Contains reports on one research project.Joint Services Electronics Programs (U.S. Army, U. S. Navy, and U. S. Air Force) under Contract DA 28-043-AMC-02536(E)U. S. Air Force Cambridge Research Laboratories Contract F19628-70-C-006

George, E. Victor

Lyon, D. L.

DSpace@MIT

V. ELECTRODYNAMICS OF MEDIAAcademic and Research StaffProf. L. J. Chu Prof. P. W. Hoff Prof. J. A. KongProf. H. A. Haus Prof. P. Penfield, Jr.Graduate StudentsB. L. Diamond H. Granek E. E. Stark, Jr.M. S. Elkind D. L. Lyon L-h. WangA. TEA CO 2 RING LASERWe have been studying the operation of transversely excited, atmospheric, CO 2lasers.1-3 In our earlier work we used gain tubes of varying lengths in Fabry-Perot,two-mirror, optical cavities. Under a wide range of operating conditions "self-locking"2 4was observed in the outputs of these lasers.4 That is, two oscillating modes werebeing supported with equal amplitudes during the duration of the giant, gain-switchedpulse from the laser.1.4 m DETECTOR\ FOR ccwHELICAL GAIN 0.45 m RADIATIONGOLD MIRROR TUBE APERTURER2 m |1 DETECTORS IRTRAN0.7 m R= 0.280% REFLECTIVITYS GOLD MIRROR'1.2 m R =6 m 0.25 mFig. V-1. Ring laser schematic.It has been suggested by E. E. Stark, Jr., and H. A. Haus that the self-locking maybe due to the existence of spatial hole-burning caused by standing-wave patterns in thegain medium. In order to investigate this hypothesis, we have constructed a "ring"laser, using three mirrors and a single gain tube (see Fig. V-i). It was our aimto simultaneously monitor the clockwise and counterclockwise radiation emitted fromthe device. We could then compare the degree of self-locking with the degree ofstanding-wave character in the cavity. The gain tube used in the experiment hadThis work was supported by the Joint Services Electronics Programs (U. S. Army,U. S. Navy, and U. S. Air Force) under Contract DA 28-043-AMC-02536(E), and in partby U. S. Air Force Cambridge Research Laboratories Contract F19628-70-C-0064.QPR No. 101Fig. V-2. Helical gain tube.(V. ELECTRODYNAMICS OF MEDIA)an over-all length of 70 cm and an active discharge length of 35 cm (see Fig. V-2).The total volume of the tube is 0. 3 liter. The anode and cathode were of helicalgeometry. The anode was made up of sixty, 1 k2 resistors spaced 0. 5 cm apart; thecathode consisted of 60 sharpened needles spaced 0. 5 cm apart. Each resistor hada pin placed diametrically across the tube from it. Pin-to-resistor separation was2. 5 cm.The helical construction was chosen because of its radial symmetry when viewedon-axis. The active discharge also has a radially symmetric shape, preferentiallyexciting radially symmetric optical modes of the cavity. With internal cavityaperturing we restricted the laser output as much as possible to the TEM00 mode.Because of the lack of reliable data concerning optimal TEA CO 2 laser opera-tion we decided to maximize several operating characteristics vs all easily vari-able parameters. Total gas pressure, ratios of component gases, total gas flowrate, discharge voltage, capacitance, discharge pulse rate, and polarity of pins andresistors were all varied. We were looking for the highest possible time-averagedoutput power, along with reproducible "nicely behaved" light pulses on a fast timebasis. In our studies we found that clean and reproducible electronic glow dischargescorrelated well with smooth, reproducible optical pulses. For this reason, we alsonoted relative amounts of arc-type vs glow-type discharging in the gain tube.To measure output power, we used a Coherent Radiation (thermal) power detec-tor followed by a Keithley microvoltmeter. Our supply voltage was set at 20 kV,capacitance at 0.01 j[F, and total gas pressure at 350 Torr. The resistors actedas the anode, pins as cathode. The pulsing rate was 8 pulses/second. We thenmeasured total power radiated as a function of the flow rates of the three compo-nent gases: CO 2 , H e , N2It was observed that total output power and pulse repeatability were both sen-sitive functions of the He/CO 2 flow rate ratio. The N 2 flow rate was a less criticalparameter. Also there was a threshold flow rate below which good pulse repeata-bility and output power were not attainable. He/CO2 flow rate ratios between 5.6and 6.0/1 maximized total output power (see Fig. V-3) at total flow rates of theorder of 2 liter-atm/min. The optimal He/N 2 ratio was found to be in the vicinityof 10/1. This ratio was not terribly critical.We then varied the supply voltage and the amount of capacitance in our triggercircuit (see Fig. V-4). These curves were taken with gas pressure equal to 350 Torr,2 liter/min flow rate, gas ratios CO : H e : N : 1. 8: 10:1, and pulse repetitionequal to 5/s.Although a capacitance value of 0. 