[Basic Optics Skills] Design and Debugging of Laser Beam Expansion System

[Basic Optical Skills] Design and Debugging of Laser Beam Expansion System

Abstract

This article provides some of the most basic skills and methods for laser practitioners on laser beam expansion technology based on the telescope principle. The basic context of the article includes the basic principles of beam expansion, debugging methods, comparison of reflective and transmissive types, criteria for laser collimation, and adjustment methods of the image transmission system.

In laser technology, the spatial size transformation of the laser beam is one of the most common operations. If the collimation of the beam is not considered, a single lens can be used to focus and diverge the beam. To obtain a nearly collimated laser beam, using an optical beam expansion (contraction) system is basically the only solution. Optical beam expansion systems are used in every corner of the laser field, such as: beam expansion between each stage of the laser amplifier to avoid optical damage and optimize the filling factor; adjusting the spot size in nonlinear transformation to obtain optimal conversion efficiency; laser image transfer to ensure beam quality; optimization of spot size and Rayleigh length in the focusing system, etc. Figure 1 shows two classic laser beam expansion systems.

Figure 1: Two classic laser beam expansion systems.

Note: This picture is only schematic. The selection of lenses in the actual beam expansion system needs to consider aberration and other factors according to the actual situation, so it is not necessarily a simple single lens form.

1. Basic Principle

The basic principle of laser beam expansion system is very simple. Generally, it can be divided into two categories according to the type of lens group used: Kepler type and Galilean type. As shown in Figure 1, the Kepler-type beam expander system consists of two positive lenses. There is a common real focus between the lenses. Ideally, the distance between the lenses is the sum of the focal lengths of the two lenses.

The Galileo-type beam expander system consists of a negative lens and a positive lens, and the distance between them is the absolute difference in the focal lengths of the two lenses. From a geometric structure point of view, the spatial distance occupied by the Galileo type is smaller than that of the Kepler type, and there is an actual focus in the Kepler type, which will penetrate the air under strong laser conditions, resulting in the deterioration of the beam spatial distribution and wavefront.

Therefore, under normal circumstances, the Galileo type will be used more for laser beam expansion. The application of Kepler type will be introduced in detail in the image transfer section below. In addition, reflective lenses with curvature (spherical mirrors, cylindrical mirrors, off-axis parabolic mirrors) are often used in experiments to expand the laser beam. For example, in strong lasers, in order to avoid nonlinear effects in the transmission beam expansion system, the reflection type is generally used for beam expansion.

However, the spherical mirror reflection type beam expansion system will introduce more aberrations, so sometimes an expensive but aberration-free off-axis parabolic beam expansion system is used. The selection of a beam expansion system often requires a comprehensive consideration of factors such as price, aberration, and whether there is any need for imaging.

2. Debugging methods and techniques

During the construction process of the beam expansion system, it is first necessary to roughly determine the position and spacing of the two lenses according to the design parameters. Simple plano-convex lenses are often used in beam expansion systems. At this time, attention should be paid to determining the direction of the convex surface of the lens so that it faces the direction of light incidence to reduce spherical aberration.

Tips for determining the convex surface: As shown in Figure 2, use a piece of clean hard paper with the edge close to the lens surface. If there are gaps on both sides, it means it is a convex surface, otherwise it is a flat surface, but you need to be careful not to scratch the lens.

Figure 2: Tips for determining the convex surface of a plano-convex lens

The next step is to ensure that the posture of the lens is correct according to the coaxial and over-center methods. One thing to note here is that the surface of the lens often has weak reflected light, which is generally called a "ghost spot." In high-energy lasers, component safety is prioritized over the optimization of aberrations, so the posture of the lens is generally artificially set away from the ideal posture to prevent damage to the upstream optical components by the return light from the lens.

During the adjustment process, you can use infrared cards or infrared night vision devices to determine the location of the ghost point and take corresponding protective measures (such as using light baffles). Next, put in the second lens in the same way, and then adjust its front and rear position with the first lens to ensure the collimated output of the light beam. The judgment of the collimated light will be explained in detail below. After collimation, if the measured distance between the two lenses deviates too much from the ideal situation, the reason may be that the initial incident light is not collimated, or it may be because the focal length of the lens is inappropriate (for example, the focal length of the corresponding wavelength was not selected when purchasing). The judgment of beam collimation is generally made by measuring the size of two light spots that are far apart (for example, use a white piece of paper to observe the size change of the light spot from near to far. If it is basically unchanged, the beam is collimated). But a more accurate and simple way is to use a shearing interferometer, as shown in the figure below, to determine the degree of collimation based on the inclination of the interference fringes. However, it should be noted that this method cannot be applied to broadband light sources with extremely short interference lengths.

Figure 3: Using an interferometer to determine the collimation of the beam

3. Image transmission

The image transmission system generally uses a Kepler-type positive lens combination to transmit the beam to the actual application position. High-quality beam expansion between each stage of the laser amplifier is most commonly used to avoid damage to optical elements caused by the diffraction structure caused by free propagation. Often used with image transfer systems are spatial filters with soft-edge diaphragms under vacuum conditions.

Figure 4: Image transfer system with spatial filtering function