A method and device for high-precision measurement of weak light power of an inter-satellite heterodyne laser interferometer

Abstract

The application discloses a kind of inter-satellite heterodyne laser interferometer weak light power high-precision measurement method and device, solve the technical problem that the existing inter-satellite heterodyne laser interferometer weak light power measurement method cannot realize the real-time accurate power solution of strong local reference light and extremely weak signal light.Method includes obtaining measurement weak light signal, reference strong light signal, completes heterodyne interference light signal beam combination;Through four quadrant photodiode photoelectric conversion output four photoelectric current signals;Through ac-dc separation circuit ac-dc separation and transimpedance amplification, output four direct current voltage signals and four alternating current beat frequency voltage amplitude;Digital data is obtained by analog-digital conversion, then preset formula is used in FPGA quadrant solution summation, and total target measurement light power value and total local reference light power value are output.

Description

Technical Field

[0001] This invention relates to the field of satellite precision measurement technology, and in particular to a method and apparatus for high-precision measurement of weak light power in an inter-satellite heterodyne laser interferometer. Background Technology

[0002] In the cutting-edge field of international precision physics measurement, space-based gravitational wave detection is one of the core topics. Typical examples include the European Space Agency's LISA (Laser Interferometer Space Antenna) program and China's "Tianqin" and "Taiji" programs. These space-based gravitational wave detection missions typically deploy three satellites in either Sun or Earth orbit, forming an equilateral triangle constellation with sides ranging from tens of thousands to millions of kilometers long. The space disturbances caused by gravitational waves are extremely weak, manifesting only as tiny changes in inter-satellite distances on the order of picometers. To accurately capture these ultra-tiny changes, the satellites need to establish heterodyne interferometric links using highly coherent lasers, serving as the core technological support for gravitational wave detection.

[0003] While inter-satellite heterodyne interferometry links built upon highly coherent lasers can meet the ranging requirements for gravitational wave detection, the laser beam experiences severe geometric divergence and diffraction losses as it propagates in a vacuum environment hundreds of thousands of kilometers away. This results in the optical power of the transmitting laser, initially at the watt (W) level, being significantly reduced to the nanowatt (nW) or even picowatt (pW) level by the time it reaches the receiving satellite. This extremely weak received optical power directly dictates that inter-satellite heterodyne laser interferometers must solve the challenge of high-precision measurement of weak optical power, which is a crucial prerequisite for ensuring the stability and accuracy of space-based gravitational wave detection links.

[0004] Existing methods for measuring weak light power in inter-satellite heterodyne laser interferometers typically use a local reference light P R Treating it as a static constant (e.g., nominal 100μW), only the AC amplitude is extracted for inverse solving. When the local light experiences a slight RIN jitter, this jitter is injected synchronously into the AC and DC probe links as common-mode interference, directly masking the true physical changes of the extremely weak signal light, making it impossible to achieve real-time and accurate power calculation for both the strong local reference light and the extremely weak signal light. Summary of the Invention

[0005] This invention provides a high-precision measurement method and device for weak light power of inter-satellite heterodyne laser interferometers, which solves the technical problem that existing methods for measuring weak light power of inter-satellite heterodyne laser interferometers cannot achieve real-time and accurate power calculation of strong local reference light and extremely weak signal light.

[0006] The first aspect of this invention provides a high-precision method for measuring weak light power in an inter-satellite heterodyne laser interferometer, comprising:

[0007] Acquire the measurement weak light signal and the reference strong light signal;

[0008] Based on the measured weak light signal and the reference strong light signal, the heterodyne interference light signal beam combining process is completed, and the combined heterodyne interference light signal is output.

[0009] A four-quadrant photodiode is used to perform photoelectric conversion on the combined heterodyne interference light signal to output four photocurrent signals.

[0010] Based on the AC / DC separation circuit, the four photocurrent signals are AC / DC separated and transimpedance amplified to output four DC voltage signals and four AC beat frequency voltage amplitudes.

[0011] Based on a high-precision analog-to-digital converter, the four DC voltage signals and the four AC beat frequency voltage amplitudes are converted from analog to digital to obtain four digital DC voltage data and four digital AC beat frequency voltage data.

[0012] Using a preset measurement optical power analysis formula and a preset reference optical power analysis formula, the FPGA performs quadrant-by-quadrant calculation and summation based on the four channels of digital DC voltage data and the four channels of digital AC beat frequency voltage data, and outputs the total target measurement optical power value and the total local reference optical power value.

[0013] Optionally, the step of performing heterodyne interference signal combining processing based on the measured weak light signal and the reference strong light signal, and outputting the combined heterodyne interference light signal, includes:

[0014] The weak light signal being measured is optically focused, and the focused light signal is output.

[0015] The reference high-intensity light signal is subjected to active frequency shifting processing to output the frequency-shifted local reference light signal;

[0016] The converged measurement optical signal and the frequency-shifted local reference optical signal are spatially combined to output the combined heterodyne interference optical signal.

[0017] Optionally, the AC / DC separation and transimpedance amplification of the four photocurrent signals based on the AC / DC separation circuit, outputting four DC voltage signals and four AC beat frequency voltage amplitudes, includes:

[0018] The four photocurrent signals are respectively input into the AC / DC separation circuit to separate four DC photocurrent components and four AC beat frequency photocurrent components.

[0019] The four DC photocurrent components are amplified transimpedantly according to a preset DC gain to obtain four DC voltage signals.

[0020] The four AC beat frequency photocurrent components are amplified transimpedancely according to the preset AC gain to obtain the four AC beat frequency voltage amplitudes.

[0021] Optionally, the step of performing analog-to-digital conversion on the four DC voltage signals and the four AC beat frequency voltage amplitudes based on a high-precision analog-to-digital converter to obtain four digital DC voltage data and four digital AC beat frequency voltage data includes:

[0022] The four DC voltage signals are respectively input into the high-precision analog-to-digital converter for high-speed digital sampling and signal digitization conversion, and the four digital DC voltage data are output.

[0023] The amplitude values ​​of the four AC beat frequency voltages are respectively input into the high-precision analog-to-digital converter for high-speed digital sampling and signal digitization conversion, and the four digital AC beat frequency voltage data are output.

[0024] Optionally, the step of using a preset measured optical power analysis formula and a preset reference optical power analysis formula to perform FPGA quadrant-by-quadrant calculation and summation based on the four-channel digitized DC voltage data and the four-channel digitized AC beat frequency voltage data, and outputting the total target measured optical power value and the total local reference optical power value, includes:

[0025] The four channels of digital DC voltage data and the four channels of digital AC beat frequency voltage data are substituted into the preset measurement optical power analysis formula and the preset reference optical power analysis formula respectively to perform optical power calculation, so as to obtain the four channels of measurement optical power value and the four channels of reference optical power value.

[0026] The total target measured optical power value is obtained by summing the measured optical power values ​​of the four channels;

[0027] The total local reference optical power value is obtained by summing the power values ​​of the four reference optical paths.

[0028] Optionally, the preset analytical formula for measuring optical power is specifically as follows:

[0029] ;

[0030] in, The measured optical power value is the value of the nth quadrant of the four-quadrant photodetector among the four measured optical power values; The current-to-optical power conversion coefficient of the four-quadrant photodetector; The area of ​​the photosensitive surface in the nth quadrant of the four-quadrant photodetector; This refers to the digital DC voltage data of the nth quadrant of the four-quadrant photodetector in the four-channel digital DC voltage data. This represents the DC gain in the nth quadrant of the four-quadrant photodetector. This refers to the digital AC beat frequency voltage data of the nth quadrant of the four-quadrant photodetector in the four-channel digital AC beat frequency voltage data. This represents the AC gain in the nth quadrant of a four-quadrant photodetector.

[0031] The preset reference optical power analysis formula is as follows:

[0032] ;

[0033] in, This represents the reference optical power value in the nth quadrant of the four-quadrant photodetector among the four reference optical power values.

[0034] The second aspect of this invention provides a high-precision measurement device for weak light power of an inter-satellite heterodyne laser interferometer, comprising:

[0035] The acquisition module is used to acquire the measurement weak light signal, the reference strong light signal, the frequency shift parameters of the acousto-optic modulator, and the beam combining parameters of the beam splitter;

[0036] The beam combining module is used to perform beam combining of heterodyne interference light signals based on the measured weak light signal, the reference strong light signal, the frequency shifting parameters of the acousto-optic modulator and the beam combining parameters of the beam splitter, and output the combined heterodyne interference light signal.

[0037] The photoelectric conversion module is used to perform photoelectric conversion on the combined heterodyne interference light signal using a four-quadrant photodiode, and output four photocurrent signals.

[0038] The amplification module is used to perform AC / DC separation and transimpedance amplification on the four photocurrent signals based on the AC / DC separation circuit, and output four DC voltage signals and four AC beat frequency voltage amplitudes;

[0039] The analog-to-digital conversion module is used to perform analog-to-digital conversion on the four DC voltage signals and the four AC beat frequency voltage amplitudes based on a high-precision analog-to-digital converter, so as to obtain four digital DC voltage data and four digital AC beat frequency voltage data.

