M2-Measurement Report

Transcription

1 Fraunhofer Institute for Laser Technology ILT Steinbachstraße Aachen Tel Aachen, July 29 th DIVERSE OE 131 Industriekleinprojekte Authors: Dr. rer. nat. Thomas Westphalen

2 Introduction Contents 1 Introduction 2 2 Beam source 2 3 Beam Quality Measurements Haas LTI BWA CAM Spiricon M2-200 FW Primes Focus Monitor Primes MicroSpotMonitor 13 4 Summary 16 1

3 Introduction 1 Introduction 2 Beam source The Beam Waist Analyzer Camera (BWA-CAM ) offered by HAAS Laser Technologies Inc.is a laser beam analyzer system which enables real-time laser beam measurement, analysis and monitoring of high power CW and pulsed lasers. The measurement technique used in the BWA-CAM-system is ISO compliant. Fraunhofer ILT offers to support HAAS Laser Technologies with reference laser beam measurements of a high power fiber laser (M 2 <1.1) comparing the BWA- CAM with three different commercial beam analyzing systems available at ILT. At least one of these systems generates ISO conformal results. Goal of the offered work is to perform measurements of the output of a single mode high-power fiber laser source and to compare the results of the BWA- CAM with the results of the current 3 leading commercial beam analyzers. For the beam quality measurements, a collimated fiber laser at Fraunhofer ILT was used. The fiber laser and collimator specifications are given in Table 1 and Table 2. For the M² measurements, the output power is set to 25% (~250W). The collimator includes a Haas LTI TLC (Thermal Lens Compensation) collimator provided by Laser Components. Fiber laser specifications Manufacturer Model Fiber connector Fiber core diameter Angle of divergence (half angle) IPG IPG Y-LS-1000 SM QBH System 14 μm 53 mrad 2

4 Beam source Beam quality M² ~1.05 to 1.10 Output power W Table 1 Fiber laser specifications Collimator specifications Manufacturer Haas LTI Model TLC 50 Focal length Fiber connector 50 mm QBH System Table 2 Collimator specifications Figure 1 Fiber laser collimator 3

5 3 Beam Quality Measurements 3.1 Haas LTI BWA CAM Measurement Setup The beam quality measurements using the Haas LTI BWA CAM system were performed on April 11 th 2016 by Michael Scaggs (Haas LTI) supported by Gilbert Haas (Haas LTI), Rene Sattler (Laser Components) and Thomas Westphalen (Fraunhofer ILT). The Haas LTI BWA CAM system consists of a near and far field analysis system (equipped with a digital GigE camera each) connected to a beam splitting and a beam attenuation unit, where the collimator is plugged from above (see Figure 2). In order to perform the beam quality measurements, additional equipment was necessary, which is listed in Table 4. The time for mounting the setup is approx.10 mins. The time for alignment and calibration amounts to approx. 15 mins, which will be done automatically in the next software version. Table 3 HAAS BWA CAM specifications Equipment 1x Laptop 1x Windows software BWA control 1x beam dump + water chiller 1x defocus lens, 2 Diameter, f=-100mm + lens mount Table 4 HAAS BWA CAM additional equipment 4

6 Figure 2 Schematic of Haas LTI BWA CAM M² measurement setup Figure 3 Picture of Haas LTI BWA CAM M² measurement setup in laboratory 5

7 Measurement Results The M² measurements were performed and displayed in real-time. The near and far field spots are shown in Figure 4 and Figure 5. The spot diameters are simultaneously calculated using the 2 nd moment method. The calculated beam caustic is shown Figure 6. The M² calculation results for the x- and y- direction are given in Table 5. Figure 4 Haas BWA CAM spots close to the waist location Figure 5 Haas BWA CAM far field measurement spots Figure 6 Haas BWA CAM beam caustic measurement using 2 nd moment calculation Results M² (2 nd Moment) 1.24 (x); 1.33 (y) 1.29 (averaged) Table 5 Haas LTI BWA CAM M² measurement results. The beam quality is calculated for x and y direction. The values are averaged in the second column. Knife edge (90%) and M2 (86%) measurements also available. 6

