Abstract:
The invention is intended to present a multi-channel optical modulator small in crosstalk, being used in a laser external modulator of an optical recording apparatus. To achieve the object, the multi-channel optical modulator comprises a plurality of first electrodes ( 13 ) provided on one side of an acoustic-optical medium ( 11 ), a plurality of piezoelectric elements ( 12 ) provided on the first electrodes ( 13 ), a plurality of second electrodes ( 14 ) provided on the piezoelectric elements ( 12 ), a plurality of first lead wires ( 15 ) connected individually to the second electrodes ( 14 ), and a plurality of second lead wires ( 16 ) connected individually to the first electrodes ( 13 ), and thereby the crosstalk is small.

Description:
TECHNICAL FIELD 
     The present invention relates to a multi-channel optical modulator used an external modulator for a laser recording apparatus such as laser printer and laser plotter. 
     BACKGROUND ART 
     A multi-channel optical modulator has a plurality of elastic wave generating sources, and the number of light beams corresponding to its number can be modulated simultaneously and independently, which allows to record at a higher speed by scanning at the same speed as when recording by a single light beam. Further, at the same recording speed, the recording density is higher than when using a single light beam. Therefore, the demand for multi-channel optical modulator is increasing along with the mounting requirement for recording at higher density and higher speed. 
     A conventional multi-channel optical modulator is described below. 
     As shown in FIG.  4  and FIG. 5, a first electrode  103  is formed on the entire surface of one side of an acoustic-optical medium  101 , and a piezoelectric element  102  is disposed thereon. Five second electrodes  104  are provided on the piezoelectric element  102 , and thereby five transducers are formed. 
     A first lead wire  105  is connected nearly to the center of each second electrode  104 , and a second lead wire  106  corresponding to each first lead wire  105  is mutually connected to both ends of the first electrode  103 . The first lead wire  105  and second lead wire  106  are connected to each driving signal source  107 . 
     In thus constructed multi-channel optical modulator, the operation is described below. 
     First, the piezoelectric element  102  is oscillated by an alternating current signal supplied from the first lead wire  105  and second lead wire  106 , and becomes an elastic wave generating source. Therefore, the acoustic-optical medium  101  has as many elastic wave generating sources as the number of transducers, that is, five. The generated elastic wave propagates vertically on the transducer mounted surface of the acoustic-optical medium  101 , and acts on the light beam passing through the propagation area, thereby generating a diffracted light. This mode is shown in FIG.  7 . Herein, “I” denotes an incident light, “I 1 ” is a diffracted light, and “I 0 ” represents a non-diffracted transmission light. Since the diffracted light intensity is proportional to the elastic wave intensity, that is, the driving signal strength, desired optical recording is realized by varying the driving signal strength depending on the recording pattern. 
     Components of this multi-channel optical modulator are expressed in a circuit diagram in FIG.  6 . 
     Herein, the first lead wires  105  and second lead wires  106  are indicated by coil symbols because they have a very slight inductance. Reference numeral  108  indicates an output impedance of the driving signal source  107 , and the driving signal sources  107  are commonly grounded by connecting among the transducers. The first electrode  103  is commonly shared among the transducers. 
     In this structure, when one transducer is driven, the voltage generated by the inductance of the second lead wire  106  may drive the output impedance  108  and the first electrode  103 , or the piezoelectric element  102  of other transducer through the first electrode  103 , thereby generating a crosstalk. 
     DISCLOSURE OF THE INVENTION 
     It is hence an object of the invention to present a multi-channel optical modulator small in crosstalk. 
     To achieve the object, the multi-channel optical modulator of the invention comprises an acoustic-optical medium, a plurality of first electrodes provided on one side of this acoustic-optical medium, a plurality of piezoelectric elements provided on the first electrodes, a plurality of second electrodes provided on the piezoelectric elements, a plurality of first lead wires connected individually to the second electrodes, and a plurality of second lead wires connected individually to the first electrodes, in which the first electrodes are independent of individual transducers, and therefore the voltage generated in the second lead wire of any transducer may not be applied to the piezoelectric element of any other transducer, so that a generation of crosstalk may be prevented. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a multi-channel optical modulator in an embodiment of the invention, 
     FIG. 2 is its side view, and 
     FIG. 3 is its circuit diagram. 
     FIG. 4 is a perspective view of a conventional multi-channel optical modulator, 
     FIG. 5 is its side view, and 
     FIG. 6 is its circuit diagram. 
     FIG. 7 is a diagram explaining the operation of a general one-channel optical modulator. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     An embodiment of the multi-channel optical modulator of the invention s described in detail below while referring to the accompanying drawings. 
     Embodiment 1 
     First, on the entire surface of one side of an acoustic-optical medium  11 , Sn is deposited by vacuum deposition method such as sputtering method or resistance heating method, and first electrodes  13  are formed. 
     The acoustic-optical medium  11  is made of tellurium dioxide, lead molybdate, or a similar material allowing to pass light at the wavelength to be modulated, and having a large figure of merit in the acousto-optic effect. The acoustic-optical medium  11  has parallel upper and lower surfaces, and the first electrodes  13  are formed on the side vertical to these upper and lower surfaces. The side opposite to the side on which the first electrodes  13  formed is not parallel to the side on which the first electrodes  13  formed. 
     On the first electrodes  13 , a piezoelectric element  12  having a narrower width than the width of the first electrodes  13  is formed. 
     The piezoelectric element  12  is made of lithium niobate which is a piezoelectric crystal, and it is compression-bonded onto the first electrodes  13  in a vacuum, and polished to a thickness for resonating at a driving frequency. 
