Optical Sources and Transmitter Engineering

Introduction and Context

An optical transmitter converts an electrical information signal into a modulated optical signal suitable for propagation through an optical fibre or free-space optical link. Its performance strongly influences link capacity, reach, bit-error rate, power consumption, dispersion penalty, and system cost. The transmitter is not merely a light source with a driver. It is an integrated electro-optical subsystem comprising a source, bias network, modulation circuit, impedance interface, thermal-control system, optical coupling arrangement, monitoring photodiode, and supervisory electronics.

The principal semiconductor sources used in communication systems are light-emitting diodes (LEDs), edge-emitting laser diodes (EELDs), vertical-cavity surface-emitting lasers (VCSELs), and tunable lasers. LEDs are inexpensive and robust but have limited bandwidth and optical power. Laser diodes provide higher speed, power, and spectral concentration. VCSELs offer low threshold current, efficient coupling to multimode fibre, and excellent array compatibility. Tunable lasers enable wavelength-selective operation in wavelength-division multiplexed (WDM) networks.

Transmitter engineering requires simultaneous consideration of optical physics, semiconductor-device behaviour, high-speed circuit design, modulation theory, thermal management, reliability, and standards compliance.

Learning Objectives

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

1. Explain spontaneous and stimulated emission and relate them to optical-source operation.

2. Compare LEDs, edge-emitting lasers, VCSELs, and tunable lasers.

3. Interpret source characteristics such as threshold current, slope efficiency, linewidth, modulation bandwidth, chirp, and relative intensity noise.

4. Explain direct and external modulation and select an appropriate transmitter architecture.

5. Analyse temperature, noise, nonlinear, and dispersion effects.

6. Design a preliminary optical transmitter using appropriate bias, modulation, coupling, and monitoring provisions.

7. Evaluate transmitter choices for practical links, including high-speed and WDM systems.

1. Semiconductor Emission Fundamentals

1.1 Radiative recombination

In a semiconductor, electrons occupy energy states in the conduction band and holes occupy states in the valence band. When an electron recombines with a hole, energy approximately equal to the band-gap energy may be released as a photon. The corresponding wavelength is approximately

\[

\lambda \approx \frac{hc}{E_g},

\]

where \(h\) is Planck’s constant, \(c\) is the speed of light in vacuum, and \(E_g\) is the band-gap energy expressed in joules. This relation is an approximation because band structure, temperature, alloy composition, and carrier confinement affect the actual emission spectrum.

In an LED, recombination is predominantly spontaneous. Photons are emitted with random phase and direction, producing relatively broad spectral and angular distributions. In a laser diode, stimulated emission is dominant above threshold. An incident photon induces an excited electron to emit a second photon with the same frequency, phase, polarization, and propagation direction. Optical feedback from a resonant cavity provides the conditions required for coherent oscillation.

1.2 Population inversion and gain

A laser requires optical gain to exceed total cavity loss. The threshold condition may be represented as

\[

\Gamma g_{th}=\alpha_i+\alpha_m,

\]

where \(\Gamma\) is the optical confinement factor, \(g_{th}\) is the material gain at threshold, \(\alpha_i\) is internal loss, and \(\alpha_m\) is mirror loss. For a cavity of length \(L\) with facet power reflectivities \(R_1\) and \(R_2\),

\[

\alpha_m=\frac{1}{2L}\ln\left(\frac{1}{R_1R_2}\right).

\]

These expressions assume a longitudinally uniform cavity and neglect gain saturation and spatial hole burning. They are useful for understanding threshold trends rather than for exact prediction of a commercial device.

The threshold current \(I_{th}\) is the injection current at which net modal gain reaches cavity loss. Below threshold, a laser diode behaves largely as a spontaneous-emission device. Above threshold, additional current produces stimulated optical output with an approximately linear characteristic over a useful operating region.

2. Optical Source Technologies

2.1 Light-emitting diodes

LEDs generally use a homojunction or heterostructure to confine carriers and optical energy. Double-heterostructure LEDs improve carrier confinement and efficiency. Surface-emitting LEDs radiate perpendicular to the junction, whereas edge-emitting LEDs emit through a stripe at the edge and can provide narrower spectra and higher radiance.

