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 23701:2023(E)
© ISO 2023

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ISO 23701:2023(E)
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© ISO 2023
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ISO 23701:2023(E)
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
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ISO 23701:2023(E)
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
committee has been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
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
editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www.iso.org/patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to
the World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see
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.
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ISO 23701:2023(E)
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.
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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
1
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ISO 23701:2023(E)
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
0
S Photothermal amplitude of test sample
S Photothermal amplitude of calibration sample
0
P, P Pump laser power W
0
ΔI, I Probe beam intensity change and dc probe intensity detected in photothermal lensing
0
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
2
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ISO 23701:2023(E)
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).
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ISO 23701:2023(E)
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
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ISO 23701:2023(E)
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
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ISO 23701:2023(E)
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
00
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.
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ISO 23701:2023(E)
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
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ISO/FDIS 23701:20222023(E)
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DATE:2023-XX
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ISO/TC 172/SC 9/WG 1
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Secretariat: DIN
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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 le mesurage et la cartographie de l'absorption des composants laser
optiques

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ISO/FDIS 23701:20222023(E)
Copyright notice
This ISO document is a working draft or committee draft and is copyright-protected by ISO. While
the reproduction of working drafts or committee drafts in any form for use by participants in the
ISO standards development process is permitted without prior permission from ISO, neither this
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Violators may be prosecuted.
© ISO 20222023 – All rights reserved
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ISO/FDIS 23701:20222023(E)
Contents Page
Foreword v
Introduction vi
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols used and units of measure . 2
5 Test method . 3
5.1 Test principle . 3
5.1.1 General . 3
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 . 4
5.2.1 Photothermal detection arrangement . 4
5.2.2 Pump laser . 6
5.2.3 Probe laser . 7
5.2.4 Translation stage . 7
5.2.5 Detection unit . 7
5.2.6 Data acquisition and processing . 7
5.2.7 Environment . 8
5.3 Preparation of test sample . 8
6 Test procedure . 8
6.1 General . 8
6.2 Measurements of photothermal amplitude and phase . 8
6.3 Maps of photothermal amplitude and phase . 9
6.4 Calibration of photothermal amplitude . 10
6.5 Assessments of the measurement . 11
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 . 14
7.4 Mapping area and spatial resolution . 14
8 Test report . 14
Annex A (informative) Theoretical and practical considerations on calibration . 16
A.1 General . 16
A.2 Theoretical consideration on calibration . 16
A.3 Practical consideration on calibration samples . 16
Annex B (informative) Separation of surface absorption and bulk absorption . 18
B.1 General . 18
B.2 Photothermal detection configurations for separation of surface absorption and
bulk absorption . 18
B.3 Determination of the surface absorption and bulk absorption . 19
Annex C (informative) Test report. 20
Bibliography. 22
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iii

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ISO/FDIS 23701:20222023(E)
Foreword . v
Introduction . vi
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols used and units of measure . 2
5 Test method . 3
5.1 Test principle . 3
5.1.1 General . 3
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 . 4
5.2.1 Photothermal detection arrangement . 4
5.2.2 Pump laser . 6
5.2.3 Probe laser . 7
5.2.4 Translation stage . 7
5.2.5 Detection unit . 7
5.2.6 Data acquisition and processing . 7
5.2.7 Environment . 8
5.3 Preparation of test sample . 8
6 Test procedure . 8
6.1 General . 8
6.2 Measurements of photothermal amplitude and phase . 8
6.3 Maps of photothermal amplitude and phase . 9
6.4 Calibration of photothermal amplitude . 10
6.5 Assessments of the measurement . 11
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 . 14
7.4 Mapping area and spatial resolution . 14
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 . 20
Bibliography . 22

