United States Patent Mi [11] 3,928,820 Pappalardo Et Al

United States Patent mi [11] 3,928,820 Pappalardo et al. [45] Dec. 23, 1975

[54] HIGH GAIN PULSED ION LASER Carbon, and Nitrogen, App. Phys. Lett., Vol. 5, No. 5, [75] Inventors: Romano G. Pappalardo, Sudbury; (Sept. 1, 1964), pp. 91-93. Robert Smith, Wilmington, both of Bridges et al., Visible and UV Laser Oscillation at 118 Mass. Wavelengths in Ionized Neori; Argon, Krypton, Xe- non, Oxygen, and other Gases, Applied Optics, Vol. 4, [73] Assignee: GTE Laboratories Incorporated, No. 5, (May 1965), pp. 573-580. Waltham, Mass.

[22] Filed: Nov. 25, 1974 Primary Examiner—William L. Sikes [21] Appl. No.: 526,646 Attorney, Agent, or Firm—Irving M. Kriegsman

[52] U.S. CI 331/94.5 G [57] ABSTRACT 2 [51] Int. CI. HOIS 3/03 A high gain pulsed ion laser is described in which a [58] Field of Search 331/94.5; 330/4.5 short discharge pulse of less than about 2.0 microseconds is applied to a discharge tube in which oxygen is [56] References Cited present in an amount sufficient to establish an oxygen UNITED STATES PATENTS pressure in the range between about 10 and 100 milli- 3,798,486 3/1974 Hernquist 331/94.5 D torr. The pulsed output from the ion laser occurs during the "afterglow" of the discharge pulse and is at- OTHER PUBLICATIONS tributed to doubly ionized oxygen. The wavelength of Heard et al., Visible Laser Transitions in Ionized Oxy- the output may be selected from certain lines in the gen, Nitrogen, and Carbon Monoxide, Proc. IEEE, visible and ultraviolet portions of the electromagnetic Vol. 52, (Oct. 1964), p. 1258. spectrum. McFarlane, Laser Oscillation on Visible and Ultravio- let Transitions of Singly and Multiply Ionized Oxygen, 11 Claims, 6 Drawing Figures U.S. Patent Dec. 23, 1975 Sheet 1 of 3 3,928,131 U.S. Patent Dec. 23, 1975 Sheet 2 of 3 3,928,820

TIME (/xSEC) FIG. 3

OXYGEN PRESSURE (IN MICRONS) FIG. 4 U.S. Patent Dec. 23, 1975 Sheet 3 of 3 3,928,820 3,927,946 1 2 peak powers measured between 0.4 and 5.9 watts, HIGH GAIN PULSED ION LASER whereas the xenon line at 364.5 nanometers was re- The Invention herein described was made in the ported to have a peak power of the order of 200 watts, course of or under a contract, or subcontract thereun- der, with the United States Navy. ... 5. SUMMARY OF THE INVENTION BACKGROUND OF THE INVENTION , f object of the present invention to provide a novel pulsed oxygen ion laser capable of The present invention is related generally to lasers operating at high gain, and is more particularly concerned with a novel high It is a second object of the invention to provide such gain pulsed ion laser having an ionized oxygen active 10 a pulsed oxygen ion laser which is capable of operating medium. at peak powers sufficiently high to be of technological Recently, a great amount of funds and effort have value, been expended in research and development projects Briefly, the invention in its broadest apect comprises involving lasers. These projects may be divided into a high gain pulsed ion laser. A cylindrical discharge two somewhat interrelated categories. First, many 15 tube is provided which has a centrally located axis projects have dealt with'the incorporation of lasers into therethrough. A pair of electrodes are disposed within industrial, medical and scientific devices and systems. the cylindrical discharge tube, one located effectively Second, a continuing search is maintained for new laser near each end of the cylindrical discharge tube. The sources. This search is done both to widen the number electrodes are open at least at the axis of the cylindrical of available wavelengths at which laser emissions may 20 discharge tube. An amount of oxygen is within the be generated and to acquire laser sources having supe- cylindrical discharge tube sufficient to establish an rior operating parameters. oxygen pressure therein in the range from about 10 to Oxygen ion lasers have been reported in the litera- about 100 millitorr. A pair of reflective elements are ture for some years. However, all such reports have disposed essentially normal to the axis of the cylindrical merely confirmed that a laser output may be attained 25 discharge tube adjacent to the respective ends thereof, from a laser utilizing ionized oxygen as the active me- The pair of reflective elements are spaced from each dium. Such prior art oxygen ion lasers have utilized other so as to establish a resonant cavity therebetween both singly and doubly ionized oxygen. Several such at a wavelength at which doubly ionized oxygen can be reports are as follows: "Laser Oscillation on Visible