Method and apparatus for compensating fiber optic lead and connector losses in a fiber optic sensor.

0001This invention relates to the field of lead loss compen­sation for fiber optic sensors, and more particularly to a dual wavelength technique for compensating losses caused by phenomenon other than the sensed condition.

0002With the rapid growth of fiber optic-based communica­tions, increasing attention is being paid to the use of fiber optic sensors for detecting such phenomena as acoustic waves, rotation rates, acceleration, pressure, magnetic and electric fields, temperature, stress and strain, etc. Such fiber optic sensors are normally based on a transducer mechanism that de­pends upon modification of the polarization of light passing through the sensor. Thus, the polarization of the light beam is modulated in accordance with the sensed phenomenon. A typical configuration for such a sensor includes a light source for providing a light beam, fiber optic cables for transmitting the light beam, a photo transducer which modu­lates the light beam in accordance with the externally applied phenomenon, additional fiber optic cables for transmitting the modulated light beam to photo detectors where the modulation of the light beam may be decoded, and a processor which out­puts a usable signal indicating the magnitude of the sensed phenomenon.

0003A significant drawback to the typical photo sensor con­figuration is that losses caused by signal attenuation in the fiber optic cables and their connectors is significant com­pared to the signal level of the light beam. In addition, power fluctuations in the light source itself also may over­shadow the signal level detected by the photo detectors. Turning to Figure 1, it can be seen that the intensity of the signal detected by the photo detectors may be hidden by fiber optic lead and connector losses. Figure 1 is directed to a photo transducer which detects stress. The X axis depicts in­creasing amounts of stress applied to the photo transducer, while the Y axis depicts the intensity of the optical signal detected by the photo detectors. Assuming that the detected optical power is I_s, it is uncertain whether the stress ap­plied to the photo transducer is S₁, S₂, or S₃. The stress is highly dependent upon the lead/connector losses of the fiber optic cable system. Such losses are inherent in any fiber optic photo sensor device employing fiber optic cables, and these losses severely restrict the potential applications for such a device. Even where such a device may be applied, the accuracy of the measured optical signal is highly suspect.

0004One solution to this problem would be to calibrate the lead/connector losses before each measurement. This solution would provide minimal improvement since such lead/connector losses can vary over time. In addition, such a calibration scheme would fail to compensate the detected signal for power fluctuations in the light source itself. In addition, such constant recalibration would consume a great deal of time for the minimal benefits achieved thereby. Therefore, a more pre­cise solution to this problem must be achieved before fiber optic sensing systems achieve wide acceptability in the sensor field.

0005One such solution is proposed in U.S. Patent 4,368,645, which discloses an optical pressure sensor which can compensate for some (but not all) lead/connector losses and light source power fluctuations. This device provides a light source which generates a light beam that is input into a fiber optic cable. The fiber optic cable directs the source light beam to a lens fiber optic cable directs the source light beam to a lens which collimates the light beam. The collimated light beam is then passed through a linear polarizer and a quarter wave plate to circularly polarize the collimated light beam. The circularly polarized light beam is then introduced into a photoelastic transducer which modulates the polarized light beam in accordance with pressure applied to the photoelastic transducer. The modulated light beam is then directed to a polarizing beam splitter which splits the modulated light beam into first and second components. Each separate component is focused by a lens into a separate fiber optic cable. These fiber optic cables then direct the first and second components to photo detector devices for detecting the intensity of the first and second components. The dependence of the intensities of the two components on the pressure applied to the photo­elastic transducer permits the measurement of that pressure in a manner that has quadratic error dependence on optical mis­alignment. The difference in the intensities of the two com­ponents is then divided by the sum of the intensities of the two components to elimate lead/connector losses in the fiber optic cable leading up to the transducer. However, such a scheme does nothing to compensate for transmission losses from the output of the transducer to the photo detector devices. Thus, the device is still highly susceptible to fiber optic lead/connector losses, and if such losses in the output fiber optic cables are significant, the device will not function properly.

0006Another solution for compensating the sensitivity varia­tions in fiber optic cables and the drift of the light source and photo detectors is proposed in U.S. Patent 4,493,995. In this solution an optical source provides a source light beam at a single wavelength which is used as a carrier wave. This carrier wave is then modulated at one or more lower frequencies in accordance with the phenomenon sensed by the photo transducer. The material in the photo transducer responds differently depending upon the modulation frequency of the light beam, not upon the carrier frequency (wavelength) itself. The response of the photo transducer material at the different modulation frequencies can then be used to determine the sensed phenomenon independently of lead and connector losses. However, such a device is complex and expensive due to the necessity of modulating the carrier wave. In addition, demodulation electronics are required, also in­creasing the complexity and cost of this device.

