An on-chip integrated microwave photonic phase-coded signal generation system

Abstract

An on-chip integrated microwave photonic phase-coded signal generation system relates to the field of microwave photonic technology. The system includes a laser, a power splitter, a radio frequency (RF) switch, a 90° hybrid coupler (HC), a dual-parallel dual-polarization Mach-Zehnder modulator (DP-DPMZM), a polarization-dependent cascaded phase modulator (PDC-PM), a polarization beam splitter, an erbium-doped fiber amplifier, and a photodetector. In this method, the local oscillator (LO) signal is split into two parts by a power splitter. One part is input into the X-DPMZM to obtain the carrier suppression +1st order sideband of the LO signal. The other part is controlled by an RF switch. When the RF switch is off, the Y-DPMZM only outputs an optical carrier; when the RF switch is on, the Y-DPMZM outputs the -1st order sideband of the LO signal. Then, two orthogonally polarized signals are input into the PDC-PM, which is driven by two independent coding signals. When the amplitude and mode of the coding signals are changed, a binary or quaternary phase-coded signal with a carrier frequency equal to or twice that of the LO signal can be obtained through a photodetector.

Description

Technical Field

[0001] The present invention proposes an on-chip integrated microwave photonic phase-coded signal generation system, and this method relates to the field of microwave photonic signal processing. Background Art

[0002] Phase-coded signals and phase-shifting techniques play an important role in modern radar systems. With the complication of the electromagnetic environment and the electronic countermeasure environment, modern radar systems are developing towards high frequency bands, large bandwidths, tunability, multi-functionality, and integration. However, due to the existence of the electronic bottleneck, there are problems such as limited bandwidth and serious electromagnetic interference in the signal transmission and processing in the traditional electrical domain, which cannot meet the increasingly urgent requirements of radar systems. In recent years, microwave photonic technology has become an effective means to solve the problems of traditional electronic technology with its advantages of large bandwidth, low loss, light weight, small volume, reconfigurability, and anti-electromagnetic interference, and is widely used in the generation, transmission, and processing of radar signals.

[0003] In a microwave photonic system, a modulator prepared based on a thin-film lithium niobate material can greatly reduce the volume of the device, significantly reduce the half-wave voltage of the device, and at the same time can achieve integration with silicon-based or Si3N4 due to its advantages of fast response speed, high modulation efficiency, good linearity, and large waveguide refractive index difference. Therefore, it provides a new idea for the development of microwave photonic systems from discrete devices to the integrated direction.

[0004] The phase-coded signal generated based on the microwave photonic phase-shifting technique not only solves the contradiction between the detection range and the range resolution in the radar system, but also enables the radar system to have a higher carrier frequency band, a larger bandwidth, and better tuning ability. However, most current microwave photonic link systems are implemented with discrete devices, resulting in a large volume and poor stability of the system, unable to ensure high reliability and consistency. At the same time, there are problems such as difficult optical phase control and poor link coherence stability in the microwave photonic system based on discrete devices. Summary of the Invention

[0005] Based on this, the present invention proposes an on-chip integrated microwave photonic phase-coded signal generation system. On the basis of generating a phase-coded signal, this method uses a thin-film lithium niobate material to replace the traditional modulator bulk material for on-chip integration, effectively reducing the volume of the system and enhancing the stability of the system.

[0006] An on-chip integrated microwave photonic phase-coded signal generation system includes a laser, a power splitter, a radio frequency (RF) switch, a 90° hybrid coupler (HC), a dual-parallel dual-polarization Mach-Zehnder modulator (DP-DPMZM), a polarization-dependent cascaded phase modulator (PDC-PM), a polarization beam splitter, an erbium-doped fiber amplifier, and a photodetector. Among them, the laser is used to provide the optical carrier for the modulator DP-DPMZM. The DP-DPMZM includes two dual-parallel Mach-Zehnder modulators (DPMZMs) (named X-DPMZM and Y-DPMZM respectively), a 90° polarization rotator (PR), and a polarization beam combiner (PBC). The local oscillator (LO) signal is divided into two parts by the power splitter. One part is input into the upper and lower arms of the upper X-DPMZM after passing through the 90° HC1. The other part is controlled by the RF switch. When the RF switch is off, the Y branch is only a DC bias. The optical carrier passes through the 90° PR and is coupled and output through the polarization beam combiner (PBC) with the polarization-orthogonal signal of the upper X-DPMZM. When the RF switch is on, the LO signal is input into the upper and lower arms of the Y-DPMZM after passing through the 90° HC2. The generated polarization-orthogonal signal is input into the PDC-PM to achieve phase encoding. The PDC-PM includes two RF ports for loading independent electrical encoding signals. By changing the amplitude and mode of the encoding signal, the switching between binary or quaternary phase-coded signals is realized. Since the PDC-PM has different modulation efficiencies for signals with different polarization states, the polarization-orthogonal optical signal is converted to one polarization state through the polarization beam splitter, and then a phase difference is formed after passing through the photodetector, and finally a fundamental frequency or doubled-frequency binary / quaternary phase-coded signal is obtained. The erbium-doped fiber amplifier is used to compensate for the loss of the entire link.

[0007] Both the dual-parallel dual-polarization Mach-Zehnder modulator (DP-DPMZM) and the polarization-dependent cascaded phase modulator (PDC-PM) are lithium niobate thin-film material optical waveguides. Since lithium niobate thin films can well combine a compact optical waveguide structure with its excellent electro-optic characteristics, and the lithium niobate optical waveguides with a high refractive index difference have a strong confinement to light and a small waveguide size, the electrode spacing can be made smaller to effectively improve the efficiency of the electro-optic effect on the premise of ensuring that the absorption loss of the metal electrode to light will not increase. In addition, by selecting a material with a low dielectric constant as the substrate of the lithium niobate thin film, it is easier to achieve the phase velocity matching between the microwave signal and the optical signal.

[0008] An on-chip integrated microwave photonic phase-coded signal generation method includes the following steps:

[0009] Step 1: The optical signal output by the laser is input into the DP-DPMZM as the optical carrier of the modulator DP-DPMZM.

[0010] Step 2: The local oscillator LO signal is input into a power divider and divided into two parts. One part passes through a 90° HC1 and then generates two parts with phases of 0 and 90°. The part with a phase of 0 is input into the upper arm of the X-DPMZM, and the part with a phase of 90° is input into the lower arm of the X-DPMZM;

[0011] Step 3: By adjusting the three DC bias voltages of the X-DPMZM, the output signal of the upper X-DPMZM is in the carrier suppression +1st order sideband state;

[0012] Step 4: The other part of the LO signal is controlled by an RF switch. When the RF switch is off, the Y-DPMZM is only modulated by a DC voltage; when the RF switch is on, the LO signal passes through a 90° HC2 and then is input into the Y-DPMZM. At this time, by adjusting the bias voltage of the Y-DPMZM, its output signal is in the carrier suppression -1st order sideband state. Since the Y-DPMZM branch contains a 90° PR, the signal of this branch is polarization orthogonal to the +1st order sideband signal of the upper branch X-DPMZM and is output after being coupled by a PBC;

[0013] Step 5: The polarization orthogonal optical signal output by the DP-DPMZM is input into the PDC-PM. For a lithium niobate electro-optic phase modulator, different polarization states have different modulation efficiencies. Two independent electrical coding signals are input into the two RF ports of the PDC-PM to achieve phase coding. When changing the amplitude and mode of the electrical coding signal, binary or quaternary phase coding signals can be obtained.

[0014] Step 6: The encoded polarization orthogonal signal is input into a polarization beam splitter to be converted into one polarization direction and finally input into a photodetector to achieve photoelectric conversion.

[0015] Advantages of the present invention: The present invention realizes phase coding signals based on a dual-parallel dual-polarization Mach-Zehnder modulator DP-DPMZM and a polarization-dependent cascaded phase modulator PDC-PM made of lithium niobate thin film, effectively reducing the volume of the entire link structure, improving the stability of the system, and laying a foundation for the future development of microwave photon integration; the PDC-PM contains two independent RF ports, supporting the generation of binary or quaternary phase coding signals using secondary coding signals generated by the digital IO interface of an FPGA, effectively avoiding the use of multi-level coding signals generated by high-speed AWGs or other DAC systems; by controlling the RF switch and the bias voltage of the Y-DPMZM, the carrier frequency of the phase coding signal can be switched between the fundamental frequency and the multiple frequency, enabling the system to have a wider frequency tuning range; by changing the amplitude and mode of the electrical coding signal, the conversion between binary and quaternary phase coding signals is realized, enabling the system to have good reconfigurability and meeting the requirements of modern radar systems for reconfigurable functions. Description of the Drawings

[0016] Figure 1 Schematic diagram of the structure of a microwave photonic phase-coded signal generation system

[0017] Figure 2 (a) Waveform diagram of a binary phase-coded signal

[0018] Figure 2 (b) Waveform diagram of a quaternary phase-coded signal

[0019] Figure 2 (c) is from Figure 2 Phase information extracted from (a)

[0020] Figure 2 (d) is from Figure 2 Phase information extracted from (b) Detailed implementation manners

[0021] The present invention will be further described below in conjunction with the accompanying drawings and mathematical derivations:

[0022] The link structure of the on-chip integrated microwave photonic phase-coded signal generation system described in the present invention is as Figure 1

[0023] shown, and it includes a laser, a power splitter, a radio frequency (RF) switch, a 90° hybrid coupler (HC), a dual-parallel dual-polarization Mach-Zehnder modulator (DP-DPMZM), a polarization-dependent cascaded phase modulator (PDC-PM), a polarization beam splitter, an erbium-doped fiber amplifier, and a photodetector. The continuous optical carrier generated by the laser is input into the DP-DPMZM. The DP-DPMZM is divided into upper and lower branches through a Y-type optical splitter, and then the signals of the upper and lower branches are coupled through a polarization beam combiner (PBC). The upper branch is denoted as X-DPMZM. The X-DPMZM includes three Mach-Zehnder modulators (MZMs), named MZM1, MZM2, and MZM3. Among them, MZM3 is composed of MZM1 and MZM2. The DC bias voltages of the MZM1, MZM2, and MZM3 are respectively denoted as V DC1 、V DC2 and V DC3 ; The lower branch includes Y-DPMZM and a 90° PR. The Y-DPMZM includes three Mach-Zehnder modulators (MZMs), named MZM4, MZM5, and MZM6. Among them, MZM6 is composed of MZM4 and MZM5. The DC bias voltages of the MZM4, MZM5, and MZM6 are respectively denoted as V DC4 、V DC5 and V DC6; Immediately following the Y-DPMZM is a 90° PR, and a polarization beam combiner PBC is located at the output port of the DP-DPMZM, which is used to couple and output the two orthogonally polarized signals. The LO signal is divided into two parts by a power splitter. One part is input into the X-DPMZM through a 90° HC1. When adjusting the bias voltage of the X-DPMZM, the sub-MZMs MZM1 and MZM2 operate at the minimum transmission point, and the main MZM3 operates at the quadrature transmission point, then the carrier suppression +1st order sideband of the LO signal can be obtained at the output end. The other part is controlled by an RF switch. When the RF switch is off, the Y-DPMZM only outputs the optical carrier controlled by the DC bias voltage. When the RF switch is on, the LO signal is input into the Y-DPMZM through a 90° HC2. By adjusting the bias voltage of the Y-DPMZM, the sub-MZMs MZM4 and MZM5 operate at the minimum transmission point, and the main MZM6 operates at the quadrature transmission point, then the carrier suppression -1st order sideband of the LO signal is generated. Then, the orthogonally polarized signals are input into a PDC-PM driven by two independent electrical coding signals. When the amplitude and mode of the coding signals are changed, binary or quaternary phase-coded signals will be obtained at the output end of the photodetector, and their carrier frequencies are equal to or twice the frequency of the LO signal.

[0024] Utilize Figure 1 The method for implementing the microwave photonic phase-coded signal shown below is as follows:

[0025] Step 1: The laser outputs an optical signal. For the convenience of explanation, assume that the angular frequency of the optical signal output by the laser is ω0 and the amplitude is E0, then the expression of the output optical signal is

[0026] E in (t) = E0exp(jω0t) (1)

[0027] where t is time and j is the imaginary unit. The above optical signal serves as the optical carrier of the DP-DPMZM modulator; after this optical signal enters the DP-DPMZM modulator, it is equally divided in power and enters the upper and lower two orthogonally polarized DPMZMs of the DP-DPMZM, namely the X-DPMZM and the Y-DPMZM.

