**4. HDWDM system experimental and simulation models**

Our experimental transmission system (see Fig. 5) employs two optical channels with external intensity modulation (IM), and non-return-to-zero (NRZ) pulse shapes. The laser is always switched on and its light waves are modulated via the electro-optic MZM by output of data pulse sequence of a pulse pattern generator (PPG), using the principles of interferometer constructive and destructive interference to achieve ON and OFF of the light waves. After the MZ modulator the signal is sent to a single-mode fibre (SMF), where optical pulses are propagating over a 40 km distance. The utilized fiber has a large core effective area of 80 μm2, attenuation *α* = 0.2 dB/km, nonlinear refractive coefficient *nk* = 2.5·10-20 cm/W and dispersion 16 ps/nm/km at the reference wavelength *λ* = 1550 nm [Kaminow et al., 2009].

the receiver's ability to correctly detect the signal. This effect can be suppressed by an optical filter placed before the optical receiver. Depending on the amplifier infrastructure used in a transmission system, the OSNR is proportional to the number of optical amplifiers and to the gain flatness of a single amplifier. This latter can be an especially critical issue in HDWDM systems, because of the gain non-uniformity in multi-span

In practice, the OSNR can be found by measuring the signal power as the difference between the total power of the signal peak and the background noise; this latter, in turn, is determined by measuring the noise contributions on either side of the signal peak. However, it is difficult to separate measurements of the signal and noise power, because the latter in an optical channel is included in the signal power. The determination of this parameter in a HDWDM system can be made by interpolating it between the adjacent

For a single EDFA with output power, *Pout*, the OSNR is given by [Jacobsen, 1994]:

����

<sup>=</sup> ���� �������������

where *NF* is the amplifier noise figure, *Gop* is the optical amplifier gain, *hv* is the photon energy, and *Bo* is the optical bandwidth found by measurement. However, OSNR does not provide good estimation to the system performance when the main degrading sources involve the dynamic propagation effects such as dispersion and Kerr

When addressing the value of an OSNR, it is important to define the optical measurement bandwidth over which the OSNR is calculated. To obtain this value, the signal power and noise power are derived by integrating all the frequency components over the bandwidth

In practice, the signal and noise power values are usually measured directly, using the optical spectrum analyzer (OSA), which does the mathematics for the users and displays the resultant OSNR versus wavelength or frequency over a fixed resolution bandwidth. The value of *Δλ* = 0.1 nm at 1550 nm, is widely used as a typical value for calculation of the

Our experimental transmission system (see Fig. 5) employs two optical channels with external intensity modulation (IM), and non-return-to-zero (NRZ) pulse shapes. The laser is always switched on and its light waves are modulated via the electro-optic MZM by output of data pulse sequence of a pulse pattern generator (PPG), using the principles of interferometer constructive and destructive interference to achieve ON and OFF of the light waves. After the MZ modulator the signal is sent to a single-mode fibre (SMF), where optical pulses are propagating over a 40 km distance. The utilized fiber has a large core effective area of 80 μm2, attenuation *α* = 0.2 dB/km, nonlinear refractive coefficient *nk* = 2.5·10-20 cm/W and dispersion 16 ps/nm/km at the reference wavelength *λ* = 1550 nm

 , (2)

���� = ����

**4. HDWDM system experimental and simulation models** 

transmissions.

channels [Binh, 2009].

nonlinearity effects.

[Kaminow et al., 2009].

OSNR.

[Rongqing & O'Sullivan, 2009].

Fig. 5. The setup used for investigation of HDWDM transmission [Bobrovs et al., 2010].

At the fibre end each channel is optically filtered with an Anritsu Xtract tunable optical filter (see Fig. 6). An essential parameter of such a filter is its centering on the signal to be extracted. Its position has to be adjusted regarding the signal harmonics [Ivanovs et al., 2010].

The Anritsu Xtract tunable optical band-pass filter covers all transmission bands of a standard single mode optical fiber. The filter operates in the range of 1450-1650 nm, covering the E, S, C and L bands and, partially, the U band. The main drawback of this BPF is 6 dB insertion losses, which is a limiting factor in realization of high-speed HDWDM transmission systems for moderate distances without optical amplifiers.

