U.S. patent application number 11/652974 was filed with the patent office on 2007-05-24 for photonic device and method for making same.
Invention is credited to Martin H. Kwakernaak.
Application Number | 20070116410 11/652974 |
Document ID | / |
Family ID | 34923261 |
Filed Date | 2007-05-24 |
United States Patent
Application |
20070116410 |
Kind Code |
A1 |
Kwakernaak; Martin H. |
May 24, 2007 |
Photonic device and method for making same
Abstract
A monolithic device for photonically coupling a first optical
waveguide to a second optical waveguide, including: an input being
optically coupled to the first waveguide; a first portion being
optically coupled to the input; a second portion being optically
coupled to the first portion; and, an output being optically
coupled to the second portion and the second waveguide; wherein,
when an optical signal is provided on the first waveguide, a given
part of the signal is provided to the second waveguide dependently
upon an angle between the first and second portions. At least one
of the waveguides may have an amorphous silicon material
coating.
Inventors: |
Kwakernaak; Martin H.; (New
Brunswick, NJ) |
Correspondence
Address: |
PATENT DOCKET ADMINISTRATOR;LOWENSTEIN SANDLER P.C.
65 LIVINGSTON AVENUE
ROSELAND
NJ
07068
US
|
Family ID: |
34923261 |
Appl. No.: |
11/652974 |
Filed: |
January 12, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11068477 |
Feb 28, 2005 |
7181109 |
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11652974 |
Jan 12, 2007 |
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60549029 |
Mar 1, 2004 |
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60549024 |
Mar 1, 2004 |
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60579383 |
Jun 14, 2004 |
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Current U.S.
Class: |
385/39 ; 385/14;
385/15; 385/50 |
Current CPC
Class: |
G02B 6/12007 20130101;
G02B 6/2813 20130101 |
Class at
Publication: |
385/039 ;
385/050; 385/014; 385/015 |
International
Class: |
G02B 6/26 20060101
G02B006/26 |
Goverment Interests
GOVERNMENT RIGHTS
[0002] The invention was made with U.S. government support, and the
U.S. Government has certain rights in the invention, as provided
for by the terms of Contract number F30602-00-C-0116 and DAAD
17-02-C-0094 (DARPA) awarded by the U.S. Army Research Laboratory.
Claims
1. A device comprising a multi-mode interference coupler at least
partially coated with an amorphous silicon material having a
selectable refractive index.
2. A monolithic device for photonically coupling a first optical
waveguide to a second optical waveguide, comprising: an input
optically coupled to the input; a first portion optically coupled
to the input; a second portion optically coupled to the first
portion; an amorphous silicon material having a selectable
refractive index coated on at least one of the first and second
portions; and an output optically coupled to the second portion and
the second waveguide, wherein when an optical signal is provided on
said first waveguide, a given part of said signal is provided to
said second waveguide dependent upon an angle between said first
and second portions.
3. The device of claim 2, wherein at least one of the first
waveguide and the second waveguide is coated with an amorphous
silicon material having a selectable index of refraction.
4. A device comprising an optical waveguide at least partially
coated with an amorphous silicon material having a selectable index
of refraction.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This Application is a divisional of U.S. patent application
Ser. No. 11/068,477, filed Feb. 28, 2005, titled "PHOTONIC DEVICE
AND METHOD FOR MAKING SAME", which in turn claims priority of U.S.
patent application Ser. Nos. 60/549,029, filed Mar. 1, 2004, titled
MODIFIED MULTIMODE INTERFERENCE COUPLERS FOR ARBITRARY COUPLING
RATIOS, 60/549,024, filed Mar. 1, 2004, titled WAVEGUIDE
ENCAPSULATION WITH HYDROGENATED AMORPHOUS SILICON NITRIDE, and
60/579,383, filed Jun. 14, 2004, entitled ELECTRO-REFRACTIVE LOW
LOSS MMI-COUPLED RING RESONATORS, the entire disclosure of each of
which is hereby incorporated by reference as if being set forth in
its entirety herein.
