Fast Adaptive Polarization and PDL Tracking in a Real-Time FPGA-Based Coherent PolDM-QPSK Receiver

Fast polarization changes of 40 krad/s and 6-dB polarization-dependent loss (PDL) are tracked in a 2.8-Gb/s real-time coherent quadrature phase-shift keying receiver. The tolerance against fast polarization changes and PDL is measured for different polarization control time constants. The sensitivity penalty of the receiver at a polarization change speed of 40 krad/s is 0.7 dB at bit-error rate (BER) of $1times 10^{-3}$, with a BER floor of $6.1times 10^{-7}$. With an additional PDL of 6 dB, these figures become 1.7 dB and $9.6times 10^{-6}$, respectively.      

Tapping light from waveguides by high-order mode excitation and demultiplexing

A novel high-performance optical tap with low wavelength-dependent loss(WDL), polarization-dependent loss (PDL), and excess loss is described. The tap is based on exciting a high-order mode by an abrupt change in the waveguide profile, and then tapping the optical power in the high-order mode by using a modal-demultiplexer. The tap can be designed to give a tapping ratio in the range of -30 to -10 dB over a wavelength range of 1510-1640 nm. In addition, it is shown that the tapping ratio is tolerant to process variations, repeatable and predictable, enabling the design to be implemented in a single process run, with no need for additional fine-tuning iterations, while achieving a high process yield. We show that measurements are in good agreement with simulated results. In the tap channel we measured 0.3 dB WDL and 0.2-dB PDL over the C- and L- bands, while the signal in the main channel was virtually uncontaminated relative to a plain single-mode reference waveguide.     

Polarization independent, linear-tuned interference filter with constant transmission characteristics over 1530-1570-nm tuning range

We present a polarization independent optical tunable filter (OTF) with nearly constant transmission characteristics over 1530-1570 nm. The tuning is achieved by horizontally sliding an interference filter, resulting in a very low polarization dependent loss (PDL)<0.1 dB and nearly constant characteristics over the tuning range. By cascading interference filters in double or triple stage, narrower bandwidth characteristics are obtained allowing for smaller channel spacing in wavelength division multiplexing (WDM) systems. Experimental results for PDL, loss, and bandwidth are presented and compared to those of angle-tuned filter. The proposed OTF has shown superior performance promising a practical solution for dense WDM applications.     

Polarization dependent loss in polymeric rib channel waveguide

Large polarization dependent loss (PDL) as high as 15 dB was observed in the rib-type channel waveguide fabricated using a birefringent polymer. The PDL mechanism was explained by the mode-mode coupling between the TM mode of a rib and the TE mode of a slab waveguides. This TM-TE mode conversion is due to nonuniform off-diagonal component of the dielectric susceptibility tensor which is generated by a thermal stress during the curing process.     

Silica-based 71-channel arrayed waveguide grating

With the rapid increase of global information capaci-ty,all optical wavelength division multiplexing(WDM)networks are very attractive because they are capable ofprocessing broadband optical signals without convertingthem to electronic signals.Large channe…     

Robustness of polarization-mode-dispersion compensation in the presence of polarization-dependent loss

The robustness of polarization-mode dispersion (PMD) compensation utilizing radio-frequency feedback in the presence of polarization-dependent loss (PDL) is investigated using a closed-loop dynamic model. Simulations using mean PMD of 30% of the bit period show that mean PDL of up to 2 dB increases the Q-penalty after PMD compensation by less than 0.1 dB. We find that the compensator can become trapped in secondary optima at high PDL, incurring significant additional penalty.     

Receiver designs and channel characterization for multi-spothigh-bit-rate wireless infrared communications

This paper addresses one of the most promising candidates for high-speed in-house wireless communications, namely, the multi-spot diffusing configuration (MSDC). Since it uses the optical infrared medium for data transmission, it has the inherent potential for achieving very high capacities. The channel characteristics in MSDC are simulated and the causes for channel distortion are analyzed. Then, conditions for creation of a virtually ideal channel are derived. It is shown that the 3-dB channel bandwidth can be extended up to beyond 2 GHz. This bandwidth comes at the cost of a poor power efficiency. In order to compensate for that, a novel receiver optical front-end design is proposed and its performance is analyzed. Taking advantage of the unique properties of the holographic optical elements, the conventional optical front-end, consisting of a concentrator and a filter, is replaced by a single holographic curved mirror. The utilization of such a holographic optical element improves the signal-to-shot noise ratio by up to 18.5 dB     

