Compact demultiplexers and spectrometers for integrated photonics

Abstract

Wavelength demultiplexing devices (WDD) disperse an input beam into a plurality of sub-beams, and are commonly used to separate optical channels at different wavelengths. Compact WDDs are of great interest for several integrated photonic applications including optical information processing, optical communications and networking, as well as integrated optical sensing such as ‘lab-on-a-chip’ biosensing systems. The compactness of these devices and their compatibility with integrated photonic and electronic platforms are among their main advantages when compared to other wavelength demultiplexing solutions. These advantages are the result of the wide range of controllable dispersive properties in a very compact structure enabled by the use of photonic crystals (PCs). Such dispersive properties are not available in conventional devices that usually rely on bulk or grating-based materials. For efficient WDD implementation, we use PCs. These structures, which consist of sub-wavelength periodic features, can be used to control the optical properties of materials: they have unique dispersive properties that are not present in naturally occurring optical materials. In particular, properly designed PCs can angularly separate optical beams of different wavelengths in widely different directions. By analogy with conventional prisms, this phenomenon is called the superprism effect and represents the primary physical effect used for the separation of wavelength channels of an optical signal as shown in Figure 1(a). Ideally, the compact spatial separation of different wavelengths would require a large angular difference between the directions of propagation of the different wavelengths. This would translate into a strong superprism effect for the selected PC. However, rapid variation of the angle withwavelength naturally Figure 1. Schematic representation of the three dispersive properties of photonic crystals used for wavelength demultiplexing: (a) the superprism effect, (b) negative diffraction, and (c) negative refraction.