Abstract:
An absorption spectrometer provides improved rejection of background radiation signal by employing a frequency-swept laser signal without frequency dithering and performing an effective differentiation of output light from a test cell to eliminate these constant or slowly varying background radiation levels.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     This invention was made with United States government support awarded by the following agencies: 
     USAF/AFOSR FA8650-08-C-2856 
     The United States government has certain rights in this invention. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to absorption spectroscopy systems and in particular to an absorption spectroscopy system having improved background light rejection. 
     Absorption spectroscopy is a tool of chemical analysis that measures light absorption by a material being investigated over a range of different frequencies. In laser absorption spectroscopy, a laser may be directed through the material, for example a gas within a test cell, while the laser frequency is tuned over the absorption range of the sample in a “sweep” of frequencies. The amplitudes of light before and after it passes through the material are compared to produce an absorption spectrum. 
     A practical problem in measuring absorption spectra using this technique is that the light detector receiving light from the test cell must be able to discriminate small intensity variations caused by absorption in the presence of the a strong “background” signal from the laser from the un-absorbed laser light striking the light detector. 
     Background laser signal may be suppressed to some extent through the use of modulation techniques such as “wavelength-modulation spectroscopy” (WMS) or “frequency-modulation spectroscopy” (FMS). In both techniques, the wavelength or frequency of the laser is “dithered” as it is swept through a range of frequencies, the dithering being a slight frequency modulation on top of monotonic modulation of the frequency sweep. Dithering serves to frequency encode the absorption bands in the material as a result of interaction of wavelength dithering with the absorption feature. This frequency encoded absorption signal may be de-modulated to eliminate the substantially unmodulated background radiation. 
     While this approach can greatly reduce the influence of background radiation, there are practical difficulties. For example, when a tunable diode laser used as the light source and the dithering is implemented by changing the injection current of the diode, a relatively small frequency range of dithering is possible; a range that may be much smaller than the absorption feature. Further, not all laser sources (for example, some lasers that may have desired frequencies for particular absorption features) permit injection current modulation or make the injection current accessible. Controlling the dithering through injection current control can be complex and it can be difficult to control the dithering in a stable manner or to manage incidental amplitude modulation that can obscure the desired signal. 
     SUMMARY OF THE INVENTION 
     The present invention provides a technique for reducing the effect of background radiation in absorption spectroscopy that works with undithered light sources. In this way, limitations on modulation depth can be overcome, the need for and complexity of injection current control is eliminated and the difficulty of managing amplitude modulation associated with such dithering reduced or eliminated. In the invention, background radiation is canceled by performing a local subtraction or differentiation of the light spectrum either with respect to time (when the light source is swept in frequency) or with respect to space (in a conventional rectilinear spectrum). 
     One embodiment of the present invention provides an absorption spectrometer having a test cell, or its functional equivalent, for holding a material for spectrographic analysis and a multi-frequency light source producing a monotonically swept frequency spanning a frequency range of a desired absorption spectrum. A splitter/delay system splits light of the multi-frequency light source into a first and second path wherein the light is delayed in the first path with respect to the second path and an analyzer receives output light from the multi-frequency light source after passage through both the test cell and the first and second path to provide an electrical signal substantially reducing a measure of background radiation from the multi-frequency light source unattenuated by the material. In one embodiment, the electrical signal may be substantially a differencing of intensity values of the output light at different adjacent spectral frequencies over successive spectral frequencies. A reconstructor then receives the electrical signal to reconstruct the absorption spectrum of the material. 
     It is thus a feature of at least one embodiment of the invention to obtain the advantages of being able to use a slowly swept light source in sensitive absorption spectroscopy. The differentiation of the intensity values eliminates the contribution of constant background radiation which either does not change between samples or changes so slowly as to be negligible. 
     The analyzer may be a balanced photodetector receiving light from the first and second light path at different ports of the balanced photodetector, the balanced photodetector taking a difference of the light at the different ports to produce the electrical signal. 
