Abstract:
In accordance with one exemplary embodiment the invention provides a multi-parameter fiber optic sensing system with an aperiodic sapphire fiber grating as sensing element for simultaneous temperature, strain, NO x , CO, O 2  and H 2  gas detection. The exemplary sensing system includes an aperiodic fiber grating with an alternative refractive index modulation for such multi-function sensing and determination. Fabrication of such quasiperiodic grating structures can be made with point-by-point UV laser inscribing, diamond saw micromachining, and phase mask-based coating and chemical etching methods. In the exemplary embodiment, simultaneous detections on multi-parameter can be distributed, but not limited, in gas/steam turbine exhaust, in combustion and compressor, and in coal fired boilers etc. Advantageously, the mapping of multiple parameters such as temperature, strain, and gas using sapphire aperiodic gratings improves control and optimization of such systems directed to improve efficiency and output and reduce emissions.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is a division of U.S. patent application Ser. No. 11/086,055 entitled “FIBER OPTIC SENSING DEVICE AND METHOD OF MAKING AND OPERATING THE SAME” filed Mar. 22, 2005, now U.S. Pat. No. 7,421,162 which is herein incorporated by reference for all purposes. 

   BACKGROUND 
   The present invention relates generally to fiber optic sensing devices, and more particularly, to a fiber optic sensing device for detecting multiple parameters in an environment or element, for example. Indeed, the present invention provides advantages related to the use of fiber optic sensing devices in harsh environments, for instance. 
   Various sensing devices are known and are generally in use. For example, thermocouples are used for measuring the temperature in components of a device, such as exhaust systems, combustors, compressors and so forth. Yet other sensing systems are employed to detect physical parameters such as, strain or temperature in an infrastructure. As one example, Bragg grating sensors are often employed. However, such conventional sensing devices are limited by the operational conditions in which they may be employed. For example, conventional sensing devices are often limited to relatively mild temperature conditions and, as such, limited operational temperature ranges. Indeed, conventional devices are limited to temperatures between +80° C. to +250° C., depending upon the fiber grating coating materials. 
   As such, it is difficult to measure temperatures for components in high-temperature environments like turbines and engines. Further, for large components, a relatively large number of discrete thermocouples may be required to map the temperatures. Such discrete thermocouples may not be scalable to meet a desired spatial resolution that is generally beneficial for accurate thermal mapping of system components, which can then used to control and optimize the operation of such systems with the objectives of improving efficiency and output. A more accurate and improved spatial resolution thermal mapping is necessary to control such systems (gas turbines, steam turbines, coal-fired boilers, etc.) with more accuracy and fidelity to meet requirements such as better efficiency and output. The sensing devices for gas components such as NOx, CO and O2 also have a similar limitation in terms of accuracy and spatial resolution. A more accurate and spatially dense gas sensing would facilitate more effective and efficient emissions control for gas turbines and coal-fired boilers. 
   Accordingly, conventional sensing devices present limitations when employed in high temperature and/or harsh environments such as, gas/steam turbine exhausts, coal-fired boilers, aircraft engines, downhole applications and so forth. For example, conventional Bragg grating sensors employ a doped or chemical grating that breaks down in high temperature settings (e.g., a gas turbine exhaust that may reach temperatures of 600° C. or higher). 
   Certain other conventional systems employ Bragg grating sensors for measuring and monitoring a parameter in an environment. Such sensors utilize a wavelength encoding within a core of the sensor to measure a parameter based upon a Bragg wavelength shift that is generated on illumination of the grating through an illumination source. Thus, environmental effects on the periodicity of the grating alters the wavelength of light reflected, thereby providing an indication of the environmental or elemental effect, such as, temperature or strain, for example. However, it is difficult to simultaneously detect multiple parameters, such as temperature and gas, through a single conventional Bragg grating sensing element. Further, multiple spectral signals at different wavelengths may be required to separate the effect of multiple sensed parameters from one another. Such separation of sensed parameters is conventionally a difficult and time-consuming process. 
   In certain conventional sensor systems, an additional grating element encapsulated in a different material is placed in series with an existing grating element for separating the effects of two different parameters, such as temperature and strain. Moreover, such systems require overwriting gratings at the same fiber location, which often present difficulties during the manufacturing the fiber grating for the sensor. In summary, conventional Bragg grating sensors do not facilitate discernment of what environmental or elemental factor influenced the sensor, rather only the physical changes in the sensor itself are readily detectable. 
   Therefore, there is a need for improved sensing devices. 
   BRIEF DESCRIPTION 
   In accordance with one exemplary embodiment, the present technique provides a fiber optic grating sensor cable. Each exemplary fiber grating includes a core having a first index of refraction and a plurality of grating elements each having an index of refraction different from the first index of refraction. The core includes a first pair of grating elements configured to reflect a first wavelength of light in phase and a second pair of grating elements configured to reflect a second wavelength of light in phase. The core also includes a third pair of grating elements configured to reflect the first wavelength of light in phase, wherein at least one grating element of the second pair of grating elements is located between at least one grating element of the first pair and at least one grating element of the third pair. The fiber optic sensor cable also includes a cladding disposed circumferentially about the core. 
   In accordance with yet another exemplary embodiment, the present technique provides a method of detecting a plurality of parameters. The method includes providing a source of light to a fiber optic sensor cable having a plurality of grating elements and comprising first, second and third portions, wherein adjacent gratings in the first and third portions are at a first distance from one another and adjacent gratings in the second portion are at a second distance from one another, and wherein the second portion is located between the first and third portions. The method also includes detecting light emitted from the fiber optic sensor cable. 
   In accordance with another exemplary embodiment, the present technique provides a distributed sensor system for sensing multiple parameters in a harsh environment. The sensor system includes a plurality of sensors disposed on a distributed fiber optic grating sensor cable, wherein each of the plurality of sensor comprises a core having a first index of refraction and a plurality of mechanically altered portions each having an index of refraction different than the first index of refraction. 

