U.S. patent application number 13/538525 was filed with the patent office on 2014-01-02 for advanced modulation formats using optical modulators.
The applicant listed for this patent is Po Dong. Invention is credited to Po Dong.
Application Number | 20140003761 13/538525 |
Document ID | / |
Family ID | 48782647 |
Filed Date | 2014-01-02 |
United States Patent
Application |
20140003761 |
Kind Code |
A1 |
Dong; Po |
January 2, 2014 |
ADVANCED MODULATION FORMATS USING OPTICAL MODULATORS
Abstract
A system, e.g. an optical modulator, includes an optical
waveguide and a plurality of optical resonators. The optical
waveguide is located along a surface of a planar substrate. The
plurality of optical resonators is also located along the surface
and coupled to the optical waveguide. Each of said optical
resonators is configured to resonantly couple to the optical
waveguide at a different optical frequency.
Inventors: |
Dong; Po; (Morganville,
NJ) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Dong; Po |
Morganville |
NJ |
US |
|
|
Family ID: |
48782647 |
Appl. No.: |
13/538525 |
Filed: |
June 29, 2012 |
Current U.S.
Class: |
385/3 ; 29/592.1;
385/2 |
Current CPC
Class: |
G02B 6/29343 20130101;
G02F 2001/212 20130101; G02F 1/2257 20130101; G02F 2203/50
20130101; Y10T 29/49002 20150115; G02F 2201/16 20130101; G02B
6/12007 20130101 |
Class at
Publication: |
385/3 ; 385/2;
29/592.1 |
International
Class: |
G02F 1/035 20060101
G02F001/035; H05K 13/04 20060101 H05K013/04 |
Claims
1. A system comprising: a substrate with a planar surface; a first
optical waveguide located along the surface; a first plurality of
optical resonators located along the surface and optically coupled
to said optical waveguide, each of said optical resonators being
configured to resonantly couple to the optical waveguide at a
different optical frequency; a second optical waveguide; a second
plurality of optical resonators optically coupled to said second
optical waveguide, wherein each optical resonator of said second
plurality is configured to resonantly couple to the second optical
waveguide at about a same optical frequency as a corresponding one
of the optical resonators of said first plurality; and an optical
combiner having first and second inputs connected to ends of
corresponding ones of the first optical waveguide and the second
optical waveguide.
2. The system of claim 1, wherein the second optical waveguide is
located along the surface.
3. The system of claim 1, wherein said first optical waveguide
end-connects a first output of an optical power splitter to a first
input of the optical power combiner and the second optical
waveguide end-connects a second output of the optical power
splitter to a second input of the optical power combiner.
4. The system of claim 1, wherein the first and second optical
waveguides and the optical resonators are configured to QPSK
optically modulate optical carriers at a sequence of
wavelengths.
5. The system of claim 1, wherein some of said optical resonators
are overcoupled to at least one of said first or second optical
waveguides.
6. The system of claim 1, wherein optical core regions of said
first optical waveguide and said second optical waveguide and said
optical resonators are formed in silicon located over a dielectric
layer.
7. The system of claim 1, further comprising an optical source
configured to output said optical signal such that said optical
signal including frequency components corresponding to resonant
frequencies of said optical resonators.
8. A system comprising: a substrate with a planar surface; an
optical waveguide located along the surface; and a plurality of
optical resonators located along the surface and optically coupled
to said optical waveguide, each of said optical resonators being
configured to resonantly couple to the optical waveguide at a
different optical frequency, wherein each optical resonator
includes a first optical phase modulator configured to enable
quasi-static optical path adjustments thereto and a second optical
phase modulator configured to enable optical path adjustments at a
frequency of at least about 1 GHz.
9. A method comprising: forming a first optical waveguide and a
plurality of optical resonators along a surface of a substrate such
that each of said resonators is adjacent to segments of and
optically coupled to said optical waveguide, each of said
resonators being configured to resonate at a different optical
frequency; forming a second optical waveguide and a second
plurality of optical resonators along said surface such that the
resonators of said second plurality are adjacent to segments of and
optically coupled to said second optical waveguide, wherein each
resonator of the second plurality is configured to resonate at
about an optical resonant frequency of a corresponding one of the
resonators of said first plurality; and making an optical combiner
having first and second inputs connected to ends of corresponding
ones of the first optical waveguide and the second optical
waveguide.
10. (canceled)
11. The method of claim 10, the forming includes making an optical
power splitter having first and second outputs connected to ends of
corresponding ones of the first optical waveguide and the second
optical waveguide.
12. The method of claim 11, wherein said first and second
pluralities of optical resonators are operable to WDM QPSK modulate
an optical carrier received by the optical power splitter.
13. (canceled)
14. The method of claim 9, wherein some of said optical resonators
are overcoupled to at least one of said first or second optical
waveguides.
15. The method of claim 9, further comprising forming controllers
capable of varying the resonant frequencies of the optical
resonators.
16. The method of claim 9, wherein optical core regions of said
first optical waveguide and said second optical waveguide and said
optical resonators are formed in silicon regions located over a
dielectric layer.
17. A method comprising: forming an optical waveguide and a
plurality of optical resonators along a surface of a substrate such
that each of said resonators is adjacent to segments of and
optically coupled to said optical waveguide, each of said
resonators being configured to resonate at a different optical
frequency, wherein each resonator includes a first optical phase
shifter configured to enable quasi-static optical path adjustments
of the each resonator and includes a second optical phase shifter
capable of varying optical pathlengths at frequencies of about 1
GHz or more.
