**2. Implementation of HDWDM transmission system**

Due to these rapidly growing capacity requirements for long-haul transmission, the optical wavelength division multiplexing systems are advancing into high data transmission rate and dense channel spacing to utilize the available bandwidth more effectively. In order to maximize the system capacity and to minimize the performance degradation caused by transmission impairments the system investigation and optimization are very important. To increase the spectral efficiency is important for building efficient HDWDM transmission systems, since this allows the optical infrastructure to be shared among many wavelengths. This approach reduces the cost per transmitted information bit in a fully loaded and optimized transmission system.

#### **2.1 Selection of HDWDM main components**

192 Optical Communications Systems

components in the telecommunication hierarchy, whose integration with standard network services and applications promotes rapid evolution of fiber optics and its wide implementation into all telecommunication branches (see Table 1 [McGloin & Reid., 2010]).

> Approx. 20 km (ITU recommendation), normally < 10 km

1310 nm; 1490 nm; 1550 nm

or CWDM CWDM WDM Depending on

internal Internal Internal

**CONNECTIONS** 

850 nm; 1310 nm

protocol type

protocol type

< 100 m

**Network CORE METRO ACCESS CROSS** 

**Laser type** DFB DFB,VCSEL DFB or Fabry-Perot VCSEL

**Bit rate** 10 Gbit/s 10 Gbit/s < 2.5 Gbit/s Depending on

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

Currently, many research topics in the field of optical transmission systems (mostly grounded on novel modulation techniques) are focused on increasing the total data transmission speed of an individual optical fiber [Abbou et al., 2008, Bhamber et al., 2007, Bobrovs et al., 2008]. An alternative − but equally valid − approach to increasing the data transmission is to decrease the wavelength division multiplexing (WDM) channel spacing to high-dense dimensions while keeping the existing data transmission speed for an exact

High performance optical filters make the groundwork for realization of high-speed highdensity WDM (HDWDM) transmission systems [Pfennigbauer & Winzer, 2006]. High channel spacing and data transmission rate set strict requirements for HDWDM filter characteristics, so any imperfections in their parameters, such as amplitude and phase responses, could become critical. The low channel separation from adjacent channels is one of these imperfections in optical filter parameters [Agrawal, 2001, Ozoliņš et al., 2009].

Due to these rapidly growing capacity requirements for long-haul transmission, the optical wavelength division multiplexing systems are advancing into high data transmission rate and dense channel spacing to utilize the available bandwidth more effectively. In order to maximize the system capacity and to minimize the performance degradation caused by transmission impairments the system investigation and optimization are very important. To increase the spectral efficiency is important for building efficient HDWDM transmission systems, since this allows the optical infrastructure to be shared among many wavelengths. This approach reduces the cost per transmitted information bit in a fully loaded and optimized transmission system.

1310 nm; 1550 nm

**Transmission** 

**Wavelength**

**Modulation** 

**Multiplexing scheme**

branches

**Length** > 100 km 10 km

1550 nm central ± 30 nm

**scheme** External External or

DWDM

channel [Ozoliņš et al., 2011, Bobrovs et al., 2009].

**2. Implementation of HDWDM transmission system** 

The complexity of a system's design in optical communications can be seen as the result of a large number of components with different parameters and operational states. The description of the interaction between the optical signal and transmission disturbances is a multi-dimensional issue, whose solution depends on the relation between different system parameters. The right approach to the optimization of system settings and derivation of design rules must take into account the interaction of effects which take place in each component. In this section, the system components needed for realization of an HDWDM transmission system are described.

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 optical 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. 2. Mach-Zehnder Modulator (MZM) principles: a) structure b) transmission function [Kaminow et al., 2009].

External modulation can be realized with a LiNbO3-based Mach-Zehnder modulator (MZM) (see Fig. 2) [Kaminow et al., 2009]. The operational principles of MZMs are based on the electro-optic effect, which is characterized by variation in the applied electrical field causing changes of the refractive index in the modulator arms. The variation of the refractive index in the modulator arms induces a change of material propagation constant *β*, resulting in different phases in both modulator arms. The input optical signal is divided by a 3-dB coupler into two equal parts – in lower and upper arm of the MZM. The external modulator is driven by an electrical signal with corresponding data rate. Depending on the electrical driving signal,

Realization of HDWDM Transmission System with the Minimum Allowable Channel Interval 195

