Efficient numerical method for predicting the polarization-dependent Raman gain in fiber Raman amplifiers

Polarization-dependent gain (PDG) of fiber Raman amplifiers (FRAs) will degrade the performance of optical communication systems. An efficient numerical model is presented to predict PDG quantitatively by substituting the polarization-dependent polarization factor for the constant one in the coupled nonlinear equations usually adopted. The simulation is carried out by estimating the polarization length by use of the average polarization-mode dispersion of the tested fiber; the results, including the Raman gain profile and the fluctuation of the PDG, are highly accordant with the experimental data reported previously. The model can aid in the design of FRAs and in the analysis of system performance.     

The gain and noise figure of Yb-Er-codoped fiber amplifiers based on the temperature-dependent model

A novel temperature-dependent model for Yb³⁺-Er³⁺-codoped fiber amplifier (EYDFA) based on the energy transfer from Yb³⁺ to Er³⁺ is established. Using appropriate fiber and energy transfer parameters, the coupled rate equations is numerically solved at 25 and 40 °C. The pumping powers are 100 and 200 mW at a pump wavelength of 1060 nm. The signal gain and noise characteristics of a 0.3 m erbium/ytterbium co-doped fiber (EYDF) in a single-pass configuration are investigated by using 1, 10 and 100 μW signals at 1535 nm. A maximum signal gain of 40.5 dB and a corresponding noise figure of 3.65 dB at the temperature of 25 °C are achieved.     

分立式光纤拉曼放大器泵浦方式的实验研究

对分立式光纤拉曼放大器的前向、后向泵浦方式进行了实验研究。拉曼增益介质为10km的色散补偿光纤。分别测量了前、后向泵浦的分立式FRA的增益、噪声指数和动态范围。实验结果表明,当泵浦功率小于500mW时,前、后向泵浦的分立式FRA的增益几乎相等。当泵浦功率大于500mW时,在前向泵浦的分立式FRA中发生了受激布里渊散射,前向泵浦的FRA的增益小于后向泵浦的FRA的增益,前者的噪声大于后者的噪声;前向泵浦的FRA不再具有增益保持特性,而后向泵浦的FRA仍具有增益保持特性。当泵浦功率大于500mW时,分立式FRA应该采用后向泵浦的方式。     

Feasibility of nonlinear Raman lidar based on stimulated Raman gain spectroscopy without a tunable laser

A novel technique of lidar for atmospheric gas detection by use of stimulated Raman gain spectroscopy without any tunable laser is proposed. Detection sensitivity and detectable range are estimated on the basis of the lidar equation for CO2, CH4, and H2 in the atmosphere. The feasibility study clearly shows that the technique has a potential for application to lidar and that, in addition, the construction of the system is simpler than those of traditional differential absorption lidars.     

不同掺杂浓度光纤拉曼增益特性的研究

研究了受激拉曼散射增益系数和增益谱与泵浦功率、光纤掺杂浓度的关系,得出增益系数和峰值频移量由光纤特性决定,而与泵浦功率无关的结论。在对二氧化硅分子建模计算后,进一步利用LM算法对光纤的拉曼增益谱进行高斯分解。分解图谱表明掺杂物破坏了二氧化硅的分子结构,并在拉曼增益范围内产生了新的峰值,从理论上证明了光纤掺杂物对拉曼增益系数和增益谱的影响。     

FRA中增益平坦与抽运光数目的数值模拟与优化

采用神经网络的方法,利用其对于输入、输出参量已知,但解析关系未知的函数具有良好逼近能力的这一特点,构造了双层(2-5-1 型)反向传播(BP)神经网络。利用数值优化技术的 LM 算法训练该网络,获得了宽带拉曼光纤放大器(FRA)中增益带宽、增益平坦度与抽运光数目配置的非线性依赖关系。模拟结果表明:在增益平坦度确定的条件下,不同增益带宽处,抽运光数目必存在下限值。     

High speed all-optical encryption and decryption using quantum dot semiconductor optical amplifiers

