Local oscillator power adaptive adjustment system and method in coherent free space light receiving facing atmospheric scintillation

Abstract

This application relates to the field of optical communication technology and discloses an adaptive local oscillator power adjustment system and method for coherent free-space optical reception in response to atmospheric scintillation. The coherent optical signal receiver includes a received signal monitoring module, a local oscillator power adjustment module, and a control module. The control module stores the minimum and maximum received optical power of the link, the maximum operating local oscillator power that ensures system performance meets a preset threshold at the minimum received optical power, and the critical received optical power that allows the analog-to-digital converter module to operate at full amplitude or the coherent optical signal receiver to reach a preset saturation criterion at the given local oscillator power. When the critical received optical power is not less than the maximum received optical power, a fixed local oscillator power is used. When it is less, the monitoring module obtains the current received optical power in real time. When the current received optical power is not greater than the critical received optical power, the local oscillator power remains at the maximum operating local oscillator power; when it is greater, it decreases accordingly. This increases the maximum tolerable received optical power while maintaining weak signal reception performance.

Description

Technical Field

[0001] This application relates to the field of optical communication technology, and in particular to a technique for adjusting the local oscillator power at the receiver in free-space coherent optical communication. Background Technology

[0002] Free-space optical communication uses the atmosphere as the transmission medium to establish an optical link between the transmitting and receiving ends. In coherent reception, the received signal light is optically mixed with the local oscillator light and then converted into an analog electrical signal. This signal is then recovered and demodulated through transimpedance amplification, analog-to-digital conversion, and digital signal processing. Unlike fiber optic links, the optical signal in a free-space link needs to traverse an atmospheric channel. Random fluctuations in the refractive index caused by atmospheric turbulence result in random fluctuations in the optical power at the receiving end over time. Even if the output optical power at the transmitting end remains constant, the received optical power will still exhibit alternating deep fading and burst enhancement, with both the amplitude and timing of these fluctuations being random.

[0003] Under the above conditions, the range of input optical power that allows the receiver to operate normally is limited in two ways: when the received optical power is low, the system performance is constrained by the receiver sensitivity; when the received optical power is high, the transimpedance amplifier (TIA) at the receiver front end may enter the saturation region, and the analog-to-digital conversion module may also reach full amplitude and experience clipping, causing the sampled waveform to be truncated and the received signal quality to deteriorate. Since optimizing one operating state often sacrifices performance in another, the receiver, under a given configuration, can usually only meet performance requirements within a narrow range of input optical power, making it difficult to adapt to the wide-range, time-varying changes in received optical power caused by atmospheric scintillation. To mitigate the effects of atmospheric turbulence, related technologies include adaptive optics, spatial diversity, and complex mode control. However, these approaches typically require the introduction of corresponding correction or diversity structures at the receiver, placing high demands on system configuration and implementation. Therefore, further improvements to these technologies are still needed. Summary of the Invention

[0004] The purpose of this application is to provide a local oscillator power adaptive adjustment system and method for coherent free-space optical reception oriented to atmospheric scintillation, so as to solve the problem that the range of usable input optical power at the receiver is limited under the condition that atmospheric scintillation causes wide time-varying received optical power.

[0005] To achieve the above objectives, this application provides a local oscillator power adaptive adjustment system for coherent free-space optical reception oriented towards atmospheric scintillation, applied to free-space coherent optical communication, comprising: a coherent optical signal transmitter for generating and transmitting an optical signal to be transmitted; a free-space transmission link for carrying the propagation of the optical signal in an atmospheric channel; and a coherent optical signal receiver for receiving the signal light transmitted via the free-space transmission link. The coherent optical signal receiver includes a received signal light monitoring module, a local oscillator light generation module, a local oscillator light power adjustment module, a control module, an integrated coherent receiving module, an analog-to-digital conversion module, and a digital signal demodulation module.

[0006] The received signal light monitoring module is used to monitor the received signal light to obtain the current received optical power. P r ( t The local oscillator light generation module is used to generate local oscillator light that is coherently mixed with the received signal light; the local oscillator light power adjustment module is used to adjust the local oscillator light power before entering the integrated coherent receiving module; the integrated coherent receiving module is used to optically mix the received signal light with the local oscillator light and complete photoelectric conversion to output an analog electrical signal; the analog-to-digital conversion module is used to convert the analog electrical signal into a digital signal; the digital signal demodulation module is used to recover and demodulate the digital signal.

[0007] The control module stores preset system design parameters, including the minimum received optical power in the free space transmission link. P low The highest received optical power in the free space transmission link. P high When the received optical power is the minimum received optical power P low The maximum operating local oscillator power that allows the system performance to meet a preset threshold under certain conditions. P LO,max ; and the local oscillator power is the maximum operating local oscillator power. P LO,max The critical received optical power corresponding to when the analog-to-digital conversion module reaches full amplitude or the coherent optical signal receiver reaches a preset saturation criterion. P full .

[0008] When the critical received optical power P full Not less than the maximum received optical power P high At that time, the coherent optical signal receiver operates with a fixed local oscillator power.

[0009] When the critical received optical power P full Less than the maximum received optical power P high At that time, the control module determines the current received optical power. P r ( t The maximum operating local oscillator power P LO,max and the critical received optical power P full The local oscillator optical power adjustment module is controlled to adaptively adjust the local oscillator optical power before it enters the integrated coherent receiving module, wherein: when the current received optical power... P r ( t The power received is not greater than the critical received optical power. P full At that time, the local oscillator optical power before entering the integrated coherent receiving module remains at the maximum operating local oscillator optical power. P LO,max When the current received optical power P r ( t () greater than the critical received optical power P full At that time, the local oscillator optical power before entering the integrated coherent receiving module is related to the current received optical power. P r ( t ) decreases as it increases.

[0010] In some embodiments, the received signal light monitoring module includes: a beam splitter for splitting a portion of the optical power from the received signal light to form a monitoring branch; an optical power detection unit for detecting the optical power in the monitoring branch; and a power conversion unit for converting the optical power in the monitoring branch into the current received optical power in the main receiving branch based on the splitting ratio of the beam splitter and a fixed link loss. P r ( t ).

[0011] In some embodiments, the local oscillator power adjustment module includes at least one of a variable optical attenuator, a laser driver adjustment unit, and a tunable amplification unit.

[0012] This application also provides a method for adaptive adjustment of local oscillator power in coherent free-space optical reception oriented towards atmospheric scintillation, comprising the following steps: obtaining the range of received optical power variation in the actual free-space link, and obtaining the minimum received optical power. P lowand maximum received optical power P high Construct a design platform identical to the actual receiving system, and install a first optical attenuation module in the received signal optical branch of the design platform, and a second optical attenuation module in the local oscillator optical branch of the design platform; use the first optical attenuation module to adjust the received signal optical power to the minimum received optical power. P low The local oscillator power is gradually increased using the second optical attenuation module until the system performance meets a preset threshold. The local oscillator power before entering the integrated coherent receiver module at this point is determined as the maximum operating local oscillator power. P LO,max Maintain the local oscillator power before entering the integrated coherent receiving module at the maximum operating local oscillator power. P LO,max The received optical power is gradually increased using the first optical attenuation module until the analog-to-digital conversion module reaches full amplitude or the coherent optical signal receiver reaches a preset saturation criterion. The received optical power at this point is then determined as the critical received optical power. P full Compare the critical received optical power. P full With the highest received optical power P high When the critical received optical power P full Not less than the maximum received optical power P high At that time, the coherent optical signal receiver operates with a fixed local oscillator power; when the critical received optical power... P full Less than the maximum received optical power P high During actual communication, the current received optical power is obtained in real time through the received signal optical monitoring module. P r ( t The control module determines the current received optical power based on this information. P r ( t The maximum operating local oscillator power P LO,max and the critical received optical power P full The local oscillator power adjustment control quantity is generated to adaptively adjust the local oscillator power before it enters the integrated coherent receiving module.

[0013] Wherein, when the current received optical power P r ( tThe power received is not greater than the critical received optical power. P full At that time, the local oscillator optical power before entering the integrated coherent receiving module remains at the maximum operating local oscillator optical power. P LO,max When the current received optical power P r ( t () greater than the critical received optical power P full At that time, the local oscillator optical power before entering the integrated coherent receiving module is related to the current received optical power. P r ( t ) decreases as it increases.

