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Abstract
We experimentally investigate two schemes of Brillouin random fiber laser (RFL) by using high-order-mode (HOM) pump in a few-mode fiber (FMF). The core-mode conversion between LP01 and LP11 modes is obtained in the FMF by cascading long period fiber gratings (LPFG) working at the same wavelength region. Different transversal modes of stimulated Brillouin scattering (SBS) can be implemented based on broadband long period fiber gratings (LPFG) and acoustically induced fiber gratings (AIFG). The RFL base on two broadband LPFGs can obtain high purity LP11 mode operating in the range of 1543 nm to 1565 nm. Moreover, the output mode can be dynamically switched between LP01 mode, LP11a mode and LP11b mode by modulating frequency shift keying (FSK) signal of the AIFG. This work has potential application prospects in the fields of mode division multiplexing systems, speckle-free imaging, free-space optical communication, laser material processing.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
In recent years, more and more scholars pay more attention to study random fiber laser (RFL) due to the characteristics of low spatial coherence, simple structure, beam quality and stable output. Some application prospects of the RFL are found in optical sensing, spectroscopy, laser imaging and medical sciences [1–4]. RFL with different structures have been proposed in these years, for example, Vatnik firstly proposed the cascaded random distributed feedback Raman fiber laser structure generation second Stokes wave operating at 1.2 µm in 2011 [5], but their threshold powers of 1st-order Stokes and 2nd-order Stokes lines are about 1.6 W and 6.5 W. In 2012, W. L. Zhang et al. reported the realization of 2nd-order random lasing in a half-opened fiber cavity and the threshold powers of 1st-order Stokes and 2nd-order Stokes lines are about 0.7 W and 2.0 W [6]. Z. N. Wang et al. firstly propose third-order random lasing operating in 1670nm spectral band in 2013 [7]. However, the RFLs based on Raman gain have obvious disadvantages, such as only a limited number of Stokes lines, high threshold power and wavelength spacing of more than 100 nm. In order to solve the above problems, the RFLs based on stimulated Brillouin scattering (SBS) were proposed. C. H. Huang et al. reported a cascaded RFL structure based on Brillouin-erbium fiber gain in 2014 [8]. The RFL can generate multi-order Stokes lines, and the threshold power of the RFL is significantly reduced compared with the previous RFL structure based on Raman gain. F. Wang et al. proposed a tunable and switchable multi-wavelength erbium–Brillouin random fiber laser in 2020 [9]. The structure can generate more than ten order Stokes lines, and the wavelength can be tunable between 1535 nm and 1550 nm. However, as the previously reported RFLs are all consist of single mode fiber (SMF), so the RFLs output mode are all fundamental mode, which limit the application of RFL in more fields.
High-order modes (HOMs) in all fiber laser have attracted a lot of attention due to its unique spatial intensity, phase and polarization distribution [10]. There are a lot of good application prospects of HOMs such as mode division multiplex (MDM), stimulated emission depletion microscopy (STED), high-resolution measurement, quantum optics [11–14]. At present, all-fiber mode conversion devices include acoustically-induced fiber grating (AIFG) [15–17], mode selective coupler (MSC) [18–20], long period fiber grating (LPFG) [21,22] and so on. In order to realize the HOM output of RFL, the mode conversion device must be added into the laser cavity. At present, the following kinds of HOM generation in RFLs have been reported, such as the lateral offset splicing [23,24], MSC [25], LPFG [26]. Nevertheless, the above RFLs structures can only implement the HOM output of a first-order Stokes line, which limit their practical applications. To the best of our knowledge, a multi-order Stokes line of RFL by using HOM pump has not yet been reported.
In this work, we propose and demonstrate two schemes of Brillouin random fiber laser by using HOM pump in a few-mode fiber (FMF). The RFL that is based on two broadband LPFGs has high purity of LP11 mode, which can continuous wavelength tunable from 1543 nm to 1565 nm. Moreover, the RFL base two AIFGs can obtain dynamic switchable modes between LP01 mode, LP11a mode and LP11b mode by adjusting the frequency shift keying (FSK) signal. The proposed RFL structures by using HOM pump have some great applications in a variety of fields, such as mode division multiplexing systems, speckle-free imaging, free-space optical communication, laser material processing [27–30].
