Optical fibres, scanning

SNOM combines the optical contrast with a high lateral resolution of SPMs [55,56]. Scanning a surface with a sharp optical fibre tip within the range of the optical near field makes it possible to overcome the optical diffraction limit that restricts the resolution of conventional optical microscopy. Moreover, the SNOM probe operates at a finite distance from the surface, so that damage and distortion of delicate samples can be eliminated. The drawback of SNOM compared to other SPM methods is its relatively low resolution - around tens of nanometers [62,63]. 

Further important technical advances include the development of devices for the automatic scanning of the depolarisation ratio, for measuring Raman CID (circular intensity differentials), for measuring difference Raman spectra (i.e. the difference between the Raman signals from solutions and solvents), for studying optical-fibre Raman spectroscopy, for rapid (i.e. picosecond) Raman spectroscopy and Raman micrography 64). 

Optical characterization methods suffer restricted spatial resolution due to the Abbe limit at approximately X/2. Near-field scanning optical microscopy (NSOM or SNOM) developed in order to overcome this limit, using the kind of sophisticated feedback mechanisms developed for AFM and STM to scan an optical fibre, drawn to an aperture with a diameter... 

Figure 11.5 Schematics of the active OCT imaging catheter, where an optical fibre is scanned by the polymer actuator and a lens focuses the light at the imaging plane. The reflected light is collected by the same optical fibre and processed by the OCT system.

Table 5.36 shows the main features of NSOM. Optical fibre tips for NSOM with high light transmission permit surface analytical and spectroscopic applications (fiuorescence imaging, Raman) with high spatial resolution (ca. 30 nm) and high chemical information content [337], NSOM overcomes critical measurement limitations of both far-fleld vibrational microscopes (low spatial resolution) and scanned probe microscopes (lack of chemical specificity). NSOM offers conventional optical characterisation and contrast mechanisms with the resolution of SPM. The spatial resolution of this relatively new technique is almost competitive with that of SEM. Consequently, NSOM is expected to become a serious alternative for SEM, since it is non-destructive if visible light is used, and allows visualisation of specimens in air. The technique holds considerable promise for the future. At the present time it is still quite expensive. 

Fluorescence spectrometers for in vivo diagnostics are commonly based on fibre optic systems [30-33], The excitation light of a lamp or a laser is guided to the tissue (e.g. some specific organ) via glass fibre using appropriate optical filters (instead of an excitation monochromator). Fluorescence spectra are usually measured either via the same fibre or via a second fibre or fibre bundle in close proximity to the excitation fibre. Scanning monochromators or OMA systems as reported above are used for emission spectroscopy. 

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