*2.3.3 Diffraction grating-based devices*

*Multiplexing*

lower rate than the optical transmission rate. It is known that transmitting data over a single fiber with higher rates is more economical than carrying lower data rates over several fibers [9]. This makes multiplexing a must, so that huge transmission capacity of the optical fibers can be supported using moderate electronic component rates. Different varieties of Mux/DeMux are available. Mostly these are reciprocal devices, hence can be used as both Mux and DeMux. They can be classified under two broad categories, diffraction-based and interference-based. Diffraction-based devices rely on angular dispersive element like diffraction gratings to decompose

Interference-based DeMux are based on optical filter and directional couplers. Filter-based Mux uses optical interference for wavelength selectivity. MZI-based filters are the most used. One arm of MZI can be made longer to induce phase difference with respect to the other arm. This phase difference is frequency dependent. The path length is adjusted such that power from two different input ports adds up

The idea is to spatially separate the different wavelength that were traversing together in the optical fiber. Each of these wavelengths can be collected in those points into individual optical fibers. Optical Mux/DeMux can be broadly classified into passive and active. Popular passive Mux/demultiplexers are based on Prims, diffraction gratings, and spectral filters. Active Mux/DeMux is usually implemented as some passive components along with tunable detector, each tuned to

These devices work based on the principle of dispersion, where different wavelengths see a different refractive index in the medium. This difference in refractive index results in some wavelengths to bend more (or less) than others which helps in separating them out. As can be seen in **Figure 5**, the incoming wavelengths are collimated and incident on the prism. Each wavelength seems a slightly different refractive index and bends differently according to Snell's law. At the output another

Superprisms employ photonic bandgap that make certain wavelength forbidden within the structure. This is achieved using special structures called photonic crystal. A photonic crystal is a periodic dielectric structure fabricated usually on Si using nanofabrication. This three-dimensional periodicity in refractive index causes periodic distribution in bands and gaps, and these can be tuned by varying the periodicity so as to make certain wavelength to propagate or not. It can act both

separate wavelength. We will see each of them in a bit more details now.

lens focuses the different wavelengths to different output fibers.

the incident light into its spectral components.

at only one output port (Mux operation).

*2.3.1 Prism-based devices*

*2.3.2 Superprism-based devices*

*Prism-based DeMuX configuration [10].*

**96**

**Figure 5.**

Diffraction elements as the name suggests use diffraction of light to separate different wavelengths. When a polychromatic light wave is incident on a diffraction grating, each wavelength is diffracted at a different angle from where they can be collected to achieve demuxing (**Figure 6**).
