GB2179146A

GB2179146A - Optical sensor

Info

Publication number: GB2179146A
Application number: GB08618479A
Authority: GB
Inventors: Masanori Watanabe
Original assignee: Sharp Corp
Current assignee: Sharp Corp
Priority date: 1985-07-31
Filing date: 1986-07-29
Publication date: 1987-02-25
Also published as: DE3625703C2; GB8618479D0; DE3625703A1; JPS6230921A; GB2179146B

Abstract

An optical sensor comprises a light source (41), a first Fabry-Pérot interferometric device (42), the characteristics of which vary with the physical quantity of an object to be measured, a second Fabry-Pérot interferometric device (43) functioning as a reference standard, the operational conditions of which are set at fixed valued, a first photodetector (46) for detecting light coming from the light source (41) through the first Fabry-Pérot interferometric device (42), a second photodetector (45) for detecting light coming from the light source (41) through the first Fabry-Pérot interferometric device (42) and the second Fabry-Pérot interferometric device (43), and a signal-processing circuit for comparing the output signal from the second photodetector (45) with the output signal from the first photodetector (46). <IMAGE>

Description

SPECIFICATION Optical sensor This invention relates to an optical sensor for detecting physical quantities. More particularly, it relates to an optical sensor using Fabry-Pérot interferometric devices.

A conventional optical sensor using Fabry-Pérot interferometric devices is provided with a pair of reflecting faces in a parallel manner so as to create an optical interference effect therebetween, the output power varying even upon a slight displacement (which is smaller than an optical wavelength) of the reflecting faces, so that the optical sensor can have excellent in sensitivity and can be used for a variety of purposes.

Fig. 4 of the accompanying drawings shows the typical structure of a conventional optical sensor using a Fabry-Pérot interferometric device, wherein a laser device 11 such as a HeNelaser oscillating device oscillates a single-color light with a stable wavelength, which travels to a Fabry-Pérot interferometric device 12. Transmitted light from the Fabry-Pérot interferometric device 12 is then changed into an electrical signal by a photodetector 13. The amount of transmitted light depends upon the optical wavelength, the optical path length of the Fabry-Pérot interferometric device 12, the reflectivity of the reflecting faces of the Fabry-Pérot interferometric device 12, etc.

The light source used in the above-mentioned simple structure must be stable. Semiconductor lasers available on the market as a small-sized laser light source are useless because of their insufficient performance.

Gas lasers, etc., can be used as a light source for the optical sensor, but they make the measuring system large, heavy and expensive.

Fig. 5 shows the structure of another conventional optical sensor using a small-sized and inexpensive light source such as a light-emitting diode (LED), etc. Light from a light source 21, which can emit light with a relative wide range of wavelengths, passes through a first Fabry Pérot interferometric device 22, the characteristics of which vary with the physical quantity of the object to be measured, and through a second Fabry-Pérot interferometric device 23, the characteristics of which do not vary with the physical quantity of the object, and reaches a photodetector 24 in which its optical intensity is changed into an electrical signal.Figs. 6(A) to 6(C) show operations of the optical sensor shown in Fig. 5, wherein Fig. 6(A) shows the radiation spectrum intensity l(A) of the light source 21 when the abscissa indicates the wavelength A, Fig. 6(B) shows the spectral transmittance T1(A) of the first Fabry-Pérot interferometric device 22, and Fig. 6(C) shows the spectral transmittance T2(A) of the second Fabry-P6rot interferometric device 23.When both T,(A) and T2(A) are represented by the same curve a, that is, the T,(A) curve corresponds to the T2(A) curve, the integral of a composed spectral transmittance T1(A).T2(A) with regard to the wavelength becomes maximum. On the other hand, when T,(A) and T2(A) are represented by curve b and curve a, respectively, that is, the peaks of the T,(A) curve are positioned between the peaks of the T2(A) curve, the integral of the composed spectral transmittance T,(A).T2(A) with regard to the wavelength becomes small.Thus, when the radiation spectrum intensity l(A) shown in Fig. 6(A) is considered in addition to the abovementioned integration, as shown in Fig. 6(D), the optical intensity at the photodetector device 24 can be represented by the area surrounded by curve Si and the abscissa when both T1(i) and T2(A) are indicated by curve a and it can be represented by the area surrounded by curve S2 and the abscissa when T,(A) and T2(A), respectively, are indicated by curves a and b. Strictly speaking, it is better that the dependence of the sensitivity of the photodetector 24 on the wavelength is considered, as well, in order to determine the optical intensity at the photodetector 24.

