JP2023500525A - Thermal stabilization of acousto-optic devices - Google Patents

Abstract

The optical device 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 multiple piezoelectric transducers is attached to the acousto-optic medium. Drive circuits are coupled to apply respective drive signals to the piezoelectric transducers, the drive signals having frequencies selected to cause acoustic waves to propagate through the acousto-optic medium at selected frequencies. , applied to transducers in the array selected to deflect the input beam at corresponding deflection angles within a range and to modulate the intensity of the deflected beam by adjusting the wavefront angle of the acoustic wave. have a phase offset between the applied drive signals.

Description

This invention relates generally to optical devices and systems, and more particularly to acousto-optic devices and methods of operating such devices.

Acousto-optic devices use sound waves to diffract light. In a typical device of this kind, a transducer, such as a piezoelectric transducer, is attached to an acousto-optic medium, usually a suitable transparent crystal or glass. The transducer is driven by an electrical signal to vibrate at a certain frequency, thus creating sound waves in the acousto-optic medium. Expansion and compression of an acousto-optic medium by acoustic waves modulates the local index of refraction, thus creating a grating structure within the medium, with a period determined by the frequency of the driving signal. Thus, a beam of light incident on this grating becomes diffracted as it passes through the device.

Various types of acousto-optic devices are known in the art. An acousto-optic deflector uses, for example, diffraction of an incident beam to steer the angle of an output beam. The deflection angle of the output beam depends on the period of the grating structure within the acousto-optic material and can therefore be adjusted by appropriately varying 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 to control the angle of the acoustic wave propagating through the acousto-optic medium, thus changing the phase between the acoustic field and the light beam to be modulated. Adjust alignment. For example, US Pat. No. 7,538,929 uses 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 to: A radio frequency (RF) phase modulation technique is described for intensity modulation of an optical wavefront. A transducer driver is connected to each electrode and is coherently phase-driven to change the angular momentum distribution of the acoustic field and to shift the phase between the optical and acoustic fields to produce the desired intensity modulation of the optical wavefront. Alternately allow or suppress matching.

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

PCT International Publication No. WO2016/075681, whose disclosure is incorporated herein by reference, describes another example of the use of phased arrays in driving acousto-optic deflectors. In this publication, an optical device includes an acousto-optic medium and an array of multiple piezoelectric transducers attached to the acousto-optic medium. A drive circuit is coupled to apply respective drive signals to the piezoelectric transducers, the drive signals being different, at respective first and second frequencies, and at each of the plurality of piezoelectric transducers. At least first and second frequency components having different respective phase offsets relative to the second frequency component.

U.S. Patent Application Publication No. 2015/0338718 U.S. Patent Application Publication No. 2005/0279807

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

An optical device is provided according to an embodiment of the present invention, the optical device receives an input beam of radiation and converts the input beam to respective first and second intensities at respective first and second beam angles. and an acousto-optic medium configured to deflect at least first and second output beams characterized by different respective first and second diffraction efficiencies. An array of multiple piezoelectric transducers is attached to the acousto-optic medium. A drive circuit is coupled to apply respective drive signals to the piezoelectric transducer, the drive signals directing the first and second output beams at respective first and second beam angles and a plurality of at different corresponding first and second frequencies to direct with different respective first and second phase offsets relative to the first and second frequency components in each of the piezoelectric transducers. for propagating acoustic waves through the acousto-optic medium at first and second frequencies with different respective first and second wavefront angles . The controller is configured to select the first and second phase offsets to compensate for the differing first and second diffraction efficiencies, thereby equalizing the first and second intensities.

Typically, the drive circuit applies at least first and second drive signals to the respective phases of the first and second drive signals to cause the acousto-optic medium to simultaneously deflect the input beam into at least first and second output beams. is configured to be applied in parallel to the piezoelectric transducer with a phase offset of .

In some embodiments, the controller adjusts the first and second drive signals such that the acousto-optic medium scans at least the first and second beams over respective first and second angular ranges. It is configured to vary at least the first and second frequencies and vary the respective phase offsets in response to the varying frequencies. In one embodiment, the controller causes the acousto-optic medium to: configured to control a drive signal applied by the drive circuit to deflect at least the first beam toward the target with a predetermined beam intensity, the first drive signal having a predetermined amplitude; with a frequency corresponding to the deflection angle of the beam during each pulse interval and a chirped frequency spectrum during each stop interval.