[0040] The summation module is used to perform FPGA quadrant-by-quadrant calculation and summation based on the four channels of digital DC voltage data and the four channels of digital AC beat frequency voltage data using a preset measurement optical power analysis formula and a preset reference optical power analysis formula, and outputs the total target measurement optical power value and the total local reference optical power value.

[0041] The third aspect of the present invention provides an electronic device, including a memory and a processor. The memory stores a computer program, and when the computer program is executed by the processor, the processor performs the steps of the high-precision measurement method for weak light power of an inter-satellite heterodyne laser interferometer as described above.

[0042] The fourth aspect of the present invention provides a computer-readable storage medium having a computer program stored thereon, wherein when the computer program is executed, it implements the high-precision measurement method for weak light power of an inter-satellite heterodyne laser interferometer as described above.

[0043] The fifth aspect of the present invention provides a computer program product, the computer program product comprising a computer program stored on a non-transitory computer-readable storage medium, the computer program comprising program instructions, wherein, when the program instructions are executed by a computer, the computer performs the steps of the high-precision measurement method for weak light power of an inter-satellite heterodyne laser interferometer as described above.

[0044] As can be seen from the above technical solutions, the present invention has the following advantages:

[0045] The above-mentioned technical solution of the present invention provides a high-precision measurement method for weak light power of an inter-satellite heterodyne laser interferometer, which acquires a measurement weak light signal and a reference strong light signal; performs heterodyne interference light signal combining processing based on the measurement weak light signal and the reference strong light signal, and outputs the combined heterodyne interference light signal; uses a four-quadrant photodiode to perform photoelectric conversion on the combined heterodyne interference light signal, and outputs four photocurrent signals; based on an AC / DC separation circuit, performs AC / DC separation and transimpedance amplification on the four photocurrent signals, and outputs four DC voltage signals and four AC beat frequency voltage amplitudes; based on a high-precision analog-to-digital converter, performs analog-to-digital conversion on the four DC voltage signals and four AC beat frequency voltage amplitudes, and obtains four digital DC voltage data and four digital AC beat frequency voltage data; and uses a preset measurement light power analysis formula and a preset reference light power analysis formula to calculate the four digital DC voltage data and four digital AC beat frequency data. Voltage data is processed and summed quadrant by quadrant using FPGA to output the total target measured optical power value and the total local reference optical power value. Based on the above scheme, this invention fully preserves and utilizes both the DC and AC components of the heterodyne interference optical signal. Instead of treating the local reference optical power as a static constant, it uses digitized DC voltage data and AC beat frequency voltage data in each quadrant to achieve common-mode subtraction of the AC and DC components through a preset analytical formula. The measured optical power and reference optical power of a single quadrant are calculated in real time and summed. This avoids the defects in existing methods where only the AC amplitude is extracted to solve the power and the reference light is treated as a constant. In such cases, power fluctuations such as the relative intensity noise of the local reference light and the wavelength noise caused by temperature drift are coupled into the signal optical power calculation, masking the real physical changes of the extremely weak signal light and affecting the long-term stability of the calculation. Thus, it achieves real-time accurate power calculation of strong local reference light and extremely weak signal light in an approximately unbiased manner. Attached Figure Description

[0046] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0047] Figure 1 This is a flowchart illustrating the steps of a high-precision measurement method for weak light power using an inter-satellite heterodyne laser interferometer, as provided in Embodiment 1 of the present invention.

[0048] Figure 2 This is a schematic diagram of the system hardware corresponding to the high-precision measurement method for weak light power of an inter-satellite heterodyne laser interferometer provided in Embodiment 1 of the present invention;

[0049] Figure 3 This is a scatter plot of the local optical power calculation residual as a function of the measured optical power, provided in Embodiment 1 of the present invention.

[0050] Figure 4 This is a graph showing the residual scatter envelope of the Monte Carlo simulation signal optical power solution and the variation of theoretical noise with signal optical power, provided in Embodiment 1 of the present invention.

[0051] Figure 5 This is a Monte Carlo simulation scatter plot with envelope of the reference optical power calculation residual as a function of the measured optical power, provided in Embodiment 1 of the present invention.

[0052] Figure 6 The Monte Carlo scatter plot with added noise provided in Embodiment 1 of the present invention reflects the square root trend of the AC and DC components as a function of signal optical power.

[0053] Figure 7 This is a scatter plot of the relative error of the measured optical power calculation residual provided in Embodiment 1 of the present invention.

[0054] Figure 8 The residual plot of the measured optical power solution after adding RIN and temperature drift according to Embodiment 1 of the present invention is used to verify the common-mode immunity of the background fluctuation of the algorithm.

[0055] Figure 9 This invention provides a comparison between the ASD calculated from the measured optical power after adding RIN and temperature drift in Embodiment 1 and the ASD calculated by existing methods.

[0056] Figure 10 This is a structural block diagram of a high-precision measurement device for weak light power of an inter-satellite heterodyne laser interferometer provided in Embodiment 2 of the present invention. Detailed Implementation

[0057] This invention provides a method and apparatus for high-precision measurement of weak light power in an inter-satellite heterodyne laser interferometer, which solves the technical problem that existing methods for measuring weak light power in inter-satellite heterodyne laser interferometers cannot achieve real-time and accurate power calculation of strong local reference light and extremely weak signal light.

[0058] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. It should be noted that in the optional embodiments of the present invention, the object information and other related data involved require the permission or consent of the object when the embodiments of the present invention are applied to specific products or technologies, and the collection, use, and processing of related data must comply with relevant laws, regulations, and standards. That is to say, if the embodiments of the present invention involve data related to the object, it needs to be obtained with the authorization and consent of the object, the authorization and consent of relevant departments, and in compliance with relevant laws, regulations, and standards. If personal information is involved in the embodiments, the acquisition of all personal information requires the consent of the individual. If sensitive information is involved, the separate consent of the information subject is required, and the embodiments also need to be implemented with the authorization and consent of the object.

[0059] Terminology Explanation:

[0060] Heterodyne Interference: This refers to spatially coinciding two coherent laser beams with different frequencies (such as the measurement beam and reference beam in this patent), and using a photodetector to beat the frequency, generating a frequency equal to the frequency difference between the two beams (i.e., the beat frequency ω). IF The technique of alternating interference signals.

[0061] AOM (Acousto-Optic Modulator): An optical device that uses the acousto-optic effect to change the frequency, phase, or intensity of a transmitted light beam. In this patent, it is used to actively shift the frequency of a local reference light to generate the beat frequency required for heterodyne interference.

[0062] BS (Beam Splitter): An optical element that splits a beam of light into two beams (or combines two beams) according to a certain ratio or polarization state. In this patent, it is used to achieve perfect spatial combining of measurement light from a distant source with local reference light.

[0063] QPD (Quadrant Photodiode): A photoelectric sensor that divides the photosensitive surface into four independent physical quadrants in a cross shape. In a spatial interferometer, it is used to convert interference light signals into current signals and simultaneously to calculate the optical power and spatial displacement of the light spot (i.e., DWS (Differential Wavefront Sensing)).

[0064] Local reference light (P) R ) and measurement light (P M ):P R This refers to the intense light (on the order of microwatts) generated by a spacecraft's locally ultrastable laser and used as an interferometric oscillator; P M This refers to the extremely weak target signal light (on the order of nanowatts) that reaches the detector after traveling hundreds of thousands of kilometers between satellites.

[0065] RIN (Relative Intensity Noise): refers to the rapid random fluctuations in the output power of a laser, usually caused by spontaneous emission and pump source fluctuations. It is one of the main sources of interference that mask extremely weak signal light, and it is also the core target of the "common-mode immunity" algorithm of this invention.

[0066] Low-frequency temperature power drift refers to the extremely low-frequency (mHz level) and large-scale (1%~5%) slow change trend of the output optical power of a local laser caused by the actual on-orbit environment (thermal expansion and contraction, etc.).

[0067] Shot noise (quantum shot limit): background current noise caused by the quantum discreteness (particle nature) of photons reaching the detector surface. In a strong local light background, it constitutes an absolute physical measurement lower limit that the system cannot overcome.

[0068] Common-mode interference: Interference signals that act simultaneously and in phase on multiple channels of a system. In this patent, RIN and temperature drift simultaneously cause proportional amplitude fluctuations in both AC and DC channels, hence the term common-mode interference.

[0069] Quantization truncation error / quantization noise: The rounding error introduced by an ADC when converting a continuous analog quantity into a discrete digital quantity due to the limitation of minimum resolution (i.e., the least significant bit (LSB)).

[0070] f 2 Effect (f) 2Noise degradation: In megahertz (MHz) level transimpedance amplifier circuits, due to the presence of detector parasitic junction capacitance, the equivalent input voltage noise of the amplifier is drastically amplified with the square of the frequency.

[0071] ASD (Amplitude Spectral Density): A physical quantity that characterizes the distribution of noise with frequency in the frequency domain, usually measured in pW / In space gravitational wave detection, it is the most authoritative indicator for measuring the low-frequency background noise of an instrument during long-duration flight.

