DE102022113109A1 - MICROPHONE AND METHOD OF OPERATING A MICROPHONE - Google Patents

Abstract

A microphone is specified, comprising: - an emitter (1) which is set up to emit electromagnetic laser radiation (20), - a detector (2) which is set up to determine an intensity (I) of the electromagnetic laser radiation (20), and - an optical element (3) whose transmittance (T) and/or reflectance (R) for the electromagnetic laser radiation (20) depends on an ambient air pressure, wherein - the optical element (3) runs along an optical path of the electromagnetic laser radiation (20 ) is arranged between the emitter (1) and the detector (2), and the optical element (3) comprises a photonic crystal (4).

Description

A microphone and a method of operating a microphone are provided.

An object of at least certain embodiments is to provide an improved diaphragmless microphone and a method of operating the improved diaphragmless microphone.

According to at least one embodiment, the microphone has an emitter that is set up to emit electromagnetic laser radiation. The emitter is designed in particular to convert an electrical current into electromagnetic laser radiation.

Electromagnetic laser radiation is created by stimulated emission. In contrast to electromagnetic radiation, which arises through spontaneous emission, electromagnetic laser radiation has a smaller spectral line width, a higher coherence length and/or a higher degree of polarization.

The emitter preferably has the smallest possible spectral line width. For example, a half-width of the electromagnetic laser radiation is at most 1 MHz, preferably at most 100 kHz and particularly preferably at most 10 kHz. The emitter is set up, for example, to emit electromagnetic laser radiation in a spectral range between infrared light and ultraviolet light.

According to at least one further embodiment, the microphone has a detector which is set up to determine an intensity of the electromagnetic laser radiation. In particular, the detector converts the electromagnetic laser radiation incident thereon into an electrical photocurrent, with an electrical current strength of the photocurrent preferably being proportional to the intensity of the electromagnetic laser radiation. The detector has, for example, a photodiode or a phototransistor.

According to at least one further embodiment, the microphone has an optical element whose degree of transmission and/or degree of reflection for the electromagnetic laser radiation depends on an ambient air pressure. Here and below, the transmittance describes a relationship between an intensity of the electromagnetic laser radiation transmitted through the optical element and an intensity of the electromagnetic laser radiation incident on the optical element. In other words, the transmittance describes a portion of the electromagnetic laser radiation that is transmitted through the optical element.

Here and below, the reflectance describes a relationship between an intensity of the electromagnetic laser radiation reflected by the optical element and an intensity of the electromagnetic laser radiation incident on the optical element. In other words, the reflectance describes a portion of the electromagnetic laser radiation that is reflected by the optical element.

Here and below, the ambient air pressure describes a pressure of ambient air. The ambient air is a medium that at least partially surrounds the optical element. In particular, sound waves propagate through the surrounding air. The microphone is set up to convert these sound waves into an optical and/or electrical output signal. In the present case, the ambient air can refer to a gaseous and/or a liquid medium in which the optical element is arranged.

According to at least one further embodiment of the microphone, the optical element is arranged along an optical path of the electromagnetic laser radiation between the emitter and the detector. In other words, the electromagnetic laser radiation generated by the emitter hits the optical element and is at least partially transmitted and/or at least partially reflected by it. The electromagnetic laser radiation transmitted or reflected by the optical element subsequently hits the detector.

According to at least one further embodiment of the microphone, the optical element comprises a photonic crystal. The photonic crystal in particular has a structuring with a spatial modulation of a refractive index. Here and below, the refractive index refers to a refractive index for the laser radiation generated by the emitter during operation. The spatial modulation of the refractive index occurs, for example, in one, two or three spatial directions. In other words, the photonic crystal has a one-dimensional, two-dimensional or three-dimensional structure.

Electromagnetic laser radiation that propagates in the photonic crystal is in particular diffracted and/or scattered by the spatial modulation of the refractive index. Through constructive and/or destructive interference of different components of the diffracted and/or scattered electromagnetic laser radiation, for example a propagation direction of the electromagnetic laser radiation can be changed.

The photonic crystal in particular has a photonic band structure with at least one photonic band gap. The photonic band gap is a forbidden energy range or a forbidden frequency range, whereby an electromagnetic wave with an energy in the forbidden energy range or with a frequency in the forbidden frequency range cannot propagate within the photonic crystal.

The degree of transmittance and/or the degree of reflection of the photonic crystal is preferably dependent on a refractive index contrast of the structuring. The refractive index contrast is in particular proportional to a difference between a maximum refractive index and a minimum refractive index of the structuring in the photonic crystal. In particular, the refractive index contrast changes due to the ambient air pressure. A change in the ambient air pressure caused by a sound wave thus results in a corresponding change in the transmittance and/or the reflectance of the photonic crystal.

• - den Emitter, der zur Emission elektromagnetischer Laserstrahlung eingerichtet ist,
• - den Detektor, der zur Bestimmung der Intensität der elektromagnetischen Laserstrahlung eingerichtet ist, und
• - das optische Element, dessen Transmissionsgrad und/oder Reflexionsgrad für die elektromagnetische Laserstrahlung vom Umgebungsluftdruck abhängt, wobei
• - das optische Element entlang des optischen Pfades der elektromagnetischen Laserstrahlung zwischen dem Emitter und dem Detektor angeordnet ist, und
• - das optische Element einen photonischen Kristall umfasst.

