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     

Modeling of gain in erbium-doped fiber amplifiers

An analytic method is described for fully characterizing the gain of an erbium-doped fiber amplifier (EDFA) that is based on easily measured monochromatic absorption data. The analytic expressions presented, which involve the solution of one transcendental equation, can predict signal gains and pump absorptions in an amplifier containing an arbitrary number of pumps and signals from arbitrary directions. The gain of an amplifier was measured over a range of more than 20 dB in both pump and signal powers. The measured theoretical results agreed to within 0.5 dB. Although the results described apply explicitly to EDFAs pumped in the 1480-nm region, they are also applicable to EDFAs pumped in the 980-nm region. The method is valid whenever the gain saturation by amplified spontaneous-emission noise can be neglected, which is typically the case for amplifiers with less than about 20 dB of gain     

Dynamic gain compensation in saturated erbium-doped fiber amplifiers

Dynamic compensation of low-frequency gain fluctuations in saturated erbium-doped fiber amplifiers is demonstrated. This compensation, based on a simple feedback-loop scheme makes it possible to reduce transient gain fluctuations efficiently across the whole amplifier bandwidth using only a low-power optical feedback signal. Such an, automatic gain control technique could be applied to suppress data packet interference due to traffic bursts in multiple-access networks, as well as in the implementation of long-haul fiber systems using erbium fiber amplifiers.<>     

On the optimization of hybrid Raman/erbium-doped fiber amplifiers

A comprehensive theoretical study on the optimal configuration of hybrid Raman/erbium-doped fiber amplifiers has been carried out yielding a closed form analysis. In order to compare different system configurations, a weight for the impact of fiber nonlinearities has been introduced. The maximum reachable distance has been evaluated as a function of the span length and nonlinear weight, given a target optical signal-to-noise ratio     

AC and DC gain tilt of erbium-doped fiber amplifiers

AC gain tilt and DC gain tilt are defined as the variable gain of an erbium-doped fiber amplifier (EDFA) with input laser wavelength chirp due to modulation, and with different static laser unmodulated wavelengths of no modulation. This paper proposes a novel experimental setup for measuring AC gain tilt. This paper also clarifies experimentally and theoretically the dependency of the AC gain tilt on the input light modulation frequency from near DC to high frequency. The measured AC gain tilt values at high frequency resemble the measured spontaneous gain tilt values for input wavelengths from 1530 to 1580 nm. The influence of the AC gain tilt effect on AM-SCM video transmission systems is evaluated     

Analysis of AC gain tilt in erbium-doped fiber amplifiers

We analyze theoretically and experimentally the dependence of AC gain tilt variations on gain variations in homogeneously broadened erbium-doped fiber amplifiers. Theoretically, we find that this dependence can be evaluated from the fiber's absorption spectrum. This is experimentally verified for gain variations induced by a pump-power reduction. For AM CATV, AC gain tilt variations limit how much the gain can vary before composite second order distortions become too large. From this point of view, for an erbium-doped alumino-germanosilicate fiber, an operating wavelength between 1542-1555 nm is more suitable than other ones in the long wavelength range of the EDFA, normally considered for AM CATV.     

Analysis of gain dynamics of erbium-doped fiber amplifiers for wavelength-division-multiplexing networks

The gain dynamics in erbium-doped fibers (EDFs) with various numerical apertures (NAs) and cutoff wavelengths used in amplifiers employed in wavelength-division-multiplexing (WDM) networks is investigated by using the time-dependent amplifier model. The calculation shows that the transient response at a time immediately after some channels have been dropped depends on the cutoff wavelength, and the cutoff wavelength for the highest transient response is independent of NA. The calculation also shows that the transient response for a constant cutoff wavelength increases with an increase in NA. Experimental results for the transient response of EDFs with similar NAs and different cutoff wavelengths agree with the calculated results for various input signal powers, input signal power changes, and surviving channel wavelengths when there is no influence of neglection of amplified spontaneous emission and EDF background loss on the calculation.     

Noise penalty of highly saturated erbium-doped fiber amplifiers in analog links

We outline the noise penalty of using a high-power erbium-doped fiber amplifier in the place of a shot-noise-limited laser for analog applications. Intensity noise measurements are presented using a highly saturated +22-dBm output-power amplifier, demonstrating the achievable signal-to-noise ratios when passive optical loss follows the amplification stage.     

The contribution to cross-phase modulation in L-band WDM systemsfrom erbium-doped fiber amplifiers

It has been suggested that the level of cross-phase modulation (XPM) produced in erbium-doped fiber amplifiers (EDFA's) may be greater than that produced in the transmission fiber. We measure and compare the levels of XPM produced in an EDFA to that produced in both dispersion-shifted fiber (DSF) and conventional transmission fiber for a 10-Gb/s L-band wavelength-division-multiplexed system. Our results show that, for a system with twenty channels, the levels of XPM produced by our amplifier was negligible compared to that from the DSF, and a factor of approximately nine smaller than for the conventional fiber     

An erbium-doped tellurite fiber amplifier for WDM systems with dispersion-shifted fibers

This letter investigates the feasibility of an erbium-doped tellurite fiber amplifier (EDTFA) with a 1581- to 1616-nm amplification band for a wavelength-division-multiplexing transmission system in the L-band that employs dispersion-shifted fiber (DSF). A 10-Gbit/s, 40-channel-transmission from 1581 to 1616 nm in the L-band, where EDTFA's are employed as a post-amplifier, repeaters, and a pre-amplifier, is error-free through a 320-km DSF. The EDTFA can increase both the transmission capacity and the allowable maximum input power launched into a DSF.     

