Method and Apparatus for Providing Reference Signals From Points Along an Optical Fiber Transmission Path

The present invention concerns methods and apparatus using optical fibers for producing a reference signal indicative of the arrival of light at a point along an optical fiber transmission path. More particularly, this invention relates to such method and apparatus in which such reference signals can be produced using an open loop single fiber or bundle of fibers, rather than a closed loop fiber or bundle of fibers as have been in use prior to this invention.

A variety of connectors, couplers and beam splitters for use with optical fibers have been discribed in the patent literature. For example, U.S. Pat. No. 3,619,030 discloses a pair of optical fiber cables connected with a partial mirror disposed between the connected ends of the cable. The assembly is intended to transmit in one direction from cathode ray tubes images which are uncontaminated by stray images without any reflection. U.S. Pat. No. 4,540,237 discloses a coupler for three optical fibers in which the ends of the fibers are cut at various angles. The coupled fibers are in contact and are said to transmit light with minimal losses. U.S. Pat. No. 4,611,884 discloses an emitter reflectance method of making a bidirectional optical fiber coupler said to have the ability to transmit light in either direction with minimal internal reflection or attenuation. The coupler comprises a molded plastic body containing a beam splitting surface for light transmitted from an optical fiber. U.S. Pat. No. 4,695,126 discloses a coupler in which the ends of the fibers form an acute angle with respect to the longitudinal axes of the fibers and the fibers are positioned at an angle to eliminate back reflection.

Various instruments are also known which comprise optical fibers. U.S. Pat. No. 3,619,068 discloses a refractometer for use in liquid chromatography in which light passes through an optical fiber cable having an end surface at an angle, the surface being in contact with the fluid whose composition is to be measured. U.S. Pat. No. 4,320,291 discloses an instrument for measuring the refractive index of a fluid using external reflectors. U.S. Pat. No. 4,421,384 discloses an optical fiber transducer in which light is reflected back along the fiber from specially angled end faces to provide a measurement of the spacing of the end faces from a nearby block of optical material. U.S. Pat. No. 4,462,699 discloses an optical fiber temperature transducer comprising coatings whose indices of refraction vary with ambient temperature. U.S. Pat. No. 4,699,511 discloses an instrument for measuring index of refraction in which a prism is used to direct light from one fiber end to another, as a function of the index of refraction of the ambient medium.

While devices of the types described in these patents have achieved a certain degree of acceptance in science and industry, a number of persistent problems have gone unsolved. In networks of optical fibers, light signals are launched into a fiber at one location for receipt at another; however, confirming at the one location that the signals have indeed arrived at the other typically has required a return loop of fiber from the other location. Thus, to simplify such networks, it has been desired to able to monitor such receipt of signals without the use of closed loops of fibers. In measurement systems using optical fibers to monitor ambient conditions, the effective use of a plurality of sensors distributed along a single fiber or bundle of fibers has been frustrated by the lack of a reliable technique for producing a reference signal from each sensor location along the fiber, as would be needed to separately detect the response of each of the plurality of sensors. In applications where two or more fibers must be coupled or connected, considerable difficulty has been experienced with reliably, consistently preparing the fiber ends for joining.

A primary objective of the present invention is to provide a method and apparatus for optically producing reference signals from points along an optical fiber transmission path.

Another primary objective of the present invention is to provide a method and apparatus for optically sensing ambient conditions in the environment along an optical transmission path.

A further objective of the present invention is to provide such a method and apparatus which do not require the use of a return or closed loop of optical fiber.

Still another objective of the present invention is to provide an apparatus for asymmetrically coupling or connecting optical fibers in such a way that light transmitted from a first direction is partially reflected and partially transmitted, while light from an opposite, second direction primarily is transmitted without significant back reflected light toward such second direction.

Yet another objective of this present invention is to provide such a coupling or connecting apparatus in which light transmitted from either direction is partially reflected and partially transmitted.

A still further objective of the invention is to provide a simple, improved method for making such a coupling or connecting apparatus.

Another objective of the invention is to provide such a method and apparatus in which recycling of light signals between successive coupler or connectors is minimized, thus reducing optical signal noise.

These objectives are given only by way of illustrative examples; thus other desirable objectives and advantages inherently achieved by the disclosed invention may occur or become apparent to those skilled in the art. Nonetheless, the scope of the invention is to be limited only by the appended claims.