0007Therefore, what is needed is an inexpensive yet precise solution to compensating photo sensor systems for fiber optic lead/connector losses and power fluctuations in the light source.

0008The present invention provides method and apparatus in which an optical source provides a source light beam having at least first and second spectral components. Each spectral component has at least one associated wavelength. Such a source light beam may be provided by a single broadband light source, or two separate light sources operating at different wavelengths.

0009A polarization-optical-transducer device then receives the source light beam and modulates it according to a detected phenomenon. This device may include a polarizer, a photo­elastic transducer, and an analyzer. The polarizer receives the source light beam and is specially selected so that the first spectral component of the source light beam is polarized while the second spectral component of the source light beam is effectively unpolarized. The light beam is then directed to the photoelastic transducer which modulates the polarized first component in accordance with the detected phenomenon. The unpolarized second spectral component remains effectively unmodulated by the transducer. The light beam is then passed through the analyzer which converts the polarization modula­tions into light intensity modulations. Thus, the analyzer provides an output light beam having modulated and unmodulated light intensity components. This output light beam is then transmitted away from the transducer device by a single fiber optic cable.

0010The output light beam is then directed to a beam splitter which splits the output light beam into first and second por­tions, each portion including the modulated and unmodulated light intensity components. The first portion is passed through a narrow passband filter which only passes the modu­lated light intensity components. These modulated light in­tensity components vary in accordance with the detected phe­nomenon and can be detected by a first photo detector. The second portion of the split beam is passed through a second filter which filters out the modulated light intensity compon­ents while passing the unmodulated components. The unmodu­lated components remain substantially unresponsive to varia­tions in the detected phenomenon. The intensity of the unmod­ulated components is then detected with a second photo detec­tor.

0011Output signals from both photo detectors are then pro­vided to a processing device which thus receives a first elec­trical signal which varies in accordance with the detected phenomenon, and a second electrical signal which does not vary in accordance with the detected phenomenon. The processing device then provides a ratio of the first and second electri­cal signals, such ratio varying in accordance with the de­tected phenomenon, but without the effects of fiber optic lead/connector losses and drift of the light source.

0012Such a device and method are inexpensive since the device is comprised of off-the-shelf components, yet completely compensate for fiber optic lead/connector losses in the fiber optic cables leading to and away from the photoelastic trans­ducer, and for power fluctuations in the optical source.

0013The features and advantages of the present invention will be more clearly understood with reference to the following de­tailed description of the presently preferred exemplary embod­iment taken together with the attached drawings in which: <ul id="ul0001" list-style="none"><li>Figure 1 is a stress versus detected optical power graph illustrating the disadvantages of fiber optic lead/connector loss on the detected signal intensity;</li><li>Figure 2 depicts the apparatus according to the presently preferred exemplary embodiment;</li><li>Figure 3 depicts an alternative optical source;</li><li>Figures 4a and 4b are two graphs showing the source opti­cal power and the polarization efficiency of the polarizer as functions of wavelength;</li><li>Figure 5 depicts the effect on the intensity of the de­tected signal of cross-polarizing the polarizer and analyzer;</li><li>Figure 6 depicts the detected optical power of the first and second components as they vary with stress; and</li><li>Figure 7 shows that the ratio of the detected optical power of the first and second components varies with stress independent of lead/coupler losses.</li></ul>

0014Briefly, the invention provides apparatus and method for use with fiber optic sensors based upon transducer mechanisms that depend upon polarization modulation of a source light beam. The essence of the technique is to employ two optical sources (or a single broadband optical source) to inject a light beam through the optical fiber and transducer, and also to use polarization optics that only function as polarizers at one of the wavelengths while being effectively non-polarizing at the other wavelength. Changes in the transmission charac­teristics of the fibers will affect both wavelengths equally, while the polarization transducer mechanism will only affect one of the wavelengths. The ratio of the signals of the two wavelengths will then vary due to the transducer mechanism, but will be relatively insensitive to losses in the fiber op­tic leads and connectors and optical source drift.

0015The present invention makes use of the photoelastic ef­fect in order to provide an optical transducer responsive to a detected phenomenon. Photoelastic transducers may be used to measure such phenomenon as acoustic waves, rotation rates, ac­celeration, pressure, magnetic and electric fields, tempera­ture, and stress and strain, etc. While the present invention will be described with reference to examples depicting pres­sure and stress sensing devices, it is to be understood that the teachings of this invention are applicable to an optical measuring device for measuring any phenomenon capable of being detected with an optical transducer.

