Multicolour spatial frequency analysis by optical multiplexing

The Fourier transforms of colour transparencies have been separated spatially according to their colour content. The use of a different spatial carrier for each colour enables simultaneous detection of the spectra as well as reconstitution of an image after different spatial filtering processes have been performed on different colour spectra.     

Pilot-based cross-phase modulation compensation for coherent optical orthogonal frequency division multiplexing long-haul optical communications systems

A pilot-based nonlinearity compensator (PB-NLC) is shown in this Letter to be an effective method for compensating cross-phase modulation (XPM) in coherent optical orthogonal frequency division multiplexing (CO-OFDM) systems. An unmodulated pilot tone is transmitted at the center of each OFDM channel to detect phase errors caused by the Kerr effect, which converts intensity fluctuations from all channels to phase errors. The pilots are then used to cancel the XPM phase errors for each OFDM channel at the receiver after each channel's self-phase modulation (SPM) has been compensated, using its intensity waveform to determine its SPM. Numerical simulations of a 58 Gb/s single polarization 2375 km system with inline dispersion compensation show that the signal quality, Q, at the optimal launch power is increased by 2.4 dB if SPM compensation is used before the PB-NLC. This contrasts with only a 0.9 dB improvement if the PB-NLC is used without an SPM compensator for the same link. This shows the PB-NLC can effectively mitigate XPM but not SPM.     

Companding scheme for peak-to-average power ratio reduction in optical orthogonal frequency division multiplexing systems

A companding method for peak-to-average power ratio (PAPR) reduction in optical orthogonal frequency division multiplexing systems has been proposed and simulated. The proposed scheme is based on the modified hyperbolic tangent transform, which can enlarge the small signals and compress the large signals while keeping the average power invariant. Simulation results verify that the proposed companding scheme can markedly decrease the PAPR, and a good bit error rate performance is obtained.     

Steepest-descent method-based adaptive frequency domain equalizer in coherent optical orthogonal frequency division multiplexing with quadrature phase shift keying modulation

An adaptive frequency domain equalizer (A-FDE) in quadrature phase shift keying-coherent optical-orthogonal frequency division multiplexing (QPSK-CO-OFDM) system is proposed in this paper. A-FDE uses steepest descent method to search the optimal equalization parameters. And through applying the adaptive control factor (ACF) to monitor the state of A-FDE, the interference from phase noise (PN) and chromatic dispersion (CD) can be mitigated. The numerical simulation shows that the performance of A-FDE has a remarkable improvement on combating PN and CD comparing with the conventional decision-directed equalizer (DDE). And the A-FDE can achieve the forward-error-correction (FEC) threshold in noisier scenario. Furthermore, because the calculation of ACF only bases on valid data instead of pilot symbols (PSs), A-FDE has low PS overhead merit.     

基于动态混沌映射的三维加密正交频分复用无源光网络

提出了一种基于混沌映射的三维加密正交频分复用无源光网络保密通信系统.该系统通过相关性检测锁定收发端混沌系统参数,实现收发双方混沌系统同步;并利用同步混沌系统生成密钥,实现符号扰动以及二重子载波加密.该加密方案的密钥空间超过10~(86),能够有效对抗穷举攻击.实验实现了13.3 Gb/s基于64进制正交幅度调制的加密正交频分复用信号在25 km标准单模光纤中的传输,并完成了信息的有效解密.     

Mode division multiplexing: from photonic integration to optical fiber transmission [Invited]

To overcome the capacity crunch of optical communications based on the traditional single-mode fiber(SMF), different modes in a few-mode fiber(FMF) can be employed for mode division multiplexing(MDM). MDM can also be extended to photonic integration for obtaining improved density and efficiency, as well as interconnection capacity. Therefore, MDM becomes the most promising method for maintaining the trend of "Moore's law" in photonic integration and optical fiber transmission. In this tutorial, we provide a review of MDM works and cutting-edge progresses from photonic integration to optical fiber transmission, including our recent works of MDM low-noise amplification, FMF fiber design, MDM Si photonic devices, and so on. Research and application challenges of MDM for optical communications regarding long-haul transmission and short reach interconnection are discussed as well. The content is expected to be of important value for both academic researchers and industrial engineers during the development of next-generation optical communication systems,from photonic chips to fiber links.     

