JP3448779B2 - Fiber optic sensor - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber sensor and an optical waveguide sensor used in the field of ultra-precise measurement of vibration amplitude or distance change. 2. Description of the Related Art Conventionally, a light interference measurement method has been used for precise measurement of vibration and distance change. The method is to irradiate a reference light to an object to be measured and cause the reflected light or transmitted light (hereinafter referred to as measurement light) to interfere with the reference light, thereby generating an interference phenomenon,
For example, distance change is measured by interference fringes or beat output. That is, if the interference fringes in one state are observed, the strength of the fringes is reversed, and the relative distance to the object to be measured is moved until the fringes become similar, the moving distance at that time corresponds to the wavelength of light. Therefore, if light having a known wavelength is used, the amount of movement of the relative distance can be known by utilizing the interference phenomenon between the reference light and the reflected light. In addition, a fiber for irradiating the target with light and a fiber for receiving reflected light are separately provided. [0003] The conventional optical interferometry is also capable of high-precision measurement because it is a phase comparison of light, but when only one interference phenomenon is observed, a change in relative distance cannot be detected. However, since the direction of change cannot be determined, a means for separately determining the moving direction was required. Therefore, even if the direction of the relative distance change is clear, it is not possible to detect whether the vibrating surface is approaching or moving away, as in the case of measuring a vibrating surface that vibrates back and forth. there were. In addition, since the fiber that irradiates light to the target and the fiber that receives reflected light are separate, a specific reflection angle is required to make the reflected light incident on the fiber on the light receiving side, and there is a limit to bringing the optical fiber sensor close to the target. However, the light receiving sensitivity was also reduced due to insufficient access. According to the present invention, four fiber type multiplexers / demultiplexers are used as means for solving the problems and connected as shown in FIG. In FIG. 1, the input path of the reference light and the input path of the measurement light are multiplexer / demultiplexers, respectively, and in this case, they operate as a demultiplexer. Therefore, the reference light is branched at the で and guided to the multiplexer / demultiplexer. Similarly, the measurement light is branched and guided to the multiplexer through the path (10). The reference light and the measurement light guided to the multiplexer are combined and output to (11) and (12). In these combined outputs, the reference light and the measurement light interfere with each other, and the intensity of the output light changes in accordance with the phase difference between them. Therefore, by applying the outputs of <11> and <12> to separate photodetectors <19> and <20>, changes in the intensity of these output lights can be detected. The present invention is characterized in that two portions for interfering the reference light and the measurement light, which are generally used in the optical interference method, are provided, and the reference light and the measurement light are guided and interfered with each other to obtain two types of beat outputs. Further, a comparison unit having a quarter-wave phase different from each other is provided so that a pair of interference outputs having a phase difference of 90 degrees can always be obtained, and whether the phase advances when a Lissajous is drawn on the XY coordinates. The direction of displacement can be detected by delaying. In addition, an optical multiplexer / demultiplexer (14) was inserted between the transmission and reception to easily receive the reflected light of the light irradiated on the target. The operation will be described with reference to FIG. Fig. 1 shows an optical fiber multiplexer / demultiplexer.
0 ▼ 11 線路 12 is an optical fiber line,
Operates as a demultiplexer, and operates as a multiplexer.
The line length of (10) is exactly the same,
Only (10) is extended by a quarter wavelength. Now, when light propagates on the line, system input light is added to the multiplexer in half at a time. The input of the next system is applied to the multiplexer at a phase difference of 90 degrees, although the amplitude is １ ／. The output of the multiplexer is guided to the fiber lines (11) and (12) and the optical detectors (19) and (20).
And is converted to a current. When two types of light are incident on the photodetector, the operations interfere with each other, resulting in two-wave beat output.