To facilitate better understanding of the invention, reference is made to FIG. 1 which shows a schematic longitudinal crossection of a typical optical fiber 10. As is understood by those skilled in the art, an optical fiber functions optimally by transmitting light almost solely along its essentially cylindrical core 12 due to total internal reflection of light at the interface between core 12 and the surrounding clad 14. If total internal reflection is not achieved, a potentially unacceptable fraction of the incident light may be lost into clad 14, rather than being transmitted along core 12. In order to ensure total internal reflection, the angle .THETA. between a ray of light 16 and a normal 18 to the core/clad interface must be greater than the critical angle for total internal reflection at the interface. For a step index fiber of the type shown, the critical angle can be determined from Snell's law, which provides that

where n.sub.1 is the index of refraction of the incident medium; n.sub.2 is the index of refraction of the medium into which the transmitted light would travel; and .THETA..sub.1 and .THETA..sub.2 are the angles of incidence and refraction respectively. From the geometry of the core/clad interface illustrated in FIG. 1, it can be shown that the minimum angle .THETA..sub.min for transmission of light along core 12 by total internal reflection is given by

The numerical aperture NA of fiber 10 is a function of the sine of the maximum acceptance angle .THETA..sub.c for light to transmit along core 12 by total internal reflection and is given by

where n.sub.o is the index of refraction of the external medium. Consideration of FIG. 1 shows that the maximum angle of refraction with respect to the longitudinal axis of fiber 10 for light entering from the external medium is given by (90.degree.-.THETA..sub.min). Applying Snell's law to the external medium/core interface yields

Calculation yields

.THETA..sub.c is the external critical angle for light entering the fiber from the external medium of index n.sub.o. A truncated cone formed at the entrance face of core 12 with an apex angle .THETA..sub.c defines the maximum acceptance cone of the fiber and substantially all light passing into the core through such a cone will transmit along the core by total internal reflection. The internal critical angle of the fiber is the angle of refraction .gamma., shown in FIGS. 1 and 2 for light transmitted into the fiber from the external medium at .THETA..sub.c.

Consider now an optical fiber having an angled, substantially planar end face 20 through which light is to be emitted, as illustrated in FIG. 2. The angle .phi. is the angle between end face 20 and a normal 22 to the longitudinal axis of the fiber. From the geometry shown in FIG. 2, it can also be shown that angle .phi. is equal to the angle between the longitudinal axis of the fiber and a normal 24 to end face 20. Light travelling along core 12 from left to right in FIG. 2 at angle .phi. with respect to the longitudinal axis of fiber 10 would strike end face 20 perpendicularly and would reflect back to the left along core 12 at angle .phi. with respect to the longitudinal axis by total internal reflection. According to the present invention, the important angle to consider in the geometry of FIG. 2 is the internal critical angle or incident angle of refraction .gamma. into core 12, which is given by (90.degree.-.THETA..sub.min). The internal critical angle .gamma. is given by

When angle .phi. is greater than the internal critical angle .gamma., essentially no light travelling from left to right in FIG. 2 will be reflected back to the left along core 12 by total internal reflection; but in such a case, reflected light will be lost into clad 14. When in accordance with the present invention, angle .phi. is less than the internal critical angle .gamma., a portion of the light travelling from left to right will be reflected back to the left along core 12 by total internal reflection; and a further portion will pass through end face 20. Thus by appropriate selection of angle .phi., one can determine the percentage of light which will be reflected back along core 12 by total internal reflection. While the foregoing analysis is specific to step index fibers, those skilled in the art will appreciate that a similar analysis applies to graded index fibers, for which an internal critical angle .gamma. also can be determined by conventional techniques and used to select the angle .phi. of the end face of the fiber.

