Advantages Of Optical Fiber Engineering Essay

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I owe my deepest gratitude to thank those who made my life become true they are none other than my mother and father who taught me what is good and bad in life.

Although far away from overseas they tried their best to encourage me in every possible way in completing my masters. They are my true mentors.

I would like to extend my regards and special thanks for my supervisor Prof. Nigel Copner whose ideas made me to gain knowledge and guided me in all the ways when ever needed and stood with me to finish the project.

I sincerely thank my supervisor for giving me a chance to work on this project.

This project helped me in learning new techniques by using available sources like library, API's and others.

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An optical fiber is a thin, flexible, transparent fiber that acts as a 'waveguide' or 'light pipe', to transmit light between two ends of the fiber. The fields which are concerned with the design and applications of optical fibers such as applied science and engineering is known as fiber optics.

Optical fibers are widely used in fiber optic communications which allows transmission over longer distances and at higher bandwidths when compared other forms of communication.

Fibers are used because signals travel along them with less loss and also unaffected to electromagnetic interference when we compare to metal wires. They are also used for illumination and are wrapped in bundles so they can be used to carry images which allow viewing in tight spaces.

Structure:

Optical fiber typically consists of a transparent core surrounded by a transparent cladding material with a lower index of refraction. Light is kept in the core by internal reflection. This causes the fiber to act as a waveguide. Multi-mode fibers are the one which support propagation paths or transverse modes and those which can support a single mode are single-mode fibers. Multi-mode fibers generally have a larger core diameter and are used for short-distance communications links and for applications where high power must be transmitted.

fiber_Intro_1

Advantages of optical fiber:

The following are some of the important advantages of an optical fiber

Light weight & Wide bandwidth

Low loss

Noise Immunity

Low Maintenance

Information Handling

Security

Wide Temperature Range

Safety

Non- Obsolence

Material availability

Stable performance

When we come to light weight & wide bandwidth 38 km of fiber can be manufactured from just one kilogram of glass material.

120,960 pairs of copper, carrying voice channels, can be replaced by only one pair of optical fiber.

Practical bandwidth of fiber is many time greater than copper.

fiber_intro_3

Low loss means greater distance between repeaters.

Low maintenance of a typical copper twisted pair is 0.1 GHz

For Typical optical fiber strand it is > 10 GHz

Theoretically Fiber bandwidth it is > 50 Tbps

Fiber optics is completely unaffected to Electrostatic and Electromagnetic interference.

fiber_intro_5

fiber_intro_4

When it comes to security copper can be easily compromised with a tap as fiber is virtually tap resistant.

Fibers can be manufactured to withstand operating temperature variation from - 40 C to + 85 C.

Silica is in abundance throughout the world and is used in manufacturing fibers.

Fiber is often known as Backbone due to following key points

High speed

High capacity

High definition

Raceway congestion

EMI consideration

Security.

HISTORY:

Fiber optics, though used extensively in the modern world, is a fairly simple and old technology. Guiding of light by refraction, the principle that makes fiber optics possible, was first demonstrated by Daniel Colladon and Jacques Babinet in Paris in the early 1840s. John Tyndall included a demonstration of it in his public lectures in London a dozen years later. Tyndall also wrote about the property of total internal reflection in an introductory book about the nature of light in 1870: "When the light passes from air into water, the refracted ray is bent towards the perpendicular. When the ray passes from water to air it is bent from the perpendicular. If the angle which the ray in water encloses with the perpendicular to the surface be greater than 48 degrees, the ray will not quit the water at all: it will be totally reflected at the surface. The angle which marks the limit where total reflection begins is called the limiting angle of the medium. For water this angle is 48°27', for flint glass it is 38°41'.

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Practical applications, such as close internal illumination during dentistry, appeared early in the twentieth century. Image transmission through tubes was demonstrated independently by the radio experimenter Clarence Hansell and the television pioneer John Logie Baird in the 1920s. The principle was first used for internal medical examinations by Heinrich Lamm in the following decade. In 1952, physicist Narinder Singh Kapany conducted experiments that led to the invention of optical fiber. Modern optical fibers, where the glass fiber is coated with a transparent cladding to offer a more suitable refractive index, appeared later in the decade.  Development then focused on fiber bundles for image transmission. The first fiber optic semi-flexible gastroscope was patented by Basil Hirschowitz, C. Wilbur Peters, and Lawrence E. Curtiss, researchers at the University of Michigan, in 1956. In the process of developing the gastroscope, Curtiss produced the first glass-clad fibers; previous optical fibers had relied on air or impractical oils and waxes as the low-index cladding material. A variety of other image transmission applications soon followed.

