A 9-shaped cavity laser with independently and continuously tunable wavelength and pulse width

Abstract

The application relates to a 9-shaped cavity laser with independently and continuously tunable wavelength and pulse width, which comprises a 9-shaped cavity oscillator and a laser parameter tuning and state monitoring feedback system. The 9-shaped cavity oscillator can realize self-starting fundamental mode locking through the introduction of a non-reciprocal device and the optimization of the positions of the devices in the 9-shaped cavity oscillator and the lengths of the optical fibers. The laser parameter tuning and state monitoring feedback system can continuously tune the wavelength and pulse width of the 9-shaped cavity oscillator, and can monitor the output state of the oscillator in real time to determine whether the oscillator is in a stable mode-locked state and control the angle of a half-wave plate and the current value of a laser diode by using a random collision recovery algorithm so that the 9-shaped cavity oscillator can always keep in a stable mode-locked state. The application can realize the independent and continuous tuning of the wavelength and pulse width of the 9-shaped cavity laser in a stable mode-locked state, and has a wide application prospect in the field of tunable lasers.

Description

Technical Field

[0001] This invention relates to a fiber mode-locked laser, specifically a 9-cavity laser whose wavelength and pulse width can be independently and continuously tuned, belonging to the field of fiber laser technology. Background Technology

[0002] Ultrafast lasers, with their narrow pulse widths and high peak power, are powerful tools in femtosecond chemistry, nonlinear optics, and biomedicine. The pulse width of ultrashort pulses is typically on the order of femtoseconds; this ultra-narrow pulse width makes it possible to observe and manipulate atoms, thereby monitoring ultrafast changes in the microscopic world. The extremely high peak power enables ultrashort pulses to be widely used in surgical procedures and femtosecond precision machining.

[0003] Mode-locking, a powerful method for generating ultrashort pulses, works by introducing a fixed phase relationship between multiple modes within a resonant cavity. This coherent and constructive relationship between the modes ultimately generates a series of ultrashort pulses in the time domain. There are three main methods for achieving mode-locking in fiber lasers: passive mode-locking based on real saturable absorbers; nonlinear polarization rotation; and nonlinear amplifying ring mirror. Mode-locking based on real saturable absorbers is currently the most mature passive mode-locking technique. Real saturable absorbers utilize their nonlinear optical effects, exhibiting a decrease in absorption of the incident light pulse as the intensity of the incident light increases, ultimately achieving saturable absorption due to Pauli blockade. Commonly used real saturable absorbers include carbon nanotubes, graphene, novel two-dimensional materials, and semiconductor saturable absorber mirrors. However, real saturable absorbers have a low damage threshold, low output energy, are easily affected by the environment, and their performance deteriorates over time, resulting in a short lifespan. Nonlinear polarization rotation technology utilizes the Kerr effect of optical fiber itself to accumulate different nonlinear phase shifts due to the different polarization of light entering the fiber, ultimately achieving an equivalent mode-locking mechanism for a saturable absorber. This mechanism offers advantages such as large modulation depth and high output power; however, its inherent mechanism makes it susceptible to environmental interference, preventing the achievement of a fully polarization-maintaining structure and resulting in a high mode-locking threshold. Nonlinear amplifying ring mirror technology equates a nonlinear amplifying ring mirror to a saturable absorber to achieve mode-locking, utilizing the accumulation of different nonlinear phase shifts within the cavity by two beams traveling in opposite directions. The figure-9 cavity laser, as a novel nonlinear amplifying ring mirror structure, incorporates non-reciprocal devices within the cavity, introducing phase bias and significantly optimizing its self-starting characteristics. Combined with fully polarization-maintaining fiber, it exhibits excellent mode-locking stability. Furthermore, its inherent low-noise characteristics make it valuable for research and application in high-stability, low-noise mode-locked lasers and optical frequency combs.

