Peak shrinking and interpolating for PAPR reduction in M-IFoF-based mobile fronthaul

We apply a Peak Shrinking and Interpolating(PSI)scheme to improve the Peak-to-Average Power Ratio(PAPR)performance in Multiple Intermediate-Frequency-over-Fiber(M-IFoF)based mobile fronthaul.The key idea is to detect the high peaks of the signal and shrink them,and then the shrunk peak values are interpolated into the original signal to reduce the PAPR.We also compare the PSI technique with the previous Tone-Reservation(TR)technique and Phase Pre-Distortion(PPD)technique in terms of PAPR reduction effect and computational complexity.The simulation results indicate that the PSI scheme can reduce the PAPR by more than 4.3 dB at 0.1%CCDF,which outperforms the two previous schemes with lower computational complexity.Furthermore,we find that altering M-IFoF system parameters has little effect on the performance of the PSI technique.

Theoretical and Experimental Investigation on a 40Gb/s FSK transmission Link

A novel Frequency Shift Keying (FSK) transmitter that can operate at 40Gb/s and above is proposed. The transmission characteristics of a FSK signal at 40Gb/s are investigated under varying dispersion management. The resilience of compensation ratio and power level is obtained. We also experimentally demonstrate transmission over 100km SMF and transparent wavelength conversion based on a semiconductor optical amplifier.

Frontier research of ultra-high-speed ultra-large-capacity and ultra-long-haul optical transmission

Ultra-high-speed, ultra-large-capacity and ultra-long-haul （3U） are the forever pursuit of optical communication. As a new mode of optical communication, 3U transmission can greatly promote next generation optical internet and broadband mobile communication network development and technological progress, therefore it has become the focus of international high-tech intellectual property competition ground. This paper introduces the scientific problems, key technologies and important achievements in 3U transmission research.

Polarization rotator-splitter covering full optical communication bands based on L-shaped cross-section waveguide

We propose an ultra-broadband and fabrication-tolerant polarization rotator-splitter （PRS） based on a waveguide with an L-shaped cross section and a Y-junction. The proposed PRS is based on the 220 nm silicon- on-insulator platform, and it shows less than 0.27 dB insertion losses and larger than 14 dB polarization extinction ratios over a wavelength range from 1200 to 1700 nm. To the best of our knowledge, the PRS working in th~ whale nntie^l enmmunien,tinn hand is nrnnased far the first time.

Full-space Cloud of Random Points with a Scrambling Metasurface

With the rapid progress in computer science,including artificial intelligence,big data and cloud computing,full-space spot generation can be pivotal to many practical applications,such as facial recognition,motion detection,augmented reality,etc.These opportunities may be achieved by using diffractive optical elements(DOEs)or light detection and ranging(LIDAR).However,DOEs suffer from intrinsic limitations,such as demanding depth-controlled fabrication techniques,large thicknesses(more than the wavelength),Lambertian operation only in half space,etc.LIDAR nevertheless relies on complex and bulky scanning systems,which hinders the miniaturization of the spot generator.Here,inspired by a Lambertian scatterer,we report a Hermitian-conjugate metasurface scrambling the incident light to a cloud of random points in full space with compressed information density,functioning in both transmission and reflection spaces.Over 4044 random spots are experimentally observed in the entire space,covering angles at nearly 90°.Our scrambling metasurface is made of amorphous silicon with a uniform subwavelength height,a nearly continuous phase coverage,a lightweight,flexible design,and low-heat dissipation.Thus,it may be mass produced by and integrated into existing semiconductor foundry designs.Our work opens important directions for emerging 3D recognition sensors,such as motion sensing,facial recognition,and other applications.

63-Tb/s(368×183.3-Gb/s) C- and L-band all-Raman transmission over 160-km SSMF using PDM-OFDM-16QAM modulation

By using PDM-OFDM-16QAM modulation, all-Raman amplification, coherent detection, and 7% forward error correction （FEC） threshold, we successfully demonstrate 63-Tb/s （368× 183.3-Gb/s） signal over 160- km standard single mode fiber （SSMF） transmission in the C- and L-bands with 25-GHz channel spacing. 368 optical channels with bandwidth spacing of 25 GHz are generated from 16 external cavity laser sources. After 160-km SSMF transmission, all tested bit error rate （BER） are under 3.8×10^-3, which can be recovered by 7% FEC threshold. Within each channel, we achieve the spectral efficiency of 6.85 bit/s/Hz in C/L band.

