Photorefractive phase conjugators

The author presents a tutorial review of various kinds of optical phase conjugators using photorefractive materials. These include externally pumped phase conjugators (EPPCs), self-pumped phase conjugators (SPPCs), and mutually pumped phase conjugators (MPPCs). He briefly describes optical phase conjugation and the photorefractive effect. He also describes wave mixing in photorefractive media. The energy transfer in two-wave mixing and the holographic nature of four-wave mixing are discussed     

Theory of photorefractive phase conjugators with mutuallyincoherent beams

Plane-wave analysis of the recently demonstrated photorefractive phase conjugators with mutually incoherent beams is presented. Based on four-wave mixing theory, two important configurations, the bird-wing phase conjugator and the mutually incoherent beam coupler are investigated by solving coupled nonlinear equations with beam depletion and absorption. Various intensity performances are theoretically analyzed. Different configurations of this kind of mutually incoherent phase conjugators are compared. Competition of different processes is discussed. Experimental data are well fitted by the theory     

Noise characteristics of fiber optical parametric amplifiers

We derive an analytical theory for the noise figure of an undepleted and lossless fiber optical parametric amplifier (FOPA). Both the signal and the wavelength converted idler are investigated. Our theory is applicable for both an ideal pump power source, as well as a noisy one. We find that a noisy pump source can severely degrade the performance at high gain due to the stochastic gain-variations the signal and idler will experience. The theory is compared with Monte Carlo simulations of the FOPA and an excellent agreement is obtained. Simulations in the gain-depleted region show the possibility to reach below quantum-limited, phase-insensitive amplification for single channel transmission.     

Nonlinear distortion generated by DSF-based optical-phase conjugators in analog optical systems

In this paper, the nonlinear distortion induced by dispersion-shifted-fiber-based optical-phase conjugators (OPCs) in intensity-modulated subcarrier multiplexing optical systems is evaluated. In this study, it is shown that the nonlinear distortion mainly arises from the four-wave mixing (FWM) process during the phase conjugation, although there also exists influences from other nonlinear effects. Closed expressions for calculating the second- and third-order harmonic distortions due to FWM, self-phase modulation, cross-phase modulation and group-velocity dispersion effects in the dispersion-shifted-fiber-based OPC are also reported for the first time. The influence of several system design parameters, such as the optical modulation index, the number of channels, the input optical powers, and the effective area of the dispersion-shifted fiber on the compensation of fiber-induced nonlinear distortions employing the optical-phase conjugation technique, is considered.     

光纤飞秒光梳频率精密控制的研究

新一代光纤飞秒光梳在精密测量和基础物理研究领域中有着广阔的前景,一种通过数字电荷泵锁相环和温控电路相结合的方法可以精密控制光纤飞秒光梳。在实验中,我们成功搭建了光纤飞秒光梳系统。光梳的重复频率为129MHz,初始频率大约为33MHz。我们通过自主研发的温控和数字电荷泵锁相环成功地把光纤飞秒光梳成功地锁定在了由Agilent PSG Analog Singal Generator提供的标准微波信号上,锁定时间至少长达1天。锁定后的重复频率的抖动标准差为0.78mHz,与基准源同一个数量级且很接近,锁定后的初始频率的抖动标准差可达8.98Hz。     

Optimization of DSF- and SOA-based phase conjugators byincorporating noise-suppressing fiber gratings

We compare the performance of dispersion-shifted-fiber (DSF) and semiconductor-optical-amplifier (SOA) based laser phase conjugators for a 10-Gb/s nonreturn-to-zero system with respect to conversion efficiency, noise figure, and distortion. Fiber gratings are used for signal extraction and amplified spontaneous emission (ASE) suppression, allowing closer wavelength spacing and reducing the conjugation noise figure by up to 12 dB. Despite the higher SOA conversion efficiency, both conjugators give similar noise figures with ASE suppression. However, the DSF-based conjugator has the advantage of distortion tolerance at higher input power     

Amplification of WDM signals in fiber-based optical parametric amplifiers

We demonstrate for the first time, experimentally, the performance of fiber-based optical parametric amplifiers in wavelength-division-multiplexed (WDM) applications. Both a 3 /spl times/ 10 Gb/s and a commercial 7 /spl times/ 2.5 Gb/s WDM system are investigated together with the parametric amplifier. Limitations due to pump depletion and four-wave mixing are quantified. Measurements showing the performance in terms of power penalty and gain versus input-output signal power are presented.     

