Intra-Cavity Loss Element Method for Measurement of the Small Signal Gain of a TEA CO 2 Laser

The small-signal gain coefficient of a TEA CO2 laser has been measured through the implementation of a variable polarization intra-cavity loss element. Charging voltage dependence of the gain property of the laser has been investigated. The advantage of this method is that no probe laser system is required. The results have been agreed with experimental data obtained by conventional oscillator-amplifier method.


INTRODUCTION
A TEA CO 2 laser as an efficient source of infrared radiation is being increasing utilized in different areas of science and technology.Developed more than 40 years of first reports about TE CO 2 lasers, and during this period some important applications found for these lasers, such as DIAL (Killinger & Menyuk, 1981), non-linear optics in midinfrared (Kildal & Deutsch, 1976), plasma generation (Haglund, Nowak, & Czuchlewski, 1981), and recently measurement of the temperature of the Alfa particles in the Tokamak (Kondoh et al., 2006).The small signal gain, (g 0 ), and saturation intensity, (I s ), characteristics are important parameters for the design and scaling of the gaseous lasers.
Although these parameters may be investigated with different methods, such as direct probe or oscillator-amplifier array, the latter technique has been used extensively usually because of the lack of availability of an appropriate tunable laser probe (Behrouzinia, Sadighi-Bonabi, Parvin, & Zand, 2004;Behrouzinia, Sadighi, & Parvin, 2003).There are two standard method for measuring g 0 of CW or pulsed CO 2 lasers; intra-cavity (variable) loss element and oscillatoramplifier methods (Heard & Zipin, 1969).The g 0 has been experimentally studied by the oscillator-amplifier method for an untunable CO 2 laser (Antropov, Silin-Bekchurin, Sobolev, & Sokovikov, 1968;P. Cheo, 1968;Tulip, 1970) at different wavelengths (Dang, Reid, & Garside, 1980;Sato & Miura, 1983) and for a TEA CO 2 laser (PK Cheo, 1967;Nath & Biswas, 1997) The intra-cavity variable loss element is easier to apply relative to the other one, whereas the measurement of the g 0 is only desired.We have been successfully used this method for measurement of  0 of a CW CO 2 laser earlier (Aram, Soltanmoradi, Ghafori, & Behjat, 2005).To our knowledge, only one report is available on a rare gas Halide pulsed laser based on using an intra-cavity variable absorption gas cell loss element (Armandillo, Kearsley, & Webb, 2000).The  0 and   can be measured at the line center of a single-longitudinal-mode laser, the result of measurements being dependent on the broadening.For CO 2 lasers operating at a normal pressure of about 10 Torr, the homogeneous line broadening dominates at vibrationrotational transitions from the upper (001) level to the lower (02 0 0, 10 0 0) levels (Aram, Soltanmoradi, & Behjat, 2004;DeMaria, 1973;Witteman, 1987).The method proposed in this work do not use a probe beam from an auxiliary laser, and therefore, errors due to the instability of the auxiliary laser are eliminated.The  0 of a TEA CO 2 laser has been measured through the implementation of a variable polarization intra-cavity loss element.The experimental results have been investigated in some different charging voltage.

EXPERIMENTAL SETUP
The laser cell with optical arrangement is shown in Fig. 1.
Fig. 1 The optical arrangement of the experimental apparatus.
The TEA cell with two Brewster windows have contains the pair of Aluminum electrodes with active volume 2 × 2 × 38  3 and two rows of ten Aluminum pins as spark array pre-ionizers.A simple capacitor using a  2 pressurized trigatron switch is used for making the electrical excitation pulse and glow discharge (Aram, Soltanmoradi, & Behjat, 2004).
The home-made polarization loss element is composed of two Brewster windows which one of them can be rotated around the optical axis of two windows, Fig. 2. Two high reflection flat mirrors (M 2 and M 3 ) are used for alignment the axis of loss element and TEA cell.Au coated concave ( = 5) mirror (M 1 ) and a flat natural Ge (M 4 ), with 180cm apart from each other, are used as back mirror and out-coupler, respectively.The output energy of laser is measured by a LMP5 pyroelectric joule meter via coherent lab master readout with 1 mj precision.The output laser line was 10(20) most of time without using any line tuning device ,and measured by a CO 2 laser grating spectrometer (Optical Engineering Co.), and it is obviously clear because of the kinetics of depopulation of CO 2 molecule energy levels in high pressure, the 00 0 1 → 10 0 0 transition is dominant to 00 0 1 → 02 0 0 one (Witteman, 1987).

RESULTS AND DISCUSSION
The variation of output energy versus charging voltage is shown in Fig .3.

Fig.3 The variation of output energy versus charging voltage.
As can be seen, the output energy is increased linearly by increasing of charging voltage in the interval of 18~ 22 .The variation of  0  versus charging voltage is shown in Fig .4.

Fig. 4
The variation of  0  versus charging voltage.
The maximum value of  0 , ( 0  ) is increased linearly so, by increasing of charging voltage in that same interval.The,  0  , is obtained as following procedure.At first, the loss element tuned at minimum loss which corresponds to the maximum output energy, then by rotating one of its windows and tracing the decreasing output energy using the threshold condition, the maximum small signal gain is derived as following formula: which  is the reflectivity of Ge out-coupler (53% total),   is minimum loss of the element, which is 10% for single pass, and mainly related to the imperfections in windows and construction of the cell, ∆ is the deviation angle from maximum output relative to threshold condition and L m is the length of active medium (45 cm).
The loss of the Brewster windows of the TEA cell isn't included because they had high quality, so the diffraction loss of tube aperture is negligible by assumption of having TEM 00 .Because the single pass time of the cavity is 6 ns and it is smaller than the duration of TEA CO 2 gain (≳ 50 ), the time variation of g 0 near the threshold is neglected.As can be seen from Fig. 4, the obtained  0  is from 6.4 %  −1 to 8.2% −1 , that is the same order with results obtained by others; (Franzen & Jennings, 1972;Singer, 1974), which give the  0  from 7 % cm -1 to 10 % cm -1 .It's obvious that the  0  has been measured at multi longitudinal mode condition, so the spatial hole burning effect is omitted.The most important point is the simplicity of having a good estimate of medium amplification capability and the minimum of reflectivity of the out coupler.

CONCLUSION
Measurement of the maximum small signal gain of a spark array UV pre-ionized TEA CO 2 laser via intra-cavity loss element is reported.The advantage of this method is that no probe laser system is required.Simplicity of polarization loss method makes it applicable for many of other lasers, too.