A single EDFA may be used for simultaneously amplifying many data channels at different wavelengths within the gain region; this technique is called wavelength division multiplexing. The average excitation level over the whole fiber length, as is relevant for the net gain spectrum, depends on the pump and signal powers, but also on the fiber length and the erbium concentration.
Relatively low-noise performance can be achieved with suitable amplifier design, taking into account particularly the erbium excitation level near the signal input end, which can be strongly influenced by, e.
For example, silica fibers can be optimized with various codopants, and fluoride fibers can offer a fairly flat gain spectrum. At longer wavelengths e.
A filter centered at nm is introduced in the middle of the EDF in a way that equalizes the gain of the two channels. Other types of fiber amplifiers, e.
For a proper choice of the material composition of the fiber corethis energy transfer can be fairly efficient. Due to the quasi-three-level nature of the erbium ions, ASE powers can be different between forward and backward direction, and the maximum ASE can occur at a wavelength which differs from that of maximum gain.
Various aspects of erbium-doped amplifiers can be analyzed with suitable fiber simulation software. The pump light is injected via dichroic fiber couplers.
The resulting quantitative understanding can be the basis for optimization of devices in terms of performance and required components.
In-line EDFAs are used between long spans of passive transmission fiber. For example, in optical fiber communications with wavelength division multiplexingthe wavelength dependence of the gain can unbalance the powers in the transmitted channels.
Only over thousands or millions of symbols, the gain adjusts itself to the average signal power level. The system layout is shown in Figure 3 and the parameters used are similar to the parameters in Table II .
Due to the not very high laser cross sectionsthe saturation power of an EDFA is fairly high compared with that of a semiconductor optical amplifier.
Its core is the erbium-doped optical fiberwhich is typically a single-mode fiber. Figure 2 equivalent to Figure 4 b  In this second case, the saturation effect of an amplifier is shown using two wavelength multiplexed signals at nm and nm.
It is more common, however, to use avalanche photodiodeswhich have some built-in signal amplification.
These functions can be realized in the telecom C and L bands. Figure 2 shows gain of pump and signal, and the power of backward and forward ASE as a function of the pump power. You will hardly find a more convenient resource.
The power of a data transmitter may be boosted with a high-power EDFA before entering a long fiber span, or a device with large losses, such as a fiber-optic splitter. Such parameters together with the choice of glass composition are used to optimize EDFAs for a particular wavelength region, such as the telecom C or L band.
It is also possible to amplify ultrashort pulses in the 1. Erbium-doped double-clad fibers can be used for generating very high output powers of tens of watts or even more. Therefore, it is common to apply methods for gain equalization, also called gain flattening.
Using multiple amplifiers in a long fiber-optic link has the advantage that large transmission losses can be compensated without a letting the optical power drop to too low levels, which would spoil the signal-to-noise ratio, and b without transmitting excessive optical powers at other locations, which would cause detrimental nonlinear effects due to the unavoidable fiber nonlinearities.
Another approach is a split-band amplifier, where a wavelength-dependent splitter distributes the signal content over two or more different fiber amplifiers, and another wavelength-dependent fiber coupler serves to recombine the spectral components. Therefore, single symbols in high bit rate data transmissions have a much too low energy to cause any significant gain saturation.
This figure is equivalent to Figure 7 in . However, stimulated emission by pump light then limits the achievable excitation level, hence also the gain per unit length, and the maximum gain occurs at longer wavelengths. Gain Spectrum The shape of the erbium gain spectrum depends on the absorption and emission cross sectionswhich depend on the host glass.Gain equalization is a technique for making the gain spectrum of an optical amplifier device flatter over a certain optical frequency range.
Perceptibly, MMZI can be considered among the categories of all-fiber wavelength filters like long-period fiber gratings (LPFGs), fiber Bragg gratings (FBGs), MZIs, fiber couplers/tapers, cascaded Hi-Bi fibers, Sagnac interferometers,, etc.
The above mentioned all-fiber devices have also been employed as gain equalization filter for. [6 ] “Gain Equalization of Erbium Doped Fiber Amplifier With Tunable Long Period Grating”, Thabiso J.
Nhlapo, Pieter L. Swart & Anatoli A. Chherbakov, Center for Optical Communication &. Abstract: We present a method for the synthesis of long-period fiber gratings (LPFGs) which is based on a genetic algorithm (GA).
The method is applied in the design of LPFG-based gain equalizers for erbium-doped fiber amplifiers (EDFAs). Differently from other works, gain equalizers designed by our. Design of a robust thin-film interference filter for erbium-doped fiber amplifier gain equalization Pierre G.
Verly. Figure 2: Amplifier gain and ASE power versus pump for an Er3+-doped fiber amplifier. Note: Figure 2 equivalent to Figure 4 b)  In this second case, the saturation effect of an amplifier is shown using two wavelength multiplexed signals at nm and nm.Download