A laser beat frequency high resolution wavelength demodulation device

Abstract

The application belongs to the technical field of optical sensing, and particularly relates to a high-resolution wavelength demodulation device based on laser beat frequency technology, the main structure of which comprises a reference light path and a sensing light path connected with a measuring light path respectively; the measuring light path, the reference light path and the sensing light path are connected with the measuring light path respectively, a gas molecule absorption line is introduced as a reference frequency, laser frequency measurement is reduced from a terahertz wave band to a low-frequency wave band which is easy to measure through an optical heterodyne detection scheme, high-resolution measurement is carried out through an oscilloscope and a frequency meter, and the device has extremely high resolution and long-term stability; the device has the advantages of simple structure, high resolution, good stability, fast response speed, high signal-to-noise ratio and the like, and is suitable for demodulating high-precision optical resonant cavity sensors.

Description

Technical fields:

[0001] This invention belongs to the field of optical sensing technology, specifically relating to a high-resolution wavelength demodulation device based on laser beat frequency technology. It has the advantages of ultra-high resolution and long-term stable measurement, and is applicable to fields such as deep-sea exploration, crustal deformation, environmental monitoring and life sciences. Background technology:

[0002] Typically, a light-emitting diode (LED) or a laser beam is used at one end of an optical fiber to transmit light pulses, while a photosensitive element is used at the other end to detect the pulses. In everyday life, optical fibers are used for long-distance information transmission because the transmission loss of light in optical fibers is much lower than that of electricity in wires. A bare optical fiber generally consists of three layers: a high-refractive-index glass core (typically 50 or 62.5 μm in diameter), a low-refractive-index silica glass cladding (typically 125 μm in diameter), and an outermost reinforcing resin coating. Light travels through the core. When the angle at which the light strikes the interface between the core and the outer layer exceeds the critical angle for total internal reflection, the light cannot pass through the interface and is completely reflected back, continuing to propagate forward within the core. The resin coating primarily serves a protective function. Existing optical fiber wavelength demodulation devices suffer from low wavelength resolution. For example, Chinese Patent 202210468243 discloses an ultra-weak fiber optic grating wavelength demodulation module, comprising a housing, a laser pulse generator, an optical circuit unit, and an embedded board. The embedded board is connected to the laser pulse generator and the optical circuit unit, respectively, and the laser pulse generator is connected to the optical circuit unit. The laser pulse generator is used to generate laser pulses with repetitive wavelength changes. The optical circuit unit is used to amplify and output the laser pulses, receive the optical signal returned by the object under test, and complete photoelectric conversion before outputting it. The embedded board is used to set commands and parameters for the laser pulse generator and configure the optical circuit unit. The embedded board is also used for grating positioning and real-time wavelength analysis, outputting the wavelength and position information of the ultra-weak grating.

[0003] Chinese Patent 202221401834 discloses a wavelength demodulation system for a tunable laser, comprising a first power module, a second power module, an FPGA, a laser current drive module, a laser temperature control module, a tunable laser, a demodulator optical path, a photoelectric conversion module, and a communication module. The FPGA is connected to the laser current drive module, the laser temperature control module, the photoelectric conversion module, and the communication module. The laser current drive module and the laser temperature control module are also connected to the tunable laser. The tunable laser is further connected to the demodulator optical path. The demodulator optical path is connected to the photoelectric conversion module and the fiber Bragg grating sensor; the FPGA is used to control the operation of the laser current drive module and the laser temperature control module; the tunable laser is used to emit wavelength-tunable laser light under the action of the laser current drive module and the laser temperature control module; the demodulator optical path is used to incident the received wavelength-tunable laser light onto the fiber Bragg grating sensor, and the fiber Bragg grating sensor will send the reflected light carrying the modulation signal through the demodulator optical path to the target optical sensor. The photoelectric conversion module is used to convert the reflected light signal into an electrical signal and output it to the FPGA. The FPGA calculates and processes the center wavelength information of the fiber Bragg grating sensor and transmits the center wavelength information of the fiber Bragg grating sensor to the host computer for display and storage through the communication module, completing the entire demodulation process of the fiber Bragg grating sensor. The first power supply module is used to power the laser current drive module. The second power supply module is used to power the FPGA, the laser temperature control module, the tunable laser, the demodulator optical path, the photoelectric conversion module, and the communication module. The first power supply module includes a DC-DC converter chip, a Zener diode, a transformer, a first resistor, a second resistor, and a third resistor. The first pin of the DC-DC converter chip is grounded, the second pin is connected to one end of the Zener diode and one end of the third resistor, the third pin is connected to the transformer, the fourth pin is connected to the other end of the Zener diode, the fifth pin is connected to the first resistor and the second resistor, and the other end of the third resistor is grounded. The second power supply module includes a buck DC-DC converter chip.

