Abstract:
An interferometer system is used to detect a wavelength of an unknown signal. The interferometer system includes a fringe pattern detection system and a power detecting system. The fringe pattern detection system measures an interference fringe pattern of the unknown signal. The power detecting system measures relative power of the unknown signal before the unknown signal enters the fringe pattern detection system. The relative power of the unknown signal is used to compensate for modulation within the unknown signal.

Description:
BACKGROUND 
     The present invention concerns signal test and measurement and pertains particularly to signal modulation compensation for a wavelength meter. 
     In a Michelson interferometer system, light from a fiber optic input is collimated and directed to the input of the interferometer. The input signal is split into two paths with a beam splitter. Both beams are then reflected by mirrors that bounce the light back toward the beam splitter. Part of the light reflected from the mirrors goes back toward the input beam. The other portion of the light is incident on a photodetector. Since there is no loss assumed in the interferometer, all of the light is directed to either the photodetector or the input beam. 
     One of the mirrors of the interferometer is stationary and one is movable. The movable mirror is movable to vary the length the beam travels before and after incidence with the movable mirror. As the mirror is moved, the amount of light reaching the photodetector will oscillate up and down because of constructive and destructive interference effects between the two paths of the interferometer. Through the analysis of these interference patterns, the wavelength of light can be calculated. 
     The beams of light can be analyzed in terms of light interfering as the path length in the interferometer changes. This is referred to as the fringe-counting description of wavelength meter operation. Alternately, if the movable mirror is moved at a constant rate, the frequency of the light in the beam is Doppler-frequency shifted. The Doppler detector then mixes the light from the moveable mirror and the stationary mirror. The beat frequency between these two signals can be used to calculate the unknown frequency of the input signal. See Dennis Derickson,  Fiber Optic Test and Measurement,  Prentice Hall, Inc., 1998, pp. 133-141. 
     A Michelson interferometer based wavelength meter measures the wavelength of an unknown signal by comparing the fringe pattern produced by the unknown signal with that of the reference (known) signal. As one arm of the Michelson interferometer is translated (i.e., the mirror is moved) the interference pattern at the photodetector oscillates between high and low irradiance. Comparing the number of fringes produced by the unknown signal with the number produced by the known signal results in a highly accurate estimate of the unknown wavelength. 
     Unfortunately, if the unknown signal is amplitude modulated it becomes difficult to accurately count the number of unknown fringes. If Fourier transform techniques are used to determine the power spectrum of the signal the modulation produces false peaks equally spaced on either side of the true frequency called sidebands. 
     SUMMARY OF THE INVENTION 
     In accordance with the preferred embodiment of the present invention, an interferometer system is used to detect a wavelength of an unknown signal. The interferometer system includes a fringe pattern detection system and a power detecting system. The fringe pattern detection system measures an interference fringe pattern of the unknown signal. The power detecting system measures relative power of the unknown signal before the unknown signal enters the fringe pattern detection system. The relative power of the unknown signal is used to compensate for modulation within the unknown signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a simplified block diagram of a Michelson interferometer based wavelength meter in accordance with a preferred embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 is a simplified block diagram of a Michelson interferometer based wavelength meter. A signal source  11  is the source of an “unknown” signal with an unknown wavelength. For example, signal source  11  is an optical fiber carrying a light signal. A collimating lens  17  collimates the light from signal source  11  to produce a beam  26 . For example, collimating lens  17  is a 7.5 millimeter (mm) focal length lens. 
     A reference source  12  is the source of a reference signal with a known wavelength. A collimating lens  18  collimates the light from reference source  12 . For example, collimating lens  18  is a 6.0 mm focal length lens. A resulting reference beam  28  is in a different plane than beam  26 . For example, beam  26  is in a higher parallel plane than reference beam  28 . A fold mirror  22  is in the same plane as reference beam  28  and reflects beam  28  to run in parallel with beam  26  along a beam path  29 . Beam  26  is not in the same plane as fold mirror  22  and so is unaffected by (i.e., passes over) fold mirror  22 . 
