WO1988006357A1

WO1988006357A1 - New waveguide design to improve waveguide laser performance

Info

Publication number: WO1988006357A1
Application number: PCT/US1987/003296
Authority: WO
Inventors: Chien Kuei-Ru; Claudio Parazzoli
Original assignee: Hughes Aircraft Company
Priority date: 1987-02-20
Filing date: 1987-12-14
Publication date: 1988-08-25
Also published as: EP0302903A1; IL84931A0; JPH01502227A

Abstract

The waveguide laser (10) has a rectangular cross-section defined by its four sidewalls (12, 14, 16, 18). The rectangular configuration gives improved gas cooling for a given cross-sectional area, requires lower voltage, which reduces arcing and has better mode selection than square or circular waveguide lasers. When a diffraction grating (28) is used, the rectangular configuration gives better line resolution than achieved by square or circular configurations.

Description

NEW WAVEGUIDE DESIGN TO IMPROVE WAVEGUIDE LASER PERFORMANCE

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally lasers. More specificallv, the invention relates to rectangular cross-sectional waveguide laser havi improved performance.

2. Description of Related Art

Early gas lasers were constructed using gla tubing of circular cross section containing the gaseo lasing medium. Glass tubing is a comparativelv poor he conductor, and so metal-ceramic waveguide lasers square cross section were developed. The squa cross-sectional waveguide laser comprises a pair spaced apart and parallel ceramic dielectric walls which a pair of spaced apart and parallel met electrodes are secured to define a waveguide cavitv square cross section.

Although the metal electrodes, forming two the waveguide laser walls, are comparativelv good he conductors, there is still a practical limit to t amount of heat which can be dissipated through the walls. Hence the conventional waveguide laser has a limit to its usable power output.

Conventional waveguide lasers contain the gaseous lasing medium under pressure. The pressure of the gas has a relationship upon the maximum output frequency tuning range of the laser. Generallv, the higher the pressure, the higher the lasing frequenr.v tuning range. There is a trade-off to increasing the pressure, however. As the pressure is increased, the excitation voltage must be increased in order to form a plasma in the laser waveguide cavity. If the voltage is increased too much, arcing can occur with damaging consequences. Hence conventional waveguide lasers also have a practical limit to the upper output frequency tuning range.

Another problem with gaseous lasers is that the gas molecules can have several vibrational states, each corresponding to a different spectral line or output frequency. Although the lines may be close to one another _r the production of more than one line can be problematic for certain sensitive applications. One technique for handling this problem is to place a grating in the laser beam path. The grating has a plurality of etched parallel grooves which selectively pass one of the lines or frequencies while reiecting or attenuating the others. In this fashion, a true monochromatic output beam is produced. Due to surface area constraints dictated by the waveguide cross-sectional area, conventional gratings have a finite resolution determined bv the finite number of etched grooves in a given lineal dimension. Because the lineal dimension of the square waveguide is fixed, the number of etched grooves for selecting a particular output frequency or line is als fixed. Hence resolution cannot be conveniently improved

For all of the above reasons, there is room fo much improvement in the design of waveguide lasers.

SUMMARY OF THE INVENTION The present invention greatly improves upo conventional waveguide lasers. The waveguide laser o the invention uses a rectangular cross-sectiona waveguide with an aspect ratio, r = a/b, in the rang from about 0.20 to 0.5. Among the advantages of th waveguide laser of the invention are better cooling better spectral line resolution, lower voltag requirements, higher power output, higher frequenc tuning range and lower probability of arcing damage. Th waveguide laser design is well-suited for RF pumpe waveguide lasers.

In accordance with the invention, the waveguid laser comprises first and second parallel sidewalls an third and fourth parallel sidewalls connected to define waveguide cavity. In the waveguide cavity is a lasin medium such as a pressurized gas including anv of th gases selected from the group consisting of CO-,, CO, N-, He, Xe and mixtures thereof. A means is provided on a least one of the sidewalls for introducing energy int the waveguide cavity to cause the medium to lase. Th first and second sidewalls are orthogonal to the thir and fourth sidewalls and the first and second sidewall are longer than the third, and fourth sidewalls, such tha the waveguide cavity is rectangular in cross section. The first and second sidewalls comprise electricall conductive electrodes while the third and fourt sidewalls comprise a dielectric material such as BeO, o 1₂0₃.

