Experimental demonstration of optical switching and routing via four-wave mixing spatial shift

Transcription

1 xperimental demonstration of optical switching routing via four-wave mixing spatial shift Zhiqiang Nie, Huaibin Zheng, Yanpeng Zhang,,* Yan Zhao, Cuicui Zuo, Changbiao Li, Hong Chang, Min Xiao Key Laboratory for Physical lectronics Devices of the Ministry of ducation & Shaanxi Key Lab of Information Photonic Technique, Xi an Jiaotong University, Xi an 749, China Department of Physics, University of Arkansas, Fayetteville, Arkansas 77, USA National Time Service Center, Chinese Academy of Sciences, Lintong 76, China * ypzhang@mail.xjtu.edu.cn Abstract: We demonstrate the shift characteristics of four-wave mixing (FWM) beam spots which are controlled by the strong laser fields via the large cross-kerr nonlinearity. The shift distances directions are determined by the nonlinear dispersions. Based on such spatial displacements of the FWM beams, as well as the probe beam, we experimentally demonstrate spatial optical switching for one beam or multiple optical beams, which can be used for all-optical switching, switching arrays routers. Optical Society of America OCIS codes: (6.8) Couplers, switches, multiplexers; (9.9) Nonlinear optics; (9.7) Kerr effect; (9.48) Multiphoton processes; (9.4) Nonlinear wave mixing; (5.675) Switching; (7.67) Coherent optical effects. References links. A. M. C. Dawes, L. Illing, S. M. Clark, D. J. Gauthier, All-optical switching in rubidium vapor, Science 8(57), (5).. A. W. Brown, M. Xiao, All-optical switching routing based on an electromagnetically induced absorption grating, Opt. Lett. (7), (5).. M. Yan,. G. Rickey, Y. F. Zhu, Observation of Absorptive Photon Switching by Quantum Interference, Phys. Rev. A 64(4), 48 (). 4. Y. P. Zhang, Z. Q. Nie, H. B. Zheng, C. B. Li, J. P. Song, M. Xiao, lectromagnetically Induced Spatial Nonlinear Dispersion of Four-Wave Mixing, Phys. Rev. A 8(), 85 (9). 5. H. Wang, D. Goorskey, M. Xiao, nhanced Kerr nonlinearity via atomic coherence in a three-level atomic system, Phys. Rev. Lett. 87(7), 76 (). 6. M. Xiao, Yq. Li, Sz. Jin, J. Gea-Banacloche, Measurement of Dispersive Properties of lectromagnetically Induced Transparency in Rubidium Atoms, Phys. Rev. Lett. 74(5), (995). 7. G. P. Agrawal, Induced focusing of optical beams in self-defocusing nonlinear media, Phys. Rev. Lett. 64(), (99). 8. J. M. Hickmann, A. S. Gomes, C. de Araújo, Observation of spatial cross-phase modulation effects in a self-defocusing nonlinear medium, Phys. Rev. Lett. 68(4), (99). 9. Y. P. Zhang, U. Khadka, B. Anderson, M. Xiao, Temporal Spatial Interference between Four-Wave Mixing Six-Wave Mixing Channels, Phys. Rev. Lett. (), 6 (9).. B. S. Ham, P. R. Hemmer, Coherence switching in a four-level system: quantum switching, Phys. Rev. Lett. 84(8), ().. J. P. Zhang, G. Hernez, Y. F. Zhu, Optical switching mediated by quantum interference of Raman transitions, Opt. xpress 6(), 9 97 (8).. R. M. Camacho, P. K. Vudyasetu, J. C. Howell, Four-Wave-Mixing Stopped Light in Hot Atomic Rubidium Vapour, Nat. Photonics (), 6 (9).. V. Boyer, A. M. Marino, R. C. Pooser, P. D. Lett, ntangled images from four-wave mixing, Science (5888), (8).. Introduction In order to develop the next generation of all-optical communication computing, certain optical elements are essential, such as all-optical switches routers. There have been several new schemes reported recently to demonstrate, in principle, such all-optically controlled switching routing functions [ ]. A weak beam was used to selectively turn on/off the # $5. USD Received 6 Nov 9; revised Dec 9; accepted 6 Dec 9; published 6 Jan (C) OSA 8 January / Vol. 8, No. / OPTICS XPRSS 899

