Our current information society reflects an unstoppable trend of increasing demand of transmission bandwidth in communication networks. This bandwidth is limited by the design and topology of the communications network. In the case of optical fiber networks there are several factors limiting the speed: distance of transmission, cable design, external physical factors, type of optical fiber, etc.. By increasing the transmission speed of WDM optical networks to values of 40 Gbps and above, limitations due mainly to the following factors take importance

•    Attenuation (dB/Km)
•    Chromatic dispersion (CD)
•    Polarization Mode Dispersion (PMD)

The higher the transmission rate, the lower the tolerance to these factors, although there are different methods of compensation that will be discussed later.

Types of single-mode fiber optics

The ITU-T (International Telecommunication Union - Telecommunication Standardization Sector) standardizes both descriptions of single-mode and multimode fiber as the definitions of parameters and associated measurement test. The optical characteristics, geometry and single-mode fiber transmission systems used in long distance communication commonly use G.652 and G.655 fibers.

G.652 Single-mode fiber

Fibers are optimized for the use around 1310nm, but can also transmit in other wavelengths. G.652.A and B fibers have an attenuation peak around 1383nm due to the presence of OH, while the G.652.C and D are free from this peak. The most recent revisions, March 2003 and May 2005, created two new categories to reduce the PMD 0.20ps/√km of the link to be able to offer transmission rates to offer high speed/distance. Also were adjusted the tolerances of some parameters to improve the performance of optical fibers.

G.652.B fiber features			Bandwidth of the transmission
Modal field diameter	Wavelength	1310 nm	1310 nm, 1550 and 1625 nm    (Bands O, C and L)
	Range	8,6 - 9,5 µm
	Tolerance	± 0,6 µm
Chromatic dispersion slope of 1300 - 1324 nm	S0max	0,092 ps/nm2.km
Cable features
Wavelength cable cutting	Maximum	1260 nm
Attenuation coefficient	Max at 1310 nm	0,40 dB/Km
	Max at 1550 nm	0,35 dB/Km
	Max at 1625 nm	0,40 dB/Km
PMD coefficient	M	20 cables
	Q	0,01%
	Max PMDq	0,20 ps/ vk
G.652.D fiber features			Bandwidth of the transmission
Modal field diameter	Wavelength	1310 nm	Wide coverage: bands O to L. Similar to G.652.B but allows the extended transmission bandwidth from 1360nm to 1530nm. Suitable for CWDM
	Range	8,6 - 9,5 µm
	Tolerance	± 0,6 µm
Chromatic dispersion slope of 1300 - 1324 nm	Range	0,092 ps/nm2.km
Cable features
Wavelength cable cutting	Maximum	1260 nm
Attenuation coefficient	1310 a 1625 nm	0,40 dB/Km
	1383 ± 3 nm	*
	1550 nm	0,30 dB/Km
PMD coefficient	M	20 cables
	Q	0,01%
	Max PMDq	0,20 ps/ √km

* Attenuation detected should be less than or equal to the value specified for the interval 1310nm to 1625nm after the hydrogen aging process according to CEI 60793-2-50 category related to the fiber

Chromatic dispersion in G.652 fibers.

In this graph we see the limits of chromatic dispersion for G.652 fibers depending on the wavelength. It is defined solely by the bands O(1260-1360nm) and C(1530-1565nm), these fibers can be used in L band (1565-1625nm) for DWDM and S+C+L (1460-1625nm), CWDM systems.

Chromatic dispersion for G.652 fiber.

G.655 single-mode fiber

The mechanical geometrical and transmission properties of the non-zero dispersion-shifted (NZDS) fiber are contained in the ITU-T recommendation G.655. These fibers are designed to transmit in the third window with low values of D, between 1530nm and 1565nm, although it is hoped that they can support transmission at wavelengths greater than 1625 and lower then 1460nm.

The chromatic dispersion (CD) is a parameter that limits the transmission capacity of fiber optics. In communication systems high-speed and long distance it is essential to control its effect, because it seriously limits the transmission capacity of the system and this requires systems using chromatic dispersion compensation.

