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Abstract
This paper presents a comprehensive experimental study of multi-stable-state output characteristics in an all-fiber laser with an acoustic-optical modulator (AOM) as the Q-switcher. For the first time, in this structure, the partitioning of the pulsed output characteristics is explored, dividing the operating status of the laser system into four zones. The output characteristics, the application prospects, and the parameter setting rules for working in stable zones are presented. In the second stable zone, a peak power of 4.68 kW with 24 ns was obtained at 10 kHz. This is the narrowest pulse duration achieved with an AOM actively Q-switched all-fiber linear structure. The pulse narrowing is attributed to the rapid release of signal power and pulse tail truncated by AOM shutdown.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Actively Q-switched all-fiber laser (AQSFL) has the advantages of high efficiency, compact structure, and controllable and adjustable repetition rate. As a single oscillator without an amplification stage, it can generate nanosecond pulse output at watt-level average power. AQSFLs are very desirable in applications such as gas detection [1], generation of THz wave radiation [2], material processing [3], and remote sensing ranging [4], and have broad development prospects [5].
In the linear laser cavity configuration using discrete devices, highly doped fiber and photonic crystal fiber have been used to improve the gain and obtain a Q-switched pulse with a duration of 10 ns [6–8]. However, Q-switched fiber lasers with discrete elements are generally unstable. For the current trend of building all-fiber structures with nanoseconds output, one approach is to use a pigtailed acoustic-optical modulator (AOM) to make an actively Q-switched laser. AOM is also commonly used to reduce the repetition frequency [9,10] and chop continuous waves, which can usually be chopped into tens of nanosecond pulses [11]. While for an AQSFL with a linear cavity, the pulse durations are typically about hundreds of nanoseconds due to the long cavity and the restricted doping concentration of small core diameter single/few-mode gain fibers. The narrow pulse duration achieved so far is usually about 50 ns [12–14]. The highest output recorded for an AQSFL all-fiber laser with a single oscillator linear cavity structure was 37 ns and 5 kW peak power [15]. Therefore, producing a narrow pulse duration is a challenge for linear cavities.
In addition, the AQSFL, where the system with multiple independent variables controls the operating states, is a laser with intracavity gain/loss modulation, and its classification is given by Ref. [16]. Such a laser can exhibit complex dynamic behaviors [17], also called generalized multi-stability, including stable states, chaotic behaviors, such as antiphase and period-doubling routes to chaos, and intermittent chaotic occurring. These complex behaviors do not arise from the external environment perturbations; they are caused by the intrinsic dynamics of operating at a specific repetition rate, which was observed in the regenerative amplification process of solid-state lasers [18–20] as well as in active/passive fiber lasers [21–23]. In such systems, the repetition rate of the system’s stable state is the same as the modulation repetition rate or its subharmonics. Also, the concept of the attractor is introduced to describe the period-doubling bifurcation routes of the system. [24,25] When a complex system evolves towards a particular steady state under parameter modulation, this steady state is called an attractor. The period related to the attractor is expressed as P with a numerical subscript to represent the number of periods. The periodicity of a particular attractor is generally the subharmonic of the modulation repetition rate; for example, PN means the N-subharmonics of the modulation repetition rate.
Although the prevalence of generalized multi-stability in fiber lasers has been observed for a long time [26], the findings have been somewhat sporadic. In the AQSFL using AOM, the P2 attractor route, which is characterized by the output frequency is the second harmonic of the modulation frequency, was first observed by Y. Wang et al. They pointed out that it can generate narrower single pulse output [27]. Y. O. Barmenkov et al. experimentally observed the P2 and P3 attractor routes and the transition state between the two attractors. They investigated the bifurcation path of the system by varying the AOM gate time and the frequency ${\textrm{f}_{\textrm{AOM}}}$ [28]. They also pointed out that the state of the system output is also strongly dependent on the pumping power [29]. Therefore, in the process of tuning the system by changing the input parameters, it is inevitable that bifurcation paths will be encountered, causing the pulse repetition frequency ${\textrm{f}_{\textrm{rep}}}$ to jump to multiple harmonics of the ${\textrm{f}_{\textrm{AOM}}}$ or become chaotic. Many studies in solid regenerative amplification systems have shown that the repetition rate instability can be suppressed by increasing the injected seed energy [30]. However, in an intracavity Q-switcher-controlled fiber laser, the pulses originate from the initial amplified spontaneous emission (ASE) in the cavity, so the appearance of chaotic paths cannot be suppressed by the same means of a regenerative amplification system. There are no detailed rules in fiber lasers to guide the system to achieve and maintain a stable state, controlled and continuously adjustable to avoid period bifurcation and chaos. Therefore, a controllable, continuously adjustable pulse repetition rate over a wide range is difficult to achieve in AQSFL.
