ADN2870(RevA) Ver la hoja de datos (PDF) - Analog Devices
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ADN2870 Datasheet PDF : 20 Pages
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THEORY OF OPERATION
Laser diodes have a current-in to light-out transfer function, as
shown in Figure 23. Two key characteristics of this transfer
function are the threshold current, Ith, and slope in the linear
region beyond the threshold current, referred to as slope
efficiency, LI.
ER =
P1
P0
P1
PAV =
P1 + P0
2
PAV
ΔP
ΔI
LI = ΔP
ΔI
P0
Ith CURRENT
Figure 23. Laser Transfer Function
DUAL-LOOP CONTROL
Typically, laser threshold current and slope efficiency are both
functions of temperature. For FP and DFB type lasers, the
threshold current increases and the slope efficiency decreases
with increasing temperature. In addition, these parameters vary
as the laser ages. To maintain a constant optical average power
and a constant optical extinction ratio over temperature and
laser lifetime, it is necessary to vary the applied electrical bias
current and modulation current to compensate for the laser
changing LI characteristics.
Single-loop compensation schemes use the average monitor
photodiode current to measure and maintain the average
optical output power over temperature and laser aging. The
ADN2870 is a dual-loop device, implementing both this
primary average power control loop and a secondary control
loop, which maintains a constant optical extinction ratio. The
dual-loop control of the average power and extinction ratio
implemented in the ADN2870 can be used successfully both
with lasers that maintain good linearity of LI transfer character-
istics over temperature and with those that exhibit increasing
nonlinearity of the LI characteristics over temperature.
Dual Loop
The ADN2870 uses a proprietary patented method to control
both average power and extinction ratio. The ADN2870 is
constantly sending a test signal on the modulation current
signal and reading the resulting change in the monitor photo-
diode (MPD) current as a means of detecting the slope of the
laser in real time. This information is used in a servo to control
the ER of the laser, which is done in a time-multiplexed manner
at a low frequency, typically 80 Hz. Figure 24 shows the dual-
loop control implementation on the ADN2870.
ADN2870
MPD
OPTICAL COUPLING
INPUT
BIAS
SHA
Gm
Φ1
VCC
Φ2
1.2V
BIAS
CURRENT
VBGAP
ERSET
IEX IPA
HIGH
SPEED
SWITCH
PAVSET
MOD
Φ2
SHA
Φ2
MOD
CURRENT 100
2
Figure 24. Dual-Loop Control of Average Power and Extinction Ratio
A dual loop is made up of an average power control loop
(APCL) and the extinction ratio control loop (ERCL), which are
separated into two time states. During Time Φ1, the APC loop
is operating, and during Time Φ2, the ER loop is operating.
Average Power Control Loop
The APCL compensates for changes in Ith and LI by varying
IBIAS. APC control is performed by measuring MPD current,
IMPD. This current is bandwidth limited by the MPD. This is not
a problem because the APCL must be low frequency; the APCL
must respond to the average current from the MPD. The APCL
compares IMPD × RPAVSET to the BGAP voltage, VBGAP. If IMPD falls,
the bias current is increased until IMPD × RPAVSET equals VBGAP.
Conversely, if the IMPD increases, IBIAS is decreased.
Modulation Control Loop
The ERCL measures the slope efficiency, LI, of the LD and
changes IMPD as LI changes. During the ERCL, IMPD is tempor-
arily increased by ΔIMOD. The ratio between IMPD and ΔIMOD is a
fixed ratio of 50:1, but during startup, this ratio is increased to
decrease settling time.
During ERCL, switching in ΔIMOD causes a temporary increase
in average optical power, ΔPAV. However, the APC loop is disabled
during ERCL, and the increase is kept small enough so as not to
disturb the optical eye. When ΔIMOD is switched into the laser
circuit, an equal current, IEX, is switched into the PAVSET
resistor. The user sets the value of IEX; this is the ERSET setpoint.
If ΔIMPD is too small, the control loop knows that LI has decreased
and increases IMPD and, therefore, ΔIMOD accordingly until ΔIMPD
is equal to IEX. The previous time state values of the bias and mod
settings are stored on the hold capacitors, PAVCAP and ERCAP.
The ERCL is constantly measuring the actual LI curve; there-
fore, it compensates for the effects of temperature and for
changes in the LI curve due to laser aging. Therefore, the laser
can be calibrated once at 25°C and can then automatically
control the laser over temperature. This eliminates expensive
and time consuming temperature calibration of the laser.
Rev. A | Page 11 of 20
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