EN ISO 13695:2024
(Main)Optics and photonics - Lasers and laser-related equipment - Test methods for the spectral characteristics of lasers (ISO 13695:2024)
Optics and photonics - Lasers and laser-related equipment - Test methods for the spectral characteristics of lasers (ISO 13695:2024)
This document specifies methods by which the spectral characteristics such as wavelength, bandwidth, spectral distribution and wavelength stability of a laser beam can be measured. This document is applicable to both continuous wave (cw) and pulsed laser beams. The dependence of the spectral characteristics of a laser on its operating conditions may also be important.
Optik und Photonik - Laser und Laseranlagen - Prüfverfahren für die spektralen Kenngrößen von Lasern (ISO 13695:2024)
Dieses Dokument legt Verfahren fest, mit deren Hilfe spektrale Kenndaten gemessen werden können, z. B. Wellenlänge, Bandbreite, spektrale Verteilung und Wellenlängenstabilität eines Laserstrahls. Dieses Dokument ist sowohl auf Dauerstrichlaserstrahlen (cw) als auch auf Pulslaserstrahlen anwendbar. Die Abhängigkeit der spektralen Kenndaten eines Lasers von den vorliegenden Betriebsbedingungen kann ebenfalls von Bedeutung sein..
Optique et photonique - Lasers et équipement associé aux lasers - Méthodes d'essai des caractéristiques spectrales des lasers (ISO 13695:2024)
Le présent document spécifie des méthodes qui permettent de mesurer les caractéristiques spectrales, telles que la longueur d’onde, la largeur spectrale, la distribution spectrale et la stabilité en longueur d’onde d’un laser. Le présent document s’applique aux faisceaux laser continu et impulsionnel. La dépendance des caractéristiques spectrales d’un laser vis-à-vis de ses conditions de fonctionnement peut être également importante.
Optika in fotonska tehnologija – Preskusne metode za spektralne lastnosti laserjev (ISO 13695:2024)
Ta dokument določa metode, s katerimi je mogoče izmeriti spektralne lastnosti, kot so valovna dolžina, pasovna širina, spektralna porazdelitev in stabilnost valovne dolžine laserskega žarka. Ta dokument se uporablja tako za trajne valove (cw) kot za pulzne laserske žarke. Pomembna je lahko tudi odvisnost spektralnih lastnosti laserja od pogojev delovanja.
General Information
Relations
Standards Content (Sample)
SLOVENSKI STANDARD
01-februar-2025
Nadomešča:
SIST EN ISO 13695:2005
Optika in fotonska tehnologija – Preskusne metode za spektralne lastnosti laserjev
(ISO 13695:2024)
Optics and photonics - Lasers and laser-related equipment - Test methods for the
spectral characteristics of lasers (ISO 13695:2024)
Optik und Photonik - Laser und Laseranlagen - Prüfverfahren für die spektralen
Kenngrößen von Lasern (ISO 13695:2024)
Optique et photonique - Lasers et équipement associé aux lasers - Méthodes d'essai des
caractéristiques spectrales des lasers (ISO 13695:2024)
Ta slovenski standard je istoveten z: EN ISO 13695:2024
ICS:
31.260 Optoelektronika, laserska Optoelectronics. Laser
oprema equipment
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
EN ISO 13695
EUROPEAN STANDARD
NORME EUROPÉENNE
December 2024
EUROPÄISCHE NORM
ICS 31.260 Supersedes EN ISO 13695:2004
English Version
Optics and photonics - Lasers and laser-related equipment
- Test methods for the spectral characteristics of lasers
(ISO 13695:2024)
Optique et photonique - Lasers et équipement associé Optik und Photonik - Laser und Laseranlagen -
aux lasers - Méthodes d'essai des caractéristiques Prüfverfahren für die spektralen Kenngrößen von
spectrales des lasers (ISO 13695:2024) Lasern (ISO 13695:2024)
This European Standard was approved by CEN on 30 November 2024.
CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this
European Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references
concerning such national standards may be obtained on application to the CEN-CENELEC Management Centre or to any CEN
member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by
translation under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC Management
Centre has the same status as the official versions.
CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway,
Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Türkiye and
United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2024 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 13695:2024 E
worldwide for CEN national Members.
