**3. Performance evaluation criteria**

196 Optical Communications Systems

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

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

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

Group delay, ps

Fig. 4. First-order amplitude transfer (*a*) and group delay (*b*) functions of different optical

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].

192.9 192.95 193 193.05 193.1

Lorentzian Raised Cosine Supergaussian

Frequency, THz

thin film filters with low refraction index modulation [Venghaus, 2006].

Lorentzian Raised Cosine Supergaussian

filters shown in the inset (with FWHM bandwidth 0.4 nm or 50 GHz). The graphs are obtained by/using OptSim simulation software.

192.9 192.95 <sup>193</sup> 193.05 193.1 -50

Frequency, THz

which are essentials for a HDWDM transmission system realization [Agrawal, 2001].

HDWDM transmission systems.




Attenuation, dB


0

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 applied.
