Lasing Without Inversion in Mercury at 253.7 nm

Experimental Setup

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 [1] 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.

Cell Geometry

Because of the 4-level scheme it is possible to cancel the Doppler effect through a proper geometric orientation of the laser beams and prevent the LWI gain spike from being washed out.


The thickness of the mercury gas cell in which the LWI process take place must be fitted to the overlapping region of the two driving-laser beams and will be about 2 mm for a beam diameter of the lasers of 2 mm.


[1] 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).

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Prof. Dr. Thomas Walther

Laser und Quantenoptik
Institut für Angewandte Physik
Fachbereich 05 - Physik
Technische Universität Darmstadt
Schlossgartenstr. 7
D-64289 Darmstadt

+49 6151 16-20831 (Sekretariat)

+49 6151 16-20834 (Fax)




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