At this time, the 1/4 wavelength delay of the input wave, that is, the 90-degree delayed phase is also output to the detector output with a 90-degree phase delay at the beat wavelength, so that the beat outputs of (19) and (20) are orthogonal to each other. Signal. Therefore, if both are applied to the X-axis and the Y-axis of the CRT oscilloscope, respectively, a circular Lissajous figure is drawn with these beat outputs, and the rotation direction of the Lissajous figure indicates the direction of the phase delay, that is, the change of the relative position with respect to the object. Will be. The length of {10} need not necessarily be longer by a quarter wavelength, but may be any length as long as it is not exactly an integral multiple of half a wavelength. The Lissajous in this case is generally an ellipse, but the rotation direction can be determined, and the operation of the present invention can be achieved. The present invention can achieve the same effect not only by using an optical fiber but also by using a solidified optical waveguide integrated circuit. In addition, by inserting an optical multiplexer / demultiplexer (14) at the tip and integrating the transmission and reception, the irradiation light and the reflected light pass through the same path, so that the relationship between the target reflection surface and the incident light is always vertical and the sensor end surface The distance between the reflector and the reflecting surface was unconditional. FIG. 2 shows an embodiment of the present invention.
The operations up to (10), (11), (12), (19), and (20) are as described above. ▲ 13 ▼ ▲ 14 ▼ ▲ 15
▼ (16), (17) and (18) are the parts added for actual measurement, (13) and (14) are optical fiber duplexers, and
Reference numeral 15 denotes a fiber line for guiding light from the light source to the tip of the sensor, and reference numeral 16 denotes a 1.55 micron DF laser light source.
A B laser was used. (17) is a target, which is set in the direction of the arrow, that is, the direction in which the reflection surface is orthogonal to the light beam and the direction of displacement is parallel to the light beam. (18) is the tip of the sensor fiber, and the light guided through the multiplexer / demultiplexer (14) is emitted from the tip of (18), reflected by the target (17) and returned again to (18).
A part of the light is guided to the demultiplexer (14) by way of. On the other hand, the light from the laser light source (16) is branched by the splitter (13) and becomes reference light. Photodetector ▲ 19 ▼ ▲ 20
The output of ▼ is connected to the X-axis and Y-axis of the CRT oscilloscope, respectively, and when the target 17 is vibrated, a circular Lissajous figure appears on the CRT surface and the amount of vibration displacement is one rotation of the Lissajous every half wavelength of the laser light source. The rotation direction of the Lissajous figure was reversed depending on the direction of the vibration displacement that was confirmed, and the expected result was obtained. From these results, the target (17)
The amount of vibration displacement or change in distance of the Lissajous can be determined by measuring the rotation angle of the above Lissajous, and whether the target (17) approaches or retreats can be determined by measuring the direction of rotation of the Lissajous clockwise or counterclockwise. Can be done. Note that a conventional method can be used for these measurements. It is further improved if an optical directional coupler is inserted into the line (15) to prevent the measurement light from returning to the light source side. Optical fiber used External shape Single-mode adaptive wavelength of 1.3 micron to 1.6 micron using 125 micron quartz as basic material Measurement wavelength 1.55 micron Light source Semiconductor laser Effect of the present invention As a result, a wide range of ultra-high-precision measurements were possible using a phase meter and counter.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a connection diagram of the present invention. FIG. 2 is an embodiment of the present invention. [Description of Symbols]: Demultiplexer: Demultiplexer: Multiplexer: Multiplexer: Reference optical transmission line: Measurement optical transmission line: Branched reference optical transmission line: Branched reference optical transmission line: Branched measurement light Transmission line {circle around (10)}: A branched measurement light transmission line which is extended by ４ wavelength. {11}: Transmission line between the multiplexer and the detector. {12}: Transmission line between the multiplexer and the detector. {Circle around (13)}: Demultiplexer for reference light <14>: Input / output coupler / demultiplexer <15>: Reference light transmission line <16>: Laser light source <17>: Target <18>: Tip of sensor fiber <▲ 19 ▼: Photodetector 1 ▲ 20 ▼: Photodetector 2

──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Yoshifumi Inoue 199-6 Kita-Iizuka, Oamishirasato-cho, Chiba Prefecture (56) References JP-A-63-47602 (JP, A) (58) Fields investigated (Int. Cl ^7, DB name) G01B 9/00 -. 11/30 G01D 5/26

Claims (1)

(57) [Claim 1] In an optical interferometer for obtaining a beat by interfering a reference light and a measurement light taken out of the same light source as the reference light and passing through a path for measurement, a reference light and a measurement are obtained. Each of the light branches,
The structure has at least two sets of combined output ends obtained by combining the branched outputs with the reference light and the measuring light, and detects a phase relationship between the two sets of combined outputs. The lengths of the four optical lines leading to the coupling portion are almost the same for the three systems, and only one system (wavelength / 4) or a length longer or shorter by an arbitrary length corresponding to exactly an integer multiple of half a wavelength is avoided. An interference type optical fiber distance sensor and an interference type optical waveguide distance sensor.