First, the laser generation mechanism
Before talking about the laser generation mechanism, let's talk about the stimulated radiation. There are three kinds of radiation processes in the light radiation. One is that the particles in the high energy state transition to the low energy state under the excitation of the external light, which is called spontaneous radiation; the other is that the particles in the high energy state are excited to the low energy state under the excitation of the external light. The transition, called the stimulated radiation; the third is that the energy of the low-energy particles absorbing the external light is called the stimulated absorption.
Spontaneous radiation, even if two particles transition from a high energy state to a low energy state at the same time, the phase, polarization state, and emission direction of the emitted light may be different, but the stimulated radiation is different, when the particles in the high energy state are outside. Under the excitation of photons, it transitions to a low-energy state, emitting light of the same frequency, phase, and polarization state as the external photons. In a laser, the radiation that occurs is stimulated radiation, and the laser it emits is exactly the same in terms of frequency, phase, and polarization state. Any excited light system, that is, stimulated radiation, also stimulated absorption, only the stimulated radiation predominates, the external light can be amplified to emit laser light. In general light sources, the stimulated absorption is dominant, and only the equilibrium state of the particles is broken, so that the number of particles in the high energy state is larger than the number of particles in the low energy state (so called the ion number inversion), and the laser can be emitted.
The three conditions for generating a laser are: achieving particle number inversion, satisfying threshold conditions, and resonance conditions. The first condition for generating stimulated emission of light is the inversion of the number of particles, in which the electrons in the valence band are pumped to the conduction band. In order to obtain the ion number reversal, the heavily doped P-type and N-type materials are usually used to form the PN junction, so that under the applied voltage, the ion number reversal occurs near the junction region - at the high Fermi level EFC The following conduction bands store electrons, while holes are stored in a valence band above the low Fermi level EFV. Achieving particle number inversion is a necessary condition for generating laser light, but it is not a sufficient condition. To generate a laser, there is also a cavity with minimal loss. The main part of the cavity is two mirrors that are parallel to each other. The stimulated radiation emitted by the activating material reflects back and forth between the two mirrors, constantly causing new The stimulated radiation makes it constantly amplified. Only the gain of the stimulated radiation amplification is greater than the various losses in the laser, that is, a certain threshold condition is met:
P1P2exp(2G - 2A) ≥ 1
(P1, P2 are the reflectivity of the two mirrors, G is the gain coefficient of the active medium, A is the loss factor of the medium, and exp is a constant) to output a stable laser. On the other hand, the laser reflects back and forth in the cavity. Only the phase difference between these two beams at the output is Δф = 2qπ q = 1, 2, 3, 4. . . . At this time, it is possible to generate enhanced interference at the output and output a stable laser. Let the length of the resonant cavity be L and the refractive index of the active medium be N, then
Δф=(2π/λ)2NL=4πN(Lf/c)=2qπ,
The above equation can be expressed as f=qc/2NL. This equation is called a resonance condition. It indicates that after the cavity length L and the refractive index N are determined, only certain frequencies of light can form a light oscillation and output a stable laser. This shows that the resonant cavity has a certain frequency selection effect on the output laser.
Second, the laser diode
Essentially a semiconductor diode, the laser diode can be divided into a homojunction, a single heterojunction (SH), a double heterojunction (DH), and a quantum well (QW) laser diode, depending on whether the PN junction material is the same. Quantum well laser diodes have the advantages of low threshold current and high output power, and are the mainstream products in the market. Compared with lasers, laser diodes have the advantages of high efficiency, small size and long life, but their output power is small (generally less than 2mW), linearity and monochromaticity are not good, so their applications in cable TV systems are affected. Very limited, can not transmit multi-channel, high-performance analog signals. In the backhaul module of the bidirectional optical receiver, the uplink transmission generally uses a quantum well laser diode as a light source.
The basic structure of a semiconductor laser diode is shown in the figure. A pair of parallel planes perpendicular to the PN junction face constitute a Fabry-Perot cavity, which may be a cleavage plane of a semiconductor crystal or a polished plane. The other two sides are relatively rough to eliminate the laser action in other directions in the main direction.
Light emission in semiconductors typically results from the recombination of carriers. When the PN junction of the semiconductor is applied with a forward voltage, the PN junction barrier is weakened, forcing electrons to be injected into the P region from the N region via the PN junction, and holes are injected from the P region through the PN junction into the N region, and these are implanted near the PN junction. The equilibrium electrons and holes will recombine to emit photons of wavelength λ, which have the following formula:
λ = hc/Eg (1)
Where: h—Planck constant; c—speed of light; Eg—the forbidden band width of the semiconductor.
The above phenomenon of luminescence due to spontaneous recombination of electrons and holes is called spontaneous emission. When the photons generated by spontaneous emission pass through the semiconductor, once they pass through the emitted electron-hole pairs, they can be excited to recombine to produce new photons, which induce the excited carriers to recombine and emit new photons. The phenomenon is called stimulated radiation. If the injection current is large enough, a carrier distribution opposite to the thermal equilibrium state is formed, that is, the population number is reversed. When the carriers in the active layer are in a large number of reversals, a small amount of spontaneously generated photons generate inductive radiation due to reciprocal reflection at both ends of the resonant cavity, resulting in selective feedback of the frequency selective resonance, or gain for a certain frequency. When the gain is greater than the absorption loss, a coherent light with a good spectral line, the laser, can be emitted from the PN junction, which is the simple principle of the laser diode.
With the development of technologies and processes, semiconductor laser diodes currently in practical use have a complicated multilayer structure.
There are two types of laser diodes commonly used: 1PIN photodiodes. It introduces quantum noise when it receives photo power to generate photocurrent. 2 avalanche photodiodes. It provides internal amplification that is much longer than the PIN photodiode, but with greater quantum noise. In order to obtain a good signal-to-noise ratio, a low-noise preamplifier and main amplifier must be connected to the photodetection device.
The working principle of a semiconductor laser diode is theoretically the same as that of a gas laser.
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 a homojunction, a single heterojunction (SH), a double heterojunction (DH) and a quantum well (QW) laser diode. Quantum well laser diodes have the advantages of low threshold current and high output power, and are the mainstream products in the market. Compared with lasers, laser diodes have the advantages of high efficiency, small size and long life, but their output power is small (generally less than 2mW), linearity and monochromaticity are not good, so their applications in cable TV systems are affected. Very limited, can not transmit multi-channel, high-performance analog signals. In the backhaul module of the bidirectional optical receiver, the uplink transmission generally uses a quantum well laser diode as a light source.
Common parameters of semiconductor laser diodes are:
(1) Wavelength: the working wavelength of the laser tube. The wavelength of the laser tube currently available for photoelectric switching is 635 nm, 650 nm, 670 nm, 690 nm, 780 nm, 810 nm, 860 nm, 980 nm, and the like.
(2) Threshold current Ith: that is, the current that the laser tube starts to generate laser oscillation. For a general low-power laser tube, the value is about tens of milliamperes, and the threshold current of the laser tube with strain multi-quantum well structure can be as low as 10 mA. the following.
(3) Working current Iop: 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 laser diode's light-emitting strip is opened in the direction of the vertical PN junction, generally around 15?~40?.
(5) Horizontal divergence angle θ ∥: The angle at which the illuminating band of the laser diode is opened in the direction parallel to the PN junction is generally about 6?~10?.
(6) Monitoring current Im: the current flowing through the PIN tube when the laser tube is at the rated output power.
Laser diodes are widely used in small-power optoelectronic devices such as optical disc drives on computers and printheads in laser printers.
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