One preferred embodiment of the invention concerns an improved optical apparatus comprising a pair of optical fibers configured at their facing ends and joined in such a manner that a first portion of the light transmitted along the first fiber is reflected back along the fiber by total internal reflection and a second portion is transmitted into the second fiber. Similarly, light transmitted in the opposite direction along the second fiber is transmitted into the first fiber. Thus the apparatus exhibits asymmetrical transmission properties, depending on the direction of travel of the light. In this embodiment, the output end of the first fiber comprises at least one substantially planar end surface or face which extends at least partially across the core of the fiber. The angle between the planar surface and a normal to the longitudinal axis of the fiber is greater than zero but less than the internal critical angle of the first fiber. The input end of the second fiber comprises at least one further substantially planar end surface or face which extends at least partially across the core of the fiber. The two fibers are positioned with these end surfaces facing and spaced from one another and with the longitudinal axes of the fibers at an angle which permits at least a portion of the light from the first fiber to follow a path within the maximum acceptance cone of the second fiber, and vice versa. The angle of the input end surface of the second fiber relative to the longitudinal axis of that fiber may be greater or less than the internal critical angle of the fiber, depending on whether or not reflection back along the second fiber is desired from that input end surface. The first and second fibers may be single optical fibers each having a core and clad or may be a plurality of such single fibers gathered into a bundle. The longitudinal axes of the two fibers may be coincident, paralled or skewed. The facing end surfaces of the fibers may comprise only one planar surface or a plurality of intersecting planar surfaces. The space between the facing end surfaces may be filled with a coupling medium transparent to the light from either of the fibers and having an index of refraction different from that of the core of at least one of the fibers. The planar end surfaces may be provided with coatings or the like for increasing their reflectivity or for modifying the optical characteristics of light transmitted through the surfaces. The angles between the end surfaces and their respective longitudinal axes neet not be identical. A second embodiment of the invention differs from that just described in that the indexes of refraction of the cores of the first and second fibers are not equal and the planar end surfaces are placed substantially in contact, thus making the use of a coupling medium unnecessary in most instances.

In accordance with a first method of the invention, reference signals are provided from a plurality of points along an optical fiber transmission path. A pulse of light is directed along the path incident from the first direction. At each of the plurality of points along the path, a first portion of the light is reflected by total internal reflection back along the path toward the first direction and a second portion is transmitted along the path toward a second direction. Similarly, at each of the plurality of points, any light travelling from the second direction is transmitted along the path toward the first direction. Thus, reflected light produced by such first portions is directed back along the path to provide reference signals indicating that light has passed each of the plurality of points and continues along the path beyond the last of such points. In accordance with a second method of the invention, no light is transmitted past the last of such points toward or from the second direction. Thus, in the second method, light is reflected back along the path to provide signals indicating that light has reached each of such points.

The foregoing and other objectives, features and advantages of the invention will be apparent from the following more particular description of the preferred embodiments of the invention, as illustrated in the accompanying drawings.

FIG. 1 shows a schematic, longitudinal crossection of an optical fiber.

FIG. 2 shows a schematic, longitudinal crossection of an optical fiber having an end face angled in accordance with the invention.

FIG. 3 shows a longitudinal crossection, partially broken away, of an optical connector or coupler according to the invention in which the end surfaces of the optical fibers are spaced from each other and at different angles to the coincident longitudinal axes of the fibers.

FIG. 4 shows a longitudinal crossection, partially broken away, of an optical connector or coupler according to the invention in which the end surfaces of the optical fibers are spaced from each other and at different angles to the laterally spaced, parallel longitudinal axes of the fiber.

FIG. 5 shows a simplified longitudinal crossection, partially broken away, of an optical connector or coupler according to the invention in which the end surface of one fiber comprises intersecting planar surfaces; and the end surfaces of the two fibers are spaced from each other at different angles to the coincident longitudinal axes of the fibers.

FIG. 6 shows a simplified longitudinal crossection, partially broken away, of an optical connector or coupler according to the invention in which the end surfaces of both fibers comprise intersecting planar surfaces; and the end surfaces of the two fibers are spaced from each other at different angles to the coincident longitudinal axes of the fibers.

FIG. 7 shows a longitudinal crossection, partially broken away, of an optical connector or coupler according to the invention in which the end surfaces of the fibers essentially are in contact and at the same angle to the longitudinal axes of the fibers.

FIG. 8 shows a longitudinal crossection, partially broken away, of an optical connector or coupler according to the invention in which the end surface of one fiber comprises a single planar surface and the end surface of the other fiber comprises intersecting planar surfaces; and the end surfaces of the two fibers are in partial contact with each other and, in part, at different angles to the coincident longitudinal axes of the fibers.

