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Abstract
In this work, for the first time, four self-sweeping regimes in a single-mode bi-directional ytterbium-doped fiber ring laser are observed by adjusting the polarization controller (PC): normal self-sweeping, reverse self-sweeping, mixed state, and wavelength stop state. In addition, regulating the PC can artificially selectively make the laser operate in normal self-sweeping or reverse self-sweeping within a certain pump power range, and their self-sweeping characteristics (e.g., sweeping rate, sweeping range, etc.) and intensity dynamics are investigated in detail, respectively. In conclusion, we can flexibly regulate the sweeping direction and sweeping characteristics of the bi-directional self-sweeping fiber ring laser in a simple approach by adjusting the PC, which is potentially valuable for its practical application.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Tunable fiber lasers are extremely valuable for applications in optical sensing [1] and optical communication [2] due to their flexible wavelength-tunable characteristics. To achieve stable periodic wavelength tunability, most of them rely on devices such as fiber Bragg gratings [3], heaters [4], and tunable filters [5], which make the laser output performance degraded and the structure complicated, and its development is therefore greatly affected. In recent years, the emerging self-sweeping fiber laser has attracted a lot of attention from researchers for its interesting spectral properties, which can achieve spontaneous, stable, periodic wavelength tunable without the use of complex tuning elements or electric drives, and its simple structure can better meet the needs of practical applications. The self-sweeping effect is attributed to the dynamic grating formed by the spatial hole-burning (SHB) effect of the gain medium in the standing wave field [6], where the dynamic variation in the net gain of the grating leads to an autonomous drift of the laser spectrum and a dynamic change in the laser output wavelength following the maximum of the gain curve, i.e., the wavelength self-sweeping is generated.
Wavelength self-sweeping was first realized in 1962 in a ruby laser [7], and about half a century later, based on the excellent waveguide medium of optical fibers, the phenomenon was re-explored by Kir’yanov et al. [8] in Russia. Since then, the self-sweeping effect has undergone rapid development. So far, most studies have reported the self-sweeping effect in 1 µm ytterbium-doped fiber lasers [9–13], but it has been observed in different spectral regions by experimenting with other doped fibers [14]: near 1.06 µm in neodymium-doped fiber lasers [15]; near 1.46 µm in bismuth-doped fiber lasers [16]; close to 1.55 µm in erbium-doped fiber lasers [17,18]; close to 2 µm in thulium-holmium co-doped [19], holmium-doped [20] and thulium-doped [21] fiber lasers. The sweeping range obtained in only ytterbium-doped and thulium-doped fiber lasers is greater than 20 nm, while in other doped fiber lasers it is shorter, especially in erbium-doped fiber lasers, the sweeping range is only 2.8 nm [18]. Usually, the intensity dynamics generated by a self-sweeping fiber laser behaves as a self-pulsed signal associated with relaxation oscillations [6]. Each pulse signal consists of multiple longitudinal modes or a single longitudinal mode, both of which depend on the variation of the cavity parameters [22], for example, changing the cavity length to limit the number of longitudinal modes can achieve a single longitudinal mode output, when the microsecond pulse signal exhibits significant periodicity and regularity. Single longitudinal mode outputs have been achieved only with ytterbium-doped [22], bismuth-doped [16], thulium-doped [21] and neodymium-doped [15] fibers, suggesting that the parameters of the gain medium (e.g., the upper energy level particle lifetime or the ratio between amplitude and phase components in dynamic gratings [23]) all have an impact on the self-sweeping effect. In the initially observed self-sweeping effect, the short wavelength drifts with time toward the long wavelength and instantaneously jumps back near the initial wavelength at the end of the sweeping region to restart the next cycle, and this sweeping direction is defined as the normal self-sweeping. Later, the researchers also realized a reverse self-sweeping with the long wavelength drifting toward the short wavelength [24] and a wavelength stop state [25]. The results show that changing the pump power can adjust the sweeping direction and normal self-sweeping, reverse self-sweeping, mixed state, and wavelength stop state are observed. What’s more, most of the originally reported doped self-sweeping fiber lasers used a Fabry-Perot linear cavity, but in subsequent developments, the self-sweeping effect was also achieved using a bi-directional ring cavity [26–28], and the reverse self-sweeping and wavelength-stopped state was observed. However, unlike the previous ones, the wavelength stop state was achieved by adjusting the PC rather than changing the pump power [26]. Nevertheless, so far, the research on bi-directional self-sweeping fiber ring lasers is still in its infancy and has not achieved the normal self-sweeping effect (to our knowledge), so it still deserves in-depth study.
