Fiber Exam Training

Well, since I like to write and I also need to study, I decided to couple both into one. I'm reviewing the notes for my BICSI fiber exam tomorrow. If you have even the least bit of a life, you'll skip this whole post! If not, you're going to be bored to death!

 

Fiber Optics Transmission Theory

Primary Elements of Optical Fiber: These are definitely on the test. (believe it or not, people miss this one)

1. Core (containts dopants to regulate the speed of light)
2. Cladding (outer layer of glass to contain light, maintains a different refractive index, to make the light reflect back down the pathway)
3. Coating (cushions and protects fiber, also known as the primary buffer)

Core Elements (What makes up the difference between the fibers)

Single Mode:

• 8.2 mµ core
• 125 mµ Cladding
• One mode of light, resulting in higher bandwidth.
• Electronics are more expensive, raising cost of the overall system.
• Must use lasers to pass light due to the tiny numerical aperature.
• Has less loss over greater distances.
• Harder to splice and terminate (smaller cores)
• Does not suffer from modal dispersion (although it does suffer from chromatic dispersion)
• Wavelengths: 1310nm and 1550nm

Multi Mode:

• 50/62.5 mµ core
• 125 mµ cladding
• Multiple modes of light, resulting in less bandwitch per mode.
• Cheaper electronics, means cheaper system costs.
• Can use LED's instead of lasers, due to the larger numerical aperature.
• Has more loss over distance, so is used for shorter distance applications.
• Has more attenuation (loss)
• Has Greater Dispersion (pulse spreading)
• Can only go short distances.
• Easier to Splice, easier to terminate. (larger cores)
• Suffers from Modal and Chromatic dispersion.
• Wavelengths: 850nm and 1300 nm

Mechanical Strength Properties of Fiber Cable

• Fiber undergoes testing at the manufacturer during the drawing process, called Proof Testing.
• Theoretically ALL fiber is fairly strong, possessing 600-750 kpsi of tensile strength (200 lateral pounds).
• ALl fiber has small inherent cracks in the cladding, and any stress directly applied to the fiber is always concentrated to the largest of the crack flaws.
• Continuous load will "stress" the flaws, making the fiber weaken and eventually snap, while dynamic stress is less harmful and bears no long-lasting effects as long as it remains within limits.
• Bending the fiber places it under strain.
• Recommended long term minimum bending radius for static installation is 1.5 inches.
• 100 kpsi proof fiber has a maximum 0.86mµ crack depth.

Waveguiding Fiber (controlling the flow of lightwaves down the path of glass at a given frequency)

• Waveguiding is done through the use of the cladding. The cladding has a different refractive index than the interior Core strand, making the light reflect back down at a different angle. Without the cladding,the light would continue straight up at an angle of no refraction and pass through the glass, leaking out, therefore not bending back down the pathway. This means: no cladding: no refraction. This process of guiding the light through cladding is known as Total Internal Reflection (THAT IS ALWAYS A TEST TERM)
• Total Internal Reflection is achieved by:
• Keeping the light signal inside the critical angle of reflection.
• The Critical Angle is a function of the "index of refraction" between the core and the cladding.
• Total Internal reflection basically means "the process of keeping ALL the light within the glass core, not letting it leak out."

Index of Refraction:

• The index of refraction is represented as "n".
• n is a ration comparing the speed of light (c) in a vaccuum to the speed of light in a medium. In fiber optics, the medium is silica glass.
• The index of refraction of silica glass is 1.45 (meaning light travels through the glass 45% slower than it does through space.)

Refractive Index (Optical Density Principles)

• Simple. Basically, the higher the number of the index of refraction, the slower light travels through the substance.
• This is in accordance to Snell's Law.
• Common Indices of refraction are:
• Water: 1.33
• Quartz Crystal: 1.54
• Glycerin: 1.47
• Diamond: 2.42
• Basic Premises of Snell's Law:Every material that light can travel through has an absolute index of refraction. This value is usually simply termed "the index of refraction" for that substance.
• The index of refraction for a substance is equal to the speed of light in free space divided by the speed of light in that substance.
• The index of refraction for a substance is also equal to the sine of the angle of incidence, in a vacuum(300,000,000 meters/second), divided by the sine of the angle of refraction, in the substance.
• For all practical purposes, in introductory physics problems like these, one can consider light to move through air much like it moves through free space. However, it actually travels a bit slower in air.

Critical Angle

Light will bend as it travels from one index of refraction to another with a different n. The bend increases as the angle of entry (numerical aperature) decreases. Eventually, the angle of light is small enough to prevent light from entering another n. This is the critical angle.

The numerical aperature is the angle of approach of light coming into the fiber, represented in a degree format.

 

INDEXING light within fiber:

There are two kinds of indexing for fiber-optic cables currently used today. Those are Stepped Index and Graded Intex. Stepped index is only used in singlemode fiber, and some rare uses of MM fiber such as home audio mm fiber, requiring communications in the ultra high wavelengths.