[0014] In some implementations, obtaining the range of received optical power variation in the actual free-space link includes: transmitting a continuous optical signal, a training optical signal, or a test optical signal equivalent to the actual service signal from the transmitter; continuously monitoring the received optical power in the actual free-space link from the receiver and statistically analyzing the changes in received optical power during the monitoring period; and determining the lowest received optical power occurring during the monitoring period as the lowest received optical power. P low The highest received optical power occurring during the monitoring period is determined as the highest received optical power. P high .

[0015] In some implementations, the system performance meets a preset threshold, including any of the following: the bit error rate is lower than the preset bit error threshold; the error vector amplitude meets a preset condition; and the Q factor meets a preset condition.

[0016] In some implementations, the preset saturation criterion includes at least one of the following: the input waveform of the analog-to-digital converter reaches full amplitude; the input waveform of the analog-to-digital converter shows clipping; the output of the coherent optical signal receiver shows a preset degree of nonlinear distortion; the transimpedance amplifier enters the saturation region.

[0017] In some implementations, when all powers are expressed in dBm, and the current received optical power... P r ( t () greater than the critical received optical power P full At that time, the local oscillator optical power before entering the integrated coherent receiving module satisfies: P LO ( t ) = P LO,max+ P full - P r ( t ) in, P LO ( t The value represents the local oscillator power before entering the integrated coherent receiving module at the current moment.

[0018] In some implementations, when there is a fixed insertion loss IL in the local oscillator branch, the control module calculates the control quantity of the local oscillator power adjustment module based on the target local oscillator power before entering the integrated coherent receiving module and the fixed insertion loss IL.

[0019] In some embodiments, the adaptive adjustment of the local oscillator optical power before it enters the integrated coherent receiving module is performed using either a continuous adjustment method or a step-by-step adjustment method; the continuous adjustment method involves the control module adjusting the current received optical power based on real-time acquisition. P r ( t The target local oscillator optical power is continuously calculated, and the local oscillator optical power adjustment module is driven to continuously change; the step-by-step adjustment method is as follows: multiple local oscillator optical power levels are preset, and when the current received optical power... P r ( t When falling into different ranges, switch to the corresponding local oscillator power.

[0020] In this embodiment of the application, in order to address the problem of wide fluctuations in received optical power over time and limited range of usable input optical power at the receiver caused by atmospheric scintillation in free-space coherent optical communication, the received optical power boundary of the actual link, the two operating parameters of the receiver, and the segmented adjustment relationship of the local oscillator power are linked to suppress the synchronous fluctuation of the electrical signal amplitude after coherent reception with the received optical power. Thus, while maintaining the receiving performance under the minimum received optical power condition, the maximum received optical power that the receiver can withstand is increased, and the working stability of the receiver under atmospheric scintillation conditions is improved.

[0021] At the system level, the receiving signal light monitoring module monitors the received signal light to obtain the current received optical power. The control module generates a local oscillator power adjustment control quantity based on the preset system design parameters stored therein. The local oscillator power adjustment module then changes the local oscillator power before it enters the integrated coherent receiving module, thus forming a control path within the receiver from received optical power monitoring to local oscillator power adjustment. Since the amplitude of the electrical signal after coherent reception is affected by both the received signal light power and the local oscillator power, maintaining the maximum operating local oscillator power within the range where the received optical power is not greater than the critical received optical power can avoid loss of reception performance due to insufficient local oscillator power in the weak signal range. Within the range where the received optical power is greater than the critical received optical power, reducing the local oscillator power as the received optical power increases can compensate for the increase on the signal side with the decrease on the local oscillator side, suppressing the continued increase of the electrical signal amplitude after coherent reception as the received optical power increases, thereby reducing the risk of front-end overload and sampling waveform clipping under strong signal conditions. The two intervals mentioned above are divided by the same critical received optical power, and this division itself is determined by the receiver's ability to withstand the maximum operating local oscillator optical power. Therefore, the performance maintenance in the weak signal interval and the amplitude suppression in the strong signal interval are not two independent measures, but rather different manifestations of the same adjustment relationship on both sides of the critical point.

[0022] At the methodological level, the minimum and maximum received optical power are first obtained in an actual free-space link. Then, on a design platform identical to the actual receiving system, but with optical attenuation modules installed in both the received signal optical branch and the local oscillator optical branch, the maximum operating local oscillator optical power is determined under the minimum received optical power condition, and the critical received optical power is determined under the same local oscillator optical power condition. This ensures that the two operating parameters correspond to the performance constraints at the weak end and the saturation constraints at the strong end, respectively, and are compared with the actual power fluctuation range of the target link using the same caliber. Since different links experience different atmospheric conditions, and different receiving systems have different performance and saturation boundaries, the parameter determination process completed under the same conditions as the actual receiving system helps to ensure that the obtained parameters correspond to the target link and the actual receiver. Based on this, comparing the critical received optical power and the maximum received optical power allows for operation with a fixed local oscillator optical power when the maximum received optical power of the actual link does not exceed the critical received optical power corresponding to the local oscillator optical power. Adaptive adjustment is only activated when there is a received optical power state that exceeds the critical point, ensuring that the activation of the adjustment function matches the power range of the link itself.

[0023] Further defined technical measures each produce corresponding additional effects. The received signal optical monitoring module uses a combination of a beam splitter, an optical power detection unit, and a power conversion unit. It can complete monitoring under the condition of extracting only a portion of the optical power, and convert the detection results of the monitoring branch into the current received optical power in the main receiving branch according to the splitting ratio and fixed link loss, so that the power value used by the control module corresponds to the actual signal optical power participating in coherent mixing. The local oscillator optical power adjustment module uses a variable optical attenuator, a laser drive adjustment unit, or a tunable optical amplification unit, so that the adjustment of the local oscillator optical power can be implemented in different ways according to the device conditions. The power boundary is obtained by transmitting continuous optical signals, training optical signals, or test optical signals equivalent to actual service signals and statistically analyzing the power changes during the monitoring period, so that the obtained boundary reflects the fluctuation of the target link under actual atmospheric conditions. The system performance is specified as meeting preset thresholds, specifically bit error rate, error vector amplitude, or Q factor. Preset saturation criteria are specified as the analog-to-digital converter reaching full amplitude or experiencing clipping, the coherent optical signal receiver output exhibiting a preset level of nonlinear distortion, and the transimpedance amplifier entering the saturation region. This provides a detectable basis for determining the two operating parameters. When each power quantity is expressed in dBm, the local oscillator power is determined by subtracting the current received optical power from the sum of the maximum operating local oscillator power and the critical received optical power. This provides a definite calculation method for adjustments above the critical point and aligns with the rule of maintaining the maximum operating local oscillator power below the critical point. In cases where the local oscillator branch has a fixed insertion loss, the control module calculates the control quantity for the local oscillator power adjustment module based on this insertion loss, ensuring that the actual local oscillator power before entering the integrated coherent receiver module corresponds to the target value. The local oscillator power adjustment can be performed continuously or in stages, providing optional implementation methods for different control conditions.

[0024] Thus, the acquisition of the actual link power boundary, the determination of the order of the two operating parameters, the determination of the operating mode, and the segmented adjustment on both sides of the critical point are all connected in the same technical chain: the result of the former constitutes the basis for the judgment and the adjustment anchor point of the latter, and the execution of the latter enables the power range characterized by the former to be actually accommodated by the receiver, thereby improving the stability of coherent reception under the condition that atmospheric scintillation causes changes in received optical power, helping to improve the overall dynamic range of the receiver, and facilitating its implementation in existing coherent free space optical receiving systems. Attached Figure Description

[0025] Figure 1 is a schematic diagram of the structure of a local oscillator power adaptive adjustment system in a coherent free space light receiver for atmospheric scintillation according to an embodiment of this application.

[0026] Figure 2This is a flowchart illustrating a method for adaptive adjustment of local oscillator power in coherent free-space light reception oriented towards atmospheric scintillation, according to an embodiment of this application.

[0027] Figure 3 This is a schematic diagram of the structure of a design platform for determining preset system design parameters according to an embodiment of this application.

