CHIN. PHYS. LETT. Vol. 26, No. 1 (2009) 014206
Characteristics of a Cataphoresis He-Ca+ Recombination Laser
*
CHEN Li(陈立), PAN Bai-Liang(潘佰良)** , WANG Ya-Juan(王亚娟), MAO Bang-Ning(毛邦宁)
Department of Physics, Zhejiang University, Hangzhou 310027
(Received 17 June 2008)
A cataphoretic input of calcium vapour into the active volume of pulsed He–Ca+ laser is designed and made.
The recombination laser at 373.3 nm and the R-M transition laser at 854.6 nm are achieved experimentally with
modified Blumlein circuit by high-frequency longitudinal pulsed discharge. The dependences of work parameters
such as the pulse frequency, the power supply voltage and the helium pressure on laser output characteristics at
373.3 nm line are measured and discussed. The maximum laser output power of 136 mW and the specific power
of 5.9 mW/cm3 are obtained, respectively.
PACS: 42. 55. Lt, 42. 60. By
The pulsed He–Ca+ laser has the characteristics
of multiple laser lines ranging from ultraviolet to infrared, the higher laser output power and the electrooptical conversion efficiency, which can find many potential applications in microelectronics, medical therapy and material sciences.[1,2] The alternate laser oscillations at 373.3 nm and 854.6 nm lines in the pulsed
He–Ca+ discharge plasma have already been realized
in experiment.[3,4] In order to attain the relative uniform distribution of metal vapours along the active
length, the traditional pulsed MVLs including the
He–Ca+ laser often put the metallic pieces uniformly
in the shallow caves or on the inner wall of a discharge tube and obtain the required metal vapour
concentrations by directly discharging self-heated or
with an attached axial heater.[5−7] However, the random discharge to metal pieces and the chemical reaction between the metal vapour and the inner surface of discharge tube often lead to the rapid loss
and variation of metal vapours for the alkaline-earth
metal vapour lasers, such as the He–Ca+ (Sr+ ) laser,
which shortens the lifetime and hence prevents the
laser from being applied in material processing, microelectronic technology, and laser medical treatment to
some extent.[8,9] The basic advantages of cataphoretic
input of metal vapours in MVLs are: the uniform
distribution of metal vapour concentrations along the
discharge tube; prevention of the channel overlapping
with the pieces of metal; the absence of the bulky external furnace; and the possibility to optimize metal
vapour pressure (in certain limits) regardless of the
discharge current. In comparison with the traditional
discharge-excited He–Ca+ laser,[3−7] the cataphoresis He–Ca+ recombination laser mainly has characteristics of the stable power, the longer lifetime, the
controllable calcium vapour density and the better
quality beam. In fact, the cataphoretic CW He–Cd+
laser shows the extensive practical applications and
the most sales rate among all MVLs.[10]
In this study, we design a cataphoretic pulsed
He–Ca+ laser based on studying the work characteristic and the kinetic process of the self-heated He–
Sr+ laser.[11−13] The relationships between the output
power of 373.3 nm recombination laser and work parameters are measured and analysed. The maximum
laser output power of 136 mW and the specific power
of 5.9 mW/cm3 are obtained, respectively, which are
the better result than those of other work. It supplies another way to realize the possibility of a longterm lifetime and high efficient He–Ca+ recombination
laser.
Heat preservation aluminum coat
External heater
M1
M2
Filter
Pump
L1
Helium
C1
C2
L1
T
+HV
Fig. 1. Schematic diagram of the experimental setup of a
cataphoresis He–Ca+ recombination laser.
The schematic diagram of the experimental setup
is presented in Fig. 1. The laser tube is made up of a
quartz basic tube with inner diameter of 12 mm and a
ceramic tube with the inner diameter of 9 mm and the
length of 36 cm. The ceramic tube is inserted into the
quartz basic tube to limit the discharge channel and
to reduce the chemical reaction between the calcium
vapour and the quartz glass. The distance between
electrodes is 46 cm. The calcium pieces of 98% purity
are put into the shallow circularity reservoir near the
anode, which is covered with a piece of molybdenum
to decrease the corruption of the quartz tube by the
calcium vapour. The strip heater is used for controlling the reservoir temperature independently so as to
* Supported
by the National Natural Science Foundation of China under Grant No 10574111.
pbl66@zju.edu.cn
c 2009 Chinese Physical Society and IOP Publishing Ltd
○
** Email:
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CHIN. PHYS. LETT. Vol. 26, No. 1 (2009) 014206
5
150
4
120
3
90
2
60
1
30
0
0
-1
0
200
400
600
Time (ns)
800
the maximum output power is measured and recorded.
After each measurement the frequency is readjusted to
the initial value to maintain the similar thermal condition. The measurement process like that repeats on
and on until the experimental result is obtained. From
Fig. 3, one can see the output power almost increases
with the pulse frequency at the constant helium pressure. This can be explained by the increase of the
input power and the uniform distribution of the calcium vapour density through the axial cataphoretic
effect. According to the formula[12]
𝛽=
𝜇𝜃𝐸𝑧 𝐿
𝜃𝑒𝑉 𝜏𝑖
𝑓 𝜏𝑖 =
𝑓,
𝐷
𝑘𝑇
(1)
where 𝛽 is the ratio between the forced diffusion coefficient and the free diffusion coefficient due to the action
of the pulsed electric field; 𝑓 is the pulse frequency; the
𝜃𝑒𝑉 𝜏𝑖
whole item
can be regarded as a constant. Thus
𝑘𝑇
the 𝛽 is proportional to the pulse frequency 𝑓 . This
means that the cataphoretic effect is increased by the
pulse frequency 𝑓 . Figure 3 also shows that the output power under the higher helium pressure is higher
than that under the lower helium pressure when the
pulse frequency is more than 13 kHz. That is because
the collision frequency between electrons and helium
atoms or ions increases with the increase of the helium pressure, resulting in faster cooling the electron
temperature, which is helpful to the excitation of the
upper level 5𝑠2 𝑆1/2 of He–Ca+ recombination laser.
