Abstract: Glasses of the present invention are tellurite and oxyhalide glasses doped with rare-earth ion, which can be applied to highly efficient optical amplifiers and lasers for optical communication. They are thermally and chemically stable during and after the fabrication processes of the optical fiber. The glass material includes 20˜70 mole % of TeO2, a heavy metal oxide, 0.001˜10 mole % of a rare earth ion dopant, 5˜30 mole % of MO, M being selected from a group consisting of Mg, Ca, Sr, Ba, Zn and Pb, and optionally 1˜20 mole % of R2O, R being selected from a group consisting of Li, Na, K, Rb and Cs. In the composition of the glass, 3˜18 mole % of MO and R2O may be substituted by the metal halides. The glasses of the present invention are similar in phonon energy to the conventional tellurite glasses not to increase the non-radiative transition rate. Further, the fluorescence lifetime is additionally increased in case of partial substitution of oxide to halide.

Abstract: A glass consisting essentially of antimony oxide. An optically active glass consisting essentially of antimony oxide and up to about 4 mole % of an oxide of a rare earth element. A rare earth-doped, antimony oxide-containing glass including 0-99 mole % SiO2, 0-99 mole % GeO2, 0-75 mole % (Al, Ga)2O3, 0.5-99 mole % Sb2O3, and up to about 4 mole % of an oxide of a rare earth element. The oxide of the rare earth element may comprise Er2O3. The glass of the invention further includes fluorine, expressed as a metal fluoride. An optical energy-producing or light-amplifying device, in particular, an optical amplifier, comprising the above-described glass. The optical amplifier can be either a fiber amplifier or a planar amplifier, either of which may have a hybrid composition. Embodiments of the glass of the invention can be formed by conventional glass making techniques, while some of the high content antimony oxide embodiments are formed by splat or roller quenching.

Abstract: A glass component in an optical system, which may be a lazing or an optical amplifying medium, comprising a silicate base glass doped with at least two Group III B elements, the glass, and a method of preventing clustering of a rare earth metal ion in the glass.

Abstract: Alkali tungstate, molybdate and vanadate glasses, and telecommunications components embodying such glasses, the compositions of the glasses consisting essentially of 15-70 mol percent of at least one oxide selected from the group consisting of WO3, MoO3 and VO2.5, 0-35% CrO3, 0-15% UO3, the total WO3 plus MoO3 plus VO2.5 plus CrO3 plus UO3 being 50-70%, 20-50% R2O where R represents at least two elements selected from the group consisting of Li, Na, K, Rb, Cs, Ag and T1, and optionally containing 0-10% MO where M is selected from the groups of elements consisting of Ca, Ba, Sr, Mg, Cd, Pb, 0-5 % X2O3 where x is at least one element selected from the group consisting of Al, Ga, In and Bi, 0-5% of at least one transition metal oxide, 0-15% P2O5 and/or TeO2 and 0-5% of a rare earth oxide selected from the lanthanide series.

Abstract: A family of alkali-tungsten-tellurite glasses that consist essentially of, as calculated in mole percent, 10-90% TeO2, at least 5% W03 and at least 0.5% R2O where R is Li, Na, K, Cs, Tl and mixtures, that may contain a lanthanide oxide as a dopant, in particular erbium oxide, and that, when so doped, is characterized by a fluorescent emission spectrum having a relatively broad FWHM value.

Abstract: A family of fluorotellurite glasses, the composition of which consist essentially of, as calculated in mole percent, 30-75% TeO2, 15-60% ZnF2 and 0.005-10% of an oxide of erbium, thulium or holmium, and amplifying optical components produced from these glasses.

Abstract: An amplifying optical fiber is proposed (particularly for the 1300 nm wavelength range) whereby a high amplification can be achieved with the smallest possible pumping power. Agglomerates comprising a number of complexes are incorporated into the core of the amplifying optical fiber. The complexes are composed of a rare-earth element and a separator substance.

Abstract: An optical fiber amplifier comprises an optical fiber for light loss which is provided in a later stage than an optical fiber amplification section or midway therein, the amount of light absorption and the wavelength dependency of the absorption of the optical fiber changing according to the power of input light; whereby the optical fiber amplifier can maintain flat gain in a wide wavelength band, the gain flatness not changing over a wide dynamic range and being maintained irrespective of variation in the power of input signal light.