[0004] Although figure-9 cavity lasers have obvious advantages, practical laser applications place higher demands on the tunability of the laser's wavelength and pulse width. The inherent structure of figure-9 cavity lasers means that they can only achieve stable mode-locking and cannot maintain a stable mode-locking state after wavelength and pulse width tuning. Therefore, this technology has the inherent defect of being unable to achieve stable mode-locking after laser parameter tuning, and there is an urgent need for a new solution to this technical problem. Summary of the Invention

[0005] This invention addresses the technical problems existing in the prior art by providing a figure-9 cavity laser with independently and continuously tunable wavelength and pulse width. By introducing non-reciprocal devices within the cavity, it facilitates the generation of the initial giant pulse required for mode-locking. Combined with optimized laser parameters, the figure-9 cavity laser can self-initiate fundamental frequency mode-locking. Through the introduction of a laser parameter tuning and state monitoring feedback system, independent and continuous tuning of wavelength and pulse width can be achieved. Simultaneously, the state of the output pulse is monitored in real time during continuous wavelength and pulse width tuning, and the mode-locking stability of the figure-9 cavity laser is ensured by controlling the waveplate angle and the laser diode current value through algorithms.

[0006] To achieve the above objectives, the technical solution of the present invention is as follows: a figure-9 cavity laser with independently and continuously tunable wavelength and pulse width. The laser includes a figure-9 cavity oscillator and a laser parameter tuning and status monitoring feedback system. The figure-9 cavity oscillator is the main loop component, and the laser parameter tuning and status monitoring feedback system is located at the linear arm and output end of the figure-9 cavity laser. The figure-9 cavity oscillator achieves self-starting fundamental frequency mode-locking through the introduction of non-reciprocal devices. The laser parameter tuning and status monitoring feedback system continuously tunes the wavelength and pulse width of the figure-9 cavity oscillator while simultaneously monitoring the oscillator output status in real time. An algorithm program controls the half-wave plate angle and the current value of the laser diode to maintain a stable mode-locked state for the figure-9 cavity oscillator.

[0007] As an improvement of the present invention, the figure-9 cavity oscillator includes a pump diode, a wavelength division multiplexer (WDM), 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 WDM. The common end of the WDM is connected to one end of the gain fiber, and the other end of the gain fiber is connected to one end of the first fiber collimator. The signal end of the WDM 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. The pump light enters the figure-9 cavity oscillator through the pump end of the WDM. The π / 2 phase shifter delays the counterclockwise light in the cavity by π / 2 phase, thus accumulating a certain nonlinear phase shift between clockwise and counterclockwise light, achieving 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 polarizing beam splitter, a reflector, a second fiber collimator, and a monitoring feedback module. The first fiber collimator converts fiber light into spatial light, which passes sequentially through the grating, the prism, and the adjustable slit. The position of the adjustable slit is along the laser spectrum unfolding direction. By horizontally adjusting the position of the adjustable slit, the laser spectrum components blocked by the adjustable slit can be controlled, thereby achieving continuous tuning of the spectral wavelength. By keeping the position of the adjustable slit unchanged and adjusting the width of the adjustable slit, the pulse width can be continuously tuned while keeping the wavelength constant. A polarizing beam splitter inputs the output spatial light into a second fiber collimator. The second fiber collimator converts the spatial light into fiber light and inputs the fiber light into a monitoring feedback module that monitors and evaluates the oscillator output status in real time. This monitoring feedback module includes a photodetector, a frequency counter, a high-speed data acquisition card, and a computer. The photodetector detects the output signal of the figure-9 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 figure-9 cavity oscillator in real time and monitors whether the repetition frequency corresponds to the cavity length of the laser. The high-speed data acquisition card inputs the acquired 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. The monitoring feedback module for real-time evaluation of the oscillator output status includes the following steps:

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

[0011] Step 3: If the mode-locked state is identified, the monitoring of the mode-locked state ends; otherwise, the random collision recovery algorithm is executed.

[0012] Step 4: Based on the search results of the random collision recovery algorithm, adjust the angle of the half-wave plate and the current value of the laser diode. After adjustment, return to step 2 to identify the fundamental frequency mode-locking state.

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

[0014] Step 3.1: During the independent and continuous tuning of wavelength and pulse width, the angle θ of the half-wave plate in the 9-cavity oscillator and the current value Cu of the laser diode are used as initial values ​​for initialization.

[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: Following Step 3.2, explore the four directions of the two dimensions of the half-wave plate angle and the laser diode current value individually. If a better target value cannot be obtained, then explore the half-wave plate angle and the laser diode current value orthogonally to obtain a better target value.

[0017] The random collision recovery algorithm uses a criterion for determining pulse mode-lock stability based on frequency counting, the root mean square of the time-domain pulse peak-to-peak value, and the spectrum of the fast Fourier transform.

