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Fiber Optic Communications

Don Koehler (KL7KN) on May 1, 2001
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(The article below is the first in a series of articles on several subjects that Don has graciously agreeed to share with the eHam community. Please see Don's bio at the end of this article. -ed.)

FUNDAMENTALS OF FIBER OPTIC COMMUNICATIONS 2001 DONALD E. KOEHLER - ALL RIGHTS RESERVED

HAMS (in my experience) have the reputation of being curious about things technical. Here is an article about fiber optics. I wrote this piece with a view to those amongst us that "do" radios for a living. Part 2 follows shortly.

Many radio providers are faced with increasingly congested sites, higher levels of RFI/EMI and co-channel interference. If this weren't bad enough, many large building owners are looking for 'seamless' internal radio communications for services such as SMR, PCS and cellular. Bringing the outside antenna and inside service user together requires a wide range of technologies, with fiber optic based packages finding increasing favor. This article, part 1 of 2 parts, will examine fiber optic basics - including how fiber 'works'; common types of fibers, connectors and transmitter/receivers; safety and testing considerations; ending with some URLs pointing to Internet sites for further study. Part 2 will focus on some specific applications and equipment.

Fiber optic cable is the application of the physics of light in a medium, in this case a coaxial fiber made of transparent material. How is this cable able to carry light energy great distances with little, if any loss? Allow me to pose this question - have you ever been outside on a sunny day and looked into a pool of water? Do you recall that part of the floor of the pool was visible and the light reflecting from the surface obscured the remainder of the floor? This is because light striking the pool surface had changed media - from air to water. A more technically correct explanation would be the density of the media in which the light is traveling changed.

The light striking the water at or below the critical angle was reflected to your eyes, light striking at or above the critical angle was refracted to impinge on the pool floor. As you move around the pool the amount of floor visible to you changes as the angle of the light changes relative to your position. What in the world does this have to do with fiber optic cable you ask? Everything. Fiber optic cable works on the principal of reflections of light caused by changes in media density.

Commercial, long haul fiber optic cable is made of ultra pure glass. During manufacture, this glass is pulled into a hair-thin core strand, then clad with an additional layer of glass - the outside layer or cladding having a different density or refractive index. See fig 1, the inner core of the glass fiber cable carries the photonic energy, the cladding provides the difference in density allowing the light to reflect further down the cable. Light must strike at an angle within a defined Numerical Aperture (NA) sometimes called a "cone of acceptance" to propagate to the distant end. For all practical purposes this has been engineered into the cable, cable connector, transmitter or receiving device used. See the sidebar for the math used to calculate the Numerical Aperture and illustrations.

Fiber optic cable comes in several 'sizes' and types. The most efficient cable is "single mode" fiber. The core of single mode fiber is approximately 5 to 8 m in diameter and the cladding is 125 m in diameter. This yields excellent characteristics: a small NA, wide bandwidth and low attenuation. Fed by a LASER device, this cable typically operates at a wavelength of 1300 or 1550 nanometers (nm). The cable covering is usually yellow in color, a holdover from when the Bell system was the primary user of this type of cable.

Multimode fiber has a core diameter of 50 to 62.5 m and a cladding diameter of 125 m. This yields a larger NA, narrower bandwidth and higher losses within the cable. Fed by LED devices, this cable typically operates at a wavelength of 850 or 1300 nm. It is usually orange in color. Finally, the lowest grade of fiber has a core diameter of 62.5 to 100 m and a cladding diameter of 125 m. It is usually gray in color. This final type of cable is suitable only for jumper cables or very short runs as it has high losses and narrow bandwidth. The fiber core and cladding are engineered to the application and can be glass over glass, glass covered by plastic cladding and even plastic over plastic.

These glass fibers, coated with plastic for protection, are then placed into a buffer tube. The buffer tubes are laced into different types of cable. A pull member of metal wire or a strand Kevlar is usually added and an outside PVC coating, colored for identification, completes the cable. These cables have at least 2 fibers or can have as many as 256 fibers, the larger cables found in CATV or data service applications. Fiber optic cable can have many different terminations or connectors.

The application or device supported by the fiber optic cable will determine the type of connector used. The most common type of connectors a technician will see are those based on a "ferrule". This component of the connector is the tip or front, which holds the fiber in place and aids in the alignment of the fiber. Usually cylindrical in shape, it has a precision hole through the center. Type designations can be two digit alpha, such as ST, FC/PC but can also be longer as in SMC or biconic. The termination of the cable with a specific connector requires specialized equipment, training and a fair amount of craftsmanship.

