1.1 Report on Work Package 3 : Applications of Cavity Soliton Lasers
The milestone of this workpackage for the year concerns Task 1:
Milestone M3.1.1 (month 12) : Observation of drifting CS in optical delay line, or identification of limiting
inhomogeneities.
1.1.1
Considerations on the state-of-the art of all-optical delay lines
All-optical delay lines are considered to be key component in future all-optical photonic networks, because
they are needed for buffering information flow through all-optical routers (e.g., Tucker et al., Electron. Lett.
41, 208, 2005; Gauthier, Physics World, Dec. 2005, p. 30). Task 1 of WP3 is devoted to the demonstration of
an all-optical delay line using cavity solitons. The basic idea behind our approach is that a serial bit stream is
injected at one location within the transverse aperture of a broad-area CS device (see Fig. 3.1). The ignited
CS drift with an adjustable speed determined by a controlled parameter gradient towards a selected part of
the device where a time-delayed version of the original bit stream can be recovered by a local detector.
Figure 3.1: Scheme illustrating the CS-based all-optical delay line in a rectangular CS-supporting photonic
device: Solitons are written at the left by “1” pulses of an incoming optical bit-stream, drift rightwards on a
uniform parameter gradient and a time-delayed version is read out at the side. The two lines show two
frames from a simulation and illustrate the spatial shift of the bit pattern with increasing time.
We remark that there is a lot of recent activity on realizing delay lines by using “slow light” in photonic
crystal waveguides, resonators, semiconductor media as well as in optical fibers. Most of these schemes rely
on the reduction of the group velocity of light in the vicinity of a resonance. Hence, it appears to be fruitful
to explore the possibilities of using a quite different mechanism as the drift of a soliton for creating the delay.
In particular, it appears that the soliton based delay line is particularly suited to produce large delays
including ones which are longer than the pulse width. The record for delay seems to be about four pulse
widths (Kasapi et al. Phys. Rev. Lett. 74, 2447, 1995; Boyd et al. Phys. Rev. A 71, 023801, 2005). However,
this experiment is based on electromagnetically induced transparency in an atomic vapor and needs very
complex and expensive laser systems to operate. More recent experiments yielding delays in the tens of ns
range were performed using stimulated Brillouin scattering in optical fibers (Gonsalez-Herraez et al., Appl.
Phys. Lett. 87, 081113, 2005), however the bandwidth is rather limited. Often the product of bandwidth
times delay is taken as a figure of merit for an optical delay line ( e.g. Tucker et al., Electron. Lett. 41, 208,
2005). The currently anticipated value of about 5 with the potential for an increase in future devices places
the CS-based systems favourable among the currently investigated schemes. In addition, we see advantages
for our scheme due to the fact that the integration of routing (packet switching) and delay functions should
be feasible due to the bistable character of CS and the cascadability brought about by the active character of
the devices.
Due to the experimental and theoretical advances made the WP has achieved its goal of
demonstrating the feasibility of drifting structures in the first year and is on a good track for a
demonstrator of an all-optical delay line.
Details on the activities are reported below.
1.1.2
Observation and preliminary analysis of dynamic and drifting structures in devices of
improved homogeneity (CNRS-INLN, UP, INFM, deliverable 3.1.1.a)
UP investigated parameters of the crystal growth to optimize the layer thickness homogeneity. The
most important parameter was identified to be the temperature offset between the lower and upper heater
filaments of the Gallium effusions cells. Several epitaxial growth runs were needed to determine the ideal
temperature offset. After optimization, the homogeneity of the dip in the reflectance spectrum was improved
from +/- 0.8% to +/- 0.3% over a 3 inch wafer (see Fig. 3.2). This homogeneity provides a great advantage to
generate optical cavity solitons, is of importance for all WPs, and in particular for WP3.
Figure 3.2: Variation
of resonance wavelength on
wafer in tangential and radial
directions after optimisation
by UP.
