Sensors that use electronics use voltage (millivolt, microvolt, nanovolt) and Analogue to Digital (ADC) conversion to convert the voltage to a digital value. Sensors that contain Fiber Bragg gratings use the wavelength of the reflected light measured in nanometers, picometers and femtometers and the equivalent of the ADC is an optical interrogator which translate the wavelength in a digital value. When designing/using electronic sensors the key parameter of sensitivity is specified as volts per minimum resolution of the measurement e.g 100 millivolt (mV) per Pascal for a pressure sensor or 30mV per gravity (30mV/g) , or 1mV per degree Celsius (1mv/°C)

Fibre Bragg gratings use a change in the wavelength of the light reflected from the FBG (Fibre Bragg Grating). The optical interrogator must resolve the wavelength change to a high resolution in order to translate the measurement into a digital value. So the important parameters of sensitivity of a sensor is picometer change in wavelength with respect to the parameter to be measured. e.g. 10 picometer (pm) per degree Celsius (10pm/C), for pressure the typical figure might be 30picometer per pascal (30 pm/Pa), and for an accelerometer it would be 30pm per gravity (30pm/g). (see S.I units documentation for lists of possible standard measurement parameters http://physics.nist.gov/Pubs/SP330/sp330.pdf )

A second parameter of interest is the sample rate of the system. In the case of instrument electronic sensors ADC are usually obtained with a sample rate of from ‘DC’ to 100ksps (100 kilo samples per second) with effective resolutions from 8bit to 20bits. The FAZ optical interrogators are designed for sample rates from ‘DC’ to 44Khz and an effective resolution of up to 20bits (120dB).

The noise floor of an electronic circuit is also an important parameter and us usually represented by volt/√Hz, the equivalent parameter for an optical interrogator is fm/√Hz which can be calculated by taking the square root of the sample frequency (1Khz) and dividing it into the resolution (e.g for a 3Khz 10nm scanning range the I4 noise floor is 18fm the noise per root Hz would then be  18fm /√3000 = 0.33 fm/√Hz). Using these calculations enable us to compare electronic and optical sensors. In seismic applications the FAZ solution matches the performance of electronic sensors with the added benefits of:

  • Do not require any power to be supplied to the sensor
  • Immune to electromagnetic interference
  • Ability to transfer measurement signals over long distances (~10 km)
  • Multiplexing of sensors to enable large sensors counts
  • Suitable for more extreme environments than electronic sensors

 

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