Although the conventional optical sensor using two Fabry-P6rot interferometric devices 22 and 23 shown in Fig. 5 detects physical quantities to be measured as a variation in the amount of light, the optical intensity at the photodetector varies with changes in light-emission intensity and/or scatter of connection losses arising when optical devices such as optical lenses, optical fibers, etc., are inserted into this optical measuring system, so that a stable-sensor output power can not be obtained by the conventional optical sensor.

It is an object of the present invention to provide an optical sensor which overcomes the above-discussed and other disadvantages and deficiencies of the known devices.

In accordance with the present invention there is provided an optical sensor comprising a light source, a first Fabry-Pérot interferometric device, the characteristics of which vary with the physical quantity of an object to be measured, a second Fabry-Pérot interferometric device functioning as a reference standard, the operational conditions of which are set at fixed values, a first photodetector for detecting light coming from the light source through the first Fabry-Pérot interferometric device, a second photodetector for detecting light coming from the light source through the first Fabry-Pérot interferometric device and the second Fabry-Perot interferometric device, and a signal-processing circuit for comparing the output signal from the second photode tector with the output signal from the first photodetector.

The optical sensor further comprises, in a preferred embodiment, an optical means for dividing a beam of light from the first Fabry-Pérot interferometric device into two elements, one of which is directed to the first photodetector and the other of which is directed to the second photodetector through the second Fabry-Pérot interferometric device.

The light source produces, in a preferred embodiment, light having a wide spectral band width.

Although output powers of the first photodetector and the second one vary in the same manner depending upon changes in light-emission intensity, changes in wavelength, and scatter of connection losses arising when various optical devices such as optical lenses, optical fibers, etc., are inserted into this sensor system, the dependence of the amount of light reaching the first photodetector on the physical quantity of the object to be measured can be, in a preferred embodiment, very much smaller than that of the amount of light reaching the second photodetector on the physical quanity.

Thus, the invention described herein makes possible the provision of an optical sensor which comprises two Fabry-Pérot interferometric devices and which gives sufficiently stable signals regardless of changes in light-emission intensity, changes in wavelength, and scatter of connection losses arising when various optical devices such as optical lenses, optical fibers, etc., are inserted into this sensor system.

The invention is described further hereinafter, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a diagram showing one embodiment of an optical sensor in accordance with the present invention; Figure 2 shows a curve illustrating the radiation spectrum of a light-emitting diode, for explaining operations of an optical sensor in accordance with this invention; Figure 3 shows characteristic curves of output powers of the photodiodes used in the optical sensor shown in Fig. 1.

Figures 4 and 5 respectively, are diagrams showing conventional optical sensors; Figures 6(AJ to 6 (D) are graphs illustrating operations of the conventional optical sensor shown in Fig. 5; and Figure 7 is a graph showing output power of the photodiode 46 of the optical sensor shown in Fig. 1.

Fig. 1 shows an optical sensor in accordance with this invention, which comprises a light source such as a light-emitting diode 41, a first Fabry-Pérot interferometric device 42, the characteristics of which vary with the physical quantity of the object to be measured, a beam splitter 44 for dividing the light from the light-emitting diode 41 into two elements, one of which is directed to a first photodetector such as a photodiode 46 and the other of which is directed to a second Fabry-Pérot interferometric device 43 functioning as a reference standard, the operational conditions of which are set at fixed values and a second photodetector 45 for detecting the light from the second Fabry-Pérot interferometric device 43, and a signal-processing circuit (not shown) for comparing the output signal from the photodiode 45 with the output signal from the photodiode 46.

