Highly efficient silicon modulator via a slow-wave Michelson structure.

The weak free carrier dispersion effect significantly hinders the adoption of silicon modulators in low-power applications. While various structures have been demonstrated to reduce the half-wave voltage, it is always challenging to balance the trade-off between modulation efficiency and the bandwidth. Here, we demonstrated a slow-wave Michelson structure with 1-mm-long active length. The modulator was designed at the emerging 2-µm wave band which has a stronger free carrier effect. A record high modulation efficiency of 0.29 V·cm was achieved under a carrier depletion mode. The T-rail traveling wave electrodes were designed to improve the modulation bandwidth to 13.3 GHz. Up to 20 Gb/s intensity modulation was achieved at a wavelength of 1976 nm.

On-chip germanium photodetector with interleaved junctions for the 2-µm wave band.

Recently, the 2-µm wave band has gained increased interest due to its potential application for the next-generation optical communication. As a proven integration platform, silicon photonics also benefit from the lower nonlinear absorption and larger electro-optic coefficient. However, this spectral range is far beyond the photodetection range of germanium, which places an ultimate limit for on-chip applications. In this work, we demonstrate a waveguide-coupled photodetector enabled by a tensile strain-induced absorption in germanium. Responsivity is greatly enhanced by the proposed interleaved junction structure. The device is designed on a 220-nm silicon-on-insulator and is fabricated via a standard silicon photonic foundry process. By utilizing different interleaved PN junction spacing configurations, we were able to measure a responsivity of 0.107 A/W at 1950 nm with a low bias voltage of -6.4 V for the 500-µm-long device. Additionally, the 3-dB bandwidth of the device was measured to be up to 7.1 GHz. Furthermore, we successfully achieved data transmission at a rate of 20 Gb/s using non-return-to-zero on-off keying modulation.

Robust, in-service, and joint monitoring of a dual-polarization transceiver IQ skew for a coherent DSCM system without channel impairment compensation.

A robust, in-service, and joint monitoring of a dual-polarization (DP) transceiver IQ skew for a coherent DSCM system is proposed and experimentally validated. Unlike traditional monitoring schemes, the proposed scheme realizes robust transceiver impairments monitoring without channel impairment compensation, including chromatic dispersion (CD), polarization variation, and carrier phase noise. This enhances the stability and precision of the monitoring process and reduces computational complexity by eliminating sophisticated DSP for impairment compensation. A complex system model for a single-tone signal is given first. Based on the model, the proposed scheme enables monitoring of the DP transmitter and the receiver IQ skew using the inserted frequency domain pilots (FPTs). Experimental results show that the proposed scheme can estimate the transceiver IQ skew within 16 ps with an estimation error of less than 0.2 ps and is robust to CD, polarization variation, phase noise, and amplified spontaneous emission noise. To the best of our knowledge, the proposed scheme achieves in-service transceiver IQ skew monitoring for coherent DSCM systems for the first time.

Receiver in-phase and quadrature skew tolerant baud-rate timing recovery scheme for short-reach coherent optical interconnection.

A baud-rate sampling timing recovery (TR) scheme with receiver IQ skew tolerance is proposed and experimentally demonstrated. The proposed scheme performs independent TR for the in-phase and quadrature (IQ) tributary signals, thereby tracking the sampling phase error while naturally compensating for receiver IQ skew. The robustness of the IQ-independent TR to frequency offset (FO) and phase noise is theoretically analyzed. To address IQ misalignment caused by the IQ-independent TR, the use of pseudo-noise (PN) sequences for IQ frame synchronization is proposed. The proposed scheme achieves accurate timing recovery with hardware-efficient baud-rate sampling in the presence of receiver IQ skew, laying the foundation for stable performance of subsequent baud-rate equalization. The performance of the scheme is validated in a 56 GBaud polarization division multiplexed (PDM) 16QAM coherent experimental system. Experimental results demonstrate that the proposed scheme achieves similar BER performance to the modified Gardner + real-valued multiple-input multiple-output (RVMIMO) (@2 SPS) scheme. Moreover, the proposed scheme exhibits robustness to arbitrary IQ skew compared to the ABSPD + RVMIMO (@1 SPS) scheme.

Robust, wide-range, and precise transmitter IQ impairment monitoring scheme for coherent digital subcarrier multiplexing systems.

