METHOD AND APPARATUS OF PERFORMING ONT WAVELENGTH
TUNING VIA A HEAT SOURCE
Cross Reference to Related Applications
[0001] This application claims benefit to provisional
application 61/416,859, entitled "Colorless ONT Laser For
Cyclic WDM PON Scheme", filed on November 24, 2010, the
entire contents of which are hereby incorporated by
reference .
Technical Field of the Invention
[0002] This invention relates to a method and apparatus of
modifying or tuning frequency/wavelength characteristics of
an optical signal, and, in particular, to tuning the laser
to a predefined wavelength by utilizing a heat source.
Background of the Invention
[0003] In general, different forms of wavelength division
multiplexing (WDM) passive optical networks (PONs) face
dilemmas with deploying networks utilizing cost effective
optical network termination units (ONTs) . Fiber optic
cabling, termination units and related hardware are
expensive and require significant amounts of precision
tuning in order to maintain optimal performance.
[0004] The ONT, in general, must be configured to operate
at a specific wavelength or wavelength group based on a
random assignment from the central office equipment. The
wavelength assignment scheme may be performed by an optical
line terminal (OLT) , which assigns the same wavelength to
more than one ONT. Alternatively, the assignment scheme
may assign one wavelength or a set of wavelengths to a
particular group of users, which, in turn, are utilized by
a corresponding group of ONTs. PONs require constant
configuration and reconfiguration for wide-scaledeployment. There is an ongoing challenge to produce
color-less laser transmitters in the ONTs of the PONs that
are cost effective, while providing flexibility with the
wavelength tuning, assignment and adjustment procedure.
[0005] When implementing wavelength division multiplexing
(WDM) wavelength assignments, the implementation must be
planned to accommodate a large number of ONTs. WDM PON
systems may not require a single wavelength per end user in
the downstream and upstream directions. Within the user
access environment, it is necessary to accommodate
continuously growing user bandwidth requirements while
maintaining a reasonable cost basis.
Summary of the Invention:
[0006] One example embodiment of the present invention may
include a method of tuning a transmitted signal received
from a first network terminal at a second network terminal.
The method may include receiving the signal at the second
network terminal, the signal operating at a first
wavelength, and determining a port used to receive the
signal at the second network terminal. The method may also
include identifying a predetermined port wavelength used as
a basis to shift the first wavelength to the predetermined
port wavelength for subsequent signals received. The
method may also include transmitting the predetermined port
wavelength information to the first network terminal to
inform the first network terminal to tune subsequent
signals to the desired wavelength for the port.
[0007] Another example embodiment of the present invention
may include an optical line terminal (OLT) configured to
tune a signal received from an optical network terminal
(ONT) . The OLT may include a receiver configured toreceive the signal operating at a first wavelength, and a
processor configured to determine a port used to receive
the signal, and identify a predetermined port wavelength
used as a basis to shift the first wavelength to the
predetermined port wavelength for subsequent signals
received. The OLT may also include a transmitter
configured to transmit the predetermined port wavelength
information to the ONT to inform the ONT to tune subsequent
signals to the desired wavelength for the port.
Brief Description of the Drawings :
[0008] FIG. 1 illustrates an example ONT system, according
to example embodiments of the present invention.
[0009] FIG. 2A illustrates an example wavelength group,
according to a conventional approach to wavelength tuning.
[0010] FIG. 2B illustrates an example wavelength group,
according to example embodiments of the present invention.
[0011] FIG. 3 illustrates an example ONT die device,
according to example embodiments of the present invention.
[0012] FIG. 4 illustrates an example network entity
configured to perform the operations of the present
invention, according to example embodiments of the present
invention .
[0013] FIG. 5 is a flow diagram of an example method of the
present invention.
Detailed Description of the Invention:
[0014] It will be readily understood that the components of
the present invention, as generally described and
illustrated in the figures herein, may be arranged and
designed in a wide variety of different configurations.
Thus, the following detailed description of the embodiments
of a method, apparatus, and system, as represented in theattached figures, is not intended to limit the scope of the
invention as claimed, but is merely representative of
selected embodiments of the invention.
[0015] The features, structures, or characteristics of the
invention described throughout this specification may be
combined in any suitable manner in one or more embodiments.
For example, the usage of the phrases "example
embodiments", "some embodiments", or other similar
language, throughout this specification refers to the fact
that a particular feature, structure, or characteristic
described in connection with the embodiment may be included
in at least one embodiment of the present invention. Thus,
appearances of the phrases "example embodiments", "in some
embodiments", "in other embodiments", or other similar
language, throughout this specification do not necessarily
all refer to the same group of embodiments, and the
described features, structures, or characteristics may be
combined in any suitable manner in one or more embodiments.