Although chromatic dispersion is reduced in the transmission window of the NZDS fibers, it can never be zero at this wavelength range (but it can at shorter wavelengths). This is because a zero value of D power nonlinear effects, which is essential to avoid in WDM applications. The appearance of nonlinear phenomena also depends on the effective area of the fiber. These are minimized with larger effective areas.
   
ITU-T Recommendation G.655 collects different subtypes NZDS fiber, which differ mainly in the following parameters: chromatic dispersion, modal field diameter (MFD) and PMD. It is a more recent recommendation that allows more variation of the parameters than the G.652, in which subtypes are clearly distinct and standardized.

The March 2003 revision created a new category with reduced link PMD 0.20ps/√km to adjust the standards to the emerging needs of higher transmission speed and distance. In the February 2006 revision the tolerances of some parameters are reduced, as well as maximum and minimum values to limit the DC between 1530 and 1565nm.

G.655.A fiber features
Modal field diameter		Wavelength		1550 nm
		Range		8 - 11 µm
		Tolerance		± 0,7 µm
Chromatic dispersion coefficient of 1530 - 1565 nm		λmin and max		1530 - 1565 nm
		Dmin		0,1 ps/nm.km
		Dmax		6,0 ps/nm,km
Wavelength cable cutting		Maximum		1450 nm
Attenuation coefficient		Max at 1550 nm		0,35 dB/Km
PMD coefficient		M		20 cables
		Q		0,01%
		Max PMDq		0,50 ps/ √km
G.655.B fiber features
Modal field diameter		Wavelength		1550 nm
		Range		8 - 11 µm
		Tolerance		± 0,7 µm
Chromatic dispersion coefficient of 1530 - 1565 nm		λmin and max		1530 - 1565 nm
		Dmin		1,0 ps/nm.km
		Dmax		10,0 ps/nm,km
		Dmax - Dmin		= 5,0 ps/nm.km
Attenuation coefficient		Max a 1550 nm		0,35 dB/Km
		Max a 1625 nm		0,4 dB/Km
PMD coefficient		M		20 cables
		Q		0,01%
		Max PMDq		0,50 ps/ √km
G.655.C fiber features
Modal field diameter		Wavelength		1550 nm
		Range		8 - 11 µm
		Tolerance		± 0,7 µm
Chromatic dispersion coefficient of 1530 - 1565 nm		λmin and max		1530 - 1565 nm
		Dmin		1,0 ps/nm.km
		Dmax		10,0 ps/nm,km
		Dmax - Dmin		= 5,0 ps/nm.km
Attenuation coefficient		Max a 1550 nm		0,35 dB/Km
		Max a 1625 nm		0,4 dB/Km
PMD coefficient		M		20 cables
		Q		0,01%
		Max PMDq		0,20 ps/ √km
G.655.D fiber features
Modal field diameter	Wavelength		1550 nm
	Range		8 - 11 µm
	Tolerance		± 0,6 µm
Chromatic dispersion coefficient of 1530 - 1565 nm	Dmin(λ):1460-1550 nm		7.00/90 (λ-1460)-4.2
	Dmin(λ):1550-1625 nm		2.97/75 (λ-1550) +2.80
	Dmax(λ):1460-1550 nm		2.91/90 (λ-1460) +3.29
	Dmax(λ):1550-1625 nm		5.06/75 (λ-1550) +6.20
Wavelength cable cutting	Maximum		1450 nm
Attenuation coefficient	Max a 1550 nm		0,35 dB/Km
	Max a 1625 nm		0,4 dB/Km
PMD coefficient	M		20 cables
	Q		0,01%
	Max PMDq		0,20 ps/ √km

Much of the installed fiber optic network currently operates over single mode fiber G.652. This standard has been improving over time and the fibers that are manufactured today are much more optimized than the ones installed in the first networks. To achieve a bandwidth of high transmission new modules are being used, compensation systems and G.655 fibers, dispersion shifted, designed to transmit in the third window. The following image shows several G.655 fiber available, with corresponding differences in the value of CD565 nm.