Further, the invention comprises a method o constructing a laser in which a pair of spaced apart an parallel dielectric sidewalls are provided. Th dielectric sidewalls have a cross-sectional dimension 2 and are spaced apart a distance 2a. A pair of space apart and parallel metal electrodes are secured to th dielectric sidewalls to define a waveguide cavitv o rectangular cross section. A lasing medium is introduce into the cavity. The dimension 2b and the distance 2 are preferably selected so that the ratio r = a/b is i the range of approximately 0.20 to 0.5. The lasin medium is selected from the group consisting of CO_, CO N₂, He, Xe and mixtures thereof. If desired, diffraction grating can be placed in the waveguide cavit in order to select a desired spectral line. The metho further comprises exposing at least one of the meta electrodes to a heat removal medium for cooling th lasing medium.

For a more complete understanding of th invention, its objects and advantages, reference mav b had to the following specification and to th accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional perspective view o the waveguide laser of the invention;

FIG. 2 is a graph depicting the effect of th aspect ratio r on the small signal gain of the laser o FIG. 1. The curves depict the gain as a function position x for different aspect ratios r under t following parameters:

?„_ - 62.5 /cm³, 4ab = 0.4 cm², o = 100 TORR, RF

Mix CO_?:CO:He:Xe(0.148:0.0747:0.74:0.0363) , EH.₁ mode, 3 (wall deactivation coefficient^ = 0.2.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, the waveguide laser of t invention is depicted generally at 10. Wavegui laser 10 comprises first and second parallel sidewalls and 14 which comprise metal electrodes. The wavegui laser further includes third and fourth sidewalls 16 a 18 which are comprised of dielectric material 20, such BeO. Attached to the first sidewall electrode 12 is electrically conductive RF feed through conductor 2 Conductor 22 is connected to a source of RF energy f pumping energv into the laser.

Sidewalls 12-18 define laser cavitv 24 whi extends longitudinally as illustrated. Sidewalls 12 a 14 are orthogonal to sidewalls 16 and 18 and sidewalls and 14 are longer than sidewalls 16 and 18, there defining a rectangular cross section. For convenience, Cartesian coordinate svstem (xyz) is superimposed FIG. 1 illustrating the orthogonality of the sidewall As illustrated, sidewalls 12 and 14 are parallel to the dimension, sidewalls 16 and 18 are parallel to the dimension, and the waveguide extends longitudinally i the z direction. Thus it will be understood that wh reference is made to the cross section of the waveguid - 6 -

laser, that cross section is taken in the plane define by the xv axis.

The rectangular cross section of the waveguid laser may be defined in terms of the aspect ratio r - a/b, where a is the distance from the center of th waveguide cavity to either sidewall 16 or 18 and b is th distance from the center of the waveguide cavitv t either sidewall 12 or 14. Thus sidewalls 16 and 18 ar spaced apart a distance 2a while metal electrod sidewalls 12 and 14 are spaced apart a distance 2b Preferably the aspect ratio r is in the range from abou 0.20 to 0.5_*

The laser cavity 24 is filled with a lasin medium 26 such as CO_ or other gases or mixtures o gases. The lasing medium is preferablv contained withi cavity 24 under pressure and the respective ends of th waveguide laser 10 are provided with windows or the lik for sealing the laser cavitv 24 in order to contain th lasing medium. It will, of course, be understood tha the ends of the waveguide laser are also provided wit one total reflector or grating and one outcoupler mirro according to customarv practice.

In use, RF energy is pumped through fee through 22 with second sidewall 14 grounded so as t establish an electric potential between the meta electrode sidewalls 12 and 14. At a sufficiently hig electric potential, discharge occurs causing the lasin medium to form a plasma, and causing lasing to occur Preferably the RF excitation is in a frequency range a approximately 100 MHz. With EH,, mode lasing bein established. In the case of a C0₂ laser, the three ato molecule has different vibrational states which produ different spectral lines in the laser output. In ma aDplications, such as in laser radar applications, sing spectral line output is preferred. In order to sele the desired spectral line and filter out the others, diffraction grating 28 mav be placed in the laser be path. The diffraction grating has a plurality of groov aligned with the x axis and permits onlv a sing spectral line in the output beam. If desired, t diffraction grating may be included as line selecti optics in the waveguide resonator.

