OPTICAL TRANSMISSION SYSTEMS
This is a continuation-in-part of application Ser. No. 395,021 filed June 22, 1982.
FIELD OF THE INVENTION
This invention relates to optical transmission systems and more particularly to systems for multiplexed transmission of different wavelengths of light sometimes called Wavelength Division Multiplexing.
In this specification the term "light" also includes light invisible to the human eye, ie. infra red and ultraviolet radiation.
BACKGROUND OF THE INVENTION
In an article entitle "Viabilities of Wavelength-Division-Multiplexing Transmission System Over an Optical Fiber Cable" published in IEEE Transactions on Communications, Vol. Com-26 No. 7 (July 1978) at 20 pages 1082 to 1087, Tetsuya Miki and Hideki-Ishio put forward a Wavelength division multiplexing (WDM) system using light emitting diodes (LEDs) having respective wavelengths of 784 nanometers, 825 nanometers and 858 nanometers. 25
The three LEDs in Miki et al are independently modulated and, because bandwidth overlap between the LEDs causes some interchannel interference, interchannel interference cancellers are used to effect a reduction in noise so caused. 30
In another paper published in the magazine Applied Optics dated Apr 15, 1979 at pages 1253-1258 a similar system employing laser diodes was discussed by Kohichi Aoyama and Jun-ichiro Minowa. In this case five laser diodes having respective wavelengths of 810 nano- 35 meters, 830 nanometers, 850 nanometers, 870 nanometers' and 890 nanometers were used.
As will be seen from these two systems laser diodes permit closer channel spacing than LEDs. This is because laser diodes have a spectrum half-width of less 40 than one-tenth of the spectrum half-width of LEDs.
Considering LEDs more carefully now it will be noted that, say, an 850 nanometer LED produces its peak power at 850 nanometers nominally but this peak power point will vary with temperature and tolerancing 45 by around plus or minus thirty nanometers. The bandwidth to the half-power point is about one hundred nanometers so with drift half power of a nominal 850 nanometer LED may extend anywhere in a range from around 730 nM up to 930 nM in a commercial device. 50
Since in WDM systems interfering signals need suppressing to about one-one thousandth power (that is thirty dB down) normal roll off separation for a successful system would require channel separations of about 350 nanometers, thus using two LEDs as an example of 55 say 850 nM and 1200 nM centre wavelength.
Another problem with LEDs is maintaining the accuracy of their centre wavelengths. Thus even if the bandwidth anr' drift problems are overcome, manufacturing LEDs with specific bandwidths requires accurate con- 60 trol of the chemical mix from which they are made. Thus whilst it is possible to manufacture or select small quantities of LEDs to accurate centre wavelength requirements, reliable commercial production of LEDs with closely spaced centre wavelengths separated by 65 say a few nanometers would require a different plant for each centre wavelength manufacturing more accurately than we currently know how.
Accordingly providing a WDM system with narrow channel separation for optical transmission is a major problem.
It is one object of the present invention to provide an optical transmission WDM system in which this problem has been overcome.
It is another object of the present invention to provide an optical transmission system which was a high radiance and efficiency, high degrees of optical isolation between wavelengths and which is rugged and compact.
SUMMARY OF THE INVENTION
According to the present invention an optical transmission system for multiplexed transmission of light comprises an array of light emitting diode elements, each diode element in said array having a broad emission spectra centred on the same wavelength and said diode elements being displaced from each other; a surface; an imagae forming means imaging and displacing spectra emitted from each diode element on said surface in an overlapping relationship, said displacement and overlap occuring with respect to the imaged spectrum of each diode element; and an optical fibre having an end located in said surface positioned in the region of overlap of the imaged spectrum of each said diode such that only a portion of each emitted spectrum is imaged and focussed on said end, and each portion being a different part of the spectrum emitted by each of said diode elements.
A plurality of arrays of light emitting diodes, the light emitting diodes in each array having their emission spectra centred on different wavelengths may be used to increase channel capacities.
BRIEF DESCRIPTION OF THE DRAWINGS
Embodiments of the invention will now be described by way of example only with reference to the accompanying drawings in which:
FIGS, la and lb illustrate schematically the principal of an optical transmission system for multiplexed transmission of light in accordance with the invention,
FIG. 2 illustrates a practical arrangement of the transmission system and,
FIG. 3 is an alternative arrangement to that of FIG.
DESCRIPTION OF THE EMBODIMENTS
FIG. la illustrates a multi-element array 10 of light emitting diodes (LED's), each element 12, 14 and 16 emitting a broad spectrum in wavelength and each element being centred on the same wavelength. The array 10 is positioned adjacent to a monochromator 18 having a wide entrance slit 20. Th monochromator 18 is illustrated in FIG. 2 and comprises a lens 22 and a dispersive element, in this case a blazed grating 24. The light from each element of the LED array is focussed by the lens 22 on to the grating 24 where it is defracted and reflected back through the lens 22 which focusses an image of the spectrum of each element adjacent to the LED array. Since the elements 12,14 and 16 are located at different positions adjacent to each other, the three resulting spectrum images are slightly displaced from one another but overlap as illustrated in FIG. lb. Mounted in the side of the monochromator 18 is an optical fibre 32 the end of which is located in the area which receives the three over-lapping spectrum images. Thus the optical fibre 32 receives three different chan