What Factors Affect Beam Quality?

Chinese name: Beam quality

Foreign name: Beam quality

In the application of strong lasers, such as laser processing of industrial materials, laser power space transport, and inertial confinement fusion fusion drives, the effect of lasers mainly depends on the power density transmitted to the target, and the power density on the target is not only related to The laser output power depends on the quality of the laser beam. Therefore, like the power, the beam quality is also an important indicator that determines the comprehensive performance of a strong laser system. For a long time, there has been no uniform standard and standardized measurement method for the evaluation of the quality of high-intensity laser beams, which has brought great difficulties to experimental identification and inconvenience to scientific research and engineering applications. Establishing uniform and standardized beam quality evaluation standards and measurement methods will not only meet the needs of testing and objective and accurate evaluation of system performance, but will also promote the development and application of high-power laser technology in China ^[1] .

For the evaluation method of laser beam quality, domestic and foreign scholars have conducted in-depth research. Although various methods for evaluating and measuring laser beam quality have been proposed, a set of evaluation parameters and measurement methods that have been generally accepted by all parties have not been established. With the continuous development and maturity of high-energy laser technology, high-energy laser systems have begun to develop in the direction of engineering. In order to more objectively evaluate the effectiveness of high-energy laser systems in energy space transportation applications, standardize evaluation standards and experimental inspections of high-energy laser systems. The standard requires an in-depth analysis of the beam quality of high-energy laser systems in order to propose a scientific, reasonable, convenient and practical evaluation method and measurement method for engineering applications such as high-energy laser system research and testing ^[2] .

Limitations of beam quality evaluation of high-energy laser beams

Commonly used definitions of beam quality include: far-field spot radius, far-field divergence angle, diffraction limit multiple U, Strehl ratio, M2 factor, power on the target surface, or surrounding energy ratio. Various beam quality definitions reflect different emphasis on beam quality for different application purposes. The quality of the beam should be evaluated according to the specific application purpose ^[2] .

1) A beam with a small far-field divergence angle has a large far-field spot radius, so it is not comprehensive enough to evaluate the beam quality solely by the far-field spot radius or far-field divergence angle ^[2] .

2) The measurement of the diffraction limit multiple U depends on the accurate measurement of the beam's far-field spot radius. Due to the factors of the strong laser and many factors in the process of the strong laser transmission, the intensity distribution of the far-field beam contains more high-order spatial frequency components. After the strong laser is attenuated, the CCD is used to measure the spot radius.It is difficult to detect the high-order component of the spot, and the relative spatial intensity distribution is difficult to reflect the high-order component of the spot. The U value obtained cannot truly reflect the beam due to the high-order Energy loss due to dispersion. Therefore, the accurate measurement of U value requires higher detection system, which brings greater difficulties for engineering applications ^[2] .

3) For low-power Gaussian laser beams, use M2 factor to define the beam energy, that is, the product of the spot radius at the beam waist and the far-field divergence angle is a constant, avoiding using only the spot radius or far-field divergence angle as the beam quality Uncertainty brought by the criteria. However, the M2 factor requires the definition of the spot radius to be a second-order moment, and it requires higher measuring instruments. The high-energy laser generated by the unstable cavity generally has an irregular output beam, and there will be no "light waist". For the energy distribution discrete beam, the spot radius calculated by the definition of second-order moment will be far from the actual. Therefore, it is inappropriate to use M2 factor to evaluate the beam quality of high-energy lasers ^[2] .

4) BQ value For energy-coupled applications, the beam quality is evaluated by combining the energy concentration of the beam on the target. The BQ value is commonly measured by different hole-limiting energy measurement methods and detection systems capable of measuring the absolute energy distribution in space, and requires a strong light array detector or target instrument that can directly receive high-energy lasers ^[2] .

Composition of high- quality laser systems with beam quality

The high-energy laser system mainly includes two subsystems, a high-energy laser and a beam director.The high-energy laser is the core of the system.Its function is to generate a laser beam with high power, high energy and good beam quality.The beam director is used to attack the target. Capture, track, and aim, and also expand, correct, and emit the high-energy laser beams output by the laser, and ensure that the emitted laser beams with better quality are as far as possible ^[2] .

