FBG sensors – strain and temperature measurement with optical fibers
In the core of an optical fibre a defined form of periodic refractive index changes is implemented. This fiber Bragg grating (FBG) is an optical interference filter, which as part of its filter bandwidth reflects the injected light. If the grid is illuminated with a spectrally broad light source, it reflects by the Bragg condition a specific wavelength.
By stretching the FBG or with a temperature change, the local grating period in the fiber (FBG) changes and the changed back-reflected wavelength gives a conclusion about the accomplished elongation. The evaluation of the reflected light occurs via an interrogator. Depending on the measurement principle typically the wavelength shift is evaluated with a spectrometer. Other FBG-applications are in the in the pressure or vibration measurement.
The advantages of the FBG technology:
- suitable for harsh environment
- no electromagnetic interference
- no EMI or insulation problems
- can be used in hazardous areas
- small size
- more load cycles possible as with strain gauges
- multiple sensors in one fiber
- long distance transmission
Photo acoustic gas spectroscopy based on micro tuning forks
The measuring principle is based on the photo-acoustic spectroscopy and is a common method for the determination of gas concentrations. The gas is introduced into a cell, where at the outer wall a microphone is attached. With a pulsed light source the gas molecules are selectively excited, through the absorption of the radiation the gas is heated and a localized acoustic wave is formed, which is detected by the microphone.
A drastic miniaturization and simplification of this measurement principle is possible if instead of the microphone a micro piezoelectric tuning fork is used for the signal detection. The light beam is focused between the two arms of the piezo tuning fork and the resulting acoustic wave generated in the quartz a piezo-voltage, which is proportional to the existing gas concentration.
The advantages of the photo acoustic measurement principle:
- small cell size
- detection possible at ambient pressure
- low cost
- offset-free measuring principle
- suitable for a lot of gases
- gases over wide concentration range can be measured