**4.3. Advanced model of mixed WDM**

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electrically filtered using Bessel electrical filters.

diagrams and BER values at 75 GHz channel spacing

**4.2. Chromatic dispersion management strategies** 

comparing to other fixed output power levels "in [3]". On the other fiber end optical signals are filtered with optical Super Gaussian filters, converted to electrical signals and then

**Figure 9.** Mixed HDWDM system 1st/2nd/3rd channel's BER correlation diagram, detected signals eye

dispersion management strategy, which will be the next sections main goal.

conjugator (OPC) can be used for chromatic dispersion compensation.

In order to achieve the greater range and information capacity, the amplifiers must be located close together with gain no greater than 10 dB and preferably less "in [2]". Amplifier spacing further increment will lead to increase of ASE noise influence and as a result BER growth for each system channel. As well as, we must take into account system's accumulated chromatic

Chromatic dispersion divides into material and waveguide dispersion.Waveguide dispersion is caused by physical structure of optical fiber core and cladding (refractive index profile), and as a result different wavelengths propagate at different velocities in the core and cladding. Material dispersion is dominant part of chromatic dispersion, and it is caused by change of optical fiber core and cladding refractive index with wavelength "in [4]".

Dispersion compensating fiber (DCF), fiber Bragg grating (FBG) and optical phase

DCF has large negative dispersion (D = -80 ps/(nm\*km), that helps to compensate chromatic dispersion. Such an optical fiber with negative dispersion is achieved by developing a complex refractive index profile. The effective core area (*Aeff*) of a DCF is much smaller than There are three different combined chromatic dispersion compensation methods and one combined method described in this work. These simple CD compensation methods are DCF and FBG, but as a combined CD compensation method for practical realization we offer the compound solution of these two methods (DCF – FBG). In this work we are studied chromatic dispersion compensation solutions that can be implemented in transmitter or receiver side, as close as possible to beginning or end of fiber optical link. In these places it is possible to access directly to the optical fiber and place dispersion compensation modules.

The aim of this research is to find the best chromatic dispersion compensation solution that can be used for implementation and adaptation in already working mixed fiber optical transmission systems to improve the performance of these systems (see Fig. 10). As

**Figure 10.** Simulation scheme of 9 – channel mixed HDWDM system and channels' transmitting and receiving parts of NRZ – OOK/ 2 – POLSK/ NRZ – DPSK modulated optical signals

performance improvement we mean the improvement of data transmission rate and / or transmission distance, simultaneously guaranteeing a stable system working condition with the recommended bit error ratio BER<10-12.

If dispersion compensation is not used in developed mixed WDM model, then performance of 1st and 2nd channel is seriously affected by accumulated dispersion. The 3rd channel is affected by accumulated CD at the same level, but this channel's tolerance to CD is much higher, because there is used NRZ – DPSK as a coding format and optical signals are transmitted with 10 Gbit/s per channel bit rate. Without dispersion compensation the BER value of the 1st and 2nd channel are high (there are many bit errors) and we can assume it is because of inter-symbol interference (ISI), which causes pulse overlapping and receiver has difficulties to separate transmitted bit sequence. In this case BER > 10-12, system performance is poor and fiber optical WDM transmission system is not able to qualitatively transmit information over distance of 50 km, until CD compensation will not be realized (see Fig. 11).

#### Realization of Mixed WDM Transmission System 251

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**Figure 10.** Simulation scheme of 9 – channel mixed HDWDM system and channels' transmitting and

performance improvement we mean the improvement of data transmission rate and / or transmission distance, simultaneously guaranteeing a stable system working condition with

If dispersion compensation is not used in developed mixed WDM model, then performance of 1st and 2nd channel is seriously affected by accumulated dispersion. The 3rd channel is affected by accumulated CD at the same level, but this channel's tolerance to CD is much higher, because there is used NRZ – DPSK as a coding format and optical signals are transmitted with 10 Gbit/s per channel bit rate. Without dispersion compensation the BER value of the 1st and 2nd channel are high (there are many bit errors) and we can assume it is because of inter-symbol interference (ISI), which causes pulse overlapping and receiver has difficulties to separate transmitted bit sequence. In this case BER > 10-12, system performance is poor and fiber optical WDM transmission system is not able to qualitatively transmit information over distance of 50 km, until CD compensation will not be realized (see Fig. 11).

receiving parts of NRZ – OOK/ 2 – POLSK/ NRZ – DPSK modulated optical signals

the recommended bit error ratio BER<10-12.

