KR20220097417A - Thermal stabilization of acousto-optical devices - Google Patents

Abstract

The 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. A plurality of arrays of piezoelectric transducers are attached to the acousto-optic medium. A drive circuit is coupled to apply a respective drive signal to the piezoelectric transducer, the drive signal causing a sound wave at the selected frequency to propagate through the acousto-optic medium, thereby deflecting the input beam at a corresponding deflection angle within range. and a phase offset between the drive signals applied to the transducers in the selected array to modulate the intensity of the deflected beam by adjusting the wavefront angle of the sound wave.

Description

Thermal stabilization of acousto-optical devices

FIELD OF THE INVENTION The present invention relates generally to optical devices and systems, and more particularly to acousto-optical devices and methods of operating such devices.

Acousto-optical 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 specific frequency, generating sound waves in an acousto-optic medium. The expansion and compression of an acoustooptic medium due to sound waves modulates the local refractive index to create a grating structure within the medium with a period determined by the frequency of the drive signal. Thus, a light beam incident on this grating is diffracted as it passes through the device.

Various types of acousto-optical devices are known in the art. For example, an acousto-optic deflector uses diffraction of an incident beam to steer the angle of the output beam. Since the deflection angle of the output beam depends on the period of the grating structure of the acoustooptic material, it can be adjusted by appropriately changing the driving signal frequency.

Some acousto-optic devices use phased array transducers to generate sound waves in an acousto-optic medium. This transducer is driven with different relative phase delays to control the angle of the sound wave propagating through the acousto-optic medium, thus adjusting the phase match between the acoustic field and the light beam to be modulated. For example, US Pat. No. 7,538,929 discloses a radio frequency (RF) (RF) modulator for performing intensity modulation of an optical wavefront using an acousto-optic bulk medium and an acousto-optic modulator comprising a transducer attached to the acousto-optic bulk medium and formed of linear array electrodes. ) to describe the phase modulation technique. A transducer driver is coupled to each electrode and is phase coherently driven to alter the angular momentum distribution of the acoustic field and alternately permit and inhibit the phase matching between the optical field and the acoustic field to produce a 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 multiple output beams at different respective angles. For example, U.S. Patent No. 5,890,789 describes a multi-beam light emitting device that splits a light beam emitted from a light source into a plurality of beams using an optical waveguide-type acousto-optic element driven by a plurality of electrical signals of different frequencies. . As another example, US Patent Application Publication No. 2009/0073544 describes an apparatus for optical splitting and modulation of monochromatic coherent electromagnetic radiation, wherein an acousto-optical element splits a beam generated by a beam source into a plurality of partial beams. do. An acousto-optic modulator disposed downstream of the acousto-optical element is supplied with a split partial beam and is driven by a further high-frequency electrical signal.

PCT International Publication WO 2016/075681 (the disclosure of which is incorporated herein by reference) describes a further example of using a phased array to drive an acousto-optical deflector. In this disclosure, an optical device includes an acousto-optic medium and a plurality of piezoelectric transducer arrays attached to the acousto-optic medium. each drive signal comprising at least first and second frequency components at different respective first and second frequencies and each having a different respective phase offset with respect to the first and second frequency components in each of the plurality of piezoelectric transducers coupled to apply to the piezoelectric transducer.

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

Accordingly, according to an embodiment of the present invention, there is provided an optical device, the optical device receiving an input beam of radiation and having the input beam having respective first and second intensities at respective first and second beam angles, respectively. an acousto-optic medium configured to deflect into at least first and second output beams, wherein the acousto-optic medium is characterized by different respective first and second diffraction efficiencies. An array of multiple piezoelectric transducers is attached to the acousto-optic medium. A drive circuit is coupled to apply a respective drive signal to the piezoelectric transducer, each drive signal having different corresponding first and second for directing the first and second output beams at respective first and second beam angles. frequency, and respective first and second phase offsets that are different for the first and second frequency components in each of the plurality of piezoelectric transducers - at respective first and second wavefront angles at which the sound waves at the first and second frequencies are different. to propagate through the acousto-optic medium; and at least first and second drive signals with A controller is configured to equalize the first and second intensities by selecting the first and second phase offsets to compensate for different first and second diffraction efficiencies.

Generally, the drive circuitry is configured to simultaneously apply at least first and second drive signals having respective first and second phase offsets to the piezoelectric transducer such that the acousto-optic medium directs the input beam to at least the first and second output beams. deflect at the same time.

