Optical modulation apparatus and method with noise compensation based on dark monitoring photodiode

Abstract

The present invention relates to a noise compensation-based optical modulation device and method using a dark monitoring photodiode, wherein a monitoring photodiode is placed at the input and output terminals of an optical modulator to control the DC bias without a pilot tone based on the calculated optical output ratio, and a dark monitoring photodiode with light blocked is symmetrically placed around the monitoring photodiode to compensate for noise included in the measured values ​​of the monitoring photodiode using dark current and thermal noise information, thereby enabling precise measurement of the optical output ratio and detection of minute optical drift even in environments with high temperatures or severe temperature changes. By using the optical output ratio, the device fundamentally prevents jitter caused by the pilot tone and has the effect of accurately identifying the operating point regardless of fluctuations in the intensity of the input light source. Furthermore, by symmetrically placing a plurality of dark monitoring photodiodes around the monitoring photodiode and averaging their measured values, the device mathematically neutralizes measurement errors caused by physical temperature gradients inside the silicon wafer, thereby enabling precise noise compensation even in environments with high temperatures or severe temperature changes. Additionally, through the calculation of a predicted bias voltage using a lookup table composed of a plurality of bins classified according to drift characteristics and a self-revining function It has the effect of enabling fast operation and stable performance.

Description

Technology Field

[0001] The present invention relates to a noise compensation-based optical modulation technology using a dark monitoring photodiode, and more specifically, to a device and method for noise compensation-based optical modulation using a dark monitoring photodiode that can improve the precision of DC bias control even in high-temperature or high-temperature changing environments by controlling the DC bias without a pilot tone based on the optical output ratio calculated by placing monitoring photodiodes at the input and output terminals of an optical modulator, respectively, and compensating for noise included in the measured values ​​of the monitoring photodiode using dark current and thermal noise information measured by symmetrically placing a dark monitoring photodiode with light blocked around the monitoring photodiode. Background Technology

[0002] Recently, Linear Drive Pluggable Optics (LPO) technology is gaining attention in optical communication systems used in AI data centers and high-performance computing environments. This technology eliminates the DSP inside the optical module and allows the Serializer / Deserializer (SerDes) of the Application-Specific Integrated Circuit (ASIC) to directly drive optical devices in order to minimize data transmission delay and power consumption. In such environments, an optical modulator capable of ultra-high-speed modulation of over 50 Gbps with high linearity is essential.

[0003] An optical modulator in an optical communication system receives continuous wave (CW) light output from a laser diode and transmits information by changing the intensity, phase, or frequency of the light according to the transmission data (RF data) to be transmitted.

[0004] The Mach-Zehnder Modulator (MZM) is known as an optical modulator used in ultra-high-speed communication utilizing LPO technology. This Mach-Zehnder modulator evenly distributes the input light into two waveguide paths and then applies different voltages to each path to create a difference in refractive index, thereby forming a phase difference between the light passing through the two paths. Subsequently, when the light from the two paths is combined again, constructive interference (brightness) or destructive interference (darkness) occurs depending on the phase difference, thereby modulating the intensity of the light.

[0005] The output optical intensity of a Mach-Zehnder modulator is the cosine square (cos 2 It represents a transmission characteristic curve of the form ). In this transmission characteristic curve, at least one of the following is used as an operating point to control the phase difference: the peak point where the output is maximum, the null point where the output is minimum, and the quadrature point (Q-Point) where the rate of change of the output is most linear. In particular, in NRZ (Non-Return-to-Zero) modulation schemes, it is common to operate at the quadrature point, which has excellent linearity.

[0006] Meanwhile, conventional Mach-Zehnder modulators control the refractive index of the optical path using materials such as Bulk LiNbO3, InP, and SiPh, but these applied devices have limitations such as relatively large size, high power consumption, and low bandwidth.

[0007] Thin-film lithium niobate (LiNbO3, TFLN), which has recently been attracting increasing interest, is a promising material capable of meeting these requirements. Compared to conventional bulk lithium niobate, TFLN has the advantages of miniaturizing device size to less than a few millimeters, lowering driving voltage to less than a few volts, and enabling ultra-high-speed modulation of over 100 GHz. However, due to the characteristics of its thin-film structure, TFLN has a problem in that it is sensitive to DC drift caused by temperature changes and charge accumulation.

[0008] DC drift refers to the phenomenon where the operating point of a modulator shifts due to changes in time, temperature, or applied voltage. For instance, even if initially set to the quadrature point, if the operating point shifts toward the peak or null point due to drift, modulation efficiency deteriorates and signal distortion occurs. In data centers, where communication equipment is integrated, temperatures frequently exceed 80°C and vary rapidly due to the operation of cooling and air conditioning systems. Therefore, to ensure the stable operation of TFLN modulators, DC bias control technology capable of compensating for drift caused by device-specific process-voltage-temperature (PVT) variations, external temperature changes, charge accumulation, and long-term aging is essential. While the degree of drift variation in TFLN modulators is significant, the same phenomenon of the operating point shifting due to temperature, characteristics, or degradation occurs in modulators using materials such as Bulk LiNbO3, InP, and SiPh; consequently, drift compensation is required for virtually all types of optical modulators.

[0009] Among the known DC bias control methods for conventional optical modulators, one method is to check for drift using a pilot tone. The pilot tone method involves superimposing a low-frequency dithering signal onto the DC bias voltage and applying it to the modulator, and tracking the operating point by lock-in detection of the component of the dithering signal in the output light.

[0010] However, the method using pilot tones has a fatal problem in that the pilot tones themselves act as jitter in the signal. In other words, a low-frequency dithering signal superimposed on the DC bias causes the modulator's operating point to vibrate slightly, resulting in unwanted fluctuations in the output optical signal and degrading signal quality.

[0011] In addition, when checking drift using only pilot tones, there is a limitation in that it is difficult to distinguish between fluctuations in the intensity of the input light source and DC drift. That is, since the pilot tone method monitors only the intensity of the output light, it is not possible to distinguish whether the change in light intensity is due to fluctuations in the output of the laser diode or a change in modulation efficiency is due to drift.

[0012] The method of applying a dither signal such as a pilot tone and performing bias voltage control considering drift based on it has a high probability of errors and inevitably has a lot of noise, so there is a limitation in that the quality decreases as the communication speed increases.

[0013] In addition, because it is a feedback control method, the bias voltage is controlled subsequently by synchronizing the pilot tone detected from the output light, so it is not easy to maintain a stable control loop.

[0014] Meanwhile, various optical modulator devices, including TFLNs, undergo a long-term burn-in test because the bias point changes rapidly when voltage is first applied after manufacturing and then stabilizes as it converges to a constant state. Control parameters are set based on information regarding drift characteristics measured during the burn-in test. In other words, the bias voltage is controlled to match the drift characteristics by changing the values ​​of the control parameters set according to the change in modulation efficiency obtained by analyzing the pilot tone included in the optical modulator output.

[0015] Therefore, long-term burn-in testing is essential, which leads to a problem of reduced mass production efficiency. Furthermore, if such drift characteristics change, long-term stability cannot be guaranteed through control parameter settings determined based on initial burn-in test results; consequently, communication quality deteriorates after a certain period of use, resulting in a shortened lifespan and incurring the burden of time, cost, and communication interruptions due to recalibration.

[0016] Therefore, there is a need for a new technology that enables accurate DC bias control without using dither signals, which are the cause of jitter, effectively responds to changes in drift characteristics, and allows for feedforward control; furthermore, it can reduce burn-in test time while controlling according to drift characteristics, or automatically change control settings to suit drift characteristics resulting from aging even if aging occurs due to use. Prior art literature

[0017] Korean Registered Patent No. 10-1070409 [Title: Mach-Zehnder Optical Modulator] Korean Registered Patent No. 10-2892845 [Title: Mach-Zehnder Optical Modulator Including Multimode Interferometer] Korean Registered Patent No. 10-2162833 [Title: Tunable U-Laser Transmitter Equipped with Integrated Mach-Zehnder Modulator] The problem to be solved

[0018] The objective of the present invention, which is to solve the above-mentioned problems, is to provide a noise compensation-based optical modulation device and method using a dark monitoring photodiode, which prevents jitter generation by controlling the DC bias without a pilot tone based on the optical output ratio calculated by measuring the optical intensity at the input and output terminals of the optical modulator, and enables accurate identification of the operating point regardless of fluctuations in the intensity of the input light source; and by symmetrically placing a dark monitoring photodiode with light blocked around the monitoring photodiode used to calculate the optical output ratio and compensating the measurement value of the monitoring photodiode with dark current and thermal noise information, thereby enabling precise measurement of the optical output ratio and detection of minute optical drift even in environments with high temperatures or severe temperature changes.

