Highly Stable Switchable Emissions of an Erbium-Doped Fiber Ring Laser Using Cascaded MZIs Based on CHCF

: A stable, single, and dual-wavelength erbium-doped ﬁber laser (EDFL), based on two Mach– Zehnder interferometers (MZIs), arranged in a cascade conﬁguration, was proposed for experimental purposes. Both MZIs were assembled by splicing a capillary hollow-core ﬁber (CHCF) section between two multimode ﬁbers (MMFs) segments. The novelty of this single and dual-wavelength EDFL is that the switchable operation of the laser is achieved by thermally tuning the interference pattern of one MZI and not by adjusting the polarization state inside the ﬁber ring cavity. The maximum measured value of SNR was 58.9 dB for the single and dual-wavelength laser emissions. Moreover, the stable output power exhibited by this EDFL, in terms of minimal power and wavelength ﬂuctuations, at 0.05 dB and 10 pm, was detected during the single and dual-wavelength operation. It is worth noticing that switching is achieved at exact wavelength locations with a separation of 1.8 nm and not randomly, as reported by other works. These features make this switchable EDFL an appealing candidate for application in optical ﬁber communication systems and ﬁber sensing.


Introduction
In the last two decades, multiwavelength erbium-doped fiber lasers (EDFLs) have become reliable light sources for different applications such as wavelength division multiplexing [1,2], optical fiber sensing [3,4], and microwave photonics [5,6], to mention but a few. As is well known, homogeneous line broadening produces high mode competition, thus causing the generation of unstable emissions in multiwavelength EDFLs [7][8][9]. One possible solution to this problem is to use fiber wavelength filters; most of them work similarly to Sagnac [10], Fabry-Perot [11], and Mach-Zehnder [12] interferometers, though other filters based on different fiber structures can also be used [13][14][15][16][17][18][19][20][21][22]. Among these devices, the Mach-Zehnder interferometers (MZIs) have attracted significant attention due to their simple structure and the ease with which they can be manufactured; therefore, different optical fibers and fiber structures have been used to implement these interferometers. For instance, Y. Lv et al. [23] proposed a Mach-Zehnder interferometer (MZI) that was based on a triple-core photonic crystal fiber in order to implement a single, dual, and triple-wavelength EDFL. In this case, the total length of the fiber structure, including the length of the multimode fiber (MMF) that acts as a modal coupler, was 5.9 cm. They in these examples is the assembly of MZI using conventional couplers, as it makes the laser more complex and bulkier.
In this paper, a stable single and dual-wavelength EDFL, based on two cascaded MZIs, was proposed and experimentally investigated. The all-fiber MZIs were constructed by splicing a section of a capillary hollow core fiber (CHCF) between two segments of MMF. The maximum length of these MZIs was 5 mm, making these interferometers attractive due to their easy construction, small size, low cost, and sturdiness. Moreover, the MZI with the longer FSR was used as the wavelength selector, whereas the MZI with the smaller FSR exhibits narrower linewidth emissions and determines the separation of switchable steps. The novelty of this single and dual-wavelength EDFL is that the switchable operation of the laser is achieved by thermally tuning the interference pattern of one MZI and not by adjusting the polarization state inside the fiber ring cavity, as in the examples mentioned above. This scheme provides a simple way to switch wavelengths over well-known wavelength locations (peak emissions), thus eliminating the randomness obtained via polarization tuning. The maximum measured value of SNR was 58.9 dB for the single and dual-wavelength laser emissions, and the linewidth for all the peaks was 30 pm. Moreover, no power and wavelength fluctuations were detected during the single and dual-wavelength operation, thus making the laser emissions of this switchable EDFL highly stable.

