Parity-time symmetry enabled ultra-efficient nonlinear optical signal processing.

Nonlinear optical signal processing (NOSP) has the potential to significantly improve the throughput, flexibility, and cost-efficiency of optical communication networks by exploiting the intrinsically ultrafast optical nonlinear wave mixing. It can support digital signal processing speeds of up to terabits per second, far exceeding the line rate of the electronic counterpart. In NOSP, high-intensity light fields are used to generate nonlinear optical responses, which can be used to process optical signals. Great efforts have been devoted to developing new materials and structures for NOSP. However, one of the challenges in implementing NOSP is the requirement of high-intensity light fields, which is difficult to generate and maintain. This has been a major roadblock to realize practical NOSP systems for high-speed, high-capacity optical communications. Here, we propose using a parity-time (PT) symmetric microresonator system to significantly enhance the light intensity and support high-speed operation by relieving the bandwidth-efficiency limit imposed on conventional single resonator systems. The design concept is the co-existence of a PT symmetry broken regime for a narrow-linewidth pump wave and near-exceptional point operation for broadband signal and idler waves. This enables us to achieve a new NOSP system with two orders of magnitude improvement in efficiency compared to a single resonator. With a highly nonlinear AlGaAs-on-Insulator platform, we demonstrate an NOSP at a data rate approaching 40 gigabits per second with a record low pump power of one milliwatt. These findings pave the way for the development of fully chip-scale NOSP devices with pump light sources integrated together, potentially leading to a wide range of applications in optical communication networks and classical or quantum computation. The combination of PT symmetry and NOSP may also open up opportunities for amplification, detection, and sensing, where response speed and efficiency are equally important. Supplementary Information: The online version contains supplementary material available at 10.1186/s43593-024-00062-w.

Integrated MLL chip-based PAM-4/DMT-16QAM photonic-wireless link in W-band for flexible applications.

To accommodate the demand of exponentially increasing global wireless traffic driven by the coming beyond 5G and 6G, wireless communication has stepped into the millimeter wave (MMW) band to exploit large available bandwidth. The future wireless application scenarios require wireless communication systems with high speed, low cost, a small footprint and simple configuration, and the integrated light source-based intensity modulation and direct detection (IM-DD) photonic-wireless system can better meet the demand than the traditional system based on bulky components. In this paper, we experimentally demonstrate a lens-free pulse-amplitude-modulation with four levels (PAM-4) and discrete multi-tone with 16-quadrature amplitude modulation (DMT-16QAM) MMW photonic-wireless transmission system in the W-band using an integrated mode-locked laser (MLL) chip and a mixer-based receiver, which could be applicable for flexible wireless applications. The integrated MLL as an on-chip single light source is used to generate W-band signals and simplify the transmitter. The signal-to-noise ratio of the generated wireless signal is improved by two coherent optical carriers both modulated with data and then beating in the photodiode. In addition, we investigate the IM-DD configuration by employing an envelope detector (ED) to receive the PAM-4 signal for further simplifying the system. The ED-based photonic-wireless system is more suitable for the applications with lower data rate and low cost. For higher data rate, the mixer-based PAM-4/DMT-16QAM systems with up to 31.75 Gbit/s net data rate are more favorable, although the cost is also higher.

High-Q titanium dioxide micro-ring resonators for integrated nonlinear photonics.

We report on the nonlinear characterizations of the titanium dioxide micro-ring resonators (TiO2 MRRs). By utilizing optimized fabrication processes, high quality factors (Q∼1.4 × 105) doubling that of the previous work are achieved here for TiO2 MRRs with high-confinement TiO2 waveguides. The four-wave mixing (FWM) experiment results with low and high signal power demonstrate that, the fabricated TiO2 MRRs can perform broadband (∼40 nm) wavelength conversion and cascaded FWMs. These achievements pave the way for key nonlinear photonic applications with TiO2 waveguides and provide an efficient platform for various integrated photonic devices.

Optimization of Probabilistic Shaping for Nonlinear Fiber Channels with Non-Gaussian Noise.