Important LED characteristics include:

• Low cost and high ruggedness: LEDs tolerate reflections and current disturbances better than lasers.
• Broad spectrum: typical spectral width is much greater than that of a laser, increasing chromatic-dispersion penalties.
• Low to moderate optical power: coupling into small-core fibre is less efficient.
• Limited modulation bandwidth: carrier recombination lifetime and parasitic capacitance restrict speed.
• No threshold current: optical output begins gradually with injected current.

LEDs remain useful in short, low-cost links, industrial sensing, visible-light communication, and systems where eye safety and simplicity are important.

2.2 Edge-emitting laser diodes

An edge-emitting laser contains a lateral waveguide and a Fabry–Pérot cavity. The cleaved or coated end facets form the optical resonator. Distributed-feedback (DFB) lasers incorporate a periodic grating that selects a longitudinal mode and provide stable narrowband operation, especially near 1.3 or 1.55 μm. Distributed Bragg reflector (DBR) lasers separate the gain and grating functions and can support tuning or improved mode control.

EELDs offer high output power, narrow spectral width, and modulation bandwidths appropriate for high-speed single-mode-fibre systems. Their disadvantages include relatively stringent temperature control, sensitivity to optical feedback, mode instability in some structures, and more complex packaging than LEDs.

The light-current characteristic above threshold is often approximated by

\[

P_o \approx \eta_d\frac{h\nu}{q}(I-I_{th}), \qquad I>I_{th},

\]

where \(P_o\) is emitted optical power, \(\eta_d\) is differential quantum efficiency, \(h\nu\) is photon energy, \(q\) is electron charge, and \(I\) is drive current. This is a local linear model; it does not include gain compression, thermal rollover, facet damage, or coupling loss. The slope efficiency is

\[

S=\frac{dP_o}{dI},

\]

with units of watts per ampere. In a transmitter specification, it is essential to distinguish internally emitted power, fibre-coupled power, and power available at the connector.

2.3 VCSELs

A vertical-cavity surface-emitting laser emits perpendicular to the wafer. Its cavity is short and bounded by highly reflective distributed Bragg reflector mirrors. The small active volume gives low threshold current and low energy per bit. Wafer-level testing and two-dimensional array fabrication reduce manufacturing cost.

VCSEL advantages include:

• low threshold and drive current;
• circular beam that couples efficiently to multimode fibre;
• high modulation bandwidth at short reach;
• suitability for parallel optical modules and data-centre interconnects;
• relatively low-cost testing and packaging.

Limitations include lower available power than many EELDs, sensitivity to mode and polarization behaviour, temperature-dependent wavelength, and restricted reach when used with multimode fibre. VCSELs are common around 850 nm, although longer-wavelength devices also exist.

2.4 Tunable lasers

A tunable laser can change its emission wavelength over a specified range. Tuning may be achieved using thermally controlled gratings, sampled gratings, distributed Bragg reflectors, Vernier-coupled cavities, or microelectromechanical elements. In a Vernier architecture, two cavities with slightly different periodicities produce a much larger effective tuning range than either cavity alone.

Tunable lasers are valuable in reconfigurable optical add/drop multiplexers, software-defined optical networks, wavelength-selective switches, coherent transceivers, and restoration systems. Important parameters are tuning range, tuning speed, output-power flatness, linewidth, side-mode suppression ratio, wavelength accuracy, settling time, and frequency repeatability.

A tunable transmitter is more flexible but generally more expensive and electronically complex than a fixed-wavelength transmitter. Calibration tables, thermal control, wavelength locking, and protection against mode hops are central engineering requirements.

3. Comparison of Sources

The source choice should follow the link requirement rather than the nominal data rate alone. LEDs are preferred for very short, inexpensive links where broad spectrum and low power are acceptable. VCSELs are particularly effective for short-reach multimode-fibre links and parallel arrays. DFB EELDs are preferred for long-reach, high-speed, single-mode links and WDM channels. Tunable DFB, DBR, or hybrid lasers are selected when wavelength agility or inventory reduction is important.

