Noise characteristics of Er3+-doped fiber amplifierspumped by 0.98 and 1.48 μm laser diodes

Measured noise characteristics of Er³⁺-doped optical fiber amplifiers pumped by 0.98- and 1.48-μm laser diodes (LDs) are reported. The noise figures estimated from the beat noise between signal and spontaneous emission are 3.2 dB for pumping by 0.98-μm LD and 4.1 dB for pumping by 1.48-μm LD. The beat noise between spontaneous emission components and the spontaneous shot noise for the 0.98-μm pumping are lower than those for the 1.48-μm pumping     

Temperature dependence of signal gain in Er3+-dopedoptical fiber amplifiers

Temperature-dependent signal gain characteristics at signal wavelengths of 1.536 and 1.552 μm in Er³⁺-doped optical fibers with a temperature range of -40 to 80°C are reported for 0.98 and 1.48 μm pumping. The temperature dependences of signal gain strongly depend on fiber length, pump wavelength, and signal wavelength. The fiber length at which signal gain temperature insensitivity occurs is found for the amplification of a 0.98-μm-pump-1.536-μm-signal, a 0.98-μm-pump-1.552-μm-signal, and a 1.48-μm-pump-1.536-μm-signal. It is confirmed theoretically that the temperature dependences result from linear changes in the fluorescence, and absorption cross sections at the signal and pump wavelengths, and a shift in the effective pump wavelength     

Modeling the gain degradation of high concentration erbium-dopedfiber amplifiers by introducing inhomogeneous cooperative up-conversion

The gain degradation of erbium-doped fiber amplifiers (EDFAs) with high erbium ion (Er³⁺) concentration at 1.48- and 0.98-μm pump wavelengths is modeled by introducing inhomogeneous cooperative up-conversion (IhCU). General formulas describing the gain degradation as a function of IhCU rate are obtained by solving rate equations for population probabilities in the relevant Er³⁺ energy levels. The experimental results, such as low gain for high Er³⁺ concentration, and higher saturated gain with counterpropagation than with copropagation pumping, which have not yet been explained theoretically, are qualitatively explained by this model. Good agreement between the measured and calculated gain is obtained. The gain degradation characteristics at 1.48- and 0.98-μm pump wavelengths are analyzed with this model. The advantage of counterpropagation pumping is determined qualitatively. The noise figure degradation is also evaluated     

A highly efficient two-stage Er3+-doped optical fiberamplifier employing an optical gate to effectively reduce ASE

Highly efficient amplification of ultrashort optical pulses is demonstrated with a two-stage Er³⁺-doped optical fiber amplifier that includes an optical gate to efficiently reduce amplified spontaneous emission (ASE) generated from the first Er³⁺-doped fiber. A gain of 49 dB, an amplified peak power 0f 105 W, and 1.05 nJ pulse energy are achieved for 2-Mb/s, 10-ps pulses at a total pumping power of 90 mW from 1.48-μm LDs     

Er3+-doped fiber amplifier pumped by 0.98 μm laserdiodes

The performance of an Er³⁺-doped fiber amplifier pumped by 0.98 μm InGaAs laser diodes (LDs) is reported. By using a fiber with low Er³⁺ content and optimizing the fiber length, a maximum signal gain of 37.8 dB at 30-mW pump power was realized at a signal wavelength of 1.536 μm. A maximum gain coefficient of 1.9 dB/mW at 14 mW pump power was achieved. It was found that the fiber amplifier pumped by the 0.98-μm LDs is twice as efficient as that pumped by 1.48-μm LDs, from the viewpoint of both required fiber length and the attained gain     

Stable operation (over 5000 h) of high-power 0.98-μm InGaAs-GaAsstrained quantum well ridge waveguide lasers for pumpingEr3+-doped fiber amplifiers

A maximum output power of 115 mW and a slope efficiency of 0.92 W/A have been achieved in 0.98-μm InGaAs strained quantum well lasers with a 3-μm-wide ridge waveguide structure for efficient fiber coupling. Stable operation of over 5000 h under 50°C constant power operation with an optical power density of 3.9 MW/cm² has been demonstrated with a degradation rate as low as 5×10^-6 per hour. These results show that this device is promising as a practical pumping source for Er³⁺-doped fiber optical amplifiers     

Optical amplification in Er3+-doped single-mode fluoridefiber

The amplification characteristics at around 1.5 μm of a 0.9-m-long, 1000-p.p.m Er³⁺-doped single-mode fluoride fiber are discussed. By using 1.48-μm laser diodes with 55-mW launched output as a pump source, a gain of 1.75 dB was obtained at 1.530 μm. A broad bandwidth of 40 nm was obtained, which may be suitable for wavelength-division multiplexing (WDM) system use     

