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Abstract
In this paper, we proposed and demonstrated two kinds of all few-mode fiber lasers with self-starting high-order mode (HOM) oscillation. The fundamental mode can be completely suppressed by using a bandpass filter with a few-mode fiber pigtail. In the continuous-wave (CW) regime, the fiber laser directly oscillates in HOM with a signal-to-noise ratio as high as 70 dB, and the slope efficiency is up to 46%. The self-starting HOM mode-locked pulse can be easily achieved by employing a saturable absorber. The HOM oscillation pulsed fiber laser stably operates at 1063.72 nm with 3dB of 0.05 nm, which can deliver cylindrical vector beams with a high mode purity of over 98%. To our knowledge, this is the first demonstration for self-starting HOM direct oscillation in stable CW and pulsed operation states without additional adjustment. This compact and stable HOM fiber laser with a simple structure can have important applications in materials processing, optical trapping, and spatiotemporal nonlinear optics. Moreover, this work may offer a promising approach to realizing high-power fiber laser with arbitrary HOMs stable output.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
In the past decades, fiber lasers have developed rapidly due to high optical conversion efficiency, excellent beam quality, and compact structure [1]. However, most of the studies focus on single-mode fiber (SMF) lasers with fundamental mode output that exhibits a gaussian intensity profile. In recent years, fiber lasers with high-order mode (HOM) output have attracted immense research interest. Compared with traditional SMF lasers, HOM fiber lasers have promising applications in optical communication [2], materials processing [3], optical trapping [4], STED imaging [5], surface plasmon excitation [6], and high-power fiber laser system [7] due to the unique polarization, helical phase, doughnut intensity distributions, and larger effective mode area [8]. Motivated by the demands of potential application value, many methods have been implemented to obtain specific HOM in all-fiber lasers, which can be classified into two categories: passive method and active method. The former means that fiber lasers oscillate in fundamental mode, and HOM can be generated by using mode converter or mode excitation including offset splicing technique [9], long-period fiber grating (LPFG) [10], few-mode fiber gratings (FMFBG) [11], acoustically induced fiber grating (AIFG) [12], mode selective coupler (MSC) [13], and photonic lantern [14]. By combining mode-locked techniques, such as saturable absorbers (SAs) materials [15,16], nonlinear polarization rotation [17,18], nonlinear amplifying loop mirror [19], pulsed HOM with high peak power and high pulse energy is realized, which can be used in materials processing [20,21] and electronic acceleration [22]. However, the passive methods usually have relatively low slope efficiency and mode purity, and essentially oscillate in the fundamental mode that has a smaller effective mode area than that of HOM. The active method means that HOM can directly oscillate in the laser cavity, which possesses the characteristics of high slope efficiency and mode quality. HOM can obtain more gain than the fundamental mode in a master oscillator power amplifier (MOPA) system, a larger effective mode area of HOM can counteract a variety of nonlinear processes and facilitate power-scaling in high-power fiber laser systems [23]. In addition, HOM with large anomalous-dispersion exhibit potential application for dispersion management of femtosecond fiber lasers [24,25]. To realize HOM direct oscillation, few-mode or multimode fibers are adopted and transverse mode-selection techniques are required. Particularly, FMFBG has been widely studied due to the mode selection feature, Liu et al. achieved LP11 mode direct oscillation fiber laser by employing two FMFBGs that are written on two different fibers, whose spectra are particularly tailored so that only the LP11 mode can oscillate in the laser cavity [26]. Li et al realized LP11 mode oscillation based on the polarization dependence of FMFBG [27]. Zhang. et al proposed an all-fiber laser oscillating on the LP11 mode by using a metal-clad transverse mode filter to suppress the LP01 mode [28]. The specially designed few-mode gain fibers and tailoring of the mode content of the pump are used for selective oscillation of high-order modes [29]. Besides, HOM direct oscillation in all-few mode fiber (FMF) pulsed lasers were demonstrated by the combination of a wavelength division multiplexing (WDM) mode selective coupler (MSC) and HOM pump. To realize HOM oscillation, a polarization-dependent isolator and two polarization controllers are utilized to control the evolution of mode polarization that induces intensity-dependent loss, consequently, the oscillation mode is chosen based on intensity-dependent loss or polarization-dependent loss by adjusting polarization controllers coalescing with HOM pumping [30,31]. The aforementioned methods also have some limitations such as precise wavelength customization, high polarization sensitivity, and special active fiber design for mode-dependent gain manipulation. However, it is difficult to realize fiber lasers with stable pulsed HOM oscillation if just rely on higher gains provided by specially designed active fiber, because the loss of other fiber components needs to be considered. Moreover, HOM direct oscillation pulsed fiber laser may suffer from mode hopping or instability due to strong polarization sensitivity and mode competition dependence in an all-FMF cavity, complex adjustments are usually required to realize HOM selection and mode-locked pulse output. Thus, there are strong motivations to develop a stable and self-starting HOM oscillation fiber laser without additional adjustment or strong polarization sensitivity.