Optical orthogonal frequency division multiplexing demultiplexer using slab star coupler-based optical discrete Fourier transform circuit

We demonstrate an optical orthogonal frequency division multiplexing (OFDM) demultiplexer with an optical discrete Fourier transform circuit fabricated using silica planar lightwave circuit technology. This compact device can process an arbitrary number of subcarriers. The operation of a ten-channel device is demonstrated by demultiplexing a 100 Gbit/s (10 subcarrier × 10 Gbits/s) OFDM signal. We also discuss a main factor affecting characteristics degradation of the device.     

Photonic ratchet superlattices by optical multiplexing

We present a method based on incremental holographic multiplexing to create a refractive index ratchet distribution into a photorefractive crystal as an example for the generation principle of such complex multiperiodic lattices. The implemented technique follows a finite optical series expansion of the desired index modulation. To analyze the induced lattice, we determine the phase retardation of a probe beam at the back face of the crystal by digital holography analysis. Our result depicts a first example to optically explore the fascinating phenomena of ratchet resembling systems.     

Sample indexed spatial orthogonal frequency division multiplexing

Optical spatial modulation(OSM) is a multiple-transmitter technique that can provide higher data rates with low system complexity as compared with single-input single-output systems. Orthogonal frequency division multiplexing(OFDM) is widely implemented to achieve better spectral efficiency in wireless channels. Asymmetrically clipped optical OFDM(O-OFDM) and DC-biased O-OFDM are two well-known O-OFDM techniques suitable for intensity-modulation direct-detection optical systems. In this work, sample indexed spatial OFDM(SIS-OFDM) is proposed to combine OSM and O-OFDM in a novel way and achieve significant performance gain. By assigning time-domain samples of the O-OFDM transmit symbol to different transmitters, SIS-OFDM achieves much better spectral efficiency and reduces computational complexity at the transmitter as compared with previous work that combines OSM with O-OFDM in the frequency domain. We also consider the impact of optical source biasing on overall performance, and the relative performance of imaging receiver(ImR) versus non-imaging receiver(NImR) design for our proposed SIS-OFDM technique. Results indicate that for an Ntx × Nrx multiple-input multiple-output configuration where Ntx = Nrx = 4, SIS-OFDM using ImR can achieve up to 135 dB of signal-to-noise ratio gain over comparable system using a NImR. Also, using Nsc number of O-OFDM subcarriers provides up to Nsc × log2(Ntx) additional bits per symbol of spectral efficiency over techniques that combine OSM and O-OFDM in the frequency domain.     

Interlacing multiplexing techniques for optical morphological correlation

We propose a novel approach to implement nonlinear morphological correlation. Previous implementation was based on a time sequential approach that consists on displaying different binary image decomposition in a joint transform correlator adding each joint power spectra sequentially. A second Fourier transformation of the sum of joint power spectra gives the correlation output. In this paper, we propose to interlace the different binary images into one single distribution. Then, we introduce the distribution in a conventional joint transform correlator. The correlation output gives the morphological correlation at a specific location. The advantage is important considering that no sequential approach is needed anymore, so the necessary number of correlations is reduced. Optical implementation results are provided.     

Chaotic wavelength division multiplexing for optical communication

We demonstrate wavelength division multiplexing with chaotic subcarriers in two pairs of one-way-coupled Nd:YVO4 microchip lasers. Two individual digital messages are encoded on two chaotic carriers with different wavelengths in two transmitters by the chaotic masking method, and the mixed signals are sent to two receivers. Dual synchronization of chaos is used to obtain the original chaotic waveforms in the receivers. Two messages are decoded by filtering the difference of the laser outputs between the transmitter and the receiver by use of a low-pass filter for each pair of lasers. The message recovery can be achieved more easily when messages with small amplitude and low frequency are used.     

Survivability strategies with backup multiplexing in WDM optical networks

In this paper, we have presented four variations of applying the same routing algorithm for primary and backup lightpaths, wavelength assignment strategy for survivability. The simulation results show that although everything is the same, yet how and when they are applied leads to variations in results in terms of number of connections accepted. The backup multiplexing technique has been incorporated to reduce the blocking probability in all the strategies. The results have been calculated both for the systems that require 100% degree of survivability, i.e. critical, and for those that do not. The variation to be used depends upon whether the application is critical or not.     

