Dual-channel all-optical wavelength conversion switching by four-wave mixing

We report an experimental demonstration of dual-channel all-optical wavelength conversion switching in hot Rb vapo r. In a four-level atomic system, a coupling field and a pump field interact with b oth87Rb and 85Rb isotopes simultaneously and facilitate the generation of t w nonlinear signals when the probe field is applied to the corresponding t ransition. Each nonlinear signal is switched on and off separately by the pum p field at different frequencies based on four-wave mixing and isotop e shifts. © 2009 Optical Society of America OCIS codes: (190.4380) Nonlinear optics, four-wave mixing; (270.4180 ) Multiphoton processes; (020.1670) Coherent optical effects References and links 1. S. E. Harris and Y. Yamamoto, ”Photon Switching by Quantum Interference,” Phys. Rev. Lett. 81, 3611-3614 (1998). 2. D. A. Braje, V. Balic, G. Y. Yin, and S. E. Harris, ”Low-ligh t-level nonlinear optics with slow light,” Phys. Rev. A 68, 041801(R) (2003). 3. M. Yan, E. G. Rickey, and Y. F. Zhu, ”Observation of absorpt ive photon switching by quantum interference,” Phys. Rev. A64, 041801(R) (2001). 4. Y. F. Chen, Z. H. Tsai, Y. C. Liu, and I. A. Yu, ”Low-light-le v l photon switching by quantum interference,” Opt. Lett. 30, 3207-3209 (2005). 5. J. P. Zhang, G. Hernandez, and Y. F. Zhu, ”All-optical swit ching at ultralow light levels,” Opt. Lett. 32, 1317-1319 (2007). 6. A. M. C. Dawes, L. Illing, S. M. Clark, and D. J. Gauthier, ”A ll-Optical Switching in Rubidium Vapor,” Science 308, 672-674 (2005). 7. A. M. C. Dawes, L. Illing, J. A. Greenberg, and D. J. Gauthie r, ”All-optical switching with transverse optical patterns,” Phys. Rev. A77, 013833 (2008). 8. C. Y. Ye, V. A. Sautenkov, and Y. V. Rostovtsev, and M. O. Scu lly, ”Fast optical switching via stimulated Raman adiabatic passage,” Opt. Lett. 28, 2213-2215 (2003). 9. X. L. Song, L. Wang, Z. H. Kang, R. Z. Lin, X. Li, Y. Jiang, and J. Y. Gao, ”Optical signal storage and switching between two wavelengths,” Appl. Phys. Lett. 91, 071106 (2007). 10. A. W. Brown and M. Xiao, ”All-optical switching and routi ng based on an electromagnetically induced absorption grating,” Opt. Lett.30, 699-701 (2005). 11. J. H. Wu, J. Y. Gao, J. H. Xu, L. Silvestri, M. Artoni, G. C. L a Rocca, and F. Bassani, ”Ultrafast All Optical Switching via Tunable Fano Interference,” Phys. Rev. Lett. 95, 057401 (2005). 12. C. Y. Wang, Y. F. Chen, S. C. Lin, W. H. Lin, P. C. Kuan, and I. A. Yu, ”Low-light-level all-optical switching,” Opt. Lett.31, 2350-2352 (2006). 13. W. H. Lin, W. T. Liao, C. Y. Wang, Y. G. Lee, and I. A. Yu, ”Low -light-level all-optical switching based on stored light pulses,” Phys. Rev. A78, 033807 (2008). 14. T. S. El-Bawab, Optical Switching (Springer, 2006). 15. Y. Li and M. Xiao, ”Enhancement of nondegenerate four-wa ve mixing based on electromagnetically induced transparency in rubidium atoms,” Opt. Lett. 21, 1064-1066 (1996) . #116247 $15.00 USD Received 26 Aug 2009; revised 13 Oct 2009; accepted 5 Nov 2009; published 4 Dec 2009 (C) 2009 OSA 7 December 2009 / Vol. 17, No. 25 / OPTICS EXPRESS 23332 16. W. Jiang, Q. Chen, Y. Zhang, and G. C. Guo, ”Optical pumpin g-assisted electromagnetically induced transparency,” Phys. Rev. A73, 053804 (2006). 17. G. Wang, College of Physics, Jilin University, Changchu n 130023, P. R. China, Y. Xue, J. H. Wu, Z. H. Kang, Y. Jiang, S. S. Liu, and J. Y. Gao are preparing a manuscript to be called ”Efficient frequency conversion induced by quantum constructive interference.” 18. S. A. Moiseev, Y. Chen and B. S. Ham, ”Numerical Analysis o f Stationary Light for Potential Applications of a Quantum Interface,” J. Korean Phys. Soc. 49, 2293-2302 (2006). 19. S. A. Moiseev and B. S. Ham, ”Quantum control and manipula tion of Multi-Color Light fields,” Optics and Spectroscopy103, 210-218 (2007). 20. S. A. Moiseev and B. S. Ham ”Generation and Manipulation o f Multi-Color Stationary Light Field Using electromagnetically induced transparency,” E-print arXiv: qu ant-ph/0606243, http://arxiv.org/abs/quant-ph/060624 3


