Patent Description:
The effectiveness of high power laser systems against non-cooperative targets is severely limited by atmospheric turbulence. One approach to mitigate the effects of turbulence uses deformable mirrors (DM) with a wave front sensor (WFS) and coherent beam combining (CBC) on the targets. However, this approach has been found to be limited to low turbulence conditions, due to the difficulty of reconstructing the phase-front in the WFS in the presence of strong turbulence. The same limitation applies to a CBC system in which multiple laser beams deform the laser wavefront without the use of a DM.

<CIT> discloses a coherent beam combination system.

A different approach, which may be more effective at high levels of turbulence, is to measure atmospheric turbulence by analyzing the interference pattern between the beam reflected by a target and a reference beam, for example, in a phase shift interferometer. <CIT> to Barchers, discloses a coherent phased array beam transmission and imaging system for end-to-end compensation of a plurality of laser beams through a turbulent medium to a non-cooperative target. The reference beam phase is modulated in order to shift the interference pattern and reduce the sensitivity to reflected intensity. The interference pattern is typically measured by a high frame rate camera, and the time-varying atmospheric aberration is deduced from shifts in the interference pattern. This approach is sensitive at low intensity levels of the reflected beam, and avoids problems such as wave-front stitching from phase gradients when using a Wave-Front Sensor (WFS), such as a Shack-Hartmann sensor. In this approach, it is necessary for the coherence length of the laser illumination to be longer than twice the distance to the target. In some applications, the required coherence length may be on the order of many kilometers.

In order to achieve high laser power, one must suppress non-linear effects, such as Stimulated Brillouin Scattering (SBS), typically by broadening the laser linewidth or enlarging the laser spectral width. However, this has the effect of greatly reducing the laser coherence length, or coherence time; for example, a <NUM> kW fiber laser may have a coherence length of only a few millimeters after SBS suppression. Thus the need to suppress non-linear effects in order to achieve a high laser power is in conflict with the requirement of a long coherence length, which is needed to compensate atmospheric turbulence by phase shift interferometry.

Pseudo-Random Binary Sequence (PRBS) modulation has been used in optical telecommunications in order to increase the power of a fiber laser, while decreasing its coherence length in a deterministic manner. The PRBS sequence is typically generated from a linear feedback shift register consisting of n bits, producing a pseudo-random pattern of <NUM>n-<NUM> (nonzero) binary numbers. The time duration of each bit is the reciprocal of the shift register clock frequency.

The invention provides a coherence reconstruction apparatus for interferometric measurement of atmospheric turbulence when illuminating a target at long range with a high-power CBC system.

According to one aspect of the presently disclosed subject matter, there is provided a coherence reconstruction apparatus in communication with a coherent beam combining (CBC) system which illuminates a target with one or more encoded and at least partially coherent beams which propagate through a turbulent atmosphere. The apparatus includes a variable delay module which receives a seed laser reference signal from the CBC system and a target time-of-flight (TOF) measurement, and a phase shift interferometer (PSI) which receives a target-reflected optical signal from the CBC system. The variable delay module generates a delayed reference optical signal which is coded; the PSI combines the target-reflected optical signal with the delayed reference optical signal to form an interference pattern; and the PSI determines at least one turbulence phase correction measurement from the interference pattern.

According to some aspects, the delayed reference optical signal includes at least a portion of a binary-phase code and/or a poly-phase code modulation.

According to some aspects, the binary-phase code modulation is a Pseudo-Random Binary Sequence (PRBS) modulation.

According to some aspects, the poly-phase code modulation is a generalized Barker code or a Frank code.

According to some aspects, a clock frequency of the code modulation is at least one gigahertz.

According to some aspects, the interferometer comprises one or more photodetectors.

According to some aspects, the interferometer comprises a photodiode array.

According to some aspects, the target-reflected optical signal is provided by a receiver in the CBC system.

According to some aspects, the apparatus further includes a controller which transmits a turbulence correction feedback signal to the CBC system.

According to some aspects, the apparatus further includes a controller which transmits dynamic tuning signals to the variable delay module.

According to some aspects, the seed laser reference signal is encoded.

According to some aspects, the seed laser reference signal is not encoded and the delay module includes a code delay module.

According to some aspects, the TOF measurement is provided by a target range tracker and/or a Target-in-the-Loop tracking system.

According to some aspects, the delay module includes an electromagnetically induced transparency (EIT) medium and/or a pump laser.

The invention is herein described, by way of example only, with reference to the accompanying drawing.

<FIG> shows a schematic drawing of an exemplary coherence reconstruction apparatus <NUM>, according to a first embodiment of the invention. Apparatus <NUM> communicates with CBC system <NUM> and includes a variable delay module <NUM> and a phase shift interferometer (PSI) <NUM>. The apparatus receives as input:.

The variable delay module uses inputs (a) and (c) to generate a delayed reference optical signal <NUM>. PSI <NUM> combines signal <NUM> with input (b) to form an interference pattern, from which one or more turbulence phase correction measurements <NUM> are determined.

The following sections describe each of the signals and signal processing functions in detail.

This signal is provided by the CBC system <NUM>. The CBC system generates and directs a multiplicity of coherent laser beams to illuminate a generally non-cooperative target <NUM>. For clarity of presentation, only one illumination beam is shown in the exemplary CBC system of <FIG>, with the understanding that the coherence reconstruction apparatus of the invention may operate independently and in parallel on more than one beam of a CBC system.

Seed laser <NUM> injects coherent light into a CBC electro-optical modulator <NUM>. Modulator <NUM> receives a phase modulation code signal 18A from a code generator <NUM>. The phase modulation corresponds to a deterministic code, such as a binary-phase or a poly-phase code. Code generator <NUM> typically includes a linear feedback shift register having a shift register clock frequency which is, for example, greater than or equal to <NUM> gigahertz (GHz).

PRBS is one example of a binary-phase code. A PRBS code consists typically of a repeating sequence of <NUM>n-<NUM> binary numbers each having (n) bits, where a bit represents a phase shift of zero or pi radians. The sequence includes all nonzero binary numbers arranged in a randomized, but deterministic, order. The value of integer (n) is typically greater than or equal to eight, in order to provide sufficient signal-to-noise ratio for coherence reconstruction.

Poly-phase codes include, for example, generalized Barker codes, Frank codes, and a variety of other modulation codes which are known to those skilled in the art of signal processing.

The encoded optical beam propagates from modulator <NUM> to beamsplitter <NUM>, along the propagation path indicated by the dashed arrow in <FIG>. The beamsplitter redirects a portion of the encoded optical signal power to form the seed laser reference signal 27A. Beamsplitter <NUM> may be implemented, for example, using fiber-optic splitters, reflective optics, and/or diffractive optical elements. Note that in the embodiment shown in <FIG>, the seed laser reference signal 27A is already encoded with the modulation code.

This signal is provided by the CBC system <NUM>. In <FIG>, beamsplitter <NUM> transmits a second optical beam along the path indicated by the dashed arrow entering into a turbulence phase corrector <NUM>. The latter receives a turbulence correction feedback signal <NUM> from the coherence reconstruction apparatus <NUM>, and applies an optical phase correction to the CBC output beams so that they combine in-phase (constructively) at the illumination spot <NUM> formed on the target surface.

The turbulence-corrected optical beam enters a beam director <NUM>, which steers the beam with high angular accuracy, typically on the order of a few micro-radians. The beam is focused, together with other illumination beams provided by the CBC system, onto an illumination spot <NUM> on the surface of the target <NUM>. A portion of the energy impinging on the target forms a reflected beam <NUM>, whose phase and intensity are distorted by time-varying turbulent perturbations as the beam propagates through moving air masses in a turbulent region <NUM> of the atmosphere.

A CBC receiver <NUM>, which typically includes a telescopic optical system with an entrance aperture, receives a portion of the light in the reflected beam <NUM>. At target ranges of several kilometers, the power of the received light may be <NUM> to <NUM> orders of magnitude smaller than the illumination power emitted by the CBC system. The receiver sends a target-reflected optical signal <NUM>, indicated by a dashed arrow, to the PSI <NUM> of the coherence reconstruction apparatus <NUM>. Signal <NUM> is typically a fiber-optic or a free-space optical signal.

This measurement is typically provided by a target range tracker or a Target-in-the-Loop (TIL) system (not shown in <FIG>). The TOF is equal to twice the target range, R, divided by the speed of light, c, in the atmosphere. For example, with c=3x10<NUM> meters/sec and R = <NUM> meters, the TOF is equal to 2R/c = 20x10-<NUM> sec, or <NUM> microseconds.

