RFLED vs. Laser: Key Differences Explained
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Both LEDs and laser diodes are semiconductor devices that emit light. However, they differ significantly in their emission characteristics, energy efficiency, working principles, applications, and safety considerations. LEDs are commonly used for general lighting and illumination, while laser diodes are employed in specific applications requiring coherent and focused light sources.
What is an LED?
An LED (Light Emitting Diode) is a junction diode made from a semiconductor compound, often gallium arsenide phosphide. LEDs used as optical fiber transmitters emit infrared radiation at a wavelength of approximately 850 nm (0.85 µm). Pulse code modulated signals from the coder supply input current to the LED, producing an equivalent stream of infrared pulses for transmission along the optical fiber system. The spectral spread of wavelengths in the output is about 30-40 nm.
LEDs are a very cheap and convenient light source. They are typically used with multimode fibers due to their low output intensity. They are employed in low data rate digital transmission systems up to speeds of about 30 Mbps, where the spreading of output pulses due to dispersion isn’t a major concern. A lens between the LED and fiber system can improve light energy transmission.
Figure 1 depicts the LED diode circuit symbol and pins. As shown, the longer lead is the anode (positive), and the shorter lead is the cathode (negative).
What is a Laser?
Laser stands for Light Amplification by the Stimulated Emission of Radiation. It produces a very intense beam of light or infrared radiation with the following properties:
- Monochromatic: Consisting of one wavelength.
- Coherent: All parts of the beam are in phase.
- Collimated: All parts travel in the same direction.
Laser diodes used in optical fiber systems are often made of gallium arsenide phosphide. A laser, even one the size of a grain of sand, can produce a power output of about 10 mWatt.
The ON/OFF switching speed of a laser is faster than that of an LED. Spectral spreading is also less than an LED, about 1 to 2 nm or even less. Therefore, dispersion is not as significant a problem with lasers compared to LEDs. This makes lasers more suitable for optical fiber systems used for single-mode and high bit rate systems.
Figure 2 depicts a Laser diode rear view and circuit symbols. The circuit symbol of a Laser diode is the same as that of an LED diode.
Differences Between LED and Laser Diodes
The following table compares LEDs and Lasers based on various factors:
| Specifications | LED | Laser |
|---|---|---|
| Working operation | Emits light by spontaneous emission. | Emits light by stimulated emission. |
| Coherent/Incoherent | Emitted light is incoherent (photons are in random phase). | Possesses a coherent beam with identical phase relation of photons. |
| Output power | Relatively low, linearly proportional to drive current. | High (few mW to GW), proportional to current above the threshold. |
| Bias/Current | Requires small applied bias and operates under low current densities. | Requires high driving power and high injected current density. |
| Coupled power | Moderate | High |
| Speed | Slower | Faster |
| Output pattern | Higher | Lower |
| Fiber Type | Multimode only | Singlemode and multimode |
| Ease of use | Easier | Harder |
| Lifetime | Longer | Long |
| Spectral width | Wider, 25 to 100 nm (10 to 50 THz) | Narrower, <10^-5 to 5 nm (<1 MHz to 2 MHz) |
| Modulation Bandwidth | Moderate, Tens of KHz to tens of MHz | High, Tens of MHz to tens of GHz |
| Available Wavelength | 0.66 to 1.65 mm | 0.78 to 1.65 mm |
| E/O Conversion Efficiency | 10 to 20 % | 30 to 70 % |
| Eye Safety | Generally considered eye-safe | Must be rendered eye-safe, especially for λ < 1400 nm |
| Cost | Low | Moderate to High |
Conclusion
In summary, LEDs are versatile light sources used in various applications for illumination and displays, while lasers offer highly focused and coherent light. The major differences between LEDs and lasers, including their emission characteristics, efficiency, operation, applications, and safety considerations, make them suitable for specific use cases in our daily lives.
