EN 61751:1998

SLOVENSKI STANDARD
01-maj-1999
Laser modules used for telecommunication - Reliability assessment (IEC
61751:1998)
Laser modules used for telecommunication - Reliability assessment
Lasermodule für Telekommunikationsanwendungen - Zuverläsigkeitsbewertung
Modules laser utilisés pour les télécommunications - Evaluation de la fiabilité
Ta slovenski standard je istoveten z: EN 61751:1998
ICS:
31.260 Optoelektronika, laserska Optoelectronics. Laser
oprema equipment
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

NORME
CEI
INTERNATIONALE
IEC
INTERNATIONAL
Première édition
STANDARD
First edition
1998-02
Modules laser utilisés pour
les télécommunications –
Evaluation de la fiabilité
Laser modules used
for telecommunication –
Reliability assessment
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Pour prix, voir catalogue en vigueur
For price, see current catalogue

61751 © IEC:1998 – 3 –
CONTENTS
Page
FOREWORD . 5
INTRODUCTION . 7
Clause
1 Scope. 9
2 Normative references. 9
3 Terms and definitions. 11
4 Laser reliability and quality assurance procedure . 11
4.1 Demonstration of product quality . 11
4.2 Testing responsibilities. 13
4.3 Quality Improvement Programmes (QIPs). 13
5 Tests. 15
5.1 Structural similarity. 15
5.2 Burn-in and screening (when applicable in the DS) . 15
6 Activities. 23
6.1 Analysis of reliability results . 23
6.2 Technical visits to LMMs . 25
6.3 Design/process changes. 25
6.4 Deliveries. 25
6.5 Supplier documentation. 25
Annex A (normative) Laser diode and laser module failure mechanisms. 27
Annex B (informative) Guide. 41
Figures
A.1 Non-linearities in laser-current characteristics. 33
A.2 “Bathtub” failure rate curve . 35
A.3 Example of cumulative failure plot showing log-normal distribution
of laser failure rate . 35
A.4 Calculated failure rates for components having a log-normal lifetime distribution,
with a median life of 10 h and dispersion in the range 0,5 to 2,0. 37
A.5 Cross-section through a typical laser module showing key components. 37
A.6 Cross-section through a typical buried heterostructure laser (bonded junction side up). 39

61751 © IEC:1998 – 5 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
_________
LASER MODULES USED FOR TELECOMMUNICATION –
Reliability assessment
FOREWORD
1) The IEC (International Electrotechnical Commission) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of the IEC is to promote
international co-operation on all questions concerning standardization in the electrical and electronic fields. To
this end and in addition to other activities, the IEC publishes International Standards. Their preparation is
entrusted to technical committees; any IEC National Committee interested in the subject dealt with may
participate in this preparatory work. International, governmental and non-governmental organizations liaising
with the IEC also participate in this preparation. The IEC collaborates closely with the International Organization
for Standardization (ISO) in accordance with conditions determined by agreement between the two
organizations.
2) The formal decisions or agreements of the IEC on technical matters express, as nearly as possible, an
international consensus of opinion on the relevant subjects since each technical committee has representation
from all interested National Committees.
3) The documents produced have the form of recommendations for international use and are published in the form
of standards, technical reports or guides and they are accepted by the National Committees in that sense.
4) In order to promote international unification, IEC National Committees undertake to apply IEC International
Standards transparently to the maximum extent possible in their national and regional standards. Any
divergence between the IEC Standard and the corresponding national or regional standard shall be clearly
indicated in the latter.
5) The IEC provides no marking procedure to indicate its approval and cannot be rendered responsible for any
equipment declared to be in conformity with one of its standards.
6) Attention is drawn to the possibility that some of the elements of this International Standard may be the subject
of patent rights. The IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 61751 has been prepared by subcommittee 47C: Optoelectronic,
display and imaging devices, of IEC technical committee 47: Semiconductor devices.
The field of this standard will henceforth be placed under the responsibility of IEC technical
committee 86: Fibre optics.
The text of this standard is based on the following documents:
FDIS Report on voting
86/115/FDIS 86/116/RVD
Full information on the voting for the approval of this standard can be found in the report on
voting indicated in the above table.
Annex A forms an integral part of this standard.
Annex B is for information only.