2. Experimental principle and setup
Figure 1 depicts the principle of SBS in an FMF based on a HOM pump. Two LPFGs are used as the core-mode conversion between LP11 and LP01 modes. Brillouin signal light is converted to LP11 mode through the LPFG2 as a HOM pump for generating the backward SBS in the FMF, which is converted back to LP01 mode after pass through the LPFG1. By the way, some of the LP11 mode continue to transmit forward and is detected by charge-coupled device (CCD), while the rest of the LP11 mode will be transmitted back to the port #3 of the circulator due to SBS. The LP11 mode is converted back to the LP01 mode after pass through the LPFG1 and is injected into LPFG2 as the signal light of the next order SBS. The whole LP modes circulate in the ring cavity to generate multi-order SBS until the threshold condition of high-order SBS is not satisfied.
The schematic of the proposed RFL by HOM pumping is shown in Fig. 2. A wavelength tunable light source with a central wavelength of 1550nm is used as Brillouin pump (BP) injection light to enter the circulator port #1. A 0.7 m long erbium-doped fiber (EDF) acts as linear gain medium, which is pump by 980 nm laser pass through a wavelength division multiplexer (WDM). A 1 km highly nonlinear fiber (HNLF) works as nonlinear gain medium to amplify SBS. When the BP is amplified by EDF and pass through the HNLF, the backward propagating first-order Stokes light will be generated, backward propagating first-order Stokes light and the Rayleigh scattered (RS) light of BP are transmitted counterclockwise in the ring cavity and amplified twice by EDF and erbium-doped fiber amplifier (EDFA). Then some of the amplified first-order Stokes light pass through the few mode fiber-circulator (FMF-CIR) and inject the 2 km FMF as a new seed light to generate the next order Stokes light propagated backwards. The remaining first-order Stokes light propagate forward and can be observed by using the optical spectrum analyzer (OSA) and CCD. The Stokes light always circulates counterclockwise in the ring cavity to generate higher-order Stokes light until the power fails to reach the threshold of the next order Stokes light. It is worth mentioning that two-stage amplification is used in the ring cavity because the HNLF being with high loss. When two AIFGs are placed in a ring cavity as mode conversion devices, a dynamic switchable RFL by HOM pump can be realized. When two broadband LPFGs are used as mode conversion devices, the RFL can achieve HOM output of wavelength tunable in the ring cavity.
Fig. 2. The schematic of the proposed random fiber laser by HOM pump base on LPFGs. BP: Brillouin pump; PC: polarization controller; RF: radio frequency; CIR: circulator; WDM: wavelength division multiplexer; EDF: erbium-doped fiber; OSA: optical spectrum analyzer; HNLF: highly nonlinear fiber; EDFA: erbium-doped fiber amplifier; RS: Rayleigh scattered ; FMF-CIR: few mode fiber-circulator.
The mode conversion of 980 nm pump and BP propagating in the cavity are shown in Fig. 3(a), first, the BP injects the LP01 mode into the circulator port #1, the LP01 mode is converted to LP11 mode by LPFG1 after passing through the EDFA. Meanwhile, first-order Stokes light occurs and backpropagates the same mode in the ring cavity. Figure 3(b) shows the mode conversion diagram of SBS, the LP11 mode is converted back to the LP01 mode by LPFG1 again due to SBS, and the LP01 mode is converted to LP11 mode by LPFG2 after passing through two EDFA, some of the LP11 mode output from the FMF-CIR port #2. At the same time, the remaining LP11 mode in the 2 km FMF again forms 2st-order Stokes light reverse propagating, from the FMF-CIR port #2 injected into the ring cavity inside. The 2st-order Stokes light propagates in the cavity as a pump of 3st-order Stokes light, and then generates 3st-order Stokes light in 2 km FMF. In the same way, higher-order Stokes light is generated until the power fails to reach the threshold.
Fig. 3. (a) Mode conversion diagram of 980 nm pump and BP propagating in the cavity. (b) Mode conversion diagram of backward HOM Brillouin scattering in the ring cavity.
