IL307724A - Laser imaging system and method - Google Patents

Description

LASER IMAGING SYSTEM AND METHOD FIELD OF THE INVENTION The invention generally relates to laser imaging of a scene.

More specifically, the invention relates to a laser system for imaging a scene via a scattering optical medium, such as via atmosphere turbulence.

BACKGROUND OF THE INVENTION Visualizing objects by laser through a diffusive medium, such as imaging close-to-the-ground objects in the presence of severe atmospheric turbulence, observing objects under the water, optical imaging below clouds, or observing objects under a camouflage net, is challenging, particularly (but not only) at long distances. In such conditions, the medium to the imaged object results in a smeared point spread function (PSF), which reduces imaging quality and results in a blurred image. To compensate for such aberrations, one may use a camera to build a wavefront sensor that operates at the multi-kHz rate (similar to the dynamics rate of the medium) . Yet, such a measurement in the absence of a clear beacon signal at the object's side is impractical in most visual applications.

Traditionally, LiDAR (Light Direction and Range) systems have been used to visualize objects in 3D, yet however, they are limited to the transverse resolution as dictated by the PSF. LiDar typically transmits short optical pulses toward the scene to acquire the distance to the object with high resolution. This distance acquisition is insensitive to the optical aberrations on the beam path to the target.

More specifically, while LiDARs typically measure the distance on a scale of meters, the atmospheric aberrations affect the LiDAR time-of-flight signal on a scale of micrometers which is several orders of magnitude below the resolution of LiDAR systems. State-of-the-art LiDARs utilize multiple (spatially incoherent) laser beamlets operating at high bandwidth and repetition rates to rapidly scan through a large area. In such systems, the target area is imaged to a detector array compatible with the illumination pattern. The entire area is raster scanned, while signals from the individual detectors are stitched to form a complete scene image. Similarly to conventional imaging systems, the LiDAR approach for imaging fails at harsh atmospheric turbulence conditions, as the reflected optical signals from the target reach the wrong detectors.

In addition, the individual sources endure a low resolution due to the propagation through the atmosphere and the (typically) small aperture used.

Assuming a shot-noise-limited operation, the signal-to- noise ratio (SNR) is mainly dictated by the amount of light collected during the integration time. Thus, the integration time and the required spatial resolution define the time it takes to cover a desired area. Conventional LiDARs apply pulsed (sub ns) laser sources that are generally limited in power, mainly if high energy (>100pJ) pulses are used. This configuration becomes a strict limitation when imaging over hundreds of kilometers, as is the case, for example, with space-borne applications.

Typical modern space-borne LiDARs use tens of millijoules pulse energy with a repetition rate of several tens of Hz, typically resulting in output power of ~10W. Such a configuration provides a very slow earth coverage rate by LiDARs (2-4% of earth's land surface in 2 years) as compared to missions containing 'passive' cameras (global coverage over 16 days) or radar (global coverage within six days).

Phase-modulated continuous wave (CW) laser systems can more easily reach high average power, thereby resolving the issue of power limitation. Typical schemes of CW LiDARs that include variants of FMCW were recently demonstrated.

Furthermore, such sources were recently combined with Coherent Beam Combining (CBC) to achieve 3D volumetric scanning while utilizing the inherent power scalability of CBC sources. Those CBC imagers combined with FMCW schemes have also been suggested as an alternative to pulsed LiDARs, mainly to resolve the blur at the imager due to turbulent mediums and to increase the scanning rate.

A typical CBC laser includes a single laser seed whose output beam is split into several (e.g., 10-200) beamlets.

The beamlets are then focused on the target. All individual beamlets are phase-locked on the target, utilizing some phase-locking technique.

Following the phase locking on the target, image acquisition is performed by raster scanning the scene via variation of the phases of the CBC laser beamlets. The back-reflected beam, typically in its electrical form, is then analyzed to form an image of the target. When necessary, the area where the target is located is further scanned by systematically changing the line of sight toward the scene.

