Research Article Stroboscopic Surface Thermal...

Research ArticleStroboscopic Surface Thermal Lensing for Fast Detection ofThermal Defects in Large-Scaled Coating Films

Chunxian Tao Dawei Zhang Ruijin Hong and Zhongfei Wang

Shanghai Key Laboratory of Modern Optical System Optical Instruments and Systems Engineering Research CenterMinistry of Education University of Shanghai for Science and Technology Shanghai 200093 China

Correspondence should be addressed to Dawei Zhang dwzhangussteducn

Received 28 January 2015 Revised 27 April 2015 Accepted 14 May 2015

Academic Editor Shoude Chang

Copyright copy 2015 Chunxian Tao et al This is an open access article distributed under the Creative Commons Attribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited

A stroboscopic surface thermal lensing (SSTL) system for the fast detection of thermal-induced defects in large-scaled opticalcoating films was constructed The SSTL signal was generated by a set of double-modulators and captured by a high speed matrixcamera respectively The spot size of both pump laser and probe laser expanded for larger detection area was finished in a singlestep Based on the STL technique both the mapping of amplitude and the phase of SSTL signal on the whole area of the coatingscan be achieved simultaneously

1 Introduction

In the laser-induced damage of coating films absorption ofthe radiated laser energy is the primary cause of damage[1 2] Thermal characterization of the radiated coating filmsis meaningful in defect identification and a supplementarymeans for laser-induced damage threshold enhancement [3]Mathematical analysis and numerical simulations of heat-ing processes of photothermal effect have been intensivelystudied [4ndash7] both for the interpretation of measurementssignal and for the development of measurement methodsapplied in the design and quality control [8ndash10] Until nowthe photothermal microscopy has been used for absorptionand defects detection in crystals or optical coatings Thesurface thermal lensing technique has become a popularmethod with the merit of high sensitivity However the STLhas been used only in small sample due to its limited speedand stability The requirements of high speed and efficiencyin measurement of large-scaled optical coating films arebecoming more and more urgent

The lock-in theory is widely used in thermography fornondestructive detection such as lock-in thermal IR imagingor thermal wave lock-in imaging [11ndash13] With the help of IRor visible charge coupled device (CCD) camera the opticallock-in has been brought out and used in topography imagery

[14 15] Due to the advantage of whole field detectionthe optical lock-in has the potential of fast and effectivedetection In this paper based on the optical lock-in methoda stroboscopic surface thermal lensing (STL) system wasconstructed and introduced into the fast defects identificationin large-scaled coating films By both pumping and probingbeam modulation configuration the mapping of the STLsignal can be captured by an array CCD camera Both theamplitude and the phase of the modulated thermal responsegiven information on the thermal-physical properties of thesample were investigated

2 Principle of Stroboscopic SurfaceThermal Lensing

When the thin film coating is irradiated by a pump lightthe absorption of optical energy by the film sample causeslocal heating which produces special physical phenomenain the sample The surface thermal lensing effect inducedby heat absorption in coating film can be interpreted asa spatial distribution of the refraction index dependent onthe energy distribution of the irradiation light beam Thefundamental mode laser of Gaussian beam or flat-toppedlight beam is used as the pump light [16 17] Based on the

Hindawi Publishing Corporatione Scientific World JournalVolume 2015 Article ID 942138 5 pageshttpdxdoiorg1011552015942138

2 The Scientific World Journal

Sampleplane

x

z

y

Detectionplane

Probewaist

120572

120573

Od

ZsdZsw

OsO

Figure 1 Schematic diagram of stroboscopic surface thermal lensing

Power meter

Probe laser

Computer

Chopper

Beam splitter

AttenuatorReflector

Transfer stageFilter

Attenuator

Sam

ple

Drivers

Focus lens

CCD

Beam expanderChopper

Pump laser

Figure 2 Schematic diagram of stroboscopic surface thermal lensing and signal process

