Optical fiber transmission with wavelength multiplexing...

Optical fiber transmission with wavelength multiplexing – faster or denser?

Ryszard S. Romaniuk

Institute of Electronic Systems, Warsaw University of Technology

ABSTRACT The subject of the paper is the immediate and not so immediate future of the DWDM systems using direct TDM multiplexing for the individual wavelength channels. The following problems and questions have been tried to be addressed: a balance and competition between the electronic and optical bandwidths in WDM systems, importance of this competition now and in the future, major factors contributing to this competition, costs of the fast and dense WDM systems, the most important price generating factors in the WDM systems, etc. Some of the answers to these questions are very probably the following: now the best WDM systems work with spectral efficiencies of 0,2bit/s/Hz for the trunk communications system solutions (5THz of aggregated bandwidth) and well below 0,1bit/s/Hz for the local communications; in the nearest decade the spectral efficiency of the WDM overcomes 0,5bit/s/Hz and 50THz of the aggregated bandwidth (ten times the aggregated bandwidth used today).

Keywords : optical fiber transmission systems, terabit ultra-broadband optical channels, DWDM, CWDM, TDM

1. INTRODUCTION The most effective, optical fiber transmission systems of very large amount of information, which are used today, rely on the combination of two multiplexing methods: a dense wavelength DWDM and time division TDM [1]. Each of the unitary transmitted signal is multiplexed in the time domain inside every wavelength channel. The wavelength channels, called transmission colors or light paths (though these notions have different meanings) have to be well separated from each other to prevent excessive crosstalk. The criterion of separation is the maximum allowable inter-channel power exchange. The inter-channel spacings, used to provide separation for the adjacent channel are useless from the point of optical spectrum usage for the transmission. They should be as narrow as possible, not to loose to much spectrum. They should be wide enough to provide proper separation. An optimal trade-off has to be find. Thus, it is impossible to use the whole optical spectrum only for the effective transmission purpose. The optical carrier of each WDM channel is modulated digitally by a TDM signal. The modulation itself, if fast enough, widens the spectrum of the optical channel. This widening, which is proportional to the speed of the TDM signal, could not exceed the nominal width of the optical channel. At some point of the electronic speed increase there appears a competition for the available optical fiber bandwidth between the electronics and the optics. A careful choice is necessary now between the number of optical channels and the electronic bandwidth. The latter is determined by the speed. In practice, one asks a question: Optical fiber WDM transmission – faster or denser? An answer to this question is a balanced alternative between the best parameters of an optical fiber transmission system. An answer is definitely not a conjunction of such parameters. The choice of one option partially excludes the choice of the other. Any particular choice has a serious consequence concerning the future development of the WDM system and its future fundamental technical as well as economical parameters [2]. It seems now, however, that this choice would not be so critical in the next decade, when the spectral efficiency will probably enter into a saturation area.

2. QUESTIONS CONCERNING DEVELOPMENT DIRECTIONS OF DWDM/TDM Let us consider here some of the fundamental questions concerning the closest future development of an optical fiber transmission system working in the DWDM/TDM standard. In which direction the DWDM/TDM technology would go in the closest future and why? Should the DWDM/TDM technology use more narrower channels (slower) or rather smaller number of wider channels (faster)? How to balance the competing requirements, of the co-operating multiplexing schemes (optical and electronic) within a single transmission system, for the optical bandwidth? Which

parameters of the DWDM are the most important from the manufacturer, vendor, telecom operator and user point of view? Is this parameter an efficiency of using the available optical transmission bandwidth? May this efficiency be expressed in the number of bits per second possible for the transmission from a unitary optical spectrum of the fiber? Where we are now with this kind of bandwidth usage efficiency in the experimental and commercial systems? Is it possible for this efficiency to reach the fundamental confinements imposed by the Shannon-Kotelnikov information theorem? Will the characteristic of the spectral efficiency saturate with the further development of relevant technologies. How does the ultimate DWDM/TDM system compare to the future analog and digital optical coherent frequency techniques (optical RF technology)? How much information may be really transmitted via an optical fiber using the DWDM, and if it is the ultimate maximum? May one expect other multiplexing techniques than DWDM to be used in the future? Is there a chance for these techniques to have better optical bandwidth usage than the DWDM? The work debates, in a concise way, some of the most fundamental DWDM system parameters, which may have influence on the answers to the above questions. Some of these debated parameters have global nature, while some of them seem now to be underestimated. The author tries to find the major factors that may answer some of these questions in the nearest future.