In this letter, two kinds of HOM direct oscillation all few-mode fiber lasers are proposed and demonstrated. By employing a few-mode bandpass filter and FMFBG, the fundamental mode is completely suppressed and HOM direct oscillation can be easily achieved. The fiber laser oscillates in HOM with a slope efficiency of as high as 46%, a signal-to-noise ratio of 70 dB, and output power of 201 mW in the continuous-wave regime. The stably mode-locked pulsed fiber laser with HOM direct oscillation is also realized based on a commercial semiconductor-saturable absorber mirror (SESAM) that served as a reliable saturable absorber. The pulsed HOM operates at 1063.72 nm with a repetition rate of 23.57 MHz. Moreover, the proposed HOM fiber laser is also capable of generating cylindrical vector beams with a mode purity of over 98% by removing the degeneracy of HOM.
2. Experimental setup and principle
The experimental setup of the proposed self-starting high-order mode fiber laser is presented in Fig. 1. Figure 1(a) and Fig. 1(b) show the schematic of the HOM fiber laser operating in the continuous wave (CW) regime and stable mode-locked pulse regime, respectively. The laser is composed of few-mode fibers (FMF) that support LP01 mode and LP11 mode at 1064 nm, as denoted by the blue line in Fig. 1. A length of 0.6 m few-mode ytterbium-doped fiber serves as a gain medium and pumped by a 974 nm laser diode (LD) through a 974/1060 nm wavelength division multiplexer (WDM) that is placed outside the cavity to reduce the insertion loss laser cavity as well as ensure high-order mode oscillating in the laser cavity. The FMFBG with a reflectivity of 98.5% is used as a highly reflective cavity mirror, which is written on the few-mode fiber (Nufern, CMS-HP-6.5/125). Another mirror consists of a 90:10 few-mode fiber coupler (FM-OC) with a few-mode fiber pigtail (Corning, SMF-28e-8.2/125, supporting LP01 and LP11 mode at 1064 nm), which has a 36% power feedback and extract 64% power from the cavity. To make the laser operating HOM, an 1064 nm bandpass filter (BPF) with SMF-28e fiber pigtails is used as a mode selector to suppress the fundamental mode completely and realize high-order mode direct oscillation without additional adjustment. The measured insertion loss of the BPF is 0.52 dB for LP01 mode and 0.82 dB for LP11 mode at 1064 nm, respectively. To obtain HOM oscillating pulse output, FM-OC in Fig. 1(a) is replaced by the commercial SESAM with a modulation depth of 18%, which is used as the reflector and a reliable saturable absorber to realize mode-locking, as shown in Fig. 1(b). Here, an output FM-OC with a 10% output is used to output the HOM pulse. Polarization controllers (PCs) in Fig. 1(a) and (b) are used to remove the degeneracy of the LP11 mode to obtain cylindrical vector beams (CVB) output and assist mode-locked pulse operation, respectively. The output spectrum power, beam profile, and pulse profile of the fiber laser are measured by an optical spectrum analyzer (Yokogawa AQ6373B), a power meter (Thorlabs PM100D), a Cincam CMOS-1201, a 4 GHz digital oscilloscope (LeCroy Wave Runner 640Zi), respectively.
Fig. 1. The schematic of the self-starting high-order mode fiber laser. (a) operating in continuous-wave regime (b) operating in pulse regime. FMFBG, few-mode fiber Bragg grating; FM-YDF, few-mode doping Yb-doped fiber; BPF, bandpass filter; FM-OC, few-mode fiber coupler; PC, polarization controller; SESAM, semiconductor-saturable absorber mirror; FMF, few-mode fiber.
