U.S. patent application number 12/432829 was filed with the patent office on 2009-11-05 for wavelength dispersing device.
Invention is credited to Sheldon MCLAUGHLIN.
Application Number | 20090273840 12/432829 |
Document ID | / |
Family ID | 41256910 |
Filed Date | 2009-11-05 |
United States Patent
Application |
20090273840 |
Kind Code |
A1 |
MCLAUGHLIN; Sheldon |
November 5, 2009 |
WAVELENGTH DISPERSING DEVICE
Abstract
A compact wavelength dispersing device and a wavelength
selective optical switch based on the wavelength dispersing device
is described. The wavelength dispersing device has a folding mirror
that folds the optical path at least three times. A focal length of
a focusing coupler of the device is reduced and the NA is
increased, while the increased optical aberrations are mitigated by
using an optional coma-compensating wedge. A double-pass
arrangement for a transmission diffraction grating allows further
focal length and overall size reduction due to increased angular
dispersion.
Inventors: |
MCLAUGHLIN; Sheldon;
(Ottawa, CA) |
Correspondence
Address: |
TEITELBAUM & MACLEAN
280 SUNNYSIDE AVENUE
OTTAWA
ON
K1S 0R8
CA
|
Family ID: |
41256910 |
Appl. No.: |
12/432829 |
Filed: |
April 30, 2009 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
61071510 |
May 2, 2008 |
|
|
|
61129136 |
Jun 6, 2008 |
|
|
|
Current U.S.
Class: |
359/569 |
Current CPC
Class: |
G01J 3/0256 20130101;
G01J 3/1804 20130101; G01J 3/0243 20130101; G02B 2005/1804
20130101; G02B 5/12 20130101; G02B 6/29313 20130101; G01J 3/0205
20130101; G02B 17/0856 20130101; G01J 3/02 20130101; G02B 6/2931
20130101; G02B 6/354 20130101; G02B 5/1814 20130101; G02B 6/29311
20130101; G02B 6/2938 20130101; G01J 3/22 20130101; G01J 3/0208
20130101; G01J 3/0291 20130101; G02B 5/1809 20130101; G02B 5/1866
20130101; G01J 3/021 20130101 |
Class at
Publication: |
359/569 |
International
Class: |
G02B 5/18 20060101
G02B005/18 |
Claims
1. A wavelength dispersing device for dispersing a light beam into
narrowband sub-beams having focal spots spaced apart along a line
of dispersion, the wavelength dispersing device comprising: an
input port for inputting the light beam; a dispersive unit
optically coupled to the input port, for dispersing the light beam
into the narrowband sub-beams; a focusing coupler having optical
power in a plane containing the line of dispersion, for receiving
the spatially separated narrowband sub-beams from the dispersive
unit and for focusing them onto the line of dispersion, so that the
focal spots of the narrowband sub-beams are disposed along the line
of dispersion; and a folding mirror disposed in optical paths
between: the input port and the dispersive unit; the dispersive
unit and the focusing coupler; and the focusing coupler and the
line of dispersion.
2. A wavelength dispersing device of claim 1, wherein the folding
mirror is disposed in an optical path between the input port and
the focusing coupler, and wherein the focusing coupler is disposed
in an optical path between the folding mirror and the dispersive
unit.
3. A wavelength dispersing device of claim 1, wherein the folding
mirror has a flat refractive surface and a flat reflective surface,
so that in operation, the spatially separated narrowband sub-beams
first impinge on the flat refractive surface, then on the flat
reflective surface, and then again on the flat refractive surface,
whereby coma of the wavelength dispersive device is lessened, so
that an average width of the focal spots along the line of
dispersion can be reduced.
4. A wavelength dispersing device of claim 3, wherein the flat
reflective surface has a dielectric reflector.
5. A wavelength dispersing device of claim 3, wherein the folding
mirror is an optical wedge having such an index of refraction that
in operation, the spatially separated narrowband sub-beams
impinging upon the flat reflective surface are reflected by total
internal reflection.
6. A wavelength dispersing device of claim 3, wherein the flat
refractive surface is tilted relative to the flat reflective
surface.
7. A wavelength dispersing device of claim 1, wherein the
dispersive unit has a transmission diffraction grating and a
retroreflector, wherein in operation, the narrowband sub-beams
impinge first on the transmission diffraction grating, then on the
retroreflector, and then back on the transmission diffraction
grating; wherein the transmission diffraction grating has parallel
grating lines, and wherein the transmission diffraction grating and
the retroreflector are disposed so that at least one of the
narrowband sub-beams is not perpendicular to the grating lines,
whereby focal spots originating from stray reflections of the
narrowband sub-beams are offset relative to the line of
dispersion.
8. A wavelength dispersing device of claim 1, wherein the
dispersive unit has a transmission diffraction grating optically
coupled to a retroreflector, so that in operation, the narrowband
sub-beams first impinge on the transmission diffraction grating,
then on the retroreflector, and then back on the transmission
diffraction grating.
9. A wavelength dispersing device of claim 1, wherein the focusing
coupler is selected from a group consisting of a concave mirror and
a lens.
10. A wavelength selective optical switch module for wavelength
selective switching of individual wavelength channels between an
input port thereof and a plurality of output ports thereof, the
wavelength selective optical switch module comprising: a wavelength
dispersing device of claim 1, wherein the input port of the
wavelength selective optical switch module is the input port of the
wavelength dispersing device, and wherein in operation, the
narrowband sub-beams carry the individual wavelength channels; and
an array of directors disposed along the line of dispersion, for
redirecting at least one of the narrowband sub-beams to propagate
back through the wavelength dispersive device towards the output
ports so as to couple into a particular of the output ports.
11. A wavelength dispersing device for dispersing a light beam into
narrowband sub-beams in a plane of dispersion and for focusing the
narrowband sub-beams into focal spots in a focal plane, the
wavelength dispersing device comprising: an input port for
inputting the light beam; a folding mirror for folding optical
paths of the light beam and of the narrowband sub-beams; a
dispersive unit for dispersing the light beam into the narrowband
sub-beams in the plane of dispersion; and a concave mirror having
optical power in the plane of dispersion, for focusing the
narrowband sub-beams into the focal spots in the focal plane;
wherein in operation, the light beam from the input port is coupled
to the folding mirror; from the folding mirror to the dispersive
unit that disperses the light beam into narrowband sub-beams that
are coupled back to the folding mirror; from the folding mirror to
the concave mirror for focusing the narrowband sub-beams; from the
concave mirror back to the folding mirror; and from the folding
mirror to the focal plane, wherein the narrowband sub-beams are
focused into the focal spots.
12. A wavelength dispersing device of claim 11, wherein on an
optical path from the folding mirror to the dispersive unit, the
light beam is first directed to the concave mirror, is collimated
thereby, and then is directed towards the dispersive unit.
13. A wavelength dispersing device of claim 11, further comprising
a coma-correcting optical wedge shaped and disposed in an optical
path of the narrowband sub-beams so as to compensate for coma of
the concave mirror, thereby reducing an average width of the focal
spots measurable along a line connecting the focal spots.
14. A wavelength dispersing device for dispersing a light beam into
narrowband sub-beams having focal spots spaced apart along a line
of dispersion, comprising: an input port for inputting the light
beam; a dispersive unit optically coupled to the input port, for
dispersing the light beam into the narrowband sub-beams; a focusing
coupler having optical power in a plane containing the line of
dispersion, for receiving the spatially separated narrowband
sub-beams from the dispersive unit and for focusing them onto the
line of dispersion, so that the focal spots of the narrowband
sub-beams are disposed along the line of dispersion; and a
coma-compensating optical element having two flat optical faces
disposed in an optical path between the focusing coupler and the
line of dispersion, for compensating coma of the focusing coupler
and for reducing an average width of the focal spots along the line
of dispersion.
