Eurepéisches
`Patentamt
`European
`Patent Office
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`EURQPEAN PATENT APPLECATEGN
`
`on
`
`E? 2 are 317% A2
`
`(43) Date of publication:
`21032014 Buiietin 2014/35
`
`(21) Application number: 141558411
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`(51)
`
`int CL:
`ooze 1i365 (2005-01)
`ooze size (2006-0?)
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`ooze 5102 {2005-01)
`H015 3I067 (mt-0?)
`
`(22) Date effiting: 19.1322814-
`
`
`(84) Designated Contracting States:
`AL AT BE [(5 CH CY CZ DE DK EE ES FE FR €353
`GR HR HUiEiSET LELT LU LV MC MK MT NL NS
`Pi. PT R0 RS SE SE SK SM TR
`Designated Extension States:
`BA ME
`
`0 Bovine, Adam
`Southampton
`50199TR (GB)
`5' Grudinin, Anatoiy Borisovich
`Southampton
`8015 flit. (GB)
`
`(74) Representative: Piotrowicz, Pawei JanAndrzej
`Venner Shtpiey LLP
`Byron House
`Cambridge Business Park
`Cowiey Road
`Cambridge C84 {3W2 (GB)
`
`
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`73
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`fl
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`7
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`9
`FIG. 7
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`(BO) Priority: 20.92.2013 GB 291302960
`
`(71)
`
`pplicent: Fianium Limited
`Southampton Hampshire $031 sit-RA (GB)
`
`inventors:
`(72)
`8 Ciowes, John Redvers
`New Miiton
`
`Hampshire BHZS STT (GB)
`
`(54)
`
`A supercontinuum source
`
`Asupercontinuum opticat puise source provides
`(57)
`a combined supercontinunm. The supercontinuurn optiu
`cat putse source comprises one or more seed pulse
`sources (13), and first and second opticat ampiifiers (7)-
`arranged along first and second respective opticat paths.
`The first and second opticai emptifiers are configured to
`arnpiify one or more opticat signals generated by said
`one or more seed pulse sources. The supercontinuum
`opticat poise source further comprises a first microstruci-
`tured tight—guiding member (9) arranged aiong the first
`optical path and configured to generate supercontinuum
`light responsive to an opticai signal propagating atong
`said first optical path, and a second microstructured iight-
`guiding member (9) arranged atong the second opticai
`path and configured to generate supercontinuum Eight
`responsive to an optical signai propagating along said
`second optical path. The supercontinuum opticat pulse
`source further comprises a supercontinoum-—-::ornbining
`member (5')- to combine supercontinuum generated in at
`toast the first and second microstructnred light-guiding
`members to form a combined supercontinuum. The so—
`perconttnuun‘r—cornbining member comprises an output
`fibre, wherein the output fibre comprises a siiicai-based
`muttimode opticat fibre supporting a ptura ity of spattai
`modes at one or more wavelengths; of the com'oined sis--
`percontinuum.
`
`
`
`E?2??{i3W3A2
`
`Printed by .Jouve, 75001 PAREE': (FR)
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`

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`E? 2 Tit) 37%) A2
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`Description
`
`Ftetd
`
`[@3601] This disctosure reiates to a supercontinuum
`source.
`
`Background
`
`Supercontinuum generation in microstructured
`[G602]
`tibre was proposed in “Visibie continuum generation in
`airsilica microstructureopticatfibresWitt anornaiousdis—
`persion at 800 nm", J.K. Ranka, RS. Windeler, and Ad.
`Stentz, Optics l_.e.:ers, 2000. Voi. 25: p. 25--27
`
`Summary
`
`[$6033 The present invention provides a supercontin—
`uum opticai pulse source to provide a combined super-
`continuum. The supercontinuum opticai
`utse source
`comprises one or more seed puise sources. and first and
`second opticai amplifiers arranged atong first and second
`respective opticai paths. The first and second opticai am-
`ptifiers are configured to amptify one or more opticai sig-
`nats generated by said one or more seed pulse sources.