FIELD OF INVENTION
[0003] The present invention relates generally to optical systems,
and more particularly to photonic devices.
BACKGROUND OF THE INVENTION
[0004] Micro-optic ring resonators are attractive for active and
passive micro-optical circuits. Rings and discs with directional
couplers have generally been realized in lateral and vertical
geometries. Coupling of ring resonators with multi-mode
interference (MMI) couplers has been demonstrated with ridge-type
waveguides on InP. Switching has been demonstrated in active discs
using electro-absorption.
[0005] It is believed to be desirable to provide devices exhibiting
lower losses and good reliabilities.
SUMMARY OF INVENTION
[0006] A monolithic device for photonically coupling a first
optical waveguide to a second optical waveguide, including: an
input being optically coupled to the first waveguide; a first
portion being optically coupled to the input; a second portion
being optically coupled to the first portion; and, an output being
optically coupled to the second portion and the second waveguide;
wherein, when an optical signal is provided on the first waveguide,
a given part of the signal is provided to the second waveguide
dependently upon an angle between the first and second portions. At
least one of the waveguides may have an amorphous silicon material
coating.
BRIEF DESCRIPTION OF THE FIGURES
[0007] Understanding of the present invention will be facilitated
by consideration of the following detailed description of the
preferred embodiments taken in conjunction with the accompanying
drawings, wherein like numerals refer to like parts and:
[0008] FIG. 1A illustrates a ring resonator according to an aspect
of the present invention;
[0009] FIG. 1B illustrates a Mach-Zehnder interferometer with a
ring resonator in one arm according to an aspect of the present
invention;
[0010] FIG. 1C illustrates a deeply etched waveguide structure
according to an aspect of the present invention;
[0011] FIG. 2A illustrates transmission characteristics of a ring
resonator, such as that illustrated in FIG. 1A;
[0012] FIG. 2B illustrates a group delay characteristic of a ring
resonator, such as that illustrated in FIG. 1A;
[0013] FIG. 3A illustrates a transmission characteristic of a
Mach-Zehnder interferometer with a ring-resonator in one arm, such
as that illustrated in FIG. 2A;
[0014] FIG. 3B illustrates a modulation gain (i.e., modulation
efficiency as compared to that of a straight waveguide of length
equal to the ring circumference) of a Mach-Zehnder interferometer
with a ring-resonator in one arm, such as that illustrated in FIG.
2A;
[0015] FIG. 4 illustrates a schematic of an angled MMI suitable for
use with the devices of FIGS. 1A, 1B and 1C according to an aspect
of the present invention;
[0016] FIG. 5 illustrates bar (solid) and cross (dashed)
transmissions of an angled MMI, such as that shown in FIG. 4;
[0017] FIG. 6 illustrates an SEM representation of ring-resonators
coupled with angled MMIs to access waveguides according to an
aspect of the present invention;
[0018] FIG. 7 illustrates coupling values for ring-resonators
coupled with angled MMIs to access waveguides according to an
aspect of the present invention;
[0019] FIG. 8 illustrates an SEM cross-section image of deeply
etched InGaAsP/InP waveguide encapsulated with hydrogenated
amorphous silicon nitride according to an aspect of the present
invention;
[0020] FIGS. 9 and 10 illustrate charts presenting achievable
refractive indices as a function of a ratio of source gases used in
a PECVD process;
[0021] FIG. 11 illustrates a schematic of sources of loss in
waveguides;
[0022] FIG. 12 illustrates a deeply etched InGaAsP/InP waveguide
with a-SiNx:H encapsulation and contacts to the p- and n-side of
the waveguide junction according to an aspect of the present
invention;
[0023] FIG. 13 illustrates transmission characteristics of a ring
resonator having the structure shown in FIG. 12;
[0024] FIG. 14 illustrates a tuning of the resonance of a ring
resonator having the structure shown in FIG. 12 in 1 V
increments;
[0025] FIG. 15 illustrates an eye diagram of a link with a ring
resonator modulator; and,
[0026] FIG. 16 illustrates a block-diagrammatic representation of a
process according to an aspect of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0027] It is to be understood that the figures and descriptions of
the present invention have been simplified to illustrate elements
that are relevant for a clear understanding of the present
invention, while eliminating, for purposes of clarity, many other
elements found in typical optical systems and methods of making and
using the same. Those of ordinary skill in the art will recognize
that other elements are desirable and/or required in order to
implement the present invention. However, because such elements are
well known in the art, and because they do not facilitate a better
understanding of the present invention, a discussion of such
elements is not provided herein.