Spectral Attenuation

It is the power loss depending on the wavelength, the factors which cause attenuation are absorption, scattering and radiation. Although the optical fiber is manufactured using high purity silicon, it may show small impurities and imperfections that make part of the power to be lost or become another type of energy.
   
The most important impurity is OH ions, from poor disposal of water. Because of this ion, a resonant frequency at 1390nm appears, making the attenuation increases at this point. In current commercial fibers this dispersion peak has been lowered, they are the LWP (Low Water Peak) fibers are under ITU-T recommendation G.652.D, as seen in the image below.

Spectral attenuation in single-mode fibers G.652.A&B (SM Conventional) y G.652C&D (SM LWP)

The Signal Noise Ratio (SNR) may vary depending on the technology used by each network, but in general this rate must increase as increasing the rate of transmission, as the following graph: 

The SNR for a system at 40 Gbps is about 12 dB, while for 160 Gbps is required SNR of 18 dB. This means working with major powers on the network and minimizing at the maximum possible the losses.
    
The G.652 and G.655 optical fibers have the following typical losses:
•    Fiber Attenuation:        G.652   0,19 dB/Km

                      G.655   0,20 dB/Km

•    Splice Attenuation:     G.652   0,20 dB

                               G.655   0,30 dB

Accordingly, G.652 fiber has slightly lower losses in terms of power in a fiber optic network, and helps to ensure the necessary SNR rate in high speed networks.

To transmit at higher speeds, greater SNR must be achieved, increasing the transmission power, which means increasing nonlinear effects (SPM and FWM) in fibers. This will unwanted increase cause worse performance of the fibers and constraints in the transmission speed.

The two non-linear effects that mainly affect WDM transmission in fiber optics are:
•    FWM (Four Wave Mixing): WDM crosstalk, this phenomenon causes the appearance of new waves at other frequencies. It influences mostly in the 0 dispersion points. It affects more to G.655 fibers, especially in the smaller effective area (broad effective area fiber to minimize this effect).
•    SPM (Self Phase Modulation): This phenomenon arises because the refractive index of the fiber has an intensity-dependent component. The nonlinear refractive index induces a phase shift that is proportional to the intensity of the pulse. Thus, the different parts of the pulse undergo different phase shifts (chirp), which modify the effects of dispersion on the pulse.

G.652 fibers have better behavior against FWM and worse with PMS, the G.655 have backward behavior. The deterioration of the signal by nonlinear phenomena is similar in both fibers, so it is not a factor in determining what type of fiber used.

Chromatic Dispersion (CD)

Chromatic dispersion of a fiber is expressed in ps/(nm*km), representing the delay, or increase of time (in ps) for a source with a spectral width of 1nm which travels through 1km of fiber. This depends on the type of fiber, and limits the bit rate or transmission distance for a good quality of service.

The widening suffered by light pulses, called dispersion, is a critical factor that limits the quality of the signal transmission over optical links. The dispersion is a consequence of the physical properties of the transmission medium. Single-mode fibers, used in fast optical networks, are subject to chromatic dispersion (CD) which causes a broadening of the pulses of light according to wavelength, and polarization mode dispersion (PMD), which causes pulse broadening according to polarization. An excessive widening causes pulses overlaping and errors in decoding.

A network that transmits at 10Gbps is 16 times more tolerant to chromatic dispersion than one that works at 40Gbps. This information allows us to have an idea of the limitation that the CD impose at high speed systems. Optical networks have a maximum cumulative CD which below that, the system is working properly. To keep the CD within the limits of each network, special equipment must be used to compensate for CD (CDFM). These devices allow to remove the limitation for CD in optical networks. The following chart shows CDFM costs depending on the bandwidth of optical transmission network:

The cost of compensation systems for G.652 and G.655 fibers is almost equal, so it is not crucial to choose a type of fiber or the other based on these costs. Importantly, the contribution of CD is critical for the functioning of the system for long distance and high transmission rates. For short distances of a few kilometers to the contribution to the total is small and does not seriously affect system behavior.

