Method and Apparatus for Subdividing a Gas Filled Glass Tube With a Laser

This invention relates to the manufacture of miniature tubular radioactive light sources and, more particularly, to a method and apparatus for subdividing a long phosphor-coated tube filled with a radioactive gas into a plurality of individual sealed segments.

Self-illuminating devices are known which utilize a glass tube coated on the inside with a phosphor and filled with a radioactive gas such as tritium or Krypton 85. In the manufacture of miniature light sources of this self-illuminating type, it has been the practice to provide a long tube which is coated inside with a luminophor, evacuated, filled with the radioactive gas, and sealed at the ends. The tube is then subdivided into segments by melting the glass, and fusing the glass in the process to seal the ends of the segments. To help form the sealed ends of the tube segments and prevent escape of the radioactive gas, a pressure higher than the gas in the tube is maintained outside the tube. When the glass of the tube melts, it collapses to form the sealed ends. Laser beams have been used to heat the glass tube in the subdividing process. U.S. Pat. Nos. 3,706,543 and 3,817,733 describe arrangements incorporating laser beams for subdividing radioactive gas-filled tubes.

Various problems have been encountered in applying the techniques described in the prior art. In the prior art, the practice has been to hold the tube in two spaced chucks which rotate the tube in the laser beam so that the beam effectively scans the whole perimeter of the tube during the melting process. In practice, it has been found difficult to rotate the two chucks exactly in unison while the cut is completed. Any slight deviation from perfectly uniform rotation tends to stress the tube, i.e., twist it or bend it, which may cause fractures. The larger the cross-sectional dimensions of the tube, the more difficulty is experienced in maintaining perfect alignment and rotation of the tube during the cutting process.

Another problem encountered is that the tube is not easily cut at more than one point at a time, since each segment must be held and rotated in exact alignment with all the other segments during the cutting process so that no stress is placed on the tube. As a result, it has been the practice to only cut one segment at a time, advancing the tube between cuts to provide successive cuts along the length of the tube. As each cut take place, the total volume of the radioactive gas is reduced and therefore the pressure is increased. As each segment is cut off, the pressure in the remaining portion of the tube increases by an incremental amount. Thus, there is a difference in gas pressure between the first segment cut from the tube and the last segment cut from the tube. This results in a nonuniformity of brightness in the successive tube segments. This increase in pressure also requires an adjustment of the outside pressure with each successive cut to maintain the pressure differential necessary to achieve proper collapse and fusion of the tube ends as each new segment is cut. Summary of the Invention

A feature of the invention is the modulation of the velocity and/or intensity of the laser beam. In the case of back and forth beam scanning across at least one side of the tube, the velocity of the beam is smaller at the extremities of the beam path than at the middle of the beam path. This transfers more heat to the ends of the tube where there is more glass to be heated. The intensity of the laser beam is also increased during the subdivision of a segment to heat the cut line uniformly to a temperature below the fusion temperature of the tube before division of the tube begins.

Another feature of the invention is a tube holder having a slot in which the long tube slidably fits during its subdivision. The long tube is held in the slot on both sides of the cut line by means that permit the tube to be pushed through the slot by a tube advancing mechanism after each cut. For tubes with a rectangular cross section, the slot preferably has a dove tail cross section.

Another feature of the invention is defocusing the laser beam after the tube divides to anneal and remove discoloration from the divided ends while still in a tube holder, which holds the tube on both sides of the division. This, in effect, eliminates the requirement for a separate annealing and discoloration removal step.

Another feature of the invention is a plurality of focused laser beams simultaneously directed onto a long, sealed, radioactive gas-filled glass tube having a longitudinal axis at spaced intervals along the longitudinal axis. Relative movement is introduced between the laser beams and the tube transverse to the longitudinal axis of the tube to heat and soften the tube along a plurality of cut lines at spaced intervals simultaneously until the tube divides along each cut line to form ends and collapses to seal the divided ends. By thus simultaneously subdividing the long tube into a number of individual sealed segments, such segments are at substantially equal pressure and, therefore, exhibit substantially uniform brightness, the pressure outside the tube does not have to be readjusted during the subdividing process, and less time is required to carry out the process. Furthermore, the beam from a single laser can be split into the plurality of laser beams required for such simultaneous subdivision.