Experimental demonstration of indoor interfered visible light communication system employing successive interference cancellation

Multipath interference induced power fading occurs when the transmission path lengths from the light emitting diodes to a single receiver are different in a visible light communication system. To solve this problem, we apply a QR-decomposition-based channel equalizer （QR-CE） to achieve successive interference cancellation for a discrete Fourier transform spreading （DFT-S） orthogonal frequency division multiplexing （OFDM） signal. We experimentally demonstrate a 200 Mb/s DFT-S OFDM over a 2 m free-space transmission. The experimental results show that a DFT-S OFDM with QR-CE attains much better bit error rate performance than a DFT-S OFDM with conventional CEs. The impacts of several parameters on a QR-CE are also investigated.

Resource-efficient quantum key distribution with integrated silicon photonics

Integrated photonics provides a promising platform for quantum key distribution(QKD)system in terms of miniaturization,robustness,and scalability.Tremendous QKD works based on integrated photonics have been reported.Nonetheless,most current chip-based QKD implementations require additional off-chip hardware to demodulate quantum states or perform auxiliary tasks such as time synchronization and polarization basis tracking.Here,we report a demonstration of resource-efficient chip-based BB84 QKD with a silicon-based encoder and a decoder.In our scheme,the time synchronization and polarization compensation are implemented relying on the preparation and measurement of the quantum states generated by on-chip devices;thus,we need no additional hardware.The experimental tests show that our scheme is highly stable with a low intrinsic quantum bit error rate of 0.50%±0.02%in a 6 h continuous run.Furthermore,over a commercial fiber channel up to150 km,the system enables the realization of secure key distribution at a rate of 866 bit/s.Our demonstration paves the way for a low-cost,wafer-scale manufactured QKD system.

Silicon-based decoder for polarization-encoding quantum key distribution

Silicon-based polarization-encoding quantum key distribution(QKD)has been extensively studied due to its advantageous characteris-tics of its low cost and robustness.However,given the difficulty of fabricating polarized independent components on the chip,previ-ous studies have only adopted off-chip devices to demodulate the quantum states or perform polarization compensation.In the cur-rent work,a fully chip-based decoder for polarization-encoding QKD was proposed.The chip realized a polarization state analyzer and compensated for the BB84 protocol without the requirement of additional hardware,which was based on a polarization-to-path conversion method utilizing a polarization splitter-rotator.The chip was fabricated adopting a standard silicon photonics foundry,which was of a compact design and suitable for mass production.In the experimental stability test,an average quantum bit error rate of 0.59%was achieved through continuous operation for 10 h with-out any polarization feedback.Furthermore,the chip enabled the automatic compensation of the fiber polarization drift when utiliz-ing the developed feedback algorithm,which was emulated by a ran-dom fiber polarization scrambler.Moreover,a finite-key secret rate of 240 bps over a fiber spool of 100 km was achieved in the case of the QKD demonstration.This study marks an important step to-ward the integrated,practical,and large-scale deployment of QKD systems.

Single-celled metasurface for labeled vortex beam generator

Vortex light is a unique beam characterized by a spiral phase as it propagates. A fundamental parameter of vortex light is the topological charge, which determines the amount of angular momentum and plays a crucial role in tailoring its behavior. However, conventional measurement methods for determining the topological charge, such as those based on interference and phase modulation, tend to be intricate and complex. In this regard, a labeled vortex beam generator is proposed, composed of a metasurface with a single-celled configuration. When the metasurface is illuminated by light of the designed wavelength, the outgoing light exhibits a vortex structure. Furthermore, the topological charge numbers can be directly observed with distinct labeled patterns when the metasurface is placed in an orthogonal-polarized optical path. With advantages such as ultra-compactness, high robustness, and exceptional precision, the proposed metasurface exhibits significant potential for applications in optical communication, light manipulation, optical sensing, etc.

Zero-order-free meta-holograms in a broadband visible range

The unwanted zero-order light accompanied by the birth of diffractive optical elements and caused mainly by fabrication errors and wavelength variations is a key factor that deteriorates the performance of diffraction-related optical devices such as holograms,gratings,beam shapers,beam splitters,optical diffusers,and diffractive microlenses.Here,inspired by the unique characteristic of nano-polarizer-based metasurfaces for both positive and negative amplitude modulation of incident light,we propose a general design paradigm to eliminate zero-order diffraction without burdening the metasurface design and fabrication.The experimentally demonstrated metahologram,which projects a holographic image with a wide angle of 70°×70°in the for field,presents a very low zero-order intensity(only 0.7%of the total energy of the reconstructed image).More importantly,the zero-orderfree meta-hologram has a large tolerance limit for wavelength variations(under a broadband illumination from520 to 660 nm),which brings important technical advances.The strategy proposed could significantly relieve the fabrication difficulty of metasurfaces and be viable for various diffractive-optics-related applications includingholography,laser beam shaping,optical data storage,vortex beam generation,and so on.

