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Fiber Laser Interrogates Fiber Bragg Grating Sensors

Photonics Spectra
Dec 2003
Breck Hitz

Researchers at Texas A&M University in College Station have designed and built a wavelength-scanned, Q-switched, erbium-doped fiber laser that is uniquely suitable for interrogating a series of multiplexed fiber Bragg grating (FBG) sensors even when the reflection spectra of some of the sensors overlap.

Fiber Bragg grating sensors are fabricated by writing a diffraction grating into a short length of optical fiber. The sensor reflects one particular wavelength and transmits all others, but the reflected wavelength varies with the sensor's temperature and with physical stress applied to the sensor. Thus, FBG sensors have found wide application in monitoring temperature or size changes. An advantage of FBG sensors is that a number of them can be multiplexed in series along a single optical fiber so that a single instrument can simultaneously monitor many individual sensors.

Normally, FBG sensors can be monitored either by illuminating the fiber with a broadband source and observing the spectral characteristics of the reflected light or by illuminating the fiber with a frequency-scanned source and monitoring the temporal characteristics of the reflected light. The sensors' reflectivities must be in different spectral regions, however, because the reflectivity of the first sensor in a given spectral region would mask a sensor farther down the fiber. The researchers have devised a technique to combine time division multiplexing with spectral division multiplexing to maximize the information that can be transferred within a given spectral range.

The Texas A&M Q-switched laser provides a sufficiently high peak power to monitor two or more fiber Bragg grating sensors in the same spectral region. The laser's resonator is formed by two wave-length-dependent reflectors: a motor-scanned grating at one end and a fiber Fabry-Perot interferometer at the other (Figure 1).

Fiber Laser Interrogates Fiber Bragg Grating Sensors
Figure 1. The fiber laser at the left side of the drawing emits a stream of wavelength-tuned, Q-switched pulses into the multiplexed fiber Bragg grating sensors on the right side. A photodetector senses the reflections from the sensors, and a computer analyzes the signal.

The reflectivity of the fiber Fabry-Perot is a series of peaks separated by the interferometer's free spectral range, while the reflectivity of the motor-scanned grating is a single peak that can be scanned across the approximately 46-nm gain bandwidth of the erbium-doped fiber. The bandwidth of the grating is about 10 times the bandwidth of the individual Fabry-Perot peaks (Figure 2).

When the grating is scanned at an appropriate rate across the Fabry-Perot peaks, a Q-switched pulse occurs near the time when two of the reflectivity peaks are aligned (Figure 2). The scanning rate must be timed so that the population inversion builds and saturates in resonance with the Q-switching frequency. The timing of the population inversion depends on pump power, so the pump power ultimately dictates the appropriate rate for scanning the motor-scanned grating.

Fiber Laser Interrogates Fiber Bragg Grating Sensors
Figure 2. The fiber laser's two end reflectors are a motor-scanned grating and a fiber Fabry-Perot interferometer. When the single peak of the grating is scanned across the multiple Fabry-Perot peaks, a Q-switched pulse is emitted near the time the grating peak lines up with each of the Fabry-Perot peaks.

When the 980-nm diode laser pumps the fiber laser with 55 mW, the latter produces a stable train of Q-switched pulses if the motor-scanned grating is scanned at approximately 20 Hz. In this case, the average power from the fiber laser is 5 mW in 120-nJ pulses with a repetition rate of approximately 4 kHz. The pulse duration is approximately 3.2 µs.

These pulses from the laser are injected into the fiber containing three separate fiber Bragg grating sensors, as shown in Figure 1. FBGa1 and FBGa2 both reflect light at the same wavelength; their reflectivities are 3 and 2 percent, respectively. FBGb reflects light at a different wavelength. The bare, cleaved end of the fiber reflects approximately 4 percent at all wavelengths.

The reflected spectrum when the laser is tuned to the region of FBGb's reflectivity is shown in Figure 3a. The regular, constant spikes are the Q-switched pulses reflected by the cleaved surface at the far end of the fiber. Their 25-µs spacing corresponds to the 4-kHz Q-switching frequency of the laser. Each of these pulses is frequency-shifted from its neighbor by the free-spectral range of the fiber Fabry-Perot in the laser; that is, by about 10.3 GHz, or 0.078 nm. As the wavelength of these pulses tunes across the reflectivity of FBGb, another set of reflected pulses appears approximately 18 µs in front of the pulses reflected from the end of the fiber. The round-trip delay time of the 900-m fiber between FBGb and the end of the fiber is approximately 18 µs, so these new pulses are reflected from FBGb.

Fiber Laser Interrogates Fiber Bragg Grating Sensors
Figure 3. These traces show the returned signal when the laser is tuned to the reflectivity of the fiber Bragg gratings. The large, constant pulses are the reflections from the end of the fiber: (a) As the laser is tuned through FBGb's reflectivity, a second set of pulses appears. Each pulse is reflected from FBGb, and each precedes the pulse reflected from the end of the fiber by approximately 18 µs. The round-trip delay of the 900 m of fiber between FBGb and the end of the fiber is approximately 18 µs. (b) When the laser is tuned across the reflectance region of FBGa1 and FBGa2, reflected pulses from both fiber Bragg grating sensors appear. The barely discernible, approximately 8-µs separation between a pulse reflected from FBGa1 and a pulse reflected from FBGa2 corresponds to the 400 m of delay line between the two. (c) When FBGa2 is heated, the set of pulses reflected from it is clearly shifted to a different frequency. But each pulse reflected from FBGa2 still precedes the pulse reflected from the end of the fiber by approximately 18 µs: the delay of the 900 m of fiber between FBGa2 and the end of the fiber.

The reflected spectrum when the laser is tuned to the region of FBGa1's and FBGa2's reflectivity is shown in Figure 3b. Here, two sets of pulses -- one set about 18 µs in front of the pulses reflected from the fiber end and the other approximately 8 µs in front of the first set -- can barely be discerned. These are the reflections from FBGa2 and FBGa1, respectively.

Figure 3b is the same as Figure 3c, except now the heat plate to which FBGa2 is attached has been turned on, and FBGa2 is operating at an elevated temperature. The set of pulses reflected from FBGa2 has clearly shifted in frequency.


GLOSSARY
optical fiber
A thin filament of drawn or extruded glass or plastic having a central core and a cladding of lower index material to promote total internal reflection (TIR). It may be used singly to transmit pulsed optical signals (communications fiber) or in bundles to transmit light or images.  
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