caused to emit laser radiation. One of the reflective and Ultraviolet Transitions of Singly and Multiply Ion- 30 elements is partially transparent at the laser wave- ized Oxygen, Carbon, and Nitrogen;" by R. A. McFar- length. A voltage source is connected across the elec- lane, Applied Physics Letters, Vol. 5, No. 5, pp. 91-93, trodes to provide a discharge pulse having a duration September 1964; "Spectroscopy of Ion Lasers," by less than about 2.0 microseconds whereby a high gain W. B. Bridges et al., IEEE Journal of Quantum Elec- pulse of light at the doubly ionized oxygen emission tronics, Vol. QE-1, No. 2, pp. 66-84, May 1965; 35 wavelength is emitted through the partially transparent "Visible and uv Laser Oscillation at 118 Wavelengths reflective element after the end of the discharge pulse, in Ionized Neon, Argon, Krypton, Xenon, Oxygen, Further objects, advantages and features of the in- and Other Gases," by W. B. Bridges et al., Applied vention will be apparent from the following detailed Optics, Vol. 4, No. 5, pp. 573-580, May 1965; and description of the preferred embodiments taken to- "New 0 II 6640-A Laser Line," by M. Birnbaum et 40 gether with the accompanying drawing. al., IEEE Journal of Quantum Electronics, Vol. BRIEF DESCRIPTION OF THE DRAWING QE-7, No. 5, p. 208, May 1971. Where the experimental conditions are reported, In Drawing: they are quite consistent, for example, McFarlane used ., FIG-1 is a partially schematic side view of a high gain a laser employing a Brewster window structure with a 45 pulsed ion laser according to the invention; discharge tube 7 millimeters i.d. and 1 meter long. FIG 2 is a schematic diagram of a source of suitable Oscillation was observed with gas pressures between 20 discharge pulses for use in the apparatus of FIG. 1. and 50 millitorr and with pulse currents in excess of FIG- 3 is a graphical representation showing the se- 500 amperes from the discharge of a 2-jj.F condenser quential relationship of current and laser pulses in the and where the pulse lasted approximately 20 microsec- apparatus of FIG. 1; onds and was repeated several times per second. Also, FIG. 4 is a graphical representation showing the ef- where reported, the reflectivities of the cavity mirrors fect of oxygen pressure on peak power output; is maintained at very high levels, e.g., 97 percent reflec- FIG 5 is a graphical representation showing the ef- tive or higher. fects on peak power output from changes in charge No report as yet has attributed performance to either 55 yoltage, oxygen pressure, and output mirror reflectiv- singly or doubly ionized oxygen which would cause the ity; and pulsed oxygen ion lasers to be considered in the first FIG-6 1S a graphical representation showing the rela- class of projects discussed above. Bridges et al., in the tionship between peak power output m the ultraviolet IEEE Journal of Quantum Electronics publication and discharge voltage for the apparatus of FIG. 1. prognosticate only that certain singly ionized lines have nPTAII Pn nPQrRIPTIOM OF THF PRFFFRRFD potential technological application. A. W. Tucker et DETAILED DESCRIPTION OF THE PREFERRED al., "Pulsed-Ion Laser Performance in Nitrogen, Oxy- EMBODIMENTS gen, Krypton, Xenon, and Argon," IEEE Journal of In referring to the various figures of the drawing Quantum Electronics, Vol. QE-10, No. 1, January hereinbelow, like reference numerals will be utilized to 1974, have reported the relative peak powers attain- refer to identical parts of the apparatus. able with a group of pulsed ion lasers operating be- Referring initially to FIG. 1, where is shown a pre- tween 350 and 460 nanometers. The group of singly ferred embodiment of a high gain pulsed ion laser ac- ionized oxygen lines touted by Bridges et al., exhibit cording to the present invention which is designated 3,928 820 3 4 generally by the reference numeral 10. The laser in- 100 percent oxygen or whether it is a mixed fill with a cludes a cylindrical discharge tube 12 having enlarged noble gas or simply air. As stated previously, the oxy- end portions 14 and 16 and a constant cross section gen pressure is maintained by the pump 44 at the de- portion 18 in the center thereof. A central axis 20 sired pressure. With the longest tube used, the best corresponding to the cylindrical axis extends longitudi- 5 pressure was typically 40 millitorr. A typical current nally through the discharge tube 12. pulse, when operating a 75 centimeter long discharge The enlarged end portions 14 and 16 of the discharge tube, has the following characteristics. On discharging tube 12 each serve to