For MultiMode (MM) purposes, a Graded index is used to help turn the sharp angles of refraction into more wave-like shapes (smooth curves). This is achieved by applying cladding to the core of the fiber using multiple layers, each with a slightly higher index of refraction as they increase in distance from the axis of the core. Utilizing this technique bends the light in a smoother wave form (sinusodial) and increases the speed at which the light travels through the fiber.

 

Modes of Propogation

Singlemode: Possesses one single mode (wave) of light for increased speed, better for longer distances.

MultiMode: possesses many waves of light, due to the wider numerical aperature. The large core(50/125 or 62.5/125) means light can travel on more wavelengths at once through the fiber.

Since SM fiber has only one mode, there is no pulse spreading, meaning more bandwidth. Conversely, MM fiber's larger core allows more pathways of light to enter the fiber, which in turn creates pulse spreading (dispersion) that limits bandwitch. The conversion of the data on the signal end is translated by a receiver on the end node, which is looking for a specific binary format.  (For example 1, 0, 1, 0, 1,0, etc....) Multimode fiber possesses more than one wave of light, meaning the modes might get overlapped (dispersed) resulting in the receiving end signal looking more like 1, 1, 1,0,0, 1, 0, 1, etc. Compensating for this modal dispersion decreases bandwidth availability.

Multimode Sizes: (50 vs 62.5)

• 50 mµ has greater bandwidth due to smaller size (less modes of light can enter the pathway = less modal dispersion)
• Most US networks utilize 62.5 currently, but are going back to 50 for newer applications.
• Splice connecting 50 mµ to 62.5 mµ is useless because core alignment issues reduce the bandwidth gains you would otherwise achieve.

Attenuation: Definition (definitely on the test)

• Attenuation is the decrease in optical power, measured in decibels (dB).
• Limits the distance a signal travels.
• Some attenuation is inherent in glass. (intrisnic annenuation)
• Some attenuation is caused by splicing, terminating, and environmental issues (extrinsic attenuation)
• (see: http://zone.ni.com/devzone/cda/ph/p/id/88)
• dB Loss Examples:
• .3 dB loss = 93% power throughput
• .4 dB loss = 91% power throughput
• 3.0 dB loss = 50% power throughput
• Two Kinds of Attenuation:

Inrinsic Attenuation

Intrinsic attenuation occurs due to something inside or inherent to the fiber. It is caused by impurities in the glass during the manufacturing process. As precise as manufacturing is, there is no way to eliminate all impurities, though technological advances have caused attenuation to decrease dramatically since the first optical fiber in 1970.

When a light signal hits an impurity in the fiber, one of two things will occur: it will scatter or it will be absorbed.

• Scattering
  
• Rayleigh scattering accounts for the majority (about 96%) of attenuation in optical fiber. Light travels in the core and interacts with the atoms in the glass. The light waves elastically collide with the atoms, and light is scattered as a result.
• Rayleigh scattering is the result of these elastic collisions between the light wave and the atoms in the fiber. If the scattered light maintains an angle that supports forward travel within the core, no attenuation occurs. If the light is scattered at an angle that does not support continued forward travel, the light is diverted out of the core and attenuation occurs.
• Some scattered light is reflected back toward the light source (input end). This is a property that is used in an Optical Time Domain Reflectometer (OTDR) to test fibers. This same principle applies to analyzing loss associated with localized events in the fiber, such as splices.
• Absorption
• 
  The second type of intrinsic attenuation in fiber is absorption. Absorption accounts for 3-5% of fiber attenuation. This phenomenon causes a light signal to be absorbed by natural impurities in the glass, and converted to vibrational energy or some other form of energy.
• Unlike scattering, absorption can be limited by controlling the amount of impurities during the manufacturing process.

Extrinsic Attenuation

 

The second category of attenuation is extrinsic attenuation. Extrinsic attenuation can be caused by two external mechanisms: macrobending or microbending. Both cause a reduction of optical power.

Macrobending
If a bend is imposed on an optical fiber, strain is placed on the fiber along the region that is bent. The bending strain will affect the refractive index and the critical angle of the light ray in that specific area. As a result, light traveling in the core can refract out, and loss occurs. (Figure 13)

A macrobend is a large-scale bend that is visible; for example, a fiber wrapped around a person's finger. This loss is generally reversible once bends are corrected.

To prevent macrobends, all optical fiber (and optical fiber cable) has a minimum bend radius specification that should not be exceeded. This is a restriction on how much bend a fiber can withstand before experiencing problems in optical performance or mechanical reliability. The rule of thumb for minimum bend radius is 1 1/2" for bare, single-mode fiber; 10 times the cable's outside diameter (O.D.) for non-armored cable; and 15 times the cable's O.D. for armored cable.

 

Microbending (pinching)
The second extrinsic cause of attenuation is a microbend. This is a small-scale distortion, generally indicative of pressure on the fiber. (See Figure 14 below.) Microbending may be related to temperature, tensile stress, or crushing force. Like macrobending, microbending will cause a reduction of optical power in the glass.

