**2. WDM link and components**

A WDM link typically used in communication system is as shown in the block schematic (**Figure 1**). There are multiple transmitters each working on its own dedicated wavelength. These individual data streams on independent wavelengths are all multiplexed together using a WDM Mux and transmitted through an optical fiber. The transmitted power is kept low enough in the link, so as not to trigger any nonlinearities in the fiber. In the absence of nonlinearities, the wavelengths do not talk to each other or induce cross talks. The C-band can be divided coarsely every 20 nm (called coarse WDM) if the link is low cost or have to support only limited number of parallel light paths. Dense WDM uses a much more tighter wavelength division scheme and needs costlier components and better lasers with lower linewidths and external modulators. It supports a greater number of channels resulting in a much higher throughput, e.g., 40 channels within 1530–1565 nm, C-band with 100 nm spacing, or 80 channels at 50 GHz spacing or even 12.5 GHz—all at the cost of significant overhead costs.

These multiplexed data will become weaker over the distance, and that's where the EDFA comes in. It's EDFA's broad bandwidth, which helps in amplifying all the channels with nearly the same gain, that paved the path for the bandwidth explosion within the fiber. After the amplification the wavelengths are demultiplexed at the DeMux and forwarded to the corresponding receivers which completes the link.

Typical WDM link consists of components like transmitters, add/drop multiplexers, and necessary detectors for the communication link. Based on the requirements, it also includes preamplifier, line amplifiers, and post amplifiers in the link. Specific WDM components like WDM/demultiplexer (Mux/DeMux) matured fast to make the systems relatively common and affordable. It also made possible to have fiber links to their maximum possible bandwidth capability.

### **2.1 Optical source**

Laser is the most widely used optical source, owing to its inherent advantages like single-frequency operation, coherence, high intensity, and directionality [2]. Laser is essentially an oscillator, having a gain medium and feedback mechanism. The gain medium is kept at an excited state by external pumping mechanisms (optical or electrical). This produces population inversion which is a necessary condition for lasing. Initial photons can be introduced into the medium by spontaneous emission, and those modes that are supported by cavity and gain spectrum are lasered out. The basic assembly of a laser is as shown in **Figure 2**.

The development of semiconductor lasers operating at room temperature (1970) provided a compact, highly efficient, and reliable optical source, which is put to great use by the communication industry [3]. Semiconductor laser needs

**93**

**Figure 3.**

*DFB laser structure [3].*

introduced [3].

**Figure 2.**

*Laser assembly block schematic.*

shows the structure of a DFB laser.

*Optically Multiplexed Systems: Wavelength Division Multiplexing*

an optical feedback mechanism for converting it from an amplifier to oscillator. Oscillations begin when the loop gain exceeds unity. Gain is obtained due to stimulated emission in optical gain medium, and the cavity formed by cleaved laser facets provides the required feedback for sustained oscillations. This configuration forms a Fabry-Perot (FP) cavity. FP laser can lase at multiple longitudinal modes that are spaced apart according to *Δ* ν*L* = *c*/2*L* where n is the refractive index (RI) and L is the cavity length. If the spacing between neighboring modes are small enough, then the laser cavity can provide almost the same gain for each of those longitudinal modes causing them to coexist. Such longitudinal modes travel with a different velocity inside the optical fiber causing group velocity dispersion and hence limits the maximum data rate through the optical fiber. So, lasers operating within a single longitudinal mode are preferred for many applications. To overcome this, distributed feedback (DFB) lasers, which achieve single longitudinal mode operation by distributing the reflection throughout cavity length, are

DFB laser developed during the 1980s is the most commonly used single-mode laser. The idea is to introduce a wavelength selective element within the laser cavity. This is achieved by introducing an etched diffraction grating within the laser waveguide structure. This can be done in two ways. If the grating layer extends through the whole of the active layer, it is called DFB, and if the gain region is in a separate planar section, the device is known as distributed Bragg reflector (DBR). **Figure 3**

In DBR lasers, the fiber Bragg gratings are used like mirrors in FP cavity with the difference that these gratings reflect only one longitudinal mode. The active layer provides cumulative gain only for the feedback wavelength, hence resulting