In the late 19th and early 20th centuries, light was guided through bent glass rods to illuminate body cavities. Alexander Graham Bell invented a 'Photo phone' to transmit voice signals over an optical beam.

It is said that, Jun-ichi Nishizawa, a Japanese scientist at Tohoku University, also proposed the use of optical fibers for communications, in 1963, as stated in his own book published in 2004 in India. Nishizawa invented other technologies that contributed to the development of optical fiber communications as well. Nishizawa later invented the graded-index optical fiber as a channel for transmitting light from semiconductor lasers.

Dispersion In optical Fiber

Material dispersion can be a desirable or undesirable effect in optical applications. The dispersion of light by glass prisms is used to construct spectrometers and spectroradiometers. Holographic gratings are also used, as they allow more accurate discrimination of wavelengths. However, in lenses, dispersion causes chromatic aberration, an undesired effect that may degrade images in microscopes, telescopes and photographic objectives.

The phase velocity, v, of a wave in a given uniform medium is given by

v = \frac{c}{n}

where c is the speed of light in a vacuum and n is the refractive index of the medium.

In general, the refractive index is some function of the frequency f of the light, thus n = n(f), or alternatively, with respect to the wave's wavelength n = n(λ). The wavelength dependence of a material's refractive index is usually quantified by an empirical formula, the Cauchy or Sellmeier equations.

Because of the Kramers-Kronig relations, the wavelength dependence of the real part of the refractive index is related to the material absorption, described by the imaginary part of the refractive index (also called the extinction coefficient). In particular, for non-magnetic materials (μ = μ0), the susceptibility χ that appears in the Kramers-Kronig relations is the electric susceptibility χe = n2 − 1.

The most commonly seen consequence of dispersion in optics is the separation of white light into a color spectrum by a prism. From Snell's law it can be seen that the angle of refraction of light in a prism depends on the refractive index of the prism material. Since that refractive index varies with wavelength, it follows that the angle that the light is refracted by will also vary with wavelength, causing an angular separation of the colors known as angular dispersion.

For visible light, most transparent materials (e.g., glasses) have:

1 < n(\lambda_{\rm red}) < n(\lambda_{\rm yellow}) < n(\lambda_{\rm blue})\ ,

or alternatively:

\frac{{\rm d}n}{{\rm d}\lambda} < 0,

that is, refractive index n decreases with increasing wavelength λ. In this case, the medium is said to have normal dispersion. Whereas, if the index increases with increasing wavelength the medium has anomalous dispersion.

APPLICATIONS:

The following are some of the applications

Optical Fiber Communication

Fiber Optic sensors

OPTICAL FIBER COMMUNICATION

Fiber- optic communication is a technique of transmitting data from one place to another by means of pulses of light through an optical fiber. The light acts carrier wave which modulated to carry information. In telecommunications industry fiber-optic communication systems have played a major role. Optical fibers have largely replaced copper wire communications in core networks in the present developed world because of its advantages over electrical transmission.

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The following are the basic steps for the process of communicating using fiber optics

Creating the optical signal involving the use of a transmitter

Relaying the signal along the fiber

Ensuring that the signal does not become too distorted or weak

Receiving the optical signal and

Converting it into an electrical signal.

HISTORY OF OPTICAL COMMUNICATION:

In 1966 Charles K. Kao and George Hockham proposed optical fibers at STC Laboratories (STL) at Harlow, England, when they showed that the losses of 1000 db/km in existing glass (compared to 5-10 db/km in coaxial cable) was due to contaminants, which could potentially be removed.

Optical fiber was successfully developed in 1970 by Corning Glass Works, with attenuation low enough for communication purposes (about 20dB/km), and at the same time GaAs semiconductor lasers were developed that were compact and therefore suitable for transmitting light through fiber optic cables for long distances.

After a period of research starting from 1975, the first commercial fiber-optic communications system was developed, which operated at a wavelength around 0.8 µm and used GaAs semiconductor lasers. This first-generation system operated at a bit rate of 45 Mbps with repeater spacing of up to 10 km. Soon on 22 April, 1977, General Telephone and Electronics sent the first live telephone traffic through fiber optics at a 6 Mbit/s throughput in Long Beach, California.

The second generation of fiber-optic communication was developed for commercial use in the early 1980s, operated at 1.3 µm, and used In GaAs P semiconductor lasers. Although these systems were initially limited by dispersion, in 1981 the single-mode fiber was revealed to greatly improve system performance. By 1987, these systems were operating at bit rates of up to 1.7 Gb/s with repeater spacing up to 50 km.

The first transatlantic telephone cable to use optical fiber was TAT-8, based on Desurvire optimized laser amplification technology. It went into operation in 1988.