FIG. 9 shows a longitudinal crossection, partially broken away, of an optical connector or coupler according to the invention in which the end surface of one fiber comprises intersecting planar surfaces and the end surfaces of two other fibers comprise single planar surfaces each facing one of the planar surfaces of the one fiber, the end surfaces being spaced from each other at different angles to the laterally spaced, parallel longitudinal axes of the fibers.

FIG. 10 shows a schematic arrangement of an apparatus according to the invention for measuring characteristics of the environment at multiple locations along an optical fiber transmission path.

FIG. 11 shows a plot of detector output versus distance from the light source for one embodiment of the apparatus of FIG. 10.

The following is a detailed description of the preferred embodiments of the invention, reference being made to the drawings in which the same reference numerals identify the same elements of structure in each of the several Figures.

FIG. 3 illustrates schematically one embodiment of the optical connector or coupler according to the invention. A first optical fiber 24 comprises in the familiar manner a cylindrical core 26 surrounded by a clad 28 which extends along the length of the fiber. A protective buffer coating or sheath 30, or both, preferably is provided around clad 28 and along the length of the fiber. For ease of illustration and description, the optical fibers shown in the Figures all are single fibers; however, those skilled in the art will appreciate that each embodiment of the invention can be used with both single fibers and groups or bundles of fibers gathered together to form a larger optical fiber transmission path. Thus, unless the context indicates otherwise, the term "fiber" should be read to include such groups or bundles of fibers. The fibers may be of the multimode or single mode variety. The fibers also may be of the same or different diameters on opposite sides of the connector. However, not every fiber in such a bundle need be configured in the manner(s) now to be discussed, so long as at least one pair of input and output fibers is so configured.

Fiber 24 comprises an output end for transmitting light which is formed by an essentially planar end surface 32 extended across core 26 which end surface may also extend across clad 28 and sheath 30. Planar surface 32 is set at an angle .phi..sub.1 to a normal 34 to the longitudinal axis of fiber 24. In order to ensure that a portion of the light transmitted through fiber 24 from the left as seen in FIG. 3 is reflected by total internal reflection back toward the left, angle .phi..sub.1 is chosen in accordance with the invention to be greater than zero and less than the internal critical angle of fiber 24. This value of angle .phi..sub.1 also ensures that a further portion of light from the left will be transmitted through planar surface 32.

A second optical fiber 36 is provided to receive light from fiber 24 and to transmit light back to fiber 24. Fiber 36 comprises a cylindrical core 38 surrounded by a clad 40 which extends along the length of the fiber. A protective buffer coating or sheath 42, or both, preferably is provided around clad 40 and along the length of the fiber. Fiber 36 comprises both an input end for light from fiber 24 and an output end for transmitting light from fiber 36, which input/output end is formed by an essentially planar end surface 44 extended across core 38 which end surface may also extend across clad 40 and sheath 42. Planar surface 44 is set at an angle .phi..sub.2 to a normal 46 to the longitudinal axis of fiber 36. Angles .phi..sub.1 and .phi..sub.2 may be less than, equal to or greater than one another, as required for a given application and as discussed with regard to the remaining embodiments of the invention.

In accordance with the invention, fibers 24,36 are positioned with planar surfaces 32,44 facing each other and with the longitudinal axes of the fibers at an angle which permits at least a portion of the light emanating from the output end of fiber 24 to follow a path within the maximum acceptance cone of fiber 36, and vice versa. The longitudinal axes may be coincident or at an angle to one another. Positioning of the fibers may be achieved with a separable connector comprising a male ferrule half 48 secured to sheath 30 of fiber 24 and a female ferrule half 50 secured to sheath 42 of fiber 36, the two ferrule halves being joined by a threaded joint 52, for example. Of course, if there is no desire to be able to separate the two fibers, ferrule halves 48,50 may be permanently joined as a fixed coupler. Whether ferrule halves 48,50 form a connector or a coupler, planar surfaces 32,44 are spaced from each other in the illustrated embodiment by a coupling volume 54. Depending on the indexes of refraction of cores 26,38, coupling volume 54 may be filled with a conventional coupling medium which is optically transparent to light transmitted into the volume from either fiber, the index of refraction of the coupling medium preferably being different from that of at least one of cores 26,38. The coupling medium also may have an index of refraction which varies transversely to the longitudinal axis of one or both fibers, if necessary for proper coupling of light between the fibers. In some applications using bundles of fibers, it may be desirable to extend some fibers of the bundle across coupling volume 54 without any break, an arrangement which is considered to be within the scope of the present invention. Either or both of surfaces 32,44 may be coated with conventional materials or provided with fully or partially reflective mirrors, not illustrated, to increase the reflectivity of the surface. Similarly, either or both surfaces may be provided with means such as filters for modifiying the optical characteristics of light transmitted through the associated planar surface. Optically active coating materials for surfaces 32,44 could be selected from the group including mirrors, metal oxides, metal fluorides, glass notch filters, band pass filters, polarizing filters and solgels. Polymeric materials could be selected from the group including polyamides, polyimides, gelatin, polymethylmethacrylate and cellulose acetate butyrate. If one or both of surfaces 32,44 is coated with such an optically active material, the indices of refraction of the two cores and the coupling medium could be the same, without departing from the scope of the invention.

