SML2108 View Datasheet(PDF) - Summit Microelectronics
Part Name
Description
MFG CO.
'SML2108' PDF : 21 Pages View PDF
SML2108
PRELIMINARY
DEVICE OPERATION
General Description
The SML2108 is an adaptive power controller for laser
diodes with an active feedback loop used to calibrate and
control the mean and modulation power of high speed,
high power laser diodes. Inherent manufacturing toler-
ances introduce variations of performance in laser diodes.
These variations, combined with parametric changes over
the laser’s extreme temperature range and laser ageing,
call for an efficient compensation solution. The SML2108,
with a minimum number of external components, is de-
signed to compensate for these tolerances using a digital
control loop and a programmable nonvolatile calibration
lookup table.
Figure 1 illustrates the usefulness of the SML2108. The
figure shows the output light power of a laser diode versus
its operating current. Depicted in the graph are typical
laser diode characteristics at two different temperatures.
At the first temperature (T1), the laser requires an average
bias current of IBIAS1. The modulation current needed to
switch the laser between its on and off state is labeled
IMOD1. The ratio of light power of its on state divided by
the light power of its off state is referred to as the extinction
ratio. Ideally the laser will maintain a constant extinction
ratio over its entire operating temperature range, as the
receiver module is calibrated to this level. Running the
laser driver at a higher extinction ratio indicates that power
is being wasted, whereas operating at a lower extinction
ratio indicates that data may possibly be lost.
When the laser is operated at a second temperature (T2)
the required bias current is shown as IBIAS2. To maintain
a constant extinction ratio as in the T1 curve the laser
requires a modulation of IMOD2. The SML2108 is the
industry’s first integrated device capable of providing a
variable modulation current based on a function of either
the bias current or an external temperature. This ability to
be able to compensate the modulation output current
enables the system designer to optimize the extinction
ratio of the laser driver module.
The SML2108 has been specified to remove the need for
any manual calibration of the laser control circuit. All
calibration values are programmed through an industry
standard 2-wire communication interface, whose protocol
and function can be controlled by most production ATE
equipment.
Bias Current — Mean Power Control
The SML2108 bias current outputs (BIASP and BIASN)
establish the average power being delivered to an external
laser diode. The output of the laser diode is separately
monitored using a local back-face diode, the output of
which is tied to the DETECT pin of the SML2108. When
coupled with the on-board integrator this feedback loop
becomes the mean power control for the laser diode. The
output block of the mean power control is shown in Figure
2.
Light
Power
(On) 1
T1
1BIAS1
T2
1BIAS2
(Off) 0
1MOD1
1MOD2
Total 1
2053 Fig01
Figure 1. Laser Current Increase Caused by Temperature Increase, Constant Light Power Out
SUMMIT MICROELECTRONICS, Inc.
2053 2.2 11/07/00
7
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