`The supercontinuum opticai pulse source further com—
`prises a first microstructured tight—guiding member ar-
`ranged atong the first opticai path and configured to gen~
`erate supercontihuum tight responsive to an opticai sig—
`nat propagating along said first optica- path, and a second
`microstructured tight-guiding member arranged along
`tne secondoopticai path and configured to generate su-
`percontinuum light responsive to an optical signat prop-
`agating along said second opticai path. The supercon—
`tinuum opticai puise source further comprises a super—
`continuum-combining member to combine supercontin-
`uurn g enerated in at least the first and second microstrucx-
`tured tight—guiding members to form a combined super-
`continuum. The supercontinuum-combining member
`comprises an output fibre, wherein the output fibre com—
`prises a sitica-based rnuitimode optical fibre supporting
`a pt-drality ot spatiai modes at one or more wavetengths
`of t.he combined supercontinuum.
`is
`[0004} The supercontinuum—combining member
`preferably passive’i ...e‘ does not amptify‘). and therefore
`provides its mu ttimode outputin comoiiance with the an
`of brightness, which dictates that the brightness of the
`output )zflhe supercontinuum-combining member cannot
`begreater than-the combined brightness of the inputs.
`{99%}
`in this way, various embodiments ofthe present
`invention sacrifice the possibility of singte mode output
`
`and instead combine a pluratity (e.g.‘muitiplet supercon—
`tinua generated atongsseeparate paths to forma mutti—
`mode output. Asufficieh.t number or paths may be chosen
`to that the combined output is at a desired power level.
`[9605] At the highest power ieveis, photodarkening
`can occur in microstructured fibre, whereby toss occurs
`due to p.hoton—induced defects, particularly in the vrsibie
`
`(51C}
`
`region of the spectrum. it is well known to mitigate against
`photodarkehing in high power supercontinuum sources
`by loading the silica of the microstructured fibre with high~
`iy reactive, mobite species such as deuterium or hydro—
`gen. These react with a defect site (dangting Si0 bond)
`to form an Si—O—D or Si-O—H bond which no tonger ab-
`sorbs radiation in the Visible or near infra red reg:ion of
`the supercontinuum spectrum.
`{999?}
`in accordance with various embodiments of the
`invention thea .Inpiifiers oi the supercontinuum source
`can be configured so that the average power propagating
`along each optical path is sut‘ficientty tow so as to sub
`stantiatty avoid the effects of photodarkehing. Any
`number of supercontinua can be generated in this way
`and combined in the supercontinuum—combining mem—
`her so as to provide a desired power tevet.
`[33008]
`in t. .is way, embodiments ofthe invention permit
`scalabitity of the output: power whiist mitigating or avoid--
`ing the effects of photodarkening.
`{coast
`in an embodiment,
`the first microstructured
`tight~guiding member comprises a first microstructured
`opticai tibre and the second microstructured light-guiding
`member .omprises a second microstructured opticai tin
`bre.
`
`room} Atternatively, the first and second mi c.oostruc-
`tured tigh ~tguiding members may be inciudedina singie
`opticai fibre, which may be integraiiy formed with the out—
`put fibre of sai d superconti.-uumcombining member.
`{99113 The one or more seed puise sources may com-
`I2rise a single seed puise source in this wey the firstand
`second opticai ampiifiers may be opticai communication
`with the same seed puise source.
`{0912] Aiternatively, the one or more seed pu'ise sourc—
`es may comprise a.:rst seed puise source in opticai com-
`.Inunicatioh with the first opticai amp.'itier and a second
`seed puise source in opticai communication with the sec—
`ond opticai amoiifi-r .
`{03313;}
`in an embodiment, the supercontinuum—com—
`'oining member comprises one or more input fibres. The
`input fibres may be multimode fibres, single mode fibres
`and/or may comprise microstructured fibres. The inputs
`fibres may have a numericai aperture above 0.3 at one
`or more of the wavetehgths of the combined supercon-
`tinuum.
`
`in an embodiment, the o'itput fibre of the super—
`{9914}
`continuumcombining member may comprise an air—ciad
`fibre.