15. A wavelength dispersing device of claim 14, wherein the
coma-compensating optical element is an optical wedge.
16. A wavelength dispersing device of claim 14, wherein the
coma-compensating optical element is disposed in an optical path
between the input port and the dispersive unit.
17. A wavelength dispersing device of claim 14, wherein the
focusing coupler is selected from a group consisting of a concave
mirror and a lens.
18. A wavelength dispersing device of claim 14, wherein the
dispersive unit has a transmission diffraction grating optically
coupled to a retroreflector for reflecting the narrowband sub-beams
dispersed by the transmission diffraction grating back to the
transmission diffraction grating, for additional dispersing by the
transmission diffraction grating.
19. A dispersive unit for spatially separating an optical beam into
narrowband sub-beams, wherein the narrowband sub-beams are
co-planar in a plane of dispersion, wherein in operation, stray
optical beams resulting from stray reflections of the narrowband
sub-beams in the dispersive unit form a non-zero angle with the
plane of dispersion, the dispersive unit comprising: a flat
transmission diffraction grating for receiving the optical beam and
dispersing the optical beam into the narrowband sub-beams, and a
flat retroreflector for reflecting the narrowband sub-beams
dispersed by the transmission diffraction grating back to the
transmission diffraction grating, for additional dispersing by the
transmission diffraction grating in the plane of dispersion;
wherein the transmission diffraction grating has parallel grating
lines, and wherein the transmission diffraction grating and the
retroreflector are disposed so that so that at least one of the
narrowband sub-beams is not perpendicular to the grating lines,
whereby in operation, the stray optical beams form a non-zero angle
with the plane of dispersion.
20. A dispersive unit of claim 19, wherein a plane of transmission
diffraction grating is not perpendicular to a plane of incidence of
the optical beam on the transmission diffraction grating.
21. A dispersive unit of claim 19, wherein a plane of the
retroreflector is not perpendicular to the plane of incidence of
the optical beam on the transmission diffraction grating.
22. A wavelength selective optical switch module for wavelength
selective switching of individual wavelength channels between an
input port thereof and a plurality of output ports thereof, the
wavelength selective optical switch module comprising: a dispersive
unit of claim 19, wherein the narrowband sub-beams are for carrying
the individual wavelength channels; a focusing coupler having a
focal length in the plane of dispersion, for receiving the
spatially separated narrowband sub-beams and for focusing them in a
focal plane disposed substantially one focal length away from the
focusing coupler; and an array of directors disposed in the focal
plane, for receiving the focused narrowband sub-beams and for
redirecting at least one of the narrowband sub-beams to propagate
back through the dispersive unit towards the output ports, so as to
couple the at least one of the narrowband sub-beam into a
particular of the output ports; wherein in operation, the stray
optical beams having the non-zero angle with the plane of
dispersion are focused by the focusing coupler such that they are
offset relative to the focused narrowband sub-beams in the focal
plane.
23. A wavelength selective optical switch of claim 22, further
comprising a light baffle disposed to intercept the stray optical
beams at a distance from the focal plane not exceeding 25% of the
focal length of the focusing coupler.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present invention claims priority from U.S. Provisional
Patent Application No. 61/071,510, filed May 2, 2008 and U.S.
Provisional Patent Application No. 61/129,136, filed Jun. 6, 2008,
which are incorporated herein by reference.
TECHNICAL FIELD
[0002] The present invention relates to wavelength dispersing
devices, and in particular to free-space optics based compact
wavelength dispersing devices for use in wavelength selective
optical switches and optical spectrometers.
BACKGROUND OF THE INVENTION
[0003] A wavelength dispersing device is an optical device for
spatially separating spectral components of light for subsequent
measurements and, or for further routing or switching of these
spectral components.
[0004] A wavelength dispersing device is a key component of an
apparatus for measuring a spectrum of light, or an optical
spectrometer. Optical spectrometers are used for remote sensing of
temperature, determining chemical composition and concentration of
chemical compounds, identifying substances, determining parameters
of optical channels in an optical communications network, and other
applications. A wavelength dispersing device is also one of the key
components of a wavelength selective optical switch for independent
wavelength-selective switching of individual wavelength channels in
an optical communications network. An optical channel equalizer for
dynamic equalization of optical power levels of the individual
wavelength channels and an optical gain equalizer for dynamic
equalization of optical gain levels of the individual wavelength
channels in an optical amplifier can also be based on a wavelength
dispersing device.
[0005] Despite proliferation of wavelength dispersing technologies
based on compact planar lightwave circuits or fiber Bragg gratings,
a technology based on free space optics such as a diffraction
grating or a dispersive prism still remains one of the most
frequently used and relied upon for high levels of performance and
high reliability. A diffraction grating diffracts a light beam
impinging thereon into a fan of narrowband sub-beams at individual
wavelengths. A diffraction grating, although invented about two
hundred years ago, has an advantage of a high achievable
diffraction efficiency and a low achievable wavefront distortion. A
low polarization sensitivity of a diffraction grating is also
attainable in some cases.
[0006] With regards to application of diffraction grating based
wavelength dispersive devices for optical communications networks,
a folded symmetrical 4-f free-space optical configuration taught in
U.S. Pat. No. 6,498,872 by Bouevitch et al., with an optional
field-flattening optical wedge taught in U.S. Pat. No. 6,760,501 by
Iyer et al., both assigned to JDS Uniphase Corporation and
incorporated herein by reference, allow construction of dynamic
gain equalizers for equalizing optical power values of individual
wavelength channels, wavelength blockers for completely blocking
any subset of a full set of the wavelength channels, and wavelength
selective optical switches for performing the abovementioned
wavelength channel switching function.
[0007] As an example, referring to FIG. 1, a prior-art optical
configuration of a wavelength selective optical switch (WSS) 10 is
shown. The optical elements of the WSS 10 are: a front end 11 for
launching and receiving free-space optical beams having a plurality
of wavelength channels, a concave mirror 12 for focusing and
collimating optical beams, a diffraction grating 13 for spatially
dispersing an input optical beam into the wavelength channels and
for combining the wavelength channels into an output optical beam,
a field-flattening wedge 14 for reducing spherical aberration of
the WSS 10, and an optical switching engine 15 for selectively
switching individual wavelength channels from an input optical port
16 to an output optical port 17, wherein both the input and the
output ports 16 and 17 are optically coupled to the front end 11.
The optical switching engine 15 has an array of beam directing
elements, or "directors", which can be either
micro-electro-mechanical system (MEMS) micromirrors or liquid
crystal (LC) pixels.
[0008] In operation, an input optical signal is launched into the
input optical port 16 of the front end 11 optically coupled by the
concave mirror 12 to the diffraction grating 13, which disperses an
incoming optical beam 18 into narrowband sub-beams 19 carrying
individual wavelength channels. Throughout the specification, the
term "narrowband" is understood as having a narrow wavelength range
as compared to a wavelength range of the light beam. By a way of
non-limiting example, a wavelength range, or a bandwidth of a
single wavelength channel could be 0.4 nm, whereas the wavelength
range of the light beam 18 could be 32 nm. The concave mirror 12
couples the narrowband sub-beams 19 to the optical switching engine
15, which spatially redirects the narrowband sub-beams 19. Upon
reflecting from the optical switching engine 15, the narrowband
sub-beams 19 are collimated by the mirror 12, recombined by the
dispersive element 13, and focused by the mirror 12 back into the
front end 11 coupled to the output optical port 17. Depending upon
the state of individual pixels, not shown, of the optical switching
engine 15, the individual wavelength channels may be attenuated,
switched to the output port 17, or suppressed. The footprint of the
WSS 10 of FIG. 1 for a 100 GHz channel spacing is approximately
2.times.3 inches. A detailed description of operation of the WSS 10
shown in FIG. 1 can be found in the abovementioned US Patent
documents.