Contents Page
European foreword . 3
European foreword
This document (EN ISO 13695:2024) has been prepared by Technical Committee ISO/TC 172 "Optics
and photonics" in collaboration with Technical Committee CEN/TC 123 “Lasers and photonics” the
secretariat of which is held by DIN.
This European Standard shall be given the status of a national standard, either by publication of an
identical text or by endorsement, at the latest by June 2025, and conflicting national standards shall be
withdrawn at the latest by June 2025.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN shall not be held responsible for identifying any or all such patent rights.
This document supersedes EN ISO 13695:2004.
Any feedback and questions on this document should be directed to the users’ national standards
body/national committee. A complete listing of these bodies can be found on the CEN website.
According to the CEN-CENELEC Internal Regulations, the national standards organizations of the
following countries are bound to implement this European Standard: Austria, Belgium, Bulgaria,
Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland,
Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Republic of
North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Türkiye and the
United Kingdom.
Endorsement notice
The text of ISO 13695:2024 has been approved by CEN as EN ISO 13695:2024 without any modification.
International
Standard
ISO 13695
Second edition
Optics and photonics — Lasers
2024-11
and laser-related equipment —
Test methods for the spectral
characteristics of lasers
Optique et photonique — Lasers et équipement associé aux lasers
— Méthodes d'essai des caractéristiques spectrales des lasers
Reference number
ISO 13695:2024(en) © ISO 2024
ISO 13695:2024(en)
© ISO 2024
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
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Email: copyright@iso.org
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Published in Switzerland
ii
ISO 13695:2024(en)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols and abbreviated terms. 7
5 Traceability . 8
6 Measurement of wavelength and bandwidth . 9
6.1 General .9
6.1.1 Preparations .9
6.1.2 Common laser types .9
6.2 Types of measurements .9
6.2.1 General .9
6.2.2 Low accuracy measurements .10
6.2.3 Medium accuracy measurements .10
6.2.4 High accuracy measurements .10
6.3 Equipment selection .10
6.4 Measurements in air .11
6.5 Measurements at low resolution . 12
6.5.1 Principle . 12
6.5.2 Measurement procedure . 12
6.5.3 Analysis . 13
6.6 Measurement at higher resolution . . 13
6.6.1 General . 13
6.6.2 Preliminary test . 13
6.6.3 Measurement with a grating spectrometer .14
6.6.4 Measurement with an interferometer .14
6.6.5 Measurement with photoelectric mixing methods . 15
−5 −4
6.6.6 Analysis for medium accuracy U /λ = U /ν in the range 10 to 10 .16
λ ν
−5
6.6.7 Analysis for high accuracy U /λ = U /ν < 10 .16
λ ν
7 Measurement of wavelength stability. 17
7.1 Dependence of the wavelength on operating conditions .17
7.2 Wavelength stability of a single frequency laser .17
8 Test report . 17
Annex A (informative) Refractive index of air .20
Annex B (informative) Criteria for the choice of a grating monochromatorand its
accessories — Calibration .21
Annex C (informative) Criteria for the choice of a Fabry-Perot interferometer .24
Bibliography .25
iii
ISO 13695:2024(en)
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).
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent
rights in respect thereof. As of the date of publication of this document, ISO [had/had not] received notice of
(a) patent(s) which may be required to implement this document. However, implementers are cautioned that
this may not represent the latest information, which may be obtained from the patent database available at
www.iso.org/patents. ISO shall not be held responsible for identifying any or all such patent rights.
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, in collaboration with the European Committee for Standardization (CEN)
Technical Committee CEN/TC 123, Lasers and photonics, in accordance with the Agreement on technical
cooperation between ISO and CEN (Vienna Agreement).
This second edition cancels and replaces the first edition (ISO 13595:2004) of which it constitutes a minor
revision.
The main changes are as follows:
— editorial changes related to the new format;
— the symbol for side-mode suppression ratio was adapted from SMS to R ;
SMS
— lg was changed to log in 3.15;
— the title of the SC 9 was updated;
— intensity was adapted to irradiance;
— in the Bibliography Reference 2 was updated and replaced by References 2 and 3.
