Research
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].

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 experiment, we use mercury to achieve LWI since there is potential for a wavelength gain.

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 435.8 nm, a weak driving laser at 546.1 nm and an incoherent repump at 404.7 nm are required.

Schematic setup of the LWI-experiment in mercury.
Schematic setup of the LWI-experiment in mercury.

435.8 nm Laser

The laser system for the 435.8 nm radiation consists of an external cavity diode laser (ECDL) which emits about 25 mW at 871.6 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 Periodically Poled Potassium Titanyl Phosphate (PPKTP) crystal with a nonlinear coefficient of 9.5pm/V in a bow-tie shaped build-up cavity 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 ultra low-noise current controller. It was possible to achieve a conversion efficiency of over 40% and reach an output power at 435.8 nm of 416 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 10mW at 546.1 nm. Considering the use of this laser as the weak coupling laser, a harmonic power of about 5mW is enough for the experiment.

404.7 nm Laser

The incoherent repump is realized with a laser diode in ECDL configuration modulated with white noise through an AOM. 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.

253.7 nm Laser

Also a laser system at the desired wavelength of 253.7 nm is used to first test AWI (amplification without inversion) which is the first step before achieving LWI. The system consists of an ECDL at the wavelength of 1014.8 nm. The infrared light is then amplified in an fiber amplifier and converted to the desired wavelength in two consecutive cavities using LBO and BBO as the nonlinear crystals. This laser system is actually used for our MOT experiment but can be shared to achieve AWI.

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.

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 435.8 nm, 546.1 nm and 404.7 nm are developed. With these laser systems and the 253.7nm system, we showed three photon resonance in mercury which is the first step for LWI. Furthermore we were able to show amplification without inversion (AWI) for the wavelength of 253.7nm.

Also a series of simulations to identify the critical parameters of the experiment was conducted.

Further improvements of the laser systems based on the simulations should improve the AWI experiment. The spectral width and power of the incoherent pump laser at 404.7nm should be increased. Also higher output power of the strong coupling laser at 435.8nm should improve the experimental results of AWI. 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.

References


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).

3) 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).

4) 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).

5) Daniel Preißler, Noah Eizenhöfer, Jens Gumm and Thomas Walther
On identifying critical parameters in an amplification without inversion setup in mercury
J. Phys. B: At. Mol. Opt. Phys. 55 244001 (2022)