WO2007141464A1 - Propagation d'impulsions optiques - Google Patents

Propagation d'impulsions optiques Download PDF

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Publication number
WO2007141464A1
WO2007141464A1 PCT/GB2006/002083 GB2006002083W WO2007141464A1 WO 2007141464 A1 WO2007141464 A1 WO 2007141464A1 GB 2006002083 W GB2006002083 W GB 2006002083W WO 2007141464 A1 WO2007141464 A1 WO 2007141464A1
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WIPO (PCT)
Prior art keywords
pulse
pulses
probe
optical
operable
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PCT/GB2006/002083
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English (en)
Inventor
Arthur Hartog
Peter Wait
Original Assignee
Schlumberger Holdings Limited
Schlumberger Canada Limitd
Schlumberger Surenco S.A.
Schlumberger Oilfield Assistance Limited
Schlumberger Overseas S.A.
Prad Research Amd Development N.V.
Schlumberger Services Limited
Services Petroliers Schlumberger
Schlumberger Technology B.V.
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Application filed by Schlumberger Holdings Limited, Schlumberger Canada Limitd, Schlumberger Surenco S.A., Schlumberger Oilfield Assistance Limited, Schlumberger Overseas S.A., Prad Research Amd Development N.V., Schlumberger Services Limited, Services Petroliers Schlumberger, Schlumberger Technology B.V. filed Critical Schlumberger Holdings Limited
Priority to PCT/GB2006/002083 priority Critical patent/WO2007141464A1/fr
Priority to GB0823038.5A priority patent/GB2469012B8/en
Publication of WO2007141464A1 publication Critical patent/WO2007141464A1/fr

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Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01M—TESTING STATIC OR DYNAMIC BALANCE OF MACHINES OR STRUCTURES; TESTING OF STRUCTURES OR APPARATUS, NOT OTHERWISE PROVIDED FOR
    • G01M11/00—Testing of optical apparatus; Testing structures by optical methods not otherwise provided for
    • G01M11/30—Testing of optical devices, constituted by fibre optics or optical waveguides
    • G01M11/31—Testing of optical devices, constituted by fibre optics or optical waveguides with a light emitter and a light receiver being disposed at the same side of a fibre or waveguide end-face, e.g. reflectometers
    • G01M11/3109—Reflectometers detecting the back-scattered light in the time-domain, e.g. OTDR
    • G01M11/3127—Reflectometers detecting the back-scattered light in the time-domain, e.g. OTDR using multiple or wavelength variable input source
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/25—Arrangements specific to fibre transmission
    • H04B10/2507—Arrangements specific to fibre transmission for the reduction or elimination of distortion or dispersion
    • H04B10/2513—Arrangements specific to fibre transmission for the reduction or elimination of distortion or dispersion due to chromatic dispersion
    • H04B10/25137—Arrangements specific to fibre transmission for the reduction or elimination of distortion or dispersion due to chromatic dispersion using pulse shaping at the transmitter, e.g. pre-chirping or dispersion supported transmission [DST]
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/25—Arrangements specific to fibre transmission
    • H04B10/2507—Arrangements specific to fibre transmission for the reduction or elimination of distortion or dispersion
    • H04B10/2513—Arrangements specific to fibre transmission for the reduction or elimination of distortion or dispersion due to chromatic dispersion
    • H04B10/2519—Arrangements specific to fibre transmission for the reduction or elimination of distortion or dispersion due to chromatic dispersion using Bragg gratings
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/25—Arrangements specific to fibre transmission
    • H04B10/2507—Arrangements specific to fibre transmission for the reduction or elimination of distortion or dispersion
    • H04B10/2537—Arrangements specific to fibre transmission for the reduction or elimination of distortion or dispersion due to scattering processes, e.g. Raman or Brillouin scattering
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/25—Arrangements specific to fibre transmission
    • H04B10/2507—Arrangements specific to fibre transmission for the reduction or elimination of distortion or dispersion
    • H04B10/2543—Arrangements specific to fibre transmission for the reduction or elimination of distortion or dispersion due to fibre non-linearities, e.g. Kerr effect
    • H04B10/255—Self-phase modulation [SPM]
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04J—MULTIPLEX COMMUNICATION
    • H04J14/00—Optical multiplex systems
    • H04J14/02—Wavelength-division multiplex systems

Definitions

  • the present invention relates to the propagation of optical pulses, particularly in optical fibres, with regard to the reduction of undesirable nonlinear effects.
  • the first limitation on the power which can be used is usually the onset of stimulated Raman scattering (SRS), a process by which power is converted from the probe wavelength to a Raman Stokes wavelength which, in silica-based optical fibres, is generally separated from the probe light by a frequency shift of around 400 cm "1 . For an incident wavelength of 1550 nm, this gives rise to a Stokes wavelength of approximately 1650 nm. SRS leads to a limitation on the peak optical power of around 1-1.5 W (depending on the fibre construction and the degree to which a small level of conversion to the Stokes wavelength is acceptable). Methods for alleviating these limitations are described in EP 0,636,868 [2] and EP 0,951,641 [3].
  • the prime limitation on the power which can be launched is usually due to stimulated Brillouin scattering (SBS).
  • SBS stimulated Brillouin scattering
  • the incident light is continuous or consists of a long series of pulses at the same frequency.
  • SBS results from the interaction between the incident light and thermally-generated vibrations of the glass forming the fibre.
  • the resultant new wavelength, the Brillouin Stokes wavelength is close in frequency to that of the incident light (a frequency shift of typically 10.7 GHz for incident light at 1550 nm).
  • the linewidth of the Brillouin process is very narrow (roughly 35 MHz).
  • the power level which can propagate before the onset of SBS is roughly proportional to the linewidth of the incident light. This is why SBS is generally significant only where the source linewidth is small.
  • a further characteristic feature of Brillouin scattering is that it is a backward process, i.e. the light converted to the Brillouin Stokes line is propagated in the opposite direction from that of the incident light.
