Linewidth Enhancement Factor of Semiconductor Lasers: Results from Round-Robin Measurements in COST 288
ABSTRACT Round-robin measurements on the linewidth enhancement factor are carried out within several laboratories participating to EU COST 288 action. The alpha-factor is measured by applying up to 7 different techniques. The obtained results are compared.
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Linewidth Enhancement Factor of Semiconductor Lasers:
Results from Round-Robin Measurements in COST 288
Asier Villafranca, Javier Lasobras, Ignacio Garces
Photonics Technology Group – University of Zaragoza, PT Walqa, Ed.1, 22197 Cuarte (Huesca) Spain, e-mail: email@example.com
Guido Giuliani, Silvano Donati
Dipartimento di Elettronica, Università di Pavia, Via Ferrata 1, I-27100 Pavia, Italy. e–mail firstname.lastname@example.org
Marek Chacinski, Richard Schatz
Royal Institute of Technology, Laboratory of Photonics and Microwave Engineering, ELECTRUM 229, SE-164 40 Kista-Stockholm, Sweden
Christos Kouloumentas, Dimitrios Klonidis, Ioannis Tomkos
Athens Information Technology Center (AIT), 19,5km Markopoulou Av., 19002, Peania, Greece
School of Electronic Engineering, Dublin City University,Dublin 9, Ireland
Aragon Photonics Labs S.L., c/ Prado 5 local, 50009 – Zaragoza, Spain
Judy Rorison, Jose Pozo
Centre for Communications Research, Department of Electronic and Electrical Engineering, University of Bristol, BS8 1UB, Bristol, U.K.
Andrea Fiore, Pablo Moreno, Marco Rossetti
Ecole Polytechnique Fédérale de Lausanne, Institute of Photonics and Quantum Electronics, Station 3, CH-1015 Lausanne, Switzerland
Wolfgang Elsässer, Jens Von Staden
Technische Universität Darmstadt, Institut für Angewandte Physik, Schloßgartenstr. 7, D-64289 Darmstadt, Germany
Department of Physics, Tyndall Institute, National University of Ireland, University College, Cork, Ireland
Mika Saarinen, Markus Pessa, Pirjo Leinonen
Optoelectronics Research Centre, Tampere University of Technology, P.O. Box 692, FI-33101 Tampere, Finland
Modulight, Inc., P.O. Box 770, FIN-33101 Tampere, Finland
SUPELEC - Campus de Metz, LMOPS, CNRS UMR 7132 - Metz et Supélec,2 Rue Edouard Belin, F-57070 Metz, France
Jan Danckaert, Krassimir Panajotov
Department of Applied Physics and Photonics (TW-TONA), Vrije Universiteit, Brussel, 1050 Brussels, Belgium
Thomas Fordell, Asa Lindberg
Accelerator Laboratory, Department of Physical Sciences, University of Helsinki, Helsinki 00014, Finland
J-F. Hayau, J. Poette, P. Besnard, F. Grillot*
FOTON-ENSSAT, CNRS UMR 6082 6 rue de Kerampont, 22300 Lannion, FRANCE (*FOTON-INSA)
Abstract: Round-Robin measurements on the linewidth enhancement factor are carried out within
several laboratories participating to EU COST 288 Action. The α–factor is measured by applying
up to 7 different techniques. The obtained results are compared.
2007 Optical Society of America
OCIS codes: 140.5960 Semiconductor lasers; 140.3490 Lasers, distributed-feedback
The linewidth enhancement factor (α–factor) , has a great importance for semiconductor lasers (SLs), as it is one of
the main features that distinguishes the behavior of SLs with respect to other types of lasers. The α–factor influences
several fundamental aspects of SLs, such as linewidth, chirp under current modulation, mode stability, laser dynamics,
laser behavior in presence of optical feedback and injection, the occurrence of filamentation in broad–area devices. A
low α–factor is generally considered good.
