- Intracavity Fiber Sensors Are Sensitive and Inexpensive
A novel approach to monitoring fiber optic sensors by placing them inside a mode-locked laser resonator has been conceived and demonstrated by researchers at Insensys Ltd. of Fareham and at Aston University, both in the UK. The technique is sensitive, and its straightforward design eliminates several expensive components required in other fiber optic monitoring systems.
Figure 1. In wavelength division multiplexing, each sensor reflects a different wavelength. The sensors can be fiber Bragg gratings, Fabry-Perot interferometers or other interferometric devices whose reflective spectrum depends on temperature or strain. The optical spectrum analyzer monitors the signal reflected from each sensor.
Fiber optic sensors find frequent application in the measurement of temperature and strain. They might be placed, for example, in an airplane wing or a highway bridge to measure the amount of strain in the wing's metal or the bridge's concrete. Two major advantages of these sensors over conventional electronic ones, such as piezoelectrics, are their imperviousness to electromagnetic interference and their ability to be multiplexed on a single fiber with a single source.
Figure 2. In time division multiplexing, a pulsed laser illuminates the sensor array, and the optical spectrum analyzer is gated to separate the signals returned from the different sensors.
Wavelength division multiplexing (WDM) of fiber optic sensors requires that each sensor reflect a different wavelength (Figure 1), while in time division multiplexing (TDM), each sensor can reflect the same wavelength (Figure 2). There are drawbacks to each approach, however. The number of sensors in WDM is limited by the bandwidth of the source. This disadvantage can be overcome with TDM, but the requirement for a pulsed laser and a gated detector drives up the system's cost and complexity dramatically.
Figure 3. The second of 10 fiber Bragg gratings (FBGs) in the sensor array is mounted on a thermoelectric cooler (TEC). If the round-trip transit time between the TEC-mounted FBG and the broadband rear reflector is equal to the period of the electrical pulses applied to the semiconductor optical amplifier (SOA), a mode-locked laser is created. The mode-locking frequency is the reciprocal of that round-trip transit time. The reflective wavelength of the FBG sensor is a function of temperature and is read by the optical spectrum analyzer (OSA). ©2004 IEEE.
The investigators developed a new approach to TDM by driving a semiconductor optical amplifier (SOA) with a pulse generator (Figure 3). In essence, they created a mode-locked laser whose gain medium was the SOA and whose resonator was formed by the fiber Bragg grating (FBG) rear reflector and one of 10 of the gratings in the sensor array. Which of the 10 array FBGs served as the resonator mirror was determined by the frequency of the pulse generator: For the laser to oscillate, the round-trip transit time of the resonator had to match the period of the electrical signal applied to the SOA. As long as the duration of the gain in the SOA was shorter than the round-trip transit time between any two sensors, only one sensor could serve as a resonator mirror.
Figure 4. Each trace is the return signal at the optical spectrum analyzer when the pulse-generator frequency has selected a different fiber Bragg grating in the array to serve as the resonator mirror. The grating has been detuned slightly from its nominal wavelength by temperature or by strain. The absence of residual signal at the nominal wavelength indicates a high degree of isolation of each sensor from its neighbors. ©2004 IEEE.
To demonstrate the high degree of isolation between detectors, the researchers detuned one of the FBGs from its nominal frequency with a thermoelectric cooler. When the pulse generator was adjusted to the resonator frequency that selected that FBG as a resonator mirror, there was no visible return from the other gratings (Figure 4).
In the field, the pulse generator would be tuned to select each FBG in the array, one at a time, to serve as the resonator mirror. The exact wavelength of that grating would be indicated by the optical spectrum analyzer, and that wavelength would measure the temperature and/or strain experienced by the FBG.
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