WO2018138472A2 - Apparatus and method for optical filtering - Google Patents

Apparatus and method for optical filtering Download PDF

Info

Publication number
WO2018138472A2
WO2018138472A2 PCT/GB2018/000014 GB2018000014W WO2018138472A2 WO 2018138472 A2 WO2018138472 A2 WO 2018138472A2 GB 2018000014 W GB2018000014 W GB 2018000014W WO 2018138472 A2 WO2018138472 A2 WO 2018138472A2
Authority
WO
WIPO (PCT)
Prior art keywords
optical
optical fibre
stop band
bending
band edge
Prior art date
Application number
PCT/GB2018/000014
Other languages
French (fr)
Other versions
WO2018138472A3 (en
Inventor
Christophe André CODEMARD
Mikhail Nickolaos Zervas
Original Assignee
Spi Lasers Uk 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 Spi Lasers Uk Limited filed Critical Spi Lasers Uk Limited
Publication of WO2018138472A2 publication Critical patent/WO2018138472A2/en
Publication of WO2018138472A3 publication Critical patent/WO2018138472A3/en

Links

Classifications

    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K26/00—Working by laser beam, e.g. welding, cutting or boring
    • B23K26/36—Removing material
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K26/00—Working by laser beam, e.g. welding, cutting or boring
    • B23K26/02—Positioning or observing the workpiece, e.g. with respect to the point of impact; Aligning, aiming or focusing the laser beam
    • B23K26/06—Shaping the laser beam, e.g. by masks or multi-focusing
    • B23K26/064—Shaping the laser beam, e.g. by masks or multi-focusing by means of optical elements, e.g. lenses, mirrors or prisms
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K26/00—Working by laser beam, e.g. welding, cutting or boring
    • B23K26/20—Bonding
    • B23K26/21—Bonding by welding
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K26/00—Working by laser beam, e.g. welding, cutting or boring
    • B23K26/352—Working by laser beam, e.g. welding, cutting or boring for surface treatment
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/02—Optical fibres with cladding with or without a coating
    • G02B6/02052—Optical fibres with cladding with or without a coating comprising optical elements other than gratings, e.g. filters
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/02—Optical fibres with cladding with or without a coating
    • G02B6/02295—Microstructured optical fibre
    • G02B6/02314—Plurality of longitudinal structures extending along optical fibre axis, e.g. holes
    • G02B6/02342—Plurality of longitudinal structures extending along optical fibre axis, e.g. holes characterised by cladding features, i.e. light confining region
    • G02B6/0238—Longitudinal structures having higher refractive index than background material, e.g. high index solid rods
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/02—Optical fibres with cladding with or without a coating
    • G02B6/02295—Microstructured optical fibre
    • G02B6/02314—Plurality of longitudinal structures extending along optical fibre axis, e.g. holes
    • G02B6/0239—Comprising means for varying the guiding properties, e.g. tuning means

