WO2002091046A2 - Structure de guide d'ondes equipee d'un treillis asymetrique et procede de production correspondant - Google Patents

Structure de guide d'ondes equipee d'un treillis asymetrique et procede de production correspondant Download PDF

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Publication number
WO2002091046A2
WO2002091046A2 PCT/IB2002/001516 IB0201516W WO02091046A2 WO 2002091046 A2 WO2002091046 A2 WO 2002091046A2 IB 0201516 W IB0201516 W IB 0201516W WO 02091046 A2 WO02091046 A2 WO 02091046A2
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Prior art keywords
waveguide structure
core
inner cladding
refractive index
cladding
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PCT/IB2002/001516
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English (en)
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WO2002091046A3 (fr
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Vladimir V. Solodovnikov
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Rayteq Photonic Solutions Ltd.
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Priority to AU2002258028A priority Critical patent/AU2002258028A1/en
Publication of WO2002091046A2 publication Critical patent/WO2002091046A2/fr
Publication of WO2002091046A3 publication Critical patent/WO2002091046A3/fr

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    • 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
    • G02B6/036—Optical fibres with cladding with or without a coating core or cladding comprising multiple layers
    • G02B6/03616—Optical fibres characterised both by the number of different refractive index layers around the central core segment, i.e. around the innermost high index core layer, and their relative refractive index difference
    • G02B6/03622—Optical fibres characterised both by the number of different refractive index layers around the central core segment, i.e. around the innermost high index core layer, and their relative refractive index difference having 2 layers only
    • G02B6/03633—Optical fibres characterised both by the number of different refractive index layers around the central core segment, i.e. around the innermost high index core layer, and their relative refractive index difference having 2 layers only arranged - -
    • 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
    • G02B6/02057—Optical fibres with cladding with or without a coating comprising gratings
    • G02B6/02076—Refractive index modulation gratings, e.g. Bragg gratings
    • G02B6/0208—Refractive index modulation gratings, e.g. Bragg gratings characterised by their structure, wavelength response
    • G02B6/021—Refractive index modulation gratings, e.g. Bragg gratings characterised by their structure, wavelength response characterised by the core or cladding or coating, e.g. materials, radial refractive index profiles, cladding shape
    • 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
    • G02B6/02057—Optical fibres with cladding with or without a coating comprising gratings
    • G02B6/02076—Refractive index modulation gratings, e.g. Bragg gratings
    • G02B6/02123—Refractive index modulation gratings, e.g. Bragg gratings characterised by the method of manufacture of the grating
    • G02B6/02133—Refractive index modulation gratings, e.g. Bragg gratings characterised by the method of manufacture of the grating using beam interference
    • 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
    • G02B6/02057—Optical fibres with cladding with or without a coating comprising gratings
    • G02B6/02076—Refractive index modulation gratings, e.g. Bragg gratings
    • G02B6/02123—Refractive index modulation gratings, e.g. Bragg gratings characterised by the method of manufacture of the grating
    • G02B6/02133—Refractive index modulation gratings, e.g. Bragg gratings characterised by the method of manufacture of the grating using beam interference
    • G02B6/02138—Refractive index modulation gratings, e.g. Bragg gratings characterised by the method of manufacture of the grating using beam interference based on illuminating a phase mask
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
    • H01S3/06—Construction or shape of active medium
    • H01S3/063—Waveguide lasers, i.e. whereby the dimensions of the waveguide are of the order of the light wavelength
    • H01S3/067—Fibre lasers
    • H01S3/0675—Resonators including a grating structure, e.g. distributed Bragg reflectors [DBR] or distributed feedback [DFB] fibre lasers
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/09—Processes or apparatus for excitation, e.g. pumping
    • H01S3/091—Processes or apparatus for excitation, e.g. pumping using optical pumping
    • H01S3/094—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light
    • H01S3/094003—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light the pumped medium being a fibre
    • H01S3/094015—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light the pumped medium being a fibre with pump light recycling, i.e. with reinjection of the unused pump light back into the fiber, e.g. by reflectors or circulators

Definitions

  • This invention relates to multicladding optical waveguide structures, and in particular, to cladding pumped fiber lasers and amplifiers, and a method of manufacturing thereof.
  • an optical fiber comprises an inner core fabricated from a dielectric material having a certain index of refraction and a cladding surrounding the core.
  • the cladding is comprised of a material having a lower index of refraction than the core.
