Capacity enhancement through entropy loading with probabilistically shaped signals in a frequency comb-based transmission system.

We demonstrate a multichannel entropy loading mechanism in an optical frequency comb-based coherent communication system. In high-capacity wavelength division multiplexing communications, the individual laser sources can be replaced by an optical frequency comb, thus reducing the complexity and energy consumption of the transmitter. However, the power variation among different comb lines will lead to performance discrepancies. Spectral flattening filters can be adopted to suppress the variation at the expense of an additional system loss. Alternatively, by applying probabilistic shaping, we have implemented multichannel entropy loading to facilitate a continuous adaptation of the source entropy. The data rate can be dynamically allocated according to the performance of each channel. Through the loading scheme, the non-uniform performances across the channels are effectively mitigated, achieving a capacity enhancement of 34.91 Gbit/s.

Optimizing the probability mass function of complex modulated signals through adaptive channel characterization.

We propose a probability mass function (PMF) optimization scheme for quadrature amplitude modulation (QAM) signals by considering the parametric characteristic of the training sequence. The training sequence for optimization is mapped in standard Maxwell-Boltzmann (M-B) distribution, and the considered characterizing parameters incorporate either the noise variance or the error matrix of the received symbols. The proposed PMF optimization is based on independent reallocation within each constellation ring, generating new distribution with almost the same entropy and transmitted power as the original distribution. This reallocation mechanism is model-free and iterative-free with very low computational complexity. By characterizing the channel in terms of constellation performance asymmetry, PMF reallocation can be effectively implemented to supplement the existing equalization algorithm. The effectiveness of this approach is experimentally verified in a 40-km transmission system with 24 Gbaud 64-QAM signals under three different scenarios. Through PMF reallocation, we achieve generalized mutual information (GMI) improvement of ∼0.06 and throughput improvement of ∼1.5 Gbit/s before forward error correction in comparison with the standard M-B distribution. The proposed mechanism provides a solution to obtain the optimal PMF in practical communication channels, which suffer from various types of noises and distortions.

Tunable optical frequency comb with hundred-GHz spacings generated on a silicon waveguide.

We demonstrate the generation of optical frequency combs with tunable spacing at the hundred-GHz range in the 1550-nm window. The widely spaced combs are realized through silicon-based cross-phase modulation. The optical pump is prepared by multiplication of a 10-GHz train of 1.7-ps pedestal-free pulses. Energy-efficient temporal Talbot processing is used to multiply the repetition rate by a factor of up to 20. In our approach, the multiplication factor can be flexibly controlled by tuning the temporal dispersion inside an optical processor. Optical frequency combs with spacings ranging from 140 to 200 GHz have been successfully generated with a maximum carrier-to-noise suppression ratio of ∼45 dB.

Enhanced CSPR for a multichannel Kramers-Kronig receiver by self-seeded stimulated Brillouin scattering.

We demonstrate carrier-to-signal power ratio (CSPR) enhancement by self-seeded stimulated Brillouin scattering to improve the performance of Kramers-Kronig (KK) detection for multichannel single-sideband (SSB) signals. By virtue of low-CSPR transmission and high-CSPR detection, our proposed scheme effectively advances system performance by reducing propagation-induced distortion while maintaining the minimum phase condition. We experimentally demonstrate the improvement in CSPR and bit error rate of 5×10-Gbaud 16-QAM SSB signals by applying the carrier recovery block after 80-km transmission. Under optimum pump power, the average Q factor improvement of all five channels is 3.0 dB. We also analyze the performances of different channels and the major limiting factor. The results verify that our scheme offers a promising solution to enhance SSB self-coherent KK detection in wavelength-division multiplexing systems.

Constellation size for probabilistic shaping under the constraint of limited ADC resolution.

We investigate the optimal constellation size for probabilistic shaping (PS) under the constraint of different effective numbers of bits (ENOB) of the analog-to-digital converter (ADC) at the receiver. For a fixed entropy, increasing the constellation size brings a larger shaping gain, while it also leads to a higher peak-to-average-power ratio, which makes the signal more sensitive to the ENOB and fiber nonlinearity. Our experimental results show that PS-44 quadrature amplitude modulation (QAM) outperforms both PS-64 QAM and uniformly shaped 32 QAM (U-32 QAM) when the ENOB of ADC is lower than 4.5. The optimal launched power and the maximum achievable distance for U-32 QAM, PS-44 QAM, and PS-64 QAM are analyzed by simulation.