[0018] This figure-9 cavity laser, whose wavelength and pulse width can be independently and continuously tuned, can serve as a research platform, providing guidance for the selection of tuning devices with specific wavelengths and pulse widths, as well as the beam splitting ratio of couplers.

[0019] Compared with the prior art, the present invention has the following advantages: 1) In a stable mode-locked 9-cavity laser, the position of the adjustable slit is along the laser spectrum unfolding direction. By horizontally adjusting the position of the adjustable slit, the laser spectrum components blocked by the adjustable slit can be controlled, and continuous tuning of the spectral wavelength can be achieved, with a tuning amount reaching 65nm; 2) In a stable mode-locked 9-cavity laser, the laser parameter tuning and state monitoring feedback system can fix the position of the adjustable slit. Under the premise that the center wavelength remains unchanged, changing the width of the adjustable slit can achieve continuous tuning of the pulse width, with a pulse width tuning range of 6.43ps-0.91ps, which is expected to realize a more compact and convenient tunable ultrashort pulse light source; 3) During the continuous independent tuning of wavelength and pulse, the monitoring feedback module monitors the output state of the 9-cavity laser in real time, uses algorithms to control the angle of the half-wave plate rotation and change the current value of the laser diode, and scans the parameter domain of the laser within a certain range to ensure stable mode-locking of the laser, which greatly improves the stability of the 9-cavity laser. Attached Figure Description

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

[0021] Figure 2 This is a schematic diagram of the monitoring feedback module.

[0022] Figure 3 This is a flowchart of the random collision recovery algorithm;

[0023] Figure 4This is a schematic diagram of the root mean square stability measurement of peak-to-peak jitter in a pulse sequence.

[0024] Figure 5 This is a schematic diagram of the spectrum obtained by the fast Fourier transform of a pulse sequence.

[0025] In the diagram: 1. Laser diode, 2. Wavelength division multiplexer, 3. Gain fiber, 4. First fiber collimator, 5. Phase shifter, 6. Laser parameter tuning and status monitoring feedback system, 6-1. Grating, 6-2. Prism, 6-3. Adjustable slit, 6-4. Half-wave plate, 6-5. Polarizing beam splitter, 6-6. Mirror, 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 Implementation

[0026] To enhance understanding of the present invention, the embodiments will be described in detail below with reference to the accompanying drawings.

[0027] Example 1: See Figure 1 A figure-9 cavity laser with independently and continuously tunable wavelength and pulse width is disclosed. The laser includes a figure-9 cavity oscillator and a laser parameter tuning and status monitoring feedback system. The figure-9 cavity oscillator is the main loop component, and the laser parameter tuning and status monitoring feedback system is located at the linear arm and output end of the figure-9 cavity laser. The figure-9 cavity oscillator achieves self-starting fundamental frequency mode-locking through the introduction of non-reciprocal devices. The laser parameter tuning and status monitoring feedback system continuously tunes the wavelength and pulse width of the figure-9 cavity oscillator while simultaneously monitoring the oscillator's output status in real time. An algorithm program controls the half-wave plate angle and the laser diode current value to maintain a stable mode-locked state for the figure-9 cavity oscillator.

[0028] The figure-9 cavity oscillator includes 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, and 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. The pump light enters the figure-9 cavity oscillator through the pump end of the wavelength division multiplexer. The π / 2 phase shifter delays the counterclockwise light within the cavity by π / 2 phase, thus accumulating a certain nonlinear phase shift between clockwise and counterclockwise light, achieving stable fundamental frequency mode locking.

[0029] A laser parameter tuning and status monitoring feedback system includes a grating, a prism, an adjustable slit, a half-wave plate, a polarizing beam splitter, a reflector, a second fiber collimator, and a monitoring feedback module. The first fiber collimator converts fiber light into spatial light, which then passes sequentially through the grating, prism, adjustable slit, half-wave plate, polarizing beam splitter, and reflector. The adjustable slit is positioned along the laser spectrum expansion direction. By horizontally adjusting the position of the adjustable slit, the laser spectral components blocked by the adjustable slit can be controlled, thus achieving continuous tuning of the spectral wavelength. By keeping the position of the adjustable slit constant and adjusting the width of the adjustable slit, continuous tuning of the pulse width while maintaining the wavelength can be achieved. A polarizing beam splitter inputs the output spatial light into a second fiber collimator, which converts the spatial light into fiber light and inputs it into a monitoring feedback module that monitors and evaluates the oscillator's output status in real time. This monitoring feedback module includes a photodetector, a frequency counter, a high-speed data acquisition card, and a computer. The photodetector detects the output signal of the figure-9 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 figure-9 cavity oscillator in real time and monitors 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 a fast Fourier transform to display its spectrum data. When it is determined that the figure-9 cavity oscillator cannot achieve stable mode locking, a random collision recovery algorithm is used to control the angle of the rotating half-wave plate and change the current value of the laser diode to scan the parameter domain of the figure-9 cavity oscillator. When stable mode locking is achieved, the adjustment of the half-wave plate is stopped.

