Polarization diversity for birefingent filters

Details Polarization diversity for birefingent filters Polarization diversity for birefingent filters

2000-03-21

2001-06-26

Spyrou, Cassandra (Department: 2872)

Optical: systems and elements

Polarization without modulation

By relatively adjustable superimposed or in series polarizers

C359S490020, C359S490020, C359S900000, C359S199200, C359S199200, C359S199200

Reexamination Certificate

active

06252711

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATIONS
Related subject matter is disclosed the concurrently filed application entitled “DOUBLE-PASS POLARIZATION DIVERSIFIED BI REFRINGENT FILTER” by J. N. Damask and C. R. Doerr, which is assigned to the same Assignee.
FIELD OF THE INVENTION
The invention relates generally to birefringent filters and, more particularly, to an improved birefringent interleaving filter for separating even and odd channels of a wavelength division multiplexed signal.
BACKGROUND OF THE INVENTION
The birefringent filter was first developed by astronomers at the turn of the century so that they could make celestial observations of the Sun through a narrow-band spectral window. The classic papers on these filters were written by Loyt [1] Evans [2] and Solc [3]. (Note in this specification, a reference to another document is designated by a number in brackets to identify its location in a list of references found in the Appendix) In 1965 Harris et al [4] published a method of filter synthesis for the birefringent filter. In the late 1980s, Buhrer [5] brought the birefringent filter into the optical communications world with his demonstration of such a filter designed for optical wavelength multiplexing and demultiplexing.
The filters built by Lyot, Evans, and Solc,
FIG. 1
a
used polarizers placed before and after the birefringent filter proper to isolate one polarization state at the input and then discriminate the state of polarization at the output of the filter. This was sufficient for their application because the Sun is an intense source and the polarization of light from the Sun is completely random.
In the late 1980s, Buhrer recognized that the use of input and output polarizers created excessive polarization-dependent loss and was therefore unsuitable for telecommunication applications. He instead created a method for polarization diversity. New input and output stages were proposed to allow the birefringent filter to handle simultaneously two orthogonal polarizations and to allow the output of the filter to couple to two complimentary ports. In his scheme, there is no loss of light, regardless of polarization. Buhrer U.S. Pat. No. 4,987,567 [6] further describes the use of a birefringent filter as an optical signal multiplexer/demultiplexer.
This invention makes material improvements to the Buhrer scheme for polarization diversity at the input and output of a birefringent filter. This invention increases the isolation between signals and decreases the chromatic dependence.
SUMMARY OF THE INVENTION
Our improved polarization diversity for birefringent filter implementation offers a decreased polarization sensitivity and an increased crosstalk rejection over prior art implementations and offers low insertion loss, high contrast, and low chromatic dispersion. As such, our birefringent filter with improved polarization diversity may be used as an interleaving filter to separate even and odd channels of a wavelength division multiplexed signal.
In accordance with the present invention, a birefringent filter apparatus comprises (1) an input optical apparatus for receiving an input optical beam and producing therefrom parallel first and second polarized optical beams offset in a first direction and having the same polarization; (2) a birefringent filter for receiving the first and second polarized optical beams and producing therefrom a first and second elliptically polarized optical beams, each including first and second orthogonal beam components having inversely varying intensities which vary periodically, over a predefined free-spectral range, with the frequency of the input optical beam; and (3) an output optical apparatus for receiving the first and second elliptically polarized optical beams and for combining together the first beam components of the first and second elliptically polarized optical beams to form a first elliptical polarized output beam and for combining together the second beam components of the first and second elliptically polarized optical beams to form a second elliptical polarized output beam.
In one embodiment, the input optical apparatus includes a birefringent beam displacer and a &lgr;/2 plate and the output optical apparatus includes a first beam displacer, a two-by-two &lgr;/2 plate, and a second beam displacer. In a second embodiment, the input optical apparatus includes a first beam displacer, a two-by-two &lgr;/2 plate, and a second beam displacer and the output optical apparatus includes a first beam displacer, a two-by-two &lgr;/2 plate, and a second beam displacer. In a third embodiment, the input optical apparatus includes a polarized beam-splitter (PBS) and a &lgr;/2 plate and the output optical apparatus includes a &lgr;/2 plate and a PBS.


REFERENCES:
patent: 4973124 (1990-11-01), Kaede
patent: 4987567 (1991-01-01), Buhrer
patent: 5606439 (1997-02-01), Wu
patent: 5867291 (1999-02-01), Wu et al.
patent: 5912748 (1999-06-01), Wu et al.
patent: 6163393 (2000-12-01), Wu et al.

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