In this Letter, we present a robust, wide-range, and precise monitoring scheme for transmitter (Tx) impairments in coherent digital subcarrier multiplexing (DSCM) systems. The proposed scheme employs frequency-domain pilot tones (FPTs) to compensate for frequency offset (FO), polarization aliasing, and carrier phase noise, thus isolating Tx impairments from channel distortions. It then implements 4 × 4 real-valued MIMO to compensate for Tx impairments by equalizing symmetric subcarriers. Tx impairment monitoring is derived from the equalizer coefficients. By considering the phase shift caused by Tx impairments, a wide-range and precise monitoring of Tx impairments including IQ skew, IQ phase, and gain imbalances is achieved. We experimentally validated our approach using a 48-GBaud, four-subcarrier, dual-polarization coherent DSCM system. The results confirm the method's capability for a wide-range, robust, and precise Tx impairment monitoring in coherent DSCM systems, maintaining performance even in the presence of ultra-fast polarization variation.

Multiplication-free and hardware-efficient baud-rate timing error detector for Nyquist and faster than Nyquist optical IM/DD systems.

Clock recovery (CR) algorithms that support higher baud rates and advanced modulation formats are crucial for short-distance optical interconnections, and it is desirable to push CR to operate at baud rate with minimal computing resources and power. In this Letter, we proposed a hardware-efficient and multiplication operation-free baud-rate timing error detector (TED) as a solution to meet these demands. Our approach involves employing both the absolute value of samples and the nonlinear sign operation to emphasize the clock tone, which is deteriorated by severe bandwidth limitation in Nyquist and faster than Nyquist (FTN) systems. Through experimental investigations based on a transceiver system with a 3 dB bandwidth of 30 GHz, the proposed baud-rate TED exhibits excellent performance. The proposed scheme successfully achieves clock synchronization of the received signals with the transmitted signals, including 50 GBaud PAM4/8, 80 GBaud PAM4, and up to 120 GBaud PAM4 FTN signals. To the best of our knowledge, the CR based on the proposed baud-rate TED is the most optimal solution for ultrahigh-speed short-reach IM/DD transmission, comprehensively considering the timing jitter, bit error rate (BER), and implementation complexity.

4-bit DAC based 6.9Gb/s PAM-8 UOWC system using single-pixel mini-LED and digital pre-compensation.

Low-cost underwater wireless optical communication (UOWC) systems are attractive for high-speed connections among unmanned vehicles or devices in various underwater applications. Here we demonstrate a high-speed and low-cost UOWC system using a low-resolution digital to analog converter (DAC), a single-pixel mini-sized light-emitting diode (mini-LED), and digital pre-compensation (DPC). The enabled DPC scheme comprises digital pre-distortion (DPD), digital pre-emphasis (DPE), and digital resolution enhancer (DRE), which pre-compensate for mini-LED nonlinearity, the bandwidth limitation of the mini-LED and avalanche photodiode detector, and DAC resolution limitation, respectively. The simulation results show that the in-band signal-to-quantization noise ratio can be increased by 6.8 dB using DRE based on a 4-bit DAC. To further improve the system capacity, we tune the level of DPE in order to optimize the trade-off between the residual inter-symbol interference and signal-to-noise ratio. With the combination of optimized DPE and DRE, we obtain a 21.1% higher data rate compared with full DPE only and demonstrate the transmission of 6.9 Gb/s PAM-8 signal over a 2-m distance underwater based on a single-pixel mini-LED and 4-bit DAC. This paper reports a cost-effective UOWC system first using a low-resolution DAC and DPC, which offers a promising path toward low-cost underwater optical wireless networks.

Net 4 Gb/s underwater optical wireless communication system over 2 m using a single-pixel GaN-based blue mini-LED and linear equalization.

High-bandwidth GaN-based mini-LEDs on the c-sapphire substrate are promising candidates for underwater optical wireless communication (UOWC) systems due to their compatibility with the mature LED fabrication process. Here we fabricate and characterize mini-LEDs based on a single-layer InGaN active region with a peak emission wavelength around 484 nm for high-speed UOWC links. Since the LED diameter affects the trade-off between the modulation bandwidth and the optical modulation amplitude, mini-LEDs with varying mesa diameters from 100 µm to 175 µm are fabricated for the measurement. The 150 µm mini-LED with a 3-dB optical bandwidth of 906 MHz performs the best and enables the transmission of a net 4 Gb/s PAM-4 signal over 2 m of underwater distance using only linear equalization. This UOWC system has achieved, to the best of our knowledge, the highest net data rate and the highest data-rate-distance product based on a single-pixel mini-LED.