[0030] Example 2: Schematic diagram of a figure-9 cavity laser with independently and continuously tunable wavelength and pulse width. Figure 1 As shown

[0031] The figure-9 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 status monitoring feedback system 6. The laser parameter tuning and status 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 polarizing beam splitter 6-5, a reflector 6-6, a second fiber collimator 6-7, and a monitoring feedback module 6-8. 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, which enters the 9-cavity oscillator through the pump end of wavelength division multiplexer 2. When the gain of the gain fiber equals the loss in the cavity, laser oscillation occurs in the fiber loop. The noise pulse in the fiber loop is reflected by a mirror and enters the main loop from the fiber collimator 4. When the width of the adjustable slit 6-3 is set to be large, the light in the laser is almost unaffected by the adjustable slit 6-3. The phase shift of phase shifter 5 is selected as π / 2. The gain fiber 3 is erbium-doped fiber. The length of fiber collimator 4 from the gain fiber 3 in the clockwise direction is set as the first preset length. The length of fiber collimator 4 from the gain fiber 3 through phase shifter 5 in the counterclockwise direction is set as the second preset length. The second preset length is greater than the first preset length. The clockwise light in the main loop is amplified first through the gain fiber, and the counterclockwise light is amplified later. A certain nonlinear phase shift difference will accumulate between the two directions in the fiber loop. After passing through the photodetector, the optical signal is converted into an electrical signal and input to the monitoring feedback module 6-8. The monitoring feedback module 6-8 monitors the mode-locking status in the cavity in real time. When the figure-9 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 figure-9 cavity laser achieves stable fundamental frequency mode-locking, the adjustment stops, and finally the stable mode-locking of the figure-9 cavity laser can be achieved.

[0033] After the 9-cavity laser achieves stable fundamental frequency mode-locking, independent and continuous tuning of wavelength and pulse width is achieved through adjusting the laser parameter tuning and state monitoring feedback system 6. The adjustable slit 6-3 primarily functions as a laser parameter tuner. The width of the adjustable slit 6-3 must be appropriately selected; too large a width will prevent it from filtering effectively, while too small a width will affect the laser's stable mode-locking. Therefore, the width of the adjustable slit 6-3 is set to 0.2 mm, corresponding to a filtering bandwidth of 4.46 nm. Keeping the width of the adjustable slit 6-3 and the driving current of the laser diode 1 constant, and with the position of the adjustable slit 6-3 aligned with the laser spectrum expansion direction, the laser spectral components blocked by the adjustable slit 6-3 can be controlled by horizontally adjusting its position. This allows tuning of the center wavelength of the spectrum, which can be tuned between 1530 nm and 1595 nm, a tuning range of 65 nm. Since the width of the adjustable slit 6-3 remains constant, the filtering bandwidth also remains essentially constant, thus ensuring that the output pulse width remains essentially unchanged. The driving current of laser diode 1 remains constant. The position of adjustable slit 6-3 is moved horizontally so that the center wavelength of the laser output is 1565nm. At this time, the width of 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] During the process of achieving 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 figure-9 cavity laser. In this case, the mode-locking status of the figure-9 cavity laser is monitored in real time by the monitoring feedback module 6-8 of the laser parameter tuning and status monitoring feedback system 6. A schematic diagram of the monitoring feedback module structure is shown below. Figure 2 As shown, the monitoring feedback module 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. The photodetector 6-8-1 converts the optical signal into an electrical signal, which is then input into 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 mode-locking is converted into an electrical signal by photodetector 6-8-1 and then connected to frequency counter 6-8-2 and high-speed data acquisition card 6-8-3. The high-speed data acquisition card 6-8-3 acquires the time domain signal and performs 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 baseband mode-locking status;

[0037] Step 3: If the mode-locked state is identified, the monitoring of the mode-locked state ends; otherwise, the random collision recovery algorithm is executed.

