SML2108
PRELIMINARY
DEVICE OPERATION
General Description
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 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-
ancesintroducevariationsofperformanceinlaserdiodes.
Thesevariations,combinedwithparametricchangesover
the laser’s extreme temperature range and laser ageing,
callforanefficientcompensationsolution. TheSML2108,
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.
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
standard2-wirecommunicationinterface,whoseprotocol
and function can be controlled by most production ATE
equipment.
Figure 1 illustrates the usefulness of the SML2108. The
figureshowstheoutputlightpowerofalaserdiodeversus
its operating current. Depicted in the graph are typical
laser diode characteristics at two different temperatures.
Atthefirsttemperature(T1), thelaserrequiresanaverage
Bias Current — Mean Power Control
bias current of IBIAS1. The modulation current needed to The SML2108 bias current outputs (BIASP and BIASN)
switch the laser between its on and off state is labeled establishtheaveragepowerbeingdeliveredtoanexternal
IMOD1. The ratio of light power of its on state divided by laser diode. The output of the laser diode is separately
thelightpowerofitsoffstateisreferredtoastheextinction monitored using a local back-face diode, the output of
ratio. Ideally the laser will maintain a constant extinction which is tied to the DETECT pin of the SML2108. When
ratio over its entire operating temperature range, as the coupled with the on-board integrator this feedback loop
receiver module is calibrated to this level. Running the becomes the mean power control for the laser diode. The
laserdriveratahigherextinctionratioindicatesthatpower output block of the mean power control is shown in Figure
is being wasted, whereas operating at a lower extinction 2.
ratio indicates that data may possibly be lost.
Light
Power
(On) 1
T1
T2
BIAS
BIAS
2
1
(Off) 0
Total
MOD
2
2053 Fig01
MOD
1
Figure 1. Laser Current Increase Caused by Temperature Increase, Constant Light Power Out
SUMMIT MICROELECTRONICS, Inc.
2053 2.2 11/07/00
7
SUMMIT [ SUMMIT MICROELECTRONICS, INC. ]
