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Grasping all-optical switching in transparent network

02 Oct 2015  | Jin-Wei Tioh, Mani Mina, Robert J. Weber, Jin-Ning Tioh

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Thermal-optical (TO) switches are based on either the waveguide thermo-optic effect or the thermal behaviour of materials. [19-22] Interferometric TO switches heat the material in one of the interferometer legs to generate a phase shift relative to the other leg. This process leads to interference effects between the two light beams when they are recombined. Digital TO switches utilise the interaction of two silica waveguides on silicon, as shown in figure 5. Heating the material changes the refractive index of the waveguide, imparting a phase difference and, thereby, altering the selectivity of the output ports. While having excellent PDL, digital TO switches consume more power due to the heating process (~70mW) and have slow switch times (ms).

Magneto-optic (MO) switches are based on the Faraday rotation of polarized light when it passes through a magneto-optic material in the direction of an applied field. [23] Changing the polarisation of an electromagnetic wave is an indirect method of controlling the relative phase of its constituent orthogonal components. One method of achieving this is by exploiting the Faraday effect in a magneto-optic material, i.e., rotating the state of polarisation by the Faraday rotation angle ΘF.

Magneto-optic switches use an interferometer to convert this phase modulation to an amplitude modulation; these switches hold the distinct advantage of having high-power-handling capability. While some work has previously been done investigating these types of switches [24], the lack of sufficiently high-quality MO materials hampered the effort. Recent advances in bismuth-substituted iron garnets and orthoferrites [25-31] have yielded materials with a high MO figure of merit, giving low ILs, ultrawide bandwidths, and more rotation for less applied field.

New implementations and results
The authors have previously proposed a Mach-Zehnder interferometer (MZI), fibre-based MO switch using a bismuth-substituted iron garnet (BIG) as the Faraday rotator (FR).[32] While showing promising performance and compatibility with contemporary optical network components, that new switch design suffered from a lowered extinction ratio due to unavoidable mismatches in the interferometer paths.

To address the shortcomings of the fibre-based MZI switch, an integrated version was recently proposed and is being actively developed. [33] As a parallel branch of investigation, a Sagnac interferometer configuration is proposed, [34-37] where a BIG FR is placed in the fibre loop, as shown in figure 6. A linearly polarized input wave (E1+) is split by a hybrid coupler into two counter-propagating waves of equal amplitude and 90° out of phase (E3-, E4-). These waves are launched into the Sagnac loop and subsequently encounter the FR. The FR then rotates their polarisation by the Faraday rotation angle ΘF that is proportional to the strength of the magnetic field applied to the FR, before arriving back at the coupler (E3+, E4+).

Due to the nonreciprocal nature of Faraday rotation, the two counter-propagating waves experience equal and opposite rotations (i.e., ΘF and -ΘF). This action is embodied in (Eq. 1) and (Eq. 2) using Jones calculus, where Ex and Ey are the x and y components of an incident wave, respectively; T is the transmission coefficient; and Φ is the phase change experienced due to the length of the Sagnac loop.

Figure 6: Implementation overview of the Sagnac switch.

Assuming the absence of an input wave at port 2, the outputs at the interferometer ports can be expressed as in (Eq .3). When no field is applied (ΘF = 0°), the input wave is returned to port 1 with a 90° phase shift. Applying a field of sufficient magnitude (ΘF = 90°) redirects the input wave to port 2.

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