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There has been renewed interest in coherent detection to provide high spectral efficiency transmission for short-reach optical interconnects. However, the expensive high-stable laser sources may preclude its potential application. To balance the high performance and low cost for short-reach optical networks, the self-coherent homodyne receiver has been investigated, where the dual-polarization (DP) signal and the remote local oscillator (LO) originating from the same laser source co-propagate over a duplex fiber, enabling a remarkable tolerance of laser linewidth. However, the fast evolution of the state of polarization (SOP) of remotely delivered LO becomes problematic for self-coherent homodyne detection systems. To combat the polarization wandering of the remote LO, the complicated adaptive/automatic polarization controller (APC) has been deployed with only up to a few hundred rad/s polarization tracking speed. In this paper, we propose a complementary polarization-diversity coherent receiver (C-PDCR) for self-coherent homodyne detection using remote LO. The proposed C-PDCR features rapid polarization tracking for remote LO utilizing electronic digital signal processing (DSP). The robustness of the proposed C-PDCR is verified and demonstrated with a 1.08-Tb/s line rate (net rate 769.4 Gb/s over 45-GHz electrical bandwidth) using DP PCS-256QAM signal under rapid polarization rotation rate up to 314 krad/s in the experiment, which is more than 3-orders improvement of magnitude compared to the prior reports. [ABSTRACT FROM AUTHOR]

Coherent detection has the primary advantage of optical field recovery, which enables channel impairment compensation and advanced modulation formats for high-capacity transmission. On the other hand, direct detection (DD) is more cost-effective due to its local oscillator (LO)-free detection. To gain the field recovery advantage while preserving the LO-free detection for DD-based receivers, carrier assisted differential detection (CADD) has been proposed to retrieve the complex-valued double-sideband (DSB) signal. In this paper, to extend the concept of CADD, we propose a simplified CADD scheme with reduced hardware complexity and present signal-to-signal beating interference (SSBI) iterative mitigation algorithms with and without symbol decision. The required number of photodetectors and analog-digital converters (ADCs) for the simplified CADD receiver becomes the same as the coherent homodyne counterpart but without needing the expensive narrow-linewidth LOs. The performance of the proposed simplified CADD receiver is evaluated by 60-Gbaud 16-QAM DSB signals. Extensive performance comparisons are made between (i) the generalized CADD receivers with and without the simplified receiver structure, (ii) SSBI iterative mitigation algorithm with and without the symbol decision, and (iii) single carrier (SC) and OFDM-modulated DSB signals for the simplified CADD receiver. [ABSTRACT FROM AUTHOR]

We propose and experimentally demonstrate the broadband power coupling and mode-order conversion based on the trapezoidal subwavelength grating (SWG) structures. The slowly varying trapezoidal SWGs alter the modal field distribution in two closely spaced waveguides and result in an enhanced and adiabatically changing coupling strength between them. As a result, high coupling efficiencies over broad bandwidths are obtained for the coupled-waveguide devices with small sizes. Simulations show that the complete power transfer and the mode-order conversion can be accomplished using the coupling structure. The power coupler has a 1-dB bandwidth of 708 nm spanning from 1353 nm to 2061 nm, while the mode-order converter has a conversion loss less than 1 dB and crosstalk below −12.5 dB over a 324-nm bandwidth from 1388 nm to 1712 nm. Both devices were fabricated and experimentally characterized. The power coupler shows a measured insertion loss of <0.25 dB and the mode-order converter exhibits a measured conversion loss of <1.2 dB and a crosstalk value of < −10 dB, over a ∼120-nm bandwidth from 1507.2 nm to 1627.8 nm that covers the whole C- and L-band. The coupling length and the total length of our devices are 12.5 μm and 62.5 μm, respectively, which are very compact for adiabatic coupling structures. [ABSTRACT FROM AUTHOR]

