Classifications

• • G—PHYSICS
  • G02—OPTICS
  • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
  • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
  • G02F1/29—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the position or the direction of light beams, i.e. deflection
  • G02F1/33—Acousto-optical deflection devices
  • G02F1/332—Acousto-optical deflection devices comprising a plurality of transducers on the same crystal surface, e.g. multi-channel Bragg cell
• • G—PHYSICS
  • G02—OPTICS
  • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
  • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
  • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
  • G02F1/11—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on acousto-optical elements, e.g. using variable diffraction by sound or like mechanical waves

Definitions

• This invention relates to modulation of light beams and more specifically to an improved acousto-optic modulator with higher packing density of the modulated laser beams.
• Multi-channel laser beam systems used for instance in laser writing applications such as imaging patterns onto photo-resist using multiple laser beams for purposes of creating electronic circuit substrates, use the well known acousto-optic modulator array.
• electrical energy is converted to acoustic waves by a piezoelectric transducer, and the acoustic waves modulate the incident laser (light) beams.
• the acoustic waves distort the optical index of refraction of the modulator body, typically made of crystalline material or glass, through which the laser beams pass. This distortion is periodic in space and time and thus provides a three dimensional dynamic diffraction grating that deflects or modulates the laser beams.
• Such acousto-optic devices are well known in broadband signal processing.
• An example of such modulator 10 is shown in Fig. 1 A illustrating the exterior of the modulator body 14.
• the light beam 16 enters from the left surface of the body 14 and passes through the body 14.
• the horizontal lines are intended to suggest diffraction grating properties; it is to be understood that the molecules in the modulator body, compressed or stretched by the presence of acoustic waves, provide the effect of a three dimensional dynamic phase grating and it is not a conventional diffraction grating.
• the electrical input signal (“input") is applied to the surface electrode 20 of the transducer body 21 which is made of a thin platelet of piezoelectric material bonded to the surface of the modulator body 14.
• Transducer body 21 is located under electrode 20.
• Light beam 16 enters the body 14 through the surfaces orthogonal to the surface to which the piezoelectric transducers 21 are bonded.
• the frequency and power of this electrical input signal determines to what extent the light beam 16 is deflected by passing through the modulator body 14 due to the presence of the resulting acoustic wave.
• an acoustic termination such as an acoustic absorber 22 is provided on the surface of the modulator body 14 opposite to the surface on which the transducer body 21 is bonded and the electrical signal is applied.
• the surface of the modulator body opposite to the surface on which transducer 21 is bonded may be cut at an angle causing incident acoustic waves to reflect off-axis and eventually be absorbed by the modulator body.
• the electrical connection with electrode 20 and (ground) electrode 24 provides an electrical input port and the voltage (signal) applied thereto creates a spatially uniform electric field in the piezoelectric active regions of transducer body 21 to cause the generation of a uniform acoustic wave traveling down the modulator body 14, which in turn, causes the intended deflection of the light beam 16.
• the actual effect is caused by appreciable variations in the refractive index of the modulator body 14 which in effect creates a moving (dynamic) diffraction grating traveling at the speed of sound with a grating strength determined by the input electrical power.
• the angle of deflection of the output light beam and its magnitude as produced by the moving diffraction grating depends on the frequency and the amplitude of the acoustic wave.
• Fig. 1A shows only a single transducer 21 with input electrode 20 for modulating a single incident light beam 16.
• Light beam in this context refers to any electro-magnetic radiation which may be so modulated, including not only visible light but also ultraviolet light and other frequencies including infra-red, etc., from a laser or other source.
• a plurality of incident laser (light) beams 16a, 16b, 16c, 16d are applied to a single modulator body.
• the modulator body has formed on its surface a corresponding number of transducer electrodes 20a, 20b, 20c, 20d, one transducer electrode for each beam to be modulated.
