U.S. patent application number 13/385780 was filed with the patent office on 2013-09-05 for high speed optical transmitter producing modulated light signals.
The applicant listed for this patent is Mehdi Asghari, Dazeng Feng, Shashank Jatar. Invention is credited to Mehdi Asghari, Dazeng Feng, Shashank Jatar.
Application Number | 20130230267 13/385780 |
Document ID | / |
Family ID | 49034719 |
Filed Date | 2013-09-05 |
United States Patent
Application |
20130230267 |
Kind Code |
A1 |
Feng; Dazeng ; et
al. |
September 5, 2013 |
High speed optical transmitter producing modulated light
signals
Abstract
An optical system includes modulators positioned on a base. Each
modulator includes a modulator waveguide that receives a light
signal and guides the received light signal through the modulator.
The system also includes drive electronics in electrical
communication with the modulators. The drive electronics apply
electrical energy to each of the modulators such that an electrical
field is generated within the modulator waveguide so as to modulate
one of the light signals into a modulated signal. The system
includes multiple drive paths that each has a length from a contact
pad on the drive electronics to a location where the electrical
field is formed in one of the modulator waveguides. The modulators
are configured such that the drive path length for each of the
modulators is less than 0.5 mm.
Inventors: |
Feng; Dazeng; (El Monte,
CA) ; Jatar; Shashank; (Pasadena, CA) ;
Asghari; Mehdi; (Pasadena, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Feng; Dazeng
Jatar; Shashank
Asghari; Mehdi |
El Monte
Pasadena
Pasadena |
CA
CA
CA |
US
US
US |
|
|
Family ID: |
49034719 |
Appl. No.: |
13/385780 |
Filed: |
March 5, 2012 |
Current U.S.
Class: |
385/2 |
Current CPC
Class: |
G02F 1/0121 20130101;
G02F 2001/0157 20130101; G02F 2202/10 20130101; H04B 10/505
20130101; G02F 1/025 20130101; G02F 2201/063 20130101 |
Class at
Publication: |
385/2 |
International
Class: |
G02F 1/035 20060101
G02F001/035 |
Claims
1. An optical system, comprising: a transmitter having waveguides
defined in a layer of light-transmitting medium positioned on a
base, the waveguide being immobilized relative to the base along
the length of the waveguide; a portion of the waveguides being
transition waveguides that each guides a different transition light
signal; modulators positioned on the base, each modulator including
a modulator waveguide that receives one of the transition light
signals and guides the received transition light signal through the
modulator, drive electronics in electrical communication with the
modulators and configured to apply electrical energy to each of the
modulators such that an electrical field is generated within the
modulator waveguide, each of the electrical fields being generated
so as to modulate one of the transition light signals into a
modulated signal, a drive path length being a length of an
electrical path from a contact pad on the drive electronics to a
location where the electrical field is formed in one of the
modulator waveguides, the drive path lengths for each of the
modulators being less than 1.0 mm.
2. The system of claim 1, wherein the modulators are arranged such
that the distance between a lateral edge of the transmitter and the
furthest point of each modulator waveguide is less than 0.5 mm.
3. The system of claim 1, wherein each of the drive paths includes
a wire that provides electrical communication between one of the
pads on the drive electronics and a contact pad included in one of
the modulator.
4. The system of claim 1, wherein the drive electronics are
configured to modulate each of the transition light signal at a
rate of at least 25 GHz.
5. The system of claim 1, wherein the modulators are arranged such
that a distance from a location on one of the modulators to the
same location on the next modulator is less than 2 mm.
6. The system of claim 1, wherein the modulators are lined up on
the base such that a line that is parallel to a direction of
propagation of each modulator waveguide can concurrently extend
through each of the modulators.
7. The system of claim 1, wherein the modulators are arranged such
that the distance between a lateral edge of the transmitter and the
furthest point of each modulator waveguide is less than 1 mm, each
of the drive paths includes a wire that provides electrical
communication between one of the pads on the drive electronics and
a contact pad included in one of the modulator. the drive
electronics are configured to modulate each of the transition light
signal at a rate of at least 25 GHz, the modulators are lined up on
the base such that a line that is parallel to a direction of
propagation of each modulator waveguide can concurrently extend
through each of the modulator waveguides, and a distance from a
location on one of the modulators to the same location on the next
modulator is less than 2 mm.
8. The system of claim 1, wherein the drive electronics are a
separate component from the transmitter.
9. The system of claim 1, wherein the modulator waveguides are each
partially defined by a ridge of an electro-absorption medium
extending upwards from slab regions of the electro-absorption
medium located on opposing sides of the ridge, a doped region
extending into a lateral side of the ridge of the
electro-absorption medium and also extending into one of the slab
regions of the ridge of the electro-absorption medium.
10. The system of claim 9, wherein the ridge of electro-absorption
medium is positioned on a portion of the layer of
light-transmitting medium located between the ridge and the
base.
11. The system of claim 10, wherein the doped region extends into
the layer of light-transmitting medium.
12. The system of claim 1, wherein an edge of the drive electronics
is substantially parallel to an edge of the transmitter.
13. The system of claim 3, wherein the edge of the drive
electronics contacts the edge of the drive electronics.
14. The system of claim 1, wherein the layer of light-transmitting
medium is not positioned between the drive electronics and the
base.
15. The system of claim 1, wherein the drive path lengths are less
than 0.5 mm.
16. The system of claim 1, wherein one or more transition
waveguides pass between a portion of the modulators and the edge of
the transmitter closest to the portion of the modulators.
17. The system of claim 1, wherein the light-transmitting medium is
silicon.
18. The system of claim 1, wherein the modulators are arranged such
that a distance between a lateral edge of the transmitter and a
furthest point of each modulator waveguide is the same.
Description
FIELD
[0001] The present invention relates to optical devices and
particularly, to optical transmitters.
