U.S. patent application number 12/198879 was filed with the patent office on 2009-06-04 for method for controlling laser power of an optical pickup unit.
Invention is credited to Chih-Ching Chen, Hsiao-Yuan Chi, Chia-Wei Liao.
Application Number | 20090141755 12/198879 |
Document ID | / |
Family ID | 40675656 |
Filed Date | 2009-06-04 |
United States Patent
Application |
20090141755 |
Kind Code |
A1 |
Chi; Hsiao-Yuan ; et
al. |
June 4, 2009 |
METHOD FOR CONTROLLING LASER POWER OF AN OPTICAL PICKUP UNIT
Abstract
A method for controlling laser power of an optical pickup unit
(OPU) includes: providing a first relationship between the laser
power and a driving parameter, wherein the driving parameter is
utilized for driving a laser diode (LD) of the OPU, and the first
relationship corresponds to a first temperature; utilizing a
temperature-related model to convert the first relationship into a
second relationship between the laser power and the driving
parameter, wherein the second relationship corresponds to a second
temperature; and storing the first relationship for being utilized
at the first temperature, and storing the second relationship for
being utilized at the second temperature.
Inventors: |
Chi; Hsiao-Yuan; (Taipei
City, TW) ; Chen; Chih-Ching; (Miaoli County, TW)
; Liao; Chia-Wei; (Hsinchu County, TW) |
Correspondence
Address: |
NORTH AMERICA INTELLECTUAL PROPERTY CORPORATION
P.O. BOX 506
MERRIFIELD
VA
22116
US
|
Family ID: |
40675656 |
Appl. No.: |
12/198879 |
Filed: |
August 26, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60991185 |
Nov 29, 2007 |
|
|
|
Current U.S.
Class: |
372/29.021 |
Current CPC
Class: |
G11B 7/126 20130101 |
Class at
Publication: |
372/29.021 |
International
Class: |
H01S 3/13 20060101
H01S003/13 |
Claims
1. A method for controlling laser power of an optical pickup unit
(OPU), the method comprising: providing a first relationship
between the laser power and a driving parameter, wherein the
driving parameter is utilized for driving a laser diode (LD) of the
OPU, and the first relationship corresponds to a first temperature;
utilizing a temperature-related model to convert the first
relationship into a second relationship between the laser power and
the driving parameter, wherein the second relationship corresponds
to a second temperature; and storing the first relationship for
being utilized at the first temperature, and storing the second
relationship for being utilized at the second temperature.
2. The method of claim 1, wherein the driving parameter represents
an LD driving voltage for controlling an LD driving current of the
LD.
3. The method of claim 1, wherein the driving parameter represents
an LD driving current of the LD.
4. The method of claim 1, wherein the step of providing the first
relationship between the laser power and the driving parameter
further comprises: applying a target command carrying a specific
value to an automatic power calibration (APC) circuit for
controlling the laser power; and when the APC circuit reaches a
steady state at the first temperature, measuring the laser power
and the driving parameter to derive the first relationship.
5. The method of claim 1, wherein the step of providing the first
relationship between the laser power and the driving parameter
further comprises: applying a target command carrying a first value
to an automatic power calibration (APC) circuit for controlling the
laser power, and when the APC circuit reaches a steady state at the
first temperature, measuring the laser power and the driving
parameter to derive a first data point; applying a target command
carrying a second value to the APC circuit, and when the APC
circuit reaches a steady state at the first temperature, measuring
the laser power and the driving parameter to derive a second data
point; and utilizing the first and second data points to derive the
first relationship.
6. The method of claim 1, wherein the temperature-related model
corresponds to curves having respective slopes with respect to
different temperatures.
7. The method of claim 1, wherein the temperature-related model
corresponds to parallel curves with respect to different
temperatures.
8. The method of claim 1, further comprising: measuring the laser
power and the driving parameter at the second temperature to derive
information of the temperature-related model.
