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Abstract
We report the peak-power-clamping (PPC) effect in a polarization-maintaining (PM) Q-switched mode locking fiber laser. The laser cavity with a compact and stable all-PM fiber configuration can clearly demonstrate three different output states including normal Q-switching, PPC Q-switching, and PPC Q-switched mode-locking (QML) with the increasing pump power. To the best of our knowledge, it is the first time that PPC effect is successfully obtained and analyzed from the Q-switching to QML. This research extends the theory of PPC in pulsed lasers and reveals the potential to achieve ultra-high pulse energy.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Pulsed fiber lasers have always been a research hotspot over the past decades, due to their great advantages of high-energy and ultra-short optical pulse generations, leading to a wide range of applications including high-speed optical communication system [1], material micro-processing [2], optical imaging [3], and spectroscopy [4]. Generally, there are two methods to realize a pulsed output, namely Q-switching and mode-locking. The mechanism of Q-switching is simple, based on the population inversion by introducing the constant intra-cavity loss more than the gain together with a fast switch, leading to the emission of a bunch of photons for a short temporal duration [5]. Normally, the pulse peak power is proportional to the incident pump power. As for the mode-locked fiber lasers, with the help of dispersion management and optical filtering, a variety of solitons have been recently developed in order to achieve larger pulse energy, ranging from conventional solitons to stretched pulses [6], similaritons [7], dissipative solitons [8,9], and the newly-discovered dissipative-soliton-resonance (DSR) [10–14]. The pulse energy has been successfully increased from tens of pico-Joules to micro-Joules. Most recently, the energy of a single DSR pulse, directly from an all-fiber laser cavity, has been boosted up to 10 µJ [15]. For most of mode-locked mechanisms, further improvement of pulse energy may be limited by multiple-pulse generation resulted from the excess of nonlinear phase shift (NPS) accumulated. However, for the DSR pulses, peak-power-clamping (PPC) is a distinguished advantage that makes it possible for pulse energy to keep growing together with the broadened pulse duration. Thus, the NPS can be dramatically reduced with a relatively low pulse peak power.
In a fiber laser, depending on the pump power, transmission from Q-switching to mode locking may be observed [16]. Specifically, a stable intermittent state, Q-switched mode locking (QML) state can be achieved in a fiber laser, where a giant bunch with internal mode-locked pulses appears [17]. The transformation between Q-switching pulses and QML pulses could also be achieved by the precise adjustment of polarization controllers (PCs) inside the cavity [18,19]. Such additional adjustment may reduce the structure stability and restrict its practical applications. The PPC effect has been investigated in numerous DSR mode-locked fiber lasers. Recently, similar phenomenon was reported in a Q-switching fiber laser [20]. However, no such effect has yet been observed in any QML fiber lasers. The evolution of QML pulses reported in previous works always follows the basic rules, where the peak power increases with pump energy enhancement [5,18–23]. In this submission, we develop an all polarization-maintaining (PM) Ytterbium-doped (Yb) fiber laser and experimentally observe, for the first time, that typical PPC can also exist for QML pulses. The laser cavity with a compact and stable all-PM fiber structure can clearly demonstrate three different output states including normal Q-switching, PPC Q-switching, and PPC QML with increasing pump power.
2. Experimental setup
The proposed fiber laser is schematically shown in Fig. 1. A commercial transmission-type semiconductor saturable absorber (SESA) mounted inside the fiber is used to achieve the saturation absorption which contributes to the Q-switched operation. The SESA has low-intensity absorption of 43% at a wavelength of 1040 nm, modulation depth of 25%, saturation fluence of 70µJ/cm2, non-saturable loss of 18%, damage threshold of 1.5 mJ/cm2, and relaxation time of 1 ps. A 976-nm laser diode (LD) with maximum output power of 350 mW acts as a pump source for a 40-cm PM Ytterbium-doped fiber (YDF, PM-YDF-5/130-VIII). An isolator is used to guarantee unidirectional propagation and to suppress detrimental reflections. The laser output is achieved by a 10:90 optical coupler (OC). We note that the isolator and the OC are integrated in a hybrid component, which also contains an in-line polarizer to make the cavity work in fast-axis-blocked condition. The hybrid component greatly simplifies the cavity structure and improves its stability. All optical components are with PM fiber pigtails, which are fusion spliced, forming a ring cavity with total length of around 5.8 m. Such an all-PM fiber structure can further ensure cavity robustness against the environment perturbation. At the output port, an optical spectrum analyzer (OSA, ANDO AQ6317B) with a resolution of 0.02 nm is used to measure the optical spectrum. Meanwhile, a real-time oscilloscope (OSC, Agilent 54855A) with a bandwidth of 6 GHz is used to monitor the temporal pulses with the help of a high-speed photodetector. Moreover, the radio-frequency (RF) spectrum is characterized by a signal source analyzer (Agilent E4446A).
