9-shaped cavity laser with wavelength and pulse width capable of being independently and continuously tuned

Abstract

The invention relates to a 9-shaped cavity laser capable of independently and continuously tuning wavelength and pulse width, which comprises a 9-shaped cavity oscillator and a laser parameter tuning and state monitoring feedback system, and is characterized in that the 9-shaped cavity oscillator can realize self-starting fundamental frequency mode locking through the introduction of a nonreciprocal device and the optimization of the position of each device in the 9-shaped cavity oscillator and the length of each optical fiber; the laser parameter tuning and state monitoring feedback system can be used for continuously tuning the wavelength and the pulse width of the 9-shaped cavity oscillator; and the output state of the oscillator is monitored in real time to judge whether the oscillator is in a stable mode-locked state, and the angle of the half-wave plate and the current value of the laser diode are controlled through a random collision recovery algorithm, so that the 9-shaped cavity oscillator is always kept in the stable mode-locked state. According to the invention, the wavelength and the pulse width of the 9-shaped cavity laser can be independently and continuously tuned in a stable mode-locked state, and the 9-shaped cavity laser has a wide application prospect in the field of tunable lasers.

Description

Technical Field

[0001] The invention relates to an optical fiber mode-locked laser, in particular to a 9-shaped cavity laser whose wavelength and pulse width can be independently and continuously tuned, and belongs to the technical field of optical fiber lasers. Background Art

[0002] Ultrafast lasers are powerful auxiliary tools for femtosecond chemistry, nonlinear optics, and biomedicine due to their narrow pulse width and high peak power. The pulse width of ultrashort pulses is usually in the order of femtoseconds. The ultra-narrow pulse width makes it possible to observe and manipulate atoms, thereby monitoring the ultrafast changes in the microscopic world; the extremely high peak power makes ultrashort pulses widely used in surgical medicine and femtosecond precision processing.

[0003] Mode locking technology is a powerful method for generating ultrashort pulses. Its mechanism is to introduce a fixed phase relationship in multiple modes in the resonant cavity, so that the various modes are coherent and constructive, and finally a series of ultrashort pulses can be generated in the time domain. There are three main methods to achieve mode locking in fiber lasers: first, passive mode locking technology based on real saturable absorbers; second, nonlinear polarization rotation technology; third, nonlinear amplifying ring mirror technology. Mode locking based on real saturable absorbers is the most mature passive mode locking technology at present. Real saturable absorbers use their own nonlinear optical effects, which show that the absorption of incident light pulses decreases with the increase of incident light intensity, and finally achieves saturable absorption effect due to Pauli blocking. Commonly used real saturable absorbers include carbon nanotubes, graphene, new two-dimensional materials, and semiconductor saturable absorber mirrors. However, the damage threshold of real saturable absorbers is low, the output energy is small, and they are easily affected by the environment. Their performance will deteriorate over time and their service life is short. The nonlinear polarization rotation technology uses the Kerr effect of the optical fiber itself to make the light entering the optical fiber accumulate different nonlinear phase shifts due to different polarizations, and finally realize the equivalent saturable absorber mode-locking mechanism. The nonlinear polarization rotation mode-locking mechanism has the advantages of large modulation depth and high output power, but its own mechanism determines that it is susceptible to environmental interference, and it is impossible to achieve a fully polarization-maintaining structure and the mode-locking threshold is high. The nonlinear amplifying ring mirror technology is to achieve mode locking by making the nonlinear amplifying ring mirror equivalent to a saturable absorber, and finally achieve mode locking by accumulating different nonlinear phase shifts in the cavity using two opposite directions of light. As a new type of nonlinear amplifying ring mirror structure, the 9-shaped cavity laser adds nonreciprocal devices to the cavity and introduces phase bias, thereby greatly optimizing its self-starting characteristics. Combined with the fully polarization-maintaining fiber, it has good mode-locking stability. In addition, its inherent low noise characteristics make it have important research and application value in the fields of high stability, low noise mode-locked lasers and optical frequency combs.

