A Tunable and Switchable Multi-Wavelength Erbium-Doped Fiber Laser Based on a Curvature Mach–Zehnder Interferometer Filter Using Thin-Core Fiber

Abstract

We propose and demonstrate a tunable and switchable multi-wavelength fiber ring laser configuration based on a Mach–Zehnder interferometer (MZI) filter. The MZI was fabricated using a core-offset splicing technique, with a 2 cm piece of thin-core erbium-doped fiber (TCEDF), with a core diameter of 2.90 µm, coupled in the central region of the MZI between two segments of single-mode fiber (SMF). By applying curvature to the MZI filter, we generated lasing single-, double-, triple-, and quadruple-emission lines with a curvature range from 2.3452 m⁻¹ to 6.0495 m⁻¹. A single-emission lasing line can be tuned from 1556.63 nm to 1564.25 nm with a tuning span of 7.62 nm and an SMSR of 49.80 dB. The laser emission can be switched to quadruple- and triple-emission lasing signals, with SMSR values of 39.96 dB and 36.83 dB, respectively. The dual-narrow emission lasing signal can be tuned from 1564.56 nm to 1561.34 nm, with an SMSR of 40.46 dB. Another lasing dual-emission signal can be tuned from 1585.69 nm to 1576.89 nm, producing an 8.8 nm tuning range, and from 1572.53 nm to 1563.66 nm, producing an 8.87 nm range, with the best SMSR of 42.35 dB.

1. Introduction

Switchable and tunable multi-wavelength fiber lasers have generated significant interest for their potential in telecommunications [1,2]. They are typically constructed from erbium-doped fiber and configured in either ring- or linear-type optical cavities [3,4,5,6,7]. In recent years, methods have been reported that can generate a stable tunable multi-wavelength and tunable range laser emission [8,9,10,11,12,13,14,15,16,17], such as polarization controllers (PCs) [18] and wavelength-selective filters (WSFs), which balance the wavelength depending on their cavity losses [8]. WSFs can be fabricated with different structures, such as fiber Bragg grating (FBG) [9] or Mach–Zehnder interferometers (MZIs) [8,10,11], and with different compounded fiber filters [12,18]. The MZI filter is fabricated by using SMF [13,14], photonic crystal fiber (PCF) [8], and thin-core fiber [10,11]. In a laser cavity, filters are used, which, by increasing curvature [8,12], temperature [9], or strain [12,15], can make a laser tunable and switchable. By utilizing three PCs, Zou, H. et al. [16] reported achieving a switchable erbium-doped fiber laser with a dual-wavelength tuning range of 3.42 nm and a side mode suppression ratio (SMSR) above 64 dB. The maximum power fluctuation and wavelength shift were 0.50 dB and 0.01 nm, respectively. Zhou et al. [18] reported a wide tuning range of 153 nm; this proposal uses a filtering mechanism in conjunction with four PCs and lasing lines with high power (57.3 dB) across single-, dual-, triple-, quadruple-, and quintuple-wavelength lines, which were adjusted using different pumping power outputs and PCs. Zhao, X. et al. [12] reported that using fiber Bragg grating together with an MZI in a laser cavity produced an emission of laser lines that can be smoothly switched from single to quadruple wavelengths from 1527.68 nm to 1550.22 nm, with an optical signal/noise ratio (OSNR) greater than 50 dB, by applying up to 258 µε of voltage to the filter. Ahmad et al. [9] present a multi-wavelength 12- and 8-line emission laser, with an OSNR of 45 dB and 47 dB, respectively, which is generated by using a Fabry–Perot interferometer (FPI) and FBG dual cavity; the tunable 8-line emission laser is obtained by using temperature increments in a span of 1 nm. Thin-core fibers (TCFs) are very efficient because they have almost no loss, as shown in the work of He, W. et al. [11], where they used one TCF to create an MZI loop and a Sagnac interferometer (SI) working in conjunction with a single-emission line tuned at 40 nm, with a 32.68 dB SMSR and a dual and triple loop with an SMSR of over 34.51 dB. A modal analysis of MZIs based on SMF and thin-core fiber was developed by Yu-Xuan et al., and high sensitivity was found in the interferometers [19]. Interferometers based on thin-core fibers have been studied in relation to curvature [20], temperature [21], and refractometers [22], among many others, with advantages in terms of low cost, simple fabrication, and relatively high sensitivity. In this work, a multi-wavelength tunable and switchable erbium-doped fiber laser based on a thin-core erbium-doped fiber (TCEDF) filter is proposed and experimentally demonstrated. By incrementing the curvature in the filter, the laser can operate as tunable for single-, dual-narrow, and dual-wavelength emissions, and it can switch between triple- and quadruple-emission wavelengths. The analysis of the stability tests for the power and wavelength fluctuation determined that the lowest were 0.67 dB and 0.10 nm; the highest were 2.86 dB and 2.57 nm, respectively. The best SMSR was 49.80 dB for a single-emission line.