Materials and Methods
A fiber ring laser cavity was assembled, as shown in Figure 1, using a pump laser diode (BL976-PAG700, Thorlabs, Newton, NJ, USA) that coupled light to a 980/1550 wavelength division multiplexer (WDM) fiber coupler. A 2 m long segment of EDF (Er80-8/125, Thorlabs, Newton, NJ, USA) was used as the gain medium, and it was pumped with the light of the laser diode at a wavelength of 976 nm. A PC and an isolator were added to the laser cavity to obtain a better SNR and to observe the unidirectional operation, respectively. To monitor the laser output, an optical fiber coupler (90/10) was set into the cavity; the 10% port was connected to the OSA, and the 90% port was used to close the cavity for continuous laser generation. Moreover, to develop a switchable EDFL, two MZIs that worked similarly to filters were placed into the laser cavity. Their fabrication process and working principle are described in the next section.
Machines 2022, 10, x FOR PEER REVIEW 3 of 15 noting that by adjusting the PC inside the cavity, multiwavelength emissions were obtained, as was also the case in the configuration where they used one interferometer. Another drawback shown in these examples is the assembly of MZI using conventional couplers, as it makes the laser more complex and bulkier. In this paper, a stable single and dual-wavelength EDFL, based on two cascaded MZIs, was proposed and experimentally investigated. The all-fiber MZIs were constructed by splicing a section of a capillary hollow core fiber (CHCF) between two segments of MMF. The maximum length of these MZIs was 5 mm, making these interferometers attractive due to their easy construction, small size, low cost, and sturdiness. Moreover, the MZI with the longer FSR was used as the wavelength selector, whereas the MZI with the smaller FSR exhibits narrower linewidth emissions and determines the separation of switchable steps. The novelty of this single and dual-wavelength EDFL is that the switchable operation of the laser is achieved by thermally tuning the interference pattern of one MZI and not by adjusting the polarization state inside the fiber ring cavity, as in the examples mentioned above. This scheme provides a simple way to switch wavelengths over well-known wavelength locations (peak emissions), thus eliminating the randomness obtained via polarization tuning. The maximum measured value of SNR was 58.9 dB for the single and dual-wavelength laser emissions, and the linewidth for all the peaks was 30 pm. Moreover, no power and wavelength fluctuations were detected during the single and dual-wavelength operation, thus making the laser emissions of this switchable EDFL highly stable.

Materials and Methods
A fiber ring laser cavity was assembled, as shown in Figure 1, using a pump laser diode (BL976-PAG700, Thorlabs, Newton, NJ, USA) that coupled light to a 980/1550 wavelength division multiplexer (WDM) fiber coupler. A 2 m long segment of EDF (Er80-8/125, Thorlabs, Newton, NJ, USA) was used as the gain medium, and it was pumped with the light of the laser diode at a wavelength of 976 nm. A PC and an isolator were added to the laser cavity to obtain a better SNR and to observe the unidirectional operation, respectively. To monitor the laser output, an optical fiber coupler (90/10) was set into the cavity; the 10% port was connected to the OSA, and the 90% port was used to close the cavity for continuous laser generation. Moreover, to develop a switchable EDFL, two MZIs that worked similarly to filters were placed into the laser cavity. Their fabrication process and working principle are described in the next section.