3. HOW TO USE EFFICIENTLY NATURAL BANDWIDTH OF OPTICAL FIBER? Transmission bandwidth of an ideally dry and clean (ultra-low-loss), singlemode, communications grade, optical fiber (i.e. having no narrow, selective, high absorption bands caused by hydronium ions and impurities – transition metals), for optical losses not increasing 0,3dB/km spans over the spectral region of 1325nm – 1675nm (350nm). The width of this spectrum (usable for the effective optical transmission) covers 500nm for the losses only slightly crossing the mark of 0,5dB/km. The transmission with acceptable losses equal to approx. 1dB/km is opened in a window of 1000nm – 1700nm. The width of the optical window is, thus, 700nm. The respective window widths for the losses equal to 2 dB/km is 900nm and for 3 dB/km exceeds slightly 1µm [3]. This dependence of the width of optical transmission window on the losses which can be accepted in the communications system was presented in fig.1 [4]. The dependence was depicted for the best grade telecom fibers available commercially today. Nowadays, most of the commercial, optical trunk telecom systems use only a tiny fraction of this bandwidth. Practically, this is only a few tens of nm (instead of hundreds of nm). Knowing the above numbers, and properly estimating their values, a question gets open, how to use optimally such a vast transmission spectrum space. The answer depends on many factors, but mainly on the fiber. The fiber provides the natural optical bandwidth, associated with the ultra-pure material. Also, the optical source has to be taken into account [5] as well as other components of the optical transmission channel [6]. Practical usage of the transmission capabilities of an optical fiber leads through a single-level or, more frequently, a multi-level division (multiplexing) of the available bandwidth. Initially, the continuous spectrum is divided into the wavelength sub-bands. The division is standardized by the International Telecommunication Union ITU and called the WDM or DWDM. The standard is also called as the ITU-T GRID (G.692). The aim of the standardization was to lower the costs of the hardware and to enable efficient usage of very broad transmission spectrum. The ITU standard introduces inter-channel separations in the following order: 200GHz, 100GHz, 50GHz, 25GHz, 12,5GHz, 10GHz, 8GHz and even smaller. The separations in the frequency domain are equivalent to the following separations in the wavelength domain: 1,6nm, 0,8nm, 0,4nm, 0,2nm, 0,1nm, 0,08nm, etc. The signal is multiplexed in the time division domain (TDM) within each wavelength channel WDM (called a color). The TDM transmission (multiplexing) speeds are normalized too. They are: 2,5Gbit/s, 10Gbit/s, 40Gbit/s. All of these speeds are available commercially. There are considered the following speeds for the closest future: 60Gbit/s, 80Gbit/s and 160Gbit/s, even 200Gbit/s or 320Gbit/s. These (electronic) speeds of the digital signal modulation have equivalents in the required optical bandwidth spectrum. The optoelectronic components for the latter speeds are essentially available, while the electronic components are not yet available for all of the listed speeds. Electronic components with the speeds well in excess of 50Gbit/s are in the research stage. The same is with the measurement equipment. Commercial optical and optoelectronic apparatus for the DWDM and TDM systems is available only up to around 50Gbit/s. The problems connected with the increase in the speed of the electronics of the TDM and the increase in optical channel density of the WDM are the following: considerable increase in the costs of the electronic and optoelectronic

