WO2021090305A1

WO2021090305A1 - Thermal stabilization of acousto-optic devices

Info

Publication number: WO2021090305A1
Application number: PCT/IL2020/051078
Authority: WO
Inventors: Oleg ERMAK; Itay Peled
Original assignee: Orbotech Ltd.
Priority date: 2019-11-05
Filing date: 2020-10-05
Publication date: 2021-05-14
Also published as: CN114631054A; KR20220097417A; JP2023500525A; JP7538861B2; JP2024149748A

Abstract

Optical apparatus includes an acousto-optic medium, which is configured to receive an input beam of radiation 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 to the piezoelectric transducers respective drive signals having frequencies selected so as to cause acoustic waves at the selected frequencies to propagate through the acousto-optic medium, which thereby deflects the input beam at corresponding deflection angles within the range, and with phase offsets among the drive signals applied to the transducers in the array selected so as to modulate an intensity of the deflected beam by adjusting a wavefront angle of the acoustic waves.

Description

THERMAL STABILIZATION OF ACOUSTO-OPTIC DEVICES

FIELD OF THE INVENTION

The present invention relates generally to optical devices and systems, and particularly to acousto-optic devices and methods for operating such devices.

BACKGROUND

Acousto-optic devices use sound waves to diffract light. In a typical device of this sort, a transducer, such as 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 certain frequency, and thus creates sound waves in the acousto-optic medium. The expansion and compression of the acousto-optic medium due to the sound waves modulate the local index of refraction and thus create a grating structure within the medium, with a period determined by the frequency of the drive signal. A beam of light that is incident on this grating will thus be diffracted as it passes through the device. Various types of acousto-optic devices are known in the art. Acousto-optic deflectors, for example, use the diffraction of the incident beam to steer the angle of the output beam. The angle of deflection of the output beam depends on the period of the grating structure in the acousto-optic material and may thus be adjusted by appropriately varying the drive signal frequency.

Some acousto-optic devices use a phased array of transducers to create sound waves in the acousto-optic medium. The transducers are driven with different relative phase delays in order to control the angle of the acoustic waves propagating through the acousto-optic medium and thus adjust the phase matching between the acoustic field and the light beam that is to be modulated. For example, U.S. Patent 7,538,929 describes a radio-frequency (RF) phase modulation technique for performing intensity modulation of an optical wavefront using an acousto-optic modulator that includes an acousto-optic bulk medium and a transducer attached to the acousto-optic bulk medium and formed as 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 an acoustic field and alternately allow and inhibit phase matching between the optical and acoustic field to produce a desired intensity modulation of the optical wavefront.

Acousto-optic deflectors may be driven with a multi frequency drive signal in order to diffract the incident beam into multiple output beams at different, respective angles. For example, U.S. Patent 5,890,789 describes a multi-beam emitting device, which splits a light beam emitted from a light source into a plurality of beams using an optical waveguide-type acousto-optic element or the like, driven with a plurality of electric signals with different frequencies. As another example, U.S. Patent Application Publication 2009/0073544 describes a device for the optical splitting and modulation of monochromatic coherent electromagnetic radiation, in which an acousto-optical element splits the beam generated by a beam source into a number of partial beams. An acousto-optical modulator disposed downstream of the acousto-optical element is fed the split partial beams and driven with additional high- frequency electrical signals.

PCT International Publication WO 2016/075681, whose disclosure is incorporated herein by reference, describes a further example of the use of a phased array in driving an acousto-optic deflector. In this publication, optical apparatus 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 to the piezoelectric transducers respective drive signals including at least first and second frequency components at different, respective first and second frequencies and with different, respective phase offsets for the first and second frequency components at each of the multiple piezoelectric transducers.

SUMMARY

Embodiments of the present invention provide improved devices and methods for acousto-optical deflection.