A silicon nitride waveguide-integrated chemical vapor deposited graphene photodetector with 38 GHz bandwidth.

We demonstrate a high-speed chemical vapor deposited graphene-on-silicon nitride waveguide photodetector. The device is designed with grating-like metal contact to reduce the channel spacing. Benefiting from the narrow channel spacing, a calculated transit-time-limited bandwidth of 111 GHz is derived. The resistance-capacitance-limited bandwidth is also improved due to the small relative permittivity of silicon nitride. At a wavelength of 1550 nm, we measured an electro-optic bandwidth of 38 GHz under zero bias and an intrinsic responsivity of 13 mA W-1 at 0.1 V reverse bias with a 6 µm detection length.

Integrated germanium-on-silicon Franz-Keldysh vector modulator used with a Kramers-Kronig receiver.

We propose an integrated vector modulator based on two compact and high-speed germanium-on-silicon Franz-Keldysh electro-absorption modulators. The proposed vector modulator is extremely compact with a total footprint of only 1800 µm×200 µm. We further experimentally demonstrate a 4-quadrature-amplitude-modulation (4-QAM) at 40 Gb/s over a 20-km standard single-mode fiber transmission. The complex signal is successfully re-constructed with a single-ended photodiode in a recently proposed Kramers-Kronig receiver for future low-cost, low-power, and low-footprint datacenter interconnect applications. The preliminary performance of the vector modulator with a 16-QAM is also investigated.

Surpassing the tuning speed limit of slow-light-based tunable optical delay via four-wave mixing Bragg scattering.

We demonstrate a tunable optical delay that surpasses the tuning speed limit of the conventional slow-light-based optical delay. A novel nonlinear optical coupler, implemented by the four-wave mixing (FWM) Bragg scattering process, is utilized to perform destructive interference of the slow-light delayed signal pulse and a nondelayed reference pulse. As a result, the Brillouin-induced frequency-dependent phase shift, as well as the group delay of the synthesized pulse, is amplified. The group delay amplification factor, determined by the coupling ratio of the nonlinear optical coupler, can be tuned through varying the FWM pump power to provide an ultrafast response. Our experimental result demonstrates that an initial 6.2 ns Brillouin-induced optical delay can be amplified and rapidly tuned within the range of -5.2 to 27.2 ns.

Carrier regeneration from a blockwise phase-switching signal for a frequency comb-based WDM system.

We demonstrate that an optical carrier can be extracted from a signal based on a blockwise phase-switching (BPS) technique for locking the frequency combs at the transmitter and the receiver in a wavelength division multiplexing (WDM) system. Two types of carrier regeneration techniques, optical injection locking and stimulated Brillouin scattering, are evaluated in the aspects of the injection ratio and pump power, respectively, under different carrier-to-signal power ratios. 7×16 Gbaud QPSK data channels with 25 GHz spacing are successfully transmitted over an 80 km fiber by the direct detection of a BPS-assisted central channel and coherent detection of the remaining channels.

Raman-enhanced optical phase conjugator in WDM transmission systems.

Optical phase conjugation (OPC) can be applied to boost the performance of long-haul transmission by mitigating the impairments from fiber nonlinearity. Unfortunately, noticeable nonlinear noise in the conjugator for optical orthogonal frequency division multiplexing (OFDM) systems often degrades the signal quality. In this paper, we demonstrate nonlinear distortion mitigation in OPC by introducing backward Raman amplification to the conjugator. Raman amplification allows a lower input signal power, thus suppressing the OPC distortion while maintaining the conjugated output power. We investigate the performances of Raman-enhanced OPC in both back-to-back (BTB) and transmission systems with 3 × 25 Gbaud optical OFDM signals. In the BTB OPC system, Raman amplification boosts the tolerance to system nonlinearity, achieving a 3-dB improvement in the output power, a 2.4-dB improvement in the Q factor, and a 6-dB improvement in the input dynamic range. In the transmission system with Raman-enhanced OPC, the optimum launched power is increased by 2 dB and the maximum Q factor is increased by 0.4 dB compared to direct transmission. Similar performances are observed in all the wavelengths, indicating that our scheme works well with WDM transmission systems.

Kramers-Kronig detection with the Brillouin-amplified virtual carrier.