After transmission through the optical fiber, a multiwavelength optical signal needs to be separated in individual channels. This is realized through implementation of band-pass filters (BPFs), which transmit optical power within a definite wavelength window only, and reflect or absorb the rest. In the case of a single-channel transmission the function of an optical BPF is to separate the channel information from the noise which has been added, e.g., by optical amplifiers. This noise is generally broadband, and can often be described as quasi-white: it has a constant level in the power spectrum [Kashyap, 2010, Venghaus, 2006]. By applying a BPF to select the wavelength channel, the useful information is retained and most of the noise is filtered resulting in an improvement of the optical-signal-to-noise ratio (OSNR). Such a filter can also be used to select a particular channel in a HDWDM application from several channels that are transmitted in a common HDWDM transmission system [Azadeh, 2009, Venghaus, 2006]. The role of an optical receiver is to detect the transmitted signal by the opto-electrical transformation of the signal received by a photodiode (e.g. PIN or APD). Furthermore, additional electrical equalization is performed together with electrical signal amplification enabling further signal-processing (e.g. clock-

In fiber optical transmission systems, the degradation effects can be categorized by the random noise and waveform distortion. For long-span HDWDM systems, signal waveform distortion can be generated by linear chromatic dispersion, polarization mode dispersion (PMD), nonlinear optical effects (NOE) in optical fibers, or their combination [Chen et al., 2006, Pan et al., 2010]. In high-speed (more than 2.5 Gbit/s) time division multiplexing (TDM) optical systems having short optical pulses and wide optical spectrum the effect of complex dispersion dominates in the system performance degradation. In multiwavelength WDM optical systems the inter-channel crosstalk originated by fiber nonlinearity, such as cross-phase modulation (XPM) and four-wave mixing (FWM), is a limiting factor. To maximize the WDM network capacity, the system's design and optimization have to take into account all the contributing factors - such as the channel data rate, transmission distance, signal optical power, fiber linear and nonlinear optical effects and, of course, the channel interval [Venghaus, 2006]. In a HDWDM system the last factor is the most

important for a high-quality solution which depends directly on the optical filters.

role in the optical domain [Venghaus, 2006, Kaminow et al., 2008].

equalization filters, all-pass filters and band-pass filters [Szodenyi, 2004].

The wavelength filters in optical transmission systems are a special subgroup of physical components defined in such a way that they select or modify parts of the signal spectrum. In fact, the optical wavelength filters are defined as referred to the modifications that they induce in the frequency spectrum. In electronic systems, relevant filters play a crucial role in numerous signal processing applications. Similarly, optical filters play an equally crucial

Multiplexing and de-multiplexing functions are performed by narrowband filters, cascaded and combined in various ways to achieve the desired result. The filters in optical HDWDM transmission systems are classified into the following types: notch filters, power

As was said above in sub-section 2.1, band-pass filters (BPFs) transmit optical power within a definite wavelength window only, and reflect or absorb the rest. The bandwidth of an optical

**2.2 Optical filters for HDWDM systems** 

recovery) and performance evaluation (e.g. quality measurements).

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 Bragg'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.

After the MZM, such a signal is sent directly to a transmission medium, where optical pulses are propagating over different distances of a single-mode fiber (SMF). For compensation of losses in the fiber and in optical components it is necessary to use the technique for amplifying optical signals. The optical amplifiers represent one of the crucial components in an optical transmission system. Despite the minimum attenuation at 1550 nm, fiber losses significantly limit the transmission performance with increased transmission distance. Optical amplification can be realized using different amplifier concepts and mechanisms, e.g. semiconductor optical amplifiers (SOA), rare-earth (erbium, holmium, thulium, and samarium) doped fiber amplifiers, or, more recently, Raman amplifiers [Kaminow et al., 2008, Binh, 2008]. All these amplifier types are based upon different physical mechanisms resulting in different device characteristics and application areas. The rare-earth-doped fiber amplifiers provide optical amplification in the wavelength region from 500 to 3500 nm. The most important representative of these amplifier types is the erbium-doped fiber amplifier (EDFA), which is widely used today in optical transmission systems since it provides efficient optical amplification in the 1550 nm region. The EDFAs present the state-of-the-art technology in conventional optical transmission systems, and they can be used as in-line amplifiers (placed every 30-80 km), power boosters (amplifiers at the transmitter side) or pre-amplifiers (amplifiers in front of the receiver) independently of the channel bit rate in the system [Thyagarajan & Ghatak, 2007].

