ADN2872(RevB) Просмотр технического описания (PDF) - Analog Devices
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ADN2872 Datasheet PDF : 20 Pages
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ADN2872
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
∆P
∆I
LI = ∆P
∆I
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 ER 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 (MPD) current to measure and maintain the average
optical output power over temperature and laser aging. The
ADN2872 is a dual-loop device, implementing both this primary
average power control loop and a secondary control loop, which
maintains a constant optical ER. The dual-loop control of the
average power and ER implemented in the ADN2872 can be
used successfully with both lasers that maintain good linearity of
LI transfer characteristics over temperature, and with those that
exhibit increasing nonlinearity of the LI characteristics over
temperature.
Dual Loop
The ADN2872 uses a proprietary patented method to control both
average power and ER. The ADN2872 is constantly sending a
test signal on the modulation current signal and reading the
resulting change in the 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 ADN2872.
Data Sheet
MPD
OPTICAL COUPLING
INPUT
BIAS
Gm
SHA
Φ1
BIAS
VCC
Φ2
1.2V
CURRENT
VBGAP
ERSET
IEX IPA
HIGH
SPEED
SWITCH
PAVSET
MOD
SHA
Φ2
Φ2
MOD
CURRENT 100
2
Figure 24. Dual-Loop Control of Average Power and ER
A dual loop is made up of an average power control loop (APCL)
and the ER control loop (ERCL), which are separated into two
time states. During Time Φ1, the APCL is operating, and
during Time Φ2, the ER loop is operating.
Average Power Control Loop
The APCL compensates for changes in the laser diode (LD), ITH
and LI, by varying IBIAS. APC control is performed by measuring
the MPD current, IMPD. This current is bandwidth limited by the
MPD. This is not a problem because the APCL must be low
frequency and the APCL must respond to the average current
from the MPD. The APCL compares IMPD × RPAVSET to the band gap
(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 laser diode
by monitoring the IMPD changes. During the ERCL, IMPD is
temporarily 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 APCL 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 resis-
tor. 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 control cycle status of the IBIAS and IMOD
settings are stored on the hold capacitors, PAVCAP and ERCAP.
The ERCL is constantly measuring the actual LI curve; it compen-
sates 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 so that it can then automatically control the laser
over temperature. This eliminates the expensive and time
consuming temperature calibration of a laser.
Rev. B | Page 10 of 20
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