**Figure 11.** Output eye diagram for all three channels of mixed WDM system without CD compensation

As one can see from obtained detected signal eye diagrams, then 3rd channel's eye is wide open and has almost ideal form. This fact let us to conclude, that in this channel it is possible an error-free transmission even without using some CD compensation schemes. As for 1st and 2nd system's channel, then CD compensation is vital necessary, because eye openings are completely closed for these transmission system channel's eye diagrams. Without CD compensation signal transmission in these channels over 50 km of SSMF with BER<10-12 is impossible.

The first realized compensation method includes the implementation of dispersion compensating fiber (DCF). We used pre- and post-compensation schemes for effective chromatic dispersion compensation in our mixed WDM system. The best proportion of LDCF1 and LDCF2 was studied, where LDCF1 is a DCF fiber length at transmitter side (precompensation) and LDCF2 is DCF fiber length at receiver side (post-compensation), in km. As shown in Fig. 12, we found that the optimal required DCF fiber length proportion is 5/5 km.

That proportion of DCF length was found by analyzing Q – value correlation diagrams. As one can see from 2nd channel's (system's worst channel) Q – value correlation diagram, then the highest - value (20.75 dB) is for fiber length proportion equal to 5/5 km. DCF proportion length was chosen on basis of 2nd channel Q – value due to the fact, that this channel is the most affected by transmission impairments and it has BER values higher than for 1st and 3rd channels. Implementation of two DCF fiber spans, with length equal to 5km each, provides the best CD compensation results for our investigated mixed fiber optical transmission system.

The second realized compensation method includes the implementation of fiber Bragg grating (FBG). At the first stage we changed FBG compensated CD value from -1000 ps/nm to -600 ps/nm with 25 ps/nm step and found that optimal compensation level providing BER<10-12 is equal to -750 ps/nm. As displayed in Fig. 13, the best BER results for the 1st and 2nd channel can be achieved, if we compensate all CD level, which is accumulated during optical signal transmission over the 50 km of SSMF (800 ps/nm). The optimal compensation level was chosen on the basis of BER results obtained from the 1st channel of mixed WDM system. If we overcompensate accumulated dispersion (if compensated CD level exceeds - 875 ps/nm then BER value grows rapidly and exceeds 10-12 value for the systems first channel.

**Figure 12.** Mixed WDM system 1st/2nd/3rd channel's Q-factor correlation diagrams, detected signals eye diagrams and BER values at 75 GHz channel spacing

At the second stage the optimal dispersion slope compensation value for FBG used in our simulation scheme was investigated. Dispersion slope value was changed from -6 ps/nm2 to -1 ps/nm2 with 0.5 ps/nm2 step. As shown in Fig. 13, the optimal dispersion slope value was chosen equal to -2.5 ps/nm2. Such a conclusion was obtained basis on the first channel BER value at above mentioned dispersion slope value. This was done due to the fact that BER values for this channel are higher than for the second channel's BER values, but BER values for the third system's channel do not vary depending on compensated CD and dispersion slope levels (see Fig. 13).

In comparison, if we use DCF pre- and post-compensation modules to compensate 750 ps/nm of accumulated CD, then the worst system's channel (1st) BER value is equal to 2.78. 10-14 (Q=17.60 dB). This value was obtained in case, if used DCF length proportion is 7:3. Numerically this means that pre-compensation module compensates 525 ps/nm, but postcompensation module compensates 225 ps/nm. As one can see, despite the fact that DCF length proportion 5:5 provides better BER results for the 2nd system's channel, it is not providing the optimal Q values (or BER values) in each system's channel. The system's 1st (worst) channel BER=2.34. 10-13 (Q=17.28 dB), if DCF length proportion, which is used in preand post-compensation modules, is equal to 5:5.

