MEMS Optical Devices

MEMS Optical Devices Figures 1-3

Visual and Infrared Optical MEMS for Alternative Small Arms Ammunition Tracers

Miniaturized solid-state lasers and light emitting diodes (LEDs) have the potential to become ubiquitous components of projectile and munitions systems. However, many stress-sensitive components of MEMS optical systems often fail under large accelerations and g-loads. At Louisiana Tech University, we are developing a new solid-state optical small arms ammunition tracing mechanism utilizing MEMS visible (VIS) and infrared (IR) laser and LED systems suitable for high-g applications.

The laser gain medium under investigation consists of a disk-shaped phosphate glass or yttrium aluminum garnet (YAG) crystal doped with rare-earth nanoparticles pumped by an infrared laser diode (Fig. 1). Neodymium nanoparticle dopants lase at 1064 nm in YAG and 1060 nm in glass. Erbium nanoparticle dopants in YAG and glass lase at 2940 nm and 1540 nm, respectively, with the 1540 nm output of Er:glass being an “eye safe” wavelength. Neodymium and erbium lasers are pumped with 808 nm and 940 nm laser diodes, respectively. Laser wavelengths in the visible range can also be produced from Nd:YAG and Nd:glass lasers by directing the output into a small non-linear crystal, such as potassium titanyl phosphate (KTP), which halves the laser wavelength to 532 nm (green). The surfaces of the disks are coated with thin (100 nm) Cr and Ti films to enhance the reflectivity and durability of the lasing medium. The gain medium, laser diode, non-linear crystals, and focusing optics are integrated into a metal package surrounded by a force-insulating polymer (Fig. 2). LEDs emitting in the VIS and IR spectrum are also an attractive option for tracing mechanisms due to their low profile, power consumption, cost, and simpler circuitry. The compact size of the optical components and high durability of the metal enclosure permit use in a wide range of novel applications, including high-g applications.

A survivability study was conducted where each of the particular components of the laser and several completed prototypes were fired through a .45 ACP rifle. A force-activated switch has been designed to power on the laser diode or LED instantly after firing. This switch has no leakage current which will significantly lengthen the shelf life of the tracer.

Bullets modified with individual laser components and complete laser tracer prototypes were fired into a 950-liter water tank, where they were then recovered. A completed glass laser tracer prototype was found to have survived firing: its glass laser medium and pumping laser diode were undamaged and remain operable (Fig. 3). We also plan to integrate KTP crystals into the laser tracer prototype for visible (green) emissions.

Power requirements for LEDs and laser diodes are also well suited to the small arms ammunition application. Initially, power will be drawn from commercially available button-cell batteries. Thin-film batteries are also a possible solution for fulfilling power requirements. Nano-wire catalytically enhanced lithium-air thin-film batteries are compact and can be designed to fit into the base of the machined cavity in the bullet. The physical and chemical properties of these batteries can be further refined to supply the exact current and voltage requirements of the LEDs and laser diodes.

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