3. Results and discussion
Figure 4(a) shows the diagram of the broadband LPFG, which can achieve mode conversion from LP01 mode to LP11 mode at a relatively wide band range. The CO2 laser is gradually irradiated to the two-mode fiber (TMF) without the coating layer, as to realize the periodic modulation of the refractive index of the fiber with period of 366 µm. By modulating the exposure time and power of the CO2 laser, the refractive index can be accurately controlled. The LP01 mode and LP11 mode group velocity of the TMF at 1.5 µm are matched, which means that the condition of dispersion turning-around point (TAP) as described in [31], so the LPFG made with the TMF can achieve a broadband mode conversion. The transmission spectra of the broadband LPFG is shown in Fig. 4(b), the LPFG can achieve a relatively wide mode conversion, with a 3dB bandwidth of about 100nm and a mode conversion efficiency of −14dB (96%). Due to the wide-band mode conversion property of the LPFG, the RFL based on the broadband LPFGs can operate over a relatively wide wavelength range between 1543nm to 1565 nm. Figure 4(c) depicts the output spectra of RFL base on broadband LPFGs, the RFL can be operated at different wavelengths by changing the central wavelength of the BP. As shown in the figure, the operating wavelength of the RFL is from 1543 nm to 1565 nm. In such a wide range of wavelengths, the RFL can achieve LP11 mode output. The main reason that restricts the tunable range of RFL wavelength is the gain bandwidth of EDF. This further enriches the working range of HOM RFL. As far as we know, it is the first experimental report about HOM RFL structure with tunable wavelength, which has a great application prospect in wavelength-division multiplexing, optical communication, laser material processing and other fields.
Fig. 4. (a) The structure of a broadband LPFG and the core-mode conversion from LP01 mode to LP11 mode. (b) Transmission spectra of the broadband LPFG. (c) The output spectra of RFL base on LPFG dependent on different BP center wavelength.
When the pump power at 980 nm is gradually increased, the output spectra of RFL base on LPFG are shown in Fig. 5. Under the premise that the BP input power is 10 mW unchanged, when the 980 nm pump power is 70 mW, the spectrum is shown as the black curve, indicating that the input power is too small to reach the threshold of first-order Stokes light. So there are only Rayleigh scattered waves. As the pump power gradually increases to 80 mW, first-order Stokes light appears, which indicates that the threshold of the Brillouin first-order Stokes light is about 80 mW. When the pump power at 980 nm increases to 100 mW, the second-order Stokes light appears, which means that the threshold of the Brillouin second-order Stokes light is about 100 mW. The green curve shows the spectrum obtained at 120 mW of the 980 nm pump, there are many peaks in the spectrum at this time, the pump power is in the critical state of third-order Stokes light, which leads to spectral instability. As the 980 nm pump increases to 210 mW, it enables the output more than the seventh-order Stokes light. The difference of the output spectra between the RFL based on the AIFGs and that RFL based on LPFG is due to the different fusion loss and insertion loss caused by optical fibers used to fabricate these two mode conversion devices.
In order to realize BRFL output of dynamic mode switch, we propose BRFL structure based on AIFG. The structure diagram of an AIFG is shown in Fig. 6(a). The AIFG is consisted of a piezoelectric transducer (PZT) and an adhered aluminum horn utilized for generating flexural acoustic waves coupled into an unjacketed FMF, which results in periodic refractive index changes of the FMF to form a dynamic LPFG. When the phase matching condition of ${{\textbf L}_{\boldsymbol B}} = \boldsymbol{\Lambda }$ is satisfied, the core-mode can achieve dynamic mode conversion from LP01 mode to LP11a/b mode in the FMF, wherein, the core/cladding radius of the FMF is 9.2/125µm. The acousto-optic coupling length is generally tens of centimeters. The AIFG period is $ \boldsymbol{\Lambda } = {(\boldsymbol{\pi} {\textbf R}{{\textbf C}_{{\boldsymbol {ext}}}}/{\boldsymbol f})^{\frac{\mathbf{1}}{\mathbf{2}}}} $ and the beat length is ${{\boldsymbol L}_{\boldsymbol B}} = {\boldsymbol{\Lambda } }/({{{\textbf n}_{\mathbf{01}}} - {{\textbf n}_{\mathbf{11}}}} )$, where R means the cladding radius of the FMF and ${\boldsymbol f}$ represents the frequency of RF signal, $ {{\textbf C}_{{\boldsymbol {ext}}}}$=5760 m/s is the velocity of acoustic wave traveling through the fiber, $\boldsymbol{\lambda }$ means the resonant wavelength, ${{\textbf n}_{\mathbf{01}}}$ and ${{\textbf n}_{\mathbf{11}}}$ represent the effective refractive index difference of LP01 mode and LP11 mode.
