RU2135963C1 - Microresonator fiber-optical converter of physical quantities - Google Patents

Abstract

FIELD: manufacture of fiber-optical converters of physical quantities, for instance, temperature, pressure, acceleration, with use of micromechanical resonators excited by light. SUBSTANCE: self-oscillating mode with oscillation frequency F coinciding with resonance frequency of i-th mode of oscillations of microresonator fi=F, where i=1, 2, ..., m , settles as result of action of measured physical quantity independent of type and parameters of microresonator. External action on microresonator changing its resonance frequency leads to change of frequency of self-oscillations in accordance with relation F=fi. EFFECT: design of converter based on laser. 3 cl, 11 dwg

Description

 The invention relates to fiber-optic converters of physical quantities (temperature, pressure, acceleration, etc.) using micromechanical resonators excited by light.

 Known works on the creation of a new class of fiber-optic sensors (VOD) of physical quantities based on the use of a micromechanical resonator (MR) and optical coherent radiation interacting with a microresonator MR. The literature reports on the development of various schemes for the optical excitation of MR oscillations and their practical implementation. In all cases, the modulation of the intensity of optical radiation occurs at the natural resonant frequency of the MR.

 When the MR of optical radiation is absorbed, its illuminated side experiences thermal expansion, as a result of which a bending moment arises in the MR, which changes in phase with the modulated optical radiation, which leads to mechanical vibrations at the natural resonant frequency of the MR.

 The external influence (temperature, pressure, acceleration, etc.) is converted into the internal mechanical stress of the MR, which leads to a change in its own resonant frequency, determined by the dimensions of the MR and its physical properties.

 Due to the small amplitude of the MR oscillations (≤ 0.1 μm) in the VOD of physical quantities, an interferometric method is used to extract information about the resonant frequency of the MR using a Fabry-Perot interferometer, the resonator of which is formed by the reflecting surface of the MR and a translucent mirror, or by the end of the fiber coupled to reflective surface mr.

 Direct communication with a digital measuring device without the need for analog-to-digital conversion, the large length of the optical transmission channel, and the high potential accuracy of resonant frequency measurements make this type of sensors promising.

 However, microresonator VODs of physical quantities, based on the photometric excitation of MR and optical detection of vibrations, have the following drawback: the position of the working point A of the Fabry-Perot interferometer is unstable and its displacement depends both on the drift of the main characteristics of the MR and on the instability of the radiation source and parameters Fabry-Perot interferometer.

 In other words, the instability of the characteristics of the excitation channel of the MR and the channel of the interferometric readout of information influences the efficiency of the operation of the VOD of physical quantities, which requires special measures to stabilize the position of operating point A.

 Closest to the proposed technical solution in terms of technical nature and the achieved result is the VOD of physical quantities published in Electronics Letters, 31 st August, 1989, vol 25, N 18, pp. 1235 ... 1236.

 The device contains a laser radiation source at a wavelength of λ = 840 nm and a power of P = 1 MW, a divider, a translucent mirror, a silicon microresonator in the form of a bridge, on the surface of which there is a silver mirror 40 nm thick, a photodetector, and a spectrum analyzer.

 The device operates as follows.

 Laser radiation through a divider, a translucent mirror is directed to the MR and brings it into an excited state at its own resonant frequency. A translucent mirror with a reflective surface MR form a Fabry-Perot interferometer. Information about the resonance frequency of MR is recorded interferometrically using a Fabry-Perot interferometer. The radiation reflected from the interferometer is sent to a photodetector connected to the information processing unit using a divider. Under the influence of external influences (temperature, pressure, acceleration, etc.), the resonance frequency of the MR changes, which is fixed using the Fabry-Perot interferometer in the measuring channel of the VOD.

 MR resonance frequency is stabilized by electronically adjusting the laser frequency in a small frequency range. To cover a given frequency range (≈ 15 GHz), the wavelength of the laser radiation source should be changed to 0.034 nm. This is achieved by varying the pump current of the laser diode to insignificant limits.

The output signal from the photodetector is divided into two parts: one part is sent to the spectrum analyzer, the other passes through a low-pass filter and provides a slight change in the pump current of the laser diode. At the same time, to ensure a self-oscillation regime for a long time, the pump diode pump voltage is subject to strict stability requirements ensuring a stable position of the operating point A on the optical characteristic Y _{A of the} Fabry-Perot resonator, as well as a careful selection of the bias voltage in accordance with the conditions where self-oscillation site.

