A laser frequency-sweeping high-resolution wavelength demodulation device

Abstract

The present invention belongs to the field of optical sensing technology, and specifically relates to a laser frequency sweeping high-resolution wavelength demodulation device based on laser frequency sweeping technology. The main structure includes a laser, a temperature controller, a beam splitter, an optical circulator, a gas absorption cell, an optical resonant cavity, a photodetector, an industrial control computer, and a current controller. The industrial control computer controls a voltage output module to periodically output an oblique triangular wave signal, which is loaded onto the current controller to change the injection current of the laser, thereby changing the output wavelength of the laser. The gas absorption cell is introduced as a reference channel, and the time delay between the sensing peak and the reference peak is calculated through a cross-correlation algorithm, effectively compensating for laser frequency drift caused by environmental fluctuations and improving demodulation stability. The device has a simple structure, high light source utilization, high resolution, good stability, and a high signal-to-noise ratio. The introduction of the gas absorption cell optical path as the reference optical path reduces sensitivity to environmental fluctuations and improves long-term stability.

Description

Technical field:

[0001] The present invention belongs to the technical field of optical sensing, and in particular relates to a laser frequency sweeping type high-resolution wavelength demodulation device based on laser frequency sweeping technology. Background technology:

[0002] Optical fiber is composed of two layers of glass with different refractive indices: the inner layer is the optical core, with a diameter ranging from a few microns to tens of microns, and the outer layer has a diameter of 0.1 to 0.2 mm. The refractive index of the inner core glass is generally 1% higher than that of the outer layer. According to the principles of light refraction and total internal reflection, when the angle at which light strikes the interface between the inner core and the outer layer is greater than the critical angle for total internal reflection, the light cannot pass through the interface and is completely reflected.

[0003] Fiber optic transmission has many outstanding advantages: wide bandwidth. The width of the bandwidth represents the size of the transmission capacity. The higher the frequency of the carrier, the wider the bandwidth of the signal that can be transmitted. In the VHF band, the carrier frequency is 48.5MHz~300Mhz, and the frequency of visible light reaches 100,000GHz, which is more than one million times higher than the VHF band. Low loss. In a system composed of coaxial cables, the best cables have a loss of more than 40dB per kilometer when transmitting 800MHz signals. In comparison, the loss of optical fiber is much smaller. When transmitting 1.31um light, the loss per kilometer is less than 0.35dB. If transmitting 1.55um light, the loss per kilometer is even smaller, reaching less than 0.2dB. Light weight. The diameter of an optical cable composed of 4 to 48 optical fibers is less than 13mm, which is much smaller than the 47mm diameter of a standard coaxial cable. This is because optical fiber is glass fiber and has a low specific gravity. It is very convenient to install; it has strong anti-interference ability. The basic component of optical fiber is quartz, which only transmits light, is not conductive, and is not affected by electromagnetic fields. The optical signal transmitted in it is not affected by electromagnetic fields. The signal transmitted in the optical fiber is not easily eavesdropped, which is conducive to confidentiality; the fidelity is high. Optical fiber transmission generally does not require relay amplification, and no new nonlinear distortion is introduced due to amplification; the working performance is reliable. The optical fiber system contains a small number of devices (unlike the cable system that requires dozens of amplifiers), so the reliability is naturally high. In addition, the lifespan of optical fiber equipment is very long, with a trouble-free working time of 500,000 to 750,000 hours. Among them, the laser in the optical transmitter has the shortest lifespan, and the minimum lifespan is more than 100,000 hours; the cost is low. The bandwidth of optical fiber information transmission doubles every 6 months, while the price decreases by 500%. Since the source of the material (quartz) for making optical fiber is very abundant, the cost will be further reduced with the advancement of technology.

[0004] Although optical fiber has many advantages, the optical fiber wavelength demodulation equipment in the existing technology still has technical problems such as low wavelength resolution, high price, and weak multi-channel demodulation capability. For example, Chinese patent 202210409148 discloses a fiber Bragg grating wavelength demodulator, which includes a broadband flat light source, a circulator, a fiber Bragg grating, a beam splitter, several narrow-band transmission filters, a photodiode, a transimpedance amplifier and a differential amplifier circuit; two adjacent narrow-band transmission filters have different tilt angles and form a wavelength demodulation channel with the corresponding amplification and differential circuits.