Fig. 6. The measured amplitude responses of the Anritsu Xtract tunable optical band-pass filter at different FWHM values.

Realization of HDWDM Transmission System with the Minimum Allowable Channel Interval 201

In compliance with the experimental model we have created a simulation scheme (see Fig. 8) using OptSim software with the real parameters of all experimental devices. The accepted method of calculation is based on solving of a complex set of differential equations, taking into account optical and electrical noise as well as linear and nonlinear effects. We have used a model where signals are propagating as time domain samples over a selectable bandwidth

The Time Domain Split Step (TDSS) method was employed to simulate linear and nonlinear behavior for both optical and electrical components. The split step method is now used in all commercial simulation tools to perform the integration of a fiber propagation equation that

> , , *Atz L N Atz*

Here *Atz* , is the optical field, *L* is the linear operator that stands for dispersion and other linear effects, and *N* is the operator that is responsible for all nonlinear effects. The idea is to calculate the equation over small spans *z* of fiber by including either a linear or a nonlinear operator [Belai et al., 2006]. For instance, on the first span only linear effects are considered, on the second – only nonlinear, on the third – again only linear ones, and so on. Two ways of calculation are possible: frequency domain split step (FDSS) and the above-mentioned time domain split step (TDSS) method. These methods differ in how linear operator *L* is calculated: FDSS does it in a frequency domain, whereas TDSS – in the time domain, by calculating the convolution product in sampled time. The first method is easy to fulfill, but it may produce severe errors during computation. In our simulation we have employed the second method,

(3)

*z*

TDSS, which, despite its complexity, ensures an effective and time-saving solution.

Fig. 9 shows the spectral and eye diagrams in a simulative HDWDM communication system with 2.5 Gbit/s transmission speed per channel after a signal's detection. It is seen there that in

Fig. 8. Simulation model of HDWDM system.

(in our case, a bandwidth that contains all channels).

can be written as [Binh, 2009]:

**5. Results and discussions** 

To evaluate the output signal characteristics, an optical direct-detection receiver was used, with an electrical fourth-order Bessel–Thomson electrical filter having a 3 db bandwidth of 7.5 GHz. In practice, a 40 km span is preferred by most network providers since it allows a compromise between the system's costs and its performance [Binh, 2009].

For the performance evaluation and optimization of the experimental HDWDM system it is necessary to analyze the optical and electrical signal quality before MZM, after MZM and after SMF. The choice of arbitrary units on the Y-axis in the eye diagrams was purposeful – to make them more general in the cases when the plotted electrical quantity is current or voltage.

As a result, we have designed a HDWDM transmission system with a variable data transmission speed up to 12.5 Gbit/s, the channel interval up to 12.5 GHz and optical power up to 23 dBm. In Fig. 7 one can see 2.5 Gbit/s HDWDM transmission systems with 18.75 GHz and 25 GHz channel interval. As follows from the results, reducing the channel interval to 18.75 GHz gives rise to Kerr's effect, which degrades the 2.5 Gbit/s signal quality. The signal eye-pattern overlaps with the mask (see Fig. 7*c*), which means that the signal quality does not ensure the BER=10–9 value. To obtain a system with an appropriate BER we should reduce the data transmission speed or increase the channel interval. As can be seen from Fig. 5, the 25 GHz channel interval ensures a good signal quality, and the signal eyepattern in this case does not overlap with the mask.

Fig. 7. Output optical signal spectra and eye-patterns with defined masks for 2.5 Gbit/s system: *a*) common optical spectra, *b*) signal optical spectrum after filtering, *c*) eye-pattern.

Fig. 8. Simulation model of HDWDM system.

To evaluate the output signal characteristics, an optical direct-detection receiver was used, with an electrical fourth-order Bessel–Thomson electrical filter having a 3 db bandwidth of 7.5 GHz. In practice, a 40 km span is preferred by most network providers since it allows a

For the performance evaluation and optimization of the experimental HDWDM system it is necessary to analyze the optical and electrical signal quality before MZM, after MZM and after SMF. The choice of arbitrary units on the Y-axis in the eye diagrams was purposeful – to make them more general in the cases when the plotted electrical quantity is current or voltage.

