Fiber Optic Stress Sensors

Superluminescent Light Emitting Diodes (SLEDs) are used in fiber optic pressure sensors for static strain (load) or dynamic strain (vibrations) measurements as well as temperature measurements in civil engineering, structural analysis and composite material manufacturing applications. Fiber optic sensors have a number of advantages over traditional technologies like electrical strain gauges. Their significantly smaller size and lower weight, their immunity to electromagnetic fields, their ability to measure at many points along a single fiber – greatly improving the ease at which sensors can be multiplexed -, and their ability to be embedded within, or bonded to, structures makes them a highly flexible solution.

Various optical system architectures exist [16] such as time-domain systems, in which propagation delays between the source and wavelength-specific fiber sensors are detected, or wavelength-domain systems, which measure the spectral change in reflectivity of such fiber sensors. Fig. 12 shows a generic setup of wavelength-multiplexed fiber sensor system using a broadband SLED, typically in the 1550-nm wavelength range (EXS210048, EXS210054, EXS210059EXS210069, EXS210071), and a reflective sensor system incorporating various wavelength-specific fiber Bragg gratings (FBGs) along the same fiber. Alternatively, the FBG sensors may also be incorporated into multiple arms for a spatially- distributed sensor system.

 

 

Fig12 Wavelength-domain

Fig. 12    Wavelength-domain distributed fiber Bragg grating (FBG) sensor
system using WDM multiplexing with a broadband SLED.

 

 

The need to deploy fiber optic sensors over extended areas requires high power sources to ensure adequate optical power after multiple splitting of the light or after very long paths (several miles) without amplification. While the required intensities are impracticable for surface emitting Light Emitting Diodes (LED), edge-emitting light sources like Superluminescent Light Emitting Diodes (SLEDs) easily provide the required intensity levels [17].

Benefits and Performance Advantages of EXALOS’ SLEDs for Fiber Optic Sensors
Fiber optic sensors for strain measurements of civil engineering structures as well as fiber optic temperature or pressure sensors in hostile surroundings benefit from specific SLED characteristics such as broadband and high-power light emission. EXALOS offers various 1550-nm SLEDs for fiber optic sensor applications that provide the wide optical spectrum and the power spectral density (PSD) required by fiber optic sensor applications, for example SLEDs with 60 to 90 nm bandwidth and output powers of 5 to 20 mW, both as cooled and uncooled modules [2]. All EXALOS’ SLEDs have been qualified according to BELLCORE GR-468-CORE.

Fiber Sensor Systems Using Swept Sources
In applications requiring higher resolution, higher speed, or higher optical power for long- distance and high-density sensor arrays, interrogation sensor systems using fast wavelength- swept laser sources in the 1550-nm wavelength range prove to be critical. Instead of an optical demultiplexer at the receiver, which separates the optical signals from the individual sensors, and instead of an array of photodetectors, a single fast receiver photodiode is used in combination with data acquisition (DAQ) electronics, as shown in Fig. 13. The optical signals of the individual sensors are detected by their timing position relative to the sweep time of the swept source [18]. Such swept source-based sensor systems can reach very long distances [18] or very high refresh rates of 100 to 200 kHz [19]. EXALOS is offering ultra-broadband swept sources [4] in the 1550- nm wavelength range with scan rates from 1 kHz (ESS320029) to 150 kHz (ESS320030) for fiber sensor or similar applications.

 

 

Fig13 Time-domai

Fig. 13    Time-domain distributed fiber Bragg grating (FBG) sensor
system using a swept source and a single receiver photodiode (PD-Rx)