Lasing Without Inversion (LWI)

LWI in Mercury at 253.7 nm

The development of CW lasers in the VUV regime and below is technically very limited because the pump power required for population inversion scales with the fourth power of the laser frequency. Current methods to produce laser light in the VUV regime are mainly based on nonlinear effects.

Lasing without inversion (LWI) provides an alternative approach based on coherent excitation of atomic transitions. The technique was already successfully demonstrated with sodium [1] and rubidium [2]. See references below.


  1. E. S. Fry, X. Li, D. Nikonov, G. G. Padmabandu, M. O. Scully, A. V. Smith, F. K. Tittel, C. Wang, S. R. Wilkinson, and S.-Y. Zhu, “Atomic coherence effects within the sodium d1 line: Lasing without inversion via population trapping,” Phys. Rev. Lett. 70, 3235–3238 (1993).
  2. A. S. Zibrov, M. D. Lukin, D. E. Nikonov, L. Hollberg, M. O. Scully, V. L. Velichansky, and H. G. Robinson, “Experimental demonstration of laser oscillation without population inversion via quantum interference in rb,” Phys. Rev. Lett. 75, 1499–1502 (1995).

The principle of LWI is to cancel the absorption at the laser transition by coherent excitation of adjacent transitions so that only a few excited atoms are sufficient for lasing. Whilst there is no more absorption for coherent light, the laser transition can still be pumped by an incoherent light source like a gas discharge lamp.

Until now there has been no LWI scheme where the laser transition had a significantly shorter wavelength than the lasers used to build up the coherence.

In our LWI experiment, we use a 4-level-scheme in mercury with a laser transition at 253.7 nm as described in [1]. For this scheme a strong driving laser at 435.6 nm, a weak driving laser at 546.1 nm and an incoherent repump at 404.7 nm are required.


  1. E. S. Fry, M. D. Lukin, T. Walther, and G. R. Welch, “Four-level atomic coherence and cw vuv lasers,” Optics Communications 179, 499 – 504 (2000).

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.


  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).

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.

Alternative 4-level scheme in Hg
Alternative 4-level scheme in Hg

The three lasers at 435.6 nm, 546.1 nm and 404.7 nm are developed and provide more than the required power of 150 mW at 435.6 nm and 0.3 mW at 546.1 nm.

In a next step, the linewidth of the two driving lasers will be measured since it constitutes a key factor for LWI. Subsequently, the lasers will be brought together in a mercury gas cell to confirm the theoretical predictions in [1] with a 253.7 nm laser system which depends on fourth-harmonic generation. Afterwards, the mercury gas cell will be set into a resonator to demonstrate LWI at 253.7 nm for the first time.

Mercury also provides a 4-level scheme which allows LWI in the VUV regime at 185 nm. For this scheme the required driving laser wavelengths are 434.8 nm and 302.8 nm which are technically accessible.


  1. E. S. Fry, M. D. Lukin, T. Walther, and G. R. Welch, “Four-level atomic coherence and cw vuv lasers,” Optics Communications 179, 499 – 504 (2000).