U.S. patent application number 14/554633 was filed with the patent office on 2016-05-26 for launch optics with optical path compensation for a wavelength selective switch.
The applicant listed for this patent is Nistica, Inc.. Invention is credited to Mitchell E. Haller, Jefferson L. Wagener.
Application Number | 20160147092 14/554633 |
Document ID | / |
Family ID | 55920080 |
Filed Date | 2016-05-26 |
United States Patent
Application |
20160147092 |
Kind Code |
A1 |
Wagener; Jefferson L. ; et
al. |
May 26, 2016 |
LAUNCH OPTICS WITH OPTICAL PATH COMPENSATION FOR A WAVELENGTH
SELECTIVE SWITCH
Abstract
An optical device includes an optical port array, a first
walk-off crystal, a first half-wave plate, a second walk-off
crystal and a segmented half-wave plate. The optical port array has
a first and second plurality of ports for receiving optical beams.
The first walk-off crystal spatially separates the beams into first
and second portions that are in first and second orthogonal
polarization states, respectively. The first portions are
walked-off by the first walk-off crystal and the second portions
pass therethrough without being walked-off. The first half-wave
plate rotates the polarization state of the first and second
portions of the optical beams. The second walk-off crystal is
oriented in an opposite direction from the first walk-off crystal
such that the second portions are walked-off by the second walk-off
crystal and the first portions pass therethrough without being
walked-off. The segmented half-wave plate receives the first or
second portions of the beams.
Inventors: |
Wagener; Jefferson L.;
(Morristown, NJ) ; Haller; Mitchell E.; (Marlboro,
NJ) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Nistica, Inc. |
Bridgewater |
NJ |
US |
|
|
Family ID: |
55920080 |
Appl. No.: |
14/554633 |
Filed: |
November 26, 2014 |
Current U.S.
Class: |
359/249 ;
349/196; 359/256; 359/489.05; 359/489.08 |
Current CPC
Class: |
G02B 1/005 20130101;
G02B 27/0068 20130101; G02B 6/354 20130101; H04Q 2011/0026
20130101; G02B 27/283 20130101; G02B 27/1026 20130101; G02F 2202/32
20130101; G02B 26/06 20130101; G02B 5/30 20130101; G02B 6/2773
20130101; G02B 27/0172 20130101; H04Q 2011/0035 20130101; G02B
26/0833 20130101; G02B 26/007 20130101; G02F 2203/12 20130101; G02B
5/3083 20130101; G02B 5/1814 20130101; G02B 6/2861 20130101; G02B
26/00 20130101; G02B 27/286 20130101; G02F 1/133606 20130101; G02F
2203/50 20130101; H04Q 11/0005 20130101; G02B 26/0816 20130101;
G02B 26/005 20130101; G02B 26/001 20130101 |
International
Class: |
G02F 1/13 20060101
G02F001/13; G02B 27/28 20060101 G02B027/28; G02B 5/30 20060101
G02B005/30 |
Claims
1. A method for compensating for a difference in optical path
length traversed by first and second optical portions of an optical
beam which are in orthogonal polarization states, comprising:
receiving the optical beam at the input port; directing the optical
beam to a first walk-off crystal for spatially separating the
optical beam into first and second optical portions that are in
first and second polarization states, respectively, the first and
second polarization states being orthogonal to one another, the
first optical portion being walked-off by the walk-off crystal and
the second optical portion passing therethrough without being
walked-off; rotating the polarization state of the first and second
optical portions; after rotating the polarization state of the
first and second optical portions, directing the first and second
optical portions to a second walk-off crystal being oriented in an
opposite direction from the first walk-off crystal so that the
second optical component is walked-off by the walk-off crystal and
the first optical component passes therethrough without being
walked-off, a thickness of the first and second walk-off crystals
being selected to adjust the optical path lengths traversed by each
of the first and second optical portions.