The present invention provides a number advantages over conventional circular or squa cross-sectional waveguide lasers. One advantage better cooling, which allows the invention to operate much higher power levels than conventional designs. understand the improvement in gas cooling, one m compare a 1 x 4 rectangular cross-sectional waveguide the invention with a 2 x 2 square conventional wave ui laser. In both instances, the cross-sectional area the same. However, in considering the gas at the cent of the waveguide cavitv, the 1 x 4 rectangular desi advantageously places the metal electrode sidewalls and 14 in much closer proximity to the center than do the conventional design. In the 1 x 4 rectangular cas the center of the waveguide cavitv is 1/2 of dimensional unit from the metal electrode sidewalls. the 2 x 2 square case, the waveguide cavity center is dimensional unit from the metal electrode sidewall Because of the closer proximity of the metal electro sidewalls to the center of the waveguide cavity, t invention can dissipate heat from the center mo efficiently than the conventional square or circul design can. This allows the laser of the invention to operate at much higher power levels than conventional designs.

Another advantage of the rectangular cross-sectional waveguide laser is better spectral line resolution. As discussed above, diffraction gratings are sometimes used to select a particular spectral line in certain applications. The intergroove spacing of the diffraction grating is dictated bv the wavelength of the spectral line to be selected. In comparing the rectangular configuration with the conventional square configuration of equal cross-sectional area, the diffraction grating of the rectangular configuration can have a considerably greater number of grooves than can a square diffraction grating, because the y dimension of the rectangular grating is greater than that of the square grating. The greater number of grooves for a given cross-sectional area gives the invention much better spectral line resolution, with the optical field of the laser able to couple to a larger number of grooves on the diffraction grating.

Still another advantage of the invention is that it requires lower voltages, due to the fact that the metal electrode sidewalls 12 and 14 are placed more closely together for a given cross-sectional area. This means that a stronger electric field can be established in the waveguide cavitv at a lower voltage. The lower voltage required, decreases the probability of arcing damage. As another consequence of the closer metal electrode sidewall spacing for a given cross-sectional area, the invention is capable of a larger frequency tuning range. The tuning frequency of a laser is controlled by the pressure of the gaseous lasing medium. As the pressure increases, however, the electric field required for lasing increases. There -is, however, a practical limit to the electric field strength which can be established in the waveguide cavitv without arcing damage due to the increased electric potential or voltage between the metal electrode sidewalls. Because the metal electrode sidewalls of the invention are more closely spaced than the conventional counterparts, the electric field can be increased to produce lasing at higher gas pressures without arcing. Another advantage of the invention is better mode selection because of increased losses on higher order modes.

All of the foregoing advantages will greatly increase the laser power and improve overall laser efficiency. Although there are manv potential uses for the invention, one important use is in laser radar, which has a demanding power requirement. The present invention, with its improved efficiencv is well suited to the laser radar application.

To further understand the theorv and performance of the invention, reference mav be had to Table I and to FIG. 2 of the drawings. The aspect ratio, r = a/b, of the waveguide cross section strongly affects the laser output. When r decreases below unitv, the average gas temperature drops, increasing the available gain. The distributed wall losses, Λ, are a function of the waveguide aspect ratio and increase with decreasing r. Hence, a value of r exists that maximizes the laser output power. In Table I,_ several laser parameters_. are calculated using an RF pumped CO- waveguide laser model. See Claudio G. Parazzoli and Kuei-Ru Chien, "Numerical Analysis of a CW RF Pumped CO_ Waveguide Laser," IEEE J. Quantum Electron., Vol. QE-22, 479, 1986. The results are given as a function of r. The distributed wall losses, , have been computed for the EH.. mode,

where ε = ε., + iε_? is the dielectric constant of the waveguide sidewalls 16 and 18.

