Abstract:
A proof mass is suspended in a cavity in a housing. The proof mass moves along a sensing axis in response to linear acceleration. Elastic support members are connected between the proof mass and the housing and are arranged to exert a reaction force on the proof mass in response to displacement of the proof mass along the sensing axis. An optical fiber is connected between the proof mass and opposite sidewall portions of the housing such that displacement of the proof mass along the sensing axis elongates a first portion of the optical fiber and shortens another portion. An optical signal source provides a broadband optical signal input to the optical fiber. A fiber optic Bragg grating is formed in the optical fiber and arranged to reflect a portion of the optical signal. Acceleration of the proof mass modulates the wavelength of the reflected optical signal.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This invention relates generally to techniques for measuring acceleration and particularly to a fiber optic device for measuring linear acceleration.  
         [0002]     Previous attempts to provide a fiber optic device that is sensitive to linear acceleration have involved microoptic techniques for fabricating individual components. Such techniques are labor intensive and therefore expensive.  
       SUMMARY OF THE INVENTION  
       [0003]     The present invention privides a highly accurate fiber optic acceleration sensor that is inexpensive to manufacture using holographic techniques.  
         [0004]     An acceleration sensor, according to the present invention comprises a housing having a cavity therein with a proof mass suspended within the cavity. The proof mass is arranged to move along a sensing axis in response to linear acceleration along the sensing axis. A plurality of elastic support members is connected between the proof mass and the housing. The elastic support members are arranged to exert a reaction force on the proof mass in response to displacement of the proof mass along the sensing axis. An optical fiber has a first portion connected between a first side of the proof mass and a first sidewall portion of the housing and a second portion connected between a second side of the proof mass and a second sidewall portion of the housing such that displacement of the proof mass along the sensing axis elongates one of the first and second portions of the optical fiber and shortens the other. An optical signal source is arranged to provide a broadband optical signal input to the optical fiber. A fiber optic Bragg grating is formed in the optical fiber and arranged to reflect a portion of the optical signal. The reflected portion has a wavelength that is modulated by acceleration of the proof mass along the sensing axis. The reflected signal may be processed to determine the acceleration of the proof mass.  
         [0005]     The acceleration sensor according to the present invention may further comprise a first fiber optic Bragg grating formed in the first portion of the optical fiber; and a second fiber optic Bragg grating formed in the second portion of the optical fiber, the first and second fiber optic Bragg gratings being arranged such that they reflect different wavelengths Λ 1  and Λ 2 , respectively, to produce a wavelength difference Λ 1 −Λ 2  that may be processed to determine the acceleration of the proof mass.  
         [0006]     A plurality of acceleration sensors according to the present invention may be combined in a variety of array structures to provide the capability of measuring acceleration at a plurality of locations with a region defined by such an array. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]      FIG. 1  is a cross sectional view showing an acceleration sensor according to the present invention;  
         [0008]      FIG. 2  illustrates a first sensor array that includes a plurality of acceleration sensors according to the present invention; and  
         [0009]      FIG. 3  illustrates a second sensor array that includes a plurality or acceleration sensors according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0010]     As shown in  FIG. 1 , a fiber optic acceleration sensor  10  includes two Fiber Bragg Gratings (FBGs)  12  and  14  working in a push pull manner. The FBGs  12  and  14  are formed in an optical fiber  16 .  
         [0011]     The FBG  12  is connected between a first side  20  of a proof mass  22  and a housing  24 . The FBG  12  is connected between the housing  24  and a second side  26  of the proof mass  22  that is opposite the first side  20 . The FBG  12  is secured to the housing  24  by any suitable means such as adhesive bonding in a passage  28 . The FBG  14  is secured to the housing  24  by any suitable means such as adhesive bonding in a passage  30 . A portion  18  of the optical fiber  16  is secured to the proof mass  22  by any suitable means such as being adhesively secured inside a passage  19  through the proof mass  22  or in a groove (not shown). The proof mass  22  is supported within the housing  24  by a plurality of elastic members  32 - 35 . The elastic members may be formed as springs as shown or as lengths of any suitable elastomeric material.  
         [0012]     Suitable structures and fabrication techniques for forming the FBGs  12  and  14  are well known in the art. The FBGs  12  and  14  may be produced by forming a periodic or aperiodic perturbation in the index of refraction in selected lengths  15  and  17  of the optical fiber  16 . The index perturbation primarily affects the core (or guiding region) of the optical fiber  16 . There are several ways in which a suitable perturbation may be generated. The most common way is to capitalize on the photosensitivity of optical fibers containing particular dopant materials. It has been discovered that germania-doped silica optical fiber is sensitive to exposure to argon ion laser radiation and that a two-photon absorption at 488 nm was responsible for the effect. The early research lead to holographic writing methods that presently are used to fabricate FBG devices as disclosed in U.S. Pat. No. 4,725,110 to Glenn, et al; U.S. Pat. No. 6,836,592 to Mead et al.; U.S. Pat. No. 6,310,996 to Byron; and U.S. Pat. No. 4,474,427 to Hill et al., the disclosures of which are incorporated by reference into the present disclosure.  
         [0013]     UV-light is caused to interfere, either by use of a phase mask, prism interferometer, or other method. The interfered light is apertured and focused on the core region of an optical fiber. The interference pattern formed on the core is a series of bright and dark bands, whose spacing can be either equidistant or chirped. The former case will form a highly period grating pattern, while the later will generate an aperiodic (or chirped) pattern. The bright bands interact with the doped core material and cause an index of refraction change to occur in the immediate area exposed to the light while the areas under the dark bands remain unaffected. It is this that gives rise to the periodic index perturbation. By changing the interference period, the grating period, Λ g , is changed in turn changing the wavelength that is reflected or transmitted through the FBG filter. The strength of the index perturbation will govern the transmission and reflection characteristics of the FBG.  
         [0014]     Referring again to  FIG. 1 , the formed FBGs  12  and  14  can then be used as reflection or rejection filters for a specific optical wavelength. The particular wavelength λ Bragg  that is acted upon by the FBG is governed by the period of the index perturbation and can be expressed to the first order as 
 
