Dual-channel microwave source phase noise testing method and device based on polarization multiplexing

Abstract

The present application proposes a dual-channel microwave source phase noise test method and device based on polarization multiplexing. The device includes: a vertical cavity surface laser for generating a required optical carrier signal, a first power divider for dividing a signal emitted by a microwave source under test into two paths, one of which is sent to a second power divider and divided into two paths by the second power divider, one of which is filtered by a low-pass filter A after passing through a phase shifter and a mixer, and the other is sent to a third power divider and divided into two paths by the third power divider, one of which is filtered by a low-pass filter B after passing through a phase shifter and a mixer, and the second power divider and the third power divider are divided into another path and enter a dual polarization intensity modulator respectively, and the light of the x polarization state after polarization beam splitting passes through a photodetector A, and the light of the y polarization state enters through a photodetector B, and then the signals amplified by the amplifier are respectively input into the corresponding mixer as radio frequency signals, and the output end of the mixer is connected to the corresponding low-pass filter to filter out high-frequency signals, and the signals output by the low-pass filter A and the low-pass filter B are input into a signal analyzer, and the phase noise of the microwave source under test can be obtained by calculation.

Description

Technical Field

[0001] The present application relates to a dual-channel microwave source phase noise testing method and device based on polarization multiplexing, belonging to the technical field of microwave source phase noise testing and microwave photonics. Background Art

[0002] Phase noise is an important indicator for measuring the short-term stability of microwave sources. It can seriously affect the performance of radar systems, wireless communication systems, multi-channel receivers and other systems. In recent years, with the continuous improvement of microwave source performance, the corresponding noise value has also been continuously reduced. In order to meet the extremely low phase noise test requirements of microwave sources with higher frequencies and larger tuning ranges, researchers have proposed and implemented many methods, such as direct spectrum technology, phase detection method, frequency detection method and dual-channel cross-correlation technology. However, these technologies have their own shortcomings. For example, direct spectrum technology cannot distinguish between amplitude noise and phase noise, and the test accuracy is low; phase detection method can distinguish between amplitude noise and phase noise, but this method requires a reference source with better noise performance than the source to be measured, and the frequency difference between the two cannot be too large; although the delay line-based frequency detection method does not require a reference source, the power loss caused by the use of cables limits the available length of the coaxial cable, thereby limiting the measurement accuracy; dual-channel cross-correlation technology improves the measurement noise floor by eliminating irrelevant noise in the system, but this solution is complex in structure, expensive and slow in test speed.

[0003] Recently, microwave photonic technology, which has been developed for decades, has been applied to the field of phase noise measurement. Thanks to the advantages of photonic technology, compared with traditional solutions, microwave photonics-based phase noise measurement technology has the advantages of lower transmission loss, larger measurement range, lower high-frequency background noise, etc. In order to overcome the high loss caused by coaxial cable, photonic delay line technology is introduced into the frequency discrimination method to obtain a longer time delay, thereby achieving higher measurement accuracy [E. Rubiola, E. Salik, S. Huang, N. Yu, and L. Maleki, "Photonic-delay technique for phase-noise measurement of microwave oscillators," J. Opt. Soc. Amer. B, vol. 22, no. 5, pp. 987–997, May 2005].In addition, microwave photonic phase shifters based on polarization modulators [D. Zhu, F. Zhang, P. Zhou, D. Zhu, and S. Pan, “Wideband phase noise measurement using a multifunctional microwave photonic processor,” IEEE Photon. Technol. Lett., vol. 26, no. 24, pp. 2434–2437, Dec. 2014] and dual-driven Mach-Zehnder modulators [WT Wang, J. Guo, HK Mei, W H Sun, and NH Zhu, “Photonic-assisted wideband phase noise analyzer based on optoelectronic hybrid units,” J. Lightw. Technol., vol. 34, no. 14, pp. 3425–3431, Jul. 2016] are respectively used, and microwave photonic phase shifters based on Mach-Zehnder modulators [D. Zhu, F. Zhang, P. Zhou, and S. Pan, “Phase noise measurement of wideband microwave sources based on a microwave photonicfrequency Microwave photonic down-converter,”Opt.Lett.,vol.40,no.7,pp.1326–1329,Apr.2015] and polarization modulator [F.Zhang,D.Zhu,and S.Pan,“Photonic-assisted wideband phase noise measurement of microwave signal sources,”Electron.Lett.,vol.51,no.16,pp.1272–1274,Aug.2015] were introduced into the photonic delay line assisted phase noise measurement system to meet the broadband measurement requirements of phase noise.In addition to these single-channel phase noise measurement systems, a dual-photon delay line cross correlation measurement system has also been proposed and experimentally demonstrated. This scheme reduces the phase noise floor by eliminating the uncorrelated noise in two independent measurement systems [E.Salik, NY u, L.Maleki, and E.Rubiola, "Dual photonic-delay line cross correlation method for phase noise measurement," in Proc.IEEE Int. Freq. Control Symp. Exp., pp. 303–306, 2004]. However, this scheme requires two identical optical delay phase noise measurement systems, including two lasers, modulators, optical fibers, etc., which are complex in structure and high in cost. In addition, in order to ensure the correct measurement results, the delay of several kilometers of optical fiber in the two channels must be the same, which is difficult to achieve in experiments. Summary of the invention

