A method and device for automatic mode locking control and detection of optical frequency comb

Abstract

The present invention discloses a method and device for automatic mode-locking control and detection of an optical frequency comb. First, the optimal mode-locking state of the optical frequency comb is manually calibrated, and the power value range of the optical frequency comb output spectrum is recorded as standard data; a microcontroller controls and gradually increases the pump current of the pump source, and at the same time, the output light of the optical frequency comb oscillator is divided into three paths, the first path is used for mode-locking detection, the second path is input to an optical power meter, and the third path is used as measurement light for subsequent measurement; the first path of light is converted into an electrical signal, the electrical signal is amplified, mixed with a reference frequency to obtain an error signal, the error signal is filtered and converted into a voltage signal, and the microcontroller is used to detect the error signal. The microcontroller samples the voltage signal to determine whether mode locking is achieved. If mode locking has not been achieved, the pump current continues to increase. Mode locking is achieved when the microcontroller calculates the cumulative sum of the absolute deviations of two adjacent sampling results among multiple sampling results and finds that the sum is less than a set threshold. At this point, the microcontroller triggers serial communication with the optical power meter, reads the real-time power of the optical power meter, and determines whether the current power is within the power value range based on standard data. If so, optimal mode-locked pulse laser output is achieved. If not, the pump current needs to be reduced until the power value measured by the optical power meter meets the calibrated optimal mode-locked power value range. This invention can achieve automatic mode locking of the optical frequency comb and effective detection of the post-mode locking state, ensuring that the optical frequency comb oscillator has good spectral characteristics and signal light quality during actual output.

Description

Technical Field

[0001] The present invention relates to, in particular to, an optical frequency comb automatic mode locking control and detection method and device. Background Art

[0002] Optical frequency combs, bridging optical and microwave frequency standards, have experienced rapid development in recent years due to their simple structure and low cost. Their implementation addresses the difficulty of measuring optical frequencies, making them the most effective tool for absolute optical frequency measurement to date. Furthermore, due to the interrelationships between frequency, time, and spatial scales, optical frequency combs provide an ideal tool for research into time-frequency transfer, absolute distance, and absolute angle measurement. They hold broad application potential in areas such as manned space flight, deep space exploration, satellite timing, and modern manufacturing.

[0003] For optical frequency combs to truly enter practical use, their oscillators must first be mode-locked. Currently, there are two main modes of mode-locking for optical frequency combs: active mode-locking and passive mode-locking.

[0004] Active mode locking is to add a modulation device in the laser cavity, modulate the modulation device with an external modulation signal, and change the amplitude and phase of the laser in the cavity. When the modulation frequency of the laser in the cavity and the longitudinal mode spacing are equal, mode locking can be completed and femtosecond optical pulse output can be achieved.

[0005] Passive mode locking utilizes all-optical nonlinear effects to achieve mode locking. A saturable absorber is added to a laser ring cavity. When laser light periodically passes through it, the absorber filters out the higher-intensity center of the pulse while gradually weakening the edges of the pulse due to their lower energy and lower transmittance. This phenomenon occurs repeatedly as the pulse passes through the absorber within the ring cavity, gradually compressing the pulse width and ultimately achieving mode locking, enabling femtosecond-scale pulse output.

[0006] Compared to active mode locking, passive mode locking eliminates the need for active modulation devices within the cavity and instead utilizes all-optical nonlinear effects to achieve mode locking. The resulting mode-locked pulses exhibit a better signal-to-noise ratio. Passive mode locking also offers advantages such as simple structure, low cost, short mode-locked pulses, and high pulse energy. Therefore, it remains the primary method for generating mode-locked pulses in optical frequency combs, particularly fiber combs.

