ОПТИКА И СПЕКТРОСКОПИЯ, 2015, том 118, № 4, с. 657-660
ЛАЗЕРЫ ^^^^^^^^^^^^
И ИХ ПРИМЕНЕНИЕ
УДК 621.373
INFLUENCE OF LASER CONDITIONING ON LASER INDUCED DAMAGE THRESHOLD OF SINGLE LAYERS OF ZrO2 WITH VARIOUS DEPOSITION CONDITIONS
© 2015 г. M. Sahraee*, H. Reza Fallah*'**, H. Zabolian*, B. Moradi*, and M. Haji Mahmoodzade*'**
* Physics department, University of Isfahan, Isfahan, Iran ** Isfahan Quantum Optics Group, University of Isfahan, Isfahan, Iran E-mail: masoome.sahraei@gmail.com Received June 18, 2014
Single layers of ZrO2 were coated at base pressure of 10-5 mbar by electron beam evaporation (EBE) technique. The influence of oxygen partial pressure on spectral properties and laser induced damage threshold (LIDT) of the samples were investigated. Spectral transmittance of the samples was measured by spectrophotometer. Laser induced damage threshold was detected according to ISO standard 11254. Laser conditioning was conducted by scanning the surface of the samples. Results showed that laser damage resistance was enhanced by increasing the oxygen partial pressure during deposition. LIDT of the samples was changed after laser conditioning. Experimental results revealed that there is enhancement of laser damage resistance of the samples with higher oxygen partial pressure after laser conditioning.
DOI: 10.7868/S0030403415040170
INTRODUCTION
Laser induced damage threshold (LIDT) of optical coatings is an important subject. Damage mechanism varies with radiation manner and laser parameters, such as pulse duration, spot size and wavelength. Laser damage testing makes it possible to monitor the quality of thin film optical coatings [1—3]. Electron beam deposited oxide coatings are frequently used for high power laser systems [4]. However, the limited radiation resistance of the thin films has been a problem for the development of high power laser. To improve the LIDT of coatings laser conditioning technique is developed [5]. Over the past decades, many investigators have reported that laser conditioning was useful for increasing the LIDT of thin films. However, the mechanism of laser conditioning is not fully understood yet. If changes in optical materials by conditioning are recognized, laser conditioning technique may be optimized [6—9]. In this work, the effect of laser conditioning on laser induced damage threshold of single layers of ZrO2 was investigated. Also the influence of oxygen partial pressure on spectral properties and LIDT of the samples were investigated. Laser induced damage threshold was detected according to ISO standard 11254. Laser conditioning was conducted by scanning the surface of the samples.
EXPERIMENTAL DETAILS
Single quarter-wave layers of ZrO2 (99% purity) at wavelength 1064 nm were deposited on BK7 glass substrates. The samples were prepared using the electron beam evaporation (EBE) technique with Balzers BAK 760 coating machine.
Deposition parameters of the samples are presented in Table 1. Before deposition, the substrates of BK7 glass were polished optically and then cleaned by ultrasonic method. In order to remove residual contaminants on their surface, then substrates were put at vacuum chamber for 10 min under glow discharge, which was helpful to clean substrates and effect of oxygen partial pressure was investigated. Spectral transmittance of the samples was measured by Shimadzu 3100 UV-VIS-NIR spectrophotometer. Experimental setup for measuring the laser damage threshold was shown schematically in Fig. 1. A high power 1064 nm Nd-YAG laser with 12 ns pulse width was used. The characteristic of the laser is presented in Table 2.
Table 1. Deposition parameters of the samples
Deposition Base Physical Rate of
temperature pressure thickness deposition
250°C 10-5 mbar 120 nm 0.3 nm/s
9
657
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Laser He-Ne 632 nm ■ Sample Microscope
Collm=ttor . Lens
Colimattor Lens
Laser Nd-YAG Beam splitter 1064 nm
4
Energy meter
Fig. 1. Experimental setup for damage testing according to ISO standard 112541.
Fig. 2. Laser beam intensity profile measured by a beam profiler from Ophir co.
The laser beam that measured by a beam profiler from Ophir co. is shown in Fig. 2. Damage testing was performed in 1-on-l regime according to ISO standard
Table 2. Characteristic of the Nd-YAG laser used in LIDT test
Pulse diameter, mm Shape of spots Energy, J Pulse width
7.5 Gaussian-like 1.64 ± 3% 12 ns
Table 3. LIDT of single layers of ZrO2 in different oxygen partial pressure
Oxygen partial Spot diameter, Fluence, Laser damage
pressure, mbar mm J/cm2 probability
1.5 x 10-4 3.4 18.08 0
3.2 20.42 0.3
3.1 21.77 0.8
1.1 x 10-4 3.4 18.08 0
3.2 20.42 0.3
3.1 21.77 0.6
8 x 10-5 3.4 18.08 0.1
3.2 20.42 0.4
3.1 21.77 0.7
4 x 10-5 3.4 18.08 0.3
3.2 20.42 0.6
3.1 21.77 0.8
11254. For measuring the single pulse (1-on-l) damage threshold, the sample is irradiated one time. Regardless of whether damage occurs or not the sample is moved. Then fluence is increased until damage occurs. The LIDT of the sample is the value of the irra-diance at which there is a none-zero chance of the sample damaging [2, 10—12]. In this work, the sample was placed near the focal point of a converging lens while the laser beam was focused on the sample. Ten sites of the sample were exposed at the same fluence and the fraction of sites, which were damaged, was recorded. A microscope of 50* magnifications was used to decide whether the radiation sites were damaged or not. Then the sample was moved and fluence was increased until damage occurred. In this work, fluence was measured 18.08, 20.42 and 21.77 J/cm2. LIDT of single layers of ZrO2 in different oxygen partial pressure is presented in Table 3.