Beam Quality Evaluation Method for High Energy Laser System

To evaluate the beam quality of high-energy laser systems, it is necessary to analyze the performance of each subsystem and the attenuation of the beam performance due to the propagation medium. Therefore, the beam quality must be determined at the interface of the subsystem. The high-energy laser system includes the laser exit interface, the beam director exit interface, and the target surface, which determines the beam quality evaluation of the high-energy laser system; the evaluation of the beam quality of the laser output beam-the evaluation of the beam quality of the high-energy laser system-considering the atmosphere Evaluation of the beam quality of the spot formed on the target surface after the action of high-energy laser ^[2] .

Beam Quality Evaluation Parameters

In the history of laser development, various parameters have been proposed for different application purposes to evaluate the beam quality of laser beams. Commonly used are the far field divergence angle , focal spot size, diffraction limit multiple , M 2 factor, Strehl ratio. And the surrounding power ratio BQ value, etc., the rationality and applicability of these evaluation standards are still quite controversial in the academic world. Moreover, the inconsistency of the beam quality evaluation parameters has caused uncertainty and even confusion in the application ^[1] .
The rationality and applicability of commonly used laser beam quality evaluation parameters are analyzed, and the results show that under the condition that the ideal beam is clearly defined, the diffraction limit multiple and the ring power ratio BQ value are ideal and practical beam quality The evaluation parameters are also the evaluation standards we recommend ^[1] . The diffraction limit multiple factor is defined as:
= / 0 (1)
In the formula, the far field divergence angle of the actual beam to be measured; 0 the far field divergence angle of the ideal beam (also referred to as the reference beam) ^[1] .
Formula (1) shows that the diffraction limit multiple uses an ideal beam as a reference standard to characterize the degree to which the beam quality of the measured laser beam deviates from the ideal beam quality under the same conditions, and its value does not change with the transformation of the beam through the ideal optical system, so it can be Essentially reflects beam quality. At the same time, as long as the beam is not too wide, accurate determination of the factor is generally more convenient ^[1] . The surrounding power ratio BQ value is defined as: BQ = P0 / P (2)
In the formula, P0 --- the ideal beam spot surrounding power within the specified size on the target; P --- the measured actual beam spot surrounding power ^[1] .
The ideal beam here is taken as a uniform beam with the same emission aperture as the measured beam, and its emitted light intensity is equal to the average intensity of the actual beam. From the above definition, the surrounding power ratio BQ value directly reflects the energy concentration of the beam on the target, so it is most suitable for evaluating the beam quality at the target ^[1] .

Beam quality evaluation procedure

In high-power laser applications, the laser output energy should be focused on the target to the maximum. Therefore, in addition to high-energy lasers, high-power laser systems also need a beam control system (also called a beam director) that can expand, correct, emit the laser beam to the far field and focus on the target. The essence of intense laser beam quality is finally reflected in the energy concentration of the beam at the far-field target. Before the laser beam is transmitted to the target, it has to go through multiple optical transformations of the beam control system and a long distance atmospheric transmission. In this process, due to the non-ideal nature of the actual transmission and transformation, the laser beam is made at each link. Degraded beam quality. Therefore, in addition to the laser itself, the factors that affect the beam quality of strong lasers, and thus determine the final effect, are the beam control system and atmospheric transmission ^[1] .
For this reason, the evaluation of the quality of a strong laser beam should include the evaluation of the output beam quality of the high-energy laser, the quality of the beam emitted by the high-power laser system, and the quality of the beam at the target. This evaluation procedure is helpful for analyzing and discovering the main factors affecting the quality of strong laser beams, and can directly reflect the performance of lasers in terms of output beam quality. The quality of the transmitted beam is a comprehensive reflection of the laser performance and the transmission performance of the beam control system. The beam quality at the target depends not only on the performance of the strong laser system, but also the effects of atmospheric turbulence and thermal halo effects on the transmission of strong lasers ^[1] .

