Feedback Control of Laser Nonlinearity


Modulation of a laser diode is useful for a variety of applications including communication. The laser output is typically modulated by modulating the input current. For an ideal system, it’s desirable to have linear dependence of laser output power on input current. But an actual laser diode always has some nonlinearity even within the modulation bandwidth which gives rise to distortion of the modulated signal.

Such distortion exists even when the input is a pure sinusoid. But it is possible to overcome this nonlinearity by suppressing the higher harmonic terms using a feedback system. This project demonstrates the feasibility of this feedback method to make the modulated laser power behave more linearly with respect to the input current.

Concept

The feedback system is emulated by adding a second harmonic to the input laser current with an appropriate phase.

Circuit diagram

  • Phase shifter with a variable resistor

  • Voltage summer with a fixed gain

Circuit on a breadboard

The relative phase of the 2kHz input varies by tuning the variable resistor (manually with a flat screwdriver).

Experimental setup

Iterative optimization of the phase shift via the phase shifter circuit and correction amplitude of the 2kHz waveform generator

Before correction

White peaks are fundamental (favorable), 2nd, and 3rd harmonics (unfavorable).

After correction

We easily achieved a reduction of 8-12 dB for the 2nd harmonic amplitude.

This is a team project for my graduate course Photonic Laboratory (EE 234) at Stanford University. The team consists of me, Sheng-Yang Tsui, Siddharth Buddhiraju, and Rituraj.