Optical domain simulation beamforming system delay and gain calibration method

Abstract

The invention discloses a time delay and gain calibration method for an optical domain simulation beamforming system, and relates to the technical field of microwaves and the field of optical communication. According to the method, a system structure is shown in a figure 1 in the specification, and the system comprises a signal source, an electric power divider, a low-noise amplifier, a directly modulated laser, an adjustable optical delay line, an adjustable optical attenuator, an optical coupler, a photoelectric detector, a frequency spectrograph and an upper computer. Signal source output is divided into N paths by an electric power divider, amplified by a low-noise amplifier and then loaded to N paths of directly modulated lasers with different optical carriers to realize electro-optical conversion, and modulated optical signals are transmitted to a delay gain control module consisting of an adjustable optical delay line and an adjustable optical attenuator through an optical fiber; the upper computer adjusts the delay and gain of the multi-path optical signals by controlling the delay gain control module, the multi-path optical signals are coupled into one path of optical signals through the optical coupler and then converted into electric signals through the photoelectric detector, and the frequency spectrograph is used for monitoring the power of the electric signals. The upper computer reads the power of the frequency spectrograph and adjusts the delay gain control module in real time so as to realize delay and gain calibration of any channel of the system, the calibration precision is high, and the operation is simple and fast.

Description

Technical Field

[0001] This invention relates to the fields of microwave technology and optical communication, and mainly to delay and gain calibration technology for optical beamforming systems. Background Technology

[0002] Each antenna receiving channel in a phased array architecture has non-time-varying errors such as circuit asymmetry, non-ideal device type, and inconsistency in assembly process, as well as changes in amplitude and phase response caused by environmental changes such as temperature and humidity in each channel. These errors will cause the broadband signals of multiple receiving channels to be uneven within the channel and inconsistent in amplitude and phase characteristics between channels, thus affecting multi-channel synthesis and system receiving performance. Therefore, multi-channel amplitude and phase calibration technology has emerged.

[0003] Beamforming technology is the core of smart antenna technology, directly affecting the gain, interference suppression, and other performance characteristics of array antennas in various directions. Electrical beamforming adds a complex weighted excitation to the radio frequency signal received by a multi-channel array antenna, and then combines the multiple signals for output. Optical beamforming first modulates the radio frequency signal onto an optical carrier through photoelectric conversion, and then performs amplitude and phase weighting on the modulated optical signal by delaying and gain control in the optical domain. Finally, the combined signal is output after photoelectric conversion by a photodetector.

[0004] Optical domain simulated beamforming technology can overcome the problems of low resolution and narrow angle scanning caused by the limited bandwidth of devices such as electric phase shifters in electrical domain beamforming technology. In addition, it has become a research hotspot in recent years due to its advantages such as low power consumption, strong anti-electromagnetic interference capability and large operating bandwidth.

[0005] In optical domain simulated beamforming systems, the varying optical power and optical path lengths of each channel lead to delay and gain mismatches between channels. Since optical domain simulated beamforming systems adjust the delay and gain of each channel to change the beam direction, delay and gain calibration are necessary before the system can operate normally. The amplitude and phase errors of the system severely affect beamforming performance. Furthermore, optical domain simulated beamforming systems cannot perform individual channel calibration using a vector network analyzer. Therefore, delay and gain calibration technology for optical domain simulated beamforming systems will play a crucial role in microwave photonic simulated beamforming systems. Summary of the Invention

[0006] To address the problems existing in the background technology, this invention proposes a method for calibrating the delay and gain of an optical domain simulated beamforming system. This method has two major advantages: First, this scheme only requires monitoring the output power of the system to calibrate the delay gain between multiple channels, without the need for repeated switching of channels, making the operation simple and convenient; Second, this scheme has high calibration accuracy, which is only limited by the accuracy of the optical domain delay attenuation device.

[0007] The technical solution adopted by this invention to solve the technical problem is as follows: the device includes a signal source, a 1-to-N power divider, N low-noise amplifiers (LNAs), N direct-modulated lasers (DMLs) with different wavelengths, N optical delay lines (VODLs), N optical attenuators (VOAs), an N-in-1 optical coupler (OC), a photodetector (PD), a spectrum analyzer, and a host computer. (See attached drawings.) Figure 1 As shown, the key feature is that: the signal source is input to an N-channel low-noise amplifier (LNA) via a one-to-N power divider, amplified, and then applied to a directly modulated laser (DML). The N signals are modulated onto N wavelength optical carriers via direct modulation. The N optical signals are then controlled by a delay-attenuation control module composed of a tunable fiber delay line (VODL) and a tunable optical attenuator (VOA) to achieve analog domain delay and gain adjustment. Subsequently, the signals are combined via an N-to-one optical coupler (OC), input to a photodiode (PD) for conversion into electrical signals, and finally, the power of the electrical signal output from the PD is observed using a spectrum analyzer. An optimization algorithm is used to search for a suitable delay-attenuation value based on the spectrum analyzer's display results. Finally, the delay-attenuation module is controlled by a host computer, thus completing the calibration of the delay and gain of the microwave photonic analog beamforming system.

[0008] The present invention includes the following steps in operation: (1) Set the signal source to output a single tone signal and connect it to a one-to-N power divider. After the N-channel electrical signals are amplified by a low-noise amplifier (LNA), they are loaded onto N direct-modulated lasers (DML) with different wavelength optical carriers. (2) Adjust the VOA of each channel to perform gain calibration for each channel. When adjusting the VOA of each channel, attenuate the VOA of all other channels to the maximum to achieve the effect of optical domain disconnection. (3) Using the first channel as the reference channel, based on step (2), the remaining channels are calibrated with the reference channel by delay. The VOA of the calibration channel and the reference channel are set to the gain calibration value. The VOA of the remaining channels other than the calibration channel and the reference channel is attenuated to the maximum, so as to achieve the effect of optical domain disconnection. This completes the calibration of the delay of each channel.

[0009] This invention proposes a method for delay and gain calibration of an optical domain simulated beamforming system. This method is a preprocessing step for the normal operation of the optical domain simulated beamforming system and has a crucial impact on the beamforming and spatial filtering operations of the microwave photonic simulated beamforming system. Furthermore, this calibration method is designed for multi-input single-output systems, has a simple system structure, and achieves high calibration accuracy. Attached Figure Description

[0010] Figure 1 This is a schematic diagram of the delay and gain calibration method of an optical domain simulated beamforming system according to the present invention; Figure 2 This is a flowchart illustrating the search principle of the host computer controlling the search for the optimal delay decay value. Figure 2 In the algorithm, P0 to P8 are nine search points, each defined by three-dimensional coordinates. X represents the delay value, Y represents the attenuation value, and Z represents the signal power displayed on the spectrum analyzer after the input link. The basic principle of the algorithm is as follows: the X-axis represents the delay parameter of a certain channel, the Y-axis represents the attenuation parameter, and Z represents the power value of the point. First, an initial point is determined. Then, the search proceeds in four directions: increasing or decreasing the delay, increasing or decreasing the attenuation, and reading the power of each new point. If the power of a new point increases, the search direction is changed; if the power decreases, the search continues in that direction until the power of the four points surrounding a given point is greater than that point. This point is then considered the optimal point. Figure 3 This diagram illustrates the power distribution of two signals of the same frequency and amplitude superimposed within the 4–18 GHz frequency range, with a delay difference of 0–250 ps. The x-axis represents the delay difference between the two signals, the y-axis represents the frequency of the two signals, and the z-axis represents the normalized power of the superimposed signal. It can be seen that when the delay difference between the two signals is 0 ps, ​​the power of the superimposed signal is consistent within the 4–18 GHz range. When the delay difference between the two signals is 50 ps, ​​the notch point appears at 10 GHz. Figure 4 To intercept Figure 3 The combined power profile of two signals with a delay difference of 100ps shows that the reciprocal of the frequency difference between the notch points of the combined power profile is the delay difference between the two signals. Therefore, the delay difference between the two channels can be determined by observing the frequency difference between the notch points of the swept signal on the spectrum analyzer. The VODL of the two channels can be adjusted according to this standard to compensate for this delay difference. Figure 5 This is a simulation diagram of synthesizing two signals at the same frequency, with a delay accuracy of 0.1 ps and an amplitude difference of 0.1 dB between the two signals, to simulate the delay adjustment accuracy and attenuation adjustment accuracy in our link. Detailed Implementation

