A system and method for compensating for latency in a multi-channel fiber optic transmission

Abstract

The application discloses a kind of multi-channel optical fiber transmission delay compensation system and method, the system includes: delay compensation unit is used to pass through optical fiber transmission to terminal receiving and reflecting unit with several optical signals, while real-time correction reference signal transmission process in coupled signal The delay error generated;Terminal receiving and reflecting unit receives several optical signals and outputs to external communication equipment, while the reference signal is returned to optical path selection unit, to obtain the reference signal with error;Optical path selection unit is used to gate each road reference signal with error, and send to digital phase discriminator;Digital phase discriminator is based on the phase of reference signal and reference signal with error, calculates delay difference, and each road reference signal corresponding delay difference is sent to phase control unit;Phase control unit is used to complete the correction of each road transmission delay error according to each road reference signal corresponding delay difference.The application can real-time correction signal transmission generated delay difference.

Description

Technical Field

[0001] This invention relates to the field of optical transmission technology, and in particular to a delay compensation system and method for multi-channel optical fiber transmission. Background Technology

[0002] Optical fiber communication and transmission are widely used in modern communications and radar industries due to their advantages such as large bandwidth, high transmission rate, strong anti-interference capability, low transmission loss over long distances, and minimal susceptibility to external environmental influences. With technological advancements, increasingly higher demands are being placed on optical fiber transmission capabilities to achieve high-precision, high-quality transmission. Therefore, long-distance optical fiber equal-phase, equal-delay transmission technology is constantly being updated and iterated.

[0003] Currently, fiber optic delay compensation technologies at home and abroad mainly include three types: frequency domain method, optical time domain back reflection method, and optical interference method. For frequency domain methods, vector network analyzers are typically used to measure time delay differences. These methods offer high accuracy, reaching the picosecond level. However, they require external photoelectric conversion equipment and introduce measurement errors. The hardware platform for time delay measurement is complex, hindering its generalization and widespread engineering application. Optical time-domain backscattering (OTDR) involves inputting a pilot light signal from the fiber optic transmitter, transmitting it through the fiber, and reflecting it back to the transmitter at the receiving end. The time delay difference is calculated by measuring the reflected signal. Such systems are generally called optical time-domain reflectometers (OTDRs). Because the entire measurement process is performed on the analog end, it is a lossy time delay compensation technique. Optical interferometry, based on the Michelson interferometer principle, offers high time delay measurement accuracy, but the test platform is difficult to build, and the accuracy is dependent on the light source quality. It is also unsuitable for measuring time delay in long-distance fiber optic transmission, thus it is rarely used in engineering.

[0004] Current academic research includes: In the paper "Research on High-Precision Remote Fiber Optic Transmission Delay Measurement System" published in *Optical Communication Technology*, the "stop pulse" signal undergoes two electro-optical conversions, two optical-electrical conversions, and one remote relay transmission in the slave unit during the correction process. These conversions introduce new delay errors, which can reach several nanoseconds or even tens of nanoseconds. Large delay errors can lead to cross-cycle phenomena. The correction method used in the system cannot solve the phase ambiguity problem and cannot achieve complete correction for cross-cycle delays. Therefore, this correction method is only suitable for delay errors within a period, and the introduced delay error must be small enough to remain within a single period, resulting in low applicability. Another example is the paper "Digital Phase Compensation Method for Fiber Optic Time and Frequency Transmission" published in *Journal of Time and Frequency*. In the article, the phase error calculation uses an analog phase detection method, mixing two signals and outputting a low-frequency signal through a filter. The phase difference is calculated by sampling the voltage of the low-frequency signal. However, due to the large error in the accuracy of the sampled voltage, the calculated phase difference will be inaccurate, making it a lossy phase detection method. Furthermore, it cannot calculate the flip period for phase reversals, so this compensation method is only applicable to phase errors within a single period and is ineffective for cross-period errors. In the article "A Phase Compensation Method for Time-Frequency Signals" published in the *Journal of Time and Frequency*, the phase changes with temperature, and a curve is fitted to this temperature-dependent phase error. This introduces fitting errors, and the phase is adjusted through discrete steps, making it a lossy phase correction method. Summary of the Invention

[0005] In view of this, the present invention provides a delay compensation system and method for multi-channel optical fiber transmission to solve the above-mentioned technical problems.

[0006] This invention discloses a delay compensation system for multi-channel optical fiber transmission, comprising:

[0007] The time delay compensation unit is used to transmit several optical signals input by the user in the coupled signal to the terminal receiving and reflecting unit through optical fiber, and at the same time correct the time delay error generated during the transmission of the reference signal in the coupled signal in real time.

[0008] The terminal receiving and reflecting unit is used to output several optical signals received from the user input to an external communication device to enable communication between the user and the outside world; at the same time, it returns the reference signal to the optical path selection unit along the original transmission path of the reference signal to obtain a reference signal with error.

