ISO 23701:2023

Optics and photonics — Laser and laser-related equipment — Photothermal technique for absorption measurement and mapping of optical laser components

This document specifies procedures for the absorption measurement and high spatial-resolution two-dimensional or three-dimensional absorption mapping of optical laser components, and upon calibration, the measurement of absolute absorptance of laser optics. The methods given in this document are intended to be used for the two-dimensional or three-dimensional absorption mapping of optical laser components, that is, measurement of absorption as a function of position, as well as absorption/absorptance measurement and mapping of laser optics used in high-power/high-energy laser systems.

Optique et photonique — Lasers et équipements associés aux lasers — Technique photothermique pour la mesure et la cartographie de l'absorption des composants laser optiques

General Information

• Status: Published
• Publication Date: 20-Apr-2023
• ICS: 31.260 - Optoelectronics. Laser equipment
• Technical Committee: ISO/TC 172/SC 9 - Laser and electro-optical systems
• Drafting Committee: ISO/TC 172/SC 9/WG 1 - Terminology and test methods for electro-optical systems
• Current Stage: 6060 - International Standard published
• Start Date: 21-Apr-2023
• Due Date: 24-May-2024
• Completion Date: 21-Apr-2023

Standards Content (Sample)

INTERNATIONAL ISO
STANDARD 23701
First edition
2023-04
Optics and photonics — Laser
and laser-related equipment
— Photothermal technique for
absorption measurement and
mapping of optical laser components
Optique et photonique — Lasers et équipements associés aux lasers
— Technique photothermique pour la mesure et la cartographie de
l'absorption des composants laser optiques
Reference number
© ISO 2023
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ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols used and units of measure .2
5 Test method . 2
5.1 Test principle . 2
5.1.1 General . 2
5.1.2 Photothermal lensing (TL) . 3
5.1.3 Photothermal deflection (PTD) . . 3
5.1.4 Rules for selecting reflected and transmitted photothermal detection
schemes . 3
5.2 Measurement arrangement and test equipment . 3
5.2.1 Photothermal detection arrangement . 3
5.2.2 Pump laser . 6
5.2.3 Probe laser . 6
5.2.4 Translation stage . 6
5.2.5 Detection unit . 7
5.2.6 Data acquisition and processing . 7
5.2.7 Environment . 7
5.3 Preparation of test sample . 7
6 Test procedure .7
6.1 General . 7
6.2 Measurements of photothermal amplitude and phase . 8
6.3 Maps of photothermal amplitude and phase . 8
6.4 Calibration of photothermal amplitude . 9
6.5 Assessments of the measurement . 10
7 Evaluation .11
7.1 Determination of absorption via photothermal measurement . 11
7.2 Determination of absorptance via photothermal calibration .12
7.3 Two-dimensional/three-dimensional maps of absorption .13
7.4 Mapping area and spatial resolution . 13
8 Test report .14
Annex A (informative) Theoretical and practical considerations on calibration .16
Annex B (informative) Separation of surface absorption and bulk absorption.18
Annex C (informative) Test report .21
Bibliography .22
iii
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical
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ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
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www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 172, Optics and photonics, Subcommittee
SC 9, Laser and electro-optical systems.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.
iv
Introduction
With the rapid development of high-power/high-energy laser technology, laser-induced damage to
optical laser components and laser-induced thermal distortion in laser components become the most
important limiting factors for the operation and applications of high-power/high-energy laser systems.
Normally, the laser-induced damages to optical laser components are caused by absorbing defects
on the surface or within the laser components which result in thermal stress or melting of the laser
components and lead to damage. The thermal distortions, which induce wavefront distortions and
therefore beam quality deteriorations to the laser beam, are caused by non-uniform thermal expansion
or refractive index change due to absorption irregularities (such as absorbing defects) inside the laser
components. To improve the laser-induced damage threshold (LIDT) and reduce the laser-induced
thermal distortion of laser components used in high-power/high-energy laser systems, there are needs
not only to measure precisely the absorptance of the laser components, but also to detect various
absorbing defects on/within the laser components, therefore to improve the performance of these laser
components via optimizing fabrication/coating processes.
