Split-beam Fourier filter

Optical waveguides – Accessories – Attenuator

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385 25, G02B 626

Patent

active

059304413

DESCRIPTION:

BRIEF SUMMARY
TECHNICAL FIELD

This invention relates to optical filtering, to means and methods for producing the filtering and to devices using such means and methods. In particular, this invention relates to optical filtering of light guided by optical waveguides such as optical fibre.


BACKGROUND ART

Many different types of filter have been demonstrated. These include various types of absorption filtering, dielectric single or multilayer filtering, interferometric filtering (Fabry-Perot, Michelson, Mach-Zehnder, Sagnac etc) and grating filtering.
A conventional (prior art) Mach-Zehnder filter consists of a beam splitter (or coupler) which splits the input light into two paths and a beam combiner (or coupler) to combine the light again. If the two paths have different lengths, the Mach-Zehnder filter has a wavelength dependent transmission characteristic.
The split-beam Fourier filter (SBFF) described in this document is a special type of filter which can give the same characteristic transmission as a Mach-Zehnder filter as well as more complex characteristics by simply splitting the beam of a fibre beam expander with an appropriate transparent element or elements so that different parts of the beam travel different optical path lengths. This gives highly stable performance and is ideally suited to incorporation in a single mode fibre beam expander.


PRINCIPLE OF OPERATION

The split-beam Fourier filter consists of a number of plates of glass appropriately positioned in a fibre beam expander. In the case of one plate of glass (single element) in one beam expander (single stage), the wavelength characteristic is that of a Mach-Zehnder filter--sinusoidal with a wavelength period dependent on the thickness of the glass plate. Excellent loss and extinction have been obtained, and mechanical tuning of the characteristic (both wavelength and extinction) is straightforward. The use of multiple elements allows more complex filter characteristics.
FIG. 1 shows the elements comprising a single stage split-beam Fourier filter. The filter is a flat plate of glass (1) with one edge (2) carefully polished perpendicular to the plate surface this edge splits the beam of a fibre beam expander. Light passing through the plate will experience a wavelength dependent phase shift compared with the light that does not pass through the plate. If the phase shift is zero or a multiple of 2.pi., then the beam is unchanged and the transmission is maximum (100%). If the phase shift is .pi., then the E field is inverted in one half of the beam compared to the other, giving an antisymmetric E field distribution in the beam. The result at the output fibre tip is an E field distribution which is the two dimensional Fourier transform of the beam E field which is also an antisymmetric function. The overlap of this distribution with the fundamental fibre mode is zero therefore no light will be launched into the fibre if the fibre is single mode. Analysis shows that the transmission, T is given by: refractive index and L is the thickness of the glass plate, .lambda. is the wavelength and s is the suppression of extinction which depends on the fraction of the beam passing through the plate (when the plate exactly bisects the beam, s=1).


BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a single element, single stage, split-beam Fourier filter.
FIG. 2 shows the transmission characteristic of a split-beam Fourier filter with various extinctions and centre wavelengths.
FIG. 3 shows the location of two plates (viewed along beam) to give a combined characteristic.
FIG. 4 shows the extension to 4 plates.
FIG. 5 shows an alternative plate distribution to achieve complex filtering characteristics.
FIG. 6 shows the two dimensional version of FIG. 5.
FIG. 7 shows the construction of an electrically tunable phase plate.
FIG. 8 shows a switchable split-beam Fourier filter.
FIG. 9 shows a split-beam mode convertor.
FIG. 10 shows an optical amplifier incorporating a split-beam Fourier filter for gain flattening.
FIG. 11 shows the saturated gain cha

REFERENCES:
patent: 4989938 (1991-02-01), Tamulevich
patent: 5276747 (1994-01-01), Pan
patent: 5781341 (1998-07-01), Lee
Kawachi et al., "Planar lightwave circuits for optical signal processing," Conference on Optical Fiber Comm. 1994; OSA, Washington, D.C. 1994; FB3, pp. 281-282.
Mollenauer et al., "Demonstration, using sliding-frequency guiding filters, of error-free soliton transmission over more than 20,000 km at 10 Gbit/s , single-channel and over more than 13,000 km at 20 Gbit/s in a two-channel WDM," Conference on Optical Fiber Comm. 1993; OSA, Washington, D.C. 1994: Paper PD8, pp. 37-40.
Poole et al., "Elliptical-Core Dual-Mode Fiber Dispersion Compensator," IEEE Photonics Technology Letters, vol. 5, No. 2, Feb. 1993, pp. 194-197.

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