TECHNICAL FIELD

The present disclosure relates to an optical microphone. The optical microphone presented in this disclosure comprises a lightguide, in particular, a diffractive waveguide. The present disclosure also relates to a method for operating the optical microphone.

BACKGROUND

Special microphones are needed for audio applications, for which a small size, a high sound quality, reliability and affordability of the microphone are key requirements. Currently, the capacitance micro-electro-mechanical systems (MEMS) microphone is the most popular microphone for such audio applications. The signal-to-noise-ratio (SNR) achievable with this capacitance MEMS microphone is around 65 dB. However, the performance of this MEMS microphone is not good enough in some situations.

Therefore, an optical microphone is suggested, which may have a better performance than the MEMS microphone due to lower noise, i.e., a higher SNR. An optical microphone can transfer an acoustic signal to an optical signal. A photodetector of the optical microphone can further convert the optical signal to an electrical signal.

An exemplary optical microphone includes a light source, a diffraction grating arranged to receive light provided by the light source, and a reflective diaphragm positioned at a distance from the diffraction grating. A first portion of the light provided by the light source is reflected from the diffraction grating, while a second portion of the light passes through the diffraction grating to the microphone diaphragm, and is reflected by the diaphragm back through the diffraction grating. The optical microphone further includes a plurality of photodetectors for sensing an intensity of light in an interference pattern, which is caused by the first portion of the light reflected from the diffraction grating interfering with the second portion of the light reflected from the diaphragm and passed through the diffraction grating. The optical microphone also includes a controller, which is configured to modulate an emission of the light from the light source. The active part of the exemplary optical microphone includes particularly a vertical-cavity surface-emitting laser (VCSEL) as the light source, and requires three photodetectors arranged next to each other besides the VCSEL. The VCSEL also needs to be tilted to some extent, in order to allow the light reflected by the diaphragm to reach the photodetectors.

Accordingly, the structure of the exemplary optical microphone is rather complex, and the exemplary optical microphone is also not sensible enough.

SUMMARY

In view of the above, embodiments of the present invention aim to provide an optical microphone that is improved over the exemplary optical microphone. An objective is to make the optical microphone less complex and more sensible.

The objective is achieved by the embodiments of the invention as described in the enclosed independent claims. Advantageous implementations of the embodiments of the invention are further defined in the dependent claims.

A first aspect of this disclosure provides an optical microphone comprising: a light source; a lightguide; an incoupling element configured to receive light from the light source and at least partly incouple the received light into the lightguide based on a light diffraction mechanism, wherein the lightguide is configured to guide the incoupled light at least partly to an outcoupling element; the outcoupling element configured to outcouple the guided light at least partly from the lightguide; a photodetector configured to receive a first portion of the outcoupled light that is outcoupled by the outcoupling element into a first direction; and a microphone diaphragm configured to receive a second portion of the outcoupled light that is outcoupled by the outcoupling element into a second direction opposite to the first direction, and to reflect the second portion of the outcoupled light at least partly towards the photodetector; wherein the photodetector is configured to detect a light intensity based on an interference between the first portion of the outcoupled light and at least a portion of the reflected light reflected by the microphone diaphragm.

Due to the use of the lightguide in the optical microphone of the first aspect, and the different paths of the light from the light source to the photodetector, the structure of the optical microphone can be less complex than that of the exemplary optical microphone, while being also more sensible. In particular, only one photodetector is needed. Further, the light source does not have to be tilted. The lightguide allows the light source 101 and the photodetector to be spatially separated, which reduces unwanted stray light.

In an implementation form of the first aspect, the microphone diaphragm is configured to receive sound, and to vibrate in accordance with the received sound; and the photodetector is configured to detect a variation of the light intensity based on a pattern of changing interference between the first portion of the outcoupled light and the at least a portion of the reflected light, when the microphone diaphragm vibrates.

Thus, the sound received by the optical microphone by means of the microphone diaphragm can be converted into a corresponding electrical signal, which is output by the photodetector in dependence of the detected light intensity.

In an implementation form of the first aspect, the incoupling element and/or the outcoupling element comprise a diffraction grating.

