U.S. patent application number 10/878834 was filed with the patent office on 2005-12-29 for waveguide structures and methods.
This patent application is currently assigned to Xerox Corporation. Invention is credited to German, Kristine A., Gulvin, Peter M., Kubby, Joel A., Lin, Pinyen, Liu, Xueyuan, Wang, Yao Rong.
Application Number | 20050286850 10/878834 |
Document ID | / |
Family ID | 35505847 |
Filed Date | 2005-12-29 |
United States Patent
Application |
20050286850 |
Kind Code |
A1 |
German, Kristine A. ; et
al. |
December 29, 2005 |
WAVEGUIDE STRUCTURES AND METHODS
Abstract
A waveguide structure has a base having a base height (h) above
a substrate and a rectangular waveguide having a waveguide height
(H) above the substrate and a waveguide width (W) between opposing
sides of the waveguide.
Inventors: |
German, Kristine A.;
(Webster, NY) ; Gulvin, Peter M.; (Webster,
NY) ; Kubby, Joel A.; (Rochester, NY) ; Lin,
Pinyen; (Rochester, NY) ; Liu, Xueyuan;
(Webster, NY) ; Wang, Yao Rong; (Webster,
NY) |
Correspondence
Address: |
Frederick W. Gibb, III
McGinn & Gibb, PLLC
Suite 304
2568-A Riva Road
Annapolis
MD
21401
US
|
Assignee: |
Xerox Corporation
|
Family ID: |
35505847 |
Appl. No.: |
10/878834 |
Filed: |
June 28, 2004 |
Current U.S.
Class: |
385/129 |
Current CPC
Class: |
G02B 6/105 20130101;
G02B 6/122 20130101 |
Class at
Publication: |
385/129 |
International
Class: |
G02B 006/10 |
Claims
What is claimed is:
1. A waveguide comprising: a substrate; and a waveguide structure
on said substrate, said waveguide structure comprising a base and a
rectangular waveguide, wherein said base has a base height (h)
above said substrate, wherein said rectangular waveguide has a
waveguide height (H) above said substrate and a waveguide width (W)
between opposing sides of said waveguide, wherein said waveguide
structure has the following features: H-4.ltoreq.(W-3).sup.2; (1)
H-1.gtoreq.(W-4).sup.2; (2) H.ltoreq.1.7*h+2.9; and (3)
H.gtoreq.0.87*h+1.8.H, (4) wherein * represents multiplication.
2. The waveguide according to claim 1, wherein said waveguide
structure provides a transverse electric-transverse magnetic
(TE-TM) wavelength shift within 0.2 nm.
3. The waveguide according to claim 1, wherein: said waveguide
height (H) is from about 2 um to about 7 um above said substrate;
said waveguide width (W) is from about 2 um to about 7 um between
opposing sides of said waveguide; and said base height (h) is from
about 1 um to about 3 um.
4. The waveguide according to claim 1, wherein: said waveguide
height (H) is from about 3 um to about 7 um above said substrate;
said waveguide width (W) is from about 4 um to about 7 um between
opposing sides of said waveguide; and said base height (h) is from
about1.5 um to about 2.5 um.
5. The waveguide according to claim 1, wherein: said waveguide
height (H) is about 5 um above said substrate; said waveguide width
(W) is about 6 um between opposing sides of said waveguide; and
said base height (h) is about 2 um.
6. The waveguide according to claim 1, wherein said waveguide
structure comprises silicon comprising one of single crystal
silicon and polycrystal silicon, wherein said silicon is one of
doped and undoped, and wherein said substrate comprises an
insulator comprising one of an oxide, a nitride, and a glass.
7. The waveguide according to claim 1, wherein said waveguide
structure comprises a polarization insensitive waveguide.
8. A waveguide comprising: a substrate; and a waveguide structure
on said substrate, said waveguide structure comprising a base and a
rectangular waveguide, wherein said base has a base height (h) from
about 1 um to about 3 um above said substrate, wherein said
rectangular waveguide has a waveguide height (H) from about 2 um to
about 7 um above said substrate above said substrate and a
waveguide width (W) from about 2 um to about 7 um between opposing
sides of said waveguide, wherein said waveguide structure has the
following features: H-4.ltoreq.(W-3).sup.2; (1)
H-1.gtoreq.(W-4).sup.2; (2) H.ltoreq.1.7*h+2.9; and (3)
H.gtoreq.0.87*h+1.8. (4)
9. The waveguide according to claim 8, wherein said waveguide
structure has the following features: H-4<(W-3).sup.2; (1)
H-1>(W-4).sup.2; (2) H<1.7*h+2.9; and (3) H>0.87*h+1.8.
