Optical waveguides – With optical coupler – Particular coupling structure
Patent
1997-09-05
1999-12-07
Ullah, Akm E.
Optical waveguides
With optical coupler
Particular coupling structure
G02B 626
Patent
active
059996730
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates in general to the coupling or transfer of light power between a light source and an optical guide, and more particularly to arrangements for improving the energy transfer efficiency between a light source and an optical fiber through an intermediate optical fiber length or pigtail, when a coaxial feeding to the final optical fiber is either undesired or impossible.
In a particular and preferred embodiment that will be illustrated in detail, the system acts as a part of an optical amplifier. In this case the light source includes a multi-mode semiconductor laser diode--or an array of such diodes--emitting around a certain wavelength and acting as a pump source, and the final optical guide includes a single-mode core carrying a travelling signal (single-mode radiation of a wavelength different from the pumping one) to be amplified.
In the above mentioned preferred embodiment the invention aims in particular to realize a highly efficient coupling between a multi-mode optical fiber pigtail and a so called single-mode double-cladding (SM-DC) optical fiber, although the invention is not to be considered restricted to such a preferred application. As Single-Mode Double-Cladding (SM-DC) optical fiber it is meant a fiber that includes an inner (Single-Mode) core with a diameter of few microns, a first cladding (or multi-mode core), and a second cladding. Actually there are a number of situations in which a fiber needs to receive light from one or more sources in a non-coaxial way, i.e. not along the longitudinal axis of the fed fiber. As an example, whenever it is requested to carry two or more different radiations along a single guide (with the radiations being different in amplitude, wavelength, signal pattern or code, and so on) an increase of the number of different sources is involved, which can be also of different kinds, and some kind of separation of the optical paths entering the final guide is required. Furthermore a non coaxial coupling arrangement could be required in a fiber laser device in which the axial pumping would require a special mirror (with selected reflectivity) which should be transparent to the pump radiation, and with a high reflectivity at the lasing wavelength.
Another situation in which a non-coaxial feeding is requested is disclosed for example in U.S. Pat. No. 5,170,458 concerning an optical amplifier obliquely receiving the pumping light.
There are known several arrangements for coupling a (additional) light source to an optical fiber that include one or more of the following coupling devices: discrete elements) micro-optic coupler (i.e. a coupler adopting microlenses, or beam splitters, or other discrete microscopic devices) or fused-fiber coupler.
The above first three examples raise considerable alignment problems in terms of mechanical tolerances, that make their realization very expensive and critical from a technological point of view, in general producing a poor coupling efficiency.
Usually, in telecommunication systems incorporating optical fiber amplifiers (OFAs), i.e. lengths of properly doped single-mode optical fiber serially connected in the communication route, these lengths of fiber are fed with light from one or more single-mode laser diodes through suitable fused-fiber couplers, each of them being in turn coupled with one or more laser diodes. Such (single-mode) couplers, formed by merging or fusing two or more pieces of fiber, can show a coupling efficiency as high as 90%. In coupling the laser chip to the pigtail. an efficiency of less than 50% is normally achieved. Due to the limited power obtainable from single mode diodes, high optical power is needed, the system has to include a large number of diodes, decreasing the reliability of the system as a whole.
In order to improve the saturated output power of the optical amplifier, without increasing the number of pump diodes involved, it has been suggested (see for example U.S. Pat. No. 4,829,529 to Kafka) to use a SM-DC optical fiber, activated by one or more diode ar
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Igor Samartsev
Valentin Gapontsev .P.
Ire-Polus Co.
Italtel SpA
Ullah Akm E.
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