Showing papers on "Wavelength-division multiplexing published in 2018"

10-Channel Mode (de)multiplexer with Dual Polarizations

Abstract: A dual-polarization 10-channel mode (de)multiplexer is proposed and realized with cascaded dual-core adiabatic tapers on a silicon-on-insulator (SOI) platform. The mode demultiplexer has a 2.3 μm-wide multimode bus waveguide, which supports six mode-channels of TE polarization and four mode-channels of TM polarization. These ten mode-channels are (de)multiplexed with five cascaded dual-core adiabatic tapers based on SOI nanowires. The widths for these dual-cores are chosen optimally according to the dispersion curves of the dual-core SOI nanowire, so that the desired highest-order modes of TE- and TM-polarizations are extracted simultaneously. These two extracted mode-channels are coupled very efficiently to the fundamental modes of TE- and TM-polarizations (TE0 and TM0) in the narrow waveguide, respectively, which are then separated by using a polarization beam splitter based on bent directional couplers. A chip consisting of a pair of 10-channel mode (de)multiplexers is fabricated and then tested with data transmission of 30Gbps/channel. The measurement results show that all TM- and TE mode-channels have low crosstalks (–15∼–25 dB) and low excess losses (0.2∼1.8 dB) over a broad wavelength band of ∼90 nm, which makes it WDM (wavelength-division-multiplexing)-compatible and thus suitable for high capacity on-chip optical interconnects.

Inverse design and demonstration of a compact on-chip narrowband three-channel wavelength demultiplexer

Abstract: In wavelength division multiplexing schemes, splitters must be used to combine and separate different wavelengths. Conventional splitters are fairly large with footprints in hundreds to thousands of square microns, and experimentally demonstrated multimode-interference-based and inverse-designed ultracompact splitters operate with only two channels and large channel spacing (>100 nm). Here we inverse design and experimentally demonstrate a three-channel wavelength demultiplexer with 40 nm spacing (1500, 1540, and 1580 nm) with a footprint of 24.75 μm2. The splitter has a simulated peak insertion loss of −1.55 dB with under −15 dB crosstalk and a measured peak insertion loss of −2.29 dB with under −10.7 dB crosstalk.

Ultra-broadband on-chip twisted light emitter for optical communications.

Abstract: On-chip twisted light emitters are essential components of orbital angular momentum (OAM) communication devices1, 2. These devices address the growing demand for high-capacity communication systems by providing an additional degree of freedom for wavelength/frequency division multiplexing (WDM/FDM). Although whispering-gallery-mode-enabled OAM emitters have been shown to possess some advantages3, 4, 5, such as compactness and phase accuracy, their inherent narrow bandwidths prevent them from being compatible with WDM/FDM techniques. Here, we demonstrate an ultra-broadband multiplexed OAM emitter that utilizes a novel joint path-resonance phase control concept. The emitter has a micron-sized radius and nanometer-sized features. Coaxial OAM beams are emitted across the entire telecommunication band from 1,450 to 1,650 nm. We applied the emitter to an OAM communication with a data rate of 1.2 Tbit/s assisted by 30-channel optical frequency combs (OFCs). The emitter provides a new solution to further increase capacity in the OFC communication scenario.

Octave-spanning coherent supercontinuum generation in silicon on insulator from 1.06 μm to beyond 2.4 μm

Abstract: Efficient complementary metal-oxide semiconductor-based nonlinear optical devices in the near-infrared are in strong demand. Due to two-photon absorption in silicon, however, much nonlinear research is shifting towards unconventional photonics platforms. In this work, we demonstrate the generation of an octave-spanning coherent supercontinuum in a silicon waveguide covering the spectral region from the near- to shortwave-infrared. With input pulses of 18 pJ in energy, the generated signal spans the wavelength range from the edge of the silicon transmission window, approximately 1.06 to beyond 2.4 μm, with a −20 dB bandwidth covering 1.124–2.4 μm. An octave-spanning supercontinuum was also observed at the energy levels as low as 4 pJ (−35 dB bandwidth). We also measured the coherence over an octave, obtaining , in good agreement with the simulations. In addition, we demonstrate optimization of the third-order dispersion of the waveguide to strengthen the dispersive wave and discuss the advantage of having a soliton at the long wavelength edge of an octave-spanning signal for nonlinear applications. This research paves the way for applications, such as chip-scale precision spectroscopy, optical coherence tomography, optical frequency metrology, frequency synthesis and wide-band wavelength division multiplexing in the telecom window. A silicon-based source that generates a wide spectrum of light, spanning the near-infrared transparency window of silicon, has been made. Supercontinuum generation involves using short, high-power pulses to generate broad continuous spectra by propagating them through nonlinear media. Supercontinuum sources are needed for applications in spectroscopy and optical coherence tomography. Silicon is an attractive medium since it is compatible with standard semiconductor fabrication processes but it suffers from losses due to nonlinear processes such as two-photon absorption. Now, Neetesh Singh of Massachusetts Institute of Technology in the USA and co-workers have realized a fully coherent supercontinuum generation in a silicon waveguide over a full octave that spans the near to shortwave infrared window. The researchers envision their source being used in applications such as chip-scale precision spectroscopy, optical frequency metrology and optical communications.

