2. Description of the Prior Art One of the most commonly used conventional microphones is an electret condenser microphone which employs electrets wherein charge is preserved in a polarized form.

Figure 1 is a diagram illustrating a conventional electret condenser microphone. The microphone in Fig. 1 comprises an electret having static electricity and a metal diaphragm adjacent to the electret, which vibrates according to acoustic signals. The variation in an electric field changes charge preserved in the electret, and the change is

measured by a junction type FET (hereinafter, referred to as JFET').

Referring to Figure 1, a JFET 12 is formed on the center portion of a base 10. A mould-type internal case 14 is formed on an outer rim of the base 10. An electret 16 is attached to the upper portion of the case 14. The electret 16 is an organic film where charges are preserved in a polarized form. A thin metal diaphragm 18 is formed on the upper portion of the electret 16 and fixed to an external case 19.

In the electret condenser microphone shown in Fig. 1, when an external acoustic signal 13 vibrates the metal diaphragm 18, the position of the diaphragm 18 changes and a static electricity field is varied. An electric signal is generated in response by the electret 16. The adjacent JFET 12 senses the change in the electret 16 and outputs an electric signal corresponding to the external acoustic signal.

Since the conventional electret condenser microphone as described above is small in size, has low production cost and is suitable for mass production, it is widely used in cell phones, telephones and computers because it has low cost and mass production.

However, the conventional electeret condenser microphone has low sensitivity, and poor characteristic at a high temperature due to organic electret film wherein charge

is preserved. In addition, the conventional electret condenser microphone is too sensitive to external signal interference because the input impedance of a JFET which transforms the change in the static electricity field into an electric signal is too high, resulting in noise generation and electromagnetic interference, especially in for radio communication terminals.

In order to overcome the above described problem, a number of methods using lights to measure small acoustic signals has been support osed. However, they are only at the conceptional level. Even if a prototype is possible, high cost and restrictive application are expected.

There are two methods using lights depending on light sources, namely LED type and laser type. The LED type is inexpensive and easy to handle. However, since light is emitted into all directions, alignment of lenses or optical fiber is required in order to condense light, resulting in low yield and high manufacturing costs. This disadvantage makes it difficult to commercialize the LED type. The laser type is expensive and difficult to control beam, wherein signals reflected from a diaphragm are impossible to distinguish. As a result, it is hardly commercialized.

Figure 2 is a schematic diagram illustrating a first example of a conventional optical microphone disclosed in the U. S. Patent No. 4,384, 858. The microphone in Fig. 2 comprises a light source 20 and a fiber optics pickup 22

located on the opposite side of the light source 20. The pickup 22 is connected to a light detector 23. An electric output signal is outputted through the light detector 23 and the AC signal processor 24. The microphone also includes a film 25 fabricated of polyethylene etc. attached to a ring 26, and is positioned between the light source 20 and the fiber optic pickup 22. When the film 25 vibrates according to acoustic signals, the amount of light passing through is changed according to the degree of variation, thereby picking up sounds.

Figure 3 is a schematic diagram illustrating a second example of a conventional optical microphone disclosed in the US patent No. 3,622, 791. When a diaphragm 31 fixed on the upper portion of a case 30 is vibrated by sound, a plane mirror 32 attached to the center of the diaphragm 31 vibrates. An optical system comprising a light source 33, a lens 34, a semitransparent cube 35 of quartz and photodiodes 36 is mounted below the mirror 32. The optical system signalizes the sound induced by light path difference in a logic circuit 37. The change of an electric field is measured by using an intergrator 39 connected to a wire 38 which is mounted below the diaphragm 31. An electric signal is generated by combining the change of the electric field with the signal measured in the logic circuit 37.

Figure 4 is a schematic diagram illustrating a third example of a conventional optical microphone described in

SPIE; International Society for Optical Engineering, Sept.

1999, Boston, MA. An LED 40 is coupled with an optical fiber 42 connected to a microphone head 44. A light from the LED 40 is projected on a diaphragm 46 and then measured in an external light receiving element 48 via the optical fiber 42.

Although the conventional optical microphones have an advantage of using light, the complicated structure of the optical system thereof required complicated manufacturing process. In addition, the optical system is too large to be fitted into electric products, and high production cost makes it difficult to commercialize. The degradation in stability of the optical system also deteriorates reliability of products.

SUMMARY OF THE INVENTION Accordingly, the present invention has an object to provide an optical microphone including a hologram plate instead of an optic fiber or a lens. The hologram plate locates a semiconductor laser and a light receiving element on the same surface by regulating a laser beam exactly. The optical microphone has a simple structure of an optical system and an easy manufacturing process. As a result, it is easy to be miniaturized. In addition, the optical microphone can sensitively detect variation of a diaphragm

because a laser is used. The reliability of high temperature can be secured because an organic film is not used in the optical microphone that can prevent the interference by electromagnetism and high frequency.