[0028] According to an aspect of the present invention, multi-mode
interference (MMI) couplers with arbitrary coupling ratios may be
provided, and a method for making the same is provided. According
to an aspect of the present invention, optical waveguides may be at
least partially coated, e.g., encapsulated, to improve their
performance. An amorphous silicon (a-Si) material may be used to
encapsulate the guides. According to an aspect of the present
invention, electro-refractive modulation may be used in combination
with ring resonators. MMI couplers may be used to provide
reproducible coupling ratios, which may be particularly useful in
electro-optic applications or when the resonators are used in
optical filters.
[0029] Referring now to FIG. 1, there is shown a ring resonator
device 10. Resonator 10 generally includes a closed optical loop 12
being operably responsive to electrode 14. Signals may be coupled
between resonator 10 and waveguide 20 via a coupler 30, such as a
multi-mode interference (MMI) based coupler.
[0030] FIG. 2 illustrates an analogous resonator 10' coupled to an
analogous waveguide 20' by an analogous MMI based coupler 30',
where waveguide 20' in included as an arm in a Mach-Zehnder
interferometer 40.
[0031] Referring now to FIGS. 1 and 3, ring resonator 10 may be
composed of an InGaAsP/InP material system, include about 25
quantum wells 16 and have a photoluminescence wavelength of about
1415 nm, by way of non-limiting example only. Waveguide 20 may take
the form of a deeply etched waveguide (about 4 .mu.m) and have a
width of about 0.8 .mu.m. MMI coupler 30 may have dimensions on the
order of about 2.7 .mu.m.times.34.5 .mu.m. Coupler 30 may be used
to couple into ring 10 with a cross-coupling ratio of 85.4% or 50%,
which ring may have a circumference of about 250 .mu.m. However, an
MMI based coupler according to an aspect of the present invention
may be used to provide for arbitrary coupling ratios as well. Ring
10, waveguide 20 and coupler 30 may be defined using direct e-beam
patterning and inductively coupled plasma (ICP) etching, for
example.
[0032] Ring resonator 10, waveguide 20 and/or MMI coupler 30 may be
coated with a dielectric 50, such a Cytop brand dielectric
available from Asahi Glass Company. Electrical contacts 14 may be
formed on top of the ring-waveguide 10 and to the n-substrate 18 to
enable modulation. Optical losses may be on the order of about
.about.1 cm.sup.-1.
[0033] Referring now also to FIG. 2, there is shown a transmission
and group delay of a ring resonator, such as resonator 10 of FIGS.
1A-1C. Transmission may be measured with a tunable laser and the
group delay extracted with a modulation phase shift method. The
dashed curves fit to the measured data. The resonator quality
factor Q is 4500. The transmission and group-delay data allows
unambiguous determination of coupling and loss, which are 51% and
8.5% (0.4 dB) per ring revolution, respectively. In this
(over-coupled) regime, the ring resonator Q is determined by the
coupling coefficient rather than the loss, and is thus defined by
the design.
[0034] Referring now also to FIG. 3A, there is shown a transmission
of an interferometer with a ring resonator in one arm, such as
interferometer 40 of FIG. 1B. Index modulation of the ring
resonator may result in an enhanced phase modulation of one
interferometer arm. Modulation response of the configuration is
shown in FIG. 3B, where the ring resonator is reverse biased, and a
1 GHz sine-signal is applied. The modulation depth of the signal
may be retrieved with a network analyzer, while the wavelength is
scanned.
[0035] According to an aspect of the present invention, an
arbitrary coupling MMI based coupler may be used for splitting and
directing light in integrated devices, such as Mach-Zehnder
interferometers (FIG. 2), optical waveguide filters, and
ring-resonators (FIG. 1), all by way of non-limiting example only.
