U.S. patent application number 15/048924 was filed with the patent office on 2016-06-16 for reducing power requirements for optical links.
This patent application is currently assigned to MELLANOX TECHNOLOGIES SILICON PHOTONICS INC.. The applicant listed for this patent is MELLANOX TECHNOLOGIES SILICON PHOTONICS INC.. Invention is credited to Mehdi Asghari, Dazeng Feng.
Application Number | 20160173203 15/048924 |
Document ID | / |
Family ID | 55643319 |
Filed Date | 2016-06-16 |
United States Patent
Application |
20160173203 |
Kind Code |
A1 |
Feng; Dazeng ; et
al. |
June 16, 2016 |
REDUCING POWER REQUIREMENTS FOR OPTICAL LINKS
Abstract
An optical system includes a device selected from a transmitter
and a receiver. The transmitter has multiple laser cavities that
each generates an optical channel at a different channel
wavelength. The transmitter is also configured to modulate the
optical channels into modulated light signals. The receiver is
configured to demultiplex modulated light signals and use the
demultiplexed light signals to generate electrical signals. The
device is configured to operate in an atmosphere having an
operational temperature range that includes a range of temperatures
extending from TL to TH. Electronics are configured to elevate the
temperature of the device when the temperature of the atmosphere is
in a first portion of the temperature range from TL to TH but not
elevate the temperature of the controlled device when the
temperature of the atmosphere is in a second portion of the
temperature range from TL to TH.
Inventors: |
Feng; Dazeng; (El Monte,
CA) ; Asghari; Mehdi; (Pasadena, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
MELLANOX TECHNOLOGIES SILICON PHOTONICS INC. |
Monterey Park |
CA |
US |
|
|
Assignee: |
MELLANOX TECHNOLOGIES SILICON
PHOTONICS INC.
|
Family ID: |
55643319 |
Appl. No.: |
15/048924 |
Filed: |
February 19, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
14280067 |
May 16, 2014 |
9312960 |
|
|
15048924 |
|
|
|
|
61825501 |
May 20, 2013 |
|
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Current U.S.
Class: |
398/140 ;
398/182 |
Current CPC
Class: |
H04B 10/516 20130101;
H04B 10/564 20130101; H04B 10/506 20130101; H04B 10/572 20130101;
H04J 14/02 20130101; H04B 10/503 20130101; H04B 10/60 20130101 |
International
Class: |
H04B 10/564 20060101
H04B010/564; H04B 10/50 20060101 H04B010/50; H04B 10/60 20060101
H04B010/60 |
Claims
1. An optical system, comprising: a transmitter including multiple
laser cavities that each generates an optical channel at a
different channel wavelength, the transmitter being configured to
modulate the optical channels into modulated light signals, the
transmitter configured to operate in an atmosphere having an
operational temperature range that includes a temperature range
extending from TL to TH; and transmitter electronics are configured
to elevate the temperature of the transmitter when the temperature
of the atmosphere is in a first portion of the temperature range
extending from TL to TH but not elevate the temperature of the
transmitter when the temperature of the atmosphere is in a second
portion of the temperature range extending from TL to TH.
2. The system of claim 1, wherein the electronics are configured to
allow the temperature of the transmitter to float when the first
temperature is above the temperature threshold.
3. The system of claim 1, wherein the transmitter electronics are
configured to heat the transmitter such that when the first
temperature is below the temperature threshold, the electronics are
configured to control the temperature of the transmitter such that
the temperature of the transmitter is less than TH.
4. The system of claim 1, further comprising: an optical receiver
in optical communication with the transmitter such that the
receiver receives the modulated signals, the receiver including a
demultiplexer for demultiplexing the received modulated signals and
DBW representing the bandwidth of the demultiplexer included in the
receiver.
5. The system of claim 4, further comprising: receiver electronics
configured to maintain the temperature of the receiver at a
constant temperature.
6. The system of claim 5, wherein the receiver is configured to
operate in an atmosphere having an operational temperature range
extending from RTL to RTH and the receiver electronics configured
to control the temperature of the receiver such that the
temperature of the receiver is above a second receiver temperature
when a first receiver temperature is above a receiver temperature
threshold and also when the first receiver temperature is below the
receiver temperature threshold, the first receiver temperature
being selected from a group consisting of a temperature of the
receiver and a temperature of the atmosphere in which the receiver
is located, the receiver temperature threshold being between RTL
and RTH and the second receiver temperature being at least equal to
the receiver temperature threshold.
7. The system of claim 1, wherein the transmitter includes
modulators for modulating the optical channels, each modulator
receiving one of the optical channels.
8. The system of claim 1, wherein the transmitter includes a
combiner configured to combine the modulated light signals into an
output light signal that exits the transmitter through a facet, the
combiner being associated with multiple center wavelengths.
9. The system of claim 8, wherein channel wavelengths are each the
same as one of the center wavelengths of the combiner at a target
temperature, the target temperature being between the temperature
threshold and TH.
10. The system of claim 8, wherein channel wavelengths are each the
same as one of the center wavelengths of the combiner at a target
temperature and at least one of the modulators has a modulation
wavelength that is the same as the channel wavelength of one of the
laser cavities, the target temperature being between the
temperature threshold and TH.
11. The system of claim 8, wherein channel wavelengths are each the
same as one of the center wavelengths of the combiner at a target
temperature and the modulators each have a modulation wavelength
that is the same as the channel wavelength of one of the laser
cavities, the target temperature being between the temperature
threshold and TH.
12. The system of claim 1, wherein the transmitter is constructed
on a silicon-on-insulator wafer.
Description
RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 14/280,067, filed on May 16, 2014, entitled
"Reducing Power Requirements for Optical Links" and incorporated
herein in its entirety; and U.S. patent application Ser. No.
14/280,067 claims the benefit of U.S. Provisional Patent
Application Ser. No. 61/825,501, filed on May 20, 2013, entitled
"Reducing Power Requirements for Optical Links" and incorporated
herein in its entirety.
FIELD
[0002] The present invention relates to optical links and more
particularly to power requirements of these optical links.
BACKGROUND
[0003] Optical communication systems often employ one or more WDM
(Wavelength Division Multiplexed) optical links that typically
include a transmitter in optical communication with a receiver. The
transmitter generates multiple modulated light signals that are
received by the receiver. The receiver demultiplexes the modulated
light signals and, in some instances, uses the demultiplexed light
signals to generate electrical signals.
[0004] The receiver and transmitter are each positioned in an
atmosphere with an operational temperature range. For instance, the
receiver and transmitter can each be positioned in a different
atmosphere and the operational temperature of that atmosphere can
vary over a wide range of temperatures. However, the optical
components on a transmitter can respond differently to changes in
temperature. As a result, a transmitter that functions very well at
a temperature where the operational wavelengths of the different
components are matched can become highly inefficient at a different
temperature. Further, the receivers are associated with the same
difficulty.
[0005] To make matters worse, the temperature of the atmosphere in
which the receiver is positioned can be very different from the
temperature of the atmosphere in which the transmitter is
positioned. These differences in temperature means that the
operational wavelengths of the components on the transmitter have
shifted differently than the operational wavelengths of the
components on the receiver. As a result, the different temperature
of the transmitter and receiver cause further drops in the
efficiency of the optical link.
[0006] The above problems have been addressed by using a common
temperature controller to maintain the transmitter and receiver at
a defined temperature. For instance, if the transmitter and
receiver are positioned in different atmospheres that each has an
operational temperature range of 0-70.degree. C., the transmitter
and receiver can be maintained at around 80.degree. C. with the use
of heating elements and the operational wavelengths of the
components on the transmitter and receiver can be configured to
match at 80.degree. C. As a result, the transmitter and receiver
efficiently work together at 80.degree. C. and the temperature of
the transmitter and receiver does not substantially shift away from
80.degree. C. Alternatively, a TEC (thermo electric cooler) device
can be used to maintain the transmitter and receiver at
temperatures between 0-70.degree. C., for instance at 55.degree. C.
Accordingly, the problems associated with temperature shifts of the
transmitter and/or receiver are eliminated. However, the energy
requirements needed to keep a transmitter and receiver at these
temperature levels are very large and not practical when large
numbers of optical links are desired. As a result, there is a need
for a more energy efficient optical link.
SUMMARY
[0007] An optical system includes a transmitter having multiple
laser cavities that each generates an optical channel at a
different channel wavelength. The transmitter is configured to
modulate the optical channels into modulated light signals. The
transmitter is configured to operate in an atmosphere having an
operational temperature range that includes a range of temperatures
extending from TL to TH. Transmitter electronics are configured to
elevate the temperature of the transmitter when the temperature of
the atmosphere is in a first portion of the temperature range from
TL to TH but not elevate the temperature of the transmitter when
the temperature of the atmosphere is in a second portion of the
operational temperature range.
[0008] In one example, the transmitter electronics elevate the
temperature of the transmitter to a temperature above a second
temperature in response to a first temperature falling below a
temperature threshold. The first temperature is selected from a
group consisting of a temperature of the transmitter and a
temperature of the atmosphere in which the transmitter is located.
The second temperature is greater than or equal to the temperature
threshold. The temperature threshold is between TL and TH and is
also greater than or equal to TH-(DBW*CS-2*tol)/(d.lamda./dT) where
DBW is a bandwidth percentage of a demultiplexer suitable for
demultiplexing the modulated light signals, CS is a wavelength
spacing between the channels wavelength, tol is a manufacturing
tolerance for the channel laser cavities, and (d.lamda./dT)
represents a shift in a channel wavelength for each optical channel
in response to temperature.
[0009] Another optical system includes a receiver having a
demultiplexer suitable for demultiplexing modulated light signals.
The receiver also includes light sensors that each uses one of the
demultiplexed light signals to an electrical signal. The receiver
is configured to operate in an atmosphere having an operational
temperature range that includes a range of temperatures extending
from TL to TH. Transmitter electronics are configured to elevate
the temperature of the receiver when the temperature of the
atmosphere is in a first portion of the temperature range from TL
to TH but not elevate the temperature of the transmitter when the
temperature of the atmosphere is in a second portion of the
temperature range from TL to TH.
[0010] One system includes multiple devices that include a
transmitter and a receiver. The transmitter is configured to
modulate the optical channels into modulated light signals. The
system also includes a receiver configured to receive and
demultiplex the modulated light signals. At least one of the
devices is a controlled device and is selected from a group
consisting of the transmitter and receiver. The controlled device
is configured to operate in an atmosphere having an operational
temperature range that includes a range of temperatures extending
from TL to TH. Electronics are configured to elevate the
temperature of the controlled device when the temperature of the
atmosphere is in a first portion of the temperature range from TL
to TH but not elevate the temperature of the controlled device when
the temperature of the atmosphere is in a second portion of the
operational temperature range.
BRIEF DESCRIPTION OF THE FIGURES
[0011] FIG. 1 is a schematic of an optical link.
[0012] FIG. 2A is a schematic of a transmitter.
[0013] FIG. 2B is another schematic of a transmitter.
[0014] FIG. 3 is a topview of a transmitter constructed according
to FIG. 2A.
[0015] FIG. 4 is a topview of a receiver constructed according to
FIG. 1.
[0016] FIG. 5A is a cross-section of a waveguide constructed on a
silicon-on-insulator wafer.
[0017] FIG. 5B is a cross-section of an echelle grating constructed
on a silicon-on-insulator wafer.
