WO1994000750A1

WO1994000750A1 - A light-wave guide serving as a gas and/or liquid sensor and an arrangement for supervising or affecting one peripheral unit among several such units from a central unit

Info

Publication number: WO1994000750A1
Application number: PCT/SE1993/000548
Authority: WO
Inventors: Hans Martin
Original assignee: Hans Martin
Priority date: 1992-06-23
Filing date: 1993-06-22
Publication date: 1994-01-06
Also published as: SE9201921L; SE9201921D0

Abstract

The present invention includes an as a gas and/or liquid sensor (2') serving light-waveguide, into which is fed, from a light transmitter, an incoming light, and from which is transmitted, to a light detector, an outgoing light, whose spectral range is evaluable in a spectral analysis device. Said light-waveguide (2') is applied to, or is formed as, a part of a carrier (15), in the form of a stable substratum, as a loop. The outer surface of the waveguide is completely or partly exposed and adjacent waveguide sub-sections (22, 23) are closely packed. The invention likewise includes an arrangement to, from a central unit, supervise and/or affect one, out of several, selectable peripheral units, suitably using a light-waveguide, by through a light-conductive waveguide (71) transmitting and receiving information in the form of coded light bundles or light-pulses.

Description

THE TITLE OF THE INVENTION:

A light-wave guide serving as a gas and/or liquid sensor and an arrangement for supervising or affecting one peripheral unit among several such units from a central unit

TECHNICAL FIELD

The invention in question includes primary an as a gas and/or liquid sensor serving light-waveguide, into which is fed, from a light transmitter, an incoming light, and from which is transmitted, to a light detector, an outgoing light, whose spectral range is evaluable in a spectral analysis device.

The invention likewise includes an arrangement to, from a central unit, supervise and/or affect one, out of several, selectable peripheral units, suitably using a light-waveguide, by through a light-conductive waveguide transmitting and receiving information in the form of coded light bundles or light-pulses.

BACKGROUND PRIOR ART

To use a light-waveguide into which is fed, from a light transmitter, an incoming light, and from which is transmitted, to a light detector, an outgoing light, whose spectral range is evaluable in a spectrai analysis device, in order to obtain a gas and/or liquid sensor is known in several previously used examples. Thus reference can be made to the "Laser Focus World" publication, issued april 1992, pages 161-168 in the article "Optical Fiber Simplifies Gas-sensing Systems".

Gas measuring (as the presence of a selected gas in a gas mixture, or the concentration of a selected gas in a gas mixture) by means of optical methods is widely used in different applications and is considered to be very reliable. There are a large number of techniques, where the unique absorption spectrum of the molecules of the selected gas is used in order to calculate and evaluate the presence and/or the molecule concentration in question.

All optical gas measuring systems or liquid measuring systems used for this purpose consist basically of three essential components;

1 ) a light source

2) an absorption volume

3) a detection system

Even though the following description will restrict the application to what is essential for a gas measuring under atmospheric conditions, where the greatest measuring need is to be found, the principles will also apply to all optical measurement situations, in gas as well as in liquid.

1. Light source.

It is known that the emission spectrum of the light source can have a wide band-width, as in for example a light bulb or a xenon high-pressure bulb, alternatively have a narrow band-width, as in a laser. A compromise between these two extremes could be a LED or an optical band pass filter in combination with a wide band-width light source, which can be made to generate light within a smaller wavelength interval.

Most measuring methods depend in some form on the fact that the molecules^' wavelength-dependent absorption of the wavelength spectrum of the light source appears in contrast against a background. The greatest possible sensivity can be achieved if a laser is used, which should, in addition, be tunable in its emitting wavelength, so that a measurement can be made differentially, with the laser wavelength in resonance as well as out of resonance with the quantum physical absorption frequencies of the measured object.

A method to automatically lock a laser wavelength prior to such measurements and to stabilize the frequency of the same during an unlimited amount of time is described in the publication PCT/SE90/00633 (WO 91/06141 ).

2. Volume of absorption.

The volume of absorption can be formulated in different ways depending on the measurement requirements. Performance must often be balanced against production costs and with the physical size of the measuring unit. Of fundamental concern is, namely, the fact that the magnitude of the molecule absorption is dependent on the time the light is present in the medium to be used for measurement, which in practice means that the size of the unit depends on the measuring distance the resonant light has passed through the measuring medium.

Absorbed power: l(z)/l(z=0) = exp. (- s_χn z);

jwhere l(z) is the incident resonant electromagnetic effect after the distance "z" in the absorbing medium with the absorption cross-section "s ", "n" is the number of absorbing molecules per unit of volume, "s^¹ is strongly wavelength dependent and gives rise to the spectrum that is charasteristic for each substance measured in the medium.

