In recent years, tunable lasers have been widely used in optical information processing^[1], fiber optic sensing^[2], and dense wavelength division multiplexing^[3]. To achieve multi-wavelength tunable and switchable output, options include semiconductor laser arrays or rare Earth element-doped fiber lasers. Due to their compact size, ease of bending, and alignment of erbium-doped fiber (EDF) emission windows with the low-loss window of fiber communication, tunable erbium-doped fiber lasers (TEDFLs) have increasingly become a focal point of research. To suppress mode competition in EDFs, which arises from their homogeneous gain broadening, and to achieve tunable lasing within a certain bandwidth, various strategies have been explored and demonstrated. These include cooling with liquid nitrogen^[4], incorporating frequency-shifted devices^[5,6], using fiber optic filters^[7], adding wavelength-dependent loss structures^[8], and leveraging the high nonlinearities of optical fibers. Fiber optic filters, in particular, are favored for their simplicity and structural versatility.

Currently, some of the fiber optic filters used in TEDFLs are based on the principles of mode or polarization interference, such as Mach–Zehnder interferometers^[9], Michelson interferometers^[10], and Sagnac ring filters^[11]. Others utilize fiber gratings, including fiber Bragg gratings and long-period gratings^[12,13]. Compared to fiber grating filters, those based on interference principles are simpler to fabricate and more cost-effective. In recent years, researchers have leveraged various interference-based fiber filters to achieve tunable, switchable outputs in erbium-doped fiber lasers (EDFLs). For instance, in 2014, Ahmad et al. achieved a tunable laser output with a bandwidth of 42 nm using a fiber optic biconical Mach–Zehnder filter structure^[14]. In 2020, Wan et al. achieved a tunable output for an EDFL ranging from 1531.92 to 1544.28 nm with an optical signal-to-noise ratio (OSNR) of about 50 dB using a fiber interferometer with a double peanut structure^[15]. In 2021, Tang et al. developed a tunable laser using multimode interference generated by a four-leaf clover suspended core fiber, with a tunable range from 1546.5 to 1581.5 nm, capable of single- and multi-wavelength switching^[16]. However, the design of current fiber optic filters is constrained by the complexity of their fabrication processes, low structural strength, and lack of ability to modulate spectral depth post-fabrication due to their fixed structures.

In this paper, we propose and demonstrate a modulatable fiber optic filter based on a hollow-core anti-resonant fiber (HC-ARF), which exploits bending-induced loss differences in orthogonal transmission directions. This filter is combined with a polarization-maintaining fiber (PMF), allowing polarized light to interfere within the bent HC-ARF and form a comb filter spectrum with an online-controllable modulation depth. We successfully constructed a tunable ring-cavity EDFL based on polarization interference. This fiber laser achieves tunable outputs ranging from 1547 to 1561 nm, with all OSNR exceeding 35 dB. Additionally, through adjustments of the polarization controller (PC), the laser can switch between single-wavelength and three-wavelength operations near 1560 nm. This technology holds significant potential for applications in optical communication, dense wavelength division multiplexing, and laser biomedicine.

The HC-ARF used to prepare the filter is depicted in Fig. 1(a), where the diameter of the fiber core is 36.4 µm, and the thickness of the cladding tube is 1.1 µm. The position of the anti-resonance window of the HC-ARF can be expressed as λγ=2tm−0.5n2−1,(1)where λγ represents the anti-resonance wavelength, t is the thickness of the cladding tube wall, n is the refractive index of the quartz glass, and m is an integer representing the resonance number of the waveguide. The variation of t corresponds to the difference in the transmission band of the HC-ARF, suggesting that the conduction band of the HC-ARF is located near 1550 nm. Figure 1(b) displays the transmission loss measurements for this HC-ARF, conducted using the truncation method for HC-ARF. The results indicate that the conduction band spans from 1400 to 1700 nm, and the transmission loss at 1550 nm is approximately 0.34 dB/m, meeting the requirements for transmission in the communication band.

The HC-ARF modulation device was assembled by fusion splicing each end of the HC-ARF to a single-mode fiber (SMF), with the HC-ARF measuring 12.5 cm in length, as depicted in Fig. 2(b). The fusion splicing was realized by a CO2 laser fusion splicer: Fujikura LZM-100.

The prepared HC-ARF modulated devices were tested using a broadband source (BBS) with an output wavelength range of 1450–1600 nm. Changes in transmitted light intensity due to varying bending diameters were observed using an optical spectrum analyzer (OSA) (Yokogawa, AQ6370C), as depicted in Fig. 2(a). Notably, we measured the transmission spectra of the HC-ARF at different lengths as shown in Fig. 2(c), which does not show any obvious mode interference. A reduction in the bending diameter of 12.5-cm-long HC-ARF only leads to an increase in transmission spectral loss, as shown in Fig. 2(d).

On this basis, linearly polarized light E0→ is introduced into the system using a polarizer. The polarization state of this light can be transformed into any desired state using a PC, as shown in Fig. 3(a). The various states of polarized light can be viewed as a combination of two orthogonal components with a phase difference Δδ, where Δδ is controlled by the PC.

