Method for monitoring laser power

Abstract

The invention discloses a method for monitoring laser power, and relates to the technical field of additive manufacturing, a system arranged in a cabin is adopted to realize independent monitoring of the laser power, the system comprises a beam splitter 1, a beam splitter 2, a dodging sheet, a diaphragm, a photoelectric detector and a light shield, the light shield rotates and moves up and down through a motor 1 and a motor 2 respectively, and the motor 1 and the motor 2 are connected with the beam splitter. The laser reflected by the galvanometer and focused by the field lens is irradiated on the beam splitter 1, and the beam splitter 1 divides the laser into two paths; according to the invention, the dodging sheet is added in the light path, and the light passing through the dodging sheet is irradiated on the detector after passing through the diaphragm, so that the problem of laser alignment is solved, and the problems that harmful capacitance or resistance is generated due to the fact that the light is irradiated on the edge of the detector and the detector is possibly invalid due to long-time use are solved; a new monitoring mode is provided, the use interval of the tested laser power is wide, and the incident position of incident light is not limited; the invention provides a structure which can prolong the service life of a system while saving cost.

Description

Technical Field

[0001] The invention relates to the technical field of additive manufacturing, and in particular to a method for monitoring laser power. Background Art

[0002] Additive manufacturing is an advanced manufacturing technology that is driven by data and forms solid parts by adding materials layer by layer. Selective laser melting (SLM) is an important research direction of additive manufacturing. SLM technology uses laser beam as a heat source and has high forming accuracy and surface finish. Figure 7 The figure shows the optical path diagram of SLM technology. The high-power laser emitted from the laser passes through the collimator and hits the scanning galvanometer. The galvanometer is then reflected to the field lens and focused on the metal powder on the substrate. The laser beam then scans point by point along the path generated by the CAD model to melt the powder on the substrate.

[0003] Appropriate laser power can provide good energy distribution, so that the powder is completely melted and the molten pool can flow fully to produce good metallurgical bonding, and finally obtain a dense molded part. Therefore, in order to improve the printing quality, the laser power is tested to prevent the laser light from being unstable, which is called independent monitoring. Independent monitoring of laser power can test the laser power in advance before using the additive manufacturing equipment to prevent the laser power from being unstable after printing starts, thereby improving printing efficiency.

[0004] The existing method of independently monitoring laser power of SLM is to use a lens to attenuate the laser power and then collect the laser power through a photoelectric detector, as shown in the attached manual. Figure 8 The defects in the prior art are:

[0005] 1. Due to the indicators of the beam splitter, in the prior art, the laser power exceeds the saturation light power of the photodetector when passing through the second beam splitter, which can easily cause the photosensitive surface of the photodetector to be damaged.

[0006] 2. Since the laser used in the test is a focused beam, the laser energy is very high. The focused spot only hits one place, which may cause the temperature of this place to rise sharply. The light block in the figure can be easily penetrated. During use, it is necessary to frequently check whether the light block can still be used. If it cannot be used, a new light block must be replaced. Therefore, the system has a short service life and will incur high costs due to frequent replacement.

[0007] In view of this, a method for monitoring laser power is provided to overcome the above problems. Summary of the invention

[0008] The object of the present invention is to provide a method for monitoring laser power to solve the problems raised in the above background technology.

[0009] In order to solve the above technical problems, the present invention provides a method for monitoring laser power, which adopts a system placed inside a cabin to realize independent monitoring of laser power, wherein the system comprises a beam splitter 1, a beam splitter 2, a homogenizer, an aperture, a photoelectric detector, and a light block, wherein the light block is respectively rotated and moved up and down by motors 1 and 2, and the laser reflected by the galvanometer and focused by the field lens hits the beam splitter 1, and the beam splitter 1 divides the laser into two paths, one of which is reflected and absorbed by the light block, and the other is transmitted and hits the beam splitter 2, and the beam splitter 2 divides the transmitted light hitting it into two paths, one of which is absorbed by the light block after reflection, and part of the other light hits the homogenizer, and the light homogenized by the homogenizer hits the aperture, and then is received by the photoelectric detector through the aperture and converted into an electrical signal, which is finally collected by the acquisition card and output.

