JPS6134646B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical circuit element that utilizes a gradient index lens (for example, the product name: SELF-OCLENS) and elastic waves propagating inside the lens.

Conventionally, this type of optical circuit element is shown in Figures 1 and 2.
I got what is shown in the figure. In FIG. 1, 1 is a glass thin film, and 2 is a substrate having a different refractive index from the glass thin film 1. In FIG. Further, in FIG. 2, 3 is a LiNbO ₃ substrate, for example, and 4 is an impurity diffusion layer such as Ti, which forms an optical waveguide. 5 is a grating with a period comparable to the wavelength of light, and L _i , L _r , and L _t are incident light, reflected light, and transmitted light, all of which are parallel light rays.

Next, the operation will be explained. In FIG. 1, it is assumed that incident light L _i is incident at an incident angle θ'. The light incident on the glass thin film 1 undergoes countless reflections as shown in the figure. The phase difference due to the optical path difference for one round trip can be expressed as δ=4πnlcosθ/λ ₀ (1). Here, λ ₀ is the wavelength of light in vacuum,
θ represents the refraction angle within the glass thin film 1, n represents the refractive index of the glass thin film 1, and l represents the film thickness of the glass thin film 1. As a result of countless reflections within the glass thin film 1,
When the total reflected light intensity and transmitted light intensity from the glass thin film 1 are expressed by reflectance and transmittance, where the total incident amount is I _i , the reflected amount is I _r , and the transmitted amount is I _t , I _r /I _i = 4Rsin ² (δ/2)/(1-R) ² +
4Rsin ² (δ/2)……(2) I _t /I _i =(1-R) ² /(1-R) ² +4Rsin
² (δ/2)……(3). Here, R is a system-specific value determined by the reflection and transmission characteristics at the air/glass thin film and glass thin film/substrate interfaces. Examples of characteristics of equations (2) and (3) are shown in FIGS. 3 and 4.

In FIGS. 3 and 4, the horizontal axis is the optical frequency, and the vertical axis is the reflectance and transmittance, and the optical frequency νm that gives the maximum value (transmittance) and minimum value (reflectance) is νm=mC ₀ / 2nl cos θ ......(4). Here m is an integer. The interval between the resonance frequencies is given by Δν=C ₀ /2nlcosθ (5). Here, C ₀ is the speed of light in vacuum.

As can be seen from FIGS. 3 and 4, by utilizing the light interference effect within the glass thin film 1, an optical band-pass filter is used for transmitted light L _t , and an optical band-stop filter is used for reflected light. operates as

Next, FIG. 2 will be explained. As shown in FIG. 2, when Ti is thermally diffused onto the surface of the LiNbO ₃ substrate 3, the refractive index of the impurity diffusion layer 4 such as Ti becomes higher than the refractive index of the LiNbO ₃ substrate 3, and it functions as an optical waveguide. When a periodic grating 5 is formed on the optical waveguide, the incident light L _i that satisfies the following equation (6) is reflected by the grating 5 in a distributed manner.

λ=2P/m (6) Here, P is the period of the grating, m is an integer, and λ is the wavelength of the incident light L _i . That is, the first
Since the light that satisfies the equation (6) is reflected and becomes reflected light L _r , considering the transmitted light L _t , this grating 5 operates as a pass blocking filter.

Conventional optical bandpass filters and optical bandstop filters are constructed as described above, so when used as an optical circuit element for optical fiber communication, the first
In the example shown in the figure, it was necessary to insert a lens between the output end and the input end of the optical fiber and the interference thin film to convert the output light into parallel light beams, or conversely to narrow down the parallel light beams. This not only increases the number of optical components but also increases the insertion loss of the entire system.
Furthermore, the example shown in FIG. 2 also has the drawback of large coupling loss between the optical fiber and the planar optical waveguide.

This invention was made in order to eliminate the drawbacks of the conventional lenses as described above. By propagating elastic waves in a gradient index lens and creating periodic changes in the refractive index, it is possible to create a compact light beam with low loss. The purpose is to provide a band rejection filter. The invention will be explained below with reference to the drawings.

FIGS. 5a and 5b are an operation explanatory diagram and a perspective view showing an embodiment of the present invention. In these figures, 11 is an optical fiber, 12 is a π-cut gradient index lens, 13 is a piezoelectric thin film transducer for excitation of acoustic waves, 14 is an electrode, 15 is an oscillator, 1
6 is an elastic wave absorber, 17 is an optical fiber, and F is an elastic wave front.