The last inspected dispersion compensation method is combined and includes the common implementation of dispersion compensating fiber (DCF) and fiber Bragg grating (FBG) in our combined fiber optical transmission system. This combined CD compensation method will be named as DCF-FBG. In pre-compensation module DCF fiber will be used, but in post-compensation module FBG will be used. For dispersion compensation we changed DCF length LDCF from 0 km to 15 km, with step 1 km to find out optimal DCF length and FBG dispersion compensation level proportion, see Fig. 13. The DCF dispersion D=-80 ps/nm/km and total accumulated dispersion amount in fiber optical link that must be compensated is -750 ps/nm to obtain BER values in each system's channel smaller than 10-12, as we found out in previous section.

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**Q value [dB]**

**21 20.75 20.5 20.25 20 19.75 19.5 19.25**

diagrams and BER values at 75 GHz channel spacing

slope levels (see Fig. 13).

(worst) channel BER=2.34.

and post-compensation modules, is equal to 5:5.

2.78.

**DCF length proportion** 

**2nd channel Q-factor correlation diagram**

**0/10 1/9 2/8 3/7 4/6 5/5 6/4 7/3 8/2 9/1 10/0**

**Figure 12.** Mixed WDM system 1st/2nd/3rd channel's Q-factor correlation diagrams, detected signals eye

At the second stage the optimal dispersion slope compensation value for FBG used in our simulation scheme was investigated. Dispersion slope value was changed from -6 ps/nm2 to -1 ps/nm2 with 0.5 ps/nm2 step. As shown in Fig. 13, the optimal dispersion slope value was chosen equal to -2.5 ps/nm2. Such a conclusion was obtained basis on the first channel BER value at above mentioned dispersion slope value. This was done due to the fact that BER values for this channel are higher than for the second channel's BER values, but BER values for the third system's channel do not vary depending on compensated CD and dispersion

In comparison, if we use DCF pre- and post-compensation modules to compensate 750 ps/nm of accumulated CD, then the worst system's channel (1st) BER value is equal to

The last inspected dispersion compensation method is combined and includes the common implementation of dispersion compensating fiber (DCF) and fiber Bragg grating (FBG) in our combined fiber optical transmission system. This combined CD compensation method will be named as DCF-FBG. In pre-compensation module DCF fiber will be used, but in

10-14 (Q=17.60 dB). This value was obtained in case, if used DCF length proportion is 7:3. Numerically this means that pre-compensation module compensates 525 ps/nm, but postcompensation module compensates 225 ps/nm. As one can see, despite the fact that DCF length proportion 5:5 provides better BER results for the 2nd system's channel, it is not providing the optimal Q values (or BER values) in each system's channel. The system's 1st

10-13 (Q=17.28 dB), if DCF length proportion, which is used in pre-

**Figure 13.** Mixed WDM system 1st/2nd/3rd channel's BER correlation diagrams, which represents channel's BER value as a function from compensated CD level and BER as a function from compensated dispersion slope value at the dispersion compensation level equal to 750 ps/nm; eye diagrams of detected signals and BER values at chosen compensated CD and slope values

Therefore we used a following formula: DFBG= -750 + 80. (LDCF), where DFBG is FBG CD compensation amount provided by FBG. Dispersion amount that can be compensated by DCF fiber can be expressed as DDCF = -80. LDCF, where LDCF is the length of used DCF.

As one can see, in Fig. 14, the optimal proportion |DDCF/DFBG| is 21/79 %. It means that 21% of -750 ps/nm must be compensated by DCF, but remaining 79% must be compensated by FBG. Numerically, in our investigated system, it means that DCF compensate 160 ps/nm (2 km of DCF are used), but FBG compensates remaining 590 ps/nm of accumulated CD amount. This proportion was found basis on the 1st system's channel BER values. This channel is the most affected by chromatic dispersion due to the fact that for optical signal modulation in this channel NRZ – OOK modulation format is used. This modulation format has smaller CD tolerance comparing to 2 – POLSK and NRZ – DPSK formats.

