CN110602617A

CN110602617A - Laser MEMS microphone

Info

Publication number: CN110602617A
Application number: CN201910836984.XA
Authority: CN
Inventors: 王鸣; 夏梵华; 郭冬梅; 夏巍; 王慧鹏; 郝辉
Original assignee: Nanjing Normal University
Current assignee: Nanjing University; Nanjing Normal University
Priority date: 2019-09-05
Filing date: 2019-09-05
Publication date: 2019-12-20

Abstract

The invention discloses a laser MEMS microphone, which comprises a light source and a microphone structure, wherein the microphone structure comprises a substrate, a back plate, a vibration sound membrane, a front cavity formed by the substrate and the vibration sound membrane, and a rear cavity formed by the vibration sound membrane and the back plate; the light source is a VCSEL, and a plurality of sound holes manufactured by an MEMS technology are arranged on the back plate and the vibration sound film; the VCSEL emits laser beams to the vibration sound film; the vibration sound film reflects part of the laser beam to the VCSEL resonant cavity; the reflected laser beam carries sound information of vibration of the vibration sound film, and the partially reflected laser beam is mixed with laser in the VCSEL resonant cavity to generate a laser self-mixing interference effect; the output power and the frequency of the VCSEL are changed, and the junction voltage on the VCSEL is extracted; after amplification and demodulation, the signals are converted into audio waveforms, and conversion from sound signals to electric signals is achieved. The laser MEMS microphone has high pick-up signal-to-noise ratio, small volume and strong anti-electromagnetic interference capability.

Description

Laser MEMS microphone

Technical Field

The invention belongs to the technical field of microphones, and particularly relates to a laser MEMS microphone for converting an acoustic signal into an electric signal through laser.

Background

A microphone is a sensor that converts sound into an electrical signal. Microphones have been used in traditional fields such as telephones, hearing aids, sound recording projects, television stations, etc., and are nowadays more used in modern fields such as speech recognition, ultrasonic sensing, automatic performance control, etc. Microphones of different principles have different ways to convert the pressure vibrations generated by sound waves into electrical signals. In a common moving-coil microphone, a vibrating diaphragm vibrates to drive a coil suspended in a magnetic field to move, and weak voltage is generated by the change of the magnetic field; the capacitance microphone uses a diaphragm as a polar plate of a capacitor, and the change of the distance between the two polar plates generates the change of output voltage. Both the two microphones rely on electric charges as media, and are easily influenced by electromagnetic interference to work normally, and the two microphones are greatly influenced by noise and have low sound pickup signal-to-noise ratio.

Microelectromechanical Systems (MEMS) or Microelectromechanical systems (MEMS) refer to electromechanical devices having dimensions in the micrometer range or even smaller. MEMS microphones are also becoming microphone chips or silicon microphones where small size, reliability and economy are key requirements. The diaphragm of a MEMS microphone is etched directly onto a silicon wafer by MEMS technology and is usually integrated with a preamplifier. However, most of the existing MEMS microphones are designed based on a condenser microphone, and compared with the conventional condenser microphone, the MEMS microphone is only small in size, and the sound pickup signal-to-noise ratio and the anti-electromagnetic interference capability are not significantly improved.

Disclosure of Invention

The purpose of the invention is as follows: the invention aims to provide a laser MEMS microphone with high pick-up signal-to-noise ratio, small volume and strong anti-electromagnetic interference capability, which can be well integrated with other digital products.

