Thermal stabilization of acousto-optic devices

Abstract

An optical device includes an acousto-optic medium configured to receive an input radiation beam and to deflect the input beam toward a target over a range of deflection angles. An array of multiple piezoelectric transducers is attached to the acousto-optic medium. A drive circuit is coupled to apply respective drive signals to the piezoelectric transducers, the frequencies of the respective drive signals being selected so as to cause acoustic waves to propagate through the acousto-optic medium at a selected frequency, the acousto-optic medium thereby deflecting the input beam at a corresponding deflection angle within the range, and wherein a phase offset among the drive signals applied to the transducers in the array is selected so as to modulate the intensity of the deflected beam by adjusting a wavefront angle of the acoustic waves.

Description

TECHNICAL FIELD

[0001] The present disclosure relates generally to optical devices and systems, and particularly to acousto-optic devices and methods for operating such devices. BACKGROUND

[0002] Acousto-optic devices use sound waves to diffract light. In a typical device of this kind, a transducer (e.g., a piezoelectric transducer) is attached to an acousto-optic medium (typically a suitable transparent crystal or glass). The transducer is driven by an electrical signal to vibrate at a particular frequency, and thus forms a sound wave in the acousto-optic medium. The expansion and compression of the acousto-optic medium due to the sound wave modulates the local refractive index and thus forms a grating structure within the medium with a period determined by the frequency of the driving signal. A light beam incident on this grating will thus be diffracted as it passes through the device.

[0003] Various types of acousto-optic devices are known in the art. For example, acousto-optic deflectors use the diffraction of an incident beam to steer the angle of an output beam. The deflection angle of the output beam depends on the period of the grating structure in the acousto-optic material and can thus be adjusted by suitably varying the driving signal frequency.

[0004] Some acousto-optic devices use a transducer phase array to form a sound wave in an acousto-optic medium. The transducers are driven with different relative phase delays in order to control the angle at which the sound wave propagates through the acousto-optic medium and thus adjust the phase matching between the sound field and the light beam to be modulated. For example, U.S. Patent 7,538,929 describes radio frequency (RF) phase modulation techniques for performing intensity modulation of an optical wavefront using an acousto-optic modulator that includes an acousto-optic bulk medium and transducers attached to the acousto-optic bulk medium and formed into a linear array of electrodes. A transducer driver is connected to each electrode and is coherently phase driven to alter the angular momentum distribution of the sound field and alternately allow and disallow phase matching between the optical field and the sound field to produce a desired intensity modulation of the optical wavefront.

[0005] An acousto-optic deflector can be driven with a multi-frequency driving signal in order to diffract an incident beam into multiple output beams at different respective angles. For example, U.S. Patent 5,890,789 describes a multi-beam launching device that uses an optical waveguide type acousto-optic element or the like driven with a plurality of electrical signals having different frequencies to split a light beam launched from a light source into a plurality of beams. As another example, U.S. Patent Application Publication 2009 / 0073544 describes a device for optical splitting and modulation of monochromatic coherent electromagnetic radiation, in which an acousto-optic element splits a beam produced by a beam source into several partial beams. An acousto-optic modulator disposed downstream of the acousto-optic element is fed with the split partial beams and driven with an additional high frequency electrical signal.

[0006] PCT International Publication WO 2016 / 075681 describes yet another example of the use of a phase array in driving an acousto-optic deflector, the disclosure of which is incorporated herein by reference. In this publication, an optical device includes an acousto-optic medium and an array of multiple piezoelectric transducers attached to the acousto-optic medium. A drive circuit is coupled to apply respective drive signals to the piezoelectric transducers, the respective drive signals including at least first and second frequency components at different respective first and second frequencies and having different respective phase offsets for the first and second frequency components at each of the multiple piezoelectric transducers. SUMMARY

[0007] Embodiments of the invention provide improved apparatuses and methods for acousto-optic deflection.

[0008] Thus, according to embodiments of the invention, there is provided an optical device including an acousto-optic medium configured to receive an input radiation beam and to deflect the input beam into at least first and second output beams having respective first and second intensities at respective first and second beam angles at which the acousto-optic medium is characterized by different respective first and second diffraction efficiencies. An array of multiple piezoelectric transducers is attached to the acousto-optic medium. A drive circuit is coupled to apply respective drive signals to the piezoelectric transducers, the respective drive signals including at least first and second drive signals at different corresponding first and second frequencies to direct the first and second output beams at the respective first and second beam angles and having different respective first and second phase offsets for first and second frequency components at each of the multiple piezoelectric transducers, the first and second phase offsets causing acoustic waves to propagate through the acousto-optic medium at different respective first and second wavefront angles at the first and second frequencies. A controller is configured to select the first and second phase offsets so as to compensate for the different first and second diffraction efficiencies, thereby equalizing the first and second intensities.

