ISO 17915:2018
(Main)Optics and photonics — Measurement method of semiconductor lasers for sensing
Optics and photonics — Measurement method of semiconductor lasers for sensing
This document describes methods of measuring temperature and injected current dependence of lasing wavelengths, and lasing spectral line width in relation to semiconductor lasers for sensing applications. This document is applicable to all kinds of semiconductor lasers, such as edge-emitting type and vertical cavity surface emitting type lasers, bulk-type and (strained) quantum well lasers, and quantum cascade lasers, used for optical sensing in e.g. industrial, medical and agricultural fields.
Optique et photonique — Méthode de mesure des lasers semi-conducteurs pour la sensibilité
General Information
Relations
Standards Content (Sample)
INTERNATIONAL ISO
STANDARD 17915
First edition
2018-05
Optics and photonics —
Measurement method of
semiconductor lasers for sensing
Optique et photonique — Méthode de mesure des lasers semi-
conducteurs pour la sensibilité
Reference number
©
ISO 2018
© ISO 2018
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.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Fax: +41 22 749 09 47
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii © ISO 2018 – All rights reserved
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Optical sensing using semiconductor lasers . 1
4.1 General . 1
4.2 Semiconductor laser. 1
4.2.1 General. 1
4.2.2 Basic structure . 2
4.2.3 Transverse mode stabilizing structure . 2
4.2.4 Mode (wavelength) selection structure . 2
4.2.5 Active layer structure . 2
4.3 Common sensing technique and equipment using semiconductor lasers . 3
4.3.1 General. 3
4.3.2 Tunable laser absorption spectroscopy (TLAS). 3
4.3.3 Cavity ring down spectroscopy (CRDS) . 4
4.3.4 Photoacoustic spectroscopy (PAS) . 5
4.4 Temperature and current dependence of wavelength . 6
4.5 Effect of current injection on lasing wavelength . 8
4.6 Effect of ambient temperature on lasing wavelength . 9
5 Measurement method for temperature dependence of wavelength .10
5.1 General .10
5.2 Description of measurement setup and requirements .10
5.3 Precautions to be observed .11
5.4 Measurement procedures.12
6 Measurement method for current dependence of wavelength .12
6.1 General .12
6.2 Description of measurement setup and requirements .12
6.3 Precautions to be observed .13
6.4 Measurement procedures.14
6.4.1 Static current dependence.14
6.4.2 Dynamic current coefficient .14
7 Measurement method of spectral line width .14
7.1 General .14
7.2 Description of measurement setup and requirements .15
7.3 Precautions to be observed .18
7.4 Measurement procedures.18
7.4.1 System employing two semiconductor lasers [shown in Figures 11 and 12] .18
7.4.2 Self-delayed heterodyne [shown in Figure 13] . .18
Annex A (informative) Essential ratings and characteristics .20
Bibliography .29
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.o rg/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 on 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 the following
URL: 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.
This first edition cancels and replaces the Technical Specification ISO/TS 17915:2013, which has been
technically revised.
The main changes compared to ISO/TS 17915:2013 are as follows:
— interband cascade semiconductor lasers have been included in 4.2.5.
— in A.3: Regarding the monitor photodiode, “option” has been inserted.
— Tables in Annex A have been separated for clarity.
iv © ISO 2018 – All rights reserved
Introduction
Sensing technologies for materials related to the environment or wellness, etc., by using lasers have
been researched and developed in academic and industrial fields. Semiconductor lasers including
quantum cascade semiconductor lasers have been widely used in sensing applications because of their
advantages of compactness and wide selectivity of lasing wavelengths. The tunable laser absorption
spectroscopy, the cavity ring down spectroscopy and the photoacoustic spectroscopy are commonly
used sensing techniques. In those sensing techniques, wavelength and/or spectrum analysis by
changing temperature or injected current is the key for determining the composition or element of the
material or the mixture to be examined. Therefore measuring methods of semiconductor lasers for
sensing applications are described with an informative annex for an example of essential ratings and
characteristics.
INTERNATIONAL STANDARD ISO 17915:2018(E)
Optics and photonics — Measurement method of
semiconductor lasers for sensing
1 Scope
This document describes methods of measuring temperature and injected current dependence of lasing
wavelengths, and lasing spectral line width in relation to semiconductor lasers for sensing applications.