   
     DRAWINGS 
     These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
       FIG. 1  is a diagrammatical representation of a fiber optic sensing system for detecting multiple parameters of an environment and/or element, in accordance with an exemplary embodiment of the present technique; 
       FIG. 2  is a diagrammatical representation of a fiber optic sensor array cable having aperiodic spaced grating structures with the refractive index modulated by a periodic or an aperiodic sequence, in accordance with an exemplary embodiment of the present technique; 
       FIG. 3  is a diagrammatical representation of the core of a fiber optic sensor cable including aperiodic grating structures; 
       FIG. 4  is a diagrammatical representation of waveforms of light generated by aperiodic grating structures of the fiber optic sensor cable of  FIG. 3 , in accordance with an exemplary embodiment of the present technique; 
       FIG. 5  is a diagrammatical representation of a Bragg grating fiber optic cable of  FIG. 3  during normal conditions, in accordance with an exemplary embodiment of the present technique; 
       FIG. 6  is a diagrammatical representation of a Bragg grating fiber optic cable of  FIG. 3  during stressed conditions, in accordance with an exemplary embodiment of the present technique; 
       FIG. 7  is a flow chart of a process for manufacturing the fiber optic sensor cable of  FIGS. 1-3 , in accordance with an exemplary embodiment of the present technique; 
       FIG. 8  is a diagrammatical representation of a distributed fiber sensor system, in accordance with an exemplary embodiment of the present technique; 
       FIG. 9  is a diagrammatical representation of a fiber optic sensor cable with a micromachined aperiodic grating structure, in accordance with an exemplary embodiment of the present technique; 
       FIG. 10  is a diagrammatical representation of a system for inscribing the fiber grating structures of  FIG. 3 , in accordance with an exemplary embodiment of the present technique; 
       FIG. 11  is a diagrammatical representation of an application having the distributed fiber sensing system of  FIG. 8  in accordance with an exemplary embodiment of the present technique; and 
       FIG. 12  is a diagrammatical representation of another application having the distributed fiber sensing system of  FIG. 8  in accordance with an exemplary embodiment of the present technique. 
   

