Handbook for Sound Engineers

458 Chapter 15

Strength members provide for better tensile load
parameters similar to coax or electrical audio cables. An
optical fiber doesn’t stretch very far before it breaks, so
the strength members must employ low elongation at
the expected tensile loads.

A common strength member used in fiber optic
cables for harsh environments is Kevlar™. Kevlar is the
material used in bulletproof vests and has the best
performance for optical fiber strength members. These
strength members are also referred to as tactical optical
fiber. They were first used for military communications
and were popularized in Operation Desert-Storm in the
Iraq and Kuwait war of 1991. These tactical optical
fiber cables are impervious to tanks, trucks and bomb
explosions. In today’s audio applications involving
broadcast sports events and news, tactical optical fiber
cables have found a niche.

15.4.3 Signal Loss

15.4.3.1 Fiber Optic Transmission Loss (FOTL)

In addition to physical changes to the light pulse which
result from frequency or bandwidth limitations, there
are also reductions in level of optical power as the light
pulse travels to and through the fiber. This optical
power loss, or attenuation, is expressed in dB/km (deci-
bels per kilometer). The major causes of optical attenua-
tion in optical fiber systems are:

1. Optical fiber loss.


2. Microbending loss.


3. Connector loss.


4. Splice loss.


5. Coupling loss.

In the ANSI/IEEE Standard 812-1984 the Definition
of Terms Relating to Fiber Optics defines attenuation
and attenuation coefficient as follows:

Attenuation. In an optical waveguide, the diminution
of average optical power. Note: In optical waveguides,
attenuation results from absorption, scattering, and
other radiation. Attenuation is generally expressed in
decibels (dB). However, attenuation is often used as a
synonym for attenuation coefficient, expressed as
dB/km. This assumes the attenuation coefficient is
invariant with length. Also see—attenuation coefficient;
coupling loss; differential mode attenuation; equilib-
rium mode distribution; extrinsic joint loss; leaky
modes; macrobend loss; material scattering; microbend

loss; Rayleigh scattering; spectral window; transmission

loss; waveguide scattering.

Attenuation Coefficient. The rate of diminution of

average optical power with respect to distance along the

waveguide. Defined by the equation

(15-9)

where,

P(z) is the power at distance z along the guide,

P(0) is the power at z = 0,

D is the attenuation coefficient in dB/km if z is in km.

From this equation,

(15-10)

This assumes that D is independent of z; if otherwise,

the definition shall be given in terms of incremental

attenuation as

(15-11)

or, equivalently,

(15-12)

15.4.3.2 Optical Fiber Loss

Attenuation varies with the wavelength of light. Win-

dows are low-loss regions, where fibers carry light with

little attenuation. The first generation of optical fibers

operated in the first window, around 820 nm to 850 nm.

The second window is the zero-dispersion region of

1300 nm, and the third window is the 1550 nm region.

A typical 50/125 graded-index fiber offers attenuation

of 4 dB/km at 850 nm and 2.5 dB/km at 1300 nm, a

30% increase in transmission efficiency. Attenuation is

very high in the regions of 730 nm, 950 nm, 1250 nm,

and 1380 nm; therefore, these regions should be

avoided.

Evaluating loss in an optical fiber must be done with

respect to the transmitted wavelength. Fig. 15-12 shows

a typical attenuation curve for a low-loss multimode

fiber. Fig. 15-13 does the same for a single-mode fiber;

notice the high loss in the mode-transition region, where

the fiber shifts from multimode to single-mode

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