As a minor side project at work I have been playing with a femtosecond laser and working to use it to do micro-machining and exotic welding.
The laser is an old model- its average output power is just one Watt, but it outputs that one Watt of power in a series of pulses- 1000 laser pulses a second, each with ~1 millijoule of energy. The laser pulses are extremely short- ~150 femtoseconds in duration. A femtosecond is 1 millionth of a nanosecond- 1*10^-15 seconds. So the laser essentially fires out 1000 of the pulses a second- if you could take a snapshot in time that pulse would look like a little pancake of light ~50 microns thick (light travels ~50 microns in 150 femtoseconds)
The output of this laser is virtually identical to the one I did my graduate work on.
Now with these small numbers (1Watt, 1 millijoule, 150 millionths of a nanosecond) it may seem this is ill-suited for machining and welding- industrial steel cutting and welding lasers are 100′s or 1000′s of watts of output power. But the short duration of the pulses gives this laser very special capabilities.
The AVERAGE power is 1 Watt, or equivalently 1 Joule of energy per second… but the average power is low because most of the time there is no light coming out of the laser- ALL of that 1 Joule of energy per second is divided among 1000 ultrashort pulses.
Therefore it makes sense to consider what the PEAK power is- the laser power within one of these short pulses… Power is just energy per unit time… each pulse has 1 milli-Joule of energy, and that energy is contained in a pulse duration of 150 femtoseconds (1.5*10^-13 seconds).
That gives a PEAK power of 0.67 *10^10 Watts or 6.7 GigaWatts… NOW we are getting somewhere!
Now consider intensity, power per unit area. The intensity of sunlight at the Earth’s surface is ~0.1 W/cm2.
If you focus one of these laser pulses onto a spot 10 microns in radius, then you have a power of 6.7 Gigawatts (6.7*10^9 Watts) in an area of 3.14*10^-6 square centimeters. This gives an intensity of 2.1*10^15 Watts/cm2! About 20-million-billion times the intensity of sunlight. Intensities like this simply do not occur in nature- not even near supernovae…
Light is an oscillating electromagnetic field- at these intensities the oscillating electric field dwarfs even the electric field binding electrons to atoms. When matter- ANY matter made of atoms- is exposed to this intensity of light, the electrons are ripped from the atoms suddenly. All of a sudden you have a bunch of positively charged ions packed WAY to close and they undergo whats called ‘Coulomb Explosion’- the positive ions strongly repel each other and explode away from each other.
Thus, by focusing these pulses down on matter, you can very precisely remove material at the micron scale. You can of course drill holes and mill away material with other types of lasers, but with pulses this short very little energy can get transferred to the surrounding material. This means there is no damage to the surrounding material… no heating and no melting… (This is why femtosecond lasers are finding their way into eye surgery). You can drill extremely precise clean holes in ANY material.
(The above pictures were taken as examples from the web)
The micro-machining properties of these femtosecond lasers is becoming widely used, and has countless applications.
We are looking at another weirder application- welding.
Normally, directly welding a piece of glass or a crystal to metal is not possible- the two materials typically have vastly different melting points, and expand with heating at different rates- so if you do manage to melt and mix the two materials together then as they cool the bond will fracture from the thermally induced stress.
So, in order to bond glasses and crystals to metals you generally must resort to using epoxies to glue them together. Unfortunately, epoxies can outgas contaminants in a vacuum- if this happens in an optical instrument in space this outgassing can degrade its performance and shorten its operational lifetime.
If you take a piece of transparent glass or crystal and place it on a piece of metal and then bring your femtosecond laser to a focus at the interface between the two materials, it turns out that you can ‘weld’ them together. It turns out that this weld can be at least as strong as epoxies. In this welding process, however, there is no heating of the bulk material… so you can weld two materials even if they have vastly different thermal expansion- for instance copper and glass, sapphire and titanium… its all done at room temperature in open air.
This potentially could allow a great reduction in the use of epoxies and thus reduce the potential contaminants in space based instruments.