Abstract: The present invention relates to a glass matrix which includes 4-70 wt. % SiO2, 0.5-20 wt. % Al2O3, 0-20 wt. % R2O, 0-30 wt. % R′O, 8-85 wt. % Ta2O5, 0-40 wt. % Nb2O5, and 0.01-1.0 wt. % R″2O3, where R2O+R″O is between about 2-35 wt. %, Ta2O5+Nb2O5 is between about 8-85 wt. %, R is selected from a group consisting of Li, Na, K, and combinations thereof, R′ is selected from a group consisting of Ba, Sr, Ca, Mg, Zn, Pb, and combinations thereof, and R″ is a rare earth element. The present invention also relates to use of the glass matrix in forming optic waveguides such as optic amplifiers. The present invention further relates to a transparent glass ceramic that contains pyrochlore, perovskite, or a combination thereof as its major crystal phase, and includes 4-40 wt. % SiO2, 1-15 wt. % Al2O3, 0-20 wt. % K2O, 0-12 wt. % Na2O, 0-5 wt. % Li2O, 8-85 wt. % Ta2O5, and 0-45 wt. % Nb2O5, wherein Ta2O5+Nb2O5 is at least about 20 wt.

Abstract: These glasses incorporate a combination of F and Al.sub.2 O.sub.3 to achieve even wider fluorescence and improved gain flatness. In addition, SPCVD incorporates large amounts of N into low-loss fiber whose high charge has an impact on rare earth behavior. The Surface Plasma Chemical Vapor Deposition (SPCVD) produces fiber preforms with high levels of F, Al.sub.2 O.sub.3, and N. These heavily fluorinated glasses provide much broader Er.sup.3+ emission than Type I or Type II silica for enhanced multichannel amplifiers. SPCVD successfully fluorinates silica with losses below 5 dB/km and increased Er.sup.3+ emission width.

Abstract: An optical amplifier having an optical waveguide structure which includes a first region in which a first dopant (for example, Er) is doped in a first glass composition (for example, Al--Si glass) and a second region in which a second dopant (for example, Er) and a third dopant (for example, Yb) are doped in a second glass composition (for example, Al--P--Si glass). The first dopant is pumped so that the optical waveguide structure may provide a first gain characteristic determined by the first glass composition and the first dopant. The third dopant is pumped so that the optical waveguide structure may provide a second gain characteristic determined by the second glass composition and the second dopant. By selectively obtaining the first and second gain characteristics, where the optical amplifier is applied to WDM (wavelength division multiplexing), the output powers of optical signals of different channels can be controlled independently of each other.

Abstract: A compact, continuous-wave blue laser is developed from a fiber made from heavy metal fluorides ("ZBLAN") doped with a rare-earth ion. The footprint required to create blue laser light is reduced because the fiber can be wound into spools of radius <25 mm and stacked one atop the other without cross talk. IR diodes (.lambda..about.790 nm and .lambda..about.1050 nm) are fiber-pigtailed to silica fiber in a conventional way. The light from the IR diodes is coupled to a single fiber through a 2.times.1 fiber coupler that has silica inputs and a ZBLAN output. The IR light optically excites the electrons of the rare-earth ions in the ZBLAN fiber host. This excitation causes the electrons to emit light at 480 nm (in the blue region of the visible spectrum) as they relax to the ground state. Dielectric mirrors feed back the emitted light. A high-reflector, high-transmitter ("HRHT") is the input coupler of the pumping light; a partial reflector, the output coupler.

Abstract: Our invention achieves spectral broadening of an input pulse by propagating it through a hollow fiber filled with a noble gas. The output pulse is then compressed to a shorter duration by passing it through an optical system consisting of phase dispersive elements. In a preferred embodiment, the input pulse is generated by a mode-locked laser, amplified and then subsequently compressed to produce an input pulse of high peak power. This input pulse is then focused into a hollow cylindrical fiber made of fused silica and mounted inside a pressure vessel filled with noble gas. The optics are designed to match the spatial mode volume of the input pulse to that of the entrance aperture of the hollow fiber. Light is constrained to propagate down the hollow fiber and generates additional bandwidth through self-phase modulation as it interacts with the gas contained therein. The spectrum of the pulse after propagation through the gas is frequency chirped.

