Basic Operation and Types of LED Light Sources Used in Fiber Optic Communications

While the previous discussion on LED transmitters focused on performance characteristics, this article takes a step back to explain LED light sources from a more foundational, structural perspective: how they physically work at the semiconductor level, and — critically — the different types of LED structures engineered for fiber optic use. Understanding these distinctions helps explain why certain LEDs are better suited to certain fiber types and applications than others.

Revisiting the Basics: What Is an LED?

A Light Emitting Diode (LED) is a semiconductor device that emits light when an electrical current passes through it, through the process of spontaneous emission. At its core, an LED is simply a diode — a p-n junction — engineered from materials chosen specifically because, when electrons and holes recombine at that junction, the released energy predominantly takes the form of light (photons) rather than heat.

The p-n Junction and Light Emission

• N-type material has an excess of free electrons.
• P-type material has an excess of “holes” (the absence of an electron, which behaves like a positive charge carrier).
• When these two materials are joined and a forward voltage is applied, electrons flow from the n-side and holes flow from the p-side into the junction region.
• At the junction, electrons and holes recombine. In materials called direct bandgap semiconductors (such as Gallium Arsenide, GaAs, and related compounds), this recombination releases energy predominantly as a photon of light, with a wavelength determined by the specific bandgap energy of the material.
• Materials like silicon are indirect bandgap semiconductors, where recombination mostly releases energy as heat (phonons) rather than light — which is why silicon, despite being the backbone of the electronics industry, is not used to make LEDs or laser diodes.

Why the Material Matters: Choosing the Wavelength

The specific semiconductor compound used to build an LED determines its emission wavelength, which must be chosen to match the low-attenuation “windows” of optical fiber:

• Gallium Arsenide (GaAs) and related compounds are commonly used for LEDs emitting around 850 nanometers, matching one of the traditional low-loss windows for multimode fiber.
• Indium Gallium Arsenide Phosphide (InGaAsP) is commonly used for LEDs emitting around 1300 nanometers, another important low-attenuation and low-dispersion window, especially relevant for longer multimode or certain single-mode-adjacent applications.

By carefully engineering the exact composition of these compound semiconductors, manufacturers can “tune” the emission wavelength of an LED to match the specific requirements of a fiber optic system.

Two Fundamental Types of LED Structures

Beyond the choice of semiconductor material, LEDs used in fiber optics come in two principal structural types, distinguished by how and where the light is emitted relative to the device geometry.

1. Surface-Emitting LED (SLED)

A Surface-Emitting LED emits light perpendicular to the plane of the semiconductor wafer, from a relatively wide emitting area on the surface of the device.

Key characteristics:

• Simple structure: relatively straightforward and inexpensive to manufacture.
• Wide emission angle: light radiates over a broad range of angles (a wide radiation pattern, often described using a “Lambertian” emission profile), leading to lower coupling efficiency into fiber.
• Larger emitting area: while this makes manufacturing easier, it also further reduces coupling efficiency, particularly into smaller-core fibers.
• Lower modulation bandwidth in typical designs, though certain optimized SLED designs (like the Burrus-type surface emitter, which uses a well etched into the substrate to bring the fiber physically closer to the active emitting region) can improve both coupling efficiency and speed somewhat.

Typical use: shorter-distance, lower-speed multimode fiber links where coupling efficiency loss is acceptable and low cost is a priority.

2. Edge-Emitting LED (ELED)

An Edge-Emitting LED emits light from the edge (side) of the semiconductor chip, guided by an internal waveguide structure built into the device — conceptually similar in geometry (though not in emission mechanism) to how an edge-emitting laser diode is structured.

Key characteristics:

• Narrower emission angle compared to surface emitters, because the internal waveguide structure helps direct the light more before it exits the device, resulting in better coupling efficiency into optical fiber.
• Smaller effective emitting area, further improving coupling efficiency, particularly beneficial when coupling into smaller-core fibers.
• Generally faster modulation response than surface emitters, because the waveguide structure and typically thinner active region can support somewhat faster carrier dynamics.
• More complex and expensive to manufacture than surface emitters, due to the more intricate internal waveguide structure required.

