IR waveguides

Hollow waveguides have emerged as a very attractive alternative to solid-core IR waveguides because of the inherent advantage of their air core. However, only a few applications in medical and industrial fields have been reported. 

W. Hartig, W. Schmidt A broadly tunable IR waveguide Raman laser pumped by a dye laser. Appl. Phys. 18, 235 (1979)... 

PT, PZT, PLZT nonvolatile memory, ir, pyroelectric detectors, electro—optic waveguide, and spatial light modulators sol—gel, sputtering... 

The construction of the optoelectronic interface can be based on a silicon photodiode since analytical and reference wavelengths are from the visible and the IR regions, respectively. The signals can be filtered out by optical filters (then two photodiodes are required) or one photodiode can be synchronised with modulation waves of the LEDs used. Finally, silica optical fibres can be used as light waveguides. The choice between single fibre or bundle is determined by the application of the sensor. 

Recent advances in instrumentation range from novel (laser) sources and highly compact spectrometers over waveguide technology to sensitive detectors and detector arrays. This, in combination with the progress in electronics, computer technology and chemometrics, makes it possible to realise compact, robust vibrational spectroscopic sensor devices that are capable of reliable real-world operation. A point that also has to be taken into account, at least when aiming at commercialisation, is the price. Vibrational spectroscopic systems are usually more expensive than most other transducers. Hence, it depends very much on the application whether it makes sense to implement IR or Raman sensors or if less powerful but cheaper alternatives could be used. 

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. 

In recent years, the evolution of the technological components required for IR sensor systems has been denoted by a significant miniaturisation of light sources, optics and detectors. Essentially, an IR sensor consists of (i) a polychromatic or monochromatic radiation source, (ii) a sensor head and (iii) a spectral analyser with a detector. As sensors where all optical elements can be included in the sensor head are the exception rather than the rule, also various optics, waveguides and filters may form essential parts of IR-optical chemical sensors. Another important building block, in particular when aiming at sensors capable of detecting trace levels, are modifications of the sensor element itself. 

Consequently, mirror optics are more common, in particular in the mid-IR. The mirrors used are usually aluminium- or gold-coated flat or curved substrates. While near-IR mirrors are usually protected by thin SiO-layers, in the mid-IR unprotected mirrors have to be used. Disadvantages of mirror optics are the elevated space consumption and the higher prices in comparison to refractive optics, especially comparing non-standard mirrors against non-standard lens. In total, mirror optics are so preferable to fibres and refractive optics, at least in the mid-IR, that in some technical applications they are used to replace waveguides to transport IR radiation between source, sensor head and spectrometer. 

The prevalently used waveguides are optical fibres. Fibre technology is standard in the UV to near-IR, but also some fibres for light transport in the mid-IR have been developed. An overview of different IR fibre materials and their characteristic performance parameters is given in Table 1. More details can be found in a number of reviews focused on the material properties of IR transmitting optical fibres26 31. For some applications, as an alternative to optical fibres also hollow waveguides may be used. 

Alaluf M., Dror J. and Croitoru N., Plastic hollow waveguides as transmitters and filters in mid-IR radiation, Proc. SPIE, 1991 1591 146-149. 

Nonconventional fluorimeters and sensors that incorporate optical waveguide coupling possess less optical attenuation in the near-IR as compared with the UV/vis-ible because of the reduction in Rayleigh scattering. The temporal dispersion is also reduced in the near-IR for the same reason, e.g., 80 psec/nm/km at 900 nm as compared with 1 nsec/nm/km at 400 nm for a single-mode fiber. 

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