components, more difficult technology of components, necessity to introduce efficiently new and relevant technological solutions, initially worse signal parameters of the components and circuits, increase in the fiber dispersion. Acceptance of a new commercial standard for the DWDM/TDM communication system depends on a compromise between the costs and system parameters. For example, an 80Gbit/s TDM system would be probably accepted for the commercial applications when the costs fall to the level equal to two 40Gbit/s systems at the comparable signal parameters. The 40Gbit/s system has been accepted for the commerce recently after it reached the production costs only 5% lower than four 10Gbit/s systems. A commercial acceptance, apart from the costs, takes into account such factors as: the effective gain or reduction in the aggregated bandwidth with the introduction of a new generation hardware, total number of the components applied for building of the system and connected with this potential increase in the system reliability (on condition that the total number of components has been reduced), comparison between the relevant signal and systems parameters, influence of fiber dispersion (chromatic and polarization). For example, during the migration process between the 10Gbit/s and 40Gbit/s and next 40Gbit/s and 80Gbit/s (or 160Gbit/s), there is a quadruple and double reduction in the number of the most expensive components which are fast semiconductor lasers. These savings on the optical sources allow for certain increase in the costs of the required much faster electronics. However, it is to remember, that the dispersion increases approximately proportionally to the square of the data transmission rate. The dispersion deteriorates considerably many of the system parameters. This deterioration has to be addressed or prevented in the faster systems.

Fig.1. Spectral width of usable transmission bandwidth B[nm] as a function of unitary optical fiber losses T[dB/km] that may be acceptable in the communications system. The curve calculated for the best commercially available singlemode communications grade optical fibers. Analy zed fibers were of the All-Wave type and equivalent.

4. REAL WIDTH OF AVAILABLE TRANSMISSION BANDWIDTH AND KINDS OF OPTICAL FIBERS

The frequency of an optical wave transmitted via the fiber in the conventional band (C) is around 200 THz. Let us assume, in the boundary condition, that the optical carrier could be modulated effectively with the frequency equal to half of that frequency, i.e. 100THz. The ITU standard assumes the following recalculation between the inter-channel distances in frequency domain (expressed in GHz) and in the wavelength domain (expressed in nm). 100GHz of the frequency bandwidth is equivalent to 0,8nm of the wavelength bandwidth. Calculating, according to this dependency, the frequency of the carrier and the width of the modulation band for different acceptable losses levels in the fiber, one obtains for 100THz frequency an optical bandwidth as wide as 800nm. It is a bandwidth available in the optical fiber at

the acceptable losses level in the range of 1,5dB/km (see fig.1.). This seems not to be acceptable for the trunk systems. 50GHz is equivalent to 400nm, with loss window of 0,35dB/km, which seems to be acceptable in some of the trunk systems. On the other hand, the ultra-broadband, passive optical networks (PON) of LAN or SAN types may accept even higher levels of unitary losses approaching 3dB/km. WDM system designer has, in such a case, the optical transmission window which is over 1µm wide. This window is available for the usable signal modulation purposes. Usage of such a vast space in a single piece is now impossible because of lack of appropriate signal processing technologies and components. Other reason is that a single signal of such spectral width would suffer from unbearable dispersion, measured in [ps/nm/km]. The results of simple calculations for the parameters of an optical modulated carrier for various transmission conditions were gathered in tables 1 and 2. These values allow us to carry out the further discussion about efficiency measures of the optical fiber bandwidth usage via the WDM system.

Table 1. Parameters of optical carrier and modulated signals in an optical fiber

in comparison with the signals of the classical RF technology (approximate values).

Frequency Period Wavelength Time Width Resolution Frequency of (optical) carrier f [Hz] or modulation bandwidth B [Hz]; interchannel separations in DWDM , spectral width of optical source ? f[Hz], transmission rate [Gbit/s]

(Optical ) Wave period T=? o/c=?/v= =1/f [s]

(Optical) Wavelength ?o=Tc [m] (approx. Values)

(Optical) Pulse rising time ? t=0,35/B [s]; Maximum allowable group delay; minimal required measurement resolution in [s]

Spectral width ? ? [m] or modulation bandwidth of carrier B [m];

minimal practically required measurement resolution in [m]

1 MHz /reference/ 1?s 300m 350ns 8fm 300 MHz 3,3ns 1m 1ns 2,4pm 1GHz 1ns 0,3m 350ps 8pm 3 GHz /laser source/ 330ps 0,1m approx. (~) 100ps 24pm ~ 1,8pm 10 GHz /chan. separation/, /transm. rate/

