LASER 101

What is a Laser?

“Laser” is an acronym for Light Amplification by Stimulated Emission of Radiation. Put more simply, a laser converts energy into light, which is then amplified through optics, before focusing that light into a high energy beam. Laser light differs from normal light in that it can be collimated, or made less prone to dispersion, and then focused to greatly increase its energy density. There are many kinds of lasers and the uses of laser light include range from entertainment, scientific endeavors, and surgery, to advanced and heavy duty industrial manufacturing.

All lasers share a basic set of components. Laser start with a gain medium which is used to amplify the power of light — laser gain mediums include gases, dyes, diodes, crystals, and optical fibers. An energy source, either an electrical current or a source of light, is then used to pump the gain medium. Once the necessary energy is generated reflective materials known as partial and total reflectors control the laser output which is then adjusted and focused as needed for the application at hand.

Laser Wavelength

Measured in nanometers (nm) or microns (µm), the wavelength of a laser is the distance between successive crests of the light wave. Laser wavelengths typically range from deep ultraviolet to mid-infrared (IR) and are visible to the human eye in the range from ~400 to ~700 nm.  

Wavelength is a critical consideration for many applications because materials often differ dramatically in how they absorb the energy of light. Materials absorb some portion of a laser beam’s energy and reflect the rest — the balance between the two can necessitate the use of a different laser wavelength. Wavelength is also of critical importance for advanced and scientific applications including microscopy, optical trapping, and ultrasonics.

Near IR wavelengths of approximately 1000 nm are used as a starting point, particularly for processing of metals. This is because near IR lasers offer higher powers, are less complex, and are often more cost-effective. Most metals absorb light in the near IR or visible range efficiently. Even metals with high IR reflectivity, such as aluminum and copper, are predominantly processed by near IR lasers which overcome material reflection with higher power densities.

Various polymers, ceramics, glass, and other non-metals are often processed by lasers with wavelengths from mid-infrared to deep ultraviolet. Clear polymers and glass are actually transparent or nearly transparent to near IR light, allowing the majority of near IR light through without being absorbed. As a result, materials that readily absorb near IR light can be processed through a polymer or glass layer.

Laser Power

Also referred to as average power, laser power is measured in watts (W). A laser’s average power represents how much energy is delivered to the target material over a period of time. Laser power requirements vary by many orders of magnitude for different applications. Many sensing, data processing, telecom, medical, or scientific applications utilize powers from a few milliwatts to tens of watts. Non-metal processing applications typically demand anywhere from a few watts to a few hundred watts of average power. Metal fabrication applications demand powers anywhere from hundreds of watts, in the case of some microprocessing applications, to dozens or more kilowatts, in the case of thick metal cutting and welding applications.

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.