• Such a device as illustrated in top view in Fig. IB has the plurality of electrodes 20a, 20b, 20c, 20d on the surface of transducer body 21 which is on the modulator body 14.
• each electrode is inversely proportional to the square of modulator bandwidth (speed) and can be very small for the case of a high speed modulator array, on the order of a few hundred micrometers by a few millimeters each for modulator bandwidth on the order of tens of megahertz. It is a common practice to form such modulator transducer arrays using conventional photolithographic means to define the small electrodes. However, to prevent a short circuit, the electrodes are made with a finite gap in between.
• the intended application is to form an array of tiny laser beam dots, modulated in time, on the imaging medium, the dots having a typical packing density of 300 to 10,000 or more dots per inch.
• Moving the modulated optical dot array in a direction nominally orthogonal to the dot array orientation, i.e. raster scanning, on optically sensitive medium will produce a recorded image of the modulating signal.
• Fig. 2A illustrates the optical output beams from a four channel linear modulator array in the prior art having finite gaps between adjacent laser beam dots, and the associated laser beam intensity (right side). These gaps can be reduced or even eliminated when the scanning direction is rotated relative to the orientation of the laser beam array as further illustrated by Fig. 2B.
• each gap between adjacent channels has the same vector quantity d. Rotation of the line image by an angle ⁇ in Fig.
• the acoustic wave must travel a certain distance to engage the incident laser beam, resulting in an acoustic delay of the signal.
• the electrical feeds for the transducer electrodes have to come in from the two sides of the modulator transducer array.
• a large number of feed conductors must be squeezed to an area of only a few millimeters, the length of a transducer electrode. If the transducer elements can be spread in the direction parallel to the incident light beams, the tight packing density can be eased resulting in a two dimensional array of modulators.
• Fig. IC is a side view of the structure of Fig. IB showing only the first electrode 20a with its connecting wire 26. (The other electrodes are exactly in line with electrode 20a and hence not visible.) The group of parallel horizontal lines suggest a diffraction grating which of course is not visible in fact.
• the wavelength of the incident laser beam 16a in air and A is the acoustic wavelength.
• the output laser beams are symmetrical to the acousto-optic diffraction grating, and have an exit angle of also ⁇ B .
• the incident laser beam 16a undergoes a bend in propagation direction in the middle of the acousto-optic modulator by the total deflection angle, 2 ⁇ B .
• the transducer electrodes are spread in the direction of laser beam propagation, allowing more spacing for the RF feed circuit.
• the transducer electrodes can be arranged two dimensionally in a plurality (at least two) of columns, each column including at least one electrode.
• This two dimensional acousto-optic modulator array allows substantial increased physical separation in both dimensions between adjacent modulator elements (electrodes). The greater physical separation makes it easier for improved electrical isolation among the feed conductors to the modulator array and allows closer effective channel to channel separation.
• a two dimensional modulator array may introduce undesirable image artifacts, due to a problem related to the bending of the optical axis in the middle of an acousto-optic modulator. Hence additional care must be given in order to eliminate print gaps between adjacent scan lines.
• a row-to-row gap G may result in the effective writing beam array.
• a correction in accordance with this invention brings the resulting beams into a distribution having uniform pitch (spacing) between all adjacent beams.
• the row-to-row gap in the image is removed by providing a beam translation device such as a tilted parallel plate in the optical path of certain elements of the beams after the modulator array. This has been found to bring the beams from all channels into a single straight line of uniform pitch.
• the incident laser beams are prearranged at their source, e.g. by a tilted parallel plate, so that they form a straight line (or have uniform pitch) upon emerging from the modulator body.
• Figs. 1 A, IB, and IC show prior art acousto-optic modulator arrays.
• Figs. 2A and 2B show a prior art one dimensional electrode array and resulting scan pattern.
• Fig. 3 shows an example of a two dimensional electrode array for a modulator array in accordance with this invention.
• Fig. 4 illustrates how a row of modulators in accordance with this invention deflects aligned incident beams to exhibit lateral displacements.
• Figs. 5A, 5B, 5C, and 5D show in accordance with this invention a two dimensional electrode array and resulting scan pattern.
