Optical field linearization modulator and method of implementing the same

Abstract

This invention discloses an optical field linearization modulator and its implementation method. The modulator includes an asymmetric dual-parallel Mach-Zehnder modulator, an electrical coupler, and a bias control unit. The electrical coupler is used to distribute radio frequency signals and connect to the radio frequency terminals of the first and second intensity modulators of the asymmetric dual-parallel Mach-Zehnder modulator. The bias control unit is connected to the bias terminal of the asymmetric dual-parallel Mach-Zehnder modulator to adjust the operating point of the asymmetric dual-parallel Mach-Zehnder modulator. The asymmetric dual-parallel Mach-Zehnder modulator is used to generate high-purity positive and negative first-order optical sidebands. This device and method are suitable for broadband multi-frequency microwave signal modulation input, can generate highly linear first-order optical sidebands, and have the advantages of large bandwidth, high linearity, and high integration.

Description

Technical Field

[0001] This invention belongs to the field of microwave photonics, specifically relating to an optical field linearization modulator and its implementation method. Background Technology

[0002] In traditional microwave photonic links, when radio frequency (RF) signals modulate electro-optic modulators, the RF signal received at the receiver after photodetector detection typically contains third-order distortion components due to the nonlinearity of the devices themselves. This limits the spurious-free dynamic range of the transmitted RF signal. To improve the spurious-free dynamic range of the link and ensure the linear performance of broadband signal transmission, scholars both domestically and internationally have proposed various linearization modulation methods to suppress the third-order intermodulation distortion of the RF signal caused by the nonlinearity of the electro-optic modulator. For example, Zhu ZH et al. proposed a linearization scheme for a simulated photonic link based on an integrated electro-optic dual-polarization modulator in 2016. This method utilizes the electro-optic anisotropy of the Mach-Zehnder modulator in the TE and TM polarization directions to simultaneously apply RF signals to two orthogonal polarization components. By adjusting the polarization angle through a polarization controller, the third-order components of the two signals cancel each other out at the photodetector after polarization beam combining, thereby improving the system's dynamic range. Wang et al. proposed a linearization structure based on two parallel Mach-Zehnder modulators combined with polarization beam combining in 2019. This scheme utilizes two orthogonal polarization paths to ensure that the third-order components generated by the two modulators have approximately equal amplitudes and opposite phases. After beam combining, the third-order components are canceled at the photodetector, thereby improving the system's spurious-free dynamic range. In 2021, Morton et al. proposed a fully optical linearization modulation scheme. Its transmitter uses a standard intensity-modulated Mach-Zehnder modulator with two optical output ports. The complementary output ports are combined with the laser carrier to construct a linearized optical local oscillator signal. When this optical local oscillator is coherently combined with the intensity-modulated signal, complete cancellation of the third-order intermodulation distortion components can be achieved at the photodetector.

[0003] Existing linearization modulator schemes are mainly used to suppress third-order intermodulation distortion generated during the modulation and detection of radio frequency signals, thereby improving the linearity of radio frequency signal transmission. However, in actual electro-optic modulation processes, second-order and third-order optical sidebands are still inevitably generated, making it difficult to meet the high-purity optical sideband requirements of applications such as linearized optical sideband generation and optical carrier frequency shifting. To solve the above problems, this invention proposes an optical field linearization modulator that can significantly suppress the generation of second-order and third-order optical sidebands during electro-optic modulation, achieving high-purity output of linearized first-order optical sidebands. Summary of the Invention

[0004] In view of the above-mentioned shortcomings in the existing technology, the present invention provides an optical field linearization modulator and its implementation method.