18. The method of claim 9, further comprising end-coupling an
optical source to said optical waveguide, the optical source being
configured to output an optical signal including frequency
components corresponding to different resonant frequencies of a
plurality of said resonators.
19. The system of claim 8, further including: a second optical
waveguide located along the substrate; and a second plurality of
optical resonators optically coupled to said second optical
waveguide, wherein each optical resonator of said second plurality
is configured to resonantly couple to the second optical waveguide
at about a same optical frequency as a corresponding one of the
optical resonators of said first plurality.
20. The system of claim 19, further including: an optical combiner
having first and second inputs connected to ends of corresponding
ones of the optical waveguide and the second optical waveguide.
21. The method of claim 17, further including: forming a second
optical waveguide and a second plurality of optical resonators
along said surface such that the resonators of said second
plurality are adjacent to segments of and optically coupled to said
second optical waveguide, wherein each resonator of the second
plurality is configured to resonate at about an optical resonant
frequency of a corresponding one of the resonators of said first
plurality.
22. The method of claim 21, further including: making an optical
combiner having first and second inputs connected to ends of
corresponding ones of the optical waveguide and the second optical
waveguide.
Description
TECHNICAL FIELD
[0001] This application is directed, in general, to optical
communications systems and methods.
BACKGROUND
[0002] This section introduces aspects that may be helpful to
facilitating a better understanding of the inventions. Accordingly,
the statements of this section are to be read in this light and are
not to be understood as admissions about what is in the prior art
or what is not in the prior art.
[0003] Optical modulators often use one or more Mach-Zehnder
interferometers. These devices typically include electro-optic
modulators. Such modulators have been implemented in various
optical media, including silicon, compound semiconductors, and
LiNbO.sub.3. While these devices are capable of high-speed
performance, they can also consume significant power, e.g. for
heating or electrically polarizing waveguide segments to modulate
the refractive index of the segments. When integrated into an
optical system, a significant portion of the power consumption of
the system may result from the optical modulators.
SUMMARY
[0004] One aspect provides a system, e.g. an optical modulator. The
system includes an optical waveguide and a plurality of optical
resonators. The optical waveguide is located along a surface of a
planar substrate. The plurality of optical resonators is also
located along the surface and is coupled to the optical waveguide.
Each of the optical resonators is configured to resonantly couple
to the optical waveguide at a different optical frequency.
[0005] Another aspect provides a method, e.g. for manufacturing an
optical system, e.g. a modulator. The method includes forming an
optical waveguide and a plurality of optical resonators along a
surface of a substrate. The forming is performed such that each of
the resonators is adjacent to segments of and optically coupled to
the optical waveguide. Each of the resonators is configured to
resonate at a different optical frequency.
[0006] Some of the above-described embodiments include a second
optical waveguide and a second plurality of optical resonators
optically coupled to the second optical waveguide. Each optical
resonator of the second plurality is configured to resonantly
couple to the second optical waveguide at about a same optical
frequency as a corresponding one of the optical resonators of the
first plurality. In some such embodiments the first optical
waveguide may end-connect a first output of an optical power
splitter to a first input of an optical power combiner and the
second optical waveguide may end-connect a second output of the
optical power splitter to a second input of an optical power
combiner. In some such embodiments the first and second optical
waveguides and the optical resonators are configured to QPSK
optically modulate optical carriers at a sequence of
wavelengths.
[0007] In any of the above-described embodiments some of the
optical resonators may be overcoupled to their associated optical
waveguide. In any embodiment the optical core regions of the
optical waveguide and the resonators may be formed in silicon
located over a dielectric layer. Any embodiment may include an
optical source configured to output the optical signal such that
the optical signal includes frequency components corresponding to
resonant frequencies of the optical resonators. In any embodiment,
each optical resonator may include a first optical phase modulator
configured to enable quasi-static optical path adjustments thereto
and a second optical phase modulator configured to enable optical
path adjustments at a frequency of at least about 1 GHz.
BRIEF DESCRIPTION
[0008] Reference is now made to the following descriptions taken in
conjunction with the accompanying drawings, in which:
[0009] FIG. 1 illustrates an embodiment of the invention in which
an optical system, e.g. a quadrature phase-shift keyed (QPSK)
modulator, is implemented using a splitter, a combiner, a pair of
two optical waveguides coupling the outputs of the splitter to the
inputs of the combiner, and a plurality of pairs of optical
microcavity resonators, e.g. ring resonators;
[0010] FIG. 2 illustrates a sectional view of an optical path of
FIG. 1 and a proximate ring resonator, according to one embodiment
of the optical system;
[0011] FIG. 3A-3D illustrates example amplitude and phase
characteristics of a waveguide coupled to a ring resonator;
[0012] FIG. 4 illustrates a single ring resonator, e.g. one of the
ring resonators of FIG. 1, with a quasi-static optical path length
adjuster, and a high-speed optical path length adjuster configured
to modulate the path length at a rate between one or a few GHz and
tens of GHz;
[0013] FIGS. 5A-5E illustrate sectional views of various
embodiments of the resonator of FIG. 4, showing features that may
be used to change the optical path length of the resonator,
including electro-optic modulators (FIGS. 5A and 5B) and thermal
phase shifters (FIGS. 5C-5E);
[0014] FIGS. 6A, 6B, 7A and 7B illustrate amplitude and phase
characteristics of, e.g. one of the waveguides of FIG. 1 with an
undercoupled ring resonator (FIGS. 6A and 6B) and an overcoupled
ring resonator (FIGS. 7A and 7B);
[0015] FIGS. 8A and 8B respectively illustrate amplitude and phase
response of four transfer characteristics associated with a
waveguide coupled to four ring resonators in FIG. 1, wherein each
ring resonator is controllable to have one of two predetermined
resonant frequencies, above and below a WDM channel frequency;
[0016] FIGS. 9A and 9B illustrate embodiments of a controller
configured to control the resonant frequencies of the ring
resonators of FIG. 1 to modulate a carrier signal; and
[0017] FIG. 10 presents a method, e.g. for manufacturing an optical
system such as described by various embodiments herein, e.g. FIGS.
