Iwao Komazaki

Optical micro encoder using a vertical-cavity surface-emitting laser

Abstract An optical micro encoder using a vertical-cavity surface-emitting laser (VCSEL) is presented in this paper. Utilizing the beam with small divergence from a VCSEL, a small optical spot is formed on a scale without using any other optical components. A quasi-sinusoidal signal is obtained when a scale with a minimum pitch of 20 μm (i.e., 10 μm line and space) is used. Resolution of nearly 1 μm can be obtained by interpolating the signal. Encoder signals are characterized when the distance between the VCSEL and the scale and their orientation are varied. In order to obtain sub-micron resolution, it is preferable to make the optical spot even smaller. Microlenses, which can eventually be integrated onto the VCSEL surface, and which can make the laser beam converge, 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 successfully observed. Although the microlens has not been integrated onto the VCSEL, the possibility of obtaining higher resolution using the microlens is clearly demonstrated.