The CD varies from other types of fiber, as typical values of reference these can be taken:

•    G.652   16.5 ps / (nm*km)
•    G.655   4.2 ps / (nm*km)

G.655 fiber has lower values of CD and CD compensation is not so necessary, in exchange, the cost per km of this type of fiber is greater than that of G.652. G.655 fiber is used on systems that are designed to work with very long distances and high transmission rates, without being so important compatibility with installed fiber.

Polarization mode dispersion (PMD)

The Polarization Mode Dispersion, PMD, is an optical scattering effect, which limits the transmission quality in fiber optic links. Its control is becoming essential as it sharply limits the transmission capacity at high speeds, especially those above 10Gbps. It is a difficult parameter to measure and compensate because of their statistical nature, and depends heavily on the physical conditions of the cable (both environmental and mechanical).

The physical origin of the PMD is essentially the birefringence of the fiber produced by differences in propagation constants in the orthogonal axes. These differences are caused by imperfections in the manufacturing process of the fiber or as a result of external forces which produce bending and tensions in the fiber. If the fiber were perfect, with a uniform geometry, uniformity in the material and void of tension, both modes will propagate at exactly the same speed and there would be no degradation on the transmitted bits.

Causes of PMD in optical fibers

In fact the fiber is not perfect and the two modes called PMD, causing a widening of the signal, increasing the uncertainty in detecting the symbols. As a result this increases the probability of bit error in transmission (BER).

Delay of the signal components

Below there is a histogram of measurements by Bellcore in the United States on the optical fiber installed in 1994, which is reflected by such high values of PMD in fibers, because until that year did not begin to control the influence of that parameter.

Measurements of PMD in installed fibers (1994)

Therefore, in meeting existing and future transmission rates, 40G, 100G, 160G... it is extremely important to assess the impact of PMD in installed networks and decrease at maximum the value for current and future fibers. Values that were acceptable a few years ago, even on international standards, as 0.5 ps/√km, must now be revised mandatory for long-distance and high speed networks. Fiber and cable manufacturers are working to develop new designs to improve this parameter.

The following chart the recommended limits can be checked for communications to be correct under the influence of the PMD. The transmission in the fiber is limited as follows:

 

Accordingly, a 40Gbps system can have an accumulative PMD up to 2.5ps, and reach link distances up to 625km. with a PMD coefficient of 0.1ps/√km.
   
The current G.652 fiber has a typical maximum PMD value of 0.2 ps/√km, this value is higher in G.655 fibers due to its higher refractive index. The PMD changes randomly with time and wavelength, so it is more difficult to predict and compensate than the CD, one solution is to use more efficient modulations.

Reducing PMD values in G.652 to 0.1 ps/√km is theoretically possible in the existing G.652 fibers, but it depends largely on the installation of the fiber and the influence of environmental factors.   

PMD of the link

According to ITU-T G.652, PMD is statistically defined, and not individually for each fiber. The requirements only refer to aspects of the link calculated from information of the cable. The manufacturer must provide a value of link design PMD, PMDq, constituting the statistical PMD superior limit coefficient of the concatenated fiber-optic cables on a link of M cable sections. The upper limit is defined in terms of low probability, Q, of a PMD coefficient value is greater than PMDq concatenated. For values of M and Q defined (M = 20 cables and Q = 0.01%), the PMD value ratio must not exceed the specified maximum PMD.

The measurements and specifications obtained from non-wired fibers are necessary but not enough to ensure specifications of the wired fiber. The maximum value for the design of specified fiber link cable will not exceed the specified fiber-wired. The relationship of the values of PMD in non-wired and wired fiber depends on the circumstances of the construction and management of cable and docking state mode fiber are not wired.