Third-generation fiber-optic systems operated at 1.55 µm and had losses of about 0.2 dB/km. They achieved this despite earlier difficulties with pulse-spreading at that wavelength using conventional In GaAs P semiconductor lasers. Scientists overcame this difficulty by using dispersion-shifted fibers designed to have minimal dispersion at 1.55 µm or by limiting the laser spectrum to a single longitudinal mode. These developments eventually allowed third-generation systems to operate commercially at 2.5 Gbit/s with repeater spacing in excess of 100 km.

The fourth generation of fiber-optic communication systems used optical amplification to reduce the need for repeaters and wavelength-division multiplexing to increase data capacity. These two improvements caused a revolution that resulted in the doubling of system capacity every 6 months starting in 1992 until a bit rate of 10 Tb/s was reached by 2001. Recently, bit-rates of up to 14 Tbit/s have been reached over a single 160 km line using optical amplifiers.

The focus of development for the fifth generation of fiber-optic communications is on extending the wavelength range over which a WDM system can operate. The conventional wavelength window, known as the C band, covers the wavelength range 1.53-1.57 µm, and the new dry fiber has a low-loss window promising an extension of that range to 1.30-1.65 µm. Other developments include the concept of "optical solitons," pulses that preserve their shape by counteracting the effects of dispersion with the nonlinear effects of the fiber by using pulses of a specific shape.

TECHNOLOGY:

An optical transmitter is used to convert an electrical signal into an optical signal to send into the optical fiber in the modern fiber optic communication systems, a cable containing bundles of multiple optical fibers that is routed through underground conduits and buildings, multiple kinds of amplifiers and an optical receiver to recover the signal as an electrical signal. The information which is transmitted is typically digital information generated by computers telephone systems and cable television companies.

Photo detector is the main component of an optical receiver which converts light into electricity using the photoelectric effect. Photo detector is a semiconductor based photodiode.

Optical amplifier is the one which amplifies the optical signal directly without having to convert the signal into the electrical domain which is made by means of doping a length of fiber with the rare earth mineral like erbium and pumping it with light from a laser with a shorter wavelength than the communications signal. Amplifiers have replaced repeaters in new installations.

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TRANSMISSION WINDOW:

Every effect that contributes to attenuation and dispersion depends on the optical wavelength. The wavelength bands that exists where these effects are weakest which are most favorable for transmission. These windows have been characterized and the following are the bands which are defined

MULTIPLEXING:

The process where multiple analog message signals or digital data streams are combined into one signal over a shared medium is multiplexing which is also known as muxing. The main aim this process is to share expensive resource.

Over a communication channel the multiplexed signal is transmitted and which may a physical medium. It divides the capacity of the low-level communication channel into several higher level logical channels.

A reverse process which can extract the original channel on the receiver side is 'demultiplexing'.

A device which performs the multiplexing is called a multiplexer and a device that performs the reverse process is called a demultiplexer.

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Types of Multiplexing:

Multiplexing technologies may be divided into several types all of which have their own significant variations.

Space Division Multiplexing

Frequency Division Multiplexing

Time Division Multiplexing

Code division Multiplexing

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Why Multiplexing

By the observation

1. Most individual data communicating devices typically require modest data rate.

2. Communication media usually have much higher bandwidth.

3. Two communicating stations do not utilize the full capacity of data link.

4. The higher the date rate, the most cost effective is the transmission facility.

When possible:

When the bandwidth of a medium is greater than individual signals to be transmitted through the channel, a medium can be shared by more than one channel of signals by using multiplexing.

For efficiency, the channel capacity can be shared among a number of communicating stations.

Most common use of multiplexing is in long haul communication using coaxial cable. Microwave & optical fiber.

Two basic approaches

1. Frequency Division multiplexing

2. Time division Multiplexing

In Analog we use frequency division multiplexing & Wavelength Division multiplexing.

WDM is the same as the FDM, in WDM it uses large amount of bandwidth which cant be used in FDM.

Operating frequencies are much higher in WDM than FDM.

It is same as the FDM, but applied to fibers. There's great potential for fibers since the bandwidth is huge fibers with different energy bands are passed through a diffraction grating prism combined on the long distance link, Split at the destination and High reliability, very high capacity.

WHY WDM?

1. Optical fiber medium provides enormous bandwidth.

2. WDM is the most viable technology that overcomes the huge opto-electronic bandwidth mismatch.

3. WDM optical fiber network comprises optical wavelength switches/ routers interconnected by point to point fiber links.

4. End users may communicate with each other through all optical WDM channels known as light paths, which may span over more than one fiber links.

Wavelength Division Multiplexing:

How filtering is done in optical domain...