Currently, the ISO 11551 standardized testing method - laser calorimetry for absorptance measurements
of optical laser components can only measure test samples with small sizes (normally less than 50 mm
in diameter and 10 mm in thickness) and has almost no capability to measure the absorptance of large-
sized laser components (100 mm in diameter and over) widely used in high-power/high-energy laser
systems. In addition, laser calorimetry has only limited capability to map the absorptance distribution
of an optical laser component.
The measurement procedures in this document have been optimized to allow the mapping of absorbing
defects of optical laser components and measurement of absolute absorptance of large-sized laser optics
actually used in high-power/high-energy laser systems using photothermal techniques which provide
absorption measurement/mapping with high sensitivity, high spatial resolution, and high reliability.
In addition to absorption measurement/mapping of optical laser components with photothermal
amplitude, the photothermal phase measurement/mapping can also find applications in thermo-
physical characterization of laser optics, which will be helpful for a better understanding of defect
properties of laser optics and laser-defect interaction that would lead to a better understanding of
laser-induced damage mechanism of laser optics.
v
INTERNATIONAL STANDARD ISO 23701:2023(E)
Optics and photonics — Laser and laser-related equipment
— Photothermal technique for absorption measurement
and mapping of optical laser components
1 Scope
This document specifies procedures for the absorption measurement and high spatial-resolution
two-dimensional or three-dimensional absorption mapping of optical laser components, and upon
calibration, the measurement of absolute absorptance of laser optics.
The methods given in this document are intended to be used for the two-dimensional or three-
dimensional absorption mapping of optical laser components, that is, measurement of absorption as a
function of position, as well as absorption/absorptance measurement and mapping of laser optics used
in high-power/high-energy laser systems.
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 14644-1, Cleanrooms and associated controlled environments — Part 1: Classification of air cleanliness
by particle concentration
ISO 80000-7, Quantities and units — Part 7: Light and radiation
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 11145 and ISO 80000-7 and
the following apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
absorption
radiant flux absorbed by the optical laser component
3.2
absorptance
ratio of the radiant flux absorbed to the radiant flux of the incident radiation
3.3
absorption map
absorptance map
measured absorption (3.1)/absorptance (3.2) as a function of sample position
Note 1 to entry: The definition of absorptance used for this document is limited to absorptance processes
which convert the absorbed energy into heat. For certain types of optics and radiation, additional non-thermal
processes may result in absorption losses which will not be detected by the test procedure described here.
4 Symbols used and units of measure
Table 1 — Symbols used and units of measure
Symbol Term Unit
A Absorptance of test sample
A Absorptance of calibration sample
S Photothermal amplitude of test sample
S Photothermal amplitude of calibration sample
P, P Pump laser power W
ΔI, I Probe beam intensity change and dc probe intensity detected in photothermal lensing
I , I Probe beam intensity detected by the two photo-detectors of a bi-cell photo-detector
1 2
in photothermal deflection
2 -1
D Thermal diffusion coefficient of test sample m s
Δφ(x, y) Photothermally induced optical phase shift to probe beam
-1
α Linear thermal expansion coefficient of test sample K
th
-1
dn/dT Temperature coefficient of refractive index of test sample K
ν Poisson ratio of test sample
f Modulation frequency of pump laser power Hz
μ Thermal diffusion length of test sample m
th
a Pump beam radius in test sample m
λ Probe laser wavelength m
z Detection distance m
T(x, y, z) Photothermally induced temperature rise distribution inside test or calibration sample K
B, C Proportional coefficients
β Slope of linear fit of the measured absorptance dependence of the power-normalized
photothermal amplitude of calibration samples
5 Test method
5.1 Test principle
5.1.1 General
Based on photothermal effects, photothermal techniques are highly sensitive for the measurement of
weak absorptance of optical laser components. In a typical photothermal experiment, a continuous-
wave or highly repetitive pulsed excitation laser (pump laser) beam is used to irradiate the sample
under test, heat is created within the sample due to optical absorption, and a temperature rise
distribution within the sample is formed. For optical laser components, a displacement is induced on
the sample surface due to thermal expansion, and a refractive index gradient is formed within the
sample due to the temperature dependence of refractive index. By employing photothermal techniques
to detect the surface displacement or refractive index gradient with a second probe laser beam, the
absorptance (absorption) of the sample can be determined. The absolute absorptance can be obtained
by calibrating the photothermal amplitude. By measuring the absorption/absorptance as a function of
position, the absorption/absorptance map of a laser component is obtained.