100ps 3cm 35ps 80pm 6pm

25 GHz /chan. separation/ 40ps 1,2cm 14ps 200pm 10-15pm 40GHz /transm. rate in [Gbit/s]/

25 ps 0,75cm ~ 9ps ~ 0,3nm 20-25pm

50 GHz /chan. separation/ 20ps 0,6cm 7ps 0,4nm 30pm 60 GHz 17ps 0,5cm ~ 6ps ~ 0,5nm 80 GHz 12ps 3,5mm ~ 4,4ps 0,65nm 100 GHz /chan. separation/ 10ps 3mm 3,5ps 0,8nm 60pm 160 GHz /transm. rate/ 6,25ps 2mm 2,2ps 1,3nm 200 GHz /chan. separation/ 5ps 1,5mm 1,8ps 1,6nm 120pm 300 GHz 3,3ps 1mm 1ps 2,4nm 1THz 1ps 0,3mm 350fs 8nm 10 THz /transm. bandw./ 100fs 30? m 35fs 80nm 30 THz 33fs 10? m 10fs 240nm 50 THz /now max. comercial badwidth/

20fs 6? m 7fs 320nm

100 THz /potential modulation bandwidth/

10fs 3? m 3,5fs 800nm

200 THz /optical carrier/ 5fs 1,5? m 1,75fs 1,6? m 300 THz /optical carrier/ 3,3fs 1? m 1fs 2,4? m 600 THz /optical carrier for visible/

1,7fs 0,5? m 0,5fs 4,8? m

1 PHz /carrier for near UV/ 1fs 0,3? m 350as 8? m

Table 2. Recalculation of frequencies to wavelengths and spectral widths for DWDM/TDM systems.

DWDM/TDM parameter \ ?, f Wavelength in optical fiber ? or width of spectrum ? ? in [nm]

Frequency f [THz], f [GHz] or transmission rate in [Gbit/s]

Optical carrier wave in communications fiber (for core refraction n=1,5)

500-2000 [nm] 600-150 [THz]; calculated from equation f=c/?=nc/?o, ?o-wavelength in the void

Optical windows IW, IIW; IIIW-S’, S, C, L, XL (Short-Conventional-Long), All-Wave

1310-1350; 1440-1490-1530-1570-1610-1650 [nm]

ITU-T-GRID standard for DWDM wavelengths and channel separations

1,6; 0,8; 0,4; 0,2; 0,1; 0,08; 0,02 [nm]

200; 100; 50; 25; 12,5; 10; 2,5 [GHz]

Standards for TDM channel rates 0,02; ~ 0,1; ~ 0,3; ~ 0,6; ~ 1,3; ~ 2,5 [nm]

2,5; 10; 40; 60; 80; 100; 160; 320 [Gbit/s]

Optimal optical fibers for the DWDM, which are spanning all the usable bandwidth, are not available on the commercial market yet. The main parameter of such a fiber is the ultimate optical transparency in the whole available bandwidth. The bandwidth is confined by the fundamental dissipation effects – Rayleigh (from short ? side) and multiphonon (from long ? side). The DWDM fiber under consideration should posses a very small but nonzero, and preferably uniform, dispersion over the whole optical band. The dispersion level should be tailored to the designed maximum transmission rate. Too small dispersion leads, in the DWDM systems, to transmission degradation (inter-channel crosstalk, total transmission loss in certain channels) via cross self phase modulation (the optical Kerr effect) and the four wave mixing - FWM (optical parametric phenomenon). Too high value of the dispersion or its inhomogeneity in the optical window, leads to a drastic confinement to transmission rate in a single color channel. It is known now quite well, which characteristics should posses an ideal DWDM optical fiber. Its manufacturing is quite difficult. For example, for a hypothetical 80Gbit/s TDM system of the next generation, the dispersion expressed by the maximum acceptable differential group delay may not overcome ~5ps for the pre -set transmission conditions (source spectral width, repeater spacing, fiber kind, method of dispersion compensation, etc.). We assume the spectral width of such a signal for 0,65nm window and 1nm together with a single neighboring inter-channel spacing for 50GHz system. Such a signal, to be transmitted efficiently in the multi-channel band of 1500-1620nm, for a distance at least 10km, requires the unitary dispersion to be lower than 0,5ps/nm/km. The classical singlemode telecom fiber does not provide this value of the dispersion. A considerable reduction in the dispersion is needed in terms of the absolute vale and inclination (first derivative). In one of the solutions to this problem, two kinds of optical fibers are concatenated together. These fibers are: NDSF (non-zero dispersion shifted fiber) and IDF (inverse-dispersion fiber). Such a pair is called DSMFP (dispersion slope managed fiber pair). A concatenated pair, with chosen lengths for the pre-set data rate, may compensate considerably the dispersion at the far end of the optical path-length. Tables 1 and 2 contain series of simple calculation results showing the confinements of DWDM/TDM technology development. The lasers, which are used for this technology must possess guaranteed frequency stability better than 3GHz, what in the wavelength domain is 1,8pm. The inter-channel spacings used now are 25GHz, and in the future 10GHz, what is equivalent to 200pm and 60pm. The measurement resolution of the above parameters are assumed to be on the level of 10-15pm and 6pm respectively. To measure effectively 40Gbi/s and next generation 80Gbit/s systems, an apparatus is needed with the bandwidth of 120GHz and 240GHz respectively.