[0038] Step 4: Based on the search results of the random collision recovery algorithm, adjust the angle of the half-wave plate 6-4 and the current value of laser diode 1. After adjustment, return to step 2 to identify the fundamental frequency mode-locking state.

[0039] Furthermore, the flowchart of the random collision algorithm is as follows: Figure 3 As shown, the specific steps are as follows:

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

[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, the current target value is saved as the starting target value for the next exploration; otherwise, it returns to the previous target value.

[0042] Step 3.3: Following Step 3.2, explore the angle of the half-wave plate 6-4 and the current value of the laser diode 1 in four directions in two dimensions. If a better target value cannot be obtained, then explore the angle of the half-wave plate 6-4 and the current value of the laser diode 1 in an orthogonal manner 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... u The current is 0.5mA. During continuous tuning of wavelength and pulse width, the adjustment range of the half-wave plate 6-4 angle is ±5°, and the adjustment range of the laser diode current 1 is ±20mA.

[0044] To accurately calculate the signal acquired by the computer, a threshold range for the target value or a target function needs to be given for calculation. After the figure-9 cavity laser is built, the cavity length is fixed, and therefore the corresponding repetition frequency is also fixed. Since no feedback control is added to the cavity length, the repetition frequency will drift somewhat in a laboratory environment. Let's assume 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 If the repetition frequency is detected outside the ±3kHz range, it is considered to be mode-locking unstable.

[0045] The time-domain signal acquired by the high-speed data acquisition card 6-8-3 is a series of pulse sequences. Figure 4 This diagram illustrates the measurement of the root mean square stability of peak-to-peak jitter in a pulse sequence. Peak-to-peak values ​​of the pulse sequence are collected within a 5000 ns range, and the average peak-to-peak value of the pulse sequence is calculated.

[0046]

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

[0048]

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

[0050]

[0051] Among them, V ave V is the average of the peak-to-peak values ​​of the pulse sequence. σLet be the standard deviation of the peak-to-peak value of the pulse sequence. Therefore, the objective function for calculating the stability of the peak-to-peak value mean square deviation of the pulse sequence can yield the corresponding target value. The smaller the target value, the more stable the mode-locking. In this case, the calculated target value is less than 5‰, indicating that the mode-locking is in a stable state.

[0052] The frequency domain signal can be obtained by performing a fast Fourier transform on a pulse sequence in the time domain. Figure 5 The diagram shows the spectrum obtained by the fast Fourier transform of the pulse sequence. The signal-to-noise ratio was measured under the condition of 1 MHz range of the fundamental frequency spectrum and 1 kHz resolution. It is assumed that the mode-locking is in a stable state when the signal-to-noise ratio is greater than 55 dB.

[0053] The stability of the 9-cavity laser is determined by simultaneously calculating the frequency information of the frequency counter 6-8-2, the root mean square stability of the pulse peak-to-peak value in the time domain, and whether the target value of the signal-to-noise ratio in the frequency domain is 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-cavity laser returns to the stable mode-locked state, the adjustment is stopped to ensure that the 9-cavity laser remains stably mode-locked throughout the entire wavelength and pulse width tuning process.

[0054] Adjusting the angle of the half-wave plate (6-4) alters the intracavity loss, thus changing the proportion of light split by the polarizing beam splitter. Therefore, this figure-9 cavity laser, with its independently and continuously tunable wavelength and pulse width, can serve as a research platform, providing guidance for selecting specific wavelength and pulse width tuning, as well as the beam splitting ratio of the coupler. It should be noted that the above embodiments are not intended to limit the scope of protection of this invention. Equivalent transformations or substitutions made based on the above technical solutions fall within the scope of protection of the claims of this invention.

