1. WP 3: Applications of Cavity Solitons
WP managers: T. Ackemann, R. Jäger
happy to be here
SUPA and
Department of Physics,
University of Strathclyde
Glasgow, UK
ULM Photonics,
Lise-Meitner-Str. 13, Ulm, Germany
31/05/2006
FunFACS review meeting, Brussels

2. Outline Task 1 of WP 3: All-optical delay line
Motivation and principal scheme
Results on optical delay line
Fabrication of device with improved homogeneity
Experimental demonstration of delay line
Theoretical analysis of drift speed
Comparison to other “slow light” systems
Supporting activities and future directions
Fabrication of top-emitters with tailored current injection
Incoherent switching
Summary

3. What is special about CS?
Properties of CS:
motion „plasticity“, „motility“
novel ingredient compared to micro-machined pixels
translational symmetry
continuous
bistable
all-optical processing
all-optical network
parallelism
optical interconnects
best „bang“ you can get from a cavity soliton should involve motility
enabling conceptionally new approaches
impact !
self-luminous
2D displays; memory or pattern recognition systems; massively parallel and reconfigurable all-optical packet
switching and routing schemes; optical delay lines; shift registers and beam fanning. To all these schemes
one can add the possibility of high-frequency self-pulsing in the case of a pulsed CSL.
Task 1 of WP 3: All-optical delay line
discrete

4. „Slow light“ and all-optical delay line
Boyd et al., OPN 17(4) 18 (2006)
Hau et al., Nature 397, 594 (1999)

5. All-optical buffers and delay lines
buffers can enhance performance of networks
future high-performance photonic networks should be all-optical
need for all-optical buffers with controllable delay
Boyd et al., OPN 17(4) 18 (2006); Gauthier, Physics World, Dec. 2005, p. 30

6. All-optical delay line based on CS
parameter gradient
read out at other side
inject train of solitons here
time delayed version of input train
all-optical delay line
buffer register
for free: serial to parallel conversion and beam fanning
note: won‘t work for non-solitons / diffractive beams
Harkness et al., USTRAT (1998)

7. Prerequisite: High homogeneity
state-of the art at the end of PIANOS:
gradient of cavity resonance 0.27 GHz / µm
quite good for university growth reactor (U Ulm)
but: limits the existence range of CS in transverse plane
 for drifting of CS over large distances higher homogeneity required !
Barland et al., Nature 419, 699 (2002)

8. Homogeneity before optimization
substrate
holder
in production reactor:
immediate improvement by a factor of about 3 and more
origin of the inhomogeneity:
beam shape of the source material
temperature inhomogeneity of the substrate
R. Jäger et al., UP, unpublished

9. Homogeneity after optimization
tangential
improve beam shape
optimize temperature difference between bottom and top heating filament of the effusions cells
improve homogeneity of substrate temperature
reduce temperature level of growth
enhance uniformity of substrate holder rings
result:
< GHz / µm
< 2.5 GHz / 200 µm
radial
R. Jäger et al., UP, unpublished

10. Set-up for all-optical delay line
spatio-temporal detection system:
6 local detectors
+ synchronized digital oscilloscopes
BW about 300 MHz
VCSEL (UP)
200 µm diameter
pumped above transparency
but below threshold
 amplifier
suitable preparation of holding beam
F. Pedaci, S. Barland, M. Giudici, J. Tredicce, INLN, Nice, unpublished

11. Preparation of holding beam
Mach-Zehnder interferometer
with 6 detectors you cannot investigate two-dimensional spatio-temporal structures
create quasi-1D situation
channeling of motion of CS
there is also a phase gradient along the stripes  drift
stripes with modulation depth of  1

12. Results: Noise-driven events
intentional (bang on table) or intrinsic perturbations trigger release of pulse
anti-phase oscillation possible interpretation: structure oscillating back and forth in a potential well
F. Pedaci, S. Barland, M. Giudici, J. Tredicce, INLN, Nice, unpublished

13. Reproducibility superposition of 50 events
deterministic propagation
noise triggered events appear at fairly random time intervals
compatible with interpretation of a noise triggered drifting CS
F. Pedaci, S. Barland, M. Giudici, J. Tredicce, INLN, Nice, unpublished

14. optically addressed drifting structure
Optical addressing
gate addressing beam with an
electro-optical modulator
rise/fall times < 1 ns
100 ns
optically addressed drifting structure
delay  12 ns
distance  25 µm
velocity  2.1 µm/ns
delay / width  2-4
this is an embryonic all – optical delay line !
F. Pedaci, S. Barland, M. Giudici, J. Tredicce, INLN, Nice, unpublished

15. Velocity
experiment suggests velocity of about 2 µm / ns = 2000 m / s = 7200 km / h
theoretical expectation here amplifier model (‚standard‘ parameters)
perturbative regime
saturation speed limit  1.5 µm/ns
semi-quantitative agreement fortuitous (at present stage)
Tissoni et al., INFM, unpublished