61751 © IEC:1998 – 7 –
INTRODUCTION
The laser modules covered by this International Standard are purchased by a system supplier
(SS) to be inserted in equipments which in turn are supplied/sold to a system operator (SO), for
example a national PTT or a network operator (see definitions in clause 3).
For the system operator to act as an informed buyer, a knowledge of the potential risks posed
by the use of critical components is required.
Optoelectronic component technology is continuing to develop. Consequently, during product
development phases, many failure mechanisms in laser modules have been identified. These
failure mechanisms, if undetected, could result in very short laser lifetime in system use.

61751 © IEC:1998 – 9 –
LASER MODULES USED FOR TELECOMMUNICATION –
Reliability assessment
1 Scope
This International Standard deals with reliability assessment of laser modules used for
telecommunication.
The aim of this standard is:
– to establish a standard method of assessing the reliability of laser modules in order to
minimize risks and to promote product development and reliability;
– to establish means by which the distribution of failures with time can be determined. This
should enable the determination of equipment failure rates for specified end of life criteria.
In addition, guidance is given on:
– the testing that a system supplier should ensure is in a place prior to procurement of a laser
module from a laser module manufacturer;
– a range of activities expected of a system supplier to verify a laser module manufacturer’s
reliability claims.
Further details concerning the rationale are given in annexes A and B.
2 Normative references
The following normative documents contain provisions which, through reference in this text,
constitute provisions of this International Standard. At the time of publication, the editions
indicated were valid. All normative documents are subject to revision, and parties to
agreements based on this International Standard are encouraged to investigate the possibility
of applying the most recent editions of the normative documents indicated below. Members of
IEC and ISO maintain registers of currently valid International Standards.
IEC 60068-2-1:1990, Environmental testing – Part 2: Tests. Tests A: Cold
IEC 60068-2-14:1984, Environmental testing – Part 2: Tests. Test N: Change of temperature
IEC 60747-1:1996, Semiconductor devices – Discrete devices and integrated circuits – Part 1:
General
Amendment 3 (1996)
IEC 60747-12-2:1995, Semiconductor devices – Part 12: Optoelectronic devices – Section 2:
Blank detail specification for laser diode modules with pigtail for fibre optic systems and sub-
systems
IEC 60749:1996, Semiconductor devices – Mechanical and climatic test methods
ISO 9000: Quality management and quality assurance standards
MIL-STD-883:1985, Test methods and Procedures for Microelectronics

61751 © IEC:1998 – 11 –
3 Terms and definitions
For the purpose of this International Standard the following definitions apply:
laser module
a packaged assembly containing a laser diode and photodiode
NOTE – The module may also include a cooler and temperature sensor to enable laser temperature to be controlled
and monitored. The optical output is normally via an optical fibre pigtail.
submount
a substrate upon which a laser diode or photodiode may be mounted for assembly into the
laser module
NOTE – Components on submounts are also subject to qualification testing.
laser module manufacturer (LMM)
a manufacturer of laser modules who provides devices meeting the requirements of the
relevant detail specification (DS) and the customer’s reliability requirements
system supplier (SS)
a manufacturer of telecommunications/data transmission equipment containing optoelectronic
semiconductor lasers, i.e. laser module customer
system operator (SO)
a network operator of telecommunications/data transmission equipment containing opto-
electronic semiconductor lasers in the transmission path
NOTE – The system may also be part of other more extensive systems, for example telecommunications, rail, road
vehicles, aerospace or weapons.
capability qualifying components (CQC)
components selected to represent critical stages of the process and limiting or boundary
characteristics of mechanical and electro-optic design
Such components should aid the identification of end product failure mechanisms to enable the
determination of activation energies.
4 Laser reliability and quality assurance procedure
4.1 Demonstration of product quality
This standard (where required by the detail specification (DS)) gives the minimum mandatory
requirements and is part of a total laser reliability and quality assurance procedure adopted by
the laser module manufacture.
It gives guidance on the activities of the system supplier, and the system operator as well as
feedback of field performance, the laser module manufacturer and the system supplier.