[0009] Typically, the drive circuit is configured to concurrently apply at least the first and second drive signals having the respective first and second phase offsets to the piezoelectric transducers, such that the acousto-optic medium simultaneously deflects the input beam into at least the first and second output beams.

[0010] In some embodiments, the controller is configured to vary at least the first and second frequencies of the first and second drive signals such that the acousto-optic medium causes at least the first and second beams to scan over respective first and second angular ranges, and to vary the respective phase offsets in response to the variations in the frequencies. In one embodiment, the controller is configured to control the drive signals applied by the drive circuit such that the acousto-optic medium deflects at least the first beam toward a target at a given beam intensity during a series of pulse intervals interspersed with block intervals in which the intensity of the first beam on the target is attenuated to less than 50% of the given beam intensity, wherein the first drive signal has a given amplitude and has a frequency during each pulse interval corresponding to a deflection angle of the beam and has a chirped spectrum during each block interval.

[0011] Additionally or alternatively, when the first diffraction efficiency is greater than the second diffraction efficiency, the controller is configured to compensate for the different first and second diffraction efficiencies by setting the second phase offset such that the acoustic wave satisfies a Bragg condition with respect to the input beam at the second frequency while deviating from the Bragg condition with respect to the input beam at the first frequency. In the disclosed embodiments, the controller is further configured to turn on and off each of the output beams by modifying the respective phase offsets while maintaining a constant power level of the respective drive signals regardless of the respective phase offsets.

[0012] According to embodiments of the present invention, there is also provided an optical device comprising an acousto-optic medium configured to receive an input beam of radiation and to deflect the input beam toward a target at a given beam intensity over an angular range during a series of pulse intervals interspersed with block intervals in which the intensity of the beam on the target is attenuated to less than 50% of the given beam intensity. At least one piezoelectric transducer is attached to the acousto-optic medium. A drive circuit is coupled to apply a drive signal to the at least one piezoelectric transducer, the drive signal having a given amplitude and having a frequency during each pulse interval corresponding to a deflection angle of the beam and having a chirped spectrum during each block interval.

[0013] In some embodiments, the chirped spectrum is selected such that during the block intervals, the intensity of the beam on the target is attenuated to less than 10% of the given beam intensity.

[0014] Additionally or alternatively, the chirped spectrum includes a sequence of discrete frequency steps applied during each blocking interval. In disclosed embodiments, the at least one piezoelectric transducer includes an array of multiple piezoelectric transducers, and the drive circuit is configured to apply respective drive signals to the piezoelectric transducers, the respective drive signals having phases selected so as to cause acoustic waves to propagate through the acousto-optic medium at the selected frequencies with different respective first and second wavefront angle during the blocking intervals, thereby to compensate for the different first and second diffraction efficiencies and to equalize the first and second intensities.

[0015] Additionally, according to embodiments of the invention, there is provided an optical device including an acousto-optic medium configured to receive an input radiation beam and to deflect the input beam toward a target over a range of deflection angles. An array of multiple piezoelectric transducers is attached to the acousto-optic medium. A drive circuit is coupled to apply respective drive signals to the piezoelectric transducers, the frequencies of the respective drive signals being selected so as to cause acoustic waves to propagate through the acousto-optic medium at the selected frequencies, the acousto-optic medium thereby deflecting the input beam at corresponding deflection angles within the range, and wherein phase offsets among the drive signals applied to the transducers in the array are selected so as to modulate an intensity of the deflected beam by adjusting a wavefront angle of the acoustic waves.

[0016] Additionally, according to embodiments of the invention, there is provided an optical device including an acousto-optic medium configured to receive an input radiation beam and to deflect the input beam toward a target over a range of deflection angles. An array of multiple piezoelectric transducers is attached to the acousto-optic medium. A drive circuit is coupled to apply respective drive signals to the piezoelectric transducers, the frequencies of the respective drive signals being selected so as to cause acoustic waves to propagate through the acousto-optic medium at the selected frequencies, the acousto-optic medium thereby deflecting the input beam at corresponding deflection angles within the range, and wherein phase offsets among the drive signals applied to the transducers in the array are selected so as to modulate an intensity of the deflected beam by adjusting a wavefront angle of the acoustic waves.

[0017] Furthermore, according to an embodiment of the present application, there is provided a method for optical scanning, comprising directing an input radiation beam to be incident on an acousto-optic medium, at least one piezoelectric transducer being attached to the acousto-optic medium. A driving signal is applied to the at least one piezoelectric transducer, the driving signal having a given amplitude and during each of a series of pulse intervals a frequency corresponding to a deflection angle of the beam and during each of a series of block intervals a chirped spectrum, the block intervals being interleaved with the pulse intervals, so as to cause the acousto-optic medium to deflect the input beam at a given beam intensity over an angle range towards a target during each of the series of pulse intervals and to attenuate the intensity of the beam on the target to less than 50% of the given beam intensity during each of the block intervals.

[0018] Furthermore, according to an embodiment of the present application, there is provided a method for optical scanning, comprising directing an input radiation beam to be incident on an acousto-optic medium, an array of a plurality of piezoelectric transducers being attached to the acousto-optic medium. Respective driving signals are applied to the piezoelectric transducers, the frequencies of the respective driving signals being selected so as to cause acoustic waves to propagate through the acousto-optic medium at the selected frequencies, thereby causing the acousto-optic medium to deflect the input beam at a corresponding deflection angle. A phase offset among the transducers in the array is set so as to modulate the intensity of the deflected beam by adjusting a wavefront angle of the acoustic waves. The present application will be more fully understood from the following detailed description of embodiments of the present application, taken together with the drawings, in which: BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1 is a schematic graphical illustration of a multi-beam deflection system according to an embodiment of the present application;

[0020] Figure 2 is a schematic graph of a frequency chirped signal applied to an acousto-optic deflector according to an embodiment of the present application;

[0021] Figure 3 is a schematic cross-sectional view of an acousto-optic deflector for generating a plurality of output beams according to an embodiment of the present application;

[0022] Figure 4 is a schematic cross-sectional view of an acousto-optic deflector driven by a transducer phase array according to an embodiment of the present application;

[0023] Figure 5 is a block diagram schematically illustrating a multi-frequency driving circuit for an acousto-optic deflector according to an embodiment of the present application;

[0024] Figure 6is a schematic plot of diffraction efficiency as a function of phase shift among transducers driving an acousto-optic deflector according to an embodiment of the application; and

[0025] Figure 7 is a schematic plot of a frequency chirp signal applied to an acousto-optic deflector according to another embodiment of the application. DETAILED DESCRIPTION

[0026] SUMMARY

[0027] Acousto-optic devices are used in many laser applications to control laser beam intensity and direction at high rates, typically in the range of 50 kHz to 1 MHz, and with high resolution. In order to precisely control the laser beam, it is important to maintain careful control over the temperature of the acousto-optic crystal, which can be affected by acoustic absorption in the crystal itself. For example, when a uniform temperature change occurs in the crystal, it will change the speed of sound (and refractive index) in the crystal, causing the deflection angle of the laser beam to drift. Non-uniform temperature changes can distort the laser beam, causing lensing and other refractive effects.

[0028] However, it is challenging to keep the acousto-optic crystal at a stable temperature, because acousto-optic modulation and deflection inherently involve changes in the RF signal used to drive the acousto-optic crystal. For example, laser pulses transmitted toward a target can be intermittently blocked by interrupting the RF signal to the crystal (an operation known as "pulse picking"); but the resulting changes in RF input power will cause temperature variations and unstable thermal behavior in the crystal. Furthermore, manipulating the laser beam direction by changing the drive frequency applied to the acousto-optic crystal can also cause temperature changes, because different drive frequencies typically require different RF power levels to achieve the same laser pulse energy, and because the acoustic absorption of the crystal is strongly dependent on frequency.

[0029] Embodiments of the application described herein provide novel techniques that can be used to keep the temperature of an acousto-optic crystal stable and uniform, despite the inherent challenges in pulse picking and beam manipulation. These embodiments enable the crystal to manipulate one or more radiation beams and alternately transmit and block the beams, while maintaining a constant RF input level to the crystal. In this context, the term "constant" means that the instantaneous RF power of the drive signal input to a piezoelectric transducer or transducers driving the crystal is maintained within predefined limits during operation of the acousto-optic device, typically changing by no more than 10%, and possibly no more than 5% (but larger or smaller limits are possible depending on application requirements), despite the manipulation and intermittent blocking of a beam or beams.

[0030] In some embodiments, an acousto-optic medium (typically a suitable crystal) is driven by at least one piezoelectric transducer attached to it during a series of pulse intervals to deflect an input beam at a given beam intensity over a range of angles toward a target. The pulse intervals are interleaved with blocking intervals in which the intensity of the beam on the target is attenuated to less than 50%, or possibly less than 10%, less than 5%, or even less than 1% of the given beam intensity. (The term "intensity" is used in its conventional sense in the context of this specification and claims to mean optical power per unit area on which the beam is incident.)

[0031] A drive circuit applies a drive signal to a piezoelectric transducer or transducers having a frequency corresponding to the deflection angle of the beam during each pulse interval and a chirped frequency spectrum during each blocking interval. The frequency chirp has a range of frequencies and a duration selected so as to cause the input beam to spread over a large area of the target, so that the intensity of the beam on the target during the blocking intervals is much lower than the intensity of the deflected beam during the pulse intervals. The drive circuit maintains a substantially same given amplitude of the drive signal in both the pulse and blocking intervals, notwithstanding the different frequency spectra.

[0032] This approach can be extended to temperature stabilization in multi-frequency operation in which a composite drive signal formed as a superposition of single-frequency drive signals is applied by an array of piezoelectric transducers to an acousto-optic medium. The composite drive signal splits the laser beam into a number of output beams. Frequency chirping can be applied as part of the drive signal to select one or more of the output beams to be blocked. Additionally or alternatively, by selecting a particular phase offset (also known as a phase delay) among the transducers for each single-frequency component, it is possible to reduce the diffraction efficiency of the selected component, and thus significantly reduce the intensity of a particular output beam. Using these approaches, it is possible to vary the number and direction of the output beams while maintaining a constant RF power flow into the device, resulting in a reduction of temperature fluctuations.

[0033] Different phase offsets among the transducers cause the acoustic waves to propagate through the acousto-optic medium at different respective wavefront angles at different frequencies. The wavefront angle at each frequency can be specifically selected to satisfy the Bragg condition, thus achieving maximum diffraction efficiency of a given RF power at this frequency, or to deviate from the Bragg condition, thus reducing the diffraction efficiency. Using this property, the phase offsets can be set so as to compensate for the inherent variation of the diffraction efficiency of the acousto-optic medium with frequency and beam angle, thus equalizing the intensities of the output beams while maintaining a constant input RF power to the acousto-optic modulator. (As with the term "constant", in this context "equalizing" means that the intensities of the output beams differ by no more than 10%, and possibly no more than 5%.)

[0034] In one embodiment, the phase offsets are modified intermittently by a sufficient amount to form a large deviation from the Bragg condition, thus turning off each of the output beams during a particular blocking interval while still maintaining constant input RF power. Additionally or alternatively, this feature can be combined with the frequency chirp described above during the blocking interval.

[0035] More generally, the intentional deviation of the wavefront angle from the Bragg condition can be used to modulate the intensity of the input beam deflected by the acousto-optic medium. In some embodiments, the drive circuit applies respective drive signals to the piezoelectric transducers, the respective drive signals having a frequency selected so as to cause the acousto-optic medium to deflect an input beam (or beams) at a corresponding deflection angle in a range and having a phase offset among the transducers in the array selected so as to modulate the intensity of the deflected beam. Thus, it is possible to modulate the deflected beam intensity (as well as turn on and off the beam) while maintaining constant RF power input to the acousto-optic medium.

[0036] System Description

[0037] Figure 1 is a schematic graphical illustration of a multi-beam deflection system 20 according to an embodiment of the present invention. A radiation source, such as a laser 22, emits a single input beam 23 of pulsed or continuous optical radiation that can include visible, ultraviolet, or infrared radiation. The input beam 23 is incident on an acousto-optic deflector 24, which splits the input beam into a plurality of output beams 30. A drive circuit 28 (also simply referred to as "driver") applies a multi-frequency drive signal to one or more piezoelectric transducers 26, which drive the deflector 24 so as to generate acoustic waves in the acousto-optic medium that split the input beam into the plurality of output beams 30.

[0038] The deflector 24 can include any suitable acousto-optic medium known in the art, including a crystalline material such as quartz, tellurium dioxide (Te02), germanium, or a glass material such as fused silica or a chalcogenide glass. A crystalline medium can be cut along a particular preferred crystal direction to obtain desired acousto-optic properties, e.g., in terms of sound velocity and birefringence. The transducers 26 can similarly include one or more pieces of any suitable piezoelectric material, such as lithium niobate, attached to the acousto-optic medium, typically via a metal bonding layer. Details of the operation of the drive circuit 28 and the drive signals it generates are presented in the following figures and the following description.

[0039] In the illustrated embodiment, scanning mirror 32 scans the output beam 30 within an angle range 38. The beam is focused onto the target surface 36 via scanning lens 34. This type of arrangement can be used in a variety of applications, such as multi-beam laser drilling and printing. The drive signal applied to the transducer by driver 28 is selected such that each of the beams 30 illuminates the target with a given beam intensity during a sequence of pulse intervals, while each of the beams can be blocked during specific corresponding blocking intervals interspersed with the pulse intervals. (As previously stated, "blocked" means that the intensity of the beam on the target is attenuated to less than 50% of the given beam intensity, and typically less than 10%, or in some cases less than 5% or even 1%.) As previously stated, this beam blocking can be achieved by changing the frequency and / or phase of the drive signal while maintaining a constant RF power level. Several types of drive signals that can be used for this purpose are described below.

[0040] Although only a single mirror 32 is shown in this figure, alternative embodiments (not shown) may employ biaxial mirrors that can be scanned together or independently and / or any other suitable type of beam scanner known in this art. In alternative embodiments, two acousto-optic deflectors may be deployed in series, one of which splits the input beam 23 into multiple output beams separated along a first direction, while the other scans the beams in orthogonal directions. All of these embodiments may utilize the various driving schemes described herein and are considered to be within the scope of the invention.

[0041] Pulse selection using chirp spectrum

[0042] Figure 2 This is a schematic graph illustrating the frequency chirp signal applied by the driver 28 to the acousto-optic deflector 24 according to an embodiment of the present invention. Instead of cutting off the RF signal for pulse blocking, this type of chirp signal can be applied during pulse blocking. The chirp in the drive signal is generated from time t... START Extending to time t STOP Within the pulse period, from the initial value f START Increase to the final value f STOP Frequency representation. For example, from f START to f STOP The frequency range can cover all or most of the typical spectral bandwidth of acousto-optic deflectors, ranging from tens of megahertz to hundreds of megahertz. From t START to t STOP The time range can be approximately equal to the sound wave travel time across the diameter of the input beam, typically a few microseconds. This chirp can be applied to a single output beam from deflector 24 or to one or more of a group of multiple output beams 30.

[0043] The chirped signal causes strong defocusing of the beam 23, so that the resulting output beams spread over a large area of the target surface 36. The laser beams will still be directed towards the target surface, with approximately the same total optical power as in the focused output beams, but the intensity will be attenuated by more than 90% and possibly up to 50 dB, depending on the optical configuration. Thus, the laser pulses will essentially have no impact on the target. Alternatively, a weaker chirp with a reduced range of frequencies can be used to defocus the laser beams in order to form large spots on the target surface with sufficient intensity, for example to pre-heat areas of the target surface that will be later irradiated by the focused spots with higher intensity.

[0044] Controlling intensity of multiple output beams

[0045] Figure 3 is a schematic cross-sectional view of an acousto-optic deflector 24 according to an embodiment of the application. This figure illustrates the effect and operation of the multi-frequency drive provided by the drive circuit 28 and the piezoelectric transducer 26. The multi-frequency drive signal from the drive circuit 28 causes the piezoelectric transducer 26 to generate acoustic waves at the multiple drive frequencies, which propagate through the acousto-optic medium in the deflector 24. Each of the different drive frequencies establishes an acousto-optic diffraction grating in the crystal at a corresponding spatial frequency, i.e. the crystal contains multiple superimposed gratings of different spatial frequencies. In the simplified example shown in Figure 3 In the simplified example shown in Fig. 1, the wavefront angles of all the gratings appear to be parallel; but in the embodiments described below, each grating has a different wavefront angle determined by the phase of the drive signal applied by the drive circuit 28.

[0046] When the input beam 23 enters the deflector 24, each of the gratings in the deflector diffracts the input beam at a different angle, depending on the grating frequency. Thus, the deflector 24 splits the input beam 23 into multiple output beams 30a, 30b, 30c, 30d,... at different angles θ1, θ2,... corresponding to the different frequencies f1, f2,.... The optics 34 focus the output beams to form a corresponding array of spots 1, 2,... on the target surface 36. By modulating the spectrum and / or phase of the signal at the corresponding frequencies in proper synchronization with the pulses of the input beam 23, the drive circuit 26 can control the intensity of the corresponding output beam 30 produced by each pulse of the input beam. Additionally or alternatively, the drive circuit 26 can modulate the component frequencies f1, f2,... in order to modulate the corresponding angles θ1, θ2,... and thus change the position of the spots on the surface 36.

[0047] More specifically, the drive circuit 28 can individually turn on and off the beams 30a, 30b, 30c, 30d,... by controlling the phase and / or spectrum of the corresponding frequency components, and can thus select the combination of output beams 30 that will be produced at each pulse. (In the example of Fig. 1, the drive circuit 28 can select the combination of beams 30a, 30b, 30c, 30d,... that will be produced at each pulse by controlling the phase and / or spectrum of the corresponding frequency components f1, f2, f3, f4,....) Figure 1 and 3In the example shown in the middle, beam 30c is turned off. Additionally or alternatively, drive circuit 28 can control the phase of the frequency components so as to compensate for variations in the diffraction efficiency of deflector 24 with angle. Thus, while there are variations in the diffraction efficiency, drive circuit 28 can equalize the intensities of beams 30a, 30b, and 30d, for example, while maintaining a constant RF power input to deflector 24.

[0048] Figure 4 is a schematic cross-sectional view of an acousto-optic deflector 24 according to an embodiment of the application, in which an array of transducers 40 is attached to the acousto-optic medium of the deflector. While transducers 26 are shown as a single block in the preceding figures, in practice, an array of transducers 40 can be used to implement all embodiments of the application in this manner.

[0049] Drive circuit 28 is conceptually illustrated as including a frequency generator 42 that drives transducers 40 through respective phase shifters 44, so that the drive signals are fed to the transducers with different respective phase shifts. Phase adjustment circuit 48 sets the phase shifts of phase shifters 44 depending on the drive frequency and the desired diffraction efficiency at this frequency. Thus, the wavefront of acoustic waves 46 propagating through the acoustic medium of deflector 24 is not parallel to the face of the medium to which transducers 40 are attached.

[0050] For maximum diffraction efficiency, the wavefront angle can be selected, typically through appropriate setting of phase shifters 44, so that the angle θ between input beam 23 and the wavefront satisfies the Bragg condition for a given drive frequency, i.e., sin θ = nλ / 2d, where λ is the wavelength of the input beam, n is the diffraction order (typically n = 1) and d is the wavelength of the acoustic wave at the given frequency. This selection of the wavefront angle improves the efficiency of the diffraction by deflector 24, particularly at frequencies away from f0(the frequency at which the Bragg condition is satisfied by setting the phase difference between adjacent transducers 40 to zero).

[0051] Alternatively, phase adjustment circuit 48 can modulate the diffraction efficiency (and thus the intensity of the generated output beam from deflector 24) by adjusting the wavefront angle to a value that deviates from the Bragg condition by a controlled amount. For example, this approach can be used to compensate for the inherent variations in the diffraction efficiency of the deflector with frequency and deflection angle, and thus to maintain a constant intensity of a deflected beam or beams while driving the deflector at a constant RF power level. Additionally or alternatively, phase adjustment circuit 48 can apply a larger modulation to the phase shift so as to destroy the diffraction efficiency and thus turn off the output beam, without changing the RF power level input to deflector 24, if desired.

[0052] In some embodiments, the drive circuit 28 applies to the piezoelectric transducers 40 respective multi-frequency drive signals having frequency components at a plurality of different frequencies. For each of these frequencies, the Bragg condition yields a different diffraction angle. Thus, in order for the deflector 24 to achieve optimal performance at all frequencies, the phase adjustment circuit 48 drives the phase shifters 44 to apply different phase offsets at each of the transducers 40 for each frequency. Thus, the acoustic waves 46 propagate through the acousto-optic medium at the frequencies with different respective wavefront angles that are selected with respect to the respective Bragg conditions for the corresponding frequencies f1, f2,... and deflection angles θ1, θ2,...

[0053] Figure 5 is a block diagram schematically illustrating the functional components of the drive circuit 28 for the acousto-optic deflector 24 according to an embodiment of the application. The digital components of the drive circuit 28 can typically be implemented in hard-wired or programmable logic, e.g., in a programmable gate array. Although the blocks in Figure 5 are shown as separate components, in practice the functions of these components can be combined in a single logic device. Alternatively, at least some of the digital components of the circuit 28 can be implemented in software running on a computer or a dedicated microprocessor.

[0054] The frequency selection block 50 selects a number of base frequencies f1, f2,... to be applied in driving the deflector 24 in order to produce the output beams 30 with corresponding deflection angles θ1, θ2,.... If the output beam angles are to be scanned laterally (as in the system 20 shown in Figure 1 ), then the block 50 can be programmed to modulate each of these frequencies by up to ±Δf over time, resulting in a modulation of the angle of each beam by up to ±Δθ. Thus, in general, the block 50 produces a sequence of frequency vectors, each vector including a number m base frequency values {f i +δθ i} and a number m deflection angle values {θ i +δf i} to be applied to the deflector 24 at a particular time in order to produce m output beams 30 at corresponding angles {θ i +δθ i and angle changes within the ranges ±Δf and ±Δθ, respectively.

