UNIT 2: Optical Fibre Communication (EC-801) - Short Notes

A. FIBER OPTICS FUNDAMENTALS & WAVE GUIDING

Numerical Aperture (NA) & Acceptance Angle

• Definition: NA measures the light-gathering ability of an optical fiber. It is the sine of the maximum acceptance angle ($$\displaystyle \theta_0 $$) from air into the fiber core.

• Derivation (Step-Index Fiber - Ray Optics):

  1. For total internal reflection (TIR) at core-cladding interface: $$\displaystyle n_1 \sin \phi_c = n_2 $$, where $$\displaystyle \phi_c $$ is the critical angle.

  2. From geometry at fiber entrance (air, $$\displaystyle n_0=1 $$): $$\displaystyle \theta_0 + \phi = 90° \Rightarrow \sin \theta_0 = \cos \phi $$.

  3. Using $$\displaystyle \cos \phi = \sqrt{1 - \sin^2 \phi} $$ and $$\displaystyle \sin \phi = n_2/n_1 $$ (from TIR condition):

$$\sin \theta_0 = \sqrt{1 - \left(\frac{n_2}{n_1}\right)^2} = \frac{\sqrt{n_1^2 - n_2^2}}{n_1}$$

4.  Therefore, **NA = $$\displaystyle \sin \theta_0 = \sqrt{n_1^2 - n_2^2} $$**.

\boxed{NA = \sqrt{n_1^2 - n_2^2}}

• For other media (refractive index $$\displaystyle n_0 $$): $$\displaystyle NA = \sqrt{n_1^2 - n_2^2} / n_0 $$.

• Significance: Higher NA ⇒ larger acceptance angle ⇒ easier coupling of light into fiber ⇒ more light-gathering ability. However, high NA often means higher modal dispersion in multimode fibers.

• Relation to Relative Index Difference ($\Delta$): For small $$\displaystyle \Delta = (n_1 - n_2)/n_1 \ll 1 $$, $$\displaystyle NA \approx n_1 \sqrt{2\Delta} $$.

[!TIP] Common mistake: Forgetting that NA is defined for the outer medium (usually air). Always check if the outer medium is specified.

Modes of Propagation

• Planar Dielectric Waveguide (Rectangular Guide): Supports discrete guided modes. Each mode has a specific electric field distribution and cutoff condition determined by waveguide thickness, refractive indices, and wavelength.

• Guided Modes in Step-Index Circular Fiber: Solutions of Maxwell's equations in cylindrical coordinates. Characterized by mode numbers (LP$$\displaystyle _{lm} $$). Number of modes depends on Normalized Frequency (V-number).

• Normalized Frequency (V-number):

  • Definition: $$\displaystyle V = k_0 a \cdot NA = \frac{2\pi a}{\lambda} \cdot NA $$, where $a$ = core radius, $\lambda$ = wavelength.

  • Physical Meaning: Represents the number of wavelengths across the core diameter. Determines the number of guided modes.

  • Cut-off for Single-Mode: $$\displaystyle V < 2.405 $$ (fundamental LP$$\displaystyle _{01} $$ mode only).

  \boxed{V_{\text{cut-off}} = 2.405 \text{ for single-mode operation}}

  • Number of Guided Modes (Multimode Step-Index): $$\displaystyle M \approx \frac{V^2}{2} $$ (for large V).

• Mode Field Diameter (MFD): For single-mode fiber, it's the diameter where the optical power intensity falls to $$\displaystyle 1/e^2 $$ of its peak value. Slightly larger than the core diameter due to field penetration into cladding.

Ray Transmission & Analysis

• Step-Index Fiber Ray Paths:

  • Meridional Rays: Pass through the fiber axis. Simple zig-zag path. Useful for basic analysis.

  • Skew Rays: Do not pass through the axis; follow a helical path. Travel longer distance per reflection ⇒ higher dispersion.

  • Axial Rays: Travel nearly parallel to the axis (very small $\phi$). Minimal dispersion.

• Comparison with Wave Theory:

  • Ray Theory: Intuitive, simple for understanding acceptance angle, TIR, and modal dispersion. Fails for small core (single-mode) where wave nature dominates.