   DETAILED DESCRIPTION 
   Referring now to drawings,  FIG. 1  illustrates an exemplary fiber optic sensing system  10  for detecting parameters of an environment and/or object  12 . Although the present discussion focuses on sensing devices and systems, the present technique is not limited to sensing field, but is also applicable to other modalities, such as, optical filters, data transmission, and telecommunications, among others. Accordingly, the appended claims should not be limited to or by the exemplary embodiments of the following discussion. The fiber optic sensing system  10  includes a fiber optic sensing device  14  that, in turn, includes a grated cable  16 . As illustrated, the cable  16  is disposed within the element  12 , causing changes in the element  12  to translate to the cable  16 . The grated cable  16  includes a core that has a plurality of grating elements arranged in an aperiodic pattern, which is described in detail below. In the present discussion, a grating element refers to a variance in the index of refraction in comparison to the index of refraction of the core. Such grating elements may be a result of a micromachining process, such as diamond saw cutting, or a chemical process, such as doping, and both processes are discussed further below. 
   Further, the fiber optic sensing system  10  includes a light source  18  that is configured to illuminate the core of the grated cable  16 . This illumination facilitates the generation of reflected signals corresponding to a grating period of the grated cable  16 . The system  10  also includes an optical coupler  20  to manage incoming light from the light source  18  as well as the reflected signals from the grated cable  16 . Indeed, the coupler  20  directs the appropriate reflected signals to a detector system  22 . 
   The detector system  22  receives the reflected optical signals from the grated cable  16  and, in cooperation with various hardware and software components, analyzes the embedded information within the optical signals. For example, the detector system  22  is configured to estimate a condition or a parameter of the object  12  based upon a diffraction peak generated from the plurality of grating elements of the grated cable  16  of the fiber optic sensing device  14 . In certain embodiments, the detector system  22  employs an optical coupler or an optical spectral analyzer to analyze signals from the fiber optic sensing device  14 . Depending on a desired application, the detector system  22  may be configured to measure various parameters in the environment  12 . Examples of such parameters include temperatures, presence of gases, strains and pressures, among others. 
   Advantageously, as discussed further below, the exemplary cable  16  generates multiple strong diffraction peaks, thereby facilitating the segregation of the various influencing parameters on the cable  16 . The information developed by the detector system  22  may be communicated to an output  24  such as, a display or a wireless communication device. Advantageously, gleaned information, such as environmental or object conditions, may be employed to address any number of concerns or to effectuate changes in the environment or object  12  itself. 
     FIG. 2  illustrates an exemplary fiber optic sensor array cable  26  having an aperiodic grated refractive index modulation, in accordance with an embodiment of the present technique. The fiber optic sensor cable  26  includes a core  28  and a cladding  30  that is disposed circumferentially about the core  28 . A portion of the cladding  30  has been removed to better illustrate the underlying core  28 . The core  28  includes a series of grated elements  32  that are configured to reflect in phase wavelengths of light corresponding to a grating period of the grated elements  32 . As illustrated, distances between adjacent gratings are arranged in an aperiodic pattern that will be described in detail below with reference to  FIG. 3 . During operation, an input broadband light signal  34  is provided to the fiber optic sensor cable  26  by the light source  18  and a portion of the input broadband light signal  34  is reflected by a respective grating element  32  in phase and corresponding to certain wavelengths of light, while remaining wavelengths are transmitted as represented by a transmitted signal  36 . 