5. USAGE EFFICIENCY OF BANDWIDTH BY DWDM/TDM The evolution of the technical possibilities to use the transmission bandwidth of an optical fiber in time, in terns of channel number and channel separations, was as follows: 1996 – from 16 to 32 used channels with 100-200GHz separations; 1999 – from 64 to 128 channels with 50 – 25 GHz separations. The year 2001 brought a 1000 channel system with separations of 12,5 GHz (published at ECOC conference in Amsterdam). The used bandwidth grew from a mere 1GHz to over 1THz during the 1990-2000 decade. The most advanced commercial systems offer now the maximum aggregated rate of over 10THz. These systems have 160 – 320 channels with 10Gbit/s each or 80 channels with 40Gbit/s each. The usual channel separations in these systems are 50GHz. A summary of the evolution during the

debated decade was presented in fig. 2. It is to remember for the DWDM that we have to divide a vast spectrum having a few hundreds of nm. This spectrum allows potentially, thanks to the normalization, for the transmission of the aggregated data rate reaching a few tens of Tb/s. Such a broad bandwidth of an optical fiber (around 500 nm) was used, in a different way, in cheaper and simpler metropolitan networks of a small territorial extent – spanning only a few km. The METRO networks, due to the costs, are build using the passive optical technology, referred to as the PON. The distribution of optical signal is done between all the nodes. There is no need in the PON to use either optical or optoelectronic selective switching components. The PON combined with the WDM or its no-normalized sub-brands like CWDM (coarse) or WWDM (wide) allow for a cheap multiplication of the transmission. Such a multiplication is very valuable in a difficult metropolitan area. Building of additional cable routes there is quite expensive. So far, the WDM METRO networks avoid strict standardization. As a result, the proprietary solutions are incompatible with each other, and the price goes down not as slow as it may be expected. The WDM METRO systems use the optical bandwidth in a much less optimal way than the trunk WDM ones. The efficiency of using the natural optical fiber bandwidth may be increased by specialized protocols for the optical and optoelectronic physical layer of the network. One of such protocols is GMPL/?S (generalized multi-protocol label/lambda switching). A label is introduced for an optical fiber, a wavelength, a group of adjacent wavelength, several groups of wavelengths, or even for an optical time slot. The label defines a bandwidth of the reserved transmission medium for a chosen data stream. The label accompanies the data stream down the optical pathway, making the traffic management more efficient. The label may also be a single wavelength. The protocol improves the traffic management but it cannot overcome the fundamental confinements combined with the physics of the optical transmission medium.

Fig.2. Development of DWDM systems during the 1990-2000 decade. The increase in the aggregated throughput is presented as well as the number of channels and their separations.

Fig.3. Bandwidth division across the transmission channels in WDM. Concatenated modulation/demodulation scheme. The efficiency of using the available, natural optical fiber bandwidth may be increased via the application of alternative configurations of the transmission system. Concatenated modulation configurations are tried, including TDM/OFDM/DWDM. In comparison with the direct WDM/TDM system an additional intermediate modulation layer

is used, which is resting on the optical frequencies. This method, similarly to the protocol optimization, increases the efficiency of the bandwidth division to smaller portions, what was presented in fig.3, but does not expand the physical limits. On the contrary, the excess cost of the increased easiness of band division is a certain lowering of the overall efficiency.

