U.S. patent application number 09/950124 was filed with the patent office on 2002-12-12 for diffractive structure for high-dispersion wdm applications.
Invention is credited to Frankel, Robert, Hoose, John, Popov, Evgeny.
Application Number | 20020186926 09/950124 |
Document ID | / |
Family ID | 27403649 |
Filed Date | 2002-12-12 |
United States Patent
Application |
20020186926 |
Kind Code |
A1 |
Hoose, John ; et
al. |
December 12, 2002 |
DIFFRACTIVE STRUCTURE FOR HIGH-DISPERSION WDM APPLICATIONS
Abstract
A grating structure useful for WDM multiplexer/demultiplexer
applications is described. The grating structure has a
substantially polarization-independent diffraction efficiency of a
predetermined useful wavelength range, for example, between 1.4 and
1.6 .mu.m. Each grating period includes a triangular section and a
flat-topped section, with the width of the flat-topped section
between 20 and 50% of the grating period. The disclosed grating
structure can be formed in any material that is substantially
transparent over the useful wavelength range and used in either a
Littrow and a Littman-Metcalf configuration.
Inventors: |
Hoose, John; (Fairport,
NY) ; Frankel, Robert; (Rochester, NY) ;
Popov, Evgeny; (Marseille Cedex, FR) |
Correspondence
Address: |
ROPES & GRAY
ONE INTERNATIONAL PLACE
BOSTON
MA
02110-2624
US
|
Family ID: |
27403649 |
Appl. No.: |
09/950124 |
Filed: |
September 10, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60286801 |
Apr 25, 2001 |
|
|
|
60300639 |
Jun 25, 2001 |
|
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|
Current U.S.
Class: |
385/37 ; 385/24;
385/36 |
Current CPC
Class: |
G02B 6/32 20130101; G02B
6/29397 20130101; G02B 6/29307 20130101; G02B 6/29373 20130101;
G02B 6/2931 20130101; G02B 6/2938 20130101 |
Class at
Publication: |
385/37 ; 385/36;
385/24 |
International
Class: |
G02B 006/34; G02B
006/293 |
Claims
We claim:
1. An optical multiplexer/demultiplexer comprising: a prism having
an input face, an output face and a grating face, a grating formed
on said grating face, a reflecting surface disposed substantially
parallel to the output face, wherein incident optical radiation
having a predetermined wavelength or wavelength range passes
through the input face and is diffracted by the grating towards the
output face, said reflecting surface retro-reflecting the
diffracted optical radiation onto the prism for an additional
diffraction by the grating, said twice diffracted light exiting the
input face of the prism at a wavelength-dependent angle relative to
a surface normal of the input face.
2. The optical multiplexer/demultiplexer of claim 1, wherein the
grating is formed integrally with the prism.
3. The optical multiplexer/demultiplexer of claim 1, wherein the
grating is formed separate from the prism and placed proximate to
the grating face of the prism.
4. The optical multiplexer/demultiplexer of claim 3, wherein the
grating is formed separate from the prism and placed in contact
with the grating face of the prism.
5. The optical multiplexer/demultiplexer of claim 1, wherein the
grating comprises a pattern alternating between a triangular-shaped
section and a flat-topped section, with a combined length of a
flat-topped section and the base of a triangular-shaped section
defining a grating period.
6. The optical multiplexer/demultiplexer of claim 1, wherein the
grating period is selected so that optical radiation entering the
input face substantially normal to the input face is diffracted in
a direction so as to impinge on the output face substantially
normal to the output face.
7. The optical multiplexer/demultiplexer of claim 1, wherein the
prism is made of a material that is substantially transparent in
the wavelength or wavelength range of the incident optical
radiation.
8. The optical multiplexer/demultiplexer of claim 7, wherein the
material is glass.
9. The optical multiplexer/demultiplexer of claim 7, wherein the
material comprises a semiconductor selected from the group
consisting of silicon, germanium and GaAs.
10. The optical multiplexer/demultiplexer of claim 1, further
comprising polarization rotation means interposed between the
reflecting surface and the output face.
11. The optical multiplexer/demultiplexer of claim 10, wherein the
polarization rotation means comprise a quarter waveplate or a
Faraday rotator.
12. The optical multiplexer/demultiplexer of claim 5, wherein the
prism is made of silicon, and the triangles have sides with a (111)
crystal orientation.
13. The optical multiplexer/demultiplexer of claim 5, wherein the
flat-topped region has a width between 20% and 50% of the grating
period.