Compared to LiDAR, a typical CBC improves image quality by overcoming scattering disturbances toward the target. Yet, the image quality is still imperfect due to turbulence and line-of-sight stabilization issues of the returning back- reflection. Furthermore, a typical CBC laser is limited to two-dimensional scanning and does not offer a 3D resolution.

It is, therefore, an object of the invention to provide a laser imaging system that improves the image quality compared to a conventional CBC by overcoming the line of sight stabilization and turbulence-related issues.

Another object of the invention is to provide a CBC-based imaging system capable of delivering a 3D resolution.

Other objects and advantages of the invention become apparent as the description proceeds.

SUMMARY OF THE INVENTION The invention relates to a method for laser imaging a target, comprising: (a) providing a laser seeder, and generating a laser beam by the seeder; (b) splitting the seeder's laser beam into a first laser beam and a second laser beam, and conveying the first laser beam into a CBC unit and the second laser beam into a reference unit; (c) identically broadening the first and second beams within the CBC unit and the reference unit, thereby forming a broadened beam within the CBC unit and another broadened beam within the reference unit; (d) given said broadened beam at the CBC unit, splitting this beam by the CBC unit to a plurality of CBC beamlets, and transmitting the CBC beamlets towards a target; (e) providing a Distance and Velocity Determination Unit (DVDU), and measuring the distance to a target; (f) given the measured distance to the target, delaying said broadened beam at the reference unit by a round trip period T to create a broadened and delayed reference beam; (g) receiving and interfering at a detecting unit a beam returned from the target and said broadened and delayed reference beam, thereby forming a combined beam, and then converting the combined beam to an electrical signal; (h) utilizing said interference component at said electrical signal to phase lock all the CBC transmitted beams on the target at a distance defined by 1; (i) utilizing an interference component within said electrical signal to construct an image of the target; and (j) continuously repeating from step (a).

In an embodiment of the invention, said returned beam from the target and said broadened and delayed reference beam are received at the detector after passing through a common inlet lens.

In an embodiment of the invention, the method further includes modifying said delay T , thereby to construct a three-dimensional image of the target.

In an embodiment of the invention, said distance is measured by utilizing one or more of said CBC beams, alternately serving as the DVDU.

In an embodiment of the invention, said detector unit comprising an array of sensing detectors.

In an embodiment of the invention, said CBC beams are transmitted to the target through a common outlet lens.

In an embodiment of the invention, said broadening utilizes a noise generator or a Pseudo Random Binary Sequence-PRBS.

In an embodiment of the invention, further comprising raster scanning that constructs an image of a larger target or an area.

The invention also relates to an imaging system, comprising: (a) a laser seeder configured to generate a laser beam, and to simultaneously convey the generated beam to a CBC unit and a reference unit; (b) a broadening generator configured to identically broaden the beam conveyed to the CBC unit and the beam conveyed to the reference unit, thereby forming a CBC broadened beam and a reference unit broadened beam; (c) a distance and velocity determination unit (DVDU) configured to measure a round trip period T between the system and the target; (d) a delay unit configured to delay the reference beam by said round trip period T , thereby forming a delayed reference beam; (e) said CBC unit is configured to split said CBC broadened beam to a plurality of CBC beamlets, and to transmit said beamlets toward a target; (f) a detecting unit configured to receive a beam reflected from the target as a result of said plurality of CBC beamlets, and said delayed reference beam, and to convert said combined beam to an electrical signal; (g) a close-loop lock module configured to phase lock all the transmitted CBC beamlets on the target, based on said combined beam; and (h) an analyser configured to receive said electrical signal, and to construct an image of the target based on said electrical signal.

In an embodiment of the invention, the system further comprising an inlet lens through which said combined beam passes, before reaching the detecting unit. - -ר In an embodiment of the invention, the system further comprising an outlet lens through which said plurality of CBC beamlets pass when leaving the system towards the target.

In an embodiment of the invention, said DVDU utilizes one or more of said CBC beamlets to determine said round trip period T .

In an embodiment of the invention, said detecting unit comprises an array of sensing detectors.

In an embodiment of the invention, said broadening generator is either a noise generator or a Pseudo Random Binary Sequebce-PRBS generator.

In an embodiment of the invention, the system further comprising a mechanism for raster scanning the target or an area.