18 km low-crosstalk OAM + WDM transmission with 224 individual channels enabled by a ring-core fiber with large high-order mode group separation

Abstract: The space domain is regarded as the only known physical dimension of lightwaves left to be exploited for optical communications. Very recently, much research effort has been devoted to using orbital angular momentum (OAM) spatial modes to increase the transmission capacity in fiber-optic communications. However, long-distance low-crosstalk high-order OAM multiplexing transmission in fiber is quite challenging. Here we design and fabricate a graded-index ring-core fiber to effectively suppress radially high-order modes and greatly separate high-order OAM mode groups. By exploiting high-order OAM mode group multiplexing, together with wavelength-division multiplexing (WDM), i.e., 12.5 Gbaud 8-array quadrature amplitude modulation (8-QAM) signals over OAM+4 and OAM+5 modes on 112 WDM channels (224 individual channels), we experimentally demonstrate 8.4 Tbit/s data transmission in an 18 km OAM fiber with low crosstalk. Multiple-input multiple-output digital signal processing is not required in the experiment because of the large high-order mode group separation of the OAM fiber. The demonstrations may open a door to find more fiber-optic communication and interconnect applications exploiting high-order OAM modes.

10.16-Peta-B/s Dense SDM/WDM Transmission Over 6-Mode 19-Core Fiber Across the C+L Band

Abstract: Space-division multiplexing (SDM) is an attractive technique for dramatically enhancing the transmission capacity in a single optical fiber. Recently, ultradense SDM transmission experiments with a spatial multiplicity of over 100 have been reported by using few mode multicore fibers (FM-MCFs). Considering the maximum capacity of around 100 Tb/s reported in single-mode single-core fiber transmission experiments, the capacity in FM-MCF transmission with more than 100 spatial channels is expected to reach 10 peta-b/s; however, the maximum capacity has been limited to 2 peta-b/s. In this paper, we demonstrate ultradense SDM transmission of 739 WDM 12-Gbd dual polarization—64-quadrature amplitude modulation (QAM)/16-QAM signals over 11.3-km 6-mode 19-core fiber using the C+L band, achieving a record fiber capacity of 10.16 peta-b/s with an aggregate spectral efficiency of 1099.9 b/s/Hz.

DSP-free `coherent-lite' transceiver for next generation single wavelength optical intra-datacenter interconnects

Abstract: We propose a DSP-free coherent-lite system that requires neither high-speed DSP nor high-resolution signal converters for deployment inside datacenters over single mode fiber links with reaches of 10 km and less. The removal of converters and DSP, in which some subsystems are fundamental for successful coherent detection, is enabled by either replacing DSP subsystems with optics having equivalent functions or by re-engineering the system. We validate in a proof-of-concept experiment the proposed DSP-free system using 50 Gbaud DP-16QAM delivering 400 Gb/s over 10 km of single mode fiber (SMF) below the KP4 forward error correction (FEC) threshold of 2.2 × 10-4. In addition, we perform a detailed experimental parametric study of the coherent-lite system in which various system parameters are swept such as baud rate, reach, laser power and laser linewidth. Our results verify that the coherent-lite system can be realized using low-cost DFB lasers with linewidths of a few hundred kHz. Moreover, we compare the performance of the coherent-lite system with that of a conventional coherent transceiver leveraging the full DSP stack. Then, we evaluate the power consumption savings achieved by the coherent-lite scheme relative to a classic DSP-based coherent system. Assuming a CMOS node ranging from 28 to 7 nm for DSP implementation, our estimate shows that the coherent-lite scheme can save 95 to 78% of the power consumed by the following subsystems: analog-to-digital converters, chromatic dispersion compensation, 2 × 2 MIMO polarization demultiplexing and carrier recovery. Finally, we compare the power consumption of the coherent-lite scheme with more standard 400G IM-DD systems utilizing either eight or four parallel WDM lanes (8 × 50G and 4 × 100G). The coherent-lite system is found to have similar module power consumption requirements as a corresponding 4 × 100G IM-DD system while bringing the benefits of coherent detection including improved sensitivity and higher spectral efficiency leading to fewer light sources per transceiver module. To the best of our knowledge, this work represents the first experimental demonstration of a DSP-free coherent-lite system for single channel 400G datacenter 10 km interconnects, a potential attractive solution due to its scalability to future 800G and 1.6T intra-datacenter optical interconnects.

