ITU-T G.681 Explained: A Light Source Selection Guide for Distributed Fiber Optic Sensing Systems

Distributed fiber optic sensing (DFOS) has become an important way to monitor pipelines, borders, subsea cables, railways, and power infrastructure — turning an ordinary strand of fiber into thousands of continuous sensing points. A major change for the market is that ITU-T G.681 is now in force. The Recommendation specifies optical interface requirements for implementing DFOS within terrestrial optical transmission systems, including wavelength allocation, interference, and allowable crosstalk between transmission and sensing channels.

For system integrators and telecom operators, a formal ITU-T Recommendation changes the conversation from “does this technology work” to “can this interrogator, including its light source, support the required system-level optical interface.” That shift has direct implications for how DFOS interrogator light sources are selected, qualified, and procured — which is what this guide walks through.

What Is ITU-T G.681, and Why It Matters Now

G.681 defines optical interface requirements for deploying distributed fiber sensing capability on terrestrial optical transmission systems, so sensing and data traffic can use the same fiber infrastructure while controlling their impact on each other. This is a meaningful step beyond DFOS deployments that depend on dedicated dark fiber.

Three things make this development worth tracking:

1. It formalizes coexistence requirements. Light sources and interrogators must be evaluated not just on sensing performance, but on how cleanly they coexist with live telecom traffic through wavelength allocation, power control, spectral excursion, timing, and crosstalk management.

2. It supports infrastructure-scale procurement. A common Recommendation gives operators, integrators, and vendors a shared reference for interface requirements and qualification discussions.

3. It makes source-related interface characteristics more visible. G.681 addresses system-level signal characteristics such as wavelength band, peak power, spectral excursion, and on/off timing. The sensing method still determines device-level needs such as linewidth, coherence, and frequency stability.

The Light Source Is the Foundation of Every DFOS System

Regardless of which DFOS technique is used, sensing performance is bounded by the light source. Several core parameters commonly recur across these methods:

• Coherence length / linewidth — determines how effectively a system can resolve distributed events and how sensitive it is to phase changes.

• Frequency and amplitude stability — directly affects the noise floor and the smallest detectable strain, temperature, or acoustic signal.

• Wavelength band and power — determines usable sensing range and whether the source can share fiber with live traffic without unacceptable interference.

• Pulse characteristics (for pulsed methods) — pulse width and extinction ratio affect spatial resolution and measurement range.

Because DFOS covers several distinct sensing techniques, the right light source is not one-size-fits-all. Below is a practical breakdown.

Matching Light Sources to DFOS Techniques

Phase-OTDR / Distributed Acoustic Sensing (DAS)
Phase-OTDR relies on coherent Rayleigh backscatter interference to detect vibration and acoustic events along the fiber in real time — used heavily for pipeline monitoring, perimeter security, and railway track monitoring. This technique demands an ultra-narrow-linewidth, highly stable single-frequency laser as the pulsed source. Frequency drift or phase noise can show up as false alarms or missed events, so source stability is often one of the biggest drivers of system performance.

Fit: Single Frequency Lasers

BOTDR / BOTDA — Distributed Temperature and Strain Sensing
Brillouin-based sensing measures temperature and strain over long distances by analyzing Brillouin frequency shift. It requires a highly stable, narrow-linewidth CW source and is often paired with a pulsed or modulated companion source for probe/pump configurations. Long-haul pipeline and power-cable monitoring applications can be especially sensitive to source linewidth and long-term frequency stability.

Fit: Single Frequency Lasers and Tunable Fiber Lasers for wavelength-agile probe/pump configurations

Interrogator and Component Testing
Before a DFOS interrogator reaches the field, optical components such as circulators, WDM filters, isolators, and splitters need to be characterized across the relevant wavelength range. A stable broadband source is useful for evaluating wavelength-dependent loss, isolation, and spectral response.

Fit: C-Band ASE Broadband Light Source and Testing Light Sources

Coexistence Qualification (G.681-specific)
G.681 requires attention to wavelength allocation, DFOS signal characteristics, and the impact on co-propagating transmission channels. Broadband or wavelength-swept sources can support component-level isolation, filter, and crosstalk characterization across the relevant bands, while final qualification must be performed at the complete system interface.

Fit: C-Band ASE Broadband Light Source and 450–2400 nm Super Broadband Light Source

A Quick Selection Framework

What This Means for Procurement Teams

If your organization is evaluating DFOS vendors or building interrogator systems for telecom, energy, or infrastructure monitoring applications, G.681 gives you a concrete reference point for system-level questions. Ask vendors: Which G.681 interface parameters apply to the proposed configuration? What linewidth does the source specify, and how was it measured? How is long-term frequency drift controlled? How was coexistence with live traffic qualified in the target wavelength band?

These are familiar engineering questions, but a named Recommendation makes them easier to put in an RFP and easier for a laser source vendor to answer clearly.

Talk to Us About Your DFOS Application

OmniWavelength supplies light sources for distributed fiber sensing, optical test, and coherent detection systems — from narrow-linewidth single-frequency lasers for phase-OTDR and BOTDR to broadband ASE sources for component and interrogator qualification. If you are specifying a light source for a DFOS system intended for deployment under G.681, our applications team can help match linewidth, wavelength band, and power requirements to your sensing range and resolution targets.

Explore related resources:

• Single Frequency Lasers

• Tunable Fiber Lasers

• C-Band ASE Broadband Light Source

• 450–2400 nm Super Broadband Light Source

• Testing Light Sources

• Fiber-Coupled Laser Sources for Sensing