TABLE I Laser Output Power vs. Aspect Ratio

Λ ^Tavg ^g(0) ^Ios ^Pout ⁿe E

(cm-¹) (°K) (cm-¹) (W/cm²) (W) (cm-³) (V/cm)

1.0 1.53 x lO^"4 464.6 9.33 x lθ^"3 =7. x 10³ 5.08 9.03 x 10¹⁰ 1.40 x 10³

0.75 1.8 x lO^-1* 436.6 1.28 x lθ^"2 =4.8 x 10³ 6.29 8.44 x 10¹⁰ 1.49 x 10³

0.50 3.25 x lO^-1* 389.4 2.07 x lθ^"2 =3.4 x 10³ 7.40 7.58 x 10¹⁰ 1.67 x 10³

0.20 1.23 x 10^"3 343.3 2.68 x lθ^"2 =2.40 x 10³ 6.11 6.67 x 10¹⁰ 1.89 x 10³

m

O oo^"

00

In Table I the losses for a BeO waveguide are given fo λ = 10.6 μ . In Table I the distributed wall losses (Λ) , the average temperature ( Tavg) , the small signal gam, g(0) , saturation intensity ( os) , power out (Pout) , electron density (ne) and electric field (E) are give for various values of aspect ratio (r) . As seen in th

Table, the output power of laser peaks at 7.40 watts fo r = 0.5. For comparison purposes, it should b recognized that the aspect ratio 1.0 is a square cros section of prior art design.

The small signal gain distribution at v = 0 an as a function of r is shown in FIG. 2. With decreasin values of r _r the gain profile changes from concave t convex, and the peak value increases. The increase i gain is due to the lower average gas temperature, as ca be seen in Table I. From the calculations presented i Table I, the 2 to 1 (r = 0.5) aspect ratio waveguid configuration gives the best performance. Table I als shows the square configuration (aspect ratio = 1) has th worst performance.

While the invention has been described i connection with the presently preferred embodiment, i will be understood that the invention is capable o certain modification and change without departing fro the spirit of the invention as set forth in the appende claims.

Claims

CLAIMSWhat Is Claimed Is:

1. A waveguide laser comprising:

first and second parallel sidewalls and thir and fourth parallel sidewalls connected to define waveguide cavitv;

a lasing medium in said waveguide cavity an means on at least one of said sidewalls for introducin energy into said waveguide cavitv to cause said medium t lase;

said first and second sidewalls bein orthogonal to said third and fourth sidewalls; and

said first and second sidewalls being longe than said third and fourth sidewalls such that sai waveguide cavitv is rectangular in cross section.

2. The waveguide laser of Claim 1 wherei said first and second sidewalls comprise electricall conductive electrodes.

3. The waveguide laser of Claim 2 wherei said means for introducing energy into said waveguid cavitv comprises a conductor coupled to one of sai electrically conductive electrodes.

4. The waveguide laser of Claim 2 wherein on of said electrically conductive electrodes is maintaine at a higher electric potential than the other of. sai electrically conductive electrodes.

5. The waveguide laser of Claim 1 wherei said third and fourth sidewalls comprise a dielectri material.

6. The waveguide laser of Claim 5 wherei said dielectric material comprises BeO or A1-0-..

7. The waveguide laser of Claim 1 wherei said lasing medium comprises a pressurized gas.

8. The waveguide laser of Claim 1 wherei said lasing medium comprises CO- gas.

9. The waveguide laser of Claim 1 wherei said lasing medium is selected from the group consistin of CO-, CO, N_, He, Xe and mixtures thereof.

10. The waveguide laser of Claim 1 wherei said rectangular cross section of said waveguide cavit has an aspect ratio r in the range from about 0.20 t 0.5; said aspect ratio being defined as r = a/b, where is the distance from the waveguide cavity center to sai third sidewall and b is the distance from the waveguid cavity center to said first sidewall.

11. A method of constructing a lase comprisinα:

providing a pair of spaced-apart and paralle dielectric sidewalls, the dielectric sidewalls having cross-sectional dimension 2b and being spaced aoart distance 2a;

securing a pair of spaced-apart and parall metal electrodes to said dielectric sidewalls to define waveguide cavity of rectangular cross section; and

introducing a lasing medium into said wavegui cavity.

12. The method of Claim 11 wherein sa dimension 2b and said distance 2a are selected such th the ratio r = a/b is in the range of approximatelv 0. to 0.5.

13. The method of Claim 11 wherein said lasi medium is selected from the group consisting of C0 , C N_, He, Xe and mixtures thereof.

14. The method of Claim 11 further comprisi placing diffraction grating in said waveguide cavity.

15. The method of Claim 11 further comprisi exposing at least one of said metal electrodes to a he removal medium for cooling the lasing medium.