  • the interaction which creates the stimulated Brillouin light only occurs in a fibre length in which the probe pulse and the resulting backscattered power overlap at any one time (this interaction length is approximately one metre for every 10 ns of pulse duration).
  • SRS is mainly a forward process and therefore, the interaction length is only marginally related to the pulse duration.
  • This marginal effect is caused by the Stokes and probe pulses having slightly different velocities. As a result, after a sufficiently long propagation distance, they no longer overlap in the same section of fibre and thus the Raman conversion ceases to occur.
  • the distance over which this walk-off effect occurs is generally several kilometres in length for pulses of a few tens of nanoseconds.
  • SPM is a process by which the optical intensity of a pulse modifies the refractive index of the medium in which it is propagating, through the optical Kerr effect.
  • the pulse then reacts to the resultant axial refractive index gradient by "chirping", i.e. its frequency varies during the pulse.
  • the resultant effect is a spectral broadening of the pulse in the immediate vicinity of a change in optical power; it is more pronounced where the intensity changes abruptly with time, i.e. the shorter the pulse, the stronger the effect.
  • SPM For a Gaussian-shaped pulse, of 250 mW peak power, SPM results in a peak frequency deviation of ⁇ 500 MHz. In certain applications, therefore, SPM limits the power which can be launched. It is important to realise that it is not the optical intensity per se which results in SPM, rather is it the variation of that intensity as a function of time which causes the spreading of the spectrum. The effect increases in proportion to the time-derivative of optical intensity.
  • OTDR spontaneous Brillouin optical time domain reflectometry
  • narrow-band pulses are launched into an optical fibre and Brillouin light backscattered all along the fibre is observed at the launch end.
  • the intensity and the frequency shift of the backscattered light are each sensitive to temperature and strain and, therefore, by measuring both of these quantities as a function of distance along the fibre (as determined by the time taken for the light to travel from the source to the point of interest and back), the temperature and strain distributions can be ascertained.
  • both the frequency and the Brillouin backscatter intensities must be measured precisely (for example the sensitivity of intensity and frequency to strain are -0.09%/°C and 4 MHz/100 microstrain, respectively).
  • SPM In Brillouin OTDR, SPM affects the measurement by broadening the spectrum, which makes the frequency determination more difficult. The alteration of the spectrum also affects the filters used to separate the Brillouin-scattered light from the
  • the spatial resolution of Brillouin OTDR is determined by the duration of the probe pulse; for a high (short-distance) resolution, it is necessary to keep this duration short, the resolution being roughly equal to one metre for every 10 ns of pulse duration. If the pulse duration is reduced and the peak power maintained, it follows that the time derivative increases. (It has been assumed here mat the pulse has been shaped already to minimise this time derivative and that the pulse is designed to have soft edges, such as a Gaussian time profile.). Therefore, as the spatial resolution is made finer, it is necessary to reduce the peak power to maintain the same level of SPM. It follows that SPM severely limits the pulse energy in high-resolution systems.
  • a first aspect of the present invention is directed to a method of propagating optical pulses, comprising: launching a composite pulse of light into a first end of an optical fibre, the composite pulse comprising at least one probe pulse with a wavelength ⁇ p , a first guard pulse with a wavelength ⁇ gl different from ⁇ p preceding the at least one probe pulse, and a second guard pulse with a wavelength ⁇ g2 different from ⁇ p following the at least one probe pulse, the at least one probe pulse and the guard pulses temporally arranged so that the composite pulse has an intensity in the temporal domain that is substantially constant over a central portion of the composite pulse corresponding to a duration of the at least one probe pulse; propagating the light within the optical fibre; and filtering light at ⁇ g i and ⁇ g2 from light subsequently emitted from the optical fibre.
  • the composite pulse allows the probe pulse to be propagated in a way that protects it from self-phase modulation (SPM), so it may travel through the optical fibre largely undistorted by this effect.
  • SPM occurs where a pulse has a variation in intensity with time, and is proportional to the rate of variation.
  • the sloping sides of the probe pulse which would usually be subject to SPM are replaced by a much more uniform intensity profile so that the probe pulse can propagate substantially unaffected.
  • a greater peak power level in the probe pulse can be used; without the invention this would increase SPM because the rate of intensity change is increased for a given pulse duration.
  • guard pulses allow them readily to be removed after propagation, leaving just the probe pulse available for detection, observation, signal processing, etc. Hence the detrimental impact of SPM on the probe pulse is removed, so that the accuracy of measurements and' signal transmission is enhanced by use of composite pulses * '
  • the method can be applied to many existing pulse propagation applications regardless of propagation direction. Therefore, in some embodiments, the light subsequently emitted from the optical fibre comprises forward propagating light emitted from a second end of the optical fibre, while in alternative embodiments the light subsequently emitted from the optical fibre comprises backscattered or backreflected light emitted from the first end of the fibre.
  • ⁇ gl is different from Xg 2 . This typically makes for simpler generation of the composite pulse, although in some cases it may be preferable to use the same wavelength for each guard pulse to simplify the filtering of the emitted light.
  • the at least one probe pulse may comprise a single probe pulse, or alternatively, the at least one probe pulse may comprise a temporal sequence of probe pulses. Further, each probe pulse in the temporal sequence of probe pulses may have a different wavelength. A sequence of probe pulses increases the amount of optical energy that can be transmitted at the probe wavelength(s) while keeping peak powers below thresholds for other unwanted nonlinear effects.
  • the method may further comprise temporally modifying the light emitted from the fibre so that the plurality of probe pulses become temporally coincident. Information carried by the various pulses can thus be accumulated for an improved signal to noise ratio.
  • a second aspect of the present invention is directed to apparatus for propagating optical pulses, comprising: an optical fibre; a source of optical pulses operable to generate a composite pulse of light and launch it into a first end of the optical fibre, the composite pulse comprising at least one probe pulse with a wavelength ⁇ p , a first guard pulse with a wavelength ⁇ gl different from ⁇ p preceding
  • the filter may be arranged at a second end of the fibre to reject light at ⁇ gl and ⁇ g2 from forward propagating light emitted from the second end of the optical fibre, or alternatively at the first end of the fibre to reject light at ⁇ gl and Xg 2 from backscattered or backreflected light emitted from the first end of the fibre.