It has appeared that during exceptional conditions and low demands on sensivity an absorption distance of a few millimeters is enough, but if a high sensivity is required in a measuring system a much longer distance is generally needed.

For measurements of trace elements (presence and concentration) out of doors, a so-called open-path measurement is used, where a transmitter, via a telescope, transmits the light several kilometers through the measuring medium. The DOAS technique is a example of this where a wide band-width light source is used and a computer analysis of the resulting absorption spectrum gives presence and concentration of several of the present trace molecules.

Diode laser systems with telescopes and retro-reflectors are examples of other sensitive open-path measuring systems for gas-analysis. In the latter case extremely small amounts of substances in the air can be detected.

Another common way of obtaining a long absorption distance is to use so- called multi-pass absorption cells. These consist of mirror arrangements, e.g. a White-cell, where the light is forced to go back and forth approximately a hundred times and a physically more compact measuring cell, with a small measuring volume, can in such a way be obtained for a given distance. This is often used in laboratories and in medical techniques, where the lack of space forces a physically compact solution of the problem of creating a long absorption distance.

3. Detection system.

Early detection systems of narrow band-width light sources consisted of spectrographs equipped with photographic film. When using wide-band light sources, spectrophotometers with CCD or diodarray detectors are used instead, or alternatively Fourier Transformation Spectrophotometers.

Different kinds of N.D.I.R. (Non Dispersive Infra-Red) systems belong to the more cost effective optical gas measuring systems. Here different wavelength intervals for different molecules are detected within the infra-red spectral range. The different masses of the molecules cause the different vibration frequencies, in many cases, to separate sufficiently not to overlap each other within this specific spectral range. Optical band pass filters select the molecules^' vibration frequencies and an IR-detector registers the amount of light that has not been absorbed.

In acousto-optical detection systems the IR-detector has been replaced with a microphone that, instead of light, detects the changes in pressure caused by the molecules absorbing the infra-red light. With lasers a great number of different measuring techniques are available. These often use the different characteristics of the laser types. In some cases molecule absorption is detected by registering the following laser-induced emission, or alternatively laser-induced ionization. In atmospheric measurements, however, pure absorption measuring dominates. With pulsing high-effect lasers and LIDAR-technique, for example, the atom and molecule absorption is studied time-resolved and thereby space-resolved, through the backward scattering that always appears when a light-pulse passes through a medium. Values can then be determined along the course of the laser pulses tenths of kilometers away from the actual transmitter. Because the lasers in question can generate a very well-defined and tunable light wavelength the relative speed of the measured object, i.e. the speed of the wind, can in some cases be determined throughout the beam by means of the so-called "Doppler effect".

The most sensitive laser systems for gas analysis are based on semi¬ conductor lasers, which emit light-waves within the infra-red spectral range where there are strong absorption bands for the majority of compounds. These lasers^' fine frequencies are tuned electronically, which can be done with a band-width of about a Gigaherz. The theoretical limit for how weak absorptions can be detected, is, among other things, dependent on the optical power that the detector registers. A typical value for this could be 0.1 mW for a semi-conductor laser system. The quantum noise in this case limits the detectability to 4 x 10 ~⁷ in a measurement of one second. In practice this sensitivity can not be obtained due to interference such as, for example, optical feedback, interference fringes and noise in the light-source.

Different electronic measuring techniques have therefore been developed, where modulation methods admit a signal treatment that reduces these interferences. Overtone-generation, "second harmonic generation", has, because of its simplicity, become the most exploited. Even better noise reduction can be obtained using high-frequency methods. Side band generation through high-frequency FM-techniques and homodyne-detection is one of the most successful methods together with the method of high¬ speed scanning through repetitive pulse-modulation. These latter methods come close to the theoretical detection limit. To use a light-waveguide into which is fed, from a light transmitter, an incoming light, and from which is transmitted, to a light detector, an outgoing light to detect and control functions is previously known within the tele¬ communications and computer areas.

Here the light-pulse used is assigned an address code and an operative code, where the address code selects and activates one, among several activatable, peripheral units while the operative code activates the above- mentioned unit to perform a pre-assigned operation.

There are, then, within this technical field, various previously known ways of, from a central unit, supervising and/or affecting one among several peripheral units, by transmitting and receiving information in the form of coded light- pulses on a light-conductive waveguide.

