Optical fibers may be doped with rare earth ions using several methods. While bulk methods such as rod-in-tube may be employed low-loss fiber requires adaptations of standard fiber fabrication processes namely modified chemical vapor deposition and vertical axial deposition. After discussion of these variations some design issues related to optical amplifiers are discussed.
The site distributions for a given Stark-level have been determined for Nd" and Er" dooed silicate glasses, preforms and fibers from fluorescence line narrowing experiments near liquid helium temperature. The total energy 5j,read increases from bulk to fibers, from 17Z ctn to 24 cn for Nd doped materials, and from 45 cs to 193 ctnt for aluminosilicate Er3' and Nd doped preforms respectively. In that case, we conclude that inhomogeneous broadening of the 300 K luminescence band is more effective for the Nd doped fiber than for the Er3' one.
Excited-state-absorption (ESA) cross sections were determined for the region between 760 and 900 nm for Er-doped fluorophosphate phosphate and silicate glasses. Measurements were performed on multimode fibers pumping at 647 nm with powers 1 . 5 Wto invert the population into the saturation regime. Over much of the 800-nm band ground-state-absorption (GSA) cross sections are equal to or greater than ESA cross sections. For comparison ESA was also measured for singlemode Al/P-doped silica fiber. The cross sections were incorporated into an amplifier model and the phosphate and fluorophosphate glasses were found to provide higher gain than silica for pumping in the 800-nm band. Photoexcited fluorozirconates were found to have substantial populations in the first four excited states and ESA transitions originating from these states are identified.
The initial predictions that fluoride glasses would find important applications as laser host materials are now becoming fulfilled. Their attraction lay in their low non-radiative decay rates, a result of their lower fundamental phonon energies (also making them transparent out to 5.0um). These low non-radiative rates promised higher efficiencies for many lasing transitions, but also the possibility of new transitions not feasible in other glass compositions. In the last three years, since the initial announcement of the first fluoride fibre laser, the field has mushroomed and to date fibrelaser operation has now been demonstrated on 20 differenttransitions ranging from 0.455 um in the blue to 2.9 um in the mid-JR. Moreover fibre optical amplifiers based on monomode fibre have also now been demonstrated and useful gains observed at 1.34, 1.54 and 2.7 um. Clearly fluoride glasses now provide an interesting alternative to silica based systems for rare-earth (lanthanide) doped fibres, and the first commercial devices are not far away.
Laser sources based on rare-earth-doped single-mode optical fibres offer considerable potential as narrow-linewidth sources. Fibre laser sources have the capacity to produce highly-coherent low-noise output pumped by laser diodes. Fibre lasers are inherently compatible with optical fibres for transmission and sensing applications. Techniques for producing narrow-linewidth and single-longitudinal-mode operation in fibre lasers are reviewed.
Stable, broadband, long-wavelength sources are required for accurate fiber sensors such as the fiberoptic gyroscope. The Er-doped superfluorescent fiber source and wavelength-swept fiber laser, which emit near 1 .55 m and can be pumped near 980 nm, are excellent candidates for this application. We discuss the design of such sources, their efficiency, pump source requirements, and the spectra they produce. The spectrum sensitivity to environmental factors such as temperature is also briefly discussed.
Characteristics of erbium-doped fiber amplifiers relevant for applications in optical communications are reviewed. Special attention is given to the relationship of saturation power to pump power the dependence of saturation characteristics on host glass erbium-doped fiber amplifiers with reduced pump requirements and a comparison of pumping at 980 nm and 1. 48 tm.