014 pF yielded the highest output power there wasa large amount (approximately 10% of the number of resistors) of high current arcdischarges. The laser output pulses were correspondingly less stable at the highestQPR No. 101- He/N 2 FLOW-RATE RATIOWAS KEPT NEAR 9/1.A N2 FLOW =0.12 LITER-ATM/MINO N 2 FLOW =0.20 LITER-ATM/MIN-AO 0 N2 FLOW =0.30 LITER-ATM/MINFig. V-3. Power vs He/CO 2 flow ratio.0.150.140D0.13Oo> 0.120.115.00 5.50 6.00He FLOW RATECO 2 FLOW RATE6.50 7.000.140.130.120.1015 16 17 18SUPPLY VOLTAGE (kV)19 20FLOW RATIO: 9:1.6:1::He:CO2 : N 2TOTAL PRESSURE: 350 TorrPower vs supply voltage.C=0.014 pFC =0.012 pFI I I I I275 300 325TOTAL GAS PRESSURE (Torr)350 375SUPPLY VOLTAGE= 19.5 kVFLOW RATIO: 9:1.6:1::He:CO2 :N 2Fig. V-5. Power vs gas pressure.-0I I I IFig. V-4.(V. ELECTRODYNAMICS OF MEDIA)capacitance value.Finally, we measured output power as a function of total gas pressure and param-etrized on capacitance values (see Fig. V-5). We had also planned to param-etrize the curves on pin-resistor polarity; however, we found that keeping theresistors positive and pins negative yielded approximately 3/2 the power under alloperating conditions than the reverse polarity.After this initial step-by-step optimization we again varied gas ratios and foundsimilar results as obtained under the slightly suboptimal conditions illustrated inFig. V-3.Our final choices for optimizing total output power and pulse repeatability werethe following.Electrical Conditions - 20 kV supply voltage0.012 jiF capacitanceresistors positive (anode); pins negative (cathode).Gaseous Conditions - 350 Torr total gas pressure2. 5 liter/min total flow rateHe: CO 2 : N2 flow ratio of 9:1.6:1.Using these parameters values, we observed the two outputs simultaneously from ourring cavity laser. Each output corresponded to the radiation traveling in either theclockwise or counterclockwise direction in the cavity. Two liquid He cooled Cu:Ge wereused to monitor the propagating 10.6 i radiation. The output current from the detec-tors is essentially a function of the magnitude of the incident light. Two oscilloscopeswith bandwidths greater than 150 MHz were used to display detector current. One was(a) (b)Fig. V-6. (a) -Type 7704 oscilloscope trace of current from detector monitoringthe counterclockwise direction. Horizontal: 2 ps/div; vertical:0. 05 V/cm.(b) Type 454 trace of response to cw traveling pulse; triggered simul-taneously with oscilloscope in (a). Same scales.QPR No. 101(V. ELECTRODYNAMICS OF MEDIA)a Tektronix Type 454 and the other a Tektronix Type 7704 main-frame with high-speedtime base and vertical amplifier plug-ins. The two oscilloscopes were triggeredsimultaneously through an attenuated feed from the discharge current pulse. In thefigures presented (see Fig. V-6) the start of the trace corresponds to the start ofthe discharge current in the gain tube.The following general behavior was observed: (i) The clockwise and counter-clockwise pulses generally had different amplitudes following a single discharge ofthe gain tube. No strict correlation between them was observed and their mean val-ues were roughly equal. (ii) Both pulses had an equal delay time (T) measured fromthe start of the gain-tube discharge. (iii) T varied around an average value of 8 is,with standard deviation equal to 1. 5 p s. (iv) In no single observation was the lightpulse ever 100% "self-locked." The output signal corresponded to the presence oftwo unequal amplitude, longitudinal modes. This observation must be contrastedwith those modes in a Fabry-Perot cavity.These results do not constitute a proof that standing-wave patterns in the gainmedium are necessary for "self-locking." They are suggestive, however, of thatstatement, and further experiments are planned to pin down the relationship. Weare considering various cavity configurations that will allow us to vary determinis-tically the degree of standing-wave character in the cavity. We are also investi-gating the possibility of an optical circulator at 10. 6 v.D. L. Lyon, E. V. GeorgeReferences1. E. V. George and L. D. Pleasance, "High-Pressure Pulsed CO 2 Laser," QuarterlyProgress Report No. 98, Research Laboratory of Electronics, M.I.T., July 15,1970, pp. 47-52.2. D. L. Lyon, E. V. George, and H. A. Haus, "Investigation of Nonlinear Effectsin High Pressure CO 2 Lasers," Quarterly Progress Report No. 99, Research Labo-ratory of Electronics, M. I. T., October 15, 1970, pp. 70-74.3. A. J. Beaulieu, Appl. Phys. Letters 16, 504 (1970).4. D. L. Lyon, E. V. George, and H. A. Haus, Appl. Phys. Letters 17, 474 (1970).QPR No. 101

Electrodynamics of Media

http://dspace.mit.edu/bitstream/1721.1/56163/1/RLE_QPR_101_V.pdf

Electrodynamics of Media

Abstract

Similar works

Full text

Available Versions

DSpace@MIT