SUMMARY OF THE PRESENT INVENTION

TECHNICAL PROBLEM

Considering the earlier state of the art, as described in the introduction above, it is obvious that in order to provide a successful analytical measuring system methods and components are required which offer cost-effectiveness and simplicity for the user; this entails that the measuring system employed must not be too expensive, too complicated to use, or require too much supervision and servicing, while at the same time many measuring systems must be robust and tolerate use in the field.

Against this background it should be regarded as a technical problem to be able to realize the significance of creating the conditions under which a used absorption volume can aquire a form that closely approaches achieved levels of efficiency, in respect of necessary control of the light source and evaluation of received light signals.

It must further be seen as a technical problem to indicate, in a simple way, a well-defined absorption volume, which can eliminate optical components, such as systems of lenses, for collecting emissions from the light source, optical components, such as systems of lenses, for concentrating light further away towards a detector, and windows for limiting absorption volume.

It must also be seen as a technical problem to indicate in another way the use of a long measuring distance, without needing to use arrangements of mirrors, as for example in a multipass cell or a telescope.

It is also a technical problem to realize the advantages of being able, artificially, to increase the length of the measuring distance, selectable to an integer multiple of the physical length of the measuring distance.

It is also a technical problem to indicate a well-defined absorption volume which has not been shown to require an initial lining up, and a service- orientated lining up, which for long measuring distances requires either an automatic servo-system or frequent manual servicing.

It is a technical problem to indicate a way of employing an absorption volume which can completely eliminate the optical components associated with it, which are expensive and must be of good quality and kept clean if the performance of the measuring instrument is to be kept at a high level.

It is a special technical problem to be able, with high resolution laser- methods, to reduce the effect of the limiting factors involved in the use of an optical feedback.

It is also a technical problem to be able to reduce the effect of noise, which in practice has limited the detection capability to a magnitude which differs from the theoretically possible limit of detectability, and to reduce the effect of the fact that the greater the number of optical components included in a radiation path, the greater the risk of interference and optical feedback.

As far as measurements are concerned which require other methods than measurements over an open distance, the system employed requires methods which transport the medium intended for measurement, such as a measuring gas, in the absorption volume.

It must therefore be regarded as a technical problem not only to be able to reduce the costs of such a system and lessen the need for service of the mechanical parts employed, but^' in addition to reduce the electrical disturbances which these mechanical parts generate in sensitive electronic apparatus.

It must in addition be regarded as a technical problem to be able to indicate a light-waveguide acting as a gas and/or liquid sensor (the presence of a chosen gas or gas mixture, a chosen liquid or liquid mixture, in an available gas mixture or liquid mixture) which with small external dimentions can provide a very long light conductor path.

It is also a technical problem to realize the significance of with such a light- waveguide adopting methods which in an effective way use the evanescent field's limited extension to define the absorption volume, which means that the light-waveguide^'s external exposable surface will be wholly or partly exposed and therefore not consist of the protective casing which is necessary in normal light conductors in order to confine the light rays within the light- waveguide.

It must also be regarded as a technical problem to realize the significance of letting the light-waveguide be orientated as a coil and of letting the sections of light conductor, positioned adjacent to each other, be orientated very close to each other in order to achieve a long measuring distance but with a short distance between them.

It must also be regarded as a technical problem to realize the significance of and effect of the form of and choice of the coil^'s design, when it is applied to a carrier.

It is also a technical problem to realize the significance of the chosen distance between the light conductor sections positioned adjacent to each other, in order to thereby create an efficiently functioning gas and/or liquid sensor.

It is also a technical problem to be able to realize the significance of letting certain light-waveguide sections be chosen with a length differing from the length of adjacent light-waveguide sections, in order to by this means lessen the effect of interference phenomena caused by cross-coupling between adjacent waveguide lengths or sections. It is a technical problem to be able to realize the significance of letting the light-waveguide be orientated folded double, with two sections of waveguide leading out, reckoning from its central point, in such a way that the end of the waveguide will be able to lie adjacent to the beginning, and thereby provide a deliberate optical cross-coupling in such a way that the incoming light along a first waveguide section will be successively switched over, in the opposite direction, to the second waveguide section lying adjacent to it.

It is a technical problem to be able to realize that in this way a deliberate directional change can occur, so that light from the light source returns to the transmitter along exactly the same fibre.