Photothermal lensing (or thermal lens - TL) and photothermal deflection (PTD) are appropriate
detection techniques for absorption measurement and mapping of optical laser components. Two
detection schemes, both reflected and transmitted probe beam detections, can be used to measure the
photothermal signal amplitude, which is linearly proportional to the absorption/absorptance of optical
laser components under test when the photothermally induced optical phase shift to the probe beam is
relatively small as compared to the probe beam wavelength (small signal approximation).
5.1.2 Photothermal lensing (TL)
In a typical TL scheme, an unfocused probe beam irradiates the pump laser-induced surface
displacement zone or the refractive index gradient zone which acts as a negative (or positive) lens. The
photothermal signal is represented by the intensity change at the centre of either the reflected probe
beam (in surface TL – STL scheme) or the transmitted probe beam (in transmitted TL – TTL scheme).
This probe beam intensity change can be detected by a pinhole photodetector (a photodetector with a
pinhole in front of it).
5.1.3 Photothermal deflection (PTD)
In a typical PTD scheme, a tightly focused probe beam irradiates the pump laser-induced surface
displacement zone or the refractive index gradient zone, the reflected probe beam is deflected due to
the slope of the surface displacement in a reflected PTD configuration, or the transmitted probe beam
is deflected due to the refractive index gradient in a transmitted PTD configuration. The photothermal
signal is represented by the probe beam deflection, which can be easily detected by a position-sensitive
photodetector (for example, a bi-cell photodetector).
5.1.4 Rules for selecting reflected and transmitted photothermal detection schemes
Selecting between the reflected or transmitted photothermal detection schemes (both TL and PTD)
should be considered as follows. As a general rule, the detection scheme with a higher detection
sensitivity should be selected for more sensitive and precise absorption measurement. For an optical
laser component with a (much) larger linear thermal expansion coefficient so that the photothermally
induced optical phase shift to the probe beam by the thermal expansion caused surface displacement
is large, a reflected photothermal detection scheme is preferable. On the other hand, a transmitted
photothermal detection scheme should be selected if the temperature coefficient of refractive index of
the component is much larger so that the optical phase shift to the probe beam caused by the temperature
rise induced refractive index change is much larger than that caused by the surface displacement.
However, if the sample under test is opaque to the probe beam, the reflected photothermal detection
scheme should be selected.
For photothermal absorption measurement of bulk samples and separation of surface absorption
and bulk absorption, the transmitted photothermal detection scheme is preferable. The reflected
photothermal detection scheme may be used for measurement of bulk samples only when the bulk
sample is homogeneous and has negligible surface absorption.
5.2 Measurement arrangement and test equipment
5.2.1 Photothermal detection arrangement
There are four photothermal detection arrangements that may be used to measure and map the
absorption of optical laser components. That is, surface thermal lens (STL), transmitted thermal
lens (TTL), reflected photothermal deflection (or photothermal displacement) (reflected PTD), and
transmitted photothermal deflection (transmitted PTD).
Figures 1 to 4 show typical experimental arrangements for the STL, TTL, reflected PTD, and transmitted
PTD detection schemes, respectively.