[0009] The optical path selection unit is used to perform time-division selection of the reference signals with errors sent by the wavelength division multiplexing unit received by each channel, and to send the reference signals with errors that are selected and returned in sequence to the digital phase detector.

[0010] The digital phase detector is used to receive a reference signal and a reference signal with error. Based on the phase of the reference signal and the reference signal with error, it calculates the phase error caused by the transmission of the reference signal through the optical fiber, converts the phase error into a time delay difference, and sends the time delay difference corresponding to each reference signal to the phase control unit.

[0011] The phase control unit is used to control the adjustable delay lines in the delay compensation unit according to the delay difference corresponding to each reference signal, so as to correct the transmission delay error of each channel.

[0012] Furthermore, it also includes:

[0013] The wavelength division multiplexing unit is used to couple the received reference signal and several optical signals input by the user and then transmit them to the time delay compensation unit so as to realize that the reference signal and several optical signals input by the user can be transmitted independently.

[0014] The reference signal generation unit is used to transmit a specified point frequency signal, i.e., a reference signal, to the digital phase detection unit and the optical signal distribution unit, respectively.

[0015] An optical signal distribution unit is used to divide the received reference signal into multiple reference signals;

[0016] The photoelectric conversion unit is used to convert the reference signal with errors sent by the optical path selection unit into an radio frequency signal, and then transmit the radio frequency signal to the digital phase detector unit.

[0017] Furthermore, the digital phase detector is also used for:

[0018] It receives the radio frequency signal output from the photoelectric conversion unit and the reference signal output from the reference signal generation unit, converts the two signals from analog signals to digital signals, realizes digital phase detection and calculates the phase error of the two signals through digital down-conversion, converts the phase error into a time delay difference, and outputs the time delay difference corresponding to each signal to the phase control unit.

[0019] The present invention also discloses a method for delay compensation system applicable to the above-described multi-channel optical fiber transmission, the method comprising:

[0020] The delay compensation unit transmits several optical signals input by the user in the coupled signal to the terminal receiving and reflecting unit through optical fiber, and at the same time corrects the delay error generated during the transmission of the reference signal in the coupled signal in real time.

[0021] The terminal receiving and reflecting unit outputs several optical signals received from the user input to external communication equipment to enable communication between the user and the outside world; along the original transmission path of the reference signal, the reference signal is returned to the wavelength division multiplexing unit to obtain a reference signal with error.

[0022] The optical path selection unit performs time-division selection on the reference signals with errors sent by the wavelength division multiplexing unit received by each channel, and sends the selected reference signals with errors to the digital phase detector.

[0023] The digital phase detector receives a reference signal and a reference signal with error. Based on the phase of the reference signal and the reference signal with error, it calculates the phase error caused by the transmission of the reference signal through the optical fiber, converts the phase error into a time delay difference, and sends the time delay difference corresponding to each reference signal to the phase control unit.

[0024] The phase control unit controls each adjustable delay line in the delay compensation unit according to the delay difference corresponding to each reference signal, thereby correcting the transmission delay error of each channel.

[0025] Furthermore, it also includes:

[0026] The wavelength division multiplexing unit couples the received reference signal and several optical signals input by the user and then transmits them to the time delay compensation unit to achieve independent transmission of the reference signal and several optical signals input by the user.

[0027] The reference signal generation unit transmits a specified point frequency signal, which is both a reference signal and a correction signal, to the digital phase detection unit and the optical signal distribution unit, respectively.

[0028] The optical signal distribution unit splits the received reference signal into multiple reference signals;

[0029] The photoelectric conversion unit converts the reference signal with errors sent by the optical path selection unit into an radio frequency signal, and transmits the radio frequency signal to the digital phase detector unit.

[0030] Furthermore, the digital phase detector receives the radio frequency signal output by the photoelectric conversion unit and the reference signal output by the reference signal generation unit. It converts the two signals from analog signals to digital signals, realizes digital phase detection of the two signals and calculates the phase error through digital down-conversion, converts the phase error into a time delay difference, and outputs the time delay difference corresponding to each signal to the phase control unit.

[0031] Furthermore, the receiving photoelectric conversion unit outputs a reference radio frequency signal with error, and simultaneously receives a reference signal output from a reference signal generation unit. The two signals are converted from analog to digital signals, and digital down-conversion is used to achieve digital phase detection and calculate the phase error between the two signals. The phase error is then converted into a time delay difference, including:

[0032] Step 1: Sample the reference signal and perform digital down-conversion on the sampled signal; obtain the initial phase of the reference signal based on the signal obtained from the digital down-conversion.

[0033] Step 2: Sample the reference signal with error, perform digital down-conversion, and calculate the phase value of the reference signal with error;

[0034] Step 3: Based on the initial phase of the reference signal and the phase value of the reference signal with error, obtain the phase error generated during the transmission of the reference signal;

[0035] Step 4: Convert the phase error obtained in Step 3 into a time delay difference.