That is, at least one of the incoupling element and the outcoupling element comprises the diffraction grating. Thus, the incoupling element and the outcoupling element, respectively, allow incoupling and outcoupling light into and from the lightguide based on a light diffraction mechanism. The diffraction gratings, in particular, allow diffracting the light into a particular direction with maximum efficiency, for instance, at an angle which is higher than the total internal reflection angle of the lightguide.

In an implementation form of the first aspect, each of the incoupling element and the outcoupling element comprises a diffraction grating, and the diffraction gratings have the same grating period.

The same grating period allows to avoid angular distortion.

In an implementation form of the first aspect, each of the incoupling element and the outcoupling element comprises a diffraction grating, and the diffraction grating of the outcoupling element is provided on the same surface of the lightguide or on an opposite surface of the lightguide than the diffraction grating of the incoupling element. In an implementation form of the first aspect, the incoupling element comprises a diffraction grating configured to diffract light received from the light source, such that an angle of the incoupled light diffracted into the lightguide satisfies a total internal reflection condition inside the lightguide.

Accordingly, the light can be trapped inside the light guide, and can be guided by the lightguide by total internal reflection, thus minimizing losses.

In an implementation form of the first aspect, a diffraction grating comprises one of a surface relief grating and a holographic optical element grating.

In an implementation form of the first aspect, the lightguide is configured to guide the incoupled light at least partly to the outcoupling element by total internal reflection.

In an implementation form of the first aspect, the lightguide is configured to pass at least a portion of the reflected light reflected by the microphone diaphragm to the photodetector.

This allows the interference of the different portions of light on the photodetector, and enables a compact optical microphone.

In an implementation form of the first aspect, the lightguide further comprises an antireflective coating provided on at least a portion of the surface of the lightguide that faces the microphone diaphragm, and the antireflective coating is arranged in the path of the at least a portion of the reflected light reflected by the microphone diaphragm to the photodetector.

The antireflective coating minimizes reflection, and thus allows as much light as possible to pass from the microphone diaphragm through the lightguide to the photodetector. This may increase the SNR of the optical microphone.

In an implementation form of the first aspect, the optical microphone further comprises a lens arranged between the light source and the incoupling element, and configured to collimate the light from the light source and to provide the collimated light to the incoupling element. By collimating the divergence angle of the light with the lens, the efficiency of the structure of the optical microphone is increased.

In an implementation form of the first aspect, the optical microphone further comprises a support structure arranged on or besides the lightguide, and configured to support the microphone diaphragm.

In an implementation form of the first aspect, the support structure is designed such that a first gap is formed between the microphone diaphragm and a region of the lightguide where the outcoupling element is located.

The gap increases the path difference between the path of the first portion of the outcoupled light and the path of the at least portion of the reflected light, which is reflected by the microphone diaphragm. In particular, the path difference may be twice the lightguide thickness and twice the size of the gap between the microphone diaphragm and the lightguide. This provides the optical microphone with a high sensitivity.

In an implementation form of the first aspect, the support structure is designed such that a second gap is formed between the support structure and at least a region of the lightguide where the incoupling element is located.

This further prevents that light exits the lightguide, due to reflection at the interface of the lightguide and the air in the second gap. Accordingly, losses are reduced.

In an implementation form of the first aspect, the optical microphone further comprises a substrate, wherein the light source and the photodetector are arranged on the substrate; and a housing enclosing the light source and the photodetector, wherein at least one of the lightguide and the support structure is carried by the housing.

The light source and the photodetector can be arranged spatially distanced from each other on the substrate. Both can be fabricated as integrated components on the substrate.

A second aspect of this disclosure provides a method for operating an optical microphone, wherein the optical microphone comprises a light source, a lightguide, an incoupling element, an outcoupling element, a microphone diaphragm, and a photodetector, and wherein the method comprises: incoupling, by the incoupling element, light received from the light source at least partly into the lightguide based on a light diffraction mechanism; guiding, by the lightguide, the incoupled light at least partly to the outcoupling element; outcoupling, by the outcoupling element, the guided light at least partly from the lightguide; receiving, by the photodetector, a first portion of the outcoupled light that is outcoupled by the outcoupling element into a first direction; receiving, by the microphone diaphragm, a second portion of the outcoupled light that is outcoupled by the outcoupling element into a second direction opposite to the first direction, wherein the microphone diaphragm reflects the second portion of the outcoupled light at least partly towards the photodetector; and detecting, by the photodetector, a light intensity based on an interference between the first portion of the outcoupled light and at least a portion of the reflected light reflected by the microphone diaphragm.