There is therefore provided, in accordance with an embodiment of the invention, optical apparatus, including an acousto-optic medium, which is configured to receive an input beam of radiation and to deflect the input beam into at least first and second output beams with 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 to the piezoelectric transducers 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 with different, respective first and second phase offsets for the first and second frequency components at each of the multiple piezoelectric transducers, which cause acoustic waves at the first and second frequencies to propagate through the acousto-optic medium with different, respective first and second wavefront angles. 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.

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

In some embodiments, 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 to vary the respective phase offsets responsively to the varying frequencies. 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 a target with a given beam intensity during a succession of pulse intervals, which are interspersed with blocking intervals in which an 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 blocking interval.

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 so that the acoustic waves at the second frequency satisfy a Bragg condition with respect to the input beam, while the acoustic waves at the first frequency deviate from the Bragg condition with respect to the input beam. In a disclosed embodiment, the controller is further configured to turn each of the output beams on and off by modifying the respective phase offsets while maintaining a constant power level of the respective drive signals regardless of the respective phase offsets.

There is also provided, in accordance with an embodiment of the invention, optical apparatus, including an acousto optic medium, which is configured to receive an input beam of radiation and to deflect the input beam toward a target with a given beam intensity over a range of angles during a succession of pulse intervals, which are interspersed with blocking intervals in which an 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 to the at least one piezoelectric transducer a drive signal having a given amplitude and having a frequency corresponding to a deflection angle of the beam during each pulse interval and a chirped frequency spectrum during each blocking interval.

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 given beam intensity during the blocking intervals.

Additionally or alternatively, the chirped frequency spectrum includes a sequence of discrete frequency steps, which are applied during each blocking interval. In a disclosed embodiment, the at least one piezoelectric transducer includes an array of multiple piezoelectric transducers, and the drive circuit is configured to apply to the piezoelectric transducers respective drive signals having phases selected so as to cause acoustic waves to propagate through the acousto-optic medium at wavefront angles that satisfy a Bragg condition with respect to the input beam during the pulse intervals, while deviating from the Bragg condition with respect to the input beam at each of the discrete frequency steps during the blocking intervals.

There is additionally provided, in accordance with an embodiment of the invention, optical apparatus, including an acousto-optic medium, which is configured to receive an input beam of radiation 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 to the piezoelectric transducers respective drive signals having frequencies selected so as to cause acoustic waves at the selected frequencies to propagate through the acousto optic medium, which thereby deflects the input beam at corresponding deflection angles within the range, and with phase offsets among the drive signals applied to the transducers in the array selected so as to modulate an intensity of the deflected beam by adjusting a wavefront angle of the acoustic waves.

There is further provided, in accordance with an embodiment of the invention, a method for optical scanning, which includes directing an input beam of radiation to be incident on an acousto-optic medium, to which an array of multiple piezoelectric transducers is attached. Respective drive signals are applied to the piezoelectric transducers, including at least first and second frequency components at different, respective first and second frequencies and with 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 with 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. The first and second phase offsets are selected so as to cause acoustic waves at the first and second frequencies to propagate through the acousto-optic medium 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.

There is moreover provided, in accordance with an embodiment of the invention, a method for optical scanning, which includes directing an input beam of radiation to be incident on an acousto-optic medium, to which at least one piezoelectric transducer is attached. A drive signal is applied to the at least one piezoelectric transducer with a given amplitude and a frequency corresponding to a deflection angle of the beam during each of a succession of pulse intervals and having a chirped frequency spectrum during each of a succession of blocking intervals, which are interspersed with the pulse intervals, so as to cause the acousto-optic medium to deflect the input beam toward a target with a given beam intensity over a range of angles during each of the succession of pulse intervals, and to attenuate an intensity of the beam on the target to less than 50% of the given beam intensity during each of the blocking intervals.

There is furthermore provided, in accordance with an embodiment of the invention, a method for optical scanning, which includes directing an input beam of radiation to be incident on an acousto-optic medium, to which an array of multiple piezoelectric transducers is attached. Respective drive signals are applied to the piezoelectric transducers with frequencies selected so as 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 corresponding deflection angles. Phase offsets among the transducers in the array are set so as to modulate an intensity of the deflected beam by adjusting a wavefront angle of the acoustic waves.

The present invention will be more fully understood from the following detailed description of the embodiments thereof, taken together with the drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a schematic, pictorial illustration of a multi-beam deflection system, in accordance with an embodiment of the present invention;

Fig. 2 is a schematic plot of a frequency-chirped signal applied to an acousto-optic deflector, in accordance with an embodiment of the invention;

Fig. 3 is a schematic sectional view of an acousto optic deflector used in generating multiple output beams, in accordance with an embodiment of the present invention;

Fig. 4 is a schematic sectional view of an acousto optic deflector driven by a phased array of transducers, in accordance with an embodiment of the present invention;

Fig. 5 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;

Fig. 6 is a schematic plot of diffraction efficiency as a function of phase offset among the transducers driving an acousto-optic deflector, in accordance with an embodiment of the invention; and

Fig. 7 is a schematic plot of a frequency-chirped signal applied to an acousto-optic deflector, in accordance with another embodiment of the invention. DETAILED DESCRIPTION OF EMBODIMENTS

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 - 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 may be affected by acoustic absorption in the crystal itself. For example, when a homogenous temperature change occurs in the crystal, it will change the acoustic velocity (and the refractive index) in the crystal, leading to drift of the deflection angle of the laser beam. A non-homogenous temperature change can distort the laser beam, giving rise to lensing and other refractive effects.

Keeping the acousto-optic crystal at a steady temperature is challenging, however, because acousto-optic modulation and deflection inherently involve changes in the RF signals that are used to drive the acousto-optic crystal. For example, the laser pulses transmitted toward a target may be intermittently blocked (an operation referred to as "pulse-picking") by interrupting the RF signal to the crystal; but the resulting changes in the RF input power will cause temperature variations and unstable thermal behavior in the crystal. Furthermore, steering of the laser beam direction by changing the driving frequency that is applied to the acousto-optic crystal can also lead to temperature changes, because different driving 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.

Embodiments of the present invention that are described herein provide novel techniques that can be used to keep the temperature of the acousto-optic crystal steady and homogeneous, notwithstanding the challenges inherent in pulse picking and beam steering. These embodiments enable the crystal to steer one or more beams of radiation, as well as 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 signals that are input to the piezoelectric transducer or transducers driving the crystal are maintained within predefined limits, typically varying by no more than 10% during the operation of the acousto optic device, and possibly no more than 5%, notwithstanding steering and intermittent blocking of the beam or beams (though larger or smaller limits are possible, depending on application requirements).

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

A drive circuit applies a drive signal to the piezoelectric transducer or transducers with a frequency corresponding to the deflection angle of the beam during each pulse interval and with a chirped frequency spectrum during each blocking interval. The frequency chirp has a frequency range and duration chosen so as to cause the input beam to be 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 that of the deflected beam during the pulse intervals. Notwithstanding the different frequency spectra, the drive circuit maintains substantially the same, given amplitude of the drive signals in both the pulse intervals and the blocking intervals.

This approach can be extended to stabilize temperature in multifrequency operation, in which a composite drive signal, formed as a superposition of single-frequency drive signals, is applied to an acousto-optic medium by an array of piezoelectric transducers. The composite drive signal splits the laser beam into several output beams. Frequency chirps 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 specific phase offsets (also referred to as phase delays) among the transducers for each single-frequency component, it is possible to reduce the diffraction efficiency for selected components, and thus to significantly reduce the intensity of the specific output beams. Using these approaches, the numbers and directions of the output beams can be varied while maintaining a constant RF power flow into the device - with a resulting reduction in temperature fluctuations.

The different phase offsets among the transducers cause acoustic waves at the different frequencies to propagate through the acousto-optic medium with different, respective wavefront angles. The wavefront angle at each frequency can be chosen specifically to satisfy the Bragg condition - thus achieving maximal diffraction efficiency for the 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 inherent variations in the diffraction efficiency of the acousto-optic medium as a function of frequency and beam angle, thus equalizing the intensities of the output beams while maintaining constant input RF power to the acousto-optic modulator. ("Equal" in this context, like the term "constant," means that the intensities of the output beams vary by no more than 10%, and possibly no more than 5%.)

In one embodiment, the phase offsets are modified intermittently by a sufficient amount to create a large deviation from the Bragg condition, thus turning each of the output beams off during certain blocking intervals while still maintain constant input RF power. Additionally or alternatively, this feature may be combined with the frequency chirp that is described above during the blocking intervals.

More generally, intentional deviation of the wavefront angle from the Bragg condition can be used to modulate the intensity of an input beam that is deflected by an acousto- optic medium. In some embodiments, a drive circuit applies respective drive signals to the piezoelectric transducers with frequencies selected so as cause the acousto-optic medium to deflect the input beam (or beams) at corresponding deflection angles within the range, and with phase offsets 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 intensities (as well as turning the beams on and off) while maintaining a constant RF power input to the acousto-optic medium.

SYSTEM DESCRIPTION

Fig. 1 is a schematic, pictorial illustration 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. 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 a multi-frequency drive signal to one or more piezoelectric transducers 26, which drive deflector 24 in order to generate acoustic waves in the acousto-optic medium that split the input beam into multiple output beams 30.

Deflector 24 may comprise any suitable acousto-optic medium that is known in the art, including crystalline materials such as quartz, tellurium dioxide (TeCb), germanium, or glass materials such as fused silica or chalcogenide glasses. Crystalline media may be cut along specific, preferred crystal directions to obtain the desired acousto-optic properties, in terms of sound velocity and birefringence, for example. Transducers 26 may similarly comprise one or more pieces of any suitable piezoelectric material, such as lithium niobate, which are typically attached to the acousto-optic medium via a metal bonding layer. Details of the operation of drive circuit 28 and of the drive signals that it generates are presented in the figures that follow and the description below.

In the pictured embodiment, a scanning mirror 32 scans output beams 30 over a range of angles 38. The beams are focused onto a target surface 36 via a scan lens 34. This sort of arrangement can be used in a variety of applications, such as multi-beam laser drilling and printing. The drive signals applied by driver 28 to transducers are chosen so that each of beams 30 impinges on a target with a given beam intensity during a sequence of pulse intervals, while each of the beams may be blocked during certain, respective blocking intervals, which are interspersed with the pulse intervals. (As noted earlier, "blocked" means that the intensity of the beam on the target is attenuated to less than 50%, and typically less than 10%, of the given beam intensity, or in some cases less than 5% or even 1%.) As noted earlier, this beam blocking can be accomplished by varying the frequencies and/or phases of the drive signals, while maintaining a constant RF power level in the drive signals. A number of types of drive signals that can be used for this purpose are described below.

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

PULSE PICKING USING A CHIRPED FREQUENCY SPECTRUM

Fig. 2 is a schematic plot of a frequency-chirped signal applied by driver 28 to acousto-optic deflector 24, in accordance with an embodiment of the invention. Instead of cutting off the RF signal for pulse-blocking, this sort of chirped signal can be applied during pulse-blocking. The chirp in the drive signal is characterized by a frequency that increases from an initial value, f_ST_ART, to a final value, fSTOP, over a pulse period extending from time tST_ART to ts_Top. For example, the range of frequencies from fST_ART to fs_Top may cover all or a large part of the spectral bandwidth of the acousto-optic deflector, which is typically on the order of tens to hundreds of Megahertz. The time range from t_ST_ART to ts_To_p can be roughly equal to the transit time of acoustic waves across the diameter of the input beam, which is typically on the order of several microseconds. Such a chirp can be applied to a single output beam from deflector 24 or to one or more of a set of multiple output beams 30.

The chirp signal causes strong defocusing of beam 23, so that the resulting output beam is spread across a large area on target surface 36. The laser beam will still be directed towards the target surface, with roughly the same total optical power as in the focused output beam, but the intensity will be attenuated by more than 90%, and possible by as much as 50 dB, depending on the optical configuration. Consequently, the laser pulse will have essentially no effect on the target. Alternatively, a weaker chirp, with a reduced range of frequencies, may be used to defocus the laser beam to form a large spot on the target surface, with sufficient intensity, for example, to pre-heat an area of the target surface that will afterwards be irradiated with higher intensity by a focused spot.

CONTROLLING INTENSITY OF MULTIPLE OUTPUT BEAMS

Fig. 3 is a schematic sectional view of acousto-optic deflector 24, in accordance with an embodiment of the present invention. This figure illustrates the effect and operation of a multi-frequency drive provided by drive circuit 28 and piezoelectric transducer 26. The multi-frequency drive signal from drive circuit 28 causes piezoelectric transducer 26 to generate acoustic waves at the multiple drive frequencies, which propagate through the acousto-optic medium in 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 superposed gratings of different spatial frequencies. In the simplified example shown in Fig. 3, the wavefront angles of all the gratings appear to be parallel; but in the embodiments described below, each grating has a different wavefront angle, which is determined by the phases of the drive signals applied by drive circuit 28. When input beam 23 enters deflector 24, each of the gratings in the deflector diffracts the input beam at a different angle, depending on the grating frequency. Thus, deflector 24 splits input beam 23 into multiple output beams 30a, 30b, 30c, 30d, ..., at different angles qi, 0₂, ..., corresponding to the different frequencies fi, f2, .... Optics 34 focus the output beams to form a corresponding array of spots 1, 2, ..., on target surface 36. By modulating the frequency spectra and/or phases of the signals at the corresponding frequencies, in appropriate synchronization with the pulses of input beam 23, drive circuit 26 may control the intensity of the corresponding output beams 30 generated by each pulse of the input beam. Additionally or alternatively, drive circuit 26 may modulate the component frequencies fi, f2, ..., in order to modulate the corresponding angles 0i, 02, ..., and thus change the locations of the spots on surface 36.

More particularly, drive circuit 28 may turn beams 30a, 30b, 30c, 30d, ..., on and off individually by controlling the phases and/or frequency spectra of the corresponding frequency components, and may thus choose the combination of output beams 30 to generate at each pulse. (In the example shown in Figs. 1 and 3, beam 30c is turned off.) Additionally or alternatively, drive circuit 28 may control the phases of the frequency components in order to compensate for variations in the diffraction efficiency of deflector 24 as a function of angle. Drive circuit 28 can thus equalize the intensities of beams 30a, 30b and 30d, for example, while maintaining a constant RF power input to deflector 24, notwithstanding the variations in diffraction efficiency. Fig. 4 is a schematic sectional view of acousto-optic deflector 24 with a phased array of transducers 40 attached to the acousto-optic medium of the deflector, in accordance with an embodiment of the present invention. Although transducer 26 is shown in the preceding figures as a unitary block, in practice all embodiments of the present invention may be implemented in this manner, using an array of transducers 40.

Drive circuit 28 is pictured conceptually as comprising a frequency generator 42, which drives transducers 40 through respective phase shifters 44, so that the drive signal is fed to the transducers with different, respective phase offsets. A phase adjustment circuit 48 sets the phase offsets of phase shifters 44 depending upon the drive frequency and the desired diffraction efficiency at this frequency. As a result, the wavefronts of acoustic waves 46 that propagate through the acoustic medium of deflector 24 are not parallel to the face of the medium to which transducers 40 are attached. For maximal diffraction efficiency, the wavefront angle can be chosen, by appropriate setting of phase shifters 44, so that the angle Q between input beam 23 and the wavefront satisfies the Bragg condition for the given drive frequency, i.e., sin0 = nX/2d, wherein l is the wavelength of the input beam, n is the diffraction order (typically n=l), and d is the wavelength of the acoustic waves at the given frequency. This choice of wavefront angle enhances the efficiency of diffraction by deflector 24, particularly at frequencies away from fo (the frequency at which the Bragg condition can be satisfied by setting the phase difference between adjacent transducers 40 to be zero).

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

In some embodiments, drive circuit 28 applies respective multi-frequency drive signals to piezoelectric transducers 40, with frequency components at multiple different frequencies. For each of these frequencies, the Bragg condition results in a different diffraction angle. Therefore, for optimal performance of deflector 24 at all frequencies, phase adjustment circuit 48 drives phase shifters 44 to apply a different phase offset for each frequency at each of transducers 40. Consequently, acoustic waves 46 at the frequencies propagate through the acousto optic medium with different, respective wavefront angles, which are chosen relative to the respective Bragg conditions for the corresponding frequencies fi, f2, ..., ...and deflection angles qi, 0₂, ..., of the output beams 30.

Fig. 5 is a block diagram that schematically illustrates functional components of drive circuit 28 for acousto-optic deflector 24, in accordance with an embodiment of the present invention. The digital components of drive circuit 28 may typically be implemented in hard-wired or programmable logic, such as in a programmable gate array. Although the blocks in Fig. 5 are shown as separate components for the sake of conceptual clarity, in practice the functions of these components may be combined in 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. A frequency selection block 50 selects a number of fundamental frequencies fi, f2, ..., to be applied in driving deflector 24, in order to generate output beams 30 with corresponding deflection angles 0i, 0₂, .... If the output beam angles are to be scanned transversely (as in system 20, shown in Fig. 1), block 50 may be programmed to modulate each of these frequencies over time by an amount up to ±Dί, resulting in angular scanning of each beam by up to ±DQ. Thus, typically, block 50 generates a sequence of frequency vectors, each vector comprising a number m of fundamental frequency values {f_± + 6f_±} that are to be applied to deflector 24 at a particular time in order to generate m output beams 30 at corresponding angles {0_± + dq_±}, wherein 6fi and dq_± are frequency and angle variations within the ranges ±Dί and ±DQ, respectively. A 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 with different, respective phase offsets. These phase offsets are chosen according to the desired wavefront angle of acoustic waves 46 in deflector 24 at each frequency. Typically, the relative phase offsets between the sample streams are not uniform over the entire frequency range, but rather increase with frequency, so that the wavefront angles likewise increase with frequency, according to the Bragg condition at each frequency, as explained above.

Specifically, to satisfy the Bragg condition (in the absence of birefringent Bragg diffraction), block 54 may set the phase offsets at the different frequencies is according to the following formula:

In this equation:

• Df(/) is the phase difference between two adjacent output channels of block 54 at frequency f;

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

• is the optical beam wavelength;

• ¾ is the acoustic velocity in the acousto-optic medium; and • /o is the applied frequency that gives zero phase difference between adjacent channels and satisfies the Bragg condition for the optical output beam.

In the present embodiment, however, block 54 may set the phase offsets at certain frequencies so as to deviate intentionally from the Bragg condition, with a phase deviation that is selected as explained above.

As noted earlier, blocks 50 and 54 are typically implemented in digital logic and/or software. The digital sample streams output by block 54 are input to respective channels of a multi-channel digital/analog converter 56, which generates corresponding output signals to drive transducers 40. (Other analog components, such as RF amplifiers between the D/A converter channels and the transducers, are omitted from the figure for the sake of simplicity.) Assuming appropriate choice of the frequency components and phase offsets, the transducers will generate a superposition of acoustic waves in deflector 24, at different fundamental frequencies and with different wavefront angles.

Fig. 6 is a schematic plot of diffraction efficiency as a function of phase offset Df between adjacent transducers 44 driving acousto-optic deflector 24, in accordance with an embodiment of the invention. Curves 60, 62 and 64 represent the diffraction efficiencies at three different driving frequencies, which are typically in the range of about 50 MHz to 150 MHz. The maximal diffraction efficiency DE_max in each curve corresponds to the phase offset Acp_ma at which the wavefront angle at the given frequency satisfies the Bragg condition. Even so, the maximal diffraction efficiency does not reach 100%, but rather varies among curves 60, 62 and 64. Away from this maximum, the diffraction DE as a function of the phase offset Df varies roughly sinusoidally:

DE = DE_max ·cos(Df — Df_max)

Phase adjustment block 54 can apply the above relation in modulating the intensity of each of the output beams from deflector 24, corresponding, for example, to the frequencies of curves 60, 62 and 64. Assuming the RF power applied to transducers 40 to be held constant, the phase offset Df that is needed to achieve a given output beam intensity I is given by the inverse cosine of T/To, wherein Jo is the intensity output at the maximal diffraction efficiency for the given drive frequency f. As explained earlier, the values of Df at the different drive frequencies can be used to compensate for the differences in the maximal diffraction efficiency at different frequencies. Alternatively or additionally, to effectively block one of the output beams, Df can be set to Dy_thac E Tΐ_I so that the diffraction efficiency will be close to zero. Further alternatively or additionally, the intensities of the output beams can be modulated, while holding the RF input power constant, by modulating the phase offsets.

Fig. 7 is a schematic plot of a multi-frequency signal applied to an acousto-optic deflector, in accordance with another embodiment of the invention. This signal has a chirped frequency spectrum, similar to the signal shown in Fig. 2, but in this case comprises a sequence of discrete frequency steps 70. A signal of this sort can be applied conveniently by a digital drive circuit, such as the drive circuit shown in Fig. 5, during the intervals in which a given output beam is to be blocked.

This frequency chirp approach can be combined advantageously with the approach based on adjustment of the phase offset between the transducers that is described above: The phases are selected in each frequency step 70 so as to cause acoustic waves to propagate through the acousto-optic medium at wavefront angles that deviate from the Bragg condition at the given frequency. For maximal attenuation of the output beam intensity, the phase offset at each frequency can be set to be approximately

+p, wherein cm_ax is the phase offset that satisfies the Bragg condition at the frequency in question.

It will be appreciated that the embodiments described above are cited 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 variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art.

Claims

1. Optical apparatus, comprising: an acousto-optic medium, which is configured to receive an input beam of radiation and to deflect the input beam into at least first and second output beams with 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 attached to the acousto-optic medium; and a drive circuit, which is coupled to apply to the piezoelectric transducers respective 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, which cause acoustic waves at the first and second frequencies to propagate through the acousto-optic medium with different, respective first and second wavefront angles, and which comprises a controller 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.

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

3. The apparatus according to 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 to vary the respective phase offsets responsively to the varying frequencies.

4. The apparatus according to 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 with a given beam intensity during a succession of pulse intervals, which are interspersed with blocking intervals in which an 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 blocking interval.

5. The apparatus according to 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 at the second frequency satisfy a Bragg condition with respect to the input beam, while the acoustic waves at the first frequency deviate from the Bragg condition with respect to the input beam.

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

7. Optical apparatus, comprising: an acousto-optic medium, which is configured to receive an input beam of radiation and to deflect the input beam toward a target with a given beam intensity over a range of angles during a succession of pulse intervals, which are interspersed with blocking intervals in which an intensity of the beam on the target is attenuated to less than 50% of the given beam intensity; at least one piezoelectric transducer attached to the acousto-optic medium; and a drive circuit, which is coupled to apply to the at least one piezoelectric transducer a drive signal having a given amplitude and having a frequency corresponding to a deflection angle of the beam during each pulse interval and a chirped frequency spectrum during each blocking interval.

8. The apparatus according to claim 7, wherein the chirped frequency spectrum is chosen so that the intensity of the beam on the target is attenuated to less than 10% of the given beam intensity during the blocking intervals.

9. The apparatus according to claim 7, wherein the chirped frequency spectrum comprises a sequence of discrete frequency steps, which are applied during each blocking interval.

10. The apparatus according to claim 9, wherein the at least one piezoelectric transducer comprises an array of multiple piezoelectric transducers, and wherein the drive circuit is configured to apply to the piezoelectric transducers respective drive signals having phases selected so as to cause acoustic waves to propagate through the acousto-optic medium at wavefront angles that satisfy a Bragg condition with respect to the input beam during the pulse intervals, while deviating from the Bragg condition with respect to the input beam at each of the discrete frequency steps during the blocking intervals.

