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WO2004027941A1 - Optical fibre with lenghtwise refractive index profile variation for brillouin scattering reduction - Google Patents

Optical fibre with lenghtwise refractive index profile variation for brillouin scattering reduction Download PDF

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Publication number
WO2004027941A1
WO2004027941A1 PCT/GB2003/004028 GB0304028W WO2004027941A1 WO 2004027941 A1 WO2004027941 A1 WO 2004027941A1 GB 0304028 W GB0304028 W GB 0304028W WO 2004027941 A1 WO2004027941 A1 WO 2004027941A1
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WO
WIPO (PCT)
Prior art keywords
optical fibre
refractive index
index distribution
length
variation
Prior art date
Application number
PCT/GB2003/004028
Other languages
French (fr)
Inventor
Malcolm Paul Varnham
Mikhail Nicholaos Zervas
Original Assignee
Southampton Photonics Limited
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Southampton Photonics Limited filed Critical Southampton Photonics Limited
Priority to AU2003267578A priority Critical patent/AU2003267578A1/en
Publication of WO2004027941A1 publication Critical patent/WO2004027941A1/en

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Classifications

    • 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
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
    • H01S3/06—Construction or shape of active medium
    • H01S3/063—Waveguide lasers, i.e. whereby the dimensions of the waveguide are of the order of the light wavelength
    • H01S3/067—Fibre lasers
    • H01S3/0675—Resonators including a grating structure, e.g. distributed Bragg reflectors [DBR] or distributed feedback [DFB] fibre lasers
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
    • H01S3/06—Construction or shape of active medium
    • H01S3/063—Waveguide lasers, i.e. whereby the dimensions of the waveguide are of the order of the light wavelength
    • H01S3/067—Fibre lasers
    • H01S3/06708—Constructional details of the fibre, e.g. compositions, cross-section, shape or tapering
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
    • H01S3/06—Construction or shape of active medium
    • H01S3/063—Waveguide lasers, i.e. whereby the dimensions of the waveguide are of the order of the light wavelength
    • H01S3/067—Fibre lasers
    • H01S3/06754—Fibre amplifiers
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/30—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range using scattering effects, e.g. stimulated Brillouin or Raman effects
    • H01S3/302—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range using scattering effects, e.g. stimulated Brillouin or Raman effects in an optical fibre

