M 2. What is it and how do you measure it?

Transcription

1 M 2 What is it and how do you measure it?

2 M 2, k-factor, or the Times-Diffraction-Limit Beam Propagation Equation M 2 4 d 0 2

3 What does M 2 mean? M 2 4 d 0 Thus d M

4 What does M 2 Mean? For any given wavelength: 4 is a constant 4

5 What does M 2 mean? M 2 is a measure of the laser s focusability Ideal Gaussian TEM00 M 2 should approach 1 M 2 cannot be <1 Measurement is only a 2% Allowable error will sometimes show 0.9X 5

6 What does M 2 mean? Practically speaking it is a way for laser manufacturers to spec their lasers 6

7 Laser Focus The larger the θ, the smaller d 0 The smaller the λ, the smaller d 0 The better the M 2, (closer to 1), the more the θ and λ control the focus 7

8 The ISO Standard

9 Observed Problems with ISO Method More data points do not improve fit Noisy or weak signal affects fit Large intensity difference between waist area and linear area Use weighted fit for more consistent results Laser must be stable over measurement time period 9

10 Most users want M 2 near 1 TEM00 lasers are easier to measure Higher order lasers can be problematic Use 4-sigma beam measurement (usually) Standard was designed for lasers, but most users want to measure systems Issues with M 2 10

11 Rayleigh Method M 2 min d z r d min 4 or M 2 2 d 2z min r 2 11

12 Rayleigh Method 12

13 Measurement Instruments Spiricon: M 2-200S Automated Measurement Photon: NanoModeScan Measurement of Any Wavelength Photon: MS-1780 Instantaneous Measurement 13

14 M 2-200S CCD Camera Based Fully Automated Operation Reports ISO Parameters M 2 Divergence Rayleigh Range Waist size and Position 14

15 M2-200S Operation 15

16 Mirror train varies beam path length M2-200S Operation Successively focuses points of beam caustic on camera Automatic attenuation applied to maintain signal levels as power density changes Reports all ISO Parameters Operates for CCD wavelengths 250nm- 1100nm Best above 350nm 266nm tends to damage CCD rapidly 16

17 Dedicated M2-200S Software Package with Ultracal 17

18 NanoModeScan 18

19 ModeScan Principles 19

20 NanoModeScan Operation Moves Scan head to measure successive points in caustic Dedicated software reports ISO Parameters Can be equipped with any Scan head to cover all wavelengths Silicon for UV-VIS ( nm) Germanium for NIR ( nm) Pyroelectric for 200nm to >20µm at power levels > ~200mW Adjustment to attenuation unnecessary making measurement fast 20 seconds for CW Pulsed lasers with rep rates >10kHz 20

21 ModeScan Report 21

22 ISO M 2 Curve Fit 22

23 ISO Measurement Window 23

24 Rayleigh Measurement Window 24

25 ModeScan 1780 Real-Time M2 Measurement First Camera System Based on Patent Concept Introduced in

26 ModeScan 1780 Real Time Camera Based Measurement Patented method - Dave Wright/John Fleischer 5 optical flats produce 10 spots on CCD 10 spots measured simultaneously Single Pulsed - CW M 2 Measurement IEEE 1394a FireWire Interface 12 Bit CCD nm 26

27 ModeScan 1780 Hardware 27

28 ModeScan 1780 Software Software Graphical User Interface All windows update in Real Time! 28

29 ModeScan 1780 Software ISO Standard M 2 Beam Parameters Reported in Real Time -M 2 Beam Propagation Ratio -Beam Waist Width -Beam Waist Location -Divergence -Rayleigh Length -Astigmatism -Beam Waist Asymmetry Divergence Asymmetry 29

30 Considerations for Measurement Instrument Geometry/Dynamic Range 10 Beam positions cover ~7.2 cm Dynamic Range of CCD allows ~3ZR Target Beam Waist Diameter 65μm Dwaist 300μm Optimal Rayleigh Range ZR cm Need to Match ZR in Test Space to Instrument Geometry/Dynamic Range 30

31 Considerations for Measurement System/Optical Setup Match ZR in Test Space to Instrument Geometry Three adjustment variables Lens Focal Length Laser-Lens Distance Laser-Instrument Distance Dependent on Laser Wavelength Laser Divergence Nominal M2 Value 31

32 ModeScan 1780 Operating Space Wavelength Range: nm (CCD Response) Divergence: f(ccd:d min, CCD:D max, M 2 ) M 2 =10 M 2 =1 32

33 Accuracy verification of new instrument technique (2% error bars) ModeScan 1780 Measurement Accuracy 33

34 ModeScan 1780 Conclusion New Instrument Features Real-Time Measurement Decrease in manufacturing QA time Statistical M 2 monitoring M 2 measurement of Single-Shot lasers In situ M 2 monitoring Use of M 2 as feedback while tuning laser cavity Monitoring M 2 during environmental changes Ease of use No Moving Parts 34

35 Conclusions There is a M2 measurement instrument best suited for your application Call for consultation and recommendations 35