As a result, the known solution is characterized by the following negative features:
- high requirements for the stability of the power of the radiation source (pump current of the laser diode) and careful control of the operating point of the Fabry-Perot interferometer due to the change in small limits of the optical power of the radiation incident on the MR;
- additional optical power loss due to the presence of the necessary discrete elements forming an additional feedback channel in the electronic circuit;
- stringent requirements for the stability of the characteristics of the Fabry-Perot resonator, as well as the characteristics of MR due to the limited possibility of their correction in the considered electronic circuit;
- limited possibilities for adjusting the operating point of the Fabry-Perot interferometer due to a change in the wavelength of the optical radiation of the laser diode when implementing a complex electronic feedback circuit.

 The problem solved by this invention is to develop a microresonator fiber optic converter of physical quantities based on a fiber optic laser with a tunable wavelength and a microresonator. In this case, one end of the single-mode fiber of the fiber-optic laser forms a Fabry-Perot interferometer with the reflecting surface of the microcavity, and the second, coupled to the photodetector, is a reflection mirror for the Mach-Zander interferometer located between the fiber-optic laser and the second end of the single-mode fiber. As a result of the occurrence of self-oscillations in the MR-fiber-optic system, the resonance frequency of the MR makes it unnecessary to introduce interferometric feedback to stabilize the position of the operating point of the Fabry-Perot interferometer in a certain spectral range of radiation Δλ of the fiber-optic laser.

 The solution of this problem is ensured by the fact that in a microresonator fiber-optic converter of physical quantities, including an optical radiation source, a translucent mirror, a microresonator, a fiber-optic laser is used as a source of optical radiation, one end of which is coupled to a microcavity, and the second is the output, when this, the Mach-Zander interferometer is located inside the fiber-optic laser cavity, and also because the microresonator is made in the form of a microbridge on the membrane not or in the form of a microbridge, as well as the fact that the microcavity is made in the form of a composite microconsole, as well as the fact that the microcavity is made in the form of a microbridge, one end of which is fixed to the base of the microcavity, and the other on the inertial mass associated with the base of the microcavity. The output signal of the sensor is modulated by the resonant frequency of the MR associated with the system of fiber-optic laser - Mach-Zander interferometer with positive feedback through the Fabry-Perot interferometer.

 The essence of the proposed technical solution lies in the development of physical quantities of VOD, in which a fiber-optic laser with a tunable wavelength using a Mach-Zander interferometer is used to excite self-oscillations and retrieve information, while the output optical signal of the sensor is modulated by the resonant frequency MR associated with the MR system fiber optic laser with positive feedback through a Fabry-Perot interferometer.

 A fiber-optic laser is a segment of a single-mode activated fiber of length L, the pumping of which can be carried out in various ways, for example, through a segment of a buffer non-activated fiber, ideally matched with the activated fiber.

Fiber optic laser has a number of unique properties that allow
- to provide effective optical matching of MR with a fiber-optic laser, thereby obtaining significant (within 0 ... 3 or more) optical feedback between them;
- to optimize the radiation wavelength λ due to the possibility of significant (up to ≈ 40 nm) tuning of the spectral range of laser generation;
- change the parameters of the laser over a wide range: output power, frequency of relaxation oscillations, resonator length of a fiber-optic laser, etc.

On the other hand, the latest MR manufacturing technology, based on the method of anisotropic etching and plasma chemistry of single-crystal materials such as Si, SiO ₂ , CaAs, makes it possible to realize MR structures with given acoustic characteristics and topology (for example, in the form of a micro-membrane, micro-bridge, micro-console, etc.) .d.), which makes it possible to implement a self-oscillating fiber-optic laser in the MR system, the resonant frequency of which depends on the influence of relevant external factors: temperature, pressure, acceleration, etc. From above zlozhennogo follows that the new properties of the MR system - Fiber-optic laser give reason to consider the system as a basis for the development of principles of construction of frequency converters of physical quantities of different designs.

Figure 1 presents a diagram of the VOD of physical quantities, which allows controlling the resonant frequencies of the main modes of acoustic self-oscillations, the magnitude of which depends on the topology and design of the MR, as well as the characteristics of the fiber-optic laser and Fabry-Perot and Mach-Zander interferometers. Here 1 is a spectrally tunable erbium fiber optical laser, 2 is a microcavity with a reflecting film n, 3 is a photodetector, 4 is a single-mode isotropic fiber L, 5 is an active single-mode fiber AB doped with ions Er _r ⁺³ , 6 is a translucent mirror M ₁ with a reflection coefficient R = 90%, 7 - a translucent mirror M ₂ , which is a fiber-air interface, with a reflection coefficient R = 3%, 9 - a spectrally tunable filter based on a Mach-Zander fiber-optic interferometer (MZI) 9 - low-Q interferome Fabry-Perot fr (IFP) with a base H ₀ formed by a mirror M ₂ and surface n of a microcavity 2, 10 — a semiconductor pump laser with λ = 0.98 μm, 11 — external action (pressure, temperature, acceleration, etc.).

