Evolution of Optical Fibres

 

Evolution of optical fibres An optical fiber is a flexible, fl exible, transparent strand of very pure glass that acts as a light pipe to transmit light between two ends of the fiber. Optical fibers have a core surrounded by a cladding layer made of dielectric material. The optical signals in the core are confined by establishing a refractive index that is greater than the cladding.

Introduction Fiber-Optic Communication is the most modern and advanced mode of data communication which has very recent roots dating back to not more than 40 years ago. Communication Scientists all over the world were in an incessant search of a wideband wi deband and low-loss medium of data communication which could be used at high data rates with wit h the least amount of lost possibl possible. e. This constant search, for such a medium, led to development of optical fiber communication. 

Attenuation The attenuation of an optical fiber measures the amount of light lost between input and output. Total attenuation is the sum of all losses. Optical losses of a fiber are usually expressed in decibel decibelss per ki lo lome mete terr (dB ( dB /km) . The expression is called the fiber’s attenuation coefficient α  and  and the expression is

where P(z) is the optical power at a position z from the origin, P(0) is the power at the origin. For a given fiber, these losses are wavelength-dependent which is shown in the figure below. The value of the attenuation factor depends greatly on the fiber material and the manufacturing tolerances, but the figure below shows a typical optical fiber ’s attenuation spectral distribution.

 

The typical fused silica glass fibers we use today has a minimum loss at 1550nm.

The signal on optical attenuates due to following mechanisms : (1) Intrinsic loss in the fiber material. (2) Scattering due to micro irregularities inside the fiber. (3) Micro-bending losses due to micro-deformation of the fiber. (4) Bending or radiation losses on the fiber. The first two losses are intrinsically present in any fiber and the last two depend on the environment in which the fiber is laid.  

There are different optical fiber losses mechanisms:   Rayleigh scattering,  ·   Absorption,  ·  Macroscopic and microscopic bends,  ·  Simulated nonlinear scattering.  ·  Material Loss

(a) Due to impurities: The material loss is due to the impurities present in glass used for making fibers. Inspite of best purification efforts, there are always impurities like Fe, Ni, Co, Al which are present in the fiber material. The Fig. shows attenuation due to various molecules inside glass as a function of wavelength. It can be noted from the figure that the material loss due to impurities reduces substantially beyond about 1200nm wavelength. (b) Due to OH molecule: In addition, the OH molecule molec ule diffuses in the material and causes absorption of light. The OH molecule has main absorption peak somewhere in the deep infra-red wavelength region. However, it shows substantial loss in the range of 1000 to 2000nm. (c) Due to infra-red absorption : Glass intrinsically is a good infra-red absorber. As we increase the wavelength the infra-red loss increases rapidly.

Dispersion The maximum bandwidth of an optical fibre is limited due to the spreading of optical pulses. This spreading increases with distance and the pulses overlap each eac h other. After some distance, the overlapping increases to such an extent that the pulses become indistinguishable. This pulse spreading phenomenon is known as dispersion. There are two main categories of o f dispersion: intermodal and intramodal. It is a wavelength dependent phenomenon.

 

Intermodal Dispersion Intermodal dispersion is applicable only to multimode fibres. It is a phenomenon between different modes in an optical fiber. In practice, light waves are composed of number of plane wave components of different frequencies. This group of waves with closely similar frequencies propagate together at group velocity. The different propagating modes have different group velocities; therefore each mode will reach at the end e nd of a fixed distance at a different time. Therefore the transmitted pulse through a multimode fibre spreads.

Intramodal Dispersion Intramodal dispersion dispersion is related to the material properties of an optical fibre. It occurs w within ithin the same mode; hence it is applicable to single-mode single- mode as well as multimode fibre. There are two distinct types of intramodal dispersion: material dispersion and waveguide dispersion.

Waveguide Dispersion

Since fiber cladding has lower refractive index than fiber core, light ray that travels in the cladding travels faster than that in the core. Waveguide dispersion is also a type of chromatic dispersion. It is a function of fiber core size, V-number, wavelength and light source linewidth. While the difference in refractive indices of single mode fiber core and cladding are minuscule, they can still become a factor over greater distances. It can also combine with material dispersion to create a large amount of single mode chromatic dispersion.  

Fiber Non-Linearity When the optical fiber becomes non-linear the pulse propagation gets significantly modified. Also new frequencies get generated gener ated inside the optical fiber. The spectrum of the output signal is not same as that of the input signal. To observe the non-linearity in the bulk medium generally a large optical power is required. Whereas, optical non-linearity can be easily observed inside an optical fiber with very low optical power of few tens of mW. This can be explained as follows: follows: The efficiency of non-linear process is proportional to the optical intensity (Optical power per unit area) and the interaction length. For a bulk medium the efficiency e fficiency is

 

bulk 

  Where

P

  P /    

=

 is the optical power and    is the optical wavelength.

On the other hand the efficiency for the optical fiber is

P =

 a 2   

  fiber 

Where P /  a

a 2

 is the core radius of the fiber and    is the attenuation constant of the fiber. Note that

 is the power density in the fiber core, and 1/    is the effective interaction length on the fiber.

