A method for testing millimeter wave signal phase noise based on super-stable laser

Abstract

The application discloses a kind of millimeter wave signal phase noise test methods based on super-stable laser, utilize common cavity Brillouin laser frequency stabilization technology to generate super-stable laser as carrier wave to carry out double sideband modulation to the millimeter wave signal to be measured, when utilizing optical fiber ring resonant cavity to excite Brillouin laser, two sidebands are different in frequency shift: the sideband of high frequency shifts down more, the sideband of low frequency shifts down less, the common cavity Brillouin laser generated is coupled with double optical sideband pump laser, separation is obtained different in frequency interval upper and lower double sideband optical signal, then beat frequency, mixed frequency generation to MHz order intermediate frequency signal, intermediate frequency signal noise characteristic directly carries the phase noise of signal to be measured, by phase noise analyzer, intermediate frequency signal is analyzed and quantitatively evaluated with noise spectral density, complete the phase noise test of millimeter wave signal to be measured, solve the cooperative optimization problem that reference source stability, system noise and signal noise to be measured are difficult to separate in super-high frequency signal noise test.

Description

A method for testing phase noise of millimeter-wave signals based on ultrastable lasers Technical Field

[0001] This invention belongs to the field of phase noise testing, and more specifically, relates to a method for testing the phase noise of millimeter-wave signals based on ultrastable lasers. Background Technology

[0002] The phase noise and frequency jitter characteristics of high-frequency signals (covering microwave, millimeter-wave, and terahertz bands) are core parameters determining the performance of modern communication, radar, electronic countermeasures, and detection systems. With the development of electronic equipment towards higher frequencies and wider bandwidths (such as 6G communication and terahertz imaging), high-precision testing of the phase noise characteristics of signals in the ≥100GHz band has become a critical requirement for improving system reliability and functional limits.

[0003] However, traditional electronic testing methods are limited by the following technical bottlenecks: (1) Local oscillator noise limitation: The local oscillator signal source on which electronic testing depends has inherent phase noise, which will alias with the noise of the signal under test in the ultra-high frequency band, resulting in a significant decrease in measurement accuracy; (2) Device nonlinear distortion: The nonlinear effect of active devices such as high frequency amplifiers and mixers will introduce additional noise, interfering with the extraction of the true noise characteristics of the signal under test; (3) Bandwidth bottleneck: The available bandwidth of traditional electronic testing equipment (such as spectrum analyzers and phase noise analyzers) is usually less than 50 GHz, which is difficult to cover the testing requirements from millimeter wave to terahertz frequency band.

[0004] To overcome the physical limitations of electronic testing, optical detection technology has gradually become a research hotspot in recent years. For example, the optical heterodyne beat frequency method can modulate high-frequency electrical signals into the optical domain, and indirectly characterize electrical signal noise by utilizing the high-frequency characteristics of light waves.

[0005] For example, Chinese invention patent application CN 113438022A, published on September 24, 2021, discloses "a microwave source phase noise measurement device and method". The method includes: splitting the microwave signal to be measured into two paths based on a microwave power divider. One path is phase-shifted by an electric phase shifter and transmitted to a first electro-optic intensity modulator. The other radio frequency signal is directly transmitted to a second electro-optic intensity modulator. A polarization multiplexing parallel light modulator receives and orthogonally multiplexes the polarization, and outputs a polarization multiplexed light signal. By adjusting the first polarization controller, the polarization multiplexed light signal can be completely separated by the polarization beam splitter. The x-polarized light enters the upper branch, and the y-polarized light enters the lower branch. By adjusting the second polarization controller and the third polarization controller, the polarization states of the upper and lower branches are orthogonal again and combined in the second polarization beam combiner. The combined signal passes through an optical bandpass filter, an analyzer, and a photodetector. The signal output by the photodetector is introduced into a fast Fourier transform analyzer to obtain the phase noise of the microwave signal under test. This method introduces a long delay τ through an electric phase shifter and a single-mode optical fiber to form two optical signals with different frequencies. Then, a photodetector beats these two signals to achieve photoelectric conversion. The signal output by the photodetector is introduced into a fast Fourier transform analyzer for data acquisition. By processing the acquired data, the phase noise of the measured microwave signal can be obtained.

