Optical device and optical scanning method

Abstract

To provide a device and a method that are improved for an acoustic optical deflection.SOLUTION: An optical device includes: an acoustic optical medium 24 that is configured so as to receive input radiation beams, attenuate the intensity of beams on a target to less than 50% of predetermined beam intensity, cause the beams to be spread by a rejection section, and deflect input beams toward a target while having predetermined beam intensity over an angle range for continuous pulse sections; a piezoelectric converter 26 mounted to the acoustic optical medium; and a drive circuit 28 that is coupled so as to apply a drive signal to the piezoelectric converter in which the drive signal has predetermined amplitude, has a frequency corresponding to a deflection angle of beams in respective pulse sections, and has a chirp type frequency spectrum in the respective rejection sections.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates generally to optical devices and systems, and more particularly to acousto-optical devices and methods of operating such devices. [Background technology]

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

[0003] Various types of acousto-optic devices are known in the art. Acousto-optic deflectors, for example, use diffraction of an incident beam to induce the angle of an output beam. The deflection angle of the output beam depends on the period of a grating structure in the acousto-optic material and can therefore be adjusted by appropriately changing the drive signal frequency.

[0004] Some acousto-optic devices use a phased array of transducers to generate acoustic waves within an acousto-optic medium. The transducers are driven with different relative phase delays to control the angle of the acoustic waves propagating through the medium, thus adjusting the phase matching between the acoustic field and the optical beam to be modulated. For example, U.S. Pat. No. 7,538,929 describes a radio frequency (RF) phase modulation technique for intensity modulation of an optical wavefront using an acousto-optic modulator including an acousto-optic bulk medium and transducers attached to the acousto-optic bulk medium and formed as a linear array of electrodes. Transducer drivers are connected to each electrode and coherently phase-driven to alter the angular momentum distribution of the acoustic field and alternately allow or suppress phase matching between the optical and acoustic fields to produce the desired intensity modulation of the optical wavefront.

[0005] The acousto-optic deflector can be driven by a multi-frequency drive signal to diffract an incident beam into multiple output beams at different angles. For example, U.S. Pat. No. 5,890,789 describes a multi-beam emission device that splits a light beam emitted from a light source into multiple beams using an optical waveguide-type acousto-optic element driven by multiple electrical signals having different frequencies. As another example, U.S. Patent Application Publication No. 2009 / 0073544 describes a device for optically splitting and modulating monochromatic coherent electromagnetic radiation, in which an acousto-optic element splits a beam generated by a beam source into several partial beams. An acousto-optic modulator located downstream of the acousto-optic element is fed with the split partial beams and driven by an additional high-frequency electrical signal.

[0006] PCT International Publication No. WO 2016 / 075681, the disclosure of which is incorporated herein by reference, describes another example of the use of a phased array in driving an acousto-optic deflector. 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 drive signals including at least first and second frequency components at different, respective first and second frequencies and having different, respective phase offsets relative to the first and second frequency components in each of the multiple piezoelectric transducers. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] U.S. Patent Application Publication No. 2015 / 0338718 [Patent Document 2] U.S. Patent Application Publication No. 2005 / 0279807 Summary of the Invention [Problem to be solved by the invention]

[0008] SUMMARY OF THE INVENTION Embodiments of the present invention provide improved devices and methods for acousto-optic deflection. [Means for solving the problem]

[0009] According to an embodiment of the present invention, an optical device is provided, the optical device including an acousto-optic medium configured to receive an input beam of radiation and deflect the input beam into at least first and second output beams at respective first and second beam angles and with respective first and second intensities, the output beams 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 drive signals comprising 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 with different, respective first and second phase offsets for the first and second frequency components at each of the multiple piezoelectric transducers, causing acoustic waves to propagate through the acousto-optic medium at the first and second frequencies and with different, respective first and second wavefront angles. The controller is configured to select the first and second phase offsets to compensate for different first and second diffraction efficiencies, thereby equalizing the first and second intensities.

[0010] Typically, the drive circuitry is configured to apply at least first and second drive signals in parallel to the piezoelectric transducer, with first and second phase offsets to each other, so that the acousto-optic medium simultaneously deflects the input beam into at least first and second output beams.

