Multiplex communications – Wide area network – Packet switching
Patent
1988-11-09
1990-05-08
Sikes, William L.
Multiplex communications
Wide area network
Packet switching
370 1, 372 50, G02B 632
Patent
active
049232703
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention concerns improvements in or relating to apparatus for optical wavelength division multiplexing, in particular laser array multiplex devices, bidirectional multiplex/demultiplex devices, and drop-and-add components suitable for use in optical transmission systems.
Wavelength division multiplexing (WDM) is a well known technique for enhancing the transmission capability of an optical fibre. Instead of just a single source (laser or LED) being used to input the fibre, two or more are used, each operating at a different wavelength. Close channel spacing is desirable for optimum transmission capability. Long term stability, is also a critical factor, in particular, immunity to thermal drift is desired.
BACKGROUND ART
Semiconductor laser diodes, for example, have been used as multiplexed sources. In these the wavelength of operation depends upon the composition of the active layer material. Diodes thus with different active layer conpositions have been used to generate the different wavelengths. The finite spectral width typical of such diodes, and the manufacturing tolerance on active layer composition, however, usually would limit the attainable minimum channel spacing to circa 30 nm. Closer channel spacing can be attained by using single frequency type lasers--for example distributed feedback (DFB) lasers or cleave-coupled-cavity (C.sup.3) lasers. Here, however, elaborate control is required to maintain the emission wavelength of each laser correct and in tune with the multiplexer used to combine all of the outputs from the different lasers.
By way of illustration, a typical apparatus configuration is presented in FIG. 1. This has the form of a ten-channel multiplexing device 1. In this arrangement, ten fixed-wavelength lasers L1 to L10, are combined in an input linear array 3 by means of coupling fibres 5. This array 3, which also includes a common output fibre 7 is located in the focal plane of Littrow mount optical assembly 9 comprised of a collimating and focussing lens 11 and a dispersive member, a reflective grating 13. Light emissions from the individual lasers L1 to L10 are collimated, diffracted and refocussed onto the common output fibre 7. It will be appreciated that in this arrangement, the emission wavelengths and array geometry must be carefully matched to ensure that all emissions are diffracted to a common focus.
Such an arrangement, as described above, is detailed in the following article: "68.3 km transmission with 1.37 Tbit km/s capacity using wavelength division multiplexing of ten single frequency lasers at 1.5 .mu.m", by Olsson, N. A. et al., published in Electronics Letters Volume 2, Number 3 pages 105-106 (1985). There, the authors describe an arrangement in which the selected ten lasers were of the hetero-epitaxially ridge overgrown (HRO) distributed feedback (DFB) laser type. These were made with a second-order diffraction grating and with both facets cleaved. The wavelength distribution of the lasers, which were selected from wafers of different design wavelengths, was between 1.529 and 1.561 .mu.m.
Wafer selection, as above, is an expensive and time-consuming procedure and clearly is not commercially practical for mass production. Long term stability also is likely to be a severe problem in this type of arrangement.
DISCLOSURE OF THE INVENTION
The present invention provides an alternative approach and is intended to obviate the need for such critical selection.
In accordance with the invention thus there is provided an apparatus for optical wavelength division multiplexing, apparatus of the type comprising: assembly; and, this assembly and arranged thus to receive light emitted from said one laser; wherein, means so to reflect light emitted by said one laser and to control thereby the resonant emission thereof.
In the apparatus aforesaid the resonant wavelength of the one laser is uniquely determined according to the spatial dispersive characteristics of the optical assembly and by the fixed position of the laser relative to this assembl
REFERENCES:
patent: 4464759 (1984-08-01), Haus et al.
patent: 4680767 (1987-07-01), Hakimi et al.
patent: 4730327 (1988-03-01), Gordon
"Design of Nondispersion Optical Feedback System Using Diffraction Girating for Semiconductor Laser Multiple Longitudinal Modes Control", by Sato et al, vol. QE-18, No. 2, Feb. 1982, pp. 155-157, IEEE Transactions of Quantum Electronics.
"Broad-Passband-Width Optical Filter for Multi-Demultiplexer Using a Diffraction Grating and a Retroreflector Prism", by Stevenage Electronics Letters, vol. 21, No. 10, 9th May 1985, pp. 423-424.
"Optical Coupling Schemes for Bidirectional 2-Wavelength Fiber Optic Systems", by Miller, Technical Digest, No. 64; Oct. 1981, pp. 41-42.
N. A. Olsson et al, "68.3 km Transmission with 1.37 Tbit km/s Capacity Using Wavelength Division Multiplexing . . . ", Electronics Let., vol. 21, No. 3, pp. 105-106, Jan. 85.
Plessey Overseas Limited
Sikes William L.
Ullah Akm E.
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