ACOUSTOOPTIC MODULATION ELEMENT AND SYSTEM

Abstract: Responsive to a plurality of parallel light beams and electric signals corresponding to the parallel light beams, an acoustic modulation element produces output light beams modulated by acoustic waves resulting from the electric signals. The element comprises a single acoustooptic medium block receiv­ ing the parallel beams on a first surface thereof and emitting the output light beams from a second surface opposite to the first surface, A plurality of transducer units are laid in parallel on a third surface of the block between the first and the second sur­ face along the respective parallel beams so as to propagate the acoustic waves into the block when activated by the electric signals. The block may have a fourth surface opposite to the third surface and arcuately or triangularly recessed toward the third surface. Preferably, the transducer units are mounted on plateaus isolated from each other by a channel formed be­ tween two adjacent ones of the plateaus, in order to avoid interferences of the acoustic waves. The element is for use in combination with a beam splitter member for producing the parallel beams in response to a single light beam.

Description: This invention relates to an acoustooptic modulation element for use in acoustooptically modulating each light beam by an acoustic wave and to an acoustooptic modulation system comprising such an acoustooptic modulation element. A conventional acoustooptic modulation element of the type described comprises an acoustooptic medium for acoustooptically modulating a single incident light beam by an acoustic wave. More particularly, the acoustooptic medium has a first surface for the incident light beam, a second surface parallel to the first surface, and a third surface between the first and the second surfaces. A transducer is attached to the third surface so as to transduce an electric signal into the acoustic wave. When the electric signal has a plurality of frequency components different from one another, the incident light beam which is given through the first surface is modulated into a plurality of output light beams by the acoustic wave resulting from the electric signal. The output light beams are emitted through the second surface in directions dependent on the frequency components of the electric signal, as well known in the art. Each of the output light beams is recorded or printed on a recording medium in a known manner. With the acoustooptic modulation element, the number of the output light beams is determined by the number of the frequency components. This means that each output light beam becomes weak in intensity with an increase of the frequency components. Accordingly, the intensity of each output light beam fluctuates with the number of the frequency components included in the electric signal. Such fluctuation of each output light beam brings about unevenness of printing and inevitably deteriorates a quality of printing. Another conventional acoustooptic modulation system comprises a beam splitter in combination with a plurality of acoustooptic modulation elements. With this system, the beam splitter splits the incident light beam into a plurality of split light beams so as to supply the acoustooptic modulation elements with the split light beams, respectively. Accordingly, each of the acoustooptic modulation elements individually modulates each split beam by an acoustic wave resulting from an electric signal. As a result, a single modulated light beam is emitted from each acoustic modulation element in a direction dependent on each electric signal. In order to produce a plurality of modulated light beams by the use of the above-mentioned system, a plurality of acoutooptic modulation elements should be arranged in parallel. The system therefore becomes bulky in structure. In addition, the acoustooptic modulation elements should individually be adjusted to the respective split light beams emitted from the beam splitter. Such individual adjustment is very troublesome. It has been proposed in US-A-4,124,281 to provide a single acoustooptic medium block having a first surface for a plurality of input light beams and a second surface, parallel to the first, for output light beams. A transducer is positioned on a third surface contiguous with the first and second surfaces and serves to transduce electrical signals into acoustic waves propagated parallel with the third surface. Signals are stored in the block in the form of index of refraction variation patterns. It is an object of this invention to provide an acoustooptic modulation element which is capable of modulating a plurality of input light beams without any fluctuation of intensities. It is another object of this invention to provide an acoustooptic modulation element of the type described, which is compact in structure. It is a further object of this invention to provide an acoustooptic modulation system which can readily be adjusted to each of split beams emitted from a beam splitter. An acoustooptic modulation element according to this invention is characterised in that the transducing means comprises a plurality of transducer units responsive to said electric signals and extended along said input light beams on said third surface substantially parallel to one another for individually propagating said acoustic waves into said block from said third