A Brillouin analysis sensor is a fibre optic distributed sensor that can measure changes in the local strain and/or temperature conditions of an optical sensing fibre through analysis of the Brillouin frequency of the sensing fibre at any point. The sensing fibre is usually attached to or embedded in a host material and is used to measure the strain and/or temperature conditions of the host. Both strain and temperature cause a shift in the Brillouin frequency. Position is determined by the round-trip transit time of the optical signal in the fibre. Reference herein to a Brillouin analysis sensor system shall mean a Brillouin analysis sensor plus an optical sensing fibre.
A Brillouin analysis sensor system operates on the principle of Brillouin amplification. In a Brillouin amplifier, a signal (or Stokes) light wave propagating in one direction experiences optical gain if its frequency falls within the Brillouin gain profile of the amplifier pump wave propagating in the opposite direction. The Stokes wave experiences gain (with a corresponding loss from the pump) in accordance with the Brillouin gain profile. The power in either wave may be monitored to determine the Brillouin gain profile; if the Stokes wave is chosen to be monitored then the system is said to operate in Brillouin gain mode (as any interaction causes an increase in the Stokes wave). Otherwise, it is said to operate in Brillouin loss mode.
Two lasers may be used to generate the two lightwaves, in which case the frequency difference between them is maintained and controlled by a control loop (often a phase-locked loop). Alternatively, a single laser source may be used and a second lightwave generated from it by optical frequency modulation (using an electronic signal of the desired frequency).
One type of Brillouin analysis sensor is a Brillouin Optical Time Domain Analysis (“BOTDA”) system. To obtain spatial information in a BOTDA system, one of the two light waves is pulsed by an electro-optic modulator (EOM) driven by an electronic pulse generator, while the intensity of the other light wave is monitored as a function of time elapsed (i.e. in the time domain) since the generation of the pulse. In this way position, z, is determined from the time-of-flight, t, as z=ct/2n, where c is the speed of light and n is the group index of refraction of the fibre. The intensity of the interaction (either gain or loss depending on which wave is monitored) is a function of the relative frequency difference between the two lasers. It is also a function of the gain profile of the fibre at the point of interaction, which moves (with the pulse) along the fibre. The frequency difference is swept over some range of frequencies of interest. Since frequency characteristics of the Brillouin profile are modified by the strain and temperature conditions of the sensing fibre, the local strain and temperature conditions can be inferred from a measurement of the Brillouin gain or loss profile. This profile is obtained by considering the Brillouin interaction at a given point in space as a function of frequency difference.
A second type of Brillouin analysis sensor is the Brillouin Optical Frequency Domain Analysis sensor (“BOFDA”). In frequency domain analysis, a sine-wave amplitude modulated light wave is used in place of the pulse modulated wave used in BOTDA. Measurements are carried out in the frequency domain by sweeping this modulation frequency and measuring the interaction with the cw wave. The frequency domain data can be mathematically transformed to the time domain to obtain the positional information which is the equivalent of the signal that would be obtained by BOTDA.
A third type of Brillouin analysis sensor is the Brillouin Optical Correlation Domain Analysis sensor (“BOCDA”). In correlation domain analysis both light waves are modulated and positional information is extracted from the correlation of the two signals. Correlation based sensors can have very high resolutions, although at the cost of limited overall sensing length.
Traditionally bright pulses have been used to interrogate the sensing fibre with BOTDA systems, however the use of dark pulses has recently been shown to have several advantages in terms of resolution, accuracy and speed.
BOTDA (and all other Brillouin analysis distributed sensors) require that the state of polarization (“SOP”) be matched (parallel) between the two light waves (pump and Stokes) at all locations in the sensing fibre. If the SOP is not matched, then the intensity of the Brillouin signal will fade at locations where the SOP is mismatched, resulting in amplitude noise and possibly locations in the sensor where the two SOPs are orthogonal and no signal can be obtained.
In prior art BOTDA sensor systems, the SOP can be kept aligned but only if special polarization-maintaining (“PM”) fibres are used as the sensing element. Such fibres have higher attenuation, are much more expensive and are more difficult to work with compared to standard single mode fibres (“SMF”).
If non-PM fibre (i.e. SMF) is used, then the state of polarization of the two lightwaves will vary with position (and possibly with time) along the fibre length. To deal with this situation, a prior art BOTDA system will often take multiple measurements on SMF, altering the lightwaves' relative polarization states between measurements, and using an average result. It has been shown that using averaged data from switching one of the waves between two orthogonal polarization states can reduce polarization fading, however the averaged signal will only be one half of the maximum Brillouin signal.
Alternatively, a set of measurements taken with three particular SOPs can improve this average to two thirds of the maximum, but requires at least triple the time to take data. Moreover, the devices typically used to alter the SOP in a BOTDA system have switching times which are slow enough to significantly add to the overall acquisition time of the sensor. Also, because the multiple measurements are made at different times, it is possible for changes to the conditions of the sensor (e.g. mechanical vibrations or motions) between measurements to alter the SOP so that the multiple measurements no longer average orthogonal SOPs, thus reducing the efficacy of polarization averaging methods.
BOTDA distributed sensor systems are therefore subject to polarization fading effects which require expensive PM fibre or polarization averaging schemes.