The light from the light-emitting diode 41 goes to the beam splitter 44 through the first Fabry Pérot interferometric device 42 and is divided into two elements in different directions. One of the two elements goes to the photodiode 46, and the other goes to the photodiode 45 through the second Fabry-Pérot interferometric device 43. The signals from the photodiodes 45 and 46 are compared with each other by the signal-processing circuit, and the physical quantity of the object to be measured can be obtained.

Instead of the light-emitting diode 41, a variety of light sources producing light with a wide spectral band width can be used. The beam splitter 44 is not essential to this system, but any optical means for dividing a beam of light into two elements (e.g., the upper half and the lower half) can be used. The light from the light source is preferably led to the photodetectors by optical fibers. The light from each of the Fabry-Pérot interferometric devices 42 and 43 is not necessarily a transmitted light, but it can be, of course, a reflected light therefrom.

Since the optical sensor of this invention is provided with a structure such as shown in Fig. 1, output powers of the photodiodes 45 and 46 vary in the same manner depending upon changes in the light-emission intensity of the light source 41 and/or changes in light transmission losses from the light source 41 to the beam splitter 44 or scatter of connection losses arising between the optical elements. However, when the characteristics of the first Fabry-Pérot interferometric device 42 vary with changes in the physical quantity of the object to be measured, output power of the photodiode 45 varies, but output power of the photodiode 46 is maintained at a fixed level. Thus, the changes in the light emission intensity of the light-emitting diode 41 and the scatter of connection losses between the optical elements can be eliminated by dividing the output power of the photodiode 45 by the output power of the photodiode 46. Alternatively, when the light emission intensity of the light-emitting diode 41 is controlled by a feedback means in order to maintain output power of the photodiode 46 at a fixed level, the output power of the photodiode 45 can be picked up, as it is, as a sensor output power.

In order to maintain output power of the photodiode 46 at a fixed level regardless of the characteristics of the first Fabry-Pérot interferometric device 42, as many peaks of the spectral transmittance Tt(Aì (i.e., Fabry-Pérot resonance peaks) as possible are required to be contained within the full-width at half maximum, hA, of the radiation spectrum of the light source. For this purpose, tA is set at a high value, or the distance t between the reflecting mirrors of the first Fabry-Pérot interferometric device 42 is set at a high value.However, in view of practical use, it is preferable that the number of Fabry-Pérot resonance peaks is as small as possible, because the full-width at half maximum, AA, is not large when a light-emitting diode is used as a light source, and moreover it is difficult to enlarge the distance t between the reflecting mirrors of a Fabry-Pérot interferometric device.

The internal optical path length of the first Fabry-Pérot interferometric device.42 is discussed below by reference to Fig. 2 in order to meet the requirement that the output power of the photodiode 46 is maintained at a fixed level regardless of the characteristics of the first Fabry PBrot interferometric device 42.

Fig. 2 shows the radiation spectrum intensity of the light-emitting diode 41 having the center wavelength A0 and the full-width at half maximum, AA, wherein the abscissa indicates the wave number, i.e., the reciprocal of the wavelength A. It is hypothesized that the spectral curve is symmetrical about the axis of ordinate which corresponds to the center value 1 /Ao of the abscissa indicating 1/A, and that the curves AB and BC are symmetrical about the point B which indicates the half maximum value of the reaction spectrum intensity.

