Abstract:
The output wavelength of a laser-diode pumped fiber source can be stabilized at its minimum value irrespective of temperature and the age of the laser diode by the seeking the minimum output wavelength of the fiber source. By introducing a dither on the temperature of the pump laser diode, a measurement feedback loop can be used to determine the optimal value of the temperature. Once determined, the temperature is used to fix the wavelength of the source.

Description:
BACKGROUND OF THE INVENTION 
     The apparatus and method described here enables one to stabilize the wavelength of the output of a fiber light source, such as an erbium-doped superfluorescent source. In turn, this will improve the scale-factor stability of an interferometric optical sensor. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a graph of the typical characteristic of the output wavelength vs. the pump source wavelength in a fiber light source; 
     FIG. 2 is a graph of the relationship between the cooler temperature and the laser diode wavelength for a laser diode pump source; 
     FIG. 3 is a graph of the relationship between the thermoelectric cooler temperature and the output wavelength of a fiber light source; 
     FIG. 4 is a graph of the relationship between the thermoelectric cooler temperature and the 2B voltage applied to a phase shifter in an optical interferometer; and 
     FIG. 5 is a schematic diagram of an optical fiber source with emission wavelength stabilization. 
    
    
     DESCRIPTION OF THE INVENTION 
     Fiber light sources are disclosed in U.S. Pat. No. 5,136,600, issued on Aug. 4, 1992, to Fidric et al. for a Stabilization Apparatus and Method for an SFS, and U.S. Pat. No. 5,177,562, issued on Jan. 5, 1993, to Wysocki et al. for a Stability Compensated Broadband Source and Fiber Interferometer, incorporated by reference herein. Optical fiber sources use a length of rare earth element-doped optical fiber energized or “pumped” by a “pump” light source. The pump light source produces light having a wavelength in the “absorption band” of the fiber. When the light from the pumped source is absorbed by the fiber, spontaneous emission occurs and light is produced in the “emission band” of the fiber. 
     The typical characteristic of the output wavelength vs. the pump source wavelength is shown in FIG.  1 . This curve shows that there is a unique emitted wavelength λ e * for a given pump source. The relationship can be expressed in equation form as: 
     
       
         λ e =a (λ p −λ p *) 2 +b  (1) 
       
     
     where λ e  is the emission wavelength; 
     λ p  is the actual pump wavelength; 
     λ p * is the characteristic pump wavelength; and 
     a and b are constants. 
     The pump source in a fiber light source is typically a laser diode mounted on a thermoelectric cooler. By varying the temperature of the cooler, the laser diode output wavelength can be altered. The relationship between the cooler temperature and the laser diode wavelength is generally linear, as illustrated in FIG. 2, and can be expressed as: 
     
       
         λ d =a′T TEC +b′  (2) 
       
     
     where a′ and b′ are constants. 
     As the laser diode ages or is exposed to different environments, its behavior may change, perhaps causing the slope and intercept of the curve of FIG. 2 to change. This will alter the wavelength of the fiber light source output. 
     Since the diode output wavelength is linearly proportional to the thermoelectric cooler temperature, thermoelectric cooler temperature can be substituted for the fiber pump wavelength. Combining FIGS. 1 and 2, as shown in FIG. 3, the output wavelength is at a minimum value at a single value of the cooler temperature, as also expressed in the following equation: 
     
       
         λ e =a″(T TEC −T TEC *) 2 +b″  (3) 
       
     
     where T TEC  is the actual thermoelectric cooler temperature; 
     T TEC * is the characteristic thermoelectric cooler temperature; and 
     a″ and b″ are constants. 
     Thus, there is one value T TEC * of thermoelectric cooler temperature at which the fiber will have a unique emission wavelength λ e *. 
     An optical interferometer with a controllable optical phase shifter can be used to determine when the unique emission wavelength has been reached. One suitable such interferometer is the Sagnac interferometer, described in U.S. Pat. No. 5,278,631, issued Jan. 11, 1994, to Hollinger et al., for a Closed Loop Fiber Optic Gyroscope with Signal Processing Arrangement for Improved Performance; U.S. Pat. No. 5,280,339 issued Jan. 18, 1994, to Hollinger et al., for a Closed Loop Fiber Optic Gyroscope with Fine Angle Resolution; and U.S. Pat. No. 5,309,220, issued May 3, 1994, to Hollinger et al., for a Closed Loop Fiber Optic Gyroscope with Reduced Sensitivity to Electronic Drift; all incorporated herein by reference. 
     The phase shift imparted in the interferometer by the phase shifter is: 
     