In use of the apparatus of FIG. 3, a portion of the light transmitted from the input end 56 of fiber 24 to its output end at planar surface 32 will be reflected by total internal reflection back toward input end 56 and a second portion of such light will be transmitted through planar surface 32 and coupled into the input end of fiber 36 at planar surface 44. When angle .phi..sub.2 is greater than the internal critical angle of fiber 36, light transmitted from the output end 58 of fiber 36 to the input end of fiber 36 will be transmitted through planar surface 44 and coupled into the output end of fiber 24; and the portion of light reflected back toward output end 58 will be vanishingly small. When angle .phi..sub.2 is less than or equal to the internal critical angle of fiber 36, a portion of light transmitted from output end 58 to the input end of fiber 36 will be reflected by total internal reflection back toward output end 58.

A plurality of couplers or connectors of the type shown in FIG. 3 can be positioned at a corresponding plurality of points along an optical fiber transmission path. Consider, for example, the system of FIG. 10 without the sensors S.sub.1 to S.sub.n. If a pulse of light is directed along the path from a first direction, at each of the plurality of points C.sub.o to C.sub.n a first portion of the light will be reflected by total internal reflection back along the path toward the first direction and a second portion of the light will be transmitted along the path toward a second direction. Similarly, at each of the plurality of points, any light travelling from the second direction will be transmitted along the path toward the first direction. Thus, a series of pulses produced by the reflected first portions is directed back along the path to provide signals indicating that light has passed each of the plurality of points and continues along the path past the last point. If the last point is provided with only a reflector, a series of pulses is directed back to indicate that light has reached each point.

Optical fibers can be provided with planar end surfaces 32,34 using simple fixtures and known polishing techniques. For one actual connector of the general type shown in FIG. 3, fibers were clamped into fixtures at small angles with respect to the fiber's longitudinal axis and polished with emery paper until an essentially planar end surface with a smooth finish was obtained. 200/240 .mu.m step index fibers from General Fiber Optics were finished using this process. Planar end surfaces at an 8.degree. angle and a 15.degree. angle were formed on the respective ends of first and second fibers. To position the ends of these fibers as in with the connector or coupler according to this invention, a 0.010 inch (0.0254 cm) diameter hole was drilled in a quartz rod. A 0.020 inch (0.0508 cm) diameter relief hole was drilled at 90.degree. to the axis of the first hole. Norland ultraviolet-curable optical adhesive was placed in the first hole to fill the gap between the two angled fiber ends. The specially polished fibers were inserted into opposite ends of the first hole in the quartz rod so that their planar end surfaces just touched at one edge. The adhesive was then cured using an ultraviolet lamp. When light was passed through the 8.degree. angle side to the 15.degree. angle side, a 3% reflectivity was measured at 820 nm; whereas, in the opposite direction, a less than 0.01% reflectivity was measured. The asymmetric transmission characteristics of the coupler or connector of the invention were clearly demonstrated.