[0004] In order to overcome the above-mentioned shortcomings, the purpose of this application is to provide a dual-channel microwave source phase noise testing method and device based on polarization multiplexing, which improves the accuracy of the measurement system while simplifying the system structure and reducing the cost of the test system.

[0005] To achieve the above objectives, the technical solutions adopted in this application are:

[0006] A dual-channel microwave source phase noise test device based on polarization multiplexing includes: a vertical cavity surface laser, a dual polarization intensity modulator and a polarization beam splitter connected in sequence along the optical path direction, wherein the vertical cavity surface laser is used to generate two optical carrier signals with different polarization states, and the dual polarization intensity modulator is used to perform electro-optical modulation on light with different polarization states.

[0007] The input end of the first power divider is connected to the microwave source to be measured, and the output end is connected to the second power divider and the third power divider respectively. The output end of the second power divider is connected to the dual polarization intensity modulator and the phase shifter A, the output end of the phase shifter A is connected to the mixer A, the output end of the third power divider is connected to the dual polarization intensity modulator and the phase shifter B, the output end of the phase shifter B is connected to the mixer B,

[0008] The dual polarization intensity modulator is connected to a polarization beam splitter through an optical fiber, a first output end of the polarization beam splitter is connected to a photodetector A, an output end of the photodetector A is connected to an input end of the mixer A, a second output end of the polarization beam splitter is connected to a photodetector B, an output end of the photodetector B is connected to an input end of the mixer B,

[0009] The output end of the mixer A is connected to a low-pass filter A, the mixer B is connected to a low-pass filter B, and the output end of the low-pass filter A and the output end of the mixer B are respectively connected to a signal analysis device.

[0010] The preferred technical solution is: the signal analysis device is a fast Fourier transform analyzer, and the fast Fourier transform analyzer receives and analyzes the low-frequency signals filtered by low-pass filter A and low-pass filter B respectively.

[0011] Preferably, the mixer A and the mixer B are double-balanced mixers.

[0012] The first power divider, the second power divider and the third power divider are all broadband one-to-two microwave power dividers.

[0013] The phase shifter A and the phase shifter B are broadband microwave phase shifters.

[0014] The photodetector A and the photodetector B are both broadband photodetectors.

[0015] The optical fiber is a polarization-maintaining optical fiber.

[0016] It is worth mentioning that the above-mentioned "A", "B", "first", "second" and "third", such as phase shifter A, phase shifter B, first power divider, second power divider and third power divider, are only numbers given to facilitate the distinction of multiple components of the same kind, and are not specific limitations on the structure, shape, and positional relationship, nor should they be understood as limitations on the order.

[0017] Beneficial Effects

[0018] Compared with the prior art, the dual-channel microwave source phase noise testing method and device based on polarization multiplexing proposed in the embodiments of the present application are:

[0019] 1) The present invention adopts polarization multiplexing technology, generates two lasers with different polarization states through a vertical cavity surface laser, uses an integrated dual-polarization intensity modulator to electro-optically modulate the light with different polarization states, and then uses a beam splitter to separate the light with different polarization states to form a dual channel, saving the use of a modulator and a laser; the two channels share a polarization-maintaining optical fiber with a length of several kilometers. This structure makes it easier to obtain the same delay between the two channels and saves a roll of polarization-maintaining optical fiber of several kilometers, thereby improving the test accuracy while optimizing the system structure and reducing the cost.

[0020] 2) The present invention uses dual-channel cross-correlation technology, which can eliminate the background noise caused by active devices such as amplifiers and photodetectors, reduce the system noise floor, and improve test accuracy.

[0021] 3) Testing phase noise using delay line technology does not require an additional reference source. It only requires mixing the microwave source to be tested with its own delayed signal. The longer the delay line used for delay, that is, the longer the optical fiber, the higher the accuracy of the phase noise test. Optical fiber has the advantages of low loss and large bandwidth, and is the best delay medium. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is a schematic diagram of the structural principle of the microwave source phase noise testing device of the present invention;

[0023] Figure 2 It is a typical structure of integrated dual-polarization intensity modulator. DETAILED DESCRIPTION

[0024] The above scheme is further described below in conjunction with specific examples. It should be understood that these examples are used to illustrate the present application and are not limited to the scope of the present application. The implementation conditions adopted in the examples can be further adjusted as the conditions of the specific manufacturer, and the unspecified implementation conditions are usually the conditions in conventional experiments.

[0025] The present application provides a dual-channel microwave source phase noise test device based on polarization multiplexing proposed in the present application, the device includes: a vertical cavity surface laser, used to generate the required optical carrier signal, a first power divider is used to divide the signal emitted by the microwave source under test into two paths, one of which is sent to the second power divider, and is divided into two paths by the second power divider, one of which is filtered by a low-pass filter A after phase shifting and mixer, and the other is sent to the third power divider, and is divided into two paths by the third power divider, one of which is filtered by a low-pass filter B after phase shifting and mixer, and the second power divider and the third power divider are divided into another path and enter the dual polarization intensity adjuster respectively, and the light of the x polarization state after polarization beam splitting passes through the photodetector A, and the light of the y polarization state enters the photodetector B, and then the signals amplified by the amplifier are input into the corresponding mixer as radio frequency signals, and the output end of the mixer is connected to the corresponding low-pass filter to filter out the high-frequency signal. The two low-frequency signals from the two independent phase noise measurement systems are respectively introduced into the dual-port fast Fourier transform analyzer for phase noise calculation, so that the working bandwidth is large, the test accuracy is high, the structure is simple, and the cost is low. The device uses an integrated dual-polarization intensity modulator to electro-optically modulate light in different polarization states, and then uses a polarization beam splitter to separate the light in different polarization states to form dual channels, saving the use of a modulator and a laser. In addition, the two channels share a polarization-maintaining optical fiber with a length of several kilometers. This structure makes it easier to obtain the same delay between the two channels and saves a roll of several kilometers of polarization-maintaining optical fiber, which improves test accuracy while optimizing the system structure and reducing costs.

[0026] Next, the microwave source phase noise testing device based on the microwave photon mixing technology proposed in the present application is described in conjunction with the accompanying drawings.

[0027] like Figure 1 The figure shows a schematic diagram of the structure of a microwave source phase noise test device based on microwave photon mixing technology, which includes:

[0028] A vertical cavity surface laser, a dual polarization intensity modulator, a polarization-maintaining optical fiber, a polarization beam splitter, two photodetectors respectively connected to two output ends of the polarization beam splitter, and

[0029] The first / second / third power divider (collectively referred to as microwave power divider), two phase shifters, two electrical amplifiers, two electrical mixers, and two low-pass filters;

[0030] The input end of the first power divider is connected to the microwave source to be measured, and the output signal of the first power divider is connected to the second power divider and the third power divider respectively.

[0031] The output end of the second power divider is connected to the dual polarization intensity modulator and the phase shifter A, and the phase-shifted signal is input into the mixer A as the local oscillator signal. The output end of the third power divider is connected to the dual polarization intensity modulator phase shifter B, and the phase-shifted signal is input into the mixer B as the local oscillator signal.

