Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
2000-01-05
2001-09-11
Shafer, Ricky D. (Department: 2872)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C359S484010, C359S490020, C359S490020
Reexamination Certificate
active
06288826
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to a multi-stage optical isolator and more particularly to an at least two stage optical isolator that requires only a single birefringent element between two non-reciprocal rotating elements.
BACKGROUND OF THE INVENTION
Optical isolators are used in a variety of applications in optical communication systems. Generally, optical isolators are used to prevent reflective portions of transmitted signals from re-entering the transmitting device. Many older prior art designs prevent reflections from re-entering a transmitting device in a polarization-selective manner. However, in certain circumstances where a transmission system causes uncontrollable changes in polarization, the polarization state of a signal may be unknown, and thus, these earlier polarization dependent designs are not considered to be practical. Thus, as of late, a large effort has been undertaken to develop an isolator that is polarization independent. It is also desired to have an optical isolator that is capable of isolating high power optical signals without compromising the longevity of the isolator.
One prior art polarization independent optical isolator is described in U.S. Pat. No. 5,033,830 issued Jul. 23, 1991 in the name of Jameson and entitled Polarization Independent Optical Isolator. Jameson describes an isolator having a single birefringent plate, a pair of stacked reciprocal rotators, a Faraday rotator, and a reflector positioned in tandem adjacent to the birefringent plate. In a forward (transmitting) direction, a lightwave signal exiting an optical fiber is split into a pair of orthogonal rays by the birefringent plate. The orthogonal rays then pass through a first reciprocal rotator and the Faraday rotator which provides 22.5° of rotation. The rotated rays are then redirected by the reflector back though the Faraday rotator. After passing through the second reciprocal rotator, the orthogonal rays re-enter the same birefringent plate where they are recombined and launched in an output fiber. Since a Faraday rotator is a non-reciprocal device, any signal traveling through the isolator in the reverse (isolation) direction will be split on both passes through the birefringent plate such that neither will intercept the input fiber. In practice, Jameson's single stage isolator described above, may provide adequate isolation; however, in some instances, increased isolation may be required. Such additional isolation has been known to be provided by using a multi-stage optical isolating device; generally these multi-stage devices are costly to manufacture often requiring nearly double the number of optical components that a single stage device requires; more importantly, aligning nearly twice as many components with one another can be difficult, time-consuming, costly, and generally increase the overall alignment error.
For example, U.S. Pat. No. 5,581,640 in the name Pan et al. Assigned to E-tek Dynamics, Inc. describes a multi-stage optical isolator wherein two polarizers in the form of a birefringent crystal wedges of lithium niobate are used as the birefringent material of the polarizers. The polarizers in prior art
FIG. 1
(shown as
FIG. 6A
of the '640 patent) are shown as spaced-apart crystal wedges having complementary slanted faces. However, it is not clear from the specification whether the space between the optical elements, shown between all of the elements in the device, is merely for the purpose of illustration. For example, if the gapped end faces of elements
64
a
and
64
b
in
FIG. 1
are air gapped, then the end faces would likely require anti-reflection (AR) coating.
The instant invention obviates both the requirement of AR coating two crystals disposed between two non-reciprocal polarization rotating elements, and, obviates using two such crystals, thereby obviating the requirement for adhesive between such crystals.
Since the instant invention obviates the requirement of two crystals disposed between two non-reciprocal polarization rotating elements, it thereby obviates the complex and difficult component alignment that is required when using two such crystals are in tandem in a multi-stage isolator.
It is an object of this invention to provide a relatively low-cost optical isolator that is particularly well suited to carrying high-power optical signals.
SUMMARY OF THE INVENTION
In accordance with the invention, there is provided, a multi-stage optical isolator having at least two stages, comprising:
an input birefringent crystal for splitting an input beam into two orthogonally polarized sub-beams;
an output birefringent crystal for combining two orthogonally polarized beams into a single output beam at an output end thereof;
first rotating means for rotating the polarization of the beams received from an output end of the input birefringent crystal;
second rotating means for rotating the polarization of the beams directed to the input end of the output birefringent crystal; and
a centrally disposed birefringent crystal substantially equal to the sum of the thicknesses of the input and the output crystals, disposed between the first and second rotating means for merging two rotated beams received from the first rotating means, and for diverging said beams after they combine therein to provide two separated beams to the second rotating means.
In accordance with the invention, a two stage optical isolator is provided having two birefringent crystals, one at each end thereof, each having a thickness of “t”, two at least non-reciprocal rotators disposed between the two birefringent crystals; and a single birefringent crystal having a thickness of substantially at disposed between the two non-reciprocal rotators.
In accordance with the invention, a two stage optical isolator, having less than four birefringent crystals, comprise a centrally disposed birefringent crystal; two non-reciprocal rotating means, one on each side of said centrally disposed birefringent crystal; and two other birefringent crystals, one on each end of the two stage optical isolator, for respectively splitting and combining two orthogonally polarized beams; wherein the centrally disposed birefringent crystal is thicker that the combined thickness of the two other birefringent crystals; and wherein the thickness of the centrally disposed crystal is such that when two separated beams orthogonally polarized by a distance “d” received from one of the other birefringent crystals are launched into the centrally disposed crystal, the two beams converge and subsequently diverge within the centrally disposed crystal to exit the centrally disposed crystal having a separation “d”.
Advantageously, the isolator in accordance with this invention is found to provide a high degree of isolation of an incoming optical signal with less overall cost per device.
Advantageously the isolator in accordance with this invention is well suited to isolating high-power optical signals.
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JDS Uniphase Inc.
Shafer Ricky D.
Teitelbaum Neil
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