Laboratory Equipment


Fizeau Interferometer	Long Trace Profilometer	Micromap 3D Surface Profiler

Summary of Instrument Measurement Ranges

Instrument	Horizontal Spatial period range	Vertical dynamic range	Approximate Measurement Limit
Micromap 570 3D Surface Profiler (Micromap Corp.)	(0.0002 - 1.25) mm	(0.010 - 150000) nm	0.03 nm RMS Height
Long Trace Profilometer (LTP) (Ocean Optics Inc.)	(2 - 1500) mm	(0.014 - 10000) mrad	10 nm RMS Height 0.45 mrad RMS Slope Error
Verifire AT Fizeau Interferometer (Zygo Corp.)	(0.039 - 150) mm (normal) (0.39 - 1500) mm (grazing)	(5 - 1000) nm	20 nm RMS Height 0.45 mrad RMS Slope Error

Micromap 570 3D Surface Profiler (Micromap Corporation)

The Micromap 570 3D surface profiler (shown above) is a microscope-based instrument that utilizes visible light interferometry to measure the surface finish (i.e. surface roughness) of optical surfaces. A CCD detector mounted on the microscope is used to measure an area (on the order of a few mm x mm, which is dependent on the microscope objective used) with a height resolution of approximately 0.03 nm.

Four different objectives are currently available: 5X, 10X, 20X, and 50X, which enable us to cover a wide range of spatial samplings (0.0002-1.25 mm) and spatial frequencies (0.0004-2.5 mm^-1).

Three measurement modes are available in software - smooth phase mode (interferometry phase measurement), wave mode (white light interferometry phase scan) and focus mode (interferometry focus scan). Smooth phase mode is ideal for very smooth surfaces (0.02 - 100) nm Sq roughness, but is limited to a maximum height measurement of 4 mm (focus depth). Wave mode is ideal for smooth surfaces with steps (maximum height measurement of 150 mm). Focus mode is ideal for rough surface textures and weak surface reflections (maximum height measurement of 150 mm).

Above is shown the 3D surface profile of a smooth mirror, taken using a 10X objective with the Micromap 570. This was taken in "smooth phase" mode using filtered white light (550 nm with a 25 nm filter bandwidth). The image represents approximately 0.5 x 0.5 mm² of the mirror.

This mirror surface has the following surface texture parameters - a root-mean-square surface roughness of 3.132 angstroms RMS (Sq), an average surface roughness of 2.481 angstroms (Sa), and a maximum peak to valley height over the sample of 37.34 angstroms (St).

Numerous striations are clearly visible in this image - they result from the lapping and polishing steps that occur in manufacturing the mirror, before the mirror is coated.

Long Trace Profilometer (LTP) (Ocean Optics Inc.)

The long trace profilometer (LTP) from Ocean Optics (shown above) measures the slope and curvature of mirror surfaces along one dimension using a zero-path difference interferometer. A solid state red laser source is used to probe the mirror under test. The CLS LTP can measure optical surfaces up to 1.5 meters in length, covering the range of spatial samplings from 2 - 1500 mm and spatial frequencies of 0.00033 - 0.25 cycles/mm.

Surfaces of virtually any shape can be measured in situ, as long as the surface slope change is within the ± 5 milliradian acceptance angle of the LTP optical system, with a sensitivity of 0.1 mrad and 0.5 nm in height. Dr. Peter Takacs and staff developed the LTP at Brookhaven National Laboratory (BNL). BNL still holds a US patent on this instrument.

The instrument is mounted on a vibration isolation table to minimize the effects from external vibrations and enclosed in a plastic curtain while measurements are taken (not shown in photograph) in order to reduce air turbulence effects on the optical head. The bearing has recently been upgraded from a crossed-roller bearing to a more precise linear air bearing.

Above is shown the height profile of a "reference" spherical mirror, taken with the LTP. From the center of the mirror, the edges are raised approximately 20 mm.

Highlighted in blue, to the left of the height profile curve, is shown the calculated radius of curvature of this mirror - in this case 9.8354 meters. Although steep radii of curvatures like this can be measured using the LTP, in general the LTP is used to measure the opposite extreme for synchrotron mirrors (where the radius of curvature is several kilometers).

Although this screen capture shows the height profile, the instrument can also display the so-called "slope error" - i.e. the difference in slope between the surface and the desired figure (e.g. cylinder, sphere).

Verifire AT Fizeau Interferometer (Zygo Corporation)

The Zygo VeriFire AT Fizeau interferometer (shown to left) measures the deviation of height of a surface from a reference surface. It accomplishes this using phase shifting interferometry. The figure error of optical surfaces up to 150 mm in diameter at normal incidence, or larger optics at grazing incidence angle ( < 1500 mm), can be measured with this instrument.

A picture of a grazing incidence measurement on a Far-IR beamline planar mirror (M3), in so-called "double pass" mode is shown below. A collimated HeNe laser beam (l=632.8 nm, 150 mm diameter) is output from the Fizeau Interferometer through a reference transmission flat at the left, reflected off the mirror under test (right) and then reflected back with a "return flat" (top right) to the detector in the interferometer. "Double pass" refers to the fact that the collimated beam is reflected twice off the test mirror.

Below is shown the 3D height profile of a transmission flat, measured in "single pass" normal incidence mode. The transmission flat under test is 150 mm in diameter.

The height profile shown (i.e. difference between the transmission flat under test and the "reference" test flat) is displayed in terms of wavelength, where l=632.8 nm (i.e. HeNe laser). Maximum peak to valley height over the test transmission flat is ~l/23.26, or 0.043l wavelength. Our best quality optical cavity, consisting of a "matched" high quality reference transmission flat and "return" flat, is approximately l/50 in quality (combined, as an optical cavity), so flatness measurements to ~l/50 are certainly possible with this instrument.

The surface/wavefront profile (curve in green) shows a slice (user selectable) through the test transmission flat (taken just off somewhat from the center line), and clearly shows that a slight concavity (i.e. raised edges and sunken center) exists in this test transmission flat.