[0004] Although the figure-9 cavity laser has obvious advantages, the actual laser application also puts forward higher requirements on the tunability of the laser wavelength and pulse width. The inherent structure of the figure-9 cavity laser can only achieve stable mode locking but cannot maintain a stable mode locking state after the wavelength and pulse width are tuned. Therefore, this technology has the inherent defect that it cannot stably lock the mode after the laser parameters are tuned. A new solution is urgently needed to solve this technical problem. Summary of the invention

[0005] The present invention is aimed at the technical problems existing in the prior art, and provides a 9-shaped cavity laser whose wavelength and pulse width can be independently and continuously tuned. By introducing a non-reciprocal device in the cavity, it is helpful to generate the initial giant pulse required for mode locking, and combined with the optimization of the laser parameters, the 9-shaped cavity laser can self-start fundamental frequency mode locking. By introducing a laser parameter tuning and state monitoring feedback system, the wavelength and pulse width can be independently and continuously tuned, and the state of the output pulse can be monitored in real time during the process of continuous wavelength and pulse width tuning. The wave plate angle and the current value of the laser diode are controlled by an algorithm to ensure the mode locking stability of the 9-shaped cavity laser.

[0006] In order to achieve the above-mentioned purpose, the technical scheme of the present invention is as follows: a 9-shaped cavity laser whose wavelength and pulse width can be independently and continuously tuned, the laser includes a 9-shaped cavity oscillator and a laser parameter tuning and state monitoring feedback system, the 9-shaped cavity oscillator is the main loop part, and the laser parameter tuning and state monitoring feedback system is located at the linear arm of the 9-shaped cavity laser and the output end of the 9-shaped cavity laser. The 9-shaped cavity oscillator realizes self-starting fundamental frequency mode locking by introducing non-reciprocal devices, the laser parameter tuning and state monitoring feedback system continuously tunes the wavelength and pulse width of the 9-shaped cavity oscillator, and monitors the oscillator output state in real time, and controls the half-wave plate angle and the current value of the laser diode through an algorithm program to keep the 9-shaped cavity oscillator in a stable mode locking state.

[0007] As an improvement of the present invention, the 9-shaped cavity oscillator includes a pump diode, a wavelength division multiplexer, a π / 2 phase shifter, a first fiber collimator, and an erbium-doped fiber. The pump diode is connected to the pump end of the wavelength division multiplexer, the common end of the wavelength division multiplexer is connected to one end of the gain fiber, the other end of the gain fiber is connected to one end of the first fiber collimator, the signal end of the wavelength division multiplexer is connected to the input end of the π / 2 phase shifter, and the output end of the π / 2 phase shifter is connected to the other end of the first fiber collimator. The pump diode is a laser diode, and the pump light enters the 9-shaped cavity oscillator through the pump end of the wavelength division multiplexer. The π / 2 phase shifter delays the counterclockwise light in the cavity by a phase of π / 2, so the clockwise and counterclockwise light will accumulate a certain nonlinear phase shift to achieve stable fundamental frequency mode locking.

[0008] As an improvement of the present invention, a laser parameter tuning and state monitoring feedback system includes a grating, a prism, an adjustable slit, a half wave plate, a polarization beam splitter, a reflector, a second fiber collimator and a monitoring feedback module. The first fiber collimator converts the fiber light into spatial light which passes through the grating, the prism and the adjustable slit in sequence. The position of the adjustable slit is along the direction in which the laser spectrum is expanded. By horizontally adjusting the position of the adjustable slit and controlling the laser spectrum components blocked by the adjustable slit, continuous tuning of the spectrum wavelength can be achieved. By keeping the position of the adjustable slit unchanged and adjusting the width of the adjustable slit, continuous tuning of the pulse width while keeping the wavelength unchanged can be achieved. The polarization beam splitter inputs the output spatial light to the second fiber collimator, the second fiber collimator converts the spatial light into fiber light and inputs the fiber light into a monitoring feedback module for real-time judgment of the output state of the oscillator. The monitoring feedback module for real-time judgment of the output state of the oscillator includes a photodetector, a frequency counter, a high-speed data acquisition card and a computer. The photodetector detects the output signal of the 9-character cavity oscillator, converts the optical signal into an electrical signal and inputs it into the frequency counter and the high-speed data acquisition card. The frequency counter reads the repetition frequency of the 9-character cavity oscillator in real time, and monitors in real time whether the repetition frequency corresponds to the cavity length of the laser. The high-speed data acquisition card inputs the collected electrical signal into the computer to display the time domain signal in real time and performs a fast Fourier transform to display its spectrum data in real time. The monitoring feedback module for real-time judgment of the output state of the oscillator includes the following steps:

[0009] Step 1: The optical signal output by the mode-locking is converted into an electrical signal by a photoelectric detector and then connected to a frequency counter and a high-speed data acquisition card. The high-speed data acquisition card collects the time domain signal and performs a fast Fourier transform to obtain a frequency domain signal.