Definitions

  • This invention relates to an optical fibre for high-power lasers and amplifiers.
  • Stimulated Brillouin scattering provides a limitation for high-power fibre lasers and optical amplifiers.
  • Light travelling down the fibre excites an acoustic wave which reflects the light, the reflected light being shifted in wavelength by the Brillouin wavelength shift.
  • Different glass materials have different Brillouin wavelength shifts and Brillouin bandwidths.
  • the stimulated Brillouin scattering threshold can be increased by varying the materials along an optical fibre, by inducing a temperature gradient along a fibre, by introducing tapers within the fibre, and by utilizing glasses having different Brillouin shifts across the cross section of a fibre.
  • an optical fibre comprising a waveguide defined by a length and a refractive index distribution, which refractive index distribution has a variation along its length such that in use stimulated Brillouin scattering is reduced.
  • the refractive index can be varied temporarily, there are significant practical advantages of providing an optical fibre that has a permanent variation in the refractive index distribution along its length.
  • the variation in refractive index distribution may be caused to vary by application of ultraviolet radiation.
  • the variation in refractive index distribution may be caused to vary by application of heat.
  • the variation in the refractive index distribution may be induced by stress.
  • the stress may be caused to vary by application of ultraviolet radiation.
  • the stress may be caused to vary by heat treatment.
  • the stress may be caused to vary by bending the fibre.
  • the variation in the refractive index distribution may be periodic.
  • the variation in the refractive index distribution may be aperiodic.
  • the variation in the refractive index distribution may be monotonic.
  • the optical fibre may be twisted along its length.
  • the twist rate may be varied along the length.
  • the optical fibre may be tapered along the length.
  • the waveguide may be tapered along the length.
  • the waveguide may comprise one or more rare earth dopants.
  • the rare earth dopant may comprise one or more Ytterbium, Erbium, Neodymium, Praseodymium, Thulium, Samarium, Holmium, Europium, Terbium, and Dysprosium.
  • the waveguide may be a so-called large mode area waveguide.
  • the invention also provides an optical amplifying device comprising the optical fibre.
  • the optical amplifying device may be an optical amplifier, a laser, a master oscillator power amplifier, or a source of amplified spontaneous emission.
  • the optical amplifying device may emit optical radiation.
  • the optical radiation may be pulsed, modulated or continuous wave.
  • the invention also provides a method for increasing the stimulated Brillouin scattering threshold of an optical fibre, which method comprises: (i) providing a waveguide of a defined length, (ii) selecting a method for varying the refractive index distribution along the length, and (iii) varying the refractive index distribution along the length such that in use stimulated Brillouin scattering is reduced.
  • the refractive index distribution may be varied by applying a variation of heat along its length.
  • the refractive index distribution may be varied by applying a variation of ultraviolet light along its length.
  • Figure 1 shows an optical fibre according to the present invention
  • Figure 2 shows an optical fibre comprising stress applying regions
  • Figure 3 shows an optical fibre comprising a photosensitive waveguide
  • Figure 4 shows an optical fibre comprising two inner claddings
  • Figure 5 shows an optical fibre configured as an amplifying optical device.
  • an optical fibre 1 comprising a waveguide 2 defined by a length 3 and a refractive index distribution 4, which refractive index distribution 4 has a variation 5 along its length 3 such that in use stimulated Brillouin scattering is reduced.
  • the waveguide 2 can comprise a core 6 and a cladding 7.
  • the variation 5 is a permanent variation in the refractive index distribution 4.
  • permanent it is meant lasting at least for hours if not years after the variation 5 has been induced.
  • Figure 2 shows an optical fibre 20 containing stress applying regions 21 which induce stress across the waveguide 2.
  • a fibre is commonly known as a polarisation maintaining fibre.
  • the stress induces a corresponding change in the refractive index distribution 4 of the optical fibre 20 via the photo-elastic effect.
  • the stress distribution of the fibre 20 can be modified by exposure to ultraviolet radiation.
  • the source of ultraviolet radiation is preferably one that is used in the manufacture of fibre optic Bragg gratings, for example a frequency doubled argon-ion laser or an excimer laser. Suitable lasers include the FreD and Sabre FreD lasers supplied by Coherent, Inc., and the Ll-FBG excimer laser manufactured by Lambda Physik.
  • the ultraviolet radiation can be applied after the fibre 20 is drawn. Alternatively, the ultraviolet radiation can be applied during the drawing process as further described in United States Patent US5400422 which is hereby incorporated herein by reference.
  • the latter patent refers to the manufacture of fibre Bragg gratings in which a fibre is exposed to ultraviolet radiation from a pair of interfering light beams.
  • the ultra violet radiation can be applied directly to the fibre 20.
  • the stress distribution of the fibre 20 can also be modified by applying heat treatment.