PMD Measurement

There are different commercially available equipment to measure PMD in the fiber, based on the injection of polarized light and measuring the delay of the signal.

The most advanced equipment combine the OTDR and a function and can achieve PMD values in different sections measuring only on one end of the fiber. By this technique it can be known if an end of the link is failing, and proceed to its replacement.

PMD and OTDR measure end to end

Optical Cable Features

Special emphasis should be made on the effects of macro and micro-bendings that can induce losses in the fibers contained in optical cables. The main causes are associated with the manufacture and installation process as well as temperature changes during operation. Micro-bending losses are avoided with proper design and process of the fibers during the application of secondary protection.

The main mechanical characteristics to be taken into account when designing a cable are: bending, tension, impact, crushing and impact. The reinforcement elements and the minimum bending radius are defined to prevent the cable from being damaged during the installation process due to inappropriate tension or bending radius.

Environmental factors that most influence are: hydrogen generation, water penetration, vibration, temperature changes and biological attacks (rodents), and also wind and ice loads in the case of overhead cables.

The geometric and optical features of the fiber (attenuation, dispersion, PMD...) are little affected by the process of wiring, if it satisfies the appropriate design conditions.

Cable Construction   

The optical fiber has a primary acrylate coating that can be easily removed for splicing the fibers. Secondary protections: loose or tight, avoid direct pressure on the optical fiber. The reinforcement elements, both central and peripheral prevent cable damage due to stresses during installation and operation.

The cables have one or two plastic, and metallic or dielectric armed covers to protect them from environmental and mechanical factors associated with: storage, installation and operation. The sheaths and its thickness is chosen according to the final application and considering constraints such as maximum diameter or maximum weight permissible.

Optical Cable Features

Special emphasis should be made on the effects of macro and micro-bendings that can induce losses in the fibers contained in optical cables. The main causes are associated with the manufacture and installation process as well as temperature changes during operation. Micro-bending losses are avoided with proper design and process of the fibers during the application of secondary protection.

The main mechanical characteristics to be taken into account when designing a cable are: bending, tension, impact, crushing and impact. The reinforcement elements and the minimum bending radius are defined to prevent the cable from being damaged during the installation process due to inappropriate tension or bending radius.

Environmental factors that most influence are: hydrogen generation, water penetration, vibration, temperature changes and biological attacks (rodents), and also wind and ice loads in the case of overhead cables.

The geometric and optical features of the fiber (attenuation, dispersion, PMD...) are little affected by the process of wiring, if it satisfies the appropriate design conditions.

Cable Construction   

The optical fiber has a primary acrylate coating that can be easily removed for splicing the fibers. Secondary protections: loose or tight, avoid direct pressure on the optical fiber. The reinforcement elements, both central and peripheral prevent cable damage due to stresses during installation and operation.

The cables have one or two plastic, and metallic or dielectric armed covers to protect them from environmental and mechanical factors associated with: storage, installation and operation. The sheaths and its thickness is chosen according to the final application and considering constraints such as maximum diameter or maximum weight permissible.

Influence of manufacture of the cable on the parameters of the optical fiber

Some of the parameters of the fiber can be affected by the manufacturing process or installation of cable: cut wavelength and PMD. A correct design and manufacturing processes allow these values to be always under the limits considered by the relevant ITU-T recommendation. The optical and geometrical characteristics are little affected by the process of wiring, which gathers prominently featured recommendations for long-span transmission installations.

Influence on the attenuation coefficient

As an example, the following table shows the values of attenuation of the optical fibers used in the manufacture of a 32-fiber cable G.652.D, both at second and third window, and the same measures after the manufacturing process. It may be noted that changes in attenuation are minimal, since the processes do not affect this parameter.