λ Bragg =2Λ g η eff   (1) 
 
 where η eff  is the effective index of refraction of the optical fiber, and Λ g  is the period of the index perturbation. 
 
         [0015]     FBGs can be fabricated as either reflective or transmissive devices. The device described here will work with either type of grating.  
         [0016]     To form the acceleration (or vibration) sensor, the FBGs  12  and  14  are used in tandem and configured in a push-pull manner. In this configuration it is not a requirement for the FBGs  12  and  14  to be matched in wavelength when in a static environment because the important element for detection is the wavelength difference between the two FBGs  12  and  14  in the dynamic environment and not the their absolute wavelength shifts.  
         [0017]     The proof mass is allowed to move within the sensor housing  24  when excited by acceleration or vibration with damping provide by the springs  32 - 35 . The FBGs  12  and  14  are rigidly attached to the proof mass  22  and the sensor case  24 . When the proof mass  22  is excited and caused to move, the FBGs  12  and  14  are alternately placed into tension and compression. Placing an FBG into tension causes the grating period, Λ g , to become larger; and, when under compression, the grating period becomes smaller. The shift in grating period therefore drives the wavelength that is filtered by the grating as can be seen by application of Equation 1.  
         [0018]     The signals that are returned for processing are modulated in wavelength. By taking the relative time-dependent wavelength differences from the two returns, the original vibration (acceleration) signature can be found. An advantage of this configuration is that the sensitivity of the device is increased by 2 over that using a single FBG. This comes about because a percentage stain in one FBG causes a corresponding percentage change in wavelength. Using the two FBGs  12  and  14  in a difference configuration yields twice the sensitivity for the same given strain. The wavelength difference signal is then 
 
Δλ=2η eff (Λ g,1 −Λ g,2 ).  (2) 
 
         [0019]     Another advantage of this configuration is that it is temperature insensitive. This again comes from the fact that only the relative difference in wavelength change between the two FBGs  12  and  14  is used and not the absolute value. The expression for the wavelength shift in an FBG due to temperature is;  
                 λ   b     ⁡     (   T   )       =     2   ⁢       Λ   g     ⁡     (     1   +     α   ⁡     [       T   1     -     T   2       ]         )       ⁢       (       n   eff     +         ⅆ     η   eff         ⅆ   T       ⁡     [       T   1     -     T   2       ]         )     .               (   3   )             
 