For example as we known using the prism, the following approach is used in optical communication

WDM in Optical Communications

"In fiber-optic communications, wavelength-division multiplexing is a technology which multiplexes multiple optical carrier signals on a single optical fiber by using different wavelengths of laser light to carry different signals. This allows for a multiplication in capacity, in addition to enabling bidirectional communications over one strand of fiber."

Wavelength-division multiplexing is commonly applied to an optical carrier (which is typically described by its wavelength), whereas frequency-division multiplexing typically applies to a radio carrier (which is more often described by frequency). Since wavelength and frequency are tied together through a simple relationship, the two terms describe the same concept.

WDM system uses a multiplexer at the transmitter to join the signals together, and a demultiplexer at the receiver to split them apart. With the right type of fiber it is possible to have a device that does both simultaneously, and can function as an optical add-drop multiplexer. The optical filtering devices used have traditionally been etalons, stable solid-state single-frequency Fabry-Pérot interferometers in the form of thin-film-coated optical glass.

The concept was first published in 1970, and by 1978 WDM systems were being realized in the laboratory. The first WDM systems only combined two signals. Modern systems can handle up to 160 signals and can thus expand a basic 10 Gbit/s fiber system to a theoretical total capacity of over 1.6 Tbit/s over a single fiber pair.

WDM systems are popular with telecommunications companies because they allow them to expand the capacity of the network without laying more fiber. By using WDM and optical amplifiers, they can accommodate several generations of technology development in their optical infrastructure without having to overhaul the backbone network. Capacity of a given link can be expanded by simply upgrading the multiplexers and demultiplexers at each end.

This is often done by using optical-to-electrical-to-optical (O/E/O) translation at the very edge of the transport network, thus permitting interoperation with existing equipment with optical interfaces.

Most WDM systems operate on single mode fiber optical cables, which have a core diameter of 9 µm. certain forms of WDM can also be used in multi-mode fiber cables (also known as premises cables) which have core diameters of 50 or 62.5 µm.

Early WDM systems were expensive and complicated to run. However, recent standardization and better understanding of the dynamics of WDM systems have made WDM less expensive to deploy.

Optical receivers, in contrast to laser sources, tend to be wideband devices. Therefore the demultiplexer must provide the wavelength selectivity of the receiver in the WDM system.

WDM systems are divided in different wavelength patterns, conventional or coarse and dense WDM. Conventional WDM systems provide up to 16 channels in the 3rd transmission window (C-Band) of silica fibers around 1550 nm. DWDM uses the same transmission window but with denser channel spacing. Channel plans vary, but a typical system would use 40 channels at 100 GHz spacing or 80 channels with 50 GHz spacing. Some technologies are capable of 25 GHz spacing (sometimes called ultra dense WDM). New amplification options (Raman amplification) enable the extension of the usable wavelengths to the L-band, more or less doubling these numbers.

CWDM in contrast to conventional WDM and DWDM uses increased channel spacing to allow less sophisticated and thus cheaper transceiver designs. To again provide 16 channels on a single fiber CWDM uses the entire frequency band between second and third transmission window (1310/1550 nm respectively) including both windows (minimum dispersion window and minimum attenuation window) but also the critical area where OH scattering may occur, recommending the use of OH-free silica fibers in case the wavelengths between second and third transmission window shall also be used. Avoiding this region, the channels 31, 49, 51, 53, 55, 57, 59, 61 remain and these are the most commonly used.

WDM, DWDM and CWDM are based on the same concept of using multiple wavelengths of light on a single fiber, but differ in the spacing of the wavelengths, number of channels, and the ability to amplify the multiplexed signals in the optical space. EDFA provide efficient wideband amplification for the C-band, Raman amplification adds a mechanism for amplification in the L-band. For CWDM wideband optical amplification is not available, limiting the optical spans to several tens of kilometers.

Coarse WDM

Originally, the term "coarse wavelength division multiplexing" was fairly generic, and meant a number of different things. In general, these things shared the fact that the choice of channel spacing's and frequency stability was such that erbium doped fiber amplifiers (EDFAs) could not be utilized. Prior to the relatively recent ITU standardization of the term, one common meaning for coarse WDM meant two (or possibly more) signals multiplexed onto a single fiber, where one signal was in the 1550 nm band, and the other in the 1310 nm band.

In 2002 the ITU standardized a channel spacing grid for use with CWDM (ITU-T G.694.2), using the wavelengths from 1270 nm through 1610 nm with a channel spacing of 20 nm. (G.694.2 was revised in 2003 to shift the actual channel centers by 1, so that strictly speaking the center wavelengths are 1271 to 1611 nm. Many CWDM wavelengths below 1470 nm are considered "unusable" on older G.652 specification fibers, due to the increased attenuation in the 1270-1470 nm bands. Newer fibers which conform to the G.652.C and G.652.D standards, such as Corning SMF-28e and Samsung Wide pass nearly eliminate the "water peak" attenuation peak and allow for full operation of all 20 ITU CWDM channels in metropolitan networks. For more information on G.652.C and .D compliant fibers please see the links at the bottom of the article.