2. MZI Fabrication and Filter Principle

We fabricate an MZI by using a core-offset fiber array as the filtering mechanism. The MZI was constructed using a standard single-mode SMF28 fiber with a length of 2 m. We utilized a 2 cm piece of the TCEDF (RightWave EDF R37003X; Furikawa, OFS Fitel, Norcross, GA, USA), with a core diameter of 2.90 µm and cladding of 80 µm, which was spliced in half using a U-shape [17,23] core-offset splice. The U-shape was achieved by holding the SMF28 to a steel ruler with masking tape (Figure 1a), applying small marks on the fiber, ruler, and tape to prevent any unintentional movement and keeping it as stable as possible during splicing to avoid any twisting. We made the splice with a 6 cm piece of the TCEDF (shown in Figure 1b), which we manually cut to a length of 2 cm using a ceramic razor (CSW12-5 Thorlabs ceramic fiber scribe; Thorlabs, Newton, NJ, USA), enabling us to control the position of the MZI segments (Figure 2b). We performed the splicing using a Fitel S178 fusion splicer and modified the SMF program parameters to a pre-fusion time of 100 ms, a 1st first-arc duration time of 400 ms, a Z pressure distance of 4 µm, a first-arc power start of +45, an arc offset of +5 µm, a cleaning arc power offset of −20, and a cleaning time duration of 100 ms, always placing the TCEDF on the right holder side and the SMF on the left holder side, with the remaining fiber attached to the steel rule. We made the first splice (Figure 2a) by aligning the cores of the SMF (left holder) and the TCEDF (right holder) and then moving the left holder of the fusion splicer 4 µm upwards on the X-axis. Figure 2b illustrates how the SMF/TCEDF tip was cut to leave a measurement of 2 cm. In Figure 2c, we show how the second splice was made by aligning the TCEDF (left holder) and SMF (right holder) cores and then moved the motor on the X-axis 4 µm down, resulting in the U-shape sensor depicted in Figure 2d and viewed from the z-side. We tested the MZI by powering it with the ASE source, using the LDC operating at 135 mW and setting the temperature controller at 25 °C.

A U-shaped Mach–Zehnder interferometer was fabricated using a segment of thin-core erbium-doped fiber (TCEDF) with a length “L”. The operation principle of the fabricated MZI is based on the light input into the SMF. At the left junction of the core offset, the device acts as a splitter, with one part of the light coupled to the core, while the other part is coupled to the cladding of the TCEDF. At the right junction of the core offset, the core and the cladding modes are recombined into the SMF to generate interference. The output intensity of the interference can be expressed by [24,25]:

$$I=I_{core}+I_{clad}+2\sqrt{I_{core}{I}_{clad}}\cos \mathrm{\varnothing }$$

(1)

where $I$, $I_{core}$ and $I_{clad}$ are the intensity of the output in the core mode and cladding modes, respectively, and $\varnothing$ is the phase difference between the core and cladding mode and is approximated as:

$$\varnothing ={\displaystyle \frac{2\pi ∆n_{eff}L}{\lambda }}$$

(2)

where $\lambda$ is the wavelength, $L$ is the length of the TCEDF, and ${∆n}_{eff}$ is the effective index difference between the core and the cladding modes of TCF. If $\varnothing$ equals ($2m+1$) in Equation (2), with $m=1,2,3,\dots ,$ The dip of the interference spectrum can be expressed as:

$${\lambda }_m={\displaystyle \frac{2{∆n}_{eff}L}{2m+1}}$$

(3)

where ${\lambda }_m$ is the monitoring wavelength.

The interference spectrum of the MZI, which uses a length $L$ of TCEDF at 2 cm, 4 cm, and 6 cm, is illustrated in Figure 3a. A spatial frequency spectrum of the interference pattern, generated through the Fast Fourier Transform (FFT) is presented in Figure 3b. In this figure, a single dominant high-order cladding mode is observed with a spatial frequency of 0.2034 nm⁻¹, 0.2670 nm⁻¹, and 0.2161 nm⁻¹, respectively. The interference emission primarily arises from the fundamental core mode and the cladding mode. However, the interference spectrum output for the 2 cm MZI is dominated by a stronger cladding mode compared to the other MZIs, with an insertion loss of 4.2 dB. This MZI configuration consisting of SMF/TCEDF/SMF was selected as a filter due to its best output power and FSR. We use TCEDF for the advantages of a TCF for MZI fabrication and the spectrum it generates once realized, not for the gain material. TCF-based interferometers have been reported to be modal exciters; due to their smaller core size, the guiding light travels through the core and much of the cladding, resulting in a larger modal field diameter. As a result, higher-order modes can be excited, which creates modal interference. Due to the difference in core diameter and changes in the cutoff wavelength, some of these modes can easily be generated from the cladding [26].

3. Schematic Configuration and Experimental Setup

The following is a diagram of the experimental setup used for the multi-wavelength thin-core erbium-doped fiber laser (MTEFL), as presented in Figure 4a. The laser operates within a 9-meter-long cavity and utilizes a 980 nm pumping laser diode (Bookham LC94ZL74-20R; Bookham, Inc., San Jose, CA, USA) together with an LDC that provides a power output of 135 mW. In the setup, a 980/1550 Wavelength Division Multiplexer (WDM) is used, along with an Optical Isolator (ISO), a 1-meter-long erbium-doped fiber (EDF) (Thorlabs ER80-8/125; Thorlabs, Inc., Newton, NJ, USA), and an Optical Coupler (OC). Within the OC, 1% of the pumping power is directed toward the Optical Spectrum Analyzer (OSA; Yokogawa AQ6370D; resolution 0.02 nm; Yokogawa Electric Corporation, Musashino, Tokyo, Japan), while the remaining 99% is retained within the ring array to facilitate lasing. Additionally, a PC is incorporated into the setup. The MZI filter is positioned at the center of a 15-gauge steel wire that measures 350 mm in length and 1.44 mm in diameter. Small pieces of adhesive tape are used to secure the MZI to a metric rod, preventing any potential rotation that could affect the optimal functioning of the filter. Two stationary bases support the optical fiber (OF). One of the bases features a one-axis motorized translation stage (MT1-Z8), which utilizes a Z812B motor and a Kinesis KDC101 motor driver. The motor driver has a resolution of 0.1 mm per step, allowing for the adjustment of the curvature of the filter, as shown in Figure 4b.

