Stability optimization method based on electro-absorption modulator in specific bias voltage interval

Abstract

The invention relates to the technical field of high-speed optical communication components and driving circuits, in particular to a stability optimization method based on an electric absorption modulator in a specific bias voltage interval, which comprises the following steps: adjusting a parallel matching resistor of an EAM modulation port from 50 ohm to 70 ohm, and forming a stable working area with TDECQ fluctuation not exceeding 0.5 dB in a bias voltage interval of-1.3 V to-1.6 V. According to the invention, on the premise of not introducing an additional compensation circuit or a signal processing algorithm, the sensitivity of TDECQ to the bias voltage is significantly reduced, and the signal integrity, production consistency and environmental robustness of the light emitting module are improved.

Description

Stability optimization method based on electroabsorption modulator in a specific bias range Technical Field

[0001] This invention relates to the field of high-speed optical communication components and driving circuit technology, and in particular to a method for optimizing the stability of an electroabsorption modulator in a specific bias range. Background Technology

[0002] In high-speed optical communication systems, electro-absorption modulators (EAMs) are widely used in high-speed optical modules such as 50G, 100G, 200G, and even 800G due to their high modulation bandwidth, low power consumption, and ease of integration with lasers. The transmission performance of EAMs is affected by various factors, including modulation bias voltage, device bandwidth, driver circuit output capability, optical nonlinearity, and radio frequency (RF) impedance matching. Among these, the transmitter and dispersion eye closure quadrature (TDECQ) is a key indicator for evaluating the signal quality of high-speed optical transmitters. Its value directly reflects the degree of signal degradation after transmission through optical fiber and has a decisive impact on the system's bit error rate.

[0003] In existing technologies, to ensure test consistency and compatibility with high-speed test equipment (such as vector network analyzers and sampling oscilloscopes), EAM chips are typically packaged in a carrier with a standard 50Ω impedance matching (such as a COC, Chip-on-Carrier structure), and connected to drivers and test instruments via 50Ω coaxial connections or microstrip lines. However, while this standardized 50Ω matching method is beneficial for high-frequency signal transmission, it causes TDECQ to be highly sensitive to the EAM bias voltage—that is, the TDECQ value fluctuates drastically with small changes in bias voltage, making it difficult to form a repeatable and predictable "performance stability range." This characteristic severely restricts the efficiency of automated calibration of optical modules during mass production, increases the dependence on bias voltage control accuracy, and raises system cost and complexity.

[0004] Further research shows that under the traditional 50Ω matching architecture, there is a strong nonlinear relationship between the modulation response of EAM and the bias voltage, resulting in significant fluctuations in the TDECQ curve and a lack of a plateau region. Therefore, even with high-precision digital control units (such as MCUs or DSPs) for bias adjustment, it is difficult to maintain stable transmission performance under wide temperature ranges or long-term aging conditions. Furthermore, attempting to mitigate this problem by adding external compensation networks or complex adaptive algorithms introduces additional insertion loss, bandwidth limitations, or power consumption burdens, which are detrimental to the miniaturization and cost reduction of high-speed optical modules.

[0005] To address the aforementioned issues, this invention proposes an optimized method for achieving stable TDECQ performance within a specific bias range based on EAM (Electronic Amplifier). By adjusting the matching resistor value connected in parallel with the EML chip (e.g., using 70Ω instead of the traditional 50Ω), it was unexpectedly discovered that a "stable range" for TDECQ can be formed within a continuous bias range (e.g., -1.3V to -1.6V), where TDECQ performance is almost unaffected by the bias voltage. This phenomenon provides new freedom in the design of optical transmitters, enabling a significant improvement in the stability and robustness of TDECQ without the need for additional matching networks or complex compensation mechanisms. This simplifies the calibration process, improves production yield, and enhances the reliability of the module in practical applications. Summary of the Invention

[0006] This invention addresses the technical shortcomings of existing Electro-Absorption Modulator (EAM) driver circuits, which, due to the use of a standard 50Ω parallel matching resistor, result in the transmitter and dispersion eye closure quadrature (TDECQ) being highly sensitive to bias voltage and lacking a stable operating range. It provides a stability optimization method based on adjusting the matching resistor value. This method replaces the 50Ω parallel matching resistor connected to the EAM modulation port with a 70Ω resistor (or a non-standard value between 60Ω and 80Ω), inducing a continuous stable operating range within a specific bias voltage range where the TDECQ fluctuation amplitude does not exceed 0.5dB. This significantly improves the signal integrity, production consistency, and environmental robustness of high-speed optical transmitter modules without introducing any additional compensation circuits, feedback control mechanisms, or signal processing algorithms.