According to a preferred embodiment, the microphone comprises:

• - the emitter, which is set up to emit electromagnetic laser radiation,
• - the detector, which is set up to determine the intensity of the electromagnetic laser radiation, and
• - the optical element, the degree of transmittance and / or degree of reflection for the electromagnetic laser radiation depends on the ambient air pressure, where
• - the optical element is arranged along the optical path of the electromagnetic laser radiation between the emitter and the detector, and
• - The optical element comprises a photonic crystal.

The microphone described here is based in particular on the idea of converting changes in the transmittance and/or in the reflectance of a photonic crystal into an optical signal and subsequently into an electrical output signal due to a sound wave striking it. In particular, the microphone described here advantageously has no mechanically moving parts. Conventional microphones, for example, include a mechanically movable membrane, the deflection of which is converted into an electrical signal. The mechanical properties of the membrane limit in particular a sound pressure range in which the microphone can be used and/or in which the microphone has a high sensitivity. Furthermore, the membrane includes a movable mass that must be deflected and therefore has a certain inertia.

Due to the lack of moving mechanical elements, the membrane-free microphone described here can be used for a very large sound pressure range without encountering mechanical limitations. Since no mass deflection of a membrane is necessary, the microphone described here advantageously has a higher dynamic range. Furthermore, a risk of a malfunction in which the membrane of a conventional microphone sticks to a counter electrode due to, for example, a high sound level is eliminated.

The microphone described here is particularly suitable for sound detection in gaseous ambient air and in liquids. Since no inert mass needs to be set in motion, the microphone described here advantageously has an improved impulse response. In addition, the converter principle described here is frequency-independent, has a very high signal-to-noise ratio, is very robust and/or can be adapted to the frequencies of the sound waves to be measured and/or to the sound levels to be measured using a wide selection of possible material systems.

According to at least one further embodiment of the microphone, the photonic crystal has a plurality of recesses in a substrate, the recesses forming a two-dimensional grid. For example, centers of recesses are arranged at intersections of a regular two-dimensional grid. Some specific intersection points of the regular two-dimensional grid can also be free of recesses. The features of a recess described below preferably apply to a majority of the recesses, particularly preferably to all recesses.

The recess is, for example, a trench or a hole that extends between two opposing main surfaces of the substrate. In a plan view of the main surface of the substrate, the recess has, for example, a circular, oval, elliptical or polygonal, in particular square, rectangular or triangular cross-sectional area. The recess preferably completely penetrates the substrate. Alternatively can the recess can be designed as a blind hole that does not completely penetrate the substrate.

A diameter of the recess is, for example, between 10 nanometers and 1000 nanometers inclusive. In the case of a recess with a non-circular cross-sectional area, the diameter refers in particular to a maximum diameter of the cross-sectional area.

The recess is filled in particular with the ambient air that surrounds the photonic crystal. The refractive index contrast of the photonic crystal is therefore proportional to the difference between the refractive index of the substrate and the refractive index of the ambient air in the recess. Sound waves propagating through the ambient air lead to a change in the ambient air pressure in the recess and thus to a change in the refractive index contrast of the photonic crystal.

The regular two-dimensional grid is, for example, a square, rectangular, triangular, hexagonal or oblique grid. Alternatively or additionally, the recesses can form a one-dimensional or a three-dimensional grid. A distance between the centers of adjacent recesses or a period of the grating is preferably adapted to a wavelength of the electromagnetic laser radiation. For example, the period of the grating corresponds to a value between 1% and 40% inclusive of the wavelength of the electromagnetic laser radiation or the period of the grating deviates by a maximum of 70% from the wavelength of the electromagnetic laser radiation.

The substrate in particular has a dielectric material which is at least partially transparent to the electromagnetic laser radiation. For example, the substrate has a glass, a semiconductor material or a polymer, or consists of one of these materials.

According to at least one further embodiment, an optical ring resonator is formed in the photonic crystal, which has a resonance frequency that depends on the ambient air pressure. The optical ring resonator in particular comprises a loop-shaped waveguide which forms a closed loop. Electromagnetic laser radiation that is coupled into the optical ring resonator can therefore pass through the loop-shaped waveguide several times. The loop-shaped waveguide has, for example, an annular, oval or flat-oval shape.

The loop-shaped waveguide of the optical ring resonator is formed, for example, in that no recesses are arranged in the substrate of the photonic crystal at the position of the loop-shaped waveguide. The loop-shaped waveguide is therefore particularly free of recesses. In other words, the loop-shaped waveguide of the optical ring resonator is formed by a line defect in the regular lattice of the photonic crystal, the line defect forming a closed loop. In particular, there are no recesses arranged in the photonic crystal along the line defect. Alternatively, recesses can be arranged along the line defect, with the recesses along the line defect having a different cross-sectional area and/or a different diameter than in the remaining photonic crystal.