14. The optical multiplexer/demultiplexer of claim 13, wherein the
flat-topped region has a width between 25% and 35% of the grating
period.
15. An optical multiplexer/demultiplexer device comprising at least
one first waveguide; at least one second waveguide; a prism
comprising an input face, an output face and a grating face, said
input face receiving from said at least one first waveguide optical
radiation having a predetermined wavelength or wavelength range; a
grating disposed on said grating surface and diffracting said
received optical radiation towards the output face; and a
reflective surface so that the diffracted optical radiation is
retro-reflected by said reflective surface towards the grating and
diffracted again by said grating, with the twice-diffracted optical
radiation exiting the prism through the input face and received by
the at least one second waveguide; wherein the grating comprises a
pattern alternating between a triangular-shaped section and a
flat-topped section, with a combined length of a flat-topped
section and the base of a triangular-shaped section defining a
grating period, wherein the grating period is substantially
identical to the wavelength propagating in the prism, and wherein
the flat-topped section has a width of between 20% and 50% of the
grating period.
16. The device of claim 15, further comprising polarization
rotation means disposed between the grating and the reflective
surface, the polarization rotation means causing a polarization
mode conversion between the received optical radiation and the
twice-diffracted optical radiation.
17. The device of claim 15, further comprising an optical element
which collimates the optical radiation received from said at least
one first waveguide and focuses the twice-diffracted optical
radiation exiting the prism onto the at least one second
waveguide.
18. An optical multiplexer/demultiplexer comprising: a prism having
at least an input face and a grating face, and a grating formed on
said grating face and having a grating pattern alternating between
a triangular-shaped section and a flat-topped section, with a
combined length of a flat-topped section and the base of a
triangular-shaped section defining a grating period, wherein
incident optical radiation having a predetermined wavelength or
wavelength range passes through the input face and is diffracted by
the grating so as to exit the input face at a wavelength-dependent
angle relative to a surface normal of the input face, and wherein
the flat-topped region has a width between 20% and 50% of the
grating period.
19. The optical multiplexer/demultiplexer of claim 18, wherein the
flat-topped region has a width between 25% and 35% of the grating
period.
20. A surface relief grating for diffracting optical radiation over
a predetermined wavelength range with a substantially
polarization-independent and wavelength-independent diffraction
efficiency, comprising: a grating pattern alternating between a
triangular-shaped section and a flat-topped section, with a
combined length of a flat-topped section and the base of an
adjacent triangular-shaped section defining a grating period,
wherein the flat-topped region has a width of between 20% and 50%
of the grating period.
Description
CROSS-REFERENCE TO OTHER PATENT APPLICATIONS
[0001] This application claims the benefit of U.S. provisional
applications No. 60/286,801, filed Apr. 25, 2001, and No.
60/300,639, filed Jun. 25, 2001 which are incorporated herein by
reference.
FIELD OF THE INVENTION
[0002] The invention is directed to an diffractive optical
structure with a high angular dispersion and low stray light, and
more particularly to a grating structure with a
polarization-independent diffraction efficiency that can be applied
in WDM applications.
BACKGROUND OF THE INVENTION
[0003] Fiber optic telecommunication systems are increasingly used
to provide high-bandwidth transmission of information to homes and
businesses. During the past decade, optical networks have become
more complex and use multiple wavelengths transmitted
simultaneously over the same fiber. This transmission method is
referred to as wavelength division multiplexing/demultiplexing
(WDM/D). The international telecommunications union (ITU) standards
body has proposed a channel allocation grid with 100 GHz channel
spacing (.about.0.81 nm at a 1550 nm wavelength) on even 100 GHz
intervals, counting nominally in both directions from a center
frequency of 193.1 THz. Newer systems are being designed to reduce
the channel spacing to 50 GHz or less. In addition, the total
wavelength range over which these devices are designed to operate
is increasing. WDM is a general term applied to the separation and
integration of information carried on these optical channels. The
technologies involved in WDM/D require efficient handling of the
optical signals propagating over fiber optic cables, and being
routed through various devices that deliver the high bandwidth
signals to the end customer.
[0004] To extract information from WDM channels, the various
optical carrier frequencies propagating, for example, in a
communication fiber, have to be separated. Wavelength-selective
optical elements, such as interference filters, fiber Bragg
gratings, arrayed waveguide gratings (AWG), and free space
gratings, e.g., surface relief diffraction gratings, are employed
for this purpose. Many of these wavelength-selective components
have a polarization-sensitive response. The free-space gratings
typically operates in a Littrow configuration. The gratings can be
ruled gratings, holographic gratings or etched gratings. Etched
gratings typically employ a crystalline, preferably a semiconductor
substrate, that exposes crystal planes with a predetermined crystal
orientation upon etching. For example, the (100) crystal planes in
Si have a preferential etch rate of 400:1 over the Si (111) planes
when etched with an aqueous solution of KOH.
