OPTICAL MICRO ENCODER USING A VERTICAL-CAVITY SURFACE-EMITTING LASER

An optical micro encoder using a vertical-cavity surface-emitting lase r (VCSEL) is presented in this paper. Utilizing the beam with small di vergence from a VCSEL, a small optical spot is formed on a scale witho ut using any other optical components. A quasi-sinusoidal signal is ob tained when a scale with a minimum pitch of 20 mu m (i.e., 10 mu m lin e and space) is used. Resolution of nearly 1 mu m can be obtained by i nterpolating the signal. Encoder signals are characterized when the di stance between the VCSEL and the scale and their orientation are varie d. In order to obtain sub-micron resolution, it is preferable to make the optical spot even smaller. Microlenses, which can eventually be in tegrated onto the VCSEL surface, and which can make the laser beam con verge, are studied. First, the diffraction pattern formed on the scale is calculated, and the effect of using a microlens is confirmed. Then , microlenses are fabricated on a glass substrate, and their profiles are characterized. Nearly spherical smooth profiles are obtained. Also microlenses are illuminated by a collimated beam from a conventional edge-emitting laser, and optical spots made by the microlenses are suc cessfully observed. Although the microlens has not been integrated ont o the VCSEL, the possibility of obtaining higher resolution using the microlens is clearly demonstrated.