The Ethernet LX-4 10 Gbit/s physical layer standard is an example of a CWDM system in which four wavelengths near 1310 nm, each carrying a 3.125 gigabit-per-second (Gbit/s) data stream, are used to carry 10 Gbit/s of aggregate data.

The main characteristic of the recent ITU CWDM standard is that the signals are not spaced appropriately for amplification by EDFAs. This therefore limits the total CWDM optical span to somewhere near 60 km for a 2.5 Gbit/s signal, which is suitable for use in metropolitan applications. The relaxed optical frequency stabilization requirements allow the associated costs of CWDM to approach those of non-WDM optical components.

CWDM is also being used in cable television networks, where different wavelengths are used for the downstream and upstream signals. In these systems, the wavelengths used are often widely separated, for example the downstream signal might be at 1310 nm while the upstream signal is at 1550 nm.

An interesting and relatively recent development relating coarse WDM is the creation of GBIC and small form factor pluggable (SFP) transceivers utilizing standardized CWDM wavelengths. GBIC and SFP optics allow for something very close to a seamless upgrade in even legacy systems that support SFP interfaces. Thus, a legacy switch system can be easily "converted" to allow wavelength multiplexed transport over a fiber simply by judicious choice of transceiver wavelengths, combined with an inexpensive passive optical multiplexing device.

Passive CWDM is an implementation of CWDM that uses no electrical power. It separates the wavelengths using passive optical components such as band pass filters and prisms. Many manufacturers are promoting passive CWDM to deploy fiber to the home.

Dense WDM

Dense wavelength division multiplexing, or DWDM for short, refers originally to optical signals multiplexed within the 1550 nm band so as to leverage the capabilities (and cost) of erbium doped fiber amplifiers (EDFAs), which are effective for wavelengths between approximately 1525-1565 nm (C band), or 1570-1610 nm (L band). EDFAs were originally developed to replace SONET/SDH optical-electrical-optical (OEO) regenerators, which they have made practically obsolete. EDFAs can amplify any optical signal in their operating range, regardless of the modulated bit rate. In terms of multi-wavelength signals, so long as the EDFA has enough pump energy available to it, it can amplify as many optical signals as can be multiplexed into its amplification band (though signal densities are limited by choice of modulation format). EDFAs therefore allow a single-channel optical link to be upgraded in bit rate by replacing only equipment at the ends of the link, while retaining the existing EDFA or series of EDFAs through a long haul route. Furthermore, single-wavelength links using EDFAs can similarly be upgraded to WDM links at reasonable cost. The EDFAs cost is thus leveraged across as many channels as can be multiplexed into the 1550 nm band.

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LIMITATIONS OF WDM:

Future higher-performance systems will be more complex than today's systems since the wavelength domain will be used to help route signals through different static or reconfigurable network paths.

In these next-generation systems, several parameters may vary which would have deleterious effects in a WDM environment, including: variable insertion losses, channel addition and deletion (i.e., add/drop multiplexing), unstable laser power. non- uniform EDFA gain, fast gain transients in EDFA cascades, and non-uniform accumulation of dispersion and nonlinearities. In order to ensure robust system operation, we discuss various dynamic schemes for compensating damaging effects so that these complex systems maintain high performance.

Additionally, optical networks will provide high-speed point-to-point connections and passive wavelength routing, but the true power of optics for high-throughput networking will require advances in high- speed optical switching. Significant functions and capabilities can be enabled by high-speed optical switches and cross-connects, but issues regarding control and routing are ripe for extensive research.

EDFA: An optical-fiber amplifier whose fiber core is lightly doped with trivalent erbium ions which absorb light at pump wavelengths of 0.98 and 1.48 micrometers and emit it at a signal wavelength around 1.5 micrometers through stimulated emission

WDM transmission distance due to cross-phase modulation induced spectral broadening in dispersion compensated standard fiber systems and chromatic dispersion.

TUNABLE FILTERS:

An electric filter in which the frequency of the pass band or rejection band can vary by adjusting its components.

FILTERS IN OPTICAL FIBRE

The red filter which is close to the source blocks some colors which are generated by the source to get a sample.

The second filter which is close to the detector blocks the colors which are not required.

This is how a filter is used in optical fiber.

Transmission based filters use a transfer function description

Iout(λ) = H(λ)Iin(λ)

For almost all optical system the response is Linear so all rules of linear systems apply

H1H2 = H2H1

HTotal = H1H2H3.....