[0036] Further, step 1 includes:

[0037] Let the reference signal s i (t i Modeling f) with amplitude A, carrier frequency f, and initial phase as The digital phase detector unit samples and discretizes the reference signal, with a sampling frequency of f. s Where m is the index of the sampling point, the sampled signal is represented as:

[0038]

[0039] The sampled signal is digitally down-converted to achieve digital phase detection, i.e.:

[0040]

[0041] The downconversion factor is expressed as:

[0042]

[0043] Selecting M points as one frame of data, the expression for digital frequency conversion accumulation of one frame of reference signal is:

[0044]

[0045] Will Expanding using vectors, we get:

[0046]

[0047] Will The expression is simplified to The initial phase of the reference signal can be accurately calculated using the simplified expression. Finally, digital phase detection of the reference signal is completed.

[0048] Further, step 2 includes:

[0049] For the k-th transmission fiber, its original length is L. k Let the speed of light be C, and the reference signal be a phase compensation introduced by the initial tunable fiber delay line be φ. kThe phase error introduced by long-distance optical fiber is PhaErr. k Then the reference signal returns to the phase when it arrives at the digital phase detector. It can be represented as:

[0050]

[0051] Further, step 3 includes:

[0052] The error reference signal returned to the phase detector via the k-th fiber is represented as:

[0053]

[0054] Digital phase detection is performed on the reference signal with error in the k-th channel to obtain the phase value. This phase value Subtract the actual fiber length L k The introduced phase delay value is then reduced by the initial phase of the reference signal. and fiber delay compensation φ k Then the phase error PhaErr introduced by the temperature change of the optical fiber can be calculated. k :

[0055]

[0056] Further, step 4 includes:

[0057] When the system's time delay error is within a single period, based on the calculated phase error PhaErr k When converted to a time delay difference, the error is corrected by controlling the time delay compensation unit, and the time delay difference τ k 'for:

[0058] τ k '=PhaErr k / (2πf)

[0059] When the system's transmission exhibits a large error that spans a single cycle, it is necessary to calculate the number of fuzzy cycles N. k Only then can the actual phase difference be calculated, and then the actual time delay difference τ can be calculated. k ,Right now:

[0060] τ k =(PhaErr) k +N k *2π) / (2πf)

[0061] Let the carrier frequency of the i-th correction reference signal be f. i , Let f be the carrier frequency of the corresponding correction reference signal for the k-th channel.i At that time, the phase error calculated by digital phase detection, Let f be the carrier frequency of the corresponding correction reference signal for the k-th channel. i The number of ambiguity cycles at different times corresponds to different actual phase differences for different carrier frequencies, i.e.:

[0062]

[0063] The maximum value of the ambiguity period N is determined based on the maximum value of the reference frequency. max .

[0064] Furthermore, step 4 also includes:

[0065] Based on the maximum ambiguity period value N of the maximum reference frequency point max By iterating through the values ​​from smallest to largest, and using the ambiguity period value of the maximum frequency point in each iteration, the ambiguity period values ​​of the remaining reference frequencies can be calculated. The actual phase of the calculated remaining correction reference frequencies is then calculated as 2πf. i τ k The ambiguous phase error compared to the corresponding digital phase detection After subtraction, take the remainder after dividing by 2π to calculate the phase error residual. Then, sum the squares of the phase error residuals calculated for each correction reference frequency point as the phase error cost function, i.e.:

[0066]

[0067] Based on the least squares principle, the minimum value of the phase error residual is calculated, and the corresponding fuzzy period is determined. This represents the actual number of fuzzy periods. With this, the fuzzy period can be solved, and the cross-period time delay error can be calculated.

[0068]

[0069] Finally, the adjustable fiber delay line is controlled by the phase control unit to compensate for the delay error of the k-th channel, thereby correcting the delay error of the k-th fiber transmission.

[0070] Because of the adoption of the above technical solution, the present invention has the following advantages:

[0071] (1) Achieve high-precision compensation for time delays across cycles.

[0072] Traditional techniques can only measure and compensate for time delay within a cycle. This invention can calculate the number of ambiguity cycles for time delay across cycles by using a phase deambiguation method with multiple correction frequency points, thereby achieving correction for time delay across cycles.

[0073] (2) Digital phase detection enables non-destructive phase error measurement.

[0074] Existing technology is based on analog phase detectors. It mixes the original reference signal and the reference signal reflected back after transmission through optical fiber, outputs the required signal through a low-pass filter, samples the voltage of the signal, and finally calculates the phase difference based on the sampled voltage. However, the analog calculation method has a large measurement error.

[0075] This invention employs digital phase detection and outputs high-precision phase error through mathematical calculation. It is a lossless phase error calculation technique that does not introduce new errors.

[0076] (3) Real-time correction of time delay error

[0077] Traditional time delay compensation systems calculate phase errors offline to compensate for delays, failing to provide real-time correction while the system is operational. This invention utilizes a wavelength division multiplexing (WDM) unit to achieve online integration of the time delay correction system and the fiber optic transmission system, ensuring that the correction system has no impact on the fiber optic transmission system while simultaneously providing real-time time delay correction.

