ADN2870 Analog Devices, Inc., ADN2870 Datasheet - Page 11

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

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.
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 lasers
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, additionally, a
secondary control loop, which maintains constant optical
extinction ratio. The dual-loop control of average power and
extinction ratio implemented in the ADN2870 can be used
successfully both with 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 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 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.
P1
P
P
AV
O
Figure 23. Laser Transfer Function
ER =
P
AV
=
P
P1
P1 + P
O
2
O
Ith
∆
CURRENT
I
∆
P
LI =
∆
∆
P
I
Rev. 0 | Page 11 of 20
A dual loop is made up of an APCL (average power control
loop) and the ERCL (extinction ratio control loop), 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 since
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 Imod as LI changes. During the ERCL, Imod is
temporarily increased by ∆Imod. The ratio between Imod and
∆Imod is a fixed ratio of 50:1, but during startup, this ratio is
increased in order to decrease settling time.
During ERCL, switching in ∆Imod causes a temporary increase
in average optical power, ∆Pav. However the APC loop is dis-
abled during ERCL, and the increase is kept small enough so as
not to disturb the optical eye. When ∆Imod 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 ∆Impd is too small, the control loop knows that LI has
decreased and increases Imod and, therefore, ∆Imod accordingly
until ∆Impd 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, therefore
it compensates for the effects of temperature and for changes in
the LI curve due to laser aging. Thus the laser may 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.
ERSET
Figure 24. Dual-Loop Control of Average Power and Extinction Ratio
INPUT
MPD
Φ
2
I
EX
PAVSET
I
PA
V
1.2V
BGAP
Gm
OPTICAL COUPLING
Φ
Φ
1
2
BIAS
MOD
SHA
SHA
CURRENT
BIAS
CURRENT
SWITCH
SPEED
MOD
HIGH
ADN2870
100
VCC
2
Φ
2