[0028] Figure 4 This is a schematic diagram comparing the received signal waveforms according to an embodiment of this application, wherein, Figure 4 a is a schematic diagram of the received signal waveform when using constant local oscillator power. Figure 4 b is a schematic diagram of the received signal waveform when the local oscillator optical power is adaptively adjusted.

[0029] Figure 5 This is a schematic diagram comparing the bit error rate with the received optical power when using a constant local oscillator power and when using adaptive adjustment of local oscillator power according to an embodiment of this application.

[0030] Figure 6 This is a schematic diagram comparing the bit error rate over time under weak atmospheric scintillation conditions using constant local oscillator power and using adaptive adjustment of local oscillator power, according to an embodiment of this application.

[0031] Figure 7 This is a schematic diagram comparing the bit error rate over time under strong atmospheric scintillation conditions using constant local oscillator power and using adaptive adjustment of local oscillator power, according to an embodiment of this application. Detailed Implementation

[0032] In the following description, many technical details are presented to help the reader better understand this application. However, those skilled in the art will understand that the technical solutions claimed in this application can be implemented even without these technical details and various variations and modifications based on the following embodiments.

[0033] Explanation of some concepts: Free Space Optical Communication (FSO) refers to a communication method that uses free space or atmospheric channels as the transmission medium and transmits information carried by optical signals.

[0034] Local oscillator (LO) refers to the optical signal generated by the receiver itself and used for optical mixing with the received signal light to achieve coherent reception.

[0035] A coherent optical signal receiver is a device installed at the receiving end for receiving signal light transmitted via a free space transmission link and performing coherent reception, analog-to-digital conversion, and digital signal demodulation; in this application, it is also referred to simply as a receiver.

[0036] A transimpedance amplifier (TIA) is a circuit located at the front end of a receiver to convert the current signal obtained from photoelectric conversion into a voltage signal and amplify it.

[0037] Atmospheric scintillation refers to the phenomenon where the intensity of received light fluctuates randomly over time due to random fluctuations in the refractive index caused by atmospheric turbulence as the light signal propagates through the atmosphere.

[0038] Received optical power (ROP) refers to the signal optical power received by the receiver, used to characterize the strength of the input signal in a free-space optical link at a certain moment or under certain conditions; in this application, it is referred to as... P r ( t This represents the current received optical power as a function of time. t Indicates time.

[0039] Minimum received optical power P low With the highest received optical power P high These refer to the lowest and highest received optical power, respectively, occurring during the monitoring period of the actual free-space link.

[0040] Receiver sensitivity refers to the minimum input optical power at which a receiver can correctly receive and demodulate signals, provided that preset performance specifications are met.

[0041] Dynamic range refers to the range of input signal power that a receiver can operate normally under the condition of meeting preset performance indicators. It is determined by the minimum input power that the receiver can correctly receive and the maximum input power that it can withstand without saturation, clipping, or severe distortion.

[0042] Full-amplitude and clipping of the analog-to-digital converter (ADC) module refer to the phenomenon that occurs when the amplitude of the input signal to the ADC module exceeds its sampling range. In this case, the ADC module cannot correctly represent the part that exceeds the range, and the sampled waveform is truncated, resulting in clipping distortion.

[0043] Preset saturation criterion refers to the condition used to determine whether the receiver front-end or analog-to-digital conversion module has reached a state where it is not advisable to continue increasing the signal amplitude.

[0044] Maximum operating local oscillator power P LO,max This refers to the determination process described in this application, where the received optical power is the minimum received optical power. P low Under certain conditions, the local oscillator power before entering the integrated coherent receiving module corresponds to the system performance meeting the preset threshold; during the adjustment process of this application, the local oscillator power before entering the integrated coherent receiving module does not exceed this value.

[0045] Critical received optical power P full , refers to the received optical power that enables the analog-to-digital converter module to reach full amplitude or the coherent optical signal receiver to reach the preset saturation criterion when the local oscillator optical power is at its maximum operating local oscillator optical power. It is used to characterize the upper limit of received optical power that the receiver can withstand under this local oscillator optical power setting.

[0046] A design platform is a device that is identical to the actual receiving system and has optical attenuation modules installed in the receiving signal optical branch and the local oscillator optical branch, respectively, to change the receiving signal optical power and the local oscillator optical power to determine the preset system design parameters.

[0047] Power Reference Plane and Unit Aperture: In this application, unless otherwise stated, all power quantities use the signal optical input port and local oscillator optical input port of the integrated coherent receiver module as a unified reference plane, i.e., the lowest received optical power. P low Maximum received optical power P high Current received optical power P r ( t (and critical received optical power) P full Both refer to the power at the signal optical input port of the main receiving branch. The detection results of the monitoring branch are converted to this reference plane according to the splitting ratio of the beam splitter and the fixed link loss; maximum operating local oscillator power. P LO,max With the local oscillator power at the current moment P LO ( t All of the above power values ​​refer to the power at the local oscillator input port. All power values ​​are expressed in dBm, and the fixed insertion loss IL is expressed in dB. In specific implementations, if... P low , P high If the received optical power of the link before the splitter is taken, it is converted to the above reference plane according to the splitting ratio of the splitter and the fixed link loss, and then compared so that the comparison relationship between the power quantities is in the same reference plane.

[0048] The following is a brief description of the problem this application aims to solve and its overall concept: In summary, this application addresses two distinct constraints existing simultaneously at the weak and strong ends of a free-space coherent optical link under atmospheric scintillation: the minimum received optical power of the received signal within the link itself. P low With the highest received optical power P high The amplitude of the received optical signal varies randomly over time, and is not solely determined by the received optical power, but is also influenced by the local oscillator power. At the weak end, this amplitude is limited by whether the system performance meets a preset threshold; at the strong end, it is limited by whether the analog-to-digital conversion module reaches full amplitude and whether the coherent optical signal receiver reaches a preset saturation criterion. This application does not eliminate flicker in the optical domain, but instead constructs a control path within the receiver from the received optical signal monitoring module through the control module to the local oscillator power adjustment module, so that the current received optical power obtained from the beam splitting monitoring is... P r ( t This is transformed into a real-time constraint on the local oscillator power, using the controllable adjustment on the local oscillator side to cope with the uncontrollable power fluctuations on the signal light side.

[0049] This pathway works because the preset system design parameters stored in the control module are not listed in parallel, but rather depend on each other in a hierarchical manner: maximum operating local oscillator optical power. P LO,max When the received optical power is set P low Under the conditions of [condition], and with the system performance meeting a preset threshold as the criterion, the weak-end constraint has been converted into a local oscillator optical power value; the critical received optical power [value]. P full Again, the local oscillator power is maintained at this level. P LO,max Under the condition of the analog-to-digital converter reaching full amplitude or the receiver reaching a preset saturation criterion, the strong-end constraint is converted into a received optical power value. Because the latter is derived based on the former, both can be placed under the same caliber as the measured power boundary of the link. P full Not less than P high At this time, all power states of the link fall within the allowable range of the existing local oscillator settings, and there is no need to change the local oscillator optical power; only when P full Less than P high At that time, Fang Yi P full As a boundary, remain below it PLO,max This ensures that the local oscillator operates within the preset threshold in the weak signal range, while above this threshold, the local oscillator optical power increases accordingly. P r ( t The signal increases and decreases as the local oscillator side decreases to compensate for the increase on the signal side. Thus, the result of the previous stage forms the basis and anchor point for the judgment of the next stage, and the execution of the next stage allows the receiver to actually accommodate the power range characterized by the previous stage, thereby compressing the amplitude fluctuation of the coherent received electrical signal.

[0050] Furthermore, through in-depth research and analysis of the free-space coherent optical link reception process, the inventors of this application discovered that the influence of atmospheric scintillation does not remain at the level of the received optical power itself, but is transmitted to the subsequent electrical signal processing link via the coherent mixing stage. Since the amplitude of the electrical signal after coherent reception is affected by both the received optical power and the local oscillator power, when the local oscillator power is fixed, the random fluctuations in the received optical power will directly manifest as synchronous fluctuations in the electrical signal amplitude: during periods of lower received optical power, the electrical signal amplitude is smaller, and the system performance is limited by the receiver sensitivity; during periods of higher received optical power, the electrical signal amplitude is too large, and the transimpedance amplifier at the receiver front end may enter the saturation region, and the analog-to-digital conversion module may also reach full amplitude and experience clipping, affecting subsequent signal recovery and demodulation.

