The optical buffer is a key component in all-optical information processing systems. Recently a number of schemes that use slow light propagation in various media and structures have been proposed as means toward implementation of all-optical buffers. We rigorously analyze the similarities and differences in approaches that use electromagnetically induced transparency (EIT) and coupled resonant structures (CRS). We introduce the figure of merit, finesse, that is common to both approaches and obtain fundamental limitations on bit rates and storage capacities of optical buffers. We show that at very low bit rates and storage capacities EIT outperforms CRS, but at rates of 10 Mbits/s and above the EIT medium becomes quite inefficient, and the situation is reversed. Two types of CRS based on high-index-contrast fiber gratings and high-index semiconductor-air photonic crystals and (or) microring resonators are found to hold promise for applications in the 1-1000-Gbits/s range, but only if the losses can be drastically reduced.
The optical buffer is a key component in all-optical information processing systems. Recently a number of schemes that use slow light propagation in various media and structures have been proposed as means toward implementation of all-optical buffers. We rigorously analyze the similarities and differences in approaches that use electromagnetically induced transparency (EIT) and coupled resonant structures (CRS). We introduce the figure of merit, finesse, that is common to both approaches and obtain fundamental limitations on bit rates and storage capacities of optical buffers. We show that at very low bit rates and storage capacities EIT outperforms CRS, but at rates of 10 Mbits/s and above the EIT medium becomes quite inefficient, and the situation is reversed. Two types of CRS based on high-index-contrast fiber gratings and high-index semiconductor-air photonic crystals and (or) microring resonators are found to hold promise for applications in the 1-1000-Gbits/s range, but only if the losses can be drastically reduced.
This paper presents an analysis of optical buffers based on slow-light optical delay lines. The focus of this paper is on slow-light delay lines in which the group velocity is reduced using linear processes, including electromagnetically induced transparency (EIT), population oscillations (POs), and microresonator-based photonic-crystal (PC) filters. We also consider slow-light delay lines in which the group velocity is reduced by an adiabatic process of bandwidth compression. A framework is developed for comparing these techniques and identifying fundamental physical limitations of linear slow-light technologies. It is shown that slow-light delay lines have limited capacity and delay-bandwidth product. In principle, the group velocity in slow-light delay lines can be made to approach zero. But very slow group velocity always comes at the cost of very low bandwidth or throughput. In many applications, miniaturization of the delay line is an important consideration. For all delay-line buffers, the minimum physical size of the buffer for a given number of buffered data bits is ultimately limited by the physical size of each stored bit. We show that in slow-light optical buffers, the minimum achievable size of 1 b is approximately equal to the wavelength of light in the buffer. We also compare the capabilities and limitations of a range of delay-line buffers, investigate the impact of waveguide losses on the buffer capacity, and look at the applicability of slow-light delay lines in a number of applications.
Research into slow light began theoretically in 1880 with the paper [1] of H A Lorentz, who is best known for his work on relativity and the speed of light. Experimental work started some 60 years later with the work of S L McCall and E L Hahn [2] who explored non-linear self-induced transparency in ruby. This field of research has burgeoned in the last 10 years, starting with the work of L Vestergaard Hau and coworkers on slow light via electromagnetically induced transparency in a Bose–Einstein condensate [3]. Many groups are now able to slow light down to a few metres per second or even stop the motion of light entirely [4]. Today, slow light – or more often `slow and fast light' – has become its own vibrant field with a strongly increasing number of publications. In broad scope, slow light research can be categorized in terms of the sort of physical mechanism used to slow down the light. One sort of slow light makes use of material dispersion. This dispersion can be the natural dispersion of the ordinary refractive index or can be the frequency dependence of some nonlinear optical process, such as electromagnetically induced transparency, coherent population oscillations, stimulated light scattering, or four-wave mixing processes. The second sort of slow light makes use of the wavelength dependence of artificially structured materials, such as photonic crystals, optical waveguides, and collections of microresonators. Material systems in which slow light has been observed include metal vapours, rare-earth-doped materials, Raman and Brillioun gain media, photonic crystals, microresonators and, more recently, metamaterials. A common feature of all of these schemes is the presence of a sharp single resonance or multiple resonances produced by an atomic transition, a resonance in a photonic structure, or in a nonlinear optical process. Current applications of slow light include a series of attractive topics in optical information processing, such as optical data storage, optical memories, quantum