© ISO 20222023 – All rights reserved
iv

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ISO/FDIS 23701:20222023(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards Formatted: English (United States)
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
Formatted: Adjust space between Latin and Asian text,
Adjust space between Asian text and numbers
through ISO technical committees. Each member body interested in a subject for which a technical
committee has been established has the right to be represented on that committee. International
Formatted: English (United States)
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
Formatted: English (United States)
ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
Formatted: English (United States)
electrotechnical standardization.
Formatted: English (United States)
The procedures used to develop this document and those intended for its further maintenance are Formatted: English (United States)
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 Formatted: English (United States)
editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directiveswww.iso.org/directives).
Formatted: English (United States)
Formatted: English (United States)
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of any Formatted: English (United States)
patent rights identified during the development of the document will be in the Introduction and/or on
the ISO list of patent declarations received (see www.iso.org/patentswww.iso.org/patents). Formatted: English (United States)
Formatted: English (United States)
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement. Formatted: English (United States)
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and Formatted: English (United States)
expressions related to conformity assessment, as well as information about ISO's adherence to the World
Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see
www.iso.org/iso/foreword.htmlwww.iso.org/iso/foreword.html. Formatted: English (United States)
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 Formatted: English (United States)
complete listing of these bodies can be found at
Formatted: Adjust space between Latin and Asian text,
Adjust space between Asian text and numbers
www.iso.org/members.htmlwww.iso.org/members.html.
Formatted: English (United States)
© ISO 20222023 – All rights reserved
v

---------------------- Page: 5 ----------------------
ISO/FDIS 23701:20222023(E)
Introduction
With the rapid development of high-power/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/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/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 Formatted: Pattern: Clear
of optical laser components can only measure test samples with small sizes (normally less than 50 mm
Formatted: Pattern: Clear
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/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.
© ISO 20222023 – All rights reserved
vi

---------------------- Page: 6 ----------------------
FINAL DRAFT INTERNATIONAL STANDARD ISO/FDIS 23701:20222023(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
Formatted: Pattern: Clear
The following documents are referred to in the text in such a way that some or all of their content
Formatted: Pattern: Clear
constitutes requirements of this document. For dated references, only the edition cited applies. For
Formatted: Pattern: Clear
undated references, the latest edition of the referenced document (including any amendments) applies.
Formatted: Pattern: Clear
ISO 11145, Optics and photonics — Lasers and laser-related equipment — Vocabulary and
Formatted: Pattern: Clear
symbols
Formatted: Pattern: Clear
ISO 11145, Optics and photonics — Lasers and laser-related equipment — Vocabulary and symbols
Formatted: Pattern: Clear
Formatted: Pattern: Clear
ISO 14644-1, Cleanrooms and associated controlled environments — Part 1: Classification of air cleanliness
Formatted: Pattern: Clear
by particle concentration
Formatted: English (United States)
ISO 80000-7, Quantities and units — Part 7: Light and radiation
Formatted: Adjust space between Latin and Asian text,
Adjust space between Asian text and numbers
ISO 80000-7, Quantities and units — Part 7: Light and radiation
Formatted: English (United States)
Formatted: English (United States)
3 Terms and definitions
Formatted: Font: Times New Roman, 12 pt, English (United
States)
For the purposes of this document, the terms and definitions given in ISO 11145 and ISO 80000-7 and
Commented [eXtyles1]: URL Validation failed:
the following apply.
https://www.iso.org/obp returns an unknown connection failure.
(connection error "Error 12031:
ISO and IEC maintain terminologicalterminology databases for use in standardization at the following
ERROR_INTERNET_CONNECTION_RESET").
addresses:
Formatted: English (United States)
— ISO Online browsing platform: available at https://www.iso.org/obphttps://www.iso.org/obp
Formatted: Adjust space between Latin and Asian text,
Adjust space between Asian text and numbers, Tab stops: Not
— IEC Electropedia: available at https://www.electropedia.org/https://www.electropedia.org/ at 19.85 pt + 39.7 pt + 59.55 pt + 79.4 pt + 99.25 pt +
119.05 pt + 138.9 pt + 158.75 pt + 178.6 pt + 198.45 pt
3.1
Formatted: English (United States)
absorption
Formatted: Hyperlink, English (United States)
radiant flux absorbed by the optical laser component
Formatted: English (United States)
Formatted: Hyperlink, English (United States)
© ISO 20222023 – All rights reserved
1