Key
1 pump laser 9 probe laser
2 variable attenuator for pump beam 10 mirror
3 beam-splitter 11 attenuator for probe beam
4 power meter 12 pinhole
5 mechanical chopper 13 photo-detector
6 lens 14 lock-in amplifier
7 sample under test 15 oscilloscope
8 translation stage
Figure 1 — Typical experimental arrangement for the STL detection scheme
Key
1 pump laser 9 probe laser
2 variable attenuator for pump beam 10 mirror
3 beam-splitter 11 attenuator for probe beam
4 power meter 12 pinhole
5 mechanical chopper 13 photo-detector
6 lens 14 lock-in amplifier
7 sample under test 15 oscilloscope
8 translation stage
Figure 2 — Typical experimental arrangement for the transmitted TL detection scheme
Key
1 pump laser 9 probe laser
2 variable attenuator for pump beam 10 mirror
3 beam-splitter 11 attenuator for probe beam
4 power meter 12, 13 lens
5 mechanical chopper 14 bi-cell photo-detector
6 lens 15 differential amplifier
7 sample under test 16 lock-in amplifier
8 translation stage
Figure 3 — Typical experimental arrangement for the reflected PTD detection scheme
Key
1 pump laser 9 probe laser
2 variable attenuator for pump beam 10 mirror
3 beam-splitter 11 attenuator for probe beam
4 power meter 12, 13 lens
5 mechanical chopper 14 bi-cell photo-detector
6 lens 15 differential amplifier
7 sample under test 16 lock-in amplifier
8 translation stage
Figure 4 — Typical experimental arrangement for the transmitted PTD detection scheme
5.2.2 Pump laser
The pump laser is a continuous-wave or highly repetitive pulsed laser used to heat the test sample.
Wavelength of the laser source, angle of incidence and state of polarization shall correspond to those
specified by the manufacturer for the use of the test sample. The state of polarization (p or s) of the laser
beam shall be selected by the polarizer. If the value ranges are acceptable for these three quantities, any
combination of the wavelength, angle of incidence and state of polarization may be chosen within these
ranges.
The (average) power of the pump laser should be sufficiently high so that the surface displacement or
refractive index gradient created by the pump beam absorption of the test sample is highly detectable.
The pump laser power should be adjustable via a variable attenuator which should not create any
change to the beam profile of the pump beam during power adjustment. The pump beam profile
shall be the same at all times, during the calibration and during the photothermal measurement at
each adjusted power. The pump laser power is periodically modulated by a mechanical chopper or an
acoustic-optic modulator. The modulation frequency is selected for optimal signal-to-noise ratio (SNR)
of the photothermal signal.
For absorption mapping performed longer than several minutes, the power stability of the pump laser
shall be monitored by a power meter or photo-detector as shown in Figures 1 to 4, and if needed its
influence on the absorption mapping shall be eliminated by normalizing the photothermal amplitude
with the monitored output power.
The pump beam is focused into the test sample to create enough temperature rise inside the test
sample. The beam size of the pump beam on/inside the test sample is optimized taking into account
the SNR of the photothermal signal, the area and spatial resolution of absorption mapping. Care shall be
taken to avoid laser damage to the test sample due to too tight focusing.
5.2.3 Probe laser
The probe laser is a continuous-wave laser used to detect the photothermal signal. Normally a highly
stable He-Ne or diode laser with a TEM mode output is used as the probe laser. The output power of
the probe laser should be low so that the heat created by the probe beam absorption of the test sample
is negligible.
For TL detection, the probe beam is normally not focused, or the probe beam size is at least five times
larger than that of the focused pump beam in the interaction zone of the two beams. For PTD detection,
the probe beam is focused. The size of the focused probe beam is at least smaller than that of the pump
beam in the interaction zone. The separation between the pump and probe beams has to be adjusted to
maximize the photothermal (lensing or deflection) amplitude for optimum SNR.