UP fabricated large area bottom-emitting VCSELs for optical cavity soliton generation. Devices with
two different active diameters were shipped to the partners INLN and USTRAT. The first type was a bottomemitting VCSEL with 80µm active diameter whereas the second type has an active area of 200 µm. Both
types were mounted on a TO56 header with a diamond heat spreader.
Delay-line experiments were started at INLN in December 2005, when the new 200 micron diameter
VCSEL structure was made available by UP, because these investigations naturally require a sufficient
transverse extension of the amplifier structure. The first step was to set-up a spatio-temporal measurement
system with sufficient bandwidth and spatial resolution. This was achieved thanks to an array of photodiodes
and careful synchronization (and the characterization thereof) of several digital oscilloscopes. At present, the
apparatus has a temporal bandwidth of 300 MHz with a spatial resolution of the order of 10 micrometer, on
six channels simultaneously. Improvements are currently under way in order to add two channels to the
previous ones.
Preliminary investigations yielded first signs of complex spatio-temporal dynamics and
demonstrated that in a certain parameter region, the dynamics of the system cannot be reduced to a spatially
extended pattern evolving in time in a fully correlated way. While some indications for propagating
structures were found at that stage, the interpretation of the time series proved to be rather difficult due to the
two-dimensional nature of the system. Therefore, the experiment was modified in order to reduce the number
of degrees of freedom of drifting cavity solitons.
A Michelson interferometer was inserted into the optical path of the holding beam in order to inject a
spatially modulated beam into the amplifier. A typical image of the near field intensity distribution is shown
in Fig. 3.3.
Figure 3.3: Typical near field intensity
distribution obtained in the VCSEL under operating
conditions with a modulated holding beam. The
stripes are running along the diagonal (lower left to
upper right). The circles indicate the positions of the
photodetectors.
INLN demonstrated that stationary CS lock at extrema of a fringe pattern (even of low modulation
depth) superimposed on the homogenous holding beam. Moving this fringe pattern, the CS can be dragged
around. If the contrast of the resulting fringes is close to unity, an eventual drifting cavity soliton should be
forced to travel along the line defined by an interference maximum.
In this configuration, INLN was able to demonstrate the propagation of a periodic wave in the
transverse dimension of the system. This wave has a period of the order of 10 nanoseconds and a wavelength
of the order of 40 micrometers, but these numbers clearly depend on parameters. While the propagation is
naturally oriented along the line defined by the holding beam spatial modulation, the origin of the direction
of propagation is still to be clarified.
Under very similar experimental conditions, INLN was also able to show the motion of a single
structure, oscillating around a fixed center (see Figs. 3.3 and 3.4). Applying an external perturbation, it was
possible to trigger the departure of this structure from its initial position and to observe its motion over a
distance of more than 30 micrometers. This structure appeared to be drifting along the line defined by the
spatial modulation of the holding beam at an average speed of about 2 µm/ns. As for the case of wave
propagation, the parameters controlling the direction and speed of motion are under investigation.
Figure 3.4: Time series of local
detectors (vertically offset for clarity).
The traces from top to bottom
correspond to the detectors indicated in
Fig. 3.3 from left to right. The two upper
lines indicate the oscillation of a
structure around some equilibrium
position, which detaches and drifts
downstream (lower lines) after a
perturbation.
INLN demonstrated also a further step towards the realization of an all-optical delay line: the
controlled excitation of a CS by a localized optical perturbations, which then drifts away from the ignition
point. For this purpose, an addressing system with a beam diameter of 10 micrometers, a pulse duration of
100 nanoseconds and rising and falling edges below 1 nanosecond was set-up.
Fig. 3.5 shows the excitation and subsequent drift of a CS using this system. The distance covered
amounts to 25 m with a delay of 12 ns. This yields a velocity of about 2.1 m/ns.
Figure 3.5: Optical ignition and
subsequent drift of a CS. The time series of
local detectors are vertically offset for
clarity, where the uppermost trace
monitors the ignition point and the
following ones monitor points
progressively downstream. The rising and
falling edges of the addressing pulse (and
possibly of the drifting structure) are
limited by the detection bandwidth of 300
MHz.
