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Table 1. Evolution of fiber optics and its wide implementation into all telecommunication branches

Table 1 Evolution of fiber optics and its wide implementation into all telecommunication branches

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Nowadays, a skyrocketing growth is observed worldwide in the bit rates of transmitted information, which is associated with development of broadband information transmission types. The annual global internet protocol (IP) traffic will exceed half a Zettabyte in four years. At just under 44 Exabytes per month, the annual run rate of traffic in the late 2012 will be 522 Exabytes per year. Driven by high-definition video and high-speed broadband penetration, the consumer of IP traffic will bolster the overall IP growth rate so that it sustains a steady growth rate through 2012, growing at a compound annual growth rate (CAGR) of 46 percent (see Fig. 1) [Cisco Systems, 2008]. In turn, to provide high-quality transmission it is necessary to develop the next generation optical networks (NGONs) that would transmit properly huge volumes of information. The optical transmission systems from the very outset have been able to offer new possibilities for solving problems of ever increasing urgency that are dictated by the need for frequency bands and transmission speed. Such networks have become one of the most important
Nowadays, a skyrocketing growth is observed worldwide in the bit rates of transmitted information, which is associated with development of broadband information transmission types. The annual global internet protocol (IP) traffic will exceed half a Zettabyte in four years. At just under 44 Exabytes per month, the annual run rate of traffic in the late 2012 will be 522 Exabytes per year. Driven by high-definition video and high-speed broadband penetration, the consumer of IP traffic will bolster the overall IP growth rate so that it sustains a steady growth rate through 2012, growing at a compound annual growth rate (CAGR) of 46 percent (see Fig. 1) [Cisco Systems, 2008]. In turn, to provide high-quality transmission it is necessary to develop the next generation optical networks (NGONs) that would transmit properly huge volumes of information. The optical transmission systems from the very outset have been able to offer new possibilities for solving problems of ever increasing urgency that are dictated by the need for frequency bands and transmission speed. Such networks have become one of the most important
The role and realization of an optical transmitter become important with increased channel data rates in the system. While the optical transmitters at lower channel data rates are less complex and easier to realize by direct modulation of a laser diode, the realization becomes more complex with the increasing channel data rate, thus raising the requirements on electrical and optical components of the optical transmitter. The conventional optica transmitter employs the amplitude/intensity modulation (AM, IM) of the laser light (better known as on-off keying (OOK)), because different signal levels for marks and spaces are characterized by the presence of optical power. The amplitude modulation can be realized by direct or external modulation of the laser diode. For the realization of transmission systems with channel data rates larger than 2.5 Gbit/s, the external modulation presents a better solution, because the impact of laser internal chirp on optical signal can be reduced efficiently, but, on the other hand, the complexity of optical transmitters increases.
The role and realization of an optical transmitter become important with increased channel data rates in the system. While the optical transmitters at lower channel data rates are less complex and easier to realize by direct modulation of a laser diode, the realization becomes more complex with the increasing channel data rate, thus raising the requirements on electrical and optical components of the optical transmitter. The conventional optica transmitter employs the amplitude/intensity modulation (AM, IM) of the laser light (better known as on-off keying (OOK)), because different signal levels for marks and spaces are characterized by the presence of optical power. The amplitude modulation can be realized by direct or external modulation of the laser diode. For the realization of transmission systems with channel data rates larger than 2.5 Gbit/s, the external modulation presents a better solution, because the impact of laser internal chirp on optical signal can be reduced efficiently, but, on the other hand, the complexity of optical transmitters increases.
Fig. 3. Simplified fiber optical transmission system. different transmission speeds can be realized. If no electrical field is applied, both signals arrive at the same time (in-phase) at the MZM output and interfere constructively. If an electrical filed is applied, signals in different arms are shifted in phase relative to each other. Depending on the phase difference between the MZM arms, the signals can interfere constructively or destructively, resulting in an amplitude modulation of the modulator input signal. In this signal generation method, the laser source acts as a continuous wave (CW) pump. In conventional systems, the CW pumps are realized with distributed feedback laser (DFB) (the most important and widely used single mode laser type for the 1550 nm region). DFB lasers are realized by the implementation of through a Brage’s grating structure inside the cavity between the reflecting surfaces of a laser [Voges & Peteramann, 2002]. The main characteristics of the DFB lasers are high side-mode suppression ratios (> 50 dB) enabling stable single-mode operation, a small spectral line width (0.8...50 MHz) and large output optical power (10...40 mW) (see Fig. 3) [Funabashi, 2001].
Fig. 3. Simplified fiber optical transmission system. different transmission speeds can be realized. If no electrical field is applied, both signals arrive at the same time (in-phase) at the MZM output and interfere constructively. If an electrical filed is applied, signals in different arms are shifted in phase relative to each other. Depending on the phase difference between the MZM arms, the signals can interfere constructively or destructively, resulting in an amplitude modulation of the modulator input signal. In this signal generation method, the laser source acts as a continuous wave (CW) pump. In conventional systems, the CW pumps are realized with distributed feedback laser (DFB) (the most important and widely used single mode laser type for the 1550 nm region). DFB lasers are realized by the implementation of through a Brage’s grating structure inside the cavity between the reflecting surfaces of a laser [Voges & Peteramann, 2002]. The main characteristics of the DFB lasers are high side-mode suppression ratios (> 50 dB) enabling stable single-mode operation, a small spectral line width (0.8...50 MHz) and large output optical power (10...40 mW) (see Fig. 3) [Funabashi, 2001].
