Semiconductor laser velocimeter

Abstract: A Doppler velocimeter (10) having a semiconductor laser (12) which sends a laser beam through a polarization beam splitter (50) onto a fractional-wavelength plate (52). The plate both reflects the beam back to the polarization beam splitter and sends the beam on through a beam expander (60) to the target. The beam is returned by the target. The returned beam passes through the fractional-wavelength plate (52) to the polarization beam splitter (50). The returned and reflected beams are directed by the splitter to a detector (56). The detector mixes the beams and sends a signal to a spectrum analyzer (64) on which the velocity of the target can be accurately determined.

Description: The present invention relates to a laser velocimeter according to the preamble of claim 1. In the related art, laser Doppler velocimeters have been restricted to the use of visible and near infrared gas lasers because of their coherency and narrow linewidth. Semiconductor or solid-state lasers would be preferred for use in laser Doppler velocimeters because of size and weight considerations. However, the semiconductor or solid state lasers have not been used in Doppler velocimeters because of their relatively large linewidth, i.e., hundreds of megahertz (MHz), which prevents the attainment of a practicable or desired resolution in velocimeters. From GB-A-2 164 172 a laser velocimeter is known which uses besides a conventional laser of relatively large bandwidth a beam splitting and polarization device which comprises a Brewster plate aligned to the emitted laser beam under the Brewster angle, two quarter-wave plates adjustable in angular rotation position and an adjustable mirror. Problems arise when this device is used in an airborne environment and mirror 20 is subject to vibration. Then the backreflected power into the laser under variable phase and frequency causes severe degradation of the velocimeter and eliminates its capability of low velocity resolution. It is, therefore, the object of the present invention to devise a stable and high resulution velocimeter under the use of a semiconductor laser. This object is achieved according to the characterizing features of claim 1. Further advantageous embodiments of the inventive velocimeter may be taken from the dependent claims. The present invention circumvents the obstacle to making high resolution semiconductor laser velocimeters with the attainment of semiconductor lasers having narrow linewidths. The resulting invention is an accurate and very compact Doppler velocimeter. The invention has the advantages of providing eye safety in that the proposed semiconductor lasers can be operable at a wavelength of 1.54 microns (µm), be multi-function sensors, be low cost and be capable of significant component miniaturization. A feature of the 1.54 µm velocimeter is the incorporation of an external cavity for linewidth reduction in an indium gallium arsenide phosphide (InGaAsP) laser. The invention concept may be extended to a multifunctional sensor capable of velocity and range determination using coherent detection. Applications of a semiconductor laser Doppler velocimeter having high resolution include that as hover sensors for helicopters, and docking sensors for space shuttles and robotic sensors. For instance, some new helicopters will require a hover sensor for the "pop up" mode in which the helicopter holds over a defined spot over a 2-3 minute time interval independent of cross winds up to 35 miles per hour. The hover hold requirements are approximately 2 centimeters per second. The present invention, having laser device design and techniques which result in narrow beam spread and short wavelength, can provide sensing meeting such accuracy requirements. The present invention has the capability to provide laser Doppler velocity resolution as fine as 0.5 centimeter/second over remote distances. In the art, semiconductor lasers have been noted to have inherently large linewidths (i.e. 100's of MHz). In the invention, the linewidth of the semiconductor laser is reduced by about 4 orders of magnitude thereby making it feasible for coherent detection. Some of the advantages of the narrow linewidth semiconductor laser are eye-safe operation, accurate velocity detection, component miniaturization and low cost. The invention satisfies a need for a stealthy ground velocity sensor that can replace presently used radar altimeters and microwave Doppler systems. With their miniaturization, e.g., tennis ball size, such systems are covert, accurate, and effectively countermeasure-proof. With respect to the figures of the attached drawings the invention shall be further described, wherein: Figure 1 is a diagram of the invention. Figure 2 is a drawing of the external cavity InGaAsP laser. Figure 3 shows the basic relationship between the passive external cavity and the active internal cavity. Figure 4 is graph showing Doppler resolution measurements of the invention utilizing the InGaAsP laser. The high precision semiconductor laser velocimeter 10 of Figure 1 has a miniaturized, external cavity InGaAsP laser 12 that produces a 1.54 micrometer (µm) wavelength light beam. Laser 12 is frequency stable with a linewidth of 40 kilohertz (kHz). Laser 12 is a continuous wave device. The linewidth of laser 12 is an improvement of about four orders of magnitude over the linewidth of a corresponding solitary laser diode. In Figure 2, external cavity laser 12 has a stable cavity 14 constructed of invar for thermal stability and acoustic isolation. Cavity 14 holds in a concentric fashion an optical beam expander 16 which has a fine adjustment 18 for the Z position. The front end of cavity 14 houses two-axis adjustment 20 having a two-axis trapezoidal fine position adjustment, an X-adjustment 22 and a Y-adjustment 24, for InGaAsP diode chip 26. InGaAsP chip 26 is mounted so as to receive cooling from Peltier electric cooler 30 mounted on block 28. Invar slab 32 is mounted so as to have two-axis angular adjustments 34 and 36. Also center threaded holder 38 provides for fine adjustment of diode 26 in the Z axis. Rear reflector-grating or mirror holder 40 is mounted on piezoelectric transducer (PZT) element 42. Holder 40 holds cavity mirror 46 in cavity 12. PZT element 42 provides a one-time phase adjustment for the phase of the optical wave of the external feedback cavity to be "in-phase" with the optical wave in the solitary diode. PZT element 42 is mounted on two-axis angular adjustment element 44 which in turn is end-mounted in invar cavity 12. Chip 26, in Figure 3, functions as an active internal cavity 23 for the laser light. The typical length of the internal cavity 23 is about .02 centimeter. Laser light is emitted from the laser diode through a partially reflective coating 27 situated at one end of the solitary diode. The emitted laser light goes through space 15 of passive external cavity 14 of about 10 centimeters in length, to a fully reflective cavity mirror 46. The laser light is reflected back through mirror 27 into laser diode 23. The laser light reflected back into cavity 23 is of a very stable frequency and is in phase with the laser light of internal cavity 23. The laser light coming from space 15 of cavity 14 provides wave reinforcement or constructive interference amounting to strong feedback of resonant external cavity space 15 to the oscillation frequency of laser diode 23. From diode 23 through partially (i.e., about 40%) reflecting mirror 25 exits narrow linewidth laser 48 through mirror 50 of Figure 1. Mirror 27 is partially reflective so as to reflect some of the laser light back within the respective cavities 14 and 23. Mirror 25 is similarly partially reflective so as to reflect some of the laser light back within diode 23. In velocimeter 10, of Figure 1, external cavity laser 12 sends out laser beam 48 having a linear polarization (S-polarization) and passes through polarization beam splitter 50 unattenuated since the S-vector is oriented in transmission direction. Beam 48 moves on to quarter-wavelength plate 52. The rear facet of plate 52 reflects beam 54 which is beam 48 with its linear polarization rotated 90 o (P-Plane). Beam 54 is reflected by polarization beam splitter 50 to detector 56. Beam 54 is the local oscillator signal to detector 56. Plate 52 converts linearly polarized beam 48 into circularly polarized beam 58 which in turn goes through beam expander 60 on to a target. The target reflects beam 58 which passes through beam expander 60 and quarter-wavelength plate 52 and becomes beam 62 linearly polarized in the same direction (P-Polarization) as beam 54. Beam 62 is reflected by polarization beam splitter 50 onto detector 56. Mixing of beams 54 and 62 takes place on the surface of detector 56. The beat spectrum of the two beams 54 and 62, due to the Doppler effect caused by the moving target, is electrically transmitted to and displayed on spectrum analyzer 64 for determination of target velocity. Polarization of the laser beam at various points of system 10, as noted above, may be removed but only at a great loss of efficiency. But the resulting inefficient system hardly allows such a velocimeter to be practicable. The performance characteristic of the InGaAsP laser has been determined with Doppler targets over laboratory distances of about 6 meters. The result is shown in Figure 4, for the 1.55 µm InGaAsP laser.