[0009] Although WSS 10 has a folded optical path as explained,
which allows an optics footprint reduction, a market pressure
exists to further reduce the size of the optics of WSS devices and
the wavelength dispersing devices they are based upon. This market
pressure is caused in part by competing planar technologies and
results from a desire of optical communication system providers to
offer higher levels of functionality at the same or smaller size
and cost of their circuit packs.
[0010] One known way to reduce the overall size of the WSS 10 is to
reduce the focal length of the concave mirror 12. However, the
spacing of optical wavelength channel sub-beams along the switching
engine 15 has also to be scaled down in proportion to the focal
length of the concave mirror 12. Switching engine technologies have
limits of the minimum practical size of the individual directors;
therefore, at a given angular dispersion of the dispersive element
13, a limit exists for the minimum focal length of the concave
mirror 12. Furthermore, to maintain a given spectral resolution
expressed as a ratio of the wavelength channel spacing to a spot
width of the sub-beams 19 at the switching engine 15, the spot
width must also scale with the focal length of the concave mirror
12. This means that the numerical aperture (NA) of the sub-beams 19
in the dispersion direction must scale inversely with the focal
length of the concave mirror 12. As the beam NA becomes larger,
optical aberrations become more problematic.
[0011] Another known footprint reduction technique of a free space
optical wavelength dispersing device is to introduce folding
mirrors into an optical layout of the wavelength dispersing device.
Turning to FIG. 2, a prior-art monochromator 20 of U.S. Pat. No.
6,597,452 by Jiang et al. is presented. U.S. Pat. No. 6,597,452 is
incorporated herein by reference. The monochromator 20 is used for
selecting one monochromatic component of a polychromatic light, for
example, in a spectrometer application. The monochromator 20 has a
front end 21, a concave mirror 22, a diffraction grating 23, and a
folding mirror 24. In operation, a diverging optical beam 25
emitted by the front end 21 impinges onto the concave mirror 22
that collimates the diverging optical beam 25 into a collimated
beam 26 and directs the collimated beam 26 towards the folding
mirror 24. The folding mirror 24 directs the collimated beam 26
towards the diffraction grating 23, which reflects one
monochromatic component 27 to propagate back towards the front end
21 for outputting from the monochromator 20. The monochromator 20
is tuned by rotating the diffraction grating 23 as indicated by
arrows 28.
[0012] Yet another known way to reduce a footprint of a free space
optical wavelength dispersing device is to double-pass light
through a transmission diffraction grating (T-DG), to effectively
double the angular dispersion of light, so that the focal length of
a focusing element of the wavelength dispersing device can be
reduced. Referring now to FIG. 3, a double pass arrangement 30 for
a T-DG 31 is shown. This arrangement is taught in U.S. Pat. No.
6,765,724 by Kramer, which is incorporated herein by reference. An
incoming beam 32 is diffracted by the T-DG 31 to form narrowband
sub-beams 33A and 33B at an angle .DELTA..THETA. therebetween. The
narrowband sub-beams 33A and 33B are reflected by a mirror 34 to
propagate back towards the diffraction grating 31, which further
diffracts the sub-beams 33A and 33B to form narrowband sub-beams
35A and 35B, respectively, at an angle 2.DELTA..THETA.
therebetween. Thus, effective wavelength dispersion of the T-DG 32
doubles upon double passing the light beam 32 through the T-DG
32.
[0013] One drawback of the approach represented by FIG. 3 is that
the T-DG 31 creates multiple reflections as a result of the
diffraction occurring both in reflection and transmission
directions. As a result of these multiple reflections, multiple
stray light beams are created, resulting in deleterious optical
cross-talk.
[0014] It is an object of the present invention to provide a
wavelength dispersing device for use in optical spectrometers and
wavelength selective optical switches, which is free from the above
mentioned drawbacks. Advantageously, a wavelength dispersing device
of the present invention achieves a high degree of space
utilization and high spectral dispersion, without associated
excessive optical aberrations or stray light-induced optical
cross-talk. This enhanced optical performance at a compact size is
attained without having to rely on a large number of additional
optical elements.
SUMMARY OF THE INVENTION
[0015] In accordance with the invention there is provided a
wavelength dispersing device for dispersing a light beam into
narrowband sub-beams having focal spots spaced apart along a line
of dispersion, the wavelength dispersing device comprising:
an input port for inputting the light beam; a dispersive unit
optically coupled to the input port, for dispersing the light beam
into the narrowband sub-beams; a focusing coupler having optical
power in a plane containing the line of dispersion, for receiving
the spatially separated narrowband sub-beams from the dispersive
unit and for focusing them onto the line of dispersion, so that the
focal spots of the narrowband sub-beams are disposed along the line
of dispersion; and a folding mirror disposed in optical paths
between:
[0016] the input port and the dispersive unit;
[0017] the dispersive unit and the focusing coupler; and
[0018] the focusing coupler and the line of dispersion.
[0019] In accordance with another aspect of the invention there is
further provided a wavelength dispersing device for dispersing a
light beam into narrowband sub-beams in a plane of dispersion and
for focusing the narrowband sub-beams into focal spots in a focal
plane, the wavelength dispersing device comprising:
an input port for inputting the light beam; a folding mirror for
folding optical paths of the light beam and of the narrowband
sub-beams; a dispersive unit for dispersing the light beam into the
narrowband sub-beams in the plane of dispersion; and a concave
mirror having optical power in the plane of dispersion, for
focusing the narrowband sub-beams into the focal spots in the focal
plane; wherein in operation, the light beam from the input port is
coupled to the folding mirror; from the folding mirror to the
dispersive unit that disperses the light beam into the narrowband
sub-beams that are coupled back to the folding mirror; from the
folding mirror to the concave mirror for focusing the narrowband
sub-beams; from the concave mirror back to the folding mirror; and
from the folding mirror to the focal plane, wherein the narrowband
sub-beams are focused into the focal spots.
[0020] According to another aspect of the present invention, there
is further provided a wavelength dispersing device for dispersing a
light beam into narrowband sub-beams having focal spots spaced
apart along a line of dispersion, comprising:
an input port for inputting the light beam; a dispersive unit
optically coupled to the input port, for dispersing the light beam
into the narrowband sub-beams; a focusing coupler having optical
power in a plane containing the line of dispersion, for receiving
the spatially separated narrowband sub-beams from the dispersive
unit and for focusing them onto the line of dispersion, so that the
focal spots of the narrowband sub-beams are disposed along the line
of dispersion; and a coma-compensating optical element having two
flat optical faces disposed in an optical path between the focusing
coupler and the line of dispersion, for compensating coma of the
focusing coupler and for reducing an average width of the focal
spots along the line of dispersion.