  • Wave Theory: Exact solution (solving wave equation). Essential for single-mode analysis, mode field diameter, waveguide dispersion, and bend loss prediction.

B. SIGNAL DEGRADATION MECHANISMS

Attenuation (Loss) Mechanisms

[!TIP] Rayleigh scattering is the fundamental loss limit in silica fibers (~0.15 dB/km at 1550 nm). OH⁻ absorption peak at 1383 nm is a key "water peak" to avoid.

Dispersion

• Intermodal Dispersion (Modal Dispersion):

  • Cause (Multimode Step-Index): Different modes travel different path lengths ⇒ different group delays.

  • Pulse Broadening (Δτ): For step-index fiber, $$\displaystyle \Delta \tau_{\text{inter}} \approx \frac{n_1 \Delta}{c} \cdot L $$, where $L$ = fiber length, $\Delta$ = relative index difference.

  • RMS Broadening: $$\displaystyle \Delta \tau_{\text{inter,rms}} \approx \frac{n_1 \Delta}{c\sqrt{3}} \cdot L $$.

  • Reduction: Use graded-index fiber (parabolic profile) to equalize mode group velocities.

• Intramodal Dispersion (Chromatic Dispersion):

  • Material Dispersion ($$\displaystyle D_{\text{mat}} $$): Wavelength dependence of core material refractive index ($$\displaystyle dn_1/d\lambda $$). Zero-dispersion wavelength $$\displaystyle \lambda_0 \approx 1310 $$ nm for standard silica.

  • Waveguide Dispersion ($$\displaystyle D_{\text{wg}} $$): Wavelength dependence of mode confinement (due to waveguide structure). Shifts zero-dispersion wavelength.

  • Total Chromatic Dispersion: $$\displaystyle D_{\text{total}} = D_{\text{mat}} + D_{\text{wg}} $$ (ps/(nm·km)).

• Overall Pulse Broadening & Bit Rate Limit:

  • Total RMS broadening: $$\displaystyle \Delta \tau_{\text{total}} = \sqrt{\Delta \tau_{\text{inter}}^2 + \Delta \tau_{\text{intra}}^2 + \Delta \tau_{\text{other}}^2} $$.

  • Maximum Bit Rate-Length Product (NRZ):

  \boxed{BL_{\text{max}} \approx \frac{0.44}{\Delta \tau_{\text{total}}}}

  • For RZ format: $$\displaystyle BL_{\text{max}} \approx \frac{0.7}{\Delta \tau_{\text{total}}} $$.

[!TIP] Single-mode fibers eliminate intermodal dispersion. Chromatic dispersion dominates. Dispersion-shifted fibers (DSF) move $$\displaystyle \lambda_0 $$ to 1550 nm.

C. FIBER FABRICATION & CHARACTERIZATION

Fiber Fabrication Techniques

• Modified Chemical Vapour Deposition (MCVD):

  1. Deposition: Reactant gases (SiCl₄, GeCl₄, etc.) flow through rotating silica tube; oxy-hydrogen torch heats tube wall → soot deposition.

  2. Sintering: High-temperature pass → porous soot densifies into glass.

  3. Collapse: Tube heated further → collapses into solid preform rod.

  • Advantages: Excellent refractive index profile control, high purity, low loss. Used for both multimode & single-mode.

• Outside Vapour Deposition (OVD) / Vapour Axial Deposition (VAD):

  • Principle: Soot (SiO₂ + dopants) deposited on a solid rod (OVD) or grown axially (VAD) from a burner.

  • Comparison: OVD/VAD can produce larger preforms faster than MCVD. MCVD offers finer index profile control. All methods achieve similar low losses today.

Optical Time-Domain Reflectometer (OTDR)

• Basic Principle: Injects short optical pulse, detects backscattered (Rayleigh) and reflected light. Time-of-flight analysis gives spatial loss profile.

• Working Mechanism:

  1. Launch high-power, narrow pulse into fiber.

  2. Backscattered/reflected light collected by directional coupler.

  3. Detector converts to electrical signal; time-gated to correlate with distance.

  4. Display: Power (dB) vs. Distance (km).

• Parameters Measured:

  • Attenuation Coefficient: Slope of backscatter curve.