Probabilistic constellation shaping is investigated in the context of nonlinear fiber optic communication channels. Based on a general framework, different link types are considered-1. dispersion-managed channels, 2. unrepeatered transmission channels and 3. ideal distributed Raman amplified channels. These channels exhibit nonlinear effects to a degree that conventional probabilistic constellation shaping strategies for the additive white Gaussian (AWGN) noise channel are suboptimal. A channel-agnostic optimization strategy is used to optimize the constellation probability mass functions (PMFs) for the channels in use. Optimized PMFs are obtained, which balance the effects of additive amplified spontaneous emission noise and nonlinear interference. The obtained PMFs cannot be modeled by the conventional Maxwell-Boltzmann PMFs and outperform optimal choices of these in all the investigated channels. Suboptimal choices of constellation shapes are associated with increased nonlinear effects in the form of non-Gaussian noise. For dispersion-managed channels, a reach gain in 2 spans is seen and across the three channel types, gains of >0.1 bits/symbol over unshaped quadrature-amplitude modulation (QAM) are seen using channel-optimized probablistic shaping.

Frequency-domain ultrafast passive logic: NOT and XNOR gates.

Electronic Boolean logic gates, the foundation of current computation and digital information processing, are reaching final limits in processing power. The primary obstacle is energy consumption which becomes impractically large, > 0.1 fJ/bit per gate, for signal speeds just over several GHz. Unfortunately, current solutions offer either high-speed operation or low-energy consumption. We propose a design for Boolean logic that can achieve both simultaneously (high speed and low consumption), here demonstrated for NOT and XNOR gates. Our method works by passively modifying the phase relationships among the different frequencies of an input data signal to redistribute its energy into the desired logical output pattern. We experimentally demonstrate a passive NOT gate with an energy dissipation of ~1 fJ/bit at 640 Gb/s and use it as a building block for an XNOR gate. This approach is applicable to any system that can propagate coherent waves, such as electromagnetic, acoustic, plasmonic, mechanical, or quantum.

All-optical OFDM demultiplexing with optical partial Fourier transform and coherent sampling.

We propose a novel scheme with a "time-lens"-based partial optical Fourier transform (OFT) and coherent sampling for high-speed complex orthogonal frequency-division multiplexing (OFDM) signal detection. Compared with all-optical OFDM demultiplexing with a matched optical filter, our proposed method replaces specialized optical filters with commercially available equipment, which relaxes stringent manufacturing and operational requirements. Our simulation shows that even with a partial OFT, theoretically, close to inter-channel interference-free performance is possible. In addition, we performed a proof-of-concept experiment of 16×10 Gbaud quadrature phase-shift keying (QPSK) all-optical OFDM detection, with all the bit error rates far below the 7% hard-overhead forward error correction limit.

12 mode, WDM, MIMO-free orbital angular momentum transmission.

Simultaneous MIMO-free transmission of 12 orbital angular momentum (OAM) modes over a 1.2 km air-core fiber is demonstrated. WDM compatibility of the system is shown by using 60, 25 GHz spaced WDM channels with 10 GBaud QPSK signals. System performance is evaluated by measuring bit error rates, which are found to be below the soft FEC limit, and limited by inter-modal crosstalk. The crosstalk in the system is analyzed, and it is concluded that it can be significantly reduced with an improved multiplexer and de-multiplexer.

Signal reshaping and noise suppression using photonic crystal Fano structures.

We experimentally demonstrate the use of photonic crystal Fano resonances for reshaping optical data signals. We show that the combination of an asymmetric Fano resonance and carrier-induced nonlinear effects in a nanocavity can be used to realize a nonlinear power transfer function, which is a key functionality for optical signal regeneration, particularly for suppression of amplitude fluctuations of data signals. The experimental results are explained using simulations based on coupled-mode theory and also compared to the case of using conventional Lorentzian-shaped resonances. Using indium phosphide photonic crystal membrane structures, we demonstrate reshaping of 2 Gbit/s and 10 Gbit/s return-to-zero on-off keying (RZ-OOK) data signals at telecom wavelengths around 1550 nm. Eye diagrams of the reshaped signals show that amplitude noise fluctuations can be significantly suppressed. The reshaped signals are quantitatively analyzed using bit-error ratio (BER) measurements, which show up to 2 dB receiver sensitivity improvement at a BER of 10-9 compared to a degraded input noisy signal. Due to efficient light-matter interaction in the high-quality factor and small mode-volume photonic crystal nanocavity, low energy consumption, down to 104 fJ/bit and 41 fJ/bit for 2 Gbit/s and 10 Gbit/s, respectively, has been achieved. Device perspectives and limitations are discussed.

Orbital angular momentum modes emission from a silicon photonic integrated device for km-scale data-carrying fiber transmission.

