Self-mixing interference based sensors for characterizing touch input

1. A method of detecting a user input on a touch input surface of an electronic device, the method comprising:

emitting a coherent light beam from a laser positioned within the electronic device, the coherent light beam emitted toward the touch input surface;

applying a sinusoidal modulation to a bias current of the laser, the sinusoidal modulation having a modulation frequency;

measuring a signal of an interferometric parameter associated with the laser;

determining a first value by demodulating the signal of the interferometric parameter at the modulation frequency;

determining a second value by demodulating the signal of the interferometric parameter at twice the modulation frequency; and

determining a displacement of the touch input surface using the first value and the second value.

2. The method of claim 1 , wherein:

the signal of the interferometric parameter is an output current of a photodetector associated with the laser; and

the laser is a vertical cavity surface emitting laser (VCSEL).

3. The method of claim 1 , further comprising determining a direction of movement of the user input based on a change in the determined displacement.

4. The method of claim 1 , wherein:

the signal is an output current of a photodetector;

the photodetector measures the signal; and

the output current varies around a fixed direct current value.

5. The method of claim 4 , wherein the output current depends on at least the modulation frequency and the displacement of the touch input surface.

6. The method of claim 4 , wherein determining the first value further comprises:

digitally sampling the output current;

multiplying the digitally sampled output current with a first sinusoid at the modulation frequency;

applying a low pass filter to the multiplied first sinusoid; and

calculating the phase of the filtered first sinusoid.

7. The method of claim 6 , wherein determining the second value further comprises:

digitally sampling the output current;

multiplying the digitally sampled output current with a second sinusoid at twice the modulation frequency;

applying a low pass filter to the multiplied second sinusoid; and

calculating the phase of the filtered second sinusoid.

8. The method of claim 7 , wherein the displacement of the touch input surface is determined using at least the calculated phase of the filtered first sinusoid and the calculated phase of the filtered second sinusoid.

9. 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.

10. The circuit of claim 9 , wherein:

the output signal generated by the photodetector is mathematically expressed as a sum of a first term, i 0 , a second term, i m sin(ω m t), and a third term, γcos(φ 0 +φ m sin(ω m t));

the first and second values are based at least in part on the third term;

the filtering subsystem reduces the first term and the second term to a scaled value; and

the filtering subsystem generates a proportional third term corresponding to the third term.

11. The circuit of claim 10 , wherein:

the extraction subsystem comprises a first low pass filter configured to receive the proportional third term and extract the first value; and

the extraction subsystem comprises a second low pass filter configured to receive the proportional third term and extract the second value.

12. The circuit of claim 11 , wherein:

the phase calculation subsystem is configured to obtain a phase from the first value and the second value; and

the phase calculation subsystem is configured to use the phase to calculate the displacement of the touch input surface.

13. The circuit of claim 9 , further comprising:

a current driver subsystem including,

a direct current (DC) voltage generator configured to generate a constant bias voltage; and

a wave generator configured to produce a single frequency sinusoid signal;

wherein the single frequency sinusoid signal is combined with the constant bias voltage and provided as a lasing current to the VCSEL.

14. The circuit of claim 13 , wherein:

the current driver subsystem further comprises:

a low pass filter configured to receive the constant bias voltage; and

a bandpass filter configured to receive the single frequency sinusoid signal;

the low pass filter reduces a variance of the constant bias voltage; and

the bandpass filter reduces noise in the output of the wave generator.

15. The circuit of claim 9 , wherein:

the output signal from the photodetector comprises a first term, i 0 , a second term, i m sin(ω m t), and a third term, γcos(φ 0 +φ m sin(ω m t));

the filtering subsystem comprises:

a differential transimpedance amplifier; and

an anti-aliasing filter; and

the differential transimpedance amplifier and anti-aliasing filter reduce the first term and the second term of the three terms.

16. 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.

17. The method of claim 16 , further comprising reducing first and second terms of the three terms to a scaled value.

18. The method of claim 16 , further comprising:

extracting a first value and a second value from the filtered signal, via an extraction subsystem, by using the proportional third term to obtain a first value and a second value.

19. The method of claim 18 , wherein calculating the displacement further comprises using the first value and the second value to determine the displacement.

20. The method of claim 16 , wherein:

generating the lasing current comprises:

generating a constant bias voltage;

producing a single frequency sinusoid signal; and

combining the constant bias voltage and the single frequency sinusoid signal.