  • optical fibers in which a core is suitably doped, for example using rare earth ions, such that the core constitutes a light amplifying medium capable of providing light amplification by the stimulated emission of radiation. Excitation occurs from the absorption of optical pumping energy by the core.
  • the optical pumping energy is within the absorption band of the active material in the core, and when the optical signal propagates through the core, the absorbed pump energy causes the amplification of the signal transmitted through the fiber core by stimulated emission.
  • Such fibers may therefore be used to provide light amplification by transmission through the fiber, or in conjunction with a resonant cavity to provide a laser.
  • a relatively high power up to several watts, can be obtained from diode pumping modules containing a set of laser diodes where their energy output is combined optically to create a single multimode laser beam (see, for example, U.S. Pat. No. 5,418,880 to Lewis, U.S. Pat. No. 5,568,577 to Hardy, et al, U.S. Pat. No. 5,802,092 and U.S. Pat. No. 5,793,783 to Endriz, U.S. Pat. No. 5,790,310 to Huang, and WO0154236 to Lebedev, et al).
  • the cladding pumping technique can be utilized for coupling the multimode output of a light radiation pumping source, including many laser diodes or a diode laser array, into the single mode core of a fiber laser.
  • a single mode fiber core containing active ions is surrounded by an undoped envelope of inner multimode cladding that is of lower refractive index than the core. This, in turn, is also surrounded by an outer cladding of yet a lower index of refraction.
  • a relatively high-power multimode-pumping light is launched and pumped into the cladding layer from a diode array and is substantially confined by total internal reflection at the interface between the inner and outer claddings and guided within the cladding.
  • the pumping light is coupled into the inner cladding, it propagates in various reflective trajectories through the inner cladding until it intersects the core. Once the light crosses over the core, pump energy is exchanged with the single mode core, and thereby provides pumping power thereto for stimulated emission amplification of the optical signal coupled into the core.
  • the absorbed multimode power is converted into a single mode laser emission within the fiber core. For many applications, this is an effective technique for supplying a relatively high-power pumping signal to a single mode fiber laser.
  • U.S. Pat. No. 4,815,079 describes two approaches that may improve the coupling efficiency.
  • a single mode core is disposed at a location which is displaced from the center ofthe cross-section ofthe cladding.
  • U.S. Pat. No. 4,815,079 teaches to perturb the modes in the inner cladding by introducing slight bends in the fiber. The perturbation introduced by the bends causes radiation from cladding modes which would not ordinarily pass through the core to couple into other cladding modes, which pass through the position occupied by the core.
  • the disadvantage of bending the fiber in this manner is that the inter-modal coupling is indiscriminate and uncontrolled. In some applications, it may not be practicable to allow the fiber to be bent.
  • the offset core configuration may also present difficulties when coupling the conventional fibers having coaxial symmetry.
  • In-line optical fiber gratings are known in the art that can be used as reflecting or filtering elements to provide narrow linewidth reflection at a given wavelength. Such a grating represents a portion of an optical fiber wherein the refractive index varies as a function of distance along the fiber.
  • U.S. Pat. No. 5,218,655 to Mizrahi describes in-line gratings that can form pump radiation reflectors in an optical fiber communication system with doped fiber amplifiers. Gratings disposed between the amplifier and the detector in such a system can be used to reflect radiation away from the detector and back through the amplifier, thus enhancing pumping efficiency.
  • In-core gratings used in the above-cited patent have a bandwidth of at least 2 nm and a peak reflectivity of at least 70%.
  • the Bragg grating was proposed for an optical fiber comprising a core and a cladding.
  • the grating comprising a spatially varying period was introduced by photochemical techniques within the fiber. The period can be independently specified in different portions of the grating.
  • the grating based on this patent may have a bandwidth in reflection with a full width greater than 12 nm at half maximum, a total intrinsic optical loss of less than 0.2 dB, and a total peak optical extinction, measured in transmission of greater than 20 dB.
  • U.S. Pat. No. 5,710,786 to Mackechnie describes a laser device for generating laser light having a wavelength in the range of 1012 nm to 1022 nm.
  • the laser device is based on the dual-cladding fiber design having a core with triply ionized ytterbium ions. Light emitting at a wavelength of 800 nm to 1070 nm is launched into an inner cladding, and as it is guided along the inner cladding ofthe fiber, it is absorbed in the core. Some of the signal light generated from the rare-earth ions is guided in the core and the laser can operate with characteristics determined mainly in the core.
  • the laser device consists of a resonant cavity having fiber Bragg gratings written into the core so as to provide optical discrimination of the emission centered in the range of 1012 nm to 1022 nm.