Definitions

  • This invention relates to an apparatus and method for optical filtering and, more especially, for optical filtering optical radiation propagating along an optical fibre.
  • the invention has particular application for power scaling in optical fibre lasers, laser beam delivery in optical fibre lasers, and equipment for laser processing of industrial materials.
  • High power lasers have important applications in the laser processing of industrial materials. Pulsed lasers, with powers exceeding lOkW, are used in marking, engraving, cutting, welding, and drilling applications. Continuous wave lasers with powers exceeding lkW are used in cutting and welding applications.
  • apparatus for optical filtering optical radiation propagating along an optical fibre which apparatus comprises the optical fibre and at least one bending mechanism for bending the optical fibre, wherein:
  • the optical fibre comprises a core, a cladding, and at least one resonant coupling structure within the cladding;
  • the resonant coupling structure is such that it couples optical radiation within a spectral bandwidth defined by a first stop band edge and a second stop band edge, the optical radiation being coupled between the core and the cladding;
  • the cladding is defined by an optical loss which causes the optical radiation that is coupled between the core and the cladding to be attenuated at wavelengths between the first stop band edge and the second stop band edge, and thereby defining a stop band having the spectral bandwidth;
  • the resonant coupling structure may comprise at least one rod placed near the core.
  • the refractive index of the rod is preferably higher than the refractive index of the core.
  • the resonant coupling structure may comprise at least one ring placed around the core. The refractive index of the ring is preferably higher than the refractive index of the core.
  • the optical loss in the cladding can be introduced by incorporating at least one region configured to absorb or scatter the optical radiation.
  • the region is preferably placed such that it interacts with the modes guided by the resonant structure much more strongly than its interaction with the fundamental mode of the core.
  • optical radiation propagates along a optical fibre that has optical loss then the optical radiation is attenuated, that is, its intensity becomes smaller as the optical radiation propagates along the optical fibre.
  • the optical radiation propagating along the optical fibre at wavelengths within the stop band will be attenuated by the optical loss in the cladding, whereas optical radiation at wavelengths that are outside of the stop band can propagate along the core of the optical fibre without being attenuated by the optical loss in the cladding.
  • Being able to tune the spectral bandwidth of the stop band by bending the optical fibre provides useful advantages over the prior art.
  • the bending mechanism may be a bimetallic strip.
  • the bimetallic strip may comprise a first metal, a second metal, and a heating element attached to the first metal.
  • the first metal and the second metal may have different thermal expansion coefficients.
  • the bending mechanism may be a piezoelectric or magnetic element, or a mechanical element such as a screw, a lever, or a ratchet.
  • the bending mechanism may be a simple bending mechanism such as bending the optical fibre over a cylinder or a mechanism designed to adjust the bend radius of a loop of the optical fibre.
  • the bending mechanism may comprise a mechanical formation onto which the optical fibre is bent by hand or other means, and then attached.
  • Bending of the optical fibre may cause a wavelength shift in the second stop band edge that is greater than a wavelength shift in the first stop band edge, thereby increasing the spectral bandwidth.
  • This is particularly useful for the suppression of stimulated Raman scattering in optical fibres because it permits signal radiation at a wavelength shorter than the first stop band edge to be transmitted with minimal loss, whilst the stimulated Raman scattering radiation generated by the signal radiation can be removed from the core of the optical fibre.
  • the ability to tune the stop band means that more than one Stokes order of the stimulated Raman scattering can be removed from the optical fibre.
  • the bending mechanism may comprise a periodic surface.
  • the bending mechanism may be configured to bend the optical fibre by squeezing the periodic surface and the optical fibre together with a squeezing force, and the second stop band edge is able to be varied by adjusting the squeezing force.
  • the periodic surface may be chirped. Varying the periodicity along the length of the bending mechanism, either monotonically or in a non-monotonic fashion, reduces the amount of squeezing force that is required to obtain the desired second stop band edge, thereby increasing reliability.
  • the bending mechanism may comprise at least two of the periodic surfaces arranged at an angle to each other. The periodic surfaces may have the same periodicity. The bending mechanism may be such that each periodic surface is able to be squeezed against the optical fibre with different squeezing forces.
  • the spatial phases of the periodic surfaces may be configured such that the optical fibre is deformed in a helical manner when the squeezing forces are applied to the two period surfaces. This arrangement provides great control over the resonant coupling.
  • the optical fibre when the optical fibre is deformed in a helical manner using the above bending mechanism, it is possible to not only control the position of the second stop band edge, but also to control the amount by which the optical radiation propagating along the optical fibre is attenuated within the stop band.
  • the optical attenuation within the stop band is also more uniform over the spectral bandwidth.
  • the bending mechanism may include an actuator.
  • the apparatus may comprise a plurality of the bending mechanisms. This reduces the required squeezing forces on each of the bending mechanisms thereby improving reliability.
  • At least one of the bending mechanisms may have a different periodicity than another of the bending mechanisms. Different periodicities cause enhanced coupling between different groups of modes of the resonant structure and the guided modes of the core.
  • the apparatus may comprise a laser.
  • the apparatus may comprise an output fibre.
  • the optical fibre may be spliced to the output fibre.
  • the apparatus may comprise at least one optical amplifier.
  • the optical fibre and the bending mechanism may be located between the laser and the optical amplifier. Alternatively or additionally, the optical fibre and the bending mechanism may be located within the optical amplifier.
  • the apparatus may comprise at least two of the optical amplifiers.
  • the optical fibre and the bending mechanism may be located between the two optical amplifiers.
  • the invention also provides a method for optical filtering optical radiation propagating along an optical fibre, which method comprises providing the optical fibre and at least one bending mechanism for bending the optical fibre, wherein:
  • the optical fibre comprises a core, a cladding, and at least one resonant coupling structure within the cladding;
  • the resonant coupling structure is such that it couples optical radiation within a spectral bandwidth defined by a first stop band edge and a second stop band edge, the optical radiation being coupled between the core and the cladding;
  • the cladding is defined by an optical loss which causes the optical radiation that is coupled between the core and the cladding to be attenuated at wavelengths between the first stop band edge and the second stop band edge, and thereby defining a stop band having the spectral bandwidth; • the bending mechanism and the resonant coupling structure are such that bending the optical fibre changes the bandwidth; and
  • the method includes the step of bending the optical fibre with the bending
  • the method of the invention may include a step or steps as required to utilize the above mentioned optional aspects of the apparatus of the invention.