2 spots in the spatial pattern of a stronger laser beam via cross-phase modulation (XPM) in a two-level atomic medium [], showing a spatial switching effect. Also, controlling the linear [] nonlinear [] optical absorptions of one laser beam by another in coherently-prepared atomic media was exploited to show all-optically controlled beam switching. Recently, it was shown that a four-wave mixing (FWM) signal beam can be spatially shifted easily by frequency detunings intensities of the dressing laser beams following a dispersion-like behavior [4]. Such electromagnetically-induced spatial dispersion (ISD) is greatly enhanced same as for the frequency (linear nonlinear) dispersions in the electromagnetically induced transparency (IT) systems [5,6], which can give large sensitive spatial displacements for the FWM the probe beams. Also, if one carefully chooses the parametric regime, the probe FWM beams can have focusing effects in a self-defocusing medium due to the strong XPM [7,8], which compensate the beam diffraction when propagating through the long atomic medium. In this Letter, we show that by making use of the ISD effect in a three-level ladder-type atomic system [4], all-optical switching/routing effects can be experimentally demonstrated. The FWM signals are generated by two coupling beams in the three- or two-level system, with an additional dressing field to shift the spatial location of the generated FWM beams. The intensities of the initial (before shifting) final (after shifting) spots of the FWM signals correspond to the off on states of the switch. Different shift directions spot locations are studied as functions of experimental parameters. Since there are two FWM beams each beam has more than one final states (spatial locations), it is possible to construct switching arrays in the current system. Such controllable spatial beam spot shifts can provide potential architectures for beam address selection routing in all-optical communication networks.. Theoretical model experimental scheme 4D / P / ω S / ω ω F ω ω ω F F F y k F k k, z k k 4 () χ k x F (a) (b) (c) Fig.. (a) (b) The diagrams of Na energy levels with different coupling schemes. The bold arrows refer to the dressing fields. (c) The experimental scheme arrangements (Inset: the spatial alignments of the incident beams). The relevant experimental system is shown in Figs. (a) (b). Three energy levels from sodium atoms (in a heat-pipe oven of length 8 cm) are involved in the experimental schemes. The pulse laser beams are aligned spatially as shown in Fig. (c). In Fig. (a), energy levels ( S / ), ( P / ) ( 4D / ) form a ladder-type three-level atomic system. Coupling field (wavelength of 568.8nm, angular frequency ω, detuning =, wave vector k, Rabi frequency G = 5.GHz ) ( ω =, k, G = 5.5GHz ) connecting the transition between level level, which are from the same near-transform-limited dye laser ( Hz repetition rate, 5 ns pulse-width.4 cm linewidth). The field in beam propagates in the opposite direction of the weak probe field (wavelength of 589.nm, ω, k, G = 4.8GHz ) in beam 4, as shown in Fig. (c), connecting the transition between to. in beam propagates in the plane (yz) having a small angle # $5. USD Received 6 Nov 9; revised Dec 9; accepted 6 Dec 9; published 6 Jan (C) OSA 8 January / Vol. 8, No. / OPTICS XPRSS 9

3 (. ) with. With the phase-matching condition, it generates a non-degenerated FWM (NDFWM) process satisfying k F = k + k k (called F for the subsystem ). Then, additional fields ( ω, k, G = 5.GHz ) ( ω, k, G ) are added, which are from the other dye laser with similar characteristics as the first one, also connecting the transition between to. adds onto beam (beam ) propagates in another plane (xz) which is perpendicular to the yz plane with a small angle relative to, as shown in the inset of Fig. (c). When are turned on simultaneously with blocking, a DFWM process is generated satisfying the phase-matching condition k F = k - k + k (called F for the subsystem ) (Fig. (b)). Here we define detuning i = Ωi ωi with the atomic resonant frequency Ω i. The are,, 5, 95.4µW, average powers of the laser beams, respectively. The laser beams ( ), ( ) (with diameters of about.59,.8.59mm, respectively) are horizontally polarized. When, are all turned on simultaneously, the NDFWM process F DFWM process