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 13695:2024(en)
Introduction
The spectral characteristics of a laser, such as its peak wavelength or spectral linewidth, are important for
potential applications. Examples are the specific application requirements of interferometry and lithography.
This document gives definitions of key parameters describing the spectral characteristics of a laser, and
provides guidance on performing measurements to determine these parameters for common laser types.
The acceptable level of uncertainty in the measurement of wavelength will vary according to the intended
application. Therefore, equipment selection and measurement and evaluation procedures are outlined for
three accuracy classes. To standardize reporting of spectral characteristics measurement results, a report
example is also included.
v
International Standard ISO 13695:2024(en)
Optics and photonics — Lasers and laser-related equipment
— Test methods for the spectral characteristics of lasers
1 Scope
This document specifies methods by which the spectral characteristics such as wavelength, bandwidth,
spectral distribution and wavelength stability of a laser beam can be measured. This document is applicable
to both continuous wave (cw) and pulsed laser beams. The dependence of the spectral characteristics of a
laser on its operating conditions may also be important.
2 Normative references
The following referenced documents are indispensable for the application 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/IEC Guide 99, International vocabulary of metrology — Basic and general concepts and associated terms (VIM)
IEC 60747-5-1, Discrete semiconductor devices and integrated circuits — Part 5-1: Optoelectronic devices — General
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 11145, ISO/IEC Guide 99 and
IEC 60747-5-1, 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
wavelength in vacuum
λ
wavelength of an infinite, plane electromagnetic wave propagating in vacuum
Note 1 to entry: For a wave of frequency f, the wavelength in vacuum is then given by λ = c/f, where c = 299 792 458 m/s.
3.2
wavelength in air
λ
air
wavelength of radiation propagating in the air and related to the wavelength in vacuum by the relationship:
λ = λ / n
air 0 air
where n denotes the refractive index of ambient air (see 6.4)
air
Note 1 to entry: The specific properties of the ambient atmosphere, such as humidity, pressure, temperature and
composition all influence n . Therefore it is better to report the wavelength in vacuum, or the wavelength in standard
air
air. These can be calculated from λ and n using the equation given in 6.4.
air air
ISO 13695:2024(en)
3.3
wavelength in dry air under standard conditions
λ
std
wavelength of radiation propagating in dry air (0 % humidity) under standard conditions and related to the
wavelength in vacuum λ by the relationship:
λ = λ / n
std 0 std
where n denotes the refractive index of air under standard conditions (see 6.4).
std
Note 1 to entry: For the purpose of this document, air under standard conditions is as defined in 6.4. Note that various
other “standard conditions” have been reported in the literature. It is therefore necessary to quote the conditions in
the test report.
3.4
spectral radiant power [energy] distribution
P (λ), [Q (λ)]
λ λ
ratio of the radiant power dP(λ) [or energy dQ(λ) in the case of a pulsed laser] transferred by laser beam in
the range of wavelength dλ to that range
dP()λ dQ()λ
P ()λ = Q ()λ =
λλ
dλ dλ
Note 1 to entry: The radiant power (energy) delivered by the laser beam in any bandwidth λ to λ is then given by
low high
the integral:
λ
high
λ
high
PP= λλddQQ= ()λλ
()
λλ
∫ ∫
λ
low
λ
low
3.5
peak-emission wavelength
λ
p
wavelength at which the spectral radiant power (energy) distribution has its maximum value
Note 1 to entry: See Figure 1.
3.6
weighted average wavelength (first moment)
λ
g
wavelength representing the centre of gravity of the spectral radiant power (energy) distribution, as defined by:
λ
max
λλS dλ
()
∫
λ
min
λ =
g
λ
max
S()λλd
∫
λ
min
where S(λ) is the spectral radiant power P (λ) in the case of a cw laser, or the spectral radiant energy
λ
distribution Q (λ) in the case of a pulsed laser
λ
Note 1 to entry: See Figure 1.