The method of the second aspect may have implementation forms, which are respectively for operating the optical microphone according to the implementation forms of the first aspect. The method of the second aspect provides the advantages of the optical microphone of the first aspect described above.

BRIEF DESCRIPTION OF DRAWINGS

The above described aspects and implementation forms will be explained in the following description of specific embodiments in relation to the enclosed drawings, in which

FIG. 1 shows an optical microphone according to an embodiment of the invention.

FIG. 2 shows an optical microphone according to an embodiment of the invention.

FIG. 3 shows an optical microphone according to an embodiment of the invention.

FIG. 4 shows examples of an incoupling element and an outcoupling element of an optical microphone according to an embodiment of the invention.

FIG. 5 shows a method for operating an optical microphone according to an embodiment of the invention. DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 shows an optical microphone 100 according to an embodiment of the invention. The optical microphone 100 comprises a light source 101, a lightguide 102, an incoupling element 103, an outcoupling element 104, a microphone diaphragm 106, and a photodetector 105.

The light source 101 is configured to provide light. The light source 101 may be a laser diode. For example, the light source 101 may be a VCSEL. The optical microphone 100 may optionally comprise a substrate 107, and the light source 101 may in this case be arranged on the substrate 107. Also the photodetector 105 may be arranged on the substrate 107, spatially distanced from the light source 101.

The incoupling element 103 is configured to receive light from the light source 101, and to at least partly incouple the received light into the lightguide 102. In particular, the light is coupled into the lightguide 102 based on a light diffraction mechanism. To this end, the incoupling element 103 may comprise a diffraction grating, as explained in detail below.

The lightguide 102 may be a waveguide, for instance, a diffractive waveguide. The diffractive waveguide may include a glass substrate and one or more gratings. The one or more gratings may comprise at least one of the incoupling element 103 and the outcoupling element 104. The grating comprising the incoupling element 103 may be responsible to diffract the input light, which may then propagate in the glass substrate due to total internal reflection. The lightguide 102 is configured to guide the incoupled light at least partly to the outcoupling element 104. Thereby, the incoupled light may be guided by total internal reflection in the lightguide 102.

The outcoupling element 104 is configured to outcouple the guided light at least partly from the lightguide 102. Thereby, the outcoupling element 104 is configured to outcouple a first portion of the outcoupled light into a first direction, and a second portion of the outcoupled light into a second direction, which is opposite to the first direction. In particular, the first portion is outcoupled towards the photodetector 105, and the second portion is outcoupled towards the microphone diaphragm 106.

Accordingly, the microphone diaphragm 106 is configured to receive the second portion of the outcoupled light, which is outcoupled by the outcoupling element 104 into the second direction. The microphone diaphragm 106 is further configured to reflect this second portion of the outcoupled light at least partly towards the photodetector 105 (i.e., into the first direction). The microphone diaphragm 106 may also be configured to receive a sound, and to vibrate in accordance with the received sound. Thus, the microphone diaphragm may be arranged such in the optical microphone 100 that it is able to freely vibrate.

The photodetector 105 is configured and arranged to receive the first portion of the outcoupled light, which is outcoupled by the outcoupling element 104 into the first direction. Further, the photodetector 105 is configured and arranged to receive at least a portion of the reflected light, which is reflected by the microphone diaphragm 106 into the first direction. This portion of the reflected light may pass through the lightguide 102 on its way to the photodetector 105. The photodetector 105 is further configured to detect a light intensity, wherein the light intensity is based on an interference between the first portion of the outcoupled light and the at least portion of the reflected light, which is reflected by the microphone diaphragm 106. In particular, the photodetector 105 may be configured to detect a variation of the light intensity, wherein the variation is based on a pattern of changing interference between the first portion of the outcoupled light and the at least a portion of the reflected light, when the microphone diaphragm 106 vibrates. Accordingly, a sound received by the microphone diaphragm 106, and causing the microphone diaphragm 106 to vibrate, may be converted via the interference phenomenon of the light of the light source 101 into an electrical signal, which may be output by the photodetector 105 and the optical microphone 100.

Notably, the optical microphone 100 may include only one light source 101, and only one photodetector 105. Further, when the light from the light source 101 propagates to the incoupling element 103, the light may be diffracted by an angle that satisfies the total internal reflection condition. To this end, the incoupling element 103 may comprise a diffraction grating (e.g., a surface relief grating or a holographic optical element grating). The light may then propagate further by total internal reflection in the lightguide 102 to the outcoupling element 104, which may also comprise a diffractive grating (e.g., a surface relief grating or a holographic optical element grating). The light may hit this diffractive grating of the outcoupling element 104 provided on the surface of the lightguide 102. The first portion of the light is then diffracted to the photodetector 105, and the second portion of the light is diffracted to the microphone diaphragm 106. The light provided to the microphone diaphragm 106 may be reflected back to the lightguide 102, and may be transmitted by the lightguide 102 towards the photodetector 105. Thus, the above-described interference can occur like in a Michelson interferometer.

Accordingly, the principle of the optical microphone 100 is based on the principle of the Michelson interferometer, wherein a path length difference, of different paths of light, is used to create an interference pattern on top of the photodetector 105. A small change in the path length difference may change the observed light intensity at the photodetector 105 substantially, so that the optical microphone 105 has a high sensitivity. To create the two different light paths, the following specific structures may be considered for the optical microphone 100.

FIG. 2 shows an optical microphone 100 according to an embodiment of the invention, which builds on the embodiment shown in FIG. 1. Same elements in FIG. 1 and FIG. 2 are labelled with the same reference signs and may be implemented likewise.

The optical microphone 100 shown in FIG. 2 comprises the light source 101 arranged on a substrate 107, on which also the photodetector 105 is arranged. In particular, a VCSEL may be used as the light source 101, any may emit light that is at least partly directed towards the incoupling element 103. As shown, the incoupling element 103 may be configured with diffractive structures, i.e., may comprise a diffractive grating (e.g., one or more surface relief gratings and/or one or more holographic gratings). With the aid of these diffractive structures, the light of the light source 101 can be coupled into the lightguide 102. Notably, grating parameters of the diffractive grating of the incoupling element 103, such as a height, a grating period, a slant angle, and/or fill factor, may be chosen in a such a way that the light arriving from the light source 101 is diffracted in a particular direction (1 ^st order) with maximum efficiency, and at an angle that is higher than the total internal reflection angle of the lightguide 102. Hence, the light can be trapped inside the lightguide 102. The lightguide 102 is then used to transport the incoupled light from one spatial position to the other, i.e., to guide the light from the incoupling element 103 to the outcoupling element 104. The main advantages of the lightguide 102 is that the light source 101 and the photodetector 105 may be spatially separated, in order to reduce unwanted stray light.

The 1 ^st order diffracted light, after successive total internal reflection, reaches the outcoupling element 104, which may also include diffractive structures as shown (again, e.g., one or more surface relief gratings and/or one or more holographic gratings). The periods of the diffractive grating of the input coupling element 103 and the diffractive grating of the outcoupling element 104 may be the same to avoid angular distortion.

The guided light interacts with the diffractive grating of the outcoupling element 104, the parameters of which may be optimized to outcouple the light. Due to the interaction, the total internally reflected light diffracts mainly into two perpendicular directions, i.e., the first direction and the second direction. The first direction is towards the photodetector 105, and the second direction is away from the photodetector 105 towards the microphone diaphragm 106.

Hence, the outcoupling element 104 produces two different light paths, which have different path lengths. The path taken by the light going into the second direction, away from the photodetector 105 towards the microphone diaphragm 106, may be modulated by the vibration of the diaphragm 106. As a result, the light detected in the photodetector plane is a superposition of two plane waves (assumption) with an extra path difference (in particular, twice the lightguide thickness and twice a first gap 203 between the microphone diaphragm 106 and the lightguide 102), wherein the vibration of the diaphragm 106 may modulate the path length difference. The difference in the path lengths results in an interference in the detection plane of the photodetector 105, as the two light paths are coming from the same light source 101 and as the extra path length may particularly be within the coherence length of the light source 101. The modulation of the path length difference results in the pattern of changing interference between the first portion of the light, which is outcoupled by the outcoupling element 104, and the at least a portion of the light, which is reflected from the microphone diaphragm 106.

