Optics Communications 283 (2010) 2728–2731

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Optics Communications

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Laser conditioning of high-reflective and anti-reflective coatings invacuum environments

Xiulan Ling, a,b,c,⁎, Yuanan Zhao a, Dawei Li a, Jianda Shao a, Zhengxiu Fan a

a Key Laboratory of Material Science and Technology for High Power Lasers, Shanghai Institute of Optics and Fine Mechanics, Shanghai 201800, Chinab Graduate School, Chinese Academy of Sciences, Beijing 100039, Chinac Department of Information Engineering, North University of China, Taiyuan 030051, China

⁎ Corresponding author. Key Laboratory of Material ScPower Lasers, Shanghai Institute of Optics and Fine MechTel.: +86 21 69918478.

E-mail address: nmlxlmiao@126.com (X. Ling,).

0030-4018/$ – see front matter © 2010 Elsevier B.V. Adoi:10.1016/j.optcom.2010.03.014

a b s t r a c t

a r t i c l e i n f o

Article history:Received 11 November 2009Received in revised form 10 February 2010Accepted 5 March 2010

Keywords:VacuumLaser-induced damageLaser conditioningDefect statistical model

Laser conditioning effects of the dielectric mirror coatings in vacuum environments were investigated. Thelaser-induced damage thresholds (LIDT) in vacuum environments before and after laser conditioning werecompared. It is found that laser conditioning in vacuum environments decrease the LIDT of the component.Laser conditioning effects in vacuum and atmosphere environments were also compared and investigated.The negative effects of laser conditioning in vacuum environments were discussed and analyzed with defectstatistical model, energy dispersive X-ray analysis and absorption measurements.

ience and Technology for Highanics, Shanghai 201800, China.

ll rights reserved.

© 2010 Elsevier B.V. All rights reserved.

1. Introduction

Laser induced damage in optical materials is a key issue for highpower laser applications, and a significant number of studies havebeen made about how to improve laser induced damage resistance ofoptical materials. Laser conditioning, pre-exposure to less thandamaging laser fluence, has been shown to be an important methodto improve the laser damage resistance of the optical materials used inatmosphere environment and many experiment results werereported about this kind of enhancement effect of laser conditioning[1–3].

At present, the laser system is widely applied in the domain ofspace [4]. Laser optics being used in space laser systems are usuallyexposed to high vacuum conditions under the absence of air oroxygen. Compared to the atmosphere conditions, the vacuumcircumstance is more complicated. It is well known that opticaldielectric coatings show a change in damage performance whenaltering the environmental condition from air to vacuum [5]. Thedeleterious effects of vacuum have been mentioned in literature.Extensive laser damage tests in vacuumhave recently been performedin the IR, VIS, and UV spectral range. These tests have consistentlyrevealed the degradation of the LIDT values for e-beam evaporateddielectric coatings under vacuum environments, which occurredindependently of wavelength and type of coating (HR or AR) and

other parameters [6]. So, it is very important that how to improve thelaser damage resistance of the optical materials used in vacuumenvironments.

In this paper, laser conditioning effects of the dielectric mirrorcoatings in vacuum environments were investigated. The laser-induced damage thresholds (LIDT) in vacuum environments beforeand after laser conditioning were compared. Laser conditioningeffects in vacuum and air environments exposed to 1064 nmnanosecond laser pulses were also compared and investigated. Thedifferences of laser conditioning effects in two environments werediscussed with defect statistical model.

2. Experimental details

Anti-reflection and high reflection coatings for the laser condi-tioning test were prepared by E-beam evaporation (EBE) and ionbeam sputtering (IBS) respectively. Two films design were [Air:2.5LH:K9] and [K9: HL(2H2L)^15 2H:Air] with a reference wavelength of1064 nm on a K9 substrate, where L denoted quarter wavelengthoptical thickness(QWOT) of SiO2 and H denoted QWOT of ZrO2 foranti-reflection coatings and Ta2O5 for high reflection coatings. Theresidual reflectance of anti-reflection for 1064 nm laser is less than0.05% and the reflectance of high reflection coatings for the samewavelength laser is more than 99.9%.