Gigahertz speed operation of epsilon-near-zero silicon photonic modulators

Abstract: Optical communication systems increasingly require electro-optical modulators that deliver high modulation speeds across a large optical bandwidth with a small device footprint and a CMOS-compatible fabrication process Although silicon photonic modulators based on transparent conducting oxides (TCOs) have shown promise for delivering on these requirements, modulation speeds to date have been limited Here, we describe the design, fabrication, and performance of a fast, compact electroabsorption modulator based on TCOs The modulator works by using bias voltage to increase the carrier density in the conducting oxide, which changes the permittivity and hence optical attenuation by almost 10 dB Under bias, light is tightly confined to the conducting oxide layer through nonresonant epsilon-near-zero (ENZ) effects, which enable modulation over a broad range of wavelengths in the telecommunications band Our approach features simple integration with passive silicon waveguides, the use of stable inorganic materials, and the ability to modulate both transverse electric and magnetic polarizations with the same device design Using a 4-μm-long modulator and a drive voltage of 2 Vpp, we demonstrate digital modulation at rates of 25 Gb/s We report broadband operation with a 65 dB extinction ratio across the 1530–1590 nm band and a 10 dB insertion loss This work verifies that high-speed ENZ devices can be created using conducting oxide materials and paves the way for additional technology development that could have a broad impact on future optical communications systems

1.72-Tb/s Virtual-Carrier-Assisted Direct-Detection Transmission Over 200 km

Abstract: Supporting the ever-increasing data-center-inter-connect traffic in a cost-effective manner is a great challenge, which requires innovative transmission and digital signal processing (DSP) techniques. Recently, single-side-band (SSB) direct-detection (DD) transmissions have been actively considered for data rates beyond 100 Gb/s per channel and distance of hundreds of kilometers due to its capability of electronic chromatic dispersion compensation. In addition, several effective DSP techniques to mitigate or suppress the signal-signal beating interference (SSBI) due to the squared-law detection of the photodiode have been intensively investigated, such as Kramers–Knonig (KK) and SSBI cancellation schemes, showing promising performance at data rates over 200 Gb/s and distance beyond 100 km. In this paper, we demonstrate that high-performance low-complexity SSB DD transmissions can be achieved by generating a digital carrier (virtual carrier) together with the complex information-bearing signal at the transmitter using only two digital-to-analog converters. Combining this transmission technique with either the KK field reconstruction or a two-stage SSBI cancellation scheme at the receiver, eight-channel WDM signals with a net data rate of 1.72 Tb/s have been transmitted successfully over a record span length of 200 km at 1550 nm.

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

Abstract: 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.

Trans-Atlantic Field Trial Using High Spectral Efficiency Probabilistically Shaped 64-QAM and Single-Carrier Real-Time 250-Gb/s 16-QAM

Abstract: We report the transmission of probabilistically shaped (PS) 64-ary quadrature amplitude modulation (QAM) at 7.46 b/s/Hz over a 5523-km in-service trans-Atlantic fiber-optic cable that consists of 65–89-km spans of Erbium-doped fiber amplifier only amplified fiber. Using a looped-back system configuration, we achieve 5.68 b/s/Hz over a trans-Pacific-equivalent distance of 11 046 km. Net spectral efficiencies are increased by 18% and 80% by using PS, at 5523 km and 11 046 km, respectively, compared to uniform square QAM. Throughout our experiments, we pay particular attention that our claims are backed by implementable forward error correction schemes. In addition, we demonstrate real-time coherent transmission of single-carrier 200 and 250-Gb/s uniform 8-QAM and 16-QAM at 4 b/s/Hz over the 5523-km cable.

