Datasheet
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R
MODSET
[W] +
265 V
I
MOD
[A]
ǒ
1 ) TC
ǒ
T[
o
C] * T
0
[
o
C]
Ǔ
Ǔ
(13)
R
MODSET
[W] +
265 V
36.3 mA
ǒ
1 )
4000 ppm
o
C
(
25
o
C * 60
o
C
)
Ǔ
+ 6.3 kW
(14)
SELECT MODTC RESISTOR
TC
LD
ƪ
1
o
C
ƫ
+
h
2
[WńA] * h
1
[WńA]
h
1
[WńA]
ǒ
T
2
[
o
C] * T
1
[
o
C]
Ǔ
10
6
(15)
TC
LD
+
0.15 mWńmA * 0.2 mWńmA
0.2 mWńmA (85
o
C * 25
o
C)
10
6
+ * 4167
1
o
C
(16)
R
MODTC
+
24 W
(
TC * 630 ppm
)
ƪ
1
o
C
ƫ
o
C
(17)
R
MODTC
+
24 W
4167 ppm*630 ppm
o
C
o
C
+ 6.8 kW
(18)
SELECT BIASMAX RESISTOR
I
BIASMAX
[A] + I
THMAX
[A]
(19)
R
BIASMAX
[W] +
343 V
I
BIASMAX
[A]
+
343 V
I
THMAX
[A]
(20)
ONET4211LD
SLLS688A – NOVEMBER 2005 – REVISED SEPTEMBER 2007
If 4000 ppm/ ° C is the desired temperature coefficient and the modulation current from the preceding example,
36.3 mA, is required at a temperature of 25 ° C, the MODSET resistor R
MODSET
is given by Equation 14 .
Note that the modulation current I
MOD
must not exceed 85 mA over the complete temperature range,
corresponding to a minimum MODSET resistor R
MODSET,MIN
= 3.1 k Ω .
The R
MODTC
resistor is used to program a modulation temperature coefficient that can be used to compensate for
the decreased slope efficiency of the laser at a higher temperature. The temperature coefficient TC
LD
of the laser
can be calculated using the slope efficiency η
1
at temperature T
1
and η
2
at temperature T
2
as shown in
Equation 15 :
As an example, for the laser in Table 2 , the slope efficiency at temperature T
1
= 25 ° C is η
1
= 0.2 mW/mA. At
temperature T
2
= 85 ° C, the slope efficiency is η
2
= 0.15 mW/mA. The corresponding temperature coefficient
TC
LD
of the laser can be calculated:
The MODTC resistor R
MODTC
can be used to compensate the laser temperature coefficient TC
LD
in order to
maintain the same optical output swing within a range of 630 ppm up to 8300 ppm. For this, R
MODTC
may be
programmed as follows:
To compensate for the decreased slope efficiency of the laser in Table 2 , TC must be 4167 ppm/ ° C.
This leads to the following MODTC resistor R
MODTC
:
The BIASMAX resistor R
BIASMAX
is used to limit the bias current applied to the laser diode.
To calculate R
BIASMAX
, the maximum threshold current at 85 ° C and end of life must be determined. The
maximum bias current for the dc-coupled interface can be approximated by Equation 19 .
R
BIASMAX
can be set by Equation 20 .
For the example laser diode, the maximum threshold current is 40 mA at 85 ° C. Therefore, R
BIASMAX
can be
approximated by Equation 21 .
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