9. The method of claim 8, wherein the step of measuring the laser
power and the driving parameter at the second temperature to derive
the information of the temperature-related model further comprises:
applying a target command carrying a specific value to an automatic
power calibration (APC) circuit for controlling the laser power;
and when the APC circuit reaches a steady state at the second
temperature, measuring the laser power and the driving parameter to
derive the information of the temperature-related model.
10. The method of claim 8, further comprising: applying a target
command carrying a first value to an automatic power calibration
(APC) circuit for controlling the laser power, and when the APC
circuit reaches a steady state at the second temperature, measuring
the laser power and the driving parameter to derive a first data
point; applying a target command carrying a second value to the APC
circuit, and when the APC circuit reaches a steady state at the
second temperature, measuring the laser power and the driving
parameter to derive a second data point; and utilizing the first
and second data points to derive the information of the
temperature-related model.
11. The method of claim 1, wherein the first and second
relationships correspond to a first channel; and the method further
comprises: utilizing the first and second relationships to derive
an additional relationship between the laser power and the driving
parameter in a second channel.
12. The method of claim 11, wherein the driving parameter
represents an LD driving voltage; and the step of utilizing the
first and second relationships to derive the additional
relationship between the laser power and the driving parameter in
the second channel further comprises: transforming a first voltage
difference into a second voltage difference; wherein the first
voltage difference represents a difference between different
voltage values of the LD driving voltage in the first channel at
the first and the second temperatures, respectively; wherein the
second voltage difference represents a difference between different
voltage values of the LD driving voltage in the second channel at
the first and the second temperatures, respectively.
13. The method of claim 11, wherein the driving parameter
represents an LD driving current of the LD; and the step of
utilizing the first and second relationships to derive the
additional relationship between the laser power and the driving
parameter in the second channel further comprises: transforming a
first current difference into a second current difference; wherein
the first current difference represents a difference between
different current values of the LD driving current in the first
channel at the first and the second temperatures, respectively;
wherein the second current difference represents a difference
between different current values of the LD driving current in the
second channel at the first and the second temperatures,
respectively.
14. A method for controlling laser power of an optical pickup unit
(OPU), the method comprising: providing a first relationship
between the laser power and a driving parameter, wherein the
driving parameter is utilized for driving a laser diode (LD) of the
OPU, and the first relationship corresponds to a first temperature;
providing a second relationship between the laser power and the
driving parameter, wherein the second relationship corresponds to a
second temperature; and storing the first relationship for being
utilized at the first temperature, and storing the second
relationship for being utilized at the second temperature.
15. The method of claim 14, wherein the driving parameter
represents an LD driving voltage for controlling an LD driving
current of the LD.
16. The method of claim 14, wherein the driving parameter
represents an LD driving current of the LD.
17. The method of claim 14, wherein the step of providing the first
relationship between the laser power and the driving parameter
further comprises: applying a target command carrying a specific
value to an automatic power calibration (APC) circuit for
controlling the laser power; and when the APC circuit reaches a
steady state at the first temperature, measuring the laser power
and the driving parameter to derive the first relationship.
18. The method of claim 14, wherein the step of providing the first
relationship between the laser power and the driving parameter
further comprises: applying a target command carrying a first value
to an automatic power calibration (APC) circuit for controlling the
laser power, and when the APC circuit reaches a steady state at the
first temperature, measuring the laser power and the driving
parameter to derive a first data point; applying a target command
carrying a second value to the APC circuit, and when the APC
circuit reaches a steady state at the first temperature, measuring
the laser power and the driving parameter to derive a second data
point; and utilizing the first and second data points to derive the
first relationship.
19. The method of claim 14, wherein the step of providing the
second relationship between the laser power and the driving
parameter further comprises: applying a target command carrying a
specific value to an automatic power calibration (APC) circuit for
controlling the laser power; and when the APC circuit reaches a
steady state at the second temperature, measuring the laser power
and the driving parameter to derive the second relationship.