surface towards said fourth surface. The invention will now be described by way of example with reference to the accompanying drawings in which:- Fig. 1 schematically shows a perspective view of an acoustooptic modulation element according to a first embodiment of this invention; Fig. 2 shows an enlarged and fragmentary front view of the acoustooptic modulation element illustrated in Fig. 1; Fig. 3 schematically shows a perspective view of an acoustooptic modulation element according to a second embodiment of this invention; Fig. 4 partially shows a perspective view of another acoustooptic modulation element according to a modification of this invention; Fig. 5 diagrammatically shows a view of an acoustooptic modulation system according to an embodiment of this invention; Fig. 6 shows a top view of a beam splitter for use in the acoustic modulation system illustrated in Fig. 5; Fig. 7 diagrammatically shows a perspective view of a system according to another modification of this invention; and Fig. 8 diagrammatically shows a view of another acoustooptic modulation system according to a further modification of this invention. Referring to Fig. 1, an acoustooptic modulation element 20 according to a first embodiment of this invention is supplied with first through n-th input light beams 21₁, 21₂, ..., 21 n substantially parallel to one another with a distance L left between two adjacent ones of the incident light beams. The acoustooptic modulation element 20 comprises a single block 22 of an acoustooptic medium, such as a single crystal of tellurium dioxide, a single crystal of lead molybdate, optical glass, or the like. Such optical glass may be, for example, SF-6, AOT-5, AOT-44B manufactured and sold by Hoya corporation. The block 22 is of a parallelpiped of 20 mm x 20 mm x 10 mm, by way of example, and has a first surface directed leftwards of Fig. 1, a second surface directed rightwards thereof, and third and fourth surfaces directed upwards and downwards, respectively. Each of the third and the fourth surfaces are opposite to the other and contiguous to the first and the second surfaces. Each of the input light beams 21₁ to 21 n is incident on the first surface in parallel along an optical axis thereof. A transducer member is attached to the third surface and is given first through n-th electric signals which are in one-to-one correspondence to the input light beams 21₁ through 21 n . When activated by the electric signals, the transducer member produces acoustic waves, namely, ultrasonic waves, respectively. The transducer member is divided into a first through an n-th transducer units 23₁ to 23 n individually operable in response to the first through the n-th electric signals. The first through the n-th transducer units 23₁ to 23 n are extended in parallel along the respective input light beams 21₁ to 21 n on the third surface to individually propagate the acoustic waves into the block 22. Referring to Fig. 2 together with Fig. 1, each of the first through the n-th transducer units 23₁ to 23 n comprises a first electrode 26, third surface, a transducer medium 27, and a second electrode 28 which are successively stacked on the third surface. The transducer medium 27 may be common to all of the first to the n-th transducer units 23₁ to 23 n , as suggested by a broken line in Fig. 2, and may be, for example, a Y plate of 36° made of a single crystal of lithium niobate. The first electrode 26 has a first electrode width transversely of each input light beam and is laid apart from an adjacent one or ones of the first electrodes with a spacing left therebetween. The second electrode 28 may have a second electrode width W somewhat narrower than that of the first electrode 26, as shown in Fig. 2. A pair of first and the second electrodes 26 and 28 are electrically isolated from the remaining electrode pairs and are individually supplied with each of the electric signals. Each of the input light beams 21 (suffixes omitted) is incident on the first surface at the Bragg angle in an incident position and is allow to pass through the block 22. As shown in Fig. 2, the incident position is present on a center line of the second electrode 28 and has a depth D from the third surface. The depth D is, for example, 1 mm. Let each of the input light beams have a spot size d . When no electric signal is supplied across the first and the second electrodes 26 and 28, each of the first through the n-th input light beams 21₁ to 21 n is not subjected to any modulation and is sent from the second surface along the optical axis of each input light beam in the form of each of zeroth-order light beams 30₁, 30₂, ..., 30 n shown by broken lines in Fig. 1. Each of the acostic wave or ultrasonic waves is propagated from each transducer unit 23 to the block 22 towards the fourth surface by supply of each of the electric signals which are previously subjected to amplitude modulation in a known manner. As a result, each input light beam 21 is subjected to modulation by each of the acoustic waves to produce first through n-th output light beams. Each of the first through the n-th output light beams includes a first-order diffracted light beam (shown by each solid line 31₁, 31₂, ..., 31 n in Fig. 1) in addition to the zeroth-order light beam 30. Such a first-order light beam 31 is sent to a printer (not shown) to be recorded on a recording medium. With this