A communicating system is a system that transmits information from one point to another, whether separated by a few kilometres distance or more. Fiber ocular communications are light moving ridge system that use optical fibres for information transmittal from one point to another, which can transport a big sum of information over longer distances compare than that of coaxal overseas telegram or wireless frequences communicating. This is based on the rule of visible radiation which stated that optical signal in glass medium can transport more information than electrical signal or wireless frequences ( RF ) signal can transport in a Cu or a wireless medium. Fiber ocular systems have been developing worldwide since 1980 and its engineering has advanced somewhat in term of public presentation, quality and applications.
Figure 1.1: Basic constellation of fiber ocular system.
The three basic elements of the fiber ocular system are transmitter, optical overseas telegram and receiving system as shown in Figure 1.1.
The sender converts an electrical signal into a light signal. The light beginning, typically a light breathing rectifying tube ( LED ) or a optical maser rectifying tube, performs the existent signal transition.
The optical overseas telegram is the transmittal medium for transporting the visible radiation.
The receiving system will accepts the light signal and converts it back into an electrical signal which is indistinguishable to the original signal Federal into the sender in most instances.
The sender and receiver circuitry can be really simple or complex depending on the application of the fiber ocular system. For more complex communications system, constituents such as multiplexer, optical amplifier and optical switch are normally added in the system. However, the sender, optical overseas telegram and the receiving system are the basic component in all fiber ocular communicating system.
Human has depended on visible radiation, particularly in signifier of fire since earliest times to direct message. For illustration, in Middle East ‘s antediluvian, human usage fire to bespeak the beginning of every month. In 1870, a British physicist John Tyndall found that visible radiation used the principal of entire internal contemplation to follow a specific way. He demonstrated an experiment utilizing a beam of visible radiation and jet of H2O that flowed from one container to other container. During 1960s, after the innovation of Ruby optical maser by T. Maiman, the involvement of scientist and physicist in optical communicating perked up. In 1964, Dr. Charles K. Kao identified a theoretical specification for long-range communicating devices, the 10 or 20 dubnium of light loss per kilometer criterion. Dr.A Kao besides demonstrated that a purer signifier of glass can assist cut down the light loss. In 1966, Dr. Kao invented optical wave guide with loss 1000dB/km.
In 1970, the first fibre ocular wire or optical wave guide fibres with fading of 20dB/km was invented by a squad of research worker from Corning Glass Corporation. They are Robert Maurer, Donald Keck, and Peter Schultz. The squad invented a Single Mode Fiber ( SMF ) with 17dB/km loss at wavelength 633 nanometer by doping the fibre nucleus with Ti component. In June 1972, they developed a Multi Mode fiber ( MMF ) , doped with Ge, with loss of 4dB/km. This germanium-doped fibre is much greater strength compared to titanium-doped fibre. In 1973, John MacChesney invented a chemical procedure for fiber industry at Bell Labs which lead to the commercial industry of fiberoptic overseas telegram. Today, optical fibre with fading even less than 0.16dB/km can be found.
By April of 1977, the universe ‘s first unrecorded telephone traffic through a fiberoptic system running at 6 Mbps was developed by General Telephone and Electronics. They tested the system in Long Beach of California. Soon in May 1977, Bell introduced an optical telephone communicating system which was installed in the business district Chicago country, covering a distance of 2.4 kilometres. Nowadays, most of the universe ‘s long-distance information is carried over optical-fiber overseas telegrams. Broadband web offering synergistic communicating services are predicted to be commonplace by the twenty-first century.
The application of fiber ocular communicating started around 1975 where the earliest fibre ocular systems so called “ first window ” in silicon oxide based optical fibre, were developed at wavelength near 850nm, as shown in Figure 1.2. This first window was attractive due to the engineering of low cost infrared LEDs and low cost Si sensors that already available at that clip. These systems were available commercially in 1980 after several field tests but become less attractive as engineering progressed due to its comparatively high loss bound of about 3dB/km. As a consequence, the system with low loss and low scattering at 1330nm part has been introduced. This part which has lower fading around 0.5dB/km normally called the “ 2nd window ” . The “ 3rd window ” has been developed by Nippon Telegraph and Telephone in late 1977. This 3rd window operated at 1550nm offers the theoretically minimal optical loss around 0.2dB/km.
