U.S. patent application number 16/641468 was filed with the patent office on 2020-07-02 for depolarizing homogenizer.
This patent application is currently assigned to NKT PHOTONICS A/S. The applicant listed for this patent is NKT PHOTONICS A/S. Invention is credited to Thomas FEUCHTER, Timur ISKHAKOV, Irnis KUBAT, Frederik NIELSEN, Dmitri OULIANOV.
Application Number | 20200209640 16/641468 |
Document ID | / |
Family ID | 60037286 |
Filed Date | 2020-07-02 |
![](/patent/app/20200209640/US20200209640A1-20200702-D00000.png)
![](/patent/app/20200209640/US20200209640A1-20200702-D00001.png)
![](/patent/app/20200209640/US20200209640A1-20200702-D00002.png)
![](/patent/app/20200209640/US20200209640A1-20200702-D00003.png)
![](/patent/app/20200209640/US20200209640A1-20200702-D00004.png)
![](/patent/app/20200209640/US20200209640A1-20200702-D00005.png)
![](/patent/app/20200209640/US20200209640A1-20200702-D00006.png)
United States Patent
Application |
20200209640 |
Kind Code |
A1 |
OULIANOV; Dmitri ; et
al. |
July 2, 2020 |
DEPOLARIZING HOMOGENIZER
Abstract
A depolarizing homogenizer including one or more lenslet arrays,
for providing a plurality of beamlets associated with different
respective parts of a received beam. The depolarizing homogenizer
includes a depolarizer comprising different areas which affect
polarization differently, the depolarizer being positioned to cause
alteration of the polarization characteristics of at least some of
the plurality of beamlets. A lens is arranged to at least partially
overlap the beamlets having the altered polarization
characteristics.
Inventors: |
OULIANOV; Dmitri; (Birkerod,
DK) ; NIELSEN; Frederik; (Birkerod, DK) ;
KUBAT; Irnis; (Birkerod, DK) ; FEUCHTER; Thomas;
(Birkerod, DK) ; ISKHAKOV; Timur; (Birkerod,
DK) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NKT PHOTONICS A/S |
Birkerod |
|
DK |
|
|
Assignee: |
NKT PHOTONICS A/S
Birkerod
DK
|
Family ID: |
60037286 |
Appl. No.: |
16/641468 |
Filed: |
August 24, 2018 |
PCT Filed: |
August 24, 2018 |
PCT NO: |
PCT/EP2018/072888 |
371 Date: |
February 24, 2020 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G02F 1/13362 20130101;
G02B 27/286 20130101; G02B 27/283 20130101; G02B 27/0961 20130101;
G02F 1/133526 20130101 |
International
Class: |
G02B 27/28 20060101
G02B027/28; G02B 27/09 20060101 G02B027/09; G02F 1/1335 20060101
G02F001/1335; G02F 1/13357 20060101 G02F001/13357 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 25, 2017 |
GB |
1713740.7 |
Claims
1. A depolarizing homogenizer, comprising: one or more lenslet
arrays, adapted for providing a plurality of beamlets associated
with different respective parts of a received beam; a depolarizer
comprising different areas which affect polarization differently,
wherein the depolarizer is positioned to cause alteration of the
polarization characteristics of at least some of said plurality of
beamlets; and a lens arranged to at least partially overlap the
said beamlets having said altered polarization characteristics, to
provide an output beam.
2. A depolarizing homogenizer according to claim 1, wherein the
depolarizer is positioned to receive said plurality of beamlets,
wherein at least some of said beamlets pass through respective
different areas of the depolarizer, thereby to alter the
polarization characteristics of said at least some beamlets.
3. A depolarizing homogenizer according to claim 1 or claim 2,
wherein the depolarizer is located between at least one of the one
or more lenslet arrays and the lens.
4. A depolarizing homogenizer according to claim 1 or claim 2,
wherein the lens is located between at least one of the one or more
lenslet arrays and the depolarizer.
5. A depolarizing homogenizer according to claim 1, wherein the
depolarizer is configured to depolarise light before the light
passes through the one or more lenslet arrays to form said
plurality of beamlets, thereby to cause alteration of the
polarization characteristics of at least some of said plurality of
beamlets.