  • U.S. Pat. No. 5,920,582 Another approach is described in U.S. Pat. No. 5,920,582, in which an index grating is written into the inner cladding of a double-clad optical fiber to provide coupling between cladding modes of pumping radiation conducted in the inner cladding.
  • the pumping radiation is launched longitudinally into the end ofthe inner cladding.
  • the index grating in U.S. Pat. No. 5,920,582 is a long-period grating, (LPG), typically having a periodicity in the range of 10 to 1000 microns.
  • LPG long-period grating
  • the periodicity of the index grating have been selected to couple energy from cladding modes having minimal overlap with the core to cladding modes having relatively high overlap.
  • the present invention satisfies the aforementioned need by providing a novel optical waveguide structure for coupling pumping radiation from the inner cladding into the core.
  • the present invention provides a waveguide structure that includes a single mode core constituted by a radiation amplifying medium doped with active elements and operable to generate and conduct radiation.
  • the core is surrounded by a multimode inner cladding operable to receive pump multimode radiation and propagate said pump multimode radiation to the core for optically pumping said radiation amplifying medium.
  • the multimode inner cladding is surrounded by a multimode outer cladding.
  • an asymmetrical refractive index grating is formed in a portion ofthe waveguide inner cladding.
  • the asymmetrical refractive index grating has a non-uniform transverse refractive index profile in the area occupied by the grating that is not symmetrical (not uniform) in its refractive index property with respect to the core length axis.
  • the asymmetrical grating can have a variable width along the waveguide structure.
  • the asymmetrical grating is operable to reflect the pump multimode radiation propagating in the inner cladding and to couple this radiation into the core.
  • the asymmetrical grating can be based on a fiber Bragg grating.
  • a core asymmetrical grating having a non-uniform transverse refractive index profile is formed in a portion ofthe core in addition the asymmetrical refractive index grating that is formed in the inner cladding.
  • the waveguide structure of the present invention has many of the advantages of the techniques mentioned theretofore, while simultaneously overcoming some ofthe disadvantages normally associated therewith.
  • the waveguide structure having asymmetrical grating based on a fiber Bragg grating with a constant period of refractive index perturbations can provide a high light reflectivity of typically 70% and a wide reflection spectral bandwidth of better than 10 nm.
  • the waveguide structure according to the present invention may be easily and efficiently manufactured and marketed.
  • the waveguide structure according to the present invention is of durable and reliable construction.
  • the waveguide structure according to the present invention may have low manufacturing cost.
  • the aforementioned need is also satisfied by providing a method for producing the novel optical waveguide structure.
  • the method includes providing a fiber waveguide structure having a single mode core, a multimode inner cladding surrounding the core and a multimode outer cladding surrounding the inner cladding.
  • an asymmetrical property i.e., an asymmetry type
  • an asymmetrical grating in the inner cladding is defined by defining a number of asymmetrical gratings and a respective position thereof within the inner cladding and the core.
  • the required number of asymmetrical gratings with a non-uniform transverse refractive index profile and constituted by perturbations in the refractive index in the inner cladding is created by exposing the waveguide structure to a interference pattern created by an illumination source. Additionally, when desired, a required number of core asymmetrical gratings are formed in the core.
  • the method described above for forming asymmetric gratings in the inner cladding of the waveguide structure can be utilized for creating refractive index perturbations in the inner cladding that is itself made of a material having a variable transverse refractive index profile, e.g. a gradient index fiber.
  • the length of the asymmetric gratings in a waveguide having a variable transverse refractive index profile should be at least equal to or longer than the length (pitch) of a gradient index fiber mode.
  • the waveguide structure of the present invention can be used in cladding-pumped laser and amplifier devices.
  • the utilization of the waveguide structure with an asymmetrical grating can provide enhanced pump radiation reflectivity in a wider wavelength range, when compared to the state-of-the-art gratings. This can significantly improve the lasers and amplifiers performances by enabling higher output energy and better energy conversion efficiency.
  • the waveguide core When in operation, the waveguide core generates a single mode radiation while the inner cladding can generate and conduct at least two-mode radiation.
  • the waveguide structure of the present invention When the waveguide structure of the present invention is utilized for a cladding-pumped laser, at least one cavity resonator should be defined at least in a portion of the core.
  • Pump radiation launched into the inner cladding of the waveguide structure can propagate in the inner cladding along a waveguide length axis in a multi-mode fashion, and thereby interact with the doped core and stimulate emission of single mode laser radiation in the core, as it travels therethrough.
