--- title: "Fibre configurations and single mode advantages" subject: "Propagation and Antenna" chapter: 6 key: "semester_6/propagation_and_antenna:fiber-configurations-single-mode" source: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/fiber-configurations-single-mode site: BCT Engineering --- # Fibre configurations and single mode advantages Propagation and Antenna · Chapter 6: Optical Fibres - Asked in: 2072 Asoj, 4+2 marks - Video: [Single-mode Optical fiber and Multi-mode Optical Fiber in Optical Fiber Communication](https://www.youtube.com/watch?v=KW3HSJthFws) — Engineering Funda - 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-configurations-single-mode - Whole chapter: https://www.bctengineer.com/md/semester_6/propagation_and_antenna/chapter-6 ## Question Compare the configuration of different types of fiber. Mention the advantages of single mode fiber. ## Short answer Step-index multimode has a uniform core (50–200 µm) and zig-zag rays causing large modal dispersion; graded-index multimode has a parabolic index and curved rays with less dispersion; single mode has a tiny core (8–10 µm) and one axial path, so no modal dispersion. Advantages of single mode: very high bandwidth, lowest attenuation (~0.2 dB/km), long repeater spacing, and supports WDM. ## Answer Optical fibres are classified by the refractive-index profile of the core and the number of modes they carry. The three standard configurations are **step-index multimode**, **graded-index multimode** and **step-index single mode**. #### Comparison of fibre configurations ```text Step-index MM Graded-index MM Single mode n n n | ___ | /\ | _ |_| |_ |__/ \__ |__| |__ core 50-200um core 50-85um core 8-10um zig-zag rays curved rays one axial path ``` | Feature | Step-index MM | Graded-index MM | Single mode | |---|---|---|---| | Core index | Uniform $n_1$ | Falls parabolically from axis | Uniform $n_1$ | | Core diameter | 50–200 µm | 50–85 µm | 8–10 µm | | Cladding diameter | 125–400 µm | 125 µm | 125 µm | | Ray path | Zig-zag, sharp reflections | Smooth curves (sinusoidal or helical) | Nearly straight along axis | | Number of modes | Many (hundreds) | Many, but equalised delay | One ($V < 2.405$) | | Modal dispersion | Large | Small | None | | Bandwidth | ~20–50 MHz·km | ~0.5–1 GHz·km | > 10 GHz·km | | Source | LED | LED or laser | Laser | | Cost / handling | Cheapest, easy | Moderate | Fibre cheap, but costly connectors and sources | | Use | Short links | LANs, campus | Long-haul, high-speed | In graded-index fibre, rays travelling far from the axis go through lower-index (faster) glass, so they arrive almost together with the axial ray. This is why modal dispersion is much lower than in step-index fibre. #### Advantages of single mode fibre - No modal (intermodal) dispersion, so very high bandwidth (Gb/s, and up to Tb/s with WDM). - Lowest attenuation (about 0.2–0.35 dB/km at 1550/1310 nm), so repeater spacing of 100 km or more. - Best for long-haul, undersea and backbone links. - Supports future upgrades (higher bit rate, DWDM) on the same cable. - Well-defined output beam, suitable for coherent and high-precision systems. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why are rays in graded-index fibre nearly synchronised? A. They travel straight B. They are absorbed C. They use TIR only D. Outer rays pass through lower-index, faster glass Answer: D. Compensates longer paths. (Key point: Graded rays) 2. Single-mode operation requires? A. V < 2.405 B. V > 2.405 C. V = 10 D. V > 50 Answer: A. Only one mode propagates. (Key point: Single-mode condition) 3. Which core diameter is single mode? A. 50-200 um B. 8-10 um C. 50-85 um D. 125 um Answer: B. Small core, small Delta. (Key point: Core sizes) 4. Which is an advantage of single mode? A. Cheapest connectors B. Works with LEDs C. No modal dispersion and lowest attenuation D. Largest core Answer: C. Suits long-haul and DWDM. 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