An Automatic Mode-Locking Detection Method and Device for Optical Frequency Combs with Wide Bandwidth

Abstract

The present invention discloses a method and device for automatically detecting mode locking of an optical frequency comb with a wide frequency band. The current of a pump source is controlled by an FPGA to enable the optical frequency comb oscillator to output optical frequency comb pulsed laser. The optical frequency comb pulsed laser is divided into two paths by a beam splitter for output. The first path serves as the measurement light for subsequent measurement. The second path is converted into an electrical signal by a photodetector I for subsequent judgment of the mode locking state. The FPGA controls and gradually increases the pump current of the pump source. Meanwhile, the electrical signal converted by the photodetector I is input into a radio frequency power detector, and a detected voltage signal is output according to the peak characteristics of the electrical signal. When it is judged through the detected voltage signal that the pulse peak value of the electrical signal output by the photodetector I is greater than 1V, the detected voltage signal is sampled multiple times. The absolute value of the difference between two adjacent sampling results is taken and accumulated. When the accumulated sum is less than the threshold set by the microcontroller, it is judged that the optical frequency comb has completed mode locking, and at this time, the increase of the pump current is stopped. The present invention can achieve automatic mode locking of the optical frequency comb and effective detection of the state after mode locking.

Description

Technical Field

[0001] The present invention relates to the technical field of frequency control, and particularly relates to a method and device for automatically detecting mode locking of an optical frequency comb in a wide frequency band. Background Art

[0002] As a bridge connecting optical frequency standards and microwave frequency standards, optical frequency combs have developed rapidly in recent years due to their simple structure and low cost. The realization of optical frequency combs first solves the problem of difficult measurement of optical frequencies and is the most effective absolute optical frequency measurement tool to date. At the same time, due to the mutual relationship between parameters such as frequency, time, and spatial scale, optical frequency combs provide an ideal research tool for time-frequency transfer, absolute distance, and absolute angle measurement, and have broad application prospects in fields such as manned spaceflight, deep space exploration, satellite time service, and modern manufacturing.

[0003] The basis for the application of optical frequency combs is to achieve effective mode locking of optical frequency combs. Currently, for optical frequency combs, the mode locking methods are mainly divided into two types: active and passive. Active mode locking usually adds a modulation device with periodic loss in the laser cavity, and realizes mode locking through the effective adjustment of the optical path by the modulation device. Passive mode locking utilizes the all-optical nonlinear effect and uses optical devices such as saturable absorbers with pulse intensity screening to complete optical pulse width compression and mode locking. Compared with active mode locking, passive mode locking does not require adding external modulation devices and can achieve mode locking by free running in the cavity. The signal-to-noise ratio of the mode-locked pulse is high. At the same time, the passive mode locking structure is simple and the cost is low, having application advantages.

[0004] For the generation of mode-locked optical pulses of optical frequency combs realized by passive mode locking, the current common method is to adjust the pump laser power after the optical oscillator is built, so that the injected optical oscillator has a high pulse energy. When the pulse energy accumulates to the mode locking threshold, mode locking can be completed. However, at this time, the mode locking state usually still needs to be adjusted, and the spectral smoothness and the presence of multiple pulses will affect subsequent use. This requires the operator to fine-tune the pump current while observing the oscilloscope and spectrometer to achieve the output of the optimal mode locking state. This mode locking method is cumbersome and time-consuming, bringing difficulties to the integrated application of optical frequency combs.

[0005] Currently, there are also methods for automatically detecting mode locking of optical frequency combs. The main method used is to combine a frequency meter and an optical power meter for measurement. The frequency meter is used to detect the frequency change of the mode locking state, the power meter is used to detect the power change in the mode locking state, and the frequency information is used to determine whether mode locking is completed. Although this method can complete mode locking detection, due to the measurement method of the frequency meter, it can only measure whether mode locking occurs and cannot detect the presence of multiple pulses. At the same time, due to the large volume of the frequency meter and the difficulty of integration, it is also difficult to truly realize miniaturized applications. Summary of the Invention

[0006] In view of this, the present invention provides a method and device for automatically detecting mode locking of an optical frequency comb with a wide frequency band, which can realize automatic mode locking of the optical frequency comb and effective detection of the state after mode locking.

[0007] The technical solution adopted by the present invention is as follows:

[0008] A method for automatically detecting mode locking of an optical frequency comb with a wide frequency band controls the current of a pump source through an FPGA, so that the optical frequency comb oscillator outputs optical frequency comb pulsed laser. The optical frequency comb pulsed laser is divided into two paths by a beam splitter for output. The first path is used as measurement light for subsequent measurement; the second path is converted into an electrical signal by a photodetector I for subsequent judgment of the mode locking state.

[0009] The FPGA controls and gradually increases the pump current of the pump source, and at the same time inputs the electrical signal converted by the photodetector I into a radio frequency power detector, and outputs a detection voltage signal according to the peak characteristic of the electrical signal. When it is judged through the detection voltage signal that the pulse peak value of the electrical signal output by the photodetector I is greater than 1V, the detection voltage signal is sampled multiple times, the absolute value of the difference between adjacent two sampling results is taken and accumulated, and when the accumulated sum is less than the threshold set by the microcontroller, it is judged that the optical frequency comb has completed mode locking, and at this time, the increase of the pump current is stopped.

[0010] The present invention also provides a device for automatically detecting mode locking of an optical frequency comb with a wide frequency band, including an FPGA, a pump source, an optical frequency comb oscillator, a beam splitter, a photodetector I, a radio frequency power detector and an analog-to-digital converter;

[0011] The FPGA is used to control the current of the pump source so that the optical frequency comb oscillator outputs optical frequency comb pulsed laser; the beam splitter is used to divide the optical frequency comb pulsed laser into two paths for output, the first path is used as measurement light for subsequent measurement; the second path is input to the photodetector I; the photodetector I is used to convert the optical signal into an electrical signal; the radio frequency power detector is used to output a detection voltage signal according to the peak characteristic of the converted electrical signal; the analog-to-digital converter is used to collect the detection voltage signal for the FPGA to judge the mode locking state of the optical frequency comb. When it is judged through the detection voltage signal that the pulse peak value of the electrical signal output by the photodetector I is greater than 1V, the detection voltage signal is sampled multiple times, the absolute value of the difference between adjacent two sampling results is taken and accumulated, and when the accumulated sum is less than the threshold set by the microcontroller, it is judged that the optical frequency comb has completed mode locking.

[0012] Beneficial effects:

[0013] 1. The present invention enables the mode locking of the optical frequency comb to get rid of the manual operation mode, improves the mode locking quality of the optical frequency comb, enables the optical frequency comb to have a wider application range, and can greatly promote the development of the optical frequency comb in engineering applications. At the same time, the device has a simple structure and an effective method, can realize the direct detection of the mode-locked optical signal of the optical frequency comb, the detection bandwidth can reach the GHz level, there is no need for frequency conversion processing for optical frequency combs with high repetition frequencies, and there is no need to strictly study whether there is temperature control in the optical frequency comb oscillator, and it has strong environmental adaptability.

[0014] 2. The present invention can detect the optimal mode locking state. After testing, during the process of realizing the automatic mode locking of the optical frequency comb, it can generate the optimal mode locking pulse within a time better than 500 ms, ensuring that the optical frequency comb oscillator has good signal light quality during actual output.

[0015] 3. The device of the present invention has a simple structure and is characterized by miniaturization and integration. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 is the working principle diagram of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0017] The present invention will be described in detail below in conjunction with the drawings and by way of examples.

[0018] The present invention provides a wide-band automatic mode locking detection method for an optical frequency comb. By controlling the current of the pump source through an FPGA, the optical frequency comb oscillator outputs optical frequency comb pulsed laser. The optical frequency comb pulsed laser is divided into two paths by a beam splitter. The first path is used as the measurement light for subsequent measurement; the second path is converted into an electrical signal by a photodetector Ⅰ for subsequent judgment of the mode locking state.

[0019] First, the FPGA controls and gradually increases the pump current of the pump laser at a rate of 100 mA / 50 ms, rapidly increasing the pump optical power injected into the optical frequency comb oscillator. At the same time, the electrical signal converted by the photodetector I is input into a radio frequency (RF) power detector to detect the pulse peak of the converted electrical signal in real time. As the FPGA controls the gradual increase of the pump current of the pump laser, the electrical signal output by the photodetector I at this time is an approximately direct current signal that is relatively flat and has certain peak characteristics. As the pump current increases, the direct current signal will gradually rise and be superimposed with a certain amount of white noise, with a small peak, on the order of mV. If the optical frequency comb completes mode locking, the intensity of the electrical signal will increase significantly, and the signal peak will become stable, with the signal intensity reaching the order of V. After inputting such an electrical signal into the RF power detector, the RF power detector outputs a detection voltage signal according to the peak characteristics of the electrical signal converted by the optical frequency comb. This detection voltage signal is acquired by the analog-to-digital converter ADC (12-bit, 4096 points) on the FPGA. Based on the acquired detection voltage signal, when it is determined through the detection voltage signal that the pulse peak of the electrical signal it represents is greater than 1 V, the FPGA continuously samples the detection voltage signal multiple times synchronously. The absolute value of the difference between the results of two adjacent samplings is taken and accumulated. When the accumulated sum is less than the threshold set by the microcontroller, it is determined that the optical frequency comb has completed mode locking. In this embodiment, the FPGA samples the voltage signal for 1 ms, samples 10 times in 10 ms, takes the absolute value of the difference between the results of two adjacent samplings and accumulates the sum. If the accumulated sum is less than 100 and the acquired peak is greater than 1 V, it is determined that the optical frequency comb has completed mode locking. At this time, the increase in the pump current is immediately stopped.

