Abstract:
Systems and methods for measuring a target radiation substance using a fiber-optic sensor. An ample method includes activating the sensor at a first power level, measuring light at a photo sensor, increasing power to a second power level, if the measured light indicates possible presence of the target radiation substance, measuring light at the photo sensor after the increase in power, and outputting an error signal, if the target radiation type was not detected based on the measured light signatures. The first power level is typically between 1-1000 μWatts and the second power level is typically between 100-10000 μWatts. In a further confirmation, the power is decreased to approximately the first power level, if the error signal is not outputted.

Description:
BACKGROUND OF THE INVENTION 
   In recent times, greater emphasis has been placed on national security and detecting threats to populations. In particular, detecting or sensing the presence of undesired chemicals or biological material in the environment has become a priority, and a variety of detection devices have been developed in response thereto. 
   In radiological sensors, if the sensor indicates the presence of a radiological substance, there have to be reliable methods of making sure the indication is correct. Otherwise, high false alarm rates will lead to the sensors being disabled or ignored by the user. One possibility is to use a fiber optic sensor, where the fiber darkens, or becomes lossy to light propagation when exposed to radiation. Situations that could lead to false alarms (indicative of radiation when there is none) could occur where a sensed loss is caused by fiber breakage, light source failures, or other failures, and thus causing false positive results. 
   Therefore, there exists a need for a radiological sensor with improved analysis capabilities and therefore a low false alarm rate. 
   SUMMARY OF THE INVENTION 
   The present invention provides systems and methods for measuring a target radiation types using a fiber-optic sensor. An example method includes activating the sensor at a first optical power level, measuring a light signal at a photo sensor and comparing it with an optical signal reference obtained in a controlled environment in the absence of radiation. If the measured optical signal is degraded based on the comparison, indicating the possible presence of the target radiation type, the optical power is increased to a second optical power level. The optical signal is then measured at the second optical power level. If the optical signal improves, as seen at the photo sensor after the increase in power, an electrical signal indicative of the presence of radiation is output. If, however, optical signal does not improve with increased power, an error signal is outputted, indicative of a sensor failure instead of presence of the target radiation type. 
   In one aspect of the invention, the first power level is between 1-1000 μWatts and the second power level is between 100-10000 μWatts. 
   In one aspect of the invention, the error signal is not outputted, if loss of the measured optical signal at the second power level is less than loss of the measured light at the first power level. 
   In still another aspect of the invention, the power is decreased to approximately the first power level after obtaining optical signal information at the second power level, if the error signal is not outputted. Presence of the target radiation type is confirmed if the round-trip loss of a fiber optic resonator measured after the decrease in power is less than the round-trip loss measured at the initial first power level. An error signal is generated and communicated if the round-trip loss of the resonator is not reduced via the above increased optical power cycling. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings: 
       FIG. 1  illustrates a schematic diagram of an example system formed in accordance with an embodiment of the present invention; 
       FIGS. 2A  and B are a flowchart of an example process performed by the system shown in  FIG. 1 ; 
       FIG. 3  illustrates a sawtooth wavelength signal that is indicative of the change in wavelength of the light output by the laser diode of the system shown in  FIG. 1 ; and 
       FIG. 4  illustrates a graph that illustrates example responses as detected at a photo diode included in the system shown in  FIG. 1 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1  illustrates an example radiological sensor system  20  formed in accordance with an embodiment of the present invention. The system  20  includes a processor  26  that is in signal/data communication with a laser diode (light source)  28  and a photo diode  32 . The laser diode and photo diode are in light communication with a fiber-optic coil, such that light generated by the laser diode  28  is transmitted through the fiber-optic coil  30  and is received by the photo diode  32  via optical components  34 . 
   A high reflectivity mirror is placed in proximity to the fiber coil to recirculate light, forming an optical resonator when combined with the coil, such that most of the light energy exiting the coil at one end is reflected back into the coil at the other end. The wavelength of the laser diode  28  is swept such that a resonance lineshape, indicative of the roundtrip loss of the optical resonator is observed, i.e. is the “light signal” on the photo diode  32 .  FIG. 1  illustrates a ring resonator configuration; however, a linear resonator may also be used. The roundtrip loss of the resonator refers to the amount of reduction in the intensity of light for one roundtrip within the resonator: that is, say, from the point where light entered the first end of the coil, propagates through the fiber coil, exits the second end, is reflected in the mirror toward the first end, and enters the first end—thus completing one trip around the closed resonator loop. 
   The processor  26  instructs the laser diode  28  to output light at a certain wavelength and power level. The photo diode  32  senses a light signal that is outputted by the optical resonator including a highly reflective mirror  38  and the fiber-optic coil  30 , and sends a sensed light signal to the processor  26 . The processor  26  determines if a radiological substance has been sensed based on a measured loss contained within the received light signal. 
   The fiber-optic coil  30  is designed for radiological sensing of particular type of radiation. The fiber-optic coil  30  includes a glass core surrounded by a cladding material. The cladding material is impermeable to substances other than radiation. The glass core is doped in order to indicate loss (i.e., darken) when a particular type of radiation is experienced. For example, the dopant is phosphorus for the fiber in the coil to sense gamma rays. 