FIG. 4 shows a second embodiment of the coupler or connector according to the invention in which the longitudinal axes of the first and second fibers are parallel rather than coincident, but light still will couple across volume 54 from one fiber to the other. It is also within the scope of the invention to place the longitudinal axes at an angle to one another. Otherwise, the embodiment of FIG. 4 is configured and functions in the same manner as that of FIG. 3. FIG. 5 shows only the optical fibers 24,36 of a third embodiment in which fiber 24 is provided with a pair of planar end faces 32a,32b which intersect at the longitudinal axis of the fiber; and fiber 36 is provided with a planar end face 44 which is perpendicular to the longitudinal axis of the fiber. Light reflects back along fiber 24 from both of faces 32a,32b when angles .phi..sub.1a and .phi..sub.1b are greater than zero but less than the internal critical angle of fiber 24. Surfaces 32a,32b need not intersect at the longitudinal axis of the fiber. FIG. 6 shows only the optical fibers 24,26 of a fourth embodiment in which fiber 24 is provided with end faces 32a,32b which intersect the longitudinal axis of the fiber; and fiber 36 is provided with similar but oppositely facing end faces 44a,44b. Light reflects back along fiber 36 from both faces 44a,44b when angles .phi..sub.2a and .phi..sub.2b are greater than zero but less than the internal critical angle of fiber 36. Surfaces 44a,44b likewise need not intersect at the longitudinal axis of the fiber.

FIG. 7 shows a fifth embodiment of the invention in which end surfaces 32,44 are essentially in contact and the indexes of refraction of cores 26,38 are not equal. In the illustrated embodiment, angles .phi..sub.1 and .phi..sub.2 are equal, making the longitudinal axes of the two fibers coincident; however, it is also within the scope of the invention to make angles .phi..sub.1 and .phi..sub.2 different and to place the longitudinal axes at an angle to bring the end surfaces 32,44 into essential contact. FIG. 8 shows a sixth embodiment of the invention in which end surface 32 on fiber 24 is in contact only with end surface 44b of fiber 36. The remainder of end surface 32 couples into volume 54. FIG. 9 shows a seventh embodiment of the invention in which end surfaces 32a,32b face and are spaced from end surfaces 44a,44b on second and third fibers 36a,36b and the longitudinal axes are parallel. Thus, the embodiment of FIG. 9 acts as a beam splitter for light fromt the left and a beam combiner for light from the right.

The couplers or connectors of FIGS. 3 to 9 find particularly important application in optical systems for measuring a characteristic of the ambient environment at multiple locations along an optical fiber transmission path. It is preferred that angle .phi..sub.2 is greater than the internal critical angle of fiber 36, to minimize back reflection. FIG. 10 shows a schematic arrangement for such a system. A conventional source or sources of pulsed light 60 and a conventional optical detector 62, or set of detectors with filters for different wavelengths, are connected to a conventional optical coupler 64 which (a) receives light from source 60 and transmits such light into a light transmitting element 66, such as an optical fiber forming part of an optical fiber transmission path and (b) receives light from element 66 and transmits such light to detector 62. Source 60 may be a narrow wavelength band source such as a laser or a broadband optical source or multiple narrow band optical sources with appropriate filters. The output from detector 62 may be passed to a general purpose computer 68 for time domain analysis in accordance with the analytical technique to be described subsequently. Element 66 transmits light to and from a coupler or connector C.sub.o according to the invention, which transmits light over a length of optical fiber to and from a conventional sensor S.sub.1 of a type which transmits light in either direction as a function of a preselected characteristic of the ambient environment.

Sensor S.sub.1 could be a fluorescence based sensor, absorbance based sensor or spectroscopic sensor. Examples of the types of characteristics of the ambient environment which are known to affect sensors of this type are humidity, temperature, flow rate, pressure, various chemicals in the ambient, various biological agents and the like. A variety of techniques have been developed for monitoring the concentration of an analyte in a fluid medium based on the effect the analyte has on the optical properties of the medium or based on the response of a sensor exposed to the analyte in the medium. For example, cobalt chloride has been used for decades in indicators of various types which rely on the change in color of the compound in response to changes in ambient humidity. Cobalt bromide, copper chloride, magnesium chloride, nickel chloride, cobalt thiocyanate, barium chloride, cobalt iodide and vanadium chloride, used alone or in various combinations, also have been demonstrated as humidity sensors whose color changes with changes in ambient humidity. Such sensing compounds have been used in combination with various carrier materials such as polymer matrices including sol gels, gelatin, polyvinylpyrrolidone, Nafion polymer and polymethylmethacrylate. Diethyaluminum hydride-2-isoquinoline complexes have also been used as humidity indicators. Rhodamine 6G in a Nafion matrix also has been used as a humidity detector since its absorbance at a wavelength of 520 nm increases with increasing ambient humidity and its fluorescence at 572 nm also changes as a function of humidity. Umbelliferon dye in a matrix of polymethylmethacrylate also has been shown to be sensitive to humidity since its absorbance at 370 nm and fluorescence at 420 nm are functions of humidity in the ambient. Other sensors depend on quenching of fluorescence, such as those using perylenedibutrate and N-substituted derivatives of perylenetetracarboxylic acid bis-imides in appropriate polymer matrices. Porous silicon oxide/titanium oxide and silicon oxide coatings have also been used as humidity sensors. Optic fibers clad with silicon oxide or plasma polymerized hexamethyldisiloxane and ammonia also have been shown to be sensitive to humidity. Various polyimide coatings are also sensitive to moisture in the ambient.