In an embodiment of the invention, said analyzer is further configured to resolve Doppler effects, based on knowledge of the distance and velocity to the target as measured by the DVDU.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings: Fig. I generally illustrates the system of the invention in a block diagram form; and Fig. 2 is a flow diagram illustrating the method of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 1 illustrates the system of the invention in a block diagram form. Laser seeder 104 generates a CW narrow-band beam 138 (split into beams 138a and 138b) . Beam 138a is conveyed into main module 108, and beam 138b is conveyed into reference module 102 (also referred to as reference unit). A broadening signal 140 (white noise, Pseudo Random Binary Sequence-PRBS or another), generated by broadening generator 114 (typically spectral broadening is done using a phase modulator) , is simultaneously conveyed into the reference module 102 and the main module 108, resulting in identically spectrally broadened main and reference beams 142 and 152, respectively. The beams' broadening reduces the coherence length of the emerging beams. Similar to a conventional CBC, the broadened main beam 142 is then split into multiple beams transmitted via individual fibers 142a- 142n, individual phase modulators 116, and lens 118 towards target 124. Lens 118 focuses the separate beamlets 142a- 142n on target 124. The individual beamlets are phase-locked on target 124, utilizing a close loop lock module 136 and phase modulators 116 (as described below). The returned back-reflected signal 126 passes through lens 130, arriving at detecting unit 132. Detecting unit 132 converts the optical signal 126 into an electrical signal 156 as required for the closed-loop lock module 136. Based on analysis of the signal 156 and based on the knowledge of the distance to the object, as determined by the DVDU 112 (as explained below), the close-loop lock module 136 manipulates at phase modulators 116 the individual phases of the output beams 142a-142n, so they all constructively interfere at the target 124.

A distance and velocity determination unit (DVDU) 1 continuously determines the distance and velocity between imager 100 and target 124. Based on the determined round trip distance of beams 142 and 126, the distance and velocity determination unit (DVDU) 112 calculates a delay period (T) 110 applied on the electrical broadening signal 140 of reference beam 138b within reference module 102.

Therefore, reference module 102 outputs a broadened and delayed reference beam 152. The delayed and broadened reference beam 152 then impinges on the same detecting unit 132 and interferes with the back-reflected returned beam 126. The interference occurs due to the defined precise delay between the two signals. This beam combination results in an interference between the two (reference 152 and returned 126) beams, which is sensed by detecting unit 132.

The interference causes a significantly amplified signal from an x-y plane at a location z, as selected and determined by delay T . The possibility to define a plane x-y at an optionally varied distance z at which the target is located, resulting in a heterodyne amplified interference signal at the detection side, is a significant advantage of the system of the invention and is the underlying mechanism of line-of-sight stabilization of the returned back-reflected signal. The amplified signal is sensed by analyzer 134, which may reproduce the target's (or a target's portion) image 146.

It should be noted that the interference component in the returned signal is much larger compared to other components that may be sensed at detecting unit 132. This fact enables the analyzer 134 to distinguish between the various components and select only the interference component while ignoring all the other components. A conventional raster scan may be applied to increase the scanned area.

As discussed above, using the DVDU 112 and adjustable delay, the system defines a virtual x-y plane and positions it at the target (longitudinal distance z from the imaging system). This structure also enables overcoming Doppler effects on the returned signal, given that the DVDU 1 determines both velocity and distance and can determine variations relating to these values. The spectrum broadening, made by the same amount to both the main beams 142 and the reference beam 152, introduces into the system a depth factor that enables a 3D (xyz volume) sectioning of the target (or several targets existing along the z-axis).

The depth can be determined by varying the delay of the reference beam. For instance, further increasing the delay T sent to the broadened reference beam would result in constructive interference between the reference and the main beam from a more distant z plane. The depth's resolution increases as the phase-broadening bandwidth applied by the phase-broadening generator 114 increases.

In one alternative, the DVDU 112 is a separate unit, as shown in the scheme of Fig. 1. In another option, one or more of the CBC beams may be utilized to perform the function of DVDU 112. In the latter case, the system may alternate between two modes of operation, a DVDU mode and an image-acquiring mode. In still another option, all the CBC beams are utilized for a limited amount of time in a DVDU mode in a periodic manner.

