22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Plasma formation during operation of a diode pumped alkali laser
A.H. Markosyan and M.J. Kushner
University of Michigan, Electrical Engineering and Computer Science Department, 1301 Beal Avenue, US-48109-2122
Ann Arbor, MI, U.S.A.
Abstract: Diode pumped alkali lasers (DPALs) achieve narrow-band lasing action on the
resonant lines of alkali atoms following pumping by broad band semiconductor lasers.
Resonant excitation of alkali vapor is known to lead to plasma formation through electron
heating by super-elastic relaxation and associative ionization. A first principles global
model was developed to investigate the operation of the Ar/Cs DPAL system and the issues
that arise if plasma formation occurs. Plasma characteristics and their impact on laser
performance are reported during high repetition rate, high power optical pumping.
Keywords: diode pumped alkali laser, DPAL, global model, laser produced plasma
1. Introduction
Semiconductor diode lasers have the advantage of high
electrical-to-optical efficiency however they suffer from
poor optical quality. The diode pumped alkali laser
(DPAL) is a method whereby the poor optical quality of
diode lasers can be converted to the high optical quality of
an atomic laser [1]. In DPAL, the resonant transition of
the alkali atom, typically the 2P 3/2 (D 2 line) is pumped by
the diode laser followed by collisional quenching to
the 2P 1/2 . Lasing then occurs on the 2P 1/2 → 2S 1/2
transition (D 1 line), which requires that the ground state
be inverted. DPALs have been investigated for their
potential as high-power lasers [2-5]. Recent studies of Rb
and K DPAL have yielded efficiencies in excess of 50%
[2, 3]. Models predict efficiencies exceeding 90%. Our
interest here is on the DPAL using Cs vapor.
LIBORS (Laser Ionization Based on Resonance
Saturation), first proposed by Measures [6] in 1970, is an
efficient means for producing plasmas in alkali vapor with
low laser intensity. Electron heating by super-elastic
relaxation of the laser produced resonant state M* can
rapidly avalanche the alkali vapor to nearly full ionization
[7]. In many alkali vapors, the process can be initiated by
associative ionization (M* + M* → M 2 + + e) by resonant
states. This process in Cs is endothermic, and so requires
states higher than Cs(62P 1/2 , 62P 3/2 ). With pre-existing or
laser generated seed electrons, super-elastic electron
heating and associative ionization during DPAL operation
may result in plasma formation through the LIBORS
process. The resulting plasma has the potential to reduce
or quench laser oscillation through electron collision
mixing.
In this paper, we discuss results from a computational
investigation of the pulsed DPAL system in Ar/Cs/C 2 H 6
mixtures. Pumping is on the 62S 1/2 → 62P 3/2 D 2 (852.35
nm) transition of Cs and lasing occurs on the
62P 1/2 → 62S 1/2 D 1 (894.59 nm) transition. The ethane,
C 2 H 6 , collisionally transfers excitation from 62P 3/2 to
62P 1/2 . This investigation focused on the formation of
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plasma and its consequences on laser action.
A parametric study was performed of the plasma
production and laser operation as a function of pump
power and cell temperature (Cs vapor pressure).
2. Description of the model
The simulation used in this investigation is Global_Kin,
a global plasma kinetics model that is described in Ref.
[8]. In Global_kin, rate equations for species densities,
temperatures, pump intensities, and laser intensities are
integrated as a function of time over successive pulsed
periods. Electron impact processes are included for
elastic and inelastic collisions, including electronic and
vibrational excitation, super-elastic collisions, ionization
and recombination. All rate coefficients for electron
impact processes are functions of electron temperature,
T e , which is determined by solving an electron energy
conservation equation. The rate coefficients are obtained
from solutions of Boltzmann’s equation for the electron
energy distribution function. The electron impact cross
sections for Cs were calculated using fully relativistic allelectron B-spline R-matrix (BSR) with pseudo-states
ansatz with 311 coupled states [9]. We determined the
Cs/Cs 2 densities by the vapor pressure based on the
temperature of the cell (350 K – 425 K). We laser cell is
5 cm long with a 98% output mirror reflectivity.
