Multimode fibres

This spread in velocity is called modal dispersion and is the principle limit to the use of multimode fibres for long-distance transmissive applications. 

Figure 2. Near field and far field of a multimode fibre. The number of speckle structures allows to roughly determine the number of modes.

If the core only corresponds to one speckle spot the fibre is monomode. Otherwise the waveguide is multimode. This back to the envelop calculation intuitively shows the origin of the mode number. Note that N is wavelength dependent the larger the wavelength, the lower the number of mode. 

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... 

From Figure 3 (distribution of the light power) it can also be seen that the largest part of the light in multimode structures is concentrated inside the fibre core and only a small part of it is in the region outside the fibre core,... 

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. 

Other intravascular-probe systems have been proposed by Abbott (Mountain View, CA)18 and by Optex Biomedical (Woodlands, TX)19, where the structure of the probe is essentially similar to the CDI one previously described i.e. three different multimode fibres, each of which is associated with the specific chemistry and charged with the detection of a single measurand. 

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). 

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. 

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). 

Fig. 7.21 Light propagation in a multimode fibre, a gradient-index fibre, and a single-mode fibre...

Multimode fibres consist of a core with a high index of refraction and a cladding with a lower index of refraction. The light is kept inside the fibre by total internal reflection. The fibres accept relatively large beam angles. Therefore multimode fibres can be used at an NA of up to 0.4. Multimode fibres made of glass and fused silica fibres eome in diameters up to 1.5 mm, plastic fibres up to 3 mm. To increase the cross section further, a large number of fibres can be combined into a fibre bundle. 

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. 

In all cases when multimode fibres or fibre bundles are used, eare must be taken that the effective NA of the light eones remains eonstant at the input and the output. The most objeetionable design is an iris diaphragm for intensity regulation in front of a fibre. However, the transmission of neutral-density filters, eolour... 

Fig. 7.23 Pulse dispersion in a multimode fibre of 1 mm diameter and 2 m length. The pulses of a 650 nm, 45 ps diode laser were sent through the fibre and detected by an R3809U MCP PMT. Left NA = 0.3, fwhm = 117 ps. Right NA < 0.1, fwhm = 54 ps...

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. 

The earliest fibres were called stepped index multimode fibres. These fibres have a large core region, allowing many modes to propagate (Figure 14.32a). The ray labelled H in Figure 14.32(a) is known as a high-order mode, whereas the ray L is a low-order mode. Stepped index multimode fibres are easy to make and join but have a lower performance compared with those described below. 

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]. 

FIGURE 9.24 Multimode Fibre Bandwidth Comparison. (Courtesy of Corning Cable Systems LLC and Corning Inc.)... 

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...

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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