The state that the resonance peaks based on the Fabry-Pérot effect are contained within the abovementioned radiation spectrum zone is discussed below: When the distance between the reflecting mirrors of the Fabry-Pérot interferometric device is t and the refractive index therebetween is n, the spectral transmittance T(A) of the Fabry-Pérot interferometric device attains a maximum value at the m order resonance wavelength Am which is represented by the following equation: A,,,=2nt/m (m=1, 2, 3,...) Alternatively, it can be represented by the reciprocal of the wavelength (i.e., the number of waves) as foilows:: 1/A,,,=m/2nt The 1/1,,, is defined herein as the wave number of the m order resonance peak. The difference between the wave number of the m order resonance peak and the wave number of the m+ 1 order resonance peak is represented by the following equation: 1/A,,.,-1/A,,,=1/2nt Point b in Fig. 2 indicates the wave number of the m order resonance peak, 1/Am. The amount of light permeating this resonance peak is presumed to be approximately proportional to the segment Bb when the resonance peak is sufficiently steep. When the position of b is shifted to the ieft or to the right, the length of the segment Bb is changed (e.g., into that of the segment B'b').However, the length of the segment B'b' plus the segment D'd' is maintained at a fixed value even though point B' is shifted in the range of A to C, given that the length of the segment b'd' indicating the difference between the wave number of the m order resonance peak and the wave number of the m+ 1 order resonance peak is maintained at a fixed value regardless of a resonance peak shift, (e.g., the length of the segment B'b' plus the segment D'd' is equal to that of the segment Bb pius the segment Dd), wherein point d indicates the wave number of the m+ 1 order resonance peak positioned at a point which is symmetrically equidistant from the center 1/leo with respect to the point b and which corresponds to the point D on the spectrum intensity curve.The point D has the same value and the same slope as B, but the slope at near D descends while the slope at near B ascends. Alternatively, even when the position of the resonance peak is unchanged and the radiation spectral distribution is shifted along the abscissa, the length of the segment B'b' plus the segment D'd' is unchanged.

The above-mentioned discussion is based on the hypotheses that the radiation spectral distribution is symmetrical about the center axis, which is an ideal state, and that the difference 1/2nt in the wave number between the m order resonance peak and the m+ 1 order resonance peak, which is indicated by the segment b'd' in Fig. 2, is approximated to be constant regardless of the shift of the resonance peaks.

It can be presumed from the above-mentioned discussion that in order for the output power of the photodiode 46 to be maintained at a fixed level regardless of changes in the optical-path length nt, the full-width at half maximum of the radiation spectrum is in accord with the distance between the above-mentioned two resonance peaks. In order to satisfy this presumption, the internal optical-path length nt of the first Fabry-Pérot interferometric device must be changed to be approximately the same as the value of Ao/2AA when a light source having the center wavelength Ao and the full-width at half maximum, M, is used.

Possible output power of the photodiode 46 is shown in Fig. 7, wherein the abscissa indicates the internal optical-path length nt and the ordinate indicates the output power of the photodiode 46.

Fig. 7 indicates the following two phenomena: Firstly, the amplitude of the output power decreases and approaches a fixed value as the internal optical-path length nt becomes large.

This is based on the fact that when the internal optical-path length nt becomes large, a number of Fabry-Pérot resonance peaks are included within the radiation spectrum zone and the influence of these peaks on the output poweris eliminated by neutralizing the difference in these peaks.

Secondly, when the internal optical-path length nt is approximately the same as the value of A02/2M and an integral multiple of the value of Ao/2AA, the output power of the photodiode 46 becomes a fixed value. This is based on the fact that, as mentioned in the discussion of Fig. 2, some of the resonance peaks within the radiation spectrum zone act to increase the output power and the others act to decrease the output power, so that increases and decreases in output power counteract each other.

Therefore, it is preferable that the desired internal optical-path length nt is approximately the same as the value of lo/2tA or more. More particularly, it is preferable that the resonance peaks are necessarily contained within the ideal radiation spectrum zone shown in Fig. 2(A), in other words, nt is required to satisfy the following equation: ntA20/4AA.