When polarized light in a specific state is transmitted through the PMF, its two orthogonal polarization modes, which have fixed phase delays, form a definite angle α with the slow axis of the PMF. This polarized light is then decomposed into two mutually perpendicular, linearly polarized beams, which propagate along the fast and slow axes of the PMF: {Ex=E0cosαEy=E0sinα.(2)

During the transmission of polarized light through the PMF, the birefringence effect causes the fast and slow axis components of polarized light to develop a phase difference δ0. The phase difference δ0 depends on the wavelength λ, the birefringence B of the PMF, and the length L. Consequently, the final magnitude of the phase difference experienced by the two polarized light components as they pass through the PMF is denoted as δ, δ=Δδ+δ0=Δδ+2πBLλ.(3)

With the bending process of the HC-ARF, significant bending along the x-axis induces strong coupling between the fiber core mode and the cladding mode. This results in higher losses along the x-axis, while the y-axis—perpendicular to the bending axis—experiences less impact, leading to a notable loss difference. Consequently, the HC-ARF can function as a polarization, with its y-axis forming a specific angle β, with the slow axis of the PMF. The intensity of the two orthogonal components of the polarized light after passing through the bent HC-ARF is {Ex′=E0cosαcosβEy′=E0sinαsinβ.(4)

Eventually, after passing through the bent HC-ARF analyzer, the two vertical polarization components with a phase difference of the two beams produce coherent superposition, forming polarization interference. The intensity of the superimposed light is calculated by E→=Ex′→+Ey′→,(5)I=E→·E*→=E2·[cos2αcos2β+sin2αsin2β+12sin2αsin2βcos(Δδ+2πBLλ)],(6)λdip=2πBL(2m+1)π−Δδ,m∈N.(7)

The resonant wavelength in Eq. (7) is derived from Eq. (6), which shows that Δδ changes when polarized light of different polarization states is incident, thus inducing a shift in the resonant wavelength. The intensity of the outgoing light depends on the wavelength λ, and the corresponding transmission spectrum is illustrated in Fig. 3(b).

We observed the effect of varying bending diameters on the transmission spectra and found that, below a certain threshold, the spectra exhibit more pronounced polarization interference, with a free spectral range (FSR) of 3.5 nm. As the bending diameter decreases, the extinction ratio (ER) increases. Consequently, we have achieved a comb-filtered spectrum with an FSR of 3.5 nm suitable for the communication band. Additionally, the depth of this filtered spectrum can be modulated by adjusting the bending.

By adjusting the angle of the three coils of the PC, we can control the phase difference Δδ, which in turn influences the filtered spectrum. Figure 3(c) illustrates the comb-filtered spectrum as modulated by the PC.

Based on the HC-ARF filter, we constructed a ring-shaped all-fiber laser cavity to generate tunable laser output in the communication band, as depicted in Fig. 4(a). A multimode laser pump source, with a center wavelength of 976 nm, directs incident laser light into the cavity through a wavelength division multiplexer (WDM). We have prepared double-clad erbium-doped fibers (EDFs) with high doping concentration using non-chemical vapor deposition (NCVD), and the fiber can achieve homogeneous broadening over a short length. The amplified spontaneous emission (ASE) of different length EDFs is shown in Fig. 4(b). We use 1 m EDF to serve as the gain medium, producing a flat ASE spectrum with enhanced emission in the C-band. This HC-ARF filter is integrated into the ring laser cavity and, combined with a PC as a wavelength selector, determines the laser output wavelength. The generated laser light is routed through a 90:10 coupler, where 10% is directed into an OSA for data analysis, and the remaining 90% continues to circulate in the cavity.

The output spectrum shown in Fig. 4(c) demonstrates that the uniform gain broadening characteristic at room temperature is suppressed by incorporating the HC-ARF filter, resulting in a single-frequency laser output with an OSNR above 35 dB at 1554 nm. Analysis of the output spectra reveals that the spectral fluctuations approximate a sinusoidal function with a period of 3.5 nm, matching the FSR of the filter’s comb spectrum.

Under the modulation of the PC, the polarization state of the light entering the HC-ARF is altered, enabling various filter functions and ultimately realizing the tunable laser output of the HC-ARF filter across different wavelengths, as depicted in Fig. 5. The tunable range spans from 1547 to 1561 nm with a step size of 3.5 nm, and the OSNR for all tunable laser outputs exceeds 35 dB.

As multi-wavelength lasers are difficult to excite at room temperature due to the uniform gain broadening of EDFs and the cross-gain saturation effect, the introduction of nonlinear polarization rotation (NPR) in the laser cavity suppresses the strong wavelength competition, which means that, by adjusting the PC, the transmittance decreases with the increase of the optical power. This is what we use to generate the intensity-dependent loss (IDL) induced by the multi-wavelength NPR. To output a stable multi-wavelength laser, we optimize the length of the EDF. We achieved multi-wavelength laser output in this ring cavity using a 2-m-long EDF as the gain medium. The transition from single-wavelength to three-wavelength output is controlled by adjusting the angle of the three coils of the PC. Because the use of 2 m less-mode EDF as the gain medium, the longer fiber length leads to the formation of a multi-interference system by combining the mode interference with the polarization interference, the wavelength spacing of the three-wavelength output is approximately 0.6 nm, and the OSNRs for all outputs exceed 25 dB, as depicted in Fig. 6.

In this paper, we propose and experimentally validate a TEDFL that utilizes polarization interferometry to achieve single-wavelength tunability and multiple-wavelength switching with a modulatable HC-ARF filter. This filter, created by fusion-splicing the ends of the HC-ARF to SMF and leveraging the orthogonal directional loss due to bending, interacts with the PMF. This setup allows the incident light of a specific polarization state to be polarized and interfered within the HC-ARF, forming a comb interference spectrum with a controllable modulation depth. Inserted into the ring laser cavity as a wavelength modulator, the HC-ARF filter enabled a tunable output ranging from 1547 to 1561 nm, with an OSNR exceeding 35 dB in all configurations. Furthermore, switching from single to three wavelengths near 1560 nm was achieved, all maintaining an OSNR greater than 25 dB. This laser enhances the application possibilities of the HC-ARF, which is of significance in the fields of optical information processing, fiber optic sensing, dense wavelength division multiplexing, and irradiation resistance.