This document is applicable to all kinds of semiconductor lasers, such as edge-emitting type and vertical
cavity surface emitting type lasers, bulk-type and (strained) quantum well lasers, and quantum cascade
lasers, used for optical sensing in e.g. industrial, medical and agricultural fields.
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 13695, Optics and photonics — Lasers and laser-related equipment — Test methods for the spectral
characteristics of lasers
3 Terms and definitions
No terms and definitions are listed in this document.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— IEC Electropedia: available at https: //www .electropedia .org/
— ISO Online browsing platform: available at https: //www .iso .org/obp
4 Optical sensing using semiconductor lasers
4.1 General
The methods described in this document shall be followed in accordance with ISO 13695.
Optical sensing using tunable semiconductor laser spectroscopy has been widely used in various
engineering fields. For example, optical sensing is being used for bio-sensing and environmental
monitoring. Semiconductor lasers are key devices for those applications and are indispensable for
building sensing equipment. Semiconductor lasers and sensing techniques are described in 4.2 to 4.6.
4.2 Semiconductor laser
4.2.1 General
A semiconductor laser is an optical semiconductor device that emits coherent optical radiation in a
certain direction through stimulated emission resulting from electron transition when excited by an
electric current that exceeds the threshold current of the semiconductor laser. Here, the mechanism of
coherent optical radiation is divided into the following two categories:
1) electron-hole recombination due to interband electron transition between conduction and valence
band (bulk type) or between two quantized states (quantum well type, see 4.2.5) and
2) intraband electron transition between two quantized states (quantum cascade type, see 4.2.5).
Edge-emitting types with single lasing modes, such as distributed feedback (DFB) lasers, have been
conventionally used in sensing equipment because of their high power and single lasing modes. Surface-
emitting types are also widely used in sensing systems because they are easy to handle. Some names
are given to those lasers from various aspects. Those lasers are briefly categorized in 4.2.2 to 4.2.5.
Optical and electrical characteristics of semiconductor lasers are complicated and should be described
precisely depending on the application (see Annex A for additional information).
4.2.2 Basic structure
a) Edge emitting type semiconductor laser: a semiconductor laser that emits coherent optical
radiation in the direction parallel to the junction plane.
b) Surface emitting type semiconductor laser: a semiconductor laser that emits coherent optical
radiation in the direction normal to the junction plane. A vertical cavity surface emitting
semiconductor laser (VCSEL) is typical.
4.2.3 Transverse mode stabilizing structure
a) Gain guiding: a semiconductor laser in which emitted light propagates along the gain region
generated by carrier injection and is amplified by stimulated emission along the gain region. Planar
type lasers are typical in gain guiding.
b) Refractive index guiding: a semiconductor laser in which a stripe-shape active layer (light emitting
layer) or junction is formed to introduce an effective refractive index difference between the stripe
and the outer region. A buried heterostructure (BH) is typical in refractive index guiding.
4.2.4 Mode (wavelength) selection structure
a) Distributed feedback (DFB) semiconductor laser: a semiconductor in which stimulated emission
is selected by a grating (equivalent to distributed mirror). This laser operates in single
longitudinal mode.
b) Distributed Bragg reflector (DBR) semiconductor laser: a semiconductor laser in which stimulated
emission is selected by a Bragg grating (equivalent to distributed mirror) jointed at a side or both
sides of the light emitting layer. This laser operates in single longitudinal mode.
c) Fabry-Perot (FP) semiconductor laser: a semiconductor laser in which stimulated emission is
generated between two mirror facets. This laser normally operates in multiple longitudinal modes.
d) External cavity controlled semiconductor laser: a semiconductor laser in which the optical cavity
is composed of one mirror and an external mirror (external grating) set on the opposite side of the
mirror. Stimulated emission is generated at the semiconductor part in the optical cavity. This laser
normally operates in single longitudinal mode.