[0006] A Chinese invention patent application, CN119291315A, published on January 10, 2025, discloses a "Microwave Signal Phase Noise Measurement System and Method," comprising: generating a continuous optical carrier; modulating a microwave signal onto the optical carrier to obtain a first modulation signal; modulating the first modulation signal through a polarization-state beam splitter input delay fiber link to obtain a modulated first modulation signal, wherein the modulation includes delay and phase orthogonality control; loading the microwave signal onto the modulated first modulation signal to obtain a second modulation signal; extracting the positive first-order sideband of the second modulation signal to obtain a light signal to be processed, wherein the light signal to be processed contains only positive first-order sideband information; converting the light signal to be processed into an electrical signal; and performing data processing on the electrical signal to obtain the phase noise of the microwave signal. This method uses a delay fiber link to re-modulate the first modulation signal, extracts the positive first-order sideband to obtain an electrical signal, and processes it to obtain the phase noise of the microwave signal.

[0007] The above methods all use fiber delay to form two optical signals with different frequencies. The difference electrical signal between the two optical signals is an electrical signal with microwave phase noise. On the one hand, the frequency of this electrical signal is still very high, reaching the GHz level. The high-frequency response of electronic devices limits the test accuracy of phase noise. On the other hand, the laser frequency output from the laser source requires high stability. If the laser frequency output from the laser source is unstable, it will also affect the test accuracy of phase noise. Summary of the Invention

[0008] The purpose of this invention is to overcome the shortcomings of the prior art and provide a method for testing the phase noise of millimeter-wave signals based on ultra-stable lasers, so as to downconvert high-frequency phase noise information to MHz-level intermediate frequency signals, and solve the problem of co-optimization of reference source stability, system noise and noise of the signal under test in ultra-high frequency signal noise testing.

[0009] To achieve the above-mentioned objectives, the present invention provides a method for testing the phase noise of millimeter-wave signals based on ultrastable lasers, characterized by comprising the following steps:

[0010] (1) Generate two pump lasers

[0011] A tunable laser is used as the laser source to generate a pump laser, which is then split into two paths via a 50:50 fiber coupler: an upper pump laser and a lower pump laser. The upper pump laser is used to generate a dual-beam sideband pump laser, while the lower pump laser outputs a PDH locking system to achieve frequency stabilization of the common cavity Brillouin laser.

[0012] (2) Generating dual-sideband pumped laser

[0013] The millimeter-wave signal to be tested is connected to the radio frequency port of the intensity modulator, and the pump laser is connected to the optical input port of the intensity modulator as an optical carrier. Under the drive of the millimeter-wave signal to be tested, the intensity modulator generates a pair of symmetrical sideband optical signals with a frequency interval of twice the frequency of the millimeter-wave signal to be tested, namely the dual-optical sideband pump laser.

[0014] (3) Generating cocavity Brillouin lasers in fiber optic rings using the Brillouin effect.

[0015] After suppressing the optical carrier power and enhancing the sideband output intensity, the dual-optical sideband pump laser is amplified by an erbium-doped fiber amplifier. Then, a 90:10 fiber coupler splits the amplified dual-optical sideband pump laser into two paths. The 90% branch is locked to the TE mode by the first polarization controller and then injected into the fiber ring resonator to generate Brillouin laser. The generated Brillouin laser is output to the polarization beam splitter by the second polarization controller. The second polarization controller is adjusted to maximize the output power of the TE port of the polarization beam splitter. The laser output from the TE port serves as the common-cavity Brillouin laser and is used as a reference source. It is coupled with the 10% branch, i.e., the dual-optical sideband pump laser, after the polarization state of the dual-optical sideband pump laser is locked to the TE mode by the third polarization controller. The laser output from the TM port is fed back to the PDH locking system to realize that the pump laser generated by the tunable laser is an ultra-stable laser and to achieve frequency stability of the common-cavity Brillouin laser.