0016Generally, the photoelastic transducer and its operation are known in the field and are more fully described in an article entitled "Multimode Fiber Optic Sensors Based On The Photoelastic Effect", written by the inventor, and D. H. McMahon. This article was presented at the SPIE Technical Symposium East, in Arlington, Virginia during April of 1983.

0017Photoelasticity is a phenomenon of stress (or strain) induced birefringence in optically transparent materials. Many of these materials are isotropic in the unstressed case. When a uniaxial stress is applied, however, a difference in index of refraction is introduced between the stress direc­tion and directions orthogonal thereto. The materials, there­fore, exhibit a birefringence that depends upon applied stress.

0018Figure 2 depicts one exemplary embodiment of the present invention. A broadband light source 10 provides a source light beam having at least first and second spectral compon­ents. Each spectral component has at least one wavelength which is different from the wavelengths of the other spectral components. As an alternative to the single broadband optical source, two optical sources 12 and 14 may be provided, see Figure 3. Optical source 12 provides a first source light beam having a spectral component with an associated wave­length. Light source 14 provides a second source light beam having a second spectral component with a wavelength different than the wavelength of the first optical source. The first and second source light beams are then provided through fiber optic cables 16 and 18 to a coupler 20 which provides a single source light beam containing both spectral components to fiber optic cable 22.

0019The source light beam then travels through fiber optic cable 22 to polarizer 26. It is to be understood that fiber optic cable 22 may actually include a number of fiber optic cables linked together by fiber optic connectors 24. The num­ber of fiber optic cables 22 and fiber optic connectors 24 will vary depending upon the application for which the optical sensor system is designed.

0020The source light beam is then projected from the fiber optic cable 22 to a polarizer 26. The polarizer 26 is a spec­ially constructed polarizer which polarizes only one of the spectral components of the source light beam while leaving the other spectral component effectively unpolarized, see Figure 4a. Such a polarizer may be a plastic polarizer spec­ially chosen depending upon the wavelengths of the first and second spectral components. For example, where the first wavelength is chosen as lying between 560 and 700 nanometers, a linear polarizing plastic polarizer provides a very good extinction ratio. On the other hand, where the second wave­length is chosen as being greater than 800 nanometers, the linear polarizing plastic polarizer provides no extinction. In addition, the two spectral components of interest in the optical source light beam must be matched to the polarizer to ensure that one spectral component is polarized while the other spectral component is not, see Figure 4b. Thus, a care­ful choice of optical sources and polarizers must be made in order to provide a first spectral component which varies in accordance with the detected phenomenon, while a second spec­tral component does not vary in accordance with the detected phenomenon (see Figures 4a and 4b).

0021Polarizer 26 may be placed so that it is generally orthogonal to the optical axis 29 of the source light beam. If the source, light beam is defined at lying along the Z axis, the polarizer 26 lies in the X-Y plane. Superimposing X and Y axes on the X-Y plane, the transmission axis of the polarizer may be disposed at π/4 with respect to the X axis. Thus, the first spectral component is polarized while the second spectral component remains effectively unpolarized. An addition to the present invention may include a quarter wave plate 28 disposed between polarizer 26 and photo transducer 30. Quarter wave plate 28 may be used to bias the linearly polarized light beam passing through polarizer 26.

0022The polarized light beam then enters photoelastic trans­ducer 30. The photoelastic transducer is disposed so that pressure (or another phenomenon of interest) impinges upon the photoelastic material in a direction orthoganal to the optical axis. For example, the pressure may be applied along the X-axis. The photoelastic transducer then acts to further polarize (modulate) the polarized component of the light beam in accordance with the pressure applied to the transducer. The unpolarized component of the light beam remains effec­tively unmodulated by the phototransducer.

0023The light beam emerging from the photoelastic transducer is then directed along its optical axis to an analyzer 32. Analyzer 32 may be a polarizer having its transmission axis displaced by -π/4 with respect to the X-axis. The analyzer acts to convert the polarized components of the light beam into light intensity components. The light beam output from the analyzer thus contains spectral components whose light intensity varies in accordance with the pressure applied to the photoelastic transducer, and spectral components whose light intensity does not vary with the pressure applied to the transducer.

0024Polarizer 26 and analyzer 32 thus function as cross-­polarizers and are specially selected so that they only function as polarizers in a given wavelength region. The optical source (a single broadband source or two individual light sources) is specifically chosen to have a spectral out­put both within and without the wavelength region over which the polarizer and analyzer function. Figure 5 depicts this relationship. Figure 5 uses a typical output for a Polaroid™ polarizing film, for example. Figure 5 assumes a single broadband optical source having spectral components having wavelengths λ₁ and λ₂. The spectral output from the broadband source is shown. Horizontal line 34 depicts spectral dependence of optical power transmitted through a polarizer/analyzer pair where their transmission axes are parallel. Curve 36 depicts the spectral dependence of opti­cal power transmitted through a polarizer/analyzer pair when their transmission axes are crossed. In this case, the spec­tral component containing wavelength λ₁ is not transmitted by the polarizer/analyzer pair thereby demonstrating the effec­tiveness of the polaroid film at this wavelength. Thus, it can be seen that the light intensity I_O(A) of the first spectral component will vary in response to the pressure ap­plied to photo transducer 30. However, dotted line 36 shows that the light intensity I_O(B) of the second spectral com­ponent containing wavelength λ₂ remains relatively unrespon­sive to the pressure applied to the photo transucer 30.

