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Abstract
We model and demonstrate a self-matching photonic lantern (SMPL) device, which is designed to address the constraint of limited transverse modes generated by fiber lasers. The SMPL incorporates a FMF into the array at the input end of a traditional photonic lantern. The few-mode fiber at the output end is specifically configured to align with the few-mode fiber at the input, therefore named as SMPL. This paper details the design and fabrication of the SMPL device, validated by both simulation and experiment. The 980nm fundamental mode, injected via 980nm single-mode fibers, selectively excites corresponding higher-order modes at the few-mode port of the SMPL. Additionally, 1550nm fundamental and higher-order modes injected at the input end into the SMPL device demonstrates mode preservation and low-loss transmission characteristics. The SMPL is well-suited for developing a ring laser system, enabling selective excitation of 980nm pump light modes and facilitating closed-loop oscillation and transmission of 1550nm laser.
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1. Introduction
Mode division multiplexing technology utilizing few-mode fibers (FMFs) has received increased interest due to its capability to enhance transmission capacity [1–3]. High-order transverse mode lasers used to control mode excitation are located at the forefront of the mode division multiplexing systems, which can serve as excellent sources for realizing mode division multiplexing. Thus there is renewed interest in producing fiber lasers operating at different transverse modes [4–6]. In addition to significantly increasing communication capacity, researchers have indicated that high-order mode laser beams have benefits in many applications, such as high-resolution imaging [7,8], fiber tweezers [9,10], optical micromachining [11,12], etc.
To produce high-order mode lasers, spatial mode multiplexers are crucial components to realize mode division multiplexing transmission, which facilitate the efficiently interfacing between single mode and multimode. Scholars have realized fiber lasers with high-order transverse mode output through a variety of mode division multiplexing components. Fiber mode converters have the advantages of small size, high stability, and good scalability, and are often used to build all-fiber lasers to excite specific high-order mode lasers. Mode converters incorporated into the fiber laser cavity to excite specific modes include few-mode fiber (FMF) Bragg gratings and long period gratings [13–16], offset-spliced fiber structures [17], mode selective couplers [18–21] and photonic lanterns [22].
For the few-mode fiber Bragg grating scheme, an additional wavelength tuning mechanism is needed to adjust the relationship between the laser emission wavelength and the output mode. Moreover, due to the resolution limit of the fiber Bragg grating, it can only excite the first few low-order modes. For the offset splicing scheme, although the structure is simple to make, this scheme cannot avoid the problems of less adjustable modes and large loss. Fully exploiting the capacity of mode division multiplexing systems requires to excite a larger number of modes. Mode selective couplers and photonic lanterns can generate more higher-order modes, but there is still a common problem, that is, the higher-order modes are generated outside the laser cavity, which further limit the mode quality and efficiency of the laser. Researchers have long been looking for a universal, robust, efficient and all-fiber integrated method to realize fiber lasers with multiple transverse mode laser outputs. In 2019, Teng Wang et al. proposed an effective method to produce high-order mode lasing in all-FMF laser cavity [21]. However, due to the adoption of a mode selective coupler for mode conversion, its high-order mode switching flexibility and scalability are limited. Photonic lanterns offer a promising solution to meet this requirement, enabling efficient and low-loss transmission between multimode systems and independent single-mode systems [23,24]. Despite valuable attempts by some researchers to combine photonic lanterns with fiber lasers to generate high-order transverse mode lasers [22,25], the limited wavelength resolution of FBG and its susceptibility to external environmental fluctuations in reflection characteristics make the grating-based scheme less conducive to expanding towards a larger number of high-order transverse mode lasers. Hence, it is also challenging to widely promote its application on a large scale. Therefore, investigating how to leverage the inherent mode conversion advantages of photonic lanterns and integrate them into fiber laser systems is of significant research importance.
