In various applications such as material processing, laser surgery, remote sensing, and notably laser marking, there exist a variety of common laser systems. Many of these laser systems share key parameters. Establishing universal terms for these parameters can prevent misrepresentation, and by understanding these terms, you can correctly specify laser systems and components to meet your application needs.
Fig. 1: Schematic diagram of a common laser material processing system, in which 10 key parameters of the laser system are represented by corresponding numbers
NO.1 Wavelength: The wavelength of the laser is a fundamental parameter that describes the spatial frequency of the emitted light wave. Lasers of different wavelengths play roles in different applications. In material processing, different materials have different absorption characteristics for different wavelengths, so the interaction with the material is also different. Shorter wavelength lasers and laser optics have advantages in creating small and precise features, with less peripheral heating. However, these devices are usually more expensive and more fragile compared to lasers with longer wavelengths.
NO.2 Power: Laser power is usually measured in watts (W), used to describe the optical power output of continuous wave (CW) lasers, or the average power of pulsed lasers. The characteristic of pulsed lasers is that their pulse energy is directly proportional to the average power and inversely proportional to the repetition rate. The unit of energy is joules (J). Therefore, pulse energy can be calculated by dividing the average power by the repetition rate.
Fig 2: a visual representation of the relationship between pulse energy, repetition rate, and average power of pulsed lasers higher power and energy lasers are generally more expensive and generate more waste heat. As power and energy increase, it becomes more and more difficult to maintain high beam quality.
NO.3 Pulse Duration: The pulse duration or pulse width of a laser is usually defined as the time it takes for the laser to reach half (FWHM) of its maximum optical power. Ultrafast lasers are characterized by short pulse durations, ranging from picoseconds (10-12 seconds) to attoseconds (10-18 seconds).
Fig 3: The Pulse Interval of a pulsed laser is the reciprocal of the repetition rate
NO.4 Repetition Rate: The repetition rate of a pulsed laser describes the number of pulses emitted per second, which is the reciprocal of the time interval between pulses. Contrary to what was mentioned earlier, the repetition rate is inversely proportional to pulse energy and directly proportional to average power. A higher repetition rate means that the thermal relaxation time of the laser optical element surface and the final focused spot is shorter, so the heating rate of the material is faster.
NO.5 Coherence Length: Lasers have coherence, which means that there is a fixed relationship between the phase values of the electric field at different times or positions. This characteristic stems from the fact that lasers are produced by stimulated emission, which is different from most other types of light sources. Although the coherence of the laser will gradually weaken during propagation, the coherence length of the laser defines the distance at which its time coherence remains at a certain level.
NO.6 Polarization: Polarization defines the direction of the light wave electric field, which is always perpendicular to the direction of propagation. In most cases, the laser is linearly polarized, that is, the emitted electric field always points in the same direction. In contrast, non-polarized light will produce electric fields pointing in many different directions. Polarization is usually expressed as the ratio of light power between two orthogonal polarization states, such as 100:1 or 500:1.
NO.7 Beam Diameter: The beam diameter of the laser describes the lateral extension of the beam, that is, the physical size perpendicular to the direction of propagation. Usually, the beam diameter is defined at the 1/e² width, that is, the point where the beam intensity reaches 1/e² (about 13.5%) of the maximum value. At this point, the electric field strength drops to 1/e (about 37%) of the maximum value. The larger the beam diameter, the larger the optical components and the entire system needed to avoid beam clipping, resulting in increased costs. However, reducing the beam diameter will increase the power/energy density, which will also bring adverse effects.
NO.8 Power or Energy Density: Power or energy density refers to the beam power or energy per unit area. The beam diameter is closely related to the power/energy density. When the power or energy of the beam remains constant, the larger the beam diameter, the smaller the power/energy density. In general, lasers with high power/energy density are the ideal final output of the system, such as in laser cutting or laser welding applications. However, lasers with low power/energy density are beneficial to the system internally, can reduce the damage caused by lasers, and prevent the high power/high energy density area of the beam from ionizing the air.
NO.9 Beam Profile: The beam profile describes the distribution intensity of the beam on the cross section. Common beam profiles include Gaussian beams and flat-top beams, and their beam profiles follow Gaussian and flat-top functions, respectively. However, because there are always a certain number of hot spots or oscillations inside the laser, no laser can produce a perfect Gaussian beam or a perfect flat-top beam that perfectly matches the ideal beam profile. The difference between the actual beam profile of the laser and the ideal beam profile is usually described by multiple measurement indicators (including the M² factor of the laser).
NO.10 Divergence: Although people usually think that the laser beam is collimated light, in fact, the laser beam will always have a certain degree of divergence. Divergence describes the degree of diffusion of the beam relative to the beam waist after long-distance propagation due to diffraction. In applications with long working distances, such as laser radar systems, where the target and the laser system may be hundreds of meters apart, divergence becomes a particularly important issue. The divergence of the beam is usually defined by the half-angle of the laser, and the divergence angle (θ) of the Gaussian beam is defined as λ is the laser wavelength, and w0 is the laser beam waist.
NO.11 Spot Size: The spot size describes the spot diameter of the focused laser beam, located at the focus of the focusing lens system. In many applications, such as material processing and medical surgery, our goal is to minimize the spot size. This can maximize the power density and create particularly fine features. Aspherical lenses are often used to replace traditional spherical lenses to reduce spherical aberrations and reduce spot size. In some types of laser systems, the laser will not eventually focus the laser into a spot, so in this case, this parameter does not apply.
Fig 5: Laser micromachining experiments at the Italian Institute of Technology show that the ablation efficiency of a nanosecond laser drilling system increases tenfold when the spot size is reduced from 220 microns to 9 microns at constant flux.
