The transmitter side of a fiber link is an electrical-to-optical converter: take a stream of bits, produce a stream of photons. Two main families of devices: LEDs and laser diodes.
5.1 LEDs: cheap, broad, slow
We met LEDs in Chapter 0. A forward-biased direct-band-gap diode emits photons by spontaneous emission. For fiber:
- Surface-emitting LED (SLED): emits roughly isotropically from the top surface. Easy to make, low coupling efficiency to fiber.
- Edge-emitting LED (ELED): emits from a narrow stripe on the cleaved edge of the chip. Better fiber coupling, slightly more directional.
LED specs for fiber communications:
- Wavelength: typically 850 or 1300 nm (GaAs and InGaAsP based).
- Spectral width: 30 to 100 nm full-width-half-max. Wide because it is incoherent spontaneous emission.
- Modulation bandwidth: under 200 MHz, typically.
- Coupling efficiency to multimode fiber: 1 to 10%.
- Coupling efficiency to single-mode fiber: less than 0.1%. Essentially unusable.
Used in: short-reach multimode links, plastic optical fiber systems, very cost-sensitive applications, and historically in 100Base-FX Ethernet (now mostly displaced by VCSELs).
5.2 Laser diodes: coherent, narrow, fast
A laser diode is a forward-biased diode operated above threshold, where stimulated emission dominates over spontaneous emission and the cleaved facets of the chip form an optical cavity. We covered the physics in Chapter 0. The key practical differences from an LED:
- Coherent: photons are in phase, not random.
- Narrow spectrum: typical 0.001 to 1 nm linewidth, depending on laser type.
- Directional: emission concentrated in a narrow beam, much better fiber coupling.
- Fast: can be modulated up to tens of GHz (directly), and to 100+ GHz with external modulators.
Below threshold, the laser acts like a noisy LED. Above threshold, output power rises linearly with current. The threshold is typically 5 to 20 mA; operating currents are 30 to 100 mA.
5.3 Laser families
Three architectures are common in fiber-optic systems, each filling a different niche.
Fabry-Perot (FP) lasers: the simplest. A straight-stripe diode with cleaved facet mirrors. The cavity supports many longitudinal modes spaced by . Multiple modes lase simultaneously, giving a few-nm spectral width. Cheap, used in 1310 nm short-reach single-mode links, but too broad for DWDM.
Distributed-feedback (DFB) lasers: a Bragg grating etched along the length of the cavity selects a single longitudinal mode. The result is single-frequency operation with linewidth of a few MHz to a few hundred kHz, spectral width well under 0.1 nm. The standard source for long-haul and DWDM systems at 1310 and 1550 nm. Tunable DFBs (with adjustable grating temperature) allow one laser to address any of 96 DWDM channels.
Vertical-cavity surface-emitting lasers (VCSELs): a totally different geometry where the laser emits perpendicular to the wafer surface, with mirrors made of distributed Bragg reflectors above and below a thin gain region. VCSELs are cheap to manufacture (test on the wafer before dicing), low-power, and emit a circularly symmetric beam that couples well to multimode fiber. VCSELs at 850 nm dominate datacenter short-reach optics: every 100/400 GbE switch in a hyperscale datacenter has dozens of VCSELs. Long-wavelength VCSELs at 1310 nm have begun to penetrate single-mode datacenter applications.
FP cavity: DFB cavity: VCSEL:
┌──────────────────────┐ ┌──────────────────────┐ top mirror
│ gain stripe │ │ ▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓ │ ───────
│ │ ←→ │ Bragg grating along │ ←→ gain
└──────────────────────┘ └──────────────────────┘ ───────
cleaved facet mirrors feedback selects 1 mode bottom mirror
│ light
▼ outTunable lasers: external-cavity lasers (ECL) and tunable DFBs let one laser cover an entire DWDM band. Coherent transponders use tunable lasers so a single SKU can be deployed to any wavelength slot in the network.
5.4 Direct vs external modulation
To put data on the laser, you have two options:
Direct modulation: vary the laser drive current at the bit rate. Simple, cheap. Limitation: changing the carrier density also chirps the wavelength (a phenomenon called frequency chirp), which interacts badly with chromatic dispersion at high bit rates and long distances. Practical limit around 10 Gbps over 100 km of standard SMF.
External modulation: keep the laser CW, then modulate the light through a separate device.
- Mach-Zehnder modulator (MZM) based on lithium niobate (LiNbO): split the light into two arms, apply a phase shift in one (controlled by an electric field through the electro-optic effect in LiNbO), then recombine. In-phase recombination gives full output; out-of-phase gives null. Bandwidths of 40 GHz and beyond, very low chirp. Standard for long-haul transmitters.
- Electro-absorption modulator (EAM) integrated alongside a DFB laser on the same InP chip (an EML, electro-absorption modulated laser). More compact and cheaper than discrete MZM, used in many 10 to 50 Gbps client-side optics.
- Silicon photonic Mach-Zehnder modulators: same MZM principle but in silicon, exploiting the plasma-dispersion effect of carriers injected into a waveguide. The basis of silicon-photonic transceivers from Intel, Cisco, Marvell.
The relevant phenomenon — that an applied voltage changes the refractive index of a crystal — is the Pockels effect or linear electro-optic effect. It is essentially instantaneous, so MZM can modulate at very high speeds.
5.5 The eye diagram and chirp
Plotting the received electrical waveform overlapped over many bit periods gives an eye diagram: ones and zeros stack into a pattern with a clear "eye" of vertical and horizontal margin. A clean transmitter and clean fiber give a wide-open eye. Chromatic dispersion, chirp, and noise close it. Much of receiver design is the art of placing decision thresholds so as to maximize tolerance to a partially-closed eye.
We will simulate an eye diagram in Section 12.