20. The method of claim 14, wherein the step of providing the
second relationship between the laser power and the driving
parameter further comprises: applying a target command carrying a
first value to an automatic power calibration (APC) circuit for
controlling the laser power, and when the APC circuit reaches a
steady state at the second temperature, measuring the laser power
and the driving parameter to derive a first data point; applying a
target command carrying a second value to the APC circuit, and when
the APC circuit reaches a steady state at the second temperature,
measuring the laser power and the driving parameter to derive a
second data point; and utilizing the first and second data points
to derive the second relationship.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Application No. 60/991,185, which was filed on Nov. 29, 2007, and
entitled "AUTO POWER CALIBRATION STRUCTURAL".
BACKGROUND
[0002] The present invention relates to power calibration of an
optical pickup unit (OPU) with respect to temperature during a mass
production phase of an optical disc drive, and more particularly,
to a method for controlling laser power of an OPU.
[0003] Regarding the control over an OPU of an optical disc drive
in the related art, a conventional automatic power calibration
(APC) circuit can be utilized for controlling the laser power of a
laser diode (LD) during a normal operation of the optical disc
drive, e.g. a reading/writing operation. When the conventional APC
circuit reaches a steady state during the normal operation
mentioned above, the laser power corresponds to a target command
that is sent to the conventional APC circuit.
[0004] FIG. 1 illustrates a relationship between the laser power
(labeled "Power" as shown on the vertical axis) and a driving
parameter such as an LD driving voltage (labeled "Voltage" as shown
on the horizontal axis) according to the related art. The LD
driving voltage is utilized for controlling an LD driving current
of the LD, where the LD driving voltage corresponds to the target
command when the steady state mentioned above is reached. As shown
in FIG. 1, a dashed line aligned to the curve corresponding to the
linear region has an offset Vth with regard to the horizontal
axis.
[0005] It is a goal for the conventional APC circuit to control the
laser power to be a specific power value corresponding to the
target command, in order that the laser power varies in accordance
with the target command. Thus, how to prepare a precise
relationship between the laser power and the target command during
a mass production phase of the optical disc drive has become an
important issue
[0006] A conventional method for deriving the relationship between
the laser power and the target command typically comprises
measuring the laser power by utilizing a power meter, and
collecting data sets of the laser power and the target command.
However, the cost of the power meter is high, and the corresponding
tooling and labor costs of a power calibration station for
implementing this method are also required. In addition, these
costs will be multiplied according to the number of production
lines. Furthermore, other issues such as the differences between
respective power calibration stations may arise.
[0007] According to the related art, an OPU vendor may design a
front-end photo diode (PD) in an OPU, and the system manufacturers
(e.g. an optical disc drive manufacturer) may use the front-end PD
as a replacement for the power meter. The measurement result from
the front-end PD is outputted through a front-end PD output (FPDO),
and can be referred to as the FPDO value. As the OPU vendor
typically provides a few data points for stating the relationship
between the laser power and the FPDO value, interpolation
operations are required for deriving the laser power corresponding
to other data points on a predicted curve passing through the few
data points mentioned above. As a result, the whole process of
deriving a precise relationship between the laser power and the
target command is slowed down due to the interpolation
operations.
[0008] Thus, no matter whether the calibration in the power
calibration station is implemented by utilizing the power meter or
the FPDO, the corresponding costs such as time, tooling and/or
labor costs are required. Moreover, there is little awareness of
the inaccuracy due to temperature variation during the normal
operation. As a result, the calibration is often performed at only
an arbitrary temperature.
[0009] Even if the inaccuracy due to the temperature variation
during the normal operation is noticed, performing the calibration
in the power calibration station with respect to different values
of temperature will be cost-ineffective for most system
manufacturers utilizing the related art methods.
[0010] Therefore, the control over the laser power will certainly
be inaccurate in a normal operation when the temperature varies. A
novel method is therefore required for solving the related art
problems, such as the inaccuracy due to the temperature variation,
and the tradeoff between the costs and the performance.