(4)
10. The waveguide according to claim 8, wherein: said waveguide
height (H) is from about 3 um to about 7 um above said substrate;
said waveguide width (W) is from about 4 um to about 7 um between
opposing sides of said waveguide; and said base height (h) is from
about 1.5 um to about 2.5 um.
11. The waveguide according to claim 8, wherein: said waveguide
height (H) is about 5 um above said substrate; said waveguide width
(W) is about 6 um between opposing sides of said waveguide; and
said base height (h) is about 2 um.
12. The waveguide according to claim 8, wherein said waveguide
structure provides a transverse electric-transverse magnetic
(TE-TM) wavelength shift within 0.2 nm.
13. The waveguide according to claim 8, wherein said waveguide
structure comprises silicon comprising one of single crystal
silicon and polycrystal silicon, wherein said silicon is one of
doped and undoped, and wherein said substrate comprises an
insulator comprising one of an oxide, a nitride, and a glass.
14. The waveguide according to claim 8, wherein said waveguide
structure comprises a polarization insensitive waveguide.
15. A method of forming a polarization insensitive waveguide, said
method comprising: forming a substrate; and forming a waveguide
structure on said substrate, said forming of said waveguide
structure comprising forming a base to a base height (h) above said
substrate and forming a rectangular waveguide to a waveguide height
(H) above said substrate and a waveguide width (W) between opposing
sides of said waveguide, wherein said forming of said waveguide
structure forms the following features: H-4.ltoreq.(W-3).sup.2; (1)
H-1.gtoreq.(W-4).sup.2; (2) H.ltoreq.1.7*h+2.9; and (3)
H.gtoreq.0.87*h+1.8, and (4) wherein * represents
multiplication.
16. The method in claim 15, wherein said forming of said waveguide
forms the following features: H-4<(W-3).sup.2; (1)
H-1>(W-4).sup.2; (2) H<1.7*h+2.9; and (3) H>0.87*h+1.8.
(4)
16. The method according to claim 15, wherein said forming of said
waveguide structure provides a transverse electric-transverse
magnetic (TE-TM) wavelength shift within 0.2 nm.
17. The method according to claim 15, wherein said forming of said
waveguide structure forms: said waveguide height (H) to be from
about 2 um to about 7 um above said substrate; said waveguide width
(W) to be from about 2 um to about 7 um between opposing sides of
said waveguide; and said base height (h) to be from about 1 um to
about 3 um.
18. The method according to claim. 15, wherein said forming of said
waveguide structure forms: said waveguide height (H) to be from
about 3 um to about 7 um above said substrate; said waveguide width
(W) to be from about 4 um to about 7 um between opposing sides of
said waveguide; and said base height (h) to be from about 1.5 um to
about 2.5 um.
19. The method according to claim 15, wherein said forming of said
waveguide structure forms: said waveguide height (H) to be about 5
um above said substrate; said waveguide width (W) to be about 6 um
between opposing sides of said waveguide; and said base height (h)
to be about 2 um.
20. The method according to claim 15, wherein said waveguide
structure comprises silicon comprising one of single crystal
silicon and polycrystal silicon, wherein said silicon is one of
doped and undoped, and wherein said substrate comprises an
insulator comprising one of an oxide, a nitride, and a glass.
Description
BACKGROUND AND SUMMARY
[0001] Embodiments herein generally relate to waveguides that are
used to direct and control light. Light has two polarizations, and
can propagate in a media (such as a silicon waveguide) at two
different speeds for the two polarizations. For a rib/ridge
waveguide, the two polarized modes are generally referred to as the
transverse electric (TE) and transverse magnetic (TM) modes. The
two modes may see the same media differently, as the effective
refractive index for the two polarizations may differ. The
difference in the effective refractive index could be the result of
the symmetry of the media, or, in the case of silicon waveguides,
the result of different boundary conditions for the two
polarizations. The difference in the refractive index of the two
polarizations can cause various adverse effects for an optical
communication system and if this difference is not properly
compensated for, it will cause adverse effects such as polarization
mode dispersion that causes bit rate errors.