Si-substrate LEDs with multiple superlattice interlayers for beyond 24 Gbps visible light communication

High-speed visible light communication(VLC)using light-emitting diodes(LEDs)is a potential complementary technology for beyond-5 G wireless communication networks.The speed of VLC systems significantly depends on the quality of LEDs,and thus various novel LEDs with enhanced VLC performance increasingly emerge.Among them,In Ga N/Ga N-based LEDs on a Si-substrate are a promising LED transmitter that has enabled VLC data rates beyond 10 Gbps.The optimization on the period number of superlattice interlayer(SL),which is a stressrelief epitaxial layer in a Si-substrate LED,has been demonstrated to be an effective method to improve Si-substrate LED’s luminescence properties.However,this method to improve LED’s VLC properties is barely investigated.Hence,we for the first time experimentally studied the impact of SL period number on VLC performance.Accordingly,we designed and fabricated an integrated 4×4 multichromatic Si-substrate wavelength-divisionmultiplexing LED array chip with optimal SL period number.This chip allows up to 24.25 Gbps/1.2 m VLC transmission using eight wavelengths,which is the highest VLC data rate for an In Ga N/Ga N LED-based VLC system to the best of our knowledge.Additionally,a record-breaking data rate of 2.02 Gbps over a 20-m VLC link is achieved using a blue Si-substrate LED with the optimal SL period number.These results validate the effectiveness of Si-substrate LEDs for both high-speed and long-distance VLC and pave the way for Si-substrate LED design specially for high-speed VLC applications.

10 Gb/s free space optical interconnect with broadcasting capability enabled by a silicon integrated optical phased array

We have proposed and experimentally demonstrated a reconfigurable free space optical interconnect with broadcasting capability based on an eight-channel silicon integrated optical phased array.By using the silicon integrated beam steering and broadcasting device,10 Gb/s on-off keying data is transmitted over 15 cm in free space for up to three receivers located in three different cards.The experimental results show that the optical phased array can be used with broadcasting capability provided to multi-receivers in the card to card optical interconnects,which can significantly reduce device size,system complexity,and total costs.

Wideband adaptive microwave frequency identification using an integrated silicon photonic scanning filter

Photonic-assisted microwave frequency identification with distinct features, including wide frequency coverage and fast tunability, has been conceived as a key technique for applications such as cognitive radio and dynamic spectrum access. The implementations based on compact integrated photonic chips have exhibited distinct advantages in footprint miniaturization, light weight, and low power consumption, in stark contrast with discrete optical-fiber-based realization. However, reported chip-based instantaneous frequency measurements can only operate at a single-tone input, which stringently limits their practical applications that require wideband identification capability in modern RF and microwave applications. In this article, we demonstrate, for the first time, a wideband, adaptive microwave frequency identification solution based on a silicon photonic integrated chip,enabling the identification of different types of microwave signals from 1 to 30 GHz, including single-frequency,multiple-frequency, chirped-frequency, and frequency-hopping microwave signals, and even their combinations.The key component is a high Q-factor scanning filter based on a silicon microring resonator, which is used to implement frequency-to-time mapping. This demonstration opens the door to a monolithic silicon platform that makes possible a wideband, adaptive, and high-speed signal identification subsystem with a high resolution and a low size, weight, and power(SWaP) for mobile and avionic applications.

High-speed and high-power germanium photodetector with a lateral silicon nitride waveguide