Typical use: applications requiring somewhat better performance (higher coupling efficiency, faster modulation) than a basic surface emitter can provide, while still remaining simpler and cheaper than a full laser diode solution.

Diagram: Surface-Emitting vs Edge-Emitting LED Structure

graph TB
    subgraph Surface-Emitting LED
    S1[Semiconductor Substrate] --> S2[Active Region]
    S2 --> S3["Wide-angle light emission<br/>(perpendicular to wafer surface)"]
    end
    subgraph Edge-Emitting LED
    E1[Semiconductor Substrate] --> E2[Active Region + Internal Waveguide]
    E2 --> E3["Narrower-angle light emission<br/>(from the edge/side of the chip)"]
    end

Comparison Table: Surface-Emitting vs Edge-Emitting LEDs

LED Emission Pattern: Understanding Lambertian Radiation

Most surface-emitting LEDs follow what’s called a Lambertian emission pattern — the light intensity is strongest directly perpendicular to the emitting surface and falls off following a cosine relationship as the viewing angle increases away from that perpendicular direction. This wide, gradually-tapering spread of light is a fundamental reason why surface-emitting LEDs couple relatively inefficiently into optical fiber compared to the tightly focused, directional output of a laser or even an edge-emitting LED.

Wavelength Windows and Why They Matter

Optical fiber has specific wavelength ranges where attenuation (signal loss per unit distance) is minimized, historically referred to as transmission “windows”:

• ~850 nm window: historically the first commonly used window, well-suited to GaAs-based LEDs and cost-effective for multimode fiber short-reach links.
• ~1300 nm window: offers lower attenuation and — importantly for LED-based systems — a point of minimal chromatic dispersion in standard fiber, making it attractive for LED sources despite their wide spectral width, since dispersion effects are naturally minimized at this wavelength in conventional fiber designs.
• ~1550 nm window: offers the lowest attenuation of all, but is used almost exclusively with laser sources (not LEDs) for long-haul, high-speed applications, since it doesn’t offer the same natural dispersion-minimization benefit that 1300nm does, and long-haul applications demand the narrow spectral width only lasers can provide.

This is precisely why LED-based systems are historically strongly associated with the 850nm and 1300nm windows, while high-performance laser-based long-haul systems dominate at 1550nm.

Manufacturing Considerations: Why LEDs Remain Cost-Effective at Scale

Part of the enduring relevance of LED light sources, even in an industry increasingly dominated by laser-based systems, comes down to manufacturing economics. Semiconductor LEDs, particularly surface-emitting designs, can be tested and characterized while still part of the full semiconductor wafer, before the wafer is diced into individual chips — a process that catches defective units early and keeps per-unit costs low at high production volumes. Laser diodes, by contrast, generally require more precise cavity structures (whether cleaved facets for edge-emitters or grown mirror structures for VCSELs) and tighter manufacturing tolerances to achieve reliable single-mode or narrow-spectral-width operation, both of which add cost. For applications where an LED’s characteristics — moderate speed, moderate distance, wide multimode coupling — are entirely adequate, this manufacturing cost advantage keeps LED-based transmitters commercially viable and, in many cost-sensitive product categories, the default choice, decades after laser technology became dominant in premium, high-speed applications.

Real-World Standards Using LED Light Sources

• 10BASE-FL (10 Mbps Ethernet over fiber): typically used 850nm LED sources over multimode fiber.
• 100BASE-FX (100 Mbps Fast Ethernet over fiber): commonly used 1300nm LED sources over multimode fiber.
• FDDI (Fiber Distributed Data Interface): used 1300nm LED sources over multimode fiber for its original specification.
• Various industrial and building automation fiber optic protocols: many continue to use simple, robust LED sources for cost and reliability reasons in applications where extreme speed isn’t required.

