Film lithium niobate MZM undisturbed frequency offset point automatic controller and control method thereof

Abstract

The invention discloses a film lithium niobate MZM disturbance-free offset point automatic controller and a control method thereof. The controller comprises a photoelectric detector, a trans-impedance amplifier, a reference source, an integrator and a driver. An optical signal is converted into an electric signal through a photoelectric detector, and the electric signal is amplified and filtered through a trans-impedance amplifier; comparing the filtered voltage with a reference voltage generated by a reference source through an integrator, and integrating the difference between the filtered voltage and the reference voltage to obtain an integrated output voltage; and the integral output voltage is converted into a driving signal required by the thin-film lithium niobate MZM through the driver, so that the output optical power of the thin-film lithium niobate MZM is changed, and feedback control on a bias point is realized. According to the invention, an innovative automatic adjustment mechanism and a feedback control link are adopted, and the bias point can be monitored and adjusted in real time so as to cope with bias point drift caused by factors such as temperature change and device aging.

Description

Technical Field

[0001] The present invention relates to the technical field of electro-optic modulation, and in particular to a thin-film lithium niobate MZM non-disturbed frequency offset point automatic controller and its control method. Background Art

[0002] The bias point control of the thin-film lithium niobate MZM is a key factor to ensure the modulator operates in the best performance state. The driving voltage range of the traditional electro-optic modulator bias point controller is about ±15V, while the controllable voltage range of the thermal tuning electrode of the thin-film lithium niobate MZM is about 5V. Direct use will cause problems such as burning out or uncontrollability of the thermal tuning electrode of the thin-film lithium niobate MZM, lacking the corresponding thermal tuning electrode drive. At the same time, the existing bias point controller adjustment methods mostly rely on manual adjustment by humans, resulting in problems of low adjustment accuracy and poor efficiency, and being easily affected by external environmental changes, leading to instability of the bias point, and further affecting the modulation effect. Therefore, the core challenge of the bias point control of the thin-film lithium niobate MZM lies in how to meet the driving requirements of the thermal tuning resistor under the constraints of small size and high stability. If the driving ability is insufficient, it will lead to insufficient temperature rise or bias point drift, affecting the modulation depth and signal-to-noise ratio.

[0003] The existing bias point control technologies mainly include the following several types:

[0004] Open-loop DC bias control: A fixed DC voltage is applied to maintain the bias point. However, due to factors such as temperature change and device aging, the bias point will gradually drift, resulting in a decline in modulation performance.

[0005] Pilot signal-based control: A low-frequency pilot signal is added to the modulation signal, and the bias point is adjusted by detecting the amplitude or phase of the pilot signal. Although this method can achieve real-time control, it will increase the complexity of the system, and the pilot signal may interfere with the main signal. Summary of the Invention

[0006] Aiming at the above deficiencies in the prior art, the thin-film lithium niobate MZM non-disturbed frequency offset point automatic controller and its control method provided by the present invention solve the problems of high complexity, high cost, insufficient bias point control accuracy, and lack of thermal tuning electrode driving ability in the prior art.

[0007] To achieve the above invention purpose, the technical solution adopted by the present invention is: A thin-film lithium niobate MZM non-disturbed frequency offset point automatic controller, including a photodetector, a transimpedance amplifier, a reference source, an integrator, and a driver;

[0008] The input end of the photodetector is the input end of the automatic controller for the film lithium niobate MZM without disturbing frequency offset points. The output end of the photodetector is connected to the input end of the transimpedance amplifier. The output end of the transimpedance amplifier is connected to the first input end of the integrator. The second input end of the integrator is connected to the reference source. The output end of the integrator is connected to the input end of the driver. The output end of the driver is the output end of the automatic controller for the film lithium niobate MZM without disturbing frequency offset points and is connected to the input end of the film lithium niobate MZM.

[0009] The present invention also provides a control method based on the automatic controller for the film lithium niobate MZM without disturbing frequency offset points, including:

[0010] Converting the optical signal into an electrical signal through the photodetector to obtain an output voltage;

[0011] Amplifying and filtering the output voltage through the transimpedance amplifier to obtain a filtered voltage;

[0012] Comparing the filtered voltage with the reference voltage generated by the reference source through the integrator and integrating the difference between the two to obtain an integrated output voltage;

[0013] Converting the integrated output voltage into a driving signal required by the film lithium niobate MZM through the driver, thereby changing the output optical power of the film lithium niobate MZM and realizing the control of the bias point.

[0014] Further, each time the bias point is controlled, the applied reference voltage is the voltage that can stabilize the phase angle of the bias point of the film lithium niobate MZM at π / 2, where π is the angle of 180°.

[0015] Further, the difference between the reference voltage and the filtered voltage is ΔV; when the phase angle of the bias point of the film lithium niobate MZM is π / 2, ΔV = 0, and at this time, the integrated output voltage of the integrator remains constant.

[0016] Further, when the phase angle of the bias point of the film lithium niobate MZM is greater than π / 2, ΔV < 0, which reduces the integrated output voltage of the integrator, thereby reducing the phase angle of the bias point of the film lithium niobate MZM.

[0017] Further, when the phase angle of the bias point of the film lithium niobate MZM is less than π / 2, ΔV > 0, which increases the integrated output voltage of the integrator, thereby increasing the phase angle of the bias point of the film lithium niobate MZM.

[0018] The beneficial effects of the present invention are as follows:

[0019] 1. For different resistance values of the thermal tuning resistor of the film lithium niobate MZM, the automatic controller for the film lithium niobate MZM without disturbing frequency offset points can stably output different driving currents to meet the driving requirements of the thermal tuning electrode.

[0020] 2. The structure of the automatic controller for the bias - free frequency offset point of thin - film lithium niobate MZM is simple, without the need to use ADC sampling and digital algorithms, making the control of the bias point more economical and efficient, and also easier to build and maintain, greatly reducing the computing resources and system costs.

[0021] 3. Through the feedback control circuit in the automatic controller for the bias - free frequency offset point of thin - film lithium niobate MZM, the bias point can be monitored and adjusted in real - time to cope with the bias point drift caused by reasons such as temperature changes and device aging.

[0022] 4. By filtering the amplified voltage, more accurate data can be obtained, enabling the thin - film lithium niobate MZM to work more stably in the quadrature bias point for a long time. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 FIG. is a schematic structural diagram of an automatic controller for the bias - free frequency offset point of thin - film lithium niobate MZM provided by the present invention;

[0024] Figure 2 FIG. is a schematic structural diagram of the Integrator;

[0025] Figure 3 FIG. is a schematic application connection diagram of the automatic controller for the bias - free frequency offset point of thin - film lithium niobate MZM in the embodiment;

[0026] Figure 4 FIG. is a schematic diagram of the phase change before bias point control;

[0027] Figure 5 FIG. is a schematic diagram of the phase change after bias point control. DETAILED DESCRIPTION OF THE INVENTION

[0028] The following describes the specific embodiments of the present invention to facilitate those skilled in the art of the present technology to understand the present invention. However, it should be clear that the present invention is not limited to the scope of the specific embodiments. For those of ordinary skill in the art of the present technology, as long as various changes are within the spirit and scope of the present invention defined and determined by the appended claims, these changes are obvious, and all inventions and creations using the concept of the present invention are within the scope of protection.

[0029] As Figure 1 shown, the automatic controller for the bias - free frequency offset point of thin - film lithium niobate MZM includes: a photodetector (PD), a trans - impedance amplifier (TIA), a reference source, an integrator (Integrator), and a driver (Drive).

