RU2771592C1 - Miniature optical microphone with resonator on whispering gallery modes - Google Patents

Abstract

FIELD: acoustics.SUBSTANCE: invention relates to acoustics, in particular to acoustic signal receivers. The optical microphone contains a movable membrane with a mirror coating on the outside and an optical unit for measuring the deflection of the membrane, including a source of monochromatic coherent optical radiation, a radiation detector. The optical unit for measuring the deflection of the membrane consists of a resonator of the whispering gallery modes, made in the form of a spherical dielectric particle with a truncated vertex, measuring at least the wavelength of the radiation used and facing the truncated vertex to the mirror surface of a movable membrane installed with a gap to it and irradiated by electromagnetic radiation from the reverse side of the truncated surface of a spherical particle with a wavelength of radiation exciting whispering gallery modes in the resonator. Quantum emitters, such as a quantum dot, are embedded in the material of the whispering gallery mode resonator. In addition, nanoparticles with a fluorescent substance are embedded in the material of the whispering gallery mode resonator.EFFECT: creation of a miniature optical microphone with high sensitivity.3 cl, 1 dwg

Description

The invention relates to acoustics and can be used in the design of miniature receivers of acoustic signals (microphones, hydrophones, vibraphones, phonendoscopes, etc.), in pressure sensors, optical-acoustic receivers.

An optoelectronic pressure sensor is known (RF patent 2006016), containing a mirror membrane and an optical unit for measuring the amount of membrane deflection, containing a radiation source and a first photodetector, interconnected using an optical channel consisting of a condenser, a raster, a lens, two mirrors and a second photodetector, in this case, the photodetectors are turned on towards each other.

The operation of the optical channel is based on the defocusing of the light flux during deflection of the mirror membrane and a change in part of the light flux entering the photodetector.

The disadvantages of this device are large dimensions, low sensitivity of the device.

Known optoelectronic pressure sensor (Udalov N.P. and others - Devices and control systems, 1981, No. 5, pp. 21-22), containing a source and receiver of radiation connected by optical fibers with an optical channel made in the form of a triangular prism, a membrane , installed with a gap relative to the prism, a signal extraction unit connected to the receiver.

The disadvantages of this device are large dimensions and low sensitivity.

A miniature optical-acoustic receiver with a capacitive displacement detector is known [Chvrier J.-B., Baert K., Slater T., An infrared pneumatic detector made by micromachining technology // J. Micromech. Microeng. 5, pp. 193-195, (1995); Marco Schossig, Volkmar Norkus, Gerald Gerlach. A pneumatic infrared detector with capacitive read-out circuit // SENSOR + TEST Conferences 2 0 1 1, IRS² Proceedings, r. 109-114], consisting of a chamber with an inlet window, a movable membrane fixed around the perimeter on the body at the outlet end of the chamber, and a capacitive displacement detector.

The disadvantage of this device is its low sensitivity.

A device of an optical microphone as part of an optical-acoustic receiver (OAP) is known (RF patent 2169911). The device contains a chamber having an entrance window, an absorbing film and a mirror membrane and an optical unit for measuring the membrane deflection value, consisting of an optical microphone. An optical microphone includes a radiation source installed in series, a main condenser, a transparent raster and a lens, the transparent raster being installed in the focal plane of the lens, as well as a beam-splitting element installed along the optical beams in front of the main condenser and photodetector.

The disadvantages of this device are large dimensions, complexity of design and use, low sensitivity.

The subsequent development of the optical-acoustic receiver was aimed at increasing the sensitivity of the receiver, expanding the recorded wavelength range, and generally reducing the noise level of the receiver. One of the solutions is the OAP, described in the publication "Modern optical-acoustic radiation receivers", Optical Journal, No. 5, 1994, p. 5, 6. In said PDA, which also contains a chamber in which electromagnetic radiation is absorbed and pressure fluctuations occur, absorption occurs non-selectively in a wide region from 1 to 3000 μm. Pressure pulsations cause vibrations of the mirror membrane, which is one of the walls of the receiving chamber. Membrane oscillations are recorded in the optical unit for measuring the membrane deflection using an optical microphone, which consists of an auxiliary radiation source, a condenser, a transparent raster, a mirror, and a photodetector. An image of the auxiliary radiation source is projected through the raster onto the membrane by a condenser, which is then directed by a mirror through the diaphragm to the photodetector through the second half of the raster and the condenser. This design improves the sensitivity of the receiver.

