Optical multiplexer

The use of hber optics and hber-optic multiplexing can increase the number of analysis points, and hence can reduce the overall costs related to a single analyzer. This approach has been used successfully with NIR instrumentation, where typically up to eight points can be handled. As noted earlier, the use of hber optics with IR Fourier transform instruments has in the past been limited. New hber materials with improved optical throughput are available, and also with the considered use of IR lasers, the role of hbers for IR applications is expected to increase. Although in the past commercial multiplexers have been available for mid-lR hber systems, their use has not been widespread. 

The Raman technique has been readily adapted for on-line process analysis, especially in the pharmaceutical industry ". It has the benefits of mid IR, e.g. the ability to identify compounds from the vibrational fundamentals, without the constraints of mid IR, e.g. the limitations of the optical materials that can be used. Its popularity is also due in part to the excellent throughput of optical fibres for the radiation required for Raman, i.e. in the Vis and NIR regions. This use of optical fibre probes (Figure 9.14) facilitates easy in-line analysis because the sample can be remote from the instrumentation, even to hundreds of metres in distance. Fibre optic multiplexers are also available, allowing many samples to be analysed sequentially. Small laser diode sources and CCD detectors can be attached to the optical fibres and changed as required, rendering the overall device small and flexible. Radiation from the laser diode light source is transmitted to the sample by optical fibre... 

These are also known as multi-channel spectroscopic methods and are based on a peculiar multiplexing mode denominated optical multiplexing or wave-length-division multiplexing . The detection system commonly used involves several Independent detectors strategically located so that the dispersed radiation strikes each of them discriminately. The optical dispersion can be accomplished with the aid of a diffraction grating or prism, or both (6chelle dispersion ). 

The advantages of these conHgurations are the electrical insulation, no electromagnetic interference small cells, multi-point measurement for all architectures, no sample transfer, little holdup and real-time information in the third architecture. The main drawbacks are associated with the lack of familiarity with the use of optical flbers in a sometimes unsuitable plant configuration, and with the youth of a technology that is still not fully under control with optical multiplexing and instrumentation coimections and costly (lacking a mass public ). 

New spectrophotometers are designed around optical fibers with wide spectral ranges (0.2-1 or 3 pm) and modular. The spectrometer from Guided Wave is already used in process control (batch). However, its design with spectral scanning and a photomultiplier makes it more similar to an analytical instrument than to a unit for plant process control. It can be associated with twelve optically multiplexed channels. The DTC 1000 spectrophotometer [34] called Spectrofip (Photonetics Society) was recently developed for remote control of nuclear materials (0.4-0.95 pm) with a resolution of 0.6 nm. It is of the video spectrometer type with photodiode arrays (1728). A spectrum is obtained in 10 ms and the minimum time between each measurement is about 0.5 s. It is an ideal device for... 

More than two decades back, EW was identified as a possible mechanism for switching light in fiber-optic applications, in which Hg droplets can be moved inside channels either to provide a reflective surface or to allow for the transmission of light in an optical multiplexer. Subsequent applications of EW in fiber optics have been... 

Machtle and Klodwig [209] have reported on improvements in instrumentation in the analytical centrifuge technique, including an eight-cell interference optics multiplexer and an eight-cell Schlieren optics multiplexer, both based on the same light source, a modulable laser, and a titanium rotor with a maximum speed of 60,000 rpm. 

One of the important features of fiber optic technology is that optical multiplexers are available to connect many sensors to the same spectrophotometer or fluorimeter. 

Sasayama, T., Taketani, N., and Asano, H., Optical multiplexed transmission system using high temperature polymer fiber, Int. Congress Expo., SAE Tech. Paper Series 890200/1-7, Feb. 27-Mar. 3, 1989. 

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