F are generated simultaneously. These two generated FWM signals have the same frequency ωf, ( = ω ), but propagate in two different directions, which are monitored by a charge coupled device (CCD) camera (Fig. (c)). In the experiment, the intensity of laser beams is about 5 times stronger than the beam, about times stronger than the beams,,. According to the insert of Fig. (c), with cross-kerr effect, such horizontal alignment of strong dressing field beams induce horizontal shift of NDFWM F DFWM F, respectively [4]. The probe beam is influenced by the combined effect of beams but mainly shifted horizontally by beam (Fig. (a)). Thus, a pair of F beams can be switched on off by beam, while one F beam can be switched on off by beam at the same time. The theoretical description of the spatial properties of the beams, F, F due to self- cross-kerr nonlinearities can be given through numerically solving the following propagation equations: i ik ( S X X = n + n + n ), z k ξ n i ik S X X 4 X 5 = ( n F + n + n + n ) F, F F F z k F ξ n i ik S X 6 X 7 X 8 = ( n F + n + n + n ) F, F F F z k F ξ n () () () where k = k F = k F = ωn / c. z ξ are the longitudinal transverse coordinates, S S respectively. n is the linear refractive index at ω. n are the self-kerr coefficients of 8, F, F X X n are the cross-kerr coefficients of,,, respectively. Generally, the Kerr coefficient can be defined by χ () D (), n = Re χ / ( ε cn ), with the nonlinear susceptibility () = ρ where D= Nµ µ / ħ ε G G, ij j ρ ( ) = ig G / η, () F F # $5. USD Received 6 Nov 9; revised Dec 9; accepted 6 Dec 9; published 6 Jan (C) OSA 8 January / Vol. 8, No. / OPTICS XPRSS 9

4 ρ ( ) = ig G / η, () F F ρ ( ) = ig G / η, () η = D D., D are the parameters related to the Rabi frequency of the dressing field, the frequency detuning, the atomic coherence rate. µ ( µ ij ) is the dipole matrix element between the states coupled by the probe beam (between i> j> ). By assuming Gaussian profiles for the input fields, qs. ()-() are solved by the split-step method.. Optical switching routing via spatial shift (a ).6 (c) N ( cm - ).6 (b) G' (GHz) (G H z) Fig.. (a) Spatial dispersion curves of F in the ladder-type three-level system versus with G = 5GHz at 5 C. (b) The spatial displacement of F versus G in the ladder-type three-level system at = 8GHz 5 C. (c) The spatial displacement of versus atomic density N with G = 5GHz at = 8GHz. The solid lines are F theoretically calculated spatial shifts the scattered points are the experimental results. When four laser beams (, ) are on, in the presence of the dressing beam, the spatial shift of F beam spot versus probe laser frequency detuning is shown in Fig. (a). The moving trace of the light spot is dispersion-like as frequency scans [4]. It means F beam can have right or left shift. There are two maximal displacements corresponding to the positive maximum nonlinear refraction coefficient the negative maximum coefficient. Without beam, the probe field F are single strong spots, as shown in Fig. (a). When the dressing field is on, the intensities of the probe F beams become weaker [9] are shifted (one to the right another to the left of the original position). Since we use one more mirror in the probe beam scheme than that of F, they have opposite direction of shift on CCD screen (Fig. (a)). In fact, in the heated pipe both two beams have right shift, as shown in Fig. (b, c). Larger spatial shift occurs with an increasing intensity, which can be understood from the expression: ϕ ( z, ξ ) = k n I NL,F exp( ξ ) z / n, (4) The nonlinear phase shift ϕ NL is directly proportional to the dressing intensity I. The component of the wave vector of the F spot δ k ξ (which we use to measure the shift effect of the optical switch) is the derivative of ϕ, i.e. δ k = ϕ / ξ, so the beam spots also move more as the dressing laser intensity increases. NL ξ NL # $5. USD Received 6 Nov 9; revised Dec 9; accepted 6 Dec 9; published 6 Jan (C) OSA 8 January / Vol. 8, No. / OPTICS XPRSS 9

