Bias control for mach-zehnder optical modulators

Abstract

The present disclosure relates to bias control for Mach-Zehnder optical modulators. An electro-optical system includes an optical source, an optical modulator, a first photodetector (PD), a second PD, and a control circuit. The optical source generates an optical signal. The optical modulator modulates the optical signal. The first PD detects the optical signal at an input to the optical modulator. The second PD detects the modulated optical signal at an output of the optical modulator. The control circuit sends a modulated bias signal including a tone to the optical modulator, determines a DC power ratio between the modulated optical signal detected by the second PD and the optical signal detected by the first PD, and sets a bias of the optical modulator as a function of both (i) the DC power ratio and (i) an amplitude of a second harmonic of the tone in the modulated optical signal at the output of the optical modulator.

Description

Technical Field

[0001] This disclosure relates to optical modulator systems, and more specifically to bias control of optical modulators such as those based on Mach-Zehnder modulators. Background Technology

[0002] Optical communication systems rely on modulators to encode electrical signals onto optical carriers for transmission over optical networks. Mach-Zehnder (MZ) modulators are commonly used in these systems because they provide high-speed modulation. MZ modulators operate by splitting the optical signal into two paths, applying a phase shift (e.g., voltage-controlled) to one or both paths, and then recombining the signals to produce constructive or destructive interference that modulates the optical output.

[0003] The performance of an MZ modulator largely depends on maintaining a proper bias point during operation. The bias point determines the operating position on the modulator's transfer function and affects parameters such as linearity, extinction ratio, and signal quality. Environmental factors, including temperature variations, component aging, and mechanical stress, can cause the optimal bias point to drift, leading to a degraded system performance.

[0004] Background information can be found, for example, in B. Yang et al.'s "Stable bias control of Mach-Zehnder modulator for arbitrary optical pulse picking via reference pulse power monitoring" (Optics Express, Vol. 33 (2025), No. 4, pp. 6689-6696), where the authors propose a stable bias control scheme for a Mach-Zehnder modulator (MZM) for optical pulse picking. The key to this method is the introduction of a reference optical pulse signal that is time-shifted relative to the electrically selected signal, thereby ensuring that the output power of the reference pulse is independent of the selection signal and reaches its minimum at the minimum bias point of the MZM.

[0005] Further background can be found in U.S. Patent Application Publication 2017 / 0059889, which describes an integrated optical modulator device including a driver module coupled to an optical modulator. The optical modulator is characterized by a raised cosine transfer function and can be coupled to a light source and a bias control module configured to apply an off-quadrature bias to the optical modulator. This bias is achieved by applying the inverse of the modulator transfer function to the optical modulator to minimize the noise variance.

[0006] The above description is presented as a general overview of the relevant technology in the art and should not be construed as an admission that any information contained herein constitutes prior art to this patent application. Summary of the Invention

[0007] The embodiments described herein provide an electro-optical system including a light source, an optical modulator, a first photodetector (PD), a second PD, and control circuitry. The light source is configured to generate an optical signal. The optical modulator is configured to modulate the optical signal. The first PD is configured to detect the optical signal at an input to the optical modulator. The second PD is configured to detect the modulated optical signal at an output of the optical modulator. The control circuitry is configured to send a modulation bias signal including tone to the optical modulator, determine a DC power ratio between the modulated optical signal detected by the second PD and the optical signal detected by the first PD, and set a bias of the optical modulator based on both (i) the DC power ratio and (i) the amplitude of the second harmonic of the tone in the modulated optical signal at the output of the optical modulator.

[0008] In some embodiments, the control circuit is configured to (a) measure the ratio between the amplitude of the second harmonic of the tone and the amplitude of the fundamental tone in the signal at the output of the optical modulator, and (b) determine the bias of the optical modulator based on the ratio.

[0009] In some embodiments, the optical modulator includes an interferometer. In an example embodiment, the optical modulator includes a Mach-Zehnder interferometer. In another embodiment, the optical modulator includes a microring resonator.

