Second, laser diodes are essentially semiconductor diodes. According to whether the PN junction materials are the same or not, laser diodes can be divided into homogeneous junction, single heterojunction (SH), double heterojunction (DH) and quantum well (QW) laser diodes. Quantum well laser diode has the advantages of low threshold current and high output power, and is the mainstream product in the market at present. Compared with lasers, laser diodes have the advantages of high efficiency, small size and long service life, but their output power is small (generally less than 2mW), linearity is poor and monochromaticity is not very good, which greatly limits their application in cable TV systems and cannot transmit multi-channel and high-performance analog signals. In the return module of bidirectional optical receiver, quantum well laser diode is generally used as the light source for uplink transmission.
The basic structure of semiconductor laser diode is shown in the figure. A pair of parallel planes perpendicular to the plane of the PN junction form a Fabry-Perot resonator, which can be a cleavage plane or a polishing plane of a semiconductor crystal. The other two surfaces are rough to eliminate the laser action in other directions except the main direction.
Light emission in semiconductors is usually the result of carrier recombination. Applying DC voltage to semiconductor PN junction will weaken the barrier of PN junction, forcing electrons to be injected from N region to P region through PN junction, and holes to be injected from P region to N region through PN junction. Unbalanced electrons and holes injected into the PN junction will recombine, thus emitting photons with wavelength λ. The formula is as follows:
λ = hc/Eg ( 1)
Where: h- Planck constant; C-the speed of light; Bandgap width of semiconductor.
The above phenomenon of luminescence due to spontaneous recombination of electrons and holes is called spontaneous radiation. When the photon produced by spontaneous emission passes through the semiconductor, once it passes near the emitted electron-hole pair, it can stimulate the two to recombine to produce new photons. This photon induces the recombination of excited carriers and emits a new photon, which is called stimulated radiation. If the injection current is large enough, the carrier distribution opposite to the thermal equilibrium state will be formed, that is, the number of particles will be reversed. When a large number of carriers in the active layer are reversed, a small number of photons generated by spontaneous emission will generate induced radiation due to the reciprocating reflection at both ends of the resonant cavity, resulting in positive feedback of frequency-selective resonance or gain at a certain frequency. When the gain is greater than the absorption loss, coherent light with good spectral line-laser can be emitted from the PN junction, which is the simple principle of laser diode.
With the development of technology and technology, the semiconductor laser diode actually used at present has a complex multi-layer structure.
There are two kinds of commonly used laser diodes: ①PIN photodiode. When it receives optical power and generates photocurrent, it will bring quantum noise. ② Avalanche photodiode. It can provide internal amplification, which has a longer transmission distance than PIN photodiode, but the quantum noise is greater. In order to obtain a good signal-to-noise ratio, a low-noise preamplifier and a main amplifier must be connected behind the photodetector.
The working principle of semiconductor laser diode is the same as that of gas laser in theory.
Laser diodes are essentially semiconductor diodes. According to whether the PN junction materials are the same or not, laser diodes can be divided into homogeneous junction, single heterojunction (SH), double heterojunction (DH) and quantum well (QW) laser diodes. Quantum well laser diode has the advantages of low threshold current and high output power, and is the mainstream product in the market at present. Compared with lasers, laser diodes have the advantages of high efficiency, small size and long service life, but their output power is small (generally less than 2mW), linearity is poor and monochromaticity is not very good, which greatly limits their application in cable TV systems and cannot transmit multi-channel and high-performance analog signals. In the return module of bidirectional optical receiver, quantum well laser diode is generally used as the light source for uplink transmission.
Commonly used parameters of semiconductor laser diode are:
(1) wavelength: the working wavelength of the laser tube. At present, the wavelengths of laser tubes used for photoelectric switches are 635nm, 650nm, 670nm, 690nm, 780nm, 8 10nm, 860nm, 980nm and so on.
(2) Threshold current Ith: the current at which the laser tube starts to generate laser oscillation. For ordinary low-power laser tube, its value is about tens of milliamps, and the threshold current of strained multi-quantum well structure laser tube can be as low as 10mA.
(3) Operating current Iop: the driving current when the laser tube reaches the rated output power, which is very important for the design and debugging of the laser driving circuit.
(4) Vertical divergence angle θ ⊥: the angle at which the light-emitting band of the laser diode opens in the direction perpendicular to the PN junction, which is generally 15? ~40? About.
(5) Horizontal divergence angle θ ‖: The angle at which the light-emitting band of the laser diode opens in the direction parallel to the PN junction, which is generally 6? ~ 10? About.
(6) Monitoring current Im: that is, the current flowing through the PIN tube at the rated output power of the laser tube.
Laser diodes have been widely used in optical disk drives on computers, print heads of laser printers, bar code scanners, laser ranging, laser medical treatment, optical communication, laser indication and other low-power photoelectric devices.