Fiber optical luminescence sensor for measuring mechanical displacement

A fiber optical measuring apparatus suitable for measuring physical quantities such as position, speed, acceleration, force, pressure, elongation, temperature, etc., comprises at least one optical fiber for conducting incident light between an electronic unit and a sensor, the sensor comprising at least one luminescent element. The measuring apparatus is characterized in that the physical quantity to be measured is arranged to influence the position of an optical spectral filter, for example of absorption or interference type, relative to the optical fiber in the sensor and/or relative to said luminescent element, and that said spectral filter is arranged to be situated, to a greater or lesser extent, between the end surface of the optical fiber and at least one of the luminescent elements.

TECHNICAL FIELD 
The present invention relates to a fiber optical measuring apparatus for 
measuring mechanical displacement of a member sensitive to a change in a 
physical quantity such as position, speed, acceleration, force, pressure, 
elongation or temperature, which apparatus comprises at least one optical 
fiber for conducting light between an electronic unit and a sensor, the 
sensor including at least one luminescent element. 
Throughout this specification the references to "light" should be taken to 
include electromagnetic radiation in the infrared and ultra violet bands 
and not just in the visible part of the spectrum. 
When measuring a change in a physical quantity by means of a luminescence 
sensor, problems arise in achieving high accuracy measurement over a 
substantial range of temperatures because of the wavelength displacement 
of the luminescence spectrum of the luminescent element when the 
temperature of the sensor changes. 
DISCUSSION OF PRIOR ART 
U.S. Patent Application Ser. No. 218,949, filed Dec. 22, 1980, now 
abandoned, discloses a luminescence-based fiber optical measuring 
apparatus for measuring changes in a mechanical quantity. In this prior 
proposal, any change in the mechanical quantity being measured causes a 
change in the position of a sensor that includes a luminescent material 
relative to the end of an optical fiber adjacent to the sensor. The 
advantage of this type of sensor, compared with a mechanical sensor which 
relies on the positional change of one or more reflecting members, is that 
since a wavelength change occurs in the sensor, optical reflections in the 
optical system of the apparatus can be eliminated by means of optical 
filters in the light emitters and/or in the light detectors of the 
apparatus. However, this known luminescence-based measuring device has 
proved to be of limited use when high demands for accuracy are made, and 
at the same time the sensor is subjected to wide temperature variations. 
One object of the invention is to provide a solution to the above-mentioned 
problems and other problems associated therewith. 
STATEMENT OF THE INVENTION 
The invention relates to a fiber optical luminescence-based measuring 
apparatus for measuring a physical quantity which does not have the 
above-mentioned drawbacks, such drawbacks being very serious in many 
situations where accurate measurement is required. The invention is 
characterized in that the physical quantity to be measured is arranged to 
influence the position of an optical spectral filter, for example a filter 
of the absorption or interference type, relative to the optical fiber in 
the sensor and/or relative to the luminescent element, and that the 
spectral filter is arranged to be situated, to a greater or lesser extent, 
between the end surface of the optical fiber and at least one of the 
luminescent elements. The invention is further characterized in that, 
instead of measuring the relative light intensities from two adjacently 
positioned luminescent elements with differing luminescent spectra, 
selectively excited luminescence with identical luminescence spectra is 
measured, which thereby eliminates the temperature dependence.

DESCRIPTION OF PRIOR ART APATUS 
FIG. 1 shows a light-emitting diode (LED) 2 supplied with operating power 
by a drive circuit 1, part of the light emitted by the LED 2 being coupled 
into an optical fiber branch 4, after passing through a filter 3, to be 
further transported via an optical fiber 5 to a measuring sensor 14. In 
the sensor 14, there are two semiconductor elements 15 and 16 which are 
fixed relative to one another and can be moved together, relative to the 
distal end of the fiber 5, in the directions of the arrows A in FIG. 1b, 
in response to changes in the physical quantity to be measured. The 
semiconductor elements 15 and 16 are chosen to have different luminescent 
spectra, so that, when the monitored physical quantity changes, the 
displacement of the semiconductor elements 15, 16 will change the spectral 
composition of the luminescent light which is fed back into the fiber 5. 
