A broadband optical vector measurement method and device based on I / Q demodulation

Abstract

The application provides a broadband light vector measurement method and device based on I / Q demodulation, which comprises the following steps: generating carrier-suppressed double-sideband linear frequency modulation light signals with fixed starting frequency difference as a measurement signal and a local oscillator signal respectively; passing the measurement signal through a light device to be measured to obtain a probe light signal; coherently mixing the probe light signal and the local oscillator signal through a 90-degree optical mixer, extracting I / Q two-way electrical signals through a balanced photodetector; constructing a complex signal to separate upper and lower sidebands and suppress mirror frequency interference, extracting fixed low-frequency components to obtain spectrum response information of the light device to be measured; obtaining the system's own spectrum response through a calibration process; eliminating the system response through complex division operation to obtain the amplitude response and phase response of the light device to be measured. The application can solve the spectrum aliasing problem in symmetrical double-sideband modulation without increasing the complexity of the system, expand the measurement bandwidth by one time, speed up the measurement, and does not need a wideband electrical amplitude and phase detector, effectively reducing the measurement cost.

Description

Technical Field

[0001] This invention relates to the fields of optical communication and microwave photonics technology, and in particular to a broadband optical vector measurement method and apparatus based on I / Q demodulation. Background Technology

[0002] Optical vector analysis is a core technology for characterizing the spectral response of optical devices. It can simultaneously measure the amplitude and phase responses of optical devices and has wide applications in optical communication, optical sensing, optical integrated circuits, and optical nanoparticle detection. In recent years, as optical systems have developed towards higher bandwidth, higher integration, and faster tunability, higher requirements have been placed on optical vector measurement technology in terms of measurement bandwidth, speed, resolution, and cost.

[0003] Traditional optical vector measurement techniques mainly rely on phase-shift modulation and optical interferometry. These methods use tunable laser sources for point-by-point frequency sweep measurements. While the principles are simple, they are limited by the finite tuning step size of the laser source, poor stability, and slow mechanical scanning speed, making it difficult to achieve high-resolution and high-speed measurements. To overcome the resolution bottleneck, researchers have proposed optical vector measurement techniques based on single-sideband modulation and double-sideband modulation. Among them, the single-sideband modulation scheme can theoretically achieve Hertz-level high resolution, but because it only uses one sideband for scanning, its measurement bandwidth is severely limited, failing to meet the testing requirements of broadband optical devices.

[0004] In contrast, optical vector measurement techniques based on double-sideband modulation can utilize both sidebands on either side of the optical carrier, theoretically doubling the measurement bandwidth compared to single-sideband schemes. However, this approach faces a critical problem: when symmetrical double-sideband signals undergo square-law detection using a photodetector, the optical carrier beats with the upper and lower sidebands at the same frequency, resulting in aliasing of image frequencies and severely impacting measurement accuracy. To address this spectral aliasing issue, researchers have attempted to introduce asymmetric double-sideband modulation, such as by using an acousto-optic modulator or stimulated Brillouin scattering to shift the optical carrier frequency, causing the two sidebands to have different frequency intervals with the optical carrier, thereby distinguishing the two beat frequency signals in the electrical domain. Figure 1 This is a block diagram of a light vector analysis system based on asymmetric double-sideband modulation. However, this type of asymmetric modulation method requires additional frequency-shifting devices or complex nonlinear effect control systems, which significantly increases the difficulty and cost of system implementation.

[0005] To circumvent the complexity of asymmetric modulation, a light vector analysis scheme based on symmetric double-sideband modulation is proposed. This scheme measures the same device under test under two different modulation conditions, such as using a phase modulator and an intensity modulator respectively. The results of the two measurements are then combined to construct a matrix equation, thereby separating the spectral information carried by the upper and lower sidebands. Figure 2The diagram shows the principle block diagram of an optical vector analysis system based on symmetric double-sideband modulation. Although this method avoids the additional hardware requirements of asymmetric modulation, it still has significant drawbacks: First, the two measurement operations are cumbersome and require extremely high stability of the optical path. Any slight environmental fluctuation or laser wavelength drift will introduce additional phase and amplitude noise, reducing measurement accuracy. Second, this method still requires the use of a broadband electric amplitude and phase detector to extract the amplitude and phase information of the device under test from the high-frequency beat signal, which is expensive. In addition, whether it is the traditional frequency sweep method or the above-mentioned improved scheme, it is essentially still a point-by-point frequency sweep measurement, which takes a long time and is difficult to adapt to rapidly changing objects under test.

[0006] In summary, existing optical vector analysis techniques have not yet achieved a good balance between measurement bandwidth, system complexity, measurement efficiency, and cost. How to solve the spectral aliasing problem in symmetrical double-sideband modulation without increasing system complexity, while simultaneously achieving fast and low-cost vector response measurement of broadband optical devices, has become a pressing technical challenge in this field. Summary of the Invention

[0007] In view of this, embodiments of the present invention provide a broadband optical vector measurement method and apparatus based on I / Q demodulation to solve the problems of spectral aliasing caused by symmetrical double-sideband modulation, increased system complexity due to asymmetric modulation, cumbersome two-stage measurement operations that are easily affected by the environment, high cost due to reliance on broadband electric amplitude phase detectors, and slow point-by-point frequency sweep measurement speed in the prior art.

[0008] On one hand, the present invention provides a broadband optical vector measurement method based on I / Q demodulation, the method comprising: A first carrier-suppressed double-sideband linear frequency modulated optical signal is generated as a measurement signal, and a second carrier-suppressed double-sideband linear frequency modulated optical signal is generated as a local oscillator signal; the measurement signal and the local oscillator signal have the same chirp rate and pulse width, and there is a preset frequency difference between the starting frequency of the linear frequency modulated signal in the measurement signal and the starting frequency of the linear frequency modulated signal in the local oscillator signal; The measurement signal is passed through the optical device under test to obtain a probe light signal carrying the spectral response of the optical device under test; The probe light signal and the local oscillator signal are input into a 90° optical mixer for coherent mixing, and the in-phase component electrical signal and the quadrature component electrical signal are extracted by a balanced photodetector respectively. The in-phase component electrical signal and the quadrature component electrical signal are subjected to analog-to-digital conversion and digital signal processing to construct a complex signal, so as to separate the upper and lower sidebands and suppress image frequency interference; the low-frequency component with a frequency of the preset frequency difference is extracted from the complex signal to obtain the spectral response information of the optical device under test carried by the upper and lower sidebands of the measurement signal, respectively. Without the measurement signal passing through the optical device under test, the measurement signal and the local oscillator signal are input into the 90° optical mixer for coherent mixing, and then the in-phase component electrical signal and the quadrature component electrical signal are extracted sequentially through the balanced photodetector. After analog-to-digital conversion and digital signal processing, a complex signal is constructed. From the complex signal, the low-frequency component with a frequency of the preset frequency difference is extracted as the spectrum response of the measurement system itself. The low-frequency component carrying the spectral response of the optical device under test is divided by the spectral response of the measurement system itself to eliminate the amplitude and phase responses of the measurement system, thereby obtaining the amplitude and phase responses of the optical device under test.