     It is thus a feature of at least one embodiment of the invention to provide a method of subtracting signals that is robust against drift or calibration differences and that avoids quantization errors inherent in performing, such as subtraction in a digital domain within a computer. 
     In one embodiment, the analyzer may include a photodetector and an optical switch may apply pulses of the light from the multi-frequency light source to the first and second light path so that light from the multi-frequency light source is received at a single photodetector after two different delay times providing, at alternating times, light at different adjacent spectral frequencies. In this case, the analyzer may further include a demodulator receiving the electrical signal from the photodetector to produce the electrical signal. 
     It is thus a feature of at least one embodiment of the invention to permit amplitude measurement with a single photodetector by timeshifting the frequency offsets and through the use of precision demodulator circuits. 
     The demodulator may be phase locked to the optical switch. 
     It is thus a feature of at least one embodiment of the invention to preserve sign information in the differentiation process to permit accurate reconstruction of the spectral shape. 
     The optical switch may be a light gate applying identical pulses of output light to the first and second light paths. 
     It is thus a feature of at least one embodiment of the invention to permit the use of a simple mechanical or electrical chopper for generating the pulses. 
     Alternatively, the optical switch may be a diverter alternating the application of light pulses to the first and second light paths, for example a Mach-Zehnder interferometer. 
     It is thus a feature of at least one embodiment of the invention to provide a simple high-speed pulse generating system with no moving parts. 
     In an alternative embodiment, the present invention may provide an absorption spectrometer using a multi-frequency light source producing a substantially constant spectrum of multiple simultaneous light frequencies over the range of an absorption spectrum, again without dithering. In this embodiment, a spectroscope receives light from the multi-frequency light source after passage through the test cell to produce a spectrum and the analyzer processes the spectrum to produce an electrical signal being substantially a differencing or differentiation of intensity values of the output light at different adjacent spectral frequencies over successive spectral frequencies. The processed spectrum is then received by the reconstructor to reconstruct the absorption spectrum of the material. 
     It is thus a feature of at least one embodiment of the invention to provide an extremely high-speed system that may process multiple frequencies simultaneously. 
     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 FIGURES 
         FIG. 1  is a block diagram of a prior art frequency modulated spectrometer using a frequency-swept light source with frequency dithering such as produces some unwanted amplitude modulation; 
         FIG. 2  is a block diagram similar to  FIG. 1  showing a first embodiment of the present invention employing a frequency-swept light source without dithering and using a beam splitter/delay line position after the test cell; 
         FIG. 3  is a block diagram similar to that of  FIG. 2 , showing an embodiment with the beam splitter/delay line positioned before the test cell; 
         FIG. 4  is a block diagram of an alternative embodiment of the invention employing a chopper and a beam splitter/delay line to create an effective dithered signal without dithering of the light source; 
         FIG. 5  is a fragmentary block diagram similar to that of  FIG. 4  showing an embodiment replacing the chopper of  FIG. 4  with a Mach-Zehnder interferometer; and 
         FIG. 6  is a figure of an alternative embodiment using a polychromatic (un-swept) light source and a slit spectrometer to provide a virtual dithering by pairwise differencing of adjacent sections of the produced spectrum. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to  FIG. 1 , a prior art wavelength or frequency modulated absorption spectrometer  10  may provide a light source  12 , such as a laser, outputting a monochromatic light  14  that may be swept monotonically in frequency over time as indicated by dotted line  16  to provide a general trend of, for example, increasing frequency with time. The light  14  may also be “dithered” with small frequency excursions to produce a frequency-swept and dithered modulation pattern  18 . During this frequency modulation, the amplitude  20  of the light (its intensity) is desirably substantially constant but in practice has small amplitude modulations caused by the dithering. 
     The output light  14  is received by a test cell  22  which should be understood herein to include a standard laboratory test cell  22  but also effective test cells such as a specially fitted combustion chamber or the like as well as environmentally open volumes used in atmospheric study. Material within the test cell  22  may have one or more characteristic absorption transitions  24  represented diagrammatically. 
