Optical waveguides – Optical fiber waveguide with cladding
Patent
1997-06-25
1999-04-06
Palmer, Phan T.H.
Optical waveguides
Optical fiber waveguide with cladding
385126, 385124, G02B 602
Patent
active
058928766
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to an optical fiber including at least one fluorescent dopant, in particular a fiber adapted to be used in optical fiber amplifiers or lasers.
Apparatuses in which a fiber is used containing a fluorescent dopant capable of interacting with an excitation signal, referred to as a "pumping" signal for the purpose of obtaining a desired output signal, i.e. an optical oscillator or an optical amplifier, are subject to a great deal of study at present. The same type of optical fiber is used both in lasers and in amplifiers: the fiber is an optical fiber of conventional structure having an optical core for guiding the majority of the lightwaves, doped by means of a fluorescent dopant, and surrounded by optical cladding. Two types of signal are conveyed by such an optical fiber: pumping signals and useful signals conveying information, referred to as "signals to be transmitted". The fluorescent dopant may be an element having three or four transition levels. By way of example it may be a rare earth such as erbium.
The operation of apparatus making use of the fluorescence effect relies on the following basic principle: ions of fluorescent dopant, referred to as "active" ions, initially in their fundamental state, absorb the photons of the pumping signal, thereby transferring them to an excited higher transition level; this phenomenon is known as "population inversion", and the dopant ions in this excited state are said to be "inverted". From this excited level, inverted ions can subsequently return to their fundamental state by emitting a photon, i.e. by means of a laser transition. In an optical amplifier or laser type apparatus, such emission is stimulated by the presence of a photon of a signal to be transmitted, and consequently the operation of such apparatus also depends on the interaction between active ions and photons in said signal at the wavelength thereof.
In order to obtain the population inversion that is essential for laser operation, it is necessary to pump at least half the active ions from their fundamental state to the excited higher level. At any particular point of the core of the optical fiber, if less than half of the active ions are inverted, then the signal to be transmitted is attenuated at said point because the non-inverted active ions absorb photons.
Consequently, in order to make best use of the pumping power injected into an optical fiber doped by means of a fluorescent dopant, it is preferable to confine the active dopant within the zone of the fiber that coincides with the peak intensity of pumping, i.e. around the axis of the fiber, and to avoid having any active ions located where the pumping intensity is weaker.
The intensity maximum of a monomode signal lies likewise on the axis of the fiber, so a monomode signal interacts effectively with the inverted ions.
A known solution for confining the fluorescent dopant to where the pumping intensity is at a peak is described in U.S. Pat. No. 4,923,279. That solution consists in subdividing the core into two zones, an active central zone containing the fluorescent dopant, and a peripheral zone in contact with the central zone but not containing fluorescent dopant.
In conventional manner, the central zone is also doped with aluminum, firstly to avoid the effect whereby erbium ions can become segregated in the core ("clustering"), where clustering considerably decrease the performance of the doped fiber for reasons that need not be explained in detail herein, and secondly as an index-raising dopant. Also, in order to obtain monomode guidance in the core, the peripheral zone is doped with one or more index-raising dopants. It is recalled that such dopants raise the refractive index relative to that of the optical cladding, which generally has an index that is substantially equal to that of pure silica.
Nevertheless, that solution is not satisfactory.
Known index-raising dopants, and in particular aluminum, germanium, and phosphorus, also cause the softening temperature of the silica in which they are incorporat
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patent: 4923279 (1990-05-01), Ainslie et al.
patent: 5121460 (1992-06-01), Tumminelli et al.
patent: 5673354 (1997-09-01), Akasaka et al.
Patent Abstracts of Japan, vol. 017, No. 320 (P-1558), 17 Jun. 1993, corresponding to JPA 05 034528 (Fujikura LTD) 12 Feb. 1993.
Desurvire Emmanuel
Le Sergent Christian
Nouchi Pascale
Alcatel Submarine Networks
Palmer Phan T.H.
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