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

**Figure 1.**

*Block schematic of a basic WDM link.*

### *Optically Multiplexed Systems: Wavelength Division Multiplexing DOI: http://dx.doi.org/10.5772/intechopen.88086*

**Figure 2.** *Laser assembly block schematic.*

*Multiplexing*

**2.1 Optical source**

**2. WDM link and components**

A WDM link typically used in communication system is as shown in the block schematic (**Figure 1**). There are multiple transmitters each working on its own dedicated wavelength. These individual data streams on independent wavelengths are all multiplexed together using a WDM Mux and transmitted through an optical fiber. The transmitted power is kept low enough in the link, so as not to trigger any nonlinearities in the fiber. In the absence of nonlinearities, the wavelengths do not talk to each other or induce cross talks. The C-band can be divided coarsely every 20 nm (called coarse WDM) if the link is low cost or have to support only limited number of parallel light paths. Dense WDM uses a much more tighter wavelength division scheme and needs costlier components and better lasers with lower linewidths and external modulators. It supports a greater number of channels resulting in a much higher throughput, e.g., 40 channels within 1530–1565 nm, C-band with 100 nm spacing, or 80 channels at 50 GHz spacing or even 12.5 GHz—all at the cost of significant overhead costs.

These multiplexed data will become weaker over the distance, and that's where the EDFA comes in. It's EDFA's broad bandwidth, which helps in amplifying all the channels with nearly the same gain, that paved the path for the bandwidth explosion within the fiber. After the amplification the wavelengths are demultiplexed at the DeMux and forwarded to the corresponding receivers which completes the link. Typical WDM link consists of components like transmitters, add/drop multiplexers, and necessary detectors for the communication link. Based on the requirements, it also includes preamplifier, line amplifiers, and post amplifiers in the link. Specific WDM components like WDM/demultiplexer (Mux/DeMux) matured fast to make the systems relatively common and affordable. It also made possible to have

Laser is the most widely used optical source, owing to its inherent advantages like single-frequency operation, coherence, high intensity, and directionality [2]. Laser is essentially an oscillator, having a gain medium and feedback mechanism. The gain medium is kept at an excited state by external pumping mechanisms (optical or electrical). This produces population inversion which is a necessary condition for lasing. Initial photons can be introduced into the medium by spontaneous emission, and those modes that are supported by cavity and gain spectrum are lasered

The development of semiconductor lasers operating at room temperature (1970) provided a compact, highly efficient, and reliable optical source, which is put to great use by the communication industry [3]. Semiconductor laser needs

fiber links to their maximum possible bandwidth capability.

out. The basic assembly of a laser is as shown in **Figure 2**.

**92**

**Figure 1.**

*Block schematic of a basic WDM link.*

an optical feedback mechanism for converting it from an amplifier to oscillator. Oscillations begin when the loop gain exceeds unity. Gain is obtained due to stimulated emission in optical gain medium, and the cavity formed by cleaved laser facets provides the required feedback for sustained oscillations. This configuration forms a Fabry-Perot (FP) cavity. FP laser can lase at multiple longitudinal modes that are spaced apart according to *Δ* ν*L* = *c*/2*L* where n is the refractive index (RI) and L is the cavity length. If the spacing between neighboring modes are small enough, then the laser cavity can provide almost the same gain for each of those longitudinal modes causing them to coexist. Such longitudinal modes travel with a different velocity inside the optical fiber causing group velocity dispersion and hence limits the maximum data rate through the optical fiber. So, lasers operating within a single longitudinal mode are preferred for many applications. To overcome this, distributed feedback (DFB) lasers, which achieve single longitudinal mode operation by distributing the reflection throughout cavity length, are introduced [3].

DFB laser developed during the 1980s is the most commonly used single-mode laser. The idea is to introduce a wavelength selective element within the laser cavity. This is achieved by introducing an etched diffraction grating within the laser waveguide structure. This can be done in two ways. If the grating layer extends through the whole of the active layer, it is called DFB, and if the gain region is in a separate planar section, the device is known as distributed Bragg reflector (DBR). **Figure 3** shows the structure of a DFB laser.

In DBR lasers, the fiber Bragg gratings are used like mirrors in FP cavity with the difference that these gratings reflect only one longitudinal mode. The active layer provides cumulative gain only for the feedback wavelength, hence resulting

**Figure 3.** *DFB laser structure [3].*

in single-wavelength operation of laser. A Bragg grating is realized by periodically varying refractive index along the length of the optical transmission. Condition for reflection of Bragg wavelength λ*B* from a grating with period Λ is given by

$$
\Lambda = m \,\,\langle \lambda\_B / 2n \rangle \tag{1}
$$

where m is an integer and n is the refractive index. Due to frequency selective nature of the feedback mechanism, the output of the laser becomes highly monochromatic. Later improvements in DFB lasers include phase-shifted DFB laser [4] and gain-coupled DFB lasers [5].

