Optical waveguides – With optical coupler – Particular coupling function
Patent
1991-09-12
1992-10-06
Gonzalez, Frank
Optical waveguides
With optical coupler
Particular coupling function
385 32, G02B 626
Patent
active
051539330
DESCRIPTION:
BRIEF SUMMARY
This invention relates to an apparatus for generating a comb of optional teeth of different wavelengths.
It is often desired to produce such combs. For example, a known sensing system is described in Electronics Letters, Aug. 4, 1988, Vol 24 No 16, in which a comb is produced by causing light from a short coherence optical source to circulate around an optical loop. The loop includes a frequency shifter, and the output signal from the loop is in the form of a comb of optical teeth of different wavelengths.
A disadvantage with this known device for producing a comb is that the amplitude of the output signal decreases rapidly and, as a result, the number of teeth in the comb having a detectable amplitude is limited to about three due to rapid decay. It is often required, however, to produce a comb having a large number of teeth all having substantially the same amplitude, for example when it is required to lock several laser sources to a single master laser.
It is an object of the present invention to overcome this disadvantage.
According to a first aspect of the present invention apparatus for generating a comb of optical teeth of different wavelengths comprises:
an optical waveguide loop including optical frequency shifting means for shifting the frequency of circulating optical signals in the loop; characterised in that said apparatus further includes:
optical amplifying means arranged in the loop for amplifying the circulating signals;
optical coupling means arranged in the loop for introducing a pulsed input coherent optical signal into the loop and for transferring a portion of the circulating signals from the loop;
a source of pulsed input coherent optical signals; and
an optical filter in the loop for limiting the number of frequency shifts of the circulating optical signals.
According to a second aspect of the present invention apparatus for generating a comb of optical teeth of different wavelengths comprises:
an optical waveguide loop including optical frequency shifting means for shifting the frequency of circulating optical signals in the loop; characterised in that said apparatus further includes:
optical amplifying means arranged in the loop for amplifying the circulating signals;
optical coupling means arranged in the loop for introducing a pulsed input coherent optical signal into the loop and for transferring a portion of the circulating signals from the loop ;
a source of pulsed input coherent optical signals; and
means for limiting the number of frequency shifts of the circulating optical signals, the limiting means comprising means for gating the optical amplifying means.
By including amplifying means in the loop, it is possible to compensate for the power being lost from the loop due to the transfer of a portion of the circulating signals from the loop, as well as due to inherent loss in the loop. This allows a comb to be built up having a large number of teeth of different frequencies and of substantially the same amplitude.
The optical waveguide loop may comprise a titanium in-diffused region in a lithium niobate body, or it could comprise an optically transparent semiconductor such as gallium arsenide or other 111-V compounds. Preferably, however, it comprises an optical fibre. By forming the loop from an optical fibre, it will be robust. In addition, coupling losses are lower and delays are easier to achieve than when the loop is formed in other ways. Further, the use of optical fibre is convenient, as a large number of existing optical components are fibre compatible.
Preferably, the frequency shifting means comprises a Bragg cell, although it could also comprise a low loss single sideband suppressed carrier modulator. By using an integrated optical Bragg cell, tooth spacings of up to tens of GHz may be achieved.
The frequency shifting means could also comprise a Mach-Zender modulator. A potential problem associated with using a Mach-Zehnder modulator as the frequency shifter in the loop is that it is not ideal and produces two frequencies. For pulse mode operation one of these fre
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patent: 4136929 (1979-01-01), Suzaki
patent: 4720160 (1988-01-01), Hicks, Jr.
patent: 4815804 (1989-03-01), Desurvire et al.
patent: 5050183 (1991-09-01), Duling, III
Electronics Letters, vol. 24, No. 16, 4 Aug. 1988, K. Arai et al.: "New Sensitivity-Enhancing Scheme for a Fibre-Optic Interferometer Utilising Two Optical Loops"-pp. 1000-1001.
Coppin Phillip
Healey Peter
Hodgkinson Terence G.
Smith David W.
Stanley Ian W.
British Telecommunications public limited company
Gonzalez Frank
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