摘要
基于电磁感应光透明的全光路由是一个原子相干过程,它是近来量子光学领域的一个热门研究课题。在量子信息传输和全光网络中,利用全光路由过程可以实现在不同光学通道间光学信息的传输以及重新分配。光学信息的路由技术和波分复用技术在未来通信领域中有着十分重要的应用价值。2007年加拿大的A.I.Lvovsky小组在铷原子蒸汽中实现了光学信息的绝热频率转换。韩国的B.S.Ham等人近年来已经在稀土掺杂晶体中实现了电磁感应光透明等一系列的原子相干效应,对于实际应用具有重要的意义。本论文就是在他们工作的基础上,研究了在稀土掺杂晶体Pr:YSO中实现基于光存储的全光路由过程。
本论文是这样安排的:
第一章介绍本论文工作的背景,即有关原子相干的概念,列举电磁感应光透明等一些典型的原子相干现象,并介绍全光路由的相关概念以及研究现状。
第二章详细地描述光和物质相互作用的半经典理论,它是本论文的理论基础。
第三章主要介绍在稀土掺杂晶体Pr:YSO中进行的全光路由实验工作。对实验中采用的晶体特性和实验装置等进行介绍,最后对实验中的实验现象进行分析说明并和理论对比。
All-optical routing by light storage in Pr:YSO crystal is a atomic coherence process based on light storage, which realizes the distribution or the total conversion of the optical information by using a series of pulses, which interact with an atomic or molecular system. We manipulate the open or the close of the control beams to realize the release of routing pulses,this is the all-optical routing process. When we change the intensity of the control beam,the energy and width of the released signals will also change.
So far, most reports on all-optical routing process are in atomic vapor. For many potential applications, a solid-state medium is preferred. However, most solid materials have relatively broad optical linewidths and short relaxation times, which limits the achievable atom coherent effects. A notable exception to this general rule is Pr~(3+):Y~2SiO~5(Pr:YSO), which has narrow spectrum structure and long relaxation times. So far, electromagnetically induced transparency (EIT), four-wave mixing (FWM), and light storage based on EIT have been realized with these crystals. To our knowledge, however, all-optical routing process based on light storage has not been investigated with these crystals.
In this thesis, we report experimentally the all-optical routing process in a Pr:YSO crystal. We consider a four-level double lambda system of Pr~(3+) irons shown in Fig 1. ~3H_4 (±3/2), ~3H_4 (±1/2), ~1D_2 (±3/2), and ~1D_2 (±1/2) are regarded as|1), |2), |3) and |4), respectively. The coupling fields ofω_1, andω_2 (with Rabi Fig 1 Energy level diagram of Pr:YSO frequenciesΩ_1 andΩ-2 interacts with the transitions |3> (?) |2>、|4> (?) |2>, respectively.The probe field interact with the transition|3> (?)|1>. The repump fieldω_r is on resonance with the transiton of~3H_4 (±5/2) (?)~1D_2(±1/2). The repump field refills theholes burned by the coupling fields, and changes the large inhomogeneous broadening.
The ionic state is initialized to|l> which can be achieved by optical pumping with these fields. We use the coupling pulseω_(c1) initially couples the two empty states, |2>and |3>. Some time later another signal pulseω_(p1) which is gauss shaped interact with the medium. After this process, the coherence is built between the the level |1?>and |2>.
The next part is our experiment. The experimental arrangement is illustrated in Fig 2. We use a Coherent dye laser 899. The dye laser is continuous wave, and its linewidth is 0.5MHz, and its maximal output power is about 700mW in the wavelength of 605.977nm. We use acousto-optic modulators (AOM) to make four different coherent laser fields as shown. The four fieldsω_(c1)、ω_(c2),ω_(p1) andω_r areupshifted 189.8MHz、185.2MHz、200Mhz and 222.1MHz from the laser frequency, respectively. F i g 2 Schemat i c d i agram of the exper i mental setup of all -opt i ca l rout i ngprocessBS, beam spliter; L, lens; AOM, acousto-optic modulator; PD, photo-diode; OS, oscilloscope.
Fig. 3—7 show the experimental results. Fig3 shows the group velocity of probe pulse is slowed down:Fig 3 slow light demonstration
In our experiment, the width of gauss pulse is 43μs .in the slow light condition,the control fieldω_(c1) and probe fieldω(p1) interact with the crystal, the control fieldω_(c2) keep closed. The slow light is observed because of the EIT effect and it is slowed 37μs .
When we we simultaneously switch on two retrieve control fields, we will see the following result:Fig 4 two retrieve control fields are simultaneously switched on (ω_(c2) i s GmW)
The process in the fig4 is the all-optical routing. When we stimultaneously swiched on control fieldsω_(c1) andω(c2) after turning down control fieldω_(ct) sometime, the optical information will be contributed to two different optical channels. Note that the released signal have different propagation direction and different carried frequency, this is because the the four beams must satisfy the phase-matching condition(?). The process that optical information is separatedfrom one channel into two different channels is the All-optical routing process.
Fig5 show the energy and temporary width of released signalω_(p2) versus theintensity of the control-2 fieldω_(c2) It is found that the energy of released signalincreases with the increment of the intensity of control-2field. This is because the intensity of the released signal is proportional to that of the associated control field. The increment of the intensity of the control-2 field leads to the result that the signal with more energy is released into the light channel with frequencyω_(p2). So the intensity of the associated control field can control the distribution ratio of the signals between different light channels. The temporal width of the signal decreases with the increment of the intensity of the control-2 field. The solid curves in fig 5 are the theoretical fits.
Fig 6 show the energy of released signalω_(p1) increases with the increment of the intensity of control-2field. Like fig 5, The temporal width of the signal also decreases with the increment of the intensity of the control-2 field. The decrement of two released signals is because the spectral width of the EIT. The original system becomes a four-level double -lambda atomic system,where the width of the EIT widows is determined by the sum of the aquares of all control Rabi frequencies.Fig 6 the energy and temporary width of released signalω_(p1) versus the intensity of the control-2 fieldω_(c2)
We study the energy ratio of the released signals in the two light channels, by varying the intensity of the retrieve control-2 field and keep the intensity of the retrieve control-1 field constant. In such an EIT four-level double-lambda atomic system, the intensity of each released signal is linearly proportional to that of the associated retrieve control field. So the energy ratio (A_(p2)/A_(p1)) of the released signalsis determined by the intensity ratio (I_(c2)/I_(c1)) of the corresponding retrieve control fields.Fig 7 the energy ratio of the two released signalω_(p2) andω(p1)
Figure 7 shows the energy ratio (A_(p2)/A_(p1)) of the released signalsω_(p2) andω(p1)as a function of the intensity of the control-2 field. We can see that the energy ratio of the releasedω_(p2) andω(p1) is linearly proportional to the intensity of the control-2field. The increment of the control-2 intensity leads to the increment of the energy of the releasedω_(p2) and decrement of the energy of the releasedω_(p1) which isconsistent with the theoretical expectation. For the control-2 intensity of 25mW, the transfer efficiencies with respect to the original in put pulse and with respect to the slowed pulse with out storage are about 0.8% and 11%,respectively.
In summary, we have experimentally demonstrated an all-optical routing based on the technique of light storage in a Pr:YSO crystal. By simultaneously switching on two retrieve control fields to rlease the stored optical information. The original optical information is distributed into two light channels. This all-optical routing by light storage may have many applications in quantum information and all-optical network.
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