• Fig. 6 shows use of a tilted parallel plate for gap compensation in accordance with this invention.
• Fig. 7 shows an alternate location for a tilted parallel plate.
• Fig. 3 shows a transducer electrode array on a surface of an acousto-optic modulator body 40 in accordance with this invention. Shown in this example are eight electrodes arranged in columns. Other features such as the wires connecting to the electrodes and the acoustic absorber are not shown since they are conventional as described above; only the arrangement of the electrodes is different. In this case there are four columns; the first column includes electrodes 44a, 44e, and the columns as shown are staggered. The separation between columns is limited by the optical depth of focus, because all acousto-optic modulators must be placed within the focus of the incident laser beams to achieve peak modulation speed and diffraction efficiency. For a typical laser beam diameter of several hundred microns, the depth of focus can be many centimeters, allowing a long row of modulators or a row of parallel columns.
• Fig. 3 shows two electrodes per columns, in other embodiments there is only one electrode per column, or more than two per column. Thus advantageously the physical separation between adjacent modulator elements
• Electrodes is increased significantly, thereby making RF signal isolation easier.
• Fig. 5A, 5B, 5C, 5D illustrate the two dimensional modulated beam array with an eight channel modulator array as in Fig. 3 having eight electrodes arranged in four columns of electrodes, where the pitch between electrodes is D.
• the optical output (beam spots) of this array as shown in Fig. 5B has two lines each of four dots with the spacing (2D sin ⁇ B ) along the x (horizontal) direction and d along the y (vertical) direction.
• Fig. 5C the laser intensity profile along the y- direction is shown by Fig. 5D which is two groups of continuous scan-lines separated by a
• a beam translation device as shown in Fig. 6.
• This device is for instance a tilted "parallel plate" 50 which is a tilted (relative to the beam axis) plate of optically flat glass which laterally translates (displaces) some of the beams (channels) indicated by the horizontal lines in Fig. 6.
• the tilt angle of this plate 50 is a function of the amount of beam displacement needed to overcome the above-described gap.
• a single tilted parallel plate 50 is introduced into the common optical path for channels 1 through 4 (the top portion) of an eight channel modulator array 40, thereby putting all eight beams into a single straight line of uniform pitch as shown in the resulting written pattern 56.
• This parallel plate 50 alternatively is located “upstream” of the transducer (modulator 40), as shown in Fig. 7 to achieve a "pre-tilt".
• multiple tilted parallel plates could be provided, depending on the modulator array arrangement. For instance additional channels may require additional parallel plate(s).
• Fig. 7 has been found advantageous because prior to entering into the modulator, the beam spots are of larger diameter and therefore the spacing between channels greater, so the criticality of the tilt angle of tilt plate 50 is less. In addition, it may be easily accomplished as part of the beam splitter array that sets up the multiple laser beams for the modulator array.
• the particular shape of the electrodes has been found not to be critical for use of the present two dimensional electrode array invention.
• a typical application uses relatively long and narrow electrodes having an aspect ratio of e.g. 10 or more, although this is not limiting.
• the electrodes may be e.g. rectangular, diamond, or other shapes.
• a typical shape and size of an electrode is an elongated rectangle of dimensions 300 ⁇ m by 6 millimeters (6000 ⁇ m), providing a 20 to 1 aspect ratio.
• a typical corresponding beam spot diameter beam is 100 to 200 microns at the center of the modulator body.

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• Physics & Mathematics (AREA)
• Nonlinear Science (AREA)
• General Physics & Mathematics (AREA)
• Optics & Photonics (AREA)
• Chemical & Material Sciences (AREA)
• Crystallography & Structural Chemistry (AREA)
• Optical Modulation, Optical Deflection, Nonlinear Optics, Optical Demodulation, Optical Logic Elements (AREA)
• Laser Beam Printer (AREA)

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• 1998
  • 1998-06-19 JP JP50867099A patent/JP2002508088A/ja active Pending
  • 1998-06-19 WO PCT/US1998/012463 patent/WO1999003016A1/en not_active Application Discontinuation
  • 1998-06-19 EP EP98930258A patent/EP0988575A1/en not_active Ceased
  • 1998-06-19 IL IL13374998A patent/IL133749A0/xx unknown
  • 1998-06-19 CA CA002296278A patent/CA2296278A1/en not_active Abandoned
  • 1998-06-19 KR KR1019997012392A patent/KR20010014272A/ko not_active Application Discontinuation

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