Abstract:
An input of an optical interferometer is a periodical optical pulse. A phase of a first half and a latter half of a reference pulse is a 90 degree (independently orthogonal) phase difference. Two interferometric outputs i 1  and i 2 , where the phase difference is 90 degrees from each other, are obtained by interference of the reference pulse and the signal pulse. θ is calculated by referring the amplitude of reference pulse and the signal pulse to remove the light intensity fluctuations. Two values of cos  θ1  and cos  θ2  are calculated and positions are determined on the cosine curve by obtaining  θ1  and θ 2  values. Δ θ1  and Δ θ2 , which are the phase increment or decrement of both  θ1  and  θ2  in a T period, are summed and becomes the sensor output signal that removes the measurement range limitation of ±90 degrees (a half wavelength of light) of the light phase.

Description:
TECHNICAL FIELD 
     The present invention relates to an optical fiber sensor utilizing a distinctive character that a phase of an optical signal passing through an optical path is changed by an external force. 
     BACKGROUND ART 
     In a conventional homodyne interferometer system, if a phase difference between a reference light (R) and a signal light (S) is represented as θ, an interferometer output (I) is represented by the following formula:
 
 I=R+S+ 2√{square root over ( )}( R·S )cos θ
 
(I, R and S are the laser power values.)
 
θ=arccosine((I−R−S)/2√{square root over ( )}(R·S))
 
     In the conventional homodyne interferometer system, a sensor output signal is affected by fluctuation of a laser signal generator output level in accordance with temperature drift, which results in variation in an optical fiber transmission path and residual errors in the output value of the sensor output signal because the output value is calculated according to the condition that the reference light and the signal light intensities are constant. 
     Although the conventional homodyne interferometer system is able to detect the phase difference of a half of light wavelength between the reference light and the signal light, it is almost impossible to keep the phase difference between the reference light and the signal light within this narrow range because the components of the conventional homodyne interferometer system do not have excellent dimension accuracy in order to keep the phase difference into the narrow range. Moreover, it is impossible to eliminate, in the conventional homodyne interferometer system, the optical path fluctuation by the influence of temperature variation in the surrounding environmental. Further, it is noted that it is noted possible to extend the this limited measurement range in the conventional interferometer system. 
     Based on the above-noted deficiencies in the conventional homodyne interferometer system, an optical fiber sensor that shows good insusceptibility to a strong electromagnetic field and an extremely high or low temperature environment is desired, in order to provide an optical fiber sensor for low transmission loss over long distances. 
     SUMMARY OF THE INVENTION 
     The present invention has been developed for solving the above-noted deficiencies in the conventional homodyne interferometer system, and the present invention realizes the optical fiber sensor which is free from the residual errors caused by the light intensity fluctuations and is not limited the measurement range limitation of the conventional homodyne interferometer system. The above object of the present invention is achieved by providing a wavelength stabilized laser source, by which a laser pulse having a period T and a pulse width  3   t  (the pulse width including parts t 1 , t 2  and t 3 ) is generated, where the laser pulse is phase modulated such that the phrase of t 1  is equal to the phase of t 2  and there is a 90 degree (independently orthogonal) phase difference between the phase of t 2  and the phase of t 3 . 
     Hereinafter the phase of t 1  is referred to as φ 1 , the phase of t 2  is referred to as φ 2 , and the phase of t 3  is referred to as φ 3 . 
     Further, the purpose of the present invention is achieved by constructing an original optical interferometer. In the present invention, the above mentioned laser pulse is first divided into two (2) laser pulses by an optical coupler, one of which is applied to a t/2 optical delay line and then is supplied to a motion sensor. The motion sensor has a weight on which laser pulse reflecting mirrors are stationed at both sides, and the weight moves according to the externally applied force, thus the sensor changes the optical path length according to the externally applied force. The reflected laser pulse is again applied to the t/2 optical delay line. Hereinafter, the phases of the delayed reflected laser pulse applied to the t/2 delay line are respectively referred to as φ 1   a , φ 2   a , and φ 3   a.    
     The other of the two laser pulses is applied to the other side of mirror on the motion sensor without time delay. Hereinafter, the phases of the non-delayed reflected laser pulse are respectively referred to as φ 1   b , φ 2   b , and φ 3   b.    
     The two reflected laser pulses are combined by the optical coupler and form a Michelson Interferometer. At the output of the Michelson Interferometer, the following five (5) different optical pulse signals are obtained: no optical signal region (Z); φ 1   b  region (R); interferometric region between φ 2   b  and φ 1   a  (I 1 ); interferometric region between φ 3   b  and φ 2   a  (I 2 ); and φ 3   a  region (S). These five signals Z, R, I 1 , I 2  and S form a Time Division Multiplex (TDM) optical signal. 
     The TDM optical signal is transformed into an electric signal by an Optic to Electricity Converter (O/E), and the components z, r, i 1 , i 2  and s are respectively obtained as the electrical derivation of the optical signals of Z, R, I 1 , I 2  and S. The value z is utilized to obtain the precise value of i 1 , i 2 , r and s by cancelling out the residual voltage appears at the output of O/E converter in the period of no optical signal (no optical pulse). 
     The Michelson Interferometer output is able to be calculated by the following formulas:
 