A scheme to realize high speed all-optical encryption and decryption using key-stream generators and an XOR gate based on quantum dot semiconductor optical amplifiers (QD-SOAs) was studied. The key used for encryption and decryption is a high speed all-optical pseudorandom bit sequence (PRBS) which is generated by a linear feedback shift register (LFSR) composed of QD-SOA-based logic XOR and AND gates. Two other kinds of more secure key-stream generators, i.e. cascaded design and parallel design, were also designed and investigated. Nonlinear dynamics including carrier heating and spectral hole-burning in the QD-SOA are taken into account together with the rate equations in order to realize all-optical logic operations. Results show that this scheme can realize all-optical encryption and decryption by using key-stream generators at high speed (~250 Gb/s).     

Measurement of gain tilt and its contribution to composite second-order distortion in the analog-digital transmission systems with Er/Yb fiber amplifiers

Fiber-optic-based analog or digital community antenna television systems experience composite second-order (CSO) distortion caused by the interaction between the gain tilt of the doped fiber amplifier and the laser chirp due to modulation. The gain tilt for an analog-digital transmission system with a high-power erbium/ytterbium-codoped fiber amplifier has been experimentally measured and its contribution to the CSO distortion of the system is evaluated. The results are in good agreement when compared with the direct measurement of the CSO distortion of the transmission system with and without the amplifier. The dependence of the gain tilt on the modulation frequency and wavelength of the input light is also investigated.     

Interferometric Raman spectrometry with fiber waveguides

We present the design and expected performance of a Fourier-transform fiber-optic Raman spectrometer that should be capable of rapidly diagnosing environmental contamination to levels approaching a few parts per thousand. The system design is predicated on fiber arms that are unequal in length initially. A voltage applied to the piezoceramic substrate hosting the shorter fiber strains it through zero path difference to a new length that exceeds the reference arm by its initial length deficiency. This approach permits one to resolve spectral features separated by 0.5 cm(-1) over an electromagnetic bandwidth that exceeds 2000 cm(-1) without using a moving mirror or introducing a second laser to provide the sampling reference. Specific expressions are given for computing the spectral resolution, modulation bandwidth, and modulated signal-to-noise ratio of the device as a function of the system's design parameters.     

L波段掺铒光纤放大器增益钳制及提高特性研究

报道了一种新型980nm激光二极管（LD）泵浦高增益的掺铒光纤放大器（EDFA）．通过加入中心波长为1550nm光纤光栅（FBG）可以明显提高增益，并分析了增益钳制的原理．实验得到其增益比无FBG结构EDFA提高8dB，并得到了小信号输入时增益25dB的有效钳制．     

Theory of Raman gain spectrum transformations

A theory describing the spectral characteristics of Raman lasers is presented. The foundation of the theory is anchored in the probabilistic nature of light–matter interactions. The Raman gain spectrum is predicted for the cases when the pump laser is oscillating in both a single longitudinal mode and multi-longitudinal mode configuration. The gain profile predicted in the two previous cases are shown to be equivalent to homogeneously and inhomogeneously broadened laser transitions, respectively. This theory is necessary to predict the behavior of Raman lasers when injection-seeding, mode-locking, or multi-frequency operation is desired. Detailed calculations are performed for the case where the pump laser medium in Nd:YAG and the Raman medium is Ba(NO₃)₂.     

Application of defect engineering to silicon Raman lasers and amplifiers

Silicon Raman lasers and amplifiers are the only silicon-based, monolithic device structures in which optical gain has been unambiguously observed. The main limitation on this gain is optical absorption due to free carriers, generated by two photon absorption. Here we explore a means to mitigate carrier effects via defect engineering. The optical and electrical properties of the defected silicon waveguides are modeled for both a uniform defect distribution and a remote, localized defect distribution. Simulation results indicate that a uniform defect distribution provides no improvement with any increase in net gain provided solely by surface modification. In contrast, for devices with remote defect volumes the reduction of carrier lifetime and limited optical absorption results in a significant improvement to net gain.     