---------------------- Page: 7 ----------------------
ISO/FDIS 23701:20222023(E)
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 Formatted: Pattern: Clear
Formatted: Pattern: Clear
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 Commented [eXtyles2]: The table "Table 1 " is not cited in
the text. Please add an in-text citation or delete the table.
Symbol Term Unit
A Absorptance of test sample
A Absorptance of calibration sample
0
S Photothermal amplitude of test sample
S Photothermal amplitude of calibration sample
0
P, P Pump laser power W
0
ΔI, I Probe beam intensity change and dc probe intensity detected in photothermal
0
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
α Linear thermal expansion coefficient of test sample -1
th K
dn/dT Temperature coefficient of refractive index of test sample -1
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 K
sample
B, C Proportional coefficients
β Slope of linear fit of the measured absorptance dependence of the power-normalized
photothermal amplitude of calibration samples
© ISO 20222023 – All rights reserved
2

---------------------- Page: 8 ----------------------
ISO/FDIS 23701:20222023(E)
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
schemes 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.
© ISO 20222023 – All rights reserved
3

---------------------- Page: 9 ----------------------
ISO/FDIS 23701:20222023(E)
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 photother
...

FINAL
INTERNATIONAL ISO/FDIS
DRAFT
STANDARD 23701
ISO/TC 172/SC 9
Optics and photonics — Laser
Secretariat: DIN
and laser-related equipment
Voting begins on:
2023-01-30 — Photothermal technique for
absorption measurement and
Voting terminates on:
2023-03-27
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
RECIPIENTS OF THIS DRAFT ARE INVITED TO
SUBMIT, WITH THEIR COMMENTS, NOTIFICATION
OF ANY RELEVANT PATENT RIGHTS OF WHICH
THEY ARE AWARE AND TO PROVIDE SUPPOR TING
DOCUMENTATION.
IN ADDITION TO THEIR EVALUATION AS
Reference number
BEING ACCEPTABLE FOR INDUSTRIAL, TECHNO-
ISO/FDIS 23701:2023(E)
LOGICAL, COMMERCIAL AND USER PURPOSES,
DRAFT INTERNATIONAL STANDARDS MAY ON
OCCASION HAVE TO BE CONSIDERED IN THE
LIGHT OF THEIR POTENTIAL TO BECOME STAN-
DARDS TO WHICH REFERENCE MAY BE MADE IN
NATIONAL REGULATIONS. © ISO 2023

---------------------- Page: 1 ----------------------
ISO/FDIS 23701:2023(E)
FINAL
INTERNATIONAL ISO/FDIS
DRAFT
STANDARD 23701
ISO/TC 172/SC 9
Optics and photonics — Laser
Secretariat: DIN
and laser-related equipment
Voting begins on:
— Photothermal technique for
absorption measurement and
Voting terminates on:
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
COPYRIGHT PROTECTED DOCUMENT
© ISO 2023
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on
the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below
or ISO’s member body in the country of the requester.
RECIPIENTS OF THIS DRAFT ARE INVITED TO
ISO copyright office
SUBMIT, WITH THEIR COMMENTS, NOTIFICATION
OF ANY RELEVANT PATENT RIGHTS OF WHICH
CP 401 • Ch. de Blandonnet 8
THEY ARE AWARE AND TO PROVIDE SUPPOR TING
CH-1214 Vernier, Geneva
DOCUMENTATION.
Phone: [phone]
IN ADDITION TO THEIR EVALUATION AS
Reference number
Email: [e-mail]
BEING ACCEPTABLE FOR INDUSTRIAL, TECHNO­
ISO/FDIS 23701:2023(E)
Website: www.iso.org
LOGICAL, COMMERCIAL AND USER PURPOSES,
DRAFT INTERNATIONAL STANDARDS MAY ON
Published in Switzerland
OCCASION HAVE TO BE CONSIDERED IN THE
LIGHT OF THEIR POTENTIAL TO BECOME STAN­
DARDS TO WHICH REFERENCE MAY BE MADE IN
ii
  © ISO 2023 – All rights reserved
NATIONAL REGULATIONS. © ISO 2023

