Abstract:
A high scan rate spectroscopic system converts a narrow-band laser pulse into a multispectral pulse, using, for example, a nonlinear fiber. The multispectral pulse is then converted to a swept frequency pulse through a second fiber impressing a frequency-dependent delay in the light beam which is then applied to the object to be tested.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is based on provisional application 60/402,492 filed Aug. 8, 2002 and entitled “High Speed Swept Frequency Spectroscopic System” and claims the benefit thereof. 

   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   BACKGROUND OF THE INVENTION 
   The present invention relates to spectroscopy systems and in particular to a high-speed spectroscopy system suitable for analyzing highly dynamic systems. 
   Spectroscopy studies the interaction between a material or materials and different frequencies of light to characterizes the spectral response of materials, such as gases, liquids, aerosols, solids, particulates, fiber-optic components etc. as may be related to physical properties of the material under test (e.g., temperature, pressure, velocity, composition, size, stress/strain. The interaction studied can be absorption, or reflectivity, scattering, fluorescence, etc. 
   The material being studied by spectroscopy may alternatively be a sensor constructed to modify particular light frequencies based on a measured parameter. One type of sensor is a fiber-Bragg grating (FBG) in which a fiber optic is treated to reflect a single frequency of light passing along its length. The reflected frequency is a function of a periodic variation in the fiber&#39;s index of refraction impressed on the length of the fiber. Physical changes in the fiber, such as caused by strain or thermal expansion, change the frequency of this reflected light allowing indirect measurements of temperature and strain. 
   High speed spectroscopy may be performed by applying a multispectral pulse to the material being studied then collecting the light modified by the material with a fiber optic which provides a variable delay in frequencies of the light pulse proportional to light frequencies. Spectroscopic analysis can then be performed by detecting the change in intensity of the light as a function of time and relating the time scale to frequency. Such a system is described in “Time of Flight Optical Spectrometry with Fiber Optic Wave Guides” by William Whitten published in Analytical Chemistry, Volume 54, Number 7, June 1982. In this device, a narrow band laser illuminates a chamber containing CCl 4  to create the multispectral pulse, which is then used to illuminate a test cell. 
   The CCl4 chamber is cumbersome and causes a loss of coherence in the light signal from the laser. Coupling the light from the test cell to the fiber is difficult. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention provides an improved spectroscopy system in which a short multi-frequency pulse is received directly by a fiber optic cable to spread the frequency in time prior to its being transmitted into a test cell. This approach significantly reduces the losses in coupling light to the optic fiber and avoids the measurement of unwanted nonlinear processes as might occur under simultaneous excitation of the test material with multiple frequencies of light. 
   In a preferred embodiment, the multifrequency light source is created by a laser pulse transmitted through a non-linear fiber to broaden its spectrum. The laser, non-linear fiber, and frequency spreading fiber may be easily joined with minimal coupling losses to produce a robust light source. 
   Specifically then, the present invention provides a spectroscopy system having a light source producing a multifrequency pulse of light followed by a frequency dependant fiber optic cable receiving the multi-frequency light pulse to produce a frequency varying light signal. A test station holding a material to be tested is positioned to receive the frequency varying light signal from the fiber optic cable and a detector receives light from the test station to characterize the light intensity as a function of time. 
   Thus, it is one object of the invention to provide an improved spectroscopic system with superior light coupling. 
   The light retarder may be a length of optical cable providing increased delay in higher frequencies of the light pulse. 
   Thus, it is another object of the invention to provide a system having optical components that are simply joined with reduced light loss. 
   The light source may be light source a laser providing a narrow frequency pulse to a non-linear optical cable. 
   Thus, it is another object of one embodiment of the invention to provide a simple wavelength agile light source for spectroscopy. 
   Alternatively, the light retarder may be a length of standard fiber optic cable or cable of the type used to compensate for frequency spreading in standard communication fibers. 
   It is another object of the invention to reduce fiber length and loss by using compensation fiber intended to compensate for spectral shifting in conventional communication optical fibers. 
   The light retarder might be a combination of at least two consecutive lengths of fiber optic cable, a first providing increased delay in lower frequencies of the light pulse, and a second providing increased delay in higher frequencies of the light pulse. 
   It is thus another object of the invention to provide a method of adjusting the linearity of frequency sweep through the selection of different cable types and lengths. 
   The system may include a spectral filter receiving the frequency-varying light signal to selectively pass only a range of frequencies of the frequency-varying light signal. 
   Thus, it is another object of the invention to provide a versatile system that may be easily adjusted to scan through different subsets of a larger range. 
   The system may include a test element receiving the frequency-varying light signal and a sensor providing an amplitude measurement of the frequency-varying light signal after passing through the test element. 