[0018] FIG. 6A through FIG. 6B illustrate a suitable interface
between a transmitter and a laser bar. FIG. 6A is a topview of the
interface and FIG. 6B is a perspective view of the interface.
[0019] FIG. 7A through FIG. 7D illustrate a laser bar that is
suitable for use with a transmitter constructed according to FIG.
6A through FIG. 6B. FIG. 7A is a bottom view of the laser bar.
[0020] FIG. 7B is a cross-section of the laser bar shown in FIG. 7A
taken along the line labeled B in FIG. 7A.
[0021] FIG. 7C is a cross-section of the laser bar shown in FIG. 7A
taken along the line labeled C in FIG. 7A.
[0022] FIG. 7D is a topview of the laser bar shown in FIG. 7A.
[0023] FIG. 8A illustrates assembly of a system using a transmitter
constructed according to FIG. 6A through FIG. 6B and a laser bar
constructed according to FIG. 7A through FIG. 7D.
[0024] FIG. 8B illustrates the transmitter as shown in FIG. 8A and
the laser bar as shown in FIG. 8A assembled in an optical assembly
according to FIG. 6A.
[0025] FIG. 9 is a perspective view of a portion of a device that
includes a Bragg grating constructed on a silicon-on-insulator
wafer.
[0026] FIG. 10 is a cross section of an optical modulator that
employs the Franz-Keldysh effect to modulate light signals.
[0027] FIG. 11 is a graph shows the amount of optical loss
resulting from modulating a particular wavelength.
DESCRIPTION
[0028] The inventors have discovered that under certain
circumstances, an optical link can maintain a desirable level of
efficiency even when the temperature of the transmitter and/or
receiver are allowed to shift over particular ranges. As a result,
an optical link that is more energy efficient can be achieved by
only controlling the temperature of the transmitter and/or receiver
over a limited portion of the operational temperature range of the
transmitter and/or receiver. For instance, when a common
temperature controller is used to control the temperature of a
transmitter and the atmosphere in which the transmitter is
positioned has a temperature below a threshold temperature, the
common temperature controller can be used to heat the transmitter
to a temperature above that threshold temperature. However, when
the atmosphere in which the transmitter is positioned has a
temperature above that threshold temperature, the common
temperature controller is not used to heat the transmitter. The
temperature threshold can be between the upper and lower limit of
the operational range associated with the transmitter. As a result,
the common temperature controller is not used when the atmosphere
in which the transmitter is positioned is in a portion of the
operational range. In this arrangement two different aspects of the
common temperature controller requires less energy. First, since
the common temperature controller is only used over a portion of
the operational range, the common temperature controller is used
less frequently than the prior art and accordingly reduces the
energy requirement of the optical link. Second, when the common
temperature controller is used, it can be used to keep the
temperature of the transmitter at a level below the upper limit of
the operational range. Since the common temperature controller
requires less energy in order to keep the transmitter at a lower
temperature, this aspect also and reduces the energy requirement of
the optical link.
[0029] FIG. 1 is a schematic of a system that includes an optical
link. The system includes a transmitter 2 in optical communication
with a receiver 4. The transmitter 2 generates an output signal
that is received at an interdevice component 6. The interdevice
component 6 guides the output signal from the transmitter 2 to the
receiver 4. The receiver receives the output signal from the
interdevice component 6 and uses it as an input signal. The
interdevice components 6 can be component or combination of
components that guides the light signal output from the transmitter
to the receiver. For instance, the interdevice component 6 can
represent something as complex as an optical network or as simple
as single optical fiber.
[0030] The transmitter 2 shown in FIG. 1 is configured to generate
multiple laser signals, modulate them and combine the result into
the output signal. The transmitter 2 includes multiple laser
cavities 3 that are each configured to output a different
wavelength laser signal on a laser waveguide 20. Each laser
waveguide 20 guides the received laser signal to a modulator 22.
The modulators 22 are configured to be in electrical communication
with transmitter electronics (not shown) that can be included on
the transmitter and/or external to the transmitter. The transmitter
electronics can operate each modulator 22 such that the received
laser signal is modulated into a modulated signal. The modulated
signals are each received on a modulated waveguide 24 that guides
the received modulated signals to a combiner 26 that combines the
modulated signals into an output signal that is received on a
common waveguide 28. The common waveguide 28 guides the output
signal to a facet through which the output signal exits from the
transmitter 2.
[0031] The receiver 4 shown in FIG. 1 is configured to convert each
of the modulated signals received from the transmitter into
electrical signals that can be further processed by receiver
electronics (not shown) that can be located on the receiver and/or
can be external to the receiver. The receiver includes an input
waveguide 8 that receives the output signal from the transmitter 2
and uses the received output signal as an input signal. The input
waveguide 8 guides the input signal to a demultiplexer 9 that
demultiplexes the input signal into sensor signals. The sensor
signals are each received on a sensor waveguide 10. Each of the
sensor waveguides 10 guides the received sensor signal to a light
sensor 11. The light sensors 11 are each in electrical
communication with the receiver electronics (not shown). The
receiver electronics are configured to operate each light sensor 11
such that the light sensor 11 outputs an electrical signal
indicating the presence and/or intensity of the sensor signal
received by the light sensor 11. In some instances, the receiver
electronics can process the electrical signals so as to extract
data that was encoded onto the modulated signals by the modulators.
Suitable demultiplexers include, but are not limited to, arrayed
waveguide gratings, and echelle gratings.
[0032] FIG. 2A is a schematic of a transmitter that is suitable for
use as a transmitter constructed according to FIG. 1. The
transmitter includes gain waveguides 12 defined in a gain medium
14. Cavity waveguides 16 each provide an optical pathway from the
gain waveguides 12 to a partial return device 18. Laser waveguides
20 each provides an optical pathway from one of the partial return
devices 18 to a modulator 22. Modulated waveguides 24 each provides
an optical pathway from one of the modulators 22 to a combiner 26.
A common waveguide 28 provides an optical pathway from the combiner
26 to a facet located at or near an edge of the transmitter. The
transmitter can then be connected to an optical fiber 40 such that
the optical fiber 40 receives light guided by the common waveguide
28.
[0033] During operation of the transmitter, the cavity waveguides
16 each guides a light signal from the gain medium 14 to the
partial return device 18. Each partial return device 18 returns a
first portion of the received light signal along its original path
and permits a second portion of the light signal to enter the laser
waveguide 20. As a result, the second portion of each light signal
serves as a laser signal output by a laser cavity.
[0034] The cavity waveguides 16 each carries the first portion of
the light signal back to one of the gain waveguides 12. The gain
waveguides 12 each guides the received first portion of the light
signal through the gain medium 14 to a reflector 41. Each reflector
41 reflects the received light signal portion such that the first
light signal portion returns to the gain waveguide 12 from which it
came. As a result, the reflected light signal portions each
eventually return to the partial return device 18 from which it
originated. Accordingly, the first light signal portions each
travels through a gain waveguide 12 twice before returning to the
partial return device 18. Since the partial return device 18 once
again returns another portion of the returned first light signal
portion, the first portion of the light signal passes through the
gain medium 14 multiple times when traveling back and forth between
one of the reflectors 41 and the associated partial return device
18. As a result, each reflector 41 and the associated partial
return device 18 define one of the laser cavities on the
transmitter.
[0035] While the first portion of the light signal is making
multiple passes through the gain medium 14, energy is applied to
the gain medium 14 so as to provide the optical gain needed to
cause lasing within the gain medium. In some instance, the energy
is electrical energy provided by the transmitter electronics but
other forms of energy can be used. The reflectors 41 can each be
highly reflective so substantially all of the first light signal
portions are returned to one of the gain waveguides 12.
[0036] The partial return devices 18 can be wavelength dependent in
that the partial return devices 18 each returns to the gain medium
14 only particular wavelengths of light while transmitting all
other wavelengths. Only the wavelengths returned to the gain medium
14 lase. Accordingly, the selection of wavelengths in each laser
signal (the second portion of the light signal that passes the
partial return device 18) are in the range of wavelengths returned
by the partial return device 18 from which the laser signal exits.
Additionally, different partial return devices 18 can be configured
to return a different range of wavelengths. As a result, each of
the different laser signals can include a different range of
wavelengths. When the range of wavelengths returned by each of the
partial return devices 18 is narrow, each of the different laser
signals can be at a different wavelength or can include a different
channel at a different channel wavelength. An example of partial
return devices 18 that can each return light in a range of
wavelengths while transmitting light of other wavelengths are
reflective optical gratings such as Bragg gratings.
[0037] The laser waveguides 20 each guide one of the laser signals
to a modulator 22. Transmitter electronics (not shown) can operate
the modulator 22 so as to modulate the laser signals. For instance,
the transmitter electronics can encode data onto one or more of the
laser signals. The modulated waveguides 24 each carries one of the
modulated signals to the combiner 26. The combiner 26 combines the
received light signals into an output signal received by the common
waveguide 28.
[0038] The common waveguide 28 guides the output signal to the
facet. Accordingly, when an optical fiber 40 is aligned with the
facet 41, the optical fiber 40 receives the amplified light signal
from the common waveguide 28.
[0039] FIG. 2A includes dashed lines showing the location of
multiple first electrical conductors 44 and a common electrical
contact 46. The first electrical conductors 44 and the common
electrical contacts 46 are each in electrical communication with
the gain medium 14 and can each be in direct physical contact with
the gain medium 14. Additionally, the gain medium 14 can be between
first electrical conductors 44 and the common electrical contact
46. Transmitter electronics (not shown) can be in electrical
communication with the first electrical conductors 44 and the
common electrical contact 46 and can be configured to use these
electrical contacts to apply electrical energy to the gain medium
14.
[0040] Four laser cavities are shown in FIG. 2A although the
transmitter can include more or less than four lasers. As is
evident from FIG. 2A, each of the first electrical conductors 44 is
associated with one of the laser cavities. As a result, the
transmitter electronics can apply electrical energy to the gain
medium 14 associated with a particular one of the laser cavities
and accordingly increase or decrease the intensity of the laser
signal produced by that laser cavity. For instance, the transmitter
electronics can increase the electrical energy applied between the
common electrical contact 46 and the first electrical conductor 44
labeled EC2 in FIG. 2A in order to increase the intensity of the
laser signal labeled L2 in FIG. 2A.
[0041] In some instances, the transmitter includes one or more
coupled waveguides 48 that are each optically coupled with the
laser waveguide 20 such that a portion of the laser signal guided
by the optically coupled laser waveguide 20 is coupled into the
coupled waveguide 48. The coupled waveguide 48 guides the tapped
portion of the laser signal to a light sensor 50. Each of the light
sensors 50 is configured to convert the received light signal to an
electrical signal. The transmitter electronics can be in electrical
communication with the light sensor 50 and can receive the
electrical signal from the light sensor 50.
[0042] During operation of a transmitter that includes coupled
waveguides 48, the transmitter electronics receive the electrical
signal from the light sensor 50. The transmitter electronics can
also adjust the level of electrical energy applied to each of the
lasers in response to the electrical signal received from the light
sensor 50 associated with that laser in a feedback loop. For
instance, in the event that the electrical signal from one of the
light sensors 50 indicates that the intensity of the laser signal
being received by the light sensor 50 is above a threshold, the
transmitter electronics can reduce the electrical energy applied to
the gain medium 14 associated with the laser that produced the
laser signal in order to reduce the intensity of the laser signal
received by that light sensor 50.