Although the 3rd window have minimized the optical signal fading in optical fibres and becomes the most involvement for long-haul and transmittal web, fiber ocular communications utilizing first and 2nd Windowss are still being deployed. First window are being used for short distance transmittal, particularly in inter-premises and Local Area Network transmittal because of its cost effectivity, while 2nd window are being used for short-haul optical web.
Figure 1.2: Fiber Optic communications runing Windowss.
Optical amplifiers were developed during the 1980s to get the better of the loss restriction job in long haul system of fiber ocular communicating. Old ages back, the used of optoelectronic repeaters to get the better of such job become complex and expensive after wavelength-division multiplexed ( WDM ) systems have been introduced in fiber ocular communicating. Optical amplifiers amplify the optical signal without require any electrical signal transition compared to optoelectronic repeater which an optical signal are converted back into electric signal by photodiode, the electrical signal is amplified and so regenerated once more utilizing sender. Optical amplifiers become widely used in long haul system during the 1990s.
There are two major categories of optical amplifiers in usage today which are semiconducting material and active fibre.
A semiconducting material optical amplifier ( SOA ) is an active medium of a semiconducting material optical maser. SOA is besides known as a optical maser rectifying tube without optical feedback. SOA operates in 1310nm and 1550nm transmittal window and it is electrically pumped by injected current. SOA is typically constructed in a little bundle ; hence it can cut down size of the system. This is one of the advantages of SOA over EDFA. However, SOA besides has its disadvantage which is high-coupling loss and a higher noise figure.
A doped fibre amplifier ( DFA ) is different from SOA. Doped fibre amplifiers have advanced more quickly due to their high-output impregnation power, high addition, polarisation insensitiveness, and long aroused province life-time that reduces noise [ 4 ] . Doped fibre amplifiers have been successfully used in the 1550nm fibre transmittal window but are non ideally suited for the 1310nm fibre transmittal window. The most popular doped fiber-type optical amplifier is the Erbium-doped fibre amplifier ( EDFA ) which has been widely used in WDM fiber ocular communicating system.
Optical amplifiers are categorized into three functional types: supporter, in-line amplifier, and preamplifier as shown in Figure 1.3.
A supporter, shown in Figure 1.3 ( a ) , is a power amplifier that amplifies a transmitter signal before directing it through a fibre to a receiving system. It is placed straight after the optical sender. This type of elaboration green goodss maximal end product power and improves a wide scope of sender features. Since the incoming signal has big signal-to-noise ratio ( SNR ) , the noise produces by the amplifier is non high.
An in-line amplifier operates in the center of the fiber ocular nexus as shown in Figure 1.3 ( B ) . It is besides known as in-line repeater. This type of amplifier give the stableness over the WDM bandwidth, maintaining the noise at the minimal degree and executing good optical interaction with transmittal fibre. The little input signal is modified and so boosted for retransmission down the fibre.
A preamplifier amplifies a signal before it reaches the receiving system as shown in Figure 1.3 ( degree Celsius ) . Year back, receiving system with sensitiveness of -30dBm was acceptable. However, presents, the demands require the sensitiveness of receiving system should be around -40dBm or -45dBm. Hence, the preamplifier is introduced. This type of elaboration plants with weaker signal and extremely unmaskings to greater noise. The noise produced by the amplifier must be minimum to maximise the standard SNR.
Figure 1.3: Functional types of optical amplifiers: ( a ) Supporter ; ( B ) in line amplifier ; ( degree Celsius ) preamplifier.
The figure of preamplifiers and supporters used for a specific web system depends on the figure of senders and receiving systems utilizations in the system while the figure of in line amplifiers needed is determined utilizing both length of fibre and web constellation. Supporters, in line amplifiers and preamplifiers normally imply the usage of EDFAs.
Chapter 1: Introduction
This chapter gives an overview of fiber ocular communicating system from the position of the of import elements in fiber ocular system and the history of optical communicating. The development of optical transmittal window and the development of optical amplifier besides discussed here. At the last of the chapter, aim of the research and the organisation of thesis are presented.
Chapter 2: Theory of EDFA and EYDFA
Erbium-Doped Fiber Amplifier ( EDFA ) is one of the optical amplifiers which amplify signals in the 3rd window at 1550nm part. Old ages back, fiber ocular communicating system used regenerative repeaters as shown in Figure 2.1 ( a ) , to hike the signal which were really complex and expensive and lead to losingss restriction job. In 1985, a squad of research worker from University of Southampton illustrated that optical fibre can exhibit laser addition at wavelength 1550nm [ 5 ] . These fibres were to a great extent doped with the rare earth component known as Er. Erbium ions ( Er3+ ) have some nature belongingss which related to radiative decay with a long de-excited life-time, approximate to 10ms, in a glass host. Their 1550nm lasing wavelength work stoppages on the 3rd window spectral part where the lowest losingss performed in optical fibres. Nowadays, Er doped fiber amplifier ( EDFA ) is really of import to optical communicating system in order to direct an information over a big distance.