  • Figure 1 shows apparatus for optical filtering and, more especially, for optical filtering optical radiation propagating along an optical fibre, the apparatus comprising an optical fibre attached to a bending mechanism;
  • Figure 2 shows a stop band provided by the optical filter
  • Figure 3 shows signal and Raman Stokes emissions
  • Figure 4 shows a bending mechanism comprising a periodic surface that bends the optical fibre periodically along its length
  • Figure 5 shows a bending mechanism in which the periodic surface is chirped along its length
  • Figures 6 to 8 show bending mechanisms that are able to bend the optical fibre in a helix
  • Figure 9 shows the tuning of the stop band in both optical attenuation and second stop band edge
  • Figure 10 shows a laser system comprising a plurality of the optical filters
  • Figure 1 1 shows an optical fibre comprising resonant structures in the form of high refractive index rods
  • Figure 12 shows an optical fibre comprising a resonant structure in the form of a high refractive index ring.
  • Figure 1 shows apparatus for optical filtering optical radiation propagating along an optical fibre 1, which apparatus comprises the optical fibre 1 and at least one bending mechanism 2 for bending the optical fibre 1, wherein:
  • the optical fibre 1 comprises a core 3, a cladding 4, and at least one resonant coupling structure 5 within the cladding 4;
  • the resonant coupling structure 5 is such that it couples optical radiation 6 within a spectral bandwidth 8 defined by a first stop band edge 1 1 and a second stop band edge 12, the optical radiation 6 being coupled between the core 3 and the cladding 4;
  • the cladding 4 is defined by an optical loss 10 which causes the optical radiation 6 that is coupled between the core 3 and the cladding 4 to be attenuated at wavelengths between the first stop band edge 11 and the second stop band edge 12, and thereby defining a stop band 7 having the spectral bandwidth 8;
  • the stop band 7 has an optical attenuation 80 at wavelengths between the first stop band edge 11 and the second stop band edge 12, and negligible attenuation outside these wavelengths.
  • the optical radiation 6 propagating along the optical fibre 1 at wavelengths within the stop band 7 will be attenuated, whereas optical radiation at wavelengths that are outside of the stop band 7 can propagate along the optical fibre 1 without substantial optical attenuation.
  • the coupling of the optical radiation 6 between the core 3 and the cladding 4 can be via the resonant coupling structure 5.
  • the attenuation may be by absorption or scattering.
  • the stop band 7 has a central wavelength 9 which as at the centre of the first stop band edge 11 and the second stop band edge 12.
  • the second stop band edge 12 is at a higher wavelength than the first stop band edge 11.
  • the optical fibre 1 can be the optical fibre 1000 shown in Figure 11.
  • the resonant coupling structures 5 are high refractive index rods 1015 placed around the core 3.
  • the rods 1015 are characterized by a diameter 1003 and a refractive index 1002.
  • the refractive index 1002 of the rods 1015 is preferably higher than the refractive index 1001 of the core 3.
  • the diameter 1003 and the refractive index 1002 are selected such that the high refractive rods 1015 support more than one optical rod mode or groups of closely spaced rod modes, of which at least one is phase matched to and, therefore, resonantly coupled to, the fundamental mode of the core 3 at the central wavelength 9.
  • At least one of the high refractive index rods 1015 is placed at a distance 1005 from the core 3 such that the fundamental mode of the core 3 overlaps the rod 1015. This enhances the resonant coupling with the rod modes of the rod 1015 at the central wavelength 9.
  • the high refractive index rods 1015 can be placed in at least one ring around the core 3 such that the fields of the optical rod modes of each rod 1015 overlaps at least one adjacent rod 1015. Increasing the number of rings of high refractive index rods 1015, such as shown in Figure 1, results in sharper stop first and second stop band edges 11 and 12 with wavelength, and thus improves the filtering characteristics.
  • the high refractive index rods 1015 can be scattered around the core 3 in a more random fashion, such as shown in Figure 4, whilst ensuring that each rod 1015 is optically coupled to at least one adjacent rod 1015.
  • the high refractive index rods 1015 can be identical and, in this case, the resonant coupling involves phase matching between the fundamental mode of the core 3 and the same rod mode or rod mode group.
  • At least one of the high refractive index rods 1015 can have a different diameter 1003 and/or refractive index 1002 than the other high refractive index rods 1015, whereupon the resonant coupling of the first rod 1015 involves phase matching between the fundamental mode of the core 3 and rod modes or rod mode groups of different order than the modes of the other rods 1015.
  • the optical fibre 1 can be the optical fibre 1 100 shown in Figure 12.
  • the resonant coupling structure 5 is in the form of at least one high refractive index ring 1010 placed around the core 3.
  • the ring 1010 is characterized by a radius 101 , a thickness 1011 and a refractive index 1012.
  • the refractive index 1012 of the ring is preferably higher than the refractive index 1001 of the core 3.
  • the high refractive ring 1010 supports more than one optical ring mode or groups of closely spaced ring modes, at least one of which is phase matched and, therefore, resonantly coupled to the fundamental mode of the core 3 at the central wavelength 9.
  • the radius 1013, thickness 101 1 and refractive index 1012 are selected to enhance the resonant coupling with the fundamental mode of the core 3 at the central wavelength 9.
  • the high refractive index ring 1010 can be circular or non-circular and placed concentrically or non-concentrically around the core 3. More than one high refractive index ring 1010 can be incorporated. Increasing the number of the high refractive index rings 1010 results in sharper first and second stop band edges 11 and 12, thereby improving the filtering characteristics of the apparatus.
  • the high refractive index rings 1010 can have identical widths 1011 and refractive indices 1012 and in this case the resonant coupling involves phase matching between the fundamental mode of the core 3 and the same ring mode or group of ring modes.
  • the high refractive index rings 1010 can have different thicknesses 101 1 and/or refractive indices 1012 and in this case the resonant coupling involves phase matching between the fundamental mode of the core 3 and ring modes or group of modes of different order.
  • the resonant coupling structures 5 can also comprise a mixture of multimode waveguides in the form of high refractive index rods 1015 and rings 1010 placed around the core 3.
  • the optical loss 10 in the cladding 4 can be introduced by incorporating at least one lossy region 1020.
  • the lossy region 1020 is shown as a concentric ring in Figures 11 and 12. Alternatively or additionally, the lossy region 1020 can be dispersed among the resonant coupling structures 5.
  • the lossy region 1020 can be doped with dopants selected to absorb the optical radiation, such as a rare earth ion, a semiconductor, or another metal ion.
  • the lossy region 1020 can comprise one or more high index leaky layers.
  • the lossy region 1020 can comprise scattering layers or segments. The scattering elements can be introduced by appropriate cladding surface modification and/or roughness.
  • the lossy region 1020 is preferably placed such that it interacts with the modes guided by the resonant structure 5 much more strongly than its interaction the fundamental mode of the core 3.
  • the fundamental mode of the core 3 does not overlap or interact with the lossy region 1020.
  • the bending mechanism 2 may be a bimetallic strip 16 as depicted in Figure 1.
  • the bimetallic strip 16 comprises a first metal 17, a second metal 18, and a heating element 19 attached to the first metal 17.
