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Abstract
A single- and dual-wavelength fiber laser with multi-transverse modes is proposed. Mode interference is realized in the core of an optical fiber by writing a long period fiber grating on a few-mode fiber to obtain the LP01 mode and the LP11 mode simultaneously. A mode interferometer based on a few-mode long period fiber grating (FM-LPFG) is used as a comb filter in the ring-cavity fiber laser. Single- and dual-wavelength outputs can be achieved in the fiber laser by adjusting a polarization controller (PC). A mode-selective photonic lantern is used to realize mode conversion, and six LP modes, LP01, LP11a, LP11b, LP21a, LP21b, and LP02, can be generated. A single- and dual-wavelength fiber laser with multi-transverse modes can be achieved by combining a mode interferometer with a mode-selective photonic lantern. This work has potential applications in mode division multiplexing (MDM) systems to enlarge the capacity of optical communications.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The excitation of high-order transverse modes in optical fibers has attracted much attention in recent years. High-order transverse modes play an important role in the generation of cylindrical vector beams [1] and orbital angular momentum beams [2] in optical fibers. High-order transverse mode fiber lasers are used in MDM systems [3], which can enlarge the capacity of optical communications. According to whether the oscillation in the laser resonator is the fundamental mode, there are two main ways to realize high-order transverse mode fiber lasers: direct generation and indirect generation. Direct generation means that high-order transverse modes oscillate in the cavity of fiber lasers [4,5]. High-order transverse mode fiber lasers in direct generation possess the characteristic of high output slope efficiency. However, the design and fabrication of few-mode active fibers such as few-mode erbium-doped fibers are complicated, which involve the matter of mode gain, mode beat, etc. Indirect generation means that the fundamental mode oscillates in the cavity of fiber lasers, and high-order modes are generated by mode excitation or mode conversion [6,7]. Mode excitation or mode conversion is realized by mode selection coupler [8], long period fiber grating [9], fiber Bragg grating [10], photonic lantern [11] and so on. High-order transverse mode fiber lasers in indirect generation have great flexibility and portability, which can combine different configurations of fiber lasers (continuous light or pulsed light) with mode converters of various performance at a fixed wavelength generally.
Compared with high-order transverse mode fiber lasers at a fixed wavelength, multi-wavelength fiber lasers with high-order transverse modes have more advantages in MDM systems to further expand the system capacity [12]. The key component for multi-wavelength fiber lasers is filters, which are generally divided into two categories. The first category is discrete filters, represented by fiber Bragg gratings [13], the number of which is used determines the amount of lasing wavelengths. Multi-wavelength fiber lasers based on these filters can switch wavelengths independently, but the insertion loss and complexity of systems worsen as the number of filters increases. The second category is comb filters, which include Mach-Zehnder interferometer [14], Fabry-Perot interferometer [15], Sagnac interferometer [16], Michelson interferometer [17], Lyot Filter [18], different types of fiber gratings cascaded [19], etc. Switchable, tunable and interval-adjustable multi-wavelength fiber lasers can be achieved by these filters. However, the transmission spectrum of these filters has characteristic of overall change, which weakens wavelength switching for multi-wavelength fiber lasers. In this paper, a few-mode long period fiber grating is used to obtain the LP01 mode and the LP11 mode, and mode interference is realized in the core of an optical fiber to form a comb filter. The LP01 mode and the LP11 mode are obtained by the core-offset splicing of fibers generally, which has low cost and simple operation. However, it is easy to decrease the mechanical strength of optical devices. Due to the excitation of high-order modes in the cladding of optical fibers, it is susceptible to external environmental such as temperature and stress for fiber lasers, which results in wavelength drift and power fluctuation to worsen system stability. At present, the LP11 mode dominates for multi-wavelength fiber lasers with high-order transverse modes [20,21], which means that the number of high-order transverse modes is relatively limited.
In this paper, a single- and dual-wavelength fiber laser with multi-transverse modes is proposed. The LP01 mode and the LP11 mode are obtained simultaneously by writing a long period fiber grating on a few-mode fiber, and mode interference is realized in the core of an optical fiber by taking advantage of the transmission difference between the LP01 mode and the LP11 mode. A mode interferometer based on a FM-LPFG is used as a comb filter in the ring-cavity fiber laser. Single- and dual-wavelength outputs can be achieved in the fiber laser by adjusting a PC. A mode-selective photonic lantern is used to realize mode conversion, and six LP modes, LP01, LP11a, LP11b, LP21a, LP21b, and LP02, can be generated. A single- and dual-wavelength fiber laser with multi-transverse modes can be achieved by combining a mode interferometer with a mode-selective photonic lantern.
2. Experimental setup and results
The experimental setup for the proposed single- and dual-wavelength fiber laser with multi-transverse modes is shown in Fig. 1. It consists of a 980 nm pump laser, a 980/1550 nm wavelength division multiplexer (WDM), a 12-m-long erbium-doped fiber (EDF), an optical isolator (ISO), a PC, a mode interferometer, a 50:50 optical coupler, and a mode-selective photonic lantern. The EDF is used as the gain medium. The optical isolator ensures unidirectional transmission of light, and the state of polarization is controlled by the PC to balance the gain and loss in the laser resonator. The mode interferometer acts as a wavelength selector. After the lasing wavelengths are output from an end of the optical coupler, they enter the mode-selective photonic lantern to realize mode conversion. The mode-selective photonic lantern is composed of six single-mode fiber input ends and one few-mode fiber output end. The lasing wavelengths enter the certain single-mode fiber and excite the corresponding LP mode, LP01, LP11a, LP11b, LP21a, LP21b, and LP02, which can be output from the few-mode fiber.