The experimental setup is shown schematically in Fig. 1. Nd:YAGlaser delivered a single longitudinal mode, Gaussian-shaped laserbeam of high spatial quality at a wavelength of 1064 nm with a pulsewidth of 12 ns. A stabilized He–Ne laser was made collinear with themain Nd:YAG beam and both beams were directed onto the sample

Fig. 1. Measurement setup of laser-induced damage in vacuum.

Fig. 2. LIDT of the unconditioned and conditioned anti-reflection coatings in vacuum.

Fig. 3. LIDT of the unconditioned and conditioned high reflection coatings in vacuumand atmosphere environments.

2729X. Ling, et al. / Optics Communications 283 (2010) 2728–2731

surface under investigation with an convex lens. The spot size of thebeam incident on the sample was 400 µm diameters at 1/e2 of themaximum intensity. The sample was fixed in a sample holder withinthe stainless steel vacuum chamber which could be moved andpositioned laterally relative to the beam with an x/y translation stagecontrolled by a PC computer. Behind the chamber, the main Nd:YAGbeam was blocked. A 2-stage vacuum pump system consisting of afore-pump and a main molecular pump allowed to achieving thevacuum of 1.2×10−3 Pa. The vacuum pressure in vacuum chamberwas measured by ionization vacuum meter.

According to the standard ISO11254-1 [7], the test method oflaser-induced damage thresholds (LIDT) was 1-on-1 mode. Laserdamage tests were carried out in vacuum and laboratory atmosphere,respectively. The measurement pressure in vacuum environment wasset about 4×10−3 Pa. Optical microscope of 100× magnification wasdeployed to confirmwhether the radiation sites were damaged or not.The LIDT (J/cm2) was defined as the incident pulse's energy densitywhen the damage occurred at 0% probability, and it could be obtainedby linear extrapolation of the damage probability data to 0% damageprobability. The total error of the LIDT measurements was within 10%.

Laser conditioning was conducted by the following steps: the LIDTof the samples were tested firstly in vacuum, and then the sample wasraster scanned with a specific fluence below its LIDT for single stepmode. In summary, the sample was translated past a stationary laserbeam to raster scan the half region of the entire sample surface. Bydefining the step size between pulses to equal the laser beamdiameter at 90% of the peak energy density, the scanned region wasexposed to the 90% peak laser energy density. To eliminate theinfluence of different samples, each sample surface wasmeasured halfwith laser conditioning and half with no conditioning. In addition, wecompared laser conditioning effect in vacuum and atmosphereenvironments.

3. Experimental results

Fig. 2 shows the LIDT of the unconditioned and conditioned anti-reflection coatings in vacuum environments. From Fig. 2, we can seethat the LIDT of anti-reflection coatings have a decreasing tread as thefluence of pre-irradiation increase from 5.2 J/cm2 to 11.2 J/cm2. Theerror bars in the figure represent errors induced by the fluenceuncertainty of the laser system. We also found, for the identical anti-reflection coatings with no conditioning, the LIDT is very different,which is mainly related to substrate characteristics and coatingmanufacturing. The LIDT of high reflection coatings before and afterlaser conditioning in vacuum and atmosphere environments areindicated in Fig. 3. Fig. 3 shows that when the laser conditioningfluence is low, the LIDT of high reflection coatings before and afterlaser conditioning almost keep constant within the test error in spiteof vacuum or atmosphere case. As the fluence of laser conditioningincrease, laser conditioning in vacuum environments decrease the

LIDT, while laser conditioning in atmosphere environment increasethe LIDT of the high reflection coatings.

4. Discussions

Laser damage of dielectric thin films in the nano-second regime ismainly initiated by nanometric absorbing defects [8–10] inherent tothe manufacturing process. A useful method to obtain information onthe damaging defects is to study the laser damage statistics: defectstatistical model. According to this statistical model, the probability ofdamage P (F) is then the probability of the presence of a defect thatreceives more energy density than its critical fluence T. Thisprobability can classically be expressed as [11–13]:

P Fð Þ = 1− exp −N Fð Þð Þ = 1− exp dST Fð Þð Þ ð1Þ

where N (F) is the number of defects under the laser spot that caninduce damage at the fluence F, d is the surface density of defects. If aGaussian beam is considered, ST (F) is the spot surface where thefluence F is higher than the defect threshold T:

ST Fð Þ = πω2

2ln

FT

� �ð2Þ

Fig. 4. The fit of laser damage probability of the unconditioned and conditioned highreflection coatings in vacuum.