1-9.
DETAILED DESCRIPTION
[0018] The inventor has determined that some limitations of
conventional modulators may be overcome by using a plurality of
controllable optical microcavity resonators, e.g. ring resonators,
optically coupled to a waveguide to modulate the phase of an
optical carrier signal at each of a plurality of wavelengths. Such
an assembly may form the basis of a compact and low power optical
modulator that can provide QPSK (quadrature phase-shift keyed)
modulation of a plurality of channels in a WDM (wavelength-division
multiplexed) communication system. Some described embodiments are
expected to be relatively robust to manufacturing variation by
providing a quasi-static adjustment of the resonant frequency of
each ring resonator to compensate for such variation. Some
described embodiments may be formed on common and inexpensive
semiconductor substrates, e.g. silicon wafers, using processing
tools commonly used in semiconductor processing. Thus some
embodiments are expected to be manufacturable for a lower cost than
similar systems using other architectures, such as those using MZIs
(March-Zehnder Interferometers).
[0019] Silicon has a relatively weak electro-optic response as
compared to LiNbO.sub.3 and III-V semiconductors. This small
response poses a significant challenge to using Si-based modulators
employing a low-voltage silicon MZI. However, the inventor has
realized that the electro-optic response of silicon may be
sufficient for realization of effective and cost-effective optical
modulators based on microcavity resonators. Moreover, low-voltage
silicon modulators have the potential to significantly reduce the
power consumption of some integrated photonic devices.
[0020] Turning to FIG. 1, an apparatus 100 is illustrated according
to one embodiment, e.g. a WDM QPSK modulator. A 1.times.2 coupler
110 receives an optical carrier signal 120 to be modulated, e.g. an
unmodulated (CW) laser output having a plurality of WDM channel
wavelengths .lamda..sub.1, .lamda..sub.2, .lamda..sub.3, . . .
.lamda..sub.M, or equivalently a plurality of WDM channel
frequencies f.sub.1, f.sub.2, f.sub.3, . . . f.sub.M. In some
embodiments the carrier signal 120 may already be modulated in a
manner that is not incompatible with additional modulation applied
by the apparatus 100. For example, the carrier signal 120 may also
include a frequency that is not a member of the set {f.sub.1,
f.sub.2, f.sub.3, . . . . f.sub.M} that has previously been
modulated to transmit data.
[0021] An optical source 125 may produce the carrier signal 120.
The optical source 125 may include optical component(s), such as
for example lasers and combiners, to produce the carrier signal
120. In some embodiments the optical source 125 produces a
frequency comb such as exemplified by a comb 127 with signal
components at channel frequencies f.sub.1, f.sub.2, f.sub.3, . . .
f.sub.M. The frequencies are not limited to any particular values,
and may be in any wavelength band used in optical communications,
e.g. in the S band (1460 nm-1530 nm), the C band (1530 nm-1565 nm)
or the L band (1565 nm-1625 nm). Furthermore, the frequency
components of the comb 127 may be spaced by a WDM grid spacing
.DELTA.f, e.g. a regular, about even spacing of the frequency
components by a same frequency difference, e.g. about 100 GHz.
[0022] The coupler 110 may, e.g., split the carrier signal 120
about equally in power, directing a first carrier portion 120a to a
first optical waveguide 130, e.g. a planar or ridge waveguide. The
coupler 110 directs a second carrier portion 120b to a second
optical waveguide 140, e.g. a planar or ridge waveguide. The core
regions of the optical waveguides 130 and 140 are surrounded by a
cladding 145 that may include, e.g., an underlying dielectric
material and/or an overlying dielectric material and/or air. In
some embodiments a phase shifter 180 has a portion located in the
optical path of the optical waveguide 140. In the illustrated
embodiment the phase shifter 180 is configured to impose a net
phase shift on the carrier portion 120b of about .pi./2, e.g.,
.pi./2.+-.20% or more generally (.pi./2.+-.10%)++n.pi., wherein
n=0, 1, 2 . . . . An optical combiner 170 receives the first and
second signal portions from the optical waveguides 130 and 140 and
combines these signal portions into an output signal 199.
[0023] The apparatus 100 includes a first set 150 and a second set
160 of microcavity resonators, e.g. ring resonators. For
convenience the first and second sets 150 and 160 may be
respectively referred to as ring resonators 150 and ring resonators
160, and a single ring resonator may be referred to a ring
resonator 150 or 160 when further differentiation is not needed.
