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Abstract
Multicore photonic crystal fiber (MC-PCF) can scale the output power with the number of cores by spatial beam combining if the in-phase mode is selected. We demonstrated simultaneous realization of phase-locked and mode-locked laser using Yb-doped 7-core MC-PCF by a semiconductor saturable absorber placed in the near-field inside a resonator. High energy 333 nJ pulses were obtained directly from a mode-locked fiber laser oscillator at a 42.4 MHz repetition rate with an average power of 14.1 W at 24 W excitation. We observed the direct output pulse width of 52 ps assuming a sech2 profile. However, it might be noise-like pulses because of no variation when we performed pulse compression. Single-pulse operation was achieved by increasing the bandwidth of the intracavity filter. In this case, 137 nJ, 42.4 MHz pulses were generated with a 5.8 W average power and the compressed output pulse width was 8.6 ps.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Fiber lasers are powerful sources for CW oscillation because of their efficiency, waveguide architectures, and compactness. Ultrafast fiber lasers have attracted considerable interest as an alternative to bulk solid-state lasers due to these characteristics. However, due to the small core diameter of fibers, high intensity pulses confined within the core induce strong nonlinear effects and/or easily exceed the breakdown threshold, which limits the output peak power [1]. The photonic crystal fiber (PCF) technology has led to the development of large core area fibers, but ultrafast fiber lasers with single-core large area fibers have also reached their power limit [2]. Therefore, coherent beam combining (CBC) of fiber lasers is an important subject for power and energy scaling.
Multicore fibers (MCFs) have multiple cores in a single fiber. The close of nearest neighbor cores causes evanescently-coupling of the beams, and CBC is realized [3,4]. MCFs excite eignenmodes called supermodes. The supermodes exist with the same number as the core. The in-phase mode, in which no phase difference among the fields in the cores generates a high-quality far-field beam profile. Therefore, the generation of high peak pulses requires the simultaneous realization of selective excitation of in-phase mode and mode-locking. Up to date, mode-locked multicore fiber laser has been reported by Fang et al. [5], where in-phase mode selection was performed by the coupling efficiency difference among supermodes and mode locking was by using both of nonlinear polarization rotation (NPR) and a semiconductor saturable absorber mirror (SESAM) with 18-core MC-PCF (180nJ, 2.6 W, 14.48 MHz). However, simultaneous realization of phase-locking and mode-locking by using only saturable absorber (SA) has not been achieved yet. In this research, we have achieved phase-locking and mode-locking in an all-normal dispersion (ANDi) cavity configuration using only an SA, and generated high energy 137 nJ, 8.6 ps compressed output pulses in single-pulse operation with an average power of 5.8 W.
2. 7-core MC-PCF and in-phase mode selection
Figure 1 shows the cross section of our Yb-doped LMA MC-PCF. There are seven cores with each core diameter of 21.2 µm and 3000 ppm Yb-doped. The clad diameter is 165 µm. The air hole diameter is 6.0 µm, and the air hole pitch is 13.2 µm. Therefore, the structural parameter (d/Λ) is 0.45, ensuring almost single-mode guidance with low bend sensitivity.
The supermodes, excited by evanescently-coupling among each single-mode cores, can maintain the mutual phase relationship. However, since there are as many supermodes as cores, we have to excite only the in-phase supermode for brightness scaling. Figures 2(a) and (b) show the supermodes of the 7-core MC-PCF, calculated by the finite element method (FEM). As shown in Fig. 2, only the in-phase mode has a strong central lobe in the far-field. In order to select the in-phase mode, various studies have been reported in the past, such as the Talbot effect [6,7], self-Fourier imaging [8], spatial filtering [9], and self-organization [10]. In this study, we use the intensity-dependent transmission characteristics of a saturable absorber to select the in-phase mode. The idea is in-phase mode selection is achieved by saturating the SA preferentially by the in-phase mode with the highest peak at the center among the multiple supermodes, and the SA works as a loss for the higher-order modes. We reported phase-locking and passive Q-switching in a MC-PCF laser using Cr4 +: YAG as a SA with this method [11]. In this study simultaneous realization of phase-locking and passive mode-locking is demonstrated by an SA with a faster response. However, in this demonstration, an SA was not placed in the far-field region but the near-field. The mechanism of the phase-locking by placing an SA in the near-field region is to be clarified in near future.
Fig. 2. (a) Calculated near-field intensity profiles and (b) far-field intensity profiles of the 7-core MC-PCF
The in-phase mode seems to have a similar profile to the Gaussian-like beam, but it is difficult to evaluate the beam quality with M2 because of the existence of side lobes. Therefore, we evaluate the in-phase mode using the Strehl ratio. The Strehl ratio is the maximum of the normalized beam profile divided by the maximum value of the normalized calculated value. In the far-field beam profile, higher-order modes other than the in-phase mode have no almost intensity at the center, so the Strehl ratio can effectively be used to calculate the in-phase mode occupancy.
