CN114631054A - Thermal stabilization of acousto-optic devices - Google Patents

Abstract

An optical apparatus includes 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. Drive circuitry is coupled to apply respective drive signals to the piezoelectric transducers, the frequencies of the respective drive signals being selected so as to cause acoustic waves to propagate through the acousto-optic medium at selected frequencies, the acousto-optic medium thereby deflecting the input beam at corresponding deflection angles within the range, and wherein phase offsets among the drive signals applied to the transducers in the array are selected so as to modulate the intensity of deflected beams by adjusting the wavefront angles of the acoustic waves.

Description

Thermal stabilization of acousto-optic devices

technical field

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

Background technique

Acousto-optic devices use sound waves to diffract light. In a typical device of this kind, a transducer (eg 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 specific frequency and thus form sound waves in the acousto-optic medium. The expansion and compression of the acousto-optic medium due to sound waves modulates the local refractive index and thus forms a grating structure within the medium with a period determined by the frequency of the drive signal. A beam of light incident on this grating will therefore diffract as it passes through the device.

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

Some acousto-optic devices use phased arrays of transducers to create sound waves in an acousto-optic medium. The transducers are driven with different relative phase delays in order to control the angle at which the acoustic wave propagates through the acousto-optic medium and thus adjust the phase matching between the acoustic field and the light beam to be modulated. For example, US Pat. No. 7,538,929 describes radio frequency (RF) phase modulation techniques for performing intensity modulation of an optical wavefront using an acousto-optic modulator comprising an acousto-optic bulk medium and attached to the acousto-optic modulator. A transducer that is optically bulk dielectric and formed as a linear electrode array. A transducer driver is connected to each electrode and is coherently phase driven to alter the angular momentum distribution of the sound field and alternately enable and disable phase matching between the light and sound fields to produce the desired intensity modulation of the optical wavefront.

The acousto-optic deflector may be driven with a multi-frequency drive signal to diffract the incident beam into a plurality of output beams at different respective angles. For example, US Pat. No. 5,890,789 describes a multi-beam emitting device that splits a beam emitted from a light source using optical waveguide-type acousto-optic elements or the like driven with a plurality of electrical signals having different frequencies into multiple beams. As another example, US Patent Application Publication 2009/0073544 describes an apparatus for optical splitting and modulation of monochromatic coherent electromagnetic radiation, wherein an acousto-optic element splits a beam generated by a beam source into several partial beams . The acousto-optic modulator arranged downstream of the acousto-optic element is fed with the split partial beam and driven with an additional high frequency electrical signal.

Yet another example of the use of phased arrays in driving acousto-optic deflectors is described in PCT International Publication WO 2016/075681, the disclosure of which is incorporated herein by reference. In this disclosure, an optical device includes an acousto-optic medium and an array of multiple piezoelectric transducers attached to the acousto-optic medium. Drive circuits are coupled to apply respective drive signals to the piezoelectric transducers, the respective drive signals including at least first and second frequency components at different respective first and second frequencies and at the plurality of voltage Each of the electrical transducers has different respective phase offsets for the first and second frequency components.

SUMMARY OF THE INVENTION

Embodiments of the present invention provide improved apparatus and methods for acousto-optic deflection.

Accordingly, in accordance with an embodiment of the present invention, there is provided an optical apparatus comprising an acousto-optic medium configured to receive an input radiation beam and to deflect the input beam into respective first and second At least first and second output beams having respective first and second intensities at beam angles at which the acousto-optic medium is composed of different respective first and second beam angles. Characterization of the second diffraction efficiency. An array of multiple piezoelectric transducers is attached to the acousto-optic medium. Drive circuits are coupled to apply respective drive signals to the piezoelectric transducers, the respective drive signals including at least first and second drive signals, the first and second drive signals being at different corresponding first and second drive signals Two frequencies to direct the first and second output beams at the respective first and second beam angles and for the first and second at each of the plurality of piezoelectric transducers The frequency components have different respective first and second phase offsets that cause the acoustic waves to propagate at different respective first and second wavefront angles at the first and second frequencies through the acousto-optic medium. A controller is configured to select the first and second phase offsets to compensate for the different first and second diffraction efficiencies, thereby equalizing the first and second intensities.

