Rejuvenation of HeliumNeon LasersKenneth W. Ehlers and Ian G. Brown Citation: Review of Scientific Instruments 41, 1505 (1970); doi: 10.1063/1.1684322 View online: http://dx.doi.org/10.1063/1.1684322 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/41/10?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Polarization of a heliumneon laser Phys. Teach. 34, 346 (1996); 10.1119/1.2344475 cw heliumneon Raman laser Appl. Phys. Lett. 48, 86 (1986); 10.1063/1.96943 Rejuvenation of helium–neon lasers Am. J. Phys. 45, 107 (1977); 10.1119/1.10894 A Simplified Construction of a Helium-Neon Visible Laser Am. J. Phys. 33, 225 (1965); 10.1119/1.1971385 Lifetime of HeliumNeon Lasers Rev. Sci. Instrum. 35, 996 (1964); 10.1063/1.1718973

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NOTES 1505

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TIME ("'SEC)

FIG. 3. Characteristics of a Channeltron during pulsation of the detector high voltage. Display (a) is the detector output for an incident beam of ions under normal voltage conditions; display (b) is the detector output when the voltage applied to the detector is that shown in display (c). The return to an apparent normal state takes about 8 msec after the full voltage is returned. Counting rate has been reduced in comparison with that shown in Fig. 2.

detector was placed in an intense beam (5X 105 counts/ sec) for several minutes with the detector supply at 1700 V; then the beam intensity was reduced to 3X 102 counts/sec, the detector voltage was increased to 2200 V, and the Channeltron output was monitored as a function of time. The gain increased by a factor of 2 over a period of 3 min. No comparable gain variation was observed for an initial beam strength of 104 counts/sec.

We have also observed that the detector gain varies for periods of minutes after the application of the Channeltron voltage (from 0 to 2200 V) and that this variation very likely has some dependence on the intensity of the beam striking the Channeltron before any detector voltage is applied.

Comparison of the above blanking systems indicates that the two detector scheme overcomes both the dead­time and gain variation effect of the voltage pulsed scheme. However, there may be experiments with time scales and counting rates which would make the voltage pulsing scheme a feasible technique.

The authors would like to thank Robert Hammerle, John Pearl, and Jens Zorn for their suggestions and com­ments and Ken Schmidt of the Bendix Corporation for the loan of several Channeltrons.

'" Work supported in part by the National Aeronautics and Space Administration.

t Present address: Department of Chemistry and Physics, Skid­more College, Saratoga Springs, N. Y. 12866.

1 D. S. Evans, Rev. Sci. Instrum. 36, 375 (1965). 2 D. P. Donnelly, J. C. Pearl, R. A. Heppner, and J. C. Zorn, Rev.

Sci. lnstrum. 40, 1242 (1969). 3 D. G. Smith, J. Sci. lnstrum. 43, 270 (1966). 4 J. Bosqued and H. Reme, Nucl. Instrum. Methods 57, 6 (1967). 6 E. C. McCullough, Nucl. Instrum. Methods 60, 351 (1968). 6 L. A. Frank, N. K. Henderson, and R. L. Swisher, Rev. Sci.

lnstrum. 40, 685 (1969). 7 R. Reed, E. Shelley, J. Bakke, T. Sanders, and ]. McDaniel,

IEEE Trans. Nucl. Sci. NS-16, No.1, 359 (1969). 8 A. Egidi, R. Marconero, G. Pizzeila, and F. Sperli, Rev. Sci.

lnstrum. 40, 88 (1969). 9 W. G. Wolber, B. D. Klettke, and H. K. Lintz, Rev. Sci. lnstrum.

40, 1364 (1969).

Rejuvenation of Helium-Neon Lasers* KENNETH W. EHLERS AND IAN G. BROWN

Lawrence Radiation Laboratory, University of California, Berkeley, California 94720

(Received 18 May 1970)

WE have recently had cause to consider the origin of the characteristic loss of power output from helium­

neon lasers as a result of aging. This deterioration is ob­served to occur independently of whether or not the laser is actually operated; typically, we find, a laser tube will suffer a loss of power by a factor of around 2 in a period of order 1-2 years. It is reasonable to assume that at least a part of the power degradation is due to irreversible effects such as contamination from electrode and wall sputtering, but in our case, the operating time was small, and we felt this effect was unlikely to be dominant. It is known that thin walled vessels of glass or quartz are particularly per­meable to helium,! and a diminution of lasing efficiency of a helium-neon laser is to be expected as the helium escapes and the gas mixture departs from optimum. This gas loss is reversible and refilling may be accomplished in a very simple manner. We have immersed the complete laser, without any dismantling, in a helium atmosphere, at atmospheric pressure, inside a suitable container of thick Pyrex or metal. The laser is left there for one to several days. Upon removal, we have measured increases in output power of up to a factor of six times that of the prefilling value, powers which are as great as or greater than the manufacturer's specification. We have in this manner renewed three of our helium-neon lasers:

(a) A Spectra-Physics model 115, ~ six years old, whose power output had fallen from 2 to 3 m W to 0.55 m W; after three days in the helium bath its power had risen to 3.0mW.