Up to now, the light coupling schemes of germanium-on-silicon photodetectors(Ge-on-Si PDs) could be divided into three main categories:(1) vertical(or normal-incidence) illumination, which can be from the top or back of the wafer/chip, and waveguide-integrated coupling including(2) butt coupling and(3) evanescent coupling. In evanescent coupling the input waveguide can be positioned on top, at the bottom, or lateral to the absorber. Here,to the best of our knowledge, we propose the first concept of Ge-on-Si PD with double lateral silicon nitride(Si_(3)N_(4)) waveguides, which can serve as a novel waveguide-integrated coupling configuration: double lateral coupling. The Ge-on-Si PD with double lateral Si_(3)N_(4) waveguides features uniform optical field distribution in the Ge region, which is very beneficial to improving the operation speed for high input power. The proposed Ge-on-Si PD is comprehensively characterized by static and dynamic measurements. The typical internal responsivity is evaluated to be 0.52 A/W at an input power of 25 mW. The equivalent circuit model and theoretical 3 dB optoelectrical(OE) bandwidth investigation of Ge-on-Si PD with lateral coupling are implemented. Based on the small-signal(S21) radio-frequency measurements, under 4 mA photocurrent, a 60 GHz bandwidth operating at-3 V bias voltage is demonstrated. When the photocurrent is up to 12 mA, the 3 dB OE bandwidth still has 36 GHz. With 1 mA photocurrent, the 70, 80, 90, and 100 Gbit/s non-return-to-zero(NRZ) and 100,120, 140, and 150 Gbit/s four-level pulse amplitude modulation clear openings of eye diagrams are experimentally obtained without utilizing any offline digital signal processing at the receiver side. In order to verify the highpower handling performance in high-speed data transmission, we investigate the eye diagram variations with the increase of photocurrents. The clear open electrical eye diagrams of 60 Gbit/s NRZ under 20 mA photocurrent are also obtained. Overall, the proposed lateral Si_(3)N_(4) waveguide structure is flexibly extendable to a light coupling configuration of PDs, which makes it very attractive for developing high-performance silicon photonic integrated circuits in the future.

Silicon photonics for high-capacity data communications

In recent years,optical modulators,photodetectors,(de)multiplexers,and heterogeneously integrated lasers based on silicon optical platforms have been verified.The performance of some devices even surpasses the traditional III-V and photonic integrated circuit(PIC)platforms,laying the foundation for large-scale photonic integration.Silicon photonic technology can overcome the limitations of traditional transceiver technology in high-speed transmission networks to support faster interconnection between data centers.In this article,we will review recent progress for silicon PICs.The first part gives an overview of recent achievements in silicon PICs.The second part introduces the silicon photonic building blocks,including low-loss waveguides,passive devices,modulators,photodetectors,heterogeneously integrated lasers,and so on.In the third part,the recent progress on high-capacity silicon photonic transceivers is discussed.In the fourth part,we give a review of high-capacity silicon photonic networks on chip.

240 Gb/s optical transmission based on an ultrafast silicon microring modulator

An ultrafast microring modulator(MRM) is fabricated and presented with V_(π)· L of 0.825 V · cm. A 240 Gb/s PAM-8 signal transmission over 2 km standard single-mode fiber(SSMF) is experimentally demonstrated. PN junction doping concentration is optimized, and the overall performance of the MRM is improved. Optical peaking is introduced to further extend the EO bandwidth from 52 to 110 GHz by detuning the input wavelength. A titanium nitride heater with 0.1 nm/m W tuning efficiency is implemented above the MRM to adjust the resonant wavelength. High bit rate modulations based on the high-performance and compact MRM are carried out. By adopting off-line signal processing in the transmitter and receiver side, 120 Gb/s NRZ, 220 Gb/s PAM-4, and240 Gb/s PAM-8 are measured with the back-to-back bit error ratio(BER) of 5.5 × 10^(-4), 1.5 × 10^(-2), and 1.4 × 10^(-2), respectively. A BER with different received optical power and 2 km SSMF transmission is also investigated. The BER for 220 Gb/s PAM-4 and 240 Gb/s PAM-8 after 2 km SSMF transmission is calculated to be 1.7 × 10^(-2), and 1.5 × 10^(-2), which meet with the threshold of soft-decision forward-error correction,respectively.

Fault monitoring in passive optical network using code division multiplex-based dual-OTDR traces comparison

A dual optical time domain reflectometry （OTDR） system, which employs two different continuous waves at the optical line terminal and a pair of fiber Bragg gratings at the end of each optical network unit, is proposed in a time-division multiplexing passive optical network （PON）. The proposed scheme accomplishes the fiber fault monitoring by comparing the different wavelength＇s testing curves. Complete complementary code is utilized to measure multiple wavelength signals simultaneously with only one receiver and to improve the dynamic range of this system. The PON system consisting of 20 km feeding fiber and a 1：16 splitter is investigated by the experiments. The experimental results show that the faulty branch can be successfully identified by using our scheme. What is more, we also demonstrate that our scheme can be applied to the multi-stage PON.

Recent progress in an ‘ultra-high speed, ultra-large capacity, ultra-long distance' optical transmission system(Invited Paper)

From 2010 to 2015, the Wuhan Institute of Posts and Telecommunications（WRI） had undertaken the national key basic research development program project ‘ultra-high speed, ultra-large capacity, ultra-long distance（3U）optical transmission based research＇ as the leading agency. Under the support of the project, we have obtained a series of achievements in scientific research and engineering. Some of the results have been widely used in commercial systems. This Review will make a preliminary summary of the achievements during the past 5 years.