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

In some embodiments, the controller is configured to vary at least the first and second frequencies of the first and second drive signals such that the acousto-optic medium causes at least the first and second beams The respective first and second angular ranges are scanned, and the respective phase offsets are changed in response to changes in the frequency. In one embodiment, the controller is configured to control the drive signal applied by the drive circuit such that the acousto-optic medium causes at least the first beam to emit at a given radiation during a series of pulse intervals the beam intensity is deflected towards the target, the pulse interval being interspersed with blocking intervals in which the intensity of the first beam on the target decays to less than 50% of the given beam intensity, wherein the first drive signal has a given amplitude and a frequency corresponding to the deflection angle of the beam during each pulse interval and a chirp 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 set the second phase offset such that the acoustic wave satisfies the second frequency at the second frequency Deviating from the Bragg condition for the input beam with respect to the sound wave at the first frequency compensates for the different first and second diffraction efficiencies. In the disclosed embodiment, the controller is further configured to switch on and off by modifying the respective phase offset while maintaining a constant power level of the respective drive signal regardless of the respective phase offset each of the output beams.

In accordance with an embodiment of the present invention, there is also provided an optical apparatus comprising an acousto-optic medium configured to receive an input radiation beam and subject the input beam to a range of angles during a series of pulse intervals is deflected towards the target at a given beam intensity, the pulse interval is interspersed with blocking intervals in which the beam intensity at the target decays to less than the given beam intensity 50%. At least one piezoelectric transducer is attached to the acousto-optic medium. A drive circuit is coupled to apply a drive signal to the at least one piezoelectric transducer, the drive signal having a given amplitude and a frequency corresponding to the deflection angle of the beam during each pulse interval and at each pulse interval There is a chirp spectrum during the blocking interval.

In some embodiments, the chirp spectrum is selected such that during the blocking interval, the intensity of the beam on the target decays to less than 10% of the given beam intensity.

Additionally or alternatively, the chirped spectrum includes a sequence of discrete frequency steps applied during each blocking interval. In disclosed embodiments, the at least one piezoelectric transducer includes an array of a plurality of piezoelectric transducers, and the drive circuit is configured to apply respective drive signals to the piezoelectric transducers, the respective drive signals have phases selected so as to cause acoustic waves to satisfy the Bragg condition for the input beam during the pulse interval and at each of the discrete frequency steps during the blocking interval A wavefront angle that deviates from the Bragg condition with respect to the input beam propagates through the acousto-optic medium.

Additionally, in accordance with embodiments of the present invention, there is provided an optical device comprising an acousto-optic medium configured to receive an input radiation beam and deflect the input beam toward a target within a range of deflection angles . An array of multiple piezoelectric transducers is attached to the acousto-optic medium. Drive circuits are coupled to apply respective drive signals to the piezoelectric transducers, frequencies of the respective drive signals selected to cause acoustic waves to propagate through the acousto-optic medium at the selected frequencies, the acousto-optical The medium thereby deflects the input beam at corresponding deflection angles within the range, and wherein the phase offset among the drive signals applied to the transducers in the array is selected for adjustment by The intensity of the deflected beam is modulated by the wavefront angle of the acoustic wave.

Furthermore, in accordance with an embodiment of the present invention, there is provided a method for optical scanning comprising directing an input radiation beam incident on an acousto-optic medium to which an array of a plurality of piezoelectric transducers is attached . applying respective drive signals to the piezoelectric transducers, the respective drive signals including at least first and second frequency components at different respective first and second frequencies and in the plurality of piezoelectric transducers have different respective first and second phase offsets for the first and second frequency components at each of , so as to cause the acousto-optic medium to deflect the input beam to be at the respective first and second At least first and second output beams having respective first and second intensities at beam angles at which the acousto-optic medium is composed of different respective first and second beam angles. Characterization of the second diffraction efficiency. The first and second phase offsets are selected to cause acoustic waves to propagate through the acousto-optic medium at different respective first and second wavefront angles at the first and second frequencies, thereby compensating for the Different first and second diffraction efficiencies and equalizing the first and second intensities.

Furthermore, in accordance with an embodiment of the present invention, there is provided a method for optical scanning comprising directing an input radiation beam 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, the drive signal having a given amplitude and having a frequency corresponding to the deflection angle of the beam during each of a series of pulse intervals and during a series of having a chirped spectrum during each of the blocking intervals interspersed with the pulse intervals such that the acousto-optic medium causes the input beam during each of the series of pulse intervals The beam is deflected towards a target at a given beam intensity over a range of angles, and the intensity of the beam on the target is attenuated during each of the blocking intervals to a different magnitude of the given beam intensity. to 50%.

Furthermore, in accordance with an embodiment of the present invention, there is provided a method for optical scanning comprising directing an input radiation beam incident on an acousto-optic medium to which an array of a plurality of piezoelectric transducers is attached . A respective drive signal is applied to the piezoelectric transducer, the frequency of the respective drive signal is selected so as to cause acoustic waves to propagate through the acousto-optic medium at the selected frequency, thereby causing the acousto-optic medium to The input beams are deflected at corresponding deflection angles. The phase offset among the transducers in the array is set so as to modulate the intensity of the deflected beam by adjusting the wavefront angle of the acoustic wave. The invention will be more fully understood from the following detailed description of embodiments in accordance with the invention, taken in conjunction with the accompanying drawings, in which:

Description of drawings

FIG. 1 is a schematic pictorial illustration of a multi-beam deflection system according to an embodiment of the present invention;

2 is a schematic graph of a frequency chirped signal applied to an acousto-optic deflector according to an embodiment of the present invention;

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

4 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;

5 is a block diagram schematically illustrating a multi-frequency drive circuit for an acousto-optic deflector according to an embodiment of the present invention;

6 is a schematic graph of diffraction efficiency as a function of phase shift among transducers driving an acousto-optic deflector according to an embodiment of the present invention; and

7 is a schematic graph of a frequency chirped signal applied to an acousto-optic deflector according to another embodiment of the present invention.

Detailed ways

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 a uniform temperature change occurs in the crystal, it will change the speed of sound (and the refractive index) in the crystal, causing the deflection angle of the laser beam to drift. Non-uniform temperature changes can distort the laser beam, causing lensing and other refractive effects.

However, maintaining acousto-optic crystals at stable temperatures is challenging because acousto-optic modulation and deflection inherently involve changes in the RF signal used to drive the acousto-optic crystal. For example, a laser pulse delivered towards the target can be intermittently blocked by interrupting the RF signal to the crystal (an operation known as "pulse picking"); but the resulting change in RF input power will result in a temperature change in the crystal and unstable thermal behavior. In addition, manipulation of the laser beam direction by changing the drive frequency applied to the acousto-optic crystal can also lead to temperature changes, since different drive frequencies typically require different RF power levels to achieve the same laser pulse energy, and because the acoustic absorption of the crystals is strongly dependent on frequency.

Embodiments of the invention described herein provide novel techniques for keeping the temperature of acousto-optic crystals stable and uniform despite inherent challenges in pulse picking and beam steering. These embodiments enable the crystal to steer one or more radiation beams and alternately pass and block the beams while maintaining a constant RF input level to the crystal. In this context, the term "constant" means the drive input to a piezoelectric transducer or transducers driving the crystal despite the manipulation and intermittent blocking of the beam or beams The instantaneous RF power of the signal remains within predefined limits during operation of the acousto-optic device, typically changing by no more than 10%, and possibly by no more than 5% (but larger or smaller limits are possible depending on application requirements) .

In some embodiments, the acousto-optic medium (usually a suitable crystal) is driven by at least one piezoelectric transducer attached to it during a series of pulse intervals such that the input beam is at a given beam intensity within a Deflection towards the target within the angular range. The pulse interval is interspersed with the blocking interval where the intensity of the beam on the target decays to less than 50% of the given beam intensity, or possibly less than 10%, less than 5%, or even in high sensitivity applications less than 1%. (The term "intensity" is used in its conventional sense in the context of this specification and the claims to mean the optical power per unit area upon which a beam is incident.)

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

This approach can be extended to temperature stabilization in multi-frequency operation where a composite drive signal formed as a superposition of a single frequency drive signal 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 for each single frequency component a specific phase offset (also referred to as a phase delay) among the transducers, it is possible to reduce the diffraction efficiency of the selected component, and thus significantly reduce the diffraction efficiency of a specific output beam. strength. Using these methods, the number and direction of output beams can be varied while maintaining constant RF power flow into the device (producing a reduction in temperature fluctuations).

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

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

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

instructions

Figure 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 laser 22, emits a single input beam 23 of pulsed or continuous optical radiation, which may include visible, ultraviolet, or infrared radiation. The input beam 23 is incident on the acousto-optic deflector 24 which splits the input beam into a plurality of output beams 30 . A drive circuit 28 (also simply referred to as a "driver") applies a multi-frequency drive signal to one or more piezoelectric transducers 26 that drive the deflector 24 to generate a The input beam is split into multiple output beams 30 of acoustic waves.

Deflector 24 may comprise any suitable acousto-optic medium known in the art, including crystalline materials such as quartz, tellurium dioxide (TeO ₂ ), germanium, or glass materials such as fused silica or chalcogenide glass. The crystalline medium can be cut along certain preferred crystallographic directions to obtain desired acousto-optic properties (eg, in terms of speed of sound and birefringence). The transducer 26 may similarly comprise one or more pieces of any suitable piezoelectric material (eg, lithium niobate) attached to the acousto-optic medium, typically via a metallic bonding layer. Details of the operation of the drive circuit 28 and the drive signals it generates are presented in the following figures and in the description below.

In the illustrated embodiment, the scanning mirror 32 scans the output beam 30 over a range of angles 38 . The beam is focused onto target surface 36 via scan lens 34 . Arrangements of this type can be used in a variety of applications such as multi-beam laser drilling and printing. The drive signals applied to the transducers by driver 28 are selected such that each of beams 30 impinges on the target at a given beam intensity during a sequence of pulse intervals, while at intervals interspersed with the pulse intervals. Each of the beams may be blocked during certain respective blocking intervals. (As stated earlier, "blocked" means that the intensity of the beam on the target is attenuated to less than 50%, and usually less than 10%, or in some cases, less than 5% of a given beam intensity, or Even 1%.) As mentioned earlier, this beam blocking can be achieved by varying the frequency and/or phase of the drive signal while maintaining a constant RF power level of the drive signal. Several 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 figure) may employ biaxial mirrors that may be scanned together or independently and/or any other suitable type of radiation known in the art beam scanner. In an alternative embodiment, two acousto-optic deflectors may be deployed in series, one of which splits the input beam 23 into a plurality of output beams separated along the first direction and the other causes the beams in orthogonal Scan in the direction. All such embodiments may utilize the various drive schemes described herein and are considered to be within the scope of the present invention.

Pulse picking using chirp spectrum

Figure 2 is a schematic graph of the frequency chirped signal applied by the driver 28 to the acousto-optic deflector 24 in accordance with an embodiment of the present invention. Instead of switching off the RF signal for pulse blocking, this kind of chirp signal can be applied during pulse blocking. The chirp in the drive signal is characterized by the frequency that increases from an initial value _fSTART to a final value _fSTOP within a pulse period extending from time _tSTART to time _tSTOP . For example, the frequency range from _fSTART to _fSTOP may cover all or most of the acousto-optic deflector's spectral bandwidth, typically on the order of tens of megahertz to hundreds of megahertz. The time range from _{tSTART to tSTOP may} be approximately equal to the acoustic transit time across the diameter of the input beam, typically on the order of a few microseconds. This chirp may be applied to a single output beam from deflector 24 or to one or more of a set of multiple output beams 30 .

The chirped signal causes a strong defocusing of the beam 23 so that the resulting output beam is spread over a large area of the target surface 36 . The laser beam will still be directed towards the target surface with approximately the same total optical power as in the focused output beam, but the intensity will be attenuated by more than 90%, and possibly as much as 50dB, depending on the optical configuration. Therefore, the laser pulses will have essentially no effect on the target. Alternatively, a weaker chirp with a reduced frequency range can be used to defocus the laser beam in order to form a large spot on the target surface with sufficient intensity, for example, to divert the target surface's light from the focused spot to Areas irradiated with higher intensity are preheated.

Control 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 figure illustrates the effect and operation of the 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 multiple drive frequencies that propagate through the acousto-optic medium in deflector 24 . Each of the different drive frequencies creates an acousto-optic diffraction grating in the crystal at the corresponding spatial frequency, ie, the crystal contains multiple superimposed gratings of different spatial frequencies. In the simplified example shown in FIG. 3 , the wavefront angles of all gratings appear to be parallel; however, in the embodiments described below, each grating has a different wavefront determined by the phase of the drive signal applied by the drive circuit 28 angle.

When the input beam 23 enters the deflector 24, each of the gratings in the deflector diffracts the input beam at a different angle (depending on the grating frequency). Thus, the deflector 24 splits the input beam ₂₃ into _{a plurality of output beams 30a, 30b, 30c, 30d, . . . at different angles θ 1 ,} θ ₂ , . Optics 34 focus the output beam to form a corresponding array of spots 1 , 2 , . . . on target surface 36 . By modulating the spectrum and/or phase of the signal at the corresponding frequency in proper synchronization with the pulses of the input beam 23, the driver circuit 26 can control the intensity of the corresponding output beam 30 produced by each pulse of the input beam. Additionally or alternatively, the driver circuit 26 may modulate the component frequencies f ₁ , f ₂ , . . . in order to modulate the corresponding angles θ ₁ , θ ₂ , .