The ring resonators 150 and 160 each resonate at one of a series of
resonant frequencies separated by the free spectral range (FSR) of
that ring resonator. The resonant frequencies of a particular ring
resonator may be determined from the optical properties of the ring
core and cladding materials, and the geometry of the particular
ring. In various embodiments the frequency range of light of the
carrier signal 120 is typically constrained to be within one FSR
period of the ring resonator having the smallest FSR. The FSR is
typically inversely proportional to the optical path length of the
ring resonator, so a smaller ring diameter will result in a larger
FSR of the ring resonators 150 and 160, easing the constraint on
the carrier signal 120 frequency range. Thus, the frequency range
of the carrier signal 120 may typically include only a single
resonant frequency of each of the ring resonators 150, 160. This
single resonant frequency may be referred to herein generally as
f.sub.r. The first set 150 includes ring resonators 150-1, 150-2,
150-3, . . . 150-M having corresponding resonant frequencies near
f.sub.1, f.sub.2, f.sub.3, . . . f.sub.M. The second set 160
includes ring resonators 160-1, 160-2, 160-3, . . . 160-M also
having corresponding resonant frequencies near f.sub.1, f.sub.2,
f.sub.3, . . . f.sub.M. By "near", it is meant that the f.sub.r of
each of the ring resonators 150 and 160 may be controlled as
described below within a narrow range, which includes a
corresponding WDM channel frequency, e.g. one of f.sub.1, f.sub.2,
. . . f.sub.M. The range may be, e.g. less than about the grid
spacing .DELTA.f. In some embodiments, the range may be no greater
than about 10% of .DELTA.f.
[0024] In some embodiments the ring resonators 150 and 160 are
organized as ring resonator pairs, as illustrated, such that a pair
includes a ring resonator from each of the sets 150 and 160 having
about a same resonant wavelength and about aligned to an axis
normal to the waveguides 130 and 140. However, embodiments are not
limited to such pairing. In the illustrated embodiment a first pair
includes the ring resonator 150-1 and the ring-resonator 160-1
having a resonant frequency near f.sub.1. A second pair includes
the ring resonator 150-2 and the ring-resonator 160-2 having a
resonant frequency near f.sub.2. A third pair includes the ring
resonator 150-3 and the ring-resonator 160-3 having a resonant
frequency near f.sub.3. An M-th pair includes the ring resonator
150-M and ring resonator 160-M having a resonant frequency near
f.sub.M. Embodiments are not limited to any particular number of
ring resonators in the first and second ring resonator sets 150 and
160. Furthermore, one or both of the sets 150 and 160 may include
one or more ring resonators that are not matched by a ring
resonator having a same resonant frequency in the other of the sets
150, 160.
[0025] FIG. 2 schematically illustrates a sectional view of the
core of the waveguide 130 and a representative ring resonator 150.
For later reference, the core of the waveguide 130 and a segment of
the optical core of the ring resonator 150 are shown as being
separated by a lateral distance D. The waveguide 130 and the ring
resonator 150 each have a width W and a height H. While the core of
the waveguide 130 and the segment of the optical core of the ring
resonator 150 are shown each having the same width, embodiments are
not limited to such cases. The cores of the waveguide 130 and the
ring resonator 150 may be formed from a semiconductor, e.g.
silicon, over a substrate 210, e.g. a silicon wafer. The cladding
145 may include a dielectric layer 220 located between the
substrate 210 and the waveguide 130, and between the substrate 210
and the waveguide ring resonator 150. The cladding 145 may also
include a dielectric 230 overlying the dielectric layer 220. The
dielectric layer 220 and the dielectric 230 serve as the cladding
for the waveguide 130 and the ring resonator 150-1 such that
optical signals are substantially confined in and guided by these
structures.
[0026] A convenient platform on which to form the apparatus 100 is
a silicon-on-insulator (SOI) wafer, but embodiments of the
invention are not limited thereto. For example, a CVD dielectric
layer, e.g. plasma oxide, could be formed on any suitable
substrate, and a silicon layer could be formed thereover by any
suitable method. Other embodiments may use a substrate formed from,
e.g. glass, sapphire or a compound semiconductor. The dielectric
230 may be a suitable dielectric material, e.g. silicon oxide,
silicon nitride, benzocyclobutene (BCB), or air. For the purpose of
this disclosure, "air" includes vacuum.
[0027] The waveguide 130 and the ring resonator 150 may be formed
from any conventional or nonconventional optical material system,
e.g. silicon, LiNbO.sub.3, a compound semiconductor such as GaAs or
InP, or an electro-optic polymer. Some embodiments described herein
are implemented in Si as a nonlimiting example. While embodiments
within the scope of the invention are not limited to Si, this
material provides some benefits relative to other material systems,
e.g. relatively low cost and well-developed manufacturing
infrastructure.
[0028] Referring again to FIG. 1, each of the ring resonators in
the set 150 is optically coupled to the waveguide 130. Each of the
ring resonators in the set 160 is optically coupled to the
waveguide 140. Herein and in the claims, a ring resonator is
defined as being optically coupled to a waveguide when that ring
resonator is overcoupled or undercoupled to that waveguide, as
further described below.
[0029] As appreciated by those skilled in the optical component
arts, light propagating within the waveguide 130 may couple, e.g.,
via evanescent coupling to the ring resonators of the set 150, and
light propagating within the waveguide 140 may couple to the ring
resonators of the set 160, e.g., via the coupling of evanescent
light. By such coupling, a portion of the optical energy
propagating in the waveguides 130, 140 couples to the ring
resonators 150, 160. The degree of coupling is dependent on, among
other factors, the wavelength of the propagating light. When the
optical path length of the microcavity resonator is an integer
multiple of the wavelength of the coupled light, a relative maximum
coupling may occur, producing a notch in the passband of the
waveguide. As discussed further below, this notch response may be
exploited for use in a low-power optical modulator.