The resonance frequency of the optical ring resonator is determined, for example, by an optical path length of the electromagnetic laser radiation in the loop-shaped waveguide. For example, the optical ring resonator has a resonance when the optical path length corresponds to a multiple of the wavelength of the electromagnetic laser radiation. The change in the ambient air pressure in the recesses of the photonic crystal changes in particular the optical path length and thus also the resonance frequency of the optical ring resonator.

According to at least one further embodiment, the optical ring resonator has a Q factor of at least 1000, preferably at least 10,000, and particularly preferably at least 100,000. The Q factor or quality factor describes in particular a relationship between the resonance frequency of the optical ring resonator and the spectral line width of the resonance of the optical ring resonator. In other words, the Q factor is proportional to an average lifetime of a resonant photon in the optical ring resonator.

According to at least one further embodiment, the optical ring resonator in the photonic crystal has a sensitivity of at least 1000 nm/RIU, preferably of at least 10,000 nm/RIU, and particularly preferably of at least 100,000 nm/RIU. Here and below, the sensitivity describes a shift in the wavelength of the resonance of the optical ring resonator in nanometers per refractive index unit (RIU). For example, a resonance wavelength shifts by 10,000 nm with a sensitivity of 10,000 nm/RIU if the refractive index in the recesses of the photonic crystal changes by the value 1.

A high Q factor in conjunction with high sensitivity advantageously allows the detection of small refractive index changes in the recesses of the photonic crystal. In particular, small changes in the ambient air pressure can be detected with a high signal-to-noise ratio by sound waves, which cause small changes in the refractive index in the recesses of the photonic crystal.

According to at least one further embodiment of the microphone, a waveguide for the electromagnetic laser radiation is formed in the photonic crystal and the waveguide is coupled to the optical ring resonator. The waveguide is designed in particular for coupling and decoupling electromagnetic laser radiation into the optical ring resonator.

For example, a distance between the waveguide and the optical ring resonator is so small that an evanescent electromagnetic field of the waveguide at least partially overlaps the optical ring resonator, and vice versa. The waveguide is formed in particular by a line defect in the photonic crystal. In particular, no recesses or recesses with a different cross-sectional area and/or a different diameter are arranged in the substrate of the photonic crystal along the line defect.

For example, the transmittance of electromagnetic laser radiation propagating in the waveguide is greatly reduced if the resonance frequency of the optical ring resonator corresponds to a frequency of the electromagnetic laser radiation.

Two or more waveguides can also be formed in the photonic crystal and coupled to the optical ring resonator. The electromagnetic laser radiation is coupled in, for example, via a waveguide and coupled out via one or two waveguides. For example, two waveguides with the optical ring resonator form an optical add-drop filter.

According to at least one further embodiment of the microphone, electromagnetic laser radiation propagating through the waveguide has a Fano resonance, in the vicinity of which the transmittance and/or the reflectance depend linearly on a wavelength of the electromagnetic laser radiation. For example, a Fano resonance can be generated by combining the optical ring resonator with a partially reflective barrier in the waveguide.

Alternatively, the electromagnetic laser radiation propagating through the waveguide has a Lorentz resonance, in the immediate vicinity of which the degree of transmittance and/or the degree of reflection depends squarely on the wavelength of the electromagnetic laser radiation. In contrast to the Lorentz resonance, the Fano resonance has in particular an asymmetrical spectral line shape. Due to the linear dependence of the transmittance and/or the reflectance on the wavelength, the Fano resonance advantageously has a higher sensitivity. In particular, a small shift in the resonance frequency due to the change in the ambient air pressure at the Fano resonance leads to a larger change in the transmittance and/or the reflectance.

According to at least one further embodiment of the microphone, the photonic crystal is set up as an optical band-edge filter and/or has a beam splitter in which at least three waveguides formed in the photonic crystal converge at one point.

Edges of the photonic band gap are referred to here and below as optical band edges. In the immediate vicinity of the optical band edge, the photonic crystal has, for example, a strong dependence of the transmittance and/or reflectance on the wavelength of the electromagnetic laser radiation. A small shift in the optical band edge due to a change in the ambient air pressure in the recesses of the photonic crystal thus advantageously leads to a large change in the transmittance and/or reflectance of the photonic crystal.

The beam splitter in particular has a waveguide for coupling in electromagnetic laser radiation and at least two waveguides for coupling out the electromagnetic laser radiation. By changing the ambient air pressure in the recesses of the photonic crystal, for example, a ratio of the intensity of the electromagnetic laser radiation in the at least two waveguides via which the electromagnetic laser radiation is coupled out shifts. Via this shift, the change in ambient air pressure can be converted into an optical signal and subsequently into an electrical output signal.

According to at least one further embodiment of the microphone, the photonic crystal can be freely flowed around by ambient air. Sound waves propagating in the ambient air can therefore change the ambient air pressure in recesses of the photonic crystal. For example, is Sound transmission through the photonic crystal is not possible due to the small diameter of the recesses. Due to the free flow around the photonic crystal, pressure equalization can advantageously take place between opposite main surfaces of the substrate of the photonic crystal. For example, the substrate of the photonic crystal has a free space or a cutout over which ambient air can flow from one main surface of the substrate to the opposite main surface of the substrate. For example, the diameter of the recesses in the photonic crystal can be so small that the ambient air cannot flow through the recesses between the opposing main surfaces or can only flow with difficulty.