The comparison involves several trade-offs. A narrow linewidth reduces chromatic-dispersion and coherent-detection penalties but does not automatically guarantee low system penalty; chirp, relative intensity noise, coupling, and receiver compatibility also matter. High optical power may improve link margin but increase nonlinear effects, facet stress, and eye-safety concerns. A low threshold reduces electrical consumption but can be accompanied by increased sensitivity to temperature or optical feedback.

4. Modulation and Transmitter Architectures

4.1 Direct modulation

In direct modulation, the injection current of the source is varied by the electrical data signal. A typical current is

\[

I(t)=I_b+i_m(t),

\]

where \(I_b\) is the dc bias current and \(i_m(t)\) is the modulation current. For a laser, the bias is normally above threshold for a non-return-to-zero (NRZ) signal, although advanced formats may use different operating points.

The optical output is approximately

\[

P(t)\approx P_b+S i_m(t),

\]

provided the source operates within its linear region and the modulation frequency is below the effective electro-optical bandwidth. \(P_b\) is the optical power at the bias point and \(S\) is local slope efficiency.

Direct modulation is compact, efficient, and inexpensive. However, changing carrier density changes the refractive index. The resulting transient change in optical frequency is called chirp. In a directly modulated laser, the optical field may be represented conceptually as

\[

E(t)=\sqrt{P(t)}\exp[j\phi(t)],

\]

where variations in \(\phi(t)\) produce instantaneous frequency changes. Chirp interacts with fibre chromatic dispersion and can broaden pulses or close the eye, especially at high bit rates and long distances.

4.2 External modulation

In an externally modulated transmitter, the laser is operated at approximately constant optical power and a separate modulator varies the light. Common devices are Mach–Zehnder modulators (MZMs), electro-absorption modulators (EAMs), and, in some systems, phase or IQ modulators.

An ideal push-pull MZM has an intensity-transfer relationship that can be written as

\[

P_o=P_{in}\cos^2\left(\frac{\pi V}{2V_\pi}+\phi_b\right),

\]

where \(P_{in}\) is input optical power, \(V_\pi\) is the voltage required to produce a relative phase shift of \(\pi\), \(V\) is the differential drive voltage, and \(\phi_b\) is the bias phase. The exact expression depends on voltage definition and modulator configuration. At quadrature bias, the response is approximately linear for small signals.

An EAM changes absorption through the electric-field-dependent absorption edge. It is compact and can be integrated with a laser, but it introduces insertion loss, residual chirp, and wavelength-dependent operation.

External modulation offers lower chirp, higher linearity, and suitability for long-haul and advanced modulation formats. Its disadvantages are additional optical loss, drive-voltage requirements, bias-control complexity, size, and cost.

4.3 Transmitter chain

A practical transmitter commonly contains:

1. a serial electrical input and clock/data recovery or retiming circuit;

2. a driver with controlled amplitude, rise time, pre-emphasis, and impedance matching;

3. a bias tee or bias-control circuit;

4. the source and, where required, an external modulator;

5. a monitor photodiode;

6. a thermoelectric cooler, thermistor, and control loop;

7. optical coupling, connectorization, and mechanical protection;

8. fault detection and automatic power control.

The electrical interconnect must be treated as a high-frequency transmission line. Reflections caused by impedance discontinuity can create deterministic jitter and amplitude distortion. Package inductance, pad capacitance, connector loss, and driver output impedance are often as important as the intrinsic source bandwidth.

5. Dynamic Response and Bandwidth

The modulation response of a semiconductor laser is governed by carrier and photon dynamics. A commonly used small-signal approximation is a second-order response with relaxation-oscillation frequency \(f_r\) and damping factor. The exact parameters depend on differential gain, photon lifetime, carrier lifetime, confinement, and bias current.

The relaxation frequency generally increases with bias above threshold, but excessive current increases heating and may reduce reliability. The usable transmitter bandwidth is determined by the combined response of the driver, source, modulator, package, and fibre-coupling interface. A nominal 3-dB bandwidth is not sufficient to predict data performance; rise time, peaking, group delay, return loss, and pattern dependence must also be considered.

