Laser Diode Parameters, Principles And Applications

The working principle of semiconductor laser diodes is theoretically the same as that of gas lasers.
The laser diode is essentially a semiconductor diode. According to whether the PN junction material is the same, the laser diode can be divided into homojunction, single heterojunction (SH), double heterojunction (DH) and quantum well (QW) laser diodes. Quantum well laser diodes have the advantages of low threshold current and high output power, and are currently mainstream products in the market. Compared with lasers, laser diodes have the advantages of high efficiency, small size, and long life. However, their output power is small (generally less than 2mW), poor linearity, and monochromaticity are not very good, which limits their application in cable TV systems. Very limited, cannot transmit multi-channel, high-performance analog signals. In the backhaul module of a bidirectional optical receiver, quantum well laser diodes are generally used as light sources for uplink transmission.

Laser Diode Essence
The laser diode is essentially a semiconductor diode. According to whether the PN junction material is the same, the laser diode can be divided into homojunction, single heterojunction (SH), double heterojunction (DH) and quantum well (QW) laser diodes. Quantum well laser diodes have the advantages of low threshold current and high output power, and are currently mainstream products in the market. Compared with lasers, laser diodes have the advantages of high efficiency, small size, and long life. However, their output power is small (generally less than 2mW), poor linearity, and monochromaticity are not very good, which limits their application in cable TV systems. Very limited, cannot transmit multi-channel, high-performance analog signals. In the backhaul module of a bidirectional optical receiver, quantum well laser diodes are generally used as light sources for uplink transmission.

The basic structure of a semiconductor laser diode is as shown in the figure. A pair of parallel planes perpendicular to the PN junction form a Fabry-Perot resonant cavity. They can be cleavage planes of the semiconductor crystal or polished planes. The remaining two sides are relatively rough to eliminate the laser effect in other directions except the main direction.

In specific operation, the P-N junction of the laser diode is formed by two doped gallium arsenide layers. It has two flat-ended structures, one mirrored parallel to the end (a highly reflective surface) and one partially reflective. The wavelength of light to be emitted is exactly related to the length of the joint. When a P-N junction is forward biased by an external voltage source, electrons move through the junction and recombine like a normal diode. When electrons recombine with holes, photons are released. These photons hit the atoms, causing more photons to be released. As the forward bias current increases, more electrons enter the depletion region and cause more photons to be emitted.

There are two commonly used laser diodes: ①PIN photodiode. When it receives optical power and generates photocurrent, it will bring quantum noise. ②Avalanche photodiode. It provides internal amplification and can transmit farther than a PIN photodiode, but has greater quantum noise. In order to obtain a good signal-to-noise ratio, a low-noise preamplifier and a main amplifier must be connected behind the photodetection device.

Commonly used parameters of semiconductor laser diodes are:
(1) Wavelength: that is, the working wavelength of the laser tube. Currently, the wavelengths of laser tubes that can be used as photoelectric switches include 635nm, 650nm, 670nm, 690nm, 780nm, 810nm, 860nm, 980nm, etc.

(2) Threshold current Ith: that is, the current at which the laser tube starts to generate laser oscillation. For general low-power laser tubes, its value is about tens of milliamperes. The threshold current of laser tubes with a strained multiple quantum well structure can be as low as 10mA. the following.

(3) Operating current Iop: That is, the driving current when the laser tube reaches the rated output power. This value is important for designing and debugging the laser driving circuit.

(4) Vertical divergence angle θ⊥: The angle at which the luminous strip of the laser diode opens in the direction perpendicular to the PN junction, generally around 15°~40°.

(5) Horizontal divergence angle θ∥: The angle at which the luminous band of the laser diode opens in the direction parallel to the PN junction, generally around 6°~10°.

(6) Monitoring current Im: that is, the current flowing through the PIN tube when the laser tube is at rated output power.

In real life, laser diodes are widely used in information science fields, such as optical fiber communications, optical disk storage, printing and copying, and medical cosmetology. For specific applications, selection needs to be combined with its main technical parameters, including wavelength, output power, operating current, operating voltage, etc. Laser diodes are also widely used in low-power optoelectronic devices such as optical disc drives on computers and print heads in laser printers.

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