The tuning and multi-line switching (multi-wavelength) are achieved by adjusting multiple wavelengths through the movements of the left motor. The lineal initial displacement is 14.70 mm to 224.70 mm, with an increment of 0.10 mm. When the curvature sensor is bent, the dip wavelength in the interference spectrum can be given by [27]:

$${\lambda }_m={\displaystyle \frac{2{∆n}_{eff}L}{2m+1}}+{\displaystyle \frac{kLb}{2m+1}}\times {\displaystyle \frac{1}{\mathrm{R}}}$$

(4)

where $k=-0.1649\times {10}^{-9} {\mathsf{\mu }\mathsf{\epsilon }}^{-1}$ is the strain refractive index coefficient, and $b$ is the distance from the cladding to the core. At each linear increment, “x” is measured and estimated by the curvature expression [28]:

$$\mathrm{C}={\displaystyle \frac{1}{\mathrm{R}}}=\sqrt{24\mathrm{x}/{\left({\mathrm{L}}_0/2\right)}^3}$$

(5)

where “R” is the curvature radio, “x” is the movement distance of the moveable fiber end, and “L₀ = 210 mm” is the distance between the edges of the two bases. The curvature range is from 2.3452 m⁻¹ to 6.0495 m⁻¹. The curvature radius is adjusted over the MZI, generating single-, double-, triple- and quadruple-line emission. We used 147 samples in a total sweep of the first and last emission lines of the MTEFL (Figure 5) at room temperature (25 °C). In state-a, we obtained a single tunable lasing line ranging from 1556.63 nm up to 1564.25 nm, with a wavelength-tunable range of 7.62 nm into a curvature range from 2.3452 m⁻¹ to 4.9323 m⁻¹. In state-b, the curvature radius is 4.9537 m⁻¹, generating a signal with quadruple lines. In state-c, we obtained two different signals with triple lasing lines, with a curvature of 4.9750 m⁻¹ and 4.9962 m⁻¹. In state-d, we could tune the dual narrow lasing line signal from 1564.56 nm to 1561.34 nm, with a wavelength-tunable range of 3.22 nm into a curvature range from 5.0173 m⁻¹ to 5.2843 m⁻¹. Lastly, in state-e, we could tune dual lasing lines, with one ranging from 1585.69 nm to 1572.53 nm and the second line from 1576.89 nm to 1563.68 nm. The wavelength-tunable range is 8.8 nm and 8.87 nm, respectively, for a curvature range from 5.3042 m⁻¹ to 6.0495 m⁻¹.

4. Experimental Investigation of MTEFL Operation

This section analyzes the tuning and switching process of the multi-wavelength thin-core erbium-doped fiber laser (MTEFL) emission lines. The analysis aims to provide a comprehensive explanation of the continuous curvature applied to the MZI sensor and the subsequent changes observed in the laser emissions. In Figure 6a, we illustrate the initial tuning process of an MTEFL. This figure displays the first and last lasing lines obtained within a wavelength range of 1556.63 nm to 1564.25 nm from 89 tuning samples for state-a. The tuning process spanned a wavelength range of 7.62 nm, corresponding to a curvature range from 2.3452 m⁻¹ to 4.9323 m⁻¹. Figure 6b shows that the initial signal (λ₁ = 1556.63 nm) had an SMSR of 40.04 dB, while the final lasing signal emission for a single line (λ₈₉ = 1564.25 nm) had an SMSR of 36.04 dB. The linewidths of 3 dB were 0.02 nm for λ₁ and 0.03 nm for λ₈₉, with a wavelength interval of approximately 0.08 nm. The SMSR difference between the first (λ₁) and the last (λ₈₉) emission lines is 4 dB. To further understand this variation, we focus on a more significant single-line emission, as depicted in Figure 6c. For this single tuning emission at wavelength λ₁, the SMSR was 40.04 dB, which increased to 49.80 dB at λ₄₆, before dropping to 36.04 dB at λ₈₉. The corresponding signal-to-noise ratios (SNRs) for these wavelengths were λ_SNR1 = 1562.46 nm, λ_SNR46 = 1566.69 nm, and λ_SNR89 = 1558.29 nm, with SNR values of 16.64 dB, 7.17 dB, and 18.74 dB, respectively. The laser SNR is defined as the ratio of the laser peak power to the adjacent ASE level monitored from the output laser [29]. The variation in SMSR during the tuning process is due to the increment in the curvature applied to the filter. This change in curvature may lead to insertion loss during tuning, which induces variations in propagation through both the core and cladding modes. As a result, these changes affect the gain and lead to variations in the gain spectrum [12]. The average peak output power obtained from 89 tuning samples was measured to be −14.76 dBm. We observed a maximum power fluctuation of 2.19 dB across the total emission tuning lines, which is below the threshold of 3.0 dB. The variation observed in the SMSR and SNR is not substantial enough to affect the peak output power. To assess the response of the MZI filter to curvature, we analyzed all 89 samples taken while tuning a single lasing emission line, as shown in Figure 6d. Our study encompasses a curvature range of 2.3452 m⁻¹ to 4.9323 m⁻¹, from which we derived a sensitivity of 2.67 nm/m⁻¹, with an R² value of 0.9982. The relationship between curvature and wavelength shift was established through the use of a polynomial fit adjustment, which yielded the equation $y=1555.7397-1.0064x+0.5571x^2$. This equation accurately represents the aforementioned relationship.

