Abstract:
Disclosed herein is an optical sensor design and method for continually interrogating that sensor to produce an accurate representation of a dynamic event (such as a change in strain, pressure or temperature) being monitored by the sensor. The sensor design preferably constitutes continuous wave optical source/detection equipment coupled in series to a first fiber Bragg grating (FBG), a long period grating (LPG), and a second FBG formed in an optical waveguide. The LPG broadly attenuates light in the vicinity of the Bragg reflection wavelength λ 2B  of the second FBG, and this attenuation profile shifts in wavelength in accordance with the dynamic event being monitored. Perturbation of the attenuation profile thus attenuates the intensity of the light reflected from the second FBG, i.e., I(λ B2 ), because such reflected light must pass (twice) through the LPG. Accordingly, continually monitoring I(λ B2 ) as a function of time allows the dynamic event to be recreated and processed accordingly. If necessary, I(λ B2 ) can be normalized by dividing it by the intensity of the Bragg reflection wavelength from the first FBG, I(λ B1 ), to discard attenuation within the system not related to the dynamic event being monitored.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
   U.S. patent application Ser. No. 10/452,124 filed Jun. 2, 2003, now U.S. Pat. No. 6,955,085, entitled “Optical Accelerometer or Displacement Device Using A Flexure System,” filed concurrently herewith, contains subject matter related to that disclosed herein, and is incorporated herein by reference in its entirety. 
   TECHNICAL FIELD 
   This invention relates to an optical sensor, and more specifically to an optical sensor for monitoring dynamic events and associated interrogation methods. 
   BACKGROUND ART 
   Optical sensors are well known in the art, and have utility in a number of different measurement applications. For example, and as shown in  FIG. 1 , a fiber Bragg grating  10  (FBG  10 ) formed in an optical fiber  12  or other optical waveguide can be used to measure pressure or temperature. A FBG, as is known, is a periodic or aperiodic variation in the effective refractive index of a core of an optical waveguide, similar to that described in U.S. Pat. Nos. 4,725,110 and 4,807,950 entitled “Method For Impressing Gratings Within Fiber Optics,” to Glenn et al. and U.S. Pat. No. 5,388,173, entitled “Method And Apparatus For Forming Aperiodic Gratings In Optical Fibers,” to Glenn, which are incorporated by reference in their entireties. 
   FBG  10 , when interrogated by broadband light from an optical source/detector  14 , will reflect a narrow band of this light (essentially a single wavelength), called the Bragg reflection wavelength, λ B , in accordance with the equation λ B ∝2n eff Λ, where n eff  denotes the index of refraction of the core of the waveguide, and A denotes the spacing of the variations in the refractive index of the core (i.e., the grating spacing). Because strain along the axis of an FBG affects its grating spacing Λ, and because temperature effects both the index of refraction n eff  and the grating spacing Λ (in the latter case, due to thermal expansion or contraction), FBG  10  can be used as either as pressure or temperature sensor by assessing the magnitude of the shift in its Bragg reflection wavelength. FBG  10  is usually partially transmissive so that a portion of the light at the Bragg reflection wavelength (and light of all other wavelengths that is not affected by the FBG  10 ) transmits through the FBG  10 , which allows further sensors along the optical fiber  12  (not shown) to be interrogated in a multiplexing approach to determine the pressures and/or temperatures present in those locations. 
   When interrogating the FBG  10 , the optical source/detector  14  can be operated in a continuous wave mode, where light is continuously fed to the FBG  10  and its reflections are continuously monitored, or the light can be pulsed. In a pulsed scheme, the frequency of the pulses needs to be sufficiently short to detect changes in the parameter being measured. For example, when measuring temperature in a given application, such as within an oil/gas well, it is noted that temperature does not change very rapidly, or at least it is usually not of interest to the well operator to detect such rapid changes if they occur. Accordingly, light pulses need to be sent from the optical source/detector  14  only occasionally, for example, every second, which provides an update of the temperature at the location of FBG  10  every second. 
