JPH0584452B2 - - Google Patents

Description

[Detailed Description of the Invention] [Industrial Application Field] This invention relates to an optical interferometer that detects phase changes in light, and in particular, converts propagation phase difference into a blinking signal with good reproducibility by modularizing it. Furthermore, since it causes light interference within a thin film optical waveguide (hereinafter referred to as a slab waveguide), it does not have a spatial propagation section for light, and as a result, a very stable interference effect can be realized. This invention relates to an interferometric sensing module that can be fully applied to hybrid integrated optical sensing devices.

[Conventional technology]

Conventionally, the detection section of an optical sensing device that converts the propagation phase difference of two lights into a blinking signal using interference and photoelectrically converts it into a blinking signal, for example, the "single mode light" in the July 1983 issue of "Sensor Technology" A common method was to detect the moving speed of interference fringes generated in the air, as disclosed in ``Temperature Measurement Using Fibers: Written by Eiji Toba''.

Specifically, the laser beam is split into two by a beam splitter, narrowed down to the diameter of an optical fiber core by a microscope objective lens, and then incident on a reference fiber and a measurement fiber, and then output in the far field at the fiber output end. The emitted light interferes in the air to form Young's interference fringes. This is projected onto a screen, and a silicon photodiode cell or the like fixed to the screen is used to detect the moving speed of the interference fringes caused by the absolute phase change of the light wave as the number of interference fringes that cross a specific point.

This method has the advantage that since the amount of phase change of light can be output as a digital amount, subsequent signal processing can be simplified and slight fluctuations in the amount of light can be ignored in signal detection. However, when laser light is emitted from an optical fiber and propagates into the air, it is diffused and interference fringes spread out in a conical shape, and the numerical aperture (hereinafter referred to as NA) is large due to the large difference in refractive index between the air and the fiber core. This results in a large detection unit. Therefore, it is not practical in terms of handling. Further, unless the entire range where interference fringes occur is detected using a silicon photodiode cell or the like, it is not possible to expect an improvement in the extinction ratio. On top of that,
It is very difficult to precisely match the pattern spacing of the interference fringes to the spacing of the filter function portion of a silicon photodiode cell, and if the adjustment precision is poor, there are problems such as poor reproducibility.

[Problem that the invention seeks to solve]

As mentioned above, the conventional method of propagating laser light through the air by detecting the number of moving interference fringes has the following problems.

First, interference fringes are spread in a conical shape due to air propagation. Furthermore, since the difference in refractive index between the core and air is large, the NA becomes large. For this reason, the signal detection section occupies a very large proportion of the interference optical system, making it difficult to handle and impractical.

Second, when it is necessary to improve the extinction ratio, it is necessary to detect interference fringes distributed over a wide range on a plane. For this purpose, a photoelectric conversion element having the same range as the interference fringes existing in a wide range is required, which increases the cost.

Thirdly, it is extremely difficult to precisely match the pattern spacing of the generated interference fringes to the filter spacing in order to perform accurate signal detection, and poor adjustment accuracy results in poor reproducibility.

This invention was made to overcome the above-mentioned situation, and has the advantages of the conventional method in that the amount of phase change of light can be output as a digital amount to simplify subsequent signal processing, and there is a slight variation in the amount of light. can be ignored in signal detection, and by modularizing it using a light introducing means, a slab waveguide, and a light guiding means, the problems of the conventional technology can be solved and high stability, high stability, The present invention provides an interferometric sensing module that is a highly accurate phase detector with a high extinction ratio, low cost, and no adjustment.

[Means and actions for solving problems]

The gist of this invention will be explained based on FIG.

A slab waveguide 2 is formed by thermally diffusing impurities such as titanium onto a highly transparent substrate 1 (for example, a lithium niobate substrate) on which a slab waveguide 2 can be formed, and both opposing end surfaces of the substrate 1 are Polish the surface to prevent light from being scattered. Two input side fibers (light introduction means) 4 are fixed to one optically polished end surface of the substrate 1 so that the optical axis is aligned with the waveguide portion of the slab waveguide 2. By doing this, the 2 that has propagated through the two input fibers 4
It becomes possible to cause the two propagating lights to interfere within the slab waveguide 2 without being propagated through the air. The two propagating lights that interfered in the slab waveguide 2 are restricted in the thickness direction of the slab waveguide 2, but not in the width direction, so the other beam of the slab waveguide 2 is optically polished. Interference light in the form of a row of bright spots is output from the end face. In addition, since the bright spot spacing of this bright spot array and the diameter in the width direction of the slab waveguide of the bright spot are determined by the length of the optical propagation path, that is, the slab waveguide, for example, the diameter of the slab waveguide in the width direction of the bright spot The optical propagation path length is adjusted so that the output side fiber core diameter becomes the diameter of the output fiber core. Furthermore, an output side fiber (light guide means) 8 is fixed onto the output end face of the bright spot array, which is the other end face of the substrate 1, so that the optical axis is aligned with the slab waveguide 2.

