It has been previously published how, using two separate Vertical-Cavity-Surface-Emitting-Lasers (VCSELs), a miniature laser-Doppler interferometer can be made for quasi-three-dimensional displacement measurements. For the use in consumer applications as PC-mice, the manufacturing costs of such sensors need to be minimized. This paper describes the fabrication of a low-cost laser-self-mixing sensor by integrating silicon and GaAs components using flip-chip technology. Wafer-scale lens replication on GaAs wafers is used to achieve integrated optics. In this way a sensor was realized without an external lens and that uses only a single GaAs VCSEL crystal, while maintaining its quasi-three-dimensional sensor capabilities.
There are many applications for non-contact measurement of the displacement and velocity of moving objects, especially
when achieved at low cost. An optical displacement sensor has been developed that can be compared to expensive laser-interferometry
sensors, however at a cost compatible with requirements for consumer products. This miniature Laser-Doppler Interferometer works on all light scattering surfaces. The first large-scale application is in PC-mice.
The measurement principle employs so-called "Laser Self Mixing", which occurs when laser light scattered on a surface,
within the coherence length, is coupled back into the laser cavity. When the object is moving, the back-scattered light is
continuously shifting in phase relative to the laser light at the laser mirror. This results in a periodic perturbation of the
feedback into the laser cavity, which causes modulations of the light intensity in the cavity. The frequency of these
modulations is proportional the speed of the object. A VCSEL, optimized for this application, is used as light source, a
photo-diode in the sensor measures the intensity fluctuations and, finally, an integrated circuit transfers the photo-diode
signal into velocity or displacement information. To determine the direction of the movement, a triangle modulation of
the laser-current is used, which modulates the laser-temperature and hence the laser frequency.
Next to the applications in PC-mice a much wider range of applications as input device in consumer products can be
envisaged. For instance menu navigation by finger movement over a sensor in remote controls, mobile phones and lap
tops. Furthermore a wide field of applications is envisaged in the manufacturing of industrial equipment, which requires
non-contact measurement of the movement of materials. The small form factor of less than 0.2 cubic centimeters allows
applications previously considered impossible.