In the optical microphone 100 shown in FIG. 2, the first path of light is accordingly from the light source 101 to the incoupling element 103, further to the outcoupling element 104, and further to the photodetector 105. The second path of light is from the light source 101 to the incoupling element 103, further to the outcoupling element 104, further to the microphone diaphragm 106, and further to the photodetector 105. The light from the first light path and the second light path, respectively, will interfere at the photodetector 105, and the light intensity measured by the photodetector 105 will vary with the microphone diaphragm 106 being vibrated by, for example, received sound.

As shown in FIG. 2, the optical microphone 100 may further include a housing 201, which encloses at least partly the light source 101 and the photodetector 105. The housing 201 may be arranged on the substrate 107, wherein walls of the housing 201 may be arranged around the light source 101 and the photodetector 105. The housing 201 may be opaque for the light of the light source 101. The housing 201 may be made of a dielectric material.

As further shown in FIG. 2, the optical microphone 100 may also include a support structure 204, which may carry the microphone diaphragm 106. The microphone diaphragm 106 may be flat and may be aligned with a top surface of the support structure 204, which is easy to fabricate. The support structure 204 may further be carried by the housing 201. Also the lightguide 102 may be carried by the housing. If the lightguide 102 is carried by the housing 201, the lightguide 102 may carry the support structure 204. For instance, as shown in FIG. 2, the support structure 204 may be attached to the lightguide 102 by means of a glue 205, or the like.

The support structure 204 may be designed to form the first gap 203 between the microphone diaphragm 106 the lightguide 102, particularly, a region of the lightguide 102 where the outcoupling element 104 is located. For instance, as illustrated in FIG. 2, the lightguide 102 may include two parts or walls that are bridged by the microphone diaphragm 106. In this way, the microphone diaphragm 106 can be stably supported by the support structure 104, while it is free to vibrate. The support structure 204 may be further designed to form a second gap 202 between the support structure 204 and the lightguide 102, particularly, a region of the lightguide 102 where the incoupling element 103 is located. As shown, the second gap 202 may also be a result of the support structure 204 being attached to the lightguide 102, for example, by means of the glue 205. That is, the glue 205 may separate and distance at least a region of the support structure 204 from the surface of the lightguide 102.

FIG. 3 shows an optical microphone 100 according to an embodiment of the invention, which builds on the embodiment shown in FIG. 2. Same elements in FIG. 2 and FIG. 3 are labelled with the same reference signs and may be implemented likewise.

As shown in FIG. 3, the optical microphone 100 may further include a lens 301 arranged between the light source 101 and the incoupling element 103, and configured to collimate the light from the light source 101, and to provide the collimated light to the incoupling element 103. The lens 301, which may be arranged in front of the light source 101 (or on the light source 101), results in the divergence angle of the light to be collimated, which will increase the efficiency of the structure of the optical microphone 100.

As further shown in FIG. 3, the optical microphone 100 may also include an antir effective coating 302 provided on at least a portion of the surface of the lightguide 102 that faces the microphone diaphragm 106. The antir effective coating 302 is, in particular, arranged in the path of the at least a portion of the reflected light, which is reflected by the microphone diaphragm 106 towards the photodetector 105. This allows the lightguide 102 to efficiently pass this reflected light from the microphone diaphragm 106 to the photodetector 105. That is, if the lightguide 102 surface close to the microphone diaphragm 106 is coated with the antir effective coating, no reflection or only minimal reflection at the lightguide 102 of the light occurs, when the light is reflected from the microphone diaphragm 106 and goes in the direction of the photodetector 105. As a consequence, an acoustic overload point (AOP) and the SNR of the optical microphone 100 can be increased.

