The basic structure of a semiconductor laser diode: a pair of parallel planes perpendicular to the PN junction form a Fabry-Perot resonant cavity. They can be the cleavage planes of the semiconductor crystal, or they can be polished plane. The remaining two sides are relatively rough to eliminate the laser effect in other directions except the main direction.
Light emission in semiconductors usually results from the recombination of carriers. When a forward voltage is applied to the PN junction of a semiconductor, the PN junction barrier will be weakened, forcing electrons to be injected from the N region through the PN junction into the P region, and holes from the P region through the PN junction into the N region. The equilibrium electrons and holes will recombine, thus emitting photons with wavelength λ. The formula is as follows:
λ = hc/Eg ⑴
Where: h—Plan Gram's constant; c—speed of light; Eg—bandgap width of semiconductor.
The above-mentioned phenomenon of emitting light due to the spontaneous recombination of electrons and holes is called spontaneous emission. When photons generated by spontaneous emission pass through a semiconductor, once they pass near the emitted electron-hole pair, they can be excited to recombine and generate new photons. This photon induces the excited carriers to recombine and emit new photons. The phenomenon is called stimulated emission of radiation. If the injected current is large enough, a carrier distribution opposite to the thermal equilibrium state will be formed, that is, the particle number is reversed. When a large number of carriers in the active layer are reversed, a small amount of photons generated by spontaneous emission produce induced radiation due to reciprocal reflection from both ends of the resonant cavity, causing positive feedback of frequency-selective resonance, or a gain at a certain frequency. When the gain is greater than the absorption loss, coherent light with good spectral lines - laser - can be emitted from the PN junction. This is the simple principle of the laser diode. With the development of technology and craftsmanship, multi-layer structures have emerged.
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.
The working principle of semiconductor laser diodes is theoretically the same as that of gas lasers. ⑴Wavelength: That is the working wavelength of the laser tube. The wavelengths of laser tubes that can be used as photoelectric switches include 635nm, 650nm, 670nm, 690nm, 780nm, 810nm, 860nm, 980nm, etc.
⑵Threshold 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 of a laser tube with a strained multiple quantum well structure The current can be as low as below 10mA.
⑶ 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.
⑷Vertical divergence angle θ⊥: The angle at which the luminous strip of the laser diode opens in the vertical direction of the PN junction, generally around 15?~40?.
⑸ 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?.
⑹Monitoring current Im: that is, the current flowing through the PIN tube when the laser tube is at rated output power.
Laser diodes are widely used in low-power optoelectronic equipment such as optical disc drives on computers, print heads in laser printers, barcode scanners, laser ranging, laser medical treatment, optical communications, laser pointing, etc. , it has also been used in high-power equipment such as stage lighting, laser surgery, laser welding and laser weapons.