To avoid expensive training for technicians, assure quality and increase production, many companies purchase finished connectors that have a 6 to 12 inch 'pigtail'. This connector 'pigtail' is then fusion spliced onto the cable ensuring constant splice to splice or splice to connector losses. Modern fusion splicing machines are small, lightweight and semi-automated. Gone are the days of heat guns, UV lamps, smelly adhesives, portable microscopes and hand lapping of connectors. For the technician in the field this means increased productivity and fewer call backs from 'bad' connectors.

Now that the cable is completed, what devices are attached. Transmitters are either Light Emitting Diodes or LASER diodes. First and probably most common are the LED based transmitters. Modulated directly by a signal they offer low power and a broad spectral dispersion, which leads to low coupling of light energy into cable. On the plus side, they are uncomplicated, inexpensive, require no cooling, use little drive power . LED transmitters offer no hazard to the technician or user. I know of no LED based amplifier systems.

LASER diode based systems, modulated by several different schemes, offer excellent (very low) spectral dispersion, and couple large amounts of energy into a fiber cable. LASER systems are more complicated, may require cooling of the LASER device and may pose a hazard to the technician or user. The plus side here is extremely long distance propagation of signal, on the order of 80 to 100 Km before amplification is required. There are several LASER pumped erbium-doped amplifiers on the market, while not something the average LMR/SMR technician will use, they are exciting as a demonstration of advanced physics in use.

Safety in fiber optic systems is straightforward. The LASER devices used by most communication products are classified under Government standards as Class 1 devices because they are considered "an enclosed working device" -- the LASER energy is contained within the fiber. The equipment should have all required warning placards, which clearly indicate where any LASER energy may be present anytime the system is energized. Safe maintenance practices on these systems are usually covered in the equipment technical manual. Safety rules covered in any manual are important - as a reminder here are some sample rules: -technicians shall not re-energize any equipment with LASER transmitter(s) until they have ensured all work on system is complete and all fiber cables are properly secured to equipment. -technicians shall not look into fiber cables, equipment apertures or connectors unless system/cable is KNOWN to be de-energized. -fiber optic connectors and cable ends must be properly cleaned before attachment to system equipment. -anti-static protective wrist straps must be worn when handling circuit packs or boards. Certain other Government and Industry standards exist for worker protection. The sidebar #2 references are included for employees who, while they may not perform direct maintenance on LASER-based units, .may be covered by Federal "Right to Know" or "Hazard Communication" regulations.

Testing in a fiber optic systems is pretty much limited to continuity checks and loss measurements. These tests are performed by using an Optical Time Domain Reflectometer (OTDR) or a calibrated source and optical power meter. Both are used to define losses in a fiber system caused by connectors, splices, cable defects and improper bends. The source/power meter is the least expensive means of testing. Several vendors, such as the John Fluke Co., make optical power sensor attachments for their meter lines. OTDR testing systems start at $20K or more, usually out of the reach of anyone not doing fiber installation or splicing as a primary business activity. For troubleshooting on larger systems, an OTDR can be rented or leased from several vendors. If you have a CATV system operator in your area, see if they utilize fiber optic cable in their system. If so, they may be a source of low-cost assistance when performing fiber work.

SIDEBAR 1 URLs for further reading http://www.fotec.com/index.html offers a "Lennie Lightwave" testing primer and an extensive line of fiber optic test equipment. While biased toward FOTEC equipment, does offer good primer material. http://corning.com/prod_svcs/index.html select the Fiber 101 to learn about the history of optic cable and how Corning glass is used to make the cable. http://www.metrotek.com/test.htm here is one source of low cost fiber optic test equipment. Further work with a search engine using "keyword search" for fiber optic, optic cable, fiber optic test equipment or glass fiber cable will give many more sites to visit - enjoy.

SIDEBAR 2 LASER safety regulations U.S. Department of Health and Human Services, Title 21 CFR, Subchapter J. Part 1040.10, Performance Standard for Laser Products, and Part 1040.11, Special Purpose Laser Products. Note: Most communication equipment falls under these rules.

OSHA regulations U.S. Department of Labor: Guidelines for Laser Safety and Hazard Assessment, OSHA Instructional PUB 8-1.7, Directorate of Technical Publications, August 19,1991

OSHA Instruction CPL 2-2.20B CH-2; Chapter 17: Laser Hazards, April 19, 1993, Directorate of Technical Support

U.S. Department of Labor, Bureau of Labor Standards, Safety and Health Regulations for Construction, Section 518.54 Non-Ionizing Radiation, Code of Federal Regulations (CFR), 36 (75): pp.7348-7349, Saturday, April 17, 1971

U.S. Department of Labor: Laser Construction Standard (non-ionizing radiation), Occupational Safety & Health Administration (DOL/OSHA): 29 CFR 1926.54

U.S. Department of Labor: Laser Eyewear Standard, Occupational Safety & Health Administration. Note: These rules are for high power, exposed LASER systems.

