OPTICAL TRANSMISSION SYSTEMS

Abstract: A system for multiplexed transmission of different wavelengths of light comprises at least two light sources whose images are formed on the end of an optical fibre. The images have displaced spectra so that a different part of the spectrum of each light source is imaged on to the end of the optical fibre and each part is transmitted by the optical fibre. The images themselves may be displaced by displacing the light sources, or the images may be coincident, their spectra being displaced by using light sources with emission spectra centred on different wavelengths.

Description: OPTICAL TRANSMISSION SYSTEMS This invention relates to optical transmission systems and more particularly to systems for multiplexed transmission of different wavelengths of light. In this specification the term "light" also includes light invisible to the human eye, ie, infra red and ultra-violet radiation. It is an object of the present invention to provide such an optical transmission system which has a high radiance and efficiency, high degrees of optical * isolation between wavelengths and which is rugged and compact. According to the present invention an optical transmission system for multiplexed transmission of light comprises at least.two light sources and means for imaging the emitted light from the at least two sources onto the end of an optical fibre, -he images having displaced spectra whereby a different part of the spectrum of each light source is imaged on to the end of the optical fibre. The light sources may be displaced from one another so that their images and hence their spectra are displace . Preferably the light sources are filtered and focussec in a mcr.ochrcmator ccmorising a lens and a dispersive element such as a prism or a blazed grating. Alternatively, the light sources may emit different wavelength bands whereby their images are not displaced " but their spectra are displaced. Preferably the at least two light sources comprise light emitting diodes. The light emitting diodes preferably have emission spectra centred on the same wavelength but they may have emission spectra centred on different wavelengths. Preferably the light sources comprise an abutting array of light emitting diodes having emission spectra centred on the same wavelength. Several arrays may be provided, each array having emission spectra centred on different wavelengths. The monochro ator may comprise a monolithic structure comprising a concave reflector in place of a lens, and a prism or grating may be used as the dispersive element. Embodiments cf the invention will now be descrioeα by way of example only with reference to the accompanying drawings in which: Figures 1a_ and lb illustrate schem ticaliy tne principal of an -Optical transmission system for multiplexed transmission of light in accordance with BAD ORIGINAL invention, Figure 2 illustrates a practical arrangement of the transmission system and Figure 3- is an alternative arrangement to that of Figure 2, Figure 1_a 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 moπochroπ^tor 18 having a wide entrance slit 20. The monochromator 18 is illustrated in Figure 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 t is defracted and reflected back through the lens 22 which focusses an image of the spectrum of eac.b element adjacent to tne 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 Figure 1b_. Mounted in the side of the monochrom tor 18 is an optical fibre 32 the end of which is located in the area which receives the e o er¬ lapping spectrum imasees IUS the ootical fibr: 32 BAD ORIGINAL receives three different channels 40 of each of the three spectrum images as illustrated in Figure 1b_ and the fibre then transmits these multiplexed channels. A similar mon-ochromator (not shown) is also used at the other end of the fibre which demultiplexes the signals (the defraction at the grating is proportional to wavelength and the three multiplexed channels are therefore separated) in conjunction with a detector array. The LED array 10 emits at high radiance and efficiency and the monochromator gives a very high degree of isolation (both optical and electrical) between the parts of the spectra transmitted along the fibre 32. The monochrc ator 18 images at the high numerical aperture of multimode .fibres (* *v -'0.2 - 0.3) with small aberrations and attenuation, has the correct dispersion characteristics and which is fabricated as a compact and rugged component. The LED multi-element array 10 may be any of several material systems including lead-tin telluride, gallium phosphide, gallium arsenide, gallium arsenide phosphide, gallium-indium-arsenide-phosphide, gallium arsenide, and also double heterostructure gallium aluminium arsenide. The two latter material systems BAD ORIGINAL ** '~ have many attractions for fibre optic applications. One example of a gallium arsenide array is a zinc diffused surface emitting array comprising eight 25x100 m w elements with 1Q0j m spacing between elements. In this case the individual elements are separated completely by chemical etching to give a • very high degree of electrical and optical isolation but positional accuracy is maintained because of a continuous gold integral heatsink pad. Each element 2 .emits at radiances around 20 watts/st/cm 'at current drives of 300mA which corresponds to 1mw output per element. Another example is a 16 element edge emitting array which is fabricated in double heterosf ucture GaAlAs material. This is a lower current device with a power output of 30ww per element at a current of 30mA. Each emission element is 20um x 1 m in size. The emitting dimensions for each element, and the number of elements for each array can be altered to suit system requirements. With the correct type of lens 32 in the monochromator 18 diffraction limited optics are straight forwardly achieved and a high quality grating 24 used at the blaze angle will reflect at 90% efficiency. Thus low overall insertion loss is achievable with such a BAD ORIGINAL c..;?ι '- ( - 6 - onochromator. This optical arrangement is set up in a 1cm long metal tube and so is rugged and compact as well as optically efficient. An alternative monolithic structure 26 is illustrate in Figure 3- In this case a concave reflector 2δ is used in place of the lens 22 and a grating 29 is used as he dispersive element. This optical structure consists of three optical parts:- A body part 27, a reflector part 31 having the curved reflector 28 and a- small angle prism 30 with a grating 29- This has the advantage of solid geometry, and as all the light rays are optically immersed- any aberrations are smaller than for an equivalent free -space configur- tion. This structure can also be made in a thin plate wave-guide form which -can be very small and manufactured in large quantities at relatively low cost. In this case light diverging from each element 12 or 14 of the LED array 10 is colli ated into a parallel beam by the reflector 28. It is diffracted at the grating 29 so that a slightly angled parallel beam is reflected towards the reflector 28 and is then re-imaged adjacent to the LED array 10. As before the fibre 32 receives light from the variously displaced elements of the LED array thus launching different BAD ORIGINAL sections of the spectrum along the fibre 32 from each element. The- multiplexing scheme of Figure 1a_ creates multiple channels 40 shown in Figure 1b_ by cutting * — ™" the spectral emission of each element 12,14 or 16 into sections but various sophistications can be .introduced to optimise the launch power. With a surface emitter- array, enhancement of coupled power by a factor of 10-20 can be achieved by fitting each element with a spherical miαrolens. Alternatively a 3-3 times improvement can be achieved by. the use of a cylindrical lens (eg. a glass fibre) which extends across all of the elements and this can be applied to both the edge and surface types of arrays. The near gaussian spectral emission of LΞD's * implies a corresponding variation in launch power for the different wavelength channels 40. This , e fect can be reduced by 'Element width tailoring' in which the end of the elements 12, 14, 16 of the array 10 are made proportionately wider so that the widths of the different channels 40 is not constant. Alternatively the current drive to the elements can be adjusted to level the launch powers. Seven elements giving seven channels 40 (a minimum requirement for one specific application) can easily BAD ORIGINAL C:,:PI be driven from one LED array and this number can be multiplied by using other similar arrays with emission spectra centred at other wavelengths. A spectrum centred at a different wavelength can provide seven further different channels and LED arrays with -wave¬ lengths centred at Q.8jU_ι, 0.85^m, 0.9\.m and 1.05 <_ι each with a bandwidth of 0.1μm can be used. The channels can then be 0.80-0.85, 0.85-0.9, 0.9-0.95 and 0.95-1.0 and used with silicon arrays as detectors. A further five LED arrays can also be used (GalnAsP/InP types) if long wavelength detector arrays are utilised- Thus potentially a 7x9 - 63 channel wavelength multiplex system can be operated over .a single fibre.