[0078] (4) Time delay error correction for multi-channel fiber optic transmission

[0079] Traditional delay compensation techniques compensate for delays in a single fiber optic transmission. This invention extends the transmission to multiple fiber optic transmissions by using an optical path selection unit and achieves real-time correction of multiple fiber optic transmissions through low-cost hardware reuse. Attached Figure Description

[0080] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments recorded in the embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings.

[0081] Figure 1 This is a schematic diagram of a delay compensation system for multi-channel optical fiber transmission according to an embodiment of the present invention;

[0082] Figure 2 This is a schematic diagram of the waveform curve of the signal delay within the period according to an embodiment of the present invention;

[0083] Figure 3 This is a schematic diagram of the waveform curve of the signal delay across cycles in an embodiment of the present invention;

[0084] Figure 4 This is a schematic diagram of the phase error cost function curve within the signal delay period according to an embodiment of the present invention;

[0085] Figure 5This is a schematic diagram of the phase error cost function curve of signal delay across cycles in an embodiment of the present invention;

[0086] Figure 6 This is a schematic diagram of the simulated waveform curve after correction using the technology of this invention in an embodiment of the invention;

[0087] Figure 7 This is a schematic diagram of the waveform curves before and after correction in an engineering process according to an embodiment of the present invention;

[0088] Figure 8 This is a schematic diagram of the phase error cost curve for long-distance transmission according to an embodiment of the present invention. Detailed Implementation

[0089] The present invention will be further described in conjunction with the accompanying drawings and embodiments. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. All other embodiments obtained by those skilled in the art should fall within the protection scope of the present invention.

[0090] This invention addresses the limitations of traditional time delay measurement systems by converting time delay into phase measurement for high-precision measurement. Since traditional techniques cannot correct cross-cycle errors, this invention proposes a phase ambiguity resolution method to correct cross-cycle time delays. Instead of traditional analog phase detection, this invention proposes a digital phase detection method for lossless calculation of phase errors without introducing new errors. Compared to the complex hardware requirements and platform construction of traditional methods, this invention proposes a general-purpose hardware implementation that is engineering-ready and can simultaneously correct multiple channels, saving hardware costs. Since traditional correction methods are offline, this invention proposes coupling the correction system into the transmission system to achieve real-time time delay correction. Ultimately, this achieves high-precision time delay correction for multiple channels, providing high-quality data transmission for subsequent time-sensitive systems.

[0091] See Figure 1 This invention provides an embodiment of a delay compensation system for multi-channel optical fiber transmission, comprising:

[0092] The time delay compensation unit is used to transmit several optical signals input by the user in the coupled signal to the terminal receiving and reflecting unit through optical fiber, and at the same time correct the time delay error generated during the transmission of the reference signal in the coupled signal in real time.

[0093] The terminal receiving and reflecting unit is used to output several optical signals received from the user input to an external communication device to enable communication between the user and the outside world; at the same time, it returns the reference signal to the optical path selection unit along the original transmission path of the reference signal to obtain a reference signal with error.

[0094] The optical path selection unit is used to perform time-division selection of the reference signals with errors sent by the wavelength division multiplexing unit received by each channel, and to send the reference signals with errors that are selected and returned in sequence to the digital phase detector.

[0095] The digital phase detector is used to receive a reference signal and a reference signal with error. Based on the phase of the reference signal and the reference signal with error, it calculates the phase error caused by the transmission of the reference signal through the optical fiber, converts the phase error into a time delay difference, and sends the time delay difference corresponding to each reference signal to the phase control unit.

[0096] The phase control unit is used to control the adjustable delay lines in the delay compensation unit according to the delay difference corresponding to each reference signal, so as to correct the transmission delay error of each channel.

[0097] This embodiment also includes:

[0098] The wavelength division multiplexing unit is used to couple the received reference signal and several optical signals input by the user and then transmit them to the time delay compensation unit so as to realize that the reference signal and several optical signals input by the user can be transmitted independently.

[0099] The reference signal generation unit is used to transmit a specified point frequency signal, i.e., a reference signal, to the digital phase detection unit and the optical signal distribution unit, respectively.

[0100] An optical signal distribution unit is used to divide the received reference signal into multiple reference signals;

[0101] The photoelectric conversion unit is used to convert the reference signal with errors sent by the optical path selection unit into an radio frequency signal, and then transmit the radio frequency signal to the digital phase detector unit.

[0102] In this embodiment, the digital phase detector is also used for:

[0103] It receives the radio frequency signal output from the photoelectric conversion unit and the reference signal output from the reference signal generation unit, converts the two signals from analog signals to digital signals, realizes digital phase detection and calculates the phase error of the two signals through digital down-conversion, converts the phase error into a time delay difference, and outputs the time delay difference corresponding to each signal to the phase control unit.