[0016] (4) Optical-electric domain noise conversion

[0017] For the optical signal output by the coupling of the common cavity Brillouin laser and the dual-sideband pump laser, a programmable filter is used to separate the upper pump sideband of the dual-sideband pump laser and the upper sideband of the common cavity Brillouin laser into one path, namely the upper double-sideband optical signal, and to separate the lower pump sideband of the dual-sideband pump laser and the lower sideband of the common cavity Brillouin laser into another path, namely the lower double-sideband optical signal.

[0018] The upper double-sideband optical signal and the lower double-sideband optical signal are injected into their respective photodetectors to complete photoelectric conversion, resulting in the upper sideband beat frequency electrical signal and the lower sideband beat frequency electrical signal, respectively. Then, the upper sideband beat frequency electrical signal and the lower sideband beat frequency electrical signal are input into a mixer to generate an intermediate frequency signal in the MHz range.

[0019] The noise characteristics of the intermediate frequency signal directly carry the phase noise of the millimeter-wave signal under test. The noise spectral density of the intermediate frequency signal is analyzed and quantified by a phase noise analyzer to complete the phase noise test of the millimeter-wave signal under test.

[0020] The objective of this invention is achieved as follows:

[0021] This invention provides a method for testing the phase noise of millimeter-wave signals based on ultrastable lasers. First, ultrastable lasers are generated using common-cavity Brillouin laser frequency stabilization technology as a low-noise optical reference. The millimeter-wave signal under test (≥100GHz) is loaded onto the sideband of the optical carrier (ultrastable laser) through an intensity modulator, realizing the mapping of electrical signal phase noise to the optical domain. Then, the modulated dual-sideband pump laser is coupled with the common-cavity Brillouin laser excited in the fiber ring resonator and generated by polarization adjustment. Next, the upper and lower double-sideband optical signals are separated using a programmable filter. Finally, optical heterodyne beat frequency is performed using a photodetector to obtain the upper and lower double-sideband beat frequency electrical signals, which are then fed into a mixer to generate intermediate frequency signals. In this way, the high-frequency phase noise information is down-converted to an intermediate frequency signal in the MHz range. This invention utilizes a common-cavity Brillouin laser as an ultra-low noise optical reference. When the Brillouin laser is excited using an ultra-high Q-value fiber ring resonator, the two sidebands have different frequency shifts (the higher-frequency sideband has a larger downward frequency shift, while the lower-frequency sideband has a smaller downward frequency shift). The generated common-cavity Brillouin laser is coupled and separated with a dual-beam sideband pump laser to obtain upper and lower double-sideband optical signals with different frequency intervals. Then, beat frequency and mixing are used to generate an intermediate frequency (IF) signal in the MHz range. The noise characteristics of the IF signal directly carry the phase noise of the signal under test. The noise spectral density of the IF signal is analyzed and quantified using a phase noise analyzer to complete the phase noise test of the millimeter-wave signal under test. This invention solves the problem of co-optimization between reference source stability, system noise, and the noise of the signal under test in ultra-high frequency signal noise testing.

[0022] Meanwhile, the millimeter-wave signal phase noise testing method based on ultra-stable laser of the present invention also has the following beneficial effects:

[0023] (1) Using fiber ring cavity as Brillouin gain medium is small in size and low in cost. Furthermore, by using the same fiber ring cavity to excite co-cavity Brillouin laser, two Brillouin lasers share the same fiber path, which effectively suppresses the path length variation effect.