In this paper, we introduce a novel self-matching photonic lantern (SMPL) device to overcome challenges in constructing fully FMF laser resonators. The SMPL combines the mode-selective capabilities of traditional photonic lanterns with the unique feature of high-order mode transmission in all-FMF ring cavity. To the best of our knowledge, SMPL represents the first mode-selective device that supports selective excitation of multiple high-order transverse modes, and capable of leveraging its inherent few-mode matching characteristics to construct a fully few-mode fiber ring resonator. The second section of this paper outlines the operating principle of the SMPL in mode selection and mode preservation. The third section details the fabrication method for the SMPL supporting the excitation and transmission of three linear polarization (LP) modes (LP01, LP11a, and LP11b). Structurally, the input end contains three SMFs and one FMF, also called 3 + 1 SMPL. Finally, taking the 3 + 1 SMPL as an example, we demonstrate the validity of our design experimentally. The proposed SMPL could serve as both a mode-selective and mode-transmission device, significantly expanding the configuration and capabilities of the photonic lantern family, offering a versatile and efficient method for generating multiple high-order transverse mode lasers.
2. Structure and operation mechanism of the SMPL
The configuration of SMPL is illustrated in Fig. 1. Structurally, SMPL comprises three distinct sections: the input fiber array, the transition region, and the output port. The input fiber array is equipped with 3 + 1 individual optical fibers, with three being SMFs designed for injecting 980nm light, and the other being a FMF for introducing 1550nm light.
Fig. 1. Schematic diagram of the SMPL. Insets: the schematic of the geometrical arrangements of the fibers at the input end (left) and the output few-mode end (right).
In the input fiber array, three SMFs with different core sizes are for selectively exciting higher-order modes. The biggest core with a diameter of 6.0 µm corresponds to the excitation of LP01 mode. The two smaller cores with diameters of 4.5 µm and 3.5 µm correspond to the excitation of LP11a and LP11b modes. The core refractive index of the 980nm SMFs is 1.4685, and the cladding refractive index is 1.4635. The geometrical arrangements of the fibers at the input end is illustrated in the left inset of Fig. 1. The FMF is additionally added in the input fiber array to realize the low-loss transmission of high-order modes at 1550nm. The FMF has a core diameter of 15.0 µm, with a core refractive index of 1.4669 and a cladding refractive index of 1.4635. At the output port, the four fibers are fused together to form the few-mode port. The schematic of the endface of the few-mode port is depicted in the right inset of Fig. 1. The outermost capillary tube of the SMPL has a refractive index of 1.4600, with an inner diameter (ID) of 450.0 µm and an outer diameter (OD) of 675.0 µm.
The transition region is formed by adiabatic tapering of the 3 + 1 fiber array inserted into a low-refractive capillary tube, to realize both the mode-selective excitation at 980nm and the mode transmission at 1550nm.
The output port is located at the end of the transition region and is a few-mode port, as shown in Fig. 1. The parameters of the output port are configured to align with the FMF at the input port (with parameters tapered to match with that of the 2-mode FMF). Consequently, it is referred to as the self-matching photonic lantern (SMPL).
The SMPL could be used in an all-FMF ring cavity laser to generate high-order transverse mode laser. The 980nm pump light, injected by three distinct SMFs of the SMPL, selectively excite high-order modes during the transmission through the transition region to the few-mode port. Subsequently, as the 980nm light undergoes amplification and conversion into 1550nm laser, the corresponding 1550nm high-order modes can achieve low-loss transmission between the FMF and the few-mode port. Therefore, SMPL is not only a mode conversion device, but also a high-order mode low-loss transmission device, which can be used in all-FMF lasers to generate high-order mode lasers simply and efficiently.
3. Simulations and analysis
In order to verify the feasibility of the SMPL, simulations were conducted. The mode analysis along the taper region, including the evolution process of effective index and mode profile, is simulated using the finite element method (FEM).
3.1 Mode selectivity simulations of the SMPL at 980 nm
In Fig. 2, the effective mode indices are graphed as a function of the taper ratio when the SMPL operates at a wavelength of 980 nm. The taper ratio is defined as the ratio of the diameter of the tapered section to the diameter before tapering [26].