Additionally or alternatively, when the first diffraction efficiency is greater than the second diffraction efficiency, the controller determines that the acoustic wave at the second frequency satisfies the Bragg condition for the input beam and the acoustic wave at the first frequency It is configured to compensate for the differing first and second diffraction efficiencies by setting a second phase offset such that the waves depart from the Bragg condition for the input beam. In the disclosed embodiment, the controller turns each of the output beams on and off by varying their respective phase offsets while maintaining constant power levels of their respective drive signals regardless of their respective phase offsets. is further configured to:

Also provided is an optical device according to embodiments of the present invention, the optical device being an acousto-optic medium for receiving an input beam of radiation, the intensity of the beam on a target being attenuated to less than 50% of a predetermined beam intensity. an acousto-optic medium configured to deflect an input beam toward a target with a predetermined beam intensity over a range of angles during successive pulse intervals interspersed by block intervals, . At least one piezoelectric transducer is attached to the acousto-optic medium. a drive circuit coupled to apply a drive signal to the at least one piezoelectric transducer, the drive signal having a predetermined amplitude and a frequency during each pulse interval corresponding to the deflection angle of the beam; Between each blocking interval has a chirped frequency spectrum.

In some embodiments, the chirped frequency spectrum is chosen such that the intensity of the beam on the target is attenuated to less than 10% of the predetermined beam intensity during the stop interval.

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

Additionally provided is an optical apparatus according to an embodiment of the present invention, the optical apparatus being an acousto-optic configured to receive an input beam of radiation and deflect the input beam toward a target over a range of deflection angles. Including media. 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 drive signals having frequencies selected to cause acoustic waves to propagate through the acousto-optic medium at selected frequencies. , applied to transducers in the array selected to deflect the input beam at corresponding deflection angles within a range and to modulate the intensity of the deflected beam by adjusting the wavefront angle of the acoustic wave. have a phase offset between the applied drive signals.

A method of optical scanning is further provided according to embodiments of the present invention, the method comprising directing an input beam of radiation to be incident on an acousto-optic medium having an array of piezoelectric transducers mounted thereon. . Respective drive signals are applied to the piezoelectric transducers, the drive signals directing the input beams, characterized by different respective first and second diffraction efficiencies, to the acousto-optic medium, respectively. at first and second frequencies, respectively, different to deflect at least first and second output beams at respective first and second intensities at first and second beam angles; and In each of the plurality of piezoelectric transducers, at least first and second frequency components having different respective first and second phase offsets with respect to the first and second frequency components. the first and second phase offsets are selected to cause acoustic waves to propagate through the acousto-optic medium at the first and second frequencies with different respective first and second wavefront angles; Thereby compensating for the different first and second diffraction efficiencies and equalizing the first and second intensities.

A method of optical scanning is further provided according to embodiments of the present invention, the method comprising 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, the drive signal having a predetermined amplitude and a frequency corresponding to the deflection angle of the beam during each successive pulse section and interspersed by the pulse sections. and has a chirped frequency spectrum during each successive stop interval so that the acousto-optic medium has a given beam intensity over a range of angles during each successive pulse interval, Deflect the input beam toward the target and attenuate the intensity of the beam on the target to less than 50% of the predetermined beam intensity during each of the stoppage intervals.

A method of optical scanning is further provided according to embodiments of the present invention, the method comprising directing an input beam of radiation to be incident on an acousto-optic medium having an array of piezoelectric transducers mounted thereon. . A respective drive signal is applied to the piezoelectric transducer, the drive signal having a frequency selected to propagate an acoustic wave at a selected frequency through the acousto-optic medium, thereby , the input beam is deflected at the corresponding deflection angle. A phase offset between transducers in the array is set 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 its embodiments, read in conjunction with the drawings.

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

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 tends to change the acoustic velocity (and refractive index) within the crystal, leading to drift in the deflection angle of the laser beam. Non-uniform temperature variations distort the laser beam and cause lensing and other refractive effects.

However, since acousto-optic modulation and deflection inherently involve changes in the RF signal used to drive the acousto-optic crystal, keeping the acousto-optic crystal at a stable temperature can be challenging. Become. For example, a laser pulse transmitted toward a target can be intermittently blocked by interrupting the RF signal to the crystal (an operation called "pulse picking"), resulting in a change in RF input power. becomes to cause temperature fluctuations and unstable thermal behavior within the crystal body. In addition, steering the laser beam direction by varying the drive frequency applied to the acousto-optic crystal is also useful, since different drive frequencies typically require different levels of RF power to achieve the same laser pulse energy. , and the acoustic absorption of crystals is strongly frequency dependent, which can lead to temperature variations.

Embodiments of the invention described herein provide a novel technique that can be used to keep the temperature of acousto-optic crystals stable and uniform despite the challenges inherent in pulse picking and beam steering. These embodiments allow the crystal to direct one or more beams of radiation and alternately pass and block the beams while maintaining a constant RF input level to the crystal. The term "constant" in this context means that the instantaneous RF power of the drive signal input to the piezoelectric transducer that drives the crystal is within predefined limits, usually regardless of beam guidance and intermittent blocking. , meaning that the variation during operation of the acousto-optic device is kept below 10%, possibly below 5% (although larger or smaller limits are possible depending on application requirements). ).

In some embodiments, an acousto-optic medium (usually a suitable crystal) is directed toward a target with a predetermined beam intensity over a range of angles by at least one piezoelectric transducer attached to it. It is driven during successive pulse intervals to deflect the beam. The pulse intervals are interspersed by stoppage intervals, during which the intensity of the beam on the target is less than 50%, or possibly less than 10%, less than 5%, or even 1 for high sensitivity applications. attenuated to less than %. (The term "intensity" is used in the context of this description and in the claims in its conventional sense to mean optical power per unit area upon which the beam is incident).

A drive circuit applies a drive signal to the piezoelectric transducer with a frequency corresponding to the deflection angle of the beam during each pulse interval and with a chirped frequency spectrum during each stop interval. The frequency chirp was chosen to spread the input beam over a large area of the target so that the intensity of the beam on target during the stop interval is much lower than the intensity of the deflected beam during the pulse interval. It has a frequency range and duration. Regardless of the different frequency spectrums, the drive circuit maintains substantially the same, predetermined amplitude of the drive signal during both the pulse and block intervals.

This approach can be extended to stabilize temperature in multi-frequency operation, where a composite drive signal formed as a superposition of single-frequency drive signals is applied by an array of piezoelectric transducers to acousto-optic applied to the medium. A composite drive signal splits the laser beam into several output beams. A frequency chirp may be applied as part of the drive signal to select one or more of the output beams to be blocked. Additionally or alternatively, by choosing a specific phase offset (also called phase delay) between the transducers for each single frequency component, the diffraction efficiency for the selected component is reduced, and thus the specific output It is possible to significantly reduce the intensity of the beam. Using these techniques, the number and direction of output beams can be varied while maintaining a constant RF power flow into the device, resulting in reduced temperature fluctuations.

Different phase offsets between the transducers cause acoustic waves at different frequencies to propagate through the acousto-optic medium with different respective wavefront angles. The wavefront angle at each frequency is specifically adjusted to satisfy the Bragg condition and thus achieve maximum diffraction efficiency for a given RF power at this frequency, or to deviate from the Bragg condition and thus reduce diffraction efficiency. ,To be elected. Using this property, the phase offset compensates for the inherent variation in the diffraction efficiency of acousto-optic media as a function of frequency and beam angle, thus maintaining a constant input RF power to the acousto-optic modulator while output The intensity of the beams can be set to be equal ("equal" in this context means, like the term "constant", that the intensity of the output beam varies by no more than 10%, possibly no more than 5%). means.).

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

More generally, a deliberate deviation of the wavefront angle from the Bragg condition can be used to modulate the intensity of an input beam deflected by an acousto-optic medium. In some embodiments, the drive circuit causes the acousto-optic medium to modulate the frequency selected to deflect the input beam and the intensity of the deflected beam at a range of corresponding deflection angles. Each drive signal is applied to the piezoelectric transducers with a phase offset between the transducers in the array. In this way it is possible to modulate the deflected beam intensity (and turn the beam off and on) while maintaining a constant RF power input to the acousto-optic medium.