[0010] Furthermore, the motor 1 drives the light block to rotate at a speed of 500 RPM during the test, and the motor 1 stops moving when the test is finished.

[0011] Furthermore, the motor 2 drives the light block to move up and down, and the moving distance range is 0-200 mm, and when setting the moving distance in the motor control software interface, it is necessary to compare with the historical moving distance data.

[0012] Furthermore, when using the monitoring system to test the stability of laser power, first determine whether the original voltage power curve of the equipment laser exists in the system. If not, use a beam quality analyzer to calibrate the light source. After calibration, test and store the initial voltage power curve, and then perform a stability test. If the original curve exists, test directly by controlling the laser output light power so that the laser emits different powers to hit the monitoring system, which is received by the photoelectric detector and converted into a voltage signal. The collected voltage signal to be tested is curve-fitted with the power, and whether the laser power is stable is determined based on the relationship between the fitting curve and the initial curve.

[0013] Furthermore, the beam splitting ratio of the beam splitter 1 is T:R=1.5%:98.5%, wherein T is the transmittance, and R is the reflectance, and the transmittance is limited by the laser damage resistance threshold of the reflective neutral density filter and the coating process.

[0014] Furthermore, the OD value of the reflective neutral density filter at 1064 nm is 3, and its transmittance is 0.1%, which is used to attenuate the laser light coming from the beam splitter 1 again to meet the saturation optical power requirement of the photodetector.

[0015] Furthermore, the light homogenizer is an optical shaping lens with an irregular surface covered with microlenses of different structures. It is insensitive to wavelength and angle changes, has a transmittance greater than 90%, and is used to shape the focused laser into a large spot and attenuate the laser again.

[0016] Furthermore, the light block is a laser safety protector made of a material with high thermal conductivity. Grooves at a certain angle are distributed on the graphite surface to reduce the impact of reflected lasers, and an air-cooling or water-cooling structure is designed on the back to increase heat dissipation.

[0017] Compared with the prior art, the present invention has the following beneficial effects:

[0018] A homogenizer is added in the optical path. The light passing through the homogenizer and then through the aperture hits the detector, which solves the laser alignment problem and does not generate harmful capacitance or resistance due to light hitting the edge of the detector, which may cause detector failure after long-term use.

[0019] A new monitoring method is provided, which has a wide range of laser power usage and no restrictions on the incident position of the incident light.

[0020] A structure is provided to extend the service life of the system while saving costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 An independent monitoring optical path diagram of a method for monitoring laser power of the present invention;

[0022] Figure 2 A front view of a light blocking structure of an optical path system in a method for monitoring laser power of the present invention;

[0023] Figure 3 A side view of a light blocking structure of an optical path system in a method for monitoring laser power of the present invention;

[0024] Figure 4 A motor control flow chart of a method for monitoring laser power of the present invention;

[0025] Figure 5 A system test flow chart of a method for monitoring laser power of the present invention;

[0026] Figure 6 It is a diagram of optical lens light power attenuation in a method for monitoring laser power of the present invention;

[0027] Figure 7 This is the optical path diagram of the existing laser selective melting technology;

[0028] Figure 8 This is the optical path diagram of the existing laser power monitoring technology. DETAILED DESCRIPTION

[0029] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0030] See also Figure 1-Figure 6 , the present invention provides a technical solution:

[0031] See also Figure 1-Figure 6 As shown, an embodiment of a method for monitoring laser power:

[0032] This method uses Figure 1 The system realizes independent monitoring of laser power. The whole system is placed inside the cabin. The laser reflected by the galvanometer is focused by the field mirror and hits the beam splitter 1. It is divided into two paths by the beam splitter 1. One path of reflection hits the light block and is absorbed by the light block, and the other path of transmission hits the beam splitter 2. The beam splitter 2 divides the transmitted light hitting it into two paths. One path hits the light block after reflection and is absorbed by the light block, and only 0.1% of the light in the other path hits the light homogenizer. The light homogenized by the light homogenizer hits the aperture and is received by the photoelectric detector through the aperture and converted into an electrical signal, which is collected by the acquisition card and output.

