Method and apparatus for generation and transmission of high energy optical pulses for long range measurements

• US 20060210269A1
• Filed: 02/11/2004
• Published: 09/21/2006
• Est. Priority Date: 02/12/2003
• Status: Active Grant

• Current Assignee: Sensornet Limited (Nova Holdings LLC)
• Original Assignee: Sensornet Limited (Nova Holdings LLC)
• Inventors: Parker, Tom Richard, Farhadiroushan, Mahmoud, Kean, Peter Neil
• Granted Patent: US 8,005,323 B2
• Time in Patent Office: Days
• US Class Current: 398/48
• CPC Class Codes: G01D 5/35364 using inelastic backscattering to detect the measured quantity, e.g. using Brillouin or Raman backscattering
• CPC Class Codes: G01K 11/32 using changes in transmittance, scattering or luminescence in optical fibres

Abstract

A method and apparatus for generating and transmitting high energy optical pulses are described. Distributed temperature sensors usually use Raman scattering in optical fibres as the means to determine the temperature. Here, light from a laser source is sent down a fibre and the small amount of light that is scattered back towards the source is analysed. As the fibre length increases, the resolutions of the temperature and loss measurements become poorer. This is because losses in an optical fibre attenuate the signal. An obvious solution to this problem is to launch more light into the fibre to compensate for the losses but stimulated Raman scattering limits how much light may be launched. The present invention solves this problem by using a pulse conversion method to maximise the resultant pulse energy while the power is kept below SRS threshold.

Claims

1-39. -39. (canceled)

40. A sensing system for sending electromagnetic signals along a waveguide and sensing conditions at one or more sensing locations along the waveguide, the system comprising a transmitting means arranged to transmit pulses of suitable shape to cause a controlled conversion of the pulses to a different wavelength by a stimulated inelastic scattering process before reaching the sensing location, the system comprising a receiving means arranged to receive signals returned from the sensing location, and to determine the conditions based on the received signals.

41. The system of claim 40, the power of the pulses being such that the different wavelength undergoes one or more further conversions to further different wavelengths such that different parts of the waveguide are illuminated by different wavelengths, and the receiving means is arranged to distinguish the returning optical signals at each of the different wavelengths.

42. The system of claim 40, having a controller arranged to carry out any one or more of the following:

adjusting the pulse power to move a transition length where the converting takes place, to a different part of the fibre;

controlling a pulse repetition rate.

43. The system of claim 40 having an arrangement to compensate for differential loss at different wavelengths.

44. The system of claim 43, the transmitting means comprising an optical source for generating the pulses;

a splitter arranged to split the pulses into two or more streams, and a convertor for converting the wavelength of one of the streams, before sending the streams along the fibre, the receiving means being arranged to compensate for differential loss by detecting an up shifted component returned from one of the pulses at a first wavelength, and a down shifted component returned from one of the pulses transmitted at a higher wavelength, down shifted to the first wavelength.

45. The system of claim 40, the receiving means being additionally arranged to receive signals returned from one or more further sensing locations, before the conversion.

46. The system of claim 40 comprising any one or more of:

a pulse shaper for lengthening the pulses before sending along the fibre;

a dispersive element for lengthening the pulses;

a step index multimode fiber element for lengthening the pulses; and

a pulse train converter for converting each pulse into a train of pulses, followed by a dispersive element for shaping the train of pulses.

47. The system of claim 40, the transmitter means being arranged to initiate one or more generations of stimulated Raman light such that different parts of the fibre can be illuminated by different wavelengths, and arranged to transmit more than one pulse in the fibre at the same time;

and the receiving means having optical filtering means to distinguish different components of backscattered light and one or more detectors for converting the filtered optical components into electrical signals.

48. The system of claim 40, the transmitting means comprising a laser, a modulator for producing pulses from the laser output, and an optical amplifier for amplifying the pulses.