[0009] In some embodiments of the present invention, generating a first carrier-suppressed double-sideband linear frequency modulated optical signal as a measurement signal and a second carrier-suppressed double-sideband linear frequency modulated optical signal as a local oscillator signal includes: The optical carrier output from the tunable laser is split into a first optical carrier and a second optical carrier by an optical beam splitter. The first linear frequency modulated signal is amplified and applied to the radio frequency input port of the first Mach-Zehnder modulator. The first Mach-Zehnder modulator is biased at the minimum transmission point, and the first optical carrier is subjected to carrier suppression double-sideband modulation to generate the measurement signal. The second linear frequency modulated signal is amplified and applied to the radio frequency input port of the second Mach-Zehnder modulator. The second Mach-Zehnder modulator is biased at the minimum transmission point, and the second optical carrier is subjected to carrier suppression double-sideband modulation to generate the local oscillator signal.

[0010] In some embodiments of the present invention, after obtaining the probe light signal by passing the measurement signal through the optical device under test, and before inputting the probe light signal and the local oscillator signal into the 90° optical mixer, the method further includes: The time delay of the branch containing the local oscillator signal is adjusted by using an optical variable delay line, so that the time delay difference between the probe light signal and the local oscillator signal is zero.

[0011] In some embodiments of the present invention, the optical field of the measurement signal is represented as follows: ; The optical field of the local oscillator signal is represented as follows: ; in, , The light fields of the measurement signal and the local oscillator signal are respectively represented. Indicates the optical carrier angular frequency; , These represent the initial angular frequencies of the first linear frequency modulated signal and the second linear frequency modulated signal, respectively. Indicates the chirp rate; Indicates the pulse width; , Let be the complex amplitude of each sideband, with subscripts -1 and +1 representing the negative first-order sideband and the positive first-order sideband, respectively.

[0012] In some embodiments of the present invention, the light field of the probe light signal is represented as follows: ; in, ; in, This represents the light field of the probe light signal; This represents the spectral response of the optical device under test; Indicates the spectral response of the measurement system; , These represent the lower sideband angular frequency and the upper sideband angular frequency of the measured signal, respectively.

[0013] In some embodiments of the present invention, analog-to-digital conversion and digital signal processing are performed on the in-phase component electrical signal and the quadrature component electrical signal to construct a complex signal, which is represented as follows: ; in, This represents the complex signal; This represents the in-phase component electrical signal; This represents the orthogonal component electrical signal; This indicates the responsivity of the balanced photodetector; and They represent and .

[0014] In some embodiments of the present invention, a low-frequency component with a frequency equal to the preset frequency difference is extracted from the complex signal to obtain spectral response information of the optical device under test carried by the upper and lower sidebands of the measurement signal, respectively, including: Two low-frequency components are extracted from the complex signal using a digital bandpass filter, and are denoted as the first measurement component and the second measurement component, respectively. The frequency of the first measurement component is the difference between the starting angular frequency of the local oscillator signal and the starting angular frequency of the measurement signal, and the frequency of the second measurement component is the difference between the starting angular frequency of the measurement signal and the starting angular frequency of the local oscillator signal. The extraction results are represented as follows: ; ; in, The first measurement component carries the spectral response information of the optical device under test corresponding to the lower sideband of the measurement signal; The second measurement component carries the spectral response information of the optical device under test corresponding to the upper sideband of the measurement signal.

[0015] In some embodiments of the present invention, extracting a low-frequency component with a frequency equal to the preset frequency difference from the complex signal as the spectral response of the measurement system itself includes: When the measurement signal does not pass through the optical device under test, two low-frequency components are extracted from the corresponding complex signal by the digital bandpass filter, which are denoted as the first calibration component and the second calibration component, respectively. The frequency of the first calibration component is the difference between the starting angular frequency of the local oscillator signal and the starting angular frequency of the measurement signal, and the frequency of the second calibration component is the difference between the starting angular frequency of the measurement signal and the starting angular frequency of the local oscillator signal. The extraction results are represented as follows: ; ; in, This represents the first calibration component; This represents the second calibration component; This represents the spectral response of the measurement system itself.

[0016] In some embodiments of the present invention, a complex division operation is performed between the low-frequency component carrying the spectral response of the optical device under test and the spectral response of the measurement system itself to obtain the amplitude response and phase response of the optical device under test, including: Perform complex division between the first measured component and the first calibration component, and perform complex division between the second measured component and the second calibration component. The calculation formula is as follows: ; ; in, This represents the spectral response of the optical device under test at the lower sideband frequency; This represents the spectral response of the optical device under test at the upper sideband frequency.

[0017] On the other hand, the present invention also provides a broadband optical vector measurement device based on I / Q demodulation, the device comprising: The signal generation module is used to generate a first carrier-suppressed double-sideband linear frequency modulated optical signal as a measurement signal and a second carrier-suppressed double-sideband linear frequency modulated optical signal as a local oscillator signal; wherein the measurement signal and the local oscillator signal have the same chirp rate and pulse width, and there is a preset frequency difference between the starting frequency of the linear frequency modulated signal in the measurement signal and the starting frequency of the linear frequency modulated signal in the local oscillator signal; The optical device under test (ODT) access port is located on the transmission optical path of the measurement signal and is used to connect the ODT, so that the measurement signal passing through carries the spectral response of the ODT and forms a probe light signal. A 90° optical mixer, with its first input terminal receiving the probe optical signal and its second input terminal receiving the local oscillator signal, is used to coherently mix the probe optical signal and the local oscillator signal; A balanced photodetector is connected to the output terminal of the 90° optical mixer to output in-phase and quadrature component electrical signals. An analog-to-digital converter module is connected to the output terminal of the balanced photodetector and is used to convert the in-phase component electrical signal and the quadrature component electrical signal into digital signals. A digital signal processing module, connected to the output of the analog-to-digital conversion module, is used to construct a complex signal from the converted in-phase and quadrature components to separate the upper and lower sidebands and suppress image frequency interference. It extracts a low-frequency component with a frequency equal to the frequency difference from the complex signal to obtain a low-frequency component carrying the spectral response of the optical device under test, and acquires a low-frequency component carrying the spectral response of the measurement system itself in a calibration state. Finally, it eliminates the amplitude and phase responses of the measurement system through complex division to obtain the amplitude and phase responses of the optical device under test.

[0018] The present invention has the following beneficial effects: This invention combines I / Q coherent detection with complex signal construction to achieve image frequency suppression and effective separation of upper and lower sidebands in the digital domain. Compared to traditional schemes that require additional devices such as acousto-optic modulators or stimulated Brillouin scattering in the optical domain to achieve asymmetric modulation, this invention eliminates the need for frequency shifting of the optical carrier and the need for two measurements and simultaneous solutions under different conditions. While simplifying the system structure and reducing implementation complexity, it fundamentally solves the inherent spectral aliasing problem in symmetric double-sideband modulation. Because the upper and lower sidebands of the measurement signal simultaneously carry the spectral response information of the optical device under test, the measurement bandwidth is doubled compared to single-sideband schemes.

[0019] Furthermore, this invention employs a linear frequency modulated signal as the modulation signal. Utilizing its high-frequency tuning speed, a single pulse can complete a frequency scan across the entire measurement bandwidth, reducing the single measurement time to the microsecond level, thus meeting the real-time testing requirements of rapidly changing measurement objects. Simultaneously, the target signal carrying the spectral response information of the optical device under test is a fixed low-frequency signal. Its frequency depends on the preset frequency difference between the measurement signal and the local oscillator signal, and can be extracted using conventional low-frequency detection devices and digital filtering. This eliminates the need for expensive broadband amplitude phase detectors, significantly reducing the hardware cost of the measurement system.

