A speckle-based CMOS sensor array for arbitrary surface movement detection Chao Wang * , Steve Tanner and Pierre-Andr´ e Farine Electronics and Signal Processing Laboratory (ESPLAB) Ecole Polytechnique F´ ed´ erale de Lausanne (EPFL) Rue Abram-Louis Breguet 2, 2000 Neuchatel, Switzerland ABSTRACT The paper presents a CMOS sensor array for the detection of speckle movements, for applications requiring accurate movement tracking of arbitrary surfaces. The array is made of eight sensors incorporating each a spatial comb filter with a pitch of 2.8 μm so as to be direction-sensitive to the movement of speckles with corresponding direction and spatial frequency, defined by the optical geometry of the system. The circuit is used with an array of micro-lenses placed between the sensor and the laser-illuminated surface. With speckle statistics (contrast, size) being independent on the surface properties, the detection works on virtually any surface. The system is operated at a sampling frequency of 64 kHz. Integrated into a 180 nm CMOS process, the circuit active area occupies 1.9 mm 2 and consumes 290 μW at full speed, allowing a maximal stable tracking speed of the surface of 0.25 m/s and a tracking accuracy of about 5 μm. Keywords: speckle, CMOS sensor, spatial comb filter, movement detection 1. INTRODUCTION If a laser source illuminates a rough surface, an interferometry pattern called speckle is formed in the image plane. These speckles show a spatial frequency distribution which depend mainly on the optical properties of the system, such as wavelength of the laser, numerical aperture and optical distance. If the rough surface is displaced with respect to the image plane, in an ideal case the observed random speckles move over the image plane without changing their shape. The tracking of their movement can therefore be used to detect the surface movement. This approach has the advantage to work on virtually any surface presenting sufficient roughness. The speckle movement can be detected by means of a photon sensor which is direction sensitive only to one spatial frequency corresponding to the speckle statistics. This motion detection principle was developed previously by Urban Schnell 1 . 2 2. SPATIAL COMB FILTERING Spatial comb filtering (Figure 1) is a technique that performs a discrete spatial Fourier transform of the speckles image at one precise spatial frequency ω 0 . The sine and cosine functions are implemented with two sets of equally spaced pixels with alternated gains of +1, -1, +1, -1... The signals are then summed together. The cosine pixels are spatially decayed by 90 ◦ (pitch p ) so as to provide a quadrature output. * E-mail:chao.wang@epfl.ch, Telephone: +41 (0)32 7183433, http://esplab.epfl.ch/