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2053 2.2 11/07/00

SML2108

SUMMIT MICROELECTRONICS, Inc.

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.

switch the laser between its on and off state is labeled

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.

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.

Figure 1. Laser Current Increase Caused by Temperature Increase, Constant Light Power Out

(Off) 0

(On) 1

Light

Power

Total

1

1

2

1

2

T1

T2

2053 Fig01