Tilted Wave Interferometer

A new technology measures aspheres and freeform optics.

01 July 2016

A new white paper from researchers at University of Stuttgart shows the evolution of a new technology to measure aspheres and freeform optics.

Fast and precise ways of measuring surface roughness and contour on these optical components are essential in the fabrication of modern photonic devices such as cameras, telescopes, head-mounted displays, laser-focusing heads, and lithography steppers. They are even needed in the efficient production of eyeglasses.

The inventors from the Institute of Applied Optics (ITO) are commercializing the tilted wave interferometer (TWI) with Mahr, the German maker of industrial dimensional metrology equipment.

An early version of the TWI, the MarSurf TWI-60, was named a finalist in the 2015 Prism Awards for Photonics Innovation.

SPIE Fellow Wolfgang Osten, one of the TWI inventors and a member of the SPIE Board of DirectorsThe demand for measurement tools for complex optical components like aspheres has grown in proportion to the growing demand for improved and high-quality devices, says SPIE Fellow Wolfgang Osten, one of the TWI inventors and a member of the SPIE Board of Directors. And “one cannot produce surfaces better than it is possible to measure them,” he says. “Systematic quality control is of the highest importance for these classes of optical functional surfaces.”

The white paper by Osten and ITO colleagues Christof Pruss and Johannes Schindler traces the development of metrology tools for high-tech industries where manufacturing processes demand nearly total elimination of defects. So-called zero-defect production “applies both to the products themselves as well as to the processes of making them,” the paper states.

Optical metrology is a “well-established and very sensitive method” to assure minimal defects and precise shapes in aspheres and freeforms, the authors say, with interferometers one of the most effective ways to monitor the surface formation and polishing processes during manufacturing.

The upgraded tilted wave interferometer from Mahr.

A typical process of measuring aspheres involves a computer-generated hologram (CGH), but fabricating them is costly and time-consuming.

The TWI is a modified Twyman-Green interferometer that can measure aspheres and freeform surfaces without CGH or stitching techniques and can be easily integrated into the workflow on the shop floor.

The new system combines the high precision and traceability of the interferometric principle with a high dynamic range of up to 10° gradient deviation from the spherical form, without the need for compensation optics and moving parts.

Like all full-field interferometric methods, it has a high lateral resolution. It also has a short measurement time, which is achieved by highly parallelized data acquisition, the authors say. “In principle, it is a single-shot technique that allows the robust measurement of the entire surface in a few seconds with a height resolution better than 1 nm and a lateral resolution of several microns.”

The researchers say the TWI extends the single spherical wavefront of a standard interferometer to an array of mutually tilted wavefronts. “All of them simultaneously impinge onto the surface under test (SUT). Thus for each area on the SUT, there is a fitting wavefront that compensates the local deviation from the best fitting sphere such that the laser light reaches the camera and produces interference fringes that contain the desired surface shape information,” the paper states. “Since all wavefronts are there simultaneously, the SUT can be measured very quickly.”

The tilted wavefronts are realized with the help of a matrix of point sources, part of the patented illumination design.

Schematic setup of the interferometer, with the central source and one exemplary off - axis source indicated, shows how the tilted wavefronts are realized with the help of a point-source array generated by a microlens array in conjunction with a pinhole array. L is the laser source; BS1 and BS2 are beam splitters; C1, C2, C3 are lenses; PSA is the point source array; AA is the aperture array; M is the mirror; O is the objective; SUT denotes the surface under test; A is the aperture; IO is the imaging optics; and C denotes the camera.

The new calibration concept, also patented, can be generalized to other types of interferometric setups and guarantees a reliable and verified precision of about λ/10 across the entire SUT, according to the white paper.

It takes the standard calibration from a two-dimensional areal calibration to a volume calibration. This allows the placement of the SUT at any position in front of the interferometer, which enhances the flexibility of the instrument.

The new technology provides continuous monitoring of the surface formation process during manufacturing, detecting form errors, waviness, local defects, footprints of the tool and surface roughness.

Download a PDF of the white paper on the tilted wave interferometer.

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