0 Photos and much of the content is courtesy of OmniGuide Communications Cambridge, Massachusetts, USA (where M. Skorobogatiy served as a theory and simulation group leader) and Prof. Yoel Fink fiber research group at MIT Applications of omnidirectional reflectivity. Communication and high power transmission through hollow Bragg (OmniGuide) fibers. Quasi - 1D systems, Bragg fibers

1 The problem: making the perfect mirror Hollow core MirrorCladding OmniGuide Cladding Core Cladding Hollow Core Conventional Hollow Metallic Conventional Dielectric Mirror Angular dependent reflectivity with very low optical loss Metallic Mirror Omnidirectional reflectivity with optical loss Omnidirectional Mirror Reflects all angles with very low loss

2 High-Energy Laser Guidance in the IR Laser Surgery, Materials Processing Fiber Devices Dispersion Compensating fibers, Tunable Cavities, Lasers, Nonlinear Devices Few applications of hollow Photonic Bandgap fibers Low loss transmission of IR signals IR Imaging Communications

3 OmniGuide/MIT hollow core fiber Output of a straight 25cm piece of fiber, =10.6  m B. Temelkuran et al., Nature 420, 650 (2002) + OmniGuide Communications

4 Spiral OmniGuide Preform Processing Stoichiometric thermal evaporation of As 2 Se 3 onto free-standing PES film Step 1: Stoichiometric thermal evaporation of As 2 Se 3 onto free-standing PES film Rolling of coated film into cladded hollow multilayer cylinder on SiO 2 tube substrate Step 2: Rolling of coated film into cladded hollow multilayer cylinder on SiO 2 tube substrate Vacuum thermal consolidation Step 3: Vacuum thermal consolidation Etching and removal of SiO 2 Step 4: Etching and removal of SiO 2 Courtecy of OmniGuide Communications

5 Step 2 Evaporation Step 1 Materials Synthesis The OmniGuide Fabrication Sequence Step 4 Fiber Drawing Step 3 Structured Preform Fabrication Courtecy of OmniGuide Communications

6 Preform-Based Fabrication Strategy Partially Drawn Preform 1 in Mirror (SEM Image) Preform 5 µm 3-30 meter draw tower Courtecy of OmniGuide Communications

7 Bragg fiber by stacking technique Silica-Air, Bragg Like fiber G. Vienne, et al. “First demonstration of air-silica Bragg fiber,” OFC, PDP25, 2003

8 Reflection form the planar dielectric mirror, modes of hollow metallic waveguide and hollow Bragg fiber "Analysis of mode structure in hollow dielectric waveguide fibers,“ M. Ibanescu, S.G. Johnson, M. Soljacic, J. D. Joannopoulos, Y. Fink, O. Weisberg, T.D. Engeness, S.A. Jacobs, and M. Skorobogatiy, Physical Review E, vol. 67, p , 2003 Modes of hollow metallic waveguide Frequency regions (gray) of omnidirectional reflection form the multilayer reflector stack Modes of hollow Bragg fiber AND =

9 Wavelength scalability. Different draw conditions shift the transmission spectrum OmniGuide FTIR spectrum Index contrast n h /n l ~2.5/1.7; R core ~200  m; Fundamental bandgap at =3  m Wavevector Courtecy of Y. Fink (MIT)

10 Colorful fibers Fibers of different draw down ratio exhibiting continuously changing position of a higher order band gap Fiber Outer Diameter decreases Y. Fink et al., Advanced Materials 15, 2053 (2003)

11 Modes of OmniGuide hollow core fiber Ultra low loss, hard to couple to Gaussian laser source Most compatible with Gaussian laser source and high power Leaky modes of a Bragg fiber are calculated using transfer matrix method Absorption losses and nonlinearities of the underlying imperfect materials are greatly suppressed as most of the field is concentrated in the hollow core

12 Modal radiation and absorption losses Index contrast n h /n l ~4.6/1.6, Rcore~15  m, bulk material loss 1dB/m, 12 mirror periods "Low-loss asymptotically single-mode propagation in large-core OmniGuide fibers,“ S.G. Johnson, M. Ibanescu, M. Skorobogatiy, O. Weisberg, T.D.Engeness, M. Soljacic, S. Jacobs, J. D. Joannopoulos and Y. Fink, Optics Express, vol. 9, pp , 2001

13 High power guiding applications HE 11 Coupling to HE 11, higher order and cladding modes Region of increased heating Beam degradation due to inter-modal scattering Beam quality M 2 degradation due to higher order mode content Coupling efficiency at the fiber input Temperature rise due to imperfect coupling Modeling tools Design and optimization Scattering/radiation due to imperfections/bends Excess heating due to bends Beam quality M 2 estimation via free space propagation HE 11 Input Transmission Region of increased heating R core ~  m Radiation, absorption loss ~ 1/R 3 core Bending loss ~ R  core /R bend M. Skorobogatiy, S.A. Jacobs, S.G. Johnson, O. Weiseberg, T.D. Engeness, Y. Fink, “Power Capacity of Hollow Bragg Fibers, CW and Pulsed Sources,” TuA4.6, Digest of the LEOS Summer Topical Meetings, pp (2003) Input

14 Components for high power guiding applications Courtecy of OmniGuide Communications

15 Imperfect coupling and heating (theory) Metal tube coupler OmniGuide fiber, =10.6  m Incoming Gaussian, m=1 mode ~80%-90% HE 11 mode Dry air cooling R c ~300  m Amplitudes of excited modes are calculated by matching transverse electric and magnetic fields of the incoming Gaussian in free space and eigen fields of the fiber/coupler, for an unoptimized coupler power in the lowest loss m=1 mode HE 11 is 80%-90%