Next, the operation will be explained. optical fiber 11
The light emitted from the lens enters the gradient index lens 12 . On the incident side end face of the gradient index lens 12, LiNbO ₃ and ZnO are used to excite elastic waves.
A piezoelectric thin film transducer 13 such as the one shown in FIG. Furthermore, metal thin film electrodes 14 are attached to both sides of the piezoelectric thin film transducer 13 . When the film thickness of the piezoelectric thin film transducer 13 is d and its longitudinal or transverse wave velocity is v _S , the 1/2 resonance frequency f ₀ is given by the following equation.

f ₀ =v _S /2d (7) Therefore, if the elastic wave phase velocity within the gradient index lens 12 is V _G , then the gradient index lens 1
The wavelength of the elastic wave within 2 is λ _G =2dv _G /v _S (8). When an elastic wave propagates within the gradient index lens 12, the elastic wave acts as a partial reflection mirror for light that travels at a distance of λ _G and a speed of v _G .
That is, light that satisfies λ=2λ _G /m (9) is reflected in a distributed manner and is not transmitted as transmitted light. On the other hand, since the medium is the π-cut graded index lens 12, the light that does not satisfy equation (9) is condensed at the center of the output end face, and is focused on the optical fiber 1.
Combined with 7. An elastic wave absorber 16 is attached to the output end face of the gradient index lens 12.
It is not attached to the central portion where the light focused by the gradient index lens 12 is coupled to the optical fiber 17. That is, the configuration shown in FIG. 5 operates as a band rejection filter that reflects light that satisfies Equation 9 and transmits other light.

In addition, in FIG. 5, if the frequency of the AC voltage applied to the piezoelectric thin film transducer 13 is changed,
Within the band of the piezoelectric thin film transducer 13,
The wavelength of elastic waves can be controlled. That is, the wavelength of the reflected light is also controlled. Therefore,
Embodiments of the invention can also be used as tunable optical bandstop filters.

Further, by utilizing the tunability of the embodiment of the present invention, it can be used as a high-speed optical switch in a wavelength division multiplexing optical transmission device. Furthermore, it can also be used as a high-speed optical switch that turns on and off the transmitted light depending on the presence or absence of elastic waves. Further, if the reflected light is coupled to the input optical fiber and the S/N ratio becomes a problem, it is also possible to use the input optical fiber with the input optical fiber slightly shifted from the center of the gradient index lens 12.

As explained above, since the present invention uses a gradient index lens as a propagation medium for elastic waves, it is possible to easily obtain a low-loss, compact optical band-stop filter that can be finely adjusted. Furthermore, since it uses a distributed reflection mechanism based on periodic changes in the refractive index created by elastic waves, it has the advantage of high wavelength selectivity of light, and can also be used as a high-speed optical switch in wavelength multiplexing optical transmission equipment. . Furthermore, a piezoelectric thin film transducer is attached to the input side end face of the gradient index lens, leaving a coupling part for the input optical fiber on both sides, and the output optical fiber is also coupled to the output side end face. Since the elastic wave absorber is attached to the remaining portions, it is possible to maintain sufficiently good coupling between the input and output optical fibers and the optical circuit element. It also has the advantage that it can be used as a high-speed optical switch that turns on and off light transmission depending on the presence or absence of elastic waves.

[Brief explanation of the drawing]

Figures 1 and 2 are schematic side views for explaining an example of a conventional optical filter, Figures 3 and 4 are transmission and reflection characteristics of the optical filter in Figure 1, and Figure 5 is a diagram showing the transmission and reflection characteristics of the optical filter in Figure 1. This shows one embodiment of the invention.
FIG. 5a is an explanatory diagram of the operation, and FIG. 5b is a perspective view. In the figure, 11 is an optical fiber, 12 is a π-cut gradient index lens, 13 is a piezoelectric thin film transducer, 14 is an electrode, 15 is an oscillator, 16 is an acoustic wave absorber, and 17 is an optical fiber. Note that the same reference numerals in the figures indicate the same or corresponding parts.

Claims (1)

[Claims] 1. Attach a piezoelectric thin film transducer with metal thin film electrodes attached to both sides, leaving the coupling part of the optical fiber for incidence, on the input side end face of the gradient index lens, 1. An optical circuit element characterized in that an acoustic wave absorber is attached to an end face of an optical fiber, leaving a coupling part of an optical fiber for output.