6. A depolarizing homogenizer according to claim 1 or claim 5,
wherein said one or more lenslet arrays is located between the
depolarizer and the lens.
7. A depolarizing homogenizer according to any one of the preceding
claims, wherein the depolarizer comprises a liquid crystal polymer
or a quartz crystal wedge depolarizer.
8. A depolarizing homogenizer according to any one of the preceding
claims, wherein the depolarizer comprises a first depolarizing
element having a first optic axis and a second depolarizing element
have a second optic axis, wherein the first and second depolarizing
elements are oriented such that the first and second optic axes are
at an angle to one another, wherein optionally the first and second
depolarizing elements are oriented such that the first and second
optic axes are perpendicular to one another.
9. A depolarizing homogenizer according to any one of the preceding
claims, further comprising a fibre bundle, wherein the fibre bundle
includes a plurality of optical fibres for guiding light, wherein
the depolarizer is located between an output of the fibre bundle
and an image plane of the lens.
10. A depolarizing homogenizer as claimed in claim 9, further
comprising a collimating lens to receive light from the fibre
bundle and to collimate the received light, wherein the one or more
lenslet arrays are arranged to receive light which has been
collimated by the collimating lens.
11. A depolarizing homogenizer as claimed in any one of the
preceding claims, wherein the one or more lenslet arrays comprise a
plurality of lenslets shaped to cause the output beam to have a
flat intensity beam profile in an image plane of the lens.
12. A depolarizing homogenizer as claimed in any one of the
preceding claims, wherein the one or more lenslet arrays comprise a
plurality of lenslets shaped to cause the output beam to have a
square or rectangular beam profile.
13. A depolarizing homogenizer as claimed in any one of the
preceding claims, wherein the one or more lenslet arrays comprise a
first lenslet array and a second lenslet array, wherein the first
and second lenslet arrays are oriented perpendicularly with respect
to one another.
14. A depolarizing homogenizer as claimed in any preceding claim,
wherein the depolarizer comprises an electronically controlled
liquid crystal depolarizer.
15. A light source comprising a depolarizing homogenizer according
to any one of the preceding claims.
16. A light source as claimed in claim 15, wherein the light source
includes a linearly polarized or partially linearly polarized
source, wherein the beam received by the one or more lenslet arrays
of the depolarizing homogenizer is derived from said linearly
polarized or partially linearly polarized source.
17. A light source as claimed in claim 15 or claim 16, wherein the
light source includes a supercontinuum source configured to
generate a supercontinuum, wherein the beam received by the one or
more lenslet arrays of the depolarizing homogenizer is derived from
said supercontinuum source.
18. A light source as claimed in claim 17, wherein the
supercontinuum includes the wavelength range 375 nm to 1200 nm.
19. A light source as claimed in claim 17 or claim 18, wherein the
supercontinuum includes the wavelength range 375 nm to 2400 nm.
20. A light source as claimed in any one of claims 17 to 19,
wherein the supercontinuum includes the wavelength range 1100 nm to
4200 nm.
21. A light source according to any one of claims 17 to 20, wherein
the light source further comprises a wavelength selector to select
one or more wavelengths from the supercontinuum.
22. A light source according to any one of claims 15 to 21, wherein
the output beam has a polarization extinction ratio of 1 dB or
less, 0.5 dB or less, 0.1 dB or less, or 0.05 dB or less.
23. A depolarization method, comprising: dividing received light
into a plurality of beamlets; causing alteration of the
polarization characteristics of at least some of said plurality of
beamlets; providing a depolarized output beam by at least partially
overlapping the beamlets having said altered polarization
characteristics.
24. The depolarization method as claimed in claim 23, comprising
altering the polarization characteristics of at least some of said
plurality of beamlets after said plurality of beamlets have been
divided from said received light.
25. A depolarization method as claimed in claim 23, comprising
depolarizing the received light before it is divided, thereby
altering the polarization characteristics of at least some of said
plurality of beamlets.
26. A depolarization method as claimed in any one of claims 23 to
25, wherein the depolarized output beam has a flat top beam
profile.
27. A depolarization method as claimed in any one of claims 23 to
26, wherein the depolarized output beam has a square beam
profile.