[0043] Although FIG. 2A shows each of the lasers using the same
gain medium 14, different lasers can employ different gain media.
For instance, FIG. 2B shows the transmitter of FIG. 2A but two of
the laser cavities use a different gain medium 14 than the other
two laser cavities. Such an arrangement may be suitable for
instances where it is desirable to create laser signals of
different wavelengths with different gain media. For instance, when
the wavelength differential between adjacent channel wavelengths
(the channel spacing) is sufficiently large that one gain medium
does not efficiently generate each of the channels, it may be
desirable to use different gain media to generate different laser
signals so that different laser cavities can each be matched to a
gain medium that efficiently generates the wavelength of laser
signal associated with that laser cavity.
[0044] FIG. 3 is a topview of a transmitter constructed according
to FIG. 2A. The transmitter is built on a silicon-on-insulator
wafer although other platforms are possible. The transmitter
employs a Bragg grating as the partial return device 18 and an
echelle grating as the combiner 26. The common waveguide 28 guides
the amplified light signal to an optical fiber 40. The optical
fiber 40 can be glued to the transmitter.
[0045] Additional information regarding the structure,
construction, fabrication, and/or operation of the above
transmitters and other transmitters that are suitable for use in
the optical link can be found in U.S. patent application Ser. No.
13/317,340, filed on Oct. 14, 2011, entitled "Gain Medium Providing
Laser and Amplifier Functionality to Optical Device," incorporated
herein in its entirety, and also in U.S. patent application Ser.
No. 13/385,780, filed on Mar. 5, 2012, entitled "High Speed Optical
Transmitter Producing Modulated Light Signals," incorporated herein
in its entirety.
[0046] FIG. 4 is a topview of a receiver constructed according to
FIG. 2A. The receiver is built on a silicon-on-insulator wafer
although other platforms are possible. An optical fiber serves as
the interdevice component 6. The receiver employs an echelle
grating as the demultiplexer.
[0047] The above transmitter and/or receivers can optionally
include secondary components in addition to the illustrated
components. Examples of suitable secondary components include, but
are not limited to, facets through which light signals can enter
and/or exit a waveguide, a taper for changing the mode size of a
light signal guide by one or more of waveguides, entry/exit ports
through which light signals can enter and/or exit a waveguide from
above or below the device, multiplexers for combining multiple
light signals onto a single waveguide, demultiplexers for
separating multiple light signals such that different light signals
are received on different waveguides, optical couplers, optical
switches, lasers that act a source of a light signal, light sensors
such as sensors that convert all or a portion of the light signal
to an electrical signal, amplifiers for amplifying the intensity of
a light signal, attenuators for attenuating the intensity of a
light signal, modulators for modulating a signal onto a light
signal, modulators that convert a light signal to an electrical
signal, and vias that provide an optical pathway from the bottom
side of a device to the top side of the device. Although not
illustrated, the devices can optionally include electrical
devices.
[0048] Another example of secondary components that can be included
on the transmitter and/or receiver is a localized temperature
control device. Localized temperature control devices are designed
to control the temperature of a temperature sensitive component on
the device (transmitter or receiver) rather than attempting to
control the temperature of the entire device. For instance, a
localized temperature control device may be configured to control
the temperature of a single component such as a laser cavity, a
modulator, a light sensor, combiner, or demultiplexer. As an
example, the transmitter of FIG. 3 shows several different dashed
lines that each represents a localized heater 51. The localized
heaters 51 are each associated with a modulator in that the heat
provided by the localized heaters 51 is intended to elevate the
temperature of the associated modulator without substantially
elevating the temperature of another one of the modulators. Because
it is often not possible or desirable to place a localized heater
51 directly in contact with the associated component, the localized
heater(s) 51 must be close enough that the zone where the
temperature is increased extends beyond the components.
Accordingly, the localized heaters 51 shown in FIG. 3 are
positioned on opposing sides of the associated modulator rather
than directly in contact with the associated modulator.
[0049] The device can also include one or more temperature sensors
(not shown) that are each positioned to sense the temperature of
the temperature sensitive component itself and/or the temperature
of a zone adjacent to the temperature sensitive component. For
instance, the transmitter of FIG. 3 can include one or more
temperature sensors (not shown) that are each positioned to sense
the temperature of one of the modulators and/or the temperature of
a zone adjacent to one of the modulators.
[0050] Electronics (not shown) can operate the one or more
localized temperature control devices and receive output from the
one or more temperature sensors. The electronics can also adjust
the level of electrical energy applied to each of the localized
temperature control devices in response to the output received from
the one or more temperature sensors in a feedback loop. For
instance, in the event that the output from the one or more
temperature sensors indicates that the temperature of a component
is below a lower threshold, the electronics can increase the heat
being generated by associated localized temperature control devices
to increase the temperature of the associated component.
Additionally or alternately, in the event that the output from the
one or more temperature sensors indicates that the temperature of
is above an upper threshold, the electronics can decrease the level
of heat being generated by the one or more localized temperature
control devices associated with that component.
[0051] In the example of FIG. 3, one or more localized heaters 51
can be associated with all or a portion of the modulators and the
electronics can be configured to control the temperature of each
modulator so as to control the wavelength that the modulator
modulates most efficiently (the modulation wavelength). For
instance, each of the modulators 22 can have the same structure and
accordingly the same modulation wavelength. However, the laser
signals each has a different wavelength. As a result, at least a
portion of the modulators are modulating laser signals at a
wavelength other than the modulation wavelength and are accordingly
not operating at optimal efficiency. The modulation wavelength of a
modulator can be shifted by changing the temperature of the
modulator. As a result, the electronics can operate the localized
heaters 51 so that the modulation wavelength of each modulator
approximates the wavelength of the laser signal modulated by that
modulator.
[0052] Suitable localized heaters for use on planar optical devices
2 include, but are not limited to, resistive heating elements. For
instance, a suitable localized heater can be a layer of
electrically conducting material with a temperature that increases
in response to the electronics applying an electrical current
across the material. Suitable temperature sensors include, but are
not limited to, thermocouples, thermistors, resistive thermal
devices (RTDs), and semiconductor temperature sensors. Although the
localized temperature control devices are disclosed as controlling
temperature by the output of heat, suitable temperature control
devices can additionally or alternately be configured to control
temperature by cooling.
[0053] As will be discussed in more detail below, the use of
localized heaters on the transmitter is optional.
[0054] The transmitter and receiver illustrated above are
positioned on a common temperature controller 49. A common
temperature controller 49 is configured to control the temperature
of the transmitter or receiver. For instance, the common
temperature controller 49 is configured to heat and/or cool more
than one of the components on the transmitter or receiver. As an
example, a common temperature controller 49 for a transmitter is
configured to heat and/or cool at least two components selected
from the group consisting of the laser cavities, modulators, and
combiner. In some instances, the common temperature controller 49
for a transmitter is configured to heat and/or cool the laser
cavities, modulators, and combiner or to heat and/or cool the
entire transmitter. In some instances, a common temperature
controller 49 for a receiver is configured to heat and/or cool at
least the demultiplexer and the light sensors or to heat and/or
cool the entire transmitter.
[0055] As noted above, the transmitter and/or receiver can include
one or more localized temperature control devices. The transmitter
and/or receiver can include the one or more localized temperature
control devices in addition to the common temperature controller
49. In these instances, the temperature across the transmitter
and/or receiver is not uniform. The electronics on the transmitter
(transmitter electronics) and/or receiver (receiver electronics)
can be in electrical communication with one or more common
temperature sensors (not shown) that are each positioned to sense
the temperature of the device (transmitter or receiver). The one or
more common temperature sensors can be positioned outside the zone
where any localized temperature control devices substantially
affect the temperature of the device. As a result, the output of
the one or more common temperature sensors indicates the
temperature across the device rather than a temperature that
characterizes a localized zone on the device. As will be described
in more detail below, the electronics can operate the common
temperature controller 49 in response to output from the one or
more common temperature sensors. Suitable common temperature
sensors include, but are not limited to, thermocouples,
thermistors, resistive thermal devices (RTDs), and semiconductor
temperature sensors.
[0056] As will become evident below, the common temperature
controller 49 is optional. In some instances, the receiver and/or
transmitter need not include a common temperature controller 49. In
addition to the common temperature controller 49 or as an
alternative to the common temperature controller, the receiver
and/or transmitter can include zone definers (not illustrated)
and/or heat sinks (not illustrated) as disclosed in U.S. patent
application Ser. No. 13/507,491, filed on Jul. 3, 2012, entitled
"System for Managing Thermal Conduction on Optical Devices," and
incorporated herein in its entirety. Suitable common temperature
controllers include, but are not limited to, thermoelectric coolers
(TEC). TECs have the ability to both cool and heat the device.
However, common temperature controllers that have only the ability
to heat or only the ability to cool can be employed. Although the
above discussion discloses a single common temperature controller
on each device, multiple common temperature controllers can be used
with a single device.
[0057] FIG. 5A illustrates a suitable construction for waveguides
on the transmitter of FIG. 1 through FIG. 3 and/or for the
waveguides on the receiver of FIG. 4. FIG. 5A is a cross section of
a waveguide on the device. For instance, FIG. 5A can be a
cross-section of the input waveguide 8, sensor waveguides 10,
cavity waveguides 16, the laser waveguides 20, modulated waveguides
24, common waveguide 28, and/or coupled waveguides 48. In one
example of the transmitter, each of the cavity waveguides 16, the
laser waveguides 20, modulated waveguides 24, common waveguide 28,
and/or coupled waveguides 48 is constructed according to FIG. 5A.
In one example of the receiver, the input waveguide 8 and each of
the sensor waveguides 10 is constructed according to FIG. 5A.
[0058] The device of FIG. 5A includes a light-transmitting medium
52 on a base 54 that includes an optical insulator 56 positioned on
a substrate 58. Suitable light-transmitting media include, but are
not limited to, silicon, polymers, silica, SIN, GaAs, InP and
LiNbO.sub.3.
[0059] The waveguide is partially defined by a ridge 60 of the
light-transmitting medium 52 extending outward from slab regions 62
of the light-transmitting medium 52. The ridge 60 and the base 54
together define a portion of the light signal-carrying region where
light signals are constrained within the waveguide. For instance,
the ridge 60 of light-transmitting medium 52 can optionally include
a cladding (not shown) with an index of refraction that is less
than the index of refraction of the light-transmitting medium 52.
Likewise, the optical insulator 56 can have an index of refraction
that is less than an index of refraction of the light-transmitting
medium 52. The drops in index of refraction causes light signals
being carried within the light signal-carrying region to be
reflected back into the light signal-carrying region. Accordingly,
the light signal is constrained between the ridge 60 and the
optical insulator 56. Suitable claddings include, but are not
limited to, silicon nitride (SiN) and silica (SiO.sub.2) and can
include one layer or more than one layer of material.
[0060] A suitable platform having a structure according to FIG. 5A
is a silicon-on-insulator wafer although other platforms can be
used. A silicon-on-insulator wafer includes a silicon layer
positioned on a base. The silicon layer serves as the
light-transmitting medium 52 through which light signals are
guided. The base includes a layer silica positioned on a silicon
substrate. The layer of silica can serve as the optical insulator
56 and the silicon substrate can serve as the substrate 58.
