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).