*2.3.7 Acousto-optic filter-based devices*

An interesting method for realizing a DeMux is shown in **Figure 9**. The device consists of an all-pass polarizer which linearly polarized the input signal. For demultiplexing these linearly polarized wavelengths, a combination of AOF and polarizing beam splitter (PBS) is used. The AOF can be controlled with electrical signal to rotate the polarization of a desired wavelength from transverse electric (TE) to transverse magnetic (TM). The PBS then reflects one of the wavelengths based on its polarization, resulting in DeMux operation.

### **2.4 Amplifier**

Optical signals produced by laser, modulated with information at the multiplexer and segregated and propagated through optical fiber, are prone to attenuation and losses arising from all these components. Optical fiber technology is so advanced now that the transmission loss is practically negligible for short-haul communications. It is the component insertion loss that causes more serious signal attenuation. Eventually signal amplitude may get small enough to fall below the receiver sensitivity, which can be prevented with the use of

**99**

*Optically Multiplexed Systems: Wavelength Division Multiplexing*

amplifiers in the optical link. Before the invention of optical amplifiers, amplification could only be done in electrical domain. But this needed conversion from optical to electrical and then back to optical conversion (O-E-O). Additionally, electrical regenerators are generally sensitive to bit rate and modulation formats, which make it less flexible for additional capacity. But optical amplifiers work in optical domain and amplify the signals without O-E-O conversion. Further, they are transparent to bit rate and modulation format changes. Now, there are amplifiers that have a wide gain-bandwidth like EDFA, Raman amplifiers that can amplify signals over a large wavelength range. These facilitate the widespread use of WDM systems, which need simultaneous amplification of a

EDFAs are the most commonly used optical amplifiers owing to their larger spectrum and high gain and simplicity. EDFA came into being by the early 1990s and has completely changed the landscape of optical communication industry. The most significant advantages of EDFA is its ability to amplify a wider bandwidth of signals, which is a big boost to WDM technique, as multiple channels can be amplified simultaneously. Other important aspects that make EDFA so mainstream is the availability of compact and high-power pump laser source, polarization insensitivity, easiness in coupling, absence of cross talk, and its

EDFA is an optical fiber with its core doped with rare earth mineral, which acts as the amplifying medium. Doping can also be done using holmium, neodymium, samarium, thulium, and ytterbium to provide gain in ranges from 500 to 3500 nm. EDFA has the capability to amplify signals in 1550-nm band, the standard telecom-

One main problem with EDFA is its nonuniform spectra. Different channels are amplified differently, and the difference builds up over a long-haul system with multiple EDFAs. Energy levels and gain spectrum of EDFA are shown in **Figure 10**.

Optical detectors are devices that convert the optical signals into electrical signals. Usually a photodetector is followed by a front-end amplifier to amplify the electrical signal, which is followed by a decision circuit that estimates the data content of the electrical signal. Decision circuit needs to know the modulation scheme used for transmission. An optional optical preamplifier section can be used in front

Photodetector works based on photoelectric effect. It is desirable for a photodetector to have "high sensitivity, fast response, low noise, low cost, and high

Several solutions have emerged in addressing this issue efficiently.

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

wide range of wavelengths.

*Acousto-optic filter configuration [10].*

**Figure 9.**

inherent simplicity [3].

munication regime [11].

**2.5 Detector**

of the photodetector.

*2.4.1 Erbium-doped fiber amplifier*

**Figure 8.** *Spectral filter-based devices [10].*

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

### **Figure 9.**

*Multiplexing*

be integrated on silica.

**2.4 Amplifier**

*2.3.6 Spectral filter-based devices*

*2.3.7 Acousto-optic filter-based devices*

• It has low losses and cross talk (insertion loss <−3 dB and cross talk <−35 dB).

• It can be fabricated on Si as photonic-integrated circuit (PIC) and can be easily

AWG suffers from drawbacks like polarization dependency and temperature

MZI-based Mux/DeMux works in the same principle of interference, where interfering coherent light of different wavelengths forms maxima at different spatial points and hence can be demuxed out. MZI-based Mux/DeMux devices can

A spectral filter inserted in the optical path can be used to sort out wavelengths and hence can be used as DeMux. These devices can be implemented in different

The first one is a fiber sandwiched at the cleaved surface of a fiber. The incident ray with two wavelengths is incident on the filter, which passes one and reflects the other. The reflected one is collected through another fiber achieving DeMux operation. Another form of filter can be implemented in a graded-index (GRIN) rod.

An interesting method for realizing a DeMux is shown in **Figure 9**. The device

consists of an all-pass polarizer which linearly polarized the input signal. For demultiplexing these linearly polarized wavelengths, a combination of AOF and polarizing beam splitter (PBS) is used. The AOF can be controlled with electrical signal to rotate the polarization of a desired wavelength from transverse electric (TE) to transverse magnetic (TM). The PBS then reflects one of the wavelengths

Optical signals produced by laser, modulated with information at the multiplexer and segregated and propagated through optical fiber, are prone to attenuation and losses arising from all these components. Optical fiber technology is so advanced now that the transmission loss is practically negligible for short-haul communications. It is the component insertion loss that causes more serious signal attenuation. Eventually signal amplitude may get small enough to fall below the receiver sensitivity, which can be prevented with the use of

sensitivity. A lot of works have been done in addressing these issues.

integrated with photodetectors as well.