  • the first metal 17 and the second metal 18 have different thermal expansion coefficients. When heated by applying electric current to the heating element 19, the bimetallic strip 16 bends the optical fibre 1 that is attached to it.
  • the bending mechanism 2 may also be a piezoelectric or magnetic element, or a mechanical element such as a screw, a lever, or a ratchet.
  • the bending mechanism 2 can be a simple bending mechanism such as bending the optical fibre 1 over a cylinder or a mechanism designed to adjust the bend radius of a loop of the optical fibre 1.
  • the bending mechanism 2 can comprise a mechanical fixture onto which the optical fibre 1 is bent by hand or other means, and then attached.
  • bending of the optical fibre 1 causes a wavelength shift in the second stop band edge 12.
  • the first stop band edge 11 is typically not shifted very much by the bending of the optical fibre 1.
  • the second stop band edge 12 shifts to the second stop band edge 21 when the optical fibre is bent, and can be shifted further to the second stop band edge 22 by an increased amount of bending.
  • the wavelength shift in the second stop band edge 12 is greater than the wavelength shift in the first stop band edge 11.
  • the first and the second stop band edges 11 and 12 result from phase matching of the fundamental mode of core 3 to different order modes of the resonant coupling structure 5.
  • the stop band edge 12 results from phase matching the fundamental mode of core 3 to a higher order mode of the resonant coupling structure 5, compared to the one responsible for the stop band edge 1 1.
  • Bending affects the phase matching conditions and mode shape.
  • the higher the order of the mode the more they are affected by the bending.
  • the effect of bending the optical fibre 1 is much more pronounced on the wavelength position of the second stop band edge 12 than on the wavelength position of the first stop band edge 11.
  • Increasing the bending of the optical fibre 1 shifts the resonance condition and therefore the second stop band edge 12 to longer wavelengths, increasing the spectral bandwidth 8.
  • Figure 3 shows the optical radiation 6 in the form of a signal 30 and the corresponding first Raman Stokes emission 31, second Raman Stokes emission 32 and third Raman Stokes emission 33 caused by stimulated Raman scattering in an optical fibre.
  • a laser signal at 1060nm is transmitted along a single mode optical fibre at a power of IkW
  • a first Raman Stokes emission appears at approximately 1120nm
  • a second Raman Stokes emission at approximately 1170nm
  • a third Raman Stokes emission at approximately 1210nm
  • the apparatus shown in Figure 1 is particularly useful for the suppression of stimulated Raman scattering in optical fibres because it permits the signal 30 which has a wavelength shorter than the first stop band edge 11, to be transmitted with minimal loss, whilst the stimulated first, second and third Raman Stokes emission 31, 32, 33 generated by the signal 30, to be removed from the core 3 of the optical fibre 1.
  • the ability to tune the stop band 7 means that more than one of the stimulated first, second and third Raman Stokes emissions 31, 32, 33 can be removed from the optical fibre 1.
  • Figure 4 shows a bending mechanism 45 comprising a periodic surface 13.
  • the periodic surface 13 is defined by a period 42 or periodicity.
  • the bending mechanism 2 is configured to squeeze the periodic surface 13 and the length of the optical fibre 1 together in order to bend the optical fibre 1 periodically along its length with a squeezing force 14.
  • the second stop band edge 12 is able to be varied by adjusting the squeezing force 14.
  • Figure 5 shows a bending mechanism 40 wherein the periodic surface 41 is chirped. Varying the period 42 along the length of the bending mechanism 40, either monotonically or in a non-monotonic fashion, reduces the amount of squeezing force 14 that is required to obtain a desired wavelength for the second stop band edge 12, thereby increasing reliability.
  • Figure 6 shows a bending mechanism 51 that comprises two periodic surfaces 52, 53 arranged at an angle 54 to each other.
  • the periodic surfaces 53, 54 may have the same period 42 shown with reference to Figures 4 and 5.
  • the bending mechanism 51 may be such that each periodic surface 52, 53 is able to be squeezed against the optical fibre 1 with different squeezing forces 14.
  • the spatial phases of the periodic surfaces 52, 53 may be configured such that the optical fibre 1 is deformed substantially in a helical manner when the squeezing forces 14 are applied to the two period surfaces 52, 53. This arrangement provides great control over the resonant coupling.
  • Figure 7 shows a bending mechanism 65 comprising three parts 61 that each have two periodic surfaces 71 and 72 shown with reference to Figure 8.
  • the periodic surfaces 71 and 72 are 120 degrees out of phase with each other.
  • Each part 61 is arranged to deform the optical fibre 1 in a substantially helical manner.
  • the optical fibre 1 when the optical fibre 1 is bent periodically along its length, and preferably in a helical deformation, it is possible to not only control the position of the second stop band edge 12, but also to control the amount by which the optical radiation 6 propagating along the optical fibre 1 is attenuated within the stop band 7.
  • the solid line shows the optical attenuation 80 and the second stop band edge 12 before increasing the bending of the optical fibre 1.
  • the dashed line shows the optical attenuation 82 and the second stop band edge 12 after increasing the bending of the optical fibre 1.
  • the optical attenuation 82 of the stop band 81 is larger than the optical attenuation 80 of the stop band 7.
  • the second stop band edge 83 is at a longer wavelength than the second stop band edge 12.
  • optical attenuation 80 and 82 within the stop band 7 and the stop band 81 is more uniform with wavelength after deforming the optical fibre 1 in a helix. This is because the helical deformation of optical fibre 1 provides bending, which shifts the second stop band edge 12 to its new position 83, and also provides stronger coupling between the fundamental mode of the core 3 to the corresponding higher order mode of the resonant coupling structure 5, thus increasing the optical attenuation 82 of the stop band 81.
  • Figure 10 shows a laser system 90 comprising a laser 91, two optical amplifiers 92, a beam delivery cable 93, and an output optic 94.
  • the laser system 90 comprises a plurality of optical filters 95 comprising the optical fibre 1 and the bending mechanisms 2 described with reference to Figures 1 to 9.
  • At least one of the optical filters 95 may comprise a bending mechanism 2 that is different from another of the bending mechanisms 2.
  • At least one of the optical filters 95 may comprise at least one bending mechanism 2 described with reference to Figures 4 to 8.
  • At least one of the optical filters 95 may comprise a periodic surface 13 with a different period 42 than the period of another one of the optical filters 95. Different periods 42 cause enhanced coupling between different groups of modes of the resonant coupling structure 5 and the guided modes of the core 3.
  • At least one of the optical filters 95 may be one that does not comprise a periodic surface 13. If an optical filter is used that does not use a periodic surface, then it is preferable that it is followed by an optical filter 99 such as shown in Figures 4 to Figure 8 that does comprise the periodic surface 13 in order to provide stronger coupling between the fundamental mode of the core 3 and the corresponding higher order modes of the resonant coupling structure 5; this will increase the optical attenuation 80 of the stop band 7.
  • Combining bending mechanisms 15 that have different periodicities provides greater control of the second stop band edge 12 and the optical attenuation 80 of the stop band 7.
  • At least one of the optical filters 95 may include an actuator 96 as shown with reference to Figure 10.
  • the optical filter 97 can be one that does not have a periodic surface 13.

