QCL mirrors, optical mirrors from hardened metals

We have summarised some of the reflective products we have been working on recently in our Laser Beam Products Newsletter , and posted it on our web site.

We are still seeing large numbers of used CO2 laser mirrors sent to us for re-polishing and coating, the last few months were no exception, notable this time were several dozen water cooled mirrors from a Trumpf laser based pipe welding machine.  On the subject of saving money, talk to us about making mirrors, mounts, and adjustment as one monolithic part. As well as costing less than the sum of the individual components, it can reduce assembly time, and later on reduce servicing costs.

We have also been developing polished mirrors from hardened tool steel, and made some Titanium mirrors.

We are always willing to perform fabrication and polishing trials on novel optical materials, in fact we rather enjoy it.

Beryllium Copper mirrors with extreme parallelism

We have developed annealing and plating techniques to make highly accurate Beryllium/Copper mirrors that are easily deformable in a controlled way, and elastic enough to return to their original surface form.  Not to be confused with pure Beryllium metal (which as a dust is highly toxic) Be/Cu is an alloy that has useful properties for elastic or deformable structures such as adaptive mirrors and deformable mirrors.

To date we have made gold coatings on these types of mirrors but we believe that our customers will find other uses and applications that we haven’t yet thought of.  If you have any ideas about how to use these products, please send us your thoughts.

Mirrors for Dental and Medical Lasers

Dental mirrors

We believe that our metal mirrors with electroplated gold coating are ideal for dental and medical lasers. We have several customers in this market sector using them successfully already, and have noticed a marked increase in enquiries from medical and dental laser manufacturers, so we thought it worth saying a bit more about the properties of these parts that make them so well-suited to these applications.

Our range of metal mirrors with electroplated gold coating are perfect for dental and medical customers because they have such high laser damage resistance.  Well-proven with high power CO2, Er:YAG and fibre lasers our electroplated gold coating can also be used successfully with high power pulsed and CW lasers from 1-20 microns wavelength.

Another benefit is that one mirror can be used with multiple wavelengths including visible light – the gold coating is 90%+ reflective in the visible and 99% reflective in the infra red. This means the mirrors can be used with a variety of laser sources including CO2, Er:YAG and HeNe/green diodes.

These mirrors are also extremely durable and won’t break or crack; they are regularly used  in high power metal cutting and welding applications because of their ability to perform for long periods in often dirty and dusty conditions.

And because they are made of solid metal, we can integrate additional design features in to the mirror, eliminating the need for customers to buy mirrors and mounts, adjusters etc separately. This speeds up and simplifies alignment during assembly and replacement ensuring greater consistency.  Over the years we’ve produced a whole range of custom-designed metal mirrors integrating features such as through-holes, grooves and alignment dowels. We’ve also made mirrors as small as 5mm diameter, 1mm thick. Here are a few examples:

Small Aluminium mirrors

Gold mirror with 2 holes

We’ve produced a leaflet about our mirrors for medical and dental lasers so if you want more information you can download it from our website, www.lbp.co.uk/Documents/Medical_Dental.pdf

Mirrors for Fiber laser

We’ve been pleased by the positive reports from a number of customers using our solid metal mirrors with fibre laser sources.

At first it seems to have been a “retrofit” emergency measure, because the original glass mirrors were found to have a very short lifetime, but now solid metal mirrors are fitted as original equipment as they are proven “in the field”.

Often it’s been our Gold coated Copper mirrors being specified, but our protected silver coated mirrors are being used as well.

The protected silver coating is really quite durable and withstands frequent cleaning, and it is a little more reflective in the 1 – 2um region where fiber lasers emit.

The picture we are building up points to two reasons why solid metal mirrors are being favoured over glass and quartz mirrors.

1) Laser Damage Resistance – both when brand new, and more importantly when used in non laboratory conditions. Dust, dirt, and small particulate debris landing on a quartz or fused silica based mirror will cause it to crack. It’s quite possible to use several glass mirrors a day, which is of course unsustainable. It appears  Silicon mirror are just as fragile. A solid Copper mirror substrate will sink the heat away from local absorbing “hot spots” of dust and debris into the mirror itself. It certainly won’t crack or fracture. It’s the same reason high power CO2 lasers uses copper mirrors, the high thermal conductivity sinks the heat away form local hot spots caused by dirt and debris

2) Broad Reflective Coatings – we have had several instances where mirrors optimised for 1064nm Nd:YAG use are poorly reflective to fiber laser, perhaps just 60% or so reflective.

It seems some dielectric coatings designed for 1064nm are so narrowly optimised they have very poor performance at 1070nm, 1080nm or other fiber laser wavelengths. Fiber lasers seem to have a fairly broad spectral bandwidth as well, which compounds the problem.

This seems to be a batch to batch problem, where some glass mirror coatings are compatible with some fiber laser spectral outputs, for some of the time. Not ideal for parts that could be replaced frequently by users.

As the debate of fiber laser versue CO2 lasers continues Laser Beam Products can supply mirrors with the very highest laser damage thresholds for both technologies.

 

Not all CO₂ lasers are the same

Trouble awaits anyone not realising that the term “CO₂ laser optics” is mostly understood to refer to optics designed for the common CW (Continuous Wave) CO2 laser types (fast axial flow, slab discharge etc). A customer with a high power TEA pulsed laser using mirrors they sourced form Germany found they completely failed within a few seconds due to the very high laser pulse energies. After waiting several months for the mirrors to be delivered, a few seconds of use was disappointing!

We have recently helped several customers using TEA (Transversely Excited Atmospheric) CO₂ lasers, which are a PULSED CO₂ laser type.