Abstract: The core of a low phonon energy glass optical fibre is doped with trivalent Dysprosium ions for application as an efficient optical fibre amplifier operating around 1.3 .mu.m. The device utilises the transition .sup.6 H.sub.9/2 -.sup.6 F.sub.11/2 to .sup.6 H.sub.15/2. The low phonon energy (high quantum efficiency) glasses include mixed halides, chalcogenides and chalcohalides. Improvements in efficiency can be achieved by co-doping of the fibre core with Eu.sup.3+ and/or Tb.sup.3+ ions.

Abstract: An optical amplifier for a high power, solid state laser which includes a slab of a solid state laser material, for example, yttrium-aluminum-garnet (YAG) crystal. One or more diode arrays may be vertically stacked and configured to provide generally uniform energy distribution in the crystal in a vertical direction. By maintaining a relatively uniform energy distribution in the crystal in a vertical direction, thermal and stress aberrations of the resulting laser beam are minimized.

Abstract: The device integrates on the same glass substrate a passive monomode guide achieved by ion exchange in the substrate and an active monomode guide with lateral confinement by the substrate formed by an active thin layer deposited on the substrate and covering a zone in the form of a channel achieved by ion exchange in the substrate. The passive monomode guide constitutes the extension of the zone in the form of a channel of the active guide. The thin layer comprises a transition zone enabling adiabatic passage of the light from the passive guide to the active guide and vice-versa. The transition zone can be formed by an edge of the layer cutting the axis of the zone in the form of a channel obliquely or by a zone of variable thickness of the thin layer.

Abstract: Disclosed are an optical fiber comprising a core and a cladding wherein the core is composed of a In-Ga-Cd-Pb halide glass and the clad composed of a halide glass possesses a refractive index of 1.515 or less, which has a large specific refractive index difference (.DELTA.n) and a large numerical aperture, and such an optical fiber as mentioned above wherein the core contains one or more activating ions as well as an optical fiber amplifier comprising a pumping source, a laser glass fiber and a means for introducing pumping light and signal light into the above laser glass fiber, wherein the laser glass fiber is the optical fiber of the present invention mentioned above.

Abstract: A waveguide optical amplifier include a substrate and a guiding or active layer. The substrate has a substrate surface and a substrate index of refraction. The guiding layer is of Zirconium dioxide (ZrO.sub.2) and/or Alumina (Al.sub.2 O.sub.3) and is carried on a cladding layer over the substrate surface. The guiding layer has a guiding layer index of refraction which is higher than the cladding index of refraction. The Zirconium dioxide guiding layer is doped with rare earth materials.

Abstract: A rare earth-doped optical fiber is supplied with a input signal light and an excitation light, so that the input signal light is amplified to be supplied to an output as an amplified signal light. At this time, an intensity of an amplification of spontaneous emission (ASE) is detected to be compared with a predetermined intensity, thereby to provide a difference signal. An amount of the excitation light is controlled by the difference signal to equalize the ASE to the predetermined intensity.

Abstract: The present invention includes an optical fiber amplifier having a core and cladding layer. The core includes Zirconium dioxide (ZrO.sub.2) and a rare earth material. The core may be co-deposited with SiO.sub.2. A co-dopant of Ytterbium (Yb) may also be used. The optical fiber amplifier is well suited for use in the 1300 nm band when pumped with an optical source of appropriate power and frequency.

Abstract: A halide glass composition contains iodide and/or bromide. In addition the halide content preferably includes chloride. The preferred metal composition includes no aluminium and 1-10 mole % of (In+Y). These compositions are used as hosts for rare earth lasing dopants, e.g, Pr.sup.3+.

Abstract: Reflectors each having a reflectance peak wavelength within a band of the wavelength characteristics of a laser amplification factor in the unsaturated state of each light source for excitation are provided between light sources for excitation and an optical coupler, and stable wavelength locking is effected by returning a portion of a small portion of excited light reflected by the reflectors into the light sources for excited light.

Abstract: An optical material comprises a host matrix (e.g. fluorozirconate glass such as ZBLANP) doped with an optical atom pair or ion pair, each pair comprising a sensitizer (e.g. Nd.sup.3+) and an activator (e.g. Pr.sup.3+). The sensitizer is capable of absorbing optical excitation energy of a single wavelength (e.g. in the 800 nm region of GaAlAs diode laser) and transferring this optical excitation energy to the activator. This causes emission of visible and/or infrared light when the activator relaxes back into any of its lower energy states. Optical devices containing the optical material, and methods for generating visible and/or infrared light involving the optical material are also disclosed.

