Telecommunications – Receiver or analog modulated signal frequency converter – Signal selection based on frequency
Patent
1987-01-05
1989-02-28
Sikes, William L.
Telecommunications
Receiver or analog modulated signal frequency converter
Signal selection based on frequency
350 9616, 350 9629, 350320, 455600, 455602, 455610, 455612, G02B 626, G02F 100
Patent
active
048079537
DESCRIPTION:
BRIEF SUMMARY
This application is related to my copending U.S. application Ser. No. 002,717 filed Jan. 5, 1987.
BACKGROUND OF THE INVENTION
The invention relates to methods and apparatus for generating a plurality of electromagnetic signals having linewidths centred on frequencies fixed relatively to a reference frequency.
The generation of a bank of frequency referenced electromagnetic signals is particularly important in the field of wide band communication, for example using optical wavelengths, where it is very important to be able to control closely the wavelengths or frequencies of the transmitted signals. Indeed, the more channels which are transmitted, the more important it is to control the frequency of the signals and in particular their spacing.
SUMMARY OF THE INVENTION
In accordance with one aspect of the present invention, a method of generating a plurality of electromagnetic signals having linewidths centered on frequencies fixed relatively to a reference frequency comprises
(a) injecting at least a portion of a reference frequency signal into a first waveguide ring, the characteristics of the signal and the form of the ring being such that scattering occurs to generate a first scattered signal travelling in an opposite direction to the reference frequency signal;
(b) injecting at least a portion of the first scattered signal into a second waveguide ring, the characteristics of the first scattered signal and the form of the second waveguide ring being such that scattering occurs to generate a second scattered signal travelling in an opposite direction to the first scattered signal;
(c) repeating step (b) n times with the second and successive scattered signals and respective waveguide rings; and
(d) guiding portions of at least two of the reference and scattered signals to respective output ports whereby the plurality of electromagnetic signals having frequencies fixed relatively to a reference frequency are provided at the output ports.
In its simplest form, n =0. In this case and in some other cases the plurality of electromagnetic signals includes a signal with the same frequency as the reference signal.
Preferably, the characteristics of the or each signal and ring are such that stimulated Brillouin scattering occurs.
In accordance with a second aspect of the present invention, apparatus for generating a plurality of electromagnetic signals having linewidths centred on frequencies fixed relatively to a reference frequency comprises a reference frequency signal source; a plurality of waveguide rings connected together in series by waveguide means, the reference frequency signal source being arranged to inject at least a portion of the reference frequency signal into a leading one of the waveguide rings, the characteristics of each signal injected into a waveguide ring and the form of the waveguide ring being such that scattering occurs in each waveguide ring to generate a scattered signal travelling in an opposite direction to the injected signal, the waveguide means being arranged to pass at least a portion of the scattered signal to the next waveguide ring in series and to pass portions of at least two of the injected and reference signals to respective output ports.
Preferably, the characteristics of each signal injected into a waveguide ring and the form of the waveguide ring is such that stimulated Brillouin scattering occurs in each waveguide ring.
It is possible that other scattering phenomena such as Raman scattering would be feasible but much higher pump powers would be required.
The resonance phenomenon known as stimulated Brillouin scattering (SBS) has until now been considered a limitation on the transmission of radiation through waveguides. It is particularly apparent in the transmission of optical wavelengths along dielectric waveguides and occurs in low loss optical fibres if narrow linewidth laser light of above a central power level threshold is injected.
The principle of SBS will now be described in connection with optical radiation. SBS can be described essentially as a coupled
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Cotter David
Smith David W.
Wyatt Richard
British Telecommunications public limited company
Healy Brian M.
Sikes William L.
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