Fabrication Process and Working Principle
The structure of these filters consisted of a spliced segment of CHCF (whose inner and outer diameters are 62.5 and 125 µm, respectively) between two pieces of 1 mm of MMF (105/125 µm), and this fiber structure was sandwiched between two conventional SMF (see Figure 1). The CHCF was composed of SiO 2 , and its coating was composed of UV-cured acrylate (not commercial fiber). It should be noted that the coatings of the CHCF and MMF were removed before starting the construction process, and the SMF was handled as a standard splice. It is crucial to note that MMF segments work similarly to mode couplers, sending light toward the hollow core and the ring cladding of the CHCF. The equation that describes the output signal of this MZI is [31] where I 1 and I 2 are the intensities of the light in the hollow core and the ring cladding, respectively. L, λ, and ∆n eff are the length of the MZI, the wavelength of the light, and the effective refractive index difference between the modes that travel in the CHCF (a detailed explanation can be found in [31]), respectively. Moreover, the FSR of the output signal of this MZI can be calculated using FSR = λ 2 /∆n e f f L, and one may observe that it is feasible to change the FSR of the interference pattern by modifying the length of the CHCF. We constructed four MZIs with different FSR values by taking advantage of this property and our capability of cutting CHCF segments with high precision. Implementing a switchable laser requires two MZIs set into a fiber laser cavity in a cascade configuration. One of these devices (MZI 1 ) had an FSR that was higher than 10 nm, and the other (MZI 2 ) had an FSR that was lower than 2 nm. For the MZI 1 , three MZIs were constructed, and their FSRs are 17.82, 13.29, and 10.98 nm (see Figure 2d), which correspond to the estimated lengths of 0.3, 0.4, and 0.5 mm, respectively. The MZIs with lengths of 0.3, 0.4, and 0.5 will be named MZI 1a , MZI 1b , and MZI 1c , respectively. For the MZI 2 , one MZI was constructed, and its FSR was 1.8 nm (see Figure 2d), corresponding to an estimated length of 3 mm. A picture of the MZIs that were used in this experiment is shown in Figure 3. Table 1 shows the essential features of the MZIs. It should be noted that we previously reported [32] on a tunable erbium-doped fiber ring laser using an MZI based on CHCF. In that work, we demonstrate that the spectral response of the MZI undergoes a redshift as the temperature increases, with a slope of 33 pm/ • C. This thermal tunability allowed us to use two MZIs in a fiber ring laser cavity, using the MZI 1 as a wavelength selector (larger FSR); conversely, MZI 2 was used as a reference device in a steady state condition. In other words, the interference pattern of the MZI 1 tunes over the spectrum of the MZI 2 to choose a peak (at a specific wavelength), or peaks (multi-wavelengths), that will emit laser light. It should be noted that the MZI 2 determines the switchable step (FSR = 1.8 nm), as well as the narrower laser linewidth when the two MZIs are connected in a cascade configuration. Moreover, all the MZIs that were used in this experiment are polarization-independent elements; this means that the amplitude and phase of their output fringing patterns do not change with the input polarization state of light.       The spectral responses of the cascade combination of MZI 1a , MZI 1b , and MZI 1c , with the MZI 2 , are shown in Figure 4. These transmission spectra were taken using the amplified spontaneous emission (ASE) of the Erbium-doped fiber (in an open cavity) when the pump power was fixed at 250 mW. In these three cases, the interference pattern of the MZI 2 was modulated by the signal generated by MZI 1a , MZI 1c , and MZI 1c , respectively (see black curves in Figure 3). For a better understanding, the peaks that show the maximum transmission in the interference pattern were produced by two cascaded MZIs, which were marked to indicate that they were highly likely to produce a laser emission. The wavelengths of the peaks were measured, and the results are shown in Table 2. MZI2 was modulated by the signal generated by MZI1a, MZI1c, and MZI1c, respectiv black curves in Figure 3). For a better understanding, the peaks that show the ma transmission in the interference pattern were produced by two cascaded MZIs, were marked to indicate that they were highly likely to produce a laser emissio wavelengths of the peaks were measured, and the results are shown in Table 2.

Results
An experimental study was carried out to analyze the switchable operation of an EDFL using two MZIs connected in a series configuration, see Figure 1. Three cases were investigated; this includes the combination of the MZI 2 with the MZI 1a , MZI 1b , and MZI 1c , respectively. It should be mentioned that changing the order of the MZI 2 and MZI 1 (in the three cases) does not affect the performance of the laser, as we observed in the experiments. The MZI 2 was set on a hot plate (Echotherm, Model IC20) at a constant temperature, 20 • C, whereas the MZI 1 was on a hot plate (Thermo Scientific™, model HP88854100, Waltham, MA, USA) that underwent different temperatures; this produced the interference pattern of the MZI 1 which suffered a redshift. In the following sections, the results of the three cases are shown.