To solve these two problems, we studied the complex multi-stable-state behaviors of the AQSFL. The rules and reasons for the variation of multi-state outputs are indicated in section 3. At a fixed AOM repetition rate, the unstable state of the system is attributed to the pump power and AOM modulation duty cycle, for which the output state of the system can be divided into four zones in the corresponding parameter space. This study gives the output characteristics of the AQSFL system in stable zones and unstable zones and summarizes the parameter-setting laws for keeping the system operating in stable zones. This has important implications for the further optimization of these fiber lasers, for the systematic understanding of the unstable pulse repetition rate phenomena, and for the knowledge of their intrinsic non-linear dynamical behaviors. After parameter optimization, we experimentally achieve a system operating with a continuously adjustable pulse repetition rate. This results in a single-peak pulsed output with a 24 ns pulse duration at a watt-level average power and a peak power of 4.68 kW. After the comparison with the works of other groups in Table 1, to our knowledge, this is the narrowest pulse duration currently achievable for this structure [5].
Table 1. Output parameters of active Q-switching all-fiber line structures laser with AOM
2. Experimental setup
The schematic experimental setup of the AQSFL linear cavity is shown in Fig. 1. The 976 nm wavelength-locked LD pump source is spliced to a beam combiner. The HR grating has a reflectivity of 99% and 2 nm bandwidth. The gain fiber length is 2 m (Nufern Yb1200-10/125 DC), and its effective absorption of the pump light is 7.4 dB/m @ 976 nm. To filter out the unabsorbed pump light, a homemade cladding power stripper (CPS) was employed after the gain fiber. The fiber-coupled AOM (CETC26th F-QSG100-1, 2.5 dB insertion loss, 45 dB extinction ratio) is located close to the OC grating side (50% reflectivity, 1 nm bandwidth) to prevent pulse distortion due to stimulated Brillouin scattering (SBS) in the cavity induced by weak reflections from the AOM [28]. The rising edge time of the AOM is a minimum of 50 ns, and the cavity length of 3.2 m was optimized for the suppression of multi-peak pulse phenomenon [40]. The output is connected directly to the oscilloscope (Keysight, DSOS404A, 4 GHz).
Fig. 1. Schematic diagram of actively Q-switched linear cavity all-fiber laser. CPS: cladding power stripper. AOM: acousto-optic modulator. RF: radio frequency.
3. Results and discussion
3.1 System output performance and state classification under variable parameters
The experiments were first operated at a fixed AOM modulation frequency ${\textrm{f}_{\textrm{AOM}}}$ of 100 kHz. By changing the pump power and AOM duty cycle, it was found that the pulse repetition frequency ${\textrm{f}_{\textrm{rep}}}$ appeared to be inconsistent with the ${\textrm{f}_{\textrm{AOM}}}$. As shown in Fig. 2(a), (b), the output states observed have P1, P2, P3, and P4 attractors, which correspond to ${\textrm{f}_{\textrm{rep}}}$, 2${\textrm{f}_{\textrm{rep}}}$, 3${\textrm{f}_{\textrm{rep}}}$ and 4${\textrm{f}_{\textrm{rep}}}$. The transition state between different attractors is uniformly referred to as the transition state. It is usually expressed as adjacent pulses with unequal energy levels. In addition, the system also has random chaotic output; no matter how many cycles are iterated, no stable output can be achieved, and a quasi-steady/steady state can never be reached.
Fig. 2. Preliminary results of the experiment: (a) variable duty cycle at 1.05 W fixed pump power, (b) variable pump power at fixed 14% duty cycle. AOM repetition frequency is set to 100 kHz.
