JPS583215B2 - Onkiyo Kougakoshi no Seizouhou - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention is directed to an acousto-optic device such as an optical deflector or optical modulator that utilizes the phenomenon of ultrasonic light diffraction, in which the ultrasonic wavefront is tilted with the frequency in order to operate in a wide frequency range. The present invention relates to a method of manufacturing an element having a designed staircase structure.

In so-called acousto-optic devices, such as optical deflection and modulation elements that utilize the optical diffraction phenomenon caused by ultrasonic waves, the frequency of the ultrasonic waves is usually kept constant while the incident angle of the light to the element is kept constant to deflect or modulate light. was doing.

However, the optical diffraction phenomenon is caused by the Bragg angle θB (where k is the wave number of light in air, n is the refractive index of the medium, and
K is the wave number of the ultrasonic wave in the medium), and θ
It is reflected by B, and when the incident angle deviates from θB, the diffraction efficiency decreases.

The dependence of the diffraction efficiency η on the angle of incidence is as follows: X=KL(θ−θB)/2 (3)(
L is the length over which light and ultrasound interact, and θ is the angle of incidence measured in the medium).

In this way, the Bragg angle θB is a function of the ultrasonic frequency, so as mentioned earlier, for a certain central ultrasonic frequency f0, the Bragg condition of equation (1) is satisfied.
When the ultrasonic frequency f is changed with the incident angle of light set, the Bragg angle changes according to the deviation from the center frequency f0, and the diffraction efficiency decreases according to equations (2) and (3).

As can be seen from equation (3), when increasing the ultrasonic frequency to operate in a wide band, that is, when increasing K,
Furthermore, when L is lengthened in order to increase the optical diffraction efficiency, the rate of decrease in the diffraction efficiency due to the deviation from f0 becomes significant, which has become a major limiting factor in practical use.

To prevent this, Kerpel et al. (Proce
-edings of the IEEE 54 (1
966) pp-1429-1437) created an element having a stepped structure as shown in FIG.

That is, one surface of the medium 1 through which the ultrasonic waves propagate and interact with the optical beats has a stepped shape, and ultrasonic transducers 2 to 5 are attached to each step.

The height h of the stairs is 1/of the ultrasonic wavelength Λ0 at the center frequency.
2, the adjacent transducer spacing S is Λ.

2/nλ (λ is the wavelength of light in air).

The transducers 2 to 5 are excited by a high frequency power source 6 so that adjacent transducers have opposite phases, and the light beam 7 is incident on the medium 1 in parallel to the step formation direction of the medium 1.

In this way, the ultrasonic wavefront is equivalently tilted so as to approximately satisfy the Plagg condition over a wide frequency range for a constant light incident angle.

As a result of experiments, it has been reported that the number of steps in this structure is at least four.

Usually, 4 to 10 stages are used.

Further, the general size of the element is such that the medium surface on which the transducer is attached is about 5 mm x 5 mm to 10 mm x 10 mm, and the height of the medium is about 5 to 25 mm.

The transducer spacing S is Λ02/n as described above.
λ is selected.

Since Λ0=v/f0 (V is the speed of sound in the medium), the transducer spacing S is inversely proportional to f02.

Deflectors typically have a center frequency f0 of 100-200MHz
Therefore, the transducer spacing S is as small as about 0.3 to 1.2 mm.

Now, in order to make such a stepped structure acousto-optic element,
Conventionally, the surface of the medium 1 is first finished into a stepped surface by mechanical polishing or chemical etching, and the transducers 2 to 5 of the desired thickness are adhered to each stepped surface, or the final target thickness is Polish thicker transducers after gluing.

In this case, the thickness of the transducer is extremely thin, and the stepped surfaces of the medium 1 to which the transducer is bonded are required to have sufficiently good flatness and parallelism.

However, its processing was extremely difficult. Especially for example number 1
When producing an element that operates at 0 MHz or higher, extremely high flatness accuracy is required, and processing difficulties tend to increase.

In addition, in order to send ultrasonic waves uniformly and efficiently to the medium 1, a considerable amount of pressure (for example, 50 kg/cm) is required during bonding.
2 or more), therefore, transducers that are polished to a target thickness from the beginning (for example, at a center frequency of 150 MHz, the thickness is usually about 20 μm) are mechanically weak and may be damaged.