The reflection and transmission spectra of FMFBG are measured, as shown in Fig. 2. Figure 2(a) shows the reflection spectrum, which has two obvious reflection peaks from self-coupling LP01 and LP11 mode. The transmission spectra are also measured under different dislocation distances, as presented in Fig. 2(b)-(d). The modulation of the spectrum comes from mode interference, and the modulation depth increases with the increase of excited higher-order mode components. It is worth noting that there are still two obvious transmission peaks at 1063.7 nm and 1066.8 nm. In our experiment, FMFBG is written in CMS-HP (Cladding mode suppression photosensitive fiber, 6.5/125µm, NA = 0.18) that has high intrinsic photosensitivity, low birefringence, and low polarization mode dispersion. The peak of cross-coupling LP01-LP11 mode is not obviously observed due to low reflectivity, which is attributed to rather small or uniform refractive index change during the fabrication process of the FMFBG [32,33], resulting in low cross-coupling coefficients. The unique properties of the fiber itself lead to the reflection spectra in Fig. 2. The detailed operation principle of high-order mode fiber laser is depicted in Fig. 3. The reflection spectrum of the FMFBG is denoted by the red line in Fig. 3. The transmission spectrum of the bandpass filter with FMF pigtail is measured by using a broadband ASE light source, as denoted in the blue line in Fig. 3. It is clear that only the reflection peak (1063.7 nm) of LP11-LP11 mode of the FMFBG locates in the transmission spectrum of the BPF, which means that only wavelength at 1063.7 nm can pass the BPF, combining with wavelength-transverse mode association features of the FMFBG [34], only LP11 mode at 1063.7 nm can be reflected into the cavity and get enough gains to oscillate in this laser cavity. Besides, the side-mode suppression ratio of the BPF is over 30 dB, which indicates that LP01 mode can be almost completely suppressed and this laser can stably oscillate in LP11 mode without mode hopping. Furthermore, as shown in Fig. 3, the reflection peak of the LP11 mode of FMFBG locates in the transmission spectrum of BPF and exceeds the left of Δλdown, at this point, the LP11 mode can directly oscillate in the cavity as long as Δλdown is less than Δλ01-11, where Δλdown is the width of the falling edge of the BPF used in our experiment (black line in Fig. 3), Δλ01-11 is the wavelength interval between reflection peaks of LP01 mode and LP11 mode of the FMFBG. In our experiment, Δλdown, is 0.6 nm, and Δλ01-11 is measured to be about 3.1 nm, which is larger than that of FMFBG written in the SMF-28e (about 2 nm). The relatively large wavelength difference between Δλdown and Δλ01-11 can completely suppress LP01 mode out of BPF bandwidth and have a wide range of wavelength tolerance for HOM direct oscillation. Even if the center wavelength of FMFBG redshifts (usually less than 1 nm) due to the change of ambient temperature or the accumulation of the thermal effect of fiber, the fiber laser still oscillates in LP11 mode. Thus, the fiber laser can self-start at high-order mode oscillation without additional adjustment and no mode hopping happens due to strong mode stripping. In addition, combined with saturable absorbers, a high-order mode fiber laser with stably mode-locked pulse output can be easily realized.
Fig. 3. Operation principle of high-order mode fiber laser. The reflection spectrum of the FMFBG (red) and the transmission spectrum of the BPF (blue).
3. Results and discussion
3.1. HOM direct oscillation in the continuous-wave regime
The output spectrum of the fiber laser is shown in Fig. 4(a) when the 90:10 FM-OC is used as a reflector, as seen in Fig. 1(a). The laser oscillates at 1063.68 nm with a 3dB linewidth of 0.03 nm. Noting that the operation wavelength coincides with the reflection peaks of LP11 to LP11 mode of the FMFBG, which means that this laser directly oscillates LP11 mode. The reflection peak of the LP01 mode at 1066.8 nm is completely suppressed due to the strong filtering effect. To further verify the oscillation of LP11 mode, the output beam profile of the fiber laser is characterized by using a CCD camera. The intensity distribution of the two distinct lobes is the typical feature of the second-order LP11 mode, as presented in the inset of Fig. 4(a). These results confirm that the proposed fiber laser is indeed directly oscillating in high-order mode (LP11 mode). Besides, to illustrate the advantages of the proposed fiber laser, the well-designed BPF is removed as shown in the blue dashed line of Fig. 1(a). At this time, the laser operates at 1066.8 nm as shown in the blue line of Fig. 4(b), which corresponds to the reflection peak of LP01 to LP01 mode of the FMFBG. The result indicates the fiber laser oscillates in LP01 mode without BPF because the high-order mode experiences greater losses than the fundamental mode in the whole laser cavity at this time. Moreover, the obvious ASE background noise appears in the output spectrum when the BPF is removed, and the side-mode suppression ratio (SMSR) of the laser is about 54 dB. By contrast, when the BPF is inserted in this linear cavity, ASE background noise can be effectively eliminated due to the filtering effect of the BPF. The proposed HOM fiber laser with BPF has lower background noise, and SMSR is as high as 70 dB, which is improved by 16 dB, as shown in the red line of Fig. 4(b). More importantly, the fiber laser with BPF can directly oscillate in HOM rather than LP01 mode, which is beneficial for power scaling to realize high-power fiber laser due to the large effective mode area of higher-order modes.
