Patent Publication Number: US-8989573-B2

Title: Sensing apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit under 35 U.S.C. §119 of U.S. Patent Application No. 61/620,440, filed Apr. 5, 2012, which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     The present disclosure relates to a sensing apparatus. 
     A sensor using an optical fiber is advantageous in that it is free of corrosion, exhibits high endurance, and has electromagnetic interference immunity. Particularly, an optical reflector such as an optical fiber Bragg grating is suitable for the use as a sensor because of its small size and high sensitivity to the temperature and strain. 
     Sensor networks using an optical reflector are classified into a wavelength division multiplexing (WDM) method in which a variation in center wavelength of each sensor is sensed with respect to strain applied from an outside and a variation in temperature, a time division multiplexing (TDM) method in which each sensor has the same center wavelength and a pulse string having a period that is longer than a distance between sensors is transmitted to the sensors to measure variation in output power reflected by each sensor, and a code division multiplexing (CDM) method in which an input light source is modulated using a pseudorandom number generator, the modulated light source is transmitted to each sensor, and then variation in autocorrelation value of a pseudorandom number returning to each sensor is measured. 
     The WDM method has been widely used in that it has a high resolution and may be easily realized compared with other systems, but has a limitation in that the number of sensors is limited because when a center wavelength of a sensor is changed by an external factor, the center wavelength of the sensor may not overlap a center wavelength of another neighboring sensor. 
     The TDM method has solved the limitation in the number of sensors by using sensors having the same center wavelength and an optical reflector having a low reflectivity to measure a variation in output power returning from each sensor. However, since the TDM method makes the design of a signal processor be complicated, it is expensive in price, and since the TDM method uses a single pulse, it has a limitation such as a low response rate, compared with other methods. 
     The CDM method may be realized by modulating a signal of an optical source by a pseudorandom code to transmit a code string to each sensor, and autocorelating the code string returning from each sensor. The CDM method may realize a demodulation circuit for a sensor system only with a low price electrical device without a high speed signal processing circuit. 
     However, since the related art CDM sensor uses a variable laser in order to increase the operation range, it requires a long scanning time and a delay for synchronizing the pseudorandom number code reflected and returning from each sensor and an originally generated pseudorandom code for autocorrelation. Also, since the variation in output power of autocorrelation value is measured, if the wavelength deviates from the center wavelength of the sensor, the measurement may be impossible. 
     BRIEF SUMMARY 
     Embodiments provide a code division multiplexing sensing apparatus that may increase the number of sensors to be measured and enhance the scanning rate by using a broadband optical source enabling a high speed modulation, enhance the measurement range of sensors by moving a variation in center wavelength of sensors to a variation in time axis and observing the moved variation, an may monitor a plurality of sensors at the same time through autocorrelation even without a delay. 
     In one embodiment, a sensing apparatus includes: a broadband optical source; a first pseudorandom number generator generating a first pseudorandom number code string to modulate the broadband optical source; at least one sensor reflecting an output of the first pseudorandom number generator at a wavelength corresponding to a center wavelength thereof when the output of the first pseudorandom number generator is inputted; a wavelength-time converter converting an output of the sensor by wavelength-time conversion; a second pseudorandom number generator generating a second pseudorandom number code string which is different in frequency from and is the same in bit length and code string as the first pseudorandom number code string; a mixer mixing an output signal of the wavelength-time converter with an output signal of the second pseudorandom number generator; and an integrator integrating an output of the mixer. 