[0011] In some embodiments, the controller is configured to vary at least first and second frequencies of the first and second drive signals and vary their respective phase offsets in response to the varying frequencies so that the acousto-optic medium scans the at least first and second beams over respective first and second angular ranges. In one embodiment, 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 the target with a predetermined beam intensity during successive 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 predetermined beam intensity, the first drive signal having a predetermined amplitude and a frequency corresponding to the deflection angle of the beam during each pulse interval and a chirped frequency spectrum during each blocking interval.

[0012] 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 at the second frequency satisfies the Bragg condition for the input beam and the acoustic wave at the first frequency deviates from the Bragg condition for the input beam. In disclosed embodiments, the controller is further configured to turn each of the output beams on and off by varying the respective phase offsets while maintaining a constant power level of the respective drive signals regardless of the respective phase offsets.

[0013] Also provided according to an embodiment of the present invention is an optical device including an acousto-optic medium configured to receive an input beam of radiation and deflect the input beam toward a target with a predetermined beam intensity over a range of angles during successive pulse intervals interspersed with blocking intervals, during which the intensity of the beam on the target is attenuated to less than 50% of the predetermined 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 predetermined amplitude and a frequency corresponding to the beam deflection angle during each pulse interval and having a chirped frequency spectrum during each blocking interval.

[0014] In some embodiments, the chirped frequency spectrum is chosen so that the intensity of the beam on the target is attenuated to less than 10% of the predetermined beam intensity during the blocking interval.

[0015] Additionally or alternatively, the chirped frequency spectrum includes a series of discrete frequency steps applied during each stop interval. In disclosed embodiments, the at least one piezoelectric transducer comprises an array of multiple piezoelectric transducers, and the drive circuit is configured to apply respective drive signals to the piezoelectric transducers having phases selected to cause acoustic waves to propagate through the acousto-optic medium at wavefront angles that satisfy the Bragg condition for the input beam during the pulse interval and deviate from the Bragg condition for the input beam at each of the discrete frequency steps during the stop interval.

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

[0017] There is also provided a method of optical scanning according to an embodiment of the present invention, the method comprising directing an input beam of radiation to be incident on an acousto-optic medium having an array of a plurality of piezoelectric transducers mounted thereon, wherein respective drive signals are applied to the piezoelectric transducers, the drive signals comprising at least first and second frequency components at different respective first and second frequencies and having different respective first and second phase offsets relative to the first and second frequency components at each of the plurality of piezoelectric transducers, such that the acousto-optic medium deflects the input beam into at least first and second output beams, at respective first and second beam angles, and with respective first and second intensities, characterized by different respective first and second diffraction efficiencies. The first and second phase offsets are selected to cause acoustic waves to propagate through the acousto-optic medium at first and second frequencies with different, respective first and second wavefront angles, thereby compensating for the different first and second diffraction efficiencies and equalizing the first and second intensities.

[0018] There is further provided a method of optical scanning according to an embodiment of the present invention, the method comprising directing an input beam of radiation to be incident on an acousto-optic medium having at least one piezoelectric transducer attached thereto, wherein a drive signal is applied to the at least one piezoelectric transducer, the drive signal having a predetermined amplitude and a frequency corresponding to a deflection angle of the beam during each successive pulse interval, and having a chirped frequency spectrum during each successive stop interval interspersed with the pulse intervals, such that the acousto-optic medium deflects the input beam towards a target with a predetermined beam intensity over a range of angles during each successive pulse interval, and attenuates the intensity of the beam on the target to less than 50% of the predetermined beam intensity during each stop interval.

[0019] There is also provided a method of optical scanning according to an embodiment of the present invention, the method comprising directing an input beam of radiation to be incident on an acousto-optic medium having an array of piezoelectric transducers mounted thereon. Respective drive signals are applied to the piezoelectric transducers, the drive signals having frequencies selected to cause acoustic waves at the selected frequencies to propagate through the acousto-optic medium, thereby causing the acousto-optic medium to deflect the input beam at a corresponding deflection angle. A phase offset between the transducers in the array is set to adjust the wavefront angle of the acoustic waves, thereby modulating the intensity of the deflected beam.

[0020] The present invention will be more fully understood from the following detailed description of the embodiments thereof, taken in conjunction with the drawings in which: [Brief explanation of the drawings]

[0021] [Figure 1] 1 is a schematic, pictorial illustration of a multi-beam deflection system, in accordance with an embodiment of the present invention; [Figure 2] 4 is a schematic graph of a frequency-chirped signal applied to an acousto-optic deflector, according to an embodiment of the present invention; [Figure 3] 1 is a schematic cross-sectional view of an acousto-optic deflector used in generating multiple output beams, according to an embodiment of the present invention. [Figure 4] FIG. 1 is a schematic cross-sectional view of an acousto-optic deflector driven by a phased array of transducers, in accordance with an embodiment of the present invention. [Figure 5] FIG. 2 is a block diagram that schematically illustrates a multi-frequency drive circuit for an acousto-optic deflector, in accordance with an embodiment of the present invention. [Figure 6] 1 is a schematic graph of diffraction efficiency as a function of phase offset between transducers driving an acousto-optic deflector, in accordance with an embodiment of the present invention. [Figure 7] 10 is a schematic graph of a frequency-chirped signal applied to an acousto-optic deflector according to another embodiment of the present invention; DETAILED DESCRIPTION OF THE INVENTION

[0022] Overview 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. For precise control of the laser beam, it is important to maintain careful control of the temperature of the acousto-optic crystal, which can be affected by acoustic absorption in the crystal itself. For example, when uniform temperature changes occur in the crystal, this changes the acoustic velocity (and refractive index) within the crystal, leading to drift in the deflection angle of the laser beam. Non-uniform temperature changes distort the laser beam and cause lensing and other refractive effects.

[0023] However, because acousto-optic modulation and deflection inherently involve changes in the RF signal used to drive the acousto-optic crystal, maintaining a stable temperature in the acousto-optic crystal can be challenging. 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"), and the resulting changes in RF input power can cause temperature fluctuations and unstable thermal behavior within the crystal. Furthermore, steering the laser beam direction by changing the drive frequency applied to the acousto-optic crystal can also lead to temperature changes because different drive frequencies typically require different levels of RF power to achieve the same laser pulse energy, and because the acoustic absorption of the crystal is strongly frequency-dependent.

[0024] The embodiments of the invention described herein provide novel techniques that can be used to keep the temperature of an acousto-optic crystal stable and uniform, despite the challenges inherent in pulse picking and beam steering. These embodiments enable the crystal to steer one or more beams of radiation, alternately passing and blocking the beams while maintaining a constant RF input level to the crystal. The term "constant" in this context means that the instantaneous RF power of the drive signal input to the piezoelectric transducer driving the crystal is maintained within predefined limits, typically varying no more than 10%, and in some cases no more than 5%, during operation of the acousto-optic device, regardless of steering and intermittent blocking of the beam (although larger or smaller limits are possible depending on the application requirements).

[0025] In some embodiments, an acousto-optic medium (typically a suitable crystal) is driven by at least one piezoelectric transducer attached thereto during successive pulse intervals to deflect an input beam toward a target with a predetermined beam intensity over a range of angles. The pulse intervals are interspersed with blocking intervals during 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% for high-sensitivity applications, of the predetermined beam intensity. (The term "intensity" is used in the context of this description and in the claims in its conventional sense to mean optical power per unit area incident on the beam.)

[0026] The drive circuit applies a drive signal to the piezoelectric transducer using a frequency corresponding to the deflection angle of the beam during each pulse interval and using a chirp-shaped frequency spectrum during each block interval. The frequency chirp has a frequency range and duration selected to distribute the input beam over a large area of ​​the target so that the intensity of the beam on the target during the block interval is much lower than the intensity of the deflected beam during the pulse interval. Despite the different frequency spectrum, the drive circuit maintains substantially the same, predetermined amplitude of the drive signal in both the pulse interval and the block interval.

[0027] This technique can be extended to stabilize temperature in multi-frequency operation, where a composite drive signal formed as a superposition of single-frequency drive signals is applied to the acousto-optic medium by an array of piezoelectric transducers. The composite drive signal splits the laser beam into several output beams. A frequency chirp 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 specific phase offset (also called a phase delay) between the transducers for each single-frequency component, it is possible to reduce the diffraction efficiency for the selected component and therefore significantly reduce the intensity of the particular output beam. Using these techniques, the number and direction of output beams can be varied while maintaining a constant RF power flow into the device, resulting in reduced temperature fluctuations.

[0028] Different phase offsets between the transducers cause acoustic waves at different frequencies to propagate through the acousto-optic medium with different, respective wavefront angles. The wavefront angle at each frequency is specifically chosen to satisfy the Bragg condition, thus achieving maximum diffraction efficiency for a given RF power at that frequency, or to deviate from the Bragg condition, thereby reducing the diffraction efficiency. Using this property, the phase offsets can be set to compensate for the inherent variation in the diffraction efficiency of the acousto-optic medium as a function of frequency and beam angle, thus equalizing the intensity of the output beam while maintaining a constant input RF power to the acousto-optic modulator. (In this context, "equal," like the term "constant," means that the intensity of the output beam varies by no more than 10%, and in some cases no more than 5%.)

[0029] In one embodiment, the phase offset is intermittently varied by a sufficient amount to create a large deviation from the Bragg condition, thus turning off each of the output beams for several stop intervals while still maintaining a constant input RF power. Additionally or alternatively, this feature may be combined with the frequency chirp described above during the stop intervals.

[0030] More generally, intentional deviations in wavefront angle from the Bragg condition can be used to modulate the intensity of an input beam deflected by an acousto-optic medium. In some embodiments, a drive circuit applies respective drive signals to the piezoelectric transducers with frequencies selected to cause the acousto-optic medium to deflect the input beam at corresponding deflection angles within a range, and with phase offsets between the transducers in the array selected to modulate the intensity of the deflected beam. In this way, it is possible to modulate the deflected beam intensity (and turn the beam off and on) while maintaining a constant RF power input to the acousto-optic medium.

[0031] System Description 1 is a pictorial schematic diagram of a multi-beam deflection system 20 in accordance with an embodiment of the present invention. A radiation source, such as a laser 22, emits a single input beam 23 of optical radiation, pulsed or continuous, which may comprise visible, ultraviolet, or infrared radiation. The input beam 23 is incident on an acousto-optic deflector 24, which splits the input beam into multiple output beams 30. A drive circuit 28 (also referred to simply as a "driver") applies multi-frequency drive signals to one or more piezoelectric transducers 26, which drive the deflector 24 to generate acoustic waves in the acousto-optic medium that split the input beam into multiple output beams 30.

[0032] Deflector 24 may comprise any suitable acousto-optic medium known in the art, including crystalline materials such as quartz, tellurium dioxide (TeO), germanium, or glass materials such as fused silica or chalcogenide glass. The crystalline medium may be cut along specific, preferred crystal directions to obtain desired acousto-optic properties, e.g., in terms of sound velocity and birefringence. Transducer 26 may similarly comprise one or more of any suitable piezoelectric materials, such as lithium niobate, typically attached to the acousto-optic medium by a metallic bonding layer. Details of the operation of driver circuit 28 and the drive signals it generates are shown in the following figures and in the following description.

[0033] In the illustrated embodiment, a scanning mirror 32 scans the output beam 30 over a range of angles 38. The beam is focused onto a target surface 36 by a scan lens 34. This type of arrangement can be used in a variety of applications, such as multi-beam laser drilling and printing. The drive signals applied to the transducer by the driver 28 are selected so that each of the beams 30 strikes the target with a predetermined beam intensity during a series of pulse intervals, while each beam can be blocked during certain respective blocking intervals interspersed with the pulse intervals. (As previously mentioned, "blocked" means that the beam intensity on the target is attenuated to less than 50%, and typically less than 10%, or in some cases less than 5% or even 1%, of the predetermined beam intensity.) As previously mentioned, this beam blocking can be achieved by varying the frequency and / or phase of the drive signal while maintaining a constant RF power level in the drive signal. Several types of drive signals that can be used for this purpose are described below.

[0034] Although only a single mirror 32 is shown in this figure, alternative embodiments (not shown) may use a two-axis mirror that can be scanned together or independently, and / or any other type of beam scanner known in the art. In an alternative embodiment, two acousto-optic deflectors may be arranged in series, one of which splits the input beam 23 into multiple output beams separated in a first direction, and the other of which scans the beam in an orthogonal direction. All such embodiments may take advantage of the various drive schemes described herein and considered within the scope of the present invention.

[0035] Pulse picking using chirped frequency spectrum 2 is a schematic graph of a frequency chirp signal applied to acousto-optic deflector 24 by driver 28, according to an embodiment of the present invention. Instead of turning 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 START From tSTOP Over a pulse period extending to START to the final value f STOP It is characterized by an increasing frequency up to f START From f STOP The frequency range from t to t can cover all or most of the spectral bandwidth of the acousto-optic deflector, which is typically on the order of tens to hundreds of megahertz. START From t STOP The time range from can be approximately equal to the travel time of an acoustic wave across the diameter of the input beam, which is typically on the order of a few microseconds. Such chirps can be applied to a single output beam from the deflector 24 or to one or more of a set of multiple output beams 30.

[0036] The chirp signal causes a strong defocusing of the beam 23, causing the resulting output beam to be dispersed over a large area on the target surface 36. The laser beam is still directed toward the target surface with approximately the same total optical power as in the focused output beam, but the intensity is attenuated by more than 90% and in some cases as much as 50 dB, depending on the optical configuration. As a result, the laser pulse has virtually no effect on the target. Alternatively, a weaker chirp having a reduced frequency range can be used to defocus the laser beam to form a large spot on the target surface with sufficient intensity to preheat an area of ​​the target surface that will later be irradiated with higher intensity by the focused spot.

[0037] Controlling the intensity of multiple output beams FIG. 3 is a schematic cross-sectional view of an acousto-optic deflector 24 according to an embodiment of the present invention. This diagram illustrates the effect and operation of 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 multiple drive frequencies, which propagate through the acousto-optic medium within the deflector 24. Each of the different drive frequencies establishes an acousto-optic diffraction grating within the crystal at a corresponding spatial frequency; i.e., the crystal contains multiple superimposed gratings of different spatial frequencies. While in the simplified example shown in FIG. 3, the wavefront angles of all the gratings appear parallel, in the embodiments described below, each grating has a different wavefront angle, which is determined by the phase of the drive signal applied by the drive circuit 28.

[0038] When input beam 23 enters deflector 24, each of the gratings within the deflector diffracts the input beam at a different angle depending on the grating frequency. Deflector 24 therefore splits input beam 23 into multiple output beams 30a, 30b, 30c, 30d, etc. at different angles θ1, θ2, etc. corresponding to different frequencies f1, f2, etc. Optical element 34 focuses the output beams to form an array of corresponding spots 1, 2, etc. on target surface 36. By modulating the frequency spectrum and / or the phase of the signals at the corresponding frequencies in appropriate synchronization with the pulses of input beam 23, driver circuit 26 can control the intensity of the corresponding output beam 30 generated by each pulse of the input beam. Additionally or alternatively, driver circuit 26 can modulate the component frequencies f1, f2, etc. to modulate the corresponding angles θ1, θ2, etc., and thus change the position of the spots on surface 36.

[0039] More specifically, driver circuitry 28 can individually turn beams 30a, 30b, 30c, 30d, etc. on and off by controlling the phase and / or frequency spectrum of the corresponding frequency components, and thus can select the combination of output beams 30 to generate in each pulse. (In the example shown in FIGS. 1 and 3, beam 30c is turned off.) Additionally or alternatively, driver circuitry 28 can control the phase of the frequency components to compensate for variations in the diffraction efficiency of deflector 24 as a function of angle. Thus, driver circuitry 28 can, for example, equalize the intensities of beams 30a, 30b, and 30d while maintaining a constant RF power input to deflector 24, regardless of variations in diffraction efficiency.

[0040] 4 is a schematic cross-sectional view of an acousto-optic deflector 24 having a phased array of transducers 40 attached to the deflector's acousto-optic medium, according to an embodiment of the present invention. While the transducer 26 has been shown as a single block in the previous figures, in fact all embodiments of the present invention can be implemented in this manner using an array of transducers 40.

[0041] The drive circuit 28 is conceptually illustrated as comprising a frequency generator 42 that drives the transducers 40 through respective phase shifters 44 such that drive signals are supplied to the transducers with different, respective phase offsets. A phase adjustment circuit 48 sets the phase offsets of the phase shifters 44 according to the drive frequency and the desired diffraction efficiency at that frequency. As a result, the wavefront of the acoustic wave 46 propagating through the acoustic medium of the deflector 24 is not parallel to the plane of the medium in which the transducers 40 are mounted.

[0042] For maximum diffraction efficiency, the wavefront angle can be chosen by appropriate setting of the phase shifter 44 so that the angle θ between the input beam 23 and the wavefront satisfies the Bragg condition for a given drive frequency: sinθ=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 choice of wavefront angle increases the efficiency of diffraction by the deflector 24, especially at frequencies away from f0 (the frequency at which the Bragg condition can be satisfied by setting the phase difference between adjacent transducers 40 to zero).

[0043] Alternatively, the phase adjustment circuit 48 may modulate the diffraction efficiency (and thus the intensity of the resulting output beam from the deflector 24) by adjusting the wavefront angle to a value that deviates from the Bragg condition by a controlled amount. This technique may be used, for example, to compensate for inherent variations in the diffraction efficiency of a deflector as a function of frequency and deflection angle, and thus maintain a constant intensity of the deflected beam while driving the deflector with a constant level of RF power. Additionally or alternatively, the phase adjustment circuit 48 may apply a larger modulation to the phase offset to the detriment of the diffraction efficiency, and thus turn off the output beam as needed, without changing the level of RF power input to the deflector 24.

[0044] In some embodiments, the drive circuit 28 applies respective multi-frequency drive signals to the piezoelectric transducers 40, each having frequency components at a plurality of different frequencies. For each of these frequencies, a Bragg condition results at a different diffraction angle. Therefore, for optimal performance of the deflector 24 at all frequencies, the phase adjustment circuit 48 drives the phase shifters 44 to apply a different phase offset to each frequency in each of the transducers 40. As a result, acoustic waves 46 at those frequencies propagate through the acousto-optic medium with different, respective wavefront angles selected for the respective Bragg conditions for the corresponding frequencies f, f, ... and deflection angles θ, θ, ... of the output beam 30.

[0045] FIG. 5 is a block diagram that schematically illustrates functional components of a driver circuit 28 for an acousto-optic deflector 24, in accordance with an embodiment of the present invention. The digital components of driver circuit 28 may typically be hardwired or implemented in programmable logic circuitry, such as in a programmable gate array. While the blocks in FIG. 5 are shown as separate components for conceptual clarity, in practice the functionality of these components may be combined into a single logic device. Alternatively, at least some of the digital components of circuit 28 may be implemented in software running on a computer or dedicated microprocessor.

[0046] A frequency selection block 50 selects several fundamental frequencies f1, f2, ... to be applied to the drive deflector 24 to generate output beams 30 having corresponding deflection angles θ1, θ2, .... If the output beam angle is to be scanned laterally (as in the system 20 shown in FIG. 1), block 50 can be programmed to modulate each of these frequencies over time by an amount of ±Δf, resulting in an angular scan of each beam by ±Δθ. Thus, typically, block 50 generates a series of frequency vectors, each vector containing m fundamental frequency values ​​{f i +δf i}, which direct the m output beams 30 at corresponding angles {θ i +δθ i}, and δf i and δθ i are the frequency and angle changes within the ranges ±Δf and ±Δθ, respectively.

[0047] 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 respective one of transducers 40 and contains the same frequency components, but has a different, respective phase offset. These phase offsets are chosen according to the desired wavefront angle of acoustic wave 46 at deflector 24 at each frequency. Typically, the relative phase offset between the sample streams is not uniform across the frequency range but increases with frequency, so that, according to the Bragg condition at each frequency, the wavefront angle also increases with frequency, as explained above.

[0048] Specifically, to satisfy the Bragg condition (in the absence of birefringence Bragg diffraction), block 54 may set the phase offsets at different frequencies according to the following equation:

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[0049] As previously mentioned, blocks 50 and 54 are typically implemented in digital logic and / or software. The digital sample stream output by block 54 is input to respective channels of a multi-channel digital-to-analog converter 56, which generates corresponding output signals to drive transducer 40. (Other analog components, such as RF amplifiers between the D / A converter channels and the transducer, have been omitted for simplicity.) Given appropriate selection of frequency components and phase offsets, the transducer will generate a superposition of acoustic waves in deflector 24 at different fundamental frequencies and with different wavefront angles.

[0050] FIG. 6 illustrates a phase offset between adjacent transducers 44 driving an acousto-optic deflector 24, in accordance with an embodiment of the present invention.

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number

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[0051] Phase adjustment block 54 can apply the above relationships in modulating the intensities of each of the output beams from deflector 24, for example, corresponding to the frequencies of curves 60, 62, and 64. Assuming that the RF power applied to transducer 40 is held constant, the phase offset required to achieve a given output beam intensity I is

number

number

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[0052] Figure 7 is a schematic graph of a multi-frequency signal applied to an acousto-optic deflector according to another embodiment of the present invention. This signal has a chirped frequency spectrum similar to that shown in Figure 2, but in this case comprises a series of discrete frequency steps 70. This type of signal may be conveniently applied by a digital driver circuit, such as the driver circuit shown in Figure 5, during intervals during which a given output beam is to be blocked.