In a semiconductor laser under forward bias, population inversion occurs, and optical gain is realized only when the injected current into the active region is greater than a minimum value known as the transparency value [3]. The input signal propagating inside the gain medium is amplified by a factor of *e gz*, where g is the gain coefficient and z is the length within the cavity. A certain portion of the generated photons is lost due to cavity losses and needs to be replenished. The optical gain must be high enough to compensate for this loss, else the photon population does not build up. This puts a minimum value of gain with which the laser should be operated to achieve lasing. This is the laser threshold, which is achieved only if the laser works above a threshold pumping level which corresponds to the threshold current.

### **2.2 Modulators**

The process of imposing data on the light stream is called modulation. At bit rates more than 10 Gb/sec, chirp effect induced by direct modulators is predominant and puts a limit on modulation bandwidth. Chirp is a phenomenon wherein the carrier frequency of transmitted pulse varies with time, causing broadening of the transmitted spectrum. Laser output acts as a carrier signal over which the input signal gets modulated with the help of modulators. These modulators are classified into electro-absorption (EA) and electro-optic modulators. The performance of an external modulator is measured based on extinction ratio and the modulation bandwidth.

In direct modulation the laser output intensity is controlled by directly modulating the injection current of laser diode in accordance with the input signal. But this can lead to chirping effect at higher frequencies. So, it is preferred to keep the laser source as itself and modulate the light output from it by keeping an external modulator in front. **Figure 4** shows the schematics of direct and external modulation schemes.

### *2.2.1 Direct modulation*

Direct modulation of laser drive current is simpler, cost effective, and gives satisfactory performance for lower-frequency-modulating signals. But as the drive current to laser is varied directly, turn on delay and oscillation can result out of the fast-changing pumping current causing frequency chirping and linewidth broadening [6]. ON and OFF operations of laser cause the gain to change rapidly in the lasing medium. The change in gain causes a change in carrier concentration which in turn changes the refractive index, and this periodic change in the refractive index results in frequency chirping (the spectrum changes with time). When a chirped pulse propagates through a dispersive medium like optical fiber, the spread in frequency causes certain portion of the wave to travel faster/slower with respect to other portions leading to intersymbol interference (ISI).

**95**

*Optically Multiplexed Systems: Wavelength Division Multiplexing*

*Modulation schemes (a) direct modulation and (b) external modulator.*

A direct modulation is not suited for very high data rate transmission due to reasons such as chirp in DFB laser and mode partition noise in FP laser. So external modulation schemes are used for high-frequency modulation as it does not affect the laser characteristics and is implemented as an additional component in front of a CW laser. But this leads to an additional insertion loss for external modulators. Large modulation bandwidth and depth, small insertion loss, lower electrical drive power, etc. far outweigh its cons for bit rates above some 10 Gb/s. Some desirable characteristics of external modulators are polarization independence, good linearity

(between drive current and modulated output), lower cost, and smaller size.

modulators are also made of the same material.

or higher with traveling wave electrode configuration [8].

**2.3 WDM multiplexer and demultiplexer**

External modulators make use of techniques like electro-optic effect, acoustooptic effect (AOF), and electro-absorption effect to modulate the information signal over the incident CW optical beam. Acousto-optic modulators are slower and hence are commonly not used for communication purposes. *Electro-absorption* modulators are usually II–V materials, which alter its absorption coefficients according to an external voltage to obtain intensity modulation directly. These modulators are generally capable of attaining an extinction ratio of 15 dB or more at bit rates up to 40 Gb/s [7]. They have further advantages of being efficient and compact in size and can also be easily integrated into the same chip along with laser, as these

Another class of external modulators are the *electro-optic modulators*, which works by altering its optical properties (mostly the refractive index (RI)) with the application of electrical field. This refractive index change may be due to Pockels effect, where the RI changes linearly with the applied electric field, or due to Kerr effect wherein the RI change is proportional to square of the electric field amplitude. When an optical signal passed through the altered RI region, it induces a phase change (or polarization rotation) in the signal, which can be converted to amplitude modulation by using Mach-Zehnder interferometer (MZI) configuration. LiNbO3 is the most widely used medium for this purpose, as it is optically birefringent and hence can be externally controlled by an applied electric field. A Mach-Zehnder-type external modulator uses phase modulation along with an integrated Mach-Zehnder interferometer to achieve intensity modulation. These modulators are capable of modulating up to 60 GHz [7]. The modulating frequency can be further extended up to 100 GHz

The inherent immense bandwidth of optical fiber systems can be tapped by the use of multiplexing techniques, which facilitates the electronics to work in much

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

*2.2.2 External modulation*

**Figure 4.**

*Optically Multiplexed Systems: Wavelength Division Multiplexing DOI: http://dx.doi.org/10.5772/intechopen.88086*

**Figure 4.**

*Multiplexing*

and gain-coupled DFB lasers [5].