[0010] Step 2: input the frequency information collected by the frequency counter and the time domain and frequency domain signals collected by the high-speed data acquisition card into a computer to identify the fundamental frequency mode-locking state;

[0011] Step 3: If the state is identified as a locked mode, the monitoring of the locked mode is terminated; otherwise, the random collision recovery algorithm is executed;

[0012] Step 4: According to the search results of the random collision recovery algorithm, the angle of the half wave plate and the current value of the laser diode are adjusted, and after the adjustment, the process returns to step 2 to identify the fundamental frequency mode-locked state;

[0013] Furthermore, the random collision algorithm specifically includes the following steps:

[0014] Step 3.1, during the independent continuous tuning of wavelength and pulse width, the angle θ of the half wave plate in the 9-shaped cavity oscillator and the current value Cu of the laser diode are initialized as initial values;

[0015] Step 3.2: Give the half-wave plate a step angle Δθ, then the angle of the half-wave plate is θ+Δθ. At this time, the computer calculates the current target value and compares it with the target value of the previous state. If the current target value is better, the current target value is saved as the starting target value for the next exploration, otherwise it returns to the previous target value.

[0016] Step 3.3: Perform a single exploration of the four directions of the two dimensions of the angle of the half-wave plate and the current value of the laser diode according to step 3.2. If a better target value cannot be obtained, then perform an orthogonal exploration of the angle of the half-wave plate and the current value of the laser diode to obtain a better target value.

[0017] Among them, in the random collision recovery algorithm, the standard for judging the stability of pulse mode locking is realized based on frequency counting, time domain pulse peak-to-peak root mean square and fast Fourier transform spectrum.

[0018] This figure-of-nine cavity laser with independently and continuously tunable wavelength and pulse width can be used as a research platform, providing guidance for the selection of tuning devices for specific wavelengths and pulse widths and coupler splitting ratios.

[0019] Compared with the prior art, the present invention has the following advantages: 1) in a stably mode-locked 9-shaped cavity laser, the position of the adjustable slit is along the direction of the laser spectrum expansion. By adjusting the position of the adjustable slit horizontally and controlling the laser spectrum components blocked by the adjustable slit, continuous tuning of the spectrum wavelength can be achieved, and the tuning amount reaches 65nm; 2) in a stably mode-locked 9-shaped cavity laser, the laser parameter tuning and state monitoring feedback system can fix the position of the adjustable slit. Under the premise that the central wavelength remains unchanged, the width of the adjustable slit can be changed to achieve continuous tuning of the pulse width. The pulse width tuning range is 6.43ps-0.91ps, and it is expected to achieve a more compact and convenient tunable ultrashort pulse light source; 3) in the process of continuous and independent tuning of wavelength and pulse, the monitoring feedback module monitors the output state of the 9-shaped cavity laser in real time, uses an algorithm to control the angle of rotating the half-wave plate and change the current value of the laser diode, and scans the parameter domain of the laser within a certain range to ensure the stable mode locking of the laser, thereby greatly improving the stability of the 9-shaped cavity laser. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 The schematic diagram of the structure of a figure-9 cavity laser whose wavelength and pulse width can be independently and continuously tuned;

[0021] Figure 2 It is a structural diagram of the monitoring feedback module;

[0022] Figure 3 Flowchart of random collision recovery algorithm;

[0023] Figure 4Schematic diagram of the peak-to-peak jitter RMS stability measurement of the pulse sequence;

[0024] Figure 5 Schematic diagram of the spectrum obtained by fast Fourier transform of the pulse sequence.

[0025] In the figure: 1. laser diode, 2. wavelength division multiplexer, 3. gain fiber, 4. first fiber collimator, 5. phase shifter, 6. laser parameter tuning and state monitoring feedback system, 6-1. grating, 6-2. prism, 6-3. adjustable slit, 6-4. half wave plate, 6-5. polarization beam splitter, 6-6. reflector, 6-7. second fiber collimator, 6-8. monitoring feedback module, 6-8-1. photodetector, 6-8-2. frequency counter, 6-8-3. high-speed data acquisition card, 6-8-4. computer. DETAILED DESCRIPTION

[0026] In order to deepen the understanding of the present invention, the present embodiment is described in detail below with reference to the accompanying drawings.