In this article, we present a single-mode bi-directional ytterbium-doped fiber ring laser that enables four self-sweeping regimes accordingly when adjusting the PC to different states: normal self-sweeping, reverse self-sweeping, mixed state (an unstable state containing normal and reverse self-sweeping) and wavelength-stop state. In the last case, the wavelength-stop output value depends on the historical process of the whole self-sweeping spectral dynamics. Besides, the PC can be adjusted to make the laser work in a relatively stable normal or reverse self-sweeping regime within a certain pump power range selectively. In achieving reverse self-sweeping, the laser acquired a self-pulsed signal, but when operating under normal self-sweeping, a continuous wave laser output with periodic burst spikes was observed, in which the old mode disappeared and the new mode was generated at some moments during the transition of two adjacent burst spikes, and this result indicates that the variation of laser frequency with the dynamics of continuous wave intensity exhibits certain self-sweeping features. In short, we have observed for the first time the normal self-sweeping in a bi-directional fiber ring laser by adjusting the PC and can flexibly control its sweeping direction and sweeping output characteristics (e.g., sweeping range, sweeping rate, etc.), which has important research significance for the future development and practical application of the self-sweeping effect.
2. Experimental setup
The schematic for realizing four different self-sweeping regimes of a single-mode bi-directional ytterbium-doped fiber ring laser is shown in Fig. 1. A 975 nm laser diode (LD) is used as the pump source with a maximum output power of 650 mW. The pump light is coupled into the cavity through a 975/1064 nm wavelength division multiplexer (WDM) to pump a section of ytterbium-doped fiber (YDF, Coractive Yb501) about 1.4 m long. A 2×2 optical coupler (OC) with a splitting ratio of 50/50 is regarded as the output coupler, and its two ends are connected to the WDM and YDF, respectively, to form a ring cavity. The cavity does not have any devices such as optical isolators that restrict the direction of light operation, so laser operation in both clockwise (CW) and counterclockwise (CCW) directions can be realized. The control of the intra-cavity polarization state depends on the PC between the YDF and OC, where the output ports are all fused with APC connectors to avoid unnecessary backside reflected light interfering with the normal operation of the self-sweeping effect. All optics, transmission fibers, and pigtails of each device in the laser cavity are of single-mode construction. The total laser cavity length is measured to be ∼4.41 m.
Fig. 1. Schematic of the single-mode bi-directional ytterbium-doped self-sweeping fiber ring laser. LD: laser diode; WDM: wavelength division multiplexer; YDF: ytterbium-doped fiber; PC: polarization controller; OC: optical coupler; CW: clockwise; CCW: counter-clockwise.
3. Results and discussion
When the pump power reaches about 40 mW, the self-sweeping operation in both CW and CCW directions can be observed by appropriately adjusting the intra-cavity PC. The effective output slope efficiency of the laser was measured using a power detector (Thorlabs PM100D), and the output slope efficiencies of the two ports of the laser, CCW and CW, were 25.33% (Fig. 2(a)) and 12.32% (Fig. 2(b)), respectively, for increasing the pump power from 40 mW to 110 mW. A reasonable explanation for the higher output slope efficiency of CCW over CW is that in our experiments we consider the introduction of the pump source and the length of the gain fiber, as well as the lower pump power. The backward ASE (CCW) is larger than the forward ASE (CW) because the forward ASE is absorbed by the active fiber. Both pass through the output coupler and are then amplified in the active fiber. However, due to absorption by the forward ASE, less light is reabsorbed in the counterclockwise direction, which ultimately results in a greater CCW power than CW power. The total bi-directional output slope efficiency of this laser can reach 37.65%, which is a high slope efficiency among self-sweeping fiber lasers, attributed to the simple structure of the bi-directional ring cavity with no insertion loss of additional components.