With this configuration, the propagating light propagates only in the solid, and the loss until the signal light is generated can be extremely reduced. Furthermore, by using a slab waveguide, the difference in refractive index with the fiber core can be reduced, the NA can be reduced, and the divergence angle can be reduced. Furthermore, by compressing the interference fringes into a row of bright spots, the light can be concentrated in a narrower range and the amount of light can be increased compared to when detection is carried out through air propagation. Furthermore, since determining the propagation path length of the slab waveguide determines the interval between the bright spots of the interference fringes, modules with the same characteristics can be manufactured with good reproducibility, and by replacing the interference optical system with a waveguide, it is stable, compact, and lightweight. Become.

〔Example〕

FIG. 1 is a configuration diagram of an embodiment of an interferometric sensing module of the present invention.

In the figure, 1 indicates a substrate on which a slab waveguide can be formed, and 2 indicates a slab waveguide. Slab waveguide 2 is 500 Å thick on lithium niobate.
It is formed by vapor-depositing titanium and performing thermal diffusion at 1050℃. 3 and 5 indicate an input side end face and an output side end face, respectively, and 15 and 16 indicate an input side ferrule and an output side ferrule for fixing the optical fiber. 4 and 8 indicate the input fiber and the output fiber. The input side fibers 4 are arranged in pairs at 400 μm intervals in the input side ferrule 15 and adhered to each other, and after optically polishing the end faces, the optically polished input side end faces 3 It is fixed to the waveguide portion of the slab waveguide 2 so that the optical axes are aligned. After optically polishing the end face of the output side fiber 8, the optical waveguide portion of the slab waveguide 2 and the light beam are placed in the portion where the propagating light from the input side fiber 4 interferes within the slab waveguide 2. It is fixed to the optically polished output side end surface 5 so that the axes are aligned. Reference numeral 9 indicates a package, and reference numerals 10 and 11 indicate fixing points at which the input fiber 4 and the output fiber 8 are fixed to the package 9, respectively. This is to prevent external force from being directly applied to the fixed portions of the input and output fibers and the slab waveguide. In order to increase the extinction ratio, the size of the slab waveguide 2 is adjusted at the design stage so that the diameter of the bright spot is equal to the core diameter of the output fiber 8.

Below, the operation of the interferometric sensing module of the present invention will be explained based on FIG.

When the light that has propagated through the two input fibers 4 enters the slab waveguide 2 on the substrate 1, it is dispersed on its surface and propagated in a fan shape, and after propagating an appropriate distance, gradually overlaps. cause interference. However, since the interference occurs within the slab waveguide 2, the interference light is constrained in the thickness direction and no planar interference fringes appear. A linear line of bright spots is formed. The bright spot interval is determined by the optical propagation path length of the slab waveguide 2, so if the optical propagation path length of the slab waveguide 2 is shortened, the bright spot interval becomes shorter, and if it is lengthened, the bright spot interval becomes longer. In addition, since this bright spot array has a shape in which the interference portion of the two lights is compressed in the thickness direction of the waveguide, it has a linear shape unlike the planar lattice fringes obtained by normal air propagation. This property is the greatest advantage of the present invention using the slab waveguide 2, and as a result, the interference fringes form a line of bright spots arranged in a straight line, making it very easy to make a detector, and the position of the line of bright spots Since it is always present at the output end of the slab waveguide 2, positioning during the production process becomes easy. Furthermore, it is possible to obtain the same characteristics with good reproducibility simply by determining the optical propagation path length of the slab waveguide 2, and because the interference fringes are compressed, the light that should spread in a conical shape is collected and the amount of light is reduced. It also has the characteristic of being suppressed.

The bright spot row moves in the row direction due to the propagation phase difference between the two lights of the input fiber 4. Therefore, the propagation path length of the slab waveguide 2 is adjusted so that the diameter of the bright spot is the same as the core diameter of the output fiber 8, and the output fiber 8 is outputted to the output end of the slab waveguide 2. When fixed at the side fiber fixing point 11, it operates as an interferometric sensing module that can convert the number of moving bright spot arrays into blinking signals.