For a first-order low-pass approximation, the 10–90% rise time is approximately

\[

t_r\approx\frac{0.35}{B_{3\text{dB}}},

\]

where \(B_{3\text{dB}}\) is bandwidth in hertz and \(t_r\) is in seconds. This relation assumes a single-pole response and should not be applied directly to a strongly peaked second-order transmitter.

6. Noise, Chirp, and Nonlinear Effects

6.1 Relative intensity noise

Relative intensity noise (RIN) describes fluctuations in optical power normalized to average optical power. It is commonly specified in dB/Hz. The source of RIN includes spontaneous-emission coupling, carrier fluctuations, mode competition, optical feedback, and drive-current noise. The integrated noise depends on the measurement bandwidth; therefore, a RIN value without bandwidth and frequency range is incomplete.

Laser phase noise is associated with spontaneous emission and finite linewidth. In coherent systems, linewidth affects the combined transmitter–local-oscillator phase error and may limit the performance of high-order modulation formats.

6.2 Chirp and dispersion

The linewidth-enhancement factor, often denoted by \(\alpha\), links carrier-induced gain changes and refractive-index changes. A large value generally produces stronger frequency chirp during intensity modulation. Chirp can be beneficial in some dispersion regimes and harmful in others, so it must be analysed together with fibre type, wavelength, modulation format, and link length.

6.3 Nonlinearity

The power-current curve is not perfectly linear. Gain compression, carrier heating, saturation, clipping, modulator transfer curvature, and driver slew limits generate harmonic and intermodulation distortion. Analogue optical links require particularly stringent linearity, while digital links are often evaluated using eye diagrams, extinction ratio, optical modulation amplitude, jitter, and bit-error rate.

The extinction ratio is

\[

ER=\frac{P_1}{P_0},

\]

where \(P_1\) and \(P_0\) are the optical powers representing logical one and zero. When expressed in decibels,

\[

ER_{dB}=10\log_{10}\left(\frac{P_1}{P_0}\right).

\]

Definitions must specify whether powers are measured at the source, after the modulator, or at the fibre interface.

7. Temperature, Reliability, and Safety

Temperature affects threshold current, slope efficiency, emission wavelength, modulation bandwidth, and lifetime. As temperature increases, threshold current usually rises and efficiency decreases. The wavelength also shifts because of changes in band gap and refractive index. In WDM systems, uncontrolled wavelength drift can cause channel overlap or filter loss.

A thermoelectric cooler (TEC) can stabilize temperature but consumes power and adds control complexity. A thermistor provides temperature feedback, while a monitor photodiode supports automatic power control. The control loop must be stable and sufficiently slow or filtered so that it does not respond incorrectly to data modulation.

Reliability mechanisms include catastrophic optical damage, gradual degradation, electromigration, bond failure, moisture ingress, and connector contamination. High optical feedback can destabilize a laser or cause mode partition effects. Optical isolators, angled interfaces, suitable facet coatings, and careful connector design reduce these risks.

Laser safety is governed by the applicable product and workplace standards. The design must account for accessible emission, wavelength, exposure duration, enclosure, interlocks, warning labels, and maintenance procedures. Safety classification is not a substitute for engineering controls.

8. Preliminary Transmitter Design Method

A practical design process is as follows:

Step 1: Define the link

Specify bit rate, modulation format, fibre type, length, wavelength window, receiver sensitivity, dispersion tolerance, allowable power consumption, and environmental range.

Step 2: Select the source

Use an LED only when bandwidth and power requirements are modest. Select a VCSEL for short-reach multimode applications or array transmission. Select a DFB laser for high-speed single-mode operation. Select a tunable source when wavelength agility, protection switching, or WDM provisioning justifies the added complexity.

Step 3: Establish the optical budget

The required transmitter launch power must satisfy

\[

P_{tx,min}=P_{rx,sens}+L_{fibre}+L_{connectors}+L_{splices}+M,

\]

when all quantities are expressed in dB or dBm consistently. Here, \(P_{rx,sens}\) is receiver sensitivity in dBm, losses are positive dB quantities, and \(M\) is engineering margin in dB. This is a power-budget relation, not a dispersion or nonlinear-performance guarantee.

