基于光纤光参量放大的多通道全光非归零/归零码转换器

提出了一种基于光纤光参量放大器(FOPA)的多通道全光非归零码(NRZ)/归零码(RZ)调制格式转换的方案.该方案中,非归零码信号与同步的时钟抽运光共同注入到高非线性光纤(HNLF)中,由高非线性光纤构成的参量放大器把非归零码信号转换为归零码信号,同时不改变信号光的波长.多通道的码型转换器以两路10 Gb/s的非归零码进行了实验论证.转换后的归零码信号的信噪比(SNR)高于7.6 dB,其脉冲宽度约为30 ps,并且具有3dB的消光比(ER)提高.根据多通道码型转换器的实现原理,该码型转换器可以应用于40 Gb/s或更高比特率的多通道码型变换操作.     

光纤传输系统中基于相位预调制的信号整型

利用相位预调制技术解决高速长距离光纤传输系统中面临的接收灵敏度降低和色散容限问题。通过在发射端对非归零(NRZ)的光信号进行比特同步相位预调制,使非归零码在传输过程中得到波形重整,演变为归零(RZ)的波形,从而提高眼图开启度。实验观测了普通非归零码和相位预调制的非归零码在不同相位调制深度和色散下的光谱、眼图和功率代价。10Gb/s的传输结果表明,链路色散绝对值小于1000ps/nm时,施加相位预调制的非归零信号功率代价小于1dB,比普通非归零信号具有更高的接收灵敏度和更低的色散功率代价。因此,基于相位预调制的信号整型技术可减轻系统对光信噪比和色散管理的要求,延长传输距离。     

Low insertion loss and low dispersion penalty InGaAsP quantum-well high-speed electroabsorption modulator for 40-Gb/s very-short-reach, long-reach, and long-haul applications

We present a metal-organic-chemical-vapor-deposition-grown low-optical-insertion-loss InGaAsP/InP multiple-quantum-well electroabsorption modulator (EAM), suitable for both nonreturn-to-zero (NRZ) and return-to-zero (RZ) applications. The EAM exhibits a dynamic (RF) extinction ratio of 11.5 dB at 1550 nm for 3 Vp-p drive under 40-Gb/s modulation. The optical insertion loss of the modulator in the on-state is -5.2 dB at 1550 nm. In addition, the EAM also exhibits a 3-dB small-signal response (S21) of greater than 38 GHz, allowing it to be used in both 40-Gb/s NRZ and 10-Gb/s RZ applications. The dispersion penalty at 40 Gb/s is measured to be 1.2 dB over /spl plusmn/40 ps/nm of chromatic dispersion. Finally, we demonstrate 40-Gb/s transmission performance over 85 km and 700 km.     

Performance degradations of 2.4 Gbit/s NRZ/RZ lightwave systems dueto reflection-induced phase-to-intensity-noise conversion

Performance degradations in 2.4-Gb/s NRZ (nonreturn to zero) and RZ lightwave systems due to phase-to-intensity-noise conversion between two connectors have been evaluated using computer simulation techniques. Both NRZ and RZ systems have approximately the same penalty if the roundtrip time delay between the two connectors is an exact integer number of bits. If the roundtrip time delay is slightly offset, however, the RZ system penalty is significantly reduced. For example, the RZ system penalty is reduced from 3 dB (roundtrip delay between the two connectors=40 b) to 1.5 dB (roundtrip delay=40.5 b) for two connectors with 8-dB return loss each     

40Gb/s光纤通信系统中不同码型传输特性的实验研究

在高速光纤通信系统中码型的选择是决定系统传输质量和光谱效率的主要因素。码型的选择和信道速率、信道波长间隔、光放大器的选择、光放大器放置间隔、光纤的类型、色散管理策略等各种因素密切相关。分析了非归零码（NRZ）、归零码（RZ）和载波抑制归零码（CSRZ）码型的产生方式及特点。采用单信道和掺铒光纤放大器（EDFA）放大方式对三种码型进行了40Gb/s的100kmG.652光纤通信传输实验。比较了三种码型的系统传输持性、最佳入纤功率和不同入纤功率下的功率代价：载波抑制归零码最佳入纤功率为9dBm，功率代价小于非归零码和归零码。结果表明，在相同的色散补偿条件下，载波抑制归零码比归零码和非归零码有更优的非线性容忍度。     

Effects of semiconductor-optical-amplifier nonlinearity on theperformance of high-speed intensity-modulation lightwave systems

