Fiber Optic Sources

In optical fiber communication systems, the choice of light source determines the bandwidth, transmission distance, and overall system performance. Two semiconductor devices are used as fiber optic sources: the Light Emitting Diode (LED) and the laser diode. While LEDs are simpler and cheaper, laser diodes are required for high-speed long-distance communication. Understanding the tradeoffs between them is important for both GATE and practical system design.

Core Concept of Light Coupling into Optical Fiber

For a light source to be effective in an optical fiber communication system, it must couple sufficient optical power into the fiber, modulate at high speeds to carry data, and maintain its spectral characteristics over the desired transmission distance. The numerical aperture (NA) of the fiber defines the acceptance cone angle for incoming light. A source with a narrow emission angle couples more power into the fiber, particularly into single-mode fiber which has an extremely small core diameter of approximately 8 to 10 micrometers.

LEDs emit light over a wide solid angle (Lambertian emission pattern) because spontaneous emission is random in direction. Only the fraction of this emission that falls within the fiber acceptance cone is coupled in. For a surface-emitting LED coupled to multimode graded-index fiber, the coupled power is proportional to (NA)^2. For a typical multimode fiber with NA = 0.2, only about 4% of the LED emission is coupled into the fiber. Edge-emitting LEDs have a narrower emission pattern and couple better to single-mode fibers than surface emitters.

Laser diodes, by contrast, emit a highly directional beam from the small active region facet. With proper lens coupling, over 80% of the laser output can be coupled into a single-mode fiber. This superior coupling efficiency, combined with much higher output power, makes laser diodes the only viable choice for long-haul single-mode fiber systems where the launched power budget determines maximum repeater spacing.

Mathematical Comparison of Bandwidth and Dispersion

The modulation bandwidth of an LED is limited by the minority carrier lifetime tau in the active region. The 3 dB electrical bandwidth is given by f_3dB = 1 / (2 * pi * tau). Typical minority carrier lifetimes in GaAs LEDs are 2 to 10 nanoseconds, giving bandwidths of 16 to 80 MHz. This limits LED-based systems to short-distance, moderate-data-rate links.

Laser diodes have far higher modulation bandwidth because stimulated emission is a much faster process than spontaneous emission. The bandwidth of a laser diode is primarily limited by relaxation oscillations, which occur at a resonance frequency f_r = (1/2*pi) * sqrt(G_N * S / tau_p), where G_N is the differential gain, S is the photon density, and tau_p is the photon lifetime. Typical laser diode bandwidths range from 5 to 30 GHz, enabling data rates well above 10 Gbit/s.

The spectral width of the source critically determines chromatic dispersion in the fiber. Chromatic dispersion causes different wavelengths to travel at different speeds, leading to pulse broadening. The pulse broadening is delta_T = D * L * delta_lambda, where D is the fiber dispersion coefficient (ps/nm.km), L is fiber length, and delta_lambda is the source spectral width. LEDs have delta_lambda of 30 to 60 nm, causing severe pulse broadening over just a few kilometers. Laser diodes have delta_lambda of 0.1 to 3 nm, enabling transmission over tens to hundreds of kilometers before dispersion becomes limiting.

Practical Understanding of LED and Laser Diode Applications

For short-distance applications such as local area networks, premises cabling, and industrial links up to 2 km at data rates below 100 Mbit/s, surface-emitting LEDs coupled to step-index multimode fiber are adequate and cost-effective. The high reliability, simple drive circuitry (no threshold current management), and longer lifetime of LEDs compared to laser diodes make them preferred in these applications.

For medium-distance multimode applications such as 1 Gbit/s Ethernet in data centers, vertical cavity surface emitting lasers (VCSELs) operating at 850 nm are the dominant choice. VCSELs combine the low cost and testability of surface emitters with the high speed and coherence of laser diodes. For long-haul single-mode applications (metro and backbone networks), distributed feedback (DFB) laser diodes operating at 1310 nm or 1550 nm are used, offering single longitudinal mode operation and extremely narrow linewidth.

The relative intensity noise (RIN) is an important practical parameter for laser diodes in analog fiber links (such as cable TV distribution). RIN is the mean square noise power normalized to the signal power, expressed in dB/Hz. Typical DFB laser RIN values are below -160 dB/Hz. LEDs have much higher RIN due to the random nature of spontaneous emission but are not used in analog links requiring high signal-to-noise ratio.

Given:
LED spectral width: delta_lambda = 40 nm
Laser diode spectral width: delta_lambda = 1 nm
Fiber dispersion coefficient D = 17 ps/(nm.km) at 1550 nm
Fiber length L = 50 km
Initial pulse width: tau_0 = 100 ps

Why this formula applies:
Chromatic dispersion broadens pulses. Pulse broadening is found from
delta_T = D * L * delta_lambda.
Total broadened pulse width (RMS): tau_out = sqrt(tau_0^2 + delta_T^2)

Formula:
delta_T = D * L * delta_lambda
tau_out = sqrt(tau_0^2 + delta_T^2)

For LED:
delta_T_LED = 17 * 50 * 40 = 34000 ps = 34 ns
tau_out_LED = sqrt((100e-12)^2 + (34e-9)^2) ≈ 34 ns (dominated by dispersion)

For Laser:
delta_T_laser = 17 * 50 * 1 = 850 ps
tau_out_laser = sqrt((100e-12)^2 + (850e-12)^2) = sqrt(1e-20 + 7.225e-19)

Calculation:
tau_out_laser = sqrt(7.325e-19) ≈ 856 ps

Final Answer:
LED output pulse width = ~34 ns (unusable at high data rates over 50 km)
Laser output pulse width = ~856 ps (acceptable for Gbit/s transmission)

Exam Tip: In GATE fiber optics problems, pulse broadening due to chromatic dispersion is delta_T = D * L * delta_lambda. A common trap is using wavelength in nm but length in km and forgetting the units come out in ps, so always track units carefully. Also remember that a narrower source spectrum directly reduces dispersion.

• LEDs produce spontaneous emission with wide spectral width (30-60 nm), leading to severe chromatic dispersion-induced pulse broadening over long fiber lengths.
• Laser diodes produce stimulated emission with narrow spectral width (0.1-3 nm), drastically reducing chromatic dispersion and enabling long-haul, high-speed transmission.
• LED modulation bandwidth is limited by carrier lifetime (f = 1/2*pi*tau), typically giving 16-80 MHz, suitable only for short links under 2 km.
• Laser diode bandwidth is limited by relaxation oscillation frequency, typically 5-30 GHz, enabling data rates above 10 Gbit/s.
• Coupling efficiency into single-mode fiber is much higher for laser diodes (over 80%) compared to LEDs (under 5%) due to directional emission from the laser.

Quick Revision

• LED: spontaneous emission, broad spectrum (30-60 nm), wide beam angle, low coupling efficiency into SMF, bandwidth ~10-100 MHz, short-range only.
• Laser diode: stimulated emission, narrow spectrum (0.1-3 nm), directed beam, high coupling into SMF, bandwidth 5-30 GHz, long-range applications.
• Pulse broadening formula: delta_T = D * L * delta_lambda. Units: ps when D in ps/nm.km, L in km, delta_lambda in nm.
• LED bandwidth: f_3dB = 1/(2*pi*tau), limited by minority carrier lifetime tau.
• VCSEL: surface-emitting laser used in 850 nm multimode data center links; combines low cost with high speed.
• DFB laser: used in 1310/1550 nm single-mode long-haul links; single longitudinal mode, very narrow linewidth.
• GATE trap: Chromatic dispersion depends on source spectral width, not just fiber. Wider source = more dispersion even if fiber is the same.