MZI 2 and MZI 1a in a Cascade Configuration
The transmission spectrum of the cascaded MZI 2 and MZI 1a is shown in Figure 4a. This indicated that the laser emission of this EDFL would probably occur at P 11 , since this peak shows the highest transmission. When these two cascade interferometers were set in the laser cavity (see Figure 1) both interferometers were at 20 • C, the polarization controller was properly adjusted, and the pump power was fixed at 250 mW to obtain the maximum SNR at room temperature. All measurements were acquired using this pump power. In this scenario, we observed a laser emission of 1563.7 nm (P 11 ), with an SNR of 58.9 dB, as shown in Figure 5a. The temperature of the MZI 1a was then increased in increments of 1 • C, from 20 • C to 29 • C, and it was observed that the single-wavelength laser emission remained fixed at 1563.7 nm (P 11 ) within this temperature range. Nevertheless, when the MZI 1a temperature reached 30 • C, another laser emission appeared at 1565.5 nm (P 12 ) with an SNR of 58.6 dB. This dual-wavelength laser emission was achieved because the peak of the MZI 1a , with a large FSR, was tuned to the point where peaks P 11 and P 12 , from the MZI 2 , which had small FSRs, exhibited the same intensity after a round trip. In such scenarios, those peaks are enhanced, and they eventually achieve laser emissions. Since this condition was achieved due to the shape of the tuning peak from MZI 1a , the dual-wavelength emission was observed within a narrower temperature range, as shown in Figure 5. This dual-wavelength laser emission continued until the MZI 1a reached a temperature of 40 • C, and at that point, at P 12 , the EDFL switched to single-wavelength laser emissions. The laser emission at P 12 remained fixed when the temperature of the MZI 1a varied between 40 to 160 • C, as can be seen in Figure 5a. The linewidth of the P 11 and P 12 was 0.03 nm. Furthermore, the wavelength stability of these laser emissions against temperature changes was measured and it is presented in Figure 5b. In the case of single-wavelength laser emissions, the peaks P 11 and P 12 did not vary their amplitudes and positions as the temperature changed from 20 to 29 • C and from 40 to 160 • C, respectively. In the case of dual-wavelength laser emissions, the peaks P 11 and P 12 did not modify their amplitudes and positions as the temperature changed from 30 to 39 • C. This means that the MZI 1a can undergo temperature fluctuations of up 9 • C, and the laser emission (single/dual) will not suffer changes in terms of their amplitudes or positions (wavelengths). This also indicates that for real applications, the temperature controller used to set the temperature on the MZI 1a does not require high accuracy. Similar behaviors are observed in the other two cases (MZI 1b and MZI 1c ) (see the following sections for more information).

MZI 2 and MZI 1b in a Cascade Configuration
Since the peak (P 21 ) shows the highest transmission, as is shown in Figure 4b, the laser emission is likely to occur at that peak. In the laser cavity, these two interferometers were 20 • C and connected in a series configuration (see Figure 1). Laser emission was detected at 1562.7 nm (P 21 ), with an SNR of 50.3 dB, as shown in Figure 6a, when the pump power was fixed at 250 mW. As with the previous case, a similar analysis was carried out by increasing the temperature of the MZI 1b in increments of 1 • C, from 20 to 160 • C (see Figure 6a,b). The linewidth of the P 21 , P 12 , and P 23 was 0.03 nm. The overall measurement results are summarized in Table 3.
positions as the temperature changed from 20 to 29 °C and from 40 to 160 °C, respectively. In the case of dual-wavelength laser emissions, the peaks P11 and P12 did not modify their amplitudes and positions as the temperature changed from 30 to 39 °C. This means that the MZI1a can undergo temperature fluctuations of up 9 °C, and the laser emission (single/dual) will not suffer changes in terms of their amplitudes or positions (wavelengths). This also indicates that for real applications, the temperature controller used to set the temperature on the MZI1a does not require high accuracy. Similar behaviors are observed in the other two cases (MZI1b and MZI1c) (see the following sections for more information).