Abstract: A solid-state laser architecture producing a beam of extremely high quality and brightness, including a master oscillator operating in conjunction with a zig-zag amplifier, an image relaying telescope and a phase conjugation cell. One embodiment of the laser architecture compensates for birefringence that is thermally induced in the amplifier, but injects linearly polarized light into the phase conjugation cell. Another embodiment injects circularly polarized light into the phase conjugation cell and includes optical components that eliminate birefringence effects arising in a first pass through the amplifier. Optional features permit the use of a frequency doubler assembly to provide output at twice optical frequencies, and an electro-optical switch or Faraday rotator to effect polarization angle rotation if the amplifier material can only be operated at one polarization.

Abstract: An optical fiber power amplifier, particularly for telecommunication lines with optical fibers, including an active fiber doped with Erbium as a fluorescing substance, and with Al.sub.2 O.sub.3 as refraction index modifying dopant. The fiber is pumped with a laser providing a wavelength higher than 520 nm and preferably, 980 nm. The amplifier has a particularly high amplification efficiency close to the theoretical maximum efficiency.

Abstract: A laser has a resonant cavity defined by a pair of mirrors butted to respective ends of a fluorozirconate optical fiber. The fiber has a numerical aperture of 0.205 and an LP.sub.11 mode cut-off of about 2.0 .mu.m. The fibre is co-doped with thulium ions to a concentration of about 0.1%, and with terbium ions to a concentration of about 1%. An optical pump source provides a pump signal at 775 nm which excites the thulium ions into the .sup.1 G.sub.4 energy level to provide lasing at about 475 nm. The pump source is preferably a high power semiconductor laser.

Abstract: Halide glasses having particular utility as hosts for praseodynmium in order to provide optical amplification by laser activity have a halide content provided as a small proportion, e.g., 1-10 percent, of choride with the remainder as fluoride. The metal content is similar to conventional ZBLAN glasses. The replacement of A1 by Y and In and/or the partial replacement of Na by Cs has synergistic benefits. PR.sup.3+ constitutes a good lasing species for amplifying telecommunication signals at 1300 nm using pumped radiation at 1020 nm.

Abstract: Rare-earth doped single-crystal amplifiers serve to simultaneously amplify member channels of an optical fiber WDM system. A tailored amplification bandwidth sufficient for this purpose is the result of a variety of "defects" which after the local crystal field as "seen" by dopant ions.

Abstract: Plural planar optical devices are simultaneously pumped by a single pumping source. Various arrangements for accomplishing such pumping are disclosed. By utilizing these arrangements, the topology and routing of integrated arrays including optical devices are simplified.

Abstract: Fluoride glass-based optical fiber for an optical amplifier which contains rare earth metal ions in the core glass has a relative refractive index difference .DELTA.n between the core and the cladding of 1.4% or more. The core glass contains PbF.sub.2 in a proportion of 25 mol % or less based on the total composition of the core glass. The fluoride glass is doped with rare earth metal ions, and part of the fluorine in the glass may be substituted by at least one halogen Pr.sup.3+, Pr.sup.3+ --Yb.sup.3+, Pr.sup.3+ --Nd.sup.3+, or Pr.sup.3+ --Er.sup.3+ can be doped as the rare earth metal ions. Chlorine, bromine or iodine may be used as the halogen.

Abstract: A fiber optical source of stimulated optical radiation comprises an optical fiber which includes a core doped with laser material having optical gain in two wavelength regions, the fiber additionally including a material in optical communication with said laser material in such a manner as to absorb radiation emitted from said laser material within one of said wavelength regions.

Abstract: An optical signal amplifier comprising an optical waveguide fiber integrated by thallium ion exchange in a boron-free silicate glass doped with up to 5 weight % erbium oxide, the glass having a base composition, as calculated in weight percent on an oxide basis, consisting essentially of______________________________________ SiO.sub.2 38-67 Al.sub.2 O.sub.3 1.5-4.5 Na.sub.2 O 0-20 ZnO 1.5-8 K.sub.2 O 0-25 Li.sub.2 O 0-1 Na.sub.2 O + K.sub.2 O 15-30 PbO 0-37 BaO 0-7 P.sub.2 O.sub.5 0-10 P.sub.2 O.sub.

Abstract: A monolithically integrated photonic circuit combining a semiconductor source of excitation light with an optically active waveguide formed on the substrate. The optically active waveguide is preferably formed of a spin-on glass to which are added optically active materials which can enable lasing action, optical amplification, optical loss, or frequency conversion in the waveguide, depending upon the added material.

