  We have characterized a semiconductor amplifier laser system which provides
up to 200mW output after a single-mode optical fiber at 780nm wavelength. The
system is based on a tapered semiconductor gain element, which amplifies the
output of a narrow-linewidth diode laser. Gain and saturation are discussed as
a function of operating temperature and injection current. The spectral
properties of the amplifier are investigated with a grating spectrometer.
Amplified spontaneous emission (ASE) causes a spectral background with a width
of 4nm FWHM. The ASE background was suppressed to below our detection limit by
a proper choice of operating current and temperature, and by sending the light
through a single-mode optical fiber. The final ASE spectral density was less
than 0.1nW/MHz, i.e. less than 0.2 % of the optical power. Related to an
optical transition linewidth of $\Gamma/2\pi=6$ MHz for rubidium, this gives a
background suppression of better than -82dB. An indication of the beam quality
is provided by the fiber coupling efficiency up to 59 %. The application of the
amplifier system as a laser source for atom optical experiments is discussed.