SIDEBAR 3 math Bandwidth of the fiber cable is a function of the spectral width (SW) of the source, dispersion (DISP) inherent in the cable itself and length (L) of the cable run.

BW= 0.187/(Disp)(SW)(L)

Numeric Aperture is a unitless measure. It is found as a function of the refractive indices of the cable in question. Find the square root of the sum [n12 - n22] where "n" is the refractive index.. Then, find the cone of acceptance with q = arcsin (NA) and thus NA = sin q - the answer is in degrees.




Donald Koehler was first licensed in 1977. His current call is KL0XK. He received his Bachelor of Science degree from the University of the State of New York. He has associates degrees in communications technology and the arts as well. He has a wide variety of industry certifications for computer, telephony, communications and holds an FCC Commercial radiotelephone license.

He is a network operations manager for a major Alaskan communications corporation.

His articles appear in a variety of publications from 73-Ham Radio Fun to Disaster Recovery Journal and he is a long time contributing editor for Mobile Radio Technology. He teaches part time with the University of Alaska system and lectures at other post-secondary institutions.

Member Comments:
This article has expired. No more comments may be added.
 
Fiber Optic Communications  
by AD6MJ on May 1, 2001 Mail this to a friend!
Nice article. Are you sure you didn't mean to have the measurements in microns on the diameters?
 
RE: Fiber Optic Communications  
by N2MG on May 1, 2001 Mail this to a friend!
I believe the single-mode wavelengths are 1550nm and 1310nm (not 1300nm).

It may be picking nits, but in the fiber business it matters!

And yes, it's microns (, 1/10^6 meters), not millimeters (mm, 1/10^3 meters).
 
Fiber Optic Communications  
by KF6TJR on May 1, 2001 Mail this to a friend!
In regards to safety, I joke I once heard while working on a SONET ring:
Do not look in to the laser with your remaining good eye.

So no matter what you tell people not to do, they will still do!
Take eye safety seriously!

KF6TJR
 
RE: Fiber Optic Communications  
Anonymous post on May 1, 2001 Mail this to a friend!
To quote from the article,

"Fed by a LASER device, this cable typically operates at a wavelength of 1300 or 1550 nanometers (nm)."

I think that he was indicating that the cable operates in the range 1300nm to 1550nm. This means that LASERs whose colors are 1310nm (or on the other end of the spectrum at 1500nm) will work with this cable.

Also, as I recall, the use of nm is correct, for nano meters.
 
RE: Fiber Optic Communications  
by AD8K on May 1, 2001 Mail this to a friend!
With out my looking, would the laser be in the visible lignt range? And what color would it be?
 
RE: Fiber Optic Communications  
by AC5WA on May 1, 2001 Mail this to a friend!
The light would be verrrry red, so much red as to be felt rather than seen. The typical red laser pointers you see around are usually fitted with diodes that emit in the 600~700 nm. visible range.
 
RE: Fiber Optic Communications  
by KL7KN on May 1, 2001 Mail this to a friend!
Sorry, in converting to plain text the um changed to mm. You are correct that micrometer is the measurement used to describe cable size.
dek
 
RE: Fiber Optic Communications  
by KL7KN on May 1, 2001 Mail this to a friend!
Thank you for your post. In single mode, single frequency systems, the data you posted is generally correct. In DWDM systems the actual wavelength is more a function of the DWDM equipment. The cable, however, is 'happy' over a range of wavelengths.
Techs rarely engineering these systems, the focus ; ) is on keeping system equipment up and running. The piece is aimed at folks interested in learning more about fiber systems.

Thanks again for your accurate comments, I'm looking forward to the feedback from the user community on eHAm!
dek
 
RE: Fiber Optic Communications  
by KL7KN on May 1, 2001 Mail this to a friend!
Good pun!
You may purchase a device which shows a spot of color to indicate if any LASER energy is present.
Always treat a LASER based system as tho it was active!

Thanks again for the pun, it is enjoyable to see people who have some humor at the end of a busy day.
dek
 
RE: Fiber Optic Communications  
by WB4QNG on May 1, 2001 Mail this to a friend!
The info is very interesting. I always wondered how it worked. One question and I think you might answer it in you next article. How can we use it in Ham radio. 73 Terry WB4QNG
 
RE: Fiber Optic Communications  
by N2MG on May 1, 2001 Mail this to a friend!
Anon wrote:
<< To quote from the article,

"Fed by a LASER device, this cable typically operates at a wavelength of 1300 or 1550 nanometers (nm)."

