---
title: "Dark Fiber Leasing and Optics Selection: What Changes When You Don't Own the Glass"
slug: "dark-fiber-leasing-optics-considerations"
type: guide
category: "Fiber & Infrastructure"
tags: [dark fiber, fiber leasing, chromatic dispersion, PMD, DWDM, dispersion compensation, SMF, fiber characterization, optical budget]
seo_focus_keyword: "dark fiber leasing optics selection dispersion"
---

Leasing dark fiber is one of those decisions that looks financially straightforward — you pay a monthly recurring cost for fiber, you put your own wavelengths on it, you retain control of the optical layer — and becomes technically complicated the moment you try to turn the first circuit up. The fiber characterization data you receive from the lessor matters enormously, and the questions you ask before signing the contract determine whether your optical equipment choices work correctly or require expensive modifications after deployment.

## What Changes With Leased Fiber

When you own the fiber plant, you know its history. You know when it was installed, which contractor pulled it, whether any spans have been reblown after rodent damage, and what the original OTDR traces showed at installation. Your OSP team has the OTDR records. You know which splices are suspect because your technician did the work in 2018 during a rainstorm.

When you lease dark fiber, you typically receive a fiber characterization report that covers: span length, total insertion loss (at 1310nm and 1550nm), connector/splice count, and possibly chromatic dispersion (CD) and polarization mode dispersion (PMD) measurements if you're fortunate enough to be dealing with a carrier that measured them. The quality of this documentation varies enormously. Some carriers provide OTDR traces, chromatic dispersion per-span measurements, and PMD summaries. Others hand you a sheet with "64.3 km, 18.2 dB total loss, 12 splices" and consider themselves done.

The missing data creates optical planning risk. Chromatic dispersion and PMD are the two parameters most likely to cause problems with high-speed optical systems, and most fiber characterization reports for leased dark fiber don't provide adequate measurement detail.

## Chromatic Dispersion: Demand the Numbers

Chromatic dispersion (CD) measures how different wavelengths travel at slightly different speeds in a fiber, causing pulses to spread over distance. Standard single-mode fiber (G.652D, the most common type) has a dispersion zero at approximately 1310nm and a dispersion coefficient of approximately 17 ps/nm/km at 1550nm. On a 100km leased span at 1550nm, accumulated CD is roughly 1700 ps/nm.

For 10G non-coherent systems (10GBASE-ER, 10GBASE-ZR), CD tolerance is approximately ±1600 ps/nm. A 100km G.652 span at 1550nm exceeds this. You need dispersion compensation.

For 100G coherent systems using DP-QPSK or DP-16QAM (typical in 100G CFP or QSFP28 coherent modules), the DSP handles electronic dispersion compensation (EDC) and can tolerate 50,000 ps/nm or more of accumulated CD. A 100km span at 1550nm is no problem; a 3000km span starts requiring more attention to DSP operating point.

For 400G ZR (DP-16QAM at 400G, as discussed in the metro DWDM article), the coherent DSP handles CD compensation automatically up to approximately 80,000 ps/nm — more than sufficient for most metro and regional spans.

Where chromatic dispersion becomes a critical variable is when you're planning to use multi-channel DWDM on leased fiber and the channels span a wide wavelength range. Channels at the edges of the C-band (1535nm and 1565nm) experience different accumulated dispersion than channels in the middle. If your ROADM or OLS (Open Line System) does not include per-channel dispersion compensation, channels at band edges may have different reach performance than the datasheet assumes. Before signing a 40-channel DWDM deployment on leased fiber, get CD measurements at multiple wavelengths across the C-band.

The measurement you should request: ITU-T G.650.1 CD measurement using the phase shift method at a minimum of three wavelengths (1310nm, 1550nm, 1625nm), reported as dispersion coefficient in ps/nm/km and total accumulated dispersion for the span. If the carrier can't provide this, budget for your own OTDR/CD measurement after turn-up.

## The PMD Surprise

Polarization mode dispersion is caused by slight asymmetry in the fiber core cross-section, which causes two orthogonal polarization states of the optical signal to travel at slightly different speeds. The result is pulse broadening, reported as Differential Group Delay (DGD) in picoseconds.

PMD is a statistical parameter — it varies with temperature, mechanical stress, and vibration — which makes it harder to predict than CD. The PMD coefficient for modern fiber (G.652D installed after 2000) is specified below 0.1 ps/√km, giving total PMD of less than 1 ps for a 100km span. Older fiber (G.652A or G.652B installed in the 1990s or early 2000s) can have PMD coefficients of 0.5 to 2.0 ps/√km, producing 5 to 20 ps of DGD on a 100km span.

At 10G NRZ, PMD tolerance is approximately 10 ps. At 100G with PDM-coherent (which actively compensates PMD using DSP), tolerance is significantly higher. At 400G DP-16QAM, the DSP can handle PMD values up to roughly 30 ps peak DGD before penalties accumulate. At 10G ER without coherent optics, a 100km span of old 0.5 ps/√km fiber will produce intermittent errors under temperature cycling that are very difficult to diagnose.

The practical danger with leased fiber is that you don't know the vintage or the PMD coefficient until you measure it. A fiber run that crosses older plant segments — often the case with inter-city routes that were built in phases — may have sections of 1990s fiber with poor PMD characteristics mixed into an otherwise modern plant. The carrier's characterization report may not flag this.

Before deploying non-coherent 10G optics on leased fiber for spans over 40km, request PMD measurements or plan to perform your own. A field PMD analyzer (instruments from EXFO or Viavi) can characterize a span in under an hour. The cost of the measurement is trivial compared to the cost of deploying equipment that produces intermittent errors under temperature extremes.

## Dispersion Compensation for Non-Coherent Systems

If you're running 10G DWDM on older leased fiber with significant CD, dispersion compensation fiber (DCF) or dispersion compensating modules (DCMs) are the standard solution. DCF has a large negative dispersion coefficient (typically -80 to -100 ps/nm/km) which cancels positive accumulated dispersion from G.652 fiber. A DCM for a 100km span correction is a coil of DCF in a passive module housing, adding 4 to 6 dB of insertion loss.

The insertion loss of a DCM must be budgeted against your optical power budget. If your span already has high splice loss and the amplifier chain is near its power budget limit, adding a 5 dB DCM may require a booster amplifier that wasn't in the original plan.

For 100G and above with coherent optics, DCMs are unnecessary — the DSP handles CD compensation in silicon without passive compensation elements. This is one of the operational advantages of coherent optics on leased fiber: you don't need to stock DCMs or negotiate span characterization requirements with the fiber lessor before deploying 100G circuits.

## Practical Pre-Contract Checklist

Before executing a dark fiber lease:

Request fiber type documentation — G.652D vs. older G.652A/B is the critical distinction for PMD risk assessment. Ask specifically whether any cable segments were installed before 2000.

Request CD measurement data at 1310nm and 1550nm per span, not just total path. Knowing which individual spans have high splice loss or anomalous dispersion lets you plan amplifier placement correctly.

Request OTDR traces for each fiber segment. The trace shows splice locations and loss, connectors, and any events (bends, damage) in the fiber path. Review for any splice with more than 0.15 dB loss, which indicates a poor mechanical splice that may degrade further.

Negotiate access rights for your own measurements. You will want to run OTDR and possibly PMD measurements after fiber delivery and before deploying DWDM equipment. Confirm this is permitted under the lease terms.

Finally, confirm fiber continuity and pair assignment before your optical equipment vendor ships. Dark fiber delivery errors — wrong pair assigned, fibers crossed between cabinet locations — are common enough that pre-deployment continuity verification should be standard practice.