*2.3.5 Mach-Zehnder interferometer-based devices*

configurations, a couple of which are shown in **Figure 8**.

based on its polarization, resulting in DeMux operation.

**98**

**Figure 8.**

*Spectral filter-based devices [10].*

*Acousto-optic filter configuration [10].*

amplifiers in the optical link. Before the invention of optical amplifiers, amplification could only be done in electrical domain. But this needed conversion from optical to electrical and then back to optical conversion (O-E-O). Additionally, electrical regenerators are generally sensitive to bit rate and modulation formats, which make it less flexible for additional capacity. But optical amplifiers work in optical domain and amplify the signals without O-E-O conversion. Further, they are transparent to bit rate and modulation format changes. Now, there are amplifiers that have a wide gain-bandwidth like EDFA, Raman amplifiers that can amplify signals over a large wavelength range. These facilitate the widespread use of WDM systems, which need simultaneous amplification of a wide range of wavelengths.

### *2.4.1 Erbium-doped fiber amplifier*

EDFAs are the most commonly used optical amplifiers owing to their larger spectrum and high gain and simplicity. EDFA came into being by the early 1990s and has completely changed the landscape of optical communication industry. The most significant advantages of EDFA is its ability to amplify a wider bandwidth of signals, which is a big boost to WDM technique, as multiple channels can be amplified simultaneously. Other important aspects that make EDFA so mainstream is the availability of compact and high-power pump laser source, polarization insensitivity, easiness in coupling, absence of cross talk, and its inherent simplicity [3].

EDFA is an optical fiber with its core doped with rare earth mineral, which acts as the amplifying medium. Doping can also be done using holmium, neodymium, samarium, thulium, and ytterbium to provide gain in ranges from 500 to 3500 nm. EDFA has the capability to amplify signals in 1550-nm band, the standard telecommunication regime [11].

One main problem with EDFA is its nonuniform spectra. Different channels are amplified differently, and the difference builds up over a long-haul system with multiple EDFAs. Energy levels and gain spectrum of EDFA are shown in **Figure 10**. Several solutions have emerged in addressing this issue efficiently.

### **2.5 Detector**

Optical detectors are devices that convert the optical signals into electrical signals. Usually a photodetector is followed by a front-end amplifier to amplify the electrical signal, which is followed by a decision circuit that estimates the data content of the electrical signal. Decision circuit needs to know the modulation scheme used for transmission. An optional optical preamplifier section can be used in front of the photodetector.

Photodetector works based on photoelectric effect. It is desirable for a photodetector to have "high sensitivity, fast response, low noise, low cost, and high

**Figure 10.** *(a) EDFA energy level diagram (b) absorption and gain spectra (codoped with Germania) [11].*

reliability" for being more effective in communication engineering. When a photon of energy, hν, which exceeds the photodetector band gap, is incident on a photodetector, the photon is absorbed, and an electron-hole pair is generated (**Figure 11**). The electric field across the junction sweeps off this excess charge, hence producing a current flow in the external circuit.

Normally a reverse bias is applied to the junction. A reverse bias adds to the junction electric field, and the photocurrent generated by the absorption of photon is proportional to the incident optical power. It should be noted that the optical power is exponentially attenuated while it passes through the semiconductor material. The energy of the incident photon should be larger than the bandgap, e.g., of the detector. The lowest such wavelength is the cutoff wavelength above which the detector cannot operate. As Si and Ge have cutoff wavelength lower than 1550 nm, they are not used for communication application. Generally, indium gallium arsenide (InGaAs) and indium gallium arsenide phosphide (InGaAsP) are used in 1550 and 1310 nm wavelength ranges.

An important characteristic of photodetector is its responsivity, R. It is defined as

$$\mathcal{R} = I\_p / P\_{in} \tag{2}$$

where *Ip* is the average photocurrent and *Pin* is the incident optical power. As *Pin*/*h* ν*e* electrons are generated by a photon of Energy *Pin* and assuming only η fraction of incident photons is actually absorbed, then *R* can be written as [9]

$$\mathbf{R} = \frac{\eta \,\lambda}{h \,\nu\_e} \tag{3}$$

**101**

**Figure 11.**

**Figure 12.**

*PIN photodiode structure.*

*Photodiode basic principle.*

*Optically Multiplexed Systems: Wavelength Division Multiplexing*

In PIN photodiode, the depletion region width is increased by the introduction of layer of very lightly doped intrinsic semiconductor material between p and n sides, hence the name PIN. **Figure 12** shows the structure of PN junction.