[0024] (2) No complex phase-locked loop is required, and the single measurement time is effectively shortened compared with the current commercial high-frequency signal noise test system, making it suitable for real-time online detection;

[0025] (3) This technology can adjust and expand the testing of phase noise of millimeter wave signals at different frequencies by selecting parameters;

[0026] (4) The phase noise detection sensitivity at the 30G test frequency reaches -144.5dBc / Hz@10kHz frequency deviation, which is a significant improvement over traditional electronic testing methods;

[0027] (5) This technology reduces the need for high-bandwidth instruments in traditional testing systems by using the optical-electric domain noise conversion method. Attached Figure Description

[0028] Figure 1 is an architecture diagram of a specific embodiment of the millimeter-wave signal phase noise testing method based on ultrastable laser of the present invention;

[0029] Figure 2 is a flowchart of a specific embodiment of the millimeter-wave signal phase noise testing method based on ultrastable laser of the present invention;

[0030] Figure 3 is a spectrum of the optical signal output from the coupling of a common-cavity Brillouin laser and a dual-beam sideband pump laser;

[0031] Figure 4 is the spectrum of the upper double-sideband optical signal;

[0032] Figure 5 is the spectrum of the lower double-sideband optical signal;

[0033] Figure 6 is a comparison between the test results of the millimeter-wave signal phase noise test method based on ultra-stable laser of the present invention and the phase noise of a 100GHz signal obtained by the currently commercially available FSWP phase noise meter + spread spectrum module. Detailed Implementation

[0034] The specific embodiments of the present invention will now be described with reference to the accompanying drawings to enable those skilled in the art to better understand the invention. It should be particularly noted that in the following description, detailed descriptions of known functions and designs that might obscure the main content of the invention will be omitted here.

[0035] Figure 1 is an architecture diagram of a specific embodiment of the millimeter-wave signal phase noise testing method based on ultrastable laser of the present invention.

[0036] In this embodiment, as shown in FIG1, the present invention comprises two parts, which can be specifically divided into a common cavity Brillouin laser generation link 1 as a reference signal, a PDH locking system 2, and an optical-electric domain noise conversion link 3.

[0037] The phase noise testing method for millimeter-wave signals based on ultrastable lasers of the present invention will be described in detail below with reference to Figure 1.

[0038] In this embodiment, as shown in Figure 2, the millimeter-wave signal phase noise testing method based on ultrastable laser of the present invention includes the following steps:

[0039] Step S1: Generate two pump lasers

[0040] In this embodiment, as shown in Figure 1, a tunable laser 101 is used as a laser source to generate a pump laser L, which is then split into two paths via a 50:50 fiber coupler 102: an upper pump laser L1 and a lower pump laser L2. The upper pump laser L1 is used to generate a dual-beam sideband pump laser LS, while the lower pump laser L2 outputs a PDH locking system to achieve frequency stabilization of the common cavity Brillouin laser.

[0041] In this embodiment, the center wavelength of the pump laser L is set to 1549.0 nm.

[0042] Step S2: Generate a dual-beam sideband pumped laser (LS)

[0043] The millimeter-wave signal S output from the signal source under test is connected to the radio frequency port of the intensity modulator 103, and the pump laser L1 is connected to the optical input port of the intensity modulator 103 as an optical carrier. Under the drive of the millimeter-wave signal S under test, the intensity modulator 103 generates a pair of symmetrical sideband optical signals with a frequency interval of twice the frequency of the millimeter-wave signal under test, namely the dual-optical sideband pump laser LS.

[0044] In the specific implementation process, the spectral characteristics of the dual-light sideband pumped laser LS are monitored in real time by a spectral analyzer, and the bias voltage of the intensity modulator 103 is adjusted to suppress the optical carrier power to the greatest extent and enhance the sideband output intensity in order to meet the input optical power requirements of the erbium-doped fiber amplifier (EDFA) 105. If the residual optical carrier power still exceeds the allowable range, a band-stop filter 104 matching the carrier wavelength can be inserted in the optical path for secondary suppression.

