Michael K. Shaffer

Photoionization in alkali lasers

We have calculated photoionization rates in alkali lasers. The photoionization of alkali atoms in the gain medium of alkali lasers can significantly degrade the laser performance by reducing the neutral alkali density and with it the gain. For a ten atmosphere Rb laser and a Cs exciplex laser, the photoionization induced alkali atom loss rates are greater than 10(5) sec(-1). These high loss rates will quickly deplete the neutral alkali density, reducing gain, and may require fast, possibly, supersonic flow rates to sufficiently replenish the neutral medium for CW operation.

Cs laser with unstable cavity transversely pumped by multiple diode lasers

We have demonstrated a Cs vapor laser with an unstable resonator transversely pumped by 15 narrowband laser diode arrays. A slope efficiency of 43%, a total optical efficiency of 31% and a maximum output power 49 W were obtained with a pump power of 157 Watts.

Scaling of diode-pumped Cs laser: transverse pump, unstable cavity, MOPA

There has been recent interest in Diode Pumped Alkali Lasers (DPALs) and their scaling to higher powers. Scaling of DPALs to high powers requires using multiple pump sources such as laser diode arrays or stacks of arrays. Coupling of multiple pump beams into the laser gain medium can be realized using a transverse pumping scheme that is most efficient for the laser operating with large mode volume. We have demonstrated Cs laser with unstable resonator transversely pumped by 15 narrowband diode laser arrays. This laser operates on lowest transverse mode with a diameter of 7 mm with an optical-to-optical efficiency higher than 30%. An alternative power scaling approach: Master Oscillator and power Amplifier (MOPA) system with transversely pumped by multiple diode lasers Cs amplifier was studied experimentally and demonstrated high optical efficiency.

Efficient potassium diode pumped alkali laser operating in pulsed mode.

This paper presents the results of our experiments on the development of an efficient hydrocarbon free diode pumped alkali laser based on potassium vapor buffered by He gas at 600 Torr. A slope efficiency of more than 50% was demonstrated with a total optical conversion efficiency of 30%. This result was achieved by using a narrowband diode laser stack as the pump source. The stack was operated in pulsed mode to avoid limiting thermal effects and ionization.

Demonstration of a diode pumped continuous wave potassium laser

This paper presents a first demonstration of a diode pumped Potassium laser. Two narrowband laser diode arrays with a linewidth about 10 GHz operating at 766.7 nm were used to pump Potassium vapor buffered by Helium gas at 600 torr. A stable laser cavity with longitudinal pumping and orthogonal polarizations of the pump and laser beams was used in this experiment. A slope efficiency about 25% was obtained.

Measurements of the gain medium temperature in an operating Cs DPAL

A Mach-Zehnder interferometer was used for contactless measurement of the temperature of the gain medium within a static cell of Cs DPAL. The maximum temperature recorded approached 700° C leading to a significant degradation of laser performance. This work also examined lasing and non-lasing heat deposition and has shown that as much as 85% of the heating in a DPAL gain medium can be attributed to quenching.

Experimental study of the Cs diode pumped alkali laser operation with different buffer gases

Abstract. Cs diode pumped alkali laser (DPAL) operation using ethane, methane, and mixtures of these hydrocarbons with the noble gases He and Ar as a buffer gas for spin–orbit relaxation was studied in this work. The best Cs DPAL performance in continuous wave operation with flowing gain medium was achieved using pure methane, pure ethane, or a mixture of ethane (minimum of 200 Torr) and He with a total buffer gas pressure of 300 Torr.

Frequency-doubling of a high power cesium vapor laser using a PPKTP crystal.

447.3 nm blue light generation was demonstrated through direct frequency doubling a continuous-wave Cesium vapor lasers 894.6 nm output using a PPKTP nonlinear crystal. The diode-pumped Cs vapor laser has an output power as large as 10 W with a very narrow emission linewidth less than 10 GHz. The PPKTP crystal is about 30 mm long and has a moderate acceptance wavelength bandwidth. The second harmonic wave generation efficiency achieved is about 4.4 %/W. Power depletion in second harmonic pulses, which relates to the absorption related thermal effects, was observed when using high pump power, long pulse duration, or high repetition rate.

Low-pressure cesium and potassium diode pumped alkali lasers: pros and cons

Abstract. This paper presents the results of our experiments on a comparative study of cesium and potassium diode pumped alkali lasers (DPALs) aimed to determine which of these two lasers has more potential to scale to high powers. For both lasers, we have chosen a “low-pressure DPAL approach,” which uses buffer gas pressure of about 1 atm for spin-orbit mixing of the excited states of alkali atoms to provide population inversion in the gain medium. The goal of this study was to determine power-limiting effects, which affect the performance of these DPALs, and find out how these limiting effects can be mitigated. We studied the performance of both lasers in CW and pulsed modes using both static and flowing gain medium and pump with different pulse duration. We observed output power degradation in time from the initial value to the level corresponding to the CW mode of operation. As a result of this study, some essential positive and negative features of both DPALs were revealed, which should be taken into account for power-scaling experiments.

Alkali Lasers - a New Type of Scalable High Power Lasers

Historically, an optically pumped alkali (potassium) vapor laser was the first laser proposed by A.L. Schawlow and C.H. Townes in 1958, but was not experimentally demonstrated at that time. In the next 45 years, many experiments with alkali vapors were performed that demonstrated stimulated emission, gain and amplified spontaneous emission. However, the real interest in alkali vapor lasers appeared in the last several years, when really efficient lasing in Rb and Cs vapors was demonstrated. The US Air Force Academy performed a variety of experiments on alkali lasers, including the demonstration of efficient Rb, Cs and K vapor lasers, power scaling experiments with multiple diode laser pumping sources and experiments on diode pumped alkali vapor amplifiers. As a result of this effort we have increased the alkali lasers output power to tens of watts in continuous wave operation. In this paper we present a review of the most important achievements in high power alkali lasers research and development, discuss some problems existing in this field.