[0004] Chinese Patent 202210682925 discloses a wavelength division multiplexing (WDM) demodulation system based on a microring resonator, comprising a microring resonator chip, a modulation signal generation circuit, a photoelectric signal conversion circuit, and a mixing and filtering circuit. The modulation signal generation circuit generates a sinusoidal oscillation modulation signal with DC bias and transmits it to the microring resonator chip and the mixing and filtering circuit. The microring resonator chip converts input optical signals of different wavelengths into wavelength-dependent optical signals and transmits them to the photoelectric signal conversion circuit. The photoelectric signal conversion circuit converts the optical signal output from the microring resonator chip into an electrical signal, amplifies it, and transmits it to the mixing and filtering circuit. The mixing and filtering circuit performs mixing and filtering of the sinusoidal oscillation modulation signal transmitted from the modulation signal generation circuit and the converted electrical signal, and outputs a demodulated voltage signal. Therefore, a high-resolution wavelength demodulation device is developed and designed based on laser beat frequency technology to achieve high resolution, good stability, fast response speed, and high signal-to-noise ratio. Summary of the Invention:

[0005] The purpose of this invention is to overcome the shortcomings of existing technologies and to develop and design a laser beat frequency type high-resolution wavelength demodulation device, which reduces the laser frequency measurement from the terahertz band to the easily measurable low-frequency band through an optical heterodyne detection scheme.

[0006] To achieve the above objectives, the main structure of the laser beat frequency type high-resolution wavelength demodulation device of the present invention includes a reference optical path and a sensing optical path respectively connected to the measurement optical path.

[0007] The main structure of the reference optical path involved in this invention includes a first laser, a first phase modulator, and a first photodetector, which are respectively connected to a first frequency-locked control system; a first optical coupler is provided between the first laser and the first phase modulator, a circulator is provided between the first phase modulator and the first photodetector, and the circulator is also connected to an optical resonant cavity.

[0008] The main structure of the sensing optical path involved in this invention includes a second laser, a second phase modulator, and a second photodetector, which are respectively connected to a second frequency-locked control system; a second optical coupler is provided between the second laser and the second phase modulator, and a gas molecule absorption cell is provided between the second phase modulator and the second photodetector.

[0009] The main structure of the measurement optical path involved in this invention includes a third optical coupler connected to a first optical coupler, a second optical coupler, and a beat frequency detector, and an oscilloscope and a frequency meter connected to the beat frequency detector.

[0010] The first and second lasers involved in this invention are both narrow linewidth lasers.

[0011] In the laser beat frequency type high-resolution wavelength demodulation device of the present invention, the light emitted by the laser enters the phase modulator through the coupler. The phase modulator modulates the phase of the laser's output light. The photodetector receives the phase-modulated light and converts it into an electrical signal. The signal is then mixed with the original radio frequency modulation signal and subjected to phase-locked amplification signal processing with low-pass filtering to generate an error signal. The laser frequency is locked to the molecular absorption line of the gas molecule absorption cell and the resonant frequency of the optical resonant cavity through the frequency-locking control system. The optical coupler combines the laser from the molecular absorption line locked to the gas molecule absorption cell and the laser from the optical resonant cavity into a single beam. The beat frequency detector receives the optical beat frequency signal, the oscilloscope monitors the optical beat frequency signal, and the frequency meter detects and records the frequency of the optical beat frequency signal.

[0012] Compared with existing technologies, this invention introduces gas molecule absorption lines as reference frequencies and reduces laser frequency measurement from the terahertz band to a low-frequency band that is easier to measure through an optical heterodyne detection scheme. High-resolution measurements are performed using an oscilloscope and frequency meter, resulting in extremely high resolution and long-term stability. Its structure is simple, with high resolution, good stability, fast response speed, and high signal-to-noise ratio, making it suitable for demodulating high-precision optical resonant cavity sensors. Attached image description:

[0013] Figure 1 This is a schematic diagram of the main structure of the present invention.