Note 2 to entry: For choosing of the integration limits λ and λ , see 6.2.2.
min max
3.7
central wavelength
λλ
ISO 13695:2024(en)
weighted average of the wavelengths of spectral lines or modes:
ii=
max
I λ
ii
∑
ii=
min
λ =
ii=
max
I
i
∑
ii=
min
where
λ is the wavelength of the ith spectral line or the ith mode;
i
I is the relative radiant power of the ith spectral line or the ith mode;
i
i , i denote extreme spectral lines or modes below and above λ .
min max p
Note 1 to entry: Usually, the summation limits are chosen such that the relative radiant power of spectral lines or
modes outside the limits remains less than 1 % of the relative radiant power of the strongest line or mode, located at λ .
p
Note 2 to entry: This definition is particularly useful in the case of a multi-mode laser.
3.8
average wavelength
λ
av
ratio of the light velocity c to the average optical emission frequency f
av
λ = c/f
av av
Note 1 to entry: The average optical emission frequency f can be measured directly, e.g. by the heterodyne
av
measurement method (see 6.6.5).
3.9
RMS spectral radiation bandwidth (second moment)
Δλ
second moment of the spectral radiant power (energy) distribution, as defined by:
λ
max 2
λλ− S()λλd
()
g
∫
λ
min
Δλ =
λ
max
S λλd
()
∫
λ
min
where S(λ) is the spectral radiant power P (λ) in the case of a cw laser, or the spectral radiant energy
λ
distribution Q (λ) in the case of a pulsed laser.
λ
Note 1 to entry: See Figure 1.
Note 2 to entry: For choosing of the integration limits λ and λ see 6.2.2.
min max
3.10
RMS spectral bandwidth
Δλ
rms
rms bandwidth is defined by:
ii=
max 2
I ()λλ−
ii
∑
ii=
min
Δλ =
rms
ii=
max
I
i
∑
ii=
min
where
ISO 13695:2024(en)
λ is the wavelength of the ith spectral line or the ith mode;
i
I is the relative radiant power of the ith spectral line or the ith mode;
i
is the central wavelength;
λ
i , i denote extreme spectral lines or modes below and above λ
min max p
Note 1 to entry: See Figure 1.
Note 2 to entry: Usually, the summation limits are chosen such that the relative radiant power of spectral lines outside
the limits remains less than 1 % of the relative radiant power of the strongest line, located at λ .
p
Note 3 to entry: This definition is particularly useful in the case of a multi-mode laser.
3.11
spectral bandwidth
FWHM
Δλ
H
maximum difference between the wavelengths for which the spectral radiant power (energy) distribution is
half of its peak value
Note 1 to entry: See Figure 1.
[SOURCE: ISO 11145:2018, 3.17, modified — The abbreviation “FWHM” has been added, “Δλ, Δν” has been
replaced by “Δλ ” and Note 1 to entry has been added.]
H
3.12
spectral linewidth
FWHM
Δλ
L
maximum difference between those wavelengths within δλ for which the spectral radiant power (energy)
distribution is half of its peak value found within δλ
Note 1 to entry: See Figure 1.
Note 2 to entry: cf. spectral bandwidth (3.11), Δλ
H
Note 3 to entry: A spectral linewidth is analogous to a spectral bandwidth (3.11), but is defined for a single (longitudinal)
mode or otherwise clearly distinguishable and labelled spectral feature contained within an interval δλ.
3.13
mode spacing
F (S )
msp msp
separation of two neighbouring longitudinal modes expressed in frequency (F ) (wavelength (S ))
msp msp
Note 1 to entry: See Figure 1.
ISO 13695:2024(en)
Key
λ wavelength
Figure 1 — Spectral characteristics of lasers — Illustration of defined parameters
3.14
number of longitudinal modes
N
m
number of longitudinal modes within a specified bandwidth, usually the rms spectral bandwidth Δλ
rms
3.15
side-mode suppression ratio
R
SMS
ratio of the relative radiant power of the most intense mode, I , located at λ , to the relative radiant power of
p p
the second most intense mode, I , located at λ :
s s
I
p
R =10log
SMS 10
I
s
Note 1 to entry: See Figure 2.
Note 2 to entry: In practice the R can be assumed to be equal to the ratio of the peak values of the spectral
SMS
distribution for the most intense and second most intense modes:
S λ
()
p
R =10log
SMS 10
S λ
()
s
ISO 13695:2024(en)
Key
λ wavelength
Figure 2 — Side-mode suppression ratio
3.16
pulse repetition rate
f
p
number of laser pulses per second of a repetitively pulsed laser
3.17
temperature dependence of wavelength
δλ
T
wavelength shift per change in temperature T of the laser:
dλ
δλ =
T
dT
3.18
current dependence of wavelength
δλ
c
wavelength shift per change in laser current I
dλ
δλ =
c
dI
3.19
Allan variance for a cw laser
σ (2,τ)
y
two sample variance of frequency fluctuations for an averaging time of τ seconds and is defined by:
[]yk()+−1 yk()
στ()2, =〈 〉
y
where
〈〉
denotes the average over an infinite set of data;
is the kth measurement of y in this set of data;
yk
()
y
is obtained by averaging y(t) over a time interval τ
Note 1 to entry: For frequency measurements, the fractional deviation y(t) is given by:
y(t) = [ν(t) − ν ]/ν
0 0
ISO 13695:2024(en)
where
ν(t) is the instantaneous frequency;
ν is the nominal frequency.
The measurement intervals all have the same duration τ and there is no dead time between subsequent
measurement intervals. For times τ < 100 s, the data set has to consist of at least 100 data. For larger times τ;
the number of data may be reduced but shall be stated in the test report.
Note 2 to entry: y may be derived from heterodyne measurements where a frequency difference Δν is integrated over
an interval τ and normalized to the oscillation frequency ν .
Note 3 to entry: Since y = Δν/ν = −Δλ/λ, σ (2,τ) is at the same time a measure of the frequency stability and of the
y
wavelength stability.
Note 4 to entry: For further details see Reference [1].
3.20
instrumental response function
R(λ,λ )
response, i.e. the output signal, of the instrument at the wavelength setting λ to a monochromatic input of
wavelength λ
Note 1 to entry: Usually, over the wavelength range of the instrument, R(λ,λ ) is nearly independent of the input
wavelength λ , and the second argument is omitted. For a properly adjusted instrument, the first moment of the
instrumental response function R(λ,λ ), as defined by:
λ
max
λλR ,λλd
()
∫
λ
min
λ =
g
λ
max
R()λλ, dλ
∫
λ
min
should be equal to the input wavelength: λ = λ .
g 0
3.21
instrumental effective spectral bandwidth
Δλ (λ )
ins 0
second moment of the instrumental response function R(λ,λ ), as defined by:
λ
max
()λλ− R()λλ, dλ
g 0
∫
λ
min
Δλ λ =
()
ins 0
λ
max
R()λλ, dλ
∫
λ
min
Note 1 to entry: If, as usually assumed, R(λ,λ ) and therefore Δλ (λ ) are approximately independent of the input
0 ins 0
wavelength λ , the effective bandwidth Δλ is used without argument.
0 ins
4 Symbols and abbreviated terms
Symbol Unit Term
F Hz mode spacing in the frequency domain
msp
f
Hz pulse repetition rate
p
N number of longitudinal modes
m
n refractive index of ambient air
air
n refractive index of dry air under standard conditions
std
P W/m spectral radiant power distribution
λ
ISO 13695:2024(en)
Symbol Unit Term
Q W⋅s/m spectral radiant energy distribution
λ
R()λλ, 1/m instrumental response function
S
m mode spacing in the wavelength domain
msp
spectral radiant power P (λ) in the case of a cw laser or the spectral radiant energy
λ
S(λ)
distribution Q (λ) in the case of a pulsed laser
λ
R dB side-mode suppression ratio
SMS
U expanded standard uncertainty for measurand x
x
δλ
m/K temperature dependence of wavelength
T
δλ m/A current dependence of wavelength
c
λ m wavelength
λ m wavelength in vacuum
λ m wavelength in air
air
λ m average wavelength
av
λ m weighted average wavelength (first moment)
g
λ
m peak-emission wavelength
p
λ m wavelength in dry air under standard conditions
std
m central wavelength
λ
Δλ m rms spectral radiation bandwidth (second moment)
Δλ m spectral bandwidth (FWHM)
H
Δλ m instrumental effective spectral bandwidth
ins
Δλ
m spectral linewidth (FWHM)
L
Δλ m measured spectral radiation bandwidth (second moment)
meas
Δλ m rms spectral bandwidth
rms
v Hz frequency of the wave
ν Hz free spectral range (FSR) of the Fabry-Perot (FP) interferometer
FSR
Allan variance characterizing the wavelength stability of a cw laser
στ()2,
Y
τ s pulse duration
H
5 Traceability
All measurement results shall be traceable to the SI Units. For example, the wavelength shall be traceable to the
meter by one of the methods recommended by the International Committee for Weights and Measures (CIPM).
NOTE For the meter, this is most commonly achieved by using a reference wavelength recommended by the CIPM,
for details see References [2][3].
ISO 13695:2024(en)
6 Measurement of wavelength and bandwidth
6.1 General
6.1.1 Preparations
Depending on the spectral characteristics, the intended use of the laser and on the required level of
uncertainty U or U (as defined in ISO/IEC Guide 98-3) in the measurement of wavelength (or frequency) of
λ ν
the laser, U /λ = U /ν, different parameters need to be tested, see 6.2.
λ ν
In the case of a laser with unknown characteristics, an operational test should be performed in order to
select well-adapted instrumentation and the best choice of parameters to be measured.
It is assumed in this document that the spectral characteristics of the laser beam are the same throughout
the spatial power (energy) distribution in the beam. If this is not the case, spatially resolved measurements
could be achieved by means of limiting apertures.
As a guideline, three testing levels are proposed, see 6.2.
6.1.2 Common laser types
The choice of parameters most suitable for characterizing the spectral characteristics of a laser depend on
the type of laser. Common types of laser are:
a) broad-bandwidth lasers, for example pulsed lasers, or multi-mode lasers showing significant and fast
mode fluctuations;
b) multi-mode lasers with a stable mode-structure over the time-scale of interest;
c) single frequency lasers.
For these three types of lasers, the use of the following parameters is recommended:
— for a broad-bandwidth laser:
the weighted average wavelength (first moment) λ , rms spectral radiation bandwidth (second moment)
g
Δλ or spectral bandwidth (FWHM) Δλ ; the dependence of the wavelength δλ and/or δλ , on the
H T c
operating parameters, temperature and/or injection current;
— for a multi-mode laser:
the central wavelength λ , the rms spectral bandwidth Δλ , the mode spacing F (frequency
rms msp
domain) or S (wavelength domain), the number of longitudinal modes within a specified bandwidth
msp
N ; the dependence of the wavelength, δλ and/or δλ , on the operating parameters, temperature and/
m T c
or injection current;
— for a single frequency laser:
the peak wavelength λ or average wavelength λ and the spectral linewidth Δλ and side mode
p av L
suppression ratio R ; the dependence of the wavelength, δλ and/or δλ , on the operating parameters,
SMS T c
temperature and/or injection current, the Allan variance σ (2,τ) as a measure of wavelength stability.
y
6.2 Types of measurements
6.2.1 General
The spectral characteristics of the lasers are assumed to be stable during the duration of the measurements,
though this may need evaluation through subsequent stability and drift tests (see Clause 7).
ISO 13695:2024(en)
6.2.2 Low accuracy measurements
−4
These measurements are useful at a typical uncertainty of U /λ = U /ν > 10 . This applies to broad
λ ν
bandwidth lasers, e.g. pulsed lasers or multi-mode cw lasers or measurements involving an instrument of
low resolution.
For these measurements, the individual modes need not be resolved and the weighted average wavelength,
λ , and the rms radiation bandwidth, Δλ, should be determined. The wavelength stability should be assessed
g
as a function of the operating parameters, i.e. δλ and/or δλ should be measured.
T c
For the determination of the weighted average wavelength, the integration limits λ and λ are usually
min max
chosen such that outside of this interval the spectral distribution remains smaller than 1 % of its maximum
value. In case other integration limits are used, these shall be reported in the test report.