This luminescent light is detected by means of two photo-diodes 8 and 9, 
each of which is associated with a different optical filter 6 and 7, 
respectively, and the photo-currents of which are amplified by amplifiers 
10 and 11. The quotient of these photo-currents is produced in a quotient 
generator 12, the output of which is fed to a measuring instrument 13. The 
reading shown on the instrument 13 is thus dependent on the position of 
the semiconductor elements 15, 16 to give the required measure of the 
physical quantity causing the movement of the semiconductor elements and 
this measure is not affected by variations in the light losses occurring 
in the fiber optical system or on changes in the light emission from the 
LED 2. 
FIG. 1b shows one example of the construction of the semiconductor elements 
15 and 16. The numerals 20 and 24 designate supporting substrates, on 
which epitaxial layers 17, 18, 19, and 21, 22, 23, respectively, have been 
grown. The luminescence is generated in the inner layers 18 and 22, 
respectively, and by giving these layers different spectral band gaps, for 
example, by employing different contents of A1 in a GaAs system, and/or by 
doping with different substances (for example Ge and Si in a GaAs system), 
different luminescence spectra from the elements 15 and 16 are obtained. 
These types of structures for fiber optical sensors are well documented in 
the patent literature (see, e.g., U.S. Pat. application Ser. No. 306,349, 
filed Sept. 28, 1981, now abandoned and U.S. Pat. application Ser. No. 
318,021, filed Nov. 4, 1981, now U.S. Pat. No. 4,473,747, the entire 
contents of which are herein incorporated by reference). 
One example of the spectral relationship of the elements 15, 16 suitable in 
apparatus according to FIG. 1a are shown in FIG. 2, where 25 designates 
the emission spectrum I of the LED 2; 26a and 26b the absorption spectra 
.alpha..sub.1, .alpha..sub.2 of the layers 18 and 22 of the elements 15 
and 16, respectively; 27 the luminescence spectrum L.sub.1 from the layer 
18; 28 the luminescence spectrum L.sub.2 from the layer 22; 29 the 
transmission spectrum T.sub.1 of the filter 6 and 30 the transmission 
spectrum T.sub.2 of the filter 7. In the case of wide temperature 
variations of the sensor 14, the luminescence spectra 27 and 28 will be 
displaced with respect to wavelength, which makes the function of the 
filter 6 critical as regards the accuracy of the measuring apparatus. 
DESCRIPTION OF EMBODIMENTS ACCORDING TO THE INVENTION 
FIGS. 3a and 3b show how the above-mentioned temperature instability 
problem can be solved by the eliminating the filter 6 together with the 
associated photo-diode 8, while at the same time introducing a second LED 
31, a new photo-detector 33 and a sensor filter 38. The same reference 
numerals are used in FIGS. 3a and 3b, as were used in FIGS. 1a and 1b, for 
similar components. The mode of operation of the measuring apparatus shown 
in FIG. 3a is as follows: 
By means of a switch 37a, the LEDs 2 and 31 are alternately connected to a 
stabilizing circuit, which comprises a regulator 36, a difference 
generator 35 and a reference signal V.sub.ref. Part of the light from the 
LEDs 2 and 31 is coupled out of the fiber 5 by a partial ray divider 32 
and is sensed by the photo-detector 33, coupled to an amplifier 34 which 
also forms part of the LED stabilizing circuit. Employing a stabilizing 
circuit in this way ensures that a constant light intensity is coupled 
into the fiber 5 irrespective of whether it is coming from the LED 2 or 
the LED 31. 
The luminescent light emitted from the sensor 14 into the fiber 5 is 
detected by the detector 9, the function of the filter 7 being to block 
out light from the LEDs 2 and 31 reflected in the optical fiber system, 
that is, the filter 7 eliminates excitation light from the luminescent 
light, which latter light constitutes the required measuring signal. The 
photo-current generated by the detector 9 is amplified in the amplifier 11 
and is coupled alternately by a switch 37b, operating synchronously with 
the switch 37a, into sample and hold circuits (S&H) 39 and 40, the 
quotient of the output signals of the S&H circuits being produced in the 
quotient generator 12 and fed as the measuring signal to the measuring 
instrument 13. 