   Thus it is another object of the invention to provide a system for absorption spectrometry. 
   Alternatively, the test element may be a sensor modifying frequencies of the frequency-varying light signal according to a sensor parameter being measured. 
   Thus it is another object of the invention to provide a light source suitable for use in interrogating sensors having predefined optical characteristics. 
   The invention enables a method of monitoring a structure, comprising the steps of attaching to the structure a plurality of light transducers, each monitoring a physical parameter and modulating received light by absorption of at least one frequency of light according to the parameter being monitored, wherein each light transducer has a different absorption frequency. The light transducers are illuminated with multiple frequencies of light, and absorption is monitored from light reflected from the structure and the light transducers to detect frequencies of absorption. 
   Thus it is another object of the invention to provide for noncontact sensing of multiple sensors, each keyed to a particular frequency in a swept frequency range, thus providing an effective frequency multiplexing of these sensors. 
   These particular objects and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a simplified block diagram of the optical path of the present invention, showing time domain and frequency domain plots of a light pulse as it passes along the path; 
       FIG. 2  is a simplified representation an upper portion of a combustion chamber of a reciprocating engine, such as may provide a test chamber for absorption spectrometry using the present invention; 
       FIG. 3  is a simplified diagram of an application of the present invention to noncontact sensing in which frequency-varying light is reflected off of a number of sensor elements on a moving object; and 
       FIG. 4  is a diagram showing the frequency-varying light before and after reflection by the sensors of  FIG. 3 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring now to  FIG. 1 , the agile laser  10  of the present invention employs a fiber laser  12  such as a passively mode-locked Er-doped fiber laser such as is sold by IMRA under the trade name Femtolite, commercially available from IMRA of Ann Arbor, Mich. 
   In the preferred embodiment, the fiber laser  12  is controlled by associated trigger electronics  20  to produce a three-hundred femtosecond pulse of 1.56-micrometer light at a repetition rate of forty-eight MHz. Each of these short pulses  14  has a narrow frequency bandwidth  22  (spectra), being substantially 0.02 micrometers in frequency width. The average power of the pulses  14  is sixty milliwatts with a peak power of five kilowatts. 
   The short pulses  14  are communicated through a fiber coupler  16  of the fiber laser  12  to a nonlinear fiber  18 . The nonlinear fiber  18  may be two-hundred meters of PM-HN-DSF fiber available from Sumimoto Electric Industries of Osaka, Japan. Nonlinear processes such as self-phase modulation, four-photon mixing, and stimulated Raman scattering act in concert to expand the spectrum  22  of pulse  14  to a broader spectrum  26  encompassing a wavelength range of 1.2 to 2.2 micrometers. After the pulses  14  pass through the nonlinear fiber  18 , they nevertheless retain their time domain characteristic as pulses  24 , although attenuation in the nonlinear fiber  18  and its coupling reduce the power to thirty milliwatts. 
   Alternatively, the fiber laser  12  and non-linear fiber may be replaced with a pulsed LED producing a multispectral pulse or a wide spectrum laser. 
   The pulses  24  are received by a standard dispersion-shifted fiber optic cable  28 . Fiber optic cable  28  is commercially available from the Corning Company of Corning, N.Y., under the trade name of MetroCor and may be 5.55 kilometers in length. The fiber optic cable  28  increases the delay of the blue end of the spectra of frequencies making up pulses  24 , causing a spreading of the pulses  24  into pulses  30  being approximately twenty nanoseconds long and having a center frequency that increases monotonically over time as indicated by frequency scan  33 . The pulses  30  exit the fiber optic cable as collimated beam  32 . 
   Alternatively, a dispersion-compensating fiber may be used for the fiber optic cable  28  in which greater dispersion is obtained at similar attenuation. Such dispersion-compensating fiber is used to correct for frequency spreading of standard fiber optic cables used for communications and the like and provide increased delay to the red end of the spectrum to produce a signal with a monotonically decreasing center frequency with time. 
   A combination of dispersion compensating and standard fibers may also be used for fiber optic cable  28  to achieve linear scans when necessary or otherwise control the time function of the frequency sweep to a desirable curve. 
   The fiber optic cable  28  may be adjusted in length to control the scan rate and effectively the line width (frequency span) of the collimated beam  32  on an instantaneous basis, this being the product of the scan rate and the laser pulse duration. Using this technique, a signal with an effective line width of 500 MHz can be scanned from 1350 to 1550 nm every 20 ns. 
