Optical Communication Foundations and System Architecture

Introduction and Context

Optical communication is the transmission of information through guided or free-space electromagnetic waves at optical frequencies. In contemporary networks, the dominant medium is optical fibre, in which information is encoded onto light and transported over distances ranging from a few metres in data centres to thousands of kilometres in submarine and terrestrial backbone systems.

The importance of optical communication arises from four characteristics: very large usable bandwidth, low transmission loss, electromagnetic immunity, and the ability to support dense wavelength-division multiplexing. These characteristics have made optical systems fundamental to fixed broadband access, mobile backhaul and fronthaul, metropolitan networks, cloud data centres, long-haul terrestrial networks, submarine cables, and selected satellite and inter-satellite links.

An optical communication system is not merely a fibre and a laser. It is a coordinated architecture involving information sources, electrical and optical interfaces, transmitters, optical channels, amplifiers or regenerators, receivers, digital signal processing, synchronization, management, and protection. System design therefore requires an understanding of both physical-layer principles and network-level requirements.

Learning Objectives

After studying this chapter, the reader should be able to:

1. identify the principal optical spectrum bands and explain their engineering significance;

2. describe the historical development of optical communication;

3. distinguish important terminology used in fibre-optic systems;

4. model an optical link from information source to destination;

5. explain the function of the principal transmitter, channel, and receiver blocks;

6. calculate basic link-budget and power-margin quantities;

7. compare access, metro, core, data-centre, satellite, and free-space optical applications;

8. relate system performance to standards, regulation, reliability, and operational constraints; and

9. evaluate contemporary developments such as coherent transmission, software-defined optical networks, and AI-assisted operations.

The Optical Spectrum and Transmission Windows

Optical communication occupies a portion of the electromagnetic spectrum extending approximately from the ultraviolet through the visible and infrared regions. Fibre systems generally operate in the near-infrared because silica fibre has low attenuation and practical optical sources and detectors are available there.

The principal bands used in fibre communications are:

• O-band: approximately 1260–1360 nm. Dispersion is relatively low near 1310 nm in standard single-mode fibre, making this band useful for short and access links. Attenuation is higher than in the 1550-nm region.
• E-band: approximately 1360–1460 nm. This region was historically avoided because of water-ion absorption in older fibres. Modern low-water-peak fibre can use it effectively.
• S-band: approximately 1460–1530 nm. It is used in some wavelength-division multiplexed systems and complements the conventional C-band.
• C-band: approximately 1530–1565 nm. It is the principal long-haul transmission band because silica loss is low and erbium-doped fibre amplifiers operate efficiently here.
• L-band: approximately 1565–1625 nm. It extends wavelength capacity beyond the C-band, using suitable amplifiers and transmission components.
• U-band: approximately 1625–1675 nm. It is used more selectively, including monitoring and specialized transmission applications.

Wavelength, frequency, and photon energy are related by

\[

f=\frac{c}{\lambda}, \qquad E=hf=\frac{hc}{\lambda},

\]

where \(f\) is optical frequency in hertz, \(\lambda\) is wavelength in metres, \(c\) is the speed of light in vacuum, \(h\) is Planck’s constant, and \(E\) is photon energy in joules. In fibre, propagation velocity is lower than in vacuum, but optical frequency remains the most natural quantity for channel spacing and spectral allocation.

A useful engineering distinction is between transmission windows and spectral bands. A window is a range selected because the combined effects of attenuation, dispersion, amplifier technology, and component availability are favourable. A band is a defined wavelength or frequency range that may contain one or more transmission windows.

Fibre attenuation is often specified in decibels per kilometre. If input and output powers are \(P_{in}\) and \(P_{out}\), the loss in decibels is

\[

A_{dB}=10\log_{10}\left(\frac{P_{in}}{P_{out}}\right).