[0005] The position and wavelength demodulation system of the cascaded fiber Bragg grating sensor array disclosed in Chinese Patent No. 202111465687 consists of a supercontinuum laser source, an optical splitter, an optical circulator, a fiber Bragg grating sensor array, a sensing light polarization beam splitter, a reference light polarization beam splitter, an X-axis optical beam combiner, a Y-axis optical beam combiner, an X-axis spectral disperser, a Y-axis spectral disperser, an X-axis CCD wavelength array detector, a Y-axis CCD wavelength array detector and a data processing unit; the supercontinuum laser source is connected to the input port of the optical splitter via an optical fiber; an output port of the optical splitter is connected to the first port of the optical circulator via an optical fiber, the second port of the optical circulator is connected to the fiber Bragg grating sensor array via an optical fiber, the third port of the optical circulator is connected to the input port of the sensing light polarization beam splitter via an optical fiber, the X-axis output port of the sensing light polarization beam splitter is connected to an input port of the X-axis optical beam combiner via an optical fiber, and the Y-axis output port of the sensing light polarization beam splitter is connected to the Y-axis light beam combiner via an optical fiber. An input port of the beam combiner; the other output port of the optical splitter is connected to the input port of the reference light polarization beam splitter via an optical fiber, the X-axis output port of the reference light polarization beam splitter is connected to the other input port of the X-axis optical beam combiner via an optical fiber, and the Y-axis output port of the reference light polarization beam splitter is connected to the other input port of the Y-axis optical beam combiner via an optical fiber; the output port of the X-axis optical beam combiner is connected to the input port of the X-axis spectral disperser via an optical fiber, the output port of the X-axis spectral disperser is directly opposite to the detection window of the X-axis CCD wavelength array detector, and the output port of the X-axis CCD wavelength array detector is connected to an input port of the data processing unit via an electrical signal line; the output port of the Y-axis optical beam combiner is connected to the input port of the Y-axis spectral disperser via an optical fiber, the output port of the Y-axis spectral disperser is directly opposite to the detection window of the Y-axis CCD wavelength array detector, and the output port of the Y-axis CCD wavelength array detector is connected to the other input port of the data processing unit via an electrical signal line.

[0006] Therefore, a high-resolution wavelength demodulation device is developed and designed based on laser scanning technology to improve stability and have positive social and economic benefits. Summary of the invention:

[0007] The purpose of the present invention is to overcome the shortcomings of the existing technology and develop a laser sweeping high-resolution wavelength demodulation device. The time delay between the sensing peak and the reference peak is calculated through a cross-correlation algorithm to effectively compensate for the laser frequency drift caused by environmental fluctuations.

[0008] In order to achieve the above-mentioned purpose, the present invention relates to a laser frequency sweeping high-resolution wavelength demodulation device, the main structure of which includes a laser, a temperature controller, a beam splitter, an optical circulator, a gas absorption cell, an optical resonant cavity, a photodetector, an industrial computer and a current controller; the laser is connected to a plurality of optical circulators and gas absorption cells respectively through the beam splitter, the optical circulators and gas absorption cells are both connected to the industrial computer through the photodetector, and the industrial computer is connected to the laser through the current controller to form a loop; in addition, the laser is also connected to the temperature controller, and the optical circulator is also connected to the optical resonant cavity.

[0009] The laser involved in the present invention is a narrow linewidth laser, including a DFB laser, which has the characteristics of narrow linewidth, single frequency, large mode hopping range and low cost, and provides a light source; the optical resonant cavity is a sensor probe.

[0010] The laser frequency sweeping high-resolution wavelength demodulation device disclosed herein is configured such that, when in use, an optical beam splitter splits laser light emitted by a laser into two beams, including but not limited to two beams. One beam passes through a gas absorption cell as a reference optical path, and the other reaches an optical resonant cavity as a sensing optical path. A photodetector receives a transmitted light signal from the gas absorption cell and a reflected light signal from the optical resonant cavity. An industrial computer controls a data acquisition module to synchronously acquire data, obtains spectral signals of the sensing optical path and the reference optical path in the time domain, and performs filtering, derivation, and cross-correlation operations to demodulate information to be measured. The industrial computer controls a voltage output module to output the information to be measured. Since the gas molecule absorption line is used as a standard frequency, a cross-correlation operation is performed on its derivative to obtain the time difference between the measurement resonance peak and the reference channel resonance peak, thereby obtaining the resonant wavelength of the optical resonant cavity. Different branch optical paths have the same wavelength-time relationship. The gas molecule absorption cell optical path serves as a reference channel, and the high-precision optical resonant cavity optical path serves as a sensing channel. The gas molecule absorption line has high frequency stability, and the gas molecule absorption peak is used to calibrate the laser's emission wavelength in real time.