[0007] This invention provides a stability optimization method for an electro-absorption modulator within a specific bias range, applicable to EML (Electro-absorption Modulated Laser) chip driving systems integrating a DFB laser and an EAM. Its core lies in reconstructing the parameters of the parallel matching resistor at the EAM modulation port. The method includes the following structure and operation steps:

[0008] S1: In the COC (Chip-on-Carrier) package structure of the EML chip, the parallel matching resistor originally connected between the EAM modulation port and ground is replaced from 50Ω to 70Ω. This resistor is directly soldered between the EAM modulation electrode and the reference ground plane to form the terminal load of the RF signal loop.

[0009] S2: Connect an external DC bias voltage source to the cathode or anode of the EAM to apply an adjustable negative bias voltage, with a bias voltage range covering -0.5V to -2.0V;

[0010] S3: The NRZ or PAM4 format modulation signal output from the high-speed arbitrary waveform generator (AWG) is input to the EAM modulation port via a 50Ω characteristic impedance RF transmission line. This signal and the DC bias voltage are superimposed inside the EAM to jointly control the light absorption coefficient to achieve light intensity modulation.

[0011] S4: The modulated optical signal output by the EML is coupled to the photodetector (PD) via a standard single-mode fiber. The PD converts the optical signal into an electrical signal and then inputs it to the digital communication analyzer (DCA).

[0012] S5: Under the condition of fixed AWG output signal parameters (including rate 53.125Gbps, amplitude, rise / fall time and pre-emphasis setting), the bias voltage is scanned in 0.1V steps by a programmable source meter, and the TDECQ value and relative frequency error (RFER) of DCA output are synchronously collected at each bias point.

[0013] S6: Plot the response curve of TDECQ as a function of bias voltage to identify whether there is a continuous bias range with TDECQ fluctuation amplitude ≤ 0.5dB;

[0014] S7: When the matching resistor is 70Ω, the TDECQ value exhibits a plateau characteristic within the bias voltage range of -1.3V to -1.6V, with fluctuations of less than 0.5dB. At the same time, the RFER is also at the optimal or suboptimal level, indicating that this range is a "TDECQ stable operating area" that can be used for emission performance optimization.

[0015] Furthermore, the resistance value of the parallel matching resistor is set to 70Ω±5Ω, that is, the resistance value range is limited to 65Ω to 75Ω. Any resistance value within this range can induce TDECQ stability characteristics within the bias range of -1.3V to -1.6V. Furthermore, when the resistance value is extended to the range of 60Ω to 80Ω, different degrees of TDECQ plateau effect can still be observed, but the plateau width and flatness gradually deteriorate as the resistance value deviates from 70Ω. Therefore, 70Ω is the optimal implementation value.

[0016] The TDECQ stable operating region is defined as follows: within this bias range, the difference between the maximum and minimum values ​​of TDECQ does not exceed 0.5dB, and the width of this range is not less than 0.3V. The existence of this range enables the system to maintain the transmitted signal quality within the specification limits when facing process deviations (such as EAM threshold voltage drift of ±0.1V), temperature changes (-10°C to +70°C), or device parameter deviations caused by long-term aging, without the need for dynamic bias adjustment or enabling compensation mechanisms.

[0017] Specifically, the technical solution of this invention does not rely on any active control unit, adaptive equalizer, pre-emphasis circuit, or digital signal processing module. The optimization effect stems entirely from the high-frequency equivalent impedance reconstruction of the EAM port caused by the change in the matching resistor value, which in turn affects the frequency domain response characteristics of the modulation signal within the EAM. Specifically, the 70Ω matching resistor changes the load conditions of the EAM modulation port, causing the reflection coefficient and phase delay to be synergistically adjusted near the critical Nyquist frequency (e.g., 26.5625GHz for 53.125Gbaud PAM4), suppressing the resonance peak formed by the package parasitic inductance, bond line capacitance, and the EAM's own capacitance, thereby reducing the eye diagram closure effect and significantly reducing the TDECQ's dependence on bias voltage.

[0018] Furthermore, the method is applicable to high-speed optical transmitter modules supporting PAM4 or NRZ modulation formats, especially EML products at 50G, 100G, 200G, and 800G speeds. These products can be packaged in various forms, including but not limited to COB (Chip-on-Board), TO-can, OSA (Optical Sub-Assembly), or silicon photonics integrated platforms. In these applications, simply changing the matching resistor from 50Ω to 70Ω during the packaging stage is sufficient to achieve stable TDECQ performance in subsequent testing and use, without requiring modifications to the driver IC design, PCB layout, or control firmware.

[0019] The bias voltage is applied by superimposing a DC bias onto the RF modulation signal, and the two are electrically connected through a Bias-Tee structure or by directly sharing the same EAM modulation electrode. There are no additional passive components in series or parallel between the 70Ω matching resistor and the RF transmission line, and its physical location is close to the EAM chip pad to minimize the influence of lead inductance on the high-frequency response.

[0020] Furthermore, the verification method for the TDECQ stability range includes: performing steps S1 to S6 under an ambient temperature of 25°C ± 2°C; then placing the module under test in a temperature chamber and repeating steps S5 to S6 at three temperature points: -10°C, 25°C, and 70°C. If the TDECQ fluctuation is ≤0.5dB within the range of -1.3V to -1.6V at all temperature points, the module is determined to have temperature robustness. This verification process can be embedded into the automated testing program of the production line for batch screening of modules that meet the stability performance requirements.

[0021] Specifically, the introduction of the 70Ω matching resistor did not lead to a significant increase in insertion loss or bandwidth compression. Actual measurements show that, under a 53.125Gbaud PAM4 signal, the -3dB bandwidth of the 70Ω matching scheme is still greater than 28GHz, meeting the transmitter bandwidth requirements of standards such as IEEE 802.3ck and OIF CEI-112G-VSR. Simultaneously, by avoiding the use of complex equalization circuitry, the overall power consumption is reduced by approximately 15% to 20% compared to the traditional 50Ω matching + pre-emphasis scheme.

[0022] The TDECQ is calculated based on the four-level eye diagram closure model defined in the IEEE 802.3 standard. It is derived from the optical eye diagram data collected by DCA after normalizing noise, jitter, and dispersion effects. The RFER is quantified based on the deviation between the receiver's decision threshold and the ideal signal level. Both of these are used as core indicators for evaluating transmission performance.

[0023] Furthermore, the method described in this invention is fully compatible with existing EML driver ICs. Mainstream driver ICs (such as those from Marvell, Semtech, and Broadcom) typically have a 50Ω output impedance. When driving a 70Ω load, although a slight impedance mismatch exists, this mismatch actually helps suppress high-frequency resonance and creates a beneficial frequency response shaping effect in the specific application scenario of this invention. Simulations and actual measurements both show that the reflected power caused by this mismatch is less than -15dB and does not lead to signal overshoot or worsening of ringing.

[0024] The matching resistor is a high-precision thin-film chip resistor with a temperature coefficient ≤ ±50ppm / °C, a rated power ≥ 1 / 16W, and a package size of 0201 or 0402 to meet the packaging requirements of high-density optical modules. The layout of the resistor on the COC substrate follows the RF design specifications to ensure the shortest grounding path and the smallest return current area to maintain high-frequency signal integrity.

[0025] In particular, the technical effects of this invention are not only reflected in TDECQ stability, but also simultaneously improve eye diagram symmetry, extinction ratio consistency, and jitter performance. Under 70Ω matching conditions, the rise / fall time difference of EAM in the stable range is reduced to less than 5ps, and the eye diagram cross-point jitter is reduced by about 20%, indicating that the modulation linearity is improved.

[0026] The beneficial effects of this invention are as follows: By adjusting the parallel matching resistor of the EAM modulation port from 50Ω to 70Ω, a stable operating region with TDECQ fluctuation ≤0.5dB is constructed within the -1.3V to -1.6V bias range. This technical solution involves only a parameter change of a single passive component, without adding any active circuits, control logic, or signal processing modules, thus achieving significant optimization of transmission performance. The method has a clear physical implementation path, a repeatable experimental verification process, and broad engineering applicability, solving the long-standing bias sensitivity problem in the field of high-speed optical communication and providing a new technical path for the design of high-density, low-cost, and high-reliability optical modules. Attached Figure Description

[0027] Figure 1 is an equivalent circuit diagram of an electroabsorption modulator (EAM) in the prior art under a COC package structure, wherein a 50Ω matching resistor is connected in parallel to the EAM modulation port;

[0028] Figure 2 is the equivalent circuit diagram of the improved COC of the present invention, wherein the matching resistor in parallel with the EAM modulation port is replaced with 70Ω;

[0029] Figure 3 is a block diagram of the test system used to verify the stability of TDECQ in an embodiment of the present invention;

[0030] Figure 4 shows a comparison curve of TDECQ as a function of bias voltage under the conditions of 50Ω and 70Ω matching resistors.

[0031] Figure 5 shows a comparison curve of RFE as a function of bias voltage under the conditions of 50Ω and 70Ω matching resistors. Detailed Implementation

[0032] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0033] Specific implementation examples are given below.

[0034] Referring to Figures 1-5, the specific embodiments of the present invention, in conjunction with the accompanying drawings, provide a detailed description of the stability optimization method for an electroabsorption modulator (EAM) within a specific bias voltage range. The figure shows an equivalent circuit diagram of an EAM in a COC package structure, where the EML chip includes a DFB laser and an EAM. The EAM modulation port is connected to a reference ground plane via a parallel matching resistor. This matching resistor has a standard resistance of 50Ω and is used for impedance matching with an external 50Ω characteristic impedance RF transmission line. A DC bias voltage source applies an adjustable negative bias voltage to the EAM via a programmable source meter. A high-speed modulation signal output from an arbitrary waveform generator is input to the EAM modulation port via the RF transmission line. The modulated optical signal is transmitted to a photodetector via a standard single-mode fiber. A digital communication analyzer then collects eye diagram data and calculates TDECQ and RFER parameters. In this configuration, TDECQ fluctuates drastically with changes in the bias voltage, lacking a usable stable operating range, which is detrimental to automated production calibration.

[0035] The improved COC equivalent circuit diagram of this invention differs primarily in that the resistance value of the parallel matching resistor is replaced from 50Ω to 70Ω. This 70Ω matching resistor is directly soldered between the EAM modulation electrode and the reference ground plane, with its physical location adjacent to the EML chip's pads to minimize the impact of lead inductance on high-frequency signals. This resistor is a high-precision thin-film surface mount resistor with a package size of 0201 or 0402, a temperature coefficient ≤ ±50ppm / °C, and a rated power ≥ 1 / 16W, ensuring resistance stability under high-temperature or long-term operating conditions. In this structure, the connection relationships of the EML chip, DFB laser, electroabsorption modulator, DC bias voltage source, arbitrary waveform generator, RF transmission line, photodetector, digital communication analyzer, programmable source meter, standard single-mode fiber, and reference ground plane are consistent with those in Figure 1, with only the resistance value of the matching resistor changing.

[0036] This is a block diagram of a test system used to verify the stability of TDECQ in an embodiment of the present invention. During testing, an arbitrary waveform generator generates a 53.125 Gbaud PAM4 format modulation signal with fixed amplitude, rise / fall time, and pre-emphasis parameters. The signal is input to the EAM modulation port of the EML chip via a 50Ω characteristic impedance RF transmission line. The DC bias voltage is provided by a programmable source meter, which is applied to the EAM cathode in 0.1V steps within the range of -0.5V to -2.0V by adjusting its output voltage. The modulated optical signal output by the EML chip is coupled to a photodetector via a standard single-mode fiber, converted into an electrical signal, and then sent to a digital communication analyzer. The DCA processes the acquired eye diagram according to the IEEE 802.3 standard, calculates the TDECQ and relative frequency error (RFER) values, and records the performance indicators corresponding to each bias point.

[0037] Under the above test conditions, COC samples with 50Ω and 70Ω matching resistors were tested respectively. Under the 50Ω matching condition, TDECQ fluctuated drastically with the bias voltage, especially in the range of -1.2V to -1.7V, where the TDECQ variation exceeded 2dB with no obvious plateau region. Under the 70Ω matching condition, TDECQ formed a clear plateau in the bias voltage range of -1.3V to -1.6V, with the difference between the maximum and minimum values ​​less than 0.5dB, which meets the definition of the "TDECQ stable operating region". At the same time, Figure 5 shows that within this stable range, RFER is also at the optimal or suboptimal level, indicating that the signal frequency response is not degraded due to impedance mismatch.

[0038] The specific implementation process of this invention includes the following steps: S1, on the COC packaging substrate of the EML chip, the original 50Ω parallel matching resistor is replaced with a 70Ω high-precision thin-film resistor. One end of the resistor is connected to the EAM modulation electrode, and the other end is directly connected to the reference ground plane. The layout follows the RF design specifications to ensure the shortest grounding path and the smallest return current area; S2, the DC bias voltage source is connected to the EAM through a Bias-Tee structure or a direct shared electrode method to apply an adjustable negative bias voltage; S3, the 53.125Gbaud output of the arbitrary waveform generator is... The PAM4 modulation signal is input to the EAM modulation port via a 50Ω RF transmission line. The modulation signal and DC bias voltage are superimposed inside the EAM to control the light absorption coefficient and achieve light intensity modulation. S4: The modulated optical signal output from the EML is coupled to the photodetector via a standard single-mode fiber, converted into an electrical signal, and then input to the digital communication analyzer. S5: Under fixed AWG output parameters, the bias voltage is scanned in 0.1V steps using a programmable source meter, and the TDECQ and RFER output from the DCA are acquired synchronously. S6: The TDECQ-bias voltage curve is plotted to identify whether there is a continuous range with fluctuations ≤0.5dB and a width ≥0.3V. S7: It is confirmed that under 70Ω matching, the range of -1.3V to -1.6V is the stable operating region of TDECQ, which can be used for transmission performance optimization.

[0039] The formation mechanism of this stable range originates from the high-frequency equivalent impedance reconstruction of the EAM port caused by the change in the matching resistor value. When the matching resistor is 70Ω, the load conditions of the EAM modulation port change, causing the reflection coefficient and phase delay near the critical Nyquist frequency (such as 26.5625GHz for 53.125Gbaud PAM4) to be adjusted in a coordinated manner. This effectively suppresses the resonance peak formed by the package parasitic inductance, bond line capacitance, and EAM itself capacitance, thereby reducing the eye diagram closure effect and significantly reducing the dependence of TDECQ on bias voltage. Although the driver IC usually has a 50Ω output impedance, which is slightly mismatched with the 70Ω load, the actual measurement shows that the reflection power caused by this mismatch is less than -15dB, which will not cause signal overshoot or ringing deterioration. Instead, it produces a beneficial frequency response shaping effect in a specific frequency band.

[0040] This invention is applicable to EML products with speeds of 50G, 100G, 200G, and 800G, and can be packaged in COB, TO-can, OSA, or silicon photonics integrated platforms. In production line applications, simply changing the matching resistor from 50Ω to 70Ω during the COC packaging stage is sufficient to obtain stable TDECQ performance in subsequent tests. To verify temperature robustness, the module can be placed in a temperature chamber, and steps S5 to S6 can be repeated at three temperature points: -10°C, 25°C, and 70°C. If the TDECQ fluctuation is ≤0.5dB within the -1.3V to -1.6V range at all temperature points, the module is considered to have environmental adaptability and can be directly used for mass production. Furthermore, actual measurements show that the -3dB bandwidth of the 70Ω matching scheme is still greater than 28GHz, meeting the requirements of standards such as IEEE 802.3ck and OIF CEI-112G-VSR. Since no pre-emphasis or equalization circuitry is required, the overall power consumption is reduced by 15% to 20% compared to traditional solutions.