`
`in various embodiments, the supermntinuum
`{(3015}
`opticai pulse source comprises:
`
`n opticai amplifiers arranged atohg n respective op—
`ticai paths, wherein said n opticai ampiifiers are con-
`figured to amptify one or more opticai signais gen~
`erated by said one or more seed puise sources, and
`n microstructuredt'ig htx-gu-iding memberssrespective--
`ly arranged atong said opticai paths and respectivety
`configured to generate supercontinuum tight respon-
`sive to an optical signai propagating along a respec-
`
`

`

`1;.)
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`the optical ampiifiers are
`lb an embodiment,
`{0925]
`configured so that in use, the power in a microstructured
`iight~guiding member over the wavelength range 400 nm
`to 700 nm is great; than 0.5 W, greater than ‘iW, or
`greater than 2 W.
`{992?}
`in an embodiment, one or more of the micro-
`structured fight—guiding members receives average opti—
`cai power that is no greater than “i Watt, or no greater
`than 2 Watts, or o greater than 4 Watts.
`{0&23} The amplifiers may be configured to generate
`pulses in the Ytterbium gain band.
`[E3029]
`in various embodiments, the one or more seed
`sources are configured to generate pulses of duration
`fess than 1 ns.
`
`{0933] The present invention also provides a super—
`continuum (S J)- source to provide a supercontinuum in-
`cluding visibie range wavetengths, comprising:
`
`one or more seed sources;
`fibres
`first and second microstructured opticai
`(MSDFs) in optical communication with said one or
`more seed sources, said first MSOF for generating
`a first SC including wavelengths from 400nm to
`700nm ("visibte range wavetengths") and said sec—
`ond MSOF for generating a second SC including vis-
`ible range wavelengths;
`a fibre optic coupier for combining the first and sec—
`ond supercontinua, said fibre optic coupler compris—
`ing an output opticai fibre, a first input fibre in optical
`communication with said first MSOF and a second
`
`input opticai fibre in optical communication with said
`second MSOF, wherein said first and second optical
`fibres are single moded at a wavelength or wave—
`lengths within the visible range waveiengths and
`wherein said output opticai fibre is multimoded at a
`waveiength or waveiengths within the visibie range
`waveiengths; and
`
`wh..rein said SC source is further adapted such that the
`optical power in the first MEI-OF over the visible range
`wavelengths does not exceed a threshoid power and
`wherein the opticai power over the visible range wave—
`iengths in the second MSOF does not exceed the thresh-
`old power, said threshold pot 'er being 1 Watt, 2 Watts
`or 4 Watts.
`
`{been The first
`MSGF.
`
`input fibre may comprise the first
`
`{$032} The first and second MSQFs may be in optical
`communication with the same seed source.
`
`{9933} At ieast one of said first and second MSOFs
`may be doped so as to act as a gain medium to provide
`optical amplification.
`[E3034] The seed source may comprise a pulsed seed
`source configured to generate pulses of duration less
`than 1 ns.
`
`(51C}
`
`tive optical path,
`
`wherein the supercontinuum—combinin member is con—
`figured to combine supercontinuum generated in said n
`microstructured iight~guiding members to form a com
`bined supercontinuum. The muitimode output fibre of
`said supercontinuum combining member may support n
`or more opticai modes.
`[9915.33
`in an embodiment, the n optical amplifiers are
`in opticai communication with the same seed puise
`source.
`
`91
`
`reater than 4. in
`ln some embodiments, n is
`[0017}
`some embodiments, n is greater than 0.
`[$6183
`in an embodiment, the supercontinuum-com—
`bining member comprises one or more input fibres which
`are singie mode at one or more of the waveiengths of
`the combined supercontinuum or at one or more of the
`wavelengths of the one or more seed puise sources.
`[33619]
`in an embodiment, the supercontinuum—com-
`pining member has one or more multimode input fibres
`which support a pluraiity oi spatial modes atone or more
`wavelengths of the combined supercontinuum, or at one
`or more of the waveiengths of the one or more seed pulse
`sources.
`
`in some embodiments, the muitimode input fi—
`{992%}
`bres oi the supercontinuum—combining member are con—
`figured to support less than four modes at one or more
`wavelengths of the combined supercontinuum, or at one
`or more of the wavelengths ofthe one or more seed pulse
`sources.
