Optical pumping
Excitation of atoms or ions to a higher energy state by absorption of pump photons. The energy source for fiber lasers, fiber amplifiers, and most solid-state lasers.
Optical pumping uses absorbed pump photons to populate the upper laser level of a gain medium, producing the population inversion required for stimulated emission. The energy difference between the pump photon and emitted signal photon is dissipated as heat (quantum defect) and is the fundamental thermal-management challenge in high-power laser systems.
Pump wavelength selection.
The pump wavelength must match an absorption band of the gain medium. The choice is constrained by the energy-level structure of the active dopant and by available pump source technology. Common pumping schemes:
| Gain medium | Pump wavelength(s) | Pump source | Notes |
|---|---|---|---|
| Er-doped fiber (EDFA) | 980 nm | Single-mode diode | Highest gain efficiency, low noise figure |
| Er-doped fiber (EDFA) | 1480 nm | Diode laser | Higher absorption, slightly higher NF, used for high-power EDFAs |
| Yb-doped fiber | 915 / 976 nm | Multimode fiber-coupled diode bar / stack | Workhorse high-power industrial laser pump |
| Tm-doped fiber | 793 nm | AlGaAs diode | 2 μm output for medical / sensing |
| Ho-doped fiber | 1.94 μm (Tm-cascade) or 1.15 μm | Tm fiber laser or diode | 2.1 μm output, deeper eye-safe |
| Nd:YAG | 808 nm | AlGaAs diode | Industrial cutting, marking, scientific |
| Yb:YAG | 940 / 968 nm | InGaAs diode | High-power thin-disk lasers |
| Ti:sapphire | 532 nm | Frequency-doubled Nd:YAG | Tunable ultrafast |
| Cr:LiSAF, Cr:ZnSe | various visible | LD or DPSS lasers | Tunable mid-IR |
| Pr:fluoride | UV / violet | Visible diode | Visible-band fiber lasers |
Pump geometry. Two principal arrangements:
- Core-pumped fiber lasers / amplifiers: the pump is launched into the same single-mode core as the signal. Used in EDFAs and low-power Yb-doped fiber sources. Pump is from a single-mode diode coupled with a WDM combiner. Maximum pump power mW.
- Cladding-pumped (double-clad fiber): the pump is launched into a multimode inner cladding surrounding the single-mode core. The pump propagates as multimode and is absorbed gradually along the fiber length where it overlaps with the doped core. Allows multi-kW pump levels from inexpensive multimode diode stacks. Standard for high-power industrial fiber lasers.
Quantum defect (Stokes shift). Energy difference between pump photon and signal photon, fractionally:
The remaining is dissipated as heat in the gain medium. Minimizing the quantum defect (resonant pumping) reduces heat load:
| Gain medium | Stokes efficiency (signal / pump) |
|---|---|
| Yb-fiber 976 → 1030 nm | 95 % |
| Er-fiber 980 → 1550 nm | 63 % |
| Er-fiber 1480 → 1550 nm | 95 % |
| Nd:YAG 808 → 1064 nm | 76 % |
| Ti:sapphire 532 → 800 nm | 67 % |
Other pumping methods.
- Electrical injection is the primary pumping mechanism for semiconductor diode lasers — directly through the p-n junction (see threshold current). This bypasses optical pumping entirely.
- Gas discharge pumps gas lasers (HeNe, argon-ion, CO, excimer) via electron-impact excitation in a high-voltage discharge.
- Flash-lamp pumping preceded diode pumping for solid-state lasers; still used in some high-energy Q-switched systems. Broadband emission means low conversion efficiency.
- Chemical pumping uses exothermic chemical reactions; rare, mostly defense / research (HF/DF chemical lasers).
- Optical pumping with another laser at the next-shorter resonant wavelength (cascaded pumping) is used in mid-IR fiber lasers (Tm pumps Ho, etc.) and in some quantum-cascade designs.