多层镜片中原生缺陷与激光诱导缺陷对多脉冲损伤的影响

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本文主要探讨了在多层镜片中，天然缺陷(native defects)和激光诱导缺陷(laser-induced defects)对多脉冲激光损伤(multi-shot laser-induced damage)的影响。研究对象是采用电子束蒸发(Electron Beam Evaporation, EBE)技术沉积的HfO2/SiO2高介电薄膜镜。实验使用355纳米脉冲激光，在单次脉冲和连续脉冲（S-on-1模式）下进行了损伤测试，每次脉冲持续时间为8纳秒。首先，实验发现单次脉冲激光损伤阈值(Single-Shot Laser-Induced Damage Threshold, SSLIDT)远高于多脉冲下的损伤阈值(Multi-Shot Laser-Induced Damage Threshold, MS-LIDT)。这表明在多脉冲照射条件下，随着激光多次作用于镜子，其损伤行为与单次脉冲时存在显著差异。单次脉冲由于能量集中，可能导致局部高温瞬态效应，使得材料能够承受更高的激光功率而不产生损伤。然而，当连续脉冲照射时，多次累积的热量可能会引发材料内部缺陷的扩展和交互作用，降低了整体的抗损伤能力。其次，文章强调了天然缺陷在多脉冲损伤中的作用。这些缺陷可能来源于材料生长过程中的不均匀性、杂质掺杂或晶格结构不完整性，它们在多次激光脉冲的作用下，可能会作为能量的聚焦点，加速损伤的发生。此外，激光诱导缺陷也随着多次曝光而加剧，如色心中心的形成，这些新产生的缺陷会进一步降低材料的稳定性。研究还指出，多脉冲激光损伤不仅受到激光参数（如波长、脉冲宽度和重复频率）的影响，还与材料的微观结构、热传导特性以及缺陷的分布和类型紧密相关。因此，优化多层镜的设计和制备工艺，减少天然缺陷的数量和分布，以及控制激光处理过程中的缺陷生成，对于提高多层镜在实际应用中的抗激光损伤性能至关重要。这篇研究为理解并改进多层镜的抗多脉冲激光损伤性能提供了深入的理论依据，这对于光学系统，特别是在高功率激光器和空间光学设备中，具有重要的工程实践意义。

COL 9(9), 093102(2011) CHINESE OPTICS LETTERS Septemb er 10, 2011

Effect of native defects and laser-induced defects on

multi-shot laser-induced damage in multilayer mirrors

Ying Wang (王王王营营营)

1,2∗

, Yuanan Zhao (赵赵赵元元元安安安)

, Jianda Shao (邵邵邵建建建达达达)

, and Zhengxiu Fan (范范范正正正修修修)

Key Laboratory of Materials for High Power Lasers, Shanghai Institute of Optics and Fine Mechanics,

Chinese Academy of Science, Shanghai 201800, China

Graduate University of Chinese Academy of Sciences, Beijing 100049, China

∗

Corresp onding author: wangying@siom.ac.cn

Received February 17, 2011; accepted April 14, 2011; posted online July 11, 2011

The roles of laser-induced defects and native defects in multilayer mirrors under multi-shot irradiation

condition are investigated. The HfO

/SiO

dielectric mirrors are deposited by electron beam evaporation

(EBE). Laser damage testing is carried out on both the 1-on-1 and S-on-1 regimes using 355-nm pulsed

laser at a duration of 8 ns. It is found that the single-shot laser-induced damage threshold (LIDT) is

much higher than the multi-shot LIDT. In the multi-shot mode, the main factor inﬂuencing LIDT is the

accumulation of irreversible laser-induced defects and native defects. The surface morphologies of the

samples are observed by optical microscopy. Moreover, the number of laser-induced defects affects the

damage probability of the samples. A correlative model based on critical conduction band (CB) electron

density (ED) is presented to simulate the multi-shot damage behavior.

OCIS codes: 310.0310, 140.3330.

doi: 10.3788/COL201109.093102.

It is widely regarded that multi-shot laser-induced dam-

age threshold (LIDT) in most optical materials decreases

with increasing pulse number because of the accumula-

tion effects

[1−3]

. Recently, many laboratories have inves-

tigated the multi-shot laser-induced damage (LID) of op-

tical materials, such as fused silica

[3]

and KTP crystal

[4]

Optical coatings are used very often in high-power laser

systems

[5]

. As a result, the investigation of multi-shot

LID of optical coatings is of high practical importance

for high-power laser applications in recent years

[6]

. In

a previous study, a model was developed to explain the

correlations between the observed multi-shot LIDT and

its dependence on the number of shots at 1 064 nm

[7]

. It

is easier to generate the color center at an intensity be-

low the damage threshold ultraviolet laser light

[8]

. The

damage behavior of 355 nm is much complicated than

that of 1 064 nm. The aim of this letter is to analyze the

accumulation effect on laser damage resistance at 355

nm. The effect of native defects and laser induce defects

on multi-shot LIDT is discussed.

The 355 high reﬂectivity (HR) coatings were prepared

by electron beam evaporation (EBE). The coating design

of the sample was (HL)

H, where H and L stand for the

high-index material (HfO

) and the low-index material

(SiO

), respectively, with one-quarter wavelength optical

thickness (QWOT).

The experimental setup for laser damage is shown

schematically in Fig. 1, in which the Nd:YAG laser sys-

tem is operated at the TEM

mode and the pulse width

is 8 ns at 355 nm. The beams were focused to a 1/e

height of 380 µm and width of 350 µm. In the S-on-1

test, the sample was tested at the frequency of 5 Hz.

The laser energy that was used to damage the sample

was obtained by the adjustment of the attenuator, and

the pulse energy was measured by an energy meter from

a split-off portion of the beam. The sample was set upon

a two-dimensional precision stage driven by a stepper

motor. The He-Ne laser was used to monitor the test.

Damage onset was detected online by a video microscopy

system.

The S-on-1 test was carried out according to ISO

11254-2

[9]

. In this letter, 1-on-1, 5-on-1, 10-on-1, 20-

on-1, and 100-on-1 LIDTs were tested to understand the

inﬂuence of the shot number on the accumulation effects.

Two representations of the damage data can be found

in Fig. 2. The S-on-1 damage probability curves with

S ranging from 1 to 100 are shown in Fig. 2(a). The

0% damage probability is summarized in Fig. 2(b). The

accumulation effects become clear in the two representa-

tions.

The data in Fig. 2(a) show the increase in damage

probability with the number of pulses and a clear de-

crease in the damage threshold with the increase in laser

pulse number. From Fig. 2(b), it can be seen that the

0% damage probability drops monotonically to a mini-

mum value.

The damage morphology of the HR coatings was ac-

cessed by optical microscope. The typical damage mor-

phology information after 1-on-1 and S-on-1 tests are

shown in Fig. 3. All the morphologies clearly permit

that the damages originated from small defects. The

delamination characteristic of optical films can be seen

in the S-on-1 test.

Due to interference effects in film, local intensity en-

hancements exist in the sample coatings. The theoretical

Fig. 1. Experimental setup of laser damage testing.

1671-7694/2011/093102(4) 093102-1

° 2011 Chinese Optics Letters

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