In some embodiments, the controller changes at least first and second frequencies of 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, and and change each phase offset in response to a change in frequency. In one embodiment, the controller is configured to control a drive signal applied by the drive circuit such that the acousto-optic medium deflects at least a first beam towards a target at a given beam intensity during a series of pulse intervals, wherein the pulse interval is the blocking interval. interspersed between, wherein at a blocking interval the intensity of a first beam on the target is attenuated to less than 50% of a given beam intensity, wherein the first drive signal has a given amplitude and corresponds to the deflection angle of the beam during each pulse interval. frequency and has a frequency spectrum chirped during each cutoff interval.

Additionally or alternatively, if the first diffraction efficiency is greater than the second diffraction efficiency, the controller is configured such that the sound wave at the second frequency meets the Bragg condition for the input beam but the sound wave at the first frequency meets the Bragg condition for the input beam. and compensating for different first and second diffraction efficiencies by setting the second phase offset to deviate. In the disclosed embodiment, the controller is also configured to turn on and off each of the output beams by correcting each phase offset while maintaining a constant power level of each drive signal irrespective of the respective phase offset.

According to an embodiment of the present invention, there is also provided an optics device, wherein the optical device is acoustically configured to receive an input beam of radiation and deflect the input beam towards a target with a given beam intensity over an angular range for a series of pulse intervals. an optical medium, wherein pulse intervals are interspersed between block intervals, wherein the intensity of the beam on the target is attenuated by less than 50% of the given beam intensity. At least one piezoelectric transducer is attached to the acousto-optic medium. A drive circuit is coupled to apply to the at least one piezoelectric transducer a drive signal having a given amplitude and a frequency corresponding to the angle of deflection of the beam during each pulse interval and a frequency spectrum chirped during each blocking interval.

In some embodiments, the chirped frequency spectrum is selected such that the intensity of the beam on the target is attenuated by less than 10% of the given beam intensity during the block interval.

Additionally or alternatively, the chirped frequency spectrum includes a series of individual frequency steps applied during each cutoff interval. In the disclosed embodiment, the at least one piezoelectric transducer comprises an array of multiple piezoelectric transducers, and the drive circuitry causes but blocks the sound wave to propagate through the acoustooptic medium at a wavefront angle that meets the Bragg condition for the input beam during the pulse interval. and apply to the piezoelectric transducer a respective drive signal having a phase selected to deviate from the Bragg condition for the input beam at each respective frequency step during the interval.

Additionally, according to an embodiment of the present invention, there is provided an optical apparatus comprising 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. the drive circuitry modulates the intensity of the deflected beam by adjusting the wavefront angle of the sound wave with the selected frequency to cause a sound wave at the selected frequency to propagate through the acousto-optic medium and thereby deflect the input beam at a corresponding deflection angle within the range coupled to apply to the piezoelectric transducers each drive signal having a phase offset between the drive signals applied to the transducers in the selected array.

Also according to an embodiment of the present invention, there is provided a method for optical scanning, the method comprising directing an input beam of radiation incident on an acousto-optical medium to which an array of a plurality of piezoelectric transducers is attached. each comprising at least a first and a second frequency component at different respective first and second frequencies and each having different respective first and second phase offsets with respect to the first and second frequency components in each of the plurality of piezoelectric transducers a drive signal is applied to the piezoelectric transducer to cause the acoustooptic medium to deflect the input beam into at least first and second output beams having respective first and second intensities at respective first and second beam angles; wherein the acousto-optic medium is characterized by different respective first and second diffraction efficiencies. Compensating for different first and second diffraction efficiencies by selecting first and second phase offsets such that sound waves at the first and second frequencies propagate through the acousto-optic medium at different respective first and second wavefront angles and equalize the first and second intensities.

Further, according to an embodiment of the present invention, there is provided a method for optical scanning, the method comprising directing an input beam of radiation incident on an acousto-optic medium to which at least one piezoelectric transducer is attached. a drive signal having a given amplitude and a frequency corresponding to the deflection angle of the beam during each of the series of pulse intervals and having a frequency spectrum chirped during each of a series of cutoff intervals interspersed between the pulse intervals is applied to the at least one piezoelectric transducer, Causes the acoustooptic medium to deflect an input beam towards a target with a given beam intensity over an angular range during each of a series of pulse intervals and attenuate the intensity of the beam on the target to less than 50% of the given beam intensity during each block interval.