[0030] Herein a ring resonator and a proximate segment of
waveguide, e.g. the adjacent segment of the ring resonator 150 and
the waveguide 130, are defined as being critically coupled when the
coupling between them is about equal to the round-trip loss in the
ring resonator. In this case, D=D.sub.c (FIG. 2). For example, if
the round trip loss is about 1 dB, the ring resonator 150 and the
waveguide 130 are critically coupled when the coupling therebetween
is also about 1 dB. In other words, about a 1 dB portion of an
optical signal propagating in the waveguide proximate the ring
resonator is transferred to the ring resonator by, e.g. evanescent
coupling. When D<D.sub.c the ring resonator and the waveguide
are overcoupled, e.g. a greater portion of the signal is coupled
from the waveguide to the ring resonator than is lost in one round
trip of the coupled signal in the ring resonator. Conversely when
D>D.sub.c the ring resonator and the waveguide are undercoupled,
e.g. the coupling is less than the round-trip loss in the ring
resonator. As further provided below, when D is greater than a
maximum coupling distance D.sub.max, the ring resonator and the
waveguide may be regarded as uncoupled.
[0031] These aspects are illustrated further by FIGS. 3A-3D, in
which the resonant frequency of an arbitrary ring resonator is
about f'. FIGS. 3A and 3B respectively show simplified and
nonlimiting amplitude and phase characteristics of a transfer
function G.sub.uc(f) of a waveguide, e.g. the waveguide 130,
coupled to an adjacent segment of a ring resonator, e.g. the ring
resonator 150-1, for the case that the waveguide and ring resonator
are undercoupled. FIGS. 3C and 3D respectively show simplified and
nonlimiting amplitude and phase characteristics of a transfer
function G.sub.oc(f) of a waveguide, e.g. the waveguide 130,
coupled to a ring resonator, e.g. the ring resonator 150-1, for the
case that the waveguide and ring resonator are overcoupled.
[0032] The amplitude characteristics of the transfer functions
G.sub.uc (f) and G.sub.oc (f) are qualitatively similar, each
having a local minimum at f' for both the undercoupled case (FIG.
3A) and the overcoupled case (FIG. 3C). For the undercoupled case
the phase of the transfer function G.sub.uc (f) (FIG. 3B) increases
from .phi..sub.o at f<<f' to a local maximum .phi..sub.max at
f'-.delta. (where .delta. is a small value, e.g. no greater than
about 5% of .DELTA.f), and from a local minimum .phi..sub.min at
f'+.delta. to .phi..sub.o at f>>f'. The phase may have an
indeterminant value at about f'. For the overcoupled case the phase
of the transfer function G.sub.oc (f) (FIG. 3D) increases smoothly
from an initial value .phi..sub.min=.phi..sub.o at f<<f' to a
final value of .phi..sub.max=.phi..sub.o+2.pi. at f>>f.sub.r.
In both FIGS. 3B and 3D .phi..sub.o is arbitrary, and .phi..sub.o
may be different in the two figures.
[0033] As discussed further below, when .phi..sub.max-.phi..sub.min
is about .pi. radians the coupling between the ring resonator 150
and the waveguide 130 may be exploited to produce BPSK (binary
phase-shift keyed) modulation on a signal propagating in the
waveguide 130. In the overcoupled case .phi..sub.max-.phi..sub.min
is expected to always meet this condition. In the undercoupled case
.phi..sub.max-.phi..sub.min may be at least about it when the
coupling between the ring resonator and the waveguide is
sufficiently strong, e.g. when D.ltoreq.D.sub.MAX.
[0034] Accordingly, herein and in the claims an optical waveguide
and a microcavity resonator are "optically coupled" when they are
overcoupled, critically coupled, or undercoupled. The term
"undercoupled" with respect to a ring resonator and a proximate
waveguide is defined as meaning that the ring resonator and the
waveguide are not overcoupled, but are sufficiently coupled to
produce a phase change in the transfer function of the waveguide of
at least about .pi. radians. A ring resonator whose core segments
are distant enough from a waveguide, e.g. D>D.sub.max such that
any phase change of the transfer function produced by the ring
resonator is less than about it radians is considered
"uncoupled".
[0035] Referring back to FIG. 1, each of the ring resonators of the
set 150 may have a different physical path length. The ring
resonators are not limited to any particular path shape. For
instance, the optical path of the ring resonators may have a
circular, elliptical, or "racetrack" shape, though a circular path
shape may be preferred to reduce losses in the ring resonator. In
the example case of a circular path, the ring resonator 150-1 may
have a radius r.sub.1, the ring resonator 150-2 may have a radius
r.sub.2>r.sub.1, the ring resonator 150-3 may have radius
r.sub.3>r.sub.2 and so on. The radius r.sub.1 may be selected
such that the ring resonator 150-1 has a physical path length that
causes the optical coupling of the waveguide 130 thereto to
resonate at about f.sub.1. The radius r.sub.2 may be selected such
that the ring resonator 150-2 has a physical path length that
causes the optical coupling of the waveguide 130 thereto to
resonate at about f.sub.2. The radius r.sub.3 may be selected such
that the ring resonator 150-3 has a physical path length that
causes the optical coupling of the waveguide 130 thereto to
resonate at about f.sub.3, and so on as illustrated.
