Monitoring a laser wavelength for strain and temperature

How a laser enables you to monitor temperature and strain variations?

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Lasers are also used in set-ups aimed for monitoring strain and temperature. In those set-ups strain and temperature variations are directly proportional to the shift of a reflected laser wavelength.

Optical Fiber Bragg Gratings (FBG) are known for many years and allow using an optical fiber as a Temperature + Strain + Pressure sensor. An optical fiber Bragg grating acts as a mirror at a specific wavelength depending on the distance between its lines, and transmits all others, or acts as a wavelength specific reflector. This mirror is achieved by creating a periodic variation in the refractive index of the fiber core. Because the Bragg wavelength is a function of the spacing between the gratings, FBGs can be manufactured with various Bragg wavelengths.

Principle of a FBG sensor: monitoring the precise shift of a reflected laser wavelength

Principle of a FBG sensor: monitoring the precise shift of a reflected laser wavelength

 

Changes in strain and temperature affect both the effective refractive index and grating period of an FBG, which results in a shift in the reflected wavelength. FBGs can therefore be used as a temperature or strain sensor. The monitoring of a FBG sensor is made with a Bragg interrogator. A Bragg interrogator is mainly composed of two elements: a large laser source and a spectrometer or interferometer. The large laser source, such as a SLED (superluminescent light-emitting diode), illuminates the fiber. The fiber may contain one or many FBG. And the spectrometer or interferometer monitors the precise shift of the reflected wavelength of the laser, also called the Bragg wavelength. This shift is directly proportional to temperature and strain variations.

This technique based on the monitoring of a laser wavelength has many advantages. FBG sensors are electrically passive, nonconductive and immune to EMI-induced noise. Also, unlike electrical sensing systems, each optical channel can measure dozens of FBG sensors. This greatly reduces the size, weight, and complexity of the strain and temperature measurement systems.


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