One example of a suitable sensor design for the measurement system of FIG. 
3a is shown in FIG. 3b. The upper sensor element 15 is identical with the 
sensor element 15 in FIG. 1b, but, in the lower sensor element 16, the 
layer 21 in FIG. 1b has been replaced with a layer 38 with a different 
absorption spectrum than that existing for the layer 21. Under the 
influence of excitation light from the LED 2, electrons are excited in the 
layer 18 only and therefore only this layer becomes luminescent, whereas 
under the influence of excitation light from the LED 31, both layers 18 
and 22 become luminescent. 
That this is the case is clear from the spectral distribution curves shown 
in FIG. 4, in which 25 again designates the emission spectrum I.sub.1 of 
the LED 2; 41 designates the emission spectrum I.sub.2 of the LED 31; 26 
designates the absorption spectrum .alpha..sub.2 of each of the layers 18 
and 22; 42 designates the absorption spectrum .alpha..sub.1 of the layer 
38; 43 designates the absorption spectrum of the layer 17; 27 designates 
the luminescence spectrum of each of the layers 18 and 22, and 30 
designates the transmission spectrum T of the filter 7. The LED intensity 
stabilizing circuit ensures that .intg.I.sub.1 
(.lambda.)d.lambda.=.intg.I.sub.2 (.lambda.)d.lambda., and the curves 25, 
41 and 42 are so selected that temperature-induced displacements of the 
curve 42, within the anticipated temperature range, will not cause the 
latter to intersect the curve 41. 
As will be clear from the FIG. 4, temperature-induced displacements of the 
spectrum 27 will not affect the detector signal, and therefore an accurate 
measurement of the displacement of the elements 15 and 16 in the 
directions of the arrows A can be made in the face of wide temperature 
changes of the sensor 14. In addition, the luminescence layers 18 and 22 
can be carefully matched, which further increases the accuracy of 
measurement and the reproducibility of such measurements. 
The measuring apparatus shown in FIG. 3a, can be used with a variety of 
different sensor designs. FIG. 5 shows a sensor in which the elements 15 
and 16 have been integrated onto a single substrate 20. If, for example, 
liquid epitaxy is used to produce the sensor, the layers 23, 22, 38, 19, 
18 and 17 are deposited, in the correct sequence, on the substrate 24. 
Thereafter, the layers 17, 18 and 19 are etched away by the use of, for 
example, a selective etching process, over part only of the surface of the 
sensor. In an Al.sub.1-x Ga.sub.x As system, the layers are distinguished 
by having different values of x, whereby the desired optical properties 
are obtained while at the same time providing conditions for selective 
etching. 
A somewhat simpler structure is shown in FIG. 6, the layers 17, 18 and 19 
being common to both sensor elements 15 and 16. In this embodiment of 
sensor, the layer 38 is removed by etching from the upper sensor element, 
so that this upper element exhibits luminescence during excitation of both 
LED 2 and from LED 31, whereas the lower sensor element only emits 
luminescent light when the LED 31 is the excitation source. The layer 38 
serves as a filter which is selective to incident light from only one of 
the sources 2 and 31 and, as shown in FIG. 7, can be detached from the 
luminescent element, the physical quantity to be measured then being 
arranged to move the filter 38 in the indicated directions between the end 
of the fiber 5 and the luminescent layer 18. 
To increase the resolution of the sensor, the layer 18 can be partially 
etched out to form a screen pattern, as shown in FIG. 8 (e.g. by using 
lithography). At the same time a second screen pattern 45 is formed on a 
transparent substrate 44 which defines the end surface of the fiber. In 
addition to increased accuracy, better reproducibility can be obtained 
with this arrangement since the modulation is produced by the movement 
being sensed over the entire surface of the sensor. Local defects in the 
sensor will therefore be of less importance. 
In those cases where the movement to be sensed influences the fiber 5, the 
filter 38 is attached to the fiber 5 and part of the luminescent layer 18 
on the substrate 20 is removed by etching as shown in FIG. 9. This 
improves the possibilities of providing a linear response from the sensor. 
The apparatus shown in FIG. 3a and the sensors shown in FIG. 3b and FIGS. 5 
to 9 can be varied in many ways within the scope of the following claims.