In order to achieve the above described object, there is provided an optical microphone comprising: a flat base; a semiconductor laser light source formed on one upper side of the base, and for emitting light into the upper side; two or more light receiving element formed on the other upper side of the base, and for generating the quantity of light changed by sound into an electrical signal by transforming light reflected from the upper side into an electrical signal; a mould-type case formed on the outside of the base, and for comprising the light source and light receiving element; a diaphragm formed on the upper portion of the case, and for reflecting light emitted from the light source in the lower surface, and transformed by external sound pressure; and a hologram plate formed between the base and the substrate, comprising a hologram grate formed on a transparent plate, projecting light emitted from the lower light source into the diaphragm using the grate at a predetermined angle, and projecting light reflected from the diaphragm in the lower light receiving element at a predetermined angle.

The optical microphone according to the present invention is characterized in that the height difference

between the light source and the diaphragm is O. lmm-lOcm and the height difference between the light receiving element and the diaphragm is O. lmm~lOcm. The light receiving element is an array element, and an arithmetic circuit and an amplifying plate for amplifying the difference of signals of each light receiving element on a semiconductor substrate identical to the array element are accumulated. The distance d between the light receiving element is l-5u. m.

The thickness of the diaphragm is O. OOOl-l00m, and the thickness of the hologram plate is O. OOl-l00mm. There is located hologram plate and an acoustic hole in the case.

There is also provided an optical microphone comprising: a flat base; a semiconductor laser light source having a sloping angle in one side of the base, and for emitting light into the upper side at a predetermined angle; two or more light receiving element having a sloping angle on the other upper portion of the base, and for detecting the change in the quantity of light according to voice, using an electric signal into which light reflected from the upper portion at a predetermined angle is transformed; a mould-type case formed on the outside of the base, and for including the light source and the light receiving element; and a diaphragm formed on the upper portion of the case, reflecting light emitted from the light source into light receiving element on the lower surface, and transformed by the external sound pressure.

The optical microphone is also characterized in that the angle between the light source and the light receiving element is 30~90°.

BRIEF DESCRIPTION OF THE DRAWINGS Preferred embodiments of the present invention will be described, by way of example only, with reference to the accompanying drawings, wherein: Figure 1 is a cross-sectional diagram illustrating a conventional electret condenser microphone; Figure 2 is a diagram of a conventional optical microphone in accordance with a first preferred embodiment; Figure 3 is a diagram of a conventional optical microphone in accordance with a second preferred embodiment; Figure 4 is a diagram of a conventional optical microphone in accordance with a third preferred embodiment; Figures 5a, 5b and 5c are diagrams for explaining a principle of an optical microphone in accordance with the present invention; Figure 6 is a diagram for explaining the transformation of a diaphragm according to an external acoustic signal; Figures 7-a and 7-b are diagrams for explaining a principle of the signal detection of array light receiving element according to the present invention;

Figure 8 is a diagram for explaining the structure of light receiving element; Figure 9 is a curve illustrating characteristics of output signals of the present invention; and Figure 10 is a cross-sectional diagram of an optical microphone in accordance with a preferred embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Figures 5a through 5c are diagrams illustrating a structure and principle of an optical microphone in accordance with the present invention. Figure 5a is a cross-sectional diagram of an optical microphone in accordance with a preferred embodiment of the present invention. Figure 5b is a schematic diagram illustrating the operating principle of the optical microphone of Figure 5a. Figure 5c is a schematic diagram illustrating a light receiving element of Figure 5a.

Referring to Figure 5a, two slope blocks 51 are attached to a base 50. Sloping sides of the slope blocks 51 are facing each other so that a light source and a light receiving element can have a predetermined angle. A semiconductor laser light source 54 is mounted on sloping side of the slope block 51. A light receiving element 56 comprising two or more components such as an array of four

photodiodes 56-1,56-2, 56-3 and 56-4 is mounted on a sloping side of the other slope block 51. A mould-type case 58 is mounted on the outer portion of the base 50. A diaphragm 52 having a reflective lower side is attached to two support s 59 mounted on the upper portion of the case 58.

Acoustic holes 57 according to the acoustic design are formed on laterals or bottoms of the case 58. The acoustic holes 57 adjusts resistance of internal air in accordance with external pressure.

Referring to Figure 5b, the microphone comprises the semiconductor light source 54 for projecting condensed light having a predetermined emitting angle, and the light receiving components 56-1,56-2, 56-3 and 56-4 for receiving the reflected light are positioned under the diaphragm 52 having a reflective lower portion. Preferably, the light source 54 is a laser such as vertical cavity surface emitting laser (hereinafter, referred to as VCSEL'). A light source with small operating current, 20mA or less for example is preferable to lower the consumption of the entire device.

