Optical waveguides – Optical fiber waveguide with cladding – Utilizing multiple core or cladding
Patent
1997-05-01
1999-07-27
Palmer, Phan T. H.
Optical waveguides
Optical fiber waveguide with cladding
Utilizing multiple core or cladding
358 37, 358 28, 358 50, G02B 602
Patent
active
059304359
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
This invention relates to optical devices such as, for example, optical filters or external cavity lasers.
Optical filters arc generally passive optical elements having a wavelength dependent transmission characteristic.
It is known to construct optical filters using dual-core, substantially lossless optical fibres. Dual-core fibres, as the name suggests, are optical fibrers having two cores in close proximity. Light injected into one core cross-couples periodically into the adjacent core via evanescent field interaction. The cross-coupling strength and the power evolution in each core depend on the geometrical parameters of the fibre such as the core size and core-to-core separation, the optical parameters of the fibre such as the fibre numerical aperture (NA) and cutoff wavelength, and the optical wavelength in use.
In previous optical filters using dual core optical fibres, an optical signal is launched in one of the cores (the input) and emerges from the same or the adjacent core (the output) depending on the length of the fibre used in the filter. Filtering is achieved by the wavelength dependence of the coupling between the two fibres, denoted by the coupling coefficient C.sub.S.
Considering each input signal separately, the optical power in that signal is periodically transferred between the two cores of a dual-core optical fibre. In fact, the transfer of power between the two cores follows a substantially sinusoidal pattern, oscillating between a peak where all or most of the power is in one core, and a peak of opposite polarity where all or most of the power is in the other core. The period of this oscillation (the cross-coupling beat length) is of the order of a few millimeters (mm).
At any point along the dual core fibre, therefore, the relative proportions of the input light in each of the two fibres depends on the current phase of the periodic transfer of light of that wavelength. By selecting a fibre of a particular length, the transfer phase of light of one wavelength can be selected so that the light of that wavelength is substantially all in one core, with light of another (undesired) wavelength being all in the other core. The filter output is taken from only one core (the core in which the desired wavelength is at a maximum at the end of the filter).
Fibre filters of this type are very sensitive to environmental variations, since their operation depends heavily on the fibre length and the phasing of the transfer between the two cores.
SUMMARY OF THE INVENTION
This invention provides an optical filter for filtering a first optical signal from a second optical signal, the first and second optical signals having different wavelengths and intensities, the filter comprising: saturation of the absorber than the second signal intensity; the second optical signal propagate in the waveguide to form respective spatial intensity distributions dependent on the wavelength of each signal.
The wavelength dependence of the spatial intensity distributions means that peak intensities in the waveguide of two signals of different wavelengths do not always coincide. The second optical signal is of an insufficient optical power to saturate the absorber on its own, and will instead be attenuated at each point along each core by an amount dependent on the saturation caused by the other (more powerful) signal. This means that at positions along the fibre where the intensity distributions of the two signals are in phase, the core absorption is saturated by the first (more powerful) signal and the second (less powerful) signal is only slightly attenuated. However, where the two signals do not coincide spatially, the second signal will be attenuated much more strongly by the unsaturated absorber at those positions.
In other words, the loss sustained by the second (weaker) signal after passing through the filter is greater than the loss sustained by the first (stronger) signal. This wavelength and power dependent loss gives rise to the filtering effect provided by the filter.
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S. J. Frisken, "Transient Bragg Reflection Gratings In Erbium-Doped Fiber Amplifiers," Optics Letters, vol. 17, No. 24, pp. 1776-1778, Dec. 15, 1992.
M. Horowitz et al., "Narrow-Linewidth, Singlemode Erbium-Doped Fibre Laser With Intracavity Wave Mixing In Saturable Absorber," Electronics Letters, vol. 30, No. 8, pp. 648-649, Apr. 14, 1994.
Y. Cheng et al., "Stable Single-Frequency Traveling-Wave Fiber Loop Laser With Integral Saturable-Absorber-Based Tracking Narrow-Band Filter," Optics Letters, vol. 20, No. 8, pp. 875-877, Apr. 15, 1995.
Laming Richard I.
Loh Wei-Hung
Payne David N.
Zervas Michael N.
Palmer Phan T. H.
University of Southampton
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