[0045] In this embodiment, the frequency of the millimeter-wave signal S under test is 100 GHz. By adjusting the bias voltage of the intensity modulator 103, the power of the optical sideband generated by the intensity modulator 103 is maximized while the power of the carrier wave is minimized. Simultaneously, a fiber Bragg grating is used as a band-stop filter 104 to further suppress the carrier wave. The grating has a center wavelength of 1549.02 nm, a bandwidth of 0.72 nm, and a reflectivity of 99.83%. The wavelengths of the dual-sideband pump laser LS after carrier suppression are 1548.2 nm and 1549.8 nm, respectively, used to excite a common-cavity Brillouin laser. An erbium-doped fiber amplifier 105 amplifies the power of the dual-sideband pump laser LS to 40 mW.

[0046] Step S3: Generate a co-cavity Brillouin laser in the fiber ring cavity using the Brillouin effect;

[0047] The dual-sideband pumped laser (LS) is amplified by suppressing the optical carrier power and enhancing the sideband output intensity before being fed into an erbium-doped fiber amplifier (105). Then, a 90:10 fiber coupler (106) splits the amplified LS into two paths. The 90% branch is locked to the TE mode polarization state by a first polarization controller (107) and then injected into a fiber ring resonator (108) to generate a Brillouin laser (LB). The generated Brillouin laser (LB) is output to a polarization beam splitter (110) via a second polarization controller (109). The second polarization controller 109 is adjusted to maximize the output power of the TE port of the polarization beam splitter 110. The laser output from the TE port is used as the common cavity Brillouin laser LE, which is used as a reference light source and coupled with the 10% branch, i.e., the dual-beam sideband pump laser LS, after the polarization state of the dual-beam sideband pump laser is locked to the TE mode by the third polarization controller 111. The laser LM output from the TM port is fed back to the PDH locking system to realize that the pump laser L generated by the tunable laser 101 is an ultra-stable laser and to achieve frequency stability of the common cavity Brillouin laser.

[0048] The generation of a common-cavity Brillouin laser (LE) is an existing technology. This invention applies it to the phase noise test of millimeter-wave signals. When the Brillouin laser is excited by the fiber ring resonator, the two sidebands have different frequency shifts (the sideband with the higher frequency shifts downward more, and the sideband with the lower frequency shifts downward less). The generated common-cavity Brillouin laser is coupled and separated with a dual-beam sideband pump laser to obtain upper and lower double-sideband optical signals with different frequency intervals.

[0049] In this invention, the PDH (Pound-Drever-Hall) locking technique is used to stabilize the frequency of the common-cavity Brillouin laser, specifically as follows:

[0050] The lower-path pump laser L2 is input to the phase modulator 201, and its frequency is adjusted by the radio frequency signal output by the microwave source 202 so that the phase of the error signal exhibits dispersive spectral characteristics. Then, the laser polarization state is locked to the TM mode by the fourth polarization controller 203 and injected into the fiber ring resonator 108. An optical isolator 204 is inserted in the middle to block the interference of the upper-path dual-beam sideband pump laser LS to the lower path by reflection from the fiber ring resonator 108.

[0051] The laser LM output from the TM port of the polarization beam splitter 110 is split through a 90:10 fiber coupler 205—the 10% branch is connected to a power meter 206 to monitor the ring cavity coupled optical power in real time, ensuring that the frequency of the dual-sideband pump laser LS resonates with the fiber ring resonator 108; the 90% branch is converted into an electrical signal by the first photodetector 207, and then compared with the radio frequency signal from the microwave source 202 through the first mixer 208 to generate an error signal. This error signal is then input to the servo controller 210 through a low-pass filter 209. The error waveform is observed through an oscilloscope 211, and the PID parameters and gain are dynamically adjusted. Finally, the servo output is fed back to the pump laser source to tune the laser 101, achieving frequency stabilization of the common-cavity Brillouin laser. PDH locking technology is existing technology and will not be described in detail here.

