Metrology

Metrology

A key element of REO’s ability to produce the world’s highest performance optical solutions is advanced metrology, to measure surface characteristics, surface quality, spectral performance, optical loss, and functional performance of optical components and assemblies. REO invests heavily in metrology equipment and expertise in order to ensure compliance with our customers’ most demanding requirements.

Laser Damage Testing

REO provides some of the highest laser damage resistant optics in the world, and is committed to the highest possible standard for testing and qualification of these optics. With our own fully automated laser damage test capability, REO not only conducts routing production testing of our products, but also continuously advances the state of the art in understanding laser damage mechanisms and test methodologies.

REO’s primary test system is comprised of a ~20ns pulse length 1064nm Laser Induced Damage Threshold (LIDT) test system for verification of high fluence operational specifications of optical substrates and their dielectric coatings according to ISO 21254-2 or other similar test protocols.

The system is fully automated, ensuring high levels of production efficiency, reliable and measurable consistency of performance, and extensive data collection to allow analysis and reference for future studies and improvements of the test system as well as optical component fabrication and performance. 

The four functional subsystems of the test station are shown in the schematic below. First, a stable high-energy laser and attenuator is used as the energy source for providing carefully measured and controlled energy doses to the Device Under Test (DUT). Next, an optical system is implemented to focus the laser to achieve a desirable spatial profile at the DUT as well as condition the beam for optimal beam waist and focal length performance, and also to split off a diagnostic beam for metrology of energy and spatial characteristics. Third, an inspection system consisting of a microscope, digital camera system, and dark-field illumination system is used to record damage events. Fourth, a motorized XYZ stage is used as a transport system to move the DUT between the test beam and the inspection station.

The control of the laser and attenuator, the transport system, and inspection system is fully automated. Also, all sensor systems are monitored and logged via a separate software system. This allows for real time automation feedback as well as minimal operator time and intervention while running a test. The operation of the system involves defining test parameters, the loading and alignment of a DUT, and initiation of the test. Analysis of the metrological and inspection data is performed by software upon completion of the testing sequence. Final reports are generated from the automated data analysis results. Human review of automatic analysis is available for improvements and more careful event analysis. 

Spectral Metrology/Laser Photometry

REO maintains an extensive set of lasers for metrology, covering more than 30 wavelengths and bands between 244nm and 10.6um. A range of standard and proprietary measurement techniques allows us to characterize reflectivity, transmission, polarization extinction, retardance and other functional characteristics across all angles of incidence.

Spectrophotometers, both dual-beam and FTIR, cover a wavelength range from 180nm to 20um, with a large number of fixtures to accommodate various sizes and geometries over the full range of incident angles. 

Surface Metrology

REO maintains 14 Fizeau interferometers for accurate measurement of surface figure (e.g. surface quality), utilizing several different configurations for both in-process and final product measurement of surface figure of flat and spherical surfaces. REO has interferometers at 633nm and 1064nm. These instruments normally have an absolute accuracy of approximately l/100. REO maintains interferometers with apertures of up to 150mm diameter, with lateral resolution of up to 100um.

For aspheric surfaces, REO utilizes stylus profilometry to provide an accurate measure of the surface form. Stylus profilometry is a contact measurement in which a stylus (typically diamond or a hard metal) is brought into very light contact with the surface and then moved laterally across the surface while its vertical position is measured accurately, often interferometrically. Though it provides only a line profile of the surface form, the measurement can be repeated at multiple angles to produce an approximation of the surface form of rotationally asymmetric surfaces.  REO’s stylus profilometry is accurate to approximately 1nm rms, and suitable for diameters of up to 200mm.

REO also uses optical profilometry to measure surface roughness. This tool provides an interferometer which is contained within the objective of a microscope, providing much higher lateral resolution than a typical Fizeau configuration. Over a lateral length scale from approximately 1um to 150um, these instruments have a height measurement resolution of approximately 0.1A rms.

REO also utilizes today’s most advanced optical microscope technology to characterize surface defects and texture, with a number of camera-equipped high power compound microscopes. These tools have a range of illumination and contrast options, including Bright Field, Dark Field, Nomarski (DIC) and Circular DIC. Objective magnification of up to 50x allows resolution and quantitative measurement of features of approximately 2um diameter, and detection of defects of only a few hundred nanometers in size.

GDD Measurement

For state-of-the-art measurement of the Group Delay Dispersion of components and subassemblies for ultrafast laser applications, REO utilizes the Chromatis White Light Interferometer from KM Labs. This tool provides GDD resolution of ±5 fs2 over a wavelength range of 500nm – 1100nm. It can be configured for dispersion measurement in reflection or transmission over a range of incident angles up to 70°. 

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