\]

For a uniform fibre of length \(L\), attenuation coefficient \(\alpha\) in dB/km gives fibre loss \(A_f=\alpha L\). This relation assumes that \(\alpha\) is approximately constant over the wavelength and distance considered. Connectors, splices, splitters, filters, and other components must be added separately.

Historical Evolution

The development of optical communication passed through several technological stages.

Early optical signalling

Human societies used fire, flags, mirrors, and semaphore systems for line-of-sight communication. These systems demonstrated the value of optical carriers but were limited by weather, visibility, manual operation, and low information capacity.

Scientific foundations

The nineteenth century established the electromagnetic theory of light. The invention of the laser in the twentieth century provided a coherent, narrow-linewidth, high-power optical source suitable for modulation. Early experiments also showed that atmospheric propagation was vulnerable to scattering, absorption, and weather.

Fibre development

The major breakthrough was the recognition that sufficiently pure glass could guide light over long distances with acceptably low loss. Early fibres had losses too high for telecommunications, but improvements in glass purification and fabrication reduced attenuation dramatically. The invention and commercialization of semiconductor lasers, photodiodes, optical connectors, and fusion splicing enabled practical systems.

From intensity modulation to coherent systems

First-generation commercial systems generally used intensity modulation with direct detection, often at 850 nm or 1310 nm. Later systems exploited the low-loss 1550-nm window and optical amplifiers. Wavelength-division multiplexing increased capacity by transmitting multiple optical channels in parallel. Modern coherent systems encode information in the amplitude, phase, and polarization of the optical field and use digital signal processing to compensate for impairments.

Network transformation

Optical communication initially replaced copper mainly in long-distance trunks. It subsequently penetrated metropolitan, access, enterprise, and data-centre environments. Current networks combine passive optical distribution, reconfigurable optical add-drop multiplexers, software control, elastic spectrum allocation, and high-speed coherent transceivers.

Essential Terminology

An optical carrier is the electromagnetic wave whose amplitude, phase, frequency, or polarization is modified to carry information. Modulation is the controlled variation of a carrier according to an information signal. In direct-detection systems, the receiver measures optical intensity; in coherent systems, a local oscillator enables recovery of phase and amplitude information.

A fibre mode is a permitted field distribution that satisfies the fibre’s boundary conditions. Multimode fibre supports many modes, whereas single-mode fibre supports essentially one spatial mode at the operating wavelength. Numerical aperture describes the light-acceptance capability of a fibre, while mode-field diameter characterizes the effective spatial extent of the fundamental mode in single-mode fibre.

Attenuation is reduction in optical power caused by absorption, scattering, bending, and component loss. Chromatic dispersion occurs because different wavelengths travel with different group velocities. Polarization-mode dispersion results from differential propagation of polarization components in an imperfect fibre. Nonlinear effects arise because the fibre refractive index depends weakly on optical intensity; examples include self-phase modulation, cross-phase modulation, and four-wave mixing.

A transceiver combines a transmitter and receiver. A transponder commonly converts a client signal into a standardized optical-channel signal, often for transport over a wavelength-division multiplexed system. A regenerator performs optical-to-electrical conversion, signal reshaping, retiming, and retransmission. An optical amplifier increases optical power without necessarily recovering the original data.

A channel is a logical or physical path allocated to a signal. A wavelength channel is an optical carrier occupying a defined spectral slot. WDM transmits multiple wavelengths through one fibre. CWDM uses relatively wide channel spacing and fewer channels; DWDM uses narrow spacing and supports high aggregate capacity.

The bit rate is the number of binary digits transmitted per second. The symbol rate is the number of modulation symbols per second. In multilevel modulation, one symbol can represent multiple bits, so bit rate may exceed symbol rate. Reach is the distance achievable under specified performance constraints without regeneration. Availability is the proportion of time for which service meets its operational specification.

Communication Models

At the most general level, an optical link can be represented as

information source → source coding and framing → electrical interface → optical transmitter → optical channel → optical receiver → digital processing → destination.