		Naked fiber attenuation					Wired fiber attenuation
Fiber color	code	Length (m)	1310 nm	1550 nm		Tube color	Lenght (m)	1310 nm	1550 nm
Green	612505900300059	50400	0,33	0,185		White 1	8245	0,33	0,184
Red	612505900400051		0,33	0,185		White 1	8246	0,33	0,184
Blue	612505900300023		0,33	0,184		White 1	8247	0,33	0,183
Yellow	612505901900018		0,33	0,183		White 1	8244	0,33	0,182
Grey	612505902200033		0,33	0,184		White 1	8248	0,33	0,185
Violet	612505900600127		0,33	0,186		White 1	8248	0,33	0,184
Brown	612505901300107		0,33	0,186		White 1	8249	0,33	0,187
Orange	612505900300113		0,33	0,183		White 1	8245	0,33	0,183
Green	612505900300060		0,33	0,184		Red 1	8256	0,33	0,183
Red	612505900400052		0,33	0,185		Red 1	8261	0,33	0,184
Blue	612505900300022		0,33	0,184		Red 1	8261	0,33	0,184
Yellow	612505900300019		0,33	0,184		Red 1	8256	0,32	0,183
Grey	612505902200021		0,33	0,186		Red 1	8262	0,33	0,185
Violet	612505900100217		0,33	0,184		Red 1	8257	0,32	0,182
Brown	612505901300108		0,33	0,183		Red 1	8256	0,32	0,183
Orange	612505900300114		0,33	0,183		Red 1	8258	0,33	0,184
Green	612505900300058		0,33	0,185		Blue 1	8246	0,33	0,184
Red	612505900400053		0,33	0,184		Blue 1	8251	0,33	0,184
Blue	612505900300024		0,33	0,185		Blue 1	8248	0,33	0,183
Yellow	612505901900017		0,33	0,184		Blue 1	8246	0,32	0,183
Grey	612505900600089		0,33	0,185		Blue 1	8246	0,33	0,183
Violet	612505902200105		0,33	0,183		Blue 1	8244	0,32	0,182
Brown	612505901300105		0,33	0,184		Blue 1	8243	0,33	0,184
Orange	612505900300116		0,33	0,183		Blue 1	8251	0,33	0,184
Green	612505900300062		0,33	0,183		Green 1	8259	0,32	0,181
Red	612505900400054		0,33	0,183		Green 1	8266	0,33	0,184
Blue	612505900300021		0,33	0,184		Green 1	8259	0,33	0,184
Yellow	612505901900020		0,33	0,184		Green 1	8259	0,33	0,183
Grey	612505900100066		0,33	0,186		Green 1	8259	0,32	0,183
Violet	612505900600128		0,33	0,186		Green 1	8261	0,33	0,185
Brown	612505901300106		0,33	0,185		Green 1	8261	0,33	0,184
Orange	612505900300115		0,33	0,185		Green 1	8256	0,33	0,183

Development of attenuation measures in installed G.652 fiber cables.

Quality improvements in the G.652.A and B fibers in recent years are clearly shown in the following charts. There are listed measures of attenuation total means (including splice losses) in 1270-1610nm measured in cables installed in two different time periods: around 2000 and 2003.
   
Statistical attenuation measures are obtained by mathematical adjustment of the mass measures with OTDR at 6 different wavelengths. Statistics of losses from attenuation splice are taken into account through an empirical model based on the results of the measurements.

Measurement of attenuation in fiber and cable splices G.652.A&B installed in 2000.

Measurement of attenuation in fiber and fiber cable splices G.652.A&B installed in 2003.

It can be seen clearly that the values of fiber installed in 2000 presented some very high attenuation increases around 1380nm, which could risk the operation of existing CWDM and DWDM.

Influence on the PMD

Currently, manufacturing processes are stringent and ensure very low PMD values for fiber. As this parameter varies with external conditions must be considered from a statistical point of view, being very important that once installed measures are carried out in different time intervals.

After several laboratory tests in which measurements are carried out on 40 reels of optical fiber, with lengths between 9 and 50 km, the results are as follows:

 

			ps/√km
Mean PMD Coef.:			0,054
Standard deviation of the PMD Coef.:			0,020
Minimum PMD Coef:			0,019
Maximum PMD Coef.:			0,121

Then proceed to make a 128-fo PKP (polyethylene-aramid-polyethylene) cable and a 64-fo PFVP (polyethylene-glass fiber-polyethylene) cable from these 40 coils measures.

