Quasi-four-level laser design and analysis of Nd:YAG operating at the 946 nm transition

Abstract

Nd:YAG, well known for its operation at 1064nm, has a weaker transition at 946nm, whose lower level is thermally populated. This dissertation describes the design and development of a diode pumped, room temperature, quasi-four-level laser operating at the 946nm transition of Nd:YAG. The design addresses two primary issues in obtaining an efficient, high energy oscillator at 946nm. These are the ground state reabsorption losses due to the thermally occupied lower laser level, and the population inversion losses incurred at the much stronger 1064nm, transition. With 55 mJ in the normal mode, and 25 mJ in the q-switched mode, the output energies obtained are the highest energies per pulse reported to date for a diode pumped, 946mn Nd:YAG laser. A quasi-four-level laser theory is developed and used to optimize oscillator parameters affected by the thermally occupied lower laser level. The laser material length and the folded V shaped cavity are selected to maximize the gain per round trip in the cavity. The availability of stacked and microlensed diode array bars, along with an efficient pump coupling technique, allows the use of an end pumped configuration which provides the high pump density required to reach threshold in quasi-four-level lasers. The oscillator design was further refined to eliminate possible parasitic lasing paths and minimize amplified spontaneous emission losses at the 1064nm, transition. A large diameter laser disk with a Samarium doped cladding, which absorbs the 1064nm, radiation, reduces the number of 1064nm, ASE paths which deplete the inversion density in the pumped volume. The cladding significantly improves the storage efficiency, and hence the q-switched efficiency, of the oscillator. Although the oscillator was developed specifically for remote sensing of atmospheric water vapor, other applications can also benefit from the development of an efficient 946nm laser source. When frequency doubled, this wavelength allows access to the blue, which is highly desirable for high density data storage, displays, biological applications, and underwater communications.