According to at least one further embodiment of the microphone, the emitter has a laser diode and the detector has a photodiode. The laser diode and/or the photodiode each comprise a semiconductor layer stack, which in particular has a III/V compound semiconductor material.

The laser diode and the photodiode preferably have a semiconductor material from the same material family. An emission wavelength of the laser diode and an absorption wavelength of the photodiode can thus be advantageously coordinated with one another.

The laser diode is, for example, a DFB laser diode (English: distributed feedback, DFB for short) and has a periodically structured active area for generating electromagnetic laser radiation. DFB laser diodes advantageously have a small spectral line width. The spectral line width of the laser diode is preferably at most a factor of two larger or smaller than the spectral line width of the resonance of the optical ring resonator.

According to at least one further embodiment of the microphone, the laser diode and the photodiode are applied to a main surface of the photonic crystal. The main surface of the photonic crystal corresponds in particular to the main surface of the substrate of the photonic crystal.

For example, the substrate of the photonic crystal comprises silicon, and the laser diode and the photodiode comprise a III/V compound semiconductor material. The laser diode and the photodiode are preferably integrated on the substrate of the photonic crystal. For example, the substrate of the photonic crystal has electrical contact surfaces on which the laser diode and/or the photodiode are soldered or attached with an electrically conductive adhesive.

According to at least one further embodiment of the microphone, the laser diode is applied to the main surface of the photonic crystal in such a way that the electromagnetic laser radiation in the photonic crystal propagates parallel to the main surface. For example, the active region for generating electromagnetic laser radiation is optically coupled to the waveguide in the photonic crystal, so that at least part of the electromagnetic laser radiation is coupled out of the active region and coupled into the waveguide.

According to at least one further embodiment, the optical element has a plurality of photonic crystals which have different degrees of transmittance and/or different degrees of reflectance for the electromagnetic laser radiation. For example, each photonic crystal has an optical ring resonator, with the resonance frequencies and/or the spectral line widths of the different optical ring resonators differing from one another.

Preferably, exactly one detector is assigned to each photonic crystal. Each photonic crystal can have an associated emitter, or the electromagnetic laser radiation from a single emitter is divided among the multiple photonic crystals. By combining several photonic crystals with different degrees of transmittance and/or degrees of reflection, the sensitivity or dynamic range of the microphone can in particular be improved.

According to at least one further embodiment of the microphone, the photonic crystal is arranged in a housing, the housing comprising a mechanical protective element that is at least partially transparent to sound waves. The mechanical protective element is, for example, a cover of the housing which has a plurality of openings through which ambient air can flow into and out of the housing. The openings are preferably so small that, for example, dust particles cannot penetrate the housing. The photonic crystal is therefore protected, for example, from mechanical forces and/or dirt. However, sound waves can penetrate through the openings into the housing and hit the photonic crystal there.

Furthermore, a method for operating a microphone is specified. The method is designed in particular to operate a microphone described here. All features of the microphone are also disclosed for the method of operating a microphone, and vice versa.

According to one embodiment of the method for operating a microphone, the photonic crystal transmits and/or reflects the laser radiation generated by the emitter during operation in such a way that a change in the ambient air pressure caused by sound waves is converted into a corresponding change in the intensity of the transmitted and/or reflected electromagnetic laser radiation . In particular, the degree of transmittance and/or the degree of reflection of the photonic crystal depends on the ambient air pressure.

According to at least one further embodiment of the method, the detector converts the change in intensity in the transmitted and/or reflected electromagnetic laser radiation into an electrical output signal.

For example, the electromagnetic laser radiation generated by the emitter has an intensity that is constant over time. Temporal oscillations in the ambient air pressure caused by sound waves are converted by the optical element into corresponding temporal oscillations in the intensity of the transmitted and/or reflected electromagnetic laser radiation. The detector finally converts the temporal oscillations of the intensity into a corresponding temporally oscillating electrical output signal.

According to at least one further embodiment of the method, a wavelength of the electromagnetic laser radiation generated by the emitter is periodically tuned in order to determine changes in the transmittance and/or the reflectance of the photonic crystal as a function of the wavelength. In particular, the wavelength of the electromagnetic laser radiation generated by the emitter is periodically changed in a predetermined wavelength range.

For example, the wavelength is changed in a sawtooth or sinusoidal manner as a function of time. By periodically tuning the wavelength of the emitter, the resonance frequency of the optical ring resonator can be determined, for example, at which the transmittance and/or the reflectance is minimum or maximum. The resonance frequency depends in particular on the ambient air pressure.

According to at least one further embodiment of the method, two sidebands are modulated onto the electromagnetic laser radiation, the sidebands being set up to determine the resonance frequency in the photonic crystal. The sidebands are generated in particular by phase modulation or frequency modulation, for example by modulating an operating current of the laser diode or by a laser modulator.

The sidebands are in particular shifted by a modulation frequency from the frequency of the electromagnetic laser radiation. A first sideband has a frequency that is lower than the modulation frequency than the electromagnetic laser radiation, and a second sideband has a frequency that is higher than the modulation frequency than the electromagnetic laser radiation.

