Fiber optics are essentially light pipes, and their basic operation may be traced back more than a century when British Physicist John Tyndall demonstrated that light could be carried within a stream of water spouting out and curving downward from a tank. A thin glass rod for optical transmission was the basis of a 1934 patent awarded to Bell Labs for a "Light Pipe." American Optical demonstrated light transmission through short lengths of flexible glass fibers in the 1950s. However, most modern advances in fiber optics grew out of Corning Glass developments in glass technology and production methods disclosed in the early 1970s.
Commercial telecommunications has evolved as the fiber optics technology driving force since the mid-'80s. Increased use of fiber optics well correlates with fiber materials developments and lower component costs. Advances in glass fibers have led to transmission improvements amounting to over three orders of magnitude since the early Corning Glass efforts. For example, ordinary plate glass has a visible light attenuation coefficient of several thousand dBs per km. Current fiber optic glasses a kilometer thick would transmit as much light as say a 1/4" plate glass pane. Table 5-1 indicates relative digital data transmission losses for copper and fiber.
Fiber Advantages
Whether used for communications or infrared temperature measurement, fiber optics offer some inherent advantages for measurements in industrial and/or harsh environments:
Like many technical developments since WWII, fiber optics programs were largely government funded for their potential military advantages. Projects primarily supported telecommunications applications and laser fiber ring gyroscopes for aircraft/missile navigation. Some sensor developments were included in manufacturing technology (Mantech) programs as well as for aircraft, missile and shipboard robust sensor developments. More recently the Dept. of Energy and NIST have also supported various fiber optic developments.
Relative Transmission Losses for Digital Data
Losses in dB/km
1.5Mb/s
6.3Mb/s
45Mb/s
26 gage twisted wire pair
24
48
128
19 gage twisted wire pair
10.8
21
56
RG 217/u coaxial cable
2.1
4.5
11
Optical fiber 0.82 um wavelength carrier
3.5
3.5
3.5
Improved glass transmissions have resulted in undersea cables with repeaters required about every 40 miles--ten times the distance required by copper. Bandwidth and robustness have led to cable service providers selecting fiber optics as the backbone media for regional multimedia consumer services. The world market for fiber optic components was in the USD$4 billion range in 1994 and is projected to reach USD$8 billion in 1998.
Figure 5-1: Fiber Optic
Probe Construction
Unaffected by electromagnetic interference (EMI) from large motors, transformers, welders and the like;
Unaffected by radio frequency interference (RFI) from wireless communications and lightning activity;
Can be positioned in hard-to-reach or view places;
Can be focused to measure small or precise locations;
Does not or will not carry electrical current (ideal for explosive hazard locations);
Fiber cables can be run in existing conduit, cable trays or be strapped onto beams, pipes or conduit (easily installed for expansions or retrofits); and,
Certain cables can handle ambient temperatures to over 300°C--higher with air or water purging.
Any sensing via fiber optic links requires that the variable cause a modulation of some type to an optical signal--either to a signal produced by the variable or to a signal originating in the sensing device. Basically, the modulation takes the form of changes in radiation intensity, phase, wavelength or polarization. For temperature measurements, intensity modulation is by far the most prevalent method used.
The group of sensors known as fiber optic thermometers generally refer to those devices measuring higher temperatures wherein blackbody radiation physics are utilized. Lower temperature targets--say from -100°C to 400°C--can be measured by activating various sensing materials such as phosphors, semiconductors or liquid crystals with fiber optic links offering the environmental and remoteness advantages listed previously.