Due to the light doping, this layer provides high resistance and most of the voltage drops across it. In effect, most of the recombination happen in the depletion region; hence, the drift current far overweighs the diffusion current. So a longer W will increase the photodiode sensitivity, but a longer depletion width also implies larger transit time for the charge carries, hence increasing the response time. So

Similar to PN photodetectors, double-heterostructure design can further improve the performance of PIN photodiodes. By choosing material of sufficiently larger bandgap as p and n regions, the absorption can be limited only to the i regions. One such example is to use InP as the p and n region while using InGaP as the intrinsic layer [12]. Such a design helps to avoid the diffusion part of photocurrent, hence increasing the efficiency to nearly 100%. Reflections from the front

Responsivity of PN diode is limited by Eq. (3). This is due to the fact that one incident photon can generate maximum of only one e-h pair. Avalanche photodiodes have internal mechanism which overcomes this and can provide larger photocurrent. They are especially preferred when the incident intensity on the

there needs to be a trade-off between sensitivity and response time.

facets can be reduced by coating with suitable dielectric layers.

photodetector is expected to be low.

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

which can be written in terms of λ as *R* = \_ 1.24where λ is expressed in *m*.

### *2.5.1 PIN photodiode*

As light is incident on the end of PN junction, the electron-hole pairs generated have to diffuse to the depletion region before getting swept away (drift) to the corresponding electrode, hence creating current in the external circuit. Diffusion velocity is slow and is in the range of 1 ns per μm. This causes the input signal to be distorted at the electrical output. Increasing the depletion region length can decrease the diffusion time. This is the idea behind PIN photodiode [3].

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

**Figure 11.** *Photodiode basic principle.*

*Multiplexing*

**Figure 10.**

reliability" for being more effective in communication engineering. When a photon of energy, hν, which exceeds the photodetector band gap, is incident on a photodetector, the photon is absorbed, and an electron-hole pair is generated (**Figure 11**). The electric field across the junction sweeps off this excess charge, hence producing

*(a) EDFA energy level diagram (b) absorption and gain spectra (codoped with Germania) [11].*

Normally a reverse bias is applied to the junction. A reverse bias adds to the junction electric field, and the photocurrent generated by the absorption of photon is proportional to the incident optical power. It should be noted that the optical power is exponentially attenuated while it passes through the semiconductor material. The energy of the incident photon should be larger than the bandgap, e.g., of the detector. The lowest such wavelength is the cutoff wavelength above which the detector cannot operate. As Si and Ge have cutoff wavelength lower than 1550 nm, they are not used for communication application. Generally, indium gallium arsenide (InGaAs) and indium gallium arsenide phosphide (InGaAsP) are used in 1550 and

An important characteristic of photodetector is its responsivity, R. It is defined as

*R* = *Ip*/ *Pin* (2)

where *Ip* is the average photocurrent and *Pin* is the incident optical power. As *Pin*/*h* ν*e* electrons are generated by a photon of Energy *Pin* and assuming only η frac-

> η λ *h* ν*<sup>e</sup>*

As light is incident on the end of PN junction, the electron-hole pairs generated

have to diffuse to the depletion region before getting swept away (drift) to the corresponding electrode, hence creating current in the external circuit. Diffusion velocity is slow and is in the range of 1 ns per μm. This causes the input signal to be distorted at the electrical output. Increasing the depletion region length can

decrease the diffusion time. This is the idea behind PIN photodiode [3].

(3)

1.24where λ is expressed in *m*.

tion of incident photons is actually absorbed, then *R* can be written as [9]

*<sup>R</sup>* = \_

which can be written in terms of λ as *R* = \_

a current flow in the external circuit.

1310 nm wavelength ranges.

*2.5.1 PIN photodiode*

**100**

**Figure 12.** *PIN photodiode structure.*

In PIN photodiode, the depletion region width is increased by the introduction of layer of very lightly doped intrinsic semiconductor material between p and n sides, hence the name PIN. **Figure 12** shows the structure of PN junction.

Due to the light doping, this layer provides high resistance and most of the voltage drops across it. In effect, most of the recombination happen in the depletion region; hence, the drift current far overweighs the diffusion current. So a longer W will increase the photodiode sensitivity, but a longer depletion width also implies larger transit time for the charge carries, hence increasing the response time. So there needs to be a trade-off between sensitivity and response time.

Similar to PN photodetectors, double-heterostructure design can further improve the performance of PIN photodiodes. By choosing material of sufficiently larger bandgap as p and n regions, the absorption can be limited only to the i regions. One such example is to use InP as the p and n region while using InGaP as the intrinsic layer [12]. Such a design helps to avoid the diffusion part of photocurrent, hence increasing the efficiency to nearly 100%. Reflections from the front facets can be reduced by coating with suitable dielectric layers.

Responsivity of PN diode is limited by Eq. (3). This is due to the fact that one incident photon can generate maximum of only one e-h pair. Avalanche photodiodes have internal mechanism which overcomes this and can provide larger photocurrent. They are especially preferred when the incident intensity on the photodetector is expected to be low.