[0055] Phase adjustment block 54 generates multiple streams of time-domain samples corresponding to the frequency components provided by blocks 50 and 54. Each stream is directed to a corresponding one in transducer 40 and contains the same frequency components but has a different corresponding phase offset. These phase offsets are selected according to the desired wavefront angle of the acoustic wave 46 in deflector 24 at each frequency. Typically, the relative phase offset between sample streams is not uniform across the entire frequency range but increases with frequency, such that according to the Bragg condition, the wavefront angle also increases with frequency at each frequency, as explained above.

[0056] Specifically, in order to satisfy the Bragg condition (in the absence of birefringence Bragg diffraction), block 54 can set the phase shift at different frequencies according to the following formula:

[0057]

[0058] In this equation:

[0059] · It is the phase difference between two adjacent output channels of block 54 at frequency f;

[0060] • S is the distance between the centers of adjacent transducers 40;

[0061] ·λ is the wavelength of the light beam;

[0062] ·V s It is the speed of sound in an acousto-optic medium; and

[0063] •f0 is the applied frequency that imparts zero phase difference between adjacent channels and satisfies the Bragg condition for the optical output beam.

[0064] However, in this embodiment, block 54 may set a phase offset at a specific frequency in order to intentionally deviate from the Bragg condition by utilizing the selected phase offset, as explained above.

[0065] As previously described, blocks 50 and 54 are typically implemented in digital logic and / or software. The digital sample stream output from block 54 is input to the corresponding channel of a multi-channel digital-to-analog converter 56, which generates a corresponding output signal to drive transducer 40. (Other analog components, such as the RF amplifier between the D / A converter channel and the transducer, are omitted in the figure for simplicity.) Assuming appropriate selection of frequency components and phase shifts, the transducer will generate superpositions of acoustic waves in deflector 24 at different fundamental frequencies and using different wavefront angles.

[0066] Figure 6 According to an embodiment of the present invention, the diffraction efficiency varies with the phase shift between adjacent transducers 40 driving the acousto-optic deflector 24. And a schematic plot of the resulting diffraction efficiency. Curves 60, 62 and 64 represent the diffraction efficiency at three different drive frequencies, typically in the range of about 50 MHz to 150 MHz. The maximum diffraction efficiency DE max corresponding to the phase shift at which the wavefront angle at the given frequency satisfies the Bragg condition. Even so, the maximum diffraction efficiency does not reach 100%, but varies among curves 60, 62 and 64. Away from this maximum, the diffraction efficiency DE varies approximately in sinusoidal form as a function of the phase shift

[0067]

[0068] Phase adjustment block 54 can apply the above relationship in modulating the intensity of each of the output beams from deflector 24 corresponding to a frequency such as curves 60, 62 and 64. In the case where the RF power added to transducer 40 remains constant, the phase shift required to achieve a given output beam intensity I is given by the inverse cosine of I / I0, where I0is the intensity output at the maximum diffraction efficiency for a given drive frequency f. As explained earlier, the value of at different drive frequencies can be used to compensate for the difference in maximum diffraction efficiency at different frequencies. Alternatively or additionally, to effectively block one of the output beams, one can set to so that the diffraction efficiency will be close to zero. Further alternatively or additionally, one can modulate the intensity of the output beam by modulating the phase shift while keeping the RF input power constant.

[0069] Figure 7 is a schematic plot of a multi-frequency signal applied to an acousto-optic deflector according to another embodiment of the present invention. Similar to the signal shown in Figure 2 , this signal has a chirped spectrum, but in this case comprises a sequence of discrete frequency steps 70. This kind of signal can be conveniently applied by a digital drive circuit, such as the drive circuit shown in Figure 5 , in which the interval in which a given output beam is to be blocked.

[0070] This frequency chirping approach can be advantageously combined with the above-described approach based on adjustment of the phase shift between transducers: the phase is selected at each frequency step 70 so as to cause the sound waves to propagate through the acousto-optic medium with a wavefront angle that deviates from the Bragg condition at the given frequency. For maximum attenuation of the output beam intensity, one can set the phase shift at each frequency to be approximately where is the phase shift at the frequency in question that satisfies the Bragg condition.

[0071] ​It will be appreciated that, by way of example, embodiments described above are referred to and the application is not limited to that specifically shown and described above. Rather, the scope of the application includes both combinations and sub-combinations of the various features described above, as well as variations and modifications thereto which will occur to persons of ordinary skill in the art upon reading the above description and which are not disclosed in the prior art.