Toward the Integration of CV Quantum Key Distribution in Deployed Optical Networks

Abstract: In this letter, we report on the advances toward the integration of our developed continuous variables (CV) quantum key distribution (QKD) system in existing optical infrastructure and wavelength division multiplexed (WDM) networks. First, we investigate the tolerance of the CV-QKD system to spontaneous Raman scattering (SRS) noise, the latter being the most dominant impairment in a WDM co-existence environment for QKD. In particular, we investigate by experiment the impact of a WDM 10 × 10 Gb/s ON-OFF-keying signal in terms of induced SRS noise in the QKD channel. The spontaneous SRS noise influence is assessed for different transmission scenarios, i.e., for various optical launch powers of the WDM signal, and for different transmission links of 20, 40, 60, and 80 km. Based on the experimental data and on the measured system's parameters, we estimate the key rates and reach capabilities of the proposed CV-QKD system. The scheme supports a key rate of 90 kbit/s over 20 km, for an ideal QKD system multiplexed with 2-mW optical power. A step further, we demonstrate for first time the use of the aforementioned CV-QKD system to encrypt a 10GE client service over deployable optical transport network legacy equipment over 20 km. Our results prove the feasibility of the integration of our proposed scheme with legacy telecom equipment in existing WDM optical networks.

Resource Allocation in Optical Networks Secured by Quantum Key Distribution

Abstract: Optical network security is attracting increasing research attention, as loss of confidentiality of data transferred through an optical network could impact a huge number of users and services. Data encryption is an effective way to enhance optical network security. In particular, QKD is being investigated as a secure mechanism to provide keys for data encryption at the endpoints of an optical network. In a QKD-enabled optical network, apart from TDChs, two additional channels, called QSChs and PIChs, are required to support secure key synchronization. How to allocate network resources to QSChs, PIChs, and TDChs is emerging as a novel problem for the design of a security-guaranteed optical network. This article addresses the resource allocation problem in optical networks secured by QKD. We first discuss a possible architecture for a QKD-enabled optical network, where an SDN controller is in charge of allocating the three types of channels (TDCh, QSCh, and PICh) over different wavelengths exploiting WDM. To save wavelength resources, we propose to adopt OTDM to allocate multiple QSChs and PIChs over the same wavelength. An RWTA algorithm is designed to allocate wavelength and time slot resources for the three types of channels. Different security levels are included in the RWTA algorithm by considering different key updating periods (i.e., the period after which the secure key between two endpoints has to be updated). Illustrative simulation results show the effects of different security-level configuration schemes on resource allocation.

Space-division multiplexing in data center networks: on multi-core fiber solutions and crosstalk-suppressed resource allocation

Abstract: The rapid growth of traffic inside data centers caused by the increasing adoption of cloud services necessitates a scalable and cost-efficient networking infrastructure. Space-division multiplexing (SDM) is considered as a promising solution to overcome the optical network capacity crunch and support cost-effective network capacity scaling. Multi-core fiber (MCF) is regarded as the most feasible and efficient way to realize SDM networks, and its deployment inside data centers seems very likely as the issue of inter-core crosstalk (XT) is not severe over short link spans (<1 km) compared to that in long-haul transmission. However, XT can still have a considerable effect in MCF over short distances,which can limit the transmission reach and in turn the data center’s size. XT can be further reduced by bi-directional transmission of optical signals in adjacent MCF cores. This paper evaluates the benefits of MCF-based SDM solutions in terms of maximizing the capacity and spatial efficiency of data center networks. To this end, we present an analytical model for XT in bi-directional normal step-index and trench-assisted MCFs and propose corresponding XT-aware core prioritization schemes. We further develop XT-aware spectrum resource allocation strategies aimed at relieving the complexity of online XT computation. These strategies divide the available spectrum into disjoint bands and incrementally add them to the pool of accessible resources based on the network conditions. Several combinations of core mapping and spectrum resource allocation algorithms are investigated for eight types of homogeneous MCFs comprising 7–61 cores, three different multiplexing schemes, and three data center network topologies with two traffic scenarios. Extensive simulation results showthat combining bi-directional transmission in dense core fibers with tailored resource allocation schemes significantly increases the network capacity. Moreover, a multiplexing scheme that combines SDM and WDM can achieve up to 33 times higher link spatial efficiency and up to 300 times greater capacity compared to a WDM solution.