[0051] These two scenarios place opposite demands on the local oscillator (LO) power: increasing LO power to improve reception performance during weak signal periods will cause the receiver to reach full amplitude or saturation earlier during strong signal periods; conversely, reducing LO power to avoid overload during strong signal periods will weaken reception performance during weak signal periods. It is difficult to achieve both simultaneously with a fixed LO power. Furthermore, different links experience different atmospheric conditions, resulting in varying fluctuation ranges in received optical power. Different receiving systems also reach performance thresholds or enter full amplitude / saturation at a given LO power at different rates. If the operating parameters do not correspond to the boundaries of the target link and the actual receiving system, there is a lack of repeatable determination criteria. Therefore, it is necessary to establish a segmented constraint relationship between the monitored received optical power and the LO power, with the critical point determined by the receiver's own capacity as the boundary. The decision to activate the LO power should be made in advance based on the actual power boundaries of the link.

[0052] To make the objectives, technical solutions, and advantages of this application clearer, the embodiments of this application will be described in further detail below with reference to the accompanying drawings.

[0053] The first embodiment of this application relates to an adaptive local oscillator power adjustment system for coherent free-space optical reception in response to atmospheric scintillation, applied to free-space coherent optical communication, and its overall structure is shown in Figure 1. The system includes a coherent optical signal transmitter 101, a free-space transmission link 102, and a coherent optical signal receiver 103. The coherent optical signal transmitter 101 generates and transmits the optical signal to be transmitted; the free-space transmission link 102 carries the propagation of the optical signal in the atmospheric channel. During propagation, the optical signal is affected by atmospheric scintillation; even if the output optical power at the transmitting end remains constant, the received optical power at the receiving end will still fluctuate over time; the coherent optical signal receiver 103 receives the signal light transmitted through the free-space transmission link 102 and adaptively adjusts the local oscillator power according to the changes in the received optical power, subsequently completing coherent reception, analog-to-digital conversion, and digital signal demodulation. Through this system, the local oscillator power before entering the receiver can be adaptively controlled under conditions of received optical power fluctuations caused by atmospheric scintillation, thereby keeping the electrical signal after beat frequency as close as possible to the range that the receiver can process.

[0054] Furthermore, the coherent optical signal receiver 103 includes a received signal light monitoring module, a local oscillator light generation module, a local oscillator light power adjustment module, a control module, an integrated coherent receiving module, an analog-to-digital conversion module, and a digital signal demodulation module. The received signal light monitoring module is used to monitor the received signal light to obtain the current received optical power. P r ( t ), t Indicates time, P r ( t The signal represents the current received optical power changing over time. In this embodiment, the monitoring is achieved by beam splitting. The local oscillator light generation module generates local oscillator light that is coherently mixed with the received signal light. The local oscillator light power adjustment module adjusts the local oscillator light power before it enters the integrated coherent receiving module. The control module generates a local oscillator light power adjustment control quantity based on the monitoring results of the received signal light monitoring module and preset system design parameters that are predetermined and stored. The integrated coherent receiving module performs optical mixing and photoelectric conversion between the received signal light and the local oscillator light, outputting an analog electrical signal. The analog-to-digital conversion module converts the analog electrical signal into a digital signal. The digital signal demodulation module recovers, demodulates, and analyzes the performance of the digital signal.

[0055] Specifically, in the signal flow direction, after the received signal light enters the coherent optical signal receiver 103, a small portion of the optical power is first diverted by the received signal light monitoring module for monitoring, while the majority of the remaining optical power enters the integrated coherent receiving module along the main receiving branch. The local oscillator light output by the local oscillator light generation module is also regulated by the local oscillator light power adjustment module before entering the integrated coherent receiving module. The received signal light and the local oscillator light undergo optical mixing and photoelectric conversion in the integrated coherent receiving module. The resulting analog electrical signal is converted into a digital signal by the analog-to-digital converter module, and then the digital signal demodulation module completes the subsequent recovery and demodulation. At the same time, the received signal light monitoring module records the current received optical power. P r ( t The signal is provided to the control module, which generates a local oscillator power adjustment control quantity based on this and the preset system design parameters, and applies it to the local oscillator power adjustment module, thereby forming a control path within the receiver from the monitoring of the received optical power to the adjustment of the local oscillator power.

[0056] The coherent optical signal receiver 103 operates as follows: Since the amplitude of the beat signal in coherent reception is affected by both the received optical power and the local oscillator power, when atmospheric scintillation causes an increase in received optical power, the amplitude of the beat signal can be suppressed from further increasing by reducing the local oscillator power, thereby mitigating the risks of transimpedance amplifier saturation and full-amplitude conversion of the analog-to-digital converter. Conversely, when the received optical power decreases, a higher local oscillator power can be maintained to ensure better reception performance. In other words, this application does not directly eliminate atmospheric scintillation, but rather compresses the amplitude fluctuations of the received electrical signal caused by scintillation by adjusting the local oscillator power, thereby improving the system's stability and dynamic range under complex received optical power conditions.

[0057] The control module stores preset system design parameters. These preset system design parameters include: the minimum received optical power in the free-space transmission link. P low The highest received optical power in a free-space transmission link P high When the received optical power is at its minimum received optical power P low The maximum operating local oscillator power that allows the system performance to meet a preset threshold under certain conditions. P LO,max ; and the local oscillator power is the maximum operating local oscillator power. P LO,max The critical received optical power at which the analog-to-digital converter module reaches full saturation or the coherent optical signal receiver reaches the preset saturation criterion. P full Among them, the lowest received optical power P lowWith the highest received optical power P high Used to characterize the received optical power fluctuation boundary of a real free-space link under atmospheric scintillation; maximum operating local oscillator power. P LO,max The critical received optical power is the upper limit of the local oscillator optical power required before entering the integrated coherent receiver module to ensure the system meets the receiving performance requirements under the weakest received optical power condition in a real link. P full Used to characterize when the local oscillator power is maintained at P LO,max The maximum received optical power that the receiver can withstand. The aforementioned preset system design parameters can be predetermined and written into the control module during the system design or deployment phase, according to the method described in the second embodiment of this application, providing parameter basis for adaptive adjustment of the local oscillator optical power during actual communication.

[0058] In terms of operation, when the critical received optical power... P full Not less than the maximum received optical power P high When, it indicates that the local oscillator power is taken P LO,max Under these conditions, the highest received optical power in the actual link is not higher than the critical received optical power corresponding to the local oscillator power. Therefore, the coherent optical signal receiver operates with a fixed local oscillator power and does not need to adjust the local oscillator power during communication.

[0059] When the critical received optical power P full Less than the maximum received optical power P high This indicates that in the actual link, there are periods of high received optical power that can cause the receiver to reach full amplitude or a preset saturation criterion. At this time, the control module adjusts the current received optical power... P r ( t Maximum operating local oscillator power P LO,max and critical received optical power P full The local oscillator optical power adjustment module adaptively adjusts the local oscillator optical power before it enters the integrated coherent receiver module. The specific adjustment rule is: when the current received optical power... P r ( t Not greater than the critical received optical power P full At that time, the local oscillator optical power before entering the integrated coherent receiver module remains at the maximum operating local oscillator optical power. PLO,max To ensure reception performance in areas with low received optical power; when the current received optical power P r ( t (Greater than the critical received optical power) P full At that time, the local oscillator optical power before entering the integrated coherent receiving module changes with the current received optical power. P r ( t The amplitude of the coherent received electrical signal decreases as the received optical power increases, thereby suppressing the continued increase in amplitude of the received electrical signal with increasing received optical power. This reduces the risk of the analog-to-digital converter module reaching full amplitude and the coherent optical signal receiver reaching the preset saturation criterion. In other words, a higher local oscillator power is maintained in the range of weaker received optical power to ensure reception performance, while the local oscillator power is reduced in the opposite direction in the range where the received optical power exceeds the critical received optical power to suppress the amplitude of the beat frequency electrical signal, thus keeping the electrical signal after coherent reception within the range that the receiver can process.