structure, the acoustic waves are internally reflected from the fourth surface. Such reflection adversely affects modulation of each input light beam. In order to avoid the reflection from the fourth surface, an acoustic absorber 33 is attached to the fourth surface. The absorber 33 may be of lead or the like. Thus, a plurality of parallel light beams substantially parallel to one another can be modulated by the use of a single acoustooptic modulation element. Further referring to Fig. 2, consideration is made about that directivity of each acoustic wave which is specified by velocity potential, as known in the art. It is generally pointed out that the velocity potential of the acoustic wave does not diverge within the acoustooptic medium block 22, as shown by broken lines 36 in Fig. 2, on condition that the block 22 and the transducer unit 23 are surrounded by a rigid body. According to the inventors' experimental studies, it has however been found out the velocity potential of the acoustic wave practically objectionably diverges within the acoustooptic medium block 22, as shown by solid lines 37 in Fig. 2, if the above-mentioned condition is not satisfied. It is assumed in the illustrated acoustooptic modulation element that the distance L is less than five times the width W of the second electrode 28 and that one of the two adjacent light beams is modulated by an acoustic wave with the other unmodulated by any acoustic wave. For brevity of description, a combination of the input and the output light beams may be simply called a light beam, as long as any confusion does not arise between them. Let the one and the other light beams be an m-th and an (m + 1)-th light beams included in the first through the n-th light beams, respectively. Under the circumstances, it has been observed that the (m + 1)-th light beam is accompanied by a spurious first-order diffracted component resulting from modulation of the m-th light beam and that an on-off ratio, namely, an extinction ratio is reduced by the spurious first-order diffracted component. Such a spurious component results from the divergence of the velocity potential of each acoustic wave. Therefore, the distance L must be longer than five times the width W of the second electrode 28 in order to completely suppress any adverse influence on the adjacent light beams. As a result, the illustrated element 20 inevitably becomes bulky in structure. Referring to Fig. 3, an acoustooptic modulation element 20 according to a second embodiment of this invention comprises similar parts designated by like reference numerals. Like in Fig. 1, an acoustooptic medium block 22' has a first or a front surface in front of this figure, a second or a back surface substantially parallel to the first surface and opposite to the first surface, and third and fourth surfaces directed upwards and downwards, respectively. As shown in Fig. 3, the block 22' has first through n-th plateaus 41₁ to 41 n separated from one another by channels 42 extended in parallel between the first and the second surfaces. Each plateau 41 has a top surface providing a part of the third surface. In other words, the whole of the top surfaces defines the third surface. Each of the channels 42 is defined by a pair of internal side surfaces contiguous to the first through the third surfaces and a recessed surface recessed from the third surface and contiguous to the internal side surfaces. The input light beams are incident on the first surface of the respective plateaus 41₁ to 41 n while the output light beams appear on the second surface of the respective plateaus. Each channel has a depth equal to or deeper than the depth D of the incident position illustrated in Fig. 2 and a width of, for example, 0.5 mm. The width of the channel is determined by a thickness of a wire saw used to form the channels 42. The first through the n-th transducer units 23₁ to 23 n are attached to the first through the n-th plateaus 41₁ to 41 n on the top surfaces thereof, respectively. Each of the first through the n-th transducers 23₁ to 23 n comprises the first electrode 26, the transducer mediums 27, and the second electrode 28 which are successively stacked on the top surfaces, as is the case with Fig. 2. Like in Fig. 2, let the second electrode 28 have a second electrode width represented by W and somewhat narrower than the first electrode 26. The second electrode width W, the beam distance L (shown in Fig. 2), and the depth D of the incident position (shown in Fig. 2) are, for example, 0.7 mm, 1.5 mm, and 1 mm, respectively. Further referring to Fig. 3, the block 22' has the fourth surface providing an arcuately recessed portion 45 recessed towards the third surface. The arcuately recessed portion 45 is extended between the first and the second surfaces. The arcuately recessed portion is nearest to the third surface in a center region of each of the first and the second surfaces and is gradually away from the third surface in both end regions laid on both sides of the center region, when each of the first and the second surfaces is divided between the third and the fourth surfaces in parallel, as shown by broken lines. Such an arcuately recessed portion serves to reflect each acoustic waves in various directions. Therefore, each acoustic wave is not substantially returned back to each of the transducer units. It is therefore possible to avoid interferences