Near-infrared laser heterodyne radiometer based on an external cavity laser
By using a near-infrared laser heterodyne radiometer based on an external cavity laser, a wide tuning range and optical power balance were achieved, solving the multi-component detection problem of traditional laser heterodyne radiometers, improving the system's signal-to-noise ratio and detection accuracy, and making it suitable for remote sensing of atmospheric composition.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- HEFEI UNIV
- Filing Date
- 2026-04-07
- Publication Date
- 2026-05-29
AI Technical Summary
Traditional laser heterodyne radiometers have a narrow tuning range, making it difficult to cover the absorption spectra of multiple gas components. The mismatch between the optical power of the signal channel and the reference channel leads to a decrease in common-mode noise suppression, and the system noise level exceeds the theoretical shot noise limit, thus limiting the detection sensitivity and accuracy.
A near-infrared laser heterodyne radiometer based on an external cavity laser is adopted. A wide tuning range of 100 cm⁻¹ is achieved by using a tunable external cavity laser. The reference optical power is adjusted by combining a variable fiber attenuator. The optical power of the signal optical channel and the reference optical channel is balanced by a balanced detector. Synchronous demodulation is performed by combining an optical chopper and a lock-in amplifier to suppress common-mode noise.
It enables simultaneous detection of multiple gases and multiple isotopes, improves spectral coverage and information acquisition efficiency, achieves the quasi-shot noise limit in system signal-to-noise ratio, and significantly improves detection stability and accuracy.
Smart Images
Figure CN122109019A_ABST
Abstract
Description
Technical Field
[0001] This application relates to the technical field of atmospheric optical remote sensing, and in particular to a near-infrared laser heterodyne radiometer based on an external cavity laser. Background Technology
[0002] Atmospheric optical remote sensing technology is an important means of obtaining information on the distribution and changes of atmospheric composition globally, and is widely used in climate change research, environmental monitoring, and weather forecasting. Among them, atmospheric absorption spectroscopy in the near-infrared band can effectively retrieve the vertical distribution information of key gases such as water vapor, carbon dioxide, and their isotopes, and has significant application value.
[0003] Laser heterodyne radiometer is a high spectral resolution remote sensing technology. Its basic principle is to use sunlight as a signal source and perform heterodyne mixing with a local oscillating laser. By measuring the intensity change of the heterodyne beat frequency signal, the atmospheric absorption spectrum is inverted. Based on the high spectral resolution of laser and the high signal-to-noise ratio of sunlight, it can achieve highly sensitive detection of trace gas components in the atmosphere.
[0004] However, traditional laser heterodyne radiometers still have shortcomings in practical applications. The tuning range of traditional lasers is narrow, making it difficult to cover the absorption spectral lines of multiple gas components, thus limiting the ability to detect multiple components simultaneously. The mismatch between the optical power of the signal optical channel and the reference optical channel leads to a decrease in the common-mode noise suppression effect, affecting the system's signal-to-noise ratio. Moreover, due to factors such as detector electronic noise and laser relative intensity noise, the actual noise level of traditional laser heterodyne radiometers is much higher than the theoretical shot noise limit, which limits the sensitivity and accuracy of detection.
[0005] Some radiometers that attempt to use dual-channel detection lack the supporting structure for real-time monitoring and precise adjustment of optical power. This makes it impossible to quantitatively monitor and dynamically balance the optical power of the signal and reference optical channels, thus hindering effective suppression of common-mode noise. Summary of the Invention
[0006] The purpose of this invention is to provide a near-infrared laser heterodyne radiometer based on an external cavity laser, thereby solving at least one of the above-mentioned problems.
