Beni AnvariPNW

Fiber Light Loss: What Actually Matters in the Field

Light loses strength as it travels through fiber. Not because the glass is defective — this is just how light behaves in any material. The question is how much loss, where it comes from, and whether there's enough signal left at the other end to do the job.

This is a simplified version of the loss concepts I use every day in the field.

The Two Numbers That Matter Most

3 dB = half your power. 10 dB = one tenth.

Every other loss number is derived from these two. A splice that loses 0.05 dB is barely a scratch. A bad connector at 1.5 dB is eating into your budget like it owes you money.

When I'm checking a link with the OTDR, I'm looking at the total loss from end to end and asking: does what I'm seeing make sense for this length of cable? If the number seems high, I start looking for the usual suspects — a dirty connector, a tight bend, a splice that took too much loss.

How Light Loses Energy in Fiber

Power decreases as light travels through glass. The technical name for this is attenuation, and in fiber we measure it in decibels per kilometer (dB/km).

Two wavelengths matter most in carrier fiber:

| Wavelength | Loss | | ---------- | ----------- | | 1310 nm | ~0.34 dB/km | | 1550 nm | ~0.19 dB/km |

The 1550 nm window loses less per kilometer. That's why long-haul and FTTH backbone routes almost always use 1550 nm — you can go farther between repeaters and amplifiers.

Why does 1550 nm lose less? Shorter wavelengths scatter more readily off microscopic variations in the glass. Think of it like this: a tennis ball (short wavelength) bounces off a gravel surface more chaotically than a basketball (long wavelength). Same surface, different outcome based on size. This scattering mechanism is called Rayleigh scattering, and it shows up everywhere in optics — including why the sky is blue.

The Decibel Scale

Decibels are just a convenient way to express ratios. They compress large numbers into smaller ones and — critically — they add instead of multiply.

Loss (dB) = 10 × log₁₀(P_in / P_out)

What this means in practice:

Loss subtracts from your budget. Gain (amplifiers, for example) adds to it. Simple.

When I look at an OTDR trace, I see loss as a downward slope. The steeper the slope, the higher the attenuation coefficient. A healthy single-mode span at 1550 nm should be a gentle decline, not a cliff.

Link Budget: Adding It All Up

A link budget is exactly what it sounds like — you add up every loss contributor and check whether your signal makes it to the receiver with enough strength to be useful.

The main contributors:

| Component | Typical Loss | | ------------------------- | ------------------------ | | Fiber (per km at 1550 nm) | ~0.19 dB/km | | Fusion splice | 0.02 – 0.10 dB | | Mechanical splice | 0.5 – 1.0 dB | | Connector (SC/LC) | 0.1 – 0.5 dB | | Tight bend | Variable — can be severe |

A well-built FTTH span might look like:

With a transmitter at +3 dBm and a receiver threshold of –28 dBm, you have 31 dB of budget. 2.35 dB of actual loss leaves you with 28.65 dB of margin. That's comfortable — but margin is what gets consumed over time by aging components, future splices for repairs, and unexpected bends.

What the OTDR Tells You

An OTDR sends a pulse down the fiber and measures what comes back. The trace plots reflected power against distance.

Always measure from both directions and average the results. A splice that looks like 0.08 dB in one direction and 0.02 dB in the other is probably closer to 0.05 dB. The true number lives in the average.

Practical Rules from the Field

The physics is straightforward once you internalize the two numbers (3 dB and 10 dB) and the two wavelengths (1310 and 1550). Everything else is just applying those basics to the specific cable plant in front of you.