[0033] Figure 2 and Figure 3 for Figure 1 The structure of the light block in the optical path system has two motors, motor 1 and motor 2 can drive the light block to rotate and move up and down respectively. Figure 2 and Figure 3 It can be seen that the light block is mounted on a structure with motor 1, and motor 1 is mounted on a linear slide with motor 2 through a fixed seat. The control process of the motor is as follows Figure 4 As shown, during the test, motor 1 drives the light block to rotate at a speed of 500RPM (to avoid interference from motor vibration to the system, the motor speed is required to be low), so that the light block will not be penetrated due to the focused laser hitting one position on the light block for a long time. The test is over and the motor stops moving. The moving distance of motor 2 is 200mm, which can drive the light block to move up and down. When setting the moving distance of 2 in the motor control software interface, it must be compared with the historical moving distance data to prevent the laser from repeatedly hitting the same position due to moving at the same position.

[0034] It is necessary to add that:

[0035] The motion control principle of the light barrier:

[0036] How the control system controls the motor movement:

[0037] Motor 2 (moving up and down): Open the control interface of motor 2 through the motor control software. When setting the moving distance, compare it with the historical moving distance data to avoid the light block being repeatedly irradiated by the laser at the same position. The moving distance range is 0-200mm (the maximum moving distance is 200mm). After starting motor 2, the light block moves to the specified position driven by motor 2. Although this control method does not mention the direct correlation with laser power and duration in the article, from the perspective of preventing local overheating and damage to the light block, changing the position of the light block during each test can avoid excessive temperature due to long-term laser irradiation at the same position. During the entire power stability test process, motor 2 moves to the specified position and remains stationary until the end of the test.

[0038] Motor 1 (rotational motion): After motor 2 moves the light block to the specified position, open the control interface of motor 1 and start motor 1. Motor 1 rotates the light block at a fixed speed of 500RPM during the test. This speed is selected to avoid interference with the system caused by motor vibration. At the same time, this rotational motion can ensure that the focused laser does not hit one position of the light block for a long time, thereby preventing the light block from being penetrated. There is no mention of changing the speed according to the laser power or duration. During the entire power stability test, motor 1 continues to rotate at a speed of 500RPM until the end of the test.

[0039] Distance and speed range for up and down movement and rotation:

[0040] Motor 2 (moves up and down):

[0041] Moving distance range: 0-200mm.

[0042] Speed: The article does not mention speed-related information. It can be inferred that the speed should be set according to the motor performance and system requirements to ensure that the light block can move to the specified position accurately and smoothly, while avoiding vibration caused by excessive speed that affects system stability.

[0043] Motor 1 (rotational motion):

[0044] Speed: 500RPM.

[0045] Rotation distance: Since it is a rotational motion, it can be measured in degrees. During the test, the rotation is continued (360° continuous rotation) until the test is completed. This continuous rotation ensures that all points on the light shield surface can be evenly irradiated by the laser to avoid local overheating.

[0046] The process of using this monitoring system to test laser power stability is as follows Figure 5As shown, when using this system to test the laser power, first determine whether there is an original voltage power curve of the laser of the device in the system. If not, it is necessary to use a beam quality analyzer to calibrate the light source. After calibration, test the initial voltage power curve and store it before performing a stability test. If the original curve already exists, you can test it directly, control the laser output light power, so that the laser emits different powers on the monitor, which are received by the photodetector and converted into corresponding voltage signals. The collected voltage signal to be tested is curve-fitted with the power to obtain the relationship between voltage and laser power. If the fitting curve is linear and consistent with the initial curve, it means that the laser power is stable. When using this monitor to test the laser power, each fitting result will be compared with the initial curve relationship. If the slope of the curve becomes larger or smaller, it also means that the laser power has changed. It is necessary to use a beam quality analyzer to debug the laser power so that the laser reaches the optimal state.