[0005] It is well known that standard single-mode fiber may not
preserve the launched state of polarization (SOP) of optical
signals propagating through the fiber. Moreover, the SOP usually
varies with time at any given point along the fiber due to small
changes in the physical environment of the fiber or in the optical
wavelengths. These random polarization fluctuations can affect
transmission systems that employ polarization-sensitive optical
components, such as optical amplifiers, coherent optical receivers
or polarization-dependent photonic switches and demultiplexers.
Polarization scramblers have recently been employed in optically
amplified transoceanic communication systems, where they are used,
for example, to eliminate anisotropic gain saturation (polarization
hole burning) in the optical amplifiers by depolarizing the
launched optical signal. Accordingly, optical components used with
optical fibers should be made polarization independent, thereby
reducing costs and complexity of the fiber-optic communications
system.
[0006] It would therefore be desirable to provide compact
wavelength-dispersive devices that can separate closely-spaced
optical channels with equal efficiency regardless of the
polarization direction of the light signal and with low crosstalk
between channels.
SUMMARY OF THE INVENTION
[0007] The invention is directed to a grating structure for an
optical multiplexer/demultiplexer which provides a substantially
polarization-independent diffraction efficiency over a
predetermined useful wavelength range. The diffraction grating has
a surface relief structure with a repeating pattern consisting of
triangular sections separated by flat sections. The structure can
be used in both Littrow and Littman-Metcalf configuration.
[0008] According to one aspect of the invention, the optical
multiplexer/demultiplexer having the grating structure according to
the invention operates in "double-pass" geometry (Littman-Metcalf
configuration) to provide a greater angular dispersion and a finer
channel spacing, allowing a larger number of optical wavelength
channels to operate over a fiber optic network. In addition, with
this structure, the received signal is cleaner as a result of
reduced contribution from stray light (reduced channel cross
talk).
[0009] The optical multiplexer/demultiplexer includes a prism
having an input face, an output face and a grating face, with a
grating being formed either directly on or alternatively applied to
the grating face. A reflecting surface is disposed substantially
parallel to the output face. Incident optical radiation with a
predetermined wavelength or wavelength range passes through the
input face and is diffracted by the grating towards the output
face. The reflecting surface retro-reflects the diffracted optical
radiation back onto the prism for an additional diffraction by the
grating. The twice diffracted light exits the input face of the
prism at a wavelength-dependent angle relative to a surface normal
of the input face.
[0010] According to another aspect of the invention, optical
multiplexer/demultiplexer device for applications in fiber-optic
communications includes at least one input optical fiber or
waveguide, at least one output optical fiber or waveguide, and a
prism with an input face, an output face and a grating face.
Optical radiation with a predetermined wavelength or wavelength
range emanating from the input optical fiber or waveguide can be
collimated by an optical element, for example a lens, before
impinging on the input face of the prism. A grating is formed
either directly on or alternatively applied to the grating face. A
reflecting surface is disposed substantially parallel to the output
face. The incident optical radiation passes through the input face
and is diffracted by the grating towards the output face. The
reflecting surface retro-reflects the diffracted optical radiation
back onto the prism for an additional diffraction by the grating.
The twice diffracted light exits the input face of the prism at a
wavelength-dependent angle relative to a surface normal of the
input face.
[0011] The grating can be formed as an immersion grating in or on a
material that is substantially transparent over the wavelength
range of interest, such as glass, Si, Ge and the like. In addition,
polarization rotation means, for example a quarter waveplate or a
Faraday rotator, can be placed between the reflecting surface and
the output face. The grating can be etched into silicon, wherein
the natural etch planes of crystalline silicon, such as the (111)
planes, form the sides of the triangles of the grating relief
structure. The flat-topped region has a width between 20% and 50%,
preferable between 25% and 35% of the grating period.
[0012] Further features and advantages of the present invention
will be apparent from the following description of preferred
embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] The following figures depict certain illustrative
embodiments of the invention in which like reference numerals refer
to like elements. These depicted embodiments are to be understood
as illustrative of the invention and not as limiting in any
way.
