Continuous-wave output at 229 nm for the application of laser cooling of Cd atoms was generated by the 4 th harmonic using two successive second harmonic generation stages. Employing a single-frequency optically pumped semiconductor laser as a fundamental source, 0.56 W of output at 229 nm was observed with a 10-mm long, Brewster-cut BBO crystal in an external cavity with 1.62 W of 458 nm input. Conversion efficiency from 458 nm to 229 nm was more than 34%. By applying a tapered amplifier as a fundamental source, we demonstrated magneto-optical trapping of all stable Cd isotopes including isotopes 111 Cd and 113 Cd, which are applicable to optical lattice clocks.OCIS codes : (140.3610) Lasers, ultraviolet; (140.3515) Lasers, frequency doubled; (190.2620) Harmonic generation and mixing; (140.3320) Laser cooling; (300.6210) Spectroscopy, atomic; (300.6320) Spectroscopy, highresolution. http://dx.doi.org/10.1364/OL.41.000705High-power, continuous-wave (CW) deep ultraviolet (DUV) lasers have numerous industrial as well as scientific applications.They include material processing such as interferometric lithography, fiber Bragg grating fabrication, Raman sensing, defect inspection on semiconductor wafers and/or reticles, and spectroscopic applications [1, 2, 3]. For the conversion into this spectral region, the materials that can be used is fairly limited, as they must be transparent at the fundamental and harmonic, and able to phasematch for the particular combination of the wavelength. These available materials have nonlinear optical coefficients of just a few pm/V. With the exception of CsLiB6O10 (CLBO), which can phasematch for the harmonic wavelength of 238 nm and longer for secondharmonic generation (SHG), the effective nonlinear coefficient becomes smaller toward the shorter wavelength because of their crystal symmetry, making the generation of shorter wavelength increasingly challenging especially in CW.In this Letter, we obtained 0.56 W of CW DUV power at 229 nm via direct SHG, without relying on more complex sumfrequency mixing normally used in this wavelength range. The feasibility and reliability of such CW DUV laser source was explored with its continuous operation of more than 100 hours. With the DUV laser source, we demonstrated magneto-optical trapping (MOT) of Cd atoms. Because of their flexibility in their operating wavelength, optically pumped semiconductor lasers (OPSLs), also known as vertical external cavity surface emitting lasers (VECSELs) are suitable for application wherein specific wavelength is needed [4]. They combine the advantages of power scalability with good beam quality, and the flexibility in the operating wavelength, allowing efficient wavelength conversion. The externally resonant harmonic conversion of OPSLs is a suitable approach for the generation at a specific wavelength in the DUV region [5]. The laser device used in this experiment is InGaAs based device with upside-down configuration; the resonant periodic gain is grown on the GaAs wafer first, followed by the Al...

We demonstrate a 30-km-long optical fiber link for frequency comparison between two strontium optical lattice clocks being developed at RIKEN and the University of Tokyo. We use a transfer laser at 1397 nm, which is twice the wavelength of the clock transition of strontium clocks. The link stability is estimated to be 1 × 10−17 for an averaging time of τ = 1 s, which is in good agreement with the theoretical limit calculated from the fiber noise spectrum. We discuss a remote clock comparison with a stability of 1 × 10−17(τ/s)−1/2 by synchronously operating two distant clocks.

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