Reagents and other materials also are known whose optical properties change in response to analytes other than moisture. The following are representative of the many examples which can be found in the literature. Oxygen in the ambient can be sensed by perylene dibutyrate dye which, when excited at 450 nm, will produce a green fluorescence at 520 nm, which fluorescence is quenched by oxygen in the ambient. Oxygen can also be sensed by pyresine butryic acid whose fluorescence also is quenched by oxygen in the atmosphere. Oxygen can be detected by 9,10 diphenyl anthracene. Ammonia can be sensed by oxazine perchlorate dye which changes color reversibly in response to ammonia in the ambient. Bromocresol purple changes its absorbance at 580 nm due to the presence of ammonia, shifting from yellow to blue. Ammonia also can be detected by indophenol with casein fluorophase whose fluorescence changes in response to ammonia in the ambient. Glucose can be sensed with fluorescein labelled dextran and concanavillin A on Sepharose. Triphenyl methane dyes change absorbance in the presence of many organic solvents. Similarly, the absorbance of 3,3,diphenyl-phthalate crystal violet lactone changes in the presence of many organic solvents. Penicillin can be detected by the enzyme penicillinase on glutaraldehyde bound with fluorescein isothiocyanate, since the enzyme binds to penicillin and quenches its fluorescence. Sodium picrate becomes blue in the presence of hydrogen cyanide. Iodine changes from purple to clear in the presence of cyanide ions. Sulphate ions can be sensed using barium chloride which changes from clear to white opaque. Bile acid can be sensed using the reagent 3.alpha.-hydroxysteroid dehydrogenase together with co-enzyme NAD; since in the presence of the reagent, bile acid converts NAD to NADH which fluoresces at 480 nm upon excitation at 340 nm. Iron can be detected by 1,10 phenanthroline which becomes red as the concentration of iron increases. Aluminum can be detected by pyrocatechol which loses its violet color as the concentration of aluminum increases. Oxazine perchlorate is sensitive to the presence of hydrazine and pyridene. Halides can be detected by acridunium and quinollinium indicators whose fluorescence is quenched by their presence. Chlorine can be sensed by orthocolidine.

Referring again to FIG. 10, sensor S.sub.1 transmits light over a length of optical fiber to and from a coupler or connector C.sub.1, which transmits light over a length of optical fiber to and from a sensor S.sub.2, which transmits light over a length of optical fiber to and from a coupler or connector C.sub.2, and so on until the desired number of sensors has been provided to monitor a given area of the environment. As a result of this arrangement, for each pulse of light from source 60, detector 62 receives light reflected back from the several couplers or connectors C.sub.o to C.sub.n, in addition to the normal back scattered light observable in optical time domain reflectometry (OTDR). In operation of the distributed sensor system shown in FIG. 10, source 60 emits a short pulse of light at time T.sub.o. The speed of light in a medium is inversely proportional to the index of refraction of the medium and it takes light a small but measurable amount of time to travel a given distance. The time between pulses from source 60 should be greater than the time required for light to travel along the entire communication path to connector C.sub.n and reflect back to detector 62. Suppose at time T.sub.o +T.sub.Co the light is incident upon connector C.sub.o. The reflected portion of the light incident upon connector C.sub.o will then reach detector 62 at time 2(T.sub.o +T.sub.Co). Similarly, if each connector C.sub.n is considered in sequence, the light from connector C.sub.n will reach detector 62 at time 2(T.sub.o +T.sub.Cn). Since sensors S.sub.1 to S.sub.n are positioned between the successive connectors C.sub.n, the effect of sensor S.sub.1 can be determined by taking the intensity ratio of the reflected light pulses at the times of arrival of the reflected pulses from connectors C.sub.o and C.sub.1. Similarly, the effect of sensor S.sub.n is found by taking the intensity ratio of the reflected light pulses from connector C.sub.n-1 to those from connector C.sub.n.