6. WHAT PAYS MORE – DENSER WDM OR FASTER TDM? Let us come back again to the question opening this paper: DWDM – faster or denser? Let us consider two cases of the WDM transmission channels of 40Gbit/s and 10Gbit/s. The 40Gbit/s channel has the width of 0,6nm=0,4nm+0,2nm together with the adjacent inter-channel spacing (for the 25GHz separation system). A single 10Gbit/s channel has the width of 0,3nm=0,1nm+0,2nm. A single 10Gbit/s channel has only two times smaller spectral width at the four-fold smaller data rate. This difference is one of the sources of different efficiencies of the fiber bandwidth usage. If our choice for the DWDM system is “faster”, then the consequences are the following: a considerable increase in the costs of electronics is observed; there may appear a need to switch from the electronics domain to the optics domain, if the speed gets too excessive; there is a possibility to use the existing bandwidth more efficiently, there is a need to use other bands outside the conventional one, i.e. the S and the L bands apart from the C band; when switching from 10Gbit/s/100GHz to 40Gbit/s/25GHz the dispersion increases 16 times; the faster system may suffer smaller unrepeatered link distance than the slower system. If the choice for the DWDM system is „denser” the consequences are: the existing systems are better used; the C band is better exploited; there in no immediate need to go into the L band before all parts of the C band are consumed up; it is easier to open the L band on a new fiber than to squeeze it into a fiber carrying the C band; more expensive optoelectronic components are needed (lasers, photodiodes, OADM multiplexers, RRR regenerators, etc.); the signal filtration in the closely spaced system channels gets more difficult; the channel crosstalk increases; The problems with the dispersion appear in both the faster and the denser systems alike.

7. SPECTRAL EFFICIENCY OF OPTICAL FIBRE DIGITAL TRANSMISSION A spectral efficiency (the ES) of the optical fiber bandwidth usage by the DWDM can be defined. It is the data rate in [bit/s] which can be obtained (transmitted through) from a unity of the optical fiber bandwidth in [Hz]. Thus, the ES is expressed in [bit/s/Hz]. The value of the ES can be calculated for an exemplary WDM system containing 160 channels, 10Gbit/s each. The total data rate is 1,6Tb/s. Total width of a single channel for ITU 50GHz is 0,4nm. Total width of the system is 64nm in the wavelength domain and occupies practically the whole C band. The whole width of the WDM system in the frequency domain is 8THz. As a result we obtain 1,6Tb/s data rate stream from 8THz of fiber bandwidth. The spectral efficiency is 0,2bit/s/Hz. Carrying out the same calculations for ITU-T grid 25GHz one obtains 0,25bit/s/Hz for the ES. Table 3 presents recalculation of the optical bandwidth for a single channel, the number of channels, throughput of the whole WDM system, and spectral efficiency for the various combinations of TDM/WDM system solutions. The spectral efficiencies ES for certain cases in table 3 are: ES(50GHz separation, 160 channels, 10Gb/s data rate)=0,16b/s/Hz; ES(25GHz, 160 channels, 10Gb/s)=0,26b/s/Hz; ES(12,5GHz, 160 channels, 10Gb/s)=0,4b/s/Hz; ES(25GHz, 40 channels, 40Gb/s)=0,5b/s/Hz. Some operators use now in this country the DWDM systems of 80 channels, 10Gbit/s each with 200GHz separation. The ES of such systems is only ES=0,05bit/s/Hz. It is known now, that these systems have no future in the trunk communications domain. The table 3 has a sort of a diagonal. It is defined for approximate rate 1:1 between the open window bandwidth and the channel separation. A case, which is difficult for practical realization, with 10Gbit/s/12,5GHz gives 2500 channels and a total throughput of 25THz (from 500nm of the optical bandwidth). The system 40Gbit/s/50GHz gives 625 channels and also a total throughput of 25THz. A similar data rate for 160Gb/s system is available for slightly over 150 channels. The spectral efficiency for all DWDM systems on the diagonal in table 3 is similar and equal to 0,4b/s/Hz. The area below this diagonal in table 3 is now a technical aim where the modern commercial system would go.

