SENSOR USING OPTICAL FIBRES

Abstract: There is provoded a sensing system using optical fibers having loss characteristics subject to variations for a signal light wave as a pumping light wave with a wavelength different from that of a pumping light wave is coupled into said optical fiber. Unlike conventional OTDRs that utilizes a light back scattering in an optical fiber, a sensing system using optical fibers according to the invention utilizes the light passing through an optical fiber as a detected signal in order to enhance the efficiency and effective­ ness of the system and therefore it has a significantly improved S/N ratio and a dynamic range of measurement as compared with those of conventional OTDRs. Moreover, continuous light may be used for the signal light wave (detected signal) to increase the rate of light emission for detection and to reduce the time required for measure­ ment providing remarkable advantages of the system when used for industrial applications.

Description: [Field of the Invention] This invention relates to a sensing system using optical fibers to measure the distribution of some measurands, such as temperature, pressure and etc., distributed along the optical fiber's longitudinal direction. [Prior Art] A technique called Optical Time Domain Reflectometry (OTDR) has been popularly used for distributed sensing of the measurands such as temperature, pressure and so on that are distributed along the optical fiber's longitudinal direction. This technique utilizes the property of optical fiber that an introduced pulse light having a given wave length that goes into an optical fiber from an end of it eventually comes back to that end with a part of the light experiencing a certain degree of back scattering at a given point along the longitudinal direction of the fiber in order to determine the location of the point as a function of the time required for the measuring to come back and to calculate the loss of the light at the given point of the optical fiber the difference between the intensity of the incident light and that of the reflected light. Conversely, if the loss of an introduced light at a point of an optical fiber varies as a function of ambient temperature, pressure and other variable factors, the physical quantities such as temperature and pressure which are specific to the point can be determined by observing the variation of the loss of the light at the point. The above described method is, however, accompanied by certain disadvantages. The ratio of back scattering light intensity relative to the light introduced into the optical fiber of an instrument using this technique is normally too small. So is the S/N ratio due to the fact that the introduced light is a pulse light and therefore the quantity of the light that can be detected per unit time is limited. The dynamic range of instrumentation of such an instrument is very small. Consequently, more than several seconds or, in some instances, more than several minutes will be required for one instrumentation as it involves tens of thousands of averaging operations when a high accuracy of measurement is essential. Since a commercially available wave length instrument has a dynamic range of 17dB to 20dB, for higher dynamic range, there have been proposed sensing systems using optical fibers that utilize a high-power semiconductor laser device that can generate alight of approximately 1W or a Nd:YAG laser device having an output of several watts. However, such asystem inevitably involves a large cost for light source and the risk of generating in the optical fiber nonlinear optical effects that can in turn cause noises in the system. A further example of prior art is given in the Article "Multiplexed and distributed optical fibre sensor systems", Journal of Physics E/Scientific Instruments 20 (1987) August No. 8, Bristol G.B. It is therefore an object of the present invention to provide a sensing system using optical fibers that utilizes a relatively large quantity of light per unit time and offers an improved S/N ratio and a widened dynamic range of instrumentation. In accordance with this invention there is provided a sensing system comprising an optical fibre, a first source for directing a signal light wave into one end of the optical fibre, and a second source for directing a pumping light wave into the other end of the optical fibre, the wavelength of the signal light wave being different from the wavelength of the pumping light wave, characterised in that the optical fibre has a loss characteristic such that the attenuation of the signal light wave is varied by the pumping light wave and is also varied as a function of an external measurand applied to the optical fibre, and means are provided at said other end of the optical fibre to detect variations in intensity, at the signal wavelength, in the signal light wave