  • the stress distribution arises because of thermal stress, and in particular because the expansion coefficient of the stress applying regions 21 is higher than the expansion coefficient of the cladding 7. If the stress applying regions 21 are made from a borosilicate glass and the cladding 7 is silica, then the thermal stress distribution within the fibre 20 can be modified by applying heat treatment to the fibre 20, for example annealing the fibre 20 at a temperature in the range 300C to 500 C. Experimental results showing an increase in the stress birefringence of a polarisation maintaining fibre can be found in A.
  • the heat treatment can be applied periodically or aperiodically along the length 3, or such that the temperature varies monotonically along the fibre 20.
  • the heat can be applied by a thermal chamber, or a laser such as a C02 laser.
  • the heat treatment can also be applied as part of the fibre drawing process, annealing the fibre 20 in a thermal chamber located below the main furnace.
  • the amount of thermal annealing can be varied by varying the temperature of the furnace, or by varying the speed at which the fibre 20 is drawn.
  • the variations in stress and hence refractive index caused by such thermal treatments are essentially permanent, that is, that is provided the fibre 20 is not heated up to similar temperatures again.
  • Figure 3 shows an optical fibre 30 comprising a photosensitive waveguide 31 having a refractive index 4.
  • the refractive index 4 may have been caused to vary by application of ultraviolet radiation such as used in the manufacture of long-period optical fibre gratings.
  • the ultraviolet radiation may also cause the internal stress distribution to vary which induces a change in the refractive index distribution via the photo-elastic effect.
  • the refractive index 4 may be caused to vary by application of heat.
  • the variation 5 in the refractive index 4 can therefore be achieved by varying the strength of the ultraviolet radiation applied to the fibre 30 along its length 3.
  • Figure 4 shows an optical fibre 40 comprising a core 41, two inner claddings 42, 43 and an outer cladding 44. At least one of the core 41, two inner claddings 42, 43 and outer cladding 44 can be photosensitive.
  • At least one of the core 41 and the two inner claddings 42, 43 can have a different thermal expansion coefficient than the outer cladding 44.
  • Application of ultraviolet light and / or heat treatment can be used to cause variation in stress along the length of the optical fibre 40 in a similar manner to that described with reference to Figure 2, this variation in stress causing a corresponding variation in the refractive index distribution along the fibre via the photo-elastic effect.
  • the variation 5 in the refractive index distribution 4 may be periodic.
  • the periodicity of the variation 5 may be in the range 1mm to 10m, but is preferably in the range 1mm to 100mm.
  • the variation 5 can be aperiodic.
  • the variation 5 can also be monotonic, that is increasing or decreasing along the length 3.
  • a periodic variation of the refractive index distribution 4 along the length 3 reduces stimulated Brillouin scattering because it dephases the acoustic photons with respect to light propagating along the fibre 1.
  • Stimulated Brillouin scattering is often likened to a grating travelling along the fibre 1 which reflects light with an associated Doppler shift.
  • the twist rate may be varied along the length 3.
  • the optical fibre 1 may be tapered along the length 3.
  • the waveguide 2 may be tapered along the length 3.
  • the waveguide 2 may comprise one or more rare earth dopants.
  • the rare earth dopant may comprise one or more Ytterbium, Erbium, Neodymium, Praseodymium, Thulium, Samarium, Holmium, Europium, Terbium, and Dysprosium.
  • the waveguide 2 may be a so-called large mode area waveguide such as described in WO 00/02290.
  • large mode area it is meant that the waveguide 2 can have a relatively low numerical aperture such that the mode field diameter of the
  • the waveguide 2 can be operated
  • the bend radius would vary with length 3.
  • the waveguide 2 can be a microstructured fibre containing longitudinally extending holes along its length.
  • Figure 5 shows an optical amplifying device 50 comprising the optical fibre 1 and a pump source 51 providing pump radiation 52.
  • the optical amplifying device 50 may be an optical amplifier, a laser, a master oscillator power amplifier, or a source of amplified spontaneous emission. In use, the optical amplifying device 50 may emit optical radiation.
  • the optical radiation may be pulsed, modulated or continuous wave.
  • the optical radiation may be single frequency, multiple frequency or broadband radiation.
  • the invention also provides a method for increasing the stimulated Brillouin scattering threshold of an optical fibre 1, which method comprises: (i) providing a waveguide 2 of a defined length 3, (ii) selecting a method for varying the refractive index distribution 4 along the length, and (iii) varying the refractive index distribution 4 along the length 3 such that in use stimulated Brillouin scattering is reduced.
  • the refractive index distribution 4 may be varied by applying a variation of heat along its length 3. Alternatively or in addition, the refractive index distribution 4 may be varied by applying a variation of ultraviolet light along its length 3. The variation may be periodic, aperiodic, or monotonic.