   The collimated beam  32  output from fiber optic cable  28  is received by an off-axis parabolic mirror  34  and directed to a tunable band pass filter  36  comprised of a diffraction grating  38  and parabolic mirror  40 . After reflection off the parabolic mirror  34 , the collimated beam  32  passes through a beam-splitter  42  to strike grating  38  and to be broken spatially into its constituent frequencies as it is reflected to the surface of parabolic mirror  40 . One or more knife edges  44  positioned at the surface of the parabolic mirror  40  may be used to control the upper and lower range of frequencies present in the collimated beam  32 , which then is reflected back from parabolic mirror  40  to grating  38 . 
   The grating  38  effects a second diffraction that recreates the collimated beam  32  and directs it back to the beam-splitter  42  that sends it ultimately to a sensor  46  as a collimated, truncated, frequency-swept beam  48 , having band-limited spectral characteristics  50 . The sensor  46  may be a balanced 1-gigahertz bandwidth InGaAs detector sampled at twenty giga samples per second. 
   The signal from the sensor  46  may be received by processing electronics  52 , which may calculate absorption (−ln(I/I 0 )) of the collimated, truncated, frequency-swept beam  48 . 
   Referring now to  FIG. 3 , for use in spectroscopy, before being received by the sensor  46 , the coherent wavelength agile beam  48  may be directed to a beam-splitter  53  and a portion  54  directed through a transparent window  56  of a chamber  58 . After passing through the chamber  58 , the portion exit a second opposed transparent window  60  to be received by sensor  46 . A second portion  55  of the signal  48  may be received by a second sensor  62 . Signals from these two sensors  46  and  62  may be compared as the values I and I 0  to calculate absorption and to eliminate the effects of variations in the spectra  26  and noise in the laser  12 . 
   The chamber  58  may be filled with a gas, liquid, or solid material and in one preferred embodiment, may be the combustion chamber of a reciprocating engine. Here, the high scan rate and sampling speed of the present invention is well suited to the dynamic environment of combusting gases. In the preferred embodiment, one thousand consecutive scans may be recorded in a twenty-microsecond time and average to produce the desired spectra. 
   Referring now to  FIG. 4 , the high frequency scanning and broad frequency range of the present invention makes possible a frequency multiplexed reading of multiple noncontact sensors. In such an application, the beam  48  is received by a beam-splitter  64 , diverting a portion  65  to beam-spreading optics  66 . The beam spreading optics  66  direct a broad beam  68  to a surface  70  of an object  72  at which measurements are to be taken. The surface  70  may expose a variety of sensors  74 , having a characteristic that they convert a desired measured quantity, for example, temperature, or strain, into absorption of received light from the broad beam  68 . The sensors  74 , for example, may be fiber-BRAGG gratings, well known for temperature and strain measurement. As mentioned above, when a fiber-BRAGG grating is expanded or compressed, its grating spectral response is changed. 
   In the application of  FIG. 4 , each of the sensors  74  exposes a cut end of a fiber-BRAGG grating to the broad beam  68  to receive the broad beam  68  into its interior. The sensors  74  are selected to each have a different absorption wavelength, and absorption wavelengths that will be nonoverlapping within the expected range of the measured parameter of stress or temperature. 
   The surface  70  will therefore reflect the broad beam  68  with variable absorption in a number of separate bands whose precise frequency is determined by the parameters measured by the sensors  74 . 
   The returning light  67  reflected off surface  70  will pass backwards through the optics  66  to beam-splitter  64  to be received by sensor  46 . A second sensor  62  may be placed to receive beam  48  directly as it passes through beam-splitter  64  for normalization purposes, as has been described above. 
   Referring now to  FIGS. 4 and 5 , the pulses  30  of beam  65  striking surface  70  will have a varying frequency content per frequency scan  33  and a substantially constant normalized on-state intensity I i . The returned light  76  will have an intensity similar to that of pulse  30  but for the absorption of particular bands  78  by the different sensors  74 . Calculation of the absorption at these different frequency bands (distinguished by their time delay after the start of the pulse  76 ) allows simultaneous noncontacting measurement of multiple sensors  74  in a brief period of time as may be necessary if surface  70  is a moving part of a machine such as a turbine blade. 
   The laser  12 , described above may be replaced with fiber-pigtailed, edge-emitting, super-luminescent light-emitting diodes, which may produce 40-nanometer-wide, 1-nanosecond pulses with peak powers of up to one hundred milliwatts. This embodiment will not produce coherent pulses. In an alternative embodiment, fiber optic cable  28  may be replaced by free space grating pairs, atomic vapor cells, or chirped filter-BRAGG gratings, known in the art. If necessary, fiber amplifiers can be used to boost the power of the scan wavelength output. It will be understood that the test cell may be placed between fibers  18  and  28  as an alternative embodiment. 
   It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein, but include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims.