LBP has the measure of mirrors and optical components


s an optical manufacturer it is essential for us to be able to measure all the optical properties of a lens or mirror

before sending it out to the customer. We regularly provide sub micron measurements on our mirrors and optical components, so need to ensure that we have the necessary metrology and expertise in-house. We use a variety of equipment and

measurement techniques, depending on the specification of the optical component.

Laser calorimetry We use laser calorimetry to determine how much heat will be generated in an optic when a high-power laser beam is passed through it – to give a measure of the ‘absorptance’ or ‘absorption’. Too much heat and the optic can change its properties or even be damaged. Using an in-house design, the measurement

involves exposing the optic to a laser beam and recording the temperature rise, followed by the subsequent temperature drop after the laser is switched off. The temperature sensor is a thermocouple connected to an amplifier and logged by a computer. The rate at which the temperature rises tells us the rate at which laser power is being absorbed, and the cooling rate tells us how quickly the optic is losing heat to the surroundings – which is also happening during the heating stage. With a knowledge of those heat gain and loss rates, the laser power and the heat capacity of the optic, the absorption can be calculated, usually expressed as a percentage.

to our customers. This demonstrates that surface form specifications such as power and irregularity have been met. We also use interferometry to measure the wedge of flat laser mirrors to ensure parallelism. Interferometry is useful to measure and align optical assemblies such as reflective beam expanders or collimators, with an interferometer. We have investigated the mounting of optics, observing the deformation that can occur to mirrors from poorly designed mounts or excessive mechanical force. We can supply mirrors complete with mounts and internal water cooling, proven by interferometry that the assembled mirror has not been deformed.

3D optical profiler Our 3D optical profiler (Filmetrics Profilm 3D) uses state-of-the-art white-light interferometry to enable us to measure surface roughness from sub-nanometre to millimetre scale – something that is essential for some of our customers, where surface finish is vitally important. Reports are easily generated and can be supplied upon request.

‘V’ configuration at 67.4° incidence angle, as shown below. To check the phase retardation produced by

a mirror, the analyser is rotated in the reflected beam and transmitted power measurements recorded with angle. For example, if the test mirror is a ¼-wave

phase retarder, the reflected beam will be circularly polarised, and the power measured will be the same at all angles.

Temperature vs time during an absorption measurement of a ZnSe high-gas pressure lens with absorption of 0.16%; initial cooling, laser heating and final cooling

Interferometry The specifications of most optical mirror surfaces involve sub micrometre dimensions, too small to measure mechanically. For the majority of our optical mirrors we use a phase shifting interferometer for quality assurance. With software analysis of the interference pattern, a hard copy of the results is available | @electrooptics

Analyser used for mirror phase retardation We also use a Brewster plate attenuator – normally used to attenuate a linearly polarised laser beam – as an analyser to assess the polarisation of a laser beam. The Brewster plates will require an ‘enhanced’ coating on one side, this increases the maximum attenuation (reduces minimum transmission) considerably. The attenuator then behaves very much like a linear polariser, closely following Malus’s law with transmission T = cos2θ, θ being the angle between the direction of polarisation and the transmission axis of the analyser/attenuator. Used with a CO2 laser, these Brewster plates are made from ZnSe and are tilted in a

Optical surface form measurements Now that optical designers are becoming more aware of the capabilities of modern day machine tools, they are increasingly using torics, cylinders and other non-rotationally symmetrical surfaces in their optical designs. It has become increasingly important for our diamond machining division to give our customers and engineers confidence that all drawing specifications have been met. The Form Talysurf PGI optics contact stylus optical profilometer, when equipped with an additional “Y” stage, is capable of measuring these advanced surfaces in X Y Z – such as spherical, diffractive, fresnel and aspheric lenses. The data is used to produce a three- dimensional map of the generated surface and calculate any deviation from the design specification. EO

June 2021 Electro Optics 23

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