[0047] Figure 3 The functions of the optical components are as follows:

[0048] Beam splitter - splits the laser into two paths, with a splitting ratio of T:R=1.5%:98.5%, where T is transmittance and R is reflectivity. The splitting ratio is limited by two factors. The first is that the T of the beam splitter is limited by the laser damage threshold of the reflective neutral density filter, which is 1500W / cm2. The calculation formula of the continuous laser damage threshold: laser damage threshold=laser power / spot area (spot diameter is 80um) shows that the maximum power allowed to hit the reflective neutral density filter is 7.5W. Therefore, based on the maximum laser power of 500W, the allowable transmittance of the beam splitter is T≤1.5%. The second factor is the coating process. Combined with the first factor, T=1.5% is selected based on the price, so the final splitting ratio is T:R=1.5%:98.5%.

[0049] Reflective neutral density filter - divides the laser into two paths. The reflective neutral density filter uses the principle of thin film interference to transmit part of the light and reflect the other part of the light. Since the saturation power of the photodetector is only 10mW, the laser reaching the reflective neutral density filter from the beam splitter is 7.5W, which far exceeds the saturation power of the detector. Therefore, in order not to damage the detector, a reflective neutral density filter is used to attenuate the laser again. In order to meet the saturation optical power of the photodetector, the parameters of the reflective neutral density filter used in this system are OD value of 3 at 1064nm. The OD value is the optical density of the lens, which indicates the light transmittance in the cut-off area. The relationship between transmittance and OD value is T = 10-OD, that is, the transmittance of the lens at 1064nm is T = 0.1%. 0.1% of the laser power is used for detector power monitoring, so the power reaching the detector is 7.5mW, which is within the saturation power range of the detector.

[0050] A homogenizer is an optical shaping lens with irregularly covered microlenses of different structures on its surface. When light is incident on its surface, each microlens projects the part of the light irradiated on it into a light spot with uniform energy distribution. The main light spot shapes are rectangular and circular, with diffusion angles of 20°, 50°, 100°, etc., and a transmittance greater than 90%. The homogenizer is insensitive to changes in the wavelength and angle of the incident light and is suitable for various lighting applications. Since the system receives a Gaussian beam focused by a field lens, the addition of a homogenizer can, on the one hand, shape the original focused laser into a large light spot, so there is no need to worry about the original focused laser point not being able to hit the target surface of the photodetector, solving the laser alignment problem. On the other hand, the power range of this system is mainly limited to the saturated light power of the photodetector. After adding a homogenizer, the laser can be attenuated again;

[0051] Derivation of the attenuation ratio range of the light homogenizer:

[0052] The article mentioned that the transmittance of the light homogenizer is greater than 90%. Assume that the power of the laser incident on the light homogenizer is P in , the power after passing through the light homogenizer is P out , then the attenuation ratio

[0053] Because the transmittance Therefore, the attenuation ratio A<(1-0.9)×100%=10%, that is, the attenuation ratio range of the light homogenizer is 0% to 10%.

[0054] Derivation of the attenuation ratio range and power range of beam splitter 1:

[0055] The beam splitting ratio of beam splitter 1 is T:R=1.5%:98.5%, where T is transmittance and R is reflectance. For the attenuation ratio of beam splitter 1, the transmitted part is mainly considered, because the reflected part is absorbed by the light block and is not in the main path of the subsequent power calculation. Therefore, the attenuation ratio of beam splitter 1 is (1-0.015)×100%=98.5%.

