Transmission Methods Using Wavelength Division Multiplexing

Signal reception: WDM systems use photodetectors such as PIN (Positive Intrinsic Negative) or APD (Avalanche Photo-Diode) to convert optical signals into electrical signals, which must be compatible with the transmitter in both wavelength and modulation characteristics.

When N channels at bit rates B 1 , B 2 , …, B N are transmitted simultaneously over a fiber of length L, then BL = (B 1 + B 2 +…+ B N )L. When the bit rates are uniform, that is, B 1 =B 2 =…=B N , the system capacity will increase by a factor of N.

The maximum capacity of WDM links depends on the allowable spacing between channels. The minimum spacing is the spacing that ensures cross-channel interference resistance between channels.

The frequency channels (or wavelengths) of WDM systems have been standardized by ITU_T, the distance between wavelength channels is 100 Ghz, the current WDM system (using EDFA- Erbium Doped Fiber Amplifier) operating in C and L bands will have 32 wavelength channels operating on each band. Thus, if the bit rate on each transmission channel is kept the same but WDM technology is used, it is enough to increase the transmission bandwidth on an optical fiber by 64 times.

WDM optical communication system has Coarse Wavelength Division Multiplexing (CWDM) and Dense Wavelength Division Multiplexing (DWDM).

CWDM: is a wavelength division multiplexing technique in which the distance between consecutive channels is greater than 20 nm and the spectral width of a channel is 2500 Ghz. The wavelength of the laser changes with temperature, but for this technique, no cooling is required because the distance between consecutive channels is large. CWDM technique is highly cost-effective for systems requiring few wavelengths.

As the system capacity increases, the number of multiplexed channels in the optical fiber increases. This makes it difficult for CWDM technology to meet the demand. Dense wavelength division multiplexing (DWDM) technology has overcome this problem. DWDM is a wavelength division multiplexing technology in which the distance between adjacent optical channels transmitted on the optical fiber is 0.8 nm at the frequency range of 1550 nm and the spectrum width of a channel is about 100 GHz. Currently, people can also multiplex wavelengths with channel spacing of 0.4 and 0.2 nm and spectrum widths of 50 and 20 GHz, respectively. When the width

As the wavelength spectrum decreases, there are many requirements that need to be solved such as: the temperature of the transmitting laser must be stable, the decoupling devices must operate more accurately. These requirements make the cost of DWDM devices increase significantly compared to the devices of the CWDM system. The comparison between CWDM and DWDM is illustrated in Table 2.2.

Table 2.2: Comparison table between CWDM and DWDM.

CWDM	DWDM
Wavelength distance	≈20 nm	≈0.8nm
Spectrum Width	2500 GHz	100 GHz
Environmental control	Are not	Have
Laser Source	DFB (no cooling)	DFB (cooling)
Data rate/channel	2.5 Gbit/s	10 Gbit/s
Concentrated bit rate	40 Gbit/s	320 Gbit/s
Channel cost	Short	High

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2.2.2. Transmission methods using wavelength division multiplexing

There are two methods of establishing a transmission system using wavelength division multiplexing (WDM), namely unidirectional and bidirectional WDM transmission.

2.2.2.1. Bidirectional WDM transmission method

R x1

T x2

R x2

The bidirectional WDM transmission method is: in the forward direction, the optical channels corresponding to the wavelengths  1 ,  2 , ...,  i are multiplexed together through a multiplexer/demultiplexer into a signal stream transmitted in one direction on a fiber. The same optical fiber, in the forward direction, the wavelengths  i+1,  i+2 ,...,  N are transmitted in the opposite direction. This method only requires using one optical fiber to establish a transmission system for both the forward and return directions. This method is shown in Figure 2.3.

λ 1 , λ 2 ,…, λ i

DE MUX

λ i+1 , λ i+2 ,…, λ N

MUX

T x1

T x N

R x N

Figure 2.3: Bidirectional WDM transmission method.

2.2.2.2 Unidirectional WDM transmission method.

Unidirectional WDM transmission method is: all optical channels on the same optical fiber are multiplexed into one signal stream and transmitted along the same path.

direction. In the forward direction, the optical channels correspond to wavelengths

 1 ,  2

,...., λ N through

λ 1 , λ 2 ,…, λ N

EDFA

λ 1 , λ 2 ,…, λ N

EDFA

T x1

MUX

DE MUX

T x2

T x N

R x1

R x2

The multiplexer combines the signals into a single stream and transmits them in one direction over an optical fiber to the receiver. At the receiver, the optical wavelength demultiplexer separates the signals with different wavelengths in the received signal stream to reach separate receivers. In the reverse direction, the transmission principle is the same as in the forward direction but on a separate optical fiber. The unidirectional transmission method is shown in Figure 2.4.

