Hybrid’ Bulk/Fiber Lasers Provide Best of Both Worlds
Eye-safe Er:YAG laser with 60 W of output is an example of hybrid potential.
A new word is working its way into the laser lexicon: hybrid. Although the word has several connotations in the English language, the new usage applies to conventional bulk solid-state lasers (i.e., those with laser rods) that are pumped efficiently with fiber lasers. They combine the fundamental advantages of fiber and bulk architectures and avoid the pitfalls of both.
For four decades, the greatest limitation of bulk solid-state lasers has been their inability to dissipate thermal loads without degrading performance. The advent of diode pumping in the 1980s reduced the magnitude of the problem but did not eliminate it. Slab lasers and, more recently, thin disks sought to reduce thermal problems with rectangular geometries that minimize the effects of thermal gradients. But these architectures introduce their own complications, often limiting their usefulness.
Fiber lasers, with their high surface-to-volume ratio, provide a viable solution to the thermal problem. Cladding-pumped fiber lasers have produced kilowatt-level outputs with excellent beam quality from small, passively cooled packages. But the problem with fiber lasers is that their small core size leads to high power densities, with the consequence of optical damage and deleterious nonlinear effects, especially when the laser is pulsed.
Enter the hybrid laser. It combines the thermal advantages and high-power operation of fiber lasers with the energy-storage capabilities and larger mode sizes of bulk lasers. The fiber laser generates radiation that pumps directly into the transition band of the bulk laser. A major advantage of this in-band pumping scheme is that nearly all the heat resulting from the quantum defect — that is, the heat generated by the spontaneous decay from the terminal pump level to the upper level of the population inversion — is dissipated in the fiber laser.
One of many examples to come to light at conferences and in the literature recently is a device demonstrated by researchers at the University of Southampton in the UK. They optically pumped an Er:YAG rod laser with an Er-Yb-doped fiber laser to produce what they believe is the highest 1.6-µm power, at the highest optical slope efficiency yet observed from an Er:YAG laser.
Figure 1. The Er:YAG laser was configured in a folded four-mirror resonator and in-band-pumped with an Er-Yb-doped fiber laser.
They arranged their Er:YAG laser in a four-mirror folded configuration that allowed two spherical mirrors to focus both the pump light and the laser mode into the YAG rod (Figure 1). As a result of the high spatial and spectral quality of the pump beam from the fiber laser, they were able to use an Er:YAG rod with a relatively low erbium concentration (~0.5 percent atomic) and still obtain a single-pass absorption of ~90 percent of the pump light. Limiting the erbium concentration minimizes a two-ion cooperative up-conversion process that depletes the population inversion (Figure 2).
Figure 2. Energy-level diagrams for two erbium ions show the 1645-nm laser transition 4I13/2à 4I15/2 (red arrow). In-band pumping at 1532 nm occurs between a different set of fine-structure levels of the same transition. A harmful two-ion up-conversion process (blue arrows) can be avoided by minimizing the erbium concentration in the rod.
A 2-m length of Er-Yb-doped double-clad fiber, pumped at both ends with up to 336 W of 975-nm radiation from spatially combined diode laser stacks. The fiber laser’s output was tuned to the 1532-nm absorption peak of Er:YAG with an intracavity diffraction grating in the Littrow configuration. It generated more than 100 W of 1532-nm power with a full-width-half-maximum bandwidth of ~1 nm and an M2 beam quality of ~1.9.
A spherical mirror focused ~82 W of this power onto the laser rod. The small amount of pump radiation that passed through the laser rod was reflected by the output coupler (whose reflectivity was 86 percent at the pump wavelength) for a second pass through the rod, resulting in an overall absorption efficiency of ~98 percent.
Figure 3. The scientists believe that both the output power (60.3 W) and the optical slope efficiency (80.7 percent) are the best yet achieved in a 1.6-μm Er:YAG laser. ©OSA.
The Er:YAG laser generated up to 60.3 W of eye-safe radiation at 1645.3 nm, with an optical slope efficiency of 80.7 percent (Figure 3). The scientists measured the output beam’s M2 to be ≤1.2 at output powers of less than 20 W and ~3.2 at the maximum output power.
Significantly, the power curve shows no rollover at the upper end, indicating that further power scaling is likely to be successful.
Optics Letters, March 15, 2006, pp. 754-756.
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