[0060] In one optional implementation, the received signal optical monitoring module includes a beam splitter, an optical power detection unit, and a power conversion unit. The beam splitter is located in the main receiving branch and is used to split a portion of the optical power from the received signal light to form a monitoring branch. The optical power detection unit is used to detect the optical power in the monitoring branch. The power conversion unit is used to convert the optical power detected in the monitoring branch into the current received optical power in the main receiving branch based on the beam splitter's splitting ratio and a fixed link loss. P r ( t Since the monitoring branch only extracts a portion of the power from the received signal light, and there may be a fixed link loss in the monitoring branch, it is necessary to convert the detection result of the monitoring branch into the actual received optical power in the main receiving branch according to the known splitting ratio and fixed link loss, and then provide it to the control module. For example, the splitter can be a 9:1, 19:1, or other fixed splitting ratio optical coupler; this application does not limit this, as long as it can achieve received optical power monitoring without significantly affecting the main signal reception.

[0061] In one optional implementation, the local oscillator power adjustment module includes a variable optical attenuator disposed in the local oscillator branch between the local oscillator light generation module and the integrated coherent receiver module. The attenuation amount is changed to adjust the local oscillator light power before it enters the integrated coherent receiver module. As a optional further implementation, the local oscillator power adjustment module may also include a laser driver adjustment unit, a tunable optical amplification unit, or a combination thereof. That is, the local oscillator power adjustment module includes at least one of a variable optical attenuator, a laser driver adjustment unit, and a tunable optical amplification unit. This application does not limit this, as long as the local oscillator light power before entering the integrated coherent receiver module can be adjusted.

[0062] In one alternative implementation, the digital signal demodulation module includes a resampling unit, an orthogonalization unit, a clock recovery unit, an equalization unit, a frequency offset estimation unit, a phase recovery unit, and a bit error analysis unit, used to recover the received signal and evaluate system performance. Its implementation can be the same as that of existing coherent receiving digital signal processing links.

[0063] The second embodiment of this application relates to an adaptive local oscillator power adjustment method in coherent free-space optical reception for atmospheric scintillation. This method can be implemented based on the system described in the first embodiment, and its flow is shown in Figure 2, including steps 201 to 207. Steps 201 to 205 belong to the receiving system design stage, used to obtain the actual received optical power variation range of the link, determine the relevant operating parameters of the local oscillator power, and determine the local oscillator power adjustment mode. Steps 206 and 207 belong to the actual communication stage, used to adaptively adjust the local oscillator power according to the current received optical power during communication. Each step is described in detail below.

[0064] Step 201: Obtain the range of received optical power variation in the actual link.

[0065] Specifically, in the target free-space optical communication link, an optical signal is transmitted by a transmitter (e.g., the coherent optical signal transmitter 101 in Figure 1), and the receiver continuously monitors the received optical power under actual atmospheric channel conditions, recording the changes in received optical power over a sufficiently long period to reflect the fluctuation range of received optical power under actual atmospheric scintillation. The changes in received optical power during the monitoring period are statistically analyzed, and the lowest received optical power occurring during the monitoring period is determined as the lowest received optical power. P low The highest received optical power during the monitoring period is determined as the highest received optical power. P high The purpose of this step is to obtain the input power boundary in the actual link, providing a basis for subsequent receiver system design and local oscillator power adjustment.

[0066] In one optional implementation, the optical signal transmitted by the transmitter in step 201 can be a continuous optical signal, a training optical signal, or a test optical signal equivalent to the actual service signal. This application does not limit this, as long as it can reflect the range of received optical power fluctuation in the target link.

[0067] Step 202: Construct a design platform identical to the actual receiving system.

[0068] Specifically, a design platform with the same configuration as the transmitter and receiver in the target free-space optical communication system is constructed. The structural diagram of this design platform is shown below. Figure 3 As shown, it includes a coherent optical signal transmitter 301 and a coherent optical signal receiver 302. The coherent optical signal transmitter 301 is used to generate and transmit the optical signal to be received, which is consistent with... Figure 1 The coherent optical signal transmitter 101 is the same as that in the previous example; the coherent optical signal receiver 302 is used for coherent reception, analog-to-digital conversion, and digital signal demodulation of the received signal. Figure 1 Compared to the actual receiving system shown, Figure 3 The design platform shown further incorporates a first optical attenuation module and a second optical attenuation module at the receiving end. The first optical attenuation module is located in the received signal optical branch and is used to adjust the signal optical power before entering the integrated coherent receiving module to simulate different received optical power conditions. The second optical attenuation module is located in the local oscillator optical branch and is used to adjust the local oscillator optical power before entering the integrated coherent receiving module. This design platform allows for the modification of the received signal optical power and local oscillator optical power while maintaining consistency with the actual system's receiving structure, providing conditions for determining the system design parameters in subsequent steps 203 and 204.

[0069] In one alternative implementation, the configuration of the integrated coherent receiver module, analog-to-digital converter module, and digital signal demodulation module in the design platform is consistent with that of the actual system, so as to ensure that the determined system design parameters can be directly used for adaptive adjustment of the local oscillator optical power in the actual link.

[0070] Step 203: Determine the maximum operating local oscillator power under low received optical power conditions.

[0071] Specifically, the first optical attenuation module is used to adjust the received signal optical power to the lowest received optical power obtained in step 201. P low Then, the local oscillator power is gradually increased using the second optical attenuation module, and the system performance after reception is tested. The system performance indicators after reception are used to determine whether a preset threshold is met. When the system performance meets the preset threshold, the local oscillator power before entering the integrated coherent receiving module is determined as the maximum operating local oscillator power.P LO,max The purpose of this step is to determine the upper limit of the local oscillator optical power required before entering the integrated coherent receiver module, under the weakest received optical power condition in the actual link, in order to ensure that the system meets the receiving performance requirements.

[0072] In one optional implementation, the system performance meeting the preset threshold includes any of the following: the bit error rate is lower than the preset bit error rate threshold; the error vector amplitude meets the preset condition; and the Q factor meets the preset condition. This application does not limit the specific type of performance indicator used.

[0073] Step 204: Determine the critical received optical power when the receiver reaches full amplitude.

[0074] Specifically, the local oscillator power before entering the integrated coherent receiver module is maintained at the maximum operating local oscillator power determined in step 203. P LO,max The received optical power is gradually increased using the first optical attenuation module until the analog-to-digital conversion module reaches full amplitude or the coherent optical signal receiver reaches a preset saturation criterion. The received optical power at this point is recorded and determined as the critical received optical power. P full The purpose of this step is to determine: when the local oscillator power is maintained at a certain value... P LO,max At what point will the received signal optical power increase to the limit that the receiver front-end or analog-to-digital conversion module will reach its limit?

[0075] In one alternative implementation, the preset saturation criterion includes at least one of the following: the input waveform of the analog-to-digital converter reaches full amplitude; the input waveform of the analog-to-digital converter exhibits clipping; the output of the coherent optical signal receiver exhibits a preset degree of nonlinear distortion; the transimpedance amplifier enters the saturation region; and other criteria that can characterize the receiver front-end reaching a state where it is not advisable to continue increasing the signal amplitude.

[0076] Step 205: Determine the local oscillator power adjustment method.

[0077] Specifically, the critical received optical power determined in step 204 P full The highest received optical power obtained in step 201 P high Comparison. When P full ≥ P high When, it indicates that the local oscillator power is taken P LO,maxUnder certain conditions, the highest received optical power in the actual link does not exceed the critical received optical power corresponding to the local oscillator power; therefore, the coherent optical signal receiver can operate with a fixed local oscillator power. P full < P high This indicates that in the actual link, there are periods of high received optical power that may cause the receiver to reach full amplitude or a preset saturation criterion. Therefore, it is necessary to adaptively adjust the local oscillator power during actual communication, i.e., execute step 206. The purpose of this step is to determine whether the adaptive adjustment function of the local oscillator power needs to be enabled based on the system design results.

[0078] Step 206: Adaptive adjustment of local oscillator optical power during actual communication.