**Figure 14.** Mixed WDM system 1st/2nd/3rd channel's BER correlation diagrams, eye diagrams of detected signals and BER values at optimal DCF-FBG dispersion compensation proportion level

This is recommended DCF-FBG proportion for optimal accumulated CD compensation in [NRZ – OOK (40 Gbit/s, 193.025 THz)] – [2 – POLSK (40 Gbit/s, 193.100 THz)] – [NRZ – DPSK (10 Gbit/s, 193.175 THz)] mixed fiber optical transmission system.

To identify channel that is a source of larger amount of interchannel crosstalk noise than the rest of system's channels six mixed systems were investigated. These systems differ from each other only with modulation formats distribution among channels. This distribution scheme is as follows: [NRZ-DPSK (1, 2, 3, 3, 2, 1)]-[NRZ – OOK (2, 1, 1, 2, 3, 3)]-[2 – POLSK (3, 3, 2, 1, 2)]. This configuration represents modulation format and channel's number where one on these formats is used. The system's channels central frequencies are anchored to 193.1 THz according to ITU-T Recommendation G.694.1 and the first channel's central frequency is equal to 193.075 THz, the second is193.100 THz and the third is193.125 THz. After this crosstalk source have been detected simulation model were updated in order to find out the optimal modulation format distribution, which provides the lowest in system's channels detected signals' BER values. For this purpose existing transmission systems model were updated to nine-channel WDM system. These channels are grouped by three and these groups have identical transmitter and receiver blocks configuration but with different channels' central wavelengths. It was specially done to take into account linear and nonlinear crosstalk influences to optical signal transmission which are experience central's group channel (1st-3rd) from adjacent groups (4th-6th and 7th-9th). For system's further analysis we will use only channels number 1-3, but 4-6 and 7-9 are used as sources of interchannel crosstalk.

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– DPSK formats.

compensated by FBG. Numerically, in our investigated system, it means that DCF compensate 160 ps/nm (2 km of DCF are used), but FBG compensates remaining 590 ps/nm of accumulated CD amount. This proportion was found basis on the 1st system's channel BER values. This channel is the most affected by chromatic dispersion due to the fact that for optical signal modulation in this channel NRZ – OOK modulation format is used. This modulation format has smaller CD tolerance comparing to 2 – POLSK and NRZ

**Figure 14.** Mixed WDM system 1st/2nd/3rd channel's BER correlation diagrams, eye diagrams of detected

This is recommended DCF-FBG proportion for optimal accumulated CD compensation in [NRZ – OOK (40 Gbit/s, 193.025 THz)] – [2 – POLSK (40 Gbit/s, 193.100 THz)] – [NRZ –

To identify channel that is a source of larger amount of interchannel crosstalk noise than the rest of system's channels six mixed systems were investigated. These systems differ from each other only with modulation formats distribution among channels. This distribution scheme is as follows: [NRZ-DPSK (1, 2, 3, 3, 2, 1)]-[NRZ – OOK (2, 1, 1, 2, 3, 3)]-[2 – POLSK (3, 3, 2, 1, 2)]. This configuration represents modulation format and channel's number where one on these formats is used. The system's channels central frequencies are anchored to 193.1 THz according to ITU-T Recommendation G.694.1 and the first channel's central frequency is equal to 193.075 THz, the second is193.100 THz and the third is193.125 THz. After this crosstalk source have been detected simulation model were updated in order to find out the optimal modulation format distribution, which provides the lowest in system's channels detected signals' BER values. For this purpose existing transmission systems model were updated to nine-channel WDM system. These channels are grouped by three

signals and BER values at optimal DCF-FBG dispersion compensation proportion level

DPSK (10 Gbit/s, 193.175 THz)] mixed fiber optical transmission system.