More specifically, the driver circuit 28 may individually turn the beams 30a, 30b, 30c, 30d, . The resulting combination of output beams 30. (In the example shown in Figures 1 and 3, beam 30c is turned off.) Additionally or alternatively, driver circuit 28 may control the phase of the frequency components in order to compensate for the variation in the diffraction efficiency of deflector 24 with angle. Thus, the drive circuit 28 may equalize the intensities of the beams 30a, 30b, and 30d, for example, while maintaining a constant RF power input to the deflector 24, despite variations in diffraction efficiency.

4 is a schematic cross-sectional view of an acousto-optic deflector 24 with a phased array of transducers 40 attached to the deflector's acousto-optic medium, according to an embodiment of the present invention. Although transducer 26 is shown as a single block in the preceding figures, in practice all embodiments of the invention may be implemented in this manner using an array of transducers 40 .

The drive circuit 28 is conceptually illustrated as including a frequency generator 42 that drives the transducers 40 through respective phase shifters 44 such that the drive signals are fed to the transducers at different respective phase offsets. The phase adjustment circuit 48 sets the phase offset of the phase shifter 44 according to the driving frequency and the desired diffraction efficiency at this frequency. Thus, the wavefront of the acoustic wave 46 propagating through the acoustic medium of the deflector 24 is not parallel to the face of the medium to which the transducer 40 is attached.

For maximum diffraction efficiency, the wavefront angle can typically be selected by appropriate setting of the phase shifter 44 such that the angle θ between the input beam 23 and the wavefront satisfies the Bragg condition for a given drive frequency, ie, sinθ=nλ/ 2d, where λ is the wavelength of the input beam, n is the diffraction order (usually n=1) and d is the wavelength of the acoustic wave at a given frequency. This choice of wavefront angle increases the efficiency of diffraction by the deflector 24, especially at frequencies away from _f0 (the frequencies at which the Bragg condition can be satisfied by setting the phase difference between adjacent transducers 40 to zero) at the frequency.

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. For example, this approach can be used to compensate for the inherent variation in the diffraction efficiency of the deflector with frequency and deflection angle, and thus maintain a constant value for a deflected beam or beams while driving the deflector at a constant RF power level. strength. Additionally or alternatively, the phase adjustment circuit 48 may apply a larger modulation to the phase shift in order to destroy the diffraction efficiency and thus turn off the output beam when needed, without changing the RF power level input to the deflector 24 .

In some embodiments, drive circuitry 28 applies respective multi-frequency drive signals to piezoelectric transducer 40 having frequency components at multiple different frequencies. For each of these frequencies, the Bragg condition produces different diffraction angles. Therefore, in order for the deflector 24 to achieve optimal performance at all frequencies, the phase adjustment circuit 48 drives the phase shifter 44 to apply a different phase offset for each frequency at each of the transducers 40 . Thus, the acoustic waves 46 propagate through the acousto-optic medium at the frequencies at different respective wavefront angles that are relative to the respective frequencies f ₁ , f ₂ , . . . and deflection of the output beam 30 The corresponding Bragg conditions of the angles θ ₁ , θ ₂ , . . . are selected.

5 is a block diagram schematically illustrating the functional components of the drive circuit 28 for the acousto-optic deflector 24 according to an embodiment of the present invention. The digital components of driver circuit 28 may typically be implemented in hardwired or programmable logic (eg, in a programmable gate array). Although the blocks in Figure 5 are shown as separate components for conceptual clarity, in practice the functionality of these components may be combined in a single logical device. Alternatively, at least some of the digital components of circuit 28 may be implemented in software running on a computer or special purpose microprocessor.

_The frequency selection block 50 selects several _fundamental frequencies _f1 , _f2 , . If the output beam angle is to be scanned laterally (as in system 20 as shown in FIG. 1 ), block 50 can be programmed to modulate each of these frequencies over time by an amount up to ±Δf, resulting in the The angular scan modulation of each beam is up to ±Δθ. Thus, in general, block 50 produces a sequence of frequency vectors, each vector comprising the number m of fundamental frequencies that will be applied to deflector 24 at a particular time to produce m output beams 30 at corresponding angles {θ _i + δθ _i } The value {f _i + δf _i }, where δf _i and δθ _i are the frequency change and angular change in the ranges ±Δf and ±Δθ, respectively.

Phase adjustment block 54 produces 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 the transducers 40 and contains the same frequency components but with different respective phase offsets. These phase offsets are selected according to the desired wavefront angle of the acoustic wave 46 in the deflector 24 at each frequency. Typically, the relative phase offset between sample streams is not uniform across the frequency range, but increases with frequency, so that at each frequency the wavefront angle also increases with frequency according to the Bragg condition, as explained above.

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

In this equation:

是在频率f下块54的两个邻近输出通道之间的相位差；·

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;

λ is the beam wavelength;

_Vs is the speed of sound in the acousto-optic medium; and

• _f0 is the applied frequency that imparts zero phase difference between adjacent channels and satisfies the Bragg condition for the optical output beam.

However, in this embodiment, block 54 may set the phase offset at a particular frequency so as to intentionally deviate from the Bragg condition with the selected phase offset, as explained above.

As mentioned earlier, blocks 50 and 54 are typically implemented in digital logic and/or software. The stream of digital samples output by block 54 is input to a corresponding channel of a multi-channel digital-to-analog converter 56 , which produces a corresponding output signal to drive transducer 40 . (Other analog components, such as RF amplifiers between the D/A converter channels and the transducers, are omitted from the figure for simplicity.) Assuming proper selection of frequency components and phase offsets, the transducers will A superposition of acoustic waves is produced in the deflector 24 at the fundamental frequency and with different wavefront angles.

而变的示意性曲线图。曲线60、62及64表示通常在约50MHz到150MHz的范围内的三个不同驱动频率下的衍射效率。每一曲线中的最大衍射效率DE_max对应于相位偏移

在所述相位偏移下在给定频率下的波前角度满足布拉格条件。即便如此，最大衍射效率未达到100％，而是在曲线60、62及64当中变化。远离此最大值，衍射DE作为相位偏移

的函数大致以正弦形式变化：6 is a graph of diffraction efficiency versus phase shift between adjacent transducers 44 driving acousto-optic deflector 24 in accordance with an embodiment of the present invention

A schematic diagram of the change. Curves 60, 62, and 64 represent diffraction efficiencies at three different drive frequencies, typically in the range of about 50 MHz to 150 MHz. The maximum diffraction efficiency DE _max in each curve corresponds to the phase shift

The wavefront angle at a given frequency at the phase offset satisfies the Bragg condition. Even so, the maximum diffraction efficiency does not reach 100%, but varies among curves 60 , 62 and 64 . away from this maximum, the diffraction DE acts as a phase shift

The function of varies roughly sinusoidally:

由I/I₀的反余弦给出，其中I₀是针对给定驱动频率f在最大衍射效率下输出的强度。如较早解释，在不同驱动频率下

的值可用于补偿在不同频率下最大衍射效率的差。替代地或另外，为了有效地阻挡输出射束中的一者，可将

设定为

使得衍射效率将接近于零。进一步替代地或另外，在使RF输入功率保持恒定的同时通过调制相位偏移可调制输出射束的强度。Phase adjustment block 54 may apply the above relationship when modulating the intensity of each of the output beams from deflector 24 corresponding to frequencies such as curves 60 , 62 and 64 . The phase shift required to achieve a given output beam intensity I, assuming that the RF power applied to the transducer 40 remains constant

is given by the arc cosine of I/I ₀ , where I ₀ is the intensity of the output at maximum diffraction efficiency for a given drive frequency f. As explained earlier, at different drive frequencies

The value of can be used to compensate for differences in maximum diffraction efficiency at different frequencies. Alternatively or additionally, to effectively block one of the output beams, the

set as

so that the diffraction efficiency will be close to zero. Further alternatively or additionally, the intensity of the output beam can be modulated by modulating the phase offset while keeping the RF input power constant.

7 is a schematic graph of a multi-frequency signal applied to an acousto-optic deflector according to another embodiment of the present invention. Similar to the signal shown in Figure 2, this signal has a chirped spectrum, but in this case includes a sequence of discrete frequency steps 70. Signals of this kind may be conveniently applied by a digital driver circuit, such as the driver circuit shown in FIG. 5, during intervals in which a given output beam will be blocked.

其中

是在所讨论频率下满足布拉格条件的相位偏移。This frequency chirping method can be advantageously combined with the method described above based on the adjustment of the phase offset between the transducers: the phase is selected at each frequency step 70 in order to cause the acoustic wave to deviate at a given frequency The wavefront angle of the Bragg condition propagates through the acousto-optic medium. For maximum attenuation of output beam intensity, the phase offset at each frequency can be set to approximately

in

is the phase offset that satisfies the Bragg condition at the frequency in question.

It will be understood that the embodiments described above are referenced by way of example and that the invention is not limited to what is particularly shown and described above. Rather, the scope of the invention includes both combinations and sub-combinations of the various features described above, as well as combinations and sub-combinations of the various features described above that would come to mind to one skilled in the art after reading the above description and which are not disclosed in the prior art. Variations and modifications of various features.

