Huai Wei

Use of a genetic algorithm to optimize multistage erbium-doped fiber-amplifier systems with complex structures.

We propose optimizing multifunctional multistage erbiumdoped fiber amplifiers (EDFAs) with complex structures by use of a genetic algorithm. With this method, we investigated optimum configurations of C- and L-band gain-flattened multistage EDFAs containing gain-flattening filters and high-loss interstage elements for dense wavelength-division multiplexing systems in detail and compared the amplifiers with various kinds of configurations under different design criteria. With the guidance of optimization results, the roles of all the factors such as pumping schemes, pump-power allocation, component position, and insertion loss in the optimization of EDFAs have been studied, and useful guidelines for optimizations have been provided.

Theoretical investigation and optimization of bi-directionally pumped broadband fiber Raman amplifiers

Abstract In this paper, we analyze the noise performance and optical signal-to-noise ratio (OSNR) tilt of a distributed broadband Raman amplifier (RA) in detail, and show that output OSNR can be improved by raising the signal power distribution at the cost of increased impact of nonlinearity. The nonlinearity performance at the same OSNR and net gain is also compared between the bi-directionally pumping scheme and another widely used method, power pre-emphasis for the first time, under the undepleted-pump approximation. Based on a complete numerical model, we propose an efficient method to optimize a bi-directionally multi-wavelength pumped RA, and in this way a required OSNR profile can be achieved without changing the original gain spectra. As applications, two bi-directionally pumped RAs are simulated with 80 and 100 nm bandwidth, respectively, and less than 0.6 dB flatness of both gain and OSNR are derived by using this method. According to the method and calculation results, some guidelines and discussions are given and a new concept called ‘dynamic OSNR equalization’ is demonstrated, which can be well realized by utilizing bi-directionally pumped RAs and will play an important role in ultralong-haul systems.

Comparison of different Raman amplification schemes in long-span fiber transmission systems with double Rayleigh backscattering

In this letter, three Raman amplification configurations, which are bidirectional pumping scheme, backward pumping with mid-way isolators, and Raman plus erbium-doped fiber amplifier hybrid amplification, respectively, are optimized and compared in long-span (> 120 km) transmission systems including the impacts of double Rayleigh backscattering (DRB). Numerical results show that a balanced bidirectional pumping scheme can achieve the best performance for most practical conditions at identical nonlinearity. However, hybrid amplification will be a better choice when DRB noise is overwhelming.

Simultaneous measurement of strain and temperature using dual-period fiber grating

This paper demonstrates a novel dual-period fiber grating sensor which can measure strain and temperature simultaneously. The dual-period fiber grating consists of a long-period fiber grating (LPG) and a fiber Bragg grating (FBG) which are written in the same section of an uncovered hydrogen-loaded fiber orderly. As is known, the Bragg wavelength of LPG and that of FBG have the different sensitivity of strain and temperature, then strain and temperature can be determined simultaneously by measuring the two transmitted Bragg wavelengths of the dual-period fiber grating. The accuracy of the sensor in measuring strain and temperature is estimated to be +/- 16 (mu) (epsilon) in a range from 0 to 1700 (mu) (epsilon) and +/- 0.8 degree(s)C from 20 to 120 degree(s)C, respectively.

Impacts of SPM/XPM on distributed Raman amplified multispan systems at identical nonlinear phase shift

The influences of self-phase modulation (SPM) and cross-phase modulation (XPM) on Raman amplified multispan systems with periodic dispersion compensation (DC) are numerically investigated at identical nonlinear phase shift. The results show that compared with lumped amplification, distributed amplification tends to enhance the SPM/XPM-induced penalties provided per span complete DC is used. However, these performance differences can be canceled out by means of optimal dispersion managements. Useful guidelines for optimizing the dispersion maps of Raman amplified systems are demonstrated.

Transient responses to slowly varying input waveforms in backward pumped Raman amplifiers

In this article, a novel numerical algorithm to simulate power transient in backward pumped Raman amplifiers is proposed. Based on this algorithm, transient response to a slowly varying input power waveform is investigated for the first time. We find that a properly designed, slowly varied input leading edge can reduce the output up-shoot drastically in a single-channel condition, while in a multi-channel condition, a gradually changed input waveform can eliminate the abrupt power up-shoot due to forward signal-to-signal Raman interactions and obtain a better gain clamping result by using pump adjustment. These conclusions will play a role in Raman amplified optical burst mode switching (or packet switching) systems and DWDM networks.

Optimal design of multistage discrete Raman amplifiers incorporating midway isolators

Multistage discrete Raman amplifiers with midway isolators to suppress double Rayleigh backscattering are fully investigated at constant nonlinearity. The results show that placing Raman pumps at the last stage only is sufficient. For most conditions, two isolators can achieve adequate performance improvement, while more isolators only lead to slightly optical signal-to-noise ratio increase. The optimum positions of the isolators are insensitive to varied Raman gains, nonlinear phase shifts, and Rayleigh coefficients, but sensitive to different fiber lengths as well as Raman pumping schemes. Moreover, backward-pumping schemes can derive better noise performance than bidirectional pumping when using midway isolators.

FBGs-based dispersion compensators and EDFA gain equalizators

32 chirped FBGs (fiber Bragg gratings)-based dispersion compensators and EDFA (Er-Doped Fiber Amplifier) gain-equalizators is demonstrated. Dispersion compensation of 600km G652 fiber and limiting amplification of 16 wavelength signals with low power penalty of<2dB can be obtained at a 10GHz/s optical communication system.

THEORETICAL ANALYSIS AND EXPERIMENT ON A NOVEL KIND OF SINGLE MODE LARGE-MODE-AREA ERBIUM-DOPED FIBER

A kind of large-mode-area (LMA) erbium-doped fiber (EDF) is required to get high output power with good beam quality. This new LMA EDF has a multi-layer-core structure which makes the mode area bigger. The key process is deposition of multi-layer-core structure by using modified MCVD to make every layer of the core with different refractive index. Furthermore, the key optical specifications of the fiber are measured. The absorption peak is 10.6 dB/m at 980 nm and 20 dB/m at 1550 nm. The cutoff wavelength is about 1300 nm. The mode field diameter (MFD) is 12.6 μm and 13.2 μm at 1550 nm according to Petermann-2 and Gaussian definitions, respectively. The core diameter is about 12 μm. The MFD of conventional standard EDFs is in a limited range ~5–8 μm, so the experimental result has made a big progress for MFD. Especially, we can improve the parameters of the multi-layer-core structure to get single mode EDF with larger mode area for high power fiber laser.

Novel design of M-profile ytterbium doped fibers for high power fiber lasers

A novel design of M-Profile Ytterbium doped Fibers (YDFs) for high power fiber Lasers was given. The output power was two times higher than that in standard double-cladding fiber under the same threshold of damage.