As a result, we have designed a HDWDM transmission system with a variable data transmission speed up to 12.5 Gbit/s, the channel interval up to 12.5 GHz and optical power up to 23 dBm. In Fig. 7 one can see 2.5 Gbit/s HDWDM transmission systems with 18.75 GHz and 25 GHz channel interval. As follows from the results, reducing the channel interval to 18.75 GHz gives rise to Kerr's effect, which degrades the 2.5 Gbit/s signal quality. The signal eye-pattern overlaps with the mask (see Fig. 7*c*), which means that the signal quality does not ensure the BER=10–9 value. To obtain a system with an appropriate BER we should reduce the data transmission speed or increase the channel interval. As can be seen from Fig. 5, the 25 GHz channel interval ensures a good signal quality, and the signal eye-

Fig. 7. Output optical signal spectra and eye-patterns with defined masks for 2.5 Gbit/s system: *a*) common optical spectra, *b*) signal optical spectrum after filtering, *c*) eye-pattern.

compromise between the system's costs and its performance [Binh, 2009].

pattern in this case does not overlap with the mask.

In compliance with the experimental model we have created a simulation scheme (see Fig. 8) using OptSim software with the real parameters of all experimental devices. The accepted method of calculation is based on solving of a complex set of differential equations, taking into account optical and electrical noise as well as linear and nonlinear effects. We have used a model where signals are propagating as time domain samples over a selectable bandwidth (in our case, a bandwidth that contains all channels).

The Time Domain Split Step (TDSS) method was employed to simulate linear and nonlinear behavior for both optical and electrical components. The split step method is now used in all commercial simulation tools to perform the integration of a fiber propagation equation that can be written as [Binh, 2009]:

$$\frac{\partial A\{t,z\}}{\partial z} = \left\{L+N\right\} A\{t,z\} \tag{3}$$

Here *Atz* , is the optical field, *L* is the linear operator that stands for dispersion and other linear effects, and *N* is the operator that is responsible for all nonlinear effects. The idea is to calculate the equation over small spans *z* of fiber by including either a linear or a nonlinear operator [Belai et al., 2006]. For instance, on the first span only linear effects are considered, on the second – only nonlinear, on the third – again only linear ones, and so on. Two ways of calculation are possible: frequency domain split step (FDSS) and the above-mentioned time domain split step (TDSS) method. These methods differ in how linear operator *L* is calculated: FDSS does it in a frequency domain, whereas TDSS – in the time domain, by calculating the convolution product in sampled time. The first method is easy to fulfill, but it may produce severe errors during computation. In our simulation we have employed the second method, TDSS, which, despite its complexity, ensures an effective and time-saving solution.

#### **5. Results and discussions**

Fig. 9 shows the spectral and eye diagrams in a simulative HDWDM communication system with 2.5 Gbit/s transmission speed per channel after a signal's detection. It is seen there that in

Realization of HDWDM Transmission System with the Minimum Allowable Channel Interval 203

Fig. 10. 10 Gbit/s HDWDM communication system with 4/8/16 channels and

diagrams are shown for the end of a 40 km SMF line.

(see Fig. 11).

31.25/37.5/37.5 GHz frequency intervals. The output spectrum of the optical signal and eye

With the number of channels increasing this value also increases. As a result, the optimal channel spacing in WDM systems with a 10 Gbit/s transmission speed per channel is 37.5 GHz; the possibility exists to provide a high-quality transmission of signals over 40 - 50 km

Fig. 11*.* Output optical signal spectra and eye-patterns with defined masks for 10 Gbit/s system: *a*) common optical spectra, *b*) signal optical spectrum after filtering, *c*) eye-pattern.

the four-channel case the allowed interval is 12.5 GHz or 0.1 nm, with BER meeting the standard (< 10-9). If we raise the number of channels to 8, the output signal quality worsens; therefore, the channel interval should be raised up to 18.75 GHz (see Fig. 9). In turn, in the 16 channel case a successful transmission is possible only using 25 GHz (or 0.2 nm) channel spacing. This is the optimal channel interval, which allows multiplexing the signals in a HDWDM system with a channel number exceeding 16. Further increase in the channel number would not change the chosen frequency interval (see Fig. 9). Considering the 25 GHz frequency interval as the chosen one, it is possible to upgrade the existing WDM communication systems with a 2.5 Gbit/s transmission speed per channel without increasing this speed while decreasing the channel interval down to the estimated value and adding signals to the freed frequency range, thus realizing an HDWDM transmission system.