   However, some parameters of interest to detect occur on much smaller time scale. For example, if the FBG  10  is used to measure a dynamic event, such as a pressure wave indicative of seismic activity occurring within the oil/gas well, sampling needs to take place more frequently. For example, a seismic pressure wave may contain frequency components as high as f=1000 Hz, and therefore would require interrogating the FBG  10  one the order of at least 2f times a second to properly resolve these higher order frequency components and to provide an accurate picture of the detected pressure wave. However, such high frequency rate pulsed sampling may not be possible in a practical application. For example, the FBG  10  will likely in an oil/gas application be wavelength-division or time-division multiplexed to other optical sensors such as flow rate meters, speed of sound meters, or other pressure or temperature sensors, and such meters or sensors may themselves contain FBGs which will produce reflections. (Examples of such other meters or sensors, and ways of multiplexing and interrogating them, are disclosed in the following U.S. patents or patent application, which are hereby incorporated by reference in their entireties: Ser. No. 09/740,760, filed Nov. 29, 2000; Ser. No. 09/726,059, filed Nov. 29, 2000; Ser. No. 10/115,727, filed Apr. 3, 2002; U.S. Pat. No. 6,354,147, issued Mar. 12, 2002). High rate sampling of FBG  10  could interfere with interrogation of the other optical sensors or meters multiplexed with FBG  10 , and/or confused the reflected signals, making it difficult to determine which reflections pertain to which meter or sensor. 
   As alluded to above, one solution to the problem of interrogating the FBG  10  to monitor dynamic events is to interrogate the FBG  10  with a continuous wave light source. Continuous wave interrogation produces a continuous reflection of Bragg wavelengths shifts from the FBG  10 , which can be monitored as a function of time. However, continually monitoring Bragg wavelength shifts is difficult in many applications, and requires detectors and signal processing schemes that are not always economical in practice. 
   Accordingly, there is room for improvement in the art of optical sensors. The art would benefit from a sensor design which can monitor dynamic events in real time, and which is interrogatable using methods that are easily implemented and reliable. 
   SUMMARY OF THE INVENTION 
   Disclosed herein is an optical sensor design and method for continually interrogating that sensor to produce an accurate representation of a dynamic event (such as a change in strain, pressure or temperature) being monitored by the sensor. The sensor design preferably constitutes continuous wave optical source/detection equipment coupled in series to a first fiber Bragg grating (FBG), a long period grating (LPG), and a second FBG formed in an optical waveguide. The LPG broadly attenuates light in the vicinity of the Bragg reflection wavelength λ 2B  of the second FBG, and this attenuation profile shifts in wavelength in accordance with the dynamic event being monitored. Perturbation of the attenuation profile thus attenuates the intensity of the light reflected from the second FBG, i.e., I(λ B2 ), because such reflected light must pass (twice) through the LPG. Accordingly, continually monitoring I(λ B2 ) as a function of time allows the dynamic event to be recreated and processed accordingly. If necessary, I(λ B2 ) can be normalized by dividing it by the intensity of the Bragg reflection wavelength from the first FBG, I(λ B1 ), to discard attenuation within the system not related to the dynamic event being monitored. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates a prior art system for monitoring a parameter using an FBG. 
       FIG. 2A  illustrates the disclosed interrogation system and sensor design which incorporates the use of a long period grating (LPG), and illustrates a dynamic event to be monitored by the system. 
       FIG. 2B  illustrates the reflection profiles of the FBGs which bind the LPG, and also shows the effect of attenuation through the LPG on the reflection profile from the second FBG. 