4.2.5 Active layer structure
a) Double heterostructure semiconductor laser: a semiconductor laser in which the active layer (light
emitting layer) is sandwiched with two heterojunctions (pn- and iso-junction).
b) Quantum well semiconductor laser: a semiconductor laser that emits coherent optical radiation
through stimulated emission resulting from the recombination of electrons and holes between
two quantized states. Here, the light emitting layer is composed of a single quantum well layer
2 © ISO 2018 – All rights reserved
or multiple quantum well layers. A quantum wire and quantum dot (box) semiconductor laser
are included in this category but the light emitting area of the quantum wire and dot is a two-
dimensional and three-dimensional structure, respectively.
c) Strained quantum well semiconductor laser: a semiconductor laser that emits coherent optical
radiation through stimulated emission resulting from the recombination of free electrons and
holes between two quantized states. Here, the light emitting layer is composed of a strained single
quantum well layer or multiple quantum well layers.
d) Interband cascade semiconductor laser: a semiconductor laser that emits coherent optical radiation
through stimulated emission resulting from the recombination of electrons and holes between two
quantized states. Here, the light emitting layer is composed of type-II (broken gap) quantum well
layers. Carriers are generated internally by a semimetallic interface.
e) Quantum cascade semiconductor laser: a semiconductor laser that emits coherent optical
radiation through stimulated emission resulting from electron transition between two quantized
states without any electron-hole recombination. The light emitting layer is composed of quantum
cascade layers.
4.3 Common sensing technique and equipment using semiconductor lasers
4.3.1 General
Semiconductor lasers including quantum cascade semiconductor lasers have various advantages:
compact size, light weight, low power consumption, easy controlling of wavelength by pulsed or
continuous wave operation, etc. Sensing techniques and equipment using such semiconductor lasers
have been researched and developed in academic and industrial fields. The main sensing techniques
are described in 4.3.2 to 4.3.4.
4.3.2 Tunable laser absorption spectroscopy (TLAS)
An absorption spectrum is monitored by scanning repeatedly the wavelength of light emitted from the
semiconductor laser as shown in Figure 1. The composition of material and mixture to be examined are
qualitatively and quantitatively analysed based on the monitored spectrum (shape, peak wavelength
and intensity). The lasing wavelength of the semiconductor laser is scanned by controlling the ambient
temperature or injected current in this technique.
Key
X wavelength
Y optical intensity
1 tunable laser diode
2 lens
3 cell
4 element to be detected
5 optical detector
Figure 1 — Basic concept of tunable laser absorption spectroscopy (two absorption peaks are
observed)
4.3.3 Cavity ring down spectroscopy (CRDS)
This technique is usually used for detecting trace elements and originated from tunable semiconductor
laser spectroscopy. Material to be analysed is introduced into the cavity built up with two mirrors
as shown in Figure 2. A light pulse (with a certain wavelength) introduced to the cavity is repeatedly
reflected between the mirror and passes through the material. A part of reflecting light escapes through
the mirror, and a pulse train with a time interval determined with the cavity length is monitored. The
trace element is qualitatively and quantitatively analysed with the decay time of the pulse train and the
wavelength of the light.
4 © ISO 2018 – All rights reserved
Key
X wavelength
Y optical intensity
A optical pulse
B optical pulse train
1 tunable laser diode
2 lens
3 cell
4 element to be detected
5 optical detector
6 mirror
Figure 2 — Basic concept of cavity ring down spectroscopy
4.3.4 Photoacoustic spectroscopy (PAS)
When material to be analysed is illuminated with laser light, the light is absorbed at the material.
The light power absorbed induces a lattice vibration, and the vibration results in the emission of a
supersonic wave as shown in Figure 3. The supersonic wave is detectable with a microphone, and the
element contained in the material is quantitatively analysed by monitoring the frequency and intensity.
Key
1 tunable laser diode
2 lens
3 cell
4 element to be detected
5 supersonic wave
6 microphone
Figure 3 — Basic concept of photoacoustic spectroscopy
4.4 Temperature and current dependence of wavelength
The lasing wavelength of semiconductor lasers is changed by various methods.
In normal semiconductor lasers, their lasing wavelength is ordinarily controlled by varying the
ambient temperature and the injected current in tunable semiconductor laser spectroscopy. These
variables correspond to a band-gap change due to ambient temperature and the band-filling effect
induced by carriers injected into the active layer of semiconductor lasers. In addition, refractive
index change of the active layer, which is induced by temperature and injected carrier density, takes
an important role of changing the lasing wavelength. The changing rate of these physical properties
determines the conventionally used temperature and current dependence of lasing wavelength. The
physical mechanisms of temperature and current control of the lasing wavelength are explained in this
subclause.
In external cavity controlled semiconductor lasers, the lasing wavelength can be selected by controlling
the angle of grating if the grating is set as an external mirror. The lasing wavelength is widely scanned
by controlling the grating angle.
Several factors govern the change in lasing wavelength of semiconductor lasers as shown in Figure 4.
A decrease (an increase) in the refractive index of the active region originates from an increase (a
decrease) in threshold carrier density and shortens (lengthens) the lasing wavelength of each Fabry-
Perot (FP)-mode in FP semiconductor lasers. This phenomenon is induced by the plasma effect related
to carrier density in semiconductors. In DFB semiconductor lasers, the lasing mode is shortened
(lengthened) with a decrease (an increase) in effective grating pitch introduced by the decrease
(increase) in the refractive index. The increase (decrease) in the refractive index is introduced by a
rising (lowering) temperature. In addition, the rising (lowering) temperature shifts the envelope of FP-
modes (gain envelope) to the longer (shorter) range. This is due to a reduction (an increase) of the band-
gap energy.
Before lasing, the peak wavelength of FP-modes shortens due to the band-filling effect, and that of DFB-
modes also shortens as the injected carrier density increases through the refractive index reduction.
After lasing, the main factor is the thermal effect because threshold carrier density is fixed at the
threshold value after lasing. Joule heating is generated and light output power changes in response to
the injected current under the constant carrier density.
6 © ISO 2018 – All rights reserved
These are basic mechanisms for changing the lasing wavelength in semiconductor lasers. Among them,
the change in lasing wavelength by controlling ambient temperature under a constant current is mainly
generated by a band-gap change in FP semiconductor lasers and a refractive index change in DFB
semiconductor lasers. Controlling the lasing wavelength with the magnitude of the injected current
also occurs by the band-gap change due to Joule heating at the active layer (or pn-junction) because the
injected carrier density is nearly constant after lasing. The temperature and current dependence of
lasing wavelength is analysed in DFB semiconductor lasers from the viewpoint of thermal conductivity
in the following parts.
Key
X wavelength
Y intensity
1 gain envelope
2 energy level change due to band filling
3 band gap change due to temperature increase
4 refractive index change due to carrier (plasma) effect
5 refractive index change due to heating
6 each lasing mode
Figure 4 — Main factors of lasing-wavelength change
Key
1 semiconductor laser
2 active layer
3 heat sink
4 package stem
5 heat flow
Figure 5 — Sample configuration
Key
X current pulse width
Y active layer temperature, in ° C
1 LD chip
2 heat sink
3 package stem
4 package
NOTE 1 Pulse height: 100 mA.
NOTE 2 The sample is a 1 300 nm-band FP semiconductor laser. The labels indicated by 1, 2, 3 and 4 indicate
the responsible parts of heat conduction for the heat generated at the active layer.
Figure 6 — Estimated temperature rise in active layer as a function of pulse width
4.5 Effect of current injection on lasing wavelength
The rate of temperature change in the active layer depends on a transient phenomenon determined
by heat conduction. As shown in Figure 5, the Joule heating generated at the active layer gradually
diffuses from the active layer to the surrounding region, and thus the change rate in lasing wavelength
strongly depends on the mounting configuration and packaging structure. Figure 6 shows an example
of active layer temperature increase as a function of the current pulse width for a 1 300 nm-band FP
semiconductor laser. The active layer temperature is estimated from the junction voltage because the
junction voltage linearly decreases with temperature. The junction voltage at 1 mA is monitored just
after turning off the 100 mA-pulsed current. The pulsed current and the monitoring current of the
junction voltage is set at 100 mA and 1 mA, respectively. Here, the value of the monitoring current is
determined so that the Joule heating due to the current is negligible. The temperature dependence of
the junction voltage is about 1 mV/°C in the 1 300 nm-band semiconductor lasers. The Joule heating due
to current injection diffuses within the laser chip and then towards the outside of the active layer, heat
sink, package stem, package, and equipment, as the pulse width was widened. This heat conduction
transient phenomenon governs the temperature of the active layer and is influenced by the laser-chip
mounting configuration (configuration of the heat-conducting path).