16 Imperfect coupling and heating (theory) Temperature rise (red) along the fiber length due to imperfect coupling (80% in HE 11 and 20% in parasitic modes) – full solution. In green, temperature distribution if 100% HE 11 mode is excited. In blue, temperature distribution ignoring the interference effects between the modes. Heat flow equation is solved with heat sources defined by amplitudes of excited parasitic modes due to imperfect coupling

17 Imperfect coupling and heating (experiment) Temperature MAX MIN Non-uniform temperature rise in a fiber under imperfect coupling Fiber Laser and coupler Courtecy of OmniGuide Communications

18 Bending loss in OmniGuide fiber (experiment) Bend loss ~ 3 dB through full “knot” of 1 cm radius B. Temelkuran et al., Nature 420, 650 (2002)

19 Bends and beam degradation (experiment) Straight – 25 cm long Bent – 360 O, 10 cm radius Courtecy of Y. Fink (MIT)

20 Bends and heating (theory) R bend =20cm R cooler R core Temperature distribution in a fiber bend Amplitudes of excited modes in a bend are found by propagating HE 11 incoming field through bend by Coupled Mode Theory Heat flow equation is solved with heat sources defined by amplitudes of excited modes

21 Transmission window and loss 10.6  m Ability to control location of transmission window for specific applications Courtecy of Y. Fink (MIT)

22 Telecommunications applications Coupling of the laser source to the fiber HE 11 or TE 01 modes Mode converter design Ultra low loss TE 01 mode (~0.1dB/km), incompatible with Gaussian Gaussian → TE 01 mode converter Gaussian → HE 11 direct launch Moderate loss HE 11 mode (~10dB/km) HE 11 TE 01 Modal losses due to absorption/radiation micro and macro bends fiber imperfections Dispersion management Signal degradation due to nonlinearities micro and macro bends fiber imperfections Polarization Mode Dispersion Modeling tools R core ~15  m HE 11 radiation, absorption loss ~ 1/R core Bending loss ~ 1/R 2 bend -1/R bend TE 01 radiation, absorption loss ~ 1/R 3 core, non-linearities ~ 1/R 7 core Input

23 Highly designable group velocity dispersion of OmniGuide modes Very high dispersion Low dispersion Zero dispersion [2  /a] [2  c/a] HE 11

24 PMD of the TE 01 and HE 11 modes TE 01 is a non-degenerate mode, and thus cannot be split PMD is zero Polarization-mode dispersion (PMD) of a doubly degenerate HE 11 mode: different group velocities: stochastic stress, imperfections… …pulse spreading! same group velocities: “single-mode” fiber HE 11 : TE 01 :

25 Challenges: coupling to Bragg fibers. HE 11 →TE 01 ”serpentine” mode converter (theory) SMF-28 silica fiber at 630nm, R c =4.1  m,  n/n c =0.36%, 7 guided modes: 1)LP 01 - HE 11 2)LP 11 - TE 01,TM 01,HE 21 3)LP 21 - EH 11, HE 31 4)LP 02 - HE 12 Amplitude of fiber wiggling  =49nm, N=35 turns, D w =512  m

26 HE 11 → TE 01 ”serpentine” mode converter (experiment) 33% LP01, 65% LP11, 2% LP21+LP % LP01 M. Skorobogatiy, C. Anastassiou, S.G. Johnson, O. Weiseberg, T.D. Engeness, S.A. Jacobs and Y. Fink, “Quantitative characterization of higher-order mode converters in weakly multimoded fibers,” Optics Express 11, 2838 (2003) HE 11 TE 01 Courtecy of OmniGuide Communications

27 Bragg fiber components and systems Device applications and functional fibers

28 Inter-Fiber Interaction 2) Bragg fiber Individual fibers are drawn. Outer polymer cladding can be removed by dissolving the polymer. 2) Stacked fiber Two closely spaced fiber cores are provisioned on the preform level. Directional coupler is then drawn from such a preform. Core 1Core 2 Drawing Cladding removal Fiber alignment B.J. Mangan, J.C. Knight, T.A. Birks, P.S. Russell, A.H. Greenaway, Electron. Lett. 36, 1358 (2000).

29 1)Cabling of several photonic band gap fibers parasitic coupling between waveguides due to the radiation leakage outside of the fiber core 2) Fiber components (directional couplers) Coupling has to be strong enough so that power transfer from one waveguide to another happens on a length scale much smaller than modal decay length (radiation loss) Coupling through radiation field resonance in the inter-fiber region M. Skorobogatiy, "Hollow Bragg fiber bundles: when coupling helps and when it hurts,” OPTICS LETTERS 29, 1479 (2004) Two related problems of directional coupling M. Skorobogatiy, K. Saitoh and M. Koshiba, "Resonant directional coupling of hollow Bragg fibers,” OPTICS LETTERS 29, 2112 (2004)

30 Functional Bragg fibers By creating a “thick” layer in the reflector, fiber transmission can be suppressed in the middle of a band gap. Application of stress offers tuning by changing defect wavelength of a resonator. Y. Fink et al., Advanced Materials 15, 2053 (2003)

31 Functional Bragg fibers Optical fibers can be integrated during drawing with “non-trivial” components such as electric wires, semiconductor devices, etc. Tin “wires” Bragg reflector Y. Fink et al., Nature 431, 826 (2004)

32 Functional Bragg fibers Optical fibers can be integrated during drawing with “non-trivial” components such as electric wires, semiconductor devices, etc. Tin “wire” Bragg reflector Semiconductor glass Y. Fink et al., Nature 431, 826 (2004)