 i 1= r+s+ 2√{square root over ( )}( r·s )cos θ1
 
 i 2= r+s+ 2√{square root over ( )}( r·s )cos θ2
 
where θ is the angle between the two laser pulses reflected by the mirror on the motion sensor.
 
     The values cos θ 1  and cos θ 2  are led by transforming the above formulas as follows:
 
cos θ1=( i 1− r−s )/2√{square root over ( )}( r·s )
 
θ1=arccosine(( i 1− r−s )/2√{square root over ( )}( r·s ))
 
cos θ2=( i 2− r−s )/2√{square root over ( )}( r·s )
 
θ2=arccosine(( i 2· r·s )/2√{square root over ( )}( r·s ))
 
     As the calculation inaccuracy of arccosine function is increased in proportion to cosine value, smaller cosine value is selected between cos θ 1  and θ 2  and angle θ of the selected cosine value is treated as measured datum θ. Then Δθ, which is the increment or decrement of θ in T period, is derived from present θ and the value period T before and arithmetic summation of Δθ is sent out as the sensor output signal. 
     Effects of the Invention 
     The laser signal level fluctuates with various factors, e.g., environmental temperature drift, atmospheric pressure change and so forth, and their electrical representation of r i 1 , i 2 , and s also fluctuate. However, the influence of such drifts are removed in the process of calculating cos θ 1  and cos θ 2  because inaccuracy in cos θ 1  and cos θ 2  is removed as a result of the calculation formula being developed to remove the inaccuracy by the calibration process in the formula by using instant measured values of r, s and i. Accordingly, the final measured datum includes very small measurement error caused from laser level drift, the optical fiber loss fluctuation or optical coupler loss change expected at an aversely severe circumstance. This contributes to reduce the system manufacturing cost as it allows the use of inexpensive components. 
     According to the present invention, cos θ 1  and cos θ 2  are obtained, which are orthogonal in phase each other, with the interferometric phenomenon between signal light and reference light. By this condition that cos θ 1  and cos θ 2  are 90 degree shifted in each other, one of two measured values always lies within ±1/√{square root over ( )}2 where good calculation accuracy from cosine to arccosine transformation is guaranteed. If the transformed value from the smaller cos θ is selected, the value is always smaller than ±1/√{square root over ( )}2 and satisfactory measurement accuracy is assured. Moreover, this does not require precise control of phase between cos θ 1  and cos θ 2 , and as a results, reduces the precision dimension required in component manufacturing. 
     The present invention provides a measurement range of multiple wavelengths because the direction of phase movement in the T period, i.e., Δθ is increased or decreased, is able to be understood by the values and phases of cos θ 1  and cos θ 2 , based on the condition that cos θ 1  and cos θ 2  are orthogonal. 
     Furthermore, the measurement dynamic range is able to be extended largely by shortening the measurement period T, i.e., higher sampling frequency larger dynamic range, because the sensor output is the summation of Δθ and the number of Δθ per second is the proportion of the sampling frequency. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings: 
         FIG. 1  is a block diagram showing an example of constitution of the optical fiber sensor according to the present invention; and 
         FIG. 2  is a time chart showing an example of optical fiber sensor operation according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is the example of the optical fiber sensor of the present invention, where a laser pulse generator  1 , an optical sensor  2 , and an arithmetic processor  3  are illustrated. 
