Spectrally Controlled Interferometry

A recent advancement in the technology of the traditional interferometer, spectrally controlled interferometry (SCI), improves the manufacturing of optics that has multiple flat surfaces.

Interferometry is a family of techniques in which waves, most often electromagnetic waves, are superimposed which causes the phenomenon of interference with the purpose to extract information. It’s uses are an important investigative technique in the fields of fiber optics, engineering metrology, oceanography, astronomy, optical metrology, seismology, spectroscopy (and its applications to chemistry), quantum mechanics, nuclear and particle physics, plasma physics, remote sensing, biomolecular interactions, surface profiling, microfluidics, mechanical stress/strain measurement, velocimetry, and optometry.Electromagnetic Waves

Widely used in science and industry, interferometers measure small displacements, refractive index changes along with surface irregularities. As is the case with most interferometers, light from a single source is split into two means that travel different optical paths, and are then combined again to produce interference.

However, measuring prismatic and flat optics with the traditional laser interferometer is difficult, if not impossible. Optical scientists, technicians and engineers continue to struggle to obtain accurate measurements on optics with two or more flat and parallel surfaces because of the mutually interfering back reflections.

In an effort to suppress back reflections and enable measurements for more than fifty years, users have tried work-arounds such as taping, painting, or adding unwanted wedge. There’s even rumor that the best color or brand of paint can be used to achieve a more accurate measurement. Such is the acceptance of the limitation imposed with the traditional interferometer.

Using these work-arounds has the potential to add hours to production, add cost of the materials such as the paint and supplies, and still leave users wondering whether the parts meet the required specification. Having to employ work-around methods can also damage parts causing them to be scrapped. At times, even the best methods don’t work, as is the case with the RBG prism combiners. Because of their millimeter dimensions – as many as five mutually interfering surfaces – there’s a completely confused interference pattern.

Historically, there’s been four methods to overcome these issues:

  • A Twyman-Green interferometer with a white light source
  • A grazing-incidence interferometer with a low-spatial-coherence laser source
  • An interferometer with a scanning or stepping laser source
  • A delay-line, coupled-cavity Fizeau white-light-source interferometer

With the advancements in all scientific disciplines, this methodology simply doesn’t work anymore.

There is a new method of isolating the measurement surface by controlling spectral properties of the source (Spectrally Controlled Interferometry – SCI). Using spectral modulation of the interferometer’s source enables formation of localized fringes where the optical path difference is non-zero. As a consequence, it becomes possible to form white-light like fringes in common path interferometers, such as the Fizeau.

The proposed setup does not require mechanical phase shifting, resulting in simpler instruments and the ability to upgrade existing interferometers. Furthermore, it allows absolute measurement of distance, including radius of curvature of lenses in a single setup with possibility of improving the throughput and removing some modes of failure.

Originally demonstrated in 1997 by professor Johannes Scwider, and commercially available since 2017, spectrally controlled interferometry (SCI) enables, without any unique preparation, the rapid measurement of flat and prismatic surfaces. It uses a standard Fizeau interferometer in which the laser has been replaced by a new light source that produces an electronically controlled spectrum. This new light source can transform any Fizeau — saving time, reducing scrap, and improving accuracy.

There are six benefits of SCI:

  • Easy setup with long coherence, like a laser Fizeau interferometer
  • Measures 100-µm-thin plates without work-arounds
  • Less than 1-sec, vibration-tolerant data acquisition
  • Electronic phase-shift control measures fixed cavities
  • Quick visual display and checking of measured surface
  • Works on any Fizeau interferometer, with a quick change on the ÄPRE S-Series

SCI has found practical applications in a number of areas:

  • Right-angle prisms (micrometer and larger)– Isolating surfaces and electronically phase-measuring blocked parts can now be done while isolating the surface of interest.
  • Cellphone cover glass: These micro-meter-thick plates influence image quality and can be measured for the first time with an interferometer.
  • Ultraparallel windows: Flat surfaces with minimal wedge can now be measured accurately without internal reflection from the backside surface.
  • Filter substrates: With SCI the part is aligned, as in a laser Fizeau, and a vibration-tolerate phase measurement is made in less than a second, front and back, and possibly internal interference — to provide total thickness variation. Total measurement time is now around two minutes, with significant cost savings and increased quality control.

Spectrally controlled interferometry capabilities are still being explored, and applicability and performance will certainly widen with future discoveries. Even now, in its infancy, SCI is advancing optical manufacturing.

Universe Optics designs, engineers and manufacturers precision optical lenses for uses in the fields of fiber optics, meteorology, oceanography, astronomy, optical metrology, along with other disciplines in which the use of spectrally controlled interferometry could be used. We stock 1000’s of standard lens assemblies and can custom design a solution for scanners, CCTV, CCD/CMOS, medical imaging, surveillance systems, machine vision and night vision systems.

Posted in UV Lens News.