Fig. 9. 22.5 Gbit/s HDWDM communication system with 4/8/16 channels and 12.5/18.75/25 GHz frequency intervals. The output spectrum of optical signal is shown after a 40 km SSMF line

The fundamental limitation in the high-speed (over 2.5 Gbit/s per channel) systems is set by the total dispersion in the fiber optical transmission (FOTS) lines. Without managing the dispersion, the FOTS operation with a 10 Gbit/s transmission speed per channel is limited to the line length from 40 km to 50 km. Fig. 10 shows the eye diagrams and output signal spectra in HDWDM communication systems with 10 Gbit/s transmission speed for different frequency intervals.

From Fig. 10 it is seen that a WDM communication system with a 10 Gbit/s transmission speed per channel and a frequency interval of 50 GHz could be optimized. In the four-channel case a decrease in the frequency interval to 31.25 GHz ensures a satisfactory BER value.

the four-channel case the allowed interval is 12.5 GHz or 0.1 nm, with BER meeting the standard (< 10-9). If we raise the number of channels to 8, the output signal quality worsens; therefore, the channel interval should be raised up to 18.75 GHz (see Fig. 9). In turn, in the 16 channel case a successful transmission is possible only using 25 GHz (or 0.2 nm) channel spacing. This is the optimal channel interval, which allows multiplexing the signals in a HDWDM system with a channel number exceeding 16. Further increase in the channel number would not change the chosen frequency interval (see Fig. 9). Considering the 25 GHz frequency interval as the chosen one, it is possible to upgrade the existing WDM communication systems with a 2.5 Gbit/s transmission speed per channel without increasing this speed while decreasing the channel interval down to the estimated value and adding

signals to the freed frequency range, thus realizing an HDWDM transmission system.

Fig. 9. 22.5 Gbit/s HDWDM communication system with 4/8/16 channels and

a 40 km SSMF line

12.5/18.75/25 GHz frequency intervals. The output spectrum of optical signal is shown after

The fundamental limitation in the high-speed (over 2.5 Gbit/s per channel) systems is set by the total dispersion in the fiber optical transmission (FOTS) lines. Without managing the dispersion, the FOTS operation with a 10 Gbit/s transmission speed per channel is limited to the line length from 40 km to 50 km. Fig. 10 shows the eye diagrams and output signal spectra in HDWDM communication systems with 10 Gbit/s transmission speed for different frequency intervals.

From Fig. 10 it is seen that a WDM communication system with a 10 Gbit/s transmission speed per channel and a frequency interval of 50 GHz could be optimized. In the four-channel

case a decrease in the frequency interval to 31.25 GHz ensures a satisfactory BER value.

Fig. 10. 10 Gbit/s HDWDM communication system with 4/8/16 channels and 31.25/37.5/37.5 GHz frequency intervals. The output spectrum of the optical signal and eye diagrams are shown for the end of a 40 km SMF line.

With the number of channels increasing this value also increases. As a result, the optimal channel spacing in WDM systems with a 10 Gbit/s transmission speed per channel is 37.5 GHz; the possibility exists to provide a high-quality transmission of signals over 40 - 50 km (see Fig. 11).

Fig. 11*.* Output optical signal spectra and eye-patterns with defined masks for 10 Gbit/s system: *a*) common optical spectra, *b*) signal optical spectrum after filtering, *c*) eye-pattern.

Realization of HDWDM Transmission System with the Minimum Allowable Channel Interval 205

Fig. 13. A 2.5 Gbit/s mixed HDWDM communication system with NRZ-RZ-NRZ signals and a 25 GHz frequency interval. The output spectrum of the optical signal and the eye

This done, the operation of a mixed HDWDM communication system with NRZ-Duobinary-NRZ signal formats was studied for a 2.5 Gbit/s transmission speed per channel. The conclusion was that it is possible to compact the signals with a 12.5 GHz frequency interval and a proper BER (< 10-9); this is two times more compact than in the NRZ-RZ-NRZ case,

Fig. 14. A 10 Gbit/s mixed HDWDM communication system with NRZ-RZ-NRZ signals and

a 50 GHz frequency interval. The output spectrum of the optical signal and the eye

which would provide a highly efficient use of the spectrum (see Fig. 15).

diagrams of the received electrical signal are shown

diagrams of the received electrical signal are shown.

