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IEEE Journal of Quantum Electronics

Relationship Between the Slowing and Loss in Optical Delay Lines

Serge Dubovitsky, William H. Steier

Published Mar 22, 2006• 6 citations
Publisher

Abstract

In this paper, we demonstrate the relation between the amount of slowing and the amount of insertion loss in an optical slow wave structure when the material used has finite absorption. The delay properties are calculated as a function of wavelength for a single Fabry-Perot resonator and a delay line consisting of multiple coupled resonators. In all of these delay lines, if the amount of delay increases, the insertion loss of the line likewise increases. For low material loss, the normalized increase in delay is equal to the normalized increase in insertion loss. We also compare a coupled resonator delay line to a simple waveguide made of the same material and with the same amount of delay. The simple waveguide always has less insertion loss than the coupled resonator line because of the unavoidable reflection from the coupled resonator line. Even in the case of an asymmetric mirror delay line with zero reflection at the input, the simple waveguide has less insertion loss.

DOI
10.1109/jqe.2006.871551

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In this paper, we demonstrate the relation between the amount of slowing and the amount of insertion loss in an optical slow wave structure when the material used has finite absorption. The delay properties are calculated as a function of wavelength for a single Fabry-Perot resonator and a delay line consisting of multiple coupled resonators. In all of these delay lines, if the amount of delay increases, the insertion loss of the line likewise increases. For low material loss, the normalized increase in delay is equal to the normalized increase in insertion loss. We also compare a coupled resonator delay line to a simple waveguide made of the same material and with the same amount of delay. The simple waveguide always has less insertion loss than the coupled resonator line because of the unavoidable reflection from the coupled resonator line. Even in the case of an asymmetric mirror delay line with zero reflection at the input, the simple waveguide has less insertion loss.
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.
The possibility of dynamically controlling the speed of light can be extremely useful in all the optical applications where a variable delay is needed, such as bit and frame synchronization and optical storage. To this aim, coupled resonator slow-wave structures (SWSs) represent a promising approach to slow down the propagation of optical signals with as large bandwidths as required by optical communication systems. We show that by suitably setting the rings' resonances, SWSs can act efficiently as tunable delay lines, with the overall time delay being continuously adjustable from zero to a maximum value, typically limited by the effects of loss and dispersion. Every ring of the structure can be used as an optical gate and optical pulses can also be trapped and released by the SWS, so as to realize an optical memory or buffer. A time delay of 300 ps and a slowing ratio of 7.5 over a 3.3 GHz bandwidth are experimentally demonstrated in SWSs realized in silicon oxynitride technology
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We analyze the transmission of light through coupled-resonator optical waveguides in the form of evanescently coupled Fabry-Perot resonator arrays. We develop a transfer matrix method to calculate the amplitude and phase responses of the arrays. We also discuss the inclusion of optical gain in the system to compensate for losses in these structures. Owing to the compact length along the propagation direction in evanescently coupled arrays, large slowing factors of the order of 102-103 can be achieved even with a weak index contrast of ∼0.1%. The large slowing factor, coupled with weak index contrast, makes this structure a promising candidate for artificial slow light system.
The insertion loss of fiber couplers in the two fiber rings waveguide system is studied. The peak power is the same by matching coupler strength of two couplers at the resonant area. The peak transmittance is inversely proportional to the group refractive index regardless of the two reflection values. Group delay increases at the expense of reduction of the peak transmissivity. The three rings waveguide structure manifests the subluminal properties accompanying normal dispersion by doping gain media. Group delay is analyzed in the frequency domain and the time domain, respectively. This study could be regarded as the significant theoretical foundation for applications in the field of controllable light velocity theory, sensors, filters, optical buffers and so on.