In practice, it has been found desirable to use at least two wavelengths of light for sensing. One wavelength should be outside the wavelength band in which the optical properties of sensors S.sub.1 to S.sub.n are influenced by the particular characteristic of the environment. This wavelength is used as a reference for determining the percentage of light transmitted through each of the connectors C.sub.o to C.sub.n. Another wavelength is used to detect changes in the properties of the sensors due to changes in the environment. In the case of fluorescence based sensors, a third wavelength is used to check for induced fluorescence. It is also within the scope of the invention to combine different sensors along a single optical fiber transmission path. Changes in the reflected pulses during specific time intervals would be related to changes in the environment of the particular sensor being interrogated. A further advantage of this type of distributed sensor system is that crosstalk among the sensors is minimized. Further, when angle .phi..sub.2 is greater than the internal critical angle of fiber 36, reflected pulses will not recirculate to create noise in the network.

The reflected pulses from the distributed sensor system of FIG. 10 can also be sampled in another manner, independent of the distances of the connectors and sensors from the source of light. In FIG. 10, the connectors C.sub.o to C.sub.n have respective reflectivities R.sub.o to R.sub.n, which can be predetermined for various wavelengths. Similarly, the sensors S.sub.1 to S.sub.n have respective transmittances T.sub.1 to T.sub.n, which are dependent on ambient conditions in the environment. Let P.sub.n be the percentage of light transmitted from source 60 which arrives at connector C.sub.n, assuming no transmission losses in the sensors. When R.sub.1 P.sub.1.sup.2 =R.sub.2 P.sub.2.sup.2 =R.sub.n P.sub.n.sup.2, the signal observed at detector 62 would be the same from each connector C.sub.n, assuming no transmission losses in the sensors. In practice, to achieve this condition, the reflectivity of the connectors would have to increase along the transmission path, to account for losses as a function of fiber length and losses in connectors C.sub.n. This has been accomplished in laboratory systems of the type shown in FIG. 10 which included two sensors and three connectors. In such a case, the observed ODTR trace would appear as in FIG. 11. Each subsequent local maximum in the trace would be the signal obtained from reflected pulses of light from each subsequent connector. When the sensors are functioning properly, the detected signal for D.sub.1 can be shown to be R.sub.1 T.sub.1.sup.2 P.sub.1.sup.2 ; and for D.sub.2 the signal would be R.sub.2 T.sub.2.sup.2 P.sub.2.sup.2 T.sub.1.sup.2, which by substitution equals D.sub.1 T.sub.2.sup.2. Thus, the detected signal D.sub.n for each sensor can be shown to be equal to (D.sub.n-1)T.sub.n.sup.2, thus simplifying calibration of the system. In the case of identical sensors in a uniform environment, the ratio between adjacent sensors would be a constant.

Whatever analytical technique is used to evaluate the output of detector 68, the mode of operation of the embodiment of FIG. 10 is that a pulse of light is directed from source 60 along the path toward connector Co. At a plurality of points Co to Cn along the path, a first portion of the light is reflected back along the path by total internal reflection toward coupler 64 and a second portion is transmitted along the path toward the next connector. After the second portion passes each connector, its optical characteristics are modified by one of sensors S1 to Sn as a function of a preselected characteristic of the ambient environment before the second portion reaches the next connector. The light transmitted in the second direction from the last connector along the path is not reflected back along the path toward coupler 64; so that, some light continues along the path. The series of pulses of light thus directed back along the path toward coupler is detected as proportional to the preselected characteristic of the ambient environment. In accordance with an alternative mode of operation of the invention, light is not transmitted in the second direction from the last connector along the path but is reflected back toward coupler 64; so that, the path ends at the last connector.

While my invention has been shown and described with reference to particular embodiments thereof, those skilled in the art will understand that other variations in form and detail may be made without departing from the scope and spirit of my invention.