[0027] Example 1: See Figure 1 , a 9-shaped cavity laser whose wavelength and pulse width can be independently and continuously tuned, the laser includes a 9-shaped cavity oscillator and a laser parameter tuning and state monitoring feedback system, the 9-shaped cavity oscillator is the main loop part, and the laser parameter tuning and state monitoring feedback system is located at the linear arm of the 9-shaped cavity laser and the output end of the 9-shaped cavity laser. The 9-shaped cavity oscillator realizes self-starting fundamental frequency mode locking by introducing non-reciprocal devices, the laser parameter tuning and state monitoring feedback system continuously tunes the wavelength and pulse width of the 9-shaped cavity oscillator, and monitors the oscillator output state in real time, and controls the half-wave plate angle and the current value of the laser diode through an algorithm program to keep the 9-shaped cavity oscillator in a stable mode locking state.

[0028] The 9-shaped cavity oscillator comprises a pump diode 1, a wavelength division multiplexer 2, a π / 2 phase shifter 5, a first fiber collimator 4, and an erbium-doped fiber 3. The pump diode 1 is connected to the pump end of the wavelength division multiplexer 2, the common end of the wavelength division multiplexer 2 is connected to one end of the gain fiber 3, the other end of the gain fiber 3 is connected to one end of the first fiber collimator 4, the signal end of the wavelength division multiplexer 2 is connected to the input end of the π / 2 phase shifter 5, and the output end of the π / 2 phase shifter 5 is connected to the other end of the first fiber collimator 4. The pump diode is a laser diode, and the pump light enters the 9-shaped cavity oscillator through the pump end of the wavelength division multiplexer. The π / 2 phase shifter delays the counterclockwise light in the cavity by a phase of π / 2, so the clockwise and counterclockwise light will accumulate a certain nonlinear phase shift to achieve stable fundamental frequency mode locking.

[0029] The laser parameter tuning and state monitoring feedback system comprises a grating, a prism, an adjustable slit, a half wave plate, a polarization beam splitter, a reflector, a second fiber collimator and a monitoring feedback module. The first fiber collimator converts the fiber light into spatial light which passes through the grating, the prism, the adjustable slit, the half wave plate, the polarization beam splitter and the reflector in sequence. The position of the adjustable slit is along the direction in which the laser spectrum is expanded. By adjusting the position of the adjustable slit horizontally and controlling the laser spectrum component blocked by the adjustable slit, continuous tuning of the spectrum wavelength can be achieved. By keeping the position of the adjustable slit unchanged and adjusting the width of the adjustable slit, continuous tuning of the pulse width while keeping the wavelength unchanged can be achieved. The polarization beam splitter inputs the output spatial light to the second fiber collimator, the second fiber collimator converts the spatial light into fiber light and inputs the fiber light into a monitoring feedback module for real-time judgment of the output state of the oscillator. The monitoring feedback module for real-time judgment of the output state of the oscillator includes a photodetector, a frequency counter, a high-speed data acquisition card and a computer. The photodetector detects the output signal of the 9-figure cavity oscillator, converts the optical signal into an electrical signal and inputs it into the frequency counter and the high-speed data acquisition card. The frequency counter reads the repetition frequency of the 9-figure cavity oscillator in real time, and monitors in real time whether the repetition frequency corresponds to the cavity length of the laser. The high-speed data acquisition card inputs the received electrical signal into the computer for real-time display and performs fast Fourier transform to display its spectrum data in real time. When it is determined that the 9-figure cavity oscillator cannot be stably locked, the angle of the rotating half-wave plate is controlled by using a random collision recovery algorithm and the current value of the laser diode is changed to scan the parameter domain of the 9-figure cavity oscillator. When stable locking is achieved, the adjustment of the half-wave plate is stopped.

[0030] Example 2: Schematic diagram of the structure of a nine-shaped cavity laser with independently and continuously tunable wavelength and pulse width Figure 1 Shown

[0031] The nine-shaped cavity laser includes a laser diode 1, a wavelength division multiplexer 2, a gain fiber 3, a first fiber collimator 4, a phase shifter 5, and a laser parameter tuning and state monitoring feedback system 6. The laser parameter tuning and state monitoring feedback system 6 includes a grating 6-1, a prism 6-2, an adjustable slit 6-3, a half-wave plate 6-4, a polarization beam splitter 6-5, a reflector 6-6, a second fiber collimator 6-7, and a monitoring feedback module 6-8, wherein the monitoring feedback module 6-8 includes a photodetector 6-8-1, a frequency counter 6-8-2, a high-speed data acquisition card 6-8-3, and a computer 6-8-4.

