Overview of Fiber Lasers

1. Core Definition & Structure:

  · A fiber laser is defined as a laser where the doped optical fiber itself acts as the gain medium, distinguishing it from systems where the laser is merely coupled into a fiber.

  · The core components are the doped fiber (commonly with Yb, Er, Tm ions) and an integrated optical resonator.

2. Core Technologies & Advantages:

  · Pumping Method: Optically pumped by laser diodes or other fiber lasers.

  · Key Component: Double-clad fibers are used for efficient coupling of pump light.

  · Resonator Implementation: Various methods, including end mirrors, fiber loop mirrors, and Fiber Bragg Gratings (FBGs).

  · Key Advantages: Benefiting from the waveguide properties of optical fibers, they offer high beam quality, high output power capability (CW/pulsed), high efficiency, excellent thermal management, and a compact design.

3. Operating Wavelengths & Applications:

  · Primary Spectral Range: Covers the near-infrared (NIR) to mid-infrared (MIR) regions.

  · Wide Applications: Fiber-optic communications, laser medicine (e.g., surgery, therapy), LiDAR, laser ranging, serving as seed sources for higher-power lasers, and dominating the high-power continuous-wave material processing market (e.g., cutting, welding, additive manufacturing).

2. Key Output Characteristics of Lasers

The coherent amplification process endows laser light with a unique set of interrelated properties that determine its suitability for various applications.

1. Wavelength:

  · Determined by the energy level transitions of the gain medium.

  · Existing lasers cover a broad spectrum from ultraviolet (UV) to far-infrared (FIR), including specialized systems for soft X-rays.

  · Wavelength flexibility via frequency conversion techniques enables coverage from short-wavelength (lithography) to long-wavelength (spectroscopy) applications.

2. Gain Bandwidth:

  · The range of wavelengths (frequencies) over which the gain medium provides amplification.

  · Varies greatly: Gas lasers (e.g., HeNe) have very narrow bandwidths (~GHz), while solid-state lasers (e.g., Ti:Sapphire) have extremely broad bandwidths (>100 THz).

  · The practically usable bandwidth is also influenced by system losses and cavity design.

3. Monochromaticity (Spectral Bandwidth):

  · Refers to the spectral purity or linewidth of the laser output.

  · Determined jointly by the gain bandwidth and the mode-selecting properties of the resonator.

  · The longitudinal mode spacing is Δν = c/(2nL). Multiple longitudinal modes can oscillate within the gain bandwidth.

  · Achieving single longitudinal mode operation (high monochromaticity): shortening the cavity (increasing Δν), or inserting intra-cavity frequency-selective elements (e.g., etalons).

  · Crucial for applications like remote sensing, spectroscopy, and frequency standards.

4. Spatial & Temporal Profile:

  · Spatial Profile (Transverse Mode): Determined by the laser cavity. The TEM?? fundamental mode (Gaussian beam) is ideal, offering the smallest divergence and best focusability.

  · Temporal Profile:

    · Continuous Wave (CW): Stable output power.

    · Pulsed Operation: Output is a train of pulses, characterized by pulse width (μs, ns, ps, fs) and repetition rate (Hz). Essential for applications requiring high peak power.

5. Collimation (Low Divergence):

  · Laser beams are highly directional with very low divergence, especially the TEM?? mode.

  · This high collimation enables long-distance transmission (e.g., lunar ranging) and allows the beam to be focused to a diffraction-limited spot, generating extremely high power densities for microscopy, micro-machining, nonlinear optics, and lithography.

6. Output Power:

  · Average Power: Directly given for CW lasers; for pulsed lasers, it is (Pulse Energy × Repetition Rate).

  · Peak Power: Critically important for pulsed lasers, equal to (Pulse Energy / Pulse Duration). Even with moderate average power, ultrashort pulses can yield extremely high peak power (e.g., GW levels).

  · The key strength of lasers is the concentration of high power/energy into a very small spatial area (small spot) and/or short time (pulse), resulting in exceptional intensity or radiance.

7. Coherence:

  · Stems from the cloned nature of stimulated emission photons, possessing fixed phase relationships.

  · Temporal Coherence: Directly related to monochromaticity (linewidth). Narrower linewidth implies longer coherence length. Essential for interferometry and holography.

  · Spatial Coherence: Refers to the phase relationship between different points across the beam's wavefront; high-quality lasers exhibit high spatial coherence.

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