There may be cases where the spectral distribution takes values not much smaller than 1 % of its maximum
value over a very wide range of wavelengths, e.g. for a narrow peak superimposed on a broad background. In
such a case, a considerable fraction of the total power may be found outside the integration limits. In addition,
for very narrow distributions the instrumental resolution may affect the measured maximum value of S(λ)
at λ , which in turn affects the integration limits. Care should be taken to ensure that the calculated value of
p
λ is not significantly influenced by this.
g
6.2.3 Medium accuracy measurements
−4 −5
These measurements are useful at a typical uncertainty U /λ = U /ν in the order of 10 to 10 . This applies
λ ν
to narrow bandwidth pulsed lasers or cw multi-mode lasers.
For these measurements, the individual modes are usually resolved and the mode spacing F (frequency
msp
domain) or S (wavelength domain), the number of longitudinal modes within a specified bandwidth N
msp m
and the side-mode suppression ratio R can be assessed. The central wavelength λ , the rms spectral
SMS
bandwidth Δλ should be determined. The wavelength stability as a function of the operating parameters,
rms
i.e. δλ and/or δλ , should be measured.
T c
6.2.4 High accuracy measurements
−5
These measurements are useful at a typical uncertainty of U /λ = U /ν < 10 . This applies to single mode
λ ν
lasers, or narrow bandwidth pulsed lasers.
For these measurements, possible side modes have to be identified and if applicable, the side-mode
suppression ratio R has to be determined.
SMS
The peak wavelength λ or average wavelength λ and the spectral linewidth Δλ , the dependence on
p av L
operating conditions δλ and/or δλ should be determined and, as a measure of the wavelength stability, the
T c
Allan variance σ (2,τ) should be measured.
y
6.3 Equipment selection
The proper equipment shall be chosen according to the required accuracy and the type of the laser. As an
example, a high resolution grating spectrometer may be capable of a practical resolving power R = λ/Δλ on
ins
5 6
the order of 10 to 10 .
In the case of a pulsed laser, interferometers can only be used if the pulse duration, τ , is large compared to
H
the inverse bandwidth of the instrument. For a Fabry-Perot interferometer with a free spectral range ν
FSR
and a finesse F, the minimum pulse duration is F/ν . For a two-wave interferometer with maximum path
FSR
difference L, the minimum pulse duration is L/c, where c is the speed of light.
The wavelength accuracy required may often be low. There may, however, be a requirement for high accuracy
in the measurement of the amplitude of the spectral power distribution, for example to determine spectral
flatness, ripple, etc. in the case of broad bandwidth sources.
Any optical component to be used to couple the laser beam to the measurement system (lenses, mirrors,
optical fibres, etc.) should be either spectrally insensitive, or spectrally characterized, within the range of
ISO 13695:2024(en)
measurement. Their possible sensitivity to the state of polarization of the laser beam should be wavelength
independent, or characterized by for instance, proper wavelength-dependent Mueller matrices (see
ISO 12005). The polarization-dependent spectral response of the measurement system shall be taken into
account. Devices such as grating monochromators are known to have a polarization-dependent transmission
curve. The same can be true for detectors or for other components of the measurement system.
For narrow bandwidth laser beams the transmission can often be considered as flat, independent of the
polarization state.
As many types of lasers are susceptible to optical feedback, any reflections of laser light back to the laser,
e.g. from optical windows, filters or lenses should be avoided by, for example, tilting the elements or by the
use of optical isolators.
6.4 Measurements in air
If λ is measured, the measurement results depend on environmental conditions such as temperature,
air
atmospheric pressure and humidity, as these influence the refractive index of air. Also, the refractive index
varies with the wavelength itself (dispersion). If a laser with known wavelength is used as a reference,
the influence of the refractive index partially cancels, and only the smaller effect of dispersion (and its
dependence on the environmental conditions) needs to be taken into account.
The calculation of the refractive index starts with the dispersion formula of dry air. Under standard
−6
conditions, at a temperature of 15 °C, a pressure of 101 325 Pa, a CO volume fraction of 450 × 10
1)
[450 ppm ] and 0 % humidity, the refractive index of air may be calculated using an updated Edlén-Equation
(see Reference [4):
2406147 15998
n −11×=08342,54+ +
()
std
2 2
130−(/1000nm λ),38 91−( 0000nm/)λ
NOTE 1 The above formula is accurate to about one part in 10 for 300 nm < λ < 1 700 nm. Within the visible range,
significantly higher accuracy is achieved with this formula.