[0019] Another objective of the present invention is to provide a noise compensation-based optical modulation device and method using a dark monitoring photodiode, which enables more precise noise compensation by mathematically neutralizing measurement errors caused by a physical temperature gradient inside a silicon wafer through the symmetrical arrangement of a plurality of dark monitoring photodiodes around a monitoring photodiode and averaging their measurement values.

[0020] Another objective of the present invention is to provide a noise compensation-based optical modulation device and method using a dark monitoring photodiode, which can shorten the initial driving time by immediately obtaining a predicted bias voltage by having a lookup table in which a DC bias voltage according to temperature, voltage application time, and optical output ratio is recorded, and improve the response speed to disturbances through feedforward control.

[0021] Another objective of the present invention is to provide a noise compensation-based optical modulation device and method using a dark monitoring photodiode that reduces mass production costs and supports stable operation throughout its lifespan without recalibration by providing a self-revining function that groups elements with similar drift characteristics into a lookup table, selects a suitable bin based on the results of an initial test over a short period of time, and automatically switches to a suitable bin when drift characteristics change by monitoring the error between the predicted bias voltage and the actual bias voltage during operation. means of solving the problem

[0022] A noise compensation-based optical modulation device using a dark monitoring photodiode according to an embodiment of the present invention comprises: a modulation unit including a laser diode that outputs an optical signal, an optical modulator that modulates and outputs light input from the laser diode according to a DC bias voltage and a transmitted data signal, and a temperature sensor that measures the temperature of the optical modulator; an input tap that branches off a portion of the input light in an optical path between the laser diode and the modulation unit; an output tap that branches off a portion of the output light in an output optical path of the modulation unit; an input monitoring photodiode that measures the intensity of the light branched from the input tap; an output monitoring photodiode that measures the intensity of the light branched from the output tap; an input dark current measuring unit including a plurality of input dark monitoring photodiodes symmetrically arranged around the input monitoring photodiode in a state where light is blocked to measure dark current and current due to thermal noise; an output dark current measuring unit including a plurality of output dark monitoring photodiodes symmetrically arranged around the output monitoring photodiode in a state where light is blocked to measure dark current and current due to thermal noise; and the input dark current measuring unit and the output dark current It may include a composite modulation control unit that compensates the measurement values ​​of the input monitoring photodiode and the output monitoring photodiode using the measurement values ​​of the measurement unit, calculates the optical output ratio based on the compensated measurement values, and calculates the DC bias voltage based on the optical output ratio and applies it to the modulation unit.

[0023] As an example related to the present invention, the input dark monitoring photodiode and the output dark monitoring photodiode may be fabricated together on the same silicon wafer using the same CMOS process as the input monitoring photodiode and the output monitoring photodiode, and may be characterized by having the same specifications as the input monitoring photodiode and the output monitoring photodiode.

[0024] As an example related to the present invention, the input dark monitoring photodiodes may be characterized by having four or more symmetrically arranged around the input monitoring photodiode, and the output dark monitoring photodiodes may be characterized by having four or more symmetrically arranged around the output monitoring photodiode.

[0025] As an example related to the present invention, the composite modulation control unit may be characterized by including a noise detection unit that calculates the average of the measurement values ​​of a plurality of input dark monitoring photodiodes of the input dark current measuring unit to determine an input noise compensation value, and calculates the average of the measurement values ​​of a plurality of output dark monitoring photodiodes of the output dark current measuring unit to determine an output noise compensation value.

[0026] As an example related to the present invention, the composite modulation control unit may be characterized by including a dual tap diagnostic unit that calculates a compensated input light intensity by subtracting the input noise compensation value from the measurement value of the input monitoring photodiode, calculates a compensated output light intensity by subtracting the output noise compensation value from the measurement value of the output monitoring photodiode, and calculates a light output ratio in real time based on the ratio of the compensated input light intensity to the compensated output light intensity.

[0027] As an example related to the present invention, the composite modulation control unit may be characterized by having a lookup table in which a DC bias voltage according to the optical output ratio, temperature, and voltage application time is stored, obtaining a predicted bias voltage by referring to the lookup table, and calculating a DC bias voltage based on the predicted bias voltage and the optical output ratio.

[0028] As an example related to the present invention, the lookup table is composed of a plurality of bins classified by groups having similar characteristics based on drift characteristics measured from a plurality of optical modulator samples, and the composite modulation control unit may be characterized by including a bias determination unit having a rebining unit that reanalyzes the drift slope and switches from the current bin to another bin when the error between the predicted bias voltage of the lookup table and the actual calculated bias voltage exceeds a reference value.

[0029] A noise compensation-based optical modulation method using a dark monitoring photodiode according to an embodiment of the present invention may include the steps of: an optical modulation device determining an input noise compensation value and an output noise compensation value using a plurality of input dark monitoring photodiodes symmetrically arranged around an input monitoring photodiode and a plurality of output dark monitoring photodiodes symmetrically arranged around an output monitoring photodiode; the optical modulation device calculating a compensated input light intensity by subtracting the input noise compensation value from the measurement value of the input monitoring photodiode and calculating a compensated output light intensity by subtracting the output noise compensation value from the measurement value of the output monitoring photodiode, and then calculating an optical output ratio as a ratio of the compensated input light intensity and the compensated output light intensity; and the optical modulation device calculating a DC bias voltage based on the optical output ratio and applying it to an optical modulator of a modulation unit.

[0030] As an example related to the present invention, the step of determining the input noise compensation value and the output noise compensation value may be characterized by determining the average of the measurement values ​​of the plurality of input dark monitoring photodiodes as the input noise compensation value and determining the average of the measurement values ​​of the plurality of output dark monitoring photodiodes as the output noise compensation value.

[0031] As an example related to the present invention, the step of calculating the DC bias voltage and applying it to the optical modulator of the modulation unit may be characterized by obtaining a predicted bias voltage by referring to a lookup table in which the DC bias voltage according to the optical output ratio, temperature, and voltage application time is stored, and calculating the DC bias voltage based on the predicted bias voltage and the optical output ratio. Effects of the invention

[0032] The noise compensation-based optical modulation device and method using a dark monitoring photodiode according to an embodiment of the present invention has the effect of improving signal quality by fundamentally preventing the occurrence of jitter caused by pilot tones by controlling the DC bias without pilot tones based on the optical output ratio calculated by measuring the optical intensity at the input and output terminals of the optical modulator, respectively.

[0033] In addition, the noise compensation-based optical modulation device and method using a dark monitoring photodiode according to an embodiment of the present invention can accurately identify the operating point without being affected by fluctuations in the intensity of the input light source by using the ratio of input light to output light, thereby having the effect of enabling stable bias control.

[0034] In addition, the noise compensation-based optical modulation device and method using a dark monitoring photodiode according to an embodiment of the present invention symmetrically arranges a dark monitoring photodiode of the same specifications with light blocked around a monitoring photodiode, and compensates the measurement value of the monitoring photodiode using dark current and thermal noise information measured by the dark monitoring photodiode, thereby enabling precise measurement of the optical output ratio even in environments with high temperatures or severe temperature changes, and thereby has the effect of accurately detecting even minute optical drift to perform precise DC bias control.

[0035] In addition, the noise compensation-based optical modulation device and method using a dark monitoring photodiode according to an embodiment of the present invention has the effect of enabling precise noise compensation because the monitoring photodiode and the dark monitoring photodiode are fabricated together on the same silicon wafer using the same CMOS process and exposed to the same temperature environment, so the dark current and thermal noise measured in the dark monitoring photodiode are very similar to the noise generated in the monitoring photodiode.

[0036] In addition, the noise compensation-based optical modulation device and method using a dark monitoring photodiode according to an embodiment of the present invention has the effect of enabling more precise noise compensation by symmetrically arranging a plurality of dark monitoring photodiodes around a monitoring photodiode and averaging their measurement values, thereby mathematically neutralizing measurement errors caused by physical temperature gradients inside a silicon wafer.

[0037] In addition, the noise compensation-based optical modulation device and method using a dark monitoring photodiode according to an embodiment of the present invention can immediately obtain a predicted bias voltage by utilizing a lookup table composed of a plurality of bins classified according to drift characteristics, thereby enabling fast operation, and provides a self-revining function that detects changes in drift characteristics during operation and automatically switches to a suitable bin, thereby enabling stable operation throughout the lifespan without recalibration due to device aging. Brief explanation of the drawing

[0038] FIG. 1 is a configuration diagram of a pilot tone-based optical modulation device according to the prior art. FIG. 2 is a basic configuration diagram for optical output ratio-based DC bias control of an optical modulation device according to an embodiment of the present invention. FIG. 3 is a flowchart illustrating a method for controlling DC bias based on the optical output ratio of an optical modulation device according to an embodiment of the present invention. FIG. 4 is a diagram showing the arrangement configuration of a dark monitoring photodiode of an optical modulation device according to an embodiment of the present invention. FIG. 5 is an overall configuration diagram of a noise compensation-based optical modulation device using a dark monitoring photodiode according to an embodiment of the present invention. Specific details for implementing the invention

[0039] It should be noted that the technical terms used in this invention are used merely to describe specific embodiments and are not intended to limit the invention. Furthermore, unless specifically defined otherwise in this invention, the technical terms used in this invention should be interpreted in the sense generally understood by those skilled in the art to which this invention pertains, and should not be interpreted in an overly broad or overly narrow sense. Additionally, if a technical term used in this invention is an incorrect technical term that fails to accurately express the concept of the invention, it should be replaced with a technical term that can be correctly understood by those skilled in the art. Moreover, general terms used in this invention should be interpreted according to their prior definitions or the context, and should not be interpreted in an overly narrow sense.

[0040] Furthermore, singular expressions used in the present invention include plural expressions unless the context clearly indicates otherwise. Terms such as "composed of" or "comprising" in the present invention should not be interpreted as necessarily including all of the various components or steps described in the invention, and should be interpreted as meaning that some of the components or steps may not be included, or that additional components or steps may be included.

[0041] Additionally, terms including ordinal numbers, such as first, second, etc., used in the present invention may be used to describe components, but the components should not be limited by the terms. The terms are used solely for the purpose of distinguishing one component from another. For example, without departing from the scope of the present invention, the first component may be named the second component, and similarly, the second component may be named the first component.

[0042] Hereinafter, preferred embodiments according to the present invention will be described in detail with reference to the attached drawings. Identical or similar components are given the same reference number regardless of the drawing symbols, and redundant descriptions thereof will be omitted.

[0043] Furthermore, in describing the present invention, detailed descriptions of related prior art are omitted if it is determined that such descriptions could obscure the essence of the invention. Additionally, it should be noted that the attached drawings are intended only to facilitate an understanding of the concept of the present invention and should not be interpreted as limiting the concept of the present invention.

[0044] FIG. 1 is a configuration diagram of a pilot tone-based optical modulation device (10) according to the prior art.

[0045] As described above, a conventional optical modulation device (10) comprises a laser diode (11), an optical modulator (12), a tap (13), a monitoring photodiode (14), a synchronization detector (15), a control unit (16), and a pilot tone generator (17).

[0046] The laser diode (11) generates a continuous wave (CW) optical signal used in optical communication and outputs it to an optical modulator (12).

[0047] The optical modulator (12) modulates the light input from the laser diode (11) according to the transmission data signal (RF signal data for transmission) and outputs it. The optical modulator (12) may be configured as a Mach-Zehnder modulator (MZM) structure, distributes the input light into two waveguide paths, forms a phase difference between the two paths due to a change in refractive index according to the applied voltage, and then combines the light from the two paths to modulate the intensity of the light through an interference phenomenon according to the phase difference.

[0048] The pilot tone generator (17) generates a low-frequency dithering signal in the range of several kHz to tens of kHz. The low-frequency signal generated by the pilot tone generator (17) is superimposed on a DC bias voltage and applied to the optical modulator (12). This low-frequency signal serves to cause the operating point of the optical modulator (12) to vibrate slightly.

[0049] The control unit (16) outputs a DC bias voltage to maintain the operating point of the optical modulator (12) at an optimal position (e.g., a quadrature point). The DC bias voltage output from the control unit (16) is combined with a low-frequency dithering signal generated by the pilot tone generator (17) and applied to the optical modulator (12). That is, the DC bias voltage, the low-frequency dithering signal, and the transmission data signal are applied together to the optical modulator (12).

[0050] A tap (13) is placed at the output end of the optical modulator (12) to divert a portion of the output light (e.g., 1 to 5%). The tap (13) may be composed of an optical fiber coupler, a beam splitter, etc., and the diverted light is transmitted to a monitoring photodiode (14), and the remaining light is transmitted to a receiving end through an optical path.

[0051] The monitoring photodiode (14) receives light branched from the tap (13) and converts it into an electrical signal proportional to the light intensity. The electrical signal output from the monitoring photodiode (14) includes a modulated data component, a component due to DC bias, and a low-frequency component due to a pilot tone.

[0052] The synchronization detection unit (15) extracts and analyzes pilot tone components from an electrical signal received from a monitoring photodiode (14). The synchronization detection unit (15) may be configured to include a lock-in amplifier and a low-frequency filter, and separates and detects the fundamental (first harmonic) component and the second harmonic component of the pilot tone.

[0053] When the operating point of the optical modulator (12) is precisely located at the quadrature point, it corresponds to the symmetric point of the transmission characteristic curve, so the fundamental component of the pilot tone is minimized (ideally 0) and the second harmonic component is maximized. On the other hand, when the operating point deviates from the quadrature point, the fundamental component increases, and depending on the phase, it is possible to determine which direction the operating point has moved. The synchronization detection unit (15) uses this principle to provide a feedback signal to the control unit (16) to make the fundamental component 0, and the control unit (16) adjusts the DC bias voltage based on this to maintain the optimal operating point.

[0054] However, this conventional pilot tone-based optical modulation device (10) has a problem in that the low-frequency dithering signal generated by the pilot tone generator (17) and superimposed on the DC bias continuously vibrates the operating point of the optical modulator (12), causing minute fluctuations corresponding to the pilot tone frequency to occur in the output optical signal, which acts as jitter in the signal and degrades signal quality. In particular, in an ultra-high-speed communication environment of 50 Gbps or higher, such jitter becomes a factor that increases the bit error rate (BER).

[0055] Additionally, since the tap (13) and the monitoring photodiode (14) only measure the intensity of the output light, it is difficult to distinguish between the change in modulation efficiency caused by drift and the fluctuation in the intensity of the input light due to the unstable output of the laser diode (11). In other words, it is impossible to distinguish whether the decrease in output light intensity is due to a decrease in input light intensity or a decrease in modulation efficiency due to a shift in the operating point, so incorrect bias control may occur.

[0056] In addition, the monitoring photodiode (14) has a problem in that the measurement reliability and precision are rapidly reduced due to irregularly variable dark current and thermal noise when the high temperature environment or external temperature environment changes rapidly. Therefore, such measurement noise of the monitoring photodiode (14) causes the quality of bias control, which must operate precisely by identifying the cause of the change in output light intensity, to deteriorate.

[0057] TFLN devices can be used as such optical modulators (12), but TFLN devices have large variations in the manufacturing process, so drift characteristics (drift speed, temperature sensitivity, etc.) differ for each individual device. Therefore, in the conventional method, a burn-in test must be performed for a long time (e.g., several hours to tens of hours) to determine the drift characteristics of each device, which increases mass production costs.

[0058] Furthermore, during long-term operation, the drift characteristics themselves may change due to the aging of the device, but it is difficult to respond to this. TFLN modulators (122) generally exhibit saturation characteristics in which the drift speed is fast at the time of shipment, but decreases and stabilizes as time passes. However, since the conventional method does not have the function to adapt to such changes in drift characteristics, the control parameters set at the beginning may not be valid after long-term operation.

[0059] Meanwhile, since the phenomenon of the operating point changing due to temperature, characteristics, or degradation occurs identically in modulators using Bulk LiNbO3, InP, SiPh, etc., instead of TFLN, virtually all types of optical modulators require drift compensation; however, there were limitations that restricted accurate drift compensation, such as jitter caused by the conventional pilot tone method and thermal noise from monitoring photodiodes.

[0060] To solve the problems of the prior art, the present invention proposes an optical modulation device (100) capable of precise DC bias control even in high temperature or high temperature change environments by controlling the DC bias without a pilot tone based on the optical output ratio calculated by measuring the optical intensity at the input and output terminals of the optical modulator, and compensating the measured value of the monitoring photodiode with dark current and thermal noise information measured by symmetrically arranging a dark monitoring photodiode with light blocked around the monitoring photodiode used to calculate the optical output ratio.

[0061] Hereinafter, an optical modulation device (100) and an optical modulation method according to an embodiment of the present invention will be described in detail with reference to FIGS. 2 to 5. Specifically, FIGS. 2 and 3 describe the optical output ratio-based DC bias control configuration and operation method of the optical modulation device (100) of the present invention, and FIGS. 4 and 5 describe the noise compensation configuration using a dark monitoring photodiode, which is a key feature of the present invention, and its application.

[0062] Although the embodiments of the present invention describe the optical modulator based on a TFLN modulator that has a large drift characteristic deviation and is sensitive to temperature, the organic technical features of the present invention, such as optical output ratio-based DC bias control, lookup table-based drift prediction, lookup table bin switching, and thermal noise removal of the monitoring photodiode, can be commonly applied to optical modulators using devices such as Bulk LiNbO3, InP, and SiPh in addition to TFLN modulators to improve modulation quality, so the invention is not limited to optical modulators using specific materials.

[0063] FIG. 2 is a basic configuration diagram for optical output ratio-based DC bias control of an optical modulation device (100) according to an embodiment of the present invention.

[0064] The optical modulation device (100) of the present invention includes a basic configuration for controlling DC bias without a pilot tone based on an optical output ratio, and a noise compensation configuration using a dark monitoring photodiode, which will be described later in FIGS. 4 and FIGS. 5. To aid in understanding the optical modulation device (100) of the present invention, FIG. 2 first describes the basic configuration for DC bias control based on an optical output ratio.

[0065] As described above, the optical modulation device (100) of the present invention comprises a laser diode (110), a modulation unit (120), an input tap (131), an output tap (132), an input monitoring photodiode (141), an output monitoring photodiode (142), a composite modulation control unit (150), and a digital-to-analog converter (160).

[0066] A laser diode (110) is a light source that generates and outputs an optical signal used in optical communication. The laser diode (110) can be composed of a distributed feedback laser diode (DFB-LD), a Fabry-Perot laser diode (FP-LD), a vertical resonant plane emitting laser (VCSEL), etc., and can output continuous wave light from one of several optical communication wavelength bands including 1310 nm and 1550 nm. The optical signal output from the laser diode (110) is transmitted to the modulation unit (120) via an input tap (131).

[0067] The modulation unit (120) includes a TFLN modulator (122) and a temperature sensor (121).

[0068] The TFLN modulator (122) modulates and outputs light input from the laser diode (110) according to the DC bias voltage and transmission data (RF data signal). The TFLN modulator (122) according to an embodiment of the present invention may be configured as a Mach-Zehnder modulator structure fabricated based on a thin-film lithium niobate material.

[0069] The TFLN modulator (122) may include an optical splitter that evenly distributes input light into two waveguide paths, a phase modulation arm placed in each path that changes the refractive index according to the applied voltage, and an optical coupler that combines the light from the two paths. The TFLN modulator (122) uses the electro-optical effect (Pockels Effect) to change the refractive index of each path according to the applied voltage, and modulates the intensity of the light by generating constructive or destructive interference through the resulting phase difference.

[0070] Compared to conventional bulk lithium niobate modulators, the TFLN modulator (122) has the advantage of being smaller in size to a few millimeters, having a lower driving voltage to a few volts, and being capable of ultra-high-speed modulation of 100 GHz or more. However, due to the characteristics of the thin film structure, DC drift caused by temperature changes and charge accumulation occurs sensitively, so precise bias control is required to compensate for this. In the present invention, this is performed by the composite modulation control unit (150).

[0071] A DC bias voltage for setting the operating point and transmission data corresponding to the data to be transmitted are applied as voltages to the TFLN modulator (122). The DC bias voltage serves to set the operating point of the TFLN modulator (122) to an optimal position on the transmission characteristic curve (e.g., a quadrature point), and the transmission data serves to modulate the intensity of light around the set operating point and transmit the data by carrying it on an optical signal.

[0072] The temperature sensor (121) of the modulation unit (120) measures the temperature of the TFLN modulator (122) in real time and provides it to the composite modulation control unit (150). The temperature sensor (121) may be composed of an NTC thermistor, a PTC thermistor, a thermocouple, an RTD, a semiconductor temperature sensor, etc. The temperature sensor (121) is placed in close proximity to the chip surface or inside the package of the TFLN modulator (122) to accurately measure the actual operating temperature of the TFLN modulator (122).

[0073] The input tap (131) is placed in the optical path between the laser diode (110) and the modulation unit (120) to divert a portion of the input light. The input tap (131) may be composed of a fiber optic coupler, a beam splitter, a directional coupler, etc. The input tap (131) diverts 1 to 3 percent of the input light and transmits it to an input monitoring photodiode (141), and transmits the remaining light to a TFLN modulator (122) of the modulation unit (120).

[0074] The output tap (132) is placed in the output optical path of the modulation unit (120) to divert a portion of the output light. Like the input tap (131), the output tap (132) can be composed of a fiber optic coupler, a beam splitter, a directional coupler, etc. The output tap (132) diverts 1 to 3 percent of the output light and transmits it to an output monitoring photodiode (142), and the remaining light is transmitted to a receiver through an optical path.

[0075] The input monitoring photodiode (141) receives light branched from the input tap (131) and measures the intensity of the input light. The input monitoring photodiode (141) may be composed of a PIN photodiode, an APD (Avalanche Photodiode), etc., and outputs a current or voltage signal proportional to the intensity of the incident light.

[0076] The output monitoring photodiode (142) receives light branched from the output tap (132) and measures the intensity of the output light. The output monitoring photodiode (142) may also be composed of a PIN photodiode, an APD, etc., and outputs a current or voltage signal proportional to the intensity of the modulated output light.

[0077] The composite modulation control unit (150) calculates the optical output ratio based on the ratio of the measured values ​​of the input monitoring photodiode (141) and the output monitoring photodiode (142), obtains a predicted bias voltage by referring to the lookup table (152), calculates a DC bias voltage based on the predicted bias voltage and the optical output ratio, and applies it to the modulation unit (120) through the digital-to-analog converter (160).

[0078] The composite modulation control unit (150) can be implemented as a combination of hardware such as an MCU (Micro Controller Unit), FPGA (Field Programmable Gate Array), ASIC (Application Specific Integrated Circuit), DSP (Digital Signal Processor) and firmware or software running on said hardware. The composite modulation control unit (150) stores a lookup table (152) and setting parameters in non-volatile memory and processes analog signals of an input monitoring photodiode (141), an output monitoring photodiode (142), and a temperature sensor (121) by converting them into digital data through an analog-to-digital converter.

[0079] In an embodiment of the present invention, the composite modulation control unit (150) includes a dual tap diagnostic unit (151), a lookup table (152), and a bias determination unit (153).

[0080] The dual tap diagnostic unit (151) receives the input light intensity (P_in) measured by the input monitoring photodiode (141) and the output light intensity (P_out) measured by the output monitoring photodiode (142), and calculates the light output ratio (R) in real time.

[0081] The optical output ratio (R) can be calculated by the formula 'R = P_out / P_in' and may be modified depending on the optical ratio of the optical path or other conditions.

[0082] The core principle of bias control using the optical output ratio (R) is as follows. When the operating point of the TFLN modulator (122) is fixed at a specific position (e.g., a quadrature point), the optical output ratio remains constant because the input light intensity and the output light intensity change at the same rate even if the output of the laser diode (110) changes. However, if the operating point moves due to DC drift, the modulation efficiency changes and the optical output ratio changes; therefore, by monitoring the optical output ratio, the movement of the operating point can be accurately detected regardless of the change in the input light source.

[0083] Meanwhile, there is a possibility that error situations may frequently occur where the DC bias cannot be properly generated in response to the direction of movement of the drift, the movement of the operating point due to the change in the characteristics of the drift, or the rapid change in modulation efficiency due to changes in the external environment, and there is a limitation that errors accumulate or the convergence of the operating point due to the bias voltage is slow because it operates only in a feedback manner. Therefore, in the present invention, in addition to the optical output ratio, the drift characteristics confirmed for the TFLN modulator (122) and the expected DC bias value considering these drift characteristics are checked and used as a standard to generate the DC bias control voltage to be applied to the TFLN modulator (122), thereby enabling the DC bias to be applied to the TFLN modulator (122) to be generated stably without error in a complex manner by using a lookup table (152) in which drift characteristic information of the TFLN modulator (122) is stored.

[0084] The lookup table (152) is a data table in which DC bias voltages are stored according to the optical output ratio, temperature, and voltage application time, and is composed of multiple bins in which devices with similar drift characteristics are grouped based on test data for multiple sample TFLN modulators. Each bin stores DC bias voltage values ​​in a three-dimensional grid structure of temperature, voltage application time, and optical output ratio.

[0085] The process of creating the lookup table (152) is described as follows.

[0086] First, prepare multiple TFLN modulator samples produced from the same process. It is desirable to use multiple samples to obtain statistically significant results.

[0087] Next, drift characteristics are measured for each sample while varying the temperature and voltage application time. For example, the temperature of the operating range is varied stepwise, and the change in bias voltage according to the voltage application time at each temperature is recorded. In addition, the DC bias voltage required to maintain the target optical output ratio under each condition is measured and mapped.

[0088] Subsequently, the collected data is analyzed to group samples with similar drift characteristics. Clustering algorithms such as k-means clustering, hierarchical clustering, and Gaussian mixture models can be applied for grouping. For example, samples can be classified into groups based on drift speed or groups with similar drift change patterns.

[0089] For each group, information is calculated by mapping a DC bias voltage (predicted bias voltage) to maintain an operating point under corresponding conditions to each grid using a three-dimensional grid structure of temperature, voltage application time, and optical output ratio, and this group-specific mapping information is divided into multiple bins and stored in a lookup table (152) of a composite modulation control unit (150).

[0090] Meanwhile, when an individual TFLN modulator (122) is produced, a short-term (e.g., 1 minute) burn-in process is performed to measure the drift slope (rate of change of bias voltage per hour). Taking into account that the drift of the TFLN device is caused by charge accumulation and redistribution phenomena, and that the time constant of this phenomenon is determined by the physical characteristics of the device, the long-term drift characteristics are predicted through a short-term initial test. That is, there is a correlation between the initial drift slope and the long-term drift characteristics, and this correlation can be modeled through regression analysis, neural network models (MLP, RNN, LSTM, etc.). Based on the drift slope measured in the initial test, the bin that best matches the drift characteristics of the device among the multiple bins of the lookup table (152) is selected, and the identification information of the selected bin is stored in non-volatile memory.

[0091] The bias determination unit (153) determines the DC bias voltage by referring to the lookup table (152) based on the optical output ratio calculated by the dual tap diagnosis unit (151), temperature information measured by the temperature sensor (121), and voltage application time information accumulated and recorded by itself.

[0092] The bias determination unit (153) includes a LUT Bin selection unit (153a), a bias calculation unit (153b), and a rebining unit (153c).

[0093] The LUT Bin selection unit (153a) manages initial selection bin information stored in non-volatile memory. When power is applied to the optical modulation device (100), the LUT Bin selection unit (153a) reads the stored bin information and sets the bin to be used currently. Additionally, when a bin change request is received from the rebining unit (153c) described later, the current bin is changed to a new bin, and the changed bin information is updated and stored in non-volatile memory.

[0094] The bias calculation unit (153b) obtains a predicted bias voltage (V_predicted) by referring to the lookup table of the bin selected in the LUT Bin selection unit (153a) based on the optical output ratio (R) calculated by the dual tap diagnosis unit (151), the current temperature (T) measured by the temperature sensor (121), and the voltage application time (t) accumulated and recorded by itself. If the coordinate values ​​do not exactly match the grid points, the predicted bias voltage can be calculated by obtaining information for interpolation and applying linear interpolation, trilinear interpolation, etc.

[0095] The bias calculation unit (153b) calculates a feedback correction value based on the difference between the current actual light output ratio (R_actual) and the target light output ratio (R_target) based on the predicted bias voltage, and calculates the final bias voltage (V_final) as follows.

[0096] V_final = V_predicted + K × (R_target - R_actual)

[0097] Here, K is the feedback gain, a parameter that determines the response speed and stability of the control loop.

[0098] The bias calculation unit (153b) transmits the calculated final bias voltage in digital form to the digital-to-analog converter (160).

[0099] In this way, when the predicted bias voltage using the lookup table (152) is checked and used as a reference, the bias voltage to be applied at the current temperature can be obtained immediately without a voltage scan process during initial operation, enabling fast operation, and when disturbances such as sudden temperature changes occur, feed-forward control is possible to preemptively adjust the bias voltage based on the lookup table (152) without feedback delay, thereby improving the response speed.

[0100] In addition, although it is difficult to determine the drift direction based solely on the light output ratio, the drift direction can be identified by comparing the predicted value of the lookup table (152) with the actual value, thereby preventing malfunction.

[0101] Meanwhile, in the case of the TFLN modulator (122), the initial drift characteristics are stabilized by performing a long burn-in process, and since the drift characteristics change due to degradation from use, the selected bin of the lookup table (152) based on the drift characteristics measured immediately after mass production cannot be continuously used. Therefore, in the present invention, the rebining unit (153c) enables automatic switching to a bin that matches the current drift characteristics among the various bins configured in the lookup table (152).

[0102] The rebining unit (153c) monitors the error between the predicted bias voltage of the lookup table (152) and the final bias voltage actually calculated in the bias calculation unit (153b), and if the error exceeds a reference value, it performs a self-rebining function of reanalyzing the drift slope and switching from the current bin to another bin.

[0103] Specifically, the rebining unit (153c) continuously monitors the error between the predicted bias voltage (V_predicted) and the actual calculated final bias voltage (V_final), but if the error exceeds a preset threshold for a certain period of time or if the error accumulates and exceeds a certain standard, it determines that the drift characteristics according to the selected bin of the current lookup table (152) do not match the characteristics of the actual device.

[0104] In this case, the rebining unit (153c) calculates the currently measured drift slope and compares it with the representative drift slope of each bin to select a candidate bin with the most similar drift characteristics and switches the bin to use.

[0105] Meanwhile, the rebining unit (153c) may apply a hysteresis technique to prevent a sudden change in bias voltage during bin switching. That is, a method of gradually switching by mixing the value of the previous bin and the value of the new bin according to weights may be used.

[0106] Additionally, the rebining unit (153c) can analyze bin switching patterns (switching cycle, switching direction, etc.) and generate an alarm signal when a pattern indicating that the device lifespan is imminent is detected, such as reaching the final bin or the switching cycle becoming shorter.

[0107] The digital-to-analog converter (160) converts the DC bias voltage information (digital value) determined by the bias determination unit (153) into an analog voltage and applies it to the TFLN modulator (122) of the modulation unit (120). That is, in the optical modulation device (100) of the present invention, unlike the conventional pilot tone method, a pure DC bias voltage in which pilot tone signals are not superimposed is applied to the TFLN modulator (122), so no jitter is generated.

[0108] FIG. 3 is a flowchart illustrating a DC bias control method based on the optical output ratio of an optical modulation device (100) according to an embodiment of the present invention.

[0109] The optical output ratio-based DC bias control method of the optical modulation device (100) of the present invention is broadly divided into a lookup table generation step, a bin selection step, and an operation step.

[0110] First, in the lookup table generation step, drift characteristics are measured from multiple TFLN modulator samples to generate a lookup table (152) composed of multiple bins.

[0111] Specifically, multiple TFLN modulator samples produced in the same process are prepared, and drift characteristics are measured for each sample while varying the temperature and voltage application time. For example, the temperature is varied from -40°C to 80°C in 5°C increments, and the amount of change in bias voltage according to the voltage application time and the DC bias voltage required to maintain the target optical output ratio are recorded at each temperature. Alternatively, the amount of change in bias voltage and the DC bias voltage required to maintain the target optical output ratio may be recorded while varying the temperature in accordance with the progress of the voltage application time.

[0112] Collected drift data is analyzed to group samples with similar drift characteristics. Clustering algorithms such as k-means clustering, hierarchical clustering, and Gaussian mixture models can be applied for grouping. For example, based on drift speed, samples can be classified into very slow group (Bin 1), slow group (Bin 2), normal group (Bin 3), fast group (Bin 4), and very fast group (Bin 5).

[0113] For each group, individual bins are created by mapping DC bias voltage values ​​to a three-dimensional grid structure of temperature, voltage application time, and optical output ratio. A lookup table (152) composed of multiple created bins is stored in the non-volatile memory of the composite modulation control unit (150).

[0114] Next, in the bin selection step, an initial test is performed on individual TFLN modulators (122) to select a suitable bin, and the selection information of the bin is stored in the composite modulation control unit (150).

[0115] Specifically, a short-term (e.g., 1 minute) burn-in process is performed on the individual TFLN modulator (122) produced, and the drift slope (rate of change of bias voltage per hour) is measured. Based on the measured drift slope, the bin that best matches the drift characteristics of the corresponding device is selected from among a plurality of bins of the lookup table (152). The identification information of the selected bin is stored in the non-volatile memory of the composite modulation control unit (150) and shipped.

[0116] Subsequently, during the operation phase, the DC bias is controlled based on the optical output ratio, and self-revining is performed if necessary.

[0117] When power is applied to the optical modulation device (100), the LUT Bin selection unit (153a) of the composite modulation control unit (150) reads the initial selection bin information stored in the non-volatile memory and sets the bin to be used currently.

[0118] When the laser diode (110) starts operating, light branched from the input tap (131) is transmitted to the input monitoring photodiode (141) to measure the input light intensity (P_in). At the same time, light output from the modulation unit (120) and branched from the output tap (132) is transmitted to the output monitoring photodiode (142) to measure the output light intensity (P_out).

[0119] The dual tap diagnostic unit (151) receives the input light intensity (P_in) and the output light intensity (P_out) and calculates the light output ratio (R = P_out / P_in).

[0120] The bias calculation unit (153b) obtains a predicted bias voltage (V_predicted) by referring to a lookup table of a selected bin based on the calculated optical output ratio (R), the current temperature (T) measured by the temperature sensor (121), and the voltage application time (t) accumulated and recorded by itself.

[0121] The bias calculation unit (153b) calculates the final bias voltage (V_final) by performing feedback correction based on the difference between the current measured optical output ratio and the target optical output ratio based on the predicted bias voltage.

[0122] The calculated final bias voltage is converted into an analog voltage through a digital-to-analog converter (160) and applied to a TFLN modulator (122). In the present invention, a pure DC bias voltage is applied without superimposing pilot tones.

[0123] The rebining unit (153c) continuously monitors the error between the predicted bias voltage and the actual calculated bias voltage. If the error continues to exceed a reference value, the rebining unit (153c) reanalyzes the drift slope and switches from the current bin to another bin. When switching bins, to prevent a sudden change in bias voltage, the values ​​of the previous bin and the new bin are mixed according to weights and switched gradually.

[0124] The above-described processes of calculating the optical output ratio, obtaining the predicted bias voltage, calculating the final bias voltage, and monitoring the error are continuously repeated while the optical modulation device (100) is in operation to maintain the optimal operating point of the TFLN modulator (122).

[0125] As such, the optical modulation device (100) of the present invention controls the DC bias without a pilot tone based on the optical output ratio, thereby solving the jitter problem of the conventional pilot tone method and the problem of distinguishing input light source variations. However, since the measurement accuracy of the input monitoring photodiode (141) and the output monitoring photodiode (142) determines the precision of the optical output ratio, the measurement accuracy of the input monitoring photodiode (141) and the output monitoring photodiode (142) is a very important factor.

[0126] In particular, an optical modulation device (100) with a TFLN modulator (122) applied is often operated in environments with high temperatures (e.g., 85 degrees Celsius or higher) or severe temperature fluctuations, such as data centers, and in such environments, serious dark current and thermal noise occur in the input monitoring photodiode (141) and output monitoring photodiode (142). To solve the problem of reduced measurement accuracy in such high-temperature or severe temperature fluctuation environments, the present invention further includes a noise compensation configuration in which a dark monitoring photodiode with light blocked is symmetrically placed around the input monitoring photodiode (141) and output monitoring photodiode (142) as shown in FIGS. 4 and 5 described later, and the measurement values ​​of the input monitoring photodiode (141) and output monitoring photodiode (142) are compensated with the measurement values ​​of the dark monitoring photodiode.

[0127] FIG. 4 is a diagram showing the arrangement configuration of a dark monitoring photodiode of a light modulation device (100) according to an embodiment of the present invention.

[0128] As described above, in the present invention, an input monitoring photodiode (141), an output monitoring photodiode (142), an input dark current measuring unit (141a), and an output dark current measuring unit (142a) are integrated together on a silicon wafer (50), and a TFLN modulator (122), an input tap (131), and an output tap (132) are mounted on the silicon wafer (50).

[0129] The silicon wafer (50) serves as a substrate for an optical integrated circuit manufactured by a silicon photonics (SiPh) process, and an input monitoring photodiode (141), an output monitoring photodiode (142), an input dark current measuring unit (141a), and an output dark current measuring unit (142a) are manufactured together by the same CMOS process. A Transimpedance Amplifier (TIA) that amplifies and converts the output current of the diode into a voltage may be configured adjacently (not shown) by the same CMOS process in each of the input monitoring photodiode (141), the output monitoring photodiode (142), the input dark current measuring unit (141a), and the output dark current measuring unit (142a).

[0130] The input dark current measuring unit (141a) includes a plurality of input dark monitoring photodiodes (Ti1, Ti2, Ti3, Ti4) symmetrically arranged around an input monitoring photodiode (141) with light blocked.

[0131] For example, as illustrated in FIG. 4, the input dark current measuring unit (141a) may be configured with a structure in which a total of four input dark monitoring photodiodes (Ti1, Ti2, Ti3, Ti4) are symmetrically arranged, one each on the upper left, upper right, lower left, and lower right sides of the input monitoring photodiode (141), with the input monitoring photodiode (141) at the center.

[0132] The input dark monitoring photodiodes (Ti1, Ti2, Ti3, Ti4) are fabricated together on the same silicon wafer (50) using the same CMOS process as the input monitoring photodiode (141) and have the same specifications as the input monitoring photodiode (141) (e.g., same area, same PN junction structure, same doping concentration, etc.). However, unlike the input monitoring photodiode (141), the input dark monitoring photodiodes (Ti1, Ti2, Ti3, Ti4) are configured with a light-blocking structure to prevent light from being incident.

[0133] For example, a light-blocking film formed of a metal thin film (e.g., aluminum, tungsten, etc.) may be laminated over the light-active area of ​​the input dark monitoring photodiodes (Ti1, Ti2, Ti3, Ti4) to block an external light source. Additionally, the input dark monitoring photodiodes (Ti1, Ti2, Ti3, Ti4) may be positioned at a distance greater than a certain distance from the optical waveguide so as not to be optically coupled with the optical waveguide.

[0134] Since the input dark monitoring photodiodes (Ti1, Ti2, Ti3, Ti4) are in a state where light is blocked, the current measured by the input dark monitoring photodiodes (Ti1, Ti2, Ti3, Ti4) is not a photocurrent generated by light, but a current generated purely by dark current and thermal noise. Since the input dark monitoring photodiodes (Ti1, Ti2, Ti3, Ti4) are manufactured together on the same silicon wafer (50) with the same specifications as the input monitoring photodiode (141) and are exposed to the same temperature environment, the dark current and thermal noise measured by the input dark monitoring photodiodes (Ti1, Ti2, Ti3, Ti4) are very similar to the dark current and thermal noise generated by the input monitoring photodiode (141).

[0135] Therefore, by subtracting the current value measured at the input dark monitoring photodiodes (Ti1, Ti2, Ti3, Ti4) from the measurement value of the input monitoring photodiode (141), the noise components caused by dark current and thermal noise included in the measurement value of the input monitoring photodiode (141) can be effectively removed, and only the pure photocurrent component can be extracted.

[0136] Likewise, the output dark current measuring unit (142a) includes a plurality of output dark monitoring photodiodes (To1, To2, To3, To4) symmetrically arranged around the output monitoring photodiode (142) with light blocked.

[0137] For example, the output dark current measuring unit (142a) may be configured with a structure in which a total of four output dark monitoring photodiodes (To1, To2, To3, To4) are symmetrically arranged, one each on the upper left, upper right, lower left, and lower right sides of the output monitoring photodiode (142), with the output monitoring photodiode (142) at the center.

[0138] The output dark monitoring photodiodes (To1, To2, To3, To4) are also manufactured together on the same silicon wafer (50) using the same CMOS process as the output monitoring photodiodes (142), just like the input dark monitoring photodiodes (Ti1, Ti2, Ti3, Ti4), and have a light-blocking structure configured with the same specifications as the output monitoring photodiodes (142).

[0139] The technical significance of the structure in which the input monitoring photodiode (141) is centered and the input dark monitoring photodiodes (Ti1, Ti2, Ti3, Ti4) are symmetrically arranged, and the output monitoring photodiode (142) is centered and the output dark monitoring photodiodes (To1, To2, To3, To4) are symmetrically arranged is as follows.

[0140] A physical temperature gradient may exist inside the silicon wafer (50) due to the heat generated by the TFLN modulator (122), the heat generated by the laser diode (110), and the external environment temperature distribution. This temperature gradient means that the temperature is distributed differently depending on the location inside the silicon wafer (50), and accordingly, even within the same silicon wafer (50), the magnitude of dark current and thermal noise may vary depending on the location.

[0141] If the input dark monitoring photodiode is simply placed on only one side of the input monitoring photodiode (141), a measurement error may occur in which the dark current and thermal noise measured by the temperature difference between the location of the input monitoring photodiode (141) and the location of the input dark monitoring photodiode differ somewhat from the actual noise of the input monitoring photodiode (141).

[0142] However, as in the present invention, if the input monitoring photodiode (141) is placed at the center and the input dark monitoring photodiodes (Ti1, Ti2, Ti3, Ti4) are symmetrically arranged and their measurements are averaged, the temperature difference between the symmetrically arranged positions is canceled out, and the average value is very close to the temperature at the position of the input monitoring photodiode (141). That is, by averaging the measurements of the symmetrically arranged input dark monitoring photodiodes (Ti1, Ti2, Ti3, Ti4), the measurement error caused by the physical temperature gradient inside the silicon wafer (50) can be mathematically neutralized.

[0143] For example, if the positions of the input dark monitoring photodiodes (Ti1, Ti2, Ti3, Ti4) are symmetrically arranged at (-x, +y), (+x, +y), (-x, -y), and (+x, -y) respectively in a two-dimensional coordinate system with the position of the input monitoring photodiode (141) as the origin, and assuming that the temperature distribution inside the silicon wafer (50) changes linearly with respect to position (x, y), the temperature deviations in the four input dark monitoring photodiodes (Ti1, Ti2, Ti3, Ti4) cancel each other out when the average is taken, and become close to zero. Therefore, the average of the measurements of the input dark monitoring photodiodes (Ti1, Ti2, Ti3, Ti4) becomes a value very similar to the dark current and thermal noise at the position of the input monitoring photodiode (141).