[0005] The solution adopted by this invention to solve its technical problem is as follows: In one aspect, this invention provides an optical field linearization modulator, comprising: an asymmetric dual-parallel Mach-Zehnder modulator, an electrical coupler, and a bias control unit. The electrical coupler is used to distribute radio frequency signals and is connected to the radio frequency terminals of the first and second intensity modulators of the asymmetric dual-parallel Mach-Zehnder modulator. The bias control unit is connected to the bias terminal of the asymmetric dual-parallel Mach-Zehnder modulator to adjust the operating point of the asymmetric dual-parallel Mach-Zehnder modulator. The asymmetric dual-parallel Mach-Zehnder modulator is used to achieve high-purity first-order optical sideband generation while suppressing second- and third-order optical sidebands. The asymmetric dual-parallel Mach-Zehnder modulator is composed of an optical beamsplitter, a first intensity modulator, a second intensity modulator, a phase modulator, and an optical beam combiner. The input optical signal is distributed to the first and second intensity modulators by an optical beam splitter according to a preset ratio. The input radio frequency signal drives the first and second intensity modulators respectively by an electrical coupler according to a preset ratio. By setting the optical power weight and the radio frequency drive amplitude weight, and controlling the phase modulator, the two modulated optical signals generated by the two intensity modulators are made to suppress the second-order optical sidebands, and the third-order optical sidebands have equal amplitudes and opposite phases. The optical beam combiner is used to combine the two modulated optical signals, so that the third-order optical sidebands cancel each other out during the combining process, thereby producing high-purity positive and negative first-order optical sidebands.

[0006] As a preferred embodiment, the input optical signal is split into two paths by an optical beamsplitter and then enters the first and second intensity modulators of an asymmetric dual-parallel Mach-Zehnder modulator, respectively. The optical power weight input to the first intensity modulator is... P (0< P <1), the optical power weight input to the second intensity modulator is 1- P This introduces an asymmetry in optical power distribution.

[0007] As a preferred embodiment, the input radio frequency signal is transmitted to the first and second intensity modulators of an asymmetric dual parallel Mach-Zehnder modulator via an electrical coupler, and the power weight of the electrical signal input to the first intensity modulator is... K The power weight of the electrical signal input to the second intensity modulator is 1- K .

[0008] As a preferred embodiment, the output optical power of the first intensity modulator in the asymmetric dual-parallel Mach-Zehnder modulator is detected, and the voltage of the bias control unit is adjusted according to the detected optical power. The bias voltage applied to the bias terminal of the first intensity modulator is gradually adjusted to the state corresponding to the minimum output optical power, thereby enabling the first intensity modulator to operate at the minimum bias point and suppressing second-order optical sidebands. The output optical power of the second intensity modulator in the asymmetric dual-parallel Mach-Zehnder modulator is detected, and the voltage of the bias control unit is adjusted according to the detected optical power. Adjustments are made to gradually adjust the bias voltage applied to the bias terminal of the second intensity modulator to the state corresponding to the minimum output optical power, thereby enabling the second intensity modulator to operate at the minimum bias point and suppressing second-order optical sidebands; the voltage of the bias control unit is adjusted accordingly by observing the optical signal at the output terminal of the optical combiner. The phase shift of the phase modulator is set to Suppress third-order optical sidebands.

[0009] In another aspect, the present invention also provides a method for implementing an optical field linearization modulator, the specific steps of which are as follows: S1. The input RF signal, after being distributed by the electrical coupler, is applied to the two intensity modulators of the asymmetric dual-parallel Mach-Zehnder modulator. The input optical signal is split into two paths by the optical beam splitter and enters the asymmetric dual-parallel Mach-Zehnder modulator for modulation; specifically as follows: The input optical signal is split into two paths by an optical beamsplitter and then enters the first and second intensity modulators of an asymmetric dual-parallel Mach-Zehnder modulator, respectively. The optical power weight input to the first intensity modulator is... P The optical power weight input to the second intensity modulator is 1- P This introduces an asymmetry in optical power distribution.

[0010] The input RF signal, after being distributed by an electrical coupler, is applied to two intensity modulators of an asymmetric dual-parallel Mach-Zehnder modulator. The distribution ratio of the electrical coupler is controlled to ensure that the power weight of the electrical signal input to the first intensity modulator is [value missing]. K The power weight of the electrical signal input to the second intensity modulator is 1- K Introducing RF drive amplitude asymmetry; weighting P and K satisfy The relationship.

[0011] S2. By detecting the output optical power of the asymmetric dual-parallel Mach-Zehnder modulator, the voltage of the bias control unit is adjusted so that the first and second intensity modulators in the asymmetric dual-parallel Mach-Zehnder modulator operate at the minimum bias point, suppressing second-order optical sidebands, and making the phase shift of the phase modulator... Suppressing third-order optical sidebands; specifically as follows: The output optical power of the first intensity modulator in the asymmetric dual-parallel Mach-Zehnder modulator is detected, and the voltage of the bias control unit is adjusted according to the detected optical power. Adjustments are made to gradually adjust the bias voltage applied to the bias terminal of the first intensity modulator to the state of minimum output optical power, so that the first intensity modulator operates at the minimum bias point.

[0012] The output optical power of the second intensity modulator in the asymmetric dual-parallel Mach-Zehnder modulator is detected, and the voltage of the bias control unit is adjusted according to the detected optical power. Adjustments are made to gradually adjust the bias voltage applied to the bias terminal of the second intensity modulator to the state of minimum output optical power, so that the second intensity modulator operates at the minimum bias point.

[0013] Observe the optical signal at the output of the optical combiner and adjust the voltage of the bias control unit accordingly. The phase shift of the phase modulator is set to Suppress third-order optical sidebands.

[0014] S3. The modulated optical signals of the first and second intensity modulators are combined into an optical field using an optical combiner. By utilizing the coherent superposition mechanism of the optical field, the third-order optical sidebands generated by the two intensity modulators undergo destructive interference at the output end, producing high-purity positive and negative first-order optical sidebands.

[0015] Due to the adoption of the above technical solution, the beneficial effects of the present invention are as follows: The present invention introduces an asymmetric configuration of optical power distribution and radio frequency drive amplitude in a dual parallel Mach-Zehnder modulator, so that the amplitudes of the third-order optical sideband components generated in different modulation branches are equal and the phases are opposite, and the coherent superposition of optical fields is used to achieve the cancellation of the third-order optical sideband components, thereby generating a high-purity first-order optical sideband; the device and method are suitable for broadband multi-frequency microwave signal modulation input, can generate high-purity optical sidebands, and have the advantages of large bandwidth, high linearity, and high integration. Attached Figure Description

[0016] Figure 1 A schematic diagram showing the structure of an optical field linearization modulator and its implementation method according to the present invention; Figure 2 This diagram illustrates the spectrum generated by the optical field linearization modulator of the present invention under modulation of two-tone signals with equal amplitude. Figure 3 A comparison diagram showing the third-order sideband suppression effect of the conventional Mach-Zehnder modulator and the optical field linearization modulator under dual-tone signals of the present invention; Figure 4 This diagram shows a comparison of the third-order sideband suppression effects of the traditional Mach-Zehnder modulator and the optical field linearization modulator under single-tone signals according to the present invention. Detailed Implementation

[0017] To more clearly illustrate the technical objectives, technical solutions, and beneficial effects of the present invention, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0018] like Figure 1 As shown, this embodiment proposes an optical field linearization modulator and its implementation method. The system includes an asymmetric dual-parallel Mach-Zehnder modulator 1, an electro-coupler 2, a bias control unit 3, and optical amplifiers and other devices that may be needed in the optical path. The asymmetric dual-parallel Mach-Zehnder modulator consists of an optical beamsplitter 1-1, a first intensity modulator 1-2, a second intensity modulator 1-3, a phase modulator 1-4, and an optical beam combiner 1-5. The radio frequency signal source outputs two signals with different powers after passing through the electro-coupler 2, which respectively drive the first and second modulators of the asymmetric dual-parallel Mach-Zehnder modulator 1. The bias control unit 2 provides a DC bias to the asymmetric dual-parallel Mach-Zehnder modulator, causing both the first and second modulators to operate at their minimum points, and the phase modulator to operate at its maximum value. Phase shift point to suppress second- and third-order optical sidebands.

[0019] The input optical signal is split into two paths by optical beam splitter 1-1 and then enters the first intensity modulator 1-2 and the second intensity modulator 1-3 of the asymmetric dual parallel Mach-Zehnder modulator 1, respectively. The optical power weight input to the first intensity modulator 1-2 is... P (0< P <1), the optical power weight input to the second intensity modulator 1-3 is 1- P This introduces an asymmetry in optical power distribution. After the input RF signal is distributed via electrical coupler 2, it is applied to the two intensity modulators of the asymmetric dual-parallel Mach-Zehnder modulator. The distribution ratio of the electrical coupler is controlled so that the power weight of the electrical signal input to the first intensity modulator 1-2 is... K The power weight of the electrical signal input to the second intensity modulator is 1- K This introduces asymmetry in the radio frequency drive amplitude; by controlling the optical power allocation weights... P Or electrical signal power weighting K , making P and K satisfy The relationship.

[0020] The output power of the first intensity modulator in the asymmetric dual-parallel Mach-Zehnder modulator 1 is detected, and the bias control unit is adjusted according to the detected optical power. The bias voltage applied to the bias terminal of the first intensity modulator 1-2 is gradually adjusted to the state of minimum output optical power, thereby making the first intensity modulator 1-2 operate at the minimum bias point and suppressing second-order optical sidebands; the output power of the second intensity modulator 1-3 in the asymmetric dual parallel Mach-Zehnder modulator 1 is detected, and the bias control unit is adjusted according to the detected optical power. Adjustments are made to gradually adjust the bias voltage applied to the bias terminals of the second intensity modulator 1-3 to the state of minimum output optical power, thereby enabling the second intensity modulator 1-3 to operate at the minimum bias point and suppress second-order optical sidebands; the optical signal at the output terminal of the optical combiner 1-5 is observed, and the bias control unit is adjusted accordingly. The phase shift of the phase modulator is set to The modulated optical signals from the first and second intensity modulators are combined into an optical field using an optical combiner. By utilizing the coherent superposition mechanism of the optical field, the third-order optical sidebands generated by the two intensity modulators undergo destructive interference at the output end, suppressing the third-order optical sidebands and thus generating high-purity positive and negative first-order optical sidebands.

[0021] In another aspect, the present invention also provides a method for implementing an optical field linearization modulator, the specific steps of which are as follows: S1. The input optical signal is split into two paths by an optical beamsplitter and then enters the first and second intensity modulators of an asymmetric dual-parallel Mach-Zehnder modulator, respectively. The optical power weight input to the first intensity modulator is: P (0< P <1), the optical power weight input to the second intensity modulator is 1- P This introduces an asymmetry in optical power distribution.

[0022] S2. After the input RF signal is distributed by the electrical coupler, it is applied to the two intensity modulators of the asymmetric dual parallel Mach-Zehnder modulator. The distribution ratio of the electrical coupler is controlled so that the power weight of the electrical signal input to the first intensity modulator is [value missing]. K The power weight of the electrical signal input to the second intensity modulator is 1- K This introduces asymmetry in the radio frequency drive amplitude.

[0023] S3, By controlling the optical power allocation weight P Or electrical signal power weighting K , making P and K satisfy The relationship.

[0024] S4. Detect the output power of the first intensity modulator in the asymmetric dual-parallel Mach-Zehnder modulator, and adjust the bias control unit according to the detected optical power. Adjustments are made to gradually adjust the bias voltage applied to the bias terminal of the first intensity modulator to the state of minimum output optical power, so that the first intensity modulator operates at the minimum bias point.

[0025] S5. Detect the output power of the second intensity modulator in the asymmetric dual-parallel Mach-Zehnder modulator, and adjust the bias control unit according to the detected optical power. Adjustments are made to gradually adjust the bias voltage applied to the bias terminal of the second intensity modulator to the state of minimum output optical power, so that the second intensity modulator operates at the minimum bias point.

[0026] S6. Observe the optical signal at the output of the optical combiner and adjust the bias control unit accordingly. The phase shift of the phase modulator is set to .

[0027] S7. The modulated optical signals of the first and second intensity modulators are combined into an optical field using an optical combiner. By utilizing the coherent superposition mechanism of the optical field, the third-order optical sidebands generated by the two intensity modulators undergo destructive interference at the output end, thereby generating positive and negative first-order optical sidebands with high purity.

[0028] The specific working principle of the optical field linearization modulator and its implementation method involved in this invention is as follows: See Figure 1 The optical carrier is split into two unequal powers by an optical beam splitter, with the power input to the first intensity modulator having a power ratio of 1:1. P (0< P <1), the ratio of optical power input to the second intensity modulator is 1- P Both the first and second intensity modulators employ a push-pull structure, and their output field expression is as follows: (1) in For the electric field of a continuous wave laser, The amplitude of the laser's electric field. For carrier frequency, For the input radio frequency signal expression, The modulator half-wave voltage, and These are the bias voltages input to the two modulators, and their values ​​are both set to... To achieve carrier-suppressed double-sideband. K The ratio of radio frequency power introduced by the coupler.

[0029] The angular frequency components are respectively and The dual-tone radio frequency signal is loaded onto the first intensity modulator, and its expression is: Then the expression for the output of the first intensity modulator is: (2) in For radio frequency signal amplitude, and Let be the angular frequency of the radio frequency signal. For modulation depth, for The first-order Bessel function of the first kind is used. Even-order sidebands have been suppressed, and optical sidebands of the fifth order and above have been omitted due to their low power.

