Diode Pumped Alkali Laser (DPAL) is one of the main candidates for development of a high power directed energy system producing laser beam from a single aperture with high spatial quality. Currently, several groups in the US and abroad demonstrated DPAL systems with kW level output power and efficiency higher than 50%. At the same time, the DPAL power scaling experiments revealed some limiting effects, which require detailed study to understand the nature of these effects and ways to mitigate them. Examples of such effects are output power degradation in time, alkali cell windows and gain medium contamination and damage that causes lasing efficiency decrease or even lasing termination. These problems can be connected to thermal effects, ionization, chemical interactions between the gain medium components and alkali cells materials. Study of all these and, possibly, other limiting effects and ways to mitigate them is very important for high power DPAL development. In this paper we present our new results of experiments on measurements of the temperature rise in the gain medium of operating DPAL leading to the output power degradation even before visible damage in the gain cell occurs. This degradation can be both recoverable and non-recoverable, depending on operation conditions and the system design.
Experiments on power scaling of Diode Pumped Alkali Lasers (DPALs) revealed some limiting parasitic effects such as alkali cell windows and gain medium contamination and damage, output power degradation in time and others causing lasing efficiency decrease or even stop lasing1 . These problems can be connected with thermal effects, ionization, chemical interactions between the gain medium components and alkali cells materials. Study of all these and, possibly, other limiting effects and ways to mitigate them is very important for high power DPAL development. In this talk we present results of our experiments on temperature measurements in the gain medium of operating Cs DPAL at different pump power levels in the range from lasing threshold to the levels causing damage of the alkali cell windows. For precise contactless in situ temperature measurements, we used an interferometric technique, developed in our lab2 . In these experiments we demonstrated that damage of the lasing alkali cell starts in the bulk with thermal breakdown of the hydrocarbon buffer gas. The degradation processes start at definite critical temperatures of the gain medium, different for each mixture of buffer gas. At this critical temperature, the hydrocarbon and the excited alkali metal begin to react producing the characteristic black soot and, possibly, some other chemical compounds, which both harm the laser performance and significantly increase the harmful heat deposition within the laser medium. This soot, being highly absorptive, is catastrophically heated to very high temperatures that visually observed as bulk burning. This process quickly spreads to the cell windows and causes their damage. As a result, the whole cell is also contaminated with products of chemical reactions.
Cs DPAL operation using Ethane, Methane and mixtures of these hydrocarbons with noble gases He and Ar as a buffer gases 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.
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.
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.
We report on a model of highly efficient static, pulsed K DPAL [Zhdanov et al, Optics Express 22, 17266 (2014)], where
Gaussian spatial shapes of the pump and laser intensities in any cross section of the beams are assumed. The model
shows good agreement between the calculated and measured dependence of the laser power on the incident pump power.
In particular, the model reproduces the observed threshold pump power, 22 W (corresponding to pump intensity of 4
kW/cm2), which is much higher than that predicted by the standard semi-analytical models of the DPAL. The reason for
the large values of the threshold power is that the volume occupied by the excited K atoms contributing to the
spontaneous emission is much larger than the volumes of the pump and laser beams in the laser cell, resulting in very
large energy losses due to the spontaneous emission. To reduce the adverse effect of the high threshold power, high
pump power is needed, and therefore gas flow with high gas velocity to avoid heating the gas has to be applied. Thus, for
obtaining high power, highly efficient K DPAL, subsonic or supersonic flowing-gas device is needed.
This paper based on the talk presented at the Security plus Defence 2015 Conference held at Toulouse, France in September 2015. In this paper we present the results of our experiments on a comparative study of Cesium and Potassium based DPALs aimed to determine which of these two lasers has better potential for scaling to high powers. For both lasers we have chosen a so called “low pressure DPAL approach”, which uses buffer gas pressure of about 1 Atm for spin-orbit mixing of the exited 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 performance of these DPALs, and find out how these limiting effects can be mitigated. The experiments were performed using both static and flowing gain medium. In our experiments, we studied the performance of both lasers in CW and pulsed modes with different pulse duration and 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, we revealed some essential positive and negative features of both DPALs, which should be taken into account for power scaling experiments.