5 Figure (b) shows the dressing field dependences of the spatial shifts based on the numerical calculation the experimental measurements. Figure (c) presents the temperature dependence (atomic density N) of the shift curves for the theoretical the experimental results, respectively. We see that increasing the atomic density equals to increasing propagation distance z, the shift of the spot becomes larger. So, as shown above the beam spots can have different spatial shifts with different experimental parameters (such as frequency, intensity, atomic density), which can correspond to different on-off combinations. The switching or routing time is the rising falling times of the switch-in switch-out signal. The cross-kerr refractive index change () ( n Re( ρ ) ) limited by the overall spin dephasing time determines the response time of the switch [,,]. The estimated switching times of F F are about ns 4 ns, respectively. Here, it should be noted that the overall spin dephasing times of the two-level (Fig. (b)) ladder-type three-level (Fig. (a)) atomic systems in sodium are determined by the transverse relaxation rates: / ( πγ ) / ( πγ ), where Γ = 4.85MHz Γ = 98kHz for transitions, respectively. However, the switching speed in Fig. is limited to a microsecond time scale by the speed of the CCD used to take the image. State (a) Time (ns). (b)... (c). 4 6 T im e (µs) Fig.. (a) Results of the optical switches the spot shifts of the probe (lower) F (upper) beams obtained from the CCD at = 8GHz. The arrows are the initial position in x direction. The spatial shift of (b) the probe (c) F beams in the ladder-type three-level atomic system with G = 4GHz at = 8GHz 5 C. Figure (a) shows the two states of the probe F beams by switching the strong laser beam off on as the laser frequency detuning is tuned to get the maximal spatial displacement. When a spot stays at its initial position, it means that the switch is in the off state. When the frequencies of the probe F beams are set at their peak shift positions, the light spots will have their largest shifts, so the switch sts at its on -state. Such two states form two ports of the optical switch. The upper spot is the F beam the lower spot is the probe beam. Initially, two spots are set at same vertical line without the dressing laser beam. As the dressing beam turns on, the upper spot moves to the left side the lower spot moves to the right side, both of which leave their initial positions completely. The switching contrast can be defined as C = ( Ioff Ion ) / ( Ion + Ioff ), where I off is the light intensity at the # $5. USD Received 6 Nov 9; revised Dec 9; accepted 6 Dec 9; published 6 Jan (C) OSA 8 January / Vol. 8, No. / OPTICS XPRSS 9

6 off -state I on is the light intensity at the on -state. The contrast derived from the experiment is about C = 9%. This experiment provides a physical mechanism to realize an all-optical switching/routing by controlling the dressing laser beam. A chopper is used to control the dressing field, subtracting the laser pulse repetition time of.s, which is considered as an idle load state. The laser pulse width is 5ns. The detected switching time is limited by the response time of the CCD, which is about µs, far larger than the laser pulse width. Thus, the switching speed in the current experiment is greatly constrained as shown in Figs. (b) (c). The on-state just lasts 5ns, followed by a µs rising time, then a 5ns off-state, followed by a µs falling time, so on. Since the spatial displacements of the probe F beams are mainly determined controlled by the large cross-kerr nonlinear coefficients of the strong laser field, the switching speed should be much faster limited by the atomic coherence time in nanosecond time scale. State Time (s) Fig. 4. The switching processes of the dressing beam (square), F (triangle), F (circle), the probe beam (diamond) in the ladder-type three-level system with G = GHz at = 8GHz 5 C. Next, when five laser beams (, ) are all on, there are interplays between the generated F, F