N-Oxide Zwitterion Functionalized Positively Charged Polyamide Composite Membranes for Nanofiltration.

N-Oxide zwitterionic polyethyleneimine (ZPEI), a new kind of aqueous phase monomer synthesized by commercially branched polyethyleneimine (PEI) via oxidation reaction, was prepared for fabrication of thin-film composite (TFC) polyamide membranes via interfacial polymerization. The main factors, including the monomer concentration and immersion time of the aqueous phase and organic phase, were investigated. Compared with PEI-TFC membranes, the obtained optimal defect-free ZPEI-TFC membranes exhibited a lower roughness (3.3 ± 0.3 nm), a better surface hydrophilicity, and a smaller pore size (238 Da of MWCO). The positively charged ZPEI-TFC membranes (isoelectric point at pH 8.05) showed higher rejections toward both divalent cationic (MgCl2, 93.0%) and anionic (Na2SO4, 96.1%) salts with a water permeation flux of up to 81.0 L·m-2·h-1 at 6 bar, which surpassed currently reported membranes. More importantly, mainly owing to N-oxide zwitterion with strong hydration capability, ZPEI-TFC membranes displayed a high flux recovery ratio (97.0%) toward a model protein contaminant (bovine serum albumin), indicating good anti-fouling properties. Therefore, the novel N-oxide zwitterion functionalized positively charged nanofiltration membranes provide an alternative for water desalination and sewage reclamation.

Real-time reception of 106 Gbps PAM-4 transmission over an 80†km SSMF link enabled by CD pre-compensation.

In this study, we report, to the best of our knowledge, the first experimental demonstration of the real-time reception of a 106 Gbps PAM-4 transmission over an 80 km dispersion uncompensated standard single-mode fiber (SSMF) link. In the transmitter, a chromatic dispersion (CD) pre-compensation, aided by an optical IQ modulator, is implemented. The optimization guideline of driver signal amplitudes and bias voltages is proposed to enable optimal CD pre-compensation. In the receiver, a real-time PAM-4 module including simple feed-forward equalization (FFE) is used. After the transmission, the required optical signal-to-noise ratio (OSNR) at a bit error rate (BER) below 3.8 × 10-3 is determined to be 35 dB for 106 Gbps PAM-4 signals. The better BER of 6.9 × 10-4 is achieved successfully compared to the previously reported off-line reception. The demonstration confirms the feasibility of 80 km DCI based on CD pre-compensation under real-time reception.

Joint, accurate and robust optical signal-to- noise ratio and modulation format monitoring scheme using a single Stokes-parameter-based artificial neural network.

A joint and robust optical signal-to-noise ratio (OSNR) and modulation format monitoring scheme using an artificial neural network (ANN) is proposed and demonstrated via both numerical simulations and experiments. Before ANN, the power iteration method in Stoke space is employed to estimate the phase difference between two orthogonal polarizations caused by fiber birefringence. Then, a three layers ANN is employed to approximate the relationship between the cumulative distribution function of a single Stokes parameter (S2) and the targeted OSNR and format information. The simulation results show that the probability of OSNR estimation error within 1dB in the proposed scheme is 100%, 99.78%, 100%, 99.78% and 98.89% for 28GS/s QPSK, 8PSK, 8QAM, 16QAM and 64QAM, respectively. Meanwhile, the proposed scheme also shows high modulation format identification accuracy in the presence of nonlinear Kerr effect and residual chromatic dispersion. With 1 dB OSNR estimation error, the proposed scheme can tolerate the residual chromatic dispersion and phase-related polarization rotation rate up to 100ps/nm and 50kHz, respectively. The experimental results also further confirm that the proposed scheme shows high modulation identification accuracy for 28GS/s QPSK, 8PSK and 16QAM under the scenarios of both back-to-back and fiber transmission. Meanwhile, with the launched power of 0dBm, the mean OSNR estimation error in our scheme is smaller than 1 dB within ±160ps/nm residual chromatic dispersion after fiber transmission.

Cost-effective and robust DSP scheme for a short-reach coherent system in the presence of transmitter IQ skew and chromatic dispersion.