[0002] In an arrayed waveguide grating system, such as a
demultiplexer, the difference in refractive index will cause the
demultiplexer wavelengths to shift away from one another for the
two polarizations. Such a shift will increase the crosstalk between
adjacent communication channels and limit bandwidth. In particular,
the difference in the refractive index for the two polarizations
will cause a relative shift for the demultiplexed wavelengths: 1 =
0 n te - n tm n te ( 1 )
[0003] where .lambda..sub.0 is the center wavelength, and n.sub.te
and n.sub.tm are the effective index for the TE and the TM mode,
respectively.
[0004] In optical communications using wavelength division
multiplexing (WDM), each wavelength channel typically uses a narrow
band of wavelengths, and the separation between the channels is in
the order of 1 nm. With increasing use of broadband, the channel
separation could even get smaller. For .lambda..sub.0=1550 nm, and,
n.sub.te.apprxeq.3.42, as typically used, a small difference in
n.sub.te and n.sub.tm could result in significant performance
deterioration of the system through channel cross talk. Many
compensation methods have been proposed. These include the
insertion of a half-wave plate in the middle of the waveguide
array, dispersion matching with adjacent diffraction orders,
special layer structures, insertion of a waveguide section that
compensates for the polarization difference in the phase array,
adding a polarization splitter at the input of the arrayed
waveguide (AWG), and making a prism-shaped region at the star
coupler (combiner) of the demultiplexer. While all these methods
reduce or eliminate the polarization dependence, they all add
significant complexity to the AWG fabrication, and for many
proposed methods, they incur considerable insertion loss.
[0005] Aspects of embodiments include forming a waveguide structure
on a substrate, where there is formed a base to a base height (h)
above the substrate and a rectangular waveguide to a waveguide
height (H) above the substrate and a waveguide width (W) between
opposing sides of the waveguide. Reference, for example, a
waveguide structure with the following features:
H-4.ltoreq.(W-3).sup.2; (1)
H-1.gtoreq.(W-4).sup.2; (2)
H.ltoreq.1.7*h+2.9; and (3)
H.gtoreq.0.87*h+1.8, (4)
[0006] where:
[0007] H is the waveguide height above the substrate;
[0008] W is the waveguide width between opposing sides of the
waveguide;
[0009] h is the base height above the substrate; and
[0010] * represents multiplication.
[0011] Examples of the base height (h) are broadly from about 1 um
to about 3um above the substrate; more narrowly from about 1.5 um
to about. 2.5 um above the substrate; and in a specific embodiment
about 2 um above the substrate. Examples of the waveguide height
(H) are broadly from about 2 um to about 7 um above the substrate;
more narrowly from about 3 um to about 7 um above the substrate;
and in a specific embodiment about 5 um above the substrate.
Examples of the waveguide width (W) are broadly from about 2 um to
about 7 um; more narrowly from about 4 um to about 7 um; and in one
embodiment about 6 um.
[0012] Light traveling through the waveguide comprises two
polarized modes known as transverse electric (TE) and transverse
magnetic (TM) modes. Producing a zero TE-TM shift was a somewhat
hit or miss proposition that varies depending upon the specific
requirements of each design. Using a trial and error method to vary
waveguide dimensions in order to obtain near zero TE-TM shift is
costly both in terms of time and resource, as each waveguide with
different dimension h and H requires different etching depth or
wafer thickness. Conventionally, there were no explicit rules
regarding what combination of different sizes would produce a zero
TE-TM shift. Indeed, as shown by the following references, which
are incorporated herein by reference, conventional wisdom abandoned
any type of formulation and instead required the inclusion of
additional structures such as a half-wave plate in the middle of
the waveguide array (H. Takashashi, Y. Hibino, and I. Nishi, Opt.
Lett., Vol. 17, p. 499-501 (1992)), dispersion matching with
adjacent diffraction orders (M. Zirngibl, C. H. Joyner, L. W.
Stulz, T. Gaigge, and C. Dragone, Electron. Lett., Vol. 29, 201-202
(1992)), a special layer structure (H. Bissessur, F. Gaborit, B.
Martin, P. Pagnod-Rossiaux, J. L. Peyre and M. Renaud, Electron.