The features of specific embodiments of the best mode contemplated of carrying out the invention are illustrated in the drawings, in which:

FIG. 1 is a schematic top view of apparatus illustrating the principles of the invention;

FIGS. 2A, 2B, and 2C are a side sectional view, a top sectional view through the plane designated 2B--2B, and a top sectional view through the plane designated 2C--2C, respectively, of the pressurized chamber depicted schematically in FIG. 1;

FIG. 3 is a graph representing the angular position of the reflector of one of the linear scanners depicted in FIG. 1 as a function of time;

FIGS. 4A and 4B are graphs representing modulation patterns of the beam intensity of the laser source depicted in FIG. 1 as a function of time;

FIG. 5 is a schematic block diagram of the circuitry for controlling the tube cutting cycle, including the beam intensity of the laser source and the mirror deflection of the linear scanners depicted in FIG. 1; and

FIG. 6 is a schematic side view of apparatus for simultaneously subdividing a long tube into a plurality of segments according to the principles of the invention.

The entire disclosure of application Ser. No. 703,809, filed on July 9, 1976, is incorporated herein by reference.

The described invention is concerned with a method and apparatus for producing miniature tubular radioactive light sources by subdividing a long, sealed, radioactive gas-filled, glass tube into sealed tube segments without loss of the gas. A long glass tube, typically one to two feet in length, is first coated on its interior surface with a luminophor, i.e., a phosphor compound such as zinc sulfide activated with copper. One end of the long tube is sealed, the tube is evacuated and filled with a radioactive gas, preferably tritium, and then the other end of the tube is sealed to confine the radioactive gas. The pressure of the radioactive gas depends upon the desired illumination, but typically varies from 10 to 40 psia. The long tube preferably has an elongated cross section with two wide sides and two narrow sides. The wide sides provide a large illuminating area for backlighting a liquid crystal display or other objects. Although the elongated cross section of the tube could be oval or oblong, it is preferably rectangular with approximately square corners so the wide sides are as flat as possible. Typically, the tube would be from 0.100 to 0.250 inches wide and from 0.028 to 0.050 inches thick, with a wall thickness from 0.005 to 0.012 inches. To obtain a reliable seal when subdividing the long tube, a rule of thumb is that the sum of the thickness of both wide side walls be equal to or less than the distance between the wide side walls. From the point of view of structural integrity, however, the side walls of the tube must be thick enough to withstand the pressure of the gas within the tube; the wider the tube, the thicker the side walls must be to provide the required structural integrity. As shown in FIG. 1, the long, sealed, phosphor-coated, gas-filled glass tube, designated 9, is loaded into a pressurized cylindrical chamber 8. The pressure of chamber 8 is adjusted to a higher level than the pressure in tube 9. Typically, the pressure difference is between 5 and 10 psi, depending upon the glass characteristics. Tube 9 is then subdivided into individual, sealed tube segments, typically between 1/2 inch and 1 inch long, by a laser source 10. Laser source 10, which is preferably a CO.sub.2 laser operated in the TEM.sub.O,O mode at a wavelength of the order to 10.6 microns in the infrared region, emits a light beam 11. A beam splitter 12 divides beam 11 into a beam 13 at right angles to beam 11, and a beam 14 in alignment with beam 11. Beam 14 is deflected by a stationary mirror 15 as a beam 16 at right angles to beam 14. Beam 13 is reflected as a beam 17 by an oscillating mirror 18 of a linear beam scanner. A lens 19 in the path of beam 13 focuses the beam onto one wide side surface of tube 9. Similarly, beam 16 is reflected as a beam 21 by an oscillating mirror 22 of a linear beam scanner. A lens 23 in the path of beam 16 focuses the beam onto the other wide side surface of tube 9. The size of the focused laser beam which is typically between 0.010 and 0.025 inches in diameter, depends upon the wall thickness of the tube, the type of glass of the tube, and the gas pressure within the tube. The longitudinal axis of tube 9, which is perpendicular to the plane of the drawing, is designated 24. Beams 17 and 21 are aligned with each other in that both lie in the same plane transverse to, and preferably perpendicular to, longitudinal axis 24. Chamber 8 has windows 25 and made of germanium 26 through which beams 17 and 21, respectively, pass to reach the wide side surfaces of tube 9.