We experimentally demonstrate orbital angular momentum (OAM) modes emission from a high emission efficiency OAM emitter for 20-Gbit/s quadrature phase-shift keying (QPSK) carrying data transmission in few-mode fiber (FMF). The device is capable of emitting vector optical vortices carrying well-defined OAM efficiently with the efficiency of the device >37%. Seven modes propagate through a 2-km two-mode and a 3.6-km three-mode FMF with measured optical signal-to-noise ratio (OSNR) penalties less than 4 dB at a bit-error rate (BER) of 2 × 10-3. The demonstrations with favorable performance pave the way to incorporate silicon photonic integrated devices as transceivers in an OAM-enabled optical fiber communication link.

Scalable WDM phase regeneration in a single phase-sensitive amplifier through optical time lenses.

Optical data regeneration is attractive, due to its potential to increase transmission reach and data throughput in communication systems, and several interesting proposals have been made. However, efficient and scalable solutions for regeneration of multiple parallel wavelength channels have been elusive, constituting a key challenge, which must be overcome for optical regeneration to have any prospect of being adapted in actual communication systems. Here we report a scalable wavelength-division multiplexing (WDM) regeneration scheme for phase only regeneration, which satisfies the multichannel requirement, using a set of optical time-lens-based Fourier processors combined with a single phase-sensitive amplifier (PSA). We describe the concept theoretically, and experimentally demonstrate simultaneous regeneration of 16 WDM channels with 50-GHz spacing, each carrying 10-Gbit/s DPSK phase-modulated data. The proposed scheme relies on ultrafast broadband optical processing and is inherently scalable in modulation speed and channel number.

Compact high-efficiency vortex beam emitter based on a silicon photonics micro-ring.

Photonic integrated devices that emit vortex beam carrying orbital angular momentum are becoming key components for multiple applications. Here we propose and demonstrate a high-efficiency vortex beam emitter based on a silicon micro-ring resonator integrated with a metal mirror. Such a compact emitter is capable of generating vortex beams with a high efficiency and small divergence angle. Vector vortex beams of various topological charges are selectively generated by the emitter at different wavelengths with an emission efficiency of up to 37%.

Pulse carving using nanocavity-enhanced nonlinear effects in photonic crystal Fano structures.

We experimentally demonstrate the use of a photonic crystal Fano resonance for carving-out short pulses from long-duration input pulses. This is achieved by exploiting an asymmetric Fano resonance combined with carrier-induced nonlinear effects in a photonic crystal membrane structure. The use of a nanocavity concentrates the input field to a very small volume leading to an efficient nonlinear resonance shift that carves a short pulse out of the input pulse. Here, we demonstrate shortening of ∼500 ps and ∼100 ps long pulses to ∼30 ps and ∼20 ps pulses, respectively. Furthermore, we demonstrate error-free low duty cycle return-to-zero signal generation at 2 Gbit/s with energy consumption down to ∼1 pJ/bit and power penalty of ∼2 dB. The device physics and limitations are analyzed using nonlinear coupled-mode theory.

Space division multiplexing chip-to-chip quantum key distribution.

Quantum cryptography is set to become a key technology for future secure communications. However, to get maximum benefit in communication networks, transmission links will need to be shared among several quantum keys for several independent users. Such links will enable switching in quantum network nodes of the quantum keys to their respective destinations. In this paper we present an experimental demonstration of a photonic integrated silicon chip quantum key distribution protocols based on space division multiplexing (SDM), through multicore fiber technology. Parallel and independent quantum keys are obtained, which are useful in crypto-systems and future quantum network.

Reconfigurable SDM Switching Using Novel Silicon Photonic Integrated Circuit.

Space division multiplexing using multicore fibers is becoming a more and more promising technology. In space-division multiplexing fiber network, the reconfigurable switch is one of the most critical components in network nodes. In this paper we for the first time demonstrate reconfigurable space-division multiplexing switching using silicon photonic integrated circuit, which is fabricated on a novel silicon-on-insulator platform with buried Al mirror. The silicon photonic integrated circuit is composed of a 7 × 7 switch and low loss grating coupler array based multicore fiber couplers. Thanks to the Al mirror, grating couplers with ultra-low coupling loss with optical multicore fibers is achieved. The lowest total insertion loss of the silicon integrated circuit is as low as 4.5 dB, with low crosstalk lower than -30 dB. Excellent performances in terms of low insertion loss and low crosstalk are obtained for the whole C-band. 1 Tb/s/core transmission over a 2-km 7-core fiber and space-division multiplexing switching is demonstrated successfully. Bit error rate performance below 10-9 is obtained for all spatial channels with low power penalty. The proposed design can be easily upgraded to reconfigurable optical add/drop multiplexer capable of switching several multicore fibers.