[0061] Suitable combiners 26 for use with the transmitter can be
wavelength dependent multiplexers such as echelle gratings or
arrayed waveguide gratings. Suitable demultiplexers for use in the
receiver can be echelle gratings or arrayed waveguide gratings. The
structure of FIG. 5A can be adapted so it also serves as an echelle
grating. For instance, FIG. 5B illustrates the ridge 60 of FIG. 5A
with sufficient width to serves as the free space region of an
echelle grating. Accordingly, the cross-section of FIG. 5B can
serves as a cross section of the combiner in the transmitter of
FIG. 3 taken along the line labeled EG and/or as a cross section of
the demultiplexer in the receiver of FIG. 4 taken along the line
labeled EG.
[0062] Suitable echelle grating structures and/or methods of
fabricating echelle gratings on a silicon-on-insulator wafer are
disclosed in U.S. patent application Ser. No. 12/800,600, filed on
May 18, 2010, and entitled "Extension of Steps in Reflective
Optical Gratings;" and also in U.S. Provisional Patent Application
Ser. No. 61/284,723, filed on Dec. 23, 2009, and entitled "Reducing
Optical Loss in Reflective Optical Gratings;" and also in U.S.
patent application Ser. No. 12/927,412, filed on Nov. 12, 2010, and
entitled "Reducing Optical Loss in Reflective Optical Gratings;"
and also in U.S. patent application Ser. No. 12/321,386, filed on
Jan. 16, 2009, and entitled "Optical Component Having Features
Extending Different Depths into a Light Transmitting Medium," each
of which is incorporated herein in its entirety. The echelle
grating structures, methods of echelle grating fabrication, and/or
operation disclosed in these patent applications can be employed in
the combiner 26 of a transmitter constructed according to FIG. 1
through FIG. 3 or in the demultiplexer of a receiver constructed
according to FIG. 1 or FIG. 4.
[0063] In some instances, the combiner 26 can be a wavelength
independent combiner such as combiners that employ waveguides that
intersect one another so as to combine light signals from different
waveguides. Wavelength dependent multiplexers can become more
desirable than wavelength independent combiners as the number of
light signals that are combined by the combiner 26 increases. The
optical loss associated with combiners increases as the numbers of
light signals that must be combined increases. In some instances,
the amount of optical loss is stronger in wavelength independent
combiners than in wavelength dependent multiplexers. Accordingly,
the use of wavelength dependent multiplexers may become more
desirable as the number of light signals combined by the combiner
26 increases.
[0064] An example of a suitable wavelength independent combiner
that employ one or more y-junctions is disclosed in U.S. patent
application Ser. No. 10/644,395, filed on Aug. 19, 2003, and
entitled "Multiplexer Having Improved Efficiency," and now U.S.
Pat. No. 7,805,037; and also in U.S. Provisional patent application
Ser. No. 10/600,748, filed on Jun. 20, 2003, and entitled
"Multiplexer Having Improved Efficiency," each of which is
incorporated herein in its entirety. The y-junction structures
and/or methods of fabrication disclosed in these patent
applications can modified to employ single mode waveguides or
multimode waveguides and can be employed as the combiner in the
transmitter of a device constructed according to FIG. 1 through
FIG. 3.
[0065] The gain media illustrated in FIG. 1 through FIG. 3 can be
included in a laser bar. FIG. 6A through FIG. 6B illustrate a
suitable interface between an optical device and a laser bar that
includes the gain medium 14. FIG. 6A is a topview of the optical
device.
[0066] The portion of the device illustrated in FIG. 6A includes
the gain waveguides 12. Since a ridge 76 for the gain waveguides 12
is on a bottom side of the laser bar and FIG. 6A is a topview, the
location of the ridge 76 for the gain waveguides 12 is shown by
dashed lines.
[0067] The portion of the device illustrated in FIG. 6A also
includes cavity waveguides 16 that each receives the first portion
of a light signal from a different gain waveguide 12 on the laser
bar. The light signals each enter and exit the cavity waveguides 16
through a facet. As evident from FIG. 6A, the facet can be angled
at less than 90.degree. relative to the direction of propagation
through the cavity waveguide 16 at the facet. The angle can reduce
performance reduction associated with back reflection.
[0068] The side of the gain medium 14 at which the first portion of
the light signal is reflected includes a material that serves as
the reflector 41. A suitable material for forming the reflector 41
includes, but is not limited to, a layer of metal on the layer of
gain medium 14. The side of the gain medium 14 through which the
first portion of the light signal is transmitted includes an
anti-reflective coating 42. A material that serves as a suitable
anti-reflective coatings 42 includes, but is not limited to,
single-layer coatings such as silicon nitride or aluminum oxide, or
multilayer coatings which may contain silicon nitride, aluminum
oxide, and/or silica.
[0069] FIG. 6B is a perspective view of a portion of the optical
device shown in FIG. 6A. The illustrated portion of the optical
device is suitable for interfacing with one of the lasers on the
laser bar or with the amplifier on the laser bar. The laser bar is
not illustrated in FIG. 6B in order to make the portion of the
optical device under the laser bar visible. A cladding 63 is shown
on the light-transmitting medium 52. While the cladding 63 is
present over the waveguides and in the trenches, the cladding 63 is
not shown in these locations in order to make certain features of
the optical device readily visible.
[0070] A recess 64 extends into the base 54 to form a laser bar
platform 66. Contact pads 68 positioned on the laser bar platform
66 can be employed for providing electrical communication with a
laser on the laser bar or with the amplifier on the laser bar. One
or more stops 70 extend upward from the laser bar platform 66. For
instance, FIG. 6B illustrates four stops 70 extending upward from
the laser bar platform 66. The stops 70 include the cladding 63
positioned on a base portion. The substrate 58 can serve as the
base portion of the stops 70 and the stop 70 can exclude the light
insulator 56 or be made of the same material as the light insulator
56. The portion of the substrate 58 included in the stops 70 can
extend from the platform up to the level of the light insulator 56.
For instance, the stops 70 can be formed by etching through the
light insulator 56 and using the underlying substrate 58 as an
etch-stop. The cladding 63 can then be formed on the first
light-transmitting medium 52 at the same time the cladding 63 is
formed on the base portion of the stops 70.
[0071] A secondary platform 72 can optionally be positioned between
the facet of the cavity waveguides 16 and the laser bar platform
66. The secondary platform 72 is elevated relative to the laser bar
platform 66. For instance, the secondary platform 72 can be above
the laser bar platform 66 and at or below the level of the light
insulator 56. The secondary platform 72 can essentially be the top
of the substrate 58 or the secondary platform 72 can be positioned
below the level of the light insulator 56 as illustrated in FIG.
6B. Alternately, the secondary platform 72 can be etched
concurrently with the base portion of the stops 70 resulting in the
secondary platform 72 and the base portion of the stops 70 having
about the same height above the laser bar platform 66. Alternately,
the secondary platform 72 may not be present at all. For instance,
the portion of the base 54 between the laser bar platform 66 and
the waveguide facet can be substantially vertical relative to the
laser bar platform 66.
[0072] The optical device includes one or more alignment marks (not
shown). Suitable marks include recesses that extend into the
optical device. An alignment recess can extend into the first
light-transmitting medium 52 and/or the base 54. In some instances,
one or more of the alignment recesses extend into the secondary
platform 72. During attachment of the laser bar to the optical
device, the alignment recesses can be aligned with secondary
alignment recesses (not shown) on the laser bar in order to achieve
horizontal alignment of the laser bar relative to the optical
device.
[0073] FIG. 7A through FIG. 7D illustrate a laser bar that is
suitable for use with an optical device constructed according to
FIG. 6A through FIG. 6B. FIG. 7A is a bottom view of the laser bar.
FIG. 7B is a cross-section of the laser bar shown in FIG. 7A taken
along the line labeled B in FIG. 7A. FIG. 7C is a cross-section of
the laser bar shown in FIG. 7A taken along the line labeled C in
FIG. 7A. FIG. 7D is a topview of the laser bar. The laser bar
includes waveguides defined in the gain medium 14 for multiple
lasers and an amplifier. Trenches 74 extending into the gain medium
14 define ridges 76 in the gain medium 14. The ridges 76 each
defines a first amplifier waveguide 30 or one of the gain
waveguides 12. Suitable gain media include, but are not limited to,
InP, InGaAsP, and GaAs.
[0074] A laser bar cladding 78 is positioned on the gain medium 14.
A first electrical conductor 44 positioned on the cladding 78
includes a contact region 80 that extends through an opening in the
laser cladding 78 into contact with a top of the ridge 76. The
first electrical conductor 44 extends from the contact region 80
across a trench 74 to a contact pad 82. The contact pad 82 can be
employed to apply electrical energy to the laser. One of the
illustrated first electrical conductors 44 can also serve as the
amplifier electrical contact 47.
[0075] One or more alignment trenches 90 are positioned between
adjacent ridges 76. For instance, FIG. 7A illustrates two alignment
trenches 90 between adjacent ridges 76 and positioned on opposing
sides of the laser bar. A secondary stop 92 extends upward from the
bottom of the alignment trench.
[0076] Although FIG. 7A through FIG. 7D illustrate a secondary stop
92 extending upward from a bottom of the alignment trench such that
walls of the secondary stop are spaced apart from walls of the
alignment trench, the bottom of the alignment trench can be
substantially flat. However, an embodiment having walls of the
secondary stop spaced apart from walls of the alignment trench may
be preferred to reduce etch induced inconsistencies on the tops of
the secondary stops.
[0077] The common electrical contact 46 is positioned under the
gain medium 14. The common electrical contact 46 can be used as a
ground for each of the lasers when applying electrical energy to a
laser and also for the amplifier when applying electrical energy to
the amplifier.
[0078] FIG. 8A illustrates assembly of the optical system using an
optical device constructed according to FIG. 6A through FIG. 6B and
a laser bar constructed according to FIG. 7A through FIG. 7D. The
optical device illustrated in FIG. 8A does not show either a
cross-sectional view or a sideview. Instead, the view of the
optical device shows the relative positions of different features
of the optical device when looking at a sideview of the optical
device. In contrast, the laser bar illustrated in FIG. 8A is a
cross-sectional view of the laser bar such as the cross section of
FIG. 7C.
[0079] The device can be assembled by moving the optical device and
the laser bar toward one another as indicated by the arrows labeled
A. Each of the stops 70 on the optical device is aligned with one
of the secondary stops on the laser bar.
[0080] FIG. 8A shows solder pads 84 positioned on the contact pads
68 on the laser bar platforms 66. The solder pads 84 can be used to
immobilize the laser bar relative to the optical device once the
laser bar is positioned on the optical device. The solder pads 84
can also provide electrical communication between the contact pads
68 on the laser platform and the contact pads 82 on the laser bar.
Accordingly, the transmitter electronics are in electrical
communication with the common electrical contact 46 and each of the
contact pads 68 on the laser platform. The transmitter electronics
can apply electrical energy to each of the lasers and/or the
amplifier by applying electrical energy across the associated
contact pad 68 and the common electrical contact 46.
[0081] FIG. 8B illustrates the optical device as shown in FIG. 8A
and the laser bar as shown in FIG. 8A assembled in an optical
assembly according to FIG. 6A. For the purposes of clarity, the
optical device is shown by the dashed lines while the laser bar is
shown by solid lines. The solder pads 84 are also removed from this
illustration. Each of the stops 70 on the optical device meets one
of the secondary stops on the laser bar. As a result, the vertical
movement of the optical device and the laser bar toward one another
is limited by the stops 70 butting against the secondary stops.
