Laser Windows Made of Sapphire

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Laser windows are simple devices. Their sole function is to let the light pass through an impenetrable wall. Impenetrable for everything but the light, that is. The light passing through a laser window has a particular energy, and a particular wavelength, which the window must not restrict. Well, may be not so simple after all.

Laser windows are typically used in systems measuring something, like distance, temperature, vibration, counting the number of particles and their color passing by on the other side of the window. Basically, every feedback system can benefit from using laser-based instrumentation. Which brings us to the original function- how does one transfer the laser light in and out of a controlled environment? This is done with a window, transparent to the particular wavelength of the laser system being used. Sapphire is a great choice for these types of windows because it transmits a broad range of light, from short wavelength blue, to long wavelength infrared. And sapphire does so without taking away much of the transmitted energy. Sapphire transmits most of the light passing through, high 90 percent of it.

Laser Window

Laser Window Impact Test

We recently had a request to fabricate laser windows with a sapphire element in them. One of the requirements was to design the window in such a way that it will not break if a steel ball weighting 1 kilogram (2.2 pounds) falls on the sapphire from the height of 400 millimeters, (15.8 inches). This was an aggressive goal for a thin piece of sapphire. Well, after some calculations and prior experience we chose a particular thickness of the sapphire disk, for a particular diameter.

Given price constrains, the thinner the sapphire, the less expensive it is. Our design was right on. During initial testing we dropped the ball onto a test sample from a low height of some 100 millimeters (approximately  4 inches). It sounded like a gun went off. Very loud, because it’s like trying putting an indestructible object in front of an unstoppable force. By the way, the metal ball was over 1.2 kilograms – much heavier than needed as we could not find one that was exactly 1.0 kg. We used McMaster Carr to find the right ball. If you know of a better source for engineering materials, please let us know. To cut the story short, the project was a success, we tested the samples, learned a great deal about sapphire diameter-thickness ratios and how all this affects sapphire strength. This was a fun test, we filmed it and posted the video on YouTube.

In summary, the samples passed the test. The ball had kinetic energy of 8314 Joules and it pushed the sapphire with a force of 2,771,440 Newtons in order to break the sapphire disk. Impressive strength. We also wrote a White Paper on it in conjunction with the mechanical engineering lab from a university in California. Sapphire is one amazing element. It has the strength of a concrete wall, cannot be scratched by anything other than another sapphire or a harder material, and does not affect the transmission of light much.  No wonder $30,000 watches have sapphire as their see-through covers.

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