arriving at said other end of the optical fibre. An embodiment of this invention will now be described by way of example only and with reference to the accompanying drawings, in which: Figure 1 is a schematic illustration of the configuration of a sensing system using optical fibers according to the invention; Figure 2 is a conceptual illustration showing different energy levels of a doping material; Figure 3 (a) is a graphic illustration showing the loss characteristics of a doping material on the ground state, Fig. 3 (b) is a graphic illustration showing the loss characteristics of a doping material on an excited energy level, under an excited condition, Fig. 4 is a graphic illustration showing the intensity response of a signal light wave coming out of the fiber (B), after optical pimping with a pumping light wave in an optical fiber sensing system according to the invention, Fig. 5 is a graphic illustration showing the intensity of a signal light wave coming out of the fiber B), after optical pumping with the pumping light wave as the the ambient temperature at a location of the optical fiber separated by distance ℓ from the fiber end (B) is varied, Fig. 6 is a graphic illustration showing the loss spectrum of an Er-doped optical fiber, Fig. 7 is conceptual illustration showing diffetent energy level of Er³⁺, Fig. 8(a) is a graphic illustration showing the loss spectrum of Er under a nonexcited condition, and Fig. 8(b) is a graphic illustration showing the loss spectrum of Er under an excited condition. DETAILED DESCRIPTION OF THE INVENTION In Fig. 1 that schematically shows the configuration of a sensing system using optical fibers according to the present invention, reference numeral 4 denotes an optical fiber having loss characteristics which are subjected to variations at the wavelength of a signal light wave when said pumping light wave is introduced into said optical fiber, the amount of said variations further varied as a fuction of externally applied physical quantities (measurandss), said optical fiber being doped with a material having two or more different energy levels. Reference numeral 9 denotes a pulse light source to emit a pumping light wave having frequency ω a (or ω b ) that corresponds to the amount of transition energy required for the dopant material to transit from the ground state Ω O to a given excited level Ω a or a more higherenergy level such as Ω b . A pulse light driven by a pulse generator 11 and a drive circuit 10 is introduced into the optical fiber from the fiber end (B) passing through a lens 6, a beam splitter 8 and another lens 5. Reference numeral 13 denotes a light detector for monitoring the intensity stability of the outgoing pulse light of the pumping light wave and establishing a desired level of the outgoing pulse light and therefore it does not constitute an essential component of the system. Reference 1 denotes a light source for being introduced into the fiber (A) through a filter 2 and a lens 3, and the light source emits the signal light wave having frequency ω a-b that corresponds to the amount of transition energy required for the dopant material to transit from said given excited energy levelΩ a to another excited energy levelΩ b which is higher than said level Ω a into one end of said optical fiber 4. Said signal light wave may be in the form of either continuous light or pulse light. Reference numeral 12 denotes a light detector for measuring the intensity of the signal light wave coming out of the fiber end (B) 4 through the lens 5, the beam splitter 8 and the lens 7. The filter 2 passes the signal light wave, while it blocks or significantly attenuates the pumping light wave and the beam splitter 8 selectively reflects the signal light wave coming from the light source 1 while it passes almost all the pumping light wave coming from the light source 9. If the doping material applied to the optical fiber has three different energy levels of the ground state Ω O and and two different excited level Ω a and Ω b as illustrated in Fig. 2, the loss characteristics of the material normally shows a large loss coefficient for frequency ω a that corresponds to the amount of transition energy required for the material to transit from the ground state Ω O to the excited level Ω a and another large loss coefficient for frequency ω b that corresponds to the amount of transition energy required to transit from the ground state Ω O to the excited level Ω b as illustrated in Fig. 3(a). While such a material is normally found on the ground state Ω O , a given portion of the material doped into the optical fiber transits to excited level Ω a when it is exposed to an intense light with frequency ω a , although it returns to the ground state Ω O when a certain period of time has elapsed after termination of the exposure to irradiation of the intense light. The period of time