[0021] According to another aspect of the present invention, there
is further provided a dispersive unit for spatially separating a
optical beam into narrowband sub-beams, [0022] wherein the
narrowband sub-beams are co-planar in a plane of dispersion, [0023]
wherein in operation, stray optical beams resulting from stray
reflections of the narrowband sub-beams in the dispersive unit form
a non-zero angle with the plane of dispersion, the dispersive unit
comprising: a flat transmission diffraction grating (T-DG) for
receiving the optical beam and dispersing the optical beam into the
narrowband sub-beams, and a flat retroreflector for reflecting the
narrowband sub-beams dispersed by the T-DG back to the T-DG, for
additional dispersing by the T-DG in the plane of dispersion;
wherein the T-DG has parallel grating lines, and wherein the T-DG
and the retroreflector are disposed so that so that at least one of
the narrowband sub-beams on the T-DG is not perpendicular to the
grating lines, whereby in operation, the stray optical beams form a
non-zero angle with the plane of dispersion.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] Exemplary embodiments will now be described in conjunction
with the drawings in which:
[0025] FIG. 1 is a plan view of a prior-art wavelength selective
optical switch;
[0026] FIG. 2 is a plan view of a prior-art monochromator having a
folding mirror;
[0027] FIG. 3 is a plan view of a prior-art double-pass arrangement
for a transmission diffraction grating (T-DG);
[0028] FIG. 4 is a plan view of an off-axis wavelength dispersing
device;
[0029] FIG. 5 is a ray fan for the wavelength dispersing device of
FIG. 4;
[0030] FIG. 6 is a plan view of a wavelength dispersing device of
the present invention, having a coma-compensating wedge;
[0031] FIG. 7 is a ray fan for the wavelength dispersing device of
FIG. 6;
[0032] FIG. 8 is a plan view of a wavelength dispersing device of
the present invention, having a coma-compensating wedge for
sub-beams only;
[0033] FIG. 9 is a ray fan for the wavelength dispersing device of
FIG. 8;
[0034] FIG. 10 is a plan view of a wavelength dispersing device of
the present invention, having a coma-compensating plano-parallel
plate;
[0035] FIG. 11 is a ray fan for the wavelength dispersing device of
FIG. 10;
[0036] FIG. 12 is a plan view of a wavelength selective optical
switch (WSS) of the present invention, having a coma-compensating
wedge;
[0037] FIGS. 13 to 16 are plan views of various embodiments of a
wavelength dispersing device and a WSS of the present invention,
having a single folding mirror;
[0038] FIG. 17 is an optical diagram showing stray optical beams
reflected, transmitted, and diffracted by a T-DG;
[0039] FIG. 18 is a plan view of a double-pass arrangement for a
T-DG of FIG. 17, showing stray optical beams reflected and
diffracted in the first order of diffraction by a T-DG;
[0040] FIGS. 19A, B, and C are side views of various embodiments of
a double-pass arrangement for a T-DG of FIG. 17;
[0041] FIG. 20 is a plan view of a section of a wavelength
dispersing device having a double-pass arrangement of FIG. 19B or
FIG. 19C; and
[0042] FIG. 21 is a plan view of a section of a wavelength
dispersing device having a double-pass arrangement of FIG. 19B or
FIG. 19C and a light baffle next to a focal plane.
DETAILED DESCRIPTION OF THE INVENTION
[0043] While the present teachings are described in conjunction
with various embodiments and examples, it is not intended that the
present teachings be limited to such embodiments. On the contrary,
the present teachings encompass various alternatives, modifications
and equivalents, as will be appreciated by those of skill in the
art.
[0044] According to the present invention, one approach to reducing
size of a wavelength dispersing device and a WSS based thereupon is
to scale down the entire device, that is, reduce the focal length
of a focusing coupler such as a concave mirror, while positioning
optical elements off-axis when required for more compact device
construction. Reduction of the focal length and, or off-axis
construction is accompanied by increasing NA of the light beams and
at the same time correcting for optical aberrations which, as has
been explained before, have a tendency to increase with increased
NA of the light beams. An aberration correcting approach of the
present invention is discussed below.
[0045] Referring to FIG. 4, a plan view of an off-axis wavelength
dispersing device 40 is presented. The wavelength dispersing device
40 is used for dispersing a light beam 41 into narrowband sub-beams
42 to 44 having focal spots 45 to 47 spaced apart along a line of
dispersion 48. The term "narrowband" is understood as having a
narrow wavelength range as compared to a wavelength range of the
light beam. The wavelength dispersing device 40 has an input port
49, a spherical mirror 107, a transmission diffraction grating
(T-DG) 108, and a retroreflector 50. In operation, the light beam
41 having a numerical aperture of 0.07 emerges from the input port
49, which is also called the "object". The beam 41 is coupled to
the spherical mirror 107 having focal length of 42.5 mm, where the
beam 41 is on the axis of the spherical mirror 107 in an XZ plane,
and off the axis by 11.degree. in a YZ plane, or the plane of FIG.
4. An off-axis reflection from a spherical mirror causes coma
aberration, in this case in the YZ plane. A collimated beam 41A is
coupled in sequence to the T-DG 108 having 966 lines per mm, then
to the retroreflector 50, and to the same T-DG 108 a second time.
The beam 41A is incident on the T-DG 108 at near the Littrow angle.
The combined effect of the T-DG 108 and the retroreflector 50 is to
produce an angular dispersion of the narrowband sub-beams 42 to 44
of the light beam 41. The narrowband sub-beams 42 to 44 have
central wavelengths of 1528 nm, 1546 nm, and 1568 nm, respectively.
The dispersed collimated narrowband sub-beams 42 to 44 are coupled
back to the spherical mirror 107, which now functions as a focusing
coupler. The off-axis incidence at the spherical mirror 107 again
causes coma in the YZ plane, and the sign of the coma is the same
as for the first reflection from the spherical mirror 107, so the
coma from the two reflections adds up. The now converging sub-beams
42 to 44 from the spherical mirror 107 are directed to the line of
dispersion 48. For a use of the wavelength dispersing device 40 as
a part of a wavelength selective optical switch (WSS), a plurality
of switching elements or directors, e.g. a micro-electro-mechanical
(MEMS) micromirror array or liquid crystal (LC) pixel array, should
be disposed at the line of dispersion 48. Ideally, in a WSS
application, the sub-beams 42 to 44 arrive at the switching
elements, not shown, at a normal incidence in the YZ plane. This
can be achieved by optimizing a distance from the T-DG 108 to the
spherical mirror 107 to make the beams at the line of dispersion 48
substantially parallel to one another, and by setting the tilt of
the line of dispersion 48 to be normal to the sub-beams 42 to 44.
Furthermore, it is desirable that the narrowband sub-beams 42 to 44
are all substantially in focus at the line of dispersion 48, which
can be achieved by optimizing the distances from the spherical
mirror 107 to the object, or the input port 49, and to the "image",
or the line of dispersion 48, to minimize the defocus variation
over wavelength.
[0046] Referring now to FIG. 5, ray fans for the wavelength
dispersing device 40 at the wavelength of 1546 nm are shown. The
wavelength of 1546 nm corresponds to the sub-beam 43 and the spot
46 in FIG. 4. The left plot in FIG. 5 shows the Y-direction lateral
ray position error E.sub.y in microns, as a function of the
Y-direction pupil coordinate P.sub.y normalized to .+-.1. This ray
fan shows the quadratic shape characteristic of an aberration known
in the art as "tangential coma". On the right-side plot of FIG. 5,
E.sub.y is plotted against the X-direction pupil coordinate
P.sub.x, showing an aberration known in the art as "sagittal coma".
The vertical scale in FIG. 5 is 100 .mu.m. The maximum values of
the tangential and sagittal coma are respectively about 60 .mu.m
and 20 .mu.m. These are quite large values of coma causing severe
penalties in channel shape and port-dependent coupling loss in a
WSS device based on the wavelength dispersing device 40.