SUMMARY
[0011] It is therefore an objective of the claimed invention to
provide a method for controlling laser power of an optical pickup
unit (OPU), in order to solve the above-mentioned problems.
[0012] An exemplary embodiment of a method for controlling laser
power of an OPU comprises: providing a first relationship between
the laser power and a driving parameter, wherein the driving
parameter is utilized for driving a laser diode (LD) of the OPU,
and the first relationship corresponds to a first temperature;
utilizing a temperature-related model to convert the first
relationship into a second relationship between the laser power and
the driving parameter, wherein the second relationship corresponds
to a second temperature; and storing the first relationship for
being utilized at the first temperature, and storing the second
relationship for being utilized at the second temperature.
[0013] An exemplary embodiment of a method for controlling laser
power of an OPU comprises: providing a first relationship between
the laser power and a driving parameter, wherein the driving
parameter is utilized for driving an LD of the OPU, and the first
relationship corresponds to a first temperature; providing a second
relationship between the laser power and the driving parameter,
wherein the second relationship corresponds to a second
temperature; and storing the first relationship for being utilized
at the first temperature, and storing the second relationship for
being utilized at the second temperature.
[0014] These and other objectives of the present invention will no
doubt become obvious to those of ordinary skill in the art after
reading the following detailed description of the preferred
embodiment that is illustrated in the various figures and
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 illustrates a relationship between laser power of an
optical pickup unit (OPU) and a driving parameter according to the
related art, where the driving parameter is utilized for driving a
laser diode (LD) of the OPU and controlling the laser power emitted
from the LD.
[0016] FIG. 2 is a flowchart of a method for controlling laser
power of an OPU according to a first embodiment of the present
invention.
[0017] FIG. 3 and FIG. 4 illustrate examples of different
temperature-related models that can be utilized by the method shown
in FIG. 2.
[0018] FIG. 5 is an exemplary functional diagram illustrating
various gains applied to different channels regarding an LD driver
(LDD) for driving an LD in the OPU according to a variation of the
first embodiment.
DETAILED DESCRIPTION
[0019] Certain terms are used throughout the following description
and claims, which refer to particular components. As one skilled in
the art will appreciate, electronic equipment manufacturers may
refer to a component by different names. This document does not
intend to distinguish between components that differ in name but
not in function. In the following description and in the claims,
the terms "include" and "comprise" are used in an open-ended
fashion, and thus should be interpreted to mean "include, but not
limited to . . . ". Also, the term "couple" is intended to mean
either an indirect or direct electrical connection. Accordingly, if
one device is coupled to another device, that connection may be
through a direct electrical connection, or through an indirect
electrical connection via other devices and connections.
[0020] Please refer to FIG. 2, FIG. 3 and FIG. 4. FIG. 2 is a
flowchart of a method for controlling laser power of an optical
pickup unit (OPU) according to a first embodiment of the present
invention, where the method can be utilized for obtaining precise
control over the laser power of the OPU. FIG. 3 and FIG. 4
illustrate examples of different temperature-related models that
can be utilized by the method shown in FIG. 2. The method for
controlling the laser power of the OPU is described as follows.
[0021] In Step 912, a first relationship between the laser power
and a driving parameter is provided, where the driving parameter is
utilized for driving a laser diode (LD) of the OPU to control the
laser power, and the first relationship corresponds to a first
temperature such as room temperature. The relationships between the
laser power (which can be simply referred to as power) and the
driving parameter (e.g. voltage) are different according to
temperature values. For example, the first relationship can be
depicted as one of the curves labeled "Middle Temperature" in the
temperature-related models respectively shown in FIG. 3 and FIG. 4.
Moreover, the slopes and the Vth of the curves with different
temperatures are different.
[0022] According to this embodiment, the vertical axis (labeled
"Power") shown in FIG. 3 and FIG. 4 depicts the laser power
mentioned above, and the horizontal axis (labeled "Voltage") shown
in FIG. 3 and FIG. 4 depicts the driving parameter mentioned above.