Claims (22)

1. An optical device comprising: an acousto-optic medium configured to receive an input radiation beam and deflect the input beam into at least first and second output radiation having respective first and second intensities at respective first and second beam angles beams, at the respective first and second beam angles, the acousto-optic medium is characterized by different respective first and second diffraction efficiencies; an array of a plurality of piezoelectric transducers attached to the acousto-optic medium; and a drive circuit coupled to apply respective drive signals to the piezoelectric transducers, the respective drive signals including at least first and second drive signals at different corresponding first and a second frequency to direct the first and second output beams at the respective first and second beam angles and at each of the plurality of piezoelectric transducers for the first and second The second frequency component has different respective first and second phase offsets that cause the acoustic wave to have different respective first and second wavefronts at the first and second frequencies angular propagation through the acousto-optic medium, and the drive circuit includes a controller configured to select the first and second phase offsets so as to compensate for the different first and second diffraction efficiencies , thereby making the first and second intensities equal. 2. The apparatus of claim 1, wherein the drive circuit is configured to concurrently apply at least the first and second drive signals with the respective phase first and second phase offsets to the voltage an electrical transducer such that the acousto-optic medium simultaneously deflects the input beam into at least the first and second output beams. 3. The apparatus of claim 1, wherein the controller is configured to vary at least the first and second frequencies of the first and second drive signals such that the acousto-optic medium causes at least the The first and second beams are scanned over respective first and second angular ranges, and the respective phase offsets are changed in response to changes in the frequency. 4. The apparatus of claim 3, wherein the controller is configured to control the drive signal applied by the drive circuit such that the acousto-optic medium causes at least the first The beam is deflected towards the target at a given beam intensity, the pulse interval being interspersed with blocking intervals in which the intensity of the first beam on the target attenuates to the given beam less than 50% of the strength, wherein the first drive signal has a given amplitude and a frequency corresponding to the deflection angle of the beam during each pulse interval and a chirp spectrum during each blocking interval. 5. The apparatus of claim 1, wherein the first diffraction efficiency is greater than the second diffraction efficiency, and wherein the controller is configured to cause the acoustic wave by setting the second phase offset Compensating the different first and first Two diffraction efficiency. 6. The apparatus of claim 5, wherein the controller is further configured to modify the respective phase offset while maintaining a constant power level of the respective drive signal regardless of the respective phase offset Each of the output beams is turned on and off. 7. An optical device comprising: an acousto-optic medium configured to receive an input radiation beam and deflect the input beam toward a target at a given beam intensity over a range of angles during a series of pulse intervals interspersed with blocking intervals , the intensity of the beam on the target attenuates to less than 50% of the given beam intensity during the blocking interval; at least one piezoelectric transducer attached to the acousto-optic medium; and a drive circuit coupled to apply a drive signal to the at least one piezoelectric transducer, the drive signal having a given amplitude and having a frequency corresponding to the deflection angle of the beam during each pulse interval and at There is a chirp spectrum during each blocking interval. 8. The apparatus of claim 7, wherein the chirped spectrum is selected such that during the blocking interval, the intensity of the beam on the target attenuates to a magnitude of the given beam intensity less than 10%. 9. The apparatus of claim 7, wherein the chirped spectrum comprises a sequence of discrete frequency steps applied during each blocking interval. 10. The apparatus of claim 9, wherein the at least one piezoelectric transducer comprises an array of a plurality of piezoelectric transducers, and wherein the drive circuit is configured to apply to the piezoelectric transducers A respective drive signal, the phase of which is selected so as to cause the acoustic wave to satisfy the Bragg condition for the input beam during the pulse interval at each of the discrete frequency steps during the blocking interval A wavefront angle that deviates from the Bragg condition with respect to the input beam propagates through the acousto-optic medium. 11. An optical device comprising: an acousto-optic medium configured to receive an input radiation beam and deflect the input beam toward a target within a range of deflection angles; an array of a plurality of piezoelectric transducers attached to the acousto-optic medium; and a drive circuit coupled to apply respective drive signals to the piezoelectric transducer, frequencies of the respective drive signals selected to cause acoustic waves to propagate through the acousto-optic medium at the selected frequencies, the acousto-optical The medium thereby deflects the input beam at corresponding deflection angles within the range, and wherein the phase offset among the drive signals applied to the transducers in the array is selected for adjustment by The wavefront angle of the acoustic wave modulates the intensity of the deflected beam. 12. A method for optical scanning, comprising: directing an input radiation beam incident on an acousto-optic medium to which an array of a plurality of piezoelectric transducers is attached; applying respective drive signals to the piezoelectric transducers, the respective drive signals including at least first and second frequency components at different respective first and second frequencies and in the plurality of piezoelectric transducers have different respective first and second phase offsets for the first and second frequency components at each of , so as to cause the acousto-optic medium to deflect the input beam to be at the respective first and second At least first and second output beams having respective first and second intensities at beam angles at which the acousto-optic medium is composed of different respective first and second beam angles. 