The imager 100 of the invention overcomes atmospheric turbulence by amplifying and distinguishing a back- reflected signal that only refers to a specific transverse x-y plane at a selected distance z from the source rather than from a larger area. In addition, it provides an improved 3D resolution compared to prior art CBC imagers.

Furthermore, the detected signal from the target is heterodyne amplified by the reference beam, significantly decreasing the effect of detector noise, making it possible to apply shorter integration times, decrease the transmitting power, and obtain a superior SNR compared to non-heterodyne-amplified designs. Moreover, using the delayed reference beam improves the performance of the CBC, as it allows a 3D phase locking of beams 142 on the target, compared to a 2D locking possible in a conventional CBC.

The physical behavior of the system is now described in more detail.

The broadened beam is described by: E = Eoe‘*W Where Eo indicates the field amplitude, i = V—T and 0(t) indicates the quickly-varying phase in the time domain due to the linewidth broadening.

The interference between the main beam and the reference beam is described by: ^measuredEcBc + Eref I2 >t=< |FCBC|2 + |Ere/|2 + 2|ECBC||fre/|cos(40(t)) >t ^0(0 — PcbcU ^TOf) ־ Pref (0 Where ECBC indicates the back-reflected field of the CBC laser, Eref indicates the field of the reference beam, t indicates the time, indicates the instantaneous phase difference between the back-reflected CBC field and the reference field, (pcec indicates the instantaneous phase of the back-reflected CBC laser, tT0F indicates the time of flight to the target and back to the system, and (pref W indicates the instantaneous phase of the reference beam.

Therefore, the detected mean intensity is: (4) ^measured ^DC ־*־ ^RF < cos(A0(t)) >t Where lmeasured indicates the detected signal, oc indicates proportionality ADC indicates a constant amplitude, and ARF indicates the amplitude of the quickly varying term in the equation.

The typical broadening bandwidth is on the order of 1- 100GHz. Hence, any integration time longer than 10-10 picoseconds with an unmatched optical path would result in no interference contrast. The only way to achieve interference with high contrast is to match the delay of the reference beam to the round trip TOF (time of flight) to the object.

As previously noted, coherent detection raises several other issues, such as a Doppler shift, maintaining a stable interference over a large distance, etc. Accurate measurement of the distance and velocity relative to the target is mandatory to resolve these issues, followed by compensation for the Doppler-induced dephasing (either full or partial compensation). The system of the present invention provides an improvement in all these issues.

It should also be noted that the invention's imager applies to distances starting from one meter or less to hundreds of kilometers or more.

The invention also relates to a method (shown in Fig. 2) for imaging a target, comprising: a. Providing a laser seeder, and generating a laser beam by the seeder (step 202); b. Splitting the seeder's laser beam into a first laser beam and a second laser beam, and conveying the first laser beam into a CBC unit (the main module 108) and the second laser beam into a reference unit (reference module 102) (step 204); c. Identically broadening the first and second beams within the CBC unit and the reference unit, thereby forming a broadened beam within the CBC unit and another broadened beam within the reference unit (step 206) ; d. Given said broadened beam at the CBC unit, splitting this beam by the CBC unit to a plurality of CBC beamlets, and transmitting the CBC beamlets towards a target (step 208); e. Providing a Distance and Velocity Determination Unit (DVDU) , and measuring the distance to a target (step 210) ; f. Given the measured distance to the target, delaying said broadened beam at the reference unit by a round trip period T to create a broadened and delayed reference beam (step 212); g. Receiving and interfering at a detecting unit a beam returned from the target and said broadened and delayed reference beam, thereby forming a combined signal, and then converting the combined signal to an electrical signal (step 214); h. Utilizing said interference component at said electrical signal to phase lock all the CBC transmitted beams on the target at a distance defined by t (step 216) ; i. Utilizing an interference component within said electrical signal to construct an image of the target (step 218); and j. Continuously repeating from step (a).