Abstract: A laser has a resonant cavity defined by a pair of mirrors (6, 10) butted to respective ends of a 3 m fluorozirconate optical fibre (14). The fibre (14) has a .DELTA.n of 0.014 and a cut-off wavelength of around 790 nm and is doped to about 500 ppm (weight) with erbium ions. An optical pump source (12) provides a pump signal at 971 nm which excites the erbium ions into the .sup.4 S.sub.3/2 energy level to provide lasing at about 546 nm. The laser may alternatively be pumped by a pump signal in the band 791 nm to 812 nm, preferably 801 nm. The pump source is preferably a high power semiconductor laser (16).

Abstract: Plural planar optical devices are simultaneously pumped by a single pumping source. Various arrangements for accomplishing such pumping are disclosed. By utilizing these arrangements, the topology and routing of integrated arrays including optical devices are simplified.

Abstract: Power laser includes a non-linear medium (NL1) within which a first beam of fixed direction (I1) and a second beam of orientable direction interfere with each other, an amplifying medium (2) placed along the direction of the first beam for amplifying the received light for transmitting an amplified beam towards the non-linear medium (NL1), which retransmits this bean in the direction of the second beam, and therefore in an orientable direction.

Abstract: Disclosed are a planar wave guide type optical amplifier for amplifying a light signal passing through an optical fiber, a method of manufacturing the above light amplifier, and a laser oscillator using the light amplifier, The planar wave guide type optical amplifier comprises a first core formed to a bar shaped on a substrate and a light amplifying region composed of a part of the first core, having a configuration extending to the longitudinal direction of the first core, and doped with a rare earth element. The method of manufacturing the planar wave guide type optical amplifier comprises the steps of (1) forming a bar-shaped core on a plane substrate, (2) forming a groove to the core which extends to the longitudinal direction thereof, (3) filling the groove with a filler doped with a rare earth element and (4) solidifying the filler.

Abstract: Embodiments of the present invention are sulfur rich glass compositions comprising germanium, gallium and sulfur, which glass compositions have a low energy phonon spectrum and which glass compositions serve as a host for active materials in fabricating light sources such as fiber laser oscillators, light amplifiers, and superluminescent sources. In particular, such a laser oscillator, light amplifier or superluminescent source is comprised of an inventive glass composition which is doped with rare earth ions such as Pr.sup.3+ or Dy.sup.3+ for producing light output at wavelengths, among others, substantially at 1.3 um. Further embodiments of the present invention are light sources such as laser oscillators, light amplifiers and superluminescent sources which have emissions substantially at 1.3 um and which are comprised of an inventive glass composition which is doped with Dy.sup.3+ and Yb3+ ions, wherein Dy.sup.3+ ions are pumped by energy transfer from Yb3+ ions.

Abstract: An optically active device comprising an optical fiber, a light source and a coupler is disclosed. The optical fiber has a core made of a silicate glass containing Rb and/or Cs oxide. The core is doped with Nd.sup.3+ as an active ion and transmits light at 1.3 .mu.m band. The light source generates excitation light at 0.8 .mu.m. The coupler directs the excitation light from the light source into the core of the optical fiber. A signal light or a spontaneous light at 1.3 .mu.m band which is transmitted in the core stimulates Nd.sup.3+ to emit light at 1.3 .mu.m band. As a result an optical function such as optical amplification can be effected at 1.3 .mu.m band.

Abstract: An optical amplifier operating at the 1.31 .mu.m wavelength for use in such applications as telecommunications, cable television, and computer systems. An optical fiber or other waveguide device is doped with both Tm.sup.3+ and Pr.sup.3+ ions. When pumped by a diode laser operating at a wavelength of 785 nm, energy is transferred from the Tm.sup.3+ ions to the Pr.sup.3+ ions, causing the Pr.sup.3+ ions to amplify at a wavelength of 1.

Abstract: A telecommunications system having erbium-doped fiber optical amplifiers for transmitting signals including at least one signal at a wavelength of about 1550 nm, wherein the amplifiers are fibers whose cores are doped with erbium and germanium and are free from aluminum; and wherein for each of the amplifiers, the length of the fiber and the pumping power which is injected therein are such that the noise generated by the fiber is generally emitted around 1550 nm. As can be seen in FIG. 4 , the signal-to-noise ratio remains satisfactory after 10,000 km of propagation without using an intermediate filter.