[0011] Compared with the prior art, the present invention controls the voltage output module through an industrial computer to periodically output an oblique triangular wave signal, which is loaded onto a current controller to change the injection current of the laser, thereby changing the emission wavelength of the laser. A gas absorption cell is introduced as a reference channel, and the time delay between the sensing peak and the reference peak is calculated through a cross-correlation algorithm, effectively compensating for the laser frequency drift caused by environmental fluctuations and improving the demodulation stability. The present invention has a simple structure, high light source utilization, high resolution, good stability, and high signal-to-noise ratio. The introduction of a gas absorption cell optical path as a reference optical path reduces sensitivity to environmental fluctuations and improves long-term stability. The present invention is suitable for demodulating high-precision optical resonant cavity probes and can be used in deep-sea exploration, crustal deformation, environmental monitoring, life sciences and other fields. Description of the drawings:

[0012] Figure 1 It is a schematic diagram of the main structure of the present invention.

[0013] Figure 2 It is a schematic diagram of the swept spectrum of the gas absorption cell and optical resonant cavity involved in the present invention.

[0014] Figure 3 The figure is a schematic diagram of the swept spectrum derivative of the gas absorption cell and optical resonant cavity involved in the present invention.

[0015] Figure 4 It is a schematic diagram of the cross-correlation results of the swept spectrum derivatives of the gas absorption cell and the optical resonant cavity involved in the present invention.

[0016] Figure 5 This is a schematic diagram comparing test results with and without gas absorption cell calibration according to the present invention. Specific implementation method:

[0017] The present invention will be further described below through examples with reference to the accompanying drawings.

[0018] Example 1:

[0019] The main structure of a laser frequency sweeping high-resolution wavelength demodulation device according to this embodiment is as follows: Figure 1As shown, it includes a laser 1, a temperature controller 2, a beam splitter 3, an optical circulator 4, a gas absorption cell 5, an optical resonant cavity 6, a photodetector 7, an industrial computer 8 and a current controller 9; the laser 1 is connected to the temperature controller 2 and the beam splitter 3 respectively, the beam splitter 3 is connected to the three optical circulators 4 and the gas absorption cell 5 respectively, the optical circulator 4 is connected to the optical resonant cavity 6 and the photodetector 7 respectively, the gas absorption cell 5 is connected to the photodetector 7, the photodetector 7 is connected to the industrial computer 8, and the industrial computer 8 is connected to the laser 1 through the current controller 9 to form a loop; the industrial computer 8 has a built-in data acquisition module 81 and a voltage output module 82, the data acquisition module 81 is connected to the photodetector 7, and the voltage output module 82 is connected to the current controller 9.

[0020] When the laser frequency sweeping high-resolution wavelength demodulation device of this embodiment is used: the industrial computer 8 controls the voltage output module 82 to periodically output an oblique triangular wave signal, which is loaded onto the current controller 9 to change the injection current of the laser 1, thereby emitting a narrow-band scanning laser;

[0021] In a scanning cycle, the laser 1 emits a narrowband laser of a single wavelength at each time point, which is different from each other. After passing through the optical beam splitter 3, the laser generates four laser beams, one of which enters the gas absorption cell 5, and the remaining three beams enter the three optical resonant cavity 6 sensing channels respectively. Based on the principle that the molecular absorption line of the gas absorption cell 5 does not change with temperature, as a frequency standard, the gas molecules absorb different wavelengths differently. After the laser passes through the gas absorption cell 5, the output light is converted into an electrical signal by the photodetector 7, and the data is collected by the data acquisition module 81 and transmitted to the industrial computer 8 for processing. The industrial computer 8 controls the voltage output module 82 to output the modulation signal while controlling the data acquisition module 81 to synchronously collect data, thereby obtaining the spectral signals of the sensing optical path and the reference optical path in the time domain, and first filters and derivates the data;

[0022] Since the laser entering the gas absorption cell 5 and the laser entering the sensing channel of the optical resonant cavity 6 have the same wavelength-time curve relationship, by performing cross-correlation calculations on their derivatives, the time difference between the measured resonance peak and the reference channel resonance peak is obtained, and then the resonant wavelength of the optical resonant cavity 6 is obtained.

[0023] The cross-correlation algorithm involved in this embodiment is a data processing algorithm with a low noise coefficient. The difference between the resonance peaks can be obtained by calculating the time delay.

