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LED vs Laser diode | Difference between LED and Laser diode

Both LED and Laser are used as optical transmitters for various applications. The following section describes them and mention difference between LED and Laser.

What is LED

An LED is a junction diode made from semiconductor compound gallium arsenide phosphide. LEDs used as optical fiber transmitters emit infrared radiation at a wavelength of about 850 nm (0.85 µm).

Pulse code modulated signals from the coder supply input current to the LED. This will produce 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.

LED diode

LEDs are very cheap and convenient source of light. They are usually used with multimode fibers due to their low output intensity. They are employed in low data rate digital transmission systems upto speed of about 30 Mbps where in spreading of output pulses because of dispersion is not a big problem. Lens between LED and fiber system will improve light energy transmission between them. This page mentions what is led and provides links to terminologies on electronic devices or components. Figure-1 depicts LED diode circuit symbol and pins. As shown in LED, long lead is anode (positive) and short lead is cathode (negative).

What is Laser Diode

Laser is derived from Light Amplification by the Stimulated Emission of Radiation. It produces a very intense beam of light or infrared radiation which is having following properties.
•  Monochromatic ( meaning consists of one wavelength)
•  Coherent (meaning all parts are in phase)
•  Collimated (meaning all parts travel in one and same direction)

Laser diode used in optical fiber systems are made of gallium arsenide phosphide. The laser having size of grain of sand can produce power output of about 10 mWatt.

laser diode

ON/OFF switching speed of laser is faster than LED. Spectral spreading is less than LED about 1 to 2 nm or even less. Hence dispersion is not a problem with laser compare to LED. Hence lasers are more suitable for optical fiber systems used for monomode and high bit rate systems. The figure-2 depicts Laser diode rear view and circuit symbols. The circuit symbol of Laser diode is same as LED diode.

This explaination on LED and Laser helps one compare difference between LED and Laser. Also refer following links on various electronic components.

Following table compares LED and Laser with respect to various comparison factors/specifications.

specification Light Emitting Diode Laser Diode
Working operation It emits light by spontaneous emission. It emits light by stimulated emission.
Coherent/Incoherent The emitted light is incoherent i.e. photons are in random phase among themselves. It possesses a coherent beam with identical phase relation of emitted photons.
Output power Emitted light power is relatively low, Linearly proportional to drive current Output power is high (Few mW to GW) , Proportional to current above the threshold
Bias/Current It requires small applied bias and operates under relatively low current densities. It requires high driving power and high injected current density is needed.
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

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