       FIG. 2C  illustrates the detector output which constitutes a recreation of the dynamic event being monitored, as normalized to subtract out system parasitic attenuation. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In  FIG. 2A , the parameter-measuring FBG  10  of  FIG. 1  has been replaced by a long period grating (LPG)  400  along the optical waveguide  12 . The dynamic event  440  being sensed effects the LPG  400 , which acts as the sensitive element as will be explained below. While capable of detecting different types of dynamic events  440 , such as temperature variations, this disclosure assumes for simplicity that the dynamic event  440  constitutes a dynamic pressure, such as a seismic pressure wave, which is the application for which the improved sensor and interrogation technique was primarily designed. The spacing Λ of the index of refraction modulation in an LPG  400  is greater than normally used in a narrow band Bragg reflector, ranging on an order of over 25 microns, e.g., about 100 microns, and stretching over a length L of approximately 2 cm. The LPG  400  provides coupling of light propagating in the waveguide to forward propagating cladding modes which are eventually lost due to absorption and scattering. The LPG  400  can be customized to couple light of specific wavelength bands into the cladding. 
   The LPG  400  is bounded by shorter reflective FBGs  410   a  and  410   b  having Bragg reflection wavelengths λ B1  and λ B2  of, for example, 1530 nm and 1550 nm respectively, and having grating spacings Λ of 0.51 and 0.52 microns respectively. Because these FBGs  410   a ,  410   b  are preferably not used in this embodiment as the pressure-sensitive elements, but rather are used merely to bind the pressure-sensitive LPG  400 , FBGs  410   a ,  410   b  are preferably isolated from the pressures being sensed. Moreover, they can be remotely located from LPG  400 , perhaps even by kilometers. Therefore, the FBGs  410   a ,  410   b  can be removed from the environment in which pressure sensing is taking place. For example, the FBGs  410   a ,  410   b  can be located near the optical source/detection equipment residing at the surface of an oil/gas well (not shown), while the LPG  400  is deployed in the well to take pressure measurements. Alternatively, the FBGs  410   a ,  410   b  can be deployed in the environment to be monitored, e.g., in the well, but isolated from the pressures or temperatures in that environment that might cause their Bragg reflection wavelengths to significantly shift. For example, the FBGs  410   a ,  410   b  could be sealed in appropriate pressure vessels, or covered with high pressure sheaths to prevent their deformation. In any event, it is not strictly necessary to isolate the FBGs  410   a ,  410   b  from the pressures being measured, and they can in some applications also be subject to the pressures being measured as will be explained below. While  FIG. 2A  shows the LPG  400  and the FBGs  410   a ,  410   b  as being formed along a common optical waveguide  12 , this is not strictly necessary, and instead these components could be coupled or spliced together. For simplicity, a grating is said to be “formed in” an optical waveguide even if it is spliced or coupled to a waveguide, and two gratings are said to be “formed in” a single optical waveguide even if they are located on two waveguides which are coupled or spliced together. 
   In a preferred embodiment, continuous wave broadband light from light source  420  enters an optical circulator  430 , which directs the light to the LPG  400  and FBGs  410   a ,  410   b . As shown in  FIG. 2B , the LPG  400  imparts an insertion loss  423  to a relatively broad spectrum of light that passes through it, and this insertion loss profile  423  preferably overlaps the Bragg reflection wavelength λ B2  of the second FBG  410   b , more preferably near the middle of one of the broadly sloped edges of the profile  423  as shown. The dynamic pressure  440  being detected changes the spacing of the index of refraction modulation for the LPG  400 , which causes every point in the transmitted spectral profile  423  to shift in wavelength, as shown at  424 . It is preferably to understand the exact shape of the insertion loss  423 , and how it responds to pressure ( 424 ) prior to its inclusion in the system, which can be determined by testing and/or computerized modeling. 
   While light reflected from the first FBG  410   a  at λ B1  is not attenuated by the LPG  400 , light reflected from the second grating  410   b  at λ B2  will be attenuated in its intensity over region  426 . (One skilled in the art will recognize that light at wavelength λ B2  is attenuated twice, because the incident light must pass to and from the second FBG  410   b , and thus will pass through the long period grating twice; this multiplicative effect on the attenuation in the reflected intensity from FBG  410   b  is not shown in  FIG. 2B  for simplicity). Because the dynamics of the insertion loss profile  423  and its response to pressure ( 424 ) are known, the attenuation or change of the intensity of light reflected from the second FBG  410   b , i.e., I(λ B2 ) can be correlated to the pressure presented to the LPG  400  at any given point in time. 