Step 4: Choose direct or external modulation

Direct modulation is normally preferred for compact, short- and medium-reach systems when chirp and linearity are acceptable. External modulation is preferred for long reach, high capacity, narrow filtering tolerance, analogue linearity, or advanced coherent formats.

Step 5: Set bias and modulation

Select a bias point above threshold for a directly modulated laser, leaving headroom below maximum safe current. Set modulation amplitude to meet the required optical modulation amplitude and extinction ratio without clipping or excessive chirp.

Step 6: Engineer the control loops

Provide automatic power control, temperature monitoring, fault shutdown, and wavelength control where required. Check loop stability across manufacturing and temperature variations.

Step 7: Verify experimentally

Measure optical spectrum, average power, extinction ratio, eye diagram, jitter, RIN, modulation response, wavelength drift, temperature performance, and bit-error rate. For WDM or coherent systems, additionally measure linewidth, side-mode suppression ratio, relative phase noise, and frequency accuracy.

Worked Example: Directly Modulated Laser Budget

A directly modulated laser has threshold current \(I_{th}=8\) mA and local slope efficiency \(S=0.20\) W/A. It is biased at \(I_b=20\) mA, and the modulation current has a peak value of 6 mA. Assume the linear model is valid and neglect coupling loss.

The bias optical power is

\[

P_b=S(I_b-I_{th})

=0.20(0.020-0.008)

=0.0024\text{ W}=2.4\text{ mW}.

\]

The optical modulation amplitude is

\[

\Delta P=S(0.006)=0.0012\text{ W}=1.2\text{ mW}.

\]

Thus, the approximate maximum and minimum powers are 3.6 mW and 1.2 mW, respectively, for a symmetric sinusoidal interpretation. For a binary current swing, the exact values depend on whether the stated modulation current is peak, peak-to-peak, or one-sided. This illustrates why transmitter specifications must define current conventions clearly. The calculated operating currents remain above threshold, but the designer must still check maximum current, thermal rollover, extinction ratio, and dynamic response.

Applied Case: WDM Metro Transmitter Upgrade

Background

Consider an illustrative metropolitan network carrying multiple 100-Gb/s channels over installed single-mode fibre. The original system uses fixed-wavelength directly modulated lasers. Increasing channel count has exposed wavelength drift, filter penalties, and limited upgrade flexibility.

Problem

The operator wants rapid wavelength provisioning, acceptable optical reach, and reduced spare inventory. The transmitter must operate over a broad environmental range and maintain stable output at WDM channel spacing.

Analysis

A fixed-wavelength VCSEL is unsuitable because the link is single-mode, the required reach is substantial, and the wavelength range is inappropriate. A directly modulated DFB laser offers lower cost but produces chirp and may suffer penalties through narrow WDM filters and accumulated dispersion. A tunable laser with an integrated external modulator provides wavelength flexibility and lower chirp, although it requires greater power, calibration, and control complexity.

Approach and solution

The proposed architecture uses a tunable narrow-linewidth laser, an integrated IQ or intensity modulator appropriate to the selected format, a wavelength locker, TEC control, monitor photodiode, and digital supervisory controller. The controller stores calibration data for tuning current, modulator bias, and output-power correction. Firmware prevents operation between stable wavelength states and reports wavelength-lock loss.

The optical budget is established first, followed by dispersion and nonlinear analysis. Laboratory characterization includes tuning repeatability, settling time, side-mode suppression, linewidth, output-power flatness, and bit-error rate across temperature. Field deployment includes connector inspection, optical safety procedures, and monitoring of wavelength drift.

Outcome

The tunable architecture reduces spare-part diversity and permits software-directed channel assignment. Its disadvantages are higher initial cost, increased electrical consumption, and the need for calibration and more sophisticated maintenance. The case demonstrates that source selection is a system-level decision: the least expensive source is not necessarily the least expensive network solution.

Learning points

1. WDM operation makes wavelength stability a primary transmitter requirement.

2. Chirp must be evaluated with fibre dispersion and filter characteristics.

3. Tunability has operational value but creates control and reliability obligations.

4. Verification must include environmental and manufacturing variation, not only nominal laboratory performance.

Contemporary Developments and Best Practices

Silicon photonics is enabling dense integration of lasers, modulators, couplers, and monitoring functions, although laser integration and thermal crosstalk remain important challenges. Heterogeneous integration combines materials selected for efficient light generation with silicon platforms suited to large-scale photonic routing. Integrated coherent transmitters increasingly use narrow-linewidth lasers, high-speed DACs, IQ modulators, and digital signal processing.