[0082] In FIG. 8A, circles show the mode of the first light signal
portion in the gain waveguides 12 and also in the mode of the
amplified light signal in the first amplifier waveguide 30. As is
evident from FIG. 8B, the modes are each aligned with the facets of
a cavity waveguide 16 or the output waveguide 36.
[0083] Additional details regarding the fabrication, structure,
and/or alignment of a laser bar with an optical device as shown in
FIG. 6A through FIG. 8B can be found in U.S. patent application
Ser. No. 12/215,693, filed on Jun. 28, 2008, granted U.S. Pat. No.
7,658,552, entitled "Interface Between Light Source and Optical
Component, and incorporated herein in its entirety.
[0084] Suitable partial return devices 18 include, but are not
limited to, a reflective optical grating such as a Bragg grating.
FIG. 9 is a perspective view of a portion of the device that
includes a Bragg grating constructed on a silicon-on-insulator
wafer. Recesses 86 extend into the top of the ridge 60 of the
waveguide. The recesses 86 are filled with a medium having a lower
index of refraction than the light-transmitting medium 52. The
medium can be a solid or a gas such as air. Accordingly, the
recesses 86 provide the variations in the index of refraction of
the waveguide that allow the recesses 86 to act as a Bragg grating.
The Bragg grating is illustrated with only four recesses 86 in
order to simplify the illustration. However, the Bragg grating can
include more than four recesses 86. The recesses 86 are arranged so
as to form a periodic pattern in the ridge 60. The range of
wavelengths reflected by a Bragg grating can be altered by changing
the depth and/or period of the grooves as well as other variables.
Accordingly, each of the laser cavities can include a Bragg grating
configured to reflect a different range of wavelengths.
[0085] Additional information regarding the structure,
construction, fabrication, and/or operation of the above laser
cavities and/or partial return devices and/or Bragg Gratings that
are suitable for use in the optical link can be found in U.S.
patent application Ser. No. 13/573,892, filed on Oct. 12, 2012,
entitled "Reduction of Mode Hopping in a Laser Cavity," and
incorporated herein in its entirety.
[0086] The modulators 22 are preferably each an intensity modulator
22 but can be other modulators 22 such as phase modulators 22. A
variety of different modulator 22 constructions are suitable for
use with waveguides on a silicon-on-insulator platform. In some
instances, the modulators 22 are constructed and operated as shown
in U.S. patent application Ser. No. 11/146,898; filed on Jun. 7,
2005; entitled "High Speed Optical Phase Modulator," and now U.S.
Pat. No. 7,394,948; or as disclosed in U.S. patent application Ser.
No. 11/147,403; filed on Jun. 7, 2005; entitled "High Speed Optical
Intensity Modulator," and now U.S. Pat. No. 7,394,949; or as
disclosed in U.S. patent application Ser. No. 12/154,435; filed on
May 21, 2008; entitled "High Speed Optical Phase Modulator," and
now U.S. Pat. No. 7,652,630; or as disclosed in U.S. patent
application Ser. No. 12/319,718; filed on Jan. 8, 2009; and
entitled "High Speed Optical Modulator;" or as disclosed in U.S.
patent application Ser. No. 12/928,076; filed on Dec. 1, 2010; and
entitled "Ring Resonator with Wavelength Selectivity;" or as
disclosed in U.S. patent application Ser. No. 12/228,671, filed on
Aug. 13, 2008, and entitled "Electrooptic Silicon Modulator with
Enhanced Bandwidth;" or as disclosed in U.S. patent application
Ser. No. 12/653,547, filed on Dec. 15, 2009, and entitled "Optical
Device Having Modulator Employing Horizontal Electrical Field;" or
as disclosed in U.S. patent application Ser. No. 12/660,149, filed
on Feb. 19, 2010, and entitled "Reducing Optical Loss in Optical
Modulator Using Depletion Region;" each of which is incorporated
herein in its entirety.
[0087] In some instances, modulators 22 that generate photocurrent
in response to the modulation of light signals are preferred
modulators 22 because they can also be used as a light sensor and
each of these modulators 22 can accordingly replace one of the
coupled waveguides 48 and the associated light sensor 50. Examples
of these modulators 22 are modulators 22 that make use of the Franz
Keldysh effect. An example of such as modulator 22 is disclosed in
U.S. patent application Ser. No. 12/653,547. FIG. 10 is a cross
section of the modulator 22 disclosed in U.S. patent application
Ser. No. 12/653,547. A ridge 100 of an electro-absorption medium
102 extends upward from a slab region 104 of the electro-absorption
medium 102. Doped regions 106 are both in the slab regions 104 of
the electro-absorption medium 102 and also in the ridge 100 of the
electro-absorption medium 102. For instance, doped regions 106 of
the electro-absorption medium 102 are positioned on the lateral
sides of the ridge 100 of the electro-absorption medium 102.
Additionally, the doped regions 106 extend from the ridge 100 into
the slab region 104 of the electro-absorption medium 102. FIG. 10
shows the transition of a doped region 106 from the ridge 100 of
the electro-absorption medium 102 into the slab region 104 of the
electro-absorption medium 102 as continuous and unbroken. When one
of the doped regions is an n-type doped region, the other doped
region is a p-type doped region.
[0088] Electrical conductors 108 are positioned on the slab region
104 of the electro-absorption medium 102. In particular, the
electrical conductors 108 each contact a portion of a doped region
106 that is in the slab region 104 of the electro-absorption medium
102.
[0089] During operation of the modulators 22 of FIG. 10, the
transmitter electronics can be employed to apply electrical energy
to the electrical conductors 108 so as to form an electrical field
in the electro-absorption medium 102. For instance, the transmitter
electronics can form a voltage differential between the doped
regions 106. The electrical field can be formed without generating
a significant electrical current through the electro-absorption
medium 102. The electro-absorption medium 102 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 102. 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 102 can absorb light signals received by
the electro-absorption medium 102 and increasing the electrical
field increases the amount of light absorbed by the
electro-absorption medium 102. Accordingly, the transmitter
electronics can tune the electrical field so as to tune the amount
of light absorbed by the electro-absorption medium 102. As a
result, the transmitter 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.
[0090] When the modulator 22 is operated as described above, a
photocurrent is generated in the modulator 22 in response to the
modulation of a light signal. Since the transmitter electronics can
be configured measure the level of photocurrent being generated,
the modulator 22 can also be used as a light sensor 50. The
transmitter electronics can adjust the level of electrical energy
applied to each of the lasers in response to amount of photocurrent
being indicated by the associated modulator 22 in a feedback loop.
For instance, if the transmitter electronics determine that the
amount of photocurrent being generated in a modulator 22 is above a
threshold, the transmitter electronics can reduce the level of
electrical energy being applied to the associated laser in an
effort to reduce the level of photocurrent being generated in the
modulator 22. These features allow modulator 22 that can also
function as light sensors to each replace one of the coupled
waveguides 48 and light sensors 50 shown on the embodiments of FIG.
1 through FIG. 6. Accordingly, the coupled waveguides 48 and the
associated light sensors 50 are optional.
[0091] Suitable electro-absorption media include semiconductors.
However, the light absorption characteristics of different
semiconductors are different. A suitable semiconductor for use with
modulators 22 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 (the modulation wavelength). For
instance, when x is zero, the modulator 22 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).
Additional details regarding the fabrication and/or structure of
modulators 22 that employ the Franz-Keldysh effect are disclosed in
U.S. patent application Ser. No. 12/653,547.
[0092] The light sensor 50 can be a conventional photodetector such
as a photodiode. Additionally or alternately, since the modulator
construction discussed in the context of FIG. 10 can also be
employed as a light sensor, the light sensors 50 of FIG. 2 through
FIG. 3 and/or the light sensors 11 of FIG. 1 and/or FIG. 4 can be
constructed as disclosed above. A variety of other light sensor 50
constructions are suitable for use with waveguides on a
silicon-on-insulator platform. For instance, the light sensor 50
can be constructed and/or operated as disclosed in U.S. patent
application Ser. No. 12/380,016, filed Feb. 19, 2009, and entitled
"Optical Device Having Light Sensor Employing Horizontal Electrical
Field;" U.S. patent application Ser. No. 12/804,769, filed Jul. 28,
2010, and entitled "Light Monitor Configured to Tap Portion of
Light Signal from Mid-Waveguide;" and/or in U.S. patent application
Ser. No. 12/803,136, filed Jun. 18, 2010, and entitled "System
Having Light Sensor with Enhanced Sensitivity;" and/or in U.S.
patent application Ser. No. 12/799,633, filed Apr. 28, 2010, and
entitled "Optical Device Having Partially Butt-Coupled Light
Sensor;" and/or in U.S. patent application Ser. No. 12/589,501,
filed Oct. 23, 2009, and entitled "System Having Light Sensor with
Enhanced Sensitivity;" and/or in U.S. patent application Ser. No.
12/584,476, filed Sep. 4, 2009, and entitled "Optical Device Having
Light Sensor Employing Horizontal Electrical Field;" each of which
is incorporated herein in its entirety.
[0093] Suitable electronics for operating the different portions of
the optical link such as the transmitter electronics and/or the
receiver electronics can include a controller. A suitable
controller includes, but is not limited to, a general purpose
processor, a digital signal processor (DSP), an application
specific integrated circuit (ASIC), a field programmable gate array
(FPGA) or other programmable logic device, discrete gate or
transistor logic, discrete hardware components, or any combination
thereof designed to perform the functions attributed to the
transmitter electronics and/or the receiver electronics. A
general-purpose processor may be a microprocessor, but in the
alternative, the controller may include or consist of any
conventional processor, microcontroller, or state machine. A
controller may also be implemented as a combination of computing
devices, e.g., a combination of a DSP and a microprocessor, a
plurality of microprocessors, one or more microprocessors in
conjunction with a DSP core, or any other such configuration.
[0094] The transmitter electronics and/or the receiver electronics
can optionally include a memory in communication with the
controller. The transmitter electronics and/or the receiver
electronics can store data for executing the functions of the
transmitter electronics and/or the receiver electronics in the
memory. The memory can be any memory device or combination of
memory devices suitable for read and/or write operations.
[0095] In some instances, the transmitter electronics and/or the
receiver electronics include a computer-readable medium in
communication with the controller. The computer-readable medium can
have a set of instructions to be executed by the controller. The
controller can read and execute instructions included on the
computer-readable medium. The controller executes the instructions
such that the transmitter electronics and/or the receiver
electronics perform one or more of the described functions. The
computer-readable medium cab be different from the memory or can be
the same as the memory. Suitable computer-readable media include,
but are not limited to, optical discs such as CDs, magnetic storage
diskettes, Zip disks, flash memories, magnetic tapes, RAMs, and
ROMs. Some functions of the transmitter electronics and/or the
receiver electronics may be executed using hardware as opposed to
executing these functions in firmware and/or software.
[0096] Although the transmitter is described in the context of each
laser cavity producing a laser signal having different wavelengths,
the transmitter can be constructed such that two or more of the
laser cavities generate laser signals having the same range of
wavelengths. The gain waveguides and/or cavity waveguides of laser
cavities configured to generate lasers signals having the same
wavelength can be optically coupled as a result of their proximity
on the device. Optical coupling of these one or more of these
waveguides can increase the power of the laser signals as described
in U.S. Patent Application Ser. No. 61/463,054, filed on Feb. 10,
2011, entitled "Laser Combining Light Signals from Multiple Laser
Cavities," and incorporated herein in its entirety. The gain
waveguides and/or cavity waveguides of laser cavities configured to
generate lasers signals of different wavelengths are optically
decoupled.
