Optical fiber basics in a nutshell

Transcription

1 Optical fiber basics in a nutshell Nuphar Lipkin, Lambda Crossing, Israel Talk outline (a taste of): (Late 70-s: 1 st phone lines, 1988: 1 st TAT, now: FTTH) Optical communication systems- basic concepts, advantages Physics of optical wave-guiding Optical fibers- most demanding transmission WG-s, types, material requirements and limitations

2 Optical communication systems Optical communication systems use light as the primary medium to carry information The light is usually guided through fibers (fiberoptic technology) Most applications use Infrared light (1.55, 1.3µm) Digital modulation bit, bit rate

3 Why optical communication? Main advantage: Most cost effective way to move huge amounts* of information (voice, data) quickly and reliably over long distances and: Light is insensitive to electromagnetic interference * ADSL: 6 Mbit/s down +640 kbit/s up to ~4-6 km FiberOptics vs. Free- space: no atmospheric interference non- straight paths Fiber: Gbit/s per single channel, ~400 Gbit/s per single fiber, ~50km

4 Transmitting more information: Different λ-s passing through the same medium do not interfere with each other (almost) Wavelength division multiplexing # of channels: -width of transmission band (opto- electronics) - width of channel, channel separation (filters, wavelength stability of components) WDM: 2-8 channels DWDM: channels Typical: channels

5 LAN: Local area networks (within building/ vehicle) Performance Price LAN

6 What determines the wavelength bands for optical communication? Silica- predominant fiber material: transmission properties Availability of opto- electronics- lasers, amplifiers, detectors A( db) = 10log I I out in C band: nm (~50 channels) Erbium doped amplifiers Optical loss of fiber- grade Silica (absorption + scattering)

7 Basic functions of an optical communication link Multiplex (WDM) Integrated/ free space optics Transmit Silica fibers Amplify (Electronic, Optical (e.g.-erbium doped)) Regenerate pulse shape (Electronic, Optical) De-multiplex (Optical) Amplifier span: km Regenerator span: km Route (Electronic, Optical)

8 All- optical networks (Potential) Advantages: No need to transfer back and forth to electronics (speed, price) Low power consumption Immunity to electromagnetic interference, ionizing radiation, temperature Size, weight, volume (= price)

9 Optical communication systems -Summary: Optical communication systems - Can carry huge amounts of information - Many wavelengths (channels) through same fiber (WDM) - Different ranges/ requirements: Long haul, metro, access, LAN - Silica fibers: wavelength bands, C- band (~1550nm) - Basic functions of a communication link - Drive for all- optical networks Physics of wave-guiding

10 Light confinement in an optical waveguide Light is confined and n core > n cladding Ray optic description: Wave optic description: θ >θ ray critic_ TIR

11 Optical Waveguides: Slab waveguide The simplest waveguideslab waveguide (dielectric layer sandwich). Low index n1 High index n2 Low index n3 Top Cladding Core Bottom Cladding Light propagating in z Layers infinite in (z,y) Claddings semi- infinite in x n core > n top/ bottom cladding

12 Optical waveguides: Wave optic description Solutions: Top cladding Core Substrate/ Bottom cladding Solutions to Maxwell s equations satisfying the condition of field continuity at the boundary between the optical media Only specific propagation modes can propagate in the waveguide. These modes are characterized by their field intensity distribution functions Mode definition: kn < βm < 0 kn 1 for guided β- Propagation constant in z m= 0,1,2 modes k ω = = c 2π λ 0

13 Optical waveguides: Mode shape The number of confined modes, and the shape of modes are a function of: n (core/ cladding) Waveguide dimensions (height for slab waveguide) Operation wavelength Higher n, larger waveguide dimensions (vs. λ) more supported modes

14 Symmetric Slab WG: strong confinement High n Large diamensions (vs. λ) Strong confinement-

15 Symmetric Slab WG: weak confinement Lower n, longer λ Weaker confinement- substantial field intensity in the cladding

16 Symmetric Slab WG: number of modes 1-st to 4-th modes of: n s =1.45 n=0.2 λ= 1.4 d=3um A 5-th mode is not supported by this WG.

17 Physics of wave guiding -Summary: -Optical communication systems - Physics of wave-guiding: - Confinement, d~λ, n core >n cladding - Ray optics description: total internal reflection - Wave optics description: Guided modes- solution to Maxwell s equations - Slab waveguide example: - High n, large diamensions (vs. λ) Stronger confinement, more supported modes - Optical fibers: - Material requirements - Silica fibers

18 Materials for optical fibers: considerations High transparency at the working λ (depending on application) Mechanical strength Environmental durability during storage/ operation Stability of optical and mechanical properties Drawing to a thin fiber with excellent uniformity Cost of material, cost of manufacturing Crystalline materials- scattering at grain boundaries, non-isotropic (sensitivity to light polarization) Polymers- Stability of properties, durability, transparency Visible 2um- Silica glass (+ polymers for non- critical applications)

19 Most demanding WG- long haul fibers. Silica fibers Preform formation Drawing How to transmit light km away? Window glass: 1dB/cm (10% left after 10cm) Fibers from optical glasses (from mineral SiO2): ~1000dB/ km (10% left after 10m) Main loss factors: absorption (λ) by traces of Fe, Cu, V, Co, Ni, Mn, Cr ions Absorption of OH scattering (λ) from irregularities in the bulk and at the core/ clad interface