   Referring now to  FIG. 3 , a grated portion of a core of a fiber optic sensor cable  38  is illustrated. As illustrated, the fiber optic sensor cable  38  is formed of a core  40  and cladding  30  that is disposed about the core  40 . The cladding  30  provides for near total internal reflection of light within the cable  38 , thereby allowing light to be transmitted by and axially through the cable  38 . The cable  38  also includes a plurality of grating elements represented generally by reference numeral  42 . In one embodiment, the core  40  comprises a fused silica fiber. In another embodiment, the core  40  comprises a sapphire fiber. The plurality of grating elements  42  has an index of refraction different from that of core  40 . In this embodiment, the index of refraction of the grating elements  42  is lower than that of the core  40 . By way of example, the core  40  may have an index of refraction of 1.48, while the grating element  42  may have an index of refraction of 1.47, for instance. As discussed below, the index of refraction of the various grating elements  42 , and the distances between these grating elements  42  defines the wavelength of light reflected in phase by the grating elements  42 . 
   The exemplary portion of the core  40  shown in  FIG. 3  includes various portions that effect how light is transmitted through the fiber optic sensor cable  38 . In the illustrated embodiment, the core  40  includes a first portion  44  where the distance between adjacent grating elements  42  is at a first distance  46 . This first distance  46  defines a first wavelength of light that will be reflected in phase by the pair of grating elements  42  in the first portion  44 . By way of example, the distance between the first pair of grating elements  42  is generally of the same order of magnitude as that of the wavelength of the reflected light, e.g., 0.775 μm, for instance. The core also includes a second portion  48  where the distance between adjacent grating elements  42  is at a second distance  50 . This second distance  50 , which is different than the first distance  46  defines a second different wavelength of light that will be reflected in phase by the pair of second grating elements  42  in the second portion  48 . In addition, the core  40  includes a third portion  52  where the distance between grating elements  42  is at the first distance  46 . Thus, the grating elements  42  in the third portion  52  reflect the first wavelength of light in phase, like the first portion  44 . In this embodiment, the second portion  48  is disposed between the first and third portions  44  and  52 . 
   As can be seen, the distances between adjacent gratings  42  have an aperiodic pattern. That is, the distances between adjacent gratings  42  along a longitudinal axis of the core  40  alternate between the first and second distance  46  and  50 . It is worth noting that the present oscillation between the first and second distances  46  and  50  to establish an aperiodic pattern is merely but one example. Indeed, a number of aperiodic patterns may be envisaged. In the illustrated embodiment, the indices of refraction of the core  40  and the gratings  42  are modulated according to Fibonacci sequence. The indices of refraction of the core  40  and the gratings  42  are modulated such that there is a relatively higher refractive index modulation with aperiodic sequence in the core  40  as compared to the refractive index modulation circumferentially surrounding the cladding  30 . Further, the grating structure of the fiber core  40  may be defined by an aperiodic sequence of blocks n a  and n b  and a constant τ. By way of example, n a  is index of refraction of 1.49 and n b  is an index of refraction of 1.45. 
   In this embodiment, the sequence for the refractive index modulation is based upon the following equation:
 