among the acoustic waves. An acoustic absorber as illustrated in Fig. 1 may be attached to the arcuately recessed portion 45 and may be laid on the internal side surfaces and the recessed surfaces. Referring to Fig. 4, the illustrated block 22" has a chevron shaped portion, namely, a triangularly recessed portion 46 on the fourth surface. The triangularly recessed portion 46 is recessed towards the third surface and is nearest to the third surface in the center region. It is possible with this structure to avoid interferences among the acoustic waves. Referring to Fig. 5, an acoustooptic modulation system according to a third embodiment of this invention comprises the acoustooptic modulation element 20 illustrated in Fig. 1. The system comprises a beam splitter 50 responsive to an incident light beam 51 for supplying the acoustooptic modulation element 20 with first through n-th split beams as the input light beams 21₁, 21₂, ..., 21 n illustrated with reference to Fig. 1, respectively. The split beams are emitted from the beam splitter 50 in parallel. Let the incident light beam 51 be produced from an argon laser as an optical source 52 and have a wavelength of 488 nm. Referring to Fig. 6 together with Fig. 5, the beam splitter 50 comprises a body 53 of quartz transparent to the incident light beam 51 and having a thickness T and front and back surfaces 54 and 55 substantially parallel to each other. The incident light beam 51 is incident on the front surface 54 at an incident portion. The front surface is coated with a reflection layer 56 except the incident portion. The incident portion is covered with an antireflection layer 57 with a part of the antireflection layer 57 superposed on the reflection layer 56. The back surface 55 is divided transversely of the incident light beam 51 into a plurality of areas which are equal in number to n and which are therefore called first through n-th areas, respectively. Each of the first through the n-th areas is partially coated with each of first through n-th layers 58₁, 58₂, ..., 58 n having reflectivities gradually reduced from the first layer 58₁ to the n-th layer 58 n . Such a reduction of the reflectivities is for emitting the first through the n-th split beams substantially equal in intensity to one another, as will later become clear, and for partially reflecting light beams internally incident on the first through the (n - 1)-th layers. For this purpose, the first through the (n - 1)-th layers 58₁ to 58 n-1 may be semitransparent or translucent and the n-th layer 58 n may be transparent. The first through the n-th areas may be completely or wholly covered with the first through the n-th layers 58₁ to 58 n , respectively. As shown in Fig. 6, the incident light beam 51 is given to the incident portion on the front surface at a predetermined incident angle ϑ ₀ and is transmitted towards the back surface. In this event, the incident light beam 51 is refracted at an angle ϑ ₁ of refraction. An internally incident light beam on the first layer 58₁ is partially transmitted through the first layer 58₁ and partially internally reflected towards the reflection layer 56. The reflection layer 56 internally reflects a light beam internally incident thereon towards the second layer 58₂. Similar operation is repeated between the second through the (n - 1)-th layers 58₂ to 58 n-1 and the reflection layer 56. Finally, the n-th split beam is emitted from the n-th layer 58 n . It should be recollected that the acoustooptic modulation element 20 illustrated in Fig. 1 is given the input light beams 21₁ to 21 n with the distance left between two adjacent ones of the input light beams 21₁ to 21 n . Accordingly, it is preferable that a distance L₁ of the first through the n-th split beams is equal to the distance L between two adjacent ones of the input light beams. Inasmuch as the distance L₁ between two adjacent ones of the split beams is calculated by: L₁ = 2Tsinϑ₁, (1) and sinϑ₁ is given in accordance with Snell's law by: sinϑ₁ = sinϑ₀/n₀ (2) where n₀ is representative of a refractive index of the body 50. Substitution of Equation (2) into Equation (1) gives: L₁ = (2T/n₀)sinϑ₀. (3) Thus, the distance L₁ between two adjacent split beams can be determined by selecting the thickness T, the refractive index n₀, and the incident angle ϑ ₀. Taking the above into consideration, the reflection layer 56, the antireflection layer 57 and the first through the n-th layers 58₁ to 58 n are properly arranged on the first and the second surfaces. Each of the reflection layer 56, the antireflection layer 57, and the first through the n-th layers 58₁ to 58 n can be formed by stacking a plurality of dielectric films. More particularly, the reflection layer 56 has a thickness different from those of the antireflection layer 57 and the first through the n-th layers 58₁ to 58 n . The antireflection layer 57 also has the thickness different from those of the first through the n-th layers 58₁ to 58 n . The reflection layer 56, the antireflection layer 57, and the first through the n-th layers 58₁ to 58 n can be obtained by individually changing the number of the dielectric films, as known in the art. As mentioned before, the reflectivities of the first through the n-th layers 58₁ to 58 n are selected so that each of the first through the n-th split beams becomes substantially equal in intensity to the remaining split