[0072] Iterative divergence: Traditional nonlinear solution algorithms (such as Newton's method) often fail to converge to the true value or tend towards infinity or errors when the signal-to-noise ratio is extremely low or the initial values ​​are poorly set. This invention completely avoids this risk by establishing a quadratic equation with a unique analytical solution.

[0073] Please see Figure 1 , Figure 1 The flowchart illustrates the steps of a high-precision measurement method for weak light power of an inter-satellite heterodyne laser interferometer provided in Embodiment 1 of the present invention.

[0074] This invention provides a high-precision method for measuring weak light power in an inter-satellite heterodyne laser interferometer, comprising:

[0075] Step 101: Obtain the measurement weak light signal and the reference strong light signal.

[0076] It should be noted that in the inter-satellite detection optical path system, the weak light signal is measured by the detection laser emitted by the laser emitting unit at the remote satellite. This laser is transmitted along the long-distance vacuum optical path between the satellites. During the propagation process, it is affected by geometric divergence and diffraction effects, resulting in a significant decrease in optical power. Finally, it arrives at the local detection end and is captured by the optical receiving structure, thus completing the acquisition of the weak light signal. The reference strong light signal is directly generated by a dedicated laser source carried by the local satellite. As the local oscillator, it does not need to be transmitted over long distances across satellites. After generation, it is directly connected to the subsequent optical path link, thus completing the acquisition of the reference strong light signal.

[0077] Step 102: Based on the measured weak light signal and the reference strong light signal, complete the heterodyne interference light signal beam combining process and output the combined heterodyne interference light signal.

[0078] It should be noted that the frequency shift modulation processing of the acquired reference strong light signal is performed based on the preset acousto-optic modulator frequency shift parameters to obtain the frequency-shifted reference strong light signal. Then, the optical path matching and spatial beam combining of the measured weak light signal and the frequency-shifted reference strong light signal are completed according to the preset beam splitter beam combining parameters, so that the two beams produce heterodyne interference coupling effect, and finally generate and output the combined heterodyne interference light signal.

[0079] Furthermore, step 102 may include the following sub-steps:

[0080] S21. Optically converge the weak light signal being measured and output the converged light signal;

[0081] S22. Perform active frequency shifting on the reference strong light signal and output the frequency-shifted local reference light signal;

[0082] S23. Spatial beam combining is performed on the converged measurement optical signal and the frequency-shifted local reference optical signal to output the combined heterodyne interference optical signal.

[0083] It should be noted that, as Figure 2 As shown, this embodiment of the invention provides a high-precision measurement device for weak light power in inter-satellite heterodyne laser interferometry ranging. The hardware system mainly consists of an optical module, a photoelectric detection module, and a digital processing module. The optical module specifically includes a telescope, an optical platform, an ultra-stable laser, an acousto-optic modulator, and a beam splitter. The measurement light from the remote end is converged by the telescope and transmitted to the optical platform. The local reference light is generated by the ultra-stable laser and frequency-shifted by the acousto-optic modulator. The two beams are combined by the beam splitter. The photoelectric detection module specifically includes a four-quadrant photodetector and an AC / DC separation transimpedance amplifier circuit. The digital processing module specifically includes an analog-to-digital converter and a field-programmable gate array (FPGA). The four-quadrant photodetector converts the combined interference light signal into a current signal. The AC / DC separation transimpedance amplifier circuit separates the current signal and converts it into a DC voltage component and an AC beat frequency voltage component. The analog-to-digital converter acquires the AC and DC voltage signals and inputs them to the FPGA for optical power calculation.

[0084] Among them, at the optical front end, the measurement light (signal light, frequency f) transmitted from the far end M After being focused by the telescope, the light enters the optical platform. Within the optical platform, a local ultrastable laser generates a reference beam (local light), which is then frequency-shifted (at a frequency of f0) by an acousto-optic modulator (AOM). R After that, the beam is combined with the incoming measurement beam at the beam splitter (BS) to obtain the combined heterodyne interference light signal.

[0085] In this embodiment, the weak light signal is optically converged to output the converged measurement light signal. In actual execution, a dedicated optical convergence component is used to converge and focus the incident weak light signal, suppressing the divergence phenomenon generated during beam propagation, thereby obtaining the converged measurement light signal. Subsequently, the reference strong light signal is actively frequency-shifted to output the frequency-shifted local reference light signal. By loading preset frequency-shifting parameters through an acousto-optic modulator, the optical frequency of the reference strong light signal is directionally shifted to generate the frequency-shifted local reference light signal with a changed frequency. Then, the converged measurement light signal and the frequency-shifted local reference light signal are spatially combined to output the combined heterodyne interference light signal. The beam splitter completes the optical axis calibration and spatial superposition of the two beams according to the predetermined combining parameters, so that the two coherent beams produce heterodyne interference, forming the combined heterodyne interference light signal. The above optical path processing optimizes the incident state of the weak light and constructs a coherent light combination with a fixed frequency difference, which can ensure that the subsequent photoelectric detection link can effectively distinguish different signal components and reduce the interference caused by local reference light jitter. Among them, the optical converging component is a combination of functional devices composed of various optical lenses. It is mainly used to converge and focus the incident light beam, which can reduce the beam divergence angle, compress the spot area, and improve the reception and coupling effect of weak light signals.

[0086] Step 103: Use a four-quadrant photodiode to perform photoelectric conversion on the combined heterodyne interference light signal and output four photocurrent signals.

[0087] It should be noted that the interference rays after beam combining then enter the photoelectric detection stage. The four-quadrant photodiode in the quadrant photodetector (QPD) converts the received light signal into four raw photocurrent signals (i1, i2, i3, i4).

[0088] Among them, the core device of the four-quadrant photodetector for photoelectric conversion of optical signals is the four-quadrant photodiode. The photodiode is a complete circular semiconductor wafer divided into four quadrants by orthogonal cross lines of equal area. Each quadrant is a 1 / 4 circle and responds to optical signals independently.

[0089] Step 104: Based on the AC / DC separation circuit, the four photocurrent signals are separated into AC and DC signals and amplified by transimpedance to output four DC voltage signals and four AC beat frequency voltage amplitudes.

[0090] It should be noted that the four photocurrent signals are simultaneously fed into the AC / DC separation circuit. The circuit first separates the AC and DC signal components, and then the two types of current components after separation are amplified by the transimpedance amplifier circuit in the four-quadrant photodetector, that is, the transimpedance amplifier performs transimpedance amplification processing on the two types of current components respectively, realizing the conversion of current signals into voltage signals, and then outputting four DC voltage signals and four AC beat frequency voltage amplitudes.

[0091] Furthermore, step 104 may include the following sub-steps:

[0092] S41. Input the four photocurrent signals into the AC / DC separation circuit respectively to separate the four DC photocurrent components and the four AC beat frequency photocurrent components.

[0093] S42. Amplify the four DC photocurrent components across impedance according to the preset DC gain to obtain four DC voltage signals.

[0094] S43. Amplify the four AC beat frequency photocurrent components across the impedance according to the preset AC gain to obtain the four AC beat frequency voltage amplitudes.

[0095] It should be noted that these four photocurrent signals enter an AC / DC separation transimpedance amplifier circuit (including an AC / DC separation circuit and a transimpedance amplifier). In this circuit, each original photocurrent is subjected to negative feedback amplification and AC / DC separation, converting it into a corresponding AC beat frequency voltage signal (V). AC1 To V AC4 ) and DC voltage signal (V DC1 To V DC4 ).

[0096] Among them, the four-quadrant photodetector at the receiver of the inter-satellite laser interferometer will simultaneously receive a relatively strong local reference light (power P). R Typically in the milliwatt or microwatt range, and extremely weak signal light (power in the P range) transmitted from a distant point. M (Typically in the nanowatt range). After the two beams of light interfere, each quadrant of the four-quadrant photodetector converts the received optical signal into a total current signal output i. tot (t). According to the principle of heterodyne interference, this current signal can be expanded into a form containing both DC and AC (beat frequency) components:

[0097] ;

[0098] In the formula, The total photocurrent signal output by a single quadrant of a four-quadrant photodetector is the total current signal containing DC and AC beat frequency components obtained by heterodyne interference of the reference strong light and the measurement weak light received in that quadrant. The total light intensity signal incident on a single quadrant of the four-quadrant photodetector; Use the angular frequency of the reference strong light signal; To measure the angular frequency of weak light signals; The initial phase of the reference strong light signal; To measure the initial phase of the weak light signal; Reference optical power value; To measure optical power;

[0099] Furthermore, the current signal is then converted into a voltage signal via an AC / DC splitter circuit and a transimpedance amplifier. Specifically, the DC photocurrent signal is amplified by a DC transimpedance amplifier circuit (gain M... DC Amplified to a DC voltage component (i.e., DC voltage signal) V DC The AC photocurrent signal is amplified by an AC transimpedance amplifier circuit (gain M). AC Amplified to AC voltage amplitude (i.e., AC beat frequency voltage amplitude) V AC :

[0100] ;

[0101] In the formula, The current-to-optical power conversion coefficient of the photodetector. This represents the photosensitive surface area of ​​a single quadrant of the photodetector. For the DC gain of the photoelectric detection circuit, This represents the AC gain of the photoelectric detection circuit.