We experimentally demonstrate on-chip modal crosstalk mitigation over two mode-division multiplexing (MDM) integrated circuits supporting four and eleven waveguide modes employing low-coherence matched detection. 30-Gbaud 8-PSK mode-multiplexed signal is successfully transmitted over the circuits without using multiple-in-multiple-output (MIMO) processing under a maximum modal crosstalk of 10 dB. Ten-mode-multiplexed on-chip transmission achieves a total capacity of 900 Gbit/s. This work shows the feasibility of using high-order modes in on-chip MDM transmission for future high-capacity optical interconnects. [ABSTRACT FROM AUTHOR]

We propose and experimentally demonstrate a Tomlinson–Harashima precoding (THP) scheme with the coefficients computed by simulations. It realizes spectrally efficient faster-than-Nyquist (FTN) signaling for quadrature amplitude modulation (QAM) format. In our scheme, the coefficients of the THP precoder are obtained by numerical simulations with an additive white Gaussian noise (AWGN) channel, in which the intersymbol interference (ISI) is induced by a digital FTN filter. Then, we experimentally apply these THP coefficients at the transmitter to mitigate the FTN-related ISI, while the ISI from the analog channel is suppressed by the receiver-side adaptive equalizer. By this means, the experimental estimation of the THP coefficients is not required, leading to a simplified system operation. We perform numerical simulations to study the impacts of FTN signaling and channel bandwidth limitation on the proposed THP scheme. A proof-of-concept experiment is conducted, in which a 28-GBaud FTN single-sideband (SSB) 16-QAM signal is transmitted over an 80-km single mode fiber (SMF). Compared to the conventional scheme with the experimentally estimated THP coefficients, a negligible sensitivity penalty is observed for the proposed scheme. We also achieve a record FTN rate of 21.73% and a high spectral efficiency of 4.30 b/s/Hz for the single-polarization 16-QAM format on a single wavelength. [ABSTRACT FROM AUTHOR]

We propose and experimentally demonstrate an automated wavelength seeking scheme for an O-band 1 × 2 silicon photonic switch based on an add-drop structure with two cascaded resonators. In the scheme, two three-layer neural networks are employed to learn the relationship between the monitored optical powers and the heating voltages of the two coupled ring resonators. Then, the two neural networks can quickly predict the required heating voltages according to the monitored optical powers. Therefore, the wavelength seeking can be realized through a training process, which is carried out only once, and a seeking process. By using this scheme, the dual-ring switch can be locked to the wavelength of the input light with a constant duration of 860 μs. The maximum seeking errors for the two rings are 0.08 and 0.09 nm, respectively. [ABSTRACT FROM AUTHOR]

Fiber-chip edge couplers are extensively used in integrated silicon photonic for the coupling of light between optical fibers and planar silicon waveguide circuits. Here, we experimentally demonstrate three kinds of edge couplers with eased fabrication process, two fork shape, and one dual-trident SWG shape, based on Silicon-on-Insulator platform. A commercial lensed fiber with mode field diameter of 6 μm is used. The coupling performance and fabrication tolerance are theoretically analyzed and verified by 3D-FDTD simulation. The experimental results show that these edge couplers pose low coupling losses and large bandwidths simultaneously. At the wavelength of 1.55 μm, the coupling losses are 1.25 dB/facet, 1.49 dB/facet, 1.82 dB/facet for fork1, fork2, and dual-trident SWG couplers, respectively. The measured wavelength bandwidths in which the loss below 2 dB are 114 nm, 102 nm, 92 nm, respectively. [ABSTRACT FROM AUTHOR]