61751 © IEC:1998 – 13 –
The laser module manufacturer shall be able to demonstrate, by means of qualification
approval of devices, technology approval or capability approval of the manufacturing process:
a) a documented and audited manufacturing process including the qualification of purchased
components in accordance with ISO 9000;
b) a performance qualification programme, including for example, accelerated life testing,
burn-in and screening of components and modules;
c) a qualification maintenance programme to ensure continuity of reliability performance;
d) a procedure to feedback reliability issues to development and production.
In addition, there are many elements which make up a comprehensive reliability assurance
programme (see annex B).
4.2 Testing responsibilities
The testing detailed in tables 1a and 1b is to be performed by the laser module manufacturer
and component suppliers (where applicable). Additional testing may be specified in the DS.
4.2.1 Recommendation (applicable to laser customer/system supplier)
The system supplier is recommended to have a programme to analyse and verify the results
including failure analysis. This programme includes an independent life test of fully packaged
laser modules, see table 1b test 2 and/or test 3 and/or test 5 (sample size >10 per test).
4.2.2 Recommendation applicable to system operator
The system operator is recommended to have a programme to monitor and report field failure
rates in sufficient detail to enable system supplier and laser module manufacturer to initiate
any necessary corrective actions at an early stage in the lifetime of a product.
Suppliers may have different approaches (i.e. to reliability concepts) during the development of
product maturity and resource limitations may dictate testing strategies.
Alternative tests and activities to those specified are permitted provided the LMM/SS/SO can
show intent to remove end-product failures and the associated failure mechanisms. However,
this will require significant data to substantiate compliance.
4.3 Quality improvement programmes (QIPs)
Quality improvement programmes (QIPs) shall be initiated with component suppliers and
customers (SOs, SSs and LMMs) to address non-compliances (including quality and reliability
problems identified during subsequent service life of the laser). The correction of non-
compliances and subsequent QIPs are a required strategy to minimize reliability risks. The
operation of QIPs should be stated in the quality approval (QA) generic and capability approval
documents.
61751 © IEC:1998 – 15 –
5 Tests
The tests described in tables 1a and 1b are designed to accelerate the main failure
mechanisms known to be reliability hazards in laser modules (see annex B). Where appro-
priate, the CQC shall demonstrate an ability to reduce end product failure mechanisms. Final
product validation is required to demonstrate that CQCs are operating at the boundaries of the
process or technology. These tests will reduce the risk of unreliable components entering
system use and will enable estimates to be made of the distribution of laser lifetimes and
hence the laser failure rates.
The sample size and level of testing may vary depending on the business volume between the
laser customer/system supplier (SS) and laser module manufacturer (LMM). This information
will be given in the capability approval (CA) document and DS where appropriate.
NOTE – It is essential that the lasers evaluated are entirely representative of standard production devices, and
have passed all the production and/or specified (where applicable in the DS) burn-in and screening procedures.
Table 1a – Initial qualification
These tests will normally be performed by the laser manufacturer as part of an initial
qualification programme.
Table 1b – Maintenance of qualification
These tests cover periodic monitoring performed on production devices to ensure that the
quality and reliability performance established during initial qualification is maintained or
improved.
5.1 Structural similarity
Where a range of laser modules is produced by a laser manufacturer, there may be some
significant structural similarity between different type codes. A combination of results from
different test programmes, where appropriate, is therefore permitted.
Consideration should be given to the fact that minor differences in technology or processing
can have a major impact on reliability, whilst not being apparent during quality assessment.
Evidence shall be presented which demonstrates that all results are directly relevant.
5.2 Burn-in and screening (when applicable in the DS)
See B.1.13 of annex B.
61751 © IEC:1998 – 17 –
Table 1 – Qualification testing
Table 1a – Initial qualifications (see notes for symbols)
n
Test Test IEC references Conditions
No.
1 Initial endurance
tests of:
1.1 a) Module with 60747-12-2, clause 8 25
Φ specified, constant power
e
thermoelectric Temperature: T = T
c op max
cooler T = T
s s nom
Duration**: 5 000 h
1.2 b) Module without
Φe specified, constant power
thermoelectric 60747-12-2, clause 8 T T 25
Temperature: c = op max
cooler Duration**: 5 000 h
NOTE – Results from the above tests shall be supplemented by a laser customer/SS independent test of fully
packaged modules in accordance with table 1b test 2 and/or test 3 (sample size ≥10 per test) see also 4.2.
1.3 Laser diode 60747-12-2, clause 8 Temperature: at least two 200
(submount) test temperatures:
Φ specified, constant power
e
1.3.1 T = T See DS
s1 s max
1.3.2 Ts2 =
Duration: >5 000 h
1.4 Photodiode 60747-12-2, clause 8 Temperature: at least two 200