It is shown that when the amplifier is driven near saturation, its inherent nonlinearity causes significant bit-pattern-dependent pulse distortion, particularly in the bit-rate range between about 2 and 32 GB/s. Without proper countermeasures, this distortion can degrade system performance appreciably due to two basic mechanisms. The first, which can result in a system power penalty of as much as 10 dB, occurs in a standard decision circuit that automatically sets the threshold voltage to the average signal level, rather than in the middle of the eye opening. The second mechanism, which occurs even with the threshold set properly, is due to the nonlinear enhancement of the simple linear intersymbol interference (ISI) within the receiver filter. For example, computations of system performance at 8 Gb/s using an RC filter that gives a quite acceptable 10% of eye closure under linear conditions show that when the amplifier is driven to its saturation output power level, this mechanism causes a system power penalty of about 1 dB, which increases to about 4.5 dB when the power is doubled. Interestingly, with the proper threshold setting, an ideal integrate-and-dump receiver, which introduces no ISI, is shown to suffer no power penalty due to amplifier nonlinearity     

All-Optical Conversion From RZ to NRZ Using Gain-Clamped SOA

An all-optical converter from return-to-zero (RZ) pulses to the nonreturn-to-zero (NRZ) format is presented. The converter operates in two stages: the laser generated in a gain-clamped semiconductor optical amplifier (SOA) is modulated with the data signal; afterwards this signal is wavelength-converted by cross-gain modulation in a common SOA. The setup is noninverting and can feature wavelength conversion. Experimental error-free conversion from 5- and 40-ps RZ pulses to NRZ format is presented at 10 Gb/s using a 2¹¹-1 bit sequence     

All-Optical Decision-Gating of 10-Gb/s RZ Data in a Semiconductor Optical Amplifier Temporally Gain-Shaped With Dark-Optical-Comb

We demonstrate a novel all-optical noninverted OC-192 return-to-zero (RZ) decision-gate by using a semiconductor optical amplifier (SOA) which is gain-controlled to achieve an extremely high cross-gain-modulation depth and a narrow gain window. A dark-optical-comb generated by reshaping the optical clock RZ data in a Mach-Zehnder intensity modulator is employed as an injecting source to temporally deplete most of the gain in the SOA. Such a dark-optical-comb injected SOA decision-gate exhibits improved 3R regeneration performances such as a timing tolerance of 33.5 ps, Q -factor of 8.1, an input dynamical tolerance of 14 dB, and an extinction ratio (ER) of 14 dB. The deviation between the wavelengths of backward injected dark-optical-comb and input RZ data for optimizing the ER of the decision-gate is determined as Deltalambda=19 nm. Under a threshold operating dark-optical-comb power of 7 dBm, such a decision-gate can recover the -18.5-dBm degraded RZ data with a bit-error-rate of less than at 10^-9 Gb/s. A negative power penalty of -4.2 dB is demonstrated for the RZ data after 50-km propagation and decision gating.     

Optimal pump wavelength in the4I15/2-4I13/2 absorption band for efficient Er3+-doped fiberamplifiers

The authors report the measured gain of a highly efficient erbium-doped fiber amplifier pumped at wavelengths between 1.46 and 1.51 μm. The optimal pump wavelength, λ_opt, was determined to be 1.475 μm. At this wavelength, the maximum gain coefficients for signals at 1.531 and 1.544 μm were 2.3 and 2.6 dB/mW, respectively. At λ_opt, high gains ranging from 32 dB at pump power P_p=20 mW up to 40 dB at P _p=80 mW were obtained. These modest pump powers are within the capabilities of currently available 1.48-μm diode lasers. The width about λ_opt for 3-dB gain variation exceeded 27 nm for P_p=10 mW and 40 nm for P_p >20 mW. With this weak dependence on pump wavelength, single-longitudinal-mode lasers do not have a significant advantage over practical Fabry-Perot multimode pump lasers     

Performance of π/4 SQAM in a hard-limited channel in thepresence of AWGN

A strategy that reduces the spectral spreading when an ideal hard-limiter is used as a first approximation to a fully saturated power amplifier is presented. This strategy combines superposed quadrature amplitude modulation (SQAM) filtering with the π/4-shift quadrature phase-shift keying (QPSK) digital transmission format adopted for the first generation of US digital cellular systems. Simulation results showed that this π/4 SQAM filtering strategy increased capacity by 35% in comparison to hard-limited π/4 QPSK. Using computer-aided design, a receive filter that would limit the degradation of E _b/N₀ to less than 1.4 dB at a bit error rate of P_e=10^-4 was selected     