 S   3   ={S   2   ,S   1   }, . . . S   n   ={S   j-1   ,S   j-2 } for  j≧ 2  (1)
 
where S 1 =n a  corresponding to core region having the first effective index of refraction; and
 
S2=n a n b  corresponding to grating elements having an index of refraction different than the first index of refraction.
 
Thus, the diffraction spectrum generated by the above defined grating structure will include a first Bragg diffraction peak that corresponds to the first wavelength of light in phase and a second Bragg diffraction peak that corresponds to the second wavelength of reflected light in phase and a plurality of diffraction peaks that are determined by a modulation periodicity and a diffraction wave vector. In this exemplary embodiment, the modulation periodicity is based upon the following equation:
 
Λ= d ( n   A )+τ d ( n   B )  (2)
 
where d(n A ) and d(n B ) are fiber lengths of the refractive index changed and unchanged areas respectively with τ being the golden mean with a value of 1.618.
 
Further, the diffraction wave vector is determined by two indices (n,m):
 
 k ( n,m )=( m+τn )/Λ  (3)
 
where Λ is a quasiperiodicity of the aperiodic grating structure and n, m are discrete wave numbers.
 
In the illustrated exemplary embodiment, the diffraction of light may occur when the discrete wave numbers satisfy the range n,m=0, ±1, ±2 . . . .
 
Further, the Bragg diffraction wavelength having relatively high intensity is given by:
 
               λ   B     ⁡     (     n   ,   m     )       =       2   ⁢     n   eff     ⁢   Λ       (     m   +     n   ⁢           ⁢   τ       )             
Advantageously, a plurality of generated diffraction peaks facilitate simultaneous multiple parameters measurements. Examples of such parameters include temperature, strain, pressure and gas.
 
