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Research Article
Vol. 55, No. 21 / July 20 2016 / Applied Optics 5583
and thus the FIR laser power. This can be compensated for by controlling the CO2 laser cavity length via 0–10 V PZT con- trol input. FIR laser cavity length also can drift, and the effect appears on a slower timescale (∼30 min). The control to counter the FIR cavity drift is the motorized translation stage, as shown in Fig. 3.
4. LASER BEAM PROPAGATION IN FREE SPACE
For designing the optics of the laser application systems, i.e., interferometer/polarimeter diagnostics, it is important to capture the laser beam propagation characteristics in free space [2]. Figure 7 shows the laser beam profiles at 124, 220, and 495 cm from the laser output coupler, respectively. They are measured by a scanning pyro electric detector with a sensor diameter of 1 mm. Small deviation from Gaussian is seen only in the near field (124 cm), and the profiles are Gaussian when the distance is greater than ∼200 cm away from the output coupler. This is the advantage of the uniform coupling of the mesh output couplers. The other reason for the good beam quality is that the transverse EH11 mode profile inside the di- electric laser waveguide can be approximately described by the Bessel function J0 2.2048r∕a , where a is the radius of the waveguide [21]. It is close to a Gaussian in shape; therefore, it couples to the fundamental Gaussian profile efficiently once entering the free space.
For each beam profile, the beam diameter at 1∕e intensity (as defined by Véron in [2]) is obtained from best fit of the data to a Gaussian function and plotted in Fig. 8 (squares) as a function of propagation distance. This propagation data is then fit to the formula describing the Gaussian beam propagation [2] to obtain a beam waist of 20 mm in diameter, located at 40 cm from the laser output coupler, as shown by the red curve in Fig. 8. Previous theoretical and experimental studies show that the ratio of the beam waist diameter to the waveguide diameter is ∼0.42 [22]. A beam waist diameter of ∼16 mm is expected
Fig. 7.
Fig. 8. Measured beam diameter (squares) as a function of the propagation distance from the laser output coupler fits well to a Gaussian beam (red curve) with beam waist diameter of 20 mm, located at 40 cm from the output coupler.
from a waveguide diameter of 38 mm. However, the output coupler meshes are stretched on the end surfaces of aluminum rings with 50 mm in diameter. These rings are ∼6.3 mm in length and are placed at ∼3–5 cm from the end of the wave- guide. The EH11 waveguide mode may have been excited in the rings, leading to the increased output beam waist diameter of 20 mm.
5. SUMMARY
The new optically pumped HCOOH vapor FIR laser operating at 433 μm has been successfully developed. High laser gain of 3 dB∕m and power conversion efficiency of 16.4% of the Manley–Rowe limit are achieved with the pump beam optimi- zation by tuning a dichroic mirror, which reflects the pump beam. A variable parallel mesh coupler enables the characteri- zation of the laser gain and optimization of the coupling coefficient. It is placed outside the vacuum boundary, making it convenient to tune up the laser. Stable laser performance is achieved by choosing meshes with a line density of ∼120–150 lines per inch. The output laser beam has near Gaussian beam quality as expected.
Acknowledgment. Management support from Michel Tuszewski and Hiroshi Gota, engineering support from the Tri Alpha Energy, Inc. (TAE) engineering team, and financial support from TAE investors are acknowledged.
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Laser beam profiles in free space measured at a distance of (a) 124 cm, (b) 220 cm, and (c) 495 cm from the laser output coupler, respectively. Squares are measured data points, and the solid curves are the corresponding least-squares fitted Gaussian functions. Beam profiles are mostly Gaussian with small higher-order transverse mode super imposed only in the near field (a).