[0016] Example embodiments of the present invention may
provide implementing a wavelength division multiplexing
(WDM) communication system with a tunable wavelength
transmitter/receiver. When operating a PON communication
system, one approach to maintaining a reasonable cost
structure may include allocating a single wavelength or a
group of wavelengths to a particular group of users. The
overall bandwidth efficiency may increase as a result of a
number of users sharing the same wavelength or wavelength
group .
[0017] FIG. 1 illustrates an example OLT and ONT system
configuration, according to example embodiments of the
present invention. Referring to FIG. 1 , an OLT 102 may belocated at a central office and may include an arrayed
waveguide grating (AWG) filters 112 and a processor 108.
Specifically, the AWG filters 112 are cyclic AWG filters.
The OLT 102 may be connected to the remote ONT 110, or may
b e in communication with the remote ONT 110, which includes
an ONT laser device.
[0018] The ONT laser device may have a tunable laser that
includes a heat source 114, sensor 116 and controller 118.
The output 120 is a laser signal that has been modified by
the heat source 114 to operate at a particular wavelength.
The ONT 110 also includes a sensor 116 and a controller
118. In operation, the signal present from the ONT at the
AWG filters 112 may detect a needed temperature adjustment
of the ONT laser device and transmit a control signal to
the ONT laser device of the ONT 110. The control signal
may be used to activate the heat source 114. The sensor
116 provides feedback regarding the temperature of the ONT
laser device, which is part of ONT 110 and provides that
feedback to the controller 118, which is also part of ONT
110. The controller 118 may then increase the amount of
heat applied to the surface of the laser transmitter of ONT
110, or, reduce the amount of heat if the temperature is
higher than necessary to modify the wavelength of the
present signal. The amount of heat applied is based on a
predefined amount of heat needed to tune the wavelength of
the ONT laser device. The amount of heat may be defined in
a table used to correlate with the tuning of the desired
wavelength of the signal, which is stored in memory (not
shown) .
[0019] According to another example embodiment of the
present invention, the sensor and heater may be integratedinto the laser die via a semiconductor fabrication
procedure. Such a laser die configuration may reduce the
thermal mass and time lag required for sensing and
controlling the laser's wavelength tuning.
[0020] The tuning of the laser device wavelength may be
performed by the heat source 114. The AWG filter 112 may
operate independently of the heat source 114. The laser
signal 120 may be tuned to fit the predefined AWG signal
passbands. A t the OLT side 102, the receiver may process a
fast relative received signal strength (RSSI) indication.
The RSSI is an indication of the power level being received
by the receiver. The RSSI may provide a control signal
that is proportional to a received burst amplitude. The
AWG filter (s) 112 is a fixed wavelength device. The heat
source 114 must be able to tune to the predefined passbands
defined by the AWG filter (s) 112. This is performed by
having the AWG filters 112 detect RSSI values and drifts,
and communicate offset values to the ONT 110 for
correction. This enables the signals detected at the AWG
filters 112 of the OLT 102 to be monitored and in turn used
to control the heat source 114 of the ONT 110.
[0021] Continuing with the tuning example, if the AWG
passband has, for example, a Gaussian-curve shape, the
passband has a minimum insertion loss at the center of
curve. Therefore, a burst in the center will receive the
highest signal amplitude. Furthermore, dithering the
wavelength can provide a way to adjust the signal to the
center of the passband.
[0022] One example may include utilizing a cyclical AWG
filter (s) 112 at the OLT 102 to select different laser
wavelengths. By determining an ONT's laser is currentlyoperating on the wrong output of the cyclic AWG 112, the
OLT 102 can then send a command to tune a laser wavelength
of the ONT within a group of predefined desired
wavelengths. The predefined wavelengths may be evenly
spaced apart within a predefined frequency range assigned
to a corresponding wavelength group. In operation, the
cyclic AWG filter 112 cycles through its outputs based on
certain groups of incoming wavelengths. The AWG filter 112
may transmit a control signal to the heat source 114 to
tune the laser wavelength to the next appropriate
wavelength of the wavelength group for the intended output
120 .
[0023] A wavelength adjustment or tuning may be required
prior to transmitting a data signal to a particular end
user. The ONT laser can be tuned by the heat source 114
alone, rather than by using a heating and cooling device,
such as, a thermo-electric cooler (TEC) . Using only
heating significantly decreases the electrical and thermal
requirements of the laser wavelength tuner configuration,
and provides a more cost effective approach than a heating
and cooling system. For a WDM PON that utilizes a cyclic
AWG filter to assign wavelengths to certain wavelength
groups, the ONT laser is able to tune to the wavelengths
included in these wavelength groups via heat-only. The
combination of the AWG filter 112 and the heat source 114
provides an effective heat-only environment for wavelength
tuning .
