435.6 nm Laser
The laser system for the 435.6 nm radiation consists of an external cavity diode laser (ECDL) which emits about 25 mW at 871.2 nm and is boosted by a tapered amplifier (TA) of up to 1.5 W optical power. The ECDL is stabilized by a novel locking scheme  which allows a mode-hop free frequency scan up to 22 GHz. For second-harmonic generation we use a potassium niobate (KNbO3) crystal with a very high nonlinear coefficient of 13.8 pm/V in a bow-tie shaped build-up cavity. The cavity is stabilized with the Pound-Drever-Hall technique. The 20 MHz frequency modulation for the PDH stabilization is directly modulated on the laser diode current through an in-house developed Opens external link in new windowultra low-noise current controller. It was possible to achieve a conversion efficiency of over 50% and reach an output power at 435.6 nm of 230 mW.
546.1 nm Laser
To produce the 546.1 nm radiation, we use a 1092.2 nm high power diode in a similar ECDL design and stabilization as the 871.2 nm laser. The radiation of the ECDL (222 mW optical power) is frequency doubled by a lithium niobate crystal with 5.5% magnesium oxide doping (MgO:LiNbO3) in a build-up cavity. Because of a poor anti reflection coating of the crystal the conversion efficiency is currently limited to 7% and we achieve an output power of 6 mW at 546.1 nm.
404.7 nm Laser
The incoherent repump is realized with a laser diode in ECDL configuration modulated with white noise through an ultra low-noise current controller. It is possible to achieve a linewidth of several 10 MHz at the desired wavelength of 404.7 nm with an output power of 12 mW.
Frequency Stabilization of the Lasers
The three lasers are absolute frequency stabilized by a switcher wavelength meter. The feedback loop is closed by a piezo driver which controls the grating of the ECDL. The software PID controller is implemented with LabView.
- T. Führer, D. Stang, and T. Walther, “Actively controlled tuning of an external cavity diode laser by polarization spectroscopy,” Opt. Express 17, 4991–4996 (2009).