Laser Mode of Operation
Lasers can emit a continuous beam of light to output a steady stream of average power – this mode is referred to as Continuous Wave (CW) and is the most common laser mode of operation. Lasers can also be used in a pulsed mode of operation. Pulsed lasers are characterized by pulses per second (repetition rate), the total energy of the laser pulse (pulse energy), the highest power achieved by the pulse (peak power, and the length of each pulse (pulse duration).
Like CW lasers, pulsed laser output over time is represented as average power. Pulsed lasers, even when their average power matches that of a CW laser, affect targeted material differently. Pulsed lasers are often used to process parts while minimizing the thermal impact on the surrounding material or when higher peak power is necessary. Long pulse quasi-continuous wave (QCW) lasers utilize pulses measured in milliseconds with high peak powers to emulate CW laser processing with less heat input and with a lower power laser. Nanosecond and ultrafast (picosecond/femtosecond) lasers take advantage of extremely short pulses for microprocessing applications where excessive heat input is not acceptable or when extremely high peak powers are required.
Generally speaking, CW lasers offer the highest average powers and, as a result, the fastest processing speeds. There are many considerations to be made when deciding between a CW laser and a pulsed laser, but balancing throughput with part quality is often the most important. Many applications, such as sheet metal cutting, benefit from a high-power CW lasers for greatly increased cutting speeds and have no need for flawless edge quality. When cutting stacks of ultra-thin foils, however, nanosecond and ultrafast pulsed lasers are typically used to ensure excellent edge quality and reduce or eliminate negative heat effects.
Left: a multi-mode beam profile with a larger spot size. Right: a single-mode beam profile with a smaller spot size.
Laser Spot Size & Beam Quality
When a laser beam comes into contact with its target material it forms an area of laser light referred to as a spot. Spot size, typically measured in µm, is a critical factor in determining how a laser interacts with its target. Spot size can be controlled in a variety of ways, including using different delivery fibers and focusing lenses, changing the distance between the beam delivery and the target, and using longer or shorter wavelengths.
Decreasing the spot size makes more efficient use of a laser’s power by concentrating the beam’s energy in a smaller area. Higher energy density is useful for increasing processing speeds by decreasing the time it takes for a laser beam to pierce the material. Small spot sizes are also essential for a variety of microprocessing applications and for parts that require fine features. For many applications like structural welding, however, increasing spot size is optimal for processing a wider area and reducing the required beam travel.
Beam quality, typically measured in M2 for single-mode lasers (typical spot size: 20 to 50 µm) and Beam Parameter Product (BPP) for multi-mode lasers (typical spot size: 100+ µm), is an important and complex laser parameter that, in practice, represents how much a laser beam can be focused. Lower M2 and BPP values correspond with higher beam qualities. A beam quality of M2 = 1 means that the beam experiences no divergence and is considered perfect. Although this is not quite achievable with actual devices, industrial fiber lasers can reliably achieve beam qualities of M2 =< 1.1. For applications that require strongly focused beams like cutting, drilling, and welding, higher beam qualities improve processing speeds and qualities. Some applications, like wide area laser heat treatment and cleaning, do not require particularly high beam qualities, instead benefitting from less focused laser energy.