  • Fault Location: Distance to reflection/break (from pulse delay).

  • Splice/Connector Loss: Step drop in backscatter level.

  • Fiber Length: Distance to fiber end (Fresnel reflection).

• OTDR Trace Interpretation:

  DiagramCANVAS: Sketch showing typical OTDR trace: distance on x-axis, loss (dB) on y-axis. Show linear backscatter slope (attenuation), sharp spike at connector (loss), large spike at fiber end (Fresnel reflection), and loss at splice (smaller step).

[!TIP] OTDR dead zone after a large reflection (like fiber end) limits ability to see events close by. Use longer pulse width for longer range but lower resolution.

D. OPTICAL SOURCES & DETECTORS

Light Emitting Diode (LED)

• Structure: p-n junction. Homojunction: Same material (e.g., GaAs). Heterojunction: Different bandgap materials (e.g., AlGaAs/GaAs) for better confinement & efficiency.

• Principle: Forward bias injects electrons & holes → recombination → spontaneous emission (isotropic, incoherent).

• Power Output & Efficiency:

  • Internal Optical Power: $$\displaystyle P_{\text{int}} = \frac{h\nu}{\tau_n} \cdot \Delta n \cdot V $$, where $$\displaystyle \tau_n $$ = carrier lifetime, $\Delta n$ = injected carrier density, $V$ = active volume.

  • External Power: $$\displaystyle P_{\text{ext}} = P_{\text{int}} \cdot (1 - r) \cdot \eta_{\text{ext}} $$, where $r$ = facet reflectivity, $$\displaystyle \eta_{\text{ext}} $$ = extraction efficiency.

  • Efficiencies: Internal ($$\displaystyle \eta_i $$), External ($$\displaystyle \eta_e $$).

• Characteristics: Broad spectrum (Δλ ~ 30-100 nm) ⇒ high chromatic dispersion. Moderate modulation bandwidth (~100 MHz). High reliability, long life.

• Comparison: Surface-emitting (Burrus type): Easy to couple to fiber, lower power. Edge-emitting: Higher power, directional, better coupling efficiency.

Lasers (Laser Diode & Principles)

• Basic Principles:

  • Stimulated Emission: Photon induces excited electron to emit identical photon (coherent, in-phase).

  • Population Inversion: More electrons in excited state than ground state (required for net gain).

  • Optical Feedback: Resonant cavity (mirrors) provides positive feedback → lasing.

  • Resonant Cavity Modes:

    • Longitudinal: Standing waves along cavity length $L$. Frequency spacing:

    \boxed{\Delta \nu = \frac{c}{2nL}}

    • Transverse: Field distribution across cavity cross-section (determined by waveguide).

• Laser Diode (LD):

  • Structure: Fabry-Perot (FP): Simple cleaved facets. DFB (Distributed Feedback): Grating along cavity for single-mode. DBR (Distributed Bragg Reflector): Gratings at ends.

  • Threshold Condition: Gain = total losses (mirror + internal). I-P curve: linear above threshold.

  • Modulation: Direct modulation: Vary pump current. Limited by relaxation oscillations and chirp (wavelength shift).

  • Advantages over LED: Higher power, narrow spectrum (Δλ ~ 1-2 nm), coherent, high modulation bandwidth (>10 GHz).

Photodetectors

• PIN Photodiode:

  • Structure: p⁺-i-n⁺. i-layer (intrinsic): Wide, depleted region → light absorption & carrier generation.

  • Working: Absorbed photon → e-h pair → drift in depletion field → current.

  • I-V Characteristics: Photovoltaic mode (zero bias): Generates voltage. Photoconductive mode (reverse bias): Faster response, lower capacitance.

  • Responsivity (R): $$\displaystyle R = \frac{I_{\text{ph}}}{P_{\text{opt}}} = \frac{\eta e}{h\nu} $$ (A/W), $\eta$ = quantum efficiency.

  • Quantum Efficiency (η): Ratio of generated carriers to incident photons.