The disadvantages of this device are large dimensions, complexity and lack of sensitivity.

Known optical microphone according to A.S. USSR N 627599 containing a housing, a membrane fixed along the perimeter on the housing, and a monochromatic light source, a focusing lens, a beam-splitting cube, a mirror, a lens and a photodetector installed inside the housing. The microphone works as follows. The acoustic wave excites mechanical oscillations of the membrane, which are converted into an electrical signal by means of an optoelectronic device. A beam of light emitted by a monochromatic light source is focused by a lens and split into two beams by a beam-splitting cube. One of the split beams is reflected from a fixed mirror, the other from the membrane. Reflected beams of light create an interference pattern that changes depending on the position of the membrane, which is expanded with a lens and projected onto the input window of the photodetector.

The disadvantages of the device are large dimensions, complexity and low sensitivity.

Known optical microphone according to the patent of the Russian Federation 1553936, consisting of a plane-parallel optically transparent plate with reflective surfaces, cantilevered in the housing with a touch of the free end of the membrane. The membrane, bending under the action of an acoustic wave, changes the inclination of the plate in a parallel light flow and changes the intensity of the light transmitted through the plates.

The disadvantages of the device are its large size, complexity, and low sensitivity.

An optical microphone is known, patent RF 2047944, containing a housing, a membrane fixed around the perimeter of the housing, an optical unit for measuring the magnitude of the deflection of the membrane, consisting of a source of monochromatic radiation, a focusing lens, a photodetector, and longitudinal grooves are made on the inner surface of the membrane in a spiral, in which fiber optic light guide. The grooves are covered with foil. The source of monochromatic radiation and the focusing lens are installed opposite the first end of the fiber optic light guide, and the photodetector is located opposite the second end of the fiber optic light guide.

The disadvantages of this device are the complexity of the design and low sensitivity.

Known optical microphone according to RF patent 2273115, containing a housing, a membrane fixed around the perimeter of the housing, a radiation source, a fiber optic light guide, a focusing lens and a photodetector. The optical microphone is equipped with a guide lens, a polarizer installed behind it, an analyzer installed in front of the focusing lens, a photomultiplier and a recorder, while the analyzer is connected through a rod to the inner surface of the membrane with the possibility of rotation using a spring-lever mechanism, and the radiation source is optically connected through a fiber-optic optical fiber guide

lens, polarizer, analyzer, focusing lens, photodetector, photomultiplier with recorder.

The disadvantages of this device are the complexity of the design, large dimensions and low sensitivity.

In addition, it is known that a decrease in the diameter of the flexible membrane leads to a significant decrease in the sensitivity of the receiving device [I. S. Gibin, P. E. Kotlyar. Matrix optical-acoustic receiver of THz radiation by nanooptoelectromechanical elements based on perforated SLG graphene // Applied Physics, 2020, no. 3, p. 76-82].

As a prototype, an optical microphone device according to the Eurasian patent EA 011914 B1 20090630 was chosen, containing a movable membrane with a mirror coating on the outside and an optical unit for measuring the amount of membrane deflection, including an auxiliary monochromatic coherent optical radiation source, a radiation detector and an optical system configured to provide passage of the auxiliary radiation from the specified source to the mirror membrane, return passage of the reflected auxiliary radiation to the radiation detector.

The disadvantage of this device are large dimensions and insufficient sensitivity.

The objective of the present invention is to eliminate these disadvantages, namely the creation of a miniature optical microphone with high sensitivity.

