Method of decoding a spectrally modulated light signal

• US 5,400,140 A
• Filed: 06/10/1994
• Issued: 03/21/1995
• Est. Priority Date: 07/30/1991
• Status: Expired due to Term

• Current Assignee: Fisher-Rosemount Limited Heath Place Bognor Regis
• Original Assignee: Fisher Rosemount Ltd (Emerson Electric Company)
• Inventors: Johnston, James S.
• Primary Examiner(s): Housel, James C.
• Assistant Examiner(s): Le, Long V.
• Application Number:
  US08/260,204
• Time in Patent Office: 284 Days
• Field of Search: 356/345, 356/357, 356/358, 356/356, 356/346, 356/355
• US Class Current: 356/482
• CPC Class Codes: G01D 5/266 by interferometric means G01D5/353 takes precedence
• CPC Class Codes: G01L 11/02 by optical means
• CPC Class Codes: G01L 9/0079 with Fabry-Perot arrangements

Abstract

In a method of analyzing a spectrally modulated light signal received from an interferometric transducer the value of an optical path difference associated with a parameter to be remotely sensed is encoded as a characteristic frequency of spectral modulation in the received light. The intensity of the received light is measured at a number of wavelengths within a selected range of wavelengths to obtain data defining a measured spectrum. Reference data is generated to represent a theoretical spectrum corresponding to a nominal optical path difference, the reference data being generated for a greater range of wavelengths than the selected range of wavelengths of the measured spectrum. The measured spectrum is correlated with successive portions of the reference data to identify a portion of the reference data exhibiting correlation with the measured spectrum, and an output signal is then generated which is representative of the location of the correlating portion within the wavelength range of the reference data. The method provides improved resolution for remote sensing of parameters such as pressure.

Claims

1. A method of analyzing a spectrally modulated light signal received from a transducer in which a value of an optical path difference associated with a parameter to be remotely sensed is encoded as a characteristic frequency of spectral modulation in the received light, the method comprising the steps of measuring the intensity of the light signal at a number of wavelengths within a selected range of wavelengths to obtain data comprising a set of measurement data values and defining a measured spectrum, generating reference data comprising a set of reference data values and being representative of a single theoretical spectrum corresponding to a nominal optical path difference, the reference data being generated for a greater range of wavelengths than the selected range of wavelengths of the measured spectrum, correlating the measured spectrum with successive portions of said reference data to identify a correlating portion of the reference data exhibiting correlation with the measured spectrum, each correlation being carried out using substantially all of the measurement data values of the measured spectrum and an equal number of said reference data values of said reference data, and generating an output signal representative of a location of the correlating portion within the wavelength range of the reference data.

2. A method as claimed in claim 1 wherein the theoretical spectrum is represented by ##EQU2## where I is received light intensity, I_o is a constant, d is the optical path difference and λ

is the wavelength of received light.

3. A method as claimed in claim 1 including the step of processing said output signal to provide an indication of the value of optical path difference and/or the parameter.

4. A method as claimed in claim 1 wherein the measured spectrum simultaneously exhibits a plurality of distinct frequencies of spectral modulation corresponding to respective values of optical path difference, the method including the step of identifying a corresponding plurality of portions of the reference data exhibiting correlation with the measured spectrum and generating output signals representative of the respective locations of the correlating portions within the wavelength range of the reference data.

5. A method as claimed in claim 4 including the step of generating a binary output signal comprising a plurality of binary digits representative of the presence in the measured spectrum of respective frequencies of spectral modulation.

6. A method as claimed in claim 1 wherein the measured spectrum is correlated with the reference data in a single chip hardware correlator.

7. A method as claimed in claim 6 including the step of storing the measured spectrum in a reference register of the correlator, generating a reference data signal which varies in time with a waveform which represents the variation with wavelength of the theoretical spectrum, passing the reference data signal in stepped manner through a sampled delay line of the correlator and performing at each step a cross-correlation calculation between the contents of the reference register and data values sampled from the delay line to produce a correlation output at each step.

8. A method as claimed in claim 7 including the step of counting the number of steps taken to reach a peak in the correlation output and outputting the number of steps as a measure of the location of a correlating portion within the wavelength range of the reference data.

9. A method as claimed in claim 1 including the step of illuminating the interferometric transducer with a pulse of light and analysing the corresponding pulsed light signal received from the transducer to sense an instantaneous value of the optical path difference.