**2.2 Modulators**

bandwidth.

tion schemes.

*2.2.1 Direct modulation*

in single-wavelength operation of laser. A Bragg grating is realized by periodically varying refractive index along the length of the optical transmission. Condition for

Λ = *m* (λ*B*/2*n*) (1)

where m is an integer and n is the refractive index. Due to frequency selective nature of the feedback mechanism, the output of the laser becomes highly monochromatic. Later improvements in DFB lasers include phase-shifted DFB laser [4]

In a semiconductor laser under forward bias, population inversion occurs, and optical gain is realized only when the injected current into the active region is greater than a minimum value known as the transparency value [3]. The input signal

coefficient and z is the length within the cavity. A certain portion of the generated photons is lost due to cavity losses and needs to be replenished. The optical gain must be high enough to compensate for this loss, else the photon population does not build up. This puts a minimum value of gain with which the laser should be operated to achieve lasing. This is the laser threshold, which is achieved only if the laser works above a threshold pumping level which corresponds to the threshold current.

The process of imposing data on the light stream is called modulation. At bit rates more than 10 Gb/sec, chirp effect induced by direct modulators is predominant and puts a limit on modulation bandwidth. Chirp is a phenomenon wherein the carrier frequency of transmitted pulse varies with time, causing broadening of the transmitted spectrum. Laser output acts as a carrier signal over which the input signal gets modulated with the help of modulators. These modulators are classified into electro-absorption (EA) and electro-optic modulators. The performance of an external modulator is measured based on extinction ratio and the modulation

In direct modulation the laser output intensity is controlled by directly modulat-

ing the injection current of laser diode in accordance with the input signal. But this can lead to chirping effect at higher frequencies. So, it is preferred to keep the laser source as itself and modulate the light output from it by keeping an external modulator in front. **Figure 4** shows the schematics of direct and external modula-

Direct modulation of laser drive current is simpler, cost effective, and gives satisfactory performance for lower-frequency-modulating signals. But as the drive current to laser is varied directly, turn on delay and oscillation can result out of the fast-changing pumping current causing frequency chirping and linewidth broadening [6]. ON and OFF operations of laser cause the gain to change rapidly in the lasing medium. The change in gain causes a change in carrier concentration which in turn changes the refractive index, and this periodic change in the refractive index results in frequency chirping (the spectrum changes with time). When a chirped pulse propagates through a dispersive medium like optical fiber, the spread in frequency causes certain portion of the wave to travel faster/slower with respect to

other portions leading to intersymbol interference (ISI).

*gz*, where g is the gain

propagating inside the gain medium is amplified by a factor of *e*

reflection of Bragg wavelength λ*B* from a grating with period Λ is given by

**94**

*Modulation schemes (a) direct modulation and (b) external modulator.*

### *2.2.2 External modulation*

A direct modulation is not suited for very high data rate transmission due to reasons such as chirp in DFB laser and mode partition noise in FP laser. So external modulation schemes are used for high-frequency modulation as it does not affect the laser characteristics and is implemented as an additional component in front of a CW laser. But this leads to an additional insertion loss for external modulators. Large modulation bandwidth and depth, small insertion loss, lower electrical drive power, etc. far outweigh its cons for bit rates above some 10 Gb/s. Some desirable characteristics of external modulators are polarization independence, good linearity (between drive current and modulated output), lower cost, and smaller size.

External modulators make use of techniques like electro-optic effect, acoustooptic effect (AOF), and electro-absorption effect to modulate the information signal over the incident CW optical beam. Acousto-optic modulators are slower and hence are commonly not used for communication purposes. *Electro-absorption* modulators are usually II–V materials, which alter its absorption coefficients according to an external voltage to obtain intensity modulation directly. These modulators are generally capable of attaining an extinction ratio of 15 dB or more at bit rates up to 40 Gb/s [7]. They have further advantages of being efficient and compact in size and can also be easily integrated into the same chip along with laser, as these modulators are also made of the same material.

Another class of external modulators are the *electro-optic modulators*, which works by altering its optical properties (mostly the refractive index (RI)) with the application of electrical field. This refractive index change may be due to Pockels effect, where the RI changes linearly with the applied electric field, or due to Kerr effect wherein the RI change is proportional to square of the electric field amplitude. When an optical signal passed through the altered RI region, it induces a phase change (or polarization rotation) in the signal, which can be converted to amplitude modulation by using Mach-Zehnder interferometer (MZI) configuration. LiNbO3 is the most widely used medium for this purpose, as it is optically birefringent and hence can be externally controlled by an applied electric field. A Mach-Zehnder-type external modulator uses phase modulation along with an integrated Mach-Zehnder interferometer to achieve intensity modulation. These modulators are capable of modulating up to 60 GHz [7]. The modulating frequency can be further extended up to 100 GHz or higher with traveling wave electrode configuration [8].

### **2.3 WDM multiplexer and demultiplexer**

The inherent immense bandwidth of optical fiber systems can be tapped by the use of multiplexing techniques, which facilitates the electronics to work in much

lower rate than the optical transmission rate. It is known that transmitting data over a single fiber with higher rates is more economical than carrying lower data rates over several fibers [9]. This makes multiplexing a must, so that huge transmission capacity of the optical fibers can be supported using moderate electronic component rates.

Different varieties of Mux/DeMux are available. Mostly these are reciprocal devices, hence can be used as both Mux and DeMux. They can be classified under two broad categories, diffraction-based and interference-based. Diffraction-based devices rely on angular dispersive element like diffraction gratings to decompose the incident light into its spectral components.

Interference-based DeMux are based on optical filter and directional couplers. Filter-based Mux uses optical interference for wavelength selectivity. MZI-based filters are the most used. One arm of MZI can be made longer to induce phase difference with respect to the other arm. This phase difference is frequency dependent. The path length is adjusted such that power from two different input ports adds up at only one output port (Mux operation).

The idea is to spatially separate the different wavelength that were traversing together in the optical fiber. Each of these wavelengths can be collected in those points into individual optical fibers. Optical Mux/DeMux can be broadly classified into passive and active. Popular passive Mux/demultiplexers are based on Prims, diffraction gratings, and spectral filters. Active Mux/DeMux is usually implemented as some passive components along with tunable detector, each tuned to separate wavelength. We will see each of them in a bit more details now.

### *2.3.1 Prism-based devices*

These devices work based on the principle of dispersion, where different wavelengths see a different refractive index in the medium. This difference in refractive index results in some wavelengths to bend more (or less) than others which helps in separating them out. As can be seen in **Figure 5**, the incoming wavelengths are collimated and incident on the prism. Each wavelength seems a slightly different refractive index and bends differently according to Snell's law. At the output another lens focuses the different wavelengths to different output fibers.

### *2.3.2 Superprism-based devices*

Superprisms employ photonic bandgap that make certain wavelength forbidden within the structure. This is achieved using special structures called photonic crystal. A photonic crystal is a periodic dielectric structure fabricated usually on Si using nanofabrication. This three-dimensional periodicity in refractive index causes periodic distribution in bands and gaps, and these can be tuned by varying the periodicity so as to make certain wavelength to propagate or not. It can act both

**97**

**Figure 7.**

DeMux.

**Figure 6.**

*Optically Multiplexed Systems: Wavelength Division Multiplexing*

almost 500 times more dispersion than normal prisms.

*2.3.4 Arrayed waveguide grating (AWG)-based devices*

as energy bandgap filters as described above and as highly dispersive media. This high-dispersion property can be used to make prism called superprisms as they have

Diffraction elements as the name suggests use diffraction of light to separate different wavelengths. When a polychromatic light wave is incident on a diffraction grating, each wavelength is diffracted at a different angle from where they can be

AWG works on the principle of interference on a specially designed structure as shown in **Figure 7**. It has two free space propagation regions (S1 and S2), an array of waveguides (Wn) in the middle and fibers for input and output. A WDM signal incident on S1 through F traverses the free space and enters the arrayed waveguide region. The length of each waveguide in the arrayed waveguide section is varied such that it introduces a wavelength-dependent phase delay in S2. This phase delay causes the interference points of each wavelength to be spatially separated, where a fiber is connected to collect each wavelength, hence attaining

AWG has some interesting features as follows which makes it very attractive.

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

*2.3.3 Diffraction grating-based devices*

collected to achieve demuxing (**Figure 6**).

• AWG has a flat spectral response.

*Diffraction grating-based Mux configuration [10].*

*AWG-based Mux/DeMux configuration [10].*

**Figure 5.** *Prism-based DeMuX configuration [10].*

as energy bandgap filters as described above and as highly dispersive media. This high-dispersion property can be used to make prism called superprisms as they have almost 500 times more dispersion than normal prisms.