  • the source of optical pulses may be configured in various ways. For example, it may be operable to generate a composite pulse in which ⁇ gl is different from ⁇ g2 ; a composite pulse in which the at least one probe pulse comprises a single probe pulse; or a composite pulse in which the at least one probe pulse comprises a temporal sequence of probe pulses. Further, each probe pulse in the temporal sequence of probe pulses may have a different wavelength.
  • the apparatus may further comprise a pulse re-timing assembly arranged to receive light emitted from the optical fibre and operable to temporally modify the light emitted from the fibre so that the plurality of probe pulses become temporally coincident.
  • a third aspect of the present invention is directed to apparatus for generating optical pulses, comprising: a light source operable to generate at least one probe pulse with a wavelength ⁇ p , a first guard pulse with a wavelength ⁇ gl different from ⁇ p, and a second guard pulse with a wavelength X g2 different from ⁇ p; and a temporal delay assembly operable to construct a composite pulse by temporally arranging the pulses so that the first guard pulse precedes the at least one probe pulse, the second guard pulse follows the at least one probe pulse, and the intensity of the composite pulse in the temporal domain is substantially constant over a central portion of the composite pulse corresponding to a duration of the at least one probe pulse.
  • the light source may be configured to generate the various pulses; a choice may be made according to parameters such as available lasers, and desired wavelengths, powers and pulse durations. Examples include a light source comprising a plurality of narrowband optical sources each operable to generate a continuous wave output at a respective one of the wavelengths ⁇ gl , Xg 2 and X n , and one or more modulators operable to receive the continuous wave outputs and modulate these outputs into pulses of light; a light source comprising a plurality of narrowband optical sources each operable to generate a pulsed output at a respective one of the wavelengths ⁇ gl , ⁇ g2 and X n ; a light source comprising a broadband optical source operable to generate a continuous wave output with a bandwidth including each of the wavelengths ⁇ gl , ⁇ g 2 and X n , and a modulator operable to receive the continuous wave output and modulate this output into pulses of light; a light source comprising a pluralit
  • the temporal delay assembly may act in one of several ways to construct the composite pulses.
  • the temporal delay assembly may receive the pulses from the light source before constructing the composite pulse.
  • the temporal delay assembly may comprise a plurality of optical fibres through which the pulses are propagated to acquire different temporal delays according to a length of optical fibre through which each pulse propagates.
  • the plurality of optical fibres have different lengths and are arranged in parallel between a wavelength division demultiplexer operable to distribute the pulses to the optical fibres according to the wavelength of the pulses, and a wavelength division multiplexer operable to receive the pulses from the plurality of optical fibres and combine the pulses into the composite pulse.
  • the plurality of optical fibres are arranged in series and separated by a plurality of reflective filters each operable to reflect a respective one of the wavelengths ⁇ gl , ⁇ g 2 and ⁇ n to allow pulses at each of these wavelengths to propagate through a length of optical fibre sufficient to give a desired temporal delay to each pulse before reflection.
  • the reflective filters may conveniently comprise fibre Bragg gratings.
  • the temporal delay assembly may comprise a triggering assembly operable to trigger generation of the pulses so that the pulses are generated with relative temporal delays.
  • ⁇ gl may be different from ⁇ g2 ; the at least one probe pulse may comprise a single probe pulse; the at least one probe pulse may comprise a plurality of probe pulses, and the temporal delay assembly is operable to arrange the plurality of probe pulses into a temporal sequence; and each probe pulse in the plurality of probe pulses may have a different wavelength.
  • a fourth aspect of the present invention is directed to a method of generating optical pulses, comprising: generating at least one probe pulse with a wavelength A 1 ,; generating a first guard pulse with a wavelength ⁇ gl different from ⁇ p; generating a second guard pulse with a wavelength ⁇ g2 different from ⁇ p; and constructing a composite pulse by temporally arranging the at least one probe pulse and the guard pulses so that the first guard pulse precedes the at least one probe pulse, the second guard pulse follows the at least one probe pulse, and the intensity of the composite pulse in the temporal domain is substantially constant over a central portion of the composite pulse corresponding to a duration of the at least one probe pulse.
  • the at least one probe pulse and the guard pulses may be generated using a light source comprising a plurality of narrowband optical sources each operable to generate a continuous wave output at a respective one of the wavelengths ⁇ gl , ⁇ and ⁇ n , and one or more modulators operable to receive the continuous wave outputs and modulate these outputs into pulses of light; or the at least one probe pulse and the guard pulses may be generated using a light source comprising a plurality of narrowband optical sources each operable to generate a pulsed output at a respective one of the wavelengths ⁇ gl , ⁇ g2 and X n ; the at least one probe pulse and the guard pulses may be generated using a light source comprising a broadband optical source operable to generate a continuous wave output with a bandwidth including each of the wavelengths ⁇ gl , ⁇ g2 and ⁇ n , and a modulator operable to receive the continuous wave output and modulate this output into pulses of light; or the
  • the composite pulse is constructed by passing the at least one probe pulse and the guard pulses through a temporal delay assembly comprising a plurality of optical fibres through which the pulses are propagated to acquire different temporal delays according to a length of optical fibre through which each pulse propagates.
  • the plurality of optical fibres may have different lengths and be arranged in parallel between a wavelength division demultiplexer operable to distribute the at least one probe pulse and the guard pulses to the optical fibres according to the wavelength of the pulses, and a wavelength division multiplexer operable to receive the pulses from the plurality of optical fibres and combine the pulses into the composite pulse.
  • the plurality of optical fibres may be arranged in series and separated by a plurality of reflective filters each operable to reflect a respective one of the wavelengths ⁇ gl , ⁇ g2 and ⁇ n to allow pulses at each of these wavelengths to propagate through a length of optical fibre sufficient to give a desired temporal delay to each pulse before reflection.