Fig. 6. (a) Schematic of an AIFG and the core-mode transformation from LP01 mode to LP11a/b mode. (b) Dual-resonant peaks at tranmission spectra dependtnt on different RF frequencies applied on the AIFG. (c) The RFL output spectra of LP01, LP11a and LP11b modes at 980 nm pump power of 130 mW.
Figure 6(b) shows the spectra of mode conversion efficiency of an AIFG, two resonant peaks represent mode conversion from LP01 mode to LP11a mode and LP11b mode, respectively. The depth of the resonant peak represents the conversion efficiency of −17dB (98%) at 1550nm. With the change of RF frequency applied on the PZT, the wavelengths of the two resonant peaks will also change. In other words, the dynamic switch between LP01 mode, LP11a mode and LP11b mode can be realized through the FSK modulation of the RF signal. When two cascaded AIFGs are connected into the RFL cavity, acting as mode conversion components, the RFL output spectra of 980 nm pump at 120 mW are shown in Fig. 6(c), the black curve represents the spectra of LP01 mode, while the red curve and the blue curve represent the spectra of LP11a mode and LP11b mode, respectively. It can be seen that the output intensity of the spectra of LP01 mode is higher than that of LP11a/b mode, the reason is that the transmission loss of LP01 mode in the whole ring cavity is lower than that of LP11a/b mode, and the difference between the spectra of LP11a mode and LP11b mode is caused by the inconsistent polarization states of the two modes. The results show that more than seven-order Stokes lines are observed in all three modes.
The 980 nm pump is a key parameter affecting the output of the RFL. As shown in Fig. 7, the picture depicts the output spectra of the RFL base on the AIFGs dependent on different pump power of 980 nm. When the 980 nm pump input is 65 mW and BP power is 10 mW, there are only Rayleigh scattered light and first-order Stokes light. As the 980 nm pump power increases to 70 mW, the threshold of second-order Stokes light is reached, meanwhile the second-order Stokes light output is obtained. As the pump power continues to increase, higher order Stokes light begins to appear. When the pump power reaches 102 mW, the spectrum is basically stable. More than seven order Stokes lines can be obtain. The Stokes lines have wavelength spacing of 0.08 nm. It is worth mentioning that at the low pump power of 980 nm, there is background noise, which is caused by amplified spontaneous emission (ASE) from the EDF.
4. Conclusion
In conclusion, we demonstrate two schemes of Brillouin random fiber laser by using high-order-mode pump in a few-mode fiber. The broadband LPFGs based RFL can obtain high purity LP11 mode operating in the range of 1543 nm to 1565 nm. Moreover, we propose an RFL structure based on the AIFGs, which can achieve dynamic mode conversion between LP01 mode, LP11a mode and LP11b mode by changing the resonance of mode conversion through the modulation of the FSK signals applied on the AIFGs. To the best of our knowledge, it is proposed for the first time to generate more than seven order Stokes lines using a HOM pumped RFL structure. These two RFL structures have low insertion loss, all-fiber structure, and each has its own characteristics, which can be selected according to the actual application of different structures. It has potential applications in optical communication, mode division multiplexing, speckle-free imaging and so on.
Funding
State Key Laboratory of Pulsed Power Laser Technology (SKL2020KF03); National Natural Science Foundation of China (61635006, 91750108); Science and Technology Commission of Shanghai Municipality (20JC1415700); 111 Project (D20031).
Acknowledgments
Zeng Xianglong acknowledges the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning.
Disclosures
The authors declare no conflicts of interest.
Data Availability
Data underlying the results presented in this Letter are not publicly available at this time but may be obtained from the authors upon reasonable request.
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