[0024] FIG. 2A illustrates a group of wavelength bands
according to a conventional approach to tuning wavelength
sub-bands. Referring to FIG. 2A, four wavelength sub-bands
Bl, B2, B3 and B4 are illustrated as having five uniquewavelengths per sub-band. By implementing a distributed
feedback (DFB) laser, an optical signal's wavelength can be
tuned by heating or cooling by approximately 0.08nm/°K.
Assuming 20° Kelvin is possible for tuning, this approach
provides a 1.6 nm tuning range for a conventional DFB.
Some amount of temperature control is required to tune the
laser wavelength to the pass-band of the filter. However
this conventional approach is not sufficient to tune over 4
wavelength sub-bands (20 nm) .
[0025] FIG. 2B illustrates a group of wavelength bands
according to an example embodiment of the present
invention. Referring to FIG. 2B, a wavelength sub-band 202
is illustrated as having four different wavelengths or
frequencies λι , λ2 , λ3 and . Each of the four wavelengths
is part of a different wavelength set 204, which also
includes four wavelengths. In the wavelength sets WS1,
WS2, WS3 and WS4, each set includes one wavelength from
each wavelength sub-band.
[0026] Tuning from one wavelength in a particular
wavelength set to another in the same set may be performed
by applying heat only. According to one example, heat may
be applied to tune between wavelengths. For example, a
laser may be tuned to a particular wavelength that is
between two of the optical wavelengths required by the ONT
110 and/or OLT 102. By applying heat to the laser, the
wavelength may adjust to a discrete wavelength that is
recognized by the optical system.
[0027] In another example, the external temperature may
cause the operating wavelength to shift away from a
particular operating wavelength. For instance, a first or
current operating wavelength may be λ 7 of WS3. The lasermay be currently operating at λ7 of WS3, however, an
increase in the external temperature may cause the laser' s
wavelength to shift upwards towards or λ . In this case,
it may be desirable to continue operating within the
predefined wavelengths of the same wavelength set WS3. A s
a result, the optical laser may apply heat via the embedded
heat source in the laser die or via an external heat source
affixed to the laser device. The heat may be controlled to
only modify the present wavelength to the next wavelength
λιι in the wavelength set WS3.
[0028] FIG. 3 illustrates an example laser die, according
to example embodiments of the present invention. Referring
to FIG. 3 , the laser structure can be modified to add a
"heater layer" in the laser epitaxial structure, or as a
metal strip on the top of the ONT laser device 400. The
epitaxial structure may be a transistor made by depositing
a thin pure layer of semiconductor material (epitaxial
layer) onto a crystalline support by epitaxy. The thin
layer may act as one of the electrode regions, such as, the
collector. The heater may also be resistor or metal
structure. The heater may also be a layer above under or
on the side of the laser device 400.
[0029] A s illustrated in FIG. 3 , the heat source 114 may
instead be a metal layer added externally to the laser die
with a floating heat source external to the die chip. This
heater layer is capable of providing a small amount of
heat, which is enough to adjust the wavelength of the laser
by approximately 1-2 nm. The heat may be just large enough
to move or shift the actual wavelength within one
wavelength assignment of a desired 10, 12, 16, etc.,
wavelength group, based on 50GHz wavelength spacing.Additionally, in order to control cost and maintain fast
thermal response, a PIN or PN junction can also be included
in a laser die of the laser to provide temperature
monitoring in the same chip as the laser itself, allowing
for ideally matched profiles. A controller may provide a
temperature regulator for increasing, decreasing and/or
cutting-off the amount of current supplied to the heat
source 114.
[0030] By providing both the heater and the temperature
sensor in the same laser die, or chip, the overall thermal
mass of the laser is reduced, facilitating quick thermal
response time and lower electrical power requirements for
temperature control. Additionally, since the laser,
heater, and the temperature sensor are all included in the
same die, or chip, the impact on cost of a TOCAN assembled
laser is low. This configuration provides a low time
constant on the order of microseconds.
[0031] The heat source 114 is capable of providing a small
amount of heat, which is enough to adjust the wavelength of
the laser by approximately l-2nm. On the laser die chip of
FIG. 3 , an additional layer includes the heat source 114
which is used for heating. The heat sensor being laid on
top of the chip will not increase the size of the chip.