Source coding removes redundancy or represents information efficiently. Framing adds structure, synchronization, error monitoring, and sometimes forward-error correction (FEC). The transmitter maps electrical symbols to an optical waveform. The channel introduces loss, dispersion, noise, nonlinear distortion, and possible interruptions. The receiver detects or coherently measures the waveform and reconstructs the transmitted data.

A network model adds nodes and control functions:

client equipment → optical transport interface → multiplexing node → fibre spans and amplifiers → switching or add-drop node → destination interface.

The OSI model remains useful for separating application, transport, data-link, and physical functions, although optical transport standards often define additional layers and management relationships. At the physical layer, the relevant questions include optical power, spectral occupancy, modulation, receiver sensitivity, dispersion tolerance, and FEC performance. At higher layers, routing, protection, service-level agreements, and security become important.

Two physical detection models dominate. In intensity modulation/direct detection (IM/DD), the transmitter changes optical intensity and the photodetector produces a current approximately proportional to received optical power. In coherent detection, the received signal is mixed with a local oscillator; the in-phase and quadrature components are sampled and processed digitally. Coherent detection supports advanced modulation, polarization multiplexing, high spectral efficiency, and electronic compensation of linear impairments, but it requires more complex and power-intensive hardware.

System Architecture and Functional Blocks

Information and electrical processing

The input may be Ethernet, storage traffic, video, mobile traffic, or a tributary from a synchronous transport system. Framing and FEC provide boundaries, overhead, error detection, and improved tolerance to noise. A gearbox or serializer/deserializer adapts the client data rate to the line interface.

Optical transmitter

A transmitter usually contains a laser, modulation mechanism, driver electronics, monitoring circuits, and control functions. Directly modulated lasers vary the drive current, whereas externally modulated systems use a continuous-wave laser followed by an electro-optic modulator. External modulation is generally preferable at very high rates or long distances because it can reduce chirp and improve transmission performance.

Important transmitter parameters include launch power, wavelength accuracy, linewidth, relative intensity noise, extinction ratio, modulation bandwidth, and eye-safety classification. Excessive launch power is not always beneficial because nonlinear effects can increase and receiver overload may occur.

Optical plant

The plant consists of fibre, connectors, splices, patch panels, splitters, multiplexers, demultiplexers, filters, amplifiers, and monitoring equipment. Single-mode fibre is dominant in access, metro, and long-haul systems. Multimode fibre remains common in short data-centre links because of lower-cost transceivers and ease of installation.

Fibre loss arises mainly from Rayleigh scattering, material absorption, and macrobending or microbending. Dispersion broadens pulses and causes intersymbol interference. In WDM systems, nonlinear interactions may transfer energy between channels or distort their phases. Optical amplifiers compensate loss but also introduce amplified spontaneous emission noise and do not restore timing or waveform quality.

Receiver

A direct-detection receiver includes an optical filter, photodiode, transimpedance amplifier, limiting or decision circuitry, and clock recovery. PIN photodiodes offer good linearity and moderate sensitivity; avalanche photodiodes provide internal gain but require careful bias and noise management.

A coherent receiver includes an optical hybrid, local oscillator, balanced photodiodes, analog-to-digital converters, and digital signal processing. Processing may perform frequency-offset estimation, polarization demultiplexing, dispersion compensation, carrier recovery, equalization, and FEC decoding.

Network control and management

Operational systems require alarms, performance monitoring, inventory, provisioning, security controls, and configuration management. Modern optical networks increasingly expose telemetry such as received power, pre-FEC bit-error ratio, optical signal-to-noise ratio, temperature, and laser bias current. Automation can use these measurements for fault localization and predictive maintenance.