In the process of investigation it has been established that the operators of telecommunication networks when using a WDM communication system's facilities are raising its total transmission speed gradually, depending on the requested information volume, adding new channels with different transmission speeds (2.5 Gbit/s or 10 Gbit/s ), different coding formats (NRZ, RZ or Duobinary) and variable frequency intervals (12.5 GHz, 25 GHz, 50 GHz, or 100 GHz); all this can result in the spectral overlapping and increased BER of the signal. We therefore have estimated the possibilities of a mixed HDWDM communication system applying different signal coding techniques (NRZ, RZ and Duo) in each channel (see Fig. 12).

Fig. 12. A mixed scheme of the WDM communication system. The NRZ, RZ and Duobinary (Duo) intensity modulation formats are applied for the 2.5 Gbit/s and 10 Gbit/s signal transmission

The design with a transmitter unit containing three combined channels is the simplest model, in which between two NRZ signals the RZ or Duo signals are located. The NRZ signal format is chosen as a base, since it is the format preferred by the majority of telecommunication operators. Further, the operation of a mixed HDWDM communication system was subjected to scrutiny, changing the transmission speed from 2.5 Gbit/s to 10 Gbit/s per channel and the channel interval in a wide range − from 12.5 GHz to 100 GHz. Fig. 13 shows the potential of NRZ-RZ-NRZ mixed HDWDM systems (with only successful signal transmission displayed). The transmission speed used for each signal is 2.5 Gbit/s. In such a case the minimum channel interval is to be equal to or greater than 25 GHz. Only under such conditions a successful realization (i.e. with BER < 10-9) is possible for a mixed HDWDM communication system with NRZ-RZ-NRZ signals.

When creating a mixed HDWDM communication system based on NRZ-RZ-NRZ formats of signals with a 10 Gbit/s transmission speed per channel it is possible to multiplex the signals with a 50 GHz frequency interval, since the quality of the output signal meets in this case the BER standard (see Fig. 14). Reducing the channel interval still further would impair the signal's characteristics. This means that the least frequency interval for the mixed NRZ-RZ-NRZ HDWDM communication system under consideration is 50 GHz.

In the process of investigation it has been established that the operators of telecommunication networks when using a WDM communication system's facilities are raising its total transmission speed gradually, depending on the requested information volume, adding new channels with different transmission speeds (2.5 Gbit/s or 10 Gbit/s ), different coding formats (NRZ, RZ or Duobinary) and variable frequency intervals (12.5 GHz, 25 GHz, 50 GHz, or 100 GHz); all this can result in the spectral overlapping and increased BER of the signal. We therefore have estimated the possibilities of a mixed HDWDM communication system applying different signal coding techniques (NRZ, RZ and

Fig. 12. A mixed scheme of the WDM communication system. The NRZ, RZ and Duobinary (Duo) intensity modulation formats are applied for the 2.5 Gbit/s and 10 Gbit/s signal

The design with a transmitter unit containing three combined channels is the simplest model, in which between two NRZ signals the RZ or Duo signals are located. The NRZ signal format is chosen as a base, since it is the format preferred by the majority of telecommunication operators. Further, the operation of a mixed HDWDM communication system was subjected to scrutiny, changing the transmission speed from 2.5 Gbit/s to 10 Gbit/s per channel and the channel interval in a wide range − from 12.5 GHz to 100 GHz. Fig. 13 shows the potential of NRZ-RZ-NRZ mixed HDWDM systems (with only successful signal transmission displayed). The transmission speed used for each signal is 2.5 Gbit/s. In such a case the minimum channel interval is to be equal to or greater than 25 GHz. Only under such conditions a successful realization (i.e. with BER < 10-9) is possible for a mixed

When creating a mixed HDWDM communication system based on NRZ-RZ-NRZ formats of signals with a 10 Gbit/s transmission speed per channel it is possible to multiplex the signals with a 50 GHz frequency interval, since the quality of the output signal meets in this case the BER standard (see Fig. 14). Reducing the channel interval still further would impair the signal's characteristics. This means that the least frequency interval for the mixed NRZ-

Duo) in each channel (see Fig. 12).

