Datasheet

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SELECT A LASER
SELECT APCSET RESISTOR
I
PD
[A] + P
AVG
[W] ò
MON
[AńW]
(7)
R
APCSET
[W] +
4.69 V
I
PD
[A]
+
4.69 V
P
AVG
[W] ò
MON
[AńW]
(8)
R
APCSET
[W] +
4.69 V
P
AVG
[W] ò
MON
[AńW]
+
4.69 V
5 mW 0.05 mAńmW
+ 18.75 kW
(9)
SELECT MODSET RESISTOR
I
MOD
[A] +
P
p*p
[W]
h[WńA]
+
2 P
AVG
[W]
r
e
*1
r
e
)1
h[WńA]
(10)
I
MOD
+
2 5 mW
6.3*1
6.3)1
0.2 mWńmA
+ 36.3 mA
(11)
I
MOD
[A] + I
MOD0
[A]
ǒ
1 ) TC
ǒ
T[
o
C] * T
0
[
o
C]
ǓǓ
(12)
ONET4211LD
SLLS688A NOVEMBER 2005 REVISED SEPTEMBER 2007
In the design example according to Figure 25 , the ONET4211LD is dc-coupled to a typical communication-grade
laser diode capable of operating at 4.25 Gb/s with the following specifications shown in Table 2 .
Table 2. Laser Diode Specifications
PARAMETER VALUE UNITS
λ Wavelength 1310 nm
P
AVG
Average Optical Output Power 5 mW
I
TH
Threshold current 10 mA
ρ
MON
Laser-to-monitor transfer 0.05 mA/mW
η Laser slope efficiency 0.2 mW/mA
When the APC loop is activated, the desired average optical output power P
AVG
is defined by characteristics of
the monitor diode and by the APCSET resistor R
APCSET
. The relation between the monitor photodiode current I
PD
and the average optical output power P
AVG
is given by Equation 7 :
The R
APCSET
resistor is calculated by Equation 8 :
For the laser diode specified in Table 2 and the desired average optical output power of 5 mW, R
APCSET
is
calculated as in Equation 9 :
Note that the monitor photodiode current I
PD
must not exceed 1.5 mA, corresponding to a minimum APCSET
resistor R
APCSET,MIN
= 3.1 k .
Modulation current I
MOD
is dependent on the required optical output peak-to-peak power P
p-p
or the average
optical power P
AVG
. I
MOD
can be calculated using the laser slope efficiency η and the desired extinction ratio r
e
:
Using the laser diode parameters from Table 2 and assuming an extinction ratio r
e
= 8 dB ( 6.3) for an average
optical power P
AVG
= 5 mW, the required modulation current results as:
The modulation current is adjustable, with a selectable temperature coefficient TC according to the relation:
where T is the ambient temperature in ° C and T
0
is the reference temperature (T
0
= 60 ° C).
The temperature coefficient TC of the modulation current is typically adjustable between 630 ppm/ ° C and 8300
ppm/ ° C.
For calculation of the required external resistor R
MODSET
for a given modulation current and a given temperature,
the formula can be modified as follows:
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