Yoichi Ohmura

Displacement sensor based on grating imaging with a cylindrical lens array and a phase grating

A novel sensor for use in displacement metrology is proposed. It is based on grating imaging, which conventionally uses two amplitude gratings to generate a sinusoidal image. In the conventional method, the poor signal-to-noise ratio of the displacement output is one of the barriers to precise measurement, because 75% of the illumination light is trapped by the two amplitude gratings. In the proposed sensor a cylindrical lens array and a sine phase grating are used as the first and the second grating, respectively. Therefore, the illumination light is intercepted by neither, so that four times higher displacement signal amplitude than that of the conventional sensor can be expected. Consequently, four times higher signal-to-noise ratio can be obtained. Furthermore, the proposed sensor generates a sinusoidal output with little distortion by using the sine phase grating with optimized conditions, so that accurate measurement can be expected. In our prototype, a cylindrical lens array with a 200-µm period and a reflective sine phase grating with a 100-µm period were used.

Imaging with a rectangular phase grating applied to displacement metrology.

We achieved displacement metrology with a high-amplitude signal by using a rectangular phase grating as the pupil in a grating imaging system. The imaging phenomenon with a pupil transmission grating that has a bilevel profile with a 50% duty ratio is discussed on the basis of the optical transfer function. By optimizing the imaging condition, we obtained high-contrast images with high light power under a magnified or demagnified imaging system. The amplitude of the signal in the displacement measurement was four times higher than that of the conventional grating imaging system with amplitude gratings.

Displacement metrology based on grating imaging with a cylindrical lens array and a phase grating

A novel sensor for use in displacement metrology is proposed. The proposed displacement sensor is based on the grating imaging, which conventionally uses two amplitude gratings with rectangular apertures of fifty percent width of the period. In the conventional way, signal to noise ratio of displacement output is one of issues to be overcome for precise measurement because about seventy five percent of the illumination light is trapped by two amplitude gratings. On the other hand, in the proposed sensor a cylindrical lens array and a phase grating are applied as the first and the second grating, respectively. Therefore, the illumination light is trapped neither by the first grating nor the second grating except absorption. In our experiments, the cylindrical lens array with 200 μm period and the reflective sine phase grating with 100 μm period are used. Experimental results demonstrate that higher position resolution and higher accuracy of the displacement measurement is feasible by our proposal.