What Is the Beam Diameter?

The beam diameter is the diameter of a specified line that is perpendicular to and intersects the beam axis. Since the beam usually does not have a defined edge, the diameter can be defined in many different ways. The beam diameter can be measured in units of length on a specific plane perpendicular to the beam axis.

The beam diameter is the diameter of a specified line that is perpendicular to and intersects the beam axis. Since the beam usually does not have a defined edge, the diameter can be defined in many different ways. The beam diameter can be measured in units of length on a specific plane perpendicular to the beam axis.

The beam diameter is usually used to characterize the electromagnetic beam in the optical state, sometimes in the microwave state, that is, the aperture of the beam exit is very large relative to the wavelength.

The beam diameter usually refers to a beam with a circular cross section, but there are special cases. For example having an oval cross section, in which case the direction of the beam diameter must be specified, for example with respect to the major or minor axis of the oval cross section. In applications where the beam does not have circular symmetry, the term "beam width" may be preferred. ^[1]

Beam diameter

If the longines ruler consists of light-shielding and light-transmitting lines, the Gaussian beam diameter can be obtained by measuring the modulation of transmitted radiation obtained by scanning the ruler through the beam interception. can prove.

The beam diameter d is given by:

Where

Is the minimum transmission power

And maximum transmission power

The ratio, W is the line width. as long as

This result is accurate to about 1%. When applying this technique to reflective situations, it is difficult to obtain light-shielding and light-transmitting lines. In this way, considering its utility, the formula (1) becomes:

Where RS is the reflectivity of the gap; RE is the reflectivity of the line;

Is the measured modulation. It should be noted that since RE and RS appear as ratios, it is not necessary to measure their absolute values.

Figure 1 is a schematic diagram of a scale used for this work. This ruler consists of two lines with a width of 10 m, ten lines with a width of 1 m, and two lines with a width of 10 m. The figure below is the composite signal and measured value of the reflected light. The advantage of using this ruler is that you can measure RS, RE,

,

, (Assuming that the reflectivity of large lines is the same as that of small lines). Using the equation (2) and the measurement values shown in FIG. 2, it is possible to obtain a beam diameter of about 1.33 m. ^[2]

Figure 1. Schematic of a scale for measuring beam diameter

Arnaud Beam diameter Arnaud et al. Knife-edge method

Document 2 describes the knife-edge technique proposed by Arnaud et al. For the total reflection boundary edge and the total transmission gap. The results are as follows:

Where

Is the maximum value of the derivative of the reflected signal. In addition, for the non-ideal boundary and clearance, the above formula must be modified. The process of deriving non-ideal results can be obtained directly from the literature (2) and the above deduction. For the clearance between the boundary edge of the reflectance RE and the reflectance RS, the radius can be given by:

Because we are measuring

Therefore, the measurement procedure shown in Figure 2 can be used for offset signals. As mentioned above, it is not necessary to know RS, RE or their ratio. FIG. 3 is a derivative of the signal of FIG. 2. This work was done on a digital oscilloscope. Using these measurements and v values, a beam diameter of about 1.24 m can be determined. ^[3]

Figure 2. Knife-edge scanning in non-ideal situations

Figure 3. Derivative of a knife-edge scan