Fig. 1. Experimental configuration. SESA: semiconductor saturable absorber; WDM: wavelength division multiplexer; YDF: Ytterbium-doped fiber; OC: optical coupler; LD: laser diode.
3. PPC Q-switching
When the pump power is increased to 60 mW, typical Q-switching pulses can be observed. The output power is around 0.4 mW. This relatively low output efficiency can be ascribed to two factors. Firstly, the OC is equipped with only 10% output port so that the output efficiency of our proposed fiber laser cannot be quite high. Moreover, as for the hybrid component, the OC is integrated after the polarizer which may also lead to some additional loss. However, since our research is mainly focused on the mechanism of PPC phenomenon instead of high-power generation, we believe such low output efficiency can be further optimized in our future research. After the continually growing pump power to 87 mW, the pulse width of Q-switching pulses becomes narrower with higher peak power. Meanwhile, the repetition rate of the pulse train is increased from 27.8 kHz to 42 kHz. These temporal evolutions agree well with the relationship of [24]
where ${t_p}$ is the pulse width, ${f_{rep}}$ is the repetition frequency, and ${P_{peak}}$ is the peak power. However, when the pump power is further increased, the evolution of Q-switching pulses become tremendously different. At such a state, the repetition rate keeps growing and the pulse width narrows slightly with the increase of pump power. We note that, the peak powers of all pulses are clamped, which almost stay at a constant value. The pulse trains at several different pump power from 60 mW to 124.5 mW are illustrated in Fig. 2(a), while Fig. 2(b) demonstrates single pulse evolution exhibiting PPC with a temporal resolution of 3 µs. In particular, the typical flat tops of pulse profiles can be regarded as an identity distinct from normal Q-switching pulse evolution. It can be noted that there exist cavity roundtrip related pulses lying on the Q-switched pulses, and they are weak and unstable. Those pulses will evolve into QML pulses with higher pump power, which will be discussed in Part 4. Figure 2(c) shows a typical ratio frequency (RF) spectrum with a 1-kHz resolution, when the incident pump power is 124.5 mW. The modulation envelope can be explained by the reciprocal of the pulse width, which is similar to the operation of the DSR. With the increasing pump power, the repetition rate of pulse trains grows from 42 kHz to 67 kHz, meanwhile the signal-to-noise ratio (SNR) increases from ∼50 dB to ∼60 dB. The inset of Fig. 2(c) shows the fundamental repetition frequencies of the PPC states with respect to the pump power.Fig. 2. Results of PPC Q-switching: (a) Pulse-train evolution with the increase of pump power; (b) single pulse envelops at different pump powers; (c) RF spectrum under the condition of 124.5 mW pump power and RF spectrum evolution with different pump powers (inset); (d) optical spectra variation with respect to the pump power.
Figure 2(d) shows the optical spectra under the PPC state. With the growing pump power, the spectral bandwidth becomes slightly wider while the spectral shape and peak wavelength stay almost unchanged. Besides the newly-emerged longitudinal modes, the self-phase modulation (SPM) also contributes to the spectral broadening described by the following equations [25]
where $\Delta \lambda$ and ${(\Delta \lambda )_0}$ represent broadened and initial spectral bandwidths, while $\gamma$, ${P_{peak}}$ and L represent the nonlinear parameter, pulse peak power and fiber length, respectively. In our cavity, ${\phi _{nl}}$ can be estimated to be ∼$1.16 \times {10^{ - 3}}$ rad, so that the optical spectrum can be broadened by only 0.1%, which agrees well with our experimental observation. In [25], the authors have obtained 8.51% spectral broadening at 2 µm wavelength with the help of a section of high nonlinear fiber (HNLF), and identified that the HNLF is the key component to realize the PPC. In our laser cavity we identify that high nonlinearity is not a compulsory condition to achieve the PPC phenomenon. In fact, both PPC Q-switching can be clearly observed under the condition of low nonlinearity.Figure 3(a) demonstrates different output parameters as the function of pump power. Apart from the average power which always evolves linearly, the pump power induced evolution of other parameters including repetition rate, pulse width, and pulse energy all have distinct turning points when the operation is switched from normal Q-switching to the PPC Q-switching. The restriction of peak power is clearly observed with blue rectangle dot and red dashed fitting line in Fig. 3(b), where there is a sudden and tremendous drop of the slope efficiency from 0.6 to only 0.1.