20 Contemporary silica fibers Breakthrough- Corning ~1970: Gaseous precursors Loss vs. Wavelength of fibergrade Silica SiCl 4 + O 2 SiO 2 +Cl 2 SiCl 4 - liquid at room temp. Boiling point: 58C. Impurities evaporate at higher temp. Loss (db) Fundamental vibrations of Silica Current performance: ~0.2 (10% left after 50km) ~ theoretical limit Rayleigh scattering Wavelength

21 Silica fibers: index profiles Doping: Ge- raises RI F - lowers RI

22 Silica fibers: index profile realization MCVD: Gases react on the inner side of a rotating tube. Energy source- heat, layered deposition (cladding first), Collapse of 2000C

23 Optical fibers (silica fibers) -Summary: - Optical communication systems - Physics of wave-guiding: - Optical fibers: - Silica fibers: - Contemporary technology: Loss brought down close to the theoretical limit (Rayleigh scattering) - Fabrication of preform from gaseous precursers - Controlling the Index profile through dopants - Types of fibers

24 Fiber structures: Core diameter (# of modes) Index profile Step index Index profile Main Fiber Types: Multimode- step index Multimode- Graded index Single mode

25 Fiber types: Step index multimode Multimode Large core- easy to couple light in

26 Multimode fiber: Modal dispersion Incoming signal excites a number of fiber modes The ability to transfer data in a multimode fiber is limited by modal dispersion

27 Partial solution to modal dispersion- Multi mode graded index fiber Modes traveling closer to the cladding travel through a medium with a lower index of refraction, and their velocity increases Used for short- range communication

28 Single mode fiber Multi- mode limitations: -Compensation for modal dispersion not full -Interference between modes (modal noise) -The technological difficulty to create a perfect index profile -Complicated and expensive manufacturing - Single mode fiber Small core- more difficult light coupling, requires connections accuracy

29 Single mode fiber limitations: loss

30 Single mode fiber limitations: Chromatic dispersion (wavelength- dependent speed*) * Signal is not monochromatic- laser linewidth, fourier transform of modulation function, etc.

31 Chromatic dispersion origins: Material dispersion n = Dielectric Constant Properties of Silica: Variation of the refractive index with frequency Speed of light in the dielectric=c/n Dielectric constant vs. frequency n(λ)- the response of various polarization mechanisms is frequency dependent (ability to respond to high frequency fields) The dielectric constant is frequency (wavelength) dependant

32 Chromatic dispersion: origins Properties of Silica: variation of the refractive index with frequency Mode shape: the more confined the mode is the higher the effective index The two contributions cancel in standard single mode step index ~1300nm

33 Single mode fiber 1300nm bandlowest Chromatic dispersion 1550 band- lowest loss (Rayleigh scattering)

34 Mid 80-s: Zero-Dispersion shifted fibers: zero- dispercion and minimum 1550nm The target: Shift the zero- dispersion point to ~1550nm, where attenuation is lower. Solution: Increase WG dispersion Un- expected problem- coherence between channels (wavelengths) can cause non- linear interactions at high laser intensities (4- wave mixing) ν = 1 + ν 2 ν 3 ν 4 e.g. for evenly spaced100ghz channels: ν + ( ν + 300GHz) ( ν 200GHz) = ν + 100GHz Noise on an existing channel

35 Non- zero Dispersion shifted fibers: Solution to four- wave mixing

36 Polarization Mode Dispersion

37 Polarization maintaining (PM) fiber Breaking the degeneration between TE and TM modes (and reducing the probability for intensity transfer) by: Shape induced birefringence (non- round core) Stress induced birefringence Coupling of light into the fiber- along one of the main axes Special connectors, alignment critical For ER > 20dB φ < 6 For ER > 30dB φ < 1.8

38 Optical fibers (silica fibers) -Summary: - Optical communication systems - Physics of wave-guiding: - Optical fibers: - Silica fibers: - Types of fibers: - Multimode fiber- easy light coupling, modal dispersion - Graded index multimode fiber- partial solution to modal dispersion - Single mode fiber - Transmission limitations: attenuation, chromatic dispersion - Controlling dispersion: shifting the zero-dispersion point to 1550nm - Polymeric fibers, Infrared fibers

39 Polymeric fibers Loss much higher than Silica (O-C, C- H bonds) Transmission fair in visible, worse in the infrared 650nm (red LED): 150dB/km for commercial fibers, 50dB/km in labs with fluorinated/ deuterated polymers Used in short range communication (inside buildings/ vehicles), for image transmission and for lighting Standard plastic fiber: PMMA core (n=1.492), Cladding (n=1.402), diameter= 85um 3mm db/m!! Stability of optical properties, environmental durability- problematic

40 Fibers for the Infra- red If the theoretical limit is Rayleigh scattering- why not longer wavelength? db/m!! Materials: Fluorozirconate fibers (ZrF 4 + BaF 4, 0.4-5um, 2.6um lab- 1dB/km) Chalcogenide glasses- S, Se compounds (3.3-11um, 0.7dB/m 5.5um) Rare and expensive Viscosity too low for fiber drawing Fibers demonstrate low strength Environmental durability: sensitivity to humidity, bases

41 Optical fibers -Summary: -Optical communication systems - Physics of wave-guiding - Optical fibers: - Silica fibers - Polymeric fibers, Infrared fibers- limited applications due to material shortcomings

42 Thank you * References (+ pictures) include: J. Hecht, Understanding Fiber Optics, Prentice Hall; 3 edition, 1998; R. G. Hunsperger, Integrated optics, Springer-Verlag, 3rd edition 1991; S. V. Kartalopoulos, Introduction to DWDM, Wiley-IEEE Press 1999