Genetic algorithm optimization of a hybrid analog/digital predistorter for RF power amplifiers

This study presents a genetic algorithm optimization of a hybrid analog/digital predistorter, in order to reduce the intermodulation distortion (IMD) caused by the nonlinear properties of the radio frequency (RF) power amplifier (PA). Designed predistorter based on polynomial work function and the coefficient of the polynomial is optimized in order to reduce IMD by spectrum monitoring. The design procedure and validation of predistorter have been carried out by Agilent-ADS2005A. In order to validate the predistorter two different modulation schemes as CDMA and 16-QAM have been used. Also in order to verifying the linearization a test power amplifier circuit has been examined including Motorola MOSFET MRF9742 showing the nonlinear characteristics with memory. Simulations have been shown that adjacent channel power ratio (ACPR) improvements were acceptable for both CDMA and 16-QAM modulation schemes.     

An optical bit-rate flexible transmission system with 5-Tb/s-kmcapacity employing multiple in-line erbium-doped fiber amplifiers

An optical bit-rate flexible transmission system using erbium-doped fiber amplifiers (EDFAs) is proposed, and the system design is discussed. An optical bit-rate flexible system using multiple in-line erbium-doped fiber amplifiers has produced a regenerative repeater spacing of 505 km at 10 Gb/s and 523 km at 5 Gb/s for direct-detection systems. This system proves that an optical bit-rate flexible system with a transmission capacity of 5.05 Tb/s-km can be feasibly constructed. System capacity is clarified both theoretically and experimentally. The power penalties involved are discussed. The related optical and electrical circuits proved operational above 10 Gb/s. The 523 km at 5 Gb/s and 505 km at 10 Gb/s transmission experiments successfully demonstrated that EDFAs effectively enhance a system's transmission capacity     

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     

1111 km, two channel IM-DD transmission experiment at 2.5 Gb/sthrough 21 in-line erbium-doped fiber amplifiers

Two-channel transmission is demonstrated in a 2.5 Gb/s intensity-modulated direct-detection system over a distance of 1111 km, using 21 in-line erbium-doped fiber amplifiers. The two channels incur virtually no penalty when they are transmitted simultaneously. Owing to the use of gain filtering in the amplifiers instead of discrete in-line optical filters, the overall bandwidth of the link is about 15 nm, for a sensitivity penalty smaller than 1 dB on one single transmitted channel     

Numerical study of passive gain equalization with twin-core fibercoupler amplifiers for WDM systems

This paper presents a numerical study of twin-core fiber coupler amplifiers for equalization of wavelength division multiplexing (WDM) channels. The propagation of WDM pulses within a coupler and a chain of couplers is analyzed. The real spectrum gain of the device is considered along with dispersion and nonlinear effects. The influence of intermodal dispersion is studied. A model is developed based on a set of linearly coupled nonlinear Schrodinger equations     

Performance of high-concentration erbium-doped fiber amplifiers

The full characteristics for two high-concentration erbium-doped fibers are reported. The comparison of the fibers characteristics indicates that design of fiber geometry can be used to partially compensate for the degradation of the amplifiers performance due to upconversion processes. For high NA fiber the 22-dB small-signal gain, and 3.5-dB noise figure are obtained from a 24-cm length of fiber. We report a photon quantum conversion efficiency of 28%, which corresponds to the highest efficiency obtained in heavily doped fibers     

Transient analysis of erbium-doped fiber amplifiers

The transient response of an erbium-doped fiber amplifier (EDFA) pumped at 1.48 μm, taking into account the gain-saturation effects due to the amplified spontaneous emission (ASE), is studied theoretically and experimentally. The theoretical model is used to predict the gain saturation and recovery times of an EDFA and its effects to the amplification of optical pulses     

Polarization dependent gain in erbium-doped fiber amplifiers:computer model and approximate formulas

A computer model for polarization dependent gain (PDG) in Er-doped fiber amplifiers (EDFA) is presented. The model assumes that each erbium ion possesses an ellipsoidal gain surface and that all ion orientations are equally likely. By dividing the ions into subsets based upon orientation and computing the inversion of each subset in the presence of polarized pump and signal waves, the model predicts the dependence of the PDG induced by this polarization hole-burning (PHB) on the design of the EDFA, the signal degree and state of polarization (SOP), and the pump SOP. For moderate gain amplifiers (made from the same fiber) with the same gain peak wavelength and the same compression level, the magnitude of the PDG is nearly independent of the EDFA gain. Internal and random fiber birefringence are included to model real fibers. In fibers which cause the signal SOP to walk rapidly around the Poincare sphere, the PDG is reduced by a factor of 2/3 when compared with a linear polarization-maintained signal. Scrambled signals and partially-polarized saturating tones are also considered. Simple rules are derived for predicting the PDG of a given EDFA     

Simultaneous distribution of multichannel analog and digital videochannels to multiple terminals using high-density WDM and a broad-bandin-line erbium-doped fiber amplifier

The simultaneous amplification of 16 distributed feedback (DFB) lasers over a 34-nm spectral range in an erbium-doped fiber amplifier is reported. Sixteen DFB lasers spaced at approximately 2-nm intervals and covering 34 nm in the 1.55-μm band were modulated with a total of 100 studio-quality analog FM-TV channels and six 622-Mb/s channels     

Average power analysis technique for erbium-doped fiber amplifiers

It is shown that an average power analysis technique similar to that used for semiconductor optical amplifiers can be used to analyze both the discrete and the distributed fiber amplifier for multiwavelength operation without the need for numerical integration. It is also shown to give results that are in excellent agreement with those from other studies. This approach gives performance insights that are not otherwise readily available