[0007] This invention provides a near-infrared laser heterodyne radiometer based on an external cavity laser, comprising an optical path module and a signal processing module, wherein the optical path module and the signal processing module are electrically connected; The optical path module includes a solar tracking optical front end, an optical chopper, a tunable external cavity laser, a beam splitter coupler, a beam combiner coupler, a variable fiber attenuator, and a balanced detector. The output of the solar tracking optical front end is connected to the input of the optical chopper, the output of the optical chopper is connected to the first input of the optical combiner, the output of the tunable external cavity laser is connected to the input of the optical splitter, the first output of the optical splitter is connected to the reference light input port of the balanced detector via the variable fiber attenuator, the second output of the optical splitter is connected to the second input of the optical combiner, and the output of the optical combiner is connected to the signal light input port of the balanced detector. The signal processing module includes a bandpass filter, a power detector, a lock-in amplifier, and a data acquisition card, which are connected in sequence. The data acquisition card is electrically connected to the computer, the RF output terminal of the balanced detector is electrically connected to the input terminal of the bandpass filter, and the lock-in amplifier is electrically connected to the optical chopper to receive the synchronous electrical signal output by the chopper. The variable fiber attenuator is used to adjust the reference optical power so that the optical power of the signal optical channel and the reference optical channel of the balanced detector are balanced, thereby achieving common-mode noise suppression.
[0008] As a further technical solution, the solar tracking optical front end includes a solar tracker and a collimator; The output of the solar tracker is connected to the input of the collimator, and the output of the collimator is connected to the input of the optical chopper. The solar tracker reflects sunlight to the collimator, which couples the received sunlight into a single-mode fiber and then outputs the sunlight to the optical chopper, thus completing the collection, fiber coupling, and transmission of sunlight.
[0009] As a further technical solution, both the optical splitter and the optical combiner are 1×2 single-mode fiber couplers, using the same splitting ratio to achieve optical signal splitting and combining. The beam splitter splits the local oscillator light output from the tunable external cavity laser into a source light (LO) and a reference light. The LO light is directly transmitted to the beam combiner, which combines the sunlight output from the optical chopper with the LO light and transmits the beam to the signal input port of the balanced detector.
[0010] As a further technical solution, the balance detector includes at least two monitoring output ports and one RF output port; The monitoring output port is used to monitor the optical power status of the signal optical channel and the reference optical channel in real time. The RF output port is directly electrically connected to the input of the bandpass filter; After the balanced detector performs photoelectric conversion and common-mode noise suppression on the received optical signal, it transmits the heterodyne beat frequency electrical signal from the RF output port to the bandpass filter for outputting the electrical signal converted from the optical signal.
[0011] As a further technical solution, the optical chopper includes an optical signal output terminal and an electrical signal output terminal; The optical signal output terminal is connected to the first input terminal of the optical coupler, and the electrical signal output terminal is electrically connected to the reference input terminal of the lock-in amplifier. The optical chopper outputs modulated sunlight from the optical signal output terminal to the optical coupler, while simultaneously outputting a corresponding modulated electrical signal to the lock-in amplifier.
[0012] As a further technical solution, the lock-in amplifier demodulates the electrical signal transmitted by the power detector according to the modulation electrical signal; The output terminal of the lock-in amplifier is connected to the input terminal of the data acquisition card. After demodulating the electrical signal that is proportional to the atmospheric absorption spectrum, the lock-in amplifier transmits it to the data acquisition card. The output terminal of the data acquisition card is connected to the computer, and the electrical signal is recorded in the computer.
[0013] As a further technical solution, the voltage output by the monitoring output port is proportional to the optical power of the corresponding channel. The variable fiber attenuator is adjusted according to the voltage of the monitoring output port, and the optical power of the reference optical channel is changed according to the adjustment of the variable fiber attenuator, so that the optical power of the reference optical channel is equal to the optical power of the signal optical channel and the monitoring output voltage. The optical power of the signal optical channel is determined by the combined beam of sunlight and local oscillator light and cannot be adjusted independently.
[0014] As a further technical solution, the tunable external cavity laser includes a gain medium, an external cavity structure, a wavelength tuning element, and an optical fiber output interface; The gain medium is used to provide spectral gain for the laser; The external cavity structure adopts a grating-selective external cavity design, which is used to cooperate with the wavelength tuning element to achieve wavelength selection; The wavelength tuning element is used to select a specific wavelength; The laser includes a software control interface, which uses a composite tuning method that combines coarse temperature tuning and fine current tuning to adjust the operating temperature and injection current of the tunable external cavity laser.