These behaviours are closely related to the rate and range of wavelength change under current
modulation. In Figure 7, the horizontal axis indicates modulation frequency and the vertical axis
corresponds to the frequency deviation, which corresponds to the wavelength variation. As modulation
frequency increases from 100 Hz, the frequency deviation decreases because the response to heat
conduction is gradually small. This behaviour is also recognized in Figure 6, in which the current pulse
width corresponds to the modulation frequency of the semiconductor laser from the viewpoint of
heat conduction. A dip appears after 100 kHz in Figure 7. After the dip, the plasma effect is dominant
and the lasing wavelength tends to be shortened (blue shift). This frequency deviation is called FM-
[4]
response or chirping in the optical fibre communication field . The frequency range used for tunable
8 © ISO 2018 – All rights reserved
semiconductor laser spectroscopy is below the dip frequency and the frequency at which the influence
of heat is dominant (red shift).
Key
X modulation frequency
Y frequency deviation, in GHz/mA
1 Joule heating (lengthening)
2 dip
3 plasma effect (shortening)
NOTE The modulation current was a 0,5 mA peak-to-peak sinusoidal wave and the DC bias was set at 60 mA.
Figure 7 — Lasing frequency (wavelength) deviation for a 1 300 nm-band DFB semiconductor
laser as a function of frequency
When the injected current is quickly changed, the increase in the temperature is not sufficient and
the increase is not saturated. The temperature difference between the active layer and the package
temperature becomes large or small in response to the magnitude of injected current when the package
temperature is set at a constant temperature. The current dependence is, therefore, not constant and
varies with the rate of current increase. Their dependences are, however, kept at fixed values if the time
intervals of monitoring are fixed at the constant values and strongly influenced by the materials used
and the chip-mount configuration.
4.6 Effect of ambient temperature on lasing wavelength
Heat is inversely transmitted from the ambient to the active layer of the semiconductor laser through
the package when the ambient temperature or package temperature is changed. The heat conductance
of the package, package stem, and heat sink is the same for the case of the diffusion of Joule heating at
the active layer, and a certain time interval is needed until the temperature of the active layer is equal
to the ambient temperature as shown in Figure 6.
The temperature dependence of wavelength and absorption peak wavelength vary depending on the
time interval of monitoring after changing the ambient temperature. If the change rate of package
temperature is set at values of more than 1 s, the temperature dependence is the same because the
change in package temperature can diffuse to the active layer (see Figure 6). Figure 8 shows a set of
changes in the absorption peak in the spectrum monitored at different scanning rate of the package the
temperature for one of the CO -gas absorption peaks. (The CO -gas pressure is set at an atmospheric
2 2
pressure, and the spectral width is broadened because of collisions and Doppler shift.) These scanning
rates correspond to the package-temperature change rate of less than 1 s. As the rate is high, the peak
position shifts to the direction of the temperature scan and the magnitude of the absorption peak tends
to be small. These phenomena are caused by the time constant of heat diffusion between the package
and active layer, and should be considered during measurement.
These dependences are governed by the change of each factor discussed in 4.4. When ambient
temperature is changed, for example, threshold current density and band-gap energy vary
simultaneously and lasing wavelength changes complicatedly. The dependences result from the overall
change in the factors. Consequently, the dependences will vary with the material used, the mounting
configuration, the monitoring time interval, etc. It can be said that the change rate of the injected
current and ambient (package) temperature has to be constant during tunable semiconductor laser
spectroscopy to eliminate wavelength error, although the dependence differs with the change rate.
Key
X package case temperature, in °C
Y light power, in a.u.
1 temperature scan
NOTE A 2 000 nm-band semiconductor laser was used in this experiment.
Figure 8 — Shape change in absorption spectrum monitored at different scanning rates of the
package temperature for one of CO -gas absorption peaks
5 Measurement method for temperature dependence of wavelength
5.1 General
As described in Clause 4, the lasing wavelength of semiconductor lasers is scanned or fixed by
temperature control when they are used for sensing application. The temperature dependence
corresponds to the magnitude of lasing wavelength shift and is a measure of the lasing wavelength
change with temperature. This characteristic is therefore important for semiconductor lasers for
sensing. A semiconductor laser used for sensing is normally a single longitudinal mode laser, and the
shift value of wavelength is, for instance, monitored with the amount of the change in peak-emission
wavelength under temperature or current variation (see 4.2 and Figure 4). The measurement method
of the temperature dependence is described in 5.2 to 5.4.