Performance of high-concentration Er3+-doped phosphatefiber amplifiers

The performances of high-concentration Er³⁺-doped phosphate fiber amplifiers are reported. The amplifiers are characterized in terms of gain, noise figure, and signal saturation power in a co-propagating pump configuration. A net gain of 21 dB and a gain per unit length 3 dB/cm are achieved in a 71-mm Er³⁺-doped phosphate fiber     

5 Gb/s optical soliton transmission experiment using Ramanamplification for fiber-loss compensation

Optical soliton transmission of 5 Gb/s over a 23-km amplification spacing using a gain-switched 1.55-μm distributed feedback laser diode and Ti:LiNbO₃ intensity modulator is discussed. An Er ⁺-doped fiber amplifier, pumped by 1.45- and 1.48-μm laser diodes, is employed for achieving intense optical pulses. Transmission fiber-loss is completely compensated for by Raman amplification using by 1.45- and 1.48-μm laser-diode pumping. A bit error rate (BER) of 2×10^-10 has been obtained     

Spectrally Resolved Approach for Modeling Short Pulse Amplification in Er$^{3+}$-Doped Fibers

We study pulse propagation in Er³⁺-doped fiber amplifiers (EDFA) within the framework of a spectrally resolved pulse rate-propagation equations model. Our model accounts for the effects of gain dispersion, gain saturation, waveguide and chromatic dispersion, and amplified spontaneous emission. This model allows us to approximate the effects of nonlinear resonant dispersion on short pulse amplification in doped fibers, without reverting to the generalized nonlinear Schroedinger equation. Numerical results of the time-dependent power spectrum of the amplified pulse demonstrate subpicosecond pulse propagation in EDFAs     

Design optimization for efficient erbium-doped fiber amplifiers

The gain and pumping efficiency of aluminosilicate erbium-doped fiber amplifiers (EDFAs) are analyzed as a function of guiding parameters and Er-doping profile for two pump wavelengths of λ _p=980 nm and λ_p=1.47 μm. Three designs of fiber-amplifier waveguides are considered: one with the same mode size as standard 1.5-μm communication fibers (type 1); one with the same mode size as standard 1.5-μm dispersion-shifted fibers (type 2); and one with mode size smaller than those of communication fibers (type 3). For the 1.47-μm pump, fundamental LP₀₁ mode excitation is assumed, while for the λ_p=980-nm pump, concurrent excitation of LP₁₁ modes is considered. It is shown that excitation of higher-order pump modes at 980 nm does not significantly affect the amplifier gain performance. The effect of concentrating the Er³⁺ doping near the center of the fiber core is shown to increase the amplifier gain coefficients by a factor of 1.5 to 2     

Comparative study on temperature-dependent multichannel gain andnoise figure distortion for 1.48- and 0.98-μm pumped EDFAs

Temperature dependence of multichannel gain flatness and noise figure (NF) was compared for different pump wavelengths of 1.48 and 0.98 μm on silica-based erbium-doped fiber amplifiers (EDFAs) through measurement-based numerical simulation. Owing to its temperature sensitive pump emission cross section, the 1.48-μm pumping showed greater temperature sensitivity (maximum 0.75-dB gain flatness distortion with 0.57-dB average gain level shift, 0.3-dB NF variation for 25°C change) than the 0.98 μm pumping (maximum 0.5-dB gain flatness distortion with 0.015-dB average gain level shift, 0.05-dB NF variation for 25°C change). However, it was also found that distortion ripple spectra mainly coming from the changes of signal cross sections and asymmetric gain temperature dependence necessitate compensation techniques in the EDFA link, irrespective of pump wavelength     

Concentration effect on gain of Pr3+-doped fluoridefiber for 1.3 μm amplification

The concentration quenching mechanism in Pr³⁺-doped fluoride fiber operating at the 1.3-μm band is investigated. It is confirmed that the cooperative upconversion process causes signal gain saturation with pump power and degrades the gain characteristics of Pr ³⁺-doped fluoride fiber at a concentration of 1000 p.p.m. The reduction in gain coefficient due to cooperative upconversion is almost suppressed at a concentration of 500 p.p.m. A Pr³⁺ concentration of 500 p.p.m. is a practical concentration for the fabrication of efficient Pr³⁺-doped fluoride fiber amplifiers     

Experimental and theoretical analysis of efficient erbium-dopedfiber power amplifiers

The efficiency of Er³⁺-doped fiber power amplifiers (EDFAs) pumped at 980 nm was experimentally investigated and quantum conversion efficiencies (QCE) up to 0.89 were achieved. The experiment was accurately simulated by a computer model using only measured input parameters. The model was further used in an analysis of power amplifiers pumped at 980 and 1480 nm that included waveguide optimization and Er³⁺ confinement. The QCE can be enhanced by increasing the numerical aperture (NA) and confining the Er³⁺ ions to the central region of the core. At pump powers typically used for packaged EDFAs (25-100 mW). QCE can be improved by up to 60% by increasing the NA from 0.15 to 0.25, and confined Er³⁺ doping can provide an improvement of up to 20%. However, NA and Er³⁺ confinement have insignificant effects on the noise figure when both the cutoff wavelength and the fiber length are optimized with respect to QCE     