On the other hand, polishing a thick transducer after bonding is very difficult to achieve high flatness and parallelism due to the stepped structure of the element, and if this is not satisfied, the characteristics of the obtained element will be adversely affected. It will have a negative impact.

The present invention aims to provide a method of manufacturing the acousto-optic element having the above-mentioned stepped structure relatively easily and with high precision.

For this reason, in the present invention, as shown in FIG. 2A, strip-shaped transducers 2 to 5 are arranged in parallel and bonded to one plane surface 1a of a rectangular parallelepiped medium 1 in which ultrasonic waves and light interact. .

In this case, the surface 1a on which the transducers 2 to 5 are attached
is polished to the required flatness.

The transducers 2 to 5 attached to this surface are polished to a predetermined thickness (usually about 20 μm), and then electrodes are deposited on the surfaces of the transducers 2 to 5 by vapor deposition or coating.

Next, the medium 1 is cut perpendicularly to the surface 1a so as to each contain one transducer, as shown by the dotted lines, to obtain pieces 8a, 8b, 8c, and 8d as shown in FIG. 2B.

This cut surface is optically polished to have high flatness and parallelism, and is finished to a predetermined dimension S.

Here, the dimensions of the piece are width 5-10mm and height 5-25m.
m, and the thickness S is about 0.3 to 1 mm.

Next, as shown in FIG. 2C, the surfaces on which the transducers 2 to 5 are attached are sequentially shifted by a predetermined value in a direction substantially perpendicular to the surface, and are assembled to form a staircase structure.

The most excellent bonding method between the pieces 8a and 8b is an optical bonding method, which allows the light beam 7 to propagate through the medium 1 almost completely without loss.

Alternatively, the pieces may be bonded together using a transparent adhesive whose refractive index matches that of the medium 1, and an anti-reflection film may be sandwiched therebetween if necessary.

Further, the adhesive 9 can be applied to the peripheral portion to reinforce the adhesive, or the adhesive 9 can be used alone to hold it.

In short, each element is optically cascaded without compromising optical quality.

There is a problem of disturbance of the ultrasonic beam at the adhesive part, but this problem can be solved by making the flat part of the stairs longer than the length of the transducer by an amount that matches the diffraction angle of the ultrasonic wave. do.

Since the diffraction of ultrasonic waves decreases in inverse proportion to the frequency, the above-mentioned length ratio can be made small at high frequencies where the manufacturing method of the present invention is particularly useful.

Further, the medium 1 is usually configured such that an ultrasonic absorber is added to the surface opposite to the surface to which the transducer is bonded so that the ultrasonic waves in the medium 1 are not reflected from that surface.

Therefore, as the propagation length for ultrasonic diffraction, only the distance between the bonded surface of the transducer and its opposing surface needs to be considered.

In addition, instead of using transducers 2 to 5 in FIG. 2A, a method may be used in which one large transducer is glued and polished, and then cut together with the medium 1 to be divided into individual pieces as shown in FIG. 2B. is also extremely effective.

This is true when the center frequency f0 is high and S is small (for example, 0
．． 5 mm or less).

In FIG. 2C, the individual pieces do not necessarily have to be in close contact with each other, but in order to obtain the smoothest possible inclination of the ultrasonic wavefront, it is better to have a narrower interval.

As explained above, in the manufacturing method of the present invention, before forming the acousto-optic medium into a stepped structure, the surface on which the transducer is attached is finished to the required parallelism and flatness, and then the transducer is attached. Since the medium is polished and then cut and assembled into a staircase structure, an acousto-optic element with an excellent staircase structure can be obtained without impairing the acoustic, optical, and acousto-optic properties.

[Brief explanation of drawings]

FIG. 1 is a perspective view showing an acousto-optic element having a staircase structure, and FIG. 2 is a process diagram showing an example of the manufacturing method of the present invention.

Claims (1)

[Claims] 1. A transducer is attached to one surface of a rectangular parallelepiped medium in which ultrasonic waves and light interact, and after polishing the transducer to a predetermined thickness, the above-mentioned Cutting the medium to form a plurality of pieces having the transducer attached to one end face, and combining the pieces so that one end face of the pieces is sequentially shifted in a direction perpendicular to the plane to form a step structure. Characteristic manufacturing method for acousto-optic elements.