After experimental measurements, we used a phase diagram like what was used in materials science to demonstrate the system's state. The output state distribution of the system was obtained in two-dimensional parameter space by using the duty cycle as the horizontal and the pump power as the vertical axes, respectively. For some parameters, the output signal is too low, or the system does not reach the laser threshold so that the photodetector is not sensitive enough to measure the system’s status, as marked by the dashed white box in Fig. 3. All the output states are plotted and illustrated in Fig. 3. The grey area means that the final output is stable in one of the attractors, the area with grey slashes shows that the system is operating in a transition state between attractors. In comparison, the area with grey dots indicates that the system is in a chaotic state. The blue part in the top right corner of Fig. 3 represents that it is not working in a purely Q-switching condition when the system is in a single cycle; due to the continuous pumping during a long gate time after the release of the Q-switched pulse, the population inversion of the system will accumulate again to reach the laser threshold, releasing a second pulse through free-relaxation oscillation.
Fig. 3. Diagram of the system state with duty cycle and pumping power as horizontal and vertical coordinates, respectively.
With all the multiple attractors, only the P1 attractor ensures that the system's population inversion and gain distribution at the end of a single cycle can be restored to the state at the start of the single cycle, and it is referred to as the stable zone. The partitioning of the stable region has resulted in the system presenting four zones, as numbered by zones I, II, III, and IV, respectively. In the following subsections, the regulations and the causes of system output variation are analyzed both vertically and horizontally, and different output characteristics and application prospects are given for the two stability zones (zone I, III), as well as rules for setting the conditions to maintain the system in the stable zones.
3.2 Physical mechanism behind multi-state operation
The traveling wave rate equations of the system in Fig. 1 can be expressed as:
N is the doping concentration. ${\textrm{N}_\textrm{1}}$ and ${\textrm{N}_\textrm{2}}$ represent the population in the lower and upper energy levels, respectively. ${\mathrm{\lambda }_\textrm{p}}$ and ${\mathrm{\lambda }_\textrm{s}}$ are the pump and signal wavelength. ${\mathrm{\Gamma}_\textrm{p}}$ and ${\mathrm{\Gamma}_\textrm{s}}$ are the pump and signal overlap factor. ${\mathrm{\sigma }_{\textrm{ap}}}$, ${\mathrm{\sigma }_{\textrm{ep}}}$, ${\mathrm{\sigma }_{\textrm{as}}}$, and ${\mathrm{\sigma }_{\textrm{es}}}$ are the absorption and emission cross section of pump and signal. ${\mathrm{\alpha }_\textrm{s}}$ and ${\mathrm{\alpha }_\textrm{p}}$ are the signal and pump loss. ${\textrm{A}_{\textrm{co}}}$ is the area of the fiber core. ${\mathrm{\nu }_\textrm{p}}$ and ${\mathrm{\nu }_\textrm{s}}$ are the pump and signal group velocity. h is the Planck constant. $\Delta {\mathrm{\lambda }_\textrm{s}}$ is the signal bandwidth. τ is the upper energy level lifetime.
The signal and pump will jointly affect the change of the population inversion in the cavity, which can eventually be reflected in the gain level (Eq. (5)) in the cavity. Therefore, the system can be viewed as a one-dimensional discrete dynamical system with gain as a variable. When we use the gain to describe the state of the endpoints of the line cavity for one cycle, the equation is as follows. At the initial moment of the low Q stage, which is the end moment of the high Q stage, the gain is ${\textrm{g}_\textrm{1}}$. At the termination moment of the low Q stage, after the constant accumulation time $\textrm{T - }{\textrm{T}_\textrm{G}}$ of the inversion particle number (let the single cycle time be $\textrm{T}$ and the gate time be ${\textrm{T}_\textrm{G}}$) the gain is ${\textrm{g}_\textrm{2}}$. ${\textrm{g}_\textrm{0}}$ is the small signal gain coefficient. ${\textrm{T}_\textrm{R}}$ is the cavity round trip time. l is the total intracavity loss. ${\textrm{E}_{\textrm{sat}}}$ is the saturation energy.
In the low Q stage, the variation of gain with time is:
In the high Q stage, the variation of gain with time is:
For Eq. (8), Eq. (9), the initial boundary conditions are $\textrm{g}$(0) = ${\textrm{g}_\textrm{2}}$ and $\textrm{E}$(0) = ${\textrm{E}_{\Delta \mathrm{\lambda }}}$. ${\textrm{E}_{\Delta \mathrm{\lambda }}}$ is the ASE component in the cavity corresponding to the reflected band of the output grating at the beginning of a single cycle and is a quantity that varies with pump intensity and accumulation time. The high Q stage has no analytical solution for g and E. In the calculation, it will exhibit the properties of a complex system with the variation of system parameters, appearing to be periodically bifurcated and chaotic. If it gets a single solution, it means that the system is in a steady state under that condition; if it gets a distribution of solutions like bifurcated branches, the system state is distinguished into different attractive sub-routes by the number of bifurcations; if it gets a chaotic distribution of solutions, the system operation state is chaotic. These solutions correspond to various zones, and when the system is affected by parameter changes, the solution of the gain changes, so that the system operating state evolves between different zones.