Lett., Vol. 30, p. 336-337 (1994)), insertion of a waveguide
section that compensates the polarization difference in the phase
array (M. Zirngibl, C. H. Joyner, and P. C. Chou, Electron. Lett.,
Vol. 31, p. 1662-1664 (1995)), adding a polarization splitter at
the input of the AWG (M. K. Smith and C. van Dam, IEEE Journal of
Selected Topics in Quantum Electronics, Vol. 5, p. 236-250 (1996)),
and/or making a prism-shaped region at the star coupler (combiner)
of the demultiplexer in order to consistently accomplish a zero
TE-TM shift (U.S. Pat. No. 5,937,113 to He et al.). Embodiments
herein go beyond simple routine experimentation and have created a
methodology that will allow virtually any design to be polarization
insensitive without requiring additional structures such as a
half-wave plate, dispersion matching, special layers, compensating
sections, a polarization splitter, prism-shaped devices, and other
devices.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 is a diagram of a cross-sectional geometry of a
rectangular rib/ridge waveguide;
[0014] FIG. 2 is a graph illustrating TE-TM wavelength shift from
Eq. (1) as a function of waveguide height and waveguide width;
[0015] FIG. 3 is a graph illustrating TE-TM-wavelength shift from
Eq. (1) as a function of waveguide height and waveguide slab
height;
[0016] FIG. 4 is a graph illustrating design domain for TE-TM
vavelength shift less than 0.2 nm; and
[0017] FIG. 5 is a graph illustrating design domain for TE-TM
wavelength shift less than 0.2 nm.
DETAILED DESCRIPTION
[0018] According to various exemplary embodiments of systems and
methods illustrated herein, a polarization insensitive design space
for waveguides and arrayed waveguide grating systems and can be
used in, for example, silicon-on-insulator (SOI) micro
electromechanical system (MEMS) technology to compensate and to
minimize bit rate errors and crosstalk. This polarization
insensitive waveguide structure is the basic building block of many
components such as an AWG and a switch. These components are the
building blocks of many optical network components such as routers
and optical add/drop multiplexers that are used by many wavelength
division multiplexing (WDM) and dense wavelength division
multiplexing (DWDM) technologies. The base geometry can be T-shaped
in cross-section, as shown in FIG. 1, and has optimum width and
height parameter ranges. By optimizing the ratio between the width
and height of the waveguide and the height of base, the TE-TM
wavelength shift is minimized to within current telecom industry
allowed drifting of 0.2 nm.
[0019] The birefringence in a SOI waveguide arises from the
different boundary condition for the TE and the TM mode. With a
proper choice of the cross section geometry, the birefringence can
be minimized or eliminated. A rectangular rib/ridge geometry is
shown in Fig, 1.
[0020] Also in various exemplary embodiments, a base 102 is formed
on a substrate 100 to a base height (h) above the substrate 100. A
rectangular waveguide 104 is then formed on, or simultaneously
with, the base 102 to have a waveguide height (H) above the
substrate 100 and a waveguide width (W) between opposing sides 106
of the waveguide. Reference, for example, a waveguide structure
having the following features:
H-4.ltoreq.(W-3).sup.2; (1)
H-1.gtoreq.(W-4).sup.2; (2)
H.ltoreq.1.7*h+2.9; and (3)
H.gtoreq.0.87*h+1.8 (4)
[0021] where:
[0022] H is the waveguide height above the substrate;
[0023] W is the waveguide width between opposing sides of the
waveguide; and
[0024] h is the base height above the substrate; and
[0025] represents multiplication.
[0026] Also reference, for example, a waveguide structure having
the following features:
H-4<(W-3).sup.2; (1)
H-1>(W-4).sup.2; (2)
H<1.7*h+2.9; and (3)
H>0.87*h+1.8 (4)
[0027] Examples of the base height (h) are broadly from about 1 um
to about 3 um above the substrate; more narrowly from about 1.5 um
to about 2.5 um above the substrate; and in a specific embodiment
about 2 um above the substrate. Examples of the waveguide height
(H) are broadly from about 2 um to about 7 um above the substrate;
more narrowly from about 3 um to about 7 um above the substrate;
and in a specific embodiment about 5 um above the substrate.
Examples of the waveguide width (W) are broadly from about 2 um to
about 7 um; more narrowly from about 4 um to about 7 um; and in one
embodiment about 6 um.