Fig. 2. Output slope efficiency of a single-mode bi-directional ytterbium-doped self-sweeping fiber ring laser with two ports. (a) CCW. (b) CW.
3.1 Four self-sweeping regimes
An optical spectral analysis (OSA, Yokogawa, AQ6370C) with a practical resolution of 0.034 nm@1µm was used to analyze the self-sweeping regime under different PC modulations, where the OSA recorded the wavelength sweeping dynamics at an acquisition rate of one data point per second. Compared to the bi-directional ytterbium-doped [27] and thulium-doped [26] self-sweeping fiber ring lasers reported in the previous two years, the self-sweeping spectral dynamics achieved by us is significantly different. As shown in Fig. 3, four different self-sweeping regimes were achieved by carefully regulating the PC to different states at the pump power of 50 mW. Figure 3(a) shows a reverse self-sweeping effect with a sweeping range of 5.04 nm and a sweeping rate of 0.388 nm/s, and the results are similar to those in [26]. In Fig. 3(b), the laser achieves an unstable mixed state containing both normal self-sweeping and reverse self-sweeping, and the sweeping direction can be changed in successive sweeps. Figure 3(c) demonstrates a relatively stable normal self-sweeping effect achieved in a bi-directional fiber ring laser for the first time, where the sweeping range of 2.49 nm and a sweeping rate of 0.311 nm/s. As can be seen in Fig. 3(d), there is no laser spectral dynamics and the central output wavelength remains essentially constant with time (the maximum wavelength fluctuation is about 0.02 nm, which may be due to external environmental fluctuations, microturbulence of the fiber or fiber thermal effects), and the stationary output can be achieved by adjusting the PC at any value in the range of 1066.04 nm to 1073.68 nm. In fact, each pump power of the laser in the self-sweeping behavior can be carefully adjusted by the PC to achieve the four self-sweeping regimes mentioned above.
Fig. 3. Four self-sweeping regimes realized by the laser under different PC modulations at pump of 50 mW. (a) Reverse self-sweeping. (b) Mixed state. (c) Normal self-sweeping. (d) Wavelength stop state.
The determination of the sweeping direction has been discussed in detail in [17,24], and the semi-empirical model based on the gain curve profile to determine the sweeping direction proposed by Peterka et al. can well explain the three self-sweeping regimes implemented by the laser: normal sweeping, reverse sweeping and mixed state. Here, we make a simple explanation, the determination of the sweeping direction depends on the dynamic gain grating generated within the gain medium, defined by the magnitude of the slope on both sides of the gain curve maximum. Theoretically, in the absence of any special circumstances, the first burn hole should appear at the gain curve maximum, then the slope of the gain curve on both sides of the burn hole will determine the position of the next burn hole. Obviously, the side of the gain curve with less tilt has more gain, so the burn hole will move along the side with the larger gain, and when the gain grating life is terminated, the burn hole moves to near the sweeping boundary and quenches instantly, after which the burn hole will reappear near the initial gain maximum and build the grating to repeat the process, and the sweeping direction is thus determined, i.e., the same direction as the burn hole movement. Throughout the gradual hole-burning process of the gain curve, we ignore the spatial distribution of the particle number inversion, and the gain curve shape remains essentially constant [24]. The mixed state corresponds to a special case of symmetry in the magnitude of the tilt of the gain curve on both sides of the peak, in which the burn-in hole selects one side to build a dynamic grating to complete a single scan cycle, and when the life of that grating is over, a second grating is built on the other side (possibly the original side) to start a second single scan cycle. When the life of that grating is over, a second grating is created on the other side (which may still be the original side) to start a second single sweeping cycle. Thus, a single-cycle sweeping mechanism in a continuous sweeping process may appear as a normal or reverse self-sweeping, with the direction of the single-cycle sweeping depending on which side of the gain curve the grating is built on. We believe that the dynamic change in the gain curve profile may be attributed to the change in the gain characteristics (e.g., gain saturation, relaxation oscillation, etc.) of the gain fiber due to the change in intra-cavity losses caused by the regulation of the PC. In addition, the wavelength stopping (sweeping rate of 0 nm/s) phenomenon may be ascribed to the fact that the relative polarization states of the two back propagation modes, CW and CCW, are orthogonal, in which case no standing wave field is generated within the gain medium [26]. Where the value of the stationary wavelength output depends on the entire self-sweeping spectral dynamics history process, in practice the laser wavelength can be stopped by adjusting the PC when the desired value is reached. The results illustrate that the sweeping direction and sweeping rate of the laser can be flexibly tuned by a simple adjustment of the PC in a bi-directional ytterbium-doped self-sweeping fiber ring laser, and the sweeping rate can be tuned to zero to obtain a stationary wavelength output, which may be very favorable for its practical application prospects.
