Semiconductor laser control method and semiconductor laser...

Details Semiconductor laser control method and semiconductor laser... Semiconductor laser control method and semiconductor laser...

2000-04-06

2002-06-18

Davie, James W. (Department: 2881)

Coherent light generators

Particular beam control device

Optical output stabilization

C372S031000

Reexamination Certificate

active

06408013

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a semiconductor laser control method and a semiconductor laser control apparatus to stabilize the output of outgoing radiation power of an apparatus that records or reproduces information on an optical information recording medium using a semiconductor laser as a light source.
BACKGROUND OF THE INVENTION
For an apparatus that reproduces or records information on an optical information recording medium using a laser light source, a semiconductor laser is used as the light source for the purposes of reducing the size, reducing power consumption and improving mass-productivity, etc.
FIG. 9
shows a current-optical output characteristic of a semiconductor laser. A current with which a semiconductor laser starts oscillation as a laser and starts to output power is called a “threshold current Ith” and the efficiency of optical output with respect to a drive current equal to or greater than the threshold current Ith is called “differential quantum efficiency &eegr;. Since this characteristic is extremely unstable, control is normally exercised to stabilize output power by detecting outgoing radiation power of a semiconductor laser and providing feedback for a semiconductor laser drive circuit.
FIG. 8
is a block diagram showing an outlined configuration of a conventional semiconductor laser control apparatus. In
FIG. 8
, reference numeral
31
denotes a semiconductor laser;
32
, a power detection means for detecting power output from the semiconductor laser
31
;
33
, an outgoing radiation power target value;
34
, a power control means for controlling outgoing radiation power of the semiconductor laser
31
to a desired power level by comparing the output value of the power detection means
32
and target value
33
;
35
, a CPU; and
36
, a control signal.
The principle of operation of the semiconductor laser control apparatus with such a configuration is explained.
Here, suppose the outgoing radiation power of the semiconductor laser
31
is P
out
, sensitivity detected by the power detection means
32
is K, the output signal of the power detection means
32
is X, the target value
33
is REF, an error signal obtained by subtracting the output signal X of the power detection means
32
from the target value
33
in the power control means
34
is Y, an amplification factor of the power control means
34
is G, and an output current of the power control means
34
is I. Then, if a feedback loop is configured only focused on a DC current, the following are obtained:
Y=REF−X  Equation 1
I=G·Y  Equation 2
 P
out
=&eegr;·(I−I
th
)  Equation 3
X=K·P
out
  Equation 4
If X, Y and I are erased from the Equations 1, 2, 3 and 4, P
out
is obtained as follows:
Equation 5
P
out
=
REF
K
-
1
1
+
G
·
η
·
K
·
REF
K
-
1
1
+
G
·
η
·
K
⁢
I
th
Generally, in a linear feedback control system, only the second term of the right side of the Equation 5 exists as a steady-state deviation of the control system. However, the semiconductor laser
31
has a current-optical output characteristic as shown in FIG.
9
and the threshold current I
th
brings about the third term of the right side of the Equation 5. If the Equation 5 is solved with respect to the target value REF, REF is given in the following Equation 6:
REF=K·P
out
+H  Equation 6
where H is a correction term and is given:
H=P
out
/G·&eegr;+I
th
/G  Equation 7
Therefore, to obtain the desired power, it is only necessary to set to the target value (REF) determined by the Equations 6 and 7. Instead of obtaining the target value by the Equations 6 and 7, the conventional method used to adjust the target value so that the semiconductor laser outgoing radiation power P
out
would consequently reach the desired power through power adjustment carried out in the manufacturing process.
However, the conventional control method had a basic problem that variations of I
th
or &eegr; due to variations in the operating temperature or deterioration of the semiconductor laser would produce an error in output power.
When the operating temperature of the semiconductor laser increases from a temperature T
1
to T
2
, the threshold current I
th
increases and the differential quantum efficiency &eegr; decreases as shown in the current-optical output characteristic in FIG.
9
. Since the correction term H of the Equation 7 includes the threshold current I
th
and the differential quantum efficiency &eegr;, the value of the correction term H will also change. Therefore, the Equation 6 also changes, with the result that the outgoing radiation power P
out
of the semiconductor laser
31
changes.
Since the adjustment of the target value in the Equation 6 takes place in the manufacturing process, the correction term H is a value at the time of adjustment. Suppose the threshold current of the semiconductor laser during the process adjustment is I
th1
; differential quantum efficiency is &eegr;
1
; outgoing radiation power of the semiconductor laser during the adjustment is P
out1
; and the correction term is H
1
, then, the predetermined value (REF
1
) is given from the Equations 6 and 7:
REF
1
=K·P
out1
+H
1
  Equation 8
and
 H
1
=P
out1
/G·&eegr;
1
+I
th1
/G  Equation 9
If Equations 8 and 9 are solved with respect to P
out1
the following expression is obtained:
Equation 10
P
out1
=
G
·
η
1
·
K
1
+
G
·
η
1
·
K
·
REF
1
K
-
η
1
1
+
G
·
η
1
·
K
⁢
I
th1
On the other hand, the characteristic of the semiconductor laser changes under temperature conditions different from those at the time of process adjustment or deterioration of life due to operation for an extended period of time. Suppose the threshold current is I
th2
and differential quantum efficiency is &eegr;
2
at this time. In this case, the outgoing radiation power P
out2
of the semiconductor laser is obtained by replacing I
th1
, and &eegr;
1
in the Equation 10 by i
th2
and &eegr;
2
respectively.
Equation 11
P
out2
=
G
·
η
2
·
K
1
+
G
·
η
2
·
K
·
REF
1
K
-
η
2
1
+
G
·
η
2
·
K
⁢
I
th2
Here, suppose the error in the outgoing radiation power of P
out2
corresponding to P
out1
is &Dgr;P=(P
out2
−P
out1
),
Equation 12
Δ
⁢
 
⁢
P
=
 
⁢
(
G
·
η
2
·
K
1
+
G
·
η
2
·
K
-
G
·
η
1
·
K
1
+
G
·
η
1
·
K
)
·
REF
1
K
-
 
⁢
(
η
2
1
+
G
·
η
2
·
K
⁢
I
th2
-
η
1
1
+
G
·
η
1
·
K
⁢
I
th1
)
Since the total loop gain G·&eegr;
1
·K and G·&eegr;
2
·K are set to values sufficiently greater than 1, the following approximations can be used:
1+G·&eegr;
1
·K≈G·&eegr;
1
·K  Equation 13
1+G·&eegr;
2
·K≈G·&eegr;
2
·K  Equation 14
If this is applied to the Equation 12, &Dgr;P is obtained as follows:
&Dgr;P=−(I
th2
−I
th1
)/G·K  Equation 15
As an example, the outgoing radiation power error AP is obtained for a semiconductor laser with a wavelength of 650 nm. Here, suppose a process adjustment is carried out at 25° C. and the threshold current is I
th1
, and differential quantum efficiency is &eegr;
1
at that time. On the other hand, suppose the actual operation is performed at 60° C. and the threshold current is I
th2
and differential quantum efficiency is &eegr;
2
at that time. Typical temperature variations are numerically expressed as follows:
I
th1
=50 (mA)  Equation 16
&eegr;
1
=0.7 (W/A)  Equation 17
I
th2
=100 (MA)  Equation 18
&eegr;
2
=0.6 (W/A)  Equation 19
Suppose a total loop gain G·&eegr;
1
·K is:
 G·&eegr;
1
·K&equa

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