[0056] Assume that the maximum output power of the laser is P max (500W), the power range of the transmitted light after passing through beam splitter 1 is 0~0.015P max , i.e. 0~7.5W (when P max =500W).

[0057] Derivation of attenuation ratio range and power range of beam splitter 2

[0058] Beam splitter 2 splits the transmitted light hitting it into two paths. One path hits the light block after reflection and is absorbed by the light block, and only 0.1% of the light in the other path hits the light homogenizer. Therefore, the attenuation ratio of beam splitter 2 for the light used for detector monitoring is (1-0.001)×100%=99.9%.

[0059] The maximum power transmitted from beam splitter 1 is 7.5W, and the power range of the light homogenizer after passing through beam splitter 2 is 1-0.001×7.5W=0~7.5mW.

[0060] In summary, the attenuation ratio of the light homogenizer is in the range of 0% to 10%; the attenuation ratio of beam splitter 1 is 98.5%, and the power range is 0 to 7.5W; the attenuation ratio of beam splitter 2 for the light used for detector monitoring is 99.9%, and the power range is 0 to 7.5mW. Determination of these ranges helps to better understand the control of laser power by each optical component in the system, as well as the power distribution of the entire monitoring system;

[0061] After attenuation, the laser power received by the photodetector is smaller than that of the existing structure, so the laser power applicable range of this system is higher than that of the existing system. Figure 6 As shown in the figure, it can be seen that when the incident laser power is 500W, the laser attenuates when passing through each lens in the system. When the laser attenuated by the neutral density filter hits the homogenizer, the laser is homogenized by the homogenizer and diffused before hitting the photodetector. The laser power reaching the photodetector is calculated as follows:

[0062] Depend on Figure 6 From the trigonometric function relationship in, we can know that the spot diameter of the laser diffused at an angle θ after reaching the photodetector at a distance of L is:

[0063]

[0064] The spot area is:

[0065]

[0066] Assuming the laser power reaching the light homogenizer is P, the optical power density is:

[0067]

[0068] The area of ​​the photodetector is:

[0069]

[0070] Then the light power received by the photodetector is:

[0071] P=LPD*s' Formula (5)

[0072] Substituting equations (1) to (4) into equation (5), we can obtain the optical power received by the photodetector as:

[0073]

[0074] In actual use, the laser power received by the photodetector can be obtained by substituting the parameter values ​​into equation (6).

[0075] Aperture is an entity that limits the light beam in an optical system, limiting the amount of light received by the photodetector in this system. It is worth noting that the size of the aperture is smaller than the chip size of the photodetector, because if the spot size hitting the photodetector is larger than the size of the photodetector, the light beam will hit the edge of the photosensitive surface of the photodetector, and the edge non-uniformity of the detector may produce harmful capacitance and resistance effects, thereby distorting the time domain response of the photodetector, which may cause signal overshoot and instability.

[0076] Photoelectric detector - receives laser light for power monitoring. To achieve photoelectric conversion, the detector converts the received laser power into a current signal through photoelectric conversion. The current signal is converted into a voltage signal output through a transimpedance amplifier circuit. The output voltage signal represents the stability of the laser power. If the voltage signal detected by the detector is stable, it means that the optical power received by the detector is stable, and thus the output optical power of the laser is stable.

[0077] Light block - is a laser safety protector used at the end of the optical system to absorb the incident laser. The material of the light block is a material with high thermal conductivity, such as graphite. The surface of the graphite is evenly distributed with grooves at a certain angle. The preferred angle of the groove is 30°, which can reduce the impact of reflected lasers. The back is designed with an air cooling or water cooling structure, which can increase the heat dissipation of the graphite to prevent damage.

[0078] Summarize:

[0079] Adding a light homogenizer in the optical path avoids the problem that the focused light spot cannot hit the photodetector exactly, and avoids the light hitting the edge of the photodetector and generating harmful capacitance and resistance effects, which would render the detector ineffective.