It is possible to employ three different transfer functions of the optical filter (see Fig. 4) for realization of HDWDM system schemes. These functions were chosen because with the Lorentzian optical filter’s transfer function we can approximate: Fabry Perrot filters, micro- ring resonators; raised cosine filters: arrayed waveguide gratings with flat tops, diffraction gratings, and particular cases of thin film filters and fiber Bragg gratings (with apodization); supergaussian filters: arrayed waveguide gratings with supergaussian transfer function, and thin film filters with low refraction index modulation [Venghaus, 2006].
It is possible to employ three different transfer functions of the optical filter (see Fig. 4) for realization of HDWDM system schemes. These functions were chosen because with the Lorentzian optical filter’s transfer function we can approximate: Fabry Perrot filters, micro- ring resonators; raised cosine filters: arrayed waveguide gratings with flat tops, diffraction gratings, and particular cases of thin film filters and fiber Bragg gratings (with apodization); supergaussian filters: arrayed waveguide gratings with supergaussian transfer function, and thin film filters with low refraction index modulation [Venghaus, 2006].
Fig. 5. The setup used for investigation of HDWDM transmission [Bobrovs et al., 2010]. Realization of HDWDM Transmission System with the Minimum Allowable Channel Interval
Fig. 5. The setup used for investigation of HDWDM transmission [Bobrovs et al., 2010]. Realization of HDWDM Transmission System with the Minimum Allowable Channel Interval
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.
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.
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. 7c), which means that the signal quality does not ensure the BER=10-° 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- pattern in this case does not overlap with the mask.
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. 7c), which means that the signal quality does not ensure the BER=10-° 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- pattern in this case does not overlap with the mask.
Fig. 8. Simulation model of HDWDM system. Realization of HDWDM Transmission System with the Minimum Allowable Channel Interval 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).
Fig. 8. Simulation model of HDWDM system. Realization of HDWDM Transmission System with the Minimum Allowable Channel Interval 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 four-channel case the allowed interval is 12.5 GHz or 0.1 nm, with BER meeting the standard (< 10-°). 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. 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.
the four-channel case the allowed interval is 12.5 GHz or 0.1 nm, with BER meeting the standard (< 10-°). 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. 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.
Realization of HDWDM Transmission System with the Minimum Allowable Channel Interval 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).
Realization of HDWDM Transmission System with the Minimum Allowable Channel Interval 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).
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).
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).
Realization of HDWDM Transmission System with the Minimum Allowable Channel Interval 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°); 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).
Realization of HDWDM Transmission System with the Minimum Allowable Channel Interval 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°); 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).
10 Gbit/s bit rate NRZ-Duobinary-NRZ mixed HDWDM system Realization of HDWDM Transmission System with the Minimum Allowable Channel Interval
10 Gbit/s bit rate NRZ-Duobinary-NRZ mixed HDWDM system Realization of HDWDM Transmission System with the Minimum Allowable Channel Interval
)ptical communications systems are very important for all types of telecommunications and networks. The onsists of a transmitter that encodes a message into an optical signal, a channel that carries the signal to lestination, and a receiver that reproduces the message from the received optical signal. This book presen 1p to date results on communication systems, along with the explanations of their relevance, from leading esearchers in this field. Its chapters cover general concepts of optical and wireless optical communication ystems, optical amplifiers and networks, optical multiplexing and demultiplexing for optical communication ystems, and network traffic engineering. Recently, wavelength conversion and other enhanced signal yrocessing functions are also considered in depth for optical communications systems. The researcher has ilso concentrated on wavelength conversion, switching, demultiplexing in the time domain and other enhar unctions for optical communications systems. This book is targeted at research, development and design sngineers from the teams in manufacturing industry; academia and telecommunications service operators/ roviders.
)ptical communications systems are very important for all types of telecommunications and networks. The onsists of a transmitter that encodes a message into an optical signal, a channel that carries the signal to lestination, and a receiver that reproduces the message from the received optical signal. This book presen 1p to date results on communication systems, along with the explanations of their relevance, from leading esearchers in this field. Its chapters cover general concepts of optical and wireless optical communication ystems, optical amplifiers and networks, optical multiplexing and demultiplexing for optical communication ystems, and network traffic engineering. Recently, wavelength conversion and other enhanced signal yrocessing functions are also considered in depth for optical communications systems. The researcher has ilso concentrated on wavelength conversion, switching, demultiplexing in the time domain and other enhar unctions for optical communications systems. This book is targeted at research, development and design sngineers from the teams in manufacturing industry; academia and telecommunications service operators/ roviders.
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