ISO/TR 22588:2005
(Main)Optics and photonics — Lasers and laser-related equipment — Measurement and evaluation of absorption-induced effects in laser optical components
Optics and photonics — Lasers and laser-related equipment — Measurement and evaluation of absorption-induced effects in laser optical components
ISO/TR 22588:2005 specifies standard measurement and evaluation techniques for determining the absorption-induced effects caused by lasers in laser optical components in order to provide useful information to reduce conflict between users and suppliers of optical components.
Optique et instruments d'optique — Lasers et équipement associé aux lasers — Mesurage et évaluation de la déformation et de la distorsion des composants optiques dans un faisceau laser
General Information
Standards Content (Sample)
TECHNICAL ISO/TR
REPORT 22588
First edition
2005-09-15
Optics and photonics — Lasers and
laser-related equipment — Measurement
and evaluation of absorption-induced
effects in laser optical components
Optique et instruments d'optique — Lasers et équipement associé aux
lasers — Mesurage et évaluation de la déformation et de la distorsion
des composants optiques dans un faisceau laser
Reference number
ISO/TR 22588:2005(E)
©
ISO 2005
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ISO/TR 22588:2005(E)
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ISO/TR 22588:2005(E)
Contents Page
Foreword. iv
Introduction . v
1 Scope . 1
2 Absorption. 1
2.1 General. 1
2.2 Measurement of absorption. 3
3 Distortion . 4
3.1 General. 4
3.2 Measurement of distortion. 4
3.3 Discussion. 4
4 Refractive index and birefringence. 5
4.1 General. 5
4.2 Measurement of birefringence . 6
4.3 Discussion. 6
5 Beam propagation . 6
5.1 General. 6
5.2 Measurement of propagation parameters. 7
5.3 Discussion. 7
6 Laser-induced damage threshold . 7
6.1 General. 7
6.2 Measurement of laser-induced damage threshold. 7
6.3 Discussion. 7
7 Discussion. 8
Bibliography . 18
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ISO/TR 22588:2005(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.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
In exceptional circumstances, when a technical committee has collected data of a different kind from that
which is normally published as an International Standard (“state of the art”, for example), it may decide by a
simple majority vote of its participating members to publish a Technical Report. A Technical Report is entirely
informative in nature and does not have to be reviewed until the data it provides are considered to be no
longer valid or useful.
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.
ISO/TR 22588 was prepared by Technical Committee ISO/TC 172, Optics and photonics, Subcommittee SC 9,
Electro-optical systems.
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ISO/TR 22588:2005(E)
Introduction
This Technical Report has been promulgated in order to highlight the problem and to specify meaningful,
standard measurement techniques in order to provide useful information to reduce conflict between users and
suppliers of optical components.
When a laser beam impinges upon an optical component (lenses, windows and mirrors) some of the energy is
absorbed. Depending on the intensity of the laser beam and the absorption properties of the component
material the component temperature will rise. Even if a uniform intensity laser beam fills the whole area of the
component, temperature gradients will be created across the aperture. Unless the material has a negligible
expansion coefficient, temperature gradients lead to differential expansion, and this will lead to distortion,
strain and a change in the birefringence properties of transmissive components. The refractive index of most
optical materials is also temperature dependent. If the optical figure of the component changes, the
transmitted and/or reflected beam will tend to change shape and/or change its divergence. If the beam path
involves a polariser, a beam splitter or a beam deflector the power/energy output and/or the beam propagation
of the laser system may change. These effects may be amplified if the component is rigidly restrained. If the
strain is high enough the component may crack.
The absorption coefficients of most materials are usually only slightly non-linear with increase in temperature.
However, transmissive components made from most semi-conducting materials exhibit a highly non-linear
absorption coefficient with a sharp threshold, significantly below the melting point of the material. This
phenomenon is termed “thermal runaway” and effectively limits the optical power loading at which these
materials can be used. This thermal runaway threshold is accompanied by a sharp increase in the absorption
and an increase in the accompanying distortion and thermal lensing. The refractive indices and linear
expansion coefficients are both temperature sensitive but do not necessarily have the same sign.
Distortion will occur when a component is irradiated nonuniformly and especially when it is held rigidly and
constrained from expanding. The material will expand because of the thermal loading and if this is not uniform
the component will bow or lens, thus changing the optical figure. In addition, in the case of transmissive
components, it is possible that the temperature dependence of the refractive index of the material will cause a
thermal lensing effect. In general, non-uniform expansion changes the focussing properties of the component.
In the case of non-linear absorbing windows and mirrors (e.g. Ge, ZnS and ZnSe used with infrared beams)
the effects have been observed to severely affect the transmitted beam divergence. In the case of planar
reflective interferometric components these have been observed to become convex. In practice, even minor
distortion of in-situ laser components leads to changes in the divergence and beam propagation ratio of the
laser beam and to loss of laser output.