[0102] In this embodiment, the four independent photocurrent signals output by the four-quadrant photodetector are synchronously connected to the AC / DC separation circuit through their respective signal channels. The frequency discrimination unit inside the circuit filters and splits each composite photocurrent signal according to the frequency characteristics of the signal, distinguishing the steady-state DC component reflecting the total incident light intensity from the alternating AC component carrying heterodyne interference information, thus obtaining four independent DC photocurrent components and four AC beat frequency photocurrent components. Subsequently, the four DC photocurrent components are transimpeded amplified according to a preset DC gain. The circuit calls the pre-calibrated DC gain parameters and linearly converts and amplifies the low-amplitude DC photocurrent into an analog DC voltage signal of the corresponding amplitude through the transimpedance amplifier, obtaining four DC... The current voltage signal is then amplified across the four AC beat frequency photocurrent components according to a preset AC gain. The circuit is matched with the preset AC gain parameters and performs targeted amplification and conversion for the weak AC beat frequency photocurrent. The current-to-voltage conversion is completed while preserving the original frequency and phase characteristics of the beat frequency signal, resulting in the amplitude of the four AC beat frequency voltages. This processing method of first separating the components and then amplifying them with differentiated gain achieves complete decoupling between the DC substrate signal and the AC interference signal, avoids crosstalk between the two types of signals, and weakens the common-mode interference caused by the jitter of the local reference light AC / DC separation circuit RIN AC / DC separation circuit. This provides a basis for accurately distinguishing the power information of strong local reference light and extremely weak measurement light in the future.

[0103] Step 105: Based on a high-precision analog-to-digital converter, perform analog-to-digital conversion on the four DC voltage signals and the four AC beat frequency voltage amplitudes to obtain four digital DC voltage data and four digital AC beat frequency voltage data.

[0104] It should be noted that the four DC voltage signals output from the AC / DC separation transimpedance amplifier circuit and the four AC beat frequency voltage amplitudes are synchronously connected to the multi-channel input port of the high-precision analog-to-digital converter. The converter performs high-speed sampling and digital quantization encoding on each analog voltage signal according to the preset sampling rate and quantization accuracy, converting the continuously changing analog voltage signal into a discrete digital signal form, thus obtaining four digital DC voltage data and four digital AC beat frequency voltage data.

[0105] Furthermore, step 105 may include the following sub-steps:

[0106] S51. Input the four DC voltage signals into the high-precision analog-to-digital converter for high-speed digital sampling and signal digitization conversion, and output four digital DC voltage data.

[0107] S52. Input the amplitude values ​​of the four AC beat frequency voltages into the high-precision analog-to-digital converter for high-speed digital sampling and signal digitization conversion, and output the four digital AC beat frequency voltage data.

[0108] It should be noted that the four DC voltage signals output from the AC / DC separation transimpedance amplifier circuit are connected to the independent analog input channels of the high-precision analog-to-digital converter. The converter is configured with a preset high sampling rate and high quantization accuracy, and performs high-speed periodic sampling, signal holding, and quantization encoding on each continuous analog DC voltage signal, converting it into a discrete digital signal sequence and outputting four channels of digital DC voltage data. Subsequently, the amplitude values ​​of the four AC beat frequency voltages are respectively input to the high-precision analog-to-digital converter for high-speed digital sampling and signal digitization conversion, outputting four channels of digital AC beat frequency voltage data. Simultaneously, the amplitude values ​​of the four AC beat frequency voltages are connected to the corresponding channels of the converter, and the appropriate sampling clock and sampling window are configured according to the frequency characteristics of the beat frequency signal. The alternating beat frequency voltage signal is synchronously sampled and quantized to fully capture the amplitude characteristics of the signal and output four channels of digital AC beat frequency voltage data.

[0109] Step 106: Using the preset measurement optical power analysis formula and the preset reference optical power analysis formula, perform FPGA quadrant-by-quadrant calculation and summation based on the four-channel digital DC voltage data and the four-channel digital AC beat frequency voltage data, and output the total target measurement optical power value and the total local reference optical power value.

[0110] It should be noted that the FPGA (Field Programmable Gate Array) AC / DC separation circuit receives and buffers the four channels of digital DC voltage data and four channels of digital AC beat frequency voltage data that have been synchronously acquired in real time. The two types of data are paired in a one-to-one correspondence of the four quadrants. For the paired data in each quadrant, the preset measurement optical power analysis formula and the preset reference optical power analysis formula are called to complete the independent optical power calculation. After all the single-channel power data in the four quadrants have been calculated, the measurement optical power data and reference optical power data in all quadrants are accumulated and summed. Finally, the total target measurement optical power value and the total local reference optical power value are summarized and output.

[0111] Furthermore, step 106 may include the following sub-steps:

[0112] S61. Substitute the four-channel digital DC voltage data and the four-channel digital AC beat frequency voltage data into the preset measurement optical power analysis formula and the preset reference optical power analysis formula respectively to calculate the optical power, and obtain the four-channel measurement optical power value and the four-channel reference optical power value.

[0113] S62. Accumulate and calculate the total target measured optical power value by summing the four measured optical power values;

[0114] S63. The total local reference optical power value is obtained by summing the four reference optical power values.

[0115] It should be noted that in the digital processing module, the analog-to-digital converter (ADC) performs high-speed sampling of the aforementioned AC and DC voltage signals and transmits the digital signals to the FPGA board. Inside the FPGA board, a pre-configured optical power calculation algorithm is executed, ultimately accurately calculating and outputting the total local reference optical power value P. R The total target measured optical power value P M .

[0116] Furthermore, such as Figure 3 As shown, during the process of changing the preset received optical power from 0 to 1nW, the DC component V DC The (square-shaped line) voltage remained relatively stable with slight fluctuations between 8.49V and 8.52V, verifying that the DC component is insensitive to changes in weak signal light. Figure 3 As shown, the AC component V AC The triangular broken line shows a trend of nonlinear (square root law) growth with the preset optical power.

[0117] To accurately calculate the optical power P of extremely weak signals M And eliminate reference optical power P R To address the impact of minute fluctuations, this application solves the two sets of voltage equations simultaneously. First, the "power sum" and "power product" of the two light paths are isolated:

[0118] ;

[0119] Introduce intermediate variables, let =A, =B, based on Vieta's formulas, a quadratic equation in one variable concerning optical power can be constructed:

[0120] ;

[0121] According to the quadratic formula, the two roots of the power are obtained as follows: Since the local reference light in this embodiment is a strong light signal, while the measured received light is an extremely weak light signal, the physical conditions must be met. > Therefore, the sign orientation of the root can be uniquely determined: the measured light takes the minus sign, and the reference light takes the plus sign, thus obtaining:

[0122] ;

[0123] ;

[0124] Substituting the actual expressions for intermediate variables A and B into the above results and extracting common factors, we finally obtain a high-precision solution model for the target received weak optical power and the reference optical power. Specifically, the preset analytical formulas for measured optical power and reference optical power are as follows:

[0125] ;

[0126] ;

[0127] in, This refers to the measured optical power value of the nth quadrant of the four-quadrant photodetector among the four measured optical power values, where the subscripts n=1,2,3,4 are the quadrant numbers; The current-to-optical power conversion coefficient of the four-quadrant photodetector; The area of ​​the photosensitive surface in the nth quadrant of the four-quadrant photodetector; This refers to the digital DC voltage data of the nth quadrant of the four-quadrant photodetector in the four-channel digital DC voltage data. This represents the DC gain in the nth quadrant of the four-quadrant photodetector. This refers to the digital AC beat frequency voltage data of the nth quadrant of the four-quadrant photodetector in the four-channel digital AC beat frequency voltage data. This represents the AC gain in the nth quadrant of a four-quadrant photodetector. This represents the reference optical power value in the nth quadrant of the four-quadrant photodetector among the four reference optical power values.

[0128] As described above, this invention substitutes the product of optical power and the sum of optical power into Vieta's formulas to construct rules, establishes a quadratic equation in one variable with the measured optical power as the unknown, and completes the validity verification, outputting a quadratic equation in one variable concerning the measured optical power; based on the quadratic equation in one variable concerning the measured optical power, it derives general analytical formulas for the measured optical power and the reference optical power with positive and negative signs; according to the preset physical constraint that the local reference optical power is much greater than the measured optical power, it determines the value of the positive and negative signs in the general analytical formulas for the measured optical power and the reference optical power, and determines the preset analytical formulas for the measured optical power and the preset analytical formulas for the reference optical power.

[0129] It is worth mentioning that, regarding the fluctuation of the reference optical power:

[0130] ;

[0131] For actual signal optical power: And then P R P M ;

[0132] In the formula, The reference optical power after fluctuation; The reference optical power fluctuation; The DC voltage signal measured during reference optical power fluctuations; The AC beat frequency voltage amplitude was measured when the reference optical power fluctuated. This represents the total conversion factor for the DC channel; is the total conversion coefficient for the AC channel; A and B are auxiliary calculation variables.