(in representative test temperatures:
package)
V l
r or r specified
1.4.1 T = 125 °C min.* See DS
s1
1.4.2 T = <(T –30 °C) See DS
s2 s1
Duration: >1 000 h
1.5 High temperature T = T of the cooler 25
stg max
storage of the
Duration: 1 000 h
thermoelectric
See tables B.1 and B.7
cooler
1.5.1 Power cycle tests Number of cycles: 20 K
cooled devices
T = T
c op max
T = T to (T – ΔT )
s c c max
1.6 High temperature T T 25
= stg max of the sensor
storage of the
See tables B.1 and B.7
thermal sensor
* Or as limited by technology.
** Provided data about the distribution of wear-out lifetime is accumulated with sufficient accuracy. Provisional
approval for product shipment shall be granted at 2 000 h. It is also recommended to continue the test until accurate
extrapolation of lifetime is possible with an upper limit of 10 000 h. Durations up to 5 000 h may be needed for
accurate lifetime prediction.
61751 © IEC:1998 – 19 –
Table 1a (concluded)
n
Test Test IEC references Conditions
No.
1.8 Fibre proof test Proof test see DS 10
Duration see DS
Min. bend radius see DS
1.9 Force = 10 N min. 10
Fibre retention
3 Rapid change 60749, chap. 3 Temperature: T = T 10
A stg min
of temperature
T = T
B stg max
Number of cycles = 50
Number of cycles = 500
Sealing Test Qk See notes
followed by test Qc
4 Shock and vibration 60749, chap. 2 See DS 10
5 High temperature 60749, chap. 3 Temperature: T = T 10
stg max
storage (not appli-
Duration: >2 000 h
cable if module life
See table B.9
test performed at
equivalent case
temperature and
submount
temperature)
6 ESDS, modules 5 per wafer
a) Lasers 60747-1, amend. 3 See B.1.11
equivalent
b) Photodiodes
7 Residual gas See B.1.12 6
analysis
8 Low temperature 60068-2-1 T = T 10
stg min
storage
Duration: >1 000 h
See B.1.4
61751 © IEC:1998 – 21 –
Table 1b – Maintenance of qualification
np
Test Test IEC references Conditions
No.
2 Ongoing reliability test 60747-12-2, Periodic testing: See notes 6
clause 8
a) Module (cooled) Test 1.1 10
b) Module (uncooled) Test 1.2 10
c) Laser diode (submount) Test 1.3  25 *
d) Photodiode Test 1.4  25 *
NOTE – Results shall be supplemented by a laser customer/system supplier (SS) independent test of fully packaged
modules in accordance with table 1b, test 2 and/or test 3 and/or test 5 (sample size ≥10 per test). See also 4.2.
3 Temperature cycling 60749, chap. 3 Temperature: 10 6
T = T
A stg min
T = T
B stg max
Sealing Test Qk followed Periodic testing: Number of
by test Qc cycles = 200 (see notes)
See notes
4 Shock and vibration 60749, chap. 2 See DS 10 12
Periodic testing: see notes
5 High temperature 60749, chap. 3 Temperature: 10 12
storage (not applicable
T = T
stg max
if module life test
Duration: >2 000 h
performed at equivalent
Periodic testing: see notes
case temperature and
and clause B.2, table B.9
submount temperature)
6 ESDS, modules 5 per wafer
a) Lasers 60747-1, amend. 3 Periodic testing: see notes
and B.1.11
b) Photodiodes MIL-STD-883,
Method 3015
7 Residual gas analysis Periodic testing: see notes See DS 6
and B.1.12
* Out of different wafers.
Letter symbols for tables 1a and 1b
T : minimum storage temperature
A
T : maximum storage temperature
B
T : module case temperature
c
T: submount temperature
s
T : recommended submount temperature
s nom
T : module minimum operating temperature
op min
T : module maximum operating temperature
op max
T : module minimum storage temperature
stg min
T : module maximum storage temperature
stg max
p: periodicity (in months)
n: sample size
61751 © IEC:1998 – 23 –
6 Activities
6.1 Analysis of reliability results
The laser module customer/system supplier (SS) shall have a programme to analyse and verify
a laser manufacturer’s reliability claims. In particular:
– life test data for the complete laser module;
– life test data for initial components, for example laser diode and photodiode;
– environmental test result, i.e. inspection requirements group B, C of the DS;
– where appropriate, see clause 5, the data and test results of appropriate CQCs.
The analysis of results should lead to reporting of the laser module reliability parameters for
each of the laser module types. Minimum reliability parameters are presented as in table 2.
Table 2 – Proforma for laser module reliability parameters
Parameter Measured value
Median life (ML) @ 25 °C: Years
see note 3
Dispersion(s)
Wear-out failure rate
FITS
at 5 years (λ)
FITS
at 10 years (λ)
FITS
at 20 years (λ)
Wear-out activation energy (E ) eV
a
Random failure rate
(λ ) @ 25 °C: see note 3 FITS
a
%
Confidence limits used:
Random failure activation energy (E ) eV
a
NOTE 1 – This table assumes a log-normal distribution of times to failure. The dispersion parameter is the
standard deviation of the logarithm to the base ‘e’ of the times to failure. See B.3.2a of annex B.
NOTE 2 – Where data reveals more than one wear-out mechanism, median life and dispersion in each case is to
be stated.
NOTE 3 – The reference temperature used for all parameters in this table is 25 °C. An alternative reference
temperature (50 °C) may be used provided activation energies are given.
NOTE 4 – The failure criteria used to derive these reliability parameters shall be agreed between the laser
customer/system suppliers (SS) and laser module manufacturer (LMM). The criteria will be stated in the (DS),
see clause B.2.
NOTE 5 – Special attention should be paid to all extrapolation models used and the justification for activation
energies employed in reliability predictions is to be stated.
Guidance on these activities is given in annex B.