The photonic crystal includes in particular the optical ring resonator and the waveguide coupled thereto, the electromagnetic laser radiation propagating through the waveguide having a transmittance with the Lorentz resonance described above. If the modulated electromagnetic laser radiation with the two sidebands is coupled into this waveguide, then the intensity of the electromagnetic laser radiation transmitted by the photonic crystal includes in particular a portion that oscillates in time with the modulation frequency.

An amplitude of this portion which oscillates over time with the modulation frequency depends in particular on the resonance frequency of the optical ring resonator and the frequency of the electromagnetic laser radiation. The amplitude is asymmetrical with respect to the resonance frequency and has an advantageously linear frequency dependence near the resonance frequency.

A small shift in the resonance frequency due to a change in the ambient air pressure caused by a sound wave thus leads to a linear change in the amplitude of the temporally oscillating component. The amplitude of the temporally oscillating component can therefore be used, for example, to convert the sound wave into an electrical output signal.

• Die 1 und 2 zeigen eine schematische Draufsicht und eine schematische Schnittdarstellung eines Mikrofons gemäß einem Ausführungsbeispiel.
• Die 3 und 4 zeigen eine schematische Draufsicht und eine schematische Schnittdarstellung eines Mikrofons gemäß einem weiteren Ausführungsbeispiel.
• Die 5 und 6 zeigen schematische Schnittdarstellungen eines Mikrofons gemäß verschiedener Ausführungsbeispiele.
• 7 zeigt eine schematische Draufsicht auf ein Mikrofon gemäß einem Ausführungsbeispiel.
• 8 zeigt einen schematischen Transmissionsgrad eines optischen Elements eines Mikrofons gemäß einem Ausführungsbeispiel.
• 9 zeigt schematisch die Intensität der elektromagnetischen Laserstrahlung als Funktion der Wellenlänge gemäß einem Ausführungsbeispiel eines Verfahrens zum Betrieb eines Mikrofons.
• Die 10 und 11 zeigen schematische Darstellungen eines optischen Elements eines Mikrofons gemäß weiterer Ausführungsbeispiele.
• Die 12 und 13 zeigen schematische Darstellungen eines Mikrofons gemäß verschiedener weiterer Ausführungsbeispiele.

Further advantageous embodiments and developments of the microphone and the method for operating a microphone result from the exemplary embodiments described below in connection with the figures.

• The 1 and 2 show a schematic top view and a schematic sectional view of a microphone according to an exemplary embodiment.
• The 3 and 4 show a schematic top view and a schematic sectional view of a microphone according to a further exemplary embodiment.
• The 5 and 6 show schematic sectional views of a microphone according to various exemplary embodiments.
• 7 shows a schematic top view of a microphone according to an exemplary embodiment.
• 8th shows a schematic transmittance of an optical element of a microphone according to an exemplary embodiment.
• 9 shows schematically the intensity of the electromagnetic laser radiation as a function of the wavelength according to an exemplary embodiment of a method for operating a microphone.
• The 10 and 11 show schematic representations of an optical element of a microphone according to further exemplary embodiments.
• The 12 and 13 show schematic representations of a microphone according to various further exemplary embodiments.

Identical, similar or identically acting elements are provided with the same reference numerals in the figures. The figures and the size relationships between the elements shown in the figures should not be considered to scale. Rather, individual elements, for example layer thicknesses, can be shown exaggeratedly large for better representation and/or better understanding.

The microphone according to the exemplary embodiment in 1 has an emitter 1 and a detector 2, which are arranged on a main surface 12 of an optical element 3. The emitter 1 comprises a laser diode, which emits electromagnetic laser radiation 20 (not shown here, see 4 ) generated. The detector 2 comprises a photodiode which is set up to measure an intensity I of the electromagnetic laser radiation 20.

The optical element 3 includes a photonic crystal 4, which is formed by a plurality of recesses 5 in a substrate 6. The substrate 6 has, for example, silicon or consists of silicon. The recesses 5 are arranged in the form of a regular square grid and filled with ambient air. Sound waves 21 (not shown here, see 4 ), which hit the photonic crystal 4, change in particular the ambient air pressure and thus the refractive index of the ambient air in the recesses 5.

An optical ring resonator 7 and a waveguide 8 are arranged in the photonic crystal. The optical ring resonator 7 and the waveguide 8 are formed by line defects in the square lattice of the photonic crystal, with no recesses 5 being arranged in the substrate 6 along the line defects. The optical ring resonator 7 is formed in particular by a circular, closed line defect. Electromagnetic laser radiation 20, which is coupled into the optical ring resonator 7, can therefore pass through it several times.

The waveguide 8 is set up to guide the electromagnetic laser radiation 20 from the emitter 1 to the detector 2. The optical ring resonator 7 is coupled to the waveguide 8, so that at least part of the electromagnetic laser radiation 20 propagating in the waveguide 8 is coupled out of the waveguide 8 and coupled into the optical ring resonator 7, and vice versa.