The diaphragm 52 has a predetermined thickness which allows vibration induced by pressure of small external sound.

The diaphragm 52 of the predetermined thickness also has high reflectivity to light. The diaphragm may be formed of metals having high reflectivity and sensitive to external pressure such as gold, Ni, Ti, Al or alloys thereof.

Referring to Figure 5c, the light receiving element 56 comprises a first light receiving component 56-1, a second light receiving component 56-2, a third light receiving component 56-3 and a fourth light receiving component 56-4 to have a predetermined distance d therebetween. The light receiving components can have various arrangements according to the design corresponding to the operating principle.

A light emitted from the light source 54 is reflected from the diaphragm 52 and then projected on the light receiving element 56. Since a semiconductor light source is approximately a point source, it has a dispersing characteristic as the distance from the light source becomes larger. An angle where the intensity of light is 50% of the total intensity is defined as emission angle 01. When the angle A1 is not 0°, the distribution area of light becomes larger according to the traveled distance. The distribution of light corresponding to an emission angle of the light reflected from the diaphragm 52 and projected on the light- receiving element 56 is determined by the angle Al of the light source 54, and paths of light from the light source 54 to the diaphragm 52 and from the diaphragm 52 to the light receiving element.

When there is no external signals and no variation the diaphragm 52, light projected onto the light receiving element is designed to be located on the center of the light receiving element. As a result, the internal light

receiving components 56-2 and 56-3 have the same support ortion, and the external light receiving components 56-1 and 56-4 are equally distributed.

Here, the light from the light source 54 has a wavelength ranging from 0.3 to 1. 5pm. The gap between the light source 54 and. the diaphragm 52 is O. lmm~lOcm, and the gap between the light receiving element 56 and the diaphragm 52 is O. lmm~lOcm. Array of light receiving components formed on the same semiconductor substrate are preferable for the light receiving element 56 to have the same characterisstics.

The light receiving element 56 comprises two or more light receiving components, and the distance d between each light receiving component is lu. m-5cm. The diaphragm 52 has thickness of 0. 001-lOOm. The angle Al between the light source 54 and the light receiving element 56 is 30~90°. The light receiving element is connected to an amplifier to amplify differential signal. In case of an array device, an amplifying terminal may be integrated to amplify the differential of signal from two light receiving components on the same substrate.

Next, the operating principle of the present invention is now described.

Referring to Figure 6, when a pressure such as an external acoustic signal is applied to the diaphragm. 52, the diaphragm 52 vibrates, which induces difference in the path of light. Here, the support 53 to which the diaphragm 52 is

attached does not vibrate. As a result, the variation of the diaphragm 52 due to the vibration is symmetric in the center of the diaphragm 52. Therefore, the vibration of diaphragm can be approximated as the dotted line shown in Figure 6. Since the diameter D'of the diaphragm 52 is at least 50 times as large as the core D where the light is projected, the variation of the diaphragm 52 can be approximated to a linear signal.

Referring to Figure 7a, when the diaphragm 52 is moved downward to h1 by an external signal, more light is projected on the first and the second light receiving components 56-1 and 56-2, and less light is projected in the third and the fourth light receiving components 56-3 and 56- 4 compared to the state before the diaphragm is varied due to the difference in the path of light. The variation hl of the diaphragm 52 induced by the external signal generates the difference in the path of light, which changes the distribution of'light projected on the light receiving element 56, thereby inducing the difference in the amount of light projected on each light receiving element to generate a signal. The difference between the amount of light projected on the second and the third light receiving components is small. The difference between the amount of light projected on the first and the fourth light receiving components is large, which results in relatively large signal. When the difference between the amount of light

projected on the first and the fourth light receiving components is divided by The difference between the amount of light projected on the second and the third light receiving components, a very small signal is amplified to an extremely large signal.

A signal according to the variation of the diaphragm is as follows: Signal = [Q (PD1) -Q (PD4)] [Q (PD2)-Q (PD3)] = [C (PD1)-C (PD4)] [C (PD2)-C (PD3)] = [I (PD1) -I (PD4)]<BR> [I (PD2) -I (PD3)] Q = amount of projected light C = measured current Referring to Figure 7b, when the diaphragm 52 is moved upward to h2 by an external signal, more light is projected on the third and the fourth light receiving components. The signal in this case is follows: Signal 2 = [Q (PD4)-Q (PD1)] [Q (PD3)-Q (PD2)] = [C (PD4)-C (PD1)] [C (PD3)-C (PD2)] = [I (PD4) -I (PD1)]<BR> [I (PD3) -I (PD2)] A signal according to the variation of the diaphragm

induced by an external sound is sensed by the difference in currents measured in the array of light receiving components 56-1,56-2, 56-3 and 56-4. This difference of signals can be precisely adjusted by regulating the distance d between two light receiving element.