We qualitatively verified the effect of gain level on the operating state of the system through simulations in Fig. 4. For ${\textrm{f}_{\textrm{AOM}}}$=100 kHz, we preliminarily calculated the solution for gain and output in 1 µs length gate times, plotted them as red and blue dots respectively. The solutions of the system in the chaotic state are densely dispersed and squeezed into red or blue color blocks when plotted. Under low gain and low initial energy conditions in Fig. 4(a), the system has a single solution state representing the stable zone at low duty cycle, and then enters the unstable zone where bifurcated and chaotic solutions exist. Under the conditions of high gain and high initial energy in Fig. 4(d), the system possesses two segments representing a single solution state in the stable zone, and between them is the unstable zone where bifurcated and chaotic solutions exist. Comparing Fig. 4(a), (c) or Fig. 4(b), (d), we can see that high initial gain causes the bifurcation point of the system to occur earlier. Comparing Fig. 4(a), (b) and Fig. 4(c), (d), the high initial ${\textrm{E}_{\mathrm{\lambda }}}$ suppresses the bifurcation complexity of the system. The general trend of the system state change is consistent with the experimental results in Fig. 1.
Fig. 4. Gain and output simulations within 1 µs AOM gate time (corresponds to a duty cycle setting of 10%). The red and blue dots represent the solution for gain and output respectively. The different initial boundary conditions are given in the upper right corner of subgraphs in (a), (b), (c), (d).
3.3 Variations of the output state in the parametric coordinate system
3.3.1 Factors that affect the system’s multi-stable state operation
AQSFL treats the following process as a single cycle: extracting the accumulated population inversion to release pulse each time the AOM is turned on and continuing to build up the population inversion after the AOM is turned off until the next time when it is turned on. This process corresponds to a multi-cycle iteration in which the result at the end of the previous cycle is used as the initial value for the next cycle. As a complex system in which the ongoing state determines the upcoming state, the diverse pulse outputs result from such an iterative run. With continuous pumping, population inversion accumulation, extraction, and gain fiber saturation all influence the single stable cycle.
The variation of the output state under the low-power and the low-duty cycle is even more complicated. Therefore, we mainly pump the system at a fixed 1.05 W and operate it at a fixed 5% duty cycle, respectively, as examples to analyze how the system output is affected by the above factors. Since the gain saturation can be ignored under a low pump condition, the continuous pumping and the accumulation of population inversion are the major influences. Ignoring the chaos and transition state, in Fig. 5(a), the process of P1-P4 appeared first because as the duty cycle increases, the accumulation time decreases, and the gain build-up through a single cycle is insufficient to make the system reach the laser threshold. When the duty cycle is large, the impact of constant pumping will be more substantial so that it comes in the process of P4-P3-P2. When the continuous pumping power is high, the duty cycle required for achieving the later process will be less, as the P2-P1 process in Fig. 5(b).
Fig. 5. Schematic diagram of the process of fixing different pump powers and increasing the duty cycle respectively; (a) system output status @1.05 W; (b) system output status @3.75 W. Schematic diagram of the process of fixing different duty cycles and increasing pump power respectively; (c) system output variations @5% duty cycle; (d) system output variations @ 10% duty cycle. The curves in the figures represent the measured output power, and the different backgrounds represent the output state at the current parameter.
The fixed 5% duty cycle means the gate time is short, leading only to several round trips for the light pulse to extract the inverted population before it is switched out. Low extraction ensures that most of the inverted population is preserved, and the system returns to its pre-cycle state. As the pumping power increases, the change in cavity gain needs to be considered to analyze the difference in the system operating conditions.
The increased gain level in the cavity would increase the extraction of reversed populations; it thus breaks the stability of the single cycle. This is the situation in the process of P1-P3 that appeared first in Fig. 5(c). However, as the pump power continues to increase until the gain fiber is saturated, that is, the gain remains unchanged, and the extraction efficiency no longer increases, the output will change from one pulse output in multiple cycles to one pulse output in a single cycle, as shown in Fig. 5(c), (d) the later P3-P1 process. When the duty cycle is higher, the pump power required for the latter process will become smaller, as shown in Fig. 5(b).