Laser conditioning is conducted by following steps: the LIDT of the sample is tested firstly and then the sample is scanned with a specific fluence below its LIDT. The scanning method is described by Sheehan et al. [5]. Then LIDT of sample is measured after laser conditioning. In this work, for laser conditioning process the repetitive frequency of the laser was 6 Hz. Fluence was measured 3.71 J/cm2 and the sample was scanned. In the next stage in order to increasing of fluence, the sample was placed in distance of1.5 cm from the focal plane of a converging lens with a focal length of 4.78 cm. In this stage fluence was measured 8.36 J/cm2 and the sample was scanned again. LIDT of single layers of ZrO2 in different oxygen partial pressure after laser conditioning was measured again and results are presented in Table 4.
Table 4. LIDT of single layers of ZrO2 in different oxygen partial pressure after laser conditioning
Oxygen partial Spot diameter, Fluence, Laser damage
pressure, mbar mm J/cm2 probability
1.5 x 10-4 3.4 18.08 0
3.2 20.42 0
3.1 21.77 0.3
1.1 x 10-4 3.4 18.08 0
3.2 20.42 0.1
3.1 21.77 0.4
8 x 10-5 3.4 18.08 0
3.2 20.42 0.3
3.1 21.77 0.6
4 x 10-5 3.4 18.08 0.3
3.2 20.42 0.7
3.1 21.77 0.8
INFLUENCE OF LASER CONDITIONING
659
Transmittance, %
Wavelength, nm
Fig. 3. Spectral transmittance measured by Shimadzu 3100 UV-VIS-NIR spectrophotometer for four samples of different oxygen partial pressures: 4 x 10-5 (1), 8 x 10-5 (2), 1.1 x 10-4 (3), 1.5 x 10-4 mbar (4).
RESULTS AND DISCUSSION
The spectral transmittance for different oxygen partial pressure was measured and shown in Fig. 3. From Fig. 3, we can see that the transmittance of the samples was increased by increasing oxygen partial pressure. In addition, from Table 3 we can see that laser damage resistance of the samples was enhanced by increasing the oxygen partial pressure during deposition. Results show that highest damage threshold of the single layers of ZrO2 is in oxygen partial pressure of 1.5 x 10-4 mbar. Furthermore, comparison between LIDT of single layers of ZrO2 in different oxygen partial pressure before and after laser conditioning is presented in Table 5 and Figs. 4—7 also. Laser damage probability in terms of fluence for the first sample in
Laser damage probability
Fig. 4. Laser damage probability in terms of fluence for the first sample in an oxygen pressure of 1.5 x 10-4 mbar before (1) and after (2) laser conditioning.
oxygen partial pressure of 1.5 x 10-4 mbar before and after laser conditioning is presented in Fig. 4. From Fig. 4, we can see that laser damage probability was decreased significantly after laser conditioning and LIDT of the sample was increased. For second sample in oxygen partial pressure of1.1 x 10-4 mbar and third sample in oxygen partial pressure of 8 x 10-5 mbar results of laser damage probability in terms of fluence before and after laser conditioning are presented in Figs. 5 and 6, respectively. Results show that for second and third samples laser damage probability was decreased but the effect of laser conditioning in compared to the first sample is lower. For the fourth sample in oxygen partial pressure of 4 x 10-5 mbar results is presented in Fig. 7. From Figs. 4—7, we can see that LIDT of the samples was changed after laser conditioning. Experimental results revealed that in all samples laser damage probability was decreased after laser
Table 5. Comparison between the results of LIDT for four samples before and after laser conditioning
Oxygen partial pressure, mbar Spot diameter, mm Fluence, J/cm2 Laser damage probability before laser conditioning Laser damage probability after laser conditioning
1.5 x 10-4 3.4 18.08 0 0
3.2 20.42 0.3 0
3.1 21.77 0.8 0.3
1.1 x 10-4 3.4 18.08 0 0
3.2 20.42 0.3 0.1
3.1 21.77 0.6 0.4
8 x 10-5 3.4 18.08 0.1 0
3.2 20.42 0.4 0.3
3.1 21.77 0.7 0.6
4 x 10-5 3.4 18.08 0.3 0.3
3.2 20.42 0.6 0.7
3.1 21.77 0.8 0.8
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Laser damage probability
Fluence, J/cm2
Fig. 5. Laser damage probability in terms of fluence for the second sample in oxygen pressure of 1.1 x 10-4 mbar before (1) and after (2) laser conditioning.
Laser damage probability
Fig. 6. Laser damage probability in term
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