Diodes used in optical modules

Optical modules primarily use laser diodes (LDs) or LEDs for transmission and photodiodes (PIN or APD) for reception.

Transmitting Diodes

The Transmitter Optical Sub-Assembly (TOSA) in optical modules converts electrical signals into optical signals using a light source. The most commonly used diodes are:

• Laser Diodes (LDs): These are the dominant choice due to their high output power, low power consumption, and efficient coupling into optical fibers. LDs produce coherent, focused light suitable for high-speed and long-distance communication .
• Light-Emitting Diodes (LEDs): Used for low-rate, short-distance applications, LEDs are cost-effective and have a longer lifespan, though they offer lower output power and broader spectral width compared to LDs .

Types of Laser Diodes

1. VCSEL (Vertical-Cavity Surface-Emitting Laser): Emits light perpendicular to the chip surface, typically at 850 nm, ideal for short-reach, high-speed links like data center interconnects. Advantages include low power consumption and cost-effectiveness .
2. FP (Fabry-Pérot) Laser: Emits multiple wavelengths in a horizontal cavity, commonly at 1310 nm or 1550 nm, suitable for medium-distance communication (up to ~20 km).
3. DFB (Distributed Feedback) Laser: Features an internal grating for single-mode, stable emission, used in long-haul networks (40 km+), offering narrow spectral width and precise wavelength control .
4. EML (Electro-Absorption Modulated Laser): Combines a laser diode with an integrated modulator, typically at 1550 nm, optimized for high-speed, long-distance data center interconnects .

Receiving Diodes

The Receiving Optical Sub-Assembly (ROSA) converts optical signals back into electrical signals using photodiodes:

• PIN Photodiodes: Standard photodetectors that convert light into current, suitable for most optical communication applications .
• Avalanche Photodiodes (APDs): Highly sensitive photodiodes that use avalanche multiplication to amplify the photocurrent, improving receiver sensitivity by 6–10 dB compared to PIN diodes, ideal for weak signal detection in long-distance or high-speed links .

Key Considerations for Photodiodes

• Wavelength Responsivity: Must match the transmitted light wavelength (e.g., 850 nm, 1310 nm, 1550 nm).
• Response Speed and Bandwidth: High-speed communication requires photodiodes with GHz-level bandwidth and nanosecond response times.
• Noise and Dynamic Range: Low dark current and thermal noise are critical for precise signal detection .

Summary

In optical modules, laser diodes (VCSEL, FP, DFB, EML) or LEDs serve as the transmitting diodes, while PIN or avalanche photodiodes are used for reception. The choice depends on distance, speed, power efficiency, and cost, with laser diodes dominating high-speed and long-distance applications, and LEDs or PIN diodes used for simpler, short-range links .