Such a laser sensor based on self-mixing interferometry (SMI) allows the measurement of velocities, vibrations and distances covering a broad range of applications. Self-mixing interference devices make use of the effect, that laser light which is scattered back from a target object and re-enters the laser cavity, interferes with the resonating radiation and thus influences the output properties of the device. When the laser is operated in a suited regime the response of the back coupled light is linear, and resulting variations in output power contain traceable information on the movement or the distance of the target object with respect to the device. The frequency of these variations in output power is measured. To this end, the laser output signal is typically collected with a photodiode or phototransistor. The distance between the target object and the laser-cavity determines the phase of the back-scattered light when re-entering the laser cavity and hence whether there is destructive or constructive interference. When the object moves, the distance and hence the phase changes at a rate proportional to the velocity of the object. Therefore constructive and destructive interference occurs at a frequency proportional to this velocity. Since this so-called Doppler frequency only gives information on the magnitude of the velocity but not on its direction, modulation techniques have to be used to determine the direction. When using a semiconductor laser as laser source, in particular a VCSEL (vertical cavity surface emitting laser), the laser can be operated with a defined current shape, e.g. a periodic saw tooth or triangular current, causing the output frequency to almost instantaneously follow these current variations due to the simultaneously changed optical resonator length. This change in resonator length is temperature induced, resulting from the dissipative heating by the laser current. The resulting difference in frequency between the resonating and the back scattered light can be evaluated in suitable electronics and can be translated back into information about the position of the target object and its direction of movement.
WO 02/37410 A1 discloses a method and device for measuring the movement of an object. The method and device use the self-mixing interference effect in order to measure the movement of the object, which in the preferred application is the movement of a finger on an input device. With such a device which is also known as Twin-Eye laser sensor, a quasi 3D displacement of the object can be measured, in particular for use in input devices like PC-mice.
The total height of such a known sensor cannot be reduced freely since a certain round-trip length is needed in order to achieve a Doppler frequency in the correct regime. Presently this results in a minimum distance of the laser source output surface, typically a VCSEL, to the measured object surface of about 5 mm and a total height of the sensor bottom to the surface of 6 mm. Although this height does not impose restrictions for use in PC mice it is prohibitive for use input devices like e.g. mobile phones or mp3 players. Also for trackballs for e.g. remote controls or notebooks the sensor height is preferably equal to the size of the ball. The large distance between the output of the laser source and the measured object also causes a significant reduction of the signal strength, such that a focusing element is needed in order to focus the light and regain signal strength. In order to measure in two directions two laser sources are used spaced away from each other and an optical element is required to deflect the beams and focus the laser light. This optical element has to be placed with high accuracy with respect to the VCSELs to meet tolerances on the accuracy of the sensor.
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