It must also be regarded as a technical problem to, in an arrangement, for the purpose of from a central unit supervising and/or affecting one among several peripheral units, suitably using a light-waveguide, providing a solution to one or several of the above mentioned technical problems, by transmitting and receiving in a light-conductive waveguide information in the form of coded light-pulses or bundles of light, realize the significance of that every bundle of light intended for supervision and/or affecting should include one or several initial light-pulses, forming an initial light code, intended to identify a selected peripheral unit and to realize the significance of there being one of several following light pulses, forming a second code, intended to initiate a required supervision or a required activation by causing the initial light code to activate and open a, the selected peripheral unit related, connecting-device, in order to transfer the following second light code to a supervising organ or organ intended to be affected or activated.

It is also a technical problem to realize the significance of letting the said organ be devised so that during the time the connecting-device is open the organ transmits information noted, via the conductor, to a device which notes and senses information, connected to the said conductor.

It is also a technical problem to be able to realize the advantages of letting the light-conductive waveguide be led in such a way that it leads past serially orientated peripheral units and of letting the respective connecting-devices interact with a material which senses radiation around the conductor. It is also a technical problem to be able to realize that when the connecting- device is open the following light-pulses are transferred to a supervising unit, and are changed by the said unit to a supervising approving or non- approving code, and reconnected to the conductor.

It is also a technical problem to, when the connecting-device is open, transfer the following ligth-pulses to an affecting unit, which generates from the light- pulses a previously determined effect, and to ensure that confirmation of the effect made is reconnected to the conductor.

In order to artificially increase the length of the measuring distance, and in this way achieve a precise gas measurement, it should be regarded as a technical problem to realize the significance of one or several following light- pulses being so devised that they pass several times along the light waveguide coil, before being fed out to the light detector.

It is also a technical problem to be able to realize the significance of the received measuring results being related to the number of times the light- pulses have passed along the coil, and that the number of times be determined by a previously determined time interval or revolution-counting unit.

SOLUTION

In order to solve one or several of the above-mentioned technical problems the invention presented here makes use of an as a gas and/or liquid sensor serving light-waveguide, into which is fed, from a light transmitter, an incoming light and from which is fed, to a light-detector, an outgoing light, whose spectral range is evaluable in a spectral analysis device.

Such a light-waveguide shall, in accordance with the invention, be positioned on, or form a part of, a carrier, taking the form of a stable substratum. The light-waveguide shall be orientated as a coil and the light-waveguide^'s outer exposable surface shall be wholly or partly exposed. Adjacently positioned light-conductor sections are orientated close to each other. As a suggested embodiment, falling within the framework of the invention^'s concept, it is pointed out that the coil shall have a zig-zag pattern or a spiral pattern.

Further it is pointed out that the distance between the adjacently positioned sections of light-conductor shall be chosen to exceed, or at least almost exceed, an absorption volume, which in practice means between 1 and 10 times the thickness of the light-waveguide, preferably between 3 and 7 times.

Further it is pointed out that if the light-waveguide is applied as a zig-zag pattern, certain sections of the light-waveguide shall be chosen with a length differing from the length of adjacent sections.

Further it is pointed out that the said waveguide and its waveguide sections shall be able to be folded double, whereby the incoming light and the outgoing light can be ordered adjacently to each other.

In order to solve one of several of the above-mentioned technical problems, the present invention shows in addition an arrangement or a device whereby from a central unit supervising and/or affecting one, among several, selectable peripheral unit, suitably using a light-waveguide according to what is indicated in the introduction, by transmitting and receiving information via a light-conductive waveguide in the form of coded light bundles or light signals.

The present invention shows that in an arrangement or a device of this sort every light bundle which is intended to supervise and/or affect shall include one or several initial light-pulses, forming an initial light code, intended to identify a selected peripheral unit, and one or several following light-pulses, forming a second light-code, intended to initiate a desired supervision or effect: The invention also shows that in a device of this sort the initial first light-code is arranged to activate and open a connecting-device, connected to the selected peripheral unit, in order to transfer the following second code to a supervising organ or to an organ intended to be affected, and that the said organ is arranged to, during the time that the connecting-device is open, transmit information noted, via the conductor, to a device which notes and senses information, connected to the said conductor. It is further pointed out, as a suggested embodiment, that the light-conducting waveguide shall be arranged in such a way that it leads past serially orientated peripheral units and that the respective connecting-devices shall interact with a means which senses radiation around the conductor.

Further it is pointed out that when the connecting-device is open the following light-pulses or codes are transferred to a control or supervising unit, and are changed by the said unit to a supervising approving or non-approving code, and reconnected to the conductor.

Further it is pointed out that when the connecting-device is open the following light-pulses are transferred to an affecting unit, which generates from the light-pulses a previously determined effect, and a confirmation of the effect made is reconnected to the conductor.