house one of a pair of electrodes 11 nF charged at 23 kV through 22 millitorr of oxygen, 22 and 24 respectively. The hollow electrodes in this the current pulse has an approximately trangular shape embodiment are of cylindrical form and are disposed 10 with a rounded top; its duration is 500 nanoseconds, coaxially within the discharge tube 12. The pair of the FWHM is 350 nanoseconds; the risetime is approxi- electrodes 22 and 24 define a discharge length L there- mately 80 nanoseconds; and the peak intensity is 1250 between. The electrodes 22 and 24 are attached to a A. However, it is within the purview of the invention to pair of feedthrough posts 26 and 28 respectively which use discharge pulses as long as about 2.0 microseconds. provide a means for externally applying a voltage that 15 A typical such pulse is shown as the bottom curve in produces a discharge pulse across the electrodes. A FIG. 3. discharge pulse source 30 is connected across the posts A strikingly high gain is exhibited by the 5592 A laser 26 and 28 and will be described more fully hereinbe- line of doubly ionized oxygen (0(111)), when the cur- low. rent pulse in the discharge is shorter than a typical In addition, the enlarged end portions 14 and 16 are 20 relaxation time of the excited electronic state, in the terminated at their distal ends by a pair of windows 32 neighborhood of 0.75 microseconds. Under these con- and 34 respectively which ate inclined to the axis 20 at ditions, lasing can even be obtained from a discharge Brewster's angle to minimize reflective losses. Outside length of 15 centimeters and with the two mirrors com- of the windows 32 and 34 are a pair of reflective ele- pleting the cavity each having a 50 percent reflectivity ments 36 and 38 which are centered generally on the 25 at the laser wavelength. For laser action to occur in this axis 20 and which intersect the axis 20 essentially nor- case, the gain constant of the oxygen plasma must be at mal thereto. Within the purview of the invention, the least 4.6% cm-1 simply to overcome the mirror losses. reflective elements 36 and 38 may have either planar or High gain is also found on some ultraviolet lines in the spherical reflective surfaces. One of the reflective ele- 3750 A region; namely, the two known lines at 3754.6 2 3 ments, e.g., element 38, is partially transparent so that 30 A and at 3759.8 A (transition 2p ( P°) 3p D2 — 2p 2 3 3 3 the output from the apparatus 10 may be coupled out ( P°)3s P,° and the corresponding D3 ->• P2° transi- of the cavity. This is represented by the arrow 40. Gen- tion). The short pulse excitation also produces two new erally, the reflective elements are dielectrically coated laser lines in the ultraviolet (0(111)) transitions 2p 2 3 2 3 mirrors selected for their performance at a specific ( P°)3p D, — 2p( P°)3s P0° at 3757.2 A and the 3 3 wavelength of interest. 35 corresponding Dt — P,° transition at 3773.8 A). The end portion 14 has a tube 42 extending there- The exceptionally high gain observed in the oxygen from to a pump 44. The pump 44 is utilized to control discharge is associated in part with the afterglow char- the pressure of the gas fill within the cylindrical dis- acter of the laser emission. In fact, laser emission both charge tube 12. 40 in the ultraviolet and visible lines of 0(111) under short The pulsed discharge through the discharge tube 12 pulse excitation occurs only after the current pulse is may be obtained from the circuit shown in FIG. 2. A completely extinguished. Since laser emission occurs capacitor bank 46 is charged at high voltages from a when the plasma is relatively cold, the losses associated source 52 via a charge resistor 48 and a bleeding resis- with the plasma scattering are minimized. The after- tor 50. In the charging process, both the discharge tube glow laser emission is due to a slow relaxation in the 12 and a triggered spark-gap 54 act as infinite imped- 45 excited state, from the states reached on electronic ances. Upon application of an external pulse to the grid excitation down to the emitting levels. of the spark-gap 54, the voltage across the capacitor The following table gives dimensions of a number of bank 46 is applied suddenly across the discharge tube discharge tubes in which successful results have been via post connections 26 and 28. Once the gas in the achieved. tube becomes ionized, the impedance of the discharge 50 tube 12 becomes much smaller than that of the bleeding resistor 50 and most of the current flows through Tube Discharge Length L Bore Electrodes the discharge tube 12. The capacitor in our