The use of alkalis as a gain element for lasers has been known for a number of years. Several years after the discovery of the laser an actual experimental demonstration of laser action in an alkali vapor occurred. However, since these early tests, there have been no convincing demonstrations of practical laser systems using alkalis.
Typically, alkali laser systems utilize the three lowest-lying electronic levels of the alkali atom for functionality. These levels include the 2S1/2 ground electronic level, and the first two 2P electronic levels, 2P1/2, and 2P3/2. These levels form a pure “three-level-laser” scheme.
In one particular alkali laser scheme, the alkali atom gain medium is excited (pumped) at a wavelength matching the wavelength of the 2S1/2→2P3/2 electric-dipole-allowed transition, also referred to as the D2 transition. After kinetic relaxation of pump excitation from the 2P3/2 level to the excited 2P1/2 electronic level, laser emission takes place on the 2P1/2→2S1/2 transition, also referred to as the D1 transition. These transition lines of the alkalis, known as the D1 and D2 lines, present textbook examples of fully allowed electric dipole transitions.
FIG. 1 shows the energy levels 100 involved in an optically pumped Rubidium (Rb) laser, in accordance with one alkali laser scheme. In this case, optical pumping on the alkali D2 transition (2S1/2→2P3/2) is followed by rapid relaxation from the 2P3/2 to the 2P1/2 level through collisions with a buffer gas, and then lasing on the D1 transition (2P1/2→2S1/2).
The problem with this scheme is the required rapid relaxation from the 2P3/2 state to the 2P1/2 state shown as the dotted line transition in FIG. 1 and commonly referred to as the fine-structure mixing transition. The rapid relaxation needed for the fine-structure mixing transition does not automatically occur in unperturbed alkali atoms. Thus, the buffer gas is typically utilized to facilitate such transition. Further, to enable laser action, the relaxation must occur very rapidly, such as on a time scale competitive with radiative transitions out of the 2P3/2 and 2P1/2 states.
Previously, it was proposed to use a small saturated hydrocarbon molecule, such as ethane, to rapidly mix via collisions with the alkali vapor the fine-structure states (2P3/2 and 2P1/2). Such small saturated hydrocarbon molecules were known to have very large fine-structure mixing cross sections and very small excitation quenching cross sections. Following that proposal, several experimental verifications of the proposed lasing scheme were undertaken, in which ethane was used as the fine-structure mixing gas.
While this proposed scheme presented an interesting possibility for a new class of lasers, the use of organic molecules such as ethane to accomplish rapid fine-structure mixing is problematic due to the molecules decomposition and subsequent deposition of carbonaceous deposits in an optical cavity. In essence, the decomposition of the organic molecule can foul the optical surfaces of a laser cell, precluding the possibility of efficient and reliable laser operation.
There is thus a need for addressing these and/or other issues associated with the prior art.