Microbending is very localized, and the bend may not be clearly visible upon inspection. With bare fiber, microbending may be reversible; in the cabling process, it may not.

 

Dispersion (also on the test)

• There are two types of dispersion. Modal and Chromatic.
• Total Dispersion  = Modal Disp. + Chromatic Disp.
• Chromatic Dispersion = Material Dispersion + Waveguide Dispersion

Modal Dispersion:

• The spreading of light pulse due to differences in the distance traveled within the core of the fiber. (Smaller waves get the to the receiver faster) These differences exist because of the many pathways of light available within the fiber.
• It can be reduced by changing the speed at which light passes through the core (utilizing a graded index).
• Dominates MM fiber.

Chromatic Dispersion: (on test)

• This is the cumulative effect of material distpersion and waveguide dispersion.
• Dominates SM fiber only.
• Standard SM fiber has a zero dispersion wavelength of 1310 nm.
• Varies with the transmitting wavelength.
• definition of chromatic disp; The spreading of the light pulse due to differences in speed in which various wavelengths of light travel through the core.
• (Remember Chromatic = COLOR and different colors travel at different speeds)

Waveguide Dispersion

• Based on the same principle as above. Different wavelengths travel at different spees. However, different wavelengths also travel at different speeds due to the differences in the index of refraction between the cladding and the core.

 

Fiber Manufacturing 1-2-3

Memorize the three step process for fx manufacturing:

1. Layout - the process by which gases are deposited as a wet "soot" upon a quartz rod by flame hydrolysis. This will ultimately create the preform for the glass core and cladding.
• These gases will be either:
• Silica Tetrachloride (SiCl4)
• Germanium Tetrachloride (GeCl4)
• And may include other chemicals (dotants) to alter the index of refraction of the glass.
2. Consolidation- the process where the bait rod is removed and the emaining silica is then heated at high temperatures to drive out the impurities and water, leaving only pure glass. This process is called "sintering."
3. Draw - The consolidated preform is loaded into a very high temperature furnace (>2000C deg. Celcius) where it becomes soft and is "drawn" down to the diameter of the cladding. During this process, the width of the beginning preform is 24 inches, then shrunk to about 8 inches, and then drawn down from 8 inches to 125 microns. (Friggin' amazing technology).

Cable Design

• LOOSE TUBE - Outdoor Cable: -40˚C to +70˚C
• TIGHT BUFFER - Indoor Cable: -20˚C to +70˚C

(Indoor cable doesn't need the loose tube design because it isn't going to be subjected to temperature extremes. Vast changes in exterior temperature require that the Loose Tube fiber allow for the fiber and other components to expand and contract during different environments, which is why they are "loosely" stacked inside the tubing.)

Loose Tube Cable: (250 mµ fibers floating in gel, decoupled from cabling stresses.) (outside to inside shown below)

• Polyethylene Jacketing
• Polyester Tape
• Rip Cord
• Aramid Yarns
• Waterblocking System
• Gel Filled Loose Tubes
• Optical Fibers
• FRP Central Member

Loose Tube Armored

• Outer Polyethlene Jacket
• Waterblocking Tape
• Coated Corrugated Steel Armoring
• Inner Polyethylene Jacket
• Polyester Tape
• Ripcords
• Amamid Yarns
• Waterblocking System
• Gel Filled Loose Tubes
• Optical Fibers
• Dielectric Central Member

UniFlex  (Small diameter, up to 24 cores maximum)

• Polyethlene Outer Sheath
• Aramid Yarns
• Loose Tube Buffer
• Optical Fibers
• Waterblocking Gel

ADSS: All Dielectric Self Supporting - Tensile Strength of 60,000 lbs.

• Polyethlene Outer Jacket
• Rip Cords
• Core Wrap
• Inner Polyethlene Jacket
• Inner Rip Cords
• Aramid Yarns
• Inner Core Wrap
• Waterblocking System
• Gel Filled Loose Tubes
• Optical Fibers
• FRP Central Member

Optical Ground Wire

• Replaces the grounding wire on electrical lines, but contains fiber cores.

Design Characteristics

Loose Tube

• PE Jacketed
• UV Resistant
• Environmentally Stable
• Dense Material - Low Drag - Abrasion Resistant
• Contains Dry Water-Swellable Tapes
• (All polyethelene contains a 2% carbon black component to resist UV brittleness and degradation.)
• Elongation and Contraction - Cores are 1.5% -2% longer than the sheathing materials.

Tight Buffer

• PVC Jacketed - Flame Resistant to NEC 770 Specifications
• Meets NEC codes for riser & plenum ratings.
• Dry cable - no gel to clean (uses powder instead)
• Buffer Coating adheres directly to the 250 mµ fiber, which:
• Increases Microbending problems
• Degades optical performance
• Is more susceptible to temperature variances

 

Ok.. giving up.. going to bed.