2. The method of claim 1 wherein the thickness of the first and
second walk-off crystals is further selected to compensate for an
optical path difference that arises from one or more upstream or
downstream optical elements that is traversed by one of the first
and second optical portions and not the other of the first and
second optical portions.
3. The method of claim 1 wherein the optical element is a half-wave
plate located downstream from the first and second walk-off
crystals.
6. The method of claim 1 wherein the first input port is angularly
offset with respect to the second input about an axis parallel to
the wavelength dispersion axis.
7. A method for directing wavelength components of an optical beam
from an input port of a port array to at least one output port of
the port array, comprising: receiving a first optical beam at a
first input port of the port array associated with a first
wavelength selective switch; receiving a second optical beam at a
second input port of the port array associated with a second
wavelength selective switch; directing the first optical beam to a
first walk-off crystal for spatially separating the first optical
beam into first and second optical portions that are in first and
second polarization states, respectively, the first and second
polarization states being orthogonal to one another, the first
optical portion being walked-off by the walk-off crystal and the
second optical portion passing therethrough without being
walked-off; directing the second optical beam to the first walk-off
crystal for spatially separating the second optical beam into third
and fourth optical portions that are in the first and second
polarization states, respectively, the third optical portion being
walked-off by the first walk-off crystal and the fourth optical
portion passing therethrough without being walked-off; rotating the
polarization state of the first, second, third and fourth optical
portions; after rotating the polarization state of the first,
second, third and fourth optical portions, directing the first,
second, third and fourth optical portions to a second walk-off
crystal being oriented in an opposite direction from the first
walk-off crystal so that the second and fourth optical portions are
walked-off by the second walk-off crystal and the first and third
optical components pass therethrough without being walked-off, a
thickness of the walk-off crystals being selected to adjust the
optical path lengths traversed by each of the first, second, third
and fourth optical portions; spatially separating the wavelength
components of the first, second, third and fourth optical portions;
focusing the spatially separated wavelength components onto a
programmable optical phase modulator so that the wavelength
components of the first and second optical beams are spatially
separated along a wavelength dispersion axis of the modulator; and
adjusting a phase shift profile of the modulator along the second
direction to selectively direct individual ones of the wavelength
components to an output port.
8. The method of claim 7 wherein the thickness of the first and
second walk-off crystals is further selected to compensate for an
optical path difference that arises from one or more upstream or
downstream optical elements that is traversed by one of the first
and second optical portions and not the other of the first and
second optical portions and that is traversed by one of the third
and fourth optical portions and not the other of the third and
fourth optical portions
9. The method of claim 7 wherein the optical element is a half-wave
plate located downstream from the first and second walk-off
crystals.
10. The method of claim 7 wherein the first input port is angularly
offset with respect to the second input about an axis parallel to
the wavelength dispersion axis.
11. An optical device, comprising: an optical port array having a
first plurality of ports for receiving optical beams and a second
plurality of ports for receiving optical beams; a first walk-off
crystal for spatially separating each of the optical beams into
first and second optical portions that are in first and second
polarization states, respectively, the first and second
polarization states being orthogonal to one another, the first
optical portions being walked-off by the first walk-off crystal and
the second optical portions passing therethrough without being
walked-off; a first half-wave plate for rotating the polarization
state of the first and second optical portions of the optical
beams; a second walk-off crystal being oriented in an opposite
direction from the first walk-off crystal such that the second
optical portions are walked-off by the second walk-off crystal and
the first portions pass therethrough without being walked-off; and
a segmented half-wave plate for receiving the second optical
portions of the optical beams or the first optical portions of the
optical beams.
12. The optical device of claim 11 wherein a thickness of the first
and second walk-off crystals being selected to adjust the optical
path lengths traversed by each of the first and second optical
portions to compensate for an optical path difference that arises
from the first optical portions or the second optical portions
traversing the segmented half-wave plate and the other of the first
and second optical portions not traversing the segmented half-wave
plate.