Abstract

Apparatus for optical radiation propagating along an optical fibre (1), which apparatus comprises the optical fibre (1) and at least one bending mechanism (2) for bending the optical fibre (1), wherein: the optical fibre (1) comprises a comprises a core (3), a cladding (4), and at least one resonant coupling structure (5) within the cladding (4); and the resonant coupling structure (5) is such that it couples optical radiation (6) within a spectral bandwidth (8) defined by a first stop band edge (11) and a second stop band edge (12), the optical radiation (6) being coupled between the core (3) and the cladding (4); and the apparatus being characterized in that: the cladding (4) is defined by an optical loss (10) which causes the optical radiation (6) that is coupled between the core (3) and the cladding (4) to be attenuated at wavelengths between the first stop band edge (11) and the second stop band edge (12), and thereby defining a stop band (7) having the spectral bandwidth (8); and the bending mechanism (2) and the resonant coupling structure (5) are such that bending the optical fibre (1) changes the spectral bandwidth (8).

Description

Apparatus and Method for Optical Filtering
Field of Invention
This invention relates to an apparatus and method for optical filtering and, more especially, for optical filtering optical radiation propagating along an optical fibre. The invention has particular application for power scaling in optical fibre lasers, laser beam delivery in optical fibre lasers, and equipment for laser processing of industrial materials.
Background to the Invention
High power lasers have important applications in the laser processing of industrial materials. Pulsed lasers, with powers exceeding lOkW, are used in marking, engraving, cutting, welding, and drilling applications. Continuous wave lasers with powers exceeding lkW are used in cutting and welding applications.
As power is scaled in optical fibre lasers, there is often a need to include additional optical amplifiers. As the powers increase, and the overall fibre length increases, stimulated Raman scattering becomes a limiting factor in the stability and reliability of the laser. Stimulated Raman scattering can also limit the maximum output power and the length of optical fibre beam delivery systems, as well as adversely affecting the ability of the laser to withstand back reflection from a work piece. The maximum length for some optical fibre beam delivery systems can be as small as lm to 2m. This places serious limitations on the design of industrial cutting and welding machines that include high power lasers. There is therefore a need for optical filters to suppress stimulated Raman scattering within and between optical amplifier stages. There is an associated need to tune one or more of the rejection and the bandwidth of the optical filter in order to cater for manufacturing tolerances.
There is a need for an apparatus and a method for optical filtering that reduces or avoids the aforementioned problems.
The Invention:
According to a non-limiting embodiment of the invention, there is provided apparatus for optical filtering optical radiation propagating along an optical fibre, which apparatus comprises the optical fibre and at least one bending mechanism for bending the optical fibre, wherein:
o the optical fibre comprises a core, a cladding, and at least one resonant coupling structure within the cladding; and
» the resonant coupling structure is such that it couples optical radiation within a spectral bandwidth defined by a first stop band edge and a second stop band edge, the optical radiation being coupled between the core and the cladding;
and the apparatus being characterized in that:
• the cladding is defined by an optical loss which causes the optical radiation that is coupled between the core and the cladding to be attenuated at wavelengths between the first stop band edge and the second stop band edge, and thereby defining a stop band having the spectral bandwidth; and
• the bending mechanism and the resonant coupling structure are such that bending the optical fibre changes the spectral bandwidth. The resonant coupling structure may comprise at least one rod placed near the core. The refractive index of the rod is preferably higher than the refractive index of the core. Alternatively or additionally, the resonant coupling structure may comprise at least one ring placed around the core. The refractive index of the ring is preferably higher than the refractive index of the core.
The optical loss in the cladding can be introduced by incorporating at least one region configured to absorb or scatter the optical radiation. The region is preferably placed such that it interacts with the modes guided by the resonant structure much more strongly than its interaction with the fundamental mode of the core.
If optical radiation propagates along a optical fibre that has optical loss, then the optical radiation is attenuated, that is, its intensity becomes smaller as the optical radiation propagates along the optical fibre. The optical radiation propagating along the optical fibre at wavelengths within the stop band will be attenuated by the optical loss in the cladding, whereas optical radiation at wavelengths that are outside of the stop band can propagate along the core of the optical fibre without being attenuated by the optical loss in the cladding. Being able to tune the spectral bandwidth of the stop band by bending the optical fibre provides useful advantages over the prior art.
The bending mechanism may be a bimetallic strip. The bimetallic strip may comprise a first metal, a second metal, and a heating element attached to the first metal. The first metal and the second metal may have different thermal expansion coefficients. When heated by applying electric current to the heating element, the bimetallic strip bends the optical fibre that is attached to it. The bending mechanism may be a piezoelectric or magnetic element, or a mechanical element such as a screw, a lever, or a ratchet. The bending mechanism may be a simple bending mechanism such as bending the optical fibre over a cylinder or a mechanism designed to adjust the bend radius of a loop of the optical fibre. Alternatively or additionally, the bending mechanism may comprise a mechanical formation onto which the optical fibre is bent by hand or other means, and then attached.
Bending of the optical fibre may cause a wavelength shift in the second stop band edge that is greater than a wavelength shift in the first stop band edge, thereby increasing the spectral bandwidth. This is particularly useful for the suppression of stimulated Raman scattering in optical fibres because it permits signal radiation at a wavelength shorter than the first stop band edge to be transmitted with minimal loss, whilst the stimulated Raman scattering radiation generated by the signal radiation can be removed from the core of the optical fibre. The ability to tune the stop band means that more than one Stokes order of the stimulated Raman scattering can be removed from the optical fibre.
The bending mechanism may comprise a periodic surface. The bending mechanism may be configured to bend the optical fibre by squeezing the periodic surface and the optical fibre together with a squeezing force, and the second stop band edge is able to be varied by adjusting the squeezing force.
The periodic surface may be chirped. Varying the periodicity along the length of the bending mechanism, either monotonically or in a non-monotonic fashion, reduces the amount of squeezing force that is required to obtain the desired second stop band edge, thereby increasing reliability. The bending mechanism may comprise at least two of the periodic surfaces arranged at an angle to each other. The periodic surfaces may have the same periodicity. The bending mechanism may be such that each periodic surface is able to be squeezed against the optical fibre with different squeezing forces. The spatial phases of the periodic surfaces may be configured such that the optical fibre is deformed in a helical manner when the squeezing forces are applied to the two period surfaces. This arrangement provides great control over the resonant coupling.
Surprisingly, when the optical fibre is deformed in a helical manner using the above bending mechanism, it is possible to not only control the position of the second stop band edge, but also to control the amount by which the optical radiation propagating along the optical fibre is attenuated within the stop band. The optical attenuation within the stop band is also more uniform over the spectral bandwidth.
The bending mechanism may include an actuator.
The apparatus may comprise a plurality of the bending mechanisms. This reduces the required squeezing forces on each of the bending mechanisms thereby improving reliability.
At least one of the bending mechanisms may have a different periodicity than another of the bending mechanisms. Different periodicities cause enhanced coupling between different groups of modes of the resonant structure and the guided modes of the core.
Combining bending mechanisms having different periodicities provides greater control of the position of the second stop band edge and the attenuation within the stop band. The apparatus may comprise a laser.
The apparatus may comprise an output fibre. The optical fibre may be spliced to the output fibre.
