With the development of technology and process, the semiconductor laser diodes currently in practical use have a complicated multilayer structure. There are two commonly used laser diodes
: ①PIN photodiode. When it receives light power to generate photocurrent, it will bring quantum noise. ②Avalanche photodiode. It can provide internal amplification, which has a longer transmission distance than PIN photodiodes, but has greater quantum noise. In order to obtain a good signal-to-noise ratio, a low-noise pre-amplifier and a main amplifier must be connected behind the optical detection device. The working principle of a semiconductor laser diode is theoretically the same as that of a gas laser. Commonly used parameters ⑴Wavelength: the working wavelength of the laser tube. The current laser tube wavelengths that can be used as photoelectric switches are 635nm, 650nm, 670nm, 690nm, 780nm, 810nm, 860nm, 980nm, etc. ⑵Threshold current Ith: the current at which the laser tube starts to generate laser oscillation. For a general low-power laser tube, its value is about tens of milliamps. The threshold current of a laser tube with a strained multiple quantum well structure can be as low as 10mA or less. ⑶ Working current Iop: the drive current when the laser tube reaches the rated output power. This value is more important for the design and debugging of the laser drive circuit. ⑷Vertical divergence angle θ⊥: The angle at which the light-emitting band of the laser diode opens in the direction perpendicular to the PN junction, generally around 15-40. ⑸Horizontal divergence angle θ∥: The angle at which the light-emitting band of the laser diode opens in the direction parallel to the PN junction, generally around 6-10. ⑹Monitoring current Im: the current flowing on the PIN tube when the laser tube is at the rated output power. Laser diodes have been widely used in low-power optoelectronic devices such as optical disc drives in computers, print heads in laser printers, barcode scanners, laser distance measurement, laser medical treatment, optical communications, laser instructions, etc., in stage lighting, laser surgery It has also been used in high-power equipment such as laser welding and laser weapons.