1) Why can lasers be "bright and accurate"
Laser comes from stimulated radiation: when incident photons interact with excited particles, they induce photons of the same frequency, phase, polarization, and direction. To form a stable laser, three conditions are required: gain medium, pump source, and optical resonant cavity. Pump sends particles to high energy levels to achieve particle number reversal; Gain medium provides amplification; The resonant cavity enhances specific wavelength and directional oscillations through positive feedback and mode selection. The output also needs to cross the threshold: intracavity gain ≥ loss. There are commonly three level and four level systems, with four level systems being more prone to inversion and typically having lower thresholds.
2) Where does' monochrome, directionality, coherence 'come from
Monochromaticity comes from the selection of gain spectral lines and resonant cavity modes, namely longitudinal mode spacing and spectral width; The directionality comes from the constraints of the endoscope and the transverse mode structure; Coherence can be described by coherence length/time coherence. In engineering, spectral width, linewidth, edge mode suppression ratio, polarization degree, and coherence length are often considered. Not all lasers are "pure monochromatic" or "pure coherent", broadband ultrafast, multi-mode semiconductor, and amplified spontaneous emission light sources are different.
3) Mode and beam quality
The transverse mode describes the spatial distribution of the light spot, and the ideal fundamental mode is close to Gaussian, denoted as TEM00; The frequency component determined by the cavity length corresponding to the longitudinal mode. The beam quality is commonly measured by the M ² factor, where M ²=1 is the ideal Gaussian; The larger the M ², the poorer the focusing ability. Actual lasers are also affected by thermal lenses, gain guidance, pump uniformity, and cavity mirror misalignment. Fiber laser is commonly observed in terms of mode field diameter, cutoff wavelength, and bending loss; Semiconductor lasers often observe fast/slow axis divergence angles, astigmatism, and smile effects.
4) Continuous and pulse: Q-switching, mode locking
Continuous laser monitoring for average power, stability, and noise. Pulse laser is used to measure single pulse energy, peak power, pulse width, repetition rate, and duty cycle. Q-switching generates nanosecond level giant pulses by mutating quantum cavity losses; Locking the mode allows for longitudinal mode phase locking, which can achieve picoseconds or even femtoseconds. The shorter the pulse width, the higher the peak power, but it is more sensitive to dispersion, nonlinearity, and damage threshold. The commonly used pulse or code modulation system for wind lidar focuses on balancing pulse width, repetition rate, peak power, coherent accumulation, and eye safe limits.
5) Gain media and typical frequency bands
Gas lasers such as He Ne, CO ₂, and excimer; Solid state lasers such as Nd: YAG/Nd: YVO ₄, Ti: sapphire, Yb series; Fiber lasers are commonly doped with ytterbium at 1030-1080 nm, erbium at 1550 nm, and thulium at 2 μ m; Semiconductor laser coverage allows for visible near-infrared, which can be made into DFB/DBR and tunable external cavity; There are also dyes, chemistry, free electron lasers, and so on. The essence of selecting a medium is to choose wavelength, efficiency, thermal management, beam quality, modulability, and cost.
6) Key engineering parameters
Looking at a laser, at least ask: wavelength and spectral width; Output power/single pulse energy/peak power; Pulse width and repetition rate; Average power stability; Spot size and far-field divergence angle M²； Polarization to extinction ratio; Noise and frequency stability; Working temperature, lifespan, and resistance to returning light; Drive current/pumping mode; And the security level. Don't just ask 'how many watts', the danger, optical design, and detector requirements for a pulse width of 10 ns and continuous 10 W are completely different for the same wattage.
7) Nonlinear and Damage
At high power, it will enter the nonlinear region: self phase modulation, four wave mixing, stimulated Brillouin/Raman scattering, optical Kerr effect, harmonic generation, optical birefringence, etc. Optical components have laser-induced damage threshold (LIDT), which involves continuous, pulsed, wavelength, and spot size; The high peak value of short pulses also depends on the self focusing and plasma formation inside the medium. In system design, it is necessary to manage the return light, debris, end caps, isolators, QBH/QD end caps, thermal lenses, and contamination.
8) Relationship with safety standards
Laser safety is not only based on "low nominal power", but also on achievable emission, wavelength, pulse width, extended source, measurement conditions, and single fault determination level. The logic of the previous GB/T 7247.1/IEC 60825-1 is as follows: use the achievable emission limit AEL to determine Class 1/1M/2/2M/3R/3B/4, and then decide on engineering protection, interlocking, labeling, and instructions.
