This application claims priority to Chinese Patent Application Ser. No. CN202210840905.4 filed on 18 Jul. 2022.
The present invention relates to a method and system for expanding a dynamic measuring range of a Mach-Zehnder sensor based on the calculation of optical length, and belongs to the technical field of optical measurement.
At present, in most methods for completing optical measurement by spectra, physical quantities to be measured are represented using single peak or valley value positions of the spectra. Many optical devices are based on the interference principle, and there is a free spectral range (FSR). A dynamic range of such a measuring instrument is limited by the FSR or a spectrum of a light source. The peak value does not have a one-to-one relationship with the physical quantity to be measured once it exceeds the FSR of the instrument. This is a common problem faced by current sensors based on spectral measurement.
To solve this problem, large dynamic range is usually achieved by reducing the sensitivity now. There are also the following problems: With the improvement of the sensor research level, the sensitivity of the sensor is also increasingly higher. Obviously, the measuring range of the sensor becomes smaller as the sensitivity increases. Therefore, in addition to the limitation to the FSR, the spectral width of a broad-spectrum light source is also limited. The spectral width of a common C-band ASE broad-spectrum light source is 40 nm to 70 nm. The spectral width of a visible light and near infrared light source will be slightly larger, but only 800 nm to 1000 nm. For many current sensors with ultra-high sensitivity, such as an ultra-high-sensitivity temperature sensor, the sensitivity reaches 50 nm/K or more, but the measuring range can only reach 20° C. even if a 1000 nm light source is used. This problem is unrelated to the FSR, and is a difficulty caused by the limited spectral line width of the broad-spectrum light source used in a test system.
Using nonlinear regression can increase the measuring range of a Mach-Zehnder sensor, but it needs to solve partial differential equations, which is difficult and inefficient.
Therefore, there is currently no simple and easy-to-operate solution for ultra-large ranges.
For the shortcomings in the prior art, the present invention provides a method for expanding a measuring range of a Mach-Zehnder sensor based on the calculation of optical length. In the present invention, under the condition that the spectral width of a light source is greater than ½ of the FSR, the dynamic range of the sensor can be theoretically expanded beyond FSR. In this embodiment, massive computing like nonlinear regression is not required, so that requirements for a peripheral circuit are lowered, and the method is simple and easy to operate.
The present invention further provides a system for expanding the dynamic range of a Mach-Zehnder sensor based on the calculation of optical length.
FSR: It means Free Spectral Range. The FSR is a distance between two peak values (or valley values) of a transmission or reflection spectrum of an optical device.
The technical solutions of the present invention are as follows.
A method for expanding the dynamic range of a Mach-Zehnder sensor based on the calculation of optical length includes:
Preferably according to the present invention, in step (1), the spectral width of the light source selected by the asymmetric Mach-Zehnder sensor is greater than half of the FSR, so that the discrete data output by the asymmetric Mach-Zehnder sensor has at least one valley value and one peak value at the same time. It is convenient to use ratios of different peaks or valleys for subsequent calculation, so that the impact of process fluctuations on an arm difference ΔL of two arms of the Mach-Zehnder sensor is eliminated; and the reliability of calculation results is improved.
Preferably according to the present invention, in step (2) and step (3), the discrete data is processed using the peak and valley synthesis algorithm to restore the diffraction order m, specifically as follows:
Ideally, the effective refractive index of the waveguide may be calculated through a waveguide structure; ΔL is a design value; spectral data is measured; and the value of m can be calculated according to any one of formulas (I), (II), and (III). However, in actual situations, the effective refractive index of the waveguide and ΔL are uncertain due to the process fluctuations, in particular ΔL; when the waveguide bends, an optical field will be deviated from a center position of the waveguide (there is no accurate theory to calculate the deviation at present, and all existing methods are approximate processing methods), resulting in a lengthened path, thus affecting the value of ΔL; and as a result, the uncertain quantity of a product of n and ΔL is easily twice the wavelength or more.