Although the driving parameter in either of the temperature-related
models of this embodiment represents an LD driving voltage (e.g.
the "Voltage" shown in FIG. 3 or FIG. 4) for controlling an LD
driving current of the LD, this is only for illustrative purposes
and is not meant to be a limitation of the present invention.
According to a variation of this embodiment, the driving parameter
represents the LD driving current of the LD.
[0023] In practice, the first relationship is typically derived by
measuring the laser power and the driving parameter at the first
temperature. According to this embodiment, a target command
carrying a specific value can be first applied to an automatic
power calibration (APC) circuit for controlling the laser power.
When the APC circuit reaches a steady state at the first
temperature, the laser power and the driving parameter are measured
in order to derive the first relationship. Typically, when partial
information of the first relationship is given (e.g. a slope or an
offset of a curve of the first relationship), only measuring a
single data point comprising a specific value of the laser power
and a specific value of the driving parameter may be enough for
deriving the first relationship, where such a data point can be
utilized for depicting a curve of the first relationship with the
horizontal and the vertical axes respectively representing the
driving parameter and the laser power.
[0024] For example, the target command carrying a specific value
V.sub.1 can be first applied to the APC circuit. When the APC
circuit reaches a steady state at the first temperature, the laser
power and the driving parameter are measured to derive the single
data point P.sub.1. Thus, the single data point P.sub.1 can be
utilized for deriving the first relationship. In this embodiment,
the single data point P.sub.1 is illustrated on the curves labeled
"Middle Temperature" in the temperature-related models respectively
shown in FIG. 3 and FIG. 4. This is for illustrative purposes only,
and is not meant to be a limitation of the present invention.
[0025] According to a variation of this embodiment, the single data
point P.sub.1 is illustrated on the curves labeled "Low
Temperature" in the temperature-related models respectively shown
in FIG. 3 and FIG. 4. According to another variation of this
embodiment, the single data point P.sub.1 is illustrated on the
curves labeled "High Temperature" in the temperature-related models
respectively shown in FIG. 3 and FIG. 4. According to another
variation of this embodiment, more than one data point can be
illustrated on one of the curves shown in FIG. 3 and FIG. 4.
[0026] Sometimes, measuring two data points may be required, where
the two data points are derived according to the same method as
that for deriving the single data point with regard to different
values of the target command, respectively. For example, the target
command carrying a first value V.sub.1-1 can be first applied to
the APC circuit. When the APC circuit reaches a steady state at the
first temperature, the laser power and the driving parameter (such
as voltage) are measured to derive a first data point P.sub.1-1.
Afterward, the target command carrying a second value V.sub.1-2 can
be applied to the APC circuit. When the APC circuit reaches a
steady state at the first temperature, the laser power and the
driving parameter (such as voltage) are measured to derive a second
data point P.sub.1-2. Thus, the first data point P.sub.1-1 and the
second data point P.sub.1-2 can be utilized for deriving the first
relationship.
[0027] In Step 914, a temperature-related model is utilized, such
as one of the two temperature-related models shown in FIG. 3 and
FIG. 4, to convert the first relationship into a second
relationship between the laser power and the driving parameter,
where the second relationship corresponds to a second temperature,
and the second temperature is different from the first
temperature.
[0028] As shown in FIG. 3, the temperature-related model
corresponds to curves having respective slopes with respect to
different temperatures. In the situation where the
temperature-related model shown in FIG. 3 is utilized, extension
lines of the linear portions of these curves (labeled "Low
Temperature", "Middle Temperature", and "High Temperature",
respectively) are substantially concurrent lines. In addition, as
shown in FIG. 4, the temperature-related model corresponds to
parallel curves with respect to different temperatures. In the
situation where the temperature-related model shown in FIG. 4 is
utilized, the offset Vth with regard to the horizontal axis varies
when the middle curve (labeled "Middle Temperature") shifts to the
left or to the right.