(b) An Optics Technology model 195, ~ one year old, whose output had fallen to 1.5 m W; after about 18 h in helium, the power was 5 m W. The manufacturer's speci­fication is 22 mW.

(c) A Spectra-Physics model 130B, of uncertain history, whose power had fallen from the rated 1 to 0.4 m W. The power rose by nearly a factor of 2 in ~ 20 h, then suffered a slow decrease, from 20 to 40 h, falling to around 0.5 m W ; in this case the optimum helium pressure was exceeded.

These observations are consistent with calculations based on the known diffusion rates of helium through quartz and through Pyrex. l The filling pressure of normal laser tubes is ~ 1 Torr helium to ~0.1 Torr neon, and the tube is either quartz or Pyrex, usually having quartz laser beam exit faces. The diffusion rate of helium through Pyrex is about an order of magnitude slower than through quartz, and the resultant loss rate is dependent on the

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1506 NOTES

precise tube structure. However, for typical tube dimen­sions, the helium pressure falls by one-half in a time of order one to several years, the observed time scale of power falloff. On the other hand, when the helium pressure difference across the tube is 1 atm, the diffusion rate is 760 times as fast. Thus the refilling operation takes a time only 1/760 of the prefilling lifetime. For maximum im­provement, the laser output power should be measured as a function of time in the helium bath; the tube should not be significantly overfilled so as to exceed the optimum helium-neon gas mixture of around 7: 1.2 We have found a bath duration roughly 1/760 of the known lifetime to be adequate.

Finally, we remark that the diffusion of neon is negligi­ble, being less than that of helium by about four orders of magnitude, and that it is possible to obtain output powers 30% or more greater with the use of 3He and 2°Ne in a 9: 1 mix.

* Work done under the auspices of the U. S. Atomic Energy Commission.

IS. Dushman, Scientific Foundations of Vacuum Techn'ique (Wiley, New York, 1958).

2 E. I. Gordon and A. D. White, App!. Phys. Lett. 3, 199 (1963).

Tungsten Microelectrode Preparations for eNS Recording'"

STEVEN J. COOl. AND M. L. J. CRAWFORD

U nh·ersity of Texas, Graduate Sclcoot of Biomedical Sciences at II ouston, and Baylor College of Medicine, Iloltston, Texas 77025

AND

IRA J. SCHEER

Ohio State University Columbus, Ohio 43210

(Received 13 April 1970'; and in final form, 26 June 1970)

T HE neophyte single-unit neurophysiologist often finds himself confused by the intricate and often

contradictory instructions that exist in the literature with respect to the production of metal microelectrodes (e.g., stir the solution while etching, or do not stir it; leave the electrode immersed in the solution while etching, or dip it in and out, etc.). Also, one finds in the literature that many small, seemingly insignificant details are left out-­details that turn out to be somewhat critical (e.g., how far below the surface of the etching solution to immerse the tip of the wire).1 After many tedious hours of trial­and-error failure, attempting to replicate this or that person's instructions, we have arrived at a technique which, although not, in all probability, unique to us, is, we believe, simpler than most methods which have been

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I'IG. 1. (A) Schematic diagram of apparatus used in etching tungsten microelectrodes. (B) Photographs of three finished electrodes actually used in recording from cat visual cortex. (C) Examples of quality and SIN ratio of recording attainable with electrodes such as those in (B). See text for further discussion.

published and yet which yields an electrode of good quality for use in extracellular recordings. While the procedure is a conglomerate of procedures used by many others, it does, we believe, remove a great deal of the art and mystery from the production of metal microelectrodes.

A 0.625 cm diam carbon rod is pushed through the bottom of a 50 ml polyethylene beaker (4 cm diameter and 6 cm tall) to a distance of about 2.5 cm and sealed in place with an epoxy cement. A 7.5 cm square piece of 0.625 cm thick Plexiglas with a 0.625 cm hole drilled in the center is used as a cover. This cover has an alligator clip attached to the top of it so that the clip may be used to hold a piece of wire vertically placed in the hole in the Plexiglas. The carbon rod and the alligator clip are con­nected to the secondary of a 6.3 V ac, 1 A transformer, which, in turn, is powered from a variable voltage source (Variac). Figure leA) is a schematic diagram of the apparatus.

The beaker is filled to within 5 mm of the top with a solution which is 70% 8-lOM NaCN and 30% 5lY NaOH. A 5 cm length of tungsten wire (127 fJ. in diameter)2 is lowered 6-7 mm into the solution and held in place by the alligator clip. The wire and the carbon rod will now be vertically aligned and their tips will be about 1.8 em apart. A 6.3 V ac potential is then applied across the alligator clip-carbon rod connections. A vigorous bubbling in the area of the immersed tungsten will be immediately obvious.

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