The apparatus may comprise at least one optical amplifier. The optical fibre and the bending mechanism may be located between the laser and the optical amplifier. Alternatively or additionally, the optical fibre and the bending mechanism may be located within the optical amplifier.
The apparatus may comprise at least two of the optical amplifiers. The optical fibre and the bending mechanism may be located between the two optical amplifiers.
The invention also provides a method for optical filtering optical radiation propagating along an optical fibre, which method comprises providing the optical fibre and at least one bending mechanism for bending the optical fibre, wherein:
• the optical fibre comprises a core, a cladding, and at least one resonant coupling structure within the cladding; and
• the resonant coupling structure is such that it couples optical radiation within a spectral bandwidth defined by a first stop band edge and a second stop band edge, the optical radiation being coupled between the core and the cladding;
and the method being characterized in that:
• the cladding is defined by an optical loss which causes the optical radiation that is coupled between the core and the cladding to be attenuated at wavelengths between the first stop band edge and the second stop band edge, and thereby defining a stop band having the spectral bandwidth; • the bending mechanism and the resonant coupling structure are such that bending the optical fibre changes the bandwidth; and
• the method includes the step of bending the optical fibre with the bending
mechanism to change the bandwidth of the stop band.
The method of the invention may include a step or steps as required to utilize the above mentioned optional aspects of the apparatus of the invention.
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 apparatus for optical filtering and, more especially, for optical filtering optical radiation propagating along an optical fibre, the apparatus comprising an optical fibre attached to a bending mechanism;
Figure 2 shows a stop band provided by the optical filter;
Figure 3 shows signal and Raman Stokes emissions;
Figure 4 shows a bending mechanism comprising a periodic surface that bends the optical fibre periodically along its length;
Figure 5 shows a bending mechanism in which the periodic surface is chirped along its length;
Figures 6 to 8 show bending mechanisms that are able to bend the optical fibre in a helix;
Figure 9 shows the tuning of the stop band in both optical attenuation and second stop band edge; Figure 10 shows a laser system comprising a plurality of the optical filters;
Figure 1 1 shows an optical fibre comprising resonant structures in the form of high refractive index rods; and
Figure 12 shows an optical fibre comprising a resonant structure in the form of a high refractive index ring.
Preferred Embodiment
Figure 1 shows apparatus for optical filtering optical radiation propagating along an optical fibre 1, which apparatus comprises the optical fibre 1 and at least one bending mechanism 2 for bending the optical fibre 1, wherein:
• the optical fibre 1 comprises a core 3, a cladding 4, and at least one resonant coupling structure 5 within the cladding 4; and
• the resonant coupling structure 5 is such that it couples optical radiation 6 within a spectral bandwidth 8 defined by a first stop band edge 1 1 and a second stop band edge 12, the optical radiation 6 being coupled between the core 3 and the cladding 4;
and the apparatus being characterized in that:
• the cladding 4 is defined by an optical loss 10 which causes the optical radiation 6 that is coupled between the core 3 and the cladding 4 to be attenuated at wavelengths between the first stop band edge 11 and the second stop band edge 12, and thereby defining a stop band 7 having the spectral bandwidth 8; and
• the bending mechanism 2 and the resonant coupling structure 5 are such that bending the optical fibre 1 changes the spectral bandwidth 8. As shown in Figure 1, the stop band 7 has an optical attenuation 80 at wavelengths between the first stop band edge 11 and the second stop band edge 12, and negligible attenuation outside these wavelengths. The optical radiation 6 propagating along the optical fibre 1 at wavelengths within the stop band 7 will be attenuated, whereas optical radiation at wavelengths that are outside of the stop band 7 can propagate along the optical fibre 1 without substantial optical attenuation. The coupling of the optical radiation 6 between the core 3 and the cladding 4 can be via the resonant coupling structure 5. The attenuation may be by absorption or scattering. The stop band 7 has a central wavelength 9 which as at the centre of the first stop band edge 11 and the second stop band edge 12. The second stop band edge 12 is at a higher wavelength than the first stop band edge 11.
The optical fibre 1 can be the optical fibre 1000 shown in Figure 11. The resonant coupling structures 5 are high refractive index rods 1015 placed around the core 3. The rods 1015 are characterized by a diameter 1003 and a refractive index 1002. The refractive index 1002 of the rods 1015 is preferably higher than the refractive index 1001 of the core 3. The diameter 1003 and the refractive index 1002 are selected such that the high refractive rods 1015 support more than one optical rod mode or groups of closely spaced rod modes, of which at least one is phase matched to and, therefore, resonantly coupled to, the fundamental mode of the core 3 at the central wavelength 9. At least one of the high refractive index rods 1015 is placed at a distance 1005 from the core 3 such that the fundamental mode of the core 3 overlaps the rod 1015. This enhances the resonant coupling with the rod modes of the rod 1015 at the central wavelength 9. The high refractive index rods 1015 can be placed in at least one ring around the core 3 such that the fields of the optical rod modes of each rod 1015 overlaps at least one adjacent rod 1015. Increasing the number of rings of high refractive index rods 1015, such as shown in Figure 1, results in sharper stop first and second stop band edges 11 and 12 with wavelength, and thus improves the filtering characteristics. Alternatively or additionally, the high refractive index rods 1015 can be scattered around the core 3 in a more random fashion, such as shown in Figure 4, whilst ensuring that each rod 1015 is optically coupled to at least one adjacent rod 1015. The high refractive index rods 1015 can be identical and, in this case, the resonant coupling involves phase matching between the fundamental mode of the core 3 and the same rod mode or rod mode group. At least one of the high refractive index rods 1015 can have a different diameter 1003 and/or refractive index 1002 than the other high refractive index rods 1015, whereupon the resonant coupling of the first rod 1015 involves phase matching between the fundamental mode of the core 3 and rod modes or rod mode groups of different order than the modes of the other rods 1015.
The optical fibre 1 can be the optical fibre 1 100 shown in Figure 12. The resonant coupling structure 5 is in the form of at least one high refractive index ring 1010 placed around the core 3. The ring 1010 is characterized by a radius 101 , a thickness 1011 and a refractive index 1012. The refractive index 1012 of the ring is preferably higher than the refractive index 1001 of the core 3. The high refractive ring 1010 supports more than one optical ring mode or groups of closely spaced ring modes, at least one of which is phase matched and, therefore, resonantly coupled to the fundamental mode of the core 3 at the central wavelength 9. The radius 1013, thickness 101 1 and refractive index 1012 are selected to enhance the resonant coupling with the fundamental mode of the core 3 at the central wavelength 9. The high refractive index ring 1010 can be circular or non-circular and placed concentrically or non-concentrically around the core 3. More than one high refractive index ring 1010 can be incorporated. Increasing the number of the high refractive index rings 1010 results in sharper first and second stop band edges 11 and 12, thereby improving the filtering characteristics of the apparatus. The high refractive index rings 1010 can have identical widths 1011 and refractive indices 1012 and in this case the resonant coupling involves phase matching between the fundamental mode of the core 3 and the same ring mode or group of ring modes. The high refractive index rings 1010 can have different thicknesses 101 1 and/or refractive indices 1012 and in this case the resonant coupling involves phase matching between the fundamental mode of the core 3 and ring modes or group of modes of different order.
The resonant coupling structures 5 can also comprise a mixture of multimode waveguides in the form of high refractive index rods 1015 and rings 1010 placed around the core 3.
The optical loss 10 in the cladding 4 can be introduced by incorporating at least one lossy region 1020. The lossy region 1020 is shown as a concentric ring in Figures 11 and 12. Alternatively or additionally, the lossy region 1020 can be dispersed among the resonant coupling structures 5. The lossy region 1020 can be doped with dopants selected to absorb the optical radiation, such as a rare earth ion, a semiconductor, or another metal ion. The lossy region 1020 can comprise one or more high index leaky layers. The lossy region 1020 can comprise scattering layers or segments. The scattering elements can be introduced by appropriate cladding surface modification and/or roughness. The lossy region 1020 is preferably placed such that it interacts with the modes guided by the resonant structure 5 much more strongly than its interaction the fundamental mode of the core 3. Preferably, the fundamental mode of the core 3 does not overlap or interact with the lossy region 1020.