Fig. 3. (a) Parameter analysis of normal Q-switching and PPC Q-switching, in terms of repetition rate, average power, pulse width, and pulse energy with respect to the pump power; (b) average power and peak power with the increase of pump power (triangle and rectangle: with and without 5-m PM fiber).
In order to investigate the influence of nonlinearity on the PPC Q-switching appearance, another section of 5-m PM fiber is inserted into the laser cavity after the YDF, indicating that more nonlinearity is introduced in the cavity. Under such condition, another PPC state is obtained and the evolution of peak power is presented in Fig. 3(b) with blue triangle dot and red solid fitting line. Compared the red solid line with dashed line, it is obvious that the peak power becomes much less than that of previous cavity, meanwhile the slope efficiencies of two regimes reduce to 0.48 and 0.08, respectively. In term of the average power, the two cavities have almost the same linear evolution feature, as shown with black solid and dashed line in Fig. 3(b), which means the cavity loss difference between the two cavities can be ignored. Therefore, for longer propagation length, PPC can appear under lower peak power. Longer fiber corresponds stronger PPC.
4. PPC QML
For the original fiber laser, when the pump power is further increased, the laser would operate at a period of unstable transition state (124.5-140 mW pump power) and then evolve to typical QML, as shown in Fig. 4(a). Figure 4(b) shows a pulse train with 28-ns spacing, which corresponds to the 5.8-m cavity length. Under such condition, the peak power of the pulse-train is still clamped when the pump power is increased to 170 mW, while the repetition rate of Q-switched envolope keeps growing and the Q-switched pulsewidth becomes broader. In particular, we observe that the pulse breaks up into multiple pulses within the Q-switched envolope, when the pump power is over 155 mW, as shown in Fig. 4(b). The pulse breaking-up behavior could be attributed to the larger pulse energy induced nonlinearity, because SPM, which starts to broaden the optical spectrum and combine with the filtering effect (gain-bandwidth of YDF in our case), may lead to the additional loss and therefore the pulse breaking up [26,27]. Figure 4(c) shows different spectra obtained at 140-mW and 170-mW pump power, respectively. With higher pump power, both the intensity and spectral bandwidth would slightly increase. Further increasing pump power, the multiple pulses would be tremendously unstable and QML can no longer be obtained. As there is no extra narrow bandpass filter added in the laser cavity, and the gain bandwidth of YDF is too wide to achieve the necessary spectrum shaping leading to the occurrence of mode-locking, it is difficult to obtain pure mode-locking operation in this cavity.
Fig. 4. Results of PPC QML: temporal pulses at 140-mW, 155-mW and 170-mW pump power with (a) 12-µs and (b) 0.2-µs time scale; (c) optical spectra at 140-mW and 170-mW pump power.
Generally, PPC QML has two distinctive features in comparison with the DSR mode-locking operation. Firstly, the repetition rate of Q-switched envelope varies continuously and considerably with the pump power, which is similar to that under the normal QML operation. Alternatively, the DSR pulses have constant peak intensity without any Q-switched envelope, and its repetition rate is only determined by the cavity length. Secondly, as for the PPC QML, when the pump power grows further, the mode-locked pulses can split into multiple to decrease the peak power. However, only pulse broadening can be observed in the DSR pulse. As for the similarity, we find that both PPC QML and DSR pulses evolve with the unchanged peak intensity when the pump power is increased. Moreover, the differences and similarities among temporal characteristics of normal Q-switching, PPC Q-switching, PPC QML and DSR can be summarized in Table 1.
Table 1. Pulse evolutions of normal Q-switching, PPC Q-switching, PPC QML and DSR with increasing pump power.
5. Conclusion
In conclusion, we have experimentally observed the PPC effect in a PM Yb-doped fiber laser with either Q-switching or QML output. Similar to a mode-locking pulse such as a DSR pulse, the peak power of both Q-switching pulses and QML pulses can be clamped with the increase of pump power while the average output power keeps rising. Such unique operation regime greatly enriches the understanding of PPC for pulsed lasers from Q-switching to mode-locking. PPC effect is still a threshold effect and the peak power of Q-switching is strong enough for its appearance. Provided that the nonlinearity is strong enough for the appearance of PPC, longer propagation may require lower pulse peak power. For a PPC QML pulse, the pulse width increases with increasing pump power. Overall, we find that PPC can be a quite general nonlinear effect in a pulsed fiber laser.
Funding
National Key Research and Development Program of China (2018YFB1801301); National Natural Science Foundation of China (61875061); Wuhan Science and Technology Bureau (No. 2018010401011297); Fundamental Research Funds for the Central Universities; China University of Geosciences, Wuhan (162301192695, No. 162301132703).
Disclosures
The authors declare no conflicts of interest.
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