Zhi Tong

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.

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.

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.

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.

A new method of efficient side-pumping for high power double fiber laser

A novel structure for efficient side-coupling of high power double-cladding fiber laser is presented. The maximum coupling efficiency of this structure is more than 90% for TM-polarization in +1 and -1 order but is only 40% for TE-polarization. Thus, a two-layer structure is introduced to obtain higher coupling efficiency for TE-polarization and maximum coupling efficiency almost 70% is demonstrated.

Optimal design of L-band EDFAs with high-loss inter-stage elements

Abstract In this paper, we theoretically investigate and optimize a multi-stage L-band EDFA including high-loss inter-stage element based on a reliable numerical model. Influences of inter-stage element as well as stage length on the amplification performance of different pumping schemes are analyzed and compared. The results show that a piece of unpumped Erbium doped fiber (EDF) should be placed before (forward pumping) or after (backward pumping) the loss element to improve the pump efficiency through ASE re-absorption, and by optimizing the EDF length of each stage properly, both high-output gain and uniform NF profile can be derived even with high-loss inter-stage devices. After optimization, only 2.4 dB gain decrease and 1.4 dB NF increase are obtained with up to 12 dB inter-stage attenuation by using a four-stage architecture, and dual-forward pumping scheme can have almost the same characteristics as forward–backward pumping. Moreover, we also consider the impact of pump adjustment on the noise performance of L-band EDFAs with inter-stage loss in case of channel add/drop, and some useful guidelines are obtained.

Design of a doubly grooved binary metallic diffraction grating For efficient side-pumping of high-power fiber lasers

A metallic binary grating with two grooves per period is applied to side-pump high-power fiber lasers for the first time. By combining a gradient algorithm and micro-genetic algorithm, the grating is optimized and the maximum coupling efficiencies of 94.77% and 77.27% for TM and TE polarization waves respectively are demonstrated.

Gain flatness performance of backward-pumped distributed fiber Raman amplifiers with pump wavelengths in different arrangement

The relationship between the gain flatness performances with the pump wavelength spacing in single-stage backward-pumped distributed fiber Raman amplifiers (B-DFRAs) were demonstrated theoretically. It was shown that the gain spectrum of B-DFRA which pump wavelengths were arranged in a geometric proportion interval sequence were flatter than one which pump wavelengths were arranged in an equal interval sequence in the same conditions.

Ultra-long-haul L-band WDM transmission over a standard single-mode fiber loop using DCF+CFBG hybrid dispersion compensation

We demonstrate a 10.7Gb/s-line-rate L-band WDM loop transmission over 1890km standard single-mode fiber (SSMF) with 100km amplifier spacing as well as non-return-to-zero (NRZ) format. For the first time, dispersion compensating fiber (DCF) plus chirped fiber Bragg grating (CFBG) is employed for hybrid inline dispersion compensation. The power penalty of each channel is less than 3dB after three loop transmission. The experimental results show that high-performance-CFBGs can be successfully used in ultra-long haul (>1000km) WDM systems. We also point out that all-CFBG compensation scheme is not suitable for re-circulating loop transmissions.

Novel method to optimize the distributed pump powers in kilowatt ytterbium-doped double-clad fiber laser

Time independent rate equations for fiber laser are solved accurately with numerical method under multipoint boundary conditions. A novel method based on genetic algorithm is firstly proposed to optimize distributed pump powers and fiber segment lengths of kilowatt YDDC fiber laser. The uniformity of temperature distribution is improved by using optimized method. The calculated results show that the lower operating temperature and better uniformity can be obtained through increasing the number of segments at the cost of decreased output signal power.