PICWave

Directly Modulated Laser (DML)

Simulation with PICWave software

PICWave's active model can give important insights into the dynamics of active devices, such as lasers and SOAs. The examples below illustrate two types of the directly-modulated laser (DML)

The first consists of a directly modulated quarter-wavelength-shifted DFB connected to an SOA at constant bias

DML + SOA device

The amplified DML output signal emerging from the SOA is shown below, in PICWave's oscilloscope analyser. The ON and OFF levels at 140 and 70 mW are clearly visible, yet it can be seen how the SOA overshoots and undershoots at the leadingedges of the ON and OFF bits, respectively, thus distorting the DML output. This arises due to the gain saturation and finite response time of the SOA.

SOA-amplified DML output - showing signal distortion caused by SOA

A better design (Finisar patent) is shown below, in which the SOA is also modulated, in sync with the laser.

Modified device: DML + modulated SOA

The amplified DML output emerging from the SOA is shown below, in PICWave's oscilloscope analyser. It can now be seen how the eye is much cleaner, with a much improved ON/OFF ratio and almost no distortion of the DML output.

SOA-amplified DML output for modified device - showing clean eye with negligible distortion and improved ON/OFF ratio