[0007] The most common method currently used to generate frequency comb mode-locked optical pulses using passive mode-locking is to first control the temperature within the oscillator cavity to maintain a relatively constant level after the frequency comb oscillator is constructed. The pump source drive current is then adjusted to inject pump laser light into the oscillator. As the drive current increases, the energy of the pump light also increases. When the drive current reaches a certain level, the short-pulse laser in the laser cavity achieves sufficient pulse energy to achieve mode-locking. However, the mode-locking state at this point is usually not optimal, requiring the operator to check the spectral characteristics of the output light from the spectrometer for smoothness. The operator also needs to perform complex manual adjustments to the pump drive current to achieve a smoother spectral characteristic, which poses difficulties in the application of frequency combs. Furthermore, this mode-locking method requires a long time. If the operator fails to operate the system properly and the lock is lost, the operation must be repeated, seriously hindering the development of frequency combs in engineering applications. Summary of the Invention

[0008] In view of this, the present invention provides a method and device for automatic mode-locking control and detection of an optical frequency comb, which can achieve automatic mode-locking of the optical frequency comb and effective detection of the post-mode-locking state, ensuring that the optical frequency comb oscillator has good spectral characteristics and signal light quality during actual output.

[0009] The technical solution adopted in the present invention is as follows:

[0010] A method for automatically controlling and detecting the mode-locking of an optical frequency comb includes manually calibrating the optimal mode-locking state of the optical frequency comb and recording the power value range of the optical frequency comb output spectrum as standard data.

[0011] The microcontroller controls and gradually increases the pump current of the pump source. At the same time, the output light of the optical frequency comb oscillator is divided into three paths. The first path is used for mode-locking detection, the second path is input into the optical power meter, and the third path is used as measurement light for subsequent measurement. The first path of light is converted into an electrical signal, which is amplified and mixed with the reference frequency to obtain an error signal. The error signal is filtered and converted into a voltage signal. The microcontroller samples the voltage signal to determine whether the mode is locked. If the mode is not locked, the pump current is continued to increase. When the microcontroller calculates that the cumulative sum of the absolute deviations of two adjacent sampling results in multiple sampling results is less than a set threshold, the mode is locked.

[0012] At this time, the microcontroller triggers serial communication with the optical power meter, reads the real-time power of the optical power meter, and determines whether the current power is within the power value range based on the standard data. If so, the optimal mode-locked pulse laser output is completed; if not, the pump current needs to be reduced until the power value measured by the optical power meter meets the calibrated optimal mode-locked power value range.

[0013] The present invention also provides an optical frequency comb automatic mode locking control and detection device, comprising a data calibration module, a microcontroller, a pump source, an optical frequency comb oscillator, a beam splitter, an optical power meter, a photodetector, a low-pass filter and a frequency-to-voltage converter;

[0014] The data calibration module is used to calibrate the optimal mode-locking state of the optical frequency comb and record the power value range of the optical frequency comb output spectrum as standard data;

[0015] The microcontroller is used to control the current of the pump source so that the optical frequency comb oscillator outputs an optical frequency comb pulse laser;

[0016] The beam splitter is used to split the optical frequency comb pulse laser into three output paths, the first path is used as a mode-locked detection input photodetector, the second path is input to an optical power meter, and the third path is used as a measurement light for subsequent measurement;

[0017] The photodetector is used to convert the optical signal into an electrical signal; the signal amplifier is used to amplify the electrical signal;

[0018] The phase detector is used to perform phase detection on the amplified electrical signal mixed with the reference frequency to obtain an error signal;

[0019] The low-pass filter is used to filter the error signal;

[0020] The frequency-to-voltage converter is used to convert the filtered error signal into a voltage signal;

[0021] The microcontroller samples the voltage signal while controlling the pump source current to determine whether the mode is locked; if the mode is not locked, the pump current is continuously increased, and the mode is locked when the cumulative sum of the absolute deviations of two adjacent sampling results calculated by the microcontroller in multiple sampling results is less than a set threshold;

[0022] After the optical frequency comb completes mode locking, the microcontroller triggers serial communication with the optical power meter. The microcontroller reads the real-time power of the optical power meter and determines whether the current power is within the optimal mode-locking power value range calibrated in advance. If so, the optimal mode-locked pulse laser output is completed; if not, the pump current needs to be reduced until the power value measured by the optical power meter is within the calibrated optimal mode-locking power value range.

[0023] Beneficial effects:

[0024] 1. This invention eliminates the need for manual mode locking of the optical frequency comb, enabling fully automated mode locking and effective detection of mode locking. Furthermore, the specific repetition frequency of the optical frequency comb after mode locking can be directly determined based on the current mode locking state, with an error range of less than 10 Hz.

[0025] 2. This invention, combined with optical power measurement, effectively improves the mode-locking quality of the optical frequency comb and the flatness of its output spectrum, broadening its application range and significantly promoting its development in engineering applications. The automatic mode-locking control and detection device for the optical frequency comb achieves optimal mode-locking pulse generation in less than 2 seconds, demonstrating excellent performance.

[0026] 3. The microcontroller of the present invention increases the pump current of the pump source at a rate of 100 mA / 100 ms, thereby effectively and quickly increasing the pump light power injected into the oscillator and accelerating the mode locking process.

[0027] 4. The present invention controls the heating or cooling of the TEC cooler through a microcontroller to achieve a constant temperature in the oscillation cavity, effectively improving the stability of the optical frequency comb. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 This is a schematic diagram of the device of the present invention. DETAILED DESCRIPTION

[0029] The present invention is described in detail below with reference to the accompanying drawings and embodiments.

[0030] The present invention provides a method for controlling and detecting automatic mode-locking of an optical frequency comb. The optical frequency comb is significantly affected by temperature. To ensure good repeatability, the oscillator temperature of the optical frequency comb oscillator must be read using a thermistor. A microcontroller employs a PID control algorithm to control the TEC driver within the cavity to maintain a constant temperature within the oscillation cavity, ensuring the cavity temperature remains within the range of 25±0.05°C. Furthermore, because optical frequency comb oscillators are currently primarily constructed manually, and the dispersion control of pulsed lasers is affected by various factors, the optimal mode-locking states of different optical frequency comb oscillators vary. Therefore, in addition to achieving temperature control, manual calibration of the optimal mode-locking state of the optical frequency comb is required. The spectral characteristics of the optical frequency comb after mode-locking are smoothed, and the corresponding optical power value range is recorded. This serves as standard data for subsequent adjustment of the optimal mode-locking state after automatic mode-locking.

[0031] Once the data is calibrated, the microcontroller gradually increases the pump current at a rate of 100mA / 100ms, effectively and rapidly increasing the pump light power injected into the oscillator and accelerating the mode-locking process. Simultaneously with the microcontroller's control of increasing the pump current, mode-locking detection is initiated. Mode-locking detection is performed as follows: the output light from the optical frequency comb oscillator is split into three paths via an optical beam splitter. One path serves as the input to a photodetector for mode-locking detection, the second path is fed into an optical power meter, and the third path serves as measurement light for subsequent measurements.

[0032] The pulsed light from the mode-locked detection is converted into an electrical signal by a photodetector. This electrical signal is then mixed with a reference frequency output by a frequency synthesizer traceable to a rubidium atomic clock in a phase detector to produce an error signal. The error signal is filtered through a low-pass filter (LPF) to remove high-frequency components and improve its quality. The filtered signal is then converted to a voltage signal by a frequency-to-voltage converter (with a 20kHz bandwidth, corresponding to 0-3.3V). This voltage signal is then collected by an analog-to-digital converter (ADC) controlled by a microcontroller (12-bit sampling resolution, 4096 points) to produce a sampled voltage signal. The microcontroller samples this voltage signal multiple times. Mode locking is achieved when the cumulative sum of the absolute deviations of two adjacent sampling results is less than a set threshold. In this embodiment, the microcontroller samples at 10ms intervals, with a total of 20 samples per cycle, resulting in an effective detection time of 200ms. Mode locking is indicated by the cumulative sum of the absolute deviations of two adjacent sampling results being less than 100.

[0033] If mode-locking is not achieved during the pump current increase, the error signal after phase detection is a chaotic signal with multiple frequency components and high noise. After acquisition, each of the 20 sample results exhibits significant deviation, and the cumulative sum of the absolute deviations between two consecutive sample results is typically greater than 1000. This indicates that the oscillator is no longer mode-locked and the pump current needs to be increased. As the pump current continues to increase, when the microcontroller calculates the cumulative sum of the absolute deviations between two consecutive sample results to be less than 100, the oscillator is mode-locked. At this point, the microcontroller immediately stops increasing the pump current, completing the mode-locking control and detection of the optical frequency comb oscillator. Once the optical frequency comb is mode-locked, the microcontroller accurately determines the repetition frequency of the current mode-locked optical frequency comb based on the current acquired frequency-to-voltage converter voltage and the signal frequency output by the frequency synthesizer controlled by the microcontroller. The measurement error is guaranteed to be within 10 Hz, effectively improving the efficiency of frequency locking and subsequent measurements.

[0034] When the optical frequency comb is mode-locked, the microcontroller triggers serial communication with the optical power meter to read the real-time power of the oscillator's output light as it passes through the beam splitter and enters the optical power meter. After the power reading is completed, the microcontroller determines whether the current power is within the optimal mode-locked power range calibrated earlier. If it is, the oscillator has achieved optimal mode-locked pulse laser output. If not, this is usually due to excessive pump current, and the pump current needs to be reduced at a rate of 10mA / 500ms until the power measured by the optical power meter falls within the calibrated optimal mode-locked power range. This ultimately achieves optimal mode-locked pulse laser output, achieving optical signal output with a flat spectrum.

[0035] The present invention also provides an optical frequency comb automatic mode locking control and detection device, such as Figure 1 As shown, the control and detection device includes a data calibration module, a microcontroller, a pump source, an optical frequency comb oscillator, a beam splitter, an optical power meter, a photodetector, a low-pass filter and a frequency-to-voltage converter;

[0036] The data calibration module is used to calibrate the optimal mode-locking state of the optical frequency comb and record the power value range of the optical frequency comb output spectrum as standard data;

[0037] The microcontroller is used to control the current of the pump source so that the optical frequency comb oscillator outputs an optical frequency comb pulse laser;

[0038] The beam splitter is used to split the optical frequency comb pulse laser into three output paths. The first path is used as the input of the photodetector for mode-locked detection, the second path is input into the optical power meter, and the third path is used as the measurement light for subsequent measurements.

[0039] The photodetector is used to convert the optical signal into an electrical signal; the signal amplifier is used to amplify the electrical signal;

[0040] The phase detector is used to mix the amplified electrical signal with the reference frequency to obtain the error signal;

[0041] The low-pass filter is used to filter the error signal;

[0042] The frequency-to-voltage converter is used to convert the filtered error signal into a voltage signal;

[0043] While controlling the pump source current, the microcontroller samples the voltage signal to determine whether the mode is locked. If the mode is not locked, the pump current continues to increase. When the microcontroller calculates that the cumulative sum of the absolute deviations of two adjacent sampling results is less than the set threshold, the mode is locked. The microcontroller can sample at an interval of 10ms, for a total of 20 times; the threshold is set to 100.

[0044] When the optical frequency comb completes mode locking, the microcontroller triggers serial communication with the optical power meter. The microcontroller reads the real-time power of the optical power meter and determines whether the current power is within the optimal mode-locking power value range calibrated in advance. If so, the optimal mode-locked pulse laser output is completed; if not, the pump current needs to be reduced until the power value measured by the optical power meter is within the calibrated optimal mode-locking power value range.

[0045] The control and detection device of the present invention also includes a temperature control unit, which includes a TEC cooler, a thermistor and a PID controller. The TEC cooler and the thermistor are both arranged in the oscillation cavity of the optical frequency comb oscillator. The thermistor is used as a temperature sensor to monitor the temperature in the cavity; the TEC cooler is used to adjust the temperature in the cavity; and the PID controller is used to control the heating or cooling of the TEC cooler.

[0046] In summary, the above are only preferred embodiments of the present invention and are not intended to limit the scope of protection of the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for automatic mode locking control and detection of an optical frequency comb, characterized in that: First, the optimal mode-locking state of the optical frequency comb is manually calibrated, and the power value range of the optical frequency comb output spectrum is recorded as standard data; The microcontroller controls and gradually increases the pump current of the pump source. At the same time, the output light of the optical frequency comb oscillator is divided into three paths, the first path is used for mode locking detection, the second path is input into the optical power meter, and the third path is used as measurement light for subsequent measurement; the first path of light is converted into an electrical signal, and after the electrical signal is amplified, it is mixed with the reference frequency generated by the frequency synthesizer to obtain an error signal, and the error signal is filtered and converted into a voltage signal through a frequency-voltage converter. The microcontroller samples the voltage signal to determine whether the mode is locked; if the mode is not locked, the pump current is continued to be increased. When the microcontroller calculates that the cumulative sum of the absolute deviations of two adjacent sampling results in multiple sampling results is less than the set threshold, the mode is locked; when the optical frequency comb is mode-locked, the microcontroller obtains the repetition frequency of the optical frequency comb after the current mode is locked according to the size of the voltage converted by the current acquisition frequency-voltage converter and the signal frequency output by the frequency synthesizer; At this time, the microcontroller triggers serial communication with the optical power meter, reads the real-time power of the optical power meter, and determines whether the current power is within the power value range based on the standard data. If so, the optimal mode-locked pulse laser output is completed; if not, the pump current needs to be reduced until the power value measured by the optical power meter meets the calibrated optimal mode-locked power value range.

2. The optical frequency comb automatic mode locking control and detection method according to claim 1, characterized in that: The frequency synthesizer is traceable to a rubidium atomic clock and is controlled by a microcontroller.

3. The optical frequency comb automatic mode locking control and detection method according to claim 1, characterized in that: The microcontroller samples the voltage signal at 10ms intervals, sampling 20 times in each cycle, and an effective detection time of 200ms; the threshold is set to 100.

4. The optical frequency comb automatic mode locking control and detection method according to claim 1, characterized in that: The microcontroller increases the pump current of the pump source at a speed of 100 mA / 100 ms.

5. The optical frequency comb automatic mode locking control and detection method according to claim 1, characterized in that: The microcontroller reduces the pump current at a rate of 10 mA / 500 ms.

6. The optical frequency comb automatic mode locking control and detection method according to any one of claims 1 to 5, characterized in that: According to the collected optical frequency comb oscillator temperature, the microcontroller controls the TEC cooler to heat or cool to achieve a constant temperature in the oscillation cavity, stabilizing it to 25±0.05℃.

7. An optical frequency comb automatic mode locking control and detection device, characterized in that: It includes a data calibration module, a microcontroller, a pump source, an optical frequency comb oscillator, a beam splitter, an optical power meter, a photodetector, a signal amplifier, a phase detector, a low-pass filter and a frequency-to-voltage converter; The data calibration module is used to calibrate the optimal mode-locking state of the optical frequency comb and record the power value range of the optical frequency comb output spectrum as standard data; The microcontroller is used to control the current of the pump source so that the optical frequency comb oscillator outputs an optical frequency comb pulse laser; The beam splitter is used to split the optical frequency comb pulse laser into three output paths, the first path is used as a mode-locked detection input photodetector, the second path is input to an optical power meter, and the third path is used as a measurement light for subsequent measurement; The photodetector is used to convert the optical signal into an electrical signal; the signal amplifier is used to amplify the electrical signal; The phase detector is used to mix and phase-detect the amplified electrical signal with the reference frequency generated by the frequency synthesizer to obtain an error signal; The low-pass filter is used to filter the error signal; The frequency-to-voltage converter is used to convert the filtered error signal into a voltage signal; The microcontroller samples the voltage signal while controlling the pump source current to determine whether the mode is locked; if the mode is not locked, the pump current is continuously increased. When the microcontroller calculates that the cumulative sum of the absolute deviations of two adjacent sampling results among the multiple sampling results is less than a set threshold, the mode is locked. When the optical frequency comb is mode-locked, the microcontroller obtains the repetition frequency of the optical frequency comb after the current mode is locked based on the magnitude of the voltage converted by the current acquisition frequency-voltage converter and the signal frequency output by the frequency synthesizer; After the optical frequency comb completes mode locking, the microcontroller triggers serial communication with the optical power meter. The microcontroller reads the real-time power of the optical power meter and determines whether the current power is within the optimal mode-locking power value range calibrated in advance. If so, the optimal mode-locked pulse laser output is completed; if not, the pump current needs to be reduced until the power value measured by the optical power meter is within the calibrated optimal mode-locking power value range.

8. The optical frequency comb automatic mode locking control and detection device according to claim 7, characterized in that: It also includes a temperature control unit, which includes a TEC cooler, a thermistor and a PID controller. The TEC cooler and the thermistor are both arranged in the oscillation cavity of the optical frequency comb oscillator. The thermistor is used as a temperature sensor to monitor the temperature in the cavity; the TEC cooler is used to adjust the temperature in the cavity; and the PID controller is used to control the heating or cooling of the TEC cooler.

9. The optical frequency comb automatic mode locking control and detection device according to claim 7 or 8, characterized in that: Sampling is performed at 10ms intervals, for a total of 20 times; the threshold is set to 100.

Images