Optical amplitude multiplexing through parametric amplification in optical fibers

We propose a new technique that uses parametric amplification in optical fibers to combine two binary signals into a single quaternary amplitude-shift keying (4-ASK) signal. We develop a theoretical model to predict the power level distribution of the 4-ASK signals as a function of the extinction ratios of the input binary signals. Computer simulation results agree with the predictions of this theoretical model within a 0.9 dB margin. We also analyze the application of this technique to the generation of quaternary-amplitude optical packets, which are attractive for allowing the label to be transmitted in the same bandwidth and simultaneously to the payload. Our results indicate that such optical packets may be propagated through lengths comparable to those involved in metropolitan area networks.     

基于光时分复用技术的高速成像系统

提出了一种基于光时分复用技术的高速成像系统。飞秒激光器中心波长1557 nm,脉冲宽度90 fs,对USAF-1951分辨率板线性扫描成像,扫描频率为38.88 MHz。在连续时间序列编码放大显微成像技术的基础上,运用光时分复用技术,复制光脉冲信号并携带检测物体相同的空间信息。原光脉冲和复制光脉冲以相同的采样率分别采样,通过相应的数据处理将两次采样数据整合在一起还原图像。实验结果表明,与传统的超快成像方法相比,成像系统利用10 GHz的数字采样设备可以达到20 GHz的采样率,采样点数是传统超快成像方法的两倍。该方法有效克服了成像系统采样率不足的问题,提高了成像系统的空间分辨率。与此同时,该系统算法复杂程度不高,有利于进一步促进超高速成像技术的发展。     

光波分复用实验的改进和扩展

利用自聚焦透镜代替集成的波分复用器件,对2路波分复用实验结构进行了改进,并在实验中增加光谱仪用于观察光纤中的光谱.通过上述改进,使学生进一步了解波分复用器内部结构,加深对复用和解复用的理解.     

基于光时分复用技术的高速成像系统北大核心CSCD

提出了一种基于光时分复用技术的高速成像系统。飞秒激光器中心波长1557nm,脉冲宽度90fs,对USAF-1951分辨率板线性扫描成像,扫描频率为38.88 MHz。在连续时间序列编码放大显微成像技术的基础上,运用光时分复用技术,复制光脉冲信号并携带检测物体相同的空间信息。原光脉冲和复制光脉冲以相同的采样率分别采样,通过相应的数据处理将两次采样数据整合在一起还原图像。实验结果表明,与传统的超快成像方法相比,成像系统利用10GHz的数字采样设备可以达到20GHz的采样率,采样点数是传统超快成像方法的两倍。该方法有效克服了成像系统采样率不足的问题,提高了成像系统的空间分辨率。与此同时,该系统算法复杂程度不高,有利于进一步促进超高速成像技术的发展。     

Novel applications of one-dimensional photonic crystal in optical buffering and optical time division multiplexing

Novel applications of one-dimensional photonic crystal in optical buffering and optical time division multiplexing are suggested. These one-dimensional photonic crystal optical buffers can store data packets temporarily for a certain period of time, which depend on the lattice parameters. By changing the lattice parameters the data hold time can be changed in a desired way. This type of optical buffer finds potential application in contention resolution in optical switch of a router, in a packet-switched optical network and in design of novel optical time division multiplexers (OTDM).     

Volume hologram scheme using optical fiber for spatial multiplexing

Speckle patterns from optical fibers are used for reference beams in writing and reading volume holograms. The photorefractive volume hologram with the scheme shows good spatial selectivity for spatial (shift) multiplexing. Experimental results are presented and compared with numerical analysis.     

Rapid updating of optical arbitrary waveforms via time-domain multiplexing

We demonstrate high-fidelity optical arbitrary waveform generation with 5 GHz waveform switching via time-domain multiplexing. Compact, integrated waveform shapers based on silica arrayed-waveguide grating pairs with 10 GHz channel spacing are used to shape (line-by-line) two different waveforms from the output of a 10-mode x 10 GHz optical frequency comb generator. Characterization of the time multiplexer's complex transfer function (amplitude and phase) by frequency-resolved optical gating permits compensation of its impact on the switched waveforms and matching of the measured and target waveforms to better than G'=5%.