... window, 30...
liquid, 34... laser, 36... light receiving element, 40.
46...Amplifier, 48...Phase shifter.

Claims (1)

[Claims] A surface acoustic wave device in which two sets of interdigital electrodes are spaced apart on a piezoelectric substrate and one end thereof is inserted into a liquid so as to be exposed to the air, and an amplifier connected in series between the two sets of interdigital electrodes. and a phase shifter, and a light source positioned in a direction transverse to the traveling direction of the ultrasonic waves in the liquid, and the modulated light is obtained as diffracted light from the ultrasonic waves in the liquid. optical modulator.