Optical waveguides – With optical coupler – Particular coupling function
Patent
1993-04-04
1994-11-08
Bovernick, Rodney B.
Optical waveguides
With optical coupler
Particular coupling function
385 24, G02B 626
Patent
active
053634560
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to mixing and/or separating optical waves having different frequencies and being guided by optical fibers. Such mixing and separation is often referred to as "multiplexing" and "demultiplexing", and the frequencies are referred to below as "low frequency" and "high frequency".
The invention applies more particularly to making long-distance data-transmission lines constituted by optical fibers. The invention then applies to mixing firstly a low-frequency data-carrying wave guided by an optical fiber which can be referred to as the "low-frequency waveguide" or the "low-frequency fiber", and secondly optical pump light constituting the high-frequency wave and guided by another optical fiber which can be referred to as the "high-frequency waveguide" or the "high-frequency fiber". The carrier wave and the pump light must be mixed in an amplifying fiber in order to amplify the carrier wave.
In which case, apparatus of the invention constitutes a mixer whose function is to perform the mixing, and which must therefore be matched as accurately as possible to the low frequency and to the high frequency. The apparatus must then also constitute a separator for the following reasons: firstly, a fraction of the carrier wave may be backscattered towards the apparatus by parts situated in the amplifying fiber or downstream therefrom. Since the apparatus is matched to the low frequency and to the high frequency, almost the entire fraction then leaves the apparatus towards the low-frequency fiber. However, if the apparatus is poorly matched to the low frequency and to the high frequency, a small fraction of the fraction leaves the apparatus towards the high-frequency fiber, and, when it leaves the high-frequency fiber, the small fraction can disturb the pump-light source. Furthermore, a portion of the pump light can also be backscattered. For similar reasons, it can pass through the apparatus not only towards the high-frequency fiber, but also towards the low-frequency fiber where it is unwanted. The apparatus must therefore constitute a separator whose function is to minimize both the fraction of the carrier wave that is returned into the high-frequency fiber, and also the fraction of the pump light that is returned into the low-frequency fiber. A quality of the separator constituted by the apparatus is therefore represented by an inter-port isolation coefficient which is the ratio of the intensity of the carrier wave that is returned into the high-frequency fiber, or the intensity of the pump light that is returned into the low-frequency fiber from the apparatus, divided respectively by the intensity of the carrier wave or of the pump light backscattered towards the apparatus. This coefficient is usually expressed in decibels, and the higher the coefficient, the higher the quality of the apparatus.
Another quality of the apparatus is represented by an insertion loss coefficient which is the ratio of the intensity of a wave or of a plurality of waves having the same frequency arriving in the apparatus, divided by the intensity of the wave(s) passed by the apparatus at the same frequency in the fiber(s) provided for guiding the wave(s). This coefficient is also usually expressed in decibels. This quality of the apparatus can be referred to as "transparency". The lower the insertion loss coefficient, the higher the transparency.
Another quality of the apparatus is passband, i.e. the width of the spectrum band that one of said waves, e.g. the carrier wave, can occupy without significantly reducing the preceding qualities.
Yet another quality of the apparatus is its power-conveying ability, i.e. the wave power that it can pass without being damaged.
Finally, another quality is the robustness of the apparatus, i.e. its reliability and its durability under severe conditions of use, e.g. in an intermediate amplifier in an under-sea transmission line.
A first prior art frequency separator-and-mixer apparatus for guided optical waves uses frequency filters and glueing between the filters and optical fi
REFERENCES:
patent: 4859016 (1989-08-01), Shaw et al.
patent: 5088095 (1992-02-01), Zirngibl
Electronics Letters, vol. 26, No. 17, Aug. 16, 1990, Stevenage, GB, pp. 1397-1398, Y. Miyajima et al.: "Efficient 1.3 .mu.-Band Amplification in a Nd3+ -Doped Single-Mode Fluoride Fibre".
Components for Fiber Optic Applications V, 1990, SPIE, vol. 1365, SPIE, Bellingham, US, P. A. Nagy et al.: "Multiplexers for Optical Fiber Amplifiers".
Alcatel Alsthom Compagnie Generale d'Electricite
Bovernick Rodney B.
Ngo John
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