[0014] FIG. 1 shows schematically a triangular grating structure,
with the triangular sections separated by flat sections;
[0015] FIG. 2 shown schematically a free space grating in Littrow
configuration;
[0016] FIG. 2a shown schematically an immersion grating with
integrally formed grating structure in Littrow configuration;
[0017] FIG. 3 shown schematically a free space grating in
Littman-Metcalf configuration;
[0018] FIG. 4 shows an embodiment of an immersion grating with
integrally formed grating structure in Littman-Metcalf
configuration;
[0019] FIG. 5 shows another embodiment of an immersion grating with
a separately formed grating structure in Littman-Metcalf
configuration;
[0020] FIG. 5 shows another embodiment of an immersion grating with
polarization rotation means in Littman-Metcalf configuration;
[0021] FIG. 7 shows a particular free space grating with reduced
polarization sensitivity;
[0022] FIG. 8 shows the diffraction efficiency of the grating of
FIG. 4 for S- and P-polarization directions (single pass);
[0023] FIG. 9 shows an immersion grating with reduced polarization
sensitivity formed in glass (n=1.5);
[0024] FIG. 10 shows the diffraction efficiency of the immersion
grating of FIG. 9 for S- and and P-polarization directions (double
pass; Littman-Metcalf configuration);
[0025] FIG. 11 shows the S- and P-polarization diffraction
efficiency of a silicon immersion grating as a function of the
length of the flat section; and
[0026] FIG. 12 shows the product of the S- and P-polarization
diffraction efficiencies of the grating of FIG. 11.
DETAILED DESCRIPTION OF CERTAIN ILLUSTRATED EMBODIMENTS
[0027] The invention is directed to a wavelength-dispersive device
suitable for multiplexing/demultiplexing optical carrier
frequencies in WDM applications, and more particularly to grating
structures with a substantially polarization-independent
diffraction efficiency and reduced crosstalk over the wavelength
range of interest for WDM.
[0028] Referring first to FIG. 1, a triangular grating structure 10
includes triangular sections 12 which are separated by flat
sections 15. Light 16 is incident on the grating structure 10 at an
angle of incidence .alpha. with respect to the surface normal N;
the diffracted light 18, 18' exits with respective angles of
diffraction .beta.,.beta.'. The angles .alpha. and .beta. are
related to one another by the formula:
M.lambda./nD=sin .alpha..+-.sin .beta.
[0029] wherein:
[0030] M=order of diffraction,
[0031] .lambda.=wavelength of interest,
[0032] n=index of refraction of material where light interacts with
groove structure,
[0033] D=distance between successive grooves (groove spacing),
[0034] .alpha.=angle of incidence measured relative to grating
surface normal, and
[0035] .beta.=angle of diffraction measured relative to grating
surface normal.
[0036] Dispersion takes place when the incident light, containing
all of the wavelengths of interest (sometimes called channels in
telecommunication), interacts with a diffraction grating. The
equation can be applied to both transmission gratings and
reflection gratings, with the "+" sign in the equation applying to
a reflection grating and the "-" sign to a transmission grating
when the incident and reflected beams are on the same side of the
grating surface normal. Otherwise, the signs are reversed. The
light beam incident on the grating surface is assumed to be
collimated, i.e., all rays of the incoming light beam arrive with
the same angle of incidence .alpha. regardless of wavelength.
[0037] After diffraction by the grating and for the same angle of
incidence .alpha., light with different wavelengths will leave the
grating at different angles .beta.. The angular dispersion
d.beta./d.lambda. obeys the following equation:
d.beta./d.lambda.=(sin .alpha.+sin .beta.)/.lambda. cos .beta..
[0038] According to this equation, the angular dispersion or
"spread" between wavelengths increases with the angle .alpha. of
the incident beam 16.
[0039] Gratings of the type depicted in FIG. 1 are known in the art
(Tsang et al., J. Appl. Phys., Vol. 46, No. 5, pp. 2163-2165
(1975)). Keller et al. (Applied Optics, Vol. 39, No. 7, pp.
1094-1105 (2000)) reported that the unblazed stripes 15 between the
grooves 12 reflect/diffract a portion of the light incident on the
grating that is proportional to the relative area covered by the
stripes. Graf et al. (Applied Optics, Vol. 33, No. 1, pp. 96-102
(1994)) presented experimental and calculated diffraction
efficiencies for an etched grating with flat groove tops and flat
groove bottoms. However, none of the references suggests that
having a relatively wide section 15 could have beneficial effects
and/or that variations in the width d of the flat sections 15 could
produce a substantially polarization-independent diffraction
efficiency over a wavelength range useful for WDM optical
communication.