Fig. 2. Mode effective RI with tapering ratio at 980nm wavelength. Insets: simulated results of the 980nm mode field evolution under different taper ratio conditions.
To break the degeneracy of the modes and make each SMF to map to an individual mode at the few-mode port of the SMPL, the SMFs are designed to have asymmetric diameters. Larger core diameters correspond to higher effective refractive indices, and higher order modes have smaller indexes. Thus, SMFs with diameters of 6.0 µm, 4.5 µm, and 3.5 µm are used to map the input 980nm fundamental modes into LP01, LP11a, and LP11b modes at the few-mode port, respectively.
The black dashed line in Fig. 2 indicates the fiber cladding RI. For taper ratios from 1 to 0.5, the SMFs shrink and are brought close together, but the effective indexes of the modes are still higher than that of the cladding modes, the lantern modes are still well confined within the SMF cores. When the refractive indexes are reduced to below the dashed line, the SMFs fail to confine modes within the cores. The light is then leaked into the cladding which gradually becomes the new FMF core. Along the transition to smaller taper ratio, light within the two smaller-sized fiber cores firstly escapes from the cores and gradually evolves into the LP11a and LP11b modes at the few-mode port (see the blue and green lines). For the larger SMF core, light spreads out of the core approximately at a taper ratio of about 0.3, progressively evolving into the LP01 mode at the few-mode port (the red line). Due to the asymmetry in the core diameter, mode selectivity is realized, as evidenced by the separation of the mode effective index across the tapered region. Finally, at a taper ratio of 0.1, LP01, LP11a and LP11b modes are effectively excited at the few-mode port of the SMPL.
The mode pattern insets in Fig. 2 illustrates the evolutionary mode profiles at different positions along the SMPL. The power launched into a certain core first slowly spreads during propagation. When the taper ratio is around 0.1, the certain fundamental mode finally evolves into a specific supermode, which is in accordance with Fig. 2.
3.2 Mode preservation simulations of the SMPL at 1550 nm
The mode transmission characteristic of the SMPL at 1550nm is a primary innovation and is of particular interest to us. With a taper ratio of 0.1, mode-selective excitation is achievable for 980nm laser. However, it remains to be investigated whether different modes of 1550nm can, as designed, preserve their respective patterns after transmission through the photonic lantern. Figure 3 illustrates the mode transmission and evolution process of the 1550nm light within the SMPL.
Fig. 3. Mode effective RI with tapering ratio at 1550nm wavelength. Insets: simulated results of the 1550nm mode field evolution under different taper ratio conditions.
Figure 3 illustrates the evolution of mode effective indices against the taper ratio, with the SMPL operated at 1550nm. The insets of Fig. 3 depict the mode distribution patterns within the SMPL at different taper ratios, with specific 1550nm LP modes (LP01, LP11a and LP11b) injected into the FMF. When the taper ratio exceeds 0.9, the LP01 (outlined in red) and LP11 (outlined in green and black) modes are confined stably in the FMF. As the taper ratio decreases, the cutoff frequency of the fibers decreases accordingly, resulting in reduced confinement of the fiber cores. This leads to LP11a and LP11b modes coupling to the claddings, which gradually forming the new core of the few-mode port. In the process where the taper ratio decreases from 0.7 to 0.1, the LP11 mode energy spreads into the cladding region and gradually evolves into new LP11 modes at the few-mode port of the SMPL. As for the LP01 mode, under conditions of larger taper ratios, the fundamental mode can still propagate in the core. When the taper ratio reaches approximately 0.3, the excessively small core diameter renders it unable to effectively support fundamental mode. The energy of LP01 mode disperses into the cladding and gradually evolves into the fundamental mode. When the taper ratio reaches 0.1, the LP01 mode's optical field is uniformly distributed at the few-mode port, achieving LP01 mode field matching with the initial FMF. Through the simulation results, it is evident that different modes within the 1550nm FMF, after evolving through the SMPL, can maintain their modal characteristics.
