We demonstrate backward-directed continuous-wave (cw) emission at 2.21 {\mu}m generated on the 4P3/2-4S1/2 population-inverted transition in Na vapors two-photon excited with resonant laser light at 589 and 569 nm. Our study of power and atom-number-density threshold characteristics shows that lasing occurs at sub-10 mW total power of the applied laser light. The observed 6 mrad divergence is defined mainly by the aspect ratio of the gain region. We find that mirrorless lasing at 2.21 {\mu}m is magnetic field and polarization dependent that may be useful for remote magnetometry. The presented results could help determine the requirements for obtaining directional return from sodium atoms in the mesosphere
Mirrorless lasing from the mesospheric sodium layer could transform laser-guide-star emission into a directional, gain-driven source. Here we convert this possibility into quantitative requirements, using a ten-level velocity-resolved rate-equation model and an exact compression--irradiance equivalence. Continuous-wave operation remains below threshold across the accessible parameter space. The transient 4D5/2-4P3/2 channel at 2339 nm reaches threshold at pulsed irradiances of 104-105 Wm-2, given six- to ten-fold velocity compression. The obstacle is therefore not laser power but momentum transfer: spontaneous scattering delivers at most one recoil per event at a bounded rate, leaving chirp-driven compression an order of magnitude short. This gap defines the technological challenge for mesospheric mirrorless lasing.
Mirrorless lasing from the mesospheric sodium layer could transform laser-guide-star emission into a directional, gain-driven source. Here we convert this possibility into quantitative requirements, using a ten-level velocity-resolved rate-equation model and an exact compression--irradiance equivalence. Continuous-wave operation remains below threshold across the accessible parameter space. The transient 4D5/2-4P3/2 channel at 2339 nm reaches threshold at pulsed irradiances of 104-105 W/m-2, given six- to ten-fold velocity compression. The obstacle is therefore not laser power but momentum transfer: spontaneous scattering delivers at most one recoil per event at a bounded rate, leaving chirp-driven compression an order of magnitude short. This gap defines the technological challenge for mesospheric mirrorless lasing.
Directed mirrorless lasing from the mesospheric sodium layer has been proposed as a way to overcome the isotropy of laser-guide-star fluorescence, with demonstrated cell-scale analogues and a demonstrated stand-off magnetometry application. Several transition paths on the Na ladder compete for the same pump photons, and which of them can sustain a population inversion---and under what pumping format---is usually settled by numerical scans of a multi-level rate model. Here the same question is answered in closed form, and three design-relevant results follow from the atomic data alone. (1) A cascade inversion on
u→l requires
τu/τl>(gu/gl)bul. The inequality reproduces the full continuous-wave classification---admitting
4P3/2→4S1/2 (2.21~\textmu m),
4P3/2→3D5/2 (9.1~\textmu m) and the fine-structure companion
4S1/2→3P1/2 (1138~nm), excluding
4D5/2→4P3/2 (2.34~\textmu m) and
3D5/2→3P3/2 (819~nm)---and shows the 2.34~\textmu m line to miss by only 9\%, which is why it is transiently accessible. (2) Velocity selectivity reverses the naive scheme ranking because a Doppler-averaged treatment, exact for one-step pumping, underestimates two-step pumping by a participation factor of order the Doppler-to-natural width ratio,
∼102; equal division of power between two pump beams is exactly optimal. (3) The transient window on 2.34~\textmu m closes after $\sim
2τ(4P)\approx210$~ns, set by the reservoir lifetime rather than by the pump. Each conclusion is traceable to a lifetime, a branching ratio or a linewidth, and transfers to another species without rerunning a model.
Population inversion on the 7S1/2− 6P3/2transition of cesium atoms is built by 455 nm diode laser pumping, and continuous-wave (cw) lasing is observed without the feedback on the gain medium from cavity mirrors. The stimulated emission at 1.47 µm infrared wavelength is restricted to a narrow angle along the direction of propagation of the pumping laser. Evidence of threshold-like characteristic is demonstrated in the cw mirrorless lasing dependence on pumping laser detuning, atomic density, and pumping light intensity. This compact structure can be used to generate mirrorless laser in a number of infrared wavelengths applied for the communication area. Moreover, it is expected to realize the narrow-linewidth optical frequency standard from an ultralow-finesse optical cavity.
Abstract We report experimental evidence of degenerate mirrorless lasing in the forward direction under excitation of a dilute atomic Rb vapor with a single linearly polarized cw laser light resonant with cycling F e > F g atomic D 2 transitions. Light polarized orthogonally to the laser light is generated for the input light intensity exceeding a threshold value of ∼3 mW cm −2 . Application of a transverse magnetic field directed along the input light polarization reveals a sharp ∼20 mG wide magnetic resonance centered at B = 0. Increasing the incident light intensity from 3 to 300 mW cm −2 , the generated light undergoes rapid amplitude increase followed by a decline and resonance broadening. Such nonlinear behavior of the observed magnetic resonance is attributed to the population inversion on optical transitions between magnetic sublevels established under linearly polarized excitation. We present observations that indicate that a combination of nonlinear-optical effects occurs in this system, including degenerate mirrorless lasing and four-wave mixing.