BACKGROUND
[0002] Optical systems are increasingly being used for a variety of
applications such as communications and communications between
electrical devices such as servers. These networks make use of
transmitters that generate the light signals at one of the
electrical devices. In some instances, these transmitters modulate
the light signals at high speeds on the order of 25 GHz. As the use
of these transmitters has increased, it has become desirable to
increase the number of light signals produced by a single device.
Increasing the number of light signals produced by a single
transmitter can increase the distance between different features of
the transmitter. This increased distance can slow down the possible
modulation speed of the light signals and increase the size of the
device. As a result, there is a need for a compact transmitter that
can generate multiple light signals that are each modulated at high
speed.
SUMMARY
[0003] An optical system includes a transmitter having waveguides
defined in a layer of a light-transmitting medium positioned on a
base. A portion of the waveguides are transition waveguides that
each guides a different transition light signal. The transmitter
also includes modulators positioned on the base. Each modulator
includes a modulator waveguide that receives one of the transition
light signals and guides the received transition light signal
through the modulator. The system also includes drive electronics
in electrical communication with the modulators. The drive
electronics apply electrical energy to each of the modulators such
that an electrical field is generated within the modulator
waveguide. Each electrical field is generated so as to modulate one
of the transition light signals into a modulated signal. The system
includes multiple drive paths. A drive path length is the length of
an electrical path from a contact pad on the drive electronics to a
location where the electrical field is formed in one of the
modulator waveguides. The modulators are constructed and arranged
on the transmitter such that the drive path length for each of the
modulators is less than 0.5 mm.
BRIEF DESCRIPTION OF THE FIGURES
[0004] FIG. 1A is a schematic of a system that includes an optical
transmitter and a receiver.
[0005] FIG. 1B is a schematic of another embodiment of a system
that includes an optical transmitter and a receiver.
[0006] FIG. 2A through FIG. 2C illustrate a transmitter that is
suitable for use as a transmitter in a system such as the system of
FIG. 1A. FIG. 2A is a topview of the transmitter.
[0007] FIG. 2B is a sideview of the transmitter taken looking in
the direction of the arrow labeled B in FIG. 2A.
[0008] FIG. 2C is a cross-section of the transmitter taken along
the line labeled C in FIG. 2A.
[0009] FIG. 3A through FIG. 3C are each a cross section of a
modulator that is suitable for use as the transmitters of FIG. 1A
through FIG. 2C.
DESCRIPTION
[0010] A transmitter includes a multiple modulators positioned
along an edge of the transmitter. Each of the modulators is
configured to guide a different light signal through a modulator
waveguide. The transmitter is used in conjunction with off board
modulator driver electronics that are configured to generate an
electrical field in each of the modulator waveguides. The modulator
driver electronics generate the electrical fields such that the
different light signals are each modulated at a rate of at least 25
GHz. In order to efficiently operate modulators at these speeds,
the electrical path from contact pads on the modulator driver
electronics to the location where the electrical field in the
modulator waveguides must have a length of less than 0.5 mm. When
prior modulator structures are combined with large numbers of
modulators and off board modulator driver electronics, these drive
path lengths become very difficult to achieve. The structure of the
modulators in the disclosed transmitter combined with the
arrangement of the modulators on the transmitter allow these drive
path lengths to be achieved even when a large number of modulators
are present on a single transmitter.
[0011] FIG. 1A is a schematic of a system that includes an optical
transmitter and a receiver. The transmitter includes multiple
waveguides, one or more lasers 10, a splitter 12, and modulators
positioned on a common platform 14. The waveguides include one or
more source waveguides 16, transition waveguides 18, and output
waveguides 20.
[0012] The system also includes laser driver electronics 22.
Although not shown in FIG. 1A, the laser driver electronics 22 are
in electrical communication with the one or more lasers 10 and are
configured to operate the one or more lasers 10 such that each of
the lasers 10 generates and outputs a light signal. The system also
includes modulator driver electronics 24. The modulator driver
electronics 24 are in electrical communication with each of the
modulators and are configured to operate the modulators such that
the modulators modulate the intensity of a light signal being
guided through the modulator. Although FIG. 1A shows the laser
driver electronics 22 and modulator driver electronics 24 as being
separate components, the laser driver electronics 22 and modulator
driver electronics 24 can be included on the same component.
[0013] During operation of the system, the laser driver electronics
22 operate the one or more lasers 10 such that they each generate a
light signal. The generated light signal(s) are each received at
one of the source waveguides 16. The source waveguides 16 carry the
light signal(s) to the splitter 12. The splitter 12 splits the
received light signal(s) into multiple transition light signals.
Each of the transition light signals is received on a different one
of the transition waveguides 18. Each of the transition waveguide
18 guides the received transition light signal to a different one
of the modulators. The modulators each include a modulator
waveguide 25 that guides the received transition light signal
through the modulator. The modulator driver electronics 24 are
configured to operate each of the modulator such that the
transition light signal being guided through the modulator
waveguide 25 is modulated into a modulated light signal. The output
waveguides 20 each receives one of the modulated light signals and
guides the received modulated light signals to a facet 28.
[0014] The system also includes multiple optical fibers 26 and a
receiver. Each of the optical fibers 26 is aligned with a facet 28
on the transmitter so as to receive a modulated light signal from
the transmitter. Each of the optical fibers 26 guides the received
modulated light signal to the receiver.
[0015] The receiver includes sensor waveguides 30 positioned on a
common platform 32. The sensor waveguides 30 are each aligned with
one of the optical fibers 26 such that each of the sensor
waveguides receives one of the modulated light signals from one of
the optical fibers 26. The receiver also includes light sensors 34
positioned on the common platform 32. Each of the sensor waveguides
30 guides the received modulated light signal to one of the light
sensors 34. The light sensors 34 are configured to convert the
received modulated light signal to an electrical signal that is
further processed by electronics (not shown) in electrical
communication with the receiver.
[0016] Although FIG. 1A shows each of the optical fibers 26 routing
a modulated light signal to a common receiver, the optical fibers
26 can route different modulated light signals to different
receivers in different locations. As a result, the transmitter can
be used to transmit modulated light signal and/or data to different
locations.