Table 3. Calculated parameters of optical fiber WDM/TDM systems. Legend: OM-Interchannel Separation in WDM according to ITU-T grid standard expressed in [GHz] or [nm], OPC- total optical spectrum of the fiber transmission system – here assumed for 500nm for the maximum acceptable losses of [0,5dB/km]; PO-optical bandwidth of a single

PO=2,6nm K=192 P=61Tb/s ES=0,98 [b/s/Hz]



PO=0,2nm K=2500 P=25T/s ES=0,4 diagonal 1:1

PO=0,12nm K=4167 P=10,4Tb/s ES=0,17 [b/s/Hz]

12,5 GHz (0,1nm)



PO=0,5nm K=1000 P=40 ES=0,64 [b/s/Hz]


PO=0,2nm K=2500 P=6,4T/s ES=0,1 [B/S/Hz]

25 GHz (0,2nm)

PO=2,9nm K=172 P=55 ES=0,88


PO=0,7nm K=715 P=28,6Tb/s ES=0,46 diagonal 1:1



50 GHz (0,4nm)





PO=0,8nm K=625 P=1,6 Tb/s ES=0,025 [b/s/Hz]

100 GHz (0,8nm)


PO=2,9nm K=172 P=27,6Tb/s ES=0,44 [b/s/Hz] diagonal 1:1




OM=200 GHz OM=1,6nm OPC=500nm OPC=62,5THz

320 Gb/s (~ 2,5nm)

160 Gb/s (~ 1,3nm)

40 Gb/s (~ 0,3nm)

10 Gb/s (~ 0,1nm)

2,5Gb/s (~0,02nm)

TDM WDM

10 GHz (0,08nm)

PO=0,1nm K=5000 P=12,5 ES=0,2 [b/s/Hz]

PO=0,2nm K=2780 P=27,8 ES=0,44 [b/s/Hz]


PO=1,4nm K=362 P=58 ES=0,92 [b/s/Hz]


2,5 GHz (0,02nm)

PO=0,04 K=12500 P=31,3Tb/s ES=0,5 diagonal 1:1

WDM channel together with a single adjacent inter-channel separation; K- number of transmission channels for OPC=500nm; P- aggregated transmission bandwidth of the whole DWDM/TDM system in [Tb/s]; ES-spectral efficiency of the WDM/TDM system in [b/s/Hz]. A diagonal in the table is defined by approximated ratio 1:1 between widths of the usable WDM channel and the inter-channel separation. The calculations were carried out according to the following simple relations: K=OPC/PO; P=K*(TDM) ; ES=P/62,5.

8. COMPLEXITY AND COSTS OF DWDM The DWDM system is unfortunately very complex and expensive. It is expected widely, that before it will be implemented very extensively outside the direct trunk communications field, i.e. in inexpensive, terabit computer networks, still a decade must pass. The components and circuits of the WDM are: optical amplifiers, tunable optical attenuators, multiplexers and demultiplexers – among them direct, interleaving, access, monitoring, switches, optical interconnects, dispersion compensators, filters, directional couplers, sources and detectors. The most important optical components, from the point of view of the asked question, are optical filters. The filters decide of the selectivity of densely packed wave channels. On the other hand the active optoelectronic components like sources and detectors are the far ends (gateways) of electronic channels and decide of the system speed. The ITU-T GRID channels are extracted using a few kinds of different filters: thin-film, Bragg fibers, or tuned active filters controlled externally. A typical characteristic of a fiber Bragg filter is sufficient for the isolation of the channels for the 50GHz system. The spectral width of the filter is 0.25nm (for the top of the characteristic) and 0,4nm (for the bottom of the characteristic) and the height is over 30dB. The respective filter parameters, for the 25GHz system, are: reserved channel width 160pm, channel separation 40pm, polarization sensitivity 10pm. On the background of these numbers, the requirements for the semiconductor laser frequency stability for the DWDM is 3GHz. There is defined a number of parameters for the WDM filters. These parameters are one of the measures of the WDM system complexity and describe ultimate possibilities of the system development in the direction of “denser”. - Insertion Loss (IL) – a minimal transmission inside the transparency window with taking into account the PDL and