Further it is pointed out that one or several light-pulses shall be so arranged that they pass several times along the light-conductor coil, before being fed out to the light-detector.

Finally it is pointed out that the number of times the light-pulses may pass along the coil is to be determined by a previously determined time interval or a revolution-counting unit.

ADVANTAGES

The advantages which primary can be regarded as associated with a light- waveguide, in accordance with the present invention, are that in this way conditions are caused for providing in a simple way a sensor, that is easy to manufacture and offers a long measuring distance. The evanescent field^'s limited extension and the length of the measuring distance are used as the absorption volume.

In addition, the measuring distance can be increased by a selected whole- number multiple.

In a device or arrangement arranged to, from a central unit, supervise and/or affect one, among several, peripheral units, by transmitting and receiving along a light-conductive waveguide information in the form of coded bundles of light, it is possible to use a light-waveguide with a specific length, and provide a measurement along a measuring distance which amounts to a multiple of the light-waveguide^'s specific length.

That which can primary be regarded as characteristic for a, as a gas and/or liquid sensor serving, light-waveguide, in accordance with the present invention, is described in the characterizing part of the succeeding patent claim 1.

That which can primary be regarded as characteristic for a device or an arrangement to, from a central unit, supervise and affect one, among several, peripheral units is described in the characterizing part of the succeeding patent claim 6.

BRIEF DESCRIPTION OF THE FIGURES

An embodiment, demonstrating the present invention^'s significant features, will now be described with reference to the attached drawings where:

Figure 1 shows in principle a known light-waveguide, for a gas and/or liquid sensor, of a first embodiment,

Figure 2 shows in principle a known light-waveguide, for a gas and/or liquid sensor, of a second embodiment,

Figure 3 shows, in a plan view, and with a chosen section enlarged, an as a gas and/or liquid sensor serving light-waveguide, acccording to a first embodiment of the present invention,

Figure 4 shows, in a plan view, an as a gas and/or liquid sensor serving light- waveguide, in acccordance with a second embodiment of the present invention, Figure 5 shows in principle a peripheral selectable unit, forming a part of a device or arrangement to, from a central unit, supervise and/or affect one, among several such peripheral, unit, using a light-waveguide according to the embodiment shown in Figure 3,

Figure 6 shows time diagrams "A" and "B" for two different examples with initial first light-code and a following second light-code,

Figure 7 shows in principle a device or arrangement to, from a central unit supervise and/or affect or control one of two peripheral units.

Figure 8 shows schematically an intensity versus time diagram for a light pulse which has passed the light path according to Figure 3 a number of times in order to evaluate a gas concentration and

Figure 9 shows in principle a peripheral, selected, unit for activating a coupling means.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to figures 1 and 2, two different light-waveguides are shown, both used as a gas and/or liquid sensor 1.

In the case of both figures, into a used light-waveguide 2 is fed, from a light transmitter 3, an incoming light 4 and from it is fed out, to a light-detector 5, an outgoing light 6, whose spectral range is evaluable in a spectral analysis device 7, where the result is shown on a display-unit 8.

The light-ray or beam 10 passing through the light-waveguide 2 is reflected against the light-waveguide^'s 2 inner surface 2a in an already known way.

For every light-waveguide, which is to evaluate the occurrence of a chosen gas or gas mixture in an available gas mixture, a measuring volume or absorption volume is required.

The measuring volume 11 in figure 1 is formed by a hollow cavity, to which also the measured molecules 11a in various ways, e.g. through diffusion, can gain access. The light 10 is reflected forward in the light-waveguide against its inner walls 2a until it finally reaches its exit, where a detector 5 is placed. A light-waveguide of this sort can be constructed of e.g. a hollow metal tube with polished inner surface, or a microwaveguide, with a rectangular cross- section, the latter reflecting light as in a hall of mirrors, like a kaleidoscope.

The second embodiment, shown in figure 2, uses as a light-waveguide 2 a glass fibre, and this glass fibre causes the light to centre itself in the light- waveguide. In this wav the light 10 follows the waveguide, but not all the light energy is concentrated to the central core, for a small part, the so-called "evanescent" field 12, is transported outside the core and can therefore react with the external gas molecules 11a, assuming that the waveguide core is not covered with too thick a protective mantle 13, which is usually the case with e.g. optic fibres used for communications.

It is evident that an exposed light-waveguide 2 (without protective casing or mantle 13) generates a greater evanescent field than a gas-impervious casing 13.

Porous casings 13 have here found a use.