Strain in crystalline components leads to induced birefringence and thus to changes in transmission/reflection
and this leads to fluctuations in the system output, to the necessity for raising the input power and to non-
linearity in the input/output characteristics of the system. Even homogeneous materials can exhibit
birefringence if the thermal loading is non-uniform and the component mounting constrains the material from
expanding. The laser output of optically thin laser rods in a flash-tube pumped close coupled or elliptical
pumping chamber is governed by the circularly symmetric induced birefringence. Changes in birefringence
commonly lead to a change of the transmission of the laser beam through the system, particularly if
polarisation sensitive elements are in the beam train.
Thermally induced strain, due to non-uniform irradiation, can occur even in a freely mounted component. Most
optical components are, however, mounted in a holder, which is used to control the angular position of the
component. If this mount does not allow differential expansion to take place, then the component will become
increasingly strained as the component is irradiated. When this strain reaches the elastic limit the component
will crack. This is perhaps one of the main causes of the unusually low laser induced damage thresholds
encountered in practice. The effect is mainly encountered in high prf (pulse repetition frequency), long pulse
and cw (continuous waves) laser systems. Thermally induced strain, due to rigidity in the component mounting
and/or lack of expansion gaps in the component/mount combination, is perhaps the greatest cause of laser
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ISO/TR 22588:2005(E)
components failing in the active system context. The induced strain either cracks the component or lowers the
thermal loading at which melting occurs.
The thermally induced effects are minimised if the component is held freely and maximised if clamped hard.
The stress involved can be positive or negative depending on the relative differences in the coefficients of
thermal expansion between the holder and the component. As this is the decisive factor it is necessary to
make the measurement with the component in its holder under, as near as possible, the system working
conditions and environment. In practice, this may make it hard/impossible to perform some of the
measurements suggested. Therefore, although the measurement of distortion is the most basic and relevant,
it may be necessary to monitor either the changes in birefringence or the changes in beam propagation ratio
of the transmitted beam. Measurement of the change in the laser induced damage threshold between free and
clamped components is not expected to be routine as it is catastrophic.
All the effects mentioned lead to a shortening of the component life and/or a change in the output
[1]
characteristics of the laser . They also form the main source of friction between the component suppliers
and the system manufacturers/users. The effects are most commonly observed in the case of high prf, long
pulse and cw laser systems (e.g. welding lasers). However they have also been seen to influence the
energy/power output of single-shot Q-switched Nd:YAG lasers operating at 1 064 µm and the transmission
and divergence of planar Ge windows under short pulse CO , 10,6 µm laser irradiation.
2
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TECHNICAL REPORT ISO/TR 22588:2005(E)
Optics and photonics — Lasers and laser-related equipment —
Measurement and evaluation of absorption-induced effects in
laser optical components
1 Scope
This Technical Report specifies standard measurement and evaluation techniques for determining the
absorption-induced effects caused by lasers in laser optical components in order to provide useful information
to reduce conflict between users and suppliers of optical components.
2 Absorption
2.1 General
Absorption is a fundamental property of a material. It is directly related to the electronic structure of the
material and the wavelength of the probe radiation. For transmitting materials the absorption is directly related
to the band gap. A schematic of the spectral transmission of a material is shown in Figure 1. The exact placing
and spacing of the different absorption edges are defined by the material structure, including impurities. In the
case of materials used for optical windows, which transmit in the visible, the absorption coefficient is small and
hardly varies with increasing temperature. In the case of both ultra-violet and infra-red transmitting materials
the absorption coefficient is non-negligible and has to be taken into account. In addition, most, but not all,
infra-red transmitting materials are semi-conducting and exhibit non-linear optical absorption. These materials
have low, usable, absorption characteristics at low/room temperatures but exhibit a thermal runaway threshold
at elevated temperatures. Above this thermal runaway temperature the absorption coefficient increases
sharply and the transmission of the window drops. In addition any non-linearity in the incident beam is
reflected in a lensing effect which changes the quality of the transmitted beam drastically.
Most optically transmitting window materials are, nowadays, homogeneous. However the absorption of a
material may be vitally affected by impurities, both localised and diffused through the lattice.
The reasons for absorption may be broken down into a variety of effects.
a) Bulk absorption
–αx
I = I e
0
This absorption may be permanent or induced. Absorption in the visible, for example, may be induced by
absorption of ultra-violet radiation, forming colour centres (trapping of electrons at negative ion vacancies).
[2]
This commonly occurs in the halides (NaCl and KCl) and in Nd:YAG laser crystal . The latter example is
the reason why many Nd:YAG lasers gradually lose output with time and why it is sensible to fit a
Nd:YAG pump cavity with an ultra-violet filter. Most instances of colour centres can be nullified by suitable
heat treatment.