[0133] Furthermore, the AC / DC combined nonlinear solution model disclosed in this invention is the core inventive point. Any scheme that employs an equivalent photoelectric sensor (such as an APD (Avalanche Photodiode) or a balanced detector) in the front-end detection stage, uses equivalent frequency division methods (such as Bias-Tee separation or ADC all-digital domain filtering separation) in the signal conditioning stage, employs an equivalent digital processor (such as a DSP (Digital Signal Processor), MCU (Microcontroller Unit), or software post-processing) on ​​the computing power carrier, or performs series approximate expansion and numerical iteration to find the root of the quadratic equation of this invention in mathematical processing, is essentially aimed at using AC / DC components to offset the common-mode interference of local optical fluctuations. The technical problems solved and the technical effects produced are the same as those of this invention. Therefore, all equivalent substitutions of the above-mentioned structures, devices, steps, or algorithm formulas fall entirely within the patent protection scope of this invention.

[0134] For comparison of technical effects, existing technologies can be used as a reference. Taking Sun Yat-sen University's "Tianqin" project as an example, the interstellar arm is approximately 1.7 × 10⁻⁶ meters long. 5 Due to long-distance propagation losses, the actual usable inter-satellite signal power at the receiver is on the order of nanowatts (nW), far lower than that of the local reference light. This weak light field environment, dominated by quantum shot noise, causes severe distortion in traditional methods for extracting weak signals against a strong reference light background. Real-time, high-precision calculation of weak light power is crucial for evaluating inter-satellite link quality, dynamically adjusting phase-locked loop parameters, and calibrating gravitational wave data.

[0135] At the receiver of the heterodyne interferometer, the system introduces a relatively high-power (typically around 100 μW) local reference beam (P0). R The two beams of light with slightly different frequencies are combined at the beam splitter (BS) and received by the four-quadrant photodetector (QPD) to generate a beat frequency current signal.

[0136] Therefore, the purpose of calculating this extremely weak measured optical power in real time and with high precision is:

[0137] (1) Signal-to-Noise Ratio (SNR) Monitoring: In space interferometers, the most fundamental noise limitation is quantum shot noise. The root-mean-square current of the shot noise is proportional to the square root of the received optical power. Shot noise is almost entirely determined by the power of the local light. To accurately assess the phase measurement accuracy (i.e., signal-to-noise ratio) of the link in real time, the P value must be precisely determined. M Real-time values.

[0138] (2) Environmental disturbance calibration: Satellite pointing error and attitude jitter will affect the received signal optical power P M A minute but real fluctuation occurs. High-precision power monitoring can provide the necessary physical calibration basis for subsequent scientific analysis of gravitational wave data.

[0139] However, in high-frequency heterodyne environments ( Under these conditions, the high-frequency f-values ​​are limited by the transimpedance amplifier (TIA). 2 Capacitive coupling voltage noise and laser instability make the accuracy of weak light calculations highly susceptible to interference.

[0140] Furthermore, the traditional single-channel extraction method assumes that the reference light P R For static constants (e.g., nominal 100μW), only the AC amplitude V is extracted. AC Using V AC ∝P R P m Direct inverse solution P m .

[0141] The specific calculation process of the existing scheme is as follows:

[0142] Based on local reference light P R Much larger than the measurement light P m Assuming that existing technology directly applies P R Consider it as an ideal static constant (such as P, which is pre-calibrated on the ground). R_const =100μW). When the circuit measures the AC voltage V AC Then, the measured optical power is directly solved using the following formula:

[0143] ;

[0144] Where V AC This represents the AC amplitude after passing through a four-quadrant photodetector (QPD). For AC transimpedance amplification factor, The responsivity of a photodiode. Let be the pixel area of ​​the photodiode.

[0145] The shortcomings of the existing solutions can be roughly divided into three parts:

[0146] (1) Unable to be immune to fluctuations at the reference light source end: In real satellite orbits, lasers not only have high-frequency relative intensity noise, but also low-frequency power drift due to changes in ambient temperature.

[0147] (2) Distorted logic: Assume P R Because the temperature change drifted by 5%, the corresponding V AC It will also fluctuate accordingly. However, because the single-channel algorithm still substitutes the preset static constant P into the denominator... R_const This will lead to the calculated P m This introduces systematic errors. For weak signals at the nanowatt level, this "false signal" caused by local light will completely mask the actual physical changes.

[0148] (3) Lack of noise immunity model: Existing technologies lack algebraic-level understanding of V DC and V AC The mechanism of dynamic coupling cannot fundamentally eliminate the fluctuations of local light as "common-mode noise".

[0149] Furthermore, while traditional similar patents may extract interference signals, they fail to utilize quadratic equations to algebraically combine the "power sum" and "power product" of AC and DC components at the mathematical level, as this invention does. Therefore, they cannot fundamentally eliminate common-mode interference caused by local optical fluctuations (RIN and temperature drift).

[0150] In summary, while existing technologies can work in a static laboratory environment, their computational accuracy cannot meet the limitations of quantum physics in the real inter-satellite environment of long flight time, variable temperature, and high noise, making it difficult to support the stringent requirements of space gravitational wave detection.

[0151] Therefore, existing technologies have the following main drawbacks in space laser interferometry:

[0152] 1. Inability to resist common-mode interference from source-end fluctuations (poor interference immunity):

[0153] Relative Intensity Noise (RIN) Masking: Locally ultrastable lasers exhibit high-frequency power fluctuations (RIN). Existing techniques typically mask this by masking the local reference beam P... R Treated as a static constant (e.g., nominal 100μW), only the AC amplitude is extracted for inverse solving. When the local light experiences a slight RIN jitter, this jitter is injected synchronously into the AC and DC detection links as common-mode interference, directly masking the true physical changes of the extremely weak signal light.

[0154] Extremely low frequency temperature power drift interference: In a real satellite on-orbit environment, the thermal effect introduced by changes in irradiated position due to satellite attitude changes can cause a large-scale (1%~5%), extremely low frequency (mHz level) slow power drift in local light. Existing technologies, because they reduce P... R Treating it as a constant, once P R Deviation, calculated measured optical power P M This will result in synchronous systematic measurement errors, completely losing the ability to accurately monitor extremely weak signals.

[0155] 2. Stability and accuracy bottlenecks in nonlinear solutions (risk of algorithm iteration divergence):

[0156] Extracting weak signals in a strong local oscillator environment involves complex nonlinear separation. If existing technologies lack effective underlying dimensionality reduction mechanisms, numerical iterative solutions performed in high-speed FPGA hardware are prone to iterative divergence due to extremely low signal-to-noise ratios, leading to program errors or solution delays.

[0157] 3. Deterioration of high-frequency electronic noise at the front end (f2 effect):

[0158] The heterodyne interference beat frequency signal is relatively high (e.g., 15MHz). In existing hardware architectures, the voltage noise of the transimpedance amplifier (TIA) couples with the detector junction capacitance, resulting in a severe f2 noise amplification effect, which further degrades the solution accuracy in the high-frequency band.

[0159] 4. Digital quantization truncation error amplification (quantization noise design mismatch):

[0160] Traditional 14-bit ADCs, when faced with a large DC bias of several volts and a noise floor of microvolts, have a range of approximately ±5V. Their coarse quantization step injects a significant digital truncation error into the system. In current technologies, this quantization noise has not been suppressed below the purely physical lower limits of shot noise and TIA thermal noise, and this error is amplified abnormally by nonlinear extraction algorithms, resulting in the final solution accuracy being limited by the digital truncation.

[0161] Based on the above, this invention mainly solves the problem of high-precision real-time power calculation for extremely weak signal light (optical power in the nanowatts nW to pW level) and strong local reference light (optical power in the hundreds of microwatts μW level) in space laser heterodyne interferometry precision measurements (such as space gravitational wave detection LISA, Tianqin, Taiji Project, etc.).

[0162] In long-baseline heterodyne laser interferometry, the beam divergence and diffraction effects introduced by long-distance inter-satellite transmission result in extremely weak signal power when the signal light emitted by the opposing spacecraft enters the local spacecraft. After heterodyne interference with the strong local reference light, the generated beat frequency interference light signal is an extremely weak AC signal superimposed on a very high DC component. The AC signal beat frequency is the frequency difference between the two laser beams after modulation.

[0163] Because the signal optical power is extremely weak, the AC amplitude loaded on the high DC component is very small. Traditional single-channel extraction methods easily couple DC component noise into the AC signal, leading to distortion in the calculation of the extremely weak signal optical power. Due to factors such as inter-satellite pointing errors and satellite attitude jitter, the optical power P of the signal light... m It changes in real time, shot noise δI shot With signal optical power P m There is δI shot ≈P m It is necessary to monitor optical power in real time to track the link signal-to-noise ratio.

[0164] Specifically, existing technologies cannot effectively overcome the following physical, electronic, and algorithmic bottlenecks that lead to measurement distortion and system crashes:

[0165] 1. Relative Intensity Noise (RIN) Masking: Locally stable lasers have high-frequency power fluctuations (RIN). Although various space gravitational wave detection projects suppress RIN noise in the target heterodyne interference band f when designing laser systems, strong RIN noise coupling still exists at the 2f harmonic. The strong local RIN noise will directly mask the actual extremely weak signal light power changes.