required for the material to transit from the excited level Ω a down to the ground state Ω O is defined as the life time of the excited level Ω a . The material shows loss characteristics as illustrated in Fig. 3(b), when the level Ω a is excited and an incoming light with frequency Ω a-b that reaches the material during the life time of the excited level Ω a . In other words, when excitation the material has a large loss coefficient for frequencyω a-b that corresponds to the amountof transition energy required for the material to transit from the excited levelΩ a to the higher excited level Ω b . Now assuming that the signal light wave having frequency ω a-b and intensity of light I SO is introduced into an fiber end (A) with length L and loss coefficient α g , the intensity I S of the signal light wave going out of the fiber end (B) is expressed by I S = I SO ·exp (-α g L). However, once a pulse light of the pumping light wave with frequency ω a is introduced into the optical fiber from the fiber end (B),the intensity of the signal light wave going out the fiber end (B) is varied as function of time (T), because said signal light wave is subjected to a further absorption by the excitation of the specific light wave. Care should be taken to establish the pulse cycle of said pumping light wave in such a way that the cycle intervals are sufficiently greaterthan the time required for a pulse to go and return from an end of the optical fiber to the other end and we must also consider the life time of excited level Ω a . Now assuming that T represents the time counted from the moment when the first pulse of the pumping light wave is coupled into the optical fiber, ℓ: the distance between the fiber end (B) and a point where the signal light meets the pumping light in the optical fiber; that is, ℓ means the length where the signal light to be detected at the fiber end (B) is subjected to an increase of absorption caused by the pumping light wave, α e (X)= α O (X) · I p (X): the loss coefficient of of the excited region in the optical fiber by exposing of the pumping light wave at the signal light wave length, where x represents a position along the optical fiber (where I p (X) represents the intensity of the pumping light wave at the position (X) in the optical fiber and depends on the location in the optical fiber and we are assuming here that the loss coefficient of α e is proportional to the intensity of the pumping light wave I p , and the proportional coefficient is α O and we defined other parameters as V a-b : the speed of propagation of the signal light wave in the optical fiber, α p : the the loss coefficient at wavelength of the pumping light wave, V a : the speed of propagation of the pumping light wave in the optical fiber, τ: the life time of excited level Ω a . The life time is defined as the time required for the excited population to be reduced to 1/ e of its initial value. If V = 1/V a + 1/V a-b is defined, T =ℓ×V is established. If the level of dopant material in the optical fiber and the neasurand along the optical fiber are homogebeous, α O (ℓ) is constant at any point in the optical fiber, then the intensity I s (T) of the signal light wave at time T, elapsed from the moment when the first pulse of said pumping light wave is coupled into the optical fiber is expressed by the following equation: I S (T)=I SO · exp (- α g L) · exp [(α O ·I pO ·τ) / (V-α p τ) × (-ℓ - T/τ · ℓ - αρℓ )]- exp (-αL)}] where I SO , α g and L are so defined mentioned earlier and I pO represents the intensity of the pumping light wave at the fiber end (B). If it can be sufficiently assumed that the relationship τ»T=ℓ V x exists, equation (1) may be expressed as follows: I S (T)=I SO ·exp(-α g ·L)·exp{[α O (ℓ)·I pO ]/ [α p ]× [e -αρℓ -1]} Fig. 4 is a graphic illustration of equation (2). This graph shows the characteristics of the case that α O (ℓ) is constant to length ℓ. However, if α O (ℓ) is varied in response to variations of measurands (physical quantities) along the optical fiber and the measurands have distribution along the optical fiber , α O (ℓ) is not constant. If the measurands along the optical fiber are not homegeneous but different at the position ofℓ, which corresponds to the time T, α O (ℓ) is also different from that at other places in the optical fiber, and so we observe the response of the output signal light wave as shown in Fig. 5, in which a small signal output change is indicated. Therefore, we can obtain the change of measurand at that point by measuring the amount of variation of the change of the signal response change and the location can be identified by the time of T. When the value of α O (ℓ) is not constant but being varied by the physical quantities (measurand), the signal response can not be determined directly by using an analytical method, although the value of α O (ℓ) may be calculated by