The linear scanners are preferably of the moving iron galvanometer type such as, for example, Model ALS-100 manufactured to Bulova Watch Company. As an electrical signal is applied to the linear scanner, the galvanometer is deflected by an amount proportional to the magnitude of the applied signal, and mirrors 18 and 22 oscillate accordingly about axes 27 and 28, respectively. For zero deflection of the linear scanners, mirrors 18 and 22 are oriented so beams 17 and 21 are reflected at a 90.degree. angle from beams 13 and 16, respectively.

As the linear beam scanners are deflected positively and negatively about the zero deflection point, mirrors 18 and 22 oscillate and beams 17 and 21 scan across the wide side surfaces of tube 9 perpendicular to axis 24. The extremities of the beam scans are represented by lines 17a and 17b and lines 21a and 21b. Beams 17 and 21 repeatedly scan back and forth across the wide side surfaces of tube 9 along a cut line transverse to and preferably perpendicular to longitudinal axis 24 to heat and soften tube 9 until tube 9 divides to form ends and collapses to seal the divided ends. Typically, the scanning frequency is from 6 to 10 Hertz, depending upon the variables, i.e., the type of glass, the gas pressure, and the tube dimensions. The higher pressure in chamber 20 than the gas pressure in tube 9 causes tube 9 to collapse along the cut line to form a reliable seal without appreciable loss of gas from within tube 9.

After the divided ends of tube 9 are sealed, a lens 29 is inserted into the path of beam 17 between chamber 8 and lens 19, and a lens 30 is inserted into the path of beam 21 between chamber 8 and lens 23 to defocus beams 17 and 21.

Alternately, lenses 29 and 30 could be inserted into the paths of beams 13 through 16, respectively. The defocused beams increase in size to impinge on the sealed ends and decrease in intensity to reduce the glass temperature. During the subdivision of tube 9 by the laser beams, the glass is discolored, i.e., blackened, by a chemical reaction between the tritium and the constituents of the glass. Defocused beams 17 and 21 scan back and forth several times across the sealed ends of tube 9, while still in its tube holder described below in connection with FIGS. 2A, 2B, and 2C, to anneal the glass of the sealed ends and remove the discoloration. The time and temperature of annealing depends upon the type of glass, and can be readily ascertained from published reference data on such glass types.

Although the preferred embodiment is to hold the long tube stationary and to move the laser beam, an alternative embodiment is to move the long tube, while holding the laser beam stationary. The movement of the tube in this alternative embodiment, however, may give rise to stresses, which might weaken or fracture the tube, and complicates the design of the equipment for automatic axial advancement of the tube after subdivision of each tube segment.

As shown in FIGS. 2A, 2B, and 2C, chamber 8 is attached to the top of a platform 34 by means not shown, and a cylindrical collector 35 for the tube segments produced by the described method is attached to the bottom of platform 34 by means not shown. Chamber 8 and collector 35 are connected by a hole 32 in platform 34. An annular seal 36 is disposed between chamber 8 and platform 34, and an annular seal 37 is disposed between collector 35 and platform 34. A source of gas 38 is supplied through valve 39 to the interior of chamber 8 to provide the desired pressure therein. Valve 39 is a 3-way valve having an exhaust port that permits exhaust of the gas from chamber 8. A tube holder 40 of hemicylindrical form is fixed within chamber 8 by means not shown. Tube holder 40 has a vertical elongated slot 41 with a dovetailed cross section designed to receive tube 9. Slot 41 is open along one side to form an open side between edges 42 and a closed side opposite the open side that is wider than the open side. The open side is several thousandths of an inch narrower than the width of tube 9, the closed side is several thousandths of an inch wider than tube 9, and the depth of slot 41 is several thousandths of an inch larger than the thickness of tube 9. As a result, tube 9 is retained by slot 41 but free to slide vertically through slot 41. Tube holder 40 has an opening 43 that passes competely through tube holder 40 in the region where the laser beams, i.e., beams 17 and 21, impinge upon the wide side surfaces of tube 9. Opening 43 is slightly wider than tube 9 so the laser beams do not touch tube holder 40. Tube holder 40 could be milled from aluminum bar stock. A portion of the bottom of tube holder 40 adjacent to one edge 42 of slot 41 is cut away for placement of a mounting block 44 which is attached to tube holder 40 by fasteners not shown. Ball plungers 45 and 46, which are mounted in bores in mounting block 44, each have a spring loaded ball that urges the adjacent short side surface of tube 9 toward the opposite slanting end of slot 41, thereby holding tube 9 in two spaced apart positions, namely, at one position above opening 43 and at one position below opening 43. The spring pressure of ball plungers 45 and 46 is not so great as to inhibit the vertical movement of tube 9 through slot 41 on application of a vertical force to tube 9. By way of example, ball plungers 45 and 46 could be Part No. CL-15-SBP-1 of Carr Lane Manufacturing Company, St. Louis, Missouri.