   This reflected light from the FBGs  410   a ,  410   b  then proceeds by way of circulator  430  to high frequency detectors  432  and  434 . Detector  432  detects light tuned to the Bragg reflection wavelength of the second FBG, λ B2  Light tuned to λ B1 , by contrast, is reflected by filter  425  and directed by circulator  430  to detector  434  where it is assessed. By comparing the intensity of this reflected signal I(λ B2 ) at detector  432  with the intensity of the signal reflected from the first Bragg grating I(λ B1 ) at detector  434 , the dynamic strain  440  imparted to the optical element  20  can be recreated in real time as shown in  FIG. 2C . Thereafter, the resulting signal can be assessed pursuant to well known signal analysis techniques; for example, the signal&#39;s frequency components can be assessed using a dynamic signal analyzer  450 , which is well known. 
   In this scheme, I(.lambda..sub.B 1 ) is used to normalize I(.lambda..sub.B 2 ), i.e., to remove attenuation losses in the system that are not due to dynamic pressure  440  impingent upon the LPG  400 . However, this is not strictly necessary, and accordingly FBG  410   a  can be dispensed with, with the variation in I(.lambda..sub.B 2 ) alone used to characterize the detected dynamic pressure. Dispensing with normalization in this fashion is particularly useful if the attenuation losses in the system are well known or characterized, or if the magnitude of the detected dynamic pressure  440  is not interesting to know with particularity. For example, in a seismology application, it may be desirable to know only the shape of the incident pressure wave, and hence its frequency components, rather than the magnitude of these components. 
   As noted earlier, this technique is beneficial in that it can operate with a continuous wave light source instead of by high rate pulsed sampling (although sampling can also be used), which allows detection of higher frequency components present in the dynamic strain  440 . The detectors  432  and  434  are accordingly preferably high frequency detectors capable of resolving the higher frequency components of interest in the dynamic pressure  440 . Either a broadband light source  420 , or at least a source containing frequency components tuned to the two FBGs  410   a ,  410   b , is suitable. One skilled in the art should note that separate detectors  432  and  434  need not be used, and that a single detector capable of sensing both FBG reflections can be used instead. Moreover, the detectors  432 ,  434 , source  420 , circulator  430 , and signal analyzer  450  can be coupled together, e.g., in a common optical source/detection unit (as in  14  of  FIG. 1 ), although they are shown separately in  FIG. 2A  to more easily understand their individual functions. 
   As noted earlier, the FBGs  410   a ,  410   b  are preferably isolated from the parameter (in this case, dynamic pressure  440 ) being sensed, although this is not strictly necessary. Should FBGs  410   a ,  410   b  be subject to dynamic pressure  440 , or other stresses in the environment being measured, such as temperature and pressure, the Bragg reflection wavelengths .lambda..sub.B 1  and .lambda..sub.B 2  will shift, but this is not deleterious and can be compensated for at the detectors  432 ,  434 , and filter  425 . For example, if it is known that the Bragg reflection wavelengths for each of the FBGs  410   a ,  410   b  can be expected to vary +/−5 nm in a given operational environment, the detectors  432 ,  434 , and filter  425  can be tuned accordingly to ensure that the detected (or filtered) signals correspond to FBGs  410   a ,  410   b . For example, the .lambda..sub.B 2  detector  432  can be designed to detect the intensity of reflections occur within a band from 1545 nm to 1555 nm. If the expected variation in the Bragg wavelength shift of these FBGs is potentially greater, their Bragg reflection wavelengths can be set a further distance apart (e.g., 1520 nm and 1560 nm) to ensure no overlap in detection of the bands of interest. In environments in which the FBGs are subject to stresses, it is particularly preferred to use normalizing FBG  410   a  to assist in subtraction of intensity-varying effects that are due to that environment, as opposed to the event in that environment being monitored. 