  • the reflective filters may comprise fibre Bragg gratings.
  • the composite pulse is constructed by triggering a light source operable to generate the at least one probe pulse and the guard pulses so that the pulses are generated with relative temporal delays.
  • ⁇ gl may be different from ⁇ ; the at least one probe pulse may comprise a single probe pulse; the at least one probe pulse may comprise a plurality of probe pulses, and constructing the composite pulse comprises arranging the plurality of probe pulses into a temporal sequence; and each probe pulse of the plurality of probe pulses may have a different wavelength.
  • a fifth aspect of the present invention is directed to a method of propagating optical pulses according to any of the above-described aspects, embodiments and examples relating thereto, and further comprising generating the composite pulse of light according to any of the above-described aspects, embodiments and examples relating thereto.
  • a sixth aspect of the present invention is directed to apparatus for propagating optical pulses according to any of the above-described aspects, embodiments and examples relating thereto, in which the source of optical pulses comprises apparatus for generating optical pulses according to any of the above-described aspects, embodiments and examples relating thereto.
  • Figure 1 shows a schematic representation of a short pulse propagation arrangement according to the prior art- "
  • Figure 2 shows a schematic representation of a short pulse propagation arrangement according to a first embodiment of the present invention
  • Figure 3 shows a schematic representation of a short pulse propagation arrangement according to a further embodiment of the present invention
  • Figures 4A and 4B show representations of a composite pulse generated according to an embodiment of the present invention in the time and frequency domain respectively;
  • Figure 5 shows a schematic representation of a first embodiment of apparatus for generating composite pulses according to the present invention
  • Figure 6 shows a schematic representation of a second embodiment of apparatus for generating composite pulses according to the present invention
  • Figure 7 shows a schematic representation of a third embodiment of apparatus for generating composite pulses according to the present invention.
  • Figure 8 shows a schematic representation of a fourth embodiment of apparatus for generating composite pulses according to the present invention.
  • Figure 9 shows a schematic representation of a first embodiment of apparatus for generating, propagating and detecting composite pulses according to the present invention
  • Figure 10 shows a schematic representation of a second embodiment of apparatus for generating, propagating and detecting composite pulses according to the present invention
  • Figure 11 shows a schematic representation of a third embodiment of apparatus for generating, propagating and detecting composite pulses according to the present invention
  • Figure 12 shows a schematic representation of a fourth embodiment of apparatus for generating, propagating and detecting composite pulses according to the present invention
  • Figure 13 shows a schematic representation of apparatus for generating, propagating and detecting short pulses according the prior art.
  • Figure 14 shows the apparatus of Figure 1 modified according to an embodiment of the present invention.
  • FIG. 1 shows a schematic representation of a simple pulse propagation system according to the prior art.
  • a short pulse of light (probe pulse, P p ) 10 at a wavelength ⁇ p is launched into a first end of an optical fibre 12, along which it propagates before being detected at the far end of the fibre 12 by a photodetector 14.
  • the probe pulse 10 is represented in the time domain as a plot of intensity I against time t. Also shown is a plot of the first derivative of the intensity with respect to time, dl/dt.
  • FIG. 2 is a schematic representation illustrating how the above issue is addressed by the present invention.
  • a probe pulse P p 10 at wavelength ⁇ p is launched into a first end of an optical fibre 12, propagated therein, and detected at a photodetector 14.
  • the pulse 10 is accompanied by two so-called "guard" pulses 16, 18, so that a first guard pulse P gl precedes the probe pulse in time, and a second guard pulse Pg 2 follows it.
  • the guard pulses 16, 18 have different wavelengths from the probe pulse 10, at ⁇ gl and X g2 .
  • the three pulses are overlapped in time to form a single composite pulse, which propagates through the fibre 12.
  • a filter 20 is positioned at the second end of the fibre 12, in front of the photodetector 14, that passes ⁇ p and blocks ⁇ gl aria 1 ⁇ g 2 so as to filter the guard wavelength(s) from forward propagating light subsequently emitted from the fibre, i.e. the emitted composite pulse, leaving only the probe pulse for detection.
  • the wavelengths of the guard pulses are preferably different from each other since this may facilitate generation of the composite pulse, but may be the same, which makes for easier filtering.
  • the shape, intensity and timing of the pulses are arranged in such a way that the total intensity in the composite pulse is substantially constant in the central section of the pulse, and in particular where the wavelength in the composite pulse transitions from that of one of the guard pulses to that of the probe pulse, i.e. over the duration of the probe pulse. In this central section, therefore, the time-derivative of intensity is substantially reduced over that which would exist if only the probe pulse were launched, as is shown in the plot of dl/dt in Figure 2. It follows that the probe pulse 10 suffers little or no SPM. SPM does, nonetheless, affect the guard pulses, on the leading and trailing edges of the composite pulse.
  • the SPM which affects them is of no consequence, provided that the wavelength separation between the guard pulses and the probe pulse is sufficient for the filter 20 to be effective in rejecting the entire spectrum of the guard pulses after propagation through the fibre (i.e. after suffering the broadening effects of SPM).
  • the guard pulses thus act to protect the probe pulse from SPM during its propagation in the fibre.
  • the overlapped arrangement of the pulses gives a total intensity that is not absolutely constant; the curved peaks of the pulses result in an intensity profile that is rippled.
  • the degree to which the ripples affect the spectral broadening scales with the power that can be launched at a given spectral distortion level.
  • the rate of change of optical power inside the composite pulse is half the value which exists on the external edges of the pulse, the power in the probe pulse can be doubled.
  • substantially constant used herein to describe the total intensity of the peak pulse is intended to be understood as meaning that the ripple amplitude does not exceed about 50% of the mean pulse amplitude. Smaller ripple values, such as 40%, 30%, 20% or 10%, will allow progressively greater probe powers to be used.