61751 © IEC:1998 – 25 –
6.2 Technical visits to LMMs
Laser module designs continue to evolve and a LMM may introduce significant changes which
impinge on reliability. Under the negotiation between customer and manufacturer, technical
visits should be performed until there is sufficient evidence of a maturing technology and
production stability. These technical meetings/visits shall contain an item on the agenda that
concerns quality and reliability. Where a LMM holds a capability approval, the frequency of
these technical visits may be reduced provided the manufacturer can demonstrate:
a) that the CQCs fully represent any relevant design, process updates and reliability issues;
b) satisfactory self-audit of the quality system.
6.3 Design/process changes
The customer/system supplier (SS) shall be informed by the laser module manufacturer (LMM)
of any design or process change which may affect the form, fit or function of the end product.
6.4 Deliveries
Laser module designs will continue to evolve and therefore each delivered lot shall be
manufactured according to a stated technology and production process.
This should be verified by the supplier/ and customer before delivery.
6.5 Supplier documentation
The laser customer/system supplier (SS) and component manufacturer or LMM shall
incorporate, wherever possible, the tests and activities described in this standard into their in-
house component qualification, or where appropriate, capability approval procedures and
purchasing specifications. This documentation will be used in reliability/technical presentations,
tender submission, marketing briefs to customers.