`
`lb some embodiments, the muitimode input fi—
`[0021]
`bres of the supercontinuum-com'oining member are con-
`figured to support more than four modes at one or more
`wavelengths of the combined supercontinuum, or atone
`or more oi the waveiengths ofthe one or more seed puise
`sources.
`
`in an embodiment, the output fibre of the super-
`[$6223
`continuum—combining member is arranged to support
`more modes than any of the input fibres of the supercon--
`tinuum—combining member. ln an embodiment, the out—
`put fibres of the supercontinuum—combining member is
`arranged to support at teast as many modes as the com-
`bined sum of modes supported by the input fibres.
`[0023}
`ln an embodiment, the supercontinuum gener—
`ated by one or more of the microstructured tight guiding
`members has a spectrum that inciudes wavelengths tail~
`ing within the visrbie/Nii‘t regions of the spectrum. As
`used herein, the term visible refers to light including a
`wavelength between 400 nm and 700 nm, and the term
`near infra-red (NlR) refers to Eight of wavelength abov
`iGOUnm.
`
`in some embodiments, the combined supercon—
`[9924}
`tinuum has a spectrum from the biue (<500 nm) to the
`transmission edge of siiica (> 2pm).
`[$6253
`in an embodiment, one or more of the micro-
`structured tight—guiding members receives average opti-
`cal power in excess of 0.5 Watt, or in excess of 1 Watt,
`or in excess of 2 Watts.
`
`

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`Brief Description of the Drawings
`
`[0035} Embodiments of the invention will now be de—
`scribed, by way of example only, with reference to the
`accompanying drawings, in which:
`
`Figure 1 is a schematic of a supercontinuum source
`according to a first embodiment;
`Figure 2 illustrates an exempiary configuration ofthe
`optical amplifier and microstructured fibre oi a su~
`percontinuum generator;
`Figure 3 illustrates another exemplary configuration
`of the optical amplifier and microstructured fibre of
`a supercontinuum generator;
`Figure 4 is a schematic of a supercontinuum source
`in which a single seed source is provided to seed
`the supercontinuum generators;
`Figure 5(a) is a schematic of a superconti. .uum
`source in which each core oia multi-core microstruc-
`
`tured fibre is pumped by amplified light from a re—
`spective optical amplifier to form a plurality oi super—
`continua;
`Figure 5th)- is a sectional view showing a single mul-
`timode output core at the output of a fibre having
`multiple—cores at its input;
`Figure 6 shows an exemplary supercontinuum—com—
`pining member;
`Figure 7’
`shows the supercontinuurn—combining
`member of Figure 6 in place in one exemplary su—
`percontinuum source;
`Figure 8 is sectional View of a multimode input fibre
`of an exemplary supercontinuum-combining mem-
`ber;
`Figure 9 is a sectional View of a single mode input
`fibre of an exemplary supercontinuum—combining
`member;
`Figure 10 shows the variation in numerical aperture
`(NA) for an exemplary nonlinear microstructured fi-
`bre;
`Figure 11 is a schematic of an exemplary supercon—
`tinuum source in which the input fibres of a super-
`continuum—combining member comprise supercon-
`tinuum—generating microstructured fibres; and
`Figure 12 shows an output fibre of an exemplary su—
`percontinuum—combining member.
`
`Detailed Description
`
`Overview
`
`is a schematic of a supercontinuum
`Figure 1
`[@3636]
`source ”i according to a first embodiment. As shown, the
`supercontinuum source 1 comprises a plurality oisuper—
`continuum generators 3 and a supercontinuum-combin-
`ing member 5. The supercontimum-combining member
`5 is configured to combine the supercontinua generated
`by the supercontinuum generators; 3 to form a combined
`supercontinuum.
`
`(51C}
`
`{0937] Although the schematic of Figure 1 shows tour
`supercontinuum gener tors 3, any number of supercon-
`tinuum generators could be provided so as to obtain a
`desired output power ievei of the combined supercontih—
`uum. The letter n is used herein to denote the number of
`
`supercontihua to be combined.
`
`Sugercontinuum generators
`
`in an embodiment. each supercontinuum gen—
`{@331
`erator 3 comprises an optical amplifier '? and a micro-
`structured light~guiding member in the form of a micro
`structured optical fibre 9. The optical ampiifier '? and
`microstructured fibre 9 are arranged along an optical path
`such that supercontinuum is generated as iight propa—
`gates along the optical path.
