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Equivalent Circuit of Fiber Optic Sensor

Equivalent Circuit of Fiber Optic Sensor

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Equivalent Circuit of Fiber Optic Sensor - BENNIE OPTICAL [PDF]

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A fiber optic sensor can be represented electrically as a combination of a light source driver, optical transmission path, photodetector, and signal conditioning circuitry.

Conceptual Equivalent Circuit

The equivalent circuit of a fiber optic sensor models the optical sensing system in electrical terms. It typically includes the following components:

  1. Light Source and Driver Circuit
    • The optical source (LED or laser diode) is represented as a current-controlled light emitter.
    • A voltage-to-current converter or driver circuit ensures stable biasing and modulation of the light intensity according to the input signal .
    • In the equivalent circuit, the LED can be modeled as a diode in series with a current source representing the modulated optical signal.
  2. Optical Fiber Transmission Path
    • The fiber itself is modeled as a lossy transmission line with attenuation and dispersion characteristics.
    • For intensity-based sensors, the fiber can be represented as a variable resistor or attenuator that changes with the sensed physical parameter (e.g., pressure, temperature, or strain) affecting light intensity .
  3. Photodetector and Receiver Circuit
    • The photodetector (photodiode or phototransistor) converts the received light back into an electrical signal.
    • In the equivalent circuit, it is modeled as a current source proportional to incident light intensity, often in parallel with a junction capacitance and shunt resistance to account for detector characteristics .
    • A transimpedance amplifier is typically included to convert the photocurrent into a voltage signal.
  4. Signal Conditioning and Processing
    • The output from the photodetector is fed into amplifiers, filters, and analog-to-digital converters for further processing.
    • These can be represented as operational amplifier blocks with gain and filtering elements in the equivalent circuit .

Block Diagram Representation

A simplified block-to-circuit mapping is as follows:

  • Input Signal → LED Driver → LED (modeled as diode + current source) → Optical Fiber (attenuator) → Photodiode (current source + capacitance) → Transimpedance Amplifier → Output Signal This equivalent circuit allows engineers to simulate the sensor response electrically, analyze signal integrity, and design appropriate conditioning circuits without modeling the optical path in full detail.

Notes

  • For interferometric sensors (e.g., Michelson or Mach-Zehnder), the equivalent circuit may include phase-dependent voltage-controlled elements to represent optical phase shifts .
  • Intrinsic fiber optic sensors can be modeled by including variable optical elements within the fiber path that modulate light according to environmental changes .
  • The equivalent circuit is particularly useful for intensity-modulated fiber optic sensors, while wavelength or phase-modulated sensors may require more complex representations. By combining these elements, the fiber optic sensor system can be analyzed using standard circuit simulation tools, bridging the optical and electrical domains effectively.
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