[0040] Returning to FIG. 1, when the grating is etched in (100) Si
(i.e., the (100) crystal axis is parallel to the surface normal N),
the apex angle .theta. is fixed at approximately 70.degree. (which
is equal to the angle formed between two (111) crystal etch
planes), and the base angle .delta. is approximately 50.degree..
The base angle can be changed by cutting the Si crystal with a
different orientation, as is known in the art. The grating period
can be selected independent of the width d of the flat section
15.
[0041] FIG. 2 depicts a free space grating 26 of the type described
in FIG. 1 in Littrow configuration 20. Unlike the grating 10 of
FIG. 1, the grating 26 in FIG. 2 has relatively wide flat-bottomed
regions 27. The benefit of the flat-topped and flat-bottomed
regions which function in an essentially equivalent manner, will be
described in greater detail below. Incident light 21 is collimated
by a lens 24 and is incident on the grating 26 at an angle .alpha.
with respect to the grating surface normal N. The angle of
incidence .alpha. is adjusted for a specified center wavelength to
be approximately equal to the angle of diffraction .beta.. The
collimated diffracted light is focused by the lens 24 to form a
focused light beam 29. The amount of angular dispersion is
d.beta./d.lambda.=2*tan(.beta.)/.lambda.. With a suitable choice of
grating design parameters, a Littrow configuration can provide a
high dispersion and a substantially polarization-independent
diffraction efficiency.
[0042] As illustrated in FIG. 2a, a Littrow configuration 22 can be
implemented by forming a grating 26a as an immersion grating on a
surface of a prism 25. The prism can be made of any suitable
material that is transparent for the wavelength range of interest.
For example, BK-7 glass (n.congruent.1.5) and Si (n.congruent.3.5)
can be used for optical communication wavelengths between
.lambda.=1.4 .mu.m and .lambda.=1.6 .mu.m.
[0043] In the embodiment of FIG. 2a, the prism 25 of
multiplexer/demultiplexer 22 has an input face 201 and a grating
face indicated by the imaginary dotted line 202. An incident beam
21 which can originate, for example, from an optical communication
fiber (not shown in FIG. 2a; see, for example, FIG. 3) having an
exit face located at point O, is collimated by a lens 24 and
strikes the input face 201 of the prism 25. The light 21 is
incident on the grating 26a at an angle .alpha. with respect to the
grating surface normal N and diffracted by the grating 26a with a
wavelength-dependent diffraction angle .beta., wherein .beta. is a
function of the wavelength. The diffracted beam exits through face
201, passes again through the collimating lens 24 and is focused
onto a respective output point P whose position depends on the
wavelength. It will be understood that the actual location of the
point P relative to the input O depends on the specific design and
orientation of the prism. One or more optical fibers (not shown)
can be placed at points P, with each fiber receiving demultiplexed
optical radiation with a specific wavelength. As mentioned above,
the dispersion of the immersion grating 26a is greater than that of
the free space grating 26 by the index of refraction of the prism
25. It has been observed that the grating in Littrow configuration
has a substantially wavelength-independent and
polarization-independent diffraction efficiency if the width of the
flat-topped regions 27 (FIG. 2) and 28 (FIG. 2a), respectively, is
between 20% and 50%, preferably between 25% and 35%, of the grating
period.
[0044] Referring now to FIG. 3, in an exemplary Littman-Metcalf
configuration 30, light 31 originating at at least one point O, for
example, an exit facet of an optical fiber or optical waveguide 37,
is collimated by a lens 34 is incident on the grating 36 at an
angle .alpha. with respect to the grating surface normal N. The
incident light is diffracted with a diffraction angle .beta. and
reflected back onto the grating 36 by a mirror 38, where the
reflected light beam is diffracted a second time. The twice
diffracted beam then passes once more through the collimating lens
34 and emerges as beam 39 with a direction that depends on the
wavelength of the diffracted beam. Light beam 39 is focused at a
point P, which can likewise be an entrance facet of at least one
other optical fiber or waveguide 38. By using a plurality of
optical fibers/waveguides 37, 38, this arrangement can be used for
multiplexing/demultiplexing optical carriers in multi-wavelength
optical transmission systems, as is known in the art. Optical
fibers and waveguides, respectively, can be placed at the focal
points of all embodiments of the diffractive structures described
with reference to FIGS. 2-6. The Littman-Metcalf configuration 30,
operating in a "double-pass" geometry, can attain a higher angular
dispersion than a corresponding Littrow configuration 20 with a
smaller blaze angle.