Comparing LED Structures to Their Laser Diode Counterparts

It’s useful to briefly connect the surface-emitting versus edge-emitting LED distinction covered in this article to the laser diode types discussed elsewhere in this series, since the geometric concepts echo each other even though the underlying emission physics differ fundamentally. Just as a VCSEL (Vertical-Cavity Surface-Emitting Laser) emits perpendicular to the wafer similar in geometry to a surface-emitting LED, and edge-emitting laser diodes (like Fabry-Pérot and DFB lasers) emit from the chip’s edge similar in geometry to an edge-emitting LED, the industry has generally found that whichever geometric approach works well for controlling and directing light output tends to be explored for both LED and laser device families, even though LEDs rely on spontaneous emission and lasers rely on stimulated emission as their fundamentally different underlying mechanisms. Recognizing this parallel can help newcomers to fiber optics build a more unified mental model of light source engineering, rather than treating LED and laser technology as entirely disconnected topics.

Best Practices

1. Choose edge-emitting LEDs over surface-emitting LEDs when coupling efficiency or slightly higher modulation speed is important, and the added cost is justified by the application’s requirements.
2. Match the LED’s wavelength to the fiber’s low-attenuation window — 850nm or 1300nm LEDs paired with multimode fiber designed for those windows, verified against the specific fiber’s datasheet.
3. Always use multimode fiber with LED sources, given their inherently wide emission angle and poor coupling efficiency into the much smaller core of single-mode fiber.
4. Don’t expect LED-based systems to compete with laser-based systems on distance or speed — they occupy a specific, valuable niche of cost-effective, reliable, shorter-distance, lower-speed connectivity.
5. For new installations requiring higher speed or distance, prefer laser-based (typically VCSEL for short multimode links, DFB for long single-mode links) transceivers rather than legacy LED technology.
6. When troubleshooting or auditing older fiber infrastructure, check documentation or datasheets to confirm whether installed transceivers are LED-based or laser-based, since this materially affects what upgrade paths and distance/speed limits apply.

Linux Example: Identifying Legacy LED-Based Interfaces via Driver/Module Info

# List detailed information about a network interface, which can sometimes reveal
# the transceiver/module type in the driver info or dmesg logs
ethtool -i eth1

# Check kernel logs for messages about the transceiver/module type detected at boot or hotplug
dmesg | grep -i sfp

# Example possibly relevant output:
# [   12.345678] sfp sfp-eth1: Host maximum power 1.0W
# [   12.349012] sfp sfp-eth1: module vendor-name (vendor-oui) rev A, 100BASE-FX

Cisco Example: Checking Legacy Fiber Port Media Type

Switch# show interfaces FastEthernet0/12

FastEthernet0/12 is up, line protocol is up
  Hardware is Fast Ethernet, address is 0011.2233.4455
  MTU 1500 bytes, BW 100000 Kbit, DLY 100 usec,
     reliability 255/255, txload 1/255, rxload 1/255
  Full-duplex, 100Mb/s, media type is 100BaseFX
  input flow-control is off, output flow-control is off

Seeing 100BaseFX on an older switch is a strong hint that the port likely uses an LED-based transmitter operating around 1300nm — useful context when planning whether the port and its cabling need upgrading for higher-speed requirements.

Python Example: Simple Wavelength-to-Application Lookup Tool

def led_wavelength_recommendation(application):
    """
    Simple educational reference tool mapping common LED-based fiber
    applications to their typical wavelength window.
    """
    mapping = {
        "10base-fl": ("850 nm", "Multimode", "Early 10 Mbps Ethernet over fiber"),
        "100base-fx": ("1300 nm", "Multimode", "100 Mbps Fast Ethernet over fiber"),
        "fddi": ("1300 nm", "Multimode", "Fiber Distributed Data Interface backbone"),
        "industrial-short-link": ("850 nm", "Multimode", "Cost-sensitive industrial/building automation link"),
    }

    result = mapping.get(application.lower())
    if not result:
        return "Application not recognized in this simple reference tool."

    wavelength, fiber_type, description = result
    return (f"Application: {application}\n"
            f"  Typical LED Wavelength: {wavelength}\n"
            f"  Fiber Type: {fiber_type}\n"
            f"  Description: {description}")


for app in ["10base-fl", "100base-fx", "fddi"]:
    print(led_wavelength_recommendation(app))
    print()