[0029] According to this embodiment, no matter whether the
temperature-related model shown in FIG. 3 or the
temperature-related model shown in FIG. 4 is utilized, measuring at
least one data point comprising a specific value of the laser power
and a specific value of the driving parameter at the second
temperature may be required for deriving sufficient information of
the temperature-related model. In this embodiment, the relationship
conversion in Step 914 may require a single data point P.sub.2 to
be measured at the second temperature in order to derive the second
relationship such as that depicted by the left curve (labeled "Low
Temperature" in FIG. 3 or FIG. 4) or that depicted by the right
curve (labeled "High Temperature" in FIG. 3 or FIG. 4).
[0030] The method for deriving the single data point P.sub.2 at the
second temperature is similar to that for deriving the single data
point P.sub.1 at the first temperature except for the ambient
temperature during the measurement. For example, the target command
carrying a specific value V.sub.2 can be first applied to the APC
circuit. When the APC circuit reaches a steady state at the second
temperature, the laser power and the driving parameter are measured
to derive the single data point P.sub.2. Thus, the data point
P.sub.2 can be utilized for deriving the sufficient information of
the temperature-related model.
[0031] According to a variation of this embodiment, measuring two
data points at the second temperature may be required in order to
derive sufficient information of the temperature-related model. In
this variation, after deriving the sufficient information of the
temperature-related model, whether the temperature-related model
shown in FIG. 3 or the temperature-related model shown in FIG. 4
should be utilized can be determined. The method for deriving the
two data points at the second temperature is similar to that for
deriving the two data points at the first temperature except for
the ambient temperature during the measurement.
[0032] For example, the target command carrying a first value
V.sub.2-1 can be first applied to the APC circuit. When the APC
circuit reaches a steady state at the second temperature, the laser
power and the driving parameter are measured to derive a first data
point P.sub.2-1. Afterward, the target command carrying a second
value V.sub.2-2 can be applied to the APC circuit. When the APC
circuit reaches a steady state at the second temperature, the laser
power and the driving parameter are measured to derive a second
data point P.sub.2-2. Thus, the first data point P.sub.2-1 and the
second data point P.sub.2-2 can be utilized for deriving the
sufficient information of the temperature-related model.
[0033] In Step 916, the first relationship for being utilized at
the first temperature is stored, and the second relationship for
being utilized at the second temperature is stored. In practice,
the various representatives of the first relationship and the
second relationship can be stored in a non-volatile memory such as
a Flash memory. For example, the representatives can be curve
coefficients of the curves representing the first relationship and
the second relationship, respectively. In addition, the
representatives in another example can be one or more data points
for each of the first relationship or the second relationship.
Additionally, the representatives in another example can be one or
more curve coefficients together with one or more data points.
[0034] FIG. 5 is an exemplary functional diagram illustrating
various gains applied to different channels regarding an LD driver
(LDD) 20 for driving the LD in the OPU according to a variation of
the first embodiment, where the functional blocks 10-1, 10-2, . . .
, and 10-N (labeled "Gain(1)", "Gain(2)", . . . , and "Gain(N)")
represent channel gain functions of the channels 1 to N,
respectively. Although the channels 1 to N are illustrated as
several paths, this is only for illustrative purposes and is not
meant to be a limitation of the present invention. In practice, the
channel gain functions Gain(1), Gain(2), . . . , and Gain(N) are
typically implemented with the same hardware circuit to be
integrated as a variable gain amplifier (VGA) 10 in the APC circuit
mentioned above.