[0045] The Littman-Metcalf configuration 30 requires precise
alignment between the orientation of the grating 36 and that of the
mirror 38.
[0046] As illustrated in FIG. 4, a Littman-Metcalf configuration 40
can be implemented by forming a grating 46 as an immersion grating
on a surface of a prism 45, which in the illustrated example has
the shape of a pentaprism. It will be understood that the prism can
also be a 3-sided prism and that the design of the prism can be
adapted to particular application, prism material and wavelength
range.
[0047] The prism can be made of any suitable material that is
transparent for the wavelength range of interest. For example, BK-7
glass (n.congruent.1.5) and Si (n.congruent.3.5) can be used for
optical communication wavelengths between .lambda.=1.4 .mu.m and
.lambda.=1.6 .mu.m.
[0048] In the embodiment of FIG. 4, the prism 45 of
multiplexer/demultiplexer 40 has an input face 401, a grating face
indicated by the imaginary dotted line 402 and an output face 403.
An incident beam 41 which can originate, for example, from an
optical communication fiber (not shown) having an exit face located
at point O, is collimated by a lens 44 and strikes the input face
401 of the prism 45. The light 41 is incident on the grating 46 at
an angle .alpha. with respect to the grating surface normal N and
diffracted by the grating 46 with a wavelength-dependent
diffraction angle .beta., .beta.', wherein .beta. and .beta.' are
different diffraction angles for different exemplary wavelengths.
The respective diffracted beams pass through output face 403 and
are retro-reflected by a mirror 48 back onto the grating 46, where
the retro-reflected light beams are diffracted a second time. The
respective twice-diffracted beams 49, 49' exit through face 401,
pass through the collimating lens 44 and are focused onto
respective output points P, P' whose position depends on the
wavelength. It will be understood that the actual location of the
points P, P' relative to the input O depends on the specific design
and orientation of the prism. Optical fibers (not shown) can be
placed at points P, P', with each fiber receiving demultiplexed
optical radiation with a specific wavelength. As mentioned above,
the dispersion of the immersion grating 46 is greater than that of
the free space grating 36 by the index of refraction of the prism
45.
[0049] The system 40 can also be used to combine (multiplex)
several wavelengths emanating from points P, P' into a common fiber
located at point O. Moreover, the mirror 48 can either be
implemented as a separate mirror or can be formed integrally (not
shown) with the prism 45. The light-transmitting surfaces of the
prism 45 and the lens 44 can be AR-coated, as is known in the
art.
[0050] As mentioned above and to be discussed below in more detail,
the diffraction efficiency of gratings typically depends on the
polarization direction of the incident light. As illustrated in the
system 50 depicted in FIG. 5, the polarization-dependent effects
can be lessened by inserting a polarization rotator 52, for example
a quarter waveplate or a Faraday rotator, in the path of the
diffracted beams between the mirror 48 and the grating 46. The beam
diffracted by the grating 46 passes through the quarter-waveplate
52, is reflected by mirror 48, passes a second time in the reverse
direction through the quarter-waveplate 52 and strikes the grating
46 again. Since the quarter-waveplate 52 (or Faraday rotator) is a
non-reciprocal device, the optical signal passing therethrough will
experience a "+45.degree." rotation during each traversal, thereby
interchanging the "S" and "P" polarization directions. Accordingly,
the diffraction efficiency of the grating for either S- or
P-polarization will be the product of the respective S- and
P-diffraction efficiencies. It will be understood that a "quarter
waveplate" includes all odd multiples of a 1/4-waveplate. Specific
examples will be given below. In all other aspects, the system 50
operates in the same manner as the system 40 of FIG. 4.
[0051] In another exemplary embodiment 60 depicted in FIG. 6,
unlike the embodiments 40 and 50 of FIGS. 4 and 5, respectively,
where the grating 46 was formed integrally with the prism face 402,
the prism face 402 of embodiment 60 is smooth, and a separate
grating structure 66 is placed in close contact with the smooth
prism surface 602. The separate grating structure 66 can be made of
another material, such as a resin or optical cement. The efficiency
of the grating in all the aforedescribed embodiments can be
improved further by coating the surface of the gratings 66, 66'
facing away from the prism surface 402 with a reflective layer,
e.g., a gold layer (not shown), as is known in the art. The
separate grating can be implemented with either with or without
using a quarter-waveplate. In all other aspects, the system 60
operates in the same manner as the system 40 of FIG. 4.