System Description FIG. 1 is a schematic illustration of a pictorial representation 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 acousto-optic deflector 24 , which splits the input beam into multiple output beams 30 . A driver circuit 28 (also referred to simply as a "driver") applies multi-frequency drive signals to one or more piezoelectric transducers 26, which are acousto-optical transducers that split an input beam into a plurality of output beams 30. A deflector 24 is driven to generate acoustic waves in the medium.

Deflector 24 comprises any suitable acousto-optic medium known in the art, including crystalline materials such as quartz, tellurium dioxide ₍ TeO2), germanium, or glass materials such as fused silica or chalcogenide glasses. obtain. Crystalline media can be cut along specific, preferred crystallographic directions to obtain desired acousto-optic properties, for example, in terms of sound velocity and birefringence. Transducer 26 may similarly comprise one or more of any suitable piezoelectric material, such as lithium niobate, typically attached to the acousto-optic medium by a metallic bonding layer. Details of the operation of drive circuit 28 and the drive signals it generates are shown in the following figures and the following description.

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

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

Pulse Picking Using a Chirped Frequency Spectrum FIG. 2 is a schematic graph of a frequency chirped signal applied to acousto-optic deflector 24 by driver 28, according to an embodiment of the present invention. Instead of interrupting the RF signal for pulse blocking, this kind of chirped signal can be applied during pulse blocking. A chirp in the drive signal is characterized by an increasing frequency from an initial value f _START to a final value f _STOP over a pulse period extending from time t _START to t _STOP . For example, the range of frequencies from f _START to f _STOP can cover all or most of the spectral bandwidth of the acousto-optic deflector, which is typically on the order of tens to hundreds of megahertz. The time range from t _START to t _STOP can be roughly equal to the travel time of the acoustic wave across the diameter of the input beam, which is typically on the order of a few microseconds. Such 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 chirp signal causes a strong defocus of beam 23 so that the resulting output beam is spread over a large area on target plane 36 . The laser beam will still be directed toward the target surface with approximately the same total optical power as in the focused output beam, but depending on the optical configuration, the intensity will be 90% It becomes attenuated more, possibly by 50 dB. As a result, the laser pulse has virtually no effect on the target. Alternatively, with a reduced frequency range, e.g. A weaker chirp can be used to defocus the laser beam to form a large spot.

Controlling the Intensity of Multiple Output Beams FIG. 3 is a schematic cross-sectional view of acousto-optic deflector 24, according to an embodiment of the present invention. This figure shows the effect and operation of the multi-frequency drive provided by drive circuit 28 and piezoelectric transducer 26. FIG. Multi-frequency drive signals from drive circuit 28 cause piezoelectric transducer 26 to generate acoustic waves at multiple drive frequencies, which propagate through the acousto-optic medium within deflector 24 . Each different drive frequency establishes an acousto-optic grating within the crystal at the corresponding spatial frequency, ie, the crystal includes 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, but in the embodiments described below each grating has a different wavefront angle, which is due to the driving circuit determined by the phase of the drive signal applied by .

As input beam 23 enters deflector 24, each of the gratings in the deflector diffracts the input beam at different angles, depending on the grating frequency. _Deflector 24 therefore _directs input beam 23 to multiple _output beams 30a, _30b , 30c, . 30d, . . . Optics 34 focus the output beams to form a corresponding array of spots 1, 2, . . . By modulating the frequency spectrum and/or the phase of the signal at corresponding frequencies in proper synchronization with the pulses of the input beam 23, the drive circuit 26 controls the corresponding output beam generated by each pulse of the input beam. 30 intensity can be controlled. Additionally or alternatively, drive circuit 26 may modulate component frequencies f ₁ , f ₂ , . . . to modulate corresponding angles θ ₁ , θ ₂ , . The position of the top spot can be changed.

More specifically, drive circuit 28 can individually turn on and off beams 30a, 30b, 30c, 30d, . . . by controlling the phase and/or frequency spectrum of the corresponding frequency components. , and thus the combination of output beams 30 may be chosen to occur in each pulse. (Beam 30c is turned off in the example shown in FIGS. 1 and 3). Additionally or alternatively, drive circuit 28 may control the phase of the frequency components to compensate for variations in the diffraction efficiency of deflector 24 as a function of angle. Thus, drive circuit 28 can, for example, equalize the intensity of beams 30a, 30b, and 30d while maintaining a constant RF power input to deflector 24 despite variations in diffraction efficiency.

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

The drive circuit 28 is conceptually illustrated as comprising a frequency generator 42, each of which has a different respective phase offset such that the drive signals are supplied to the transducers with different respective phase offsets. drives the converter 40 through a phase shifter 44 of . A phase adjust circuit 48 sets the phase offset of the phase shifter 44 according to the driving frequency and the desired diffraction efficiency at this frequency. As a result, the wavefront of acoustic wave 46 propagating through the acoustic medium of deflector 24 is not parallel to the plane of the medium to which transducer 40 is attached.

For maximum diffraction efficiency, the wavefront angle is the angle θ of the phase shifter 44 such that the angle θ between the input beam 23 and the wavefront satisfies the Bragg condition, i. 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 wavefront angle selection is particularly useful for deflector 24 at frequencies away from f ₀ (the frequency at which the Bragg condition can be satisfied by setting the phase difference between adjacent transducers 40 to be zero). Increase the efficiency of diffraction.

Alternatively, phase adjust circuit 48 adjusts the wavefront angle to a value that deviates from the Bragg condition by a controlled amount, thereby increasing the diffraction efficiency (and thus the intensity of the resulting output beam from deflector 24) to can be modulated. This approach compensates for the inherent variation in the diffraction efficiency of the deflector, e.g., as a function of frequency and deflection angle, and thus the deflection of the deflected beam while driving the deflector with a constant level of RF power. It can be used to maintain constant strength. Additionally or alternatively, phase adjust circuit 48 may apply a greater modulation to the phase offset to impair diffraction efficiency, and thus reduce the level of RF power input to deflector 24 as needed. The output beam can be turned off without changing .

In some embodiments, drive circuit 28 has frequency components at multiple different frequencies to apply respective multi-frequency drive signals to piezoelectric transducer 40 . For each of these frequencies, Bragg conditions result at different diffraction angles. Therefore, for optimum performance of deflector 24 at all frequencies, phase adjust circuit 48 drives phase shifters 44 to apply different phase offsets for each frequency in each of transducers 40 . As a result, acoustic waves 46 at those frequencies are for each Bragg condition for corresponding frequencies f ₁ , f ₂ , . . . and deflection angles θ ₁ , θ ₂ , . Propagating through the acousto-optic medium with different respective wavefront angles chosen.

FIG. 5 is a block diagram that schematically illustrates functional components of drive circuitry 28 for acousto-optic deflector 24, according to an embodiment of the present invention. The digital components of drive circuit 28 may typically be implemented in programmable logic circuits, such as those in hardwired or programmable gate arrays. Although the blocks of FIG. 5 are shown as separate components for conceptual clarity, in practice the functionality of these components may be combined into 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 dedicated microprocessor.

A frequency selection block 50 selects a number of fundamental frequencies f ₁ , which are to be applied to the drive deflector 24 to generate an output beam 30 having corresponding deflection angles θ ₁ , θ ₂ , . Select f ₂ , . If the output beam angle is to be laterally scanned (as in system 20 shown in FIG. 1), block 50 modulates each of these frequencies over time by an amount of ±Δf. , resulting in an angular scan of each beam up to ±Δθ. Block 50 therefore generally generates a series of frequency vectors, each vector comprising m fundamental frequency values {f _i +δf _i }, which direct the m output beams 30 to corresponding angles { θ _i +δθ _i }, where δf _i and δθ _i are the frequency and angle changes within the ranges ±Δf and ±Δθ, respectively.

Phase adjust 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 the acoustic wave 46 at the deflector 24 at each frequency. Typically, the relative phase offset between the sample streams is not uniform across the frequency range, but increases with frequency, resulting in the wavefront angle as well, subject to the Bragg condition at each frequency, as explained above. increases with frequency.

この式において、
・

は、周波数ｆにおけるブロック５４の２つの隣接した出力チャネルの間の位相差、
・Ｓは、隣接した変換器４０の中心の間の距離、
・λは、光ビーム波長、
・Ｖ_ｓは、音響光学媒体内の音響速度、および
・ｆ_０は、隣接したチャネルの間にゼロの位相差を与え、光出力ビームに対するブラッグ条件を満たす、印加される周波数である。
しかし本実施形態において、ブロック５４は、上記で説明されたように選択される位相偏移によって、ブラッグ条件から意図的に逸脱させるように、ある一定の周波数での位相オフセットを設定し得る。 Specifically, to satisfy the Bragg condition (in the absence of birefringent Bragg diffraction), block 54 may set phase offsets at different frequencies according to the following equations.