[0036] FIG. 4 illustrates a nonlimiting embodiment of a single ring
resonator 410 with a resonant frequency f' that may be
representative of any of the ring resonators 150, 160. The ring
resonator 410 has an optical path length l, and includes two
electrically or thermally controllable optical path length
adjusters, e.g. phase shifters, 420 and 430. A controller 440
provides an appropriately configured signal to control the
electrically or thermally controllable phase shifter 420. A
controller 450 provides an appropriately configured signal to
control the electrically or thermally controllable phase shifter
430 via a control line 460.
[0037] The controllable phase shifter 420 may be configured to
provide a quasi-static, e.g. relatively slow, adjustment to the
optical path length l, while the adjuster 430 may be configured to
provide a relatively fast adjustment of the same optical pathlength
l. For example, the controllable phase shifter 420 may be a thermal
phase shifter with a response time on the order of one second. Such
a slow phase shifter may be useable for the purpose of fine tuning
the optical path length l to account for, e.g. a manufacturing or
operating temperature variation of the optical path length l. The
f.sub.r of each resonator may be tuned to be about equal to one of
the WDM channel frequencies f.sub.1 . . . f.sub.M, e.g., by setting
such slow phase shifters appropriately. The controllable phase
shifter 430 may be an electro-optic phase shifter that is useable
for the purpose of rapidly modulating the optical path length
between one of two predetermined values. For example, the
controller 450 may modulate the optical path length l at a rate
between one or a few GHz and tens of GHz to provide for data
modulation of an optical carrier. As described further below the
modulation may cause the ring resonator 410 to rapidly switch
between a resonant frequency of f.sub.n-.delta. and
f.sub.n+.delta., where n=1, 2, 3, . . . m to thereby impart data on
an optical carrier signal having one of the channel frequencies
f.sub.1, f.sub.2, f.sub.3, . . . f.sub.M wherein the carrier signal
is propagating in an adjacent and optically coupled or couplable
waveguide.
[0038] Referring back to FIG. 1, each of the ring resonators 150-1,
150-2, 150-3 . . . 150-M includes a corresponding control line
155-1, 155-2, 155-3, . . . 155-M. Similarly each of the ring
resonators 160-1, 160-2, 160-3 . . . 160-M includes a corresponding
control line 165-1, 165-2, 165-3, . . . 165-M. Each of the control
lines 155 and 165 may be configured to provide a modulation signal
to the corresponding ring resonator as described with respect to
the control line 460 in FIG. 4.
[0039] FIGS. 5A-5E illustrate without limitation cross sections of
several examples of ring resonators formed from a semiconductor,
e.g. silicon, and configured to have adjustable resonant
frequencies. While the embodiments of FIGS. 5A-5E are presented as
examples of suitable structures for enabling variable control of
the resonant frequency of the ring resonators 150 and 160,
embodiments of the invention are not limited to any particular type
of resonant frequency control, which may be implemented by any
conventional or future-discovered method.
[0040] FIGS. 5A and 5B illustrate ring resonators whose resonant
frequencies are controllable by electro-optic modulation. These
structures may provide high frequency switching, and thus may be
suitable for the controllable phase modulator 430. The ring
resonator in FIG. 5A includes an optical core region that has
n-doped and p-doped portions that form a p-n junction, e.g. a p-n
diode. Heavily doped n.sup.+ and p.sup.+ regions provide electrical
contact to the core region. The core regions are constructed so
that the refractive index of the semiconductor core is dependent on
the electron concentration. The electron concentration may be
modulated by applying a variable back-bias on the p-n junction. By
changing the refractive index the optical path length of the ring
resonator, the ring resonator's resonant frequency is changed. The
ring resonator in FIG. 5B includes an optical core region formed
from an intrinsic semiconductor. Doped n.sup.+ and p.sup.+ regions
provide electrical contact to the intrinsic region and form a p-i-n
diode. As described with respect to FIG. 5A, the resonant frequency
of the ring resonator of FIG. 5B may also be modulated by varying
the electron density of the intrinsic region through the modulation
of a back-bias across the p-i-n diode.
[0041] FIGS. 5C-5E illustrate in sectional view embodiments of ring
resonators in which the resonant frequencies may be changed by
heating the ring-like waveguide core therein. These structures
provide relatively slow optical path length adjustments, and thus
may be suitable for the controllable phase shifter 420. In FIG. 5C
the core region of the ring resonator's waveguide is formed from a
p-type semiconductor, and heavily doped p.sup.+ regions provide
electrical contact to the core region. FIG. 5D illustrates a
similar embodiment in which the core region is formed from an
n-type semiconductor and heavily doped n.sup.+ regions provide
electrical contact to the core. In each of these embodiments the
waveguide of the ring resonator, or a segment thereof, may be
heated by passing current through the core region via the heavily
doped regions. In response, resistive heating will warm the ring
resonator, which changes the refractive index thereof by the
thermo-optic effect, thereby changing the resonant frequency. FIG.
5E shows another embodiment that relies on heating, but for this
embodiment, the heating is provided by a resistive heater element
510 formed over a cladding layer 520. Those skilled in the art are
familiar with forming resistive heater elements.
