Sep 17, 2015

Exploring the Technologies of Multiplexing and Demultiplexing

In Part III of Precision’s Mux/Demux blog series, we will discuss the several Mux and Demux technologies, including: 1) Fiber Bragg Grating 2) Thin Film Filter 3) Array Waveguide Grating 4) Diffraction Grating Filter 1) Fiber Bragg Gratings (FBG): A Bragg Grating is a short length of fiber where the index of refraction is modulated at a periodic rate for a short length along the fiber. What this implies is that for any WDM sequence {λ1, λ2, … , λB , λN}, all wavelengths other than λB will be passed through the fiber unchanged. λB will be completely reflected back to the source. (λB = 2ηeΛ where ηis the effective refractive index in the fiber core and Λ is the grating period.) The Bragg Grating requires an optical circulator to complete the mux and demux. A circulator is a non-reciprocal 3-port optical device that passes any input signal, Si , from port Pi and outputs the signal SO = Si to port PO = (Pi mod 3) +1. Circulator Diagram Multiplexer Diagram  Demultiplexer Diagram   2) Thin Film Filters: Thin Film Filters are multiple layers of very thin materials that are deposited on glass substraights. The materials are selected so that they have tuned refractive index values. Incident light is mostly reflected from the device; however, the tuned wavelength is allowed to pass through the device in either direction (this is opposite of Fiber Bragg Grating). The multiplexer has N thin film filters tuned to each wavelength. The input wavelength passes through the filter and into the output fiber. The signal is then cascade combined with the other N-1 thin film filters as shown. Thin Film Filter Diagram  Mux/Demux Diagrams   Thin film technology has excellent spectral shaping at a low cost. It is typically used with systems of less than 40 channels, is bidirectional, and is resistant to temperature changes. Thin Film Filters are completely passive with low insertion loss. 3) Array Waveguide Gratings (AWG): Array Waveguide Gratings consist of an optical splitter/combiner, a series of varying length fibers, and an optical coupler. The coupler has N inputs, one for each λ{I ε 1, N}. A given input is divided and presented to each of the fiber inputs (individual optical signals each having its own λare incident to the optical coupler). From there, signals run down each variable length fiber where they end at the optical combiner. The fiber lengths for this technology are tuned such that the phase relationships of each signal constructively or destructively interfere with one another. The output of the combiner is the linearly summed signals from the input of the optical coupler. When demultiplexing, the demux takes the mixed signal and presents it to the N varying length fibers. Each fiber has its own tuned length, therefore each output is phase delayed by some calculated amount. The delayed signals are then presented to each output fiber where they constructively and destructively interfere. Hence, demultiplexing occurs. Array waveguide gratings introduce polarization mode dispersion, and can have high insertion loss. They are very temperature dependent; generally they require temperature control to maintain proper phase relationships. AWG devices also support large channel counts and narrow channel spacing. They are suitable for larger channel numbers and have satisfactory filter response. 4) Diffraction Grating Filters  A diffraction grating filter consists of a grated lens that bends light according to its wavelength. Diffraction, unlike refraction, occurs when a wave encounters any object in free space that is compatible its wavelength. If we assume that an incident wave reaching an obstacle (such as a slit in a diffraction grating) acts according to the Huygens-Fresnel principle, then a series of monopole radiators will be present equally spaced according to the incident wavelength in the slit. These monopole radiators will interfere both constructively and destructively on the opposite side of the slit; this creates a pattern that is dependent upon the wavelength of the signal. Using this principle, an incident wave consisting of multiple λ’s is spectrally spread to a series of output fibers. As demonstrated below (figure 1.1), the center lobe contains most of the signal intensity; however, the side lobes are periodic according to the Bessel function of the first kind (J1). The argument of the Bessel function is proportional to ka, the wave number and the diameter of the aperture. If we fix the aperture size (the size of the diffraction grating), then the spread given by the Bessel function will be completely wavelength dependent (figure 1.2 shown below in red) Figure 1.1  Figure 1.2  A diffraction grating can be built on either a transmissive or reflective material. It generally has low temperature sensitivity, are bidirectional, and has low insertion loss. This technology is purely passive, and supports variable channel bandwidths/large channel counts.