One wavelength assignment in a group of wavelengths may be
part of a group of ten different wavelengths based on 50
GHz wavelength spacing. In order to maintain fast thermal
response, a PIN or PN junction can also be included in the
laser die to provide temperature monitoring in the same
chip as the laser itself, allowing for ideally matched
profiles .[0032] The operations of a method or algorithm described
in connection with the embodiments disclosed herein may be
embodied directly in hardware, in a computer program
executed by a processor, or in a combination of the two. A
computer program may be embodied on a computer readable
medium, such as a storage medium. For example, a computer
program may reside in random access memory ("RAM") , flash
memory, read-only memory ("ROM") , erasable programmable
read-only memory ("EPROM") , electrically erasable
programmable read-only memory ("EEPROM") , registers, hard
disk, a removable disk, a compact disk read-only memory
("CD-ROM"), or any other form of storage medium known in
the art .
[0033] An exemplary storage medium may be coupled to the
processor such that the processor may read information
from, and write information to, the storage medium. In the
alternative, the storage medium may be integral to the
processor. The processor and the storage medium may reside
in an application specific integrated circuit ("ASIC") . In
the alternative, the processor and the storage medium may
reside as discrete components. For example, FIG. 4
illustrates an example network element 400, which may
represent any of the above-described components of the
previous drawings .
[0034] A s illustrated in FIG. 4 , a memory 410 and a
processor 420 may be discrete components of the network
entity 400 that are used to execute an application or set
of operations. The application may be coded in software in
a computer language understood by the processor 420, and
stored in a computer readable medium, such as, the memory
410. The computer readable medium may be a non-transitorycomputer readable medium that includes tangible hardware
components in addition to software stored in memory.
Furthermore, a software module 430 may be another discrete
entity that is part of the network entity 400, and which
contains software instructions that may be executed by the
processor 420. In addition to the above noted components
of the network entity 400, the network entity 400 may also
have a transmitter and receiver pair configured to receive
and transmit communication signals (not shown) .
[0035] An example method of modifying a first signal is
disclosed. The method may include tuning a signal received
from a first network terminal at a second network terminal.
The method provides receiving the signal at the second
network terminal, the signal operating at a first
wavelength, at operation 501. The method may also include
determining a port used to receive the signal at the second
network terminal, at operation 502. The method may further
include identifying a predetermined port wavelength used as
a basis to shift the first wavelength to the predetermined
port wavelength for subsequent signals received, at
operation 503. The method may also include transmitting
the predetermined port wavelength information to the first
network terminal to inform the first network terminal to
tune subsequent signals to the desired wavelength for the
port, at operation 504.
[0036] While preferred embodiments of the present
invention have been described, it is to be understood that
the embodiments described are illustrative only and the
scope of the invention is to be defined solely by the
appended claims when considered with a full range ofequivalents and modifications (e.g., protocols, hardware
devices, software platforms etc.) thereto.Claims
WHAT IS CLAIMED IS:
1 . A method of tuning a transmitted signal received
from a first network terminal at a second network terminal,
the method comprising:
receiving the signal at the second network terminal,
the signal operating at a first wavelength;
determining a port used to receive the signal at the
second network terminal;
identifying a predetermined port wavelength used as a
basis to shift the first wavelength to the predetermined
port wavelength for subsequent signals received; and
transmitting the predetermined port wavelength
information to the first network terminal to inform the
first network terminal to tune subsequent signals to the
desired wavelength for the port.
2 . The method of claim 1 , wherein the predetermined
port wavelength information is determined via a cyclic
arrayed waveguide grating (AWG) filter.
3 . The method of claim 1 , wherein the signal is an
optical signal.
4 . The method of claim 3 , wherein the first network
terminal is an optical network terminal (ONT) and the
second network terminal is an opti cal line terminal (OLT) .5 . The method of claim 1 , wherein responsive to
receiving the predetermined port wavelength information,
the first network terminal performs applying a heat source
to a transmitter to modify the first wavelength for
subsequent signal transmissions.
6 . The method of claim 5 , wherein responsive to
receiving the predetermined port wavelength information,
the first network terminal performs generating a second
signal having a second wavelength different from the first
wavelength based on the heat applied from the heat source.
7 . The method of claim 6 , wherein responsive to
receiving the predetermined port wavelength information,
the first network terminal performs transmitting the second
signal .
8 . The method of claim 5 , wherein applying the heat
source comprises applying a predefined amount of heat
corresponding to a target wavelength used as a basis to
modify the first wavelength and create the second signal
having the second wavelength.
9 . The method of claim 8 , further comprising sensing
the amount of heat supplied by the heat source and
providing feedback to a controller.
10. The method of claim 8 , further comprising
modifying the amount of heat supplied to the transmitter to
tune the second wavelength of the second signal.