Gain-flattened tellurite-based EDFA with a flat amplificationbandwidth of 76 nm

We describe a tellurite-based Er³⁺-doped fiber amplifier (EDFA) with a flat amplification bandwidth of 76 nm and a noise figure of less than 7 dB. Furthermore, a parallel-type amplifier composed of this EDFA and a 1.45-μm-band Tm³⁺-doped fluoride fiber amplifier achieved a flat amplification bandwidth of 113 nm     

Anomalous dispersion in Er3+ and Yb3+-dopedfibers

In experimental and theoretical study of anomalous dispersion in Er³⁺and Er³⁺-Yb³⁺-doped fibers has been developed. Anomalous time delay caused by both absorption and emission at 1.535 μm has been theoretically calculated and experimentally measured. A pump power dependence of anomalous time delay in rare-earth-doped fibers has been theoretically calculated and experimentally investigated. It has been shown that pump power fluctuations lead to propagation time jitter in Er³⁺-doped fiber amplifiers. The pulse interaction due to refractive index change caused by gain saturation is predicted. It has been shown that for Er ³⁺-doped fibers with SiO₂-GeO₂ core composition, the anomalous dispersion per 1-dB gain is twice that of fibers with SiO₂-Al₂O₃ core, which is caused by gain curve form difference. A scheme of mutual compensation of intrinsic fiber dispersion and anomalous dispersion caused by Er³⁺ in the region 1.532-1.537 μm has been suggested     

3.6 Gb/s all laser-diode optical soliton transmission

3.6-Gb/s optical soliton transmission using a gain-switched 1.55-μm distributed-feedback laser diode and a Ti:LiNbO₃ intensity modulator is demonstrated. An Er³⁺-doped fiber amplifier and a Raman amplifier, both pumped by 1.48-μm laser diodes, are used for achieving intense optical pulses and fiber-loss compensation, respectively. The intensity-modulation direct-detection optical receiver of a commercial F-1.6 G system is used to measure the bit-error rate     

A 980 nm pseudomorphic single quantum well laser for pumpingerbium-doped optical fiber amplifiers

The authors have fabricated ridge waveguide pseudomorphic InGaAs/GaAs/AlGaAs GRIN-SCH SQW (graded-index separate-confinement-heterostructure single-quantum-well) lasers, emitting at 980 nm, with a maximum output power of 240 mW from one facet and a 22% coupling efficiency into as 1.55-μm single-mode optical fiber. These lasers satisfy the requirements on efficient and compact pump sources for Er³⁺-doped fiber amplifiers     

Theoretical characterization of Raman oscillation with intracavitypumping of fiber lasers

Theoretical results relating to the generation of continuous-wave (CW) output from fiber lasers that are internally pumped with light generated from the stimulated Raman effect are presented. This investigation establishes the important fiber and resonator parameters, such as the fiber length and glass composition, dopant concentration, and pump power required to realize this new form of fiber laser arrangement. Three examples are studied: the Ho³⁺-doped silica fiber laser that is pumped at a wavelength of 1.15 μm, the Er ³⁺-doped silica fiber laser which is pumped at 1.48 μm and, the Tm³⁺-doped silica fiber laser which Is pumped at 1.625 μm. These three examples cover first Stokes pumping, second Stokes pumping, and first Stokes pumping with direct dopant absorption of the pump light, respectively. The simulations involve the use of simple numerical models comprising the spatially dependent field propagation equations (under the slowly varying field approximation) and the rate equations for the population densities. It is established that intracavity Raman pumping of fiber lasers with first Stokes radiation is efficient when the losses at the pump, Stokes and laser wavelengths are kept low (<10 dB/km). It is also established that second Stokes pumping is, even with direct absorption of the pump light, theoretically quite efficient and, as a result, the Er³⁺-doped silica fiber laser which is pumped with second Stokes radiation at 1.48 μm may provide the best demonstration of intracavity Raman pumping     

Analysis on the transient response of 1.55-μm/1.4-μmdual-wavelength pumped thulium-doped fiber amplifiers

The transient response of S- and S⁺-band thulium-doped fiber amplifiers with 1.4-μm/1.5-μm dual-wavelength pumping scheme was investigated both experimentally and theoretically. In contrast to conventional Erbium-doped fiber amplifiers, two characteristic time-constants related with ³F₄ and ³H ₄ level lifetime were observed, implying the need of much complex transient control algorithm for the future applications. The amount of surviving channel gain, gain excursion, and related time constants showed changes in their response characteristics, depending on the combinations of main pump, subsidiary pump, and signal powers. A simplified numerical approach for the analysis of transient response in thulium-doped fiber amplifiers under average inversion framework, will be provided