[0052] As mentioned above, providing a grating structure with a
substantially polarization-independent diffraction efficiency over
a predetermined spectral range of interest is important for WDM
optical communication applications. Although the diffraction
efficiency of etched gratings with flat-topped grooves has been
reported for S-polarized, P-polarized and unpolarized light (Graf
et al., cited above), there has been no suggestion for providing
useful grating design parameters and grating configurations that
substantially eliminate polarization-dependent diffraction
effects.
[0053] Referring now to FIG. 7, an exemplary cross-sectional
profile of particular free space grating structure 40 which
provides the desired polarization-independent high diffraction
efficiency when operated in a double-path Littman-Metcalf
configuration (see FIG. 3) in air, is composed a triangular section
72 and a flat-topped section 76. The combination of sections 72 and
76 form the grating period which repeats periodically to form, for
example, the grating 36 (see FIG. 2). The dimensions of the
optimized free space grating 70 are as follows:
1 Rising side 73 of triangle: Length (projected distance across
grating surface): 1.00 .mu.m, Height (from baseline 76): 0.84
.mu.m; Falling side 73 of triangle: Length (projected distance
across grating surface): 0.125 .mu.m, Height (from baseline 76):
0.84 .mu.m; Flat section 76: Length (across grating surface): 0.38
.mu.m; Grating period: 1.50 .mu.m (or .about.660 grooves/mm).
[0054] The corresponding groove angle (blaze angle) is 40.degree.
and the apex angle 58.5.degree. This grating can be fabricated, for
example, by conventional ruling techniques.
[0055] FIG. 8 shows the diffraction efficiency of the grating
structure of FIG. 7 in the wavelength range between 1.4 .mu.m and
1.6 .mu.m for two orthogonal polarization directions TE and TM. The
angle of incidence .alpha. is 46.degree. which is close to the
calculated blaze angle of .about.40.degree.. As seen from FIG. 8,
the diffraction efficiency is approximately 0.6-0.65 and
substantially independent of the polarization (TE/TM) over the
useful wavelength range of 1.4-1.6 .mu.m.
[0056] FIG. 9 shows an optimized grating profile when the grating
is operated as an immersion grating (illustrated in FIGS. 3-5)
instead of the free space grating depicted in FIG. 3. The
dimensions of the groove and the flat-topped sections scale
inversely with the index of refraction of the material of the prism
65, i.e., the linear dimensions are decreased by a factor 1.5 for a
BK-7 glass prism relative to those of FIG. 7.
[0057] The dimensions of the optimized free space grating 90 are as
follows:
2 Rising side 93 of triangle: Length (projected distance across
grating surface): 0.67 .mu.m, Height (from baseline 96): 0.56
.mu.m; Falling side 73 of triangle: Length (projected distance
across grating surface): 0.08 .mu.m, Height (from baseline 76):
0.56 .mu.m; Flat section 76: Length (across grating surface): 0.25
.mu.m; Grating period: 1.0 .mu.m (or .about.1,000 grooves/mm).
[0058] The groove angle and apex angles are identical to those of
FIG. 7 . As seen from FIG. 10, the diffraction efficiency is
substantially independent of the polarization directions (TE/TM)
between 1.4 .mu.m and 1.6 .mu.m.
[0059] When a grating structure is etched into silicon, the apex
angle is defined by the etch pattern of the silicon crystal, such
as the angle enclosed between two (111) crystal planes, which is
70.5.degree.. As described above, the groove angle (blaze angle)
can be varied by cutting the crystal at a suitable angle from the
(100) crystal orientation.
[0060] Referring now to FIG. 11, the diffraction efficiency of a Si
immersion grating (n.congruent.3.5) with a grating period of 0.45
.mu.m was modeled in double-path (Littman-Metcalf) configuration
(see FIGS. 4-6) for incident optical radiation in the wavelength
range between 1.4 .mu.m and 1.6 .mu.m. The grating period of 0.45
.mu.m is approximately equal to the average free space wavelength
of .lambda..sub.air=1.5 .mu.m in Si
(.lambda..sub.Si=.lambda..sub.air/n=1.5/3.5=0.43 .mu.m). The
exemplary symmetric groove structure is similar to that depicted in
FIG. 4 (which corresponds to the inverted structure of FIG. 1),
with an apex angle of .theta.=70.5.degree. and two identical groove
angles of .delta.=54.75.degree.. An angle of incidence of
.alpha.=28.96.degree. was selected, yielding an approximately
identical angle .beta. of the diffracted beam of
.beta.=28.degree..