[0032] Laser diode 1 emits pump light, and the pump light enters the 9-shaped cavity oscillator through the pump end of wavelength division multiplexer 2. When the gain of the gain fiber is equal to the loss in the cavity, laser oscillation occurs in the fiber loop, and the noise pulse of the fiber loop enters the main loop from the fiber collimator 4 through reflection-mirror reflection. When the width of the adjustable slit 6-3 is set to be larger, the light in the laser is almost unaffected by the adjustable slit 6-3. The phase shift amount of the phase shifter 5 is selected to be π / 2, and the gain fiber 3 is an erbium-doped fiber. The length from the fiber collimator 4 to the gain fiber 3 in the clockwise direction is set to a first preset length, and the length from the fiber collimator 4 to the gain fiber 3 in the counterclockwise direction through the phase shifter 5 is set to a second preset length, wherein the second preset length is greater than the first preset length. The clockwise light in the main loop is first amplified by the gain fiber, and the counterclockwise light is amplified later. A certain nonlinear phase shift difference will accumulate in the two directions in the fiber loop. After passing through the photodetector, the optical signal is converted into an electrical signal and input into the monitoring feedback module 6-8. The monitoring feedback module 6-8 monitors the mode-locking state in the cavity in real time. When the 9-shaped cavity laser is not mode-locked, the monitoring feedback module 6-8 adjusts the angle of the half-wave plate 6-4 and the current value of the laser diode 1 through an algorithm. When the 9-shaped cavity laser achieves stable fundamental frequency mode-locking, the adjustment is stopped, and finally the stable mode-locking of the 9-shaped cavity laser can be achieved.

[0033] After the 9-shaped cavity laser achieves stable fundamental frequency mode locking, the wavelength and pulse width are independently and continuously tuned by adjusting the laser parameter tuning and state monitoring feedback system 6. The adjustable slit 6-3 mainly plays the function of laser parameter tuning. The adjustable slit 6-3 should select a suitable width. If the width is too large, it cannot play the role of filtering. If the width is too small, it will affect the stable mode locking of the laser. Therefore, the width of the adjustable slit 6-3 is set to 0.2mm, and the corresponding filter bandwidth is 4.46nm. Keep the width of the adjustable slit 6-3 and the driving current of the laser diode 1 unchanged. The position of the adjustable slit 6-3 is along the direction of the laser spectrum expansion. By adjusting the position of the adjustable slit 6-3 horizontally, the laser spectrum components blocked by the adjustable slit 6-3 are controlled, and the tuning of the center wavelength of the spectrum can be achieved. The center wavelength can be tuned between 1530nm-1595nm, and the tuning range is 65nm. Since the width of the adjustable slit 6-3 remains unchanged, the filter bandwidth is also basically unchanged, and the output pulse width remains basically unchanged. The driving current of the laser diode 1 remains unchanged, and the position of the adjustable slit 6-3 is moved horizontally so that the central wavelength of the laser output is 1565nm. At this time, the width of the adjustable slit 6-3 is adjusted so that the bandwidth of the spectrum varies from 0.64nm to 4.26nm, and the corresponding pulse width varies from 6.43ps to 0.91ps.

[0034] In the process of realizing independent and continuous tuning of wavelength and pulse width, changing the width and position of the adjustable slit 6-3 may affect the stable mode locking of the 9-shaped cavity laser. At this time, the monitoring feedback module 6-8 of the laser parameter tuning and state monitoring feedback system 6 monitors the mode locking state of the 9-shaped cavity laser in real time. The structural diagram of the monitoring feedback module is shown in FIG. Figure 2 As shown, the monitoring feedback module includes a photoelectric detector 6-8-1, a frequency counter 6-8-2, a high-speed data acquisition card 6-8-3 and a computer 6-8-4. The photoelectric detector 6-8-1 converts the optical signal into an electrical signal and then connects it to the frequency counter 6-8-2 and the high-speed data acquisition card 6-8-3. The specific steps of the monitoring feedback module are as follows:

[0035] Step 1: The optical signal output by the mode-locking is converted into an electrical signal by the photoelectric detector 6-8-1 and then connected to the frequency counter 6-8-2 and the high-speed data acquisition card 6-8-3. The high-speed data acquisition card 6-8-3 collects the time domain signal and performs a fast Fourier transform to obtain the frequency domain signal;

[0036] Step 2, input the frequency information collected by the frequency counter 6-8-2 and the time domain and frequency domain signals collected by the high-speed data acquisition card 6-8-3 into the computer to identify the fundamental frequency mode-locking state;