A slider 50, which could also be milled from aluminum bar stock, has a tongue 51 with a dovetailed cross section matching that of slot 41. One end of a rod 52 is attached to slider 50. Rod 52 extends vertically upward from slider 50 through an O-ring seal 53 in the top of chamber 8. Above chamber 8 the other end of rod 52 is attached to a connecting block 54. A lead screw 55 passes through stationary supporting blocks 56 and 57, which have threads engaging the threads of lead screw 55. The lower end of lead screw 55 is attached to connecting block 54. A stepping motor 58 drives a gear 59 having teeth that engage the threads of lead screw 55. Thus, as motor 58 operates, gear 59 rotates to translate lead screw 55 in a vertical direction and drive slider 50 through slot 41 of tube holder 40. By way of example, the function of lead screw 55, supporting blocks 56 and 57, and gear 59 could be provided by a unislide motor drive assembly, Part No. B2515P10J of Velmex, Inc., East Bloomfield, New York, and the function of motor 58 could be provided by Part No. M092FD08 of Superior Electric Company, Bristol, Connecticut. After each subdivision of tube 9 by the laser beams in the region of opening 43, ball plunger 46 holds the resulting tube segment, and ball plunger 45 holds the remainder of tube 9 until motor 58 drives slider 50 downwardly an increment equal to the length of the resulting tube segment. This pushes the resulting tube segment downwardly through hole 32 into collector 39, and pushes the remainder of tube 9 downwardly to the position for the next subdivision.

The velocity and/or intensity of laser beams 17 and 21 are preferably modulated during the process of subdividing tube 9 into individual segments. The velocity of the laser beams scanning across the wide side surfaces of the long tube is preferably modulated so the scanning velocity is lower at the extremities of the beam scans than at the middle of the beam scans. This is accomplished by applying a sinusoidal signal to the linear scanners to produce a sinusoidal oscillation of mirrors 18 and 22. As represented in FIG. 3, the angular position, .theta., of each of mirrors 18 and 22 varies sinusoidally as a function of time, T. Typically, .theta. is of the order to two or three degrees. Tube 9 is shown in cross section in FIG. 3 in its proper position relative to the angular position of mirrors 18 and 22. As depicted, the rate of change of the angular position, .theta., i.e., the scanning velocity of beams 17 and 21 is zero at the edges of the wide side surfaces of tube 9 and increases to a maximum at the center thereof. Such modulation of the scanning velocity of beams 17 and 21 results in greater heat transfer of the beam energy to the glass at the edges of the wide side surfaces where more glass is present, namely, the glass forming the narrow sides of tube 9. This provides more uniform heating along the entire cut line and, therefore, produces a more reliable seal.

In addition to the described modulation of the scanning velocity, it is also preferable to modulate the intensity of beams 17 and 21 during each cutting cycle, i.e., each division of tube 9 into individual sealed segments. The desideratum is first to uniformly preheat tube 9 along the cut line to a temperature below the fusion temperature, i.e., melting temperature, of the glass in a large number of beam scans, and then to heat the glass along the cut line to a temperature above the fusion temperature in one, two, or a few beam scans to form the sealed ends. The slow preheating step minimizes glass stresses, and accordingly produces a more reliable seal. The rapid final heating step improves the glass distribution at the sealed ends, thereby further improving the reliability of the seal. The pattern of the intensity modulation depends upon the type of glass of the tube, the gas pressure in the tube, and the tube dimensions.