Fresnel diffraction theory the reflected electric field of probebeam (fundamental Gaussian mode) on the detection planeis expressed as [9]

119864 (119903 119905) =119888119890119894 arctan(119911sw119891)+119894119896(minus119911sw+119911sd)

119894120582119911sd1205961

sdot int

infin

minusinfin

int

infin

minusinfin

119890(minus1198941198962119902)(1205722+1205732)minus2119894119896119906(119903119905)+(1198941198962119911sd)[(119909minus120572)2+(119910minus120573)2]119889120572119889120573

(1)

where 119888 is a constant 119896 is the wave number 120582 is thewavelength 119911sw is the distances from the probe beam waistto the sample plane and 119911sd is the distances from the sampleplane to the detection planeThe heat deformation due to theabsorption of the film is expressed as 119906(119903 119905) and the centralhighness is proportional to the desired absorptance 119860 Theschematic diagram of the surface thermal lensing is shown inFigure 1

Although there aremanymethods to demodulate the STLsignal as described in (3) it would be desirable to have asynchronous detection to all the points in a larger excitedarea To this purpose optical lock-in system of stroboscopicphotography with large pump laser spot size was constructedwith the matrix camera There is an additional modulator 2of probe light used for realization of the double-modulationof optical-locking in And thus the stroboscopic signal of theSTL can be captured by the CCD which is controlled by thesame counter in the sequence The schematic of stroboscopicsurface thermal lensing is shown in Figure 2

According to the correlation function of optical lock-insignal the lock-in device multiplies the observed signal withtwo reference functions at the frequency119891 10Hz respectivelywhich are quadrature with each other And then it integratesboth products by integrator to give the in-phase (V

119904sin 120579119904) and

quadrature phase (V119904cos 120579119904) components [18] Therefore 120579

119904

and V119904of the detecting signal are derived which represents

the RMS amplitude and the phase difference respectively Inour SSTL system the modulation frequency 119891 and phasesdifference Δ120593(119905) between the probe and pump are controlledby the counter driver (NI PCI6601) The digital phase-shiftdevice composed of two electronic fast shutters is triggered byspecified pulse trains generated by a digital pulse generator

The four-point correlation meets our requirement invirtue of popular modulation mode more noise-immunityonline operation and high efficiency Generally in sinusoidalwave four frames of equidistant signal data are enough toderive the phase shift 120579

119904and the amplitude V

119904as

120579119904= arctan(

1198781 minus 11987831198782 minus 1198784

)

V119904= radic(1198781 minus 1198783)

2+ (1198782 minus 1198784)

2

(2)

where 1198781 1198782 1198783 and 119878

4are stroboscopic STL signals on the

pixel (119909 119910) of the film with phase shift 1205872 as shown inFigure 3(c) To obtain these four kinds of signal the phaseshift between the pump and probe triggers pulse sequencesspecified as in Figures 3(a) and 3(b) And thus at least four

The Scientific World Journal 3

OFF ON OFF ON OFFONOFF

Pumppulse

Probepulse

Cameracapture

ON

(a)

(b)

(c)

Δ1205931 = 1205872 Δ1205932 = 120587 Δ1205933 = 31205872Δt

S1 S2 S3 S4

Counter

CCD arraySSTL

Modulator 2

Modulator 1

Signal recorder

Pum

pPr

obe

NfΔ1205930 = 0

Figure 3 Pulse sequence of double-modulation of four-point correlation for stroboscopic surface thermal lensing

phase differences called in-phase and quadrature phase areobtained

Δ120601 (119905) = 120601probe minus120601pumb = 2120587119891119905119894=

0 120587

120587

2

31205872

(3)

To obtain maximum output phase differences of integralmultiple quarter wave are the best choice Consequently theselected four detecting time points of CCD camera are

119905119894=

0 12119891

14119891

34119891

(4)

The theoretical calculation of the SSTL measurementdistribution based on this system is shown in Figure 4 Twotypical absorption defects (Au particles in nm scale) areembedded in the subsurface of the Ta