Claims: WHAT WE CLAIM IS : - 1. An optical transmission system for multiplexed transmission of light comprising at least two light sources an 'means for imaging the emitted light from ' 5 the ^t least two sources on to the end of an optical fibre, the images having displaced spectra whereby a different part of the spectrum of each light source is imaged on to the end of the optical fibre. 2. An optical transmission system as claimed in claim 10 1 in which the light sources are displaced from one another so their images and hence their spectra are dis- •* placed. 3. An optical transmission system as claimed in claim 1 in which the light sources emit different wavelength 15 bands whereby their images are not displaced ' but their spectra are displaced. 4. An optical transmission system as claimed in any preceding claim in which the at least two light sources comprises at least two light emitting diodes. 20 5. An optical transmission system as claimed in claim 4 in which the at least two light emitting diodes have emissions spectra centred on different wavelengths. 6. An optical transmission system as claimed in claims 4 in which the at least two light sources comprises an 25 abutting array of light emitting diodes having emission spectra centred on the same wavelength. 7. An optical transmission system as claimed in claim 4 in which the at least two light sources comprises a plurality of arrays of light emitting diodes, each array having emission spectra centred on a different wavelength. 8. * An optical transmission system as claimed in any preceding claim in which the light sources are filtered and focussed in a monochromator. 9- .An optical transmission system as claimed in claim 8 in which the monochromator comprises a lens and a dispersive element. 10 ' . An optical transmission system as claimed in claim 8 in which the monochromator comprises a concave reflector and a dispersive element. 11. An optical transmission system as claimed in claim 9 or claim 10 in which the dispersive element comprises a prism. 12. An optical transmission system as claimed in claim 9 or claim 10 in which the dispersive element comprises a grating. 13- An optical transmission system as claimed in any of claims 8 to 12 in which the monochromator comprises a monolithic structure. 14. An optical transmission system constructed and adapted to operate substantially as described with reference to Figure 1 and Figures 2 or 3 of the accompanying drawings.

Inventors: CARTER ANDREW C, GOODFELLOW ROBERT C
Applicants: CARTER ANDREW C, GOODFELLOW ROBERT C, PLESSEY OVERSEAS, PLESSEY OVERSEAS LTD
Application year: 1981
Main classification: H04J15
IPC codes: G02B6, H04B10, H04J14