Abstract: A tellurite glass particularly usable for an amplifier or oscillator utilizing an optical fiber or other guided wave structure. In approximate terms, the glass contain between 58 and 84 molar % of TeO.sub.2, up to 24 molar % Na.sub.2 O, and between 10 and 30 molar % of ZnO. Other alkali and divalent metals may be substituted for the Na and Zn respectively. Combinations of these tellurite glasses can be formed as an optical fiber (10) having a core (12) with a higher refractive index than that of the cladding (14). The tellurite glass of the core, when composed of at least 0.05 molar % Na.sub.2 O, can be doped with large amounts of Er, Pr, or Nd to act as a fiber amplifier at 1.5 or 1.3 .mu.m when pumped with light of a specified shorter wavelength. The core can be doped with other rare-earth metals which would provide optical amplifiers or oscillators at wavelengths appropriate to their lasing characteristics.

Abstract: Er and Ti are co-diffused under controlled conditions into a single crystal of LiNbO.sub.3. The co-diffusion is used to produce a light amplifier in LiNbO.sub.3 that is suitable for use in integrated optic devices.

Abstract: An optical fiber for amplifying or sourcing a light signal in a single transverse mode. The fiber comprises a host glass doped with erbium (Er) and a sensitizer such as ytterbium (Yb) or iron (Fe). Preferably the host glass is doped silicic glass (e.g., phosphate or borate doped). Electrical energy is provided to diode lasers that pump the Nd laser rod, which in turn pumps the fiber. Such a configuration for pumping the fiber provides a high energy transfer from the diodes to the Nd laser rod, which in turn enables high pumping powers to be coupled into the single-mode co-doped fiber. Based on the amplification characteristics of the co-doped fiber and the efficient coupling of power from the laser diodes, the amplifier provides power and small signal gains comparable to the best observed, while requiring only conventional and readily available diode-based pump sources.

Abstract: An optical amplifier in the 1.26 .mu.m to 1.34 .mu.m spectrum range, comprising a solid substrate of fluoride glass doped with praseodymium in which a three-dimensional monomode waveguide is formed having an index difference .DELTA.n relative to the index of the fluoride glass lying in the range 4.times.10.sup.-3 and 8.times.10.sup.-2, said waveguide being associated by coupling means to an optical pump having a wavelength equal to 1.02 .mu.m .+-.0.1 .mu.m.

Abstract: This invention is drawn to heavy metal oxide glasses exhibiting high nonlinear susceptibility and infrared transmission consisting essentially, in weight percent, of 42-48% PbO, 33-44% Bi.sub.2 O.sub.3, 10-15% Ga.sub.2 O.sub.3, and up to 15% total of at least one member of the group consisting of up to 5% SiO.sub.2 and/or GeO.sub.2 and up to 15% Tl.sub.2 O. This invention also comprehends the fabrication of light guiding fibers from those glasses.

Abstract: An optical signal amplifier with a waveguide path integrated into a glass body that is doped with optically active material. The signal to be amplified is transmitted through the waveguide and the pump power is coupled into the waveguide at one end. The waveguide comprises closely spaced adjacent guide lengths in the form of a spiral, zigzag paths between mirrors, or parallel paths with connecting semicircles at alternating ends of succeeding pairs of paths.

Abstract: In a transmission system having concatenated optical amplifiers, gain saturation of the optical amplifiers at any stage is caused by the extraction of power from the amplifier by the ASE and amplification of the signal and of the ASE from previous stages. Three methods of operating transmission systems are classsified as free-running, constant-total power and constant-signal power. Differences in performance of the three systems arise from the propagation of signal power and the buildup of the ASE. It is now disclosed that the buildup of ASE noise in a system of either lumped or distributed concatenated optical amplifers can be limited by operating at least a plurality of the optical amplifers to provide unsaturated gain which is greater than the loss of the system and/or each optical amplifier is operated in its saturated state.

Abstract: An optical fibre for use in fibre lasers has the lasing additive eg Er.sup.3+, concentrated in center of the core. Preferably the core has an inner region which contains the additive and an outer region which is dopant free. The concentration of the dopant reduces the pump threshold for a laser and improves the gain performance for a given pump power. The fibre is conveniently made in MCVD. The use of Al.sub.2 O.sub.3 in the inner zone appears to reduce loss of dopant during tube collapse.