For absorptance measurements and two-dimensional absorption mapping, the angle of incidence of the
probe beam with respect to the sample normal should be small, for example smaller than 10 degrees.
For three-dimensional absorption mapping or separation of surface absorption and bulk absorption,
the angle of incidence of the probe beam with respect to the pump beam should be adjusted taking into
consideration depth resolution and SNR of photothermal signal. The angle of incidence of the probe
beam should be documented.
A detailed description on the separation of surface and bulk absorption is given in Annex B.
5.2.4 Translation stage
A two-dimensional/three-dimensional translation stage is used to move the test sample in order to
measure the absorption as a function of position, so that a two-dimensional or three-dimensional
absorption map is obtained. The translation stage should be motorized with a position resolution better
than 10 µm and controlled by a computer software.
5.2.5 Detection unit
For TL detection, the detection unit consists of a pinhole and a photo-detector, which is appropriate
for the probe laser wavelength, and a lock-in amplifier. As a general rule, the size of the pinhole is
comparable to that of the surface displacement or refractive index gradient zone.
For PTD detection, the detection unit consists of a position-sensitive photo-detector (e.g. a bi-cell photo-
detector), a differential amplifier, and a lock-in amplifier. The position-sensitive photo-detector should
be appropriate for the probe laser wavelength, and the total detection area should be larger than the
probe beam size to make sure that all probe power is detected by the photo-detector. A positive lens
may be used in between the sample and photo-detector to adjust the probe beam size on the detection
area of the photo-detector. If a lens is used, it shall be ensured that no imaging of the probe beam spot at
the sample onto the detector is established. A differential amplifier is used to delete the dc level of the
probe beam deflection signal.
In both cases the reference frequency of the lock-in amplifier should be the same as the modulation
frequency of the pump laser power, and the time constant of the lock-in amplifier should be set taking
into consideration the temporal resolution and SNR of photothermal signal.
5.2.6 Data acquisition and processing
At each position of the test sample, the amplitude and phase of the modulated photothermal signal
is recorded via the lock-in amplifier for a certain time and averaged over a certain number of repeat
measurements. The photothermal measurement may also be repeated at several different sample
positions. The reported absorption/absorptance value is the average of repeat measurements
performed at a certain position or the average of measurements performed at the different positions.
For absorption mapping, the test sample is moved by controlling the motorized translation stage and
photothermal amplitude and phase are measured as a function of sample position. By choosing an
appropriate scan step and a measurement area, a map of photothermal amplitude is obtained, which
represents the absorption map. In addition, a map of photothermal phase is obtained, which gives
useful information on the thermo-physical characteristic distribution of the test sample.
5.2.7 Environment
The environment of the testing place should consist of dust-free filtered air with 40 % to 60 % relative
humidity. The residual dust shall be reduced in accordance with the clean-room Class 7 as specified in
ISO 14644-1.
5.3 Preparation of test sample
In general, test samples should have flat surfaces, for example un-coated, highly reflective (HR) or anti-
reflective (AR) coated plane substrates. The absorption/absorptance of samples with curved surfaces
could also be measured. However, care shall be taken for absorption mapping of samples with curved
surfaces as the surface curvature shall be considered to keep identical alignment at each position
during position scanning.
Storage, cleaning and preparation of the test samples shall be carried out in accordance with the
instructions of the manufacturer.
6 Test procedure
6.1 General
Either STL, TTL, reflected PTD, or transmitted PTD configuration shall be chosen to measure and map
the absorption/absorptance of the optical laser component. The amplitude of the photothermal signal
shall be measured to determine the absorption, and upon calibration, to determine the absorptance of
the test sample. The sample position has to be scanned to map the absorption/absorptance of the test
sample.
The incident angle of pump beam to the test sample shall be set according to the manufacturer's
instruction. For a sample with normal incidence, the absorption measurement should be performed
with the incident angle in between 1° to 10°, which shall be documented.
6.2 Measurements of photothermal amplitude and phase
The absorption of the test sample is
...