Performance Measures and Link Engineering

Power budget

The received power can be estimated by

\[

P_r=P_t-A_{total}+G,

\]

when powers and losses are expressed in dBm and gains in dB. Here \(P_t\) is transmitter output power, \(A_{total}\) is the sum of fibre and component losses, and \(G\) is the sum of amplifier gains. A basic passive-link loss is

\[

A_{total}=\alpha L+N_cA_c+N_sA_s+A_{split}+A_{misc},

\]

where \(\alpha\) is fibre attenuation in dB/km, \(L\) is length in km, \(N_c\) and \(N_s\) are numbers of connectors and splices, \(A_c\) and \(A_s\) are their individual losses, and the remaining terms represent splitter and miscellaneous losses.

The power margin is

\[

M=P_r-P_{sens},

\]

where \(P_{sens}\) is receiver sensitivity in dBm. A positive margin indicates that the received power exceeds the specified sensitivity, but practical engineering reserves are required for ageing, repairs, temperature, connector contamination, and future reconfiguration.

Worked example: passive link budget

Consider a 40-km single-mode link with transmitter power \(+2\) dBm and receiver sensitivity \(-18\) dBm. Let fibre attenuation be 0.25 dB/km, with four connector interfaces at 0.5 dB each, eight fusion splices at 0.1 dB each, and engineering reserve of 3 dB. The fibre loss is

\[

0.25\times40=10\text{ dB}.

\]

Connector loss is \(4\times0.5=2\) dB, and splice loss is \(8\times0.1=0.8\) dB. Including reserve, total design loss is

\[

10+2+0.8+3=15.8\text{ dB}.

\]

The estimated received power is

\[

P_r=2-15.8=-13.8\text{ dBm}.

\]

Therefore, the power margin is

\[

M=-13.8-(-18)=4.2\text{ dB}.

\]

The result indicates adequate power under the stated assumptions. It does not by itself prove successful operation, because dispersion, nonlinearities, extinction ratio, receiver bandwidth, and FEC requirements must also be checked.

Quality and capacity measures

The bit-error ratio (BER) is the ratio of incorrectly received bits to total received bits over a defined observation interval. Modern systems often specify a pre-FEC BER and a post-FEC error performance. The optical signal-to-noise ratio (OSNR) compares signal power with optical noise power over a stated reference bandwidth; the reference bandwidth must always be specified.

The quality factor, eye opening, error vector magnitude, and generalized mutual information may be used depending on the modulation and receiver architecture. Capacity is influenced by symbol rate, bits per symbol, number of polarizations, number of wavelength channels, coding overhead, and available optical bandwidth. A simplistic multiplication of nominal rates can be misleading because guard bands, FEC overhead, protection capacity, and utilization reduce net payload capacity.

System performance is constrained by a combination of power budget and impairment budget. A link may have sufficient received power but fail because of dispersion or OSNR degradation. Conversely, a high-quality waveform may be unusable if the received optical power is below detector sensitivity.

Network Domains and Applications

Access networks

Access networks connect homes, businesses, and mobile sites to aggregation nodes. Passive optical networks use an optical line terminal, passive splitters, and optical network units or terminals. Downstream traffic is broadcast through the splitter, while upstream transmissions require controlled time sharing to avoid collisions. Access systems prioritize low cost, optical distribution efficiency, reach, split ratio, operational simplicity, and coexistence of multiple generations.

Common families include gigabit-class passive optical networks and 10-gigabit-class systems standardized through ITU-T recommendations. Fibre-to-the-home, enterprise access, and mobile transport are major applications.

Metro networks

Metro networks aggregate access traffic across cities or regions. They require flexible switching, ring or mesh protection, moderate reach, and efficient support for Ethernet and packet services. Reconfigurable optical add-drop multiplexers and coherent pluggable optics increasingly enable metro systems to use fewer electrical regenerations.

Core and long-haul networks

Core networks carry large aggregated traffic over hundreds or thousands of kilometres. DWDM, optical amplifiers, coherent modulation, FEC, and ROADMs are central technologies. Design must address span loss, amplifier noise accumulation, nonlinear interference, wavelength continuity, restoration time, and network survivability.

