**3. Challenges to be addressed in WDM systems**

As with any maturing technology, WDM too had a lot of challenges to be tackled as it progresses. For example, [13] discusses a hybrid multiplexer which can be used for WDM and MDM. Also WDM have found applications beyond communication, which also brings about additional challenges. Like in [14], WDM is used for optical beam steering, which is archived using photonic crystal waveguide and an integrated version of WDM in coupled micro-ring Muxs. Another field where WDM has found application is inter-chip links, as in [15] where micro-ring wavelength demultiplexers are used.

This section discusses about some of the realization challenges like EDFA transients and unequal link output power, which are not desirable in WDM-based systems. In this section, these effects are analyzed under different test cases and validated experimentally. Transient analysis is based on variations with input power, pump power, duty cycle, cascading stages, and multiplexed configurations.

### **3.1 Unequal channel power and its equalization**

As seen before, EDFA is a major component in a WDM link for providing amplification in wavelength range around 1550-nm optical communication band. The non-flat gain spectrum of EDFA leads to uneven amplification levels for various WDM channels. This becomes more serious as more and more amplifiers are added in the link. Another serious problem for WDM networks is the wavelength-dependent gain saturation of EDFAs. Because of this effect, the loss or removal of one or more channels at the input of an EDFA can cause large changes in the output powers of the remaining channels. This effect is more predominant in CWDM systems than DWDM systems with a smaller number of wavelengths because of wider wavelength spacings when a single "C"-band EDFA is used for amplification.

To overcome this problem, an L-band EDFA in combination with a C-band EDFA can be used to flatten the EDFA gain spectrum. By using EDFA with longer fiber length or heavy doping concentration, EDFA gain characteristics can be altered [9]. So an L-band EDFA with a longer fiber length was considered. In the proposed configuration, multiplexed signal is split and passed through C- and L-band EDFAs. **Figure 13** shows the variation of gain spectra for L-band EDFA. It is suggested that if wavelengths are beyond 1565 nm, L-band EDFAs are better for practical applications [9]. CWDM configuration where the signals are splitted first and then amplified using two separate EDFAs is shown in **Figure 14**. These signals are recombined later.

### **3.2 EDFA transients**

Due to slow gain recovery of EDFA gain, the low bit rate signals passing through EDFA undergo saturation and recovery effects during level transitions. The characteristic saturation and recovery times are, for typical operating conditions, in the range of 100 μs to 1 ms. As a result, EDFAs are intrinsically immune to the effects of cross talks at high data rates [16, 17]. The recovery time is of few hundred microseconds, hence do not affect the high bit rate signal amplification, as the erbium concentration is not significantly altered by the high bit rate signal during its short ON time period. But this is not the case with low bit rate signals, which is ON for enough time for reducing the population inversion, hence reducing the gain. The effect can be seen in **Figure 15**, when EDFA is input (1550 nm) with square optical pulse of 2 KHz and varying duty cycle, pumped with a 980-nm laser at 70 mW. The transient effects are sensitive to input signal duty cycle, signal power, and pump power of EDFA configuration. The following section focuses on EDFA transients applicable to multiplexed fiber links.

**103**

(**Figure 17**).

**Figure 14.**

**Figure 13.**

*L-band EDFA gain spectrum (30 m).*

*3.2.1 Compensation using complementary pulse*

*Block schematic diagram with C- and L-band EDFAs for power equalization.*

*Optically Multiplexed Systems: Wavelength Division Multiplexing*

EDFA transients disappear with increasing bit rate as shown in **Figure 16** (bit rate 10 KHz and 1 MHz). It also decreases with lower signal and pump power

Transient effects can produce a negative impact as the pulse shape at the output of the link is heavily distorted, it can lead to misinterpretation of data at the receiver side. Also, in cascaded EDFA applications, the transients can accumulate over length and can cause problems at detector stage. A few compensation techniques are described below.

To accomplish this, another complementary signal is multiplexed into the link at a different wavelength which ensures the EDFA input power remains constant. The block schematic and results are as shown in **Figure 18**. As the wavelength of compensation signal approaches the original wavelength of the signal pulse, the distortion is reduced, i.e., the closer the compensation wavelength, the better the compensation.