information devices, and optical communication systems in which the use of slow light will allow all-optical processing with less wasted heat. To implement these applications, devices such as buffers, memories, interferometers and switches that utilize slow light need to be developed. Future challenges include the need for improved coupling of light into slow light modes, overcoming propagation losses, and mitigating the influence of large dispersion of the group velocity. The collection of papers in this special issue of Journal of Optics features a broad spectrum of articles that highlight actual developments in many of the material types and schemes described above. It represents therefore an excellent up to date snapshot of the current state of the field of slow light research. References [1] Lorentz H A 1880 Uber die Beziehung zwischen der Fortpflanzung des Lichtes und der Körperdichte Wiedemann Ann. 9 641–64 [2] McCall S L and Hahn E L 1967 Self-induced transparency by pulsed coherent light Phys. Rev. Lett. 18 908–11 [3] Vestergaard Hau L, Harris S E, Dutton Z and Behroozi C H 1999 Nature 397 594 [4] Philips D F, Fleischhauer A, Mair A, Walsworth R L and Lukin M D 2001 Storage of light in atomic vapor Phys. Rev. Lett. 86 783–6
Optical resonators with controllable Q factors are key components in many areas of optical physics and engineering. We propose and investigate a Q -factor controllable system composed of two directly coupled microring resonators, one of which is tunable and coupled to dual waveguides. By shifting the resonance of the controllable microring, the Q factor of the system as well as the other microring changes significantly. We have demonstrated wide-range controllable Q factors based on this structure in silicon-on-insulator, for example. The influences of spectral detuning and coupling strength between two resonators on the variation of Q factors are studied in detail experimentally. Then, we explore its applications in optical buffering. Tunable fast-to-slow/slow-to-fast light has been carried out by switching the system between the high- Q state and low- Q state. Moreover, optical pulse capture and release are also achievable using this structure with dynamic tuning, and the photon storage properties are investigated. The proposed Q -factor tunable system is simple, flexible, and realizable in various integrated photonic platforms, allowing for potential applications in on-chip optical communications and quantum information processing.
This is a detailed study of slow light based on quantum effects in semiconductor nanostructures. We propose to implement an optical buffer using semiconductor quantum dots and quantum wells based on coherent population oscillation and spin related electromagnetically induced transparency. We identify different pump-and-probe schemes and develop comprehensive theoretical models which include important physical mechanisms in semiconductors. Semiconductors have unique properties which are inaccessible to other slow light media, and these enable the possibilities of controlling the semiconductor-based optical buffer in addition to the optical method. We investigate how to electrically control group index (slowdown factor) in quantum dots by using reverse bias voltage and forward injection current. We also develop a model to use the anisotropy of the light-hole exciton to vary the slowdown factor via two different mechanisms (coherent population oscillation and spin related electromagnetically induced transparency). Finally, we propose an idea to use the strain to control the spin relaxation time in semiconductors, which can help spin-dependent slow light in semiconductors. This research shows that the semiconductor is a promising candidate to demonstrate an optical buffer and can provide more flexible control than other slow light materials.
The analogue of electromagnetically induced transparency in optical methods has shown great potential in slow light and sensing applications. Here, we experimentally demonstrated a coupled resonator induced transparency system with three cascaded ring coupled resonators in a silicon chip. The structure was modeled by using the transfer matrix method. Influences of various parameters including coupling ratio of couplers, waveguide loss and additional loss of couplers on transmission characteristic and group index have been investigated theoretically and numerically in detail. The transmission character of the system was measured by the vertical grating coupling method. The enhanced quality factor reached 1.22 × 10⁵. In addition, we further test the temperature performance of the device. The results provide a new method for the manipulation of light in highly integrated optical circuits and sensing applications.
Electromagnetically induced transparency (EIT) is a phenomenon where a narrow transparency window appears within an absorption band of a material due to quantum interference. This effect has significant applications in various fields, including slow light devices, optical filters, and sensors. This paper compares three prominent EIT implementation resonator structures: single direct-coupled, double coupler add-drop, standard double resonator (SDR), coupled ring reflector (CRR) and self-coupled optical waveguide (SCOW). We analyze their underlying mechanisms, performance characteristics, and suitability for different applications, supported by recent research from 2020 onwards. We also present simulated transmission spectra, using Lumerical Interconnect, for each configuration to visually illustrate their distinct features.