measure- (cm) (mm) ments has been varied from 3.5 to 11 nF. The charge A 75 g a voltage has been varied from 10 to 30 kV. The laser 55 B 33 8 b C 15 8 a emission wavelength is measured by means of a Jarrel- D 21 4 b Ash one meter grating spectrometer, while the tem- E 13 2 a poral development of the laser pulse is measured with a F 16 2.4 b fast photodiode (ITT FW 114) and a double trace (a) Oxide coated cathodes in tube Ride arms. oscilloscope. The current pulse is measured with a 60 (b) Stainless steel, hollow, cylindrical electrodes, coaxial to tube. Pearson current transformer Model 411 with a 0.1 V/A sensitivity and a nanosecond risetime. The spark-gap Referring now to FIG. 3 of the drawing, there are used in our preferred arrangement is an EGG model shown two curves representing oscilloscope traces of HY-32 spark-gap. the current discharge pulse, lower curve, and the re- The gas fill within the discharge tube 12 has at least 65 sulting laser output pulse, upper curve. This result is a portion thereof which is oxygen. The oxygen pressure achieved at the various emission wavelengths associ- in the discharge tube 12 is within the range from about ated with doubly ionized oxygen. These wavelengths 10 to about 100 millitorr whether the fill is essentially are in the yellow-green portion of the visible spectrum, 3,928,820

5592 A, and in the ultraviolet, 3754.6 A, 3757.2 A, 3759.8 A, and 3773.8 A. ; ' ' : ' ^ ' The laser pulse Starts generally when the current Example • Fill Gases Oxygen Pressure Added Gas (Millitorr) Pressure pulse is completely extinguished; therefore, the laser (Millitorr) 5 action is an "afterglow" effect. This is shown in FIG. 3 . 1 0,Ne . 10 100 for the longest tube (A). With the shorter tube (B), 2 O.Ar -.•••". 50 10 contraction by roughly a factor of 2 in the duration of 3 0,Xe 50. 10 4 0,Xe • 25 5 the current pulse causes a corresponding shortening in 5' . O.He 30 30 the laser pulse, but not in the time delay between the JQ 6 O.He . 12.5 12.5 7 Air 20 80 current pulse peak and the laser pulse peak. The peak power of FIG. 3, namely 96 watts, corresponding to a pulse energy of 50 microjoules is obtained in tube A In such mixed fill arrangements, generally a sequen- with one broadband (465 to 690 nanometer) maximum tial laser pulse effect is achieved. That is, the two com- reflectivity mirror and a 5 percent transmitting output 15 ponent gases can be made to lase during the same dis- mirror. The optimum pressure under these conditions charge pulse. In a typical case of xenon-oxygen mix- was around 40 millitorr, as shown in FIG. 4, where the tures, xenon starts lasing in the green Xe(IV) lines at effect of gas fill pressure on peak output power is plot- the peak of the current pulse, while laser emission from ted at constant excitation energy. No systematic opti- oxygen occurs later in the afterglow. Similar effects mization of the output mirror reflectivity has been 20 were found in the argon-oxygen mixtures, although in carried out at any tube length and can be expected to the case of argon the emitting species is the singly ion- improve the results already attained. The output power ized atom and not the triply ionized atom, as in the case generally does not show saturation with tube length, of xenon. A discharge in these gas mixtures can be used but continued to increase, at least up to the tube 25 directly in kinetic spectral studies, with the laser pulse lengths used. FIG. 5 of the drawing illustrates the com- from the added gas constituents acting as an excitation bined effects of the variance of a number of parame- pulse and the delayed laser emission from oxygen acting as a probe beam. ters, namely oxyen pressure, charge voltage, and output mirror reflectivity. It should be noted here that, at Furthermore, the apparatus of the present invention 50 millitorr oxygen pressure, the output vs. voltage 30 can be operated at repetition rates at least as high as curves have an extended linear range, different slopes, 100 pulses per second in both the visible and ultraviolet. and the output with a 50 percent transmitting mirror always exceeds that with a 97 percent transmitting . While there have been shown and described what are presently considered to be the preferred embodiments mirror. of the invention, it will be obvious to those of ordinary In the ultraviolet, generally, two laser lines are ob- skill in the art that various changes and modifications served, one at 3754.6 A and another at 3759.8 A. Two may be made therein without departing from the spirit additional lasing lines at 3757.2 A and 3773.8 A are of the