[0015] In summary, this application includes at least one of the following beneficial technical effects: 1. This invention uses a tunable external cavity laser as the local oscillator light source to achieve a 100cm... -1Its wide tuning range can simultaneously cover H2O, CO2 and their isotopes (HDO, CO2 ... 13 Multiple characteristic absorption peaks of CO2 can be detected simultaneously for multiple gases and multiple isotopes without changing the light source or adjusting the equipment, which significantly improves the information acquisition efficiency and spectral coverage of atmospheric composition remote sensing detection.
[0016] 2. This invention adjusts the reference optical power using a variable fiber attenuator, ensuring precise balance between the signal and reference optical power of the balanced detector. This effectively suppresses the relative intensity noise and other common-mode noise of the laser. Combined with the synchronous demodulation of the optical chopper and lock-in amplifier, the relative intensity noise and circuit noise of the laser are further filtered out. The system can reach the quasi-shot noise limit performance, significantly improving the detection signal-to-noise ratio and providing a reliable signal basis for the accurate inversion of the total gas column and isotope ratio.
[0017] 3. The optical path module adopts an all-fiber design, and is equipped with a 1×2 single-mode fiber coupler to achieve efficient beam splitting and combining of optical signals. The optical signal transmission loss is low, the coupling efficiency is high, and the optical path mismatch problem of discrete optical paths is effectively avoided. The invention also combines a solar tracker and a collimator, avoiding the complex alignment and debugging process of traditional space optical paths. The overall structure is compact, highly integrated, and has strong resistance to external environmental interference, ensuring the stability of the detection process and data consistency. Attached Figure Description
[0018] Figure 1 This is a structural diagram of a near-infrared laser heterodyne radiometer based on an external cavity laser. Detailed Implementation
[0019] The embodiments of this application are described in detail below, and examples of the embodiments are shown in the accompanying drawings.
[0020] In the description of this specification, the references to "certain embodiments," "one embodiment," "some embodiments," "illustrative embodiment," "example," "specific example," or "some examples" refer to specific features, structures, materials, or characteristics described in connection with the described embodiment or example, which are included in at least one embodiment or example of this application. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.
[0021] This application discloses a near-infrared laser heterodyne radiometer based on an external cavity laser, employing an all-fiber balanced detection structure, which can realize the absorption spectrum measurement of gaseous components such as water vapor, carbon dioxide and their isotopes in sunlight, such as... Figure 1As shown, it includes an optical path module and a signal processing module, wherein the optical path module and the signal processing module are electrically connected; The optical path module completes the collection, transmission, beam splitting or combining of optical signals, and photoelectric conversion of sunlight. The signal processing module filters, demodulates, acquires and processes the electrical signals after photoelectric conversion, and finally realizes data recording and inversion analysis through a computer.
[0022] The optical path module includes a solar tracking optical front end, an optical chopper, a tunable external cavity laser, a beam splitter coupler, a beam combiner coupler, a variable fiber attenuator, and a balanced detector. The output of the solar tracking optical front end is connected to the input of the optical chopper, the output of the optical chopper is connected to the first input of the optical combiner, the output of the tunable external cavity laser is connected to the input of the optical splitter, the first output of the optical splitter is connected to the reference light input port of the balanced detector via the variable fiber attenuator, the second output of the optical splitter is connected to the second input of the optical combiner, and the output of the optical combiner is connected to the signal light input port of the balanced detector. In this embodiment, the tunable external cavity laser includes a gain medium, an external cavity structure, a wavelength tuning element, and an optical fiber output interface; the tunable external cavity laser is a near-infrared tunable external cavity semiconductor laser with a center wavelength of 1.56 nm. m, tuning range is 100cm -1 The maximum output power is 5mW, the experimental operating power is set to 1.2mW, and the wavelength scanning step size is 0.005nm; The tunable external cavity laser employs a composite tuning method that combines coarse temperature tuning with fine current tuning. The laser's operating temperature and injection current are adjusted via a software control interface to achieve the selection of a specific wavelength. In this embodiment, an external field experiment was conducted in Dunkirk using the tunable external cavity laser, and the absorption spectra of multiple gas components were measured simultaneously. The specific absorption peak positions and corresponding molecules are shown in the table below:
[0023] The gain medium is used to provide spectral gain for the laser; The external cavity structure adopts a grating-selective external cavity design, which is used to cooperate with the wavelength tuning element to achieve wavelength selection; The wavelength tuning element is used to select a specific wavelength; The laser includes a software control interface, which uses a composite tuning method that combines coarse temperature tuning and fine current tuning to adjust the operating temperature and injection current of the tunable external cavity laser.