R x N

Figure 2.4: Unidirectional WDM transmission method.

Both transmission methods have their own advantages and disadvantages. Assuming current technology allows transmission of N wavelengths on a single fiber, the two methods can be compared as follows:

First is capacity: the two-fiber bidirectional transmission method has twice the capacity of the one-fiber bidirectional transmission method, but the number of optical fibers required is twice as many.

Next, in the event of a cable break, the two-fiber bidirectional transmission system does not require an automatic protection switching mechanism because both ends of the link are capable of instantly recognizing the failure.

In addition, when designing the network: bidirectional systems are more difficult to design because we have to consider interference factors due to the fact that there are more wavelengths transmitted on an optical fiber than a unidirectional system, ensuring routing and wavelength distribution so that the two directions on the optical fiber do not use the same wavelength.

Finally, the amplifiers in bidirectional systems are usually more complex in structure than in unidirectional systems. But because the number of amplified wavelengths in bidirectional systems is halved in each direction, amplifiers in bidirectional systems will provide larger optical output power than unidirectional systems.

2.3. DEVICES USED IN WDM SYSTEM

The structure of a WDM optical wavelength division multiplexing system includes the following devices: transmitter and receiver, interleaver, optical cross-connect, wavelength converter, router, optical amplifier. Next, we will learn about some types of devices used in the WDM system.

2.3.1. Optical Add/Drop Multiplexer (OADM)

Optical interleavers are used in WDM systems when the system needs to split or multiplex one or more channels while preserving the integrity of the other channels. The role of an optical interleaver can be clarified through the following example:

Node A Node B Node C

a Add/Drop

Node A Node B Node C

b Add/Drop

Figure 2.5: Role of the OADM.

Consider a network of three nodes A, B, and C. Node A communicates with node C through node B, assuming that the links are full duplex. Suppose the traffic requirements are as follows: one wavelength between A and B, and three wavelengths between A and C. Point-to-point WDM systems are deployed to provide this traffic requirement. There are two solutions as shown in Figure 2.5.

Solution 1: There are two point-to-point systems, one between A and B, one between B and C. Each point-to-point link uses an OLT at the end of the link (an optical line terminal is a device used at the end of a point-to-point link to multiplex and demultiplex wavelengths. The OLT consists of three elements: a repeater, a wavelength multiplexer, and an amplifier). Each node has four wavelengths, so

four transponders are needed. But only one wavelength is for node B, the remaining transponders are used for communication between nodes A and C. Therefore, six of the eight transponders at node B will be used for traffic control, which is very expensive. Solution 1 is shown in Figure 2.15(a).

Solution 2: do not use a point-to-point WDM system but use a wavelength routing network. At each node A and C, an OLT is used, and node B uses an OADM optical splitter/multiplexer. The OADM will split one of the four wavelengths of node B, the remaining three wavelengths go through the optical domain without the need for transponders, thus only two transponders are needed instead of eight as in solution 1, thus reducing costs. Solution 2 is illustrated in Figure 2.15(b).

There are many proposed architectures for OADMs, the simplest of which is to use one or more filters, a MUX/DEMUX. But there are two common structures: parallel and series.

2.3.1.1. Parallel structure

In the parallel topology, all signal channels are demultiplexed/multiplexed, then an arbitrary number of channels are demultiplexed, and the remaining channels are configured to pass through appropriately (see Figure 2.6(a)). An arbitrary set of channels can be demultiplexed, so there are no constraints on the channels that are interleaved and demultiplexed. Therefore, this configuration has the fewest constraints on the lightpath configuration in the network. In addition, the loss through the OADM is fixed and independent of the number of channels interleaved/demultiplexed. However, this configuration is not cost-effective for handling a small number of demultiplexed channels, because every time a wavelength needs to be demultiplexed, all the other wavelengths need to be demultiplexed and remultiplexed. So, it costs money to split and multiplex all incoming channels, and it also increases the loss, since all channels are split and multiplexed at every OADM, each path has to go through multiple filters before reaching the destination. But this configuration is more efficient when there is a large number of split channels and the flexibility to add or remove any channel.

λ 1 , λ 2 ,…, λ w

λ 2

λ 1 , λ 2 ,…, λ w

(a)

1 Drop

Add

λ 1 , λ 2 ,…, λ w

(b)

Band 1

Band 4

λ 1 , …, λ 4

Figure 2.6: Parallel OADM structure.