Introduction
All-optical switch is an important component in high-speed optical communication networks and has potential applications in quantum information systems.In the past decade, all-optical ultrafast switching has been a subject of numerous studies and many interesting proposals toward its realization have been carried out.Several promising proposals, e.g. based on quantum interference, are demonstrated with different mechanisms, such as electromagnetically induced transparency (EIT) [1,2,3,4,5], transverse optical pattern [6,7], stimulated raman adiabatic passage (STIRAP) [8,9], electromagnetically induced absorption grating (EIG) [10], Fano interference [11], and stored light pulses [12,13], etc.Quantum interference based all-optical switching exhibits such favorable advantages as high response speed and low switching power compared with electro-optical switching and the ones of using silicon waveguides or fiber-based systems.
In our knowledge, all-optical switch is usually implemented by turning on or off one light beam with another light beam (defined here as one-way switch).Compared with one-way switch, it is obvious that multi-way switch, where two or more light beams are controlled synchronously or asynchronously by a single light beam, is more advantageous in improving the communication capacity [14].In this letter, we report an experimental observation of dualchannel all-optical switching based on four-wave mixing (FWM) in a four-level atomic system.Of interest to this letter is that two light signals with different wavelengths can be switched on or off respectively by a single pump field.The key point is that 87 Rb and 85 Rb isotopes are simultaneously driven by the applied coupling and pump fields, which then result in the generation of two FWM signals with different wavelengths as the probe fields are sent into the atomic sample.We show in particular that, in this dual-channel optical switch, intensities of the two FWM signals can be well controlled by modulating the pump frequency.

Energy Structure and Theoretical Basis
The atom-light interaction scheme for the proposed all-optical switching is depicted in Fig. 1.For 87 Rb atoms, a standard three-level Λ-type EIT configuration is formed when a strong coupling field Ω 1 with the detuning of δ 1 and a weak probe field Ω p with the detuning of δ p are applied on the medium to drive respectively transitions |2 ↔ |3 and |1 ↔ |3 .A strong pump field Ω 2 with the detuning of δ 2 drives transition |2 ↔ |4 to facilitate the generation of a FWM signal Ω f with the detuning of δ f on transition |1 ↔ |4 .Here the detuned probe and coupling fields are kept in two-photon resonance δ p = δ 1 , thus another two-photon resonant condition δ f = δ 2 is also satisfied due to the energy-conservation and phase-matching requirements.On the other hand, for 85 Rb atoms, the coupling field Ω 1 drives transition |2 ′ ↔ |3 ′ with the detuning of δ ′ 1 = δ 1 − 913 MHz and the pump field Ω 2 drives transition |2 ′ ↔ |4 ′ with the detuning of δ ′ 2 = δ 2 − 1218 MHz.When the probe field Ω ′ p is applied on transition |1 ′ ↔ |3 ′ with the detuning of δ ′ p and the two-photon resonance δ ′ p = δ ′ 1 is satisfied, the second generated FWM signal Ω ′ f with the detuning of δ ′ f = δ ′ 2 may be observed due to the energy-conservation phasematching requirements.The frequency detunings for the respective transitions are defined as δ is the angular frequency of the corre- sponding laser field].
Fig. 1. (Color online) Diagrams of the four-level double-Λ system of 85 Rb and 87 Rb atom for strong nonlinear interactions.The two laser fields Ω 1 and Ω 2 drive the transitions in 85 Rb and 87 Rb atom as shown.When the probe field Ω p (or Ω ′ p ) is applied to the transition in 87 Rb (or 85 Rb) atom and two-photon resonance is formed in lower-Λ configuration, a FWM signal Ω f (or Ω ′ f ) is observed at the end of the vapor cell.
It is well known that, within an EIT window, the linear susceptibility χ (1) can be substantially reduced as a result of destructive quantum coherence while the nonlinear coefficient χ (3)  associated with the nonlinear-optical generation process may be greatly enhanced [15].But when a pump field sharing the same lower-level with the coupling field is resonantly applied on an auxiliary transition as shown in Fig. 1, the depth of the EIT window will be remarkably reduced due to the two-photon absorption [16].This then leads to a reduction of the nonlinear coefficient χ (3) as well as an attenuation of the generated FWM signal.Fortunately, this attenuation can be compensated with a large pump detuning [17] leading to the improved the nonlinear coefficient χ (3) in off-resonance regions, which means that the FWM signal inten- sity can be well controlled by modulating the pump detuning.We should note that, however, the pump detuning corresponding to the signal peak generated in 87 Rb atoms is quite different from that in 85 Rb atoms because of the isotope shifts between 85 Rb and 87 Rb atoms.That is, we can separately enhance or reduce intensities of the two FWM signals by simply adjusting the pump frequency.Thus our scheme may also be used as a wavelength-conversion [18,19,20] quantum switch, where the pump field turns on or off the FWM signals at different frequencies.Moreover, during the process of switching, informations carried by the probe field will be reserved and converted into the generated FWM field with a different wavelength [17].This is true, for instance, if informations are loaded onto the probe field by amplitude modulation, because the intensity of the output FWM field depends linearly on the intensity of the input probe field.

