--- title: "Absorption, scattering and bending losses" subject: "Propagation and Antenna" chapter: 6 key: "semester_6/propagation_and_antenna:absorption-scattering-bending-losses" source: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/absorption-scattering-bending-losses site: BCT Engineering --- # Absorption, scattering and bending losses Propagation and Antenna · Chapter 6: Optical Fibres - Asked in: 2075 Baisakh, 8 marks - Video: [Losses Associated with Optical Fibers | Skill-Lync](https://www.youtube.com/watch?v=luF4mg9FGPU) — Skill Lync - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/absorption-scattering-bending-losses - Whole chapter: https://www.bctengineer.com/md/semester_6/propagation_and_antenna/chapter-6 ## Question Discuss loss or signal attenuation in an optical fiber with respect to absorption, scattering and bending losses. ## Short answer Fibre attenuation arises from absorption (OH⁻ ions at 1380 nm, UV/IR tails), scattering (Rayleigh ∝ 1/λ⁴, dominant below 1.3 µm) and bending losses (macrobending leaks light in sharp curves, microbending from random core deviations). Low-loss windows at 1310 and 1550 nm minimise these combined effects. ## Answer **Attenuation** (signal loss) in an optical fibre is the decrease of optical power with distance. It limits the repeater spacing and is expressed in dB/km: ```text α (dB/km) = (10/L) · log10(Pin/Pout) ``` ```text loss dB/km |\ | \ Rayleigh OH peak | \ (1/λ⁴) /\ IR | \ / \ absorption | \____ ________/ \_____ / | \/ \___\__/ +----+--------+--------+--------+---> λ (nm) 850 1310 1380 1550 ``` The low-loss windows at 850, 1310 and 1550 nm come from the combination of the losses below. #### 1. Absorption losses Light energy is absorbed by the glass and converted to heat. - **Intrinsic absorption:** by the pure silica itself; electronic absorption in the ultraviolet and molecular vibration absorption in the infrared (beyond ~1.6 µm). It sets the long-wavelength limit. - **Extrinsic absorption:** by impurities such as transition metal ions (Fe, Cu, Cr) and especially **OH⁻ (water) ions**, which give absorption peaks near 950, 1240 and 1380 nm. - **Atomic defect absorption:** defects in the glass structure, e.g. caused by radiation. #### 2. Scattering losses Light is scattered out of the guided path by small non-uniformities in the glass. - **Rayleigh scattering:** by density and composition fluctuations much smaller than λ; loss ∝ 1/λ⁴. It is the dominant loss at short wavelengths and the main reason long links use 1310 and 1550 nm. - **Mie scattering:** by larger imperfections (core–cladding irregularities, bubbles), reduced by careful manufacture. - Non-linear scattering (stimulated Brillouin and Raman) appears only at high optical power. #### 3. Bending (radiation) losses - **Macrobending:** bends with radius much larger than the fibre diameter (e.g. cable round a corner). On the outer side of a sharp bend the ray angle falls below the critical angle, so light leaks into the cladding. Loss rises steeply once the bend radius goes below a critical value. - **Microbending:** small random bends of the fibre axis (µm scale) caused by uneven cabling pressure or temperature changes. They couple light from guided to radiating modes. Bending loss rises exponentially when the bend radius falls below a critical radius (for multimode fibre Rc ≈ 3n1²λ / (4π(n1² − n2²)^(3/2))). Microbending is reduced with soft-plus-hard dual coatings and loose-tube cables, macrobending by keeping cables above their minimum bend radius. #### Summary | Loss | Mechanism | Remedy | |---|---|---| | Intrinsic absorption | UV/IR absorption by silica | Work at 1.3–1.6 µm | | Extrinsic absorption | OH⁻, metal ions | Purer, dry glass | | Rayleigh scattering | Density fluctuations, ∝ 1/λ⁴ | Longer wavelength | | Macrobending | Sharp curves | Keep radius large | | Microbending | Small axis deviations | Good coating, cabling | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What causes intrinsic absorption in a fibre? A. OH- impurities B. Metal ions C. Microbends D. The pure silica itself: UV electronic and IR molecular vibration absorption Answer: D. Intrinsic absorption sets the long-wavelength limit beyond about 1.6 um. (Key point: Intrinsic absorption) 2. Where do OH- ions cause absorption peaks? A. Near 950, 1240 and 1380 nm B. Near 850, 1310 and 1550 nm C. Only near 1550 nm D. Only in the UV Answer: A. The water peaks separate the low-loss windows. (Key point: OH- peaks) 3. Rayleigh scattering is dominant at which wavelengths and why? A. Long wavelengths, since loss is proportional to lambda^4 B. Short wavelengths, since loss is proportional to 1/lambda^4 C. All wavelengths equally D. Only at high optical power Answer: B. It is caused by fluctuations much smaller than lambda. (Key point: Rayleigh scattering) 4. Which pairing of bend and remedy is correct? A. Macrobending: dual coating; microbending: minimum radius B. Both: longer wavelength C. Macrobending: keep cables above minimum bend radius; microbending: dual coating and loose-tube cable D. Both: higher OH- content Answer: C. Macrobends come from routing; microbends from pressure and temperature stress. 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