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

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

**Figure 13.** *L-band EDFA gain spectrum (30 m).*

*Multiplexing*

**3. Challenges to be addressed in WDM systems**

**3.1 Unequal channel power and its equalization**

As with any maturing technology, WDM too had a lot of challenges to be tackled as it progresses. For example, [13] discusses a hybrid multiplexer which can be used for WDM and MDM. Also WDM have found applications beyond communication, which also brings about additional challenges. Like in [14], WDM is used for optical beam steering, which is archived using photonic crystal waveguide and an integrated version of WDM in coupled micro-ring Muxs. Another field where WDM has found application is inter-chip links, as in [15] where micro-ring wavelength demultiplexers are used. This section discusses about some of the realization challenges like EDFA transients and unequal link output power, which are not desirable in WDM-based systems. In this section, these effects are analyzed under different test cases and validated experimentally. Transient analysis is based on variations with input power, pump power, duty cycle, cascading stages, and multiplexed configurations.

As seen before, EDFA is a major component in a WDM link for providing amplification in wavelength range around 1550-nm optical communication band. The non-flat gain spectrum of EDFA leads to uneven amplification levels for various WDM channels. This becomes more serious as more and more amplifiers are added in the link. Another serious problem for WDM networks is the wavelength-dependent gain saturation of EDFAs. Because of this effect, the loss or removal of one or more channels at the input of an EDFA can cause large changes in the output powers of the remaining channels. This effect is more predominant in CWDM systems than DWDM systems with a smaller number of wavelengths because of wider wavelength spacings when a single "C"-band EDFA is used for amplification.

To overcome this problem, an L-band EDFA in combination with a C-band EDFA can be used to flatten the EDFA gain spectrum. By using EDFA with longer fiber length or heavy doping concentration, EDFA gain characteristics can be altered [9]. So an L-band EDFA with a longer fiber length was considered. In the proposed configuration, multiplexed signal is split and passed through C- and L-band EDFAs. **Figure 13** shows the variation of gain spectra for L-band EDFA. It is suggested that if wavelengths are beyond 1565 nm, L-band EDFAs are better for practical applications [9]. CWDM configuration where the signals are splitted first and then amplified using two separate EDFAs is shown in **Figure 14**. These signals are recombined later.

Due to slow gain recovery of EDFA gain, the low bit rate signals passing through EDFA undergo saturation and recovery effects during level transitions. The characteristic saturation and recovery times are, for typical operating conditions, in the range of 100 μs to 1 ms. As a result, EDFAs are intrinsically immune to the effects of cross talks at high data rates [16, 17]. The recovery time is of few hundred microseconds, hence do not affect the high bit rate signal amplification, as the erbium concentration is not significantly altered by the high bit rate signal during its short ON time period. But this is not the case with low bit rate signals, which is ON for enough time for reducing the population inversion, hence reducing the gain. The effect can be seen in **Figure 15**, when EDFA is input (1550 nm) with square optical pulse of 2 KHz and varying duty cycle, pumped with a 980-nm laser at 70 mW. The transient effects are sensitive to input signal duty cycle, signal power, and pump power of EDFA configuration. The following section focuses on EDFA transients

**102**

**3.2 EDFA transients**

applicable to multiplexed fiber links.

**Figure 14.** *Block schematic diagram with C- and L-band EDFAs for power equalization.*

EDFA transients disappear with increasing bit rate as shown in **Figure 16** (bit rate 10 KHz and 1 MHz). It also decreases with lower signal and pump power (**Figure 17**).

Transient effects can produce a negative impact as the pulse shape at the output of the link is heavily distorted, it can lead to misinterpretation of data at the receiver side. Also, in cascaded EDFA applications, the transients can accumulate over length and can cause problems at detector stage. A few compensation techniques are described below.

### *3.2.1 Compensation using complementary pulse*

To accomplish this, another complementary signal is multiplexed into the link at a different wavelength which ensures the EDFA input power remains constant. The block schematic and results are as shown in **Figure 18**. As the wavelength of compensation signal approaches the original wavelength of the signal pulse, the distortion is reduced, i.e., the closer the compensation wavelength, the better the compensation.