Experimental Setup
The experiment is done with hot 85 Rb (natural abundance ∼ 72.17%) and 87 Rb (∼27.83%)atoms in a 7.5-cm-long temperature-stabilized vapor cell filled with 0.4Torr Ne buffer gas.The temperature of the cell is set to ∼62 • C (atomic density of ∼2.2×10 11 cm −3 ) and the optical depth Nσ L is ∼311 for the incident probe field and ∼300 for the generated FWM field.An experimental setup is depicted in Fig. 2 where a Ti:sapphire ring laser (Coherent 899 ring laser system) with a power of ∼24mW, acting as the coupling field Ω 1 , simultaneously drives transition |2 ↔ |3 of 87 Rb atoms and |2 ′ ↔ |3 ′ of 85 Rb atoms.An external cavity diode laser (ECDL1, DL100) with a power of ∼0.5mW, acting as the probe field Ω p (Ω ′ p ), is scanned across the D 1 line of 85 Rb and 87 Rb atoms.Another external cavity diode laser (ECDL2, DL100) with a power of ∼10mW, acting as the pump field Ω 2 , simultaneously drives transition |2 ↔ |4 of 87 Rb atom and |2 ′ ↔ |4 ′ of 85 Rb atoms.All laser beams are linearly polarized and collinearly propagate inside the vapor cell with the help of a half-wave plate and a polarization beam splitter (PBS1).The collinear laser beams are also focused inside the vapor cell by a lens to allow the probe beam to be completely contained in the coupling and pump beams so that all probed atoms are coherently prepared.After leaving the vapor cell, most of the coupling field Ω 1 and the generated FWM signal Ω f (Ω ′ f ) will be reflected by a polarization beam splitter (PBS2), while the pump field Ω 2 and the probe field Ω p (Ω ′ p ) will pass through PBS2 due to their perpendicular polarizations relative to the coupling field and the FWM signal.A grating with a groove density of 1200 lines/mm is used to spatially separate the pump field Ω 2 at 780nm and the probe field Ω p (Ω ′ p ) at 795nm.Photodiode D1 monitors the transmission spectrum of the probe beam Ω p (Ω ′ p ) or the pump beam Ω 2 when it is scanned in frequency.Another grating, same as the first one, is used to spatially separate the coupling field Ω 1 and the FWM signal Ω f (Ω ′ f ).Photodiode D2 is used to detect the generated FWM signal.