Since    is very small for the optical fiber, the interaction length is few Km of a typical single mode optical fiber. Also since the core radius is very small, few micrometer, the optical intensity inside inside the core is very high. The ratio of the fiber to bulk efficiency (assuming a =

4μ m,



=

0.3dB/km = 3. 3.456 10 5 nepers/m, −

  fiber        bulk     a 2  =

9



10

  = 1550nm ) is

 

That means the non-linear efficiency inside a fiber is about billion times more than that in the bulk medium. It is therefore easy to get non-linearity inside an optical fiber than in a bulk medium. Since the non-linearity affects the signal signal propagation as a whole, whole, there may be two approaches to handle it. 1.  Avoid non-linearity in the system by keeping low optical power levels. 2.  Understand the non-linear signal propagation and make intelligent use of it for increasing the transmission capability of the fiber. The first option does not seem very feasible since the optical power has be increased for long distance communication. Also in a multi-channel system like WDM, even if each channel has has low power, the total power of all channels together is large enough to drive the optical o ptical fiber into non-linear regime. The second option therefore is more appropriate and desirable. This option is interesting and at the same time very challenging because the non-linear pulse propagation is a very complex phenomenon. It requires far more in-depth understanding of the wave propagation. Nevertheless we will investigate some of the aspects of the non-linear pulse propagation on an optical fiber. The saving grace is, the non-linear effects are week and therefore t herefore certain approximations can be rightly made in the analysis.

Kerr Non-linearity

 

  In the presence of intense optical field, field, the induced electric polarization inside the material can be generally written as 

P =  0  (1)  E +  (2) : EE + Where

 

0

 is the free-space permittivity, and  

(1)



(3)

 EEE

+

....  

,  (2) ,  (3)  are the first, second, and third order

susceptibilities of the medium. The susceptibilities are in general tensors. (1)

Quantity 1 +   

 essentially is the dielectric constant of the medium. The first order susceptibility

represents linear property of the medium. The higher order susceptibilities give non-linear effects. These susceptibilities depend upon the (2)

molecular structure of the material. For Pure silica glass, the   

 is very small.

The lowest order non-linear susceptibility which affects the optical fibers is the third order susceptibility (3)

  

. This susceptibility can provide many effects like, like, third harmonic generation, Four Wave mixing

(FWM), and non-linear refraction. Let investigate here the phenomenon of non-linear refraction. Due to third order susceptibility, the refractive re fractive index of the medium can be written as n

=

n0

+

2

n2  | E |    

For an electromagnetic wave the t he Poynting vector (power density) is proportional to the square of the magnitude of the electric field. The refractive index of the medium medium has two components, n0  , the linear 2

refractive index which could be a function of frequency, and n2  | E  | , the non-linear refractive index which is a function of optical power density. This non-linearity is called the Kerr Non-linearity and the effect is called the Kerr-effect. The non-linear index coefficient

n

2

  3.2 1 10 0 For glass  glass  n2 =

2

3 =

12

−

n

8n0

 is related to the third order susceptibility through a relation (3)

  

 

2

m /W .

Signal to noise ratio

 

The signal-to-noise ratio (SNR) of the receiver which w hich is defined as the ratio of the useful signal power in the received signal to the unwanted noise power in the received signal, at the receiver of the data communication system. Optical Signal to Noise Ratio (OSNR) [dB] is the measure me asure of the ratio of signal power to noise power in an optical channel. OSNR is important because it suggests a degree of impairment when the optical signal is carried by an optical transmission system that includes optical amplifiers.

The most common use of the decibel scale occurs for power ratios. For instance, the signal-to-noise ratio (SNR) of an optical or electrical signal is given by  / PN  PN ) ,  , (A.2) SNR = 10 log10(PS  where PS and PN are the signal and noise powers, p owers, respectively. The fiber loss can also  be expressed in decibel units by noting that the loss corresponds to a decrease in the optical power during transmission and thus can be b e expressed as a power ratio. For example, if a 1-mW signal reduces to 1 W after transmission over 100 km of fiber, the 30-dB loss over the entire fiber span translates into a loss of 0.3 dB/km. The same technique can be used to define the insertion loss of any component. For instance, a 1-dB loss of a fiber connector implies that the optical power is reduced by 1 dB (by about 20%) when the signal passes through the connector. The bandwidth of an optical filter is defined at the 3-dB point, corresponding co rresponding to 50% reduction in the signal  power. The modulation bandwidth of ight-emitting diodes (LEDs) in Section 3.2 and of semiconductor lasers in Section 3.5 is also defined at the 3-dB point, at which the modulated powers drops by 50%. Since the losses of all components in a fiber-optic communication systems are expressed in dB, it is useful to express the transmitted and received powers also by using a decibel scale. This is achieved by using a derived unit, denoted as dBm and defined as  power (in dBm) = 10 log10 1 mW where the reference level of 1 mW is chosen simply because typical values of the transmitted power are in that range (the letter m in dBm is a reminder of the 1-mW 1 -mW reference level). In this decibel scale for the absolute ab solute power, 1 mW corresponds to 0 dBm, whereas powers below 1mWare expressed as negative numbers. For example, a 10-W power corresponds to −20 dBm. The advantage of decibel units becomes clear when the power budget of lightwave systems is considered in Chapter 5. Because of the logarithmic nature of the decibel scale, the power budget can be made simply by subtracting various losses from the transmitter power expressed in dBm units.