The optical ring resonator 7 is set up to change the transmittance T of the photonic crystal 4 for the electromagnetic laser radiation 20 depending on the ambient air pressure. The optical ring resonator 7 has a resonance frequency f _R which changes as a function of the ambient air pressure in the recesses 5. For example, the transmittance T for the electromagnetic laser radiation 20, which propagates through the waveguide 8, is greatly reduced when the frequency of the electromagnetic laser radiation 20 corresponds to the resonance frequency f _R. In particular, the transmittance T changes sharply in the vicinity of the resonance frequency f _R. A change in the ambient air pressure in the recesses 5 by a sound wave 21 leads to a shift in the resonance frequency f _R and, as a result, to a change in the intensity I of the electromagnetic laser radiation 20 at the detector 2. The sound wave 21 is thus converted into an electrical output signal at the detector 2 .

2 shows a schematic side section view of the in connection with 1 described embodiment of a microphone. The recesses 5 of the photonic crystal 4 extend from the main surface 12 of the substrate 6 to an opposite main surface of the substrate. In particular, the recesses 5 penetrate the substrate 6 completely.

The laser diode of the emitter 1 and the photodiode of the detector 2 are applied to the main surface 12 of the substrate 6 in such a way that active areas for generating or absorbing the electromagnetic laser radiation 20 in the laser diode or in the photodiode are coupled to the waveguide 8. The electromagnetic laser radiation 20 propagates parallel to the main surface 12 of the substrate 6 in the photonic crystal 4.

3 shows another embodiment of a microphone compared to that in connection with 1 described embodiment additionally has a carrier 17 with electrical connection contacts 18 and a housing 13. The carrier 17 is, for example, a circuit board. The housing 13 is set up to mechanically protect the photonic crystal 4. The emitter 1 and the detector 2 are each connected to the electrical connection contacts 18 via two bonding wires 19 and electrical contact surfaces 16 on the substrate 6. The emitter 1 and the detector 2 are, for example, soldered onto the contact surfaces 16 of the substrate 6 or attached to it with electrically conductive adhesive.

The substrate 6 of the photonic crystal 4 has two free spaces 15, which are designed as cutouts in the substrate 6. The ambient air can flow freely around the photonic crystal 4 via the free spaces 15 and, for example, create a pressure equalization between the two opposing main surfaces 12.

4 shows a schematic side section view of the in connection with 3 described embodiment of a microphone. The substrate 6 of the photonic crystal 4 is applied to the carrier 17 via spacers 22. The housing 13 is connected to the carrier 17, for example via an adhesive connection, and encloses the substrate 6.

The carrier 17 has an opening 23 through which ambient air can flow through the carrier 17 to the substrate 6 of the photonic crystal 4. Sound waves 21 can thus impinge on the photonic crystal 4 unhindered. The photonic crystal 4, the emitter 1 and the detector 2 are protected inside the housing 13, for example from mechanical forces.

The laser radiation 20 generated by the emitter 1 during operation is coupled into the photonic crystal 4 and propagates parallel to the main surface 12 of the substrate to the detector 2.

This in 5 illustrated embodiment of a microphone, in contrast to that in connection with 4 Microphone described has a mechanical protective element 14, which is arranged via spacers 22 on a main surface 12 of the substrate 6 opposite the emitter 1 and the detector 2. The mechanical protective element 14 has a plurality of openings 23 through which ambient air can flow into the microphone and in particular into the photonic crystal 4. Sound waves 21 can thus propagate to the photonic crystal 4, while the photonic crystal 4 is protected, for example, from mechanical forces.

A carrier 17, in particular a circuit board, with electrical connection contacts 18 is arranged on a main surface 12 of the substrate 6 facing the emitter 1 and the detector 2. The electrical connection contacts 18 are set up for electrical contacting of the emitter 1 and the detector 2. The carrier 17 is structured in such a way that, together with the mechanical protective element 14, it forms a protected free space 15 in which the photonic crystal 4 is arranged and ambient air can flow around it freely.

This in 6 illustrated embodiment of a microphone, in contrast to that in connection with 5 described microphone has a structured substrate 6, so that the emitter 1 and the detector 2 are arranged on side surfaces 24 of the photonic crystal 4. Thus, for example, the electromagnetic laser radiation 20 of an edge-emitting laser diode can be coupled directly into the photonic crystal 4 and propagate parallel to its main surface 12.

7 shows a top view of the mechanical protective element 14 in connection with the 5 and 6 described embodiments of a microphone. The mechanical protection element 14 has a plurality of openings 23 with a square cross-sectional area, the openings 23 forming a regular square grid. For example, the cross-sectional area of the openings is so small that dust particles cannot penetrate through the openings into the interior of the microphone, while ambient air can flow in and out unhindered. The shape of the cross-sectional area of the openings 23 and the arrangement of the openings 23 can be arbitrary.

8th shows schematically the transmittance T of a photonic crystal 4 as a function of the frequency f of the electromagnetic laser radiation 20 according to one of the in connection with 1 until 6 described embodiments of a microphone. In particular, the transmittance T of the waveguide 8, which is coupled to the optical ring resonator 7, is shown.

The transmittance T has, for example, a Fano resonance 9 or a Lorentz resonance 10. In the case of the Lorentz resonance 10, the transmittance T is greatly reduced in the immediate vicinity of the resonance frequency f _R. The Lorentz resonance is symmetrical with respect to the resonance frequency f _R . In particular, the transmittance T at the Lorentz resonance 10 changes quadratically as a function of the frequency f for small deviations from the resonance frequency f _R.