16. Bandwidth and bit rate velocity: 2 µm / ns
CS diameter typically 10 µm  a local detector would see a signal of length µm/(2 µm/ns) = 5 ns  bit rate 100 Mbit/s
not great, but certainly a start
limit: time constant of medium (carriers) typically assumed to be about 1 ns  d-response some ns µm / 3 ns = 3.3 µm /ns  probably origin of numerically observed saturation behaviour
even this makes sense with experiment (HWHM  3ns)

17. „Slow media“: Non-instantaneous Kerr cavity
g  0.01  semiconductor
velocity determined by response time
saturation for instantaneous medium 
faster medium will speed up response !
log (velocity / gradient)
slope 1
response time can be engineered by growers: low-temperature growth, ion implantation, QW close to surface, quantum dots
need to pay for it by increase of power
log (g)
A. Scroggie, USTRAT, unpublished
(1D, perturbation analysis)

18. „Conventional“ approaches to slow light
modification of group velocity in vicinity of a resonance
two-level atom
electro-magnetically induced transparency
cavity resonance
....
large effect needs steep slope, narrow resonance
bandwidth limited by
absorption
high-order dispersion
Hau et al., Nature 397, 594 (1999)

19. Intermediate résumé: CS-based delay line
it works !
drifting CS are a quite different approach to slow light  pros and cons should be assessed
potentially very large delays
potentially simple combination with processing/routing (bistability!)
in a cavity soliton laser there are (at least) two other twists
relaxation oscillations are faster than carrier decay time and modulation frequency of modern SC lasers is certainly faster (at least 10 Gbit/s)  drift velocity might not be limited simply by material response time
possibility of fast spontaneous motion
N. N. Rosanov, e.g. Spatial hysteresis and optical patterns, Springer, 2002

20. Future and supporting activities
lot‘s of things to be done
theory: saturation behaviour of velocity material response t N = - A N – B N2 – C N new features in lasing regime
fabrication: homogeneity, purpose made devices
experiment: control gradients, improve ignition, larger distances ...
ongoing at INLN: set-up and characterization of liquid-crystal two-dimensional spatial light modulator
other activities in first year to enhance delay-line activities
speed-up of laser simulations by improved adiabatic elimination technique
optimization of current injection
incoherent switching of CS

21. Optimization of current injection
standard broad-area top-emitting VCSEL (below threshold)
IL max
100µm A
IL min B
strongly inhomogeneous (1:2) carrier density due to current crowding at device perimeter (oxide aperture)
corroborated by device simulations
Fontaine et al., LAAS, unpublished

22. Results: VCSEL with ITO layer
ITO layer (250 nm)
Anode
current & carrier density profiles in MQW
P-DBR
Oxide
Cavity + MQW
N-DBR
Cathode
reduction of inhomogeneity to about a factor of 1.6
taken alone, not sufficient
Fontaine et al., LAAS, unpublished

23. Results: Patterned injection
Cavity + MQW
Anode
Cathode
P-DBR
N-DBR
Localized
oxides
reduction of inhomogeneity to about a factor of 1.06

24. Design for patterned injection
patterning with oxide apertures and/or metal mesh
Dimensions of the mesh:
d = 4µm e = 2µm
d = 5µm e = 3µm
d = 20µm e=5µm
soliton should average over mesh
coupled solitons
„discrete solitons“ ?! d e
100µm
Camps et al., LAAS, unpublished

25. Design for delay line
create quasi-1D situation for all-optical delay line: rectangular VCSEL
channeling
minimize electrical and optical power requirements
dimensions:
d = 5µm, 10µm, 20µm
L = 320µm L d
explore possibility of electrically controlled gradient
Camps et al., LAAS, unpublished

26. Towards incoherent manipulation of CS
previous CS manipulation in SC amplifier system relied on phase coherence
switch-on: constructive interference between holding beam and writing beam
switch-off: destructive interference between holding beam and writing beam
phase-sensitive interactions do not tend to be robust
motivation to study feasibility of incoherent control!
especially for a cavity soliton laser !
feasibility demonstrated by
simulations in laser with saturable absorber
first experiments on VEGSEL
experiments on optically pumped amplifier

27. Setup: Optically pumped amplifier
Barbay et al., Opt. Lett. 31, 1504 (2006)

28. demonstration of incoherent switch-on
quite fast switch
but long delay
thermal effects ? on
demonstration of incoherent switch-on
Barbay et al., Opt. Lett. 31, 1504 (2006)

29. demonstration of incoherent switch-off
same pulse can switch CS off at slightly different bias level
quite fast switch
demonstration of incoherent switch-off
full control would give robust optical flip-flop
Barbay et al., Opt. Lett. 31, 1504 (2006)

30. Summary experimental demonstration of embryonic all-optical delay line
speed about 2 µm/ns in accordance with expectation
milestone fully reached
novel approach to slow light
well-placed among other slow light systems (large delays)
further achievements: incoherent switching, improved device design
future work directed on better understanding and optimization
on good track for robust, phase-insensitive soliton-based delay-line