Measurement of beam quality

Measurement method of light spot:

Whether it is the measurement of the diffraction limit multiple value or the measurement of the ring power ratio BQ value, it finally comes down to the measurement of the light intensity distribution of the focused spot (see the subsequent analysis). To this end, the measurement method of the intense laser spot is first discussed. There are many methods. The following mainly introduces and reviews the ablation method, CCD measurement method, and special intense laser spot area array detector ^[1] .
(1) Ablation method uses the measured laser to irradiate a material with a known ablation energy within a certain period of time, measuring the ablation distribution on the material, and calculating by combining the ablation depth, irradiation time, material density and ablation energy Laser light intensity distribution on the material. From the mass of the ablated material, the output power of the laser can also be obtained by calibration. It can be seen that using this method requires a material with known ablation heat under irradiation conditions, and the material is preferably highly one-dimensional in terms of ablation mechanism. For example, for CO 2 laser and hydrogen fluoride, deuterium fluoride chemical laser, organic glass can be used as the ablation material. Another problem with this method is that the calibration is more complicated ^[1] .
(2) CCD measurement method When using infrared CCD to measure the strong laser spot, usually the strong laser light is sampled and further attenuated, and then the infrared CCD is used to directly receive the beam measurement to obtain the low-power light intensity distribution, which is then analyzed and processed by the image processing system. Get various beam characteristic parameters. In addition, the absolute light intensity distribution can also be obtained after calibration. The disadvantage of this method is that after the strong laser light is greatly attenuated, a large number of high-order components of the light intensity distribution are filtered out, so that the complete light intensity distribution and accurate spot size cannot be obtained, the measurement error is very large, and an infrared CCD can also be used. The camera captures the spot of intense laser light on the diffuse reflection screen to obtain the relative spatial light intensity distribution. If it is calibrated, it can also obtain the absolute light intensity distribution. In addition to the shortcomings of the above-mentioned CCD direct-receiving measurement method, this measurement method also has the problems of measurement errors caused by uneven reflection in all directions and more difficult calibration. The U.S. Air Force Weapons Laboratory has developed a special metal target disk for measuring the intensity distribution of intense laser light. A thin metal target disk with known thermal conductivity is irradiated with the measured laser light. By measuring the temperature and irradiation time of each point on the rear surface of the target disk, The laser intensity distribution on the target can be obtained. The requirements for the target disk material are able to withstand strong laser irradiation and fast response. The heat conduction is highly one-dimensional during the data sampling time, so that the response of any point on the rear surface of the target disk can directly correspond to the corresponding point on the front surface Radiation is linked. For a 30 m thick target disc, when the intensity of the absorbed laser beam is less than 1.4 kW / cm2, a steel disc (SS304) can be used. When the absorption is 2 kW / cm2, a nickel disc (Ni200) can be used. If the disk absorption exceeds 7 kW / cm2, the method of tilting the target disk can be used. For different light intensity levels, different coatings are used on the front surface of the target, such as graphite. The range of light intensity that can be measured using target disk technology is 50W / cm2 to 912 kW / cm2. The measurement error of the standard target disk increases with the increase of the peak power density. When the laser power density is 50 to 400 W / cm2, the measurement error is 7% to 9.5% ^[1] .
(3) Spot array detector A special calorimetric intense laser spot array detector has been developed in China. The detector array consists of 252 detection units, which can be used to directly measure the energy distribution of intense laser spots. In addition, a prototype of a 32-unit fast-response strong laser test system was also developed. The photodiode detection was used to attenuate the strong laser to improve the response speed, so that the instantaneous light intensity distribution can be measured ^[1] .

Measurement of high-energy laser beam quality :
When actually measuring the diffraction limit multiple of a beam, a near-field method is usually used, that is, a focusing optical system is used to focus the laser beam to be measured or a beam expansion focusing system is used to expand and focus the beam, and then the beam width wf is measured on the focal plane. Use: = wf / f (3) to obtain the far-field divergence angle (where f is the focal length of the focusing optical system), and then calculate the diffraction limit multiple ^[1] according to the definition.
However, for high-energy lasers, because the direct output laser power is too high, set the power to 104 W and the beam diameter to 100 mm, the corresponding average light intensity will be about 120 W / cm2, which will increase by about 8 orders of magnitude after focusing. For such a light intensity level, any optical components and detectors are burned out, so the measurement cannot be focused directly, but the output beam must be sampled and further attenuated before focusing. Depending on the level of light intensity after attenuation focusing, an infrared CCD can be used to directly receive light spots on the measurement focal plane or light spots on the diffuse reflection screen at the focal plane (to avoid detector saturation, sometimes it is necessary to use an attenuation sheet to further diffuse light reflection It can be measured only after attenuation), and finally the focal spot radius is obtained by analyzing and processing the low-power spot image through an image processing system ^[1] .