     A beam splitter  25  splits beam  26  and reference beam  28  so that a portion of each beam runs along a beam path  30 , is reflected by a corner cube mirror  21  and returns to beam splitter  25  along beam path  30 . The remaining portion of each beam runs along a beam path  32 , is reflected by a corner cube mirror  23  and returns to beam splitter  25  along beam path  32 . The beams switch planes at the corner cube mirrors. 
     After returning to beam splitter  25 , a portion of each beam travels back along beam path  29  and a portion of each beam is directed to beam path  31  through imaging lenses  20  to detectors  13 . In the preferred embodiment, lenses  20  are two lenses, one for each beam, and detectors  13  are two detectors, one for each beam. For example, each of imaging lenses  20  is a 25.4 mm focal length lens. 
     A portion of the beam in the lower plane is reflected by fold mirror  22  towards collimating lens  18 . This beam in the lower plane originated from signal source  11 . A high pass filter  15  blocks the reflected beam from reaching collimating lens  18 . 
     A portion of the beam in the higher parallel plane passes over fold mirror  22  and continues towards collimating lens  17 . This beam in the higher parallel plane originated from reference source  12 . A low pass filter  16  blocks this beam from reaching collimating lens  17 . 
     The beams of light detected by detectors  13  can be analyzed in terms of light interfering as the path length in the interferometer changes. As corner cube mirror  23  is translated (moved), as represented by an arrow  24 , the interference patterns at detectors  13  oscillate between high and low irradiance. Comparing the number of fringes produced by the unknown signal with the number produced by the reference signal results in a highly accurate estimate of the wavelength of the unknown signal. 
     As discussed above, if the unknown signal is amplitude modulated it becomes difficult to accurately count the number of unknown fringes. If Fourier transform techniques are used to determine the power spectrum of the signal the modulation produces false peaks equally spaced on either side of the true frequency called sidebands. 
     In the preferred embodiments of the present invention, additional entities are added to make fringe counting more accurate and eliminate spurious signals in the Fourier transform of the data array. Specifically, a detector  14  and an imaging lens  19  are added to the Michelson interferometer based wavelength meter. For example, imaging lens  19  is a 25.4 mm focal length lens. Detector  14  detects the input power of beam  26  before beam  26  reaches beam splitter  25 . The power is detected based on reflections from beam  26  resulting from beam  26  passing through low pass filter  16 . It is not required that power is split away by low pass filter  16 . Power can be split away by a separate item added specifically to perform the splitting. 
     The measured input power is used to compensate for the modulation within the signal from signal source  11 . This is done by dividing the fringe pattern for the unknown signal, detected by detector  13 , by the relative power of the unknown signal, as measured by detector  14 . 
     The power (P) of the fringe pattern at detector  13  is represented by Equation 1 below:                P        (     x   ,   t     )       =       P   1          A        (   t   )            (       1   +     γcos        [       4      π                 x       λ   unknown       ]         2     )               Equation                 1                                
     In Equation 1 above, A(t) represents the amplitude modulation of the unknown signal, γ is a fringe contrast constant, x represents a location of corner cube mirror  23 , t represents time, P 1  is a constant that represents the maximum power that would arrive at detector  13  if the signal were unmodulated and for γ=1, and λ unknown  is the wavelength of the unknown signal. Detector  13  measures P(x,t) and detector  14  measures P 2 A(t). This allows A(t) to be removed by dividing P(x,t) measured at detector  13  by P 2 A(t) as measured at detector  14 . The result for P′ is given in Equation 2 below:                  P   ′          (   x   )       =         P        (     x   ,   t     )           P   2          A        (   t   )           =       (       1   +     γcos        [       4      π                 x       λ   unknown       ]         2     )            P   1       P   2                   Equation                 2                                
     P 2  is a constant that represents the maximum power that arrives at detector  14 . By using the value of P 2 A(t) detected at detector  14  to remove the modulation from the signal detected at detector  13 , this increases the accuracy of fringe counting and helps to eliminate spurious signals in the Fourier transform of the data array. 
     The foregoing discussion discloses and describes merely exemplary methods and embodiments of the present invention. As will be understood by those familiar with the art, the invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. Accordingly, the disclosure of the present invention is intended to be illustrative, but not limiting, of the scope of the invention, which is set forth in the following claims.