[0032] The photodetector A is connected to the amplifier A. The signal amplified by the amplifier A is input into the mixer A as the RF signal. The output of the mixer A is connected to the low-pass filter A to filter out the high-frequency signal.

[0033] The photodetector B is connected to the amplifier B, and the signal amplified by the amplifier B is input into the mixer B as a radio frequency signal. The output end of the mixer B is connected to the low-pass filter B to filter out the high-frequency signal.

[0034] In this embodiment, a vertical cavity surface laser (VESEL) and a microwave source (microwave source to be measured) are used to generate the required optical carrier signal and the microwave signal to be measured, respectively; a one-to-two microwave power splitter is used to split the microwave signal to be measured output by the microwave source into two paths, a dual polarization intensity modulator is used to realize electro-optical modulation, and a polarization beam splitter is used to separate light of different polarization states.

[0035] Continue to refer Figure 1, the microwave signal to be measured is divided into two paths by power divider 1, and any branch is further divided into two paths by power divider 2 / 3. One path is introduced into the microwave photon link to realize electro-optical modulation and time delay, and the other path is sent to the matching mixer to achieve precise phase adjustment through the phase shifter (phase shifter A / B) to maintain phase orthogonality with the RF signal. In the microwave photon link, the x-polarization state and y-polarization state light from the vertical cavity surface laser are introduced into the dual polarization intensity modulator for electro-optical modulation, and its output is transmitted through the polarization-maintaining fiber. The output end of the polarization-maintaining fiber is connected to the polarization beam splitter, which is used to separate the light of different polarization states, such as: the x-polarization state light enters the upper branch (via photodetector A), and the y-polarization state light enters the lower branch (via photodetector B), and then the photoelectric conversion is performed by photodetector A / B respectively. The output signal of photodetector A / B is amplified by a matching electrical amplifier and mixed with the local oscillator signal in the corresponding mixer. The output intermediate frequency signal is filtered out of the high frequency part through a low-pass filter (low-pass filter A / B). Finally, the two low-frequency signals from the two independent phase noise measurement systems are respectively introduced into a dual-port fast Fourier transform analyzer for phase noise calculation. In a preferred embodiment, the microwave power divider is preferably a broadband microwave power divider. In a preferred embodiment, the phase shifter is preferably a broadband microwave phase shifter; the electro-optic modulator is preferably a broadband electro-optic modulator; and the photodetector is preferably a broadband photodetector so as to improve the working bandwidth of the system.

[0036] In order to make the public understand the technical solution of the present invention, Figure 1 and Figure 2 To describe the test mechanism of the above test device:

[0037] Figure 1 The typical structure of a dual-channel microwave source phase noise measurement solution based on polarization multiplexing is shown. It includes two independent photon-assisted phase noise measurement systems. The light from the x-polarization state and y-polarization state of the vertical cavity surface laser is introduced into the dual-polarization intensity modulator for electro-optical modulation. Since the dual-polarization intensity modulator is an integrated device and the polarization beam splitter is integrated at the optical carrier input end, the input optical carriers of different polarization states can be electro-optically modulated separately to form a dual-channel structure. And the polarization beam combiner is integrated at the end of the device, so the long optical fiber that provides a longer delay time can be used for two channels at the same time to obtain the same time delay. For a single channel, first, as Figure 2As shown, assuming that the microwave signal output by the first power divider is E0(t), the two microwave signals output by the second microwave power divider are E1(t) and E2(t), respectively, where E1(t) = E2(t) = Vcos[ωt+φ(t)], and the output signal of the third microwave power divider is the same as the output signal of the second microwave power divider, that is, E4(t) = E5(t) = E1(t). The optical carrier E of the x-polarization state and the y-polarization state output by the vertical cavity surface laser x (t) = E y (t) = E c exp(jωt). After E2(t) is phase-shifted by the phase shifter, we get E3(t)=Vcos[ωt+φ(t)+φ]. The first beam splitter inputs the x-polarized optical carrier into the first Mach-Zehnder modulator, which is driven by E1(t). Then the signal at the output of the first Mach-Zehnder modulator is:

[0038]

[0039] Among them, V B is the DC bias voltage applied to the first Mach-Zehnder modulator, V π is the half-wave voltage of the first Mach-Zehnder modulator.

