**1. Introduction**

Since its advent in the mid-1960s, optical technologies and components have been changing the landscape of communication as such. The constant push for higher data rates ensured that optical components matured fast, enabling the terabits of data rates we are enjoying today. It all started with extremely lossy optical fibers coupled with a broadband source, which could transmit only a few mbps over a few meters. The scenario drastically changed over half a century, reaching data rates of even terabit/sec possible over a single fiber. Now we have the luxury of external cavity lasers (ECL) which can easily give linewidths below 1 MHz, Mach-Zehnder modulators (MZM) which can easily operate at 40 Gbps and above, low attenuation dispersion managed fibers, dispersion compensation fibers, low-noise high-gain optical amplification systems, very high-speed detectors, and extremely fast digital signal processing (DSP) capabilities which make many compensation hardware redundant.

A commonly overlooked component of this lot is the multiplexer, without which the entire system is only fast enough as any of its electronic counterpart. Multiplexer is the one which helps in combining (and splitting) the data from different sources so that the tremendous bandwidth of the system could be utilized. As with other components, Mux/DeMux came a long way into maturing and providing the kind of precise work it does today. So, this chapter is dedicated for giving the readers a quick understanding of the different types and techniques for the implementation of wavelength division multiplexing (WDM) schemes.

Multiplexing is the process of combining multiple signals into a shared channel used for tapping the full potential of the optical links. It facilitates networking with advanced topologies supported with redundancy features. Historically, multiplexing had been used to share the limited bandwidth of the medium between different transmitters, but with optical systems it is more about making full use of the huge available bandwidth. This is where wavelength division multiplexing comes in where different channels are multiplexed into a single fiber. It divides the huge bandwidth of optical fiber into various logical channels of lower bandwidth that can be filled with electronically achievable data rates. The advent of coherent optical links and advanced multiplexing techniques used in wireless communication raised the achievable bandwidth limit of fiber links. But the proposed chapter focuses on one of the most common and important multiplexing techniques, wavelength division multiplexing.

The advent of erbium-doped fiber amplifiers (EDFAs) in the late 1980s proved to be a great impetus for multichannel system implementation. EDFA, with its capability to amplify the C-band, proved to be a vital component that could facilitate WDM in long-haul links. As the different WDM channels could traverse the fiber without cross talk, and EDFA can amplify these signals simultaneously, it increased the transmission rates exponentially. This ushered in the need of multiplexers, specifically wavelength division multiplexers. A few popular optical multiplexing techniques are discussed later in this chapter. Also, it should be noted that being bidirectional, most of these devices in these schemes can be used as Mux or DeMux.

### **1.1 Other optical multiplexing techniques**

With the optical components maturing and becoming very reliable and accurate, almost all multiplexing techniques possible in the wireless communications are viable in optics now. Though a few are truly developed, the others are expected to mature in the near future. A few of these techniques are discussed here.

### *1.1.1 Time-division multiplexing*

Probably the most used scheme in electrical and wireless systems, optical timedivision multiplexing (OTDM) does not have that much widespread use, probably because of the large bandwidth already available with optical fibers and the widespread use of WDM but is still used in different applications. Pulse interleaving is used to allow different optical data streams to use the full capacity of the fiber, albeit for a limited time. Different optical data pulses thus share the same optical channel with its full capacity for a limited time and passes on to the next pulse and so on so forth. Thus, the overall data rate in the fiber is improved despite the fact that the individual data streams are still operating at the same speed. But the optical pulses have to be compressed, to fit in the time slot per bit. A train of ultrashort pulses can be helpful in this regard, along with an optical delay line for combining all the pulses. So, for a good OTDM link, we need pulses with high extinction, short duration, and low timing jitter.

**91**

*Optically Multiplexed Systems: Wavelength Division Multiplexing*

electronics can also be shared between different channels too.

