By goodvin | 09 July 2026 | 0 Comments
Optical Switch Applications in Fiber Optic Sensing: FBG and DTS Interrogation
Introduction
This article systematically introduces the fiber optic automatic protection system based on 1×2 optical switches, elaborates on the impact of fiber breaks on services, the principles and applicable scenarios of three protection architectures (1+1, 1:1, 1:N), as well as the working mechanisms of key system components (optical power monitor, mechanical optical switch, control logic) and the critical performance requirements of the ITU-T G.8131 standard.
Fiber Optic Sensing: Why Optical Switching is Essential
Fiber optic sensors (FBG, Brillouin, Raman) can measure strain and temperature at hundreds of points along a single fiber. But most FBG interrogators have only 4–16 channels — while a large infrastructure project might need 64–512 sensor points. This is where 1×N optical switches are indispensable: they multiply the channel count of a single interrogator by N, enabling cost-effective multi-point sensing.FBG Sensor Interrogation with 1×N Switches
Fiber Bragg Grating (FBG) sensors reflect a specific wavelength that shifts with temperature and strain (Δλ/λ = 1.2 pm/με + 10 pm/°C). The interrogator measures the reflected wavelength to calculate strain and temperature at each FBG location.A 1×N switch extends the interrogator from 4 channels to 4×N channels:
Configuration example (32-point system): 1×8 mechanical switch × 4 units = 4 switch groups. Each group connects to one interrogator channel. Each switch group handles 8 FBG sensors (one per fiber). Total: 4 groups × 8 sensors = 32 measurement points on one interrogator.
The switch sequentially connects each fiber to the interrogator, measures all FBGs on that fiber, then moves to the next fiber. At 100 Hz sampling rate and 1 ms switch time, the 8-fiber cycle completes in 8 ms — each fiber is sampled at 125 Hz, well within structural health monitoring requirements (typically 50–100 Hz for bridges).
Application 1: Bridge Structural Health Monitoring
Large bridges (span >200m) require 32–128 FBG strain sensors and 16–32 temperature sensors. Typical system configuration:- Main span: 16 FBG strain sensors (8 on top flange, 8 on bottom flange)
- Piers: 8 FBG strain sensors per pier × 4 piers = 32 sensors
- Temperature: 8 FBG temperature sensors (thermal compensation)
- Total: 56 FBG sensors. Interrogator: 4 channels × 1×16 switch = 64-point capacity.
- Sampling rate: 50 Hz (structural dynamics). Alarm latency: <1 second.
- Data: Real-time strain maps, modal analysis, overload detection.
Application 2: Dam and Levee Monitoring
Large dams and levees require continuous monitoring of:- Strain at multiple levels (upstream/downstream faces, core)
- Temperature gradients (thermal cracking risk)
- Seepage (using FBG flow meters)
- Crack opening displacement (FBG crackmeters)
- Total sensors: 64–256 per dam. Interrogator: 8 channels × 1×32 switch = 256-point capacity.
Application 3: Pipeline Monitoring
Oil and gas pipelines span thousands of kilometers. FBG sensors detect:- Third-party interference (excavation, anchor drag) — strain spikes
- Geohazard events (landslide, subsidence) — curvature changes
- Internal pressure and temperature — process monitoring
- Configuration: 1×64 switch × 4 channels = 256-point pipeline monitoring system. Typical installation: one FBG sensor every 500m = 2,000 sensors per 1,000km pipeline section.
Application 4: Wind Turbine Foundation Monitoring
Offshore wind turbine monopile foundations require monitoring of:- Bending moments at mudline (foundation-soil interface)
- Corrosion (FBG pH sensors in concrete)
- Impact from ships and ice
- Typical: 16–32 FBG sensors per monopile. Interrogator with 1×8 switch: sufficient for 64 sensors.
- Offshore environment requires marine-grade switches (IP68, –40°C to +85°C operating range).
Distributed Temperature Sensing (DTS) with Optical Switches
DTS (Raman OTDR, Brillouin OTDR) measures temperature along the entire fiber length (up to 50 km). A 1×N switch enables multiple DTS units to share fiber infrastructure:- Multiple wells in an oil field share one DTS unit (1×8 switch)
- Multiple zones in a district heating network share one DTS unit (1×16 switch)
- Multiple fire zones in a tunnel share one DTS unit (1×8 switch)
Key Specifications for Sensing Applications
Switching Time: <1 ms for >50 Hz sampling; <5 ms for 10–50 Hz samplingLoss Uniformity: ±0.3 dB across all ports — critical for balanced FBG measurements
Operating Band: C-band (1510–1590 nm) for FBG; O-band (1260–1360 nm) for some DTS
Repeatability: <0.1 dB IL variation across repeated switch cycles — ensures measurement stability
Fiber Type: SMF-28 for telecom FBG; bend-insensitive G.657 for tight-radius routing in structures
Environmental: IP65 minimum for outdoor; IP68 for underground/underwater; –40°C to +85°C
Conclusion
The optical fiber protection system achieves transparent disaster recovery for fiber interruptions through automatic switching within 50 milliseconds. Among these, the 1+1 architecture offers the fastest restoration (<25 ms) but has the lowest bandwidth utilization, while the 1:N architecture provides optimal cost efficiency but requires handling priority conflicts. Proper configuration of the hold time and adoption of heterogeneous routing are key measures to prevent false triggers and ensure protection effectiveness against simultaneous dual-fiber interruptions.
OPELINK FBG reflectors for fiber optic sensing networks:FC/APC FBG Reflector,SC/APC FBG ReflectorFrequently Asked Questions
Q1: How many FBG sensors can one optical switch interrogate?
The theoretical maximum = (Interrogator channels) × (Switch ports). For a 4-channel, 100 Hz interrogator with four 1×16 switches: 4 × 16 = 64 sensors at 100 Hz. However, practical limits: (1) Switch time per fiber (1 ms × 16 ports = 16 ms cycle; 1000/16 = 62.5 Hz per fiber); (2) Each FBG on a fiber must be wavelength-separated — maximum 20–25 FBGs per fiber (C-band, 1528–1565 nm, 1.5 nm spacing). Practical maximum: 4 channels × 16 ports × 20 FBGs = 1,280 sensors per interrogator.Q2: What happens if the optical switch fails in a sensing network?
In most sensing applications, switch failure means one fiber group goes offline (16–32 sensors). Unlike telecom protection (where a fiber cut causes service outage), sensing networks typically log data for later analysis — a momentary switch failure causes a data gap, not a safety failure. For critical infrastructure monitoring (dams, nuclear plants), specify dual-redundant switches: two 1×N switches in parallel, automatic failover on switch failure. This doubles the channel count and adds 1:1 redundancy.Q3: Can I use a MEMS switch instead of a mechanical switch for FBG interrogation?
Yes, for high-speed applications (≥500 Hz sampling). MEMS switches have <1 ms switching time vs. 5–10 ms for mechanical. This enables 500+ Hz FBG interrogation for dynamic testing (earthquake response, blast loading). However, for static monitoring (bridges, dams, pipelines at 50 Hz), mechanical switches are preferred: lower cost ($400 vs. $2,000), higher isolation (60 dB vs. 40 dB), and more than sufficient switching speed. Use MEMS only when the sampling rate requirement exceeds what mechanical switches can deliver.Related Guides
Optical switch specifications
Optical switch classification
Smart city fiber network
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