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.
Overcoming bandwidth limitations is of immense interest in optical signal processing. We propose to use a Mach-Zehnder modulator inside an integrated ring, which multiplies the same signal multiple times with sampling pulses of different time shifts, leading to higher sampling rates. In proof-of-concept experiments with a fiber ring, we have realized the sampling of an optical periodic pulse train (signal) by a multiplication with sampling pulses with 2.7 times the bandwidth of the modulator driving signal. This was achieved by applying an electrical multitone signal to the modulator with a frequency spacing close to the fundamental resonance of the ring. The modulator converts the electrical multitone signal into optical sampling pulses and the consecutive multiplication of these pulses with the signal to sample at slightly shifted positions enables very short pulses. The frequency mismatch between the multitone signal and the ring resonance ensures that the frequency components of the sampling pulses are not distorted by the frequency selectivity of the ring. The setup operates like a row of cascaded modulators driven with time-shifted sampling pulses. The method might enable an accurate waveform characterization for high-bandwidth optical periodic pulse trains.
With an ever increasing amount of end user devices connected to the internet, the global data traffic, especially within data centers, increases significantly. In order to keep pace, the current 100 Gbps standard (4 lines x 25 Gbps) needs to be upgraded. There are several possibilities to increase the data rate per line. The easiest way is to use multilevel modulation formats such as PAM4 with 2 bit per symbol or PAM8 or 16. Furthermore, optical multiplexing should be taken into account to maximize the bandwidth usage. Especially, optical signal processing with Nyquist pulses shows no inter-symbol interference, exhibit a rectangular spectrum and enables transmission at the maximum possible symbol rate for a given bandwidth. Here we present a simplified concept for ideal Nyquist pulse generation and simultaneous data modulation using just a single modulator per channel. Thereby, a laser source is split into three branches and the data signal is mixed electrically with a sine wave and then transferred into the optical domain, leading to a modulated Nyquist pulse train. The time delay between each Nyquist Pulse sequence for multiplexing is realized by a simple phase shift of the sinusoidal signal. With 3 time-domain channels, the proposed method achieved an aggregate baud rate, which corresponds to the full optical bandwidth of the modulators. On the contrary, the electronics require only 1/3 of the bandwidth. Due to the simple setup, integration on a silicon photonics platform might be straight forward. Preliminary simulation results show data transmission with PAM4 modulation at low bit error rates.
Robust and economic but precise and high-resolution analysis of optical spectra is of immense interest in optical communications, spectroscopy, sensing, and many other fields. Conventional optical spectrum analyzers utilize either movable gratings, interferometers, heterodyning or Brillouin scattering. Besides the large size, limited robustness and high costs, spectrometers with a high resolution, like interferometers and Brillouin scattering, usually measure only limited bandwidths, while grating-based spectrometers can measure a large bandwidth but, with limited resolution. Here we present preliminary results of a silicon integrated optical spectrum analyzer with wide operational range and high resolution. The device utilizes two spectral filtering devices in succession, namely an integrated high-Q microring resonator with tunable resonance frequency and a wavelength division demultiplexer. The narrow resonances of a tunable ring resonator enable high resolution measurements of the power distribution of unknown signals at multiple positions according to the free spectral range. Subsequently, the individual resonances are separated by a wavelength division demultiplexer with a much broader bandwidth. The narrow and equidistant resonances are placed so that each resonance falls within a separate channel of the demultiplexer. The power of each subchannel is monitored during tuning of the ring by simple low bandwidth photodiodes. If one scan corresponds to the tuning of the ring resonances over the full width of one channel of the demultiplexer, the combination of all channels will represent the whole spectrum of the signal under test. Preliminary experiments show a resolution of 98 MHz.
The observation of ultrafast signals by expanding them to a time scale that enables the measurement with conventional high-speed systems is of considerable interest in many applications. Usually, a time-lens can be used for this purpose. Like a lens in optics, a time lens expands the signal in time. This can be accomplished by a strong first order dispersion. However, higher order dispersion leads to a distortion of the signal and an integration of elements with a strong first order dispersion is challenging. Here we present a dispersion-less time-lens with an integrated ring resonator. Several replicas of a single input signal are generated by a microring resonator having a free spectral range (FSR) much less than the bandwidth of the input signal. These copies are then subjected to a coupled Mach-Zehnder intensity modulator (MZM) system driven by a single sinusoidal radio frequency (RF) signal to generate copies of the input spectrum. In the time-domain this can be seen as a multiplication of the input signal with a sinc-pulse sequence. The sinc-pulse sequence is tunable by the single sinusoidal radio frequency. By choosing a suitable radio frequency, the signal waveform can be sampled at a different position for each copy, so that an expanded waveform with a configurable stretching factor determined by the input RF can be achieved. This time lens system can be fully integrated into a photonic integrated circuit and requires neither an optical source nor a dispersive medium. In first preliminary experiments we present a sampling rate of around 110 GSa/s.
According to the sampling theorem, bandwidth limited signals can be seen as superposition of time shifted sinc pulses weighted with the sampling values. Since sinc pulses are orthogonal to each other, bandlimited signals can be perfectly sampled by an integration over the product between them and a sinc pulse with the correct time shift and bandwidth. Because sinc pulses have an infinite time length, they cannot be realized experimentally. Instead, generating a periodical sinc pulse sequence is straightforward. For a low duty cycle the pulses in such a sequence come close to single sinc pulses and thus the sampling might come closer to ideal sampling. In the frequency domain, this nearly ideal sampling is represented by a convolution between the signal spectrum and a rectangular frequency comb with many lines. The bandwidth of the comb corresponds to the sampling rate, while a bigger number of comb lines reduces the duty cycle and might enhance the sampling quality. We present the generation of a flat frequency comb with up to 33 lines in the optical domain as well as how to convolve it with an optical input spectrum for optical sampling. Already with one Mach- Zehnder modulator driven with m equidistant radio frequencies, the sampling with a comb consisting of 2m+1 lines can be realized. Additionally, with a second Mach-Zehnder modulator driven with n equidistant radio frequencies, the comb line number can be enhanced to (2m+1)(2n+1).
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