[0104] Based on the above embodiments, the present invention provides an embodiment of a delay compensation method for multi-channel optical fiber transmission. In this embodiment:

[0105] Assuming the system uses a signal bandwidth of 200MHz and its signal frequency is between 275MHz and 475MHz, to distinguish it from the system's signal frequency, five point-frequency signals with frequencies of 530MHz, 550MHz, 570MHz, 590MHz, and 600MHz are selected as reference signals. These reference signals are the system's correction signals, and phase ambiguity periods are resolved using five coprime frequency points across five carrier cycles. Since the wavelength of the correction signal differs from that of the user signal within the 275MHz–475MHz bandwidth, they can be coupled together in the same optical fiber using a wavelength division multiplexing (WDM) unit for independent transmission. The correction signal is reflected by the terminal receiving and reflecting unit and returns along the original optical signal path. Therefore, the transmission path of the correction signal is twice that of the user signal, and the phase error caused by fiber optic transmission is also twice that of the user signal transmission error.

[0106] This embodiment transmits a correction signal along the system user signal path and returns along the transmission path by reflection. Therefore, this correction method can simultaneously correct the time delay caused by the change in refractive index with temperature and the time delay caused by the change in fiber length with temperature.

[0107] Assuming there are 10 fiber optic transmission channels, the system sequentially outputs five point-frequency signals (530MHz, 550MHz, 570MHz, 590MHz, and 600MHz) from the frequency source built into the reference signal generation unit. These correction signals are then power-divided and output to the digital phase detector unit. The phase detector unit branch serves as the original, unaffected reference signal for the signal, and is simultaneously output to the optical signal distribution unit. The optical signal distribution unit converts the RF signal into an optical signal and distributes it into 10 channels, each outputting to a wavelength division multiplexing (WDM) unit. The WDM unit couples the 10 user-used optical signals within a bandwidth of 275MHz to 475MHz with the 10 correction optical signals, fusing the corresponding signals into a single fiber and outputting the 10 fused signals to the delay compensation unit.

[0108] The merged user signal and correction signal pass through the time delay compensation unit. According to the adjustable fiber delay line set in the previous cycle, the signal is time-delayed and corrected. After the 10 merged signals are corrected by the time delay compensation unit, they are output to 1000m long-distance optical fibers for transmission. Finally, they are output to the terminal receiving and reflection unit. This unit outputs the user signal to the back end to realize the user's communication or transmission. At the same time, it reflects the correction signal containing errors so that it returns along the original path.

[0109] In the optical path selection unit, 10 returning optical signals are selected sequentially using a gating method and output to the photoelectric conversion unit. The photoelectric conversion unit converts the optical signals into radio frequency signals and outputs them to the digital phase detection unit. This unit samples the initial correction signal and the error-containing correction signal returned after twice the fiber path. To satisfy the Nyquist sampling theorem and the maximum sampling frequency of 600MHz, the system selects a sampling rate f. s The frequency is 1400MHz. The analog signal is converted into a digital signal, and digital phase detection is implemented in the digital domain, ultimately outputting the phase error without loss. Digital phase detection is the core of this invention, and the relevant derivation and calculation of digital phase detection will be performed below.

[0110] Let the reference signal s i (t i Modeling f) with amplitude A, carrier frequency f, and initial phase as The analog-to-digital converter module of the digital phase detector unit samples and discretizes the continuous time-domain signal, with a sampling period of T. s The AD sampling frequency f of the digital phase detector unit is specified. s Then the sampling period T s =1 / f s Let m be the sampling point number, then the complex signal form is shown in expression (1):

[0111]

[0112] The sampled signal is digitally down-converted to complete digital phase detection. The down-conversion factor is shown in expression (2):

[0113]

[0114] The sampled signal is digitally frequency converted, as shown in expression (3):

[0115]

[0116] M points are selected as one frame of data. The windowed data is digitally down-converted. By performing coherent accumulation of the signal, the detection sensitivity of the system is improved, and the initial phase of the reference signal can be determined more stably. The expression for digital frequency conversion accumulation of a frame of correction signal. See (4):

[0117]

[0118] Digital frequency conversion expression Expand it using vectors, as shown in expression (5):

[0119]

[0120] The expression for the accumulation of a frame of reference signal through digital frequency conversion is simplified. The initial phase of the reference signal can be accurately calculated using this expression. The digital phase detection was finally completed.