Application of defect engineering to silicon Raman lasers and amplifiers

Silicon Raman lasers and amplifiers are the only silicon-based, monolithic device structures in which optical gain has been unambiguously observed. The main limitation on this gain is optical absorption due to free carriers, generated by two photon absorption. Here we explore a means to mitigate carrier effects via defect engineering. The optical and electrical properties of the defected silicon waveguides are modeled for both a uniform defect distribution and a remote, localized defect distribution. Simulation results indicate that a uniform defect distribution provides no improvement with any increase in net gain provided solely by surface modification. In contrast, for devices with remote defect volumes the reduction of carrier lifetime and limited optical absorption results in a significant improvement to net gain.     

Performance comparison of Raman/erbium-doped-fiber hybrid-amplification-based long-distance fiber Bragg grating sensor systems

A study regarding the performance comparison of three different types of long-distance fiber Bragg grating (FBG) sensors based on bidirectional Raman and erbium-doped fiber (EDF) amplification has been carried out to determine the most power-efficient and accurate configuration. The three sensor system configurations studied were a bidirectional hybrid Raman amplifier with (Type I) an EDF located after the distributed Raman amplification section, (Type II) an EDF located before the distributed Raman amplification section, and (Type III) without an EDF. The FBG sensor systems are based on a demodulation scheme employing radio frequency power measurements of a beat signal, in which a sine-modulated amplified spontaneous emission from a directly modulated reflective semiconductor optical amplifier is employed as a broadband light source. The results of our experimentation showed that the sensor system with the EDF located prior to the Raman amplification section provided the best performance in regards to its pump power efficiency, electrical signal-to-noise ratio, and measurement accuracy.     

Method of developing all-optical trinary JK, D-type, and T-type flip-flops using semiconductor optical amplifiers

To meet the demand of very fast and agile optical networks, the optical processors in a network system should have a very fast execution rate, large information handling, and large information storage capacities. Multivalued logic operations and multistate optical flip-flops are the basic building blocks for such fast running optical computing and data processing systems. In the past two decades, many methods of implementing all-optical flip-flops have been proposed. Most of these suffer from speed limitations because of the low switching response of active devices. The frequency encoding technique has been used because of its many advantages. It can preserve its identity throughout data communication irrespective of loss of light energy due to reflection, refraction, attenuation, etc. The action of polarization-rotation-based very fast switching of semiconductor optical amplifiers increases processing speed. At the same time, tristate optical flip-flops increase information handling capacity.     

Gas Raman sensing with multi-opened-up suspended core fiber

Gas sensing and fluid-guiding response properties of a suspended core fiber Raman analyzer with side-opened and strut microfluid-guiding array are explored. A Raman sensing model is introduced for effective mode area optimization and normalized intensity overlap enhancement between Raman sensing light and analyte. Calculations predict that there is a trade-off between the overlap and the effective mode area, while the optimal trade-off depends on the refractive index of the background material, core diameter, and strut's thickness. Furthermore, the multi-opened-up structure ensures a fast gases diffusing into/out of each hole for real-time Raman sensing. Simulation results confirm a limited gas sensing response time of less than 6 s could be feasible and, thus, a new approach to real-time gas sensing applications is identified.     

Modeling and optimization of add-drop dynamics in gain-clamped fiber amplifiers

A new dynamic model for gain-clamped fiber amplifiers is presented. The model can simulate the transient behavior of a gain-clamped erbium-doped fiber amplifier as a function of its controlling parameters. Results show the detailed dependence of the amplifier gain dynamics on launched pump power and reveal the pump-power penalty incurred in implementing gain control with minimal transients to power disturbances. The effect on the transients of varying the laser cavity feedback coupling is also presented. Model accuracy is verified by the excellent quantitative agreement with dynamic measurements over a range of operating conditions. Results show how an increase in pump power can effectively suppress both the dynamic and the steady-state signal power transients in wavelength-division multiplexing channel add-drop operations.