[0144] This symmetrical arrangement and average calculation method is applied equally between the output monitoring photodiode (142) and the output dark monitoring photodiodes (To1, To2, To3, To4).

[0145] Meanwhile, although the present invention illustrates an example in which four input dark monitoring photodiodes (Ti1, Ti2, Ti3, Ti4) and four output dark monitoring photodiodes (To1, To2, To3, To4) are arranged, it is not limited thereto. The number of input dark monitoring photodiodes and output dark monitoring photodiodes can be changed to 2, 6, 8, etc., respectively, and the arrangement method can also be diverse, such as a cross arrangement or a radial arrangement. When a larger number of dark monitoring photodiodes are arranged to improve statistical reliability, the average of the measured values ​​becomes more stabilized, and the effect of improving the precision of noise compensation can be obtained.

[0146] Furthermore, since the measurements of the dark monitoring photodiodes exhibit a pattern according to temperature, the estimated temperature (wafer temperature distribution) based on the measurements of multiple dark monitoring photodiodes (Ti1, Ti2, Ti3, Ti4, To1, To2, To3, To4), the temperature of the modulator estimated based on the arrangement configuration with the modulator on the wafer (for verifying the temperature sensor of the modulation part), and the temperature of the monitoring photodiode placed at the center of the multiple dark monitoring photodiodes (environment of the measurement sensor) can be estimated, and thus these temperature information can be additionally utilized as inputs for a lookup table.

[0147] FIG. 5 is an overall configuration diagram of a noise compensation-based thin-film lithium niobate optical modulation device (hereinafter referred to as the 'optical modulation device') (100) using a dark monitoring photodiode according to an embodiment of the present invention.

[0148] As described, the optical modulation device (100) of the present invention is configured such that, in addition to the basic configuration for optical output ratio-based DC bias control described with reference to FIG. 2, the arrangement configuration of the dark monitoring photodiode described with reference to FIG. 4 and the noise compensation configuration accordingly are added.

[0149] Specifically, the optical modulation device (100) of the present invention comprises a laser diode (110), a modulation unit (120), an input tap (131), an output tap (132), an input monitoring photodiode (141), an output monitoring photodiode (142), an input dark current measuring unit (141a), an output dark current measuring unit (142a), a composite modulation control unit (150), and a digital-to-analog converter (160).

[0150] The laser diode (110), modulation unit (120), input tap (131), output tap (132), input monitoring photodiode (141), output monitoring photodiode (142), and digital-to-analog converter (160) are the same as those described above with reference to FIG. 2, so a redundant description is omitted.

[0151] As described with reference to FIG. 4, the input dark current measuring unit (141a) includes a plurality of input dark monitoring photodiodes symmetrically arranged around the input monitoring photodiode (141) with light blocked, and provides the measurement values ​​of the input dark monitoring photodiodes to the composite modulation control unit (150).

[0152] As described with reference to FIG. 4, the output dark current measuring unit (142a) includes a plurality of output dark monitoring photodiodes symmetrically arranged around the output monitoring photodiode (142) with light blocked, and provides the measured values ​​of the output dark monitoring photodiodes to the composite modulation control unit (150).

[0153] As illustrated in FIG. 5, the composite modulation control unit (150) of the optical modulation device (100) of the present invention further includes a noise detection unit (154) in the configuration of the composite modulation control unit (150) of FIG. 2, and the dual tap diagnosis unit (151) of FIG. 2 is replaced by a dual tap diagnosis unit (155) with an added noise compensation function. That is, the composite modulation control unit (150) of the optical modulation device (100) of the present invention includes a noise detection unit (154), a dual tap diagnosis unit (155), a lookup table (152), and a bias determination unit (153).

[0154] The noise detection unit (154) receives measurement values ​​from multiple input dark monitoring photodiodes of the input dark current measurement unit (141a) to determine input noise compensation values, and receives measurement values ​​from multiple output dark monitoring photodiodes of the output dark current measurement unit (142a) to determine output noise compensation values.

[0155] For example, the noise detection unit (154) can calculate an input noise compensation value (N_in) by averaging the measurement values ​​of multiple input dark monitoring photodiodes of the input dark current measurement unit (141a), and calculate an output noise compensation value (N_out) by averaging the measurement values ​​of multiple output dark monitoring photodiodes of the output dark current measurement unit (142a).

[0156] The noise detection unit (154) calculates the average of the measurements of the input dark monitoring photodiode as the input noise compensation value and calculates the average of the measurements of the output dark monitoring photodiode as the output noise compensation value, thereby mathematically neutralizing the measurement error caused by the physical temperature gradient inside the silicon wafer (50) as described with reference to FIG. 4.

[0157] In addition, for example, the noise detection unit (154) may apply a method of calculating the standard deviation of the measurements of the input dark monitoring photodiode and the output dark monitoring photodiode, not just a simple average, to detect and remove outliers, and then calculate the average. That is, the noise detection unit (154) determines that measurements that deviate from the average by more than a certain standard deviation among the measurements of the input dark monitoring photodiode and the output dark monitoring photodiode are outliers and excludes them, and then determines the average of the remaining measurements as the noise compensation value, thereby minimizing the impact of measurement errors caused by defects or disturbances in some input dark monitoring photodiodes or some output dark monitoring photodiodes.

[0158] Additionally, the noise detection unit (154) may reduce the influence of temporary measurement noise by calculating the input noise compensation value and the output noise compensation value as a moving average or exponentially weighted moving average over a certain period of time, rather than as simple immediate values.

[0159] The dual tap diagnostic unit (155) compensates the measurement values ​​of the input monitoring photodiode (141) and the output monitoring photodiode (142) using the input noise compensation value and the output noise compensation value calculated by the noise detection unit (154), and calculates the optical output ratio (R) in real time based on the compensated measurement values.

[0160] For example, the dual tap diagnostic unit (155) calculates a compensated input light intensity (P_in_comp) by subtracting the input terminal noise compensation value (N_in) calculated by the noise detection unit (154) from the input light intensity (P_in) measured by the input monitoring photodiode (141).

[0161] P_in_comp = P_in - N_in

[0162] Additionally, the dual tap diagnostic unit (155) calculates a compensated output light intensity (P_out_comp) by subtracting the output terminal noise compensation value (N_out) calculated by the noise detection unit (154) from the output light intensity (P_out) measured by the output monitoring photodiode (142).

[0163] P_out_comp = P_out - N_out

[0164] Afterwards, the dual tap diagnostic unit (155) calculates the light output ratio (R) by dividing the compensated output light intensity by the compensated input light intensity.

[0165] R = P_out_comp / P_in_comp

[0166] The optical output ratio (R) calculated by the dual-tap diagnostic unit (155) is a value calculated based only on the pure photocurrent component from which dark current and thermal noise components included in the measurements of the input monitoring photodiode (141) and the output monitoring photodiode (142) have been removed, so even minute optical drift can be accurately detected even in environments with high temperatures or severe temperature changes.

[0167] The dual tap diagnostic unit (155) provides the compensated optical output ratio (R) to the bias determination unit (153).

[0168] The operation of the lookup table (152), the bias determination unit (153), and the LUT Bin selection unit (153a), bias calculation unit (153b), and rebining unit (153c) included therein is the same as described above with reference to FIG. 2. However, the bias calculation unit (153b) differs in that it obtains a predicted bias voltage and calculates a final bias voltage based on the compensated optical output ratio (R) calculated by the dual tap diagnosis unit (155), rather than the optical output ratio calculated by the dual tap diagnosis unit (151).

[0169] That is, the optical modulation device (100) of FIG. 5 performs the same optical output ratio-based DC bias control operation as the optical modulation device (100) of FIG. 2, but when calculating the optical output ratio, the measured value of the monitoring photodiode is compensated with dark current and thermal noise information measured through the dark monitoring photodiode to calculate the compensated optical output ratio, thereby enabling precise DC bias control even in environments with high temperatures or severe temperature changes.

[0170] In the operation phase of the optical modulation device (100), when power is applied to the optical modulation device (100), the LUT Bin selection unit (153a) reads the initial selection bin information stored in the non-volatile memory and sets the bin to be used currently. When the laser diode (110) starts operating, the input monitoring photodiode (141) and the output monitoring photodiode (142) each measure the input light intensity (P_in) and the output light intensity (P_out), respectively, and at the same time, the multiple input dark monitoring photodiodes (Ti1, Ti2, Ti3, Ti4) of the input dark current measurement unit (141a) and the multiple output dark monitoring photodiodes (To1, To2, To3, To4) of the output dark current measurement unit (142a) each measure the dark current and current caused by thermal noise at the input and output terminals, respectively. The noise detection unit (154) calculates an input noise compensation value and an output noise compensation value by averaging the measured values ​​of the dark monitoring photodiode, and the dual tap diagnosis unit (155) calculates the compensated light output ratio after compensating the measured values ​​of the monitoring photodiode with the noise compensation value. Subsequently, the bias calculation unit (153b) obtains a predicted bias voltage and calculates a final bias voltage by referring to the lookup table (152) based on the compensated light output ratio, temperature, and voltage application time, and the rebining unit (153c) monitors the error between the predicted bias voltage and the actual calculated bias voltage and switches bins if necessary.

[0171] As described above, the optical modulation device (100) according to the embodiment of the present invention controls the DC bias without a pilot tone based on the optical output ratio calculated from the measurement results of the input monitoring photodiode (141) and the output monitoring photodiode (142), and calculates a precise optical output ratio by compensating for noise caused by dark current and thermal noise of the input monitoring photodiode (141) and the output monitoring photodiode (142) using the measurement value of a dark monitoring photodiode symmetrically placed around the input monitoring photodiode (141) and the output monitoring photodiode (142), thereby enabling accurate detection of even minute optical drift even in environments with high temperatures or severe temperature changes, thus increasing the yield of the optical modulation device (100) and significantly improving the modulation quality.

[0172] In particular, the optical modulation device (100) of the present invention can be commonly applied to various optical modulators using devices such as Bulk LiNbO3, InP, and SiPh in addition to TFLN modulators to improve modulation quality.

[0173] A person skilled in the art to which the present invention pertains will be able to make modifications and variations to the foregoing without departing from the essential characteristics of the present invention. Accordingly, the embodiments disclosed in the present invention are intended to explain, not limit, the technical concept of the present invention, and the scope of the technical concept of the present invention is not limited by these embodiments. The scope of protection of the present invention shall be interpreted by the claims below, and all technical concepts within an equivalent scope shall be interpreted as being included within the scope of rights of the present invention.

[0174] Furthermore, the optical modulation device may be implemented as a hardware component, a software component, and / or a combination of the hardware component and the software component.

[0175] Additionally, the components described in the embodiments may be implemented using one or more general-purpose computers or special-purpose computers, such as, for example, a processor, a controller, an arithmetic logic unit (ALU), a digital signal processor, a microcomputer, a field programmable gate array (FPGA), a programmable logic unit (PLU), a microprocessor, or any other device capable of executing and responding to instructions.

[0176] Software may include a computer program, code, instructions, or a combination of one or more of these, and may command the optical modulation device to operate as desired, either independently or collectively.

[0177] The various devices and components described in this specification may be implemented by hardware circuits (e.g., semiconductor-based logic circuits), optical systems (e.g., lenses, optical fibers, optical benches, semiconductor-based optical paths, etc.), firmware, software, or a combination thereof. For example, they may be implemented using transistors, logic gates, and electronic circuits in the form of various electrical structures, and lenses, optical paths, optical interfaces, filters, etc. in the form of optical structures. Explanation of the symbols

[0178] 10: Conventional optical modulation device 11: Laser diode 12: Optical modulator 13: Tap 14: Monitoring photodiode 15: Synchronization detector 16: Control unit 17: Pilot tone generator 50: Silicon wafer 100: Optical modulator 110: Laser diode 120: Modulation unit 121: Temperature sensor 122: TFLN modulator 131: Input tab 132: Output tab 141: Input monitoring photodiode 141a: Input dark current measurement unit 142: Output monitoring photodiode 142a: Output dark current measurement unit 150: Complex modulation control unit 151: Dual Tab Diagnostic Unit 152: Lookup Table 153: Bias determination unit 153a: LUT Bin selection unit 153b: Bias calculation unit 153c: Revining unit 154: Noise detection unit 155: Dual tap diagnostic unit 160: Digital-to-Analog Converter

Claims

Claim 1 A modulation unit comprising: a laser diode that outputs an optical signal; an optical modulator that modulates and outputs light input from the laser diode according to a DC bias voltage and a transmitted data signal, and a temperature sensor that measures the temperature of the optical modulator; an input tap that branches off a portion of the input light in the optical path between the laser diode and the modulation unit; an output tap that branches off a portion of the output light in the output optical path of the modulation unit; an input monitoring photodiode that measures the intensity of the light branched from the input tap; an output monitoring photodiode that measures the intensity of the light branched from the output tap; an input dark current measurement unit comprising a plurality of input dark monitoring photodiodes symmetrically arranged around the input monitoring photodiode in a state where light is blocked to measure dark current and current due to thermal noise; and an output dark current measurement unit comprising a plurality of output dark monitoring photodiodes symmetrically arranged around the output monitoring photodiode in a state where light is blocked to measure dark current and current due to thermal noise. A noise compensation-based optical modulation device using a dark monitoring photodiode having the same specifications as the input monitoring photodiode and the output monitoring photodiode, comprising a composite modulation control unit that compensates the measurement values ​​of the input dark current measuring unit and the output dark current measuring unit using the measurement values ​​of the input dark current measuring unit and the output dark current measuring unit, calculates an optical output ratio based on the compensated measurement values, and calculates a DC bias voltage based on the optical output ratio and applies it to the modulation unit, wherein the input dark monitoring photodiode and the output dark monitoring photodiode are fabricated together on the same silicon wafer using the same CMOS process as the input monitoring photodiode and the output monitoring photodiode. Claim 2 delete Claim 3 A noise compensation-based optical modulation device using a dark monitoring photodiode according to claim 1, characterized in that four or more input dark monitoring photodiodes are symmetrically arranged around the input monitoring photodiode, and four or more output dark monitoring photodiodes are symmetrically arranged around the output monitoring photodiode. Claim 4 A noise compensation-based optical modulation device using a dark monitoring photodiode according to claim 1, wherein the composite modulation control unit calculates one of an arithmetic mean, a moving average, and an exponentially weighted moving average of the measurement values ​​of a plurality of input dark monitoring photodiodes of the input dark current measuring unit to determine an input noise compensation value, and calculates one of an arithmetic mean, a moving average, and an exponentially weighted moving average of the measurement values ​​of a plurality of output dark monitoring photodiodes of the output dark current measuring unit to determine an output noise compensation value. Claim 5 A noise compensation-based optical modulation device using a dark monitoring photodiode according to claim 4, wherein the composite modulation control unit calculates a compensated input light intensity by subtracting the input terminal noise compensation value from the measured value of the input monitoring photodiode, calculates a compensated output light intensity by subtracting the output terminal noise compensation value from the measured value of the output monitoring photodiode, and calculates a light output ratio in real time as the ratio of the compensated input light intensity to the compensated output light intensity. Claim 6 A noise compensation-based optical modulation device using a dark monitoring photodiode according to claim 1, wherein the composite modulation control unit comprises a lookup table storing a DC bias voltage according to the optical output ratio, temperature, and voltage application time, obtains a predicted bias voltage by referring to the lookup table, and calculates a DC bias voltage based on the predicted bias voltage and the optical output ratio. Claim 7 A noise compensation-based optical modulation device using a dark monitoring photodiode according to claim 6, wherein the lookup table is composed of a plurality of bins classified by groups having similar characteristics based on drift characteristics measured from a plurality of optical modulator samples, and the composite modulation control unit includes a rebining unit that reanalyzes the drift slope and switches from the current bin to another bin when the error between the predicted bias voltage of the lookup table and the actual calculated bias voltage exceeds a reference value. Claim 8 A noise compensation-based optical modulation device using a dark monitoring photodiode according to claim 6, wherein the lookup table includes a temperature estimate based on the measurement values ​​of the dark monitoring photodiodes as a temperature level. Claim 9 A light modulator determines an input noise compensation value and an output noise compensation value using the measurement values ​​of a plurality of input dark monitoring photodiodes symmetrically arranged around an input monitoring photodiode and the measurement values ​​of a plurality of output dark monitoring photodiodes symmetrically arranged around an output monitoring photodiode; the light modulator calculates a compensated input light intensity by subtracting the input noise compensation value from the measurement value of the input monitoring photodiode, calculates a compensated output light intensity by subtracting the output noise compensation value from the measurement value of the output monitoring photodiode, and then calculates a light output ratio as the ratio of the compensated input light intensity to the compensated output light intensity. The optical modulation device comprises the step of calculating a DC bias voltage based on the optical output ratio and applying it to an optical modulator of a modulation unit, wherein the step of determining the input noise compensation value and the output noise compensation value comprises determining the average of the measured values ​​of the plurality of input dark monitoring photodiodes as the input noise compensation value and determining the average of the measured values ​​of the plurality of output dark monitoring photodiodes as the output noise compensation value. Claim 10 delete Claim 11 A noise compensation-based optical modulation method using a dark monitoring photodiode according to claim 9, wherein the step of calculating the DC bias voltage and applying it to the optical modulator of the modulation unit comprises obtaining a predicted bias voltage by referring to a lookup table storing the DC bias voltage according to the optical output ratio, temperature, and voltage application time, and calculating the DC bias voltage based on the predicted bias voltage and the optical output ratio.

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