  • Li addition it can be arranged for the . outer edges of the composite pulse (comprising the leading edge of the front guard pulse P gl and the trailing edge of the rear guard pulse Pg 2 ) to have substantially longer rise/fall times (lower dl/dt values) than the edges of the probe pulse. This further reduces the potential impact of SPM on the guard pulses and thus relieves the constraints on the selection of the wavelengths ⁇ g i and ⁇ g2 relative to that of the probe pulse, ⁇ p , both as regards the required performance of the filter and any distortion or interference between the broadened guard pulse spectra and the probe spectrum.
  • the method may also be implemented using a backscatter detection arrangement (used for OTDR and distributed temperature sensing techniques, for example) in which light pulses are launched into a first end of the fibre, and resulting backscatter is detected as it returns to the first fibre end.
  • a backscatter detection arrangement used for OTDR and distributed temperature sensing techniques, for example
  • Figure 3 shows a schematic representation of simple apparatus that can be used for this.
  • the fibre 12 has an associated directional device 24 at its first end, which is used both to couple the composite pulse 22 into the fibre 12, and to couple the backscattered output out of the fibre towards the filter 20 and detector 14, which are positioned at the first (launch) end of the fibre 12.
  • the directional device 24 may be a coupler, a circulator or any optical device capable to separating forward from backward propagating power.
  • the invention may be usefully extended by including two or more probe pulses in the composite pulse. If the probe pulses each have the same wavelength, this can be used to deliver a larger amount of optical energy at a single wavelength while keeping peak powers below the thresholds for various nonlinear effects. However, probe pulses of different wavelengths may also be ⁇ bf use for some applications.
  • this embodiment of the present invention may usefully be combined with a prior art method for circumventing to some extent the limitation of power resulting from non-linear effects such as stimulated Raman scattering in applications such as narrow band OTDR [3].
  • This method addresses this by launching a train of pulses at different wavelengths into the OTDR fibre, such that each pulse is below the power level at which the non-linear effect occurs. Cumulatively, the energy contained in the series of pulses can be significantly higher than could be propagated in a single pulse.
  • Backscattered optical power is returned to the launch end of the fibre and passed through a re-timing network which cancels the relative delays between the backscatter traces, thus providing a multi-wavelength backscatter trace with the same spatial resolution as that of a single pulse, but having a signal level corresponding to the total energy contained in the entire series of pulses.
  • the benefits of the present invention in reducing SPM effects and of the prior art in boosting usable power levels can both be gained by providing a composite pulse which contains a pair of guard pulses flanking a plurality of probe pulses arranged in a temporal sequence, each at different wavelengths and each having a peak power preferably below the limit at which non-linear effects, including SRS, occur.
  • the timing and the shape of the pulses is arranged in such a way that the power distribution as a function of time within the central part of the composite pulse corresponding to the duration of the probe pulse sequence is essentially constant.
  • Only the first and last pulses, i.e. the guard pulses, may be subject to SPM in the fibre, but these are discarded by filtering before detection, and the backscattered power originating from these pulses is not cumulated with that of the probe pulses. In this way, the SPM can affect the first and last pulse, but not any of the central pulses.
  • Figure 4A illustrates such a composite pulse in the time domain, where a first guard pulse P gl and a second guard pulse P g2 respectively precede and follow a train or sequence of probe pulses P pl to P p5 . Only five probe pulses in addition to the necessary guard pulses are shown for the sake Of clarity; however, a smaller, or generally larger, number of probe pulses may be used instead.
  • Figure 4B illustrates the composite pulse of Figure 4A in the frequency domain, to show how the five probe pulses each have a different wavelength Xp 1 to ⁇ p 5 , which are each different from the wavelengths of the guard pulses ⁇ gl and Ig 2 .
  • the guard pulse wavelengths are sufficiently separated from the probe pulse wavelengths to allow efficient filtering.
  • a pulse train consists often pulses (including the guard pulses), formed into a composite pulse and launched at the SRS power limit.
  • the backscattered power which is passed to the detector is eight times that of the SRS limit and 48 times that set by SPM for a single pulse (this latter factor arises from the product of the number of pulses, eight in this example) and a factor of 6 derived from a calculation for specific spatial resolution and allowable spectral broadening, using, in this case, a 14 ns pulse duration and a maximum of 400 MHz peak frequency excursion).
  • FIG. 5 shows a schematic representation of a first embodiment of apparatus being a source of optical pulses with which the composite pulses described above may be generated.
  • three narrowband optical sources 30, 32, 34 for example lasers, that generate wavelengths ⁇ p , ⁇ gl and ⁇ g2 are operated as continuous wave (cw) sources.
  • the outputs from each source 30, 32, 34 are combined in a combiner 36, which may take, for example, the form of a wavelength-division multiplexer, which is preferred, or a series of directional couplers.
  • the combined cw multi- wavelength output from the combiner 36 is passed through an optical modulator 38 which is operable to modulate the outputs from the optical sources to define a pulse of a duration and shape desired for the probe pulse P p .
  • an optical modulator 38 which is operable to modulate the outputs from the optical sources to define a pulse of a duration and shape desired for the probe pulse P p .
  • the single pulse is passed through a fibre-optic temporal delay-line assembly 40 that delays the relative timing of the pulses so as to achieve the desired composite pulse 42, in which the only substantial change in optical intensity occurs at the leading edge of the first guard pulse and at the trailing edge of the second guard pulse.
  • the delay line assembly 40 consists of an optical circulator 44 that receives the single pulse at port 1 and feeds it to port 2, into a plurality of optical fibres 46a, 46b, 46c arranged as a linear series.
  • the fibres are separated by reflective filters 48a, 48b, 48c designed to reflect only the first guard pulse, the probe pulse and the second guard pulse, respectively.
  • these filters may take the form of fibre Bragg gratings.
  • the first guard pulse wavelength ⁇ fi is reflected from the grating 48a closest to the circulator 44 and the second guard pulse wavelength Xg 2 is reflected from the grating 48c furthest from the circulator 44, so that the second guard pulse experiences the greatest delay.
  • the three pulses thus propagate through different lengths of fibre and are reflected back to port 2 of the circulator 44 with different temporal delays so that together they form a composite pulse with the desired temporal arrangement.