   The illustrated aperiodic pattern of the gratings  42  of the fiber optic sensing device  38  enables the fiber optic sensing device  38  to generate a plurality of diffraction peaks simultaneously from emitted light from the core  40 . In this exemplary embodiment, the plurality of diffraction peaks is representative of a plurality of sensed parameters such as, temperature, strain and so forth. The grated cable of the fiber optic sensing device  38  is configured to generate first and second diffraction peaks that contain embedded information representative of first and second sensed parameters. Such first and second diffraction peaks are then detected by the detector system  22  (see  FIG. 1 ) for estimating the first and second sensed parameters. Advantageously, the grated cable allows the fiber optic sensing device  38  to generate the first and second diffraction peaks to appear in fiber low-loss transmission windows and also with substantially comparable efficiencies. The first and second diffraction peaks may be employed for simultaneously measuring the first and second sensed parameters such as temperature and strain. These first and second diffraction peaks corresponding to first and second sensed parameters are described below with reference to  FIG. 4 . 
     FIG. 4  illustrates exemplary waveforms  54  of light generated by the aperiodic grated cable of the fiber optic sensing device of  FIG. 3 . The abscissa axis  58  of the waveforms  54  represents a wavelength of the light signal and the ordinate axis  60  of the waveforms  54  represents an intensity of the light signal. In the illustrated embodiment, an input broadband light signal is represented by a waveform  56  and a reflected signal from the grated cable is represented by reference numeral  62 . As can be seen, the reflected signal  62  from the core of the grated cable includes first and second diffraction peaks  64  and  66  that may be processed by the detector system  22  (see  FIG. 1 ) to estimate the first and second sensed parameters. Further, the transmitted signal is represented by a reference numeral  68  that transmits wavelengths that are not corresponding to the grating period of the grated cable  32 . Thus, the aperiodic grated structure facilitates the generation of strong first and second diffraction peaks  64  and  66  with comparable diffraction efficiencies that can be detected by a single detector. These detected diffraction peaks  64  and  66  may then be processed to detect multiple parameters of the environment or object  12  (see  FIG. 1 ). Advantageously, these diffraction peaks  64  and  66  can be maintained over relatively long lengths of cable without signal deterioration due to losses. 
   Referring now to  FIG. 5 , the Bragg grating fiber optic sensor cable  70  of  FIG. 3  during normal conditions is illustrated. In the illustrated embodiment, the distance between a first pair of grating elements  42  is at a first distance  72  and the distance between a second pair of grating elements  42  is at a second distance  74 . As described earlier, the illustrated aperiodic pattern of the distance between adjacent grating elements enables generation of two diffraction peaks from the fiber optic sensor cable  70  that are representative of two sensed parameters. In operation, when the fiber optic sensing device  70  is subjected to a stress for example, a temperature, or a strain, the distance between the adjacent grating elements changes in response to the applied stress as can be seen in  FIG. 6 . 
     FIG. 6  illustrates an exemplary Bragg grating fiber optic sensor cable  76  of  FIG. 3  during stressed conditions. In this embodiment, the length of the fiber optic sensor cable  76  changes in response to an environmental condition, such as an applied stress and temperature. Therefore, the distance between the first pair of elements  42  changes from the first distance  72  (shown in  FIG. 5 ) to a new distance  78 , and this new distance  78  may be greater or lesser than the original distance  72 . Similarly, the distance between the second pair of elements  42  changes from the second distance  74  (shown in  FIG. 5 ) to a new distance  80 . Again, this new distance  80  may be greater or lower than the second distance  74  depending on the influence of the environmental factors on the cable  76 . On illumination of the fiber optic sensing device  76  through an illumination source, diffraction peaks are generated from the light emitted from the fiber optic sensing device  76 . These diffraction peaks correlate to a change in length of fiber optic sensing device such as represented by the change in first and second distances  78  and  80  to estimate parameters corresponding to the diffraction peaks. 
   Similarly, in a gaseous environment, gases may interact with the fiber cladding, causing a change in the index of refraction resulting in cladding modes wavelength shifts that may be detected by the fiber optic sensing device  76  to simultaneously distinguish the temperature and gas effects. In this embodiment, the gases in the environment are detected from the optical properties variation of a sensing film that is coated on the grating elements of the fiber optic sensing device  76 . The absorption and adsorption properties of a gas varies the cladding absorption properties and thereby the index of refraction. Thus, the reflectance and transmittance spectra associated with the light through the grating of the sensing device  76  enables the separation of the environmental effects of the temperature and strain from gas sensing. That is to say, the effects of the environment change the wavelengths of light reflected in phase by the cable  76 . Further, by comparing this change with the diffraction peaks of the cable in its quiescent state, the magnitude of the environment effects can be determined. 
