A scheme to perform ultrahigh-Q and single passband microwave photonic filter (MPF) with tunability is proposed and experimentally demonstrated. The single passband of the MPF is implemented by the stimulated Brillouin scattering (SBS) in an optical fiber. The ultrahigh-Q of the MPF is realized due to the ultra-narrow resonant linewidth of the fiber ring resonator, which compresses the bandwidth of the SBS dramatically. A MPF with a single passband and an ultra-narrow 3 dB bandwidth of $825\pm 125\mathrm{kHz}$ over the wide center frequency tuning range of 2–16 GHz are achieved. The MPF also shows high out-of-band suppression exceeding 25 dB and small amplitude fluctuation of $\pm 1.8\mathrm{dB}$ within the tuning range. The maximum Q-factor of the MPF is 17778. To the best of our knowledge, this is the highest Q single-passband MPF reported to date, with wide tunability, high out-of-band suppression, and a simple structure.

Taking the advantage of ultrafast optical linear and nonlinear effects, all-optical signal processing (AOSP) enables manipulation, regeneration, and computing of information directly in optical domain without resorting to electronics. As a promising photonic integration platform, silicon-on-insulator (SOI) has the advantage of complementary metal oxide semiconductor (CMOS) compatibility, low-loss, compact size as well as large optical nonlinearities. In this paper, we review the recent progress in the project granted to develop silicon-based reconfigurable AOSP chips, which aims to combine the merits of AOSP and silicon photonics to solve the unsustainable cost and energy challenges in future communication and big data applications. Three key challenges are identified in this project: (1) how to finely manipulate and reconfigure optical fields, (2) how to achieve ultra-low loss integrated silicon waveguides and significant enhancement of nonlinear effects, (3) how to mitigate crosstalk between optical, electrical and thermal components. By focusing on these key issues, the following major achievements are realized during the project. First, ultra-low loss silicon-based waveguides as well as ultra-high quality microresonators are developed by advancing key fabrication technologies as well as device structures. Integrated photonic filters with bandwidth and free spectral range reconfigurable in a wide range were realized to finely manipulate and select input light fields with a high degree of freedom. Second, several mechanisms and new designs that aim at nonlinear enhancement have been proposed, including optical ridge waveguides with reverse biased PIN junction, slot waveguides, multimode waveguides and parity-time symmetry coupled microresonators. Advanced AOSP operations are verified with these novel designs. Logical computations at 100 Gbit/s were demonstrated with self-developed, monolithic integrated programmable optical logic array. High-dimensional multi-value logic operations based on the four-wave mixing effect are realized. Multi-channel all-optical amplitude and phase regeneration technology is developed, and a multi-channel, multi-format, reconfigurable all-optical regeneration chip is realized. Expanding regeneration capacity via spatial dimension is also verified. Third, the crosstalk from optical as well as thermal coupling due to high-density integration are mitigated by developing novel optical designs and advanced packaging technologies, enabling high-density, small size, multi-channel and multi-functional operation with low power consumption. Finally, four programmable AOSP chips are developed, i.e., programmable photonic filter chip, programmable photonic logic operation chip, multi-dimensional all-optical regeneration chip, and multi-channel and multi-functional AOSP chip with packaging. The major achievements developed in this project pave the way toward ultra-low loss, high-speed, high-efficient, high-density information processing in future classical and non-classical communication and computing applications. Graphical Abstract

Microwave photonic filters (MPFs) with only one ultra-narrow passband are able to provide high frequency selectivity and wide spectral range, and they are of great importance in radio-frequency (RF) signal processing. However, currently all MPFs are limited by trade-offs between key parameters such as spectral resolution and range, tunability, and stability. Here, we report the first demonstration of a single passband MPF with unprecedented performance including ultrahigh spectral resolution of 650 kHz, 0–40 GHz spectral range, and high stability of center frequency drifting within ±50 kHz. This record performance is accomplished by breaking the amplitude equality of a phase-modulated signal via a Brillouin dynamic grating (BDG) which has an ultra-narrow reflection spectrum of sub-MHz. The results point to new ways of creating high performance microwave photonic systems, such as satellite and mobile communications, radars, and remote-sensing systems.

An optical format interconversion scheme between on-off keying (OOK) and quadrature phase shift keying (QPSK) is proposed and verified in this paper. The conversion system mainly consists of a coherent vector combiner and a reconfigurable two-dimensional (2D) vector mover. As a key element of the proposed conversion system, the 2D vector mover is implemented by a non-degenerate phase-sensitive amplifier (PSA). The operating principle and theoretical derivations of the PSA-based 2D vector mover are fully introduced. The reconfigurable transfer characteristics of the vector mover are analyzed under different parameter settings to exhibit the flexible 2D moving function. The signal constellations, eye diagrams, spectrum, error vector magnitudes, and bit error ratios are estimated and depicted to validate the proposed idea. With the input signal-to-noise ratios of 20 dB and 25 dB, error-free conversions are achieved between 50G Baud OOK and QPSK. The scheme proposed in this paper fills the lack of the one-to-one interconversion between OOK and QPSK, and has potential applications in optical interconnect nodes, across-dimensional optical transmissions, and flexible optical transceivers.

Optical format conversion technologies have attracted a lot of attention to bridge the different optical networks employing different formats. In this paper, the advances of optical format conversion technologies are introduced and reviewed. Through analyzing the optical network architecture and modulation technology development, we focus on the format conversions between the signals with one dimensional and two dimensional constellations. The important enabling technologies for flexible constellation manipulation are introduced in detail, including the linear and nonlinear optical effects-based functions. Then the key format conversion researches are reviewed in four categories: one-to-one conversions, optical aggregations, optical de-aggregations, and optical interconversions. Finally, the current issue and future trend of this research field is analyzed and discussed.

An all-optical format conversion (AOFC) scheme for star-octal quadrature amplitude modulation (star-8QAM) signals is proposed and numerically simulated based on nonlinear effects in the high nonlinear fiber (HNLF). In the first stage AOFC processing, the input 30 Gbit/s star-8QAM signal is firstly de-aggregated into one 20 Gbit/s quadrature phase shift keying (QPSK) signal and one 10 Gbit/s typical on-off keying (OOK) signal by deploying the self-phase modulation (SPM), cross-phase modulation (XPM) and parametric amplification (PA) effects. In the second stage AOFC processing, the 20 Gbit/s QPSK signal and the 10 Gbit/s typical OOK signal are aggregated into one 30 Gbit/s star-8QAM signal again by utilizing the XPM and PA effects. Moreover, the amplitude ratio (AR) between the outer ring and inner rings of the aggregated star-8QAM signal can be adjusted flexibly by changing the QPSK and OOK signals power. Other octal modulation formats can also be generated through changing the QPSK and OOK signals power, including the rectangular 8QAM signals, $\pi$ /4-8QAM signals and 8-ary phase shift keying (8PSK) signals. Error vector magnitude (EVM) and bit error ratio (BER) for star-8QAM signals before and after AOFC are measured to analyse the scheme performance. The AOFC scheme provides the flexibility to adopt the appreciate modulation formats at the optical nodes to connect different optical networks.