--- title: "Fibre losses; multimode vs single mode" subject: "Propagation and Antenna" chapter: 6 key: "semester_6/propagation_and_antenna:fiber-losses-multimode-vs-single" source: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/fiber-losses-multimode-vs-single site: BCT Engineering --- # Fibre losses; multimode vs single mode Propagation and Antenna · Chapter 6: Optical Fibres - Asked in: 2072 Magh, 1+4+5 marks - Video: [Advantages & Disadvantages of Single mode & Multi mode Fiber Unit 4 l Fiber optics l Engineering Phy](https://www.youtube.com/watch?v=6qb9N9VAPEE) — AKTU Online - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/fiber-losses-multimode-vs-single - Whole chapter: https://www.bctengineer.com/md/semester_6/propagation_and_antenna/chapter-6 ## Question What is an optical fiber? What are the different types of losses you can visualize in optical fiber communication? What are the advantages and disadvantages of multimode fiber over the single mode fiber? ## Short answer An optical fibre is a thin glass/plastic strand with a higher-index core and lower-index cladding that guides light by total internal reflection. Losses include absorption, Rayleigh scattering, bending, coupling, and splice/connector losses. Multimode fibre has a larger core and accepts cheap LEDs but suffers higher modal dispersion and attenuation, limiting it to short links; single mode has low loss and no modal dispersion for long-haul. ## Answer #### Optical fibre An **optical fibre** is a thin, flexible strand of very pure glass or plastic, with a core of higher refractive index surrounded by a cladding of lower index, that guides light over long distances by total internal reflection. #### Types of losses in optical fibre communication 1. **Absorption loss:** intrinsic (UV and IR absorption by silica) and extrinsic (OH⁻ ions, peak near 1380 nm; metal ions such as Fe, Cu). 2. **Scattering loss:** mainly Rayleigh scattering from microscopic density variations, ∝ 1/λ⁴; also Mie scattering from larger defects. 3. **Bending loss:** macrobending (sharp curves of the cable) and microbending (small random axis deviations from cabling stress). 4. **Coupling loss:** at the source-to-fibre and fibre-to-detector interfaces, due to NA and area mismatch. 5. **Splice and connector loss:** lateral, angular and gap misalignment, end-face quality, core/NA mismatch and Fresnel reflection. Attenuation is measured as α = (10/L)·log(Pin/Pout) dB/km. (Dispersion does not remove power but spreads pulses, which limits bandwidth.) #### Multimode fibre compared with single mode fibre **Advantages of multimode fibre:** - Large core (50–200 µm) and large NA make it easy to launch light, so cheap **LEDs** and VCSELs can be used. - Connectors, splicing and alignment are easier and cheaper; tolerances are relaxed. - Lower overall system cost for short links (LANs, buildings, data centres). - Several fibres can be joined with simple tools in the field. **Disadvantages of multimode fibre:** - Large **intermodal dispersion** (different modes arrive at different times), so much lower bandwidth–distance product. - Higher attenuation (about 2.5–3 dB/km at 850 nm compared with 0.2–0.35 dB/km for single mode). - Limited to short distances (a few hundred metres to ~2 km at high data rates). - Not suited to optical amplifiers and DWDM long-haul systems. | Feature | Multimode | Single mode | |---|---|---| | Core diameter | 50–200 µm | 8–10 µm | | Modes | Many | One | | Source | LED / VCSEL | Laser diode | | Bandwidth | Low–medium | Very high | | Loss | Higher | Lowest | | Distance | Short | Very long | | Cost of system | Lower | Higher | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is an optical fibre? A. A copper conductor B. A coaxial cable C. A radio waveguide D. A thin glass or plastic strand with a higher-index core in a lower-index cladding that guides light by TIR Answer: D. Light guiding is by TIR. (Key point: Fibre definition) 2. Which is NOT a power loss mechanism? A. Dispersion B. Absorption C. Scattering D. Bending Answer: A. Dispersion spreads pulses without removing power. (Key point: Loss types) 3. What is an advantage of multimode fibre? A. Lowest attenuation B. Large core and NA allow cheap LEDs and easy connectors C. Highest bandwidth D. Best for DWDM long-haul Answer: B. Relaxed tolerances reduce system cost. (Key point: Multimode advantage) 4. Why is multimode limited in distance? A. Small core B. Small NA C. Large intermodal dispersion and higher attenuation D. Needs a laser Answer: C. Bandwidth-distance product is lower. (Key point: Multimode disadvantage) ## Related in Propagation and Antenna - [Multimode vs single mode; NA derivation](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Amultimode-vs-single-na-derivation) — Propagation and Antenna, chapter 6 - [Optical sources and fibre losses](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Aoptical-sources-fiber-losses) — Propagation and Antenna, chapter 6, asked 2× - [Fibre configurations and single mode advantages](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Afiber-configurations-single-mode) — Propagation and Antenna, chapter 6 - [Optical fibre system vs wireless](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Aofc-working-vs-wireless) — Propagation and Antenna, chapter 6 - [Short note: Multimode graded index fibre](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Amultimode-graded-index-short-note) — Propagation and Antenna, chapter 6 - [Properties for selecting optical fibre](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Afiber-selection-properties) — Propagation and Antenna, chapter 6 ## Related in other subjects - [Optical fiber, NA, single vs multimode](https://www.bctengineer.com/question.md?key=semester_1%2Fengineering_physics%3Aoptical-fiber-na-acceptance-mode-comparison) — Engineering Physics, chapter 6 - [Short note: Multi mode optical fiber](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Amultimode-fiber-short-note) — Data Communication, chapter 4 - [Communication system; optical fiber merits](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Adefine-comm-optical-merits) — Basic Electronics Engineering, chapter 5 - [Advantages of optical fiber over copper](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Aoptical-fiber-advantages) — Instrumentation II, chapter 5, asked 3×