     FIGS. 2A  and B illustrate an example process  50  performed by the system  20  of  FIG. 1 . First at a block  52 , the system  20  is turned on. The laser diode  28  generates light that propagates through many passes of the fiber-optic coil  30  and is then sensed by the photo diode  32  that supplies a return signal to the processor  26 . The light is sensed over a period of time in which the laser diode frequency is swept. The processor  26  then determines the loss of the optical resonator, block  54  via observation of a finesse of a resonance lineshape. The finesse is a measure of the sharpness of the resonance lineshape, and is highly sensitive to the round-trip resonator loss. Higher loss broadens the resonance lineshape, and lowers its finesse. Lower loss narrows the resonance lineshape (with respect to input light frequency) and increases the finesse. The resonator loss, or finesse, is measured over a first time period (e.g., time needed to sweep the laser frequency over two free spectral ranges and observe resonance lineshapes) at a first power level (e.g., 1-1000 μWatts) of the laser diode  28 . If the measured resonator loss is not greater than a predefined threshold amount, see decision block  56 , then the process  50  returns to measuring the loss at block  54 . However, if at the decision block  56  the loss measured (finesse of the resonator) is greater than a first predefined threshold amount, the processor  26  commands the laser diode  28  to increase power to a second level (e.g., 100-10000 μWatts) that is greater than the first level, see block  60 . For example, the resonator loss may give a finesse of &gt;300 when it is calibrated at the factory (i.e., prior to delivery). That is, in the event that the resonator finesse is below 300, the loss is determined to be above the threshold, and a “yes” answer is generated in block  56 , causing the light power to be increased. Next at a block  62 , the processor  26  again measures the finesse/loss of the resonator via the signal received from the photo diode  32  after a period of time (e.g. 10 sec-10 min) has expired. At a decision block  64 , the processor  26  determines if the second measured loss (block  62 ) of the resonator is a decreased amount of loss as compared to the first measured loss (block  54 ). The decision at block  64  may also determine if the second measured loss shows a reduced rate of increase of loss as compared to the rate of loss at block  54 . If the second measured loss does not show a decrease as compared to the first measured loss or the rate of increase of loss is not reduced, then the processor  26  outputs an error signal, block  68 . This indicates that a radiological substance was not detected and the result is a false positive that might be due to some other failure, such as fiber-optic coil or light source failure. This indicates that there is a malfunction of the sensor. However, if at the decision block  64  the second measured loss does show a decrease as compared to the first measured loss of the resonator or shows a reduction in the rate of increase of loss of the resonator, then the process  50  continues to block  76 . If the result of decision block  64  is positive, then the phenomenon of annealing or photo-bleaching is occurring. 
   At block  76  the processor  26  instructs the laser diode  28  to reduce power of the light outputted by the laser diode  28 —similar to the first power level. Then, at decision block  78 , the processor  26  determines if this new condition of reduced power confirms the presence of the target radiation (e.g. gamma rays, beta particles, etc.). If at the decision block  78  the reduced power condition fails to confirm presence of the target radiation, the process  50  returns to block  68 . However, if at the decision block  78  the reduced power condition does confirm the presence of the target radiation, at block  80 , the processor  26  confirms the sensed radiation (blocks  56 ,  64 ). 
   In one embodiment, the first measured loss (block  54 ) may be outputted as the sensed value when a positive result is determined at decision block  64 . The double check performed at blocks  76 - 80  need not be performed. 
     FIG. 3  illustrates a graph of wavelength λ of the light generated by the laser diode  28 . In this example, the wavelength λ is ramped in a sawtooth pattern over time as instructed by the processor  26 . The wavelength is ramped to cover two free spectral ranges. This guarantees that the produced light signal sensed at the photo diode  32  experiences at least one low loss dip, when in a clean environment. 
     FIG. 4  illustrates an example graph of light intensity (I) sensed by the photo diode  32  as the wavelength of the light source is swept during a no-radiation period (line  100 ) and a sensed radiation period (line  102 ). Dips in light intensity (optical power) occur when an integer number of wavelengths fits into the resonator round-trip optical pathlength. The source wavelength difference between the two dips shown in  FIG. 4  is the free spectral range. In the no-radiation period (line  100 ) the resonance dips are narrow and sharp, indicative of high finesse and low resonator loss. When the resonator loss increased, the dip becomes broader and shallower, line  102 . The loss situation indicates a possible presence of the target radiation. The power of the light signal is increased to confirm the loss increase, indicated by the broadened width of the resonance, is due to the target radiation. If the lineshape in line  102  becomes less broad, then sensing of the target radiation is confirmed because annealing is occurring. The presence is further confirmed, if after the power of the light signal is reduced, the lineshape is less broad than that in shape line  102 . Note that after the power is reduced, there may be a trend toward lineshape-broadening indicative of continuing radiation exposure, but if the lineshape initially is narrower than that shown in line  102  it indicates that annealing had occurred, and the original roundtrip resonator loss increases in the resonator have been due to radiation exposure. 
   While the preferred embodiment of the invention has been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. Accordingly, the scope of the invention is not limited by the disclosure of the preferred embodiment. Instead, the invention should be determined entirely by reference to the claims that follow.