Fibre optic sensors

Fibre optic sensors

1. What a Fibre Optic Sensor Is

Fibre optic sensor = sensor where optical fibre itself or attached fibre transduces a physical measurand (temperature, strain, pressure, vibration, chemical concentration) into change in light intensity, phase, wavelength, or polarization, which is detected.

Advantages over electronic sensors: EMI immunity, small, lightweight, remote (km), multiplex many sensors on one fibre, works in harsh/high-voltage/explosive environments.

2. Two Classes

• Intrinsic (all-fibre): fibre itself is sensing element (measurand directly modulates light inside fibre) — e.g., FBG strain sensor.
• Extrinsic: fibre just carries light to/from external sensing head — e.g., fibre endoscope carrying image to CCD.

3. Sensing Mechanisms (4 types — exam list)

Modulated Property	Principle	Example Sensor
Intensity	Bending/microbending loss varies with pressure	Pressure diaphragm that bends fibre
Phase (interferometric)	Path difference $Δφ = (2π/λ)2ΔL$ from strain/temp modulates interference (Mach-Zehnder or Fabry-Perot)	Hydrophone, accelerometer — nε sensitivity
Wavelength (FBG)	Fibre Bragg Grating reflects

where $Λ$ grating period; strain $ε$ and temp $ΔT$ shift

| Strain/temperature in bridges, pipelines, wings (most commercial) |
| Polarization | Birefringence change with pressure/temp rotates polarization | Current via Faraday rotation |

• Fibre Bragg Grating (FBG): periodic index modulation inscribed by UV laser; acts as wavelength-selective mirror; wavelength shifts linearly with strain/temp — interrogated by broadband source + spectrometer. Many FBGs at different $λ_B$ multiplexed on one fibre.
• Interferometric: splitting fibre into sensing + reference arm, recombining gives intensity $I∝ cosΔφ$, sensitive to nm displacement.

4. Typical Applications

• Civil: strain in bridges, dams, tunnels (FBG arrays along fibre).
• Oil & gas: downhole temp/pressure (km fibre).
• Aerospace: wing strain, health monitoring.
• Biomedical: FBG catheter pressure, temperature.
• Security: intrusion detection via phase change from vibration.

5. Worked Principle

FBG temperature sensor: Λ≈530 nm, n_eff≈1.45 ⇒ λ_B≈1537 nm; temp coefficient ~10 pm/°C ⇒ 50°C shift 0.5 nm easily detected.
Pressure via diaphragm: pressure bends fibre ⇒ loss ΔI ∝ pressure.

6. Exam Points

• Define fibre optic sensor and advantage vs electronic — 3-5 marks, frequent.
• Distinguish intrinsic vs extrinsic — 3 marks, frequent.
• List four modulation types with one example — 5-7 marks, very frequent (FBG is compulsory detail).
• Draw and explain FBG

and shift — 5 marks, very frequent.