3. Experiments and results
Figure 3 shows the cavity configuration. The length of the Yb-doped 7-core MC-PCF was 2.3 m. The fiber was rolled by one turn around a bobbin with a radius of 24 cm. The fiber was cladding-pumped by a laser diode at 975 nm. The pump side of the fiber was normal cleaved and the other side was angle cleaved by 8° to suppress parasitic oscillation. The dichroic mirrors were used to separate the pump light from laser radiation. A polarizing beam splitter (PBS) was inserted in the cavity to filter the vertical polarization. In addition, a 5 mm thick quartz phase plate was placed behind the PBS as a birefringence filter for wavelength selection. It was designed to have a center wavelength of 1042 nm and a transmission bandwidth of 6 nm. We use a saturable output coupler (SOC, BATOP GmbH, Germany), a kind of SESAM. The SOC has a reflectance of 58%, a transmittance of 19%, a modulation depth of 9.5%, and a saturation fluence of 40 µJ/cm2. The beam was focused on the SOC.
Fig. 3. Cavity setup. LD: laser diode, DM: dichroic mirror, PBS: Polarizing Beam Splitter, SOC: Saturable Output Coupler
Figure 4 shows the temporal waveform at 24 W excitation and the output power as a function of launched pump power. The slope efficiency was about 58%. The maximum average power achieved in the experiment was 14.1 W, corresponding to 333 nJ of pulse energy at a repetition rate of 42.4 MHz. The laser operation changed from Q-switching to CW mode-locking around 15 W excitation.
Fig. 4. (a), (b) Temporal waveforms with two time scales and (c) output power as a function of launched pump power
In order to investigate the selective excitation of the in-phase mode by the SA, the SOC was changed to an HR mirror and CW operation was obtained. Figure 5 shows the beam profile in the CW operation and those in 24 W excitation when mode locking was obtained. During CW operation, multiple modes were excited, forming a complicated beam profile. On the other hand, when the saturable absorber was placed in the cavity, the beam profile shows the in-phase mode as the basic mode. The far-filed beam profiles showed a strong central lobe and the near-filed beam profile showed the strong intensity in the central core, indicating that the in-phase mode was dominant. The Strehl ratio was maximum 34% at 21W excitation and 24% at 24W excitation, ndicating mode-locking was achieved with a few supermodes excited. Wright et al. have recently reported spatiotemporal mode-locking (STML) [12], where mode-locking can be achieved in the presence of multiple transverse modes by spatial filtering and splicing different fibers for reducing mode dispersion. In the near future, we will clarify the mechanism by which STML can take place in our setup.
Fig. 5. (a) Far-field profile and near-field profile in CW operation, (b) far-field profile and near-field profile in mode-locked operation
Figure 6 shows the spectrum and the autocorrelation trace at 24 W excitation. Although some structure is present in the spectrum, we observed a broad spectrum with a width of about 1 nm. The direct output pulse width was 52 ps, assuming a sech2 profile, and then the peak power was estimated to be 6.4 kW. The presence of structure in the autocorrelation trace may be due to the intensity modulation in the pulses.
The autocorrelation trace showed a single pulse with no coherent spikes. However, when dispersion compensation was performed outside the resonator, there was no variation in the pulse width, indicating that this resonator configuration might produce noise-like pulses at this power. The mode-locking behavior in the ANDi regime depends on the shape and the bandwidth of the spectral filter (SF) [13]. Therefore, we changed the SF to one with a wider transmission bandwidth.
The new filter was designed to a center wavelength of 1029 nm and a bandwidth of 15 nm. In this case, the mode-locking operation was changed and the average power was 5.8 W at 17 W excitation, and the pulse energy was 137 nJ. We also implemented a grating pair for pulse compression. The group delay dispersion was -2.1 ps2 . Figure 7 shows the far-field beam profile, the autocorrelation trace before compression, the spectrum and the autocorrelation trace after compression at 17 W excitation. The Strehl ratio was 22% and the in-phase mode was preferentially selected. We observed a spectrum with a width of about 0.25 nm with some structures. The compressed output pulse width was 8.6 ps, assuming a sech2 profile, and then the peak power was estimated to be 16 kW. Since the pulse width was shortened by dispersion compensation outside the cavity, a single pulse mode-locking operation is indicated. The pulse width before compression was 42 ps, and thenconstriction of the pulse width by ∼1/5 was observed.
4. Conclusion
We achieved the phase-locked and mode-locked multicore fiber laser using only a saturable absorber for the first time. As a result, we observed a high energy pulse of 137 nJ directly from a mode-locked fiber laser oscillator. We also obtained a high energy pulse of 333 nJ from the resonator, although it may be a noise-like pulse. Our results show that coherent beam combining of a laser array is one of the most useful methods to generate high peak power pulses from fiber lasers.
Funding
Japan Society for the Promotion of Science (JP18H01896); Ministry of Education, Culture, Sports, Science and Technology Quantum Leap Flagship Program (JPMXS0118067246).
Disclosures
The authors declare no conflicts of interest.
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