These results demonstrate the principle of an embryonic all-optical delay line fully satisfying the
milestone. Details are currently under investigation, but obviously the CS-based delay line has the potential
to create large delays, i.e. delays considerably larger than the pulse widths at reasonable speeds/bit rates.
For a more flexible control of phase and intensity gradients an optically addressable spatial light
modulator operating at 980 nanometers is currently set-up. It is addressed by a 650 nanometer beam, whose
intensity is in turn prepared by a computer-controlled liquid crystal display.
Motivated by these exciting experimental results, INFM performed numerical calculation on the drift
of CS in vertical-cavity amplifiers with a uniform phase gradient on the injection field. Velocities around 1.5
m/ns were found in qualitative agreement with the preliminary experimental measurements. Future work
will address the specific experimental situation.
1.1.3
Theoretical analysis of the motion of CS and effects of spatial variations of the background
(USTRAT, deliverable 3.1.1.a)
The utilisation of CS in VCSEL and VECSEL for practical applications in information technology such as
those proposed in WP3 crucially depends on the possibility of controlling their spatial position and an
understanding of the effects of intentional and parasitic spatial variation of the background state on selforganization and CS. In any real device, some spatial variations will be unavoidable due to growth
imperfections.
Previous theories and numerical simulations have shown that CS move upward in phase gradients
and can be located at maxima of phase modulated backgrounds (W.J. Firth and A.J. Scroggie, Phys. Rev.
Lett. 76, 1623, 1996; T. Maggipinto et al., Phys. Rev. E 62, 8726, 2000). These predictions have been
experimentally verified in vertical-cavity amplifiers within the project by INLN.
A general theory was developed and verified on several models where optical CS have been
predicted (Scroggie, et. al., Phys. Rev. E 71, 046602, 2005). At difference with previous approaches, we
found that CS can either climb or descend background modulations depending on the parameter values and
crucially on the wave-vector of the background modulation. This has obvious consequences for the design of
corresponding experiments. The theory is so general that it can also be applied to other spatial structures
such as patterns (Scroggie et al., App. Phys. B 81, 963, 2005). By expanding these ideas, we have also
formulated a general theory of the spatial response of cavity systems to input pump modulations (Scroggie
et al., Phys. Rev. A 72, 023824, 2005). Finally, modulations due to photonic crystals inside the optical
cavity have been shown to strongly modify the output spatial structure of nonlinear optical devices and to
create novel localised structures (D. Gomila and G.-L. Oppo, Phys. Rev. E 72, 016614, 2005).
The theoretical and numerical approach developed to study the motion of CS in modulated
background can be extended to investigate the behaviour of CS in presence of inhomogeneities due to
material fabrication being a superposition of many Fourier components. This represents essential work for
the utilisation of drifting CS in a delay-line and for the development of a CS microscope. Preliminary work
performed on Kerr cavities clearly shows that the CS velocity field induced by spatial noise of different
bandwidths filters out high spatial frequency components. The determination of the cut-off condition is
presently under investigation. At the same time, a theoretical formalism is pursued, by which the motion of a
CS under the influence of irregular, noisy backgrounds can be described, which is the typical experimental
situation.
One important aspect of the dynamics of photonic devices based on semiconductor materials is that
the carrier lifetime is considerably longer than the photon lifetime in the cavity and thus the limiting time
constant for dynamical phenomena. Hence, it is likely to be also the limiting factor for the drift velocity (see,
e.g. Tissoni et al., J. Opt. Soc. Am. B 16, 2095, 1999). Indeed, calculations performed for a model medium
with a Kerr nonlinearity with a non-instantaneous response yield that the drift velocity per unit gradient is
inversely proportional to the time constant in the perturbative limit. This shows that the speed of the CS and
thus the bit-rate in a delay line can be enhanced compared to the current values by reducing the carrier
lifetime. These results need to be analysed more systematically and extended to the non-perturbative case in
order to access systematically the limits for the drift velocity.