After transmission through the optical fiber, a multiwavelength optical signal needs to be separated in individual channels. This is realized through implementation of band-pass filters (BPFs), which transmit optical power within a definite wavelength window only, and reflect or absorb the rest. In the case of a single-channel transmission the function of an optical BPF is to separate the channel information from the noise which has been added, e.g., by optical amplifiers. This noise is generally broadband, and can often be described as quasi-white: it has a constant level in the power spectrum [Kashyap, 2010, Venghaus, 2006]. By applying a BPF to select the wavelength channel, the useful information is retained and most of the noise is filtered resulting in an improvement of the optical-signal-to-noise ratio (OSNR). Such a filter can also be used to select a particular channel in a HDWDM application from several channels that are transmitted in a common HDWDM transmission system [Azadeh, 2009, Venghaus, 2006]. The role of an optical receiver is to detect the transmitted signal by the opto-electrical transformation of the signal received by a photodiode (e.g. PIN or APD). Furthermore, additional electrical equalization is performed together with electrical signal amplification enabling further signal-processing (e.g. clockrecovery) and performance evaluation (e.g. quality measurements).

In fiber optical transmission systems, the degradation effects can be categorized by the random noise and waveform distortion. For long-span HDWDM systems, signal waveform distortion can be generated by linear chromatic dispersion, polarization mode dispersion (PMD), nonlinear optical effects (NOE) in optical fibers, or their combination [Chen et al., 2006, Pan et al., 2010]. In high-speed (more than 2.5 Gbit/s) time division multiplexing (TDM) optical systems having short optical pulses and wide optical spectrum the effect of complex dispersion dominates in the system performance degradation. In multiwavelength WDM optical systems the inter-channel crosstalk originated by fiber nonlinearity, such as cross-phase modulation (XPM) and four-wave mixing (FWM), is a limiting factor. To maximize the WDM network capacity, the system's design and optimization have to take into account all the contributing factors - such as the channel data rate, transmission distance, signal optical power, fiber linear and nonlinear optical effects and, of course, the channel interval [Venghaus, 2006]. In a HDWDM system the last factor is the most important for a high-quality solution which depends directly on the optical filters.

#### **2.2 Optical filters for HDWDM systems**

194 Optical Communications Systems

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 Bragg'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

After the MZM, such a signal is sent directly to a transmission medium, where optical pulses are propagating over different distances of a single-mode fiber (SMF). For compensation of losses in the fiber and in optical components it is necessary to use the technique for amplifying optical signals. The optical amplifiers represent one of the crucial components in an optical transmission system. Despite the minimum attenuation at 1550 nm, fiber losses significantly limit the transmission performance with increased transmission distance. Optical amplification can be realized using different amplifier concepts and mechanisms, e.g. semiconductor optical amplifiers (SOA), rare-earth (erbium, holmium, thulium, and samarium) doped fiber amplifiers, or, more recently, Raman amplifiers [Kaminow et al., 2008, Binh, 2008]. All these amplifier types are based upon different physical mechanisms resulting in different device characteristics and application areas. The rare-earth-doped fiber amplifiers provide optical amplification in the wavelength region from 500 to 3500 nm. The most important representative of these amplifier types is the erbium-doped fiber amplifier (EDFA), which is widely used today in optical transmission systems since it provides efficient optical amplification in the 1550 nm region. The EDFAs present the state-of-the-art technology in conventional optical transmission systems, and they can be used as in-line amplifiers (placed every 30-80 km), power boosters (amplifiers at the transmitter side) or pre-amplifiers (amplifiers in front of the receiver)

independently of the channel bit rate in the system [Thyagarajan & Ghatak, 2007].

optical power (10...40 mW) (see Fig. 3) [Funabashi, 2001].

Fig. 3. Simplified fiber optical transmission system.

The wavelength filters in optical transmission systems are a special subgroup of physical components defined in such a way that they select or modify parts of the signal spectrum. In fact, the optical wavelength filters are defined as referred to the modifications that they induce in the frequency spectrum. In electronic systems, relevant filters play a crucial role in numerous signal processing applications. Similarly, optical filters play an equally crucial role in the optical domain [Venghaus, 2006, Kaminow et al., 2008].

Multiplexing and de-multiplexing functions are performed by narrowband filters, cascaded and combined in various ways to achieve the desired result. The filters in optical HDWDM transmission systems are classified into the following types: notch filters, power equalization filters, all-pass filters and band-pass filters [Szodenyi, 2004].