beams. Let the reflectivities of the first through the n-th layers 58₁ to 58 n be R₁, R₂, ..., R n , respectively. It is assumed that the reflectivity, of the reflection layer 57 and internal absorption of the body 50 are equal to 1 and 0, respectively. In order to render the intensities of the split beams equal to one another, the reflectivities R₁ to R n should have relationships given by: (1 - R₁) = (1 - R₂)R₁ = (1 - R₃)R₁R₂ ... = (1 - R n )R₁R₂ ... R n-1 . (4) It is readily possible to form by the use of a known technique the first through the n-th layers 58₁ to 58 n so that Equation (4) holds. Thus, the illustrated beam splitter can emit from the back surface the split beams substantially equal in intensities to one another. In addition, the split beams are substantially parallel to one another and sent from the beam splitter as the first through the n-th input light beams 21₁ to 21 n to the acoustooptic modulation element 20. Referring back to Fig. 5, the acoustooptic modulation element 20 comprises the first through the n-th transducer units 23₁ to 23 n attached to the third surface of the acoustooptic medium block 22, as is the case with Fig. 1. The first through the n-th transducer units 23₁ to 23 n are activated by first through n-th electric signals supplied from an electric circuit 60, respectively. The electric circuit 60 comprises an oscillator 62 generating an oscillation signal of 80 MHz. The oscillation signal is delivered to first through n-th modulators 64₁ to 64 n supplied with first through n-th modulation signals S₁ to S n from external signal sources (not shown), respectively. The first through the n-th modulators 64₁ to 64 n carry out amplitude modulation to produce first through n-th amplitude modulated signals, respectively. The first through the n-th amplitude modulated signals are supplied through amplifiers 66₁ to 66 n as the first through the n-th electric signals to the first through the n-th transducer units 23₁ to 23 n , respectively. The acoustooptic modulation element 20 individually modulates the first through the n-th input light beams 21₁ to 21 n by the acoustic waves propagated from the first through the n-th transducer units 23₁ to 23 n in the manner described in conjunction with Figs. 1 and 2. As a result, the first through the n-th output light beams depicted at 68₁ to 68 n in Fig. 5 are emitted from the acoustooptic modulation element 20. The acoustooptic modulation element 20 illustrated in Fig. 3 may be substituted for the element 20 illustrated in Fig. 5. Referring to Fig. 7, the acoustooptic modulation element 20 is for use in combination with first and second optical systems 71 and 72 opposite to the first and the second surfaces of the element 20 with spacings left between the first optical system 71 and the first surface and between the second optical system 72 and the second surface, respectively. Each of the first and the second optical systems may be a cylindrical lens, an elliptic lens, or a spherical lens. The first through the n-th split beams emitted from the beam splitter 50 are sharpened or reduced in spot sizes through the first optical system 71 to be given as the first through the n-th input light beams 21₁ to 21 n . Likewise, each of the first through the n-th output light beams 68₁ to 68 n is sharpened by the second optical system 72. In general, an acoustooptic modulation element has a modulation bandwidth inversely proportional to spot sizes of each input light beam. Accordingly, the first and the second optical systems 71 and 72 serve to widen the modulation bandwidth of the acoustooptic modulation element 20 by reducing the spot sizes of the input light beams 21₁ to 21 n . Referring to Fig. 8, an acoustooptic modulation system according to a modification of this invention is similar to that illustrated in Fig. 5 except that a combination of a grating 75 and an elliptic lens 76 is substituted for the beam splitter 50 illustrated in Fig. 5. The grating 75 is made of optical glass and has slits of 12,000/inch and a lattice constant of 21,000 angstroms. As well known in the art, the incident light beam 51 is diffracted in directions determined by the grating 75. As a result, a plurality of diffracted light beams which are equal in number to n are rendered parallel to one another through the elliptic lens 76 and are thereafter sent as the first through the n-th input light beams 21₁ to 21 n to the acoustooptic modulation element 20. Each of the first through the n-th input light beams 21₁ to 21 n is subjected in the acoustooptic modulation element 20 to modulation in cooperation with the electric circuit 60 in the manner described in conjunction with Fig. 5. The combination of the grating 75 and the elliptic lens 76 may be called a beam splitter member. The grating 75 may be of a transmission type or a reflection type. A fiber grating may be substituted for the grating 75. Such a fiber grating is known in the art and comprises a plurality of optical fibers arranged in parallel with slits left between two adjacent ones of the optical fibers. While this invention has thus far been described in conjunction with a few embodiments thereof, it will readily be possible for those skilled in the art to put this invention into practice in various manners. For example, the first and the second optical systems 71 and 72 illustrated in Fig. 7 may be used in the system shown in Fig. 8.