[0166] 2. Extremely Low Frequency Temperature Power Drift Interference: In a real satellite-on-orbit environment, temperature variations in the scientific instruments (local laser) caused by changes in the satellite's irradiated surface result in a large-scale (1%~5%), extremely low-frequency (mHz) power drift in the local reference light. This introduces systematic errors into the calculation of signal light power and local light power. Existing techniques typically treat the reference light as a static constant for calculation, causing any jitter in the local light (including RIN noise and temperature drift) to be incorrectly coupled into the final result, leading to severe measurement distortion.

[0167] 3. Limitations of traditional hardware anti-interference schemes: To suppress the above common-mode interference, traditional methods often rely on laser hardware power-stabilized closed-loop feedback systems or add additional beam splitting monitoring links. This not only significantly increases the payload complexity and power consumption of spacecraft, but also introduces additional optical losses, reduces the already meager signal power, and further reduces the signal-to-noise ratio.

[0168] 4. Stability and accuracy bottlenecks in nonlinear solutions: Under strong local oscillator conditions, the extraction of weak signals involves complex nonlinear separation. Existing single-channel AC extraction techniques, lacking effective underlying dimensionality reduction mechanisms, are prone to iterative divergence or computational lag in high-speed FPGA operations, and struggle to avoid the error amplification effect caused by digital quantization truncation.

[0169] To address the shortcomings of the existing technology, the present invention aims to provide a method and apparatus for high-precision measurement of weak optical power that is completely isomorphic to the existing heterodyne interferometric differential wavefront sensing optical path, requiring no additional optical hardware and exhibiting zero additional optical power loss, as detailed below:

[0170] 1. Completely eliminate common-mode interference at the algorithm level:

[0171] By utilizing the nonlinear mixing effect of heterodyne interference, an algebraic mapping relationship between AC / DC voltage components and optical power is established at the algebraic level of the algorithm. By simultaneously solving the equations of "power sum" and "power product", a univariate quadratic solution model capable of eliminating local optical fluctuations in common mode is constructed, thereby eliminating common-mode interference caused by local strong reference light micro-fluctuations (RIN) and large-scale low-frequency temperature drift from a mathematical perspective.

[0172] 2. Ensure the absolute stability of hardware solutions by incorporating prior physical conditions:

[0173] Creatively introducing the concept that "the local reference optical power is much greater than the measured optical power" The inherent physical constraint of the space interferometer is "(P)". This constraint is used to uniquely determine the sign of the effective analytical solution (P) of the quadratic equation's root-finding formula. M It must be a minus sign for the small root, P R (It must be a large root plus sign) to completely eliminate the divergence risk brought about by complex iterative algorithms and ensure the absolute stability of the algorithm when running at high speed on hardware computing platforms such as FPGA.

[0174] 3. Approaching the physical limit of quantum shot noise:

[0175] Utilizing the nonlinear physical attenuation characteristics of errors using an analytical model (front-end AC electronic noise transmitted to P) M The coefficient is approximately P M P R (compressed by approximately several hundred times), greatly compressing the f of the front-end transimpedance amplifier circuit. 2 Electronic noise.

[0176] By combining 18-bit high-precision quantization to eliminate the truncation effect, and by reasonably setting the AC / DC separation transimpedance amplifier circuit to reduce the ADC range, the digital quantization noise is strictly suppressed below the front-end physical quantum background.

[0177] Ultimately, this allows the system's solution accuracy to overcome the limitations imposed by power fluctuations at the laser end, and the solution residuals to converge strictly to the physical quantum shot limit, meeting the stringent requirements of space gravitational wave detection for monitoring optical power limits.

[0178] Specifically, in space-based gravitational wave detection and next-generation gravity satellite missions, inter-satellite laser interferometry (LIA) is the core technology for achieving ultra-high precision inter-satellite distance measurements. Taking a typical space-based gravitational wave detection mission as an example, the European Space Agency's LISA uses an equilateral triangular constellation with inter-satellite arms approximately 2.5 × 10⁻⁶ meters long. 6 Each satellite emits approximately 2W of 1064nm laser light, with a distance of km, but the actual signal power received by the remote telescope is only 250pW; the arm length of Sun Yat-sen University's "Tianqin" project is approximately 1.7×10 km. 5 km, simulation and optical design show that the optical power of the remote received signal is approximately 78nW; the baseline arm length of the Chinese Academy of Sciences' "Taiji" scheme is approximately 3×10 km. 6 The local laser power at the telescope entrance is approximately 2W, while the received signal power is approximately 100~300pW. It is evident that for space gravitational wave constellations such as LISA, Tianqin, and Taiji, even with watt-level lasers at the transmitting end, the actual usable inter-satellite signal power at the receiving end is generally in the nanowatt (nW) or even picowatt (pW) range due to severe beam divergence and diffraction losses caused by propagation over hundreds of thousands to millions of kilometers. This is far lower than the mW-level optical power in conventional laboratory interferometers, causing the inter-satellite laser link to operate under extremely low photon count and "weak light" conditions for extended periods.

[0179] Under such extremely low light conditions, the necessity of this invention for high-precision calculation of low light power is reflected in the following two aspects:

[0180] First, the signal light power is extremely weak, and shot noise dominates. Considering the received light power from the aforementioned space gravitational wave project, the dominant noise source in the photoelectric conversion stage is no longer electronic noise, but rather the shot noise of the photons themselves. This weak light field environment, dominated by shot noise, causes severe distortion in traditional methods of directly extracting weak signal light power against a strong reference light background, making it difficult to accurately separate the contribution of extremely weak signal light. Therefore, a novel method is urgently needed to extract weak light power with high precision through the calculation of AC and DC components.

[0181] Second, because shot noise is dominant, the system must accurately calculate the signal optical power in real time. For the ideal photocurrent I... total Its shot noise current variance satisfies the formula Where e is the elementary charge, To measure bandwidth. Due to photocurrent I total With received optical power P R Proportional to the square root of the shot noise current, we can deduce that the root mean square value of the shot noise current is proportional to the square root of the optical power, i.e., δI.Shot ≈P R Therefore, it can be concluded that the phase measurement signal-to-noise ratio of the system approximately increases with P under the shot noise limit. R Changes. In actual on-orbit operation, due to factors such as inter-satellite pointing errors and attitude jitter, the weak signal optical power P... R The noise level will change dynamically in real time, directly altering the background level of the system's shot noise. Without real-time monitoring, the static power values ​​estimated from the ground alone will not reflect the true on-orbit noise level. Therefore, in order to assess the quality of inter-satellite links online, dynamically adjust phase-locked loop parameters, and provide a reliable basis for subsequent gravitational wave data analysis and noise spectrum modeling, it is essential to perform real-time, high-precision calculation and monitoring of changes in the power of extremely weak signal light.

[0182] Specifically, this invention first acquires heterodyne interference optical signals: the measurement light is focused through a telescope and spatially combined with the local frequency-shifted reference light. After photoelectric conversion and AC / DC separation and transimpedance amplification, digital acquisition is performed to obtain the amplitudes of the DC voltage component and AC beat frequency voltage component of the original observation signal. Features are extracted and equations are constructed: based on the heterodyne interference beat frequency physical model, a mapping relationship is established between the DC voltage component and the "power sum" of the reference light and the measurement light, as well as a mapping relationship between the amplitude of the AC beat frequency voltage component and the "power product" of the two beams. Thus, a univariate quadratic solution model for the measured weak light power is constructed. The target weak light power is solved: using the univariate quadratic solution model, and combined with the prior physical condition that the local reference light is strong and the received measurement light is extremely weak (i.e., the reference light power is greater than the measurement light power), the sign orientation in the root-solving formula is uniquely determined, thereby solving the precise analytical expression of the target received measurement light power. The analytical expression is executed using a digital solution platform such as a field-programmable gate array (FPGA), and the measured light power value received by the target is calculated and output in real time.

[0183] As can be seen from the above, the beneficial effects of the present invention include at least the following:

[0184] Eliminating common-mode interference using nonlinear mixing effects: This application utilizes the nonlinear mixing effect in the heterodyne interference principle to establish an algebraic mapping relationship between AC / DC voltage components and optical power at the algorithm level. By simultaneously solving the equations for "power sum" and "power product", the common-mode interference caused by the small fluctuations of the local strong reference light is directly eliminated through mathematical derivation, breaking through the bottleneck of separating extremely weak nanowatt-level signals in the context of strong local oscillator.

[0185] Achieving highly reliable analytical solutions by incorporating prior physical conditions: This method transforms the complex nonlinear separation problem into a standard quadratic equation problem and creatively introduces the inherent physical constraint of the space interferometer: "the local reference optical power is much greater than the measured optical power." This condition uniquely determines the effective solution of the root-finding formula, ensuring the stability of the algorithm during high-speed operation on hardware computing platforms such as FPGAs, and avoiding the divergence risk of traditional iterative measurement algorithms.

