Abstract:
Apparatus for minimizing the effects of radiation induced attenuation on a sense coil in a fiber optic rotation sensor includes apparatus for injecting photobleach light at a frequency selected to remove radiation-induced color centers. Wavelength division multiplexing optical couplers are used to introduce the photobleach light into the fiber optic rotation sensor system and then remove the photobleach light from the gyroscope optical circuit without effecting the gyro signal.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to fiber optic rotation sensors and particularly to reducing sensitivity of such sensors to radiation environments. Still more particularly, this invention relates to photobleaching the sense coil of a fiber optic rotation sensor system to reduce sensitivity to radiation. 
     A typical fiber optic gyro uses a long length of polarization maintaining (PM) fiber as a sense element for rotation. It has been shown that this type of fiber is sensitive to radiation environments. When exposed to ionizing radiation, the fiber develops color centers that attenuate light propagating through the sense coil. If enough attenuation occurs, the random walk of the gyroscope will increase significantly. When the attenuation is extremely high due to large doses of ionizing radiation, the gyroscope will lose all signals. 
     SUMMARY OF THE INVENTION 
     The present invention minimizes the effects of radiation induced attenuation by photobleaching the sense coil. This is accomplished by injecting enough light at a frequency selected to remove the induced color centers. Since this light is at a different frequency from the light the gyroscope typically uses to sense rotation it must be introduced and removed from the gyroscope optical circuit without effecting the gyro signal. 
     Apparatus according to the present invention for photobleaching a fiber optic sense coil in a fiber optic rotation sensor system, comprises a photobleach light source and an optical fiber arranged to guide the photobleach light. A wavelength division multiplexing apparatus is connected to the optical fiber and arranged to introduce photobleach light into the fiber optic sense coil to anneal out color centers therein. The invention further includes apparatus arranged for removing the photobleach light from the fiber optic rotation sensor system. 
     The apparatus for removing the photobleach light from the fiber optic rotation sensor system includes a second wavelength division multiplexing optical coupler. The wavelength division multiplexing couplers may be formed to include the fiber optic leads of the sensing coil. Alternatively, the wavelength division multiplexing couplers may be formed to include the fiber optic sense coil. 
     The apparatus according to the present invention preferably further includes apparatus for dispersing the photobleach light that has been coupled out of the gyro. 
    
    
     An appreciation of the objectives of the present invention and a more complete understanding of its structure and method of operation may be had by studying the following description of the preferred embodiment and by referring to the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates apparatus according to a first embodiment of the present invention for photobleaching a fiber optic sense coil; 
     FIG. 1A is an expanded view illustrating photobleaching apparatus connected to a fiber optic sense coil; 
     FIG. 2 illustrates a wavelength division multiplexer that may be included in the apparatus of FIG. 1; 
     FIG. 3 illustrates a second embodiment of the present invention; and 
     FIG. 4 illustrates a third embodiment of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     This disclosure describes an apparatus and a method for isolated photobleaching fiber optic rotation sensor coils. Specific details are disclosed to provide a thorough description of the invention. However, it will be apparent that the present invention may be practiced without these specific details. Well-known components of the optical signal source according to the present invention are shown in block diagram form, rather than in detail, to avoid unnecessarily obscuring the invention. 
     FIG. 1 illustrates a reverse pumped, single-pass optical signal source  10  arranged to provide optical signals to a fiber optic rotation sensor system  12 . The particular optical signal source  10  disclosed is representative of a suitable type of optical signal source that may be used in practicing the present invention. The invention is not limited to the particular optical signal source  10  described herein. Any optical signal source suitable for use in a fiber optic rotation sensor may be used in the present invention. 
     The optical signal source  10  includes a pump light source  14 , which preferably is a laser diode, arranged to output pump light at a fiber optic pigtail  16 . A splice  17  connects an optical fiber  18  to the fiber optic pigtail  16 . A wavelength division multiplexer (WDM)  20  has a fiber optic pigtail  18 . The WDM  20  is a fiber optic coupler arranged to separate light beams of different wavelengths. The WDM  20  has a fiber optic pigtail  24  connected to a gain fiber  26  via a splice  28 . The pump light propagates through the optical fiber  16  and the fiber optic pigtail  24  to the WDM  20 . The WDM  20  allows the pump light to propagate to the gain fiber  26 . 