In contrast to the Lorentz resonance 10, the Fano resonance 9 is asymmetrical with respect to the resonance frequency f _R. In particular, the transmittance T at the Fano resonance 9 changes linearly as a function of the frequency f for small deviations from the resonance frequency f _R. Thus, with the Fano resonance 9, small fluctuations in the resonance frequency f _R due to changes in the ambient air pressure, for example, can be detected more easily and with a higher signal-to-noise ratio than with the Lorentz resonance 10.

The transmittance T has a Fano resonance 9 if, for example, a partially reflecting barrier for the electromagnetic laser radiation 20 is arranged in the waveguide 8. The barrier can be generated, for example, by changing a structure of the recesses 5 in the photonic crystal 4. In particular, the barrier is arranged in a region of the waveguide 8 which is coupled to the optical ring resonator 7.

In the exemplary embodiment of the method for operating a microphone in 9 electromagnetic laser radiation 20 with constant intensity I is coupled into the photonic crystal 4. In particular shows 9 an intensity I of the electromagnetic laser radiation 20 and a transmittance T or reflectance R of the photonic crystal 4 as a function of the wavelength λ. The electromagnetic laser radiation 20 has a fixed wavelength λ _L with a spectral line width Δλ, which in this exemplary embodiment is slightly larger than the spectral line width Δλ of the resonance of the transmittance T or reflectance R.

If no sound wave 21 impinges on the photonic crystal 4, the wavelength of the electromagnetic laser radiation λ _L corresponds to the wavelength λ _R at which a resonance occurs in the transmittance T of the photonic crystal (λ _L = λ _R , left part of 9 ).

Changes in the ambient air pressure in the recesses 5 of the photonic crystal 4 caused by a sound wave 21 lead, for example, to a small shift in the resonance frequency f _R of the optical ring resonator 7. This also results in the wavelength λ _R at which the resonance in the transmittance T or reflectance R of the photonic Crystal 4 occurs, shifted accordingly (right part of the 9 ). The intensity I of the electromagnetic laser radiation 20 measured at the detector 2 changes, for example, proportionally to the transmittance T or reflectance R at the wavelength λ _L of the electromagnetic laser radiation 20. The sound wave 21 is thus converted into a change in the intensity I of the electromagnetic laser radiation 20 at the detector 2 .

10 shows a schematic top view of a photonic crystal 4, which has a beam splitter 11. Three waveguides 8 are brought together at one point in the photonic crystal 4. Electromagnetic laser radiation 20 is coupled in through the left waveguide 8, divided at the beam splitter 11, and coupled out via the two right waveguides 8.

By changing the ambient air pressure in the recesses 5 of the photonic crystal 4, a division ratio of the intensities I of the electromagnetic laser radiation 20, which is coupled out via the two waveguides 8, changes in particular. By determining the relative change in intensity of the electromagnetic laser radiation 20 coupled out via the two waveguides 8, the sound wave 21 is converted in particular into an optical signal and subsequently into an electrical output signal. The two partial beams can also be interfered with, for example.

11 shows a schematic representation of a photonic crystal 4, which, in contrast to the exemplary embodiment in 10 has more than two waveguides 8 for coupling out the electromagnetic laser radiation 20. In particular, the photonic crystal 4 has a waveguide 8 for coupling in and four waveguides 8 for coupling out the electromagnetic laser radiation 20. For example, the photonic crystal 4 is a multimode interference block-based power splitter (MMI). By determining the relative intensity changes and/or by interference of some or all of the four coupled-out partial beams of the electromagnetic laser radiation 20, the sound wave 21 can be converted in particular into an electrical output signal.

12 shows a schematic top view of an exemplary embodiment of a microphone that has an optical element 3 with three different photonic crystals 4, which are formed in a common substrate 6. The photonic crystals 4 each have a differently designed optical ring resonator 7, whose resonance frequencies f _R and/or spectral line widths are different. The various optical ring resonators 7 in particular have closed line defects of different lengths, and/or the recesses 5 in the photonic crystals 4 have different diameters.

The optical ring resonators 7 are each coupled to an associated waveguide 8, which guides the electromagnetic laser radiation 20 from an emitter 1 to a detector 2. With the microphone in 12 Each photonic crystal 4 has an associated emitter 1 and an associated detector 2. Alternatively, a common emitter 1 can also be arranged on the substrate 6, the electromagnetic laser radiation 20 of which is divided between the various photonic crystals 4.

By combining several, differently designed photonic crystals 4, the sensitivity and/or the dynamic range of the microphone can be expanded and/or improved in the smallest possible installation space.

13 shows a schematic sectional view of a microphone according to an exemplary embodiment, with the electromagnetic laser radiation 20 propagating perpendicular to the plane of the drawing. An emitter 1 is applied to the main surface 12 of a substrate 6 of a photonic crystal 4.