[0030] The input end of the photodetector is the input end of the thin-film lithium niobate MZM non-disturbed frequency offset point automatic controller. The output end of the photodetector is connected to the input end of the transimpedance amplifier. The output end of the transimpedance amplifier is connected to the first input end of the integrator. The second input end of the integrator is connected to the reference source. The output end of the integrator is connected to the input end of the driver, and the output end of the driver is the output end of the thin-film lithium niobate MZM non-disturbed frequency offset point automatic controller. Among them, as Figure 2 shown, the integrator consists of an operational amplifier and an integration capacitor.

[0031] In an embodiment of the present invention, as Figure 3 shown, the optical signal emitted by the laser diode (LD) and the radio frequency signal (RF) serve as the input signals of the thin-film lithium niobate MZM (TFLN MZM). After electro-optic modulation by the thin-film lithium niobate MZM, the output optical signal is split by an optical coupler. One path of light (accounting for 10% of the original light beam) enters the bias point controller (Bias Controler, that is, the thin-film lithium niobate MZM non-disturbed frequency offset point automatic controller provided by the present invention) for feedback control, and one path of light (accounting for 90% of the original light beam) is output to the optical power meter. The control effect of the bias point controller is judged by measuring the change in optical power.

[0032] The control method of the thin-film lithium niobate MZM non-disturbed frequency offset point automatic controller provided in this embodiment is specifically as follows: The optical signal entering the bias point controller enters the photodetector (PD). The optical signal is converted into an electrical signal by the photodetector to obtain the output voltage. The output voltage is amplified and filtered by the transimpedance amplifier (TIA). Since the light output from the laser passes through optical fiber transmission and then through thin-film lithium niobate MZM modulation, it contains noise information. Therefore, a filter needs to be used in the link to filter out the high-frequency jitter information. Therefore, the transimpedance amplifier uses a second-order active low-pass filter to filter the amplified voltage to obtain the filtered voltage. The filtered voltage is compared with the reference voltage (Vref) generated by the reference source by the integrator, and the difference between the two is integrated to obtain the integrated output voltage. The integrated output voltage is converted into the drive signal required by the thin-film lithium niobate MZM by the driver, thereby changing the output optical power of the thin-film lithium niobate MZM and realizing the control of the bias point.

[0033] Among them, each time bias point control is performed, the applied reference voltage is the voltage that can stabilize the phase angle of the bias point of the thin-film lithium niobate MZM at π / 2, where π is the angle of 180°. The difference between the reference voltage and the filtered voltage is ΔV. When the phase angle of the bias point of the thin-film lithium niobate MZM is π / 2, ΔV = 0, and the integral output voltage remains constant. When the thin-film lithium niobate MZM is disturbed by mechanical stress (such as vibration, etc.) or environmental temperature changes, it will affect the refractive index of the optical waveguide in the thin-film lithium niobate MZM, resulting in a shift of the bias point. When the phase angle of the bias point is greater than π / 2, the output optical power of the thin-film lithium niobate MZM increases. At this time, ΔV < 0, which further reduces the integral output voltage of the integrator, thereby reducing the phase angle of the bias point of the thin-film lithium niobate MZM and stabilizing it at π / 2. When the phase angle of the bias point of the thin-film lithium niobate MZM is less than π / 2, the output optical power of the thin-film lithium niobate MZM decreases. At this time, ΔV > 0, which further increases the integral output voltage of the integrator, thereby increasing the phase angle of the bias point of the thin-film lithium niobate MZM, realizing the stable operation of the thin-film lithium niobate MZM at the quadrature bias point.

[0034] The effect of bias point control is judged by measuring the change in optical power. The calculation formula for optical power is:

[0035]

[0036] In the formula, T MZM is the transfer function of the thin-film lithium niobate MZM, cos represents the cosine function, represents the original phase difference, Δv(t) is the radio frequency interpolation, and V π is the half-wave voltage of the thin-film lithium niobate MZM; P represents the optical power offset, represents the phase offset.

[0037] By converting the optical power change into a phase change through the above formula, the phase change of the bias point of the thin-film lithium niobate MZM is detected within a certain period of time. As Figure 4 , 5 shown, Figure 4 is a schematic diagram of the phase change before bias point control using the device and method provided by the present invention, Figure 5 is a schematic diagram of the phase change after bias point control using the device and method provided by the present invention. It can be found that before the bias point controller is controlled, during the entire monitoring period, under a certain bias interference, the offset of the phase angle is about ±10.5°. After the bias point control, during the entire monitoring period, under the same bias interference, the offset of the phase angle always remains within ±0.36°.

[0038] In addition, the typical parameter value of the single thermal tuning electrode of the thin-film lithium niobate MZM is about 200 Ω. Within one working cycle of the modulator, the adjustable voltage range on the thermal tuning resistor is 0 V to 5 V (and the corresponding adjustable current range can be obtained as 0 mA to 25 mA according to Ohm's law). With an input of 5 V, the bias point controller can stably output a current of 30 mA, which can meet the driving requirements of the thermal tuning electrode of the thin-film lithium niobate MZM, and at the same time can cover the line loss and resistance fluctuation. For different resistance values of the thermal tuning resistor of the thin-film lithium niobate MZM, the bias point controller can stably output different driving currents to meet the driving requirements of the thermal tuning electrode.

[0039] In summary, the present invention can meet the driving requirements of the thermal tuning electrode of the thin-film lithium niobate MZM, and at the same time can achieve automatic calibration under long-term operation, greatly reducing the manpower requirement. In addition to the above advantages, the present invention is simple, easy to build and maintain, does not require ADC sampling and digital algorithms, and even more does not require the combination of software and hardware to achieve high-precision bias point control, reducing the system complexity and at the same time reducing the cost and saving resources.

Claims

1. A thin film lithium niobate MZM non-disturbance bias point automatic controller, characterized in that: It includes a photodetector, a transimpedance amplifier, a reference source, an integrator and a driver; The input end of the photodetector is the input end of the thin film lithium niobate MZM non-disturbance bias point automatic controller, the output end of the photodetector is connected to the input end of the transimpedance amplifier, the output end of the transimpedance amplifier is connected to the first input end of the integrator; the second input end of the integrator is connected to the reference source; the output end of the integrator is connected to the input end of the driver; the output end of the driver is the output end of the thin film lithium niobate MZM non-disturbance bias point automatic controller, and is connected to the input end of the thin film lithium niobate MZM.

2. A control method based on the thin film lithium niobate MZM non-disturbance bias point automatic controller according to claim 1, characterized in that: include: The optical signal is converted into an electrical signal through a photodetector to obtain an output voltage; The output voltage is amplified and filtered by a transimpedance amplifier to obtain a filtered voltage; The filtered voltage is compared with the reference voltage generated by the reference source through an integrator, and the difference between the two is integrated to obtain an integrated output voltage; The integrated output voltage is converted into a driving signal required by the thin-film lithium niobate MZM through a driver, thereby changing the output optical power of the thin-film lithium niobate MZM and realizing the control of the bias point.

3. The method according to claim 2, characterized in that Each time the bias point is controlled, the reference voltage applied is a voltage that stabilizes the phase angle of the bias point of the thin-film lithium niobate MZM at π / 2, where π is an angle of 180°.

4. The method according to claim 3, characterized in that: The difference between the reference voltage and the filtered voltage is ΔV; when the phase angle of the thin film lithium niobate MZM bias point is π / 2, ΔV=0, and the integrated output voltage of the integrator remains constant.

5. The method according to claim 4, characterized in that When the phase angle of the bias point of the thin-film lithium niobate MZM is greater than π / 2, ΔV<0, so that the integral output voltage of the integrator decreases, thereby reducing the phase angle of the bias point of the thin-film lithium niobate MZM.

6. The method according to claim 5, characterized in that When the phase angle of the bias point of the thin-film lithium niobate MZM is less than π / 2, ΔV>0, so that the integral output voltage of the integrator increases, thereby increasing the phase angle of the bias point of the thin-film lithium niobate MZM.

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