[0011] The embodiments of the present invention will now be described in detail with reference to the accompanying drawings. These embodiments are implemented based on the technical solution of the present invention, and detailed implementation methods and specific operation processes are provided. However, the scope of protection of the present invention is not limited to the following implementation examples: like Figure 1The device shown includes a signal source, an 8-way power divider, eight directly modulated lasers of different wavelengths, eight low-noise amplifiers (LNA1-LNA8), eight tunable optical delay lines (VODL1-VODL8), eight tunable optical attenuators (VOA1-VOA8) forming a delay attenuation module, an 8-in-1 optical coupler (OC), a photodetector (PD), a spectrum analyzer, and a host computer. The signal source output is divided into eight paths by the 8-way power divider. Each path is amplified by a low-noise amplifier (LNA) and then fed onto a directly modulated laser (DML). The eight signals are modulated onto eight optical carriers of different wavelengths using direct modulation. The delay gain of each of the eight optical signals is controlled by the tunable fiber delay lines (VODL) and tunable optical attenuators (VOA). The signals are then combined by the 8-in-1 optical coupler (OC) and finally input to the PD to be converted into electrical signals. These electrical signals are then input to the spectrum analyzer for spectrum monitoring. The host computer reads the power from the spectrum analyzer and then... Figure 2 The search algorithm flow shown is used to calibrate delay and gain. The specific steps are as follows: Step 1: As shown in the attached diagram Figure 1 With the link connected as shown, the RF source is set to -40dBm, 10GHz single-tone signal output, and the VOA of channel one is set to a1 = 3dB. The VOA of channels two, three, four, five, six, seven, and eight is set to 30dB. At this time, it can be assumed that the optical paths of channels two, three, four, five, six, seven, and eight are open. Record the 10GHz signal power on the spectrum analyzer as P1 dBm at this time. Step 2: Set the VOA of the second channel to 0dB, and set the VOA of channels 1, 3, 4, 5, 6, 7, and 8 to 30dB. Similarly, treat these channels as open circuits. At this time, observe the 10GHz signal power on the spectrum analyzer, and then adjust the VOA value of the second channel until the 10GHz signal power on the spectrum analyzer is equal to P1 dBm. Record the VOA value of the second channel as a2 dB at this time. Step 3: Repeat the operation in Step 2 and record the gain calibration values ​​of channels 3, 4, 5, 6, 7 and 8 respectively, which are a3 dB, a4 dB, a5 dB, a6 dB, a7 dB and a8 dB. At this time, the gain calibration of channels 1 to 8 is considered to be completed. Step 4: Based on Step 3, select channel 1 as the reference channel, and perform delay calibration on channels 2, 3, 4, 5, 6, 7, and 8 respectively, relative to the reference channel. At this point, set the VOA of channel 1 to 3dB, the VOA of channel 2 to a2dB, and the VOA of channels 3, 4, 5, 6, 7, and 8 to 30dB, assuming these channels are optically unblocked. Perform coarse delay calibration by sweeping the frequency and observing the notch points, such as... Figure 4As shown, when the delay is fixed, the reciprocal of the frequency difference between the two notch points generated on the spectrum analyzer after the sweep signal passes through the link is the delay difference between the two channels. Then, fine calibration of the delay is performed using the principle of cancellation between two signals of the same frequency. The signal of the reference channel is sin(w(t+τ1)), and the signal of the calibration channel is sin(w(t+τ2)). By adjusting the delay difference between the two signals to make them out of phase, cancellation between the two signals can be achieved. When using a 10GHz RF signal as the detection signal, the VODL of the two channels is increased by 50ps on the basis of coarse calibration. Figure 5 As shown, with a delay accuracy of 0.1 ps and an attenuation accuracy of 0.1 dB, the relationship between the suppression power of the detection signal and the delay difference between the two channels can be obtained. When the suppression of the detection signal power exceeds 32 dB compared to the single-channel signal power, the delay difference between the two channels can be considered to be less than 0.1 ps. Subsequently, 50 ps is subtracted to obtain the fine calibration value of the delay. Based on this delay calibration principle, according to... Figure 2 The search algorithm shown searches for the optimal suppression point Pmin, records the delay attenuation value at this point, and subtracts 50ps from the delay value. At this time, the signal power displayed on the spectrum analyzer reaches the maximum value Pmax. When Pmax-Pmin≥38dB, the fine calibration of this channel can be considered complete. At this time, the VODL of the two channels is the delay calibration value, recorded as b2 ps. Step 5: Following the procedure in Step 4, perform delay calibration on channels 3, 4, 5, 6, 7, and 8, and record the delay calibration values ​​as b3, b4, b5, b6, b7, and b8ps.