13. The optical device of claim 11 further comprising: a dispersion
element receiving the first and second optical portions of the
optical beams and spatially separating the first and second optical
portions into a plurality of wavelength components along a
wavelength dispersion axis, a port axis being orthogonal to the
wavelength dispersion axis, the first plurality of ports extending
in a first plane that includes the port axis and the second
plurality of ports extending in a second plane that includes the
port axis, the first and second planes being parallel to one
another and offset from one another along the wavelength dispersion
axis, the first plurality of ports being angularly offset with
respect to the second plurality of ports about an axis parallel to
the wavelength dispersion axis; a focusing element for focusing the
plurality of wavelength components; and a programmable optical
phase modulator for receiving the focused plurality of wavelength
components, the modulator being configured to steer the wavelength
components received from any one of the first plurality of ports to
a selected one of the first plurality of ports and being further
configured to steer the wavelength components received from any one
of the second plurality of ports to a selected one of the second
plurality of ports.
14. The optical device of claim 11 wherein the first plurality of
ports are interleaved with the second plurality of ports along the
port axis.
15. The optical device of claim 11 wherein the programmable optical
phase modulator includes a liquid crystal-based phase
modulator.
16. The optical device of claim 15 wherein the liquid crystal-based
phase modulator is a LCoS device.
17. The optical device of claim 13 wherein the dispersive element
is selected from the group consisting of a diffraction grating and
a prism.
18. The optical device of claim 13 further comprising an optical
system for magnifying the optical beam received from the optical
port array and directing the magnified optical beam to the
dispersion element.
19. The optical device of claim 18 wherein the optical system has a
first magnification factor in a first direction and a second
magnification factor in a second direction orthogonal to the first
direction, the first magnification factor being different from the
second magnification factor.
20. The optical device of claim 19 wherein the first direction is
parallel to a wavelength dispersion axis along which the optical
beams are spatially separated, the first magnification factor being
less than the second magnification factor.
Description
BACKGROUND
[0001] Optical networks use Wavelength Selective Switches (WSS) to
dynamically route optical wavelength signals from a source to a
destination. WSS devices often rely on wavelength manipulation
elements such as liquid crystal on silicon (LCoS) devices or
micro-electromechanical (MEMS) mirror arrays to perform the
routing.
[0002] LCoS devices include a liquid crystal material sandwiched
between a transparent glass layer having a transparent electrode,
and a silicon substrate divided into a two-dimensional array of
individually addressable pixels. Each pixel is individually
drivable by a voltage signal to provide a local phase change to an
optical signal, thereby providing a two-dimensional array of phase
manipulating regions. Manipulation of individual spectral
components is possible once an optical signal has been spatially
separated by a diffractive element such as a diffraction grating.
The spatial separation of spectral components is directed onto
predetermined regions of the LCoS device, which can be
independently manipulated by driving the corresponding pixels in a
predetermined manner.
SUMMARY
[0003] A method and device is provided for compensating for a
difference in optical path length traversed by first and second
optical portions of an optical beam which are in orthogonal
polarization states. In accordance with the method, the optical
beam is received at the input port and directed to a first walk-off
crystal for spatially separating the optical beam into first and
second optical portions that are in first and second polarization
states, respectively. The first and second polarization states are
orthogonal to one another. The first optical portion is walked-off
by the walk-off crystal and the second optical portion passes
therethrough without being walked-off. The polarization state of
the first and second optical portions is rotated. After rotating
the polarization state of the first and second optical portions,
the first and second optical portions are directed to a second
walk-off crystal that is oriented in an opposite direction from the
first walk-off crystal so that the second optical component is
walked-off by the walk-off crystal and the first optical component
passes therethrough without being walked-off. Thicknesses of the
first and second walk-off crystals are selected to adjust the
optical path lengths traversed by each of the first and second
optical portions.
[0004] In one particular implementation, an optical device includes
an optical port array, a first walk-off crystal, a first half-wave
plate, a second walk-off crystal and a segmented half-wave plate.