HDWDM communication system with NRZ-RZ-NRZ signals.

RZ-NRZ HDWDM communication system under consideration is 50 GHz.

transmission

Fig. 13. A 2.5 Gbit/s mixed HDWDM communication system with NRZ-RZ-NRZ signals and a 25 GHz frequency interval. The output spectrum of the optical signal and the eye diagrams of the received electrical signal are shown

This done, the operation of a mixed HDWDM communication system with NRZ-Duobinary-NRZ signal formats was studied for a 2.5 Gbit/s transmission speed per channel. The conclusion was that it is possible to compact the signals with a 12.5 GHz frequency interval and a proper BER (< 10-9); this is two times more compact than in the NRZ-RZ-NRZ case, which would provide a highly efficient use of the spectrum (see Fig. 15).

Fig. 14. A 10 Gbit/s mixed HDWDM communication system with NRZ-RZ-NRZ signals and a 50 GHz frequency interval. The output spectrum of the optical signal and the eye diagrams of the received electrical signal are shown.

Realization of HDWDM Transmission System with the Minimum Allowable Channel Interval 207

Fig. 16. A 10 Gbit/s mixed HDWDM communication system with NRZ-Duobinary-NRZ signals and an 18.75 GHz frequency interval. The output spectrum of the optical signal and

At the same time, the increase in the transmission speed from 2.5 Gbit/s to 10 Gbit/s in a mixed NRZ-Duo-NRZ HDWDM communication system leads to frequency interval rising up to 18.75 GHz, which, as compared with the results for a NRZ-RZ-NRZ system provides a

Our results have proved once more that HDWDM is a powerful technique for increasing the capacity of fiber optics transmission systems. It may be crucial for enabling technology of ultra-high capacity on-chip optical interconnects, as well as chip-to-chip optical interconnects in massively parallel different optical systems. It has been shown that the BER and eye-diagram technique is a good means for evaluating the system performance that

In contrast to the conventional high speed approach of increasing WDM transmission capacity, we have demonstrated the minimal allowed channel spacing in HDWDM systems, and provided we are able to provide recommendations for future HDWDM solutions.

In the measurements, different optical filter FWHM values (from 0.15 nm to 0.7 nm) were used. The best results were obtained for 0.15 nm, when the eye pattern was opened wider. For evaluation of the signal quality a visual method was employed, in which the eye pattern was evaluated visually in the electric signal analyser varying the quasi-rectangular optical

At reducing the channel interval to 18.75 GHz the Kerr effects (self-phase modulation, crossphase modulation, and four-wave mixing) degrades the 2.5 Gbit/s HDWDM system. The signal eye-pattern overlaps with the mask, which means that the signal quality does not

the eye diagrams of the received electrical signal are shown

highly efficient exploitation of the spectrum (see Fig. 16).

allows HDWDM system to be optimized for different parameters.

**6. Conclusions** 

filter FWHM value.

**2.5 Gbit/s bit rate NRZ-Duobinary-NRZ mixed HDWDM system**

Fig. 15. A 2.5 Gbit/s mixed HDWDM communication system with NRZ-Duobinary-NRZ signals and a 12.5 GHz frequency interval. The output spectrum of the optical signal and the eye diagrams of the received electrical signal are shown.

Fig. 16. A 10 Gbit/s mixed HDWDM communication system with NRZ-Duobinary-NRZ signals and an 18.75 GHz frequency interval. The output spectrum of the optical signal and the eye diagrams of the received electrical signal are shown

At the same time, the increase in the transmission speed from 2.5 Gbit/s to 10 Gbit/s in a mixed NRZ-Duo-NRZ HDWDM communication system leads to frequency interval rising up to 18.75 GHz, which, as compared with the results for a NRZ-RZ-NRZ system provides a highly efficient exploitation of the spectrum (see Fig. 16).