This paper presents the results of our experiments on development of the efficient hydrocarbon free Diode Pumped
Alkali Laser based on potassium vapor buffered by He gas at 600 Torr. We studied the performance of this laser
operating in pulsed mode with pulses up to 5 ms long at different pulse energies and cell temperatures. A slope
efficiency of more than 50% was demonstrated with total optical efficiency about 30% for the pump pulses with duration
about 30 μs. For the longer pump pulses the DPAL efficiency degraded in time with a characteristic time in the range
from 0.5 ms to 4.5 ms depending on the pump pulse energy and cell temperature. The recorded spectrum of the side
fluorescence indicates that multi-photon excitation, energy pooling collisions and ionization may be strong candidates
for explaining the observed performance degradation.
Alkali vapor lasers are under extensive research and development during the past decade because of their potential for scaling to high powers while maintaining a good beam quality. Also, a possibility of using efficient diode lasers for pumping alkali vapor promises high total wall plug efficiency for a Diode Pumped Alkali Laser (DPAL). Since the first DPAL demonstration with output power of 130 mW in 20051, a significant progress in this field was achieved. The output power of about 1 kW in continuous wave (CW) operation with optical efficiency close to 50% was recently demonstrated for a Cs DPAL2. Also, the DPALs based on other alkali metals (Rubidium and Potassium) were demonstrated3,4 . In spite of these significant achievements, there are still several problems in DPAL power scaling exist that must be addressed. Among them are the thermal5 and photoionization6 issues that become important even at power level about several tens of watts. In this paper we present a historical review of the alkali laser research and development, discuss the most important achievements and future perspectives in this field of research.
In this review we present an analysis of optically pumped alkali laser research and development from the first proposal in 1958 by Schawlow and Townes to the current state. In spite of the long history, real interest in alkali vapor lasers has appeared in the past decade, after the demonstration of really efficient lasing in Rb and Cs vapors in 2003 and the first successful power scaling experiments. This interest was stimulated by the possibility of using efficient diode lasers for optical pumping of the alkali lasers and by the fact that these lasers can produce a high quality and high power output beam from a single aperture. We present a review of the most important achievements in high power alkali laser research and development, discuss some problems existing in this field, and provide future perspectives in diode pumped alkali laser development.
Diode pumped alkali lasers attract growing attention during the past several years because they have demonstrated
potential to compete and, possibly, replace the best existing high power laser systems. In spite of the fact that an
optically pumped alkali (potassium) vapor laser was first proposed by A.L. Schawlow and C.H. Townes in 1958, the
intensive research and development of alkali vapor started only in 2003, when really efficient lasing in Rb and Cs
vapors was demonstrated. The interest to this research was stimulated by the possibility of using efficient diode lasers
for optical pumping of the alkali gain medium that promises high overall efficiency of the device. 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 were
performed during the past several years. 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 and future perspectives in
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.
An examination of the efficiencies of three commonly used nonlinear crystals (PPKTP, LBO, and BiBO) when
generating second harmonic of a Cesium laser is presented. The experiment investigates both the intracavity and single
pass second harmonic generation of 895 nm Cs laser light when operating in quasi-CW and in CW modes and pumped
by several watts. A degradation of the conversion efficiencies for each crystal was observed when high fundamental
powers or a high duty cycle of the pump were used. For a Cs laser operating at 894nm, PPKTP is found to be the optimal
crystal for intracavity SHG in both pulsed and CW modes when operating at SHG powers of several watts. At higher
powers, however, the increased absorption coefficient of PPKTP at 447nm, compared to that of BiBO or LBO, may
become significant to where another crystal will be more appropriate for this application. Maximum blue light power
obtained with PPKTP crystal was about 1.5W in CW mode and 2.5W in QCW.
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.
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.
Scaling of alkali lasers to higher powers requires combining beams of multiple diode laser pump sources. For
longitudinal pumping this can be very complicated if more than four beams are to be combined. In this paper we report a
first demonstration of a transversely pumped Cs laser with fifteen laser diode arrays. The LDA pump beams were
individually collimated with a beam size of about 1 x 4 cm as measured at a 1 m distance from the diodes. All these
beams were incident on a cylindrical lens to be focused and coupled through the side slit of a hollow, cylindrical diffuse
reflector which contained the Cs vapor cell. We measured the output power and efficiency of the Cs laser for pump
powers up to 200 W at different cell temperatures. Although the values of output power and slope efficiency obtained for
this laser system were less than those for a longitudinally pumped alkali laser, these recent results can be significantly
improved by using a more optimal laser cavity design. The demonstrated operation of Cs laser with transverse pumping
opens new possibilities in power scaling of alkali lasers.