Absorption is a function both of the electronic structure of a material and the wavelength at which it is
irradiated. Single-photon absorption will occur if the photon energy is great enough to bridge the energy
gap between the valence band and the conduction band. This is independent of the energy density
(Beer's law) but is crucially dependent on the wavelength of the probe radiation. Two-photon absorption
can occur if the two photons arrive simultaneously and the sum of the energies exceeds the band gap. At
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constant pulse length this process is linearly dependent on energy density. Multi photon absorption can
occur as long as, again, the photons arrive simultaneously and the sum of the photon energies exceed
the band gap. This process becomes more likely as the pulse length decreases but the total energy
density remains constant. There is also an intermediate absorption path where electrons elevated to
energy levels within the band gap can then absorb a second photon and thus populate the conduction
[3]
band . Figure 2 shows a comparison of the absorption behaviour of CaF at 248 nm, 193 nm and
2
157 nm. The lowest trace (measured using 248 nm radiation) indicates a constant absorption indicating
an absence of 2- and 3-photon absorption. At 193 nm there is a strong energy dependent absorption
indicating two-photon absorption. At 157 nm only weak two-photon absorption was shown as the
combined energy of two photons exceeds the vacuum level, however the linear absorption at 157 nm is
about three times that at 193 nm. It will be noticed from Figure 2, where data from two different samples
are shown, that the absorptions are strongly sample dependent. This is confirmed by the measurements
shown in Figure 3 where the β data from 19 different samples are plotted versus their corresponding A
eff 0
values. It will be noticed that there is a linear relationship. This points to a dominant two-step absorption
process as opposed to an intrinsic two-photon absorption. Ratification of this analysis is that the
comparison of β versus pulse length, τ, also shows a linear relationship (see Figure 4).
eff
b) Surface absorption
Scratches, digs and adsorbed contaminants all offer the possibility of increased absorption. It is not
always possible to clean this absorption off the surface without damaging it. Figure 5 shows the
[2]
absorption measured as a function of the resistivity, for n-doped single-crystal germanium . The upper
trace shows the absorption measured in air while the lower trace shows the absorption measured under
vacuum. Analysis indicated that the germanium surface in air had a 100 µm layer of water adsorbed on
its surface. Further experimentation indicated that this water-absorbing layer could be eliminated
temporarily by drying or cleaning but that the water layer grew back fairly fast if the component was left in
a humid atmosphere.
c) Sub-surface absorption
Machining and cutting the surface of a material involve straining the surface and, it has been proved,
leaving a damaged layer under the visible surface. A number of measurement programmes have come
up with a figure for the depth of the damaged layer, under tightly controlled machining conditions, of 100.
Polishing, etching and annealing can do a lot to remove this disordered, polycrystalline layer but there is
usually a remnant of the effect left. Some polishing materials, if chosen wrongly, may also leave
absorbing material lodged just underneath the surface.
d) Localised absorption
Some materials contain particulate material dispersed throughout the bulk. When this is sub-micron the
absorption related to this material simply raises the bulk absorption. When the particles are larger they
may absorb radiation without heating the lattice. In these cases differential strain will occur and
catastrophic damage result. Platinum inclusions were a common occurrence in the early Nd-doped laser
glass. Dust particles and misoriented crystallites are the chief problem in solution grown materials and in
natural crystals.
e) Transient absorption
[4]
Colour centres may give rise to intensity dependant absorption. This has been observed in fused silica ,
[5, 6] [2] [3]
Ruby , Nd:YAG laser crystal and MgF . The colour centre absorption may only be low at the
2
wavelength of operation but it may give rise to absorption, which is only effective while the radiation is
high. Figure 6 shows the transient absorption (lower trace) measured, at 0,63 µm, when a Nd:YAG laser
rod was flash-pumped (upper trace). The extra absorption could only be measured during the pulse
duration.
[3]
The induced absorption in MgF measured at 193 nm as a function of probe intensity is shown in
2
Figure 7. This Figure indicates both the energy dependent two-photon absorption and the increase in this
absorption with irradiation level.
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f) Non-linear absorption
Both stimulated Brillouin and Raman scattering or multi-photon absorption may occur at high optical
intensity levels. These effects occur in many optical fibres and effectively limit the transmission of these
materials at the wavelength of incoming laser light.
g) Free-electron absorption
This, non-linear absorption, occurs in all semi-conducting materials and results in a thermally non-linear
absorption and thermal runaway. The thermal runaway temperature is usually well below the melting
point of the semi-conducting material but effectively acts as a limit to the temperature at which the
material can be used. Figure 8 shows traces of the absorption, measured at 10,6 µm, as a function of the
ambient temperature for both germanium and zinc selenide.
h) Conduction electron absorption
The absorption in electronically conducting materials, e.g. metals, is a function of the plasma frequency
and is therefore affected by the temperature of the material. The skin-depth and the thermal conductivities
dominate the absorption coefficients of metal mirrors although it has also been shown that the surface
[2]
roughness has a marked effect (see Figure 9). This indicates that as the surface is heated the
absorption rises. This gives rise to extra heating, distortion and a lowering of the laser-induced damage
threshold (LIDT). However if the substrate is cooled sufficiently these effects do not occur.