Fig. 1. Experimental setup for the proposed single- and dual-wavelength fiber laser with multi-transverse modes.
Filters play a vital role in laser resonators, whose performance directly determines optical properties of fiber lasers. The mode interferometer is used as a wavelength selector in this experiment as shown in Fig. 2. A chirped long period fiber grating with a start period of 1100µm, an end period of 1104µm, and a step length of 0.1µm is written on a few-mode fiber, and the LP01 mode and the LP11 mode are obtained simultaneously. The chirped type aims to minimize the power difference between two modes in wide wavelength range, which makes two modes interfere well. When two modes are transmitted in a few-mode fiber, their relative phases change during the propagation process. Mode interference occurs at the splicing point between the few-mode fiber and the single-mode fiber, which forms a Mach-Zehnder mode interferometer in the core of an optical fiber. The transmission spectrum of the FM-LPFG is shown in Fig. 3. The maximum power difference between the LP01 mode and the LP11 mode is 1dB within the gain wavelength range of the EDF from 1530nm to 1570nm. The transmission spectrum of the mode interferometer is shown in Fig. 4. The power of each peak and the free spectral range (FSR) between adjacent peaks formed by mode interference are not always equal as the power of the LP01 mode and the LP11 mode are not always equal within the gain wavelength range of the EDF.
The mode-selective photonic lantern used in this experiment is composed of six single-mode fiber input ends and a few-mode fiber output end. When the phase matching condition is met, the fundamental mode of the single-mode fiber input end can be converted to high-order modes of the few-mode fiber output end. The lasing from the optical coupler is input to different single-mode fiber ends of the mode-selective photonic lantern, and six LP modes, LP01, LP11a, LP11b, LP21a, LP21b, and LP02, can be output from the few-mode fiber end of the mode-selective photonic lantern. At the few-mode fiber end of the mode-selective photonic lantern, CCD is used to capture the mode profile of different modes, and OSA is used to observe the wavelength lasing of the fiber laser under different modes.
Single- and dual-wavelength outputs can be achieved in the fiber laser by adjusting a PC. In this paper, the mode interferometer is used as a comb filter. The amplified spontaneous emission (ASE) spectrum of the EDF is not flat, and the EDF spectrum is modulated by the comb spectrum of the mode interferometer. The gain and loss of different wavelengths in the EDF spectrum can be balanced by the PC. Lasing is formed when certain wavelengths get enough gain. There is randomness at a certain degree for the tuning of the PC, which gives an explanation to the experimental results that the lasing wavelength of this fiber laser is not completely equal to the FSR. Even so, the repeatability of this fiber laser is determined by the optical components and working principle.
The mode profiles and the wavelength lasing under different modes at the few-mode fiber end of the mode-selective photonic lantern are shown in Fig. 5–10. By switching the six single-mode fiber input ends of the mode-selective photonic lantern, lasing with different modes can be output from the few-mode fiber end of the mode-selective photonic lantern. Under the LP01 mode, single-wavelength lasing can be achieved at 1560.41nm and 1560.81nm, and dual-wavelength lasing can be achieved at 1560.43nm and 1560.81nm. The output spectrums of the fiber laser under the LP11a, LP11b, LP21a, LP21b, and LP02 mode are shown in Fig. 6–10. Compared with the lasing of single-wavelength and dual-wavelength under different modes, it is found that there is a slight shift in wavelength. According to the laser principle, the lasing wavelength $\lambda $ is determined by the formula ${n_{eff}}L = m\lambda /2$, where ${n_{eff}}$, $L$ and m are the effective refractive index of the transverse mode, the geometric cavity length, and the number of longitudinal modes. Since the effective refractive index of different modes is different, there is a slight deviation in lasing wavelengths for different modes. Under the LP11a, LP11b, LP21a, LP21b, and LP02 mode, single- and dual-wavelength outputs can be achieved in the fiber laser, whose linewidth is less than 0.08nm, and optical signal-to-noise ratio is more than 40dB.
3. Conclusion
In this paper, a single- and dual-wavelength fiber laser with multi-transverse modes is proposed. The LP01 mode and the LP11 mode are obtained simultaneously by writing a long period fiber grating on a few-mode fiber, and mode interference is realized in the core of an optical fiber by taking advantage of the transmission difference between the LP01 mode and the LP11 mode. A mode interferometer based on a FM-LPFG is used as a comb filter in the ring-cavity fiber laser. Single- and dual-wavelength outputs can be achieved in the fiber laser by adjusting a PC.A mode-selective photonic lantern is used to realize mode conversion, and six LP modes, LP01, LP11a, LP11b, LP21a, LP21b, and LP02, can be generated. A single- and dual-wavelength fiber laser with multi-transverse modes can be achieved by combining a mode interferometer with a mode-selective photonic lantern, which can be used in MDM systems to enlarge the capacity of optical communications and make full use of fiber bandwidth.
Funding
National Natural Science Foundation of China (62075080); Department of Science and Technology of Jilin Province (20190302014GX, 20200401051GX).
Disclosures
The authors declare there are no conflicts of interest.
Data availability
No data were generated or analyzed in the presented research.
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