The technical scheme is as follows: the laser MEMS microphone comprises a light source and a microphone structure, wherein the microphone structure comprises a substrate, a back plate, a vibration sound film, a front cavity formed by the substrate and the vibration sound film, and a rear cavity formed by the vibration sound film and the back plate; the light source is a VCSEL; the substrate is used as a mechanical support; the back plate is positioned right above the vibrating sound film; the vibration sound film is positioned right above the VCSEL light-emitting plane; the back plate and the vibrating sound membrane are respectively provided with a plurality of sound holes manufactured by the MEMS technology, and the vibrating sound membrane vibrates between the front cavity and the rear cavity;

the VCSEL emits a laser beam toward the diaphragm; the vibration sound film reflects part of the laser beam to the VCSEL resonant cavity; the reflected laser beam carries sound information of vibration of the vibration sound film, and the partially reflected laser beam is mixed with laser in the VCSEL resonant cavity to generate a laser self-mixing interference effect; the output power and the frequency of the VCSEL are changed, and the junction voltage on the VCSEL is extracted; after amplification and demodulation, the signals are converted into audio waveforms, and conversion from sound signals to electric signals is achieved.

Preferably, when the VCSEL emits the laser beam, a PN junction in the VCSEL has voltage; when the reflected part of the laser beam is mixed with the laser in the VCSEL resonant cavity, the voltage at the PN junction is changed.

Preferably, the vibrating diaphragm in the microphone structure is responsive to the sound signal in the medium to generate vibrations of corresponding frequency and amplitude with sound signals of different frequencies and loudness.

Preferably, the light source emits a laser beam under the drive of a precise constant current source.

Preferably, the air in the front and rear chambers is compressed; the sound hole on the back plate allows the compressed air in the back cavity to flow out, and the vibrating sound film moves towards the back cavity; the diaphragm sound hole allows the air compressed in the front chamber to flow out, and the diaphragm moves toward the front chamber.

Preferably, the VCSEL is supported by a substrate, and the VCSEL light exit plane coincides with the substrate plane.

Preferably, the substrate is made of silicon nitride or silicon oxide.

Preferably, the material of the back plate is silicon dioxide.

Has the advantages that: compared with the prior art, the invention has the beneficial effects that: (1) according to the laser MEMS microphone provided by the invention, the sound signal is sensed by using the laser self-mixing interference effect, the system noise can be effectively reduced based on the extremely narrow line width of the laser, and the detection capability of weak sound is improved; (2) the microphone structure design optimizes the acoustic performance of the microphone on the signal to noise ratio, and obtains flat correspondence on the whole audio band; (3) the laser MEMS microphone provided by the invention has the advantages of high pickup signal-to-noise ratio, small volume, strong anti-electromagnetic interference capability, convenience for practicability and easiness for integration on digital products such as mobile phones and the like.

Drawings

FIG. 1 is a schematic diagram of a laser MEMS microphone according to the present invention;

FIG. 2 is a schematic diagram of a laser MEMS microphone vibrating diaphragm vibration of the present invention;

FIG. 3 is a VCSEL self-mixing interference schematic of the laser MEMS microphone of the present invention;

fig. 4 is a system block diagram of a laser MEMS microphone of the present invention.

Detailed Description

The invention will be described in further detail with reference to the drawings and the detailed description.

As shown in fig. 1 and 4, the laser MEMS microphone provided by the present invention includes a light source 1 and a microphone structure, where the microphone structure includes a substrate 2, a back plate 3, a vibrating sound membrane 4, a front cavity 5 composed of the substrate 2 and the vibrating sound membrane 4, and a back cavity 6 composed of the vibrating sound membrane 4 and the back plate 3; in this embodiment, the light source 1 is a VCSEL (vertical-cavity surface-emitting laser), and the characteristics of small threshold current, small divergence angle, stable frequency, and the like are suitable for application in the compact structure of the invention. In this embodiment, a VCSEL having a center wavelength of 850nm, a typical power of 0.5mW, and a divergence angle of 10 ° is specifically selected. The light source 1 emits a laser beam by being driven by a driving power supply. In this embodiment, the driving power source is a precise constant current source, which can drive the VCSEL to emit a laser beam with laterally uniform distribution and stable frequency.