This task is achieved by the fact that an optical microphone containing a movable membrane with a mirror coating on the outside and an optical unit for measuring the amount of membrane deflection, including a source of monochromatic coherent optical radiation, a radiation detector, is new in that the optical unit for measuring the amount of membrane deflection consists of a resonator whispering gallery mode, made in the form of a spherical dielectric particle with a truncated top or in the form of a truncated circular cylinder, with dimensions not less than the wavelength of the radiation used and the truncated top facing the mirror surface of the movable membrane, installed with a gap to it and irradiated with electromagnetic radiation from the reverse side of the truncated surface of a spherical particle with the emission wavelength of the whispering gallery mode exciting in the resonator. In addition, quantum emitters, such as a quantum dot, are embedded in the material of the whispering gallery mode resonator. In addition, nanoparticles with a fluorescent substance are embedded in the material of the whispering gallery mode resonator.

The authors are not aware of technical solutions containing similar distinguishing features and their use for the stated purpose - increasing the sensitivity of the optical microphone and reducing the size of the device.

For the first time, the possibility of creating electromagnetic resonators using whispering gallery modes (WGMs) arising from total internal reflection from the surface of an axially symmetric body was pointed out in 1939 by Robert Richtmyer [R.D. Richtmyer. Dielectric resonators // J. Of Applied Physics. 10:391-398, 1939].

Known optical sensors that use high-quality natural resonances ("whispering gallery" modes) excited in symmetrical dielectric structures [Zheng Y, Wu Z F, Shum P P, Xu Z L, Keiser G et al. Sensing and lasing applications of whispering gallery mode microresonators // Opto-Electronic Advances 1, 180015 (2018)], for example, ring sensors [P. Zhang, C. Zhang, Z. Yan, Simultaneous measurement of the refractive index and the pressure by mode splitting in concentric triple microring resonators with a single opening // Appl. Opt. 60, 2958-2966 (2021).], disc [T. Ma, J. Yuan, L. Sun, Z. Kang, B. Yan, X. Sang, K. Wang, Q. Wu, H. Liu, and J. Gao, Simultaneous measurement of the refractive index and temperature based on microdisk resonator with two whispering gallery modes // IEEE Photon. J. 9, 6800913 (2017)], multi-ring [K. De, I. Bartolozzi, E. Schacht, P. Bienstman, and R. Baets, Silicon-on-Insulator microring resonator for sensitive and label-free biosensing // Opt. Exp. 15, 7610–7615 (2007).], “bottled” [F. Gu, F. Xie, X. Lin, S. Linghu, W. Fang, H. Zeng, L. Tong , S. Zhuang, Single whispering-gallery mode lasing in polymer bottle microresonators via spatial pump engineering // Light Sci. Appl. 6, e17061 (2017)] and spherical WGM microcavities [A.R. Ali, Micro-optical vibrometer/accelerometer using dielectric microspheres // Appl. Opt. 58, 4211-4219 (2019)]. WGM is an optical resonance localized close to the outer edge of the resonator with a high quality factor, i.e. narrow line width.

The principle of operation of WGM sensors, as a rule, consists in measuring the transmission of a special optical line (optical fiber or strip) used to excite and receive a signal from the WGM resonator, while scanning the excitation frequency with a tunable laser. Within the WGM circuit, the transmission of the fiber drops sharply, which is an indicator of tuning to resonance. Any mechanical or thermal effect on the resonator changes its characteristics and the operating frequency of the WGM, which is detected using a spectrometer. The magnitude of the spectral shift of the WGM can be used to judge the level of mechanical load on the resonator.

The disadvantage of optical sensors based on the WGM resonator is the need to provide mechanical contact between the sensor (membrane) and the WGM resonator that is sensitive to external load and limits their scope, as well as the durability of the structure.

The radiative quality factor of the WGM resonator grows exponentially with increasing ratio of the resonator radius to the wavelength and therefore does not prevent the achievement of arbitrarily high values of the quality factor [A.N. Oraevsky. Whispering gallery waves // Quantum Electronics 32 (5), 2002, 377-400].