[0024] The signal processing module includes a bandpass filter, a power detector, a lock-in amplifier, and a data acquisition card, which are connected in sequence. The data acquisition card is electrically connected to the computer, the RF output terminal of the balanced detector is electrically connected to the input terminal of the bandpass filter, and the lock-in amplifier is electrically connected to the optical chopper to receive the synchronous electrical signal output by the chopper. The variable fiber attenuator is used to adjust the reference optical power. It is a manually adjustable attenuator connected in series between the first output terminal of the optical splitter and the reference optical input port of the balanced detector. It is manually adjusted according to the voltage value of the monitored output port of the balanced detector to balance the optical power of the signal optical channel and the reference optical channel of the balanced detector, thereby achieving common-mode noise suppression.
[0025] In this embodiment, the solar tracking optical front end includes a STR-21G solar tracker and an F810APC-1550 collimator; The output of the solar tracker is connected to the input of the collimator, and the output of the collimator is connected to the input of the optical chopper. The solar tracker reflects sunlight to the collimator, which couples the received sunlight into a single-mode fiber and then outputs the sunlight to the optical chopper, thus completing the collection, fiber coupling, and transmission of sunlight.
[0026] In this embodiment, both the optical splitter and the optical combiner are 1×2 single-mode fiber couplers, using the same splitting ratio to achieve optical signal splitting and combining, with the same splitting ratio being 50:50. The beam splitter splits the local oscillator light output from the tunable external cavity laser into a source light (LO) and a reference light. The LO light is directly transmitted to the beam combiner, which combines the sunlight output from the optical chopper with the LO light and transmits the beam to the signal input port of the balanced detector.
[0027] In this embodiment, the balanced detector is a balanced InGaAs detector, which is the core photoelectric conversion component of the optical path module. It includes two fiber optic input ports, namely a signal light input port and a reference light input port; it also includes at least two monitoring output ports and one RF output port, with a transimpedance gain of 250×10⁻⁶. 3 V / A, monitoring channel transimpedance is 10kΩ, bandwidth is 75MHz; The monitoring output port is used to monitor the optical power status of the signal optical channel and the reference optical channel in real time. The RF output port is directly electrically connected to the input of the bandpass filter; The balanced detector performs photoelectric conversion on the received signal light and reference light, and achieves common-mode noise suppression under the condition of balanced optical power, and transmits the heterodyne beat frequency electrical signal from the RF output port to the signal processing module. In this embodiment, the voltage output by the monitoring output port is proportional to the optical power of the corresponding channel. The variable fiber attenuator is adjusted according to the voltage of the monitoring output port, and the optical power of the reference optical channel is changed according to the adjustment of the variable fiber attenuator, so that the optical power of the reference optical channel is equal to the optical power of the signal optical channel and the monitoring output voltage. The optical power of the signal optical channel is determined by the combined beam of sunlight and local oscillator light and cannot be adjusted independently; The method for balancing the detector based on the monitored voltage includes:
[0028] in, For output voltage, To balance the DC photocurrent of the detector, To balance the transimpedance gain of the detector, ; Set the local oscillator laser power to 1.2mW and observe the two monitoring output voltages. and .
