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Abstract
A highly stable figure-9 Yb-doped fiber laser with all polarization-maintaining (PM) double-cladding fiber is demonstrated. Through leveraging the saturable absorption effect of a nonlinear amplifying loop mirror, both the Q-switched and mode-locked operation are realized by adjusting the pump power. With increasing the pump power from the threshold to the maxima, the repetition rate of the Q-switched pulses is linearly increased from 14.9 kHz to 138.0 kHz with the pulse duration accordingly reduced from 3.9 µs to 970 ns. The corresponding maximum average power and pulse energy are respectively 2.34 W and 17 µJ, which are more than ten times larger than the common material-based Q-switched all-fiber lasers. In addition, in the process of increasing and decreasing the pump power, an optical bistability that manifested as a significant power jumping effect is observed, while its effect on the pulse repetition rate and duration is trivial. Whereas for the single pulse mode-locked operation, a maximum output power of 56.3 mW with a fundamental repetition rate of 12.5 MHz is realized, corresponding to a pulse energy of 4.5 nJ. To the best of our knowledge, it is much higher than the most of previous works concerning figure-9 all-PM-fiber lasers of which the emitted pulse energy is generally less than 1 nJ. After being compressed by a pair of diffraction grating, a minimum pulse width of 378 fs and a maximum peak power of 9.76 kW are respectively obtained. In addition, through characterizing the spectral and temporal properties of the laser source, the excellent stability of both the Q-switched and mode-locked operations is verified.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Pulsed fiber lasers have attracted widespread research interests, driven by diverse applications in for example material processing [1,2], biomedicine [3], and spectroscopy [4,5], to name a few. Depending on the specific application, laser pulses with duration ranging from microsecond down to femtosecond are generally required, along with other dedicatedly engineered properties, among which high pulse energy is always desired for enhancing its versatility [6–8]. Thanks to the merits of high efficiency and low quantum loss, ytterbium-doped fiber lasers (YDFLs) have been intensively investigated in generating high-energy pulses at the 1.0 µm range [9–11]. In addition, the abundant fiber components that are commercially available at this spectral range render the fiber lasers can be implemented with an all-fiber format, even operated in the high power/energy regime, showing advantages of compact structure, stable and reliable operation, which are hardly realized with a free space coupled system [12,13].
To obtain pulsed laser source, the common scheme is Q-switching or mode-locking a fiber laser with the aid of the saturable absorption effect, which can initiate and maintain stable pulse operation in a passive manner, and endows the laser with advantages of simple structure, cost-effective and self-starting. Generally, the saturable absorbers (SAs) that adding intensity dependent loss to the transmitting light mainly consist of the material-based real SAs [14–16] and the fiber nonlinearity based artificial SAs [17–20]. With regarding to the Q-switched operation, the well-developed two-dimensional material SAs including Graphene [21], black phosphorus [22] and transition metal dichalcogenides [23] have been successfully utilized to generate nanosecond or microsecond pulses from fiber lasers through passively switching the cavity loss. Nevertheless, as the material SAs are prone to be damaged under high laser intensity, the maximum pulse energy generated from an all-fiber Q-switched YDFLs have not surpassed 1 µJ yet [24,25]. In terms of the artificial SAs, it does not have the concern of damage and should be beneficial for achieving high-energy pulse operation. However, the realization of stable Q-switched pulse generation using artificial SAs was rarely reported, mostly because the formation of artificial SAs generally involves nonlinear effects, which can be hardly driven by the nanosecond pulses with relatively low peak power [26]. As for the mode-locked operation, it is commonly realized with the material and artificial SAs, enabling the generation of picosecond or femtosecond pulses. The shortening of pulse width brings significantly high peak power, which would also potentially damage or degrade the material SAs over time [27]. The artificial SAs that routinely exploited to implement a mode-locked fiber laser are the nonlinear polarization evolution (NPE) and nonlinear amplifying loop mirror (NALM) [28,29]. Although the NPE-based fiber laser can realize a high energy output with an all-fiber structure, it suffers greatly from the random fiber birefringence, which can result in even a complete loss of mode-locking under environmental perturbations. Whereas the NALM mode-locked laser holds a considerably high damage threshold, and has the advantages of an all-polarization maintaining (PM) structure for compact and robust operation [30,31]. In addition, the NALM has the advantages of more stable and lower intrinsic noise owing to the dynamical interaction between the transmission curve of the NALM and the pulse peak power fluctuations [32–34], which makes it a favorable choice as a mode-locker. A shortcoming of all-PM fiber lasers with a figure-8 configuration is the difficulty of self-starting the mode-locking operation [29,35]. This is circumvented with the proposal of the figure-9 fiber laser, in which a non-reciprocal phase shifter is introduced in the NALM loop to generate an optical phase bias, rendering the mode-locking easier to be started [36]. In addition, compared with the figure-8 configuration, the number of components required for constructing the figure-9 laser is significantly reduced, making the laser structure simpler and more flexible for optimization [37].