Whispering gallery resonators are made in the form of a sphere, cylinder, disk or polygon and in the optical wavelength range can have characteristic dimensions of about a few microns [A. I. Sidorov, "Fundamentals of Photonics: Physical Principles and Methods for Converting Optical Signals in Photonics Devices". Tutorial. St. Petersburg: FGBOU VPO "SPb NRU ITMO", 2014 - 148 pages].

WGM excitation is possible by near-surface waves of grazing incidence on the surface of a ball - a spherical WGM resonator [A.N. Oraevsky. Whispering gallery waves // Quantum Electronics 32 (5), 2002, 377-400]. Such waves can be generated, for example, using a total internal reflection prism, a circular or rectangular dielectric waveguide, or a beveled fiber waveguide. Surface fields in a total internal reflection prism and waveguides decrease exponentially with distance from the surface of the prism (waveguide). By changing the distance from the prism (waveguide) to the surface of the ball, one can control the degree of excitation by whispering gallery modes.

At the same time, WGM excitation in bulk microstructures can be carried out not only by damped electromagnetic fields using a conical fiber or prism, but also when the microparticle is illuminated with direct radiation [L. Cai, J. Pan, S. Hu. Overview of the coupling methods used in whispering gallery mode resonator systems for sensing // Optics and Lasers in Engineering 127, 105968 (2020)]. Despite the lower excitation efficiency, this method has undeniable advantages in that it does not require precise positioning of the element that transmits optical excitation and the WGM resonator. It is important that the efficiency of WGM excitation in a microparticle by direct radiation can be significantly increased using various methods, for example, side illumination with a structured focused beam [A.A. Zemlyanov, Yu.E. Geints. Efficiency of excitation of the spatial resonant configurations of the internal optical field of spherical microparticles by focused laser beams // Atmospheric and Oceanic optics 13, 412-422 (2000)], etc.

WGM resonators are known in the form of a truncated sphere or cylinder (the so-called Janus particles). In such a resonator, sharp resonances are provided depending on the depth of the remote segment of the sphere or cylinder. These resonances are associated with excited whispering gallery waves caused by truncation. This is a new field localization mechanism. Optimizing this effect for cylinders makes it possible to achieve super-resolution in line thickness and has the maximum line emission intensity [Igor V. Minin, Oleg V, Minin, Yinghui Cao, Bing Yan, Zengbo Wang, Boris Luk'yanchuk. Photonc lenses with whispering gallery waves at Janus particles // preprint, https://arxiv.org/abs/2012.09489].

The invention is illustrated by drawings.

On FIG. 1 schematically shows the device.

Designations: 1 - source of monochromatic coherent radiation, 2 - radiation irradiating the WGM resonator, 3 - WGM resonator in the form of a spherical dielectric particle with a truncated top, 4 - transmitted and reflected radiation from a mirror membrane, 5 - flexible membrane with a mirror coating on the outside, 6 – radiation detector.

The operation of the device is as follows. As a source of monochromatic coherent optical radiation 1 can be, for example, a laser. The electromagnetic radiation 2 of the source 1 irradiates the WGM resonator 3 and excites the whispering gallery modes in it, the radiation transmitted through the resonator 3 is reflected from the flexible membrane with a mirror coating on the outer side 5. one of the eigenmodes of the resonator. In this case, the spatial structure of the internal field is rearranged, leading to a sharp increase in the intensity and localization of the field near the surface of the WGM 3 dielectric resonator with a truncated top, with the formation of ring periodic structures in the form of standing waves. The WGM resonator 3 has a truncated apex facing the mirror surface of the movable membrane 5.

As a result of the interference of the resonance modes of the "whispering gallery" (WGM), excited in a dielectric spherical microparticle with a truncated top (or a cylinder with a truncated top) 3 by direct and reflected from the flexible mirror 5 by optical radiation. In this case, WGM interference can be constructive or destructive, depending on the phase difference of the exciting WGM waves, which will lead to a change in the amplitude of the resulting resonant field in the particle. In turn, the phase difference between the direct and reflected waves depends on the amount of deflection of the mirror membrane 5, which occurs under the action of the excess pressure of the analyte located on the reverse side of the mirror of the device.