I think that he was indicating that the cable operates in the range 1300nm to 1550nm. This means that LASERs whose colors are 1310nm (or on the other end of the spectrum at 1500nm) will work with this cable. >>

I don't think that's what he was indicating... The article said that the cable was operating at two specific wavelengths, 1300nm and 1550nm. Single mode LASERs are generally 1310nm and 1550nm. That was my point. Of course the fiber can pass more than *just* 1310 and 1550...

Anon also wrote:

<< Also, as I recall, the use of nm is correct, for nano meters. >>

Yes, nm (nanometers) is the correct unit for LASER wavelengths, but that's not what I was correcting. The article gave dimensions for optical fiber using millimeters when it should have used microns.

73 Mike N2MG
 
RE: Fiber Optic Communications  
by N2MG on May 1, 2001 Mail this to a friend!
Optical fiber operates at the very red, indeed! It's called "infrared". One danger of LASERs operating at these wavelengths is that the eye cannot see the emissions, so one cannot tell the device is even on. Therefore there is no squinting reflex or urge to look away. One can easily, therefore, sustain permanent damage long before realizing it.
 
RE: Fiber Optic Communications  
by N2MG on May 1, 2001 Mail this to a friend!
Visible light is 700nm (red) to 400nm (violet). Sometimes used are 7000 (Angstroms) to 4000. The mnemonic (that you all should have learned in school), ROY G. BIV, is short for Red Orange Yellow Green Blue Indigo Violet which it the order of the colors of light (by wavelength). This works in a rainbow and in the resistor color code.

Just past the ends of the visible range are infrared (700nm to around 1mm) and ultraviolet (about 400nm to 100nm - shorter than 100nm are X-rays, then gamma rays). These infrared and ultraviolet regions are broken up further, as one might guess. Terms like UV/A, UV/B and near-infrared are often used.

What was confusing to this radio guy (me) when first entering the optical world, was that "frequency" was almost never used when describing LASERs or other optical devices - it was always "wavelength". Frequencies would easily be in the hundreds (or more)of Terahertz (10^12 Hz).

73 Mike N2MG
 
RE: Fiber Optic Communications  
by KL7KN on May 2, 2001 Mail this to a friend!
Terry/WB4QNG

The article lead indicated a focus on the hams who are also commercial radio types as well as the hams who want to know more.
There are several types of equipment that go from "light" to RF - used for example, as antenna system extenders - and you are correct - part 2 has some of this type of information.
For the casual users, I have listed sources for kits. If you ever wanted to isolate your rig from your PC, F/O is one way to do it. Knowing how that control head extension works is kinda neat as well.
Thanks for reading the article and for your feedback.
Have fun, that is what the hobby is all about!
dek
 
RE: Fiber Optic Communications  
by N5ZVP on May 3, 2001 Mail this to a friend!
I enjoyed this article.

It would be neat to have the radios near the antenna and not have to worry about them being near the operator and various computers. That way you reduce feedline loss, cut RF exposures, reduce lightning damage to the house and reduce noise from those darn multisync monitors. Fiber seems to be the perfect choice for control and audio signals.

What I have been looking for is a VHF/UHF kit with the quality of the Elecraft HF rigs that would allow something like this to be expermented with. Something like the Ten-Tec or Icom PC based receivers, but with a fiber control link and the ability to transmit. Possible canidates for mods are the Ramsey and the Ten-Tec 2m kits and maybe W7PUA's DSP-10 software radio. The control head doesn't have to be a full PC, there are lots of small microcontrollers out there that would do the job.

Any suggestions folks?

Chris N5ZVP




 
RE: Fiber Optic Communications  
by KL7KN on May 5, 2001 Mail this to a friend!
Chris
The next of the series lists a set of suppliers who sell kits which - I think - Will allow you to interface a simple CW transmitter like the RAamsey.
Thanks for reading the article.
72
dek
 
Excellent Job !  
by VE2XLT on May 5, 2001 Mail this to a friend!
As i finished a course in Telecommunication at DAWSON
college I find the notes a very usefull addition to a brief fiber introduction we had there.Will keep a TEXT copy in my computer. Thank You, Don, for sharing the knowledge!
 
Fiber Optic Communications  
by KF2HC on May 6, 2001 Mail this to a friend!
"These cables have at least 2 fibers or can have as many as 256 fibers" - Now days fiber optic cables with 864 fibers are very common. It's amazing how quickly this technology is taking over the communications industry.



 
RE: Excellent Job !  
by KL7KN on May 6, 2001 Mail this to a friend!
Yuri
Good luck at DAWSON College, I glad you found the piece of value. Part two will have interesing things to say about how to get hands-on play with real F/O equipment.

Once again- Thanks
Don
 
RE: Excellent Job !  
by N5NJ on May 7, 2001 Mail this to a friend!
The mm vs m issue should be corrected now.
 
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