Patent Description:
The present invention relates to a coherent beam combination (CBC) system and to a control method thereof.

As is known, coherent beam combination is a technique that is used to obtain a high power laser beam from a low power laser source.

A known coherent beam combination comprises a laser source that generates a primary laser beam, a splitter that splits the primary laser beam into N secondary beams, an amplifying body having N channels, one for each secondary beam, and a recombination unit that recombines the N secondary beams, thereby forming an output beam focused on a target.

In fact, the possibility to obtain a high-power laser beam from the amplification of a single laser source is limited by non-linear optical effects and thermal effects.

On the other hand, in a coherent beam combination system, the N channels are individually amplified and then recombined with each other.

This allows to use the recombination of the amplified beams to obtain a high power output.

If the amplified beams are coherent one with the other, the amplified beams interfere with each other. In particular, it is desired that the amplified beams interfere constructively with each other.

In fact, in a theoretical case, if the amplified beams are coherent with each other and have a mutual phase-shift equal to zero or a multiple of 2π, then the recombined beam has a peak intensity proportional to N<NUM>, wherein N is the number of channels of the CBC system.

On the other hand, if the beams are not coherent with each other, the intensity of the recombined beam is just proportional to N.

However, maintaining the amplified beams coherent with each other and phase-locked with each other require an accurate control of the phase of the amplified beams.

The Applicant has verified that the known CBC systems have a low efficiency with respect to the theoretical case.

Examples of known CBC systems are disclosed for example in:.

However, the known CBC systems do not allow to achieve beam steering in a simple and effective way.

The aim of the present invention is to overcome the disadvantages of the prior art.

The present invention relates to a coherent beam combination system and to a control method thereof, as claimed in the appended claims.

The following description is provided to enable a person skilled in the art to make and use the invention. Various modifications to the embodiments will be readily apparent to those skilled in the art, without departing from the scope of the claimed invention. Thus, the present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope defined in the appended claims.

Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the embodiments disclosed belongs. In the case of conflict, the present specification, including definitions, will control. In addition, the examples are illustrative only and not intended to be limiting.

For the purposes of promoting understanding of the embodiments described herein, reference will be made to certain embodiments and specific language will be used to describe the same. The terminology used herein is for the purpose of describing particular embodiments only, and is not intended to limit the scope of the present disclosure.

<FIG> shows a block diagram of a coherent beam combination (CBC) system <NUM> configured to provide an output recombined beam <NUM> having a high peak intensity, for example up to tens of kW, directed onto a target T.

The CBC system <NUM> may be used for example for space debris removal, spectroscopy and laser-shaping applications, Point-to-Point communication in air (such as air-air, ground-air, air-space, ground-space), as a counter for Unmanned Aerial System, improvised explosive devices or as a dazzling system.

The CBC system <NUM> comprises a laser source <NUM>, a beam broadener <NUM> and a splitter <NUM>, optically coupled with each other, in particular through an optical fibre.

The laser source <NUM> generates a primary laser beam <NUM> having a narrow linewidth, for example below <NUM>, the beam broadener <NUM> generates a broadened beam <NUM> from the primary laser beam <NUM>, and the splitter <NUM> splits the broadened beam <NUM> into N secondary beams, of which here only a first, a second, a third and a fourth secondary beam 12A, 12B, 12C, 12D are shown.

The CBC system <NUM> further comprises a main body <NUM>, optically coupled with the splitter <NUM>, and a focusing optics <NUM>, optically coupled with the main body <NUM>.

The main body <NUM> comprises a plurality of channels, one for each secondary beam. In detail, with reference to <FIG>, the main body <NUM> has a first, a second, a third and a fourth channel 20A, 20B, 20C, 20D, each receiving a respecting secondary beam 12A, 12B, 12C, 12D and providing a respective intermediate beam 21A, 21B, 21C, 21D.

The focusing optics <NUM> receives the intermediate beams 21A-21D and is configured to recombine the intermediate beams 21A-21D and generate the recombined output beam <NUM> directed onto the target T.

The focusing optics <NUM>, as discussed in detail hereinafter with respect to <FIG>, directs part of the intermediate beams 21A-21D towards an intensity sensor, here a photodiode <NUM>, and towards an image sensor, here a CCD camera <NUM>.

In this embodiment, the photodiode <NUM> is coupled to a motor <NUM>, for example a piezoelectric actuator, which is configured to move the photodiode <NUM> along one or more axis, in particular here along three orthogonal axis X, Y, Z.

The CBC system <NUM> further comprises a control unit <NUM> including a phase-locking unit or module <NUM> and a delay compensation unit or module <NUM>.

In this embodiment, the laser source <NUM> is a fibre laser, in particular a single-mode DFB fibre laser having a low-intensity noise and a high beam quality, for example with an M<NUM> factor smaller than <NUM>.

The laser source <NUM> is substantially a monochromatic laser, for example having a wavelength of <NUM>. However, the wavelength of the primary beam <NUM> generated by the laser source <NUM> may be different, depending on the specific application.

According to an embodiment, the laser source <NUM> may be configured to provide a variable-wavelength the primary beam <NUM>.

The laser source <NUM> may be a continuous wave laser source or a pulsed laser source, here a continuous wave laser source.

The beam broadener <NUM>, for example a chirp modulator, a sinusoidal modulator or a noise modulator, enlarges the linewidth of the primary beam <NUM>. For example, the broadened beam <NUM> may have a linewidth of about tens of GHz, in particular up to <NUM>.

<FIG> shows an embodiment of the beam broadener <NUM>, here a noise-modulated broadener, comprising a phase modulator <NUM>, in particular an electro-optical modulator, coupled between the laser source <NUM> and the splitter <NUM> and driven by an rf input signal RFin.