Then NRZ – DPSK, 2 – POLSK and NRZ – OOK modulated optical signals are combined, optically preamplified with fixed output power erbium-doped fiber amplifier (EDFA) and send over 50 km of single mode optical fiber. There are two different types of single mode fiber used in this research: standard single mode fiber or SSMF (according to ITU – T Recommendation G.652 D) and non-zero dispersion shifted fiber or NZ – DSF (according to ITU – T Recommendation G.655). Then optical signals are filtered with Super Gaussian optical filters, converted to electrical signals and then electrically filtered using Bessel electrical filters. Fiber span length was chosen equal to 50 km in order to avoid increase of amplified spontaneous emission (ASE) noise. Larger amplifier spacing would require gain greater than 10 dB, but this in a prohibitive leads to growth of ASE noise "in [14]". The aim of this task was to investigate optimal configuration for mixed WDM systems where differently modulated optical signals are transmitted. To achieve this goal several objectives must be solved.

Firstly it is necessary to identify channel in [1st: NRZ – DPSK, 10 Gbit/s, 193.075 THz]-[2nd: 2 – POLSK, 10 Gbit/s, 193.100 THz]-[3rd: NRZ – OOK, 10 Gbit/s, 193.125 THz] mixed WDM FOTS that is a source of larger amount of interchannel crosstalk noise than the rest of channels.

For this purpose six different systems were studied. These systems differ from each other only with modulation formats that are used in each particular system's channels. These systems have following configurations:

> [NRZ-DPSK]-[NRZ-OOK]-[2-POLSK]; [NRZ-OOK]-[NRZ-DPSK]-[2-POLSK]; [NRZ-OOK]-[2-POLSK]-[NRZ-DPSK]; [2-POLSK]-[NRZ-OOK]-[NRZ-DPSK]; [2-POLSK]-[NRZ-DPSK]-[NRZ-OOK]; [NRZ-DPSK]-[2-POLSK]-[NRZ-OOK].

In each systems channel were determined signals BER values that further were used for system's performance analyze. The obtained results are summarized below (see Table 2). Using these results for each configuration system's average detected signals BER values were calculated. As one can see, sufficiently smaller BER value is for the third mixed system configuration then it is for the rest of possible configuration. The third configuration is as follows: [1st: NRZ – OOK\193.075 THz]-[2nd: 2 – POLSK\193.100 THz]-[3rd: NRZ – DPSK\193.125 THz]. After careful analysis of these obtained results it was found that investigated mixed system channel, where for optical signals modulation NRZ-DPSK

modulation format is used, is a source of larger amount of interchannel crosstalk than channels, where NRZ – OOK or 2 – POLSK format is used. This was concluded based on obtained NRZ – OOK and 2 – POLSK channels BER results for different system's configurations. This become evident if we analyze obtained BER values for the fourth, fifth and sixth system.


**Table 2.** BER values for different mixed systems channels

Firstly let's focus to the fifth system's BER values. As one can see from this configuration scheme then in this case NRZ – DPSK modulated optical signals are transmitted in central system's channel. As a result detected signals BER values in adjacent channels are sufficiently higher than they are in cases, when NRZ – OOK or 2 – POLSK modulated optical signals are located further from NRZ – DPSK channel as it is in the sixth system. Comparing BER results obtained for 2 – POLSK modulated signals in the fourth and sixth system (1×10-40 and 9×10-14 respectively), we can conclude that in mixed system detected signals BER value decreases if channel, where these signals are transmitted, is located further from NRZ – DPSK channel.

To assess NRZ-DPSK channel created crosstalk impact to optical signals transmission in all others mixed system's channels previously studied three-channel mixed systems model was modified and supplemented with 2 × 3 channels that have appropriate system's configuration. As before, in system channels detected signals BER values were obtained for six different mixed system configurations (see Table 3).

As well as using these data two different channels average BER values were calculated: system's average BER that takes into account all system channels (1st-9th); central group channels' average BER that takes into account only channels number one to three.


