Key Parameters of Laser Systems

Key Parameters of Laser Systems

There are a wide range of general-purpose laser systems used in applications as diverse as materials processing, laser surgery, and remote sensing, but many have key parameters in common. Establishing common terminology for these parameters can prevent communication errors, and understanding them allows the laser system and components to be properly specified to meet application needs.

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.

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 µm)

The wavelength of a laser describes the spatial frequency of the emitted light waves. The optimal wavelength for a given use case is highly application dependent.

Different materials will have unique wavelength-dependent absorption properties during material processing, resulting in different interactions with the material. Similarly, atmospheric absorption and interference affect certain wavelengths differently in remote sensing, and in medical laser applications, various complexes absorb certain wavelengths differently. 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 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), which 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 directly proportional to the average power and inversely proportional to the repetition rate of the laser (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. As power and energy increase, maintaining high beam quality becomes increasingly difficult.

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

Figure 3: Pulses of a pulsed laser are separated in time by the reciprocal of the repetition rate

4. petition 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 interval between pulses (Figure 3). As mentioned before, the repetition rate is inversely proportional to the pulse energy and directly proportional to the average power. Although the repetition rate usually depends on the laser gain medium, in many cases it can vary. Higher repetition rates result in shorter thermal relaxation times at the laser optics surface and final focus point, which results in faster heating of the material.

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

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 the electric field of a light wave, which is always perpendicular to the direction of propagation. In most cases, the laser light 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. Degree of polarization is usually expressed as the ratio of optical powers 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 determined by the beam intensity up to 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 overall system need to be to avoid beam truncation, which increases cost. However, reduction in beam diameter increases 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)

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. At the final output of the system (such as in laser cutting or welding), high power/energy density is often desirable, but within the system, low power/energy concentration is often beneficial to prevent laser-induced damage. This also prevents the high power/energy density regions of the beam from ionizing the air.

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 it 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 distributed intensity across the cross-section of the beam. Common beam profiles include Gaussian beams and flat-top beams, whose beam profiles follow Gaussian functions 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 characteristic function, because there is always a certain number of hot spots or fluctuations inside the laser. The difference between a laser's actual beam profile and its ideal beam profile is often described by a metric including the laser's M2 factor.

Figure 4: Comparison of the beam profiles of a Gaussian beam and a flat-top beam with the same average power or intensity shows that the peak intensity of a Gaussian beam is twice that of a flat-top beam

10. Divergence (typical unit: mrad)

Although laser beams are generally considered collimated, they always contain a certain amount of divergence, which describes the extent to which the beam diverges at increasing distances from the laser 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, 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 its output.

11. Spot size (typ. unit: µm)

The spot size of a focused laser beam describes the diameter of the beam 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 particularly sophisticated features (Figure 5). Aspheric lenses are often used instead of traditional spherical lenses to reduce spherical aberration and produce smaller focal spot sizes. Some types of laser systems end up not focusing the laser into the spot, in which case this parameter does not apply.

Figure 5: Laser micromachining experiments at the Italian Institute of Technology show that the ablation efficiency of a nanosecond laser drilling system increases by a factor of 10 when reducing the spot size from 220 μm to 9 μm under constant flow.

12. Working distance (typical units: µm to 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.