The curvature was incremented to 4.9537 m⁻¹, leading to a wavelength switch that resulted in the division of a single lasing signal into quadruple-emission lines (state-b), as shown in Figure 7a. These lines were identified as λ₁ = 1557.99 nm, λ₂ = 1558.89 nm, λ₃ = 1564.02 nm, and λ₄ = 1564.78 nm. The spectral power distribution of these lines was measured, revealing SMSR values of 29.07 dB, 31.98 dB, 39.96 dB, and 34.11 dB for each emission line. Additionally, the 3 dB line widths were determined to be 0.03 nm, 0.05 nm, 0.04 nm, and 0.05 nm, respectively. The adjacent peaks were separated by distances of 0.90 nm, 5.13 nm, and 0.78 nm. Among these peaks, λ₃ exhibited the highest power, with the power differences between each lasing emission line being less than 10.89 dB, as shown in Figure 7b.

In state-c, the curvature was incremented to 4.9750 m⁻¹, resulting in the generation of triple-emission lasing lines, as shown in Figure 8a. Specifically, these triple-emission lines produced maximum peaks at λ_1A = 1558.62 nm, λ_1B = 1564.22 nm, and λ_1C = 1564.53 nm. The SMSR for these peaks was measured at 27.5 dB, 30.44 dB, and 24.71 dB, with 3 dB linewidths of 0.03 nm, 0.04 nm, and 0.02 nm, respectively. The adjacent peaks were separated by 5.60 nm and 0.31 nm. Among these wavelengths, λ_1B exhibited the highest power, with the peak power difference between each lasing line being less than 5.73 dB, as depicted in Figure 8b. An MZI with a thin-core has the advantages mentioned above [10,11,20]. But, as EDF has a gain medium [30], the system has a fluctuation that can be seen when increasing the curvature (4.9537 m⁻¹ to 4.9962 m⁻¹) as in the b- and c-states. From this curvature range, the laser has a switch behavior, and we believe that the high-order modes generated in the cladding influence the emission spectrum of the filter, and, therefore, the wavelength lasing oscillations compete significantly with the overall homogeneity of the gain medium’s laser spectrum, as reported by L.A. Herrera-Piad et al. [31]; therefore, the spacing between the wavelengths observed in Figure 7 (0.90 nm, 0.78 nm) and Figure 8 (0.31 nm and 0.58 nm) is generated. When the curvature was increased to 4.9962 m⁻¹, a second triple-emission lasing line was obtained, as illustrated in Figure 8c. This new triple-emission line produced maximum peaks at λ_2A = 1558.27 nm, λ_2B = 1558.85 nm, and λ_2C = 1564.81 nm. The associated signal had SMSR values of 29.15 dB, 31.16 dB, and 36.83 dB, with 3 dB linewidths of 0.02 nm, 0.04 nm, and 0.02 nm, respectively. The adjacent peaks were separated by 0.58 nm and 5.96 nm. Among these lines, λ_2C exhibited the highest power, and the peak power difference between each lasing line was less than 8.51 dB, as shown in Figure 8d.