28. A depolarization method as claimed in any one of claims 23 to
27, wherein the depolarized output beam has a polarization
extinction ratio of idB or less, 0.5 dB or less, 0.1 dB or less, or
0.05 dB or less.
29. A light source, comprising: a supercontinuum source for
generating a supercontinuum; a wavelength selector for selecting
light at one or more wavelengths from the supercontinuum; a fibre
bundle comprising a plurality of optical fibres arranged to guide
light selected by the wavelength selector, and to output light at
the one or more selected wavelengths; a depolarizing homogenizer
configured to receive light which has been output by the fibre
bundle, wherein the depolarizing homogenizer comprises: one or more
lenslet arrays, for providing a plurality of beamlets associated
with different respective parts of a beam received at the one or
more lenslet arrays; a depolarizer comprising different areas which
affect polarization differently, wherein the depolarizer is
positioned to cause alteration of the polarization characteristics
of at least some of said plurality of beamlets; and a lens arranged
to at least partially overlap the said beamlets having said altered
polarization characteristics, to provide an output beam.
30. A light source as claimed in claim 29, wherein the
supercontinuum includes the wavelength range 375 nm to 1200 nm.
31. A light source as claimed in claim 29 or claim 30, wherein the
supercontinuum includes the wavelength range 375 nm to 2400 nm.
32. A light source as claimed in any one of claims 29 to 31,
wherein the supercontinuum includes the wavelength range 1100 nm to
4200 nm.
Description
FIELD
[0001] This specification relates to a depolarizing homogenizer,
and to a light source comprising a depolarizing homogenizer. In
some examples the light source includes a supercontinuum
source.
BACKGROUND
[0002] Depolarizers in the form of patterned retarders or
quartz-wedge depolarizers alter the polarization characteristics of
a received beam of light to provide a depolarized output beam. Such
depolarizers convert the received beam into a pseudo-random
polarized beam, i.e. the polarization varies spatially in a cross
section of the beam perpendicular to the beam path.
[0003] This variation of the polarization characteristics across
the beam profile may not be desired in some applications which
employ depolarized beams.
SUMMARY
[0004] This specification provides a depolarizing homogenizer. The
depolarizing homogenizer includes one or more lenslet arrays,
adapted for providing a plurality of beamlets associated with
different respective parts of a received beam. The depolarizing
homogenizer also includes a depolarizer comprising different areas
which affect polarization differently. The depolarizer is
positioned to cause alteration of the polarization characteristics
of at least some of said plurality of beamlets. The depolarizing
homogenizer also includes a lens arranged to at least partially
overlap the said beamlets having said altered polarization
characteristics.
[0005] By at least partially overlapping beamlets having altered
polarization characteristics, an output beam is produced having a
high degree of depolarization. In particular, the degree of
depolarization may be substantially uniform over a cross section of
the beam perpendicular to the beam path (or a substantial region
thereof). In other words, the degree of depolarization may be
substantially uniform over a substantial region of the beam
profile.
[0006] The received beam may be coherent, incoherent or partly
coherent (temporally and/or spatially). In the case of a coherent
beam, the depolarizer may additionally help reduce interference
fringes (speckle effect), because beams from individual lenslets
having different polarizations may not interfere in the image plane
of the lens.
[0007] The depolarizer may comprise a liquid crystal polymer or a
quartz crystal wedge depolarizer. In some embodiments the
depolarizer may comprise an electronically controlled liquid
crystal depolarizer, which may be configured to change polarization
characteristics of different parts of the depolarizer in time.
[0008] The depolarizer may comprise a first depolarizing element
having a first optic axis and a second depolarizing element have a
second optic axis, wherein the first and second depolarizing
elements are oriented such that the first and second optic axes are
perpendicular to one another. In the case of a quartz crystal wedge
depolarizer comprising two quartz crystal wedges, one thicker than
the other, the optic axis may be defined by the optic axis of the
thicker wedge. In the case of a liquid crystal polymer depolarizer
comprising retardation lines, the optic axis may be defined by the
retardation lines. Thus, two liquid crystal polymer depolarizers
may be oriented such that their respective first and second optic
axes are perpendicular by orienting the two depolarizers so that
equal retardation lines of respective depolarizers are
perpendicular to one another.