`[33039] The optical paths for each supercontinuum gen-
`erator are arranged in parallel. it will be appreciated that
`the expression "in paraiiel" is used in relation to the optical
`paths to distinguish theirconfiguration from an "in series"
`configuration, and not to describe the physical relation—
`ship between the paths.
`in practice, the optical paths,
`whilst being (2 )nfigured in parallel, may run in any appro--
`priate direction, may be curved, may wind or otherwise
`take a circuitous path, and may be respectively defined
`by optical fibres which may overlap or be wound around
`one another one.
`
`{Wild} Each optical am pi itier7 may comprise an optical
`fibre amplifier. The optical fibre amplifier may have two
`or more stages oioptical ampliiier cascaded to incremen—
`taliy provide gain to the optical signal. The output of the
`optical fibre amplifier may comprise a large mode area,
`singie mode iibre.
`{9941}
`Figures 2 and 3 iliustrate exemplary configura-
`tions of an optical amplifier 7’ and a microstructured op—
`tical fibre of a supercontinuum generator 3.
`{opera}
`in the example of Figure 2, a microstructured
`optical fibre 9 is located downstream of the optical am—
`plitier “i so as to receive an amplified signal generated
`by the amplifier 7. The amplifier output fibre and iicro-
`structured optical fibre may be joined together at a splice
`it.
`
`in the alternative exampie of Figure 3, an optical
`{99433
`ampliiier 7 includes a microstructured optical fibre 9.
`which may be doped so as to act as a gain medium of
`the amplifier.
`{0944]
`lb either case, the opticai amplifier can be con—
`
`
`figured such that the power pro ‘ded to the microstruo—
`tured fibre is high enough to cause supercontinuum spec~
`trai broadening. in embodiments, the power provided to
`the microstructured fibre is above a first threshold so as
`
`to generate a supercontinuum, and below a second
`threshold so as to substantially avoid the etiects oi pho~
`todarltening.
`{OMS} The second threshold may be no greater than
`1 Watt of power, or no greater than 2 Watts of power, or
`no greater than 4 Watts of power, where the threshold
`refers to power in the spectral range from 400 nm to 700
`
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`nm (referred to herein as the visibte range of wave-
`iengths). "Power" means average optical power. it has
`been found that higher powers can resuit in photodark—
`ening in the 550nm to 700 nm spectrat range by more
`than 50% over 100 hours continuous operation.
`[9946} Thefirst threshotd can be greater than 0.5 Watt,
`or greater than 1 Watt, or greater than 2 Watts.
`[004?] Ratherthan a single microstructured fibre, a su—
`percontinuum generator may comprise a pturatity of
`rnicrostructured fibres arranged one after the other to de—
`tine a tight propagation path along which supercontinuum
`is generated.
`[99483 The microstructured fibres 5.3 may be formed of
`sitica. Alternativeiy, one or more (or at!) of the microstrucx-
`tured fibres may be fabricated from other giasses, tor
`exampie soft glasses such as ftuoride, chalcogenide or
`tetiuride.
`
`in various embodiments, each of the supercon-
`[$6493
`tinuurn generators may produce a diffraction iimited out-
`put. Aiternativeiy, one or more of the supercontinuum
`generators may produce a muitimode output.
`
`Seed source(sl
`
`in some embodiments, supercontinuum source
`{995%}
`1 inciudes a plurality of seed sources to seed the ampii—
`tiers 7 of respective supercontinuum generators 3. in an
`embodiment, the seed sources are configured to gener—
`ate short opticat puises. To this end, the seed sources
`may respectiveiy comprise an osciiiator such as a gain
`switched laser diode or modeiocked fibre taser. The
`
`puised signal generated by the seed sources is ampiitied
`by respective amptitiers 7 such that pulsed supercontin-
`uum is generated by tight propagating along microstruc—
`tured opticai fibre 9 in each supercontinuum generator.
`The waveiength ofthe pulses outputfrom the seed sourc-
`es may be forexampie a wavetength within the gain band-
`width of Ytterbium. The puises output from the seed
`sources may comprise a waveiength of about 1064nm.
