--- title: "Elements of optical communication system" subject: "Propagation and Antenna" chapter: 6 key: "semester_6/propagation_and_antenna:optical-communication-elements" source: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/optical-communication-elements site: BCT Engineering --- # Elements of optical communication system Propagation and Antenna · Chapter 6: Optical Fibres - Asked in: 2080 Asoj, 6 marks; 2073 Bhadra, 8 marks; 2072 Asoj, 8 marks (asked 3 times) - Video: [Lec08: Optical communication system](https://www.youtube.com/watch?v=c0AqYjK1EA4) — NPTEL-NOC IITM - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/optical-communication-elements - Whole chapter: https://www.bctengineer.com/md/semester_6/propagation_and_antenna/chapter-6 ## Question What are the various elements of an optical communication system? Explain each element in brief. ## Short answer An optical communication system consists of a transmitter (electrical-to-optical conversion using LED/laser), optical fibre channel with connectors, splices and amplifiers, and a receiver (photodetector and electrical amplifier) for optical-to-electrical conversion. ## Answer An **optical communication system** sends information as modulated light through an optical fibre. Its main elements are the transmitter (electrical-to-optical conversion), the channel (fibre, with connectors, splices and repeaters) and the receiver (optical-to-electrical conversion). #### Block diagram ```text +--------+ +-----------+ +---------+ | Info |-->| Electrical|-->| Optical | | source | | Tx (drive)| | source | +--------+ +-----------+ +----+----+ | optical fibre cable v ===[connector]====[splice]====[repeater]=== | +--------+ +-----------+ +----v----+ | Dest- |<--| Electrical|<--| Photo- | | ination| | Rx (amp) | | detector| +--------+ +-----------+ +---------+ ``` #### 1. Information source Provides the message: voice, video or data. Analog signals are usually digitised (PCM) and multiplexed (TDM) before transmission. #### 2. Electrical transmitter (drive circuit / modulator) Encodes and shapes the signal and supplies the drive current that modulates the light source. Most systems use **intensity modulation** (on–off keying); coherent systems modulate phase as well. #### 3. Optical source Converts the electrical signal into light at 850, 1310 or 1550 nm. - **LED:** cheap, incoherent, wide spectrum; short multimode links. - **Laser diode:** coherent, narrow spectrum, high power and speed; long-haul single-mode links. #### 4. Source-to-fibre coupler A lens or direct butt coupling launches the light into the fibre core within its acceptance cone. #### 5. Optical fibre cable (transmission channel) A glass core surrounded by a cladding of lower refractive index guides light by **total internal reflection**. Signal degradations are **attenuation** (absorption, scattering, bending; ~0.2 dB/km at 1550 nm) and **dispersion** (pulse spreading). The cable includes strength members and a protective jacket. #### 6. Connectors and splices Join fibre lengths and equipment. Splices are permanent (fusion ~0.05 dB, mechanical ~0.2 dB); connectors are demountable (~0.3 dB). #### 7. Repeaters / optical amplifiers On long routes the signal is boosted every 50–100 km. A regenerator detects, retimes and retransmits the signal; an **optical amplifier** (EDFA) amplifies light directly at 1550 nm without conversion. #### 8. Optical detector Converts received light back into current. A **PIN photodiode** (low noise, cheap) or an **avalanche photodiode** (internal gain, more sensitive) is used. #### 9. Electrical receiver A low-noise preamplifier and main amplifier, an equaliser, a filter, and a decision circuit with clock recovery restore the original digital pulses, which are then decoded and demultiplexed. #### 10. Destination The user equipment (telephone, computer, TV) that receives the recovered message. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. The main parts of an optical communication system are: A. Antenna, ionosphere, receiver B. Modulator only C. Source and mirror D. Transmitter, fibre channel, receiver Answer: D. Electrical to optical, fibre, then optical to electrical. (Key point: Main parts) 2. The fibre guides light by: A. Total internal reflection at the core–cladding boundary B. Reflection from metal C. Diffraction D. Absorption Answer: A. Core index n1 exceeds cladding n2. (Key point: Fibre guidance) 3. An EDFA differs from a regenerator in that it: A. Modulates the source B. Detects and retimes the signal C. Amplifies the light directly without conversion D. Splices fibres Answer: C. EDFAs amplify at 1550 nm optically. (Key point: Optical amplifier) 4. The electrical receiver contains: A. Only connectors B. A modulator C. Only a laser D. Preamplifier, equaliser, filter and decision circuit with clock recovery Answer: D. They restore the digital pulses. (Key point: Receiver) ## Related in Propagation and Antenna - [Short note: Light sources in optical communication](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Alight-sources-short-note) — 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 - [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 - [Five-element Yagi design for 100 MHz](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Ayagi-design-100mhz) — Propagation and Antenna, chapter 3, asked 2× - [Two-element array: broadside and end fire](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Atwo-element-array-broadside-endfire) — Propagation and Antenna, chapter 2 - [OFC system; pros and cons over metal wire](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Aofc-system-vs-metal-wire) — Propagation and Antenna, chapter 6 ## Related in other subjects - [Digital communication system elements; Shannon-Fano coding](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Adcs-block-shannon-fano) — Communication Systems, chapter 6 - [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 - [Elements of communication system; modulation is a must](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Acomm-elements-modulation-justify) — Communication Systems, chapter 1 - [Non-verbal communication and its elements](https://www.bctengineer.com/question.md?key=semester_5%2Fcommunication_english%3Anonverbal-communication-elements) — Communication English, chapter 4