JPH0230493B2 - - Google Patents

Description

[Detailed Description of the Invention] [Technical Field] The present invention relates to an acousto-optic element that has a function as an optical deflector that changes the traveling direction of light or a modulator that changes the intensity of light. .

[Prior art]

Acousto-optic elements use acoustic waves, such as ultrasonic waves, to periodically create density variations in a material, and these density variations act as a diffraction grating for light, changing the direction of light passing through the material. It is something that makes you Utilizing this phenomenon, acousto-optic elements are used as deflectors that change the traveling direction of light, and are also used as digital/analog modulators of light. Recently, it has also been applied as a directional coupler that constitutes an optical pulse tester, which is a measuring device for searching for fault points in optical fibers.

Materials for acousto-optic elements include liquid, glass,
Among these, crystals have a large figure of merit (index of merit) that indicates the performance of an acousto-optic device. However, due to its anisotropy, crystals have different figures of merit depending on the polarization direction of incident light, resulting in differences in diffraction efficiency. Therefore, when deflecting or modulating light whose polarization state changes over time, the diffracted light or undiffracted light changes in accordance with the polarization state of the incident light. This is a particularly big problem when using analog modulation.

Further, when an acousto-optic element is applied to a directional coupler of an optical pulse tester, the function of the optical pulse tester is impaired because optical power proportional to the light intensity cannot be extracted by diffraction. In order to explain this problem, first, an optical pulse tester using an acousto-optic element as a directional coupler will be described with reference to FIG.

In FIG. 1, 1 is a pulse generator, 2 is a laser light source, 3 is a drive circuit for an acousto-optic element, 4, 5 and 6 are lenses, 7 is an acousto-optic element, 8 is an optical fiber, 9 is a light receiver, 10 11 is an amplifier, 11 is a display device, and 12 is a transducer for generating ultrasonic waves.

FIG. 2 shows signal waveforms at various parts in FIG. 1.

A laser light source 2 driven by a pulse generator 1 emits the laser light shown in FIG. 2A. This laser light passes through the lens 4, the acousto-optic element 7, and the lens 5, and then enters the optical fiber 8.
The signal light shown in FIG. 2B is reflected from the optical fiber 8 and returns to the lens 5. The large-amplitude signal at the tip of this signal light is a Fresnel reflection reflected at the fiber input end face, and the subsequent exponentially decreasing signal is scattered backward within the optical fiber 8. This is Rayleigh scattered light.

When these reflected lights return to the acousto-optic element 7, the acousto-optic element 7 is operated via the transducer 12 by the RF signal supplied from the drive circuit 3 as shown in FIG. 2C, and the reflected light is reflected. The signal light is diffracted in the direction of the light receiver 9. At that time, by appropriately adjusting the operating time of the RF signal shown in FIG.
The Fresnel reflected light that saturates the optical fiber 8 is not diffracted, and only the signal that has been Rayleigh scattered and returned from the optical fiber 8 is diffracted in the direction of the light receiver 9. Therefore, the diffracted light has a waveform shown in FIG. 2D. Since this signal light is very weak, the receiver 9
After the signal is converted into an electrical signal, it is amplified by an amplifier 10 and displayed on a display device 11.

However, when the optical fiber 8 is a single mode optical fiber, the propagation constant of the light returning from the optical fiber 8 generally differs slightly depending on the x-axis and y-axis polarization directions in the fiber cross section. Due to the phase difference corresponding to the length of the optical fiber 8, the polarization state of the backscattered light repeats linear polarization, elliptical polarization, and circular polarization over time, as shown in FIG. When such backscattered light is diffracted by the acousto-optic element 7, which has a different diffraction efficiency depending on the polarization direction of the incident light, the diffracted light will have a waveform different from the waveform shown in FIG. 2D, but as shown in FIG. This results in a waveform with fluctuations as shown in . It is difficult to distinguish this phenomenon from the presence or absence of a failure in the optical fiber 8 itself, and it is also impossible to evaluate the loss of the optical fiber from the attenuation rate of backscattered light.
This is a major problem in the configuration of the optical pulse tester.

〔the purpose〕

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to solve these drawbacks by appropriately tilting the end face on which light enters in the crystal constituting the acousto-optic element with respect to the propagation direction of ultrasonic waves. An object of the present invention is to provide an acousto-optic element in which the diffraction efficiency of the acousto-optic element is made the same.

[Structure of the invention]

In order to achieve such an object, the present invention causes a periodic refractive index change caused by acoustic waves in a crystal having an acousto-optic effect, thereby changing the traveling direction of light passing through the crystal. The acousto-optic element has a transducer that generates the acoustic wave in the crystal, the length of the transducer is L, the frequency of the acoustic wave is f, the velocity of the acoustic wave is v, ordinary light and abnormal light. The refractive index of the crystal for light is n _p and n _e respectively, and the figure of merit of the acousto-optic element for ordinary light and extraordinary light is respectively
M ₁₁ and M ₁₃ , when the angle α [rad] between the end face of the crystal and the propagation direction of the acoustic wave is M ₁₁ ≧M ₁₃ , approximately , and when M ₁₁ < M ₁₃ , approximately However, here, sinc ^-1 (X) is sin(Y)/Y (however, 0Y
It is characterized by being an inverse function of π).