[0010] In some embodiments, the control circuitry includes (i) a first loop that sets an offset based on a DC power ratio and an offset, and (ii) a second loop that adjusts the offset based on the second harmonic of the tone. In an example embodiment, the control circuitry is configured to adapt the second loop more slowly than the first loop.

[0011] According to the embodiments described herein, an electro-optical system is also provided, comprising a light source, an optical modulator, a first PD, a second PD, and control circuitry. The light source is configured to generate an optical signal. The optical modulator is configured to modulate the optical signal. The first PD is configured to detect the optical signal at an input to the optical modulator. The second PD is configured to detect the modulated optical signal at an output of the optical modulator. The control circuitry is configured to send a modulation bias signal including a tone to the optical modulator to measure, in the signal at the output of the optical modulator, the ratio between the amplitude of the second harmonic of the tone and the amplitude of the fundamental tone, and to set the bias of the optical modulator according to the ratio.

[0012] According to the embodiments described herein, a method is also provided, the method comprising generating an optical signal and modulating the optical signal using an optical modulator. An optical signal at the input of the optical modulator and a modulated optical signal at the output of the optical modulator are detected. A modulation bias signal including a tone is sent to the optical modulator. A DC power ratio between the modulated optical signal detected at the output of the optical modulator and the optical signal detected at the input of the optical modulator is determined. The bias of the optical modulator is set based on both (i) the DC power ratio and (i) the amplitude of the second harmonic of the tone in the modulated optical signal at the output of the optical modulator.

[0013] According to the embodiments described herein, a method is further provided, which includes generating an optical signal and modulating the optical signal using an optical modulator. The optical signal at the input of the optical modulator and the modulated optical signal at the output of the optical modulator are detected. A modulation bias signal including a tone is sent to the optical modulator. In the signal at the output of the optical modulator, the ratio between the amplitude of the second harmonic of the tone and the amplitude of the fundamental tone is measured. The bias of the optical modulator is set according to the ratio. Attached Figure Description

[0014] This disclosure will be more fully understood from the following detailed description of embodiments thereof, taken in conjunction with the accompanying drawings, in which:

[0015] Figure 1 This is a schematic diagram illustrating an optical system including modulator bias control according to an embodiment disclosed herein.

[0016] Figure 2 This is a schematic diagram illustrating a calibration system according to an embodiment disclosed herein;

[0017] Figure 3 This is a schematic illustration of a flowchart of a method for calibrating an optical modulator system according to an embodiment disclosed herein;

[0018] Figure 4 A block diagram illustrating the MZ bias control flow according to embodiments disclosed herein is shown schematically; and

[0019] Figure 5 A flowchart illustrating a method for MZ bias control according to embodiments disclosed herein is shown schematically. Detailed Implementation

[0020] Overview

[0021] Mach-Zehnder modulators in optical communication systems require precise bias control to maintain optimal performance, but existing control methods face trade-offs between measurement speed and accuracy. For example, when relying on single-parameter control, environmental factors and component aging can cause bias drift, leading to a degraded system performance.

[0022] The embodiments described below provide fast and accurate bias control using a two-parameter bias control system that combines a DC power ratio measurement with an AC ratio measurement of the second harmonic to the fundamental tone. In these embodiments, this method provides both fast response and long-term accuracy by utilizing a fast control loop for immediate adjustment and a slower correction loop for higher precision.

[0023] The calibration system embodiment described below establishes reference parameters during lifetime start-up testing (and / or at any other time when recalibration is required) by optimizing modulator performance and storing initial bias conditions, utilizing respective DC and AC ratios.

[0024] The control flow implementation demonstrates a hierarchical control loop that operates on different time scales to balance speed and accuracy requirements. The fast loop can provide instantaneous bias correction based on DC power measurements, while the slow loop can adjust the fast loop's target based on harmonic analysis.

[0025] The bias control method embodiment describes a flowchart for maintaining optimal modulator bias during system operation. This method continuously monitors both DC and AC signal characteristics to detect bias drift and apply appropriate correction.

[0026] System Description

[0027] The following detailed description presents an exemplary embodiment of a Mach-Zehnder modulator bias control system that addresses the challenge of maintaining an optimal bias point in optical communication systems. The primary objective is to provide a robust control method that combines the speed advantages of input-output power ratio measurement with the accuracy advantages of tone-based harmonic ratio measurement (AC ratio).

[0028] The disclosed system utilizes a fast control loop based on the ratio of optical signal input to output power (DC ratio) to provide fast, finite-precision bias adjustment, while simultaneously employing a slower control loop based on a more accurate ratio of second harmonic to fundamental tone modulation to correct the operation of the fast control loop. Hereinafter, we will refer to the ratio of optical signal input to output power as the DC ratio (or DCR), and the ratio of second harmonic to fundamental tone modulation as the AC ratio (or ACR).

[0029] The slower control loop updates the bias set by the fast control loop by adding an offset. Two variations are described: fast correction, where the offset is updated immediately on each new calculation; and asymptotic correction, where abrupt changes in the offset are low-pass filtered.

[0030] Figure 1 This is a schematic block diagram illustrating an optical system 100 including modulator bias control according to embodiments disclosed herein. The optical system includes an optical source 102, an optical modulator 104, a first photodetector 106, an input ADC 108, a second photodetector 110, an output ADC 112, and a digital controller 114. In embodiments, the optical source may include a laser to generate an optical signal that is sent to the optical modulator. The optical modulator receives both the optical signal from the optical source and a modulated RF signal from an external source. In embodiments, the optical modulator 104 may include a Mach-Zehnder (MZ) interferometer that modulates the optical signal based on the modulated RF signal to generate a modulated optical signal. In another embodiment, the optical modulator 104 may include a microresonator, and in other embodiments, other suitable types of optical modulators may be used.

[0031] A portion of the optical signal from optical source 102 is directed to first photodetector 106, which monitors the input optical signal and sends a corresponding first electrical signal to input ADC 108. Input ADC converts the first electrical signal into a first digital signal and sends the digital signal to digital controller 114.

[0032] Similarly, a portion of the modulated optical signal output by the optical modulator is directed to the second photodetector 110, which converts the optical signal into a second electrical signal; then, the output ADC 112 converts the second electrical signal into a second digital signal and sends the digital signal to the digital controller 114.

[0033] To control the bias of the optical modulator, the optical system 100 also includes a tone synthesizer 116, a tone digital-to-analog converter (DAC) 118, a tone injection circuit 120, and a bias DAC 122.

[0034] according to Figure 1 In the example embodiment illustrated, a tone (e.g., a 100kHz sine wave) is injected into an optical modulator. A tone synthesizer 116 uses digital synthesis techniques to generate a precise tone. A tone DAC 118 converts the synthesized tone into an analog signal, and a tone injection circuit 120 injects the tone into the optical modulator.

[0035] Digital controller 114 receives signals from input ADC 108, output ADC 112, and tone detector circuit 124, which detects the second harmonic of the tone in the modulated output signal. The digital controller processes these inputs to control the bias point of the optical modulator, thereby implementing both power ratio-based and tone-based control schemes to maintain optimal modulator performance. In some embodiments, the digital controller outputs a digital bias, which is converted to an analog signal by bias DAC 122. The analog bias is input to the optical modulator. The tone sent to the optical modulator by the tone injection circuit is added to this bias.

[0036] Therefore, according to Figure 1 In the example embodiment illustrated, the tone injection circuit adds tone to the MZ bias; the digital control circuit monitors the optical power at the input and output of the MZ modulator and adjusts the bias voltage to improve MZ performance based on both the ratio of input to output power and the ratio of second harmonic power to tone power.

[0037] Figure 1 The configuration of the optical system 100 illustrated and described above is referenced by way of example. Other configurations may be used in alternative embodiments. For example, in some embodiments, the optical system does not include a synthesizer; instead, the digital controller uses a sine table that can be stored in read-only memory (ROM) to generate tones.

[0038] Initial calibration

[0039] Figure 2 A block diagram of a calibration system 200 according to an embodiment disclosed herein is schematically illustrated. The calibration system 200 includes a computing engine 202, a data source 204, an optical engine 206, an optical feedback path 208, and a data verifier 210.

[0040] The computational engine serves as the central processing unit coordinating the calibration operation. It connects to a data source configured to generate the test signals (e.g., pseudo-random binary sequences, PRBS) required for the calibration process. These test signals are input to an optical engine that includes an optical modulator (e.g., an MZ). The optical modulator uses the test signals to modulate the light source.

[0041] Optical feedback path 208 forms a feedback loop and sends the modulated light back to the optical engine, which also includes an optical receiver / demodulator to decode the modulated signal. Data verification unit 210 compares the transmitted signal with the received signal and sends the comparison result to the computation engine. Therefore, the computation engine can test the optical system with various bias voltages and find the optimal performance (e.g., the bias voltage that produces the best comparison result).

[0042] therefore, Figure 2 The calibration system 200, illustrated in the figure and described above, allows the optical system to perform calibration measurements and establish initial parameters for subsequent bias control operations.

[0043] Figure 2 The configuration of the calibration system 200 illustrated and described above is referenced by way of example. Other configurations may be used in alternative embodiments. For example, in one embodiment, the optical system to be calibrated includes only the transmitter, and calibration is performed by optically coupling the calibration system to the optical receiver system; in this case, the data verifyer may be in the optical receiver system, and the verification result may be fed back to the calibration system, for example, via an electrical connection.

[0044] Figure 3 This is a schematic illustration of a flowchart of a method 300 for calibrating an optical modulator system according to an embodiment disclosed herein. The method comprises a calibration system 200 (… Figure 2 ) and executed by its subunits.

[0045] The method begins with a traffic injection operation 302, in which traffic such as PRBS (pseudo-random binary sequence) is injected into the system. The method then proceeds to an initial bias operation 304, in which the bias applied to the modulator of the optical engine 206 is preset to a pre-calibration initial value.

[0046] Following the initial bias operation, the flowchart enters a loop to find the optimal (or sufficiently good) bias voltage. This loop includes a quality measurement operation 306 (where the calibration system measures modulation quality standards such as bit error rate (BER)), a check for optimal quality operation 308, and a bias modification operation 310. In some embodiments, the bias modification operation 310 sequentially scans all possible bias values; in other embodiments, a more efficient search method may be used, where the calibration system determines the next bias voltage in operation 310 based on the quality measurements from previous iterations of the loop. In some embodiments, instead of searching for the optimal quality measurement, the calibration system compares the quality measurement to a preset minimum value and stops the loop when the minimum value is reached.

[0047] When optimal quality measurement is achieved, the flowchart proceeds to measurement ratio operation 312, where the calibration system measures the DC ratio and AC ratio at the final bias voltage. These measurements characterize the optical signal properties at the optimized operating point. The flowchart then proceeds to storage operation 314, where the computation engine stores the bias value, DC ratio, and AC ratio in non-volatile memory (NVM).

[0048] In some embodiments, the optical system does not include an NVM; alternatively, the ratio is sent to an external database for storage in an information file, which may be sent along with the physical components.

[0049] Figure 4 The diagram schematically illustrates the MZ bias control flow 400 according to the embodiments disclosed herein. The bias to AC ratio control function F2(s) 402 defines a combined bias to AC ratio transfer function and an actual measurement (mostly delay) transfer function.

[0050] The bias and DC ratio control function F1(s)404 defines the combined bias to DC ratio transfer function and any measurement errors, such as photodiode mismatch.

[0051] The bias-to-AC ratio control function and the bias-to-DC ratio control function receive a bias input. The bias-to-AC ratio control function generates an output that is input to subtractor 406, which compares the measured AC ratio with a target AC ratio.

[0052] The difference between the measured AC ratio and the target AC ratio is input to the transfer function G(s) 408 (e.g., a PID controller). The output of the G(s) function 408 is the offset used to correct the DC-based bias control. Adder 410 adds the sum to the target DC ratio; this sum is input to subtractor 412, which subtracts the sum from the output of F1(s) 404 to form the bias B of the MZ interferometer.