[0121] For the reference optical signal returned after passing through the terminal receiving and reflecting unit, the single-path fiber length of the k-th path is L. k The propagation of the reference signal in the optical fiber causes a phase delay; twice the optical path length results in twice the transmission phase delay, which can be calculated. For the k-th reference signal, assume that the phase compensation introduced by its initial adjustable fiber delay line is φ. k The phase error introduced by long-distance optical fiber is PhaErr. k Then the phase of the reference optical signal when it returns to the digital phase detector This can be expressed as expression (6), where C is the speed of light:

[0122]

[0123] The signal returning to the phase detector through the k-th optical fiber is shown in expression (7):

[0124]

[0125] For the k-th channel, digital phase detection is performed using the same down-conversion factor as the reference signal, and the phase value is finally obtained. This phase value Subtract the designed fiber length L k The introduced phase delay value is then removed by subtracting the initial phase of the reference signal. and fiber delay compensation φ k Then the phase error PhaErr introduced by the temperature change of the optical fiber can be calculated. k See expression (8):

[0126]

[0127] When the system's time delay error is within a single period, the comparison curve between its time delay error waveform and the reference waveform is shown below. Figure 2 As shown, based on the calculated phase error PhaErr k When converted to a time delay difference, the error is corrected by controlling the time delay compensation unit, and the time delay τ k See expression (9):

[0128] τ k '=PhaErr k / (2πf) (9)

[0129] When the system transmission exhibits a large error that spans a single cycle, the comparison curve between its delay error waveform and the reference waveform is shown below. Figure 3 The calculated phase difference is therefore ambiguous, so it is necessary to calculate the number of ambiguity periods N. k Only then can the actual phase difference be calculated, and then the actual time delay difference τ can be calculated. k See expression (10):

[0130] τ k =(PhaErr) k +N k *2π) / (2πf) (10)

[0131] The carrier frequency of the i-th correction reference signal is defined as f. i , Let f be the carrier frequency of the corresponding correction reference signal for the k-th channel. i The phase error calculated at that time, Let f be the carrier frequency of the corresponding correction reference signal for the k-th channel. i The number of fuzzy cycles at different times corresponds to different actual phase differences for different carrier frequencies, as shown in expression (11):

[0132]

[0133] Based on the principle that higher frequencies correspond to shorter wavelengths, the maximum value of the ambiguity period N is determined according to the maximum value of the reference frequency. max Furthermore, given a fixed phase extraction accuracy, higher frequencies yield higher calculated delay accuracy. For ordinary G652 optical fiber, its temperature coefficient ranges from 30 ps / (km·℃) to 200 ps / (km·℃). This means that for a 1000m long transmission fiber, the maximum delay change within a temperature range of -10℃ to +45℃ is approximately 11 ns. For a 600MHz correction reference signal, the phase difference changes within a range of 2376°. Therefore, the maximum value of the ambiguity period number N is N0. max It is 7.

[0134] The number of correction reference signal frequency points is specified as I, and the correction frequency point values ​​are sorted in ascending order. The correction reference signal is then used as the reference signal with the maximum frequency point f. I It is necessary to set N from [0 N] max The system iterates through each ambiguity period within the specified range. For each ambiguity period, the actual phase 2πf corresponding to the maximum frequency of the correction reference signal for that ambiguity period can be calculated. I τ k As shown in expression (12), the actual phase 2πf of the remaining correction reference frequencies is then calculated sequentially. i τ k See expression (13):

[0135]

[0136]

[0137] The actual phase 2πf of the remaining correction reference frequencies is calculated. i τ k The ambiguous phase error compared to the corresponding digital phase detection After subtraction, take the remainder after dividing by 2π to calculate the phase error residual. Sum the squares of the phase error residuals calculated at each correction reference frequency point as the phase error cost function, as shown in expression (14):

[0138]

[0139] Based on the least squares principle, the minimum value of the phase error residual is calculated, and the corresponding fuzzy period is determined. This represents the actual number of fuzzy cycles, thus solving for the fuzzy cycles and finally calculating the time delay error across cycles.

[0140]

[0141] Finally, the adjustable fiber delay line is controlled by the phase control unit to compensate for the time delay error of the k-th channel, thereby correcting the transmission time delay error of the k-th fiber. For Figure 2 The time delay within the period shown is used to obtain the phase error cost function curve using this correction technique. Figure 4 As shown in the figure, the phase residual is minimized when the number of fuzzy periods is 0, which is close to the actual result. Therefore, the fuzzy period of the error within a single period is 0, meaning it does not cross any periods, which is consistent with reality. For Figure 3 The phase error cost function curve for the cross-cycle time delay shown is obtained from the curve. Figure 5 As shown, from Figure 3 As can be seen, the error spans 1 cycle, while the corresponding Figure 5 The phase residual is minimized when the number of intermediate periods is 1, which is in line with the correction expectation.

[0142] The correction technique in this embodiment is used to compensate and simulate the delayed waveform. A comparison between the corrected simulated waveform and the reference waveform is shown below. Figure 6 As shown, this correction technique is used to compensate for time delay on a time delay compensation device. The corrected waveform is compared with the reference waveform. Figure 7 As shown, due to quantization errors and system random errors in the engineering process, there is still a residual error of about 10ps after correction. Therefore, there is a certain error between the corrected waveform curve and the reference curve, but a correction accuracy of 10ps is sufficient for normal use.