100 Gb/s to 1 Tb/s Based Coherent Passive Optical Network Technology

Abstract: Following on from current 1 Gb/s to 10 Gb/s based passive optical networks (PON), IEEE 802.3ca has commenced discussion of the first 100 Gb/s-based PON standard in the form of 100 G Ethernet PON (100G-EPON), in anticipation of growing bandwidth demand by emerging applications such as fixed-mobile convergence for 5G and beyond 5G (B5G) where mobile front-haul and mobile back-haul (MFH/MBF) will require greatly increased capacity of up to 1 Tb/s. In the move toward these 100 Gb/s to 1 Tb/s class PONs, research into the latest coherent technology based PON systems is being pursued due to its high receiver sensitivity at much higher bit rates, superior to current direct-detection based PON systems, aided by the rapid progress of the latest small-scale digital signal processing (DSP) suitable for access spans of less than a few tens of km. In this paper, we review the recent progress of such 100 Gb/s to 1 Tb/s class coherent PON technology. As the key successful factor for realizing 100 Gb/s to 1 Tb/s based coherent PON systems, the latest trends in coherent DSP and the accompanying embedded coherent transceivers are summarized from the point of view of miniaturization. In addition, highlighting the strong progress with the upcoming coherent PON systems, the first demonstration of a 100 Gb/s/λ × 8 wavelengths (800 Gb/s) based real-time wavelength division multiplexing (WDM)-PON system using a simplified DSP suitable for access spans is presented. A 100 Gb/s/λ based time division multiplexing (TDM)-PON system is also introduced, addressing the burst-mode coherent reception technique which will be the main issue for achieving practical TDM-based coherent PON systems.

Frequency Comb-Based WDM Transmission Systems Enabling Joint Signal Processing

Abstract: We review the use of optical frequency combs in wavelength-division multiplexed (WDM) fiber optic communication systems. In particular, we focus on the unique possibilities that are opened up by the stability of the comb-line spacing and the phase coherence between the lines. We give an overview of different techniques for the generation of optical frequency combs and review their use in WDM systems. We discuss the benefits of the stable line spacing of frequency combs for creating densely-packed optical superchannels with high spectral efficiency. Additionally, we discuss practical considerations when implementing frequency-comb-based transmitters. Furthermore, we describe several techniques for comb-based superchannel receivers that enables the phase coherence between the lines to be used to simplify or increase the performance of the digital carrier recovery. The first set of receiver techniques is based on comb-regeneration from optical pilot tones, enabling low-overhead self-homodyne detection. The second set of techniques takes advantage of the phase coherence by sharing phase information between the channels through joint digital signal processing (DSP) schemes. This enables a lower DSP complexity or a higher phase-noise tolerance.

Silicon photonics-based 100 Gbit/s, PAM4, DWDM data center interconnects

Abstract: In this paper we discuss the nature of and requirements for data center interconnects. We then demonstrate a switch-pluggable, 4.5 W, 100 Gbit/s, siliconphotonics- based, PAM4, QSFP-28 module to transport Ethernet data directly over DWDM for layer 2/3 connection between switches at data centers up to 120 km apart, thereby eliminating the need for a separate optical transport layer. The module, based on the direct detect modulation format, is of much reduced complexity, power, and cost compared to the coherent systems that are currently being deployed for this application.

Integrated (de)multiplexer for orbital angular momentum fiber communication

Abstract: The quickly increasing data transfer load requires an urgent revolution in current optical communication. Orbital angular momentum (OAM) multiplexing is a potential candidate with its ability to considerably enhance the capacity of communication. However, the lack of a compact, efficient, and integrated OAM (de)multiplexer prevents it from being widely applied. By attaching vortex gratings onto the facets of a few-mode fiber, we demonstrate an integrated fiber-based OAM (de)multiplexer. A vortex grating fabricated on the fiber facet enables the direct multiplexing of OAM states at one port and the demultiplexing of OAM states at the other port. The measured bit error rate of the carrier signal after propagating through a 5-km few-mode fiber confirms the validity and effectiveness of the proposed approach. The scheme offers advantages in future high-capacity OAM communication based on optical fiber.