 The device operates as follows.

As a result of the influence of the measured physical quantity, irrespective of the type and parameters of MR, a self-oscillating regime is established in the system with an oscillation frequency F that coincides with the resonant frequency of the i-th mode of MR oscillations: f _i = F, where i = 1, 2, ... m. To do this, the following conditions must be met:
- the frequency of relaxation oscillations of the fiber-optic laser, depending on the pump power, should be close to the relaxation frequency of the MR;
- the optical coupling coefficient ξ exceeds the critical value ξ = 10 ^-3 ;
- at a given wavelength of the fiber-optic laser λ, the self-oscillation zones in the parameter H _{0 are} discrete, periodic with the period of the Fabry-Perot interferometer. When the working point of the Fabry-Perot interferometer is shifted, a corresponding reconstruction of the fiber-optic laser generation line is carried out, so that with the tuning range Δλ exceeding the free dispersion region of the Fabry-Perot λ ² / 2H _o interferometer, the proposed excitation method admits a self-oscillating regime when the initial value of H ₀ .

 At the output of the photodetector, the signal-to-noise ratio, as a rule, is 30 ... 60 dB, in some cases - 90 dB.

External influence on MR, changing its resonant frequency, leads to a change in the frequency of self-oscillations in accordance with the relation F = f _i . This allows us to consider this system, consisting of MR, a fiber-optic laser and a photodetector, as a basic microresonator fiber-optic converter of physical quantities.

 Figure 2 presents the options for the design of the sensitive elements of the microresonator converters of physical quantities: pressure, temperature, acceleration (position I is a top view, position II is a side view, in section AA of the same MR).

In FIG. 2 (a), (b), (c) presents the designs of microresonators for measuring pressure in the form of a microbridge on the membrane (a), a microbridge (b), (c). Here 1 is a single-mode fiber waveguide, the core diameter of which is d _c , 2 is a reflective coating of MR (microbridge, membrane) of thickness h _n , 3 is a Fabry-Perot resonator with a base of H, 4 is an MR made of silicon.

где f₁(Р), f₁(P₀) - резонансные частоты микромостика при давлениях P и P₀ соответственно;
E, ν - модуль Юнга и коэффициент Пуассона кремния;
l, d, h, h_s, r - геометрические размеры ЧЭ (чувствительного элемента).The principle of operation of the pressure transducer, the MR of which is made in the form of a microbridge on the membrane and is shown in FIG. 2a is based on the fact that pressure P causes deformation of the membrane on which the microbridge is located. As a result of this deformation, tensile (or compressive) stresses arise in the bridge, leading to a change in the resonant frequency of the microbridge, which is described by the formula

where f ₁ (P), f ₁ (P ₀ ) are the resonant frequencies of the microbridge at pressures P and P _0, respectively;
E, ν — Young's modulus and Poisson's ratio of silicon;
l, d, h, h _s , r are the geometric dimensions of the SE (sensitive element).

Возможность вариации геометрических размеров микрорезонаторных структур позволяет в соответствии с выражением (I) изменять диапазон измеряемых давлений и коэффициент преобразования K_p в широких пределах.For l = 300 μm, d = 30 μm, h = 5 μm, h _s = 6 μm, 2r = 1000 μm, we have the conversion coefficient

The possibility of varying the geometric dimensions of the microresonator structures allows, in accordance with expression (I), to change the range of measured pressures and the conversion coefficient K _p over a wide range.

 In FIG. 2 (b), (c) presents microresonator structures in the form of a microbridge, differing in various configurations, topologies, and acoustic characteristics.