Experimental Results and Discussions
The measured FWM signals as a function of the pump detuning are shown in Fig. 3.For these measurements, the coupling field Ω 1 is always resonant with transition |2 ↔ |3 of 87 Rb atoms (δ 1 = 0) while the probe frequency ω p is fixed to form the two-photon resonance δ 1 = δ p or δ ′ 1 = δ ′ p , separately.For the FWM signal generated in 87 Rb atoms (red square), a peak is found in this curve due to the improved nonlinear coefficient χ (3) and the reduced resonant interaction between the pump field and transition |2 ↔ |4 .The pump detuning corresponding to the FWM signal peak is ∼-1.127GHz(relative to transition |2 ↔ |4 of 87 Rb atoms).The profile of the FWM signal generated in 85 Rb atoms (black circle) is similar to that in 87 Rb atom.But the pump detuning corresponding to the FWM signal peak is ∼-96MHz, which is quite different from that in 87 Rb atoms.The difference in pump frequency opens a way to implement a dualchannel optical switching.Let's pay more attention to the tick points A, B, C, and D in Fig. 3.
With the gradual departure from the pump resonance, four different output statuses are shown in turn.With a small pump detuning, a maximal FWM signal of 85 Rb atoms is shown at point A while the FWM signal of 87 Rb is small enough to be negligible.With increased pump detunings, the FWM signal intensity of 85 Rb atoms decreases while that of 87 Rb atoms increases instead.This leads to the same FWM signal intensities for both 85 Rb and 87 Rb atoms at point B, as well as the phenomena of the maximum FWM signal for 87 Rb atoms and the negligible FWM signal for 85 Rb atoms at point C.As the pump detuning is larger enough, both FWM signals become even hard to be seen at point D.
For a clear sight on this dual-channel optical switching, we present now the experimental results as viewed from the channel statuses.A series of measured FWM signals as a function of the probe detuning are plotted in Fig. 4. For these measurements, the pump detuning is fixed sequentially at the tick points A, B, C, and D as shown in Fig. 3.The channel I (the FWM signal of 85 Rb atoms depicted as the left part in Fig. 4) is switched on (Fig. 4A) and off (Fig. 4D) when the pump detuning is set as -96MHz (point A) and -1.826GHz (point D) in turn, with frequency conversion efficiencies η = I f (z = L)/I p (z = 0) respectively to be ∼65% and ∼2%.
During the process of switching on and off channel I, the output of channel II (the FWM signal of 87 Rb atoms depicted as the right part in Fig. 4) is always small (η ≤ 2%).On the other hand, the channel II is switched on (Fig. 4C) and off (Fig. 4D) when the pump detuning is set as -1.127GHz (point C) and -1.826GHz (point D) in turn, with frequency conversion efficiencies η respectively to be ∼72% and ∼2%.During this switching process, the output of channel I is also very small (η ≤ 5%).For the case where channel II is on, the absorption coefficient for the probe field is estimated to be 0.26cm −1 .This estimation is based on the fact that the transmissivity is ∼14.38% at the EIT window center and the output probe field has an intensity ∼0.115mW corresponding to the total light losses ∼ 5%.Furthermore, both channel I and channel II are switched on (Fig. 4B, η ≃ 30%) when the pump detuning is set as -838MHz (point B), and are switched off (Fig. 4D) when the pump detuning is set as -1.826GHz (point D).It is convinced that the output of the dual-channel optical switch is the combination of two FWM signals Ω f and Ω ′ f with different frequencies because the applied laser beams can interact with either 87 Rb atoms or 85 Rb atoms when the probe detuning is continuously scanned.As discussed above, this dual-channel optical switch has four distinct statuses: only channel I only channel II open, both channels open, and both channels closed, which are mainly determined by the pump detuning.Finally, we note that the FWM signal intensity depends critically on the buffer gas pressure.In the presence of buffer atoms, the spectral line broadening occurs and the atom-light resonant interaction is enhanced, which have led in fact to the broadened spectra of the FWM signals.Additionally, this will further result in the unclosed channel I at the pump detuning -1.127GHz if the buffer gas pressure is much higher than 0.4Torr.It is expected that intensities of the two FWM signals will be negligibly small at the pump detuning -838MHz if the buffer gas pressure is very low compared with the present one.

Conclusion
In conclusion, we have demonstrated in experiment a dual-channel all-optical wavelengthconversion switching scheme utilizing the four-wave mixing process in a hot vapor cell containing both 85 Rb and 87 Rb isotopes.We find that two channels of this all-optical switching at different probe frequencies can be separately switched on or off by simply modulating the the pump frequency.

Fig. 3 .
Fig. 3. (Color online) The output FWM signal spectra of 87 Rb atom (red square) and 85 Rb atom (black circle) versus the pump detuning δ 2 .The red and black solid curves are the guideline for the measured data.The points A, B, C, and D demonstrate the four statuses of this proposed optical switching, which are marked by the dash lines at different pump detuning.

Fig. 4 .
Fig. 4. (Color Online) The output FWM signal spectra of 85 Rb atom and 87 Rb atom versus the probe detuning δ 1 .These FWM signals (from upper to lower) denote the four switching statuses corresponding to the marked point A, B, C, and D in Fig. 3.