We present results demonstrating that Er doped silica fibres may be operated as efficient high power oscillators and high gain power amplifiers in the 1i5um telecommunications window. Investigation of the saturation behaviour of a number of different fibre types is undertaken. The best of these provides 140mW of amplified output for a 1mW input signal at 1. SSpm pumped with 180mW at 1. 48pm. Pumping with higher powers at 980nm provides in excess of 500mW output for the same input signal power. Quantum efficiencies greater than 80 are achieved for both pump wavelengths. A 980nm contra-directionally pumped grating tuned oscillator is demonstrated giving a tunable output of over 250mW for lasing wavelengths between 1. 52jm and 1. S7jm and a launched power of 540mW. This represents an internal quantum efficiency in excess of 95. The results of two systems experiments utilising the power amplifiers are also presented. The first demonstrates a ten channel WDM network broadcasting to over 7 users111 and the second an unrepeatered 250km 2. 4Gbit/s transmission system12
The dependence of gain in Erdoped fibre on Vvalue refractive index dip confinement of Er atoms and diffusion at the core cladding interface is presented. The absorption of the pump light and the gain of the signal light are calculated by solving the population density equations as a function of radial and axial position in the fibre. When the pump wavelength is 980 nm the optimum Vvalue at signal wavelength 1530 nm of a step index fibre increases from 1. 16 to 1. 29 with the pump power increasing from 2 mW to 8 mW. When the pump wavelength is 1490 nm the optimum Vvalue is 1. 37 without pump power dependence. With the confinement of the dopant atoms the gain can be improved as much as 68 . The confinement increases the optimum Vvalue to around 1 . 44. The gain is decreased 37 by an index dip which is 75 of the core diameter and 100 of the index difference. The diffusion of the dopants at the interface between the core and the cladding decreases the calculated gain 17 when the core radius increases 40 due to the diffusion measured at 10 index difference level.
To explore the fundamental limits on Nd3-doped fiber amplifier performance at 1 . 3 m an analysis of the smallsignal pump efficiency saturation properties and signal/noise ratio has been performed. Ignoring ESA the pump efficiency (dB/mW) is found to be only moderately sensitive to the choice of glass host indicating that ESA is the critical parameter distinguishing experimental results reported for different materials. For fundamental reasons the pump efficiency for a linear Nd3 amplifier without ESA is an order of magnitude less than for an Er amplifier and degrades still further if ESA is included. Despite the general expectation of quantum-limited performance for a four-level amplifier the noise figure is found to degrade significantly if ESA is introduced. From this analysis we conclude that applications as power amplifiers are more promising because high power conversion efficiencies can be obtained and there is less sensitivity to noise figure. Nevertheless acceptable performance in any application requires operation at wavelengths where the ESA cross section is only a small fraction of the stimulated emission cross section.
The present understanding of colour centres in germanosilicate glass fibres and the diverse effects attributed to colour centre activity are reviewed. Drawing on a wide range of up-to-date research results an attempt is made to piece together as far as possible a unified picture of the defect processes behind second harmonic generation nonlinear transmission and photorefractive grating formation in optical fibres.
We have investigated several techniques for generating ultrashort pulses from fiber lasers. The actively modelocked erbium-doped fiber laser has produced pulses as short as 4 psec at a wavelength of 1530 nm. We have also generated pulses as short as 160 fsec in a synchronously pumped fiber Raman soliton laser. The importance of group velocity dispersion in these laser systems and the advantages of each modelocking technique are reviewed.
The trivalent thulium ion is an interesting activator for silica fiber lasers because of the near infrared transition which is broadband tunable ( diode pumpable can be operated with photon conversion efficiencies greater than 100 and has yielded in excess of 1W output power when pumped by a cw Nd:YAG laser. The paper will review progress on this system and indicate some potential future developments.
Simultaneous laser oscillation near 2m and 2. 3jm on the 3H4 -3H6 and 3F4 - 3H5 transitions respectively has been demonstrated in a thulium-doped fluorozirconate fiber when pumping at 79mm. Diode-pumped operation of the 3H4 - 3H6 transition has also been achieved. Pulsed laser emission at 1 . 72jm has been observed on the 3/2 ''9/2 transition in an erbium-doped fluorozirconate fiber when pumping at 488nm or 5l4nm.
