Traditionally, optical telescopes are mainly used to observe distant celestial bodies, such as those in the universe. Optical telescopes can be divided into two categories: refractive telescopes and reflecting telescope. Refractive telescopes make full use of lenses to refract or bend light, while reflective telescopes use mirrors to reflect light.
Refractive telescopes can be divided into two categories: Kepler telescope and galileo telescope. Kepler telescope consists of lenses with positive focal lengths, which are separated by the sum of focal lengths (Figure 1). The lens closest to the observed object or source image is called the objective lens, and the lens closest to the human eye or imaging is called the imaging lens.
Figure 1: Kepler telescope
Galileo telescope consists of a positive lens and a negative lens, which are also separated by the sum of focal lengths (Figure 2). But because one of the lenses is a negative lens, the distance between the two lenses is shorter than that of Kepler telescope. The approximate total length can be obtained by using the effective focal length between the two lenses, and the most accurate length can be obtained by using the focal length.
Figure 2: galileo telescope
The magnification or reciprocal of the magnification of a telescope is equal to the ratio of the focal length of the objective lens to the focal length of the eyepiece.
If the magnification is greater than 1, the telescope will be enlarged. If the magnification is less than 1, the telescope will shrink.
In the laser beam expander, the positions of the objective lens and the imaging lens are reversed. The Kepler beam expander aims to focus the collimated input beam on a point between the objective lens and the imaging lens, thus forming a region where the laser energy is focused (Figure 3). This concentrated point will heat the air between the lenses and refract the light in the optical path, which may cause wavefront error. In high power laser applications, air ionization at the focal point may also be a problem. In view of this, most beam expanders choose to use Galileo design or its variant (Figure 4). However, Kepler designs are still very useful in laser applications that require spatial filtering, because they provide a focus and can easily place spatial filters.
Figure 3: Kepler beam expander has an internal focus, which is not conducive to high-power applications, but suitable for spatial filtering in low-power applications.
Figure 4: Galileo beam expander has no internal focus and is very suitable for high power laser applications.
When Kepler or Galileo designed the laser beam expander, it is very important to calculate the divergence of the output beam. This determines the deviation from perfectly parallel light. The beam divergence depends on the input laser beam diameter and the output laser beam diameter.
Magnification (MP) can now be expressed in terms of beam divergence or beam diameter.
By solving Equation 4 and Equation 5, it can be found that the output beam divergence (θO) decreases with the increase of the output beam diameter (DO), and vice versa. Therefore, if the beam is narrowed by the beam expander, the beam diameter will decrease, but the beam divergence of the laser will increase. The price of a small beam is to form a large divergence angle.
In addition, it is extremely important to be able to calculate the output beam diameter at a specific working distance (L). The output beam diameter is a function of the input beam diameter and beam divergence after a specific working distance (L) (Figure 5).
Figure 5: The input beam diameter and divergence of laser can be used to calculate the output beam diameter at a specific working distance.
The divergence of the laser beam is represented by a half angle, so the second term of Equation 6 requires a factor of 2.
The beam expander amplifies the input beam and reduces the input divergence through amplification. Substituting Equations 4 and 5 into Equation 6, the result is as follows:
The beam expander increases the beam area by the square of the magnification without significantly affecting the total energy contained in the beam. This will reduce the power density and irradiance of the beam, thus prolonging the life of the laser module, reducing the possibility of damage caused by laser, and allowing the use of more economical coatings and optical components.
Although this may not seem intuitive, increasing the diameter of the laser by using a beam expander may lead to a smaller beam diameter far from the laser aperture. Due to a specific beam expander power, the beam expander will increase the input laser beam, and also reduce the beam divergence due to the same beam expander power, thus forming smaller parallel beams at a greater distance.
example
Explore the laser beam expander equation described above with numerical examples;
initial parameter
Calculation parameter
Output beam diameter
Using Equation 6, this value can be compared with the beam diameter without beam expander.