Data-centre networks

Data centres use very short links with stringent requirements for cost, density, latency, energy consumption, and operational automation. Multimode fibre is common within buildings, while single-mode fibre is increasingly used for longer reaches and high-rate interconnection. Parallel optical lanes, VCSEL-based transceivers, silicon photonics, and pluggable modules are important approaches.

Satellite and free-space optical links

Free-space optical communication can provide high data rates without a fibre cable. It is attractive for inter-satellite links because the vacuum environment avoids atmospheric turbulence. Ground-to-satellite links, however, are affected by clouds, rain, scintillation, pointing errors, and atmospheric absorption. Accurate acquisition, pointing, and tracking are essential. Optical satellite links can complement radio-frequency systems rather than universally replace them.

Standards, Regulation, and Interoperability

Standards provide common definitions, optical interfaces, safety requirements, management models, and conformance targets. The ITU-T develops recommendations for transport, passive optical networks, and optical interfaces. IEEE standards define many Ethernet physical interfaces. The Optical Internetworking Forum has contributed implementation agreements for interoperable optical components and coherent interfaces. IEC standards address fibre, cables, testing, safety, and environmental requirements. Regional and national regulators control spectrum use, electromagnetic and laser safety, rights of way, and infrastructure deployment.

A standard does not guarantee interoperability in every deployment. Vendors may implement optional features differently, and parameters such as FEC, dispersion tolerance, optical power class, connector type, and management protocol must be verified. Engineers should consult the current revision of the applicable standard rather than rely on nominal data rates alone.

Laser safety is governed by hazard classifications and safe operating procedures. Fibre fragments, invisible infrared radiation, and active connectors present hazards. Optical ports should be treated as potentially active, and inspection microscopes must not be used without appropriate safety controls. Network operators must also address lawful access, privacy, cybersecurity, supply-chain risk, and protection of critical infrastructure.

Applied Case: Illustrative Metro-to-Access Upgrade

Background

A regional operator serves residential subscribers through a legacy passive optical network and aggregates traffic into a metro ring. Video, cloud applications, remote work, and 5G transport have increased peak demand. The operator must increase capacity while preserving existing outside-plant fibre where possible.

Problem

The operator faces three constraints: limited duct space, a requirement to maintain service during upgrades, and a business need to avoid replacing every customer terminal simultaneously. A simple increase in downstream line rate would not solve congestion at metro aggregation points.

Analysis

Traffic measurements show that access splitters and customer drops are physically serviceable, but feeder links and metro aggregation wavelengths are approaching utilization limits. Optical measurements indicate adequate power margin on most routes, while a small number of old connectors have excessive insertion loss. The upgrade therefore requires both capacity expansion and plant remediation.

Approach and solution

The operator deploys a coexistence-capable higher-rate passive optical system using additional wavelengths while retaining legacy services during migration. Splitter loss and optical power classes are checked for every distribution segment. At the metro layer, coherent pluggable optics and ROADM-based wavelength routing are introduced selectively on high-capacity paths. Existing fibre is tested using optical time-domain reflectometry, connector end faces are cleaned and inspected, and defective splices are repaired.

Traffic engineering separates residential, enterprise, and mobile-transport services through appropriate service classes and protection policies. Telemetry from optical terminals and aggregation equipment is integrated into the operations system. A staged migration begins with low-risk customers and includes rollback procedures.

Outcome

The architecture increases aggregate capacity without immediate replacement of all customer equipment. The operator reduces truck rolls through remote diagnostics, improves resilience at aggregation points, and gains a migration path toward additional wavelengths and higher-rate terminals.

Learning points

The case demonstrates that optical upgrades are system-engineering projects rather than isolated transceiver replacements. Power budget, coexistence, standards, operations, service continuity, and business timing must be considered together. Measured plant condition can be as important as nominal equipment specifications.

