KEYWORDS: Signal generators, Modulators, Analog electronics, Oscillators, Signal to noise ratio, Telecommunications, Interference (communication), Error analysis
The bandwidth and resolution of the electronic digital-to-analog converters (DAC) and analog-to-digital converters (ADC) of modern-day communication systems defines the link capacity to a large extent. For high analog bandwidths, the performance of state-of-the-art DACs is limited in terms of the effective number of bits (ENOB). A drastic improvement in ENOB might be realizable with photonic based DAC by employing integrated Mach- Zehnder modulators (MZM) and time-domain interleaving. Especially, the optical signal processing of Nyquist pulses with MZM might provide a possible solution to achieve high analog bandwidths with relatively low required electronic and photonic bandwidth. By using optical time interleaving and pulses synthesized by an MZM with a bandwidth of 100 GHz for the modulator and the electronics, sampling rates of 300 GS/s can be achieved. Thus, with standard silicon components available on the market, a compact and low-cost integrated photonic DAC module can easily be realized. The ENOB of such a system is limited by the quality of the Nyquist pulses, which in turn is affected by the jitter of the used signal generator (SG) and MZM nonlinearities. Here we present analytically that an ENOB of more than 8 can be achieved for analog bandwidths greater than 100 GHz by using a low phase noise SG. With experimental validation, we analyze the upper operation limit of such photonic DACs and their dependence on non-idealities of the Nyquist pulses.
For increasing the data rates in digital communication networks, high-speed signal generation is required. To generate these high-speed signals, electronics-based arbitrary waveform generators (AWGs) are the key components. However, most of the commercially available high-speed electronic AWGs are subject to linearity and resolution limitations. Photonics-based AWG, instead, might offer high bandwidth with better resolution and phase noise. Several photonic techniques have been proposed in recent years but with increased system complexity and limited dynamic range. We have recently proposed a photonics based architecture for high-speed arbitrary waveform generation using low-speed electronics, which is based on optical Nyquist pulse sequences and time-domain interleaving to obtain high-quality waveforms. Within this system, a single laser source is split into N branches. A Nyquist pulse sequence is generated by an integrated modulator driven by a single electrical sinusoidal frequency in each branch. Subsequently, they are modulated and multiplexed to obtain the targeted waveforms. The time delay between the pulse sequences is realized by a simple electrical phase shift of the sinusoidal driving signal. Here, a theoretical validation for the N channel system is presented along with simulation and experimental results for a three-branch photonic AWG. Using an integrated silicon Mach-Zehnder modulator saw-tooths, sinusoidal and some bandwidth-limited analog waveforms are generated. With available 100 GHz integrated modulators, the maximum possible sampling rate of 300 GS/s can be achieved. The mathematical proof validates that this simple concept can generate bandwidth limited user-defined waveforms with very high precision.
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