Waveguides advantages

The materials to be investigated have to be incorporated into electrochemical cells in such a way as to permit the influx and the reflection of microwaves. The electrodes have to be adjusted to the microwave techniques that will be used for the investigation. Basically three different measurement approaches can be distinguished (Fig. 3). The simplest technique for microwave conductivity studies [Fig. 3(a)] is to place the sample directly at the exit of an ordinary waveguide. This setup has the advantage of being very simple and relatively transparent with respect to the phenomena occurring. Microwave power is reflected from the sample... 

The majority of currently deployed IR sensors operate in the near-IR. Although near-IR sensors suffer from limited selectivity and sensitivity due to the relatively unstructured broadband absorptions in this frequency range, the easy availability of waveguides and other instrumentation give this spectral range a significant advantage over the mid-IR. Main application areas involve substance identification and process control. 

A hollow waveguide (HWG) is essentially a hollow tube that transports light from one end to the other either by multiple mirror reflection or by total internal reflection. The hollow structure gives them several advantages (i) a high power threshold, (ii) low insertion losses, (iii) no end reflections, (iv) a small beam divergence, (v) robustness and - especially important for sensor applications - (vi) a wide spectral transmission range. 

Perhaps the most effective demonstration of the advantages of evanescent wave interrogation is provided by the optical biosensor platform depicted in Figure 5. The platform consists of a multimode slab waveguide on the upper surface of which antibodies have been immobilised. 

While planar optical sensors exist in various forms, the focus of this chapter has been on planar waveguide-based platforms that employ evanescent wave effects as the basis for sensing. The advantages of evanescent wave interrogation of thin film optical sensors have been discussed for both optical absorption and fluorescence-based sensors. These include the ability to increase device sensitivity without adversely affecting response time in the case of absorption-based platforms and the surface-specific excitation of fluorescence for optical biosensors, the latter being made possible by the tuneable nature of the evanescent field penetration depth. 

The advent of integrated optics has highlighted advantages of using waveguiding structures for non-linear interactions and these have been exploited by a number of researchers for optical SHG and mixing. These advantages are ... 

The advantages described above of using waveguiding structures can cancel out if the field overlap integral of the interacting modes is small. This arises since the SHG efficiency is proportional to it. 

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