Miniature Incandescent Lamps as Fiber-Optic Light Sources

These lamps can be used without coupling optics.

Miniature incandescent lamps of a special type have been invented to satisfy a need for compact, rapid- response, rugged, broadband, power- efficient, fiber-optic- coupled light sources for diverse purposes that could include calibrating spectrometers, interrogating optical sensors, spot illumination, and spot heating. A lamp of this type (see figure) includes a re-entrant planar spiral filament mounted within a ceramic package heretofore normally used to house an integrated-circuit chip. The package is closed with a window heretofore normally used in ultraviolet illumination to erase volatile electronic memories. The size and shape of the filament and the proximity of the filament to the window are such that light emitted by the filament can be coupled efficiently to an optical fiber without intervening optics.

The components used for fabricating a lamp of this type are, more specifically, the following:

  • The ceramic package is an appropriately sized commercially available leadless chip carrier containing gold contact pads. The package is chosen to have a contact-pad spacing of about 0.25 in. (≈6 mm) so that the filament fits between two of the pads.
  • The window is part of a windowed lid for the leadless chip carrier, supplied with a preform made of gold/tin solder. The preform is to be used subsequently in bonding (by soldering) the lid to the leadless chip carrier.
  • The filament is formed by chemical etching or laser ablation of a 25-μm-thick sheet of tungsten or a tungsten/ rhenium alloy.
  • Two commercially available contactpad brazing preforms made of a silver/ copper/indium/titanium alloy that has a liquidus temperature of 715 °C are needed for attaching (by brazing) the filament to two contact pads.

Once the aforementioned components have been prepared, the lamp is assembled as follows:

  1. The brazing preforms are placed on two opposing contact pads.
  2. The outer ends of the filament are placed on the brazing preforms.
  3. The assembly as described thus far is placed in either a vacuum furnace at a pressure of 10–7 torr (1.3 × 10–5 Pa) or a furnace containing an inert atmosphere, and heated to ≈ 800 °C or until brazing alloy melts and wets the filament.
  4. The assembly is cooled to harden the braze, then the furnace is opened to room air and the assembly is removed from the furnace.
  5. Optionally, at this point, the assembly can be placed in a vacuum chamber, wherein the filament can be baked out by applying operating power to it. The assembly is then removed from the vacuum chamber.
  6. A small nick is made in the solder preform on the lid to allow air to escape during step 8.
  7. The lid is placed on the ceramic package, held in place by a weight or a clip. The package is placed in a vacuum furnace.
  8. The vacuum furnace is pumped down to the desired vacuum for the interior of the lamp.
  9. The furnace is heated to the eutectic temperature of the solder to melt and reflow the solder, then is cooled back to room temperature, then opened to air.

Lamps of this type containing tungsten and tungsten/rhenium filaments have been operated in laboratory tests at temperatures up to 2,650 and 2,725 °C, respectively. At an input power of ≈2 W, each lamp generates a luminous flux of about 1.5 lumens.

This work was done by Margaret Tuma of Glenn Research Center; Joe Collura of the Lighting Innovations Institute; Henry Helvajian of the Aerospace Corp.; and Michael Pocha, Glenn Meyer, Charles F. McConaghy, and Barry L. Olsen of Lawrence Livermore National Laboratory.

Inquiries concerning rights for the commercial use of this invention should be addressed to NASA Glenn Research Center, Innovative Partnerships Office, Attn: Steve Fedor, Mail Stop 4–8, 21000 Brookpark Road, Cleveland, Ohio 44135. Refer to LEW-17820-1.

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