U.S. patent application number 12/620745 was filed with the patent office on 2011-05-19 for tracking injection seeding power based on back facet monitoring (bfm) of an injection seeded laser.
This patent application is currently assigned to NORTEL NETWORKS LIMITED. Invention is credited to Douglas James BECKETT, Bin CAO, Rong CHEN, Tom LUK.
Application Number | 20110116521 12/620745 |
Document ID | / |
Family ID | 43928315 |
Filed Date | 2011-05-19 |
United States Patent
Application |
20110116521 |
Kind Code |
A1 |
CAO; Bin ; et al. |
May 19, 2011 |
TRACKING INJECTION SEEDING POWER BASED ON BACK FACET MONITORING
(BFM) OF AN INJECTION SEEDED LASER
Abstract
A method of estimating an injection power of seed light injected
into an injection-seeded transmitter. A back face monitoring (BFM)
response of the injection-seeded transmitter is determined, and
data representative of the BFM response stored in a memory. During
run-time, a controller of the injection-seeded transmitter, detects
a temperature of the injection-seeded transmitter and an
instantaneous BFM current. BFM response data is obtained from the
memory based on the detected temperature, and the seed light
injection power estimated based on the obtained data and the
detected instantaneous BFM current.
Inventors: |
CAO; Bin; (Kanata, CA)
; BECKETT; Douglas James; (Kanata, CA) ; LUK;
Tom; (Ottawa, CA) ; CHEN; Rong; (Ottawa,
CA) |
Assignee: |
NORTEL NETWORKS LIMITED
St. Laurent
CA
|
Family ID: |
43928315 |
Appl. No.: |
12/620745 |
Filed: |
November 18, 2009 |
Current U.S.
Class: |
372/29.021 ;
372/38.01 |
Current CPC
Class: |
H01S 5/0683 20130101;
H01S 5/06804 20130101 |
Class at
Publication: |
372/29.021 ;
372/38.01 |
International
Class: |
H01S 3/13 20060101
H01S003/13 |
Claims
1. A method of estimating an injection power of seed light injected
into an injection-seeded transmitter, the method comprising:
preliminarily determining a back face monitoring (BFM) response of
the injection-seeded transmitter, and storing data representative
of the BFM response in a memory; and during run-time, a controller
of the injection-seeded transmitter: detecting a temperature of the
injection-seeded transmitter and an instantaneous BFM current;
obtaining BFM response data from the memory based on the detected
temperature; and estimating the seed light injection power based on
the obtained data and the detected instantaneous BFM current.
2. The method as claimed in claim 1, wherein determining a back
face monitoring (BFM) response of the injection-seeded transmitter
comprises: determining a BFM response surface of the injection
seeded laser as a function of bias current and seed injection
power; analysing the BFM response surface to compute a BFM response
slope curve defining the slope of the BFM response surface,
relative to bias current, as a function of seed injection power;
wherein data representative of the BFM response slope curve is
stored in the memory.
3. The method as claimed in claim 2, wherein a respective BFM
response slope curve is computed for each one of a set of
predetermined temperature values of the injection seeded
transmitter, respective data representative of each BFM response
slope curve being stored in the memory.
4. The method as claimed in claim 3, wherein obtaining BFM response
data from the memory comprises selecting data representative of one
BFM response slope based on the detected temperature.
5. The method as claimed in claim 3, wherein obtaining BFM response
data from the memory comprises computing the BFM response data from
data representative of two or more BFM response slopes stored in
the memory, based on the detected temperature.
6. The method as claimed in claim 1, wherein estimating the seed
light injection power comprises: determining an instantaneous slope
of the BFM response; and comparing the determined instantaneous
slope to the obtained BFM response data.
7. The method as claimed in claim 6, wherein determining the
instantaneous slope of the BFM response comprises: dithering the
bias current; and detecting changes in the instantaneous BFM
current corresponding to the bias current dither.
8. The method as claimed in claim 1, wherein determining a back
face monitoring (BFM) response of the injection-seeded transmitter
comprises: determining a BFM response surface of the injection
seeded transmitter as a function of bias current and seed injection
power; analysing the BFM response surface to compute a set of
normalized BFM response curves defining the BFM response to seed
injection power, as a function of bias current; wherein data
representative of the set of normalized BFM response curves is
stored in the memory.
9. The method as claimed in claim 8, wherein a respective set of
normalized BFM response curves is computed for each one of a set of
predetermined temperature values of the injection seeded
transmitter, respective data representative of each set of
normalized BFM response curves being stored in the memory.
10. The method as claimed in claim 9, wherein obtaining BFM
response data from the memory comprises selecting data
representative of one normalized BFM response curve based on the
detected temperature and an instantaneous value of the bias
current.