In detail, the beam broadener <NUM> comprises a noise generator <NUM> that generates a noise signal NS, for example a noise waveform or a pseudorandom binary sequence (PRBS), an rf amplifier <NUM> amplifying the noise signal NS and a low-pass filter <NUM> that filters the amplified noise signal, thereby generating the rf input signal RFin.

The low-pass filter <NUM> allows to set the bandwidth of the rf input signal RFin and, therefore, the optical bandwidth modification of the primary beam <NUM>.

According to an embodiment, the low-pass filter <NUM> may also comprise signal-shaping modules to modify the spectral shape of the rf input signal RFin, depending on the specific application.

The beam broadener <NUM> also comprises a termination load <NUM>, which receives an rf output signal RFout generated by the phase modulator <NUM> starting from the rf input signal RFin. The termination load <NUM> may be used for impedance matching and as a heatsink, to dissipate the heat generated by the rf input signal RFin in the phase modulator <NUM>.

The channels 20A-20D of the main body <NUM> each comprise an amplifier <NUM>, a phase modulator <NUM>, an optical delay line <NUM>, and an aperture combiner <NUM>, optically coupled with each other, in particular here through a respective optical fibre.

The amplifier <NUM> of each channel 20A-20D is coupled to a respective optic fibre extending from the splitter <NUM> and carrying the respective secondary beam 12A-12D. The amplifiers <NUM> of each channel 20A-20D amplify the respective secondary beam 12A-12D.

The amplifiers <NUM> each have a respective gain, for example fixed or variable, comprised, for example, between <NUM> and <NUM><NUM>.

The control unit <NUM>, in particular here the delay equalization unit <NUM>, may provide a signal S to the amplifiers <NUM>, which control one or more parameters of the amplifiers <NUM>.

In detail, the signal S comprises a plurality of beam-control signals s<NUM>, s<NUM>, s<NUM>, s<NUM>, one for each amplifier <NUM>.

For example, the beam-control signals s<NUM>, s<NUM>, s<NUM>, s<NUM> may each control the gain of the amplifier <NUM> of a respective channel 20A-20D.

According to an embodiment, the beam-control signals s<NUM>, s<NUM>, s<NUM>, s<NUM> may each command the switching on and the switching off of the respective amplifier <NUM>, thereby commanding the activation and de-activation of the respective channel 20A-20D. The phase modulators <NUM>, for example each formed by an electro-optical modulator or a fibre stretcher, receive a phase control signal U, for example a voltage signal, from the phase-locking unit <NUM>.

In detail, the phase modulator of each channel 20A-20D receives a respective phase control signal u<NUM>, u<NUM>, u<NUM>, u<NUM>, for example a voltage signal, from the phase-locking unit <NUM>.

The phase control signals u<NUM>, u<NUM>, u<NUM>, u<NUM> control the phase variations undergone by the secondary beams 12A-12D that, after being amplified by the respective amplifier <NUM>, propagates through the respective phase modulator <NUM>.

The phase modulators <NUM> may for example be manufactured as a waveguide, by using a proton-exchanged process, in order to obtain a high stability even at high optical power.

The optical delay lines <NUM> are variable delay lines that set the length of the optical path of the respective channel 20A-20D and receive a delay control signal D from the delay equalization unit <NUM>. In detail, the optical delay lines <NUM> receive each a respective delay control signal d<NUM>, d<NUM>, d<NUM>, d<NUM> from the delay equalization unit <NUM>.

In practice, the delay control signals d<NUM>, d<NUM>, d<NUM>, d<NUM> may each tune the physical length of the respective optical delay line <NUM> and/or may change the refractive index of the respective optical delay line <NUM>.

For example, the optical delay lines <NUM> may be fibre stretchers or folded delay lines.

According to an embodiment, the optical delay lines <NUM> may each comprise a fibre input coupled to the respective phase modulator <NUM>, a fibre output coupled to the beam combiner <NUM> and a movable opto-mechanical element, such as a retroreflector, arranged between the fibre input and the fibre output that reflects the respective secondary beam 12A-12D coming from the fibre input towards the fibre output. By moving the movable retroreflector it is possible to change the length of the path travelled by the respective secondary beam 12A-12D and, therefore, the length of the optical path of the respective channel 20A-20D.

For example, if the optical delay lines <NUM> comprise said movable opto-mechanical element, the delay control signals d<NUM>, d<NUM>, d<NUM>, d<NUM> may control an actuator, for example a piezoelectric actuator, configured to move the movable opto-mechanical element.

The beam combiner <NUM> has a back coupling portion 45A, receiving the secondary beams 12A-12D propagating from the optical delay lines <NUM>, and a front coupling portion 45B having a plurality of apertures <NUM>, each providing a respective intermediate beam 21A-21D.

The apertures <NUM> are arranged, on the front coupling portion 45B, in a tiled-aperture configuration, in particular in a honeycomb configuration, which allows a high scalability in the number of channels.

However, the apertures <NUM> may be arranged, on the coupling portion 45B, in a different configuration, depending on the specific application and/or on the desired filling factor.

By way of example only, <FIG> shows an example of the coupling portion 45B, in a case wherein the CBC system <NUM> has nineteen apertures <NUM> arranged in a honeycomb configuration.

The apertures <NUM> have a circular cross section having a diameter d defining the beam waist of the intermediate beams 21A-21D.

In the honeycomb configuration, two adjacent apertures 46A, 46B are arranged at a distance <NUM>, for example measured between the centres of the two adjacent apertures 46A, 46B.

Still with reference to the exemplificative configuration of <FIG>, the apertures <NUM> are arranged along an axis X in order to form a plurality of rows mutually spaced along an axis Y perpendicular to the axis X.

