Spherical mirrors for collimation
Collimator Mirror
For collimating infrared lasers (like CO₂ flying optics cutting systems) our reflective beam expanders are robust devices. They are simple to align and adjust, and with careful optical design inexpensive spherical mirrors can be used, and still give near diffraction limited results. The design of a mirror collimator needs two separate considerations.
Firstly Gaussian beam propagation, factors such as waist location, 1/e2 diameters, M2, Rayleigh Range.
Small copper mirrors, and lighter aluminium mirrors
It’s the way of the world, technology makes products smaller and lighter. We can make copper mirrors as small as 5mm diameter and 1mm thick. When Gold coated they are finding their way into medical and dental laser systems based on Er:YAG lasers.
The high laser damage threshold is the key enabler, and the good reflectivity of Gold in the visible is useful when aligning articulated arms with a red or green laser. Of course the same mirror can be used with CO2 lasers, so a just one articulated arm can be used for all three wavelengths, 633nm, 2.94um, and 10.6um.
Not such a big issue for dental laser arms is the weight of the mirrors, but for other applications the solution of making copper mirrors thinner and thinner becomes futile, as the loss of rigidity and stiffness means the mirrors are difficult to manufacture, and almost impossible to mount. Aluminium mirrors are around 30% of the weight of copper ones, they offer good laser damage resistance, and can have exactly the same coatings made on them as copper mirrors do. We have a page on our web site dedicated to Aluminium mirrors.
A good rule of thumb is that a laser mirror should have a diameter to thickness ratio of 5:1, like our standard 50mm diameter 10mm thick parts. On request 12:1 is possible, such as our 3″ diameter 1/4″ thick mirrors, but please no thinner!
Focus a laser with a spherical mirror ?
We often get asked this question, the attraction of using a spherical mirror to focus a beam is obvious. Spherical mirrors are low cost, readily available, easy to test, easy to mount and align. Some laser users understand that the scheme on the left, of using a mirror at 45 degree incidence to turn and focus a beam, is probably not quite as easy as it seems. And so it turns out to be. A spherical mirror surface is a perfect surface for imaging a point located exactly at it’s radius of curvature, exactly back to it’s radius of curvature ! . This is why, even for the simple imaging of a far away object, rather than one located at the mirrors radius of curvature such as a star, a parabolic surface is required. Isaac Newton who studied a few miles from our factory in Cambridge knew this, and made his own parabolic mirror for his telescope. A customer asked us if a CO2 laser was focused using a spherical mirror at 45 degrees incidence what would the focal spot look like.
This can be modelled with a lens design programme like Zemax, which we did. To even be in with a chance of having something useful we looked at a long focal length, 300mm. The size and shape of the spot above and below focus can be seen on our web site, a rough guess of 5mm would be about right, and nastily elliptical. A better spot diagram at 254mm away from the mirror is on our website too. Obviously astigmatic spot would be hopeless for laser cutting, but for crude surface heat treatment, or this case firing the beam into an instrument entrance it was good enough.
Volume orders
We spend quite a lot of time talking about our ability to produce one-off or low volume orders for R&D and prototype development, at a cost-effective price. This is an important part of our business, but it’s also worth remembering that we can, and do, regularly ship orders of hundreds of pieces, often for companies with sophisticated production scheduling systems.
We accept blanket orders with pre-arranged delivery dates, often spread over several months or for military contracts as long as 2 – 3 years. We work with customers using ‘Kanban’ systems, and can integrate our deliveries with users of SAP software like defence prime contractors, or those using other ERP systems.
Deliveries can start usually within a few weeks, and we often use customer-supplied packaging and labelling for ease of stock control. We can even package several parts together into a “kit” to be issued to the shopfloor.
Individual test results for each part, such as interferograms or calorimeter measurements are available electronically and all parts are individually inspected prior to packing and despatch.
There’s more information in our April 2012 newsletter on Laser Beam Products download page
Over payment scams
We have had several enquiries from the Middle East that at first seem to be from genuine bonafide organisations. The enquiry is plausible, and has a level of technical knowledge beyond that of the average fraudster telling you about a bogus lottery win, or secret Iraqi gold bullion. They seem to be targeting laser and optical suppliers just now, several customers have had the same experience too.
Annoyingly it takes some time to respond to the enquiry, offer a quotation, and process an order; only to find out it is an “overpayment scam” You will be told that instead of the say 5,000 Pounds you were expecting in payment for goods, a far larger amount like 52,000 Pounds has been transferred by mistake, along with some pleas like……
“We appreciate your understanding, the error on payment was caused in accordance of total orders the client placed with our company while some were purchased from other companies, the client advised that we should proceed with the order regardless of the mismatch in the amount transfered, any charges incurred should be deducted from the payment so we don’t have a delay in receipt of the ordered product, then we supply you with the account details for the refunds after which the shipment will then be completed…..
If you don’t know how this then goes on to con you, the Royal Bank of Scotland has a short explanation on their web site.
