Abstract:
A method for monitoring changes in a gap which corresponds to changes in a particular environmental parameter using a tunable laser and interferometer at high frequency is disclosed. The laser light provided to the interferometer is swept through a small range of wavelengths. Light modulated by the interferometer is detected and a non-sinusoidal light intensity output curve is created, a reference point on the curve identified and subsequent sweep of the laser performed. The difference in time, wavelength, or frequency at the occurrence of the reference point between the two sweeps allows for measuring the relative changes in the gap and, as a result, the change in the environmental parameter.

Description:
This application claims priority from U.S. Provisional Patent Application Ser. No. 60/637,967, filed Dec. 21, 2004. 

   FIELD AND BACKGROUND OF THE INVENTION 
   The present invention is generally related to tunable lasers, and more particularly to tunable lasers used for optical measurements in a dynamic environment. 
   Among other things, tunable lasers find utility in measurements relying upon the observation of light reflected or transmitted from a sensor using a Fabry-Perot interferometer. In such methods, the Fabry-Perot gap will move through a range of displacements, and the displacement range of the sensor is defined by the tuning range of the laser. Sensors with small gaps require lasers with a large tuning range and sensors with large gaps require lasers with a small tuning range. 
   For example, in co-pending U.S. patent application Ser. No. 11/105,651, entitled Method and Apparatus for Continuous Readout of Fabry-Perot Fiber Optic Sensor, Applicant disclosed a tunable laser with 40 nm tuning range as the light source to use for measurement of the absolute gap distance of a Fabry-Perot sensor with gaps ranging from 60 to 80 μm. The time required for a laser to sweep through a 40 nm tuning range is acceptable for making absolute measurements of static environmental parameters such as downhole oil and gas applications but far too slow for making relative measurements of dynamic environmental parameters such as engines and turbines. In the current prior art, time periods greater than 100 ms are needed to tune a laser through a 40 nm range. Thus any process to be measured with a Fabry-Perot sensor that changes in times shorter than 100 ms (10 Hz) cannot be measured with such a laser. 
   In U.S. Pat. No. 5,276,501 to McClintock et al. another tunable laser method is described. In this case, the method requires observation of two separate references points on the output curve of the detector associated with the interferometric sensor as a tunable laser is swept through its range of wavelengths. Moreover, McClintock assumes that this curve must be sinusoidal in nature (according to a two-beam interferometric model). As shown in the description of one embodiment of the invention, the teachings and assumptions in McClintock ultimately limit the operation and capabilities of the system and method described in McClintock. 

   
     DESCRIPTION OF THE DRAWINGS 
     Objects and advantages together with the operation of the invention may be better understood by reference to the following detailed description taken in connection with the following illustrations, wherein: 
       FIG. 1  is a schematic representation of sensor and optoelectronics of the present invention. 
       FIG. 2  is a graphical representation of tuning ranges versus time for two different lasers. 
       FIGS. 3   a - 3   d  are graphical representations of non-sinusoidal light intensity output curves generated by sensor (shown in the lower plot of each Figure) as the laser is tuned through the corresponding range (shown in the upper plot of each Figure). In  FIG. 3   a , a low finesse sensor with a static gap is used and the minima occur at the same wavelength and time in each sweep of the laser. In  FIG. 3   b , a high finesse sensor is used with a static gap, and the minima occur at the same wavelength and time in each sweep of the laser. In  FIG. 3   c , a low finesse sensor is shown where the gap is changing, and the minima occur at a different wavelength and time in each sweep of the laser. In  FIG. 3   d , a high finesse sensor has a changing gap, and the minima occurs at a different wavelength and time in each sweep of the laser. 
       FIG. 4  is a schematic representation of a long gap sensor with an embedded reflector. 
       FIG. 5  is a schematic representation of a long gap sensor coupled to a collimating lens. 
   

