Coherent light generators – Raman laser
Patent
1987-01-05
1988-10-25
Scott, Jr., Leon
Coherent light generators
Raman laser
372 6, 372 94, 350 9613, 350 9615, H01S 330
Patent
active
047808764
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention relates to methods and apparatus for generating coherent radiation, for example optical radiation.
There have been many proposals in the past for methods and apparatus for generating coherent radiation, in particular lasers. There is now a requirement for the generation of relatively narrow linewidth coherent radiation and to achieve this it has been necessary to construct highly accurate and therefore expensive radiation sources.
SUMMARY OF THE INVENTION
In accordance with one aspect of the present invention, apparatus for generating coherent radiation comprises a source of coherent radiation having a relatively broad linewidth; a waveguide ring into which radiation from the source is injected, the radiation generated by the source and the form of the waveguide ring being such that the radiation is scattered in use to generate a relatively narrow linewidth, coherent wave travelling in an opposite direction to the broad linewidth radiation; and separation means for separating the narrow linewidth wave from the injected radiation.
In accordance with a second aspect of the present invention, a method of generating relatively narrow linewidth radiation comprises injecting relatively broad linewidth radiation into a waveguide ring, the characteristics of the relatively broad linewidth radiation and the form of the waveguide ring being such that the radiation is scattered to generate a relatively narrow linewidth, coherent wave travelling in an opposite direction to the broad linewidth radiation; and separating the narrow linewidth wave from the injected radiation.
The invention is based on the use of resonance phenomena such as stimulated Brillouin scattering (SBS) and Raman scattering which until now have been considered a limitation on the transmission of radiation through waveguides. These effects are particularly apparent in the transmission of optical wavelengths through dielectric waveguides and occur in low loss optical fibres if narrow linewidth laser light of above a certain power level threshold is injected. SBS is preferable to Raman scattering because much lower pump powers are required.
The principle of SBS will now be described in connection with optical radiation. SBS can be described essentially as a coupled three-wave interaction involving the incident light wave (pump), a generated acoustic wave, and the scattered light wave (Stokes). The pump creates a pressure wave in the medium due to electrostriction and the resultant variation in density changes the optical susceptibility. Thus the incident light wave pumps the acoustic wave which scatters it and the scattering creates the Stokes wave.
The three waves obey the energy conservation law which relates the three frequencies by: frequencies respectively. Maximum power transfer occurs when the wave-vector mismatch is zero:
There are two important consequences of these two equations. Firstly, the Stokes wave experiences maximum gain when the pump and Stokes wave vectors are parallel and counter-directional. Thus in a monomode fibre SBS generates a backward-travelling Stokes wave. Secondly, the Stokes wave is shifted to a lower frequency with respect to the pump by an amount equal to the acoustic frequency.
Stimulated Brillouin scattering is most apparent for narrow linewidth coherent waves. The invention makes use of this property by generating a narrow linewidth wave from relatively broad linewidth radiation. It is comparatively straight forward to generate broad linewidth radiation using for example single mode diode lasers such as distributive feedback (DFB) lasers.
As has been mentioned above, the narrow linewidth wave will be frequency shifted from the central wave length of the broad linewidth radiation. Typically, the broad linewidth radiation may have a linewidth between 1 MHz and 100 MHz, for example 10 MHz while the narrow linewidth radiation will have a linewidth less than 1 MHz typically of the order of kHz.
The characteristics of the broad linewidth radiation and the form of the waveguid
REFERENCES:
patent: 4107628 (1978-08-01), Hill et al.
patent: 4530097 (1985-07-01), Stokes et al.
Light Signal Amplifier, vol. 6, No. 22, (P-101), Feb. 9, 1982, Nippon Deshin Denwa Kosha (72) Sunao Uesugi(1)-1 page.
"All-Fiber Stimulated Brillouin Ring Laser with Submissiwatt Pump Threshold", by L. F. Stokes et al.-vol. 7, (1982), Oct., No. 10, New York, USA Optics Letters, pp. 509-511.
"Raman Amplification of Recirculating Pulses in a Reentrant Fiber Loop", by E. Desurvire et al., Optics Letters, vol. 10, No. 2, Feb. 1985, Optical Society of America, pp. 83-85.
"Stimulated Brillouin Scattering in Monomode Optical Fibre", Journal of Optical Communications 4(1983), 1, 10-19, D. Cotter.
Cotter David
Smith David W.
Wyatt Richard
British Telecommunications public limited company
Jr. Leon Scott
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