invention as defined in the appended claims. observed in the longest tube. The oxygen pressure We claim: range for lasing is similar to that for the visible line, 40 1. A high gain pulsed ion laser comprising although in the longest tube optimum pressures were a cylindrical discharge tube having a centrally lo- lower, namely about 20 millitorr. The two intense com- cated axis therethrough and being closed at the ponents of the lasing group are found for all tubes. ends, The time development of the laser pulse follows the a pair of electrodes within the cylindrical discharge same pattern observed for the 0(111) laser transition at 45 tube, one electrode located effectively near each 5592 A; namely, lasing starts only when the current end of the cylindrical discharge tube, the elec- pulse is extinguished completely. The laser pulse delay trodes being open at least at the axis of the cylindri- (time lag between current pulse peak and laser pulse cal discharge tube, peak) is about 1.6 microseconds near the threshold and an amount of oxygen within the cylindrical discharge decreases gradually above the treshold to 0.75 micro- 50 tube sufficient to establish an oxygen pressure in seconds. The peak output power of the ultraviolet laser the discharge tube in the range from about 10 to pulse for tube A is rather insensitive to pressure in the about 100 millitorr, 20 to 40 millitorr range. When both cavity mirrors have a pair of reflective elements disposed essentially nor- a 95 percent reflectivity at approximately 3750 A, the mal to the axis of the cylindrical discharge tube 55 adjacent to the respective ends thereof, the pair of output of one side of the cavity is plotted in FIG. 6 for reflective elements being spaced from each other a 22 millitorr oxygen pressure. For most of the range of so as to establish a resonant cavity therebetween at excitation voltages, the peak power increases linearly a wavelength at which doubly ionized oxygen can with applied voltage. Beyond 25 kV the curve slope be caused to emit laser radiation, and one of the decreases. 60 reflective elements being partially transparent at In addition, the fill may, within the purview of the the laser wavelength, and invention, be a mixed fill of oxygen and other gases. a voltage source connected across the electrodes to Successful operation of the devices is attained when the produce a discharge pulse having a duration less mixed fill is used with oxygen and a gas selected from than about 2.0 microseconds whereby a high gain the group consisting of neon, argon, xenon, helium, and 65 pulse of light at the doubly ionized oxygen emission nitrogen (generally in the form of air). The following wavelength is emitted through the partially trans- table gives pressure values for a typical selection of parent reflective element after the end of the dis- mixed fills. charge pulse. 3,928 ,820 7 8 2. A high gain pulsed ion laser according to claim I, the cylindrical discharge tube and the cylindrical dis- wherein the wavelength is 5592 A. charge tube is closed at the ends by a pair of windows 3. A high gain pulsed ion laser according to claim 1, transparent at the wavelength and inclined to the axis wherein the reflective elements are spaced such that at Brewster's angle. light at the wavelengths of 3754.6 A, 3757.2 A, 3759.8 5 8. A high gain pulsed ion laser according to claim 1, A, and 3773,8 A may be coupled out of the laser. wherein the discharge pulse has a duration of about 0.5 4. A high gain pulsed ion laser according to claim 1, microseconds. wherein the cylindrical discharge tube also contains a 9. A high gain pulsed ion laser according to claim 1, mix gas selected from the group consisting of argon, wherein the oxygen pressure is in the range from about xenon, neon, helium, and air. 10 20 millitorr to 40 millitorr. 5. A high gain pulsed ion laser according to claim 1, 10. A high gain pulsed ion laser according to claim 1, wherein the partially transparent reflective element is wherein the source includes means for repetitively at least 50 percent reflective. applying a voltage across the electrodes at a rate up to 6. A high gain pulsed ion laser according to claim 5, f5 about 100 Hertz. wherein the reflective elements are dielectrically 11. A high gain pulsed ion laser according to claim 1, coated spherical mirrors. wherein the electrodes are cylindrical and are disposed 7. A high gain pulsed ion laser according to claim 5, coaxially in the cylindrical discharge tube. wherein the reflective elements are located external to *****

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