Contributions of USTRAT-EXP to WP1
USTRAT_Exp is working in Task 2 of WP1 on the experimental realization of a cavity soliton laser using a VCSEL
coupled to an external cavity with a diffraction grating as a frequency-selective element (VEGSEL = vertical extended
cavity grating surface-emitting laser). Activities included
1) Characterization of solitary lasers
Bottom-emitting VCSELs with an emission wavelength around 980 nm and an oxide aperture of 80 µm were delivered
by UP. The characteristics of these devices (spectra, intensity distribution in near and far field) were investigated in
dependence on substrate temperature and driving current. The devices show on-axis emission, which corresponds to
some large-scale emission spots close to the perimeter of aperture, and off-axis emission, which corresponds to smallscale fringe patterns in the near field. Quantitative analysis of the data is ongoing. However, it is already obvious from
the distribution of the spots and the length scales within the aperture that the cavity resonance of the lasers is
considerably more homogeneous than the lasers used in the PIANOS project. This was expected due to the
improvement of the MBE growth conditions by UP and provides a promising start for the further investigations.
2) Design of external cavity
The figure below shows the developed setup for the experiments in the external cavity.
Fig.: Setup developed for the
realization of a VEGSEL.
The grating is mounted in a so-called Littrow configuration, in which the diffracted beam is directly retroflected for the
chosen wavelength. Due to a intra-cavity telescope, the external cavity is self-imaging for the Littrow-wavelength, thus
preserving the (potentially high) Fresnel number of the VCSEL. Due to the grating, the propagation and diffraction
within the external cavity cannot be treated with the usual 22 ABCD matrices, but an extension to 33 matrices
proposed by Martinez (O. E. Martinez, IEEE J. Quantum Electron. 24, 2530, 1988) needed to be implemented.
3 ) Experiments in external cavity configuration
Tuning the Littrow-wavelength by rotation of the grating, different emission states in the laser can be excited, which
differ in their emission wavelength and the length scale. The quantitative analysis of these data is ongoing. If the
Littrow-frequency is tuned close to the frequency of the longitudinal resonance of the laser, a rather localized emission
spot was observed. If the current is ramped up and down, hysteresis is obtained between a situation, in which the spot is
present, and one, in which it is absent (see Fig.). Hence, the spot is bistable. This makes it a good candidate for a cavity
soliton. The results confirm the previous preliminary experiments performed at the University of Muenster, but with a
much better control of diffraction in the external cavity, and indicate that bistability of localized spots is indeed a robust
feature in the VEGSEL scheme.
Fig.: Emission power in dependence
on current displaying hysteresis for a
VEGSEL with an aperture of 80 µm.
In addition, the corresponding near
field near distributions are shown.
In order to establish that this spot is a self-localized cavity soliton, it is necessary to control it by external means and to
establish the existence and independent control of at least two of them. For this, a tunable laser is currentlyl be set up,
from which an addressing beam for the solitons can be derived.
First results on this were presented at the “Nonlinear Guided Waves and Their Applications” in Dresden (September 69, 2005).
4) Collaborations with USTRAT-Theory and Dr. Loiko on modelling
T.A. has had (and still has) collaboration with the group of Dr. N. A. Loiko, Belorussian Academy of Science, Minsk,
on the modelling of VCSEL with feedback from an external grating. The model does not consider spatial degrees of
freedom and hence is only valid for small-area lasers. However, it displays the possibility of bistability between the
non-lasing off-state and a high-amplitude emission state. The origin of this bistability is the interplay of phaseamplitude coupling (described by the linewidth enhancement factor ) and the frequency-selection by the grating. The
results will appear soon in Opt Commun. (A. Naumenko, N. A. Loiko, M. Sondermann, K. F. Jentsch, and T.
Ackemann. Abrupt turn-on and hysteresis in a VCSEL with frequency-selective optical feedback, Opt Commun., in
press). Ongoing work is on finding optimal conditions for bistability which are considered to serve as guidelines for
finding bistability and possibly solitons in the more complex models including space.
Such a description is currently developed in collaboration with USTRAT-Theory.