In state-d, when the curvature is applied to the filter that goes from 5.0173 m⁻¹ to 5.2843 m⁻¹, a tunable narrow-emission dual-wavelength line is achieved, with a tuning range of 2.24 nm and a wavelength interval of approximately 0.16 nm (see Figure 9a). Throughout the tuning process, a total of 14 samples were analyzed, labeled as λ₁, λ₂, …, λ₁₄. The initial λ₁ (represented by the blue line) features a dual wavelength at λ_1A = 1563.71 nm and λ_1B = 1564.56 nm, with an SMSR of 39.12 dB and 3 dB linewidth of 36.59 dB, corresponding to 0.97 and 0.41 nm, respectively. The λ_1A showed the highest power, with the peak power difference being less than 2.53 dB. The final dual-wavelength sample, λ₁₄ (represented by the orange line), has wavelengths λ_14A = 1561.34 nm and λ_14B = 1562.27 nm, with an SMSR of 33.75 dB and a linewidth of 0.98 nm and 0.41 nm for each, respectively. The λ_14B wavelength also exhibited the highest power, with the peak power difference being less than 1.45 dB. The adjacent peaks in both the initial and final samples are separated by approximately 0.89 nm. Figure 9b illustrates the most significant dual-narrow-emission lines throughout the tuning process, highlighting samples λ₁, λ₃, λ₁₂, and λ₁₄. We can see that λ₁₂ showed the highest power, with the peak power difference of each lasing line being less than 2.63 dB. The total peak power difference across all lasing lines was less than 6.10 dB, with 5.09 dB for λ_As and λ_Bs, respectively. Furthermore, a linear relationship between the shifts in wavelength spectra and curvature was observed (see Figure 9c). The data show a linear response from 1564.56 nm to 1562.27 nm and from 1563.71 nm to 1561.40 nm, with sensitivities of −8.4008 nm/m⁻¹ and −8.7734 nm/m⁻¹, respectively, along with an R² value of 0.9912 and 0.99126.

In state-e, a tunable dual-wavelength emission line was achieved by applying curvature to the MZI filter, resulting in a tuning range of 8.81 nm and a wavelength interval of approximately 0.22 nm, with curvature in a range from 5.3042 m⁻¹ to 6.0495 m⁻¹ (see Figure 10). The tuning process involved a total of 41 samples, labeled λ₁, λ₂, …, λ₄₁. The initial λ₁ (represented by the red line in Figure 10a) exhibited a dual wavelength at λ_1A = 1572.53 nm and λ_41B = 1585.69 nm, with respective SMSR values of 42.35 dB and 41.31 dB. The final dual-wavelength λ₄₁ (depicted by the violet line in Figure 10a) was recorded at λ_41A = 1563.68 nm and λ_41B = 1576.88 nm, showing SMSR values of 29.21 dB and 42.35 dB, respectively. Figure 10b illustrates that the adjacent peaks for the initial and final wavelengths were separated by 13.16 nm and 13.21 nm. During the tuning process, it was observed that the wavelength interval between λ₂₆ and λ₂₇ was 1.08 nm for side A (λ^s) and 0.48 nm for side B (λ^s). The adjacent peaks for λ₂₆ and λ₂₇ were separated by 12.06 nm and 12.76 nm, respectively. Figure 3a shows an FSR of 9.58 nm resulting from the sensor interference pattern. This FSR is smaller than the distance between adjacent peaks. This discrepancy is due to the fact that the TCF core diameter is significantly smaller than the SMF. As a result, the ability to confine guided mode light is reduced, and the mode field diameter is considerably larger, leading to the excitation of higher-order modes and the formation of modal interference [32,33]. As the curvature increases and due to the misalignment of the core diameter and the change in the cutoff wavelength, some modes of the cladding are easily excited [34]. This condition leads to energy absorption and transitions between different modes due to fluctuations in the system. We believe that the differences in the emission spectrum of the filter modes create a variance in the wavelengths of the laser, thus creating a separation between the peaks and effectively expanding its operating range [35]. This analysis provides a thorough explanation of the continuous curvature applied to the MZI sensor and the resulting changes observed in the laser emission signals. Upon analyzing the wavelengths λ₂₈ to λ₄₁, it was found that the separation of adjacent peaks and their wavelength intervals fell within the initially measured tuning lines. Figure 10c illustrates the wavelength lines with higher and lower peak power emissions for the A and B sides λ^s, demonstrating a peak power variation of 10.41 dB for side A λ^s and 7.71 dB for side B λ^s. The variation in the wavelength interval between λ₂₆ and λ₂₇ and the peak power in the span tuning process can be attributed to the increment in curvature applied to the filter. This may have been induced by the change in insert loss during tuning, leading to variations in the core and cladding modes and ultimately affecting the gain spectrum variation [8]. Finally, a linear curvature response was observed, with a sensitivity of −12.2451 nm/m⁻¹ and −11.777 nm/m⁻¹ and R² values of 0.9923 and 0.9744, respectively, as shown in Figure 10d.