As was said above in sub-section 2.1, band-pass filters (BPFs) transmit optical power within a definite wavelength window only, and reflect or absorb the rest. The bandwidth of an optical

Realization of HDWDM Transmission System with the Minimum Allowable Channel Interval 197

The right choice of the performance evaluation criteria for characterizing the optical transmission lines is one of the key issues in designing efficient high-speed systems. The evaluation criteria should provide precise determination and separation of dominant system limitations, which is crucial for suppressing the propagation disturbances. They should also provide comparison of experimental and numerical data to verify the numerical models

The bit error ratio (BER) evaluation is a straightforward and relatively simple method for performance evaluation based on counting the errors in the received bit streams. The error counting in a practical system with a transmission speed greater than 1 Gbit/s can be a long process, especially for realistically low BER values (< 10–9). For investigation of performance of an optical transmission system by simulation, several effective statistical methods have

Conventional methods of Q-factor and hence BER calculation are based on the assumption of Gaussian noise distribution. However, new methods relying upon statistical processes with account for the distortion dynamics of optical fibers are necessary in order to include

The former statistical technique employs the expected maximization theory in which the *pdf* of the detected electrical signal is approximated as a mixture of multiple Gaussian

The latter technique is based on the generalized extreme values (GEV) theorem [Bierlaire et al., 2007, Markose & Alentorn, 2007]. Although this theorem is well-known in other fields (financial forecasting, meteorology, material engineering, see e.g. [Kotz & Nadarajah, 2000] to predict the probability of occurrence of extreme values, it has not yet been applied in

Exactly as the BER set used in experimental transmission, the BER in the simulation of a particular HDWDM system configuration is calculated. In this case the BER is the ratio of the occurrence of errors (Nerror) to the total number of transmitted bits (Ntotal) and given as:

The Monte-Carlo method offers a precise picture via the BER metric for all modulation formats and receiver types. The optical system configuration under a simulation test must include all the sources of impairments imposing on signal waveforms, including fiber

The optical signal-to-noise ratio (OSNR) is a widely used evaluation criterion for characterizing the system performance in already deployed transmission lines. The optical noise created by transmission media and devices around an optical signal reduces

ൌ ܴܧܤ

impairments and amplified spontaneous emission (ASE) [Binh, 2009].

**3.2 Optical signal-to-noise ration (OSNR)** 

(1)

ேೌ

**3. Performance evaluation criteria** 

**3.1 Evaluation of bit error ratio (BER)** 

been developed [Binh, 2009].

the common patterning effects.

optical communications.

distributions.

applied.

BPF typically depends on the optical transmission system type [Venghaus, 2006]. For example, in HDWDM the sharpness of the optical BPF amplitude transfer function is of great importance, while in the coarse wavelength division multiplexing (CWDM) it is a minor factor because of a wide frequency interval between the adjacent channels. In these systems the major role is played only by the optical BPF bandwidth, and in the DWDM systems also the shape of the amplitude and phase transfer function should be taken into account. Although different kinds of filters are necessary in a HDWDM transmission system, BPFs are by far the most important, since they are prerequisite for add and drop, multiplex, interleave and routing functionalities which are essentials for a HDWDM transmission system realization [Agrawal, 2001].

Travelling through a multiple optical BPF, the optical signal experiences spectral narrowing due to temperature instability of filtering devices and to central frequency fluctuations of light sources, which could be the main factor of degradation in future transmission systems. Therefore, it is necessary to find out the minimal filter's full width half maximum (FWHM) which ensures appropriate quality of transmitted data signals. Still, the filter bandwidth is not the exclusive parameter of which we need to be aware: the phase transfer function of optical band-pass filters is also of great importance for transmitting information via HDWDM transmission systems.

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, microring 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. 4. First-order amplitude transfer (*a*) and group delay (*b*) functions of different optical filters shown in the inset (with FWHM bandwidth 0.4 nm or 50 GHz). The graphs are obtained by/using OptSim simulation software.

As is seen from Fig. 4*b*, the greater group delay is for the Raised Cosine optical band-pass filter whose amplitude characteristics are the closest to an ideal filter's amplitude parameters. The ideal amplitude transfer function of a band-pass filter has an almost rectangular shape, providing a perfect transmission (without distortion) of the whole signal within the filter bandwidth, and cutting undesired signals out of the band [Venghaus, 2006].
