M2

What is it and how do you measure

it?

2

M2, k-factor, or the Times-Diffraction-Limit

Beam Propagation Equation

0

2

4dM

3

What does M2 mean?

Thus

42

0 Md

0

2

4dM

4

What does M2 Mean?

For any given wavelength:

4

is a constant

5

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

6

What does M2 mean?

• Practically speaking it is a way for laser

manufacturers to “spec” their lasers…

7

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

8

The ISO Standard 11146

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

10

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

11

Rayleigh Method

4min

2 min

dzd

Mr

22

2

min2

zd

Mr

or

12

Rayleigh Method

13

Measurement Instruments

• Spiricon: M2 -200S

– Automated Measurement

• Photon: NanoModeScan

– Measurement of Any Wavelength

• Photon: MS-1780

– Instantaneous Measurement

14

M2-200S

• CCD Camera Based

• Fully Automated

Operation

• Reports ISO

Parameters

– M2

– Divergence

– Rayleigh Range

– Waist size and

Position

15

M2-200S Operation

16

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

17

Dedicated M2-200S Software Package with

Ultracal®

18

NanoModeScan

19

ModeScan Principles

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 (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

21

ModeScan Report

22

ISO M2 Curve Fit

23

ISO Measurement Window

24

Rayleigh Measurement Window

25

ModeScan 1780 Real-Time M2 Measurement

First Camera System Based on Patent Concept

Introduced in 2007

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 M2 Measurement

• IEEE 1394a “FireWire” Interface

• 12 Bit CCD

• 250-1100nm

27

ModeScan 1780 Hardware

28

ModeScan 1780 Software

Software Graphical User Interface

All windows update in Real Time!

29

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

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

– 1.2—1.8cm

• Need to Match ZR in Test Space to Instrument

Geometry/Dynamic Range

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

32

ModeScan 1780 Operating Space

Wavelength Range: 250-1100 nm (CCD Response)

Divergence: f(CCD:Dmin, CCD:Dmax, M2)

M2=1

M2=10

33

Accuracy verification of new instrument

technique (2% error bars)

ModeScan 1780 Measurement Accuracy

34

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

35

Conclusions

• There is a M2 measurement instrument best

suited for your application

• Call for consultation and recommendations