[0053] This frequency chirp technique can be advantageously combined with the technique based on adjusting the phase offset between transducers described above. The phase is selected at each frequency step 70 to cause the acoustic wave to propagate through the acousto-optic medium at a wavefront angle that deviates from the Bragg condition at the given frequency. For maximum attenuation of the output beam intensity, the phase offset at each frequency should be approximately

number

number

[0054] It will be understood that the above-described embodiments are described by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as modifications and variations thereof which may occur to those skilled in the art upon reading the above description and which are not disclosed in the prior art.

Claims

1. 1. An optical device, comprising: an acousto-optic medium configured to receive an input beam of radiation and deflect the input beam toward the target with a predetermined beam intensity over a range of angles during successive pulse intervals separated by blocking intervals, wherein the intensity of the beam on the target is attenuated to less than 50% of the predetermined beam intensity; at least one piezoelectric transducer attached to the acousto-optic medium; a drive circuit coupled to apply a drive signal to the at least one piezoelectric transducer, the drive signal having a predetermined amplitude and a frequency during each pulse interval corresponding to a deflection angle of the beam and a chirp-shaped frequency spectrum during each stop interval, the chirp-shaped frequency spectrum being characterized by a frequency that increases from an initial value fSTART to a final value fSTOP over a period extending from time tSTART to time tSTOP, the range of frequencies from the initial value fSTART to the final value fSTOP covering all or most of the spectral bandwidth of the acousto-optic medium, and the period from time tSTART to time tSTOP equaling a travel time of an acoustic wave across a diameter of the input beam; An optical device comprising:

2. 2. The apparatus of claim 1, wherein the chirped frequency spectrum is selected such that the intensity of the beam on the target is attenuated to less than 10% of the predetermined beam intensity during the blocking interval.

3. 2. The apparatus of claim 1, wherein the chirped frequency spectrum comprises a series of discrete frequency steps applied between each stop interval.

4. 4. The apparatus of claim 3, wherein the at least one piezoelectric transducer comprises an array of piezoelectric transducers, and the drive circuit is configured to apply respective drive signals to the piezoelectric transducers having phases selected to cause acoustic waves to propagate through the acousto-optic medium at wavefront angles that satisfy the Bragg condition for the input beam during the pulse interval and deviate from the Bragg condition for the input beam at each of the discrete frequency steps during the stop interval.

5. 1. A method of optical scanning, comprising: directing an input beam of radiation so that it is incident on an acousto-optic medium having at least one piezoelectric transducer mounted thereon; applying a drive signal to the at least one piezoelectric transducer, the drive signal having a predetermined amplitude and a frequency corresponding to a deflection angle of the beam during each successive pulse interval, and having a chirp-shaped frequency spectrum during each successive stop interval separated by the pulse intervals, the chirp-shaped frequency spectrum being characterized by a frequency that increases from an initial value fSTART to a final value fSTOP over a period extending from time tSTART to time tSTOP, the range of frequencies from the initial value fSTART to the final value fSTOP covering all or most of the spectral bandwidth of the acousto-optic medium, and the period from time tSTART to time tSTOP equal to the travel time of an acoustic wave across a diameter of the input beam, thereby causing the acousto-optic medium to deflect the input beam towards a target with a predetermined beam intensity over a range of angles during each successive pulse interval, and attenuating the intensity of the beam at the target to less than 50% of the predetermined beam intensity during each of the stop intervals; A method comprising:

6. 6. The method of claim 5, wherein the chirped frequency spectrum is selected such that the intensity of the beam on the target is attenuated to less than 10% of the predetermined beam intensity during the blocking interval.

7. 6. The method of claim 5, wherein the chirped frequency spectrum comprises a series of discrete frequency steps applied between each stop interval.

8. 8. The method of claim 7, wherein the at least one piezoelectric transducer comprises an array of a plurality of piezoelectric transducers, and applying the drive signal comprises applying respective drive signals to the plurality of piezoelectric transducers having phases selected to cause an acoustic wave to propagate through the acousto-optic medium at a wavefront angle that satisfies the Bragg condition for the input beam during the pulse interval and deviates from the Bragg condition for the input beam at each of the discrete frequency steps during the stop interval.

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