[0017] The transmitter can include more than one laser 10. For
instance, FIG. 1B is a schematic of a system that includes an
optical transmitter having multiple lasers 10. The laser driver
electronics 22 operate the one or more lasers 10 such that they
each generate a light signal. Different light signals can be at the
same wavelength or different light signals. The generated light
signal(s) are each received at a transition waveguide 18. Each of
the transition waveguides 18 guides the received transition light
signal to a different one of the modulators. The modulators each
include a modulator waveguide 25 that guides the received
transition light signal through the modulator. The modulator driver
electronics 24 are configured to operate each of the modulator such
that the transition light signal being guided through the modulator
waveguide 25 is modulated into a modulated light signal. The output
waveguides 20 each receives one of the modulated light signals and
guides the received modulated light signals to a facet 28.
[0018] The system also includes multiple optical fibers 26 and a
receiver. Each of the optical fibers 26 is aligned with a facet 28
on the transmitter so as to receive a modulated light signal from
the transmitter. Each of the optical fibers 26 guides the received
modulated light signal to the receiver.
[0019] The receiver includes sensor waveguides 30 positioned on a
common platform 32. The sensor waveguides 30 are each aligned with
one of the optical fibers 26 such that each of the sensor
waveguides receives one of the modulated light signals from one of
the optical fibers 26. The receiver also includes light sensors 34
positioned on the common platform 32. Each of the sensor waveguides
30 guides the received modulated light signal to one of the light
sensors 34. The light sensors 34 are configured to convert the
received modulated light signal to an electrical signal that is
further processed by electronics (not shown) in electrical
communication with the receiver.
[0020] Although FIG. 1B shows each of the optical fibers 26 routing
a modulated light signal to a common receiver, the optical fibers
26 can route different modulated light signals to different
receivers in different locations. As a result, the transmitter can
be used to transmit modulated light signal and/or data to different
locations.
[0021] FIG. 2A through FIG. 2C illustrate a transmitter that is
suitable for use as a transmitter in a system such as the system of
FIG. 1A. FIG. 2A is a topview of the transmitter. FIG. 2B is a
sideview of the transmitter taken looking in the direction of the
arrow labeled B in FIG. 2A. FIG. 2C is a cross-section of the
transmitter taken along the line labeled C in FIG. 2A. The
transmitter is within the class of optical devices known as planar
optical devices. These devices typically include one or more
waveguides immobilized relative to a substrate or a base. The
direction of propagation of light signals along the waveguides is
generally parallel to a plane of the device. Examples of the plane
of the device include the top side of the base, the bottom side of
the base, the top side of a substrate included in the base, and/or
the bottom side of the substrate.
[0022] A suitable platform for building a transmitter according to
FIG. 2A through FIG. 2C includes a light-transmitting medium 40
positioned on a base 42. The waveguides guide the different light
signals through the light-transmitting medium 40. FIG. 2C is a
cross-section of a source waveguide 16; however, the transition
waveguides 18 and output waveguides 20 can also be constructed as
shown in FIG. 2C. The light-transmitting medium 40 includes a ridge
44 defined by trenches 46 that extend into the light-transmitting
medium 40 on opposing sides of the ridge 44. In FIG. 2A, only the
portion of the trench 46 adjacent to the laser 10 is shown for the
purposes of simplifying the illustration. The ridge 44 defines an
upper portion of the waveguide. Accordingly, the waveguides include
a ridge 44 of the light-transmitting medium 40 extending upward
from slab regions of the light-transmitting medium 40 located on
opposing sides of the ridge 44.
[0023] The portion of the base 42 adjacent to the
light-transmitting medium 40 is configured to reflect light signals
being guided in the ridge 44 back into the ridge 44 in order to
constrain light signals in the waveguide. For instance, the portion
of the base 42 adjacent to the first light-transmitting medium 40
can be an optical insulator 48 with a lower index of refraction
than the light-transmitting medium 40. The drop in the index of
refraction can cause reflection of a light signal from the
light-transmitting medium 40 back into the light-transmitting
medium 40. The base 42 can include the optical insulator 48
positioned on a substrate 50.
[0024] In one example, the platform is a silicon-on-insulator
wafer. A silicon-on-insulator wafer includes a silicon layer
positioned on a base 42. The layer of silicon serves as the
light-transmitting medium 40. The base 42 of the
silicon-on-insulator wafer also includes a layer of silica
positioned on a silicon substrate. The layer of silica serves as
the optical insulator 48 while the silicon substrate serve as a
substrate 50 for the base 42.
[0025] The transmitter includes a laser chip 52. The illustrated
laser chip 52 includes a single laser 10 although it is possible to
build the transmitter that makes use of multiple lasers. Suitable
lasers 10 include Fabry-Perot lasers. The laser 10 includes a ridge
54 that extends upwards from a platform and at least partially
defines a laser waveguide on the laser chip 52. The laser chip 52
is positioned in a recess 56 that extends into at least the
light-transmitting medium 40. In some instances, the recess 56
extends into the base 42. The laser chip 52 is inverted in that the
ridge 54 defining the laser waveguide is positioned between the
platform of the laser chip 52 and the base 42 of the transmitter.
Accordingly, the location of the ridge 54 is shown by dashed lines
in FIG. 2A. Laser driver electronics 22 (not show) are in
electrical communication with the laser 10 and are configured to
operate the laser 10 such that the laser 10 generates and outputs a
light signal.
[0026] The laser chip 52 is placed in the recess 56 such that the
laser 10 is aligned with the source waveguide 16. As a result,
during operation of the transmitter, the source waveguide 16
receives the light signal output by the laser 10. Suitable methods,
structures, and configurations for mounting a laser chip 52 on a
silicon-on-insulator 48 wafer with the proper alignment are
disclosed in U.S. patent application Ser. No. 08/853,104, filed on
May 8, 1997, entitled "Assembly of an Optical Component and an
Optical Waveguide, now issued as U.S. Pat. No. 5,881,190, and also
in U.S. patent application Ser. No. 12/215,693, filed on Jun. 28,
2008, entitled "Interface Between Light Source and Optical
Component," each of which is incorporated herein in its entirety.