3.2 Normal self-sweeping regime
In the process of carefully adjusting the PC, we can selectively make the laser work relatively stable in normal or reverse self-sweeping regimes and analyze both in detail. It was found that when the PC was fixed to a suitable position, the normal self-sweeping could achieve stable operation within a certain pump power range, the self-sweeping threshold power was 40 mW, and the sweeping phenomenon disappeared and the spectrum became chaotic and unstable when it exceeded 85 mW. Figure 4(a)-(d) show the dynamics of normal self-sweeping spectra at pump powers of 50 mW, 55 mW, 65 mW, and 70 mW, corresponding to the sweeping ranges (sweeping rates) of 3.35 nm (0.837 nm/s), 3.51 nm (0.877 nm/s), 3.28 nm (1.091 nm/s), and 3.26 nm (1.089 nm/s). Besides, adjusting the PC not only selects the sweeping direction of the laser, but also regulates the self-sweeping output characteristics such as sweeping range, sweeping rate, as shown in Fig. 3(c) and Fig. 4(a), where the laser is at the same pump power (50 mW), but under different PC modulation.
Fig. 4. Normal self-sweeping spectral dynamic at different pump powers (a) 50 mW. (b) 55 mW. (c) 65 mW. (d) 70 mW.
A photodetector (Thorlabs DET08CFC) connected to a digital storage oscilloscope (Agilent Technologies DSO9104A) and a radio frequency spectrum analyzer (Keysight N9000A) was used to analyze the time-domain intensity dynamics of the laser. When modulating the PC to achieve a normal self-sweeping effect, the time-domain intensity dynamics as shown in Fig. 5(a) was obtained and a continuous-wave laser output with periodic burst spikes was observed, a result similar to [18] but reported for the first time in a bi-directional ring cavity, as shown in Fig. 5(a), where a continuous wave laser output with periodic burst spikes was observed, a result similar to that in [18] but the first report in a bi-directional ring cavity. This intensity dynamics consists of a pure sinusoidal intensity signal (Fig. 5(b), red dashed box and the part pointed by the red dashed arrow) and periodic burst spikes (Fig. 5(c), green dashed box and the part pointed by the green dashed arrow). The Fourier analysis of the signal reveals that the continuous wave output between two adjacent burst spikes consists of two longitudinal modes with an inter-mode beat frequency interval of about 46.8 MHz, which is displayed as a single peak in the radio frequency (RF) spectrogram, as shown in Fig. 5(d). Figure 5(e) indicates that not only the fundamental frequency signal peak but also an additional two-fold frequency signal peak (∼93.6 MHz) is observed in the RF spectrogram when the burst spike is generated, and the generation of two signal peaks is attributed to the interference of at least three adjacent longitudinal modes. That is, it can be seen that under some moments of transition from one burst spike to another adjacent burst spike, the old mode disappears and the new mode arises. The result indicates that the laser frequency under the self-sweeping effect exhibits some self-sweeping behaviors with the dynamics of the continuous wave laser intensity, and we believe that the change of mode dynamics in the time-domain intensity can better characterize the self-sweeping effect.