Therefore, in this solution, the diffraction order m is calculated using the peak and valley synthesis algorithm, that is, using the ratios of different peak wavelengths or valley wavelengths, so that the impact of the process fluctuations on ΔL is eliminated; and the reliability of calculation results is improved. This method requires that the spectrum of the light source at least covers ½ of the FSR, that is, at least one peak value and valley value need to be acquired at the same time. In addition, it is required that there is only one polarization state.
When the acquired discrete data contains both a peak wavelength λ2 and a valley wavelength Δ1, and λ1<λ2:
In formulas (IV) and (V), m is the diffraction order; ΔL is the arm difference between the two arms of the Mach-Zehnder sensor; nλ1 is an effective refractive index of a waveguide corresponding to wavelength λ1; nλ2 is an effective refractive index of a waveguide corresponding to wavelength λ2;
Similarly, when the acquired discrete data contains both a peak wavelength λ1 and a valley wavelength λ2, and λ1<λ2:
Formulas (VI) and (VII) are the core principles for calculating the diffraction order on the basis of peaks and valleys provided by the present invention, where λ1 and λ2 are acquired and measured by the optical measuring device in step (2). There are mature empirical formulas for relationships between the refractive indexes and wavelengths of commonly used optical waveguide materials (Si, SiO2, LiNbO3, and the like); variations of the effective refractive indexes of the waveguides can be approximately considered to be equal to changes of the refractive indexes of the materials. Since the refractive indexes of two wavelengths are on the denominator and the numerator respectively, and the process fluctuations have the same impact on the refractive indexes at different wavelengths, formula (VI) can also greatly offset the fluctuations of the refractive indexes by the process.
Adjacent peak wavelengths and valley wavelengths in the discrete data, as well as nλ1 and nλ2 are substituted into formula (VI) or formula (VII), thus obtaining the diffraction order m.
Preferably according to the present invention, in step (2) and step (3), the specific process of calculating the optical length value of the known parameters or the unknown parameters is as follows: first performing translation and scaling transformation on the discrete data such that the amplitude of the discrete data is ±1; and taking an arc-cosine function to obtain a phase value; and superimposing am to obtain a total optical length value.
The method provided by the present invention is extremely high in fault tolerance, which is proved as follows:
Error E is recorded as:
A lithium niobate asymmetric Mach-Zehnder structure is taken as an example. The effective refractive index of the waveguide of the structure is about 2.17; the arm difference is 21.73 μm; and the light source uses a C-band ASE broad-spectrum light source:
n
eff
=n
1.55
+d
n·(λ−1.55) (IX)
In formula (IX), n1.55 is the effective refractive index of the waveguide at 1550 nm, and dn is a wavelength-varying coefficient of the effective refractive index of the waveguide.
As a comparative example, if only formula (I) is used to calculate m without using peaks and valleys, and if exact values are: n1.55=2.22, wavelength=1550 nm and arm difference=22.34 microns, m=32. It is assumed that the refractive index has an error of 0.01; the wavelength has no error; and the arm difference has an error of 0.3 microns (which is a standard tolerance of contact lithography). In this way, calculated m=32.5758; and if m exceeds 32.5, it will be determined that m=33, which is a calculation failure. A larger m indicates larger ΔL. If formula (I) is used for calculation, the requirement for the tolerance of ΔL is higher. At this time, the advantage of this method is more obvious. In addition, m of a general integrated optical waveguide device is about 30-200.
However, if the method provided by the present invention is adopted, E is 0.0001467 under the same conditions. Emax of 0.00023 is 64% of a misjudgment threshold, and there is still a margin. The main reason is that this method eliminates the impact of ΔL, and puts the remaining similar parameters on the numerator and denominator at the same time, which can offset the fluctuations of various numerical values to an extremely large extent, so the fault tolerance is quite good.
To sum up, the method has high operability and can accurately restore the current diffraction order of the Mach-Zehnder sensor. With the diffraction order, the physical quantity to be measured can be calculated. This method is not limited by the FSR, and can achieve measurement as long as the sensor is not damaged under measuring conditions such as high temperature or high pressure.