2.2 Measurement of absorption
It has been shown that, as long as the measurement rules are followed, it is possible to make corroborative
measurements of the absorptance of optical components at different laboratory locations. ISO/TC 172/SC 9
[21]
has produced a standard procedure document for these measurements (ISO 11551 ) and also, in the
process of doing so, made a range of comparative measurements.
The main problems with obtaining agreement on the absorption of specified components have been:
⎯ Attempting to make measurements with too large or too small a probe spot size. Radiation scattered
around the edge of a component will add to the measured absorption. If there is any particulate matter or
surface scratches present they may cause an increase in the absorption, particularly if the probe spot is
comparable with the discontinuity. Unless the optical component under measurement is uniformly
homogeneous the probe spot size and placement may be critical.
⎯ Differences in the calibration. It is important that the “standard” sample, where used, is well characterised
and that it is tested at all laboratories in the investigation.
⎯ Measurements may be made using either pulsed or cw probe radiation. It must be realised that the
conductivity of the sample may affect the temperature measured. If the more accurate pulsed
measurement procedure is used it must be noted that it is sensible to make measurements at a range of
different probe pulse lengths and/or pulse energy levels to ensure that the differences between the
thermal diffusivity of the sample and the standard do not lead to wrong measurements.
⎯ The absorption measurement must be made under identical ambient atmospheric conditions and at the
same wavelengths. Measurements made under vacuum and in air may give radically different results
because most surfaces adsorb water. Water vapour absorbs strongly at specific wavelengths in the
infrared. In practice, it is recommended that the measurement should be made under the identical
conditions that pertain in the system in which the components are to be used.
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3 Distortion
3.1 General
When an optical component is irradiated by a laser beam, the material on the axis of the laser beam will
expand, due to heating because of absorption of radiation. The periphery of the component is likely to remain
cooler due to conduction of the heat away by the rest of the system.
A good example of the (reversible) thermal lensing effect in fused silica during 193 nm irradiation has been
[3]
published . This measurement was performed with a Shack-Hartmann wavefront sensor, using the
transmission of a collimated 193 nm probe beam through the centre of the sample.
If the component is properly located but allowed to expand, the distortion may be minimised (the whole
component expanding uniformly). If the component is held in a constricting holder distortion may well arise
both from the non-uniform expansion and because of the restriction imposed by the holder. If there is a
requirement to gauge the effect of irradiation on a component in a given system then it is necessary to
measure the distortion under the same radiation geometry and power levels as will be expected in the final
system. There are three different possibilities:
a) Measurement of distortion under free standing conditions.
This is relatively easy, as long as the irradiation power levels and geometry can be duplicated. However,
except as a means of measuring the absolute maximum power that could be placed on the component if
the holder was perfect, this is not very useful in practice.
b) Measurement of distortion in a sample holder of the correct dimensions, under the same irradiation levels
that occur in practice.
c) Measurement of distortion of the component inside the complete system.
This can be an extremely difficult measurement. It is however the final case on which the performance of
the component depends.
The justification for undertaking this measurement lies in the possibility of helping the optics manufacturer and
the user agreeing on a specification (including manufacturing tolerances) for the optical components for use in
a given system. It is therefore in the interests of both the optics manufacturer and the systems manufacturer to
agree on a measurement system which fulfills their requirements.
3.2 Measurement of distortion
The optics manufacturer and the systems buyer must identify an interferometer in which the distortion
[25]
measurements can be made (see ISO 14999 series ). The component holder must be as close as possible
to the system holder while allowing the measurement to be made. A simple schematic of a Michelson
interferometer is shown in Figure 10 with the optical component in place of one of the standard reflecting
components. The object of the measurement is to allow the measurement of the change of shape of the
optical component when it is irradiated under the identical conditions as it would be in the final system. The
procedure is to monitor the change of shape of the component under increasing irradiation levels, up to that
expected in the system.
3.3 Discussion
Measurements of the distortion induced when a laser component is irradiated are not well documented. It is
surmised that most manufacturers do not realise that this effect may be important or do not want to advertise
the fact that their systems are not temperature insensitive. The best documented results are those from the
manufacturers of He-Ne gas lasers in the 1980s where a lot of effort was put in to ensure that the output of the
lasers did not degrade due to thermal expansion. Earlier than this He-Ne laser outputs were startlingly
sensitive to the temperature of the components and most insisted that a warm up time of (e.g.) 30 min was
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necessary to achiev
...
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