There are a wide range of general-purpose laser systems in various applications such as material processing, laser surgery, and remote sensing, but many laser systems share key parameters in common. Establishing common terms for these parameters prevents miscommunication, and understanding them allows the laser system and components to be correctly specified to meet application requirements.
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/um)
The wavelength of the laser describes the spatial frequency of the emitted light wave. 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 have different effects on certain wavelengths in remote sensing, and in medical laser applications, various complexes will have different absorption on certain wavelengths. Shorter wavelength lasers and laser optics facilitate the creation of small, precise features with minimal peripheral heating because the focal spot is smaller. However, they are generally more expensive and more prone to damage than longer wavelength lasers.
2. Power and energy(Typical units: W/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 repetition rate of the laser. Energy is measured in joules (J).
Higher power and energy lasers are generally more expensive, and they produce more waste heat. As power and energy increase, it becomes increasingly difficult to maintain high beam quality.
3. Pulse duration(Typical units: fs/ms)
The laser pulse duration or pulse width is usually defined as the half-peak full width (FWHM) of the laser light power and time. Ultrafast lasers have many advantages in a range of applications, including precision material processing and medical lasers, and are characterized by short pulse durations of about picoseconds (10-12 seconds) to attoseconds (10-18 seconds).
4. Repeat rate(Typical units: Hz/MHz)
The repetition rate or pulse repetition rate of a pulsed laser describes the number of pulses emitted per second or the reverse time pulse interval. As mentioned earlier, the repetition rate is inversely proportional to the pulse energy and proportional to the average power. Although the repetition rate usually depends on the laser gain medium, it can vary in many cases. The higher repetition rate results in a shorter thermal relaxation time for the surface and final focus of the laser optics, which leads to faster heating of the material.
5. Coherence length(Typical units: mm/m)
Lasers are coherent, which means there is a fixed relationship between the phase values of the electric field at different times or locations.This is because unlike most other types of light sources, laser light is produced by stimulated emission.Coherence degrades throughout propagation, and the coherence length of a laser defines the distance over which the temporal coherence of the laser remains of a certain quality.
6. Polarization
Polarization defines the direction of a light wave's electric field, which is always perpendicular to the direction of propagation. In most cases, the laser will be linearly polarized, meaning that the emitted electric field always points in the same direction. Unpolarized light will have electric fields pointing in many different directions. Polarization is usually expressed as the ratio of the focal strength of the light in 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/cm)
The beam diameter of a laser characterizes the transverse extension of the beam, or its physical size perpendicular to the direction of propagation. It is usually defined as a 1/e2 width, which is defined by the beam intensity reaching 1/e2 (≈ 13.5%). At 1/e2, the field intensity drops to 1/e (≈ 37%). The larger the beam diameter, the larger the optics and the overall system need to be to avoid the beam being truncated, thus increasing the cost. However, a reduction in the beam diameter increases the power/energy density, which can also be harmful.
8. Power or energy density(Typical units:W/cm2,MW/cm2 or µJ/cm2,J/cm2)
The beam diameter is related to the power/energy density of the laser beam or the optical power/energy per unit area. The larger the beam diameter, the smaller the power/energy density of a beam with constant power or energy. In the final output of a system (e.g. in laser cutting or welding), a high power/energy density is usually ideal, but inside the system, a low power/energy concentration is usually beneficial to prevent laser-induced damage. This also prevents the high power/energy density region of the beam from ionizing the air. For these reasons, among others, laser beam extenders are often used to increase the diameter, thereby reducing the power/energy density inside the laser system. However, care must be taken not to expand the beam too much so that the beam is obstructed from the pores in the system, resulting in wasted energy and potential damage.
9. Beam profile
The beam profile of a laser describes the intensity distribution across the beam cross-section. Common beam profiles include Gaussian beam and flat-topped beam, whose beam profiles follow Gaussian function and flat-topped function respectively. However, no laser can produce a perfectly Gaussian or perfectly flat top beam whose beam profile exactly matches its characteristic function, because there is always a certain number of hot spots or fluctuations inside the laser. The difference between the actual beam profile of a laser and the ideal beam profile is usually described by a metric that includes the M2 factor of the laser.
10. Divergence (typical unit: mrad)
Although laser beams are generally thought of as collimating, they always contain a certain amount of divergence, which describes the extent to which the beam diverges over an increasing distance from the waist of the laser beam due to diffraction. In applications with long working distances, such as liDAR systems, where objects may be hundreds of meters away from the laser system, divergence becomes a particularly important problem. Beam divergence is usually defined by the half Angle of the laser, and the divergence (θ) of a Gaussian beam is defined as:
θ═λ/πw0
λ is the wavelength of the laser, and w0 is the waist of the laser.
These final parameters describe the performance of the laser system at output.
11. Spot size (Typical unit: µm)
The spot size of the focused laser beam describes the beam diameter at the focal point of the focusing lens system. In many applications, such as material processing and medical surgery, the goal is to minimize spot size. This maximizes power density and allows for the creation of particularly fine-grained features. Aspherical lenses are often used instead of traditional spherical lenses to reduce spherical aberrations and produce a smaller focal spot size. Some types of laser systems do not end up focusing the laser to the spot, in which case this parameter does not apply.
12. Working distance (typical unit: µm / m)
The working distance of a laser system is generally 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. Some applications, such as medical lasers, often seek to minimize the working distance, while other applications, such as remote sensing, often aim to maximize their working distance range.
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