[0097] The transmitter and receiver in above optical link are
configured to operate over an operational temperature range. For
instance, the above optical link should be able to continue
operating when the transmitter and receiver are each positioned in
an environment having temperatures extending from TL to TH.
Further, since the transmitter and receiver are positioned in
different locations that are at different temperatures, the optical
link should be able to operate when the transmitter is at TL and
the receiver is at TH or the transmitter is at TH and the receiver
is at TL. In some instances, TL is below 0.degree. C., 10.degree.
C., or 20.degree. C. and/or TH is greater than 50.degree. C.,
70.degree. C., or 80.degree. C. The values of TH and TL associated
with the receiver can be the same as or different from the values
of TH and TL associated with the transmitter. The operational
temperature range is generally defined as part of the specification
for the transmitter and/or receiver. In general the operational
temperature range is designed so the transmitter and/or receiver
meets customer requirements. The operational temperature range is
generally the same for the transmitter and receiver.
[0098] Components on the transmitter generally produce heat during
operation. For instance, the lasers, modulators, any local heaters
on the transmitter, and any associated electronics produce heat
during operation. The heat produced by the components on the
transmitter can increase the temperature of the transmitter above
the temperature of the atmosphere in which the transmitter is
positioned by TC during operation of the transmitter. Accordingly,
the temperature of the transmitter is generally equal to about
TA+TC where TA is the temperature of the atmosphere in which the
transmitter is positioned. As a result, in order for the link to
operate in the above range of atmospheric temperatures, the
transmitter should be operated at temperatures up to TH+TC.
[0099] Components on the receiver also produces heat during
operation. For instance, the light sensors and any associated
electronics produce heat during operation of the receiver. The heat
produced by the components on the receiver can increase the
temperature of the receiver by RC during operation of the receiver.
Accordingly, the temperature of the receiver is generally equal to
about TA+RC where TA is the temperature of the atmosphere in which
the receiver is positioned. As a result, in order for the link to
operate in the above range of atmospheric temperatures, the
receiver should be operate at temperatures up to TH+RC.
[0100] During operation of the optical link, the transmitter and
receiver are each positioned in an atmosphere that can range from
TL to TH. During a portion of this temperature range, the
transmitter is operated without the use of common temperature
control and during another portion of this temperature range the
transmitter is operated with common temperature control. For
instance, suppose that the transmitter is located in an atmosphere
that can range from 0.degree. C. to 70.degree. C. When the
temperature of the atmosphere is from 40.degree. C. to 70.degree.
C., the common temperature controller is not employed to control
the temperature of the transmitter, however, when the temperature
of the atmosphere is from 0.degree. C. up to 40.degree. C., the
common temperature controller is used. Accordingly, the portion of
the atmospheric temperature between 40.degree. C. and 70.degree. C.
serves as the uncontrolled portion of the temperature range and the
portion of the atmospheric temperature from 0.degree. C. up to
40.degree. C. serves as the controlled portion of the temperature
range. When the common temperature control is used, the temperature
of the transmitter is kept above the lower limit of the
uncontrolled portion of the temperature range (TUTR+TC). For
instance, in the above example, when the atmospheric temperature is
below 40.degree. C., (TUTR=40.degree. C.) the common temperature
controller is operated so as to keep the temperature of the
transmitter at or above (40+TC).degree. C. In some instances, TH
serves as the upper limit of the uncontrolled portion of the
temperature range as is illustrated in the above example, so that
the temperature of the transmitter is operated at a maximum
temperature of (TH+TC).degree. C.
[0101] The different components on the transmitter respond to
temperature changes differently. For instance, the wavelength of
light produced by a particular laser cavity changes with
temperature changes. Similarly, the wavelength that is most
efficiently modulated by a modulator also changes in response to
changes in temperature. However, the responses to temperature
changes are different for different components. As a result, the
modulator may be able to efficiently modulate the laser light
signal received by that modulator when the transmitter is at a
particular temperature, however, as the temperature of the
transmitter changes, the modulator may no longer be able to
efficiently modulate the laser light signal received by that
modulator. The wavelengths that are most efficiently multiplexed by
the multiplexer also change with changes in temperature adding even
more complexity to the effects of temperature on the
transmitter.
[0102] The change in the modulation wavelengths output by the
transmitter are largely a result of the channel wavelength output
by each of the laser cavities shifting in response to changes in
temperature. The channel wavelength is the wavelength that the
laser cavity is designed to output or the most intense wavelength
output by a laser cavity at a particular temperature. Since the
laser cavities determine the wavelengths of the modulated light
signals output by the transmitter, the total possible range of
wavelengths output by the transmitter when the transmitter is
operated between TUTR+TC and TH+TC can be determined by
((TH-TUTR)*(d.lamda./dT)+2*tol) (Expression 1). In this above
expression, tol represent the manufacturing tolerance of the laser
cavities. For instance, when a laser cavity is designed to produce
a channel wavelength of XI, tol represents 3 times the standard
deviation of the result (3-sigma). As an example, a laser cavity
designed to produce a channel wavelength of XI will generally
result in a laser cavity that produces a channel wavelength of
.lamda.t+/-tol. The manufacturing tolerance for distributed
feedback lasers (DFBs) can be around +/-1 and can be as high as
+/-3 nm. The laser cavities disclosed above are external cavity
lasers (ECLs) and generally have a lower tolerance of around +/-0.5
nm. In the above expression, d.lamda./dT represents the change in
the channel wavelength output by a laser cavity in response to
temperature changes. For a DFB laser or an ECL laser working in the
C-band range of wavelengths (1530-1565 nm), d.lamda./dT is
generally greater than 0.07 nm/.degree. C., or 0.075 nm/.degree. C.
and/or less than 0.085 nm/.degree. C., or 0.095 nm/.degree. C. For
a DFB laser or an ECL laser constructed as disclosed above, the
d.lamda./dT is about 0.08 nm/.degree. C. for C-band
wavelengths.
[0103] The mutliplexer is associated with several center
wavelengths. The center wavelengths of the multiplexer are the
wavelengths at the center of the bandwidth for each waveguide that
provides input to the multiplexer. In other words, each of the
center wavelengths is one of the wavelengths that is most
efficiently multiplexed by the multiplexer. The center wavelengths
of the multiplexer also changes in response to temperature changes.
However, the multiplexers disclosed above generally have a
temperature dependence that tracks the temperature dependence of
the laser cavities. For instance, the center wavelengths of the
multiplexer generally shifts about 0.085 nm/.degree. C. for C-band
wavelengths. Because the 0.085 nm/.degree. C. for the multiplexer
approximates the 0.08 nm/.degree. C. d.lamda./dT for the lasers
disclosed above, the value of d.lamda./dT used in the above
expression can be d.lamda./dT for the laser cavities or for the
center wavelength of the multiplexer. However, in general, the
smaller of the two values is used in the Expression 1.
[0104] The wavelengths that can be efficiently processed by the
receiver also shift with changes in temperature. Light sensors
constructed as disclosed above are able to sense light across a
broad range of wavelengths. As a result, the total range of
wavelengths over which the receiver can operate without common
temperature control is controlled by the demultiplexer.
[0105] Since the shift in wavelengths that can be efficiently
processed by the receiver is essentially controlled by the
demultiplexer, the total range of wavelengths that a receiver held
at constant temperature can process efficiently can be expressed as
DBW*CS (Expression 2). In expression 6, DBW represents the
bandwidth of the demultiplexer. The bandwidth of the demultiplexer
is the range over which the modulation wavelength of the modulated
signals can shift before the demultiplexer is unable to efficiently
demultiplex the light signals. The bandwidth, DBW, is expressed as
a percentage of the channel spacing, CS. For instance, if the
channel wavelengths produced by the laser cavities (channels) are
each separated by 8 nm (CS=8 nm) and the modulation wavelengths of
the modulated signals can each shift over a range of 6 nm before
the multiplexer is unable to efficiently multiple the light
signals, the DBW is 75% (6 nm/8 nm). The DBW for the demultiplexers
constructed as discussed above and included in a receiver is
generally about 65%, 50%, or 30%. The DBW for the demulitplexer
included in a receiver can be higher than the BW for the
multiplexer included in the transmitter because the multiplexer
input waveguides included in the transmitter are preferably single
mode where the demulitplexer output waveguides included in the
receiver can optionally be multi-mode. The ability of the
demultiplexer output waveguides to be multi-mode provides for a
broader bandwidth than can be achieved with a single mode
demultiplexer for no significant additional optical loss.
[0106] The DBW for a demuliplexer can be determined by a
calculation of the overall loss budget of the optical link.
Generally, demultiplexer bandwidths are given to 1 dB loss points
in that over the bandwidth of the demultiplexer the insertion loss
will not vary by more than 1 dB. If there is sufficient loss budget
for the link, then the bandwidth may be widened by defining it with
respect to higher loss points such as 2 dB. In some instances, the
maximum loss over the bandwidth is 2 dB, and the overall system
loss budget must be able to accommodate this additional loss. Loss
factors to be considered in calculating a loss budget are the
losses in the receiver and transmitter chips including the
modulators, the coupling losses of the optical fiber to the
transmitter and receiver chips, the optical loss in the optical
fiber, the laser output power and the detector sensitivity at the
receiver. If the total optical power arriving at the detector is
below the detector sensitivity, then the optical link will not be
viable. In addition to the loss factors, there are other
considerations such as fiber dispersion and sources of noise within
the system that in most cases can be considered as equivalent to
additional loss. Since some of these factors vary with environment
and age, most systems will specify a loss margin of for example 3-6
dB. For instance, the overall loss budget must allow for an
additional 3-6 dB loss from variable sources or from system
aging.
[0107] Expression 1 and expression 2 can be compared to determine
the largest possible uncontrolled temperature range. For instance,
note that the largest possible uncontrolled temperature range
occurs when the total range of each modulation wavelength that can
be output by the transmitter is equal to the total range of
wavelengths that a receiver held at constant temperature can
process efficiently. Accordingly, the largest possible uncontrolled
temperature range occurs when ((TH-TUTR)*(d.lamda./dT)+2*tol)
(Expression 1)=DBW*CS (Expression 2). We can solve this for TH-TUTR
to find that the largest possible uncontrolled temperature range
occurs when TH-TUTR=(DBW*CS-2*tol)/(d.lamda./dT). As a result, the
lowest possible TUTR that will allow the receiver to efficiently
process each of the modulated wavelengths output by the transmitter
when the transmitter is operated at any temperature from TUTR to TH
without the use of a common temperature controller is
TUTR=TH-(DBW*CS-2*tol)/(d.lamda./dT). The value of TUTR in this
equation may or may not represent the actual value of TUTR used
during the operation of the optical link because the TUTR in this
equation represents the lower limit of possible values for TUTR. As
a result, this expression is better written as
TUTR.sub.lim=TH-(DBW*CS-2*tol)/(d.lamda./dT) (Expression 3).
Accordingly, the transmitter can be operated without the use of a
common temperature controller when the atmosphere in which the
transmitter is placed is in a range that extends from TH down to a
temperature that is equal to or greater than TUTR.sub.lim.