     The pump light has a wavelength of 1480 nm in a preferred embodiment of the invention. The gain fiber  26  typically is an erbium-doped fiber, which is well-known in the art. The erbium-doped gain fiber  26  has a three level transition that produces a broad band optical signal having a wavelength centered at 1560 nm. U.S. Pat. No. 5,231,465, issued Feb. 8, 1991 to Phillips et al. and U.S. Pat. No. 5,119,229 issued Jun. 2, 1992 to Grasso et al. disclose the use of erbium-doped cores to provide optical signal in an optical fiber. The disclosures of U.S. Pat. No. 5,231,465 and U.S. Pat. No. 5,119,229 are hereby incorporated by reference into this disclosure. 
     The gain fiber  26  absorbs part of the pump light and emits light propagating lengthwise in both directions in the gain fiber  26 . Light emitted in the direction of propagation direction of the pump light is referred to as forward light. The forward light exits the gain fiber  26  via an angled capillary  30 , which terminates the gain fiber  26 . Light emitted by the gain fiber  26  opposite in direction to the direction of propagation of the pump light is referred to as reverse light. The reverse light enters the WDM  20  via the pigtail  24  and exits the WDM  20  at a fiber optic pigtail  31 . A splice  32  connects the pigtail  31  to an optical fiber  33 . 
     The WDM  20  thus directs reverse emitted light from the gain fiber  26  into the optical fiber  33 . The light coupled from the WDM  20  into the optical fiber  33  is the optical signal output from the optical signal source  10  to the fiber optic rotation sensor system  12 . 
     A multiplexer  34  has a first fiber optic pigtail  33  connected to the optical fiber  31  via a splice  32 . The multiplexer  34  has a second fiber optic pigtail  40  connected to a fiber optic pigtail  42  that extends from a multifunction integrated optics chip (MIOC)  44 . The source light propagates through the multiplexer  34  to the MIOC  44 , which includes well-known components (not shown) used to form and process the counter-propagating waves used in fiber optic rotation sensor systems. Suitable MIOC structures are disclosed in U.S. Pat. No. 4,915,503 (Pavlath); U.S. Pat. No. 4,997,282 (Pavlath); U.S. Pat. No. 5,037,205 (Pavlath), all of which are assigned to Litton Systems, Inc. The disclosures of U.S. Pat. Nos. 4,915,503; 4,997,282; 5,037,205 are hereby incorporated by reference into this disclosure. 
     The MIOC  44  then provides counter-propagating light beams to a fiber optic sense coil  50 . Rotation of the fiber optic rotation sensor system  12  about a line perpendicular to the plane of the sense coil  50  produces a phase difference in the counter-propagating light beams by means of the Sagnac effect. After traversing the sense coil  50 , the counter-propagating light beams combine in the MIOC  44  and form an interference pattern. The combined counter-propagating light beams then exit the MIOC  44  at the pigtail  42  and enter the multiplexer  34  via the pigtail  40 . The combined counter-propagating light beams exit the multiplexer  34  via a fiber optic pigtail  54 . The fiber optic pigtail  54  is connected to an input pigtail  56  of a photodetector  60  via a splice  62 . The photodetector  60  produces electrical signals that indicate the light intensity in the interference pattern produced by combining the light beams that have propagated through the sense coil. 
     The return light from the sense coil  50  contains the rotation signal of the fiber optic rotation sensor system  12 . It is this light that can be attenuated when the sense coil  50  is exposed to significant levels of ionizing radiation such as gamma radiation. After the sense coil  50  is removed from the source of the ionizing radiation the fiber optic sense coil  50  will start to recover. Many applications of fiber optic rotation sensors cannot afford the time necessary for the sense coil  50  to recover to a level were it is usable. Photobleaching according to the present invention increases the rate of recovery. Photobleaching under certain conditions can significantly reduce generation of color centers. Typically, high energy photons (shorter wavelengths) work best for photobleaching. 
     FIG.  1  and FIG. 1A illustrate apparatus for introducing and removing light for the purpose of photobleaching the fiber optic sense coil  50 . The photobleaching apparatus includes a pair of wavelength division multiplexers (WDMs)  70  and  72  that are added to the leads  74  and  76 , respectively, of the sense coil  50 . An optical fiber  78  is connected between the WDM  70  and a high power laser  80 . An optical fiber  82  extends from the WDM  70  to terminate in an angle capillary  84 . An optical fiber  86  is connected between the WDM  72  and an angle capillary  88 . An optical fiber  90  extends from the WDM  72  to terminate in an angle capillary  92 . In some applications the angle capillary  88  may be replaced by a high power laser similar to the laser  80 . 
     Each of the WDMs  70  and  72  is preferably a fused fiber optic coupler comprising two input legs and two output legs with the ability to separate light beams of different wavelengths. The WDMs  70  and  72  are designed to operate at specific wavelengths. Typically two wavelengths enter a WDM from one of the input legs. One wavelength exits from one of the output legs. The second wavelength is designed to be cross coupled to a second output leg of the WDM as shown in FIG.  2 . 
