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second waveguide section 302 and the ends of the resistive heating pad 304 are aligned (in a direction parallel to the direction of propagation of the light). In other examples (not shown in the figures), however, the length of the second waveguide section 302 (as measured along the direction of propagation of the light) may be longer (e.g. only slightly longer) than the length of the resistive heating pad 304, such that the resistive heating pad 304 terminates before the end(s) of the second waveguide section 302. In the first two examples, as shown in FIGs. 3B and 3C, a multi-layer waveguide structure is formed, with the second waveguide section 302 being in a plane (or layer) which is parallel to, but spaced from, the plane (or layer) of the first and third waveguide sections 301, 303. Light is coupled vertically from the end of the first waveguide section 301 and into the second waveguide section 302 (i.e. into a proximate end of the second waveguide section 302) and then coupled vertically from the other end of the second waveguide section 302 and into the third waveguide section 303 (i.e. into a proximate end of the third waveguide section 303). 

<img src='content_image/954516.jpg'>