[0061] Curve 1102 of FIG. 11 represents the mean value (averaged
over the 1.4-1.6 .mu.m wavelength range) of the diffraction
efficiency for P-polarized incident light (TE) as a function of the
width d of the flat-topped section 14. As seen in FIG. 11, the mean
diffraction efficiency for P-polarization has an approximately
constant value of 0.93-0.95 for 0<d<0.18 .mu.m. Conversely,
the diffraction efficiency for S-polarized incident light (TM) as a
function of d, depicted in curve 1104, has a relatively sharp
maximum of 0.82 for d=0.148 .mu.m and decreases precipitously
towards both smaller and greater values of d.
[0062] Curve 1106 depicts the absolute value of the difference
between the maximum and minimum diffraction efficiency for
P-polarization, abs(max(TE)-min(TE)), whereas curve 1108 depicts
the absolute value of the difference between the maximum and
minimum diffraction efficiency for S-polarization,
abs(max(TM)-min(TM)), both averaged over the 1.4-1.6 .mu.m
wavelength range. As seen from an inspection of curves 1102 and
1104, the mean diffraction efficiency for P-polarization is
identical to the mean diffraction efficiency for S-polarization to
within 15% for d=0.148 .mu.m. Equally important, as seen from an
inspection of curves 1106 and 1108, the P- and S-diffraction
efficiency is substantially independent of wavelength, varying only
by approximately 0.03 (=4%) or for S-polarization and 0.04 (=4%)
for P-polarization over the 1.4-1.6 .mu.m wavelength range. In
other words, when the diffraction efficiency for a Si immersion
grating with the above parameters is plotted as a function of
wavelength between 1.4 and 1.6 .mu.m, the diffraction efficiency
curves for S- and P-polarized light will look very similar to the
corresponding curves plotted in FIG. 10. Any residual difference
between the S- and P-polarization diffraction efficiencies of the
grating can be further compensated in a manner known in the art by
placing, for example, a tilted glass plate (not shown) in the
optical path between the lens 24, 34, 64 and the grating 26, 36,
66, respectively.
[0063] Referring now to FIG. 12, curve 1202 represents the average
value of the means of the S- and P-polarization efficiencies
(mean(P)+mean(S))/2 of curves 1102 and 1104. Curve 1204 of FIG. 12
represents the absolute difference between the means of the S- and
P-polarization efficiencies abs(mean(P)-mean(S)) of curves 1106 and
1108. Curve 1202 has a pronounced maximum for a width of the
flat-topped section of d=0.148 .mu.m, with curve 1204 exhibiting a
pronounced minimum at the same value of d. The shape of these
curves 1202 and 1204 support the conclusion reached above, that an
immersion grating can be etched in silicon with a diffraction
efficiency that is substantially constant and independent of the
polarization direction over a wavelength range useful for optical
communication. Such grating design is compact and suitable for a
Littman-Metcalf-type WDM multiplexer/demultiplexer.
[0064] It should be pointed out that the optimum performance is
achieved with a width d of the flat-topped section 15 that is
approximately 1/3 of the grating period. The width of the
flat-topped section of previously reported etched silicon gratings
did not exceed approximately 10% of the grating period. Wide
flat-topped sections were previously deemed undesirable because the
unblazed stripes tend to degrade the overall diffraction
efficiency. However, the present invention demonstrates that a
grating with a combination of triangular sections and comparatively
wide flat-topped regions can have a substantially constant and
polarization-independent diffraction effciency.
[0065] In summary, exemplary grating structures having a high and
substantially polarization-independent diffraction efficiency over
a broad wavelength range have been described. The grating
structures can be employed in WDM/D applications either in
single-pass or double-pass configuration. The structure can be made
compact if the grating is formed as an immersion grating in a
medium with a high index of refraction that is substantially
transparent to optical radiation in the desired wavelength range,
such as glass, silicon, germanium or GaAs. The increased free space
dispersion on an immersion grating in Littman-Metcalf configuration
can also decrease the channel spacing in fiber optic cables. This
can be achieved without increased cost or complexity of
polarization control devices.
[0066] While the invention has been disclosed in connection with
the preferred embodiments shown and described in detail, various
modifications and improvements thereon will become readily apparent
to those skilled in the art. Accordingly, the spirit and scope of
the present invention is to be limited only by the following
claims.
* * * * *