[0143] This embodiment can also extend the fiber optic length to hundreds of kilometers by simply increasing the number of fuzzy periods searched. Figure 8 The figure shows the phase residual cost function curves when the maximum fiber delay spans 32 cycles and the actual phase error spans 23 cycles. It can be seen from the figure that the phase residual is the smallest corresponding to the 23rd cycle, which verifies that the system can adapt to error correction for long-distance transmission.

[0144] The correction method of this invention can overcome the shortcomings of traditional time delay compensation in terms of technical principles. It achieves lossless phase error measurement through digital phase detection, and high-precision compensation for cross-cycle time delays through phase ambiguity resolution. Furthermore, it couples the time delay correction system and the transmission system together to achieve online real-time correction. The optical path selection unit enables time delay correction for multi-channel fiber optic transmission, saving correction resources and costs. The hardware design of this invention is simple and universal, scalable to most fiber optic transmission systems, easy to implement, and highly operable. From both technical principles and hardware implementation, it has significant advantages over traditional time delay compensation systems and has excellent application and promotion value.

[0145] Although the illustrative specific embodiments of the present invention have been described above to enable those skilled in the art to understand the invention, it should be understood that the invention is not limited to the scope of the specific embodiments. For those skilled in the art, various changes are obvious as long as they are within the spirit and scope of the invention as defined and determined by the appended claims, and all inventions utilizing the concept of the present invention are protected.

Claims

1. A delay compensation system for multi-channel optical fiber transmission, characterized in that, include: The time delay compensation unit is used to transmit several optical signals input by the user in the coupled signal to the terminal receiving and reflecting unit through optical fiber, and at the same time correct the time delay error generated during the transmission of the reference signal in the coupled signal in real time. The terminal receiving and reflecting unit is used to output several optical signals received from the user input to an external communication device to enable communication between the user and the outside world; at the same time, it returns the reference signal to the optical path selection unit along the original transmission path of the reference signal to obtain a reference signal with error. The optical path selection unit is used to perform time-division selection of the reference signals with errors sent by the wavelength division multiplexing unit received by each channel, and to send the reference signals with errors that are selected and returned in sequence to the digital phase detector. The digital phase detector is used to receive a reference signal and a reference signal with error. Based on the phase of the reference signal and the reference signal with error, it calculates the phase error caused by the transmission of the reference signal through the optical fiber, converts the phase error into a time delay difference, and sends the time delay difference corresponding to each reference signal to the phase control unit. The phase control unit is used to control the adjustable delay lines in the delay compensation unit according to the delay difference corresponding to each reference signal, so as to correct the transmission delay error of each channel. Also includes: The wavelength division multiplexing unit is used to couple the received reference signal and several optical signals input by the user and then transmit them to the time delay compensation unit so as to realize that the reference signal and several optical signals input by the user can be transmitted independently. The reference signal generation unit is used to transmit a specified point frequency signal, i.e., a reference signal, to the digital phase detection unit and the optical signal distribution unit, respectively. An optical signal distribution unit is used to divide the received reference signal into multiple reference signals; The photoelectric conversion unit is used to convert the reference signal with error sent by the optical path selection unit into an radio frequency signal, and transmit the radio frequency signal to the digital phase detection unit; The digital phase detector is also used for: It receives the radio frequency signal output from the photoelectric conversion unit and the reference signal output from the reference signal generation unit, converts the two signals from analog signals to digital signals, realizes digital phase detection and calculates the phase error of the two signals through digital down-conversion, converts the phase error into a time delay difference, and outputs the time delay difference corresponding to each signal to the phase control unit.

2. A method for delay compensation in the multi-channel optical fiber transmission system of claim 1, characterized in that, The method includes: The delay compensation unit transmits several optical signals input by the user in the coupled signal to the terminal receiving and reflecting unit through optical fiber, and at the same time corrects the delay error generated during the transmission of the reference signal in the coupled signal in real time. The terminal receiving and reflecting unit outputs several optical signals received from the user input to external communication equipment to enable communication between the user and the outside world; along the original transmission path of the reference signal, the reference signal is returned to the wavelength division multiplexing unit to obtain a reference signal with error. The optical path selection unit performs time-division selection on the reference signals with errors sent by the wavelength division multiplexing unit received by each channel, and sends the selected reference signals with errors to the digital phase detector. The digital phase detector receives a reference signal and a reference signal with error. Based on the phase of the reference signal and the reference signal with error, it calculates the phase error caused by the transmission of the reference signal through the optical fiber, converts the phase error into a time delay difference, and sends the time delay difference corresponding to each reference signal to the phase control unit. The phase control unit controls each adjustable delay line in the delay compensation unit according to the delay difference corresponding to each reference signal, thereby correcting the transmission delay error of each channel.