Pulsed Laser Damage

Our Gold coating works with both high power CW and Megawatt pulsed TEA lasers. For this customer we were able to take the used failed mirrors, remove the remains of the ablated dielectric coating, Gold coat them, and return them, taking just a few days.

Interestingly the first true high power TEA CO₂ laser was developed in Baldock, England, just 5 miles away from our factory here in Biggleswade.

Our Gold coated Copper mirrors have been independently certificated as having an LIDT of 46J/cm² in a 80ns pulse.

Versatile Stainless Steel

Stainless Steel Mirrors

We make mirrors from many different materials at Laser Beam Products, including uncoated Stainless Steel (or Inox).  Stainless Steel mirrors are extremely versatile with uses in many industries, such as pharmaceutical, brewing, medical and nuclear. Because Stainless Steel is corrosion resistant, it is perfect for use in outdoor applications, and in humid or dusty conditions.

The hard polished mirror surface can be used as a reference for other optics, or for equipment to be mounted on. The ready availability of Stainless Steel, and the ease of machining means we can offer these mirrors at low prices and with quick delivery times, even for single prototype mirrors.

Stainless Steel Prism

Over the years Laser Beam Products have made circular, rectangular and even multifaceted prisms from Stainless Steel, with laser quality polished reflective surfaces. The typical reflectivity of Stainless Steel in the visible spectrum and infra red is 60%. Often that’s enough to allow alignment with a visible laser diode, or to detect and reflect an optical signal.

If your industry can only accept stainless steel as a material, we offer an option to integrate high quality, laser grade reflective mirrors into your equipment.

Metal Laser Mirrors with through holes

We regularly make metal laser mirrors with one or more through holes in them for our customers. They are easy to make from metals, and with our own unique polishing techniques we can keep the surface form accuracy right up to the hole.

In addition, our gold coating uniformly covers the sides and internal surfaces of through holes, so all surfaces are coated with gold.

The hole in this mirror was used to pass a visible laser beam through. A visible laser diode was placed behind the mirror, and the beam passed through the central hole. The laser diode is protected from back reflections and scattered radiation by the mirror face.

We have also made mirrors with slots at the rear to accept a solid state laser bar that injects light into a resonant cavity, and mirrors with through holes for  in line monitoring applications.

Mulitipass cells for chemical sensing, such as Herriott cells and White cells, often use Gold coated mirrors depending on the gas being sensed.

TeraHertz, Infrared, or something in between

My university education was as a chemist, a while ago I admit. We didn’t think about the wavelength or frequency of electromagnetic radiation in the infrared, tending to use the standard units for chemists of wavenumbers (cm-1) .

In the VUV though we swapped to Electron Volts, but did use wavelength for the visible spectrum.

Nowadays I’ve converted my mind to use wavelength across the spectrum, only to find that frequency (Terahertz) is being used for the far infrared and millimetric waves.

I’ve just spent a fair bit of time answering an enquiry from Japan for metal mirrors to work with “Terahertz”, which to me means 100um – 300um wavelength. I should have checked, as I now find out its 20THz, or as I now think of it 15um wavelength. Actually our Gold coated Copper mirrors do work well in the region of 100 – 200um, or should I say the 1.5 – 3 THz spectrum, maybe longer.

This is a nice conversion tool at a US University to convert frequency, wavelength & energy

Take care cleaning Uncoated Copper laser mirrors

Uncoated copper mirrors are popular in high power CO₂ laser welding systems. Having no coating at all,  there is almost nothing to “go wrong” with the mirror and they can withstand very high laser powers.

Unfortunately Copper reacts with the atmosphere and uncoated Copper mirrors will lose reflectivity over  time. Exactly how long it takes and the amount of tarnish and corrosion is dependent on the local conditions such as the humidity and the local air quality. A similar problem happens with Copper printed circuit boards where they can become difficult to solder.

Many laser users clean the copper mirror surface with a super fine abrasive polish, but it’s very easy to inadvertently remove copper from the centre of the mirror . This creates a considerably concave mirror out of what was once a flat mirror. The contour map above shows such a mirror with a regular concave surface. Each circle corresponds to a drop or “sag”of 0.3 microns, so this mirror is nearly 4 microns concave in the centre. That is about 10 times out of specification and indeed this mirror did turn the laser beam elliptical and astigmatic, just as a genuine weakly concave mirror would.

This will result in an oval shaped focussed spot giving uneven performance depending on if it’s the minor or major axis of the oval that is parallel to the direction of travel. It is possible to form what appears to be a circular spot with an astigmatic beam. Either side of this “circle of least confusion” though, the spot is elliptical.

Glass mirrors and CO2 lasers don’t mix…

We recently had a call from a slightly distressed customer whose toolbox had been hit by an 8o Watt beam from his CO2 laser, and had caught fire! After a short conversation it became clear what had happened.

He had a low power Chinese made CO2 laser system, fitted with what appeared to be gold coated glass mirrors. We’ve seen this before several times, and it can be extremely dangerous.  Even in low power systems,  these mirrors can heat up, the mirror will crack and fall apart, and the laser beam can then pass straight through the mirror aperture.  I guess the mirror mount looked like this one that is being sold on Amazon

This is dangerous

It’s  for CO2 lasers, although they wrongly call it a lens mount. All that stops someone being burnt, or blinded, is a self tapping screw and a small piece of glass.

There doesn’t appear to be any rules, regulations or legislation to prevent  this happening (please respond if you know differently).

Copper mirrors may cost slightly more, but they are  much better suited, and safer, for use with CO2 lasers of any power.