The measuring volume 11 is the evanescent field outside the waveguide and the length of said waveguide. Since only a small part of the light energy, of the order of 1-20%, is in this case transported in the surrounding measuring medium 11 , the measuring length in figure 2 must be longer, 5 to 100 times longer, in order to maintain the same signal strength as in the case of an absorption measurement according to figure 1.

In both embodiments, according to figures 1 and 2, the light source 3 and the receiver 5 do not need to be directly connected to the waveguide 2 itself, since the light can be transported via other waveguides, by optic fibres or similar methods, over long distances to and from the actual measurement site.

When weak absorptions are to be measured long absorption times are required, and therefore long absorption distances. It is therefore obvious that a waveguide according to figure 1 can not be employed for practical reasons. The embodiment shown in figure 2, even though it requires a considerably longer absorption distance, is more practical because a very long light- waveguide 2, such as an optic fibre, can be coiled into very small outer dimensions.

It is evident that such an optic fibre must be a very great deal longer, in order to compensate for the lower degree of utilization of the light, in view of the limited extension of the evanescent field.

The invention is based on the principles described under the form shown in figure 2.

With reference to the present invention it is pointed out, according to figure 3, that the aforementioned light-waveguide 2^' shall use optic fibre or fibres and that these shall be applied to, or be formed as, a part of a carrier 15, in the form of a stable substratum.

The light-waveguides 2^', as shown in figure 3, consists of waveguide sections 20, 21 , 22, 23, and 24, 25 and these shall be orientated as a coil, which means that section 20 is connected via a connecting device, not shown in figure 3, with section 25.

In every case the outer exposed surfaces of sections 20-25 shall be wholly or partly exposed, and the light-waveguide sections 22. 23, positioned adjacently to each other, shall be orientated close to each other with a short distance between them.

Since the light-waveguide^'s sections, according to the present invention, shall not have a protective casing (or in any case a very thin transparent one), a light-waveguide of this sort must be protected against mechanical damage.

It is suggested here that waveguide 2^' be manufactured on a mechanically stable substratum with micro-optic techniques. It should be possible, using already known techniques and with micrometer-precision, to manufacture a wave-conductor of titanium implanted lithiumniobate Tr.LiNbOβ. In a waveguide plate of this sort as shown in figure 3, the light beam or pulses can be made to traverse a predetermined surface in a zig-zag pattern with high density, for example a 5 micrometer diameter waveguide with 25 micrometer space between. In this way a geometric path is obtained of, in this case, 330 m/dm² single-sided sensor surface.

It should be noted here that if the space between the waveguides in the pattern is chosen too small, then there is a risk of an optical cross-coupling between them, which can give rise to interference problems.

The influence of cross-couplings and resonance phenomena can be reduced by letting the different waveguide lines or wave-conductor sections 22,23 have an almost continually variable length, so that the phase position for the cross-coupling varies between the different sections of waveguide and the net result of the total cross-coupling is nil.

It is obvious that the number of parallel light conductor sections is very great here, and only a few have been designed reference numerals.

Within the framework of this invention the possibility also exists, as figure 4 is intended to show, of using the cross-coupling effect to change the direction of the light. By letting the waveguide 2" consist of a plate 15^' and letting its two sections of waveguide lie folded double, reckoning from its centre, 32, so that the one waveguide section's 30 end 30b lies adjacent to the other waveguide section's 31 beginning 31a, a deliberate cross-coupling can be made in such a way that the incoming light 31a will succesively be coupled in the opposite direction along the adjacent waveguide section. Thus a deliberate change of direction can occur so that the information-carrying light from the transmitter 3 goes in exactly the same fibre back to the receiver 5 as that which the fibrelight was transported forward in.

In the case of a light-waveguide 2^', designed in the way shown in figure 3, with a sensor of a square decimeter^'s size, the light will need 2,4 microseconds to pass along the measuring length (refraction index n=2.2).

By means of known pulse sweep technique a molecule absorption measurement can be performed with the help of laser pulses lasting under a microsecond. It should therefore be most advantageous to construct the sensor, preferably in the way described in figure 3, in the form of a closed coil so that light and light-pulses can be switched into or out of the coil according to need. The length of the light pulse must be adopted so that the total length is within the optical length of the sensor.

A switching mechanism of this sort for a peripheral unit 50 can be constructed with electro-optical switches and micro-optics according to already known techniques.

In figure 5 is shown the use of an optical switch with reference number 52 with a selected unit 50 having a loop 20-25.

If the time at which an optical measuring-pulse is connected into the coil 20, 25 is known, then light-pulse or measuring pulses can be allowed to go round the coil an arbitrary number of times, before the light-pulse is reconnected out of the sensor for analysis.