11. The method as claimed in claim 9, wherein obtaining BFM
response data from the memory comprises computing the BFM response
data from data representative of two or more normalized BFM
response curves, based on the detected temperature and an
instantaneous value of the bias current.
12. The method as claimed in claim 8, wherein estimating the seed
light injection power comprises: analysing the obtained BFM
response data to determine a BFM current for zero seed injection
power; determining a difference between an instantaneous value of
the BFM current and the determined BFM current for zero seed
injection power; and comparing the determined difference to the
obtained BFM response data.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This is the first application filed in respect of the
present invention
FIELD OF THE INVENTION
[0002] The present application relates generally to controlling
injection seeded lasers and, more specifically, to tracking
injection seeding power based on Back Facet Monitoring (BFM) of an
injection seeded transmitter.
BACKGROUND OF THE INVENTION
[0003] In the field of optical communications, it is well known to
use semi-conductor laser diodes to generate a narrowband optical
signal onto which data is modulated for transmission through an
optical medium such as an optical fibre link. In order to obtain
desired characteristics of the optical signal (such as center
wavelength, line width, signal reach, for example) the output power
of the laser diode must be maintained within narrow tolerances.
Because different laser diodes have different output power
characteristics in response to a given driving current, it is
desirable to monitor the output power from each laser diode, and
adjust the driving current as needed to maintain the output power
at a desired level. FIG. 1 schematically illustrates a feedback
control loop 2 for this purpose.
[0004] In the feedback control loop 2 of FIG. 1, a semiconductor
laser diode 4 is typically constructed as a multi-layer doped
semiconductor structure defining a laser cavity in which light is
generated in response to a drive current 6 supplied by a controller
8. In the case of a direct modulation transmitter, the drive
current 6 will comprise a bias current and a modulation current
derived from data being transmitted. In the case of an external
modulation transmitter, the drive current 6 will typically comprise
only the bias current. Reflective front and back facets 10, 12
define the respective front and back boundaries of the laser
cavity. The front facet 10 is designed to be only partially
reflective. Light emitted through the front facet forms the optical
signal 14 onto which data is modulated for transmission. The
optical power level of the optical signal 14 emitted by the front
facet 10 is considered to be the output power of the laser diode
4.
[0005] By contrast, the back facet 12 is normally designed to be a
highly reflective surface, so as to minimize "leakage" of light
through the back facet 12, and thereby maximize the output power of
the laser diode 4. However, the leakage of light through the back
facet 12 is not zero, so that back facet light 16 leaks through the
back facet 12 of the diode 4. The power level of the back facet
light 16 is known to be proportional to the power level of the
optical signal 14 emitted by the front facet 10. This relationship
between laser output and back facet light 16 affords the
opportunity to monitor the output power from the front facet 10 by
detecting the back facet light 16.
[0006] Typically, a photodetector 18 is placed proximal the back
facet 12 of the laser diode 4 to detect the back facet light 16
emitted through the back facet 12. The output current 20 of the
photodetector 18 is proportional to the power level of the back
facet light 16, and thus is also proportional to the output power
of the optical signal 14 emitted through the front facet 10 of the
laser diode 4. The controller 8 can then use various techniques
known in the art, to control the output power of the laser 4 by
adjusting the laser drive current 6 based on the monitored
photodetector current 20. For this reason, the photodetector
current 20 may conveniently be referred to as Back Facet Monitoring
(BFM) current I.sub.BFM.
[0007] Back Facet Monitoring is commonly used for controlling
non-injection seeded lasers, as described above with reference to
FIG. 1. It would be desirable to also utilize Back Facet Monitoring
to control injection seeded transmitters, including injection
seeded lasers and reflective semiconductor optical amplifiers
(RSOAs). As may be seen in FIG. 1b, an injection seeded laser 22
receives a seed light 24, which is used in combination with the
drive current 6 to generate the output optical signal 14. However,
in this case, the back facet light 16 emitted from the back facet
12 includes a first optical component 26 due to the drive current,
and a second optical component 28 due to the seed light 24.
Consequently, the BFM current 20 is highly dependent on the
injection seed light power. This raises a difficulty in that the
power level of the injection seed light 24 is unknown, and may
change rapidly with time. As a result, conventional BFM techniques
cannot be used to control injection seeded lasers. RSOAs suffer the
same limitation, and thus cannot be controlled using conventional
BFM techniques.
[0008] Techniques that overcome the above-noted limitations in the
prior art remain highly desirable.