3.3 Simulation of the SMPL’s transmission efficiency
Subsequently, we performed transmission simulations for the designed SMPL using beam propagation method (BPM). The simulation results are shown in Fig. 4. Laser beams were individually introduced into SMFs 1, 2, and 3, exciting the LP01, LP11a, and LP11b modes. As observed in the figure, the SMPL has excellent performance in mode transmission efficiency.
Fig. 4. (a-c) Coupling efficiency curves of LP01, LP11a and LP11b modes in SMPL, (d) calculated mode crosstalk matrix.
Figure 4(a-c) shows the coupling efficiency diagrams of LP01, LP11a and LP11b modes under different coupling lengths of SMPL respectively. Clearly, each mode can only get the largest excitation efficiency from a special corresponding input port. As shown in Fig. 4(a), during the gradual tapering process of the SMPL, the LP01 mode can achieve selective excitation. When the length of the transition region reaches about 40mm, the injected fundamental mode nearly entirely evolves into the LP01 mode at the output end, achieving a coupling efficiency of up to 99.5%, with minimal inter-mode crosstalk. Figures 4(b-c) illustrate the coupling efficiency curves for the LP11a and LP11b modes. It can be observed that, some crosstalk occurs during the coupling process. Nevertheless, selective excitation of these modes is still achievable, with coupling efficiencies of 96.8% and 97.1% for LP11a and LP11b modes, respectively, when the length of the transition region reaches 40mm.
Mode selectivity is defined as the ratio of the power measured in the desired mode over the power measured in all the crosstalk modes [27]. In order to verify the mode selectivity of the SMPL, the transfer matrix is provided in Fig. 4(d) (expressed in dB), showing that most of the energy is located on the diagonal. Obviously, each mode can only obtain the lowest loss from the specific input and output port, which is consistent with the results of the coupling efficiency curve.
4. Fabrication
The fabricating process of SMPL involves the gradual melting and thinning of the fiber bundle through the tapering method, resulting in a newly formed few-mode waveguide at the output port. The specific production process of SMPL is divided into the following four steps:
- i. Two of the 980nm SMFs were pre-tapered to diameters of 62.5 µm and 48.5 µm over a length of about 1.6cm and 2.0cm, corresponding to the excitation LP11a mode and LP11b mode respectively. The remaining 980nm SMF is not pre-tapered and corresponds to the excitation LP01 mode.
- ii. Insert the standard 980nm SMF, along with two pre-tapered 980nm SMFs and one 1550nm FMF, into the low-index capillary according to the designed arrangement (shown in Fig. 1). The low-index capillary has an OD and ID of 673/450µm.
- iii. Place the assembled fiber bundle on the tapering machine, fix it with clamps. Then using the modified flame brushing technique to perform two-step tapering (the two-step tapering parameters are given in Table 1).
- iv. Finally, the new structure is cleaved at a taper ratio of 0.1 at the tapering waist, and the side view of the fabricated SMPL is shown in Fig. 5(a-b).
Fig. 5. (a) The micrograph of the fabricated SMPL, (b) the overall picture of the SMPL, (c) packaging picture of the SMPL-FMF, inset: microscopic image of the fusion joint between the few-mode port of the SMPL and the FMF.
Table 1. Fabricating parameters of the SMPL
In order to verify the main feature of the SMPL, which involves integrating an FMF into the input array and matching it with the few-mode port formed by tapering, we fused the fabricated SMPL with a segment of FMF. The fusion joint was then encapsulated and protected using a sleeve, as depicted in Fig. 5(c). The inset shows a microscopic image of the fusion joint between the few-mode port of the SMPL and the FMF. Following the fusion of the FMF, the mode transmission characteristics of the SMPL-FMF assembly will be tested in the next section.