〔Example〕

The present invention will be described in detail below with reference to the drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Prior to showing embodiments of the present invention, the principle of the present invention will be explained using the drawings.

FIG. 4 shows an acousto-optic device of the present invention (hereinafter referred to as
Abbreviated as AO element. ) represents the state of light diffraction at 27. The incident light 13 on the AO element 27 is diffracted by the ultrasonic wave 14 generated by the transducer 12 driven by the drive circuit 3, and becomes diffracted light 15, so that the traveling direction is changed. The angle of incidence of the light 13 entering this AO element 27 is θ _p , the angle that the light refracted within the AO element 27 makes with the traveling direction of the ultrasonic wave 14 is π/2 - θ, and the refractive index of the AO element 27 is n. Then, according to Snell's law, n・sin(θ−α)=sinθ _p (1).

In reality, due to the anisotropy of the AO element 27, the refractive index n differs depending on the polarization direction of the incident light 13. When the optical axis of the crystal coincides with the traveling direction of the ultrasonic wave 14, and when the polarization direction of the incident light 13 is perpendicular to the traveling direction of the ultrasonic wave 14 (perpendicular to the plane of the paper in FIG. 4), the refractive index n is The refractive index n _p of the AO element 27 for ordinary light, and also in the parallel case, the refractive index n corresponds to the refractive index n _e of the AO element 27 for extraordinary light. Therefore, equation (1) becomes the following equation (2).

n _p sin (θ⊥−α)= _ne sin (θ−α)=sinθ _p (2) Here, θ⊥ and θ are the progress of the ultrasonic wave 14 in the AO element 27 when the ordinary light and the extraordinary light respectively Represents the angle made with the direction.

On the other hand, in order to efficiently diffract light by the AO element 27, the following Bragg conditions must be satisfied.

n _p sinθ⊥=fλ/2v (3.1) n _e sinθ=fλ/2v (3.2) where f and v are respectively
where λ is the wavelength of the light.

As can be seen from equation (2) and equations (3.1) and (3.2), when α≠0, equations (3.1) and (3.2) do not hold simultaneously. Now, let θ⊥ and θ that satisfy equations (3.1) and (3.2) be θ _B ⊥ and θ _B , respectively. Also, when θ=θ _B , finding the deviation angle Δθ=θ⊥−θ _B ⊥ from θ⊥ and θ _B ⊥, we get (2)
From equations (3.1) and (3.2), Δθ=α(1− _ne /n _p ) (4). However, here, θ⊥−α≪1 and θ−
α<<1.

AO element 27 when there is such an angular deviation
The diffraction efficiency η is approximately given by the following equation (5).

η=CMP(sinx/x) ² x=πfL/2vΔθ (5) Here, C is the proportionality constant, M is the figure of merit of the AO element 27, P is the acoustic wave power, and L is the transducer that generates the acoustic wave. The length is 12.

The figure of merit M generally has different values from M ₁₁ and M ₁₃ for ordinary and extraordinary rays, respectively. Now, when M ₁₁ > M ₁₃ and the inclination angle α of the end face of the AO element 27 is changed, this AO element 27
The relationship between the diffraction efficiency and the incident angle θ _p is schematically shown in FIGS. 5A to 5D. Here, the solid line and the broken line represent the diffraction efficiency for the ordinary light and the extraordinary light (η _p and η _e , respectively), and A = CM ₁₃ P and B = CM ₁₁ P are the diffraction efficiency for the ordinary light and the extraordinary light. Indicates maximum value. As shown in Figure 5A,
When α=0, except when the diffraction efficiency is zero, η _p ≠
η _e , but as shown in Figures 5B to D, α>0
When , there exists an angle of incidence such that η _p =η _e .

Figures B and D show the case where η _p = (maximum value of η _e ) x 1/2, the inclination angle α at that time is α ₁ and α ₃ , respectively, and the incident angle θ ₀ is θ ₁ and θ ₃ . Further, FIG. 5C shows a case where η _p =(maximum value of η _e ), and the inclination angle α at that time is α ₂ and the incident angle θ ₀ is θ ₂ . Needless to say, the maximum diffraction efficiency is obtained in the case of FIG. 5C (α=α ₂ ), and η _p =
It is possible to set η _e . Furthermore, when α ₁ ≦α≦α ₃ , η _p ≧1/2η _e , and it is practically important to set the inclination angle α within this range. Tilt angle α ₁ , α ₂ ,
The values of α ₃ and the incident angles θ ₁ , θ ₂ , and θ ₃ are obtained from equations (1) to (5) as follows.

Here, sinc ^-1 (X) is sin(Y)/Y(0Y
π), and γ=sinc ^-1 (0.5)≒
It is 1.3915.

The above is a case where M ₁₃ < M ₁₁ , but M ₁₁ < M ₁₃
It can be obtained similarly in the case of , and in this case, n _p and n _e and M ₁₃ and M ₁₁ can be exchanged with each other in equation (6).