    Derivation: $$\displaystyle \eta = \frac{\text{collected carriers/sec}}{\text{incident photons/sec}} = \frac{I_{\text{ph}}/(e)}{P_{\text{opt}}/(h\nu)} = \frac{R h\nu}{e} $$.

    In terms of lifetimes: $$\displaystyle \eta = \frac{\tau_c}{\tau_c + \tau_t} $$, where $$\displaystyle \tau_c $$ = carrier lifetime, $$\displaystyle \tau_t $$ = transit time.

• Avalanche Photodiode (APD):

  • Principle: High reverse bias → impact ionization → carrier avalanche multiplication (internal gain).

  • Multiplication Factor (M): $$\displaystyle M = \frac{I_{\text{ph,APD}}}{I_{\text{ph,PIN}}} $$. Depends on bias voltage.

  • Excess Noise Factor (F): $$\displaystyle F(M) \approx M^x $$ (x = 0.3-1, depends on ionization ratio).

  • Advantages: High sensitivity (gain 10-1000x). Disadvantages: Higher noise (excess noise), higher bias voltage, temperature sensitive.

  • Application: Long-haul, low-power systems where receiver sensitivity is critical.

[!TIP] PIN: Simple, low noise, moderate speed. APD: High gain but noisy. Choice depends on required sensitivity vs. bandwidth trade-off.

E. OPTICAL AMPLIFIERS

Erbium-Doped Fiber Amplifier (EDFA)

• Principle: Pump laser (980 nm or 1480 nm) excites Er³⁺ ions to higher energy levels → population inversion between metastable level (~1550 nm) and ground state. Signal photons (1550 nm) stimulate emission → amplification.

• Structure:

  DiagramCANVAS: Block diagram: Input signal + pump laser → WDM combiner → Er-doped fiber → isolator → amplified output. Show energy level diagram inset: pump absorption (980/1480), signal stimulated emission at 1550 nm.

• Derivations:

  • Amplifier Gain (G): $$\displaystyle G = \frac{P_s^{\text{out}}}{P_s^{\text{in}}} = \exp[(\sigma_e N_2 - \sigma_a N_1) \cdot L_{\text{eff}}] $$, where $$\displaystyle \sigma_e, \sigma_a $$ = emission/absorption cross-sections, $$\displaystyle N_1, N_2 $$ = population densities, $$\displaystyle L_{\text{eff}} $$ = effective length.

  • Conversion Efficiency (η): $$\displaystyle \eta = \frac{P_s^{\text{out}} - P_s^{\text{in}}}{P_{\text{pump}}} $$.

• Characteristics:

  • Gain Spectrum: ~1530-1565 nm (C-band), ~1565-1625 nm (L-band).

  • Noise Figure (NF): Typically 4-6 dB (due to amplified spontaneous emission - ASE).

  • Saturation Power: Output power at which gain compresses (~10-20 dBm).

Raman Amplifier

• Principle: Stimulated Raman Scattering (SRS). High-frequency pump photon scatters off fiber phonon → lower-frequency Stokes photon (signal) + phonon. Frequency shift ~13 THz (~100 nm at 1550 nm).

• Types:

  • Disributed Raman Amplifier (DRA): Pump co-propagates with signal over long fiber length → distributed gain.

  • Discrete Raman: Separate Raman gain module.

• Comparison with EDFA:

  | Feature | EDFA | Raman | | :--- | :--- | :--- | | Gain Bandwidth | Fixed (C/L-band) | Flexible (any band via pump λ) | | Noise Figure | Higher (4-6 dB) | Lower (theoretically 0 dB, practically ~3-5 dB) | | Polarization Dependence | Low | High (requires polarization diversity) | | Pump Power | Moderate (~100 mW) | High (>500 mW) | | Application | In-line amplification | Distributed gain, extended reach, multi-band |

F. OPTICAL LINK DESIGN & SYSTEM PERFORMANCE

Link Power Budget Analysis

Design Steps:

1. Requirements: Bit rate (B), distance (L), BER.

2. Total Loss (α_total): $$\displaystyle \alpha_{\text{total}} = \alpha_f \cdot L + \alpha_{\text{conn}} \cdot N_{\text{conn}} + \alpha_{\text{splice}} \cdot N_{\text{splice}} + \alpha_{\text{margin}} $$.