5. Characterization of the SMPL
In order to confirm the effectiveness of the fabricated SMPL, we need to test its mode selectivity characteristics at a wavelength of 980nm and the mode-maintaining transmission characteristics at a wavelength of 1550nm.
5.1 Mode selectivity characteristics of the SMPL at 980nm
First, the mode selective performance at 980nm of the SMPL is tested. A 980nm laser source is used to emit 980nm laser into the SMFs at the input array. The light is output from the few-mode port after being transmitted through SMPL. The output light is focused onto an infrared detector (Ophir, sp620) through a 20x microscope objective. The observed near-field intensity distribution at the few-mode output is shown in Fig. 6(a). Figure 6(a) include the corresponding few-mode port mode field distribution diagrams when 980nm laser is injected into different SMFs, respectively. As anticipated, the near-field mode profiles at the few-mode port clearly demonstrate independent excitation of LP01, LP11a and LP11b modes. Additionally, we connected two SMPLs through a short segment of FMF and tested the back-to-back transmission matrix of the SMPL, which is shown in Fig. 6(b), the unit is in dB.
Fig. 6. (a) The near-field mode profiles (from left to right: LP01, LP11, LP11b modes) at the few-mode port of the SMPL. (b) back-to-back transmission matrix of the SMPL.
5.2 Mode preserving characteristics of the SMPL at 1550nm
As the key innovation of the SMPL, its 1550nm mode preserving feature is tested and verified. Because the few-mode port of the SMPL ultimately needs to be connected to the FMF, to test the mode profile after transmission through FMF, we fused the few-mode port of the SMPL with a segment of FMF. To generate distinct modes at 1550nm, a simple fiber misalignment structure is employed. The 1550nm laser is firstly injected into a SMF. Following this, the SMF and the FMF at the input fiber array of the SMPL were fusion spliced with a misalignment, as shown in Fig. 7. By manipulating the offset between the SMF and FMF and controlling the polarization state, the desired LP01, LP11a and LP11b modes can be achieved, as shown in the bottom left of Fig. 7. Inject each mode into the input FMF of the SMPL, and observe the corresponding outputs with an infrared camera (Beam, 2000/1550) at the FMF. After the LP01, LP11a and LP11b modes are injected into the FMF, the corresponding modes are sequentially observed in the few-mode port through SMPL diffraction transmission. The detected mode patterns are depicted at the bottom right of Fig. 7. As anticipated, the near-field mode profiles at the output of the few-mode port clearly demonstrate the low-loss and independent transmission of fiber modes. Thus the mode preserving characteristics of the SMPL at 1550nm have been experimentally validated.
Fig. 7. Schematic diagram of experimental setup for generating 1550nm high-order modes. Bottom insets: input and output mode patterns of the SMPL.
6. Conclusions
In this paper, we introduce an SMPL device that introduces a FMF based on the conventional photonic lantern. The congruence in parameters between the input FMF and the output few-mode port in SMPL renders it suitable for constructing a complete FMF ring laser cavity. The dual-band multiplexing features of SMPL, involving mode-selective excitation at 980nm and mode pattern maintenance at 1550nm, as evidenced by simulations and experiments, empower SMPL to selectively excite high-order modes of 980nm pump light and simultaneously facilitate closed-loop oscillation and transmission of the 1550nm laser within the complete few-mode fiber ring. The ability of SMPL to extend to more modes is of significant importance for high-capacity space division multiplexing transmission systems. Theoretically, SMPL exhibits unlimited scalability, and investigations into its scalability will be reported in forthcoming research endeavors. In addition, this paper focuses on the theoretical validation and experimental tests of the SMPL, subsequent research will introduce its performance when applied to fiber lasers.
Funding
National Natural Science Foundation of China (61963038, 62065001, 62105279); Applied Basic Research Foundation of Yunnan Province (202201AU070047); Yunnan Provincial Department of Education (2023J0588); Yunnan Young and Middle-aged Academic and Technical Leaders Reserve Talent Project (202205AC160001).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
Data availability
The 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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