How can I select the right laser source for my application?
Any Laser Source has 3 key components: Gain Media, Pump Source, and Resonator
|Typical Laser Source||Gain Media||Pump Source||Resonator|
|CO2 laser, HeNe laser||Gas||Glow discharge||Optic sets in cavity|
|Nd:YAG laser, Nd:Vanadate Laser||Solid: Crystal||Arc Lamp or Diode||Optic Sets in cavity|
|Fiber Laser||Solid: Coated Fiber||Diode||Fiber ends coating|
For the laser equipment buyer or system end user, there is no need to dig out all physical mechanism details regarding a laser source. All they need to make an informed decision regarding what laser source to use for their project are the following basic points:
Gain media determines the wavelength of the laser beam
The first basic rule for selecting a laser source for material processing is to know the wavelength of the laser beam, because different materials have different absorption rates at different wavelengths.
One commonly held rule is that the 1064nm wavelength laser beam of an Nd:YAG (Neodymium-doped Yttrium Aluminum Garnet) laser is absorbed well by Aluminum and Steel, and the 10600nm wavelength laser beam of a CO2 (Carbon Dioxide) laser is absorbed well by organic materials like paper, wood, leather, plastic, and cloth.
Today, fiber and CO2 laser systems are the two most popular styles of laser source. If there are no special notes, fiber lasers refer to 1064nm wavelength lasers, and CO2 laser refer to 10600nm lasers. However, it is important to note that fiber lasers actually have outputs of 1060, 1500, 2000, 3000, or 5000nm depending on the trace elements mixed into the fiber. A CO2 laser may generate a 10600, 10300 or 930nm laser beam, depending on the precise mixture of the gas (mainly the ratio of CO2) used.
A laser application engineer may not name a laser source by its gain media, but by its wavelength. For instance a 1064nm laser is called a NIR (Near InfraRed) laser, while a 10600nm laser is called a FIR (Far InfraRed) laser. Matching the wavelength of the laser source to the material that will be processed is key to ensuring effective laser operations.
Pump source resonator design influences the maintenance costs
When you talk with a laser technician, they often will call refer to a laser source by its pump source, or resonator style, or by the cooling requirements. For instance, the “lamp pumped” laser, “fiber” laser, “glass tube” laser, “air cooled” or “water chilled” laser.
The older generation glass tube CO2 lasers and lamp pumped Nd:YAG lasers were very popular in the market, but their pump sources – the arc lamp or the gas-filled glass tube – are consumables. Every 500 to 1000 working hours, you would have to stop the machine entirely and replace the lamp or glass tube. The even older style of CO2 laser used a constant flow of gasses through the resonator, resulting in high operating costs as none of the gasses could be recaptured. The optical resonator cavities of Nd:YAG and Co2 lasers are made with gold or ceramic coated reflectors and specially coated mirrors. The ratio between energy expended in the pump source and energy absorbed by the gain medium is so low that the system requires active cooling, most commonly with a water-based heat exchanger, which brings more maintenance challenges and expenses.
The fiber laser entered the industrial market in the late 1990s, and quickly came to occupy 80% of that market within 20 years. This rapid growth in market share was mainly because of two very attractive features of the new technology: being almost entirely maintenance free, and having a very long long working life. These features are because of the unique style of laser resonator used by a fiber laser, in which all of the optics are part of an effectively continuous fiber optic cable.
CW vs. Pulsed laser (compress the energy by time dimension)
Laser sources can be categorized into either CW (Continuous Wave) or Pulsed by its beam output mode. We can use the following simple diagram to explain how a pulsed beam output is formed.
Assume we set a 90% reflective mirror at one end of the lamp-pumped laser cavity, and a highly reflective mirror at the other end. When the arc lamp irradiates the Nd:YAG rod with light, the rod emits light at a wavelength of 1064nm from either end which heads towards the mirrors. These photons of light bounce back and forth between the two mirrors, pulling additional photons along with them each time they pass through the Nd:YAG rod. Because the one mirror is only 90% reflective, a percentage of the photons can also pass through this mirror, which is the usable laser energy for marking.
If we place a switching element between the Nd:YAG rod and one of the two mirrors (this is generally an Acusto-Optic Modulator, or ‘Q-Switch’) and then ‘close’ the switch to prevent photons from reflecting off of the mirror, there will be a build up of energy within the laser rod. When the switch is ‘opened’ and the photons are allowed to flow through again, this energy will be stripped off the rod with the initial surge of photons, creating a high-energy pulse.
Now, looking at the following chart, we can see the effect of pulsing the laser beam.
The energy-over-time (continuous power) is identical between the two modes; however, when the laser is pulsed, we build up a very large spike of power which is followed by a correspondingly lower power section.
When the pulse duration is modulated into a time frame measured in the nanosecond, picosecond, femtosecond, or even attosecond, the peak power of a pulsed laser may be thousands or millions of times more intense than the average power output. Doing this allows even a low power beam to vaporize or ablate the workpiece material instead of simply melting it.
For cutting and welding applications most lasers used will be CW, but for micro-machining, marking, and engraving applications most lasers are pulsed.
Average power vs. Energy Density
Along with pulsing the laser beam in intervals of different length, you can also send the laser beam through a focusing optic. This will reduce the size of the beam when it makes contact with the workpiece. Doing so puts a large amount of energy into a very small area, concentrating its effect. However, the output of a laser cannot be shrunk smaller than its wavelength, nor can a laser with a poor quality output create a high quality spot at its focal point, no matter how good of a focusing optic is used. This creates a hard limit on the size of any laser spot size (e.g. a CO2 laser cannot focus down smaller than 10600nm, or roughly .01mm, and this would require a perfect beam and perfect optics to accomplish).
However, not every application benefits from a smaller spot size. For instance, laser welding, laser surface cleaning, laser sintering, and laser surface hardening need a fairly large spot size to ensure performance, and laser annealing can also be done more easily with a larger spot size.
As can be seen from the chart above, laser micromachining, marking, and engraving all benefit from a smaller spot size; laser cutting applications benefit from a medium spot size; while in laser welding and surface treatment, the bigger spot size is better.
The x-axis of the chart shows which power range fits which application scope, which is roughly summarized as follows:
- Laser micromachining applications needs less than 50 watts of average power but with higher peak power levels and energy density so that the HAZ (Heat Affected Zone) can be very finely controlled.
- For laser marking and engraving, the average power range will be 20 to 100 watts. More average power can help to improve the cycle time when doing deep engraving.
- Laser cutting applications for most non-metal (organic) workpieces will have an average power range of 30 to 400 watts. Generally this will be done with a CO2 laser for optimal beam absorbtion. For laser cutting of metal workpieces, the power range will be anywhere from 700 to 10,000 watts, depending on the thickness of the workpiece. Cutting applications generally will use an assist-gas to blow out melted material while the laser is running.
- Laser welding and surface treatment applications use 500 to 10,000 watts for metal workpieces, and 10 to 500 watts for organic materials.