When phases are synchronized between paired WGMs, the resulting eigenmode is excited with a multiple of greater intensity than in the classical situation of unidirectional illumination of a particle by a plane wave or a wide beam.

Photodetector 6 (photodiode, PMT) converts the optical signal of the intensity of the whispering gallery mode into an electrical signal proportional to it in amplitude.

When the whispering gallery modes are excited in resonator 3, near-surface fields arise on its surface and, through damped oscillations, interact with the membrane from the mirror surface 5, reflecting from it and changing the intensity of the modes in a spherical dielectric resonator with a truncated top 4. The WGM resonator 3 is installed with a gap from the mirror the surface of the movable membrane 5, which determines the maximum possible displacement of the flexible membrane. By measuring the intensity of the mode with a radiation detector 6, one can judge the amplitude of the membrane oscillations and, consequently, the amplitude of the acoustic wave incident on the membrane or the pressure that causes the membrane to shift.

Thus, the device of a miniature optical microphone based on the effect of WGM excitation in a dielectric microsphere with a truncated top and with dimensions not less than the radiation wavelength (mesowave particle). A feature of the proposed device is the method of excitation of high-quality natural resonances (direct illumination) and the non-contact type of placement of a pressure-sensitive element - a flexible mirror membrane. This simplifies the device in comparison with WGM excitation by near-surface grazing-incidence waves. In this case, due to the presence of optical reflection at the membrane, the WGM sphere with a truncated apex is double excited, first by direct and then by reflected radiation. The optical intensity of the resulting WGM field is determined by the result of the interference of the direct and reflected WGM and depends on the position of the flexible loaded membrane.

A microsphere with a truncated top or a circular truncated cylinder can be made of a dielectric material that is transparent to illuminating radiation.

The mirror coating of the flexible membrane on the outside can be made of a metal, for example, gold or a dielectric with a high refractive index in the required radiation range.

If inside the resonator, in the volume occupied by the resonant mode, we place quantum emitters, such as a quantum dot [T. Huber, M. Davanco, M. Müller, Y. Shuai, O. Gazzano, and G.S. Solomon, Filter-free single-photon quantum dot resonance fluorescence in an integrated cavity-waveguide device // Optica 7, 380-385 (2020)], spaser [M.A. Noginov, G. Zhu, A. Belgrave, R. Bakker, V.M. Shalaev, E.E. Narimanov, S. Stout, E. Herz, T. Suteewong, U. Wiesner. Demonstration of a spaser-based nanolaser // Nature 460, 1110–1112 (2009)], or nanoparticles with a fluorescent substance [D. Lu, M. Pedroni, L. Labrador-Páez, M.I. Marqués, D. Jaque, and P. Haro-González, Nanojet trapping of a single sub-10 nm upconverting nanoparticle in the full liquid water temperature range // Small 17, 2006764 (2021)], then the intensity of their glow, received by the photodetector, will also vary depending on the deflection of the sensitive mirror membrane.

The high quality factor of the whispering gallery modes makes it possible to create, on the basis of resonators, a WGM connected to a source of monochomatic coherent radiation for measuring the deflection (displacement) of a high-sensitivity membrane.

Claims (3)

1. An optical microphone containing a movable membrane with a mirror coating on the outside and an optical unit for measuring the amount of membrane deflection, including a source of monochromatic coherent optical radiation, a radiation detector, characterized in that the optical unit for measuring the amount of membrane deflection consists of a whispering gallery mode resonator made in the form of a spherical dielectric particle with a truncated apex or in the form of a truncated circular cylinder, with dimensions not less than the wavelength of the radiation used and the truncated apex facing the mirror surface of the movable membrane, installed with a gap to it and irradiated by electromagnetic radiation from the reverse side of the truncated surface of the spherical particle with length radiation waves of the whispering gallery mode exciting in the resonator. 2. An optical microphone according to claim 1, characterized in that quantum emitters of the quantum dot type are embedded in the material of the whispering gallery mode resonator. 3. An optical microphone according to claim 1, characterized in that nanoparticles with a fluorescent substance are embedded in the material of the whispering gallery mode resonator.

Images