Optical: systems and elements – Optical amplifier – Optical fiber
Patent
1994-01-12
1995-11-21
Moskowitz, Nelson
Optical: systems and elements
Optical amplifier
Optical fiber
359337, 385126, 385131, 372 6, H01S 307, G02B 622
Patent
active
054692920
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention relates to an optical amplifier which amplifies signals at a certain signal wavelength and consists of an active optical wave guide of simple mode geometry at the signal wavelength, and a light source, e.g. a pump laser for obtaining inverted population between the energy levels involved and coupling devices for coupling pump light and signal light in the active optical wave guide. The wave guide is doped with active ions which are substantially positioned in a cylindrical shell-shaped area around the axis of symmetry of the wave guide.
The use of optical amplifiers constitutes a technically interesting field, because optical power can be amplified with these directly in an optical fiber or wave guide without conversion of the optical energy to electric energy.
Such optical amplifiers are used or may possibly be used in fiber optical communications systems, for fiber lasers, for fiber optical sensor systems or the like. Also amplifiers manufactured in plane wave guide technology can be used within the above-mentioned fields.
Optical amplifiers and in particular fiber optical amplifiers operate by the process "stimulated emission", where a material can emit light with the same wave vector and phase as incoming light, it being energy-supplied (pumped) with light from a strong pump source, typically a laser having a lower wavelength than the signal. Amplification in a material capable of amplifying by stimulated emission depends on the dipole moment of the quantum transition considered, on the wavelength of the signal and on the size of the relative population of the two quantum states directly involved. Where the upper one of the two levels has a greater population than the lower one, so-called inverted population, it is possible to obtain amplification. This inverted population is generated via absorption of pump light.
The materials capable of forming the basis for stimulated emission can generally be divided into two groups: 3-level systems and 4-level systems.
The European patent application 368 196 discloses a fiber amplifier in which the active Er.sup.3+ ions are distributed along the axis of symmetry of the fiber in a cylinder shell which is contained completely within the core region. The optical fiber amplifier is pumped with energy distributed on several transversal wave types, and the cylinder shell is therefore spaced from the axis of symmetry corresponding to the distance where the pump light has maximum intensity. This design is not optimal for amplification of light by means of 4-level systems with amplified spontaneous emission (ASE) at lower wavelengths than the signal.
The invention also concerns a method of amplifying an optical signal with an optical amplifier, which is stated in claim 1. Hereby it is possible e.g. to obtain discrimination between the formation of spontaneous photons at a shorter wavelength (e.g. 1050 nm) by formation of signal photons at signal wavelength (e.g. 1340 nm) by stimulated emission. Amplification of signal photons will be increased considerably hereby, because the pump light creating inverted population for the levels involved is utilized better for the 1300 nm transition than for the 1050 nm transition. The invention can e.g. be worked in connection with effective amplifiers based on Nd.sup.3+ doped ZBLAN glasses, but can be utilized by any 4-level optical amplifier to which it applies that the signal wavelength is greater than the wavelength of the dominating amplified spontaneous emission.
One of the crucial problems of e.g. Nd.sup.3+ doped fibres is thus that a large amount of ASE is formed at considerably lower wavelengths than the wavelength of the signal to be amplified. The invention reduces this problem by using a special design of the active fiber. When the fiber is manufactured such that doping is in a ring around the axis of symmetry, the formation of the ASE having a low wavelength will be reduced relatively more than the stimulated amplification of signal light, and amplification at the signal wavelength
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Bjarklev Anders O.
Rasmussen Thomas P.
Lycom A/S
Moskowitz Nelson
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