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Characteristics of a Cataphoresis He-Ca+ Recombination Laser

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Characteristics of a Cataphoresis He-Ca+ Recombination Laser *

CHEN Li(^u), PAN Bai-Liang(SM^)**, WANG Ya-Juan(iM^), MAO Bang-Ning(^^)

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 136mW 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 electro-optical 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.t8'9] 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 long-term lifetime and high efficient He-Ca+ recombination laser.

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. "Email: pbl66@zju.edu.cn

© 2009 Chinese Physical Society and IOP Publishing Ltd

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 cataphore-sis effect and the slowly flowing helium buffer gas. The excitation circuit is the modified Blumlein circuit (Ci:C2=500nF:300nF), 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.

150 120

-90 <;

400 600

Time (ns)

Fig. 2. Experimental current pulse and laser pulse.

The relationships between the output power and the pulse frequency are plotted in Fig. 3, on the conditions of 5.4-5.5 kV power supply voltage and 19.9533.25 kPa helium pressure. In doing the experiment, we set a lower frequency of 9 kHz as the initial thermal balance state with less calcium vapour, then the frequency is increased about 1 kHz each time as quickly as possible to avoid overheating the laser tube, finally,

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]

. m0EZ L 0eVn ft = -Fi— fTi = f,

where ft 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

whole item —can be regarded as a constant. Thus kl

the ft 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 5s2S'1/2 of He-Ca+ recombination laser. Thus the output power could be further improved via the way of increasing the frequency and the helium pressure.

3 100 £

-■- 33.25 kPa -O- 27.93 kPa -A- 19.95 kPa

12 14 16

Frequency (kHz)

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 power supply voltage are plotted in Fig. 4 on the condition that the pulse frequency is 13.4-16.1 kHz and the helium pressure is 19.95-33.25 kPa. The experimental method is similar to that mentioned above, and the voltage is increased about 0.3 kV every time. From Fig. 4, it can be seen that the output power increases with the increasing power supply voltage at the

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 cataphore-sis He-Ca+ recombination laser.

CD £ О P<

5.2 5.4 5.6 Voltage (kV)

Fig. 4. Dependences of the output power on the power supply voltage at different helium pressure in experiment.

a so s

_I_I_I_I_I_L

760 780

Temperature (K)

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-

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.

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