   The fiber optic sensing device of  FIGS. 1-3  may be manufactured by an exemplary process as represented by reference numeral  82  in  FIG. 7 . The process  82  begins at step  84  where a core is provided. In this embodiment, the core has a first index of refraction. At step  86 , a first pair of grating elements is provided wherein the distance between adjacent grating elements of the first pair is at a first distance. In the illustrated embodiment, the first pair of grating elements is configured to reflect a first wavelength of light in phase. At step  88 , a second pair of grating elements is provided that is configured to reflect a second wavelength of light in phase. The distance between adjacent grating elements of the second pair is at a second distance. In this embodiment, the second distance is different than the first distance. As represented by step  90 , a third pair of grating elements that is configured to reflect the first wavelength of light in phase is provided. In this embodiment, at least one grating element of the second pair of grating elements is located between at least one grating element of the first pair and at least one grating element of the third pair. As illustrated by step  92 , a cladding is disposed circumferentially about the core. In one embodiment, the first, second and third pair of grating elements are provided through an ultraviolet light exposure laser inscribing technique. In another embodiment, the first, second and third pair of grating elements are provided by disposing an optical coating on the core and subsequently selectively removing portions from the core along a longitudinal axis of the core. In certain embodiments, the optical coating on the core may be etched through a slit pattern mask to provide the first, second and third pair of grating elements. 
   The fiber optic sensing device manufactured by exemplary process of  FIG. 7  may be employed for sensing parameters in a distributed environment.  FIG. 8  illustrates an exemplary distributed fiber sensor system  100  for sensing parameters over a relatively large environment  102 . In the illustrated embodiment, the sensor system  100  includes a plurality of sensors  104  disposed on a distributed cable  106 . Further, each of the plurality of sensors  104  includes a grated cable  108 . In certain embodiments, the grated cable  108  comprises a plurality of grating elements arranged in a periodic pattern. In certain other embodiments, the grating cable  108  comprises a plurality of grating elements arranged in an aperiodic pattern as described above. In the distributed sensor system  100 , data regarding different locations of the environment can be obtained by evaluating the changes in the diffraction peaks reflected by the various sensors  104 . 
   In operation, the distributed fiber sensor system  100  may be placed in the environment  102  for detecting parameters of environment such as temperature, strain and so forth. The distributed fiber sensor system  100  is illuminated by a light source  110  as represented by the reference numeral  112  and respective reflective and transmitive signals  114  and  116  are then received by an optical spectral analyzer  118 . A coupler  20  may be coupled to the light source  110  and to the optical spectral analyzer  118  to combine the input and the reflected signal. The optical spectral analyzer  118  measures the wavelength spectrum and intensity of the received signals to estimate a parameter of the environment  102 . Finally, the detected signals representative of the sensed parameters are transmitted to a data acquisition and processing circuitry  120 . 
   Referring now to  FIG. 9 , a fiber optic sensor cable  122  with a micro machined or mechanical structure is illustrated. The fiber optic sensor cable  122  includes a core  124  and a plurality of mechanically altered portions  126 . Additionally, the fiber optic sensor cable  122  includes a cladding  128  disposed about the core  124  and the mechanically altered portions  126 . In the illustrated embodiment, each of the mechanically altered portions comprises portions with diameter different than the diameter of the core. In certain embodiments, a micromachining process, such as diamond saw cutting process, may be employed for selectively removing portions of the core  124  to form the mechanically altered portions  126  by altering the diameter of the core  124 . This micromachining process enables to create areas of refractive index that is different than the index of refraction of the core  124  and this variance in index of refraction functions as grating elements for the fiber optic sensor cable  122 . 
   In certain exemplary embodiments, the distance between adjacent grating elements, such as represented by reference numeral  130 , may vary along a longitudinal axis of the core  124  to form an aperiodic grating structure (as shown in  FIG. 9 ). As described above with reference to  FIG. 3 , the aperiodic grating structure may be defined by an aperiodic sequence of blocks n a  and n b  and a constant τ and the sequence is based upon the following equation:
 