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  • Physics & Mathematics (AREA)
  • Electromagnetism (AREA)
  • Engineering & Computer Science (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Signal Processing (AREA)
  • Plasma & Fusion (AREA)
  • Optics & Photonics (AREA)
  • Lasers (AREA)

Abstract

An optical fibre (1) comprising a waveguide (2) defined by a length (3) and a refractive index distribution (4), which refractive index distribution (4) has a variation (5) along its length (3) such that in use stimulated Brillouin scattering is reduced. The waveguide (2) can comprise a core (6) and a cladding (7). The variation (5) in the refractive index distribution (4) can be varied by the application of ultraviolet radiation or by thermal heat treatment.

Description

OPTICAL FIBRE WITH LENGHTWISE REFRACTIVE INDEX PROFILE VARIATION FOR BRILLOUIN SCATTERING REDUCTION
Field of Invention
This invention relates to an optical fibre for high-power lasers and amplifiers. Background to the Invention
Stimulated Brillouin scattering provides a limitation for high-power fibre lasers and optical amplifiers. Light travelling down the fibre excites an acoustic wave which reflects the light, the reflected light being shifted in wavelength by the Brillouin wavelength shift. Different glass materials have different Brillouin wavelength shifts and Brillouin bandwidths.
It is known that the stimulated Brillouin scattering threshold can be increased by varying the materials along an optical fibre, by inducing a temperature gradient along a fibre, by introducing tapers within the fibre, and by utilizing glasses having different Brillouin shifts across the cross section of a fibre.
It is an aim of the present invention to increase the stimulated Brillouin scattering threshold of an optical fibre.
Summary of the Invention
According to a non-limiting embodiment of the present invention there is provided an optical fibre comprising a waveguide defined by a length and a refractive index distribution, which refractive index distribution has a variation along its length such that in use stimulated Brillouin scattering is reduced.
Although the refractive index can be varied temporarily, there are significant practical advantages of providing an optical fibre that has a permanent variation in the refractive index distribution along its length.
The variation in refractive index distribution may be caused to vary by application of ultraviolet radiation. The variation in refractive index distribution may be caused to vary by application of heat.
Alternatively or additionally, the variation in the refractive index distribution may be induced by stress. The stress may be caused to vary by application of ultraviolet radiation. The stress may be caused to vary by heat treatment. The stress may be caused to vary by bending the fibre.
The variation in the refractive index distribution may be periodic. The variation in the refractive index distribution may be aperiodic. The variation in the refractive index distribution may be monotonic.
The optical fibre may be twisted along its length. The twist rate may be varied along the length.
The optical fibre may be tapered along the length. Alternatively or additionally the waveguide may be tapered along the length.
The waveguide may comprise one or more rare earth dopants. The rare earth dopant may comprise one or more Ytterbium, Erbium, Neodymium, Praseodymium, Thulium, Samarium, Holmium, Europium, Terbium, and Dysprosium. The waveguide may be a so-called large mode area waveguide.
The invention also provides an optical amplifying device comprising the optical fibre. The optical amplifying device may be an optical amplifier, a laser, a master oscillator power amplifier, or a source of amplified spontaneous emission. In use, the optical amplifying device may emit optical radiation. The optical radiation may be pulsed, modulated or continuous wave.
The invention also provides a method for increasing the stimulated Brillouin scattering threshold of an optical fibre, which method comprises: (i) providing a waveguide of a defined length, (ii) selecting a method for varying the refractive index distribution along the length, and (iii) varying the refractive index distribution along the length such that in use stimulated Brillouin scattering is reduced.
The refractive index distribution may be varied by applying a variation of heat along its length. The refractive index distribution may be varied by applying a variation of ultraviolet light along its length.
Brief Description of the Drawings
Embodiments of the invention will now be described solely by way of example and with reference to the accompanying drawings in which:
Figure 1 shows an optical fibre according to the present invention; Figure 2 shows an optical fibre comprising stress applying regions; Figure 3 shows an optical fibre comprising a photosensitive waveguide; Figure 4 shows an optical fibre comprising two inner claddings; and Figure 5 shows an optical fibre configured as an amplifying optical device. Detailed Description of Preferred Embodiments of the Invention
With reference to Figure 1, there is provided an optical fibre 1 comprising a waveguide 2 defined by a length 3 and a refractive index distribution 4, which refractive index distribution 4 has a variation 5 along its length 3 such that in use stimulated Brillouin scattering is reduced. The waveguide 2 can comprise a core 6 and a cladding 7.
Preferably, the variation 5 is a permanent variation in the refractive index distribution 4. By permanent, it is meant lasting at least for hours if not years after the variation 5 has been induced.
Figure 2 shows an optical fibre 20 containing stress applying regions 21 which induce stress across the waveguide 2. Such a fibre is commonly known as a polarisation maintaining fibre. The stress induces a corresponding change in the refractive index distribution 4 of the optical fibre 20 via the photo-elastic effect.
The stress distribution of the fibre 20 can be modified by exposure to ultraviolet radiation. The source of ultraviolet radiation is preferably one that is used in the manufacture of fibre optic Bragg gratings, for example a frequency doubled argon-ion laser or an excimer laser. Suitable lasers include the FreD and Sabre FreD lasers supplied by Coherent, Inc., and the Ll-FBG excimer laser manufactured by Lambda Physik. The ultraviolet radiation can be applied after the fibre 20 is drawn. Alternatively, the ultraviolet radiation can be applied during the drawing process as further described in United States Patent US5400422 which is hereby incorporated herein by reference. Note that the latter patent refers to the manufacture of fibre Bragg gratings in which a fibre is exposed to ultraviolet radiation from a pair of interfering light beams. For the present invention, there is no need to provide interference fringes and the ultra violet radiation can be applied directly to the fibre 20.
It may be advantageous to select a coating on the optical fibre 20 that allows the ultraviolet wavelength to be written through the coating. Alternatively, it may be advantageous to apply the ultraviolet light during the fibre drawing process.