[0012] In summary, this invention provides a method for delay and gain calibration of an optical domain simulated beamforming system. This method utilizes a signal source, a power divider, a spectrum analyzer, and a host computer to perform delay and gain calibration of the optical domain simulated beamforming system. The device has a simple structure and is easy to implement. The host computer controls the entire system by controlling the VOA and VODL within the optical domain simulated beamforming system. The entire process has high repeatability and clear adjustment logic, and adaptive adjustment and calibration can be performed using relevant algorithms, ensuring the flexibility and convenience of this method.

[0013] In summary, the above-described embodiments are merely examples of the present invention and are not intended to limit the scope of protection of the present invention. It should be noted that for those skilled in the art, several equivalent modifications and substitutions can be made to the content disclosed in the present invention. The center frequency of the radio frequency signal, the power of the radio frequency signal, and the number of channels of the optical domain analog beamforming system can all be changed. These equivalent modifications and substitutions, as well as the adjustment of the frequency range, should also be considered within the scope of protection of the present invention.

Claims

1. A delay and gain calibration method suitable for optical domain analog beamforming systems, the method comprising a signal source, a power divider, a low-noise amplifier, an adjustable optical delay line, an adjustable optical attenuator, an optical coupler, a photodetector, a spectrum analyzer, and a host computer, characterized in that, The signal source output is split into N paths by a power divider, amplified by a low-noise amplifier, and then loaded onto N directly modulated lasers with different optical carriers to achieve electro-optic conversion. The modulated optical signal is transmitted through optical fiber to a delay-gain control module composed of an adjustable optical delay line and an adjustable optical attenuator. The host computer controls the delay-gain control module to adjust the delay and gain of the multiple optical signals. The multiple optical signals are coupled into one optical signal by an optical coupler and then converted into an electrical signal by a photodetector. The spectrum analyzer is used to monitor the power of the electrical signal. The host computer reads the power from the spectrum analyzer and adjusts the delay-gain control module in real time to achieve delay and gain calibration of any channel in the system. The adjustment method is as follows: For gain calibration, set all adjustable optical attenuators to their maximum values ​​(e.g., 30dB), select channel 1 as the reference channel, and adjust the adjustable optical attenuator of this channel to 'a' (set according to the amplitude flatness between channels, e.g., 3dB). Observe and record the power using a spectrum analyzer. Then adjust the adjustable optical attenuator of this channel to its maximum value (e.g., 30dB). Select channel i as the measurement channel, i = 2, ..., N, and adjust the adjustable optical attenuator of this channel to 0dB. Observe the power using a spectrum analyzer, and adjust the adjustable optical attenuator of this channel until the output power of the spectrum analyzer matches the output power of the reference channel. Record the optical power attenuation value of each channel, find the minimum value 'b', and subtract 'b' from the optical power attenuation value of all channels to obtain the gain calibration result for each channel. For delay calibration, select channel 1 as the reference channel, set the adjustable optical attenuators of the remaining channels to their maximum value (e.g., 30dB), and select channel i as the calibration channel (i = 2, ..., N). First, set the RF source output to a broadband swept signal, observe the spectral envelope of the spectrum analyzer output signal, calculate the delay difference between the two channels based on the interval between the two troughs of the spectrum, and adjust the corresponding delay using an adjustable optical delay line to complete the coarse calibration. Then, change the RF source output signal to a single-tone signal, calculate the delay t′ required for a 180-degree phase shift based on the single-tone signal frequency, and further adjust the delay t′ using an adjustable optical delay line on top of the coarse delay calibration. At this point, observe the power of the spectrum analyzer output signal, and achieve the minimum power of the spectrum analyzer output signal by fine-tuning the delay. Finally, subtract t′ from the delay value of the adjustable optical delay line to obtain the fine delay calibration result.

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