Patent
1988-06-20
1990-10-16
Lee, John D.
350 9616, 350 9629, G02B 626
Patent
active
049629875
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention relates to an optical device for use for example as an optical amplifier or logic element.
BACKGROUND AND SUMMARY OF THE INVENTION
A paper entitled "Nonlinear antiresonant ring interferometer" in Optics Letters, Vol. 8, No. 9, pages 471-473, by Kenju Otsuka, describes an interferometer comprising a beam splitter to split an optical beam into portions of different intensities, a pair of mirrors on to respective ones of which the portions impinge, and a block of non-linear material positioned in the optical path between the mirrors. Light is split at the beam splitter into the different intensity portions which are then caused to pass in opposite directions through the non-linear medium which has the effect of imparting different phase shifts on the light portions due to its non-linear refractive index property. The phase shifted portions are recombined at the beam splitter.
The Otsuka device relies on cross-interaction between the two counter propagating optical fields and is dependent on interference between the fields producing a non-linear index grating within the non-linear medium. In these circumstances the counter propagating fields in the device are necessarily of a duration which exceeds the propagation period within the non-linear medium. The device operation requires the optical fields to be coincident in the non-linear medium thereby necessitating precise location of the non-linear medium at the mid-point of the optical path around the device. One of the problems with this device is therefore the need for accurate positioning of the various components. For example, the mirrors also have to be very accurately aligned with the non-linear medium and with each other.
In addition, to avoid problems which would arise from field divergence within an extended non-guiding medium, the length of the non-linear medium itself is restricted. There is a further risk of diffraction problems because the optical fields are laterally unconstrained during propagation around the device i.e.. the fields could spread out laterally which would reduce their intensities.
It is an object of the present invention to provide an optical device which substantially overcomes or at least mitigates the aforementioned problems and restrictions. It is a further object of the present invention to provide a method of operation of such a device.
In a first aspect the present invention provides an optical device comprising a coupling means having first and second pairs of optical communication ports, in which portions of an optical signal received at a port of one pair are coupled into each port of the other pair in a predetermined coupling ratio; and an optical waveguide at least a portion of which includes a first material having a non-linear refractive index, the optical waveguide coupling together the first pair of ports; the coupling ratio and appropriate waveguide parameters being selected such that in use the portions of an optical signal at a working intensity received at one of the second pair of ports of the coupling means and coupled into each end of the waveguide return with an intensity dependent relative phase shift after travelling around the waveguide.
In contrast to the known device described above, the present invention makes use of an optical waveguide including a material having a non-linear refractive index. This not only enables previously encountered alignment and diffraction problems to be avoided, but furthermore provides more flexibility in operation and avoids the constructional limitations of the earlier device. Further, the present device does not require cross-interaction between counter propagating fields, nor the establishment of an interference grating. Thus, in contrast to the device of Otsuka, the present device enables an intensity dependent relative phase shift to be achieved where the duration of an input signal is shorter than the signal transit time through the non-linear medium or material. The non-linear material may also be conveniently distributed thro
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British Telecommunications public limited company
Lee John D.
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