As the realization of Q-switching and mode-locking operations are all dependent on the saturable absorption effect, it is intuitive to explore the feasibility of realizing both operation regimes from a single laser cavity. Up to now, the reports of pulse fiber lasers which can generate both Q-switched and mode-locked pulses are mainly based on real material SAs, of which the saturable absorption effect is uncorrelated with fiber nonlinearities [23,38,39]. In this work, we demonstrate an all-PM YDFL that generated Q-switched and mode-locked pulses through exploiting the NALM effect with a figure-9 structure. The laser cavity adopts double-clad large mode area (LMA) fiber which can withstand higher pump power than conventional single-mode fiber. Highly stable Q-switched and mode-locked pulses have been respectively obtained through simply adjusting the pump power. The Q-switched pulse delivers 2.34 W maximum output power and 17 µJ single pulse energy, with the repetition rate and duration respectively varying from 14.9 to 138.0 kHz and from 3.9 µs to 970 ns. The realized power and energy parameters are more than ten times larger than the maximal of the real material-based Q-switched fiber lasers [24,25]. In terms of the mode-locked operation, a maximum output power/pulse energy of 56.3 mW/4.5 nJ are obtained. To the best of our knowledge, it is much higher than the most of previous works concerning figure-9 all-PM-fiber lasers of which the emitted pulse energy is generally less than 1 nJ. [37,40–42]. After been compressed by a pair of diffraction grating, a minimum pulse width of 378 fs and a maximum peak power of 9.76 kW are respectively obtained. In addition, the short- and long-term stability of the laser pulses are measured and the results consistently show an excellent performance of the proposed laser source.
2. Experimental setup
The schematic design of the figure-of-9 fiber laser cavity is illustrated in Fig. 1. In the NALM-module as shown in the left ring, a 0.8 m long PM-YDF with a core/cladding diameter of 10/125 µm and an absorption coefficient of 4.9 dB/m at 975 nm was employed to provide the optical gain, with the pumping of a 976 nm laser diode through a pump/signal combiner. A PM phase shifter with π/4 phase bias was utilized to shift the transmission curve of the NALM to ensure sufficient difference of the nonlinear phase shift of the NALM for self-starting the stable pulsed operation. The π/4 phase bias of the phase shifter was realized with a λ/16 waveplate, a Faraday rotator, and a reflection mirror. A splitting ratio of the PM fiber coupler of 40:60 was chosen to obtain a high output power given that the asymmetry degree of the NALM is high enough. The linear arm module at the 60% port of the coupler as shown in the right part contains a band-pass filter with a 3 dB bandwidth of 11 nm centered at 1030 nm to facilitate a dissipative soliton mode-locking in the all-normal dispersion regime, and a high-reflection fiber mirror to form the cavity [43]. It is noted that the whole cavity is composed of all-PM fiber with a core/cladding diameter of 10/125 µm, numerical aperture of 0.075 and birefringence of $3\times10^{-4}$), and the corresponding total fiber length is approximately 12.2 m. Finally, the remainder 40% port of the coupler was used to output the laser pulses through a PM isolator.
Fig. 1. Schematic design of the figure-of-9 fiber laser. LD: laser diode pump; YDF: ytterbium-doped fiber; ISO: isolator; BPF: band pass filter; HR: high-reflection mirror.