  • the circulator 44 then feeds the composite pulse 42 to its port 3, where it can be coupled into an optical fibre (not shown) for transmission/propagation.
  • the pulses may advantageously be amplified using optical amplifiers 50a, 50b situated respectively between the modulator 38 and the circulator 44, and between the circulator 44 and the transmission fibre.
  • the source of optical pulses may conveniently be viewed as comprising two parts, one being a light source operable to generate the various pulses, and the other being a temporal delay assembly that constructs the composite pulse by temporally arranging the pulses produced by the light source into the appropriate temporal sequence.
  • T is the transmission of the modulator and takes values between 0 and 1 and t represents time
  • Many functions fulfil this criterion, such a raised cosine, the Heaviside step function and the error function. With this criterion satisfied, provided that appropriate relative timings between the guard and probe pulses are achieved, the centre of the composite pulse will have little or no variation with time and the probe pulse is thus protected from SPM.
  • FIG. 6 shows a schematic representation of composite pulse-generating apparatus that comprises optical sources 30, 32, 34, a combiner 36 and a modulator 38 as in the embodiment of Figure 5.
  • the delay line assembly 40 comprises a parallel arrangement of fibres of different lengths, in which the pulsed output from the modulator 38 enters a wavelength division demultiplexer 52, which splits the single pulse into its constituent wavelengths ⁇ p, ⁇ gl and ⁇ g2 and launches each into a separate delay line fibre 54a, 54b, 54c, the lengths of said fibres being arranged to produce the desired relative temporal delays between the pulses at each wavelength.
  • ⁇ gl passes through the shortest fibre to arrive at the front of the composite pulse and ⁇ gi passes through the longest fibre to arrive at the back of the composite pulse.
  • the time-delayed pulses are then re-combined to form the composite pulse 42 in a wavelength-division multiplexer 56, which takes the outputs of the delay line fibres 54a, 54b, 54c and launches them all into a transmission fibre (not shown).
  • Other devices which fulfil the function of splitting and recombining the light can be used in place of the multiplexer and/or the demultiplexer, such as splitter and filter combinations, although the wavelength division multiplexer approach is preferred as being potentially more efficient.
  • each optical source is delivered directly to individual delay line fibres 66a, 66bj 66c of different lengths so that the front guard pulse at ⁇ gl has a short fibre for least delay and the rear guard pulse at ⁇ has a long fibre for greatest delay.
  • the delay lines fibres 66a, 66b, 66c are each coupled to a wavelength division multiplexer 68 where the delayed pulses are combined to form the composite pulse 42, for delivery to a transmission fibre (not shown).
  • the outputs of the optical sources 60, 62, 64 may be combined and coupled into a delay assembly of the type described with respect to the first embodiment.
  • the delays introduced by either arrangement of the delay line fibres can be enhanced or replaced by relative triggering of the optical sources 60, 62, 64, so that, for example, the temporal delay assembly may comprise a triggering assembly 70 to control the pulse generation of each optical source, so that the pulses are directly emitted with relative temporal delays.
  • FIG 8 shows a schematic representation of a fourth embodiment of the apparatus that uses injection locking to construct the composite pulses 42.
  • Injection locking is a process used to force a laser to oscillate at a frequency other than its free- running frequency [4].
  • the output from a master laser 150 is injected into a slave laser 152. This causes the slave laser 152 to oscillate at the frequency of the master laser 150, provided that the master laser's frequency is within approximately 1 GHz of the slave laser's free-running frequency.
  • the output from the master laser 150 is received at port 1 of an optical circulator 154 and is fed to the slave laser 152 connected to port 2 of the optical circulator 154.
  • the composite pulse 42 is output from port 3 of the optical circulator.
  • An optical isolator 156 ensures that there is no feedback from the slave laser 152 into the master laser 150.
  • the components of this embodiment are connected using polarisation maintaining components.
  • the master laser 150 and the slave laser 152 are assumed to be semiconductor distributed feedback lasers, although other types of lasers may also be used.
  • the master laser 150 is driven with a sinusoidally varying injection current, which causes the formation of a plurality of side bands (i.e. discrete peaks), in addition to the usual plurality of peaks in the master laser's output spectrum.
  • the modulation of the master laser's injection current can be designed to cause the plurality of peaks in the master laser's output spectrum to occur at frequencies corresponding to the wavelengths ⁇ g i, ⁇ and ⁇ p.
  • the multiple- frequency output from the master laser is injected into the slave laser 152.
  • the output frequency of a semiconductor distributed feedback laser is determined by its operating temperature, injection current and the properties of its internal Bragg grating.
  • varying the injection current of the slave laser will vary its free-running frequency. This allows the slave laser 152 to be locked onto any one of the plurality of peaks in the output spectrum of the master laser 150 by varying the slave laser's injection current so that its free-running frequency shifts within approximately 1 GHz of the chosen peak. If the peaks in the master laser's output spectrum are separated by a frequency of approximately 1-2 GHz, then further variation of the slave laser's injection current can cause the slave laser 152 to lock to different peaks in the master laser's output spectrum.
  • the injection locking occurs in a very short time period (approximately 1 ns).
  • the composite pulse 42 can be generated directly using the apparatus of this embodiment of the invention, by varying the slave laser's injection current so that its output wavelength switches from ⁇ gl , to generate the first guard pulse, ⁇ p to generate the probe and ⁇ g2 to generate the second guard pulse.
  • the pulse widths are controlled by the timing of the switches between different wavelengths. Multiple pulses at different wavelengths may also be generated.
  • a delay line assembly 40 is not absolutely necessary for the construction of composite pulses 42 by the apparatus of this embodiment of the invention. However, a delay line assembly may be used to further alter the timing of the pulses within the composite pulse 42.
  • Varying the injection current of the slave laser causes a consequent variation in its output power. However, only a small variation of the injection current is required for injection locking. Hence the output power of the slave laser 152 varies by only a few percent, which is an acceptable power level variation for the purposes as described herein.