As reported by Osinski and Buus , several different techniques have been proposed to measure the α–factor,
without any comparison between the results achieved by different methods. This statement is still valid nowadays.
Moreover, the number of the proposed measuring methods has increased, and several SL novel designs have become
commonplace, for which the determination of the α–factor can be particularly critical (e.g., VCSELs, Quantum Dot
Lasers, Quantum Cascade Lasers).
2. Round–Robin Measurement within COST 288 Action
The COST 288 Action “Nanoscale and Ultrafast Photonics”  is an initiative sponsored by the European
Commission and the European Science Foundation, within the frame COST – “European Cooperation in the field of
Scientific and Technical Research”. In Working Group 2 “Physics of Photonic Devices” of COST 288, it was decided
to set–up a Round–Robin (RR) measurement activity on the SL α–factor, with the main goal of comparing different
measurement methods and assessing their consistence and repeatability, using the same set of laser devices circulating
in sequence among different Laboratories. The continuous flow of experimental results is supplying interesting results
that will be completed by mid–year 2007. In the present work we summarize the results achieved to date, by
illustrating the measurement techniques that have been implemented in the RR, and by comparing preliminary data.
3. Experimental Results
The different measuring methods that have been considered for the RR are hereby listed.
1. The Hakki–Paoli method  relies on direct measurement of the refractive index change (measured through
detection of the frequency shift of longitudinal Fabry–Perot mode resonance) and the differential gain as the carrier
density is varied by slightly changing the current of a SL in sub–threshold operation.
2. The Linewidth method relies on the measurement of SL linewidth, and on fitting the results to known SL
parameters, so that the α–factor can be extracted by applying the Henry formula (equation (26) of )
3. The Modified Linewidth method relies on the measurement of SL linewidth as a function of emitted power in the
threshold region, and the ratio of the slope of the curve linewidth vs inverse power gives directly the α value 
4. The FM/AM method  relies on high–frequency SL current modulation which generates both amplitude (AM)
and optical frequency (FM) modulation. The ratio of the FM over AM gives a direct measurement of the α–factor.
Corrections for the so–called adiabatic chirp shall be taken into account .
5. The Fiber Transfer Function method exploits the interaction between the chirp of a high–frequency modulated SL
and the chromatic dispersion of an optical fiber, which produces a series of minima in the amplitude transfer
function vs. modulation frequency. By fitting the measured transfer function, the α–factor can be retrieved .
6. In the Optical Injection method, light from a master SL is injected into the (slave) SL under test, causing locking of
the slave optical frequency to that of the master. The locking region is characterized in terms of the injected power
level and frequency detuning, showing an asymmetry in frequency due to the non–zero α–factor [9,10].
7. The Optical feedback method is based on the self–mixing interferometry configuration and, according to the Lang–
Kobayashi theory, the α–factor is determined from the measurement of specific parameters of the resulting
interferometric waveform, without the need to directly measure the feedback strength .
The above methods have been applied to two specimen of commercial high–power DFB SLs (JDSU CQF935/808)
that come in a butterfly package with optical isolator. In addition to the comparison of methods, other relevant aspects
are being studied, such as the high-power behavior of the α-factor due to non-linear effects , which can be studied
with methods 4–7, the influence of the adiabatic chirp  on methods involving modulation (4,5) or the interaction
with other non-linear effects.
While the complete results will be made available on the COST 288 website , preliminary results to–date can be
summarized as follows. (i) The Hakki–Paoli method is difficult to be applied to commercial DFBs, due to the lack of
knowledge of laser parameters. (ii) The FM/AM method requires modulation well above the relaxation frequency of
the SL, and it can be difficult to be implemented with some SL types. (iii) The Fiber Transfer Function method
appears to be the most reliable, provided a precise measurement of fiber dispersion is preliminarly made using a
LiNbO3 M–Z modulator, and the power along the fiber span is kept below +3 dBm to avoid non–linear effects. (iv)
Methods 6–7 can be applied to a variety of devices, and give the effective α in operating conditions above threshold.
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