Improving sodium laser guide star brightness by polarization switching

Abstract

Optical pumping with circularly polarized light has been used to enhance the brightness of sodium laser guide star. But the benefit is reduced substantially due to the precession of sodium atoms in geomagnetic field. Switching the laser between left and right circular polarization at the Larmor frequency is proposed to improve the return. With ESO's laser guide star system at Paranal as example, numerical simulation shows that the return flux is increased when the angle between geomagnetic field and laser beam is larger than 60°, as much as 50% at 90°. The proposal is significant since most astronomical observation is at angle between 60° and 90° and it only requires a minor addition to the delivery optics of present laser system.

Figure 1. Schematic diagram of the polarization switching method for improving the sodium excitation.

(a) the sodium D₂ transitions and energy levels. (b) Population dynamics on the five F = 2 ground state magnetic sublevels when the angle between laser beam and magnetic field θ is π/2. τ is the period of Larmor precession. (c) The timing diagram of laser polarization switching.

Figure 2. Calculated return flux for different conditions.

(a) Specific return flux ψ(I) for different polarization conditions and different angles between laser beam and magnetic field θ. CP stands for circular polarization, and PS stands for polarization switching. (b) The calculated return flux ψ versus θ at a laser power intensity of 10 W/m² with repumping power fraction of 0.1.

Figure 3. Occupation probabilities of the eight ground state’s magnetic sublevels as functions of the normalized time  = t/τ for θ = π/2 and I = 10 W/m².

(a) Polarization switching at Larmor frequency of 159.207 kHz, (b) cw laser of single polarization state.

Figure 4. Specific return flux in case of θ = π/2 and I = 10 W/m².

(a) return flux ψ as function of repumping fraction q, and (b) calculated return flux ψ versus angle between laser beam and magnetic field for both q = 0.1 and q = 0.2. Occupation probabilities evolution of the eight ground state’s magnetic sublevels at (c) q = 0.1 and (d) q = 0.2. The figure (c,d) share the same legend as in Fig. 3(b).

Figure 5. ψ as a function of the polarization switching frequency at I = 10 W/m² and θ = π/2.