[0037] Step 3: If the state is identified as a locked mode, the monitoring of the locked mode is terminated; otherwise, the random collision recovery algorithm is executed;

[0038] Step 4: According to the search results of the random collision recovery algorithm, the angle of the half wave plate 6-4 and the current value of the laser diode 1 are adjusted, and after the adjustment, the process returns to step 2 to identify the fundamental frequency mode-locked state;

[0039] Further, the random collision algorithm flow chart is as follows: Figure 3 The specific steps are as follows:

[0040] Step 3.1, during the independent continuous tuning of wavelength and pulse width, the angle θ of the half wave plate 6-4 in the 9-shaped cavity oscillator and the current value Cu of the laser diode 1 are initialized as initial values;

[0041] Step 3.2, give the half wave plate 6-4 a step angle Δθ, then the angle of the half wave plate 6-4 is θ+Δθ, at this time the computer calculates the current target value and compares it with the target value of the previous state, if the current target value is better, then save the current target value as the starting target value for the next exploration, otherwise it still returns to the previous target value;

[0042] Step 3.3, according to step 3.2, a single exploration is performed on the angle of the half wave plate 6-4 and the current value of the laser diode 1 in four directions of two dimensions. If a better target value cannot be obtained, then an orthogonal exploration is performed on the angle of the half wave plate 6-4 and the current value of the laser diode 1 to obtain a better target value;

[0043] Furthermore, in the random collision algorithm, the step angle Δθ of the half wave plate 6-4 is set to 0.1°, and the step current value ΔC of the laser diode 1 is set to u It is 0.5mA. During the continuous tuning of wavelength and pulse width, the adjustment range of the angle of the half wave plate 6-4 is ±5°, and the adjustment range of the laser diode current 1 is ±20mA.

[0044] In order to accurately calculate the signal collected by the computer, it is necessary to give a threshold range of the target value or a given target function to calculate the target value. After the 9-shaped cavity laser is built, the cavity length is fixed, and the corresponding repetition frequency is fixed. Since there is no feedback control on the cavity length, the repetition frequency will drift to a certain extent in the laboratory environment. Assume that the repetition frequency at this time is F rep At this time, the target value threshold range collected by the frequency counter is set to F rep ±3kHz. When the repetition frequency is detected outside this range, it is considered that the mode locking is unstable.

[0045] The time domain signal collected by the high-speed data acquisition card 6-8-3 is a series of pulse sequences. Figure 4 The peak-to-peak value of the pulse sequence is measured in the RMS stability diagram. The peak-to-peak value of the pulse sequence is collected within the range of 5000ns, and the average value of the peak-to-peak value of the pulse sequence is calculated to be

[0046]

[0047] Where n is the pulse count in the range of 5000ns, A i is the amplitude of the pulse. The standard deviation of the peak-to-peak value of the pulse train is

[0048]

[0049] Where n is the pulse count in the range of 5000ns, A i is the pulse amplitude, V ave is the average value of the peak-to-peak value of the pulse sequence. Therefore, the objective function of the peak-to-peak mean square error stability of the pulse sequence can be obtained as follows:

[0050]

[0051] Among them, V ave is the average value of the peak-to-peak value of the pulse train, V σis the standard deviation of the peak-to-peak value of the pulse sequence. Therefore, the objective function of calculating the peak-to-peak mean square error stability of the pulse sequence can obtain the corresponding target value. The smaller the target value, the more stable the mode locking. At this time, when the calculated target value is less than 5‰, the mode locking is in a stable state.

[0052] The frequency domain signal can be obtained by fast Fourier transforming the pulse sequence in the time domain. Figure 5 This is a schematic diagram of the spectrum obtained by fast Fourier transform of the pulse sequence. At this time, the signal-to-noise ratio is measured under the conditions of 1 MHz range of the baseband spectrum and 1 kHz resolution. At this time, it is considered that when the signal-to-noise ratio is greater than 55dB, the mode locking is in a stable state.

[0053] Whether the 9-shaped cavity laser is in stable fundamental frequency mode-locking is determined by simultaneously calculating whether the frequency information of the frequency counter 6-8-2, the time domain pulse peak-to-peak root mean square stability, and the frequency domain signal-to-noise ratio target values ​​are within the threshold range. When any of the above three target values ​​is not within the range, the random collision recovery algorithm is executed to adjust the half-wave plate 6-4 and the laser diode 1, and scan the parameter domain of the laser. When the 9-shaped cavity laser returns to a stable mode-locking state, the adjustment is stopped to ensure that the 9-shaped cavity laser is always stably mode-locked during the entire wavelength and pulse width tuning process.