[0053] The Laplace space bias function can be derived from the MZ bias control flow 400:

[0054] By using a suitable G(s) function, stable behavior with good, fast-converging unit step response can be achieved.

[0055] Figure 5 This is a schematic illustration of a flowchart of a method 500 for MZ bias control according to an embodiment disclosed herein. The method comprises an optical system 100 (… Figure 1 )implement.

[0056] Method 500 begins with a calibration data loading operation 502, where the calibration bias point and DCR / ACR target are loaded. The method then proceeds to an initialization bias operation 504, where digital control 112 ( Figure 1 The MZ bias voltage is initialized based on the value loaded in operation 502.

[0057] Next, the method proceeds to DCR measurement operation 506, where digital control measures the DC ratio (DCR) based on the outputs of photodetector 1 106 and photodetector 2 108. Then, the method enters bias correction operation 508, where digital control corrects the MZ bias voltage based on the DCR and the sum of the target DCR and the offset value. After operation 508, the method returns to operation 506.

[0058] The sequence of operations on 506 and 508 forms a fast loop, which is typically executed whenever power is applied to the circuit. The fast loop controls the MZ bias based on DCR and on the correction offset.

[0059] In parallel with the fast loop, the method includes a slow loop comprising a measurement-ACR operation 510, which is followed by a calculated offset based on an ACR operation 512, and optionally, by a PID offset modification operation 514. The fast loop controls the offset value based on the ACR.

[0060] Therefore, the bias control method 500 implements a dual-loop control architecture, which includes a fast loop that provides fast bias adjustment by measuring DCR operation 506 and correcting bias operation 508, while a slow loop that includes measuring ACR operation 510, calculating offset operation 512 and optional offset PID control operation 514 provides long-term correction to compensate for drift or measurement errors.

[0061] Figure 5 The configuration of method 500 illustrated in the figures and described above is referenced by way of example. Other configurations may be used in alternative embodiments. For example, in one embodiment, the bias voltage calculated in operation 508 is considered more accurate than the bias found during calibration (e.g., component aging may have occurred after the initial calibration), and therefore, the bias derived in operation 408 may be stored in the NVM instead of the calibration data.

[0062] exist Figures 1 to 5The configurations (including all their sub-circuits), control flow 400, calibration method 300, and bias control method 500 of the optical system 100 and calibration system 200 illustrated and described above are example configurations, control flows, and methods shown merely for conceptual clarity. Any other suitable configurations, control flows, and methods may be used in alternative embodiments. Different components of the optical system 100 and calibration system 200 may be implemented in integrated circuits, such as application-specific integrated circuits (ASICs) or field-programmable gate arrays (FPGAs). Some components of the optical system 200 and calibration system 300 may be implemented in software, in hardware, or in a combination of software and hardware components.

[0063] Digital controller 112 ( Figure 1 ) and / or Computing Engine 202 ( Figure 2 Typically, this includes a general-purpose processor, which is programmed with software to perform the functions described herein. For example, the software can be downloaded to the processor electronically via a network, or alternatively or additionally, the software can be provided and / or stored on a non-transitory tangible medium such as magnetic, optical, or electronic memory.

[0064] Although the embodiments described herein primarily deal with the setting of bias voltages in optical modulators, the methods and systems described herein can also be used in other applications, such as optical multiplexing and demultiplexing.

[0065] It should be noted that the above embodiments are cited by way of example, and the invention is not limited to what has been specifically shown and described above. Rather, the scope of disclosure herein includes both combinations and sub-combinations of the various features described above, as well as variations and modifications thereof that would occur to those skilled in the art upon reading the foregoing description and which are not disclosed in the prior art. Documents incorporated herein by reference are to be considered part of this application; however, where a definition of any term in such incorporated documents conflicts with a definition expressly or implicitly made herein, only the definition in this specification shall be considered.

Claims

1. An electro-optical system, comprising: An optical source is configured to generate an optical signal; An optical modulator is configured to modulate the optical signal; A first photodetector PD is configured to detect the optical signal at the input to the optical modulator; The second PD is configured to detect the modulated optical signal at the output of the optical modulator; as well as Control circuit, the control circuit being configured to: A modulation bias signal, including tone, is sent to the optical modulator; Determine the DC power ratio between the modulated optical signal detected by the second PD and the optical signal detected by the first PD; as well as The bias of the optical modulator is set according to both (i) the DC power ratio and (i) the amplitude of the second harmonic of the tone in the modulated optical signal at the output of the optical modulator.

2. The electro-optical system of claim 1, wherein the control circuit is configured to: In the signal at the output of the optical modulator, the ratio between the amplitude of the second harmonic of the tone and the amplitude of the fundamental tone is measured; and The bias of the optical modulator is determined based on the ratio.

3. The electro-optical system according to claim 1, wherein the optical modulator comprises an interferometer.

4. The electro-optical system of claim 1, wherein the optical modulator comprises a Mach-Zehnder interferometer.

5. The electro-optical system of claim 1, wherein the optical modulator comprises a microring resonator.

6. The electro-optical system according to claim 1, wherein, The control circuit includes: (i) a first circuit that sets the bias according to the DC power ratio and according to the offset, and (ii) a second circuit that adjusts the offset according to the second harmonic of the tone.

7. The electro-optical system of claim 6, wherein the control circuit is configured to adapt to the second circuit more slowly than the first circuit.

8. An electro-optical system, comprising: An optical source is configured to generate an optical signal; An optical modulator is configured to modulate the optical signal; A first photodetector PD is configured to detect the optical signal at the input to the optical modulator; The second PD is configured to detect the modulated optical signal at the output of the optical modulator; as well as Control circuit, the control circuit being configured to: A modulation bias signal, including tone, is sent to the optical modulator; In the signal at the output of the optical modulator, the ratio between the amplitude of the second harmonic of the tone and the amplitude of the fundamental tone is measured; as well as The bias of the optical modulator is set according to the ratio.

9. The electro-optical system of claim 8, wherein the optical modulator comprises an interferometer.

10. The electro-optical system of claim 8, wherein the optical modulator comprises a Mach-Zehnder interferometer.

11. The electro-optical system of claim 8, wherein the optical modulator comprises a microring resonator.

12. A method comprising: Generate optical signals; The optical signal is modulated using an optical modulator; The optical signal is detected at the input of the optical modulator; The modulated optical signal is detected at the output of the optical modulator; A modulation bias signal, including tone, is sent to the optical modulator; Determine the DC power ratio between the modulated optical signal detected at the output of the optical modulator and the optical signal detected at the input of the optical modulator; as well as The bias of the optical modulator is set according to both (i) the DC power ratio and (i) the amplitude of the second harmonic of the tone in the modulated optical signal at the output of the optical modulator.

13. The method of claim 12, wherein setting the bias comprises: In the signal at the output of the optical modulator, the ratio between the amplitude of the second harmonic of the tone and the amplitude of the fundamental tone is measured; as well as The bias of the optical modulator is determined based on the ratio.

14. The method of claim 12, wherein the optical modulator comprises an interferometer.

15. The method of claim 12, wherein the optical modulator comprises a Mach-Zehnder interferometer.

16. The method of claim 12, wherein the optical modulator comprises a microring resonator.

17. A method comprising: Generate optical signals; The optical signal is modulated using an optical modulator; The optical signal is detected at the input of the optical modulator; The modulated optical signal is detected at the output of the optical modulator; A modulation bias signal, including tone, is sent to the optical modulator; In the signal at the output of the optical modulator, the ratio between the amplitude of the second harmonic of the tone and the amplitude of the fundamental tone is measured; as well as The bias of the optical modulator is set according to the ratio.

18. The method of claim 17, wherein the optical modulator comprises an interferometer.

19. The method of claim 17, wherein the optical modulator comprises a Mach-Zehnder interferometer.

20. The method of claim 17, wherein the optical modulator comprises a microring resonator.

Images