3. The method according to claim 2, characterized in that, Also includes: The wavelength division multiplexing unit couples the received reference signal and several optical signals input by the user and then transmits them to the time delay compensation unit to achieve independent transmission of the reference signal and several optical signals input by the user. The reference signal generation unit transmits a specified point frequency signal, which is both a reference signal and a correction signal, to the digital phase detection unit and the optical signal distribution unit, respectively. The optical signal distribution unit splits the received reference signal into multiple reference signals; The photoelectric conversion unit converts the reference signal with errors sent by the optical path selection unit into an radio frequency signal, and transmits the radio frequency signal to the digital phase detector unit.

4. The method according to claim 2, characterized in that, The digital phase detector receives the radio frequency signal output by the photoelectric conversion unit and the reference signal output by the reference signal generation unit. It converts the two signals from analog signals to digital signals, realizes digital phase detection of the two signals and calculates the phase error through digital down-conversion, converts the phase error into a time delay difference, and outputs the time delay difference corresponding to each signal to the phase control unit.

5. The method according to claim 4, characterized in that, The receiver receives the reference radio frequency signal with error output from the photoelectric conversion unit, and simultaneously receives the reference signal output from the reference signal generation unit. It converts both signals from analog to digital, performs digital down-conversion to achieve digital phase detection and calculate the phase error, and converts the phase error into a time delay difference, including: Step 1: Sample the reference signal and perform digital down-conversion on the sampled signal; obtain the initial phase of the reference signal based on the signal obtained from the digital down-conversion. Step 2: Sample the reference signal with error, perform digital down-conversion, and calculate the phase value of the reference signal with error; Step 3: Based on the initial phase of the reference signal and the phase value of the reference signal with error, obtain the phase error generated during the transmission of the reference signal; Step 4: Convert the phase error obtained in Step 3 into a time delay difference.

6. The method according to claim 5, characterized in that, Step 1 includes: Let the reference signal be Modeled as amplitude A, carrier frequency f Initial appearance The digital phase detector unit samples and discretizes the reference signal, with a sampling frequency of . Where m is the index of the sampling point, the sampled signal is represented as: The sampled signal is digitally down-converted to achieve digital phase detection, i.e.: The downconversion factor is expressed as: Selecting M points as one frame of data, the expression for digital frequency conversion accumulation of one frame of reference signal is: Will Expanding using vectors, we get: Will The expression is simplified to The initial phase of the reference signal can be accurately calculated using the simplified expression. Finally, digital phase detection of the reference signal is completed.

7. The method according to claim 6, characterized in that, Step 2 includes: For the k-th transmission fiber, its original length is Let the speed of light be C, and the reference signal be an initial tunable fiber delay line with phase compensation of C. The phase error introduced by long-distance optical fiber is Then the reference signal returns to the phase when it arrives at the digital phase detector. It can be represented as: 。 8. The method according to claim 7, characterized in that, Step 3 includes: The error reference signal returned to the phase detector via the k-th fiber is represented as: Digital phase detection is performed on the reference signal with error in the k-th channel to obtain the phase value. The phase value Subtract the actual fiber length The introduced phase delay value is then reduced by the initial phase of the reference signal. And fiber optic delay compensation Then the phase error introduced by the temperature change of the optical fiber can be calculated. : 。 9. The method according to claim 8, characterized in that, Step 4 includes: When the system's time delay error is within a single period, based on the calculated phase error When converted to a time delay difference, the error is corrected by controlling the time delay compensation unit. for: When the system's transmission exhibits a large error that spans a single cycle, it is necessary to calculate the number of fuzzy cycles. Only then can the actual phase difference be calculated, and then the actual time delay difference can be calculated. ,Right now: Setting the first i The carrier frequency of each correction reference signal is , Represented as the first k The corresponding correction reference signal carrier frequency is At that time, the phase error calculated by digital phase detection, Represented as the first k The corresponding correction reference signal carrier frequency is The number of ambiguity cycles at different times corresponds to different actual phase differences for different carrier frequencies, i.e.: The maximum value of the ambiguity period is determined based on the maximum value of the reference frequency. .

10. The method according to claim 9, characterized in that, Step 4 also includes: Based on the maximum ambiguity period value of the maximum reference frequency By iterating through the values ​​from smallest to largest, and using the ambiguity period value of the maximum frequency point in each iteration, the ambiguity period values ​​of the remaining reference frequencies can be calculated. The actual phase of the calculated remaining correction reference frequencies is then used to... The ambiguous phase error compared to the corresponding digital phase detection After doing the difference Taking the remainder, the phase error residual is calculated. The sum of the squares of the phase error residuals calculated at each correction reference frequency is used as the phase error cost function, i.e.: Based on the least squares principle, the minimum value of the phase error residual is calculated, and the corresponding fuzzy period is determined. This represents the actual number of fuzzy periods. With this, the fuzzy period can be solved, and the cross-period time delay error can be calculated. Finally, the adjustable fiber delay line is controlled by the phase control unit to compensate for the delay error of the k-th channel, thereby correcting the delay error of the k-th fiber transmission.

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