  • the waveguide structure is used for amplifying a first optical signal in a first wavelength band coupled to the single mode core for propagating the first optical signal in a longitudinal direction therethrough.
  • the core ofthe waveguide structure can absorb pump radiation in a second wavelength band provided from the inner cladding and generate radiation in the first wavelength band in response thereto, thereby amplifying the first optical signal.
  • a optical waveguide structure comprising:
  • a multimode inner cladding surrounding the core and operable to receive pump multimode radiation and propagate said pump multimode radiation to the core for optically pumping said radiation amplifying medium;
  • a multimode outer cladding surrounding the inner cladding, characterized by an asymmetrical grating having a non-uniform transverse refractive index profile formed at least in a portion of the inner cladding, said asymmetrical grating being operable to at least partially reflect said pump multimode radiation propagating in said inner cladding and to couple said pump multimode radiation into said single mode core.
  • the waveguide structure according to the present invention allowing for an increase in the efficiency of cladding-pumped laser and amplifier devices.
  • a method for producing an optical waveguide structure comprising: (a) providing a fiber waveguide structure having:
  • a single mode core constituted by a radiation amplifying medium and operable to generate and conduct radiation
  • a multimode outer cladding surrounding the inner cladding
  • Fig. 1 is a schematic view of a longitudinal cross-section of a waveguide structure, according to one embodiment ofthe preset invention
  • Fig. 2 is a schematic diagram illustrating a method for producing an optical wave structure with asymmetric reflecting gratings.
  • Fig. 3A and Fig. 3B illustrate set-ups for forming asymmetric gratings into a waveguide
  • Fig. 4A and Fig. 4B illustrate examples of a cross section of a double-clad waveguide structure with symmetrical and asymmetrical transverse profiles of the refractive index ofthe gratings, respectively;
  • Fig. 5 illustrates is schematic view of a system for recording a transmission spectrum ofthe grating
  • Fig. 6 illustrates an example of a transmission spectrum of an optical fiber having a symmetrical grating formed in the inner cladding of a waveguide structure
  • Fig. 7A and Fig. 7B illustrate examples of transmission spectra of an optical fiber having one and two asymmetrical gratings, respectively;
  • Fig. 8 is a schematic diagram illustrating the relationship between the length of an asymmetrical grating formed in to the inner cladding of the waveguide structure and the length of a gradient index fiber mode ofthe amplified radiation.
  • Fig. 9 illustrates a schematic diagram of a cladding-pumped laser with the reflective component based on an asymmetrical grating formed in the inner cladding ofthe waveguide structure (10 in Fig. 1), according to one embodiment of the present invention.
  • Fig. 10 illustrates a schematic diagram of a fiber amplifier with the reflective component based on an asymmetrical grating formed in the inner cladding ofthe waveguide structure (10 in Fig. 1), according to one embodiment of the present invention.
  • Fig. 1 shows a schematic view of a longitudinal cross-section of a waveguide structure 10 constituted by an optic fiber, according to one embodiment ofthe preset invention.
  • the waveguide structure 10 includes a single mode core 11, a multimode inner cladding 12 surrounding the core 11, a multimode outer cladding 13 surrounding the inner cladding 12, and an asymmetrical grating 14 formed at least in a portion 15 ofthe inner cladding 12.
  • the multimode inner cladding 12 is operable to receive a pump multimode radiation 18 and propagate the pump radiation to the core 11 for optically pumping the radiation amplifying medium ofthe core 11.
  • the asymmetrical grating 14 has a non-uniform transverse refractive index profile in the area occupied by the grating that is not symmetrical in its refractive index property with respect to the core length axis.
  • the asymmetrical grating 14 is operable to at least partially reflect the pump multimode radiation 18 from an optical pumping source S propagating in the inner cladding 12 and to couple a reflected radiation 19 into the single mode core 11.
  • the asymmetrical grating 14 is constituted by perturbations in the refractive index in the inner cladding 12.
  • An example of the asymmetrical grating 14 includes but is not limited to a Bragg grating having an asymmetric transverse refractive index profile.
  • the optical pumping source S is an optical source of known design, such as a diode laser array module, which has the desired output wavelength.
  • An example ofthe optical pumping source S includes, but is not limited to, a SuperFocus 1, commercially available from Rayteq Photonic Solutions Ltd., Rehovot, Israel.
  • a core asymmetrical grating 16 having a non-uniform transverse refractive index profile is formed in a portion 17 ofthe core 11 in addition the asymmetrical grating 14 that is formed in the inner cladding.