61751 © IEC:1998 – 27 –
Annex A
(normative)
Laser diode and laser module failure mechanisms
A.1 Introduction
Much of the published laser reliability data (and also reliability data from laser manufacturers)
is from the service life testing of laser chips bonded onto submounts or special headers. The
results usually show increasing threshold or operating currents leading to eventual failure.
However, other laser characteristics can also degrade and should be monitored during life
testing, for example, light-output spectrum.
Practical laser transmitters, as used in fibre transmission systems, contain several other
important piece parts and components which are also vulnerable to failure. For example,
reduced fibre output power, due to instability in the fibre to laser chip alignment, is a significant
failure mechanism in laser modules. Less information is available on the stability of the output
from receptacle packages.
A schematic cross-section through a typical laser module is shown in figure A.5 in which the
laser chip is mounted on a submount within a dual-in-line package with a fibre pigtail. the
temperature of the laser submount is often controlled using a Peltier cooler, with a thermistor
as a temperature sensor. Some distributed feedback laser modules for use in high bit-rate
optical fibre systems also contain optical isolators to prevent reflected optical power from
adversely affecting the laser operation. Advanced modules containing integrated circuits for
some control functions are also available.
A.2 Description of the main failure mechanisms which affect laser diodes
and laser modules
A.2.1 Laser diodes
A cross-section through a typical InGaAsP/InP buried heterostructure laser is shown in figure A.6.
A wide range of failure mechanisms have been identified in laser diodes associated with
material defects in the semiconductor material, facet degradation, both p and n-side
metallizations and with the bond to the heatsink. These failure mechanisms are discussed in
more detail below.
a) Degradation due to the growth of material defects
A common cause of rapid failure in early lasers was the growth of dark line defects (DLDs)
and dark spot defects (DSDs) – network of dislocations leading to localized regions of strong
non-radiative recombination, and hence increased threshold currents or even complete loss
of light output. The defects could be observed as dark lines or spots when viewed in a
scanning-electron-microscope using cathode-luminescence or electron-beam-induced current
(EBIC). They were a particular problem in early GaAIAs (850 nm) lasers, in which they were
associated with defects threading up through the epitaxial layers from the substrate. Defect
growth is accelerated by stress within the laser, for example caused by bonding. Penetration of
copper from the laser submount has also been seen to contribute to the growth of arrays of
dark spot defects. In 1 300 nm and 1 550 nm lasers fabricated from InGaAsP/InP, dislocation
networks can grow as a result of lattice mismatch between quaternary or ternary material and
the indium phosphide substrate.

