Wavelength, power and energy, repetition rate, coherence length, etc., laser terminology
Publish：Box Optronics  Time：2024-05-08  Views:682
Wavelength (common units: nm to µm):
The wavelength of a laser describes the spatial frequency of the emitted light wave. The optimal wavelength for a specific use case depends heavily on the application. During material processing, different materials will have unique wavelength absorption characteristics, resulting in different interactions with the materials. Likewise, atmospheric absorption and interference can affect certain wavelengths differently in remote sensing, and in medical laser applications, different skin colors will absorb certain wavelengths differently. Shorter wavelength lasers and laser optics have advantages in creating small, precise features that generate minimal peripheral heating due to smaller focused spots. However, they are generally more expensive and more susceptible to damage than longer-wavelength lasers.
Power and energy (common units: W or J):
Laser power 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. In addition, the characteristic of pulsed laser is that its pulse energy is directly proportional to the average power and inversely proportional to the pulse repetition rate. The unit of energy is Joule (J).
Pulse energy = average power repetition rate Pulse energy = average power repetition rate.
Lasers with higher power and energy are generally more expensive and produce more waste heat. As power and energy increase, maintaining high beam quality becomes increasingly difficult.
Pulse duration (common units: fs to ms):
Laser pulse duration or (i.e.: pulse width) is generally defined as the time it takes for the laser to reach half its maximum optical power (FWHM). Ultrafast lasers are characterized by short pulse durations, ranging from picoseconds (10-12 seconds) to attoseconds (10-18 seconds).
Repetition rate (common units: Hz to MHz):
The repetition rate of a pulsed laser, or pulse repetition frequency, describes the number of pulses emitted per second, which is the reciprocal of the sequential pulse spacing. 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 the repetition rate can vary. The higher the repetition rate, the shorter the thermal relaxation time at the surface of the laser optics and final focused spot, allowing the material to heat up faster.
Coherence length (common units: mm to cm):
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 laser light is produced by stimulated emission, unlike most other types of light sources. Coherence gradually weakens throughout propagation, and the coherence length of a laser defines the distance over which its temporal coherence maintains a certain quality.
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, laser light is linearly polarized, meaning that the emitted electric field always points in the same direction. Unpolarized light produces electric fields that point 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 diameter (common units: mm to cm):
The beam diameter of a laser represents the lateral extension of the beam, or the physical size perpendicular to the direction of propagation. It is usually defined at 1/e2 width, that is, the point at which the beam intensity reaches 1/e2 (≈ 13.5%) of its maximum value. At the 1/e2 point, the electric field strength drops to 1/e (≈ 37%) of its maximum value. The larger the beam diameter, the larger the optics and overall system required to avoid beam clipping, resulting in increased cost. However, reducing the beam diameter increases the power/energy density, which can also have detrimental effects.
Power or energy density (common 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 (that is, the optical power/energy per unit area). When the power or energy of the beam is constant, the larger the beam diameter, the smaller the power/energy density. High power/energy density lasers are usually the ideal final output of the system (such as in laser cutting or laser welding applications), but low The power/energy density of the laser is often beneficial within the system, preventing laser-induced damage. This also prevents the high power/high energy density regions of the beam from ionizing the air. For these reasons, beam expanders are often used to increase the diameter, thus reducing the power/energy density inside the laser system. Care must be taken, however, not to expand the beam so much that it gets clipped within the system‘s aperture, resulting in wasted energy and possible damage.
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