Functional Description

SML2120

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2066 6.3 1/22/04

Summit Microelectronics

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FUNCTIONAL DESCRIPTION

The SML2120 is an adaptive power controller for laser

diodes. The device contains an active feedback loop used

to calibrate and control the mean and modulation power of

high-speed high-power laser diodes.

Inherent manufacturing tolerances introduce variations of

performance in laser diodes. These variations, combined

with parametric changes over the laser's extreme operating

temperature range and laser aging, require an efficient

compensation solution. The SML2120, together with a

minimum number of external components, is designed to

compensate for these variations using a digital control loop

and dual programmable nonvolatile calibration lookup

tables.

Figure 2 shows the output light power of a typical laser

diode versus its operating current. Depicted in the graph are

laser diode characteristics at two different temperatures. At

switch the laser between its ON and OFF states is labeled

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 requires a constant extinction ratio over its

entire operating temperature range, as the receiver module

is calibrated to this level. Operating 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.

dual lookup table architecture (Figure 3), is capable of

providing variable output currents based on a function of

either the bias current or the external temperature, and

thereby enables the system designer to optimize the

extinction ratio of the laser driver module.

The SML2120 eliminates the need for any manual

calibration of the laser control circuit. All calibration values

communication interface whose protocol and functions can

be controlled by automated test equipment (ATE).

Figure 2. Laser Current Increase Caused by Temperature Increase, Constant Light Power Out