[0020] As an improvement, after the mode-locking of the optical frequency comb is completed, the FPGA triggers the adjustment of the optimal mode-locking pulse. The pulsed laser of the optical frequency comb will also output a third path through the beam splitter for the detection of the optimal mode-locking state; the third path of light is converted into an electrical signal by the photodetector II. After the mode-locking of the optical frequency comb is completed, the FPGA reads the mode-locking peak output by the radio frequency power detector, and then sets the digital-to-analog converter to output a comparison voltage 10 mV lower than the mode-locking peak according to the mode-locking peak; this comparison voltage (DC signal) is compared with the mode-locking pulse signal generated by the photodetector II, and the comparison result is a square wave signal, which is captured by the FPGA and triggers the FPGA in real time. If the FPGA is triggered evenly, there are no multi-pulses, and at this time, it is the output of the optimal mode-locking pulse. When the FPGA is triggered unevenly and is triggered twice in a very short time (about 1 ns), it indicates that there are multi-pulses at this time. The existence of multi-pulses is usually caused by a relatively large pump current. At this time, the FPGA controls the pump source to gradually reduce the pump current at a speed of 10 mA / 50 ms until the FPGA is triggered evenly, thereby completing the adjustment of the optimal mode-locking pulse. At this time, the optical frequency comb completes the output of the optimal mode-locking state. When the optical frequency comb completes the output of the optimal mode-locking pulse, the system state at this time is uploaded to the host computer through the FPGA for the operator to perform remote monitoring.

[0021] The present invention also provides an optical frequency comb wide-band automatic mode-locking detection device for the above-mentioned optical frequency comb wide-band automatic mode-locking detection method, as Figure 1 shown, including an FPGA, a pump source, an optical frequency comb oscillator, a beam splitter, a photodetector I, a radio frequency power detector, an analog-to-digital converter, a photodetector II, a comparator, and a digital-to-analog converter.

[0022] The FPGA is used to control the current of the pump source so that the optical frequency comb oscillator outputs pulsed laser of the optical frequency comb; the beam splitter is used to divide the pulsed laser of the optical frequency comb into three paths for output. The first path is used as the measurement light for subsequent measurement, the second path is input to the photodetector I for subsequent judgment of the mode-locking state, and the third path is input to the photodetector II for detection of the optimal mode-locking state;

[0023] The photodetector I is used to convert the optical signal into an electrical signal;

[0024] The radio frequency power detector is used to output a detection voltage signal according to the peak characteristics of the converted electrical signal;

[0025] The analog-to-digital converter is used to collect the detected voltage signal for the FPGA to judge the mode-locking state of the optical frequency comb. When it is judged through the detected voltage signal that the pulse peak value of the electrical signal output by the photodetector I is greater than 1V, the detected voltage signal is collected multiple times. The absolute value of the difference between the results of two adjacent samplings is taken and the sum is processed. When the sum is less than the threshold set by the microcontroller, it is judged that the optical frequency comb has completed mode-locking. Sampling can be done 10 times in 10ms, and the threshold is 100. That is, the absolute value of the difference between the results of two adjacent voltage signal samplings is taken and accumulated. If the sum is less than 100, combined with the current pulse peak value being greater than 1V, it is judged that the optical frequency comb has completed mode-locking.

[0026] The photodetector II is used to convert the third optical signal into an electrical signal;

[0027] The digital-to-analog converter is used to output a comparison voltage lower than the mode-locking peak value after the optical frequency comb has completed mode-locking;

[0028] The comparator is used to compare the comparison voltage with the mode-locking pulse signal generated by the photodetector II, and the comparison result is a square wave signal;

[0029] The square wave signal is captured by the FPGA to trigger the FPGA in real time. Whether the FPGA is triggered evenly or not is used to judge whether it is the best mode-locking pulse output. If the FPGA is triggered evenly, it is the best mode-locking pulse output. If it is not even and is triggered twice in 1ns, the FPGA controls the pump source to gradually reduce the pump current until the FPGA is triggered evenly, and the optical frequency comb completes the output of the best mode-locking state.

[0030] In summary, the above is only the preferred embodiment of the present invention and is not used to limit the protection scope of the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. An automatic mode-locking detection method for an optical frequency comb with a wide frequency band, characterized in that, The current of the pump source is controlled by an FPGA, so that the optical frequency comb oscillator outputs optical frequency comb pulsed laser. The optical frequency comb pulsed laser is divided into two paths by a beam splitter. The first path is used as the measurement light for subsequent measurements. The second path is converted into an electrical signal by a photodetector I for subsequent judgment of the mode-locking state. The FPGA controls and gradually increases the pump current of the pump source. At the same time, the electrical signal converted by the photodetector I is input to a radio frequency power detector, and a detection voltage signal is output according to the peak characteristics of the electrical signal. When it is judged by the detection voltage signal that the pulse peak of the electrical signal output by the photodetector I is greater than 1V, the detection voltage signal is sampled multiple times. After taking the absolute value of the difference between two adjacent sampling results and performing an accumulation and processing, when the accumulated sum is less than the threshold set by the microcontroller, it is judged that the optical frequency comb has completed mode-locking, and at this time, the increase of the pump current is stopped. The optical frequency comb pulsed laser also outputs a third path through the beam splitter for the detection of the optimal mode-locking state. The third path of light is converted into an electrical signal by a photodetector II. After the optical frequency comb completes mode-locking, the FPGA reads the mode-locking peak output by the radio frequency power detector, and then the FPGA sets a digital-to-analog converter to output a comparison voltage lower than the mode-locking peak according to the mode-locking peak. The comparison voltage is compared with the mode-locking pulse signal generated by the photodetector II, and the comparison result is a square wave signal. The square wave signal is captured by the FPGA and triggers the FPGA in real time. If the FPGA is triggered evenly, then this is the output of the optimal mode-locking pulse at this time. If the FPGA is triggered unevenly and is triggered twice within 1ns, the FPGA controls the pump source to gradually reduce the pump current until the FPGA is triggered evenly, and the optical frequency comb completes the output of the optimal mode-locking state.

2. The method for automatically detecting mode locking of an optical frequency comb with a wide frequency band according to claim 1, characterized in that The pump source includes a voltage-controlled constant current source and a pump laser. The voltage-controlled constant current source is used to provide a driving current for the pump laser, and the FPGA controls the voltage-controlled constant current source to adjust the pump current of the pump laser.

3. The optical frequency comb wideband self-mode locking detection method according to claim 1, wherein The detection voltage signal is collected by an analog-to-digital converter on the FPGA, and the FPGA judges the mode-locking state of the optical frequency comb according to the collected detection voltage signal.

4. The optical frequency comb wide-band self-mode-locking detection method according to claim 3, wherein The FPGA collects the voltage signal for 1ms, samples 10 times in 10ms, takes the absolute value of the difference between two adjacent sampling results and accumulates them. If the accumulated sum is less than 100 and combined with the current pulse peak being greater than 1V, it is judged that the optical frequency comb has completed mode-locking.

5. The method for automatically detecting mode locking of an optical frequency comb with a wide bandwidth according to claim 1, characterized in that The comparison voltage is 10mV lower than the mode-locking peak.

6. An optical frequency comb wide-band automatic mode-locking detection device, characterized in that It includes an FPGA, a pump source, an optical frequency comb oscillator, a beam splitter, a photodetector I, a radio frequency power detector, and an analog-to-digital converter; The FPGA is used to control the current of the pump source so that the optical frequency comb oscillator outputs optical frequency comb pulsed laser; the beam splitter is used to divide the optical frequency comb pulsed laser into two paths for output, the first path is used as the measurement light for subsequent measurement, and the second path is input to the photodetector I; the photodetector I is used to convert the optical signal into an electrical signal; the RF power detector is used to output a detection voltage signal according to the peak characteristics of the converted electrical signal; the analog-to-digital converter is used to collect the detection voltage signal for the FPGA to judge the mode-locking state of the optical frequency comb; when it is judged through the detection voltage signal that the pulse peak value of the electrical signal output by the photodetector I is greater than 1V, the detection voltage signal is sampled multiple times, the absolute value of the difference between adjacent two sampling results is taken and accumulated and processed, and when the accumulated sum is less than the threshold set by the microcontroller, it is judged that the optical frequency comb has completed mode-locking; It further includes a photodetector II, a comparator, and a digital-to-analog converter; The photodetector II is used to convert the third optical signal into an electrical signal; the digital-to-analog converter is used to output a comparison voltage lower than the mode-locking peak value after the optical frequency comb completes mode-locking; the comparator is used to compare the voltage with the mode-locking pulse signal generated by the photodetector II and output a square wave signal for triggering the FPGA; if the FPGA is triggered evenly, then this is the best mode-locking pulse output at this time; if the FPGA is triggered unevenly and is triggered twice within 1ns, the FPGA controls the pump source to gradually reduce the pump current until the FPGA is triggered evenly and the optical frequency comb completes the output of the best mode-locking state.

7. The optical frequency comb wideband self-mode-locking detection device according to claim 6, characterized in that, The comparison voltage is 10mV lower than the mode-locking peak value.

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