Troubleshooting Guide

Case Study: Auditing a Legacy Industrial Fiber Network

A manufacturing plant running a 20-year-old FDDI-derived industrial fiber network for its factory floor sensors and PLCs began experiencing intermittent communication faults as production expanded and additional equipment was added to the same fiber backbone. The maintenance team initially suspected the fiber cable itself had degraded, but a careful audit using an optical power meter at each LED-based transmitter revealed the actual problem: several of the original edge-emitting LED transmitters, now approaching two decades of continuous operation, were producing measurably lower output power than their original datasheet specification, gradually eroding the link power budget margin as more splices and connectors had been added to the network over the years to accommodate plant expansion.

Rather than replace the entire network — a costly undertaking for a facility with limited downtime windows — the team took a targeted approach: they used optical power meter readings to identify the specific LED transmitters whose output had degraded closest to the minimum usable threshold, replaced just those units with new-manufacture equivalents, and used the freed budget to also clean and re-terminate connectors on the longest fiber runs, recovering additional margin. This selective approach restored full network reliability without a wholesale rip-and-replace, and it highlights a genuinely useful lesson about LED light sources: their long but finite operational lifespan, combined with gradual power degradation rather than sudden failure, means periodic power budget audits — not just reactive troubleshooting after a failure — are the right maintenance strategy for LED-based industrial fiber infrastructure.

Frequently Asked Questions

Can an LED transmitter be upgraded to a laser transmitter without replacing the fiber? Often yes, provided the existing fiber is multimode and the new laser-based optic (commonly VCSEL-based) is designed for multimode fiber and the same wavelength window; however, distance and power budget should always be recalculated, since laser-based optics behave differently from LED-based ones even over the same physical fiber.

Why don’t manufacturers just always use edge-emitting LEDs since they perform better than surface emitters? Cost and manufacturing complexity — surface-emitting LEDs remain attractive for high-volume, cost-sensitive applications where the performance gap doesn’t matter, such as short industrial links or basic building automation, where edge-emitting LEDs’ added expense isn’t justified by the application’s requirements.

Is it true that LEDs can’t be damaged by looking directly at them, unlike lasers? Not entirely — while LEDs are generally lower risk than lasers due to lower power density and wider beam divergence, some higher-power LED sources can still exceed safe viewing thresholds, so the universal safety habit of never staring directly into any energized fiber connector still applies regardless of source type.

What’s the practical maximum data rate for an LED-based fiber link today? While historically LED-based standards topped out around 100-155 Mbps for common standards like 100BASE-FX and early FDDI, some specialized modern LED designs optimized specifically for speed can reach into the several-hundred-Mbps to low-Gbps range in short-reach applications like certain plastic optical fiber systems, though this remains well below what VCSEL or laser-based systems achieve.

How do I know if an unlabeled legacy fiber port uses an LED or a laser source? Check the interface’s documented media type (such as 100BASE-FX, which is historically LED-based) and, where possible, verify with an optical power meter — LED sources typically show noticeably lower output power (often below -10 dBm) compared to laser-based optics operating in the same port form factor.

Conclusion

LED light sources, while technologically simpler than their laser diode cousins, represent a thoughtfully engineered and still-relevant category of fiber optic transmitter. Understanding the distinction between surface-emitting and edge-emitting LED structures, the role of semiconductor material choice in determining emission wavelength, and how these factors interact with fiber type and application requirements gives network professionals the foundation needed to correctly specify, deploy, and troubleshoot LED-based fiber optic systems — technology that, despite the dominance of laser-based high-speed networking today, remains a practical and cost-effective choice for a meaningful slice of real-world applications.

Further Reading and References

• ITU-T G.651: Characteristics of Multimode Optical Fibre Cables
• IEEE 802.3 Ethernet Standards (10BASE-FL, 100BASE-FX)
• TIA-568-C Optical Fiber Cabling Standards
• Fiber Optic Association Technical Reference
• Cisco Legacy Fast Ethernet Fiber Interface Documentation
• Linux ethtool documentation