The optical port array has a first plurality of ports for receiving
optical beams and a second plurality of ports for receiving optical
beams. The first walk-off crystal spatially separates each of the
optical beams into first and second optical portions that are in
first and second polarization states, respectively. The first and
second polarization states are orthogonal to one another. The first
optical portions are walked-off by the first walk-off crystal and
the second optical portions pass therethrough without being
walked-off. The first half-wave plate rotates the polarization
state of the first and second optical portions of the optical
beams. The second walk-off crystal is oriented in an opposite
direction from the first walk-off crystal such that the second
optical portions are walked-off by the second walk-off crystal and
the first portions pass therethrough without being walked-off. The
segmented half-wave plate receives the second optical portions of
the optical beams or the first optical portions of the optical
beams.
BRIEF DESCRIPTION OF THE DRAWINGS
[0005] FIG. 1 shows a single input port to an optical arrangement
such as a WSS that employs a LCoS device.
[0006] FIG. 2 shows one example of an optical path compensator.
[0007] FIGS. 3A and 3B are top and side views respectively of one
example of a simplified optical arrangement such as a free-space
that may employ the optical path compensator shown in FIG. 2.
DETAILED DESCRIPTION
[0008] FIG. 1 shows a single input port 110 to an optical
arrangement such as a WSS that employs a LCoS device. The input
beam 115 to such a device can sometimes be highly astigmatic. In
the representation shown in FIG. 1, the beam 115 has a small waist
along the y-axis and a large waist along the z-axis. The beam 115,
which is randomly polarized, first enters a walk-off crystal 120 to
spatially separate the beam into two orthogonally polarized beams,
a walked-off beam 150 and a pass-through beam 160. In FIG. 1 one
polarization component (e.g., a vertical or v-component) is denoted
by a vertical arrow, and the other polarization component (e.g., a
horizontal or h-component) is denoted by a horizontal arrow. The
walk-off direction of walk-off crystal 120 and the directions of
rotation caused by the half-wave plate 130 will be described with
respect to polarization components of light beams propagating in
the forward or downstream direction, i.e., positive
z-direction.
[0009] In order to place both the two spatially separated beams 150
and 160 exiting the walk-off crystal 120 into the same polarization
state the walked-off beam 150 passes through a half-wave plate 130
that rotates the walked-off beam from the h-polarization state to
the v-polarization state. The v-polarized beam 160 does not pass
through the half-wave plate 130. As a result the beams 150 and 160
are both in the same polarization state.
[0010] One problem with the arrangement shown in FIG. 1 is that the
two beams 150 and 160 propagate over different optical path lengths
because of the walk-off crystal 120 and the half-wave plate 130.
This can be a problem if the incoming astigmatic beam has a small
waist. For instance, if the beam waist is about 3.5 microns its
Raleigh length or range is about 30 microns. The path length
differential experienced by the two beams is generally desired to
be less than this distance.
[0011] By way of illustration, in the example of FIG. 1 the
walked-off beam 150 may travel an effective propagation distance of
about 200 microns compared to the pass-through beam 160. Likewise,
when traversing the half-wave plate 130 the walked-of beam 150 may
travel an effective propagation distance of about -30 microns
compared to the pass-through beam 160. Thus, the difference in the
total effective propagation distance traveled by the two beams is
about 170 microns. Clearly, this distance is large in comparison to
the Rayleigh range of the beam.
[0012] One way to address this problem is to compensate for this
difference in the total effective propagation distance by using two
walk-off crystals that are oriented in opposite directions from one
another. In this case a beam in one polarization state accumulates
an effective propagation distance differential in the first
walk-off crystal but not in the second walk-off crystal while the
beam in the other polarization state accumulates an effective
propagation distance differential in the second walk-off crystal
but not the first walk-off crystal. FIG. 2 shows one example of an
optical path compensator that is operable in this manner.