The basic idea is to have different channels separated spatially. This spatial separation can be brought out in different ways; the simplest is with the use of multiple fibers. But this requires most of the channel infrastructure to be duplicated for each fiber, hence not most economical. But still there could be some cost improvements as one may choose to use a high-power laser along with splitters to pump different optical amplifiers corresponding to each fiber. Or multiple fibers can be integrated to form one fiber cable or use a fiber ribbon. Another possible way of cost reduction is by using a vertical-cavity surface-emitting laser (VCSEL) array at the source along with the integration of different receivers into the same chip. Also, some

A better implementation of SDM will be using multicore fibers. These are single optical fibers having multiple cores, each core carrying one communication channel. The cores are usually separated enough that there is no coupling and cross talk. But this limits the number of cores, as increasing the cladding diameter tends to make the fiber more brittle. Additionally, this makes fiber splicing more difficult. A work around of this is to keep the core together allowing the different modes to couple to form a supermode (coupled SDM fiber) and then later electronically process the detected data using multiple input/multiple output (MIMO) techniques in detection process. MIMO techniques have already matured in wireless communication. Photonic crystal fibers are a very good candidate for these kinds of coupled SDM fibers. But there are a lot of challenges that are to be dealt with before this becomes commercial; this includes difficulties in splicing multicore fibers, maintaining the correct rotation orientation, developing spatial multiplexers and fiber

amplifiers, providing sufficient gain uniformity over different modes, etc.

Another potential method for the MIMO setup is the use of few-mode fibers, using a technique called mode division multiplexing (MDM). This has some advantages like easier fiber connections and splices. But as the cross talk with different fiber modes are high and there is a significant difference in the group velocity of the different modes, MIMO receivers are much more complex than the above techniques [1].

Generally used together with phase modulation or QAM, the idea is to modulate different information on orthogonal polarizations of the same frequency effectively doubling the data rate. PDM signal can be transmitted over normal WDM infrastructure expanding its capacity. But here the challenges are the drifts in the polarization state of the fiber-optic system over time. Also polarization mode dispersion, polarization-dependent losses, and cross-polarization modulation create additional challenges. But the DSP techniques at the receiver are becoming faster and more

The orbital angular momentum (OAM) of the light waves is used to carry orthogonal information. OAM can in theory have huge number of parallel channels, bound only by practical limitations. OAM states of light is not supported on conventional single-mode fiber, instead few-mode or multimode fibers are to be used. Additionally, conventional fiber suffers from mode coupling which changes the orbital angular momentum when the fiber is bent or stressed causing mode instability. But coherent detection along with signal processing techniques can be used to correct mode mixing

*DOI: http://dx.doi.org/10.5772/intechopen.88086*

*1.1.3 Polarization-division multiplexing*

efficient in compensating these impediments.

*1.1.4 Orbital angular momentum multiplexing*

in fiber. These coherent systems are normally complex in nature.

*1.1.2 Space-division multiplexing*

## *1.1.2 Space-division multiplexing*

*Multiplexing*

wavelength division multiplexing.

**1.1 Other optical multiplexing techniques**

*1.1.1 Time-division multiplexing*

duration, and low timing jitter.

A commonly overlooked component of this lot is the multiplexer, without which the entire system is only fast enough as any of its electronic counterpart. Multiplexer is the one which helps in combining (and splitting) the data from different sources so that the tremendous bandwidth of the system could be utilized. As with other components, Mux/DeMux came a long way into maturing and providing the kind of precise work it does today. So, this chapter is dedicated for giving the readers a quick understanding of the different types and techniques for the implementation of wavelength division multiplexing (WDM) schemes. Multiplexing is the process of combining multiple signals into a shared channel used for tapping the full potential of the optical links. It facilitates networking with advanced topologies supported with redundancy features. Historically, multiplexing had been used to share the limited bandwidth of the medium between different transmitters, but with optical systems it is more about making full use of the huge available bandwidth. This is where wavelength division multiplexing comes in where different channels are multiplexed into a single fiber. It divides the huge bandwidth of optical fiber into various logical channels of lower bandwidth that can be filled with electronically achievable data rates. The advent of coherent optical links and advanced multiplexing techniques used in wireless communication raised the achievable bandwidth limit of fiber links. But the proposed chapter focuses on one of the most common and important multiplexing techniques,

