Experimental characterization of a dynamically gain-flattened erbium-doped fiber amplifier

We have designed and experimentally characterized an erbium doped fiber amplifier (EDFA) which possesses a wavelength-independent gain spectrum, independent of the operating level of the gain (dynamic gain flatness), and without requiring any gain-level-dependent control of any parameters. In the wavelength range 1542-1552 nm, the gain was flat to within the experimental uncertainties of /spl plusmn/0.3 dB even as the gain level changed by 17 dB. The EDFA was based on a low-Al-content alumino-germanosilicate EDF and a Mach-Zehnder filter. We believe that this type of EDFA, which has not been demonstrated before, can significantly simplify the design of amplified wavelength-division multiplexing (WDM) transmission systems and increase the robustness of long-distance WDM transmission.     

Special erbium fiber amplifiers for short pulse switching, lasers,and propagation

We show that specially tailored erbium-doped fiber amplifiers (EDFA's) can be useful for short-pulse soliton lasers, switching, and propagation. EDFA properties include group-velocity dispersion, birefringence, doping level, and doping distribution; and changing each of these properties enables novel applications. Polarization-maintaining EDFA's can Re used in erbium-doped fiber lasers to avoid intensity-dependent or temperature-dependent state of polarization. We have demonstrated a passively mode-locked erbium-doped fiber laser that generates nearly transform-limited 320 fs pulses with 40 pJ energy using a polarization-maintaining EDFA near the zero dispersion wavelength and a bulk InGaAsP saturable absorber. Moderately birefringent EDFA's can be used to control the walkoff and interaction between orthogonally polarized solitons in all-optical switches. For example, through numerical simulations, we design an all-optical cascadable logic gate with a fanout of 2.7 and energy contrast of 5.5 based on interactions in a soliton period long EDFA. Furthermore, distributed EDFA's can be used for long-distance soliton propagation to shepherd the pulse using bandwidth-limited gain. For picosecond soliton pulses, we show that soliton self-frequency shift and Gordon-Hans effects limit propagation to below 100 km even for low-dispersion fibers. Bandwidth-limited amplification in EDFA may counteract frequency shift due to the soliton self-frequency shift, while frequency filters may suppress the Gordon-Hans effect. Also, because the gain spectral profile for the three-level EDFA changes with pump intensity, complications arise from pump attenuation, and the corresponding changes in the gain spectral profile along the length of the fiber     

Gain flattening of an erbium-doped fiber amplifier using ahigh-birefringence fiber loop mirror

We propose a flexible method for flattening the gain profile of an erbium-doped fiber amplifier (EDFA). The method is based on a high-birefringence fiber loop mirror (HiBi-FLM), which contains a number of high-birefringence fiber sections and polarization controllers. We show that, by setting the polarization controllers properly, the reflection spectrum of the loop mirror can be adjusted to compensate for the variations in the gain profile of the EDFA. The method is demonstrated experimentally by using a single-section HiBi-FLM and a two-section HiBi-FLM, respectively, With a two-section HiBi-FLM, we are able to flatten the gain profile of an EDFA to within ±0.9 dB over a bandwidth of 33 nm at the center wavelength of 1543 nm, under different operating conditions of the EDFA     

Segmented-clad fiber design for tunable leakage loss

This paper proposes a novel segmented-clad fiber (SCF) design in which the spectral variation of leakage loss of the fundamental mode can be finely tuned by varying the fiber parameters. A fiber with such optimized spectral variation of leakage loss should find application in the realization of inherently gain-flattened optical fiber amplifiers, wavelength filters, wavelength-flattened attenuators, etc. In this paper, the authors present SCF designs optimized for realizing inherently gain-flattened S-band erbium-doped fiber amplifier (EDFA) and high-gain discrete Raman fiber amplifier (RFA). Amplifier characteristics have been modeled in both cases, and simulations show that a 20-dB gain with /spl plusmn/0.9 dB of gain ripple over a 30-nm bandwidth in S-band is achievable with the designed EDFA based on the optimized SCF. In case of discrete RFA based on SCF, a 20-dB net gain with /spl plusmn/0.5-dB ripple is shown to be achievable over a 28-nm wavelength range in S-band.     

Inherent enhancement of gain flatness and achievement of broad gainbandwidth in erbium-doped silica fiber amplifiers

We report new methods to inherently increase the flatness and bandwidth of erbium-doped silica fiber amplifiers from three perspectives: fiber design, pump-signal WDM coupler optimization, and amplifier structure. First, to achieve inherent control of the gain spectrum, a new type of composite fiber structure with an Er-doped core and a Sm-doped cladding ring is proposed and experimentally demonstrated. Interaction of the optical field with the Sm-doped cladding to produce evanescent wave filtering is modeled, which provides an in-line control of gain fluctuation in the erbium-doped flier amplifier (EDFA) C band, 1530-1560 nm. Second, the effect of the spectral characteristics of WDM couplers over the L band of an EDFA is explored. A fused taper fiber coupler for a 1480-nm pump is optimized for signals in the wavelength range of 1570-1610 nm by measuring the small-signal gain, gain tilt, and noise figure in an L-band EDFA. Finally, a new all-fiber structure for a wide-band EDFA, where the L and C bands were coupled serially, is demonstrated with optimized pump-signal couplers. Further optimization of the new composite fiber structure and the transient effects in the serially coupled EDFAs are also discussed     

