1. Field of the Invention
The present invention relates generally to fiber laser acoustic sensors, and specifically to a system for multiplexed high resolution measurement of frequency variations in fiber laser acoustic sensors.
2. Description of Related Art
Since the development of amplifiers for optical signals, efforts have been made to improve the optical amplifying fiber into which an optical signal to be amplified and pump light are propagated, causing amplification of the optical signal. Such optical amplifying fibers are commonly known as "active" fibers. An active fiber is a fiber doped with one or more of the rare earth family of elements, such as erbium, which generates a light source by introducing an excitation signal into the doped fiber which in turn causes the fiber to emit a light characteristic of the dopant. For discussion purposes, we will describe a fiber doped with erbium, although it is understood that the invention covers a fiber doped with any of the elements which have similar characteristics to those described herein. When erbium is pumped with a laser at the appropriate wavelength, it emits a light in the 1525 to 1560 nanometer (nm) wavelength. When an erbium doped fiber is supplied with a source of energy being pumped into the fiber, such as for example a wavelength of 1480 nm generated by a pumping laser diode, the electrons in the erbium absorb the energy and jump to a higher energy state. This energy may later be released as coherent laser light on which a measurand signal is encoded.
An active sensor is formed by positioning the erbium doped fiber between a pair of wavelength-matched Bragg gratings, where the Bragg gratings reflect a certain wavelength while allowing all other wavelengths to pass through. Thus, a resonant cavity is formed in the doped fiber between the Bragg gratings. The stored energy from the doped fiber is eventually released as a wavelength between 1525 to 1560 nm traveling down the fiber. Active sensors may be used for both strain and temperature measurements, since these elements affect the optical properties of the active sensors. The length of the doped fiber changes in conjunction with changes in temperature and pressure acting on the fiber, which in turn changes the length of the resonant cavity. By changing the length of the resonant cavity, the lasing wavelength of the active sensor similarly changes. The wavelength emitted by the active sensor can be measured to determine the length of the resonant cavity which, in turn, represents the strain acting on the active sensor from the acoustic pressure or temperature.
Active sensors may be formed having either single or multiple longitudinal modes, where the length of the resonant cavity and the doping material used determine the number of modes resulting in an active sensor. Multimode active sensors have longer resonant cavities which make them easier to construct and operate, because more gain is available in the longer resonant cavities and cavity length requirements are not very stringent. The output spectrum of multimode active sensors, however, spans several gigahertz, which degrades the signal-to-noise ratio in readout devices that analyze the active sensor frequency shifts. For high resolution analysis of frequency shifts in an active sensor, it is generally desirable to suppress all but the dominant longitudinal mode. Thus, it is desirable to operate in a single lasing mode with a single frequency emitted. Erbium is typically used in most active sensors, because erbium has a very broad gain bandwidth and will amplify a number of frequencies. The active fiber sensor must be made as small as possible in order to operate with as few as possible wavelengths. To produce a single resonant frequency, the resonant cavity must be very short, resulting in a small amount of erbium or other doping material being present in the cavity. This tends to produce a fairly weak signal that cannot be transmitted over long distances. With an extremely short resonant cavity, a large amount of pump power must delivered to the active sensor to produce a signal from the active sensor strong enough to be transmitted more than a minute distance.
There is a need for a multimode active sensor which does not possess a degraded signal-to-noise ratio associated with the large output spectrum of prior active sensors. Moreover, there is a need for a multimode active sensor allowing for a high resolution analysis of frequency shifts occurring in the active sensor to be obtained similar to that of a single mode active sensor, without suffering the stringent operating and manufacturing requirements of a single mode active sensor.