Neuromorphic photonics that aims to process and store information simultaneously like human brains has emerged as a promising alternative for next generation intelligent computing systems. The implementation of hardware emulating the basic functionality of neurons and synapses is the fundamental work in this field. However, previously proposed optical neurons implemented with SOA-MZIs, modulators, lasers or phase change materials are all dependent on active devices and pose challenges for integration. Meanwhile, although the nonlinearity in nanocavities has long been of interest, the previous theories are intended for specific situations, e.g., self-pulsation in microrings, and there is still a lack of systematic studies in the excitability behavior of the nanocavities including the silicon photonic crystal cavities. Here, we report for the first time a universal coupled mode theory model for all side-coupled passive microresonators. The excitability of microresonators resulting from the subcritical Andronov-Hopf bifurcation is categorized as Class 2 neural excitability and can be used to mimic the Hodgkin-Huxley neuron model. We demonstrate that the microresonator-based neuron can exhibit the typical characteristics of spiking neurons: excitability threshold, leaky integrating dynamics, refractory period, cascadability and inhibitory spiking behavior, paving the way to realize all-optical spiking neural networks. [ABSTRACT FROM AUTHOR]

Heterogeneous integration of III-V lasers on Silicon on Insulator (SOI) is crucial for next-generation optical interconnects. However, reducing the footprint of the couplers between III-V laser sources and silicon chips has been a challenge for dense photonic integrated circuit (PIC). Here, we propose a slot coupler and a bridge-SWG coupler with coupling lengths of only 5.5 and 5 μm respectively, to heterogeneously integrate III-V lasers on silicon chips. For such compact coupling structures, they also show high coupling efficiencies. At the wavelength of 1550 nm, the coupling efficiencies of the fundamental TE mode can reach 93.7% and 95.5% for the slot coupler and the bridge-SWG coupler, respectively. Moreover, these coupling structures possess excellent fabrication tolerance. In addition, we optimize the taper adiabatic coupler with a total coupling length of only 4 μm. [ABSTRACT FROM AUTHOR]

In this article, we demonstrate a push-pull silicon dual-ring modulator based on two cascaded rings which are coupled with each other through two parallel bus waveguides. In the transmission spectrum of the modulator, a narrow transparency peak exists in the center of the broad dip based on the electromagnetically induced transparency (EIT)-like effect. Benefiting from both the EIT-like effect and push-pull drive configuration, a low insertion loss (IL) of 1.4 dB and an extinction ratio (ER) over 6.4 dB are obtained at a bit rate of 10 Gb/s with a peak-to-peak driving voltage (Vpp) of 5 V. The corresponding maximum optical modulation amplitude (OMA) is 0.56, which is almost twice that of a single micro-ring modulator with the same driving voltage. Moreover, the maximum OMA is 0.47 at 20 Gb/s, which is 81% larger than that of a single micro-ring modulator. [ABSTRACT FROM AUTHOR]

We experimentally demonstrate a 56-Gb/s vestigial sideband (VSB) modified duobinary (MDB) 4-level pulse amplitude modulation (PAM4) signal transmission over an 80-km single-mode fiber (SMF) in an intensity-modulation and direct-detection (IM/DD) system without optical dispersion compensation. Compared to the conventional PAM4 signal without pulse shaping, the MDB PAM4 signal exhibits a higher spectral efficiency (SE) due to the modified duobinary precoding. Moreover, if VSB modulation is employed to mitigate the chromatic dispersion (CD)-induced power fading, the VSB MDB PAM4 signal suffers less image interference resulting from VSB filtering and direct detection (DD), thanks to its lower components at the carrier frequency. A proof-of-concept experiment is performed, in which the performances of the conventional VSB PAM4 signal, the VSB Nyquist PAM4 signal, and the VSB MDB PAM4 signal are investigated and compared comprehensively. The experimental results show that only the VSB MDB PAM4 signal can reach the bit error ratio (BER) below the 20% soft-decision forward error correction (SD-FEC) threshold of 2.4 × 10−2 at the 56-Gb/s rate after the 80-km SMF transmission, due to the mitigated image interference. [ABSTRACT FROM AUTHOR]