The bending mechanism 2 may be a bimetallic strip 16 as depicted in Figure 1. The bimetallic strip 16 comprises a first metal 17, a second metal 18, and a heating element 19 attached to the first metal 17. The first metal 17 and the second metal 18 have different thermal expansion coefficients. When heated by applying electric current to the heating element 19, the bimetallic strip 16 bends the optical fibre 1 that is attached to it.
The bending mechanism 2 may also be a piezoelectric or magnetic element, or a mechanical element such as a screw, a lever, or a ratchet. The bending mechanism 2 can be a simple bending mechanism such as bending the optical fibre 1 over a cylinder or a mechanism designed to adjust the bend radius of a loop of the optical fibre 1.
Alternatively or additionally, the bending mechanism 2 can comprise a mechanical fixture onto which the optical fibre 1 is bent by hand or other means, and then attached.
As shown in Figure 2, bending of the optical fibre 1 causes a wavelength shift in the second stop band edge 12. The first stop band edge 11 is typically not shifted very much by the bending of the optical fibre 1. The second stop band edge 12 shifts to the second stop band edge 21 when the optical fibre is bent, and can be shifted further to the second stop band edge 22 by an increased amount of bending. The wavelength shift in the second stop band edge 12 is greater than the wavelength shift in the first stop band edge 11. The first and the second stop band edges 11 and 12 result from phase matching of the fundamental mode of core 3 to different order modes of the resonant coupling structure 5. The stop band edge 12 results from phase matching the fundamental mode of core 3 to a higher order mode of the resonant coupling structure 5, compared to the one responsible for the stop band edge 1 1. Bending affects the phase matching conditions and mode shape. The higher the order of the mode, the more they are affected by the bending. As a result, the effect of bending the optical fibre 1 is much more pronounced on the wavelength position of the second stop band edge 12 than on the wavelength position of the first stop band edge 11. Increasing the bending of the optical fibre 1 shifts the resonance condition and therefore the second stop band edge 12 to longer wavelengths, increasing the spectral bandwidth 8.
Figure 3 shows the optical radiation 6 in the form of a signal 30 and the corresponding first Raman Stokes emission 31, second Raman Stokes emission 32 and third Raman Stokes emission 33 caused by stimulated Raman scattering in an optical fibre. Thus for example, if a laser signal at 1060nm is transmitted along a single mode optical fibre at a power of IkW, a first Raman Stokes emission appears at approximately 1120nm, a second Raman Stokes emission at approximately 1170nm, and a third Raman Stokes emission at approximately 1210nm, the exact figures being dependent on the types of glasses within the optical fibre. These Raman Stokes emissions can cause instability in lasers and amplifiers, and are very undesirable. The apparatus shown in Figure 1 is particularly useful for the suppression of stimulated Raman scattering in optical fibres because it permits the signal 30 which has a wavelength shorter than the first stop band edge 11, to be transmitted with minimal loss, whilst the stimulated first, second and third Raman Stokes emission 31, 32, 33 generated by the signal 30, to be removed from the core 3 of the optical fibre 1. The ability to tune the stop band 7 means that more than one of the stimulated first, second and third Raman Stokes emissions 31, 32, 33 can be removed from the optical fibre 1.
Figure 4 shows a bending mechanism 45 comprising a periodic surface 13. The periodic surface 13 is defined by a period 42 or periodicity. The bending mechanism 2 is configured to squeeze the periodic surface 13 and the length of the optical fibre 1 together in order to bend the optical fibre 1 periodically along its length with a squeezing force 14. The second stop band edge 12 is able to be varied by adjusting the squeezing force 14.
Figure 5 shows a bending mechanism 40 wherein the periodic surface 41 is chirped. Varying the period 42 along the length of the bending mechanism 40, either monotonically or in a non-monotonic fashion, reduces the amount of squeezing force 14 that is required to obtain a desired wavelength for the second stop band edge 12, thereby increasing reliability.
Figure 6 shows a bending mechanism 51 that comprises two periodic surfaces 52, 53 arranged at an angle 54 to each other. The periodic surfaces 53, 54 may have the same period 42 shown with reference to Figures 4 and 5. The bending mechanism 51 may be such that each periodic surface 52, 53 is able to be squeezed against the optical fibre 1 with different squeezing forces 14. The spatial phases of the periodic surfaces 52, 53 may be configured such that the optical fibre 1 is deformed substantially in a helical manner when the squeezing forces 14 are applied to the two period surfaces 52, 53. This arrangement provides great control over the resonant coupling. Figure 7 shows a bending mechanism 65 comprising three parts 61 that each have two periodic surfaces 71 and 72 shown with reference to Figure 8. The periodic surfaces 71 and 72 are 120 degrees out of phase with each other. Each part 61 is arranged to deform the optical fibre 1 in a substantially helical manner.
As shown in Figure 9, when the optical fibre 1 is bent periodically along its length, and preferably in a helical deformation, it is possible to not only control the position of the second stop band edge 12, but also to control the amount by which the optical radiation 6 propagating along the optical fibre 1 is attenuated within the stop band 7. The solid line shows the optical attenuation 80 and the second stop band edge 12 before increasing the bending of the optical fibre 1. The dashed line shows the optical attenuation 82 and the second stop band edge 12 after increasing the bending of the optical fibre 1. The optical attenuation 82 of the stop band 81 is larger than the optical attenuation 80 of the stop band 7. The second stop band edge 83 is at a longer wavelength than the second stop band edge 12. Experimentally, it is found that the optical attenuation 80 and 82 within the stop band 7 and the stop band 81 is more uniform with wavelength after deforming the optical fibre 1 in a helix. This is because the helical deformation of optical fibre 1 provides bending, which shifts the second stop band edge 12 to its new position 83, and also provides stronger coupling between the fundamental mode of the core 3 to the corresponding higher order mode of the resonant coupling structure 5, thus increasing the optical attenuation 82 of the stop band 81.
Figure 10 shows a laser system 90 comprising a laser 91, two optical amplifiers 92, a beam delivery cable 93, and an output optic 94. The laser system 90 comprises a plurality of optical filters 95 comprising the optical fibre 1 and the bending mechanisms 2 described with reference to Figures 1 to 9. At least one of the optical filters 95 may comprise a bending mechanism 2 that is different from another of the bending mechanisms 2. At least one of the optical filters 95 may comprise at least one bending mechanism 2 described with reference to Figures 4 to 8. At least one of the optical filters 95 may comprise a periodic surface 13 with a different period 42 than the period of another one of the optical filters 95. Different periods 42 cause enhanced coupling between different groups of modes of the resonant coupling structure 5 and the guided modes of the core 3. At least one of the optical filters 95, such as the optical filter 97 or the optical filter 98, may be one that does not comprise a periodic surface 13. If an optical filter is used that does not use a periodic surface, then it is preferable that it is followed by an optical filter 99 such as shown in Figures 4 to Figure 8 that does comprise the periodic surface 13 in order to provide stronger coupling between the fundamental mode of the core 3 and the corresponding higher order modes of the resonant coupling structure 5; this will increase the optical attenuation 80 of the stop band 7.
Combining bending mechanisms 15 that have different periodicities provides greater control of the second stop band edge 12 and the optical attenuation 80 of the stop band 7.
At least one of the optical filters 95 may include an actuator 96 as shown with reference to Figure 10.
The optical filter 97 can be one that does not have a periodic surface 13.
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. Individual components shown in the drawings are not limited to use in their drawings and they may be used in other drawings and in all aspects of the invention. The invention also extends to the individual components mentioned and/or shown above, taken singly or in any combination.