Major difference between filters in electronics and those in optics is that for intensity the phase is not defined. The linear assumption and the phase assumption aren't strictly true.

Three Types of filters

Neutral Density Filter: changes intensity but not wavelength

Color Glass Filter: block some wavelengths but not others.

Interference Filter: very narrow band filters for specific wavelengths.

Limitations in tunable filters:

Acousto-optic tunable filters (AOTF) transmit and process information which modulates wavelength of the light passed through the device. AOTF information possibilities can be described by specific parameters such as information transmission capability. The reasons of these parameter limitations have been analyzed. Among the factors influencing the AOTF transmission capability, such factors as size and form of initial light source as well as features of the medium modulating the light beam by wavelength, can be considered very important. Theoretical considerations clarifying the sources of information losses in AOTF have been stated. A set of experiments has been performed in which the dependences of AOTF information characteristics on the system configuration have been measured. The studied devices were designed with tellurium dioxide single crystals in the configuration providing the best recognition of adjacent wavelengths.

Tunable Filter:

Here we are going to consider 2-port optical tunable filter (optoplex).

Optoplex's Tunable Optical Filter, also known as Optical Tunable Filter or Tunable Band pass Filter, is an integrated module, consisting of micro optics and electronics. When receiving a stream of optical signals of a plurality of wavelengths from the Input-Port (IN), the 2-port tunable optical filter directs a selected channel to the Output-Port (OUT). The selected channel can be varied (tuned) within the operating wavelength (frequency) range.

Optoplex's Tunable Optical Filter is based on a patented micro-optic design and thin-film interference filter coating technology. The thin-film filter used in tunable optical filter is very similar to those already widely used in DWDM devices. The wavelength tuning is achieved by varying the incident angle of the incoming light beam on the thin-film filter. Each single device is optimized to cover either C- or L-band wavelengths. The standard tunable optical filter product family supports 100- and 50-GHz channel-spacing. 

"Here we are going to consider 50GHz channel spacing"

Wavelength Tuning Range C- band 1528-1562 nm

Wavelength tuning Resolution 10 pm THz

Pass band width @ 1 dB >16 GHz

Pass bandwidth @ 3.0 dB 25 GHz

Pass bandwidth @ 20 dB <85 GHz

Peak Insertion Loss (without connector) <5 dB

Polarization Dependent Loss <0.3 dB within CW±5GHz

Polarization Mode Dispersion 0.5 ps

Chromatic Dispersion <± 100 ps/nm within CW ±5GHz

Wavelength setting error <±4 GHz

Wavelength Repeatability <± 1 GHz

Wavelength Temperature Dependence <± 1 pm/c

Return Loss >40 dB

Maximum Input optical power 300mW

Tuning speed <10 sec

Tuning power consumption <1800 (peak); <300(idle) mw

Tuning voltage 5v (DC)

Operating temperature 0 to 65 0C

Storage temperature -40 to 85

OPTICAL FIBER:

Corning SMF-28 single mode fiber is considered as the regular optical fiber for telephony, cable television, submarine and private network applications in the transmission of data, voice and video services.

This fiber is used in the wavelength region '1310 nm' and the information carrying capacity of the fiber is at its highest in this transmission window and it is also where dispersion is the lowest.

SMF-28 fiber is also used effectively in the c- band region 1550 nm wavelength region.

Optical specifications

Maximum attenuation of Wavelength for 1550nm is 0.17- 0.31 dB/ km

Cable cutoff wavelength is less than or equal to 1260 nm

The mode field diameter of wavelength 1550 nm is 10.7± 0.5

Dispersion value for wavelength 1550 nm is <18.0 ps/ (nm*km)

Today and the Future

Corning SMF-28 is the fiber choice for widespread deployment in rapidly growing and vigorously changes in access networks.

Low attenuation combined with comprehensive environmental specification throughout the 1285 nm to 1625 nm wavelength range provide full spectrum capability. Fiber enables flexible network designs such that customers can be confident that they will be able to access networks to the current and emerging wavelength division multiplexing systems increasingly being used for high band width, multiple services or multi- protocol applications.

FIBER DISPERSION:

Dispersion is a measure of the temporal spreading that occurs when a light pulse propagates through an optical fiber. Dispersion is sometimes referred to as delay distortion in the sense that the propagation tome delay causes the pulse to broaden. The broaden pulse overlaps with its neighbors, eventually becoming indistinguishable at the receiver input. This effect is known as intersymbol interference.

INTERMODAL DISPERSION:

Intermodal or modal dispersion arises due to difference in the propagation times of modes with the slowest and fastest velocities. In SI fibers, where each mode has approximately the same velocity, it is the result of varying path lengths among the different modes. This dispersion mechanism creates the fundamental difference in the overall dispersion for the three types of fibers.