[0030] Similarly, the expression for the signal generated by the second intensity modulator after passing through the phase modulator is: (3) In practical applications, the frequency component is and The third-order sideband is far from the first-order sideband, therefore the focus is on the frequency components. and The third-order sideband components. For example... Figure 2 As shown, in order to eliminate these third-order sideband components, and The third-order sideband components should have equal amplitudes and opposite phases, and the splitting ratio should be equal. P and power attenuation ratio K The following formula should be satisfied: (4) When the modulation depth is small, formula (4) can be simplified to: (5) In this embodiment, we compared and analyzed a traditional Mach-Zehnder modulator and the linearized modulator. Under the same RF signal input, we compared and analyzed the third-order sideband suppression ratio (the power ratio of the first-order optical sideband and the third-order optical sideband) generated by the modulator to verify the ability of the linearized modulator to generate a high-purity first-order optical sideband.

[0031] Both modulators have a half-wave voltage of 4V, and the splitting ratio of the optical field linearization modulator is... P Set to 0.1, RF power ratio K The value is set to 0.68 according to the formula. First, a two-tone signal is used for testing; the frequencies of the two-tone signal are... =12 GHz =10 GHz. At 14 GHz ( ) and 8 GHz ( The power of the third-order intermodulation component is observed at point (). For example... Figure 3 As shown, compared to traditional Mach-Zehnder modulators, the optical field linearization modulator exhibits a significant improvement in third-order sideband suppression ratio across the modulation depth range of 0.2–1.8, with a 36 dB improvement at a modulation depth of 0.4. Secondly, single-tone signal testing was conducted with an input frequency of… =12 GHz, at 36 GHz ( Observe the power of the third-order sideband at point (). For example... Figure 4 As shown, compared to traditional Mach-Zehnder modulators, the optical field linearization modulator significantly improves the third-order sideband suppression ratio within the modulation depth range of 0.2–1.8, and by 39 dB at a modulation depth of 0.4. Comparative results show that the optical field linearization modulator achieves higher third-order sideband suppression ratios and produces purer first-order optical sidebands in both single-tone and two-tone signal modulation.

[0032] The specific embodiments provided herein are intended to enable those skilled in the art to fully understand and implement this solution. Although the specific embodiments have been described in detail above, those skilled in the art should understand that reasonable adjustments can be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features. All such modifications or substitutions, provided they do not depart from the essential technical concept of this solution, should be considered to fall within the scope of protection defined by the claims of this solution.

Claims

1. A linearization modulator for an optical field, characterized in that, include: Asymmetric dual parallel Mach-Zehnder modulator, electrical coupler and bias control unit; The electrical coupler is used to distribute radio frequency signals and connect to the radio frequency terminals of the first and second intensity modulators of the asymmetric dual parallel Mach-Zehnder modulator. The bias control unit is connected to the bias terminal of the asymmetric dual parallel Mach-Zehnder modulator to adjust the operating point of the asymmetric dual parallel Mach-Zehnder modulator. The asymmetric dual parallel Mach-Zehnder modulator is used to realize the generation of first-order optical sidebands while suppressing second-order and third-order optical sidebands.

2. The optical field linearization modulator according to claim 1, characterized in that, The asymmetric dual-parallel Mach-Zehnder modulator consists of an optical beamsplitter, a first intensity modulator, a second intensity modulator, a phase modulator, and an optical beam combiner. The input optical signal is distributed to the radio frequency (RF) terminals of the first and second intensity modulators by the optical beamsplitter according to a preset ratio. The input RF signal drives the first and second intensity modulators respectively by an electrical coupler according to a preset ratio. By setting the optical power weight and the RF drive amplitude weight, and controlling the phase modulator, the two modulated optical signals generated by the two intensity modulators are made to suppress the second-order optical sidebands, and the third-order optical sidebands have equal amplitudes and opposite phases. The optical beam combiner is used to coherently combine the two modulated optical signals, so that the third-order optical sideband components cancel each other out during the combining process, generating positive and negative first-order optical sidebands.

3. The optical field linearization modulator according to claim 2, characterized in that, The input optical signal is split into two paths by an optical beamsplitter and then enters the first and second intensity modulators of an asymmetric dual-parallel Mach-Zehnder modulator, respectively. The optical power weight input to the first intensity modulator is: P ,0< P <1, the optical power weight input to the second intensity modulator is 1- P This introduces an asymmetry in optical power distribution.

4. The optical field linearization modulator according to claim 2, characterized in that, The input radio frequency signal is transmitted to the first and second intensity modulators of an asymmetric dual-parallel Mach-Zehnder modulator via an electrical coupler. The power weight of the electrical signal input to the first intensity modulator is... K ,0< K <1, the power weight of the electrical signal input to the second intensity modulator is 1- K .

5. The optical field linearization modulator according to claim 1, characterized in that, The output optical power of the first intensity modulator in the asymmetric dual-parallel Mach-Zehnder modulator is detected, and the voltage of the bias control unit is adjusted according to the detected optical power. Adjustments are made to gradually adjust the bias voltage applied to the bias terminal of the first intensity modulator to the state of minimum output optical power, so that the first intensity modulator operates at the minimum bias point and suppresses second-order optical sidebands. By detecting the output power of the second intensity modulator in the asymmetric dual-parallel Mach-Zehnder modulator, and adjusting the voltage of the bias control unit based on the detected optical power, Adjustments are made to gradually adjust the bias voltage applied to the bias terminal of the second intensity modulator to the state of minimum output optical power, so that the second intensity modulator operates at the minimum bias point and suppresses second-order optical sidebands. By observing the optical signal at the output of the optical combiner, the voltage of the bias control unit is adjusted accordingly. Set the phase shift of the phase modulator to Suppress third-order optical sidebands.

6. A method for implementing an optical field linearization modulator, used to implement the optical field linearization modulator according to any one of claims 1 to 5, characterized in that, Includes the following steps: S1. The input radio frequency signal is distributed by the electrical coupler and applied to the two intensity modulators of the asymmetric dual parallel Mach-Zehnder modulator. The input optical signal is split into two paths by the optical beam splitter and enters the asymmetric dual parallel Mach-Zehnder modulator for modulation. S2. By detecting the output optical power of the asymmetric dual parallel Mach-Zehnder modulator, the voltage of the bias control unit is adjusted so that the first intensity modulator and the second intensity modulator in the asymmetric dual parallel Mach-Zehnder modulator work at the minimum bias point, thereby suppressing the second-order optical sidebands. S3. The modulated optical signals of the first and second intensity modulators are combined into an optical field using an optical combiner. The optical field coherent superposition mechanism is used to cause the third-order optical sidebands generated by the two intensity modulators to undergo destructive interference at the output end, generating positive and negative first-order optical sidebands.

7. The method for implementing an optical field linearization modulator according to claim 6, characterized in that, The specific implementation process of step S1 is as follows: The input optical signal is split into two paths by an optical beamsplitter and then enters the first and second intensity modulators of an asymmetric dual-parallel Mach-Zehnder modulator, respectively. The optical power weight input to the first intensity modulator is... P The optical power weight input to the second intensity modulator is 1- P This introduces an asymmetry in optical power distribution; The input RF signal, after being distributed by an electrical coupler, is applied to two intensity modulators of an asymmetric dual-parallel Mach-Zehnder modulator. The distribution ratio of the electrical coupler is controlled to ensure that the power weight of the electrical signal input to the first intensity modulator is [value missing]. K The power weight of the electrical signal input to the second intensity modulator is 1- K This introduces asymmetry in the radio frequency drive amplitude; Weight P and K satisfy The relationship.

8. The method for implementing an optical field linearization modulator according to claim 7, characterized in that, The specific implementation process of step S2 is as follows: The output optical power of the first intensity modulator in the asymmetric dual-parallel Mach-Zehnder modulator is detected, and the voltage of the bias control unit is adjusted according to the detected optical power. Adjustments are made to gradually adjust the bias voltage applied to the bias terminal of the first intensity modulator to the state of minimum output optical power, so that the first intensity modulator operates at the minimum bias point; The output optical power of the second intensity modulator in the asymmetric dual-parallel Mach-Zehnder modulator is detected, and the voltage of the bias control unit is adjusted according to the detected optical power. Adjustments are made to gradually adjust the bias voltage applied to the bias terminal of the second intensity modulator to the state of minimum output optical power, so that the second intensity modulator operates at the minimum bias point; Observe the optical signal at the output of the optical combiner and adjust the voltage of the bias control unit accordingly. The phase shift of the phase modulator is set to Suppress third-order optical sidebands.

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