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Role of Optical Fiber in Short Distance Communication

>> Origin of Optical Fiber

This idea is very simple. Let us fill up a container with water and shone a light into it. In a darkened room, then pull out the bung. The light shone out of the hole and the water gushed out. It is expected that the light would shine straight out of the hole and the water would curve downwards, as in the diagram.

But the light stayed inside the water column and follows the curved path. Nature had found a way to guide light. What was expected and what actually happened here lead to the basic foundation of Optical Fiber.

The basic requirements still remain the same today, a light source and a clear material (usually plastic or glass) for the light to shine through. The light can be guided around any complex path. Being able to guide light along a length of optic fiber has given rise to two distinct areas of use, light guiding and communications.



>> Modern Day Optical Fiber

Modern day optical fiber is oriented towards faster rate of communicating data between source and destination. Fiber might not to be in a line of sight, now light can pass through the complex loop as shown in the figure.

This property of fiber to conduct even on bending made it more and more possessive towards new area of research.



>> Why is OFC on such a hype

Optical fiber had a property of communicating even when bent without much attenuation and on short versions of data communication had no or negligible data loss, which opted it more in medical and IC chip designing technology.

Conduction with or without amplifier at the later stage and its tiny structure had some commanding influence in the area of data communication.



>> How to provide data

The angles of the rays are measured with respect to the normal. This is a line drawn at right angles to the boundary line between the two refractive indices, core and cladding region. The angles of the incoming and outgoing rays are called the angles of incidence and refraction respectively.



>> Stephen William Hawking

“Optical Fiber will lead an example for all lossless communication in mere future, including IC technology” And now the worlds fastest calculating machine. Blue Gene/L ”, Developed at Lawrence Livermore National lab, is capable of calculating 280.6 trillion calculations/sec which uses Optical fiber for its internal connection has done the miracle as stated above by Stephan Hawking.



>> Types of Optical Fiber



>> Single Mode Step-Index Fiber


  • Minimum dispersion: all rays take same path, same time to travel down the cable. A pulse can be reproduced at the receiver very accurately.
  • Less attenuation, can run over longer distance without repeaters.
  • Larger bandwidth and higher information rate


  • Difficult to couple light in and out of the tiny core
  • Highly directive light source (laser) is required.
  • Interfacing modules are more expensive


>> Multimode Fiber

Multimode step-index Fibers:

  • Inexpensive; easy to couple light into Fiber result in higher signal distortion; lower TX rate

Multimode graded-index Fiber:

  • intermediate between the other two types of Fibers


>> Acceptance Cone & Numerical Aperture


Acceptance Angle, θc, is the maximum angle in which external light rays may strike the air/Fiber interface and still propagate down the Fiber with <10 dB loss.


Numerical Aperture (NA)



>> Optical Fiber Link



>> High Speed Optical Fiber Communication ICs Based on INP HEMT

High-speed integrated circuit technology is the key to realizing large-capacity optical fiber communication systems. This paper describes the present status of 0. l-pm-gate InP HEMT ICs for the next-generation 40-Gbit/s/ch. systems. As an advanced IC technology, this paper also describes a 4OMbit/s OEIC that is monolithically fabricated with a uni-traveling-carrier photodiode and the 0.1-pm InP HEMTs.


>> OEIC Technology

The capability of monolithic integration with a photodiode is another great merit of the InP HEMT. The EDFA relaxes the gain requirement for the electrical amplifier in the optical receiver. Furthermore, a photodiode that has broad bandwidth and high saturation output power, such as the UTC-PD makes direct driving possible at the characteristic impedance of 50 ohms with a voltage swing of 1 Vp-p.



>> IC Design and Results

The basic 40-Gbit/s optical sender (OS) and receiver (OR) configurations. The functions required for optical communication ICs are basically time-division multiplexing, reshaping, retiming, regenerating, and time-division demultiplexing. Reshaping is performed by the Er-doped fiber amplifier (EDFA), photo detector (PD), preamplifier (Pre) and baseband amplifier (Base).



>> Questions of Strength

One common misconception about optical fiber is that it must be fragile because it is made of glass. In fact, research, theoretical analysis, and practical experience prove that the opposite is true. While traditional bulk glass is brittle, the ultrapure glass of optical fibers exhibits both high tensile strength and extreme durability.

How strong is fiber?

Figures like 600 or 800 thousand pounds per square inch are often cited, far more than copper’s capability of 100 pounds per square inch. That figure refers to the ultimate tensile strength of fiber produced today. Fiber’s real, rather than theoretical; strength is 2 million pounds per square inch.


>> ILD versus LED


  • More focused radiation pattern
  • Smaller fiber
  • Much higher radiant power
  • Longer span
  • Faster ON, OFF time
  • Higher bit rates
  • Possible monochromatic light
  • Reduces dispersion


  • Much more expensive
  • Higher temperature
  • Shorter lifespan


>> Light Detectors

PIN Diodes

  • Photons are absorbed in the intrinsic layer, sufficient energy is added to generate carriers in the depletion layer for current to flow through the device

Avalanche Photodiodes (APD)

  • Photogenerated electrons are accelerated by relatively large reverse voltage and collide with other atoms to produce more free electrons avalanche multiplication effect makes APD more sensitive but also more noisy than PIN diodes


>> Advantages of Fiber Optics

Less expensive – Several miles of optical cable can be made cheaper than equivalent lengths of copper wire.

Higher carrying capacity – Because optical fibers are thinner than copper wires, more fibers can be bundled into a given-diameter cable than copper wires. This allows more phone lines to go over the same cable.

Low power – Because signals in optical fibers degrade less, lower-power transmitters can be used instead of the high-voltage electrical transmitters needed for copper wires.

Digital signals – Optical fibers are ideally suited for carrying digital information, which is especially useful in computer networks.


>> Conclusion

40-Gbit/s ICs for next-generation optical fiber communication systems have been developed using 0.1 – pm InP HEMT technology. These ICs have sufficient speed margins for the 40-Gbit/s data rate. An optoelectronic decision IC monolithically integrated with a UTC-PD and the InP HEMTs was also confirmed to operate at 40 Gbit/s, and an optical receiver using the OEIC offered high receiver sensitivity.

Use of well-developed signal processing techniques and algorithms to design these optical devices is a wise use of existing technology.


>> Next Generation Optical ICs




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