To reduce the cost of the above design, we have implemented the method shown in Figure 2.6(b) in two stages: the first stage is to separate the wavelengths into bands, the second stage is to separate the bands into individual wavelengths. For example, a system with 16 channels can be divided into 4 bands, each band consisting of 4 wavelengths. If only 4 channels are separated at a node, the remaining 12 channels can remain in the bands instead of being separated into individual channels. In addition, separating the channels into bands allows the signal to pass through with lower attenuation and better attenuation uniformity.

2.3.1.2. Serial structure

In the serial structure, each channel is interleaved/deleted from a main channel in turn and can be called by another name as a single channel OADM (Single Channel OADM). To interleave/delet multiple channels, SC-OADMs are connected in series as shown in Figure 2.7.

SC-OADM

Drop λ 1

Add

λ 2

λ 1 ,λ 2 ,…λ w λ 1 ,λ 2 ,…λ w

Figure 2.7: Serial OADM structure.

This structure is in many ways the opposite of the parallel structure. The interleaving/detaching of channels affects the existing channels. Therefore, it is necessary to plan which wavelength sets should be taken at each location to minimize this effect. This structure is only effective when a small number of channels are interleaving/detaching, it is not effective when the number of channels to be interleaving/detached is large, the cost can increase significantly because of the number of separate devices that have to be connected together. In addition, it increases the attenuation when there are many channels to be interleaving/detached, requiring additional amplifiers, thus increasing the cost of the system. The increase in attenuation with the number of channels to be interleaving/detached plays an important role in the OADM system.

continued. Suppose, the path budget allows a transmission between the receiver and the transmitter to be 25 dB. Consider the case where a transmission from node B to node D is carried out with a loss of nearly 25 dB as shown in Figure 2.8(a), suppose, that an additional channel with a different wavelength is needed from node A to node C, an OADM needs to be installed at node C to separate this new transmission. This OADM adds 3 dB of loss to the channels passing through node C. The addition of this OADM increases the loss from B to D to 28 dB as shown in Figure 2.8(b), thus, it is inefficient. To overcome this problem, try to restore the path of C by separating it, passing it through the restorer and re-assembling it. This requires an additional OADM at node C, and adds 3 dB of loss to the channels passing through node

C. This may in turn disrupt other paths passing through C as shown in Figure 2.8(c). Therefore, the insertion or removal of channels affects all other paths in the network. Using optical amplifiers along with careful path construction can partially overcome this. In a serial structure, the channels do not have to pass through any filters, so each path only passes through two filters at the source and destination nodes.

X-3 dB

SC-

(a)

X-3 dB

SC-

X-6 dB

SC-

(c)

Figure 2.8: Impact of network traffic changes using serial OADM.

To take advantage of the advantages of parallel structure and series structure, there is also a combined structure as shown in Figure 2.9.

OADM

Drop λ 1 , λ 2 , λ 3 , λ 4 Add

λ 1 , λ 2 ,…, λ w λ 1 , λ 2 ,…, λ w

Figure 2.9: Combined OADM structure.

In this architecture, a fixed set of channels is interleaved/demultiplexed from the main transmission channel. This set is passed through a further interleave/demultiplexing layer to divide it into separate channels. The additional channels are usually combined with simple multiplexers and added to the through channels. Typically, four consecutive channels are separated among the 32 channels using a bandpass filter.

The hybrid structure is a compromise between parallel and serial structures. The maximum number of channels that can be split is determined by the bandpass filter used. Within this group of channels, the insertion/detachment of additional channels does not affect other transmissions in the network. However, it is complex and imposes many constraints on wavelength assignment because only a fixed number can be split at each location. For example, if wavelength λ 1 is added at one node and taken out at the next node, all other wavelengths: λ 2 , λ 3 , λ 4 in the same wavelength band as λ 1 will also be added at that node and taken out at the next node. Once a split wavelength belongs to a band, it needs to be regenerated before it can be inserted back into the network. Therefore, in this example, wavelengths λ 2 , λ 3 , λ 4 need to be regenerated at both nodes. Therefore, it is difficult to construct a link budget that allows optical capacity for these wavelengths without regeneration. This problem can be overcome by using multiple types of OADMs, each focusing on a different set of wavelengths. This is a complex task, but if the wavelengths to be split can be pre-arranged and the network remains fixed, this is acceptable, but for networks where traffic varies over time, this is not easy.

2.3.1.3. Reconfigured OADM structure

Reconfiguration is essential for OADM. It allows the selection of wavelengths to be inserted/deleted, without having to plan and deploy the equipment accordingly. This allows the service provider to be flexible in planning the network and allows lightpaths to be established and terminated as required by the users in the network. The reconfigurable OADM architecture is illustrated in Figure 2.10.

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