The current evidence for clustering of erbium ions in silica glass is reviewed, including experiments on fiber amplifier efficiency, excited state decay, and nonsaturable absorption in Er³⁺ pumped at 980 nm. Experiments performed by the authors on nonsaturable excited state absorption in Er³⁺ pumped at 980 nm add further evidence for clustering. It is found that the degree of clustering is much reduced with the addition of Aluminum to the glass. However, it is also found that ion-ion interactions can potentially limit amplifier efficiency even when no clustering is present.

11Erbium Doped Fiber Laser (EDFL) experimental slope efficiencies are used in conjunction with a paired ion model to estimate the percentage of paired ions in a fiber as a function of the erbium concentration. These percentages are compiled with other published results to derive a dependence of percentage of pairs on the aluminum co-dopant concentration. A mole ion concentration ratio of 20 aluminum to 1 erbium is found to greatly reduce pairing. Higher ratios are found to have minimal added benefit.

An in-line scheme of an Er³⁺-doped fiber laser with self-starting passive mode locking using the effect of polarization ellipse self-rotation is suggested. The laser has been studied experimentally. For the pump radiation with a power of 150 mW and wavelength of 532 nm, a self-starting generation of pulses with 3 - 3.5 ps duration has been obtained.

The defect driven properties of erbium doped fiber amplifier devices are reviewed examining the role of the host dopants, aluminum, phosphorus and germanium, typically used in these applications. The effects of added loss on the performance of amplifiers are shown by way of modeling. Measurements of the gamma and UV radiation induced absorption are presented to show the generic host glass composition effects on radiation induced loss and their relative magnitude. In contrast to these `bad' defects, the UV induced effects in H<sub>2</sub> impregnated fiber are discussed in their role as `good' defects, allowing the more efficient writing of UV induced gratings.

Erbium-doped optical fibers have been manufactured with a vapor phase technique. A `hot line' has been designed and realized which allows us to fabricate low and high alumina contents fibers, to make laser sources and optical amplifiers working in the third telecommunication window.

We present rate equations describing the effects of clusters on Nd-doped fibers. Two models are presented, based on formalisms found in the literature. The first one, valid for any cluster size, describes the interaction between an excited ion and a ground state ion in proximity to each other as a non-radiative process referred to as cross relaxation. The second model, valid only for large clusters, describes this interaction as cooperative upconversion. The effects of clustering on two experimental applications are explored: nonlinear phase shift in all-optical fiber switches and cw transmission saturation behavior of clustered fibers. Both measurements suggest that in some fibers as many as 60% of the available dopant ions are clustered. The extent of clustering is experimentally linked to the concentration of Nd in the fibers tested, and perhaps to the use of co-dopants as well.

A passive WDM channel equalizer using a twin-core EDFA is studied in detail. Gain saturation limits the range of input signal powers from approximately -20 dBm to approximately 0 dBm. Channel equalization rates as high as 0.35 dB/dB are predicted. When used in cascade, twin-core EDFAs are shown to provide channel power stabilization and self- healing against additional channel losses.

Simulations are used to examine the effects of Er³⁺ pairs on the characteristics of erbium-doped fiber amplifiers. As the percentage of pairs is increased, the most significant effect is that the small-signal gain steadily drops. On the other hand, the optimum fiber length (for maximum gain at a given pump power) and the saturation power (at optimum length and constant gain) vary minimally up to pair concentrations of 40%. The noise figure shows degradation with increasing pair concentrations that is due entirely to the reduction in gain, i.e., at constant gain a paired and unpaired amplifiers are predicted to have the same noise figure. The main effect of pairs is therefore to increase the amplifier pump power requirement.

The influence of the center wavelength of fused-taper WDM couplers for use in EDFAs with pumping around 1480 nm is investigated. This is done by computer simulations of the EDFA using measured coupler characteristics of three manufactured WDM couplers. It is shown that using a WDM coupler with a broadened channel spacing a higher bandwidth of the gain and the noise figure spectrum is obtained due to lower excess and insertion losses. In addition, the manufacturing process of these WDM couplers is highly reliable.

The optical properties of praseodymium-doped glasses have attracted considerable attention recently for their potential application as a 1.3 micron optical amplifier. We report here on our spectroscopic evaluation of a series of low-phonon-energy glasses based on halides and sulphides. These results, though driven by the desire for a practical amplifier, provide insight into the application of these glasses not only for telecommunications applications, but also an understanding of the overall optical properties of a low-phonon-energy glass. Using Raman spectroscopy, the vibrational characteristics of the glass host are determined. Absorption measurements across the visible and infrared allow evaluation of the intrinsic loss of these glasses when in fiber form, as well as providing an indication of glass purity. Fluorescence of Pr³⁺-doped glasses, through excitation of the ³P₀, ¹D₂ and ¹G₄ levels, is measured along with the fluorescence lifetimes. These radiative properties are compared to those predicted by a Judd-Ofelt analysis, which has been performed on all glasses. In this way, this work provides an overall spectroscopic evaluation of the optical properties of low-phonon-energy glasses, leading the way towards a practical device.