[0020] Furthermore, the calibration and measurement processes of this invention share the same hardware system, which can be completed simply by changing the access status of the optical device under test. This simplifies the operation and avoids system errors caused by inconsistencies between the calibration and measurement paths, effectively ensuring the accuracy and consistency of the measurement results.

[0021] Additional advantages, objects, and features of the invention will be set forth in part in the description which follows, and will also become apparent in part to those skilled in the art upon studying the text, or may be learned by practice of the invention. The objects and other advantages of the invention can be realized and obtained by means of the structures specifically pointed out in the description and drawings.

[0022] Those skilled in the art will understand that the objectives and advantages achievable with the present invention are not limited to those specifically described above, and that the above and other objectives achievable with the present invention will become clearer from the following detailed description. Attached Figure Description

[0023] The accompanying drawings, which are included to provide a further understanding of the invention and form part of this application, are not intended to limit the scope of the invention. In the drawings: Figure 1 This is a block diagram of a light vector analysis system based on asymmetric optical double-sideband modulation in one embodiment of the present invention.

[0024] Figure 2This is a block diagram of a light vector analysis system based on symmetrical optical double-sideband modulation in one embodiment of the present invention.

[0025] Figure 3 This is a schematic diagram of the steps of a broadband optical vector measurement method based on I / Q demodulation in one embodiment of the present invention.

[0026] Figure 4 This is a schematic diagram of a broadband optical vector measurement device based on I / Q demodulation in one embodiment of the present invention.

[0027] Figure 5 This is a carrier-suppressed double-sideband signal spectrum at the output terminals of the first Mach-Zehnder modulator and the second Mach-Zehnder modulator in one embodiment of the present invention.

[0028] Figure 6 This is a spectrum diagram of a complex signal combined in a DSP using I / Q demodulation technology in one embodiment of the present invention.

[0029] Figure 7 The above diagram shows the amplitude-frequency response and phase-frequency response of a phase-shifted fiber optic grating measured using the method of this invention in one embodiment of the invention. Detailed Implementation

[0030] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the embodiments and accompanying drawings. Here, the illustrative embodiments and descriptions of this invention are used to explain the invention, but are not intended to limit the invention.

[0031] It should also be noted that, in order to avoid obscuring the invention with unnecessary details, only the structures and / or processing steps closely related to the solution according to the invention are shown in the accompanying drawings, while other details that are not closely related to the invention are omitted.

[0032] It should be emphasized that the term "including / comprises" as used herein refers to the presence of a feature, element, step, or component, but does not exclude the presence or addition of one or more other features, elements, steps, or components.

[0033] It should also be noted that, unless otherwise specified, the term "connection" in this article can refer not only to a direct connection, but also to an indirect connection involving an intermediary.

[0034] In the following description, embodiments of the invention will be illustrated with reference to the accompanying drawings. In the drawings, the same reference numerals represent the same or similar parts, or the same or similar steps.

[0035] It should be emphasized here that the step markers mentioned below are not a limitation on the order of the steps, but should be understood as meaning that the steps can be executed in the order mentioned in the embodiments, or in a different order than in the embodiments, or several steps can be executed simultaneously.

[0036] To address the problems of spectral aliasing caused by symmetrical double-sideband modulation, increased system complexity due to asymmetric modulation, cumbersome two-step measurement operations that are susceptible to environmental influences, high cost due to reliance on broadband amplitude phase detectors, and slow point-by-point frequency sweep measurement speed in existing technologies, this invention provides a broadband optical vector measurement method based on I / Q demodulation.

[0037] Figure 3 This diagram illustrates the steps of the broadband optical vector measurement method based on I / Q demodulation of this invention. Figure 3 As shown, the method includes the following steps S101~S106: Step S101: Generate a first carrier-suppressed double-sideband linear frequency modulated (LFM) optical signal as a measurement signal, and a second carrier-suppressed double-sideband LFM optical signal as a local oscillator signal. The measurement signal and the local oscillator signal have the same chirp rate and pulse width, and there is a preset frequency difference between the starting frequency of the LFM signal in the measurement signal and the starting frequency of the LFM signal in the local oscillator signal.

[0038] Step S102: Pass the measurement signal through the optical device under test to obtain a probe light signal carrying the spectral response of the optical device under test.

[0039] Step S103: Input the probe light signal and the local oscillator signal into a 90° optical mixer for coherent mixing, and extract the in-phase component electrical signal and the quadrature component electrical signal respectively through a balanced photodetector.

[0040] Step S104: Perform analog-to-digital conversion and digital signal processing on the in-phase and quadrature component electrical signals to construct a complex signal, so as to separate the upper and lower sidebands and suppress image frequency interference; extract the low-frequency component with a preset frequency difference from the complex signal to obtain the spectral response information of the optical device under test carried by the upper and lower sidebands of the measurement signal, respectively.

[0041] Step S105: Without the measurement signal passing through the optical device under test, the measurement signal and the local oscillator signal are input into a 90° optical mixer for coherent mixing. The in-phase component electrical signal and the quadrature component electrical signal are extracted sequentially through a balanced photodetector. After analog-to-digital conversion and digital signal processing, a complex signal is constructed. The low-frequency component with a preset frequency difference is extracted from the complex signal as the spectrum response of the measurement system itself.

[0042] Step S106: Perform a complex division operation between the low-frequency component carrying the spectral response of the optical device under test and the spectral response of the measurement system itself to eliminate the amplitude and phase responses of the measurement system, and obtain the amplitude and phase responses of the optical device under test.

[0043] In step S101, a measurement signal and a local oscillator signal are generated.

[0044] The purpose of step S101 is to generate two carrier-suppressed double-sideband linear frequency modulated optical signals with a specific frequency relationship, which will serve as the measurement signal and local oscillator signal in the subsequent measurement process, respectively. These two signals are the basic signal sources of the entire measurement system, and their parameter design directly determines the measurement bandwidth, resolution, and the effectiveness of subsequent I / Q demodulation.

[0045] First, a tunable laser generates a continuous optical carrier beam with a stable center frequency. The center frequency of this tunable laser can be selected according to the operating wavelength of the optical device under test.

[0046] In some embodiments, the laser is configured to generate an optical carrier with a center frequency of 193.55 THz, which corresponds to a common wavelength in the C-band of optical communication.

[0047] The optical carrier is then fed into an optical beamsplitter, which splits the input optical carrier into two paths based on power: the first path is used to generate the measurement signal, and the second path is used to generate the local oscillator signal. This beamsplitting method ensures that the two optical signals originate from the same laser source and have a coherent phase relationship, laying the foundation for subsequent coherent mixing.

[0048] At the same time, two linear frequency modulation (FM) signals are generated by an arbitrary waveform generator: a first FM signal and a second FM signal.

[0049] Among them, the two linear frequency modulation signals have the following strict parameter relationship: Same chirp rate: Ensure that the frequency scanning rates of the two signals are consistent, which is a prerequisite for generating a fixed frequency difference signal by subsequent beat frequency.

[0050] Same pulse width: Ensures that the two signals have the same duration in the time domain, guaranteeing that the frequency scanning process of the two signals is completely synchronized throughout the entire measurement cycle.

[0051] There is a preset frequency difference between the starting frequencies. For example, the first linear frequency-modulated signal has a starting angular frequency. The second linear frequency modulated signal has a starting angular frequency. The difference between the two This is the angular frequency difference corresponding to the preset frequency difference.

[0052] The frequency difference remains constant throughout the pulse duration because the two signals have the same chirp rate. At any given moment, the instantaneous angular frequencies of the two signals are respectively... and The difference is always .

[0053] In some embodiments, the first linear frequency modulated signal is set to a start frequency of 0 MHz, an end frequency of 10 GHz, and a pulse width of 5 μs, and the second linear frequency modulated signal is set to a start frequency of 100 MHz, an end frequency of 10.1 GHz, and a pulse width of 5 μs, with a 100 MHz start frequency difference between the two. This 100 MHz frequency difference is the target frequency for subsequent fixed low-frequency detection, which is far below the frequency range required to be covered by a broadband amplitude phase detector in the electrical domain, thus allowing the use of low-cost low-frequency detection devices to replace expensive broadband devices.

[0054] In some embodiments, the first linear frequency modulated signal is amplified by a low-noise amplifier and then applied to the RF input port of the first Mach-Zehnder modulator. The first Mach-Zehnder modulator is biased at its minimum transmission point to perform carrier-suppressed double-sideband modulation on the first optical carrier, generating a measurement signal. Similarly, the second linear frequency modulated signal is amplified and applied to the second Mach-Zehnder modulator, which is also biased at its minimum transmission point to modulate the second optical carrier, generating a local oscillator signal. When the Mach-Zehnder modulator is biased at its minimum transmission point, the optical carrier component is suppressed to the maximum extent, and the output optical signal mainly consists of positive first-order sidebands and negative first-order sidebands, achieving carrier-suppressed double-sideband modulation.

[0055] In some embodiments, the optical field of the generated measurement signal after the above modulation can be expressed as formula (1): ; (1) The optical field of the generated local oscillator signal can be expressed as formula (2): ; (2) in, , These represent the optical fields of the measured signal and the local oscillator signal, respectively. Indicates the optical carrier angular frequency; , These represent the initial angular frequencies of the first linear frequency modulated signal and the second linear frequency modulated signal, respectively. Indicates the chirp rate; Indicates the pulse width; , Let be the complex amplitude of each sideband, with subscripts -1 and +1 representing the negative first-order sideband and the positive first-order sideband, respectively.

[0056] In step S102, the measurement signal is passed through the optical device under test to obtain a probe light signal carrying the spectral response of the optical device under test.

[0057] The purpose of step S102 is to pass the measurement signal generated in step S101 through the optical device under test, so that the amplitude and phase of the measurement signal change accordingly according to the spectral response characteristics of the optical device under test, thereby "loading" the spectral response information of the optical device under test onto the measurement signal to form a probe light signal.

[0058] Connect the optical device under test into the optical path for transmitting the measurement signal.

[0059] In some embodiments, a dedicated access port for the optical device under test is provided, which is located between the output of the first Mach-Zehnder modulator in the signal generation module and the first input of the 90° optical mixer.

[0060] The optical device under test can be any optical element or photonic system whose spectral response needs to be characterized, such as fiber gratings, optical filters, optical amplifiers, optical waveguide devices, functional units in photonic integrated circuits, etc. In one specific embodiment, the optical device under test is modeled as an ideal phase-shifted fiber grating with a center frequency of 193.55 THz and a bandwidth of 20 GHz.

[0061] When the measurement signal passes through the optical device under test (DUT), the DUT applies different amplitude attenuation (or gain) and phase shift to different frequency components. The negative first-order sideband and positive first-order sideband in the measurement signal scan through the DUT at different frequency positions, and are therefore modulated by the spectral response of the DUT at the corresponding frequencies.

[0062] After the measurement signal passes through the optical device under test, it forms a probe light signal carrying the spectral response of the optical device under test. The light field of this probe light signal can be expressed by formula (3): ; (3) in, This can be expressed as formula (4): ; (4) in, The light field representing the probe light signal; This represents the spectral response of the optical device under test; Indicates the spectral response of the measurement system; for and The product of these terms represents the total spectral response that the measured signal actually experiences after passing through the optical device under test. , These represent the lower sideband angular frequency and the upper sideband angular frequency of the measured signal, respectively.

[0063] It should be noted that in the optical field expression for the probed optical signal, the complex amplitude of the negative first-order sideband is changed from the original... Become The complex amplitude of the positive first-order sideband changes from the original Become This change reflects the modulation effect of the optical device under test on different frequency components.

[0064] It should be noted that the measurement signal passes through other optical components in the optical path (such as optical beam splitters, optical mixers, and fiber optic connectors) before and after passing through the optical device under test. These components themselves also have certain frequency response characteristics, collectively referred to as the spectral response of the measurement system. Therefore, what can be directly extracted from the probe optical signal is the product of the spectral response of the optical device under test and the spectral response of the system. In order to obtain the true spectral response of the optical device under test. The impact of this on the system response needs to be eliminated through a calibration process in subsequent steps.

[0065] In step S103, the probe light signal and the local oscillator signal are input into a 90° optical mixer for coherent mixing, and the in-phase component electrical signal and the quadrature component electrical signal are extracted by a balanced photodetector respectively.

[0066] The purpose of step S103 is to perform 90° coherent mixing in the optical domain between the probe optical signal carrying the spectral response of the optical device under test (S102) and the local oscillator signal generated in step S101, linearly converting the amplitude and phase information in the optical signal into electrical signals, and outputting in-phase (I-channel) and quadrature (Q-channel) electrical signals respectively. This step is the core of I / Q demodulation, providing the necessary signal form for subsequent digital signal processing to separate the upper and lower sidebands and suppress image frequency interference.

[0067] First, the probe optical signal output from the upper branch and the local oscillator signal output from the lower branch are respectively sent to the two input terminals of the 90° optical mixer. The probe optical signal is connected to the first input terminal (signal optical port) of the 90° optical mixer, and the local oscillator signal is connected to the second input terminal (local oscillator optical port) of the 90° optical mixer.

[0068] In some embodiments, before sending the two signals into the 90° optical mixer, it is necessary to ensure time delay matching between the two signals. An optical variable delay line is set on the branch where the local oscillator signal is located. By adjusting the optical variable delay line, the time delay of the local oscillator signal is compensated so that the time delay difference between the probe light signal and the local oscillator signal is zero.

[0069] Among them, the 90° optical mixer is an optical device capable of simultaneously outputting multiple interference optical signals with specific phase relationships. In this invention, the 90° optical mixer mixes the input probe optical signal and the local oscillator signal to output four optical signals. The phase relationships between the optical signals are 0°, 180°, 90° and 270°, specifically expressed as formula (5): ; (5) in, and These are a pair of in-phase optical signals with a phase difference of 180°. and These are two pairs of orthogonal optical components with a phase difference of 180°. These two pairs of optical signals respectively contain in-phase interference information and orthogonal interference information between the probe optical signal and the local oscillator signal.

[0070] Will and Connect to the first balanced photodetector, and and A second balanced photodetector is connected. The balanced photodetector performs differential detection on the two input optical signals, that is, it performs photoelectric conversion on each signal and then takes the difference. Balanced detection plays an important role in suppressing common-mode noise, eliminating DC components, and eliminating beat frequency interference. Specifically, the beat frequency terms of the probe optical signal and the local oscillator signal are canceled out in the differential output, and only the crossbeat frequency term between the probe optical signal and the local oscillator signal is retained, thereby improving the signal-to-noise ratio.

[0071] The first balanced photodetector outputs an in-phase component electrical signal, and the second balanced photodetector outputs a quadrature component electrical signal. By using two balanced photodetectors with approximately equal responsivity, they achieve essentially identical responsivity.

[0072] After differential detection and photoelectric conversion by the balanced photodetector, the in-phase and quadrature component electrical signals each contain multiple frequency components generated by the beat frequencies between the sidebands of the probe light signal and the local oscillator signal. Specifically, the in-phase and quadrature component electrical signals contain frequencies generated by the corresponding sidebands (i.e., the beat frequency between the negative first-order sideband of the measurement signal and the negative first-order sideband of the local oscillator signal, and the beat frequency between the positive first-order sideband of the measurement signal and the positive first-order sideband of the local oscillator signal). and The low-frequency components also include frequencies generated by the cross sidebands (i.e., the beat frequency between the negative first-order sideband of the measurement signal and the positive first-order sideband of the local oscillator signal, and the beat frequency between the positive first-order sideband of the measurement signal and the negative first-order sideband of the local oscillator signal). The high-frequency chirping components.

[0073] It is important to emphasize that the phase relationship between corresponding frequency components in the in-phase electrical signal I and the quadrature electrical signal Q exhibits orthogonality. This orthogonality is the physical basis for subsequent image frequency suppression achieved through the construction of I / Q complex signals: when using only a single I or Q signal, it is impossible to distinguish frequencies with a specific frequency. The positive frequency component and frequency are The information carried by the negative frequency components of the two signals are mirror images of each other. However, when the I-channel and Q-channel signals are combined into a complex signal, one of the mirror frequency components is eliminated, thereby achieving effective separation of the upper and lower sideband information.

[0074] In step S104, analog-to-digital conversion and digital signal processing are performed on the in-phase component electrical signal and the quadrature component electrical signal to construct a complex signal, so as to separate the upper and lower sidebands and suppress the image frequency interference; low-frequency components with a preset frequency difference are extracted from the complex signal to obtain the spectral response information of the optical device under test carried by the upper and lower sidebands of the measurement signal, respectively.

[0075] The purpose of step S104 is to digitally acquire the in-phase and quadrature electrical signals obtained in step S103, construct a complex signal using digital signal processing techniques to separate the upper and lower sidebands of the measurement signal and suppress image frequency interference, and then extract the fixed low-frequency component carrying the spectral response information of the optical device under test through digital filtering. This step achieves the upper and lower sideband separation function in the digital domain, which traditional solutions require complex asymmetric modulation or two measurements in the optical domain, fundamentally solving the spectral aliasing problem in symmetric double-sideband modulation.

[0076] First, the in-phase component electrical signal I and the quadrature component electrical signal Q are synchronously sampled by the analog-to-digital conversion module, and the analog electrical signal is converted into a digital signal.

[0077] The I-channel and Q-channel digital signals after analog-to-digital conversion are sent to the digital signal processing module to construct a complex signal in the digital domain. This complex signal completely preserves all frequency components in the I-channel and Q-channel signals.

[0078] In some embodiments, the complex signal is represented by formula (6): (6) in, Indicates a complex signal; Indicates the in-phase component of the electrical signal; Indicates orthogonal component electrical signals; Indicates the responsivity of a balanced photodetector; and They represent and .

[0079] Based on formula (6), it can be seen that complex signals contain multiple frequency components, which can be divided into two categories according to their frequency characteristics and sources: The first category consists of fixed low-frequency components, comprising two items. The first item is... Its angular frequency is It is generated by the beat frequency of the negative first-order sideband (lower sideband) of the measurement signal and the negative first-order sideband of the local oscillator signal, and carries the spectral response of the optical device under test at the lower sideband frequency. The second item is... Its angular frequency is It is generated by the beat frequency of the positive first-order sideband (upper sideband) of the measurement signal and the positive first-order sideband of the local oscillator signal, and carries the spectral response of the optical device under test at the upper sideband frequency. The frequencies of these two low-frequency components depend on the preset frequency difference between the measured signal and the local oscillator signal, and remain constant throughout the pulse duration.

[0080] The second category consists of high-frequency chirp components, comprising two items. These are... and These two parameters are generated by the cross-beat frequency of one sideband of the measurement signal and another sideband of the local oscillator signal, and their instantaneous angular frequency is... It changes rapidly over time.

[0081] It should be noted that in the case of non-I / Q detection (i.e., using only single-channel detection), the angular frequency of the electrical signal output by the photodetector is... and Both components exhibit positive frequency signals, completely overlapping and indistinguishable in the spectrum, resulting in mirror frequency interference. This is also the core challenge faced by symmetrical double-sideband modulation schemes in the background technology.

[0082] In the I / Q coherent detection scheme adopted in this invention, by constructing the I-channel and Q-channel signals as complex signals, the two low-frequency components are separated in the spectrum of the complex signal: the response information carried by the lower sideband of the measurement signal is located at the angular frequency. At that point, the response information carried by the upper sideband is located at the angular frequency. The two are located at different positions on the positive and negative half-axis of the complex spectrum and are independent of each other. Image frequency interference is thus suppressed, and the spectral response information carried by the upper and lower sidebands is completely preserved.

[0083] After constructing the complex signal, two low-frequency components are extracted from the complex signal using a digital bandpass filter. The specific steps are as follows: Two low-frequency components are extracted from the complex signal using a digital bandpass filter, denoted as the first measurement component and the second measurement component, respectively. The frequency of the first measurement component is the difference between the starting angular frequency of the local oscillator signal and the starting angular frequency of the measurement signal, i.e., The frequency of the second measurement component is the difference between the starting angular frequency of the measurement signal and the starting angular frequency of the local oscillator signal, i.e. .

[0084] In some embodiments, the extraction results are represented by formulas (7) and (8): ; (7) ; (8) in, This represents the first measurement component, carrying the spectral response information of the optical device under test corresponding to the lower sideband of the measurement signal; The second measurement component represents the spectral response information of the optical device under test corresponding to the upper sideband of the measurement signal.

[0085] The center frequencies of the digital bandpass filters are respectively set at... and Because the instantaneous frequency of the high-frequency chirp component changes rapidly over time, its frequency range differs significantly from that of the fixed low-frequency component. A digital bandpass filter can effectively filter out the high-frequency chirp component, retaining only the two fixed low-frequency components. These two fixed low-frequency components correspond to the spectral response information carried by the upper and lower sidebands of the measurement signal after passing through the optical device under test.

[0086] In step S105, without the measurement signal passing through the optical device under test, the measurement signal and the local oscillator signal are input into a 90° optical mixer for coherent mixing. The in-phase component electrical signal and the quadrature component electrical signal are extracted sequentially through a balanced photodetector. After analog-to-digital conversion and digital signal processing, a complex signal is constructed. The low-frequency component with a preset frequency difference is extracted from the complex signal as the spectrum response of the measurement system itself.

[0087] Step S105 is the calibration step in the method of this invention. In actual measurements, the measurement signal is affected not only by the modulation of the optical device under test (DUT) but also by the frequency response characteristics of other optical components in the optical path. These effects are collectively referred to as the spectral response of the measurement system. To obtain the true spectral response of the DUT, the system's own spectral response must be obtained separately through the calibration process so that it can be eliminated in subsequent steps.

[0088] The optical device under test (DUT) is removed from the measurement optical path, allowing the measurement signal to be transmitted directly to the first input of the 90° optical mixer without passing through the DUT. In this case, the measurement signal is only affected by the frequency response of the measurement system itself during transmission, without the modulation effect of the DUT.

[0089] During the calibration process, except for the removal of the optical device under test, the remaining optical path structure, device settings, and operating parameters remain consistent with the measurement process described in steps S101 to S104. Specifically, the tunable laser, arbitrary waveform generator, Mach-Zehnder modulator, optical variable delay line, 90° optical mixer, balanced photodetector, and analog-to-digital converter module all maintain the same settings and operating conditions as in the measurement process.

[0090] Under calibration conditions, the measurement signal and local oscillator signal generated by the signal generation module are exactly the same as in step S101. After passing through the measurement system (excluding the optical device under test), the measurement signal is transmitted to the first input of the 90° optical mixer, and the local oscillator signal is transmitted to the second input of the 90° optical mixer. After coherent mixing in the 90° optical mixer, the in-phase and quadrature components of the electrical signal are extracted by a balanced photodetector and then converted into digital signals by an analog-to-digital converter.

[0091] In the digital signal processing module, the I-channel and Q-channel digital signals obtained in the calibration state are constructed into complex signals in the same manner as in step S104. This calibration complex signal has the same frequency component structure as the measurement complex signal, the difference being that it does not contain the spectral response of the optical device under test, but only contains the spectral response of the measurement system itself.

[0092] In some embodiments, during calibration, two low-frequency components are extracted from the calibration complex signal using a digital bandpass filter, denoted as the first calibration component and the second calibration component, respectively. The frequency of the first calibration component is the difference between the starting angular frequency of the local oscillator signal and the starting angular frequency of the measurement signal, and the frequency of the second calibration component is the difference between the starting angular frequency of the measurement signal and the starting angular frequency of the local oscillator signal.

[0093] The extraction results are expressed as formulas (9) and (10): ; (9) ; (10) in, Indicates the first calibration component; Indicates the second calibration component; This represents the spectral response of the measurement system itself.

[0094] In step S106, the low-frequency component carrying the spectral response of the optical device under test is divided by the spectral response of the measurement system itself to eliminate the amplitude and phase responses of the measurement system, thereby obtaining the amplitude and phase responses of the optical device under test.

[0095] The purpose of step S106 is to perform a complex division operation between the measurement component carrying the spectral response of the optical device under test obtained in step S104 and the calibration component of the spectral response of the measurement system itself obtained in step S105. By eliminating the influence of the measurement system response, the true amplitude and phase responses of the optical device under test are finally solved. This step completes the full data processing closed loop from the original measurement data to the final measurement result.

[0096] As analyzed in steps S104 and S105, the first measurement component contains the product of the spectral response of the optical device under test (DUT) and the system spectral response, while the first calibration component contains only the system spectral response. Performing a complex division operation on both components cancels out the system response and common coefficients, yielding the spectral response of the DUT at the lower sideband frequency. Similarly, performing a complex division operation on the second measurement component and the second calibration component yields the spectral response of the DUT at the upper sideband frequency.

[0097] In some embodiments, a complex division operation is performed between the first measurement component and the first calibration component, and a complex division operation is performed between the second measurement component and the second calibration component, with the calculation formulas shown in formulas (11) and (12), respectively: ; (11) ;(12) in, This represents the spectral response of the optical device under test at the lower sideband frequency. This represents the spectral response of the optical device under test at the upper sideband frequency.

[0098] In summary, through steps S101 to S106, this invention provides a broadband optical vector measurement method based on I / Q demodulation. This method generates carrier-suppressed double-sideband linear frequency-modulated optical signals with a fixed starting frequency difference as the measurement signal and the local oscillator signal, respectively. The measurement signal is passed through the optical device under test (DUT) to carry its spectral response. Coherent mixing is performed using a 90° optical mixer and a balanced photodetector to extract the I / Q signals. A complex signal is constructed in the digital domain to separate the upper and lower sidebands and suppress image frequency interference. A fixed low-frequency component carrying the DUT's spectral response is extracted using a digital bandpass filter. The spectral response of the measurement system itself is obtained in the same manner under calibration conditions after removing the DUT. Finally, the system response is eliminated through complex division to obtain the true amplitude and phase responses of the DUT.

[0099] Corresponding to the above method, the present invention also provides a broadband optical vector measurement device based on I / Q demodulation. Figure 4 This is a schematic diagram of the broadband optical vector measurement device based on I / Q demodulation according to the present invention. Figure 4 As shown, the device includes: The signal generation module is used to generate a first carrier-suppressed double-sideband linear frequency modulated (LFM) optical signal as a measurement signal and a second carrier-suppressed double-sideband LFM optical signal as a local oscillator signal. The measurement signal and the local oscillator signal have the same chirp rate and pulse width, and there is a preset frequency difference between the starting frequency of the LFM signal in the measurement signal and the starting frequency of the LFM signal in the local oscillator signal.

[0100] The optical device under test (ODT) access port is located on the optical path of the measurement signal transmission and is used to connect the ODT, so that the passing measurement signal carries the spectral response of the ODT and forms a probe light signal.

[0101] A 90° optical mixer has a first input terminal that receives a probe optical signal and a second input terminal that receives a local oscillator signal, and is used to coherently mix the probe optical signal and the local oscillator signal.

[0102] A balanced photodetector is connected to the output of a 90° optical mixer to output in-phase and quadrature electrical signals.

[0103] The analog-to-digital converter module is connected to the output of the balanced photodetector and is used to convert in-phase and quadrature component electrical signals into digital signals.

[0104] The digital signal processing module, connected to the output of the analog-to-digital converter module, is used to construct a complex signal from the converted in-phase and quadrature components of the electrical signal to separate the upper and lower sidebands and suppress image frequency interference. It extracts a low-frequency component with a preset frequency difference from the complex signal to obtain a low-frequency component carrying the spectral response of the optical device under test, and in the calibration state, it acquires a low-frequency component carrying the spectral response of the measurement system itself. It then eliminates the amplitude and phase responses of the measurement system through complex division to obtain the amplitude and phase responses of the optical device under test.

[0105] The present invention will be further described below with reference to a specific embodiment.

[0106] This embodiment utilizes the broadband optical vector measurement system and method for I / Q demodulation provided by the present invention to measure the amplitude and phase responses of the optical device under test. The system mainly comprises two parts: a signal generation module and an optical vector measurement module based on I / Q coherent detection. The signal generation module generates a broadband carrier-suppressed double-sideband linear frequency modulated signal; the optical vector measurement module measures the spectral response of the optical device under test. The main components of the system include: a tunable laser, an optical beam splitter, an arbitrary waveform generator, a low-noise amplifier, a first Mach-Zehnder modulator, a second Mach-Zehnder modulator, an access port for the optical device under test, an optical variable delay line, a 90° optical mixer, a balanced photodetector, an analog-to-digital converter, and a digital signal processing module.

[0107] A tunable laser generates an optical carrier with a center frequency of 193.55 THz. This optical carrier is split evenly by an optical beamsplitter and transmitted to a first Mach-Zehnder modulator and a second Mach-Zehnder modulator. Simultaneously, a high-performance arbitrary waveform generator generates two linear frequency modulated (FM) signals. The first FM signal is set to a start frequency of 0 MHz, an end frequency of 10 GHz, and a pulse width of 5 μs; the second FM signal is set to a start frequency of 100 MHz, an end frequency of 10.1 GHz, and a pulse width of 5 μs. Both FM signals have the same chirp rate and pulse width, and their start frequencies have a preset frequency difference of 100 MHz.

[0108] The first linear frequency-modulated (FM) signal, amplified by a low-noise amplifier, is applied to the RF input of the first Mach-Zehnder modulator. The first Mach-Zehnder modulator, biased at its minimum transmission point, performs carrier-suppressed double-sideband modulation on the first optical carrier to generate a measurement signal. The second FM signal, amplified by a low-noise amplifier, is applied to the RF input of the second Mach-Zehnder modulator. The second Mach-Zehnder modulator, also biased at its minimum transmission point, performs carrier-suppressed double-sideband modulation on the second optical carrier to generate a local oscillator signal. At the output of the Mach-Zehnder modulator biased at its minimum transmission point, the optical carrier component is suppressed to the maximum extent, and the output optical signal mainly consists of positive first-order sidebands and negative first-order sidebands, achieving carrier-suppressed double-sideband modulation. Figure 5 The image shows the carrier-suppressed double sideband (CS-DSB) signal spectrum at the outputs of the first and second Mach-Zehnder modulators.

[0109] In the upper branch, the measurement signal passes through the optical device under test (DUT) connected to its access port, carrying its spectral response to form a probe signal. In the lower branch, an optical variable delay line is installed. By adjusting the optical variable delay line, time delay compensation is performed on the branch containing the local oscillator signal, ensuring that the time delay difference between the probe signal and the local oscillator signal is zero. The signals from both branches are then input to a 90° optical mixer for coherent mixing. The output of the 90° optical mixer is connected to a balanced photodetector, which converts the optical signal into an electrical signal, outputting an in-phase component electrical signal I and a quadrature component electrical signal Q.

[0110] The analog-to-digital converter (ADC) synchronously samples the I-channel and Q-channel electrical signals, converts them into digital signals, and then sends them to the digital signal processing (DSP) module. In the DSP module, the I-channel and Q-channel digital signals are constructed into complex signals to separate the upper and lower sidebands and suppress image frequency interference. When the frequency difference between the first and second linear frequency modulated (FM) signals is 100MHz, the 100MHz signal component in the complex signal carries the required spectral response information of the optical device under test. Figure 6 The image shows the spectrum of the complex signal combined in the Digital Signal Processing (DSP) module using I / Q demodulation technology. (a) shows the spectrum of the complex signal combined in the DSP module during the measurement process, with the 100MHz signal component carrying the required spectral response information of the optical device under test. (b) shows the corresponding complex signal spectrum during the calibration process. Figure 6 As can be seen, the high-frequency chirped component and the 100MHz fixed low-frequency component are clearly separated in the spectrum, and the target signal can be effectively extracted by a digital bandpass filter.

[0111] To eliminate the measurement system's own spectral response, a calibration process is performed: the optical device under test (DUT) is removed from the access port, preventing the measurement signal from passing through it. The mixing, detection, acquisition, and digital signal processing steps described above are repeated to obtain the measurement system's own spectral response. The digital signal processing module performs a complex division operation between the low-frequency component carrying the DUT's spectral response and the measurement system's own spectral response, eliminating the measurement system's amplitude and phase responses, ultimately obtaining the true amplitude and phase responses of the DUT.

[0112] In the simulation of this embodiment, an ideal phase-shifted fiber grating is used as the optical device under test. This phase-shifted fiber grating has a center frequency of 193.55 THz, a bandwidth of 20 GHz, an effective refractive index of 1.47, and a first fiber length of 0.2 × 10⁻⁶. -3 m, the secondary fiber length is 3.2×10 m. -3 m, with a phase shift of π / 2. Vector measurements were performed on the phase-shifted fiber grating using the measurement method provided in this embodiment. The measurement results show that the measured amplitude-frequency response and phase-frequency response are very close to the spectral response of an ideal phase-shifted fiber grating. Figure 7 To obtain the amplitude-frequency response and phase-frequency response diagrams of a phase-shifted fiber grating using the method of this invention. (Refer to...) Figure 7 The solid blue line represents the spectral response of an ideal phase-shifted fiber grating, and the dashed red line represents the simulation measurement results using the broadband optical vector measurement method provided by this invention. Figure 7As can be seen, the measurement results are in high agreement with the constructed spectral response, verifying that the optical vector measurement method provided by this invention can achieve high-precision measurement of the optical device under test. Furthermore, the measurement range is doubled compared to optical vector measurement schemes based on optical single-sideband modulation, and a single measurement time is only 5 μs.

[0113] It should be clarified that the present invention is not limited to the specific configurations and processes described above and shown in the figures. For the sake of brevity, detailed descriptions of known methods are omitted here. In the above embodiments, several specific steps are described and shown as examples. However, the method process of the present invention is not limited to the specific steps described and shown. Those skilled in the art can make various changes, modifications, and additions, or change the order of steps, after understanding the spirit of the present invention.

[0114] In this invention, features described and / or illustrated for one embodiment may be used in the same or similar manner in one or more other embodiments, and / or combined with or in place of features of other embodiments.

[0115] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, various modifications and variations of the embodiments of the present invention are possible. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A broadband optical vector measurement method based on I / Q demodulation, characterized in that, The method includes: A first carrier-suppressed double-sideband linear frequency modulated optical signal is generated as a measurement signal, and a second carrier-suppressed double-sideband linear frequency modulated optical signal is generated as a local oscillator signal; the measurement signal and the local oscillator signal have the same chirp rate and pulse width, and there is a preset frequency difference between the starting frequency of the linear frequency modulated signal in the measurement signal and the starting frequency of the linear frequency modulated signal in the local oscillator signal; The measurement signal is passed through the optical device under test to obtain a probe light signal carrying the spectral response of the optical device under test; The probe light signal and the local oscillator signal are input into a 90° optical mixer for coherent mixing, and the in-phase component electrical signal and the quadrature component electrical signal are extracted by a balanced photodetector respectively. The in-phase component electrical signal and the quadrature component electrical signal are subjected to analog-to-digital conversion and digital signal processing to construct a complex signal, so as to separate the upper and lower sidebands and suppress image frequency interference; the low-frequency component with a frequency of the preset frequency difference is extracted from the complex signal to obtain the spectral response information of the optical device under test carried by the upper and lower sidebands of the measurement signal, respectively. Without the measurement signal passing through the optical device under test, the measurement signal and the local oscillator signal are input into the 90° optical mixer for coherent mixing, and then the in-phase component electrical signal and the quadrature component electrical signal are extracted sequentially through the balanced photodetector. After analog-to-digital conversion and digital signal processing, a complex signal is constructed. From the complex signal, the low-frequency component with a frequency of the preset frequency difference is extracted as the spectrum response of the measurement system itself. The low-frequency component carrying the spectral response of the optical device under test is divided by the spectral response of the measurement system itself to eliminate the amplitude and phase responses of the measurement system, thereby obtaining the amplitude and phase responses of the optical device under test.

