Optical Beam Scanner

Optical scanning is usually accomplished by one of two simple mechanical means. These are, a moveable mirror which produces a scan angle, which is twice the mirror movement angle, and a moveable prism which provides a similar but non linear beam scan function. Optical energy from an optical source impinges on the mirror or the prism and is redirected as the moveable member oscillates. Both of these methods require a moving optical element to accomplish the angular beam scan, which results in inherent limitations on upper scan frequency as well as suffering from the effects of friction and wear. A description on prisms as dispersors is found in Williams and Becklund, "Optics, a Short Course for Engineers and Scientists", Wiley-Interscience 1972, pages 251-254.

FIGS. 1 and 2 show a typical circuit for and the phenomena associated with the inverse Stark effect. The inverse Stark effect utilizes absorption spectra instead of emission spectra as in the Stark effect. The apparatus in FIG. 1 is comprised of a line spectra optical source 10, such as a laser. The output light beam 12 traverses Stark cell 14 and emerges as output 16. The Stark cell is filled with an optical medium which has an absorption line at the frequency .nu..sub.0, near the source emission frequency at .nu..sub.1. Closing switch 18 applies voltage from battery or power supply 20 to capacitor plates 21 and 22, thus producing an electric field across the Stark cell. This electric field splits the absorption line of the cell medium and the resulting shift in absorption frequency is typically used to absorb the input light and thereby amplitude modulate the output light. Switch 18 may obviously be replaced with an electronic switch to provide higher frequency modulation. This prior art is well known to those technically versed in the field.

FIG. 2 illustrates the physical phenomena of the inverse Stark effect. The Stark cell medium 14 possesses an absorption line spectra, one line of which is shown at A. The index of refraction, .mu., near this absorption line is shown at B; the negative slope portion of this curve is known as anomalous dispersion. In an electric field, this absorption line of A may be made to split as shown in C. In general, the stronger the applied voltage the wider the separation of these two absorption lines. The index of refraction is also altered as shown at D. The symbol .nu. represents the optical wave number (proportional to frequency). Anomalous dispersion and the Stark effect are described in Jenkins and White, "Fundamentals of Optics", McGraw-Hill, 1957.

As shown in FIG. 2, if light of frequency .nu..sub.1 or .nu..sub.2, is provided by source 10 and Stark cell 14 has the absorption lines of A and C in the absence and presence respectively of an electric field, strong amplitude modulation is produced at output 16. Prior art provides for mechanical beam scanning with moving elements such as mirrors or prisms and for amplitude modulation of narrow spectral sources using the tunable selective absorption of Stark cells.

The optical beam scanner is a no-moving-parts beam scanner using the variable refractive index properties of the Stark cell. This allows scanning to be accomplished simply by varying the applied voltage across the cell.

FIG. 1 is a schematic of typical inverse Stark effect apparatus established in the prior art.

FIG. 2 illustrates the physical phenomena of the inverse Stark effect.

FIG. 3 is a partial schematic of a Stark cell cross section adapted for independent modulation from two sources.

FIGS. 4-8 show the output beams of the apparatus of FIG. 3 as modulated voltage sources vary.

FIG. 9 is a preferred embodiment of the optical beam scanner for scanning a single beam with no moving parts.

FIG. 10 is an embodiment of the Stark cell having two sets of parallel plates disposed for scanning an optical beam in two planes.

Referring now to the drawings wherein like numbers represent like parts, FIG. 3 shows the configuration of a modified Stark cell which was used in the discovery of practical beam bending effects. This cell 30 has split capacitors 32 and 34 to independently modulate with conventional AM modulation the right and left halves of a laser beam. Varying the electric field developed between the capacitor plates allows the beam to be modulated. An input laser beam 36 shown in dashed lines is passed through the Stark absorbing medium 38. A direct current (DC) power source 40 and an AC power source 42 are coupled to capacitor 32. A similar power system (not shown) is coupled to capacitor 34. Optical energy was provided from a CO.sub.2 wave guide laser. Ammonia gas was utilized as the Stark absorbing medium. The CO.sub.2 laser used a carbon isotope to shift the laser line to 10.7 microns in order to be compatible with the Ammonia inverse Stark effect. The two capacitors received a DC bias voltage (approximately 1,000 volts) to shift the Stark absorbing line nearer the laser line, plus an AC voltage (approximately 100 volts) to provide conventional on-off modulation independently in the two halves of the laser beam. This allowed two separate and distinct output frequencies (those of the two AC power sources) to be generated. As DC bias voltages and AC modulating voltages were adjusted, the unexpected phenomena of FIGS. 4-8 resulted. The left most beam L (first frequency) in FIG. 4 moved to the right, and the right most beam R (second frequency) moved toward the left as shown in FIG. 5 with an increase in one of the DC voltages. Further increase in this DC voltage superimposed the dual modulated beam (FIG. 6), and still further increases caused the reversed orientations of FIGS. 7 and 8. This angular scanning of the output beam portions results from variation of the index of refraction gradient induced in the Stark cell medium by variation of the electric field gradient caused by the two DC power supply voltages. The AC power supplies were used to separately modulate the two halves of the laser beam, and thus allow far field identification of each beam half. This separate AC modulation is unnecessary to a single beam scanning device, and a single power supply, which may be variable, is employed.

A preferred embodiment of an optical scanner is shown in FIG. 9. A prism housing 46 is filled with an appropriate medium 48, such as ammonia gas, for Stark cell operation and is placed between the capacitor plates 50. Typically, housing 46 may have the shape of a pentahedron. A voltage source 52 is coupled across the capacitor and an optical source 54 is adapted for directing optical energy such as laser light, through the prism 46. As the voltage on the capacitor plates vary, the index of refraction varies, thus varying the prism dispersion. Voltages used are slightly lower than those used for AM modulation, so only a slight absorption occurs on the skirt of the absorption band. A very simple and effective scanner results. Absorption losses are thus minimized by use of normal dispersion rather than anomalous dispersion portions of the refractive index curve.

In operation, optical energy from source 54 is coupled through prism housing 46 to provide an output beam at an angle with respect to the line of the input beam. The variable power supply is adjusted to provide a strong electrical field across capacitor 50 and the Stark medium 48 within prism 46. As the power supply is varied back and forth across a center or reference voltage output V.sub.0 the electric field varies across the Stark cell. The Stark medium shifts the absorption lines (and thus the refractive index near the optical source frequency) as the voltage changes incrementally causing the output beam to scan back and forth across an angle in response to changes in the voltage applied across the cell. Scan angles of several degrees are readily achieved.

FIG. 10 shows a Stark cell housing 56 disposed between two sets of parallel plates, plate 50A and 50B and plates 51A and 51B and filled with medium 48. This allows the field to be developed from different planes across the Stark cell and thereby allows the beam to be scanned in either one of the two planes 60 or 62 controlled by the parallel plates. Typically, for this particular embodiment, housing 56 will have the shape of a tetrahedron having sides 56A and 56B perpendicular and the remaining two sides sloped to complete the housing, surface 56A being parallel to capacitive plate 51A and surface 56B being parallel to capacitive plate 50B.

Although the present invention has been described with reference to the preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention. Accordingly, the scope of the invention should be limited only by the claims appended hereto.