Absorption of light is a fundamental process in imaging. The optical properties of atoms are thoroughly understood, so a
single atom is an ideal system for testing the quantum limits of absorption imaging. Here we report the first absorption
imaging of a single isolated atom, the smallest and simplest system reported to date. Contrasts of up to 3.1(3)% were
observed in images of a laser cooled <sup>174</sup>Yb<sup>+</sup> ion confined in vacuum by a radio-frequency Paul trap. This work
establishes a new sensitivity bound for absorption imaging with a 7800x improvement over the contrast previously
observed in imaging a single molecule.
Phase-coupled stripe-array diode lasers show a strong double-lobed far-field because adjacent stripes tend to operate in
the anti-phase supermode. One way to achieve a stable phase relationship and global coupling of the emitters of such a
stripe-array is off-axis feedback. In this work several off-axis external cavity designs are discussed. A 400 μm wide
emitter stripe array consisting of 40 stripes with a pitch of 10 μm was investigated. By operating this device in a Littrow
type off-axis external cavity, more than 2 W of output power of near diffraction-limited, single longitudinal mode
emission with a brightness as high as 88 MW/cm<sup>2</sup>-str could be achieved. The technique of off-axis feedback was also
adapted to realize spectral beam combining of 25 emitters of a laser bar. The experimental data are compared with
numerical simulations using a new theoretical model including feedback.
A new setup for efficient blue light generation that consists of two passively coupled optical resonators is presented. The
first resonator is based on a broad area laser diode (BAL) in a Littrow external cavity with a special off-axis design. This
external cavity diode laser provides more than 450 mW diffraction limited and narrow bandwidth emission at 976 nm. A
compact cavity design with 40 mm length could be realized. The second resonator is a monolithic high finesse ring
cavity containing a 10 mm bulk periodically poled lithium niobate (PPLN) crystal for resonant second harmonic
generation. This ring resonator consists of four small mirrors with appropriate reflectivities and two GRIN lenses for
stability reasons. All parts of this ring cavity are mounted monolithically on a glass substrate with a size of 19.5 mm x
8.5 mm. First experiments showed good passive matching of both cavities without any active closed-loop control. With
this setup efficient SHG was achieved. A maximum optical output power of 70 mW blue light at 488 nm was obtained.
The conversion efficiency was better than 15%.
The combination of high brightness laser diodes and periodically poled (PP) waveguide crystals for the generation of blue light at the technically interesting wavelength of 488 nm is promising. Although PPKTP has a lower nonlinear coefficient compared to PPLN it is of interest for the use in such devices. Because of its higher photorefractive damage threshold, it is well suited for operation at room temperature. In this work, a DFB laser as well as a tunable external cavity enhanced broad area diode laser (ECDL) are used for second harmonic generation using a waveguide PPKTP crystal. Both lasers yield several hundred Miliwatts of diffraction limited light around a center wavelength of 976 nm with excellent spectral properties. The ECDL system is further tunable over a broad range of 40 nm. The PPKTP crystal has a length of 12 mm and the 4 μm x 8 μm waveguides are manufactured by ion exchange followed by a patented submount poling technique. By using a DFB laser diode as pump source a laser to waveguide coupling efficiency of more than 55% could be achieved. A maximum output power of 66.7 mW could be generated out of 220 mW infrared light inside the waveguide channel at room temperature. This results in a conversion efficiency of more than 260%/W.