[0013] As shown in FIG. 2 the walked-off beam 235 undergoes an
effective propagation distance differential of 90 microns in the
first walk-off crystal 220 relative to the pass-though beam 225. A
half-wave plate 250 rotates the polarization state of the beam 225
from the horizontal to the vertical so that it will be walked-off
by the second walk-off crystal 240. Likewise, the half-wave plate
250 rotates the polarization state of the beam 235 from the
vertical to the horizontal. As a consequence, the beam 225
undergoes an effective propagation distance differential of 120
microns in the second walk-off crystal 240 relative to the beam
235. Additionally, the beam 235 undergoes an additional effective
propagation distance differential of 30 microns relative to the
beam 225 since the beam 225 traverses the half-wave plate 230.
Accordingly, the beam 225 undergoes a total effective propagation
distance differential of 120 microns and the beam 235 also
undergoes a total effective propagation distance differential of
120 microns (i.e., 90+30 microns). As a result both beams propagate
over the same path length.
[0014] As the example of the optical path compensator shown in FIG.
2 illustrates, the thickness of the second walk-off crystal 240 is
chosen to compensate for the effective propagation distance
differential of both the first walk-off crystal 220 and the
half-wave plate 230 through which only one of the beams pass. The
thicknesses of the two walk-off crystals 220 and 240, which
determine the additional path lengths traversed by the walked-off
beams that respectively pass therethrough, are therefore different
from one another.
[0015] FIG. 2 shows a single optical input port of an optical
arrangement. The optical beams received by devices employing
multiple ports may be processed in a similar manner to ensure that
the two orthogonally polarized, spatially separated beams into
which each incoming beam is divided propagate over the same path
length. In some cases these optical arrangements incorporate the
functionality of multiple wavelength switches may share a common
set of optical elements such as lenses, dispersion elements and a
spatial light modulator. One example of such a wavelength selective
switch that employs the methods and techniques herein will be
presented below in connection with FIGS. 3A and 3B.
[0016] FIGS. 3A and 3B are top and side views respectively of one
example of a simplified optical arrangement such as a free-space
WSS 100 that may be used in conjunction with embodiments of the
present invention. Light is input and output to the WSS 100 through
optical waveguides such as optical fibers which serve as input and
output ports. A fiber collimator array 101 includes a first series
of fibers 120, which are associated with a first WSS, and a second
series of fibers 130, which are associated with a second WSS. Each
individual fiber is associated with a collimator 102, which
converts the light from each fiber to a free-space beam.
[0017] As best seen in FIG. 3B, the fibers 320.sub.1, 320.sub.2,
320.sub.3 and 320.sub.4 in the first fiber series 320 are
interleaved with the fibers 330.sub.1, 330.sub.2 and 330.sub.3 in
the second fiber series 330. Moreover, as also seen in FIG. 3B, the
fibers in the fiber series 320 are angularly offset from fibers in
the second fiber series 330. This angular offset causes the
wavelengths associated with the two different WSSs to be spatially
offset from one another on the LCoS device 21 in the y-direction
(the port axis), as shown in FIG. 2.
[0018] An optical path compensator of the type shown in FIG. 2
receives the optical beams from each fiber/collimator pair. In
FIGS. 2 and 3 like elements are denoted by like reference numerals.
Two representative beams are illustrated in FIG. 3B, a first beam
that is received by fiber 320.sub.1 (associated with the first WSS)
and a second beam that is received by adjacent fiber 330.sub.1
(associated with the second WSS). As shown, after exiting the first
walk-off crystal 220 the optical beam received by fiber 320.sub.1
is divided into two beams 323 and 325 which are in orthogonal
polarization states with respect to one another. Likewise, after
exiting the second walk-off crystal 240 the optical beam received
by fiber 330.sub.1 is divided into two beams 333 and 335 which are
also in orthogonal polarization states with respect to one
another.