[0054] By adjusting the angle of the half-wave plate 6-4 to adjust the intracavity loss, the proportion of light separated by the polarization beam splitter changes. Therefore, this 9-shaped cavity laser with independently and continuously tuned wavelength and pulse width can be used as a research platform to provide guidance and suggestions for the selection of specific wavelength and pulse width tuning and the splitting ratio of the coupler. It should be noted that the above embodiments are not used to limit the protection scope of the present invention. Equivalent changes or substitutions made on the basis of the above technical solutions fall within the scope of protection of the claims of the present invention.

Claims

1. A nine-shaped cavity laser with independently and continuously tunable wavelength and pulse width, characterized in that: The laser comprises a 9-shaped cavity oscillator and a laser parameter tuning and state monitoring feedback system. The 9-shaped cavity oscillator is a main loop part. The laser parameter tuning and state monitoring feedback system is located at the linear arm of the 9-shaped cavity laser and the output end of the 9-shaped cavity laser. The 9-shaped cavity oscillator realizes self-starting fundamental frequency mode locking by introducing a non-reciprocal device. The laser parameter tuning and state monitoring feedback system continuously tunes the wavelength and pulse width of the 9-shaped cavity oscillator, and monitors the oscillator output state in real time. The 9-shaped cavity oscillator is controlled by an algorithm program to keep the 9-shaped cavity oscillator in a stable mode locking state.

2. The nine-shaped cavity laser with independently and continuously tunable wavelength and pulse width according to claim 1, characterized in that: The nine-figure cavity oscillator comprises a pump diode (1), a wavelength division multiplexer (2), a π / 2 phase shifter (5), a first optical fiber collimator (4) and a gain optical fiber (3); the pump diode (1) is connected to a pump end of the wavelength division multiplexer (2); the common end of the wavelength division multiplexer (2) is connected to one end of the gain optical fiber (3); the other end of the gain optical fiber (3) is connected to one end of the first optical fiber collimator (4); the signal end of the wavelength division multiplexer (2) is connected to an input end of the π / 2 phase shifter (5); the output end of the π / 2 phase shifter (5) is connected to the other end of the first optical fiber collimator (4); the pump diode is a laser diode; and the π / 2 phase shifter delays the counterclockwise light in the cavity by a π / 2 phase.

3. The nine-shaped cavity laser with independently and continuously tunable wavelength and pulse width according to claim 2, characterized in that: The laser parameter tuning and state monitoring feedback system (6) comprises a grating (6-1), a prism (6-2), an adjustable slit (6-3), a half wave plate (6-4), a polarization beam splitter (6-5), a reflector (6-6), a second fiber collimator (6-7) and a monitoring feedback module (6-8). The first fiber collimator (4) converts the fiber light into spatial light and sequentially passes through the grating (6-1), the prism (6-2), the adjustable slit (6-3), the half wave plate (6-4), the polarization beam splitter (6-5), the reflector (6-6), the second fiber collimator (6-7) and the monitoring feedback module (6-8). A wave plate (6-4), a polarization beam splitter (6-5) and finally a reflector (6-6) form a loop. The position of the adjustable slit (6-3) is along the direction in which the laser spectrum is expanded. By adjusting the position of the adjustable slit (6-3) horizontally, the laser spectrum component blocked by the adjustable slit (6-3) is controlled to achieve continuous tuning of the spectrum wavelength. The position of the adjustable slit (6-3) is kept unchanged. By adjusting the width of the adjustable slit (6-3), the wavelength is kept unchanged and the pulse width is continuously tuned.

4. The nine-shaped cavity laser with independently and continuously tunable wavelength and pulse width according to claim 3, characterized in that: In the laser parameter tuning and state monitoring feedback system, the monitoring feedback module comprises a photoelectric detector (6-8-1), a frequency counter (6-8-2), a high-speed data acquisition card (6-8-3) and a computer (6-8-4); a polarization beam splitter (6-5) inputs the output spatial light to a second optical fiber collimator (6-7); the second optical fiber collimator (6-7) converts the spatial light into optical fiber light and inputs the optical fiber light into a monitoring feedback module for real-time judgment of the output state of an oscillator; the photoelectric detector (6-8-1) detects the output signal of a 9-character cavity oscillator, converts the optical signal into an electrical signal and inputs it into a frequency counter (6-8-2) and a high-speed data acquisition card (6-8-3); the frequency counter (6-8-2) reads the repetition frequency of the 9-character cavity oscillator in real time; the high-speed data acquisition card (6-8-3) inputs the collected electrical signal into a computer (6-8-4) for real-time display and performs fast Fourier transformation to display its spectrum data in real time.