We introduce and experimentally demonstrate a modified Kramers-Kronig (KK) receiver with direct detection (DD) to realize an accurate optical field reconstruction at low carrier-to-signal power ratio (CSPR) condition. In our scheme, an exponential operation is applied to a single-sideband (SSB) signal, rather than a continuous-wave tone adding method in the conventional KK scheme. By this means, the minimum phase condition (MPC) is inherently satisfied, thus the optical field can be reconstructed accurately through KK relation at a low CSPR, leading to a high receiver sensitivity in a DD system. A proof-of-concept experiment is conducted, in which an SSB signal is generated based on an 8-GBaud 16-quadrature amplitude modulation (QAM) data stream and then is transmitted over an 80-km single mode fiber (SMF). By using the modified KK technique, 1.1- and 0.9-dB sensitivity improvements are achieved in the optical back-to-back and after the fiber transmission cases, respectively. The proposed and the conventional KK schemes show similar BER performances at the optimum CSPR value with a high received optical power. In addition, we perform numerical simulations to study the impacts of bandwidth limitation and chromatic dispersion on the proposed minimum phase signal. [ABSTRACT FROM AUTHOR]

Switching can be performed with multiple physical dimensions of an optical signal. Previously optical switching was mainly focused in the wavelength domain. In this paper we discuss the general architecture of integrated silicon photonic switches by exploiting multi-dimensions in wavelength, polarization, and mode. To route a data channel from one input port to an arbitrary output port in a network node, three basic functions are required: de-multiplexing, switching, and multiplexing. The multiplexing and de-multiplexing processes can be realized in any one physical dimension. The capacity of a switch can be effectively scaled by using joint physical dimensions. As two examples, we first present a wavelength switch based on dual-nanobeam cavities with high quality factors, a low power consumption, and a compact footprint. We then propose a design of a mode-polarization-wavelength selective switch by leveraging three physical dimensions, and experimentally demonstrate the building blocks and key functionalities. [ABSTRACT FROM AUTHOR]

We propose and experimentally demonstrate an approach to carrier frequency and phase recovery of a microwave photonic signal based on carrier period measurement (CPM). In the approach, a coarse carrier frequency offset (CFO) estimation is first performed, which is realized by measuring the crossing period of the microwave signal with its mean level. Then, an envelope detection method is used to simultaneously estimate the residual CFO and the carrier phase. Through the two steps, wideband and high-accuracy carrier recovery can be realized. The proposed approach is validated by an experiment. Effective carrier recovery of a microwave photonic signal is achieved with a carrier frequency varying from 7 to 37 GHz carrying a 2-Gbaud on–off-keying or quadrature phase shift keying baseband signal. The CFO estimation range is from −21 to +9 GHz, and the CFO estimation errors are less than 0.1 MHz. [ABSTRACT FROM AUTHOR]

We propose and experimentally demonstrate a microwave photonic phase shifter based on a tunable silicon photonic interleaver. This device consists of a microring resonator, a tunable interleaver, and a $2\,\times \,1$ multimode interferometer (MMI). By thermally tuning the interleaver, a linear phase shift of 188° is realized with a linearity of 0.08 and a radio-frequency (RF) power variation within 1 dB for a 40-GHz signal. [ABSTRACT FROM AUTHOR]

Here, we demonstrate a silicon reconfigurable electro-optical directed-logic circuit enabled by a single-wavelength light input based on optical switches and photodetectors (PDs). Each switch consists of a silicon micro-ring resonator with an embedded p-n junction for logic input and a micro-heater for reconfiguration. The PDs have integrated p-i-n junctions for photocurrents generation which can eliminate the optical interference in the signal-wavelength calculation. This scalable circuit can be reconfigured to perform any combinational logic operation with a single wavelength light input. For a proof of concept demonstration, arbitrary two-input combinational logic functions with speeds up to ~270 Mb/s were implemented successfully. [ABSTRACT FROM AUTHOR]

