Reflective apertures, stops and shutters

Sometimes a high power infra red laser needs to have its power attenuated, or have strongly diverging modes removed from the beam. At Laser Beam Products we manufacture mirrors with precision through holes or with sharp knife edges that are very cost effective, even when made to your design as a one-off prototype.

We usually advise customers that the length of the hole is no more than the hole diameter, to avoid having a “tunnel” with an aperture at either end. We can make tapered holes to avoid this problem. Because our Gold coating is applied electrochemically the internal bore of any hole is also Gold coated.GoldPrismHoleFlippedIf an acousto-optic modulator (AOM) is being used, a reflective aperture can be used to manipulate one of the beam orders to be reflected and the other to be transmitted. We can make through holes as small as 1 mm diameter very accurately.

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Automotive customer saves $15,000

Some of our aerospace and automotive customers insist we offer a RTV (Return To Vendor) repair programme to reduce the overall cost of ownership of laser equipment. It saves time, money and valuable materials, and is well worth doing.  The reworked mirrors are as good as new so there is no loss of quality or productivity. It can also contribute to ISO 14001 Environmental Management Here’s a great case study…

One of our customers, a European automotive manufacturer, sent us a collection of used mirrors from their production line that had accumulated minor burns and scratches, and had been swapped out.  They were large, complex water cooled beam delivery mirrors which we repolished and recoated to a condition that was as good as new. The mirrors were repaired and returned in under 3 weeks.

The customer was delighted and told us the mirrors were working perfectly, and had saved them $15,000 on the cost of new mirrors!. They have since had them reworked three more times.

Here’s an example of another customer’s mirrors that we reworked to a ‘good as new’ condition. These were from a gas sensing cell made by a company who stopped supporting their equipment. You can find out more on our website.

Reworked Mirror

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.

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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

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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.
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   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.