     In another embodiment, a sensing apparatus includes: a broadband optical source; a first pseudorandom number generator generating a first pseudorandom number code string to modulate the broadband optical source; a optical amplifier amplifying the modulated broadband optical source; at least one sensor reflecting an output of the first pseudorandom number generator at a wavelength corresponding to a center wavelength thereof when the output of the first pseudorandom number generator is inputted; a wavelength-time converter converting an output of the sensor by wavelength-time conversion; a light detector converting an output of the wavelength-time converter to an electrical signal; an electrical signal amplifier amplifying an output of the light detector; a second pseudorandom number generator generating a second pseudorandom number code string which is different in frequency from and is the same in bit length and code string as the first pseudorandom number code string; a mixer mixing an output signal of the electrical signal amplifier with an output of the second pseudorandom number generator; and an integrator integrating an output of the mixer. 
     In further another embodiment, a sensing apparatus includes: a broadband optical source; a first pseudorandom number generator generating a first pseudorandom number code string to modulate the broadband optical source; at least one sensor reflecting an output of the first pseudorandom number generator at a wavelength corresponding to a center wavelength thereof when the output of the first pseudorandom number generator is inputted; a second pseudorandom number generator generating a second pseudorandom number code string which is different in frequency from and is the same in bit length and code string as the first pseudorandom number code string; a mixer mixing an output signal of the sensor with an output signal of the second pseudorandom number generator; and an integrator integrating an output of the mixer. 
     According to the present invention, the number of sensors to be measured may be increased and the scan rate may be enhanced by using a broadband optical source enabling a high speed modulation, the measurement range of sensors may be enhanced by moving a variation in center wavelength of sensors to a variation in time axis and observing the moved variation, an a plurality of sensors may be monitored at the same time through autocorrelation even without a delay. 
     The details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a sensing apparatus according to a preferred embodiment of the present invention. 
         FIGS. 2 and 3  are schematic views for explaining the sliding autocorrelation in  FIG. 1 . 
         FIG. 4  is a graph showing a variation in autocorrelation value with time in the sensing apparatus of  FIG. 1 . 
         FIG. 5  is a graph showing example 1 in which a variation in autocorrelation value is observed after a pressure is applied to the second sensor of  FIG. 1 . 
         FIG. 6  is a graph showing example 2 in which a variation in autocorrelation value is observed after a pressure is applied to the second sensor of  FIG. 1 . 
         FIG. 7  is a graph showing a variation in wavelength when a strain is applied to the second sensor of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to the embodiments of the present disclosure, examples of which are illustrated in the accompanying drawings. 
     Hereinafter, a sensing apparatus according to an embodiment will now be described. 
       FIG. 1  is a block diagram of a sensing apparatus according to an embodiment. 
     Referring to  FIG. 1 , a sensing apparatus according to an embodiment includes: a broadband optical source  101 ; a first pseudorandom number generator  102  generating a first pseudorandom number code string to modulate the broadband optical source; a optical amplifier  103  amplifying the modulated broadband optical source; at least one sensor  105  reflecting an output of the optical amplifier  103  at a wavelength corresponding to a center wavelength thereof when the output of the optical amplifier  103  is inputted; a wavelength-time converter  106  converting an output of the sensor  105  by wavelength-time conversion; an electrical signal amplifier  108  amplifying an output of the light detector  107 ; a second pseudorandom number generator  109  generating a second pseudorandom number code string which is different in frequency from and is the same in bit length and code string as the first pseudorandom number code string; a mixer  110  mixing an output signal of the electrical signal amplifier  108  with an output of the second pseudorandom number generator  109 ; and an integrator  111  integrating an output of the mixer  110 . 
     The elements of the sensing apparatus having the above-mentioned configuration according to the embodiment will be described in detail. 
     The broadband optical source  101  is preferably a light source oscillating by broadband spectral light, such as a high luminance light emitting diode (SLED), a semiconductor optical amplifier (SOA), or a reflective semiconductor optical amplifier (RSOA). While the embodiment exemplarily describes that the broadband optical source  101  is a reflective semiconductor optical amplifier, the present invention is not limited thereto. Any broadband source will be possible if it may perform a high speed modulation. 
     The amplification factor of the reflective semiconductor optical amplifier (RSOA) may be obtained by using the standard theory of Fabry-Perot and is given by Equation 1. 
     
