Laser flashtube triggering arrangement

[57] ABSTRACT

The trigger transformer for the disclosed laser excitation arrangement includes a spool-like ceramic core having a pair of radially outwardly extending annular end wall portions and defining an elongated laser cavity along its axis. A primary winding and a plurality of secondary windings are coaxially wound about the core at different radial locations, with the secondary windings disposed radially inwardly of the primary winding and with an electrical conductor interconnecting the longitudinally opposite ends of each pair of successive secondary windings. A layer of plastic is disposed between each secondary winding and the adjacent interconnecting conductor and between the outermost secondary winding and the primary winding, while epoxy resin potting surrounds the primary and secondary windings and the interconnecting conductors. A laser rod and a flashtube are mounted parallel to one another within the laser cavity. Electrical excitation circuitry includes an RC pulse-forming network coupled to the flashtube and a triggering network including a silicon controlled rectifier coupled to the transformer primary winding.

Claims, 1 Drawing Figure

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PATENTED JU1251972

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3,679,998

This invention relates generally to laser excitation, and more particularly it relates to a highly compact arrangement 8^eneratJⁿg and applying triggering pulses to the pumping ilasntube of a laser. ⁶

Ti.³¹! ^{schemes for} triggering laser flashtubes have included both senes and shunt triggering arrangements. In series tnggenng, the tnggering pulse (which ionizes the flasthtube gas) is applied directly to an electrode ofthe flasthtube for example by means of a trigger transformer having its secondary winding connected in series with the flashtube electrodes, since with series triggering arrangements the transformer secondary winding provides a return path for flashtube discharge current, the transformer must be designed to handle relatively large currents such as several hundred amperes. Iransformers having this capability are usually large, heavy and expensive. Moreover, since some flashtubes require triggering voltages of 25,000 volts or more, electrical leacL between the transformer secondary and the flashtube electrodes must be insulated to withstand voltages of this magnitude in order to insure against arcing between the flashtube leads ana the laser pumping cavity.

In shunt triggering arrangements of the prior art, the flashtube gas is initially ionized by an electric field generated from ²⁵current flow in a triggering wire wound about the outer surface of the flashtube. The triggering wire is connected to the secondary winding of a trigger transformer located externally ot the laser pumping cavity. Since the flashtube discharge current does not flow through the trigger transformer in a shunt tnggenng arrangement, considerably smaller and lighter transformers can be used than with series triggering arrangements. Nevertheless, since triggering voltages of 25,000 volts or more are still required for certain flashtubes, conductors which carry the triggering potential into the laser cavity must be insulated to withstand these high potentials. Thus, even with shunt triggering arrangements, provision ofthe necessary lead insulation greatly adds to the size, weight and cost ofthe overall laser device.

Accordingly, it is an object of the present invention to provide a laser flashtube triggering arrangement which, for comparable tnggering potential levels, is considerably smaller, lighter and less costly than triggering arrangements of the prior art.

It is a still further object of the present invention to provide a compact and reliable laser flashtube triggering arrangement which does not require the transporting of a high voltage internally of the laser cavity, thereby eliminating the need for elaborate and expensive insulation schemes within or adjacent sn to the laser cavity.

An arrangement according to the invention includes a transformer having a spool-like core member of electrically insulating material defining an elongated laser cavity along its axis. A primary winding and at least one secondary winding are coaxi- 55 ally wound about the core member with the secondary winding disposed radially inwardly ofthe primary winding. An element of laser material and a flashtube are mounted parallel to one another within the laser cavity. Electrical excitation circuitry includes a pulse-forming network coupled to the flash- 60 tube and a circuit for applying trigger pulses to the primary winding ofthe transformer.

Additional objects, advantages and characteristics features ofthe present invention will become readily apparent from the anoae-. tollowing detailed description of a preferred embodiment of 65 nected the invention when considered in conjunction with the accompanying drawing wherein the sole FIGURE is a longitudinal sectional view, partly in circuit schematic form, illustrating an arrangement in accordance with the invention fhJhSttΪ ^{figUre With greater a} — ⁷⁰ ’^^S^towWchhipirtttiggerp^sM ilasntube triggering arrangement according to the invention «·».·.- - es P includes a transformer 10 wound on a spool-like electrically insulating core 12. The transformer core 12 is preferably of a ceramic material such as beryllia or alumina, although other is provided with a pair of radially outwardly extending annular wall portions 13 and 14, respectively, at opposite ends thereof and defines an elongated axial hole 15 which functions as the laser cavity for the device. The cavity 15 may have the configuration of a circular cylinder, an elliptic cylinder, or a near elliptic cylinder, for example. If desired, a coating of a highly reflective material such as gold or silver may be provided on the wall of cavity 15.