---------------------- Page: 2 ----------------------
ISO/FDIS 23701:2023(E)
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 Two­dimensional/three­dimensional 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
© ISO 2023 – All rights reserved

---------------------- Page: 3 ----------------------
ISO/FDIS 23701:2023(E)
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
committee has been established has the right to be represented on that committee. International
organizations, governmental and non­governmental, in liaison with ISO, also take part in the work.
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
editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www.iso.org/patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to
the World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see
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
  © ISO 2023 – All rights reserved

---------------------- Page: 4 ----------------------
ISO/FDIS 23701:2023(E)
Introduction
With the rapid development of high-power/energy laser technology, laser-induced damage to optical
laser components and laser­induced thermal distortion in laser components become the most important
limiting factors for the operation and applications of high-power/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 laser­induced damage threshold (LIDT) and reduce the laser­induced thermal distortion
of laser components used in high-power/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 large­sized laser
optics actually used in high-power/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 laser­defect interaction that would lead to a better understanding of
laser­induced damage mechanism of laser optics.
v
© ISO 2023 – All rights reserved

---------------------- Page: 5 ----------------------
FINAL DRAFT INTERNATIONAL STANDARD ISO/FDIS 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
two­dimensional or three­dimensional 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 two­dimensional 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 14644­1, Cleanrooms and associated controlled environments — Part 1: Classification of air cleanliness
by particle concentration
ISO 80000­7, 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
1
© ISO 2023 – All rights reserved

---------------------- Page: 6 ----------------------
ISO/FDIS 23701:2023(E)
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
0
S Photothermal amplitude of test sample
S Photothermal amplitude of calibration sample
0
P, P Pump laser power W
0
ΔI, I Probe beam intensity change and dc probe intensity detected in photothermal lensing
0
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
2
  © ISO 2023 – All rights reserved

---------------------- Page: 7 ----------------------
ISO/FDIS 23701:2023(E)
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 schemes 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).
3
© ISO 2023 – All rights reserved

---------------------- Page: 8 ----------------------
ISO/FDIS 23701:2023(E)
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 beam­splitter 11 attenuator for probe beam
4 power meter 12 pinhole
5 mechanical chopper 13 photo­detector
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 beam­splitter 11 attenuator for probe beam
4 power meter 12 pinhole
5 mechanical chopper 13 photo­detector
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
4
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---------------------- Page: 9 ----------------------
ISO/FDIS 23701:2023(E)
Key
1 pump laser 9 probe laser
2 variable attenuator for pump beam 10 mirror
3 beam­splitter 11 attenuator for probe beam
4 power meter 12, 13 lens
5 mechanical chopper 14 bi­cell photo­detector
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 beam­splitter 11 attenuator for probe beam
4 power meter 12, 13 lens
5 mechanical chopper 14 bi­cell photo­detector
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
© ISO 2023 – All rights reserved

---------------------- Page: 10 ----------------------
ISO/FDIS 23701:2023(E)
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 He­Ne or diode laser with a TEM mode output is used as the probe laser. The output power of
00
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 two­dimensional 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 three­dimensional 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 two­dimensional/three­dimensional translation stage is used to move the test sample in order to
measure the absorption as a function of position, so that a two­dimensional or three­dimensional
absorption map is obtained. The translation stage should be motorized with a position resolution better
than 10 µm and controlled by a computer software.
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ISO/FDIS 23701:2023(E)
5.2.5 Detection unit
For TL detection, the detection unit consists of a pinhole and a photo­detector, 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 position­sensitive photo­detector (e.g. a bi­cell photo­
detector), a differential amplifier, and a lock-in amplifier. Th
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