Repeater span

 

 

span length  length is assumed to be 100 km in the capacity study presented earlier. In The fiber span terrestrial optical networks, span lengths much shorter than 100 km are not unusual. In submarine optical transmission, spans lengths are typically between 50 and 60 km. With shorter span length, the OSNR after transmission over a fixed distance with fixed signal launch power is increased. Based on the above discussion, discu ssion, we can define a figure of merit (FOM) for comparing the th e OSNR obtained after a given transmission distance under a fixed total mean nonlinear phase shift sh ift as, FOM(dB)=−α·L+10log(Aeff ·L/L ·L/Leff ), ), where α is the loss the loss coefficient in units of dB/km, A eff  is  is fiber effective area in μm2, and L and Leff  are  are respectively the span length and effective span length in km. For the SSMF case with α=0.22 dB/km, Aeff =80 =80 μm μm2, L=100 km, we have FOM100 km,SSMF=4.1

dB. For the PSCF case with α=0.17 dB/km, Aeff =120 =120 μm2, L=100 km, we have

FOM100 km,PSCF=9.8 dB, indicating a remarkable FOM gain of 5.7 dB compared with the SSMF case. When the span length is reduced to 80 km, we have FOM 80 km,PSCF=12.3 dB, which is 2.5 dB more than that obtained with 100 km span length. It is useful to link the FOM gain to the increase in transmission distance. Assuming a simplified yet representative scenario where the fiber nonlinear effect is solely sol ely dependent on Φ NL, a FOM gain of G-dB could be used to increase the transmission distance by G/2 dB when signal launch  power is lowered by G/2 dB. This is because the OSNR and Φ NL values after the increased distance with the G-dB FOM gain remain the same as those after the nominal transmission without the FOM gain. Repeater spacing for commercial systems is as follows:

1.  Long-haul systems –  systems –  600  600 km repeater spacing. 2.  Ultra-long-haul-systems Ultra-long-haul-systems –   –  2,000  2,000 km repeater repeater spacing (Raman + EDFA amplifier amplifiers, s, forward error correction coding, fast external modulators). 3.  Metro systems –  systems –  100  100 km repeater spacing. 4.  State-of-the-art in DWDM –  DWDM –  channel  channel spacing 50 GHz, 200 carriers, speed 10 Gb/s, repeater spacing few thousand km.

Bandwidth

 

Bandwidth is referred as the range of frequency of a particular fiber optic cable. In reality, different cables have different bandwidth capacity of transmitting data data from one point to another. another. For instance, there those which are most suitable in carrying large quantity quantity data over long distanc es within a short time (1 second). Consequently, there are those most suited in transmitting large quantity of data over short distances. In this case, th e cable made to transmit data over long distance cannot be used to transmit the same data over a s hort distance and if that is done, then the frequency will lower. For that r eason, it is very important to study t he Bandwidth of a particular fiber optic cable.

Discussion Single mode fiber is used for both interbuilding and intrabuilding backbone cable. At distances up to 3 km, single mode fiber will deliver data rates up to 10 Gbps with a bandwidth of 20Ghz. Its operating wavelengths are 1310 nm and 1550 nm.

Using the ITU standard 50 GHz dense wavelength division multiplexing grid, not only can you easily do 80 x 10 Gbps channels in a single fiber pair, but recent advances in modulation technology mean that with QPSK, 4QAM or 16QAM modulation, 1/80th of a dark fiber pair can carry a 100 Gbps signal in the optical space previously occupied by a single long distance 10 Gbps circuit.

Bandwidth Capacity Capacity of Fiber Optics Optics fiberss are moving toward, a10-Gbps signal has the ability to transmit To give perspective to the incredible capacity that fiber any of the following per second: 1000 books 130,000 voice channels 16 high -definition TV (HDTV) channels or 100 HDTV channels EE4367 Telecom. Telecom. Switching & Transmission Prof. Prof. Murat Torlak 16 high -definition TV (HDTV)channels or 100 HDTV channels using compression techniques. (an HDTV channel requires a much higher  bandwidth than today’s standard television).  television).  

Conclusion The cost of fiber cables varies with the Bandwidth Capacity of Fiber Optic Cable. You can find that, the fiber cables with efficiently and faster Bandwidth capacity is more expensive than others. Research showed that, the greater the frequency and larger the Bandwidth of a fiber optic cable the more efficient is the cable. Since different people have got different reasons to purchase a fiber cable, the variation of Bandwidth has indeed been beneficial to many. Therefore, consider the Bandwidth Capacity of Fiber Optic Cable before purchasing it.