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Pulsed fiber laser sources
How to choose the right pulsed fiber laser ?
The pulse energy, pulse duration, wavelenght and beam quality are the most important factors.
Together with the pulse energy comes the repetition rate and final output power. The beam quality is related to the source quality and the collimator fiber optical connector type :
Collimator/end of fiber connection : is a optical device consisting of two lenses at a precise distance from each other to collimate the outcoming cone shape light beam from the fiber end. A collimator can be fixed to the fiber or having a QHB type connector with separate collimator. The collimator will give a parallel output beam with a defined diameter.Depending on the intrinsic incoming light quality the beam path will have a certain divergence. High quality beams will have a lower divergence.
QHB : is a standard water/air cooled optical connector at the end of a fibre.The light output is a cone shape and this light beam has to be collimated for further use.With this type of fiber connector you can choose your own collimator and change it.
Isolator : an optical isolator is typical for a high quality beam and consists of a collimator and a magnet to filter out back reflection. The risk to damage the laser source is higher with high quality beams and high reflective materials like copper.
QCS: is a small collimator (14-17 mm) without optical isolator and used with lower quality beams a for cleaning purpose.
Pulse energy : this is the total amount of energy stored in one pulse. You can compare it with the weight of a hammer to hit the surface.The unit is mili joule or mJ. With pulsed fiber laser this quantity can be adapted by changing the frequency of pulses.Like a simple water pump a laser source can give low energy pulses at high frequencies and high puls energy at low frequency.
Puls frequency: the quantity of pulses pro second gives the frequency output in Khz. The nominal frequency is the max pulse energy at nominal frequency.
Laser output power : the laser frequency multiplied with the pulse energy gives the total output power in Watt. If the lasers gives 100 Khz pulses of 1mJ energy then we have 100 watt output power.
If we ask pulses at 200Khz we can only get 0,5 mJ energy pulses.Laser sources have a max puls energy they can deliver and are also restricted in max Khz.In between you can regulate everything over a laser source control interface.
Pulse duration : this is the time span in which the pulse energy is delivered and giving the max peak power in Kw. In this regard we have mainly 3 categories : nano-; pico- and femto second lasers. The puls duration is related to the grade off thermal effect at the surface.This can be a melting pool or a cold ablation in al possible variations on the specific material surface. The pulse duration is a specific setting in the laser and can restrict the total laser output due to fiber energy traveling limitations causing the fiber glass material to behave non linear or causing thermal damage inside the fiber light path. A 1mJ pulse at 100 nanoseconds would deliver a whopping 10Kw peak power.
Beam quality : seen the difficult nature of light scientist tried to quantify a laser beam to compare laser sources and to select the right optics. In general we can say the better we can focus a beam the higher the light quality.The theoretical highest quality is M2=1,4. This laser has a low divergence and can travel a long way without increasing his beam diameter. For working on copper being a high IR reflecting material we need the best possible beam quality to be able to melt down the surface. To clean a surface from oxides no high beam quality is needed only a certain puls energy and a moderate pulse duration between 50 and 120 nanoseconds.
Energy distribution : if a round light pulse is hitting the surface the energy distribution can be compared with hitting the surface with a hammer or bullet. Some bullets are sharp other hammers are flat.In laser terms this is noted as Gaussian an homogene Flat top energy distribution. Its clear depending on the application this will be important.
Single mode versus multi mode lasers : Single mode lasers are delivering the highest quality but are more difficult to produce.They use very small, high quality fiber diameters and one single mode of light is traveling through the fiber resulting in minimal power loss and high quality beam output.With multimode lasers we inject/create a lot of different light waves trough the fiber. they will mix up together and create a more powerfull beam output but at lower beam quality. Cleaning lasers can be multimode but to get white cleaned metal a single mode is better.
Choosing a laser source is a difficult task without any reference in a proper laser lab.The final result on the surface is only visible after a test with the complete system : source +beam path optics +scanner.
If you have a tough contaminant to remove or a high reflective surface a high beam quality source will be needed. If you do'n't want to damage the substrate you need to find a balance between lenses and beam quality.If also you need big surfaces to handle you will go towards high power with high puls energy and less beam quality.
Watercooled sources can handle more power, puls energy then air cooled ones, but comes with an extra chiller unit.
Puls duration variation is less important. Depending on the task you can use most standard laser sources if your base material is sensitive to thermal impact/influence then shorter puls duration will help.
With nano second lasers you will still have a heat affected zone, but not to compare with a CW laser.If you want minimal heat effect in your base material when it is very thin you can move to femto and pico second lasers at a high price.
Facts when choosing a laser source :
The highest laser power is better?
Not necessary, a high power unit can deliver more pulses at lower puls energy till the repetition limit of the system has been reached. Mostely the maximum optical peak energy limit of the used glass fiber.
A 1000 watt laser can deliver 10khz pulse ratew with 100mj pulse energy or could deliver 1000khz 1mj pulses. Mostly this are different laser source series seen the different cooling needs. The 100mj source will be watercooled and the 1 mj can be air cooled. A water cooled laser is more stable then an air cooled.
High puls energy will remove every contamination ?
High puls energy will help but more important is the laser spot energy density on the surface. If the density is high you will have more chance to remove, mark, cut, weld the base material. The density will be given by your focal lens and the outcoming beam diameter. The bigger the beam diameter the better you can concentrate the incoming beam towards a concentrated spot.
The laser fiber can be unlimitted in lenght ?
The lenght of the fiber is limited and for high quality laser sources. A fiber is in fact a glass tube which can contain impurities and cracks witch will affect the out coming laser beam. Puls duration can be lowered to reduce the amount of energy trough the glass fiber. The fiber can handel a certain amount of energy. 3-5 m is a normal standard lenght for small laser sources from 20-300 watt . Once you have high puls energy lasers with big fiber diameter you can go to 60 m. 100 meter can already cause quality loss.
Connecting and disconnecting fibers is not a common job, it can destroy your fibre when a small dust particle is located at the fiber end.
A Yag laser has more maintenance ?
Absolutely a YAG laser demands high attention to keep thes type of laser running. Depending on the running time you need to re calibrate fiber inputs and cavity mirror to get the maximum power out of it. This laser is thermal not stable and will change its output depending on the temperature.Nice in the lab but not on the industrial floor. The optical efficiency is 27% against 35-40% with a fiber laser.
The cooling is absolute critical in this device.