Also according to an embodiment of the present invention, there is provided a method for optical scanning, the method comprising directing an input beam of radiation incident on an acousto-optical medium to which an array of a plurality of piezoelectric transducers is attached. A respective drive signal having a selected frequency is applied to the piezoelectric transducer to cause a sound wave at the selected frequency to propagate through the acousto-optic medium so that the acousto-optic medium deflects the input beam at a corresponding deflection angle. By adjusting the wavefront angle of the sound wave, we set the phase offset between the transducers in the array to modulate the intensity of the deflected beam.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be more fully understood from the following detailed description of embodiments in conjunction with the drawings:
1 is a schematic diagram of a multi-beam deflection system according to an embodiment of the present invention.
2 is a schematic diagram of a frequency chirped signal applied to an acousto-optical deflector according to an embodiment of the present invention.
3 is a schematic cross-sectional view of an acousto-optical deflector used to generate multiple output beams in accordance with an embodiment of the present invention;
4 is a schematic cross-sectional view of an acousto-optical deflector driven by a phased array transducer according to an embodiment of the present invention.
5 is a block diagram schematically illustrating a multi-frequency driving circuit for an acousto-optical deflector according to an embodiment of the present invention.
6 is a schematic diagram of diffraction efficiency as a function of phase offset between transducers driving an acousto-optical deflector according to an embodiment of the present invention;
7 is a schematic plot of a frequency chirped signal applied to an acousto-optical deflector according to another embodiment of the present invention.

survey

Acousto-optical devices are used in many laser applications to control laser beam intensity and direction at high speeds (typically in the 50 kHz - 1 MHz range) and high resolution. For precise control of the laser beam, it is important to carefully control the temperature of the acousto-optic crystal, which can be affected by the acoustic absorption of the crystal itself. For example, when a uniform temperature change occurs in a crystal, this changes the speed of sound (and refractive index) of the crystal, resulting in a drift in the deflection angle of the laser beam. Non-uniform temperature changes can distort the laser beam, causing lens effects and other refractive effects.

However, maintaining an acousto-optic crystal at a stable temperature is difficult 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 sent towards a target can be intermittently blocked by blocking the RF signal to the crystal (an operation called "pulse peaking"). However, the resulting change in RF input power will cause temperature changes and unstable thermal behavior in the crystal. Additionally, steering the laser beam direction by changing the driving frequency applied to the acousto-optic crystal can also lead to temperature changes, as different driving frequencies typically require different RF power levels to achieve the same laser pulse energy. This is because the acoustic absorption of the crystal is highly dependent on the frequency.

The embodiments of the invention described herein provide a novel technique that can be used to keep the temperature of an acousto-optic crystal stable and uniform despite the problems inherent in pulse peaking and beam steering. Such an embodiment allows the crystal to steer one or more radiation beams and alternately pass and block beams while maintaining a constant RF input level to the crystal. In this context, the term “constant” means that the instantaneous RF power of a driving signal input to a piezoelectric transducer or transducers driving a crystal is maintained within a predefined limit despite steering and intermittent blocking of the beam or beams, and is generally This means that during operation of the acousto-optic device it varies within 10%, possibly within 5% (larger or smaller restrictions are possible depending on application requirements).

In some embodiments, the acousto-optic medium (typically a suitable crystal) is driven by at least one piezoelectric transducer attached thereto during a series of pulse intervals to deflect the input beam towards the target with a given beam intensity over an angular range. Pulse intervals are interspersed between block intervals, in which the intensity of the beam on the target is attenuated to less than 50%, possibly less than 10%, less than 5% of a given beam intensity, or for high-sensitivity applications. cases are even attenuated by less than 1%. (The term "intensity" is used in its ordinary sense in the context of the specification and claims to mean light output per unit area upon which a beam is incident.)

The drive circuit applies to the 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 frequency spectrum during each blocking interval. Since the frequency chirp has a frequency range and duration chosen to cause the input beam to spread over a large area of the target, the intensity of the beam on the target during the cutoff interval is much lower than that of the deflected beam during the pulse interval. Despite the different frequency spectra, the drive circuitry keeps a given amplitude of the drive signal substantially the same in both the pulse interval and the cutoff interval.

This approach can be extended to stabilize temperature in multi-frequency operation, in which a composite drive signal formed by the 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 may be applied as part of the drive signal to select one or more output beams to be blocked. Additionally or alternatively, by selecting a specific phase offset (also called phase delay) between the transducers for each single frequency component, the diffraction efficiency for the selected component can be reduced, thereby significantly reducing the intensity of a specific output beam. have. Using this approach, the number and direction of output beams can be changed while maintaining a constant RF power flow to the device (and consequently reducing temperature fluctuations).