[0047] Turning to FIG. 6, a plan view of an off-axis wavelength
dispersing device 60 of the present invention is shown. Similar to
the wavelength dispersing device 40 of FIG. 4, the wavelength
dispersing device 60 is used for dispersing the light beam 41 into
the narrowband sub-beams 42 to 44 having focal spots 65 to 67
spaced apart along the line of dispersion 48. One difference
between the wavelength dispersing device 60 of FIG. 6 and the
wavelength dispersing device 40 of FIG. 4 is that an optical wedge
61 is disposed in an optical path of the diverging light beam 41
between the input port 49 and the spherical mirror 107 and in an
optical path of the converging narrowband sub-beams 42 to 44
between the spherical mirror 107 and the line of dispersion 48. The
optical wedge 61 of FIG. 6 is used for compensating coma of the
spherical mirror 107. Using the wedge 61 results in the focal spots
65 to 67 of FIG. 6 being much narrower, on average, than the
corresponding focal spots 45 to 47 of FIG. 4. The optical wedge 61
is made of fused silica and has two flat faces 62 and 63 disposed
at an angle of 22.degree. to one another. During optimization of
the optical model, the distance between the line of dispersion 48
and the optical wedge 61 has been fixed, and object and image
distances have been re-optimized to minimize the defocus over
wavelength. The optical wedge 61 is preferably oriented at the
minimum deviation angle for the optical beam 41, to minimize
sensitivity of optical beams direction to orientation of the
optical wedge 61.
[0048] Referring now to FIG. 7, ray fans for the wavelength
dispersing device 60 at a wavelength of 1546 nm is shown. The
wavelength of 1546 nm corresponds to the sub-beam 43 and the spot
66 in FIG. 6. The left plot in FIG. 7 shows the Y-direction lateral
ray position error E.sub.y in microns as a function of the
Y-direction pupil coordinate P.sub.y normalized to .+-.1. On the
right-side plot of FIG. 7, E.sub.y is plotted against the
X-direction pupil coordinate P.sub.x. Note that the vertical scale
in FIG. 7 is now ten times finer, that is 10 .mu.m. The maximum
values of the Y-direction ray error E.sub.y are now less than 4
.mu.m, and primarily due to spherical aberration, which is evident
from the cubic form of E.sub.y vs. P.sub.y in the left side plot of
FIG. 7. Thus, with a wedge angle of 22.degree., the coma at 1546 nm
resulting from the two passes of optical beams through the optical
wedge 61 nearly exactly compensates the coma due to two off-axis
reflections from the spherical mirror 107. As a result, an average
width of focal spots 65 to 67 of the narrowband sub-beams 42 to 44,
respectively, measured along the line of dispersion 48 connecting
the focal spots 65 to 67, is reduced. The tilted surfaces of the
wedge 61 do introduce some astigmatism into the wavelength
dispersing device 60, but this is often not a major concern. When
the wavelength dispersing device 60 is used as a part of a
monochromator or a spectrograph, an input slit is extended in the
X-direction, and in the case of a WSS, highly elliptical beams are
preferably used, making the system relatively insensitive to the
X-direction defocus. An embodiment of a WSS based on the wavelength
dispersing device 60 will be described below, during discussion of
FIG. 12.
[0049] Turning to FIG. 8, a plan view of an off-axis wavelength
dispersing device 80 of the present invention is shown. Similar to
FIGS. 4 and 6, the wavelength dispersing device 80 is used for
dispersing the light beam 41 into the narrowband sub-beams 42 to 44
having focal spots 85 to 87 spaced apart along the line of
dispersion 48. One difference between the wavelength dispersing
device 80 of FIG. 8 and the wavelength dispersing device 60 of FIG.
6 is that an optical wedge 81 is disposed only in an optical path
of the converging narrowband sub-beams 42 to 44 between the
spherical mirror 107 and the line of dispersion 48, and the beam 41
impinges directly onto the mirror 107 in FIG. 8. The optical wedge
81 of FIG. 8 is used for compensating coma of the spherical mirror
107. Using the wedge 81 results in the focal spots 85 to 87 in FIG.
8 being narrower than the corresponding focal spots 45 to 47 of the
narrowband sub-beams 42 to 44 of FIG. 4. The optical wedge 81 is
made of fused silica and has two flat faces 82 and 83 disposed at
an angle of 48.degree. to each other. To arrive at the arrangement
of FIG. 8, the distance between the line of dispersion 48 and the
optical wedge 81 has been fixed, and object and image distances
have been re-optimized to minimize the defocus over wavelength. The
optical wedge 81 is preferably oriented at the minimum deviation
angle for the narrowband sub-beams 42 to 44, to minimize
sensitivity of direction of the sub-beams 42 to 44 to orientation
of the optical wedge 81. The wedge angle of the wedge 81 of the
wavelength dispersing device 80 of FIG. 8 is larger than the
corresponding wedge angle of the wedge 61 in the wavelength
dispersing device 60 of FIG. 6, because there is only one pass
through the wedge 81 to effect the required coma compensation.
[0050] Turning now to FIG. 9, ray fans corresponding to the
wavelength dispersing device 80 of FIG. 8 are presented. The ray
fans are at the wavelength of 1546 nm, which corresponds to the
sub-beam 43 and the spot 86 in FIG. 8. The ray fans of FIG. 9 show
that the maximum Y-direction ray error E.sub.y is reduced to less
than 3 .mu.m. The vertical scale in FIG. 9 is 10 .mu.m. The optical
wedge 81 nearly exactly compensates the coma from two off-axis
reflections from the spherical mirror 107, so that an average width
of focal spots 85 to 87 of the narrowband sub-beams 42 to 44,
respectively, measured along the line of dispersion 48, is reduced.
It will be obvious to those skilled in the art that some degree of
coma compensation could be also be achieved with an optical wedge
of this second embodiment disposed in the diverging optical beam 41
instead of the converging narrowband sub-beams 42 to 44.
[0051] Referring to FIG. 10, a plan view of an off-axis wavelength
dispersing device 100 of the present invention is shown. One
difference between the wavelength dispersing device 100 of FIG. 10
and the wavelength dispersing device 80 of FIG. 8 is that a
plano-parallel plate 99 is used instead of the optical wedge 81.
During optical modeling, the object and image distances have been
optimized to minimize defocus over wavelength, and the angle and
thickness of the plano-parallel plate 99 in this example have been
optimized to 40.degree. and 15 mm, respectively, to substantially
compensate the coma from the rest of the wavelength dispersing
device 100.
[0052] Turning now to FIG. 11, corresponding ray fans for the
wavelength dispersing device 100 at 1546 nm are presented. The
vertical scale in FIG. 11 is 10 .mu.m. The maximum value of
Y-direction ray error E.sub.y is now less than 5 .mu.m. Compared to
the embodiment of FIG. 6, this embodiment may be less convenient in
practice due to the large angles of incidence at the plano-parallel
plate 99, and the large required thickness of the plano-parallel
plate 99. It will be obvious to those skilled in the art that some
degree of coma compensation could also be achieved with a tilted
plano-parallel plate disposed in the diverging object beam 41 only,
or the converging image beams 42 to 44 only.
