Characterizing laser beams in general and laser spots for LID experiments on targets - a comparison

Transcription

1 INDLAS 2013, LID-LBC Symposium, Bran, 05/22/13 Characterizing laser beams in general and laser spots for LID experiments on targets - a comparison George Nemeş 1,2, Aurel Stratan 1, Alexandru Zorilă 1,3 1 ISOTEST Laboratory, NILPRP, 409 Atomiştilor Str., Măgurele - Bucharest, Romania 2 ASTiGMAT TM, 3409 Pecky Cedar Ct., Sacramento, CA 95827, USA; gnemes98@hotmail.com 3 "Politehnica" University of Bucharest, 313 Splaiul Independenţei, Bucharest, Romania

2 Acknowledgments - Sponsors: POS CCE "Investments for your future" Ministry of National Education, Romania - Co-sponsor: European Fund for Regional Development (EFRD/FEDR) - Invited guests - supporting LID-LBC mini-symposium - "Horia Hulubei" NIPNE/IFIN, Magurele - Bucharest - sponsoring invited guests - Audience - interest Official project number and title: Project POS CCE No. 172 / 2010 "Facility for laser beam diagnosis and ISO characterization / certification of behavior of optical components/materials subjected to high power laser beams / ISOTEST"

3 OUTLINE 1. Introduction 2. Beams and spots comparison of relevant ISO standards 3. Results - spatial beam and spatial spot characterization 4. Results - measuring effective pulse duration 5. Conclusion

4 1. Introduction Motivation ISOTEST Project Involvement in "beam diagnosis" "Beam diagnosis" to measure laser beams and for LID (laser spots) - different Beam characterization Specific ISO standards Energetic characteristics (ISO 11554:2006) - Average power: P ave (W) - Pulse energy: E p (J) - Pulse peak power: P pk (W) Temporal characteristics (ISO 11554:2006) - Pulse repetition frequency, f rep (Hz) - Pulse shape - Pulse duration, t FWHM (s) Spatial characteristics (ISO ,2,3:2004,2005) + some tutorial - Formal beam definition second-moments matrix (Nemes, Siegman, Serna) - Beams - geometrical classification: ST, ASA, RSA, GA (Arnaud, Kogelnik, Nemes, Siegman) - Beams - intrinsic invariants at propagation intrinsic classification (Nemes, Siegman)

5 Introduction Spatial characteristics - Beam profiling at different transverse locations along z - Determining the geometrical symmetry of the beam: ST, ASA, RSA, GA For ST and ASA beams, main quantities of interest: - Spot sizes, waist sizes: D, D x, D y, D 0, D 0x, D 0y (mm) - Waists locations: z 0, z 0x, z 0y (mm) - Rayleigh ranges (lengths): z R, z Rx, z Ry (mm) - Divergences: θ, θ x, θ y (rad) - Beam propagation ratios (Siegman, ISO ): M 2, M x2, M y 2 For all types of beams: ST, ASA, RSA, GA intrinsic invariants at propagation - Effective beam propagation ratio (Nemes + Siegman, ISO ,3): M eff4 1 - Intrinsic astigmatism (Nemes + Siegman, ISO ,3): a 0 - Maximum intrinsic astigmatism (Nemes): a M M eff 4 (not in ISO standard); 0 a a M IS a = 0 Type I (a = 0, a M = 0) Type II (a = 0, a M > 0) IA a > 0 Type III (a > 0, a M > 0; a M > a) Type IV (a > 0, a M > 0; a M = a)

6 Introduction Tutorial: formal beam definition 1 Paraxial θ h θ M = 1/π rad = M 2 w 0 /λ 2 Minimum size >> λ w 0 /λ 10 3 Uncertainty relation M Extension - < z < + ST light distributions: B - beams; Q-B - quasi-beams; N-B - non-beams

7 Introduction Tutorial: geometrical beam classification ST ASA RSA GA Uses matrix properties and free-space spot symmetry Free-space spot symmetry alone - too weak as criterion GA pseudo-symmetric beams

8 Introduction Tutorial: intrinsic beam classification All types of beams: ST, ASA, RSA, GA invariants P - beam matrix (2MM) Δ = detp > 0; T = tr[(pj) 2 ] < 0; k = 2π/λ a = k 2 ( T - 4Δ 1/2 ) 0 M eff4 = 4k 2 Δ 1/2 1 a M = (1/2)(M eff4-1) 2 a - Effective beam propagation ratio (Nemes + Siegman, ISO ,3): M eff4 1 - Intrinsic astigmatism (Nemes + Siegman, ISO ,3): a 0 - Maximum intrinsic astigmatism (Nemes): a M M eff 4 (not in ISO standard); 0 a a M IS: a = 0 Type I (a = 0, a M = 0) Type II (a = 0, a M > 0) IA: a > 0 Type III (a > 0, a M > 0; a M > a) Type IV (a > 0, a M > 0; a M = a) (a, a M ) (M x2, M y2 ) only for ASA / ST beams - Each pair (a, a M ) set of equivalent beams