A cost-effective and robust digital signal processing (DSP) scheme is proposed and demonstrated experimentally in a coherent 61 GBaud PDM 16QAM system. In our scheme, multi-stage DSP blocks are used to deal with channel effects, transceiver in-phase and quadrature (IQ) skew, and phase noise. A 4×4 real-valued multiple-input multiple-output (RV-MIMO) with N1 taps is for polarization recovery and receiver IQ skew calibration. After frequency offset compensation, two 2×2 RV-MIMO with N2 taps are used to compensate for chromatic dispersion (CD), inter-symbol interference, transmitter IQ skew, and phase noise. Finally, the residual phase noise is eliminated by the maximum likelihood (ML) estimator. The experimental results indicate that the proposed scheme provides better received optical power sensitivity and CD tolerance than the existing simplified DSP schemes. In addition, the proposed scheme can tolerate transmitter IQ skew up to 7 ps in a 10 km case, which outperforms both simplified and conventional DSP schemes. Meanwhile, the proposed scheme can keep the same transceiver IQ skew and CD tolerance and has reduced complexity by 25% after 10 km links, compared to 4×4 RV-MIMO followed by a transmitter skew compensator. To the best of our knowledge, the proposed scheme is the most cost-effective solution for a high baud rate datacenter interconnects where transmitter IQ skew and CD have to be dealt with.

Rapid and precise phase retrieval from two-frame tilt-shift based on Lissajous ellipse fitting and ellipse standardization.

A rapid and precise phase-retrieval method based on Lissajous ellipse fitting and ellipse standardization is demonstrated. It only requires two interferograms without pre-filtering, which reduces its complexity and shortens the processing time. The elliptic coefficients obtained by ellipse fitting are used for ellipse standardization. After compensating phase-shift errors by ellipse standardization, the phase distribution is extracted with high precision. It is suitable for fluctuation, noise, tilt-shift, simple and complex fringes. This method is effective for the number of fringes less than 1. The reliability of the method is verified by simulations and experiments, indicating high accuracy and less time consumption.

Orbital angular momentum density characteristics of tightly focused polarized Laguerre-Gaussian beam.

The orbital angular moment (OAM) of light has been proved to be useful in plenty of applications. By transmitting the OAM of the focused light field to a particle, it will be orbited around the optical axis. Therefore, it is necessary to study the OAM distribution of the focused light field used to manipulate the particles. In this application, the widely used paraxial approximation is no longer sufficient due to the tightly focused beam. We employ the higher-order Poincaré sphere to represent the Laguerre-Gaussian (LG) beams with arbitrary polarization. Then the Rayleigh-Sommerfeld integral method and the q-parameter method are used to derive the analytical expression of the light field on the focal plane. Based on this, the OAM density expression of the tightly focused LG beam is derived. In the numerical simulation, we study and analyze the unique intensity distributions and OAM distributions of tightly focused linear polarized, radial polarized, and circular polarized LG beams. The results could be leveraged to further explore the applications of the polarized vortex beam.

Low complexity, modulation-transparent and joint polarization and phase tracking scheme based on the nonlinear principal component analysis.

A low complexity, modulation-transparent and joint polarization and phase tracking scheme based on the nonlinear principal component analysis (NPCA) is proposed and demonstrated via both simulation and experiment. Based on high-order statistics, NPCA can achieve joint polarization and phase tracking successfully without any prior information of modulation format. Meanwhile, owing to fact that the estimated matrix is constrained to be a unitary matrix, NPCA can avoid the singularity problem. Compared with the format dependent scheme such as CMA/MMA + VVPE, NPCA shows comparable BER performance under the back-to-back case and shows fast-tracking capability over wide polarization rotation rates ranges. Moreover, for 16QAM signals, the proposed NPCA-based scheme has reduced around 30% computation resources compared with the format dependent scheme, which confirms the advantage of low implementation complexity.

Modulation-format-transparent IQ imbalance estimation of dual-polarization optical transmitter based on maximum likelihood independent component analysis.