Two-dimensional distributed-phase-reference protocol for quantum key distribution.

Quantum key distribution (QKD) and quantum communication enable the secure exchange of information between remote parties. Currently, the distributed-phase-reference (DPR) protocols, which are based on weak coherent pulses, are among the most practical solutions for long-range QKD. During the last 10 years, long-distance fiber-based DPR systems have been successfully demonstrated, although fundamental obstacles such as intrinsic channel losses limit their performance. Here, we introduce the first two-dimensional DPR-QKD protocol in which information is encoded in the time and phase of weak coherent pulses. The ability of extracting two bits of information per detection event, enables a higher secret key rate in specific realistic network scenarios. Moreover, despite the use of more dimensions, the proposed protocol remains simple, practical, and fully integrable.

640 Gbit/s return-to-zero to non-return-to-zero format conversion based on optical linear spectral phase filtering.

We propose a novel approach for all-optical return-to-zero (RZ) to non-return-to-zero (NRZ) telecommunication data format conversion based on linear spectral phase manipulation of an RZ data signal. The operation principle is numerically analyzed and experimentally validated through successful format conversion of a 640 Gbit/s coherent RZ signal into the equivalent NRZ time-domain data using a simple phase filter implemented by a commercial optical waveshaper.

Ultrafast all-optical modulation using a photonic-crystal Fano structure with broken symmetry.

We experimentally demonstrate ultrafast all-optical modulation using an ultracompact InP photonic-crystal Fano structure. In contrast to symmetric configurations previously considered, the use of a structure with broken symmetry in combination with a well-engineered Fano resonance is shown to suppress patterning effects as well as lower the energy consumption. These properties enable the achievement of error-free 10 Gbit/s modulation with low pump energy using realistic pseudorandom binary sequence patterns. At 20 Gbit/s, the bit error ratio remains well below the limit for forward error correction.

4 × 160-Gbit/s multi-channel regeneration in a single fiber.

Simultaneous regeneration of four high-speed (160 Gbit/s) wavelength-division multiplexed (WDM) and polarization-division multiplexed (PDM) signals in a single highly nonlinear fiber (HNLF) is demonstrated. The regeneration operation is based on four-wave mixing in HNLF, where the degraded data signals are applied as the pump. As a result, the noise on both '0' and '1' levels can be suppressed simultaneously in our scheme. The stimulated Brillouin scattering (SBS) from the continuous wave (CW) is suppressed by cross-phase modulation (XPM) from the data pump, relieving the requirement of external phase modulation of the CW light. Mitigation of the inter-channel nonlinearities is achieved mainly through an inter-channel 0.5 bit slot time delay. Bidirectional propagation is also applied to relieve the inter-channel four-wave mixing. The multi-channel regeneration performance is validated by bit-error rate (BER) measurements. The receiver powers at the BER of 10(-9) are improved by 1.9 dB, 1.8 dB, 1.6 dB and 1.5 dB for the four data channels, respectively.

Polarization-insensitive wavelength conversion of 40 Gb/s NRZ-DPSK signals in a silicon polarization diversity circuit.

Polarization insensitive wavelength conversion of a 40 Gb/s non-return-to-zero (NRZ) differential phase-shift keying (DPSK) data signal is demonstrated using four-wave mixing (FWM) in a silicon nanowire circuit. Polarization independence is achieved using a diversity circuit based on polarization rotators and splitters, which is fabricated by a simple process on the silicon-on-insulator (SOI) platform. Error-free performance is achieved with only 0.5 dB of power penalty compared to the wavelength conversion of a signal with well optimized input polarization. Additionally, data transmission over 161 km standard single-mode fiber (SSMF) is demonstrated at 40 Gb/s using optical phase conjugation (OPC) in the proposed circuit.

Ultrafast all-optical clock recovery based on phase-only linear optical filtering.

We report on a novel, efficient technique for all-optical clock recovery from RZ-OOK data signals based on spectral phase-only (all-pass) optical filtering. This technique significantly enhances both the recovered optical clock quality and energy efficiency in comparison with conventional amplitude optical filtering approaches using a Fabry-Perot filter. The proposed concept is validated through recovery of the optical clock from a 640 Gbit/s RZ-OOK data signal using a commercial linear optical waveshaper.