   DETAILED DESCRIPTION 
   While the present invention is described with reference to the preferred embodiment, it should be clear that the present invention should not be limited to this embodiment. Therefore, the description of the preferred embodiment herein is illustrative of the present invention and should not limit the scope of the invention as claimed. Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. 
   Reference will now be made in detail to the preferred embodiment of the invention as illustrated in the accompanying drawings. The present invention discloses a new approach for using a tunable laser source  10  to make measurements of a Fabry-Perot gap  12  that is dynamically changing. The Fabry-Perot sensor  14  and optoelectronic configuration  16  is shown in  FIG. 1  and the laser-tuning characteristic is shown in  FIG. 2 . As illustrated in  FIG. 1 , the gap  12  in the Fabry-Perot sensor  14  is formed by a pair of at least partially reflective elements  18 ,  20 . In one embodiment, the reflective elements  18 ,  20  may be partially reflective mirrors separated by the gap  12 . 
   This approach requires a tunable laser source  10  such as a tunable laser that can provide rapid switching in fine increments or continuous sweeping over a narrow band of wavelengths with temporal repeatability. The laser  10  may be tunable over any spectral band such as the infrared spectral band from 1500 to 1600 nm. 
   Notably, the sensor gap  12  is configured to be long so that a laser with a small tuning range may be used and the sweep rate may be rapid. Thus, the system described herein is capable of operating at much faster rates than previously known systems increasing the frequency response from 2 Hz to over 1000 Hz. Although it would be necessary to tune the laser  10  through at least two minima in the interference pattern if an absolute measurement were desired, the laser  10  only need be tuned through one minimum to make a relative measurement of changes in the gap  12 . 
   Note that the tuning range varies inversely with the length of the Fabry-Perot gap  12 :
 
Δν= c/ 2 G   (1)
 