Fig. 5. Intensity dynamic under normal self-sweeping at pump of 65 mW. (a) Full view. (b) Pure sinusoidal signal. (c) Periodic burst spikes. (d) Radio frequency spectrum of pure sinusoidal signal. (e) Radio frequency spectrum of periodic burst spikes.
The normal sweeping output characteristics of the laser at pump power from 45 mW to 85 mW are analyzed in Fig. 6(a)-(c), respectively. Among them, Fig. 6(a) shows that the average sweeping rate v increases with increasing pump power P, i.e., it satisfies $v = 0.25\sqrt P - 0.77$, and the rate range is 0.837 nm/s-1.316 nm/s. From Fig. 6(b), it can be seen that the repetition frequency of periodic burst spikes increases with increasing pump power from 0.88 kHz to 1.04 kHz, which combined with Fig. 6(a) indicates that the repetition frequency and the sweeping rate vary with pump power in the consistent relationship. In Fig. 6(c), the sweeping range gradually increases with increasing pump power, which may be attributed to the widening of the laser spectral range due to the elevated gain curve profile, as shown in Fig. 6(d), and thus more longitudinal modes of different frequencies can participate in the laser process to generate SHB, and then the output wavelength can sweep in a wider range [19]; after reaching the maximum value, the sweeping range gradually decreases or even disappears due to the interference of nonlinear effects such as stimulated Brillouin scattering (SBS), stimulated Raman scattering (SRS) [8]. The maximum sweeping range is only 3.51 nm, which is most likely due to the narrow pump power range.
Fig. 6. Output characteristics of normal self-sweeping. (a) Sweeping rate versus pump power. (b) Repeat ratio of periodic spikes versus pump power. (c) Sweeping range versus pump power. (d) Sketch of gain contours versus frequency at different pump powers.
3.3 Reverse self-sweeping regime
The same fine adjustment of the PC allows the laser to achieve a stable reverse self-sweeping effect at a pump power of 45 mW-109 mW. The dynamics of the self-sweeping spectra at pump powers of 50 mW, 60 mW, 90 mW, and 100 mW are shown in Fig. 7(a)-(d), corresponding to the sweeping ranges (sweeping rates) of 5 nm (0.385 nm/s), 5.56 nm (0.327 nm/s), 4.08 nm (0.51 nm/s), and 3.84 nm (0.547 nm/s), respectively. Comparing with Fig. 4(a)-(d), it can be seen that the laser output spectral dynamics under reverse self-sweeping is more stable (less fluctuation at the sweeping boundary) than the normal self-sweeping.
Fig. 7. Reverse self-sweeping spectral dynamic at different pump powers. (a) 50 mW. (b) 60 mW. (c) 90 mW. (d) 100 mW.
Figure 8 shows the time-domain intensity dynamics of the bi-directional ytterbium-doped fiber ring laser achieving the reverse self-sweeping effect with the observed microsecond pulse signal, which is basically similar to the [27]. Among them, Fig. 8(a) demonstrates the pulse intensity signal in the 800 µs range, which reveals that the laser operates in the self-pulsing regime and that the microsecond pulses are modulated with inter-mode beat frequency and associated with relaxation oscillations. Figure 8(b) indicates the amplified longitudinal mode beat frequency detail with a beat frequency interval $\Delta v$ of about 46.79 MHz. The laser cavity length L is determined by the longitudinal mode beat frequency interval: $\Delta v = c/nL$, where c is the speed of light in vacuum (∼3×108 m/s), n is the effective refractive index (∼1.45) and L is calculated to be about 4.42 m, which roughly coincides with our measured cavity length.
Fig. 8. Intensity dynamic under reverse self-sweeping at pump of 65 mW. (a) Pulse train. (b) The longitudinal mode beating detail in zoom view.