In this formula,
・

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 light beam wavelength,
• V _s is the acoustic velocity in the acousto-optic medium, and • f ₀ is the applied frequency that gives 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 certain frequencies to deliberately deviate from the Bragg condition by phase shift selected as described above.

As previously mentioned, blocks 50 and 54 are typically implemented in digital logic and/or software. The digital sample stream output by block 54 is input to each channel of a multi-channel digital-to-analog converter 56 (D/A converter channels and converters and Other analog components such as RF amplifiers between are omitted for simplicity.). Assuming proper selection of frequency components and phase offsets, the transducer will produce superposition of acoustic waves in the deflector 24 at different fundamental frequencies and with different wavefront angles.

の関数としての回折効率の概略グラフである。曲線６０、６２、および６４は、通常は約５０ＭＨｚから１５０ＭＨｚの範囲内である、３つの異なる駆動周波数における回折効率を表す。各曲線における最大回折効率ＤＥ_ｍａｘは、所定の周波数での波面角がブラッグ条件を満たす、位相オフセット

に対応する。それでも、最大回折効率は１００％に到達せず、曲線６０、６２、および６４の間で変化する。この最大から離れると、回折ＤＥは、位相オフセット

の関数として、おおよそ正弦的に変化する。

FIG. 6 illustrates the phase offset between adjacent transducers 44 driving acousto-optic deflector 24, according to an embodiment of the present invention.

4 is a schematic graph of diffraction efficiency as a function of . Curves 60, 62, and 64 represent diffraction efficiency at three different drive frequencies, typically in the range of about 50 MHz to 150 MHz. The maximum diffraction efficiency DE _max for each curve is the phase offset for which the wavefront angle at a given frequency satisfies the Bragg condition.

corresponds to Still, the maximum diffraction efficiency does not reach 100% and varies between curves 60, 62 and 64. Away from this maximum, the diffraction DE has a phase offset of

varies approximately sinusoidally as a function of .

は、Ｉ／Ｉ_０の逆余弦によって与えられ、ここでＩ_０は、所定の駆動周波数ｆに対する、最大回折効率で出力される強度である。前に説明されたように、異なる駆動周波数での

の値は、異なる周波数での最大回折効率における差を補償するために用いられ得る。代替としてまたは追加として、出力ビームの１つを効果的に阻止するために、回折効率がゼロに近くなるように、

は

に設定され得る。さらに代替としてまたは追加として、出力ビームの強度は、位相オフセットを変調することによって、ＲＦ入力パワーを一定に保持しながら、変調され得る。 Phase adjustment block 54 may apply the above relationship, for example, in modulating the intensity of each of the output beams from deflector 24 corresponding to the frequencies of curves 60 , 62 and 64 . Assuming that the RF power applied to transducer 40 is kept constant, the phase offset required to achieve a given output beam intensity I is

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

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

teeth

can be set to Further alternatively or additionally, the intensity of the output beam may be modulated while holding the RF input power constant by modulating the phase offset.

FIG. 7 is a schematic graph of multi-frequency signals applied to an acousto-optic deflector, according to another embodiment of the invention. This signal has a chirped frequency spectrum similar to that shown in FIG. 2, but this time with a series of discrete frequency steps 70 . A signal of this kind may conveniently be applied by a digital drive circuit, such as the drive circuit shown in FIG. 5, during intervals during which a given output beam is to be blocked.

に設定されることができ、ここで

は、当該の周波数におけるブラッグ条件を満たす位相オフセットである。 Advantageously, this frequency chirp technique can be combined with the technique based on adjusting the phase offset between the transducers described above. The phase is chosen to cause, at each frequency step 70, the acoustic wave to propagate through the acousto-optic medium at wavefront angles that deviate from the Bragg condition at the given frequency. For maximum attenuation of the output beam intensity, the phase offset at each frequency is approximately

where

is the phase offset that satisfies the Bragg condition at the frequency of interest.

It will be understood that the above-described embodiments have been described by way of example and that the present invention is not limited to that specifically shown and described hereinabove. Rather, the scope of the present invention may occur to those skilled in the art, both in combination and subcombination of the various features set forth hereinabove, as well as upon reading the foregoing description and disclosures in the prior art. not including modifications and variations thereof.

Claims (22)

An optical device,
configured to receive an input beam of radiation and to deflect said input beam into at least first and second output beams at respective first and second beam angles with respective first and second intensities; an acousto-optic medium characterized by different respective first and second diffraction efficiencies;
an array of piezoelectric transducers attached to the acousto-optic medium;
A drive circuit coupled to apply respective drive signals to the piezoelectric transducers, the drive signals directing the first and second output beams to respective first and second beam angles. and in each of the plurality of piezoelectric transducers, different corresponding first phase offsets for the first and second frequency components to direct with different respective first and second phase offsets. and at least first and second drive signals at a second frequency, which drive acoustic waves at said first and second frequencies with different respective first and second wavefront angles. , propagating through the acousto-optic medium, and the drive circuit for compensating for the differing first and second diffraction efficiencies, thereby equalizing the first and second intensities. and a drive circuit comprising a controller configured to select a phase offset of two. 2. The apparatus of claim 1, wherein the drive circuit is adapted to cause the acousto-optic medium to simultaneously deflect the input beam into at least the first and second output beams. drive signals having first and second phase offsets in said respective phases in parallel to said piezoelectric transducer. 2. The apparatus of claim 1, wherein the controller causes the acousto-optic medium to scan at least the first and second beams over respective first and second angular ranges. An apparatus configured to vary at least said first and second frequencies of first and second drive signals and to vary said respective phase offsets in response to said varying frequencies. 4. The apparatus of claim 3, wherein the controller controls successive pulse intervals interspersed by stop intervals in which the intensity of the first beam on target is attenuated to less than 50% of a predetermined beam intensity. while the acousto-optic medium has the predetermined beam intensity and controls the drive signal applied by the drive circuit to deflect at least the first beam toward the target. configured to
The first drive signal has a predetermined amplitude, a frequency corresponding to the deflection angle of the beam during each pulse interval, and a chirped frequency spectrum during each stop interval. device to 2. The apparatus of claim 1, wherein the first diffraction efficiency is greater than the second diffraction efficiency, and the controller determines that the acoustic wave at the second frequency satisfies the Bragg condition for the input beam. and the different first and second diffraction efficiencies by setting the second phase offset such that the acoustic wave at the first frequency deviates from the Bragg condition for the input beam. An apparatus characterized in that it is configured to compensate. 6. The apparatus of claim 5, wherein the controller maintains a constant power level of the respective drive signal regardless of the respective phase offset, while varying the respective phase offset by: An apparatus further configured to turn on and off each of the output beams. An optical device,
An acousto-optic medium that receives an input beam of radiation and is angularly divided between successive pulse intervals interspersed by stoppage intervals in which the intensity of the beam on the target is attenuated to less than 50% of the predetermined beam intensity. an acousto-optic medium configured to deflect the input beam toward the target over a range with the predetermined beam intensity;
at least one piezoelectric transducer attached to the acousto-optic medium;
A drive circuit coupled to apply a drive signal to said at least one piezoelectric transducer, said drive signal having a predetermined amplitude and corresponding to a deflection angle of said beam during each pulse interval. and a drive circuit having a frequency and having a chirped frequency spectrum between each stop interval. 8. The apparatus of claim 7, wherein said chirped frequency spectrum is such that said intensity of said beam on said target is attenuated to less than 10% of said predetermined beam intensity during said blocking interval. A device characterized by being selected. 8. The apparatus of claim 7, wherein the chirped frequency spectrum comprises a series of discrete frequency steps applied between each stop interval. 10. The apparatus of claim 9, wherein the at least one piezoelectric transducer comprises an array of piezoelectric transducers and the drive circuit directs acoustic waves to the input beam as Bragg's during the pulse interval. phases selected to propagate through the acousto-optic medium at wavefront angles that satisfy the condition and deviate from the Bragg condition for the input beam at each of the discrete frequency steps during the stop interval. and applying a respective drive signal to the piezoelectric transducer. An optical device,
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 piezoelectric transducers attached to the acousto-optic medium;
A drive circuit coupled to apply respective drive signals to the piezoelectric transducers, the drive signals selected to propagate acoustic waves through an acousto-optic medium at selected frequencies. frequency, thereby deflecting the input beam at a corresponding deflection angle within the range, and modulating the intensity of the deflected beam by adjusting the wavefront angle of the acoustic wave. and a drive circuit having a phase offset between the drive signals applied to the transducers in the array. A method of optical scanning, comprising:
