User’s Guide to Fiber Optic Video Transmission – Snell’s Law– Part 2

Snell’s Law

Early fiber optics exhibited high loss that limited transmission distances. To correct this, glass fibers were developed that included a separate glass coating. The innermost region of the fiber, the core, carried the light, while the glass coating or cladding prevented the light from leaking out of the core by refracting the light back into the inner boundaries of the core. Snell’s Law explained this concept. It states that the angle at which a light reflects as it passes from one material to another depends on the refractive indices of the two materials.

FIGURE 2 Light wave refraction principles. The refraction index of the core, n1, is always less than that of the cladding, n2. Light incident on the boundary at less than the critical angle, ϕ1, propagates through the boundary, but is refracted away from the normal to the boundary (a) at the critical angle, ϕC, along the boundary (b). Light incident on the boundary at angles ϕ1 above the critical angle is totally internally reflected (c). (Adapted from Force, Inc., illustration used with permission.)

In the case of fiber optics, this is the refractive index between the core and the cladding. Figure 2 illustrates the equations for Snell’s Law. In this figure, the upper region of the frame, n1, indicates a higher refractive index than the lower region n2. The refractive index of the upper region is designated as n1 while the lower region refractive index is n2. The figure on the top shows the case with the angle of the indices less than the critical angle. Note that the angle of the light changes at the interface between the higher refractive index, in region 1, and the lower refractive index, in region 2. In the center figure, the angle of indices has increased to the critical angle. At this point all the refracted light rays travel parallel to the interface region. In the figure on the bottom, the angle of indices has increased to a value greater than the critical angle. In this case 100% of the light refracts at the interface region.

Advancements in laser technology next elevated the fiber-optics industry. Only the light-emitting diode or its higher powered counterpart, the laser diode, had the potential to generate large amounts of light in a focused beam small enough to be useful for fiber optic transport.

Communications engineers quickly noticed the importance of lasers and their higher modulation frequency capabilities. Light has the capacity to carry 10,000 times more information than radio frequencies. Because environmental conditions, such as rain, snow, and fog, disrupt laser light, a transmission scheme other than free space was needed. In 1966, Charles
Kao and Charles Hockham, working at the standard Telecommunications Laboratory, presented optical fibers as an ideal transmission medium, assuming fiber optic attenuation could be kept under 20 dB per kilometer. Optical fibers of the day exhibited losses of 1,000 dB/km or more. At a loss of 20 dB/km, 99% of the light would be lost over only 1000 meters (3300 ft).

Scientists theorized that the high levels of loss were due to impurities in the glass and not the glass itself. At the time in 1970, an optical loss of 20 dB/km was within the capabilities of electronics and opto- electronic components for short distances (less than 1 km) but not for longer distances (greater than 1 km). Dr. Robert Maurer, Donald Keck, and Peter Schultz of Corning succeeded in developing a glass fiber that exhibited attenuation at less than 20 dB/km, the limit for making fiber optics a usable technology. Other advances of the day, such as semiconductor chips, optical detectors, and optical connectors, initiated the true beginnings of the fiber-optic communications industry.

Click for Part 3 on Optical Windows and Spectrum

About Jim Jachetta

Jim Jachetta, EVP & CTO of VidOvation Corporation, has over 25 years of experience in designing, integrating and delivering video audio and data communications systems over wireless, IP networks and fiber optics. Jim drives VidOvation to create solutions using world class technology to make the “impossible” and “never been done before” a cost effective, everyday solution aligned with your business goals – modern technology, creatively implemented to meet your business needs, easy to support, and delivered at a price point that leads the industry.

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4 Responses to User’s Guide to Fiber Optic Video Transmission – Snell’s Law– Part 2

  1. Pingback: Users Guide to Fiber Optic Video Transmission – Introduction to Fiber Optics – Part 1 | VidOvation Blog

  2. I am making these comments to give a bit more explanation regarding fibre
    Most fibres are now graded index(as opposed to step index) the core has a refractive index which decreases with increase radial distance from the optical axis of the core .In this type of fibre the paths are not reflected, but are bent
    With step index fibre rays which are reflected down it have a longer path length than rays straight down the centre resulting in lower bandwidth .With graded index the bent rays travel at a higher speed when not in the centre .The speed of light is the speed in free space c divided by the refractive index about 1.5 for glass .Rays which travel longer will be faster and arrive at the far end at approximately the same time resulting in a higher bandwidth.
    Diamond has the highest refracted index of all which is why it sparkles so much I wish you could dope glass to do this it would save us men a fortune.
    For higher bandwidth the core is made much smaller from 62.5 Multi Mode (MM) to 9 microns Single Mode (SM)
    A problem with fibre in broadcast systems is the coding used for SDI Ethernet uses an 8b10b code which has a penalty as it results in a 25% increase in bit rate, but it is dc balanced and easy to decode, scrambling is only used on twisted pair copper for EMC considerations.
    Broadcast scrambling for video result in pathological patterns with long periods without transitions Ethernet SFPs (they convert electrical to light) will not pass these so special types have been developed, but they cost about 8 times as much (internally there is very little difference), nor can it be passed by magnetics.
    I would hope that for the higher data rate systems proposed we could standardize on a code system such as 8b10b to use the high data rate ethernet SFPs now available . The SFP is the major cost in media converters
    Many links can be sent down 1 fibre using CWDM (different wave lengths),I shall be putting a discussion entitled ” to mux or not to mux that is the question “of this on our website ,were you will find Volamps wide range of fibre products including camera back fibre systems.
    I have been involved with fibre optic for over 35 years .Volamp is still making the defacto standard RS232 line driver from that period

  3. Pingback: User’s Guide to Fiber Optic Video Transmission by Jim Jachetta | VidOvation Blog

  4. Pingback: User's Guide to Fiber Optic Video Transmission by Jim Jachetta - VidOvation BlogVidOvation Blog

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