A method and system for detecting mode-locking of optical fiber frequency combs

Abstract

This invention provides a method and system for detecting mode-locked optical fiber frequency combs. The method involves calculating the theoretical repetition frequency of the optical fiber frequency comb based on the cavity length of the optical system. Pump light is injected into the optical system, and the output light is converted into an electrical signal by a photodetector. This electrical signal is then filtered out by a bandpass amplifier to obtain the fundamental frequency signal of the optical fiber frequency comb, which is then amplified. The signal is then down-divided to the kHz level by a frequency divider / amplifier signal processing unit, and finally converted into a square wave signal by a voltage comparator. The current division level and division ratio of the frequency divider / amplifier signal processing unit are determined by a microcontroller based on the theoretical repetition frequency of the optical fiber frequency comb. A rubidium atomic clock provides a clock reference for the microcontroller, which determines whether the optical fiber frequency comb has achieved mode-locked operation based on the measurement error of the time interval between the square wave signal periods. This invention can accurately detect the mode-locked state of optical fiber frequency combs with relatively low repetition frequencies.

Description

Technical Field

[0001] This invention relates to the field of ultrafast optics technology, specifically to a fiber optic comb mode-locking detection method and system. Background Technology

[0002] As the core bridge connecting optical and microwave frequency standards, optical frequency combs have achieved rapid development in recent years due to their significant advantages of simple structure and low cost, and have become the most effective absolute optical frequency measurement tool to date. Based on the inherent relationship between parameters such as frequency, time, and spatial scale, optical frequency combs provide ideal technical support for time-frequency transmission, absolute distance, and absolute angle measurement, showing broad application prospects in key fields such as manned spaceflight, deep space exploration, satellite timing, and modern manufacturing.

[0003] Currently, optical frequency combs mainly include three types: fiber optic frequency combs, electro-optical combs, and microcavity optical combs. Among them, fiber optic frequency combs have the highest design and technological maturity, and have become a research hotspot in this field. However, due to the presence of fiber pigtails in fiber optic frequency combs, the repetition frequency is relatively low, with commonly used products having repetition frequencies below 500MHz. Furthermore, for optical frequency combs to achieve practical engineering applications, their oscillators must complete mode-locking. In existing technologies, mode-locking of optical frequency combs is usually achieved by adjusting the pump current after the optical system has been debugged. However, the mode-locking process is significantly affected by environmental factors: temperature changes, mechanical vibrations, and other external conditions can cause fluctuations in the mode-locking threshold of the pump current, making it impossible for the optical frequency comb system to achieve stable mode-locking under different environments by fixing the pump threshold. Precise control of the pump current itself is also one of the key factors affecting mode-locking quality. In addition, changes in ambient temperature or mechanical vibration can cause deformation of components such as mechanical structures, optical fibers and lens frames, which in turn leads to a decrease in the coupling efficiency of the optical system. This not only further increases the mode-locking threshold of the pump current and reduces the mode-locking quality, but in severe cases, it can also cause mode-locking failure. Therefore, accurate judgment of the mode-locking state during the mode-locking process of the optical frequency comb system is crucial for its stable operation.

[0004] The existing methods for determining the mode-locking status of optical frequency combs are mainly divided into two categories: one is optical means, which directly measures the spectral changes during the mode-locking process using a spectrometer. When the spectrum shows broadening and flattening characteristics, it indicates that mode-locking is complete. The other is electrical means, which detects parameters related to the repetition frequency of the optical frequency comb using measuring equipment such as frequency counters and spectrum analyzers, and indirectly reflects the mode-locking status.

[0005] As optical frequency comb technology matures, its development trend is gradually moving towards miniaturization and integration. However, existing mode-locking judgment methods have significant technical bottlenecks. On the one hand, judgment schemes relying on external measuring instruments such as spectrometers and frequency counters are difficult to adapt to the integrated design requirements of optical frequency combs. For low-repetition-frequency optical frequency combs like fiber optic frequency combs, such detection methods are complex to operate and involve cumbersome procedures, requiring manual intervention and failing to achieve intelligent operation, thus failing to meet the actual needs of engineering applications. On the other hand, optical frequency combs can only perform nonlinear transformations such as amplification, spectral spreading, and frequency doubling after mode-locking is completed. Performing these operations without mode-locking will cause irreversible damage to the system. Existing measuring instruments can only provide feedback to the operator and cannot transmit the results to the pump circuit of the optical frequency comb in real time. When external environmental factors cause mode-locking to fail, the pump circuit cannot shut down immediately. If other optical modules are still in operation at this time, it will cause serious damage to the optical frequency comb system, greatly limiting the large-scale application of optical frequency combs in engineering fields. Summary of the Invention

