Technical parameters of the optical transmitter

Optical transmitters convert electrical signals into optical signals, with key specifications including light source type, wavelength, output power, modulation method, and operating voltage.

Light Source

Optical transmitters use either laser diodes (LDs) or light-emitting diodes (LEDs) as the light source. Laser diodes are preferred for high-speed, long-distance transmission due to their higher output power, narrower spectral width, and better coupling efficiency, while LEDs are suitable for low-rate, short-distance applications because of their lower cost and longer lifespan .

Wavelength

The operating wavelength depends on the fiber type and application. Common wavelengths include 650 nm for plastic optical fibers, and 850 nm, 1310 nm, or 1550 nm for single-mode or multimode glass fibers. The wavelength affects attenuation and dispersion in the fiber .

Output Power

The launched optical power is a critical parameter, typically expressed in dBm. It determines how much fiber loss can be tolerated and ensures reliable signal transmission over the desired distance. Laser-based transmitters generally provide higher output power than LEDs .

Modulation Method

Transmitters modulate the light source to encode the electrical signal onto the optical signal. Common modulation techniques include direct modulation of the laser diode or external modulation using an optical modulator. The modulation rate defines the data transmission speed, which can reach several gigabits per second .

Operating Voltage and Power

Typical optical transmitters operate at +3 to +5 V, with low dissipation current to maintain efficiency and steady light output. Integrated driver circuits are used to control the light source and ensure proper modulation .

Additional Components

Modern transmitters often include a Transmitter Optical Sub-Assembly (TOSA), which integrates the light source, optical interface, monitoring photodiode, housing, and electrical interface. An automatic optical power control (APC) circuit is commonly included to maintain consistent output power .

Summary

Key specifications to consider when selecting or designing an optical transmitter include:

• Light source type: Laser diode or LED
• Wavelength: 650 nm, 850 nm, 1310 nm, 1550 nm
• Output power: Typically in dBm, higher for lasers
• Modulation method: Direct or external modulation
• Operating voltage: +3 to +5 V
• Additional features: TOSA, APC, efficient coupling, low power dissipation These specifications collectively determine the performance, efficiency, and suitability of an optical transmitter for a given fiber optic communication system .