Polarization dependent loss in a Ti:LiNbO/sub 3/ polarization scrambler/controller

We have investigated the polarization dependent loss (PDL) of a waveguide-type polarization scrambler/controller consisting of a half-wave plate (HWP) and a quarter-wave plate (QWP) in series fabricated in x-cut/z-propagating Ti-diffused LiNbO/sub 3/ optical waveguides. Based on the polarization dependences of both the propagation loss coefficient and mode-mismatch loss with fiber at 1.55 /spl mu/m combined with a numerical analysis, the PDL of the fabricated device is estimated to be 0.063 dB. It is shown that the low PDL is a direct consequence of a special anisotropy relation between the propagation loss and waveguide-fiber mode-mismatch loss.     

Design of free space WDM router based on holographic concavegrating

A design for a 91×91 router with 0.33 nm channel spacing using a stationary anastigmatic Littrow mount of a holographic concave grating is reported. A simplified method of concave grating analysis is developed for the evaluation of the wavelength router performance. The coupling losses for all routed channels are within -0.9 dB and the power penalty due to coherent crosstalk caused by aberrations and diffraction at the aperture is negligibly small     

Tunable Polarization-Dependent-Loss Element Based on Rotatable Birefringent Prisms and a Half-Wave Plate

A novel tunable polarization-dependent-loss (PDL) element is realized using two rotatable birefringent prisms and a half-wave plate. The design is favorable for interferometric stability and is operable over a wide wavelength range. The PDL can be tuned from 0 to 35 dB for a wavelength of 780 nm. An interference visibility of 0.98 between the two principal polarizations for 0-dB loss is achieved by compensating for thickness mismatch between the prisms.      

A low power 8th order elliptic low-pass filter for a CMMB tuner

This paper presents an 8th order active-RC elliptic low-pass filter（LPF） for a direct conversion China Mobile Multimedia Broadcasting（CMMB） tuner with a 1 or 4 MHz -3 dB cutoff frequency(f_-3dB).By using a novel gain-bandwidth-product（GBW） extension technique in designing the operational amplifiers（op-amps）, the proposed filter achieves 71 dB stop-band rejection at 1.7f_-3dB to meet the stringent CMMB adjacent channel rejection（ACR） specifications while dissipates only 2.8 mA/channel from a 3 V supply,its bias current can be further lowered to 2 mA/channel with only 0.5 dB peaking measured at the filter’s pass-band edge.Elaborated common-mode（CM） control circuits are applied to the filter op-amp to increase its common-mode rejection ratio （CMRR） and effectively reject the large signal common-mode interference.Measurement results show that the filter has 128 dBμV_rms in-band IIP3 and more than 80 dB passband CMRR.Fabricated in a 0.35-μm SiGe BiCMOS process,the proposed filter occupies a 1.19 mm² die area.     

High-performance all-fiber Fabry-Perot filters with superimposed chirped Bragg gratings

The performance of large-band Fabry-Perot (FP) filters is investigated through numerical simulations and experimental realization. The limitations of the filters are studied theoretically to determine the impact of the chirped fiber Bragg grating (FBG) imperfections on spectral variations of the free spectral range (FSR), finesse, transmission loss, and polarization dependent loss (PDL). All-fiber FP devices are realized experimentally with finesse reaching 240 and covering a spectral band of 26 nm. For finesse up to 80, the filters present acceptable performance in terms of spectral variations of FSR (/spl plusmn/3%), finesse, attenuation (/spl plusmn/0.5 dB), PDL (<0.2 dB), and insertion loss (<3 dB).     

Moment-Generating Function Method Used to Evaluate the Performance of a Linear Optical Communication System

The moment-generating function (MGF) of the received photoelectric current is evaluated for a linear optical communication system consisting of distributed amplified spontaneous emission (ASE), polarization-mode dispersion (PMD), and polarization dependent loss (PDL). Using this function, optical performance characterization based on the bit error rate (BER), ${rm Q}$-factor, and signal-to-noise ratio (SNR), can be evaluated. As an example of the applicability to binary differential phase-shift keying (DPSK) systems with defined PDL, the BER results predicted by linked model and lumped model are compared. Our results indicate that the difference can be orders of magnitude when the PDL is larger than 2.5 dB. Additionally, random PDL induced statistical feature of the BER is entirely different for these two models. Finally, relations between the statistical variations of other performance parameters ( ${rm Q}$-factor and SNR) and link model parameters (input signal polarization, average PDL value, and link number $K$) are also investigated.      