   • $$\displaystyle \alpha_f $$: fiber attenuation (dB/km)

   • $$\displaystyle \alpha_{\text{conn}} $$, $$\displaystyle \alpha_{\text{splice}} $$: per connector/splice loss (typical 0.3-0.5 dB)

   • $$\displaystyle \alpha_{\text{margin}} $$: safety margin (3-6 dB).

3. Transmitter Power (P_t): Must be ≥ Receiver Sensitivity (P_r) + α_total.

4. Power Margin: $$\displaystyle P_t - (P_r + \alpha_{\text{total}}) \geq 0 $$ dB.

Numerical Example:

• L = 50 km, α_f = 0.2 dB/km, N_conn = 4 (0.5 dB each), N_splice = 2 (0.1 dB each), margin = 3 dB.

• α_total = (0.2 × 50) + (0.5 × 4) + (0.1 × 2) + 3 = 10 + 2 + 0.2 + 3 = 15.2 dB.

• If P_r = -28 dBm, then required P_t ≥ -28 + 15.2 = -12.8 dBm.

System Performance & Rise-Time Budget

• Causes of Pulse Broadening:

  • Transmitter rise time ($$\displaystyle \tau_{\text{Tx}} $$)

  • Fiber dispersion ($$\displaystyle \tau_{\text{fiber}} $$)

  • Receiver rise time ($$\displaystyle \tau_{\text{Rx}} $$)

• Rise-Time Budget (RMS):

  \boxed{\frac{1}{\Delta \tau_{\text{total}}^2} = \frac{1}{\Delta \tau_{\text{Tx}}^2} + \frac{1}{\Delta \tau_{\text{fiber}}^2} + \frac{1}{\Delta \tau_{\text{Rx}}^2}}

  • $$\displaystyle \Delta \tau_{\text{fiber}} $$ = dispersion-induced broadening (from Section B).

• Maximum Bit Rate: $$\displaystyle B_{\text{max}} \approx \frac{0.7}{\Delta \tau_{\text{total}}} $$ (for NRZ, 70% rule).

Example Problem (May 2023):

• Given: $$\displaystyle \tau_{\text{Tx}} = 4 $$ ns, $$\displaystyle \tau_{\text{inter}} = 5 $$ ns, $$\displaystyle \tau_{\text{intra}} = 1 $$ ns, $$\displaystyle \tau_{\text{Rx}} = 2 $$ ns.

• $$\displaystyle \Delta \tau_{\text{fiber}} = \sqrt{\tau_{\text{inter}}^2 + \tau_{\text{intra}}^2} = \sqrt{25 + 1} = \sqrt{26} \approx 5.1 $$ ns.

• $$\displaystyle \Delta \tau_{\text{total}} = \sqrt{4^2 + 5.1^2 + 2^2} = \sqrt{16 + 26 + 4} = \sqrt{46} \approx 6.78 $$ ns.

• $$\displaystyle B_{\text{max}} \approx 0.7 / 6.78 \times 10^{-9} \approx 103 $$ MHz.

Power Penalties

• Definition: Additional power required above theoretical sensitivity to maintain BER due to non-ideal effects.

• Causes:

  • Modal Noise: Due to speckle pattern from multimode source.

  • Dispersion: Pulse broadening reduces eye opening.

  • Reflection: From connectors/air gaps → coherence-induced noise.

  • Interference (MPN): Mode partition noise in multimode lasers.

• Included in power budget: $$\displaystyle P_t \geq P_r + \alpha_{\text{total}} + \text{penalties} $$.

Digital Receiver Performance

• Q-Factor: $$\displaystyle Q = \frac{\mu_1 - \mu_0}{\sigma_1 + \sigma_0} $$, where $\mu$ = mean, $\sigma$ = std. dev. of "1" and "0" noise distributions.