[0040] A beam combiner is used to combine the optical signal in the x polarization state with the optical signal in the y polarization state. After the optical signal is delayed by τ through the polarization-maintaining fiber, a beam splitter is used to separate the optical signals in different polarization states. The light in the x polarization state is injected into the photodetector in the upper branch. The optical signal before being injected into the photodetector in the upper branch is:

[0041]

[0042] After passing through the photoelectric detector, the microwave signal output by the upper branch can be expressed as:

[0043]

[0044] The signal after the microwave phase shifter phase shifts φ is expressed as:

[0045] E3(t)=Vcos(ωt+φ(t)+φ) (4)

[0046] Adjust the microwave phase shifter to keep the signals at the two input ends of the mixer orthogonal, that is, The signals in equation (3) and equation (4) are mixed in the double-balanced mixer of the upper branch and the voltage signal after being filtered by the low-pass filter can be expressed as:

[0047]

[0048] Where k is the calibration factor, and the phase noise information of the microwave source to be measured is included in equation (5). Then the power spectrum of the output signal can be expressed as:

[0049]

[0050] Among them, S φi (f) is the double-sideband phase noise power spectrum of the microwave signal to be measured, then according to the definition, the single-sideband phase noise can be written as:

[0051]

[0052] like Figure 2 As shown, in this structure, the microwave signal to be measured is divided into two paths and input into two independent photon-assisted phase noise measurement systems respectively, and then the output signals of low-pass filter A and low-pass filter B are input into a dual-channel FFT analyzer for cross-correlation processing. Assuming that the background noises of the two channels are a(t) and b(t) respectively, and the phase noise of the microwave source to be measured is c(t), then the noise signals to be measured at the output ends of the two channels will be added with the background noises of each channel. The working principle of the system is as follows:

[0053]

[0054] Among them, A(f), B(f), C(f), X(f), and Y(f) are the Fourier transform forms of a(t), b(t), c(t), x(t), and y(t), respectively. Assuming that the background noise of the two channels and the phase noise of the microwave source to be measured are both stable and ergodic, then a(t), b(t), and c(t) are statistically independent of each other, so the cross-power spectrum S of the noise signals at the output of the two channels is xy for:

[0055]

[0056] Since a(t), b(t) and c(t) are statistically independent of each other, E{CB *}=0,E{AC *}=0,E{AB *}=0. Therefore, the background noise in a single channel can be eliminated by using the dual-channel cross-correlation technology, thereby reducing the noise floor of the entire measurement system.

[0057] The above derivation is a theory under the condition of ideal and continuous signal. The actual signal processing is all done under discrete conditions. For example, the data tested by the dual-channel FFT analyzer, as well as the fast Fourier transform and cross-correlation are all processed under discrete conditions. The following is a re-derivation of the continuous signal in the above theoretical derivation with a discrete signal. The process is as follows:

[0058]

[0059] In equation (10), Represents the background noise of the entire test system, which is a function of the correlation number N. As the correlation number N increases, the background noise of the test system has less impact on the phase noise test results, that is, the noise floor of the system decreases.

[0060] The above embodiments are only for illustrating the technical concept and features of the present application, and their purpose is to enable people familiar with the technology to understand the content of the present application and implement it accordingly, and they cannot be used to limit the protection scope of the present application. Any equivalent transformation or modification made according to the spirit of the present application shall be included in the protection scope of the present application.

