Fotech Solutions Limited

Nortel Networks Corporation

Sotiris Emil Kanellopoulos

Alan John Rogers

Sergey Vladimir Shatalin

Vincent Handerek

Sergey Shatalin

Andrew L Reynolds

Ricardo Feced

Bruce R Napier

A partially coherent Optical Time Domain Reflectometry (OTDR) apparatus has a light source comprising a directly modulated semiconductor Distributed FeedBack (DFB) laser diode for transmitting partially coherent light pulses along a monomode optical fibre. Light Rayleigh backscattered from the light pulses as they travel along the optical fibre is output from the end of the fibre into which the light pulses are transmitted to a Fibre Bragg Grating (FBG) filter. The FBG filter reduces the supectral width of light received at a photodetector. In one embodiment, the supectral width of the FBG filter is around one fifth of the supectral width of the light pulse after it has travelled around 1 km along the optical fibre. As a consequence of reducing the supectral width of the light received at the photodetector, the FBG filter increases the temporal coherence of the light. So, the FBG filter can ensure that the detected light is sufficiently coherent that a temporal supeckle pattern can be detected at the photodetector. At the same time, the light traveling in the optical fibre can be relatively supectrally broad so that non-linear effects in the optical fibre, such as Brillouin scattering, can be reduced.

Detecting a disturbance in the phase of light propagating in an optical waveguide

A pressure sensing apparatus has a light source for transmitting pulses of light along a monomode optical fiber. The polarization of light backscattered from the light pulses in the optical fiber is detected by a polarization processing unit (PPU) and a photo detector. The optical fiber is adapted to deform asymmetrically under the influence of applied external isotropic pressure, e.g. from a fluid. The deformation causes the birefringence of the optical fiber to change proportionally to the applied pressure. The change in birefringence can be determined from the detected polarization of the backscattered light, allowing detection of pressure distribution in the fluid. Importantly, the construction of the optical fiber is such that the birefringence beat length of the optical fiber at the wavelength of light propagated by the fiber remains more than twice the spatial length of the light pulses transmitted along the optical fiber. Applications of the invention include detection of fluid flow; location of an interface between two fluids of different density; and tsunami detection.

Method and apparatus for detecting pressure distribution in fluids

An optical time domain reflectometry apparatus has a laser and light modulator for producing coherent light pulses, each having two sections of higher intensity separated by a gap of lower or substantially zero intensity. As the light pulses propagate along the optical fibre, light is continuously Rayleigh backscattered by inhomogeneities of the optical fibre. A photodetector generates backscatter signals representing the intensity of light Rayleigh backscattered in the optical fibre as each light pulse travels along the optical fibre. The PC uses these backscatter signals to derive a difference signal representing a change dI in intensity between signals generated from two successive pulses. The PC then calculates the Root Mean Square (RMS) of the difference signal averaged over the interval between the two sections of the light pulses. Next, the PC averages the backscatter signal generated from the first of the pulses over the same interval and normalises the RMS difference signal using the averaged signal to obtain a compensated difference signal that depends only on differences in the rate of change of phase of light of the light pulses as they travelled along the waveguide. This is repeated at different wavelengths to allow the compensated difference signal to be adjusted to represent the magnitude of the differences.

Detecting a disturbance in the propagation of light in an optical waveguide

Compensating for polarization mode dispersion in a birefringent optical transmission fibre is achieved by controlling the birefringence of the fibre. The difference in group velocity of the orthogonal polarization states of an optical signal transmitted over the fibre is monitored to generate an error signal representing the difference. The birefringence of the fibre is adjusted accordingly to minimize the difference and thereby provide dynamic compensation. Birefringence control may be achieved by a non-linear fibre grating written into the fibre to impose a differential time delay. The fibre may be a side hole fibre (SHF), a holey fibre (HF), a photonic crystal fibre (PCF), or any other suitable microstructure fibre. The fibre may have stressing rods, may be tapered along its length and may be controlled electrically, mechanically, acoustically or thermally by spaced heating elements.

Compensating for polarisation mode dispersion in optical transmission fibers

A method of writing a diffraction grating into a photosensitive optical transmission medium is disclosed, in which a first phase mask is used to generate an interference pattern in a second phase mask substrate and using photolithographic techniques. This produces a second phase mask, which is then use to write a diffraction grating into a photosensitive optical transmission medium. This method enables the line placement in a phase mask to be improved by averaging out the line placement errors.

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London, United Kingdom

Sotiris Emil Kanellopoulos