Free space optical communication system

Optical waveguides – With optical coupler – Plural

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G02B 628

Patent

active

057488134

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to optical communication systems.
2. Related Art
It is well known to use, for example, an infrared source to pass information, for example, a television remote control apparatus, or an infrared transmission system in a theatre for transmitting signals to a headset receiver for use, for example, by people with impaired hearing. Optical free-space transmission is the new alternative to radio and microwave communications for future mobile computer networks. It is capable of offering far greater bandwidth and, where it is used inside rooms where it is used, it does not interfere with other systems. These new infra-red communications systems could be based on very low-cost consumer optoelectronic components such as 850 nm sources and silicon detectors. The projected cost of optical free-space wireless systems is therefore much lower than microwave-based systems. Low cost is fundamental to mass exploitation of communication systems for use with mobile computers.
There are, however, several problems with such systems. Firstly they require power to be provided to the transmitter itself, and secondly, in some circumstances, it may be possible by the use of a lens, for example a binocular lens, (the diameter of the lens is defined in the standard to be 50 mm) to focus the infrared radiation from the transmitter on to the skin, or worse still, the retina and to possibly damage the skin or retina.
An important factor is the size of the optical source--the lens of the eye may focus the light from a point source to a small intense spot on the retina, whilst the same optical energy from a large area source will give a much larger spot on the retina and will thus result in less of a hazard.
Various national and international standards on the safety of lasers (such considered as safe. In these standards, products are classified according to the hazard, with class 1 being defined as inherently safe (even when viewed with optical instruments such as binoculars). It will be necessary for optical wireless applications to fall into this category.
The increasing exploitation of wavelengths in the near-infra-red range for communications and other applications has stimulated more detailed examination of the biological damage levels for these wavelengths. The relevant optical wavelengths for LAN systems are about 850 nm, 1300 nm, 1480 nm and 1550 nm or other wavelengths where suitable emitter and detector components are available. In general, the maximum safe-power levels will be relatively low, near the visible part of the spectrum, because the eye is particularly efficient at focusing and absorbing the radiation. At wavelengths greater than 1400 nm the retina is less at risk, but skin damage is still possible. Proposed changes, in Amendment 2 of IEC825, will incorporate relaxations to the safe-power levels at the relevant wavelengths. These changes will result in increases in maximum power for class 1 at 850 nm from 0.24 mW to 0.4 mW, at 1300 nm from 0.6 mW to 8 mW, and at 1480 nm and 1550 nm from 0.8 mW to 10 mW. In addition, further analysis of the effect of light from large area sources has resulted in a new, more accurate, method of assessing the hazard, which is also planned to be included in Amendment 2.
Reference may be made to our copending EP 521 710A which is hereby incorporated by reference.


SUMMARY OF THE INVENTION

The present invention provides an optical communication system for free space communication comprising an optical source including means to modulate the optical output to provide an optical signal, said optical source being connected to an optical fibre to pass said signal therealong, the optical fibre including antenna means comprising an optical fibre transmit portion comprising a core, and a cladding, said cladding including at least one substantially flat surface extending along its length and means to couple light out of the optical fibre through said substantially flat surface(s), and scattering means to incoherent

REFERENCES:
patent: 4929050 (1990-05-01), Wilson
Patent Abstracts of Japan, vol. 13, No. 497 (M-889) 1989 & JP,A,01 197 902 (ASAHI GLASS).
Achurch, "Optical Tap Based on D-Fibre", Electronics Letters, vol. 26, No. 22, 25 Oct. 1990, Stevenage GB, pp. 1909-1910.
Smyth et al, "Optical Wireless Local Area Networks--Enabling Technologies", BT Technol J. vol. 11, No. 2, Apr. 1993, pp. 56-64.
Zyskind et al., "Erbium-Doped Fiber Amplifiers and the Next Generation of Lighwave Systems", 8010 AT&T Techical Journal , 71 (1992) Jan./Feb., No. 1, Short Hills, NJ, US, pp. 53-62.
Closs et al, "Wireless Connection Between A Controller and a Plurality of Terminals", IBM Technical Disclosure Bulletin, vol. 20, No. 7, Dec. 1977, p. 2843.

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