MZI2 and MZI1b in a Cascade Configuration
Since the peak (P21) shows the highest transmission, as is shown in Figure 4b, the laser emission is likely to occur at that peak. In the laser cavity, these two interferometers were 20 °C and connected in a series configuration (see Figure 1). Laser emission was detected at 1562.7 nm (P21), with an SNR of 50.3 dB, as shown in Figure 6a, when the pump power was fixed at 250 mW. As with the previous case, a similar analysis was carried out by increasing the temperature of the MZI1b in increments of 1 °C, from 20 to 160 °C (see Figure  6a,b). The linewidth of the P21, P12, and P23 was 0.03 nm. The overall measurement results are summarized in Table 3.

MZI 2 and MZI 1c in a Cascade Configuration
The transmission spectrum of the cascaded MZI 2 and MZI 1c is shown in Figure 4c. The peak that shows the highest transmission is likely to produce the laser emission of the EDFL; in this case, P 31 . In the laser cavity, these two interferometers were connected in a series configuration (see Figure 1). The pump power was fixed at 250 mW and both interferometers were at 20 • C. Laser emission was observed at 1560.0 nm (P 31 ), with an SNR of 54.5 dB, as shown in Figure 7a. An analysis similar to that of the previous cases was performed by increasing the temperature of the MZI 1c in increments of 1 • C, from 20 to 160 • C (see Figure 7a,b). The linewidth of P 31 , P 32 , P 33 , and P 34 was 0.03 nm. The general results of the measurement are shown in Table 3. It is worth noting that as the FSR of MZI 1 was reduced, the wavelength switching range increased, as can be observed from Figures 5-7. First, it is necessary to say that the temperature sensitivity of the three MZI 1 is around 33 pm/ • C; therefore, the wavelength shift as a function of temperature for these filters is the same. From Figure 2a-c, one may observe that the MZI 1a , with the larger FSR, also exhibits a larger plateau at its peak, compared with the MZI 1b and MZI 1c . Since the plateau is larger, we need a more significant wavelength shift to observe laser switching, which translates to a small number of laser switching transitions for a given temperature range. On the other hand, in the case of MZI 1c , which has a smaller FSR and a sharper peak (smaller plateau), the number of laser switching transitions increases significantly for the same temperature range; therefore, a smaller FSR (i.e., sharper spectrum) provides more laser transitions.
we need a more significant wavelength shift to observe laser switching, which translates to a small number of laser switching transitions for a given temperature range. On the other hand, in the case of MZI1c, which has a smaller FSR and a sharper peak (smaller plateau), the number of laser switching transitions increases significantly for the same temperature range; therefore, a smaller FSR (i.e., sharper spectrum) provides more laser transitions.