We propose and experimentally demonstrate a modulation-format-transparent dual-polarization (DP) transmitter (Tx) in-phase/quadrature (IQ) imbalance estimation scheme based on maximum likelihood independent component analysis (ML-ICA). The proposed scheme can separate Tx IQ imbalance from polarization crosstalk and phase noise and achieve accurate IQ imbalance estimation without training data and the information of modulation format. Firstly, the complex-ML-ICA is used to implement format-transparent polarization de-multiplexing to remove polarization crosstalk; then the real-ML-ICA is employed to estimate inverse IQ mixing matrix and compensate Tx IQ imbalance/phase noise on each polarization channel. Inverse IQ mixing matrix contains the information of phase noise and Tx IQ imbalance; Finally, Tx IQ imbalance is derived from the inverse matrix by analytic method. The impact of Tx IQ imbalance on polarization demultiplexing and carrier phase recovery (CPE) is investigated by numerical simulation from three aspects of Jones space, Stokes space, and Kurtosis. The simulation results demonstrate the proposed scheme has strong robustness to phase noise, quantization noise, and amplified spontaneous emission (ASE) noise. The proposed ML-ICA algorithm is verified experimentally in polarization division multiplexing (PDM) quadrature phase-shift keying (QPSK)/8 quadrature amplitude modulation (QAM)/16QAM/64QAM systems. The experimental results show the scheme can accurately estimate Tx IQ imbalance within wide range in a format transparent manner.

Stokes space modulation format classification based on non-iterative clustering algorithm for coherent optical receivers.

A Stokes-space modulation format classification (MFC) technique is proposed for coherent optical receivers by using a non-iterative clustering algorithm. In the clustering algorithm, two simple parameters are calculated to help find the density peaks of the data points in Stokes space and no iteration is required. Correct MFC can be realized in numerical simulations among PM-QPSK, PM-8QAM, PM-16QAM, PM-32QAM and PM-64QAM signals within practical optical signal-to-noise ratio (OSNR) ranges. The performance of the proposed MFC algorithm is also compared with those of other schemes based on clustering algorithms. The simulation results show that good classification performance can be achieved using the proposed MFC scheme with moderate time complexity. Proof-of-concept experiments are finally implemented to demonstrate MFC among PM-QPSK/16QAM/64QAM signals, which confirm the feasibility of our proposed MFC scheme.

Order-of-magnitude multiphoton signal enhancement based on characterization of absorption spectra of immersion oils at the 1700-nm window.

To enhance signal levels in multiphoton microscopy (MPM) at the deep-tissue excitation window (1600-1820 nm) with oil immersion, we demonstrate: First, the absorption spectra of several commonly immersion oils are characterized, which were unknown before. Second, new material with lower absorption based on mixing is proposed. Third, optimal selection of excitation wavelength within this window is proposed based on absorption spectra characterization. Second and third harmonic generation imaging of mouse tissue corroborate our selection: 1600-nm excitation leads to notable orders-of-magnitude increase in MPM signal, compared with 1700-nm excitation, enabling 200-µm imaging depth of mouse skin while 1700-nm excitation could resolve virtually no structure.

112 Gb/s transmission over 80 km SSMF using PDM-PAM4 and coherent detection without optical amplifier.

Polarization-division-multiplexed (PDM) four-level pulse amplitude modulation (PAM4) with coherent detection is a promising low cost solution for 80 km inter-datacenter transmissions at 100 Gb/s and beyond. In this paper, three modified adaptive equalization algorithms for the PDM-PAM4 optical coherent systems, i.e. signal-phase aid least-mean-square (SP-LMS) algorithm, training multi-modulus algorithm (TMMA) and cascaded four-modulus algorithm (CMMA-4), are proposed and compared. Based on the proposed algorithms, 112 Gb/s PDM-PAM4 transmission over 80 km standard single mode fiber (SSMF) in C-band for a bit error rate (BER) below 3.8e-3 is successfully demonstrated without optical amplifier, chromatic dispersion (CD) pre-compensation and extra carrier recovery operations.

Frequency spacing switchable multiwavelength Brillouin erbium fiber laser utilizing cascaded Brillouin gain fibers.

A new hybrid Brillouin erbium fiber laser scheme that employs cascaded multiple Brillouin gain fibers in a ring cavity to realize multiwavelength laser output with switchable frequency spacing is proposed and experimentally investigated. The multiple frequency downshifting processes introduced by multiple stimulated Brillouin scattering (SBS) effects in one round-trip of the cavity make it possible to realize multiwavelength output with frequency spacing that is an integer multiple of the SBS frequency shifting. With two cascaded SBS fibers, the frequency spacing can be switched between single and double SBS frequency shifting by properly adjusting the Brillouin pump power. Multiwavelength outputs with triple or quadruple SBS frequency spacing are also demonstrated by employing three or four SBS gain fibers, respectively.