There is a wide range of common laser systems used in a variety of applications such as material processing, laser surgery, and remote sensing, but many laser systems have common key parameters. Establishing common terminology for these parameters prevents communication errors, and understanding them allows the laser system and components to be correctly specified to meet application requirements.
Figure 1: Schematic diagram of a common laser material processing system, where each of the 10 key parameters of the laser system is represented by a corresponding number
Basic Parameters
The following basic parameters are the most basic concepts of laser systems, and are also critical to understanding more advanced points
1: Wavelength (typical units: nm to um)
The wavelength of a laser describes the spatial frequency of the emitted light wave. The optimal wavelength for a given use case is highly application dependent. Different materials will have unique wavelength-dependent absorption properties in material processing, resulting in different interactions with the material. Similarly, atmospheric absorption and interference will affect certain wavelengths differently in remote sensing, and various complexes will absorb certain wavelengths differently in medical laser applications. Shorter wavelength lasers and laser optics are beneficial for creating small and precise features with minimal peripheral heating because the focal spot is smaller. However, they are generally more expensive and more susceptible to damage than longer wavelength lasers.
2: Power and Energy (Typical Units: W or J)
The power of a laser is measured in Watts (W) and is used to describe the optical power output of a continuous wave (CW) laser or the average power of a pulsed laser. Pulsed lasers are also characterized by their pulse energy, which is proportional to the average power and inversely proportional to the laser's repetition rate (Figure 2). Energy is measured in Joules (J).
Figure 2: Visual representation of the relationship between pulse energy, repetition rate, and average power of a pulsed laser
Higher power and energy lasers are generally more expensive, and they generate more waste heat. Maintaining high beam quality also becomes increasingly difficult as power and energy increase.
3: Pulse Duration (Typical Units: fs to ms)
Laser pulse duration or pulse width is usually defined as the full width at half maximum (FWHM) of the laser optical power versus time (Figure 3). Ultrafast lasers offer many advantages in a range of applications including precision materials processing and medical lasers. They are characterized by short pulse durations of the order of picoseconds (10-12 seconds) to attoseconds (10-18 and less
P(W)
1/Repetitfion Rate
Purchase Public Account Time(s)
Figure 3: The pulses of a pulsed laser are separated in time by the inverse of the repetition rate
4: Repetition rate (typical units: Hz to MHz)
The repetition rate or pulse repetition frequency of a pulsed laser describes the number of pulses emitted per second or the inverse time pulse interval (Figure 3). As mentioned earlier, the repetition rate is inversely proportional to the pulse energy and directly proportional to the average power. While the repetition rate is generally dependent on the laser gain medium, it can vary in many cases. Higher repetition rates result in shorter thermal relaxation times at the surface of the laser optics and at the final focus, which results in faster material heating.
5: Coherence Length (Typical Units: Millimeters to Meters)
The laser is coherent, which means that electrical currents at different times or locations are coherent. There is a fixed relationship between the field phase values. This is because lasers, unlike most other types of light sources, are produced by stimulated emission. The coherence length defines a distance over which the temporal coherence of the laser light remains constant throughout the propagation of the laser light, without degradation during the process.
6: Polarization
Polarization defines the direction of the electric field of the light wave, "it is always perpendicular to the direction of propagation. In most cases, laser light will be linearly polarized, meaning that the emitted electric field always points in the same direction. Unpolarized light will have an electric field pointing in many different directions. The degree of polarization is usually expressed as the ratio of the optical power of two orthogonal polarization states, such as 100:1 or 500:1.
Beam parameters
The following parameters characterize the shape and quality of the laser beam .
7: Beam Diameter (Typical Units: mm to cm)
The beam diameter of a laser characterizes the lateral extension of the beam, or its physical size perpendicular to the direction of propagation. It is usually defined as the 1/e2 width, which is the width of the beam intensity up to 1/e2 (=13.5%). At the 1/e2 point, the electric field intensity drops to 1/e (=37%). The larger the beam diameter, the larger the optics and the entire system need to be to avoid beam truncation, which increases cost. However, a reduction in beam diameter increases the power/energy density, which can also be detrimental.
8: Power or Energy Density (Typical Units: W/cm2 to MWicm2 or uJ/cm2 to J/cm2)
Beam diameter is related to the power/energy density of the laser beam. Energy density, or the amount of optical power/energy per unit area. The larger the beam diameter, the lower the power/energy density of the beam for a constant power or energy. High power/energy density is often desirable at the final output of the system (for example in laser cutting or welding), but low power/energy concentrations are often beneficial inside the system to prevent laser-induced damage. This also prevents the high power/energy density areas of the beam from ionizing the air. For these reasons, among others, laser beam expanders are often used to increase the diameter and thus reduce the power/energy density inside the laser system. However, care must be taken not to expand the beam too much so that it is blocked from apertures in the system, resulting in wasted energy and potential damage.
9: Beam Profile
The beam profile of a laser describes the distributed intensity over the cross-section of the beam. Common beam profiles include Gaussian beams and flat-top beams, whose beam profiles follow the Gaussian function and the flat-top function, respectively (Figure 4). However, no laser can produce a completely Gaussian or completely flat-top beam with a beam profile that exactly matches its characteristic function, because there is always a certain amount of hot spots or fluctuations inside the laser. The difference between the actual beam profile of a laser and the ideal beam profile is often described by metrics including the laser's M2 factor
Gaussian and flat Top Beam Profiles
Figure 4: Comparison of the beam profiles of a Gaussian beam and a flat top beam of equal average power or intensity shows that the peak intensity of the Gaussian beam is twice that of the flat top beam
10: Divergence (typical units: mrad)
While laser beams are often considered collimated, they always contain a certain amount of divergence, which describes the degree to which the beam diverges at increasing distances from the laser's beam waist due to diffraction. In long working distance applications, such as LiDAR systems, where objects may be hundreds of meters away from the laser system, divergence becomes a particularly important issue. Beam divergence is often defined by the laser's half angle, and the divergence of a Gaussian beam (0) is defined as:
W is the wavelength of the laser and w0 is the beam waist of the laser
Final system parameters
These final parameters describe the performance of the laser system at the output
11: Spot size (typical units: um)
The spot size of a focused laser beam describes the beam diameter at the focus of the focusing lens system. In many applications such as material processing and medical surgery , the goal is to minimize the spot size. This maximizes power density and allows the creation of particularly fine features (Figure 5). Aspheric lenses are often used instead of traditional spherical lenses to reduce spherical aberrations and produce smaller focal spot sizes. Some types of laser systems do not ultimately focus the laser to a spot, in which case this parameter does not apply.
Figure 5: Laser micromachining experiments at the Italian Institute of Technology show a 10-fold increase in ablation efficiency in a nanosecond laser drilling system when the spot size is reduced from 220um to 9um at a constant flow rate
12: Working distance (typical units: um to m)
The working distance of a laser system is typically defined as the physical distance from the final optical element (usually a focusing lens) to the object or surface on which the laser is focused. Certain applications, such as medical lasers, typically seek to minimize the working distance, while others, such as remote sensing, typically aim to maximize their working distance range.