       
         
           
             
               
                 
                   
                     
                       
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     where R 1  and R 2  are the input and output facet reflectivity, ν m  represents the cavity resonance frequency, Δν L  is the longitudinal mode spacing, and G(ν) is the frequency dependent single pass amplification factor defined by Equation 2.
 
 G (ν)= e   g(v)L ,  [Equation 2]
 
     where L is the length of the cavity, and g(ν) is the amplification factor gain. 
     For an ideal SOA, R 1 =R 2 =0 and then the gain is equal to the single pass gain. 
     For an ideal RSOA, the photon traveling length is double that of the cavity length, then L ROSA =2L SOA  and may be expressed as Equation 3.
 
 G   RSOA ( n )= e   [g(n)L     ROSA     ]   =e   [g(n)2L     SOA     ]   ={G   SOA ( n )} 2 .  [Equation 3]
 
     The first pseudorandom number generator  102  generates a first pseudorandom number code string to module the broadband source. In the description of the present embodiment, the first pseudorandom number generator  102  generates a first pseudorandom number code string having, for example, the frequency of 500.1 [MHz] and the bit length of 2 7 −1 (=127) to module the broadband source. 
     The optical amplifier  103  amplifies the broadband source modulated by the first pseudorandom number code string and allows the modulated and amplified broadband source to be inputted to the respective sensors through a circulator  104  and a distributor (not shown). 
     Each of the sensors  105  may be configured to include an optical reflector (not shown) reflecting a wavelength corresponding to the center wavelength thereof when the output of the optical amplifier  103  is inputted. The optical reflector is a Bragg grating or a thin dielectric filter connected to an output terminal of the broadband optical source and having single wavelength full reflection or partial reflection characteristics. 
     In the description of the present embodiment, it is exemplified that the sensing apparatus is provided with three sensors  105  including first to third sensors  105   a ,  105   b , and  105   c , the center wavelengths of the first to third sensors  105   a ,  105   b , and  105   c  are 1550.9 [nm], 1552.6 [nm], and 1555.8 [nm], and the reflectivity is equal to 99[%]. 
     The sensors  105   a ,  105   b , and  105   c  may be arranged in series, in parallel, in a ring pattern, in a star pattern, or in a bus pattern, and may be arranged variously within the scope that does not deviate from the gist of the present invention. 
     The wavelength-time converter  106  converts a spectrum variation in each sensor (i.e., a variation in center wavelength) to a time shift by using wavelength to time conversion when the outputs of the sensors  105   a ,  105   b , and  105   c  are inputted through the circulator  104 . An example of the wavelength-time converter  106  is a dispersion compensating fiber (DCF). The DCF converts a shift in center wavelength of each of the sensors generated due to variations in temperature and pressure applied to the respective sensors to the time shift. In the present embodiment, it is exemplified that the dispersion value of the DCF is −1344.8 [ps/nm] at 1550 [nm]. 
     The optical detector  107  is provided to the output terminal of the wavelength-time converter  106  to convert the output of the wavelength-time converter  106  to an electrical signal. 
     A band pass filter (not shown) which has a wavelength corresponding to the wavelength of each sensor and a wider pass band may be further provided between the wavelength-time converter  106  and the optical detector  107 , thereby minimizing amplification of a noise generated in the optical amplifier. Also, by using the broadband optical source having a high output power, it will be possible to monitor the sensors without the band pass filter. 
     The electrical signal amplifier  108  is provided to an output terminal of the optical detector  107  to amplify the output of the optical detector  107 . 
     The second pseudorandom number generator  109  generates a second pseudorandom number code string which is different in frequency from and is the same in bit length and code string as the first pseudorandom number code string. In description of the present invention, it is exemplified that the second pseudorandom number generator  109  generates a second pseudorandom number code string having a frequency of 500 [MHz] and a bit length of 2 7 −1 (=127). 
     The second pseudorandom number code string outputted from the second pseudorandom number generator  109 , and the output of the electrical signal amplifier  108  are autocorrelated through the mixer  110  and the integrator  111 . An input terminal of the mixer  110  is connected the an output terminal of the second pseudorandom number generator  109  and an output terminal of the electrical signal amplifier  108 , an output terminal of the mixer  110  is connected to an input terminal of the integrator  111 , the mixer  110  mixes and outputs the second pseudorandom number code string generated in the second pseudorandom number generator  109  and the output of the electrical signal amplifier  108 , and the integrator  111  integrates and outputs the output of the mixer  110 . 
     The sensing apparatus according to the embodiment may monitor the sensors by observing the output variation of the integrator  111  with time. In the description of the present embodiment, it is exemplified that an oscilloscope  112  is provided for observing the output of the integrator  111 . 
       FIGS. 2 and 3  are schematic views for explaining the sliding autocorrelation performed through the mixer  110  and the integrator  111  in the sensing apparatus according to the preferred embodiment of the present invention. 
     Specifically,  FIG. 2  exemplarily shows that code 1 and code 2 that have the same bit length and the same code type but have slightly different frequencies from each other are connected to the oscilloscope. When code 1 is taken by a trigger, since code 2 is the same in type and bit length as but is slightly different in operation frequency from code 1, code 2 will appear slide. Thus, sliding autocorrelation may be performed through the above method. Herein, the sliding speed is determined by a frequency gap between the two codes, and the repetition of correlation is defined as Equation 4.
 
