Fiber Performance

Optical Fiber performance can be broken down into two key factors: Attenuation and Dispersion.

Attenuation

Attenuation is the loss of signal strength over distance.  It impacts the ability of the receiver to receive sufficient power to properly process the signal. It is expressed in decibels of power loss (dB) per unit distance.  It is most common in the industry to use dB/kilometer (km), but here in the US, dB/mile or dB/1000 ft are used.

Decibel loss can significantly reduce the efficiency of a cable as the “divorce law” states for every 3dB loss of optical power, you lose 50% of what you had before.   

Attenuation has two components: intrinsic and extrinsic.

Intrinsic Attenuation is the loss of signal strength from internal influences. The two main causes of intrinsic attenuation are absorption and Rayleigh scattering.

Absorption is when a photon will give up kinetic energy when it interacts with an electron and excites it to a higher energy level.

Raleigh Scattering is when light is scattered as the result of inhomogeneities and defects in the glass.  These imperfections are microscopic and happen during production.

Extrinsic Attenuation is the loss of signal strength from external influences. The two main casues of extrinsic attenuation are Macrobends and Microbends.

A Macrobend is caused by a large-scale bend of the fiber which is visible and less than the fiber’s safe minimum bending radius.

This type of loss is generally reversible once the bend is removed.

A Microbend is a small-scale distortions in the geometry of the fiber core. It can be caused during the manufacturing process, or by fiber “cross-overs” inside a tube.

A microbend is usually reversible, unless the core has been permanently deformed.

Dispersion

Dispersion is the spreading out of a light pulse as it travels through a fiber. There are three types of dispersion: Chromatic Dispersion, Modal Dispersion, and Polarization Mode Dispersion (PMD).

Chromatic Dispersion is the phenomenon of pulse spreading due to the different colors of light (wavelengths) travelling at slightly different speeds through the fiber.

Modal (a.k.a. Intermodal) Dispersion in Multimode (MM) Fiber is the spreading of signal pulses as they travel down the fiber (May cause pulses to overlap as they arrive at the receiver, and cause bit errors)

Polarization Mode Dispersion (PMD)

Single-Mode optical fiber consists of one propagation mode, which in turn, is comprised of two orthogonal polarization modes, as shown in the graphic below. Asymmetrical differences in the fiber introduce small refractive index variations between the two modes. This is known as birefringence. This is caused by ovality of the core caused by a few different factors: the manufacturing process, internal stress (cabling), and external stress. Combating PMD becomes very important as transmission speeds increase.                                       

Dispersion leads to bit error; The graphic below illustrates how dispersion impacts the pulse input. Higher dispersion makes it harder to discern between the 0s and 1’s:

Applying the Performance Factors

Attenuation – recall: loss of signal strength

• Very much affected by cable manufacturing processes
• Cannot get better (lower), only worse (higher)

Typical maximum, individual, final values for single-mode fiber

• At 1310 nm the typical maximum dB loss is .35 dB/km while with 1550 nm it is .25 dB/km loss
• Best practice = 0.34 and 0.20 dB/km loss

Typical maximum, individual, final values for Multimode Fiber

• At 850 nm the typical maximum dB loss is 3.5 dB/km while with 1300 nm it is 1.5 dB/km loss
• Best practice = Unknown, we do not work as much with multimode fiber so we advise to stick to typical maximum values

Dispersion – recall: spreading of signal pulses. Overwhelmingly from the fiber manufacturing process

• Specified by the fiber supplier. Not usually affected by cable design or manufacturing process

Therefore, typically not affected by the cable design or manufacturing processes

• “It is what it is” in a finished cable (unless a cable is very poorly made?)
• Finished cable limits not used, as long as you have good fiber and good cable, dispersion is easily manageable

Advanced Fiber Types

Increases in bandwidth demand and transmission distances have led to two special types of single-mode (SM) fiber:

Non-Zero Dispersion Shifted (NZDS) SM Fiber:

Utilized to correct Chromatic Dispersion through controlling how light bends. This is similar to how eyeglasses are used to correct vision. The standard fiber is ITU-T G.655

NZDS Fiber is designed for use with Dense Wavelength Division Multiplexing (DWDM) to boost bandwidth which allows for higher data rates to be transmitted at a longer distance:

Data Rates:

• Standard SM fiber: Commonly 10Gbit/s, as much as 40 Gbit/s
• NZDS SM fiber: 100 Gbit/s and more!

Transmission Distances:

• Standard SM fiber: 60 – 90 miles
• NZDS SM fiber: up to 250 miles

Cut-Off Shifted SM Fiber or “G654” SM Fiber:

An “ultra-low-loss” type fiber that was originally used for transoceanic submarine cables. The current G.654.E fiber allows even higher data rates: 400 Gbit/s up to 1 Tbit/s. Additionally, it allows for data to be transmitted at even longer distances too: Up to 900 km (560 miles). Essential for keeping the world connected.

Optimized for use between 1550 – 1625 nm

Typical attenuation limits:

• 1550nm ≤ 0.17dB/km
• 1625nm ≤ 0.19dB/km