Noise characteristics in line-narrowed semiconductor lasers with optical feedback

Effects of optical-feedback-induced line-narrowing on low-frequency amplitude and phase noise of semiconductor lasers are reported. Free-running laser noise is compared with that of lasers operating with external mirror feedback. While less than 1 dB difference in the amplitude noise with and without feedback is observed, line-narrowed lasers exhibit 15?20 dB reduction in low-frequency wavelength instability, or phase noise.     

Noise characteristics of nonlinear semiconductor optical amplifiersin the Gaussian limit

This paper addresses noise properties of nonlinear semiconductor optical amplifiers. From a basic point of view, noise properties of nonlinear optical amplifiers are sufficiently different from those of linear amplifiers to warrant detailed modeling which has not been formulated previously. From a practical point of view, nonlinear semiconductor optical amplifiers are important for future all-optical signal-processing applications which may involve the operation of these devices in a saturated regime. Nonlinear amplifiers are also common in systems operating near 1300 nm and in integrated booster amplifiers. Under nonlinear operating conditions, amplifier noise contains a narrow-band contribution that comes about due to the nonlinear coupling of noise and gain. The more conventional broadband spontaneous noise also changes as the inversion factor becomes power-dependent and varies along the amplifier axis. We analyze noise in nonlinear amplifiers in the Gaussian limit (meaning, for fields consisting of large photon numbers) for CW or NRZ modulated signals and separately for short pulses. We consider the case of a single input as well as configurations of multi-input signals interacting via four-wave mixing. Using a specific detection system for the calculations of electronic signal-to-noise ratios, we demonstrate a reduction in the narrow-band electronic noise due to saturation in the single input case. We also demonstrate a vast advantage of using short pulses in four-wave-mixing applications     

Noise characteristics of high-frequency superposed laser diodes for optical disc systems

Excess noise peaks in an index-guided single-mode laser can be suppressed by superposing high-frequency current. By this method, index-guided lasers have stable noise characteristics and the noise level is about 7?12 dB lower than that in gain-guided multimode lasers at output power of about 3 to 5 mW.     

直接检测光OFDM相位特性分析

理论分析了直接检测光正交频分复用(DDO-OFDM)传输系统中信号的相住特性.分别讨论了与频率有关的非随机性噪声和其他随机性噪声对信号的影响;并在此基础上提出了一种简便的相位均衡方法.推导了系统Q参数与光信噪比之间的关系式.仿真验证了此相位特性和均衡方法的可行性.     

Nonlinear spectral shaping and optical rogue events in fiber-based systems

We provide an overview of our recent work on the shaping and stability of optical continua in the long pulse regime. Fibers with normal group-velocity dispersion at all-wavelengths are shown to allow for highly coherent continua that can be nonlinearly shaped using appropriate initial conditions. In contrast, supercontinua generated in the anomalous dispersion regime are shown to exhibit large fluctuations in the temporal and spectral domains that can be controlled using a carefully chosen seed. A particular example of this is the first experimental observation of the Peregrine soliton which constitutes a prototype of optical rogue-waves.     

Nanofilms on hollow core fiber-based structures: an optical study

The optical characteristics of one multimode fiber (MMF)-hollow core fiber (HCF)-structure when a nanofilm is deposited on it has been theoretically and experimentally studied. The electrostatic self-assembly method has been used as the deposition technique, and the polymers chosen are polydiallyldimethylammonium and Poly R-478. Two different types of HCF have been used for the fabrication of the devices: 10/150 and 50/150 /spl mu/m inner and outer diameters, respectively. Depending on several design parameters, the transmitted optical-power characteristic of the device experiences important changes that could be interesting towards development of several practical optical devices. The length and thickness of the HCF segment, the refractive index of the material deposited, the angle of the light when it reach the HCF section, and the wavelength of the light source will be analyzed.     

Applications of large-scale optical 3D-MEMS switches in fiber-based broadband-access networks

Applications of non-blocking large-scale optical switches based on three-dimensional micro-electro-mechanical system (3D-MEMS) technology with small size and low power consumption are described for fiber-based broadband access networks. The low-loss and fast-switching 3D-MEMS switches offer remotely reconfigurable and automated operational solutions for access networks such as fiber management, preventative maintenance, monitoring, testing, and troubleshooting of a large number of end customers. Furthermore, the wavelength, data rate, and protocol-transparent nature of 3D-MEMS switches results in a future-proof optical distribution network design for future higher speed and higher capacity wavelength division multiplexing overlay upgrades over the passive optical network (PON). We show that large-scale 3D-MEMS switches deployed in PON environments can offer over an order of magnitude in capital and operational savings in comparison to manual patch panels (in deployed fiber hardware, real estate, and manual labor) with minimal impact on the overall network design.     