[0079] Specifically, in step 205, it is determined that adaptive adjustment is needed (i.e. P full < P high In the case of [missing information], during actual communication, the current received optical power is obtained in real time through the received signal optical monitoring module. P r ( t The control module determines the current received optical power based on the received optical power. P r ( t Maximum operating local oscillator power P LO,max and critical received optical power P full The local oscillator optical power adjustment control quantity is generated to control the local oscillator optical power adjustment module to adaptively adjust the local oscillator optical power before it enters the integrated coherent receiver module. The adjustment rule is: when P r ( t ) ≤ P full At that time, the local oscillator optical power before entering the integrated coherent receiver module remains at the maximum operating local oscillator optical power. P LO,max ;when P r ( t ) > P full At that time, the local oscillator optical power before entering the integrated coherent receiving module changes with the current received optical power. P r ( t The amplitude of the electrical signal after coherent reception decreases as the received optical power increases, thereby suppressing the continued increase of the amplitude of the electrical signal after coherent reception with the increase of the received optical power, reducing the risk that the analog-to-digital conversion module will reach full amplitude and the coherent optical signal receiver will reach the preset saturation criterion.

[0080] Furthermore, when all powers are expressed in dBm, and the current received optical power... P r ( t (Greater than the critical received optical power) P full At that time, the local oscillator power before entering the integrated coherent receiver module satisfies: P LO ( t ) = P LO,max + P full - P r ( t (1) in, P LO ( t The value represents the local oscillator power before entering the integrated coherent receiver module at the current moment. P r ( t The received optical power at the current moment is ). P LO,max For maximum operating local oscillator power, P full The critical received optical power is given, and all power quantities are expressed in dBm. From equation (1), it can be seen that when... P r ( t In more than P full Subsequently, for every 1 dB increase, the local oscillator optical power before entering the integrated coherent receiver module decreases by 1 dB, thereby suppressing the continued increase in the beat frequency electrical signal amplitude as the received optical power increases; and, when P r ( t ) = P full At that time, the local oscillator power calculated according to equation (1) is exactly... P LO,max ,and P r ( t ) ≤ P full Keep P LO,max The adjustment rules are connected at the critical point.

[0081] In another alternative implementation, when the power is expressed in linear units, the local oscillator power before entering the integrated coherent receiver module satisfies: P LO ( t ) = ( P LO,max × P full ) / P r ( t (2) The physical quantities represented by each symbol are the same as those in equation (1). The difference is that all power quantities in equation (2) are expressed in linear power units, while all power quantities in equation (1) are expressed in dBm. Equations (1) and (2) are the same regulation relationship written in different power units. They are essentially equivalent. This application does not limit the power representation format used.

[0082] Furthermore, the adaptive adjustment of the local oscillator optical power in step 206 can be performed using either a continuous adjustment method or a step-by-step adjustment method. The continuous adjustment method involves the control module adjusting the current received optical power based on the real-time acquired data. P r ( t The system continuously calculates the target local oscillator optical power and drives the local oscillator optical power adjustment module to continuously change it. The adjustment method is as follows: multiple local oscillator optical power levels are preset, and the current received optical power... P r ( t When the signal falls into different ranges, the local oscillator power is switched to the corresponding level. Whether continuous adjustment or level adjustment is used, as long as the local oscillator power can be adaptively controlled according to the changes in the received optical power, this application does not limit this.

[0083] Step 207: Calculate the adjustment amount based on the fixed loss of the local oscillator branch.

[0084] Specifically, in one optional implementation, when a fixed insertion loss IL exists in the local oscillator branch due to the introduction of devices such as optical attenuators, the control module calculates the control quantity of the local oscillator power adjustment module based on the target local oscillator power before entering the integrated coherent receiver module and the fixed insertion loss IL. For example, when the local oscillator power adjustment module is located after the local oscillator generation module and before the integrated coherent receiver module, and the fixed insertion loss is expressed in dB, the target local oscillator power output by the control module to the local oscillator power adjustment module can be expressed as: P LO,src ( t ) = P LO ( t ) + IL(3) in, PLO,src ( t The target local oscillator power is the power at the corresponding location of the local oscillator power adjustment module in the local oscillator branch. P LO ( t Let be the target local oscillator power before entering the integrated coherent receiver module at the current moment, and IL be the fixed insertion loss in the local oscillator branch, expressed in dB. Through the above conversion, the fixed insertion loss in the local oscillator branch can be compensated, ensuring that the actual local oscillator power before entering the integrated coherent receiver module reaches the target value. The control module can further determine the target attenuation amount applied to the variable optical attenuator based on the conversion result.

[0085] To verify the technical effects of the above-described embodiments of this application, an experimental system was built and verified according to the system configuration shown in Figure 1. The coherent optical signal transmitter 101 generates and transmits the optical signal to be transmitted; the free-space transmission link 102 introduces atmospheric scintillation effects from the actual link; and the coherent optical signal receiver 103 adjusts the local oscillator power according to changes in the received optical power, and completes subsequent coherent reception, analog-to-digital conversion, and digital signal demodulation. In the following verification results, the method of receiving with a fixed local oscillator power is denoted as constant LO, and the method of receiving with adaptive local oscillator power adjustment as described in this application is denoted as adaptive LO.

[0086] Figure 4 The diagram shows the received signal waveforms obtained by using constant LO and adaptive LO at the receiver, respectively. Figure 4 a represents the signal waveform when using a constant LO. Figure 4 b represents the signal waveform when using adaptive LO. Figure 4 It can be seen that under constant LO conditions, the electrical amplitude of the received signal fluctuates significantly with changes in received optical power. During some time periods, the waveform amplitude is too large, easily approaching the receiver's full-amplitude range, while during other time periods, the waveform amplitude is too small, which is detrimental to subsequent analog-to-digital conversion and digital signal processing. However, after adopting adaptive LO, the overall amplitude of the received signal waveform is significantly more balanced, the electrical amplitude in the strong signal segment is effectively compressed, and the electrical amplitude in the weak signal segment is maintained to a certain extent. This phenomenon indicates that the local oscillator optical power adjustment scheme described in this application can effectively suppress the fluctuations in electrical signal amplitude caused by atmospheric scintillation, keeping the electrical signal entering the subsequent receiving link within a more suitable range.

[0087] Figure 5 The diagram illustrates the relationship between bit error rate and received optical power when using constant LO and adaptive LO, where the dashed line represents... The error threshold. (By...) Figure 5It can be seen that under constant LO conditions, as the received optical power increases from low to high, the system bit error rate first decreases and then increases. This is because under low received optical power conditions, the system is limited by the receiver sensitivity, while under high received optical power conditions, it is affected by receiver front-end saturation and analog-to-digital converter clipping. Therefore, it can only meet the bit error rate threshold requirement within a relatively narrow input power range. In contrast, under adaptive LO conditions, the system can maintain a low bit error rate over a wider range of received optical power, indicating that by adjusting the local oscillator power according to changes in received optical power, the system's tolerable input power range can be effectively extended. In the verification results of this embodiment, after adjusting the local oscillator power, the receiver dynamic range can be extended from approximately 13 dB to over 25 dB.

[0088] Figure 6 The results show a comparison of bit error rate versus time when using constant LO and adaptive LO under weak atmospheric scintillation conditions, where the dashed line also represents... The bit error rate (BER) threshold was determined. It can be seen that when using a constant LO, the BER fluctuates significantly over time, exceeding the BER threshold at certain moments. This indicates that under weak flicker conditions, a fixed local oscillator power is still insufficient to adapt to changes in received optical power. However, with an adaptive LO, the BER remains low for the vast majority of moments, with only a few transient instances showing a relatively high BER, but still below the threshold. In the measurement results of this embodiment, approximately 3% of the measurement points using a constant LO exceeded the BER threshold. The threshold is set, but with adaptive LO, the bit error rate of all measurement points remains below this threshold.

[0089] Figure 7 This paper presents a comparison of bit error rate (BER) versus time under strong atmospheric scintillation conditions, using constant LO and adaptive LO. Figure 7 It can be seen that under strong flicker conditions, the bit error rate (BER) fluctuations in the constant LO mode are further aggravated, with a significant increase in the number of points exceeding the threshold. This indicates that the fixed local oscillator power mode cannot guarantee stable reception when high and low received optical power alternate. However, with the adaptive LO, although the BER may increase relatively at a few extremely low received optical power moments, the overall BER level is significantly reduced and can be stably maintained at a low level most of the time. In the measurement results of this embodiment, the BER exceeding the threshold when using the constant LO mode is... The threshold measurement points exceeded 20%, but after adopting adaptive LO, the bit error rate of all measurement points was reduced to below the threshold.

