I. Semiconductor Lasers and Their Principles
Semiconductor laser devices can be classified into several types, including homojunction, single heterojunction, and double heterojunction lasers. Homojunction lasers and single heterojunction lasers are mostly pulsed devices at room temperature, while double heterojunction lasers can achieve continuous operation at room temperature.
According to the band structure theory of solid-state materials, the energy levels of electrons in semiconductor materials form energy bands. The higher energy band is the conduction band, and the lower energy band is the valence band, separated by the band gap. When non-equilibrium electron-hole pairs recombine in a semiconductor, the released energy is radiated out in the form of light emission; this is called carrier recombination luminescence.
There are two main categories of semiconductor materials commonly used: direct bandgap materials and indirect bandgap materials. Direct bandgap semiconductor materials such as GaAs (gallium arsenide) have a much higher radiative transition probability and a much higher luminous efficiency than indirect bandgap semiconductor materials such as Si.
The necessary conditions for semiconductor recombination luminescence to achieve stimulated emission (i.e., laser generation) are: ① Population inversion distribution: When the carrier density injected into the active region from both the P-type and n-type sides is very high, the number of electrons occupying the conduction band states exceeds the number of electrons occupying the valence band states, thus forming a population inversion distribution. ② The optical resonant cavity: In a semiconductor laser, the resonant cavity consists of mirrors at both ends, called a Fabry-Perot cavity. ③ High gain: This is used to compensate for optical losses. The optical losses of the resonant cavity are mainly the losses from light emitted outward from the reflecting surface and the light absorption of the medium.
Semiconductor lasers operate by injecting charge carriers, and three basic conditions must be met to emit laser light:
(1) To generate a sufficient population inversion distribution, that is, the number of particles in the high-energy state must be sufficiently greater than the number of particles in the low-energy state;
(2) A suitable resonant cavity can act as a feedback mechanism, causing stimulated emission photons to multiply, thereby generating laser oscillation;
(3) A certain threshold condition must be met so that the photon gain is equal to or greater than the photon loss.
The working principle of a semiconductor laser is based on excitation. It uses the transition of electrons between energy bands in a semiconductor material to emit light. Two parallel reflective surfaces formed by the cleavage planes of the semiconductor crystal are used as reflectors to form a resonant cavity, causing the light to oscillate and feedback, generating radiation amplification of the light, and outputting laser light.
Advantages of semiconductor lasers: small size, light weight, reliable operation, low power consumption, and high efficiency.
II. Differences between semiconductor lasers and fiber lasers
1. The difference between fiber lasers and semiconductor lasers lies in the medium they use to emit laser light. Fiber lasers use optical fibers as the gain medium, while semiconductor lasers use semiconductor materials, typically gallium arsenide (GaAs) or indium gallium phosphate (IGaS).
2. Different Light Emission Mechanisms: The light emission mechanism of semiconductor lasers involves the transition of particles between the conduction band and valence band, producing photons. Because they are semiconductors, they can be directly electro-optically converted using electrical excitation. In contrast, optical fibers cannot directly achieve electro-optical conversion; they require light to pump the gain medium (typically using laser diodes), resulting in optical-optical conversion.
3. Different heat dissipation performance: Fiber lasers have good heat dissipation and can generally be cooled by air. Semiconductor lasers are greatly affected by temperature, and water cooling is required when the power is high.
4. Key Differences in Main Characteristics: Fiber lasers are characterized by their small size and flexibility. They offer a wide range of output spectral lines, excellent monochromaticity, and a broad tuning range. Furthermore, their performance is independent of the light polarization direction, resulting in low coupling loss between the device and the fiber. They also boast high conversion efficiency and a low laser threshold. The geometry of the fiber contributes to its low volume and surface area, and in single-mode, the laser and pump can be fully coupled. Semiconductor lasers, on the other hand, are easily integrated with other semiconductor devices. Their characteristics include direct electrical modulation; easy optoelectronic integration with various optoelectronic devices; small size and light weight; lower drive power and current; high efficiency and long lifespan; compatibility with semiconductor manufacturing technology; and mass production capability.
5. Applications of Fiber Lasers: Fiber lasers are mainly used in laser fiber optic communication, laser long-distance space communication, industrial shipbuilding, automobile manufacturing, laser engraving, laser marking, laser cutting, printing roller manufacturing, metal and non-metal drilling, cutting, and welding (copper brazing, water quenching, cladding, and deep welding), military and national defense security, medical equipment, large-scale infrastructure construction, and as pump sources for other lasers, etc. Semiconductor lasers have wide applications in laser ranging, lidar, laser communication, laser simulated weapons, laser warning, laser guidance and tracking, ignition and detonation, automatic control, and detection instruments.