Fig. 6. The stoichiometry of the damaged area with andwithout conditioning in vacuumenvironments compared to air environments.

2730 X. Ling, et al. / Optics Communications 283 (2010) 2728–2731

The probability law that results from relations (1) and Eq. (2) canbe written as:

p Fð Þ = 1−ðFTÞ−

πdω2

2

!,ð3Þ

So, the shape and slope of the fit curves of damage probability arerelated to the density, the threshold of defects and to the spot size.Given the spot size, we can know the density and the threshold ofdefects.

In Figs. 4 and 5, the laser damage probability curves ofunconditioned and conditioned high reflective film irradiated invacuum and atmosphere environments are plotted with the defectstatistical model. The analysis of the curve slopes (signature of thedefect density) points out differences between the unconditioned andconditioned cases in two conditions. For the conditioned case, thecurve is steeper than the unconditioned case in vacuumenvironments,while it is in diametrical opposition in atmosphere environments.Within the framework of the statistical defect model, these results

Fig. 5. The fit of laser damage probability of the unconditioned and conditioned highreflection coatings in atmosphere environments.

can be understood as a significant increase and decrease in defectdensity of film after laser conditioning in vacuum environments andatmosphere environments separately. Indeed, for the atmosphereenvironments, as has already been observed, laser conditioning canreduce sensitive defect density or stabilize defect and are used toimprove the LIDT of the component. This is in agreement with ourresults. However, for the vacuum case, we observed the negative effectof laser conditioning which increase the sensitive defect densityinduced damage and decrease the LIDT of the component.

Our previous study [14] showed that laser irradiation in vacuumresulted in much more loss of oxygen and formation of the sub-stoichiometry defect in oxide thinfilms in the course of laser irradiation.Based on these previous results, we think that formation of the sub-stoichiometry defect is possible responsible for the increase of thesensitive defect density induced damage in course of laser conditioningin vacuum environments. So, energy dispersive X-ray analysis wasperformed using 15 Kev electrons on damaged region of the HR filmswith and without conditioning. The Ta, Si and O contents at each pointare reported as the element atomic ratio. In Fig. 6, we contrast O, Ta andSi contents of damaged area of the sample with and withoutconditioning in air and vacuum environments. The major difference ofthe laser-induced damaged region irradiated in vacuum as opposed toair environments is reduced signal for oxygen for no conditioning

Fig. 7. Absorption of sample with conditioning in air and vacuum environmentscompared to that of no conditioning.

2731X. Ling, et al. / Optics Communications 283 (2010) 2728–2731

sample and more reduced signal for oxygen for conditioning sample.This is agreementwith our previous study. It iswell known that the sub-stoichiometry defect is an important factor resulting in absorptionof thefilms. For this reason, absorptionmeasurementsof the sampleswith andwithout conditioning in air and vacuum environments were madethrough surface thermal lensing technique (STL). Fig. 7 showed thatlaser conditioning with 14.6 J/cm2 laser fluence in atmosphereenvironment reduced absorption of the film while laser conditioningwith the same laser fluence in vacuum environment increasedabsorption of the film as opposed to no conditioning films. Hence, asignificant increase in sub-stoichiometry defect density of film afterlaser conditioningwhich increased absorption of thefilm resulted in thenegative effect of laser conditioning in vacuum environments.

5. Conclusion

Laser conditioning effects of the dielectricmirror coatings in vacuumenvironments were investigated. The laser-induced damage thresholds(LIDT) in vacuum environments before and after laser conditioningwere compared. Laser conditioning effects in vacuum and atmosphereenvironments were also compared and investigated. The differences oflaser conditioning effects in two environments were discussed withdefect statisticalmodel. It is found that laser conditioning in atmosphereenvironment can reduce sensitive defect density and improve the LIDTof the component. However, for the vacuum case, the negative effect of

laser conditioning were observed, which increase the sensitive defectdensity induced damage and decrease the LIDT of the component.

Acknowledgement

The work is supported by the National Natural Science Foundationof China (No 60708004).

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