   where Δν is the tuning range in Hz for a gap G=30 μm. 
   For a laser  10  operating at 1550 nm, a tuning range of 40 nm in wavelength corresponds to a tuning range Δν=5×10 12  Hz. If the gap  12  is increased by 40 times from 30 μm to 1200 μm=1.2 mm, then the laser tuning range is reduced 40 times so that Δν=125×10 9  Hz. In wavelength space, assuming a 1550 nm light source, the corresponding tuning range would be reduced from 40 nm to 1 nm. The sensor gap  12  may consist of a fiber  22  with an embedded reflector  24  as shown in  FIG. 4  or a fiber  26  coupled to a collimating lens  28  as shown in  FIG. 5 . 
   In an embodiment, as illustrated in  FIG. 4 , the optical fiber  22  may be a single mode fiber and the embedded reflector  24  may be a fiber Bragg grating  24 . The optical fiber  22  may have a numerical aperture of 0.1. A single mode light containment waveguide  30  may extend from the fiber Bragg grating  24 , and a reflective element  32  may be positioned a distance  34  away from the free end of the waveguide  30 . In an embodiment, the distance  34  between the free end of the waveguide  30  and the reflective element  32  may be 20 μm or less. The Fabry-Perot gap  12  may be formed between the fiber Bragg grating  24 , which acts as the first reflective surface of the gap  12 , and the reflective element  32 , which acts as the second reflective surface of the gap  12 . In an embodiment, the gap  12  may be approximately 1.2 mm. 
   In an embodiment, as illustrated in  FIG. 5 , the optical fiber  26  may be either a multimode or single mode fiber. The optical fiber  26  may have a numerical aperture equal to 0.2. A substrate  36 , such as a glass window, is positioned proximate to the end of the optical fiber  26 . A reflector  38  may be located on one side of the substrate  36  to form the first reflective surface of the Fabry-Perot gap  12 , and a reflective element  40  may be positioned a distance away from the reflector  38  to form the second reflective surface of the Fabry-Perot gap  12 . In an embodiment, the Fabry-Perot gap  12  may be approximately 1.2 mm. The collimating lens  28  may be positioned to direct light from the optical fiber  26  to the Fabry-Perot gap  12  and to receive light reflected from the Fabry-Perot gap  12 . 
   Lasers are available and can be swept over the range at 1 k Hz rates with a 200 pm tuning range and 0.2 pm resolution and with a 1000 pm range and 1 pm resolution. Thus, use of a high-speed tunable laser makes possible the application of Fabry-Perot sensors in processes where changes are taking place on millisecond time scales (1000 Hz rates). 
   The trade-off with high-speed tunable lasers is the uncertainty in the laser wavelength and this uncertainty determines the resolution of the system. If a laser is tuned over 1 nm in 1 millisecond and the desired resolution is 0.1%, then the laser wavelength must be repeatable to better than 1 pm. 
   A schematic of this configuration  16  is shown in  FIG. 1 . Infrared light from the laser  10  is injected into a multimode (or single mode) optical fiber  42 . It passes through a power splitter  44  and to a Fabry-Perot sensor  14  and returns to a photodiode detector  46  where the light signal is converted to a photocurrent and amplified for processing in a signal conditioner (not shown) connected to the detector  46 . The photodiode material may be InGaAs for detection of infrared light at 1550 nm. 
     FIG. 2  shows exemplary plots of the preferred laser tuning. Notably, the laser tuning must be controllable and repeatable. That is, each sweep of the desired wavelength range from λ 1  to λ 2  must start and stop at times that are known precisely. Notably, λ 1  and λ 2  should be selected to maximize the speed of operation. In an embodiment illustrated by  FIG. 2 , a plot  48  of the wavelength of laser light over time shows a range of 1000 pm, i.e., λ 1  equal to 1550.0 nm and a λ 2  1551.0 nm, with a resolution of 1 pm. The sweep through the 1000 pm range is 1 ms in duration and may be continuously repeated. In another embodiment illustrated by  FIG. 2 , a plot  50  of the wavelength of laser light over time shows a range of 200 pm, i.e., λ 1  equal to 1550.0 nm and a λ 2  1550.2 nm, with a resolution of 0.2 pm. The sweep through the 200 pm range is 1 ms in duration and may be continuously repeated. While exemplary values for λ 1  and λ 2  are disclosed herein, other values are possible depending upon the particulars of the intended application. 
   The light intensity reflected back to the signal conditioner from the Fabry-Perot gap  12  is modulated as the gap  12  changes. The ratio of the incident-to-reflected intensity IR is a function of both the laser frequency and the gap  12  and is given by: 
                   IR   ⁡     (     v   ,   G     )       =       F   ⁢           ⁢       sin   2     ⁡     [       (     2   ⁢   π   ⁢           ⁢   vG     )     /   c     ]           1   +     F   ⁢           ⁢       sin   2     ⁡     [       (     2   ⁢   π   ⁢           ⁢   vG     )     /   c     ]                     (   2   )               
where:
         c=Δν is the velocity of light   ν=1.93×10 14  Hz is the frequency of the infrared light   λ=1550×10 −9  m (1550 nm) is the wavelength   G is the Fabry-Perot gap distance   F=4R/(1−R) 2      R=(R1*R2) 1/2  is the composite reflectance       
     FIGS. 3   a - 3   d  show exemplary plots of the non-sinusoidal light intensity output curve (IR(ν,G)) in varying situations, along with the corresponding plot of the laser light tuning. As the laser is tuned through its range (see the upper plot in each figure), for any given gap G, the reflected intensity ratio measured by the photodiode will appear as in the lower plot. Although the minimum intensity value is the preferred feature, software can be instructed to locate any feature (i.e., a reference point) on the intensity output curve. For each sweep of the laser, the precise time after the beginning of the sweep at which the feature occurs in the intensity ratio IR(ν,G) is used to determine the relative gap of the sensor. Time correlates with laser wavelength/frequency, and time, wavelength, or frequency may be used to determine the relative gap of the sensor. The change in the gap between any two sweeps can be determined by measuring the difference in precise location of the feature in time, wavelength, or frequency. The absolute value of the gap is not important in this application, only changes that occur in the gap between each subsequent sweep of the laser. The system output consists of only dynamic changes in gap where the frequency response is as fast as the laser sweep rate (1 kHz in this example). 
   The smallest change (resolution) in the gap δG that can be measured is determined from Equation 3, where G is the gap.
 
δ G/G=δν/ν   (3)
 
   The incremental change (resolution) in laser frequency is (1/1000) of the tuning range, which is 125 GHz in this example. The laser frequency ν=193.5×10 12  and using Equation 3, δG is calculated to be:
 
δ G =(1.2×10 −3 )(1.25×10 8 )/(193.5×10 12 )
 
δG=775 pm
 
   
     
       
             
             
             
           
         
             
                 
                 
             
           
           
             
                 
               Tuning Range 
               1.25 × 10 11  Hz or 1000 
             
             
                 
                 
               pm at 1550 nm wavelength 
             
             
                 
               Tuning Resolution 
               1.25 × 10 8  Hz or 1 pm 
             
             
                 
                 
               at 1550 nm wavelength 
             
             
                 
               Gap Displacement Range 
               775 nm 
             
             
                 
               Gap Resolution 
               775 pm