[0052] Step S4: Optical-Electrical Domain Noise Conversion

[0053] Step S4.1: Select beat frequency

[0054] For the optical signal LES output by the coupler 301 of the common cavity Brillouin laser LE and the dual-sideband pumped laser LS, the programmable filter 302 splits the upper pump sideband of the dual-sideband pumped laser LS and the upper sideband of the common cavity Brillouin laser LE into one path, namely the upper double-sideband optical signal LES1, and splits the lower pump sideband of the dual-sideband pumped laser LS and the lower sideband of the common cavity Brillouin laser LE into another path, namely the lower double-sideband optical signal LES2.

[0055] Step S4.2: Generate the intermediate frequency signal to be tested

[0056] The upper double-sideband optical signal and the lower double-sideband optical signals LES1 and LES2 are injected into their respective photodetectors, namely the second and third photodetectors 303 and 304, to complete photoelectric conversion, thereby obtaining the upper sideband beat frequency electrical signal SB1 and the lower sideband beat frequency electrical signal SB2, respectively. Then, the upper sideband beat frequency electrical signal SB1 and the lower sideband beat frequency electrical signal SB2 are input into the second mixer 305 to generate an intermediate frequency signal SM in the MHz range.

[0057] In this embodiment, before the upper band beat frequency electrical signal SB1 and the lower band beat frequency electrical signal SB2 are input to the second mixer 305, they are respectively amplified by the first and second radio frequency amplifiers 306 and 307 to increase the power.

[0058] Step S4.3: Test the intermediate frequency signal noise

[0059] The intermediate frequency (IF) signal's SM noise characteristics directly carry the S-phase noise of the millimeter-wave signal under test. The noise spectral density of the IF signal is analyzed and quantified using a phase noise analyzer to complete the S-phase noise test of the millimeter-wave signal under test.

[0060] In this embodiment, the parameters of the programmable filter 302 are as follows: Channel 1: wavelength 1549.843nm, bandwidth: 0.3nm; Channel 2: wavelength 1548.243nm, bandwidth 0.3nm. Channel 1 is used for splitting the upper double-sideband optical signal LES1, and Channel 2 is used for splitting the lower double-sideband optical signal LES2. The spectrum before filtering (splitting) is shown in Figure 3. From Figure 3, we can see that the common-cavity Brillouin laser LE has a lower frequency (increased wavelength) compared to the two-beam sideband pumped laser LS.

[0061] The resulting spectra after filtering (separation) are shown in Figures 4 and 5. In this embodiment, "up" and "down" refer to wavelengths, with longer wavelengths at the top and shorter wavelengths at the bottom.

[0062] In this embodiment, the millimeter-wave signal S to be measured from the dual-beam sideband pumped laser LS is twice the measured signal, 200 GHz. The upper sideband beat frequency electrical signal SB1 output by the second photodetector 303 has a frequency of 10.721 GHz, and the lower sideband beat frequency electrical signal SB2 output by the third photodetector 304 has a frequency of 10.733 GHz (the lower sideband frequency of the common-cavity Brillouin laser LE is higher and the offset is greater, so the lower sideband beat frequency electrical signal SB2 has a higher frequency). After mixing, the required 12 MHz intermediate frequency signal SM is obtained. The 12 MHz intermediate frequency signal SM is input into a phase noise analyzer, and the phase noise of the measured signal is obtained by subtracting 6 dB from the measured phase noise.

[0063] To better illustrate the technical effects of this invention, a comparative experiment was conducted using R&S's FSWP series high-end signal analyzer, currently the highest-performing commercial analyzer available. For measuring signal noise above 50 GHz, this phase noise analyzer can be expanded with an external mixer via optional components. Combined with external FS-ZXX series mixers for various millimeter-wave frequency bands, and employing mixer cross-correlation, stable test results are obtained. The comparative phase noise test results for a 100 GHz millimeter-wave signal under test are shown in Figure 6. The phase noise test results remain consistent within a 1-1 MHz frequency offset, and this scheme does not require complex high-bandwidth electrical components, thus demonstrating the advantages of this invention through optical-electric domain noise conversion.

[0064] 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 method for testing the phase noise of millimeter-wave signals based on ultrastable lasers, characterized in that, The process includes the following steps: (1) Generating two pump lasers: A tunable laser is used as the laser source to generate pump lasers, which are then split into two paths via a 50:50 fiber coupler: an upper pump laser and a lower pump laser. The upper pump laser is used to generate a dual-sideband pump laser, and the lower pump laser outputs a PDH-locked system to achieve frequency stabilization of the common-cavity Brillouin laser. (2) Generating dual-sideband pump lasers: The millimeter-wave signal to be measured is connected to the RF port of the intensity modulator. The upper pump laser is used as an optical carrier and connected to the optical input port of the intensity modulator. Under the drive of the millimeter-wave signal to be measured, the intensity modulator generates a frequency twice that of the millimeter-wave signal to be measured. (3) Using the Brillouin effect to generate a co-cavity Brillouin laser in the fiber ring cavity, the dual-optical sideband pump laser is sent to an erbium-doped fiber amplifier for amplification after suppressing the optical carrier power and enhancing the sideband output intensity. Then, the amplified dual-optical sideband pump laser is split into two paths using a 90:10 fiber coupler. The 90% branch is locked to the polarization state of the dual-optical sideband pump laser as TE mode by the first polarization controller. Then, it is injected into the fiber ring resonator to generate Brillouin laser. The generated Brillouin laser is output to the polarization beam splitter through the second polarization controller. The second polarization controller is adjusted to make the TE mode of the polarization beam splitter... The port output power is the largest. Among them, the laser output from the TE port is used as the common cavity Brillouin laser. As a reference light source, it is coupled with the 10% branch, i.e., the dual-light sideband pump laser, after the polarization state of the dual-light sideband pump laser is locked to the TE mode by the third polarization controller. The laser output from the TM port is fed back to the PDH locking system to realize that the pump laser generated by the tunable laser is an ultra-stable laser and to realize the frequency stability of the common cavity Brillouin laser; (4) Optical-electric domain noise conversion For the optical signal output by the coupling of the common cavity Brillouin laser and the dual-light sideband pump laser, the upper pump sideband of the dual-light sideband pump laser and the upper sideband of the common cavity Brillouin laser are used as one channel by a programmable filter. The upper double-sideband optical signal is split off, and the lower pump sideband of the dual-sideband pump laser and the lower sideband of the common-cavity Brillouin laser are used as another path, namely the lower double-sideband optical signal. The upper double-sideband optical signal and the lower double-sideband optical signal are injected into their respective photodetectors to complete photoelectric conversion, resulting in the upper sideband beat frequency electrical signal and the lower sideband beat frequency electrical signal, respectively. Then, the upper sideband beat frequency electrical signal and the lower sideband beat frequency electrical signal are input into a mixer to generate an intermediate frequency signal in the MHz range. The noise characteristics of the intermediate frequency signal directly carry the phase noise of the millimeter-wave signal under test. The noise spectral density of the intermediate frequency signal is analyzed and quantified by a phase noise analyzer to complete the phase noise test of the millimeter-wave signal under test.

2. The method for testing the phase noise of millimeter-wave signals based on ultrastable lasers according to claim 1, characterized in that, The spectral characteristics of the dual-light sideband pumped laser are monitored in real time by a spectral analyzer. The bias voltage of the intensity modulator is adjusted to suppress the optical carrier power to the greatest extent and enhance the sideband output intensity in order to meet the input optical power requirements of the erbium-doped fiber amplifier. If the residual optical carrier power still exceeds the allowable range, a band-stop filter matching the carrier wavelength is inserted into the optical path for secondary suppression.

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