`The puise energy and peak power of pulses from the
`seed sourcei's) can be tow, e.g.: of the order of 100 Pt-
`cojouies and 10 Watts respectiveiy.
`{995?}
`Figure 4 iilustrates an embodiment in which a
`singie seed source 13 seeds more than one of the su—
`percontinuum-generators 3. This approach has the ben-
`etit of reducing the cost and compiexity of the system.
`Moreover, in embodiments :n which the seed source pro-
`
`vid “; a pulsed output, the supercontinuum output puises
`of the super'continuum~generator 3 are advantageously
`output synchronous to the same ctock as the seed
`source. Synchronous output is important for exampie is
`applications in which a beam is detected based on a pre—
`detined frequency (e.g.:
`to identify tight from a given
`source if the frequency is known). Synchronous output
`can also be important for biomedicai appiications (e.g.:
`for tifetime measurements). it will be understood that the
`spiit tines 14 in Figure 4 represent opticat spiitters.
`[0052]
`it wilt be appreciated that in any of the ernbod—
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`irnents described herein, the supercontinuum—genera—
`tors 3 may be seeded with a singte seed source, or at-
`ternatively with one seed source for each supercontinu~
`um generator 3. The number of seed sources could be
`between one and n. where n is the number of supercon—
`tinuum generators 3.
`[Whit]
`in embodiments, the opticat paths of respective
`supercontinuum generators 3 define separate opticat
`paths from the one or more seed sources to the super--
`continuum—combining member 5.
`
`Supercontinuum generation in multi-core microstruc—
`tured fibre
`
`in some embodiments
`{0954] As described above,
`each supercontinuum generator 3 may comprise a micro-
`structured tight-guiding member in the form of a micro-
`structured opticat
`fibre 9.
`l-towever, atternatively,
`the
`microstructured tight-guiding members of the supercon—
`tinuum generators 3 may be provided as separate cores
`of a singie muiti—cored noniinear microstructured fibre.
`Figure 5(a)- shows an exampie in which each core 15 ot
`a microstructured fibre 17 is pumped by ampiitied Eight
`from a respective opticat amplifier 7 to form a piuraiity of
`supercontinua, which are combined in a supercontinu-
`um~combining member 5 to form a combined supercon—
`tinuum output.
`{0&55} A rnicrostructured member, such as a micro—
`structured opticat fibre, inctudes a guiding region (e.g., a
`core ofa fibre)- and iongitudinaiiy extending features (e.g..
`air hotes) disposed about the guiding region, where the
`features have an index of refraction that is different than
`
`that of the material surrounding the features. The tea—
`tures facilitate tight guidance via one or more mecha-
`nisms. such as for example, index guidance or via the
`creation of a photonic bandgap. i‘viicrostructured opticat
`fibres include to caiied holey fibres, photonic crystai ti-
`bres, and air ctad tibres.
`{($056}
`Figure 5(a) shows each opticat amplifier 7 seed-
`ed by a single seed source 13. As in as in Figure 4-, the
`iunctions M where tines split represent optical sptitters.
`{0957] As previously indicated, each opticat ampiifier
`T may aiternativeiy be seeded by a respective seed
`source, or the number of seed sources couid be between
`one and n where n is the number of supercontinuum
`generators 3.
`
`Supercontinuum-combining member
`
`{9953} The supercontinuum—combining member 5 is
`configured to combine supercontinua generated by the
`supercontinuum generators 3 into a muttimode opticat
`fibre which supports one or more spatiai modes a one
`or more i. 'ayetengths of the combined supercontinuum.
`{MESS} The super-continua g enerated by the supercon-
`tinuum generators 3 may be coupied into the supercon—
`tinuum-combining member 5 by way of one or more fibre
`spiices between the supercontinuum generators 3 and
`
`(51CI
`
`

`

`9
`
`EP 2 Til) 37%) A2
`
`10
`
`(51
`
`70
`
`15
`
`20
`
`30
`
`the supercontinuum-combiningmemberfi,oralternative-
`ly by free—space coupling, for example using lenses
`and/or mirrors.
