**5. Conclusion**

In conclusion, three tunable high-power narrow-spectrum red diode laser systems based on tapered semiconductor optical amplifiers in Littrow external-cavity are demonstrated. The epitaxial structruce and the geometry of these tapered devices are described, and the data on the gain media of the devices are presented and compared.

For diode laser system A at 668 nm, an output power of 1.38 W is obtained with an injected current of 2.0 A, and the laser system is tunable from 659 to 675 nm with output power over 800 mW. This is to our knowledge the highest output power from a tunable diode laser system in this wavelength range. The spectral bandwidth of the output beam is less than 0.07 nm.

Both diode laser system B and C are tunable in a 20 nm range centered at 675 nm, and the spectral bandwidth of the output beam for both diode laser systems is less than 0.07 nm in their tunable ranges. The maximum output power is 1.25 W obtained from laser system B at the wavelength of 675.3 nm, and the maximum output power from laser system C is 1.05 W obtained at the wavelength of 675.6 nm. The beam quality factor *M*2 is 2.07 with the output power of 1.0 W for laser system B, and the *M*2 value is 1.13 with the output power of 0.93 W for laser system C. Laser system C is used as the pump source for the generation of UV light by single-pass frequency doubling in a BIBO crystal. An output power of 109 µW UV light at 337.6 nm, corresponding to a conversion efficiency of 0.026%W-1 is attained.