[0053] In this foregoing embodiments, near-Littrow spectrometer
configurations were presented using a single spherical mirror 107
as a focusing coupler having optical power in a plane containing
the line of dispersion 48. The object of a coma-correcting prism or
wedge, such as the wedges 61, 81, or the plano-parallel plate 99,
is to produce a coma of opposite sign to the net coma from the rest
of the optical system, which is frequently dominated by the coma of
the spherical mirror 107. It will be obvious to one skilled in the
art that the invention is also effective in compensating coma
produced by any focusing coupler, such as an aspherical mirror, or
a toroidal mirror, or a cylindrical mirror in place of the
spherical mirror 107. Furthermore, a transmissive lens of
spherical, aspherical, toroidal, cylindrical, gradient index, or
other form may produce coma when used off-axis, and the disclosed
invention is effecting in correcting for coma from such a lens as
well. Further, a diffraction grating or a diffractive optical
element having a curved surface may produce coma, and the disclosed
invention is effective in correcting for coma from such a
diffractive optical element. A complete spectrometer optical system
may contain one or more curved mirrors, or one or more lenses, or
one or more diffractive surfaces, which together perform an imaging
function. Since one object of the invention is to counteract the
net system coma, the invention is effective in coma compensation
for such a multi-surface imaging system. In particular, the
invention can be applied to coma present in Czerny-Turner,
Ebert-Fastie, Monk-Gillieson, Rowland circle, or Wadsworth
spectrometer configurations.
[0054] Referring now to FIG. 12, a plan view of a WSS 120 of the
present invention is shown. The WSS 120 is based on the wavelength
dispersing device 60 of FIG. 6. The WSS 120 has a front end 101
having an input port 111 and a plurality of output ports 112.sub.1
to 112.sub.N, a first turning mirror 102, a first cylindrical lens
103, a second turning mirror 104, a second cylindrical lens 105, a
coma-compensating wedge 121, the concave mirror 107, a dispersive
unit 301 having the T-DG 108 and a retroreflector 109, and an array
of directors 110. The WSS 120 functions to switch wavelength
channels between the input port 111 and any particular of the
output ports 112.sub.1 to 112.sub.N independently on each other.
The retroreflector 109 is an alternative embodiment of the
retroreflector 50 of FIGS. 4, 6, 8, and 10. The light beam 41 in
this embodiment carries the wavelength channels of the WSS 120. The
light beam 41 is launched into the input port 111, is directed by
the first and the second turning mirrors 102 and 104, respectively,
and is focused by the first and the second cylindrical lenses 103
and 105, respectively. Then, the beam 41 passes through the
coma-compensating wedge 121 and is directed by the concave mirror
107 toward the T-DG 108. The T-DG 108 disperses the beam 41 into a
fan of the narrowband sub-beams 42 to 44 carrying the individual
wavelength channels. Again, the term "narrowband" is understood as
having a narrow wavelength range as compared to a wavelength range
of the light beam. By way of a non-limiting example, a wavelength
range, or a bandwidth of a single wavelength channel is about 0.4
nm, whereas the wavelength range of the light beam 18 is about 32
nm. The narrowband sub-beams 42 to 44 dispersed by the T-DG 108 are
reflected by the retroreflector 109 back to the T-DG 108, for
additional dispersing by the T-DG 108, after which the narrowband
sub-beams 42 to 44 are directed back towards the concave mirror
107, which reflects them and focuses them on the array of directors
110 through the coma-compensating wedge 121. The function of the
coma-compensating wedge 121 is to compensate for coma of the
concave mirror 107, thereby reducing an average width of focal
spots 125 to 127 of the narrowband sub-beams 42 to 44,
respectively, on the director array 110. The width is measured
along a line 116 connecting the focal spots 125 to 127. The array
of directors 110 redirects the narrowband sub-beams 42 to 44 to
propagate back through the WSS 120 towards the output ports so as
to couple each of the narrowband sub-beams 42 to 44 into a
particular of the output ports 112.sub.1 to 112.sub.N, depending
upon an angle at which the narrowband sub-beams 42 to 44 are
redirected by the array of directors 110.
[0055] Note that the actual wavelengths of the sub-beams 42 to 44
may deviate from the previously stated values of 1528, 1546, and
1568 nm. It is to be understood that, for a WSS application, the
wavelengths of the narrowband sub-beams 42 to 44 are selected to
correspond to the International Telecommunications Union (ITU)
wavelengths grid.
[0056] Further, various obvious modifications of the optical
dispersing devices 40, 60, 80, 100, and of the WSS 120 are of
course possible. For example, the concave mirror 107 can be
replaced with any other focusing coupler that has optical power,
i.e. a capability to focus light, in a plane containing the line of
dispersion 48, or in a dispersion plane of the T-DG 108. With
regards to the latter, any other dispersive unit, such as a
single-pass or a double-pass reflection diffraction grating, may be
used. Double passing light through a diffraction grating is
preferable because it doubles the angular dispersion, which is
usable for optics size reduction as explained above.
[0057] According to another aspect of the present invention, one
can reduce the size of a wavelength dispersing device and a WSS
based thereupon by providing a single folding mirror which, due to
its location in the optical train, folds the optical path at least
three times.
[0058] Referring now to FIG. 13, a wavelength dispersing device 130
has a folding mirror 131 for folding the optical path four times in
this case, a prism 132 for correcting some aberrations, a
diffraction grating 133, as well as the previously introduced
concave mirror 107 and the input port 49 for inputting the light
beam 41. The folding mirror 131 is disposed in optical paths
between: 1) the input port 49 and the concave mirror 107; 2) the
concave mirror 107 and the diffraction grating 133; 3) in another
optical path, the diffraction grating 133 and the concave mirror
107; and 4) the concave mirror 107 and the line of dispersion 48.
Therefore, the folding mirror 131 folds the total optical path four
times.
[0059] With the array of directors 110, the wavelength dispersive
device 130 can be used as a WSS, which is more compact than the
prior-art WSS 10. Indeed, in the WSS of the present invention based
on the wavelength dispersing device 130 has the optical path folded
four times by the use of the folding mirror 131, whereby a
significant size reduction in comparison to a size of the prior-art
WSS 10 is achieved. The wavelength dispersing device 130 is similar
to the wavelength dispersing device 80 of FIG. 8, wherein coma
mostly due to off-axis reflections from the concave mirror 107 is
compensated by the coma compensating optical wedge 81. In FIG. 13,
the wedge 132 is constructed so as to compensate for the coma due
to off-axis reflections from the concave mirror 107. Thus, the
wedge 132 is not equivalent to the prior-art wedge 14 in FIG. 1,
because the wedge angle of the prior-art wedge 14 is selected so as
to compensate for on-axis spherical aberration or field curvature,
whereas the wedge angle of the wedge 132 is selected to compensate
for coma.
[0060] In operation, the light beam 41 emitted by the input port 49
is coupled to the folding mirror 131; from the folding mirror 131
to the concave mirror 107, is collimated thereby, and then is
directed, after another reflection from the folding mirror 131,
towards the diffraction grating 133 that disperses the light beam
41 into narrowband sub-beams 42 to 44. The narrowband sub-beams 42
to 44 are coupled back to the folding mirror 131; from the folding
mirror 131 to the concave mirror 107 that makes the narrowband
sub-beams 42 to 44 parallel to each other and focuses them. From
the concave mirror 107, the narrowband sub-beams 42 to 44 propagate
back to the folding mirror 131 for the fourth time; and from the
folding mirror 131 to a focal plane containing the line of
dispersion 48, wherein the narrowband sub-beams 42 to 44 are
focused into focal spots 135 to 137, respectively.
