A Phase Measuring Radial Shear Interferometer For Measuring The Wavefronts Of Compact Disc Laser Pickups

Abstract

A phase measuring radial shear interferometer has been developed that enables the user to measure the wavefront of Compact Disc or other optical storage laser pickups. The instrument is designed to operate over the wavelength range from 700nm to 830nm. It will accept the diverging wavefront of any optical storage laser pickup, and it includes a holder for the thin coverplate necessary to compensate for the plastic or glass overcoat on the optical storage media. The interferometer is equal path and has been achromatized over the wavelength range so that the source coherence length can be less than one millimeter. Compensation for variations in light source intensity is accomplished by a rotating variable neutral density disk driven by a servo system. This compensation system permits input light levels to vary from 0.1mw to as great as 100mw. Data processing options include: peak-to-valley and root-mean-square wavefront distortion computation with isometric and contour plotting as well as Zernike polynomials, point spread function, and modulation transfer function. System accuracy depends on the numerical aperture of the input source. With the standard collimating optics the input numerical aperture can be as large as 0.65. For this numerical aperture the instrument is typically better than 1/5 wave at 780nm. For 0.5 numerical aperture (typical for most Compact Disc pickups) the system performance is better than 1/10 wave. If more accurate measurements are required, the wavefront of the instrument must be stored and subtracted from the measurement of the pickup. Results of measurements of Compact Disc heads will be presented and other applications of the instrument will be discussed.