[0041] Furthermore, this invention improves TDECQ stability while simultaneously enhancing eye diagram symmetry and jitter performance. Under 70Ω matching conditions, the rise / fall time difference of EAM within the stable range is reduced to less than 5ps, and the jitter at the eye diagram crossover point is reduced by approximately 20%, indicating improved modulation linearity. This effect stems from the optimization of carrier distribution and electric field intensity within EAM by impedance reconstruction, making the optical absorption response closer to ideal linear modulation. Since this solution involves only parameter changes of a single passive component and does not rely on any active control unit, adaptive algorithm, or digital signal processing module, it has extremely high engineering feasibility and cost advantages, making it particularly suitable for large-scale optical module manufacturing scenarios.

[0042] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A stability optimization method for an electro-absorption modulator within a specific bias range, applicable to an EML chip driving system integrating a DFB laser and an electro-absorption modulator, characterized in that... Includes the following steps: In the COC package structure of the EML chip, the resistance of the parallel matching resistor connected between the modulation port of the electroabsorption modulator and the reference ground plane is set to 70Ω; a DC bias voltage source is connected to the electroabsorption modulator to apply an adjustable negative bias voltage in the range of -0.5V to -2.0V. The high-speed modulation signal output from the arbitrary waveform generator is input to the modulation port of the electroabsorption modulator via a 50Ω characteristic impedance RF transmission line; the modulation optical signal output from the EML chip is coupled to the photodetector via a standard single-mode fiber and then input to the digital communication analyzer; under the condition of fixed output signal parameters of the arbitrary waveform generator, the bias voltage is scanned in 0.1V steps by a programmable source meter, and the TDECQ value output by the digital communication analyzer is acquired simultaneously. Identify whether there exists a continuous bias range where the TDECQ fluctuation amplitude does not exceed 0.5dB and the bias range width is not less than 0.3V.

2. The stability optimization method based on an electroabsorption modulator in a specific bias range as described in claim 1, characterized in that, The resistance value of the parallel matching resistor ranges from 65Ω to 75Ω.

3. The stability optimization method based on an electroabsorption modulator in a specific bias range as described in claim 2, characterized in that, The parallel matching resistor is a high-precision thin-film chip resistor with a temperature coefficient of no more than ±50ppm / °C and a rated power of no less than 1 / 16W.

4. The stability optimization method based on an electroabsorption modulator in a specific bias range as described in claim 1, characterized in that, The parallel matching resistor is directly soldered between the modulation electrode of the electroabsorption modulator and the reference ground plane, and its physical location is close to the pad of the EML chip.

5. The stability optimization method based on an electroabsorption modulator in a specific bias range as described in claim 1, characterized in that, The high-speed modulation signal is a 53.125 Gbaud PAM4 format signal.

6. The stability optimization method based on an electroabsorption modulator in a specific bias range as described in claim 1, characterized in that, The bias voltage range corresponding to the stable operating region of the TDECQ is -1.3V to -1.6V.

7. The stability optimization method based on an electroabsorption modulator in a specific bias range as described in claim 1, characterized in that, The DC bias voltage source and the arbitrary waveform generator are electrically connected by sharing the same electroabsorption modulator modulation electrode.

8. The stability optimization method based on an electroabsorption modulator in a specific bias range as described in claim 1, characterized in that, The package size of the parallel matching resistor is 0201 or 0402.

9. The stability optimization method based on an electroabsorption modulator in a specific bias range as described in claim 1, characterized in that, The method is applicable to EML chips in COB, TO-can, OSA, or silicon photonics integrated platform packages.

10. The stability optimization method based on an electroabsorption modulator in a specific bias range as described in claim 1, characterized in that, There are no additional series or parallel passive components between the parallel matching resistor and the radio frequency transmission line.

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