5. MTEFL Experimental Results and Discussion

Stability tests were conducted to evaluate the performance of the MTEFL ring-type array at room temperature (25 °C). A total of 31 recordings were taken over 90 min to assess stability. The results of the stability test for the corresponding tuning and switchable emission lines are summarized for single, dual-narrow, and dual configurations in

Table 1. Tunable laser emission lines, stability testing guidance and highlights.

State	Figure	Curvature (m−1)	Emission Lines		Wavelength (nm)	Max Wavelength Fluctuation (nm)	Max Peak Power Fluctuation (dB)
a	Figure 11	2.5871	Single	λ1	1556.40	0.10	0.87
				λ89	1564.50	0.26	0.78
d	Figure 12	0.2670	Dual narrow	λ1A	1573.70	0.20	0.78
				λ1B	1564.50	0.10	1.11
				λ14A	1561.30	0.09	0.81
				λ14B	1562.20	0.08	0.78
e	Figure 13	0.7453	Dual	λ1A	1572.53	2.00	0.67
				λ1B	1585.69	2.26	0.68
				λ41A	1563.68	1.97	1.01
				λ41B	1576.88	2.57	0.78

and for quadruple and triple lines in

Table 2. Switchable laser emission lines, stability testing guidance and highlights.

State	Figure	Emission Lines		Wavelength (nm)	Max Wavelength Fluctuation (nm)	Max Peak Power Fluctuation (dB)
b	Figure 14	Quadruple	λ1	1557.99	0.33	1.69
			λ2	1558.89	0.53	2.68
			λ3	1564.02	0.49	2.85
			λ4	1564.78	0.41	2.34
c	Figure 15	Triple	λ2A	1558.27	0.20	2.47
			λ2B	1558.85	0.35	1.00
			λ2C	1564.81	0.54	1.57
			λ1A	1558.62	0.68	1.84
			λ1B	1564.22	0.37	2.06
			λ1C	1563.53	0.36	2.86

. For the tunable emission lines (Figure 11, Figure 12 and Figure 13), stability analysis focuses on the initial and final lines. In Table 1, we observe that during the tuning process, the fluctuations in power and wavelength are as follows: for single lines, less than 0.87 dB and 0.26 nm; for dual-narrow lines, 1.11 dB and 0.2 nm; and for dual lines, 1.01 dB and 2.57 nm. In Table 2, the stability measurements for the switchable lines (Figure 14 and Figure 15) show power and wavelength fluctuations of less than 2.85 dB and 0.53 nm for quadruple lines and 2.86 dB and 0.68 nm for triple lines, respectively.

Table 3. Comparison between similar works using a sensor in a laser cavity to generate switchable and tunable multi-wavelength emission lines.