Next, as a specific design example of the acousto-optic device of the present invention, an example using TeO ₂ crystal will be shown. TeO ₂ crystals have a very large figure of merit, but the values are slightly different for ordinary and extraordinary light.
The acousto-optic parameters of TeO ₂ crystal are as follows.

At λ = 0.633 nm, M ₁₁ = 34.5 x 10 ^-18 S ³ /g M ₁₃ = 25.6 x 10 ^-18 S ³ /g n _p = 2.260, n _e = 2.412 v = 4200 m/s and f = 120 MHz, Assuming L=18mm,
From equation (6), α ₁ = 2.81×10 ^-3 [rad] = 0.16 [deg] α ₂ = 1.72×10 ^-2 [rad] = 0.98 [deg] α ₃ = 5.75×10 ^-1 [rad] = 3.29 [deg] θ ₁ = 1.90×10 ^-3 [rad] = −0.11 [deg] θ ₂ = −3.23×10 ^-2 [rad] = −1.85 [deg] θ ₃ = −1.25×10 ^-1 [rad ] = −7.19 [deg].

An example of the present invention based on this design value is shown in the sixth example.
As shown in the figure.

Here, TeO ₂ is used as the AO element 27,
The length L of the transducer 12 attached to this is
It was set to 18mm. The incident angle of light θ ₀ is −1.85°, AO element 2
The inclination angle α of the end face of No. 7 was set to 0.98°. Light is guided from the laser light source 2 to the lens 4 through an optical fiber 16,
Light is guided from the lens 6 to the light receiver 9 through an optical fiber 17. Here, these optical fibers 8,
Optical fibers 16 and 17 may be either multimode fibers or single mode fibers.

The insertion loss of the directional coupler for an optical pulse tester constructed in this manner showed no polarization dependence at all. In addition, in FIG. 6, the AO element 2
Although the end surface 27b of 7 is drawn as a plane parallel to the end surface 27a, in order to eliminate the polarization dependence of the diffraction efficiency in the operation of the directional coupler shown in FIG. Inclination angle α
is important, but the end face 27b is not relevant. Therefore, end surface 27b does not need to be parallel to end surface 27a.

〔effect〕

As explained above, the acousto-optic element according to the present invention can eliminate the polarization dependence of the diffraction efficiency by tilting its entrance end face with respect to the direction of propagation of the ultrasonic wave. There are advantages.

(1) There is no need to control the polarization direction of light incident on the acousto-optic element. Therefore, the acousto-optic element can be effectively applied even to light emitted from an optical fiber whose polarization direction is difficult to control.

(2) Even for polarized light, such as backscattered light from a single mode optical fiber, light can be extracted by diffraction in proportion to the intensity of the incident light. Intensity sampling measurements can be made accurately.

(3) Low-distortion analog modulation is possible even for light whose polarization direction and state change over time.

In addition, although the case where the acousto-optic element of the present invention uses ultrasonic waves as the acoustic waves has been described above, the present invention is not limited to the use of ultrasonic waves, but can be effectively applied to a wide range of acoustic waves. .

[Brief explanation of drawings]

Figure 1 is a configuration diagram showing an example of the configuration of an optical pulse tester using an acousto-optic element as a directional coupler.
Figures A to E are signal waveform diagrams showing the signal waveforms at each part in Figure 1, Figure 3 is an explanatory diagram of the polarization state of backscattered light from a single mode optical fiber, and Figure 4 shows the oblique end face according to the present invention. Figures 5 A to D are diagrams showing the relationship between the inclination angle of the oblique end face and the diffraction efficiency of ordinary light (solid line) and extraordinary light (broken line). FIG. 2 is a configuration diagram showing an example in which the acousto-optic device of the present invention is applied to a directional coupler constituting an optical pulse tester. 1...Pulse generator, 2...Laser light source, 3...
...Acousto-optic element drive circuit, 4, 5, 6... Lens, 7... Acousto-optic element, 8... Optical fiber, 9
... Light receiver, 10 ... Amplifier, 11 ... Display device, 27 ... Acousto-optic element according to the present invention, 12 ...
...Transducer for generating ultrasonic waves, 13...Incoming light, 14...Propagating ultrasonic waves, 15...Diffracted light, 1
6,17...Optical fiber.

Claims (1)

[Scope of Claims] 1. An acousto-optic element in which a periodic refractive index change is caused by an acoustic wave in a crystal having an acousto-optic effect to change the traveling direction of light passing through the crystal, The crystal has a transducer that generates the acoustic wave, the length of the transducer is L, the frequency of the acoustic wave is f, the velocity of the acoustic wave is v, and the crystal is resistant to ordinary light and extraordinary light. When the refractive index is n _p and n _e , and the figures of merit of the acousto-optic element for ordinary light and extraordinary light are M ₁₁ and M ₁₃ , respectively, the angle α between the end face of the crystal and the propagation direction of the acoustic wave is [ red〕
However, when M ₁₁ ≧ M ₁₃ , approximately , and when M ₁₁ < M ₁₃ , approximately However, here, sinc ^-1 (X) is sin(Y)/Y (however, 0Y
An acousto-optic element characterized by being an inverse function of π).