The substrate 2 is used as a mechanical support to fixedly support the VCSEL, so that the optical path of the VCSEL is aligned as required, and laser is vertically incident on the surface of the vibrating sound film 4. The backplate 3 and the diaphragm 4 are both fixed to the substrate 2, and the substrate 2 may be made of silicon nitride or silicon oxide, in this embodiment, the substrate 2 is made of silicon nitride. Silicon nitride is a white, high melting point solid with good thermal stability and inert chemical properties. The good thermal stability and chemical stability are beneficial to keeping the stability of the structure and performance of the whole laser MEMS microphone.

The back plate 3 is positioned right above the vibrating sound film 4, and the back plate 3 and the vibrating sound film 4 form a microphone rear cavity 6. The backplate 3 is provided with a plurality of sound holes 7 manufactured by the MEMS technology, and in this embodiment, the diameter of the sound holes 7 is 10 um. The compressed air in the back chamber 6 flows out through the sound holes 7 in the back plate 3 and the vibrating diaphragm 4 moves towards the back chamber 6. In this embodiment, the material of the backplate 3 is silicon dioxide, which also has stable chemical properties, and maintains the structure of the back cavity 6, so that the acoustic performance is not affected by the environment.

The vibrating diaphragm 4 is located directly above the VCSEL exit plane 9. The substrate 2 and the diaphragm 4 form a microphone front volume 5. The diaphragm 4 is also provided with a plurality of sound holes 7 formed by MEMS technology, and in the present embodiment, the sound holes 7 have a diameter of 10 um. The compressed air in the front chamber 5 flows out of the sound holes 7 of the diaphragm 4, and the diaphragm 4 moves toward the front chamber 5. The back plate 3 and the diaphragm 4 are provided with sound holes 7, which allow air to flow in the front cavity 5 and the back cavity 6 to form an acoustic structure, so that the diaphragm 4 responds to acoustic signals more sensitively to generate corresponding vibration.

As shown in fig. 1 and 4, the laser MEMS microphone is placed in a sound receiving environment, the precise constant current source provides constant current drive for the VCSEL until the VCSEL reaches a current above a light emitting threshold, the VCSEL emits a laser beam with stable frequency toward the vibrating sound film 4, and at this time, a PN junction 8 of the VCSEL has a voltage, and a junction voltage at the PN junction 8 is measured;

when the laser beam reaches the end face of the vibration sound film 4, the vibration sound film 4 is positioned right in front of the light-emitting end face of the VCSEL, so that the vibration sound film 4 reflects part of the emitted light beam to the VCSEL resonant cavity;

when sound waves are transmitted to the vibrating sound film 4, the vibrating sound film 4 generates corresponding periodic mechanical deformation due to the sound pressure, and the distance between the vibrating sound film 4 and the VCSEL changes. The reflected laser beam carries sound information for vibrating the vibrating sound film 4, and part of the reflected laser beam returns to the laser resonant cavity and is mixed with the laser in the VCSEL resonant cavity to generate laser self-mixing interference effect;

when the reflected part of the laser beam is mixed with the light in the cavity of the VCSEL, the output power and frequency of the VCSEL change, extracting the voltage at the PN junction 8 of the VCSEL at that moment, which change is linear with the acoustic signal. According to the intensity demodulation principle, the reconstruction of the motion of the vibrating sound film 4 is realized by extracting, amplifying and demodulating the junction voltage and converting the junction voltage into an audio waveform, and data fitting and calibration calculation are carried out to finally realize the conversion from the sound signal to the electric signal.