[0024] Example 2:

[0025] During the test of a laser frequency-sweeping high-resolution wavelength demodulation device involved in this embodiment, a 10 mm long polyimide-coated π-PSFBG was used as a temperature probe, and the HCN gas absorption line was used as a frequency standard to compensate for laser frequency fluctuations caused by drift and environmental noise in real time. The output beam of the laser 1 was split into two paths by a beam splitter 3 and simultaneously transmitted to the sensing optical path and the reference optical path. The error signals of the π-PSFBG resonance peak and the HCN gas absorption line were cross-correlated to obtain the time delay between the two, and then the frequency difference between the two was derived. The temperature probe was placed on a semiconductor cooler (TEC) set in a constant temperature box, and the reflection spectrum of the π-PSFBG at 18°C was measured using a spectrometer. A negative slant three-wavelength laser with a period of 0.625s and an amplitude of 1.5V was used to measure the reflection spectrum of the π-PSFBG at 18°C. The angular scanning signal and the sinusoidal wave modulation signal with a frequency of 10kHz and an amplitude of 10mV act together on the current controller 9. During the laser scanning process, both photodetectors 7 generate time-varying spectra. The zero point position of the error signal reflects the resonance peak position of the π-PSFBG. The spectrum of the HCN gas absorption line measured by the spectrometer has many narrow absorption lines between 1530-1560nm, and there is a gas molecule absorption line near the resonance peak of the π-PSFBG. Since the wavelength corresponding to the gas molecule absorption line does not change with temperature and environmental fluctuations, the wavelength of the swept laser can be calibrated in real time to compensate for the drift of the laser frequency, reduce the measurement error, and improve the measurement accuracy. The actual measured swept spectrum of the gas absorption cell 5 and the optical resonant cavity 6 is shown in Figure 2. Figure 2 As shown, the derivative of the swept spectrum is Figure 3 As shown, the cross-correlation results of the swept spectrum derivatives are as follows Figure 4 The test results with and without gas absorption cell calibration are shown in Figure 5 As shown in the figure, it can be seen that in a stable environment, when there is no real-time calibration of the gas absorption cell 5, the difference of the demodulated resonance peaks drifts over time. This is due to the influence of external noise and temperature, and the performance parameters of the laser 1 drift, resulting in measurement errors. When there is real-time calibration of the gas absorption cell 5, except for the noise, the cross-correlation result remains unchanged over time, indicating that the difference between the two resonance peaks is relatively stable, which is consistent with the actual situation. This is because the gas absorption cell 5 is used as the reference frequency. When the laser 1 drifts, the reference signal and the measurement signal will also drift in the same way. The cross-correlation results of the two cancel the drift, effectively reducing the measurement error caused by the laser frequency drift and improving the measurement accuracy. Except for the noise, the frequency difference between the two is relatively stable, with a standard deviation σ of 3.696 MHz and a temperature sensitivity of about 1.42 GHz / ℃, corresponding to a temperature resolution of 2.6×10 -3 ℃.

Claims

1. A laser frequency sweeping high-resolution wavelength demodulation device, the main structure of which includes a laser, a temperature controller, a beam splitter, an optical circulator, a gas absorption cell, an optical resonant cavity, a photodetector, an industrial computer and a current controller; characterized in that: The laser is connected to several optical circulators and gas absorption cells through a beam splitter. The optical circulators and gas absorption cells are connected to an industrial computer through a photoelectric detector. The industrial computer is connected to the laser through a current controller to form a loop. The laser is also connected to a temperature controller. The optical circulator is also connected to an optical resonant cavity. When in use, the optical beam splitter divides the laser emitted by the laser into two beams, including but not limited to two beams. One beam passes through the gas absorption cell as a reference light path, and the other reaches the optical resonant cavity as a sensing light path. The photoelectric detector receives the transmitted light signal from the gas absorption cell and the reflected light signal from the optical resonant cavity. The industrial computer controls the data acquisition module at the same time. Data is collected step by step to obtain the spectral signals of the sensing optical path and the reference optical path in the time domain, and filtering, derivation and cross-correlation operations are performed to demodulate the information to be measured. The industrial computer controls the voltage output module to output the information to be measured. Since the gas molecule absorption line is used as the standard frequency, its derivative is cross-correlated to obtain the time difference between the measurement resonance peak and the reference channel resonance peak, and the resonant wavelength of the optical resonant cavity can be obtained. Different branch optical paths have the same wavelength-time relationship. The gas molecule absorption cell optical path is used as the reference channel, and the optical resonant cavity optical path is used as the sensing channel. The gas molecule absorption peak is used to calibrate the emission wavelength of the laser in real time.

2. The laser frequency sweeping high-resolution wavelength demodulation device according to claim 1, characterized in that: The laser is a narrow linewidth laser, including a DFB laser, which provides a light source.

3. A laser frequency sweeping high-resolution wavelength demodulation device according to claim 1 or 2, characterized in that: The optical resonant cavity is the sensing probe.

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