11. Optical apparatus, comprising: an acousto-optic medium, which is configured to receive an input beam of radiation and to deflect the input beam toward a target over a range of deflection angles; an array of multiple piezoelectric transducers attached to the acousto-optic medium; and a drive circuit, which is coupled to apply to the piezoelectric transducers respective drive signals having frequencies selected so as to cause acoustic waves at the selected frequencies to propagate through the acousto-optic medium, which thereby deflects the input beam at corresponding deflection angles within the range, and with phase offsets among the drive signals applied to the transducers in the array selected so as to modulate an intensity of the deflected beam by adjusting a wavefront angle of the acoustic waves.

12. A method for optical scanning, comprising: directing an input beam of radiation to be incident on an acousto-optic medium, to which an array of multiple piezoelectric transducers is attached; applying to the piezoelectric transducers respective drive signals comprising at least first and second frequency components at different, respective first and second frequencies and with 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 with 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, which cause acoustic waves at the first and second frequencies to propagate through the acousto-optic medium with 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.

13. The method according to claim 12, wherein applying the respective drive signals comprises applying at least the first and second drive signals, with the respective phase first and second phase offsets, to the piezoelectric transducers concurrently, so that the acousto-optic medium deflects the input beam into at least the first and second output beams simultaneously.

14. The method according to claim 12, wherein applying the respective drive signals comprises varying 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 wherein selecting the first and second phase offsets comprises varying the respective phase offsets responsively to the varying frequencies.

15. The method according to claim 14, wherein applying the respective drive signals comprises controlling the drive signals so that the acousto-optic medium deflects at least the first beam toward a target with a given beam intensity during a succession of pulse intervals, which are interspersed with blocking intervals in which an 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 blocking interval.

16. The method according to claim 12, 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 so that the acoustic waves at the second frequency satisfy a Bragg condition with respect to the input beam, while the acoustic waves at the first frequency deviate from the Bragg condition with respect to the input beam.

17. The method according to claim 16, wherein selecting the first and second phase offsets comprises turning each of the output beams on and off by modifying the respective phase offsets while maintaining a constant power level of the respective drive signals regardless of the respective phase offsets.

18. A method for optical scanning, comprising: directing an input beam of radiation to be incident on an acousto-optic medium, to which at least one piezoelectric transducer is attached; and applying to the at least one piezoelectric transducer a drive signal having a given amplitude and having a frequency corresponding to a deflection angle of the beam during each of a succession of pulse intervals and having a chirped frequency spectrum during each of a succession of blocking intervals, which are interspersed with the pulse intervals, so as to cause the acousto-optic medium to deflect the input beam toward a target with a given beam intensity over a range of angles during each of the succession of pulse intervals, and to attenuate an intensity of the beam on the target to less than 50% of the given beam intensity during each of the blocking intervals.

19. The method according to claim 18, wherein the chirped frequency spectrum is chosen so that the intensity of the beam on the target is attenuated to less than 10% of the given beam intensity during the blocking intervals.

20. The method according to claim 18, wherein the chirped frequency spectrum comprises a sequence of discrete frequency steps, which are applied during each blocking interval.

21. The method according to claim 20, wherein the at least one piezoelectric transducer comprises an array of multiple piezoelectric transducers, and wherein applying the drive signal comprises applying to the multiple piezoelectric transducers respective drive signals having phases selected so as to cause acoustic waves to propagate through the acousto-optic medium at wavefront angles that satisfy a Bragg condition with respect to the input beam during the pulse intervals, while deviating from the Bragg condition with respect to the input beam at each of the discrete frequency steps during the blocking intervals.

22. A method for optical scanning, comprising: directing an input beam of radiation to be incident on an acousto-optic medium, to which an array of multiple piezoelectric transducers is attached; applying to the piezoelectric transducers respective drive signals having frequencies selected so as 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 corresponding deflection angles; and setting phase offsets among the transducers in the array selected so as to modulate an intensity of the deflected beam by adjusting a wavefront angle of the acoustic waves.