Claims

1. An optical apparatus comprising: an acousto-optic medium configured to receive an input radiation beam and to deflect the input beam into at least first and second output beams having respective first and second intensities at respective first and second beam angles, the acousto-optic medium being characterized by different respective first and second diffraction efficiencies at the respective first and second beam angles; an array of multiple piezoelectric transducers attached to the acousto-optic medium; and a drive circuit coupled to apply respective drive signals to the piezoelectric transducers, the respective drive signals including at least first and second drive signals at different corresponding first and second frequencies to direct the first and second output beams at the respective first and second beam angles and having different respective first and second phase offsets for first and second frequency components at each of the multiple piezoelectric transducers, the first and second phase offsets causing acoustic waves to propagate through the acousto-optic medium at different respective first and second wavefront angles at the first and second frequencies, and the drive circuit including a controller configured to select the first and second phase offsets so as to compensate for the different first and second diffraction efficiencies, thereby to equalize the first and second intensities.

2. The apparatus of claim 1, wherein the drive circuit is configured to apply at least the first and second drive signals with the respective first and second phase offsets concurrently to the piezoelectric transducers so that the acousto-optic medium simultaneously deflects the input beam into at least the first and second output beams.

3. The apparatus of claim 1, wherein the controller is configured to vary at least the first and second frequencies of the first and second drive signals so that the acousto-optic medium scans at least the first and second beams over respective first and second angular ranges and varies the respective phase offsets in response to the variations in the frequencies.

4. The apparatus of claim 3, wherein the controller is configured to control the drive signals applied by the drive circuit so that the acousto-optic medium deflects at least the first beam toward a target at a given beam intensity during a series of pulse intervals interspersed with block intervals in which the intensity of the first beam on the target is attenuated to less than 50% of the given beam intensity, wherein the first drive signal has a given amplitude and has a frequency corresponding to a deflection angle of the beam during each pulse interval and has a chirped frequency spectrum during each block interval.

5. The apparatus of claim 1, wherein the first diffraction efficiency is greater than the second diffraction efficiency, and wherein the controller is configured to compensate for the different first and second diffraction efficiencies by setting the second phase offset so that the acoustic waves satisfy a Bragg condition with respect to the input beam at the second frequency while the acoustic waves deviate from the Bragg condition with respect to the input beam at the first frequency.

6. The apparatus of claim 5, wherein the controller is further configured to turn on and off each of the output beams by modifying the respective phase offsets while maintaining a constant power level of the respective drive signals regardless of the respective phase offsets.

7. A method for optical scanning, comprising: directing an input radiation beam to be incident on an acousto-optic medium to which an array of multiple piezoelectric transducers is attached; applying respective drive signals to the piezoelectric transducers, the respective drive signals including at least first and second frequency components at different respective first and second frequencies and having different respective first and second phase offsets for the first and second frequency components at each of the multiple piezoelectric transducers, so as to cause the acousto-optic medium to deflect the input beam into at least first and second output beams having respective first and second intensities at respective first and second beam angles at which the acousto-optic medium is characterized by different respective first and second diffraction efficiencies; and selecting the first and second phase offsets that cause acoustic waves to propagate through the acousto-optic medium at the first and second frequencies at different respective first and second wavefront angles so as to compensate for the different first and second diffraction efficiencies, thereby equalizing the first and second intensities.

8. The method of claim 7, wherein applying the respective drive signals includes concurrently applying at least first and second drive signals having the respective first and second phase offsets to the piezoelectric transducers so that the acousto-optic medium simultaneously deflects the input beam into at least the first and second output beams.

9. The method of claim 7, wherein applying the respective drive signals includes changing at least the first and second frequencies of the first and second drive signals so that the acousto-optic medium scans at least first and second beams over respective first and second angular ranges, and wherein selecting the first and second phase offsets includes changing the respective phase offsets in response to the changing of the frequencies.

10. The method of claim 9, wherein applying the respective drive signals includes controlling the drive signals so that the acousto-optic medium deflects at least the first beam toward a target at a given beam intensity during a series of pulse intervals interspersed with blocking intervals in which the intensity of the first beam on the target is attenuated to less than 50% of the given beam intensity, wherein the first drive signal has a given amplitude and has a frequency corresponding to a deflection angle of the beam during each pulse interval and has a chirped spectrum during each blocking interval.

11. The method of claim 7, wherein the first diffraction efficiency is greater than the second diffraction efficiency, and wherein selecting the first and second phase offsets comprises compensating for the different first and second diffraction efficiencies by setting the second phase offset such that the acoustic wave satisfies a Bragg condition with respect to the input beam at the second frequency while the acoustic wave is off the Bragg condition with respect to the input beam at the first frequency.

12. The method of claim 11, wherein selecting the first and second phase offsets comprises turning on and off each of the output beams by modifying the respective phase offset while maintaining a constant power level of the respective drive signal regardless of the respective phase offset.

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