Further, large scale integrated photonic circuits may incorporate
such couplers, which are well suited to split and couple the light
in a controllable and repeatable fashion.
[0036] One may recognize that conventional directional couplers and
multi-mode interference couplers (MMI) may typically be used for
splitting and directing light in integrated devices. However,
directional couplers are typically sensitive to geometrical
dimensions (gaps between the waveguides) and therefore tend to have
generally irreproducible coupling ratios. This is particularly true
for waveguide platforms, which involve narrow, highly confined
waveguides--waveguide types suitable for narrow bends required in
ultra-compact devices. Conventional multi-mode interference
couplers also present drawbacks, such as only having select
coupling ratios (50%:50% or 14.6%:85.4%).
[0037] Referring now also to FIG. 4, according to an aspect of the
present invention, an MMI based 2.times.2 coupler 300 may be used
to provide coupling ratios beyond those typically obtained with
MMI's. Coupler 300 may be based on two MMI couplers 310, which each
provide a coupling ratio of 14.6%:85.4%. The dimensions of each MMI
310 may be designed according to J. H. Heaton, R. M. Jenkins,
"General Matrix Theory of Self-Imaging in Multimode Interference
(MMI) Couplers", IEEE Photonics Technology Letters, 11(2): 212-214
(1999), to provide an N=3, Q=2 coupler, by way of non-limiting
example only. According to an aspect of the present invention, two
MMI couplers 310 may be cascaded in series at a small angle a to
provide a coupler 300 with a coupling ratio that depends upon the
angle a at the joint between MMI couplers 310.
[0038] When a is 0 degrees, coupler 300 is functionally equivalent
to a 50%:50% MMI coupler; a coupler widely used in planer optical
circuits. When an angle other than 0 is present, however, arbitrary
coupling ratios may be provided in a package no larger the 50%:50%
coupler (N=3, Q=4 coupler).
[0039] By way of further, non-limiting example only, the transfer
function of the (N=3, Q=2) MMI coupler is given by J. H. Heaton, R.
M. Jenkins, "General Matrix Theory of Self-Imaging in Multimode
Interference (MMI) Couplers", IEEE Photonics Technology Letters,
11(2): 212-214 (1999) as: T 3 , 2 = 1 / 2 .function. [ 2 - 2 e j
.times. 5 8 .times. .pi. 2 + 2 e j .times. 1 8 .times. .pi. 2 + 2 e
j .times. 1 8 .times. .pi. 2 - 2 e j .times. 5 8 .times. .pi. ] . (
1 ) ##EQU1## The angled region 320 between the two (N=3, Q=2) MMIs
310 provides a phase-difference to the connecting ports. The
corresponding transfer matrix is: T .alpha. = [ exp .function. (
j.phi. ) 0 0 exp .function. ( - j.phi. ) ] ( 2 ) ##EQU2## with
.phi. = 2 .times. .pi. .lamda. .times. n eff .times. sin .function.
( .alpha. ) .times. d 2 , ##EQU3## where .alpha. is the joint
angle, d is the separation of the waveguides where they enter the
MMI section (center to center separation), n.sub.eff is the
effective slab index and .lamda. is the wavelength.
[0040] The transfer matrix, which characterizes the entire coupler,
is given by: T = T 3 , 2 T .alpha. T 3 , 2 = e j .times. .pi. 4
.function. [ 2 2 .times. cos .function. ( j.phi. ) - j .times.
.times. sin .function. ( j.phi. ) 2 2 .times. cos .function. (
j.phi. ) 2 2 .times. cos .function. ( j.phi. ) 2 2 .times. cos
.function. ( j.phi. ) - j .times. .times. sin .function. ( j.phi. )
] . ( 3 ) ##EQU4##
[0041] The power-coupling coefficients are given by: T 2 = 1 / 4
.function. [ ( 3 - cos .function. ( 2 .times. .phi. ) ) ( 1 + cos
.function. ( 2 .times. .phi. ) ) ( 1 + cos .function. ( 2 .times.