Challenges, Limitations, and Contemporary Developments

Optical fibre is not lossless, unlimited, or maintenance-free. Civil construction is expensive, cables can be damaged, and repairs may require specialized skills. Capacity is constrained by nonlinear propagation, available optical spectrum, transceiver power consumption, component cost, and operational complexity. Amplifiers accumulate noise, and high-order modulation reduces tolerance to impairments.

Coherent technology and digital signal processing continue to increase capacity and reach. Pluggable coherent modules reduce the distinction between traditional transport platforms and packet equipment. Flexible-grid ROADMs allocate spectrum more efficiently than fixed-grid systems. Open and disaggregated optical networking can reduce vendor lock-in but increases integration and testing responsibility.

Machine learning and AI are relevant when used carefully for anomaly detection, traffic forecasting, fault localization, and predictive maintenance. They do not remove the need for physical models, calibrated measurements, or engineering accountability. Training data may be incomplete or biased toward previously observed faults; automated actions therefore require confidence thresholds, human oversight, auditability, and safe rollback.

Best practice includes maintaining accurate fibre records, using standardized measurement procedures, documenting patching and splice data, applying adequate design margins, monitoring both optical and digital indicators, validating interoperability, planning protection and restoration, and reviewing safety and cybersecurity controls throughout the asset life cycle.

Chapter Summary

Optical communication uses light, principally in the near-infrared, to transport information through fibre or free space. The O-, E-, S-, C-, L-, and U-bands differ in attenuation, dispersion, amplifier availability, and component maturity. Historical progress from basic optical signalling to laser transmitters, low-loss fibre, WDM, optical amplification, and coherent DSP has transformed communications infrastructure.

An optical system comprises information processing, a transmitter, optical plant, receiver, digital processing, and management functions. IM/DD systems offer relative simplicity, whereas coherent systems provide greater spectral efficiency and impairment compensation at higher complexity. Link engineering requires both a power budget and an impairment analysis. Fibre loss, component loss, receiver sensitivity, OSNR, BER, dispersion, nonlinearities, and operational margin all affect performance.

Access, metro, core, data-centre, and satellite networks impose different requirements. Standards from organizations such as ITU-T, IEEE, OIF, and IEC support interoperability and safety, but deployment decisions must examine detailed specifications. Contemporary optical networks emphasize coherent pluggables, flexible-grid switching, automation, telemetry, and carefully governed AI-assisted operations.

Key Terms

Optical carrier: Electromagnetic wave used to transport information.

Wavelength: Spatial period of an optical wave, commonly expressed in nanometres.

O-band: Optical band near 1260–1360 nm.

C-band: Conventional low-loss and erbium-amplifier band near 1530–1565 nm.

Single-mode fibre: Fibre designed to guide essentially one spatial mode.

Multimode fibre: Fibre that supports multiple spatial propagation modes.

Attenuation: Reduction of optical power during propagation or component passage.

Chromatic dispersion: Pulse spreading caused by wavelength-dependent group velocity.

Polarization-mode dispersion: Differential delay between polarization components.

WDM: Wavelength-division multiplexing of multiple optical channels.

DWDM: Dense WDM using closely spaced optical channels.

Transceiver: Equipment combining optical transmission and reception.

Optical amplifier: Device that increases optical power without full signal regeneration.

Coherent detection: Detection using a local oscillator to recover optical field information.

OSNR: Ratio of optical signal power to optical noise power over a stated bandwidth.

BER: Ratio of incorrectly received bits to total received bits.

Power budget: Accounting of transmitter power, losses, gains, and receiver sensitivity.

ROADM: Reconfigurable optical add-drop multiplexer.

FEC: Forward-error correction using redundant coding to correct selected errors.

Passive optical network: Access architecture using passive splitters between central and customer equipment.