     First, a laser pulse of  3   t  width in every T period is generated in the laser pulse generator  1 , with which the light phase of front  2   t  portion is set to 0 degree (°) and last It portion 90 degree (°) in each other. The laser pulse is divided into two (2) by the optical coupler and one of which is applied to an optical sensor  2  through a t/2 optical delay line. The other half of laser pulse is applied to the other input of the optical sensor  2 . Both laser pulse phases are shifted in proportion to the external force applied to the optical sensor  2 . 
     The reflected laser pulses, one through t/2 optical delay line and the other directly, are combined by the optical coupler and the overall optical circuit forms a Michelson interferometer. The interferometer output laser pulse, i.e., the output optical pulse from the optical coupler is applied to the arithmetic processor  3  through the optical directional coupler in the laser pulse generator  1 . The arithmetic processor  3  detects the optical pulse amplitude and calculates cos θ 1  and cos θ 2 , then transforms cos θ 1  and cos θ 2  into θ 1  and θ 2 , and further obtains θs as the arithmetic processor  3  final output. 
     The output optical pulse of the optical sensor  2  is the Time Division Multiplex (TDM) signal. The TDM signal shows a series of amplitude data, i.e., Z, R, I 1 , I 2  and S, which represent no optical signal region (Z), the φ 1   b  region (R), the interferometric region between φ 2   b  and φ 1   a  (I 1 ), the interferometric region between φ 3   b  and φ 2   a  (I 2 ), and the φ 3   a  region (S). In the arithmetic processor  3 , z, r, i 1 , i 2  and s are obtained as the electrical derivation of the optical signals of Z, R, I 1 , I 2  and S, and cos θ 1  and cos θ 2  are calculated. 
     Next, the operation theory of the optical fiber sensor shown in  FIG. 1  is explained by referring a time chart of the optical fiber sensor  FIG. 2 . The structure of the optical fiber sensor is first described in detail. The sensor is comprised of a laser pulse generator  1 , an optical sensor  2  and an arithmetic processor  3 . 
     In the laser pulse generator  1 , a laser signal generated by a wavelength stabilized laser generator  11  is amplitude modulated by an intensity modulator  13  with a driving pulse signal “a” from a pulse generator  12 , whose driving pulse makes the intensity modulator “on” and “off”, and makes a laser pulse “b” of  3   t  width in every T duration. The laser pulse “b” is further phase modulated by an optical phase modulator  14 , with which the first part of laser pulse t 1  and middle part of it t 2  phases are set to zero (0) degree and the last part of it t 3  is set to 90 degree (π/2) by the driving signal “c”, so that both t 1 , t 2  and t 3  is 90 degree different in relative phase. (Both t 1 , t 2  and t 3  relative phase is orthogonal in each other.) 
     The output laser pulse “d” from the optical phase modulator  14  is sent to the optical sensor  2  through an optical directional coupler  15  as “e 1 ”. Consequently, the laser pulse generator  1  sends out the laser pulse of  3   t  length (the first part t 1 , the second part t 2  and the third part t 3 ), whose light phase is φ 1  at t 1 , φ 2  at t 2  and φ 3  at t 3 . The phase relation is 
     θ1=φ2, φ2 is orthogonal to φ 3 . 
     The laser pulse of  3   t  width from laser pulse generator  1  is applied to an optical coupler  21  in the optical sensor  2  as “e 2 ”. One of the divided laser signals “f 1 ” generated by the optical coupler  21  is led to a t/2 optical delay line  22  and then to a collimator  23 , which transmits the laser signal to a mirror  24  as “f 2 ”. The collimator is used as a beam parallelizing apparatus to send the laser signal to the mirror and receive the laser signal reflected by the mirror in a good condition. 