In this embodiment, two rows that are adjacent along the axis Y are arranged at a distance h measured in a direction parallel to the axis Y. For example, the distance h may be measured between the centres of two apertures of two adjacent rows.

The distance h may be expressed as a function of the distance l by the formula: h = <MAT>.

Again with reference to <FIG>, the beam combiner <NUM> comprises a plurality of opto-mechanical elements <NUM>, one for each channel 20A-20D, which couples the respective secondary beam 12A-12D propagating from the delay line <NUM> to the aperture <NUM>.

In detail, each opto-mechanical element <NUM> comprises a fibre connector <NUM> and an optical element <NUM>, arranged at the respective aperture <NUM>.

The fibre connector <NUM> is coupled to the optical fibre extending from the optical delay line <NUM>. The secondary beams 12A-12D coming from the optical delay lines <NUM> propagate in free space between the fibre connector <NUM> and the respective optical element <NUM>.

The optical element <NUM>, for example a converging lens, collimates the respective secondary beam 12A-12D propagating from the fibre connector <NUM>, thereby generating the respective intermediate beam 21A-21D.

For example, the fibre connector <NUM> may be placed at the focus plane of the respective optical element <NUM>.

Moreover, in this embodiment, each opto-mechanical element <NUM> further comprises an intensity mask <NUM> arranged between the respective fibre connector <NUM> and the respective optical element <NUM>.

The intensity mask <NUM> may reduce the beam waist of the beam propagating from the fibre connector <NUM> and the optical element <NUM>, so that only a portion of the secondary beam 21A-21D, for example comprised between <NUM>% and <NUM>%, in particular of about <NUM>%, is transmitted and forms the respective intermediate beam 21A-21D, while the remaining portion, for example comprised between <NUM>% and <NUM>%, in particular of about <NUM>%, of the respective secondary beam 21A-21D is blocked.

The intensity mask <NUM> allows to optimise the filling factor of the apertures <NUM> on the front coupling portion 45B of the beam combiner <NUM>.

<FIG> shows the CBC system <NUM>, wherein a detailed embodiment of the focusing optics <NUM> is illustrated.

The focusing optics <NUM> defines a primary optical path <NUM>, which directs a first portion of the intermediate beams 21A-21D towards the target T, and a secondary optical path <NUM>, which directs a second portion of the intermediate beams 21A-21D towards the photodiode <NUM> and the CCD camera <NUM>.

In detail, the primary optical path <NUM> of the focusing optics <NUM> forms a two-lens optical system comprising a convex lens <NUM> having a focus length f<NUM> and arranged in front of the front coupling face 45B of the beam combiner <NUM>, and a concave lens <NUM> having a focus length f<NUM> and optically coupled to the first lens <NUM>.

The first and the second lenses <NUM>, <NUM> allow the mutual recombination of the intermediate beams 21A-21D, thereby forming a recombined beam <NUM>.

The recombined beam <NUM> is generated by the interference of the intermediate beams 21A-21D. Therefore, the wavefront of the recombined beam <NUM> forms an interference pattern having a main lobe and one or more secondary lobes (as for example shown in <FIG>).

Moreover, the position of the concave lens <NUM> along the primary optical path <NUM> may be changed, in use, so that the two-lens system formed by the convex lens <NUM> and the concave lens <NUM> has a variable focal length.

For example, the focal length of the two-lens system may be changed depending on the distance of the target T from the CBC system <NUM>.

For example, the concave lens <NUM> may be coupled to a DC actuator, here not shown, configured to move the concave lens <NUM> in order to reduce or increase the distance between the convex lens <NUM> and the concave lens <NUM>.

In this embodiment, the focusing optics <NUM> further comprises a first mirror <NUM>, arranged along the primary optical path <NUM> between the first lens <NUM> and the second lens <NUM>, and a second mirror <NUM>, arranged along the primary optical path <NUM> between the concave lens <NUM> and the target T.

In practice, the first and the second mirrors <NUM>, <NUM> are arranged so that the primary optical path <NUM> is folded, thereby reducing the occupancy of the CBC system <NUM>.

The focusing optics <NUM> also comprises a beam splitter <NUM> arranged along the primary optical path <NUM>, in particular here between the concave lens <NUM> and the second mirror <NUM>.

The beam splitter <NUM> splits the recombined beam <NUM> propagating from the concave lens <NUM>, thereby forming a sample beam <NUM> propagating along the secondary optical path <NUM>.

The beam splitter <NUM> samples a small portion, for example <NUM>% or even less, of the recombined beam <NUM>, depending on the power of the recombined beam <NUM>, and the maximum optical power sustained by the photodiode <NUM> and the CCD camera <NUM>.

In detail, the secondary optical path <NUM> comprises a beam splitter <NUM>, a CCD lens <NUM>, a mirror <NUM> and a photodiode lens <NUM>.

The beam splitter <NUM> further splits the sample beam <NUM> so that a first portion is focused by the CCD lens <NUM> on the CCD camera <NUM> and a second portion is directed towards the photodiode <NUM> by the mirror <NUM> and focused thereto by the photodiode lens <NUM>.

The CCD lens <NUM> and the photodiode lens <NUM> may be chosen depending on a desired size of the main lobe of the portion of the recombined beam <NUM> that is directed towards the CCD camera <NUM> and, respectively, the photodiode <NUM>. For example, the CCD lens <NUM> and the photodiode lens <NUM> may be chosen so that the size of the main lobe of the recombined beam <NUM> is equal to or smaller than the active area of the photodiode <NUM> and, respectively, of the CCD camera <NUM>.

In this embodiment, the secondary optical path <NUM> also comprises a pinhole <NUM> arranged between the photodiode lens <NUM> and the photodiode <NUM>. The pinhole <NUM> has an aperture approximately equal to the size of the main lobe of the beam propagating from the photodiode lens <NUM>. For example, the aperture of the pinhole <NUM> may be comprised between <NUM> and <NUM>, in particular of about <NUM>.

In practice, the photodiode <NUM> may measure only the intensity of the main lobe of the beam propagating from the photodiode lens <NUM>.

The photodiode <NUM> provides an intensity signal INT, which is indicative of the intensity of the recombined beam <NUM>, in particular here of the main lobe of the recombined beam <NUM>. In fact, by knowing the splitting characteristics of the first and the second beam splitters <NUM>, <NUM>, the intensity signal INT may be used to obtain the intensity of the main lobe of the recombined beam <NUM>.

In use, the phase-locking unit <NUM> receives the intensity signal INT from the photodiode <NUM> and provides the phase-control signals U = {u<NUM>, u<NUM>, u<NUM>, u<NUM>} to the phase modulators <NUM> of the channels 20A-20D.

The phase-locking unit <NUM> performs a closed-loop optimization algorithm that modifies the phase-control signal U so to maximise the intensity measured by the photodiode <NUM>.

<FIG> shows a flow chart of a method <NUM> performed by the phase-locking module <NUM> to maximise the intensity measured by the photodiode <NUM>.

At a step <NUM>, the phase-locking module <NUM> receives a detection signal, here the intensity signal INT from the photodiode <NUM>.

At a step <NUM>, the phase-locking module <NUM> calculates a cost function from the detection signal, wherein the cost function is a function of the intensity detected by the photodiode <NUM>.

At a step <NUM>, the phase-locking module <NUM> performs an optimisation algorithm that is configured to maximise the intensity measured by the photodiode <NUM>.

At a step <NUM>, the phase-locking module <NUM> provides a plurality of updated phase control signals to the phase modulators <NUM>, based on an output of the optimization algorithm.

According to an embodiment, as shown in <FIG>, the phase-locking unit <NUM> performs, as optimization algorithm, a method <NUM> based on a Stochastic Parallel Gradient Descent (SPGD) algorithm.

In an initialization step <NUM>, the phase-locking unit <NUM> initializes the phase values of the secondary beams 12A-12D at an initial phase. In detail, the phase-locking <NUM> provides an initial phase signal U<NUM> to the phase modulators <NUM>. For example, the initial phase signal U<NUM> may provide the same phase signal to the modulator <NUM> of each channel 20A-20D, i.e. U<NUM> = {u<NUM>, u<NUM>, u<NUM>, u<NUM>}.

However, the phase-locking unit <NUM> may apply a different phase values to the phase modulators <NUM>, depending on the specific application.

The method <NUM> is an iterative method. Each iteration will be indicated by the index k. Moreover, in the following, the index j will be used to identify any one of the channels 20A-20D.

At each iteration k, the phase-locking unit <NUM> generates, step <NUM>, a phase-perturbation vector δu(k) = {δu<NUM>, δu<NUM>, δu<NUM>, δu<NUM>} comprising a plurality of perturbation voltages δu<NUM>, δu<NUM>, δu<NUM>, δu<NUM>, one for each phase modulator <NUM>.

In detail, the perturbation voltages δu<NUM>, δu<NUM>, δu<NUM>, δu<NUM> are generated according to a Bernoulli distribution having values v<NUM> and v<NUM> wherein v<NUM> is different from v<NUM> and wherein P(δuj = v<NUM>) = p and P(δuj = v<NUM>) = <NUM>-p.

Therefore, each perturbation voltage δuj may have either the value v<NUM> or v<NUM> with a probability p and, respectively, <NUM>-p.

According to an embodiment, the values v<NUM> and v<NUM> have the same modulus and opposite sign, i.e. v<NUM> = -v<NUM>.

According to an embodiment, p=<NUM>, so that P(δuj = v<NUM>) = P(δuj = v<NUM>) = <NUM>.

According to an embodiment, v<NUM> = -v<NUM> and p=<NUM>.

Then, step <NUM>, the phase phase-locking unit <NUM> provides a phase control signal U = U(k-<NUM>) + δu(k) = {u<NUM>(k-<NUM>) +δu<NUM>, u<NUM>(k-<NUM>)+δu<NUM>, u<NUM>(k-<NUM>)+δu<NUM>, u<NUM>(k-<NUM>)+δu<NUM>} to the phase modulators <NUM>.

In practice, the phase-locking unit <NUM> sums the phase-perturbation vector δu(k) to the phase control signal U(k-<NUM>) = {u<NUM>(k-<NUM>), u<NUM>(k-<NUM>), u<NUM>(k-<NUM>), u<NUM>(k-<NUM>)} that has been determined in the previous iteration k-<NUM>.

At the first iteration, i.e. for k=<NUM>, the phase-perturbation vector δu(k) is summed to the initial phase signal U<NUM>.

Therefore, at step <NUM> the phase modulators <NUM> change the phase values of the secondary beams 12A-12D propagating in the respective channels 20A-20D, with respect to the phase values provided in the previous iteration k-<NUM>.

Since the phases of the secondary beams 12A-12D have been changed with respect to the previous iteration k-<NUM>, also the interference pattern formed by the recombination of the intermediate beams 21A-21D changes. Accordingly, the intensity of the main lobe measured by the photodiode <NUM> changes.

The photodiode <NUM> measures a positive intensity I+, k, which is indicative of the intensity change in the recombined beam <NUM> caused by the phase control signal U(k-<NUM>) + δu(k).

The phase locking unit <NUM>, step <NUM>, receives the intensity signal INT from the photodiode <NUM>.

The phase-locking unit <NUM> calculates, step <NUM>, a positive cost function J+,k given by I+, k/Imax, wherein Imax is the maximum intensity that may be achieved if the intermediate beams 21A-21D are perfectly matched, i.e. if the mutual phase difference among the intermediate beams 21A-21D is <NUM> or an integer multiple of 2π.

Then, step <NUM>, the phase phase-locking unit <NUM> provides a phase control signal Uc = U(k-<NUM>) - δu(k) = {u<NUM>(k-<NUM>)-δu<NUM>, u<NUM>(k-<NUM>)-δu<NUM>, u<NUM>(k-<NUM>)-δu<NUM>, u<NUM>(k-<NUM>)-δu<NUM>} to the phase modulators <NUM>.

In practice, the phase-locking unit <NUM> subtracts the phase-perturbation vector δu(k) from the phase control signal U(k-<NUM>) that has been determined in the previous iteration k-<NUM>.

At the first iteration, i.e. for k=<NUM>, the phase-perturbation vector δu(k) is subtracted to the initial phase signal U<NUM>.

Therefore, at step <NUM> the phase modulators <NUM> change the phase values of the secondary beams 12A-12D propagating in the respective channels 20A-20D, with respect to the phase values provided in the previous iteration k-<NUM> and with respect to the phase provided at step <NUM>.

The photodiode <NUM> measures a negative intensity I-, k, which is indicative of the intensity change in the recombined signal caused by the phase control signal U(k-<NUM>) - δu(k).

The phase-locking unit <NUM> calculates, step <NUM>, a negative cost function J-,k given by I-, k/Imax.

Then, step <NUM>, the phase-locking unit <NUM>, calculates a new phase control signal U(k) by updating the phase control signal U(k-<NUM>) based on the phase-perturbation vector δu(k) and the positive and negative cost functions J+,k, J-,k.

In detail, in this embodiment, the new phase control signal U(k) is calculated as U(k) = U(k-<NUM>) + δu(k) · γ · δJ(k), wherein γ is a gain value and δJ(k) is the difference between the positive and the negative cost functions J+,k, J-,k e.g. δJ(k) = J+,k - J-,k.

The gain value γ may be chosen by a user of the CBC system <NUM>, for example during the calibration of the CBC system <NUM>.

The phase-locking unit <NUM>, step <NUM>, provides the new phase control signal U(k) to the phase modulators <NUM>.

Therefore, at step <NUM> the phase modulators <NUM> update the phase of the secondary beams 12A-12D, based on the new phase control signal U(k).

The photodiode <NUM> measures a corrected intensity Icorr, k, which is indicative of the change of intensity of the recombined signal <NUM> caused by the new phase control signal U(k).

The phase-locking unit <NUM> then calculates, step <NUM>, a corrected cost function Jcorr,k as Icorr, k/Imax.

The phase-locking unit <NUM> verifies, step <NUM>, a convergence condition of the optimization method <NUM>.

In detail, in this embodiment, the phase-locking unit checks if the corrected cost function Jcorr,k is equal to or higher than a convergence threshold Jth, which may be for example chosen by a user during a calibration step of the CBC system <NUM>.

If the convergence condition is not verified, i.e. here if the corrected cost function Jcorr,k is lower than the convergence threshold Jth (branch N output from step <NUM>), the phase-locking unit <NUM> returns to step <NUM> and generate a new random perturbation vector δu(k+<NUM>) for the next iteration k+<NUM>.

The phase locking unit <NUM> then repeats all steps from <NUM> to <NUM>.

On the other hand, if the convergence condition is verified, branch Y output from step <NUM>, the phase-locking unit <NUM> returns to step <NUM> and repeats steps <NUM>, <NUM> and <NUM>.

In practice, if the convergence condition is verified, the phase-locking unit <NUM> keeps monitoring the cost function by acquiring the intensity signal INT (step <NUM>) and by calculating the associated cost function (step <NUM>), until the convergence condition is not verified anymore.

For example, in response to the convergence condition being verified (branch Y output from step <NUM>), the phase-locking unit <NUM> may immediately return to step <NUM> or may wait a time interval, which may be chosen depending on the specific application, before returning to step <NUM>.

During use, the phases of the secondary beams 12A-12D may be subject to unwanted changes caused by external factors. For example, a temperature drift may change the length of the optical fibres wherein the secondary beams 12A-12D propagate, thereby causing an unwanted phase shift among the secondary beams 12A-12D, which may degrade the mutual phase locking thereof.

The method <NUM> allows to adjust the phases of the secondary beams 12A-12D in a closed loop, so that the intensity of the main lobe of the recombined beam <NUM> is kept at a maximum value.

Moreover, the Applicant has verified that the method <NUM> allows also to reduce the power noise of the recombined beam <NUM> caused by optical phase fluctuations, in particular in a frequency range of said fluctuations comprised between <NUM> and <NUM>.

Moreover, the fact that the phase-locking unit <NUM> keeps monitoring the cost function even after the convergence condition has been satisfied (branch Y from step <NUM>), allows the method <NUM> to achieve a high speed of convergence and at the same time to keep high the performance of the CBC system <NUM>.

The method <NUM> is also used as a method to steer the output recombined beam <NUM>, for example to track the target T if the target T has moved to a different position (as for example indicated by a dashed line in <FIG>).

<FIG> shows an example of a schematic top plan view of the photodiode <NUM>, wherein the beam spot <NUM> of the portion of the recombined beam <NUM> focused by the photodiode lens <NUM> falls completely within an active area <NUM> of the photodiode <NUM>. In this case, by supposing that the phase-locking unit <NUM> has verified the convergence condition, the mutual phase shift among the secondary beams 12A-12D is optimised and the main lobe of the recombined beam <NUM> has a maximum intensity.

If, as shown in <FIG>, the photodiode <NUM> is moved along a first and a second axis X, Y, the beam spot <NUM> may fall only in part within the active area <NUM>.

The movement of the photodiode <NUM> is controlled by the motor <NUM>, for example a piezoelectric actuator having a high accuracy, e.g. able to cause a displacement of the photodiode <NUM> comprised between <NUM> and <NUM>.

The motor <NUM> may be controlled by the control unit <NUM>.

In response to the displacement of the photodiode <NUM>, the photodiode <NUM> detects a reduction in the measured intensity.

Accordingly, when the phase-locking unit <NUM> acquires the intensity signal INT (step <NUM>) and calculates the cost function J (step <NUM>), the convergence condition may not be verified anymore (step <NUM>). Therefore, the phase-locking unit <NUM> returns to step <NUM> and performs one or more new iterations (from step <NUM> to step <NUM>) until the convergence condition is satisfied.

In fact, by changing the phase applied by the phase modulators <NUM>, the phase-locking unit <NUM> is able to change the position of the recombined beam <NUM>, in particular is able to move the main lobe of the recombined beam <NUM>, for example until the beam spot <NUM> falls again completely within the active area <NUM>.

In practice, by moving the photodiode <NUM>, it is possible to steer the recombined beam <NUM> and, therefore, the output recombined beam <NUM>.

The Applicant has verified that the method <NUM> allows to achieve a very fast and accurate beam steering, for example to accurately control the position of the output recombined beam <NUM> at the target T. For example, even when the target T is placed at a distance of about <NUM> from the CBC system <NUM>, by moving the photodiode <NUM> with the motor <NUM>, the CBC system <NUM> may be able to adjust the position of beam, at the location of the target T, even by few micrometres.

According to an embodiment, the optical delay lines <NUM> may be variable delay lines. <FIG> shows a flow chart of a method <NUM> performed by the delay optimization unit <NUM> for equalizing the optical paths of the secondary beams 12A-12D in the channels 20A-20D, according to an embodiment.

The method <NUM> starts, step <NUM>, if the delay optimization unit <NUM> verifies that the optical paths of the channels 20A-20D need to be equalized.

For example, the method <NUM> may be performed during a calibration of the CBC system <NUM>, for example before a first use of the CBC system <NUM>, or may be performed periodically, during use, for example upon verification of a specific condition.

The delay optimization unit <NUM> selects, step <NUM>, one of the channels 20A-20D to be equalized and sets one of the channels 20A-20D as reference channel.

For example, hereinafter, the first channel 20A is taken as the reference channel and the second channel 20B as the channel to be equalized.

However, any of the channels 20A-20D may be taken as reference channel. For example, if the channels 20A-20D are arranged in a honeycomb structure in the aperture combiner <NUM>, the channel whose aperture is arranged at the centre of the honeycomb structure may be taken as reference channel.

At step <NUM>, the delay optimization unit <NUM> selects the first channel 20A and the second channel 20B by turning off the third channel 20C and the fourth channel 20D.

For example, the third channel 20C and the fourth channel 20D may be turned off by stopping the emission of the corresponding amplifier <NUM>. For example, the delay optimization unit <NUM> may provide the signals s<NUM> and s<NUM> so that the respective amplifiers <NUM> block the propagation of the third and fourth intermediate beams 21C and 21D.

Then, step <NUM>, the delay optimization unit <NUM> determines a coarse estimate of the optical path difference between the second channel 20B and the reference channel 20A. For example, the coarse estimate may have an accuracy comprised between few centimetres and several meters of the optical path difference.

In this embodiment, the delay optimization unit <NUM> performs a Frequency Modulation Continuous Wave (FMCW) technique to find the coarse estimate of the optical path difference between the second channel 20B and the reference channel 20A.

In detail, the delay optimization unit <NUM> provides a chirp signal CHIRP to the laser source <NUM> and, in response thereto, acquires the intensity signal INT from the photodiode <NUM>.

The chirp signal CHIRP has a chirp frequency α that causes a temporal modulation of the wavelength of the primary beam <NUM>, in particular causes the wavelength of the primary beam <NUM> to follow a triangular ramp.

If there is a delay OPD<NUM> between the reference channel 20A and the second channel 20B, the intensity signal INT has a beat note at a beat frequency fb.

The relation between the delay OPD<NUM> and the beat frequency fb is: <MAT> wherein c is the speed of light in vacuum and n is the refractive index of the medium through which the first and the second secondary beams 12A, 12B.

Then, step <NUM>, the delay optimization unit <NUM> provides the delay control signal d<NUM> to the optical delay line <NUM> of the second channel 20B, in order to compensate for the delay OPD<NUM>.

In practice, the delay control signal d<NUM> shortens or stretches the optical path of the optical delay line <NUM> of the second channel 20B, in order to compensate for the delay OPD<NUM>.

For example, if the optical delay line <NUM> of the second channel 20B has a movable opto-mechanical element configured to change the length of the optical path of the optical delay line <NUM>, then the delay control signal d<NUM> may control an actuator, for example a piezoelectric actuator, configured to move the optical delay line <NUM> of the second channel 20B.

Then, the delay optimization unit <NUM> finds a fine estimate of the optical path difference between the second channel 20B and the reference channel 20A.

In detail, step <NUM>, the delay optimization unit <NUM> measures a fringe visibility V<NUM> from the image IMG received from the CCD camera <NUM>.

At step <NUM>, only the first channel 20A and the second channel 20B are activated; therefore, the recombined beam <NUM> is formed by the interference between the first and the second intermediate beams 21A, 21B.

Accordingly, the image IMG acquired by the CCD camera <NUM> represents the interference pattern between the first and the second intermediate beams 21A, 21B.

The fringe visibility V<NUM> may be defined as: <MAT> wherein I+ is the maximum value of intensity on the image IMG, e.g. the peak value of the main lobe of the interference pattern, and I- is the minimum value of intensity on the image IMG, e.g. a zero of the interference pattern.

The delay optimization unit <NUM> checks, step <NUM>, if the fringe visibility V<NUM> satisfies an interference-quality condition. In this embodiment, the delay optimization unit <NUM> checks if the fringe visibility V<NUM> is equal to or higher than a fringe visibility threshold Vth, which may be chosen for example by a user during a calibration of the CBC system <NUM>.

In the negative case, branch N at output from step <NUM>, the delay optimization unit <NUM> updates, step <NUM>, the delay control signal d<NUM> that is provided to the optical delay line <NUM> of the second channel 20B, in order to increase the fringe visibility V<NUM>.

For example, the delay control signal d<NUM> is updated by using a known search or optimisation algorithm, such as a bisection algorithm.

If the optical delay line <NUM> of the second channel 20B has a movable opto-mechanical element, then the delay control signal d<NUM> is updated so that the optical delay line <NUM> of the second channel 20B is moved by the respective actuator, in order to shorten, or stretch, the optical path of the secondary beam 12B accordingly.

Then, the delay optimization unit <NUM> repeats step <NUM> to measure the updated fringe visibility V<NUM>, and step <NUM> to check if the interference-quality condition has been satisfied.

When the interference-quality threshold has been reached, i.e. here when V<NUM> ≥ Vth, branch Y at output from step <NUM>, the delay optimization unit <NUM> returns to step <NUM> by selecting the j+<NUM>-th channel. The, in the example considered, the delay optimization unit <NUM> selects the third channel 20C.

Therefore, the delay optimization unit <NUM> activates the third channel 20C and deactivate the second channel 20B.

According to this embodiment, the first channel 20A is still used as reference channel.

The delay optimization unit <NUM> then repeats the steps from <NUM> to <NUM> for the third channel 20C.

After also the optical path of the third channel 20C has been equalized, the steps from <NUM> to <NUM> are repeated also for the fourth channels 20D.

<FIG> shows an experimental example of a 3D representation of the spatial distribution of the intensity of the portion of the sample beam <NUM> acquired by the CCD camera <NUM>, in use. By taking into account the splitting characteristics of the first and the second beam splitters <NUM>, <NUM>, said distribution of intensity is indicative of the distribution of intensity of the recombined beam <NUM> and, therefore, of the output recombined beam <NUM> directed onto the target T.

By measuring the peak intensity of the main lobe, either from the image IMG acquired by the CCD camera <NUM> or by the intensity signal INT provided by the photodiode <NUM>, it is possible to find the efficiency of the CBC system <NUM> by calculating the ratio of the peak intensity of the recombined beam <NUM> over the peak intensity of a single intermediate beam 21A-21D, e.g. η = Imax,CBC/Imax, SB.

Theoretically, the efficiency of the CBC system <NUM> should be equal to N<NUM>, with N being the number of channels.

The Applicant has verified that the efficiency of the CBC system <NUM> may achieve a high value, close to the theoretical value.

In particular, for a CBC system having a number of channels N=<NUM>, the Applicant has found an experimental efficiency of <NUM>, with respect to the theoretical value of N<NUM>=<NUM>. Therefore, the CBC system may have an overall efficiency of about <NUM>% with respect to the theoretical value.

According to an embodiment, as shown in <FIG>, the CBC system <NUM> may comprise also a temperature analysis unit or module <NUM> configured to perform a method, illustrated in <FIG> and indicated by <NUM>, for simulating the effect of temperature variations on the CBC system <NUM>, in particular of the temperature variations induced by the high optical power of the recombined beam <NUM>.

In detail, the method <NUM> may be performed on a specific component of the CBC system <NUM>, in order to optimise the parameters of said specific component.

In detail, the method <NUM> may be performed for any of the optical elements of the focusing optics <NUM>. In fact, the recombined beam <NUM> may reach high optical power values, for example around tens of kW, that may cause high temperature variations in the optical elements of the focusing optics <NUM>.

According to the method <NUM>, at a step <NUM>, the temperature analysis unit <NUM> receives data indicative of the properties of the component under test (hereinafter c. The component under test may be, for example, the convex lens <NUM> or the concave lens <NUM>, or any other of the optical components of the focusing optics <NUM> shown discussed with reference to <FIG>.

data may comprise, for example, the geometry of the component under test, the optical properties, in particular absorption, and thermal properties of the substrate material of the component under test.

At a step <NUM>, the temperature analysis unit <NUM> receives laser data indicative of the properties of the laser beam to be analysed, i.e. here of the recombined beam <NUM>, such as optical power, spot size and wavelength.

Then, step <NUM>, the temperature analysis unit <NUM> uses the c. data and the laser data as input to solve a 3D partial differential heat equation of the component under test and provides at output a temperature map representing the temperature variations induced in the component under test by the propagation of the laser beam.

At a step <NUM>, the temperature analysis unit <NUM> uses the temperature map as input to calculate the local variations of the refractive index of the component under test that are induced by the temperature variations. In detail, the unit <NUM> provides at output a map of the updated refractive index n(x, y, z, ΔT), e.g. given by n(x, y, z, ΔT) = n<NUM>(x, y, z) + Δn(x, y, z, ΔT), wherein the variation Δn of the refractive index as a function of a temperature variation ΔT depends on the substrate material of the component under test.

Then, step <NUM>, the unit <NUM> uses the updated refractive index n(x, y, z, ΔT) to find the phase variations Δϕ induced by the variations in the refractive index. In detail, in this embodiment, the unit <NUM> calculates a 2D map of the phase variation through the formula: <MAT>, wherein z is the propagation direction of the laser beam within the component under test and L is the length of the component under test along the propagation direction.

The unit <NUM> converts, step <NUM>, the phase variation Δϕ(x, y, ΔT) in polar coordinates Δϕ(r, θ, ΔT).

In detail, the unit <NUM> decomposes the phase variation by using the Zernike polynomials Z as: <MAT> wherein the indexes n, m refer to the radial and, respectively, the angular behaviour of the phase variation.

The Zernike polynomials may also be expressed in terms of a single index <MAT>, so that each Zernike polynomial Zi is associated to a typical optical aberration and the corresponding coefficient represents the weight of said optical aberration in the decomposed wavefront. For example, i=<NUM> represents the piston phase offset, i=<NUM>,<NUM> the wavefront tilt, i=<NUM>,<NUM> the astigmatism, i=<NUM> the defocus, etc..

Then, step <NUM>, the unit <NUM> performs a simulation algorithm of the optical propagation of the laser beam in the CBC system <NUM>. The algorithm for optical propagation, per se known in the art, receives as input a model of the CBC system <NUM>, which for example may be previously stored in the unit <NUM> and provided by a user, and the Zernike polynomials Zi of the component under test.

The simulation algorithm provides at output data of the optical properties of the recombined beam <NUM>, such as wavefront properties of the recombined beam <NUM>, after propagation in the CBC system <NUM>, in particular after propagation through the component under test.

Based on said output data, the unit <NUM> determines, step <NUM>, the c. data, for example the material of the component under test, that optimise the optical properties of the recombined beam <NUM>, for example that guarantee a lowest distortion of the wavefront of the recombined beam <NUM> and a lowest absorption of the recombined beam <NUM>, depending on the specific application.

In detail, the methods <NUM>, <NUM>, <NUM> and <NUM> all contribute to improve the performance of the CBC system <NUM> and to achieve a value of efficiency close to the theoretical value.

Finally, it is clear that modifications and variations may be made to what has been described and illustrated herein, without thereby departing from the scope of the present invention, as defined in the annexed claims.

For example, the number N of channels may be different from what discussed with reference to <FIG>; in particular, the CBC system <NUM> may have up to nineteen channels.

For example, the phase-locking unit <NUM> may perform a different algorithm, such as a LOCSET algorithm.

For example, the phase-locking unit <NUM> may be implemented using an FPGA, a multichannel DAC unit or a different hardware. For example, the phase-locking unit <NUM> may comprise a multichannel DAC unit, be coupled to the CCD camera and configured to extract the peak intensity from the image IMG.

The sample beam <NUM> may be split directly from the intermediate beams 21A-21D, before being recombined. In this case, the focusing optics would comprise one or more beam splitters arranged between the front coupling portion 45B and the convex lens <NUM>.

In alternative, each opto-mechanical element may also comprise a respective fibre splitter that extracts a small portion of laser power, for example below <NUM>%. In this case, each intermediate beam comprises a first portion that propagates through the aperture and is focused by the focusing optics on the target, and a second portion extracted by the fibre splitter that is focused by the focusing optics on the image sensor and/or the intensity sensor.

The focusing optics <NUM> may have different optical elements with respect to what shown in <FIG>; for example, the mirror <NUM> and/or <NUM> may be absent, so that the optical path <NUM> is unfolded. Moreover, the lenses <NUM> and <NUM> may have different focal lengths and/or different shapes.

The present coherent beam recombination system is configured to provide an output recombined beam and comprises:.

Claim 1:
A coherent beam recombination system (<NUM>) configured to provide an output recombined beam (<NUM>), comprising:
- a laser source (<NUM>) providing a source beam (<NUM>) having a linewidth;
- a beam broadener (<NUM>) coupled to the laser source and configured to provide a broadened beam (<NUM>) having a larger linewidth than the source beam;
- a splitter (<NUM>) configured to split the broadened beam into a plurality of secondary beams (12A-12D);
- a plurality of channels (20A-20D) coupled to the splitter, each channel being configured to receive a respective secondary beam (12A-12D) and to provide a respective intermediate beam (21A-21D), each channel comprising an optical amplifier (<NUM>), a phase modulator (<NUM>), an optical delay line (<NUM>) and an opto-mechanical element (<NUM>) that provides the respective intermediate beam;
- an optical sensor (<NUM>, <NUM>) configured to provide a detection signal (INT, IMG) indicative of an intensity of a received optical beam;
- a focusing optics (<NUM>) configured to receive the intermediate beams (21A-21D), to provide the output recombined beam (<NUM>) from a first portion of each intermediate beam, and to provide a sampled recombined beam (<NUM>) to the optical sensor from a second portion (<NUM>) of each intermediate beam; and
- a control unit (<NUM>) coupled to the optical sensor and the plurality of channels,
wherein the control unit comprises a phase-locking module (<NUM>) configured to:
- provide a plurality of phase control signals (u<NUM>, u<NUM>, u<NUM>, u<NUM>), each to the phase modulator (<NUM>) of a respective channel (20A-20D),
- receive the detection signal (INT) from the optical sensor, the detection signal being indicative of an intensity of the sampled recombined beam;
- calculate a cost function (J) from the detection signal (INT), the cost function being a function of the intensity of the sampled recombined beam;
- perform an optimization algorithm of the cost function, the optimization algorithm being configured to maximise the intensity of the sample recombined beam; and
- provide a plurality of updated phase control signals, based on a result of the optimization algorithm,
characterized in that the coherent beam recombination system further comprises an actuator (<NUM>) coupled to the optical sensor (<NUM>) and configured to cause a displacement of the optical sensor for steering the output recombined beam (<NUM>).