Ref.	Main Characteristics	Emission Lines	Tuning (nm)	SMSR (dB)
[7]	Cacade structure—Sagnac loop filter (SLF)/twin core photonic crystal fiber (TCPCF)	1	23.04	57
		2	19	40
[8]	MZI (SMF/PCF/SMF)	1	24	30
		2	3.43
		3	-	28
[11]	Sagnac loop and MZI filter—SMF/TCF/SMF	1	40	32.68
		2	13	33.5
		3	-	34.51
[36]	Double SMF tapers/thulium-doped fiber (TDF) from the ASE	1	-	51
		2	12.6	55.2
		3	-	52
		4	-	48
		5	-	48
[37]	MZI—SMF/with a segment of select cutoff fiber (SCF)/SMF	1	4.88	55.3
[38]	SMF/TCPCF/SMF—MZI	1	23.04	45
		2	33.82	42
This work	MZI—core-offset SMF/TCEDF/SMF	1	7.62	49.8
		2-narrow	2.33	40.36
		2	8.8	42.35
		3	-	36.83
		4	-	39.96

presents a comparison of the reported performance of fiber lasers with other studies on tunable and multi-wavelength fiber lasers. Wei He et al. [11] reported single- and dual-line emissions using a thulium-doped ring-cavity fiber laser based on a Sagnac loop. This setup involved 1.6 m of TCF in an MZI filter, achieving a tunable range of 40 nm and 13 nm, with an SMSR of 32.68 dB and 33.5 dB, respectively. In another study, Ibarra-Escamilla, B. et al. [36] achieved a range of 12.6 nm for a single line, boasting an SMSR of 55.2 dB, and they reported five emission lines. The primary characteristics of our MTEFL include its switchable and tunable line emissions with an MZI filter, which was fabricated through a simple process using only 2 cm of TCEDF for the interferometer. We reported tunable ranges of 7.62 nm, 2.33 nm, and 8.8 nm, with SMSRs of 49.8 dB, 40.36 dB, and 42.35 dB for single-, dual-narrow, and dual-line emissions, respectively. Although our tunable range is smaller, our SMSRs are superior.

6. Conclusions

In this work, we presented a novel switchable and tunable multi-wavelength ring fiber laser that uses a structure comprising SMF/TCEDF/SMF as a WSF. The proposed ring cavity design features a filter, enabling tunable and switchable laser line emissions that go from single to quadruple wavelengths, achieved by adjusting the curvature on the MZI. Our findings indicate a tuning range of 7.62 nm with an SMSR of 49.80 dB for a single lasing line emission, complemented by a peak power and wavelength fluctuation of less than 0.87 dB and 0.26 nm, respectively. Furthermore, we successfully obtained quadruple lasing lines at specific wavelengths, 1557.99 nm, 1558.89 nm, 1564.02 nm, and 1564.78 nm, with SMSR values of 29.07 dB, 31.98 dB, 39.96 dB, and 34.11 dB, respectively. Additionally, for triple-emission lines at 1558.27 nm, 1558.85 nm, and 1564.81 nm, the SMSR values were 29.15 dB, 31.16 dB, and 26.83 dB, respectively. We also observed a dual-narrow wavelength lasing line at λ_1B = 1564.56 nm and λ_1A = 1563.71 nm, with a tuning range of 3.22 nm and SMSR values of 36.59 dB and 39.12 dB, respectively. Lastly, a dual wavelength with λ_1A = 1572.53 nm and λ_41B = 1585.69 nm and an SMSR of 36.59 dB and 39.12 dB was achieved. The intrinsic flexibility, versatility, and adaptability of these multi-wavelength lasers make them essential tools across multiple sectors, including telecommunications, sensors, materials science, medicine, and research. In particular, they are highly invaluable in studies due to their ability to provide a stable range of emissions across several tunable wavelengths. Our study demonstrates the consistent repeatability in tuning and switching among the multiple lines of the proposed laser using the WSF.