[0080] The existing method does not shape the incident focused light spot, so when using it for testing, the incident laser must be aimed at the center of the light port to ensure that the incident light hits the detector target surface. However, after adding a light homogenizer, when using the device to measure laser power, there is no restriction on the incident position of the incident laser, as long as it hits the light port.

[0081] Focusing the light spot on different parts of the light barrier helps to better dissipate heat.

[0082] The light block does not need to be replaced because the high-energy laser hits one part of the light block for a long time and damages it, thus saving costs.

[0083] There is no need to frequently disassemble the device to replace the light block, which ensures that dust does not enter the system due to being opened, thus requiring the lens to be cleaned, extending the service life of the lens and thus extending the service life of the entire device.

[0084] The laser power usage range of the system is increased.

[0085] Although it is not possible to directly test the optical power, it is possible to monitor whether the power of the laser is accurate after long-term use.

Claims

1. A method for monitoring laser power, characterized in that: A system placed inside the cabin is used to realize independent monitoring of laser power. The system includes beam splitter 1, beam splitter 2, light homogenizer, aperture, photoelectric detector and light block. The light block is rotated and moved up and down by motor 1 and motor 2 respectively. The laser reflected by the galvanometer and focused by the field lens hits beam splitter 1. Beam splitter 1 divides the laser into two paths. The reflected path is absorbed by the light block and the transmitted path hits beam splitter 2. Beam splitter 2 divides the transmitted light hitting it into two paths. One path is absorbed by the light block after reflection and part of the other path hits the light homogenizer. The light homogenized by the light homogenizer hits the aperture, and then is received by the photoelectric detector through the aperture and converted into an electrical signal, which is finally collected by the acquisition card and output.

2. A method for monitoring laser power according to claim 1, characterized in that: During the test, the motor 1 drives the light block to rotate at a speed of 500 RPM, and the motor 1 stops moving when the test is finished.

3. A method for monitoring laser power according to claim 1, characterized in that: Motor 2 drives the light block to move up and down, and its moving distance range is 0-200mm. When setting the moving distance in the motor control software interface, it must be compared with the historical moving distance data.

4. A method for monitoring laser power according to claim 1, characterized in that: When using the monitoring system to test the laser power stability, first determine whether the original voltage power curve of the equipment laser exists in the system. If not, use a beam quality analyzer to calibrate the light source. After calibration, test and store the initial voltage power curve, and then perform a stability test. If the original curve exists, test directly by controlling the laser output light power so that the laser emits different powers to hit the monitoring system, which is received by the photoelectric detector and converted into a voltage signal. The collected voltage signal to be tested is curve-fitted with the power, and whether the laser power is stable is determined based on the relationship between the fitting curve and the initial curve.

5. A method for monitoring laser power according to claim 1, characterized in that: The beam splitting ratio of the beam splitter 1 is T:R=1.5%:98.5%, where T is the transmittance and R is the reflectance. The transmittance is limited by the laser damage resistance threshold of the reflective neutral density filter and the coating process.

6. A method for monitoring laser power according to claim 1, characterized in that: The OD value of the reflective neutral density filter at 1064 nm is 3, and its transmittance is 0.1%. It is used to attenuate the laser coming from the beam splitter 1 again to meet the saturation optical power requirement of the photodetector.

7. A method for monitoring laser power according to claim 1, characterized in that: The light homogenizer is an optical shaping lens with an irregular surface covered with microlenses of different structures. It is insensitive to changes in wavelength and angle, and has a transmittance of more than 90%. It is used to shape the focused laser into a large spot and to attenuate the laser again.

8. A method for monitoring laser power according to claim 1, characterized in that: The light block is a laser safety protector made of a material with high thermal conductivity. There are grooves at a certain angle on the graphite surface to reduce the impact of reflected lasers, and an air-cooling or water-cooling structure is designed on the back to increase heat dissipation.

Images