# A Review of DSP-Based Enabling Technologies for Cloud Access Networks
## 3 CAN Network Elements
### Spectral Converters
#### 3.3.2 Experimental Analysis of Spectral Converter Performance  
To verify the theoretically predicted performance [11], the spectral converter's performance has been experimentally demonstrated when subject to varying operating conditions. The experiment employed a DP-MZM-based spectral converter system, as shown in Figure 11. The input optical signal was a 1550.118 nm continuous wave light source at a power of 13 dBm, which was intensity modulated via an MZM with a 10 Gb/s, 16-QAM, OFDM signal, generated via an arbitrary waveform generator (AWG). The modulated optical signal power first passed through an EDFA and a variable optical attenuator before injection into an integrated DP-MZM (12.5 dB insertion loss and 22 dB extinction ratio) where the output optical signal is subject to the spectral conversion effect. The RF driving signal is an amplified 12.5 GHz sinusoid, which is divided into four separate signals, each RF signal then passes through a phase shifter to independently control its phase, before it drives one of the four DP-MZM electrodes. According to the identified optimum operating conditions, the four RF driving signals must have the same amplitude, the required amplitude being determined by the order of the desired converted optical component. The phases of the four RF driving signals must also be adjusted for a spacing of 90\({}^{\circ}\), thus the relative phases are 0\({}^{\circ}\), 90\({}^{\circ}\), 180\({}^{\circ}\) and 270\({}^{\circ}\). The output optical signal containing the converted optical frequency components is then optically filtered by a tunable optical filter to select the desired optical component. The optical signal is subsequently directly-detected by a 12 GHz photodetector, the resultant electrical signal is digitized and captured by a digital storage oscilloscope (DSO) and offline processing then demodulates the signal and analyses its BER performance. For a 7.9 dBm, 10 Gb/s OOFDM signal at the converters input, Figure 12a shows the unfiltered converted optical spectrum at the DP-MZM's output, where the operating conditions have been optimized for the first spectral component giving a detuning range of 12.5 GHz. To show the effect of non-optimized operating conditions, Figure 12b shows the corresponding optical spectrum when the DC bias voltages are arbitrarily set and not optimized. It can be clearly seen that when optimized operating conditions are employed the power of the first spectral component is significantly enhanced  
Figure 11: Spectral converter structure. whilst the power of the other spectral components is diminished. The experiment demonstrated that the optimum operating conditions reduced the power of 75% of the parasitic optical components to more than 35 dB below the power of the desired optical component. Furthermore, the experimentally measured conversion efficiency of \(-17.4\) dB closely agreed with the theoretically predicted value. For the first and third converted optical components, each obtained under its optimum operating conditions, BERs as a function of optical launch power are plotted in Figure 13, thus the spectral converter-induced power penalties are shown in relation to a spectral conversion-free optical signal. The spectral conversion induced power penalty at the FEC limit of 1E-3 is approximately 16.2 dB and 19.1 dB for the first and third converted optical components, respectively. The power penalties are due to the DP-MZM insertion loss effect and the frequency detuning range dependent conversion efficiency, thus the third converted optical component has a higher penalty, due to its higher frequency detuning range. Frequency detuning range-independent spectral conversion efficiency is however feasible by utilizing a variable driving RF frequency and keeping the targeted optical component index constant. This first experimental demonstration of the recently proposed spectral converter clearly shows the technical viability of the spectral conversion technique.