Realization of Mixed WDM Transmission System 257

**Table 3.** BER values for different 9-channel mixed systems' channels

and sixth system.

modulation format is used, is a source of larger amount of interchannel crosstalk than channels, where NRZ – OOK or 2 – POLSK format is used. This was concluded based on obtained NRZ – OOK and 2 – POLSK channels BER results for different system's configurations. This become evident if we analyze obtained BER values for the fourth, fifth

f (THz) 1st system 2nd system 3rd system

193.075 3×10-24 2×10-8 1×10-40

193.100 9×10-12 9×10-25 1×10-18

193.125 1×10-40 1×10-13 3×10-27

Average 3×10-12 7×10-9 5×10-19

f (THz) 4th systems 5th system 6th system

193.075 1×10-40 1×10-21 4×10-27

193.100 2×10-8 6×10-25 8×10-14

193.125 3×10-25 6×10-12 1×10-40

Average 5×10-9 2×10-12 3×10-14

Firstly let's focus to the fifth system's BER values. As one can see from this configuration scheme then in this case NRZ – DPSK modulated optical signals are transmitted in central system's channel. As a result detected signals BER values in adjacent channels are sufficiently higher than they are in cases, when NRZ – OOK or 2 – POLSK modulated optical signals are located further from NRZ – DPSK channel as it is in the sixth system. Comparing BER results obtained for 2 – POLSK modulated signals in the fourth and sixth system (1×10-40 and 9×10-14 respectively), we can conclude that in mixed system detected signals BER value decreases if channel, where these signals are transmitted, is located

To assess NRZ-DPSK channel created crosstalk impact to optical signals transmission in all others mixed system's channels previously studied three-channel mixed systems model was modified and supplemented with 2 × 3 channels that have appropriate system's configuration. As before, in system channels detected signals BER values were obtained for

As well as using these data two different channels average BER values were calculated: system's average BER that takes into account all system channels (1st-9th); central group

channels' average BER that takes into account only channels number one to three.

**Table 2.** BER values for different mixed systems channels

six different mixed system configurations (see Table 3).

further from NRZ – DPSK channel.

As one can see from obtained data (see Table 3), then the lowest average BER values for 1st till 3rd and 1st till 9th channel are for the third mixed system configuration and they are equal to BER1st-3rd = 4 × 10-11and BER1st-9th = 2 × 10-11 respectively. But the highest BER values are for the second configuration and they are equal to BER1st-3rd = 7 × 10-9 and BER1st-9th = 1 × 10-8. So, BER difference between the best and worst case scenario, corresponding to [(NRZ – OOK)\193.075 THz]-[(2 – POLSK)\193.100 THz]-[(NRZ – DPSK)\ 193.125 THz] and [(NRZ – OOK)\193.075 THz]-[(NRZ – DPSK)\193.100 THz]-[(2 – POLSK)\ 193.125 THz] configuration respectively, is approximately three orders.

In these both cases channel with highest detected signal error probability is the first one, where by the way NRZ-OOK modulated optical signals are transmitted. Comparing BER values obtained for NRZ-OOK and 2-POLSK modulated optical signals for these two systems configuration, we have to conclude that these values differ by no more than two orders (1 × 10-10 and 2 × 10-8 in NRZ – OOK case and 4 × 10-15 and 3 × 10-13 for 2 – POLSK channels). As for NRZ – DPSK channel then the resulting BER values differences in both cases are not significant: 4 × 10-23 and 2 × 10-23 (see Fig. 15).

**Figure 15.** Nine-channel mixed system's with the third configuration output spectrum and eye diagrams in case of 10 Gbit/s per channel bitrates and 25 GHz channel spacing, and nine-channel mixed system's with the second configuration output spectrum and eye diagrams in case of 10 Gbit/s per channel bitrates and 25 GHz channel spacing.

As a result, for further research of optimal mixed system configuration will be used as a starting point nine-channel mixed WDM system with the third configuration.

Previously it has been detected that channel, where NRZ – DPSK modulated optical signals are transmitted, is larger amount of interchannel crosstalk source than NRZ – OOK or 2 – POLSK channels. So, to reduce that type of noise it has been decided to decrease optical power level radiated by distributed feedback lasers (DFB) in continuous wavelength (CW) regime that are used in these channels.

As previously, using these BER results for each system channel average BER value for central channels were calculated. It revealed that in system channels detected signals average BER values are below 10-12 if NRZ – DPSK channels' lasers output power level is in the range from 3.5 to 4.5 dBm. The lowest average channels' BER value is reached if these lasers output power is equal to 3.5 dBm. In this case BER1st-3rd = 3 × 10-18 and the worst channel is the second one (2 – POLSK) and its BER2nd = 1 × 10-17 (see Fig. 16).

