fiber optics

Flexible transparent fiber devices, sometimes called lightguides, used for either image or information transmission, in which light is propagated by total internal reflection. In simplest form, the optical fiber or lightguide consists of a core of material with a refractive index higher than the surrounding cladding. The optical fiber properties and requirements for image transfer, in which information is continuously transmitted over relatively short distances, are quite different than those for information transmission, where typically digital encoding of information into on-off pulses of light (on = 1, off = 0) is used to transmit audio, video, or data over much longer distances at high bit rates. Another application for optical fibers is in sensors, where a change in light transmission properties is used to sense or detect a change in some property, such as temperature, pressure, or magnetic field.

There are three basic types of optical fibers. Propagation in these lightguides is most easily understood by ray optics, although the wave or modal description must be used for an exact description. In a multimode, stepped-refractive-index-profile fiber, the number of rays or modes of light which are guided, and thus the amount of light power coupled into the lightguide, is determined by the core size and the core-cladding refractive index difference. Such fibers, used for conventional image transfer, are limited to short distances for information transmission due to pulse broadening. An initially sharp pulse made up of many modes broadens as it travels long distances in the fiber, since high-angle modes have a longer distance to travel relative to the low-angle modes. This limits the bit rate and distance because it determines how closely input pulses can be spaced without overlap at the output end.

A graded-index multimode fiber, where the core refractive index varies across the core diameter, is used to minimize pulse broadening due to intermodal dispersion. Since light travels more slowly in the high-index region of the fiber relative to the low-index region, significant equalization of the transit time for the various modes can be achieved to reduce pulse broadening. This type of fiber is suitable for intermediate-distance, intermediate-bit-rate transmission systems. For both fiber types, light from a laser or light-emitting diode can be effectively coupled into the fiber

A single-mode fiber is designed with a core diameter and refractive index distribution such that only one fundamental mode is guided, thus eliminating intermodal pulse-broadening effects. Material and waveguide dispersion effects cause some pulse broadening, which increases with the spectral width of the light source. These fibers are best suited for use with a laser source in order to efficiently couple light into the small core of the lightguide and to enable information transmission over long distances at very high bit rates.

A special class of single-mode fibers comprises polarization-preserving fibers. In an ideal, perfectly circular single-mode fiber core, the polarization state of the propagating light is preserved, but in a real fiber various imperfections can cause birefringence; that is, the two orthogonally polarized modes of the fundamental mode travel at different speeds. For applications such as sensors, where controlling the polarization is important, polarization-maintaining fibers can be designed that deliberately introduce a polarization. This is typically accomplished by using noncircular cores (shape birefringence) or by introducing asymmetric stresses (stress-induced birefringence) on the core.

The attenuation or loss of light intensity is an important property of the lightguide since it limits the achievable transmission distance, and is caused by light absorption and scattering. Optical fibers based on silica glass have an intrinsic transmission window at near-infrared wavelengths with extremely low losses. Glass fibers, intrinsically brittle, are coated with a protective plastic to preserve their strength.

similarities between your eye and the camera

Both the human eye and a camera use something called a lens. In fact, they both use the same type of lens - a converging lens. Converging lenses are like the ones in magnifying glasses - they work to make an image look bigger. This is why you can see the details in something even if it’s on the other side of the room. One thing about lenses is that they can only focus on things that are the same distance away from them. This is why your eyes can focus on things that are close to you or far from you; just not at the same time. Cameras are the same way - you can only focus them on things that are the same distance away. In the camera, the lens focuses the light onto a piece of film. The film has chemicals in it that basically trap the image on it, making it permanent. Instead of film, your eye uses something called a ’retina.’ The retina has lots of little tiny things called ’rods’ and ’cones’ all over it. These are basically tiny antennae that tell your brain about the light that hits them. The rods tell your brain if there’s light in a certain spot or not (a bit like a black and white photo) and the cones tell your brain what color the light is. There is one spot on the retina, though, that has no antennae at all. This is the spot where the nerve leaves your eye to go to your brain. At this spot, you can’t see anything at all - it’s called your ’blind spot.’ This is one of the reasons that you have two eyes; what you can’t see with one eye you can see with the other. Unlike your eyes, cameras have no blind spot, so they only need to have one lens. Another important thing about seeing light is that you have to be able to control how much light gets in... otherwise, you couldn’t see things in a brightly-lit room because you’d be overwhelmed by how much light there is. Your eye controls how much light gets in by changing the size of the pupil - the dark spot in the center of your eye. The more light there is, the smaller your pupil becomes, and the less light gets in. Many (but not all) cameras also can adjust to let different amounts of light in. This way, your outdoor pictures don’t look washed out and your indoor pictures don’t look too dark.