JPS6340868A - Optical integration spectrum analyser - Google Patents

Abstract

PURPOSE:To stabilize capacity, by constituting the title analyser of a refractive index distribution type optical waveguide and a piezoelectric substrate having a refractive index lower than that of said waveguide. CONSTITUTION:A slab lens having refractive index distribution only in an x-axis direction is laminated to a piezoelectric substrate 1 and the thickness thereof is reduced to a wavelength of a surface elastic wave 13 or less by cutting and grinding to form a waveguide 15. By properly setting a refractive index and a length etc., an input terminal 16 and an output terminal 17 can be mutually brought to image forming positional relation. The beam emitted from semiconductive laser 3 and guided is condensed to the point A at the terminal 17 when no high frequency electric signal is applied to a transducer (Tr)6 and the emitting region of the laser 3 is formed into an image. Tr6 is positioned at the almost middle between the terminals 16, 17 and, when a high frequency electric signal is applied to Tr6, the elastic wave 13 is excited and a part of the beam guided through the waveguide 15 is diffracted to be condensed to a point B. As mentioned above, by constituting the wave guide only by the processing of a plane without performing the cutting and grinding of a curved surface, the facilitation of manufacturing and the stabilization of capacity are attained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an optical integrated spectrum analyzer that analyzes the spectrum of a high-frequency electrical signal by utilizing the interaction between surface acoustic waves and light.

[Conventional technology]

Figure 4 Bar, Margerian (D, Mergerian)
Literature Force International Conference on Ingray Tirado Obtics and et al.
Optical Fiber Communication, Tokyo, Technical Digest 2so-2etH.

1983 (4th Int, Conf, on In
tegrated 0ptics and 0ptics
al Fiber Con+municatlon,
Tokyo, Technical Digest, pp.
, 260-261.1983) is a configuration diagram of an optical integrated spectrum analyzer reported in 1983).

In the figure, l is a piezoelectric substrate such as LiNb0i, 2 is a two-dimensional optical four-wave path created by depositing metal such as TI or Ni on the surface of the piezoelectric substrate 1 and then thermally diffusing the same, and 3 is a four-wavelength optical wave path.
Semiconductor lasers 4 and 5 attached to the end face of wave path 2
are the first and second geodesic lenses created on the optical waveguide 2; 6 and 7 are the first and second geodesic lenses created on the optical waveguide 2, and the first geodesic lens 4 and the second geodesic lens 5;
8 is a photodetector array attached to the end face of the optical waveguide 2 facing the semiconductor laser 3, 9 is a diverging light emitted from the semiconductor laser 3, and 10 is a parallel light. , 11 is undiffracted light, 12
is a diffracted light, and 13 is a surface acoustic wave excited by the transducer 6.

Next, the operation will be explained. Diverging light 9 emitted from the semiconductor laser 3 and guided to the optical waveguide 2 is converted into parallel light 10 by the first geodesic lens 4, enters the second geodesic lens 5, is further converted into convergent light, and is subjected to photodetection. The light is focused on the device array 8. Here transducer 6
When a high frequency electric signal is applied to the optical waveguide 2, the transducer 6 excites a surface acoustic wave 13 having a period corresponding to the frequency of the high frequency electric signal.

The frequency Δ is given by the first equation, where vl is the speed of the surface acoustic wave 13 propagating in the optical waveguide 2, and Δf is the frequency of the high-frequency electric signal.

After the surface acoustic wave 13 crosses the parallel light 10, it is absorbed by the damper 7. When the surface acoustic wave 13 crosses the parallel light 10, the surface acoustic wave 13 acts as a periodic diffraction grating for the parallel light 10. Also, since the parallel light 10 and the surface acoustic wave 13 are crossed so as to satisfy the Bragg condition, a part of the parallel light 10 is diffracted at an angle θ given by the second equation.

λ Here, λ is the wavelength of the light emitted from the semiconductor laser 3, n me
t is an effective refractive index for light guided into the optical waveguide 2. That is, the parallel light 10 is composed of the undiffracted light 11 and the diffracted light 12.
The light is divided into two parts, each focused by a second geodesic lens 5, and focused on points A and B on the photodetector array 8. The distance l between the condensing points A and B is given by the third equation, where f2 is the focal length of the second geodesic lens 5.

1-ft ・θ1 ... (3
) Here, θ1 is the angle shown in the second equation. By knowing i from the third equation, Δf can be determined.