using an equation (3). And when we can determine the change of the measurand from the change of α O (ℓ) by using the measurement results of I SO and I S . α O (ℓ)=(1/I pO )·e - α p ·T/V [-(V/τ){log(I S /I SO )+α g ·L}-V(d/dt)(log I S /I SO ) where (d/dt) represents to differentiate by time. Now the present invention will be described further by way of an embodiment of the invention. An optical fiber that can be suitably used for the purpose of the present invention can be prepared by a rear-earth element doped into the core of a silicate glass optical fiber. Fig. 6 illustrates the loss characteristics of an optical fiber prepared by Er³⁺ doping with a concentration of 790 (wt)ppm into the silcate glass optical fiber. Er³⁺ has an energy configuration as illustrated in Fig. 7, and a remarkable absorption of energy found at and around 1.5 µm in Fig. 6 corresponds to transition energy from the ground state ⁴I 15/2 to the exited energy level ⁴I 13/2 . Two other remarkable absorptions are also shown in Fig. 6 at and around 0.8 µm and 0.65µm, they are respectively caused by transistions from the ground state ⁴I 15/2 to the excited energy level ⁴I 9/2 and from the ground state ⁴I 15/2 to the excited energy level ⁴F 9/2 . When an intense light (semiconductor laser beam) having a wavelength of 1.5 µm or so is introduced into the fiber as excitation, the spectrum between 0.6 µm and 0.8 µm comes to show, as illustrated in Fig. 8(b), an increased level of absorption at or around 0.7 µm and also at 0.8 µm which are not found in the previous loss spectrum (under a nonexcited condition) when the semiconductor laser beam with a wavelength of 1.5 µm or so is not introduced into the optical fiber as illustrated in Fig. 8(a). These increased absorptions are respectively based on the transitions from the excited level ⁴I 13/2 to the level ⁴F 7/2 and from the level ⁴I 13/2 to the level ⁴H 11/2 and correspond to ω a-b of Figs. 2 and 3. When a signal light (or the signal light wave) with a wavelength of 0.72 µm which is obtained by spectroscopically processing a light from a white light source by means of a monochronometer is introduced into said optical fiber 4 from the fiber and (A) thereof, the instensity of the light coming out from it shows a variation as illustrated in Fig. 4 as it is measured by a light detector 12 arranged at the other end thereof. Alternatively, a light emitted from an LED or a semiconductor laser beam having the same wavelength may also be used for the signal light. The lift of the excited energy level ⁴I 13/2 proved to be as long as 14 m when measured in an experiment and this fact indicates that equation (2) is good for use when the light ofan optical fiber is less than several hundred kilometers. Since the concentration of Er³⁺ in this measurement is high,the loss coefficient (that corresponds toαp in equation (2)) of the pulse light (the pumping light wave) generated by a semi-conductor laser device should be very large at or around 1.5µm and therefore the optical fiber will become ineffective as the intensity of the pulse light coming from a semiconductor laser device for exciting the fiber should become weak at or around 1.5 µm, if the optical fiber is too long. In view of this fact, an optical fiber which is 30m long is used for this embodiment, although a longer optical fiber may be used, if the Er³⁺ concentration is too small. For practical purpose, an optical fiberas long as 5 km may be feasible, the Er³⁺ concentration is as low as several ppm or less. Apart from this, it is known that the rate of absorption of ions of a rare-earth element due to transition between the excited energy levels is varied by variation of ambient temperature. (See "The Tempera-ture Dependency of Electron Transition in Rare Earth Ions": M.C. Farries et al, Electron Lett, Vol. 22, No. 8, p.418, 1989) When the temperature at a point separated by distance ℓ from the other end of the optical fiber was subjected to changes in an experiment, it produced a result similar to the loss characteristicsas illustrated in Fig. 5. Therefore, the temperature at this point can beknown by the change that took place on the line of the loss characteristics of Fig. 5. A high speed detector utilizing Si was used for the light detector 12 in the experiment. A dye laser device that emits light with a wavelength of 0.73 µm and an intensity of light which is greater than that of a white light source is used for the light source 1 to produce a dynamic range of approximately 30 dB or more. The loss coefficient α O (ℓ)·I p of a signal light having a wavelength of 0.7 µm due to introduction of the pumping light wave can be increased by augumenting the intensity of the pulse light of the wavelength of 1.5 µm coming from the optical fiber, although the maximum feasible length of the optical fiber inevitably diminishes as a result of such manipulation. Therefore, when