CONCLUSIONS
In this thesis a Quasi-Asynchronous optical sampler has been analized and implemented evaluating and testifying its performances and efficacy with different setups and optical probe signals.
At first, the main sampling techniques already developed in literature and their working principles have been presented and briefly characterized in order to explain the reason why the mixed techinque (used in the Quasi-Asynchronous sampler) seems to be the most advantageous among synchronous or asynchronous sampling. As a matter of fact the Quasi-Asynchronous sampler has the advantages of both synchronous and asynchronous samplers: there are no limitations for the temporal scanning range that is equal to the period of the signal under test (as for the asynchronous sampler) and there isn’t post processing to the sample sequence for samples reordering (as for the synchronous sampler).
The Quasi-Asynchronous sampler behaviour has been also simulated through a Matlab script in order to predict several informations in terms of sampling operation bandwidth and nonlinearities efficiency as well. The sampler simulated in the third chapter is based on nonlinear interaction (polarization rotation induced by XPM) in optical fiber between the signal to be resolved and a sampling ultra-short pulse train whose frequencies are locked to a fixed frequency difference. It has been possible to
CONCLUSIONS
notice that, when the signal pulse FWHM is comparable with the sampling pulse FWHM, the acquired signal enlarges itself. However, a signal pulse with a FWHM of 1ps has been sampled with a sampling pulse with the same FWHM, and even if the acquired signal was a little more wide than the original signal, it has been possible to appreciate that the shape of the signal was perfectly reconstructed. Simulations brought out the fact that the impact of the choice of ∆T value on the resolution limitation is much less decisive than the sampling pulsewidth. In fact it is not a problem to experimentally obtain a ∆T value of 500 or 250fs, but the acquired signals in the two different cases basically fitted together, even reconstructing a 1ps-signal pulse.
In the last chapter the Quasi-Asynchronous sampler operation was reported. First of all, a continuous wavelength λs modulated by a pattern generator clock signal through a Mach-Zender modulator, has been used as signal under test which was sampled by means of ultra-short pulses generated by the PicoSource. The wavelengths used in this first experiment were: λs=1553.48 nm, λc=1550.44 nm and λFWM=1547.38 nm. The signals frequencies were: fc=10GHz and fLO=500KHz, while fs is imposed by the PLL, so fs=fc-fLO=9.999995GHz. It has been induced FWM between the two optical signals into HNLF, so every pulse of the sampling signal generated a pulsed FWM component, whose power is proportional to the instantaneous power of the signal under test in the corresponding interaction time (and to the square power of the sampling pulse). It has been possible to reconstruct the sampled signal using a slow photodiode (bandwidth of 125 MHz) and a low-bandwidth oscilloscope to obtain just the envelope of the samples train.
Then it has been sampled a RZ signal (fs=10GHz) exploiting XPM-based polarization rotation, which occurs when the signal under test and the sampling signal are launched together through a highly non-linear fiber (HNLF). In order to maximize the efficiency of the non linear effect, the signal under test and the sampling signal were chosen with a difference of 45° in the polarization state. The two signals wavelengths have been kept 20nm apart: it didn’t affect XPM and reduced the spectral crosstalk.
After the polarizer, the optical samples were then amplified, filtered, photo-detected and viewed on a 600MHz real-time oscilloscope.
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Analisi e realizzazione di un campionatore ottico quasi-asincrono basato su cross-phase modulation per segnali ultra-veloci
NRZ signals were sampled using both HNLF and SOA. The possibility to make the Quasi-Asynchronous sampler “integrable” consists in exploiting SOA nonlinearities instead of HNLF. Furthermore, it could be possible to solve the problem of the spectral cross-talk, exploiting a sufficiently short SOA in a counter-propagating configuration.
Injecting signal and pump with opposite directions into che SOA, it has been avoided the use of an output filter to remove the pump so as to make whole the C-band available for the signal (signal and pump wavelengths can be equal). Using SOAs it has been observed a minimum acquired pulsewidth of 9ps, even injecting a 4ps input pulsewidth.
It means that SOA dynamics (slower than Kerr effects) limit the bandwidth of this kind of optical sampler around 110 GHz. The next step in order to decrease this limited resolution of this solution consists in exploiting shorter SOAs (SOAs by Kamelian have a length of 500 µs), trying to overcome the problem of their lower nonlinearities.
In the CNIT-CEIIC laboratories, researchers are working on the Quasi- Asynchronous sampler prototype (based on XPM polarization rotation) in order to reconstruct a signal bit rate between 10 and 40 Gb/s, with a long temporal scanning range (up to 3.2 ns), an acquisition time smaller than 20.48µs and a resolution of 500 fs.
Nowadays there is only an optical sampler on sale implemented by PicoSolve with a temporal resolution of 1 ps, able to characterize and monitor high speed transmission from 40 Gb/s up to 100Gb/s.
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