JPS6394188A - Optical fiber utilizing device - Google Patents

Abstract

PURPOSE:To eliminate the need for a 1/4-wavelength plate and an analyzer and to greatly remove noise components by setting the length of an optical fiber longer than the half of the interference distance of light from a light source. CONSTITUTION:This device consists of a sensor head 20 and a measurement part 10 and a single mode optical fiber 30 is provided between both. Then, a part of incident light from a laser 11 is reflected by the base end surface of a coupling lens 13, the base end surface of the optical fiber 30. A light intensity signal which returns through the optical fiber 30 is set <=1/2 as long as the coherence distance of this light and the light intensity signal only reciprocates by longer than the coherence distance of the light and the light intensity signal reciprocates in the optical fiber and is projected, so when the light is projected from the base end eventually is propagated by >= coherence interference distance and never interferes with its reflected wave. Namely, the reflected light only superposed as a constant-level Dc on the output signal of a photodetector 15.

Description

DETAILED DESCRIPTION OF THE INVENTION Summary of the Invention In an optical fiber-based sensor device, for example, an optical fiber displacement sensor applying a Fizeau interferometer.

Using an optical fiber with a length of more than half of the coherence length of this laser light for a laser light source with a relatively short coherence length,
By transmitting the signal through this optical fiber, interference between the signal and noise light was prevented and the noise component of the signal was removed.

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to various measuring devices using optical fibers, such as optical fiber displacement sensors using Fizeau type optical interferometers.

Prior invention In this type of optical fiber displacement sensor, light is introduced from the base end of an optical fiber, and a portion of this propagated light is reflected at the tip of the optical fiber (reference light). The light that is emitted without being reflected at the tip of the optical fiber is reflected by a mirror surface interlocked with the object to be measured and returns to the optical fiber from the tip (signal light). The reference light and the signal light that return within the optical fiber float on each other, and the tip of this light has a light intensity that corresponds to the phase difference between these lights.

This interference light emitted from the base end of the optical fiber is photoelectrically converted and the amount of displacement of the constant object is determined by counting the change in intensity.

The applicant has already made several proposals regarding such optical fiber displacement sensors. Special application 1986-967
In No. 83, the applicant proposes a fiber optic displacement sensor with improved signal-to-noise ratio. That is, a half mirror for obtaining a reference light is placed between the mirror surface that moves with the object to be measured and the tip of the optical fiber, and a quarter wavelength plate is placed between the tip of the optical fiber and the half mirror. Furthermore, an analyzer is provided at the base end of the optical fiber to allow the interference light to pass through. The light reflected from the base end of the optical fiber or the coupling lens surface for inputting light to the base end of the optical fiber becomes noise, but this noise component does not pass through the photons described in 2, increasing the S/N ratio. .

However, this device requires a 1/4 wavelength plate and an analyzer, and there is a problem in that fine angle adjustments of these elements must be made.

Summary of the Invention Object of the Invention The present invention provides an optical fiber-based sensor device that eliminates the need for a quarter-wave plate or analyzer and can significantly remove noise components.

Structure and Effects of the Invention The present invention is directed to guiding light from a light source to an optical sensor head through an optical fiber, and guiding intensity-modulated n1 constant light from the optical sensor head to a signal processing device through the optical fiber. The optical fiber is characterized in that the length of the optical fiber is longer than half the coherence length of the light from the light source.

Many noise components occur in the portion where light from the light source is introduced into the optical fiber. That is, they include light reflected at the input end (the base end) of the optical fiber, light reflected by a coupling lens placed there, and the like.

According to the invention, the intensity-modulated measurement light (e.g., a light intensity signal due to interference) that is guided from a light source through an optical fiber to a photosensor head and returned from the photosensor head through an optical fiber is an optical The fiber will go back and forth. The length of the optical fiber is more than 1/2 of the coherence length (coherence length) of the light from the light source, and this measurement light has propagated beyond the coherence distance at the base end of the optical fiber, so as described above. Does not interfere with reflected light. Therefore, this M1 constant light is not further intensity-modulated by the reflected light, which is a noise component. This noise component is only added to the measurement light as a DC component whose light intensity is constant. This improves the S/N ratio and makes it possible to perform various measurements with relatively high accuracy.

According to this invention, simply increasing the length of the optical fiber eliminates the need for a 1/4 wavelength plate or analyzer, and also eliminates the need for angle adjustment, improving work efficiency.

DESCRIPTION OF THE EMBODIMENTS FIG. 1 shows the entire optical fiber displacement sensor device.

This device consists of a sensor head 20 and a measuring part 10, between which a single mode optical fiber 30 is provided.

In the measuring section 10, the laser beam output from the laser beam ilk is passed through an isolator 12 and a beam splitter 1.
3, is condensed by the coupling lens 14 (see FIG. 2), and is input to the base end of the optical fiber 30. The isolator 12 includes a beam splitter 13 and a lens 14 . This is to prevent the reflected light from the base end of the optical fiber 30 from returning to the laser light source If.

For example, a laser diode is used as the laser light source 11. Generally, the coherence length of laser light from a laser light source is several meters to several tens of meters. single·
The length of the mode optical fiber 30 is set to be longer than 1/2 of the coherence length of the laser light emitted from the laser light source used.

An outline of the configuration of the sensor head 20 is shown in FIG. This type of sensor head 20 is disclosed in earlier applications by the applicant, Japanese Patent Application Nos. 6O-9G781 and 130-96782.