Referring to Figure 8, the difference of current sensed in light receiving components according to the difference in the amount of light projected on light receiving components is as follows: AI= [I (PD1)-I (PD4) ]<BR> [I (Pd2) -I (PD3)] Therefore, the variation of the diaphragm 52 by the external sound can be obtained as an electric signal.

Since the first, the second, the third and the fourth light receiving components 56-1, 56-2,56-3 and 56-4 are required to have almost the same characteristics in order to effectively obtain the minute difference, it is preferable that light receiving element be fabricated on the same substrate in the same process, having a structure of an array. The amplitude of vibration of the diaphragm is varied according to the degree of variation of external signals. The difference in the path of light corresponding to this variation induces the change the distribution of light which can be retrieved and transformed this variation into electric signals within the range of linearity.

Specifically, the variation of signals detected from light

receiving element can be amplified before it is exposed to noise by integrating a low noise amplifier circuit 70, for example, a transistor or an amplifier, and an arithmetic circuit 71 on the same semiconductor substrate as light receiving element so that the signal difference between each light receiving component is amplified near the light receiving element.

Referring to Figure 9, a linearized characteristic curve 60 plotted on acoustic signal vs. output signal plane is illustrated. It is preferable that the initial amount of distribution of light be symmetric to make the difference of two signals to have a linear characteristic. However, when the initial amount of the distribution of light is asymmetric due to errors in the fabrication process of light devices, a function for adjusting off-set may be added to an output terminal integrated with the light receiving element or may be connected to the output terminal to obtain linear characteristics. Signals may be passed through a capacitor connected to output terminal to obtain pure AC acoustic signal without a DC signal corresponding to the off-set.

Although above-described example embodies full operation of the optical microphone according to the present invention, the assembling process of the microphone may be complicated because a semiconductor laser and an array of light receiving element is positioned and aligned to have an angle of 45°.

Figure 10 is a diagram of an optical microphone in accordance with another preferred embodiment of the present invention. The operation principle of the microphone is same as that of Fig. 5. A laser and light receiving element are horizontally mounted on the surface of the base for the purpose of easier assembly. Referring to Figure 10, a light source 82 consisting of VCSEL is attached to one side of a base 80. An array of light receiving components is attached to the other side of the base 80. The microphone also comprised on a detecting circuit (not shown) for detecting signals of light receiving element 81.

A mould-type case 88 having a support 86 on its upper portion is mounted on the outer rim of the base 80. A diaphragm 85 having reflective lower side is mounted on the support 86. A transparent plate having hologram grating 84- 1 and 84-2, i. e. a hologram plate 83, is mounted between the light source 82 and the diaphragm 85. The hologram plate 83 has a thickness ranging from O. OOL to lOOmm.

A laser beam from the light source 82 is diffracted to have a predetermined angle a when passed through the hologram grating 84-1. As a result, the emitting angle of the laser beam can be adjusted. The diffracted beam is projected on the diaphragm 85 at a predetermined angle and then reflected. The reflected beam passes through the hologram grating 84-2 and diffract again. By adjusting these angles, an optical system having the array of light

receiving element 81 and a VCSEL light source is horizontally attached to the base can be constituted to have a symmetrical distribution of light.

The hologram gratings of the hologram plate 83 is designed in consideration of wavelength of a laser beam such as VCSEL and the distance of light path. Specifically, the positions of the hologram gratings are determined so that a laser beam passed through the grating be projected on light receiving element. The hologram area is sufficiently larger than the area where the beam passes, which allows easy assembly of the optical microphone. The light source 82, the diaphragm 85 and the light receiving element 81 may be formed using a similar method as described in Figure 5.

In addition, acoustic holes 87 may be employed in the hologram plate 82 and the case 88.

As described above, in the optical microphone of the present invention, the degree of vibration of a diaphragm to which a light having a predetermined angle is projected is measured using a semiconductor laser and a hologram. The optical microphone employs a plurality of light receiving components for converting light reflected from the diaphragm into the current change. Since fine vibration of the diaphragm is converted into current change generated by the array of light receiving components, the optical microphone provides high sensitivity. In addition, since a VCSEL light source and an array of light receiving components are

mounted on the same plane, it is easy to assemble and suitable for commercial production.

Moreover, the reliability of devices is improved because an acoustic signal is detected by light with simple optical system. The noise problem due to the interference with signals such as high frequency wave and electromagnetic field is eliminated. The optical microphone is a semiconductor device and a high temperature soldering process can be used so that the microphone can be assembled on the PCB at high temperature in electronic devices, which provides facility for designing a product compared to a conventional microphone which must be mounted after the assembly of all the parts due to its organic film electret which has poor characteristics at high temperature.