Technology > Interrogation Techniques
To use an FBG as a sensor, it is illuminated by a light source with a broad spectrum and the reflected wavelength is measured and related to the local measurands of interest. Shifts in the Bragg wavelength can be monitored by any of the following techniques:
- An interferometer may be used to convert wavelength shifts into phase shifts, which can be detected by measuring variations in the light intensity as the path difference in the interferometer is varied. This technique potentially allows for very high sensitivity, but the equipment to do it is expensive and prone to environmental interference.
- A sloped optical filter, which may be another Bragg grating, can be used to convert wavelength shifts directly into intensity changes. If the filter is designed to have a known pass/reject ratio which varies with wavelength, then the wavelength of a narrowband reflection from a single grating can be determined simply by measuring and comparing the passed and rejected intensities. For the filter with a transmission spectrum shown on the left in the figure below, as the Bragg wavelength increases from λ1 to λ2, the transmitted intensity It decreases and the reflected or rejected intensity Ir increases correspondingly. This is the simplest and cheapest way of demodulating FBG, but it has the important disadvantage that it can address only one grating at a time.
Demodulating FBG with a passive filter. Wavelength shifts (left) are converted into intensity changes (right).
- A widely-employed approach is to illuminate the FBG with a narrowband tunable light source. This is the basis of Smart Fibres' current products. This method will be dealt with in more detail in the following section, "Wavelength Division Multiplexing"
Wavelength-Division Multiplexing (WDM)
The principle behind WDM is simple. Many gratings can be combined on a single fibre and addressed simultaneously provided each has a different Bragg wavelength. This is achieved in practise either by using either:
- a broadband light source and a spectrometer for detection
- or an agile tunable or swept-wavelength light source and simple photodiode detectors.
Smart Fibres employs the latter method in its interrogation units and the diagrams below will help to illustrate how they work.
Schematic and operating principle of WDM equipment.
Key: a) light source, b) scanning filter, c) scan generator, d) coupler network for channels 1-4, e) FBG arrays, f) photo-detectors,
g) processor and h), time varying output of the detector on channel 4, showing times ti converted into Bragg wavelengths λλi.
The scan generator tunes the light source, sweeping it back and forth across its range such that at any given instant the wavelength of light being transmitted down the fibres is known. When this wavelength coincides with the Bragg wavelength of an FBG, light is reflected back down the fibre to a photodetector. The scan generator also supplies a timing signal to the processor, allowing it to convert the intensity vs. time information into a spectrum. Further processing is performed to identify peaks in this spectrum, find their peak positions and convert these to strain or temperature.
Time-Division Multiplexing (TDM)
A TDM system employs a pulsed broadband light source and identifies different gratings by the time taken for their return signals to reach a detector. The pulses from closer gratings are received before those from more distant FBG. The figure below shows an array of FBG at different distances l from the interrogation unit. The time ti required for a pulse return from an FBG at li is given by:
ti = 2li n/c
where c is the speed of light in vacuo and n is the fibre refractive index.
Having established the position of a grating in an array, a system of passive sloped filters, as described previously, can be used to determine the wavelength of each pulse as it arrives. Alternatively a high-speed spectrometer can be used.
Schematic and operating principle of TDM equipment. Top: Pulses from light source (a) pass through coupler (b), which is also connected to detector (c), to fibre (d) containing FBG (e). Bottom: pulses emanating from source at time t0 are reflected from FBG at l1, l2 and l3, and return at t1, t2 and t3 respectively.
At the time of writing, Smart Fibre's position is that tuneable or swept laser WDM technology offers the best price/performance ratio available.