Figure 3.6: Drift velocity per unit gradient
of a CS (in normalized units) versus , the
ratio between the decay rate of the medium
and the one of the intracavity field (  0.01
for a semiconductor mircrocavity. The
existence of a scaling region with slope 1 in
a double-logarithmic plot shows that the
medium controls the drift velocity for small
. For large  (corresponding to an
instantaneously responding medium) the
velocity saturates at a value related to the
cavity response time.
1.1.4
Long-Term Spatio-Temporal Dynamics of Lasers and VCSELs (USTRAT, deliverable 3.1.1b)
Several numerical models describing the generation and dynamics of cavity solitons (CS) in semiconductor
lasers suffer of so-called ‘stiffness’. ‘Stiffness’ means that dynamical variables (here the optical field inside
the cavity, the carrier density and the dielectric polarization) evolve on very different time scales so that the
numerical implementation is very inefficient. Ten years ago the USTRAT group discovered that the
straightforward elimination of fast variables in laser models with diffraction can lead to spurious instabilities
and that more advanced techniques are needed for a correct description (Oppo et al., Phys. Rev. A 44, 4712,
1991) of lasers’ spatio-temporal dynamics. Semiconductor laser modelling is more complex than the one for
‘standard’ laser devices (e.g. based on solid-state materials). The difficulty of obtaining simple reduced
models for broad ranges of parameter values appeared to be insurmountable for a long time. In fact, during
the first part of the FunFACS collaboration several groups have used VCSEL models where the dynamics of
fast variables had to be simulated in spite of the large separation of the time scales.
Recently, USTRAT established that one of the main difficulties in applying the Centre Manifold
(CM) technique to the elimination of fast variables can be circumvented by using higher order expansions
and non-trivial terms coming from the CM theory. New models for standard two-level (e.g. solid-state) lasers
and VCSELs have been derived and tested. With typical decay times of the polarization, the cavity field and
the population differences being of around 100 fs, 10 ps and 1 ns, respectively, a carefully reduced model
can achieve the same asymptotic result as the original simulation but 1000 times faster. Because of the much
larger number of operations per step required in the reduced model, we have achieved a gain factor of 400 in
the simulation time of the same spatial structures for both standard lasers and VCSELs. The final equations
of the reduced models are quite complicated and are not amenable of analytic manipulations. However they
represent a powerful tool for future numerical investigation of CS in a variety of experimental laser
configurations ranging from injected signals to saturable absorbers relevant for WP1 and WP3. Fig 3.7
illustrates the power of the method for calculating the switch-on transient of a laser.
a)
b)
Figure 3.7: Time evolution of snapshots after the switch-on of a laser using the full equations
(a) and the reduced equations (b). The latter show a remarkable agreement with the former
ones with a computational effort reduced by a factor of 400.
A similar system of reduced equations was obtained by an independent method by INFM and is
reported in WP1. The comparison between the two approaches and the resulting systems of equations is
ongoing and will lead to a joint deliverable about this issue in the report for the second year.
1.1.5
Advancing cavity soliton manipulation towards applications by incoherent writing and erasure
of cavity solitons (CNRS-LPN, deliverable 3.1.1c)
The possibility to excite a CS with an incoherent beam is appealing since it eliminates the need for a
controlled phase matching between the excitation and the driving field. It also constitutes a first step in the
manipulation of cavity solitons. In the following, we report on the incoherent and coherent writing and
erasure of CS in a broad-area, optically-pumped, vertical-cavity semiconductor optical amplifier.
This result has been obtained in an optically pumped microresonator thanks to a novel cavity design.
The cavity is composed of two aperiodic AlAs/AlGaAs mirrors designed for efficient optical pumping at
800nm. As a result, a pump window created on the high energy side of the multilayer mirror stop-band
allows optical pumping over a 20 nm spectral width with a maximum of pump absorption. The active zone is
composed of a bulk GaAs layer, surrounded by two AlGaAs absorbing spacers. The total cavity has a
resonance around 880nm. The cavity is first grown upside-down and then bonded onto a SiC substrate. The
sample temperature is controlled by a Peltier element and a feedback loop at temperatures close to 275K. A
800nm high-power fiber-coupled laser diode array provides a uniform pump intensity of several Watts. The
near-field of the sample is re-imaged on a CCD camera and a fast detector (90 ps risetime) is used for local
dynamical measurements.