Assuming that we are dealing with one sector of ultra-long haul backbone optical network, it was decided to supplement this model of mixed WDM system with additional optical element is fixed output power optical amplifier. It allowed take into an account ASE noise arising from EDFA which is the most widely used optical amplifier. To find out optimal amplifier output power level, that provides minimal channels' BER values, BER correlation diagram for each were obtained. It represents in systems channels detected signals BER values as a function from amplifier fixed output power level (see Fig. 17). Let is note, that in this case NRZ-DPSK channel laser output power level remains unchanged as it was in initial mixed WDM system model "in [2]".

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**Figure 15.** Nine-channel mixed system's with the third configuration output spectrum and eye diagrams in case of 10 Gbit/s per channel bitrates and 25 GHz channel spacing, and nine-channel mixed system's with the second configuration output spectrum and eye diagrams in case of 10 Gbit/s per

starting point nine-channel mixed WDM system with the third configuration.

channel is the second one (2 – POLSK) and its BER2nd = 1 × 10-17 (see Fig. 16).

As a result, for further research of optimal mixed system configuration will be used as a

Previously it has been detected that channel, where NRZ – DPSK modulated optical signals are transmitted, is larger amount of interchannel crosstalk source than NRZ – OOK or 2 – POLSK channels. So, to reduce that type of noise it has been decided to decrease optical power level radiated by distributed feedback lasers (DFB) in continuous wavelength (CW)

As previously, using these BER results for each system channel average BER value for central channels were calculated. It revealed that in system channels detected signals average BER values are below 10-12 if NRZ – DPSK channels' lasers output power level is in the range from 3.5 to 4.5 dBm. The lowest average channels' BER value is reached if these lasers output power is equal to 3.5 dBm. In this case BER1st-3rd = 3 × 10-18 and the worst

Assuming that we are dealing with one sector of ultra-long haul backbone optical network, it was decided to supplement this model of mixed WDM system with additional optical element is fixed output power optical amplifier. It allowed take into an account ASE noise arising from EDFA which is the most widely used optical amplifier. To find out optimal amplifier output power level, that provides minimal channels' BER values, BER correlation diagram for each were obtained. It represents in systems channels detected signals BER values as a function from amplifier fixed output power level (see Fig. 17). Let is note, that in

channel bitrates and 25 GHz channel spacing.

regime that are used in these channels.

As one can see from Fig. 17, then BER value for the system's first channel varies around 10-11 value, for the second channel around 10-16 and for the third is10-24. Knowing that the worst mixed system's channel is the second one, where 2 – POLSK modulated optical signals are transmitted, and then was decided to choose amplifier output power level that provides minimal BER value exactly in this channel.

**Figure 16.** BER as a function from NRZ – DPSK channel laser radiated output power level, and BER as a function form optical amplifier fixed output power level.

Consequently, optical amplifier fixed output power level equal to 4 dBm was chosen. This level provides in the second system's channel detected signals BER2nd = 1 × 10-17.

If both these optimal parameters are used in mixed system model configuration, then in system channels detected signals BER values are well below the maximal acceptable BER threshold 10-12, that is defined for 10 Gbit/s per channel bitrate (see Table 4). Channels BER values for revealed optimal system configuration were obtained for two types of single mode fiber. The first fiber was standard single mode fiber (SSMF) according to ITU-T Recommendation G.652 D and the second was non-zero dispersion shifted fiber (NZ – DSF) according to ITU – T Recommendation G.655.


**Table 4.** BER value for different system configurations

For these two cases BER results as well as detected signals eye diagrams were compared one to another (see Fig. 17).