8 3.2 Spiricon M2-200 FW Measurement Setup The Spiricon M2-200 FW is designed for measuring horizontal beams, therefore the collimator and the complete setup is mounted on the optical bench as shown in Figure 7 and Figure 8. The M2-200 FW is designed for low power beams (max. 15μW/cm²), this means that for beam quality measurements on high power beams additional equipment and beam attenuators are required. For beam attenuation, two wedged prisms made from fused silica were chosen which are mounted on adjustable mirror mounts. The prisms reduce the optical power to approx. 0.5W. Additional ND filters are necessary to decrease the optical power below camera saturation. To prevent thermal effects from the ND filters a stack of increasing neutral density (ND0.1 + ND0.5 + ND1.0 + ND2.0 + ND3.0) was chosen. The low power beam then enters the M2-200 FW device via a focus lens with 450 mm focal length. The estimated beam waist diameter is approx. 126 μm (the minimum spot size is 44 μm). The setup time amounts to 90 minutes. The mirror mounts are subsequently adjusted until the beam is parallel to the internal optical axis of the M2-200 FW. The alignment time amounts to approx. 30 minutes. Equipment 1x Notebook Fujitsu LIFEBOOK S710 CI5-460M, 4 GB RAM, (2.53 GHz CoreI5), Windows 7 Professional 64 bit 1x Firewire to PCMCIA adapter with ext. power supply 2x 4 Wedged Prism, uncoated Fused Silica, 1 diameter, 2x 3-Axis kinematic Mirror mount for 1 diameter, Thorlabs 1x Beam dumps + 1x water chiller 1x ND Filter set (ND 0.1 N4.0) 1x Focus lens, 1 Diameter, f=450mm 1x Defocus lens, 2 Diameter, f=-100mm 1x Mount for Collimator 1x Software Spiricon M2-200s FW v4.94 for Windows 7. Table 6 Additional equipment for Spiricon M2-200 FW measurements 7

9 Figure 7 Spiricon M2-200 FW setup Figure 8 Picture of the Spiricon M2-200 FW setup in laboratory 8

10 Measurement Results The measurement results are directly displayed in the Spiricon software. Faulty measurement points can be manually excluded from the beam quality calculation. The measurement time strongly depends on the z-axis resolution. In this case the smallest step size of 1 mm is necessary to fulfill the ISO compliance. This results in a measurement time of 7-10 minutes. Figure 9 Spiricon M2-200 FW measurement results using 2 nd moment calculation The beam waist diameter is calculated in x- and y- axis using the 2 nd moment and 90% method. Also the beam caustic and the beam quality are calculated for both directions as shown in Figure 9 and Table 7. Results M² (2 nd Moment) 1.34 (x); 1.38 (y) 1.36 (averaged) M² (90%, Knife edge) 1.24 (x); 1.26 (y) 1.25 (averaged) Table 7 Spiricon M2-200 FW M² measurement results. The beam quality is calculated for x and y direction. The values are averaged in the second column. 9

11 3.3 Primes Focus Monitor Measurement Setup The Primes FocusMonitor is designed for vertical beam direction, therefore the collimator is mounted above the optical bench. The downwards directed beam is then focused via a focus lens with 500 mm focal length. The estimated waist diameter is approx. 140 μm which is close to the lower spot size limit of the device (typ. 150 μm). The FocusMonitor is manually aligned close to the waist location using a pre-alignment mask. A defocusing lens and a water cooled beam dump are positioned below the FocusMonitor in order to capture the passed radiation. The FocusMonitor requires an external power supply and a bus box for serial communication with a Laptop or PC. In this case, an intermediate RS232-to-USB-adapter was necessary. The complete equipment which was necessary to perform the measurements is given in Table 8. The setup time amounts to approx. 30 mins. The alignment time amounts to approx. one minute. Equipment 1x Notebook Fujitsu LIFEBOOK S710 CI5-460M,(4 GB RAM, 2.53 GHz, CoreI5), Windows 7 Professional 64 bit 1x RS232 to USB converter 1x collimator mount 1x focus lens f=500 mm fused silica, 2 diameter 1x alignment mask 1x defocus lens, f=-100mm, 2 diameter 2x lens mounts 1x water cooled beam dump 1x water chiller Table 8 Additional equipment for Primes Focus Monitor measurements 10