In this review we present the analysis of optically pumped alkali lasers research and development from their first proposal in 1958 to the current state. Main achievements and problems existing in this field of research are discussed and possible solutions of the problems are proposed. Detailed description of the most important experiments and their results are presented. We have tried to provide an extensive list of references on this subject.
The Laser and Optics Research Center of the US Air Force Academy started a research program on optically pumped
alkali lasers in 2004. We demonstrated the first diode pumped alkali (cesium) vapor laser, the first optically pumped
potassium laser, the most efficient (slope efficiency higher than 80%) cesium laser, and diode pumped Rb and Cs lasers
with highest output powers (17 W and 48 W respectively). We have developed an efficient Cs amplifier with a small
signal amplification factor of 145 and tunable single mode Cs laser for scientific applications. In this paper we present a
review of our main results and recent achievements in high power alkali laser development, discuss some problems
existing in this field and ways to solve them.
Diode pumped alkali vapor lasers developed during the last several years have the potential to achieve high power.
Efficient operation of Rubidium, Cesium and Potassium lasers has been demonstrated. Laser slope efficiencies higher
than 80% have been achieved. A diode laser pumping can provide high overall efficiency of these devices. A diode
pumped continuous wave 10 W Cs laser and continuous wave 17 W Rb laser were demonstrated. In this paper we review
the main results and recent achievements in high power alkali lasers development, discuss some problems existing in this
field and ways to solve them.
Optically pumped alkali vapor lasers have been developed during last several years. Efficient operation of Cesium,
Rubidium and Potassium vapor lasers has been demonstrated. Laser slope efficiencies higher than 80% have been
achieved. In this paper we present the latest achievements in this field, discuss the main directions and problems in high
power alkali lasers development and possible solutions of these problems.
An efficient Cesium vapor laser pumped with a continuous wave narrowband Laser Diode Array (LDA) has been
demonstrated. To obtain a high lasing efficiency, it is necessary to narrow the linewidth of the pumping LDA to match
the Cs atom absorption line. An external cavity with holographic grating was used to narrow the linewidth of a
commercially available LDA to a value of 11 GHz that matches the Cs vapor absorption line broadened by a buffer gas
at atmospheric pressure. The developed pump source was used for pumping a Cs vapor laser, which operated at 894 nm.
Preliminary experiments yielded 400 mW output power and about 20% slope efficiency. The laser efficiency can be
significantly increased by optimizing the cell and cavity design and matching the pump beam to the cavity mode size.
An extensive development of high power lasers for military and civilian applications has resulted in several highly successful laser systems based on chemical lasers. These lasers have some undesirable features, most notably their use of dangerous chemicals and excessive size. Alternative laser systems such as solid state, fiber lasers and semiconductor lasers have not achieved the necessary powers and beam quality. In this paper we present the results of our work on optically pumped cesium vapor laser development. We demonstrated efficient cesium laser operation at wavelength 894 nm with diode laser pumping. The measured optical efficiency was more than 32% with an overall electrical to optical efficiency of 15%. With an improved cavity design and narrowband pump source we have demonstrated Cs laser with slope efficiency of 81% and overall optical efficiency of 63%. This laser can be scaled to higher powers by increasing the volume of the active medium and number of narrowband pump diode lasers.
Collision-induced absorption spectra have been measured in a wide temperature (80 - 300 K) and pressure (10 - 150 atm) range for the a ← X and b ← X absorption bands of oxygen. The peak and integrated absorption coefficients and monomolecular and multimolecular collision-induced cross-sections are measured for an oxygen density range of 4x1020 cm-3 to liquid. No temperature dependence was found in the absorption of the high density oxygen.
A kinetics model to describe the behavior of singlet delta O2 in optically pumped liquid oxygen has been developed and tested against experimental data. The model was developed to study alternative methods of O2(a1Δg) production for the Chemical Oxygen Iodine Laser (COIL), and has been used in conjunction with experimental data to determine a value for the pooling rate constant in liquid oxygen of 1± 0.5 x 10-17 cm3molecule-1s-1.
We have developed a new senior-level undergraduate laboratory course at the US Air Force Academy. The students perform six experiments that include; optical modulators, waveguiding, laser kinetics, CO2 lasers, harmonic generation and the measurement of ultrashort laser pulses. These experiments were chosen so that there is an integrated theme of lasers and optics, to teach experimental methods and reinforce fundamental physics concepts.