Slow and fast light in a microsphere–optical fiber system.pdf
《Slow and fast light in a microsphere–optical fiber system.pdf》由会员分享,可在线阅读,更多相关《Slow and fast light in a microsphere–optical fiber system.pdf(6页珍藏版)》请在三一文库上搜索。
1、Slow and fast light in a microsphereoptical fi ber system Kouki Totsuka and Makoto Tomita Department of Physics, Faculty of Science, Shizuoka University, 836 Ohya, Suruga-ku, Shizuoka, Japan 422-8529 Received May 12, 2006; revised June 30, 2006; accepted July 4, 2006; posted July 10, 2006 (Doc. ID 7
2、0903) The dispersion relation in a system of a fi ber taper coupled with a microsphere is investigated. On the under- coupling condition, in which the coupling strength between the sphere and the fi ber is weak compared with the round-trip loss in the sphere, the system shows anomalous dispersion. O
3、n the other hand, on the overcoupling condition, in which the coupling strength is strong compared with the loss, the system shows normal disper- sion. We performed pulse propagation experiments and observed both the slow light and the fast light relevant to the normal and anomalous dispersions in t
4、he single microspherefi ber taper system, controlling the cou- pling strength between the sphere and the fi ber. The observed pulse propagation times show good agreement with theoretical calculations based on a directional coupling theory. 2006 Optical Society of America OCIS codes: 350.5500, 260.20
5、30, 290.4020, 030.1670. 1. INTRODUCTION The control of light velocity has been attracting much in- terest not only from fundamental physics but also from the dispersion-engineering point of view. Superluminal and negative light velocities, as well as ultraslow pulse propagation, have been observed i
6、n various systems. Among them, gaseous atoms may be one of the most fun- damental systems because they are the basic unit of ma- terials and less infl uenced by the disturbance from the surrounding environments. At the center frequency of a simple atomic absorption line, anomalous dispersion appears
7、,1and fast light is expected.2Usually, the anoma- lous dispersion in the absorption lines is accompanied by strong attenuation in the transmitted intensity, although a gain-assisted linear anomalous dispersion induced by double Raman lines is also employed, and the superlumi- nal pulse propagation h
8、as been demonstrated without anyattenuationor amplifi cationorseriouspulse distortions.3Slow light is also developed in atomic sys- tems. In an electromagnetically induced transparency system,4quantum interference induced by a strong coher- ent light creates ultrafi ne spectral structures in refract
9、ive index at a probe light frequency, and the resultant linear dispersion has been used in the reduction of the light ve- locity. The ultraslow velocity of 17 m/s was realized in cold Na atoms. When a light pulse traveling in free space enters such a medium, the pulse front decelerates while the pul
10、se end still propagates at the speed of c; conse- quently, a shrinking of the pulse occurs, and the whole light pulse could be contained in the atomic cloud.5This technique has been developed for coherent optical infor- mation storage or the freezing of light.6,7 The atomic system is ideal in the de
11、monstration of fun- damental physics and the relevant dispersion effects; however, it is diffi cult to integrate this system into de- vices in communication and information technologies. It is also disadvantageous that the available frequencies in atomic systems are limited to the intrinsic and disc
12、rete atomic transitions. For practical applications, dielectric nanostructures could be the most promising system for the design of photonic systems with desired dispersion curves. Normal and anomalous dispersions have been ob- served in one-dimensional and two-dimensional photonic crystal structure
13、s.811 Photonic crystal fi ber12,13and the coupled resonator optical waveguide1416have been at- tracting recent interest because they can vary the disper- sion properties beyond a range of conventional optical waveguides. The structural dispersion induced by a mi- crosphere also has high potential fo
14、r the control of light velocity.17The dielectric sphere traps light in the circula- tionorbitsnearthespheresurface,theso-called whispering-gallery mode (WGM), and can act as an excel- lent optical cavity with ultrahigh-Q factors. A quality fac- toroverQ=1010hasalreadybeenobtained experimentally,18an
15、d Q=1021is predicted theoretically.19 Such a quality factor corresponds to the frequency width of the order of 10 kHz, which is as narrow as resonance lines in atomic and molecular systems. In this paper, we examined extremely large normal and anomalous disper- sions induced by a resonance line of a
16、 single microsphere fi ber taper system with a quality factor of the order of Q =107. This value of Q factor is 3 orders of magnitude higher than that used in the coupled resonant optical waveguide.14,15In our system, the dispersion can be changed to a large extent through the coupling strength betw
17、een the fi ber taper and the microsphere. We per- formed pulse propagation experiments and observed both the slow light and the fast light relevant to the normal and anomalous dispersions in a single microspherefi ber taper system, controlling the coupling strength between the sphere and the fi ber.
18、 This controllability of the disper- sion in the system is important in applications when the coupling strength is dynamically controlled by electro- optical effects or nonlinear optical effects. 2. THEORY Figure 1 shows a schematic illustration of the waist re- gion of a microspherefi ber taper sys
19、tem. The fi ber has a 2194J. Opt. Soc. Am. B/Vol. 23, No. 10/October 2006K. Totsuka and M. Tomita 0740-3224/06/102194-6/$15.00 2006 Optical Society of America taper structure in which the diameter is reduced gradu- ally from the normal optical fi ber diameter of 125?m to several micrometers or submi
20、crometers.20,21In contrast to the conventional optical fi ber, which has a coaxial core- clad structure, the fi ber taper has an air-clad structure. Since the core is exposed directly to the open space, the traveling light along the fi ber taper is not confi ned in the glass core but emerges out of
21、the core as an evanescent wave. The strength of the evanescent wave decays quasi- exponentially from the surface of the fi ber. When a micro- sphere is moved into this evanescent wave region, the light traveling though the fi ber taper is coupled into WGMs. The light transferred into the WGM circula
22、tes in orbits near the sphere surface, repeating total internal re- fl ection at the boundary of the sphere. The circulating light also emerges out of the sphere surface as an evanes- cent wave and couples back again into the fi ber taper. A part of the incident light does not couple with the sphere
23、 and directly appears at the output of the system. We refer to this component as ballistic light. The total electric fi eld at the output of this system can be represented as the sum of the circulated and ballistic light. The stationary inputoutput characteristics can be analyzed on the basis of a d
24、irectional coupling theory.22 The electric fi elds in the fi ber at the input and output points are denoted A0and A, respectively, and the electric fi elds inside the sphere at the circulation starting and ending points are B and B0, respectively. The directional coupling theory gives A = ?1 ?1/2?A0
- 配套讲稿:
如PPT文件的首页显示word图标,表示该PPT已包含配套word讲稿。双击word图标可打开word文档。
- 特殊限制:
部分文档作品中含有的国旗、国徽等图片,仅作为作品整体效果示例展示,禁止商用。设计者仅对作品中独创性部分享有著作权。
- 关 键 词:
- Slow and fast light in microsphereoptical fiber system microsphere optical
链接地址:https://www.31doc.com/p-5116954.html