2. The broadband optical vector measurement method based on I / Q demodulation according to claim 1, characterized in that, Generating a first carrier-suppressed double-sideband linear frequency modulated optical signal as a measurement signal, and a second carrier-suppressed double-sideband linear frequency modulated optical signal as a local oscillator signal, includes: The optical carrier output from the tunable laser is split into a first optical carrier and a second optical carrier by an optical beam splitter. The first linear frequency modulated signal is amplified and applied to the radio frequency input port of the first Mach-Zehnder modulator. The first Mach-Zehnder modulator is biased at the minimum transmission point, and the first optical carrier is subjected to carrier suppression double-sideband modulation to generate the measurement signal. The second linear frequency modulated signal is amplified and applied to the radio frequency input port of the second Mach-Zehnder modulator. The second Mach-Zehnder modulator is biased at the minimum transmission point, and the second optical carrier is subjected to carrier suppression double-sideband modulation to generate the local oscillator signal.

3. The broadband optical vector measurement method based on I / Q demodulation according to claim 1, characterized in that, After obtaining the probe light signal by passing the measurement signal through the optical device under test, and before inputting the probe light signal and the local oscillator signal into the 90° optical mixer, the method further includes: The time delay of the branch containing the local oscillator signal is adjusted by using an optical variable delay line, so that the time delay difference between the probe light signal and the local oscillator signal is zero.

4. The broadband optical vector measurement method based on I / Q demodulation according to claim 2, characterized in that, The optical field of the measurement signal is represented as follows: ； The optical field of the local oscillator signal is represented as follows: ； in, , The light fields of the measurement signal and the local oscillator signal are respectively represented. Indicates the optical carrier angular frequency; , These represent the initial angular frequencies of the first linear frequency modulated signal and the second linear frequency modulated signal, respectively. Indicates the chirp rate; Indicates the pulse width; , Let be the complex amplitude of each sideband, with subscripts -1 and +1 representing the negative first-order sideband and the positive first-order sideband, respectively.

5. The broadband optical vector measurement method based on I / Q demodulation according to claim 4, characterized in that, The optical field of the probe optical signal is represented as follows: ； in, ； in, This represents the light field of the probe light signal; This represents the spectral response of the optical device under test; Indicates the spectral response of the measurement system; , These represent the lower sideband angular frequency and the upper sideband angular frequency of the measured signal, respectively.

6. The broadband optical vector measurement method based on I / Q demodulation according to claim 1, characterized in that, The in-phase and quadrature components of the electrical signal are subjected to analog-to-digital conversion and digital signal processing to construct a complex signal, which is represented as follows: ； in, This represents the complex signal; This represents the in-phase component electrical signal; This represents the orthogonal component electrical signal; This indicates the responsivity of the balanced photodetector; and They represent and .

7. The broadband optical vector measurement method based on I / Q demodulation according to claim 6, characterized in that, Extracting the low-frequency component with a frequency equal to the preset frequency difference from the complex signal yields the spectral response information of the optical device under test, corresponding to the upper and lower sidebands of the measurement signal, respectively, including: Two low-frequency components are extracted from the complex signal using a digital bandpass filter, and are denoted as the first measurement component and the second measurement component, respectively. The frequency of the first measurement component is the difference between the starting angular frequency of the local oscillator signal and the starting angular frequency of the measurement signal, and the frequency of the second measurement component is the difference between the starting angular frequency of the measurement signal and the starting angular frequency of the local oscillator signal. The extraction results are represented as follows: ； ； in, The first measurement component carries the spectral response information of the optical device under test corresponding to the lower sideband of the measurement signal; The second measurement component carries the spectral response information of the optical device under test corresponding to the upper sideband of the measurement signal.

8. The broadband optical vector measurement method based on I / Q demodulation according to claim 7, characterized in that, Extracting the low-frequency component with a frequency equal to the preset frequency difference from the complex signal as the spectral response of the measurement system itself includes: When the measurement signal does not pass through the optical device under test, two low-frequency components are extracted from the corresponding complex signal by the digital bandpass filter, which are denoted as the first calibration component and the second calibration component, respectively. The frequency of the first calibration component is the difference between the starting angular frequency of the local oscillator signal and the starting angular frequency of the measurement signal, and the frequency of the second calibration component is the difference between the starting angular frequency of the measurement signal and the starting angular frequency of the local oscillator signal. The extraction results are represented as follows: ； ； in, This represents the first calibration component; This represents the second calibration component; This represents the spectral response of the measurement system itself.

9. The broadband optical vector measurement method based on I / Q demodulation according to claim 8, characterized in that, The amplitude and phase responses of the optical device under test are obtained by performing a complex division operation between the low-frequency component carrying the spectral response of the optical device under test and the spectral response of the measurement system itself, including: Perform complex division between the first measured component and the first calibration component, and perform complex division between the second measured component and the second calibration component. The calculation formula is as follows: ； ； in, This represents the spectral response of the optical device under test at the lower sideband frequency; This represents the spectral response of the optical device under test at the upper sideband frequency.

10. A broadband optical vector measurement device based on I / Q demodulation, characterized in that, The device includes: The signal generation module is used to generate a first carrier-suppressed double-sideband linear frequency modulated optical signal as a measurement signal and a second carrier-suppressed double-sideband linear frequency modulated optical signal as a local oscillator signal; wherein the measurement signal and the local oscillator signal have the same chirp rate and pulse width, and there is a preset frequency difference between the starting frequency of the linear frequency modulated signal in the measurement signal and the starting frequency of the linear frequency modulated signal in the local oscillator signal; The optical device under test (ODT) access port is located on the transmission optical path of the measurement signal and is used to connect the ODT, so that the measurement signal passing through carries the spectral response of the ODT and forms a probe light signal. A 90° optical mixer, with its first input terminal receiving the probe optical signal and its second input terminal receiving the local oscillator signal, is used to coherently mix the probe optical signal and the local oscillator signal; A balanced photodetector is connected to the output terminal of the 90° optical mixer to output in-phase and quadrature component electrical signals. An analog-to-digital converter module is connected to the output terminal of the balanced photodetector and is used to convert the in-phase component electrical signal and the quadrature component electrical signal into digital signals. A digital signal processing module, connected to the output of the analog-to-digital conversion module, is used to construct a complex signal from the converted in-phase and quadrature components to separate the upper and lower sidebands and suppress image frequency interference. It extracts a low-frequency component with a frequency equal to the frequency difference from the complex signal to obtain a low-frequency component carrying the spectral response of the optical device under test, and acquires a low-frequency component carrying the spectral response of the measurement system itself in a calibration state. Finally, it eliminates the amplitude and phase responses of the measurement system through complex division to obtain the amplitude and phase responses of the optical device under test.

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