3.3.2 Pulse partially truncated phenomenon
It is worth noting that the transition state from P2 to P1 in Fig. 5(b) (duty cycle 11%-15%) will cause an output power drop. Select the output state of it to analyze the cause of the power drop; the process is shown in Fig. 6. It can be seen that during the transition from P2 to P1 in the system, the energy is gradually transferred from the adjacent high-energy pulse to the new low-energy pulse; At the same time, low-energy vibrations often start to establish when the AOM is turned off due to the low accumulation level of system population inversion, resulting in most of the pulses being cut off by the AOM turn-off behavior and thus the energy was wasted. As the energy transfer between adjacent pulses increases, more energy is wasted being turned off (occurs at duty cycle 11%-15% as shown in Fig. 6(a)-(d). It continued until the single pulse energy of adjacent pulses was almost the same in Fig. 6(d). At the same time, the pulse completes the establishment and releases most of the energy before the AOM is turned off (shown in Fig. 6(c)), and the system enters the second stable zone.
Fig. 6. Output during two Q switching cycles at different duty cycles: (a) 11%; (b) 13%; (c)14%; (d) 15%. AOM repetition frequency is set to 100 kHz, rise time 50 ns. To clearly express the variation of pulse intensity in the 11%-15% duty cycle stage, the pulse in (a) under the 11% duty cycle condition is selected as the standard to normalize (b), (c), (d). Since the state at 12% duty cycle is basically the same as 13%, the figure does not show the 12% state.
In addition, we found that the system’s output power does not increase linearly with the increase of pump power, but there is a platform in Fig. 5(c), (d). This process corresponds to the power drop phenomenon in Fig. 5(b) of Section 3.3.1; pulse energy transfer and cutoff appear in these regions.
3.4 Output characteristics and parameter selection for operation in the stable zone
The operation state is divided by the system’s ability to form a single stable cycle (P1 attractor) as partitioning criteria. As can be seen that there are two cases of stable single-cycle situations: One is the first stable zone I in the lower left corner in Fig. 3, which occurs when the duty cycle is small, and the pumping power does not go too high. At a specific duty cycle and pumping power, the higher the ${\textrm{f}_{\textrm{AOM}}}$, the easier it is to achieve the system operating in the first stable region. A high repetition rate (usually several hundreds of kHz) and a narrow pulse duration (around tens of ns) characterize the output of the first stable zone. The limited pulse duration arises from fast oscillations in a short gate time to extract the population inversion accumulated in the cavity, release the pulse before the gate closes, and cut out part of the pulse to get narrower by shutting down the AOM.
The above analysis of the zone I was verified experimentally in a low-doped gain fiber AQSFL using the same cavity structure as illustrated in Fig. 1. (Took 1.75 m length of LMA-YDF-10/125-9 M, 5.7 dB/m @976 nm. And the OC grating reflectance is 10%, and the total cavity length is 3 m.) For a fixed duty cycle and pumping power, there is a corresponding minimum operation frequency ${\textrm{F}_{\textrm{min}}}$ with ${\textrm{f}_{\textrm{AOM}}}$ satisfies ${\textrm{f}_{\textrm{AOM}}}\textrm{ } \ge \textrm{ }{\textrm{F}_{\textrm{min}}}$, where ${\textrm{F}_{\textrm{min}}}$ is required for the system to operate in the first stable zone. For example, with a pump power of 7 W and a 1% duty cycle, maintaining operation in zone I requires ${\textrm{f}_{\textrm{AOM}}}\textrm{} \ge \textrm{}{\textrm{F}_{\textrm{min}}}$ = 300 kHz. For other duty cycle conditions > 1%, a higher ${\textrm{f}_{\textrm{AOM}}}$ is required.
Another stable single-cycle operation is in the second stable zone III. At a fixed duty cycle and pumping power, the ${\textrm{f}_{\textrm{AOM}}}$ is set to be $\le \textrm{}{\textrm{F}_{\textrm{max}}}$ for the system to operate in zone III. When the above conditions are satisfied, the system repetition rate is controlled and continuously adjustable up to the ${\textrm{F}_{\textrm{max}}}$ With a pump power of 7 W and a 1% duty cycle, for example, maintaining operation in the second stable zone requires ${\textrm{f}_{\textrm{AOM}}}\textrm{} \le \textrm{}{\textrm{F}_{\textrm{max}}}$ = 55 kHz.
The pulse repetition rate that can be maintained under zone III was measured and is shown in Fig. 7(a). The Y-axis is the experimentally measured maximum AOM repetition rate ${\textrm{F}_{\textrm{max}}}$ that is desirable under different conditions. The typical output pulse duration in zone III is usually around hundred nanoseconds. To achieve a narrow pulse duration and high-power output in this state, the duty cycle needs to be reduced at high pumping, which means that the ${\textrm{F}_{\textrm{max}}}$ to satisfy the steady state is also reduced. The output power (peak power and average power) of the system from 10 kHz to 55 kHz for a 1% duty cycle setting at the pump power of 7 W is given in Fig. 7(b). As the ${\textrm{f}_{\textrm{AOM}}}$ increases, the peak power continues to decrease. However, the average output power increases to a maximum flat value and eventually decreases as the ${\textrm{f}_{\textrm{AOM}}}$ further increases. A lower repetition rate would result in higher peak power and narrower pulse duration as shown in Fig. 7(b), (c).
Fig. 7. (a) The maximum Q-switching repetition rate ${\textrm{F}_{\textrm{max}}}$ which is sufficient to maintain the system in the second stable one, as a function of pumping power and duty cycle; (b) plots of peak and average system power with different ${\textrm{f}_{\textrm{AOM}}}$ at 7 W pump and 1% duty cycle. (c) Plots of pulse width with different ${\textrm{f}_{\textrm{AOM}}}$ at 7 W pump and 1% duty cycle. (d) The output pulse of the system at 10 kHz at 7 W pump and 1% duty cycle.
Considering that the operating conditions for the parameters in the second stable zone are more relaxed than those in the first stable zone, we finally chose to operate the system in the second stable zone. With a pumping power of 7 W and a 1% duty cycle setting, the output state is held in the second stable zone and keeping ${\textrm{f}_{\textrm{Laser}}}\textrm{=}{\textrm{f}_{\textrm{AOM}}}$. A narrow pulse duration output of 1.14 W, 24 ns@10 kHz is experimentally obtained in Fig. 7(d). The signal change rate is mainly determined by the product of the proportion of population inversion and the signal, so the faster the rate of change, the steeper the rising edge of the pulse and the narrower the pulse. Another reason for narrow pulse duration is that when the pulse is released, part of the tail of the pulse is truncated when the AOM turns off. Further output peak power increases are directly limited by the power that the device can withstand, as well as the heat generation due to AOM exit losses.
4. Conclusion
We detailed the complex behaviors of an actively Q-switched all-fiber laser based on the AOM technique. We also investigated the origins of the instability and characteristics of the laser. The system output states are divided into four zones. A system with stable operation and a controllable and continuously adjustable repetition rate can only be realized in two stable zones (I & III). When the system is operated in the zone I, there is a minimum value of the AOM repetition rate ${\textrm{F}_{\textrm{min}}}$ required for a given pumping power and duty cycle. And there is a maximum value of the AOM repetition rate ${\textrm{F}_{\textrm{max}}}$ needed for the system to operate at zone III. The typical pulse duration in zone I (tens of nanoseconds) is generally narrower than in zone III (hundreds of nanoseconds). For applications requiring narrow pulse durations, it can be realized in zone I with high repetition rate pulses or zone III with a low repetition rate. After optimization of all parameters, a narrow pulse duration of 24 ns was produced with a peak power of 4.68 kW when it operated at 10 kHz; This is the narrowest pulse duration ever reported with this all-fiber AOM Q-switched linear laser cavity structure. The results of this paper will provide a reference for both the fundamental and the applied research in the field of actively Q-switched all-fiber lasers and give the reader a deeper understanding of the system dynamics of this type of laser.
Funding
CAS Project for Young Scientists in Basic Research (No. YSBR-065); National Natural Science Foundation of China (No. U2033211, No.62175230, No.62175232, No.62225507, No.62275244); Changchun Science and Technology Planning Project (No.21SH08).
Acknowledgments
The authors would like to thank Professor Jing-Yuan Zhang for his precious time and valuable suggestions in revising the English version of this paper.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available but may be obtained from the authors upon reasonable request.
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