ISO/TR 20811:2017

TECHNICAL ISO/TR
REPORT 20811
First edition
2017-08
Optics and photonics — Lasers and
laser-related equipment — Laser-
induced molecular contamination
testing
Qualification des composants optiques laser pour les applications
spatiales
Reference number
©
ISO 2017
© ISO 2017, Published in Switzerland
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ii © ISO 2017 – All rights reserved

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols and abbreviated terms . 2
5 Test method . 2
5.1 Test setup . 2
5.1.1 Vacuum chamber . 2
5.1.2 Laser source and optical beam line . 3
5.2 Setup for data acquisition . 3
5.2.1 Beam profile monitoring . 3
5.2.2 Measurement of pulse energy and determination of transmission . 3
5.2.3 Laser-induced fluorescence monitoring . 4
5.3 Test preparation . 4
5.3.1 Bakeout . 4
5.3.2 Pretreatment of optical samples . 4
5.3.3 Blank test . 4
5.4 LIMC test . 5
5.4.1 Test parameters . 5
5.4.2 Individual steps for LIMC test . 5
5.5 Evaluation of test results . 6
Bibliography .10
Foreword
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This document was prepared by Technical Committee ISO/TC 172, Optics and photonics, Subcommittee
SC 9, Electro-optical systems.
iv © ISO 2017 – All rights reserved

Introduction
Laser technique is becoming increasingly important for space applications. Complex laser systems
are used both for Earth observation and for planetary exploration. For long-term operations, optical
components have to satisfy stringent requirements concerning precision and reliability. Before being
used in space, all optical components have to be tested extensively. For standardized determination
of laser damage threshold, ISO 21254 (all parts) should be applied. For characterization of optics for
space applications, corresponding tests should be performed under vacuum conditions. In addition to
laser damage issues, laser-induced molecular contamination (LIMC) should be taken into account. LIMC
denotes the interaction of laser radiation, especially in case of high fluences and short wavelengths
with volatile molecules and the resulting formation of deposits on optical components. LIMC proved
to be particularly critical, if the laser system is operated under vacuum conditions and could
considerably reduce the functionality of the whole laser system. Molecular contamination is mainly
caused by organic materials and silicones, e.g. glues, adhesives, insulating material or circuit boards
due to stronger outgassing rates compared to inorganic materials. The outgassing can be reduced but
not totally prevented by selection of suitable materials and preconditioning, e.g. bake-out at elevated
temperature well above the planned operating temperature. The outgassing behaviour of materials
is generally characterized by these parameters: collected volatile condensable material (CVCM), total
mass loss (TML), recovered mass loss (RML), volatile condensable material (VCM) and water vapour
regained (WVR). Definitions and corresponding measuring specifications for these quantities can be
found in ECSS-Standard Q-ST-70-02C, ASTM-E595-07 and ASTM-E1559.
This document outlines the test procedure for investigations of laser-induced molecular contamination
in order to compare the growth of laser-induced depositions on optical surfaces for different molecular
contamination materials.
TECHNICAL REPORT ISO/TR 20811:2017(E)
Optics and photonics — Lasers and laser-related equipment
— Laser-induced molecular contamination testing
1 Scope
This document describes the setup, test procedure and analysis of measured data for investigation of
laser-induced molecular contamination (LIMC) for space and vacuum applications.
LIMC is the formation of depositions on optical surfaces due to interaction of intense light radiation with
outgassing molecules especially from organic materials. It is a phenomenon of molecular contamination
and it is distinguished from particle contamination, which can occur during manufacturing, assembly,
integration or test of the optical components.
Formation of laser-induced depositions can lead to deterioration of the performance of an optical
system. Phase distortion, scattering and absorption can be increased by LIMC. LIMC is of particular
relevance, if a laser system is operated in vacuum at short wavelength and short pulse duration. In such
−5
a case, even small partial pressure of contamination material in the range of 10 hPa could have strong
negative impact on optical performance. It was also shown that the laser-induced damage threshold
could be reduced by a factor of 10 and more if laser-induced depositions are involved.
Laser-induced molecular contamination and laser-induced damage are both phenomena, for which the
interaction of laser radiation with optical surfaces plays a major role, in case of LIMC with additional
molecular contamination. Therefore, this document is treated in relation to ISO 21254 (all parts) which
specifies the test methods for the determination of laser-induced damage thresholds.
This method was derived to evaluate qualitatively, whether the material under investigation causes
deposits on optical surfaces in a low-pressure environment in the presence of high-energy nanosecond
pulsed laser irradiation at a wavelength of 355 nm. Due to the nature of photochemical surface reactions,
this result cannot be directly transferred to scenarios where the properties of the irradiation are altered
(especially wavelength, repetition rate, pulse duration, etc.). Due to the non-linear growth of the laser-
induced contamination and its detection methods, this technique does not provide quantitative means
to evaluate the deposit and, therefore, it should be seen as a means to compare materials relatively with
respect to their laser-induced contamination behaviour.
Furthermore, it is out of the scope of this method to select representative quantities of contamination
materials — representative with respect to the material partial pressure present in the vicinity of the
optical surface in a real laser system. This is carefully derived with other methods and is a mandatory
parameter to be fixed before applying this method.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 11145, Optics and photonics — Lasers and laser-related equipment — Vocabulary and symbols
ISO 21254 (all parts), Lasers and laser-related equipment — Test methods for laser-induced damage
threshold
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 11145 and ISO 21254 (all
parts) and the following apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— IEC Electropedia: available at http:// www .electropedia .org/
— ISO Online browsing platform: available at http:// www .iso .org/ obp
3.1
laser-induced deposition
material growth on optical surfaces as a result of photochemical or photothermal mechanisms triggered
by the interaction of laser radiation with volatile molecules from outgassing process of especially
organic materials
3.2
laser-induced fluorescence
light emission from any substance that has been excited to singlet states by absorption of
electromagnetic radiation
4 Symbols and abbreviated terms
Symbol Unit Term
α rad Angle of incidence
d m Beam diameter
N — Number of pulses per site
p
H J/m Peak energy density
peak
τ s Pulse duration
f Hz Pulse repetition rate
p
E J Pulse energy
p Pa Pressure
T K Contamination temperature
c
λ m Wavelength
5 Test method
5.1 Test setup
5.1.1 Vacuum chamber
A typical setup for tests of anti-reflective optics (angle of incidence: 0°) is shown in Figure 1. The main
part is an ultra-high vacuum chamber. It should be ensured that no organic materials are present in
this chamber. Especially an oilfree pumping system and metallic sealings instead of plastic o-rings
should be used. In particular, the use of silicone-based materials (e.g. pump oils) should be absolutely
avoided. In general, silicones have a very low vapour pressure and it is very laborious to get rid of it
once the test chamber is contaminated with it. Moreover, silicones are known to produce laser-induced
depositions even at ambient or oxygen atmosphere. In contrast to this, for hydrocarbons, the formati
...