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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Slow light in photorefractive phase-engineered index structures

Wolfgang Horn, Jan v Bassewitz, Cornelia Denz

Published Jun 1, 2009• 0 citations
Publisher

Abstract

We investigate all-optical tunable index structures, induced in photorefractive materials for the observation of room-temperature slow light. The potential of this technique lies in the possibility of reconfiguration in different spectral windows at low pump intensities and transmission properties comparable to fiber-based resonant delay-lines [1]. Volume-holographic gratings offer low insertion loss and narrow bandwidth selectivity but are nearly insensitive to photo-induced damage in the NIR, thus making them suitable for telecom applications, e.g. WDM filters [2]. The narrow spectral features of the thick volume gratings produce a steep slope in the dispersion which can be considered as the basis of all slow-light experiments. Investigations of increased group index of single band-stop configurations in photorefractive materials without gain have proven this concept [3,4].

DOI
10.1109/cleoe-eqec.2009.5196407

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We investigate all-optical tunable index structures, induced in photorefractive materials for the observation of room-temperature slow light. The potential of this technique lies in the possibility of reconfiguration in different spectral windows at low pump intensities and transmission properties comparable to fiber-based resonant delay-lines [1]. Volume-holographic gratings offer low insertion loss and narrow bandwidth selectivity but are nearly insensitive to photo-induced damage in the NIR, thus making them suitable for telecom applications, e.g. WDM filters [2]. The narrow spectral features of the thick volume gratings produce a steep slope in the dispersion which can be considered as the basis of all slow-light experiments. Investigations of increased group index of single band-stop configurations in photorefractive materials without gain have proven this concept [3,4].
We reviewed the recent progress on slow and fast lights in solids at room temperature based on moving and stationary refractive index gratings. A dispersive photorefractive phase coupling associated with moving gratings results in slow and fast lights. In principle, such phase-coupling-induced slow and fast lights can be observed in any nonlinear wave mixing process with a dispersive phase coupling effect. The slow and fast lights in the stationary gratings are also discussed. One advantage of the stationary gratings is the possibility to engineer the dispersion slope of the grating through designing the grating structure and parameters. As an example, we show that the dispersion slope of the gratings is enhanced significantly by stratifying a series of identical volume index gratings with homogeneous optical buffer layers sandwiched between every two neighboring grating layers. The slow and fast lights, therefore, can be controlled more effectively in such specifically designed grating structures than in the homogeneous gratings. Another advantage is the high transparency of the slow and fast lights with appropriate grating structure and parameters. Issues such as the pulse broadening effect and the pulse distortion are addressed. The slow and fast light techniques have many important potential applications such as optical delay lines and optical buffers.
The group velocity at which light pulses propagate through a dispersive material system is very different from the vacuum speed of light c, One refers to light as being "slow" for v g << c (Boyd & Gauthier, 2009) or "fast" for v g > c or v g <0 (Stenner et al, 2003 ).For v g <0, the pulse envelope appears to travel backward in the material (Gehring et al, 2006), and hence it is sometimes referred to as "backward light."The subject of slow light has caused keen interest in the past decade or more, and it is possible to control the group velocity of light pulses in the dispersive materials.Interest in slow and fast light dates back to the early days of the 20th century.Sommerfeld and Brillouin (Sommerfeld & Brillouin, 1960) were intrigued by the fact that theory predicts that v g can exceed c, which leads to apparent inconsistencies with Einstein's special theory of relativity.Experimental investigations of extreme propagation velocities were performed soon after the invention of the laser (Faxvog and et al, 1970).In 1999, Harris's group research work greatly stimulated researchers' interests, which showed that light could be slowed down to 17m/s.The result was obtained in ultra cold atom clouds with the use of electromagnetically induced transparency (EIT), which induces transparency in a material while allowing it to retain strong linear and nonlinear optical properties (Hau et al, 1999).Slow light can also be obtained through the use of the optical response of hot atomic vapors (Philips et al, 2001).These early research works require hard conditions and the slow light cannot operate in room temperature.Recently, researchers found ways to realize slow light operating in room temperature and solid-state materials, which are more suited for many practical applications, namely slow