Self-mixing interference based sensors for characterizing touch input

A circuit for determining a displacement of a touch input surface, comprising:

• a vertical cavity surface emitting laser (VCSEL) configured to:
• generate light;
• receive light;
• and
• self-mix the generated light and the received light to generate self-mixing interference light;
• a photodetector configured to:
• detect the self-mixing interference light;
• and
• generate an output signal on which a first value corresponding to a first harmonic of the signal and a second value corresponding to a second harmonic of the signal are based;
• a filtering subsystem configured to:
• receive the output signal from the photodetector;
• and
• output a filtered signal proportional to a portion of the output signal generated by the photodetector;
• an extraction subsystem configured to extract the first value and the second value;
• and
• a phase calculation subsystem configured to calculate the displacement of the touch input surface using the first value and the second value.

A method for determining a displacement of a touch input surface, comprising:

• generating a lasing current;
• emitting light from a vertical cavity surface emitting laser (VCSEL) in response to the VCSEL receiving the lasing current;
• generating self-mixing interference light via the (VCSEL);
• generating an output signal in response to detecting the self-mixing interference light, wherein the output signal comprises three terms;
• filtering the signal to generate a filtered signal proportional to a third term of the three terms;
• and
• determining the displacement of the touch input surface via a phase calculation performed using the filtered signal.