Machine learning can assist with predictive maintenance, wavelength-lock optimization, calibration reduction, and anomaly detection. It should not replace physical protection limits or validated control logic. Training data must cover temperature, ageing, component variation, and abnormal conditions; otherwise, an apparently intelligent controller may fail outside its training domain.

Best practices include:

• specify optical power, extinction ratio, and bandwidth at a defined reference plane;
• distinguish source limitations from package and driver limitations;
• characterize chirp and dispersion together;
• use calibrated instruments and document resolution bandwidth for spectral and RIN measurements;
• design thermal and automatic-power-control loops for worst-case conditions;
• include eye-safety, electromagnetic compatibility, and reliability requirements from the beginning;
• validate production tolerances, ageing, vibration, humidity, and temperature cycling;
• avoid operating a source near absolute current, voltage, or temperature limits;
• maintain traceable calibration and clear fault-reporting procedures.

Challenges and Limitations

No source simultaneously provides minimum cost, maximum power, zero chirp, unlimited bandwidth, perfect wavelength stability, and negligible power consumption. Semiconductor parameters vary with fabrication and ageing. The linear equations used in preliminary design become inaccurate near threshold, saturation, thermal rollover, clipping, and large-signal modulation. Datasheet values may refer to different measurement planes or test conditions. Furthermore, a transmitter that performs well in isolation may fail when connected to a particular fibre, filter, receiver, or package.

Consequently, transmitter engineering must combine analytical models, simulation, measurement, statistical tolerance analysis, and system-level validation. The appropriate design objective is not the best individual source metric but the best verified end-to-end performance at acceptable cost, power, reliability, and safety.

Chapter Summary

Optical transmitters convert electrical information into controlled optical signals. LEDs use spontaneous emission and provide low-cost, robust operation with limited bandwidth and broad spectra. Edge-emitting lasers provide high power and narrow spectra for demanding single-mode links. VCSELs are efficient, compact, and well suited to short-reach multimode and parallel optical systems. Tunable lasers add wavelength agility for WDM and reconfigurable networks.

Laser operation depends on stimulated emission and the condition that modal gain exceed internal and mirror losses. Important transmitter parameters include threshold current, slope efficiency, modulation bandwidth, extinction ratio, linewidth, chirp, RIN, wavelength stability, and thermal sensitivity. Direct modulation is compact and efficient but produces carrier-induced chirp. External modulation reduces chirp and improves linearity at the cost of insertion loss, drive complexity, and expense.

Reliable design requires source selection from link requirements, power and dispersion budgeting, impedance-controlled electrical design, thermal management, automatic power control, optical monitoring, safety controls, and environmental verification. Contemporary transmitters increasingly integrate photonics, electronics, calibration, and software supervision. Final performance must be assessed at the complete system level rather than from a single source specification.

Key Terms

Threshold current: Minimum laser injection current required for net optical gain to equal cavity loss.

Slope efficiency: Change in optical output power per unit change in drive current.

Spontaneous emission: Random photon emission caused by carrier recombination without an inducing photon.

Stimulated emission: Coherent photon emission induced by an incident photon.

LED: Semiconductor light source based primarily on spontaneous emission.

Edge-emitting laser: Laser that emits parallel to the semiconductor junction through a waveguide edge.

VCSEL: Vertical-cavity surface-emitting laser emitting perpendicular to the wafer surface.

DFB laser: Laser using a distributed grating to select and stabilize a longitudinal mode.

Tunable laser: Laser whose emission wavelength can be electronically or thermally varied.

Direct modulation: Modulation achieved by varying the source injection current.

External modulation: Modulation performed by a device separate from the continuous-wave laser.

Chirp: Time-dependent change in optical frequency during modulation.

RIN: Relative intensity noise, describing normalized optical-power fluctuations.

Linewidth: Spectral width associated with the optical phase-noise process.