[0108] The performance of the modulators will become less efficient
as the temperature of the transmitter's atmosphere fluctuates. For
instance, the most efficient wavelength modulated by a modulator is
the modulation wavelength for that modulator. The modulation
wavelength for a modulator also shifts with wavelength. However,
unlike the multiplexer, the shift in modulation wavelength does not
track the shift in channel wavelength of the laser cavities. For
instance, for modulators the change in modulation wavelength with
temperature d.lamda./dT is generally greater than 0.6 nm/.degree.
C., or 0.7 nm/.degree. C. and/or less than 0.8 nm/.degree. C., or
0.9 nm/.degree. C. For a modulator constructed as disclosed above,
the change in modulation wavelength with temperature is about 0.76
nm/.degree. C. (.about.0.8 nm/.degree. C.). However, as discussed
above, for a DFB laser or an ECL laser constructed as disclosed
above, the d.lamda./dT is about 0.08 nm/.degree. C. As a result,
the response of the modulation wavelength to temperature changes
can be nearly an order of magnitude higher than the response of the
channel wavelength.
[0109] Since the modulation wavelength changes with temperature
more than the wavelength output by the laser cavity, the modulator
efficiency decreases in response to temperature changes. This
concept is illustrated in FIG. 11 for a modulator constructed as
disclosed above. FIG. 11 shows the amount of optical loss that
results from modulating a particular wavelength. As is evident from
FIG. 11, a modulator can modulate over a range of about 45 nm while
incurring optical loss of less than 1 dB or can modulate over a
range of about 65 nm while incurring optical loss of less than 2
dB.
[0110] Suppose all of the modulators have the same construction and
accordingly have the same modulation wavelength when they are at
the same temperature. For instance, suppose that the modulators all
have a modulation wavelength of 1580 nm at 50.degree. C. Also
suppose that the transmitter includes four laser cavities and that
at 50.degree. C. the channel wavelengths are 1568 nm, 1576 nm, 1584
nm, and 1592 nm. Further, suppose that TH is 70.degree. C. and TUTR
is 30.degree. C. In this arrangement, when the temperature is at
30.degree. C. the largest wavelength that will have to be modulated
is 1592 nm-20.degree. C.*0.08 nm/.degree. C.=1590.4 nm but the
modulator will have a modulation wavelength of 1580-20.degree.
C.*0.76 nm/.degree. C.=1564.8 nm. When the temperature is at
70.degree. C. the lowest wavelength that will have to be modulated
is 1568 nm+20.degree. C.*0.08 nm/.degree. C.=1569.6 nm but the
modulator will have a modulation wavelength of 1580+20.degree.
C.*0.76 nm/.degree. C.=1595.2 nm. As a result, the modulators must
modulate light signals where the difference between the modulation
wavelength and the actual wavelength being modulated ranges from
+25.6 nm to -25.6 nm for a total range of 51.2 nm.
[0111] The above range of 51.2 nm does not take into consideration
the manufacturing tolerance of the modulators. When a modulator is
designed for a modulation wavelength of .lamda.m, mtol represents 3
times the standard deviation of the result. As an example, a
modulator designed for a modulation wavelength of .lamda.m will
generally result in a modulator with a modulation wavelength of
.lamda.m+/-mtol. The manufacturing tolerance for modulators that
are fabricated as disclosed above is around +/-3 nm and can be as
high as +/-5 nm. Adding a factor of 2*mtol to the above range
corrects for manufacturing tolerance. As a result, the modulators
must modulate light signals where the difference between the
modulation wavelength and the actual wavelength being modulated
ranges from 51.2 nm+2*mtol or 51.2 nm+2*5 nm or 61.2 nm. FIG. 11
shows that a range of 65 nm is associated with a loss of around 2
dB. As a result, if a loss of 2 dB is acceptable within the loss
budget of the link, this arrangement of modulators is
acceptable.
[0112] As noted above, rather than having the same construction,
each of the modulators can be configured such that the modulation
wavelength matches the channel wavelength at a particular
temperature. For instance, different modulators can be constructed
differently and/or different modulators can include localized
heaters that are operated so as to change the modulation wavelength
to a particular level. Now consider the above example but with each
of the modulators constructed to have a modulation wavelength
matched to the received channel wavelength at 50.degree. C. For
instance, suppose that the transmitter includes four laser cavities
and that at 50.degree. C. the channel wavelengths are 1568 nm, 1576
nm, 1584 nm, and 1592 nm and that the associated modulation
wavelengths are respectively 1568 nm, 1576 nm, 1584 nm, and 1592
nm. Further, suppose that TH is 70.degree. C. and TUTR is
30.degree. C. In this arrangement, when the temperature is at
30.degree. C. the largest wavelength that will have to be modulated
is 1592 nm-20.degree. C.*0.08 nm/.degree. C.=1590.4 nm but the
associated modulator will have a modulation wavelength of
1592-20.degree. C.*0.76 nm/.degree. C.=1576.8 nm. When the
temperature is at 70.degree. C. the lowest wavelength that will
have to be modulated is 1568 nm+20.degree. C.*0.08 nm/.degree.
C.=1569.6 nm but the modulator will have a modulation wavelength of
1568+20.degree. C.*0.76 nm/.degree. C.=1583.2 nm. As a result, the
modulators must modulate light signals where the difference between
the modulation wavelength and the actual wavelength being modulated
ranges from +13.6 nm to -13.6 nm for a total range of 27.2 nm. When
we correct this range for manufacturing tolerance, the modulators
must modulate light signals where the difference between the
modulation wavelength and the actual wavelength being modulated
ranges from 27.2 nm+2*mtol or 27.2 nm+2*5 nm or 37.2 nm. Because
this 37.2 nm range is substantially below the 45 nm range disclosed
in FIG. 11, modulators in this arrangement will be responsible for
a very low level of optical loss. However, when localized heaters
are used on the modulators in order to achieve this result, there
becomes a balance between the power savings that results from
limiting the temperature range over which the common temperature
controller is used and the increased power required by the
localized heaters.
[0113] The above discussion assumes that a common temperature
controller maintains the receiver at a constant temperature. In
these circumstances, the common temperature controller generally
maintains the receiver at a temperature that is DTR above the top
of the operational range (TH). For instance, the common temperature
controller generally maintains the receiver at TH+DTR. In some
instances, DTR is a constant that is greater than or equal to RC.
For instance, DTR can be 10.degree. C., 15.degree. C., or
20.degree. C. For instance, when TH is 70.degree. C., the receiver
is generally maintained at a temperature of about 80.degree. C.
[0114] In many circumstances, it is possible to operate the
receiver with an uncontrolled receiver temperature range. For
instance, when TUTR is greater than TUTR.sub.lim, the temperature
of the receiver can be allowed to float for at least portion of the
operational temperature range (TL to TH). The receiver is also
positioned in an atmosphere that can range from TL to TH
independent of the temperature of the transmitter. During a portion
of this temperature range, the receiver can be operated without the
use of common temperature control and during another portion of
this temperature range the transmitter is operated with common
temperature control. For instance, suppose that the receiver is
located in an atmosphere that can range from 0.degree. C. to
70.degree. C. When the temperature of the atmosphere is from
60.degree. C. to 70.degree. C., the common temperature controller
is not employed to control the temperature of the receiver,
however, when the temperature of the atmosphere is from 0.degree.
C. up to 60.degree. C., the common temperature controller is used
to control the temperature of the receiver. Accordingly, the
portion of the atmospheric temperature between 60.degree. C. and
70.degree. C. serves as the uncontrolled receiver portion of the
temperature range and the portion of the atmospheric temperature
from 0.degree. C. up to 60.degree. C. serves as the controlled
receiver portion of the temperature range. When the common
temperature control is used, the temperature of the receiver is
kept at or above the lower limit of the uncontrolled portion of the
temperature range (TRUTR). For instance, in the above example, when
the atmospheric temperature is below 60.degree. C., the common
temperature controller is operated so as to keep the temperature of
the receiver at or above 60.degree. C. and below 70.degree. C.+RC.
In some instances, TH serves as the upper limit of the uncontrolled
receiver portion of the temperature range as is illustrated in the
above example.
[0115] As noted above, when TUTR is greater than TUTR.sub.lim, the
operational temperature range for the atmosphere of the receiver
can include an uncontrolled receiver temperature range extending
from TH down to TRUTR. The value of TRUTR can be decreased until
the range of modulation wavelengths that the receiver can process
at both TH and TRUTR is at least equal to the range of modulation
wavelengths that each modulation signal can have during operation
of the transmitter while the atmosphere of the transmitter varies
to TL to TH. In some instances, it may be possible to bring TRUTR
all the way down to TL while retaining this condition. As a result,
in some instances, the uncontrolled receiver portion of the
temperature range can extend from TH to TL. In these instances, a
common temperature controller may be optional for the receiver.
[0116] In some instances, the components on the transmitter and
receiver are designed to be wavelength matched at a particular
temperature. For instance, the operational range for the
transmitter and receiver is generally about 0.degree. C. to
70.degree. C. As a result, the components on the transmitter are
receiver are designed to be wavelength matched at a target
temperature between 0.degree. C. to 70.degree. C. such as
60.degree. C. For instance, the modulation wavelengths can be
matched to the center wavelengths at the target temperature.
Alternately, the modulation wavelengths can be matched to the
center wavelengths and one of the modulation wavelengths can be
matched to a channel wavelength at the target temperature.
Alternately, the modulation wavelengths, the channel wavelengths,
and the center wavelengths can be matched at the target
temperature. Matching of these wavelengths provides more efficient
operation of the transmitter over a larger range of temperatures.
In some instances, the target temperature is included in the
uncontrolled portion of the operational temperature range of the
transmitter.
[0117] As noted above, the transmitter electronics employ a common
temperature controller to control the temperature of the
transmitter in response to output from one or more common
temperature sensors. For instance, when the output of the one or
more common temperature sensors indicates that the temperature of
the transmitter is below TUTR (or TUTR+TC), the transmitter
electronics can operate the common temperature controller such that
the temperature of the transmitter is at or above TUTR, or at or
above TUTR+TC. In some instances, when the output of the one or
more common temperature sensors indicates that the temperature of
the transmitter is below TUTR (or TUTR+TC), the transmitter
electronics can operate the common temperature controller such that
the temperature of the transmitter is at or above TUTR, or at or
above TUTR+TC and is also less than or equal to TH, TH+TC, or
TH+DTT where DTT is a constant that is greater than TC. In
contrast, when the output of the one or more common temperature
sensors indicates that the temperature of the transmitter is above
TUTR (or above TUTR+TC), the transmitter electronics can allow the
temperature of the transmitter to float and/or can operate the
common temperature controller such that the temperature of the
transmitter is less than or equal to TH, TH+TC, or TH+DTT where DTT
is a constant that is greater than TC.