2O5coating film The

SSTL has the ability of identifying the defects on the wholelaser pump field at a time and provides a great help formeasurement of large-scaled optical films The test area isabout 10 times as much as that of the single point of STLdetermined by the distance between the laser source andthe sample the energy density and the beam expanderThis mapping process is a development of (3) to the wholeradiation area of pump beam such as top-hat beam [19]Comparedwith the single pixel detection the image sensor ofcamera ismade up of amounts of small photosensor elementseach of them may act as the required lock-in detector arrayConsequently large amounts of points of the sample can bedetected simultaneously

3 Analysis of Detection Errors

This solution of (3) can be interpreted as strobe frequencyor stroboscopic photography When an oscillatory sample atfrequency 119891 with initial phase 120572 is irradiated by a pulsedlight source at 119891pulse once the condition Δ119891 equiv 119891pulse minus 119891 =

0 is satisfied the phase 120572 of the vibration will be frozenAnd thus the image of the sample with this fixed phase canbe repeatedly captured by the CCD camera Consequentlyfirst the demand on high capture speed can be overlookedSecondly integration in multiple capture cycle is helpful toreduce random noise

The measuring accuracy is another key character formeasurementThe captured images of the sample in119873 cyclescan be expressed as the product of the vibration function ofthe pumping probe beam and the pulse array of the probingbeam

1199041015840= int

119873119891

0

1 + 119890119894(2120587119891119905+120572)

2

119873

sum

0120575 (119905 minus 1199050) 119889119905

=1 + 119890119894(21205871198911199050+120572)

2

(5)

where 1199050is the point in the capturing time Since the

integration is in a period of time pulse width Δ119905 the practicalsignal can be expressed as

119904 = int

1199050+Δ119905

1199050

11990410158401198891199050 =

12

119873

sum

0(Δ119905 +

119890119895(2120587119891Δ119905)

minus 12119895120587119891

119890119895(21205871198911199050+120572))

asymp

119873

sum

0

1 + 119890119895(21205871198911199050+120572)

2Δ119905

(6)


0010

0005

0000

minus0005

0005

0005

0000

0000minus0005

minus0005

0004

0002

0000

minus0004

minus0002

000400020000minus0004 minus0002

Figure 4 Mapping of the stroboscopic surface thermal lensing in large-scaled radiation area

From (6) we find that the measuring accuracy is onlya function of the pulse width With the fixed modulationfrequency the pulse width of image acquisition time shouldbe reduced as much as possible which can greatly improvethe accuracy of measurement

4 Conclusions

The stroboscopic and the surface thermal lensing was inte-grated for fast defects detection of optical coatings andcrystals Using a matrix CCD camera with high frame ratethe mapping of amplitude and phase of SSTL on a largearea of thin coatings could be obtained simultaneously Thestroboscopic STL technique being proposed to realize fastmeasurement of weak absorption and defects identificationin large-scaled optical coating films can provide a new andeffective diagnostic tool for laser conditioning on large-scaledthin coating films

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

Acknowledgments

This work was partially supported by the NationalScience Instrument Important Project (2011YQ140147042011YQ15004003) National High Technology Researchand Development Program of China (863 Program)(2013AA030602) and Shanghai Ultra-Precision OpticalManufacture and Testing Center Programs from ShanghaiCommittee of Science amp Technology (11DZ2290301)

References

[1] B Bertussi J-Y Natoli and M Commandre ldquoHigh-resolutionphotothermal microscope a sensitive tool for the detection ofisolated absorbing defects in optical coatingsrdquo Applied Opticsvol 45 no 7 pp 1410ndash1415 2006

[2] R Chow J R Taylor and Z L Wu ldquoAbsorptance behaviorof optical coatings for high-average-power laser applicationsrdquoApplied Optics vol 39 no 4 pp 650ndash658 2000