[0042] Now considering FIGS. 6A and 6B, illustrated are amplitude
and phase characteristics of the transfer function of a waveguide,
e.g. the waveguide 130, which is undercoupled to a ring resonator,
e.g. the ring resonator 150-1. The following discussion is
presented with reference to the ring resonator 150-1. Based on this
description, it will be immediately apparent to those skilled in
the art that the described principles also may be applied to the
other ring resonators. The ring resonator 150-1 is configured to
switch between a resonant frequency of f.sub.1-.delta. and
f.sub.1+.delta. in response to application of control signals
thereto. For the ring resonator 150-1, an amplitude characteristic
610 and a phase characteristic 620 are associated with the lower
resonant frequency at f.sub.1-.delta. (i.e., indicated as f.sub.-),
and a different amplitude characteristic 630 and a different phase
characteristic 640 are associated with the higher resonant
frequency at f.sub.1+.delta. (i.e., indicated as f.sub.+).
[0043] Considering first the case in which the resonant frequency
of the ring resonator is f.sub.1-.delta., when the optical signal
portion 120a having a frequency of f.sub.1 propagates within the
waveguide 130, the signal is attenuated by the amplitude
characteristic 610 to a value 650. The frequency of the optical
signal is considered to correspond to the resonant frequency of the
ring resonator by virtue of being about equal to the frequency at
which the amplitude characteristics 610 and 630 intersect, e.g.
f.sub.1. The phase of the signal is shifted by the phase
characteristic 620 to a value 660 indicated as .phi..sub.-. Now
when the resonant frequency of the ring resonator is
f.sub.1+.delta., the signal is again attenuated by the amplitude
characteristic 630 to about the same value 650. However, the phase
of the signal is shifted to a value 670 indicated as .phi..sub.+ by
the phase characteristic 640. The size of .delta. can be set such
that the relative phase shift .phi..sub.--.phi..sub.+ is about .pi.
radians. Thus the propagating signal may be BPSK modulated by
controllably switching the resonant frequency of the ring resonator
between f.sub.1-.delta. and f.sub.1+.delta..
[0044] FIGS. 7A and 7B illustrate amplitude and phase
characteristics for an example case in which a waveguide, e.g. the
waveguide 130, is overcoupled to a controllable ring resonator,
e.g. the ring resonator 150-1 during two different modulation
states thereof. An amplitude characteristic 710 and a phase
characteristic 720 respectively describe the amplitude reduction
and phase shift produced by the coupling between the ring resonator
and the waveguide when the resonant frequency f.sub.r of the ring
resonator 150-1 is f.sub.1-.delta. (i.e., indicated as f.sub.-). An
amplitude characteristic 730 and a phase characteristic 740
respectively describe the amplitude reduction and phase shift
produced by the coupling between the ring resonator and the
waveguide when f.sub.r=f.sub.1+.delta. (i.e., indicated as
f.sub.+). For both the amplitude characteristics 710 and 730 the
propagated signal amplitude is reduced to a value 750 at the
carrier frequency f.sub.1. For the case that
f.sub.r=f.sub.1-.delta., the phase shift of an optical carrier, at
the frequency f.sub.1, is shown by the phase characteristic 720 as
being .phi..sub.- at reference 760. For the case that
f.sub.r=f.sub.1+.delta., the phase shift of an optical carrier, at
the frequency f.sub.1, is shown by the phase characteristic 740 as
.phi..sub.+ at reference 770. As described previously, the value of
.delta. may be selected such that the relative phase shift
.phi..sub.--.phi..sub.+ for the two different modulation states is
about .pi. radians. Thus, the optical carrier signal at carrier
frequency f.sub.1 and propagating in the waveguide 130 may be BPSK
modulated by such an operation on an over coupled ring
resonator.
[0045] FIGS. 8A and 8B respectively schematically illustrate
attenuation and phase characteristics of a waveguide, e.g. the
waveguide 130, when overcoupled to a plurality of ring resonators,
e.g. the ring resonators 150-1, 150-2, 150-3, . . . 150-M. The
waveguide may transmit optical carriers propagating at a plurality
of frequencies, e.g. f.sub.1, f.sub.2, f.sub.3, . . . f.sub.M, as
produced by the optical source 125, e.g., a WDM multi-channel
optical source or a wavelength tunable optical source. By
appropriately switching the modulation states of the ring
resonators 150, any signal component of the optical carrier portion
120a at f.sub.1, f.sub.2, f.sub.3, . . . f.sub.M may be
independently BPSK modulated with a series of ring resonators whose
coupling characteristics to the waveguide 130 are as shown in FIGS.
8A-8B. Similarly, by appropriately modulating the states of the
series of ring resonators 160 any signal component of the carrier
portion 120b at f.sub.1, f.sub.2, f.sub.3, . . . f.sub.M may be
BPSK modulated.
[0046] Recalling FIG. 1, the phase shifter 180 in line with or
coupled to the waveguide 140 (FIG. 1) may apply about a relative
.pi./2 phase shift to the carrier portion 120b. Then, when the
combiner 170 recombines the carrier portions 120a and 120b, a QPSK
modulated output signal 199 will result such that the waveguide 130
and the waveguide 140 produce the respective in-phase and
quadrature components of the QPSK modulated output signal 199.
Because the ring resonators 150 and 160 may independently modulate
multiple wavelength components of the carrier signal 120, the
apparatus 100 provides the ability to perform WDM QPSK.