[0019] The beams from the two different WSSs are angled so that
they cross in the plane of the patterned half-wave plate 260 having
half-wave plate segment 230. Accordingly, the location of the
patterned half-wave plate determines the angle at which the beams
need to be launched from the optical ports. In the example of FIG.
3B, the beam 323 from the port 320.sub.1 associated with the first
WSS and the beam 333 from the port 130.sub.1 associated with the
second WSS cross at the half-wave plate segment 230. Likewise, beam
325 from the port 3201 associated with the first WSS and the beam
335 from the port 330.sub.1 associated with the second WSS cross in
the plane of the half-wave plate segment 230. In one alternative
implementation, the beams 325 and 335 may be directed to the
half-wave plate 230 and not the beams 323 and 333.
[0020] Following the optical path compensator, a pair of telescopes
or optical beam expanders magnifies the free space light beams from
the port array 101. A first telescope or beam expander is formed
from optical elements 106 and 107 and a second telescope or beam
expander is formed from optical elements 104 and 105.
[0021] In FIGS. 3A and 3B, optical elements which affect the light
in two axes are illustrated with solid lines as bi-convex optics in
both views. On the other hand, optical elements which only affect
the light in one axis are illustrated with solid lines as
plano-convex lenses in the axis that is affected. The optical
elements which only affect light in one axis are also illustrated
by dashed lines in the axis which they do not affect. For instance,
in FIGS. 3A and 3B the optical elements 102, 108, 109 and 110 are
depicted with solid lines in both figures. On the other hand,
optical elements 106 and 107 are depicted with solid lines in FIG.
3A (since they have focusing power along the y-axis) and with
dashed lines in FIG. 3B (since they leave the beams unaffected
along the x-axis). Optical elements 104 and 105 are depicted with
solid lines in FIG. 3B (since they have focusing power along the
x-axis) and with dashed lines in FIG. 3A (since they leave the
beams unaffected in the y-axis).
[0022] Each telescope may be created with different magnification
factors for the x and y directions. For instance, the magnification
of the telescope formed from optical elements 104 and 105, which
magnifies the light in the x-direction, may be less than the
magnification of the telescope formed from optical elements 106 and
107, which magnifies the light in the y-direction.
[0023] The pair of telescopes magnifies the light beams from the
port array 101 and optically couples them to a wavelength
dispersion element 108 (e.g., a diffraction grating or prism),
which separates the free space light beams into their constituent
wavelengths or channels. The wavelength dispersion element 108 acts
to disperse light in different directions on an x-y plane according
to its wavelength. The light from the dispersion element is
directed to beam focusing optics 109.
[0024] Beam focusing optics 109 couple the wavelength components
from the wavelength dispersion element 108 to a programmable
optical phase modulator, which may be, for example, a liquid
crystal-based phase modulator such as a LCoS device 110. The
programmable optical phase modulator produces a phase shift at each
of its pixels which gives rise to a phase shift profile across its
surface. As shown in FIG. 3, the wavelength components are
dispersed along the x-axis. Accordingly, each wavelength component
of a given wavelength is focused on an array of pixels 19 extending
in the y-direction. By way of example, and not by way of
limitation, three such wavelength components having center
wavelengths denoted .lamda..sub.1, .lamda..sub.2 and .lamda..sub.3
are shown in FIG. 3A being focused on the LCoS device 110 along the
wavelength dispersion axis (x-axis).
[0025] As best seen in FIG. 3B, after reflection from the LCoS
device 110, each wavelength component can be coupled back through
the beam focusing optics 109, wavelength dispersion element 108 and
optical elements 106 and 107 to a selected fiber in the port array
101. Accordingly, appropriate manipulation of the pixels 19 in the
y-axis allows selective independent steering of each wavelength
component to a selected output fiber.
[0026] Although the subject matter has been described in language
specific to structural features and/or methodological acts, it is
to be understood that the subject matter defined in the appended
claims is not necessarily limited to the specific features or acts
described above.
* * * * *