Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Reexamination Certificate
1999-06-01
2002-04-30
Pyo, Kevin (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
C250S227230, C385S013000
Reexamination Certificate
active
06380534
ABSTRACT:
The present invention relates to an apparatus for the simultaneous measurement of temperature and strain in optical waveguides and, in particular, relates to strain and temperature measurements in monomode optical fibres.
There is a need to monitor distributions of temperature and/or strain in many fields of engineering from power station steam pipes to smart structures and aircraft bodies. The sensors often need to cover large volumes, to be integrable into complex structures and to be immune from interference by electromagnetic fields. Optical-fibre sensors should be able to fulfil these requirements. Commercial sensing systems are available which use optical fibres to measure temperature distributions. However, these systems are not capable of measuring distributed strain. If strain could be measured then the fibres could be embedded into critical structures. such as aircraft components, bridges and dams and be used to give advance warning of possible failure. The measurement of strain would also allow the measurement of pressure, since the act of squeezing a fibre extends it. A particular application for such a system is in the field of oil exploration where the fibre can be used as a distributed pressure (and temperature) sensor down a bore hole.
Optical-fibre sensors which measure variations of a parameter along the fibre length can be divided into two classes: quasi-distributed and fully distributed. In quasi-distributed sensing, certain sections of the fibre are modified and used for local measurement and the rest is used simply to carry the light from sensor to sensor and, finally, to the detector. In fully distributed sensing, the fibre is unmodified and the whole length is used for light transmission and sensing. Fully distributed sensors are the more flexible and can be used with fibres already in place, allowing applications such as diagnostics of optical-fibre communication networks.
A possible technique for the fully distributed measurement of strain/temperature is based on the Brillouin effect. Here, a fraction of the optical power launched down a fibre is scattered at some point in the fibre, causing it to change optical wavelength and return towards the optical source. Critically, the wavelength of the returning light depends upon the strain/temperature of the fibre at the point at which the light was scattered. The wavelength of the returning light can then be measured to yield the strain/temperature in the fibre from the point at which the returning light was generated. If the incident light is launched as a short pulse, then by recording the wavelength of the returning light as a function of time, the strain/temperature at all points along the fibre can be measured.
There are two techniques which can be used to detect strain only, or temperature only, from the measurement of the Brillouin shift: (1) using a pump-probe system to produce frequency-dependent Brillouin power; (2) using optical heterodyning to detect the Brillouin signal. However, both methods require special arrangements of fibres to deconvolve the temperature and strain information. For example, one solution that has been suggested is, to pass a single fibre twice over the same region in such a way that one section is isolated from the strain and only affected by temperature. This approach makes the installation of the sensing optical fibre cable complicated and cannot be used with the existing optical fibre networks.
If it is desired to measure strain and temperature simultaneously along the same length of a single optical fibre, then both the amplitude and frequency shift of the Brillouin backscattered light has to be detected. Technique (1) would fail, as the stimulated Brillouin power produced would be practically independent of the temperature and strain due to the non-linear interaction of the pump and probe beams. Technique (2) would have difficulties due to polarisation noise-induced signal fading.
Another technique (3), used in the detection of temperature, is to measure the power of the Brillouin backscattered signal using a narrow bandwidth optical filter. However, this is not practical as: 1) amplitude variations due to drift in the optical filter response and the optical source wavelength; 2) strain cannot be determined from measuring only the Brillouin backscatter power.
We have devised a distributed sensing system which overcomes these difficulties.
The technique we have devised provides for referencing to overcome any optical filter or source frequency drift and to accurately detect both the amplitude and frequency of the Brillouin scattered light as a finction of time and, therefore, allowing simultaneous measurement of strain and temperature distributions along the same length of optical fibre.
According to the present invention there is provided apparatus for simultaneous measurement of temperature and strain distributions, which apparatus comprises a light source for generating pulses of light, a sensing network which comprises at least one optical fibre down which can pass pulses of light generated by the light source, a conversion means adapted to convert physical parameters into changes of strain or temperature along the sensing optical fibre thereby modifying the spectral response of the backscattered light passing back down the optical fibre (Brillouin backscatter and Rayleigh backscatter signals), a reference section subjected to known magnitude of the physical parameters, a receiver means in which a portion of returned light is passed on to a scanning optical filter able to resolve the Rayleigh peaks and Brillouin peaks and in which the optical signals are converted to electrical signals which are then fed into a processor means; the scan rate of the scanning optical filter being slower than the repetition of the optical pulses allowing the spectral light of the backscattered light to be recorded along the length of the optical fibre and both the amplitude and frequency shift of the Brillouin peaks relative to the Rayleigh peaks to be accurately measured from which temperature and strain distributions along the same length of optical fibre can be determined.
Light is transmitted down the optical fibre and backscattered light is transmitted back down the fibre. This light will be predominately of the same wavelength as the transmitted light (the Rayleigh peak), but some of the light will have a frequency shift due to the interaction of the energy of the vibrational state of the optical fibre and the light (the Brillouin peaks). There can be either addition of energy to the light, which gives backscattered light of a shorter wavelength (anti-Stokes scattering) or there can be removal of energy from the light which gives backscattered light of longer wavelength (Stokes scattering). The amplitude of the Brillouin peaks and the frequency shift of the Brillouin peaks compared with the Rayleigh peak is a measure of the strain and the temperature of the optical fibre at the position from where the light is backscattered.
The light source preferably generates coherent light in the visible or infrared spectrum, e.g. it is a laser, and conventional lasers can be used. The light source can be a narrow linewidth laser and it can be a solid-state laser, semiconductor laser diode or fibre laser source and it can include an external cavity for controlling the linewidth and the operating wavelength.
The modulating means pulses the light from the light source so that light is transmitted down the optical fibres in pulses, the light can be modulated using Q-switched, modelocked or direct modulation techniques or it may be modulated by an external modulator such as an acoustic optic modulator or an integrated optics modulator.
Preferably there is an amplifying means which can amplify the backscatter light and optical pulses. Optical amplifiers may be used to amplify the optical signals at the transmitter, receiver and in the sensing network means and the amplifiers may be solid state semiconductor or optical fibre amplifiers.
The conversion means converts the parameter to be measured
Farhadiroushan Mahmoud
Parker Tom Richard
Bartlett & Sherer
Pyo Kevin
Sensornet Limited
Sherer Ronald B.
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