The reaction mechanism includes 629 reactions and the
following 38 species: e, Cs(62S 1/2 , 62P 1/2 , 62P 3/2 , 52D 3/2 ,
52D 5/2 , 72S 1/2 , 72P 1/2 , 72P 3/2 , Ryd), Cs+, Cs 2 , Cs 2 +, Cs 2 *,
CsAr(A2Π 1/2 , A2Π 3/2 , B2Σ+ 1/2 ), CsAr(X)+, CsAr(A)+, Ar,
Ar(1s 2 , 1s 3 , 1s 4 , 1s 5 , 4p, 4d), Ar+, Ar 2 *, Ar 2 +, C 2 H 6 ,
C 2 H 6 (v13, v24), C 2 H 6 +, C 2 H 6 +, C 2 H 6 -, φ pump ,
φ laser (894.59 nm), φ laser (852.35 nm).
3. Lasing and plasma formation
As a base case, the diode laser pump pulses at 852.35
nm are 1μs long with a 1 kHz frequency. The pump
radiation propagates through a quartz cell heated to 375 K
containing 600 Torr of Ar/C 2 H 6 /Cs = 83/17/1.1 × 10-6.
1
A pulse-periodic steady state is reached after 10-20 pump
pulses with electron densities of (0.9-1.4) × 1011 cm-3.
The electron densities over successive pulses of
10 kW cm-2 are shown in Fig. 1. The electron density
increases over tens of pulses to 1011 cm-3 due to electron
impact following super-elastic heating and associative
ionization. The densities of species responsible for lasing
are shown in Fig. 1 for the 100th pulse for a 7.5 kW cm-2
pump. Laser oscillation occurs for nearly the entire pump
period. The laser transition is saturated, as indicated by
the equal densities of the 62P 1/2 and 62S 1/2 states. The
positive charge is mainly concentrated in Cs+ and Cs 2 +, as
there is little ionization of either Ar. or C 2 H 6 since the
electron temperature is no greater than 0.1 eV. During the
pump pulse a significant amount of Cs(72P 1/2 , 72P 3/2 )
created, which enables associative ionization and creation
of Cs+ (e.g. Cs(62P 1/2 ) + Cs(72P 1/2,3/2 ) → Cs+ + Cs + e).
slope efficiency. That is, more pump power produces
more laser intensity. At high pump power or low vapor
density, laser intensity is limited by the absolute Cs
density available to absorb the pump power. This causes
saturation in the laser intensity with increasing pump
power.
Fig. 2. D 1 (894.59 nm) laser intensity as a function of
pump power for various cell temperatures and vapor
pressures.
Lasing efficiency as a function of pump power for
various cell temperatures and Cs vapor pressures is shown
in Fig. 3. Laser efficiency is the time integrated laser
intensity (laser pulse energy) divided by the time
integrated pump intensity (pump energy).
Due to
saturation of the laser intensity at high pump powers or
low vapor densities the laser efficiency decreases. On the
other hand, the higher laser power (and efficiency) can be
“bootstrapped” by raising the cell temperature (and so Cs
vapor pressure) with pump rate. Other limiting factors for
efficiency at high pump rate are associative ionization,
photo-ionization of higher lying excited states of Cs and
electron quenching of the laser levels.
Fig. 1. (Top) Electron density after 100 pulses of 10 kW
cm-2. (Bottom) The densities of Cs excited states and ions
at pump power of 7.5 kW cm-2.
Laser intensity as a function of pump power is shown in
Fig. 2 for various cell temperatures and Cs vapor
densities. At low pump power or high vapor density,
laser action is limited by pump rate resulting positive
2
4. Concluding Remarks
Using a global model, a computational parametric study
was performed of the DPAL system in Ar/C 2 H 6 /Cs
mixture with lasing on Cs(62P 1/2 ) → Cs(62S 1/2 ) (894.59
nm) transition. Plasma formation is unavoidable when
populating resonant states of alkali atoms (i.e., LIBORS)
given seed electrons. Due to lack of associative
ionization, which naturally provides seed electrons,
significant plasma formation occurs only at higher pump
rates. For example, in high repetition rate operation with
high pump intensities produces plasma densities exceeds
1012 cm-3. These densities decrease laser intensity by
electron collisional mixing.
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modeling an excimer-pumped alkali laser”. Plasma
Sources Sci. Technol., 23 035011 (2014)
Fig. 3. D 1 Laser efficiency as a function of pump power
for various cell temperatures and Cs vapor pressures.
5. Acknowledgements
This work was supported by the US Department of
Defense High Energy Laser Multidisciplinary Research
Initiative.
6. References
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