Extinction ratio improvement and wavelength conversion based onfour-wave mixing in a semiconductor optical amplifier

We have demonstrated an extinction ratio improvement of more than 3 dB at 2.5 Gb/s with a method based on four-wave mixing in a semiconductor optical amplifier. A wavelength conversion and a data format conversion from nonreturn-to-zero (NRZ) to return-to-zero (RZ) have been simultaneously achieved     

2500 km-10 Gbps RZ transmission system based on dispersion compensation CFBGs without electric regenerator

The characteristics of chirped fiber Bragg gratings (CFBGs) are optimized so that the ripple coefficient of the power reflectivity spectrum and group time delay are less than 1 dB and |± 15| ps, group delay is about 2600 ps/nm, polarization module dispersion is very small, PMD<2 ps, -3 dB bandwidth is about 0.35 nm, and insertion loss is about 4-5 dBm. Using dispersion compensation CFBG, a 2500 km-10 Gbps RZ optical signal transmission system on G.652 fiber was successfully demonstrated without an electric regenerator by optimizing dispersion management and loss management. The RZ optical signal was generated through a two-stage modulation method. At 2081 km, the power penalty of transmission is about 3 dB (conditions: RZ signal, BER = 10-12, PRBS = 1023 - 1); At 2560 km, the power penalty is about 5 dB. It is superior to the system using NRZ under the same conditions.     

Optical and noise characteristics of amorphous Si/SiC superlatticereach-through avalanche photodiodes

In order to improve the performance of the a(amorphous)-Si:H/SiC:H superlattice avalanche photodiode (APD), a-Si:H/SiC:H superlattice reach-through APDs (SRAPDs) have been fabricated on ITO(indium tin oxide)/glass substrates by plasma-enhanced chemical vapor deposition (PECVD). For a typical electron-injection SRAPD, the ratio of room-temperature electron and hole impact ionization rates (α/β) is 10.2 at an electric field 3.33×10⁶ V/cm, the optical gain is 506 at an applied reverse-bias V_R=18 V and an incident power P_in=5 μW emitted from a He-Ne laser, the rise time is 1 μs at a load resistance R_L 1 kΩ, and the excess noise factor is 6.53 at a multiplication M=48     

A Taylor synthesis for principal solution patterns ofmultiplicative antenna systems

A multiplicative (cross-correlation) receiving antenna system with a linear aperture can have a power pattern P₀(u ) (the so-called principal-solution power pattern) whose spatial frequency transfer function (SFTF) is uniform over the entire spatial frequency (SF) bandwidth. A modified principal solution system which retains the uniform SFTF except for smooth transitions at the ends of the SF passband is described. The transitions are due to a change in the original pattern P₀(u), which suffers from high sidelobes, to a Taylor (1955) synthesis pattern P_T (u) which involves a slowly varying envelope pattern. All of the slowly varying envelope sidelobes of P_T(u ) are set at the same appropriate low level, e.g. -30 dB. The aperture weighting distributions are free of singularities, unlike those for P₀(u), and can be sampled to provide the current weightings for a linear multiplicative array     

2.5 Gb/s laser-driver GaAS IC

A laser-diode driver GaAs IC incorporating an optional NRZ/RZ (non-return-to-zero/return-to-zero) conversion facility, having ECL (emitter-coupled logic) and SCFL (source-coupled FET logic)-compatible inputs and providing a 0-60-mA adjustable output current into a 50-Ω/5-V termination at bit rates up to 2 Gb/s NRZ and maintaining a clear eye opening of 50 mA at 2.5 Gb/s NRZ bit rate has been designed, using a commercial 1-μm gate-length (F_τ=12 GHz) GaAs MESFET foundry service. The high maximum output current is obtained by implementing the output driver as a cascode differential amplifier. The logic circuitry implemented using a novel, DCAL (diode-clamped active-load) SCFL family, which is based on gate-width scaling rather than on absolute values, so that the on-chip logic voltage swing is less sensitive to process variations than conventional SCFL. A 60% improvement in noise margin is also obtained. To verify laser driving performance a back-to-back optical-fiber transmission experiment was performed, giving good optical eye diagrams at 2.5 Gb/s. The electrooptical interplay between laser-diode driver and laser-diode has been demonstrated using SPICE simulations     