Indoor broadband optical wireless communications: optical subsystems designs and their impact on channel characteristics

With proper system design, infrared multispot diffusing (MSD) configuration communications links promise several orders of magnitude higher bit rates than radio links. Essential to the communications system are the optical subsystems: transmitter and receiver optics. Preliminary experiments on fabrication of beamshaping optical elements for the transmitter and receiver optical front-end have been conducted. The impact of optical subsystems on channel characteristics is investigated, and the results undoubtedly prove the great potential of MSDC. Use of holographic optical elements at both transmitter and receiver increases the signal-to-noise ratio by at least 11 dB and at the same time significantly improves the power budget of the system by reducing path loss by more than 6 dB.     

Polarisation independent tunable liquid-crystal Fabry-Perotinterferometer filter module with double-pass configuration

A novel tunable liquid-crystal Fabry-Perot interferometer (LC-FPI) filter module with a Faraday rotator and double-pass configuration has been fabricated. The filter module has a 3 dB bandwidth of ~0.4 nm, a 20 dB bandwidth of 1.5 nm, and a polarisation dependent loss (PDL) <0.8 dB over a tuning range of 56 nm. More than 49 dB isolation from the output port to the input port has been achieved     

Variable optical attenuator based on ion-exchange technology in glass

A thermooptic variable optical attenuator (VOA) based on the Mach-Zehnder interferometer principle was fabricated in glass by an ion-exchange technique. The thermooptical effect was reached via heating electrodes on the side of the Mach-Zehnder arms. The insertion loss of the device was 1 dB, the dynamic range was 38 dB, maximal power consumption was 138 mW, and the polarization-dependent loss (PDL) was 0.2 dB/0.6 dB at 10 dB/20-dB attenuations, respectively.     

Accurate spectral characterization of polarization-dependent loss

Building on previous work, a rapid automated nonmechanical measurement system for spectral characterization of polarization-dependent loss (PDL) has been developed. A deterministic fixed-states Mueller-Stokes method in conjunction with realtime calibrated spectral information is used to derive wavelength-dependent Mueller matrix elements. Voltage-modulated liquid-crystal variable retarders set the input polarization states. A narrow voltage-tuned filter provides a wavelength sweep following a broadband source; the sweep wavelength is calibrated in realtime by hydrogen cyanide reference lines. This rapid measurement system can measure PDL over a wavelength range of 15 nm in 5 s. A complete uncertainty analysis has been conducted for PDL in the range of 0.05 to 0.3 dB with an expanded uncertainty of 0.0098 dB over the range of 1535 to 1560 nm. Performance was verified using a Fresnel reference. Finally, design and performance results from all-fiber artifact calibration standards are presented.     

Calculation of penalties due to polarization effects in a long-haulWDM system using a Stokes parameter model

We derive a Stokes parameter model to calculate the penalties due to the combination of polarization mode dispersion (PMD), polarization dependent loss (PDL), and polarization dependent gain (PDG) in long-haul, dense wavelength division multiplexed (WDM) systems. In this model, we follow the Stokes parameters for the signal and the noise in each channel instead of following the full time domain behavior of each channel. This approach allows us to determine the statistical distribution of penalties with up to 10⁵ fiber realizations and 40 channels. We validate this model to the extent possible by comparison to full numerical simulations. Using this model, we find that the interaction of PMD and PDL is the major source of penalties and that the effect of PDG is negligible in WDM systems with more than ten channels     

盲均衡化在体全息数据存储中的应用

研究了盲均衡化方法在体全息存储信道中的应用，提出采用非线性盲均衡化的方法改善误码率(BER)，从而提高存储密度。数值分析与初步实验结果表明，盲均衡化方法可以使体全息存储系统的BER得到很大改善，使原始BER从10^-2下降到10^-3。     

Si纳米线阵列波导光栅制备

采用绝缘层上Si(SOI)材料设计制备了3×5纳米线阵列波导光栅(AWG),器件大小为110μm×100μm。利用简单传输法模拟了器件的传输谱,并采用二维时域有限差分(FDTD)模拟中心通道输出光场的稳态分布,模拟结果表明,器件的通道间隔为11 nm,通道间的串扰为18 dB。通过电子束曝光(EBL)和感应耦合等离子(ICP)刻蚀制备了所设计的器件,光输出谱测试分析表明,器件中心通道的片上损耗为9 dB,通道间隔为8.36～10.40 nm,中心输出通道的串扰为6 dB。在误差允许范围内,设计和测试的结果一致。