  • a location ofthe core asymmetrical grating 16 on the core can be under the asymmetrical grating 14 or shifted aside along the core 11.
  • the core 11 may be made of any material typically used in optical fibers (e.g. silica glass) and is constituted by a radiation amplifying medium doped with active elements and operable to generate and conduct radiation.
  • the active elements are ionized rare earth elements, e.g., erbium, ytterbium, neodymium, thulium, etc.
  • a schematic diagram of a method for producing an optical waveguide structure with asymmetric reflecting gratings is illustrated.
  • the method includes the step of providing a fiber waveguide structure having a single mode core, a multimode inner cladding surrounding the core, and a multimode outer cladding surrounding the inner cladding.
  • an asymmetrical property i.e. an asymmetry type
  • an asymmetrical grating is obtained by defining a number of asymmetrical gratings and a respective position thereof within the inner cladding and the core.
  • the required number ofthe asymmetric gratings constituted by perturbations in the refractive index in the inner cladding is created by exposing the waveguide structure to a interference pattern created by an illumination source. Additionally, when desired, a required number of core asymmetrical gratings can also be formed in the core.
  • the waveguide structure before the illumination UV light exposure, can be placed in a chamber with an H 2 atmosphere at the pressure of 150 bar and temperature of 100°C, where it should be kept over 24 hours. After the UV light exposure, the waveguide structure is annealed in the air atmosphere at the temperature of around 100°C over 24 hours. Under these conditions, most of the hydrogen molecules leave the waveguide structure, thus the structure, inter alia, improves its splicing ability required, for example, for assembling a fiber laser.
  • the method for producing an optical structure may include the step of providing a protective coating over the outer cladding.
  • the materials selected for the protective coating include, but are not limited to, polymers, plastics, etc.
  • Fig. 3A and Fig. 3B examples of set-ups for forming gratings in a waveguide structure are illustrated. According to these schemes, a required number of gratings can be written into the inner cladding and the core doped with refractive index modifying dopants by exposing these components of the waveguide to a interference pattern created by an illumination source.
  • refractive index modifying dopants include, but are not limited to, boron (B), aluminium (Al), nitrogen (N), fluorine (F), phosphor (P), germanium (Ge), titanium (Ti), and tin (Sn), or a combination thereof.
  • a first part 31a of a cross-section of an original high-coherence beam 31 of a UV illumination light is reflected by a mirror 32 to form a beam 34, while a second part 31b ofthe cross-section ofthe beam 31 passes close by the mirror 32 to form a beam 35.
  • the beams 34 and 35 (obtained by splitting the original beam 31) interfere with each other to form in an interference zone 33.
  • a portion 15 of the waveguide 10 can be placed in the interference zone 33, to expose the portion 15 to a interference pattern ofthe interference zone 33.
  • Fig. 3B illustrates another set-up for writing gratings into the waveguide structure.
  • a low-coherence beam 21 of a UV illumination light produced, for example, by an excimer laser diffracts as it passes through a phasemask 22.
  • the diffracted beams are reflected by mirrors 24 and interfere in an interference zone 23.
  • the mirrors 24 have been arranged downstream of the optic flow and face each other by their reflective surfaces.
  • a portion 15 of the waveguide 10 should be placed in the interference zone 23, to expose the portion 15 to a interference pattern ofthe interference zone 23.
  • the periodicity of the perturbations provided by the set-ups shown in Fig. 3A and Fig. 3B can be uniform along the length of the grating.
  • the value ofthe periodicity has been varied in the range of 0.2 micron to 0.5 micron.
  • the illuminating light utilized in the schemes shown in Fig. 3A and Fig. 3B for forming gratings may, for example, be a laser light in the UV range from 180 to 300 nm; the exposure light energy may be in the range from 100 to 200 miliJoule per cm ; the grating typical length is between 1 and 10 mm; the light exposure time may be in the range from 1 to 15 min; the concentration of the refractive index modifying dopant (e.g., Ge) in the inner cladding is in the range from 1 vol. % to 10 vol. %.
  • the refractive index modifying dopant e.g., Ge
  • a non-uniform transverse refractive index profile in the area ofthe gratings can be predominantly achieved by modifying the transverse refractive index by reacting the light of the interference patterns with the refractive index modifying dopants.
  • the modified transverse refractive index profile can become more elevated in the part of the waveguide structure (i.e., in the inner cladding and the core) proximal to the light than in the part distal thereto, due to the non-uniform light absorption.