the uniformity of the passband channel; - Insertion Loss Uniformity (ILU) – a difference between the channel insertion losses for the best and the worst

cases; - Passband Uniformity (PU) – a maximal value of which the transmission may change with polarization and

wavelength inside a window of a chosen channel; - Polarization Dependent Loss (PDL) – a maximal change in the transmission for all polarizations at the pre-set

wavelength for the whole passband window; - Adjacent Crosstalk (AC) – the biggest transmission inside the adjacent window, referenced to the minimal

transmission inside a chosen window; - Non Adjacent Crosstalk (NAC) – the biggest transmission inside a non-adjacent window referenced to the

minimum transmission in a chosen window. The biggest and the smallest transmissions are determined for the arbitrary polarization states in the transmission window.

- Maximum integrated crosstalk of channel (MIC) – a sum of the maximal values of the crosstalks from all other channels. MIC concerns channels which are very narrow spectrally.

- Average integrated crosstalk of channel (AIC) – a sum of average crosstalks from all other channels. Average channel crosstalk is a medium value from the maximum transmissions in the channel window referenced to the average from minimal transmis sion of the chosen channel. The average crosstalk represents the worst case of the whole crosstalk which may affect the signal of uniform power distribution in the transparency channel. The average integrated crosstalk is an appropriate measure for the signals of great rates, because they have spread spectrum.

- Chromatic Dispersion of Channel (CD) – the worst case of the dispersion inside the channel window for all polarization states of the photonic functional component (here a filter).

- Return Losses of Channel (RL) – maximum back reflected signal inside the transmission window for all polarization states referenced to the level of the forward signal. The parameter includes description of input channel in respect to the all output channels.

- Channel Directionality (D) – a maximum signal from this channel measured in each other channels for all polarization states referenced to the level of the forward signal.

- Differential Group Delay (DGD) – a maximal difference in the group delays between all polarization states inside the channel window.

Some of these definitions were illustrated in fig. 4.

Fig.4. Parameters of optical DWDM filters. They define ultimate development possibility in the direction of “denser”.

9. SUMMARY, WHAT IS THE FUTURE OF DWDM? The spectral efficiency of practically used today the best commercial trunk DWDM systems is around ES=0,2bit/s/Hz and the used bandwidth reaches a few THz. It is predicted, that the trunk systems will migrate to ES=0,5bit/s/Hz value of the spectral efficiency during the nearest decade. At this spectral efficiency, reached via a balanced choice between the faster and the denser options (see table 3), the used bandwidth may be ten times more than used today and cross the mark of 50THz (at the carrier frequency of 200THz). Apart from the obvious applications in the trunk communications, embracing today also the fast backbone optical network of the Internet, the DWDM systems have a great potential in the local multigigabit or even terabit computer LAN networks. However, now the great system costs prevent proliferation of this effective technology. In these fields, much cheaper systems are used of much lower value of the spectral efficiency ES (ten and even hundred times lower). It seems now that the ES parameter is now a major factor building the price of the WDM system.

REFERENCES [1] R.S.Romaniuk, DWDM, Technology, measurements, exploitation, development, Committee of Electronics and

Telecommunications, Polish Academy of Sciences, Warsaw 2001, 300 pages (in Polish) [2] D.C. McCarthy, Faster vs. denser: Networks reach another crossroad, Photonics Spectra, September 2001 [3] J.Wójcik, Development of communication optical fibre technology in the beginning of XXI century,

Telecommunications Review and News, No 3/2002, pp. 135-143 (in Polish) [4] www.optics.org; www.fibers.org; www.corning.com; [5] B.Mroziewicz, Tunable semiconductor lasers: perspectives of applications in optical networks, Telecommunications

Review and News, No 3/2002, pp. 143-147 (in Polish) [6] K.Jedrzejewski, Bragg gratings – a new component in optical fiber technology, Telecommunications Review and

News, No 3/2002, pp. 148-152 (in Polish)

Optical fiber transmission with wavelength multiplexing...

Documents

Transcript of Optical fiber transmission with wavelength multiplexing...