Mode-division multiplexing on an integrated photonic chip is critical for future optical networks. High-order mode multiplexing is desired to increase the transmission capacity. Here, we propose and experimentally demonstrate a silicon on-chip high-order mode (de)multiplexer using subwavelength grating (SWG) structure, which supports 11-mode (de)multiplexing on a TE-polarized light (TE0–TE10). The proposed mode (de)multiplexer comprises three directional couplers and seven SWG-based directional couplers. Measurement results show that all the 11 channels have low crosstalk values (−15.4 to −26.4 dB) and low insertion losses (0.1 to 2.6 dB) at 1545 nm. To the best of our knowledge, our device achieves the highest-order-mode (de)multiplexing on a silicon photonic chip. [ABSTRACT FROM AUTHOR]

We present the design and experimental demonstration of a compact tunable silicon photonic interleaver with a wide spectral range. The interleaver consists of an interfering loop containing a Fabry–Perot cavity formed by two Sagnac loops. The transmission coefficients of the directional couplers are calculated based on the maximally flat criterion. The impacts of the transmission coefficient variations on the extinction ratio and filter sharpness are investigated. The wavelength dependences of the directional couplers are analyzed to increase the spectral range of the interleaver. The device was fabricated on a silicon-on-insulator platform. The spectral ranges of the measured transmission and reflection spectra are increased to 60 nm. By thermal tuning the waveguide connecting the two Sagnac loops, the central wavelength can be shifted over one free spectral range with a wavelength-tuning efficiency of ∼0.08 nm/mW. [ABSTRACT FROM PUBLISHER]

We propose and demonstrate via both simulation and experiment a scheme for a low-bit resolution digital-to-analog convertor (DAC) to improve the signal-to-quantification noise ratio (SQNR) of an intensity-modulated direct-detection (IM-DD) orthogonal frequency division multiplexing (OFDM) signal. In this scheme, the discrete output levels of a DAC are nonuniform and properly assigned based on the distribution of the OFDM signal. Such nonuniformity allows a better exploitation of output levels for the OFDM signal so that low-level waveforms that appear much more frequently experience mitigated quantization noise. To calculate these optimized output levels with the distribution of an OFDM signal, a numerical algorithm is proposed and verified. Our proposed scheme is proven effective in the simulations for Gaussian-distributed signals and brings over 3.6-dB and ∼2.5-dB improvements on SQNR and receiver sensitivity, respectively. The feasibility is also verified by a proof-of-concept experiment with ∼7.4-Gbps quadrature phase shift keying (QPSK)-modulated OFDM signals output by a 3-bit DAC. Our scheme enables transmission of such signals with no further optimization with ∼−25-dBm receiver sensitivity, while it is impossible using a conventional way. Both simulation and experimental results validate our scheme as a promising way to be implemented in low-bit resolution DACs for IM-DD OFDM applications. [ABSTRACT FROM PUBLISHER]

In this paper, we use a dynamic mask lithography system to fabricate a polymer-stabilized blue-phase Fresnel lens. Fresnel-zone patterned light is directly generated by a programmable spatial light modulator and then projected onto the blue phase precursor. As blue phase is cured, a Fresnel-zone refractive index distribution is achieved simultaneously. The fabricated Fresnel lens features polarization-insensitivity, a broad temperature range, submillisecond response and high spatial frequency. With easy fabrication and programmable flexibility, this method has great potential for various optical applications. [ABSTRACT FROM PUBLISHER]

We propose and experimentally demonstrate a photonic generation scheme for multiband ultra-wideband (MB-UWB) signal by employing a dual-drive Mach–Zehnder modulator (DDMZM). In this scheme, four electrical binary pulse streams with different bit rates are combined to drive one radio frequency (RF) port of the DDMZM, while the other RF port is modulated by the sum of their 1-b delayed duplicates. By properly adjusting the bit rate and the amplitude of each pulse stream, as well as the bias voltage of the DDMZM, an MB-UWB signal with four sub-bands can be obtained and the transmission capacity can be quadrupled. The feasibility of the proposed scheme is verified by a system demonstration, where the signal modulated by amplitude-phase-shift-key format reaches a high bit rate of 2 Gb/s. [ABSTRACT FROM PUBLISHER]