Ultra-broadband multimode 3dB optical power splitter using an adiabatic coupler and a Y-branch.

Abstract: As an essential component of mode division multiplexing (MDM) system, a multimode 3dB power splitter with low loss, high power balance, and low mode crosstalk is highly desired. In this paper, we propose an ultra-broadband on-chip multimode 3dB optical power splitter using an adiabatic coupler and an S-bend based Y-branch. As an example, a splitter for the four-lowest modes of a rib waveguide on silicon on insulator (SOI) platform is designed. Simulation results show that the device exhibits < 0.12dB insertion losses, within ± 0.38dB power imbalances, and < -18.5dB mode crosstalks for the four-lowest modes within a large operating wavelength range of 165 nm (from 1400 nm to 1565 nm). The fabrication tolerance of gap size at the output end of the adiabatic coupler is also analyzed.

Hybrid Photonic-Plasmonic Nonblocking Broadband 5 × 5 Router for Optical Networks

Abstract: Photonic data routing in optical networks is expected to overcome the limitations of electronic routers with respect to data rate, latency, and energy consumption. However, photonics-based routers suffer from dynamic power consumption, and nonsimultaneous usage of multiple wavelength channels when microrings are deployed and are sizable in footprint. Here, we show a design for the first hybrid photonic-plasmonic, nonblocking, broadband $5\times 5$ router based on 3-waveguide silicon photonic-plasmonic $2\times 2$ switches. The compactness of the router (footprint $ ) results in a short optical propagation delay (0.4 ps) enabling high data capacity up to 2 Tb/s. The router has an average energy consumption ranging from 0.1 to 1.0 fJ/bit depending on either DWDM or CDWM operation, enabled by the low electrical capacitance of the switch. The total average routing insertion loss of 2.5 dB is supported via an optical mode hybridization deployed inside the $2\times 2$ switches, which minimizes the coupling losses between the photonic and plasmonic sections of the router. The router's spectral bandwidth resides in the S, C, and L bands and exceeds 100 nm supporting wavelength division multiplexing applications since no resonance feature is required. Taken together this novel optical router combines multiple design features, all required in next-generation high data-throughput optical networks and computing systems.

High-order mode direct oscillation of few-mode fiber laser for high-quality cylindrical vector beams.

Abstract: Generation of high-order modes with high quality is important for the application of cylindrical vector beams in fibers. We experimentally demonstrated high-order LP11 mode generation and amplification with a broad bandwidth in an all few-mode fiber laser. A wavelength-division-multiplexing (WDM) mode selective coupler (MSC) is proposed to achieve efficient mode conversion from LP01 mode to LP11 mode, but also combine high-order LP11 modes at the wavelengths of 980/1550 nm. To the best of our knowledge, this is the first report on the high-order mode oscillation in an all few-mode fiber laser. LP11 mode and cylindrical vector beams including radially and azimuthally polarized beams are obtained with high modal purity. The purity of the generated high-order modes are all in excess of 95%.

Multiterabit Transmission Over OM2 Multimode Fiber With Wavelength and Mode Group Multiplexing and Direct Detection

Abstract: In this paper, 5 Tb/s bidirectional transmission (2.5 Tb/s in each direction) over 2.2 km of OM2 fiber is demonstrated using selective excitation of four mode groups, wavelength division multiplexing, and direct detection. Twenty wavelengths per mode group are used, each wavelength is modulated using discrete multitone scheme with 1024 subcarriers with external lithium niobate Mach–Zehnder modulator (MZM). A minimum bit rate of 68.8 Gb/s per channel is obtained for a bit error rate of 3.8 × 10−2 using a 65 Gs/s digital to analog converter (DAC). Same bit rate is achieved over 4.4 km using 88 Gs/s DAC. Electroabsorption modulated laser (EML) with a 3 dB bandwidth of 18 GHz is also used in the same mode group division transmission. 1 dB power penalty is measured with the EML compared to MZM. A minimum of channel bit rate of 68.5 Gb/s over 2.2 km of OM2 fiber with the EML and 88 Gs/s DAC is reported. Therefore, we demonstrate the transmission of the same throughput over multimode fiber using a low-cost EML instead of an external MZM.