The method of fabrication, operation, and mounting disclosed in
U.S. patent application Ser. No. 08/853,104 and/or Ser. No.
12/215,693 can be use din conjunction with the transmitter of FIG.
1A through FIG. 2C.
[0027] The splitter 12 need not be a wavelength dependent splitter
12. For instance, FIG. 2A shows the source waveguide 16 guiding the
light signal to a series of y-junctions that serve as the splitter
12. Y-junctions are example of splitters 12 that do not split up an
incoming light signal into transition light signals that each has a
different selection of wavelengths. Other examples of suitable
wavelength independent splitters 12 include, but are not limited
to, Multimode Interference couplers (MMIs), and directional
couplers. In some instances, the splitter 12 is a wavelength
dependent splitter 12 that splits the incoming source signal into
transition light signals that each has a different selection of
wavelengths. Examples of suitable wavelength dependent splitters 12
include, but are not limited to, arrayed waveguide gratings,
echelle gratings, and bragg gratings.
[0028] The transmitter includes modulators positioned along an edge
of the transmitter. The transition waveguides 18 each guides one of
the transition light signals to a different one of the modulators.
In order to simplify FIG. 2A, the details of the modulator
construction are not shown in FIG. 2A. However, the modulator
construction is evident from other illustrations such as FIG. 3A
through FIG. 3C. The modulators each include a modulator waveguide
25 configured to guide the received transition light signal through
an electro-absorption medium 61. For instance, a ridge 60 of the
electro-absorption medium 61 can extend upward from slab regions 62
of the electro-absorption medium 61. Accordingly, the modulator
waveguides 25 are each partially defined by the top and lateral
sides of the ridge 60 of the electro-absorption medium 61.
[0029] The modulators each include a first contact pad 63 and a
second contact pad 64 for providing electrical communication
between the modulator and the modulator driver electronics 24. The
modulator driver electronics 24 can be "off board" as shown in FIG.
2A. For instance, the modulator driver electronics 24 can be
included on a component that is in addition to and/or separate from
the transmitter and the additional component can be positioned
adjacent to the transmitter as shown in FIG. 2A. This "off board"
arrangement is in contrast to the "on board" arrangements where the
modulator driver electronics 24 are integrated directly onto the
transmitter or are included on a flip chip bonded on the top of the
transmitter.
[0030] The modulator driver electronics 24 can also include drive
pads 66. Suitable drive pads 66 include contact pads. An electrical
conductor such as a wire 68 can provide electrical communication
between the first contact pad 63 and the second contact pad 64 of a
modulator and the drive pads 66. An electrical conductor such as a
wire 68 can be connected to the first contact pad 63 and the second
contact pad 64 of a modulator and the drive pads 66 using
technologies such as wire bonding.
[0031] The modulator driver electronics 24 are configured to apply
electrical energy to the drive pads 66 such that an electrical
field is formed in the modulator waveguide 25. The modulator driver
electronics 24 vary the electrical field so as to modulate the
transition light signal traveling through the modulator waveguide
25. This modulation of the transition light signal results in the
generation of a modulated light signal that exits from the
modulator. The modulator driver electronics 24 can modulate each of
the transition light signals such that different modulated light
signal are the same or different. For instance, different modulated
light signals can be modulated at different frequencies or at the
same frequency. Further, different modulated light signals can be
modulated to include different data or the same data.
[0032] The modulated light signals are each received by one of the
output waveguides 20. The output waveguides 20 each guides one of
the modulated light signals to a facet 28 through which the
modulated light signal can exit the transmitter.
[0033] The length of the electrical path from the drive pads 66 to
the location where the electrical field is formed in the modulator
waveguide 25 (the drive path length) affects the speed at which the
modulators are able to modulate the modulated light signal. For
instance, increasing the drive path length for the first contact
pad 63 and/or the second contact pad 64 associated with a single
modulator reduces the modulation speeds that are possible for a
given power level. It is generally desirable to modulate the
modulated light signal in the RF range (frequency in a range of 3
kHz to 400 GHz). For communications applications, it is generally
desirable to modulate the modulated light signal in a range of 100
MHz to 400 GHz. However, the Applicant has found that in order to
effectively modulate a modulated light signal at a rate of 25 GHz,
the drive path length for the first contact pad 63 and the second
contact pad 64 both need to be less than 1 mm.
[0034] It becomes more difficult to achieve the require drive path
lengths as the number of modulators on the transmitter increases.
For instance, increasing the number of modulators can be achieved
by staggering the locations of the modulators on the transmitter.
Staggering the locations of the modulators means the drive path
lengths will be different for different modulators and that the
drive path length can become undesirably large as the number of
modulators increases. The arrangement of FIG. 2B overcomes these
challenges. The modulators are positioned along an edge of the
transmitter in order to reduce the distance between the modulators
and the modulator driver electronics 24. This reduced distance
shortens the drive path lengths. In some instances, the modulators
are arranged such that the distance between the edge of the
transmitter and the furthest point of each modulator waveguide 25
(labeled D in FIG. 2A) is less than 1 mm, 0.5 mm, or 0.25 mm. In
cases where the modulator waveguides 25 are partially defined by a
ridge 60 extending upward from slab regions 62, the furthest point
of each modulator waveguide 25 is the furthest portion of the ridge
60 from the edge of the transmitter.
[0035] The modulators are lined up along the edge of the
transmitter. For instance, the modulator waveguides 25 are arranged
so the direction of light signal propagation through each modulator
waveguide 25 is parallel to a common line (labeled C in FIG. 2A).
Accordingly, the length of each modulator waveguide 25 is also
parallel to the common line. Additionally, the modulator waveguides
25 are arranged so that common line can concurrently pass through
each of the modulator waveguides 25. Further, the modulator
waveguides 25 are arranged so the common line can concurrently pass
through the same location in each of the modulator waveguides 25.
