Optical Inline-Sagnac Current Sensor, Part I: Theory

A study is made of the optical inline-Sagnac interferometer current sensor/transducer with active phase modulation and with spun polarization maintaining fiber. A brief review of the common sensor types is made to give context to the discussion of the optical inline-Sagnac current sensor theory. A new discussion is made of the limitations of the optical inline-Sagnac interferometer current sensor caused by the spun polarization maintaining fiber, the polarizing fiber design, and the temperature effects of each type of fiber. These combined temperature effects limit the sensor to a practical scale factor temperature sensitivity range of about 80 to 200 ppm/°C. The fiber component optical-wavelength effects require a broad-band light source to have a wavelength stability of <0.65 nm to achieve a ±0.1% sensor scale factor stability. The opto-mechanical effects from available spun polarization maintaining fibers limit the measurement head to diameters >30 mm. The transit-time of the light through the sensor is the ultimate bandwidth limit for the device. However, most practical sensors have −3 dB bandwidths >20 kHz. In utility applications the isolation of the sensor to nearby currents is estimated to be −96 dB.