SUMMARY OF THE INVENTION
[0009] An aspect of the present invention provides a method of
estimating an injection power of seed light injected into an
injection-seeded transmitter. A back face monitoring (BFM) response
of the injection-seeded transmitter is determined, and data
representative of the BFM response stored in a memory. During
run-time, a controller of the injection-seeded transmitter detects
a temperature of the injection-seeded laser and an instantaneous
BFM current. BFM response data is obtained from the memory based on
the detected temperature, and the seed light injection power
estimated based on the obtained data and the detected instantaneous
BFM current.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] Further features and advantages of the present invention
will become apparent from the following detailed description, taken
in combination with the appended drawings, in which:
[0011] FIGS. 1a and 1b schematically illustrate conventional laser
feedback control loops known in the prior art;
[0012] FIG. 2 illustrates a representative response surface of an
injection seeded laser;
[0013] FIG. 3 illustrates representative BFM response slope curves
for an injection seeded laser at a set of different temperatures,
in accordance with a first embodiment of the present invention;
[0014] FIG. 4 schematically illustrates principal elements and
operations of a feedback control loop for an injection seeded
laser, in accordance with an embodiment of the present invention;
and
[0015] FIG. 5 illustrates a representative set of normalized BFM
response curves for an injection seeded laser for a set of
different bias current values, in accordance with a second
embodiment of the present invention.
[0016] It will be noted that throughout the appended drawings, like
features are identified by like reference numerals.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0017] The present invention provides techniques for tracking
injection seeding power based on Back Facet Monitoring (BFM) of an
injection seeded transmitter. A representative embodiment is
described below with reference to FIGS. 2-5.
[0018] In very general terms, the injection seed light power by can
be tracked (monitored) by first characterising the BFM response of
the injection seeded transmitter. This information can be used,
during run-time, to determine the respective instantaneous power
levels of the injection seed light and the laser output light.
This, in turn enables the laser output to be controlled by
adjusting the laser bias current. In the following description, two
alternative techniques are described. In the first technique
described below, the slope of the BFM response is used to estimate
the injection seed power level. In the second technique described
below, the injection seed power level is estimated from the
difference between the BFM response curve for the case of zero seed
injection power and the corresponding response curve for the case
of a non-zero seed injection power.
[0019] As noted above, in the first technique, the injection seed
power level is estimated using the slope of the BFM response
curve.
[0020] FIG. 2 is a chart showing the BFM current as a function of
seed injection power and bias current, for a typical injection
seeded laser. As may be seen in FIG. 2a, the BFM current is an
approximately linear function of bias current, and a non-linear
function of seed light injection power. In addition, the BFM
current is a non-linear function of laser temperature, so that the
total BFM response of any given laser may be characterised by
determining the respective response surface (FIG. 2) at each one of
a set of laser temperature values.
[0021] In accordance with the first technique, the BFM response
surface (FIG. 2) is analysed to determine the slope of the BFM
response, relative to bias current, as a function of seed injection
power. One method for performing this computation is to calculate
the partial derivative
.differential. I BFM .differential. I BIAS , ##EQU00001##
(where I.sub.BFM is the BFM current and I.sub.BIAS is the bias
current) at each one of a selected set of seed injection power
levels. The computed BFM slope values for a desired range of seed
light injection power values yields a response slope curve defining
the slope of the BFM response as a function of seed light injection
power. FIG. 3 is a chart showing a family of response slope curves
generating by repeating the above computation for each one of a set
of laser temperature values.
[0022] As may be seen in FIG. 3, for all temperature values, the
response slope increases with increasing injection power, and this
effect becomes more pronounced with increasing laser temperature.
It is therefore possible to estimate the seed injection power,
during run-time, by determining the instantaneous slope of the BFM
response, relative to the bias current. FIG. 4 schematically
illustrates a representative laser control system implementing this
approach.
[0023] In the embodiment of FIG. 4, the injection seeded laser 22
is constructed and operates in the same manner as described above
with reference to FIG. 1. Accordingly, semiconductor laser 22 is
typically constructed as a multi-layer doped semiconductor
structure defining a laser cavity in which light is generated in
response to a drive current 6 supplied by a controller 30.
Reflective front and back facets 10, 12 define the respective front
and back boundaries of the laser cavity. The front facet 10 is
partially reflective and thus emits optical signal 14. The back
facet 12 is highly reflective, so as to minimize "leakage" of light
through the back facet 12, and thereby maximize the output power of
the optical signal 14. The power level of the back facet light 16
emitted through the back facet 12 is detected by a photodetector 18
is placed proximal the back facet 12 of the laser 22. The back
facet monitoring (BFM) current 20 output from the photodetector 18
is supplied to the controller 30. In addition, a temperature sensor
32 (such as, for example, a thermocouple) detects the temperature
of the laser 22, and supplies a corresponding temperature signal 34
to the controller 30.