For instance, the common line can concurrently pass through the
left side of the ridge 60 that defines each of the modulator
waveguides 25 or the common line can concurrently pass through the
right side of the ridge 60 that defines each of the modulator
waveguides 25. Alternately, the common line can concurrently pass
through the center of the ridge 60 that defines each of the
modulator waveguides 25. In this arrangement, the modulator
waveguides 25 are optically aligned. For instance, the modulated
light signal that exited from the lowest modulator shown in FIG. 2A
would pass through each of the modulator waveguides 25 in the line
of modulators if there were not other components (other waveguides)
between those modulators that interfered with the path of the
modulated light signal.
[0036] When the modulators are lined up along the edge, the
distance between the first contact pad 63 and the edge of the
transmitter remains the same for each of the different modulators
and the distance between the second contact pad 64 and the edge of
the transmitter remains the same for each of the different
modulators as is evident from FIG. 2A. As a result, the drive path
lengths for the different modulators can be substantially constant.
For instance, the drive pads 66 on the modulator driver electronics
24 can be arranged such that each drive pads 66 is about the same
distance from an edge of the modulator driver electronics 24 as
shown in FIG. 2A. As a result, the drive path length for each of
the first contact pads 63 is about the same and the drive path
length for each of the first contact pads 63 is about the same.
[0037] The distance between the drive pads 66 and the edge of the
modulator driver electronics 24 can also affect the drive path
length as is evident from FIG. 2A. The distance between the drive
pads 66 and the edge of the modulator driver electronics 24
(labeled E in FIG. 2A) is typically less than 300 .mu.m, or 100
.mu.m, or 10 .mu.m. Although the modulator driver electronics 24
are "off board" and are a separate component from the transmitter,
the edge of the modulator driver electronics 24 can be
substantially parallel to the edge of the transmitter. In some
instances, the edge of the modulator driver electronics 24 contacts
the edge of the transmitter and is parallel to the edge of the
transmitter as is shown in FIG. 2A. The modulator driver
electronics 24 can optionally be immobilized relative to the
transmitter. For instance, the modulator driver electronics 24 can
optionally be epoxied to the transmitter
[0038] In FIG. 2A, a portion of the modulators include transition
waveguides 18 located between the modulator and the edge of the
transmitter that is closest to the modulators. These transition
waveguides 18 can alternately be positioned on the opposing side of
the modulators; however, these transition waveguides 18 would then
cross one or more output waveguides 20 before being connected to
the desired modulator. Since waveguide intersections are a source
of optical loss, the arrangement of FIG. 2A may be more
desirable.
[0039] Although the transmitter of FIG. 2A is shows with four
modulators, the transmitter can include other numbers of modulators
and the associated waveguides. In some instances, the transmitter
includes more than three modulators arranged as shown in FIG. 2A or
more than more than five modulators arranged as shown in FIG.
2A.
[0040] A schematic of the transmitter of FIG. 2A is in accordance
with FIG. 1A; however, the transmitter of FIG. 2A can be modified
to have a schematic in accordance with FIG. 1B by replacing the
single laser 10, source waveguide 16, and splitter 12 of FIG. 2A
with multiple different lasers.
[0041] Suitable modulators for satisfying the above size
limitations are Franz-Keldysh modulators. Accordingly, in some
instances the modulators shown in FIG. 2A are each a Franz-Keldysh
modulator. FIG. 3A is a cross section of a Franz-Keldysh modulator
that can serve as the modulators of FIG. 2A through FIG. 2C. As
will become evident from the following discussion, the modulator
includes multiple doped regions 72. In the cross section of FIG.
3A, the perimeter of portions of the doped regions 72 are
illustrated with dashed lines to prevent them from being confused
with interfaces between different materials. The interfaces between
different materials are illustrated with solid lines. The modulator
is configured to apply an electric field to the electro-absorption
medium 61 in order to intensity modulate the transition light
signals received by the modulator.
[0042] A ridge 60 of electro-absorption medium 61 extends upward
from a slab region 62 of the electro-absorption medium 61.
Accordingly, the modulator waveguide 25 is partially defined by the
top and lateral sides of the ridge 60 of electro-absorption medium
61. The slab regions 62 of the electro-absorption medium 61 and the
ridge 60 of the electro-absorption medium 61 are both positioned on
a seed portion 70 of the light-transmitting medium 40. As a result,
the seed portion 70 of the light-transmitting medium 40 is between
the electro-absorption medium 61 and the base 42. In some
instances, the seed portion 70 of the light-transmitting medium 40
is continuous with the portion of the light-transmitting medium 40
included in the transition waveguide 18 from which the modulator
receives the transition light signals. In these instances, when a
transition light signal travels from a transition waveguide 18 into
the electro-absorption medium 61, a portion of the transition light
signal enters the seed portion 70 of the light-transmitting medium
40 and another portion of the transition light signal enters the
electro-absorption medium 61. Accordingly, the seed portion 70 of
the light-transmitting medium 40 is included in the modulator
waveguide 25 in the sense that the modulator waveguide 25 extends
from the base 42 to the top of the ridge 60 of the
electro-absorption medium 61. During fabrication of the modulator,
the electro-absorption medium 61 can be grown on the seed portion
70 of the light-transmitting medium 40.
[0043] Doped regions 72 are both in the slab regions 62 of the
electro-absorption medium 61 and also in the ridge 60 of the
electro-absorption medium 61. For instance, doped regions 72 of the
electro-absorption medium 61 are positioned on the lateral sides of
the ridge 60 of the electro-absorption medium 61. In some
instances, each of the doped regions 72 extends up to the top side
of the electro-absorption medium 61 as shown in FIG. 3A.
Additionally, the doped regions 72 extend away from the ridge 60
into the slab region 62 of the electro-absorption medium 61. The
transition of a doped region 72 from the ridge 60 of the
electro-absorption medium 61 into the slab region 62 of the
electro-absorption medium 61 can be continuous and unbroken as
shown in FIG. 3A.