Claims

1. A 9-cavity laser with independently and continuously tunable wavelength and pulse width, characterized in that, The laser includes a figure-9 cavity oscillator and a laser parameter tuning and status monitoring feedback system. The figure-9 cavity oscillator is the main loop part, and the laser parameter tuning and status monitoring feedback system is located at the linear arm and output end of the figure-9 cavity laser. The figure-9 cavity oscillator achieves self-starting fundamental frequency mode-locking through the introduction of non-reciprocal devices. The laser parameter tuning and status monitoring feedback system continuously tunes the wavelength and pulse width of the figure-9 cavity oscillator, and monitors the output status of the oscillator in real time. The algorithm program controls the half-wave plate angle and the current value of the laser diode to keep the figure-9 cavity oscillator in a stable mode-locked state. The laser parameter tuning and status monitoring feedback system (6) includes a grating (6-1), a prism (6-2), an adjustable slit (6-3), and a half-wave plate (6-4). -4), polarizing beam splitter (6-5), reflector (6-6), second fiber collimator (6-7) and monitoring feedback module (6-8), the first fiber collimator (4) converts fiber light into spatial light and passes it sequentially through grating (6-1), prism (6-2), adjustable slit (6-3) and half-wave plate (6-4). The polarizing beam splitter (6-5) finally forms a loop through the reflector (6-6). The position of the adjustable slit (6-3) is along the laser spectrum unfolding direction. 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 continuous tuning of the spectral wavelength is achieved. The position of the adjustable slit (6-3) remains unchanged. By adjusting the width of the adjustable slit (6-3), the pulse width is continuously tuned while the wavelength remains unchanged.

2. The 9-cavity laser with independently and continuously tunable wavelength and pulse width according to claim 1, characterized in that, The 9-shaped cavity oscillator includes a pump diode (1), a wavelength division multiplexer (2), a π / 2 phase shifter (5), a first fiber collimator (4), and a gain 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). 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. The π / 2 phase shifter causes the counterclockwise light in the cavity to be delayed by π / 2 phase.

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

4. The figure-9 cavity laser with independently and continuously tunable wavelength and pulse width according to claim 3, characterized in that, The monitoring and feedback module includes the following steps: Step 1: The optical signal output by mode-locking is converted into an electrical signal by a photodetector (6-8-1) and then connected to a frequency counter (6-8-2) and a high-speed data acquisition card (6-8-3). The high-speed data acquisition card (6-8-3) acquires the time domain signal and performs a fast Fourier transform to obtain the 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 baseband mode-locking status; Step 3: If the mode-locked state is identified, the monitoring of the mode-locked state ends; otherwise, the random collision recovery algorithm is executed. Step 4: Based on the search results of the random collision recovery algorithm, adjust the angle of the half-wave plate (6-4) and the current value of the laser diode (1), and return to step 2 to identify the fundamental frequency mode-locked state.

5. The 9-cavity laser with independently and continuously tunable wavelength and pulse width according to claim 4, characterized in that, The random collision recovery algorithm includes the following steps: Step 3.1, during the independent and continuous tuning of wavelength and pulse width, the angle θ of the half-wave plate (6-4) and the current value Cu of the laser diode (1) in the 9-cavity oscillator 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, the current target value is saved as the starting target value for the next exploration; otherwise, it will return to the previous target value. Step 3.3: Following step 3.2, explore the angle of the half-wave plate (6-4) and the current value of the laser diode (1) in four directions in two dimensions. If a better target value cannot be obtained, then explore the angle of the half-wave plate (6-4) and the current value of the laser diode (1) in an orthogonal manner to obtain a better target value.

6. The 9-cavity laser with independently and continuously tunable wavelength and pulse width according to claim 5, characterized in that, The random collision recovery algorithm judges the stability of pulse mode-locking based on frequency counting, the root mean square of the peak-to-peak value of the pulse in the time domain, and the spectrum of the fast Fourier transform.

7. The 9-cavity laser with independently and continuously tunable wavelength and pulse width according to claim 6, 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 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 If the repetition frequency is detected outside the ± 3 kHz range, it is considered to be mode-locking unstable. The high-speed data acquisition card (6-8-3) acquires a series of pulse sequences in the time domain. The peak-to-peak values ​​of these pulse sequences are collected within a 5000 ns range, and the average peak-to-peak value is calculated. in, For pulse counting in the 5000 ns range, Let be the amplitude of the pulse, and the standard deviation of the peak-to-peak value of the pulse sequence be . in, For pulse counting in the 5000 ns range, The amplitude of the pulse. Let the average peak-to-peak value of the pulse sequence be the objective function for the stability of the peak-to-peak value mean square error of the pulse sequence. in, This is the average of the peak-to-peak values ​​of the pulse sequence. The standard deviation of the peak-to-peak value of the pulse sequence is used to calculate the objective function for the stability of the peak-to-peak value mean square deviation of the pulse sequence, and the corresponding objective value is obtained. The smaller the objective value, the more stable the mode locking. At this time, the calculated objective value is less than 5‰ and the mode locking is in a stable state.

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