     The mirror  24  makes the phase change according to the moving distance of mirror by an externally applied force “+j” and the laser pulse signal is returned back to the optical coupler  21  as “f 3 ”, whose  3   t  component phases are respectively referred to as φ 1   a , φ 2   a  and φ 3   a . The other one of the divided laser signals “f 1 ” generated by the optical coupler  21  is led to an another collimator  25  and then transmitted into air so that the transmitted laser signal “g 2 ” is reflected by an another mirror  26 . The reflected laser signal from the mirror  26  is supplied to the optical coupler  21  through the collimator  25  as “g 3 ”, whose  3   t  component phases are respectively referred to as φ 1   b , φ 2   b  and φ 3   b . The mirror  26  receives the external force “j”, opposite from the mirror  25 , as the mirrors  24  and  26  are installed on one (1) moving weight back to back. 
     The reflected laser pulse signals “f 3 ” and “g 3 ” are combined in the optical coupler  21  and to form a Time Division Multiplex (TDM) laser signal “h 1 ”. The laser signal “f 3 ” from the mirror  24  has the delay time of t compared with the reflected laser signal “g 3 ”, because “f 3 ” passing through the t/2 delay line twice. The TDM signal “h 1 ” has five (5) components of amplitude data, i.e., Z, R, I 1 , I 2  and S, which represent a no optical signal region (Z), a φ 1   b  region (R), an interferometric region between φ 2   b  and φ 1   a  (I 1 ), an interferometric region between φ 3   b  and φ 2   a  (I 2 ), and a φ 3   a  region (S). 
     The TDM signal “h 1 ” is applied back to the optical directional coupler  15 . The optical directional coupler  15  makes the routing of the TDM signal “h 1 ” to an O/E converter  301  in the arithmetic processor  3 . The O/E converter  301  transforms the laser pulse signal into an electric signal and the resulted TDM electric signal as “h 3 ”, i.e., z, r, i 1 , i 2  and s are obtained as the electrical derivation of the optical signals of Z, R, I 1 , I 2  and S. The electric signals z, r, i 1 , i 2  and s are supplied to A/D converters  312 ,  313 ,  314 ,  315  and  316  through analog switches  302 ,  303 ,  304 ,  305  and  306  and further through low pass filters  307 ,  308 ,  309 ,  310 ,  311  and  312 . The analog switches distribute the electric signals to the designated low pass filters and the low pass filters filter the high frequency components contained in the electric signals and provide necessary frequency band limitation. 
     The A/D converters  312 ,  313 ,  314 ,  315  and  316  convert z, r, i 1 , i 2  and s into digital values and the resulting digital values are applied to a cosine processor  317 , which makes the calculation necessary to obtain cosine values. In the cosine processor  317 , first r, i 1 , i 2  and s voltages are calibrated by the value of z in order to remove the uncertainty of measured voltage. Then, the processor  317  calculates the values cos θ 1  and cos θ 2  from r, i 1 , i 2  and s. The calculated values of cos θ 1  and cos θ 2  from the processor unit  317  are further transmitted to a Δθ processor  318 . In the Δθ processor  318  first θ 1  and θ 2  are calculated from cos θ 1  and cos θ 2  and then Δθ 1  and Δθ 2 , which are the finite difference of θ 1  and θ 2  in a period of time T. The values Δθ 1  and Δθ 2  are transferred to the next processor, a Σθ processor  319 , an appropriate one is selected from Δθ 1  and Δθ 2  and the selected one&#39;s value is added to θs in each T period. The value θs is the final optical fiber sensor output. 
     In the present invention, the calculations of cos θ 1  and cos θ 2  are done by the Formula 1 and Formula 2 as follows:
 
