Surface quality sensing using self-mixing interferometry

1. An electronic device, comprising:

a housing;

a set of one or more self-mixing interferometry (SMI) sensors attached to the housing and including,

a set of one or more electromagnetic radiation emitters having a set of one or more resonant cavities and configured to emit a set of one or more beams of electromagnetic radiation; and

a set of one or more detectors configured to generate indications of self-mixing within the set of one or more resonant cavities; and

a processor configured to,

characterize, using the indications of self-mixing, an optical field speckle of a target; and

characterize, using the characterization of the optical field speckle, a surface quality of the target.

2. The electronic device of claim 1 , wherein:

the processor is further configured to,

determine, contemporaneously with at least one of characterizing the optical field speckle or characterizing the surface quality, and using the indications of self-mixing, at least one of,

a posing of the housing with respect to the target;

a distance of the housing to the target; or

a velocity of movement between the housing and the target.

3. The electronic device of claim 1 , wherein the processor is configured to characterize the optical field speckle by:

performing a time domain analysis of the indications of self-mixing;

performing a frequency domain analysis of the indications of self-mixing; and

determining, from at least one of the time domain analysis or the frequency domain analysis, at least one of:

an optical field speckle contrast;

a phase shift; or

an optical field speckle correlation length.

4. The electronic device of claim 1 , wherein:

the set of one or more SMI sensors includes at least a first SMI sensor and a second SMI sensor having different,

angles of incidence;

numerical apertures;

working distances;

polarizations; or

emitted electromagnetic radiation wavelengths.

5. The electronic device of claim 1 , further comprising:

a set of optics disposed over an SMI sensor in the set of one or more SMI sensors; wherein,

the processor is configured to tune the set of optics, the tuning providing diversity in the indications of self-mixing.

6. The electronic device of claim 1 , wherein characterizing the surface quality comprises determining at least one of:

surface roughness;

surface waviness;

subsurface scattering within the target; or

a refractive index of the target.

7. The electronic device of claim 1 , further comprising:

a motion sensor; wherein:

the processor is further configured to characterize the surface quality responsive to an output of the motion sensor.

8. The electronic device of claim 1 , wherein:

the housing defines a stylus body having a tip;

the set of one or more beams of electromagnetic radiation has a set of one or more known relationships to the tip; and

the processor is further configured to characterize the surface quality of the target using the set of one or more known relationships.

9. The electronic device of claim 1 , wherein:

the housing defines a wearable device; and

at least a subset of the set of one or more beams of electromagnetic radiation is directed outward from an exterior of the wearable device.

10. The electronic device of claim 1 , wherein:

the housing defines a wearable device; and

at least a subset of the set of one or more beams of electromagnetic radiation is directed inward from an interior of the wearable device.

11. The electronic device of claim 1 , wherein the surface quality comprises a surface roughness.

12. The electronic device of claim 1 , wherein the surface quality comprises a grain height and at least one of a grain width or a grain spacing.

13. The electronic device of claim 1 , further comprising:

a set of optics; wherein,

an SMI sensor in the set of one or more SMI sensors,

emits a beam of electromagnetic radiation from a resonant cavity and into the set of optics; and

receives a returned portion of the beam of electromagnetic radiation from the set of optics and into the resonant cavity; and

the emitted beam of electromagnetic radiation and the returned portion of the beam of electromagnetic radiation have non-reciprocal optical paths through the set of optics.

14. A method, comprising:

receiving indications of self-mixing from a set of one or more self-mixing interferometry (SMI) sensors;

determining, using the indications of self-mixing, at least one of a distance or a directional velocity with respect to a target;

characterizing, using the indications of self-mixing and the at least one of the distance or the directional velocity, an optical field speckle of the target; and

characterizing, using the characterization of the optical field speckle, a surface quality of the target.

15. The method of claim 14 , further comprising:

rendering a stroke on a display in accord with the surface quality.

16. The method of claim 14 , further comprising:

animating, in an augmented reality or virtual reality environment, an interaction between a virtual object and the target; wherein,

the animating is performed responsive to the surface quality of the target.

17. The method of claim 14 , further comprising:

causing an actuator to provide, to a user, feedback indicative of the surface quality.

18. The method of claim 14 , further comprising:

causing an actuator to provide, to a user, feedback that masks the surface quality.

19. A method of characterizing surface quality, comprising:

receiving indications of self-mixing from a set of one or more self-mixing interferometry (SMI) sensors;

characterizing, using the indications of self-mixing, an optical field speckle of a target; and

characterizing, using the characterization of the optical field speckle, a surface quality of the target.

20. The method of claim 19 , wherein characterizing the surface quality comprises determining at least one of:

surface roughness;

surface waviness;

subsurface scattering within the target; or

a refractive index of the target.