Cavity Solitons positioning and drift in presence of a phase gradient

Transcription

1 Cavity Solitons positioning and drift in presence of a phase gradient F. Pedaci, S. Barland, E. Caboche, P. Genevet, M. Giudici, J. Tredicce Institut non linéaire de Nice Acknowledge: FunFACS CEE project

2 Plan of the presentation Introduction Experimental set-up and CS addressing Control of CS by parameters gradients Optical reconfiguration of CS arrays Controlling CS motion Mapping device defects Conclusions

3 Introduction OPERATIVE DEFINITION Cavity solitons are localized single peaks of light in the transverse plane of an optical system (optical resonator or other) that can be individually switched on and off by external addressing beams NECESSARY CONDITIONS Spatially Extended System (Large Fresnel number in Optics) to guarantee independence of transverse boundaries. This is equivalent to say that the correlation length must be much smaller than the size of the system Coexistence of a pattern state and an homogeneous state

4 PROPERTIES AND APPLICATIONS Addressability by a local beam coherent with the injected field information encoding in the transverse plane of the resonator : optical memory array Parameter gradient induces a movement of CS all-optical reconfiguration of the memory array shift register; serial-parallel converter These applications become very attractive if implemented in semiconductors which allow for fast response and miniaturization

5 EXPERIMENTAL REALISATION broad-area (> 150 μm diameter) vertical cavity semiconductor lasers (VCSELs) allows for high Fresnel numbers Optical injection of a coherent field (holding beam) engenders bistability Theoretical simulations indicate that CS exists in this system Holding beam VCSEL Nonlinear medium χ nl Output field

6 Experimental set up

7 The Vertical Cavity Surface Emitting Lasers Engineered and Manufactured by ULM University (R. Jaeger, T. Knoedl, M. Miller) Grabherr Jaeger, Miller Thalmaier, Heerlein, Michalzik, Ebeling, Phot. Tech.Lett. 10, 1061 (98) 980 nm μm diameter Bottom emitting configuration Operated as amplifier Very Low Thermal Resistance GaAs Substrate Bragg reflector Active layer (MQW) Bragg reflector CRITICAL POINT Device homogeneity : Cavity length 60 GHz/ 150 μm (13 nm/cm) today negligible Current crowding Device defects: dislocations, local change of device roughness

8 CS adressing by a writing beam: first observation in VCSELs Device homogeinity : 60 GHz/ 150 mm (13 νm/cm) Holding Beam (HB) : ~ 3 mw, waist 200 μm ( ~ 5 W/ cm 2 ) Writing Beam (WB) : ~ 1 mw, 15 μm focused waist (~ 0.3 W/cm 2 ) VCSEL current between ma Barland et al. Nature 419, 722 (2002)

9 CS switching The switch-on time of a CS after application of the WB is composed by the CS rise time (τ) and a lethargic time (Δt) between the WB application and the beginning of the CS rising front τ is not affected by the parameters of the system: we measured τ = 520 ± 50 ps Δt depends on the system parameters; best result obtaine for the switch on time: 800 ± 50 ps Intensity (Arb. Units) Intensity (Arb. Units) φ=1.22 φ=1 φ= Time (ns) Delay Time (ns) Hachair et al, PRA72, (2005)

10 Experimental Problems Device spurious gradients The Today cavity delivered gradient VCSELs confine do the not CS show existence cavityin a gradient narrow band anymore, and it while blows local the defects CS in the are still patterned present region Current crowding at the borders introduce a current gradient with radial symmetry Surface roughness and impurities They can trap CS. This allows CS observation even in the presence of gradients but they affect the controlling of CS motion and positioning

11 CSs control by parameters gradient in the HB A gradient (intensity or phase) in the HB makes CSs to move towards the maxima This effect stems from space invariance of the system: the CS acquire a velocity proportional to the gradient strength Can we overcome the device defects and unpin CSs?

12 Optical reconfiguration of CS arrays We want to introduce a phase landscape on the HB in order to locate the CSs at fixed points In order to control the HB phase we need a phase spatial modulator. Our solution is a LCLV 640 nm and 980. We modulate the intensity of the writing beam by a LCD computer controlled Computer generated pattern on LCD Interferogram of the phase nm Phase gradient : 0.1 π/μm

13 Experimental set-up

14 Starting from a situation where CSs are randomly distributed, pinned by local defects and some drifting Appling the phase landscape we obtain a regular array of CSs located at some of the phase maxima Some pinning defects were not overcome Square grid landscape of phase maxima

15 Guiding CS motion In order to channel CS movement in the transverse plane we can modulate the intensity of the holding beam in form of fringes. The motion will be induced by tilting the HB direction with respect the resonator axis A stripe like HB can be created by selfinterference of the HB (Mach Zender interferometer) or by using a cylindrical lens in front of the VCSEL Along one fringe fast detectors can be placed in order to detect the passage of the CS

16 Controlling cavity soliton motion A CS can appear spontaneously in a point induced by the noise of the system We put an HB fringe on this point and a phase gradient along the fringe in order to induce the movement and to channel the direction Once appeared, the evolution of CS is always the same: CS drift CS movement: superposition of 50 events of CS generation

17 CS can be created on-demand by applying the writing beam at a point A fringe is injected onto the target point for channeling the CS direction A phase gradient is introduced along the fringe by tilting the HB direction Once removed the WB the CS starts to drift The movement is monitored by the detectors a-e CS drifts of 36 μm in 7.5 ns which makes an average speed of 4.7 μm/ns VCSEL near field. Cylindrical lens. INPUT writing pulse gradients delayed OUTPUT

18 Application: All-optical CS delay line Amplitude gradient Writing bits Phase gradient Reading bits Serial to parallel conversion Time delayed version of input train Speed is proportional to gradient: delay is tunable Figure of merit (BW X delay)=2.5

19 CS gradient-induced drift as alternative to slow light buffers can enhance performance of networks future high-performance photonic networks should be all-optical need for all-optical buffers with controllable delay State-of-the-art methods rely on the existence of resonance for modifying longitudinal group velocity Our method is based on modification of transverse group velocity Our figure of merit is competitive with other methods Good potential for improving performances From Boyd et al., OPN 17(4) 18 (2006)

20 Mapping the device defects strong lateral confinement by intensity gradient and no phase gradient each soliton is free to move along the intensity channel defects deviate the trajectory

21 Mapping the device defects strong lateral confinement by intensity gradient and no phase gradient each soliton is free to move along the intensity channel defects deviate the trajectory

22 How to build a map? We sweep the fringes along a large number of directions covering the full space (0-2π), summing and normalizing all the images, one obtains the map of the most visited regions A perfect, defect-free cavity would give a homogeneously gray map. white = attractive black = repulsive

23 Conclusions CS are promising objects for the all-optical treatment of information They form an array of light pixels that can be reconfigured by a phase landscape They drift under the action of a phase gradient following the imposed path: all-optical delay line They are affected by local defects and they can be used for probing device purity: CS microscope