[0044] Each of the doped regions 72 can be an N-type doped region
72 or a P-type doped region 72. For instance, each of the N-type
doped regions 72 can include an N-type dopant and each of the
P-type doped regions 72 can include a P-type dopant. In some
instances, the electro-absorption medium 61 includes a doped region
72 that is an N-type doped region 72 and a doped region 72 that is
a P-type doped region 72. The separation between the doped regions
72 in the electro-absorption medium 61 results in the formation of
PIN (p-type region-insulator 48-n-type region) junction in the
modulator.
[0045] In the electro-absorption medium 61, suitable dopants for
N-type regions include, but are not limited to, phosphorus and/or
arsenic. Suitable dopants for P-type regions include, but are not
limited to, boron. The doped regions 72 are doped so as to be
electrically conducting. A suitable concentration for the P-type
dopant in a P-type doped region 72 includes, but is not limited to,
concentrations greater than 1.times.10.sup.15 cm.sup.-3,
1.times.10.sup.17 cm.sup.-3, or 1.times.10.sup.19 cm.sup.-3, and/or
less than 1.times.10.sup.17 cm.sup.-3, 1.times.10.sup.19 cm.sup.-3,
or 1.times.10.sup.21 cm.sup.-3. A suitable concentration for the
N-type dopant in an N-type doped region 72 includes, but is not
limited to, concentrations greater than 1.times.10.sup.15
cm.sup.-3, 1.times.10.sup.17 cm.sup.-3, or 1.times.10.sup.19
cm.sup.-3, and/or less than 1.times.10.sup.17 cm.sup.-3,
1.times.10.sup.19 cm.sup.-3, or 1.times.10.sup.21 cm.sup.-3.
[0046] The first contact pad 63 and the second contact pad 64 are
each positioned on the slab region 62 of the electro-absorption
medium 61. In particular, the first contact pad 63 and the second
contact pad 64 each contact a portion of a doped region 72 that is
in the slab region 62 of the electro-absorption medium 61.
Accordingly, the each of the doped regions 72 is doped at a
concentration that allows it to provide electrical communication
between an electrical conductor and one of the doped regions 72 in
the electro-absorption medium 61. As a result, the modulator driver
electronics 24 can apply electrical energy to the first contact pad
63 and the second contact pad 64 in order to apply the electric
field to the electro-absorption medium 61.
[0047] During operation of the modulators of FIG. 3A, the modulator
driver electronics 24 apply electrical energy to the first contact
pad 63 and the second contact pad 64 so as to form an electrical
field in the electro-absorption medium 61. For instance, the
electronics can form a voltage differential between the doped
regions 72. The electrical field can be formed without generating a
significant electrical current through the electro-absorption
medium 61. The electro-absorption medium 61 can be a medium in
which the Franz-Keldysh effect occurs in response to the
application of the electrical field. The Franz-Keldysh effect is a
change in optical absorption and optical phase by an
electro-absorption medium 61. For instance, the Franz-Keldysh
effect allows an electron in a valence band to be excited into a
conduction band by absorbing a photon even though the energy of the
photon is below the band gap. To utilize the Franz-Keldysh effect
the active region can have a slightly larger bandgap energy than
the photon energy of the light to be modulated. The application of
the field lowers the absorption edge via the Franz-Keldysh effect
and makes absorption possible. The hole and electron carrier
wavefunctions overlap once the field is applied and thus generation
of an electron-hole pair is made possible. As a result, the
electro-absorption medium 61 can absorb light signals received by
the electro-absorption medium 61 and increasing the electrical
field increases the amount of light absorbed by the
electro-absorption medium 61. Accordingly, the electronics can tune
the electrical field so as to tune the amount of light absorbed by
the electro-absorption medium 61. As a result, the electronics can
intensity modulate the electrical field in order to modulate the
light signal. Additionally, the electrical field needed to take
advantage of the Franz-Keldysh effect generally does not involve
generation of free carriers by the electric field.
[0048] Suitable electro-absorption media 61 include semiconductors.
However, the light absorption characteristics of different
semiconductors are different. A suitable semiconductor for use with
modulators employed in communications applications includes
Ge.sub.1-xSi.sub.x (germanium-silicon) where x is greater than or
equal to zero. In some instances, x is less than 0.05, or 0.01.
Changing the variable x can shift the range of wavelengths at which
modulation is most efficient. For instance, when x is zero, the
modulator is suitable for a range of 1610-1640 nm. Increasing the
value of x can shift the range of wavelengths to lower values. For
instance, an x of about 0.005 to 0.01 is suitable for modulating in
the c-band (1530-1565 nm).
[0049] Additional details about the fabrication, structure,
incorporation into an optical device such as the transmitter, and
operation of a modulator having a cross section according to FIG.
3A can be found in U.S. patent application Ser. No. 12/653,547,
filed on Dec. 15, 2009, entitled "Optical Device Having Modulator
Employing Horizontal Electrical Field," and incorporated herein in
its entirety.
[0050] The modulator of FIG. 3A can be modified as shown in FIG.
3B. FIG. 3B is a cross section of another embodiment of a suitable
Franz-Keldysh modulator. The perimeter of portions of doped regions
72 shown in FIG. 3A are illustrated with dashed lines to prevent
them from being confused with interfaces between different
materials. The interfaces between different materials are
illustrated with solid lines. A first doped zone 80 and a second
doped zone 82 combine to form each of the doped regions 72. In some
instance, the first doped zone 80 is located in the
light-transmitting medium 40 but not in the electro-absorption
medium 61 and the second doped zone 82 is located in the
electro-absorption medium 61. The first doped zone 80 can contact
the second doped zone 82 or can overlap with the second doped zone
82. In some instances, the first doped zone 80 and the second doped
zone 82 overlap and at least a portion of the overlap is located in
the light-transmitting medium 40. In other instances, the first
doped zone 80 and the second doped zone 82 overlap without any
overlap being present in the electro-absorption medium 61.