3. Results and discussion
3.1 Q-switched operation
Figure 2 shows the overall output power evolution of the figure-of-9 fiber laser in the process of continuously increasing and decreasing pump power. In the experiment, self-starting Q-switched pulse trains was obtained at an output/pump power of 0.12/1.5 W, below which the laser is operated in the continuous-wave regime. The linear increasing of output power depicted by black circles is interrupted when the pump reaches 11.38 W, at which the output power suddenly jumps from 0.63 W to 1.62 W. After that, the output power continues to exhibit a linear relationship with the enhancement of the pump power, and a maximum output power of 2.34 W was realized at the pump power of 18 W. When further increasing the pump power, the laser would switch to the unstable mode-locked state, resulting in the output of pulses exhibiting random fluctuated amplitude. In addition, the power evolution of the Q-switched laser was further examined with decreasing the pump power, and a linear reducing of the average power from the maxima to 0.74 W was observed at the pump power of 4 W, below which the power jumps to 0.22 W at the pump power of 3 W. With further decreasing the pump power, the measured output power roughly coincides with that obtained during the power increasing process. Such a hysteresis loop behavior of the output power has been investigated in NALM based fiber laser that operates in the telecommunication band, and is attributed to the saturable absorption of the gain fiber [44]. Moreover, it is noted that the phenomenon of jumping takes place at an average power of around 0.7 W, as illustrated by the dotted line in Fig. 2, and the intrinsic mechanism might be related with the dynamic transmission function of the NALM, however a detailed explanation needs further theoretical analysis that unfortunately exceeds our current capability.
Fig. 2. Measured Q-switched signal power from the output ports versus the pump power. Black circles and red squares respectively denote increasing and decreasing pump power.
The optical spectra and corresponding oscilloscope traces of the Q-switched laser pulse with power increasing and decreasing were recorded at selected pump powers, and the results are shown in Fig. 3. It is noted that during the measurement, the laser power was adjusted to make sure a roughly consistent spectral and temporal amplitude, in this way it can be observed that the spectral profile and the temporal waveform are hardly changed during the power increasing and decreasing process. The slight deviation at pump powers at 7 W and lower is attributed to the adjustment accuracy of the attenuator that used for controlling the laser power that injecting into the measurement devices. The results indicate that the power jumping effect has minor impact on the overall characteristics of the Q-switched laser pulse.
Fig. 3. Characteristics of the Q-switched operation: (a) output spectra and (b) corresponding oscilloscope traces versus the pump power. Black and red curves respectively denote the process of increasing and decreasing pump power.
Figure 4 demonstrates the repetition rate, pulse width, pulse energy and peak power evolution of the Q-switched laser with increasing and decreasing the pump power. It can be seen that the repetition rate shown in Fig. 4(a) changes linearly from 14.9 kHz to 138.0 kHz with adjusting the pump power, and the measurement results coincide with each other precisely in the process of increasing and decreasing the pump power. Whereas for the pulse duration, it monotonically narrows/broadens with increasing/decreasing the pump power, and a minimum pulse width of 0.97 µs was obtained. Considering that the temporal characteristics of the Q-switched pulse are directly related to the pump power, the power stability of the LD at a selected operating power of 9 W was tested for half an hour and an RMS value of around 0.12% was obtained. In addition, through recording and comparing 60 oscilloscope traces of 5 pulses with a time interval of 30 seconds (as shown in the inset of Fig. 4(a)), a consistent repetition rate and pulse duration was obtained, indicating that the Q-switched laser can maintain a favorable temporal stability under the pumping of a commercial LD without any extra control. However, while the pulse duration does not change when increasing/decreasing the pump power at the range of 6 W-18 W, it apparently reduced in the power decreasing process at pump powers lower than 6 W, and a maximum pulse width of respectively 3.7 µs and 3.9 µs were obtained for the two processes, as demonstrated in Fig. 4(b). A possible explanation for this discrepancy is that in the power decreasing process, the hysteresis effect shown in Fig. 2 induces a pulse narrowing effect at relatively low power, for maintaining stable Q-switched operation. As to the single pulse energy, it respectively keeps at around 7 µJ and 17 µJ before and after the power jumping, otherwise it is unaffected by increasing or decreasing the pump power, as shown in Fig. 4(c). Figure 4(d) depicts the calculated peak power, which increases/decreases linearly with a constant efficiency before and after the power jumping, and the minimum/maximum peak power is 2.1 W/17.5 W. In addition, at pump power higher than 11.38 W the peak power is hardly changed in the process of increasing and decreasing the pump power, whilst at pump power lower than 3.1 W the peak power is slightly higher with power decreasing than that with power increasing, resulting from the pulse width narrowing effect as shown in Fig. 4(b). An interesting point that should be noted is that at an output peak power of 2 W, the difference of nonlinear phase shift inside the NALM is very weak and even can be neglected, however a stable Q-switched pulse operation is initiated and maintained in our experiment. As in convention NALM mode-locked fiber lasers the intracavity peak power is generally hundreds of watts or even higher [45], it is then intuitive to draw a conclusion that the phase shifter that inserted into the NALM drives the saturable absorption effect and thus the Q-switching. Therefore, a NALM incorporated with a biased phase shifter could be an effective Q-switcher for fiber lasers, whilst without the assistance of the strong fiber nonlinear effects. The time stability of the Q-switched pulse under maximum output power was characterized by utilizing a 1/9 coupler to monitor the spectrum and output power synchronously for over 1.5 hours, and the results are respectively shown in the inset of Fig. 5(a) and Fig. 5(b). From the figure an average power fluctuation of 0.84% (RMS) was obtained. Furthermore, the amplitude of 10000 pulses was counted and normalized relative with respect to the maxima, and the corresponding calculated histogram is demonstrated in Fig. 5(a), which indicates that there is a normal distribution in a quite narrow range from 0.95 to 1. While the spectra recorded every 5 seconds throughout 1.5 h in Fig. 5(b) manifest consistent evolution of the laser spectrum.
Fig. 4. Characteristics of the Q-switched operation: (a) repetition rate, inset: 60 overlapped oscilloscope traces of five pulses recorded with a time interval of 30 seconds; (b) pulse width, (c) pulse energy and (d) peak power as a function of the incident pump power. Black and red curves respectively denote the process of increasing and decreasing pump power.
Fig. 5. Characteristics of the Q-switched operation under maximum output power: (a) the statistical result of 10000 pulse amplitudes with respect to the maxima, inset: output power and (b) spectrum evolution over 1.5 h.
3.2 Mode-locked operation
When the pump power of the Q-switched laser exceeds 18 W, the pulse train was instantly switched to an unstable pulsing at the fundamental repetition rate of the laser, and then changed to the multi-pulse state with the pump power continuously reduced to around 11 W, indicating that the laser was operated in the unstable mode-locked regime. In addition, the multiple pulses gradually merge with further decreasing the pump power, and a stable single pulse operation with a temporal period of 80 ns (fundamental repetition rate of 12.5 MHz) was realized when the pump power drops to 0.81 W, which is lower than the starting threshold of the Q-switched operation. With further reducing the pump power to be less than 0.6 W, the laser is out of lock and the pulsed operation disappears. It is noted that the repeatability of the switching of the figure-of-9 fiber laser operation from Q-switching to multi-pulse mode-locking and then single pulse mode-locking has been verified in the experiment.
Figure 6(a) shows the output power and the pulse energy as a function of the incident pump power of the single pulse mode-locked laser. With the enhancement of the pump power, the output average power is monotonically increased from 36.5 mW to 56.3 mW, and the corresponding maximum pulse energy is 4.5 nJ. As the laser was constructed with all normal dispersion fiber, it should be operated in the dissipative soliton regime and the output pulse is temporally chirped, which was verified through measuring its autocorrelation trace and a pulse width of around 10 ps was obtained at the maximum output power. The laser pulse was then de-chirped by a pair of diffraction grating (1000 lines/mm), and the pulse width and peak power of the compressed pulse under different pump powers are illustrated in Fig. 6(b). It is observed that the pulse width ranges at around 400 fs-500 fs, and a minimum of 378 fs was realized at the pump power of 0.73 W. Whereas for the peak power, it first increases from 5.43 kW to 9.76 kW with increasing the pump power and then slightly decreased to 8.62 kW at the pump power of 0.81 W, mostly caused by the broadened pulse width after compression as induced by uncompensated nonlinearities.