The stress distribution of the fibre 20 can also be modified by applying heat treatment. The stress distribution arises because of thermal stress, and in particular because the expansion coefficient of the stress applying regions 21 is higher than the expansion coefficient of the cladding 7. If the stress applying regions 21 are made from a borosilicate glass and the cladding 7 is silica, then the thermal stress distribution within the fibre 20 can be modified by applying heat treatment to the fibre 20, for example annealing the fibre 20 at a temperature in the range 300C to 500 C. Experimental results showing an increase in the stress birefringence of a polarisation maintaining fibre can be found in A. Ourmadz et al, "Thermal Properties of High-Birefringence Optical Fibers and Preforms", Applied Optics, Volume 22, pages 2374 to 2379, 1983, and A. Ourmadz et al, "Enhancement of Birefringence in Polarisation-Maintaining Fibres by Thermal Annealing", Electronics Letters, Volume 19, pages 143-144, 1983. The heat treatment can be applied periodically or aperiodically along the length 3, or such that the temperature varies monotonically along the fibre 20. The heat can be applied by a thermal chamber, or a laser such as a C02 laser. The heat treatment can also be applied as part of the fibre drawing process, annealing the fibre 20 in a thermal chamber located below the main furnace. The amount of thermal annealing can be varied by varying the temperature of the furnace, or by varying the speed at which the fibre 20 is drawn. The variations in stress and hence refractive index caused by such thermal treatments are essentially permanent, that is, that is provided the fibre 20 is not heated up to similar temperatures again.
Figure 3 shows an optical fibre 30 comprising a photosensitive waveguide 31 having a refractive index 4. The refractive index 4 may have been caused to vary by application of ultraviolet radiation such as used in the manufacture of long-period optical fibre gratings. The ultraviolet radiation may also cause the internal stress distribution to vary which induces a change in the refractive index distribution via the photo-elastic effect. The refractive index 4 may be caused to vary by application of heat. The variation 5 in the refractive index 4 can therefore be achieved by varying the strength of the ultraviolet radiation applied to the fibre 30 along its length 3.
Figure 4 shows an optical fibre 40 comprising a core 41, two inner claddings 42, 43 and an outer cladding 44. At least one of the core 41, two inner claddings 42, 43 and outer cladding 44 can be photosensitive.
Application of ultraviolet light and / or heat treatment can be used to cause variation in refractive index along the length of the optical fibre 40 in a similar manner to that described with reference to Figure 2.
At least one of the core 41 and the two inner claddings 42, 43 can have a different thermal expansion coefficient than the outer cladding 44. Application of ultraviolet light and / or heat treatment can be used to cause variation in stress along the length of the optical fibre 40 in a similar manner to that described with reference to Figure 2, this variation in stress causing a corresponding variation in the refractive index distribution along the fibre via the photo-elastic effect.
Referring to Figures 1 to 4, the variation 5 in the refractive index distribution 4 may be periodic. The periodicity of the variation 5 may be in the range 1mm to 10m, but is preferably in the range 1mm to 100mm. The variation 5 can be aperiodic. The variation 5 can also be monotonic, that is increasing or decreasing along the length 3.
Without attempting to restrict the applicability of the invention, it is believed that a periodic variation of the refractive index distribution 4 along the length 3 reduces stimulated Brillouin scattering because it dephases the acoustic photons with respect to light propagating along the fibre 1. Stimulated Brillouin scattering is often likened to a grating travelling along the fibre 1 which reflects light with an associated Doppler shift. By causing the relative phase and/or travelling speed of the grating and light to vary along the length of the fibre 1 by variations in the refractive index 4, the build up of stimulated Brillouin scattering can be reduced.
Further reductions in stimulated Brillouin scattering can be obtained by twisting the optical fibre 1 along its length, either during manufacture to provide a frozen-in twist, or subsequently. The twist rate may be varied along the length 3. The optical fibre 1 may be tapered along the length 3. The waveguide 2 may be tapered along the length 3.
The waveguide 2 may comprise one or more rare earth dopants. The rare earth dopant may comprise one or more Ytterbium, Erbium, Neodymium, Praseodymium, Thulium, Samarium, Holmium, Europium, Terbium, and Dysprosium.
The waveguide 2 may be a so-called large mode area waveguide such as described in WO 00/02290. By large mode area, it is meant that the waveguide 2 can have a relatively low numerical aperture such that the mode field diameter of the
fundamental mode is in the range 1 Oμm to 40μm. The waveguide 2 can be operated
such that it is effectively single moded. This can be achieved by bending a low- numerical aperture, multimode waveguide such that higher modes leak away leaving the fundamental mode in place. Preferably the bend radius would vary with length 3.
The waveguide 2 can be a microstructured fibre containing longitudinally extending holes along its length.
Figure 5 shows an optical amplifying device 50 comprising the optical fibre 1 and a pump source 51 providing pump radiation 52. The optical amplifying device 50 may be an optical amplifier, a laser, a master oscillator power amplifier, or a source of amplified spontaneous emission. In use, the optical amplifying device 50 may emit optical radiation. The optical radiation may be pulsed, modulated or continuous wave. The optical radiation may be single frequency, multiple frequency or broadband radiation.
The invention also provides a method for increasing the stimulated Brillouin scattering threshold of an optical fibre 1, which method comprises: (i) providing a waveguide 2 of a defined length 3, (ii) selecting a method for varying the refractive index distribution 4 along the length, and (iii) varying the refractive index distribution 4 along the length 3 such that in use stimulated Brillouin scattering is reduced.
The refractive index distribution 4 may be varied by applying a variation of heat along its length 3. Alternatively or in addition, the refractive index distribution 4 may be varied by applying a variation of ultraviolet light along its length 3. The variation may be periodic, aperiodic, or monotonic.
It is to be appreciated that the embodiments of the invention described above with reference to the accompanying drawings have been given by way of example only and that modifications and additional components may be provided to enhance performance. The present invention extends to the above mentioned features taken singly or in any combination.