[0051] The first doped zone 80 and the second doped zone 82
included in the same doped region 72 each includes the same type of
dopant. For instance, the first doped zone 80 and the second doped
zone 82 in an n-type doped region 72 each includes an n-type
dopant. The first doped zone 80 and the second doped zone 82
included in the same doped region 72 can have the same dopant
concentration or different concentrations.
[0052] Although FIG. 3A and FIG. 3B illustrates the slab regions 62
of the electro-absorption medium 61, the slab regions 62 of the
electro-absorption medium 61 may not be present. For instance, the
etch that forms the slab regions 62 of the electro-absorption
medium 61 may etch all the way through the slab regions 62. In
these instances, the first doped zone 80 and the second doped zone
82 are both formed in the light-transmitting medium 40.
[0053] Although FIG. 3B shows the first doped zone 80 not extending
down to the optical insulator 48, the first doped zone 80 can
extend down to the optical insulator 48 or into the optical
insulator 48.
[0054] The modulator of FIG. 3A can be modified as shown in FIG.
3C. FIG. 3C presents another embodiment of a suitable Franz-Keldysh
modulator. The perimeter of portions of doped regions 72 shown in
FIG. 3C are illustrated with dashed lines to prevent them from
being confused with interfaces between different materials. The
interfaces between different materials are illustrated with solid
lines.
[0055] The doped regions 72 each includes a portion that extends
into the ridge 60 of electro-absorption medium 61 and another
portion that extends into the slab region 62 of the
electro-absorption medium 61. The doped region 72 extends further
into the slab region 62 of the electro-absorption medium 61 than
the doped region 72 extends into the ridge 60 of the
electro-absorption medium 61. For instance, the portion of each
doped region 72 in the slab region 62 of the electro-absorption
medium 61 is thicker than the portion in the ridge 60. Reducing the
extension of the doped region 72 into the ridge 60 reduces the
interaction between the doped region 72 and a light signal being
guided through the ridge 60. As a result, a reduced extension of
the doped region 72 into the ridge 60 reduces optical loss.
Extending the doped region 72 further into the slab regions 62
allows the electrical field formed between the doped regions 72 to
move closer to the base 42. As a result, the extension of the doped
regions 72 further into the slab increases the portion of the light
signal that interacts with the electrical field. Accordingly,
problems associated with increasing the thickness of the slab
regions 62 do not arise because they can be addressed by extending
the doped regions 72 further into the slab regions 62.
[0056] A suitable thickness for the portion of the doped region 72
in the ridge 60 (labeled T.sub.R in FIG. 3C) includes a thickness
greater than 0.01, 0.075, 0.1, or 0.125 .mu.m and/or less than
0.175, 0.2, or 0.5 .mu.m. A suitable thickness for the portion of
the doped region 72 40 in the slab region 62 of the
electro-absorption medium 61 27 (labeled T.sub.S in FIG. 3C)
includes a thickness greater than 0.175, 0.2, or 0.225 .mu.m and/or
less than 0.275, 0.3, 0.325, or 0.8 .mu.m. A suitable thickness
ratio (ratio of thickness of portion of doped region 72 in the slab
region 62: thickness of portion of doped region 72 in the ridge 60)
includes ratios greater than 1, 1.25, or 1.5 and/or less than 2.0,
2.5, and 3.
[0057] The doped regions 72 can each be a result of combining a
first doped zone 80 (not shown in FIG. 3C) and a second doped zone
82 (not shown in FIG. 3C). The first doped zone 80 can be located
in the slab region 62 of the electro-absorption medium 61 and the
second doped zone 82 can be located both in the ridge 60 and in the
slab region 62 of the electro-absorption medium 61. The first doped
zone 80 and the second doped zone 82 included in the same doped
region 72 each includes the same type of dopant. For instance, the
first doped zone 80 and the second doped zone 82 in an n-type doped
region 72 each includes an n-type dopant. The first doped zone 80
and the second doped zone 82 included in the same doped region 72
can have the same dopant concentration or different concentrations.
Additionally, the first doped zone 80 can contact the second doped
zone 82 so as to form the doped region 72 or can overlap with the
second doped zone 82 so as to form the doped region 72. In some
instances, the first doped zone 80 and the second doped zone 82
overlap and at least a portion of the overlap is located in slab
region 62 of the electro-absorption medium 61.
[0058] Although FIG. 3C shows the doped region 72 not extending
down to the optical insulator 48, the doped region 72 can extend
down to the optical insulator 48 or into the optical insulator
48.
[0059] The modulator driver electronics 24 can operate the
Franz-Keldysh modulators of FIG. 3B and FIG. 3C in a manner that is
analogous to the operation of the Franz-Keldysh modulators of FIG.
3A. Additional details about the fabrication, structure,
incorporation into an optical device such as the transmitter, and
operation of a modulator having a cross section according to FIG.
3A through FIG. 3C can be found in U.S. patent application Ser. No.
13/385,099, filed on Feb. 1, 2012, entitled "Optical Component
Having Reduced Dependency on Etch Depth," and incorporated herein
in its entirety.
[0060] The modulators of FIG. 3A through FIG. 3C are suitable for
use as the modulators of FIG. 2A because of their compact size. For
instance, the width of the slab regions 62 in each of FIG. 3A
through FIG. 3C (labeled W, slab region) can be less than 40 .mu.m,
30 .mu.m, or 20 .mu.m. Additionally or alternately, the distance
between the outermost end of the contact pad and the ridge 60 of
the electro-absorption medium 61 (labeled d in FIG. 3A) can be less
than 40 .mu.m, 30 .mu.m, or 10 .mu.m. These small dimensions allow
these modulators to have a very close proximity to other features
on the transmitter. For instance, these dimension allow the
modulators to be positioned near the edge of the transmitter. The
ability to position these modulators near the edge of the
transmitter makes it possible to achieve the small drive path
lengths disclosed above. Another example of transmitter features
that can be positioned close to these modulators are transition
waveguides 18. As discussed above, a portion of the modulators
include transition waveguides 18 located between the modulator and
the edge of the transmitter that is closest to the modulators. The
ability to place the modulators close to these transition
waveguides 18 increases the number of transition waveguides 18 that
can be positioned between the edge and the modulators without
affecting the modulation speed.
