We report an experimental demonstration of dual-channel all-optical wavelength conversion switching in hot Rb vapor. In a four-level atomic system, a coupling field and a pump field interact with both 87 Rb and 85 Rb isotopes simultaneously and facilitate the generation of two nonlinear signals when the probe field is applied to the corresponding transition. Each nonlinear signal is switched on and off separately by the pump field at different frequencies based on four-wave mixing and isotope shifts.
© 2009 Optical Society of America
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 differentmechanisms, 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) , Fano interference , 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 comparedwith 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 . In this letter, we report an experimental observation of dual-channel all-optical switching based on four-wavemixing (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.
2. 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 Ω1 with the detuning of δ 1 and a weak probe field Ωp with the detuning of δp are applied on the medium to drive respectively transitions |2〉↔|3〉 and |1〉↔|3〉. A strong pump field Ω2 with the detuning of δ2 drives transition |2〉↔|4i to facilitate the generation of a FWM signal Ωf with the detuning of δf on transition |1〉↔|4〉. Here the detuned probe and coupling fields are kept in two-photon resonance δp=δ 1, thus another two-photon resonant condition δf=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 Ω1 drives transition |2′〉↔|3′〉 with the detuning of δ′1=δ 1-913MHz and the pump field Ω2 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 phase-matching requirements. The frequency detunings for the respective transitions are defined as δ 1=ω 1-ω 23, δ 2=ω 2-ω 24, δp=ωp-ω 13, δf=ωf-ω 14, δ′1=ω 1-ω2′3′, δ′2=ω 2-ω 2′4′, δ′p=ω′p-ω 1′3′, δ′f=ω′f-ω 1′4′[ωi(i=1,2, p, p′, f, f′) is the angular frequency of the corresponding laser field].
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 . 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 . 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  leading to the improved the nonlinear coefficient χ (3) in off-resonance regions, which means that the FWM signal intensity 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 . 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.
3. 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 ~2.2×1011cm-3) and the optical depth NσL 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 Ω1, 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 Ωp (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 Ω2, 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 Ω1 and the generated FWM signal Ωf(Ω′f) will be reflected by a polarization beam splitter (PBS2), while the pump field Ω2 and the probe field Ωp (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 Ω2 at 780nm and the probe field Ωp (Ω′p) at 795nm. Photodiode D1 monitors the transmission spectrum of the probe beam Ωp (Ω′p) or the pump beam Ω2 when it is scanned in frequency. Another grating, same as the first one, is used to spatially separate the coupling field Ω1 and the FWM signal Ωf (Ω′f). Photodiode D2 is used to detect the generated FWM signal.
4. 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 Ω1 is always resonant with transition |2〉↔|3〉 of 87 Rb atoms (δ 1=0) while the probe frequency ωp is fixed to form the two-photon resonance δ 1=δp 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 dual-channel 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 FWMsignal 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=If(z=L)/Ip(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 h 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 Ωf 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 open, 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.
In conclusion, we have demonstrated in experiment a dual-channel all-optical wavelength-conversion 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.
This work is supported by NSFC (Grant No. 10774059, 10904047), the National Basic Research Program (Grant No. 2006CB921103), the doctoral program foundation of institution of High Education of China (Grant No. 20060183046).
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