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TEM₀₀, Transverse Mode
Definition of laser transverse modes
Lasers travel back and forth within the resonant cavity, eventually stabilizing to form a steady distribution of the output laser—this stable state is known as the laser mode. Typically, the stable intensity distribution (the "spot") of the laser beam in the transverse cross-section perpendicular to its direction of propagation is referred to as the transverse mode. Transverse mode patterns are usually denoted by the symbol TEM. pq It states that p and q represent the number of dark regions in the horizontal and vertical directions, respectively, across the beam's cross section. When p = q = 0, the mode is referred to as the fundamental mode (TEM₀₀), while all other transverse modes are called higher-order transverse modes. Figure 1 shows photographs illustrating the intensity distributions of different transverse modes. For instance, the axially symmetric TEM mode is depicted in Figure 1-b. 10 Indicates one dark region in the horizontal direction and no dark regions in the vertical direction; TEM 21 This indicates there are 2 dark areas in the horizontal direction and 1 dark area in the vertical direction.
Figure 1-a: Rotational Symmetry Transverse Mode Diagram | Figure 1-b: Axially Symmetric Transverse Mode Pattern |
Transverse Mode TEM ₀₀
The laser transverse mode we commonly encounter is the TEM₀₀ fundamental mode, which has no dark regions in either the horizontal or vertical directions. It features a small divergence angle, higher brightness, and a Gaussian distribution of energy. As shown in Figure 2 below, this illustrates the Gaussian-shaped beam profile of the TEM₀₀ fundamental mode, captured using a laser profile analyzer.
Figure 2: Fundamental Transverse Mode TEM ₀₀ Spotlight Mode
Measurement of Laser Spot Dimensions
Light spot measurement is primarily used to analyze the shape, size, and energy distribution of transverse mode patterns, making it crucial for evaluating laser performance. The main methods for detecting laser light spots include the following:
(1) CCD- or CMOS-based vision systems: These systems use high-resolution CCD or CMOS cameras to capture images of laser spots, which are then analyzed by image-processing software to determine characteristics such as spot size, shape, and energy distribution. A widely adopted method defines the boundary at 13.5% of the peak energy (1/e²), offering high precision, excellent resolution, and a more intuitive approach to measurement.
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Figure 3.1: Contour Analyzer | Figure 3.2: Laser Spot Test Analysis Software |
(2) Knife-edge Method: Gradually block the laser beam with a knife edge, measure the changes in optical power using a power meter, and determine the spot size. Position the laser source to be tested so that its beam passes just beyond the edge of the knife. Mark the coordinate x1 where the knife blocks the beam to reduce the optical power to 10% of the total, and mark the coordinate x2 where the beam is blocked down to 90% of the total power. The spot diameter is calculated as 1.561 × |x1 - x2| (with 1.561 being the fitted value).
Figure 4: Measuring the Spot Diameter Using the Knife-Edge Method
(3) Thermal Imaging Method: This technique uses thermal imaging equipment, such as infrared cameras, to capture the thermal effects generated when a laser beam is directed onto the target surface. By analyzing the resulting thermal images, spot parameters can be detected.
(4) Laser Spot Ablation Method: This technique involves ablating the target surface with a laser beam to create visible ablation marks, which are then measured in terms of size and shape to infer the characteristics of the laser spot. However, it suffers from low precision, significant limitations, and can cause damage to the target surface.
CNI offers high-precision laser profile analyzers and laser power meters—click www. for details.
Changchun New Industries Optoelectronics Technology Co., Ltd.
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