[0009] The depolarizer may be positioned to receive said plurality
of beamlets, wherein at least some of said beamlets pass through
respective different areas of the depolarizer, thereby to alter the
polarization characteristics of said at least some beamlets. The
depolarizer may be located between the one or more lenslet arrays
and the lens. Alternatively, the lens may be located between the
one or more lenslet arrays and the depolarizer.
[0010] Further alternatively, the depolarizer may be configured to
depolarise light before the light passes through the one or more
lenslet arrays to form said plurality of beamlets, thereby to cause
alteration of the polarization characteristics of at least some of
said plurality of beamlets. In some example configurations the one
or more lenslet arrays may be located between the depolarizer and
the lens.
[0011] The depolarizing homogenizer may further comprise a fibre
bundle, wherein the fibre bundle includes a plurality of optical
fibres for guiding light. The depolarizer may be located between an
output of the fibre bundle and the image plane of the lens. The
depolarizing homogenizer may include a collimating lens to receive
light from the fibre bundle and to collimate the received light.
The one or more lenslet arrays may be arranged to receive light
which has been collimated by the collimating lens.
[0012] A lenslet array may comprise a plurality of lenslets
arranged in a plane which is perpendicular to the direction of the
received beam.
[0013] The lenslets may comprise microlenses, e.g. cylindrical
microlenses.
[0014] The focal lengths of the lenslets in a lenslet array may be
the same and/or one or more lenslets of one lenslet array may have
the same focal length as one or more lenslets of a second lenslet
array.
[0015] The one or more lenslet arrays may comprise a plurality of
lenslets shaped to cause the output beam to have a flat top beam
profile.
[0016] The one or more lenslet arrays may comprise a plurality of
lenslets shaped to cause the output beam to have a square or
rectangular beam profile.
[0017] The one or more lenslet arrays may comprise a first lenslet
array and a second lenslet array. The first and second lenslet
arrays may be spaced apart. The spacing may be less than the focal
length of one of more lenslets in the first and/or the second
array.
[0018] The first and second lenslet arrays may be oriented at an
angle (e.g. perpendicular) to one other. In the case of first and
second lenslets arrays having respective non-symmetric lenslets
(e.g. cylindrical microlenses), the first and second lenslet arrays
may be oriented with respect to one another so that the lenslets of
the respective arrays are oriented at an angle (e.g. perpendicular)
to one another.
[0019] The beamlets may be collimated.
[0020] The lens may be configured to form overlapping images of the
lenslet array cells in an image plane. The image plane may comprise
a work surface.
[0021] This specification also provides a light source comprising
the depolarizing homogenizer. The light source may include a
linearly polarized or partially linearly polarized source. The beam
received by the one or more lenslet arrays of the depolarizing
homogenizer may be derived from said linearly polarized or
partially linearly polarized source.
[0022] In some embodiments the light source includes a
supercontinuum source configured to generate a supercontinuum,
wherein the beam received by the one or more lenslet arrays of the
depolarizing homogenizer is derived from said supercontinuum
source. In various embodiments the supercontinuum spectrum that is
generated includes the wavelength range 375 nm to 1200 nm. In some
embodiments the supercontinuum io spectrum that is generated
includes the wavelength range 375 nm to 2400 nm. In some
embodiments the supercontinuum that is generated may comprise a
mid-infrared supercontinuum which may include the wavelength range
1100 nm to 4200 nm. The light source may further comprise a
wavelength selector to select one or more wavelengths from the
supercontinuum.
[0023] In various embodiments the output beam may have a
polarization extinction ratio of 1 dB or less, 0.5 dB or less, 0.1
dB or less, or 0.05 dB or less.
[0024] This specification also provides a depolarization method,
comprising dividing received light into a plurality of beamlets,
causing alteration of the polarization characteristics of at least
some of said plurality of beamlets, and providing a depolarized
output beam by at least partially overlapping the beamlets having
said altered polarization characteristics.