By the way, in the first geodesic lens 4 and the second geodesic lens 5, a hemispherical depression is formed on the surface of the piezoelectric substrate 1, and a metal such as Ti or Ni is coated on the surface of the depression in the same manner as in the production of the optical waveguide 2. An optical waveguide layer is produced by thermal diffusion, and the optical waveguide layer has a refractive index distribution similar to that of the optical waveguide 2, and is connected from the optical waveguide 2 to the first and second geodesic lenses 4. When the guided light is incident on and 5, the guided light travels along the depression. At this time, the guided light travels along the shortest optical path according to Fermat's principle, so the guided light is bent by the depression, and the depression acts as a lens.

FIG. 5(a) shows how the parallel light 10 is bent by the second geodesic lens 5 and converted into convergent light. FIG. 5 (bl is a cross-sectional view of FIG. 5(a) taken along a line passing through the center of the second geodesic lens 5.
In Figure (b), reference numeral 14 represents a four-wave optical layer formed in the recess, and is connected to the optical waveguide 2. The first geodesic lens 4 also has a similar structure.

[Problem that the invention seeks to solve]

Since the conventional optical integrated spectrum analyzer is configured as described above, two geodesic lenses 4 and 5, the first and second geodesic lenses, are required.

However, the first and second geodesic lenses 4
and No. 5 required advanced processing technology because the depressions had to be cut on the order of 1 micron. The above processing is performed using a computer-controlled diamond cutting tool, etc., but the recesses required for geodesic lenses are smooth curved surfaces. Such a curve is divided into steps as shown in FIG. 6('b). However, if the optical waveguide layer 14 has a step as shown in FIG. 6('b), the light propagating through the optical waveguide layer 14 will suffer a large loss. FIG. 7 shows the relationship between the amount ΔX of the step and the loss of light generated at the step. The figure shows the case where TE mode light is guided through the light waveguide layer 14 made by thermally diffusing Ti into LiNb0°, and C, D and E in the figure indicate the light guides, respectively. 1.28 μm from the surface of the wave layer 14,
The above TE has a depth of 1.09 μm and 0.95/Jm,
This shows the case where the mode propagates. For example, in E in the figure, if the slip Δx of the step is 0.4 μI, a loss of about 1 dB will occur, and if there are many steps as described above, a very large loss will occur. Therefore, after cutting, the particle size was 0.3.
It is essential to perform polishing using an abrasive of approximately μm size to reduce the amount ΔX of the step. However, it is difficult to polish uniformly over the entire surface of the depression,
Since insufficient polishing or excessive polishing occurs in some parts, not only the above-mentioned step cannot be completely eliminated, but also the required curved surface shape is deformed. For this reason, the first and second geodesic lenses 4 and 5 have the problem of not only large losses but also large aberrations. Furthermore, since machining the curved surface portion as described above requires advanced machining techniques and a lot of machining time, there is a problem in that the geodesic lens described above becomes extremely expensive.

This invention was made in order to solve the above-mentioned problems, and by giving the optical waveguide itself a lens function that focuses the light incident from one end face onto the opposite end face, the above-mentioned problem can be solved. The purpose of the present invention is to obtain an optical integrated spectrum analyzer that does not require the cutting and polishing of curved surfaces required for processing geodesic lenses, and can be configured with inexpensive lenses with small aberrations.

[Means for solving problems]

In the optical integrated spectrum analyzer according to the present invention, the refractive index distribution of the optical waveguide is uniform in the depth direction (two directions), and the optical axis direction (y direction) of the output light of the semiconductor laser and the depth of the optical waveguide are uniform. In the direction (X direction) perpendicular to the direction (two directions), the input end face and the output end face of the opposing optical waveguides are distributed so that they are almost in an imaging positional relationship with each other, and the refractive index of the piezoelectric substrate is Furthermore, the surface acoustic wave is made to propagate approximately midway between the input side end face and the output side end face of the optical waveguide.

[Effect]

In this invention, the optical waveguide itself is a gradient index optical waveguide having a lens function, and the refractive index of the optical waveguide is higher than that of the piezoelectric substrate, and the optical waveguide is attached to the input side end face. Since the light emitted from the semiconductor laser is guided and the light emitted from the semiconductor laser is imaged at the output end facet, a surface that is excited by the transducer and propagates approximately midway between the input end facet and the output end facet. The guided light diffracted by the elastic wave is imaged at a position different from the imaged position of the undiffracted light on the output side end face,
The frequency of the electrical signal applied to the transducer can be determined from the difference between the imaging positions of the diffracted light and the undiffracted light, making it possible to eliminate the need for a geodesic lens that requires cutting and polishing of curved surfaces.