Claims

Claims
1. Apparatus for optical filtering optical radiation propagating along an optical fibre, which apparatus comprises the optical fibre and at least one bending mechanism for bending the optical fibre, wherein:
• the optical fibre comprises a core, a cladding, and at least one resonant coupling structure within the cladding; and
• the resonant coupling structure is such that it couples optical radiation within a spectral bandwidth defined by a first stop band edge and a second stop band edge, the optical radiation being coupled between the core and the cladding;
and the apparatus being characterized in that:
• the cladding is defined by an optical loss which causes the optical radiation that is coupled between the core and the cladding to be attenuated at wavelengths between the first stop band edge and the second stop band edge, and thereby defining a stop band having the spectral bandwidth; and
• the bending mechanism and the resonant coupling structure are such that bending the optical fibre changes the spectral bandwidth.
2. Apparatus according to claim 1 wherein the bending mechanism is a bimetallic strip comprising a first metal, a second metal, and a heating element attached to the first metal.
3. Apparatus according to claim 1 or claim 2 wherein bending of the optical fibre causes a wavelength shift in the second stop band edge that is greater than a wavelength shift in the first stop band edge, thereby increasing the spectral bandwidth.
4. Apparatus according to any one of the preceding claims wherein the bending mechanism comprises a periodic surface.
5. Apparatus according to claim 4 wherein the bending mechanism is configured to bend the optical fibre by squeezing the periodic surface and the optical fibre together with a squeezing force, and the second stop band edge is able to be varied in wavelength by adjusting the squeezing force.
6. Apparatus according to claim 4 or claim 5 wherein the periodic surface is chirped.
7. Apparatus according to any one of claims 4 to claim 6 wherein the bending
mechanism comprises at least two of the periodic surfaces arranged at an angle to each other.
8. Apparatus according to claim 7 wherein the periodic surfaces have the same
periodicity.
9. Apparatus according to claim 7 or claim 8 wherein the bending mechanism is such that each periodic surface is able to be squeezed against the optical fibre with different squeezing forces.
10. Apparatus according to any one of claims 7 to 9 wherein the spatial phases of the periodic surfaces are configured such that the optical fibre is deformed in a helical manner when the squeezing forces are applied to the two period surfaces.
11. Apparatus according to any one of claims 4 to 12 wherein the bending mechanism includes an actuator.
12. Apparatus according to any one of claims 4 to 11 wherein the apparatus comprises a plurality of the bending mechanisms.
13. Apparatus according to claim 12 wherein at least one of the bending mechanisms has a different periodicity than another of the bending mechanisms.
14. Apparatus according to any one of the preceding claims wherein the apparatus comprises a laser.
15. Apparatus according to any one of the preceding claims wherein the apparatus comprises an output fibre.
16. Apparatus according to claim 15 wherein the optical fibre is spliced to the output fibre.
17. Apparatus according to claim 15 or claim 16 and comprising at least one optical amplifier.
18. Apparatus according to claim 17 wherein the optical fibre and the bending
mechanism are located between the laser and the optical amplifier.
1 . Apparatus according to claim 17 wherein the optical fibre and the bending
mechanism are located within the optical amplifier.
20. Apparatus according to any one of claims 17 to 19 and comprising at least two of the optical amplifiers.
21. Apparatus according to claim 20 wherein the optical fibre and the bending
mechanism are located between the two optical amplifiers.
22. A method for optical filtering optical radiation propagating along an optical fibre, which method comprises providing an optical fibre and at least one bending mechanism for bending the optical fibre, wherein: • the optical fibre comprises a comprises a core, a cladding, and at least one resonant coupling structure within the cladding; and
• the resonant coupling structure is such that it couples optical radiation within a bandwidth defined by a first stop band edge and a second stop band edge, the optical radiation being coupled between the core and the cladding;
and the method being characterized in that:
• the cladding is defined by an optical loss which causes the optical
radiation that is coupled between the core and the cladding to be attenuated at wavelengths between the first stop band edge and the second stop band edge, and thereby defining a stop band having the spectral bandwidth;
• the bending mechanism and the resonant coupling structure are such that bending the optical fibre changes the bandwidth; and
• the method includes the step of bending the optical fibre with the bending mechanism to change the bandwidth of the stop band.
PCT/GB2018/000014 2017-01-27 2018-01-26 Apparatus and method for optical filtering WO2018138472A2 (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
GB1701449.9 2017-01-27
GBGB1701449.9A GB201701449D0 (en) 2017-01-27 2017-01-27 Apparatus and method for optical filtering

Publications (2)

Publication Number Publication Date
WO2018138472A2 true WO2018138472A2 (en) 2018-08-02
WO2018138472A3 WO2018138472A3 (en) 2018-09-27

Family

ID=58462683

Family Applications (1)

Application Number Title Priority Date Filing Date
PCT/GB2018/000014 WO2018138472A2 (en) 2017-01-27 2018-01-26 Apparatus and method for optical filtering

Country Status (2)

Country Link
GB (1) GB201701449D0 (en)
WO (1) WO2018138472A2 (en)

Family Cites Families (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4912523A (en) * 1987-04-10 1990-03-27 At&T Bell Laboratories Optical fiber communication system comprising mode-stripping means
JP3214684B2 (en) * 1994-09-30 2001-10-02 日本電信電話株式会社 Reflection type fiber grating filter
US7778500B2 (en) * 2005-04-05 2010-08-17 Agency For Science, Technology And Research Optical fiber strain sensor
EP2321678B1 (en) * 2008-08-18 2018-12-26 NKT Photonics A/S Optical fiber with resonant structure of cladding features connected to light sink
JP5597005B2 (en) * 2010-03-24 2014-10-01 株式会社フジクラ Optical fiber type optical filter
CN105372754A (en) * 2015-11-27 2016-03-02 西北工业大学 Graphene-assisted all-optical driving fiber grating tuner and calibration method

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
None

Also Published As

Publication number Publication date
WO2018138472A3 (en) 2018-09-27
GB201701449D0 (en) 2017-03-15

Similar Documents

Publication Publication Date Title
EP1236061B1 (en) Large diameter optical waveguide, grating, and laser
US8711471B2 (en) High power fiber amplifier with stable output
JP7316791B2 (en) Monolithic visible wavelength fiber laser
JP5662151B2 (en) Glass large core optical fiber
US11681094B2 (en) Optical fiber filter of wideband deleterious light and uses thereof
EP1191365A1 (en) Optical fiber grating element, production method thereof and optical fiber filter
KR20010098469A (en) Mechanically induced, long period optical fiber gratings
US11050209B2 (en) Apparatus and method for optical isolation
WO2018138472A2 (en) Apparatus and method for optical filtering
WO2002075395A1 (en) Optical waveguide with blazed grating
EP1333299A2 (en) Optical fiber and slanted optical fiber grating
US6810178B2 (en) Large diameter optical waveguide having blazed grating therein
US6766080B2 (en) Optical waveguide type defraction grating device and a method of manufacturing thereof
US6563984B1 (en) Long-period grating device
CN113383251A (en) Optical fiber apparatus and method for suppressing Stimulated Raman Scattering (SRS)
KR102436516B1 (en) femtosecond laser device
JP4237115B2 (en) Band-variable optical filter
US11646541B2 (en) Femtosecond laser device
Lobo et al. N-fold symmetric gratings as gain-flattening filters
Tran et al. Effects of fiber cladding diameter on cladding-mode coupling in fiber Bragg gratings
Yu et al. Design and simulation of cascade long-period fiber gratings for flattening EDFA's gain
Endo et al. Optical add/drop multiplexers using slanted fiber Bragg gratings
JP2007147800A (en) Optical loss device
Song et al. All-fiber band pass filter based on long period fiber grating pair and null core fiber for applications in 2 µm band
Komukai et al. Fabrication and wavelength tuning of a high‐quality fiber Bragg grating

Legal Events

Date Code Title Description
121 Ep: the epo has been informed by wipo that ep was designated in this application

Ref document number: 18704282

Country of ref document: EP

Kind code of ref document: A2

NENP Non-entry into the national phase

Ref country code: DE

122 Ep: pct application non-entry in european phase

Ref document number: 18704282

Country of ref document: EP

Kind code of ref document: A2