[3] D-W Zhang Y-SHuang Z-J Ni S-L Zhuang J-D Shao andZ-X Fan ldquoPreparation of high laser induced damage thresholdantireflection film using interrupted ion assisted depositionrdquoOptics Express vol 15 no 17 pp 10753ndash10760 2007

[4] Z L Wu C J Stolz S C Weakley J D Hughes and Q ZhaoldquoDamage threshold prediction of hafnia-silica multilayer coat-ings by nondestructive evaluation of fluence-limiting defectsrdquoApplied Optics vol 40 no 12 pp 1897ndash1906 2001

[5] S Wu and N J Dovichi ldquoFresnel diffraction theory for steady-state thermal lens measurements in thin filmsrdquo Journal ofApplied Physics vol 67 no 3 pp 1170ndash1182 1990

[6] Y Han Z L Wu J S Rosenshein M Thomsen Q Zhao andK Moncur ldquoPulsed photothermal deflection and diffractioneffects numerical modeling based on Fresnel diffraction the-oryrdquo Optical Engineering vol 38 no 12 pp 2122ndash2128 1999

[7] S H Fan H B He Z X Fan J D Shao and Y A ZhaoldquoTheory and experiment of surface thermal lens technique usedin absorption measurement of thin filmsrdquo Acta Physica Sinicavol 54 no 12 pp 5774ndash5777 2005

[8] L Gallais and M Commandre ldquoSimultaneous absorptionscattering and luminescence mappings for the characterizationof optical coatings and surfacesrdquo Applied Optics vol 45 no 7pp 1416ndash1424 2006

[9] J W Fang and S Y Zhang ldquoModeling for laser-induced surfacethermal lens in semiconductorsrdquo Applied Physics B vol 67 no5 pp 633ndash639 1998


[10] Z L Wu P K Kuo Y S Lu S T Gu and R KrupkaldquoNon-destructive evaluation of thin film coatings using a laser-induced surface thermal lensing effectrdquo Thin Solid Films vol290-291 pp 271ndash277 1996

[11] L C Malacarne F Sato P R B Pedreira et al ldquoNanoscalesurface displacement detection in high absorbing solids bytime-resolved thermal mirrorrdquo Applied Physics Letters vol 92no 13 Article ID 131903 2008

[12] O Breitenstein M Langenkamp F Altmann D Katzer ALindner and H Eggers ldquoMicroscopic lock-in thermographyinvestigation of leakage sites in integrated circuitsrdquo Review ofScientific Instruments vol 71 no 11 pp 4155ndash4160 2000

[13] S Grauby B C Forget S Hole and D Fournier ldquoHigh res-olution photothermal imaging of high frequency phenomenausing a visible charge coupled device camera associated with amultichannel lock-in schemerdquo Review of Scientific Instrumentsvol 70 no 9 pp 3603ndash3606 1999

[14] P M Mayer D Luerssen R J Ram and J A HudgingsldquoTheoretical and experimental investigation of the thermalresolution and dynamic range of CCD-based thermoreflectanceimagingrdquo Journal of the Optical Society of America A Optics andImage Science and Vision vol 24 no 4 pp 1156ndash1163 2007

[15] A Dubois A C Boccara and M Lebec ldquoReal-time reflectivityand topography imagery of depth-resolved microscopic sur-facesrdquo Optics Letters vol 24 no 5 pp 309ndash311 1999

[16] B Li S Martin and E Welsch ldquoIn situ measurement onultraviolet dielectric components by a pulsed top-hat beamthermal lensrdquo Applied Optics vol 39 no 25 pp 4690ndash46972000

[17] B Li S Xiong and Y Zhang ldquoFresnel diffraction model formode-mismatched thermal lens with top-hat beam excitationrdquoApplied Physics B Lasers and Optics vol 80 no 4-5 pp 527ndash534 2005

[18] E Beaurepaire A C Boccara M Lebec L Blanchot and HSaint-Jalmes ldquoFull-field optical coherence microscopyrdquo OpticsLetters vol 23 no 4 pp 244ndash246 1998