FIG. 2 shows an experimental system for the interferometric sensing module of the present invention, and FIG. 3 shows an oscilloscope waveform photograph showing the experimental results. This experiment
The laser beam output from the Ne laser oscillator 20,
The BS (beam splitter) 21 divides the laser beam into two equal parts, and the two parts of the interferometric sensing module 26 use the objective lenses 22 and 23 to send the divided laser beam into two equal parts.
I made it so that each input was made to the fiber on the input side of the book. A tungsten wire 2 with a diameter of 20 μm is attached to one fiber on the input side of the interference type sensing module 26.
4 was wound several times, and the tungsten wire 24 was connected in such a way that a rectangular voltage pulse was applied to the tungsten wire 24 by a pulse generator 25 to generate heat in a pulsed manner. In this way, subtle thermal fluctuations were applied to the input fiber to change the propagation phase difference of the light propagating within the fiber. And the interferometric sensing module 2
The output light outputted from the output side fiber 6 is photoelectrically converted by a photomultiplier 27, and the signal after the photoelectric conversion is outputted from an output terminal 28 to the outside. This experimental system is an example in which the interferometric sensing module 26 detects a propagation phase difference in the input fiber caused by subtle thermal fluctuations.

The upper waveform in FIG. 3 is the applied pulse signal, and the lower waveform is the output signal obtained by photoelectrically converting the output light from the output fiber. From the experiment shown in Figure 3, the phase is reversed depending on the on/off of the rectangular pulse, and the output light amount is a sine wave, so the interferometric sensing module converts the phase difference into a blinking signal. This can be confirmed.

FIG. 4 is a configuration diagram showing an example of a sensor to which the present invention is applied.

Instead of the output side fiber of the interferometric sensing module 33, a filter (light guiding means) 39 with the same pattern spacing as the bright spot spacing is fixed directly to the output end of the waveguide, and then a photodiode 40 is fixed, resulting in hybrid integrated light. The detection section of the sensing device can be easily configured.

A laser beam 31 output from a laser diode 30 is made into parallel light by a collimating lens 32. These parallel light beams are narrowed down and input to the input fibers 34 and 35 of the interferometric sensing module 33 through self-occurring lenses 36 and 37. The input fiber 35 is connected to a sensor section 38, and when a phase change occurs in the sensor section 38, the interference type sensing module 33
The interference fringes move inside. The movement of the interference fringes is output as a movement of a bright spot array at the output end of the slab waveguide and converted into a blinking signal. The flashing signal is photodiode 4
After being photoelectrically converted at 0, it is output to the outside. In this way, a hybrid integrated sensor in which the light source and the detection section are integrated can be manufactured. This was possible because the interference optical system was realized using a slab waveguide, which made it small and lightweight and enabled signal detection with good reproducibility.

〔Effect of the invention〕

As explained above, since the interference optical system is modularized and a slab waveguide is used inside it,
We were able to create a structure in which light propagates only through a solid body without propagating through the air. Therefore, leakage of propagated light is extremely reduced, making it possible to manufacture a highly efficient signal detection section. In addition, by using a slab waveguide,
A row of bright spots compressed in the thickness direction of the waveguide can now be obtained, making it easier to connect output fibers, filters, and other light output means, and improving the extinction ratio compared to when interference fringes are propagated in the air. did. As a shape feature, it is possible to reduce the size and weight, and it has become possible to perform hybrid integration of optical sensing devices, improving reproducibility and stability. Furthermore, since the bright spot interval is determined by the optical propagation path length of the waveguide, the design and adjustment of the filter becomes easy, and an improvement in industrial productivity can be expected.

[Brief explanation of the drawing]

Fig. 1 is a block diagram of an embodiment of the interferometric sensing module of the present invention, Fig. 2 is a block diagram of an experimental system for confirming the operation of the interferometric sensing module due to thermal fluctuations, and Fig. 3 is the block diagram of an embodiment of the interferometric sensing module of the present invention. FIG. 4 shows an oscilloscope waveform photograph showing the results of an experiment using the experimental system, and FIG. 4 shows a configuration diagram of a hybrid integrated sensor using the interferometric sensing module of the present invention. In the figure, 1 is a substrate, 2 is a slab waveguide (thin film optical waveguide), 3 is an input side end face, 4 is an input side fiber (light introduction means), 5 is an output side end face, 8 is an output side fiber (light guide means) ), 9 is a package, 10 is an input side fiber fixing point, 11 is an output side fiber fixing point, 15 is an input side ferrule, and 16 is an output side ferrule.

Claims (1)

[Claims] 1. A substrate 1 having a thin film optical waveguide 2 on its surface and having both opposing end surfaces optically polished, and one optically polished end surface of the substrate provided so that the waveguide portion of the thin film optical waveguide and the optical axis are aligned. multiple light introducing means 4
and the other end surface receives interference light in the form of a bright spot array on the optically polished other end surface of the substrate, which is generated by interference of a plurality of lights incident from the plurality of light introduction means within the thin film optical waveguide. light guiding means 8 provided in
An interferometric sensing module with