[0029] Manually rotate the adjustment knob of the variable fiber optic attenuator to change the optical power of the reference optical channel, and observe the two monitoring output voltages in real time until... = At this point, the DC photocurrents of the two channels are equal, satisfying the equilibrium condition.
[0030] In this embodiment, to better complete performance verification, the optical power balance and optical path matching are optimized so that the system noise mainly comes from shot noise, achieving the quasi-shot noise limit performance and significantly improving the signal-to-noise ratio. The method is as follows:
[0031]
[0032] in, It is single-channel shot noise. To balance the total shot noise under detection, To detect bandwidth, =1MHz, ; hour, ; At different local oscillator laser powers, the monitoring voltage of the balanced detector was measured, the DC photocurrent was calculated, and the theoretical shot noise was compared with the measured total noise. The experimental data are shown in the table below:
[0033] Based on the principle of laser heterodyne and the characteristics of balanced detection, the factors that may lead to discrepancies are shown in the table below:
[0034] Based on the table above, the following conclusions can be drawn: When the local oscillator power is low, the measured noise is much higher than the theoretical value. The system noise is mainly dominated by electronic noise and laser relative intensity noise. As the local oscillator power increases, the measured noise gradually approaches the theoretical value. At the experimental operating point of 1.2mW, the measured noise is 123nA, and the ratio of this to the theoretical shot noise of 8.77nA is 14.0, indicating that the system has entered the quasi-shot noise limit. The measured background noise of 141 nA when there is no light input indicates that the noise difference mainly comes from the detector's electronic noise, residual relative intensity noise, thermal noise, and the superposition effect of various noise sources. Balanced detection has effectively suppressed most of the common-mode noise.
[0035] In this embodiment, an MC2000B optical chopper is used, which includes an optical signal output terminal and an electrical signal output terminal. The optical signal output terminal is connected to the first input terminal of the optical coupler, and the electrical signal output terminal is electrically connected to the reference input terminal of the lock-in amplifier. The optical chopper outputs the modulated sunlight from the optical signal output terminal to the optical coupler, and synchronously outputs the corresponding modulated electrical signal to the lock-in amplifier as a reference signal for lock-in demodulation. The modulation frequency is set to 5kHz.
[0036] In this embodiment, an SR830 lock-in amplifier is used, which is connected to the electrical signal output terminal of the optical chopper to receive the synchronous modulation electrical signal. The time constant is set to 30ms and the roll-off is set to 24dB / octave. The electrical signal transmitted by the power detector is demodulated according to the modulation electrical signal. The output terminal of the lock-in amplifier is connected to the input terminal of the data acquisition card. After demodulating the electrical signal that is proportional to the atmospheric absorption spectrum, the lock-in amplifier transmits it to the data acquisition card. The output terminal of the data acquisition card is connected to the computer, and the electrical signal is recorded in the computer.
[0037] Although embodiments of this application have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting this application. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of this application.
Claims
1. A near-infrared laser heterodyne radiometer based on an external cavity laser, characterized in that, It includes an optical path module and a signal processing module, wherein the optical path module and the signal processing module are electrically connected; The optical path module includes a solar tracking optical front end, an optical chopper, a tunable external cavity laser, a beam splitter coupler, a beam combiner coupler, a variable fiber attenuator, and a balanced detector. The output of the solar tracking optical front end is connected to the input of the optical chopper, the output of the optical chopper is connected to the first input of the optical combiner, the output of the tunable external cavity laser is connected to the input of the optical splitter, the first output of the optical splitter is connected to the reference light input port of the balanced detector via the variable fiber attenuator, the second output of the optical splitter is connected to the second input of the optical combiner, and the output of the optical combiner is connected to the signal light input port of the balanced detector. The signal processing module includes a bandpass filter, a power detector, a lock-in amplifier, and a data acquisition card, which are connected in sequence. The data acquisition card is electrically connected to the computer, the RF output terminal of the balanced detector is electrically connected to the input terminal of the bandpass filter, and the lock-in amplifier is electrically connected to the optical chopper to receive the synchronous electrical signal output by the chopper. The variable fiber attenuator is used to adjust the reference optical power so that the optical power of the signal optical channel and the reference optical channel of the balanced detector are balanced, thereby achieving common-mode noise suppression.