〔Example〕

An embodiment of the present invention will be described below with reference to the drawings. 1st
The figure is a configuration diagram showing an embodiment of the present invention. In the figure, 1 is a piezoelectric substrate such as a fused crystal plate, 3 is a semiconductor laser, 6 is a transducer, 7 is a damper, 8 is a photodetector array, 11 is undiffracted light, 12 is diffracted light, and 13 is surface acoustic wave. , 15 is a gradient index optical waveguide.

First, the gradient index optical waveguide 15 will be explained. FIG. 2 is a diagram showing only the gradient index optical waveguide 15 in FIG. 1, and 16 is an input (+
1. +1 end face 17 is the output side end face to which the photodetector array 8 is attached. The X-axis direction of the coordinate axes in the figure is a direction parallel to the surface of the gradient index optical waveguide 15 in a plane parallel to the input-side end surface 16, the y-axis direction is a direction perpendicular to the input-side end surface 16, and the Z-axis direction is a direction parallel to the surface of the gradient index optical waveguide 15. This is the depth direction of the gradient index optical waveguide 15.

FIG. 3 shows the refractive index distribution in each axis direction of the optical waveguide 15, and FIG. 3(a) shows the refractive index distribution n(x
), Figure 3 (bl is the refractive index distribution (y) in the y-axis direction,
Figure (C) shows the refractive index distribution n (x) in the z-axis direction, and 2
Xo, yo, and 2zo are the width, length, and depth of the four refractive index gradient optical wave paths 15, respectively.

The refractive index distribution type optical waveguide 15 is an optical waveguide having a refractive index distribution only in the X-axis direction, as shown in FIG.

For example, the refractive index distribution n (x) in the X-axis direction is expressed by the fourth formula % formula %
(4) Here, no is the maximum refractive index, and g is a constant representing the refractive index gradient. By appropriately setting no and the length yo of the gradient index optical waveguide 15 in the fourth equation,
The input side end face 16 and the output side end face 17 can be in an imaging positional relationship with each other. Note that the four-wavelength optical waveguide having the refractive index distribution as described above is sold by HOYA Co., Ltd. under the trade name of "unidirectional refractive index distribution type slab lens."

This lens has a depth of about 5B in the two directions shown in Figure 2.
Here, after bonding the lens to a piezoelectric substrate l made of fused quartz, the lens is cut and polished until the thickness is equal to or less than the wavelength of the surface acoustic wave 13 excited by the transducer 6. A wave path 15 can be formed. Note that the cutting and polishing processes described above are relatively simple since they are performed only on the flat surface. By the way, in the gradient index optical waveguide 15, the refractive index of the molten crystal is n.
When q, at least the fifth equation (2) must be satisfied, and in order to further reduce the loss of guided light, the sixth equation must be satisfied.

no > n, ...(
5) n(±xo)>n,...
(6) Now, no = 1.470. g=0.088
When using the above unidirectional gradient index slab lens, n is -1,457, so Xo = 1.2 weight, y
By setting o-35, 6, it is possible to realize a gradient index optical waveguide 15 that satisfies the 6th condition.

Next, the operation will be explained. When no high-frequency electrical signal is applied to the transducer 6, the light emitted from the semiconductor laser 3 attached to the input side end face 16 of the gradient index optical waveguide 15 and guided through the gradient index optical waveguide 15 is The light is focused at the output side end face 17 at the heat input point shown in FIG. The transducer 6 is located approximately midway between the input side end face 16 and the output side end face 17, and when a high frequency electric signal is applied, excites the surface acoustic wave 13, thereby causing light to be guided through the gradient index optical waveguide 15. A part of the light is diffracted and focused on point B in FIG. Distance a between the above points A and B and diffraction angle θ1
The relationship is given by the seventh equation by replacing +2 with y0/2 in the third equation.

The first and second equations that relate the distance β in the seventh equation to the frequency Δf of the high-frequency electric signal applied to the transducer 6 hold true as they are, and by knowing the distance β, the above Δf can be determined.