Claims

Claims
1. An optical fibre comprising a waveguide defined by a length and a refractive index distribution, which refractive index distribution has a variation along its length such that in use stimulated Brillouin scattering is reduced.
2. Apparatus according to claim 1 in which the variation is a permanent variation in the refractive index distribution.
3. An optical fibre according to claim 1 or claim 2 in which the variation in refractive index distribution has been caused to vary by application of ultraviolet radiation.
4. An optical fibre according to any one of claims 1 to 3 in which the refractive index distribution has been caused to vary by application of heat.
5. An optical fibre according to any one of the preceding claims in which the variation in the refractive index distribution is induced by stress.
6. An optical fibre according to claim 5 in which the stress has been caused to vary by application of ultraviolet radiation.
7. An optical fibre according to claim 5 or claim 6 in which the stress has been caused to vary by heat treatment.
8. An optical fibre according to any one of claims 5 to 7 in which the stress has been caused to vary by bending the fibre.
9. An optical fibre according to any one of the preceding claims in which the variation in the refractive index distribution is periodic.
10. An optical fibre according to any one of claims 1 to 8 in which the variation in the refractive index distribution is aperiodic.
11. An optical fibre according to any one of 1 to 8 in which the variation in the refractive index distribution is monotonic.
12. An optical fibre according to any one of the preceding claims in which the optical fibre is twisted along its length.
13. An optical fibre according to claim 12 in which the twist rate is varied along the length.
14. An optical fibre according to any one of the preceding claims in which the optical fibre is tapered along the length.
15. An optical fibre according to any one of the preceding claims in which the waveguide is tapered along the length.
16. An optical fibre according to any one of the preceding claims in which the waveguide comprises one or more rare earth dopants.
17. An optical fibre according to claim 16 in which the rare earth dopant comprises one or more of Ytterbium, Erbium, Neodymium, Praseodymium, Thulium, Samarium, Holmium, Europium, Terbium, and Dysprosium.
18. An optical fibre according to any one of the preceding claims in which the waveguide is a large mode area waveguide.
19. An optical fibre according to any one of the preceding claims configured as an optical amplifying device.
20. An optical fibre according to claim 19 in which the optical amplifying device is an optical amplifier, a laser, a master oscillator power amplifier or a source of amplified spontaneous emission.
21. An optical fibre substantially as herein described with reference to the accompanying drawings.
22. A method for increasing the stimulated Brillouin scattering threshold of an optical fibre, which method comprises: (i) providing a waveguide of a defined length, (ii) selecting a method for varying the refractive index distribution along the length, and (iii) varying the refractive index distribution along the length such that in use stimulated Brillouin scattering is reduced.
23. A method according to claim 22 in which the refractive index distribution is varied by applying a variation of heat along its length.
24. A method according to claim 22 or claim 23 in which the refractive index distribution is varied by applying a variation of ultraviolet light along its length.
25. A method for increasing the stimulated Brillouin threshold of an optical fibre substantially as herein described with reference to the accompanying drawings.
PCT/GB2003/004028 2002-09-20 2003-09-19 Optical fibre with lenghtwise refractive index profile variation for brillouin scattering reduction WO2004027941A1 (en)