Dynamic gain and output power control in a gain-flattenederbium-doped fiber amplifier

We demonstrate an erbium-doped fiber amplifier (EDFA) that has a flat-gain spectrum over an 18-nm band and a constant per-channel output power regardless of the input powers and the number of channels. The gain flatness and the constant output are obtained by changing the laser diode pump power and the attenuation of the voltage-controlled attenuator (VCA) dynamically. The response times of pump control circuit and attenuator control circuit are 650 ns and 9 ms, respectively. The power excursion of surviving channel in an EDFA with fast control circuit is ~1% of that without fast control circuit when channels are added or dropped     

Very flat gain erbium-doped fiber amplifier using samarium-doped fiber

A two-stage erbium-doped fiber amplifier (EDFA) with extremely flat gain (/spl plusmn/0.1 dB) over 17-nm bandwidth is demonstrated. The gain flatness is achieved by incorporating samarium-doped fiber in the mid section of the amplifier.     

中间级接入掺铒光纤放大器及其自动增益校准

为了根据外界条件的变化自动进行增益调整,引入了中间级接入掺铒光纤放大器,它具有两级放大功能,增益在一定范围内可调,且能自动根据系统的变化调整自身的增益,使其满足不同条件的应用需求。采用自动增益校准的方法,对其原理进行了理论分析和实验验证,取得了中间级接入掺铒光纤放大器的增益校准及验证数据。结果表明,自动增益校准数据具有较高的准确性。这一结果对掺铒光纤放大器实现快速、准确的自动增益控制是有帮助的。     

S-band erbium-doped fiber amplifiers with a multistage configuration /sub e/sign, characterization, and gain tilt compensation

We report an S-band erbium-doped fiber amplifier (EDFA) with a multistage configuration in terms of its design, gain, and noise characteristics for various pump powers and input signal powers, the temperature dependence of the gain spectra, and gain tilt compensation for changes in input signal power and temperature change. We show that there is a tradeoff between low noise and efficiency in the S-band EDFA and describe the development of an S-band EDFA with a flattened gain of more than 21 dB and a noise figure of less than 6.7 dB. We also show that there is a change in the gain spectra with changes in the pump power and input signal power that is different from that observed in C- and L-band EDFAs, and that our EDFA has a temperature-insensitive wavelength. Furthermore, we develop a gain tilt compensated S-band EDFA that can cope with changes in input signal power and temperature.     

Optical gain control of erbium-doped fiber amplifiers with asaturable absorber

Optical gain control (OGC) can be used to lock the inversion of an erbium-doped fiber amplifier (EDFA), assuming that the gain medium is homogeneous. However, EDF's exhibit certain degrees of spectral hole burning. As a result, when the OGC laser power and the spectral-hole depth at the laser wavelength change, the inversion of the EDFA changes accordingly with a fixed OGC cavity loss. In this work, a saturable absorber is placed in the OGC laser cavity to adjust the cavity loss dynamically and compensate the gain tilt caused by the OGC laser spectral hole burning. It is demonstrated that the steady state gain variation of a surviving channel in an optically gain controlled EDFA is improved from 1.3 dB (with a fixed loss) to 0.4 dB (with a saturable absorber in the cavity). The transient response of this gain control scheme is also discussed     

Analysis of optical gain enhanced erbium-doped fiber amplifiersusing optical filters

Erbium-doped fiber amplifiers (EDFAs) with enhanced optical gain obtained by incorporating narrow-bandpass optical filters into the amplifier length are studied. It is shown in theory that it is possible to increase optical gain by more than 10 dB for optical signals around the wavelength of 1.55 μm, compared with conventional EDFAs without filters. It is also shown that the gain improvement at longer wavelengths away from the amplifier gain peak is much higher than that of the EDFA with an optical isolator within the amplifier length. The optimum filter position is found to be around 42% of the total amplifier length from the input end. The effects of filter insertion loss and pump loss are discussed. This amplifier can be used as an optical preamplifier in a receiver for a wide range of wavelengths     

All optical gain-locking in erbium-doped fiber amplifiers usingdouble-pass superfluorescence

All optical gain-locking in an erbium-doped fiber amplifier (EDFA) is demonstrated. A double-pass superfluorescence is created by using a broad-band fiber reflector centered at 1530 nm at the output of the EDFA, to lock the gain at 21 dB. Experiments on an eight-channel wavelength-division-multiplexing system shows promise with gain variation between channels of less than 0.6 dB over the input signal power range. While, gain variation with input signal power is about 0.2 dB for all channels     