61751 © IEC:1998 – 29 –
Rapid failures due to DLDs and DSDs have largely been eliminated by the use of low-defect
density substrates and greatly improved epitaxial material growth. A rigorous burn-in screen
can effectively remove any individual lasers which may still suffer from this problem. Although
rapid failures due to material problems have largely been eliminated, lasers in general still
show gradual long-term degradation under normal operating conditions, leading to a slow rise
in threshold current or change in efficiency. The manner in which degradation occurs is
dependent on the laser structure, and the rate (and hence the laser lifetime) is very dependent
on the quality of the material growth and on batch-to-batch processing variations.
In buried heterostructure (BH) lasers, defects tend to grow along the side walls of the active
layer which are exposed during the growth of the blocking layers. These defects lead to
increased non-radiative recombination and hence to an increase in threshold current. The
slope efficiency is generally unchanged (at a given current). Two stages of degradation have
been reported, a rapid first stage which saturates, followed by a much lower rate of long-term
degradation. A short period of high temperature and current stress, applied as a burn-in, will
saturate the first stage degradation. The user should therefore only observe the gradual long-
term increase in threshold or operating current.
With ridge waveguide lasers the active layer is not cut during processing, with the result that
side walls are not exposed during overgrowth. Ridge lasers do not in general therefore exhibit
the two-stage degradation exhibited by BH lasers, but tend only to show gradual degradation
after an initial settling down period.
The cause of the gradual long-term increase in threshold current, which continues after any
first stage has saturated, is not clearly understood, but is thought to be associated with the
accumulation or generation of point defects within the active region which give rise to
increased non-radiative recombination.
b) Blocking layer leakage
Increased leakage currents in the blocking layers of buried heterostructure lasers have been
reported leading to increased threshold currents. However, blocking layer degradation is not a
general problem.
c) Facet degradation
Laser facets are vulnerable to catastrophic damage due to high-current transients, and even
slight transient damage has been shown to lead to increased degradation rates in GaAIAs
lasers, and hence reduced laser lifetimes. InGaAsP lasers are generally somewhat less
sensitive to face damage than GaAIAs lasers. The facets of all types of lasers can be damaged
by handling during assembly.
Oxidation of the facet, leading to increased threshold currents, was observed to be a problem
in early GaAIAs (850 nm) lasers, but was largely suppressed by the use of coatings such as
Al O .
2 3
InGaAsP lasers are far less vulnerable to this problem and, under normal operation, facet
degradation is generally insignificant.
Contamination within a laser package can lead to build-up of contaminates (for example
carbon, chlorine, copper) along the line of the active region on the facets and hence to reduced
light output.
d) Laser metallization and bonding
A common cause of failure in early GaAIAs lasers was increased thermal impedance due to the
formation of indium/gold intermetallics in the laser die bond. This was due to the use of indium
solder in conjunction with gold layer metallization or gold-plated submounts. This problem can
be minimized by careful control of gold layer thickness, but operation of lasers with this
bonding system at temperatures greater than 50 °C is still risky. Indium solder is no longer
widely used for InGaAsP lasers, but is often required for GaAIAs lasers where a low-stress
solder is essential.
61751 © IEC:1998 – 31 –
Laser failures have been seen to be due to metal penetration into the active layer, including
gold from metallization and copper submounts. Effective barrier metals are therefore essential
in both lasers and submounts, for example TiPtAu for laser p-side metallizations, and NiAu for
copper submounts.
Sudden laser failures have been observed because of short-circuits of whisker growth, but
these can be eliminated by the corrected choice of solder. Both AuSn (80:20) and PbSn can
give reliable bonds for InGaAsP.
A.2.2 Monitor photodiode
Several kinds of photodiode are used as back-facet monitors in the laser module. For 850 nm
operation, silicon pin photodetectors are used, and for long wavelengths either Germanium or
III-V pin detectors can be used. There are two-main types of InGaAs/InP photodiode available,
having either a mesa or a planar structure.
The dominant cause of failure in photodiodes is increased dark (leakage) current. Mesa
structures, which have an exposed p-n junction at the surface, are particularly vulnerable to
increased surface leakage. An improvement in the stability of the mesa pin can be obtained by
the use of an organic passivation, but the best reliability is normally achieved by planar
devices.
A.2.3 Peltier cooler and thermistor
Peltier coolers are constructed from a series of p and n doped bismuth telluride elements
soldered to copper bus-bars within a sandwich of ceramic plates. They are relatively fragile
devices and vulnerable to mechanical stresses arising from mounting them within the package
and from thermal mismatch with other package materials. Diffusion of metal ions into the
elements from the solder or metallization can lead to loss of cooling efficiency, and
metallurgical reactions in solders can lead to weakened joints and cracked elements.
Changes in the thermistor resistance can occur due to reactions within the metallization and
solder. Increased laser drive currents then follow as the laser submount is controlled at a
higher temperature than intended.
A.2.4 Packaging and optical fibre
A critical alignment is required between the fibre tip and laser facet in order to maintain a
constant light output from the fibre pigtail. For lasers coupled to single-mode fibres, alignment
is required to within a few tenths of a micrometer, unless lenses are used to reduce the
alignment tolerance.
Some very early failures have been observed in laser module service life tests due to fibre
alignment instability and consequent loss of fibre light output. Failures due to loss of fibre
output have also been observed during temperature cycling testing. Temperature cycling
testing can also reveal vulnerability to fibre breaks due to shrinkage of the fibre pigtail. As with
other hermetic packages, a dry inert gas atmosphere is required within the package to avoid
problems such as metallization corrosion. Therefore hermeticity and gas analysis testing are
required. Contamination, for example residual chlorine from solvent residues, resulting from
inadequate cleaning can exacerbate corrosion problems.