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GB0221858.4 2002-09-20
GB0221858A GB0221858D0 (en) 2002-09-20 2002-09-20 An optical fibre for high power lasers and amplifiers

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Cited By (8)

* Cited by examiner, † Cited by third party
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US7555186B2 (en) 2006-12-04 2009-06-30 Draka Comteq B.V. Optical fiber
US7623747B2 (en) 2005-11-10 2009-11-24 Draka Comteq B.V. Single mode optical fiber
US7889960B2 (en) 2008-05-06 2011-02-15 Draka Comteq B.V. Bend-insensitive single-mode optical fiber
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EP1891471A4 (en) * 2005-06-15 2017-11-22 Corning Incorporated High sbs threshold optical fiber
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Cited By (18)

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EP1891471A4 (en) * 2005-06-15 2017-11-22 Corning Incorporated High sbs threshold optical fiber
EP1762867A1 (en) 2005-09-09 2007-03-14 Draka Comteq B.V. Optical fiber with reduced stimulated brillouin scattering
EP1764633A1 (en) 2005-09-09 2007-03-21 Draka Comteq B.V. Optical fiber with reduced stimulated brillouin scattering
US7437040B2 (en) 2005-09-09 2008-10-14 Draka Comteq B.V. Optical fiber with reduced stimulation Brillouin scattering
US7995889B2 (en) 2005-11-10 2011-08-09 Draka Comteq, B.V. Single mode optical fiber
US7623747B2 (en) 2005-11-10 2009-11-24 Draka Comteq B.V. Single mode optical fiber
US8837889B2 (en) 2005-11-10 2014-09-16 Draka Comteq, B.V. Single mode optical fiber
US7899293B2 (en) 2006-04-10 2011-03-01 Draka Comteq, B.V. Single-mode optical fiber
US8103143B2 (en) 2006-04-10 2012-01-24 Draka Comteq, B.V. Single-mode optical fiber
US7894698B2 (en) 2006-12-04 2011-02-22 Draka Comteq B.V. Optical fiber
US7555186B2 (en) 2006-12-04 2009-06-30 Draka Comteq B.V. Optical fiber
US8385705B2 (en) 2007-11-09 2013-02-26 Draka Comteq, B.V. Microbend-resistant optical fiber
US8145027B2 (en) 2007-11-09 2012-03-27 Draka Comteq, B.V. Microbend-resistant optical fiber
US8145025B2 (en) 2008-05-06 2012-03-27 Draka Comteq, B.V. Single-mode optical fiber having reduced bending losses
US8428414B2 (en) 2008-05-06 2013-04-23 Draka Comteq, B.V. Single-mode optical fiber having reduced bending losses
US7889960B2 (en) 2008-05-06 2011-02-15 Draka Comteq B.V. Bend-insensitive single-mode optical fiber
US8131125B2 (en) 2008-05-06 2012-03-06 Draka Comteq, B.V. Bend-insensitive single-mode optical fiber
US10294146B2 (en) 2016-08-31 2019-05-21 Corning Incorporated Single mode optical fibers with Brillouin frequency-shift management

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