How to use CGH to do the test of aspheric mirrors?

CGH stands for computer generated hologram. It is an optical element that is used in interferometric measurement of complex surfaces, such as aspheric optics or freeform optics. CGH is designed using computer algorithms and then manufactured via optical lithography. The holographic interference fringes contained within the CGH element can generate an exact replica of the intended wavefront it is designed to test, thus serving as a reference for testing and measurement of complex optics.

Due to its wide application range, high precision in testing, and short production cycle, CGH is increasingly being used in Aspheric Mirror testing. The following is an introduction to the adjustment method and steps of CGH to test the aspheric surface.

Preparation: interferometer, aspheric optics to be tested, CGH null corrector, adjusting table and fixtures. First, install the interferometer on the adjusting table, install the CGH null corrector, and the aspheric optics.

1.Positioning of the interferometer, CGH component, and aspheric mirror to be tested:

First, place the interferometer, CGH null corrector and the measured aspheric optics according to the CGH marking information or the distance parameter information given in the manual. Turn on the laser interferometer without installing trasnsmission sphere, so that the center of the interferometer spot area is roughly aligned with the center of the CGH and the center of the aspheric optics. 

CGH null correctors manufactured by MG Optics are given the mirror name and aperture in the identification area, as well as the F-number of the interferometer's transmission sphere used, the distance from the interferometer focal point to the front surface of the CGH and the distance from the CGH back surface to the mirror center.

2.CGH position adjustment:

Install the transmission sphere on the interferometer. First, loosen the screws and make sure the right position of the transmission sphere. confirm the front, back, and direction of the TS. After installation, rotate the lens and tighten the screws, then loosen the TS. Not tighten the locking screws too much.

CGH is designed with an alignment area, and the position adjustment of the CGH is completed based on the standard that the alignment area returns to the fringes of the interferometer without inclination and defocus (zero fringes).

First, roughly adjust the CGH position. This ensures that the spherical light waves reflected by the alignment zone make their way back into the interferometer. Turn off the lighting in the room, and place a cardboard with a small hole between the interferometer and the CGH. The spherical wave emitted by the interferometer enters the surface of the CGH through the hole in the paper, and returns to the interferometer through the hole after reflection. If it is not returned to the interferometer intact, you should be able to see the real image spot of the CGH alignment area on the paper (or a larger peripheral area). At this time, adjust the twist and pitch of the CGH to make the spot enter the interferometer lens.

Secondly, accurately adjust the position of the CGH. After rough adjustment, the spot of the CGH can be seen on the Align camera display of the interferometer. Move the spot to align with the center of the interferometer crosshair. Switch to the interferogram (View mode) to see the stripes formed by the CGH alignment area (spherically shaped reflective area): adjust the up, down, left, and right translation to make the CGH at the center position of the stripe display area; adjust the tilt, pitch, or rotation to minimize the stripe tilt; adjust the front and back translation to minimize the defocus of the stripes (power).

3. Position adjustment of the aspheric mirror being tested

Rough adjustment. The CGH is designed with an alignment area for aligning the mirror, which will converge the beams of multiple crosshairs of the interferometer near the mirror. The specific position coordinates are given by the designer. Adjust the position of the mirror so that the coordinates of the transmitted crosshairs basically match the coordinates given by the designer.

Adjust the pitch and tilt of the mirror so that the reflected light from the surface of the optical element being tested can pass back through the CGH and return to the interferometer. First, use a white paper or other screen to receive the actual image of the surface of the optical element being tested around the CGH, and adjust the tilt and pitch of the tested element to make the actual image enter the CGH. Then, move the screen between the interferometer lens and the CGH for observation, and continue to adjust the tilt and pitch until the focal point of the actual image coincides with the focal point of the standard lens, and the converged point enters the receiving range of the Align camera of the interferometer.

Delicate adjustment. After rough adjustment, the spot of the tested surface can be seen on the Align camera display of the interferometer. If the spot does not converge, adjust the front and back translation of the tested surface to make it converge at a point. At this time, there are multiple spots with different convergence degrees. Carefully identify the spot that meets the diffraction order of the CGH. The spot that meets the conditions coincides with the center of the interferometer crosshairs. Switch to View mode, adjust the Focus and Zoom of the interferometer View camera to obtain appropriate and clear stripes. Measure the surface accuracy, adjust the CGH and the relative position of the mirror according to the surface accuracy distribution characteristics, adjust the tilt and pitch of the mirror to make the interference fringes sparse, and then measure it again. In general, misalignment of coaxial or nearly coaxial mirrors will result in spherical aberration, while off-axis mirrors will result in coma.