Optical frequency comb carrier envelope offset frequency locking system based on closed-loop servo control

Abstract

The invention discloses an optical frequency comb carrier envelope offset frequency locking system based on closed-loop servo control, which is widely applied to the fields of astronomical spectrum calibration, frequency measurement and the like. The system is mainly composed of a femtosecond laser light source, an f-2f module, a photoelectric phase discriminator, a low-pass / band-pass filter, a low-noise amplifier, a digital frequency divider, a digital phase discriminator, a servo controller, a laser diode driver and the like. The initial carrier offset frequency of the optical frequency comb generated by the laser is converted into an electric signal through the f-2f module and the photoelectric phase discriminator, and then digital phase discrimination is carried out on the electric signal and a frequency reference. And the obtained phase error signal is processed by the self-developed servo circuit and then is fed back to the pumping end of the laser so as to modulate the pumping current, so that the high-precision synchronization of the carrier envelope offset frequency of the optical frequency comb and the frequency reference is realized. The system is simple in structure, can remarkably improve the stability of the carrier envelope offset frequency of the optical frequency comb, and has important application value.

Description

Technical Field

[0001] The present invention relates to a high-precision optical frequency comb locking technology, in particular to an optical frequency comb carrier envelope offset frequency locking system based on closed-loop servo control, which is applied to the fields of astronomical spectrum calibration and frequency metrology. Background Art

[0002] Optical Frequency Comb (OFC) is an optical frequency signal generated by a mode-locked laser, with a series of evenly spaced frequency components. Optical frequency comb plays an important role in the fields of high-precision optical frequency measurement, time-frequency transfer and precision spectral analysis, and is an important tool for modern scientific research. The main advantage of fiber frequency comb is that it can provide a high-stability and high-precision optical frequency reference, which greatly promotes the development of frequency metrology and astronomical spectrum calibration.

[0003] Although optical frequency combs have shown significant application potential, they still have many shortcomings. The carrier envelope offset frequency instability is usually as high as 10 -9 This may be acceptable in some applications, but it is still insufficient for scientific experiments and industrial applications that require extremely high measurement accuracy. In long-term observations, a small drift in frequency may lead to significant errors. In addition, the system is very sensitive to environmental changes. Changes in temperature, humidity, etc. can cause frequency fluctuations, which significantly affect frequency stability.

[0004] Traditional frequency locking methods usually use open-loop control and lack effective resistance to external disturbances. Such methods have a lag effect in the phase error feedback process and cannot respond quickly to frequency changes, resulting in measured frequency drift. Although some technologies (such as proportional differential integral control) have been used to try to compensate, the results are often unsatisfactory in complex application environments.

[0005] In view of this, it is particularly urgent to develop a new frequency locking system. The present invention proposes an optical frequency comb carrier envelope offset frequency locking system based on closed-loop servo control. The system can monitor the output frequency of the laser in real time and dynamically adjust the pump current according to the feedback information, thereby achieving effective frequency locking and significantly reducing frequency drift. This method improves the system's anti-interference ability to temperature changes and other environmental factors, and enhances the overall stability and reliability. Summary of the invention

[0006] The invention proposes an optical frequency comb carrier envelope offset frequency locking device based on closed-loop servo control.

[0007] In order to solve the problems mentioned in the background technology such as insufficient stability of the optical frequency comb and susceptibility to environmental interference, the purpose of the present invention is to provide an optical frequency comb carrier-envelope offset frequency locking system, which can achieve the locking of the optical frequency comb carrier-envelope offset frequency and improve its stability.

[0008] To achieve the above object, the present invention provides the following technical solutions: An optical frequency comb stabilization system for high-precision astronomical spectrum calibration, comprising: (1) A mode-locked fiber laser whose gain medium is an ytterbium-doped fiber, comprising a pump source, a gain fiber, a wavelength division multiplexing collimator, a free-space polarization wave plate, and a dispersion-controlled grating pair connected in sequence, for generating a periodic optical frequency comb; (2) The f-2f module obtains the carrier-envelope offset frequency signal by frequency doubling and beating the optical comb’s own spectrum; (3) Photodetector (PD), used to convert the optical frequency comb signal output by the laser into an electrical signal; (4) A filter / amplifier cascade module, which is used to amplify the carrier envelope offset frequency signal and filter out the out-of-band noise to improve the signal quality; (5) Digital frequency divider, used to divide the carrier envelope offset frequency signal to facilitate the subsequent digital phase detection processing; (6) A digital phase detector for obtaining the error signal between the optical signal and the microwave frequency reference; (7) a servo control circuit for receiving an error signal and implementing PID (proportional-integral-differential) control based on the error signal; (8) A laser pump source, which adjusts the laser pump current according to the output of the servo control circuit, thereby achieving synchronization between the laser carrier envelope offset frequency and the frequency reference.

[0009] Preferably, the filtering / amplifying cascade module comprises two low-pass filters, two low-noise amplifiers and a band-pass filter.

[0010] Preferably, the digital frequency divider is composed of a direct digital synthesis module and an output filtering module. The direct digital synthesis module synthesizes the divided signal based on the input signal as the frequency reference, and the divided signal is used as the first input of the digital phase detection after passing through the output filtering.

[0011] Preferably, the digital phase detector uses an operational amplifier and a digital phase detector chip to detect a phase error signal between the optical frequency comb carrier envelope offset frequency and the standard frequency.

[0012] Preferably, the servo control circuit can digitally adjust the loop on / off and PID parameters, and limit the output amplitude to obtain a feedback control signal.

[0013] Preferably, the laser pump source adjusts the output current according to the input feedback control signal, changes the carrier envelope offset frequency of the optical frequency comb, and thus achieves frequency locking.

[0014] The present invention adopts the above technical solution and has the following advantages: Multi-stage amplification and filtering: The multi-stage amplification and filtering scheme further improves the performance of the system, ensuring higher signal quality and more accurate output.

[0015] Customized servo design: The self-designed servo system has a large adjustment range, lower noise level, and a high degree of customization, which makes it easy to remotely adjust the bandwidth and PID coefficient according to actual needs. This ensures the flexibility and adaptability of the system, enabling it to better meet the needs of different application scenarios.

[0016] Real-time monitoring and dynamic adjustment: Through the closed-loop servo control system, the output frequency of the laser is monitored in real time, and the pump current is dynamically adjusted according to the feedback information, achieving effective frequency locking and significantly reducing frequency drift. This method improves the system's ability to resist temperature changes and other environmental factors, enhances overall stability and reliability, and is particularly suitable for high-precision astronomical spectrum calibration and frequency measurement. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 It is a schematic diagram of the overall structure of the optical frequency comb carrier envelope offset frequency locking system of the present invention. DETAILED DESCRIPTION

[0018] The technical solution of the present invention is described clearly and completely below in conjunction with the accompanying drawings. The accompanying drawings are provided only to assist in understanding the present invention and do not constitute a limitation of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in the field without creative work belong to the protection scope of the present invention.

[0019] Embodiment 1:

[0020] See attached Figure 1 The embodiment provides an optical frequency comb carrier envelope offset frequency locking device based on closed-loop servo control, including: a mode-locked laser, an f-2f optical path, a photodetector, a filter / amplifier cascade module, a digital frequency divider, a digital phase detector, an initial frequency servo module and a pump diode.

[0021] Among them, the pump diode, mode-locked laser, f-2f optical path, and photodetector are connected by optical fiber; the filter / amplifier cascade module, digital frequency divider, digital phase detector, and primary frequency servo module are connected by radio frequency transmission line; in addition, in order to realize remote monitoring and control, the main control board and the host computer are connected by wireless connection, such as Figure 1 Indicated by yellow dotted line.

[0022] See attached Figure 1 , the 10 MHz signal generated by the rubidium atomic clock is used as the reference signal of the signal generator, providing a stable and reliable clock reference for the entire system. The signal generator uses the rubidium atomic clock signal as a reference, and the output reference signal is compared with the carrier envelope offset frequency signal output by the laser to achieve calibration and locking of the carrier envelope offset frequency. After the laser output signal passes through the f-2f optical path, the carrier envelope offset frequency signal is generated by the photodetector (PD), which is then connected to the frequency counter for data measurement to analyze the frequency stability of the system. At the same time, the carrier envelope offset frequency signal output by the laser is converted into an electrical signal by the photoelectric phase detector and connected to the spectrum analyzer for monitoring for subsequent debugging and locking. The carrier envelope offset frequency signal after frequency division matches the output frequency of the signal generator, and the two signals are compared by the phase detector to form an error signal. After filtering and amplification, the error signal is input into the servo circuit, and the other is connected to the oscilloscope for debugging and monitoring. The error signal processed by the servo circuit is input into the pump laser diode to modulate its current, thereby enabling the Ytterbium-doped fiber optical comb to achieve narrow linewidth locking of the carrier envelope offset frequency, further reducing phase noise.

[0023] Embodiment 2: This embodiment provides an optical frequency comb carrier envelope offset frequency locking method based on the locking system of embodiment 1, and the specific steps are as follows: Step 1: After powering on all parts of the system, turn on the pump diode, adjust the mode-locked laser and f-2f module, and ensure that the laser outputs a stable optical frequency comb power; Step 2: Use a photodetector to detect the carrier envelope offset frequency of the optical pulse, adjust the output optical power of the mode-locked laser and the f-2f module so that the detector output signal has a high signal-to-noise ratio, and adjust the pump diode so that the carrier envelope offset frequency is in the range of 100 MHz to 120 MHz; Step 3: Input the carrier envelope offset frequency signal output by the photodetector into the filter / amplifier cascade module, and adjust the pump current so that the output signal amplitude is about 500 mV to improve the signal processability; Step 4: Send the output of the cascade module to the digital frequency division module for frequency division, and use the signal generator to output a signal synchronized with the frequency reference, and send the two signals to the digital phase detector for phase detection; Step 5: Use an oscilloscope to observe the output of the digital phase detector module. If the output is a square wave with a frequency higher than 600 kHz, adjust the pump current to make the frequency lower than 600 kHz. Then send the signal to the servo control circuit, and input the servo output signal to the modulation port of the pump diode to start the frequency locking process; Step 6: In the frequency locking stage, the modulated carrier envelope offset frequency is connected to the system. At this time, the phase error signal output by the digital phase detector is input into the servo control circuit, the PID parameters and the output voltage amplitude are adjusted, and the error control signal output by the PID is continuously monitored to ensure system stability; Step 7: When the error control signal approaches the DC level, it indicates that the system is close to the locked state. If there is still a small phase error, it is recommended to adjust the PID parameters until the error control signal presents a stable zero level. This state indicates that the carrier envelope offset frequency of the optical frequency comb has been effectively locked.

[0024] The above embodiments are only used to illustrate the present invention, wherein the structure and connection mode of each component can be adjusted and optimized according to actual needs. All equivalent transformations and improvements based on the technical solution of the present invention should be regarded as within the protection scope of the present invention.

Claims

1. A fiber femtosecond laser frequency comb stabilization system for high-precision astronomical spectrum calibration, characterized in that: The system comprises an optical path part, a signal preprocessing part and a feedback control part.

2. The fiber femtosecond laser frequency comb stabilization system according to claim 1, characterized in that: The optical path portion comprises: (1) Mode-locked fiber laser using ytterbium-doped fiber as gain medium; (2) f-2f module, which generates a carrier-envelope offset frequency signal by frequency doubling and beating the optical frequency comb spectrum; (3) Photodetector (APD), which is used to convert the carrier-envelope offset frequency of the optical frequency comb from an optical signal to an electrical signal.

3. The fiber femtosecond laser frequency comb stabilization system according to claim 1, characterized in that: The signal preprocessing part comprises three parts: a filtering / amplifying cascade module, a digital frequency divider and a digital phase detector.

4. The signal preprocessing part according to claim 3, characterized in that: The filtering / amplifying cascade module comprises two low-pass filters, a band-pass filter and two low-noise amplifiers.

5. The signal preprocessing part according to claim 3, characterized in that: The digital frequency divider comprises a direct digital synthesis module and an output filtering module. The direct digital synthesis module uses the input signal as a frequency reference to synthesize the divided signal. The divided signal is used as the first input of the digital phase detector after passing through the output filtering.

6. The signal preprocessing part according to claim 3, characterized in that: The digital phase detector uses an operational amplifier and a digital phase detector chip to detect a phase error signal between an optical frequency comb carrier envelope offset frequency and a standard frequency.

7. The fiber femtosecond laser frequency comb stabilization system according to claim 1, characterized in that: The feedback control part includes two modules: a servo control circuit and a laser pump.

8. The feedback control according to claim 7, characterized in that: The servo control circuit can digitally adjust the loop on-off and PID parameters, and limit the amplitude of the output signal to obtain a feedback control signal.

9. The feedback control according to claim 7, characterized in that: The laser pump source adjusts the output current according to the input feedback control signal, changes the carrier envelope offset frequency of the optical frequency comb, and thus realizes frequency locking.

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