Page 353 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

FIGURE 13.17
13.2 Transmission 345
Clear water
Clear water
e
Below critical angle
Above critical angle
Light source
Light sourc
Basic function of Colladon and Babinet’s “light fountain.”
The waveguide, or “light pipe,” principle can easily be demonstrated with a common bucket, some
clear liquid, and a flashlight (a laser flashlight works best). Fill the bucket with 10
15 L of water, place
a puncture hole at 0.5
1.0 cm in the bucket just above the bottom of the bucket (on its side), and shine
the laser horizontally through the hole (if the bucket is opaque, mount a transparent window opposite the
hole so that the light can shine down the outflow stream) as the water flows out of the bucket. The
refractive index (the scientific notation for refractive index through a medium (e.g., air, water, and glass)
is n) of water is 1.33, making the critical angle (θC) for the water
air interface as (arcsin of 1/1.33 in 
degrees) 49 . With the bucket full, the water pressure keeps the stream coming out of the bucket at an
angle below 49 from the horizontal. However, as the water level decreases due to decreased water pres-
sure, the angle of the water streaming out of the bucket will slowly increase until the “critical angle” of
49 when the light can no longer follow the water’s path. At that point, the light will escape the water
stream (Figure 13.18(a)). The same principle works for optical glass fiber (Figure 13.18(b)), only the 
critical angle changes to 41 .
The refractive index of transparent materials varies based upon the wavelength/frequency of the
light passing through the medium. A prime example of this is white light passing through a prism (Figure 13.19) whereby the light is dispersed into its various component colors.
Some representative refractive indexes (n) and critical angles (θc) through various materials at a wavelength of 589 nm are provided in Table 13.1.
To compute the critical angle between two transparent media (e.g., air/water, water/glass, glass/ glass (of different optical properties), etc.) simply compute the arcsine between the two refractive indexes and convert into degrees.
Optical fiber data transmission was developed by physicists as a superfast method of data trans- mission over long distances. The bandwidth of fiber-optic cabling can be measured in the terahertz (THz) range! Fiber has such huge frequency response that the entire RF spectrum can be conducted over one strand approximately the diameter of a human hair. The data rates achieved are such that one serial bit stream can be transmitted at 10 Gbps (gigabits per second) or through wave division multiplexing (WDM) at greater than 100 Gbps. Further, a single fiber can transmit more than a whole bundle of copper.
With optical fiber, light waves are conducted through extremely high-purity glass or plastic (due to its internal refractive properties), allowing the light to be channeled to its destination.
Light refracts
Light escapes