  • FIG. 4B illustrate examples of cross sections 21a and 21b ofthe waveguide structure 10 in the area ofthe gratings obtained by utilizing the method of the present invention and which have symmetrical and asymmetrical transverse profiles 22a and 22b of the refractive index, respectively.
  • the transverse refractive index profile of the waveguide structure is a gradient index fiber profile.
  • a variation value of the refractive index An in the inner cladding 12 obtained as a result of exposing the inner cladding to an illuminating source has a constant value.
  • the multimode inner cladding 12 has a non-uniform transverse refractive index profile.
  • a non-uniform transverse refractive index perturbations profile ofthe inner cladding 12 is defined by a monotonous function.
  • a variation value of the refractive index between the two sides of the inner cladding along the diameter may be in the range from 10 "3 to 10 "2 , preferably from 2 x 10 "3 to 8 x 10 "2 .
  • Fig. 5 a schematic view of a system for the experimental recording of a transmission spectrum ofthe reflective component is illustrated.
  • the system includes a white light source 51, the waveguide structure 10 constituted by an optic fiber including a grating 53 that is written into the inner cladding of the structure 10, and a spectrometer 52.
  • the grating 53 can be a symmetrical grating or an asymmetrical grating.
  • the broadband light source 51 is a conventional device which can produce a broadband output spectrum of amplified spontaneous emission (ASE) radiation ranging, for example from 520 nm to 1610 nm.
  • the spectrometer is a conventional spectra analyzer, e.g., A DO, model AQ-6317B commercially available from ANDO Ltd., Tokyo, Japan, which ensures the reliable resolution ofthe grating spectra.
  • the formation of the grating 53 in the inner cladding of the waveguide structure 10 has been obtained by controlling a test signal ofthe white light passing through the inner cladding.
  • the output of a signal transmitted, for example, through a waveguide having the length of 10 meters can be estimated as a product of the abso ⁇ tion coefficient, the waveguide length, the ratio between the spectral range ⁇ P , and the reflection bandwidth ⁇ grat, to wit: -1.5 dB/m x 10 m x ⁇ P / ⁇ grat
  • FIG. 6 an example of the transmission spectrum of an optical waveguide having a symmetrical Bragg grating formed in the inner cladding is illustrated.
  • the formation ofthe symmetrical grating have been carried out by using an Excimer laser having the power of 100 milliwatts and operating in the UV range with the wavelength of 193 nanometers. Exposure time have been from about 10 min to 15 min.
  • the waveguide structure thereby obtained has the following parameters. Diameter ofthe inner cladding - 40 microns; Diameter ofthe core - 6 microns;
  • this symmetrical Bragg grating has the following abso ⁇ tion properties: - pump abso ⁇ tion at half-maximum - 1 dB;
  • a symmetrical Bragg grating has been formed in the inner cladding characterized by the similar physical parameters to the previous example with the sole difference in the amplitude of the refractive index modulation or perturbation ⁇ n.
  • ⁇ n 2.5 x 10 " .
  • This grating has the following abso ⁇ tion properties:
  • Fig. 7A illustrates an example of the experimental spectrum of the transmission output versus wavelength of an optical waveguide having an asymmetrical Bragg grating.
  • the formation of the symmetrical grating have been carried out by using an Argon laser having power 100 milliwatts and operating in the UV range with the wavelength of 244 nanometers. Exposure time have been from about 10 min to 15 min.
  • this asymmetric Bragg grating has the following abso ⁇ tion properties:
  • Fig. 7B illustrates an example of the transmission spectrum of an optical waveguide having two concatenated asymmetrical Bragg gratings of the example shown in Fig. 7A, which are shifted by 2 nm with respect to each other
  • such an element including two concatenated asymmetric Bragg gratings, has the following abso ⁇ tion properties: - pump abso ⁇ tion at half-maximum - 4.5 dB;
  • FIG. 7A A comparison between the spectra shown in Fig. 7A and Fig. 7B shows that the utilization of two shifted asymmetrical Bragg gratings can provide better reflectivity and reflective bandwidth, when compared with those of a sole asymmetrical Bragg grating.
  • the described above method of forming asymmetric gratings in the inner cladding 12 ofthe waveguide structure 10 can be utilized for creating refractive index perturbations in the inner cladding that is itself made of a material having a variable transverse refractive index profile, e.g. a gradient index fiber.
  • a technique for manufacturing such a material having a variable transverse refractive index profile ofthe inner cladding is know per se. For example, it can be achieved by modifying the transverse refractive index of the inner cladding by reacting a transverse illuminating light having a transverse decreasing intensity (due to abso ⁇ tion primarily in the inner cladding) across the waveguide.