The emitter 1 is a laser diode that includes a semiconductor layer stack 25 with an active region 26 for generating electromagnetic laser radiation 20. The active area 26 has, for example, a pn junction. The active region 26 is arranged so close to the main surface 12 of the substrate 6 that the electromagnetic laser radiation 20 from the active region 26 at least partially couples into the waveguide 8 in the photonic crystal 4 and propagates there. For example, an evanescent electromagnetic field of the electromagnetic laser radiation 20 in the active region 26 overlaps with the waveguide 8.

The semiconductor layer stack 25 has a III/V compound semiconductor material, while the substrate has, for example, silicon. The laser diode is, for example, soldered or glued onto the substrate 6. A p-contact 27 and an n-contact 28 are set up for electrically contacting the semiconductor layer stack 25.

The invention is not limited to these by the description based on the exemplary embodiments. Rather, the invention encompasses every new feature and every combination of features, which in particular includes every combination of features in the patent claims, even if this feature or this combination itself is not explicitly stated in the patent claims or exemplary embodiments.

Reference symbol list

1

Emitter

2

detector

3

optical element

4

photonic crystal

5

recess

6

Substrate

7

optical ring resonator

8th

waveguide

9

Fano resonance

10

Lorentz resonance

11

Beam splitter

12

main area

13

Housing

14

mechanical protection element

15

free space

16

Contact surface

17

carrier

18

electrical connection contact

19

Bonding wire

20

electromagnetic laser radiation

21

sound wave

22

Spacers

23

opening

24

side surface

25

Semiconductor layer stack

26

active area

27

p-contact

28

n-contact

I

intensity

T

Transmittance

R

reflectance

f

frequency

fr

Resonance frequency

λ

wavelength

λL

Wavelength of electromagnetic laser radiation

λR

Wavelength of resonance

Δλ

spectral line width

Claims (16)

Microphone, comprising: - an emitter (1) which is set up to emit electromagnetic laser radiation (20), - a detector (2) which is set up to determine an intensity (I) of the electromagnetic laser radiation (20), and - an optical element (3), the transmittance (T) and/or reflectance (R) for the electromagnetic laser radiation (20) depends on an ambient air pressure, whereby - the optical element (3) is arranged along an optical path of the electromagnetic laser radiation (20) between the emitter (1) and the detector (2), and - The optical element (3) comprises a photonic crystal (4). Microphone according to the preceding claim, wherein the photonic crystal (4) has a plurality of recesses (5) in a substrate (6), and the recesses (5) form a two-dimensional grid. Microphone according to one of the preceding claims, wherein an optical ring resonator (7) is formed in the photonic crystal (4) and has a resonance frequency (f _R ) which depends on the ambient air pressure. Microphone according to the preceding claim, wherein the optical ring resonator (7) has a Q factor of at least 10,000. Microphone after one of the Claims 3 or 4 , wherein a waveguide (8) for the electromagnetic laser radiation is formed in the photonic crystal (4) and the waveguide (8) is coupled to the optical ring resonator (7). Microphone according to the preceding claim, wherein electromagnetic laser radiation (20) propagating through the waveguide (8) has a Fano resonance (9), in the vicinity of which the transmittance (T) and / or the reflectance (R) linearly depend on a wavelength (λ _L ) depends on the electromagnetic laser radiation (20). Microphone according to one of the preceding claims, wherein the photonic crystal (4) is set up as an optical band edge filter and / or the photonic crystal (4) has a beam splitter (11), in which at least three waveguides (8) formed in the photonic crystal are at one point converge. Microphone according to one of the preceding claims, wherein ambient air can flow freely around the photonic crystal (4). Microphone according to one of the preceding claims, wherein the emitter (1) has a laser diode and the detector (2) has a photodiode. Microphone according to the preceding claim, wherein the laser diode and the photodiode are applied to a main surface (12) of the photonic crystal (4). Microphone according to the preceding claim, wherein the laser diode is applied to the main surface (12) in such a way that the electromagnetic laser radiation (20) propagates in the photonic crystal (4) parallel to the main surface (12). Microphone according to one of the preceding claims, wherein the optical element (3) has a plurality of photonic crystals (4) which have different degrees of transmittance (T) and/or different degrees of reflectance (R) for the electromagnetic laser radiation (20). Microphone according to one of the preceding claims, wherein the photonic crystal (4) is arranged in a housing (13) and the housing (13) comprises a mechanical protective element (14) that is at least partially transparent to sound waves (21). Method for operating a microphone according to one of Claims 1 until 13 , wherein - the photonic crystal (4) transmits and/or reflects the laser radiation (20) generated by the emitter (1) during operation in such a way that a change in the ambient air pressure caused by sound waves (21) results in a corresponding change in the intensity (I) of the transmitted and/or reflected electromagnetic laser radiation (20), and - the detector (2) converts the change in intensity (I) in the transmitted and/or reflected electromagnetic laser radiation (20) into an electrical output signal. Procedure according to Claim 14 , wherein a wavelength (λ _L ) of the electromagnetic laser radiation generated by the emitter (1) is periodically tuned to change the transmittance (T) and / or the reflectance (R) of the photonic crystal (4) as a function of the wavelength (λ _R ) to determine. Procedure according to Claim 14 , whereby two sidebands are modulated onto the electromagnetic laser radiation (20), and the sidebands are set up to determine a resonance frequency (f _R ) in the photonic crystal (4).

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