     At this time, the piezoelectric element  12  is formed so that its width direction end may not coincide with the width direction end of the first electrodes  13 , that is, it is deviated in position so that the second lead wires  16  may be formed on the first electrodes  13  in a later process. 
     On the piezoelectric element  12 , further, five second electrodes  14  are formed at specific intervals. The second electrodes  14  have a three-layer structure, formed of Ni—Cr layer, Cu layer, and Au layer sequentially from the piezoelectric element  12  side. 
     Afterwards, four split grooves  18  are formed in the first electrodes  13  and acoustic-optical medium  11  by using a dicer between the adjacent second electrodes  14 , and five transducers having independent piezoelectric elements  12  and first electrodes  13  are fabricated. 
     The first lead wire  15  is connected electrically by soldering to the second electrode  14  of each transducer. At this time, the first lead wire  15  is alternately connected to the different end from the adjacent second electrode  14 . Further, the second lead wire  16  is soldered to the first electrode  13  at the soldering side of the first lead wire  15  of each transducer. At this time, for the ease of soldering of the second lead wires  16 , a plating layer  19  of two layers made of lower layer of Cu and upper layer of Au is formed at the soldering position of the second lead wire  16  of the first electrode  13 . 
     Consequently, in each transducer, the first lead wire  15  and second lead wire  16  are connected to a driving signal source  17 . 
     Thus, the multi-channel optical modulator having five transducers of the same shape as shown in FIG.  1  and FIG. 2 is formed. 
     FIG. 3 is a circuit diagram of this multi-channel optical modulator, in which the first lead wires  15  and second lead wires  16  are indicated by coil symbol because they have, if very slight, an inductance. Reference numeral  20  is an output impedance of the driving signal source  17 , and the grounding of the driving signal source  17  connected to each transducer is common. 
     Each transducer of this multi-channel optical modulator individually has an independent first electrode  13 , and the signal of a certain transducer will not be applied to any other transducer through the output impedance  20  or first electrode  13 . 
     Besides, since the distance between the first lead wires  15  of adjacent transducers is long, the induction of the inductance is small, so that the crosstalk may be decreased. The characteristic points of the invention are explained below. 
     (1) The transducer forming surface of the acoustic-optical medium  11  and its opposite side are preferred to be mirror-finished. surfaces in order to obtain a stable modulation operation. 
     (2) The first electrodes  13  are preferred to be formed by using Sn or In in order to obtain matching of acoustic impedance between the piezoelectric element  12  and acoustic-optical medium  11 . 
     (3) The split groove  18  is formed so that its section may be in a U-form. It is because, if there is a sharp corner in the split groove  18 , a stress is formed in the area to cause a crack in the acoustic-optical medium  11 . Therefore, the surface forming the split groove  18  of the acoustic-optical medium  11  is formed in a curved surface. 
     (4) Usually, the light beam to be modulated enters in a range of 1 to 3 mm from the transducer. The elastic wave generated from the transducer is propagated radially, and the angle of radiation of the elastic wave is wider as the interval of the transducers becomes narrower. 
     Accordingly, when the transducer interval is narrow, the incident light is diffracted by the elastic wave generated by the adjacent transducer, thereby causing an crosstalk. 
     Therefore, by setting the depth of the split groove  18  deeper than the light beam incident position, if the transducer interval is narrow, it is possible to prevent crosstalk generated by the elastic wave transmitted from the adjacent transducers. 
     Incidentally, if free from effects of diffraction by the elastic wave from the adjacent transducer, the depth is not particularly defined as far as each transducer may exist independently electrically. 
     (5) The transducer generates heat by electro-mechanical conversion loss proportional to the electric input. Hence, deviation of temperature distribution depending on the holding structure and a shape of the acoustic-optical medium  11  may be generated in the acoustic-optical medium  11  to cause distortion. 
     On the other hand, the transducer is lowered in the modulation efficiency for a specific electric input if increased in the width in the vertical direction. 
     Therefore, in the case of the multi-channel optical modulator having a plurality of transducers, the transducer in the portion lower in the temperature of the acoustic-optical medium  11  is, as compared with the transducer in the portion higher in temperature, set larger in the width in the vertical direction, larger in the electric input for obtaining an equivalent efficiency, and more in the heat generation due to electro-mechanical conversion loss. Therefore distortion of the acoustic-optical medium  11  can be prevented without deviation of temperature distribution in the acoustic-optical medium  11  without varying the modulation efficiency of each transducer. 
     (6) In order to release heat in the acoustic-optical medium  11 , it is preferred to form cooling plates on the upper and lower surfaces of the acoustic-optical medium  11 . The shape of the cooling plate is not particularly specified as far as the surface contacting with the acoustic-optical medium  11  is flat. 
     (7) In this embodiment, by forming the split groove  18 , a plurality of transducers having independent electrodes  13  are formed, but instead of forming the split groove  18 , the first electrodes  13  and piezoelectric elements  12  may be preliminarily formed independently. 
     (8) The interval of the second electrodes  14  of the adjacent transducers should be as equal as possible. This is because if the interval of the second electrodes  14  is different, the crosstalk varies depending on the transducers. 
     (9) In the foregoing embodiment, five transducers are provided, but the number of transducers may be determined freely depending on the number of desired channels. 
     INDUSTRIAL APPLICABILITY 
     According to the invention, by forming the first electrodes of the transducers independently electrically, an excellent multi-channel optical modulator small in crosstalk is realized.