[0028] Step 2: Input the local oscillator (LO) signal into a power splitter and divide it into two parts. Let ω LO and V LO be the angular frequency and amplitude of the LO signal respectively, then the LO signal can be expressed as:

[0029]

[0030] A part of the LO signal passes through the 90° HC1 and is then split into two parts with phases of 0 and 90°. Among them, the part with a phase of 0 is input into the sub-MZM1 in the X-DPMZM, and the part with a phase of 90° is input into the sub-MZM2.

[0031] Step 3: By adjusting the three DC bias voltages of the X-DPMZM, make the bias voltages V DC1 and V DC2 work at the minimum transmission point, and the bias voltage V DC3 work at the quadrature transmission point. At this time, the output signal of the X-DPMZM is in the carrier-suppressed single-sideband state, which is specifically expanded as follows:

[0032] The output expression of the X-DPMZM is

[0033]

[0034] In the formula, m LO =πV LO / V π is the modulation depth of the Mach-Zehnder modulator MZM i (i = 1, 2,..., 6), where MZM i represents the 6 sub-Mach-Zehnder modulators included in the DP-DPMZM, V π is the half-wave voltage of the DP-DPMZM, is the phase difference between the two MZMs introduced by the DC bias voltage V DCl , and V DCl (l = 1, 2, 3) respectively correspond to the 3 DC bias voltages of the X-DPMZM.

[0035] Using Bessel functions to expand the above formula, the output optical field of the X-DPMZM can be expressed as

[0036]

[0037] In the formula, J n is the Bessel function of the first kind of order n. By adjusting the DC bias voltages of MZM1 and MZM2 to make them work at the minimum transmission point MZM3 works at the quadrature transmission point Then, under small-signal modulation, since the optical sideband power of the second order and above is relatively small and can be ignored, only the optical sidebands below the second order are considered. The output optical field of the X-DPMZM is:

[0038]

[0039] Step 4: Another part of the LO signal is controlled by the RF switch. When the RF switch is off, the Y-DPMZM is only modulated by the DC voltage at this time. By adjusting the three DC bias voltages of the Y-DPMZM, it is operated at the maximum transmission point. Then the output expression of the Y-DPMZM is:

[0040]

[0041] When the RF switch is on, the LO signal is input into the Y-DPMZM through the 90° HC2. By adjusting the bias voltage, MZM4 and MZM5 are operated at the minimum transmission point. MZM6 is operated at the quadrature transmission point. Then, under small-signal modulation, since the optical sideband power of the second order and above is small and can be ignored, only the optical sidebands below the second order are considered. The output optical field of the Y-DPMZM is:

[0042]

[0043] Where are the phase differences introduced in the MZM by adjusting the three DC bias voltages (V DCl (l = 4, 5, 6)) of the Y-DPMZM respectively.

[0044] Since the Y-DPMZM path contains a 90° PR, after polarization coupling by the PBC, the output expressions of the DP-DPMZM in the fundamental frequency and second harmonic modes are respectively

[0045]

[0046]

[0047] Where and represent the unit vectors of the X and Y polarization states respectively.

[0048] Step 5: This polarization multiplexed signal is input into the polarization-dependent cascaded phase modulator PDC-PM. For a lithium niobate electro-optic phase modulator, different polarization states have different modulation efficiencies. Therefore, the output optical signal of the PDC-PM can be expressed as

[0049]

[0050]

[0051] Where V n and b n (t) (n = 1, 2) are the amplitudes and sequence patterns of the two independent coding signals respectively, and V π,PM represents the half-wave voltage of the phase modulator.

[0052] Step 6: Use a polarizing beam splitter to convert the polarization-orthogonal optical signals to the same polarization direction, and then input them into a photodetector for photoelectric conversion. Assume the responsivity of the photodetector is Since the microwave signal contained in the signal is expected to be obtained in the experiment, a DC blocker can be used to ignore the DC component without affecting the experimental results. Then the photocurrent output by the photodetector is

[0053]

[0054]

[0055] It can be seen from the above formula that by controlling the bias voltages of the RF switch and the Y-DPMZM, a fundamental frequency or frequency-doubled phase-coded signal can be generated. And by setting the amplitude and coding rules of the electrical coding signal as shown in Table 1 and Table 2, binary and quaternary phase-coded signals can be obtained. There are two schemes for generating the binary phase-coded signal. Scheme 1 is to set the coding modes of the two electrical coding signals to be exactly the same, and use an electrical amplifier to amplify the amplitude to 3V π,PM / 4. At this time, a phase difference of π will be generated between "0" and "1". Scheme 2 is to set one of the coding signal modes to "0", and the other coding signal mode changes between "0" and "1", and the amplitude is amplified to 3V by an electrical amplifier π,PM / 2. At this time, a phase jump of π will also be generated, but this scheme has higher requirements for the electrical amplifier. Therefore, adopting Scheme 1 in the experiment can reduce the performance requirements for the electrical amplifier. To obtain the quaternary phase coding, the amplitudes of the two coding signals need to be amplified to 3V π,PM / 2 and 3V π,PM / 4. When the coding modes of the two coding signals are both "0", the generated phase is 0. When the modes are "0" and "1" respectively, the generated phase is π / 2. When the modes are "1" and "0" respectively, the generated phase is π. When the modes are "1" and "1" respectively, the generated phase is 3π / 2. It can be seen that by setting the amplitudes and modes of the two coding signals, the generated phase-coded signal has 4 phase jumps and the phase interval is π / 2.

[0056] Table 1. Amplitude and coding sequence pattern rules of binary phase-coded signals

[0057]

[0058] Table 2. Amplitude and coding sequence pattern rules of quaternary phase-coded signals

[0059]

[0060] The binary and quaternary phase-coded signal waveforms obtained according to the above rules are as shown in Figure 2 (a) and 2(b), and the phase information recovered using the Hilbert transform is as shown in Figure 2 (c) and 2(d), from which it can be seen that there are two and four phase steps respectively.

[0061] In summary, the present invention realizes an on-chip integrated microwave photonic phase-coded signal. By integrating a dual-parallel dual-polarization Mach-Zehnder modulator and a polarization-dependent phase modulator onto a lithium niobate thin film, the system volume is effectively reduced and the system stability is improved; by using a polarization-dependent cascaded phase modulator with two independent RF ports, the use of multi-level coded signals generated by high-speed AWG or other DAC systems is effectively avoided; by controlling the bias voltages of the RF switch and the Y-DPMZM, the carrier frequency of the phase-coded signal can be switched between the fundamental frequency and the harmonic frequency, increasing the frequency tuning range of the system, reducing the requirement for the RF source frequency, and enabling the system to break through the frequency bandwidth limitation of the device; by changing the amplitude and mode of the electrical coded signal, the switching between binary and quaternary phase-coded signals is realized, making the system have good reconfigurability and meeting the requirements of modern radar systems for reconfigurable functions.

Claims

1. An on-chip integrated microwave photonic phase-encoded signal generation system, comprising a laser, a power splitter, a radio frequency (RF) switch, a 90° hybrid coupler (HC), a dual-parallel dual-polarization Mach-Zehnder modulator (DP-DPMZM), a polarization-dependent cascaded phase modulator (PDC-PM), a polarization beam splitter, an erbium-doped fiber amplifier, and a photodetector, wherein, The laser is used to provide the optical carrier for the modulator DP-DPMZM; the DP-DPMZM includes two dual-parallel Mach-Zehnder modulators DPMZM, a 90° polarization rotator PR, and a polarization beam combiner PBC. Among them, the two dual-parallel Mach-Zehnder modulators DPMZM are named X-DPMZM and Y-DPMZM respectively; the LO signal is divided into two parts by a power splitter. One part is input into the X-DPMZM through a 90° HC1. When adjusting the bias voltage of the X-DPMZM, the sub-MZM1 and MZM2 work at the minimum transmission point, and the main MZM3 works at the quadrature transmission point, then the carrier suppression +1st order sideband of the LO signal can be obtained at the output end. The other part is controlled by an RF switch. When the RF switch is off, the Y-DPMZM only outputs the optical carrier controlled by the DC bias voltage. When the RF switch is on, the LO signal is input into the Y-DPMZM through a 90° HC2. By adjusting the bias voltage of the Y-DPMZM, the sub-MZM4 and MZM5 work at the minimum transmission point, and the main MZM6 works at the quadrature transmission point, then the carrier suppression -1st order sideband of the LO signal is generated. Since the Y branch includes a 90° PR, the DP-DPMZM outputs the polarization-orthogonal optical signal coupled by the PBC; then, this polarization-orthogonal signal is input into the PDC-PM driven by two independent electrical coding signals. When the amplitude and mode of the coding signal are changed, a binary or quaternary phase-coded signal will be obtained at the output end of the photodetector, and its carrier frequency is equal to or twice the frequency of the LO signal.

2. A method for generating an on-chip integrated microwave photonic phase-encoded signal, characterized in that It includes the following steps: Step 1: The optical signal output by the laser is input into the DP-DPMZM as the optical carrier of the modulator DP-DPMZM; Step 2: The local oscillator LO signal is input into the power splitter and divided into two parts. One part generates two parts with phases of 0 and 90° after passing through a 90° HC1. The part with a phase of 0 is input into the upper arm of the X-DPMZM, and the part with a phase of 90° is input into the lower arm of the X-DPMZM; Step 3: When adjusting the bias voltage of the X-DPMZM, the sub-MZM1 and MZM2 work at the minimum transmission point, and the main MZM3 works at the quadrature transmission point, then the carrier suppression +1st order sideband of the LO signal can be obtained at the output end; Step 4: The other part of the LO signal is controlled by an RF switch. When the RF switch is off, the Y-DPMZM is only modulated by the DC voltage; when the RF switch is on, the LO signal is input into the Y-DPMZM after passing through a 90° HC2. At this time, by adjusting the bias voltage of the Y-DPMZM, the sub-MZM4 and MZM5 work at the minimum transmission point, and the main MZM6 works at the quadrature transmission point, then the carrier suppression -1st order sideband of the LO signal is generated; since the Y-DPMZM branch contains a 90° PR, the signal of this branch is polarization-orthogonal to the +1st order sideband signal of the upper branch X-DPMZM and is output after being coupled by the PBC; Step 5: Input the polarization-orthogonal optical signal output by the DP-DPMZM into the PDC-PM. For a lithium niobate electro-optic phase modulator, different polarization states have different modulation efficiencies. Input two independent electrical coding signals into the two RF ports of the PDC-PM to achieve phase coding. When changing the amplitude and mode of the electrical coding signal, a binary or quaternary phase coding signal is obtained; Step 6: Input the encoded polarization-orthogonal signal into a polarization beam splitter to convert it into one polarization direction, and finally input it into a photodetector to achieve photoelectric conversion.

3. A method for generating an on-chip integrated microwave photonic phase-coded signal according to claim 2, characterized in that: The dual-parallel dual-polarization Mach-Zehnder modulator and the polarization-dependent cascaded phase modulator are designed on a lithium niobate thin film; based on the polarization-dependent cascaded phase modulator having two independent RF ports, the conversion between binary and quaternary phase-encoded signals can be achieved by changing the amplitude and mode of the electrical coding signal, and the specific description is as follows: There are two schemes for generating binary phase-encoded signals. Scheme 1 is to set the coding modes of the two electrical coding signals to be exactly the same, and use an electrical amplifier to amplify the amplitude to 3V π,PM / 4, at this time a phase difference of π will be generated between "0" and "1"; Scheme 2 is to set one of the coding signal modes to "0", and the other coding signal mode changes between "0" and "1", and the amplitude is amplified to 3V by an electrical amplifier π,PM / 2, at this time a phase jump of π will also be generated; To generate a quaternary phase encoding, the amplitudes of the two coding signals need to be amplified to 3V π,PM / 2 and 3V π,PM / 4. When the modes of the two coding signals are both "0", the generated phase is 0. When the modes are "0" and "1" respectively, the generated phase is π / 2. When the modes are "1" and "0" respectively, the generated phase is π. When the modes are "1" and "1" respectively, the generated phase is 3π / 2. It can be seen that by setting the amplitudes and modes of the two coding signals, the generated phase-encoded signal has 4 phase jumps and the phase interval is π / 2; Adjust the bias voltages of the RF switch and the Y-DPMZM to obtain the fundamental frequency or the second harmonic phase-encoded signal.

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