We proposed a novel scheme for realizing Kramers-Kronig detection by utilizing the narrow-band and high-gain characteristic of stimulated Brillouin scattering. At the receiver, the weak virtual carrier located at the edge of the signal spectrum is Brillouin amplified by the output of a slave laser, which is injection locked by a weak pump seed provided by the transmitter. More than a 2.7-dB optical signal-to-noise ratio sensitivity improvement is experimentally obtained compared with the traditional virtual carrier assisted scheme after an 80-km transmission. The effective number of bits requirement and the performance gain in the nonlinear transmission are both analyzed by the simulation.

High-performance chemical vapor deposited graphene-on-silicon nitride waveguide photodetectors.

Waveguide photodetectors integrated with graphene have demonstrated potential for ultrafast response and broadband operation. Here, we demonstrate high-performance chemical vapor deposited graphene-on-silicon nitride waveguide photodetectors by enhancing the absorption of light propagating in the transverse-magnetic mode through a metal-graphene junction. A doubling in responsivity is experimentally observed. In our zero-biased metal-graphene junction, a 15 mA W-1 intrinsic responsivity and a 30 GHz bandwidth are achieved at ∼1550 nm. The results are comparable to those obtained from the best pristine graphene-based photodetectors. Our work enables new architectures for high-performance optoelectronic devices based on the graphene-on-silicon nitride platform.

Brillouin optical time domain analyzer sensors assisted by advanced image denoising techniques.

We have experimentally analyzed and compared the performance of Brillouin optical time-domain analyzer (BOTDA) sensors assisted by non-local means (NLM) and wavelet denoising (WD) techniques in terms of measurement accuracy and experimental spatial resolution, respectively. Degradation of the measurement accuracy and experimental spatial resolution after denoising by NLM and WD are observed, which originate from the fact that higher signal-to-noise ratio (SNR) improvement is achieved at the expense of sacrificing the details of BOTDA data, and smaller data sampling point number (SPN) gives rise to insufficient redundant information for denoising. The two parameters degrade to different extents depending on the amount of SNR improvement and SPN adopted in data acquisition. Compared with WD, NLM relies more on the features of the raw data, which makes its performance highly dependent on the level of neighbouring data similarity. Also, for the first time we propose and demonstrate a BOTDA assisted by advanced Block-Matching and 3D filtering (BM3D) denoising technique, which minimizes the degradation of the two parameters even under higher SNR improvement and smaller SPN. BM3D takes the advantage of NLM and WD and utilizes the spatial-domain non-local principle to enhance the denoising in the transform domain, thus it shows the least degradation of measurement accuracy/experimental spatial resolution after denoising. Thus the BOTDA assisted by BM3D maintains the best measurement accuracy/experimental spatial resolution compared with those by NLM and WD. We also show that BM3D has the advantage of temperature independent performance, unlike NLM where the accuracy is affected by the temperature value. We believe BM3D would be an excellent denoising technique for state-of-the-art BOTDA sensors. In addition, this work is also valuable for practical applications of image denoising techniques in BOTDA sensors with respect to the appropriate choice of image denoising techniques, design of SNR improvement and the adoption of SPN to maintain optimal measurement accuracy/experimental spatial resolution/data acquisition speed.

Support vector machine assisted BOTDA utilizing combined Brillouin gain and phase information for enhanced sensing accuracy.

Benefiting from both Brillouin amplitude and phase spectral responses during Brillouin scattering, a support vector machine (SVM) assisted Brillouin optical time domain analyzer (BOTDA) enabling the improvement of sensing accuracy with only a slight sacrifice of processing speed has been proposed and demonstrated. Only one SVM model, i.e. SVM-(g + p), is required to effectively combine the Brillouin gain and phase information in the training and testing phases, which avoids separate Brillouin gain spectrum (BGS) and Brillouin phase spectrum (BPS) fitting, and hence saves the processing time. Both simulation and experiments using different parameters were conducted to evaluate the improved performance of SVM-(g + p). Compared with the case of using BGS only or BPS only, SVM assisted BOTDA using combined BGS and BPS enhances the accuracy of temperature extraction by about 30% over a wide range of simulation and experiment parameters, only at a slight expense of the processing speed. Although the processing of both gain and phase information takes extra time, SVM-(g + p) assisted BOTDA still has a processing speed 80 times faster than that of using a conventional curve fitting method like Lorentzian curve fitting (LCF). The improved accuracy, together with fast processing speed, is crucial for future high-speed and accurate BOTDA sensors based on both Brillouin gain and phase detection.