An implementation system for expanding a measuring range of a Mach-Zehnder sensor based on the calculation of optical length includes a light source, an asymmetric Mach-Zehnder sensor, a discrete data acquisition module, an optical length calculation module, and a physical-quantity-to-be-measured calculation module which are connected in sequence.
The discrete data acquisition module includes an optical measuring device, used for measuring acquired discrete data; and the optical measuring device includes a spectrometer or an optical power meter.
The optical length calculation module is used for processing the discrete data using a peak and valley synthesis algorithm, restoring a diffraction order m, and calculating an optical length value of parameters.
The physical-quantity-to-be-measured calculation module is used for restoring unknown parameters according to a calibrated relationship curve between the optical length value and the parameter, thus calculating a physical quantity to be measured.
The present invention has the beneficial effects below.
The present invention is further described below in combination with the embodiments and the drawings of the specification, but is not limited to this.
A method for expanding a measuring range of a Mach-Zehnder temperature sensor based on the calculation of optical length includes the following steps:
As shown in
When the temperature is 26.5° C., the peak wavelength of the discrete data is λ1=1528.3 nm, and the valley wavelength is λ2=1552.4 nm; when the temperature is 28.6° C., the peak wavelength of the discrete data is λ1=1545.9 nm, and the valley wavelength is λ2=1570.6 nm; and nλ1 and nλ2 are calculated according to formula (X). λ2=1552.4 nm and λ2=1570.6 nm, as well as nλ1 and nλ2, are brought into
After optimization, when n1.55=2.17, dn=−0.025 RIU/μm, 13° C., and dt=−0.128 RIU/° C., the errors at the various diffraction orders m are as shown in
The optical length value of the known parameters is calculated according to the diffraction order m, and then the correction relationship curve between the optical length value and the measured parameter is obtained, thus completing the calibration of the asymmetric Mach-Zehnder sensor.
(3) Measurement is performed. Temperature t is unknown at this time. For an unknown parameter, i.e. temperature t, discrete data is acquired first using the asymmetric Mach-Zehnder sensor; the discrete data is processed using the peak and valley synthesis algorithm according to relevant parameters of the effective refractive index obtained in step (2), so as to restore a diffraction order m; and an optical length value of the unknown parameter is calculated to restore the unknown parameter, i.e. temperature t.
In this embodiment, in step (2), a measuring curve can be obtained by calibrating 26.5° C. and 28.6° C.
In step (3), m is first determined through the spectrum of the unknown temperature; the optical length is then calculated; and the correction measuring curve obtained in step (2) is queried, thus obtaining the temperature value.
In this embodiment, the sensitivity of the sensor is about 16 nm/° C., and the spectral width of the light source is 50 nm. If the temperature is determined according to the traditional method and a peak position, the measuring range is only 50/16=3.125° C. However, by using the method provided in the present invention, the measuring range can exceed this limit. In this embodiment, only the measuring range of 4° C. is shown. It is unnecessary to describe a larger measuring range.
A method for expanding a measuring range of a Mach-Zehnder pressure sensor based on the calculation of optical length is different from Embodiment 1 in that:
A method for expanding a measuring range of a Mach-Zehnder refractive index sensor based on the calculation of optical length is different from Embodiment 1 in that:
A system for expanding a measuring range of a Mach-Zehnder sensor based on the calculation of optical length is used for implementing the method for expanding the measuring range of the Mach-Zehnder sensor based on the the calculation of optical length provided in any one of Embodiments 1-3, and includes a light source, an asymmetric Mach-Zehnder sensor, a discrete data acquisition module, an optical length acquisition module, and a physical-quantity-to-be-measured acquisition module which are connected in sequence.
The discrete data acquisition module includes an optical measuring device, used for measuring acquired discrete data; and the optical measuring device includes a spectrometer or an optical power meter.
The optical length calculation module is used for processing the discrete data using a peak and valley synthesis algorithm, restoring a diffraction order m, and calculating an optical length value of parameters.
The physical-quantity-to-be-measured calculation module is used for restoring unknown parameters according to a calibrated relationship curve between the optical length value and the parameter, thus calculating a physical quantity to be measured.
Number | Date | Country | Kind |
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202210840905.4 | Jul 2022 | CN | national |