Photonic crystal tapers for ultracompact mode conversion

Optics Letters (Impact Factor: 3.29). 08/2001; 26(14):1102-4. DOI: 10.1364/OL.26.001102
Source: PubMed


We have studied the coupling of a classic ridge waveguide with a two-dimensional photonic crystal (PC) waveguide, using finite-difference time-domain calculations. The ridge waveguide exhibits only a weak refractive-index confinement of light, as it is commonly used in buried-heterostructure or ridge-waveguide lasers. The light is coupled to a PC waveguide that consists of one missing row along the ?K direction in a triangular lattice of air cylinders in AlGaAs. We compare various designs for PC tapers with that of a classic taper and for butt coupling. The calculation yields high coupling efficiency that exceeds 80% for a 2.5-microm-long PC taper. In addition, the dependence of the efficiency on the PC air-fill factor and on alignment tolerances is analyzed.

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    • "A variety of techniques have been employed to select a taper shape for coupling to periodic waveguides. Most of this work examines cases operating far from any band edge (so the group velocity is not small) and focusses on simple linear (constant-rate) tapers [9] [10] [11] [12] [13] [14] [15] [16] [17] or families of quadratic shapes [18] [19] [20]. Genetic algorithms have also been employed to design couplers using arbitrarily placed scattering cylinders [21] [22]. "
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    ABSTRACT: We consider the design of tapers for coupling power between uniform and slow-light periodic waveguides. We describe new optimization methods for designing robust tapers, which not only perform well under nominal conditions, but also over a given set of parameter variations. When the set of parameter variations models the inevitable variations typical in the manufacture or operation of the coupler, a robust design is one that will have a high yield, despite these parameter variations. We introduce the ideas of successive refinement, and robust optimization based on multi-scenario optimization with iterative sampling of uncertain parameters, us-ing a fast method for approximately evaluating the reflection coefficient. We compare our robust design results to a linear taper, and to optimized tapers that do not take parameter variation into account. Finally, we verify the robust performance of our de-signs using an accurate, but much more expensive, method for evaluating the reflection coefficient.
    Preview · Article · Jan 2007 · Engineering Optimization
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    • "Therefore, a large variety of coupling techniques for efficient interfacing conventional waveguides with PhC waveguides have been proposed. PhC tapers have shown high coupling efficiencies and small coupling lengths [2]. Recently, different configurations of PhC tapers have been experimentally demonstrated. "
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    ABSTRACT: The experimental demonstration of a high efficiency coupling technique based on setting a single defect within a photonic crystal (PhC) taper is reported. The samples were fabricated on a Silicon-on-insulator substrate and a 3-μm-wide ridge waveguide was used to couple the light into and out of a single-mode PhC waveguide. Transmission efficiencies higher than 70% for wavelengths at 1.55 μm are demonstrated which sharply improves the transmission efficiency achieved with butt-coupling and conventional PhC tapers.
    Preview · Article · Nov 2004 · IEEE Photonics Technology Letters
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    • "A large variety of coupling techniques and structures have recently been proposed for efficient interfacing fiber and wide dielectric waveguides to the narrower photonic crystal waveguide. Among all of them, photonic crystal tapers are a promising approach due to the small coupling length and high coupling efficiencies achieved in a broad frequency range [4]–[9]. However, an efficient interfacing between dielectric and photonic crystal waveguides with a similar width may also be important if photonic crystal circuits have to be inserted on a chip with other blocks relying on traditional index-contrast guiding. "
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    ABSTRACT: We present a detailed analysis of butt coupling from conventional dielectric waveguides into photonic crystal waveguides. Closed-form expressions for the reflection and transmission matrices based on an eigenmode expansion technique are derived and validated by means of simulations. We use them to investigate butt-coupling losses in two kinds of photonic crystal structures: one formed by rods with a higher refractive index than the surrounding medium and the other formed by air holes inserted in a high-refractive-index medium. The origin and difference of coupling losses between the two photonic crystal structures is analyzed and discussed. We show that, although the coupling efficiency is much worse in the former structure, it can be significantly improved by choosing the optimum interface position that minimizes the mode impedance mismatch. Furthermore, the dependence of coupling efficiency on frequency is also analyzed. Finally, we also relate some traditionally used approximate formulas to our rigorous expressions.
    Preview · Article · Jun 2004 · IEEE Journal of Quantum Electronics
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