[0025] The method may comprise altering the polarization
characteristics of at least some of said plurality of beamlets
after said plurality of beamlets have been divided from said
received light. Alternatively, the method may comprise depolarizing
the received light before it is divided, thereby altering the
polarization characteristics of at least some of said plurality of
beamlets.
[0026] The specification also provides an optical arrangement
configured to carry out the method.
[0027] This specification also provides a light source. The light
source can comprise a supercontinuum source for generating a
supercontinuum, a wavelength selector for selecting light at one or
more wavelengths from the supercontinuum, and a fibre bundle
comprising a plurality of optical fibres arranged to guide light
selected by the wavelength selector, and to output light at the one
or more selected wavelengths, and a depolarizing homogenizer
configured to receive light which has been output by the fibre
bundle.
[0028] As will be understood by those skilled in the art, the term
"light" as used herein is not limited to visible light and instead
includes any suitable electromagnetic radiation such as infrared
light (including near and far infrared light), visible light and
ultraviolet light.
BRIEF DESCRIPTION OF THE FIGURES
[0029] Example embodiments will now be described with reference to
the accompanying figures, in which:
[0030] FIG. 1 is a schematic illustration of a light source
according to one example embodiment;
[0031] FIG. 2(a) is a schematic illustration showing components of
a depolarizing homogenizer in accordance with one example
embodiment;
[0032] FIG. 2(b) is a schematic illustration showing components of
a depolarizing homogenizer in accordance with an example
embodiment;
[0033] FIG. 2(c) is a schematic illustration showing components of
a depolarizing homogenizer in accordance with an example
embodiment;
[0034] FIG. 3 shows the polarization distribution of a linearly
polarized monochromatic beam after it has been passed through a
quartz crystal depolarizer;
[0035] FIG. 4 shows the result of a beam profile measurement of the
output beam of the system of FIG. 2(a) after has been passed
through a polarizer; and
[0036] FIG. 5a shows the results of polarization extinction ratio
measurements.
[0037] FIG. 5b is a close-up of a region of the graph of FIG.
5a.
DETAILED DESCRIPTION
[0038] FIG. 1 shows a light source 1 according to one example. As
shown, the light source includes a supercontinuum source 10, a
wavelength selection module 20, a coupling module 25, a fibre
bundle 30 and an optical head 40.
[0039] The supercontinuum source 10 comprises a source which
generates a broadband spectral output in the form of a
supercontinuum. Supercontinuum generation is known per se and will
not be described in detail here. Reference is directed to "Visible
continuum generation in air silica microstructure optical fibres
with anomalous dispersion at 800 nm", J. K. Ranka, R. S. Windeler,
and A. J. Stentz, Optics Letters, 2000. Vol. 25: p. 25-27. An
exemplary commercially-available supercontinuum source is the
SuperK Extreme EXU-6 source from NKT Photonics A/S. In a
supercontinuum source, the temporal coherence may be low and the
spatial coherence may be high. That is, the supercontinuum light
may be spatially coherent and temporally incoherent.
[0040] The wavelength selection module 20 is configured to select
one or more wavelengths w from the broadband output produced by the
supercontinuum source 10. In the example of FIG. 1, the wavelength
selection module is a tunable multi-wavelength filter based on an
acousto-optic filter. An exemplary commercially available tunable
multi-wavelength filter based on acousto-optic tunable filter
technology is the SuperK SELECT product from NKT Photonics A/S. The
output of the wavelength selection module may be linearly
polarized. The temporal coherence of the output may be low, e.g.
the output may be temporally incoherent. The supercontinuum
spectrum that is generated may include the range 375 nm to 1200 nm.
In some example implementations the bandwidth of the supercontinuum
may extend from 375 nm to 2400 nm. In some example implementations
the supercontinuum that is generated may comprise a mid-infrared
supercontinuum which may extend from 1100 nm to 4200 nm.
[0041] The light source 1 further comprises a coupling module 25
arranged to couple the one or more wavelengths selected by the
wavelength selection module 20 into a fibre bundle 30. The coupling
module 25 may include a homogenizing light pipe (HLP) in the form
of an elongated rod or tube capable of propagating light and of
homogenizing the intensity distribution of the propagated light.
