ADN2870 Analog Devices, Inc., ADN2870 Datasheet - Page 12

• Manufacturer Part Number: ADN2870
• Description: 3.3 V Dual-loop, 50 Mbps To 3.3 Gbps Laser Diode Driver
• Manufacturer: Analog Devices, Inc.

ADN2870
Operation with Lasers with Temperature-Dependent
Nonlinearity of Laser LI Curve
The ADN2870 ERCL extracts information from the monitor
photodiode signal relating to the slope of the LI characteristics
at the optical 1 level (P1). For lasers with good linearity over
temperature, the slope measured by the ADN2870 at the optical
1 level is representative of the slope anywhere on the LI curve.
This slope information is used to set the required modulation
current to achieve the required optical extinction ratio.
Some types of laser have LI curves that become progressively
more nonlinear with increasing temperature (see Figure 25). At
temperatures where the LI curve shows significant nonlinearity,
the LI curve slope measured by the ADN2870 at the optical 1
level is no longer representative of the overall LI curve. It is
evident that applying a modulation current based on this slope
information cannot maintain a constant extinction ratio over
temperature. However, the ADN2870 can be configured to
maintain near constant optical bias and extinction ratio with a
laser exhibiting a monotonic temperature-dependant nonlinear-
ity. To implement this correction, it is necessary to characterize
a small sample of lasers for their typical nonlinearity by
measuring them at two temperature points, typically 25°C and
85°C. The measured nonlinearity is used to determine the
amount of feedback to apply. Typically one must characterize 5
to 10 lasers of a particular model to get a good number. Then
the product can be calibrated at 25°C only, avoiding the expense
of temperature calibration. Typically the microcontroller
supervisor is used to measure the laser and apply the feedback.
This scheme is particularly suitable for circuits that already use
a microcontroller for control and digital diagnostic monitoring.
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
0
Figure 25. Measurement of a Laser LI Curve Showing
RELATIVELY LINEAR LI CURVE AT 25°C
Laser Nonlinearity at High Temperatures
20
CURRENT (mA)
40
NONLINEAR LI CURVE AT 80°C
60
80
100
Rev. 0 | Page 12 of 20
The ER correction scheme, while using the average nonlinearity
for the laser population, in fact, supplies a corrective measure-
ment based on each laser’s actual performance as measured
during operation. The ER correction scheme corrects for errors
due to laser nonlinearity while the dual loop continues to adjust
for changes in the Laser LI.
For more details on maintaining average optical power and
extinction ratio over temperature when working with lasers
displaying a temperature dependant nonlinearity of LI curve,
see Application Note AN-743.
CONTROL
The ADN2870 has two methods for setting the average power
(PAV) and extinction ratio (ER). The average power and
extinction ratio can be voltage-set using a microcontroller’s
voltage DACs outputs to provide controlled reference voltages
PAVREF and ERREF. Alternatively, the average power and
extinction ratio can be resistor-set using potentiometers at the
PAVSET and ERSET pins, respectively.
VOLTAGE SETPOINT CALIBRATION
The ADN2870 allows interface to a microcontroller for both
control and monitoring (see Figure 26). The average power at
the PAVSET pin and extinction ratio at the ERSET pin can be
set using the microcontroller’s DACs to provide controlled
reference voltages PAVREF and ERREF. Note that during power
up, there is an internal sequence that allows 25 ms before
enabling the alarms; therefore the customer must ensure that
the voltage for PAVREF and ERREF are active within 20 ms.
where:
R
P
I
P
ER is the desired extinction ratio (ER = P1/P0).
In voltage setpoint, RPAV and R
a 1% tolerance and a temperature coefficient of 50 ppm/°C.
MPD_CW
CW
AV
SP
is the monitor photodiode responsivity.
is the average power required.
is the dc optical power specified on the laser data sheet.
PAVREF
ERREF
is MPD current at that specified P
=
=
R
P
ERSET
AV
×
R
×
SP
I
MPD
×
P
RPAV
CW
_
CW
ERSET
(Volts)
×
must be 1 kΩ resistors with
ER
ER
−
+
1
1
CW
×
.
P
AV
(Volts)