Noise power spectra of optical two-beam interferometers induced by the laser phase noise

Dynamic range of many optical signal processing and sensing devices incorporating two-beam interferometers (such as Mach-Zehnder and Michelson interferometers) can be limited by random phase fluctuations of the optical source emission field. This paper is concerned with the intensity fluctuations, originating from the laser source phase noise, at the output of a two-beam interferometer. Closed-form theoretical expressions are presented for the autocovariance function, variance, and noise power spectral density of the instantaneous output intensity which are valid for any optical biasing phase and any source coherence time. Application of the results in the noise performance evaluation of optical devices incorporating two-beam interferometers will also be shown.     

Noise characteristics of a single-mode laser diode subject to strong optical feedback

An experimental study has been performed of the relative intensity noise (RIN) of a semiconductor laser in optical feedback regimes I to V. At low bias current, a low RIN is observed with low feedback ratio, the RIN increased in the coherence collapse regime (regime IV) and decreased in regime V. The RIN in regime V is lower than that of the solitary laser. For higher bias current, a higher feedback ratio is needed for the semiconductor laser to transit from regime IV to V. The measurements are found to be in good qualitative and quantitative agreement with theoretical predictions.     

Noise and distortion characteristics of semiconductor lasers in optical fiber communication systems

The distortion and noise characteristics of semiconductor lasers in connection with optical fibers are reviewed. In particular, the intrinsic distortions and noise of semiconductor lasers together with the partition noise are discussed followed by a discussion on the influence of reflections. Modal noise phenomena due to the interference pattern at the endface of optical fibers are treated with respect to noise and distortions. Finally, the influence of polarization coupling in single-mode fibers on the resulting transmission behavior is discussed.     

Modulation and detection characteristics of optical continuous phase FSK transmission system

The modulation and differential detection characteristics of optical CPFSK transmission systems are investigated both theoretically and experimentally. The error rate expressions of differentially detected CPFSK are derived by considering phase noise of LD's. It is clear that the linewidth requirement is less than0.68 mpercent of the bit rate, wheremis modulation index. The performances of CPFSK are then experimentally presented at 400 Mbit/s using external optical feedback DFB LD's as the optical source. A beat spectral linewidth of less than 200 kHz for the transmitter and local oscillator LD's is achieved. The frequency response nonuniformity of frequency modulation efficiency is compensated by electrical circuits within 3 dB and 60°. To reduce IF thermal noise, a resonance-type preamplifier is used, with a 4.8 pA/sqrt{Hz}average input noise current density, and a receiver sensitivity 1.3 dB better than the conventional preamplifier. Differential detection of the 400-Mbit/s CPFSK modulation is performed. The generation of CPFSK is confirmed by good correlation between the output spectrum and theory. The average received optical power at a 10^-9bit error rate is -49.9 dBm which improves direct detection by 10.3 dB. No additional power penalties due to 290-km transmission exist.     

Noise in optical low-coherence reflectometry

It is theoretically and experimentally shown that Fresnel end reflection at a waveguide under test can degrade the sensitivity of an optical low-coherence reflectometer (OLCR) with a balanced detection scheme. Optical mixing of the local oscillator (LO) light and the end reflection produces beat noise whose current noise spectral density is represented by 2(1+P²)〈I₁〉〈I₂ 〉/δν, where P and δν are the degree of polarization and the effective linewidth of the light, respectively, and 〈I₂〉 and 〈I₂〉 are the total mean photocurrents of the LO light and the end reflection at the balanced mixer, respectively. The balanced detection technique suppresses the intensity noise of the light and the beat noise becomes the dominant source of sensitivity degradation. The minimum detectable reflectivity is derived which includes the effect of sensitivity degradation caused by beat noise     

Noise in phototransistor optical isolators

Noise measurements were performed on several commercially available phototransistor optical isolators in order to examine the signal detection limits of typical devices. It was found that, in general, phototransistor optical isolators are very noisy devices exhibiting all of the common types of noise usually found in bipolar junction transistors. A large number of devices exhibited burst noise which dominated their low-frequency noise performance. The noise performance of devices without burst noise was dominated by 1/f noise or flicker noise at low frequencies and by shot noise at high frequencies. Experimental data indicates that in most cases the electrical noise contribution of the LED is negligible and that the dominant source of noise is the phototransistor.