5.2 Description of measurement setup and requirements
The measurement setup is depicted in Figure 9.
10 © ISO 2018 – All rights reserved
Key
1 LD driver
2 device (semiconductor laser) being measured
3 lens
4 spectrometer
5 attenuator
6 optical detector
7 thermoelectric cooler
8 controller of power supply
9 power supply of thermoelectric cooler
Figure 9 — Basic measurement setup of temperature dependence of lasing wavelength
The Laser-Diode (LD) driver supplies current to the semiconductor laser, and if the laser is not capable
of operating in continuous wave throughout measurement, supplies pulsed current to the lasers, such
as quantum cascade ones.
The spectrometer (spectrophotometer or spectroscope) resolves the spectral components of input light
in corresponding to the wavelength by wavelength-tunable optical filter and outputting light of the
individual spectral components within a certain wavelength range. A diffraction grating or Fabry-Perot
interferometer is used as the tunable optical filter.
The optical spectrum analyser is applicable to monitor, instead of the spectrometer and optical
detector, if the spectrum analyser is corrected. Besides the diffraction-grating-based and Fabry-
Perot interferometer-based optical spectrum analysers, a Michelson interferometer-based spectrum
analyser, which outputs the autocorrelation function of the input light signal is also applicable.
5.3 Precautions to be observed
Care should be taken so that the optical output power does not exceed the linearity range of the optical
detector.
The optical power sensitivity of the optical detector shall be calibrated over the required
wavelength range.
The wavelength resolution and the bandwidth of the spectrometer shall be such that the measurement
is carried out with adequate accuracy.
For measurement, light reflected into the laser shall be minimized to ensure that the spectral response
is not significantly affected.
The temperature monitoring point should be set at the device being measured as close as possible.
The rate of temperature change has to be set at a constant value throughout measurement, unless the
monitoring data are deviated and scattered. The constant rate should be determined under taking the
time constant of heat conductance between the active layer and package of device being measured.
5.4 Measurement procedures
The specified current, cw or pulse, is applied to the device being measured.
The wavelength of the spectrometer is adjusted within the required range until the maximum reading
on the optical detector has been achieved. The wavelength corresponding to this peak value is recorded.
This is the peak-emission wavelength.
The temperature of the device being measured is changed with a constant rate. The peak-emission
wavelength is continuously monitored with a constant temperature interval.
The change in the peak-emission wavelength is assumed to be linearly proportional to the temperature
of the device being measured.
The slope of the change in the peak-emission wavelength to the temperature is calculated and the
temperature dependence of the peak-emission wavelength is obtained. If the relationship is not linear,
the calculation has to be performed within the linear range.
The wavelength temperature tuning range is determined and limited by the kink point or the point at
which the linear relationship is deviated at the low and high temperature range.
6 Measurement method for current dependence of wavelength
6.1 General
The lasing wavelength of semiconductor lasers is scanned with injected current when they are used
for sensing application. The current dependence corresponds to the magnitude of lasing wavelength
shift under current control and is a measure of the lasing wavelength change with current. This
characteristic is therefore important for semiconductor lasers for sensing. The measurement method
for the current dependence is described in 6.2 to 6.4.
6.2 Description of measurement setup and requirements
The measurement setup is depicted in Figure 10.
12 © ISO 2018 – All rights reserved
Key
1 LD driver
2 device (semiconductor laser) being measured
3 lens
4 spectrometer
5 attenuator
6 optical detector
7 thermoelectric cooler
8 controller of LD driver
9 power supply of thermoelectric cooler
Figure 10 — Diagram of measurement setup for current dependence of lasing wavelength
The LD driver supplies current to the semiconductor laser, and if the laser is not capable of operating
in continuous wave throughout measurement, supplies pulsed current to the lasers, such as quantum
cascade ones.
The spectrometer (spectrophotometer or spectroscope) resolves the spectral components of input light
in corresponding to the wavelength by wavelength-tunable optical filter and outputting light of the
individual spectral components within a certain wavelength range. The diffraction grating or Fabry-
Perot interferometer is used as the tunable optical filter.