Claims

1. A dual-channel microwave source phase noise test device based on polarization multiplexing, characterized by: include: A vertical cavity surface laser, a dual polarization intensity modulator and a polarization beam splitter are sequentially connected along the optical path direction, wherein the vertical cavity surface laser is used to generate two optical carrier signals with different polarization states, and the dual polarization intensity modulator is used to perform electro-optical modulation on light with different polarization states. The input end of the first power divider is connected to the microwave source to be measured, and the output end is connected to the second power divider and the third power divider respectively. The output end of the second power divider is connected to the dual polarization intensity modulator and the phase shifter A, the output end of the phase shifter A is connected to the mixer A, the output end of the third power divider is connected to the dual polarization intensity modulator and the phase shifter B, the output end of the phase shifter B is connected to the mixer B, Light including x polarization state and y polarization state from the vertical cavity surface laser is introduced into the dual polarization intensity modulator for electro-optical modulation, and is transmitted to the polarization beam splitter through the polarization-maintaining optical fiber. The first output end of the polarization beam splitter is connected to the photodetector A, and the output end of the photodetector A is connected to the input end of the mixer A. The second output end of the polarization beam splitter is connected to the photodetector B, and the output end of the photodetector B is connected to the input end of the mixer B. The output end of the mixer A is connected to a low-pass filter A, the mixer B is connected to a low-pass filter B, and the output end of the low-pass filter A and the output end of the mixer B are respectively connected to a signal analysis device; The polarization beam splitter separates light of different polarization states. The light of x polarization state enters the upper branch, and the light of y polarization state enters the lower branch. The photoelectric detector A and the photoelectric detector B based on the upper branch and the lower branch respectively perform photoelectric conversion, and the converted signals are output to the amplifier A and the amplifier B respectively, and are mixed with the local oscillator signal in the corresponding mixer A and the mixer B. The intermediate frequency signal mixed and output by the upper branch through the mixer A is sent to the low-pass filter A to filter out the high-frequency part, and the intermediate frequency signal mixed and output by the lower branch through the mixer B is sent to the low-pass filter B to filter out the high-frequency part. The signal analysis device receives the low-frequency signals of the low-pass filter A and the low-pass filter B to calculate the phase noise.

2. The dual-channel microwave source phase noise test device based on polarization multiplexing according to claim 1, characterized in that: The signal analysis device is a fast Fourier transform analyzer, which receives and analyzes low-frequency signals filtered by low-pass filter A and low-pass filter B respectively.

3. The dual-channel microwave source phase noise test device based on polarization multiplexing according to claim 1, characterized in that: The mixer A and the mixer B are double-balanced mixers.

4. The dual-channel microwave source phase noise test device based on polarization multiplexing according to claim 1, characterized in that: The first power divider, the second power divider and the third power divider are all broadband one-to-two microwave power dividers.

5. The dual-channel microwave source phase noise test device based on polarization multiplexing according to claim 1, characterized in that: The phase shifter A and the phase shifter B are both broadband microwave phase shifters.

6. The dual-channel microwave source phase noise test device based on polarization multiplexing according to claim 1, characterized in that: The photodetector A and the photodetector B are both broadband photodetectors.

7. A dual-channel microwave source phase noise test method based on polarization multiplexing, characterized in that: The method comprises: The microwave signal emitted by the microwave source to be tested is divided into two paths based on the first power divider. Based on the second power divider and the third power divider, one of the two signals will be divided into two signals. One of the two paths separated by the second power divider and the third power divider is sent to the corresponding mixer after phase adjustment by the phase shifter so as to maintain phase orthogonality with the RF signal. Another path introduces a microwave photonic link to achieve electro-optical modulation and time delay. In the microwave photonic link, the light from the vertical cavity surface laser, including the x-polarization state and the y-polarization state, is introduced into the dual-polarization intensity modulator for electro-optical modulation and transmitted to the polarization beam splitter through the polarization-maintaining optical fiber. Based on the polarization beam splitter, light of different polarization states is separated. The x-polarized light enters the upper branch. The light with y polarization enters the lower branch. Then, the photoelectric detectors based on the upper branch and the lower branch perform photoelectric conversion respectively, and the converted signals are output to the amplifier for amplification and mixed with the local oscillator signal in the corresponding mixer. The intermediate frequency signal mixed and output by the mixer of the upper branch is sent to the low-pass filter A to filter out the high-frequency part, and the intermediate frequency signal mixed and output by the mixer of the lower branch is sent to the low-pass filter B to filter out the high-frequency part. The signal analysis device receives the low-frequency signals of the low-pass filter A and the low-pass filter B to perform phase noise calculation.

8. The dual-channel microwave source phase noise testing method based on polarization multiplexing according to claim 7 is characterized in that: The upper branch includes: a photodetector A, an amplifier A and a mixer A which are electrically connected in sequence; the lower branch includes: a photodetector B, an amplifier B and a mixer B which are electrically connected in sequence.

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