Abstract
A direct detection LIght Detection and Ranging (“LIDAR”) system for instantaneous measurement of target velocity and distance uses principles of dichroic atomic vapor absorption in a closed feedback loop. In one or more embodiments, the system includes a laser light source to transmit laser light toward a target; a Dichroic Atomic Vapor Laser Locking (“DAVLL”) system including a gas cell in a magnetic field, wherein the DAVLL system is coupled to receive the laser light after being reflected by the target and output an error signal that can be used to calculated the relative velocity between the emitter and the target; and a feedback control configured to determine respective gas absorption rates of the LCP and RCP light beams in the gas cell, determine a difference of the respective gas absorption rates, and control the laser light source to adjust the frequency of the transmitted laser light in accordance with the determined ratio.
Claims
1. A LIDAR system, comprising: a laser light source to transmit laser light toward a target; a DAVLL system including a gas cell in a magnetic field, wherein the DAVLL system is coupled to receive the laser light after being reflected by the target and output an error signal related to the frequency offset between the transmitting laser and the gas absorption line center and the frequency offset between the received light from a target and the gas absorption line center; and a feedback control configured to determine respective gas absorption rates of the LCP and RCP light beams in the gas cell, determine a difference of the respective gas absorption rates, and control the laser light source to adjust the frequency of the transmitted laser light in accordance with the determined ratio.
2. The LIDAR system of claim 1, further comprising a processor configured to compute a speed of the target in accordance with the determined ratio.
3. The LIDAR system of claim 2, wherein the processor is further configured to compute a distance to the target in accordance with the determined ratio.
4. The LIDAR system of claim 1, wherein the feedback control is configured to control the laser light source to adjust the frequency of the transmitted laser light to the frequency at the gas absorption line center.
Description
DESCRIPTION OF DRAWINGS
[0006] FIG. 1 illustrates an example of a DAVLL system that may be employed in one or more embodiments described herein.
[0007] FIGS. 2a-2b illustrate an example of a DAVLL LIDAR system in accordance with one or more embodiments described herein.
[0008] FIG. 3 illustrates an error signal produced by a pulsed laser and a target with selected combinations of laser frequency and target Doppler offset frequency.
[0009] FIG. 4 illustrates the implementation of the DAVLL system of the present invention in the Scheimpflug condition.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0010] The following description presents one or more embodiments of a direct detection LIDAR system for instantaneous or nearly instantaneous measurement of target speed and distance using features and principles of dichroic atomic vapor absorption.
[0011] FIG. 1 illustrates an example of a LIDAR system 100 in accordance with at least one embodiment. The illustrated system leverages DAVLL to stabilize a narrow linewidth laser 110 at a desired frequency. As employed in one or more of the embodiments disclosed herein, transmitted light output by laser 110 may be held to a desired frequency with closed loop control. For example, a laser 110 can be passed through a fiber optic circulator 180 which sends the majority of the transmitted laser signal to the transmit/receive optics 190 but also leaks some of the laser 110 signal to the DAVLL gas cell 120. This signal, either from leakage from the circulator 180 or from another optical arrangement where a sample of the laser 110 is collected and directed through the gas cell 120, serves to produce an error signal during operation that is used in a closed loop to keep the laser 110 tuned as close as possible to the center of the gas absorption line.
[0012] In one or more embodiments, LIDAR system 100 may include a gas cell 120 arranged in a magnetic field to receive the laser beam output by laser 110 which is initially linearly polarized. When an appropriate gas, such as Rubidium, is chosen that has an absorption line that splits due to the Zeeman effect is placed in the magnetic field of the cell, light that is shifted in frequency to one side of the absorption line will have its right hand circularly polarized component get absorbed more strongly than its left hand component and vice versa if the frequency is shifted to the opposite side of the absorption line. A quarter wave plate 130 combined with a polarizing beam splitter 140 separates and outputs beams through optics 142, 141 onto detectors 151 and 152, respectively, that correspond to the left hand and right hand circularly polarized components of the initial laser beam from the laser 110 prior to entering the gas cell 120. The graphs in FIGS. 2a-2b show an error signal 160 (see FIG. 1) generated by the difference between the signals from the detectors 151 and 152 being inputted into a difference amplifier 161 to generate the error signal 160, which is then inputted into a closed loop system in accordance with the difference in the gas absorption rate of laser light of the left hand circularly polarized (LCP) light to that of the right hand circularly polarized (RCP) light. As such, the error signal 160 can be analyzed to indicate a deviation of the laser output frequency from a desired frequency (e.g., the frequency at the atomic gas absorption line center 170 shown in FIGS. 2a-2b) and the corresponding need for an adjustment or “correction” to the frequency of the laser light as indicated by the difference.
[0013] Thus, referring to FIGS. 2a-2b, if the frequency of the laser light drifts from its frequency at gas absorption line center 170, the LCP light will be more strongly absorbed than the RCP. The difference of the gas absorption rates of the RCP light to LCP light results in the error signal 160 which may then serve as the feedback signal inputted into the laser drive electronics 163 in a closed loop system (FIG. 1). That is, when the gas absorbs the LCP and RCP components of the light equally, the laser 110 is considered to be “locked” to the frequency at gas absorption line center 170. When the gas absorbs the LCP or RCP components more strongly depending on the laser frequency, the feedback control resulting from the error signal 160 being used as a feedback signal into the laser drive electronics 163 corrects the laser frequency of the laser 110 in the appropriate direction to bring it once again to the frequency corresponding to the gas absorption line center, as illustrated in FIGS. 2a-2b. Features of one, nonlimiting configuration that may be used in principle for generating an error signal and performing a correction using a DAVLL, in general, are described in Corwin, et al., U.S. Pat. No. 6,009,111, the disclosure of which is incorporated herein by reference in its entirety.
[0014] In one or more embodiments, the DAVLL system 100 is configured to be sensitive to frequency changes from less than 1 MHz to hundreds of MHz, which corresponds to the range of Doppler shift frequencies in which light is shifted from targets moving at conventional motor vehicle speeds. In the example embodiment shown in FIG. 1, light outputted by laser transmitter/receiver optics 190 is scattered by a target 210, and a portion of the scattered light is collected by the transmitter/receiver optics 190 where it is then input to the DAVLL system 100 by the circulator 180. In one or more embodiments, the DAVLL system 100 may be contained within the same housing (not shown) as the laser source. In principle, however, a common housing for all components is not necessarily required.
[0015] Referring to FIG. 1, the narrow line width frequency laser 110 is controlled by the laser drive electronics 163 based on the error signal 160 as a feedback control signal. The laser beam 111 is directed to the circulator 180 where the laser beam is directed to the transmit/receive optics 190. If a target 210 is illuminated by the transmitted light from the laser transmitter/receiver optics 190, some of that light will be reflected and collected by the transmit/receive optics 190 and directed to the circulator 180 to a collimating lens 154 that collimates the light. Following the collimating lens 154 is a polarizer 153 and a narrow bandpass filter 155. The narrow bandpass filter is used to reduce background illumination. The light then goes through the gas cell 120. The gas cell 120 is inside a magnetic field 121 that is produced by magnets 122. The magnets 122 could be either conventional or electro magnets with the electro magnets having the option of changing the magnetic field strength by varying the current to the electro magnets. Light that is not absorbed by the gas cell 120 goes to the quarter wave plate 130 and then on to the polarizing beam splitter 140. The corresponding outputs of the polarizing beam splitter 140 are then focused by lenses 142 and 141 onto detectors 151 and 152, respectively. Detectors 151 and 152 convert the respective light signals to electrical signals that go to the difference amplifier 161. The output of the difference amplifier 161 is the error signal 160 that is sent to the laser drive electronics 163 and the range and velocity computation system 162.