We first set the system in a parameter region where bistable localized spots exist. Bistability is
observed by varying either the holding beam intensity or the pump beam intensity. By setting the system at
the center of the hysteresis cycle, we are able to excite a cavity soliton by sending a short (60 ps) pulse on the
sample, whose wavelength is different from those of the pump and the holding beam. Triggerable writing of
cavity solitons by a single pulse has been demonstrated (see Fig. 3.8). A closer look at the switch-on
dynamics showed that the transient consists of a ‘lethargic time’, where the system remains close to its initial
state, and a ‘switching time’, where abrupt switching occurs. The first one can be of the order of hundreds of
nanoseconds while the second is much faster, of the order of 1 ns. The physical origin of the lethargic time is
currently under investigation, but we mention that, in this type of excitation, the pulse creates carriers at high
energy that cascade very rapidly to the bottom of the bands. In contrast to the case of coherent excitation,
writing is initiated by a local excess of carriers.
Surprisingly, we could also erase incoherently the CS. However, in that case switch-off is obtained
by setting the pump bias apart from the centre of the hysteresis cycle, closer to the switch-off threshold, and
we could not in the same experiment repeatedly switch-on and off CS. The exact mechanism by which
switching-off occurs is not completely understood at the moment.
a)
b)
a)
a)
Figure 3.8: Time evolution of the power emitted by a vertical-cavity amplifier (black line) after
being perturbed a short pulse of a Ti:Sa laser (red curve). Panel a) shows the switch-on of a
CS, panel b) the switch-down.
The localized spots generally appear in the central zone of the cavity, where a pump-induced
transverse thermal gradient is present. The center of the pumped zone, where heating is more pronounced,
has thus a red shifted cavity resonance which translates into a maximum of the field-cavity detuning,
attracting the localized spot. We could independently excite two nearby localized spots in sequence by an
incoherent excitation, thus demonstrating their CS character. However, the cavity resonance gradient present
in the sample combined with the relatively narrow interval in parameter space where CS exist, presently
does not the writing and erasure of CS at arbitrary transverse positions of the cavity.
In conclusion, we have shown the fast incoherent switch-on and switch-off of cavity solitons in a
vertical-cavity optical amplifier. Incoherent writing and erasure of CS opens new perspectives for optical
information processing with CS as it greatly reduces the requirements on the writing beam. It also permits a
wavelength conversion associated with the writing process, since the CS has a wavelength imposed by the
holding beam wavelength while writing of a CS can occur in a very broad range of wavelengths.
1.1.6
Technology for top-emitting devices and design of optimised structures for current injection
(CNRS-LAAS in collaboration with CNRS-LPN, CNRS-INLN and USTRAT)
LAAS performed technological work on wafer growth and homogeneity of current injection in VCSELs,
which is described in detail here, but which is also of high importance for WP 1.
Epitaxial growth of an optically-pumped VCSEL structure designed by LPN was planned to be
performed during the first year. This could not be achieved, yet, due to a failure of one of the aluminium
effusion cells. For the structures grown after careful calibrations, one could not get a reasonable agreement
between the measured reflectivity spectra and those calculated for the designed VCSEL structures.
Furthermore, one observed a lack of reproducibility of the growth process, which prevented us from any
improvement. After a thorough reverse analysis by scanning electron microscopy, a failure of the Al cell
was detected. The vacuum chamber was opened which confirmed this analysis and led to the change of the
defective cell. Growth will then be performed again in view of providing the optically-pumped VCSEL
structures to LPN.
Regarding the growth and processing of electrically driven structures, after a discussion, the
Consortium has decided that LAAS would get standard VCSEL structures from UP. These wafers will be
processed in LAAS using the novel techniques described below in order to improve carrier injection in topemitting VCSELs. The electrical characteristics of these multilayer structures will be also better than the
ones which could be grown currently at LAAS because they use a Beryllium effusion cell for p-type doping
which causes diffusion. These problems could be addressed by the use of a Carbon effusion cell installed in a
new MBE chamber of which the acquisition is in discussion in LAAS.