     In applying this operation to the fiber optic rotation sensor system of FIG. 1, the WDM  70  is used in the reverse fashion where the output leads serve become the input and the input leads serve as the output. Referring to FIG. 2, output  1  serves to provide a path for the fiber optic gyro light to be introduced into the fiber sense coil. Output  2  of the WDM  70  serves to introduce the photobleach light into the sense coil  50 . In FIG. 1, output  2  of the WDM  70  is connected to a high power laser diode with a different wavelength than the gyroscope wavelength. Typically the photobleach wavelength is 980 nanometers and the gyroscope operating wavelength is 1560 nanometers. 
     The WDM  72  is used to remove the photobleaching light from the sense coil  50  by separating the wavelengths into its two output legs. If the light that is used to photobleach the coil is not removed from the detection circuitry, then the gyro will exhibit significant bias and scale factor errors. Only the 1560 nanometer gyroscope light is allowed to stay in the optical circuit for the gyro. The photobleaching light (980 nanometer) is coupled out of the optical circuit where it is scattered out of the system by the action of the angle capillary  88 . In this way only the gyro light reaches the photodetector  60 . 
     Typically WDMs are made with single mode non-polarization maintaining fiber. However, the fiber sense coil  50  typically is made with PM fiber. In order to ensure that there is no polarization drift associated with the single mode non-polarization maintaining sections, the WDMs  70  and  72  should be made with PM fiber. 
     The fast axes of the PM fibers used in the WDMs  70  and  72  are aligned with the fast axis of the PM fiber used in the sense coil  50 . In this case the photobleaching laser diode  80  launches light into an arbitrary axis of the fiber  78 . This light is then removed by the action of the WDM  72 . However the gyro light is coupled in the sense coil  50  along the fast axis of the WDM  72  and coupled into the sense coil  50  fast axis. 
     As shown in FIG. 1A, the photobleaching light propagates in the counter-clockwise direction in the sense coil  50 . In a preferred embodiment of the invention, the photobleaching light has a wavelength of 980 nm. The sense coil  50  guides the counter-propagating waves from the optical signal source  10 . The counter-propagating waves are indicated by the right and left pointing arrows adjacent the WDMs  70  and  72 . The 980 nm photobleaching light is coupled from the optical fiber  78  into the sense coil  50 . The photobleaching light then traverses the length of the sense coil  50  and enters the WDM  72 . The WDM  72  couples the photobleach light out of the gyro to the angle capillary  88  shown in FIG. 1, which is designed to prevent the photobleach light from reaching the MIOC  44 . 
     As shown in FIG. 3, the present invention may further be practiced with a pair of PM WDMs  102  and  110  integrated with the sense coil  50 . In this case the photobleaching laser diode is pigtailed with PM fiber. The light from the photobleaching laser diode  108  is coupled into the PM pigtail  104  along the slow axis of the fiber. This light is then coupled into the fiber sense coil  50  via WDM  102  along the slow axis, which is the perpendicular axis to the gyro light. The gyro light propagates through the sense fiber coil  50  along the fast axis. This photobleaching light is then removed by the action of WDM  110 . Any residual photobleaching light not removed by WDM  110  is in the wrong polarization orientation to pass through the integrated optics chip  44 . The photobleaching light ( 980  nanometer) is coupled out of the optical circuit where it is scattered out of the system by the action of angle capillaries  106 ,  114  and  118  in the same manner as the angle capillary  88  of FIG.  1 . The MIOC  44  has a distributed polarizer (not shown) which rejects light not in the fast axis. This will isolate the gyro light from the photobleaching light by an additional 40 to 70 dB depending on the quality of the polarizer. 
     As shown in FIG. 4, the present invention may be practiced with a single WDM  120  integrated with the sense coil  50 . The WDM  120  includes a PM fiber  122  arranged to couple photobleach light into the sense coil  50 . A narrow bandpass filter  124  is placed in the optical circuit between the multiplexer  34  and the photodetector  60  to prevent the photobleach light from reaching the photodetector  60 . 
     An exemplary embodiment of the invention is disclosed herein to explain how to make and use the invention. The described embodiments are to be considered in all respects as exemplary and illustrative rather than restrictive. Therefore, the appended claims rather than the foregoing descriptions define the scope of the invention. All modifications to the embodiments described herein that come within the meaning and ranges of equivalence of the claims are embraced within the scope of the invention.