Optical fibres multimode

Cherif K., Hleli S., Abdelghani A., Jaffrezic-Renault N., Matejec V., Chemical detection in liquid media with a refractometric sensor based on a multimode optical fibre, Sensors 2002 2 195-204. 

Trouillet, A. Ronot Trioli, C. Veillas, C. Gagnaire, H., Chemical sensing by surface plasmon resonance in a multimode optical fibre, Pure Appl. Opt. 1996, 5, 227 237... 

In one common method used with multimode fibres (Paik and Sung, 1994, Powell et al., 1995, 1998), the cladding is stripped from the optical fibre and the polymerizing network is sensed by the evanescent wave (Figure 3.43). 

Optical fibres are convenient for sending light from one place to another. Once the light is coupled into one end of a fibre it arrives at the other, regardless of how the fibre is bent or moved. Optical fibres come in three versions - multimode fibres, gradient-index fibres, and single-mode fibres (Fig. 7.21). 

Another point to be considered is the pulse width of the light source and the pulse dispersion in the optical system. Multimode fibres or fibre bundles used at high NA can easily add a few hundred ps to the IRF widths. It is, of course, not necessary to use a detector that has an IRF width shorter than 30-50% of the pulse dispersion of the optical system. 

Instead of absorption, weak emission lines can also be detected with the intracavity techniques [26]. If this light is injected into specific modes of the multimode laser, the intensity of these modes will increase for observation times t mode-coupling with other modes. Cavity-enhanced spectroscopy in optical fibres have been reported in [32, 33]. 

Optical Fibres - Part 2-10 Product Specifications - Sectional Specification for Ca tegory A1 Multimode Fibres, International Electrotechnical Commission, 2002. 

Optical fibres of the single-mode type have a core diameter of some micrometers at maximiun, whereas multimode fibres can have a diameter of up to 1 mm (Fig. 8.4). The terms are a consequence of an optical theory which considers permitted field distributions along the fibres. 

Figure 8.4. Top single-mode optical fibre, bottom multimode fibre...

Pask, C. and Snyder, A. W., (1974) Illumination of multimode optical fibres-leaky ray analysis. Opto-electronics, 6, 297-304. 

Snyder, A. W., Mitchell, D. J. and Pask C. (1975) Pulse propagation in multimode optical fibres. Electron. Lett., 11, 275-6. 

Snyder, A. W. (1976) Pulse distortion in the ultimate multimode optical fibre. Appl. Opt., 15, 129CM. 

Advanced computerisation and sensorisation and developments in the field of multielement optical detectors (CCD and PDA) and fibre optic remote spectroscopy have added modularity and flexibility. Silica-silica fibres used for spectroscopy applications are multimode with core diameters from 50 to 1000 p,m. The application of new technologies to optical instrumentation (e.g. improved gratings in spectrographs, the use of... 

A cracial part of optical tomography instruments are the fibres or fibre bundles used to transmit the light to the sample and back to the detectors. The problem of the fibres is mainly pulse dispersion. The pulse dispersion in multimode fibres increases with the numerical aperture (NA) at which they are used. In particular, the detection fibre bundles, which have to be used at high NA, can introduce an amount of pulse dispersion larger than the transit time spread of the detectors [326, 443]. If the length of the bundles exceeds 1 or 2 meters, a tradeoff between time resolution and NA must often be made. 

Due to their high throughput eapability, multimode fibres are frequently used to transmit light in optical systems for TCSPC. Figure 7.22 shows how NA and pulse dispersion can be traded against fibre diameter. 

Figure 7 Use of fibre optics. L light source D detector S fibre splitter/combiner 1, 2 output ports. (A) single mode fibre (B) multimode fibre (cf. text).

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