The optical spectrum analyser is applicable to monitor, instead of the spectrometer and optical
detector, if the spectrum analyser is corrected. Besides the diffraction-grating-based and Fabry-
Perot interferometer-based optical spectrum analysers, a Michelson interferometer-based spectrum
analyser, which outputs the autocorrelation function of the input light signal is also applicable.
6.3 Precautions to be observed
Care should be taken so that the optical output power does not exceed the linearity range of the optical
detector.
The optical sensitivity of the optical detector shall be calibrated over the required wavelength range.
The wavelength resolution and the bandwidth of the spectrometer shall be such that the measurement
is carried out with adequate accuracy.
For measurement, light reflected into the semiconductor laser shall be minimized to ensure that the
spectral response is not significantly affected.
The temperature monitoring point should be set at the device being measured as close as possible.
The rate of current change has to be set at a constant value throughout measurement, unless the
monitoring data are deviated and scattered. The constant rate should be determined under taking the
time constant of heat conductance between the active layer and package of device being measured.
6.4 Measurement procedures
6.4.1 Static current dependence
The specified injected current, cw or pulse, is applied to the device being measured.
The specified temperature is set to the device being measured.
The wavelength of the spectrometer is adjusted within the required range until the maximum reading
on the optical detector has been achieved. The wavelength corresponding to this peak value is recorded.
This is the peak-emission wavelength.
The injected current of the device being measured is changed with a constant rate. The peak-emission
wavelength is monitored continuously or with a constant current interval.
The change in the peak-emission wavelength is assumed to be linearly proportional to the magnitude of
the injected current of the device being measured.
The slope of the change in the peak-emission wavelength to the magnitude of the injected current is
calculated and the current dependence of the peak-emission wavelength is obtained. If the relationship
is not linear, the calculation has to be performed within the linear region.
The wavelength current tuning range is determined and limited by the kink point or the point at which
the linear relationship is deviated at the small and large magnitude of injected current.
If the laser is not capable of operating in continuous wave near room temperature, the specified pulsed
current is set to the device being measured. The current pulse height is changed with a constant pulse
width and duty ratio. The other procedures are the same as described above.
6.4.2 Dynamic current coefficient
The specified injected current, cw, is applied to the device being measured.
The specified temperature is set to the device being measured.
An etalon plate or a Fabry-Perot etalon, replacing the spectrometer in Figure 10, is adjusted within
the required finesse [free spectral range (FSR)/FWHM of resonance peak]. The optical detector can
monitor cavity modes with the finesse.
A small signal current, sinusoidal wave, is biased to the laser under test in addition to the specified
current.
The width of the peak deviation is divided with the peak height of injected current, and the dynamic
current dependence is calculated in unit of Hz/mA.
7 Measurement method of spectral line width
7.1 General
Lasing spectral line width is a device characteristic of semiconductor lasers with single mode, and
therefore the measurement method is described only for semiconductor lasers operating under
continuous wave. The lasing spectral line width is uncritical in most cases of sensing but is important
when narrow absorbing lines at low pressure are monitored. If the spectral line width is wider than
that of absorption line shapes, the line shape measured is broadened and vague. This characteristic is
14 © ISO 2018 – All rights reserved
therefore important for semiconductor lasers for some sensing applications. The measurement method
for lasing spectral line width is described in 7.2 to 7.4.
7.2 Description of measurement setup and requirements
The measurement setup is depicted in Figure 11, Figure 12 and Figure 13.
Key
1 device being measured
2 optical isolator
3 polarization controller
4 single mode fibre coupler
5 optical detector
6 rf spectrum analyser
7 single mode optical fibre
8 local oscillator
Figure 11 — Lasing spectrum line width measurement system: Optical heterodyne fibre system
Key
1 device being measured
2 lens
3 optical isolator
4 optical beam combiner
5 optical detector
6 rf spectrum analyser
7 mirror
8 local oscillator
Figure 12 — Lasing spectrum line width measurement system: Optical heterodyne system
16 © ISO 2018 – All rights reserved
Key
1 device being measured
2 optical isolator
3 single mode fibre coupler
4 optical fibre for phase delay
5 optical frequency modulator
6 polarization controller
7 single mode fibre coupler
8 optical detector
9 rf spectru
...
Questions, Comments and Discussion
Ask us and Technical Secretary will try to provide an answer. You can facilitate discussion about the standard in here.
Loading comments...