[0090] Depend on Figure 6 and Figure 7Furthermore, the local oscillator power adaptive adjustment scheme described in this application does not simply improve reception performance under weak signal conditions. Instead, it controls the electrical signal after frequency beat within a more reasonable range by reducing the local oscillator power when the received optical power is high and maintaining a relatively high local oscillator power when the received optical power is low. Therefore, under strong signal conditions, it can suppress receiver front-end overload and analog-to-digital converter clipping, while under weak signal conditions, it can maintain relatively good reception performance, thereby improving the overall stability and dynamic range of the system under atmospheric scintillation conditions.

[0091] comprehensive Figures 4 to 7 The verification results show that the adaptive local oscillator power adjustment system and method for coherent free-space optical reception oriented to atmospheric scintillation described in this application can effectively compress the amplitude variation range of the received electrical signal under the condition that the received optical power fluctuates with time, reduce the risk of saturation and clipping under strong signal conditions, and improve the reception stability under weak signal conditions, so that the system can maintain good reception performance over a wider range of received optical power, and is especially suitable for coherent reception scenarios that are significantly affected by atmospheric scintillation.

[0092] The above embodiments have the following technical effects: (1) Adapting to received optical power fluctuations caused by atmospheric scintillation. This application addresses the problem of drastic changes in received optical power over time caused by atmospheric scintillation in free-space optical communication. A received signal monitoring module, a control module, and a local oscillator (LO) power adjustment module are incorporated into the coherent optical signal receiver. The LO power is adjusted based on changes in the current received optical power before entering the integrated coherent receiving module. Since the amplitude of the beat frequency electrical signal in coherent reception is affected by both the received signal optical power and the LO power, reducing the LO power when the received optical power increases and maintaining a higher LO power when the received optical power decreases can compress the amplitude fluctuations of the beat frequency electrical signal caused by scintillation, keeping the electrical signal within a more suitable range, thereby improving the system's adaptability in actual atmospheric channels.

[0093] (2) Improve the overall dynamic range of the system. The fixed local oscillator optical power receiving method can only ensure the normal operation of the system within a narrow input power range. This application maintains the maximum operating local oscillator optical power when the received optical power does not exceed the critical received optical power to ensure the receiving performance. When the received optical power exceeds the critical received optical power, the local oscillator optical power is reduced in reverse to reduce the risk of transimpedance amplifier saturation and full-width and clipping of the analog-to-digital conversion module. Under the premise of maintaining the receiving performance under the minimum received optical power condition, the maximum received optical power that the receiver can withstand can be increased, so that a wider range of received optical power can be effectively received, thereby improving the overall dynamic range of the system.

[0094] (3) The design process is complete and easy to implement in engineering. This application provides a complete process from obtaining the actual link received optical power variation range, determining the maximum working local oscillator optical power and the critical received optical power, to determining the fixed or adaptive working mode, and then controlling the local oscillator optical power according to the segmented adjustment rules in the actual communication process. It establishes a clear relationship between the actual link received optical power range and the local oscillator optical power adjustment. All parameters are determined through the above process, and the adjustment relationship is simple and easy to implement in engineering.

[0095] (4) The system structure is relatively simple and easy to deploy. The scheme of this application is mainly implemented at the receiving end, without the need for complex modifications to the transmitting end, and does not rely on complex atmospheric wavefront correction structures. Compared with methods that improve the effects of turbulence through adaptive optics, spatial diversity, or complex mode control, this application only needs to set up a received signal light monitoring module, a control module, and a local oscillator light power adjustment module in the receiver to achieve adaptive control of the local oscillator light power. Moreover, the local oscillator light power adjustment can be achieved through devices such as variable optical attenuators. The structure is clear and the implementation method is direct, which is convenient to deploy in existing coherent free space light receiving systems and is conducive to improving the receiving stability and engineering practicality of the system under atmospheric scintillation conditions.

[0096] This concludes the description of an implementation method for an adaptive local oscillator power adjustment system and method in coherent free-space light reception for atmospheric scintillation.

[0097] It should be noted that in this application, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one" does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element. In this application, if it refers to performing an action according to an element, it means performing the action at least according to that element, including two cases: performing the action only according to that element, and performing the action according to that element and other elements. Expressions such as "multiple," "repeatedly," and "various" include two, two times, two kinds, and more than two, more than two times, and more than two kinds.

[0098] The following provides exemplary descriptions of several implementation details of this application. The following content is merely illustrative for the purpose of understanding this application and represents optional implementation methods. It does not constitute a limitation on this application, nor is it intended to limit the scope of protection of the claims. Those skilled in the art can make corresponding adjustments based on actual system conditions, device conditions, and link conditions, and may also adopt specific practices different from the examples below.

[0099] For example, the maximum operating local oscillator power P LO,max This can be determined as follows: Maintain the received optical power at the minimum received optical power. P low Under these conditions, the local oscillator power before entering the integrated coherent receiver module is gradually increased from low to high, and the local oscillator power that first makes the system performance meet the preset threshold is taken as... P LO,max During the subsequent adaptive adjustment of the local oscillator power, the local oscillator power before entering the integrated coherent receiver module will not exceed this value. As another example, within the allowable operating range where system performance meets a preset threshold but has not yet reached a preset saturation criterion, and considering factors such as the device safety upper limit and the stopping condition that further increases in local oscillator power do not significantly improve system performance, the usable local oscillator power within this range can be selected as the threshold. P LO,max .

[0100] For example, in the process of gradually increasing the received signal optical power according to the above procedure, the received optical power corresponding to the first attainment of any one of the listed preset saturation criteria can be used as the critical received optical power. P full Alternatively, the primary criterion can be that the input waveform of the analog-to-digital converter reaches full amplitude, with other criteria used as auxiliary verification. These criteria can be detected at appropriate locations, such as the sampled waveform of the analog-to-digital converter, the demodulation result of the digital signal demodulation module, or the output of the transimpedance amplifier.

[0101] For example, the critical received optical power is determined according to the above process. P full At that time, the maximum received signal optical power that the design platform can provide in the received signal optical branch is not lower than the specified value. P full When the received signal optical power provided by the design platform is insufficient to enable the analog-to-digital converter module to reach full amplitude or the coherent optical signal receiver to reach the preset saturation criterion, calibration can be completed first under lower local oscillator optical power conditions, and then the calibration results can be converted to a local oscillator optical power of dBm according to the adjustment relationship expressed in dBm as described above. P LO,max conditions.

[0102] For example, when the critical received optical power P full Not less than the maximum received optical power P high When a coherent optical signal receiver operates with a fixed local oscillator power, this fixed local oscillator power can be taken as the maximum operating local oscillator power. P LO,max ; or you can P LO,max On this basis, a certain safety margin should be reserved, and the value should be lower than P LO,max The local oscillator power, for example, is used when the highest received optical power is close to the critical received optical power.

[0103] For example, the local oscillator optical power before entering the integrated coherent receiving module is reduced to a level required to ensure coherent reception; when the target local oscillator optical power calculated according to the above adjustment relationship is lower than the adjustable lower limit of the local oscillator optical power adjustment module, it can operate according to that adjustable lower limit. The above-mentioned adaptive adjustment of local oscillator optical power can achieve the lowest received optical power. P low Up to the highest received optical power P high The operation is performed within the defined range of received optical power. When the current received optical power exceeds this range, the operation can be performed according to the local oscillator optical power corresponding to the boundary of this range. When using the stepped adjustment method, a hysteresis interval or minimum dwell time can also be set at the boundary of the interval between adjacent steps to reduce frequent switching when the current received optical power fluctuates near the boundary.

[0104] For example, the update cycle of the received signal light monitoring and the local oscillator power adjustment is shorter than the typical fluctuation time of atmospheric scintillation of the target link, so that the adjustment of the local oscillator power can track the changes in received light power caused by scintillation; accordingly, the response time of the monitoring branch, the update cycle of the control module and the response speed of the local oscillator power adjustment module can be selected according to the above relationship.