The development of EDFA application and engineering has brought to the development and betterment of wavelength-division multiplexed ( WDM ) fiber ocular communications systems. With the development of EDFAs, the losingss restriction job occurred in old optical system could be overcome by straight magnify the signal utilizing EDFA without any conversional repeater as shown in Figure 2.1 ( B ) .
( a )
( B )
Figure 2.1: ( a ) Optical communicating system with regenerative repeaters ;
( B ) Optical communicating system with erbium-doped fibre amplifiers ( EDFAs ) .
The basic EDFA includes an active fibre, a pump optical maser, a WDM coupling and an isolator with a filter as shown in Figure 2.2.
Erbium-doped fibre is an active medium where its nucleus is to a great extent doped with Erbium ( Er3+ ) ions. Higher concentration of Er3+ ion in silica fibre is needed since the elaboration procedure is done by these ions. To accomplish higher figure of elaboration efficiency, the maker reduces the nucleus diameter of the erbium-doped silicon oxide fibre and concentrates the Er3+ ions in the centre of the nucleus [ 2 ] .
Pump optical masers are semiconductor optical maser rectifying tubes which have high radiation power as their basic characteristic. This optical maser radiates either at 980nm or at 1480nm, or both.
WDM coupling combines a pump with the signal wavelength into a fibre.
An isolator prevents the amplifier fibre from reflected visible radiation while the filter flats the addition and removes the pump wavelength from the end product signal.
A coupling, an isolator, and a filter are known as inactive constituents. These inactive constituents are necessary for fiber ocular communicating system, and assist bettering the public presentation of the EDFA unit. Figure 2.2 ( a ) and ( B ) show the two general constellation of an EDFA which are co-propagating pump constellation and counter-propagating pump constellation severally.
Figure 2.2: Erbium-doped fibre optical amplifiers ( a ) Co-propagating pump ;
( B ) Counter-propagating pump.
Pumping is done by a pump optical maser radiating at 980nm or 1480nm, or both while the information signal is transmitted at 1550nm. Optical information signal and pumping signal are joined together in one fibre utilizing coupling and so propagate together along the active fibre. At this point, the pump signal losingss its power while the information signal is amplified. This is because the information signal receives power from the pumping signal. A pumping signal can co-propagate or counter-propagate with an information signal.
A co-propagating pump, or sometimes known as forward pumping provides lower end product power and lower noise.
A counter-propagating, besides known as backward pumping features higher end product power but greater noise.
Bi-directional pump which consists both co-propagating and counter-propagating is normally used for commercial amplifier. To take the residuary pumping visible radiation from the transmittal fibre, the 2nd coupling is needed. An isolator blocks the backreflected visible radiation from come ining the active fibre in order to forestall the visible radiation from being amplified, therefore prevent the amplifier from working as a optical maser. A filter separate any light power that remains in the information signal.
Amplification in an EDFA occurs through the mechanism of stirred emanation where the signal from the pump excites Er3+ ions to the upper energy province while the information signal stimulates passage of the aroused Er3+ ions to the lower energy province, ensuing radiation of photon with the same energy and wavelength the input signal has. Figure 2.3 shows the energy diagram of Er3+ ions in EDFA. Free Er3+ ions exhibit distinct energy degrees. When the Er3+ ion are doped into a fibre, their energy degrees splits into a figure of closely related degrees which are considered as energy set. This will gives the EDFA the ability to magnify a set of wavelength. It besides eliminates the demand to ticket melody a pumping wavelength [ 2 ] .
The constellation of the EDFA is shown in Fig. 1 ( a ) . A piece of EDF of 17m long is used as a addition medium. The experiment is besides carried out utilizing a backward pumping strategy. Backward pumping is obtained by altering the location of MMC to the point after the EDF section.. For comparing intent, the experiment is besides repeated for double-pass EDFA constellation as shown in Fig. 1 ( B ) . % .
