Optical: systems and elements – Polarization without modulation – Polarization using a time invariant electric – magnetic – or...
Patent
1995-06-21
1998-06-16
Shafer, Ricky D.
Optical: systems and elements
Polarization without modulation
Polarization using a time invariant electric, magnetic, or...
359495, 359497, 359282, 372703, 385 6, 385 11, G02B 530, G02F 109
Patent
active
057680159
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to an optical isolator, and in particular, an isolator which is wavelength selective.
Optical isolators are used in optical communications systems to restrict, or isolate, the direction of travel of optical signals to one direction over a broad wavelength band. It has also recently been discovered that there is application for isolators which are wavelength selective and restrict a first wavelength band to one direction of travel, and a second wavelength band to the opposite direction, as discussed in International Patent Application No. PCT/AU93/00258 for a bidirectional isolator.
The present invention, more specifically, relates to using the wavelength dispersion characteristics of optical rotator materials, such as Faraday rotator materials, to provide isolator functions, and, in some instances, enhance performance.
Faraday rotators rotate polarised signals of a wavelength band .lambda. by a selected number of degrees in a set clockwise or anticlockwise direction, regardless of whether the signals travel through the rotator in a forward or reverse direction. Faraday rotators are normally constructed from YIG, which is Yttrium based, or BIG, which is based on Bismuth substituted YIG. Optical rotators can also be provided by multi-order half-wave plates, which can be cut from quartz and used to rotate optical signals of wavelength .lambda. by a predetermined number of degrees. Unlike Faraday rotators, the half-wave plates will rotate a signal of wavelength .lambda. in one direction as it passes therethrough in a forward direction but will perform a reciprocal rotation, by rotating the signal in the opposite direction, when it passes through the wave plate in the reverse direction. Reciprocal optical rotators can also be fabricated from optically active material which continuously rotates polarised light along its length in a linear polarization state. Half-wave plates rotate linearly polarized light by changing it to an elliptical polarization state and then back to a linear polarization state. Half-wave plates normally can achieve a desired rotation in a much shorter length of material than optically active material, which can also be cut from quartz.
The term wavelength dispersion is used herein to describe a characteristic of a device which processes differently or has a different effect on signals of different wavelengths. Similarly, the term polarization dispersion is used to refer to a characteristic of a device which processes differently or has a different effect on signals of different polarizations.
Previously the dispersion characteristic of a Faraday material in a cascaded isolator assembly has only been used to achieve isolation of a broader signal wavelength band, as discussed in Kazuo Shiraishi and Shorjiro Kawakami, "Cascaded optical isolator configuration having high-isolation characteristics over a wide temperature and wavelength range" Optics Letters, Volume 12, No. 7, July 1987, pages 462 to 464.
In accordance with the present invention there is provided an optical isolator comprising two polarizer means, two input/output ports formed respectively on said polarizer means, and optical rotator means disposed between said polarizer means, said optical rotator means including Faraday rotator means and being selectively configured so that the isolator performs one of a plurality of isolator functions.
Advantageously the wavelength dispersion characteristics of said optical rotator means may determine said one of said isolator functions for at least two wavelength bands.
Preferred embodiments of the present invention are hereinafter described, by way of example only, with reference to the accompanying drawings, wherein:
FIG. 1 is a side view of a first preferred embodiment of an isolator;
FIG. 2 is polarization diagrams for the isolator of FIG. 1 for light of wavelength .lambda..sub.2 ;
FIG. 3 is polarization diagrams for the isolator of FIG. 1 for light of wavelength .lambda..sub.1 ;
FIG. 4 is a side view of a second preferred embodiment of an isolator;
FIG. 5
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Shafer Ricky D.
Telstra Corporation Limited
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