Optics: measuring and testing – Velocity or velocity/height measuring – With light detector
Reexamination Certificate
2001-01-18
2002-05-14
Buczinski, S. C. (Department: 3662)
Optics: measuring and testing
Velocity or velocity/height measuring
With light detector
C073S657000, C356S477000, C356S005090
Reexamination Certificate
active
06388739
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to ladar systems and, more particularly, to a self-referencing microdoppler ladar receiver and an associated detection method.
BACKGROUND OF THE INVENTION
Lidar, laser ladar, optical ladar, and ladar (hereinafter collectively termed “ladar”) are all names for “ladar” systems utilizing electromagnetic radiation at optical frequencies. The radiation used by ladar is at wavelengths which are 10,000 to 100,000 times shorter than that used by conventional radar. Nonetheless, radiation in the form of photons is scattered by the target and is collected and processed to yield information about the target and/or the path to the target.
Ladar uses the same principle as radar, i.e., the ladar system transmits optical signals to a target, the transmitted optical signals interact with the target, and some of the optical signals are reflected or scattered back to the ladar system where the backscattered signals can be analyzed. The change in the properties of the backscattered signals enables some property of the target to be determined. For example, the round trip time required for the optical signals to travel to the target and back to the ladar system is commonly used to determine the range to the target.
One type of ladar system is a Doppler ladar system that is used to measure the velocity of a target. When the optical signals transmitted from the ladar system strike a target moving towards or away from the ladar system, the wavelength of the light reflected or scattered off the target will be changed slightly. This change is known as a Doppler shift—hence the term Doppler ladar. If the target is moving away from the ladar system, the return light will have a longer wavelength (sometimes referred to as a red shift) while the target is moving towards the ladar system, the return light will have a shorter wavelength (producing so-called blue shift).
As described by U.S. Pat. Nos. 5,847,816; 5,847,817 and 5,867,257, the contents of each of which are incorporated herein in their entirety, a microdoppler ladar system can be utilized to detect and to obtain the vibration signature of a number of targets. For example, a microdoppler ladar system can obtain the vibration signature of various military targets for target classification, damage assessment, intelligence gathering and the like. By way of further example, a microdoppler ladar system can be utilized to measure the vibrational spectrum of bridges, buildings, pipelines, pumps, aircraft, volcanoes and the like. Accordingly, a microdoppler ladar system can assist in determining the mechanical status of machinery for a variety of purposes. Moreover, a microdoppler ladar system may be able to monitor the vital signs of a remotely located person, such as a witness during a deposition or a lie detector examination.
A conventional microdoppler ladar system includes a transmitter and a coherent receiver. The transmitter includes a master oscillator and an associated power amplifier for generating a primary laser beam that illuminates the target. The coherent receiver is responsive to backscattered signals produced by the interaction of the transmitted laser beam and the target. The coherent receiver can include a phase locked loop for receiving both the backscattered signals and the primary laser beam generated by the transmitter. By phase locking the backscattered signals and the primary laser beam generated by the transmitter, the phase locked loop can generate signals indicative of the range, the velocity and a characteristic signature of the target. Therefore, a conventional microdoppler ladar system requires that the coherent receiver not only detect the backscattered signals, but also be provided with a sample of the primary laser beam generated by the transmitter for purposes of phase locking with the backscattered signals.
It is oftentimes advantageous for microdoppler ladar systems to detect targets at relatively long ranges. However, the range of conventional microdoppler ladar systems is primarily limited by two factors. First, the transmitter must provide a primary laser beam that has sufficient power to obtain useful backscattered signals. Secondly, the master oscillator of the transmitter must be selected such that the coherence length of the master oscillator is somewhat longer than the cumulative distance from the transmitter to the target and then to the receiver in order for the coherent receiver to properly combine the backscattered signals and the primary laser beam. As such, for a conventional microdoppler ladar system in which the transmitter and receiver are colocated, the microdoppler ladar system cannot reliably detect target vibrations if the target is spaced from the master oscillator by a distance that is more than one-half of the coherence length of the master oscillator.
The coherence length l
c
of a master oscillator is related to the frequency linewidth &Dgr;&ngr; of the master oscillator as follows: l
c
=&pgr;(c/&Dgr;&ngr;) wherein c is the speed of light. In order to have the long coherence lengths required to detect remote targets, the master oscillator must therefore be designed to have an extremely narrow linewidth. For example, microdoppler ladar systems onboard spacecraft that are designed to detect targets on the earth would be required to have a master oscillator with an extremely narrow linewidth. Likewise, ground-based microdoppler ladar systems designed to detect targets disposed in space would also be required to have a master oscillator with an extremely narrow linewidth. Similarly, the detection and classification of long range airborne targets would also require a master oscillator having an extremely narrow linewidth since the range of the microdoppler ladar system would have to be in excess of 500 km in some situations. Unfortunately, master oscillators, such as fiber optic sources, semiconductor lasers and diode pumped solid state lasers, having linewidths that are sufficiently narrow for these long range applications are not readily available and, even if available, would greatly increase the cost of the resulting microdoppler ladar system.
SUMMARY OF THE INVENTION
A self-referencing microdoppler ladar receiver and an associated detection method are therefor provided for detecting the vibration of a target based upon an analysis of backscattered signals without a local reference derived from the transmitter. As such, the self-referencing microdoppler ladar receiver and the associated detection method can detect targets at long ranges since the analysis of the backscattered signals from the target does not require a comparison or phase locking to the primary laser beam emitted by the transmitter. The self-referencing microdoppler ladar receiver and associated detection method is particularly useful for space-to-earth, earth-to-space and long range air-to-air, ground-to-air and air-to-ground applications.
The self-referencing microdoppler ladar receiver includes a frequency shifter for receiving a backscattered signal from the target and for controllably shifting the frequency of the backscattered signal. The frequency shifter can include, for example, an acoustooptic frequency shifter for shifting the frequency spectrum of the backscattered signals. The self-referencing microdoppler ladar receiver also includes an interferometer, such as a Mach Zender interferometer, for directing portions of the frequency shifted signals along first and second paths of unequal length and for combining the portions of the frequency shifted signals to produce first and second output signals. According to one embodiment, the first path of the interferometer includes a delay loop for delaying the respective portion of the frequency shifted signals by a predetermined time relative to the other portion of the frequency shifted signals. In addition, the interferometer can include a coupler for combining the portions of the frequency shifted signals following their propagation along the first and second paths to produce the first and second outpu
Alston & Bird LLP
Buczinski S. C.
The Boeing Company
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