The advent of erbium-doped fiber amplifiers (EDFAs) in the late 1980s proved to be a great impetus for multichannel system implementation. EDFA, with its capability to amplify the C-band, proved to be a vital component that could facilitate WDM in long-haul links. As the different WDM channels could traverse the fiber without cross talk, and EDFA can amplify these signals simultaneously, it increased the transmission rates exponentially. This ushered in the need of multiplexers, specifically wavelength division multiplexers. A few popular optical multiplexing techniques are discussed later in this chapter. Also, it should be noted that being bidirectional, most of these devices in these schemes can be used as Mux or DeMux.

With the optical components maturing and becoming very reliable and accurate,

Probably the most used scheme in electrical and wireless systems, optical timedivision multiplexing (OTDM) does not have that much widespread use, probably because of the large bandwidth already available with optical fibers and the widespread use of WDM but is still used in different applications. Pulse interleaving is used to allow different optical data streams to use the full capacity of the fiber, albeit for a limited time. Different optical data pulses thus share the same optical channel with its full capacity for a limited time and passes on to the next pulse and so on so forth. Thus, the overall data rate in the fiber is improved despite the fact that the individual data streams are still operating at the same speed. But the optical pulses have to be compressed, to fit in the time slot per bit. A train of ultrashort pulses can be helpful in this regard, along with an optical delay line for combining all the pulses. So, for a good OTDM link, we need pulses with high extinction, short

almost all multiplexing techniques possible in the wireless communications are viable in optics now. Though a few are truly developed, the others are expected to

mature in the near future. A few of these techniques are discussed here.

**90**

The basic idea is to have different channels separated spatially. This spatial separation can be brought out in different ways; the simplest is with the use of multiple fibers. But this requires most of the channel infrastructure to be duplicated for each fiber, hence not most economical. But still there could be some cost improvements as one may choose to use a high-power laser along with splitters to pump different optical amplifiers corresponding to each fiber. Or multiple fibers can be integrated to form one fiber cable or use a fiber ribbon. Another possible way of cost reduction is by using a vertical-cavity surface-emitting laser (VCSEL) array at the source along with the integration of different receivers into the same chip. Also, some electronics can also be shared between different channels too.

A better implementation of SDM will be using multicore fibers. These are single optical fibers having multiple cores, each core carrying one communication channel. The cores are usually separated enough that there is no coupling and cross talk. But this limits the number of cores, as increasing the cladding diameter tends to make the fiber more brittle. Additionally, this makes fiber splicing more difficult.

A work around of this is to keep the core together allowing the different modes to couple to form a supermode (coupled SDM fiber) and then later electronically process the detected data using multiple input/multiple output (MIMO) techniques in detection process. MIMO techniques have already matured in wireless communication. Photonic crystal fibers are a very good candidate for these kinds of coupled SDM fibers. But there are a lot of challenges that are to be dealt with before this becomes commercial; this includes difficulties in splicing multicore fibers, maintaining the correct rotation orientation, developing spatial multiplexers and fiber amplifiers, providing sufficient gain uniformity over different modes, etc.

Another potential method for the MIMO setup is the use of few-mode fibers, using a technique called mode division multiplexing (MDM). This has some advantages like easier fiber connections and splices. But as the cross talk with different fiber modes are high and there is a significant difference in the group velocity of the different modes, MIMO receivers are much more complex than the above techniques [1].

### *1.1.3 Polarization-division multiplexing*

Generally used together with phase modulation or QAM, the idea is to modulate different information on orthogonal polarizations of the same frequency effectively doubling the data rate. PDM signal can be transmitted over normal WDM infrastructure expanding its capacity. But here the challenges are the drifts in the polarization state of the fiber-optic system over time. Also polarization mode dispersion, polarization-dependent losses, and cross-polarization modulation create additional challenges. But the DSP techniques at the receiver are becoming faster and more efficient in compensating these impediments.
