Optical: systems and elements – Polarization without modulation – Polarization using a time invariant electric – magnetic – or...
Reexamination Certificate
2001-03-30
2003-09-16
Epps, Georgia (Department: 2873)
Optical: systems and elements
Polarization without modulation
Polarization using a time invariant electric, magnetic, or...
C359S494010, C359S490020, C372S703000
Reexamination Certificate
active
06621630
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to composite optical elements, optical isolators, and optical attenuators for applications in optical communications and optical measurements, and also to processes for producing the same.
BACKGROUND OF THE INVENTION
Japanese Patent Application Kokai No. 11-2725 describes a composite optical element comprising a Faraday rotator and at least a first and a second birefringent regions joined to one plane of the rotator, with or without at least a third and a fourth birefringent regions joined to the opposite plane of the rotator, and also describes optical isolators, optical circulators, and optical switches utilizing such optical elements. In order to achieve a desired optical isolation or the like, it is essential that the first and second birefringent regions and, where provided, the third and fourth birefringent regions should be precisely equal in thickness. According to the cited teachings, the first and second birefringent regions are made by: first forming grooves in plate materials, equidistantly in parallel with one another to a predetermined depth by etching or other similar method, leaving ridges or lands in between; the grooved sides of the plate materials are faced to each other, with the grooves and lands of one side being fitted in the lands and grooves of the other, and joined integrally with the aid of adhesive; and then the both outer sides of the joined body are machined to the bottoms of the grooves and finished by polishing. The cited method is effective in strictly equalizing the thicknesses of the first and second birefringent regions so joined. The same applies to the joining of the third and fourth birefringent regions.
Adoption of a birefringent element thus made renders it possible to fabricate an optical isolator, for example, a polarization-independent optical isolator as a composite optical element comprising a Faraday rotator with a Faraday rotation angle of 45° (although it is common to add an external magnetic field, spontaneous magnetization sometimes takes place in the absence of a magnetic field), at least a first and a second birefringent regions joined to one side of the rotator, and at least a third and a fourth birefringent regions joined to the opposite side of the rotator, wherein:
the light that has been transmitted through the first birefringent region passes through the third birefringent region;
the light that has been transmitted through the second birefringent region passes through the fourth birefringent region;
the optical axis of the first birefringent region and that of the second birefringent region intersect orthogonally;
the optical axis of the third birefringent region and that of the fourth birefringent region intersect orthogonally;
the optical axis of the first birefringent region and that of the third birefringent region make an angle of about 45° with respect to each other;
the both principal planes of the first and second birefringent regions have the same surfaces flush with each other;
the both principal planes of the third and fourth birefringent regions have the same surfaces flush with each other; and
the first, second, third, and fourth birefringent regions are of the same material quality and have the same thickness d. Thus all light beams in the forward direction pass through the composite element without dependence upon the direction of polarization, whereas return light beams are all diffracted without dependence upon the polarization direction and are unable to return to the incident side.
The principles of the optical isolator described in the cited literature are as follows.
Out of the light beams incoming in the forward direction, the light of linear polarization parallel to the optical axis of the first birefringent region is transmitted as extraordinary light (refractive index ne) through the first birefringent region. As it further passes through the Faraday rotator the plane of polarization rotates 45°, and then the light passes as ordinary light (refractive index no) through the third birefringent region. [The optical path length is (ne+no)d]. Meanwhile light as ordinary light (refractive index no) is transmitted through the second birefringent region. As it further passes through the Faraday rotator the plane of polarization rotates 45°, and then the light passes as extraordinary light through the fourth birefringent region. [The optical path length is (no+ne)d].
Of the light incoming in the forward direction, light of linear polarization perpendicular to the optical axis of the first birefringent region passes as ordinary light through the first birefringent region. As it further passes through the Faraday rotator the plane of polarization rotates 45°, and then the light passes as extraordinary light through the third birefringent region. [The optical path length is (no+ne)d]. It then travels as extraordinary light through the second birefringent region and, after 45° rotation of the polarized surface by the Faraday rotator, it passes as ordinary light through the fourth birefringent region. [The optical path length is (no+ne)d]. Thus all the optical paths are equal in length and light travels straightly forward without diffraction.
Of the light incident from the reverse direction, the light of linear polarization parallel to the optical axis of the third birefringent region is transmitted as extraordinary light through the third birefringent region. As it further passes through the Faraday rotator the plane of polarization rotates 45°, and then the light passes as ordinary light through the first birefringent region. [The optical path length is (ne+ne)d]. Meanwhile light as ordinary light is transmitted through the fourth birefringent region. As it further passes through the Faraday rotator the plane of polarization rotates 45°, and then the light passes as extraordinary light through the second birefringent region. [The optical path length is (no+no)d]. Here if d is set so that the optical path difference is 2(no−ne)d=(M+½)&lgr; (where &lgr; is the wavelength of the light and M is an arbitrary integer), light will all be diffracted.
Of the light incident from the reverse direction, the light of linear polarization perpendicular to the optical axis of the third birefringent region is transmitted as ordinary light through the third birefringent region. As it further passes through the Faraday rotator the plane of polarization rotates 45°, and then the light passes as ordinary light through the first birefringent region. [The optical path length is (no+no)d]. Meanwhile light as extraordinary light is transmitted through the fourth birefringent region. As it further passes through the Faraday rotator the plane of polarization rotates 450, and then the light passes as extraordinary light through the second birefringent region. [The optical path length is (ne+ne)d]. Here because the optical path difference is 2(no−ne)d=(M+½)&lgr; (where &lgr; is the wavelength of the light and M is an arbitrary integer), light will all be diffracted.
According to the method of Patent Application Kokai No. 11-2725, as illustrated in
FIG. 1
, a composite optical element is fabricated by disposing a first birefringent region
1
and a second birefringent region
2
on one side of a Faraday rotator
7
and disposing a third birefringent region
3
and a fourth birefringent region
4
on the other side of the rotator and joining them through layers of adhesive
8
. The arrangement necessarily forms a fifth region
5
and a sixth region
6
of the adhesive used, in addition to the two birefringent regions each on both sides of the Faraday rotator, resulting in deteriorated optical properties. The problem arises from the fact that a blank sheet of birefringent material must be formed with a sufficient number of grooves to make a plurality of composite optical elements and, in order to bring the two grooved shee
Drinker Biddle & Reath LLP
Epps Georgia
Harrington Alicia M
TDK Corporation
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