[0186] Zero additional optical loss and ultimate measurement accuracy: This solution eliminates the need for additional beam splitting or detection devices in the existing ranging optical path, completely avoiding secondary attenuation of precious, extremely weak signal light. System simulation tests have verified that the algorithm exhibits excellent linearity within the received optical power range of 0.05~1nW; its solution residual is strictly limited to the picowatt (pW) level (maximum deviation not exceeding 3pW), and the relative error across the entire range remains within 0.65%, fully meeting the stringent accuracy requirements for real-time monitoring of optical power in space gravitational wave detection under extremely low photon number environments.

[0187] Compared with traditional technologies, the present invention has the following significant advantages:

[0188] First, the algorithm eliminates common-mode interference at the underlying level, achieving high-precision real-time calculation in extremely weak light conditions while ensuring absolute stability of the FPGA. This invention abandons the traditional approach of treating the reference light as a static constant. By constructing a nonlinear algebraic mapping model between AC / DC voltage components and optical power (i.e., simultaneously solving the "power sum" and "power product"), it achieves real-time accurate power calculation for strong local reference light and extremely weak signal light. In terms of causality, this algebraic combination perfectly offsets and eliminates common-mode interference caused by fluctuations in local strong reference light power (such as high-frequency RIN and low-frequency temperature drift) from a mathematical perspective. More importantly, when solving the quadratic equation, this invention creatively introduces the inherent physical constraint that "the reference light power is much greater than the measured light power," thereby uniquely determining the sign orientation of the analytical solution. This transforms the originally complex nonlinear separation process into an explicit analytical calculation, ensuring absolute stability during high-speed calculations on hardware platforms such as FPGAs, and completely avoiding the risk of iterative divergence and system crashes that traditional numerical iterative algorithms are prone to at extremely low signal-to-noise ratios.

[0189] Second, the hardware architecture is completely isomorphic to existing devices, achieving "zero additional optical cost" and avoiding beam splitting loss. In missions such as space gravitational wave detection, the received signal light has already attenuated to the nanowatt or even picowatt level, and beam splitting loss will further reduce the signal-to-noise ratio of the back-end sampling data. The measurement device of this invention is completely isomorphic to existing inter-satellite heterodyne interferometry devices, that is, it relies purely on deep digital mining of the existing AC and DC current signals output by the standard four-quadrant detector (QPD) and transimpedance amplifier. This purely digital solution does not require the insertion of any additional beam splitters, waveplates, or independent optical power meters into the original precision optical path, thereby completely avoiding secondary beam splitting loss of the extremely precious weak signal light, and achieving optical power monitoring that reaches the physical limits without increasing the complexity of any spacecraft optical payload.

[0190] In addition, such as Figures 4-9 As shown, Figure 4 The diagram shows the envelope of the residual points from the Monte Carlo simulation of signal optical power and the variation of theoretical noise with signal optical power. The simulation results verify that the trend of the residual change with the preset measured optical power conforms to basic physical laws, and that shot noise is proportional to the measured optical power by the square root, which is consistent with the theory. Furthermore, the Monte Carlo simulation points of the residual are distributed within the theoretical envelope in a 3-fold pattern, which verifies the budget allocation of various power residual noises in this embodiment during the solution process.

[0191] Figure 5 The Monte Carlo simulation scatter plot with envelope plot shows the variation of the reference optical power calculation residual with the measured optical power. It can be seen that the calculation residual of the reference optical power can maintain a relative calculation error of less than 2% above the preset measured optical power of 0.5nW, which verifies the ability of the calculation algorithm to resist local optical RIN noise and low-frequency temperature drift noise.

[0192] Figure 6 The Monte Carlo scatter plots with added noise show the square root trend of the AC and DC components as a function of signal optical power. It can be seen that the scatter plots of the Monte Carlo simulation are in close agreement with the theoretical mean curve, which confirms the noise resistance and solution accuracy of the solution algorithm.

[0193] Figure 7 The scatter plot of the relative error of the optical power measurement residual is shown. The relative error of the optical power measurement is extremely small, below 0.003%.

[0194] Figure 8 The residual plot of the measured optical power solution after adding RIN and temperature drift is shown, which verifies the common-mode immunity to background fluctuations of the algorithm.

[0195] Figure 9The invention demonstrates a comparison between the ASD calculation residual after adding RIN and temperature drift mitigation and the ASD calculation of existing methods; it can be seen that the invention is significantly superior to existing solutions in resisting optical power calculation noise caused by low-frequency fluctuations.

[0196] In this embodiment of the invention, the above-mentioned technical solution provides a high-precision measurement method for weak light power of an inter-satellite heterodyne laser interferometer. The method involves: acquiring the original light signal from the inter-satellite heterodyne laser interferometer by focusing the measurement light transmitted from a distant point using a telescope and spatially combining it with a frequency-shifted local reference light; photoelectric conversion and conditioning: receiving the combined interference light signal using a photodetector and converting it into a current signal, then separating and amplifying the current signal to obtain the amplitudes of the DC voltage component and the AC beat frequency voltage component; digital acquisition and power calculation: digitally acquiring the DC voltage component and the AC beat frequency voltage component, constructing a weak light power calculation model based on the amplitudes of the DC voltage component and the AC beat frequency voltage component using the heterodyne interferometer beat frequency physical model; and calculating the measured light power value received by the target using the weak light power calculation model. The acquisition of the original optical signal from the inter-satellite heterodyne laser interferometry and its photoelectric conversion specifically includes: after the measurement light enters the optical platform, it is combined with the reference light generated by the local ultra-stable laser and frequency-shifted by the acousto-optic modulator through a beam splitter; the interference optical signal is converted into a current signal output by a four-quadrant photodetector; the current signal is then separated and amplified, specifically including: using an AC / DC separation transimpedance amplifier circuit to separate the current signal into AC and DC signals, extracting the DC current signal and the AC beat frequency current signal respectively, and then converting them into a DC voltage signal V through transimpedance amplification. DCn With AC beat frequency voltage signal amplitude V ACn The digital acquisition and power calculation process specifically includes: DC voltage signals and AC beat frequency voltage signals are acquired by an analog-to-digital converter and then input into a field-programmable gate array (FPGA) board. The FPGA board executes a weak light power calculation model. This model isolates the power sum and product of the two light sources by solving simultaneous voltage equations, constructing a quadratic equation. Based on the prior physical condition that the local reference light power is greater than the remote measured light power, the sign of the root-solving formula is determined, ultimately calculating the total reference light power P received by the four-quadrant photodetector. R With the measurement of total optical power P M .

[0197] Based on the above scheme, this invention acquires inter-satellite heterodyne interference optical signals, extracts the DC voltage component and AC beat frequency voltage component amplitude of the original observation signal through photoelectric conversion and AC / DC separation; based on the heterodyne interference beat frequency physical model, a univariate quadratic solution model for the measured weak light power is constructed; combined with the physical prior condition that the local reference light power is greater than the measured light power, the measured light power received by the target is accurately analyzed. This application utilizes nonlinear mixing effects to eliminate common-mode interference caused by local strong light fluctuations at the algorithm level, without adding additional optical hardware and losses. It exhibits excellent linearity under extremely weak light conditions at the nanowatt (nW) level, with the solution residual controlled within 3pW and the relative error across the entire range less than 0.65%, meeting the stringent requirements for light power monitoring in extremely low photon number environments such as space gravitational wave detection.

[0198] Please see Figure 10 , Figure 10 This is a structural block diagram of a high-precision measurement device for weak light power of an inter-satellite heterodyne laser interferometer provided in Embodiment 2 of the present invention.

[0199] This invention provides a high-precision measurement device for weak light power of an inter-satellite heterodyne laser interferometer, comprising:

[0200] Acquisition module 1001 is used to acquire the measurement weak light signal and the reference strong light signal;

[0201] The beam combining module 1002 is used to perform beam combining of heterodyne interference light signals based on the measured weak light signal and the reference strong light signal, and output the combined heterodyne interference light signal.

[0202] The photoelectric conversion module 1003 is used to perform photoelectric conversion on the combined heterodyne interference light signal using a four-quadrant photodiode, and output four photocurrent signals.

[0203] Amplification module 1004 is used to perform AC / DC separation and transimpedance amplification on four photocurrent signals based on AC / DC separation circuit, and output four DC voltage signals and four AC beat frequency voltage amplitudes.

[0204] The analog-to-digital conversion module 1005 is used to perform analog-to-digital conversion on four DC voltage signals and four AC beat frequency voltage amplitudes based on a high-precision analog-to-digital converter, so as to obtain four digital DC voltage data and four digital AC beat frequency voltage data.

[0205] The summation module 1006 is used to perform FPGA quadrant-by-quadrant calculation and summation based on four channels of digital DC voltage data and four channels of digital AC beat frequency voltage data using a preset measurement optical power analysis formula and a preset reference optical power analysis formula, and outputs the total target measurement optical power value and the total local reference optical power value.

[0206] Those skilled in the art will clearly understand that, for the sake of convenience and brevity, the specific working process of the above-described device and module can be referred to the corresponding process in the foregoing method embodiments, and will not be repeated here.

[0207] This invention also provides a computer device, including a memory and a processor, wherein the memory stores a computer program; when the computer program is executed by the processor, the processor performs the steps of the low-precision measurement method for inter-satellite heterodyne laser interferometers as described in the above embodiments.