 The expression for the wavelength difference between the two FBGs can be written as  
                 Δλ   ⁡     (   T   )       =       2   ⁢       Λ     g   ,   1       ⁡     (     1   +     α   ⁡     [       T   1     -     T   2       ]         )       ⁢     (       n   eff     +         ⅆ     η   eff         ⅆ   T       ⁡     [       T   1     -     T   2       ]         )       -         Λ     g   ,   2       ⁡     (     1   +     α   ⁡     [       T   1     -     T   2       ]         )       ⁢     (       n   eff     +         ⅆ     η   eff         ⅆ   T       ⁡     [       T   1     -     T   2       ]         )           ,           (   4   )             
 
 where α is a temperature expansion coefficient of the FBG. Equation 4 can be simplified to  
               Δλ   ⁡     (   T   )       =     2   ⁢     (       Λ     g   ,   1       -     Λ     g   ,   2         )     ⁢     (     1   +     α   ⁡     [       T   1     -     T   2       ]         )     ⁢       (       n   eff     +         ⅆ     η   eff         ⅆ   T       ⁡     [       T   1     -     T   2       ]         )     .                             
 
 where the temperature terms behave only a static offset to the wavelength differences, therefore not affecting the dynamic performance of the sensor. 
 
         [0020]      FIG. 2  shows a first sensor array  40  that may include a plurality of fiber optic acceleration sensors A 1 , A 2 , . . . A N  formed accordance with  FIG. 1  and the foregoing description thereof. The array  40  is a linear array that receives an optical signal  42  from a broadband optical signal source  44 . The input optical signal  42  propagates through an optical fiber  46  to an optical isolator  48  that prevents propagation in the reverse direction.  
         [0021]     The input optical signal  42  then propagates to an optical coupler  50  that is arranged to have ports P 1 -P 4 . The input optical signal  42  is input to port P 1  of the optical coupler  50 . Part of the optical signal  42  input to the optical coupler  50  is cross-coupled to be output at port P 4  where the cross-coupled signal is absorbed by and absorber  52 . The portion of the input optical signal  42  that remains in the optical fiber  46  is output from the optical coupler  50  at port P 3  for input to the acceleration sensors A 1 , A 2 , . . . A N . Each of the acceleration sensors A 1 , A 2 , . . . A N  returns a wavelength doublet signal back to the optical coupler  50 . Each doublet signal returned indicates acceleration of the corresponding acceleration sensor.  
         [0022]     The doublet signal returns are guided by the optical fiber back to the optical coupler  50 , which couples the doublet signal returns from port P 3  for output to an optical fiber  54  at port P 2 . The optical fiber  54  guides the doublet signal returns to an optical wavelength interrogator  56  for wavelength processing to extract the desired acceleration information.  
         [0023]      FIG. 3  shows a second sensor array  60  that includes a linear array  62  that is similar to the array  40  of  FIG. 1  and a linear array  64 , which is also similar to the array  40 . A broadband optical signal source  66  provides an optical signal  68  to an optical fiber  70  that is arranged to guide the input signal to an optical isolator  72 . The input optical signal propagates through the optical isolator  72  to an optical coupler  74  that has ports P 1 -P 4 . A first portion  75  of the input optical signal remains in the optical fiber  70  and is output from the optical coupler at port P 3  for input to the array  62  that includes a plurality of acceleration sensors A 1 , A 3 , . . . A N . A second portion  76  of the input optical signal cross-couples from port P 1  to port P 3  into an optical fiber  77  for input to the array  64  that includes a plurality of acceleration sensors A 2 , A 4 , . . . A 2N .  
         [0024]     The array  62  produces a first set of doublet signal returns that propagate back to the optical coupler  74  where they are cross-coupled to port P 2  and into the optical fiber  77 . The array  64  produces a second set of doublet signal returns that return to the optical coupler  74  where they propagate from port P 4  to port P 2 . Both sets of doublet signal returns propagate in the optical fiber  77  to an optical interrogator  78  that processes the doublet signal returns to obtain numerical data for the acceleration at each acceleration sensor in the arrays  62  and  64 .