Working Principle and Parameter Description of Semiconductor Lasers ​

Working Principle and Parameter Description of Semiconductor Lasers

Semiconductor lasers, also known as laser diodes (LD), are a type of laser that uses semiconductor materials as the working medium to achieve stimulated emission. Commonly used materials include gallium arsenide (GaAs), cadmium sulfide (CdS), indium phosphide (InP), and zinc sulfide (ZnS). The excitation methods include electrical injection, electron beam excitation, and optical pumping.

Semiconductor laser devices can generally be classified into homojunction, single heterojunction, and double heterojunction types. Homojunction and single heterojunction lasers mostly operate in pulsed mode at room temperature, while double heterojunction lasers can achieve continuous operation at room temperature. Their advantages lie in their compact size, lightweight, reliable operation, low energy consumption, high efficiency, long lifespan, and high-speed modulation. As a result, they are widely used in fields such as laser communication, optical storage, optical gyroscopes, laser printing, laser medical applications, laser ranging, lidar, automatic control, and detection instruments.

Working Principle of Semiconductor Lasers

Through specific excitation methods, a population inversion of non-equilibrium carriers is achieved between the energy bands (conduction band and valence band) of semiconductor materials or between the energy bands and impurity levels (acceptor or donor). When a large number of electrons and holes in the population-inverted state recombine, stimulated emission occurs.

Semiconductor lasers primarily employ three excitation methods: electrical injection, electron beam excitation, and optical pumping. Electrically injected semiconductor lasers are typically fabricated as surface-junction diodes using materials such as GaAs (gallium arsenide), InAs (indium arsenide), or InSb (indium antimonide). Forward bias current injection is used for excitation, generating stimulated emission in the junction plane region.

Electron beam-excited semiconductor lasers generally use N-type or P-type semiconductor single crystals (e.g., PbS, CdS, ZnO) as the gain medium, excited by externally injected high-energy electron beams.

Optically pumped semiconductor lasers usually employ N-type or P-type semiconductor single crystals (e.g., GaAs, InAs, InSb) as the active material, with laser light from another laser serving as the optical pump.

Currently, among semiconductor laser devices, the most widely used and highest-performing type is the electrically injected GaAs double heterostructure diode laser.

The operating wavelength of semiconductor optoelectronic devices depends on the type of semiconductor material. Semiconductor materials have a conduction band and a valence band—electrons move freely in the conduction band, while holes move freely in the valence band. Between the conduction and valence bands lies a forbidden band (bandgap). When electrons absorb light energy and jump from the valence band to the conduction band, light energy is converted into electrical energy. Conversely, when energized electrons drop back from the conduction band to the valence band, electrical energy is converted back into light. The width of the material's bandgap determines the operating wavelength of the optoelectronic device.

Key Parameters of Semiconductor Lasers

Wavelength (nm): The operating wavelength of the laser, such as 405nm, 532nm, 635nm, 650nm, 670nm, 690nm, 780nm, 810nm, 860nm, and 980nm.

Threshold current (Ith): The minimum current required for a laser diode to initiate lasing oscillation. For low-power lasers, this value typically ranges from tens of milliamperes.

Operating current (Iop): The driving current at which the laser diode achieves its rated output power. This parameter is particularly important for designing and debugging laser driving circuits.

Vertical divergence angle (θ⊥): The angular spread of the laser beam in the direction perpendicular to the PN junction plane, generally ranging between 15° to 40°.

Horizontal divergence angle (θ∥): The angular spread of the laser beam in the direction parallel to the PN junction plane, typically ranging from 6° to 10°.

Monitoring current (Im): The current flowing through the PIN photodiode when the laser diode operates at its rated output power.

The development of semiconductor lasers is progressing in two main directions: one focuses on information transmission (information-type lasers), while the other emphasizes high optical power output (power-type lasers).

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Low-power semiconductor lasers (information-type lasers) are primarily used in information technology applications, including distributed feedback lasers (DFB-LD) and dynamic single-mode lasers for fiber optic communication and optical switching systems, narrow-linewidth tunable lasers, as well as visible-wavelength lasers (405nm, 532nm, 635nm, 650nm, 670nm) for optical storage and information processing. These devices are characterized by single-frequency narrow linewidth, high speed, tunability, short wavelengths, and optoelectronic monolithic integration.

High-power semiconductor lasers (power-type lasers) are mainly employed as pump sources, in laser processing systems, the printing industry, and biomedical applications.

Driven by applications such as solid-state laser pumping, high-power semiconductor lasers have achieved breakthrough progress, evidenced by significantly increased output power. Future development trends will focus on high-speed broadband lasers, high-power lasers, short-wavelength lasers, mid-infrared lasers, and related technologies.