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Defense & Security

Advanced optical fuzing technology

Recent advances have provided new ways to set off ordnance with greater precision and at lower cost.  There may also be civilian applications.
7 February 2006, SPIE Newsroom. DOI: 10.1117/2.1200601.0066

Optical fuzing (OF) is a promising alternative to current remote (stand-off) fuzing techniques, which traditionally use radio-frequency (RF) or radar sensing techniques. It is especially useful for sensing highly-directional point targets.1 The direct and narrow laser beam used in OF systems makes them suitable for use with direct-fire munitions, where they can provide precision fuzing for both long and short stand-off engagements with extremely high accuracy.

The OF system uses advanced opto-electronic (OE) components including high-power vertical-cavity surface-emitting lasers (VCSELs), PIN (positive-intrinsic-negative) or metal-semiconductor-metal (MSM) photodetectors, custom semiconductor driver circuits, and miniature optics. The sensor is compact, so it can replace conventional, costly assemblies based on discrete lasers, photodetectors and bulky optics. Because it will be possible to make the sensors in large volumes at low cost civilian automotive, robotics, and aerospace applications may also open up. In addition, the technology may be applied to robotic ladar and short-range 3D imaging.

When used as a fuze on the front of a projectile, the laser transmits a highly collimated beam that is amplitude-modulated with a chirped RF signal at a particular frequency. Photoreceivers on the side of the projectile have their electrical bias modulated at the same frequency as the transmitted optical signal. As the photoreceivers pick up the optical signal reflected from the target, the on-board signal processor mixes it with a portion of the delayed transmitted waveform. The intermediate frequency this generates corresponds to the time delay due to the travel time of the light, which yields the range to the target.

We used advanced high-power VCSELs and a high-bandwidth SiGe driver as the optical transmitter. MSM photodetectors were used as the photoreceiver and as part of the mixing process. Putting optical detection and RF mixing in one system brings greater simplicity and better signal-to-noise ratio than alternative approaches.2

 
Figure 1. Illustration of an Optical Fuzing system in a gun-fired projectile.
 

Several types of high-power 980nm VCSELs, fabricated by Sandia National Laboratories and other industrial companies, were investigated in our development. The VCSELs had aperture sizes ranging from tens to hundreds of microns and delivered optical output power from tens of millwatts to a few watts in continuous-wave mode. Medium-power devices demonstrated a high modulation bandwidth. InGaAs MSM photodetectors with resonant cavities and interdigitated fingers were designed and fabricated at the Army Research Laboratory and integrated in the OF sensor system.3

A typical implementation of an OF for gun-fired projectiles is illustrated in Figure 1. The intended targets (both area and point) can be covered by an extremely narrow spot or a large-area beam, depending on the application. The major advantage of the OF technique is its ability to keep a focused and directed targeting source, which enables a direct-fire feature difficult to accomplish with other proximity-fuzing schemes.

The OF system is integrated using a series of OE and electronic modules to create a compact sensor package that can fit into gun-fired projectiles. A custom driver circuit, based on SiGe technology, was designed by the University of Delaware and fabricated at the IBM foundry to drive high-power VCSELs. The driver can provide a few amperes of current and modulate the VCSEL at a high rate. A special PC board was also designed and made to host the VCSEL transmitter and MSM photodetector. The current system can fit in an elongated mortar proximity-fuze housing. In this application, the fuze sets off the warhead when the projectile comes within a desired range from the target, where it will produce maximum effect.

During this OF development we have evaluated the performance of high-power VCSELs, MSM photodetectors, and other optical components. Some components have been flight-tested and air-gun tested to evaluate their ability to survive under high acceleration. Our goal is to use the VCSELs and MSM photodetector technology to produce a compact, low-cost, OF system that is rugged enough to use in a gun. We are now testing the performance of the entire system and reducing the electronics footprint. We are also testing the ruggedization of the OF system in simulated and actual flight environment conditions.


Authors
Christian von der Lippe
Armament Research, Development & Engineering Center
Adelphi, MD USA
Mr. von der Lippe received his Masters and Bachelors degree in Electrical Engineering in 1985 and 1983 respectively from The Catholic University of America in Washington D.C. He is a project and team leader of optical fuze and sensor systems at the U.S. Army Research Development Engineering Command ARDEC Fuze Division at Adelphi, MD. He holds a U.S. patent in the optical sensor area. He serves as a committee member on the DOD/DOE Technology Coordination Group 13, Sensors and Signal Processing. He has over 20 years experience working in the fuzing area on Army, Air Force, and Navy fuze programs. Mr. von der Lippe serves as a committee member for the SPIE Optical Technologies for Arming, Safing, Fuzing, and Firing Group. He served as a session chair at the 2005 SPIE Photonics Conference and presented in the Sensors and Systems session.
J. Jiang Liu
Armament Research, Development & Engineering Center
Department of Army RDECOM ARDEC, USA
Dr. Liu received his Ph.D. degree in solid state physics in 1991 from The Pennsylvania State University. He is a project leader and principal investigator in the development of high-bandwidth multi-channel optoelectronic interconnects and optical sensor systems at U.S. Army Research Laboratory. He has authored and co-authored over 70 technical papers and book chapters in U.S. and internationally circulated scientific journals. He has been an invited speaker and chairperson for many international technical conferences. He holds two U.S. patents in the microelectronics and optoelectronics area. He serves as a reviewer for Journal of Applied Physics, Applied Physics Letters, and Journal of Vacuum Sciences and Technology. He also serves on Ph.D. advisory committees for graduate students in University of Maryland and University of Delaware.