Fig. 6. Characteristics of the mode-locked operation: (a) output power and pulse energy and (b) pulse width and peak power after temporal compression as a function of the incident pump power.
The spectral and temporal properties of the mode-locked laser was further examined at the pump power of 0.73 W, and the results are depicted in Fig. 7. In Fig. 7(a) and its inset the RF spectrum at the fundamental repetition frequency of 12.5 MHz and at a broad frequency range of 1 GHz are respectively shown, and a signal to noise ratio (SNR) of >76 dB for the fundamental repetition frequency, as well as its harmonics without any spurious peaks were obtained, confirming the stable operation of the laser. The uniformly distributed pulse train is depicted in Fig. 7(b), with a pulse interval of 80 ns that corresponds to the fundamental repetition frequency of the cavity. Figures 7(c) and 7(d) respectively show the optical spectrum with a central wavelength of 1030.16 nm and a 3 dB bandwidth of 5.94 nm, and the autocorrelation trace of the laser after compression with a Sech2 function fitted duration of 378 fs. The calculated time-bandwidth product (TBP) of the de-chirped pulse is 0.634, which is higher than the transform limit, owing to the uncompensated nonlinearities that accumulated inside the cavity. The long-term stability of the mode-locked pulse under maximum output power was tested in the same way as that demonstrated for the Q-switched pulse. As demonstrated in the inset of Fig. 8(a) and Fig. 8(b), an output power fluctuation of less than 0.78% (RMS) and consistent evolution of the laser spectrum over 1.5 hours were obtained. In addition, the histogram of the normalized amplitude of 7000 pulses shown in Fig. 8(a) indicates that the distribution of pulse intensities is mainly concentrated at around 0.9 times of the maxima. Figure 8(b) consists of more than 1100 sets of spectral data with a recording time interval of 5 seconds.
Fig. 7. Characteristics of the mode-locked operation at the pump power of 0.73 W: (a) RF spectrum in a small frequency range, inset: RF spectrum in a large frequency range; (b) pulse train; (c) output spectrum; (d) autocorrelation trace after compression.
Fig. 8. Characteristics of the mode-locked operation under maximum output power: (a) the statistical result of 7000 pulse amplitudes with respect to the maxima, inset: output power and (b) spectrum evolution over 1.5 h.
4. Conclusion
In conclusion, we have demonstrated Q-switching and mode-locking operations of an all-PM double-cladding Yb-doped fiber laser based on NALM. The repetition rate of the Q-switched pulses was varied from 14.9 to 138.0 kHz with the pulse width accordingly varied from 3.9 µs to 970 ns, with the pump power increasing from the threshold to the maxima. The maximum output power was 2.34 W, and the corresponding maximum single-pulse energy was 17 µJ with a peak power of 17.5 W, more than ten times larger than the maximal of the real material-based Q-switched fiber lasers. In addition, an optical bistable behavior of the output power was observed in the process of increasing and decreasing the pump power, and its effect on the pulse repetition rate and duration was negligible. As for the single-pulse mode-locked operation, it was obtained through switching the Q-switched operation into the unstable mode-locked regime by increasing the pump power, and then decreasing the pump power to a level that is lower than the threshold of the Q-switching. The maximum output power of the mode-lock pulse was measured to be 56.3 mW, corresponding a pulse energy of 4.5 nJ. To the best of our knowledge, it is much higher than the most of previous works concerning figure-9 all-PM-fiber lasers of which the emitted pulse energy is generally less than 1 nJ. With the compression of a diffraction grating pair, a minimum pulse width of 378 fs and a maximum peak power of 9.76 kW were obtained. In addition, through characterizing the spectral and temporal properties of the laser, both the short- and long-term stability of the Q-switched and mode-locked operation were verified. It is believed that this highly-stable all-PM double-cladding figure-of-9 fiber oscillator can provide versatile laser sources that promise many important scientific and industrial applications.
Funding
State Key Laboratory of Pulsed Power Laser Technology (SKL2020ZR02).
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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