Understanding the key parameters of a laser system

There is a wide range of common laser systems for a variety of applications such as materials processing, laser surgery, and remote sensing, but many laser systems share common key parameters. Establishing common terminology for these parameters prevents miscommunication, and understanding them allows for proper specification of laser systems and components to meet application requirements.

Basic Parameters

The following basic parameters are the most fundamental concepts of a laser system and are essential to understanding the more advanced points.

 

1: Wavelength (typical units: nm to µm)

The wavelength of a laser describes the spatial frequency of the emitted light wave. The optimal wavelength for a given use case is highly dependent on the application. 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 facilitate the creation of small, precise features with minimal peripheral heating because the focal spot is smaller. However, they are typically more expensive and more easily damaged 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 characterize 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 (Figure 2). The energy is measured in joules (J).

Higher power and energy lasers are usually more expensive and they produce more waste heat. Maintaining high beam quality also becomes more difficult with increasing power and energy.

 

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 light power versus time (Figure 3). Ultrafast lasers offer many advantages in a range of applications including precision materials processing and medical lasers, and are characterized by short pulse durations of about picoseconds (10-12 seconds) to attoseconds (10-18 seconds).

 

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 reverse 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 usually depends 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 point, which leads to faster material heating.

 

5: Coherence length (typical units: millimeters to meters)

Lasers are coherent, meaning 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, lasers are produced by excited emission. Coherence degrades throughout the propagation process, and the coherence length of a laser defines a distance over which the temporal coherence of the laser is maintained at a certain quality.

 

6: Polarization

Polarization defines the direction of the electric field of a light wave, 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 an electric field that points in many different directions. The degree of polarization is usually expressed as the ratio of the focal lengths of light in two orthogonally polarized states, e.g. 100:1 or 500:1.

Beam parameters

The following parameters characterize the shape and quality of a 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 dimension perpendicular to the direction of propagation. It is usually defined as the 1/e2 width, which is reached by the beam intensity at 1/e2 (≈ 13.5%). At the 1/e2 point, the electric field strength drops to 1/e (≈ 37%). The larger the beam diameter, the larger the optics and the entire system need to be to avoid truncation of the beam, which increases the 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 MW/cm2 or µJ/cm2 to J/cm2)

Beam diameter relates to the power/energy density of the laser beam or the optical power/energy per unit area. The larger the beam diameter, the lower the power/energy density of a beam with constant power or energy. At the final output of the system (e.g., in laser cutting or welding), a high power/energy density is often desirable. Still, within the system, a low power/energy concentration is often beneficial to prevent laser-induced damage. This also prevents ionization of the air in the beam's high power/energy density region. For these reasons, among others, laser beam expanders are often used to increase the diameter and thereby reduce the power/energy density inside the laser system. However, care must be taken not to expand the beam so much that the beam is obscured from the apertures in the system, resulting in wasted energy and potential damage.

 

9: Beam Profile

The beam profile of a laser describes the intensity distribution in the beam cross-section. Common beam profiles include Gaussian and flat-top beams, whose beam profiles follow the Gaussian and flat-top functions, respectively (Figure 4). However, no laser can produce a completely Gaussian or completely flat-top beam with a beam profile that exactly matches its eigenfunction, because there are 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 usually 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 applications with long operating distances, 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 usually defined by the half angle of the laser, and the divergence (θ) of a Gaussian beam is defined as:

λ 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 output.

 

11: Spot size (typical unit: µm)

The spot size of a focused laser beam describes the beam diameter at the focal point of the focusing lens system. In many applications, such as materials processing and medical surgery, the goal is to minimize spot size. This maximizes power density and allows the creation of exceptionally fine features (Figure 5). Aspherical lenses are often used instead of traditional spherical lenses to minimize spherical aberration and produce smaller focal spot sizes. Some types of laser systems do not ultimately focus the laser to the spot, in which case this parameter is not applicable.

12: Working distance (typical units: µm to m)

The working distance of a laser system is usually defined as the physical distance from the final optical element (usually the focusing lens) to the object or surface on which the laser is focused. Some 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.