Thus the output power could be further improved via
the way of increasing the frequency and the helium
pressure.
Current (A)
Laser intensity (arb. units)
adjust the amount of the calcium vapour. This kind
of experimental setup has a greater improvement than
before, because it can avoid the optical path overlapping with calcium particles and decrease the laser instability caused by the discharge to the calcium pieces.
Finally the calcium vapour distributes uniformly in
the active area with the help of the pulsed cataphoresis effect and the slowly flowing helium buffer gas.
The excitation circuit is the modified Blumlein circuit
(C1 :C2 =500 nF:300 nF), and the buffer gas is helium
with 99.999% purity. The mirror M1 with 3m radius
of curvature served as a total reflector to the recombination laser at 373.3 nm has 99.5% reflectivity, and
the mirror M2 is an output flat mirror of 87% reflectivity. As the laser components focus on the 373.3 nm
lines, the design of cataphoretic He–Ca+ laser is helpful to the recombination laser whatever on the excitation circuit or the optical cavity, however the weaker
R-M transition laser at 854.6 nm still appears due to
the high gain. The output power is measured by a
THORLABS model PM100 power meter. The waveforms of the discharge current and the laser pulses
are detected by a Pearson model 410 current transformer and a THORLABS model DET710 plane photoelectric diode, respectively. All pulses are displayed
on a Tektronix TDS 754C oscilloscope. The experimental current pulse and the laser pulse of 373.3 nm
and 854.6 nm are presented in Fig. 2, and the peak
of self-terminating laser pulse at 854.6 nm appears at
the rising edge of the current pulse, while that of the
recombination laser pulse at 373.3 nm starts at the
early afterglow period of complete recombination in
the discharge plasma. It agrees well with the output
characteristic of the self-terminating and recombination laser.
-30
1000
Fig. 2. Experimental current pulse and laser pulse.
Fig. 3. Dependences of the output power on the pulse
frequency at the different helium pressure in experiment.
The relationships between the output power and
The relationships between the output power and
the pulse frequency are plotted in Fig. 3, on the conthe power supply voltage are plotted in Fig. 4 on the
ditions of 5.4–5.5 kV power supply voltage and 19.95–
condition that the pulse frequency is 13.4–16.1 kHz
33.25 kPa helium pressure. In doing the experiment,
and the helium pressure is 19.95–33.25 kPa. The exwe set a lower frequency of 9 kHz as the initial thermal
perimental method is similar to that mentioned above,
balance state with less calcium vapour, then the freand the voltage is increased about 0.3 kV every time.
quency is increased about 1 kHz each time as quickly
From Fig. 4, it can be seen that the output power inas possible to avoid overheating the laser tube, finally,
creases with the increasing power supply voltage at the
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CHIN. PHYS. LETT. Vol. 26, No. 1 (2009) 014206
constant helium pressure, the higher output power is
obtained on the higher helium pressure at the same
power supply voltage. When the helium pressure is
high, the influence of the voltage variety on the output power is smaller than that under the low helium
pressure. Thus the helium pressure has a significant
meaning for the stable output power of the cataphoresis He–Ca+ recombination laser.
Fig. 4. Dependences of the output power on the power
supply voltage at different helium pressure in experiment.
ists a narrower working temperature range than other
MVLs and has the optimal calcium vapour pressure
under certain discharge conditions. Too much calcium
vapour concentrations produced by overheating the
reservoir will lead to rapidly increase the peak current and make the current pulse appear the second
oscillation in the early afterglow, which worsens the
output power of recombination laser.[13] As the external heater can independently adjust the tube wall
temperature of the reservoir in a certain range, the
pulsed cataphoresis He–Ca+ recombination laser has
the advantage to optimize the calcium vapour pressure
and maintain a long stable operation.
In conclusion, a cataphoretic input of the metal
calcium vapour into the active length for pulsed He–
Ca+ laser has been designed. For the small-size cataphoresis He–Ca+ recombination laser at 373.3 nm
with the discharge channel of 9 mm i.d. and active
length of 36 cm, the maximum laser output power of
136 mW and the specific power of 5.9 mW/cm3 are obtained, respectively. The dependences of working parameters such as the pulse frequency, the power supply
voltage and the helium pressure on laser output characteristics at 373.3 nm are measured and discussed. It
provides another way to further develop a long-term
lifetime and high efficient pulsed He–Ca+ recombination laser.
References
Fig. 5. Dependences of the output power and the peak
current on the reservoir temperature in experiment.
Figure 5 describes the changes of the output power
and the peak current with the reservoir temperature under the condition of 27.93 kPa helium pressure,
5.4 kV power voltage and 9.5 kHz frequency. As is
shown in Fig. 5, the peak current rises up with the
increasing temperature, especially when the temperature is higher than 780 K. However, the output power
firstly increases with the temperature from 730 K to
775 K, then drops sharply at the temperature more
than 775 K. It shows that the pulsed He–Ca+ laser ex-
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