   The above-disclosed approach provides a simple way to recreate the detected dynamic pressure without the need for high rate pulsed sampling, and without the inconvenience of continuous wave spectral monitoring approaches used in the prior art. For example, and as discussed above, were a continuous wave source to interrogate the pressure-sensitive FBG  10  in  FIG. 1 , the optical source/detector  14  would need to determine the Bragg wavelength shift and track that shift as a function of time, a relatively demanding task. By contrast, using the disclosed sensor design incorporating the long period grating, the detector(s) need only measure intensity at one (or two) wavelengths (or at relatively narrow bands around those wavelengths). Intensity is easily determined by simply monitoring the detector current at those tuned wavelengths, and thus can be performed without the need to spectrally process the reflected signal. 
   Although FBGs  410   a ,  410   b  are preferred, it is not strictly necessary to use FBGs to bind the LPG  400 . Any device, such as a tuned reflector, capable of reflecting light at a given wavelength (i.e., λ B1  and λ B2 ) or in discrete bands can be used in lieu of these components. 
   The disclosed sensor structure and method for interrogating the reflections therefrom can benefit and improve a wide variety of optical sensors, and particularly those that are used to measure dynamic events. An example of a sensor benefited by the disclosed approach is disclosed in U.S. patent application Ser. No. 10/452,124, entitled “Optical Accelerometer or Displacement Device Using A Flexure System,” which is concurrently filed herewith and which is incorporated herein by reference in its entirety. Other examples of sensors in which the disclosed technique can be employed include static strain or temperature sensors, electrical current sensors, chemical analysis sensors, vibration sensors, liquid level sensors, etc. The LPG can also be made sensitive to the external index of refraction which will allow its use for chemical and presence of liquids. 
   As disclosed in the above-referenced application U.S. patent application Ser. No. 10/452,124, the LPG  400  can be placed in the narrowed portion of a relatively large diameter “cane” waveguide, with the FBGs  410   a ,  410   b , being placed at larger diameter portions of the cane waveguide, in a so-called “dog bone” structure. Alternatively, the LPG  400  (and or the FBGs  410   a ,  410   b ) can all be placed in a large diameter cane based waveguide, without utilizing a narrowed portion. Further information concerning cane based waveguides can be found in U.S. patent application Ser. No. 10/371,910, filed Feb. 21, 2003, which is incorporated herein by reference. 
   If desirable, further loss can be imparted to the waveguide over and beyond that provided by the LPG  400 . For example, the LPG could be replaced by notches in the waveguide, or air gaps, which would generally act to broadly attenuate light passing therethrough. Other techniques for purposefully imparting loss to the LPG, or to optical waveguide more generally, could also be used. 
   As used herein, “fiber Bragg grating” or “FBG” do not necessary imply that the grating is contained within a fiber, i.e., a standard communications optical fiber. Any suitable grating for simplicity, and consistent with common nomenclature, is referred to herein as an “fiber Bragg grating” or “FBG” even if it is contained within larger diameter waveguides (e.g., cane-based waveguides) or other optical waveguides which are not optical fibers, such as those disclosed herein and preferably used in connection with the optical sensing element  20 . 
   “Long period grating” of “LPG” should not be understood to encompass gratings having traditional grating spacings A for reflecting light near the visible portion of the electromagnetic spectrum, e.g., from 400 to 800 nm. Instead, a “long period grating” or “LPG” should be understood as having grating spacings approximately at least 100 times larger than such typical grating spacing values. 
   “Coupled” as used in this disclosure should not necessarily be interpreted to require direct contact. Thus, two elements can be said to be “coupled” from a functional standpoint even if an intermediary element intervenes between them. 
   “Light” as used herein does not necessarily constitute visible light, but instead for simplicity constitutes any portion of the electromagnetic spectrum useable to interrogate the disclosed sensors. 
   Although the disclosed sensors are described as being interrogated by assessing reflection therefrom, those of skill in the art will recognize that assessing transmission of light through the sensors is equally feasible. 
   Although designed as particularly useful for measuring seismic activity in oil/gas well applications, the disclosed sensor and techniques can be used to sense dynamic and constant forces in any number of applications, including other industrial sensing applications.