• BER (Gaussian Approximation):

  \boxed{BER = \frac{1}{2} \text{erfc}\left(\frac{Q}{\sqrt{2}}\right)}

  • For $$\displaystyle Q=6 $$, BER ≈ $$\displaystyle 10^{-9} $$.

• Q in terms of SNR: $Q \approx \sqrt{SNR/2}$ for dominant noise.

• Noise Sources: Shot noise, thermal noise, relative intensity noise (RIN) from source.

G. MULTIPLEXING & NETWORK ARCHITECTURES

Wavelength Division Multiplexing (WDM)

• Principle: Combine multiple wavelengths (channels) onto single fiber → increases capacity.

• Components:

  • Multiplexer/Demultiplexer: AWG (Arrayed Waveguide Grating): Planar lightwave circuit, precise wavelength routing. Thin-film filter: Interference coatings. Grating: Bulk optic, dispersive.

• System Architecture: Transmitter (wavelength-specific laser) → MUX → EDFA/Raman amp → Fiber → DEMUX → Receiver.

• Applications: Capacity upgrade, wavelength routing in optical networks (OXC), analog video distribution.

Passive Optical Networks (PON)

• Architecture: Point-to-multipoint. OLT (Optical Line Terminal) at central office → ODN (Optical Distribution Network) (passive splitter) → multiple ONU/ONT (Optical Network Unit/Terminal) at user premises.

• Types:

  • APON/BPON: ATM-based, 155 Mbps down / 155 Mbps up.

  • EPON: Ethernet-based, 1 Gbps symmetric.

  • GPON: GFP framing, 2.5 Gbps down / 1.25 Gbps up.

• Multiple Access: TDMA (Time Division Multiple Access): ONUs transmit in assigned time slots to avoid collision.

SONET / SDH

• Architecture & Hierarchy:

  • Basic Frame: STS-1 (SONET) / STM-1 (SDH) = 810 bytes, 125 µs.

  • Rate Hierarchy: OC-n (SONET) / STM-n (SDH): OC-1=51.84 Mbps, OC-3=155.52 Mbps, OC-12=622 Mbps, OC-48=2.5 Gbps, OC-192=10 Gbps.

  • Frame Structure:

    DiagramCANVAS: Sketch STS-1/STM-1 frame: 9 rows × 90 columns. Highlight: Section Overhead (first 3 columns), Line Overhead (4th column), Path Overhead (last column), Payload (remaining).

• Working & Features:

  • Synchronous Multiplexing: Byte-interleaved from lower rates.

  • Network Elements: ADM (Add-Drop Multiplexer): Insert/drop specific channels. DCS (Digital Cross-Connect): Cross-connect STS-1s. DXC (Digital Exchange): Higher-order cross-connect.

  • Protection Schemes: Path protection (end-to-end), Line protection (1+1 or 1:1), Subnetwork connection protection.

  • Virtual Tributaries (VT): Sub-STS-1 containers (VT1.5, VT2, VT3, VT6) for mapping lower-rate signals (PDH, ATM).

[!TIP] SONET (US) and SDH (International) are nearly identical. Key is understanding frame structure, overhead bytes (for management, error monitoring), and protection switching.

H. CONNECTORS, SPLICES & PASSIVE COMPONENTS

Fiber Connectors

• Purpose: Provide temporary, repeatable connection between fibers.

• Requirements: Low loss (<0.3 dB), low back reflection (< -30 dB), environmental stability.

• Types:

  • SC (Subscriber Connector): Push-pull, snap-in, square ferrule. Common in datacom.

  • ST (Straight Tip): Bayonet-lock, round ferrule. Common in telecom.

  • FC (Ferrule Connector): Screw-on, high precision. Used in high-vibration environments.

  • LC (Lucent Connector): Small form-factor (like RJ-45), high density.

  • MTP/MPO: Multi-fiber (12, 24 fibers) rectangular connector. For high-density, parallel optics.

• Connector Loss Mechanisms: Misalignment (lateral, angular), Gap (Fresnel reflection), Ferrule end-face quality (scratches, dirt).

Fiber Splicing Techniques

• Classification:

  • Fusion Splicing: Permanent, low loss (<0.1 dB). Uses electric arc to melt fiber ends together.