Discussion
The experimental results regarding the switchable operation of the EDFL for the three possible combinations of the MZI 2 with the MZI 1a , MZI 1b , and MZI 1c , respectively, are summarized in Table 3. Regarding single-wavelength laser emissions, they were detected twice in the first combination of MZI 2 and MZI 1a , at peaks P 11 and P 12 ; in the second combination of MZI 2 and MZI 1b , they were identified three times, at peaks P 21 , P 22 , and P 23 , and in the third combination of MZI 2 and MZI 1c , it was detected four times, at peaks P 31 , P 32 , P 33 , and P 34 . For the case of dual-wavelength laser emissions, it was detected once in the first combination of MZI 2 and MZI 1a , at peaks P 11 and P 12 . In the second combination of MZI 2 and MZI 1b , it was observed twice, at P 21 and P 22 , and P 22 and P 23 . In the third combination of MZI 2 and MZI 1c , it was identified three times, at P 31 and P 32 , P 32 and P 33 , and P 33 and P 34 . Regarding dual-wavelength switchable operations, they were detected by decreasing the FSR of the MZI 1 and noting the number of times that the dual operation increased in the same temperature range. Similar behavior was observed in the single-wavelength switchable operation (see Table 3). The output power of the laser emissions is another interesting feature of this EDFL that needs to be analyzed. The output power of the laser emissions decreased as the laser switched from single to dual operation, and the power increased when the laser switched from dual to single operation. This is because the power is redistributed on the peaks. When MIZ 1a was used in the laser, a maximum power change of 2.2 dBm was measured when switching from single to dual laser emissions, and a maximum power change of 2.7 dBm was measured when switching from dual to single laser emissions. In the case of the MZI 1b , maximum power changes of 3.2 and 2.7 dBm were measured when switching from single to dual laser emissions, and from dual to single laser emissions, respectively. In the case of the MZI 1c , the maximum power changes when switching from single to dual laser emissions and from dual to single laser emissions were the same, at 3.7 dBm.
It should be noted that the most significant characteristic of the proposed EDFL is that it can be switched from lasing single to dual wavelengths by increasing the temperature of the MZI 1 . This performance offers an advantage over the switchable laser based on polarization [10][11][12][13]23], not only because of its simple straight-line operation but also because of its stability against temperature fluctuations. Switchable lasers based on polarization have a problem with temperature fluctuations, and these fluctuations produce changes in the polarization state; therefore, small adjustments need to be applied to the polarization controller to keep the wavelength and amplitude of the laser emission. However, the laser emissions of this switchable laser exhibited fluctuations in wavelength and power when the temperature of the MZI 1 was changed within a certain temperature range. These experimental results would appear to indicate that a highly accurate temperature controller is not required to regulate the temperature of the MZI 1 , since temperature oscillations of up to 9 • C do not change the wavelength or amplitude of the single or dual wavelengths, and in some cases, this temperature value can be even larger (see Table 3). This can be achieved due to the small size of both MZIs. It is worth noting that the level linearity and wavelength linearity of the OSA are 0.05 dB and ±10 pm, respectively; this means that our equipment is not able to detect power or wavelength fluctuations that are lower than the values mentioned above. It is important to highlight two vital features of this proposed switchable EDFL, which both come from the use of the MZI 2 . The first one is related to the linewidth of 0.03 nm for all the laser emissions, although the other interferometers (MZI 1a , MZI 1b , and MZI 1c ) have different FSR values. The second is the fact that the wavelength spacing between dual-wavelength laser emissions is always 1.8 nm, which is the FSR of the MZI 2 . Table 4 was expanded upon to compare the performance of our laser with that of similar switchable lasers, using recently published studies. It is worth noting that in most of these works (from 1 to 7), a fiber interferometer was constructed using conventional fiber couplers, thus causing the system to become more complex and bulkier. The laser linewidth of our switchable laser is narrower than 2, 6, and 7, but wider than 3, 4, and 8.
Regarding the SNR, its value is higher than all the lasers in Table 4. These striking results demonstrate the good performance of our switchable laser.

Conclusions
This paper has proposed and experimentally demonstrated the use of a single and dual-wavelength EDFL, incorporating two MZIs, in a series configuration. The all-fiber MZIs were constructed by splicing a section of a capillary hollow core fiber (CHCF) between two segments of MMF. These MZIs exhibit good characteristics, such as ease of construction, small size, low cost, and robustness when being handled. Moreover, their interference patterns have contrast well and do not change with the input polarization's state of light (polarization-independent element). Moreover, an MZI with a longer FSR, was used as a wavelength selector, whereas an MZI with a shorter FSR made the linewidths narrower and determined the size of the switchable step. The novelty of this single and dual-wavelength EDFL is that the switchable operation of the laser is achieved by thermally tuning the interference pattern of the MZI, and not by adjusting the polarization state inside the fiber ring cavity. The maximum measured value of SNR was 58.9 dB for the single and dualwavelength laser emissions, and the linewidth of all the peaks was 30 pm. Furthermore, no power or wavelength fluctuations were detected during the single and dual-wavelength operation, making the laser emissions of this switchable EDFL highly stable. The good performance of this switchable EDFL makes it a candidate for potential application in optical fiber communications systems and fiber sensing.