9 Tutorial Geometrical, intrinsic, and combined beam classification

10 2. Beams and spots comparison of relevant ISO standards Beam characterization deals with beams Spatial properties along z (ISO ,2,3:2004,2005) Temporal properties (ISO 11554:2006) Energetic properties (ISO 11554:2006) LID experiments spot (on target) (ISO :2011) - for surface damage Spatial properties at z = constant plane - normal beam incidence on sample z(x, y) - non-normal (oblique) incidence on sample Temporal properties (ISO 11554:2006) Energetic properties (ISO 11554:2006) Combined properties (ISO 13694:2000/Corr:2005) Laser power / energy density distribution

11 Beams and spots Spatial beam characterization transverse and longitudinal beam profile (ISO 11146) Spatial spot characterization transverse spot profile (ISO 21254) Laser power/energy density distribution transverse spot profile (ISO 13694) Spot concept clip level (threshold) of peak fluence to define the spot: η H CL = H η = ηh pk ; 0 η < 1 Comparison beam vs. spot - Beams "propagation" (behavior along z ) is important - Spots "propagation" (behavior along z ) is not important - Some confusing names do exist between the last two standards dealing with spots - Our approach: rename the confusing quantities and use them in LIDT standards

12 Spots Relevant concepts / quantities for LIDT (ISO ) - Pulsed laser spot - A eff : Effective area of pulsed laser spot main quantity for fluence LIDT evaluation A eff = Q/H pk (cm 2 ) Q = H(x,y)dxdy H(x,y) local fluence distribution in the spot (J/cm 2 ) H(x,y) H pk y Q total pulse energy (J) A eff H pk peak (maximum) laser fluence in the spot (J/cm 2 ) - d eff : Effective spot diameter (mm) Derived quantity from A eff : A eff = (π/4)d eff 2 x A eff Effective diameter concept Effective area concept d eff

13 Spots Related quantities defined in ISO 13694: A η : Effective irradiation area area within the spot for which the fluence exceeds a threshold fluence, H η Confusing name! -A η is measured by CCD beam profilers, A eff is not - Rename A η as A CL clip-level excess area H(x,y) H pk H η = H CL = ηh pk clip-level fluence - Effective (clip-level excess) energy, Q η = Q CL pulse energy evaluated over spot coordinates x, y, for which H(x,y) H CL - Average effective (clip-level excess) fluence, H ηave A η = A CL Q CL y H CLave H η = ηh pk = H CL x H ηave = H CLave = Q η /A η = Q CL /A CL - Fractional energy, f η = Q CL /Q Quantities of ISO Flatness factor, F η = H CLave /H pk

14 Defining new relevant spot concepts / quantities Motivation - Avoiding confusion: A eff A η - both named "effective area" - Not all spots are round (Ex: excimer lasers; diode stack lasers) - Spots with the same A eff can have different fluence profile different damaging properties Results - Using 2MM - defined spot shape/sizes and rescaling for ideal distributions Effective spot boundary (elliptical - aligned or tilted; round; rectangular; square; doughnut) Effective spot sizes (appropriate for the spot shape)

15 Defining new relevant spot concepts / quantities Results - Using idealized fluence distributions to approximate the real one with: Cylindrical (uniform) Conical trunk Gaussian Conical - Using the flatness factor, F η, to characterize the real fluence distribution: F = Q real distribution above CL /Q flat-top above CL = Q real distribution above CL /(A CL H pk ) Need A CL Measured with CCD or simpler See next

16 3. Results - spatial beam and spatial spot characterization Measuring clip-level (threshold) spot area, A CL Principle and schematic Results ε = (A CL-Microscope -A CL-CCD )/A CL-CCD

17 Results: Measuring A eff and d eff Schematic CCD camera measures Q and H pk Noise influence Matlab simulation: theoretical profiles (supergaussians) + noise (zero-mean + offset) Findings - Accuracy - affected by zero-mean noise; less affected by background offset (contrary to second-moments defined spot size) - Precision - less affected by both - Spot size / pixel size important Results - CCD Beam profiler 14 bit, 1200 x 1600 pixels, 4.6 μm pixel size - Spot size - hundreds of pixels - Expanded uncertainty for A eff : 6%; 3%

18 Results: spatial beam characterization - Measured different lasers: He-Ne, several Nd:YAG, Ti:sapphire - Conventional schematic: Laser + focusing lens + longitudinally moving CCD camera - Preliminary results on a new method, using rotating cylindrical optics Several contributing papers on these measurements

19 4. Results - measuring effective pulse duration + Definition: t eff = Q/P pk ; Q = P(t)dt Q - total energy of the pulse incident on the detection system aperture P(t) - instantaneous power of the incident pulse P pk - pulse peak power of the incident pulse Typical temporal profile, 1064 nm t eff = 5.8 ns; t FWHM = 4.4 ns Definition Temporal pulse profile (a.u.) Time (fs) Measuring method Typical temporal profile: t eff = 280 fs; t FWHM = 250 fs

20 5. Conclusion - Laser beam characterization and laser spot characterization - different - Implemented ISO standard methods for beam and spot characterization - Compared different ISO standards used to characterize laser spots - We suggest that existing quantities to be "imported" to other ISO standards - We suggest more appropriate names for them - New useful, measurable quantities were defined - Implemented ISO methods to characterize laser beams and laser spots - Performed temporal, energetic, and spatial beam and spot characterization - Implemented new method for spatial beam characterization promising results