12 Figure 10 Primes Focus Monitor setup Figure 11 Primes Focus Monitor setup in laboratory 11

13 Measurement Results The measurement results including a caustic plot are directly displayed in the Primes software and can be printed in a datasheet as shown in Figure 12. Single sliced can be selected and manually re-measured e.g. with higher resolution or refitted calculation window. The measurement time of a single slice amounts to approx. one minute, which is mostly delayed by the slow serial interface. A low resolution measurement of a complete caustic takes approx. 5 minutes, a high resolution measurement including window size corrections takes up to 30 mins. Figure 12 Primes Focus Monitor measurement results using 2 nd moment calculation The beam width is calculated using the 2 nd moment and 86% method. The M² beam quality results using both methods are given in Table 9. Compared to the M2-200 FW and the BWA CAM system, the beam quality is not separately calculated for x- and y- direction. Results M² (2 nd Moment) 1.18 M² (86%) 1.11 Table 9 Primes Focus Monitor M² measurement results 12

14 3.4 Primes MicroSpotMonitor Measurement Setup Similar to the FocusMonitor, the Primes MicroSpotMonitor MSM 35 supplied by Fraunhofer ILT is designed for vertically beams, the beam quality measurement setup is similar. Due to the fact, that the FSM 35 is equipped with an integrated water cooled beam dump, the defocusing lens and the external beam dump are not necessary as shown in Figure 13 and Figure 14. The collimated fiber laser beam is vertically focused into the MSM 35 using a focus lens with 500 mm focal length. The MSM 35 is connected to a Windows Laptop via Ethernet interface using the PRIMES LaserDiagnoseSoftware V The additional equipment is listed in Table 10. The time for mounting the setup amounts to approx. 20 mins. The alignment is simplified by using an alignment tool, the alignment time amounts to less than one minute. The maximum movement range along the optical axis amounts to 35 mm. Equipment 1x Notebook Fujitsu LIFEBOOK S710 CI5-460M,(4 GB RAM, 2.53 GHz, CoreI5), Windows 7 Professional 64 bit 1x Software PRIMES LaserDiagnoseSoftware V x collimator mount 1x focus lens f=500 mm fused silica, 2 diameter 1x lens mount 1x water chiller Table 10 Additional equipment for Primes Micro Spot Monitor measurements 13

15 Figure 13 Measurement setup using the Primes MicroSpotMonitor Figure 14 Picture of the Primes MicroSpotMonitor measurement setup 14

16 Measurement Results Similar to the FocusMonitor, the software shows the measurements results including a caustic plot, which is printed as datasheet shown in Figure 15. Figure 15 Primes MicroSpotMonitor measurement results using 2 nd moment calculation The time for performing a measurement with 17 slices amounts to 2 minutes. The M² measurement results calculated using the 2 nd moment and 86% method are given in Table 11. Results M² (2 nd Moment) 1.37 M² (86%) 1.27 Table 11 Primes MicroSpotMonitor M² measurement results The measurement error depends on the relation between spot diameter and measurement window size, which is automatically set during the measurements. The measurement error due to faulty offset calculations amounts approx. 10%. 15

17 Summary 4 Summary Fraunhofer ILT performed beam quality measurements on a high power fiber laser (250W) and a thermal lens corrected collimation optic using four different commercial beam analyzers from Haas LTI, Spiricon and Primes. The Haas LTI BWA-CAM, Spiricon M2-200-FW and Primes MicroSpotMonitor MSM35 comply with ISO The M² beam quality results using different beam width calculation methods are given in Table 12. M² calculation method 2 nd Moment 86% 90% Haas LTI BWA CAM* 1.29 Spiricon M2-200FW Primes FocusMonitor Primes MicroSpotMonitor MSM Table 12 Overview of M² calculation results *Knife edge (90%) and M2 (86%) measurements also available. The Haas LTI BWA CAM and Primes MicroSpotMonitor require few equipment and alignment, therefore the setup time was less than 20 mins in this specific case. The Spiricon M2-200 FW is designed for low power measurements, therefore the performed high power measurements using this device required the most additional equipment for attenuation and alignment. The Haas LTI BWA CAM is the only tested device which enables real time beam quality measurements. An overview of further specifications of all four devices is given in the appendix. The Haas LTI TLC Collimator was used for all testing due to thermal lensing of the original collimator. Collimator thermal lensing measured by Haas LTI BWA-CAM instantaneous measurement prior to testing. 16

18 Summary 17