Different phase offsets between the transducers cause sound waves at different frequencies to propagate through the acousto-optic medium at different respective wavefront angles. The wavefront angle at each frequency is specially chosen to either meet the Bragg condition (thus achieve maximum diffraction efficiency for a given RF power at this frequency) or deviate from the Bragg condition (thus reduce the diffraction efficiency) can be Using this property, the phase offset can be set to compensate for inherent variations in the diffraction efficiency of the acousto-optic medium as a function of frequency and beam angle, thus maintaining a constant input RF power to the acousto-optic modulator while maintaining the intensity of the output beam. can be equalized. (In this context, "same" as with the term "constant" means that the intensity of the output beam varies within 10%, possibly within 5%.)

In one embodiment, the phase offset is intermittently modified by a sufficient amount to deviate significantly from the Bragg condition to turn off each output beam during a specified blocking interval while still maintaining a constant input RF power. Additionally or alternatively, this function may be combined with the frequency chirp described above during the cutoff 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 that is deflected by the acousto-optic medium. In some embodiments, the drive circuitry is configured such that the acoustooptic medium has a phase offset between the transducers in the array selected to modulate the intensity of the deflected beam and a frequency selected to deflect the input beam (or beams) at a corresponding deflection angle within range. A drive signal is applied to the piezoelectric transducer. Thus, it is possible to modulate (beam on and off) the deflected beam intensity while maintaining a constant RF power input to the acousto-optic medium.

System Description

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

The deflector 24 may comprise any suitable acousto-optic medium known in the art including a crystalline material such as quartz, tellurium dioxide (TeO ₂ ), germanium, or a glass material such as fused silica or chalcogenide glass. can The crystalline medium can be cut along certain preferred crystal directions to obtain desired acousto-optic properties with respect to, for example, the speed of sound and birefringence. Transducer 26 may similarly comprise one or more pieces of any suitable piezoelectric material, such as lithium niobate, which is generally attached to an acousto-optic medium through a metal bonding layer. Details of the operation of the driving circuit 28 and the driving signals it generates are set forth in the following figures and the description below.

In the illustrated embodiment, a scanning mirror 32 scans the output beam 30 over an angular range 38 . The beam is focused onto a target surface 36 through a scan lens 34 . This kind of arrangement can be used in a variety of applications such as multi-beam laser drilling and printing. The drive signal applied to the transducer by the driver 28 causes each beam 30 to strike the target with a given beam intensity during a series of pulse intervals, but during each specific blocking interval, with each beam interspersed between pulse intervals. chosen to be blocked. (As noted above, "blocked" means that the intensity of the beam on the target is attenuated by less than 50%, typically less than 10%, or in some cases less than 5% or even less than 1% of a given beam intensity. .) As mentioned above, this beam blocking can be achieved by changing 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) may include dual axis mirrors that can be scanned together or independently and/or any other suitable type known in the art. A beam scanner may be used. In an alternative embodiment, two acousto-optical deflectors may be arranged in series, one of which splits the input beam 23 into a plurality of output beams that are split along a first direction and the other beams in an orthogonal direction. scan the All such embodiments may use the various actuation schemes described herein and are considered to be within the scope of the present invention.

chirped Pulse using the frequency spectrum picking

2 is a schematic diagram of a frequency chirped signal applied to an acousto-optical deflector 24 by a driver 28 in accordance with an embodiment of the present invention. Instead of blocking the RF signal for pulse blocking, this kind of chirped signal can be applied during pulse blocking. The chirp of 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 time t _STOP . For example, the frequency range from f _START to f _STOP can cover all or most of the spectral bandwidth of an acoustooptic deflector, which is typically on the order of tens to hundreds of megahertz. The time range from t _START to t _STOP can be approximately equal to the travel time of the sound wave across the diameter of the input beam, which is typically on the order of a few microseconds. This chirp may be applied to one or more of a single output beam from deflector 24 or a set of multiple output beams 30 .

The chirped signal causes 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 light output as in the focused output beam, but the intensity will be attenuated by more than 90%, possibly up to 50 dB, depending on the optical configuration. As a result, the laser pulse will essentially not affect the target. Alternatively, using a weaker chirp with a reduced frequency range, for example, to preheat an area of the target surface that will later be irradiated with a higher intensity by a focused spot, a large enough intensity is applied to the target surface. The laser beam can be defocused to form a spot.