a long optical fiber is used, the intensity of the pumping light wave to be introduced into the optical fiber should bereduced. Lanthanoids such as Ho, Tm and Sm and actinoids such as U as well as alkali metal ions and alkaline earth metal ions that havesimilar effects as an alternation change by the optical pumping which is observed in rare-earth elements may be used for doping an optical fiber in place of Er. (Only the clad area of the optical fiber may be doped for the purpose of the present invention.) Organic compounds such as dyes can be used for doping when the optical fiber 4 has a core made of a liquid material. Photochromic glass and other materials having a phototropy effect may also be used for doping. Absorption due to defects of the glass structure of an optical fiber may also be utilized for the purpose of the present invention. It should be noted that the approximation ofτ» T in the analytical process described earlier is not essential and a sensing system using equation (1) may be also feasible. While the loss coefficient which is determined on the basis of the incoming signal light wave is a linear function of the intensity of the pumping light wave α O (ℓ)·I p in the above analytical process, it may not necessarily be so and, if such is the case, sensing operations can be conducted by comparing the situation where the environmental physical quantities to be determined are constant and the situation where they are partially varied, although the relationship as illustrated in Figs. 4 and 5 are not applicable in such a case. As is apparent from the above description, an sensing system using optical fibers according to the present invention, unlike the conventional OTDRs that utilize back scattering of a signal light wave in an optical fiber, utilizes the light wave passing through an optical fiber as signal in order to enhance the efficiency and effectiveness of the system and therefore it has a significantly improved S/N ratio and a dynamic range of measurement as compared with those of the conventional OTDRs. Moreover, continuous light may be used for the signal light wave to increase the rate of light emission for detection and to reduce the time required for measurement, providing remarkable advantages of the system when used for industrial applications.

Claims: A sensing system comprising an optical fibre (4), a first source (1) for directing a signal light wave into one end (A) of the optical fibre (4), and a second source (9) for directing a pumping light wave into the other end (B) of the optical fibre, the wavelength (ω a-b ) of the signal light wave being different from the wavelength (ω a ) of the pumping light wave, characterised in that the optical fibre (4) has a loss characteristic such that the attenuation of the signal light wave is varied by the pumping light wave and is also varied as a function of an external measurand applied to the optical fibre (4), and means (7,12) are provided at said other end (B) of the optical fibre (4) to detect variations in intensity, at the signal wavelength (ω a-b ), in the signal light wave arriving at said other end (B) of the optical fibre. A sensing system as claimed in claim 1, characterised in that said first source (1) is arranged so that said signal light wave is continuous. A sensing system as claimed in claims 1 or 2, characterised in that said second source (9) is arranged so that said pumping light wave is pulsed and each pulse is emitted for a period of time longer than the time required for the light wave to travel from said other end (B) of the optical fibre (4) to said one end (A) and back. A sensing system as claimed in claim 3, characterised in that said second light source (9) is arranged so that the interval between successive pulses of the pumping light wave is longer than the time required for the light to travel from said other end (B) of the optical fibre (4) to said one end (A) and back. A sensing system as claimed in claim 1, characterised in that said second light source (9) is arranged so that the pumping light wave is pulsed and each pulse has a width greater than the time required for the pumping light wave to go from said other end (B) of the optical fibre (4) to said one end (A) and subsequently for the signal light wave to go from said one end (A) to said other end (B). A sensing system as claimed in any preceding claim characterised in that said optical fibre (4) comprises silicate glass doped with rare earth elements, alkali metal ions, alkaline earth metal and/or transition metal ions. A sensing system as claimed in any of claims 1 to 5 characterised in that said optical fibre (4) comprises silicate glass doped with Er3+.

Inventors: NAKAMURA, KAZUNORI
Applicants: THE FURUKAWA ELECTRIC CO., LTD.
Application year: 1989
Main classification: G01D5
IPC codes: G01D21, G01M11, G01J5, G01K11, G01D5, G02B6