Fig. 4. (a) Output spectrum of the HOM fiber laser with BPF in CW regime. (b) Measured wide spectra of the fiber laser with BPF (red line) and without BPF (blue line) in the CW regime.
The output power of the HOM fiber laser with BPF is measured by a power meter (Thorlabs PM100D). Due to the coupler with a 90:10 ratio providing 36% power feedback, the threshold of the laser is as low as 76 mW, as shown in the black line of Fig. 5. The output power almost linearly increases with a slope efficiency of 23.8% when the pump power is above the laser threshold. The maximum output power is 113 mW at the maximum pump power of 540 mW. Besides, when the coupler at the right is replaced by the cleaved fiber end with a reflectivity of 4%, the maximum output power is as high as 201.4 mW. The laser threshold is increased to 113 mW due to higher cavity loss, and the slope efficiency goes up to 46%, as shown in the red line of Fig. 5. It should be noted that the reflectivity of 4% and 0.6 m length of YDF may not be the most appropriate, the slope efficiency may be further increased. Compared to the HOM fiber laser using a special gain medium or accurate grating pairs [26,35], the proposed HOM fiber laser has a clear net gain difference in different transverse modes while precise grating pairs not, which provides great flexibility for higher order mode direct oscillation. Furthermore, the proposed HOM fiber laser has low ASE background noise and high SMSR.
3.2. HOM direct oscillation in mode-locked pulse regime
To obtain pulsed HOM operation, the mode-locked technique is applied to our experiment, as shown in Fig. 1(b). A commercial SESAM with a modulation depth of 18% is used as a reliable saturable absorber to realize stable mode-locking pulse output. The 10% port of FM-OC is engaged as the laser output. With the pump power increasing to 144 mW, an unstable mode-locked pulse appears. Just slightly adjusting the PC, a stable mode-locking pulse train could be realized. Figure 6 shows the output characteristics of the stable mode-locked pulse with HOM operation at the pump power of 185 mW. The corresponding pulse train is monitored with a span of 1000 ns by an oscilloscope, as shown in Fig. 6(a). The interval between adjacent pulses is 42.4 ns, which corresponds to the round trip time of the cavity length of about 4.4 m. The inset shows the relationship between the output power of pulsed HOM and pump power, which present a linear increase with a slope efficiency of 3.3%. The slope efficiency can be improved by using a higher output ratio coupler. The fiber laser oscillates at 1063.72 nm with a spectrum width of 0.05 nm at 3 dB, and the SMSR is as high as 54 dB, as shown in Fig. 6(b). Due to wavelength-transverse mode association features of the FMFBG [34], the wavelength at 1063.7 nm only reflects back LP11 mode, and only LP11 mode could be amplified by the gain medium. Thus, the laser directly oscillates in LP11 mode and outputs LP11 mode. The beam profile of the inset in Fig. 6(b) also confirms the LP11 mode operation. The temporal profile single pulse is illustrated in Fig. 6(c), which has a full width at half maximum (FWHM) of 532.5 ps. To measure pulses more accurately, an autocorrelator (APE PulseCheck) with a measurement range from 12 fs to 150 ps is used, but no pulse can be observed because the actual pulse width is out of range. Figure 6(d) shows the radio frequency spectrum of the output pulse within a 3 MHz range with a resolution of 100 Hz, the signal-to-noise ratio is as high as 63 dB at the fundamental frequency peak of 23.57 MHz, which is in agreement well with the cavity length of 4.4 m. Meanwhile, the insert presents a wider RF spectrum span of 500 MHz, and there are no other frequency components or modulation, indicating good stability of pulsed HOM fiber laser. Besides, to evaluate the self-starting performance of the mode-locking operation, we have rebooted the fiber laser many times when keeping the state of the PC unchanged, this laser can self-start once the pump power increases to the threshold value of mode-locking.