.phi. ) ) ( 3 - cos .function. ( 2 .times. .phi. ) ) ] . ( 4 )
##EQU5##
[0042] The bar-coupling is 1/4(3-cos(2.phi.)) and the cross
coupling is 1/4(1+cos(2.phi.)). Accordingly, these values may be
chosen to be in the range of about 0% to 50% (for cross coupling)
by selecting a corresponding angle a .
[0043] Referring now also to FIG. 5, there are shown power coupling
values as a function of .phi., for bar coupling and cross coupling
of a 2.times.2 MMI with an angled joint, such as that shown in FIG.
4.
[0044] Referring now also to FIG. 6, there is shown an SEM image of
ring-resonators coupled to an access waveguide with arbitrary
coupling ratio couplers. The illustrated ring resonators and
waveguides may be analogous to those discussed with regard to FIGS.
1A-1C. The illustrated MMI based coupler may be analogous to that
discussed with regard to FIG. 4. Exemplary values for coupling and
resonator losses associated with the construction of FIG. 6 are
shown in FIG. 7. Resonator losses, which include one pass waveguide
loss and excess coupler-loss, are about 10%. The dependence of
coupling values on the intra-coupler joint angle is consistent with
that discussed herein, such as is shown in FIG. 5.
[0045] According to an aspect of the present invention, amorphous
silicon (a-Si) materials may be used to provide semiconductor
waveguides with improved loss and bending properties. According to
an aspect of the present invention, amorphous silicon (a-Si)
materials, including a-Si:H based materials, may be used to provide
for better performing waveguide type devices. For example,
waveguides and lasers with tailored spot-shape and properties may
be provided. According to an aspect of the present invention, a-Si
material may be used to encapsulate waveguides so as to improve the
performance thereof. Such a material has a selectable refractive
index (n), that may be selected so as to achieve the desired
performance improvement.
[0046] By way of non-limiting example, the amorphous silicon (a-Si)
material may take the form of a-SiC.sub.x where 0<x<1,
a-SiN.sub.y where 0<y<1.33, a-SiO.sub.z where 0<z<2 and
a-SiGe.sub.w where 0<w<1. Hydrogenated or fluorinated
materials may be used.
[0047] By way of further non-limiting example only, and referring
now also to FIG. 9, there is shown a chart illustrating achievable
refractive indices as a function of a ratio of CH.sub.4 to
SiH.sub.4 used in a plasma enhanced chemical vapor deposition
(PECVD) process to form a coating including a-SiC.sub.x. Referring
now also to FIG. 10, there is shown a chart illustrating achievable
refractive indices as a function of a ratio of N.sub.2 to SiH.sub.4
used in a PECVD process to form a coating including a-SiN.sub.y.
Reference may be drawn to U.S. Pat. No. 6,788,721, entitled
PHOTONIC INTEGRATED CIRCUIT (PIC) AND METHOD FOR MAKING SAME, the
entire disclosure of which is hereby incorporated by reference
herein, for more information regarding tuning the refractive index
of a-Si based materials. The present invention will be further
discussed as it relates to a-SiN.sub.x:H, for non-limiting purposes
of explanation only. Other materials may be used though.
[0048] Referring now also to FIG. 8, PECVD deposited hydrogenated
amorphous silicon nitride 810 may be used as a semiconductor
waveguide 800 encapsulation to provide increased design flexibility
and better performance of semiconductor waveguide based photonic
circuits, such as those discussed with regard to FIGS. 1-7 hereof.
Waveguide 820 may take the form of an InGaAsP/lnP based waveguide,
for example. By way of further example, resonator 10, waveguide 20
and/or coupler 30 or 300 (FIGS. 1A-1C, 4) may be at least partially
encapsulated with an amorphous silicon based material, such as
hydrogenated amorphous silicon nitride.