[0035] According to this variation, a method for transforming a
voltage difference .DELTA.V.sub.X.sup.(T1, T2) corresponding to a
channel X (e.g. one of the channels 1 to N) into a voltage
difference .DELTA.V.sub.Y.sup.(T1, T2) corresponding to a channel Y
(e.g. another of the channels 1 to N) is further provided, in order
to make sure the method mentioned above can be widely applied. As a
result, some special situations can be covered. The voltage
differences .DELTA.V.sub.X.sup.(T1, T2) and .DELTA.V.sub.Y.sup.(T1,
T2) mentioned above respectively have two indexes T1 and T2
representing two different values of temperature T, and are
typically defined as follows:
.DELTA.V.sub.X.sup.(T1,
T2)=Volt.sub.X.sup.(T2)-Volt.sub.X.sup.(T1); and
.DELTA.V.sub.Y.sup.(T1,
T2)=Volt.sub.Y.sup.(T2)-Volt.sub.Y.sup.(T1);
where Volt.sub.X.sup.(T) represents a voltage value of the LD
driving voltage in the channel X at temperature T, and
Volt.sub.Y.sup.(T) represents a voltage value of the LD driving
voltage in the channel Y at temperature T.
[0036] According to this variation, the voltage difference
.DELTA.V.sub.Y.sup.(T1, T2) can be derived by the following
equation:
.DELTA.V.sub.Y.sup.(T1,
T2)=(Gain(X)/Gain(Y))*.DELTA.V.sub.X.sup.(T1, T2);
where Gain(X) and Gain(Y) represent the channel gain functions of
the channels X and Y, respectively.
[0037] For example, the voltage difference .DELTA.V.sub.X.sup.(T1,
T2) is derived from the method shown in FIG. 2 with T1 and T2
respectively representing the first and the second temperatures
mentioned above. When the voltage value Volt.sub.Y.sup.(T1) is
known, the voltage value Volt.sub.Y.sup.(T2) can be derived as
follows:
Volt Y ( T 2 ) = .DELTA. V Y ( T 1 , T 2 ) + Volt Y ( T 1 ) = (
Gain ( X ) / Gain ( Y ) ) * .DELTA. V X ( T 1 , T 2 ) + Volt Y ( T
1 ) ; ##EQU00001##
where the above equation can be utilized for deriving an additional
relationship between the laser power and the driving parameter in
the channel Y.
[0038] As the channels X and Y mentioned above may represent any
two of the channels 1 to N, the relationships between the channel
gain functions Gain(1), Gain(2), . . . , and Gain(N) and the
voltage differences respectively corresponding to the channels can
be expressed, in general, by utilizing the following equation:
.DELTA.V.sub.channel 1*Gain(1)=.DELTA.V.sub.channel 2*Gain(2)= . .
. =.DELTA.V.sub.channel N*Gain(N);
where .DELTA.V.sub.channel 1, .DELTA.V.sub.channel 2, . . . , and
.DELTA.V.sub.channel N represent the voltage differences
corresponding to the channels 1 to N, respectively.
[0039] In another variation of the first embodiment, the voltage
values Volt.sub.X.sup.(T) and Volt.sub.Y.sup.(T) of the LD driving
voltage can be replaced with corresponding current values
Curr.sub.X.sup.(T) and Curr.sub.Y.sup.(T) of the LD driving current
mentioned above, and the voltage differences
.DELTA.V.sub.X.sup.(T1, T2) and .DELTA.V.sub.Y.sup.(T1, T2) can be
replaced with corresponding current differences
.DELTA.C.sub.X.sup.(T1, T2) and .DELTA.C.sub.Y.sup.(T1, T2),
respectively.
[0040] As a result of applying the method mentioned above, the
present invention indeed provides proper control over laser power
in the normal operation when the temperature varies. In contrast to
the related art, by performing the relationship conversion and
measuring of a few data points as disclosed in the embodiment(s)
and/or variations mentioned above, the present invention method
greatly saves the costs required for the calibration in the power
calibration station, and therefore solves the tradeoff between the
costs and the performance.
[0041] It is another advantage of the present invention that, when
the laser power control should be implemented with respect to a
large number of values of temperature, the relationships
corresponding to these values of temperature can be efficiently
derived according to the embodiments and/or variations mentioned
above.
[0042] Those skilled in the art will readily observe that numerous
modifications and alterations of the device and method may be made
while retaining the teachings of the invention. Accordingly, the
above disclosure should be construed as limited only by the metes
and bounds of the appended claims.
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