Cross-sectional view of the cables manufactured

On the terminated cables, 192 fibers have been measured distributed in several reels and the result is the following:

 

			ps/√km
Mean PMD Coef.:			0,038
Standard deviation of the PMD Coef.:			0,012
Minimum PMD Coef:			0,026
Maximum PMD Coef.:			0,097

From the above tables, we see that the manufacturing process, in well-designed cables, does not increase the coefficient of PMD, but rather achieves a slight mean decrease (about -0.02 ps/√km). This is because the measurement of PMD in reeled fiber is higher due to the reeling tension.
   
The following graph represents in pink color the PMD in the 40 original fiber reels testing and in blue the PMD in fiber cables manufactured by matching each fiber with the corresponding original. It is noted that considering the steps individually, we cannot ensure direct relationship between the value of the PMD in reels and cables, said the fact of the instantaneous variation of the PMD. Although it is statistically noted, as the above tables, that the wired PMD measured without tension is lower than in the reels of fiber from the manufacturer.

If we have a look at the following charts, the standard deviation is higher in raw material fiber reels, again because of the reeling tension, leading to a wider distribution than the wired fiber.

Distribution of PMD in raw fiber reels

Distribution of PMD in wired fiber

The following histogram shows the 192 measures of the wired fiber and observed that 94% of values are below 0.06 ps/√km:

Based on the measured values of PMD in wired fiber, and using the link design tool Link PMD Calculator v.1.0 from Photon Kinetics, It has been calculated the PMDQ value, known as link design value. PMDQ value is as follows for different values of Q:
   
            Q = 0,0001 (0,01%),      PMDQ = 0,057 ps/√km
            Q = 0,001 (0,1%),          PMDQ = 0,054 ps/√km
            Q = 0,01 (1%),               PMDQ = 0,050 ps/√km

These values are obtained by simulating a link with M=20 random individual lengths of the 192 measures on reels of wire, and a probability that the value of PMD is indicated above the 0.01%, 0, 1% and 1% respectively.

However, based on the tests described in this report made in our factory and, thanks to the high quality of our certified fiber suppliers, we can now ensure a coefficient of individual PMD=0.10 ps/√km.

In fact we found that over 90% of the measures will be below 0.06 ps/√km, but do not forget that the emphasis should be on the statistical coefficient PMDQ.

Under these conditions, and with the variable M=20 and Q=0.01% can be achieved a PMDQ value = 0.05 ps/√km.

This low value of link design undoubtedly means a big step for the design and construction of the future renewal of the network, since it means halving the most stringent requirements of PMDs so far.

References

•    “ECOC2005. 40 Gb/s uncompensated 8-channel CWDM system over 30 km of non-zero dispersion shifted fibre”
•    “ECOC2005. 160 Gbit/s-based field transmission experiments with single polarization RZ-DPSK signals and simple PMD compensator”
•    “ECOC2005. 8x85.4 Gbit/s WDM Field Transmission over 421 km SSMF Link Applying an 85.4 Gbit/s ETDM Receiver”
•    “ECOC2005. Long Haul Field Transmission Experiment of 8x170 Gbit/s over 421 km Installed Legacy SSMF Fiber Infrastructure”
•    “ECOC2005.  FGB 40Gx820km Proximinion”
•    “ECOC2006 Upgrade from 10G to 40G Alcatel”
•    “Evolution of the ITU-T Standarization of Optical Fibres and Cables” IWCS 2005
•    “ITU-T G.652. Características de las fibras y cables ópticos monomodo.” 2005   
•    “ITU-T G.655. Características de fibras y cables ópticos monomodo con dispersión desplazada no nula.” 2003 y 2006.
•    “Stability of Low Water Peak SMF against Hydrogen Aging”. IWCS 2002.