Figure 9 depicts the output characteristics of the reverse self-sweeping. As can be seen from Fig. 9(a), the average sweeping rate is 0.33 nm/s-0.55 nm/s with increasing pump power and shows a trend of first decreasing and then steadily increasing, which is significantly different from that described by [27]. We have collected several experimental data that are consistent with this result, so we believe that in a bi-directional ytterbium-doped self-sweeping fiber ring laser, the sweeping rate of the reverse self-sweeping with pump power does not necessarily drop to zero and then switch to the normal self-sweeping, but also may switch to the faster sweeping rate of the reverse self-sweeping. However, since there are too few studies on bi-directional ring cavities [26–28], which are still at a preliminary stage, the fundamentals of their sweeping rate variation are still unclear and need to be studied in more depth in the future. Figure 9(b) indicates that the average pulse repetition frequency increases with pump power from 35.15 kHz to 96.97 kHz. In addition, the sweeping range varies with pump power in line with the previous one, up to 5.56 nm, as shown in Fig. 9(c). Figure 9(d) visualizes the output spectral information of the laser at a pump power of 65 mW with the center wavelength at 1069.236 nm, and it can be seen that the optical signal-to-noise ratio (OSNR) is significantly greater than 46 dB. The 3 dB linewidth of the laser measured by OSA is about 0.04 nm, which exceeds the actual resolution of OSA.
Fig. 9. Output characteristics of reverse self-sweeping. (a) Sweeping rate versus pump power. (b) Average pulse repetition frequency versus pump power. (c) Sweeping range versus pump power. (d) The spectral information at a pump power of 65 mW.
It should be noted that each of the above experimental results was achieved by carefully adjusting the PC to a certain fixed state, and if the PC was regulated, the self-sweeping state would change accordingly. However, compared with other PC modulation states, the experimental results given in this paper are the best.
Table 1 summarizes the stable normal and reverse self-sweeping characteristics achieved by the laser under different PC modulations. Compared to the Fabry-Perot linear cavity, we achieve stable normal and reverse self-sweeping regimes at lower pump power but higher output slope efficiency (up to 37.65%), which seems to indicate that the bi-directional ring cavity is more likely to work in a self-sweeping regime. In addition, in the bi-directional ring cavity, the laser operating in the reverse self-sweeping regime has a wider pump power range, a larger sweeping range, and is relatively more stable spectral dynamics compared to the normal self-sweeping, which seems to be more prone to reverse self-sweeping when analyzed from the perspective of these results. More in-depth study and improvement of the bi-directional ring cavity is desired in future work.
Table 1. Summary of sweeping characteristics of normal self-sweeping and reverse self-sweeping
4. Conclusion
In this paper, we present a single-mode bi-directional ytterbium-doped self-sweeping fiber ring laser that can operate in four self-sweeping regimes by carefully adjusting the PC: normal self-sweeping, reverse self-sweeping, mixed state, and wavelength stop state. In the last case, the laser wavelength can achieve a stationary output at any value, depending on the historical course of the overall self-sweeping spectral dynamics. Furthermore, the PC can be adjusted to selectively stabilize the laser to operate in normal or reverse self-sweeping regimes over a range of pump powers, the former of which is reported for the first time in a bi-directional ring cavity. The time-domain intensity dynamics indicates two different wavelength selection regimes, one is the reverse self-sweeping represented by microsecond pulse signals; the other is the normal self-sweeping represented by a continuous wave laser with periodic burst spikes, in which the old mode disappears and the new mode arises at some moments during the transition between two adjacent burst spikes, and this result indicates that the laser frequency shows a certain self-sweeping feature with the change of continuous wave intensity dynamics. In conclusion, we can flexibly regulate the sweeping direction and sweeping characteristics such as sweeping range, sweeping rate in the bi-directional ytterbium-doped self-sweeping fiber ring laser by controlling the PC according to the actual situation required, and this simple approach can greatly expand the application prospects of the self-sweeping effect.
Funding
National Major Scientific Research Instrument Development Project of China (51927804); National Natural Science Foundation of China (61905193).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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