By pumping the EDF with 1440nm optical maser rectifying tube, the Er ions are excited to the upper degree province ( 2F5/2 ) . The energy of the Yb ions is following transferred to erbium ions, which are excited to the 4I11/2. The aroused Erbium ion decays nonradiatively to make 4I13/2 degree, which is the metastable degree. Stimulated emanation is prompted by the reaching of the input visible radiation, an extra photon is created with the same optical stage and way as the incident photon and therefore elaboration is achieved. Spontaneous decay of the aroused ions which do non take part in elaboration becomes a beginning of optical noise. This noise gets amplified along with the incident visible radiation and consequences in amplified self-generated emanation ( ASE ) . The EDFA operates in impregnation and hence the input signal power is fixed at -10dBm and 5dBm in this experiment. The input signal is provided by a tunable erbium-doped fibre optical maser that runing in C-band part. An optical circulator is used in double-pass EDFA to route the amplified signal into the power metre ( or optical spectrum analyzer, OSA ) for optical power ( or optical spectrum ) measuring.
( a )
( B )
Figure 3.1: Experimental set-up for the EYDFA ( a ) single-pass with a forward pumping strategy ( B ) double-pass constellation.
Figure 3.2 shows the attenuated end product spectrum at different constellations of 17m long EDF. The single-pass EDFA of 17m long with the backward pumping shows the lowest ASE degree which indicates the lowest noise figure. The double-pass constellation shows the highest power of ASE, which translates to the highest noise figure. The noise figure in figure increase is due to the double-pass strategy which increases the ASE degree at the input portion of the amplifier and therefore increases the noise of the amplifier. The peak power to resound degree ratio is observed to be more than 15dB as shown in the inset figure. This indicates that the measured end product power is chiefly from the peak part of the amplified signal.
High power doped fibre amplifiers are happening applications as supporter amplifier for long draw repeaterless optical links, head terminal amplifiers for CATV distribution architectures and as amplifiers for 1xN lossless splitters [ 1-5 ] . The high end product power can be achieved by researching efficient, high power pump visible radiation beginnings, co-dopants and fresh fibre designs for effectual soaking up of pump power [ 6-10 ] . Optimization of fiber stuff to better the effectivity of soaking up can be achieved by co-doping Ytterbium ions into the erbium-doped fibre as a sensitiser and a dual facing fibre construction for spread outing the aperture of the pump light [ 11 ] . Such a fibre is called Erbium-Ytterbium doped fiber ( EYDF ) and has several advantages [ 12-13 ] . Co-doping with Ytterbium prevents the formation of Erbium bunchs and efficaciously controls the up transition from 4I13/2 degree ; a higher doping degree can therefore be used. The brace induced energy transportation from Ytterbium to erbium provides an efficient indirect pumping mechanism. Ytterbium ions, which have a high soaking up at wavelengths between 900 nanometers and 1000 nanometer, absorb pump visible radiation and reassign their energy to erbium ions through a non-radiative cross-relaxation consequence. The dual clad construction increases the sum of pump power coupled to the fibre. The first facing provides a multimode wave guide for the pump visible radiation, therefore leting the usage of high power multiple laser rectifying tubes for pumping. The pump visible radiation propagates in the fibre and is absorbed in the nucleus part for the elaboration of the signal.
In an earlier work, Sun et. Al. [ 12 ] demonstrated a semiconducting material optical amplifier ( SOA ) based multi-wavelength fiber optical maser utilizing a fibre cringle mirror which is fabricated utilizing a double-clad EYDF. In another work, Peng et. Al. [ 13 ] demonstrated a pulse fibre optical maser with an end product power of 6W utilizing a double-clad Erbium Ytterbium doped fiber amplifier ( EYDFA ) . The EYDFA operates expeditiously utilizing multi-stage constellations. In this research, a high power Erbium Ytterbium doped fiber amplifier ( EYDFA ) is demonstrated utilizing a dual clad EYDF in concurrence with 927nm pumping. The double-pass strategy is proposed in this work to increase the pump transition efficiency of the amplifier alternatively of utilizing multi-stage constellation. The double-pass operation is achieved utilizing a broadband fibre Bragg grate ( FBG ) .