[0049] Hydrogenated amorphous silicon nitride (a-SiN.sub.x:H) can
provide a refractive index of about 2 to about 3.7, with low
optical absorption at telecommunication wavelengths (1300 nm, 1550
nm windows). This range of refractive indices is not typically
covered by traditional materials used for semiconductor waveguide
encapsulation/passivation, such as silicon oxide/nitride, polymers
and Benzocyclobutene (BCB). Using a-SiN.sub.x:H as a complementary
material to form semiconductor-based waveguides is particularly
attractive due to the large refractive index range that can be
exploited. Choice of the refractive index of a waveguide
encapsulation material over the achievable index range provides a
valuable design tool for waveguide and photonic devices. Further,
a-SiN.sub.x:H can be deposited over semiconductor structures using
plasma enhanced chemical vapor deposition (PECVD) at low
temperatures (200 deg C.), for example.
[0050] Referring now also to FIG. 11, semiconductor waveguides 1100
made of III-V semiconductors such as InGaAsP/InP typically provide
guiding of the light in the vertical dimensions due to proper
design of the epitaxially grown layer structure. Lateral guiding is
obtained by etching the structure to a certain depth. The lateral
guiding properties are typically determined by etch depth,
waveguide shape and the refractive indices of the remaining
semiconductor and surrounding material. According to an aspect of
the present invention, encapsulation of semiconductor waveguide
1100 with an a-Si based material, such as a-SiN.sub.x:H, provides
for better and direct control of the lateral guiding properties
dependently upon the selectable refractive index of the a-Si based
material.
[0051] Referring again to FIG. 8 and now also to FIG. 12, there are
shown a deeply etched InGaAsP/InP waveguide 800 with an
a-SiN.sub.x:H encapsulation layer 810. The semiconductor is etched
entirely through an epitaxially grown vertical waveguide structure.
Lateral guiding is determined by the waveguide width W-W and the
a-SiN.sub.x:H layer 820 refractive index value. Using this
geometry, one may design single-mode waveguides dependently upon
the a-SiN.sub.x:H refractive index, that are wider and more
practical than single mode waveguides obtained with traditional
encapsulation materials with indices below 2 (which typically
require waveguides narrower than about 0.4 .mu.m).
[0052] According to an aspect of the present invention, deeply
etched waveguides can provide a platform for compact photonic
circuits due to achievable tight bending radii, which can be
obtained with negligible optical loss. Bending radii around 4 .mu.m
and larger may generally be used with refractive indices in the
range of about 2 to about 3.7 to enable low loss waveguides.
[0053] Tighter bending radii may be used, but scattering losses
resulting from sidewall roughness and the use of encapsulation
indices below 2 may result. Scattering loss scales approximately
with .DELTA.n.sup.2, where .DELTA.n is the difference of the
effective index of the vertical mode in the semiconductor and the
encapsulation material. A choice of n=2.65 as opposed to n=2 will
reduce the scattering loss four times, and provide for waveguide
bends around 8 .mu.m, for example.
[0054] Referring still to FIG. 12, waveguide 800 may include
include quantum wells 805 between p- and n-type regions, and be
formed on an n-InP substrate 820 having an n-contact 830. A
p-contact 840 may also be provided. By way of further, non-limiting
example only, waveguide width W-W may be on the order of about 1
.mu.m, where waveguide height H-H is on the order of about 4
.mu.m.
[0055] Further yet, the performance of waveguide based photonic
components, such as MMI-couplers (30, 300, FIGS. 1A-1C, 4), are
typically sensitive to the dimensions of the fabricated devices.
Fabrication errors, which are generally inevitable, and even
tolerances, thus result in performance degradation or differences.
In the case of MMI-couplers, the width of the MMI waveguide is
typically the most sensitive dimension. The performance of the
device is determined by the effective width of the guide. According
to an aspect of the present invention, penetration of the optical
field into a surrounding dielectric material, which results in an
effective width wider than the physical width of the device, may be
leveraged. This effect is called the Goos-Hanchen shift. The
magnitude of the Goos-Hanchen shift is determined by the index
difference of the waveguide material and the encapsulating
dielectric index.