`
`Figure 5 shows an example of a supercohtlnu-
`[99693
`um~combihing member 5 comprising a fibre—coupled de—
`vice having a plurality of input fibres 19 and an output
`iibre 21. The output iibre comprises 2a multimode optical
`fibre which supports one or more spatial modes at one
`or more wavelengths of the combined supercontinuum.
`The device may comprise a fused fibre device tag: a
`tapered fused fibre bundle coupler).
`[0061}
`Figure 7 shows the fibre combiner 5 in 2a partic—
`ular supercontihuum source in which a plurality of super-
`continuum generators 3 are see-26'by a single seed
`source 13, and in which each supercohtinuum generator
`comprises a microstructured fibre 9 located downstream
`of an optical amplifier '2’. wherein respective microstruc—
`t-ired fibres are in this example spliced to respective in-
`puts of the fibre combiner. it will however be understood
`that this example is not intended to be limiting and that
`the iibre combiner 5 could be used as a supercontinuum—
`combining member in any of the embodiments described
`above with reference to Figures 1-5, or indeed with any
`of the supercontinuum sources disclosed herein
`[9952} Moreover, although the fibre combiner 5 of Fig—
`ure tits shown itrvitl four inputs and one output it \. ill be
`appreciated that the number of inputs may be more or
`less than four to accommodate a desired number n of
`
`supercohtinua to be combined, and in some embodi—
`ments'....e number of outputs may be greater than one
`[0553]
`in an embodiment, the inputfibres 19 oftheflbre
`combiner 5 respectively comprise multimode fibre (e.g.:
`multimode step—index fibre). Such a fibre combiner 5 may
`be formed as a multimode tapered fibre bundle. Figure
`8 shows a cross sectional view o a multimode input fibre
`19 comprising a multimode core 23 of area A1 and clad-
`ding 25. The refractive index of the cladding 25 is low -
`than that ofthe core 23 to produce a core numerical ap-
`erture NM. The output fibre is alcr a multimode fibre
`having a core and cladding, but has numerical aperture
`MAE: and core area A2. The brightness in the output fibre
`cannot be greater than the combined brightness of the n
`input fibres and the brightness law is obeyed, i.e
`
`n * (A1) * (New _: {A2} * (NAzlfi
`
`[0554] The foregoing formula is for the case where the
`input fibres have the same NA and core area and there
`is one output fibre; more generally, the input fibres can
`have different NAs and/or core areas, and there can be
`more than one ou.trputiibre,2aind..-such a case the sum
`of brightnesses of the input fibres must be less than or
`equal to the sum of the brightnesses of the output fibres.
`in some embodiments, the microstructured light-guiding
`members 9 ol the supercontinuum generators 3 are re—
`spectively spliced to niput fibres of the supercontinuum—
`
`(51CI
`
`is
`it
`lh this way,
`combining member at a splice point.
`possible to have n supercontihuum outputs, each with a
`high spatial brightness and in some examples diffra tion
`limited beam quality, combined in a combiner 5 support—
`ing many modes (for examIzie many tens of mod es), to
`produce a highly degraded beam quality. Thatis, through
`the use ofa many modeIt combiner, the brightness of
`the..-supercontinua is not well maintained through the
`beam combination.
`
`{WES} Consider for example the case of nt==7, and the
`combiner 5 (which may be referred to herein as a "pump
`combiner") comprising a 7:1 pump combiner with input
`fbres having lOGpm core diameters and numerical ap—
`
`errture NMm 0.15, and an output fibre 21 havir-o core
`diameter 125 pm and numerical aperture NA2=O‘.t-5 .By
`filling the modes of the combiner, the brightness of the
`combined supercontlnuum light can be degraded by or-
`o'ers of magnitude.
`3:
`{01355}
`in another embodiment, the input fibres 19 o:
`the fibre combiner 5 comprise single mode fibre. The
`multimode output fibre 21 supports at least n modes.
`where n is the number of supercontinua to be combined.
`{WEST} Figuregshowsa cross sectional viewofasingle
`mode input fibre 19 comprising a single mode core 27.
`Supercontihuum light generated by respective supercon—
`tihuum generators 3 may be launched into the inputfibres
`15.3, for example by way of respective splices between
`the microstructured light~guiding members 9 of the su—
`percontinuum generators 3 and the input fibres 19 ofthe
`fibre combiner 5.
