Hybrid lasers for directed energy applications

There are several military applications that require high-power lasers that can propagate for large distances through the atmosphere. Missile defense systems are perhaps the best known ‘directed energy’ examples. As the beam must arrive at the target with sufficient localization to deliver a high flux to a small area, these applications demand very high beam quality.

Due to the stringent co-requirements for power and beam quality, gas-phase chemical lasers have been the first choice in directed energy development.¹ Gas-phase lasers can achieve continuous operation at megawatt (MW) powers, with remarkably high beam quality. A primary advantage of these devices is that heat dissipation is readily handled using gas flow, so that thermal and refractive index gradients within the medium can be minimized. However, from the perspective of deployment, chemical lasers present significant technical and logistical problems since their reagents are both hazardous and rapidly consumed during operation.

Solid-state devices, such as semiconductor and fiber lasers, are efficient, compact, and relatively easy to deploy. The downside for solid-state materials is that it is difficult to meet the power and beam quality requirements. The construction of a single-element MW class solid-state laser poses technical challenges that are beyond the reach of current technology. An alternative approach is to combine the outputs from an array of solid-state lasers into a single beam. Attainment of the required beam quality requires coherent beam combination, so phase control of multiple lasers sources becomes the key issue. Options include both active and passive phase control methods, but the former presents significant complications. For passive methods, the most attractive scheme is to use optical pumping of a gas, thereby merging the best characteristics of gas-phase and solid-state lasers.

Figure 1. Three level laser scheme for an optically pumped atomic gas.

At present, the development of hybrid solid-state/gas-phase laser systems is focused on diode-pumped alkali vapor lasers (DPALs).^{2, 3} Alkali atoms are preferred because their absorption wavelength matches the output of commercial diode sources, and their emission wavelength is suitable for long-range atmospheric propagation. In the energy level scheme (see Figure 1), the excitation or pumping (3←1) of a DPAL is via the ²P_3/2←²S_1/2 transition, while lasing (2→1) happens through the ²P_1/2→²S_1/2 emission line (which corresponds to 770.1, 795.0 and 894.6 nm for potassium, rubidium and cesium, respectively). Rapid population transfer between the ²P_3/2 and ²P_1/2 levels is accomplished through collisional relaxation with a buffer gas (usually methane or ethane). The development of DPALs has advanced to the point where a 1kW cesium device was reported recently.⁴

Figure 2. The apparatus used to demonstrate lasing of optically pumped, metastable rare gas atoms. The pump source (labeled OPO System) produces light with wavelength λ₁, which optically excites the rare gas mixed with helium (Rg/He) in the discharge chamber (labeled Excimer). The lasing wavelength λ₂ is measured by the photodiode. A pulsed delay generator controls the timing of the laser pulses from the pump source with respect to the electric discharges within the chamber.

Of course, DPALs are not without problems. The metals must be heated to produce adequate vapor pressures, and the hot vapors are chemically aggressive. Reactions with the hydrocarbon buffer gases currently consume the reagents and produce particulate contaminants. The vapor must be kept away from the gas cell windows to avoid photochemically induced surface damage.

Our research explores the possibility of developing DPAL-like laser systems using inert atomic gas media.⁵ As commonly known, a low-power electrical charge can generate long-lived excited states of the rare gases (neon, argon, krypton, and xenon). These excited states have a single electron in the outermost s-orbital, and their optical properties strongly resemble those of the alkali metals. The lowest energy metastable state has the electronic configuration np⁵(n+1)s³P₂, and transitions to higher states, i.e. (n+1)p←(n+1)s, can induce optically pumped lasing. The energy level scheme from Figure 1 has 1=³P₂, 2=³S₁ and 3=³D₃. The main advantages of this system are that the lasing species is a gas at room temperature, and that the critical excited state relaxation process (3→2) can be accomplished using helium. The typical rate constants for this relaxation step are such that the helium pressure needed is on the order of 0.5 to 1.5 atmospheres.

We measured the gain and lasing for metastable rare gas atoms using the experimental arrangement shown in Figure 2. To produce the metastable states, we employed the discharge chamber of a commercial excimer laser in which the optical cavity consisted of a total reflector and an uncoated window. We filled the chamber with a mixture of a heavy rare gas in an excess of helium. A tunable pulsed laser (labeled OPO in Figure 2) provided longitudinal optical excitation. We timed the pump pulses so that they arrived in the chamber several microseconds after the termination of the discharge pulse. With this system, we demonstrated pulsed lasing for neon (703.2nm), argon (912.6nm), krypton (893.1nm) and xenon (980.2nm). We observed peak powers as high as 27kW, indicating that scaling to higher powers should be feasible.

In our most recent experiments,⁶ we substituted the pump pulse laser with a line-narrowed, continuous wave diode laser. Despite having lower power input, the system was still able to optically excite the metastable state, which in this case was that of argon. This shows that the power available from a small diode device is sufficient to achieve lasing (the threshold was 3.5W for the focused pump beam). The initial indications are that optically pumped rare gas lasers may offer the same desirable performance characteristics as DPALs, without the chemical complexities. Our future work will be directed towards developing continuous discharge sources for metastable production and conducting studies of power scaling.

This work was supported by the Joint Technology Office through the Air Force Office of Scientific Research (AFOSR) under contract FA9550-07-1-0572.