light via stimulated Brillouin scattering(SBS), slow light via coherent population oscillations (CPO), tunable time delays based on group velocity dispersion or conversion/ dispersion(C/D), slow light in fiber Bragg gratings and so on.In this chapter, we describe some of the physical mechanisms that can be used to induce slow and fast light effects in room-temperature solids (Bigelow et al, 2003) and some of the exotic propagation effects that can thereby be observed.We also survey some applications of slow and fast light within the fields of quantum electronics and photonics. Fundamentals of slow and fast lightSlow and fast light refer to the group velocity of a light wave.The group velocity is the velocity most closely related to the velocity at which the peak of a light pulse moves through an optical dispersive material (Milonni, 2005), and is given by the standard result www.intechopen.comRecent Progress in Optical Fiber Research 82where n is the refractive (phase) index and ω is the angular frequency of the carrier wave of the light field.One refers to light as being slow light for v g << c, fast light for for v g > c, and backwards light for v g <0 or v g is negative.Extreme values of the group velocity invariably rely on the dominance of the second contribution to the group index of Equation ( 1).This contribution of course results from the frequency dependence of the refractive index, and for this reason extreme values of the group velocity are usually associated with the resonant or near-resonant response of material systems.According to this theory, slow light is expected in the wings of an absorption line and fast light is expected near line center, and the spatial dispersion, that is, the non-locality in space of the medium response, is another mechanism that can lead to slow light, as has been predicted (Kocharovskaya et al,2001) and observed (Strekalov et al, 2001).We catalogue the main methods to realize slow light in room temperature solid, namely slow light via stimulated Brillouin scattering(SBS), slow light via coherent population oscillations (CPO), tunable time delays based on group velocity dispersion or conversion/ dispersion(C/D), slow light in fiber Bragg gratings and so on.We will describe CPO and SBS slow light in more details.
As a potential enabling technology, slow light has captured much research interest over the past few years for achieving a continuously tunable optical delay line [1]. In principle, slow light is generated by tailoring an enhanced group-index resonance within a given nonlinear medium. This effective refractive index change is experienced by the data stream passing through and thus induces a controllable group delay onto the signal. Themain feature of a slow-light-based optical delay line is the capability for fine-grain temporal manipulation of optical pulses. This fine tunable delay element is envisioned to be useful for various high-bandwidth signal processing functions and applications.
We experimentally propose a technique to demonstrate an optical slow-light system using the two-wave mixing (TWM) process in a photorefractive (PR) crystal at room temperature. This technique uses the refractive index variation that occurs in the PR material at room temperature. In this paper, we show that the time delay and the bandwidth of the transmitted pulses can be easily tuned by varying both laser beam intensities and the widths of the input pulses. In addition, time delays of short light pulses are observed with modest pulse distortions at the output of the PR crystal using a pulsed laser at the nanosecond regime.
We demonstrate a technique for generating tunable all-optical delays in room temperature single-mode optical fibers at telecommunication wavelengths using the stimulated Brillouin scattering process. This technique makes use of the rapid variation of the refractive index that occurs in the vicinity of the Brillouin gain feature. The wavelength at which the induced delay occurs is broadly tunable by controlling the wavelength of the laser pumping the process, and the magnitude of the delay can be tuned continuously by as much as 25 ns by adjusting the intensity of the pump field. The technique can be applied to pulses as short as 15 ns. This scheme represents an important first step towards implementing slow-light techniques for various applications including buffering in telecommunication systems.
Liquid crystal media are characterized by large and tunable dispersive properties and hence allow achievement of large group delays. At the same time, liquid crystals provide large areas and are easily reconfigurable and highly sensitive devices; they are, therefore, well adapted for interferometric applications. Two different ways of achieving slow light in liquid crystals are presented. The first method consists of exploiting photoisomerization-induced transparency in dye-doped chiral liquid crystals, and the second method makes use of two-wave mixing optical resonance in pure nematics. In both mechanisms, two beams are sent to the medium, where they create a grating, either of absorption or of refractive index. Both physical mechanisms are elucidated in the context of slow light, then, as examples of sensing applications, Doppler shift measurements and adaptive holography are presented.