[0118] In some instances, the transmitter electronics are also in
electrical communication with one or more atmosphere temperature
sensors. The one or more atmosphere temperature sensors can be
positioned such that the output of the one or more atmosphere
temperature sensors indicates the temperature of the atmosphere in
which the transmitter is positioned. The transmitter electronics
can operate the common temperature controller in response to output
from the one or more common temperature sensors and the one or more
atmosphere temperature sensors. For instance, when the output of
the one or more atmosphere sensors indicates that temperature of
the atmosphere falls below TUTR (or TUTR+TC), the transmitter
electronics can use the output of the one or more common
temperature sensors to operate the common temperature controller
such that the temperature of the transmitter is at or above TUTR,
or at or above TUTR+TC. In some instance, when the output of the
one or more atmosphere sensors indicates that temperature of the
atmosphere falls below TUTR (or TUTR+TC), the transmitter
electronics can use the output of the one or more common
temperature sensors to operate the common temperature controller
such that the temperature of the transmitter is at or above TUTR,
or at or above TUTR+TC and is also less than or equal to TH, TH+TC,
or TH+DTT where DTT is a constant that is greater than TC. In
contrast, when the output of the one or more atmosphere sensors
indicates that temperature of the atmosphere is at or above TUTR
(or TUTR+TC), the transmitter electronics can allow the temperature
of the transmitter to float and/or can operate the common
temperature controller such that the temperature of the transmitter
is less than or equal to TH, TH+TC, or TH+DTT where DTT is a
constant that is greater than TC. Suitable atmosphere temperature
sensors include, but are not limited to, thermocouples,
thermistors, resistive thermal devices (RTDs), and semiconductor
temperature sensors.
[0119] As noted above, in some instances, the receiver electronics
employ a common temperature controller to control the temperature
of the receiver in response to output from one or more common
temperature sensors. For instance, when the output of the one or
more common temperature sensors indicates that the temperature of
the receiver is below TRUTR (or TRUTR+RC), the receiver electronics
can operate the common temperature controller such that the
temperature of the receiver is at or above TRUTR, or at or above
TRUTR+RC. In some instance, when the output of the one or more
common temperature sensors indicates that the temperature of the
receiver is below TRUTR (or TRUTR+RC), the receiver electronics can
operate the common temperature controller such that the temperature
of the receiver is at or above TRUTR, or at or above TRUTR+RC and
is also less than or equal to TH, TH+RC, or TH+DTR where DTR is a
constant that is greater than RC. In contrast, when the output of
the one or more common temperature sensors indicates that the
temperature of the receiver is at or above TRUTR (or TRUTR+RC), the
receiver electronics can allow the temperature of the receiver to
float and/or can operate the common temperature controller such
that the temperature of the receiver is less than or equal to TH,
TH+RC, or TH+DTR where DTR is a constant that is greater than
RC.
[0120] In some instances, the receiver electronics are also in
electrical communication with one or more atmosphere temperature
sensors. The one or more atmosphere temperature sensors can be
positioned such that the output of the one or more atmosphere
temperature sensors indicates the temperature of the atmosphere in
which the receiver is positioned. The receiver electronics can
operate the common temperature controller in response to output
from the one or more common temperature sensors and the one or more
atmosphere temperature sensors. For instance, when the output of
the one or more atmosphere sensors indicates that temperature of
the atmosphere falls below TRUTR (or TRUTR+RC), the receiver
electronics can use the output of the one or more common
temperature sensors to operate the common temperature controller
such that the temperature of the receiver is at or above TRUTR, or
at or above TRUTR+RC. In some instance, when the output of the one
or more atmosphere sensors indicates that temperature of the
atmosphere falls below TRUTR (or TRUTR+RC), the receiver
electronics can use the output of the one or more common
temperature sensors to operate the common temperature controller
such that the temperature of the receiver is at or above TRUTR, or
at or above TRUTR+RC and is also less than or equal to TH, TH+RC,
or TH+DTR where DTR is a constant that is greater than RC. In
contrast, when the output of the one or more atmosphere temperature
sensors indicates that the temperature of the receiver is at or
above TRUTR (or TRUTR+RC), the receiver electronics can allow the
temperature of the receiver to float and/or can operate the common
temperature controller such that the temperature of the receiver is
less than or equal to TH, TH+RC, or TH+DTR where DTR is a constant
that is greater than RC.
[0121] The above discussions set forth circumstances where
electronics allow the temperature of the receiver or transmitter to
float. When the electronics allow the temperature of the receiver
or transmitter to float, the electronics may refrain from employing
the common temperature controller to control the temperature of the
receiver or transmitter. As a result, the temperature of the
receiver or transmitter reflects the temperature of the atmosphere
in which the receiver or transmitter is positioned. For instance,
the temperature of a floating transmitter will generally be equal
to TA+TC and the temperature of a floating receiver will generally
be equal to TA+RC. These equations assume that the TA is constant
or changes slowly enough that the receiver or transmitter remain in
thermal equilibrium with the atmosphere. In the event that the
temperature atmosphere changes more quickly than the temperature of
the receiver or transmitter, there may be time lag before the
temperature of a floating transmitter becomes TA+TC or the
temperature of a floating becomes TA+RC.
[0122] As is also discussed above, in some instances, the receiver
electronics operate the common temperature controller such that the
temperature of the receiver is held at a substantially constant
temperature. For instance, the receiver electronics can use the
output of the one or more common temperature sensors to hold the
temperature of the receiver at TH+RC or TH+DTR where DTR is a
constant that is greater than RC.
[0123] In some instances, TUTR is higher than 10, 20, or 30.degree.
C. and/or is 10, 20, or 30.degree. C. more than the lower limit of
the operational range, TL, for the transmitter and the lower limit
of the operational temperature range for the transmitter is greater
than -40, -20, or 0.degree. C.
Example 1
[0124] An optical link is configured to operate with the receiver
and transmitter in different environments that can each vary from
TL=0.degree. C. to TH=70.degree. C. The receiver includes a common
temperature controller that is operated so as to retain the
temperature of the receiver constant above TH+RC at 80.degree. C.
The transmitter is configured to generate four modulated signals
with a channel spacing (CS) of 8 nm. The channel wavelengths are
1515 nm, 1523 nm, 1531 nm, and 1539 nm at 60.degree. C.
Accordingly, the combiner is configured to have center wavelengths
of 1515 nm, 1523 nm, 1531 nm, and 1539 nm at 60.degree. C. The
modulators can be constructed such that each modulation wavelength
is matched to the channel wavelength at 60.degree. C. Alternately,
the modulators can each have the same construction and can
accordingly have the same modulation wavelength at 60.degree. C.
For instance, the modulators can each have a modulation wavelength
of 1521 nm at 60.degree. C.
[0125] The bandwidth (DBW) for the demultiplexers constructed as
discussed above can be as high as 65% and the external cavity
lasers (ECLs) disclosed above generally have a manufacturing
tolerance of around +/-0.5 nm. For a DFB laser or an ECL laser
constructed as disclosed above, the d.lamda./dT is about 0.08
nm/.degree. C. and the d.lamda./dT for the center wavelength of the
above multiplexers is about 0.08 nm/.degree. C. Substituting these
numbers into TUTR.sub.lim=TH-(DBW*CS-2*tol)/(d.lamda./dT) shows
that the lowest possible value for TUTR (TUTR.sub.lim) is
30.degree. C. Accordingly, when the receiver is held at a constant
temperature, the uncontrolled temperature range for the atmosphere
of the transmitter can extend from 30.degree. C. to 70.degree. C.
(TUTR=30.degree. C.). Further, using a higher TUTR such as
40.degree. C. would allow the operational range of the receiver to
include an uncontrolled receiver temperature range.
[0126] A single gain medium can be used for each of the laser
cavities. Additionally, when each of the modulators has the same
structure and composition and has a modulation wavelength of 1521
nm at 60.degree. C., a loss of 2 dB is expected to be associated
with the modulators. However, when each of the modulators is
tailored to have a modulation wavelength that is matched to the
channel wavelength at 60.degree. C., the expected loss drops to
around 1 dB.
[0127] As discussed above, one technique for constructing the
modulators such that each modulation wavelength is matched to the
channel wavelength at 60.degree. C. is to place localized
temperature controllers on the transmitter such that each modulator
is associated with one or more localized temperature controllers.
The localized temperature controllers can then be operated such
that the modulation wavelength of all or a portion of the
modulators matches the channel wavelength received at 60.degree. C.
In one arrangement, each of the modulators is constructed such that
at 60.degree. C., the modulation wavelengths are all 1515 nm;
however, the localized temperature controllers are operated such
when the transmitter is in a 60.degree. C. atmosphere, the
modulation wavelength of three of the modulators is shifted to 1523
nm, 1531 nm, and 1539 nm. Operating these modulators in this manner
will generally be associated with about a 90 mW power demand.
Example 2
[0128] An optical link is configured to operate with the receiver
and transmitter in different environments that can each vary from
TL=0.degree. C. to TH=70.degree. C. The receiver includes a common
temperature controller that is operated so as to retain the
temperature of the receiver constant above TH+RC at 80.degree. C.
The transmitter is configured to generate four modulated signals
with a channel spacing (CS) of 20 nm. The channel wavelengths are
1515 nm, 1535 nm, 1555 nm, and 1575 nm at 60.degree. C.
Accordingly, the combiner is configured to have center wavelengths
of 1515 nm, 1535 nm, 1555 nm, and 1575 nm at 60.degree. C. The
modulators cannot each have the same construction because a single
modulator construction does generally not have the bandwidth to
efficiently modulate wavelengths from 1515-1575 nm varied over
temperatures from 0-70.degree. C. However, the modulators can be
constructed such that each modulation wavelength is matched to the
received channel wavelength at 60.degree. C. For instance,
different levels of silicon in the electro-absorption media of
different modulators combined with localized temperature
controllers can be employed to shift the modulation wavelengths to
the desired levels. Additionally, due to the larger range of
wavelengths, it may not be desirable to use a single gain medium
for each of the laser cavities. As a result, the transmitter may
need to include multiple gain media.
[0129] The bandwidth (DBW) for the demultiplexers constructed as
discussed above can be as high as about 65% and the external cavity
lasers (ECLs) disclosed above generally have a manufacturing
tolerance of around +/-0.5 nm. For a DFB laser or an ECL laser
constructed as disclosed above, the d.lamda./dT is about 0.08
nm/.degree. C. and the d.lamda./dT for the center wavelength of the
above multiplexers is about 0.08 nm/.degree. C. Substituting these
numbers into TUTR.sub.lim=TH-(DBW*CS-2*tol)/(d.lamda./dT) shows
that the lowest possible value for TUTR (TUTR.sub.lim) is around
-70.degree. C. When the value of TUTR is below TL (0.degree. C.),
then a common temperature controller is not necessary and the
uncontrolled portion of the operational temperature range for the
transmitter can be 100% of the operational temperature range for
the transmitter. Further, when TUTR is below the 0.degree. C. lower
limit of the operational range for the transmitter, a demultiplexer
with a much lower bandwidth can be employed. For instance, a
demultiplexer with a bandwidth ratio of 35% would provide a
TUTR.sub.lim that is still below 0.degree. C. (-7.5 0.degree. C.).
Additionally, or alternately, the operational range of the receiver
could include an uncontrolled receiver temperature range.
Example 3
[0130] An IEEE standard LAN employs channel wavelengths around 1.3
.mu.m with a channel spacing around 4.5 nm. At these wavelengths,
d.lamda./dT is about 0.07 nm/.degree. C. and the bandwidth ratio
(DBW) for the demultiplexers is about 65%. The different
environments for the receiver and transmitter can each vary from
TL=0.degree. C. to TH=70.degree. C. The receiver includes a common
temperature controller that is operated so as to retain the
temperature of the receiver constant above TH+RC at 80.degree. C.
Substituting these numbers into
TUTR.sub.lim=TH-(DBW*CS-2*tol)/(d.lamda./dT) shows that the lowest
possible value for TUTR (TUTR.sub.lim) is around 40.degree. C.
[0131] 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.
* * * * *