directing an input beam of radiation to be incident on an acousto-optic medium having an array of piezoelectric transducers mounted thereon;
applying respective drive signals to the piezoelectric transducers, the drive signals directing the acousto-optic medium to the acousto-optic medium characterized by different respective first and second diffraction efficiencies; different respective second beam angles 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 least a first phase offset having different respective first and second phase offsets for said first and second frequency components at first and second frequencies and at each of said plurality of piezoelectric transducers; and a second frequency component;
To compensate for the different first and second diffraction efficiencies, thereby equalizing the first and second intensities, acoustic waves are applied at the first and second frequencies to different respective first and second frequencies. selecting said first and second phase offsets to propagate through said acousto-optic medium with a second wavefront angle. 13. The method of claim 12, wherein applying the respective drive signals causes the acousto-optic medium to simultaneously deflect the input beam into at least the first and second output beams. applying said first and second drive signals concurrently to said piezoelectric transducer having first and second phase offsets in said respective phases. 13. The method of claim 12, wherein applying the respective drive signals causes the acousto-optic medium to direct at least the first and second beams over respective first and second angular ranges. Selecting the first and second phase offsets includes varying at least the first and second frequencies of the first and second drive signals in a scanning manner, wherein selecting the first and second phase offsets varies the varying frequencies. and varying said respective phase offsets in response to . 15. The method of claim 14, wherein applying the respective drive signal is interspersed with blocking intervals, wherein the intensity of the first beam on target is attenuated to less than 50% of a predetermined beam intensity. controlling the drive signal such that during successive pulse intervals the acousto-optic medium has the predetermined beam intensity and deflects at least the first beam toward the target. including
The first drive signal has a predetermined amplitude, a frequency corresponding to the deflection angle of the beam during each pulse interval, and a chirped frequency spectrum during each stop interval. how to. 13. The method of claim 12, wherein the first diffraction efficiency is greater than the second diffraction efficiency, and 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 and the acoustic wave at the first frequency deviates from the Bragg condition for the input beam; A method, comprising compensating for said different first and second diffraction efficiencies. 17. The method of claim 16, wherein selecting the first and second phase offsets includes maintaining a constant power level of the respective drive signals regardless of the respective phase offsets: and turning each of said output beams on and off by changing said respective phase offset. A method of 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;
applying a drive signal to the at least one piezoelectric transducer, the drive signal having a predetermined amplitude and a frequency corresponding to the deflection angle of the beam during each successive pulse interval; and has a chirped frequency spectrum during each successive stop interval interspersed by said pulse intervals, such that said acousto-optic medium, during each successive pulse interval, over a range of angles, Deflect the input beam toward a target with a predetermined beam intensity, and attenuate the intensity of the beam on the target to less than 50% of the predetermined beam intensity during each of the stop intervals. and applying. 19. The method of claim 18, wherein the chirped frequency spectrum is such that the intensity of the beam on the target is attenuated to less than 10% of the predetermined beam intensity during the blocking interval. A method characterized by being selected. 19. The method of claim 18, wherein the chirped frequency spectrum comprises a series of discrete frequency steps applied between each stop interval. 21. The method of claim 20, wherein the at least one piezoelectric transducer comprises an array of piezoelectric transducers, and applying the drive signal causes an acoustic wave to generate the selected to propagate through the acousto-optic medium at wavefront angles that satisfy the Bragg condition for the input beam and deviate from the Bragg condition for the input beam during the stop interval, at each of the discrete frequency steps. and applying respective drive signals having phases adjusted to the plurality of piezoelectric transducers. A method of optical scanning, comprising:
directing an input beam of radiation to be incident on an acousto-optic medium having an array of piezoelectric transducers mounted thereon;
applying respective drive signals to the piezoelectric transducers, the drive signals having frequencies selected to cause acoustic waves at the selected frequencies to propagate through the acousto-optic medium; applying to the acousto-optic medium, thereby deflecting the input beam at a corresponding deflection angle;
setting a phase offset between the transducers in the array selected to modulate the intensity of the deflected beam by adjusting the wavefront angle of the acoustic wave. and how.

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