An exemplary intensity modulation pattern which is particularly effective for tubes having a width in the order of 0.180 inches is illustrated in FIG. 4A. FIG. 4A is a graph of beam intensity, I, as a function of time, T, which designates specific values of beam intensity, i.sub.1 and i.sub.2, and specific values of time, t.sub.1 through t.sub.7. The cycle begins at t.sub.1 with zero beam intensity, i.e., no beam. From t.sub.1 to t.sub.2, the beam increases in intensity to an intermediate value of i.sub.1. Typically, the interval between t.sub.2 and t.sub.1 is of the order of a second or less, and i.sub.1 is of the order of 4 to 8 watts. The beam intensity remains at i.sub.1 until time t.sub.3 , during which the glass is preheated. Typically, the interval between t.sub.3 and t.sub.2 is of the order of 3 to 4 seconds. The beam intensity increases rapidly between t.sub.3 and t.sub.4 to a high value of i.sub.2. Typically, the interval between t.sub.4 and t.sub.3 is of the order of a second or less, and i.sub.2 is of the order of 28 to 30 watts. The beam intensity remains at i.sub.2 from t.sub.4 to t.sub.5, which is the time interval required to divide and seal the ends of the tube and to anneal the tube, typically of the order of one or two seconds. During the interval between t.sub.4 and T.sub.5, after the tube is subdivided and its ends are sealed, the laser beams are defocused to anneal the sealed ends and remove the discoloration. At t.sub.5 the laser beam is turned off. In the interval between t.sub.5 and t.sub.6, the sealed ends of the tube cool off sufficiently so they will not fuse upon contact. At t.sub.6 the long tube is advanced axially until the next cut line perpendicular to longitudinal axis 24 is aligned with beams 17 and 21, while the newly formed sealed end of the long tube pushes the newly formed individual tube segment from the tube holder into collector 35 (FIG. 2A). At t.sub.1 the described cutting cycle repeats. The period of the described cutting cycle, i.e., the interval between successive occurrences of t.sub.1 on the graph is of the order of 7 seconds.

In the case of a long tube having a width of 0.250 inches, the modulation pattern illustrated in FIG. 4B is particularly effective. The beam intensity rises continuously and at a constant rate from zero at t.sub.1 to a value of i at t.sub.2 to first preheat the glass of the long tube along the cut line and then divide and seal its ends in a few beam scans. Typically, i would be between 28 and 30 watts and the time interval between t.sub.2 and t.sub.1 would be 5 to 6 seconds. During the interval from t.sub.2 to t.sub.1, the sealed ends of the tube are annealed, if desired, and after cooling the sealed ends, the long tube is axially advanced.

Reference is made to FIG. 5 for circuitry that automatically controls the described process of subdividing tube 9 into individual sealed tube segments using the intensity modulation pattern of FIG. 4A. A sine wave generator 70 producing an output signal identical to the waveform shown in FIG. 3 at a frequency from 6 to 10 Hertz is connected to the linear scanners of which mirrors 18 and 22 are a part, designated 71 in FIG. 5. Sine wave generator 70 is also connected to a counter 72, which controls the timing of the intensity modulation, beam defocusing, and axial advancement of tube 9. Thus, the timing of these operations is synchronized to the beam scanning. In particular, it is desirable to have the beam scans and changes in intensity start and stop at the edges of the wide side surfaces. To this end, the state of counter 72 advances responsive to each positive peak of the output of sine wave generator 70. If the frequency of the output of sine wave generator 70 is 6 Hertz and the period of the cycle for producing individual tube segments is 7 seconds, counter 72 would have 42 states and would comprise 6 binary stages. Depending upon the desired timing, different states of counter 72 are assigned to t.sub.1, t.sub.2, t.sub.3, t.sub.4, t.sub.5, and t.sub.6. At t.sub.1, counter 72 triggers a ramp generator 73. At t.sub.2, counter 72 triggers a step generator 74 and turns ramp generator 73 off. At t.sub.3, counter 72 triggers a ramp generator 75. At t.sub.4, counter 72 triggers a step generator 76 and turns ramp generator 75 off. At t.sub.5, counter 72 turns step generator 76 off. The outputs of ramp generators 73 and 75 and step generators 74 and 76 are applied to a summing network 77. The output of summing network 77, which duplicates the modulation pattern shown in FIG. 4A, is applied to laser source 10 of FIG. 1 to adjust its bias current, thereby controlling the intensity of the beam emitted thereby. Assuming that the ends of the long tube are divided and sealed by the time the beam intensity reaches i.sub.2, counter 72 also actuates defocusing lens inserting mechanisms 78 at t.sub.4. Each of mechanisms 78 is a conventional solenoid actuated, translatable shuttle that supports one of defocusing lens 29 and 30. Mechanisms 78 translate in one direction at t.sub.4 to insert the defocusing lenses carried by mechanisms into the laser beam path. At t.sub.5, mechanisms 78 translate in the other direction to remove the defocusing lenses from the laser beam path. Alternatively, mechanisms 78 could be actuated at a point in time between t.sub.4 and t.sub.5. At t.sub.6, counter 72 actuates stepping motor 58 to axially advance tube 9 until the next cut line is aligned with beams 17 and 21. After advancement of tube 9, stepping motor 79 emits a pulse, which is applied to counter 72 to reset it. Thereupon, counter 72 begins to advance once again through its states repeating the cycle.