This process can be controlled either by measuring the time from start and comparing it with earlier readings or calculated values, or else by using a detector 51 to read a small fraction of the light pulse and use it as a revolution-counter, and compare the current revolution count with the preselected revolution count.

After a previously selected number of revolutions the swich 52 is reopened and the optical pulse is steered towards the detector 5 for evaluation in device 7.

In this way the geometrical measuring length has been increased to "n" times the sensor^'s total waveguide length 20-25. After analysis of the signal at the device 7 the number of revolutions may be increased or reduced depending upon the intensity of the absorption.

Apart from the dynamic of the detector signal at light detector 5, one can also now use a distinct variable measuring length in order to thereby determine molecule concentrations over an even greater number of measured revolutions. The dynamic measuring range has thus been increased a hundredfold.

Referring again to figure 5, it is shown there that the light 4 can be switched in or out of a loop or coil, where the light-waveguide sections 20 and 25 are connected with each other via light conductor sections 53 and 54 according to need.

Figure 6 shows the structure of an optical light-pulse, where in figure 6A the optical address code 61 forms a train of pulses distinct from the measuring pulse 62. Alternatively, according to figure 6B, the address code 61 can be included in the initial phase of the measuring pulse 62.

Figure 7 shows in principle a device or an arrangement for from a central unit 70 supervising and/or affecting one of two peripheral units. 50 or 60, both of a design shown in figure 5.

The conductor 71 is connected so that the units 50 and 60 or the sensor modules are connected in series.

The number of units can be considerably larger and via address code generation in the central unit 70 only the sensor unit 50 with the right address code will be activated. In this way a central unit 70, consisting of a transmitter 73, a detector 75 and an evaluating unit 77 can serve a large number of sensor modules situated in different measuring areas. Other functions 79 can also be addressed and selectively activated in a similar way, e.g. alarm sirens, illumination, microphones or other apparatus.

The embodiment according to figure 7 not only offers the possibility of being able to evaluate the occurrence of and/or concentration of a chosen gas or gasmixure in a measuring volume by evaluating the form of the spectral scan of the light-pulses when they are received in the detector, but also the possibility of, at every peripheral unit, activating or deactivating a device in the unit in question via light-pulses.

It is also possible to let every peripheral unit generate a light-pulse which is significant for the device^'s position. For all applications it is necessary to provide the sensor design, shown in figure 5, with a pulse-detector 55 which senses small fractions 56 of the light- pulse outside the coil or loop 4 itself, and also to let the optical switch 52 open dependent of the result of a decoder 57 connected to the detector 55, so that the sensor can be activated with an optical address code 61.

The address coding 61 makes it possible to connect to a larger distribution coil 71 of optic fibre, a number of different sensor modules 50, 60, where only the sensor module with the right address code is activated.

In this way a central unit consisting of a transmitter 73, a detector 75, and an evaluating unit 77, can serve a large number of sensor modules in different measuring locations.

Complete flexibility is possible when all these measuring areas can be addressed individually. All information about control and evaluation is transported by means of a single optic fibre 71. The different sensor modules require only a power supply. It might even be possible to rationalise away the power supply, in the event that it can be provided by photo-electric effect. In that case the power supply too could be managed through the optic fibre 71.

The distribution system shown in figure 7 can be installed with the help of available tele-communications material and components, assuming that laser wavelengths suitable for the purpose are used for concentration measurements. Service, splicing and the like are done by techniques which are today well-established. The sensor modules can easily be connected into the system as the need arises. These modules can be standardised and manufactured in series in one piece at low cost.

The central unit is the unit from where everything is steered and controlled, and can therefore need a certain amount of servicing. This unit can in its turn be in contact via modem or other communications equipment with even larger supervising centres. The central unit can also be extended later and complemented with laser transmitters for sensing of new substances. Wavelength multiplexing and time multiplexing make it possible to extend the sensor system almost unlimitedly. This can be done merely by upgrading the central unit. One of the big areas of application for the sensor system described above is to be found in the area of environment measurement and control. For example, carbon dioxide content provides an excellent measurement of the ventilation requirements in areas occupied by people. A sudden increase in carbon dioxide content is also a very sensitive and reliable indication of a imminent fire. Carbon monoxide is on account of its toxicity necessary to detect in garages and cellar spaces, or in other spaces where oxidisation occurs. Methane is, on account of the risk of explosion, important to detect in e.g. buildings which have gas stoves. All these gases, together with many other substances, can be detected with sufficient accuracy (ppm range) with the help of laser light sources, which emit within the wavelength band (1.3 and 1.55 micrometers) where the ordinary optical communications systems function.