       
         φ=2πnL/λ  (4) 
       
     
     where φ is the imparted phase shift; 
     n is the waveguide index of refraction; 
     L is the length of the phase shifter region; and 
     λ is the wavelength of the fiber. 
     The phase shifter alters the phase by changing the index of refraction in response to an input voltage. The relationship between the input voltage and the phase shift can be stated as: 
     
       
         Δn=KΔV  (5) 
       
     
     where K is a constant describing the phase shifter transfer function. 
     The phase shift can then be stated as a function of input voltage by combining Eqs. 4 and 5: 
     
       
         Δφ=2πKΔVL/λ  (6) 
       
     
     For a phase shift of 2π, the voltage shift (known as the “2π voltage”) is: 
     
       
         ΔV 2π =λ/(KL)  (7) 
       
     
     Thus, the voltage applied to the phase shifter to obtain a 2π phase shift will be proportional to the wavelength of the source. The relationship between the 2π voltage and the cooler temperature can be determined by substituting λ e  in Equation 3 for λ in Equation 7: 
     
       
         V 2π =[a″(T TEC −T TEC *) 2 +b″]/(KL)  (8) 
       
     
     where V 2π  is the voltage applied to the phase shifter. 
     The curve in FIG. 4 illustrates this relationship. 
     The voltage V 2π  can be monitored to provide an indication of the fiber emission wavelength. Thus, to find λ e * regardless of the condition of the pump laser diode, one need only determine when V 2π  is at a minimum, i.e., V 2π *. A circuit arrangement that will achieve this is illustrated in FIG.  5 . 
     A laser diode  10  excites a fiber light source  20 . The temperature of the laser diode  10  is maintained by a thermoelectric cooler  12  controlled by a thermoelectric cooler controller  14 . In the system of FIG. 5, the light output of the fiber light source  20  passes through a directional coupler  30  into an optical interferometer  40  with a controllable optical phase shifter  42 . 
     The output of the interferometer  40  passes through the coupler  30  to a photo detector  50 . The photo detector  50  produces an error voltage proportional to the phase difference in the interferometer. This error voltage is processed by a peak-to-peak detector  60 , an integrator  70 , and a ramp generator  80 , the last component generating an error signal for the phase shifter  42 . The interferometer  40 , the photo detector  50 , the peak-to-peak detector  60 , the integrator  70 , the ramp generator  80 , and the phase shifter  42  collectively comprise a rate feedback loop  90 . A dither signal generator  92  introduces a square wave dither signal in the loop at a summing node  94  and also provides a synchronization signal for the peak-to-peak detector  60 . 
     The voltage V 2π  is generated by a scale factor stability loop  100 , discussed and shown in detail in FIG. 5 of U.S. Pat. No. 5,278,631. In FIG. 5, the scale factor stability loop  100  comprises a V 2π  calculator  110 . The value V 2π  is the voltage required for the phase shifter  42  to impart a 2π phase shift to the light traveling through it. Since the output of the interferometer  40  is characterized by a cosine function, a phase change of 2π results in no change in the output. Additionally, a change in the wavelength of the light entering the interferometer  40  will be reflected in the output of the photo detector  50  and ultimately in the detected value ΔV 2π . Therefore, changes in this value can be used to track changes in wavelength of the fiber light source  20 . 
     A ΔT generator  200  generates a square wave signal that dithers the voltage passing through summing node  210  and applied to the thermoelectric cooler controller  14 , causing the temperature of the cooler  12  to dither between a high temperature state “a” and a low temperature state “b.” This causes the wavelength λ of the output of the laser diode  10  to vary proportionately and, in turn, the output wavelength λ e  of the fiber light source  20  varies. 
     Assuming constant rate input to the interferometer  40 , the rate feedback loop  90  stabilizes, but the value of V 2π  varies proportionately with the output wavelength λ e  of the light source  20  (see Eq. 7 above). The V 2π  voltage is then demodulated in a V 2π  peak-to-peak detector  220  against the output of the ΔT generator  200 , and the difference is integrated by an integrator  230 , yielding the cumulative error over time. The error signal is fed back to the thermoelectric cooler controller  14  to drive the error to zero. When the value of T TEC *, the characteristic thermoelectric cooler temperature, reaches the null point, ΔV 2π  will be zero, and the wavelength will then be stabilized. At this value of T TEC *, the value of V 2π  is stabilized and the output wavelength λ e  of the fiber light source  20  is at its desired value, and therefore the operating temperature for the thermoelectric cooler  12  is optimal.