Implementations of the apparatus 100 may be very compact, e.g.,
integrated optical devices, thereby providing, in some embodiments,
small and low-cost WDM QPSK optical modulators.
[0047] FIGS. 9A and 9B illustrate two representative and
nonlimiting embodiments of electrical modules for mapping input
data to the control lines 155-1, 155-2, 155-3, . . . 155-M and
165-1, 165-2, 165-3, . . . 165-M as shown in FIG. 1. FIG. 9A
illustrates an electrical switching module 910. The switching
module 910 is configured to receive data streams at inputs 920-1,
920-2, 920-3 . . . 920-M for corresponding ones of the control
lines 155-1, 155-2, 155-3, . . . 155-M. The switching module 910 is
further configured to receive data at inputs 930-1, 930-2, 930-3 .
. . 930-M for corresponding ones of the control lines 165-1, 165-2,
165-3, . . . 165-M. The electrical switching module 910 is
controllable to permute the mapping data streams at the inputs
920-1-920-M and 930-1-930-M onto the set of control lines
155-1-155-M and 165-1-165-M. For that reason, the switching module
910 enables different mappings of received WDM data streams onto
the channels modulated by the apparatus 100 of FIG. 1. FIG. 9B
illustrates an electrical power-splitter module 940 configured to
receive a single electrical modulation signal at input 950 and to
power split the signal to produce therefrom individual electrical
control signals for each of the control lines 155-1, 155-2, 155-3,
. . . 155-M, and each of the control lines 165-1, 165-2, 165-3, . .
. 165-M.
[0048] With the switching module 910, each of the ring resonators
150 and 160 may be treated as a modulator for a single frequency
channel that may be modulated independently of the other frequency
channels of a WDM optical carrier. The module 910 enables
rearrangements of the separate data streams over the set of optical
modulation channels. With the module 940, a data stream received
via the input 950 may be power-divided between the control lines
155 and 165 to modulate the ring resonators 150 and 160 in a
coordinated fashion to transmit the received data stream. Those
skilled in the art will appreciate that the ring resonators 150 and
160 may be operated by modules such as the modules 910, 940 and
variants thereof to provide various combinations of independent and
coordinated modulation to transmit data. Each of the modules 910
and 940 may include any combination of electronic components as
needed to implement the desired mapping of the received data to the
control outputs. The type of electrical output may correspond to
the type of control signal appropriate to the modulation structure
of the ring resonators 150, 160, e.g. as illustrated in FIG. 4.
[0049] Turning now to FIG. 10, a method 1000 is described, e.g. for
forming an optical device according to various embodiments. The
steps of the method 1000 are described without limitation by
reference to elements previously described herein, e.g. in FIGS.
1-9. The steps of the method 1000 may be performed in another order
than the illustrated order, and in some embodiments may be omitted
altogether and/or performed concurrently or in parallel groups. The
method 1000 is illustrated without limitation with the steps
thereof being performed in parallel fashion, such as by concurrent
processing on a common substrate. Other embodiments, e.g. those
utilizing multiple substrates, may perform the steps partially or
completely sequentially and in any order.
[0050] The method 1000 begins with an entry 1001. In a step 1010 an
optical waveguide, e.g. the waveguide 130, and a plurality of
optical resonators, e.g. the resonators 150, are formed along a
surface of a substrate. The forming is performed such that each of
the resonators is adjacent to segments of and optically coupled to
the optical waveguide. Each of the resonators is configured to
resonate at a different optical frequency.
[0051] Some embodiments of the method 100 include a step 1020 in
which a second optical waveguide is formed, e.g. the waveguide 140,
and a second plurality of optical resonators is formed, e.g. the
resonators 160, along the surface. The resonators of the second
plurality are adjacent to segments of and optically coupled to the
second optical waveguide. Each resonator of the second plurality is
configured to resonate at about an optical resonant frequency of a
corresponding one of the resonators of the first plurality.
[0052] Some embodiments of the method 1000 include a step 1030 in
which controllers are formed, e.g. the controllers 440 and 450. The
controllers are capable of varying the resonant frequencies of the
resonators. Some embodiments of the method 1000 include a step 1040
in which an optical source, e.g. the optical source 125, is
end-coupled to the optical waveguide. The optical source is
configured to output an optical signal including frequency
components corresponding to different resonant frequencies of a
plurality of the resonators.
[0053] In any embodiment of the method 1000 the forming may include
making an optical power splitter, e.g. the splitter 110. The
splitter has first and second outputs connected to ends of
corresponding ones of the optical waveguides. In any such
embodiment the first and second pluralities of optical resonators
may be operable to WDM QPSK modulate an optical carrier received by
the optical power splitter.
[0054] In any embodiment of the method 1000, the forming may
include making an optical combiner, e.g. the combiner 170. The
combiner has first and second inputs connected to ends of
corresponding ones of the optical waveguides. In any embodiment,
some of the resonators may be overcoupled to the optical waveguide.
In any embodiment optical core regions of the optical waveguide and
the resonators may be formed silicon regions located over a
dielectric layer. In any embodiment each resonator may include a
first optical phase shifter configured to enable quasi-static
optical path adjustments of the each resonator, and may include a
second optical phase shifter capable of varying optical pathlengths
at frequencies of 1 GHz or more.
[0055] Those skilled in the art to which this application relates
will appreciate that other and further additions, deletions,
substitutions and modifications may be made to the described
embodiments.
* * * * *