  • the pulses generated using the apparatus of this embodiment are separated by a wavelength change of only a few tenths of a nanometre. Such a small separation between the wavelengths of the pulses mitigates the effects of chromatic dispersion, and thus reduces dispersion of the different wavelengths in the composite pulse 42 during propagation in the optical fibre.
  • Figure 9 shows a schematic representation of apparatus suitable for implementing the use of a composite pulse with multiple probe pulses such that the total intensity within the composite pulse is essentially constant between the leading edge of the first guard pulse and the trailing edge of the second guard pulse, in conjunction with the prior art method for addressing the limitation of power resulting from non-linear effects such as stimulated Raman scattering [3], as described above.
  • the apparatus as described includes the transmission fibre and the filtering for removing the guard pulses before detection.
  • the apparatus of Figure 9 combines aspects of apparatus described in EP
  • a broadband, high power pulsed optical source 80 produces an output of broadband pulses that is directed through a directional fibre coupler 82 to a temporal delay assembly in the form of a fibre network 84 comprising a linear series of reflective filters 86 (exemplarily represented as fibre Bragg gratings in the Figure) that respectively reflect a sequence of wavelengths designated for the first guard pulse wavelength ⁇ gl , the first probe pulse wavelength ⁇ pi, the second probe pulse wavelength ⁇ p2 , further probe pulse wavelengths up to the final probe pulse wavelength ⁇ pn , and finally the second guard pulse wavelength ⁇ g2 .
  • a linear series of reflective filters 86 exemplarily represented as fibre Bragg gratings in the Figure
  • Reflection from these various filters breaks the broadband pulse up into the separate narrower band guard and probe pulses.
  • the reflective filters 86 are coupled by lengths of optical fibre 88, the lengths of which are arranged so that the relative delays caused by pulse propagation in each of the fibres 88 and through the reflecting filters 86 gives the desired temporal spacing between pulses, for example, a delay equal to one pulse width.
  • a pulse train is reflected from the network 84 that consists of n+2 pulses (n probe pulses and two guard pulses) each having a different wavelength defined by the reflectivities of the filters 86.
  • the relative timing of the pulses is such that the intensity is substantially constant across the probe pulse part of the pulse train, so that the train forms the desired composite pulse.
  • the composite pulse is directed by the directional coupler 82 through an isolator 90 and a second coupler 92 to a transmission fibre 94, in this case for distributed parameter sensing.
  • a portion of the backscattered light returning from the fibre 94 is directed through the second coupler 92 to a second fibre network 96 that operates as a pulse re-timing assembly.
  • This network also comprises a series of reflective filters 98 for the wavelengths ⁇ p i to ⁇ pn connected by lengths of fibre 100.
  • the order of the filters is reversed from that of the first network 84, so that the second network 96 acts to temporally modify the emitted light so as to cancel the relative delays between the wavelengths in the returned signal so that the probe pulses become temporally coincident and return to the state of a single multi- wavelength short pulse.
  • the second network 96 contains only n reflective filters and does not reflect the guard pulses. Thus, the guard pulses are substantially removed from the signal.
  • the reflected re-timed signal is directed to the second directional coupler 92 through a pair of filters 102 (in this case, fibre Bragg gratings) designed to eliminate any residual power at the guard wavelengths ⁇ gl and ⁇ g2 .
  • the signal is then detected by a detector 106.
  • a modulator 104 is placed in front of the detector which is timed to eliminate any outgoing pulse power that passes directly through the second coupler 92 (and hence bypasses the sensing fibre 94) which may otherwise saturate the detector 106.
  • Figure 10 shows a schematic representation of apparatus alternative to that of Figure 9, in which the first directional coupler 82 and the isolator 90 are replaced by an optical circulator 108.
  • This receives the output of the optical source 80 at a first port and circulates it to the first fibre network 84 arranged at a second port of the circulator.
  • the pulse train returned from the network 84 is then circulated to a third port, for transmission to the second directional coupler 92 for launching into the sensing fibre 94.
  • the second directional coupler 92 may also be replaced with a circulator.
  • Use of a second circulator in this way removes the need for the modulator 104 since the operation of the circulator 110 ensures that all the composite pulse power is coupled into the sensing fibre.
  • an optical amplification stage such as an erbium doped fibre amplifier 112
  • an optical amplification stage such as an erbium doped fibre amplifier 112
  • a first example illustrates the improvement of a direct detection method for the measurement of spontaneous Brillouin scattering in relation to the intensity of the Rayleigh scattering, this ratio — which is known as the Landau-Placzek ratio — being directly related to temperature.
  • the Brillouin measurement requires a high-power, narrowband probe pulse to be launched into the fibre, and for the Brillouin signal to be extracted from the components of the backscatter spectrum.
  • a separate measurement, using a different probe pulse, is made of the Rayleigh backscatter.
  • a broadband source is used to reduce the so- called coherent Rayleigh noise, an effect caused by interference between backscatter originating from different portions of the fibre within the distance illuminated * simultaneously by the probe pulse.
  • FIG 13 shows a schematic representation of a prior art arrangement used for this measurement [5].
  • An optical source 120 comprises a narrowband continuous- wave laser 122 (a distributed-feedback laser, DFB) with an output that is modulated by a first modulator 124 (such as an acousto-optic device).
  • the resultant probe pulse is amplified by an erbium doped fibre amplifier 126, and a second modulator 128 is used to eliminate any amplified spontaneous emission, except during transmission of the pulse itself.
  • the amplified probe pulse leaves the source 120 and enters a first port of a circulator 130, for direction to a second port where it is reflected from a first fibre Bragg grating Gl and returned to the circulator 130, from where it exits through a third port to be launched into a measurement fibre 132.
  • the first grating Gl is temperature-tuned so that its peak reflection coincides with the peak output of the source 120, and serves to remove spurious wavelengths that may be emanating from the source 120.
  • Backscattered light returned from the measurement fihre 132 is directed through the circulator 130 from the third port to a fourth port, where it passes through a filter 134.