  • the length of the asymmetric gratings in a waveguide having a variable transverse refractive index profile should be at least equal to or longer than the length (pitch) of a gradient index fiber mode.
  • this mode hereinafter will be referred to as the control mode.
  • a schematic diagram illustrates the relationship between a length L gr of an asymmetrical grating 14 formed in the inner cladding 12 ofthe waveguide structure 10 and a length of a gradient index fiber mode.
  • the mode 104 is referred to as a control single mode having a length L m .
  • the length of the asymmetric grating 14 is related to the length L m of the control mode in accordance with the following condition: L gr > L m .
  • different modes propagating in a gradient index fiber have substantially equal periods (pitches), although their phases are typically shifted, as illustrated in Fig 8 for the modes 104 and 106.
  • the refraction properties of the asymmetric grating 14 are substantially equal for all the pumping radiation modes propagating in the gradient index cladding 12, and all of them will be similarly reflected from the grating 14.
  • the waveguide structure of the present invention can be used in cladding-pumped laser and amplifier devices.
  • Fig. 9 illustrates a schematic diagram of a cladding-pumped laser 100 with a reflective component based on an asymmetrical grating 14 formed in the inner cladding 12 of the waveguide structure (10 in Fig. 1), according to one embodiment of the present invention.
  • the cladding-pumped laser 100 includes a cavity resonator 103 formed in a portion ofthe core 11.
  • the cavity resonator 103 is defined by two spaced gratings 101 and 102 written into the core 11 which at least partially reflect light propagating inside the core at a desired wavelength range.
  • the gratings 101 and 102 are constituted by Bragg gratings comprising a plurality of periodic perturbations in the refractive index formed within the core.
  • the pump multimode radiation 18 from an optical pumping source S is coupled to the waveguide structure 10 for propagating in the inner cladding 12.
  • the optical pumping source S is an optical source of known design, such as a diode laser array module, which has the desired output power and wavelength.
  • An example of the optical pumping source S includes, but is not limited to, a SuperFocus 1, commercially available from Rayteq Photonic Solutions Ltd., Rehovot, Israel.
  • the asymmetrical grating 14 at least partially reflects the pump radiation 18 and couples the reflected radiation 19 to the portion 103 of the core 11.
  • the reflected radiation 19 interacts with the core constituted by a radiation amplifying medium and stimulates single mode laser radiation that is amplified in the cavity resonator 103, as the radiation travels therethrough.
  • the amplified optical radiation 104 is then output for using in any desired application.
  • Table 1 illustrates examples of results measured in a 10-nm band of relative performance parameters of gratings and cladding-pumped Yb-doped lasers (lasing at 980 nm) utilizing different configurations of these gratings.
  • the results for the relative increase in the output laser power are based on the experimental data ofthe relative grating reflectivity obtained for the above described symmetrical and asymmetrical Bragg gratings written into a waveguide structure.
  • a comparison between the reflectivity properties of the symmetrical and asymmetrical Bragg grating illustrates that the asymmetrical Bragg grating has a significantly higher value of reflectivity.
  • the reflectivity can be further increased by utilizing the double asymmetric grating element including two concatenated asymmetrical gratings.
  • the utilization of such gratings for producing cladding-pumped lasers can significantly increase the laser output power, when compared to the lasers based on symmetrical Bragg gratings.
  • asymmetrical gratings can provide better energy conversion efficiency and increased lasing or enhanced laser emission power, when compared to the state-of-the-art laser devices.
  • the amplifier includes the waveguide structure 10 used for amplifying a first optical signal 111 in a first wavelength band coupled into the single mode core 11 of the structure 10 for propagating the first optical signal in a longitudinal direction therethrough.
  • the first optical signal 111 is coupled into the core 11 at a coupler 112, which is an optical coupler of known design.
  • the optical coupler 112 is also used to couple the pump radiation signal 18 in a second wavelength band from an optical pumping source S into the inner cladding 12.
  • the waveguide structure 10 includes at least one asymmetrical grating 14 having a non-uniform transverse refractive index profile formed in the inner cladding 12 of the structure 10.
  • the asymmetrical grating 14 is operable to at least partially reflect the pump multimode radiation propagating in the inner cladding 12 and to couple a reflected radiation 113 into the core 11.
  • the core 11 is operable to absorb pump radiation in the second wavelength band provided from the inner cladding and to generate radiation in the first wavelength band in response thereto, thereby amplifying the first optical signal 111.
