Optical fiber

Presented By-Er. Sanyam S.

SainiME (I&CE) (Regular)

2012-14

Presented To-Prof. M.S. Narkhede

NITTTR, Chandigarh

Question Based on University Exam

1. Enlist the material used for Fluoride Glass fabric. Discuss the difficulties involve in manufacturing of such fiber using conventional techniques.

2. State & Explain the various Losses in Optical Fiber. Mention the wavelength at which attenuation is minimum?

Answer #01Basic Requirements

It must be possible to make a long ,thin and flexible fibers from the materials.

Materials having slightly different refractive indices for the core and cladding must be available.

For the fiber to guide light efficiently, the material must be transparent at a particular wavelength.

Silica

Silica exhibits fairly good optical transmission over a wide range of wavelengths.

Silica fiber has high mechanical strength .

In the near-infrared (near IR) portion of the spectrum, particularly around 1.5 μm, silica can have extremely low absorption and scattering losses of the order of 0.2 dB/km.

Silica is also relatively chemically inert.

Silica is not hygroscopic.

Silica

Silica glass can be doped with various materials. The purpose of doping is to raise the refractive index or to lower it.

To lower the RI, doping materials are: fluorine or B2O3

To raise the RI, doping materials are: (GeO2) or (Al2O3))

Silica fiber also exhibits a high threshold for optical damage.

Because of these properties silica fibers are the material of choice in many optical applications, such as communications , fiber lasers, fiber amplifiers, and fiber-optic sensors.

Fluorides Fluoride glass is a class of non-oxide optical quality glasses composed

of fluorides of various metals.

As heavy metal fluoride glasses exhibit very low optical attenuation, they are not only difficult to manufacture but they have poor resistance to moisture and other environmental attacks.

Because of their low viscosity, it is very difficult to completely avoid crystallization while processing it through the glass transition.

An example of a heavy metal fluoride glass is the ZBLAN glass group, composed of zirconium, barium, lanthanum, aluminum , and sodium fluorides. They are advantageous especially in the mid-infrared (2000–5000 nm) range.

The optical applications of fluoride fibers include mid-IR spectroscopy, fiber optic sensors, thermometry, imaging etc.

Absorption in

Infrared Region

Absorption

Atomic DefectsExtrinsic Intrinsic

Absorption

Absorption in

Ultraviolet Region

Attenuation

Scattering Losses

Compositional fluctuations

Homogeneitiesin fiber

Bendinglosses

Macroscopic bends

Microscopic bends

Answer #02

Attenuation

Signal attenuation (fiber loss) largely determines the maximum Repeater less separation between optical transmitter & receiver.

Signal distortion cause that optical pulses to broaden as they travel along a fiber, the overlap between neighboring pulses, creating errors in the receiver output, resulting in the limitation of information-carrying capacity of a fiber.

Attenuation Power loss along a fiber:

Z=0P(0) mW

Z= llpePlP )0()(

zpePzP )0()(The parameter is called fiber attenuation coefficient in a units of for example [1/km] or [nepers/km]. A more common unit is [dB/km] that is defined by:

p

]km/1[343.4)()0(log10]dB/km[ plP

Pl

Fiber loss in dB/km

]km[]dB/km[]dBm)[0(]dBm)[( lPlP

Attenuation v/s Wavelength

Absorption

1- Impurities in fiber material: from transition metal ions & particularly from OH ions with absorption peaks at wavelengths 2700 nm, 400 nm, 950 nm & 725nm

Absorption is caused by three different mechanisms:

2- Intrinsic absorption (fundamental lower limit): electronic absorption band (UV region) & atomic bond vibration band (IR region) in basic SiO2.

3- Radiation defects

Intrinsic Absorption

• Less significant than extrinsic absorption. For a pure (no impurities) silica fiber a low loss window exists between 800 nm and 1600 nm.

• Graph shows attenuation spectrum for pure silica glass.• Intrinsic absorption is very loother forms of loss.• It is for this reason that fibers are made up of silica and optical

communications systems work between about 800 to 1600 nm.

Extrinsic Absorption (metallic ions)• Extrinsic absorption is much more significant than intrinsic• Caused by impurities introduced into the fiber material during

manufacture• Iron, nickel, and chromium• Caused by transition of metal ions to a higher energy level• Modern fabrication techniques can reduce impurity levels below

1 part in 1010.

• For some of the more common metallic impurities in silica fibre the table shows the peak attenuation wavelength and the attenuation caused by an impurity concentration of 1 in 109

Scattering Losses in Fibre• Scattering is a process whereby all or some of the optical power

in a mode is transferred into another mode.• Frequently causes attenuation, since the transfer is often to a

mode which does not propagate well. (also called a leaky or radiation mode).

Bending Loss (Macro bending & Micro bending)

• The curvature of the bend is much larger than fiber diameter. Light wave suffers sever loss due to radiation of the evanescent field in the cladding region.

• As the radius of the curvature decreases, the loss increases exponentially until it reaches at a certain critical radius.

• For any radius a bit smaller than this point, the losses suddenly becomes extremely large.

• Higher order modes radiate away faster than lower order modes.

Macro bending Loss:

Micro bending Loss

• Microscopic bends of the fiber axis that can arise when the fibers are incorporated into cables.

• The power is dissipated through the micro bended fiber, because of the repetitive coupling of energy between guided modes & the leaky or radiation modes in the fiber.

Micro bending Loss:

Thank You