The constellation of the proposed high power EYDFA is shown in Fig. 1 ( a ) . A piece of double-clad EYDF of 10m long is used as a addition medium. The EYDF has a star form inner cladding with the nucleus and outer cladding diameters of 6 and 130 Aµm severally. The inner facing has an soaking up coefficient of about 0.5dB/m at 915 nanometer for Ytterbium ion. The Er extremum soaking up is 40dB/m at wavelength of 1535 nanometer. Pump beams from 927nm multimode laser rectifying tube are launched into the EYDF via multimode combiner ( MMC ) . The splice part between the EYDF and MMC is covered by low-index gel so that the multimode pump beam is efficaciously guided into the facing of the EYDF. The experiment is besides carried out utilizing a backward pumping strategy which is obtained by altering the location of MMC to the point after the EYDF subdivision. Optical isolator is used in the single-pass constellations to forestall the specious contemplation from hovering in the system. For comparing intent, the experiment is besides repeated for double-pass EYDFA constellation as shown in Fig. 1 ( B ) . A broadband FBG is merger spliced to end product terminal of the EYDF to move as a reflector for the trial signal. The FBG used operates at wavelength part from 1527nm to 1567nm with the mean coefficient of reflection of about 100 % .
By pumping the EYDF with 927nm optical maser rectifying tube, the Yb ions are excited to the upper degree province ( 2F5/2 ) . The energy of the Yb ions is following transferred to erbium ions, which are excited to the 4I11/2. The aroused Erbium ion decays nonradiatively to make 4I13/2 degree, which is the metastable degree. Stimulated emanation is prompted by the reaching of the input visible radiation, an extra photon is created with the same optical stage and way as the incident photon and therefore elaboration is achieved. Spontaneous decay of the aroused ions which do non take part in elaboration becomes a beginning of optical noise. This noise gets amplified along with the incident visible radiation and consequences in amplified self-generated emanation ( ASE ) . The EYDFA operates in impregnation and hence the input signal power is fixed at -10dBm and 5dBm in this experiment. The input signal is provided by a tunable erbium-doped fibre optical maser that runing in C-band part. An optical circulator is used in double-pass EYDFA to route the amplified signal into the power metre ( or optical spectrum analyzer, OSA ) for optical power ( or optical spectrum ) measuring.
Multimode 927nm optical maser rectifying tube
Power Meter /OSA
Splice point coated by low index polymer
Multimode 927nm optical maser rectifying tube
( a )
( B )
Figure 4.1: Experimental set-up for the EYDFA ( a ) single-pass with a forward pumping strategy ( B ) double-pass constellation.
Figure 4.2 shows the end product power spectra of the amplified signal against the signal wavelength of the EYDFA for both single-pass and double-pass constellations. The input signal and 927nm pump power are fixed at -10dBm and 3.1W severally. For the single-pass constellations, the backward pumping provides higher end product power than the forward pumping because the stronger pump power at the end product delays the oncoming of addition impregnation. On the other manus, the dual base on balls EYDFA has a higher addition and end product power compared to the single-pass due to the double-propagation of the trial signal in the addition medium. The level addition of 33.5dB is observed with a addition fluctuation of less than 1dB at wavelength part from 1545 to 1566nm as shown in Figure 4.2. The maximal end product power of 23.6dBm that corresponds to the addition of 33.6dB is obtained at wavelength of 1557.4nm.
Figure 4.3 shows the end product power against the input pump power for the double-pass EYDFA at assorted input signal wavelengths and powers. As shown in Figure 4.3, the higher input signal power, the higher end product is obtained. As the input signal additions from -10dBm to 5dBm, the maximal end product power additions from 320mW to 390mW. The maximal end product power of 390mW is obtained at the maximal pump power of 4.1W at 1557.4mW. This translates to a power transition efficiency of approximately 10 % . Inset of Figure 4.3 shows the attenuated end product spectrum at different constellations. The single-pass EYDFA with the forward pumping shows the lowest ASE degree which indicates the lowest noise figure. The double-pass constellation shows the highest power of ASE, which translates to the highest noise figure. The noise figure increase is due to the double-pass strategy which increases the ASE degree at the input portion of the amplifier and therefore increases the noise of the amplifier. The peak power to resound degree ratio is observed to be more than 15dB as shown in the inset figure. This indicates that the measured end product power is chiefly from the peak part of the amplified signal. The proposed EYDFA can be used to construct oculus safe high power light beginning so called maestro oscillator power amplifier ( MOPA ) that is besides utile for short optical maser pulse elaboration.
Figure 4.2: The end product power of the amplified signal against the signal wavelength
Figure 4.3: The end product powers of the amplified signal against 927nm pump powers. Inset shows the end product spectrum of the signal at different constellations.