[0105] For example, the aforementioned monitoring period can cover the typical operating hours and weather conditions of the target link, and the minimum received optical power. P low With the highest received optical power P high The extreme values ​​observed during the monitoring period can be taken directly; alternatively, after removing abnormal observations, the power boundary can be determined according to preset statistical rules (e.g., taking quantiles at a certain confidence level).

[0106] For example, the integrated coherent receiving module may include an optical mixer and a balanced photodetector. After the received signal light and the local oscillator light are optically mixed in the optical mixer, the photoelectric conversion is completed by the balanced photodetector. In the implementation using an unbalanced detection method, the DC component generated when the received optical power increases may also constitute a saturation source. In this case, the influence of the DC component can be considered in the aforementioned preset saturation criterion.

[0107] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application, and not to limit them; although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or make equivalent substitutions for some of the technical features; and these 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 this application.

Claims

1. A local oscillator power adaptive adjustment system for coherent free-space optical reception oriented towards atmospheric scintillation, applied to free-space coherent optical communication, characterized in that, include: A coherent optical signal transmitter is used to generate and transmit optical signals to be transmitted. A free-space transmission link is used to carry the propagation of the optical signal in an atmospheric channel; A coherent optical signal receiver is used to receive signal light transmitted through the free space transmission link. The coherent optical signal receiver includes a received signal light monitoring module, a local oscillator light generation module, a local oscillator light power adjustment module, a control module, an integrated coherent receiving module, an analog-to-digital conversion module, and a digital signal demodulation module. The received signal light monitoring module is used to monitor the received signal light to obtain the current received optical power. P r ( t The local oscillator light generation module is used to generate local oscillator light that is coherently mixed with the received signal light; the local oscillator light power adjustment module is used to adjust the local oscillator light power before it enters the integrated coherent receiving module; the integrated coherent receiving module is used to optically mix the received signal light with the local oscillator light and complete photoelectric conversion to output an analog electrical signal; the analog-to-digital conversion module is used to convert the analog electrical signal into a digital signal; the digital signal demodulation module is used to recover and demodulate the digital signal. The control module stores preset system design parameters, including the minimum received optical power in the free space transmission link. P low The highest received optical power in the free space transmission link. P high When the received optical power is the minimum received optical power. P low The maximum operating local oscillator power that allows the system performance to meet a preset threshold under certain conditions. P LO,max ; and the local oscillator power is the maximum operating local oscillator power. P LO,max The critical received optical power corresponding to when the analog-to-digital conversion module reaches full amplitude or the coherent optical signal receiver reaches a preset saturation criterion. P full ; When the critical received optical power P full Not less than the maximum received optical power P high At that time, the coherent optical signal receiver operates with a fixed local oscillator power; When the critical received optical power P full Less than the maximum received optical power P high At that time, the control module determines the current received optical power. P r ( t The maximum operating local oscillator power P LO,max and the critical received optical power P full The local oscillator optical power adjustment module is controlled to adaptively adjust the local oscillator optical power before it enters the integrated coherent receiving module, wherein: when the current received optical power... P r ( t The power received is not greater than the critical received optical power. P full At that time, the local oscillator optical power before entering the integrated coherent receiving module remains at the maximum operating local oscillator optical power. P LO,max When the current received optical power P r ( t () greater than the critical received optical power P full At that time, the local oscillator optical power before entering the integrated coherent receiving module is related to the current received optical power. P r ( t ) decreases as it increases.

2. The system according to claim 1, characterized in that, The received signal optical monitoring module includes: A beam splitter is used to separate a portion of the optical power from the received signal light to form a monitoring branch. An optical power detection unit is used to detect the optical power in the monitoring branch; The power conversion unit is used to convert the optical power in the monitoring branch into the currently received optical power in the main receiving branch based on the splitting ratio of the optical splitter and the fixed link loss. P r ( t ).

3. The system according to claim 1, characterized in that, The local oscillator optical power adjustment module includes at least one of a variable optical attenuator, a laser driver adjustment unit, and a tunable amplification unit.

4. A method for adaptive adjustment of local oscillator power in coherent free-space light reception for atmospheric scintillation, characterized in that, Includes the following steps: Obtain the range of received optical power variation in the actual free-space link to obtain the minimum received optical power. P low and maximum received optical power P high ; Construct a design platform identical to the actual receiving system, set a first optical attenuation module in the receiving signal optical branch of the design platform, and set a second optical attenuation module in the local oscillator optical branch of the design platform; The first optical attenuation module is used to adjust the received signal optical power to the minimum received optical power. P low The local oscillator power is gradually increased using the second optical attenuation module until the system performance meets a preset threshold. The local oscillator power before entering the integrated coherent receiver module at this point is determined as the maximum operating local oscillator power. P LO,max ; Maintain the local oscillator power before entering the integrated coherent receiving module at the maximum operating local oscillator power. P LO,max The received optical power is gradually increased using the first optical attenuation module until the analog-to-digital conversion module reaches full amplitude or the coherent optical signal receiver reaches a preset saturation criterion. The received optical power at this point is then determined as the critical received optical power. P full ; Compare the critical received optical power P full With the highest received optical power P high When the critical received optical power P full Not less than the maximum received optical power P high At that time, the coherent optical signal receiver operates with a fixed local oscillator power; When the critical received optical power P full Less than the maximum received optical power P high During actual communication, the current received optical power is obtained in real time through the received signal optical monitoring module. P r ( t The control module determines the current received optical power based on this information. P r ( t The maximum operating local oscillator power P LO,max and the critical received optical power P full The local oscillator power adjustment control quantity is generated to adaptively adjust the local oscillator power before it enters the integrated coherent receiving module; Wherein, when the current received optical power P r ( t The power received is not greater than the critical received optical power. P full At that time, the local oscillator optical power before entering the integrated coherent receiving module remains at the maximum operating local oscillator optical power. P LO,max When the current received optical power P r ( t () greater than the critical received optical power P full At that time, the local oscillator optical power before entering the integrated coherent receiving module is related to the current received optical power. P r ( t ) decreases as it increases.

5. The method according to claim 4, characterized in that, The process of obtaining the range of received optical power variation in the actual free-space link includes: The transmitter emits continuous optical signals, training optical signals, or test optical signals equivalent to actual service signals. The receiving end continuously monitors the received optical power in the actual free space link and statistically analyzes the changes in received optical power during the monitoring period. The lowest received optical power occurring during the monitoring period is determined as the lowest received optical power. P low The highest received optical power occurring during the monitoring period is determined as the highest received optical power. P high .

6. The method according to claim 4, characterized in that, The system performance meets the preset threshold, including any of the following: the bit error rate is lower than the preset bit error threshold; the error vector amplitude meets the preset condition; the Q factor meets the preset condition.

7. The method according to claim 4, characterized in that, The preset saturation criterion includes at least one of the following conditions: the input waveform of the analog-to-digital converter reaches full amplitude; the input waveform of the analog-to-digital converter shows clipping. The output of the coherent optical signal receiver exhibits a preset level of nonlinear distortion; the transimpedance amplifier enters the saturation region.

8. The method according to claim 4, characterized in that, When all powers are expressed in dBm, and the current received optical power P r ( t () greater than the critical received optical power P full At that time, the local oscillator optical power before entering the integrated coherent receiving module satisfies: P LO ( t ) = P LO,max + P full - P r ( t )； in, P LO ( t The value represents the local oscillator power before entering the integrated coherent receiving module at the current moment.

9. The method according to claim 4, characterized in that, When there is a fixed insertion loss IL in the local oscillator optical branch, the control module calculates the control quantity of the local oscillator optical power adjustment module based on the target local oscillator optical power before entering the integrated coherent receiving module and the fixed insertion loss IL.

10. The method according to claim 4, characterized in that, The local oscillator optical power before entering the integrated coherent receiving module is adaptively adjusted, using either a continuous adjustment method or a step-by-step adjustment method. The continuous adjustment method is as follows: the control module adjusts the current received optical power based on the real-time acquired data. P r ( t The target local oscillator power is continuously calculated, and the local oscillator power adjustment module is driven to continuously change the power. The graded adjustment method is as follows: multiple local oscillator optical power grades are preset, and when the current received optical power is... P r ( t When falling into different ranges, switch to the local oscillator power of the corresponding level.

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