End-Pumped Laser
End-pumped laser (also called longitudinally pumped) is an optically pumped laser in which the pump light is injected along the same axis as the circulating laser beam, rather than from the side. The pump beam and the resonator mode therefore travel roughly collinearly through the gain medium. This geometry is the usual choice for compact, efficient diode-pumped solid-state lasers (DPSSLs) of low to moderate power.
How it works:
A pump source—most often a fiber-coupled laser diode or diode bar—is focused through one (or both) end faces of the gain medium (typically a crystal such as Nd:YAG, Nd:YVO₄, Yb:YAG, or Tm:YLF). One resonator mirror is usually a dichroic coating that transmits the pump wavelength and reflects the laser wavelength, so the pump can enter through a cavity mirror.
Because the pump intensity is concentrated inside the volume of the fundamental (TEM₀₀) resonator mode, two things follow:
Higher-order modes see little gain and often stay below threshold, so the output can be diffraction-limited.
Unused pump light outside the laser mode is minimized, which raises optical-to-optical efficiency and reduces wasted heat.
The sides of the crystal remain free for heat sinking, which simplifies cooling compared with side-pumped rods. Typical pump wavelengths match strong absorption lines of the dopant (e.g., ~808 nm for Nd³⁺, ~940–980 nm for Yb³⁺).
Advantages and limitations versus side pumping:
Advantages -
Excellent spatial overlap of pump and laser mode → high slope efficiency and good beam quality (often M² ≈ 1).
Compact layouts; small crystals such as Nd:YVO₄ become practical.
Relatively simple cooling of the barrel of a rod.
Limitations -
Pump power can enter from at most two ends, so scaling to multi-hundred-watt or kilowatt levels is harder.
The pump source must have reasonably good beam quality so the pump beam stays confined over the absorption length of the crystal.
Absorption is strongest near the entrance face, producing a strong axial temperature gradient and thermal lensing that must be managed (composite undoped end caps, dual-end pumping, or optimized mode matching help).
Side pumping is therefore preferred when very high average power from lamps or many diode bars is needed and beam quality can be sacrificed. End pumping dominates when efficiency and beam quality matter more than raw power.
Technical details:
Gain media: short rods or slabs a few millimeters to a few centimeters long.
Pump delivery: free-space focusing optics or multimode fiber (common core diameters 100–400 µm).
Resonator: often a simple two-mirror cavity; one end of the crystal may itself be coated as a high reflector.
Power range: milliwatts to tens of watts (occasionally >100 W with dual-end pumping and careful thermal design).
Variants: dual-end pumping, fiber-laser pumping of another fiber or crystal, and “end-pumped” thin-disk or single-crystal-fiber geometries that still inject pump along the optical axis.
Applications:
Precision micromachining, PCB/flex drilling, resistor trimming, and marking, where a clean TEM₀₀ beam and good focusability are required.
Frequency-doubled green and UV sources (the high brightness of an end-pumped TEM₀₀ beam improves nonlinear conversion).
Pump lasers for Ti:sapphire, OPOs, and Raman lasers.
Scientific and medical systems that need compact, stable, single-mode output.
Low- to mid-power industrial and laboratory DPSS lasers in general.
End pumping trades some power-scaling headroom for mode matching, efficiency, and beam quality—the combination that made modern compact diode-pumped solid-state lasers practical.