A laser illuminator system has been developed for use in automated tracking of objects. In its original application, the system illuminates a 5° conical region expected to contain a small satellite at a distance up to 750 m from the space shuttle. If the satellite comes within this region, then a receiving optoelectronic system aboard the space shuttle actively tracks the satellite by locking onto the laser illumination reflected by small mirrors on the satellite. The laser illuminator system could be adapted to other outer-space and terrestrial applications that involve illumination with or without tracking, or could be used as a beacon on a moving platform for tracking by an optoelectronic system located on another moving or stationary platform.
The laser illuminator system is intended to replace a white light source that has been used for tracking. The white light source provides a relatively weak return signal, and this signal is readily contaminated by noise from background light. The laser illuminator system provides a stronger return signal, and since the laser light occupies a very narrow wavelength band, background light can readily be filtered out to increase the signal-to-noise ratio.
The source of light is a commercially available array of AlGaAs semiconductor laser diodes in a fiber-optic-coupled package. This source emits continuous-wave optical power of 5 W from the outer end of a sheathed optical fiber 1 m long with a core diameter of 400 µm. The beam as emitted from the bare outer end of the fiber is multimode (non-diffraction-limited), with nearly uniform intensity across a divergence of < 50° full width at half maximum. The full length of the optical fiber is used to ensure adequate "mode-scrambling" to make the intensity distribution as nearly uniform as possible.
The optical fiber is flexible and can easily be moved to illuminate locations or directions that might otherwise be difficult to illuminate. The bare outer end of the fiber is covered by a commercial fiber-optic connector that has been modified to contain a single molded glass aspherical lens and a monitor photodiode. The lens reduces the divergence of the beam emitted from the bare fiber to narrow the field of illumination to the required 5° cone. This optical assembly at the outer end of the fiber is only 10 mm in diameter and therefore introduces only minimal obscuration into the telescope optics of the receiving optoelectronic system.
The system consumes a total power of about 20 W. Active feedback control circuitry ensures that the current to the laser remains constant. The laser current can also be adjusted manually by remote control. Additional electronic circuitry amplifies the output of the monitor photodiode for remote determination of the health of the system.
This work was done by Donald M. Cornwell, Jr., of Goddard Space Flight Center, Jimmie D. Fitzgerald of Allied Signal Corp., and Valerie Dutto of USRA. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Electronics & Computers category. GSC-13823