[19] B C Li X X Chen and Y Gong ldquoAnalysis of surface thermallens signal in optical coatings with top-hat beam excitationrdquoJournal of Applied Physics vol 103 no 3 Article ID 0335182008

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Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

High Energy PhysicsAdvances in

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014


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Atomic and Molecular Physics

Journal of



Advances in Condensed Matter Physics

OpticsInternational Journal of



AstronomyAdvances in

International Journal of


Superconductivity


Statistical MechanicsInternational Journal of


GravityJournal of


AstrophysicsJournal of


Physics Research International


Solid State PhysicsJournal of

Computational Methods in Physics

Journal of



Soft MatterJournal of

Hindawi Publishing Corporationhttpwwwhindawicom

AerodynamicsJournal of

Volume 2014


PhotonicsJournal of


Journal of

Biophysics


ThermodynamicsJournal of


Sampleplane

x

z

y

Detectionplane

Probewaist

120572

120573

Od

ZsdZsw

OsO

Figure 1 Schematic diagram of stroboscopic surface thermal lensing

Power meter

Probe laser

Computer

Chopper

Beam splitter

AttenuatorReflector

Transfer stageFilter

Attenuator

Sam

ple

Drivers

Focus lens

CCD

Beam expanderChopper

Pump laser

Figure 2 Schematic diagram of stroboscopic surface thermal lensing and signal process

Fresnel diffraction theory the reflected electric field of probebeam (fundamental Gaussian mode) on the detection planeis expressed as [9]

119864 (119903 119905) =119888119890119894 arctan(119911sw119891)+119894119896(minus119911sw+119911sd)

119894120582119911sd1205961

sdot int

infin

minusinfin

int

infin

minusinfin

119890(minus1198941198962119902)(1205722+1205732)minus2119894119896119906(119903119905)+(1198941198962119911sd)[(119909minus120572)2+(119910minus120573)2]119889120572119889120573

(1)

where 119888 is a constant 119896 is the wave number 120582 is thewavelength 119911sw is the distances from the probe beam waistto the sample plane and 119911sd is the distances from the sampleplane to the detection planeThe heat deformation due to theabsorption of the film is expressed as 119906(119903 119905) and the centralhighness is proportional to the desired absorptance 119860 Theschematic diagram of the surface thermal lensing is shown inFigure 1

Although there aremanymethods to demodulate the STLsignal as described in (3) it would be desirable to have asynchronous detection to all the points in a larger excitedarea To this purpose optical lock-in system of stroboscopicphotography with large pump laser spot size was constructedwith the matrix camera There is an additional modulator 2of probe light used for realization of the double-modulationof optical-locking in And thus the stroboscopic signal of theSTL can be captured by the CCD which is controlled by thesame counter in the sequence The schematic of stroboscopicsurface thermal lensing is shown in Figure 2

According to the correlation function of optical lock-insignal the lock-in device multiplies the observed signal withtwo reference functions at the frequency119891 10Hz respectivelywhich are quadrature with each other And then it integratesboth products by integrator to give the in-phase (V

119904sin 120579119904) and

quadrature phase (V119904cos 120579119904) components [18] Therefore 120579

119904

and V119904of the detecting signal are derived which represents

the RMS amplitude and the phase difference respectively Inour SSTL system the modulation frequency 119891 and phasesdifference Δ120593(119905) between the probe and pump are controlledby the counter driver (NI PCI6601) The digital phase-shiftdevice composed of two electronic fast shutters is triggered byspecified pulse trains generated by a digital pulse generator

The four-point correlation meets our requirement invirtue of popular modulation mode more noise-immunityonline operation and high efficiency Generally in sinusoidalwave four frames of equidistant signal data are enough toderive the phase shift 120579

119904and the amplitude V

119904as

120579119904= arctan(

1198781 minus 11987831198782 minus 1198784

)