Reference is made to FIG. 6 for a schematic side view of an embodiment of the invention, in which tube 9 is simultaneously divided along a plurality of cut lines. The beam emitted by laser source 10 is divided by a beam splitter 80. A portion of the beam continues to travel horizontally and is directed as shown in FIG. 1 onto both wide side surfaces of tube 9 along a cut line designated A. Another portion of the beam travels vertically and is reflected from a stationary mirror 81 to a horizontal plane lying above cut line A. This portion is divided by a beam splitter 82. A portion of the beam continues to travel horizontally and is directed as shown in FIG. 1 onto both wide side surfaces of tube 9 along a cut line designated B. Another portion of the beam travels vertically and is reflected from a stationary mirror 83 to a horizontal plane lying above cut line B. This portion of the beam continues to travel horizontally and is directed as shown in FIG. 1 onto both wide side surfaces of tube 9 along a cut line designated C. The beams directed onto both wide side surfaces of tube 9 along cut lines A, B, and C repeatedly scan back and forth across the wide side surfaces of tube 9 simultaneously to heat and soften tube 9 until tube 9 divides to form ends along each cut line and collapses to seal the divided ends. Thus, a number of subdivisions of tube 9 are made simultaneously with a single laser source. The individual tube segments resulting from such subdivisions are at substantially equal pressure and, therefore, exhibit substantially uniform brightness. Furthermore, the pressure in chamber 8 does not have to be readjusted during the subdividing process.

With reference to FIGS. 2A, 2B, and 2C, the embodiment of FIG. 6 would have a tube holder with a pair of ball plungers, such as ball plungers 45 and 46, for each cut line and a slot, such as slot 43. Otherwise, the description of FIGS. 2A, 2B, and 2C is applicable to the embodiment of FIG. 6. After the plural simultaneous cuts are made by the laser beams, slider 50 pushes all the resulting tube segments and the remainder of tube 9 downwardly in tandem to dump the resulting tube segments one after the other into collector 35 and to position the remainder of tube 9 for the next plurality of cuts. Alternatively, if the entire length of tube 9 is to be cut simultaneously, slider 50 as well as the described apparatus for driving it could be eliminated.

The described embodiments of the invention are only considered to be preferred and illustrative of the inventive concept; the scope of the invention is not to be restricted to such embodiments. Various and numerous other arrangements may be devised by one skilled in the art without departing from the spirit and scope of this invention as set forth in the following claims. For example, with a stationary long tube the invention contemplates other types of laser beam movement than a back and forth scan. The invention also contemplates defocusing the laser beam to anneal the sealed ends, directing a plurality of laser beams simultaneously at spaced apart intervals along the longitudinal axis of the long tube to effect multiple subdivisions, and beam intensity modulation to slowly preheat and rapidly divide the long tube independent of the nature of the relative movement between the laser beam and the long tube during the subdividing process. Instead of modulating the beam velocity to transfer more heat to the edges of the wide side surfaces of the long tube, the same result could be achieved by modulating the beam intensity. If the radioactive gas pressure in the long tube is less than 14.7 psia, the long tube can be subdivided without a pressurized chamber. It may also be possible to cut tubes having a circular cross section with a back and forth scan if the velocity of the beam scan is modulated to uniformly heat the perimeter of the cut line.

Reference is made to application Ser. No. 916,936, filled June 19, 1978, for a continuation-in-part of the present application which claims subject matter disclosed in the present application.