[0014] Figure 2 This is a schematic diagram of the frequency locking control system involved in the present invention.

[0015] Figure 3 This is a schematic diagram of the beat frequency spectrum of the optical resonator involved in this invention at different temperatures.

[0016] Figure 4 This is a schematic diagram of the Allan deviation of the beat frequency signal when the temperature is stable, as per the present invention. Detailed implementation method:

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

[0018] Example 1:

[0019] The main structure of a laser beat-frequency type high-resolution wavelength demodulation device involved in this embodiment is as follows: Figure 1As shown, the system includes a reference optical path, a sensing optical path, and a measurement optical path, with the reference optical path and sensing optical path connected to the measurement optical path respectively. The main structure of the reference optical path includes a first laser 11, a first optical coupler 12, a first phase modulator 13, a circulator 14, an optical resonant cavity 15, a first photodetector 16, and a first frequency-locking control system 17. The first laser 11 is connected to the first phase modulator 13 via the first optical coupler 12. The first phase modulator 13 is connected to the circulator 14. The circulator 14 is connected to the optical resonant cavity 15 and the first photodetector 16. The first photodetector 16 is connected to the first frequency-locking control system 17. The first frequency-locking control system 17 is connected to the first laser 11 and the first phase modulator 13, forming a loop. The main structure of the sensing optical path includes a second laser 21, a second optical coupler 22, and a second... The optical path includes a phase modulator 23, a gas molecule absorption cell 24, a second photodetector 25, and a second frequency locking control system 26. A second laser 21 is connected to a second phase modulator 23 via a second optical coupler 22. The second phase modulator 23 is connected to the gas molecule absorption cell 24, which is connected to the second photodetector 25. The second photodetector 25 is connected to the second frequency locking control system 26. The second frequency locking control system 26 is connected to both the second laser 21 and the second phase modulator 23, forming a loop. The main structure of the measurement optical path includes a third optical coupler 31, a beat detector 32, an oscilloscope 33, and a frequency meter 34. The third optical coupler 31 is connected to the first optical coupler 12 and the second optical coupler 22, and then to the beat detector 32. The beat detector 32 is connected to the oscilloscope 33 and the frequency meter 34.

[0020] The structures of the first frequency locking control system 17 and the second frequency locking control system 26 involved in this embodiment are as follows: Figure 2 As shown, each includes a lock-in amplifier consisting of a mixer 41 and a low-pass filter 42, a proportional-integral-differential controller 43, and a signal generator 44. The mixer 41 is connected to the first photodetector 16 or the second photodetector 25 and then to the low-pass filter 42. The low-pass filter 42 provides a connection between the proportional-integral-differential controller 43 and the first laser 11 or the second laser 21. The signal generator 44 is located between the first phase modulator 13 or the second phase modulator 23 and the mixer 41.

[0021] When using the laser beat-frequency type high-resolution wavelength demodulation device described in this embodiment:

[0022] The light sources provided by laser 11 and laser 21 are split into two beams by coupler 12 and coupler 22. One beam is used for frequency locking, and the other is used for beat frequency. The light used for frequency locking enters the optical resonant cavity 15 and the gas molecule absorption cell 24, respectively. Phase modulator 13 and phase modulator 23 modulate the phase of the frequency-locked light. Photodetector 16 receives the light signal reflected back from the optical resonant cavity 15 through circulator 14 and converts it into an electrical signal. Photodetector 25 receives the transmitted light signal from the gas molecule absorption cell 24 and converts it into an electrical signal.

[0023] The electrical signal is demodulated by the lock-in amplifier to obtain the error signal. After receiving the error signal, the proportional-integral-differential controller 43 calculates the corresponding feedback signal and feeds the feedback signal back to the drivers of the first laser 11 and the second laser 21. The frequencies of the first laser 11 and the second laser 21 are locked at the resonant frequency of the optical resonant cavity 15 and the molecular absorption line of the gas molecule absorption cell 24, respectively, so that the output frequency of the first laser 11 and the second laser 21 is consistent with the resonant frequency of the resonant peak.

[0024] Optical coupler 14 combines the laser beam locked at the resonant frequency of optical resonant cavity 15 and the laser beam on the molecular absorption line of gas molecule absorption cell 24 into a single beam. Beat frequency detector 32 receives the beat frequency signal of this beam, oscilloscope 33 monitors the beat frequency signal, and frequency meter 34 detects and records the frequency of the beat frequency signal. The resonant wavelength of optical resonant cavity 15 is demodulated using the frequency of the beat frequency signal.