  • This comprises a pair of fibre Bragg gratings G2, G3 separated by an isolator 136, and is designed to eliminate Rayleigh scattering resulting from propagation of the probe pulse in the fibre 132 (a single grating may suffice if sufficient rejection and a narrow spectrum can be achieved).
  • the remaining light which is the Brillouin backscatterf is transmitted to an optical receiver 138 for detection.
  • a separate measurement of the Rayleigh backscatter is made using a pulse from a second optical source 140, being a broadband source with has sufficient spectral width not only to avoid coherent Rayleigh noise but also to pass through each of the gratings Gl, G2, G3 largely unaffected.
  • a probe pulse from this source 140 is launched into the measurement fibre 132 by being coupled into the second port of the circulator 130, which passes the light to the fibre 132 at the third port.
  • Measurement electronics 142 connected to the receiver 138 make the necessary comparison between the Brillouin and Rayleigh signals.
  • Figure 14 shows a schematic representation of the prior art apparatus of Figure 13 modified to make use of the present invention
  • the narrow band optical source 120 is modified to include three DFBs 122 that respectively generate cw outputs at ⁇ g i, ⁇ p and ⁇ g2 . These outputs are combined prior to reaching the first modulator 124.
  • the source 120 thus outputs a single pulse containing each of three wavelengths. These are arranged to form a composite pulse by providing two further narrowband reflectors 142a, 142b at each of the guard pulse wavelengths, one on each side of the probe pulse reflector Gl at the port 2 output of the circulator 130.
  • the gratings are spaced to give appropriate time delays between the three wavelengths. This arrangement is comparable to that of Figure 5.
  • a further two gratings 144 reflecting the guard wavelengths ⁇ gl and ⁇ g2 , are included in the optical path between the circulator and the optical receiver, preferably before the filter 134.
  • the gratings 142 at the second port to have a narrow spectrum to give narrow bandwidth pulses
  • those gratings 144 that filter the guard pulses must be of sufficient spectral width also to reject any Brillouin scattering originating from the guard pulses.
  • the width of the response of these filters should include the spectral broadening which will accompany the pulse propagation through the fibre at high intensity.
  • An alternative prior art arrangement [6] also involves launching narrowband pulses into the measurement fibre.
  • a coherent detection approach in which the detector or receiver is illuminated not only with the backscattered light, but also with a continuous- wave optical input at the same frequency as the probe pulse (or at a known frequency offset from the probe pulse).
  • the latter signal is known as a local oscillator and the resulting detector photocurrent is proportional to the product of the electric field of the Brillouin scattered light and that of the local oscillator.
  • the benefit of coherent detection is a stronger signal since the local oscillator can be made arbitrarily large which can lead to a better signal-to- noise ratio.
  • the present invention may be used to adapt this arrangement by the addition of the guard pulses, and the elimination thereof prior to detection, in a similar manner to that described with respect to Figures 13 and 14.
  • the invention may be used to increase the probe power in any of the arrangements described to date for the measurement of Brillouin backscatter.
  • any of the various apparatus described above may be readily adapted for the production of composite pulses with any number of probe pulses, for example by altering the number of narrowband optical sources and using an appropriate number of matching reflecting elements such as gratings, or by using a broadband source with a suitable series of gratings to produce separate pulses at each wavelength.
  • the invention may be used in conjunction with other prior art methods for reducing the impact of other non-linear effects. These methods allow the threshold for non-linear effects, in particular Brillouin and Raman effects, to be circumvented in certain circumstances so that higher powers can be used. Since the present invention provides a way of avoiding the limitations of SPM, combinations of the present invention and the prior art methods allow the use of much higher powers still.

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Abstract

La présente invention concerne un procédé de propagation d'impulsions optiques courtes qui comprend l'élaboration d'une impulsion composite (22) comprenant une impulsion sonde (10) encadrée par une paire d'impulsions de protection (16, 18) d'une longueur d'onde différente de l'impulsion sonde. Ces impulsions ont des retards temporels respectifs afin que l'impulsion composite comporte une partie centrale dans laquelle l'intensité est pratiquement constante au cours du temps. Ceci protège l'impulsion sonde d'une auto-modulation de phase (SPM) au cours de la propagation de l'impulsion composite à travers une fibre optique; seuls les côtés externes des impulsions de protection subissent une SPM. Après la propagation, les impulsions de protection sont éliminées par filtrage, laissant ainsi une impulsion sonde non distordue pour la détection. Dans certains modes de réalisation, l'impulsion composite contient une pluralité d'impulsions sondes, qui peuvent avoir des longueurs d'onde différentes. Divers appareils peuvent être utilisés pour la génération de l'impulsion sonde.
PCT/GB2006/002083 2006-06-07 2006-06-07 Propagation d'impulsions optiques WO2007141464A1 (fr)

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US7859654B2 (en) 2008-07-17 2010-12-28 Schlumberger Technology Corporation Frequency-scanned optical time domain reflectometry
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Cited By (8)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US7668411B2 (en) 2008-06-06 2010-02-23 Schlumberger Technology Corporation Distributed vibration sensing system using multimode fiber
US7859654B2 (en) 2008-07-17 2010-12-28 Schlumberger Technology Corporation Frequency-scanned optical time domain reflectometry
EP2606328A1 (fr) * 2008-07-21 2013-06-26 Qorex LLC Système de détection brillouin de contrainte-température à longueur d'onde double
EP2606328A4 (fr) * 2008-07-21 2014-07-16 Qorex Llc Système de détection brillouin de contrainte-température à longueur d'onde double
US8401401B2 (en) 2008-10-14 2013-03-19 Schlumberger Technology Corporation Polarization-diverse, heterodyne optical receiving system
US9110018B2 (en) 2010-12-08 2015-08-18 Fotech Solutions Limited Distributed optical fibre sensor
US9244009B2 (en) 2010-12-08 2016-01-26 Fotech Solutions Limited Distributed optical fibre sensor
JP7385867B2 (ja) 2020-06-12 2023-11-24 鹿島建設株式会社 ひずみ測定装置、ひずみ測定方法及びひずみ測定プログラム

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