[0208] This invention also provides a computer-readable storage medium storing a computer program / instruction thereon, which, when executed by a processor, implements the steps of the high-precision measurement method for weak light power of an inter-satellite heterodyne laser interferometer as described in the above embodiments.

[0209] This invention also provides a computer program product, including a computer program stored on a non-transitory computer-readable storage medium. The computer program includes program instructions, wherein when the program instructions are executed by a computer, the computer performs the steps of high-precision measurement of weak light power of an inter-satellite heterodyne laser interferometer as described in the above embodiments.

[0210] In the several embodiments provided in this application, it should be understood that the disclosed apparatus and methods can be implemented in other ways. For example, the apparatus embodiments described above are merely illustrative; for instance, the division of units is only a logical functional division, and in actual implementation, there may be other division methods. For example, multiple units or components may be combined or integrated into another system, or some features may be ignored or not executed. Furthermore, the coupling or direct coupling or communication connection shown or discussed may be through some interfaces; the indirect coupling or communication connection between apparatuses or units may be electrical, mechanical, or other forms.

[0211] The units described as separate components may or may not be physically separate. The components shown as units may or may not be physical units; that is, they may be located in one place or distributed across multiple network units. Some or all of the units can be selected to achieve the purpose of this embodiment according to actual needs.

[0212] Furthermore, the functional units in the various embodiments of the present invention can be integrated into one processing unit, or each unit can exist physically separately, or two or more units can be integrated into one unit. The integrated unit can be implemented in hardware or as a software functional unit.

[0213] If the integrated unit is implemented as a software functional unit and sold or used as an independent product, it can be stored in a computer-readable storage medium. Based on this understanding, the technical solution of the present invention, in essence, or the part that contributes to the prior art, or all or part of the technical solution, can be embodied in the form of a software product. This computer software product is stored in a storage medium and includes several instructions to cause a computer device (which may be a personal computer, server, or network device, etc.) to execute all or part of the steps of the methods of the various embodiments of the present invention. The aforementioned storage medium includes various media capable of storing program code, such as USB flash drives, portable hard drives, read-only memory (ROM), random access memory (RAM), magnetic disks, or optical disks.

[0214] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A high-precision method for measuring weak light power in an inter-satellite heterodyne laser interferometer, characterized in that, include: Acquire the measurement weak light signal and the reference strong light signal; Based on the measured weak light signal and the reference strong light signal, the heterodyne interference light signal beam combining process is completed, and the combined heterodyne interference light signal is output. A four-quadrant photodiode is used to perform photoelectric conversion on the combined heterodyne interference light signal to output four photocurrent signals. Based on the AC / DC separation circuit, the four photocurrent signals are AC / DC separated and transimpedance amplified to output four DC voltage signals and four AC beat frequency voltage amplitudes. Based on a high-precision analog-to-digital converter, the four DC voltage signals and the four AC beat frequency voltage amplitudes are converted from analog to digital to obtain four digital DC voltage data and four digital AC beat frequency voltage data. The FPGA performs quadrant-by-quadrant calculation and summation based on the four channels of digital DC voltage data and the four channels of digital AC beat frequency voltage data using a preset measurement optical power analysis formula and a preset reference optical power analysis formula, and outputs the total target measurement optical power value and the total local reference optical power value.

2. The high-precision measurement method for weak light power of an inter-satellite heterodyne laser interferometer according to claim 1, characterized in that, The step of performing heterodyne interference signal combining processing based on the measured weak light signal and the reference strong light signal, and outputting the combined heterodyne interference light signal, includes: The weak light signal being measured is optically focused, and the focused light signal is output. The reference high-intensity light signal is subjected to active frequency shifting processing to output the frequency-shifted local reference light signal; The converged measurement optical signal and the frequency-shifted local reference optical signal are spatially combined to output the combined heterodyne interference optical signal.

3. The method for high-precision measurement of weak light power in an inter-satellite heterodyne laser interferometer according to claim 1, characterized in that, The AC / DC separation circuit performs AC / DC separation and transimpedance amplification on the four photocurrent signals, outputting four DC voltage signals and four AC beat frequency voltage amplitudes, including: The four photocurrent signals are respectively input into the AC / DC separation circuit to separate four DC photocurrent components and four AC beat frequency photocurrent components. The four DC photocurrent components are amplified transimpedantly according to a preset DC gain to obtain four DC voltage signals. The four AC beat frequency photocurrent components are amplified transimpedancely according to the preset AC gain to obtain the four AC beat frequency voltage amplitudes.

4. The high-precision measurement method for weak light power of an inter-satellite heterodyne laser interferometer according to claim 1, characterized in that, The high-precision analog-to-digital converter performs analog-to-digital conversion on the four DC voltage signals and the four AC beat frequency voltage amplitudes to obtain four channels of digital DC voltage data and four channels of digital AC beat frequency voltage data, including: The four DC voltage signals are respectively input into the high-precision analog-to-digital converter for high-speed digital sampling and signal digitization conversion, and the four digital DC voltage data are output. The amplitude values ​​of the four AC beat frequency voltages are respectively input into the high-precision analog-to-digital converter for high-speed digital sampling and signal digitization conversion, and the four digital AC beat frequency voltage data are output.

5. The method for high-precision measurement of weak light power in an inter-satellite heterodyne laser interferometer according to claim 1, characterized in that, The process employs a preset measurement optical power analysis formula and a preset reference optical power analysis formula to perform FPGA quadrant-by-quadrant calculation and summation based on the four channels of digitized DC voltage data and the four channels of digitized AC beat frequency voltage data, outputting the total target measurement optical power value and the total local reference optical power value, including: The four channels of digital DC voltage data and the four channels of digital AC beat frequency voltage data are substituted into the preset measurement optical power analysis formula and the preset reference optical power analysis formula respectively to perform optical power calculation, so as to obtain the four channels of measurement optical power value and the four channels of reference optical power value. The total target measured optical power value is obtained by summing the measured optical power values ​​of the four channels; The total local reference optical power value is obtained by summing the power values ​​of the four reference optical paths.

6. The high-precision measurement method for weak light power of an inter-satellite heterodyne laser interferometer according to claim 5, characterized in that, The preset analytical formula for measuring optical power is as follows: ； in, The measured optical power value is the value of the nth quadrant of the four-quadrant photodetector among the four measured optical power values; The current-to-optical power conversion coefficient of the four-quadrant photodetector; The area of ​​the photosensitive surface in the nth quadrant of the four-quadrant photodetector; This refers to the digital DC voltage data of the nth quadrant of the four-quadrant photodetector in the four-channel digital DC voltage data. This represents the DC gain in the nth quadrant of the four-quadrant photodetector. This refers to the digital AC beat frequency voltage data of the nth quadrant of the four-quadrant photodetector in the four-channel digital AC beat frequency voltage data. This represents the AC gain in the nth quadrant of a four-quadrant photodetector. The preset reference optical power analysis formula is as follows: ； in, This represents the reference optical power value in the nth quadrant of the four-quadrant photodetector among the four reference optical power values.

7. A high-precision measurement device for weak light power of an inter-satellite heterodyne laser interferometer, characterized in that, include: The acquisition module is used to acquire the measurement weak light signal and the reference strong light signal; The beam combining module is used to perform beam combining of heterodyne interference light signals based on the measured weak light signal and the reference strong light signal, and output the combined heterodyne interference light signal. The photoelectric conversion module is used to perform photoelectric conversion on the combined heterodyne interference light signal using a four-quadrant photodiode, and output four photocurrent signals. The amplification module is used to perform AC / DC separation and transimpedance amplification on the four photocurrent signals based on the AC / DC separation circuit, and output four DC voltage signals and four AC beat frequency voltage amplitudes; The analog-to-digital conversion module is used to perform analog-to-digital conversion on the four DC voltage signals and the four AC beat frequency voltage amplitudes based on a high-precision analog-to-digital converter, so as to obtain four digital DC voltage data and four digital AC beat frequency voltage data. The summation module is used to perform FPGA quadrant-by-quadrant calculation and summation based on the four channels of digital DC voltage data and the four channels of digital AC beat frequency voltage data using a preset measurement optical power analysis formula and a preset reference optical power analysis formula, and outputs the total target measurement optical power value and the total local reference optical power value.

8. An electronic device, characterized in that, The system includes a memory and a processor. The memory stores a computer program, which, when executed by the processor, causes the processor to perform the steps of the low-light power measurement method for inter-satellite heterodyne laser interferometers as described in any one of claims 1-6.

9. A computer-readable storage medium having a computer program stored thereon, characterized in that, When the computer program is executed, it implements the high-precision measurement method for weak light power of inter-satellite heterodyne laser interferometer as described in any one of claims 1-6.

10. A computer program product, characterized in that, The computer program product includes a computer program stored on a non-transitory computer-readable storage medium, the computer program including program instructions, wherein when the program instructions are executed by a computer, the computer performs the steps of the high-precision measurement method for weak light power of an inter-satellite heterodyne laser interferometer as described in any one of claims 1-6.

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