Intensity control of multiple output beams

3 is a schematic cross-sectional view of an acousto-optical deflector 24 according to an embodiment of the present invention. This figure shows the effect and operation of the multi-frequency driving provided by the driving circuit 28 and the piezoelectric transducer 26 . The multi-frequency drive signal from the drive circuit 28 causes the piezoelectric transducer 26 to generate sound waves at multiple drive frequencies, which propagate through the acoustooptic medium of the deflector 24 . Each of the different driving frequencies sets an acousto-optic diffraction grating in the crystal at the corresponding spatial frequency, ie, the crystal comprises a plurality of 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 embodiment described below, each grating is determined by the phase of the drive signal applied by the drive circuit 28 . have different wavefront angles.

As the input beam 23 enters the deflector 24, each of the gratings of the deflector diffracts the input beam at different angles depending on the grating frequency. Thus, the deflector 24 directs the input beam ₂₃ to _a plurality _of output beams 30a, 30b, 30c, 30d, …). Optics 34 focus the output beam to form an array 1 , 2 , ... of corresponding spots on target surface 36 . By modulating the frequency spectrum and/or phase of the signal at a corresponding frequency that is appropriately synchronized with the pulses of the input beam 23, the drive circuit 28 generates a corresponding output beam 30 generated by each pulse of the input beam. ) can be controlled. Additionally or alternatively, the drive circuit 28 may modulate the component frequencies f ₁ , f ₂ , ... to modulate the corresponding angles θ ₁ , θ ₂ , … and thus the surface 36 . You can change the position of the spot on the image.

More specifically, the drive circuit 28 can individually turn on and off the beams 30a, 30b, 30c, 30d, ... by controlling the phase and/or frequency spectrum of the corresponding frequency component, and thus each It is possible to select a combination of output beams 30 to generate in the pulse. (In the example shown in FIGS. 1 and 3 , beam 30c is turned off.) Additionally or alternatively, drive circuit 28 compensates for variations in the diffraction efficiency of deflector 24 as a function of angle. In order to do this, the phase of the frequency component can be controlled. Thus, the drive circuit 28 can equalize the intensities of the beams 30a, 30b, 30d while maintaining, for example, a constant RF power input to the deflector 24 despite variations in diffraction efficiency.

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

The drive circuit 28 is conceptually shown as including a frequency generator 42 , which drives the transducer 40 via a respective phase shifter 44 , such that the drive signal with a different respective phase offset is generated in the transducer. is supplied to 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. As a result, the wavefront of the sound wave 46 propagating through the acousto-optic medium of the deflector 24 is not parallel to the plane of the medium to which the transducer 40 is attached.

For maximum diffraction efficiency, since the wavefront angle can be selected by an appropriate setting of the phase shifter 44, the angle θ between the input beam 23 and the wavefront is the Bragg condition for a given driving frequency, i.e. sinθ=nλ/2d where λ is the wavelength of the input beam, n is the diffraction order (typically n=1), and d is the wavelength of the sound wave at a given frequency. This choice of wavefront angle improves the diffraction efficiency by the deflector 24, especially at frequencies far from f ₀ (the frequency at which the Bragg condition can be satisfied by setting the phase difference between adjacent transducers 40 to zero).

Alternatively, the phase adjustment circuit 48 may modulate the diffraction efficiency (and thus the intensity of the output beam generated from the deflector 24) by adjusting the wavefront angle to a value that deviates from the Bragg condition by a controlled amount. This approach can be used, for example, to compensate for inherent variations in the diffraction efficiency of a deflector as a function of frequency and deflection angle, thus maintaining a constant intensity of the deflected beam or beams while driving the deflector at a constant RF power level. have. Additionally or alternatively, the phase adjustment circuit 48 may apply a larger modulation to the phase offset to compromise the diffraction efficiency, so that the output beam when desired without changing the RF power level input to the deflector 24 . can be turned off.

In some embodiments, the drive circuit 28 applies each multi-frequency drive signal having a frequency component of a number of different frequencies to the piezoelectric transducer 40 . For each of these frequencies, the Bragg condition produces a different diffraction angle. Thus, for optimal performance of the deflector 24 at all frequencies, the phase adjustment circuit 48 drives the phase shifter 44 to apply a different phase offset for each frequency to each transducer 40. . As a result, the sound wave 46 at frequency propagates through the acoustooptic medium at different respective wavefront angles, which correspond to the respective Bragg conditions for the corresponding frequencies f ₁ , f ₂ , ... and the output beam 30 . is chosen for the deflection angles θ ₁ , θ ₂ , ….

5 is a block diagram schematically showing the functional components of a drive circuit 28 for an acousto-optical deflector 24 according to an embodiment of the present invention. The digital components of drive circuit 28 may be implemented in hardwired or programmable logic, such as in generally programmable gate arrays. Although the blocks in FIG. 5 are shown as separate components for conceptual clarity, in practice the functions of these components may be combined into a single logic device. Alternatively, at least some of the digital components of circuit 28 may be implemented in software running on a computer or dedicated microprocessor.

When the frequency selection block 50 drives the deflector 24, it selects a number of fundamental frequencies f ₁ , f ₂ , … to be applied to output with corresponding deflection angles θ ₁ , θ ₂ , … Create a beam 30 . If the output beam angle is to be scanned transversely (as in system 20 shown in Figure 1), block 50 can be programmed to modulate each of these frequencies over time by an amount up to ±Δf. and, as a result, can result in angular scanning of each beam by up to ±Δθ. Thus, in general, block 50 generates a sequence of frequency vectors, each vector comprising m fundamental frequency values {f _i + δf _i } to be applied to deflector 24 at a particular time. Produces m output beams 30 at an angle {θ _i + δθ _i }, where δf _i and δθ _i are the frequency and angular change within the ±Δf and ±Δθ ranges, respectively.

Phase adjustment block 54 generates multiple streams of time domain samples corresponding to the frequency components provided by block 50 . Each stream is directed to a respective converter 40 and contains the same frequency components but with different respective phase offsets. This phase offset is selected according to the desired wavefront angle of the sound wave 46 in the deflector 24 at each frequency. In general, the relative phase offset between sample streams is not uniform over the entire frequency range, but rather increases with frequency, so the wavefront angle increases with frequency, as described above, with the Bragg condition at each frequency.

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

In this formula:

는 주파수 f에서 블록(54)의 2개의 인접한 출력 채널 간의 위상차이고; ·

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 speed of sound in the acousto-optic medium;

· f ₀ is the applied frequency that makes the phase difference between adjacent channels zero 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 to intentionally deviate from the Bragg condition with a selected phase deviation as described above.

As noted above, 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 , which produces a corresponding output signal for driving converter 40 . (Other analog components, such as the RF amplifier between the D/A converter channel and the converter, are omitted from the figure for simplicity.) Assuming proper selection of frequency components and phase offsets, the converter is It will create a superposition of sound waves with different frequencies and different wavefront angles.

6 is a schematic diagram of diffraction efficiency as a function of the phase offset Δφ between adjacent transducers 40 driving an acousto-optical deflector 24 according to an embodiment of the present invention. Curves 60, 62 and 64 show the diffraction efficiencies at three different driving frequencies, typically in the range of about 50 MHz to 150 MHz. The maximum diffraction efficiency DE _max of each curve corresponds to the phase offset Δφ _max at which the wavefront angle at a given frequency satisfies the Bragg condition. Nevertheless, the maximum diffraction efficiency does not reach 100% and differs between curves 60, 62 and 64. Moving away from this maximum, the diffraction efficiency DE as a function of the phase offset Δφ changes approximately sinusoidally.

Phase adjustment block 54 may apply the above relationship, for example, in modulating the intensity of each output beam from deflector 24 corresponding to the frequency of curves 60 , 62 and 64 . Assuming that the RF power applied to the converter 40 remains constant, the phase offset Δφ required to achieve a given output beam intensity I is given as the inverse cosine of I/I ₀ , where I ₀ is the given drive frequency f is the intensity output at the maximum diffraction efficiency for As described above, Δφ values at different driving frequencies can be used to compensate for differences in maximum diffraction efficiency at different frequencies. Alternatively or additionally, in order to effectively block one of the output beams, Δϕ can be set to Δϕ _max + π, so that the diffraction efficiency will be close to zero. Additionally, alternatively or additionally, the intensity of the output beam may be modulated by modulating the phase offset while keeping the RF input power constant.

7 is a schematic plot of a multi-frequency signal applied to an acousto-optic deflector according to another embodiment of the present invention. This signal has a chirped frequency spectrum similar to the signal shown in FIG. 2 , but in this case includes a series of discrete frequency steps 70 . A signal of this kind can be conveniently applied by a digital drive circuit such as the drive circuit shown in FIG. 5 for an interval during which a given output beam is blocked.

This frequency chirping approach can be advantageously combined with the approach based on adjusting the phase offset between the transducers described above. The phase is selected at each frequency step 70 to cause the sound wave to propagate through the acousto-optic medium at a wavefront angle that deviates from the Bragg condition at a given frequency. For maximum attenuation of the output beam intensity, the phase offset at each frequency may be set to approximately Δφ _max +π, where Δφ _max is the phase offset that satisfies the Bragg condition at the corresponding frequency.

It is to be understood that the embodiments described above are taken by way of example, and that the invention is not limited to what has been particularly shown and described above. Rather, the scope of the present invention includes combinations and subcombinations of the various features described above, as well as variations and modifications not disclosed in the prior art that would occur to those skilled in the art upon reading the foregoing description.

Claims (22)

An optical device comprising:
an acousto-optic medium configured to receive an input beam of radiation and deflect the input beam into at least first and second output beams having respective first and second intensities at respective first and second beam angles, the acoustic the optical 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 a respective drive signal to the piezoelectric transducer, each drive signal comprising a different corresponding second for directing the first and second output beams to the respective first and second beam angles. first and second frequencies, and respective first and second phase offsets that are different with respect to the first and second frequency components in each of the plurality of piezoelectric transducers, each of which the sound waves at the first and second frequencies are different at least first and second drive signals to propagate through the acousto-optic medium at first and second wavefront angles, the first and second drive signals to compensate for the different first and second diffraction efficiencies and a controller configured to select two phase offsets to equalize the first and second intensities.
comprising, an optical device. 2. The acousto-optic medium of claim 1, wherein the drive circuit is configured to simultaneously apply the at least first and second drive signals having the respective first and second phase offsets to the piezoelectric transducer such that the acousto-optic medium directs the input beam. simultaneously deflecting the at least first and second output beams. 2. The apparatus of claim 1, wherein the controller is configured to: the at least first and second of the first and second drive signals such that the acousto-optic medium scans at least first and second beams over respective first and second angular ranges. and change the frequency and change the respective phase offset in response to the change in frequency. 4. The system of claim 3, wherein the controller is configured to control the drive signal applied by the drive circuit such that the acousto-optic medium deflects the at least first beam towards a target at a given beam intensity during a series of pulse intervals and , wherein the pulse intervals are interspersed between block intervals, wherein the intensity of the first beam on the target is attenuated to less than 50% of the given beam intensity,
wherein the first drive signal has a given amplitude and a frequency corresponding to the deflection angle of the beam during each pulse interval and has a chirped frequency spectrum during each block interval. The method of claim 1 , wherein the first diffraction efficiency is greater than the second diffraction efficiency, and the controller is configured to determine that the sound wave at the second frequency meets a Bragg condition for the input beam but the sound wave at the first frequency is configured to compensate for the different first and second diffraction efficiencies by setting the second phase offset to deviate from the Bragg condition for the input beam. 6. The method of claim 5, wherein the controller is further configured to turn on and off each of the output beams by modifying each phase offset while maintaining a constant power level of each drive signal irrespective of the respective phase offset. , optical devices. An optical device comprising:
an acousto-optic medium configured to receive an input beam of radiation and deflect the input beam toward a target at a given beam intensity over an angular range over a series of pulse intervals, the pulse spacing interspersed between the blocking intervals where the beam intensity on the target is attenuated to less than 50% of the given beam intensity;
at least one piezoelectric transducer attached to the acousto-optic medium; and
a drive circuit coupled to apply to the at least one piezoelectric transducer a drive signal having a given amplitude and having a frequency corresponding to the angle of deflection of the beam during each pulse interval and a frequency spectrum chirped during each blocking interval
comprising, an optical device. 8. The optical apparatus of claim 7, wherein the chirped frequency spectrum is selected such that the intensity of the beam on the target is attenuated by less than 10% of the given beam intensity during the block interval. 8. The apparatus of claim 7, wherein the chirped frequency spectrum comprises a series of discrete frequency steps applied during each cutoff interval. 10. The acousto-optic medium 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 oriented at a wavefront angle at which a sound wave meets a Bragg condition for the input beam during the pulse interval. and apply to the piezoelectric transducer a respective drive signal having a phase selected to propagate through but out of the Bragg condition for the input beam at each of the respective frequency steps during the blocking interval. An optical device comprising:
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 attached to the acousto-optic medium; and
modulates the intensity of the deflected beam by adjusting the wavefront angle of the sound wave with a selected frequency to cause a sound wave at a selected frequency to propagate through the acousto-optic medium and thereby deflect the input beam at a corresponding deflection angle within range a drive circuit coupled to apply to the piezoelectric transducers each drive signal having a phase offset between the drive signals applied to the transducers in the array selected to
comprising, an optical device. A method for optical scanning, comprising:
directing an input beam of radiation incident on an acousto-optic medium to which an array of multiple piezoelectric transducers is attached;
at least first and second frequency components at different respective first and second frequencies and having different respective first and second phase offsets with respect to the first and second frequency components in each of the plurality of piezoelectric transducers; applying a respective drive signal to the piezoelectric transducer, wherein the acoustooptic medium has at least first and second output beams having respective first and second intensities at respective first and second beam angles, respectively, the input beam ; wherein the acousto-optic medium is characterized by different respective first and second diffraction efficiencies; and
selecting the first and second phase offsets such that sound waves at first and second frequencies propagate through the acousto-optic medium at different respective first and second wavefront angles, the first and second different Compensating for diffraction efficiency and equalizing said first and second intensities -
A method for optical scanning comprising: 13. The method of claim 12, wherein applying each of the drive signals comprises simultaneously applying to the piezoelectric transducer at least first and second drive signals having the respective phase first and second phase offsets, and the acousto-optic medium causes the input beam to simultaneously deflect the at least first and second output beams. 13. The method of claim 12, wherein applying each of the drive signals comprises changing at least first and second frequencies of the first and second drive signals, such that the acousto-optic medium comprises each of the first and second drive signals. scan at least the first and second beams over two angular ranges;
wherein selecting the first and second phase offsets includes changing each phase offset in response to the frequency change. 15. The method of claim 14, wherein applying each drive signal comprises controlling the drive signal such that the acousto-optic medium directs the at least first beam toward a target at a given beam intensity during a series of pulse intervals. wherein the pulse spacing is interspersed between blocking intervals, wherein the intensity of the first beam on the target is attenuated by 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 has a chirped frequency spectrum during each block interval. 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 causing the sound wave at the second frequency to satisfy a Bragg condition for the input beam. compensating for the different first and second diffraction efficiencies by setting the second phase offset such that the sound wave at the first frequency deviates from the Bragg condition for the input beam. A method for scanning. 17. The method of claim 16, wherein selecting the first and second phase offsets turns on each of the output beams by modifying the respective phase offsets while maintaining a constant power level of each drive signal irrespective of the respective phase offsets. and turning off. A method for optical scanning, comprising:
directing an input beam of radiation incident on an acousto-optic medium to which at least one piezoelectric transducer is attached; and
to the at least one piezoelectric transducer a drive signal having a given amplitude and a frequency corresponding to the angle of deflection of the beam during each of a series of pulse intervals and having a frequency spectrum chirped during each of a series of cutoff intervals interspersed between the pulse intervals. applying - cause the acousto-optic medium to deflect the input beam towards a target at a given beam intensity over an angular range during each of the series of pulse intervals and measure the intensity of the beam on the target during each of the blocking intervals to the given beam to be attenuated to less than 50% of the intensity -
A method for optical scanning comprising a. 19. The method of claim 18, wherein the chirped frequency spectrum is selected such that the intensity of the beam on the target is attenuated by less than 10% of the given beam intensity during the block interval. 19. The method of claim 18, wherein the chirped frequency spectrum comprises a series of discrete frequency steps applied during each cutoff 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: a sound wave at a wavefront angle that meets a Bragg condition for the input beam during the pulse interval. applying to the plurality of piezoelectric transducers a respective drive signal having a phase selected to propagate through the acousto-optic medium but deviate from the Bragg condition for the input beam at each of the respective frequency steps during the blocking interval. A method for optical scanning. A method for optical scanning, comprising:
directing an input beam of radiation incident on an acousto-optic medium to which an array of multiple piezoelectric transducers is attached;
applying a respective drive signal having a selected frequency to the piezoelectric transducer to cause a sound wave at the selected frequency to propagate through the acousto-optic medium so that the acousto-optic medium deflects the input beam at a corresponding deflection angle; and
establishing 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 sound wave;
A method for optical scanning comprising a.

Images