[0025] Example 2:

[0026] This embodiment relates to a laser beat frequency type high-resolution wavelength demodulation device. The laser locked to the absorption line of gas molecules is used as the standard frequency. The resonant wavelength of the sensing probe, optical resonant cavity 15, is obtained through optical heterodyne method. This converts the optical frequency measurement to a low-frequency band, and high-resolution measurement is performed using a frequency meter 34. This device features good stability and high resolution. During actual measurement, the first laser 11 locked to the optical resonant cavity 15 and the second laser 21 locked to the gas absorption cell 24 beat frequency. The outputs of the first laser 11 and the second laser 21 are split into two branches through a 90 / 10 coupler: 90% of the optical power is coupled to the optical resonant cavity 15 or the gas absorption cell 24, and 10% of the optical power is extracted from each branch and combined through a third optical coupler 31 before entering the beat frequency detector 32. A beat frequency signal is generated in the radio frequency (RF) range. The frequency meter 34 measures the beat frequency signal, and the resulting beat frequency signal spectrum of the optical resonant cavity 15 at different temperatures is shown below. Figure 3 As shown, it exhibits good sensing response; the Allan deviation of the beat frequency signal obtained when the temperature is stable is as follows: Figure 4As shown, the wavelength resolution reaches 200kHz, and it exhibits good long-term stability.

Claims

1. A laser beat-frequency type high-resolution wavelength demodulation device, the main structure comprising a reference optical path and a sensing optical path respectively connected to a measurement optical path; characterized in that, The reference optical path and the sensing optical path are respectively connected to the measurement path; the main structure of the sensing optical path includes a second laser, a second phase modulator, and a second photodetector, which are respectively connected to the second frequency-locked control system; a second optical coupler is set between the second laser and the second phase modulator, and a gas molecule absorption cell is set between the second phase modulator and the second photodetector.

2. The laser beat frequency type high-resolution wavelength demodulation device according to claim 1, characterized in that, The main structure of the reference optical path includes a No. 1 laser, a No. 1 phase modulator, and a No. 1 photodetector, which are respectively connected to the No. 1 frequency-locking control system. A No. 1 optical coupler is provided between the No. 1 laser and the No. 1 phase modulator, and a circulator is provided between the No. 1 phase modulator and the No. 1 photodetector. The circulator is also connected to the optical resonant cavity.

3. The laser beat frequency type high-resolution wavelength demodulation device according to claim 2, characterized in that, The main structure of the measurement optical path includes a third optical coupler connected to the first optical coupler, the second optical coupler, and the beat frequency detector, as well as an oscilloscope and a frequency meter connected to the beat frequency detector.

4. The laser beat frequency type high-resolution wavelength demodulation device according to claim 2, characterized in that, Both laser number one and laser number two are narrow linewidth lasers.

5. A laser beat-frequency type high-resolution wavelength demodulation device according to claim 2 or 3, characterized in that, Both the No. 1 and No. 2 frequency locking control systems include a lock-in amplifier consisting of a mixer and a low-pass filter, a proportional-integral-differential controller, and a signal generator. The mixer is connected to either the No. 1 or No. 2 photodetector and then to the low-pass filter. The low-pass filter provides a connection between the proportional-integral-differential controller and either the No. 1 or No. 2 laser. The signal generator is located between the No. 1 or No. 2 phase modulator and the mixer.

6. The laser beat frequency type high-resolution wavelength demodulation device according to claim 5, characterized in that, In use, the laser beat frequency type high-resolution wavelength demodulation device transmits light emitted from the laser through a coupler to a phase modulator. The phase modulator modulates the phase of the emitted laser light. The photodetector receives the phase-modulated light and converts it into an electrical signal. This signal is then mixed with the original radio frequency modulated signal and subjected to phase-locked amplification signal processing with low-pass filtering to generate an error signal. The frequency-locking control system locks the laser frequency to the molecular absorption line of the gas molecule absorption cell and the resonant frequency of the optical resonant cavity, respectively. The optical coupler combines the laser from the molecular absorption line locked in the gas molecule absorption cell and the laser from the optical resonant cavity into a single beam. The beat frequency detector receives the optical beat frequency signal, the oscilloscope monitors the optical beat frequency signal, and the frequency meter detects and records the frequency of the optical beat frequency signal.

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