Compared with the same laser without beam expander, the output beam diameter using 10X beam expander is reduced by 100m, which is more than 5 times.
The spot size usually refers to the radial distance from the center point of the maximum irradiance to the point where the intensity drops to the initial value 1/e2 (Figure 6). The focal spot size of an ideal lens can be calculated by using the wavelength (λ), the focal length (f) of the lens, the input beam diameter (di), the refractive index (n) of the lens and the M2 factor of the beam (representing the degree of variation from the ideal Gaussian beam).
Fig. 6: The spot size is usually measured when the intensity I(r) drops to 1/e2 of the initial value I0.
The spot size is basically determined by the combination of diffraction and aberration, which are indicated by red and blue respectively in fig. 7. Generally speaking, when focusing a laser beam, spherical aberration is considered as the only and main aberration type, which is why equation 1 1 only considers spherical aberration. In diffraction, the shorter the focal length, the smaller the spot. More importantly, the larger the input beam diameter, the smaller the spot size.
By expanding the beam in the system, the input diameter (d) is increased by using the factor m, while the divergence is reduced. When the beam is focused into a small spot, the spot with a factor of m is smaller than the ideal diffraction limit spot of the unexpanded beam. However, because the spherical aberration increases with the increase of the input beam diameter, it needs to be weighed.
Figure 7: For small input beam diameter, the focused spot size is limited by diffraction. With the increase of the input beam diameter, spherical aberration begins to control the spot size.
In practical application, adjustable laser beam expander is often used to standardize the size of laser beam. The laser produces a specified beam diameter, but the diameter also has a certain tolerance. In order to achieve a fixed beam diameter extending further along the optical path in multiple systems, an adjustable beam expander can be used to compensate for the laser-to-laser variation in beam size.
When selecting the beam expander application, certain conditions must be determined to obtain appropriate performance.
The structures used to focus the beam expander or change the magnification of the adjustable beam expander are usually divided into two types: sliding and rotating. A rotating focusing mechanism, such as a threaded focusing tube, rotates the optical element during translation. Because of its simple structure, its cost is lower than that of the sliding focusing structure, but the rotation of the element may lead to beam drift (Figure 8).
Fig. 8: Enlarged explanation of beam drift possibly caused by rotating focusing mechanism.
A sliding focusing structure, such as a spiral tube, translates the internal optical element without rotation, thereby minimizing beam drift. However, this requires a more complicated structure than the rotary focusing mechanism, which will increase the system cost. A poorly designed sliding optical element may also have excessive freedom of movement in mechanics. Although the pointing error in these poorly designed designs will not lead to rotation after adjustment, it will be larger than the rotating optical element or the correctly designed sliding optical element.
Kepler beam expander contains internal focus, which may cause problems in high power systems. Dense focused light spots will ionize air, or lead to wavefront error due to thermal deflection of light. So most beam expanders are Galileo to avoid the complicated problems caused by internal focusing. However, some applications need spatial filtering, which can only be achieved by Kepler design with internal focusing ability.
The reflective beam expander uses a curved mirror instead of a transmissive lens to expand the beam (Figure 9). Reflective beam expander is far less common than transmissive beam expander, but some advantages make it the correct choice for some applications. There is no chromatic aberration in reflective beam expander, but the amplification and output beam collimation of transmissive beam expander are related to wavelength. Although this has nothing to do with many laser applications, it may be crucial in broadband applications because lasers are usually emitted at a single wavelength. Multi-laser system, some tunable lasers and ultrafast lasers need achromatic characteristics of reflective beam expander. Because the pulse duration of ultrafast laser is extremely short, its natural wavelength range is wider than other lasers. Quantum cascade lasers also benefit from reflective beam expanders because there may be no transmission options at their operating wavelengths.
Figure 9: Unlike the transmission beam expander, the curved mirror of the Karnops reflection beam expander can expand the incident laser beam. The hole on the side of beam expander has integrated installation function.