The carrier injection non-uniformity into the active zone remains the major problem for a large part
of microcavity devices and a great challenge for research. Its solution would also open the way to new
possibilities for this class of devices. This problem is particularly critical in the case of the large area topemitting VCSELs as those used in FunFACS. Within the FunFACS project, the main contribution of LAAS
is to work on the alternative ways to improve this characteristic on standard epitaxial structures. The upper
semiconductor layer and the metallic deposit must be thus optimised.
-3
carrier density (cm )
A modelling study has been performed using Silvaco ATLAS software. The use of an Esaki junction or a
transparent metallic ITO layer has been compared to standard annular contact on a P+ cap layer. The nonuniformity in the carrier density at the QWs between the edge and the center of a 100µm diameter VCSEL
structure with an annular metallic contact on the upper P+ DBR layer has been found to be of the order of 2
(see Fig. 3.9, black line). An Esaki junction has been shown to improve this parameter to 1.56, but
simultaneously has been measured on test-structures to degrade the series resistance by a factor of 2.65;
therefore, this solution does not seem to be the most appropriate to solve the electrical problem in VCSEL
devices.
Modelling of an ITO electrode leads to a non-uniformity of the same order: 1.6 (see Fig. 3.9, blue line). This
solution has been retained. We have equipped a sputtering reactor with an ITO target and performed our first
ITO deposits. Up to now, the calibrations of growth rate and layer uniformity have been carried out. The
optical (transmission of 98% around 850nm) and electrical characteristics of the ITO have been obtained.
Next, these conducting layers will be applied onto test-structures, and the improvement in the carrier
injection will be finally characterized.
1x10
19
9x10
18
8x10
18
7x10
18
6x10
18
5x10
18
4x10
18
3x10
18
Figure 3.9: Variation of carrier density
along the radial coordinate of a VCSEL with
an aperture of diameter of 100 m. Black
line: standard top-emitter with a P+ cap
laye, blue line: ITO layer, red line:
patterned injection.
0
10
20
30
40
50
60
radial position ( m)
In order to further improve the carrier injection uniformity, we have modelled a novel contact design. This
consists of a mesh for carrier injection, which is a continuous ITO deposit upon a semiconductor layer with
localized conducting apertures with predetermined sizes. These conducting apertures can be obtained thanks
to localized distributed dielectric patterns on the surface. The preliminary modelling study of this solution
shows a clear improvement of the carrier density uniformity (1.06, see Fig. 3.9, red line). This work will be
pursued by applying this geometry to test-structures to experimentally validate its positive effect.
LAAS proposed a VCSEL structure with a rectangular gain region which would be optimal for the
realization of a delay-line due to the channelling of CS motion. This is supported by the latest experiments of
INLN. Electrodes along the long sides of the aperture will enable quite homogenous current injection. These
devices will be fabricated during the second year of the project.
1.2 WP3 Deliverables
Deliverable D3.1.1: Report on status of delay line experiments and theoretical analysis. It is composed of
several subparts:
Deliverable D3.1.1a A. Scroggie, G.-L. Oppo, W. J. Firth, T. Ackemann, G. Tissoni, F. Prati, M. Brambilla,
L.A. Lugiato, F. Pedaci, S. Barland, M. Giudici, J. R. Tredicce, R. Jäger. Report on the status of all-optical
delay line: Experiments and theoretical analysis
Deliverable D3.1.1b G.-L. Oppo, Long-Term Spatio-Temporal Spatio-Temporal Dynamics Dynamics of
Lasers and of Lasers and VCSELs
Deliverable D3.1.1c S. Barbay, Y. Menesguen, X. Hachair, L. Leroy, I. Sagnes and R. Kuszelewicz,
Incoherent and coherent writing and erasure of cavity solitons in an optically pumped semiconductor
amplifier, Opt. Lett. 31, 1504-1507 (2006)