2 diffraction efficiency characterization; and The first and second phase offsets are selected that cause acoustic waves to propagate through all of the first and second frequencies at different respective first and second wavefront angles. the acousto-optic medium to compensate for the different first and second diffraction efficiencies, thereby equalizing the first and second intensities. 13. The method of claim 12, wherein applying the respective drive signals comprises concurrently applying to the piezoelectric transducers at least first and second drive signals having first and second phase offsets of the respective phases. A transducer such that the acousto-optic medium simultaneously deflects the input beam into at least the first and second output beams. 14. The method of claim 12, wherein applying the respective drive signals comprises varying at least the first and second frequencies of the first and second drive signals such that the acousto-optic medium causes at least a first and a second beam scan over respective first and second angular ranges, and wherein selecting the first and second phase offsets includes changing the respective phase offsets in response to changes in the frequency. 15. The method of claim 14, wherein applying the respective drive signals comprises controlling the drive signals such that the acousto-optic medium causes at least the first beam to operate at a given beam during a series of pulse intervals the intensity is deflected towards the target, the pulse interval is interspersed with a blocking interval in which the intensity of the first beam on the target decays to less than 50% of the given beam intensity, wherein the first drive signal has a given amplitude and a frequency corresponding to the deflection angle of the beam during each pulse interval and a chirp spectrum during each blocking interval. 16. The method of claim 12, wherein the first diffraction efficiency is greater than the second diffraction efficiency, and wherein selecting the first and second phase offsets comprises by setting the second phase offset , such that the acoustic wave satisfies the Bragg condition for the input beam at the second frequency and the acoustic wave deviates from the Bragg condition for the input beam at the first frequency to compensate for the difference the first and second diffraction efficiencies. 17. The method of claim 16, wherein selecting the first and second phase offsets comprises by modifying the respective phase offsets while maintaining the respective drive signals constant regardless of the respective phase offsets power level to turn each of the output beams on and off. 18. A method for optical scanning, comprising: directing the input radiation beam to be incident on an acousto-optic medium to which at least one piezoelectric transducer is attached; and A drive signal is applied to the at least one piezoelectric transducer, the drive signal having a given amplitude and a frequency corresponding to the deflection angle of the beam during each of a series of pulse intervals and during a series of having a chirped spectrum during each of the blocking intervals interspersed with the pulse intervals such that the acousto-optic medium causes the input beam during each of the series of pulse intervals The beam is deflected towards a target at a given beam intensity over a range of angles, and the intensity of the beam on the target is attenuated during each of the blocking intervals to a different magnitude of the given beam intensity. to 50%. 19. The method of claim 18, wherein the chirped spectrum is selected such that during the blocking interval, the intensity of the beam on the target attenuates to a magnitude of the given beam intensity less than 10%. 20. The method of claim 18, wherein the chirped spectrum comprises a sequence of discrete frequency steps applied during each blocking interval. 21. The method of claim 20, wherein the at least one piezoelectric transducer comprises an array of a plurality of piezoelectric transducers, and wherein applying the drive signal comprises applying the drive signal to the plurality of piezoelectric transducers A respective drive signal is applied, the phase of the respective drive signal is selected so as to cause the acoustic wave to satisfy the Bragg condition for the input beam during the pulse interval and in the discrete frequency steps during the blocking interval Each propagates through the acousto-optic medium at a wavefront angle that deviates from the Bragg condition with respect to the input beam. 22. A method for optical scanning, comprising: directing an input radiation beam incident on an acousto-optic medium to which an array of a plurality of piezoelectric transducers is attached; A respective drive signal is applied to the piezoelectric transducer, the frequency of the respective drive signal is selected to cause acoustic waves to propagate through the acousto-optic medium at the selected frequency, thereby causing the acousto-optic medium to cause the acousto-optic medium to cause the the input beam is deflected at the corresponding deflection angle; and A phase offset among the transducers in the array is set, the phase offset being selected to modulate the intensity of the deflected beam by adjusting the wavefront angle of the acoustic wave.

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