Distortion of laser mirrors

It’s not widely appreciated how easy it is to distort a laser mirror with mechanical forces that can be generated from inadvertently over-tightening mounts, or water fittings. Although our experience is mostly with metal mirrors such as copper or aluminium, we know glass mirrors are just as sensitive to distortion. Sometimes the distortion is elastic, so the surface figure is restored when the mechanical force is removed. Often though the distortion can be permanent.  We recently repaired a water cooled mirror for a customer that was very thick ( 33mm ) and looked to be in good condition.

pip_in_mirrorBy reflecting a high mode quality visible laser beam from the mirror at 45 degrees incidence we could see the distortion the mirror introduced into the reflected beam profile. Using our phase shifting interferometer we could resolve a 2-3mm diameter convex ‘pip’, perhaps just a micron or so high, in the centre of the mirror. This is where the surface had been ‘punched through’ by the mounting screw in the rear of the mirror.


There was 16 mm thickness of solid copper between the bottom of the screw hole and the mirror face. Despite this though, it was still possible for the mechanical force from tightening of the mounting screw to permanently distort the mirror face.

The hot spot created in the reflected beam by the mirror distortion when used with a CO2 laser would have likely caused damage to other optics, such as the focus lens.



Diffuse infrared reflector


Gold coated diffuse reflector

We have recently developed the gold coating of carefully roughened metal substrates to be used as diffuse reflectors. This particular example, based on Aluminium, has a consistent surface roughness of Ra=6um and would be useful in the near infra red.

The surface roughness needs to be random enough to allow the surface to function as an isotropic diffuse reflector for infra red wavelengths. Additionally the magnitude of the surface roughness needs to be high enough so the reflectance will be perfectly diffuse and have no enhanced reflectance in the specular direction. We are happy to coat customer supplied material and we can provide measurements of the surface texture.


Optical polishing and finishing

Article recently published in Electro Optics magazine (issue 257, October 2015)

As our customers are starting to work with visible light as well as the more traditional infrared,  optics are increasingly required to perform well over a broader range of wavelengths. This means we need to look at how the mid-spatial frequency surface roughness affects the reflection of light in addition to the Ra surface roughness value and scratch-dig specifications.

A lot of applications are becoming more and more broadband and the days of having a mirror specified just for the infrared (IR) are becoming numbered; people want a multifunctional mirror that will work not just in the IR but perhaps for the visible as well.

What we are finding is that although the final application of the mirror might be for an IR application such as CO2 laser cutting (where surface roughness values in the range of tens of nanometres are often adequate) it may not be possible to test or align the part because these techniques often involve the use of visible light. For example you might be using terahertz or CO2 lasers – long wavelength applications where the quality of the mirrors doesn’t matter too much. But you have to align it or test it and that generally requires some sort of visible laser or visible technique. Then there will be scatter and diffraction of the visible light and although this might not matter when it is installed on the customer’s premises, if you can’t line it up in the factory then you are a bit stuck.

This is something that people overlook. We’ve had customers who think the mirrors don’t need to be that high a quality for their terahertz application, but then when they try to align their equipment with a visible laser they can’t do it.

Metal mirrors used for CO2 and terahertz applications are often produced by Single Point Diamond Turning (SPDT) whereby a flat, spheric, aspheric or even a freeform reflective surface is machined directly onto the mirror.

However, a simple surface roughness value and/or a scratch-dig specification have been found to be inadequate for SPDT metal mirrors used with visible or near infrared radiation. Using white light interferometric testing, we produced a cross section profile of a typical SPDT surface. Several families of grooves could be observed with different spacings and amplitudes; this significantly reduces the amount of specularly reflected visible light.
   unpolished                  Polished_newsdec13   polished

So although the SPDT parabolic mirror had a good surface roughness value of Ra = 5nm, when the mirror surface was analysed by spatial frequency a regularly repeating set of grooves could be seen with a spatial frequency in the order of 50–100 lines per mm. This repetitive mid spatial frequency surface roughness rendered the mirror unusable at 1um wavelength and produced a large amount of scattered and diffracted light at 633nm.

As a result of our chemical polishing, the mid spatial frequencies of the surface roughness of the same mirror were reduced considerably.

If you want good quality reflection from a mirror surface, then surface roughness Ra doesn’t tell the whole story. It’s the roughness and the important mid-spatial frequencies content which control the quality you get. So simply having a surface roughness specification of Ra = 5nm is just not sufficient.

Laser World of Photonics New Delhi 9-11 September 2015

We’re busy preparing for the next Laser World of Photonics show in India. We were G21_5impressed with the show last year in Bangalore so we’re hoping it will be just as good this year in New Delhi.

If you’re visiting come and visit us in the UK pavillion, Booth number G21. We’ll be showing a range of our products including gold coated copper and aluminium mirrors with a range of optical coatings and sizes from as small as 5mm diameter.

Resistant to harsh environments our mirrors have high laser damage thresholds and are used in many applications including gas sensing, medical and dental lasers, fiber lasers, high power pulsed and CO2 laser cutting and infra red counter measures.

LBP manufacture a wide range of standard and bespoke parts with a range of optical coatings covering the spectrum from UV to THz as well as infrared. Full details are available on our website.

Laser Beam Products exhibiting at Laser World of Photonics Munich, June 22-25

35x15mm_en (2)

We will be in Munich again this year showcasing our range of all-metal mirrors for science and research, as well as our now well-established mirror reworking service.

We’ll be showing a wide range of our mirrors including Gold coated Copper, Aluminium, Molybdenum, Stainless Steel and Tungsten. Most of these materials have excellent broad reflectivity across the mid infra red, and are ideal for many infra red applications. They are highly durable and resistant to harsh environments such as those found in gas sensing and infrared spectroscopy.

With an excellent range of reflective coatings covering the spectrum from UV to THz, we have an unrivalled range of products which can be seen, and handled, at this years show. Suitable for CO2 laser cutting and welding, fiber laser applications, Dental and Medical Er:YAG lasers, QCL sources, fiber lasers and even Terahertz applications our mirrors are regularly supplied for both low volume prototype and high volume production parts. Customers include industrial laser users, OEMs, Academics and Research Institutes around the world.

LBP’s customers worldwide are also regularly saving thousands by repairing mirrors back to operational condition after damage by exposure to chemicals and harsh environments. Examples of ‘before’ and ‘after’ mirrors will be on display.