 S   3   ={S   2   ,S   1   }, . . . S   n   ={S   j-1   ,S   j-2 } for  j≧ 2  (1)
 
where S 1 =n a  that corresponds to core region having the first index of refraction; and
 
S2=n a n b  that corresponds to grating elements having an index of refraction different than the first index of refraction.
 
   It should be noted that, the mechanically altered portions  126  of the grating structure manufactured by the micromachining processes enable the fiber optic sensor cable  122  to be employed in harsh environments such as a gas turbine exhaust, a steam turbine exhaust, a coal-fired boiler, an aircraft engine, a down hole application and so forth where the temperatures reach 600° C. and above, for instance. 
   The aperiodic grating structure described above may also be formed by employing a point-by-point laser inscribing technique. Referring now to  FIG. 10 , a system  132  for inscribing Bragg grating is illustrated. The system  132  includes a phase mask  134  disposed on a fiber core  136 . The system  132  also includes a UV laser  138  to generate a beam that is directed towards the phase mask  134 . Further, the beam generated from the UV laser  138  may be focused with a plane cylindrical lens  140  towards the fiber core  136 . In this embodiment, the phase mask  134  is employed to spatially modulate and diffract the UV beam from the UV laser  138  to form an interference pattern. The interference pattern induces a refractive index modulation that creates a Bragg grating structure  142  in the fiber core  136 . The system  132  also includes a broadband light source  144  and an optical spectrum analyzer  146  for detecting the Bragg wavelength of the light wave that is received by the optical spectrum analyzer  146  from the fiber core  136 . 
   The various aspects of the technique described above may be used for sensing multiple parameters such as, temperature, strain, pressure and fossil fuel gas in a variety of environments. In certain embodiments, the technique is employed for detecting parameters in a harsh environment such as those subjected to high temperatures. For example, the technique may be used for providing a thermal mapping in an exhaust system by measuring the temperature at multiple grating locations that are dispersed circumferentially about the exhaust system, or a combustor, or an output stage of a compressor of a jet engine.  FIG. 11  illustrates an exemplary application  150  having the distributed fiber sensor system of  FIG. 8 . 
   Referring now to  FIG. 11  the exemplary application  150  includes a gas turbine  152  having various components, such as a compressor  154 , a multi-chamber combustor  156  and a turbine  158  disposed about a shaft  160 . As illustrated, a distributed fiber sensor system  162  is coupled to the turbine  158  for providing a spatially dense exhaust temperature measurement  164  from the turbine  158 . The temperature measurements  164  obtained by the distributed fiber sensor system  162  may be utilized for a substantially accurate control of the performance of the gas turbine  152 . In the illustrated embodiment, the temperature measurements  164  may be utilized for determining combustor firing temperatures  166  through a model-based estimation technique or an empirical method  168 . The model based estimation technique  168  utilizes an inverse of a physics based model that maps the combustor firing temperatures  166  to the exhaust temperatures  164 . Alternatively, the empirical method  168  may utilize unload data of the turbine  158  to facilitate mapping of the exhaust temperatures  164  to the individual combustor chambers of the multi-chamber combustor  156 . Advantageously, the spatially denser and more accurate exhaust temperature profile facilitated by the present exemplary embodiment increases the accuracy and the fidelity of the model based estimation and the empirical method. 
   The estimated combustor firing temperatures  166  may be utilized for adjusting fuel distribution tuning  170  to ensure that all chambers of the multi-chamber combustor  156  are firing uniformly and have dynamic pressures and emissions that are within pre-determined thresholds. Advantageously, the technique provides a substantial reduction in combustion chamber-to-chamber variation for emissions and firing temperatures  166 . 
     FIG. 12  illustrates another exemplary application  172  having the distributed fiber sensor system of  FIG. 8 . As illustrated, the application  172  includes a coal fired boiler  174  for providing steam to steam turbines for power generation. The coal fired boiler  174  includes a distributed fiber sensor system  176  for measuring parameters such as temperatures, exhaust gases and so forth. In this embodiment, the measurement of parameters via the distributed fiber sensor system  176  may be employed to model relationship between inputs such as air and fuel flows to a burner  178  of the coal fired boiler  174  and exhaust parameters  180  from the boiler  174 . Further, a model  182  may be employed to correlate the burner air and fuel flow to the exhaust parameter measurements  180  and to tune the air and fuel flows  184  via a model based optimization method  186 . Advantageously, the exemplary sensor can provide more accurate and dense temperature profiles, CO and O 2  measurements in the exit plane of the boiler  174 . Thus, air and fuel flows  184  may be adjusted to reduce exit temperatures, gas emissions such as CO while meeting desired output requirements. 
   Similarly, the technique may be used for mapping a temperature distribution in components such as gas turbines, steam turbines, distillation columns and so forth. The fiber optic sensing device as described above may be employed to estimate multiple parameters in an aircraft engine such as, a compressor blade tip clearance, compressor exit temperature, combustor temperature and for detecting exhaust tail pipe fire. For example, in detecting an exhaust tail pipe fire, the exemplary sensor described above can differentiate between abnormally high temperatures from normal elevated temperatures in locations that are not observable by the crew but are accessible by the sensor. Further, the fiber optic sensing device may be also employed for monitoring downhole sensing in an oil and gas drilling rig. The fiber optic sensing device may be employed in various other applications such as a narrow band reflector, a broadband mirror, a wavelength division multiplexing (WDM) filter, a differential photonic sensor and so forth. 
   Advantageously, in accordance with embodiments of the present technique, mapping temperature and fossil fuel gas species helps improve turbine and engine power production efficiency, thereby saving energy. For example, the exemplary periodic and aperiodic sapphire fiber-grating sensor array will simultaneously distinguish the temperature and gas by monitoring cladding modes wavelength shifts. The need for temperature sensing in industrial sensing and control applications has to simultaneously measure emissions (NOX, CO, O2, H2, etc.) of coal-fired boilers, gas turbines or steam turbine. The combined sensing helps optimize the turbine energy usage efficiency. The speed, accuracy and the spatial resolution of available extractive (offline) or non-extractive (online) gas sensing systems available today are limited. There is a need to improve speed of response, accuracy, and spatial resolution of such sensing systems so as to enable for instance better and possibly active closed loop emissions control for power generation equipment. For that matter, any control/optimization application that needs accurate and/or spatially denser gas sensing is a good candidate application for the multi-parameter sensors described herein. 
   While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.