FIBER LASER ADVANTAGES, LIFE EXPECTANCIES AND M2 VALUES
Over the last 10-15 years, the laser industry has been somewhat transformed by the continued development of Fiber lasers. They are now in wide use across multiple sectors of manufacturing and are steadily becoming the main laser used for processing/marking metal (as well as many plastics). This rise in use can be attributed to any number of reasons, not the least of which is that in general, fiber lasers are far more reliable than conventional YAG, CO2, YV04 and standard diode-pumped lasers.
There are no internal optics utilized in fiber lasers. This eliminates the need for alignments of front & rear mirrors or delivery (“beam bending”) mirrors. This also eliminates internal optical contamination issues experienced with other laser types. In cases where contamination exists, lenses will heat up due to beam absorption by the anti-reflective coatings applied to the lens for specific wavelengths. This heating can cause a thermal lensing effect resulting in a possible focal shift as well as efficiency degradation of energy transmission and eventual lens failure.
Fiber lasers are very efficient when compared to the above-mentioned laser sources. In particular, YAG lasers are typically 4-6% efficient while fiber lasers, in general, are between 22-30% efficient. The increased efficiencies and elimination of additional optical surfaces allow for simple air cooling (no chiller required) with far less thermal lensing and stability issues. Long warm-up stabilizing routines are eliminated which also means that a safety shutter and/or water-cooled beam dump is no longer required.
Certain laser manufacturers use a design known as MOPA (Master Oscillator Power Amplifier) as opposed to Q-switching. Q-switched lasers depend on an RF source, an electro-mechanical transducer and an opto-acoustical crystal. This technology has been improved over the years and certain laser manufacturers have used these improvements to both enhance the reliability and life of the Q-switch as well as increase the achievable pulse frequency. The most reliable of the Q-switched pulsed lasers can operate from 20 – 200Khz while less advanced Q-switch lasers are limited to 10 – 100Khz or 20 – 80Khz.
In contrast, a MOPA design utilizes a seed laser to create an initial amount of low power and creates gains by switching sections of a monolithic fiber on and off. These sections are known to be saturable absorbers, meaning they can hold a certain amount of energy and then must be discharged. The rate at which the fiber sections can be turned on and off is much greater than the rate at which an RF driver or Q-switched laser can be pulsed. This gives the MOPA configuration an advantage in certain applications where the exposed material requires much higher frequency pulses (as is the case with numerous plastics and certain metal finishes).
The MOPA laser also allows the user to select or vary the pulse width (3 – 500ns) to match the pulse frequency, where the Q-switch style laser has a locked pulse width (typically 90-120ns or ~200ns). Keep in mind that approximately 75-80% of all applications can be marked using 20 – 80Khz with an average pulse width of 100ns, making the Q-switch laser a very capable tool at a lower cost point.
The MOPA laser uses multiple single emitter diodes (depending on the desired average power output) and couples the diode output to the fiber. The use of single emitter diodes is relatively new to the laser industry. YAG lasers utilize consumable flash lamps, CO2 lasers use a consumable gas mixture and standard diode-pumped lasers use diode bars or diode stacks that have a somewhat limited life due to cooling requirements and temperature fluctuations. These single emitter diodes have been widely utilized in the telecom industry and have an established MTTF (Mean Time to Failure) rating from 100,000 to 350,00 hours (depending on the manufacturer). (Note: SPI utilizes the highest-rated diode source which has a 350,000 MTTF). Computer modeling studies have shown that if the highest-rated lasers were outputting power 24/7/365 and never shut off, the laser should last for somewhere between 4.5-6 years minimum before any power degradation is realized. Running the laser in a more normal production environment, estimating 20 hours of actual power output per day, 6 days a week…the laser could be expected to run somewhere from 8-14+ years before any degradation would be visible.
Additionally, the laser architecture is such that some headroom is built into the system (more diodes than needed) so as the diodes age or become less efficient the drive voltage can be increased and thereby compensate for any standard losses. In some instances, the system is self-calibrating and requires no manual intervention to compensate for diode degradation as the laser itself will automatically look to even the drive voltage across the pump banks. In other cases, a calibration routine or manual adjustment can be initiated. Eventually, the laser will reach a point where the diodes are at their max drive voltage and can no longer fully compensate for degradation. This should be many years down the road (8-14+ years) however, once it occurs, an optical sub-module replacement may be an option.
Another key factor in determining the overall effective usefulness of a given laser is the M2 value. The closer the M2 value is to 1.1 or less (single mode), the higher the beam quality and the greater the energy density of the beam. It also means that the energy will be more evenly distributed across the spatial profile of the beam. This will allow a lower-powered laser with a lower M2 number to seem more powerful or “bright” than a higher power laser with a higher M2 (within a certain reasonable range). The range of the M2 number varies from 1.1 to as high as 3+ in pulsed fiber lasers, depending on the manufacturer.
The result of these advancements has basically eliminated the need for required routine laser maintenance and the stocking of consumable items.
The above data indicates a huge step forward in reliability for Fiber lasers vs any conventional laser currently in use and can be summarized below: