GHz high repetition frequency mode-locked pulse laser

Abstract

The invention relates to the technical field of optical fiber lasers, in particular to a GHz high repetition frequency mode-locked pulse laser. In the invention, a 980nm narrow linewidth laser is used as a pumping source to provide resonant cavity excitation, a section of erbium-doped fiber is used as an intra-cavity gain medium, a Mach-Zehnder phase modulator is used as an intra-cavity mode locking device, and the loss in the resonant cavity is periodically modulated through the Mach-Zehnder phase modulator, so that the resonant cavity is formed. In addition, the cavity length of the laser is about 8.45 m, and the corresponding intra-cavity fundamental frequency is 24.4 MHz, so that the laser can realize harmonic mode locking at the integral multiple of 24.4 MHz, the laser can realize harmonic mode locking with the stable duration longer than 6 hours at the 41st-order harmonic wave and the 42nd-order harmonic wave respectively, the laser is further matched with an external TEC temperature control platform, and the laser can be used for controlling the temperature of the TEC. And the stability and harmonic suppression ratio of the active mode-locked laser can be effectively improved.

Description

Technical Field

[0001] The invention relates to the technical field of optical fiber lasers, in particular to a GHz high repetition rate mode-locked pulse laser. Background Art

[0002] Before the 21st century, human measurement of optical frequency was mainly limited by the accuracy of optical wavelength meters. In order to meet higher measurement requirements, people used 133 The main frequency of Cs atoms near 9.2GHz is used as the reference frequency, connecting the microwave and optical frequency domains through a series of frequency doubling and phase-locked oscillators. However, due to the large size and high complexity of the system, the frequency doubling chain can only provide one or two accurate optical frequency measurement values ​​each year, which greatly limits the development of optical frequency measurement technology. In order to seek more efficient optical frequency measurement methods, researchers at home and abroad are committed to developing more advanced light source technology.

[0003] At the beginning of the 21st century, the invention of optical frequency comb technology opened a new door to solving the problem of frequency measurement. Optical frequency comb technology gives people ultra-high time resolution and ultra-precise frequency measurement, and has gradually become the core technology for studying light sources. As a cutting-edge technology in today's laser and time-frequency science, it effectively links the optical frequency and microwave frequency in the electromagnetic spectrum, providing a high-precision frequency source in a simple and ingenious way. As an important tool for precision frequency measurement and control, it has shown great application potential in time-frequency standards, spectrum analysis, and frequency synthesis. In many applications, high-repetition-rate mode-locked optical frequency combs are urgently needed. The 1GHz mode-locked optical frequency comb can maintain the advantage of high repetition rate while being compatible with ordinary RF devices, becoming a hot topic in the field of microwave photonics in recent years.

[0004] Since the repetition frequency of passive mode-locked lasers is limited by the cavity length design and the frequency tunability is poor, it is difficult to achieve high repetition rate mode-locked pulses above GHz. The advantage of active mode-locking technology is that it can generate high repetition rate, tunable mode-locked optical pulses, and the cavity structure is flexible. However, active mode-locking technology still has some problems:

[0005] First, since actively mode-locked fiber lasers are difficult to operate in a stable state, changes in the polarization state, temperature, or cavity length drift in the cavity can cause the laser to lose lock.

[0006] Second, there is a lot of noise in the laser, such as the spontaneous radiation noise of EDFA and the supermode noise in the cavity, which will cause fluctuations in pulse power and reduce the signal-to-noise ratio.

[0007] Therefore, in order to solve the above two pain points, the present invention proposes a GHz high repetition rate mode-locked pulse laser, which, in conjunction with an external TEC temperature control platform, can effectively improve the stability and harmonic suppression ratio of the active mode-locked laser. Summary of the invention

[0008] The purpose of the present invention is to provide a GHz high repetition rate mode-locked pulse laser to solve the problem that the change of polarization state, temperature change or cavity length drift in the cavity may cause the laser to lose lock and there is a lot of noise in the laser.

[0009] To achieve the above object, the present invention provides a GHz high repetition rate mode-locked pulse laser, comprising a pump source, a fiber ring resonator and an external microwave signal generator, wherein the fiber ring resonator comprises a wavelength division multiplexer, a gain fiber, an isolator, a bandpass filter, an electro-optic modulator and an optical coupler, and the external microwave signal generator is an RF signal generator.

[0010] As a further improvement of the technical solution, the output end of the pump source acts on a wavelength division multiplexer, and the output end of the RF signal generator acts on an electro-optical modulator.

[0011] As a further improvement of the technical solution, the gain fiber is placed on one side of the wavelength division multiplexer, the isolator and the bandpass filter together constitute an optical bandpass filter and isolator integrated device, the optical bandpass filter and isolator integrated device is placed on the side of the gain fiber away from the wavelength division multiplexer, the optical coupler is placed on the side of the wavelength division multiplexer away from the gain fiber, the electro-optical modulator is placed on the side of the optical coupler away from the wavelength division multiplexer, and the side of the electro-optical modulator away from the optical coupler is close to the side of the optical bandpass filter and isolator integrated device away from the gain fiber.

[0012] As a further improvement of the technical solution, the resonant cavity excitation of the pump source is provided by a 980nm narrow linewidth laser.

[0013] As a further improvement of the technical solution, the gain optical fiber is an erbium-doped optical fiber.

[0014] As a further improvement of the technical solution, the electro-optic modulator is a Mach-Zehnder phase modulator, which serves as a mode-locked device in a fiber ring resonator. The Mach-Zehnder phase modulator is used to periodically modulate the loss in the fiber ring resonator to achieve the output of mode-locked optical pulses.

[0015] As a further improvement of the technical solution, the cavity length of the optical fiber ring resonator is 8.45m, corresponding to the cavity fundamental frequency of 24.4MHz.

[0016] As a further improvement of the technical solution, the time-frequency domain parameters of the laser at 994 MHz are characterized as follows:

[0017] In the frequency domain, with a sweep width of 1GHz and a bandwidth resolution of 10kHz, the signal-to-noise ratio (SNR) is >64dB and the harmonic suppression ratio is >55dB.

[0018] At a sweep width of 10 GHz and a bandwidth resolution of 100 kHz, the minimum signal-to-noise ratio (SNR) is > 50 dB.

[0019] When the pulse interval is 1.006ns, the corresponding frequency difference interval is 994MHz;

[0020] The 3dB spectrum is 0.23nm, and the time domain pulse width of the mode-locked pulse reaches the transformation limit pulse width.

[0021] As a further improvement of the technical solution, the laser increases the repetition rate to above 1 GHz, achieves stable mode locking at a frequency of 1.048 GHz, and has a mode locking time of 3 hours in the free-running state. The corresponding frequency domain corresponds to 42nd-order harmonic mode locking. When observed by a spectrum analyzer with a sweep width of 1 GHz and a bandwidth resolution of 10 kHz, the signal-to-noise ratio (SNR) is >50 dB.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] 1. In this GHz high repetition rate mode-locked pulse laser, the mode-locking scheme adopted is to insert a loss modulation device controlled by an external signal into the laser resonant cavity. The applied external modulation signal periodically changes the amplitude or phase of the oscillation mode in the resonant cavity at a certain modulation frequency. When the applied modulation frequency is equal to the longitudinal mode interval, the modulation of each mode will produce a side frequency, and its frequency is consistent with the frequency of the two adjacent longitudinal modes. Due to the interaction between the modes, all modes are synchronized at the appropriate modulation frequency to form a mode-locked sequence pulse, so that the laser can be combined with an external TEC temperature control platform to effectively improve the stability and harmonic suppression ratio of the active mode-locked laser.

[0024] 2. In this GHz high repetition rate mode-locked pulse laser, a 980nm narrow linewidth laser is used as a pump source to provide resonant cavity excitation, and a section of erbium-doped fiber is used as the intracavity gain medium. At the same time, a Mach-Zehnder phase modulator is used as an intracavity mode-locked device. The Mach-Zehnder phase modulator is used to periodically modulate the loss in the resonant cavity to achieve the output of mode-locked optical pulses.

[0025] 3. In this GHz high repetition rate mode-locked pulse laser, the laser cavity length is 8.45m, corresponding to the intracavity fundamental frequency of 24.4MHz. Therefore, the laser can achieve harmonic mode-locking at integer multiples of 24.4MHz, so that the laser can achieve harmonic mode-locking at the 41st and 42nd harmonics with a stable duration of more than 6 hours. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 This is a schematic diagram of the internal structure of the GHz high repetition rate mode-locked pulse laser fiber ring resonator of the present invention.

[0027] Figure 2 This is a schematic diagram of the 1 GHz spectrum of the 41st-order harmonic mode locking of the present invention.

[0028] Figure 3 This is a schematic diagram of the 10 GHz spectrum of the 41st-order harmonic mode-locking of the present invention.

[0029] Figure 4 It is a time domain schematic diagram of the 41st-order harmonic mode locking of the present invention.

[0030] Figure 5 It is a schematic diagram of the 41st-order harmonic mode-locking spectrum of the present invention.

[0031] Figure 6 This is a schematic diagram of the 42nd harmonic mode-locked spectrum of the present invention when observed by a spectrum analyzer with a sweep width of 1 GHz and a bandwidth resolution of 10 kHz. DETAILED DESCRIPTION

[0032] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0033] The mode-locking scheme adopted by a GHz high repetition rate mode-locked pulse laser in the present invention is to insert a loss modulation device controlled by an external signal into the laser resonant cavity, and the applied external modulation signal periodically changes the amplitude or phase of the oscillation mode in the resonant cavity at a certain modulation frequency. When the applied modulation frequency is equal to the longitudinal mode interval, the modulation of each mode will produce a side frequency, and its frequency is consistent with the frequency of two adjacent longitudinal modes. Due to the interaction between the modes, all modes are synchronized at a suitable modulation frequency to form a mode-locked sequence pulse.

[0034] In a specific embodiment, Figure 1 As shown in the figure, a GHz high repetition rate mode-locked pulse laser is composed of a pump source, a fiber ring resonator and an external microwave signal generator, wherein the pump source uses a 980nm narrow linewidth laser to provide excitation, which has high stability and is suitable for long-term operation. The laser signal of the pump source enters the fiber ring resonator through a wavelength division multiplexer, and the gain fiber generates gain under the excitation of the pump source. Under the regulation of the electro-optic modulator, the loss modulation of the oscillation mode inside the resonator is achieved by applying an RF signal, so that the laser can form a stable mode-locked output at a specific frequency.

[0035] The fiber ring resonator includes a wavelength division multiplexer, a gain fiber, an isolator, a bandpass filter, an electro-optic modulator and an optical coupler. The fiber is circulated to enhance the laser output, and the output end of the pump source acts on the wavelength division multiplexer.

[0036] The external microwave signal generator is an RF signal generator, and its output signal is used to modulate the phase of the electro-optic modulator to achieve mode-locked operation.

[0037] At the same time Figure 1 As shown, the gain fiber is placed on one side of the wavelength division multiplexer, the isolator and the bandpass filter together constitute an optical bandpass filter and isolator integrated device, the optical bandpass filter and isolator integrated device is placed on the side of the gain fiber away from the wavelength division multiplexer, the optical coupler is placed on the side of the wavelength division multiplexer away from the gain fiber, the electro-optic modulator is placed on the side of the optical coupler away from the wavelength division multiplexer, and the side of the electro-optic modulator away from the optical coupler is close to the side of the optical bandpass filter and isolator integrated device away from the gain fiber.

[0038] A GHz high repetition rate mode-locked pulse laser of the present invention uses a 980nm wavelength narrow linewidth laser as a pump source to provide the required excitation light and provide effective energy input for the gain medium in the resonant cavity. At the same time, a section of erbium-doped fiber (EDF) is selected as the gain medium in the cavity, which has extremely high gain characteristics and can greatly increase the output power of the laser under appropriate pumping conditions. This configuration not only improves the efficiency of the laser, but also ensures the stability and reliability of its output signal.

[0039] At the same time, in the design of the laser, a Mach-Zehnder phase modulator (MZM) is also used as a mode-locking device in the cavity. By precisely adjusting the Mach-Zehnder phase modulator, the optical loss in the resonant cavity can be periodically modulated, and this process enables the generation and output of mode-locked optical pulses. The application of this technology greatly improves the mode-locking performance of the laser, enabling it to operate in a stable and efficient state.

[0040] The total cavity length of the laser is 8.45 meters, and the corresponding intracavity fundamental frequency is 24.4MHz. This means that the laser can achieve harmonic mode locking at integer multiples of 24.4MHz. This feature is crucial for generating ultrashort laser pulses and high-frequency optical signals. Through such a design, the laser can achieve stable mode locking at the 41st and 42nd harmonics, and the duration is more than 6 hours, showing excellent stability and long-term performance output.

[0041] In practical applications, long-term stable mode-locking performance has significant practical value in optical communications, scientific experiments, industrial applications, etc. This stability and high efficiency enable the laser to meet the requirements of high-precision measurement and long-term operation, broadening its application range in the field of modern optics.

[0042] At the same time, in the performance test of a GHz high repetition rate mode-locked pulse laser of the present invention, the time-frequency domain parameters of the laser at a frequency of 994 MHz were characterized in detail. The specific results are as follows: Figure 2 , Figure 3 , Figure 4 and Figure 5 shown.

[0043] First, in terms of frequency domain signal parameters, the sweep width is set to 1GHz and the bandwidth resolution is set to 10kHz for detection. Under this condition, the signal-to-noise ratio (SNR) of the laser exceeds 64dB. This high signal-to-noise ratio value indicates that the optical signal output by the laser is significantly enhanced compared to the background noise, ensuring the clarity and reliability of the signal. In addition, the harmonic suppression ratio has also reached greater than 55dB, which means that while the laser outputs the main frequency signal, it can effectively suppress the interference of higher harmonics, further improving the signal quality.

[0044] Further performance tests were conducted with a sweep width of 10 GHz and a bandwidth resolution of 100 kHz. In this case, the minimum signal-to-noise ratio (SNR) was still greater than 50 dB. This result shows that the laser maintains good signal quality over a wide frequency range and is suitable for high-precision signal transmission and measurement under various complex conditions.

[0045] In terms of pulse characteristics, it can be observed that the laser pulse interval is 1.006ns, which means that the laser can operate at a relatively high pulse repetition rate. Based on this pulse interval, the corresponding frequency difference interval is calculated to be 994MHz, further verifying the effective working state of the laser at this frequency point.

[0046] In addition, the 3dB spectrum width is 0.23nm, indicating that the spectrum range emitted by the laser is narrow, which is crucial for accuracy and resolution in optical applications. Combined with the calculation of the time-bandwidth product, it is found that the mode-locked pulse width is about 11ps. This short pulse width not only meets the requirements for high time resolution, but also makes the laser more competitive in applications such as optical communications and optical information processing.

[0047] In general, the time-frequency domain parameter characterization results at a frequency of 994 MHz clearly demonstrate the excellent performance of the laser of the present invention, enabling it to achieve higher signal quality and stability in practical applications, and providing strong support for users' needs in various high-precision optical applications.

[0048] In addition, if Figure 6 As shown, in a GHz high repetition rate mode-locked pulse laser of the present invention, the repetition rate is significantly improved, thereby effectively improving the performance and application potential of the laser. Specifically, the laser is adjusted to a repetition rate exceeding 1 GHz, and stable mode locking is achieved at this frequency point.

[0049] At a frequency of 1.048 GHz, the laser exhibits extremely high frequency stability and can maintain mode locking in a free-running state. This stability has been confirmed by actual tests, with the mode locking duration reaching 3 hours. This long-term stable output is an important advantage of the laser in various applications, especially in the fields of precision measurement and signal transmission.

[0050] The mode locking in this state corresponds to the 42nd harmonic, which means that the laser can not only generate stable fundamental laser output but also output higher harmonics when working. The generation of higher harmonics is of great significance for enhancing the spectrum characteristics of the laser and improving the tunability and resolution of the signal.

[0051] In the specific spectrum analysis, a spectrum analyzer with a sweep width of 1GHz and a bandwidth resolution of 10kHz was used for monitoring. After testing, the signal-to-noise ratio (SNR) of the laser under this condition reached more than 50dB. This high SNR indicates that the output signal of the laser is very clean and has relatively few interference signals, ensuring the quality and availability of the signal.

[0052] Therefore, the laser of the present invention shows excellent performance by increasing the repetition rate to more than 1 GHz and achieving stable mode locking at 1.048 GHz. This design not only meets the basic application requirements, but also expands its potential applications in high-precision measurement and complex optical systems, and has broad market prospects and practical application value.

[0053] In summary, the GHz high repetition rate mode-locked pulse laser provided by the present invention can cooperate with an external TEC temperature control platform to effectively improve the stability and harmonic suppression ratio of the active mode-locked laser. And it can avoid the problem that the polarization state change, temperature change or cavity length drift in the cavity in the prior art will cause the laser to lose lock and there is a lot of noise in the laser. By using a 980nm narrow linewidth laser as a pump source to provide resonant cavity excitation, and a section of erbium-doped fiber as the intracavity gain medium, and using a Mach-Zehnder phase modulator as an intracavity mode-locked device, the loss in the resonant cavity is periodically modulated by the Mach-Zehnder phase modulator to achieve the output of mode-locked optical pulses. And its laser cavity length is 8.45m, corresponding to the cavity fundamental frequency of 24.4MHz, so the laser can achieve harmonic mode locking at integer multiples of 24.4MHz, so that the laser can achieve harmonic mode locking with a stable duration of more than 6 hours at the 41st and 42nd harmonics respectively.

[0054] The above shows and describes the basic principles, main features and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited by the above embodiments. The above embodiments and descriptions are only preferred examples of the present invention and are not intended to limit the present invention. Without departing from the spirit and scope of the present invention, the present invention may have various changes and improvements, which fall within the scope of the present invention. The scope of protection of the present invention is defined by the attached claims and their equivalents.

Claims

1. A GHz high repetition rate mode-locked pulse laser, characterized in that: The invention comprises a pump source, a fiber ring resonator and an external microwave signal generator. The fiber ring resonator comprises a wavelength division multiplexer, a gain fiber, an isolator, a bandpass filter, an electro-optic modulator and an optical coupler. The external microwave signal generator is an RF signal generator.

2. The GHz high repetition rate mode-locked pulse laser according to claim 1, characterized in that: The output end of the pump source acts on a wavelength division multiplexer, and the output end of the RF signal generator acts on an electro-optic modulator.

3. The GHz high repetition rate mode-locked pulse laser according to claim 1, characterized in that: The gain optical fiber is placed on one side of the wavelength division multiplexer, the isolator and the bandpass filter together constitute an optical bandpass filter and isolator integrated device, the optical bandpass filter and isolator integrated device is placed on the side of the gain optical fiber away from the wavelength division multiplexer, the optical coupler is placed on the side of the wavelength division multiplexer away from the gain optical fiber, the electro-optical modulator is placed on the side of the optical coupler away from the wavelength division multiplexer, and the side of the electro-optical modulator away from the optical coupler is close to the side of the optical bandpass filter and isolator integrated device away from the gain optical fiber.

4. The GHz high repetition rate mode-locked pulse laser according to claim 1, characterized in that: The resonant cavity excitation of the pump source is provided by a 980nm narrow linewidth laser.

5. The GHz high repetition rate mode-locked pulse laser according to claim 1, characterized in that: The gain optical fiber is erbium-doped optical fiber.

6. The GHz high repetition rate mode-locked pulse laser according to claim 1, characterized in that: The electro-optic modulator is a Mach-Zehnder phase modulator, which is used as a mode-locking device in a fiber ring resonator. The Mach-Zehnder phase modulator is used to periodically modulate the loss in the fiber ring resonator to achieve the output of mode-locked optical pulses.

7. The GHz high repetition rate mode-locked pulse laser according to claim 1, characterized in that: The cavity length of the optical fiber ring resonator is 8.45 m, corresponding to a fundamental frequency of 24.4 MHz.

8. The GHz high repetition rate mode-locked pulse laser according to claim 1, characterized in that: The time-frequency domain parameters of the laser at 994 MHz are characterized as follows: In the frequency domain, with a sweep width of 1GHz and a bandwidth resolution of 10kHz, the signal-to-noise ratio (SNR) is >64dB and the harmonic suppression ratio is >55dB. At a sweep width of 10 GHz and a bandwidth resolution of 100 kHz, the minimum signal-to-noise ratio (SNR) is > 50 dB. When the pulse interval is 1.006ns, the corresponding frequency difference interval is 994MHz; The 3dB spectrum is 0.23nm, and the time domain pulse width of the mode-locked pulse reaches the transformation limit pulse width.

9. The GHz high repetition rate mode-locked pulse laser according to claim 1, characterized in that: The laser increases the repetition rate to above 1 GHz and can achieve stable mode locking at a frequency of 1.048 GHz. The laser can be mode locked for more than 3 hours in a free-running state at room temperature. The mode locking frequency corresponds to the 42nd-order harmonic mode locking. When observed by a spectrum analyzer with a sweep width of 1 GHz and a bandwidth resolution of 10 kHz, the signal harmonic suppression ratio is >50 dB.

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