It is explained in detail in No. 01-189604 and the like. A mirror 40 is attached to a member attached to or in contact with the object to be measured itself or the δIII constant object to be measured. A rod lens 31 is arranged between the tip of the optical fiber 30 and the mirror 40.

Au is deposited on the tip of the rod lens 31, which makes the half mirror 3 have a reflectance of about 30%.
2 are configured. A rod is attached to the tip of the optical fiber 30.
The position of the lens 31 is determined so that the focal plane of the lens 31 is located. As a result, the light emitted from the tip of the optical fiber 30 is collimated by the lens 31. The distance between the tip of the optical fiber 30 and the lens 31 does not necessarily have to be equal to the focal length of the lens 31. The tip of the optical fiber 30 and the lens 31 are fixed by appropriate means.

As described above, the light that has entered the proximal end of the optical fiber 30 and propagated through the optical fiber 30 is emitted from the distal end of the optical fiber 30' in a spreading manner, but is collimated by the rod lens 31. A part of this collimated light is reflected by the half mirror 32, is condensed as it returns inside the rod peak lens 31, enters the optical fiber 30 from its tip, and propagates through the optical fiber 80 toward the base end. go. This is the reference light. The remaining portion of the collimated light traveling through the rod lens 31 exits from the lens 31, is reflected by a mirror 40, returns to the rod lens 31, is further condensed, and enters the optical fiber 30. This is the signal light.

The distance between the reference beam reflecting surface, ie, the half mirror 32, and the signal beam reflecting surface, ie, the mirror 40, is approximately several tens of square meters, and this is well within the coherence distance of the laser beam, so that the reference beam The signal light and the signal light interfere with each other, and return within the optical fiber 30 as light (light intensity signal: interference light) having an intensity representing their phase difference.

A light intensity signal emitted from the base end of the optical fiber 30 passes through a lens 14, is deflected by a beam splitter 13, is received by a light receiving element 15, for example, a photodiode, and is converted into an electrical signal corresponding to the light intensity.

The interference light intensity changes λ with respect to the change in the optical path difference between the signal light and the reference light.
It changes sinusoidally with a period of (wavelength of light).

Since the signal light travels back and forth between the half mirror 82 and the mirror 40, the optical path difference is equal to twice the amount of displacement of the mirror 40. The light intensity signal is converted into an electric signal by the light receiving element 15, and then inputted to the processing circuit 16, where the level is discriminated at an appropriate threshold level and binarized.

The rise and/or fall of this binary signal is counted by a counter. Therefore, the amount of displacement of mirror 40 is measured in units of λ/2 or λ/4. For example, if a laser diode with a wavelength of λ-0.8 μm is used as the light source 11, displacement can be measured in units of 0.4 μm or 0.2 μm.

Now, as shown in an enlarged view in FIG. 2, a portion of the incident light from the laser 11 reaches the lens surface of the coupling lens 14.

It is reflected at the base end of the optical fiber 30, etc. optical fiber 30
The returned light intensity signal (interference light) also exits from the base end of the optical fiber 30 and passes through the lens 14, so this light intensity signal mutually interferes with the reflected light. However, according to this invention.

The length of the optical fiber 30 is longer than 1/2 of the coherence length of this light. On the other hand, the optical intensity signal enters from the base end of the optical fiber, travels back and forth through the optical fiber, and exits from the same base end.
After all, when the light is emitted from the same base end, it will have propagated beyond the coherence distance.

The light intensity signal does not interfere with the reflected light. The reflected light is only included in the output signal of the light receiving element 15 as a DC component of a partial level. In this way, the reflected light component that becomes noise only appears as an increase in the DC component, so that measurement accuracy can be improved.

FIG. 3 shows another example of the optical coupling means on the base end side of the optical fiber 30. An optical adhesive 18 is attached to the base end of the optical fiber 30 to one end of the rod lens 17 as a coupling lens.
The other end surface of the rod lens 17 is coated with an anti-reflection coating 19. The refractive index of the optical adhesive 18 (
1, 5) are values between the refractive index (1, 48) of the optical fiber 30 and the refractive index (L, S) of the rod lens 17, then the reflectances of the rod lens end face and the optical fiber end face are respectively o, ots%, 0.1%, and the amount of reflected light can be considerably reduced.

Since the reflectance of the non-reflection coat 19 is 1 to 2%, it is possible to reduce the overall reflected light in this embodiment.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing the entire optical fiber displacement sensor device, FIGS. 2 and 3 are diagrams showing an example of optical coupling means on the proximal end side of the optical fiber, and FIG. 4 is a diagram showing the configuration of the sensor head. It is shown schematically. 10...M1 fixed part, 11...Laser light source. 1G...Signal processing circuit, 20...Sensor head. 30...Optical fiber. Patent applicant Tateishi Electric Co., Ltd. Agent Patent attorney Kenji Ushiku (and one other person) Figure 2 Figure 3

Claims (1)

[Claims] Light from a light source is guided to an optical sensor head through an optical fiber, and intensity-modulated measurement light from the optical sensor and head is guided to a signal processing device through the optical fiber, wherein the length of the optical fiber is defined as the light source. A sensor device using an optical fiber, characterized in that the coherence distance of light from the optical fiber is longer than half of the coherence distance of light from the optical fiber.