[0061] Turning to FIG. 14, a plan view of an embodiment of a
compact WSS 140 of the present invention is shown. The WSS 140 is
based on a wavelength dispersing device having a Littrow mount of a
double-passed transmissive diffraction grating 143. In the WSS 140
of FIG. 14, the light beam 41 emitted by the front end 101 is
coupled to the first lens 103 for an angle-to-offset transformation
in the XZ plane; to the first turning mirror 102; to the second
turning mirror 104; and to a negative cylindrical lens 105A that
re-images the light beam 41 in the YZ plane forming a virtual image
behind, or upstream of, the lens 105A. Then, the light beam 41
passes through a first wedge 141 that corrects for some aberrations
and impinges on the folding mirror 131, after which it impinges on
the spherical mirror 107 that collimates the light beam 41. Then,
the light beam 41 propagates back to the folding mirror 131; to a
second wedge 142; to the T-DG 143 that produces an additional to
the second wedge 142 angular dispersion of the light beam 41 into
the narrowband sub-beams 42 to 44. Then, the narrowband sub-beams
42 to 44 propagate to a retroreflector 144 that has a mirrored
surface on its back; back to the T-DG 143; back to the second wedge
142; back to the folding mirror 131; back to the spherical mirror
107, which now functions as a focusing means; back to the folding
mirror 131; and back to the first wedge 141. Then, the narrowband
sub-beams 42 to 44 propagate to the director array 110, which is
preferably a MEMS switching engine. Note that the first and the
second turning mirrors 102 and 104 each turn a path of the optical
beam 41 once, whereas the folding mirror 131 folds the entire
optical path between the front end 101 and the director array 110
four times. The narrowband sub-beams 42 to 44 redirected by the
director array 110 propagate back through the WSS 140, being
reflected from the folding mirror 131 four more times, and finally
get coupled into a particular one of output ports, not shown, of
the front end 101. The retroreflector 144 is another embodiment of
the retroreflector 109 of FIG. 12.
[0062] Referring now to FIG. 15, a plan view of an embodiment of a
compact WSS 150 of the present invention is shown. The WSS 150
operates similar to the WSS 140 of FIG. 4, with two main
differences. First, a transmission grating 153 does not have a
prism attached to it, and second, a folding mirror 151 has a flat
refractive surface 151-1 and a flat reflective surface 151-2, so
that in operation, the light beam 41 and the narrowband sub-beams
42 to 44 first impinge on the flat refractive surface 151-1, then
on the flat reflective surface 151-2, and then again on the flat
refractive surface 151-1. As a result, coma of the WSS 150 is
lessened, so that an average width of focal spots 155 to 157
measured along the line of dispersion 48 can be reduced. Coma of
the WSS 150 is dominated by coma of the concave spherical mirror
107. The mechanism of coma reduction in the WSS 150 of FIG. 15 is
the same mechanism as the one used in the wavelength dispersing
device 60 of FIG. 6 or in the WSS 120 of FIG. 12. A dielectric
reflector is preferably used for the flat reflective surface 151-2.
A total internal reflection (TIR) can also be employed. In the
latter case, the folding mirror 151 is manufactured as an optical
wedge having such an index of refraction that the optical beam 41
and the narrowband sub-beams 42 to 44 are reflected from the
surface 151-2 by TIR. The surface 151-1 is preferably
antireflection (AR) coated.
[0063] Referring now to FIG. 16, a plan view of an embodiment of a
compact WSS 160 of the present invention is shown. The WSS 160 is
similar to the WSS 130 of FIG. 13, except it does not have a
coma-correcting prism or wedge, and the optical beam 41 is directed
by the folding mirror 131 not to the concave mirror 107, and then
to the diffraction grating 133, but straight to the diffraction
grating 133. In the WSS 130 of FIG. 13, the concave mirror 107
functions to collimate the optical beam 41, whereas in the WSS 160
of FIG. 6, the function of collimation is performed by a separate
lens 105B. In operation, the light beam 41 emitted by the front end
101 is coupled, after being refracted by the lenses 102 and 105B,
to the folding mirror 131; from the folding mirror 131 to the
diffraction grating 133 that disperses the light beam 41 into
narrowband sub-beams 42 to 44. The narrowband sub-beams 42 to 44
are coupled back to the folding mirror 131, and from the folding
mirror 131 they are coupled to the concave mirror 107 that makes
the narrowband sub-beams 42 to 44 parallel to each other and
focuses them. From the concave mirror 107, the narrowband sub-beams
42 to 44 propagate back to the folding mirror 131, and from the
folding mirror 131 to a focal plane containing the line of
dispersion 48, wherein the narrowband sub-beams 42 to 44 are
focused into focal spots 165 to 167, respectively. Thus, the
folding mirror 131 is disposed in optical paths between: 1) the
front end 101 and the diffraction grating 133; 2) the diffraction
grating 133 and the concave mirror 107; and 3) the concave mirror
107 and the line of dispersion 48. The folding mirror 131 folds the
optical path between the front end 101 and the director array 110
three times. A dielectric reflector is preferably used to provide a
high reflectivity of the folding mirror 131.
[0064] According to another aspect of the present invention, one
can reduce size of a wavelength dispersing device and a WSS based
thereupon by providing a double passed diffraction grating, in
particular a double-passed T-DG. Increased wavelength dispersion
due to the double pass arrangement allows reduction of the focal
length of a concave mirror, whereby the size of the entire device
can be further reduced. As has been pointed out above, however, the
prior-art double-pass approach suffers from the drawback of
multiple stray reflections, which deteriorate optical performance
of a wavelength dispersing device.
[0065] Turning now to FIG. 17, an optical diagram showing various
optical beams reflected and diffracted by a T-DG 170 is presented.
A beam 171 is incident on the T-DG 170 in the YZ plane at an angle
of incidence .theta..sub.0 relative to a grating normal 172. It is
assumed that the T-DG 170 has parallel grating lines and only
supports the zeroth and a "-1", herein called "the first", order of
diffraction. The beam 171 is split by the T-DG 170 into a
"reflection-diffracted" beam 173, a reflected zeroth order beam
174, a transmitted zeroth order beam 175, and a desired
"transmission-diffracted" beam 176. The two zeroth order beams 174
and 175 are at an angle .theta..sub.0 to the grating normal 172,
while the two diffracted beams 173 and 176 are at an angle
.theta..sub.1 to the grating normal 172. The magnitude of the
diffraction angle .theta..sub.1 may be calculated from the
following grating equation:
sin .differential..sub.1=sin .differential..sub.0-.lamda./d (1)
[0066] wherein .lamda. is wavelength of the incoming light beam
171, and d is groove spacing of the T-DG 170.
[0067] From the viewpoint of building a spectrometer using the T-DG
170, the zeroth order beams 174 and 175, as well as the
reflection-diffracted beam 173 are stray beams that need to be
blocked. In many cases, these stray beams are widely separated from
the incident beam 171 and the desired transmission-diffracted beam
176, so they can be easily blocked. However, in a practically
important case when the angle of incidence .theta..sub.0 is near
the Littrow angle as is the case in FIG. 17, the
reflection-diffracted beam 173 returns with only a small angle to
the incident beam 171, making it difficult to block the
reflection-diffracted beam 173.
[0068] The problem of suppressing stray beams becomes more
complicated when the T-DG 170 is used in a double-pass
configuration desirous in the present invention for size-reduction
purposes as is explained above. Turning now to FIG. 18, a plan
view, in the YZ plane, of a dispersive unit 180 for spatially
separating a optical beam into narrowband sub-beams is presented.
The dispersive unit 180 has the T-DG 170 and a retroreflector 181
optically coupled to the T-DG 170. In the following discussion, the
zeroth order beams are neglected. In operation, the incoming
optical beam 171 is diffracted by the T-DG 170 in two first-order
diffracted beams: the transmission-diffracted beam 176 and the
reflection-diffracted beam 173. The transmission-diffracted beam
176 is reflected by the retroreflector 181 to impinge again on the
T-DG 170, at which point it splits again into an output
transmission-diffracted beam 177 and into a stray
reflection-diffracted beam 178, which is reflected again by the
retroreflector 181 to form a stray beam 179 that is
transmission-diffracted again by the T-DG 170 to form a stray beam
182. The output transmission-diffracted beam 177 has an angle of
diffraction of .theta..sub.b, whereas the reflected and transmitted
stray beams 173 and 182 have angles of diffraction of .theta..sub.a
and .theta..sub.c, respectively. When
.theta..sub.a.apprxeq..theta..sub.b.apprxeq..theta..sub.c, as is
the case in FIG. 18, the stray beams are difficult to intercept.
