sensor classification intensity modulation sensors phase modulation sensors frequency modulation...

Download Sensor classification Intensity modulation sensors Phase modulation sensors Frequency modulation sensors Polarization modulation sensors Wavelength distribution

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  • Slide 1
  • Sensor classification Intensity modulation sensors Phase modulation sensors Frequency modulation sensors Polarization modulation sensors Wavelength distribution sensors Mode number sensors 4 Optical Fiber Sensors
  • Slide 2
  • Intensity modulation sensors MModulation mechanisms Fig. 4.1: Simple sensor based on the position-dependent coupling between two fires which move relative to each other (a) lateral (b) longitudinal. Position dependent coupling between two fibers
  • Slide 3
  • Absorption based sensor Fig. 4.2
  • Slide 4
  • Example (a C 2 H 2 sensor)
  • Slide 5
  • Without sample gas
  • Slide 6
  • With 9% of acetylene (C 2 H 2 )
  • Slide 7
  • Measured C 2 H 2 absorption
  • Slide 8
  • Evanescent field sensor Fig. 4.3(a) Fig. 4.3(b): Photograph of a section of D fiber.
  • Slide 9
  • Evanescent field
  • Slide 10
  • Detection of intensity modulation : The intensity variation can be converted into an electric signal (current or voltage) by a light detector (e.g., PIN photo-detector).
  • Slide 11
  • Phase modulation sensors Modulation mechanisms The total phase ( = L = 2 n eff L / ) of the light path along an optical fiber depends on three properties of the fiber guide: Its total physical length L The refractive index and the index profile (affect n eff ) The geometrical transverse dimensions of the guide (affect n eff ) The total physical length of an optical fiber may be modulated by: Application of a longitudinal strain Thermal expansion Application of a hydrostatic pressure causing expansion via Poissons ratio
  • Slide 12
  • The refractive index varies with: Temperature Pressure and longitudinal strain via the photoelastic effect The guide dimensions varies with Radial strain in a pressure field Longitudinal strain through Poissons ratio Thermal expansion
  • Slide 13
  • Fig. 4.4: An all-fibre Mach Zehnder interferometer using homodyne detection incorporating the required quadrature bias via modulation of the feedback voltage to the PZT modulator The light intensity at the photo-detector is given by: (4-1) Detection of Phase Modulation Optical Interferometers 1. Mach-Zehnder Interferometer
  • Slide 14
  • I - 90 bias point (operation point) Fig. 4.5 Limitation : Operating point not stable Not suitable for static measurement
  • Slide 15
  • All fiber Mach Zehnder has approved to be very useful for high accuracy dynamic phase measurement. Assuming there is dynamic phase modulation (t) = Xsin t, the output of the interferometer may be written as (4-2) If can be kept to /2, for small phase modulation, i.e., X

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