Measurement of the quality of the transmitted beam <br /> Because the transmitting telescope itself is a beam expanding focusing system, it can be used to measure the diffraction limit multiple of the beam at the transmitting telescope. During the measurement, in order to eliminate the atmospheric influence as much as possible, the focal length of the transmitting telescope is adjusted to the shortest. The spot is measured on the focal plane and the spot radius is obtained. Use formulas (1) and (3) to obtain , and f in formula (3) is for this purpose. The focal length of a time-transmitting telescope system. The value at this time is determined by the performance of the laser and the optical quality of the beam control system, which marks the quality of the emitted beam of the strong laser system. For the above-mentioned method for measuring the quality of the transmitted beam, due to the limited focusing range of the transmitting telescope, its shortest focal length is still a certain length. Atmospheric turbulence and thermal halo effects on this optical path will obviously affect the beam quality, so the measurement results need to be Corrected ^[1] .

Measurement of the quality of the beam at the target <br /> In the application of strong laser, usually the focus of the launch system is on the target, so as to obtain the maximum power density on the target to achieve the maximum effect. Therefore, it is similar to measuring the quality of the emitted beam. In principle, the beam width can be obtained by measuring the intensity distribution of the focal spot on the target. Using formulas (1) and (3), the diffraction limit multiple of the beam at the target can be obtained.At this time, the focal length of the telescope That is, the beam transmission distance. The focal spot size at this time is the focal spot size obtained after the laser beam is transmitted through the long-distance atmosphere, and includes the beam expansion caused by atmospheric turbulence and thermal halo effects, so the corresponding diffraction limit multiple reflects the effect of atmospheric transmission on the beam quality. Affect ^[1] .
Although the diffraction limit multiple can still be used to measure the beam quality in principle at the target, in practical applications, on the one hand, because the focal spot size increases with the transmission distance, when the target distance is far away, the focal spot The size is much larger than the receiving surface of the detector; on the other hand, due to the non-ideality of the cavity mode of the high-energy laser, the non-ideal transformation of the beam control system, and various linear and non-linear effects in the atmospheric transmission process, resulting in beam expansion at the far-field target Very serious, the light field distribution is extremely complicated, which contains a large number of high-order spatial frequency components. The intensity of these higher-order components is much smaller than the peak intensity.Using common light intensity distribution measurement methods, such as ablation method, CCD measurement method, or a special intense laser spot area array detector, due to sensitivity, measurement dynamic range, and detection surface size It is practically impossible to detect, but all these higher-order components add up to a considerable proportion of the total energy of the beam. In this case, it is impossible to obtain a complete light intensity distribution, nor is it possible to obtain an accurate focal spot size and the corresponding diffraction limit multiple . Therefore, it is actually impossible to accurately measure the diffraction limit multiple at the far-field target.
For this reason, only the BQ index can be used when evaluating the beam quality at the far-field target ^[1] . In fact, the biggest advantage of using the BQ value when evaluating the quality of the beam at the target is that its measurement only needs to measure the energy value within a certain standard size on the target, without the energy distribution information of the entire spot, which is easier than measuring the diffraction limit multiples. many. Therefore, when evaluating the beam quality at long-distance targets, it is recommended to uniformly use the surrounding power ratio BQ value of a certain standard size as the evaluation index. There are many factors that determine the laser power density on the far-field target, such as laser emission power, beam quality, and transmission distance. In addition, different applications have different requirements for light intensity levels. Therefore, the laser light intensity distribution at the target should be measured by a method or area array detector that can detect the corresponding light intensity level according to the specific situation and the actual light intensity level ^[1] .