SBS-enhanced FWM for polarization division multiplexed signals in coherent communication systems.

Stimulated Brillouin scattering (SBS) is applied to improve the performance of polarization insensitive four-wave mixing (FWM) realized with two orthogonal pumps. By manipulating the phase matching condition with SBS-induced phase change, we achieve conversion efficiency improvement up to 6 dB with negligible polarization dependence. Our approach is implemented in a coherent communication system where polarization division multiplexing (PDM) quadrature phase shift keying (QPSK) signal is applied. The quality of the converted idler is evaluated in both back-to-back (BtB) and transmission systems. Our experimental results show that the converted signal enjoys an improved conversion efficiency with negligible polarization-induced distortion. The SBS-enhanced Q factor leads to an extension of the transmission distance by 240 km. The results indicate that SBS-enhanced FWM can accommodate the advanced modulation formats and multiplexing schemes in today's optical networks.

Photonically assisted microwave waveform generation by gain-transparent SBS-induced carrier processing: erratum.

In our previous work [Opt. Lett.42, 3852 (2017)OPLEDP0146-959210.1364/OL.42.003852], there is an error in Fig. 1. That error is addressed here.

Photonically assisted microwave waveform generation by gain-transparent SBS-induced carrier processing.

We propose and experimentally demonstrate a new approach to generate microwave waveforms by phase modulation and optical carrier phase processing based on gain-transparent stimulated Brillouin scattering (SBS). By properly adjusting the SBS-induced-phase shift on the carrier and the following group velocity dispersion, microwave waveforms with controllable repetition rates and temporal profiles can be generated. In the experiment, we have successfully generated triangular waveforms at repetition rates of 5.64 and 7.87 GHz, and rectangular waveforms at repetition rates of 5.04 and 7.01 GHz.

Enhanced performance in serial-to-parallel data conversion via Raman-assisted time lens processing.

We have demonstrated a new approach to enhance the uniformity of conversion efficiency in serial-to-parallel data conversion via time lens processing. In our approach, Raman amplification is applied to enhance four-wave mixing in a highly nonlinear fiber. By carefully selecting the pump wavelength, the Raman gain profile can be exploited to compensate the roll-off in conversion efficiency resulted from the varying phase mismatch between the linearly chirped pump and the signal. With Raman amplification, improvement of sensitivity up to 6.8 dB has been experimentally obtained. The variation of sensitivity across the output channels has been reduced from 8.4 to 2.0 dB.

Cavity-enhanced thermo-optic bistability and hysteresis in a graphene-on-Si3N4 ring resonator.

Cavity-enhanced thermo-optic bistability is studied in a graphene-on-Si3N4 ring resonator. By engineering the coverage of the monolayer graphene on top of the Si3N4 ring resonator, we observed a two-fold enhancement in the thermo-optically induced resonance shift rate and an 18-fold increase in the effective thermal nonlinear refractive index compared with the devices without graphene. The thermo-optic hysteresis loop was also characterized in this hybrid structure, where the experimental results agree well with the theoretical calculations. This Letter paves the way for graphene-on-Si3N4-based high-speed photodetectors, modulators, and devices for on-chip nonlinear optical applications.

Synergistic Effects of Plasmonics and Electron Trapping in Graphene Short-Wave Infrared Photodetectors with Ultrahigh Responsivity.

Graphene's unique electronic and optical properties have made it an attractive material for developing ultrafast short-wave infrared (SWIR) photodetectors. However, the performance of graphene SWIR photodetectors has been limited by the low optical absorption of graphene as well as the ultrashort lifetime of photoinduced carriers. Here, we present two mechanisms to overcome these two shortages and demonstrate a graphene-based SWIR photodetector with high responsivity and fast photoresponse. In particular, a vertical built-in field is employed in the graphene channel for trapping the photoinduced electrons and leaving holes in graphene, which results in prolonged photoinduced carrier lifetime. On the other hand, plasmonic effects were employed to realize photon trapping and enhance the light absorption of graphene. Thanks to the above two mechanisms, the responsivity of this proposed SWIR photodetector is up to a record of 83 A/W at a wavelength of 1.55 µm with a fast rising time of less than 600 ns. This device design concept addresses key challenges for high-performance graphene SWIR photodetectors and is promising for the development of mid/far-infrared optoelectronic applications.