It is suitable for the adjustable beam expander with various wavelengths, and the infinitely adjustable beam expander can ensure the high precision required in laser material processing.
1x-8x adjustable beam expander is a part of SilverlineTM high-power lens series, which is specially designed for laser applications with wavelength range of 355 or 1030- 1080nm.
The optical system of the objective lens is designed to reach the diffraction limited image quality in the whole extended range. The optimized coating can ensure the highest transmittance and minimum thermal effect in the wavelength range of 355 and1030–1080 nm. The use of all-time ensures a high damage threshold, so the lens can even withstand high-power laser applications.
The innovative frame concept means that the structure of the beam expander is very compact and strong. These lenses are very suitable for SilverlineTM series F-theta objective lens. They are used to make microstructures, marks and inscriptions of different materials.
A firm 1x-4x adjustable beam expander-a very powerful integrated timing system.
Steady 1x-4x adjustable beam expander has an innovative mechanical design: the moving optical components are stably guided along a straight line. This reduces the influence of mechanical manufacturing tolerances and improves the beam stability when changing the magnification and/or divergence. The beam expander achieves high beam stability less than 1 milliradian.
You can also lock the settings on the newly designed beam expander. This minimizes the influence of vibration and acceleration of the system, is simple and safe to use, and can minimize the solidification time.
1x-4x adjustable beam expander-fusion timing system
The 1x-4x adjustable beam expanders for 355nm, 532nm and1030-1080nm can provide high image quality and very compact length. All versions can be continuously adjusted between 1 and 4 times the expansion coefficient. The larger the expanded beam, the smaller the structure that F-Theta lens can produce. The focusing ring can adjust the divergence of the laser beam in a targeted way, thus compensating the focal length tolerance in the whole system. The position of the image plane can also be changed according to the scanning system.
Through the optimized structural design, the beam expander is very firm and compact, so the lens will not rotate when the setting changes. This improve that stability of the light beam. The beam expander adopts diffusion limiting design, and the temperature can be adjusted for the corresponding application wavelength of 355 nm, 532 nm or1030–1080 nm. You can use the zoom and focus scales directly engraved on the beam expander to adjust the settings.
2x- 10x adjustable beam expander-large zoom range
Adjustable beam expanders with wavelengths of 355 nm, 532 nm and1030–1080 nm have a particularly large zoom range. All versions can be continuously adjusted between 2 and 10 times the expansion coefficient. These beam expander mirrors are specially designed for the beneficial application of greatly expanding the beam diameter. The larger the expanded beam, the smaller the structure that F-Theta lens can produce.
The focusing ring can adjust the divergence of the laser beam in a targeted way, thus compensating the focal length tolerance in the whole system. The position of the image plane can also be changed according to the scanning system. Through the optimized structural design, the beam expander is very firm and compact, so the lens will not rotate when the setting changes. This improve that stability of the light beam. The beam expander adopts diffusion limiting design, and the temperature can be adjusted for the corresponding application wavelength of 355 nm, 532 nm or1030–1080 nm. You can use the zoom and focus scales directly engraved on the beam expander to adjust the settings.
1x-8x new electric beam expander-automatic configuration and setting of intelligent beam expander
1x-8x new electric beam expander simplifies the production steps of laser material processing, and the beam expander can be continuously adjusted from 1x to 8x by using software commands.
The optimum spot size can be adjusted during operation. The divergence of laser beam can be accurately adjusted to realize tolerance compensation of the whole system, such as thermal effect compensation. Controlling divergence can also change the position of the work plane, for example, for 3D processing. When the laser beam shrinks or increases, the lens does not rotate, but moves in the linear guide rail, thus achieving excellent beam stability and consistent high quality. These functions also reduce the setup time and improve the production efficiency. The new electric beam expander can be easily integrated into any laser material processing system.