In fact, when a light source having the A0 (=850 nm) and the AA (=50 nm) is used, the value of Ao/2AA is 7.2 ,um and the Fabry-Pérot interferometric device used is designed in such a manner that the internal optical path length nt can be changed to be about 7.2 ,um or more.

However, since the above-mentioned discussion about the output power of the photodiode 46 is carried out based on certain hypotheses, the value of 7.2 ,um obtained herein is not a condition on which the output power of the photodiode 46 is strictly maintained at a fixed level, but it is only a measure of such a condition. However, the condition that the resonance peaks are contained within the ideal radiation spectrum zone shown in Fig. 2(A) (i.e., ntA02/4AA=3.6 Am) should be satisfied.

In the above-mentioned example, an optical sensor of this invention shown in Fig. 1 was used as a temperature sensor. A0 and AA of the light emitting diode 41 were 850 nm and 50 nm, respectively. The Fabry-Pérot interferometric devices 42 and 43, respectively, were provided with semitransparent mirrors on both sides of an organic film having a relatively high thermal expansion coefficient, wherein the optical-path length nt of the organic film was 6.8 Am. The first Fabry-Pérot interferometric device 42 was operated at the temperature of the object to be measured and the second Fabry-Pérot interferometric device 43 was maintained at a fixed temperature of 25"C. Output power of the photodiode 46 was indicated by curve 1, shown in Fig. 3 and output power of the photodiode 45 was indicated by curve 12 shown in Fig. 3. The abscissa in Fig. 3 indicated the temperature of an atmosphere surrounding the first Fabry-Pérot interferometric device 42.

It can be seen from Fig. 3 that the dependence of curve Ii on temperature is sufficiently smaller than that of curve 12, so that measurement of the temperatures can be carried out by comparing output power of curve 12 with output power of curve 11.

Although the above-mentioned example only discloses a temperature sensor using materials with a high thermal expansion coefficient, it is not limited thereto. This invention is, of course, applicable to a moisture sensor or a dew condesation sensor using materials which expand by moisture, a micro-displacement sensor wherein one of the semitransparent mirrors of a Fabry Pérot interferometric device is disposed on the object to be measured and the displacement of the mirror is detected, a dynamic sensor such as a pressure sensor, a sound sensor, a vibration sensor, a load sensor, etc., using a Fabry-Pérot interferometric device, the inside of which is void, can be used.

Moreover, if a Fabry-Pérot interferometric device is designed to change the distance between its reflecting mirrors based on a variation in other physical quantities such as electricity, magnetism, etc., any kind of physical quantity will be able to be detected.

It is understood that various other modifications will be apparent to and can be readily made by those skilled in the art without departing from the scope of this invention as defined by the appendent claims.

Claims

1. An optical sensor comprising a light source, a first Fabry-Pérot interferometric device, the characteristics of which vary with the physical quantity of an object to be measured, a second Fabry-Pérot interferometric device functioning as a reference standard, the operational conditions of which are set at fixed values, a first photodetector for detecting light coming from the light source through the first Fabry-Pérot interferometric device, a second photodetector for detecting light coming from the light source through the first Fabry-Pérot interferometric device and the second Fabry-Pérot interferometric device, and a signal-processing circuit for comparing the output signal from the second photodetector with the output signal from the first photodetector.

2. An optical sensor as claimed in claim 1, which further comprises an optical means for dividing a beam of light from the first Fabry-Pérot interferometric device into two elements, one of which is directed to the first photodetector and the other of which is directed to the second photodetector through the second Fabry-PBtot interferometric device.

3. An optical sensor as claimed in claim 1 or 2, wherein said light source produces light having a wide spectral band width.

4. An optical sensor as claimed in claim 1, 2 or 3, wherein the dependence of the amount of light reaching the first photodetector on the physical quantity of the object to be measured is very much smaller than that of the amount of light reaching the second photodetector on the physical quantity.

5. An optical sensor substantially as hereinbefore described with reference to and as illustrated in Figs. 1, 2, 3 and 7 of the accompanying drawings.