     Light  26  output from the test cell  22  will have a modulated amplitude  28  caused by the interaction of the dithered frequency of the light  14  and the absorption peak  24 . The modulated amplitude  28  of light  26  may be measured by a photodetector  30  to produce signal  31  being generally the electrical equivalent of modulated amplitude  28 . 
     Generally the signal  31  will be offset above zero by the intensity of the light  14  passing through the test cell  22  to be received by the photodetector  30 . This intensity, termed “background radiation”, limits the amount of amplification of the signal detected by the photodetector  30  without saturation of a downstream amplifier or other device. 
     Accordingly, the signal  31  may be communicated to an analyzer  32  which may be a high pass filter/amplifier  34  or its equivalent removing any constant term of the modulated amplitude  28  thereby essentially eliminating the background radiation to produce an amplified substantially offset-free amplitude modulated signal  36  approximately centered about zero voltage to be compatible with further amplification. 
     A demodulator  38  may receive the offset-free amplitude modulated signal  36 , for example, using a phase-locked demodulation linked to the frequency and phase of the dithering  17  to reconstruct the absorption peak  24  and display the same on a display  40  according to techniques generally understood in the art. The process of modulation thus allows the component of the light signal related to the absorption peak  24  to be separated from the background radiation to provide better amplification of the absorption component. 
     As discussed above, a drawback to this approach is that typically the mechanisms for producing the dithering  17  (for example, modulating a signal driving a tunable diode laser) also impose a slight amplitude modulation on amplitude  20  of the light  14 . This modulation passes through the high pass filter/amplifier  34  and thus competes with and corrupts the amplitude modulated signal  36  used to produce the absorption peak  24  ultimately distorting the reconstruction of the absorption spectrum  24 ′. 
     Referring now to  FIG. 2 , an absorption spectrometer  42  of the first embodiment of the present invention employs a light source  44  providing for a swept frequency  46  without dithering and with substantially constant or slowly varying amplitude  47 . Significantly the light source  44  avoids the amplitude modulation associated with dithering. 
     Generally, the swept frequency  46  of light  48  from the light source  44  moves monotonically from a low to a high frequency or vice versa spanning the desired range of the absorption peaks  24 . Monotonically, as used herein, means that successive frequencies of the light  14  either consistently increase or decrease but not both as measured in the range of the spectrum. 
     Light  48  from the light source  44  passes through test cell  22  and is received by a beam splitter  50  which separates the beam into a first optical fiber providing first light path  52  and a second optical fiber providing second light path  54 . First light path  52  includes a delay line  56  (for example a coil of fiber). Light exiting the first fiber will therefore show an absorption feature  60  that is delayed with respect to the absorption feature  62  exiting the second fiber. Otherwise absorption features  60  and  62  are essentially identical. 
     Light from the first light path  52  and second light path  54  are then received by different ports of a balanced photodetector  58  which subtracts the intensity or amplitude of the two signals to produce a differentiated signal  64  being a point-by-point difference between features  62  and  60  as a function of time. As a result of the delay line  56 , the differentiated signal  64  may be understood to be a time-derivative of the light signal received by the beam splitter  50 . 
     The differentiated signal  64  may simply be integrated, for example by analyzer  66 , to produce an output of an absorption spectrum  24 ′ in display  40 . Analyzer  66  may also receive a signal from the light source  44  indicating the frequency function of the light for calibration purposes (e.g. assigning specific frequencies to the absorption spectrum  24 ′) and/or correcting for nonlinearities in the sweeping process. Alternatively this calibration signal may be derived from measurement of the actual produced light  48 . 
     Referring now to  FIG. 3 , in an alternative embodiment which provides simpler collimation optics, the beam splitter  50  may receive light  48  directly from the light source  44  to provide light along the first light path  52  and second light path  54  communicating directly with the test cell  22 . Separate light paths through the test cell  22  may be maintained to the two input ports of the balanced photodetector  58 , the latter which may communicate with analyzer  66  to process the absorption spectrum  24 ′ as previously described. 