  • the amplified signal is then output at an output terminal 115, where it is used in any desired application.
  • the inner cladding 12 can have a noncircular cross-sectional shape.
  • any number of gratings formed in the inner cladding 12 can be concatenated to result in an even wider reflection bandwidth.
  • the waveguide structure of the present invention can be used in a optical communication network for other applications, e.g., free space communications, satellite communications, Q-switch lasers for medical applications and material processing, etc. Also, it is to be understood that the phraseology and terminology employed herein are for the pu ⁇ ose of description and should not be regarded as limiting.

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  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Optics & Photonics (AREA)
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Abstract

L'invention porte sur une nouvelle structure de guide d'ondes optique et sur un procédé de production correspondant. Cette structure comporte un noyau monomode fabriqué par un dispositif d'amplification de rayons, une gaine interne multimode qui entoure le noyau, et un treillis asymétrique constitué de perturbations dans l'indice de réfraction dans la gaine interne et comportant un profil d'indice de réfraction transversal non uniforme formé dans au moins une partie de la gaine interne. Cette gaine asymétrique sert à refléter des rayons multimode de pompe qui se propagent dans la gaine interne et à relier ces rayons au noyau monomode. Le procédé selon l'invention de production de structure de guide d'ondes optique consiste à fournir une structure de guide d'ondes de fibre équipée d'un noyau monomode, une gaine interne multimode entourant le noyau, et une gaine externe multimode entourant la gaine interne ; à obtenir une propriété asymétrique d'un treillis asymétrique par la définition d'un nombre de treillis asymétriques et d'une position respective de ce dernier à l'intérieur de la gaine interne ; et à exposer la gaine interne multimode à un schéma d'interférences créé par une source d'éclairage.
PCT/IB2002/001516 2001-05-10 2002-05-06 Structure de guide d'ondes equipee d'un treillis asymetrique et procede de production correspondant WO2002091046A2 (fr)

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US60/289,805 2001-05-10

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Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2017027862A1 (fr) * 2015-08-13 2017-02-16 Nufern Fibres optiques à mode de mélange et procédés et systèmes les utilisant
US10630040B2 (en) 2016-02-05 2020-04-21 Nufern Mode mixing optical fibers and methods and systems using the same
US20210367391A1 (en) * 2020-05-20 2021-11-25 UNIVERSITé LAVAL Pump reflectors for cladding-pumped optical fiber systems
CN114079220A (zh) * 2022-01-19 2022-02-22 北京凯普林光电科技股份有限公司 一种具有泵浦光反射功能的光纤及其制作和测试方法

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US5694248A (en) * 1992-12-23 1997-12-02 Lucent Technologies Inc. Spatially-varying distributed Bragg reflectors in optical media
EP0918382A2 (fr) * 1997-11-21 1999-05-26 Lucent Technologies Inc. Structures à fibre pompée à travers le gainage
US5920582A (en) * 1996-12-19 1999-07-06 Northern Telecom Limited Cladding mode pumped amplifier

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Publication number Priority date Publication date Assignee Title
US5694248A (en) * 1992-12-23 1997-12-02 Lucent Technologies Inc. Spatially-varying distributed Bragg reflectors in optical media
US5920582A (en) * 1996-12-19 1999-07-06 Northern Telecom Limited Cladding mode pumped amplifier
EP0918382A2 (fr) * 1997-11-21 1999-05-26 Lucent Technologies Inc. Structures à fibre pompée à travers le gainage

Cited By (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2017027862A1 (fr) * 2015-08-13 2017-02-16 Nufern Fibres optiques à mode de mélange et procédés et systèmes les utilisant
US10761267B2 (en) 2015-08-13 2020-09-01 Nufem Mode mixing optical fibers and methods and systems using the same
US11287572B2 (en) 2015-08-13 2022-03-29 Nufern Mode mixing optical fibers and methods and systems using the same
US10630040B2 (en) 2016-02-05 2020-04-21 Nufern Mode mixing optical fibers and methods and systems using the same
US20210367391A1 (en) * 2020-05-20 2021-11-25 UNIVERSITé LAVAL Pump reflectors for cladding-pumped optical fiber systems
CN114079220A (zh) * 2022-01-19 2022-02-22 北京凯普林光电科技股份有限公司 一种具有泵浦光反射功能的光纤及其制作和测试方法
CN114079220B (zh) * 2022-01-19 2022-06-07 北京凯普林光电科技股份有限公司 一种具有泵浦光反射功能的光纤及其制作和测试方法

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