Comparison of Laguerre-Gaussian and Donut modes for MDM-WDM in OFDM-Ro-FSO transmission system

Abstract: Radio-over-free-space-optics (Ro-FSO) technology may pave the way towards a ubiquitous platform for seamless integration of radio and optical networks without expensive optical fiber cabling. In this paper, to increase the capacity of Ro-FSO, mode division multiplexing (MDM) of two modes has been capitalized in a three-channel WDM system spaced by 1 nm over a FSO link of 80 km, resulting in a 120 Gbps six-channel Ro-FSO system. The SNR and received power of MDM of two Laguerre-Gaussian modes LG00 and LG01 is compared with respect to MDM of two transverse donut modes. The performance of four-level quadrature amplitude modulation (QAM) for orthogonal frequency division multiplexing (OFDM) of radio subcarriers in the WDM-MDMs system is investigated for mitigation of frequency-selective fading under strong atmospheric turbulence.

Surface-illuminated photon-trapping high-speed Ge-on-Si photodiodes with improved efficiency up to 1700 nm

Abstract: In this paper, high-speed surface-illuminated Ge-on-Si pin photodiodes with improved efficiency are demonstrated. With photon-trapping microhole features, the external quantum efficiency (EQE) of the Ge-on-Si pin diode is >80% at 1300 nm and 73% at 1550 nm with an intrinsic Ge layer of only 2 μm thickness, showing much improvement compared to one without microholes. More than threefold EQE improvement is also observed at longer wavelengths beyond 1550 nm. These results make the microhole-enabled Ge-on-Si photodiodes promising to cover both the existing C and L bands, as well as a new data transmission window (1620–1700 nm), which can be used to enhance the capacity of conventional standard single-mode fiber cables. These photodiodes have potential for many applications, such as inter-/intra-datacenters, passive optical networks, metro and long-haul dense wavelength division multiplexing systems, eye-safe lidar systems, and quantum communications. The CMOS and BiCMOS monolithic integration compatibility of this work is also attractive for Ge CMOS, near-infrared sensing, and communication integration.

A Simple Nonlinearity-Tailored Probabilistic Shaping Distribution for Square QAM

Abstract: A new probabilistic shaping distribution that outperforms Maxwell-Boltzmann is studied for the nonlinear fiber channel. Additional gains of 0.1 bit/symbol MI or 0.2 dB SNR for both DP-256QAM and DP-1024QAM are reported after 200 km nonlinear fiber transmission.

Spin-Dependent Optical Geometric Transformation for Cylindrical Vector Beam Multiplexing Communication

Abstract: Given by the Shannon theorem, the data rate in a single mode fiber is approaching the capacity limit of 100 Tbit/s, which even applies to all existing wavelength division multiplexing and advanced modulation formatting techniques. Optical vortex beams, including orbital angular momentum (OAM) beams with phase singularities and cylindrical vector beams (CVBs) with polarization singularities, are orthogonally structured light beams providing new degrees of freedom for multiplexing optical communication, for which the multiplexer is the key component. Although there are various OAM detection approaches such as the optical geometric transformation and vortex grating, CVB sorting with high efficiency and large dynamic range has not been demonstrated before. In this work, we propose and demonstrate an efficient approach for multiple coaxial CVB sorting based on the spin-dependent optical geometric transformation using the Pancharatnam–Berry optical element device fabricated with the photoaligned liquid crystal....

Silicon-based hybrid demultiplexer for wavelength- and mode-division multiplexing

Abstract: A silicon-based hybrid demultiplexer for wavelength-division multiplexing (WDM) and mode-division multiplexing (MDM) is proposed and demonstrated by integrating an M-channel-mode demultiplexer and N-channel WDM filters based on microring resonators (MRRs) with box-like responses. For the mode demultiplexer, the 2k-th output port is connected with the (2k+1)-th output port through the bus waveguide for the k-th MRR array, so that each MRR-based optical filter works bi-directionally and provides two drop ports. As an example, a 32-channel hybrid MDM-WDM demultiplexer is realized by integrating a 4-channel mode demultiplexer based on dual-core adiabatic tapers and two bi-directional MRR-based WDM filters with eight wavelength-channels. For the fabricated hybrid demultiplexer, the excess loss is 0.5–5 dB, the intermode cross talk is −16.5 to −23.5 dB, and the cross talks between the adjacent and nonadjacent wavelength channels are about −25 dB and −35 dB, respectively.