Claims: A high accuracy laser Doppler velocimeter comprising: a laser for generating a first light beam; a polarization analyzing means (50) for of passing said first light beam; a dual-functional fractional wavelength means (52) for receiving said first light beam, reflecting a second light beam, transmitting a third light beam to a target, receiving said third light beam from the target and transmitting a fourth light beam to said analyzing means; said polarization analyzing means (50) serving to receive and to reflect said second and fourth light beams; and a detector (56) for receiving said second and fourth light beams comparing said fourth light beam with said second light beam, and determining a velocity of the target, characterized in that said polarization analyzing means is a polarization beamsplitter (50); said dual-functional fractional wavelength means is a quarter wavelength plate (52) having a coating to reflect a proper amount of light as a local oscillator, and said laser is an external cavity semiconductor laser which comprises: an active internal cavity (23) having a first partially reflecting mirror (25) at a first end of said internal cavity, having a second partially reflecting mirror (27) at a second end of said internal cavity, and having said first and second partially reflecting mirrors parallel to each other; and a passive external cavity (15) having said active internal cavity (23) at a first end and having a fully reflecting mirror (46) at a second end of said external cavity, wherein said active internal cavity and the fully reflecting mirror are positionally alignable for generating the first light beam having a narrow line width. Velocimeter according to claim 1, further characterized by: a first mount (40) attached to the second end of said external cavity (15) and attached to and supporting the fully reflecting mirror (46) having a piezoelectric length adjustment alement (42) sensitive to temperature and having a two-axis angular adjustment element (44) for compensating variations of external cavity length due to temperature variations and for providing proper phase adjustment; and a second mount (28) attached to the first end of said passive external cavity (23), having a temperature stabilizer (30) and two-axis angular adjustment elements (22,24) for supporting, aligning and stabilizing said active internal cavity. Velocimeter according to claim 2, characterized in that said active internal cavity (23) has a laser light being reflected between the first and second ends of said active internal cavity; said passive external cavity (15) has some of the laser light emitted from the second end of said active internal cavity and reflected by the fully reflecting mirror back through the second end of said active internal cavity, in a continuous fashion, stabilizing the frequency of the laser light reflected between the first and second ends of said active internal cavity; and said active internal cavity emits through the first end of said internal cavity some of the laser light reflected between the first and second ends of said internal cavity, as said first light beam. Velocimeter according to claim 3, characterized in that said first light beam has a first polarization; said second light beam has a second polarization; said third light beam has a third polarization; and said fourth light beam has a fourth polarization. Velocimeter according to claim 4, characterized in that the determining of the velocity of the target by said detector is a result of a Doppler frequency shift of said fourth light beam from a frequency of said second light beam. Velocimeter according to claim 5, characterized in that said external cavity further comprises a beam expander, situated between the first end and the second end of said external cavity, having a position adjustment (18). Velocimeter according to claim 6, characterized in that the wavelength of said first light beam has a narrow linewidth of less than 100 kilohertz. Velocimeter according to claim 7, characterized in that said active internal cavity comprises an InGaAsP laser diode (26). Velocimeter according to claim 8, characterized in that said external cavity (15) further comprises a housing comprising invar.

Inventors: BJORK PAUL E, MOCKER HANS W, BJORK, PAUL E., MOCKER, HANS W.
Applicants: HONEYWELL INC, HONEYWELL INC.
Application year: 1989
Main classification: G01S17
IPC codes: G01S17, G01S7