While some embodiments of the invention have been described by way of illustration, it will be apparent that the invention can be carried into practice with many modifications, variations, and adaptations, and with the use of numerous equivalent or alternative solutions that are within the scope of persons skilled in the art, without departing from the spirit of the invention or exceeding the scope of the claims.

Claims (16)

- 15- CLAIMS

1. A method for laser imaging a target, comprising: a. providing a laser seeder, and generating a laser beam by the seeder; b. splitting the seeder's laser beam into a first laser beam and a second laser beam, and conveying the first laser beam into a CBC unit and the second laser beam into a reference unit; c. identically broadening the first and second beams within the CBC unit and the reference unit, thereby forming a broadened beam within the CBC unit and another broadened beam within the reference unit; d. given said broadened beam at the CBC unit, splitting this beam by the CBC unit to a plurality of CBC beamlets, and transmitting the CBC beamlets towards a target; e. providing a Distance and Velocity Determination Unit (DVDU), and measuring the distance to a target; f. given the measured distance to the target, delaying said broadened beam at the reference unit by a round trip period T to create a broadened and delayed reference beam; g. receiving and interfering at a detecting unit a beam returned from the target and said broadened and delayed reference beam, thereby forming a combined beam, and then converting the combined beam to an electrical signal; h. utilizing said interference component at said electrical signal to phase lock all the CBC transmitted beams on the target at a distance defined by 1; - 16- i. utilizing an interference component within said electrical signal to construct an image of the target; and j. continuously repeating from step (a).

2. The method of claim 1, wherein said returned beam from the target and said broadened and delayed reference beam are received at the detector after passing through a common inlet lens.

3. The method of claim 1, further modifying said delay T , thereby to construct a three-dimension image of the target.

4. The method of claim 1, wherein said distance is measured by utilizing one or more of said CBC beams, alternately serving as the DVDU.

5. The method of claim 1, wherein said detector unit comprising an array of sensing detectors.

6. The method of claim 1, wherein said CBC beams are transmitted to the target through a common outlet lens.

7. The method of claim 1, wherein said broadening utilizes a noise generator or a Pseudo Random Binary Sequence- PRBS.

8. The method of claim 1, further comprising raster scanning that constructs an image of a larger target or an area.

9. An imaging system, comprising: - 17- a. a laser seeder configured to generate a laser beam, and to simultaneously convey the generated beam to a CBC unit and a reference unit; b. a broadening generator configured to identically broaden the beam conveyed to the CBC unit and the beam conveyed to the reference unit, thereby forming a CBC broadened beam and a reference unit broadened beam; c. a distance and velocity determination unit (DVDU) configured to measure a round trip period T between the system and the target; d. a delay unit configured to delay the reference beam by said round trip period T, thereby forming a delayed reference beam; e. said CBC unit is configured to split said CBC broadened beam to a plurality of CBC beamlets, and to transmit said beamlets toward a target; f. a detecting unit configured to receive a beam reflected from the target as a result of said plurality of CBC beamlets, and said delayed reference beam, and to convert said combined beam to an electrical signal; g. a close-loop lock module configured to phase lock all the transmitted CBC beamlets on the target, based on said combined beam; and h. an analyser configured to receive said electrical signal, and to construct an image of the target based on said electrical signal.

10. The system of claim 9, further comprising an inlet lens through which said combined beam passes, before reaching the detecting unit. - 18-

11. The system of claim 9, further comprising an outlet lens through which said plurality of CBC beamlets pass when leaving the system towards the target.

12. The system of claim 9, wherein said DVDU utilizes one or more of said CBC beamlets to determine said round trip period T .

13. The system of claim 9, wherein said detecting unit comprises an array of sensing detectors.

14. The system of claim 9, wherein said broadening generator is either a noise generator or a Pseudo Random Binary Sequebce-PRBS generator.

15. The system of claim 9, further comprising a mechanism for raster scanning the target or an area.

16. The system of claim 9, wherein said analyzer is further configured to resolve Doppler effects, based on knowledge of the distance and velocity to the target as measured by the DVDU. תכירע םיטנטפ 10012 . tn עבש-ראב 1400101