  • Mechanical Splicing: Temporary, higher loss (~0.3 dB). Uses alignment sleeve (V-groove) and index-matching gel/epoxy.

• Fusion Splicing Process Steps:

  1. Fiber Preparation: Strip coating, clean, cleave (perfect 90° end-face).

  2. Alignment & Pre-fusion: Align cores (automatic alignment via cameras), pre-heat to clean.

  3. Arc Fusion & Cooling: Fuse with controlled arc, cool slowly.

• Splice Loss Parameters:

  • Insertion Loss: Power loss at splice (dB).

  • Return Loss: Reflection back toward source (dB). Fusion splices have high return loss.

Optical Couplers & Splitters

• Parameters:

  • Splitting Ratio: Power division (e.g., 50:50, 90:10).

  • Excess Loss: Total power lost in coupling process (dB).

  • Insertion Loss: Loss from input to a specific output port (dB).

  • Crosstalk (Isolation): Unwanted power from other channels (dB). Critical in WDM.

  • Polarization Dependence: Variation in loss with input polarization state (dB).

• Types:

  • Fused Biconical Taper (FBT): Two fibers fused & tapered → evanescent coupling. Used for 2×2 couplers, splitters.

  • Planar Lightwave Circuit (PLC): Waveguides on silica substrate. High stability, precise ratios, multi-channel (e.g., 1×N splitters for PON).

I. ADVANCED TOPICS & SYSTEM INTEGRATION

Micro-Electro-Mechanical Systems (MEMS) in OXC

• Principle: Micromirrors (2D or 3D arrays) on silicon substrate. Electrostatic actuation tilts mirrors → routes wavelengths to different ports.

• Benefits for WDM:

  • Low insertion loss (~1-2 dB).

  • High scalability (hundreds of ports).

  • Dynamic reconfiguration (wavelength routing, add/drop).

  • Polarization independent.

• Application: Optical Cross-Connect (OXC) nodes in core networks.

Eye Pattern Analysis

• Use: Assess digital transmission quality (dispersion, noise, jitter) on oscilloscope by overlaying many bits.

• Interpretation:

  • Eye Opening: Vertical (noise margin) & horizontal (timing jitter margin). Larger opening ⇒ better performance.

  • Jitter: Horizontal eye closure → timing errors.

  • Noise: Vertical eye closure → amplitude errors.

  • Distortion: Asymmetry, curvature.

  • BER Estimation: Q-factor from eye opening: $$\displaystyle Q \approx \frac{\text{vertical opening}}{\text{noise peak-to-peak}} $$.

• [[DIAGRAM: CANVAS: Ideal eye diagram (large open rectangle) vs. degraded eye (reduced opening, jitter, noise). Label key features: crossing points, eye height, eye width, jitter, noise.]

Case Studies / Historical Examples

• Dispersion Case History: Early 1980s multimode links (850 nm) limited to ~2 km due to intermodal dispersion. Solution: Graded-index fiber (Δ optimized) extended reach to ~10 km. Single-mode at 1300/1550 nm eliminated modal dispersion.

• Real-World Link Design Considerations:

  • Safety: Laser safety classes (1, 1M, 2, 2M, 3R, 3B, 4) → required labeling, interlocks.

  • Installation: Cable pulling tension, bend radius, temperature range.

  • Maintenance: OTDR testing, splice documentation, spare fiber management.

Summary of High-Yield Topics for Exams:

1. NA Derivation & Calculations (Always asked)

2. Link Power Budget (Step-by-step design problem)

3. SONET/SDH Architecture (Frame, overhead, hierarchy)

4. EDFA Principle & Derivation (Gain, efficiency)

5. Dispersion Types & Formulas (Intermodal, intramodal, BL product)

6. Bending Losses (Critical radius derivation)

7. OTDR Working & Trace Interpretation

8. LED vs. Laser (Structure, characteristics)

9. PIN vs. APD (Quantum efficiency, multiplication)

10. Raman vs. EDFA (Comparison)

11. Splicing Techniques (Fusion steps)

12. WDM Components & Applications

13. V-number & Single-mode Condition

14. Ray vs. Wave Theory