What is it and how do you measure it?
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Transcript of What is it and how do you measure it?
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M2
What is it and how do you measure
it?
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M2, k-factor, or the Times-Diffraction-Limit
Beam Propagation Equation
0
2
4dM
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What does M2 mean?
Thus
42
0 Md
0
2
4dM
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What does M2 Mean?
For any given wavelength:
4
is a constant
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What does M2 mean?
• M2 is a measure of the laser’s focusability
• Ideal Gaussian TEM00 M2 should approach 1
• “M2 cannot be <1”
– Measurement is only a 2%
– Allowable error will sometimes show 0.9X
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What does M2 mean?
• Practically speaking it is a way for laser
manufacturers to “spec” their lasers…
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Laser Focus
• The larger the θ, the smaller d0
• The smaller the λ, the smaller d0
• The better the M2, (closer to 1), the more the
θ and λ control the focus
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The ISO Standard 11146
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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
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Issues with M2
• Most users want M2 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
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Rayleigh Method
4min
2 min
dzd
Mr
22
2
min2
zd
Mr
or
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Rayleigh Method
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Measurement Instruments
• Spiricon: M2 -200S
– Automated Measurement
• Photon: NanoModeScan
– Measurement of Any Wavelength
• Photon: MS-1780
– Instantaneous Measurement
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M2-200S
• CCD Camera Based
• Fully Automated
Operation
• Reports ISO
Parameters
– M2
– Divergence
– Rayleigh Range
– Waist size and
Position
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M2-200S Operation
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M2-200S Operation
• Mirror train varies beam path length
• 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
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Dedicated M2-200S Software Package with
Ultracal®
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NanoModeScan
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ModeScan Principles
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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 (200-900nm)
– Germanium for NIR (700-1800nm)
– 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
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ModeScan Report
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ISO M2 Curve Fit
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ISO Measurement Window
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Rayleigh Measurement Window
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ModeScan 1780 Real-Time M2 Measurement
First Camera System Based on Patent Concept
Introduced in 2007
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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 M2 Measurement
• IEEE 1394a “FireWire” Interface
• 12 Bit CCD
• 250-1100nm
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ModeScan 1780 Hardware
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ModeScan 1780 Software
Software Graphical User Interface
All windows update in Real Time!
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ModeScan 1780 Software
ISO 11146 Standard M2 Beam Parameters
Reported in Real Time -M2 Beam Propagation Ratio
-Beam Waist Width
-Beam Waist Location
-Divergence
-Rayleigh Length
-Astigmatism
-Beam Waist Asymmetry
Divergence Asymmetry
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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
– 1.2—1.8cm
• Need to Match ZR in Test Space to Instrument
Geometry/Dynamic Range
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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
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ModeScan 1780 Operating Space
Wavelength Range: 250-1100 nm (CCD Response)
Divergence: f(CCD:Dmin, CCD:Dmax, M2)
M2=1
M2=10
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Accuracy verification of new instrument
technique (2% error bars)
ModeScan 1780 Measurement Accuracy
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ModeScan 1780 Conclusion
New Instrument Features
Real-Time Measurement Decrease in manufacturing QA time
Statistical M2 monitoring
M2 measurement of Single-Shot lasers
In situ M2 monitoring
Use of M2 as feedback while tuning laser cavity
Monitoring M2 during environmental changes
Ease of use
No Moving Parts
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Conclusions
• There is a M2 measurement instrument best
suited for your application
• Call for consultation and recommendations