5. The nine-shaped cavity laser with independently and continuously tunable wavelength and pulse width according to claim 4, characterized in that: The monitoring feedback module comprises the following steps: Step 1, the optical signal output by the mode-locking is converted into an electrical signal by the photoelectric detector (6-8-1) and then connected to the frequency counter (6-8-2) and the high-speed data acquisition card (6-8-3). The high-speed data acquisition card (6-8-3) collects the time domain signal and performs a fast Fourier transform to obtain a frequency domain signal; Step 2, input the frequency information collected by the frequency counter (6-8-1) and the time domain and frequency domain signals collected by the high-speed data acquisition card (6-8-3) into the computer (6-8-4) to identify the fundamental frequency mode-locking state; Step 3: If the state is identified as a locked mode, the monitoring of the locked mode is terminated; otherwise, the random collision recovery algorithm is executed; Step 4: According to the search results of the random collision recovery algorithm, the angle of the half wave plate (6-4) and the current value of the laser diode (1) are adjusted, and after the adjustment, the process returns to step 2 to identify the fundamental frequency mode-locked state.

6. The nine-shaped cavity laser with independently and continuously tunable wavelength and pulse width according to claim 5, characterized in that: The random collision recovery algorithm includes the following steps: Step 3.1, during the independent continuous tuning process of wavelength and pulse width, the angle θ of the half wave plate (6-4) in the 9-shaped cavity oscillator and the current value Cu of the laser diode (1) are initialized as initial values; Step 3.2, give the half wave plate (6-4) a step angle Δθ, then the angle of the half wave plate (6-4) is θ+Δθ, at this time the computer (6-8-4) calculates the current target value and compares it with the target value of the previous state, if the current target value is better, then save the current target value as the starting target value for the next exploration, otherwise it still returns to the previous target value; Step 3.3: Perform a single exploration of the four directions of the two dimensions of the angle of the half wave plate (6-4) and the current value of the laser diode (1) according to step 3.

2. If a better target value cannot be obtained, then perform an orthogonal exploration of the angle of the half wave plate (6-4) and the current value of the laser diode (1) to obtain a better target value.

7. The random collision recovery algorithm of the figure-9 cavity laser with independently and continuously tunable wavelength and pulse width according to claim 6 is characterized in that: The criterion for judging the stability of pulse mode locking is realized based on frequency counting, time domain pulse peak-to-peak root mean square and fast Fourier transform spectrum.

8. The nine-shaped cavity laser with independently and continuously tunable wavelength and pulse width according to claim 5, characterized in that: In the random collision algorithm, the step angle Δθ of the half wave plate (6-4) is set to 0.1°, and the step current value ΔCu of the laser diode is 0.5 mA. During the continuous tuning of the wavelength and pulse width, the adjustment range of the angle of the half wave plate (6-4) is ±5°, and the adjustment range of the current of the laser diode (1) is ±20 mA. Let the repetition frequency at this time be F rep At this time, the target value threshold range collected by the frequency counter (6-8-1) is set to F rep ±3kHz. If the repetition frequency is outside this range, it is considered that the mode locking is unstable. The time domain signal collected by the high-speed data acquisition card (6-8-3) is a series of pulse sequences. The peak-to-peak value of the pulse sequence is collected within the range of 5000ns, and the average value of the peak-to-peak value of the pulse sequence is calculated to be Where n is the pulse count in the range of 5000ns, A i is the pulse amplitude, and the standard deviation of the peak-to-peak value of the pulse train is Where n is the pulse count in the range of 5000ns, A i is the pulse amplitude, V ave is the average value of the peak-to-peak value of the pulse sequence, so the objective function of the peak-to-peak mean square error stability of the pulse sequence can be obtained as Among them, V ave is the average value of the peak-to-peak value of the pulse train, V σ is the standard deviation of the peak-to-peak value of the pulse sequence. The objective function of the peak-to-peak mean square error stability of the pulse sequence is calculated to obtain the corresponding target value. The smaller the target value is, the more stable the mode locking is. At this time, when the calculated target value is less than 5‰, the mode locking is in a stable state.

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