Fig. 6. Output results of mode-locked fiber laser with HOM direct oscillation. (a) Pulse train with an interval of 42.4 ns. Inset: output power versus pump power (b) The spectrum of laser output and the intensity distribution of output mode (inset). (c) single pulse profile. (d) RF spectrum in the 3 MHz range with a resolution of 100 Hz. Inset: wider RF spectrum with a 500 MHz span.
To evaluate the long-term stability of the proposed mode-locking pulsed fiber laser with HOM direct oscillation, the output spectra of the mode-locked pulse are recorded every 10 minutes in an hour at the pump power of 800 mA, as shown in Fig. 7. The center wavelength of the mode-locked fiber laser can always keep at 1063.72 nm and the fluctuation of the intensity is also inconspicuous. More importantly, no mode hopping is observed. These results indicate the mode-locked pulse with HOM operation has good long-term stability.
It should be noted that the proposed method for self-starting HOM direct oscillation based on a BPF and FMFBG has some important advantages. The higher SMSR can be obtained due to the strong filter effect of the BPF that can eliminate obvious ASE background noise. The relatively large wavelength matching range for HOM selection ensures the stable operation of HOM fiber laser even if the center wavelength of FMFBG shifts or mode gain changes in the condition of the variation of ambient temperature, fiber bending, or disturbance. The reason is that HOM can oscillate in the laser cavity once Δλdown is less than Δλ01-11. The proposed method for achieving HOM oscillation in a fiber laser is polarization insensitive and a pair of FMFBG with the same reflection wavelength of LP11 mode is not required, which may be applied to other spectral regimes such as visible light and mid-infrared bands. In addition, mode-locked pulsed fiber laser with HOM direct oscillation can be also easily realized. This flexibility may make the fiber laser oscillates in arbitrary HOMs of all-few mode or multimode fiber cavity. Moreover, HOM is beneficial for high-power fiber laser systems and ultrafast fiber lasers due to its large effective mode area and unique dispersion property [7].
Finally, to generate a cylindrical vector beam (CVB), a polarization controller is added outside the laser cavity. The doughnut-shaped pattern can be observed by carefully adjusting the PC to eliminate the degeneracy of HOM (LP11 mode), as shown in Fig. 8. To further reveal the polarization property of the output doughnut beam, a linear polarizer is placed before the CCD camera. By rotating the polarizer to four different orientations, the radially polarized beam (TM01) and azimuthally polarized beam (TE01) are confirmed, and the corresponding intensity profiles are shown in Fig. 8(a) and 8(b), respectively. Moreover, using the fiber bend method [11], the measured mode purity of the TM01 and TE01 modes are 98.1% and 98.3%, respectively.
Fig. 8. (a) Intensity profile of TM01 mode and corresponding intensity distribution after a linear polarizer. (b) Intensity profile of TE01 mode and corresponding intensity distribution after a linear polarizer. The white arrows indicate the polarization orientation of a linear polarizer.
4. Conclusion
In summary, we propose and experimentally demonstrate a simple and efficient approach for HOM direct oscillation in an all-FMF laser that can operate in CW and pulsed states. Mode stripper composed of a BPF and FMFBG can completely suppress the fundamental mode in the cavity and the fiber laser directly oscillates at HOM with a slope efficiency of 46% and maximum output power of 201 mW. An SMSR as high as 70 dB is obtained using the BPF to remove the ASE noise. Besides, mode-locked pulsed HOM fiber laser also can be easily achieved based on a SESAM. The pulsed HOM fiber laser operates at 1063.72 nm with a 3dB linewidth of 0.05 nm, a pulse duration of 532.5 ps, and shows good stability in an hour. By eliminating the degeneracy of HOM, CVB with a purity of over 98% is obtained. The proposed HOM fiber laser can self-start due to polarization insensitivity and large mode-matching tolerance. This compact fiber laser with HOM oscillation in CW and pulsed regimes can be applied in optical trapping, materials processing, and biomedical imaging. Moreover, this work may offer an efficient method for the development of arbitrary HOMs oscillation fiber lasers, which facilitates a high-power fiber laser system.
Funding
Open Project of Advanced Laser Tenchnology Laboratory of Anhui Province (AHL2021ZR02); National Key Research and Development Program of China (2021YFF0307804).
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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