[0006] In view of this, the present invention provides a method and system for detecting the mode-locking state of an optical fiber frequency comb, which can accurately detect the mode-locking state of an optical fiber frequency comb with a relatively low repetition frequency.

[0007] The technical solution adopted in this invention is as follows: A method for detecting mode-locking of an optical fiber frequency comb, which calculates the theoretical value of the repetition frequency of the optical frequency comb based on the cavity length of the optical system of the frequency comb; Pump light is injected into the optical frequency comb system. The output light is converted into an electrical signal by a photodetector. The electrical signal is filtered out by a bandpass amplifier to extract the fundamental frequency signal of the optical frequency comb repetition frequency and amplified. Then, it is down-frequencyd to the KHz level by a frequency division amplification signal processing unit and then enters a voltage comparator to be converted into a square wave signal. The current division level and division ratio of the frequency division amplification signal processing unit are determined by the microcontroller based on the theoretical value of the optical frequency comb's theoretical repetition frequency. The rubidium atomic clock provides a clock reference for the microcontroller. The microcontroller determines whether the optical frequency comb has completed mode locking based on the measurement error of the time interval between the square wave signal periods. If the measurement error is greater than the set value, the optical frequency comb is not mode locked; if the measurement error is not greater than the set value, the optical frequency comb has completed mode locking.

[0008] Furthermore, the frequency division amplification signal processing unit includes two or more frequency dividers and amplifiers, with the number of frequency dividers and amplifiers being the same, and one frequency divider and one amplifier constituting one unit.

[0009] Furthermore, the microcontroller controls the frequency division amplification signal processing unit through the frequency division amplification management unit.

[0010] Furthermore, the frequency divider amplification management unit is an RF switch, which manages the number of stages of the currently selected frequency divider and amplifier by controlling the on / off state of the RF switch.

[0011] Furthermore, the system's mode-locking status detection result is output through logic levels, where 0 indicates no mode-locking and 1 indicates mode-locking.

[0012] Furthermore, after the initial detection of mode-locking, the microcontroller continues to detect the time interval between the rising edges of the square wave signal to continuously determine whether the optical frequency comb is truly mode-locked.

[0013] Furthermore, pump light is injected into the optical frequency comb optical system through the pump source circuit; If the measurement error of the time interval between square wave signal periods is still greater than the set value when the pump source circuit increases to the maximum output power, then the optical frequency comb is not mode-locked.

[0014] Furthermore, when the optical frequency comb loses its mode-locked state, the microcontroller detects this by the change in the time interval between the two square wave signals and inputs this result to the pump source circuit through a logic level, which then shuts down the pump light.

[0015] Furthermore, the system mode-locking status detection result is indicated by an indicator light: not mode-locked displays the first designated color; mode-locked displays the second designated color; if the optical frequency comb optical system is malfunctioning, the indicator light flashes alternately between the first and second designated colors.

[0016] The present invention also provides a fiber optic comb mode-lock detection system, including a fiber optic comb optical system, a photodetector, a bandpass amplifier, a frequency divider amplification signal processing unit, a voltage comparator, a microcontroller, and a rubidium atomic clock; The optical frequency comb optical system is used to receive pump light to complete mode locking; the photodetector is used to convert the optical signal of the output light of the optical frequency comb optical system into an electrical signal; the bandpass amplifier is used to filter out the fundamental frequency signal of the optical frequency comb repetition frequency from the electrical signal and amplify it. The frequency division amplification signal processing unit is used to down-frequency the amplified optical frequency comb repetition frequency base frequency signal to the KHz level; The voltage comparator is used to convert the down-frequency optical frequency comb repetition frequency base frequency signal into a square wave signal and send it to the microprocessor. The microcontroller is used to determine the current division level and division ratio of the frequency division amplification signal processing unit based on the theoretical value of the optical frequency comb's theoretical repetition frequency; and to determine whether the optical frequency comb has completed mode locking based on the measurement error of the time interval between square wave signal periods. If the measurement error is greater than the set value, the optical frequency comb is not mode locked; if the measurement error is not greater than the set value, the optical frequency comb has completed mode locking. The rubidium atomic clock is used to provide a clock reference for the microcontroller.

[0017] Beneficial effects: 1. This invention can detect the mode-locking state of a low-repetition-frequency fiber optic comb system, directly process the signal, convert the frequency-divided signal into a square wave signal through a voltage comparator, detect the edge of the square wave signal to determine the mode-locking state, and utilize the clock synchronization of a rubidium atomic clock to achieve extremely high precision in square wave edge detection, resulting in accurate detection results.

[0018] Furthermore, depending on the repetition frequency of the optical frequency comb, the microcontroller controls the frequency division amplification management unit to achieve frequency division and frequency division amplification stage control, so as to adapt to different low repetition frequencies and different optical frequency comb systems for mode-locked detection. In addition, the microcontroller strictly sets the frequency division ratio of the frequency division amplification signal processing unit according to the theoretical value of the optical frequency comb's theoretical repetition frequency, which can improve the signal-to-noise ratio of the optical frequency comb's repetition frequency after frequency division and strictly ensure detection accuracy.

[0019] 2. This invention converts the detection of the mode-locked state of the optical frequency comb into the detection of square wave signals down to the KHz level. The KHz level is usually a low frequency. This conversion method greatly reduces the requirements for the microcontroller, improves the processing speed, and can well adapt to the system design requirements.

[0020] 3. The device used in this invention can be integrated into a size better than 5×5cm. Compared with traditional measuring devices such as spectrometers, frequency meters or spectrum analyzers for detecting the mode-locking state of optical frequency combs, it has the advantages of small system size, high reliability, and the system mode-locking state detection result is output through logic level, which greatly improves the signal anti-interference ability, the ease of interpretation and compatibility with digital systems.

[0021] 4. This invention can detect the specific reasons why the optical frequency comb cannot achieve mode locking. Specifically, by detecting the mode locking state and the setting state of the pump source current, it can be determined whether the inability to achieve mode locking is caused by insufficient pump light intensity or by changes in the optical system structure.

[0022] 5. In addition to achieving high-reliability detection of the optical frequency comb's mode-locking status, this invention can also achieve continuous monitoring of the optical frequency comb's mode-locking status. Once the optical frequency comb loses mode-locking due to external environmental factors, the system can transmit the loss of lock status to the pump source circuit via logic level within 1ms. The pump source circuit will then immediately shut down the pump light to prevent damage to the optical frequency comb system and achieve the purpose of protecting the optical frequency comb system. Attached Figure Description

[0023] Figure 1 This is a schematic diagram of the device structure of the present invention. Detailed Implementation

[0024] The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0025] This invention provides a method for detecting mode-locking of an optical fiber frequency comb, the method being as follows: First, once the optical frequency comb system is assembled, the theoretical range of its repetition frequency values ​​is determined, specifically by the following formula:

[0026] in, v c The speed of light in a vacuum is 299,792,458 m / s. f r The repetition frequency, L This is the laser cavity length of the optical frequency comb system.

[0027] Once the laser cavity length is determined, the microcontroller can calculate the theoretical value of the optical frequency comb repetition frequency using a formula, and record this value as a reference value for the optical frequency comb repetition frequency.

[0028] Next, pump light is injected into the optical frequency comb optical system through the pump source circuit to provide energy for the pulsed laser within the optical frequency comb optical system to perform optical oscillations within the cavity, thereby supporting the optical frequency comb oscillator to complete mode locking. During the process of the pump source circuit injecting pump light into the optical frequency comb optical system, the output light of the optical frequency comb optical system is converted into an electrical signal by a photodetector. The electrical signal is then filtered out by a bandpass amplifier to obtain the fundamental frequency signal of the optical frequency comb repetition frequency, and the signal is amplified to meet the power requirements of subsequent devices such as frequency dividers.

[0029] Then, the fundamental frequency signal of the optical frequency comb repetition frequency after passing through the bandpass amplifier is input to the frequency divider amplification signal processing unit, which performs frequency down-amplification processing on it, reducing it to the KHz level.

[0030] This frequency division and amplification signal processing unit includes two or more frequency dividers and amplifiers, with the number of frequency dividers and amplifiers being the same; one frequency divider and one amplifier constitute one unit. In this embodiment, two frequency dividers and two amplifiers are used. The two frequency dividers are a first-level frequency divider and a second-level frequency divider, with basic division ratios of 64, 128, and 256 times, respectively, and an input frequency of up to 2.4 GHz. The amplifier used has a 3 GHz bandwidth and a gain of 24 dB, meeting the current design requirements for the repetition frequency of fiber optic frequency combs.

[0031] The frequency divider amplification signal processing unit is controlled by the frequency divider amplification management unit controlled by the microcontroller. The frequency divider amplification management unit is actually an RF switch, which manages the number of stages of the currently selected frequency divider and amplifier by controlling the on / off state of the RF switch.

[0032] The specific number of stages used in the frequency division amplification signal processing unit is as follows: the microcontroller allocates the number of frequency division stages currently used based on the theoretical value of the optical frequency comb repetition frequency, so that the frequency signals output by the frequency division amplification signal processing unit are all reduced to the KHz level. In this embodiment, the frequency of the optical frequency comb repetition frequency fundamental signal is reduced to below 10KHz. The specific selection of the optical frequency comb repetition frequency and the frequency division factor is shown in the table below:

[0033] To enhance the signal-to-noise ratio of the optical frequency comb repetition frequency during the frequency division process, the microcontroller needs to strictly control the number of frequency division stages to achieve reliable mode-locking state judgment.

[0034] Based on the theoretical value of the current optical frequency comb repetition frequency, the microcontroller determines the number of frequency division stages of the frequency division amplification signal processing unit in the current optical frequency comb system. The frequency division amplification signal processing unit converts the output signal into a square wave signal through a voltage comparator. This square wave signal is detected by the input capture function of the microcontroller. In order to improve the capture accuracy of the microcontroller, the clock reference of the microcontroller is connected to the rubidium atomic clock.

[0035] During the mode-locking process, the microcontroller continuously measures the square wave signal output by the voltage comparator and determines whether mode-locking is complete through the following process: As the pump current increases, mode competition occurs in the optical frequency comb when mode-locking is not completed within the oscillator cavity of the optical frequency comb system. As a result, the repetition frequency of the optical frequency comb is an unstable signal (fluctuating on the order of hundreds of kHz or even MHz). The frequency value changes greatly. When the low-frequency signal after frequency division is converted into a square wave signal, the time interval between each pair of multiple square wave signals detected by the microcontroller changes significantly, indicating that the optical frequency comb has not completed mode-locking at this time.

[0036] As the pump current of the optical frequency comb continues to increase, once mode-locking occurs, the mode within the optical frequency comb cavity becomes fixed, resulting in a relatively stable optical frequency comb repetition frequency signal. The variation in the optical frequency comb repetition frequency signal typically increases with the increase in the repetition frequency value, but the maximum variation will not exceed the order of hundreds of Hz. With frequency division by the frequency divider, the jitter of the optical frequency comb repetition frequency after mode-locking will also be below 1 Hz. At this point, the microcontroller detects that the time intervals between pairs of multiple square wave signals become stable, indicating that the optical frequency comb has completed mode-locking. The microcontroller determines whether the optical frequency comb has completed mode-locking based on the measurement error of the time interval between square wave signal periods. If the measurement error is greater than a set value, the optical frequency comb is not mode-locked; if the measurement error is not greater than the set value, the optical frequency comb has completed mode-locking. In this embodiment, the set value is 10 μs, meaning that when the measurement error of the time interval between square wave signal periods is not longer than 10 μs, the optical frequency comb has completed mode-locking; when it is greater than 10 μs, the optical frequency comb has not completed mode-locking.