61751 © IEC:1998 – 33 –
IEC  396/98
Figure A.1a – Kinks in radiant power – forward current (L/I) curve
IEC  397/98
Figure A.1b – Snap-on of radiant power output
Figure A.1 – Non-linearities in laser-current characteristics

61751 © IEC:1998 – 35 –
IEC  398/98
Figure A.2 – “Bathtub” failure rate curve
IEC  399/98
Figure A.3 – Example of cumulative failure plot showing log-normal
distribution of laser failure rate

61751 © IEC:1998 – 37 –
IEC  400/98
Figure A.4 – Calculate failure rates for components having a log-normal lifetime distribution
with a median life of 10 h and dispersion in the range 0,5 to 2,0
IEC  401/98
Figure A.5 – Cross-section through a typical laser module showing key components

61751 © IEC:1998 – 39 –
IEC  402/98
Figure A.6 – Cross-section through a typical buried heterostructure laser
(bonded section side up)
61751 © IEC:1998 – 41 –
Annex B
(informative)
Guide
B.1 Guidance on testing
B.1.1 Service life tests – general
To demonstrate the long-term stability of laser modules, accelerated ageing is required.
Thermally accelerated testing is the most widely used method of providing component reliability
data in a test of reasonable duration, and is also appropriate for laser diodes and photodiodes.
For thermal overstress, the relationship between lifetime and temperature is derived from the
Arrhenius relationship:
t /t = exp [(E /k)(1/T – 1/T )] (B.1)
1 2 a 1 2
where
t and t are the lifetimes at temperatures T and T respectively;
1 2 1 2
k is Boltzmann’s constant;
T and T are absolute temperatures, in kelvins;
1 2
E is the activation energy for the failure mechanism.
a
In order to obtain an estimate of the reliability of laser modules, life testing of the laser diodes
is not sufficient. Many kinds failure mechanisms which cause field failures are associated with
packaging and therefore life tests as well as environmental testing of complete modules are
essential. The results of life tests of laser diodes on submounts, monitor photodiodes, or other
included components, provide necessary supporting data on the reliability of the key active
devices. As a matter of fact, on such components, life tests can be performed over a wider
temperature range without the limitations imposed by packaging materials. Such life testing is
most readily performed by the component manufacturer. However, the laser customer system
supplier should perform an independent test of complete modules (sample size > 10 per test,
table 1b, test 2 and/or test 3 and/or test 5).
To obtain valid results, all life test components have to be representative of the standard
production processes, including burn-in and screening tests (where appropriate, see DS).
B.1.2 Scale of testing
The scale of reliability testing will be dependent on the system requirements and system
operator application and, in particular, the failure rate (or life time) and the confidence level
required. The sample size selected should enable the total failure rate (wear-out + random
failure rate) to be determined with sufficient accuracy for the system construction. To
demonstrate a low total failure rate to a high level of confidence, accumulated component
hours on many hundreds of components may be required (see clause B.3). Field data and
water validation and burn-in results may be used to life test results to give increased
confidence. Periodic testing on a smaller sample size is required to ensure that predictions
remain valid.
61751 © IEC:1998 – 43 –
B.1.3 Laser module life tests containing thermoelectric coolers
(for example Peltier, test 1.1 of table 1a)
With laser modules containing thermoelectric coolers it is difficult to provide a significant
degree of overstress to all key components simultaneously. During “normal operation”, the
laser submount temperature is usually controlled at Ts = 25 °C. However, for a life test with a
case temperature of Tc = Top max, a useful stress can be obtained for the laser diode, fibre
fixing, photodiode and thermal sensor if the cooler is operated at a relatively high current to
maintain a submount temperature of T = T .
s s nom
Some additional testing of the cooler is recommended, for example T = T and T = T – 10 °C.
c op max s s
Table B.1 – Recommended life test conditions for laser modules
containing Peltier coolers
Case temperature T
op max
Laser submount temperature T T
s = s nom
Optical power Fibre output set to P at start of life test
max
(using monitor circuit)
Laser current To maintain constant monitor output
Monitor current Normal bias
Thermal sensor current Normal bias
Cooler current To maintain constant thermistor resistance
(or sensor conditions)
Duration >5 000 h
B.1.4 Laser module life tests – uncooled module (test 1.2, table 1a)
For modules without thermoelectric coolers (for example Peltier), life tests can be readily
performed over a range of temperatures up to the recommended maximum operating
temperature for the module. During initial qualification, service life tests at two or more
temperatures, for example T = T and T 40 °C to 50 °C, are recommended.
c op max c
An additional life test at low temperature (duration >2 000 h at T ) may be required for
op min
modules containing epoxies or organic materials.
If only a single life test is to be performed, for example during maintenance of qualification
testing, the following conditions are recommended:

61751 © IEC:1998 – 45 –
Table B.2 – Recommended life test conditions for uncooled laser modules
Case temperature T
op max
Optical power Fibre output set to P at start of life test
max
(using monitor circuit)
Laser current To maintain constant monitor photocurrent
Monitor photocurrent Normal bias
Duration >5 000 h
B.1.5 Laser diode life tests on submounts (test 1.3, table 1a)
The laser life test shall be performed with the laser operating at constant light output, as it
would be in normal operation, unless otherwise agreed with the ONS. Temperatures in the
range T = 50 °C to 80 °C are often used. The acceleration in the rate of degradation, relative
s
to normal operation, is therefore relatively small. The maximum temperature at which life tests
can be performed under lasing operating conditions is usually in the range T = 70 °C to
s
100 °C. However, constant current/life tests at temperatures up to T = 150 °C can be useful in
s
studying the reliability of contact metallizations. Actual failures do not often occur in well
screened laser diodes tested at temperatures T <90 °C. In order to estimate the laser life
s
some extrapolation is required to predict when the threshold or operating current will exceed
the pre-determined failure criterion. To obtain a reasonable increase in operating current, a life
test duration greater than 5 000 h is required.
If a single life test is to be performed, for example during maintenance of qualification testing,
the conditions in table B.3 are recommended:
Table B.3 – Recommended laser diode life test conditions
Temperature T = 70 °C
s
Optical power Maximum specified
Bias To maintain constant monitor output
Duration >5 000 h
B.1.6 Monitor photodiode life tests (test 1.4, table 1a)
Photodiode life
...