2. A near-infrared laser heterodyne radiometer based on an external cavity laser according to claim 1, characterized in that, The solar tracking optical front end includes a solar tracker and a collimator; The output of the solar tracker is connected to the input of the collimator, and the output of the collimator is connected to the input of the optical chopper. The solar tracker reflects sunlight to the collimator, which couples the received sunlight into a single-mode fiber and then outputs the sunlight to the optical chopper, thus completing the collection, fiber coupling, and transmission of sunlight.
3. A near-infrared laser heterodyne radiometer based on an external cavity laser according to claim 1, characterized in that, Both the splitter and combiner are 1×2 single-mode fiber couplers, using the same splitting ratio to achieve beam splitting and combining of optical signals. The beam splitter splits the local oscillator light output from the tunable external cavity laser into a source light (LO) and a reference light. The LO light is directly transmitted to the beam combiner, which combines the sunlight output from the optical chopper with the LO light and transmits the beam to the signal input port of the balanced detector.
4. A near-infrared laser heterodyne radiometer based on an external cavity laser according to claim 3, characterized in that, The balance detector includes at least two monitoring output ports and one RF output port; The monitoring output port is used to monitor the optical power status of the signal optical channel and the reference optical channel in real time. The RF output port is directly electrically connected to the input of the bandpass filter; After the balanced detector performs photoelectric conversion and common-mode noise suppression on the received optical signal, it transmits the heterodyne beat frequency electrical signal from the RF output port to the bandpass filter for outputting the electrical signal converted from the optical signal.
5. A near-infrared laser heterodyne radiometer based on an external cavity laser according to claim 3, characterized in that, The optical chopper includes an optical signal output terminal and an electrical signal output terminal; The optical signal output terminal is connected to the first input terminal of the optical coupler, and the electrical signal output terminal is electrically connected to the reference input terminal of the lock-in amplifier. The optical chopper outputs modulated sunlight from the optical signal output terminal to the optical coupler, while simultaneously outputting a corresponding modulated electrical signal to the lock-in amplifier.
6. A near-infrared laser heterodyne radiometer based on an external cavity laser according to claim 5, characterized in that, The lock-in amplifier demodulates the electrical signal transmitted by the power detector according to the modulation electrical signal; The output terminal of the lock-in amplifier is connected to the input terminal of the data acquisition card. After demodulating the electrical signal that is proportional to the atmospheric absorption spectrum, the lock-in amplifier transmits it to the data acquisition card. The output terminal of the data acquisition card is connected to the computer, and the electrical signal is recorded in the computer.
7. A near-infrared laser heterodyne radiometer based on an external cavity laser according to claim 4, characterized in that, The voltage output by the monitoring output port is proportional to the optical power of the corresponding channel. The variable fiber attenuator is adjusted according to the voltage of the monitoring output port. The optical power of the reference optical channel is changed according to the adjustment of the variable fiber attenuator, so that the optical power of the reference optical channel is equal to the optical power of the signal optical channel and the monitoring output voltage. The optical power of the signal optical channel is determined by the combined beam of sunlight and local oscillator light and cannot be adjusted independently.
8. A near-infrared laser heterodyne radiometer based on an external cavity laser according to claim 1, characterized in that, The tunable external cavity laser includes a gain medium, an external cavity structure, a wavelength tuning element, and an optical fiber output interface. The gain medium is used to provide spectral gain for the laser; The external cavity structure adopts a grating-selective external cavity design, which is used to cooperate with the wavelength tuning element to achieve wavelength selection; The wavelength tuning element is used to select a specific wavelength; The laser includes a software control interface, which uses a composite tuning method that combines coarse temperature tuning and fine current tuning to adjust the operating temperature and injection current of the tunable external cavity laser.