V119904= radic(1198781 minus 1198783)

2+ (1198782 minus 1198784)

2

(2)

where 1198781 1198782 1198783 and 119878

4are stroboscopic STL signals on the

pixel (119909 119910) of the film with phase shift 1205872 as shown inFigure 3(c) To obtain these four kinds of signal the phaseshift between the pump and probe triggers pulse sequencesspecified as in Figures 3(a) and 3(b) And thus at least four



Pumppulse

Probepulse

Cameracapture

ON

(a)

(b)

(c)

Δ1205931 = 1205872 Δ1205932 = 120587 Δ1205933 = 31205872Δt

S1 S2 S3 S4

Counter

CCD arraySSTL

Modulator 2

Modulator 1

Signal recorder

Pum

pPr

obe

NfΔ1205930 = 0




0 120587

120587

2

31205872

(3)


119905119894=

0 12119891

14119891

34119891

(4)


2O5coating film The






1199041015840= int

119873119891

0

1 + 119890119894(2120587119891119905+120572)

2

119873

sum

0120575 (119905 minus 1199050) 119889119905

=1 + 119890119894(21205871198911199050+120572)

2

(5)



119904 = int

1199050+Δ119905

1199050

11990410158401198891199050 =

12

119873

sum

0(Δ119905 +

119890119895(2120587119891Δ119905)

minus 12119895120587119891

119890119895(21205871198911199050+120572))

asymp

119873

sum

0

1 + 119890119895(21205871198911199050+120572)

2Δ119905

(6)


0010

0005

0000

minus0005

0005

0005

0000

0000minus0005

minus0005

0004

0002

0000

minus0004

minus0002

000400020000minus0004 minus0002



4 Conclusions




Acknowledgments


References


























FluidsJournal of


Journal of










Superconductivity




GravityJournal of








Journal of






Volume 2014


PhotonicsJournal of


Journal of

Biophysics





Pumppulse

Probepulse

Cameracapture

ON

(a)

(b)

(c)

Δ1205931 = 1205872 Δ1205932 = 120587 Δ1205933 = 31205872Δt

S1 S2 S3 S4

Counter

CCD arraySSTL

Modulator 2

Modulator 1

Signal recorder

Pum

pPr

obe

NfΔ1205930 = 0




0 120587

120587

2

31205872

(3)


119905119894=

0 12119891

14119891

34119891

(4)


2O5coating film The






1199041015840= int

119873119891

0

1 + 119890119894(2120587119891119905+120572)

2

119873

sum

0120575 (119905 minus 1199050) 119889119905

=1 + 119890119894(21205871198911199050+120572)

2

(5)



119904 = int

1199050+Δ119905

1199050

11990410158401198891199050 =

12

119873

sum

0(Δ119905 +

119890119895(2120587119891Δ119905)

minus 12119895120587119891

119890119895(21205871198911199050+120572))

asymp

119873

sum

0

1 + 119890119895(21205871198911199050+120572)

2Δ119905

(6)


0010

0005

0000

minus0005

0005

0005

0000

0000minus0005

minus0005

0004

0002

0000

minus0004

minus0002

000400020000minus0004 minus0002



4 Conclusions




Acknowledgments


References


























FluidsJournal of


Journal of










Superconductivity




GravityJournal of








Journal of






Volume 2014


PhotonicsJournal of


Journal of

Biophysics




0010

0005

0000

minus0005

0005

0005

0000

0000minus0005

minus0005

0004

0002

0000

minus0004

minus0002

000400020000minus0004 minus0002



4 Conclusions




Acknowledgments


References


























FluidsJournal of


Journal of










Superconductivity




GravityJournal of








Journal of






Volume 2014


PhotonicsJournal of


Journal of

Biophysics



















FluidsJournal of


Journal of










Superconductivity




GravityJournal of








Journal of






Volume 2014


PhotonicsJournal of


Journal of

Biophysics



Research Article Stroboscopic Surface Thermal...

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