Claims: An acoustooptic modulation element capable of responding to a plurality of input light beams substantially parallel to one another and electric signals equal in number to said input light beams for producing output light beams, respectively, said acoustooptic modulation element comprising a single acoustooptic medium block (22) having a first surface for said input light beams, a second surface for said output light beams, a third surface contiguous to said first and said second surfaces, and a fourth surface contiguous to said first and said second surfaces and opposite to said third surface; and transducing means (23) attached to said third surface and responsive to said electric signals for transducing said electric signals into acoustic waves to produce the output light beams modulated by said acoustic waves when said electric signals are given to said transducing means; characterised in that the transducing means comprises a plurality of transducer units (23₁ to 23 n ) responsive to said electric signals and extended along said input light beams on said third surface substantially parallel to one another for individually propagating said acoustic waves into said block from said third surface towards said fourth surface. An acoustooptic modulation element as claimed in Claim 1, wherein said block comprises; a plurality of plateaus (41₁ to 41 n ) equal in number to said transducer units and extended between said first and second surfaces along said input light beams with a channel (42) left between two adjacent ones of said plateaus; each of said plateaus having a top surface defining that part of said third surface. An acoustooptic modulation element as claimed in Claim 1, wherein said fourth surface has an arcuately recessed portion (45) recessed towards said third surface and extended between said first and said second surfaces. An acoustooptic modulation element as claimed in Claim 3, each of said first and said second surfaces being divisible into a center region and a pair of end regions adjacent to said center region on both sides thereof, said center region and said end regions being extended between said third and said fourth surfaces in parallel, wherein said arcuately recessed portion is nearest to said third surface in said center region and is gradually away from said third surface in said end regions An acoustooptic modulation element as claimed in any one of the preceding claims, wherein said fourth surface has a triangularly recessed portion (46) recessed towards said third surface and extended between said first and said second surfaces. An acoustooptic modulation element as claimed in any one of the preceding claims, further comprising acoustically absorbing means (33) attached to said fourth surface for absorbing said acoustic waves. An acoustooptic modulation system responsive to a single incident beam and a plurality of electric signals for producing output light beams, said acoustooptic modulation system comprising beam splitting means (50) for splitting said incident light beam into a plurality of light beams substantially parallel to one another; and a single acoustooptic modulation element (20) in accordance with any one of the preceding claims. An acoustooptic modulation system as claimed in Claim 7, wherein said beam splitting means comprises a body (53) having front and back surfaces substantially parallel to each other, said front surface partially having an incident portion on which said incident light beam is incident, said back surface being divisible into a plurality of partial areas arranged in parallel to one another; a reflection layer (56) having a predetermined reflectivity and covered on said front surface except said incident portion; and a plurality of semitransparent layers (58₁ to 58 n ) covered on said partial areas and having variable reflectivities lower than said predetermined reflectivity so that each of said parallel light beams is rendered substantially equal in intensity to the remaining parallel light beams. An acoustooptic modulation system as claimed in Claim 7, wherein said beam splitting means comprises a grating member (75) for diffracting said incident light beam to produce a plurality of diffracted light beams; and an optical member (76) responsive to said diffracted light beams for producing said parallel light beams.

Inventors: AMANO, SATORU, HORIUCHI, SHIGENORI, SHINOHARA, TAKERU
Applicants: HOYA CORPORATION
Application year: 1983
Main classification: G02F1
IPC codes: G02F1