signals [9] we can control the shifts of the probe, F F beams, to achieve a triple binary optical switch. The initial locations of the spots are the off states the switches are considered to come to their on states when the spots shift away to new locations. The repetition frequency of the chopper is much longer than 5 ns pulse-width of the dressing laser, so the on -state lasts several 5ns intervals then turns to the off -state. In Fig. 4, at = 8GHz for the self-focusing side temperature 5 C, when is on, the probe F beams have right shift due to the beam via the cross-kerr nonlinear coefficients. At the same time, the F beam is shifted to the left by the dressing field. When is off, all the beams come back to their original position X 4 X 6 X ( off -state). Since the cross-kerr nonlinear coefficients n n ( n ) of the F F (probe) beams induced by the dressing fields are all positive, respectively, the spots of F F ( ) beams are shifted to the opposite directions, as shown in Fig. 4. According to the nonlinear phase shifts = X 6 k / ( X ϕ k n A z / n ϕ n A z n F F X 4 k / ϕ = n A z n F F = ) induced by the dressing fields # $5. USD Received 6 Nov 9; revised Dec 9; accepted 6 Dec 9; published 6 Jan (C) OSA 8 January / Vol. 8, No. / OPTICS XPRSS 94

7 , respectively, we can use two controllable parameters, i.e. the frequency intensity of the laser, to control the different shifts of the three spots. Such simultaneous optical switching for three beams can perform the functions of choosing different addresses in data transmissions can be used as the optical routings, the multiplexer or all-optical switching arrays for all-optical networks. In the above discussion, we have controlled the probe, F F by two dressing fields, respectively. In that case, F F are shifted towards the opposite directions (Fig. 4). Actually, such three beams can also be shifted to the same direction when the sign of the cross Kerr-nonlinear coefficient of the F signal is opposite to those of the F (probe) beams at the proper laser detuning. So, each spot can have left right locations. Including the initial position, every spot has three possible spatial locations. Totally there are controllable spatial positions. It can such achieve a switch array. The advantages of solids include high density of atoms, compactness, absence of atomic diffusion, but with relatively broad optical linewidths fast decoherence rates. However, there are still many advantages to study all-optical switching, especially spatial all-optical switching routing, using multi-level atomic media via IT (or atomic coherence) related effects. The current atomic experiment has several easily tunable experimental parameters (such as laser intensities, frequency detunings, atomic density), which provide a much better platform (compared to the solid systems) to study the formation dynamics of the novel spatial optical switch router protocols. Also, there is a narrow linewidth (compared to the solid systems) in an atomic media. 4. Conclusion In conclusion, we experimentally demonstrated the spot shifts of the FWMs probe laser beams, which can be used as the on off states of the spatial all-optical switch. Several experimental parameters (such as frequency detunings, intensities temperature) have been used to optimize the beam shift distances directions, so the extinction ratio for the on/off states can be optimized. At the same time, the opposite-direction shifting has been realized simultaneously for different FWM beams, which could be employed to construct switching/routing arrays. The current experiment also opens the door for spatial manipulations of FWM signal beams in optical imaging storage [], quantum correlation [], all-optical computation, future all-optical networking. Acknowledgments This work was supported by NSFC (No. 9745, No ), FANDD (No. 9), RFDP (No ), FYTFYTIHC (No. 6), NCT (No. 8-4), 9xjtujc8. # $5. USD Received 6 Nov 9; revised Dec 9; accepted 6 Dec 9; published 6 Jan (C) OSA 8 January / Vol. 8, No. / OPTICS XPRSS 95