Review Questions

Short Answer Questions

1. Why is the C-band widely used in long-haul optical communication?

2. Distinguish attenuation from chromatic dispersion.

3. What is the principal difference between IM/DD and coherent detection?

4. Define optical signal-to-noise ratio and state why its reference bandwidth matters.

5. What functions are performed by an optical amplifier, and what functions does it not normally perform?

Long Answer Questions

1. Explain the historical evolution of optical communication from early signalling systems to coherent WDM networks.

2. Describe the functional blocks of an optical communication system and the role of each block.

3. Compare access, metro, core, and data-centre optical networks in terms of reach, architecture, cost, and performance priorities.

4. Explain the main physical impairments in fibre-optic systems and discuss methods used to manage them.

5. Discuss the role of standards, safety regulation, interoperability testing, and network management in optical-network deployment.

Analytical / Application Questions

1. A link has a transmitter output of 0 dBm, 30 km of fibre with attenuation 0.3 dB/km, six connectors at 0.4 dB each, and ten splices at 0.1 dB each. If the receiver sensitivity is −19 dBm and a 3-dB reserve is required, calculate the received power and determine whether the power margin is positive.

2. An operator must upgrade a congested access and metro network while retaining existing subscribers. Propose a migration architecture and identify the measurements and safeguards required before service activation.

3. A long-haul WDM route has adequate received power but fails its error-performance target after several amplifier spans. Analyse likely causes and recommend measurements or design changes to distinguish OSNR degradation, dispersion, and nonlinear impairment.

Multiple Choice Questions with Answers

Question 1

Which optical band is most strongly associated with erbium-doped fibre amplifier operation in conventional long-haul systems?

A. O-band

B. C-band

C. U-band

D. Visible band

Correct Answer: B. C-band

Question 2

For a uniform fibre, which expression gives loss when α is in dB/km and L is in km?

A. α/L

B. L/α

C. αL

D. 10 log10(αL)

Correct Answer: C. αL

Question 3

What is the main function of a passive splitter in a passive optical network?

A. Convert optical signals into microwave signals

B. Distribute optical power among multiple branches

C. Regenerate timing and reshape data

D. Increase optical power through stimulated emission

Correct Answer: B. Distribute optical power among multiple branches

Question 4

Which receiver architecture uses a local oscillator?

A. Coherent receiver

B. Basic PIN direct-detection receiver

C. Passive splitter

D. Optical attenuator

Correct Answer: A. Coherent receiver

Question 5

A transmitter launches +2 dBm and total link loss is 15 dB. What is the estimated received power, ignoring amplifier gain?

A. +17 dBm

B. +13 dBm

C. −13 dBm

D. −17 dBm

Correct Answer: C. −13 dBm

Question 6

Which impairment is caused by wavelength-dependent group velocity?

A. Connector contamination

B. Chromatic dispersion

C. Laser safety hazard

D. Amplifier gain

Correct Answer: B. Chromatic dispersion

Question 7

What does WDM increase by transmitting several optical wavelengths through one fibre?

A. Aggregate channel capacity

B. Fibre cladding diameter

C. Receiver dark current only

D. Connector insertion loss

Correct Answer: A. Aggregate channel capacity

Question 8

Which network domain generally prioritizes low cost, high density, and short-reach links?

A. Submarine long-haul network

B. Data-centre network

C. Intercontinental core network

D. Satellite ground segment only

Correct Answer: B. Data-centre network

Question 9

If received power is −13.8 dBm and receiver sensitivity is −18 dBm, what is the power margin?

A. −31.8 dB

B. −4.2 dB

C. +4.2 dB

D. +31.8 dB

Correct Answer: C. +4.2 dB

Question 10

Which statement about an optical amplifier is correct?

A. It always removes all accumulated noise

B. It increases optical power but does not normally restore data timing

C. It operates only by converting every signal to an electrical format

D. It eliminates nonlinear fibre effects

Correct Answer: B. It increases optical power but does not normally restore data timing