An elongated element 16 of laser material and a pumping flashtube 18 are mounted (by means not shown) parallel to one another within the laser cavity 15. Although the laser element 16 is preferably a rod of solid material such as Nd:YAG, Ndiglass, ruby, etc, any laser medium capable of being optically pumped may be employed. The flashtube 18 is selected in accordance with the particular laser excitation desired and usually contains krypton or xenon gas. As a specific example when the laser material 16 is Nd, an L909-001 xenon flash lamp manufactured by ILC of Sunnyvale, California may be employed for the flashtube 18.

One electrode of the flashtube 18 is connected to a pulseforming network 20, while the other electrode ofthe flashtube 18 is connected to a level of reference potential illustrated as ground. The pulse-forming network 20 includes an inductor 22 connected between the adjacent electrode ofthe flashtube 18 and a power supply terminal 24 furnishing a DC voltage +Vj, and a capacitor 26 connected between terminal 24 and the ground level. As a specific example, when the aforementioned L909-001 xenon flash lamp is employed, inductor 22 may provide an inductance of around 50 ph, capacitor 26 may have a capacitance of about 20 pf, and V, may be around 1000 volts.

The transformer 10 includes a primary winding 30 and at least one secondary winding 32 wound coaxially with and radi₃₅ ally inwardly of the primary winding 30. In order to achieve high voltage step-up and at the same time minimize the electric field externally of the transformer 10, it is preferred to employ a plurality of series connected secondary windings 32. The windings 32 are coaxially wound about the core 12 at difj ferent radial locations, with the longitudinally opposite ends of each pair of successive secondary windings 32 being interconnected by means of a conductor 34. It is pointed out that although three secondary windings 32 are illustrated, any practical number may be employed depending upon particular design requirements. As a specific example, a primary winding 30 of 50 turns and three secondary windings 32 of 1,000 turns each would be appropriate for a transformer 10 designed to step up an input voltage pulse of 500 volts amplitude to a 30,000 volt level at the far end 36 of the innermost secondary winding 32. A layer 38 of electrically insulating material such as plastic is disposed between each secondary winding 32 and the adjacent interconnecting conductor 34 as well as between the outermost secondary winding 32 and the primary winding 30, and outwardly of the primary winding 30. The remaining space between the transformer core end wall portions 13 and 14 is provided with a suitable high voltage potting 40, of epoxy resin for example, in order to further insulate the windings 30 and 32 and electrical conductors 34.

One end of the outermost secondary winding 32 and the adjacent end of the primary winding 30 are electrically connected together and to the ground level. The other end of primary winding 30 is connected to a triggering network 42. The network 42 includes a current limiting resistor 44 and the anode-cathode path of a silicon controlled rectifier 46 connected in series between a power supply terminal 48 and ground. The power supply terminal 48 furnishes a DC voltage +V₂ which, for the aforementioned transformer parameter values, may be around 500 volts, for example. The silicon controlled rectiner 46 has a gate electrode connected to a ter· oo—.----J are applied. A capacitor i&gt;2, which for the aforementioned transformer parameter values may provide a capacitance of around 1.0 pf, is connected between the adjacent end of primary winding 30 and electrically insulating materiais^al^^^^ 75 £T™

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In the operation of the aforedescribed flashtube triggering arrangement, the silicon controlled rectifier 46 is initially nonconductive of current, and capacitor 52 is charged to the voltage level V₂. When a triggering pulse is applied to the input terminal 50, the silicon controlled rectifier 46 fires, and capacitor 52 discharges through the anode-cathode path of the rectifier 46 and the primary winding 30 of transformer 10. A voltage is induced in the transformer secondary windings 32 creating an electric field within the laser cavity 15. This electric field produces sufficient ionization of the gas within the flashtube 18 to establish an electric discharge in the flashtube 18 which is maintained for a duration of time determined by the time constant of the pulse-forming network 20. As the discharge proceeds, the flashtube 18 emits pumping radiation which excites the laser medium 16 to a lasing state.

It may be seen that when the aforementioned specific parameter values are employed in an arrangement according to the invention, a triggering voltage of around 30,000 volts is produced at end 36 of the transformer secondary winding 32. However, this high voltage is insulated from the laser cavity 15 not only by the transformer core 12, but by the potting 40 as well. Moreover, no special insulated conductors or other arrangements are needed for transporting the triggering voltage into the laser cavity 15. Thus, a laser flashtube triggering arrangement is provided which is compact light, and inexpen- 25 sive, and in which reliable flashtube triggering is insured without the danger of arcing within the laser cavity.

Although the invention has been shown and described with reference to a particular embodiment, nevertheless various changes and modifications obvious to one skilled in the art to 30 which the invention pertains are deemed within the spirit, scope and contemplation of the invention.