     Referring now to  FIG. 4 , in an alternative embodiment, the light source  44  may produce light  48 , similar to that described above, that is received by a chopper  68  shown here as a mechanical rotating vane that converts the light into a series of pulses, preferably having a 50% duty cycle. It will be understood that the chopper  68  could also be an electro-optical switch such as a Kerr cell or the like. The pulses produced by the chopper  68  are received by beam splitter  50  passing the pulses to both the first light path  52  and second light path  54 . The output of these light paths  52  and  54  will provide a set of pulses  69  that can be interleaved by proper selection of the delay of delay line  56  to equal the pulse “on time” defined by the chopper  68 . As so interleaved, the pulses  69  from the first light path  52  and second light path  54  may be recombined by an optical combiner  70  to produce a light  71  that shows an overall monotonic frequency sweeping per the light  48  with an effective frequency-dithering  17  imposed by the delay line  56  in light path  54  which introduces a time discontinuity between pulses  69 . 
     The dithered light  71  may be passed to the test cell  22  and be received by the photodetector  72  to provide a modulated amplitude light signal  73  similar to amplitude modulated signal  36  described above in the prior art albeit in a discretized form. 
     Amplitude modulation in light signal  73  imposed on dithered light  71  after passage through the test cell  22  may be demodulated by analyzer  74  using a phase locked demodulator  75  receiving phase information from the chopper  68  to provide a difference signal  64  being generally the derivative of the spectral peak  24 . This difference signal  64  may be processed, for example, by integrator  78  to produce an absorption spectrum  24 ′. 
     Referring now to  FIG. 5 , the action of the chopper  68  which simultaneously applies pulses to both the first light path  52  and second light path  54  may be replaced by a Mach-Zehnder interferometer  80  having to output ports to which the light  48  is alternately directed according to its frequency. In one embodiment, the Mach-Zehnder interferometer  80  may have one port connected to splitter  50  of  FIG. 4  to serve in lieu of chopper  68 , albeit without moving parts. 
     Alternatively, as shown, the Mach-Zehnder interferometer  80  may apply alternate pulses to a first light path  52  and second light path  54 . Delay line  56 , in this case, may be adjusted to delay light through the first light path  52  by a full cycle of the pulsing to produce interleaved, time discontinuous pulses at the output of combiner  70 . As described above, this dithered light  71  will be both frequency-swept and dithered. The analysis of the dithered light  71  may be otherwise identical to the description of  FIG. 4 . 
     Referring now to  FIG. 6 , in an alternative embodiment a light source  81  may provide for a polychromatic light  84  having a constant amplitude  82  and multiple simultaneous frequencies in a frequency band  85 . The light  84  may be received by the test cell  22  and, exiting the test cell  22 , received by a spectrometer  90  shown here as a split spectrometer using a prism, this depiction intended to symbolically represent any spectrometer that produces a classic optical spectrum  92  in which various light components are spatially separated along a least one axis, for example by a grating or other optical technique. The spectrum  92  may be read by a CCD camera  94  having different pixel detectors  96  associated with different frequencies of the spectrum  92 . Each pixel detector  96  thus detects light in a different spectral segment of the spectrum  92 . 
     Signals from pairwise adjacent pixel detectors  96  are subtracted one from another by means of summing junctions  98 , for example operation amplifiers configured as subtractors, each producing a different point in an equivalent of signal  64  being a derivative of the absorption spectrum  24 ′. The signal  64  may be processed by analyzer  100  to produce the absorption spectrum  24 ′ according the techniques described above with respect to  FIG. 1 . 
     It should be understood that the invention is not limited in its application to the details of construction and arrangements of the components set forth herein. The invention is capable of other embodiments and of being practiced or carried out in various ways. Variations and modifications of the foregoing are within the scope of the present invention. It also being understood that the invention disclosed and defined herein extends to all alternative combinations of two or more of the individual features mentioned or evident from the text and/or drawings. All of these different combinations constitute various alternative aspects of the present invention. The embodiments described herein explain the best modes known for practicing the invention and will enable others skilled in the art to utilize the invention.