A theoretical analysis of the erbium-doped fiber amplifier completely describes this device. This modeling is developed within the most attractive configurations : 1. 48im and O. 98im pump wavelengths. The original approach used to take into account the amplified spontaneous emission (ASE) enables us to fully characterize the amplifier at any pump and signal regime. Thus we may quantitavely predict the influence of optogeometrical parameters erbium distribution and spectroscopic properties pump and signal powers and wavelengths and amplifier lengths over the amplifier performances in terms of gains residual pump powers co- and counter-propagating ASE frequency components. The agreement between predictive calculations and experiments appears truly excellent. Moreover this original ASE modeling results in noise figures compelling with published experiments and this analysis may also be used for superfluorescent sources design. Due to its quantitative nature this numerical tool can be used for erbium-doped fiber optimization optimal set-up conditions for amplifier modules transmission link evaluations and network applications.
Advances in the field of erbium doped fiber amplifiers have stimulated interest in their application to future undersea systems. In this paper we summarize analytical models that suggest the feasibility of 2. 5 Gbit/s transmission over trans-oceanic systems using erbium doped fiber amplifiers as repeaters. We also discuss recent results of high bit-rate fiber amplifier experiments.
Experimental results will be presented which demonstrate linear and nonlinear transmission over distances of up to 100km. The optical pulse source is a prototype packaged mode-locked semiconductor laser operating around 1560nm at a repetition rate of 8GHz. Both erbium-doped fibre amplification and Raman gain in the transmission fibre itself have been used to access the fibre nonlinearity. A model has been developed which accurately predicts the transmission over 100km by including the chirp on the signal laser. This model has been used to compare the performance of discrete and distributed fibre amplifiers in this application over transmission distances of up to 1000km.
A large signal model of three-level superfluorescent sources (SFS''s) is presented. It is found that due to the ground state signal absorption present in three-level systems theoretical backward signal quantum efficiencies well in excess of the limit of 0. 5 exhibited by four-level systems can be achieved. Feedback is also studied and it is shown that when used in high feedback applications SFS ''s may require the use of an optical isolator.
The A. M. and F. M. noise mechanisms in a number of rare-earth doped fiber devices are reviewed. Spontaneous emission noise presents an ultimate limit to the performance of a number of fiber laser and amplifier devices and the paper concentrates primarily on this area. Important concepts are reviewed and experimental and theoretical data are presented.
The spectral thermal stability of broadband rare-earth-doped fiber sources makes them attractive for fiber sensor applications. We quantify the mean wavelength variation of both Nd- and Er-doped fiber sources operating as superfluorescent fiber lasers. Besides the intrinsic variation of such sources the effects of pump power and pump wavelength are also considered since both the power and wavelength of conventional laser diode pump sources are temperature sensitive. Other types of rare-earth-doped sources are also briefly considered.
A CWpumped Nd-doped single-mode double-clad fiber laser coiled around a resonant piezo-electric ceramic ring and coupled to an empty external cavity with strong feedback is described. Intensity laser pulses with duration in the 1-2 range are observed as the pump power or the ring drive voltage is increased yielding peak powers up to 25 times greater than average powers. The effect is attributed to strong phase modulation of the fiber laser by the piezo-electric ring which induces FM laser oscillation the external cavity acts as a frequency dependent reflector thus effectively cavity dumping the fiber laser as the oscillation frequency is swept through the narrow transparent windows of the external cavity.
A comprehensive model is used to analyze gain and noise performance for Erbium doped fibre amplifiers pumped at 980 and 1480 nm. A minimum noise figure is shown to be given by emission to absorption cross section ratios at both pump and signal wavelength. The deterioration from the minimum noise figure in each pumpband is shown mainly to be due to the presence of the counter propagating Amplified Spontaneous Emission. It is shown that for the preamplifier the fibrelength for optimum receiver sensitivity is up to 50 % smaller than the maximum gain fibre-length