As shown in fig. 1 and 2, since the periphery of the vibrating sound diaphragm 4 is fixed in the substrate 2, the vibrating sound diaphragm 4 vibrates under the driving of the sound wave signal, and the form of the substrate 2 remains unchanged and is in a stable state. According to the principle of elasticity mechanics, for a membrane with uniform thickness and small deformation with the maximum deformation amount not exceeding 30% of the thickness of the membrane, a circular thin plate fixedly supported on the periphery has the relationship between the central deflection y and the pressure P:

where P is the pressure exerted on the circular vibrating diaphragm 4, r is the radius of the diaphragm, E is the young's modulus, h is the thickness of the diaphragm, upsilon is the poisson's ratio, and Y is the central deflection of the diaphragm 4. In general, the deflection at the center of the diaphragm satisfies y < < h/2, so the relationship can be approximated as:

as shown in fig. 1 and 3, the VCSEL is supported by the substrate 2, and the light exit plane 9 is aligned with the plane of the substrate 2, so that the normal direction of the vibrating diaphragm 4 coincides with the optical axis direction of the VCSEL. When part of the reflected laser beam is mixed with the light in the VCSEL resonant cavity, a laser self-mixing interference effect is generated. The laser self-mixing interference can be explained by a three-mirror cavity model, when the vibration amplitude is small, the external cavity feedback coefficient is considered to be unchanged, and the laser self-mixing interference signal intensity can be obtained:

P＝P₀[1+m cos(ωτ)]

where P0 is the initial light intensity at the time of optical feedback, m is the intensity modulation factor, and τ is the laser round-trip time at the outer cavity.

Claims

1. A laser MEMS microphone, comprising: the microphone structure comprises a light source (1) and a microphone structure, wherein the microphone structure comprises a substrate (2), a back plate (3), a vibration sound film (4), a front cavity (5) formed by the substrate (2) and the vibration sound film (4), and a rear cavity (6) formed by the vibration sound film (4) and the back plate (3); the light source (1) is a VCSEL; the substrate (2) is used as a mechanical support; the back plate (3) is positioned right above the vibrating sound film (4); the vibrating sound film (4) is positioned right above the VCSEL light-emitting plane (9); a plurality of sound holes (7) manufactured by an MEMS technology are formed in the back plate (3) and the vibrating sound membrane (4), and the vibrating sound membrane (4) moves between the front cavity (5) and the rear cavity (6);

the VCSEL emits a laser beam toward the diaphragm (4); the vibrating sound film (4) reflects part of the laser beam to the VCSEL resonant cavity; the reflected laser beam carries sound information of vibration of the vibration sound film (4), and the reflected laser beam is mixed with laser in the VCSEL resonant cavity to generate a laser self-mixing interference effect; the output power and the frequency of the VCSEL are changed, and the junction voltage on the VCSEL is extracted; after amplification and demodulation, the signals are converted into audio waveforms, and conversion from sound signals to electric signals is achieved.

2. The laser MEMS microphone of claim 1, wherein: when the VCSEL emits laser beams, a PN junction (8) in the VCSEL has voltage; when the partially reflected laser beam is mixed with the laser light in the cavity of the VCSEL, the voltage at the PN junction (8) changes.

3. The laser MEMS microphone of claim 1, wherein: the vibrating sound film (4) in the microphone structure responds to sound signals in the medium and generates vibration with corresponding frequency and amplitude along with the sound signals with different frequencies and loudness.

4. The laser MEMS microphone of claim 1, wherein: the light source (1) emits laser beams under the driving of a precise constant current source.

5. The laser MEMS microphone of claim 1, wherein: the air in the front cavity (5) and the rear cavity (6) is compressed; the sound hole (7) on the back plate (3) allows the air compressed in the back cavity (6) to flow out, and the vibrating sound film (4) moves towards the back cavity (6); the sound hole (7) on the vibrating sound film (4) allows the air compressed in the front cavity (5) to flow out, and the vibrating sound film (4) moves towards the front cavity (5).

6. The laser MEMS microphone of claim 1, wherein: the VCSEL is fixedly supported by the substrate (2), and the VCSEL light-emitting plane (9) is superposed with the plane of the substrate (2).

7. The laser MEMS microphone of claim 1, wherein: the substrate (2) is made of silicon nitride or silicon oxide.

8. The laser MEMS microphone of claim 1, wherein: the back plate (3) is made of silicon dioxide.