cos θ1=( i 1− r−s )/2√{square root over ( )}( r·s )  Formula 1
 
cos θ2=( i 2− r−s )/2√{square root over ( )}( r·s )  Formula 2
 
     In the above formulas, r represents φ 1   b  of g 3 , s φ 3   a  of f 3 , i 1  interferometric output between φ 2   b  of g 3  and φ 1   a , i 2  interferometric output between φ 3   b  of g 3  and φ 2   a . Moreover, as the laser signals φ 2   b  and φ 3   b  are orthogonal in each other, the angle difference between θ 1  and θ 2  is 90 degree (°). The Δθ processor  318  makes calculations and procedures listed in (1) to (7) below. 
     (1) Identify the smaller one in the absolute value from cos θ 1  and cos θ 2 . 
     (2) Distinguish the polarities of cos θ 1  and cos θ 2 . 
     (3) The values of cos θ 1  and cos θ 2 , and the conditions of (1) and (2), decide the θ point on the cosine curve. 
     (4) Calculate θ 1  and θ 2  from the result of (3) 
     (5) Name obtained θ 1  and θ 2  as θ 1   a  and θ 2   a    
     (6) Calculate the difference Δθ
 
Δθ1=θ1−θ1 a  
 
Δθ2=θ2−θ2 a  
         where Δθ is the difference between θ 1  of the previous datum and the new datum (θ 1   a  or θ 2   a ).
 
(7) Select the smaller absolute value of Δθ from Δθ 1  and Δθ 2 .
       

     Send out selected Δθ to the Σθ processor  319 . 
     The selected Δθ is added to the θs in the Σθ processor  319  and the θs is the optical fiber sensor output signal. There is an important assumption that the instant phase change of sensed laser signals f 3  and g 3  shall not exceed ±π/2 in a sample period of T. 
     Operation and Effects 
     According to the present invention, cos θ 1  and cos θ 2  can be correctly positioned on a cosine curve if Δθ, which is the increment or decrement of θ in T period, is within 90 degrees. Further, as a precise phase change can be measured, if the increment or decrement of θ in T period is kept within 90 degrees, the present invention can extend the dynamic range by increasing the integration number and overcomes the limit of ±90 degree, which the conventional system cannot. 
     The present invention assumes the case that the example of the phase difference of reference light is independently orthogonal. The same effect can be obtained if the phase difference of signal light is independently orthogonal. Also, the discussion until now is made with the assumption that the phase difference of the reference light is ±90 degree, but the same effect can also be obtained by the phase difference is fixed to approximately ±90 degree. 
     Moreover, the discussion until now is made with the assumption that the delay time of the optical fiber is t/2, but the same effect can be achieved by a delay time of around t/2. In the example of optical sensor  2 , two mirrors are used to increase the sensor sensitivity. However, the system can be modified so as to use one (1) mirror. 
     The aforementioned embodiment, the construction of the interferometer of the sensor section is explained with the Michelson interferometer system, but the similar operation is possible using the Mach-Zehnder interferometer system. 
     INDUSTRIAL APPLICABILITY 
     The present invention is applied for the sensor system which is installed at the remote location, the strong electromagnetic field, extremely high and low temperature environment and the area which cannot have power supply. 
     EXPLANATION OF LETTERS OR NUMERALS 
     
         
           1  laser pulse generator 
           2  optical sensor 
           3  arithmetic processor 
           11  wavelength stabilized laser generator 
           12  pulse generator 
           13  intensity modulator 
           14  optical phase modulator 
           15  optical directional coupler 
           21  optical coupler 
           22  optical delay line 
           23 , 25  collimator 
           24 , 26  mirror 
           301  O/E converter 
           302 - 306  analog switch 
           307 - 311  low pass filter 
           312 - 316  A/D converter 
           317  cosine processor 
           318  Δθ processor 
           319  Σθ processor