There are two ways to reduce the modal dispersion. One is to use so called GI fiber. This type of fiber has a core whose RI is highest on the axis and tapers off roughly parabolically towards the core cladding interface. In such fibers rays do not travel the zig zag paths rather helical path.

There is an alternative to GI fiber for reducing pulse spreading. One can use a fiber with a very small core diameter. In such only a single mode is allowed to propagate. Hence these fibers exhibit the least pulse broadening and have the greatest and have the greatest possible bandwidth.

POLARIZATION DISPERSION:

Single- mode fibers support two orthogonally polarized degenerate modes. In a practical SM fiber, various perturbations in the fiber geometry and composition are present. The perturbations may occur at the time of fiber manufacturing and cabling and are difficult to eradicate. These perturbations plus environmental disturbances such as strain can remove the degeneracy of modes. In that case, orthogonally polarizes modes will have different propagation constants which will lead to pulse broadening. This form of dispersion is called polarization mode dispersion δTp

GAUSSIAN BEAM:

In optics, a Gaussian beam of electromagnetic radiation who's transverse electric field and intensity distributions are well approximated by Gaussian functions.

Light beams where the electric field profile in a plane perpendicular to the beam axis can be described with Gaussian function, possibly with an added parabolic phase profile.

Gaussian beams are the simplest and often the most desirable type of beam provided by a laser source. They are well characterized and the evolution is smooth and easily predicted. The amplitude function representing a Gaussian beam can be deduced from the boundary conditions of the optical resonator where the laser radiation is produced. The geometrical characteristics of the resonator determine the type of laser emission obtained. For stable resonators neglecting a small loss of energy, the amplitude distribution is self-reproduced in every round trip of the laser through the resonator. Unstable resonators produce an amplitude distribution more complicated than in the stable case. Besides, the energy leaks in large proportion for every round trip. For the sake of simplicity, we restrict this first analysis to those laser sources producing Gaussian beams. The curvature of the mirrors of the resonator and their axial distance determine the size and the location of the region showing the highest density of energy along the beam. The transversal characteristics of the resonator allow the existence of a set of amplitude distributions that are usually named as modes of the resonator. The Gaussian beam is the lowest-degree mode, and therefore it is the most commonly obtained from all stable optical resonators.

The Gaussian beam is the simplest case of laser beams actually appearing in practical optical systems. The parameters defined for Gaussian beams are: the width, which informs about the transversal extension of the beam; the divergence, which describes the spreading of the beam in the far field; and the radius of curvature, which explains the curvature of the associated wave front. There also exist some other derived parameters, such as the Rayleigh range, which explains the extension of the beam waist along the propagation axis, and the Guoy phase shift, which describes how the phase includes an extra p phase shift after crossing the beam waist region. Although simple, Gaussian beams exhibit a great variety of realizations when 3-D beams are studied. They can be rotated, displaced, and twisted. To properly evaluate such effects, some other parameters have been defined by accounting for the elasticity of the irradiance pattern, the longitudinal astigmatism, and the twisting of the irradiance profile.

The value q(z) determines the entire wave function at z

The evolution of q describes the propagation of Gaussian Beam through optical systems, including lens and Dielectric interfaces.

REPRESENTATION OF GAUSSIAN BEAM:

The beam has a plane wave front at the beam waist position i.e. Z = 0

Half angle beam divergence of the beam divergence of the beam will be given by

Where l is the wavelength of the light.

Note the smaller the beam waist the larger the divergence

Numerical Aperture:

Multimode optical fiber will only propagate light that enters the fiber within a certain cone, known as the acceptance cone of the fiber. The half-angle of this cone is called the acceptance angle, θmax. For step-index multimode fiber, the acceptance angle is determined only by the indices of refraction:

n \sin \theta_\max = \sqrt{n_1^2 - n_2^2},

where n1 is the refractive index of the fiber core, and n2 is the refractive index of the cladding. OF-na.svg

When a light ray is incident from a medium of refractive index n to the core of index n1, Snell's law at medium-core interface gives

n\sin\theta_i = n_1\sin\theta_r.\

From the above figure and using trigonometry, we get:

\sin\theta_{r} = \sin\left({90^\circ} - \theta_{c} \right) = \cos\theta_{c}\

where  \theta_{c} = \sin^{-1} \frac{n_{2}}{n_{1}}is the critical angle for total internal reflection, since

Substituting for sin θr in Snell's law we get:

\frac{n}{n_{1}}\sin\theta_{i} = \cos\theta_{c}.

By squaring both sides

\frac{n^{2}}{n_{1}^{2}}\sin^{2}\theta_{i} = \cos ^{2}\theta_{c} = 1 - \sin^{2}\theta_{c} = 1 - \frac{n_{2}^{2}}{n_{1}^{2}}.

