Beam splitting device

Details Beam splitting device Beam splitting device

2001-06-25

2003-07-01

Dang, Hung Xuan (Department: 2873)

Optical: systems and elements

Single channel simultaneously to or from plural channels

C359S494010, C359S490020, C385S011000

Reexamination Certificate

active

06587273

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to the field of optics and in particular to a polarisation beam splitting device whose incorporation into optical systems that require polarisation beam splitters, can significantly reduce their overall dimensions.
BACKGROUND
Beam splitting devices are commonly used in the field of optics when it is required to separate two spatially overlapping beams of light or two polarised components of a single beam. The prior art teaches of various methods for achieving such a result that employ either blocks of birefringent material, polarisation dependent coatings or other polarisation effects.
When requiring the polarisation components of a light beam to be resolved, the most efficient manner is to employ a block of birefringent material. A birefringent material is one that is optically anisotropic in that the optical properties it exhibits depend upon the polarisation and propagation direction of the incident light. Many crystalline substances, such as rutile, calcite or yttrium orthovanadate, exhibit such birefringent properties and so provide ideal media from which to develop polarisation beam splitting devices. Such crystal structures are so suited for producing compact birefringent medium, as they comprise high-density structures that lend themselves to cutting, so producing incident surfaces and optic axis of the required predetermined orientations. Nicol prisms and Glan-Foucault prisms are examples taught in the prior art of birefringent crystals employed as beam splitters.
FIG. 1
presents a side elevation of a typical block of birefringent material
103
as taught in the prior art. Here an unpolarised incident beam
140
is incident on the block of birefringent material
103
, thereby being resolved into two light beams having orthogonal linear polarisations. For reference a propagation axis L is defined corresponding to the axis of an input beam
140
. With this particular orientation, beam
140
a
corresponds to the ordinary beam while beam
140
b
corresponds to the extraordinary beam. As is typical in optical systems, components are designed such that where possible input and output faces are perpendicular to the central axis L. Therefore, with the incident beam
140
perpendicular to the block of birefringent material
103
the resulting ordinary beam
140
a
passes without deviation through the block
103
while the extraordinary beam
140
b
is refracted as shown.
An inherent disadvantage of such a splitting of the ordinary and extraordinary component beams is that when incorporated into an optical system, such blocks of birefringent material
103
introduce an asymmetric beam splitting. It is normally advantageous for the emerging ordinary
140
a
and extraordinary beams
140
b
to be parallel and equidistant from the propagation axis L. The dimensions of the other optical elements of an optical system are then directly dependent on the block of birefringent material
103
.
By way of example such blocks of birefringent material
103
are considered herein as incorporated with an optical circulator. However, as will be obvious to those skilled in the art, the problem of reducing the dimensions of an optical system that ernploys such a block of birefringent material
103
as a beam splitter, is not limited solely to optical circulators. Such optical systems also include for example, optical isolators and polarisation beam splitters/combiners.
An optical circulator is a device that has at least three ports for accepting optical fibres. Light that enters the circulator through the first port exits through the second port; light that enters through the second port exits through the third. The optical circulator is an inherently non-reciprocal device. If light enters through the first port it exits through the second, but if that light is subsequently reflected back into the second port, it does not retrace its path back to the first port, but exits through the third port instead.
Circulators are necessary, for example, to use the same fibre for both receiving and transmitting data. The first port may be connected to a data transmitter, and the second port to a long distance optical fibre. In that case, data can be sent from the transmitter to the fibre. At the same time, incoming optical data from the long distance fibre enters the circulator through the second port and is directed to the third port where a receiver may be connected.
An optical circulator found in the prior art is that taught by Li et al in U.S. Pat. No. 5,930,039, see
FIG. 2
, the contents of which are incorporated herein by reference.
This document teaches of an optical circulator
100
that employs reciprocal and non-reciprocal polarisation rotators
130
a
and
130
b
, birefringent optical components
103
,
108
and
111
, and a polarisation dependent refraction element
150
comprising of two tapered birefringent plates
106
and
107
. In the preferred embodiment the optical circulator
100
has its optical components aligned such that effects of the birefringent optical components occur in the vertical plane while the effects of the polarisation dependent refraction element occur in the horizontal plane.
The first and third fibres
100
a
and
100
b
are inserted in parallel and adjacent to each other into a glass capillary
101
which is followed by a first lens
102
. Together the glass capillary
101
and the lens
102
comprise a first collimator
120
a
. A first block of birefringent material
103
, a first compound polarisation rotator
130
a
, a light guiding device
150
, a second birefringent block
108
, a second compound polarisation rotator
130
b
and a third block of birefringent material
111
are then located along a longitudinal axis L of circulator
100
. A second collimator
120
b
comprising a second lens
112
and a second glass capillary
113
which holds the second fibre
114
are found at the opposite end of device
100
.
FIG. 3
provides alternative elevations of the optical circulator
100
. In particular
FIG. 3
a
presents a side profile of the circulator
100
presenting light propagating in the z-y plane from the first fibre
100
a
to the second fibre
114
. Initially the light propagates through the first lens
102
and into the first birefringent block
103
Walk off within the block
103
in the z-y plane then produces two mutually orthogonal linearly polarised beams,
140
a
and
140
b
, as shown. These linearly polarised beams
140
a
and
140
b
then propagate through the first compound polarisation rotator
130
a
before continuing on through the optical circulator
100
until they are recombined by the third birefringent block
111
and focused by second lens
113
into second the fibre
114
.
For the optical circulator
100
to work correctly it requires that any light entering the device at the second fibre
114
exits the optical circulator
100
via the third fibre
100
b
, and not via the first fibre
100
a
. The non-reciprocal nature of the device lies in the inherent properties of the compound polarisation rotators
130
a
and
130
b
. To illustrate these features
FIG. 3
b
presents a side profile in the z-y plane of the circulator
100
presenting light propagating from the second fibre
114
to the third fibre
100
b.
Comparison of the orientations of the linearly polarised electric field components after propagating through the compound polarisation rotators
130
a
and
130
b
shows how the polarisation orientation of an electric field depends on which direction it has propagated through the compound polarisation rotators
130
a
and
130
b
. The origin of this non-reciprocity lies in the inherent properties of the Faraday rotators
105
and
110
. Unlike the half wave plates
104
a
,
104
b,
109
a
and
109
b
which reverse the rotation experienced by a linearly polarised electric field on reversal of its propagation direction, a Faraday rotator is designed to always rotate a linearly polarised electric field in the same sense irrespective of propagation direction.
FIG. 3
c
shows the x-y pla

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