`
`[9058] The multimode output fibre may support.
`modes. The brightnessis maintainedif.. each ofthe n input
`fibres ofthe..f-
`re excites one of n modesin the combiner
`
`output fibre.
`[@3069] However, if the h singie mode input fibres of the
`combiner are combined into an output fibre having more
`than n modes, but not substantially more than n modes,
`the brightness is nonetheless fairly well maintained.
`{(1070}
`in some embodiments,
`the .upercontinuum—
`combining member may be integral with the supercon-
`tinuurn--ge nerators. For example, the multi-cored nonlin-
`ear micrositructured fibre 17 of Figure 5(a)- may be mod—
`ifled at the fibre output tofiorm a single multimode fibre
`with a multi mode core by collapsing central holes and
`leaving surrounding microstructure intact. Figure 5 (b)
`shows a cross sectional view of the fibre output 17a, il—
`iustrating the st.hgle multimode output core 131).
`{(1071} Alternatively, such a multi-cored microstruc-
`tured fibre may be provided as a linear, rather t'nan a
`nonlinear device to act as a supercontihuum—combinlng
`member. ln cases in which supercontinua are geneated
`in plurality of microstructured fibres (such as described
`above with reference to i-igures 2 and 3) each micro~
`structured fibre may be arranged so th2t its output is
`launched intr a respective combiner core.
`{@972}
`in an embodiment, the supercontinuum—com—
`bining member 5 comprises a multimode fibre having a
`plurality of single mode cores within the multimode fibre,
`
`

`

`“
`
`EP2 770 379 A2
`
`12
`
`where the number of cores is denoted by N N single
`mode fibres can be spliced to the multimode combiner,
`with each of the n—single mode fibres spliced to excite
`the mode of the N single mode cores of the combiner.
`The output ofthe multimode. multi—core combiner can be
`tapered down, resulting in an output fibre in the form of
`a multimode fibre supporting lJl modes, wherein M is
`greater than or equal to N.
`[09733 As discussed above, various embodiments of
`the present disclosure involve combining multiple super~
`continuum outputs into a single multimode optical fibre.
`According to some embodiments, high brightness may
`be advantageously maintained. Asis well known to those
`skilled in the art, the brightness law states that the bright-
`ness ofa source cannot be increased by passive means
`(i.e.: without amplification). Brightness of a source is pro--
`portional to the power of the source and inversely pro—
`portional to the beam area and square oftIie divergence
`(NAZ).
`it may
`in combining supercontinuum outputss,
`[9074}
`also beimportant to minimise loss, regardless ofwhether
`one wishes to maintain high brightness. For supercon—
`tinuum sources, owing to their extremely broad ban-:3"-
`width and the nature of nonlinear microstructured fibres
`
`that generate supercontinuum, minimising loss is not
`straightforward.
`[09753
`in accordance with various embodiments ofthe
`invention, multiple tibre outputs can be combined into a
`single fibre output using 1) all—fibre arrangements such
`asa taperedfbre bundle or 2)treespace methods usi ng
`lenses and/or mirrors. ln either case, the component that
`generates the supercontinuum may comprise a micro-
`structured fibre such as a holey fibre or F’Cl: comprising
`a solid core surrounded by a series of air holes. in such
`tibres the air holes act to produce a reduced effective
`index ofthe cladding so as to form an optical waveguide.
`Unlike step index fibres, the numerical aperture (NA) of
`this wavegUide is not constant. instead, the effective in-
`dex of the cladding region and hence the NA of the
`waveguide is a function of wavelength. Figure 10 shows
`the variationin NA foran exemplarynonlinoarn-icrostruc—
`tu.ed fibre with core diameter of approximately 4pm,
`which is designed for generating supercontinuum when
`pumped at a pump wavelength in the region of turn. The
`fibre NA varies from approximately 0.06 at the blue end
`of the supercontinuum (400 nm) to more than 0.85 nm
`at-..he red end of the supercontinuum (2.4 am).
`{0075} The variatio n of the fibre NA can be modified to
`some extent by gradual collapsing of the holes in the
`microstructured fibr