# Why are lasers used for concentrated light applications instead of incoherent light sources?

Some laser applications consist simply of concentrating light into a small spot. Two example applications are laser welding and cutting. In these cases a CO2 laser is often used which needs a regulated power supply, a water cooling system, and a supply of CO2 gas.

Why do these applications use a laser instead of a simpler (i.e. incoherent) light source such as an AC powered arc lamp?

• What exactly do you mean by "a simpler light source"? – Paul May 31 '15 at 18:42
• the wiki on lasers discusses the advantages of a spatially coherent light source as a means for producing highly concentrated spots of light. Perhaps you could expand or clarify your question in the context that information. – Dan May 31 '15 at 19:48
• @Paul imagine an industrial heating application. One alternative is to use a gas laser, with its regulated power supply, a water cooling system, a vacuum pump, a gas supply, etc. Another alternative is to use a simple AC arc lamp powered with a simple transformer and air cooled. – user3368561 May 31 '15 at 20:21
• @user3368561, can please include the alternatives in the comment section to the question. This will improve the question as well a improve the chance of receiving a good response. Also delete the comment too. thanks – Mahendra Gunawardena May 31 '15 at 23:00

There are many reasons why highly monochromatic light, such as that emitted by a laser, is useful for delivering a large amount of power to a small spot.

First of all, incoherent light sources such as a lamp are extended sources which means that they are emitting light from a piece of material which takes up a finite amount of space. When focusing this light to a point, the focal spot is limited by the size of your source multiplied by the magnification of your imaging system. This may sound like a small effect, but if you want to focus the light to a spot size which is on the order of the wavelength ($~1\ \mu\text{m}$) it becomes important. Lasers, on the other hand, act like true point sources and can be imaged to spot sizes smaller than the wavelength of the light.

A second issue with incoherent light sources is that they generally emit light in all directions. So, even if you can generate an equivalent amount of optical power, it is much more difficult to gather it all up into a collimated source which is needed prior to focusing to a spot.

A final advantage of lasers is their monochromaticity. This is useful because the wavelength can be precisely chosen to match a particular application. CO2 lasers, for instance, emit at $10.6\ \mu\text{m}$; which is absorbed well in a wide range of materials but not all that well in metals. Nd:YAG lasers and its harmonics (the three wavelengths labeled as 'solid-state laser' in the image below) are, however, absorbed well in metals and can be used for metal processing. With incoherent sources the light is emitted over a broad range of wavelengths which may or may not exhibit desirable properties in the material you are trying to process.