BER value for the first system channel sufficiently dropped from 1 × 10-10 to 5 × 10-26 if SSMF is used and to 9 × 10-20 if NZ-DSF is used. Exactly for this first channel, where NRZ – OOK modulated optical signals are transmitted, experiencing the most radical BER value improvement comparing to 2 – POLSK and NRZ – DPSK channels. In these channels detected signal BER values do not improve so noticeably. The second channel's BER value decreases from 4 × 10-15 to 7 × 10-19 for SSMF and to 6 × 10-21 for NZ – DSF, but the third channel's BER value variation is not essential from 4 × 10-23 to 6 × 10-22 if SSMF is used. But if in this system instead of SSMF NZ – DSF is used then it is possible to obtain lower BER values for NRZ – DPSK channels. In this channel detected signals BER value decreases to 1 × 10-40. In addition, coherence between detected signals BER values and channels' central frequency position in C-band (191.6-195.9 THz) was investigated. As well as, for each system channel the worst and the best position in C-band, that provides the highest and the lowest possible signals BER values, respectively, for previously found optimal mixed system's configuration, was revealed (see Table 4). As previously, this research was held for two types of optical fiber: SSMF (G.652 D) and NZ – DSF (G.655).

**Figure 17.** Optimal configuration nine-channel mixed WDM system output optical spectrum and eye diagrams: (a) SSMF; (b) NZ – DSF.

It showed that depending on channel central frequency the first channel's BER value varies around nominal value of 10-20 if SSMF is used and around 10-30 if NZ – DSF. But BER values obtained for the second channel and NZ – DSF are for several orders worse comparing to transmission over SSMF. They vary around 10-20 and 10-25, respectively.

260 Optical Communication

BER value for the first system channel sufficiently dropped from 1 × 10-10 to 5 × 10-26 if SSMF is used and to 9 × 10-20 if NZ-DSF is used. Exactly for this first channel, where NRZ – OOK modulated optical signals are transmitted, experiencing the most radical BER value improvement comparing to 2 – POLSK and NRZ – DPSK channels. In these channels detected signal BER values do not improve so noticeably. The second channel's BER value decreases from 4 × 10-15 to 7 × 10-19 for SSMF and to 6 × 10-21 for NZ – DSF, but the third channel's BER value variation is not essential from 4 × 10-23 to 6 × 10-22 if SSMF is used. But if in this system instead of SSMF NZ – DSF is used then it is possible to obtain lower BER values for NRZ – DPSK channels. In this channel detected signals BER value decreases to 1 × 10-40. In addition, coherence between detected signals BER values and channels' central frequency position in C-band (191.6-195.9 THz) was investigated. As well as, for each system channel the worst and the best position in C-band, that provides the highest and the lowest possible signals BER values, respectively, for previously found optimal mixed system's configuration, was revealed (see Table 4). As previously, this research was held for

**Figure 17.** Optimal configuration nine-channel mixed WDM system output optical spectrum and eye

diagrams: (a) SSMF; (b) NZ – DSF.

two types of optical fiber: SSMF (G.652 D) and NZ – DSF (G.655).

In addition, as one can see from Table 5, then BER value obtained for NZ-DSF and the worst case of 2nd channel central frequency is approximately for six orders larger comparing to the worst case of SSMF.

These let us conclude that 2-POLSK modulated signals are not suitable for transmission over NZ – DSF fiber in [NRZ – OOK]-[2 – POLSK]-[NRZ – DPSK] mixed WDM systems and they are sufficiently distorted at appropriate channel central frequency.


**Table 5.** Best and worst channels positions in C-band and their BER values

As result system's average BER value is significantly higher than it might be in case of SSMF (see Table 6). In same time the lowest 1st-3rd channels average BER value is gained with NZ – DSF and it is about two orders lower than in case of SSMF. As for the third channel, then in case of SSMF and different channel central frequencies obtained BER values vary somewhere around 10-23. Whereas in case of NZ – DSF these values remain constant and approximately equal to 10-40 in all C-band. It allows to judge about NRZ – DPSK modulated optical signals transmission suitability over NZ – DSF single mode optical fiber.

Let is note, that Corning LEAF non-zero dispersion shifted fiber characteristics and parameters were used in OptSim in order to obtain mathematical model of a NZ-DSF. This fiber is the world's most widely deployed NZ-DSF and is specially optimized for high-speed and high capacity long-haul and metro networks.


**Table 6.** Minimal and maximal in system's 1st-3rd channel detected signals BER values
