Fiber-Optic Sensors - Applied Optics Fundamentals and Device Applications - page 207

integrating image recording devices such as CCDs. A trade between angular
precision and field of view is necessary.
4.19.5 False Alarms and Laser Discrimination
Potential naturally occurring sources of false alarm include direct sunlight,
sun glint, and lightning. The sun is by far the strongest broadband source in
the visible and near-infrared. Artificial false alarm sources, also broadband,
may include searchlights, flares, and munition explosions.
Lasers are often distinguishable from nonlaser sources by their temporal
characteristics as well as their narrow linewidths. Although short pulse (tens
of nanosecond) laser systems have been discussed thus far, it is conceivable
that longer pulsed or quasi-CW lasers could be encountered, and discrimi-
nation against typically millisecond duration false alarms could then be dif-
ficult. Consideration must also be given to the false alarms, or false bearing
indications, generated through reflection of laser radiation off local terrain,
clouds, and other vehicles. Discrimination on the basis of spatial coherence
analysis may be possible, subject to the complicated effects of atmospheric
turbulence and scatter on spatial coherence.
4.19.6 System Summary
Some unique features of the system concept described herein include the
following:
• Unique fiber bundle collection apertures for low-ambiguity wide
field of view coverage of multiple wavelength laser threats
• High-resolution optical wavelength analyzer using custom holo-
graphic dispersion grating and fiber coupler
• Novel optical phase delay coding for precise angle of arrival
determination
• Custom algorithms for retrieval and identification of laser threat data
References
1. Mentzer, M. A. 1988. Fiber sensors. Paper Presented in Proceedings Investigator's
Meeting on Semiconductors, Optoelectrics and Magnetic Optic Materials , Watertown
Materials Technology Laboratory, January, Watertown, MA.
2. Ibid.
3. OFC/IOOC 1987 Technical Digest. March 1987. Reprinted in Lightwave .
4. Krohn, D. A. 1988. Fiber Optic Sensors: Fundamentals and Applications . Research
Triangle Park, NC: Instrument Society of America.