As described above, the optical integrated spectrum analyzer of this embodiment does not require advanced processing techniques such as cutting and polishing curved surfaces to create a geodesic lens, and can be inexpensive because it only requires processing a flat surface.

In addition, slab lenses with a refractive index distribution in only one direction used in the gradient index optical waveguide 15 are stably manufactured with good performance by various manufacturers using the molecular stuffing method or the ion exchange method. Even if the above-mentioned plane processing is applied to the lens, its function as a lens is not impaired in any way, and an optical integrated spectrum analyzer with stable performance can be obtained.

In the above embodiment, the piezoelectric substrate 1 made of fused quartz is used, but any piezoelectric substrate may be used as long as it has a refractive index lower than the gradient index optical waveguide 15. Furthermore, in the above embodiments, the refractive index distribution in the X-axis direction is given by No. 4, but the refractive index gradually decreases in the positive and negative directions of the X-axis.
Any distribution may be used as long as the input side end surface 16 and the output side end surface 17 are in an imaging positional relationship with each other. Furthermore, in the above embodiment, a case was explained in which the semiconductor laser and the photodetector laser were directly attached to the input side end face and the output side end face.
This may be coupled to the input end face and the output end face via an imaging optical system.

〔Effect of the invention〕

As described above, according to the present invention, in the optical integrated spectrum analyzer, the optical waveguide is constructed using a gradient index optical waveguide which itself has a lens function and a piezoelectric substrate having a lower refractive index than the optical waveguide. Therefore, the device can be made inexpensive, have good performance, and be stable.

[Brief explanation of drawings]

FIG. 1 is a configuration diagram showing an optical integrated spectrum analyzer according to an embodiment of the present invention, FIG. 2 is a diagram showing coordinate axes set in a gradient index optical waveguide, and FIG. 3 is a diagram showing refraction of the gradient index optical waveguide. Figure 4 is a diagram showing the configuration of a conventional optical integrated spectrum analyzer, Figure 5 is a diagram showing the function and configuration of a geodesic lens, Figure 6 is a diagram showing the processing state of the curved surface of the geodesic lens, FIG. 7 is a diagram showing the relationship between steps and loss in geodesic lens processing. In the figure, 1 is a piezoelectric substrate, 2 is an optical waveguide, 3 is a semiconductor lens, 4 is a first geodesic lens, 5 is a second geodesic lens, 6 is a traverse juicer, 7 is a damper, 8
is a photodetector array, 9 is a diverging light, 10 is a parallel light, 11 is a non-diffracted light, 12 is a diffracted light, 13 is a surface acoustic wave, 14 is a light waveguide layer, 15 is a gradient index optical waveguide, 16 is an input side end face, and 17 is an output side end face. Note that the same reference numerals in the figures indicate the same or equivalent parts.

Claims (2)

[Claims] (1) A two-dimensional optical waveguide having an input side end face and an output side end face facing each other that are substantially parallel, and a guided light provided between the input side end face and the output side end face and extending from the input side end face to the output side end face. a piezoelectric substrate, the surface of which is provided with a transducer that excites surface acoustic waves that cross obliquely and diffract a portion of the guided light, and a damper that absorbs surface acoustic waves that arrive obliquely with the guided light; , a photodetector array that receives the guided light diffracted by the surface acoustic wave from the input end face directly or via an optical system, and a light receiving position of the diffracted guided light on the photodetector array; In an optical integrated spectrum analyzer that analyzes the frequency of an electric signal applied to the transducer from the input side, When the depth direction of the optical waveguide is taken as the z coordinate, the optical waveguide has a uniform refractive index distribution in the y and z coordinate directions, and the refractive index n_0 at the origin is the maximum in the x coordinate direction. Using a refractive index distribution that gradually decreases in the positive and negative directions of the coordinates and that makes the input side end face and the output side end face approximately in an imaging positional relationship with each other, the piezoelectric substrate is provided with: An optical integrated spectrum analyzer characterized in that an optical integrated spectrum analyzer is used that has a refractive index n_q satisfying the relationship n_q<n_0 with respect to the maximum refractive index n_0 of the optical waveguide. (2) In the piezoelectric substrate, where n_s is the minimum refractive index of the optical waveguide, n_q≦
2. The optical integrated spectrum analyzer according to claim 1, characterized in that an optical integrated spectrum analyzer having a refractive index n_q having the relationship n_s is used.