[0028] Thus, in an exemplary embodiment, a value equal to the
waveguide height (H) minus 4 is less than or equal to the waveguide
width (W) minus 3 all squared; and the waveguide height (H) minus 1
is greater or equal to than the waveguide width minus 4 all
squared. Additionally, the waveguide height (H) is less than or
equal to 1.7 times the base height (h) plus 2.9; and the waveguide
height (H) is greater than or equal to 0.87 times the base height
(h) plus 1.8.
[0029] In one example that applies the foregoing, if the waveguide
width (W) is selected to be 6 um and the base height (h) is
selected to be 2 um, the range for the waveguide height (H) can be
calculated as follows:
H.ltoreq.(6-3).sup.2+4, which results in H.ltoreq.13 um; (1)
H.gtoreq.(6-4).sup.2+1, which results in H.gtoreq.5 um; (2)
H.ltoreq.1.7*2+2.9, which results in H.ltoreq.6.3 um; and (3)
H.gtoreq.0.87*2+1.8, which results in H.gtoreq.3.54 um. (4)
[0030] Therefore, in the above example where the waveguide width
(W) is selected to be 6 um and the base height (h) is selected to
be 2 um, the range for the waveguide height (H) that produces less
than 0.2 nm TE-TM wavelength shift is between 5 um and 6.3 um.
[0031] The waveguide 104 and base 102 can comprise, for example,
silicon (single crystal silicon or polycrystal silicon, doped or
undoped) and the substrate 100 comprises an insulator (such as an
oxide, a nitride, glasses, and other similar materials), thereby
forming a silicon-on-insulator (SOI) waveguide structure. The
substrate 100, base 102, and rectangular waveguide 104 can be
formed using many different conventional processing techniques
including, for example, silicon deposition processing, oxidation,
photolithographic masking and etching processing, and others. For
example, see the manufacturing methods discussed in U.S. Pat. No.
6,690,863, the disclosure of which is fully incorporated herein by
reference. With the foregoing features, the waveguide structure
shown in FIG. 1 allows a maximum of 0.2 nm TE-TM wavelength shift.
In one embodiment, the waveguide height (H) can be from about 2 um
to about 7 um, the waveguide width (W) can be from about 2 um to
about 7 um, and the base height (h) can be from about I um to about
3 um.
[0032] FIGS. 2 and 3 illustrate how these systems and methods
provide a design space for AWGs that are made from SOI rib/ridge
waveguides which is polarization insensitive. More specifically,
FIGS. 2 and 3 are graphs illustrating TE-TM wavelength shift from
Eq. (1) as a function of waveguide height and waveguide width for
different base heights. In FIG. 2, the slab height (base height)
and waveguide height are shown using (h) and (H) adjacent each of
the different curves 200-208. In FIG. 3, all base heights (h) are
2.2 um and the waveguide heights (H) are as shown for curves
300-308. These figures illustrate that it is possible to produce a
design that compensates for birefringence. For example, in FIG. 2,
in curve 204, which represents a waveguide height (H) of 3.5 um and
a base height of 1.35 um, when the waveguide width is 3 um, the
TE-TM shift is zero. Similarly, for curve 308, which has a base
height of 2.2 um and a waveguide height (H) of 4.9 um, when the
waveguide width is 2 um, the TE-TM shift is zero.
[0033] Thus, FIGS. 2 and 3 illustrate that there are a number of
features that produce nearly zero TE-TM shift. However,
conventionally there is not believed to be a specific formulation
that would permit a designer to consistently produce a design that
resulted in the zero TE-TM shift.
[0034] FIG. 4 and 5 are graphs illustrating design domains for
TE-TM wavelengths that shift less than 0.2 nm. The squares in FIGS.
4 and 5 represent criterion that satisfy this 0.2 um maximum TE-TM
shift requirement and are consistent with the embodiments shown
herein, while the diamonds represent criteria outside this maximum
that do not comply with the embodiments herein. The compilation of
data herein provides that rectangular silicon-on-insulator
structures that comply with the features of the embodiments herein
will consistently produce an approximate zero TE-TM shift.
[0035] While the foregoing has been described in conjunction with
various exemplary embodiments, it is to be understood that many
alternatives, modifications and variations would be apparent to
those skilled in the art. Accordingly, Applicants intend to embrace
all such alternatives, modifications and variations that follow in
the spirit and scope of this invention.
* * * * *