Laser beam quality is a crucial aspect of laser characterization, determining how tightly a laser beam can be focused under specific conditions. The quality of the beam influences its performance in various applications, such as in laser cutting, communication, and imaging systems.
What is Laser Beam Quality?
Laser beam quality is generally quantified using two main metrics:
Beam Parameter Product (BPP):
This is the product of the beam radius at the beam waist and the far-field beam divergence angle.M² Factor:
The M² factor is a ratio that compares the beam parameter product of a laser beam with the corresponding product for a diffraction-limited Gaussian beam (which is ideal). The M² value is a measure of how close the beam is to a perfect Gaussian beam, where M² = 1 corresponds to an ideal beam (fundamental mode, TEM00).
The divergence of a pure Gaussian TEM00 beam can be expressed as:
where D00 is the diameter of the beam waist, and λ is the wavelength. Higher-order mode beams typically start with a larger beam waist and/or have a faster divergence. For higher-order modes, the equation becomes:
where Θ0 and D0 are the divergence and waist of a higher mode beam, M^2 is greater than 1 and represents the Beam Propagation Ratio (per the ISO 11146 standard). When a Gaussian laser beam is focused, the focused spot diameter is defined by
where D00 is the ideal focused spot diameter, f is the focal length of the focusing lens, and D00 is the input beam waist and is placed one focal length from the lens as shown in the figure. However, when a multimode beam is focused, Equation becomes
Difference between Fundamental Mode and High Order Mode Laser Beam
With a given divergence angle (i.e. knowing the focal length of the lens), the fundamental mode alone produces the theoretically smallest possible beam waist (green curve). If beam quality worsens (red curve), the beam waist increases. If divergence is fixed, beam waist increases linearly by the factor M2 compared to the underlying Gaussian.
The appropriate power density at z0 is reduced by a factor (M²)². Also the Rayleigh Length increases by a factor of M².
Why Does Laser Beam Quality Degrade During Propagation?
The quality of a laser beam can degrade during propagation due to several factors:
Existence of Higher-Order Modes:
Beams that are not purely Gaussian tend to have higher-order modes, which lead to a larger beam waist.Amplitude and Phase Distortions:
Inhomogeneities in the gain medium of lasers can cause amplitude and phase distortions, resulting in a non-Gaussian beam profile.Optical Component Imperfections:
Scrambling of the optical wavefronts, caused by spherical aberrations in lenses, thermal effects in the gain medium, or diffraction at apertures, can degrade the beam quality. These distortions cause the beam to spread out, increasing the beam waist compared to an ideal Gaussian beam.Parasitic Reflections:
Reflections from optical surfaces or components that are not properly aligned can also cause degradation in the beam quality.Restoration of Beam Quality:
In theory, phase masks can be used to compensate for wavefront distortions and restore beam quality. However, this approach is difficult in practice. A more flexible solution is the use of adaptive optics combined with a wavefront sensor, which can actively correct distortions in the beam.
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