High power waveguide laser

Details High power waveguide laser High power waveguide laser

2000-07-10

2004-09-07

Wong, Don (Department: 2828)

Coherent light generators

Particular active media

Gas

C372S055000, C372S061000, C372S081000, C372S087000

Reexamination Certificate

active

06788722

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method and system for increasing the output power of gas lasers and more particularly to increasing the output power of sealed-off, diffusion cooled, CO
2
waveguide lasers utilizing radio frequency (RF) excitation
2. Prior Art
The output power per unit cross sectional area for diffusion cooled CO
2
lasers scales inversely as the square of the diameter of the discharge region and directly with the product of the mean free path and the thermo molecular speed of the CO
2
molecules within the discharge region. CO
2
diffusion cooled lasers have the advantages of smaller, size, longer sealed-off life time, and lower maintenance requirements below 500 to 1000 W of output power; while convectively cooled lasers, utilizing the fast flowing of the gas through the discharge region, have the advantage of higher power output capability ranging up to several tens of thousands of watts. When the product of the discharge diameter and the gas flow velocity is smaller than the product of the mean free path and the thermo molecular speed of the CO
2
molecule within the discharge, higher power output per cross sectional area of the discharge is obtained with diffusion cooled lasers than with convectively cooled laser. (Review of CW High Power CO
2
Laser, by Anthony J. DeMaria, Proceeding of the IEEE, pages 731 to 748 June 1973, which is incorporated herein by reference).
It is well know that diffusion cooled lasers utilize the collision of gas molecules, which have given up photons into the laser feedback cavity by stimulated emission but have not been completely de-excited to the ground state, with the walls of the housing containing the discharge to cool the gas within the discharge by de-exciting them to the ground state. This is especially true with CO
2
molecules in typical CO
2
:N
2
:He discharges used in CO
2
lasers. These wall collisions de-excite these CO
2
molecules that have contributed a photon to the laser process down to the ground state, thereby cooling the discharge. The discharge containing housing is in turn cooled externally by either liquid or air cooling techniques. Air-cooling is utilized for lower power lasers that typically operate below 50 Watts of output power. It is known that if the cross section of the gas discharge section is large, the time required for CO
2
molecules, e.g., in the center of the discharge, to diffuse to the cooled walls and became de-excited to where they can again participate in the stimulated emission laser process, is long. Consequently, the gas-cooling rate will be lower for diffusion cooled lasers that utilize large diameter discharges than for CO
2
diffusion cooled laser whose discharges have smaller cross section. This results in lower power per laser beam cross sectional area as the cross sectional area of the discharge CO
2
diffusion cooled laser is increased. The power output for diffusion cooled circular discharges scales as the inverse of the discharge tube diameter. As a result, the output power of diffusion cooled lasers with circular or square discharges can, to first order, only be increased by increasing the length of the discharge (Compact Distributed Inductance RF Excited Waveguide Gas Lasers by Leon A. Newman, John T. Kennedy, Richard A. Hart, U.S. Pat. No. 4,787,090, Nov. 22, 1988; Extended Multiple Folded Optical Path Laser, by Armando Cantoni, U.S. Pat. No. 5,610,936 issued Mar. 11, 1997, which are incorporated herein by reference).
Increasing the discharge length of diffusion cooled lasers beyond a convenient and practical length is usually accomplished by folding the discharge into some form of a closely packed zigzag pattern to obtain small, compact, rugged, and rigid laser head packages (Recent Research and Development Advances in Sealed-Off CO
2
Lasers, by Leon A. Newman and Richard A. Hart, Laser Focus/Electro-Optics, June 1987, which is incorporated herein by reference). Utilizing the concept of U.S. Pat. No. 5,610,936, Armando Cantoni extended this concept of multiple folded optical path square waveguide shaped laser configuration to an unfolded single mode waveguide length of approximately six meters. With this six meter length, approximately 200 watts of output power was obtained with approximately thirtyfive optical bounces off multiple folding mirrors. Unfortunately, the impedance difference seen by the solid state RF source driving the large area discharge before the discharge is ignited compared to after it is ignited is so large that lighting the discharge and maintaining the discharge with one phase matching network structure is difficult. Distributed induction for tuning out the capacitance is used in Tuned Circuit RF Excited Laser, by Peter P. Chenausky, Errol H. Drinkwater, Lanny M. Laughman, U.S. Pat. No. 4,363,126 issued Dec. 7, 1982, which is incorporated herein by reference. U.S. Pat. No. 4,787,090 utilized spiral distributed inductors to achieve the tuning out of the capacitance taught by U.S. Pat. No. 4,363,126.
The output power of diffusion cooled lasers can also be increased by utilizing a rectangular discharge containing section. The two closely spaced walls of the rectangular discharge configuration provides good diffusion cooling while the other two walls of the rectangular discharge housing that are located far apart providing an increase in gas volume. This increase is gas volume yields higher output powers for a given length of laser. These rectangular discharge lasers are called slab lasers (Power Scaling of Large Area Transverse RF Discharge CO
2
Lasers, by K. M. Abranski, A. D. Colley, etc., Applied Physics Letters, Volume 54 page 1833, 1989, which is incorporated herein by reference). CO
2
slab laser technology has been responsible for pushing the average power output of diffusion cooled lasers to approximately the 1000 W range. Slab lasers normally yield multimode, large divergent beams unless the use of more complex optical feed back resonators, such as unstable resonators, are utilized to discriminate against the higher order modes.
Referring now to prior art
FIGS. 1A-1D
, the general types of RF excited diffusion cooled laser discharge configurations known today and normally found in presently commercially available CO
2
laser heads with the exception of
FIG. 1D
are illustrated.
FIG. 1A
illustrates the cylindrical discharge configuration, which usually utilizes either a glass or ceramic tube
2
a
. This configuration was the first to be utilized in diffusion cooled lasers dating back to 1972 for RF excited discharges and dating back to the mid 1960's for DC excited discharges. In general, RF excitation has advantages over DC excitation predominantly because (i) the electrodes
4
a
,
6
a
are external to the discharge region
10
, (ii) low voltages are utilized, and (iii) RF excitation is more compatible with solid state electronics. For the RF excited discharge arrangements, electrodes
4
a
,
6
a
are placed opposite one another down the outside length of the tube
2
a
across which an RF voltage is applied to excite the discharge. Larger diameters result in multiple modes unless more complex optical resonators are used, while smaller diameters (about several millimeters or less) result in waveguideing action that yield single mode beams with simple optical resonators configuration. CO
2
diffusion cooled laser operation in a BeO capillary was reported in 1972 (BeO Capillary CO
2
lasers by E. G. Burkhardt, T. J. Bridges, and P. W. Smith, Optical Communication, Volume 6 pages 193-1951, October 1972, which is incorporated herein by reference). Larger diameter tubes result in lower output power per unit discharge cross-sectional area because of the 1/D
2
power output scaling characteristics mentioned previously. D is the tube diameter. For waveguide lasers, flat mirrors in closed proximity to the ends of the waveguide are normally used which greatly simplify the optical resonator.
The ground electrode
4
a
,
4
b
,
4
c
,
4
d
is normally part of the metal vacuum tight housing f

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