Optics: measuring and testing – Velocity or velocity/height measuring – With light detector
Reexamination Certificate
1998-11-13
2001-10-02
Buczinski, Stephen C. (Department: 3662)
Optics: measuring and testing
Velocity or velocity/height measuring
With light detector
C356S141100
Reexamination Certificate
active
06297878
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a non-scanning three-axis laser air data sensing system, including heterodyne operation for obtaining complete velocity information (i.e. direction and relative speed) with reference to particles or surfaces reflecting the provided laser beams.
At the present time, light detection and ranging (LIDAR) techniques for determining relative velocities between a laser source and airborne particulates that reflect light from the laser are known. To obtain information for more than one axis of velocity, these systems require either a scanning device for scanning the laser beam source to positions for sensing in different axes, or multiple sources and telescopes to provide the multiple axes. Scanners will increase the complexity of the circuitry used as well as the complexity of the packaging. Also, there are reliability issues for the moving parts involved. For multiple lasers and telescopes, the cost is increased drastically over a single telescope/laser system.
The technique to measure velocity involves Doppler principles where a frequency shift in the reflected signal is used for determining the relative fluid velocity. U.S. Pat. No. 5,164,784 illustrates schematically a device that uses a continuous wave Doppler Lidar with an enhanced signal to noise ratio, but this operates only in a single axis. The readout circuitry disclosed in U.S. Pat. No. 5,164,784, for analyzing the return signal of reflected light from a particle in order to determine the relative velocity of the particle, can be used with return signals obtained with the present invention to providing the output information. The present invention utilizes three separate laser sources to provide three axis information while eliminating the need for scanning. “Non-scanning”, as used herein, means the laser beams have a pre-defined and non-moving path.
SUMMARY OF THE INVENTION
The present invention relates to a three-axis laser air data sensing system that permits accurate determination of relative velocity between laser sources and particles or other surfaces that reflect the laser beams from the sources, without the need for scanning, and without providing an additional frequency source for heterodyning the reflected signals.
The invention comprises a plurality of separate sources, as shown, three, that have beams which are focused to a small diameter and volume, and therefore higher power density to get a higher percent of reflectivity off particles passing through the volume at the focal region. The reflected light exhibits a Doppler shift in frequency, and is reflected back along the same path as the source light is emitted. The reflected beam is used in connection with a sensing system to obtain a signal that indicates relative velocity between the laser source and the reflecting particle. The beams are directed along separate axes so velocity in the orthogonal axes can be resolved.
The present invention has a focusing arrangement to focus the laser beams to a small focal volume, so that the volume at the focal region will on average contain only one particle at any one time. A single particle in the tightly focused focal region provides for a stronger reflection than in larger volumes even when multiple particles are present. However, if desired, this invention can be used with larger volume, pulsed LIDAR systems. Appropriate changes to the source and design of the focusing optics will allow these changes. In fact, a range gaged collimated system could also be designed using the principals presented in this invention.
When the coherent light is reflected from an aerosol particle and received back at the instrumentation, it is intercepted by an optical switch which will turn the reflected beam, while the source beam passes through unimpeded in the opposite direction.
In order to heterodyne the reflected beams, a reference is provided from a different one of the source beams and the reference is combined with the return signal from an adjacent source. About only 1% of the respective source signal is tapped for the reference signal for the heterodyne operation, and since there are three laser sources, typically, source one would provide the reference for return signal two, source two would provide the reference for return signal three and source three would provide the reference for return signal one.
The frequencies of the plurality of laser sources are separated from each other by an amount greater than the frequency shift caused by the Doppler effect of the reflected beam, so that heterodyne detection is possible with unambiguous velocity direction and magnitude determination at each signal, path.
Standard optical components are used for obtaining these results, when they are arranged in the appropriate way. The frequencies of the laser sources can be adjusted to accommodate known ranges of relative velocity. If the relative velocity is high, the source frequency separation is set higher. In the preferred embodiment, laser diodes are used, so that consumed power can be relatively low. The present focusing arrangement, which provides reflection from a single particle passing through the beam focal region, also provides a return signal that is much stronger than with larger focal regions where reflections from multiple particles can be present at once. Again, the focal region is selected so that the volume of the light at the focal region will contain only one particle.
In the present system, the non-scanning, three-dimensional laser system has a support that preferably mounts three laser diodes, as shown spaced 120° around a central axis, and which direct the collimated, coherent light onto separate expander mirrors that are spaced from the laser sources a selected amount. The expander mirrors expand the individual beams and reflect the respective beams back toward a concave focusing mirror, which will receive all of the expanded beams and cause the beams to be reflected in separate directions away from the laser sources, to refocus the separate beams at a specified distance from the focusing mirror.
The expander mirrors can be specially designed, to remove any spatial aberrations introduced by the offset alignment in the form shown.
The beams will be reflected by the concave focusing mirror past the supports for the expander mirrors and into space. As an aerosol particle passes through the focal region of the respective beams, light is reflected back along the same path as the outgoing light for that beam, to the concave focusing mirror, back to the respective expander mirror, and back to the instrumentation where the reflected light is passed into a detection system. As shown, the detection system includes an optical combiner to combine a reference signal and the reflected signal for heterodyning, and passing this heterodyned signal to a detector circuit of conventional design.
The detector circuit provides an electrical output containing information on the Doppler shift of the reflected signal, which is used to compute a velocity in a known processing circuit. The three velocities from the three axes are geometrically combined to provide velocities in the three orthogonal axes.
With the present laser diodes used, the overall size of the instrumentation unit can be kept relatively small. For example, with a 2 mm diameter source beam, the expander mirrors can be 2 cm in diameter and the focusing mirror may be in the range of 8 to 10 cm. Also, laser diodes do not consume large amounts of power, so cooling is not needed, and the laser diodes provide a tightly focused beam. Solid state, gas or other lasers can be used if desired.
While a three-axis system is shown, a two-axis system also can be used, with the principle of using a, portion of the signal from one of the sources for a, reference signal for heterodyning the reflected signal from the other source.
It also is helpful to keep the laser sources equally spaced around a central axis for simplifying the calculations for orthogonal velocities, but different geometric configurations can be made if Cartesi
Buczinski Stephen C.
Rosemount Aerospace Inc.
Westman Champlin & Kelly P.A.
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