[0061] Modulator types other than Franz-Keldysh modulators can be
employed in the above transmitter. However, where Franz-Keldysh
modulators directly modulate intensity, many other modulator types
modulate phase and are accordingly incorporated into a Mach-Zehnder
interferometer in order to modulate intensity. Modulators that
manipulate a depletion region in a waveguide are an example of a
phase modulator that is typically incorporated into a Mach-Zehnder
interferometer. Since the waveguides associated with a Mach-Zehnder
interferometer require more space on the transmitter than a
Franz-Keldysh modulator, the Franz-Keldysh modulator may provide a
more compact transmitter. For instance, the length of the
Franz-Keldysh modulators can be selected such that a distance from
a location on one of the modulators to the same location on the
next modulator (labeled P) in FIG. 2A is less than 2 mm, 1 mm, or
0.5 mm.
[0062] In the modulators of FIG. 3A through FIG. 3C, the region of
the light-transmitting medium 40 or electro-absorption medium 61
between the doped regions 72 can optionally be undoped or lightly
doped as long as the doping is insufficient for the doped material
to act as an electrical conductor that electrically shorts the
modulator.
[0063] In the modulators of FIG. 3A through FIG. 3C, the electrical
field is essentially formed between the portions of the doped
region 72 located in the ridge 60 of the electro-absorption medium
61. As a result, the drive path length extends from the drive pads
66 to the location where the doped region 72 of the active ends
under the ridge 60. For instance, in the modulator of FIG. 3A, one
of the drive path lengths extends from one of the drive pads 66 to
the location labeled DPL. In the case shown in FIG. 3A, the drive
path length consists of the length of the wire 68 from the drive
pad 66 to the contact pad and the distance between the location
where the wire 68 is bonded to the contact pad to the location
labeled DPL.
[0064] Although FIG. 2A through FIG. 3C illustrate the first
contact pad 63 being positioned on an opposite side of the ridge 60
of the electro-absorption medium 61 from the second contact pad 64,
the first contact pad 63 and the second contact pad 64 can be
positioned on the same side of the ridge 60 of the
electro-absorption medium 61. In these instances, the transmitter
can include a metal trace that extends from one of the contact
pads, across the ridge 60 of the electro-absorption medium 61
and/or across the ridge 44 of light-transmitting medium 40, and
into contact with the doped region 72 on that side of the ridge 60
of the electro-absorption medium 61. Accordingly, the metal trace
provides electrical communication between a contact pad on one side
of the modulator waveguide 25 and a doped region 72 on the other
side of the modulator waveguide 25. In these instances, the metal
trace is part of the drive path length. As a result, the length of
the metal trace can affect the modulation speed.
[0065] In one example of the transmitter, the waveguides (source
waveguides 16, transition waveguides 18 and output waveguides 20)
are single mode waveguides. The single mode waveguides can also be
large core single mode waveguides. For instance, each of the source
waveguides 16, transition waveguides 18 and output waveguides 20
can have a cross section according to FIG. 2C and can have a ridge
44 width (labeled w in FIG. 2C) greater than 1 .mu.m or 2 .mu.m
and/or less than 4 .mu.m or 5 .mu.m, a ridge 44 height (labeled h
in FIG. 2C) greater than 0.5 .mu.m or 1 .mu.m and/or less than 2
.mu.m or 2.5 .mu.m, and a thickness (labeled T in FIG. 2C) greater
than 1 .mu.m or 2 .mu.m and/or less than 4 .mu.m or 5 .mu.m. In
this same example of the transmitter, the modulators can each be
constructed according to FIG. 3A, FIG. 3B, or FIG. 3C with a
modulator waveguide 25 having a ridge 60 width (labeled wr in FIG.
3A) greater than 0.2 .mu.m or 0.4 .mu.m and/or less than 0.8 .mu.m
or 1 .mu.m, a ridge 60 height (labeled hr in FIG. 3A) greater than
1 .mu.m or 2 .mu.m and/or less than 2.8 .mu.m or 3 .mu.m, and a
waveguide thickness (labeled Tr in FIG. 2C) greater than 1 .mu.m or
2 .mu.m and/or less than 4 .mu.m or 5 .mu.m.
[0066] Using the above transmitter construction and the above
modulators, the transmitter and modulator driver electronics 24 can
be constructed so as to provide drive path lengths less than 1 mm,
0.5 mm, or 0.25 mm.
[0067] The transmitter can include components in addition to the
components shown in FIG. 1A through FIG. 2C. For instance, the
transmitter can include a tap or tap waveguide configured to tap
off a portion of one of the light signals (transition signal,
source signal, modulated signal). The transmitter can also include
a light sensor or monitor that receives the tapped portion of the
light signal. The light sensor or monitor can convert the received
light signal to an electrical signal. Electronics can use the
electrical signal to adjust the intensity of light being generated
by the one or more lasers 10 in a feedback loop. In one example,
the transmitter is constructed according to FIG. 2A and a tap
waveguide taps off a portion of the light signal from the source
waveguide 16. The tap waveguide guides the tapped portion of the
light signal to a light sensor and electronics adjust the output
from the laser 10 in response to output from the light sensor.
Additionally or alternately, the transmitter can include a combiner
or multiplexer. For instance, the transmitter can include a
combiner or multiplexer that combines two or more of the modulated
signals into a single output signal that is processed further by
the transmitter and/or is received by an optical fiber 26. In one
example, the transmitter of FIG. 2B includes a multiplexer that
combines multiple modulated signals into an output signal that is
an optical fiber 26. In this instance, the lasers 10 that are the
source of the combined modulated signals can each have a different
wavelength so the output signal can be demuliplexed later.
[0068] Other embodiments, combinations and modifications of this
invention will occur readily to those of ordinary skill in the art
in view of these teachings. Therefore, this invention is to be
limited only by the following claims, which include all such
embodiments and modifications when viewed in conjunction with the
above specification and accompanying drawings.
* * * * *