Thus,

n \sin \theta_{i} = \sqrt{n_1^2 - n_2^2},

from where the formula given above follows.

This has the same form as the numerical aperture in other optical systems, so it has become common to define the NA of any type of fiber to be

\mathrm{NA} = \sqrt{n_1^2 - n_2^2},

Where n1 is the refractive index along the central axis of the fiber. Note that when this definition is used, the connection between the NA and the acceptance angle of the fiber becomes only an approximation. In particular, manufacturers often quote "NA" for single-mode fiber based on this formula, even though the acceptance angle for single-mode fiber is quite different and cannot be determined from the indices of refraction alone.

The number of bound modes, the mode volume, is related to the normalized frequency and thus to the NA.

"Here we are taking NA 0.13 which is 0.999 degrees "

Beam Waist:

http://www.emeraldinsight.com/content_images/fig/0330270307003.png

Here we are taking focal length of ½ mm & beam waist of 4.88µm at 1550nm because were considering c band.

Gaussian beam parameters:

Beam Width:

For a Gaussian beam propagating in free space, the spot size w(z) will be the minimum value w0 at one place along the beam axis known as beam waist.

For a beam of wavelength λ at a distance z along the beam from the beam waist, the variation of the spot size is given by

w(z) = w_0 \, \sqrt{ 1+ {\left( \frac{z}{z_\mathrm{R}} \right)}^2 } \ .

Where the origin of the z-axis is defined, without loss of generality, to coincide with the beam waist, and where

z_\mathrm{R} = \frac{\pi w_0^2}{\lambda}

is called the Rayleigh range.

http://upload.wikimedia.org/wikipedia/commons/thumb/5/53/GaussianBeamWaist.svg/350px-GaussianBeamWaist.svg.png

Rayleigh range and co focal parameter

At a distance from the waist equal to the Rayleigh range zR, the width w of the beam is

w(\pm z_\mathrm{R}) = w_0 \sqrt{2} \,

The distance between these two points is called the co focal parameter or depth of focus of the beam:

b = 2 z_\mathrm{R} = \frac{2 \pi w_0^2}{\lambda}\ .

Radius of curvature

R(z) is the radius of curvature of the wave fronts comprising the beam. Its value as a function of position is

R(z) = z \left[{ 1+ {\left( \frac{z_\mathrm{R}}{z} \right)}^2 } \right] \ .

Beam divergence

The parameter w(z) approaches a straight line for z \gg z_\mathrm{R}. The angle between this straight line and the central axis of the beam is called the divergence of the beam. It is given by

\theta \simeq \frac{\lambda}{\pi w_0} \qquad (\theta \mathrm{\ in\ radians.})

The total angular spread of the beam far from the waist is then given by

\Theta = 2 \theta\ .

Because of this property, a Gaussian laser beam that is focused to a small spot spreads out rapidly as it propagates away from that spot. To keep a laser beam very well collimated, it must have a large diameter. This relationship between beam width and divergence is due to diffraction. Non-Gaussian beams also exhibit this effect, but a Gaussian beam is a special case where the product of width and divergence is the smallest possible.

Since the Gaussian beam model uses the paraxial approximation, it fails when wave fronts are tilted by more than about 30° from the direction of propagation. From the above expression for divergence, this means the Gaussian beam model is valid only for beams with waists larger than about 2λ/π.

LOSSES:

Considering the following

Focal Length of the lens = 0.5 mm

Wavelength (λ) = 1550 nm

Beam waist = 4.88 µm

Diameter 'D' = fÆŸ

Where Ɵ = λ/πωout = 0.099678

ÆŸ in radians = 0.0172

D = 0.0498 mm

GAUSSIAN BEAM COUPLING LOSS

When considering ÆŸ in the above equation we need to vary the ÆŸ to find the coupling loss.

Arc of a second = 4.8481368 * 10-6 rad

Calculating the values for coupling loss by changing arc of the second ÆŸ.

Changing for every 10th arc of second

3.43655e-5

12.7462e-5

30.92895e-5

54.9848

85.91375

123.7158

168.39095

219.9392

278.36055

343.6558

415.82255

494.8632

580.77695

673.5638

773.22325

879.7568

993.148

1113.422

1240.5945

1374.6200

1515.518155

1663.2902

1817.9349

1979.4528

2147.8437

2323.1078

2505.2449

2694.2552

2890.1385

3092.895

By plotting the graph

For Defocusing

When considering z in the above equation we need to vary the z to find the defocusing

Calculating the values for defocusing by changing the value of Z that is by changing the distance of the lens from one to another.

Plotting the graph