laser diode (fiber optics) (LD)
Laser diodes (LD) are semiconductor lasers for optoelectronics used in optical transmission technology. They consist of doped semiconductor layers, often a substrate made of a compound semiconductor of indium- gallium or aluminum-gallium, and are used to generate light signals that are transmitted in optical waveguides.
Most laser diodes are edge-emittinglasers, orEELs, which use stimulated emission to generate light. They modulate the light signal in the intensity and form of amplitude modulation, frequency modulation or phase modulation, with which transmission frequencies of about 10 GHz are achieved.
Laser diodes have a much higher output power compared to light-emitting diodes(LEDs), exceeding 5 mW, and are therefore more suitable for long optical transmission distances. The wavelength of light emission is mainly determined by the band gap and the semiconductor material. For example, the wavelength range of InGaN semiconductor lasers in the visible light range is 380 nm to 470 nm, AlGaInP semiconductor lasers emit wavelengths of 635 nm, 650 nm and 670 nm, and AlGaAs laser diodes used in CD drives have emission wavelengths of 720 nm to 850 nm. In addition, there are InGaAs semiconductor lasers and InGaAsP semiconductor lasers for the infrared range. The emission wavelength of the former is between 900 nm and 1,100 nm and of the latter between 1,000 nm and 1,650 nm. The wavelength of green-emitting laser diodes is 532 nm, which is exactly in the range used for displays and projections.
The spectral width with which laser diodes emit their strongly focused light is only 1 nm; in the case of the distributed feedback laser( DFB), it is even only 0.1 nm. In the case of laser diodes used in optical transmission technology, the radiation area has a diameter of only 5 µm. This small emission area and the enormously strong beam focusing are ideal prerequisites for use in single mode fibers. VCSEL lasers are used for the optical window at 850 nm, and Fabry Perot lasers( FPL) are used at 1,300 nm.
The modulation frequencies can be 10 GHz, the rise time is 10 ps, and the lifetime of 10,000 hours is much lower than that of LEDs. Disadvantages are the higher circuit complexity compared to LEDs as well as the temperature dependence and the sensitivity to backscattering.