We demonstrate experimentally that a self-sustained radio frequency (RF) signal delivered from our most-compact RF circuit generator is efficiently integrated into a CO2 waveguide laser slab, impedance-matched electrode. The whole circuit uses a common source MOSFET transistor that will be modified for self-sustained oscillations terminated at 50Ω. RF power is directly integrated into the laser resonant cavity of the laser ceramic-metal slab (the electrode). Measurement showed the power efficiency of the oscillator circuit to be more than 70%, and the overall electrical-to-optical power efficiency conversion was larger than the standard 10%.
©2005 Optical Society of America
A typical slab waveguide CO2 laser system consists of a RF power supply and the laser head. Low-to high-power laser systems are broadly used in industry as manufacturing tools such as for cutting, marking, and engraving materials. CO2 laser systems have been in use since their invention by Bell Labs (1964) and have been thoroughly investigated as a source of mult-ikilowatt optical powers [1–3]. Later developments have been achieved by pumping the laser amplifying media with a RF electrical signal that substitutes for the high-voltage DC sources of earlier designs. The optimal RF frequency for laser excitation and gas mixture has been also investigated in previous works by Hall et al. . In order to achieve a versatile design of a laser system that can be easily mounted and transported, it is necessary to integrate the electrical RF power source into the laser head. A compact version of a slab waveguide laser with 0–10 Watts of output power can be achieved by the appropriate length of the waveguide. Generally, for this kind of slab waveguide CO2 laser, the RF power unit is impedance-matched to 50Ω and delivers RF power to the laser head via a suitable 50Ω cable. By use of a ‘T’ or ‘?’ network at the laser head one can impedance-match the laser discharge circuit to 50Ω. For RF-excited CO2 slab lasers it would be advantageous to combine the RF power unit and laser head. This would enable the MOSFET transistor to be impedance-matched directly to the laser-head electrodes, making the RF power supply circuitry straightforward. Furthermore, the RF transistors may be mounted on the laser head, removing the need for separate cooling of the transistors and laser discharge plates. With the RF power supply combined within the laser head, no external RF cabling is necessary. In this case, the only electrical connection required is a DC power line.
2. Metal-ceramic waveguide construction
Generally, the electrical circuit, equivalent to the experimental laser head, is a passive element that may exhibit loss. The hollow waveguide optical structure for the laser amplifying media has to be stabilized in terms of the gas ionization. To have a uniform intensity distribution in the discharge volume, it is necessary to have a uniform RF power distribution along the entire electrode length. To achieve this, it was necessary to treat the whole waveguide as an inductive-capacitive circuit by inserting parallel distributed inductors. The waveguide slab length is set to 375 mm with a rectangle cross section of 20×2.5 mm. The separation of the electrodes and the discharge width were set using ceramic spacers (dielectric constant=9), 375 mm long and 2.5 mm of thickness placed side by side at 20 mm apart. This configuration forms an equivalent circuit of three capacitors put in parallel: C1, C2 and C3. Capacitors C1 and C3 are identical, formed by the metallic electrodes with the ceramic spacers, whereas C2 is the capacitor formed by the metallic electrodes and the gas mixture inside the hollow waveguide. The final waveguide incorporates an inductance, L, which is parallel distributed to satisfy the resonance frequency of 81 MHz for the LC circuit with the aim of transferring the net input power from the RF generator to the laser head. With the use of a BIRD400 Antenna Tester a coupling efficiency better than 96% at the resonant frequency of 81 MHz  was reached. Regarding the optical resonator coupled to the slab waveguide structure, and operating at the appropriate temperature, the optimum optical output coupling is close to 9%. In our case, the optical resonator’s radii of curvature are R1=42.99 cm and R2=38.79 cm, accordingly, having the resonator lateral magnification of 1.108. Optical coupling losses depend on the mirror separation from the hollow waveguide. For our experimental setup, the calculated coupling losses for a 6 mm gap between the waveguide and the mirrors are less than 1%.
3. RF power oscillator design
The design for the matching networks and common-source amplifier was based on the s-parameter characteristics of the MRF151 MOSFET transistor. For maximum power transfer, the impedance of the laser-oscillator system was set to 50Ω. Figure 1 shows the proposed circuit design. There are few components in the design, which can be tuned for the required laser pumping frequency near 80–90 MHz. The circuit is mounted on a PCB board with less than 10×10 cm2 of surface area. The components include one MOSFET power transistor, mica tunable capacitors, RF inductor coils and magnetite bead coils that work as RF chokes. The power efficiency of the oscillator circuit has been measured at greater than 70%. The DC biasing source has provided a DC power of 150 W, which has been converted to an RF power ~100 W.
4. RF and optical measurements
We proceed to do several ionization tests by using different gas pressures and mixtures. We observed that the best performance, seen in terms of the corresponding optical power, is seen at a pressure of 45 Torr as shown in Fig. 2. The laser cavity was filled with a gas mixture 1:1:3 of CO2, N2, and He, respectively, and the internal pressure ranged from 38 to 50 Torr. In this case we reach the maximum optical power at 45 Torr and 100W RF input feed from our RF circuit unit. The discharge plasma was lit using the oscillator as an electric source, and light amplification was obtained by means of the unstable optical resonator. The highest optical power reached at the output of the resonator was 12.42 W. Under these optimized gas mixture conditions, the electrical-to-optical power efficiency was in excess of 12%. Given the simplicity of our circuit design, these results are encouraging for the investigation for new structure configurations that would permit the incorporation of more RF block units integrated directly to the slab waveguide, with the aim of having a more-compact CO2 laser with more optical output power.
The present study has introduced and demonstrated that a simple circuit design for a RF generator (oscillator-amplifier) can be directly impedance-matched connected to a CO2 slab waveguide electrode. The electrical RF power achieved from this circuit exceeds 100 W, and its power is injected into the metallic-waveguide cavity by using a ‘T’ network at the laser head, which is impedance-matched to 50Ω. The latter is to minimize reflections back to the RF circuit. The optical signal generated inside the cavity has been amplified by means of a negative unstable mirror resonator extracting slightly less than 13 W at 45 Torr and ~100 W RF input feed from our RF unit. As a final remark, with the RF power supply fused within the laser head no external RF cabling is necessary. In this case, the only electrical connection required is a DC power line. Thus, important research extensions of this work would include the integration of several RF unit oscillators to multiply the overall output power and the characterization and modification of the discharge plasma by means of external coil inductors, to extend its power in a range of several hundred watts with the intention of having CO2 slab-waveguide lasers—the most compact and the most powerful.
R. Villagómez is grateful to CONACYT, México, for research grant number 42944 and to colleagues for their assistance, in particular Ricardo Chavez, and Benjamin Ramirez for his advice on measurements and design.
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