OPTIC FIBERSOPTIC FIBERS

XIAOLI ZHOUXIAOLI ZHOU

April 12April 12

What is optic fiber?What is optic fiber?Concept: Optical fiber (or "fiber optic") refers to the medium and the technology that uses glass (or plastic) threads (fibers) to transmit data.

Basic idea of fiber communication:

Electrical Signal

FiberOptical signal

Electrical Signal

Light Source

Receiver

Figure 1.Fiber-optic system components

Basic structureBasic structure

• 1)Core

-----higher refractive index• 2)Cladding

-----less refractive index• 3)Buffer coating

-----protect optic fiber

Figure 2.Basic structure of optic fiber

Light guidingLight guiding• Light is guided in the core of

an optical fiber by total internal reflection of the boundary of the lower-index cladding.

For nr/ni= 0.99 θc=820

Confinement angle: 80

-----light can be confined in the core if it strikes the interface with the cladding at this angle or less.

Figure 3.Light guiding in optic fiber.

acceptance angle:acceptance angle: The acceptance angle is measured outside the fiber; it

differs from the confinement angle in the core material. Light must fall inside this angle to be guided in the fiber.

The acceptance angle is measured as numerical aperture (NA) which for light entering a fiber from air is approximately: cnnnNA sinsin 0

21

20

θc---confinement angle

Cladding---n1

Core---n0

Light must fall inside this angle to be guided in the fiber core

Fiber AxisWhole Acceptance Angle (2θ) Half acceptance

angleθ

Light guiding Light guiding (Cont.)(Cont.)

Figure 4. Acceptance angle of optic fiber.

Fiber transmissionFiber transmission 1)Attenuation Transmission of light by optical fibers is not 100% efficient. Some

light is lost, causing attenuation of the signal.

Measurements of attenuation are made in decibels (dB): logarithmic units measuring the ratio of output to input power.

Loss (attenuation) in decibels is defined as:

2) Dispersion

Dispersion is the spreading out of light pulses as they travel along a fiber. It occurs because the speed of light through a fiber depends on its wavelength and the propagation mode.

Some fibers allow light to travel in many distinct paths called “modes”, so they have high dispersion.

)(log10)( 10in

out

P

PdBL

Types of optic fiberTypes of optic fiber1) Single-mode fiber Single mode fiber is optical fiber that is designed for the transmission

of a single ray or mode of light as a carrier and is used for long-distance signal transmission.

125μm

Index profile

9μm

Figure 5. Single-mode fiber

2) Multimode fiber Multimode fiber is optical fiber that is designed to carry multiple

light rays or modes concurrently, each at a slightly different reflection angle within the optical fiber core.

a) Step-index Multimode Fibers

140μm

100μm

Index profile

Types of optic fiber (Cont.)Types of optic fiber (Cont.)

Figure 6. Step-index multimode fiber

Types of optic fiber (Cont.)

b) Graded-Index Multimode Fibers

125μm

50μm

Index profile

Figure 7. Graded-index multimode fiber

Nonlinear effectsNonlinear effects The interaction between light wave and the material transmitting them

can affect optical signals. These processes are called nonlinear effects because their strength typically depends on the square of intensity rather than simply on the amount of light.

Nonlinear effects are comparatively small in optical fibers transmitting a single optical channel. They become much larger when dense wavelength-division multiplexing packs many channels into a single fiber.

Dense Wavelength-division Multiplexing (DWDM): The transmission of many of closely spaced wavelengths in the 1550 nm region over a single optical fiber. It can increase the capacity signal of fiber. This increase means that the incoming optical signals are assigned to specific wavelengths within a designated frequency band, then multiplexed onto one fiber. This process allows for multiple video, audio, and data channels to be transmitted over one fiber while maintaining system performance and enhancing transport systems.

Multiplex--- to combine multiple signals for transmission over a single line or media.

Nonlinear effects (Cont.)Nonlinear effects (Cont.)• Second harmonic

generation The incident field E1 at

frequency ω excites a weak nonlinear polarization at the double frequency 2ω. This emission is only efficient when the refractive index at ω and 2ω are identical.

ω in 2ω out

Brillouin Scattering

Four-Wave Mixing Figure 8. Laser beam enters a crystal of ammonium dihydrogen phosphate as red light and emerges as blue.

Holey optical fibersHoley optical fibers While conventional optical

fiber is composed of a core surrounded by a cladding, the core of holey fiber contains air holes arranged like a crystal lattice. Representative examples include hole-assisted fiber (highly refractive core and a few holes), photonic crystal fiber (silica glass core and several dozen holes), and photonic band-gap fiber (hollow core and dozens of holes).

Figure 9. Holey optical fibers

Holey optical fibers Holey optical fibers (Cont.)(Cont.)

• The basic idea is to create a material that cannot transmit light at certain wavelengths.

• Suppose, that a uniform and closely spaced array of tiny holes was drilled through a block of glass. Light of certain wavelengths would hit the holes in phase throughout the material, so they would be scattered out of the “holey” zone. Thus light of those wavelengths could not travel through the material, and would fall in a photonic bandgap.

• Through design of the size, number, separation, and arrangement of the holes, holey fiber has achieved many optical characteristics impossible with conventional fiber such as single-mode in an extremely wide range of wavelengths, a dramatic reduction in bending loss, and high polarization maintenance.

Holey optical fibers (Cont.)Holey optical fibers (Cont.)• Photonic crystal fibers filled with

liquid crystals create a high-performance optical switch.*

Liquid-crystal-filled photonic crystal fibers (LC-PCF) that guide light by the photonic bandgap effect make ideal all-optical signal processing devices.

Researcher’s work exploits these phase changes to control the wavelength of the PCF’s photonic bandgap.

The researchers filled a 20-mm long section of PCF with a cholestric LC and heated it from 77 °C to 94 °C using a hotplate. As the temperature increased, the researchers saw that the color of the guided mode changed from green to yellow, then into an off-state and then blue.

Figure 10. LC-PCF

These devices will hopefully find applications within tuneable PCF devices for example to tune dispersion in nonlinear processes.

*Optics Express 11 2589, http://optics.org/articles/news/9/10/23/1

Fiber ManufactureFiber ManufactureMaking optical fibers requires the following steps: 1.Making a perform glass cylinder

2.Drawing the fibers from the preform

3.Testing the fibers

                                                                                  

                                    

Figure 11.modified chemical vapor deposition process for making the preform blank Figure 12.Diagram of a fiber drawing tower used to draw

optical glass fibers from a preform blank

Optic fiber in the futureOptic fiber in the future• Promising• The biggest challenge

remaining for fiber optics is economic. Terminal equipment remains too expensive to justify installing fibers all the way to homes, at least for present services.