Continuing with the description of the coherence reconstruction apparatus <NUM> of <FIG>, the variable delay module <NUM> receives the seed laser reference signal 27A and the target TOF measurement. An exemplary embodiment for module <NUM> incorporates a coherent control art device, which includes an electromagnetically induced transparency (EIT) medium and a pump laser to tune the group velocity of the seed laser reference signal <NUM> in the medium. Module <NUM> generates a delayed reference optical signal <NUM>.

The PSI <NUM> receives signal <NUM> and the target-reflected optical signal <NUM>, both of which are typically of fiber-optic or free-space optical signals. The PSI forms an interference pattern between the two signals <NUM> and <NUM> using one or more photodetectors, such as for example, a photodiode array. In the absence of atmospheric turbulence, the two signals would be in phase, to within a constant phase offset, and there would be no interference pattern; the presence of an interference pattern is caused solely by atmospheric turbulence. PSI <NUM> converts the interference pattern into one or more turbulence measurement signal(s) <NUM>. Generally, the use of interferometry greatly improves the signal-to-noise ratio of the turbulence measurement signals, especially when the reflected beam passes through a strong turbulence regime, characterized by large fluctuations in the intensity of the reflected beam.

<FIG> shows a schematic drawing of an exemplary coherence reconstruction apparatus <NUM>, according to a second embodiment of the invention. This embodiment differs from that in <FIG> in the following respects.

In CBC system <NUM> of <FIG>, a beamsplitter <NUM> is positioned between the seed laser <NUM> and the CBC modulator <NUM>, which is different from the placement of beamsplitter <NUM> in <FIG>. The beamsplitter <NUM> sends an unencoded seed laser reference signal 17A to the coherence reconstruction apparatus <NUM>.

In apparatus <NUM>, a variable delay module <NUM> includes a code delay module <NUM> and an optical delay module <NUM>. Code delay module <NUM> receives a phase modulation code signal from code generator <NUM> and introduces a time delay equal to the received target TOF measurement in order to form a delayed code signal <NUM>. Delay module <NUM> uses, for example, a digital-to-analog converter (DAC) or a voltage-controlled oscillator (VCO) followed by an RF amplifier, to generate a delayed code signal <NUM>.

Optical delay module <NUM> receives the signal <NUM> and the unencoded seed laser reference signal 17A. Variable delay module <NUM> typically generates an analog signal representing the code modulation, using componentry similar to that used in CBC modulator <NUM>, but with the additional input of delayed code signal <NUM>. The analog signal drives an electro-optical (EO) modulator, such as a lithium niobate EO modulator. The latter generates the delayed reference optical signal <NUM>, which is sent to the PSI <NUM>.

Additional embodiments of the reconstruction apparatus <NUM> and/or the reconstruction apparatus <NUM> include a controller <NUM>, which fulfills any one of several functions. One function is to generate the turbulence correction feedback signal <NUM> and to send it to the turbulence phase corrector <NUM> of the CBC system. A second function of controller <NUM> is to generate and transmit dynamic tuning signals <NUM> to the variable delay module. The tuning signals typically contain TOF offsets that are determined by analyzing incremental shifts in the interference pattern formed in the PSI.

Although the embodiments of the present disclosure have been described within the context of binary-phase and poly-phase modulation, the principles of the present disclosure may be equally applicable to implementations that use other types of phase modulation.

Claim 1:
A coherence reconstruction apparatus (<NUM>) in communication with a coherent beam combining, CBC, system (<NUM>) which is configured to illuminate a target with one or more encoded and at least partially coherent beams which propagate through a turbulent atmosphere (<NUM>), the apparatus comprising:
a variable delay module (<NUM>) which is configured to receive a seed laser reference signal (27A) from the CBC system and a target time-of-flight, TOF, measurement; and
a phase shift interferometer, PSI, (<NUM>) which is configured to receive a target-reflected optical signa (<NUM>) from the CBC system;
wherein
the variable delay module is configured to generate a delayed reference optical signal (<NUM>) which is coded;
the PSI is configured to combine the target-reflected optical signal with the delayed reference optical signal to form an interference pattern; and
the PSI is configured to determine at least one turbulence phase correction measurement from the interference pattern.