[0037] As an improvement, the system's mode-lock status detection result is output via logic levels, with 0 indicating no mode-lock and 1 indicating mode-lock. Simultaneously, the system's mode-lock status detection result is indicated by an indicator light: no mode-lock displays a first specified color (which can be red); mode-lock displays a second specified color (which can be green); if the optical frequency comb system malfunctions, the indicator light flashes alternately between the first specified color (red) and the second specified color (green). That is, when mode-locked, the mode-lock status indicator light is red, and the output value is 0. When mode-locked, the mode-lock status indicator light turns green, and the output value is 1.

[0038] To improve the accuracy of the judgment results, it is usually necessary to perform multiple checks on the rising edge time interval of the square wave signal after the initial detection of mode locking, continuously judging whether mode locking is truly achieved, thereby improving the robustness of the detection system. The detection results can be directly output via logic levels and fed back to the pump source circuit. Replacing signal communication with logic levels prevents communication errors and improves the reliability of system detection.

[0039] In special circumstances, when the pump source circuit increases to its maximum output power, if the optical frequency comb mode-lock detection system still fails to detect mode-locking, it indicates that the optical path coupling of the optical frequency comb optical system has deviated due to factors such as ambient temperature and mechanical vibration. In this case, the system mode-locking status indicator light will flash red and green alternately, prompting the operator to adjust the optical system. When the optical system is properly adjusted, the mode-locking status indicator light will turn green upon detecting that the optical frequency comb optical system has completed mode-locking.

[0040] When the mode-locked state of the optical frequency comb suddenly drops due to external environment or accidental touch, the microcontroller immediately detects the mode-locking drop by observing the change in the time interval between two square wave signals. This result is then input to the pump source circuit via a logic level, and the pump source circuit immediately shuts off the pump light to prevent damage to the optical frequency comb system.

[0041] This invention also provides a fiber optic comb mode-locking detection system, such as... Figure 1 As shown, it includes an optical frequency comb optical system, a photodetector, a bandpass amplifier, a frequency divider amplification signal processing unit, a voltage comparator, a microcontroller, and a rubidium atomic clock; The optical frequency comb system is used to receive pump light to complete mode locking; Photodetectors are used to convert the optical signal output from an optical frequency comb system into an electrical signal; A bandpass amplifier is used to filter out the fundamental frequency signal of the optical frequency comb repetition frequency from an electrical signal and amplify it; The frequency division amplification signal processing unit is used to down-frequency the amplified optical frequency comb repetition frequency base frequency signal to the KHz level; The voltage comparator is used to acquire the down-frequency fundamental frequency signal of the optical frequency comb, convert it into a square wave signal, and send it to the microprocessor; The microcontroller is used to determine the current division level and division ratio of the frequency division amplification signal processing unit based on the theoretical repetition frequency of the optical frequency comb; and to determine whether the optical frequency comb has completed mode locking based on the measurement error of the time interval between the square wave signal periods. If the measurement error is greater than the set value, the optical frequency comb is not mode locked; if the measurement error is not greater than the set value, the optical frequency comb has completed mode locking.

[0042] Rubidium atomic clocks are used to provide clock references for microcontrollers, thereby providing ultra-high precision time and frequency standards.

[0043] Furthermore, the automatic mode-locking detection system also includes a pump source circuit, a frequency divider amplifier management unit, status indicator lights, and a rubidium atomic clock.

[0044] The pump source circuit is used to provide the necessary initial energy input for the laser to generate laser light in the optical frequency comb optical system.

[0045] The frequency divider amplification management unit is used to control the frequency divider amplification signal processing unit under the control of the microcontroller, and to manage the frequency divider and amplifier stages currently selected by the frequency divider amplification signal processing unit.

[0046] The status indicator light is used to indicate the system's mode-locking status detection result: mode-locked, mode-locked, or abnormal. Mode-locked displays the first specified color; mode-locked displays the second specified color; if the optical frequency comb system is abnormal, the indicator light flashes alternately between the first and second specified colors. The first specified color can be red, and the second specified color can be green.

[0047] Based on this fiber optic comb mode-locking detection system, the above-mentioned detection method can be used to detect the mode-locking status of a low-repetition-frequency fiber optic comb system, and the judgment result is output in logic level, so the detection result has high reliability.

[0048] In summary, the above are merely preferred embodiments of the present invention and are not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for detecting mode-locking of an optical fiber frequency comb, characterized in that, The theoretical value of the repetition frequency of the optical frequency comb is calculated based on the cavity length of the optical system of the optical frequency comb. Pump light is injected into the optical frequency comb system. The output light is converted into an electrical signal by a photodetector. The electrical signal is filtered out by a bandpass amplifier to extract the fundamental frequency signal of the optical frequency comb repetition frequency and amplified. Then, it is down-frequencyd to the KHz level by a frequency division amplification signal processing unit and then enters a voltage comparator to be converted into a square wave signal. The current division level and division ratio of the frequency division amplification signal processing unit are determined by the microcontroller based on the theoretical value of the optical frequency comb's theoretical repetition frequency. The rubidium atomic clock provides a clock reference for the microcontroller. The microcontroller determines whether the optical frequency comb has completed mode locking based on the measurement error of the time interval between the square wave signal periods. If the measurement error is greater than the set value, the optical frequency comb is not mode locked; if the measurement error is not greater than the set value, the optical frequency comb has completed mode locking.

2. The fiber optic comb mode-locking detection method as described in claim 1, characterized in that, The frequency division amplification signal processing unit includes two or more frequency dividers and amplifiers, with the number of frequency dividers and amplifiers being the same. One frequency divider and one amplifier constitute one unit.

3. The fiber optic comb mode-locking detection method as described in claim 1, characterized in that, The microcontroller controls the frequency division amplification signal processing unit through the frequency division amplification management unit.

4. The fiber optic comb mode-locking detection method as described in claim 3, characterized in that, The frequency divider amplifier management unit is an RF switch that controls the on / off state of the RF switch to manage the number of stages of the currently selected frequency divider and amplifier.

5. The fiber optic comb mode-locking detection method according to any one of claims 1-4, characterized in that, The system mode-locking status detection result is output through logic levels, where 0 indicates no mode-locking and 1 indicates mode-locking.

6. The fiber optic comb mode-locking detection method as described in claim 1, characterized in that, After the initial detection of mode-locking, the microcontroller continues to detect the time interval between the rising edges of the square wave signal to continuously determine whether the optical frequency comb is truly mode-locked.

7. The fiber optic comb mode-locking detection method as described in claim 5, characterized in that, Pump light is injected into the optical frequency comb system through the pump source circuit; If the measurement error of the time interval between square wave signal periods is still greater than the set value when the pump source circuit increases to the maximum output power, then the optical frequency comb is not mode-locked.

8. The fiber optic comb mode-locking detection method as described in claim 7, characterized in that, When the mode-locked state of the optical frequency comb is lost, the microcontroller detects this by the change in the time interval between the two square wave signals and inputs this result to the pump source circuit through a logic level, and the pump source circuit shuts down the pump light.

9. The fiber optic comb mode-locking detection method as described in claim 5, characterized in that, The system's mode-locking status detection result is indicated by an indicator light: not mode-locked displays the first specified color; mode-locked displays the second specified color; if the optical frequency comb system malfunctions, the indicator light flashes alternately between the first and second specified colors.

10. A fiber optic comb mode-locking detection system, characterized in that, It includes an optical frequency comb system, a photodetector, a bandpass amplifier, a frequency divider amplification signal processing unit, a voltage comparator, a microcontroller, and a rubidium atomic clock; The optical frequency comb optical system is used to receive pump light to complete mode locking; the photodetector is used to convert the optical signal of the output light of the optical frequency comb optical system into an electrical signal; the bandpass amplifier is used to filter out the fundamental frequency signal of the optical frequency comb repetition frequency from the electrical signal and amplify it. The frequency division amplification signal processing unit is used to down-frequency the amplified optical frequency comb repetition frequency base frequency signal to the KHz level; The voltage comparator is used to convert the down-frequency optical frequency comb repetition frequency base frequency signal into a square wave signal and send it to the microprocessor. The microcontroller is used to determine the current division level and division ratio of the frequency division amplification signal processing unit based on the theoretical value of the optical frequency comb's theoretical repetition frequency; and to determine whether the optical frequency comb has completed mode locking based on the measurement error of the time interval between square wave signal periods. If the measurement error is greater than the set value, the optical frequency comb is not mode locked; if the measurement error is not greater than the set value, the optical frequency comb has completed mode locking. The rubidium atomic clock is used to provide a clock reference for the microcontroller.

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