Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Patent
1991-09-17
1993-06-01
Nelms, David C.
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
359580, 257431, H01J 4014
Patent
active
052162370
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND
I. Field of the Invention
This invention relates to optical detectors, in particular to optical detectors incorporating optical filters, and more particularly to such detector and filter combinations suitable for use in optical communications.
II. Prior Art and Other Considerations
Optical communications systems employing optical fibres as the transmission medium are increasingly widely used. Such systems are now routinely being incorporated in national and international telecommunications networks.
There is an increasing desire to employ wavelength division multiplexing (WDM) techniques to increase the useful capacity of optical fibre links. WDM transmitters routinely comprise a plurality of narrow-linewidth semiconductor lasers, the operating wavelengths of the different lasers being suitable spaced apart. WDM demultiplexers or receivers employ either gratings or dielectric transmission filters to separate the various optical wavelengths. A disadvantage of such demultiplexers is that precise alignment of their many components is required, and this, together with the high cost of components such as gratings, makes such demultiplexers unsuitable for mass production, thereby inhibiting the widespread adoption of WDM systems.
The present invention seeks to facilitate the mass production of WDM demultiplexers and receivers.
SUMMARY
According to a first aspect of the present invention, there is provided an optical detector having an integral wavelength selective filter, the filter comprising a plurality of dielectric layers.
The detector may be an avalanche photodiode.
A WDM optical communications system may comprise a plurality of detectors according to the first aspect of the present invention, the wavelength selective filter on two or more of said detectors having passbands whose centre wavelengths differ by 20 nm or more.
According to a second aspect of the present invention, there is provided a demultiplexer for WDM optical communications system, comprising at least two optical detectors each having an integral wavelength selective filter, the wavelength selective filters each comprising a plurality of dielectric layers, and each of said wavelength selective filters having a different pass band.
According to a third aspect o the present invention, there is provided a WDM optical communications system comprising at least one optical fibre, a transmitter for launching WDM optical signals into said fibre, and a receiver arranged to receive said optical signals, wherein the receiver comprises at least two optical detectors each having an integral wavelength selective filter, the wavelength selective filters each comprising a plurality of dielectric layers, and each of said wavelength selective filters having a different pass band.
According to a fourth aspect of the present invention, there is provided a method of making a tuned optical detector, the method comprising the steps of: filter comprising a plurality of layers of dielectric material, the refractive indices of the various layers being chosen to provide a desired filter characteristic; and the filter being so arranged that, in use, light passed by the filter is detected by said photodetector.
Preferably, the semiconductor substrate comprises a III-V semiconductor.
According to a fifth aspect of the present invention, there is provided a method of making a detector according to the first aspect of the present invention, which method comprises the step of forming an optical filter on a body of a semiconductor, the filter comprising a plurality of layers of dielectric material, and a lift-off step in which a thickness of 7 microns or more is removed.
The thickness removed may be between 7 and 10 microns inclusive.
In a method according to the fifth aspect of the present invention, a multi-layered structure of silicon and silicon monoxide having a thickness of 7 microns or more may be removed in a lift-off step.
In a preferred embodiment, the filter comprises dielectric layers. The filter may consist essentially of alternate semiconductor
REFERENCES:
patent: 3996461 (1976-12-01), Sulzbach et al.
patent: 4158133 (1979-06-01), Spaeth et al.
Learmouth Mark D.
Ritchie Simon
Spurdens Paul C.
Allen Stephone
British Telecommunications plc
Nelms David C.
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