49. The system of claim 40, the receiving means having any one or more of the following:

filters arranged such that a desired backscattered signal is collected at a slightly offset wavelength compared to a wavelength of the transmitted pulses before wavelength conversion;

filters arranged such that Raman Stokes and/or Raman anti-Stokes and/or Rayleigh signal components backscattered from one section of the fibre are collected without significant overlap from equivalent components backscattered from other sections;

a non-linearity correction arrangement arranged such that one of, two of or all of the returning Raman Stokes, Raman anti-Stokes and Rayleigh signal components are compared to correct for any non-linearity in the signals, and a non linearity correction arrangement arranged to use a combination of the returning Raman Stokes, and Raman anti-Stokes components which are in a saturated, non linear region of their relationship to the sensed condition, to compensate for the non linearities.

50. A sensing system for sending electromagnetic signals along a waveguide and sensing conditions at one or more sensing locations along the waveguide, the system having a receiving means and a transmitting means having a source for generating a stream of pulses, a splitter arranged to split the pulses into two or more streams, and a convertor for converting the wavelength of one of the streams, before sending the streams along the fibre.

51. The system of claim 50, the source and splitter being arranged to provide sufficient power in the pulses reaching the convertor such that the conversion is carried out by stimulated inelastic scattering.

52. The system of claim 50, further comprising any one or more of:

a pulse shaper for lengthening the pulses before sending along the fibre;

a dispersive element; and

a pulse train converter for converting each pulse into a train of pulses, followed by a dispersive element for shaping the train of pulses.

53. A pulse shaper for shaping an optical input pulse, the shaper having a pulse train generator for generating a train of pulses from the input optical pulse, the generator comprising a means for splitting the input optical pulse between two or more paths with different delays, to create delayed pulses, and a combiner for combining the delayed pulses to create the train, and a dispersive element located before or after the generator to spread the input pulse or the train respectively.

54. The apparatus of claim 53 having one or more optical switches for selecting which of the delayed pulses form the train.

55. Apparatus for generating and transmitting high energy optical pulses along a fiber which provides a scattering medium which apparatus comprises an optical pulse generator for generating the high energy optical pulses, an optical arrangement for lengthening a duration of each pulse to reduce a peak power such that stimulated Raman light is suppressed while maintaining pulse energy for transmission along the fiber, and a receiver for measuring different components of backscattered light from the fiber for remote sensing.

56. Apparatus as claimed in claim 55 in which the optical fibre comprises lengths of different fibres to provide different Raman shifts down the length or to change the modal characteristics of the propagating light.

57. Apparatus as claimed in claim 55 in which the amount of lengthening corresponds to a spatial resolution required.

58. Apparatus as claimed in claim 55 in which the arrangement for lengthening comprises any one or more of the following:

a dispersive element;

a step index multimode fiber element;

a pulse train converter for converting each pulse into a train of lower power pulses, followed by a dispersive element for shaping the train of pulses into a lengthened pulse;

one or more sets of splitters and recombiners each of which sends the input light pulse down two or more paths of different optical lengths and recombines the light to form a pulse train; and

optical fibre and optical fibre components;

an arrangement in which the pulse train duration and pulse train pattern resultant from a pulse conversion may be fixed or may be controlled by use of optical switches which select or deselect a number of pulse converters used and/or the paths used in the pulse converters.

59. Apparatus as claimed in claim 55 in which any one or more of the pulse train width, pattern, and energy may be varied to help to correct for non-linearities.

60. Apparatus as claimed in claim 55 in which any one or more of the returning Raman Stokes, Raman anti-Stokes and Rayleigh signals may be compared to correct for any non-linearity in the signals.

61. A sensing system for sending electromagnetic signals along a waveguide and sensing conditions at one or more sensing locations along the waveguide, the system having a transmitting means for sending pulses along the waveguide, a receiving means and the waveguide, the waveguide comprising a number of sections having different characteristics such that backscatter from the pulses has different optical properties, the receiving means being arranged to use the different optical properties to distinguish the backscatter from the different sections.

62. The sensing system of claim 61, the sections having waveguides providing any one or more of the following:

different Raman shifts to provide backscatter components at different wavelengths; and

different changes in the modal characteristics of the propagating light, to provide backscatter components with different modal characteristics.

63. The sensing system of claim 61 comprising any one or more of:

a pulse shaper for lengthening the pulses before sending along the fibre;

a dispersive element for lengthening the pulses;

a step index multimode fiber element for lengthening the pulses; and

a pulse train converter for converting each pulse into a train of pulses, followed by a dispersive element for shaping the train of pulses.