Fizeau interferometers for surfaces with different reflectivity

Dec 18, 2007
Intensities
Intensities of the interferences of common Fizeau interferometers, in this example calculated for a reference surface with a reflectivity of 0.5 and test sample surfaces with varying reflectivity between 0 and 100% and a variation of the distance d between test sample and reference piece over a wavelength (λ = 633 nm). The new method avoids the strong dependence of the intensity profile and the contrast on the reflectivity of the test sample's surface. Credit: PTB

Due to their stable design, Fizeau interferometers are used to determine the topography of surfaces such as, e.g., plane surfaces. PTB (Physikalisch-Technische Bundesanstalt, Germany) has now developed a method which makes it possible to analyse surfaces with different reflectivities in a simple way. This method can also be extended to dynamic measurements.

Fizeau interferometers generate an interference between the surface of a test sample and a reference surface that is brought close to the test sample. The interference image is recorded and analysed by an imaging optics system. The contrast and the shape of the interference signals depend, however, on the reflectivity of the test samples. The time and effort required for measuring and analysing the topographies of differently reflecting test pieces is therefore significantly increased.

The method of separating the wavefronts of the reference surface and the test sample surface in the plane of the reference surface – for which a patent has been applied for – uses a new generation of commercially available beam splitters, also called "on-axis beam splitters", which cause a separation of the polarisation directions of the incident light along the optical axis.

In comparison with common Fizeau interferometers, this has various advantages. On the one hand, through the generation of polarised light, the measurement can be traced back to a pure two-beam interference. The analysis of the signal is thus significantly facilitated and improved. Classic Fizeau interferometers, however, are based on the analysis of the multiple-beam interference.

On the other hand, by varying the direction of polarisation, a maximum contrast can be set, independent of the reflectivity of the test sample. In contrast to this, in conventional Fizeau interferometers, different reference surfaces with adapted reflectivity must be held available in case the reflectivity of the test samples varies strongly.

In order to increase the accuracy of analysis of common Fizeau interferometers, a variable phase is generated by varying mechanically the distance between the test sample and the reference surface (phase-shifter interferometry). Another advantage of the new method is that such a phase shifting becomes possible through the use of electro-optical components and thus without using mobile parts. Thanks to the increased measuring dynamics achieved in this way, it is, for example, possible to carry out topographical measurements in environments which are subjected to vibrational strain.

The new method allows a facilitated and improved analysis of the measurement data, requires only one calibrated reference surface instead of several, and opens up new possibilities of application in the field of dynamic interferometry. It is particularly suited for measurements on structured or unstructured surfaces with different reflectivities – especially in the optical and semiconductor industry.

Source: Physikalisch-Technische Bundesanstalt (PTB)

Explore further: New approach to form non-equilibrium structures

add to favorites email to friend print save as pdf

Related Stories

Progress on detecting glucose levels in saliva

Jun 03, 2014

Researchers from Brown University have developed a new biochip sensor that can selectively measure concentrations of glucose in a complex solution similar to human saliva. The advance is an important step ...

Space science stories to watch in 2014

Jan 02, 2014

There's an old Chinese proverb that says, "May you live in interesting times," and 2013 certainly fit the bill in the world of spaceflight and space science. The past year saw spacecraft depart for Mars, ...

Asteroid deflection

Sep 02, 2013

Potential asteroid impact on Earth can have disastrous consequences. In order to prevent such collisions, earthbound space objects must be deflected. This can be accomplished using a space probe to impact ...

Astronomers gear up to discover Earth-like planets

Jun 06, 2013

If one looks only for the shiniest pennies in the fountain, chances are one misses most of the coins because they shimmer less brightly. This, in a nutshell, is the conundrum astronomers face when searching ...

Recommended for you

New approach to form non-equilibrium structures

3 hours ago

Although most natural and synthetic processes prefer to settle into equilibrium—a state of unchanging balance without potential or energy—it is within the realm of non-equilibrium conditions where new possibilities lie. ...

Nike krypton laser achieves spot in Guinness World Records

5 hours ago

A set of experiments conducted on the Nike krypton fluoride (KrF) laser at the U.S. Naval Research Laboratory (NRL) nearly five years ago has, at long last, earned the coveted Guinness World Records title for achieving "Highest ...

Chemist develops X-ray vision for quality assurance

9 hours ago

It is seldom sufficient to read the declaration of contents if you need to know precisely what substances a product contains. In fact, to do this you need to be a highly skilled chemist or to have genuine ...

The future of ultrashort laser pulses

9 hours ago

Rapid advances in techniques for the creation of ultra-short laser pulses promise to boost our knowledge of electron motions to an unprecedented level.

IHEP in China has ambitions for Higgs factory

Jul 23, 2014

Who will lay claim to having the world's largest particle smasher?. Could China become the collider capital of the world? Questions tease answers, following a news story in Nature on Tuesday. Proposals for ...

User comments : 0