Indeed, turning to FIG. 19A, which is a side view, or XZ-plane
view, of an embodiment 190A of the double-pass arrangement 180 of
FIG. 18, all the beams coincide in the side view of FIG. 19A and
are parallel to the grating normal 172 and to a mirror normal 191
and are perpendicular to a direction of grating lines denoted at
198A.
[0069] According to the present invention, the problem of
suppressing stray optical beams is ameliorated by introducing a
tilt of either the T-DG 170, or the retroreflector 181, or both. An
equation for out-of-plane diffraction by the T-DG 170 disposed in
the XY plane can be recorded as
k.sub.my=k.sub.ly+m2.pi./d; k.sub.mx=k.sub.lx (2)
[0070] where the grating lines are parallel to the X axis, k.sub.x,
k.sub.y, and k.sub.z are the wavevector components, "i" indicates
"incidence", and m is an order of diffraction. The case when
k.sub.x is zero is most commonly used, and is known as the
"classical diffraction mount". The case when k.sub.x is non-zero is
known as the "conical diffraction mount", since the directions of
the diffracted orders are disposed on a cone. For small k.sub.x and
a limited range of angles of diffraction, one can make a
simplifying approximation that the diffracted directions still lie
in a plane that is tilted with respect to the grating lines.
[0071] Referring now to FIG. 19B, a view in the XZ-plane of an
embodiment 190B of the dispersive unit 180 of FIG. 18 is presented.
In the embodiment 190B, the T-DG 170 is tilted about the Y-axis at
an angle .gamma. with respect to the incoming beam 171. The
transmission-diffracted beams 176 and 177 will remain parallel to
the incoming beam 171, while the stray beams 173 and 182 that were
reflection-diffracted form the T-DG 170 at least once, will exit
the T-DG 170 at an angle of 2.gamma. to a plane of dispersion 199,
which is parallel to the YZ plane. Note that the incoming beam 171
is not perpendicular to a direction of grating lines 198B.
[0072] Referring now to FIG. 19C, a view in the XZ-plane of an
embodiment 190C of the dispersive unit 180 of FIG. 18 is presented.
In the embodiment 190C, the T-DG 170 is not tilted with respect to
the incoming beam 171, but the retroreflector 181 is tilted about
the Y-axis by the angle .gamma. with respect to the incoming beam
171. Since the retroreflector 181 is tilted by .gamma., the
transmission-diffracted beam 176 will be reflected at an angle of
2.gamma. as a beam 183, and upon transmission-diffracting the
second time, will exit as the output transmission-diffracted beam
177 in the plane of dispersion 199. As can be gleaned from FIG. 19C
by comparing the angles of reflection, both stray beams 173 and 182
form an angle of 2.gamma. with respect to the output beam 177 and
the corresponding plane of dispersion 199. Therefore, in both
embodiments 190B and 190C, the stray beams 173 and 182 exit at
angles of 2.gamma. with respect to the plane of dispersion 199,
whereby they can be easily suppressed. Note that the beam 183 is
not perpendicular to a direction of grating lines 198C.
[0073] Therefore, in both cases of the embodiments 190B and 190C of
FIGS. 19B and 19C, at least one of the narrowband sub-beams
impinging on the T-DG 170 is not perpendicular to the grating lines
as denoted at 198B and 198C. In case of the embodiment 190B of FIG.
19B, such a narrowband sub-beam is in the incoming beam 171, and in
case of the embodiment 190C of FIG. 19C, it is the reflected beam
183. To fulfill the condition of non-perpendicularity, a plane of
T-DG 170 should not be perpendicular to the plane of incidence of
the incoming optical beam 171 on the T-DG 170. Alternatively, a
plane of the retroreflector 181 should not be perpendicular to the
plane of incidence of the incoming optical beam 171 on the T-DG
170. Both conditions, of course, may apply as well.
[0074] The embodiments 190B and 190C of FIGS. 19B and 19C are
preferable over the embodiment 190A of FIG. 19A, because they allow
for easier stray light rejection. In the embodiments 190B and 190C,
stray optical beams resulting from stray reflections from the
diffraction grating 180 form a non-zero angle with the plane of
dispersion 199, which is parallel to the output beam 177 and the YZ
plane in FIG. 19B, and is parallel to the output beam 177 in FIG.
19C.
[0075] Stray light rejection according to the present invention
will now be illustrated. Referring to FIG. 20, a focusing part 200
of a WSS is shown wherein the output beam 177 propagating in the
plane of dispersion 199 is focused together with the stray beams
173 and 182 propagating, before the concave mirror 107, at angles
.DELTA..theta. with respect to the plane of dispersion 199, onto
the director array 110. The output beam 177 will focus into a focal
spot 201. The stray beams 173 and 182 will form stray focal spots
202 and 203 on the director array 110. The focal spots 202 and 203
are spaced apart from the spot 201 of the output beam 177 by
approximately F.DELTA..theta., wherein F is the focal length of the
concave mirror 107.
[0076] According to the present invention, light baffles can be
used to block the stray beams in a WSS or in a wavelength
dispersing device. Referring now to FIG. 21, a focusing part 210 of
a WSS is shown having a baffle 211 for blocking the stray beams 173
and 182. The baffle 211 is preferably disposed close to a focal
plane 212 of the concave mirror 107 having a focal length in the XZ
plane. Preferably, the baffle 211 is positioned at a distance from
the focal plane 212 not exceeding 25% of the focal length of the
concave mirror 107.
[0077] According to the present invention, the embodiments 190B and
190C of the double-pass dispersive unit 180 are preferably used in
the wavelength dispersing devices 40 of FIG. 4, 60 of FIG. 6, 80 of
FIG. 8, 100 of FIG. 10, and 130 of FIG. 13; as the unit 301 of the
WSS 120 of FIG. 12; and in the WSS devices 140 and 150 of FIGS. 14
and 15, respectively. Corresponding baffle arrangements, such as
the baffle 211 of FIG. 21, should preferably be used to block stray
reflections, which enables the double-pass arrangement of a
transmission diffraction grating, which, in its turn, allows for
further size reduction of a wavelength dispersing device.
[0078] It is to be understood that even though the WSS devices are
described herein as preferred embodiments of the invention, the
wavelength dispersing devices that these WSS devices are based upon
are also a part of this invention. A skilled artisan will
understand that these wavelength dispersing devices can be used for
other applications that involve dispersing a light beam into
narrowband sub-beams having focal spots spaced apart along a line
of dispersion, for example, for a spectrograph having a
monochromator and a slit, or for a spectrograph having a
"polychromator" and a detector array.
[0079] Further, the preferred embodiments are described herein as
having specific features. These features can be easily combined
and, or substituted by those skilled in the art. For example, the
above-described wavelength dispersing devices and WSS devices have
the concave spherical mirror 107 as a focusing coupler; however,
spectrometers and WSS devices can also use toroidal mirrors, which
have different radiae of curvature in X and Y-directions, or
cylindrical mirrors. Spherical, cylindrical, or even toroidal
lenses can also be used in place of mirrors. All of these focusing
couplers have optical power, or a focal length, in the plane of
dispersion.
* * * * *