Amorphous silicon/silicon carbide superlattice avalanchephotodiodes

An a-Si/SiC:H superlattice avalanche photodiode (SAPD) has been successfully fabricated on an ITO/glass substrate by plasma-enhanced chemical vapor deposition. The room-temperature electron and hole impact ionization rates, α and β, have been determined for the a-Si/SiC:H superlattice structure by photocurrent multiplication measurements. The ratio α/β is 6.5 at a maximum electric field of 2.08×10⁵ V/cm. Avalanche multiplications in the superlattice layer yields an optical gain of 184 at a reverse bias V_R=20 V and an incident light power P_in=5 μW. An LED-SAPD photocouple exhibited a switching time of 4.5 μs at a load resistance R-1.8 kΩ     

Tunable dispersion compensation at 40-Gb/s using a multicavity etalon all-pass filter with NRZ, RZ, and CS-RZ modulation

We present a multichannel tunable dispersion compensator (TDC) based on multicavity all-pass etalons that is capable of operation at 40 Gb/s. The device has a tuning range of +200/-220 ps/nm with a group delay ripple < /spl plusmn/5 ps over a channel bandwidth of 80 GHz, an overall loss of < 5.2 dB, very low insertion loss ripple, and can operate on any channel on a 200-GHz grid over the C-band. In addition, we present system performance results at 40 Gb/s using NRZ, RZ, and CS-RZ modulation, compensating up to 45 km of nonzero dispersion shifted fiber (NZDSF). Our results show that this device introduces very little excess system penalty with signal frequency drifts of up to 20 GHz when operated near the center of its tuning range. For single channel experiments with fiber, the system penalty increase versus signal detuning is more significant, but can be reduced by dynamically optimizing the device dispersion during detuning. Finally, we demonstrate simultaneous compensation of 4 channels across the C-band over 25 km of NZDSF.     

NRZ versus RZ in 10-40-Gb/s dispersion-managed WDM transmissionsystems

We compare nonreturn-to-zero (NRZ) with return-to-zero (RZ) modulation format for wavelength-division-multiplexed systems operating at data rates up to 40 Gb/s. We find that in 10-40-Gb/s dispersion-managed systems (single-mode fiber alternating with dispersion compensating fiber), NRZ is more adversely affected by nonlinearities, whereas RZ is more affected by dispersion. In this dispersion map, 10- and 20-Gb/s systems operate better using RZ modulation format because nonlinearity dominates. However, 40-Gb/s systems favor the usage of NRZ because dispersion becomes the key limiting factor at 40 Gb/s     

Nonlinear effects in coherent multichannel transmission throughoptical fibers

Various nonlinear optical interactions in single-mode fibers that are used in coherent FDM (frequency division multiplexed) transmission systems are examined. It is these nonlinearities that lead to crosstalk between channels, power losses, and deleterious fluctuations, which in turn limit the power of the transmitted light and the number of allowed channels, and dictate the channel allocations. It is shown that, for long-haul transmission systems with fiber lengths exceeding 100 km, typical channel separation of 10 GHz, and few channels, the maximum allowed input power per channel, P_max, is limited by SBS (stimulated Brillouin scattering) to about 5 dBm. As the number of channels increases, FWM (four wave mixing) becomes the limiting process with P_max of about -5 dBm, whereas above several hundred channels SRS (stimulated Raman scattering) becomes dominant with P_max of about -5 dBm. For local area networks with shorter lengths, the results are similar, except that the values of P_max are uniformly higher by about 5 dB     

8-element linear array monolithic p-i-n MODFET photoreceivers usingmolecular beam epitaxial regrowth

An 8-element linear array of single-stage integrating front-end photoreceivers using molecular beam epitaxial (MBE) regrowth was investigated. Each element consisted of a p-i-n In_0.53Ga_0.47As photodiode integrated with a selectively regrown pseudomorphic In_0.65Ga_0.35As/In_0.52Al_0.48 As MODFET. Cutoff frequencies of 1.0-μm discrete regrown MODFETs were f_t=24 GHz and f_max=50 GHz. Transconductance of the regrown MODFETs was as high as 495 mS/mm with a current density (I_ds) of 250 mA/mm. The 3-dB bandwidth of the photoreceiver was measured to be 1 GHz. The bit rate sensitivity at 1 Gb/s was -31.8 dBm for BER 10^-9 using 1.55 μm excitation for a photoreceiver with an anti-reflection coating. The single-stage amplifier exhibited up to 25 dB flatband gain of the photocurrent, and a two-stage amplifier was up to 31 dB of gain. Good uniformity between each photoreceiver element in the array was achieved. Electrical crosstalk between photoreceiver elements was estimated to be ~-34 dB