The HLP may comprise a light-guiding region comprising a
transparent medium with cross-sectional dimensions greater than the
wavelength(s) of the guided light. The HLP may be formed by
appropriately shaping a dielectric medium (e.g. forming a polygonal
glass rod), or by providing a tubular wall with a reflective inner
surface, which defines the light-guiding region and which typically
has an appropriate rotationally asymmetric (i.e. non-circular)
cross-section that homogenizes the light irradiance by multiple
reflections off the boundary or boundaries of the HLP.
[0042] Light from the wavelength selection module 20 may be coupled
into an input end of the HLP of the coupling module, e.g. via free
space optics and/or via an optical fibre. The exit end of the HLP
may be butt coupled to the common packed input end of the fibre
bundle 30. The fibre bundle 30 comprises a plurality of optical
fibres (e.g. a plurality of multimode fibres) which guide light at
the wavelengths selected by the wavelength selector.
[0043] The fibre bundle 30 is terminated with the optical head 40.
Components of the optical head are shown in FIG. 2(a). As show, a
fiber bundle ferule 41 is located at the input of the optical head.
The fiber bundle ferule emits a divergent beam, which is received
at a collimating lens 42 which collimates the beam. The collimated
beam then passes through two lenslet arrays in the form of
cylindrical microlens arrays 43a , 43b . An imaging lens 44 (which
may also be referred to herein as a "Fourier lens" or a "focusing
lens"), is arranged to form an image in image plane 46. A
depolarizer 47 is located between the lenslet arrays and the
imaging lens 44.
[0044] The depolarizer 47 may comprise a patterned retarder in the
form of an achromatic liquid crystal polymer depolarizer.
Alternatively, or in addition, the depolarizer may comprise a
quartz crystal wedge depolarizer. Achromatic liquid crystal polymer
depolarizers and quartz crystal wedge depolarizers are known per se
and will not be described in detail here: suitable depolarizers are
commercially available from Thorlabs under product numbers DPP25-A
and DPU25-A. In general, such depolarizers comprise different areas
which affect polarization differently. Hence, if a linearly
polarized monochromatic beam having a beamwidth which spans
multiple of these areas is passed through such a depolarizer, the
polarization of different regions of the beam profile will be
altered in different ways, resulting in variation of the
polarization in a cross section perpendicular to the beam
direction; this is illustrated schematically in FIG. 3.
[0045] In contrast, in the arrangement of FIG. 2(a), the
microlenses of the cylindrical microlens arrays 43a , 43b are
configured to form a plurality of beamlets associated with
different respective parts of the received beam. In FIG. 2(a), the
depolarizer is positioned after the microlens arrays 43a , 43b so
that beamlets pass through respective different areas of the
depolarizer and are therefore subject to different polarization
retardation. The imaging lens 44 is configured to form overlapping
images of the microlens array cells in the same region of the image
plane. Thus, the beamlets, which have been subjected to different
polarization retardation, are scrambled in the image plane,
resulting in an output beam having a high degree of depolarization
uniformly distributed across the beam. In one embodiment,
contributions to depolarization are also provided by changes in
polarization characteristics resulting from propagation through the
fiber bundle, and other optical components of the light source
1.
[0046] In addition, the arrangement of the cylindrical microlens
arrays 43a , 43b and imaging lens 44 acts as an intensity
homogenizer, i.e. it homogenizes the intensity across the beam
profile. This is because the imaging lens 44 is configured to form
overlapping images of the beamlets, and therefore the intensity
profiles of individual beamlets associated with different parts of
the received beam are "averaged out" in the image plane, resulting
in an output beam with a homogenized intensity profile.
[0047] The beam profile of the output beam is determined by the
shape of the lenslets. For example, the lenslets in the lenslet
arrays may be shaped so that the image which is formed in the image
plane is of a flat top (e.g. square) homogenized beam. The
homogenizing light pipe discussed above also contributes to the
degree of homogenization of the output beam.
[0048] FIG. 4 shows the result of a beam profile measurement of the
output beam of the system of FIG. 2(a) after has been passed
through a polarizer. As shown a homogenized intensity distribution
is obtained. In contrast, significant intensity variations would be
expected if there were significant polarization change across the
beam.