 T   R   =n/Δf,   [Equation 4]
 
     where n is the code bit length and Δf is the frequency gap. The bandwidth of the autocorrelation peak is also given by Equation 5.
 
 T   width =2 /Δf.   [Equation 5]
 
     Referring to  FIG. 3 , when two pseudorandom number codes having different frequencies of 100 [MHz] and 100.01 [MHz] and the same code bit length of 31 are autocorrelated using the mixer and the integrator, the repetition time and the bandwidth calculated by Equations 4 and 5 are 3.10 [ms] and 0.20 [ms], respectively, which show approximate coincidence with the measured values of 3.09 [ms] and 0.23 [ms]. 
       FIG. 4  is a graph showing a variation in autocorrelation value with time in the sensing apparatus according to the preferred embodiment. 
     Referring to  FIG. 4 , it may be seen that the period of pulse is 1.27 [ms]. Since the period of pulse is determined by the center wavelength and the bit length, the time (i.e., the period of pulse) may be shortened or extended. 
     As in the sensing apparatus according to the present embodiment, since the number of measurable sensors in the CDM method is determined by the bit length, it is possible to monitor about 100 sensors when the pseudorandom number generator generating the pseudorandom number code string of 2 7 −1 (=127) is used. 
       FIG. 5  is a graph showing example 1 in which a variation in autocorrelation value is observed after a pressure is applied to the second sensor  105   b  among the first to third sensors  105   a ,  105   b , and  105   c  shown in  FIG. 1 . 
     Referring to  FIG. 5 , when a pressure is applied to the second sensor  105   b  to move the center wavelength and then correlation between the second sensor  105   b  and the third sensor  105   c  is observed, it may be seen that the third sensor  105   c  is independent on the variation in autocorrelation value with time by the variation in center wavelength of the second sensor  105   b . Herein, the sensitivity of the sensors  105   a ,  105   b , and  105   c  is determined through the sensitivity of the wavelength-time converter  106  and the adjustment of an interval between the center wavelengths. 
       FIG. 6  is a graph showing example 2 in which the variation of autocorrelation value was observed after a pressure was applied to the second sensor  105   b  among the first to third sensors  105   a ,  105   b , and  105   c  shown in  FIG. 1 , and  FIG. 7  is a graph showing the variation of wavelength when a strain was applied to the second sensor  105   b  of  FIG. 1 . 
     Referring to  FIGS. 6 and 7 , it may be seen that the autocorrelation time was shortened due to strain. That is, the autocorrelation time was shortened by 0.05 ms when the strain applied on the second sensor  105   b  by increasing 0.36 [μ∈] step, and the time shift (sliding) of the second sensor  105   b  occurred by high negative dispersion slop of DCF. The first sensor  105   a  and the third sensor  105   c  have a time shift deviation of 6.4 [μs] and 8.6 [μs] when the strain was applied on the second sensor  105   b . Thus, the first sensor  105   a  and the third sensor  105   c  are independent with variation of the second sensor  105   b  so the sensing apparatus according to the present invention has a low crosstalk and high reliability. 
     Although embodiments have been described with reference to a number of illustrative embodiments thereof, it should be understood that numerous other modifications and embodiments can be devised by those skilled in the art that will fall within the spirit and scope of the principles of this disclosure. More particularly, various variations and modifications are possible in the component parts and/or arrangements of the subject combination arrangement within the scope of the disclosure, the drawings and the appended claims. In addition to variations and modifications in the component parts and/or arrangements, alternative uses will also be apparent to those skilled in the art.