We report on recent progress towards the application of both mixed cadmium halides and sulphide-based glasses as host materials for a Pr³⁺-doped 1.3 micron optical fiber amplifier. Both of these materials offer the potential for higher gains than can be currently achieved in a Pr³⁺-doped ZBLAN fiber. Fundamental glass properties, including optical and thermal characteristics, are compared. Losses of these glasses in fiber form have been estimated and spectroscopic measurements are summarized. From these studies we show that quantum efficiencies over an order of magnitude higher than those demonstrated with Pr³⁺-doped ZBLAN amplifiers are in principle obtainable. Measured efficiencies of 11% and 52% for the Cd halide and Ga sulphide, respectively, are achieved, while 25% and 80% are predicted. Numerical modelling allows comparison of the expected amplifier performance. Thermal analysis has identified the challenges which remain for the drawing of single mode fibers and the results of preliminary fiber drawing trials are described. The relative merits of each of the various glasses are considered and the challenges before a practical amplifier is achieved are outlined.

We report the noise and gain characteristics of a 1.3 micrometers PDFFA, in both the small and large signal regime. The small-signal noise performance shows a value of 5 dB at 1.3 micrometers , reducing to < 4 dB for wavelengths < 1.28 micrometers , and increasing to > 7 dB for wavelengths > 1.32 micrometers . No noise penalty has been observed in saturation for output powers as high as + 17 dBm, corresponding to 10 dB gain compression. Crosstalk measurements show a 3 dB rollover frequency of 6 kHz.

We present a simple rule for predicting the peak gain wavelength of an Er-doped fiber amplifier. For a given fiber type, the peak gain wavelength is determined solely by the operating gain per unit length. Using this rule coupled with a simple Er-doped amplifier model and measured modeling parameters, the gain peak is predicted for a particular Er-doped fiber. The result is verified by direct measurement in a fiber loop, using polarization scrambling to eliminate instabilities. We demonstrate that the gain peak does not vary with pump power, pump wavelength, or signal power as long as the gain per unit fiber length is fixed. The theory is then extended to roughly estimate the pump power, signal power, compression level, and other design parameters. Finally, the inclusion of the wavelength dependences of other components in an amplifier chain is also discussed and demonstrated.

We report on optically induced, sub-microsecond optical switching in an elliptical core neodymium doped, two-mode fiber. The improved response time of this switch, which is three orders of magnitude faster than the 400 microsecond(s) metastable lifetime of Nd³⁺, was provided by rapid cross relaxation within neodymium clusters.

Distortions induced by erbium doped fiber amplifiers on optical AM signals are shown, experimentally and theoretically, to originate from the so called locked-inversion gain tilt of the amplifier. The locked-inversion gain tilt may be measured by simple methods, or it may be calculated from a standard steady-state numerical model. It is demonstrated by experiment and calculation that the gain-tilt, and hence the harmonic distortion, may be reduced to acceptable levels by proper design of the fiber amplifier without the need for separate equalization.

In this paper, we investigate soliton propagation in long fibers using lumped amplifiers with nonideal gain. If the gain is invariant, but less than the ideal value, the perturbation analytical results can be found under the condition that the difference between actual gain and ideal value is small by introducing the concept of equivalent loss coefficient. If the gain fluctuates around ideal value, the numerical results indicate that the soliton can still propagate stably over ultra- long distance when the standard deviation is less than 1% of the average gain.

A fluorozirconate-derivative glass has been identified having superior optical properties for the purpose of fabricating high NA, low loss fiber used in the Pr³⁺-doped fiber amplifier. In the most promising glass, the AlF₃ in traditional ZBLAN has been replaced with YF₃ and InF₃, and NaF has been partially replaced with CsF for improved stability. The ¹G₄ lifetime of Pr³⁺ in this new glass is almost 25% longer than in ZBLAN. The properties of partially chlorinated glasses are also presented.

We report the development of a simple self starting passively mode locked diode pumped laser oscillator utilizing erbium doped fiber and a minimum number of readily available components. With pump powers as low as 8.0 mw, the oscillator generates stable pulses of 1.2 ns width and rates of 5.0 MHz at wavelengths around 1.55 micrometers . Evidence exists for substructure as short as 35 ps within the 1.2 ns pulses. A mechanism for the observed characteristics of the oscillator based on the all optical Kerr effect is proposed.

In this presentation, preliminary observations and measurements of increased switching speeds in resonantly enhanced nonlinear systems are shown. The method uses stimulated down- pumping of the ⁴F_3/2 metastable state in neodymium, but is applicable to any four- level system, or pseudo-four-level system such as erbium. Since the stimulated down-pumping is only initiated at the end of each switching period, the lifetime of the metastable state is not affected. Thus high up-pumping efficiencies are still possible. This allows both low power operation and fast relaxation to be achieved in the same switching system. The relaxation time has been reduced from approximately 390 microsecond(s) to less than 120 microsecond(s) , and it is shown that the reduction is dependent on the power of the down-pumping source.

We describe the design and performance of single pump 980 nm pre and post Er³⁺ doped fiber amplifiers plug-in modules. We demonstrate the potential and robustness of these amplifiers in a 980 nm-based rack mounted repeaterless 5 Gb/s system with a 54 dB optical power margin system. Excellent long term bit error ratio measurements (< 10^-14) are reported for various system configurations. In particular, error free transmission is obtained with suppression of stimulated Brillouin scattering through 60 km of non-dispersion fiber.