基于单个光纤光栅反射技术的高性能L波段EDFA

基于单个光纤光栅反射技术提出一种高性能L波段EDFA。用一个光纤布拉格光栅(FBG)反射EDFA产生的一部分C波段放大自发辐射(ASE)噪声,该部分ASE噪声被重新注入到掺铒光纤中以提高增益效率。用静态均衡器平坦输出的增益谱,在L波段范围内,增益被箝制在25.5dB,增益不平坦度小于0.5dB,噪声指数小于5.5dB,为DWDM系统提供了一项有效的解决方案。     

Performance optimization of radio-on-fiber systems employing erbium doped fiber amplifier for optical single sideband signals considering intermodulation distortion

The performances of radio on fiber (RoF) systems with a dual-electrode Mach-Zehnder modulator and an erbium-doped fiber amplifier (EDFA) are optimized by numerical equations including the third order intermodulation (IM3) as well as amplified spontaneous emission (ASE) noise. We investigate a signal-to-noise-and-distortion ratio (SNDR) considering fiber dispersion with respect to an input signal power and an EDFA gain in both noise-dominant and third order intermodulation (IM3)-dominant cases. We also verify that the numerical analysis results are well matched with those of a commercial simulator, VPItransmissionMaker. In the analysis results, the optimum input signal power for the maximum SNDR of a RoF system with EDFA was reduced over 8 dB compared with that without EDFA. The dramatic reduction of IM3 power at a receiver was resulted from this decrement of input signal power. Thus, the maximum SNDR of the system with EDFA was obtained over 17 dB at 40 km fiber compared with that of the system without EDFA. In addition, the results showed that the SNDR was efficiently improved by EDFA in the noise-dominant case, while the SNDR improvement was negligible by EDFA in the IM3 dominant case.     

Polarization dependent loss induced gain ripple statistics inerbium-doped fiber amplifiers

A Monte-Carlo model is used to simulate the wavelength-dependent polarization dependent loss (PDL) statistics and PDL induced gain-ripple statistics for a chain of concatenated optical elements in an erbium-doped fiber amplifier (EDFA). Simulations indicate that unlike polarization-mode dispersion (PMD), the quadrature sum of PDL overestimates the mean of the PDL distribution. We show that EDFA total PDL variance scales linearly with the individual component PDL variance. Moreover, the relative impact of PDL on the gain ripple is reduced in the presence of the PDL independent gain ripple in the EDFA     

Actively gain-flattened erbium-doped fiber amplifier over 35 nm byusing all-fiber acoustooptic tunable filters

We demonstrate an actively gain-flattened erbium-doped fiber amplifier (EDFA) using an all-fiber gain-flattening filter with electronically controllable spectral profiles. A good gain flatness (<0.7 dB) over a broad wavelength span (>35 nm) is achieved for a wide range of operational gain levels as well as input signal and pump powers     

高频CO2激光脉冲写入的新型长周期光纤光栅及其在光通信中的应用

报道了一种利用频率为几千赫兹的高频CO2激光脉冲的热冲击效应制作长周期光纤光栅的新技术。实验中，作者首次发现这种新型的长周期光纤光栅具有一些独特的光学特性，比如弯曲特性和横向负载特性等，这主要是因为用高频CO2激光脉冲写成的长周期光纤光栅的横截面折射率分布不对称所致。基于这种新型长周期光纤光栅，设计了一种用于减小掺铒光纤放大器噪声系数的ASE噪声滤波器和平坦掺铒光纤放大器增益谱的增益均衡器，此外，还利用这种新型长周期光纤光栅独特的弯曲和压力特性，研究设计了两种用于动态平坦掺铒光纤放大器增益谱的动态增益均衡器。     

Experimental investigation of the gain flatness characteristics for1.55 μm erbium-doped fluoride fiber amplifiers

The flat gain over a broad spectral width offered by fluoride-based Erbium-Doped Fiber Amplifiers (EDFA) appears very attractive for wavelength-multiplex systems operating near 1550 nm. The relevant parameters (input pump and signal powers, fiber length) that determine the gain flatness characteristics of fluoride-based EDFA have been experimentally investigated. It is found that hat gain operation requires an optimal, though not critical, population inversion within the doped fiber, which is somewhat lower than the maximum population inversion achievable with 1.48 μm-pumping. Implications on the design and operating conditions of fluoride-based EDFA are discussed     

Gain-clamped fiber amplifier with a short length ofpreamplification fiber

Although gain-clamping an erbium-doped fiber amplifier (EDFA) by a counterpropagating ring resonator prevents oscillation light from being output, it degrades the amplifier's noise figure. The noise figure degradation can be reduced by using a length of EDF, that is short enough not to cause any gain saturation, for preamplification. Experiments confirm that the prestage EDFA reduces the noise figure degradation under constant gain operation     

Transmission line with distributed erbium-doped fiber amplifier

We show the advantages of distributed erbium-doped fiber amplifiers (EDFAs) over lumped EDFAs. Using the distributed EDFA for 50%-100% of the transmission line lessens the signal degradation compared to the conventional transmission line. We also show the feasibility of the gain-flattened d-EDFA transmission line. An experiment shows that the gain peak is shifted and flattened at low pump powers