[0049] In order to measure the polarization extinction ratio (PER)
of the system of FIG. 2(a), the output of the optical head 40 was
directed through a polarizer into a power meter. By rotating the
polarizer the maximum and minimum powers (Pmax and Pmin) were
measured, and the polarization extinction ratio, given by
10Log.sub.10(Pmax/Pmin) was calculated. The procedure was repeated
at different wavelengths by selecting different wavelengths using
the wavelength selection module 20. The results are shown in FIGS.
5a and 5b, which show measurement results for 3 cases: no
depolarizer, liquid crystal polymer depolarizer (DPP25-A) and
quartz crystal wedge depolarizer (DPU25-A). As shown, using a
quartz crystal wedge depolarizer, a polarization extinction ratio
of less than 0.04 dB was achieved in the range 400 nm to 700 nm
[0050] Although FIG. 2(a) shows the depolarizer located between the
microlens arrays and the imaging lens, alternatively the imaging
lens may be located between the microlens arrays and the
depolarizer. This alternative configured is illustrated in FIG.
2(c). Positioning the depolarizer immediately after the imaging
lens (rather than before the imaging lens) did not affect the
measurement results shown in FIGS. 5a and 5b.
[0051] In a further alternative embodiment, the depolarizer may be
located upstream of the microlens arrays, between the collimating
lens and the microlens arrays. In this case light passes through
the depolarizer before it passes through the one or more microlens
arrays to form said plurality of beamlets. In this configuration
the depolarizer provides the beam with a polarization which is
different in different regions of the beam profile, before the beam
is divided into beamlets by the microlens arrays. In this way, the
presence of the depolarizer causes alteration of the polarization
characteristics of the beamlets, such that different beamlets have
different polarization characteristics. The beamlets are then
brought into overlap and in the image plane by the imaging lens 44
so as to obtain a high degree of depolarization uniformly
distributed across the beam profile.
[0052] As will be understood from the foregoing, in various
embodiments a depolarizing homogenizer is provided comprising one
or more lenslet arrays (e.g. one or more microlens arrays), a
depolarizer, and a lens. The lenslet array(s) are adapted for
providing a plurality of beamlets associated with different
respective parts of a received beam. The depolarizer is positioned,
either upstream or downstream of the lenslet array(s), to cause
alteration of the polarization characteristics of at least some of
the plurality of beamlets. The lens is arranged to at least
partially overlap the beamlets having said altered polarization
characteristics, to provide an output beam. In various embodiments
the output beam has a high degree of depolarization uniformly
distributed across the profile of the beam
[0053] Although two successive lenslet arrays may be employed in
some embodiments (e.g. as shown in FIG. 2(a)), in other embodiments
a single lenslet array may be employed (e.g. as shown in FIG. 2(b).
Moreover although a single depolarizing element may be employed in
some embodiments, in some embodiments an even higher degree of
depolarization may be achieved by placing two depolarizing elements
with optical axis perpendicular to each other. The depolarizer may
thus comprise one or more liquid crystal polymers, one or more
quartz crystal wedge depolarizers, or a combination of one or more
liquid crystal polymers and one or more quartz crystal wedge
depolarizers.
[0054] Although FIG. 2(a) shows a depolarizing homogenizer used in
conjunction with a supercontinuum source having a wavelength
selection module, a coupling module and a fiber bundle, the
depolarizing homogenizer may be employed in conjunction with any
suitable supercontinuum source. For example in some embodiments one
or more, including all, of the wavelength selection module,
coupling module and fiber bundle may be omitted. For example, in
some embodiments the depolarizing homogenizer io may be used in
conjunction with a supercontinuum source which does not employ a
wavelength selector, but employs a coupling module and/or fiber
bundle.
[0055] Moreover depolarizing homogenizers according to various
embodiments may alternatively be used in conjunction with other
linearly or partly linearly polarized sources such as a narrowband
single wavelength, tunable or other broadband light sources. For
example, in various embodiments the depolarizing homogenizer may be
used in conjunction with a single wavelength laser, tunable laser,
LED, or other suitable source.
[0056] Many further modifications and variations will be evident to
those skilled in the art, that fall within the scope of the
following claims:
* * * * *