[0024] As an initial step, the BFM response of the laser 22 is
determined, as described above with reference to FIGS. 2 and 3, and
data representative of a set of BFM slope curves (FIG. 3) stored in
a memory 36 of the controller 30. Since the laser 22 and its
feedback control loop will typically be manufactured and sold as an
integrated package, this operation can conveniently be performed by
the manufacturer of the laser integrated package, for example as
part of product inspection and calibration processes.
[0025] During operation of the laser 22, the controller 30 can use
the detected temperature of the laser 22 to select the appropriate
BFM slope curve stored in the memory 36. In some embodiments, the
controller 30 may use the data stored in the memory 36 to compute
(e.g. by interpolation) a set of BFM slope values for the detected
laser temperature. This approach can be used to obtain BFM slope
values for laser temperatures lying between the specific
temperature values for which data is stored in the memory 36.
[0026] In addition, the controller 30 can estimate the
instantaneous slope of the BFM response, by alternately offsetting
the bias current a predetermined amount above and below its present
value, and detecting the corresponding changes in the BFM current.
Once the instantaneous slope of the BFM response has been estimated
in this manner, the controller 30 can use the selected (or
computed) BFM slope data to estimate the seed injection power.
[0027] As may be appreciated, the technique described above enables
the seed injection power to be estimated during run time. However,
this technique requires that the bias current be repeatedly offset
(or dithered), in order to monitor the instantaneous BFM response
slope. In some cases, dithering the bias current in this manner may
be undesirable. The second technique, which avoids this difficulty,
is described below.
[0028] In the second technique, the injection seed power level is
estimated from the difference between the BFM response for the case
of zero seed injection power and the corresponding response for the
case of a non-zero seed injection power. Referring back to FIG. 2,
it may be seen that, for the case of zero seed injection power, the
BFM response is an approximately linear function of bias current.
For any given (fixed) value of bias current, increasing the seed
injection power from zero produces a corresponding increase in the
BFM current.
[0029] In accordance with the second technique, the BFM response
surface (FIG. 2) is analysed to determine a normalized BFM response
to seed injection power, as a function of bias current. One method
for performing this computation is to calculate the difference
.DELTA.I.sub.BFM=I.sub.BFM(x)-I.sub.BFM(0), where I.sub.BFM(x) is
the BFM current for a given non-zero seed injection power level and
I.sub.BFM(0) is the BFM current for zero seed injection power
level, for each one of a selected set of bias current levels.
Repeating this calculation for each value of bias current yields a
family of BFM response difference curves that define the normalized
BFM response as a function of seed light injection power. FIG. 5 is
a chart showing a normalized BFM response a given laser
temperature. The total normalized BFM response can be derived by
repeating the above calculations for each one of a set of laser
temperature values.
[0030] As may be seen in FIG. 5, the BFM current increases with
increasing seed injection power, and this trend hold true for all
values of the bias current. It is therefore possible to estimate
the seed injection power, during run-time, by correlating the
instantaneous BFM current with the bias current.
[0031] As an initial step, the BFM response of the laser 22 is
determined, as described above with reference to FIGS. 2 and 3, and
data representative of a set of BFM difference curves (FIG. 5)
stored in the memory 36 of the controller 30. Since the laser 22
and its feedback control loop will typically be manufactured and
sold as an integrated package, this operation can conveniently be
performed by the manufacturer of the laser integrated package, for
example as part of product inspection and calibration
processes.
[0032] During operation of the laser 22, the controller 30 can use
the detected temperature of the laser 22 and the known bias current
value the appropriate BFM difference curve stored in the memory 36.
In some embodiments, the controller 30 may use the data stored in
the memory 36 to compute (e.g. by interpolation) a set of BFM
difference values for the detected laser temperature and
instantaneous bias current. This approach can be used to obtain BFM
difference values for laser temperature and/or bias current values
lying between the specific values for which data is stored in the
memory 36.
[0033] The controller 30 can use the selected (or computed) BFM
difference data and the instantaneous BFM current to estimate the
seed injection power. For example, for the known bias current
value, the BFM current I.sub.BFM(0) for zero seed injection power
level can be retrieved from memory and subtracted from the
instantaneous value of the BFM current 20. The resulting BFM
difference value can then be compared to the selected (or computed)
BFM difference data from the memory 36 to estimate the seed
injection power level.
[0034] In the foregoing description, the invention is described by
way of example embodiments in which the injection seeded
transmitter is a laser. However, the person of ordinary skill in
the art will recognise that the same techniques may equally be used
to control an injection seeded Reflective Semiconductor Optical
Amplifier (RSOA). Thus it will be appreciated that the present
invention is not limited to injection seeded lasers.
[0035] The embodiments of the invention described above are
intended to be illustrative only. The scope of the invention is
therefore intended to be limited solely by the scope of the
appended claims.
* * * * *