имеет вид

где α_s,n, γ_s,n, β_s,n - соответственно коэффициент линейного расширения, относительные изменения модуля Юнга Е и плотности ρ для кремния (Si) и материала покрытия (n);
α_эфф - эффективный коэффициент линейного расширения слоистой структуры;
M = E_n • h_n/E_s•h_s;
N = ρ_n•h_n/ρ_s•h_s
В соответствии с формулой (2) для составного консольного МР со слоем из вольфрама с отношением h_n/h_s = 0,1 имеем K_t ≈ -6 • 10^-3% K^-1.In FIG. Figure 2 (d) shows the microcavity in the form of a composite microconsole used as a temperature transducer T. This choice is explained by the fact that for the MR of this topology, the residual thermal stresses in the structure arising during technological processes do not lead to features in the dependence f (T) ( nonmonotonicity, ambiguity, nonlinearity of the transformation function). In addition, the advantages of a console MR compared with other types are that it is practically insensitive (except for temperature) to other types of external influences, for example, pressure, etc. For the considered MR conversion coefficient

has the form

where α _{s, n} , γ _{s, n} , β _{s, n} are the linear expansion coefficient, the relative changes in Young's modulus E and density ρ for silicon (Si) and coating material (n);
α _eff is the effective coefficient of linear expansion of the layered structure;
M = E _n • h _n / E _s • h _s ;
N = ρ _n • h _n / ρ _s • h _s
In accordance with formula (2) for a composite cantilever MP with a tungsten layer with a ratio of h _n / h _s = 0.1, we have K _t ≈ -6 • 10 ^-3 % K ^-1 .

The threshold sensitivity of the converter under consideration is ΔT _min ≈ 0.2 K. Due to the weak temperature dependence of the terms in formula (2), the coefficient K _{t is} practically independent of temperature, therefore, the conversion function (T) is linear.

In FIG. 2 (d) shows an embodiment of a microcavity linear acceleration converter a. The converter contains an NN 'microbridge with dimensions l • d • h _s , one end of which N is fixed on the base of the MP, and the other N' on the inertial mass with dimensions c • b • y, attached to the base of the MP using the holder D.

 The principle of operation of the acceleration converter is based on the fact that the presence of an inertial mass M during accelerated MR motion leads to the appearance of tensile or compressive stresses G in the microbridge, depending on the direction of acceleration, which changes its resonant frequency.

где

- деформация микромостика.

Where

- deformation of the microbridge.

Подстановка численных значений параметров рассматриваемого преобразователя l = 500 мкм, c = 1500 мкм, b = 2000 мкм, y = 200 мкм, y₀ = 100 мкм, d = 15 мкм, h_s = 5 мкм приводит к значению K_a≈ 5 • 10^-2% (м/сек²)^-1, пороговая чувствительность преобразователя a_min = 5 • 10^-4 м/сек².MR conversion coefficient is

Substitution of the numerical values of the parameters of the converter under consideration l = 500 μm, c = 1500 μm, b = 2000 μm, y = 200 μm, y ₀ = 100 μm, d = 15 μm, h _s = 5 μm leads to the value K _a ≈ 5 • 10 ^-2 % (m / s ² ) ^-1 , the threshold sensitivity of the transducer a _min = 5 • 10 ^-4 m / s ² .

The speed of such converters is limited by the frequency band 0 ... f / Q, which is associated with the finiteness of the time for the establishment of the oscillations of the MR. The acoustic quality factor of the MR Q can vary depending on the vacuum conditions of the MR structures over a wide range, for example, in a vacuum of 10 ^-3 mm Hg. may be Q ≥ 10 ⁴ .

 Thus, a new principle is proposed for constructing microresonator fiber-optic converters of physical quantities into a frequency change based on the effect of excitation of self-oscillations in a fiber-optic laser-microcavity system. The possibility of developing fiber optic pressure, temperature, and acceleration sensors with corresponding estimates of their characteristics under normal conditions on the basis of the system under consideration with different microresonator structures is shown.

Claims (4)

 1. Microresonator fiber-optical converter of physical quantities, including a source of optical radiation, a translucent mirror, a microcavity, characterized in that a fiber-optic laser is used as a source of optical radiation, one end of which is coupled to a microcavity, and the second is an output, while inside fiber laser resonator placed a Mach-Zander interferometer.  2. Microresonator fiber-optic converter of physical quantities according to claim 1, characterized in that the microresonator is made in the form of a microbridge on the membrane or in the form of a microbridge.  3. The microresonator fiber-optic converter of physical quantities according to claim 1, characterized in that the microresonator is made in the form of a composite micro-console.  4. Microresonator fiber-optic converter of physical quantities according to claim 1, characterized in that the microresonator is made in the form of a microbridge, one end of which is fixed to the base of the microresonator, and the other to the inertial mass associated with the base of the microresonator.

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