Extinction ratio: Ratio of optical one power to optical zero power.

MZM: Mach–Zehnder modulator based on interference between two optical paths.

EAM: Electro-absorption modulator controlled through electrically varied absorption.

Optical confinement factor: Fraction of optical mode power overlapping the active gain region.

Automatic power control: Feedback system maintaining specified transmitter optical power.

Wavelength locker: Control subsystem that stabilizes a transmitter at a target optical wavelength.

Review Questions

Short Answer Questions

1. Distinguish spontaneous emission from stimulated emission.

2. Define threshold current and slope efficiency.

3. State two advantages and two limitations of VCSELs.

4. What is laser chirp, and why can it be harmful in fibre links?

5. Why is a monitor photodiode used in a practical transmitter?

Long Answer Questions

1. Compare LEDs, edge-emitting lasers, VCSELs, and tunable lasers for optical communication applications.

2. Explain the construction and operation of a DFB laser and discuss its use in WDM systems.

3. Describe direct and external modulation, including their performance and implementation trade-offs.

4. Discuss the effects of temperature, optical feedback, noise, and nonlinearities on transmitter performance.

5. Develop a complete preliminary design procedure for a high-speed optical transmitter.

Analytical / Application Questions

1. A laser has \(I_{th}=10\) mA, slope efficiency 0.25 W/A, and bias current 30 mA. Estimate its bias optical power using the linear model, and state the assumptions involved.

2. Select a source and modulation architecture for a 10-km single-mode WDM link requiring narrow spectral width and wavelength agility. Justify the selection against at least two alternatives.

3. A directly modulated transmitter shows acceptable eye opening at room temperature but severe closure at high temperature. Analyse the likely physical and circuit causes and propose a verification plan.

Multiple Choice Questions with Answers

Question 1

Which emission process is dominant in an LED?

A. Stimulated emission

B. Spontaneous emission

C. Thermionic emission

D. Field emission

Correct Answer: B. Spontaneous emission

Question 2

What is the principal function of a DFB grating?

A. Increase connector loss

B. Select and stabilize a longitudinal mode

C. Convert optical power into current

D. Provide electrical isolation from the driver

Correct Answer: B. Select and stabilize a longitudinal mode

Question 3

Which source is especially suitable for short-reach multimode-fibre links and optical arrays?

A. VCSEL

B. Tunable external-cavity laser only

C. LED with no waveguide

D. Photodiode

Correct Answer: A. VCSEL

Question 4

In direct modulation, which quantity is varied to modulate laser output?

A. Fibre length

B. Injection current

C. Receiver responsivity

D. Connector reflectivity only

Correct Answer: B. Injection current

Question 5

What is chirp?

A. A change in optical frequency during modulation

B. A permanent fibre break

C. A reduction in photon energy to zero

D. A type of optical connector

Correct Answer: A. A change in optical frequency during modulation

Question 6

For a laser with threshold current 8 mA, bias current 20 mA, and slope efficiency 0.20 W/A, what is the linear-model bias optical power?

A. 0.24 mW

B. 2.4 mW

C. 24 mW

D. 240 mW

Correct Answer: B. 2.4 mW

Question 7

Which device generally provides lower chirp than direct laser modulation?

A. External Mach–Zehnder modulator

B. Unbiased LED

C. Fibre connector

D. Thermistor

Correct Answer: A. External Mach–Zehnder modulator

Question 8

What does RIN describe?

A. Normalized optical-power fluctuations

B. Fibre attenuation only

C. Laser cavity length

D. The electrical supply voltage

Correct Answer: A. Normalized optical-power fluctuations

Question 9

Why is thermal control important in a WDM transmitter?

A. It eliminates the need for a receiver

B. It stabilizes wavelength and operating characteristics

C. It increases fibre core diameter

D. It changes single-mode fibre into multimode fibre

Correct Answer: B. It stabilizes wavelength and operating characteristics

Question 10

In the optical power budget, engineering margin is included primarily to accommodate

A. Uncertainty, ageing, and additional impairments

B. The speed of light in vacuum

C. The semiconductor band gap only

D. The number of wavelength channels without any loss

Correct Answer: A. Uncertainty, ageing, and additional impairments