A central unit could for example supervise and control all the rooms in an office complex, a hotel, or a large apartment building, steer the ventilation individually or according to current needs in all the supervised rooms, and at the same time discover fires, toxic gases and explosion risks and in addition give danger warnings for each one of these dangers. The latter can be achieved by connecting modules for light or sound sirens to the system, where special codes are provided for activating these to the desired degree. In this way it is possible with the invention presented here to construct cost- effective solutions for total environment supervision, for an optimally comfortable and secure indoor existence.

JJsing the embodiment shown in figure 5 it is possible to detect the presence and/or the concentration of a certain gas. In figure 8A a pulse waveform is shown after one single pass in the loop, in figure 8B a pulse waveform is shown for a double pass in the loop, and in figure 8C is a pulse waveform shown after a triple pass in the loop.

Already, one can notice the build-up of the characteristic wavelength dependent molecular absorption dip.

After several, but uniquely determined, loop counts the pulse 62^' gets the shape she i in figure 8D. Using the embodiment shown in figure 9 a supervising and/or an affecting device can be realized.

Through a pulse detector 55^' and a decoder 57' the optical switch 52 can be affected.

When the switch is activated, the light code 62 is passed to a photo-receiver 91 , which interprets an instruction associated to the particular code. This is transformed and affects electrically a coupling device (or a supervising device) that transmits a coded light pulse back, using an optical transmitter 93, as status indication to be evaluated in the central unit 70.

The invention is of course not limited to the design form described above as an example, but can be modified within the framework of the invention concept illustrated in the following patent application.

Claims

1. As a gas and/or liquid sensor serving light-waveguide, into which is fed, from a light transmitter, an incoming light and from which is fed, to a light detector, an outgoing light, whose spectral range is evaluable in a spectral analysis device, characterized in, that said light-waveguide is applied to, or included as a part of, a carrier, being in the form of a stable substratum, that the light-waveguide is orientated as a loop or coil, that the light- waveguide^'s outer surface is wholly or partly exposed and that the adjacent light-waveguide sections are orientated close to each other.

2. Light-waveguide according to claim ^characterized in, that the loop or coil has a zig-zag pattern or a spiral pattern.

3. Light-waveguide according to claim 1 or 2, c h a r a c t e r i z e d in, that the chosen distance between the adjacently positioned light conductor sections is small, suggested beween 1 and 10 times the light-waveguide^'s thickness, preferably between 3 and 7 times.

4.. Light-waveguide according to claim 2 with zig-zag pattern, characterized in, that certain sections of the waveguide are chosen with a length differing from the length of the adjacent waveguide sections.

5.. Light-waveguide according to claim 2, characterized in, that said waveguide is folded double, whereby the light input and the light output are orientated adjacent to each other.

6. An arrangement to, from a central unit, supervise and/or affect one, among several, peripheral unit, suitably using a light-waveguide according to one of the succeeding claims, by in a light-conductive waveguide transmitting and receiving information in the form of coded bundles of light, characterized in, that every bundle of light, intended for supervision or activating effect, includes one or several inital light-pulses, forming a first light code, intended to identify a selected peripheral unit, and one or more following light-pulses, forming a second light code, intended to initiate a required supervision or required effect, that the first light code is devised to activate and open a connecting-device connected to the selected peripheral unit in order to transfer the following second code to a supervising organ or to an organ intended for effect and that the said organ is devised to, during the time that the connecting-device is open, transmit to the conductor information noted, and that to the said conductor is connected a device for sensing and noting noted information.

7. Arrangement according to claim 6, characterized in, that the light- conductive waveguide is devised to serially connect respective peripheral unit and that the connecting-device interacts with a material that senses radiation around the conductor.

8. Arrangement according to claim 6 or 7, characterized in, that when the connecting-device is open the following light-pulses are transferred to a supervising unit, and changed by the said unit to a supervising approving or disapproving code and then reconnected to the conductor.

9. Arrangement according to claim 6 or 7, c h a r a c t e r i z e d in, that when the connecting-device is open the following light-pulses are transferred to an affecting unit, which from the light-pulses generates a previously selected effect and information about the effect made is reconnected to the conductor.

10. Arrangement according to claim 6, characterized in, that one or more following light-pulses are devised to pass several times through the light-waveguide coil before the said light-pulses are fed out to the light- detector.

11. Arrangement according to claim 10, characterized in, that the number of times is determined by a previously selected time or by a revolution-counting unit.