### **Figure 15.**

*EDFA response to duty cycle variation (single input pulse).*

**Figure 16.** *EDFA response to 10 KHz (left) and 1 MHz (right) with different duty cycles.*

**Figure 17.** *EDFA transient response to signal (left) and pump variations (right).*

### *3.2.2 Compensation using delayed pulse*

In the case of 50% duty cycle pulse, two more additional suppression techniques are proposed. One uses electrical delay and the other uses optical delay. **Figure 19** shows the block schematic used for transient suppression of 50% duty cycle signal using optical delay line. **Figure 19** shows the experimental result of the transient suppressed output. The delay introduced should be equivalent to the ON/OFF time of transmitted signal. In this case, only one laser source is required. Optical delay can be introduced by using fiber spools of longer length. The delay can be applied electrically too; schematic and results are shown in **Figure 20**.

**105**

of this chapter.

**Figure 20.**

**Figure 18.**

**Figure 19.**

**4. Conclusion**

*Optically Multiplexed Systems: Wavelength Division Multiplexing*

*Compensation using complementary pulse. Block schematic (left) and results (right).*

*Compensation using delayed pulse. Schematic (left) and experimental result (right).*

*Compensation using electrical delayed pulse. Schematic (left) and result (right).*

EDFA transients affect WDM systems too in a similar manner by distorting the transmitted signal [17]. The description of the same is not included within the scope

The chapter introduces the concept of optical multiplexing with special focus on wavelength division multiplexing. Other multiplexing methods are also briefly described highlighting the operation and potential applications. A WDM link is

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

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

**Figure 18.**

*Multiplexing*

**104**

**Figure 16.**

**Figure 15.**

**Figure 17.**

*3.2.2 Compensation using delayed pulse*

*EDFA transient response to signal (left) and pump variations (right).*

In the case of 50% duty cycle pulse, two more additional suppression techniques are proposed. One uses electrical delay and the other uses optical delay. **Figure 19** shows the block schematic used for transient suppression of 50% duty cycle signal using optical delay line. **Figure 19** shows the experimental result of the transient suppressed output. The delay introduced should be equivalent to the ON/OFF time of transmitted signal. In this case, only one laser source is required. Optical delay can be introduced by using fiber spools of longer length. The delay can be applied

electrically too; schematic and results are shown in **Figure 20**.

*EDFA response to 10 KHz (left) and 1 MHz (right) with different duty cycles.*

*EDFA response to duty cycle variation (single input pulse).*

*Compensation using complementary pulse. Block schematic (left) and results (right).*

**Figure 19.** *Compensation using delayed pulse. Schematic (left) and experimental result (right).*

**Figure 20.** *Compensation using electrical delayed pulse. Schematic (left) and result (right).*

EDFA transients affect WDM systems too in a similar manner by distorting the transmitted signal [17]. The description of the same is not included within the scope of this chapter.

## **4. Conclusion**

The chapter introduces the concept of optical multiplexing with special focus on wavelength division multiplexing. Other multiplexing methods are also briefly described highlighting the operation and potential applications. A WDM link is

explained by going into detail of the different components making up the link. The chapter also includes a few challenges which degrade the performance of the link and potential methods to overcome those effects.

With the WDM Mux/DeMux described above, adding or dropping an unplanned channel may require the traffic in the entire link be suspended. But with a reconfigurable optical add-drop multiplexer (ROADM), an operator can remotely reconfigure the multiplexing so that data in the other channels are not interrupted. Several technologies are developed for achieving this.

Another interesting development is the emerging of super-channels, which reduces the channel gap close to the Nyquist bandwidth. The idea is to combine multiple coherent carriers to create a unified channel, called a super-channel, which will operate at the maximum data rate supported by the analog-to-digital convertor (ADC) at the receiver. The absence of guard channels and coherent detection ensures high spectral efficiency. Some techniques include orthogonal frequency division multiplexing (OFDM), orthogonal band multiplexed (OBM), no-guard-interval (NGI)-OFDM, multichannel equalization (MCE)-WDM, Nyquist WDM, etc.

These WDM links are widely used in various regimes of communication. At present, majority of the links are made with discrete components. When a greater number of channels are required to be transmitted, a small form factor solution is preferable. Currently many researches are being carried out to bring these components to a photonic-integrated circuit form which can reduce the size to a greater extent. It is quite sure that with the latest advancements in nanotechnology, more components can be integrated resulting in a very-small-factor WDM chips.
