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doping) and this modifies the local resistivity of this part of the structure formed from the second material. As described above, the first and third waveguide sections 301, 303 are formed from a first material having a first value of dn/dT, Sl, and the second (or centre) waveguide section 402 is formed from a second material having a second value of dn/dT, S2. The second material is more thermo-optically sensitive than the first material, i.e. S2>Sl. 

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