NOTE 2 n (633 nm) = 1,000 276 5, n (532 nm) = 1,000 278 2, n (1 530 nm) = 1,000 273 3.
std std std
— If the accepted level of uncertainty in the measurement of wavelength (or frequency) of the laser, U /λ = U /ν
λ m v m
−4
is larger than 10 , the atmospheric conditions need not be taken into account explicitly.
— If the accepted level of uncertainty in the measurement of wavelength (or frequency) of the U /λ = U /ν is less
λ m v m
−4
than or equal to 10 , measurement results shall be corrected by the following formula (see Reference [4]):
−5
1,04063221× 0 p
21− 0
nn−11=− × ×+11ε ×+x −−×f 3,,73450−×04011(/000nm λ) ×10
() () () ()
airstd
10+×,0036610 T
where
n is the refractive index in air;
air
n is the refractive index in dry air under standard conditions, see above, at the measurement wavelength;
std
T is the temperature, in °C;
f is the partial pressure of water vapour, in Pa;
p is the total atmospheric pressure, in Pa;
and the correction terms are defined as follows:
1) The use of ppm is deprecated. 1 ppm = 1 μl/l.
ISO 13695:2024(en)
−8
1 + ε is the higher order, p and T, correction term, and 1 + ε = 1 + 10 × p × (0,601 − 0,009 72 × T);
−6
1 + x is a term taking into account deviations of CO volume fraction, φ , from 450 × 10 [450 ppm], and 1 +
2 CO2
x = 1 + 0,54 × (φ − 0,000 45).
CO2
Both correction terms may be taken to be equal to 1, if the accepted level of uncertainty in the wavelength, U /λ ,
λ m
−6
is larger than 10 .
φ is the CO volume fraction, in air.
CO2 2
−7
NOTE 3 The refractive index n is changed by approximately 1 × 10 by each of the following changes in the
air
−6
environmental conditions: temperature: ΔT = 0,1 °C, pressure: Δp = 30 Pa (or 0,3 mbar), CO -content: Δφ = 600 × 10
2 CO2
[600 ppm], humidity: Δf = 250 Pa.
NOTE 4 The above equations assume normal composition of the atmosphere. Enclosed apparatus may contain
−7
vapours of oils or solvents changing the refractive index by 1 × 10 or more. Particularly in the near infrared, the
wings of infrared absorption lines of water vapour, CO or other gases may need to be taken into account.
Further details may be found in Annex A.
If the accepted level of uncertainty in the measurement of wavelength (or frequency) of the laser, U /
λ
−7
λ = U /ν , is smaller than 10 , wavelength measurements shall be performed in vacuum or by frequency
m v m
measurements by heterodyne methods.
6.5 Measurements at low resolution
6.5.1 Principle
For unknown sources, a preliminary low-resolution measurement of the weighted average wavelength and
spectral radiation bandwidth shall be done in order to determine the required instrumentation.
For this test a grating monochromator of moderate size (focal length of the order of 30 cm) is appropriate.
A single instrument may be used for all kinds of laser devices, but the choice of some components and
accessories shall be made according to the spectral domain of the laser radiation.
The aspects of the choice of the instrument and the accessories are given in the informative Annex B.
6.5.2 Measurement procedure
The laser beam to be measured, or a fraction of this beam extracted from an appropriate beamsplitter,
shall be directed onto the input of the instrument e.g. the entrance slit of the monochromator. The aperture
ratio of the instrument should be matched to the beam by means of an appropriate optical system. This
usually requires focusing of the laser beam. It should be remembered that the instrument, e.g. the lips of the
entrance slit, can be damaged if too high a power density is used, and attenuators can be used if necessary.
The value of the effective spectral bandwidth of the instrument, Δλ , shall be checked using, as a reference,
ins
the beam of a narrow-linewidth laser adjusted to form a beam following approximately the same geometry.
For this test a narrow-linewidth laser can be any laser device known to provide a beam of spectral bandwidth
and of wavelength drift fluctuation at least 10 times smaller than the required value for Δλ (see 6.5.3). In
ins
many cases, a 633 nm free-running He-Ne laser will be adequate.
If the instrument is scanned over the wavelength range of inter
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