[0056] Referring now also to FIG. 16, there is shown a process
according to an aspect of the present invention. Where a deviation
in a characteristic of a fabricated device differs from the target
characteristic, one may compensate for the difference by tuning the
encapsulating material index value. The index can be adjusted such
that a Goos-Hanchen shift will be obtained which results in an
effective waveguide width closer to that originally targeted.
Again, a-Si materials may be used as the encapsulating
material.
[0057] According to an aspect of the present invention, the core
device may be fabricated 1610. A difference between a design
characteristic of the device and the actual fabricated device may
then be determined 1620. The index value can be determined 1630
after fabrication 1610, e.g., etching the waveguides, dependently
upon the determined difference 1620. For example, the width of a
fabricated MMI may be compared to a target width, and a refractive
index for an encapsulating a-Si material chosen to facilitate high
performance devices and performance repeatability, even in the case
of limited width control and repeatability during fabrication. The
encapsulated material may be of any suitable thickness, such as
0.5-1 .mu.m thick.
[0058] According to an aspect of the present invention,
ring-resonator modulator arrays for wavelength division
multiplexing (WDM) applications may be provided to facilitate
reduced size, power consumption and cost. The resonant cavity of
the ring resonator results in a substantial decrease of the V.sub.p
of the modulator for a given electrode length. As a consequence,
compact devices with low electrical parasitics can be used for
efficient modulation. The wavelength selective nature of the
resonators allows modulation at one or multiple, specific
wavelengths without requiring filtering devices, such as AWG's.
[0059] The ring resonators may be based on deeply etched 1 .mu.m
wide waveguides on InGaAsP/InP (e.g., FIGS. 1A-1C, 8, 12). Smooth,
highly vertical etched sidewalls may be used to provide for low
optical losses. Etching that provides for good vertical wall
smoothness, and that may be suitable for use with the present
invention, is described in "CHARACTERIZATION OF SIDEWALL ROUGHNESS
OF INP/INGAASP ETCHED USING INDUCTIVELY COUPLED PLASMA FOR LOW LOSS
OPTICAL WAVEGUIDE APPLICATIONS", J. Vac. Sci. Technol. B 21,6,
November/December 2003. A quantum-well active region analogous to
that presented in U.S. patent application Ser. No. 10/792,585,
entitled, IN-P PHASE MODULATORS AND METHODS FOR MAKING AND USING
THE SAME, the entire disclosure of which is also hereby
incorporated by reference herein, may be used to provide a high
dn/dV.
[0060] Accurate control over the coupling value to the access
waveguide (e.g., FIGS. 1A-1B) may be provided for device designs
involving multiple rings (e.g., FIG. 6), allowing one to obtain
specific filter functions or multi-wavelength resonator arrays, for
example. A coupler (FIGS. 1A, 1B, 4) may include two cascaded MMI
sections, which join at a small angle (FIG. 4). Each section may
take the form of a 2.times.2 MMI with a coupling ratio of
14.6%:85.4%. The coupling ratio of the combined coupler may be
selected by the angle at the joint. This MMI scheme results in
reproducible arbitrarily selectable coupling ratios and does not
require the elaborate processing associated with vertical coupling
schemes.
[0061] Referring now also to FIG. 13, there is shown an optical
transmission of a ring resonator with a quality factor of 20,000
and a finesse of 36. The ring circumference is 200 .mu.m and the
coupling to the access waveguide is 7%. Values for the coupling and
resonator losses are shown FIG. 7.
[0062] Referring now also to FIG. 14, there is shown an
electro-optic response of a ring resonator according to an aspect
of the present invention, with a reverse bias from 0 to 10 V. A
tuning coefficient df/dV of 6 GHz/V may be obtained. With
appropriate biasing, the close to critically coupled ring-resonator
may act as a modulator with an equivalent V.sub.p of 2 V. An eye
diagram at 1 Gb/s corresponding to a link incorporating such a ring
modulator is shown in FIG. 15.
[0063] It will be apparent to those skilled in the art that various
modifications and variations may be made in the apparatus and
process of the present invention without departing from the spirit
or scope of the invention. Thus, it is intended that the present
invention cover the modification and variations of this invention
provided they come within the scope of the appended claims and
their equivalents.
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