[0016] As noted above, FIGS. 2a-2b illustrate graphically a relationship between the error signal 160 and the relative velocity of the target 210. FIG. 2a plots error signal 160 vs. frequency offset from laser line center of a signal passing through the system 100. For a measured error signal, FIG. 2a provides the relationship between the error signal 160 and the offset frequency which for a transmitted laser 110 locked onto the line center of the error signal 160 scattered from a target, can be converted through FIG. 2a to a Doppler shift from the target induced by relative motion between the target and the emitter. FIG. 2b illustrates a portion of the error signal 160 that is single valued 165 that provides an unambiguous relationship between error signal 160 and Doppler shift which can then be converted to relative velocity via Δv=λ*Δf where Δv is the component of the relative velocity between the emitter and the target along the laser beam axis, λ is the laser wavelength, and Δf is the change in frequency calculated using FIG. 2a to convert the measured error to a frequency offset. Once the laser 110 is locked to a gas absorption line (frequency), the light scattered from target 210 can be transmitted through the same gas cell 120. The error signal 160 from the difference of the RCP to LCP signals scattered from the target 210 is related to the relative speed of the target 210 to the LIDAR system 100. If calibrated properly, this difference combined with the time of flight of the return signal from target 210 provides instantaneous or nearly instantaneous information about the distance to target 210 as well as its component of its velocity along the LIDAR beam. An example of what an error signal vs time for the embodiment in FIG. 1 with a pulsed laser 110 is shown in FIG. 3. For one laser pulse emitted, a portion of the outgoing laser pulse is sent through the gas cell assembly and an error signal e.sub.0 is produced. A signal r.sub.0 from a scattering target is measured a time later depending on the distance to the target 210 is also measured. The difference in the error signal for e.sub.0 and r.sub.0 corresponds to a difference in frequency Δf.sub.0 which is then used for relative velocity calculations as discussed earlier. Thus, with a single pulse, the time of flight between e.sub.0 and r.sub.0 and the Δf provides simultaneous ranging and velocity measurements of a target 210. Multiple laser pulses are emitted (e.sub.1, e.sub.2, e.sub.3) and received (r.sub.1, r.sub.2, r.sub.3) over time. The laser center frequency per pulse can be actively locked with the emitted error signals vs. time, and the Δf per pulse can be used to record the target relative component velocities vs. time.
[0017] In another embodiment, the laser light scattered from targets along the beam is imaged through the gas cell assembly onto two camera detectors 164. In this arrangement, the laser beam from the laser 110 is put in focus at all ranges by tilting the camera detectors 164 into the Scheimpflug condition. A calibration of the instrument maps the pixel on the camera detectors 164 to a distance from at least the transceiver of the LIDAR system 100. The camera detectors may be implemented using line cameras, CCDs or other detector arrays (e.g., 1D or 2D arrays) known in the art. All other elements of the present invention not otherwise mentioned for purposes of this embodiment remain the same in structure and function. For a given distance, the pixel intensity on each camera pixel corresponding to the LHP and RHP signals is analyzed to compute the error signal and thus the Doppler shift. In this embodiment, continuous wave lasers can be used in place of higher priced pulsed lasers, and line cameras can measure the backscattered intensity instead of more costly high speed photodetectors such as photomultiplier tubes.
[0018] Embodiments of a LIDAR system have been described in the context of a target speed detector that does not require multiple measurements of target position or mixing of light scattered from the target with emitted laser light. Such advantages are merely illustrative and the disclosed embodiments may enjoy one or more of these advantages as well as other advantages. Moreover, the disclosed LIDAR system is not limited to detecting any particular type of target, but may be used to detect targets of various size, shape, composition, and/or velocity.
[0019] Various changes and modifications to the disclosed LIDAR system will be apparent to those skilled in the art. All such changes and modifications that rely on the basic teachings and principles through which the invention has advanced the state of the art are to be understood as included within the spirit scope of the present invention.