For all the embodiments of the optical microphone 100 shown in the FIGs. 1-3, it is possible to use several different structures for the lightguide 102 as shown in FIG. 4, which comprise the incoupling element 103 and the outcoupling element 104. In particular, each of the incoupling element 103 and the outcoupling element 104 may comprise a diffraction grating. Thereby, the diffraction grating of the outcoupling element 104 may be provided on the same surface of the lightguide 102 than the diffraction grating of the incoupling element 103, or may be provided on an opposite surface of the lightguide 102 than the diffraction grating of the incoupling element 103. Each diffraction grating may further comprise one of a surface relief grating (SRG) and a holographic optical element (HOE) grating, so that at least the four structures in the table below are possible: Structure 1 is shown in FIG. 4(a), wherein the incoupling element 103 and the outcoupling element 104 both comprise a SRG and are arranged on the same surface (same side) of the lightguide 102. Structure 2 is shown in FIG. 4(b), wherein the incoupling element 103 and the outcoupling element 104 both comprise a SRG and are arranged on opposite surfaces (opposite sides) of the lightguide 102. Structure 3 is shown in FIG. 4(c), wherein the incoupling element 103 and the outcoupling element 104 both comprise a HOE and are arranged on the same surface (same side) of the lightguide 102. Structure 4 is shown in FIG. 4(d), wherein the incoupling element 103 and the outcoupling element 104 both comprise a HOE grating and are arranged on opposite surfaces (opposite sides) of the lightguide 102. The HOE gratings may be provided in a material layer 401 provided on the respective surface of the lightguide 102.

FIG. 5 shows a method 500 according to an embodiment of the invention. The method 500 is for operating an optical microphone 100 according to an embodiment of the invention, as for instance shown in the FIGs. 1-3. The method 500 may accordingly be performed by the optical microphone 100. In particular, the method 500 may be performed when the optical microphone 100 receives a sound. As described above, the optical microphone 100 includes the light source

101, the lightguide 102, the incoupling element 103, the outcoupling element 104, the microphone diaphragm 106, and the photodetector 105. The optical microphone 100 may further comprise a controller, processor, or control circuitry to control at least one of the light source 101 and the photodetector 105. That is, the controller, processor, or control circuitry may be used for operating the optical microphone 100 according to the method 500.

The method 500 comprises a step 501 of incoupling, by the incoupling element 103, light received from the light source 101 at least partly into the lightguide 102 based on a light diffraction mechanism. Further, the method 500 comprises a step 502 of guiding, by the lightguide 102, the incoupled light at least partly to the outcoupling element 104. Thereby, the light may be guided by total internal reflection. Then, the method 500 comprises a step 503 of outcoupling, by the outcoupling element 104, the guided light at least partly from the lightguide

102. At step 504 of the method 500, the photodetector 105 receives a first portion of the outcoupled light that is outcoupled by the outcoupling element 104 into a first direction. At step 505 of the method 500, the microphone diaphragm 106 receives a second portion of the outcoupled light that is outcoupled by the outcoupling element 104 into a second direction opposite to the first direction. The microphone diaphragm 106 also reflects the second portion of the outcoupled light at least partly towards the photodetector 105. The method 500 further comprises a step 506 of detecting, by the photodetector 105, a light intensity based on an interference between the first portion of the outcoupled light and at least a portion of the reflected light reflected by the microphone diaphragm 106. In case that the microphone diaphragm 106 of the optical microphone 100 receives a sound, and thus vibrates in accordance with the received sound, the step 506 of the method 500 comprises detecting, by the photodetector 105, a variation of the light intensity based on a pattern of changing interference between the first portion of the outcoupled light and the at least a portion of the reflected light.

The present disclosure has been described in conjunction with various embodiments as examples as well as implementations. However, other variations can be understood and effected by those persons skilled in the art and practicing the claimed subject matter, from the studies of the drawings, this disclosure and the independent claims. In the claims as well as in the description the word “comprising” does not exclude other elements or steps and the indefinite article “a” or “an” does not exclude a plurality. A single element or other unit may fulfill the functions of several entities or items recited in the claims. The mere fact that certain measures are recited in the mutual different dependent claims does not indicate that a combination of these measures cannot be used in an advantageous implementation.