0025Figure 6 depicts the interrelationship of the light in­tensity of the first spectral component containing λ₁ with the light intensity of the second spectral component contain­ing λ₂. It can be seen that the light intensity of wavelength λ₁ varies in accordance with the stress or pressure applied to photo transducer 30, whereas the light intensity according to wavelength λ₂ remains invariant to such pressure. The present invention makes use of this relationship in order to provide an output signal whose response is relatively independent of fiber optic lead/connector losses and power fluctuations in the light source.

0026Returning to Figure 2, the light beam exiting analyzer 32 is injected into a single fiber optic cable 38. This output light beam contains light intensities corresponding to the first and second spectral components, as depicted in Figure 6.

0027Fiber optic cable 38 may include a plurality of cables coupled together using connectors 40.

0028The output light beam is directed from fiber optic cable 38 to beam-splitter 42. Beam-splitter 42 splits the output light beam into first and second portions. First portion 44 is directed to a narrow passband filter 48 which filters out the light intensity components which do not vary in accordance with the pressure applied to photo transducer 30. Photo de­tector 50 thus detects a light intensity which varies in ac­cordance with the pressure applied to the photo transducer 30.

0029The second portion 46 of the split beam is directed to a filter 52 which filters out the light intensity components that vary with the pressure applied to the photo transducer 30, leaving those light intensity components which remain sub­stantially invariant to the pressure applied to photo trans­ducer 30. Photo detector 54 thus detects light intensity com­ponents which are not responsive to the pressure applied to photo transducer 30. Thus, photo detector 50 detects light intensity components which vary in accordance with the applied pressure, and in accordance with transmission losses in the fiber optic cables and connectors and also power fluctuations in light source 10. On the other hand, photo detector 54 de­tects light intensity components which vary only in accordance with transmission losses and power fluctuations. Both photo detectors 50 and 54 provide electrical output signals to pro­cessor 56 corresponding to the light intensities detected by each photo detector.

0030The electrical signals from photo detectors 50 and 54 are provided to processor 56 which provides an electrical output signal that is proportional to the ratio of the optical inten­sities affected and unaffected by the photo transducer 30. This ratio is then insensitive to changes in optical intensity due to non-transducer effects. The signal output by processor 56 is depicted in Figure 7. As shown therein, the output sig­nal varies with the stress applied to the photo transducer and is independent of lead/coupler losses, or losses due to the drift in the optical source.

0031In sum, even though photo transducer 30 acts on the polarization state of all light passing through it, this change will not appear as an intensity fluctuation unless its polarization state is analyzed by a polarizing element 32. As shown in Figure 6, the light intensity I_λ₂ does not change with variations of the detected phenomenon due to the fact that the portion of the broadband optical source that is transmitted through the narrow passband filter 52 is not af­fected by polarizer 26 and analyzer 32 (i.e. it is some frac­tion of the optical power I_O(B) shown in Figure 4). The detected intensity signal I_λ₁ does change with the varia­tion of the detected phenomenon due to the fact that the por­tion of the broadband source ouput that is transmitted through filter 48 is affected by the polarizer 26 and the analyzer 32 (i.e. it is some fraction of the optical power I_O(A) that is a function of the detected phenomenon shown in Figure 5).

0032Thus, the technique according to the present invention is based upon an optical source which is carefully selected to have spectral components which straddle the wavelength region of the effectiveness of the polarizer 26 and analyzer 32. Narrow band filtering of the optical signal allows both refer­ence and transducer-varying output to be detected simultane­ously.

0033Thus, what has been described are apparatus and method for precisely sensing a phenomenon of interest without incur­ring losses due to signal attenuation by fiber optic cables and connectors, and by fluctuations in the optical source.

0034While the invention has been described in connection with what is presently considered to be the most practical and pre­ferred embodiments, it is to be understood that this invention is not limited to the disclosed embodiment, but, on the con­trary, is intended to cover various modifications and equiva­lent arrangements included within the spirit and scope of the appended claims. It is to be understood that all such equiva­lent structures are to be included within the scope of the following claims: