1 Introduction

Multiwavelength erbium-doped fiber (EDF) laser sources have attracted great interest in recent decades due to their potential applications in dense wavelength-division-multiplexed (DWDM) fiber communication systems, optical sensing and microwave photonics generation. But, the homogeneously broadened nature of EDF gain media normally leads to strong mode competition and unstable lasing. To achieve stable multiwavelength operation at room temperature, different techniques have been proposed to suppress the homogeneous broadening of EDF, for instance, utilizing polarization hole burning [1, 2], providing frequency shift feedback [3, 4], introducing intensity-dependent loss [5, 6], combining four-wave-mixing effect [7] and incorporating the cascaded stimulated Brillouin scattering (SBS). Comparing with other reported techniques, multiwavelength Brillouin-erbium fiber laser (MW-BEFL) has the advantages of great tunability, narrow linewidth and stable dense spacing around 10 GHz [8]. Typically, to achieve great tunability in the MW-BEFL, a tunable filter is required to suppress the self-lasing cavity modes that are caused by high erbium gain [9–11].

In recent years, twin-core fiber (TCF) has received much attention to fiber lasers and fiber sensing due to the compact configuration that two cores are integrated in one single fiber. Generally, the TCF-based comb filter can be divided into two types, TCF-based in-fiber Mach–Zehnder interferometer (MZI) and TCF coupler. As for the TCF-based in-fiber MZI, there is no mode coupling occurring between the cores, and each core is regarded as a separate waveguide. This TCF-based in-fiber MZI has been applied in the multiwavelength laser [12–14] and fiber sensor [15]. As for the TCF coupler, optical power flows periodically between two cores along the fiber due to evanescent field coupling. Rare earth-ion-doped TCF coupler shows inhomogeneous broadening property for multiwavelength lasing [16, 17]. In addition, this kind of TCF coupler be also employed for constructing narrow-band filters [18, 19]. But for the previously reported applications, several meters of TCF are required to achieve dense wavelength spacing, which induce high insertion loss. Moreover, the wavelength tuning range is quite short by applying tension along the TCF.

In this paper, we propose and demonstrate a tunable MW-BEFL using a twin-core fiber (TCF) coupler. The broadly tunable TCF coupler is all-fiber-based component and simply tuned by mechanical bending. A stable multiwavelength laser comb with up to 40 nm wavelength tuning range is achieved by continuously tuning the TCF coupler within the EDF gain bandwidth in conjunction with modifying BP. The wavelength spacing between two adjacent lasers is 0.0876 nm. Besides, the impact of the free spectral range (FSR) of the TCF on the output characteristics of the MW-BEFL is also investigated.

2 Principle of TCF coupler

As shown in Fig. 1a, TCF coupler consists of a section of TCF and two pieces of standard SMF spliced at two ends. The homemade TCF is fabricated by means of groove--stack--draw method. The cross section of TCF is given in Fig. 1b. The diameters of the two cores are 6.3 μm. Two cores share the same cladding and distributed symmetrically around the center of the cross section. Light wave propagates in each core has only single transverse mode. The two cores have the same effective refractive index, n = 1.44902. The separation distance d between two cores is about 14 μm; thus, the mode field associates with each core stretch-out, and the strong coupling effect can be achieved [20]. Compared with TCF-based in-fiber MZI filter, the TCF coupler has higher sensitivity of bending, which provides wider tuning range [21]. Two pieces of standard single-mode fiber (SMF) are aligned with one of the two cores and spliced by using a commercial splicer (Ericssion FSU-952). Broadband light source connects to one end. The aligning process is monitored by a power meter placed at another end. Due to the high refractive index coating, only the light coupled into the cores of TCF can propagate with low loss; otherwise, the light in the cladding modes that are excited at the splicing point between SMF and TCF will be attenuated. Thus, maximum coupling output from another end indicates that two pieces of SMF are properly aligned with the arbitrary core of TCF. As for this case that two SMFs are aligned with the same core, the transmission of the TCF coupler can be expressed as:

$$T=1-\frac{K_{12}K_{21}}{\delta^{2}+K_{12}K_{21}}\sin^{2} \sqrt{\delta^{2}+K_{12}K_{21}}L$$
(1)

Here, 2δ is the differential propagating constant between two cores; K ij (i, j = 1 or 2) is coupling coefficient from core i to core j. L is the length of TCF. Therefore, the maximum transmission must satisfy the condition:

$$\sqrt{\delta^{2}+K_{12}K_{21}}L=N\pi$$
(2)

Here, N is integer. Both δ and K ij are the differential core index \(\Updelta n\) and wavelength λ-dependent parameters. Thus, the transmission spectrum of TCF coupler is comblike. And the FSR around λ would be:

$$\hbox{FSR}\approx\frac{\pi}{L}\left\{\frac{\partial\varphi(\Updelta n,\lambda)}{\partial\lambda}\right\}^{-1}$$
(3)

Here,

$$\varphi(\Updelta n, \lambda)=\sqrt{\delta(\Updelta n, \lambda)^{2}+K_{12}(\Updelta n, \lambda)\cdot K_{21}(\Updelta n, \lambda)}$$
(4)
Fig. 1
figure 1

The schematics of TCF coupler

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Equation (3) shows that a longer length of TCF results in a smaller FSR. Changing the differential core index also changes the wavelength of transmission peaks. According to this mechanics, the tuning setup was depicted in Fig. 1c. The TCF coupler was fixed on two one-dimensional stages with the TCF hanging in the middle. The curvature of the TCF, defined as C = 1/R, was adjusted by moving one stage along the fiber. When the bending is applied on the TCF, the two cores suffered different strains. Due to the elasto-optical effect, the refractive index of the upper-patch core in Fig. 1c becomes lower. In contrast, the refractive index of the lower-patch core becomes higher. The tuning process can be described as follows [22]:

$$\frac{\Updelta n}{n}\approx \frac{c_{2}n^{2}d\cdot C}{2}$$
(5)

Here, the parameter c 2 = 0.204 is a constant value. This refractive index profile change induces an increase in δ. In this TCF, K ij increases with the operating wavelength λ. To satisfy the condition as shown in Eq. (2), the resonance dips should shift toward short wavelength to give a smaller coupling coefficient. Figure 2 shows the tuning spectra of the TCF coupler with L = 15 cm. The SuperK TM white light source was used in the measurement. The insertion loss of TCF coupler was 1.8 dB, which was induced by the mode field mismatch loss. The FSR of the TCF coupler around 1,550 nm was 27 nm. When the curvature increased, the blueshift in the spectra was observed. The tuning process was continuous but nonlinear.

Fig. 2
figure 2

The bending characteristics of TCF coupler

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3 Experimental setup and principle

The configuration of the MW-BEFL using a TCF coupler is illustrated in Fig. 3. The MW-BEFL cavity adopted linear structure, where two optical circulators were placed at two ends of cavity to provide two loop mirrors separately. A 7-m EDF was used to provide linear gain; a 980/1,550 wavelength division multiplexer (WDM) was used to couple the 980-nm pump laser into the EDF; a 3.5-km SMF-28 fiber was served as Brillouin gain medium, and a polarization controller (PC) was used to adjust the polarization state to obtain the optimized output. For high-efficiency SBS generation, the BP with linewidth narrower than Brillouin gain bandwidth was chosen. The BP was provided by a tunable laser source (TLS, Agilent TM  81600B) with a maximum power of 10 mW and a bandwidth of 100 kHz and launched into laser cavity via 3-dB coupler in one loop mirror. The laser output coupled from one port of 3-dB coupler was monitored by an optical spectrum analyzer (OSA,  YOCOGAWA,  AQ6375) with a resolution of 0.05 nm.

Fig. 3
figure 3

The proposed setup of MW-BEFL

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When BP was launched into the cavity, it was preamplified in 7-m EDF to suppress the ASE noise floor, which could improve the OSNR of multiwavelength lasers [23]. The TCF coupler was adjusted to reshape the net gain profile and move the peak gain wavelength close to the preamplified BP. Due to the injected locking effect, self-lasing cavity mode was suppressed [24]. Due to bidirectional propagation characteristics of Brillouin Stokes in 3.5-km SMF, lower-order Brillouin Stokes would generate higher-order Brillouin Stokes when Brillouin threshold was satisfied. By using this mechanics, stable multiwavelength lasing with a spacing of 0.08 nm was formed after many round-trips.

4 Results and discussion

In the experiment, the stability of output spectrum was degraded when the self-lasing cavity mode of EDFL was oscillated. Therefore, the tuning range of MW-BEFL was defined as the range in the absence of self-lasing cavity modes. To study the suppression of self-lasing cavity mode by using TCF coupler, the output spectra were measured when the 980-nm pump power was set at 150 mW, and the output power of TLS was 1 mW. Without TCF coupler, the self-lasing cavity mode was generated around 1,562 nm, and quite unstable. As shown in Fig. 4a, the tuning range of MW-BEFL was only around 6 nm. The wavelength of self-lasing cavity mode was depended on the length of EDF in use, and the tuning range was quite rigorous with the flatness of EDF gain. Under the same pumping condition, Fig. 4b shows the output spectrum of MW-BEFL with 15-cm TCF coupler in use. The use of TCF coupler in the design can move self-lasing cavity mode close to the BP. The self-lasing cavity mode was suppressed by BP injection. In Fig. 4b, the pass band of TCF coupler was much broader than the bandwidth of multiwavelength laser comb; thus, the wavelength and the number of the Stokes were low sensitive to the vibration of TCF coupler.

Fig. 4
figure 4

The output laser spectra a without TCF coupler, b with TCF coupler

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In Fig. 4, the Brillouin Stokes could not separate with each other clearly due to the low OSA resolution of 0.05 nm. The spectrum of the laser output was remeasured by Apex TM OSA with a resolution of 0.16 pm. The input signals were attenuated before launched into the high-resolution OSA, whose maximum input power was 0 dBm. Figure 5 presents the enlarged output spectrum at 1,560 nm. Ten wavelengths were oscillating within 10 dB bandwidth. The spectrum Brillouin shift is 0.0876 nm, corresponding to a frequency spacing of 10.8 GHz. The OSNR of the 10 wavelengths was higher than 40 dB. Besides, a few anti-Stokes lines were also generated by four-wave mixing [25]. But due to weak four-wave mixing effect in SMF, the anti-Stokes lines were quite weak and treated as noise floor in low-resolution OSA. To study the stability of the MW-BEFL, we monitored the first-order Stokes in Fig. 5 in 30 min. The maximum wavelength drift was 14 pm, which was related to the stability of the TLS. The power fluctuation was about 0.56 dB. The results show good stability of the MW-BEFL.

Fig. 5
figure 5

The enlarged spectrum measured by high-resolution OSA

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In the experiment, the tuning process of TCF coupler in this MW-BEFL was also studied. The 15-cm TCF was used in the TCF coupler. The laser spectrum in the tuning process is illustrated in Fig. 6a. The tuning process was achieved by continuously adjusting the transmission peak of TCF coupler in conjunction with modifying BP. In Fig. 6b, the transmission peak of TCF versus the BP wavelength was shown. The 980-nm pump power was set at 100 mW, and the output power of TLS was 3 mW. From 1,545 to 1,564 nm, the transmission peak was following the BP. However, beyond this range, the self-lasing cavity modes oscillated at the adjacent transmission peak were observed. That is, owing to the comblike transmission of TCF coupler, the self-lasing cavity modes can take place at any transmission peak with highest net gain. In this case, the adjacent transmission peak that is close to the peak gain of EDF around 1,562 nm has the highest net gain; thus, the injection-locking effect of the BP cannot completely avoid the emission of the self-lasing cavity modes at the adjacent transmission peak. To suppress the self-lasing cavity modes, the proper wavelength offset of TCF coupler was applied to reduce the net gain around 1,562 nm, and the injection-locking effect of the BP synchronized the wavelength of the lasing comb. Therefore, the tuning process of TCF coupler was no longer synchronized with the BP wavelength. In Fig. 6, it is also found that the tuning range of TCF coupler was close to one period of its transmission spectrum.

Fig. 6
figure 6

a Five typical output spectra of MW-BEFL in the tuning process when 15-cm TCF coupler was used; b the transmission peak of TCF versus the BP wavelength in the tuning process

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To further study the impact of TCF coupler on the output characteristics of MW-BEFL, we compared three lengths of TCF in the same laser cavity. The lengths of the TCF were 5, 15 and 25 cm, corresponding to the FSR of 74, 27 and 17 nm, respectively. The relationship between the FSR and TCF length was agreed well with Eq. (3). Figure 7 depicts the number of generated Stokes against the BP wavelength when the BP power and 980-nm pump power were set at 3 and 130 mW, respectively. The BP wavelength was tuned from 1,530 to 1,580 nm. The tuning ranges of these three TCF couplers were 42, 26 and 16 nm, respectively. The number reduction of the output lasers at 1,566 nm was due to the low gain of EDF. The tuning range of MW-BEFL increased with the reducing length of TCF. Due to the comblike transmission of TCF coupler, the mode competition between two adjacent transmission peaks was also observed. The smaller FSR means that two adjacent transmission peaks have stronger mode competition. The tuning range was limited due to the oscillation of self-lasing cavity modes at adjacent transmission band. By using 15- and 25-cm TCF coupler, the number of Stokes in MW-BEFL was almost the same. It indicates that the broad pass band of TCF coupler would not restrict the emission of high-order Stokes. The maximum output channels of MW-BEFL using 5-cm TCF coupler were less than using the former two TCF couplers. It is because of the higher insertion loss of this 5-cm TCF coupler which is induced by the interference between the core mode and the residual cladding modes. Besides, the flat transmission band of 5-cm TCF coupler was much broader than that of other two TCF couplers, which resulted in less suppression of self-lasing cavity modes. In order to suppress the self-lasing cavity modes at long wavelength, larger offset from BP wavelength was required to provide enough cavity loss. Consequently, the number of the output lasers was reduced when BP wavelength was shorter than 1,552 nm. The above results show that, as for this MW-BEFL, the 15-cm TCF coupler has better property of both tunability and the number of output lasers among these three TCF couplers.

Fig. 7
figure 7

The number of lasing wavelengths

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To further investigate the number of lasers and wavelength tuning range, the multiwavelength laser outputs with different BP power and 980-nm pump power were measured. Figure 8 illustrates the impact of BP power and 980-nm pump power on tuning range. The maximum tuning range about 40 nm was achieved when the BP launched was 5 mW and 980-nm pump power was 60 mW. In Fig. 8, the tuning range increases with the BP power and decreases with the 980-nm pump power. That is, due to the injection-locking effect that the injected BP consumed most of the excited Er3+, the self-lasing cavity modes are suppressed. It is indicated that the higher BP power is launched and the wider tuning range can exceed the FSR of the TCF coupler.

Fig. 8
figure 8

Tuning range of the MW-BEFL against 980-nm pump power at different BP powers

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In contrast, the tuning range decreased with the increasing of 980-nm pump power. The mode competition between self-lasing cavity modes and the Stokes become severe at high EDF gain. Therefore, the system could be tuned wider if the 980-nm pump power was maintained low enough to minimize this mode competition. However, both increasing BP power and decreasing 980-nm pump power would reduce the number of the Stokes. As shown in Fig. 9, the high BP power will cause gain suppression, which results in the low gain for high-order Stokes. The more EDF gain will be needed to generate the same number of Stokes. The trade-off between tuning range and the number of Stokes should be considered in practical use.

Fig. 9
figure 9

The number of lasing wavelengths against 980-nm pump power at different BP powers

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5 Conclusion

We have proposed and demonstrated the broadly tunable MW-BEFL using a TCF coupler. By tuning the TCF coupler, the peak net gain was drifted close to the BP, and up to 40 nm tuning range was achieved. Different from the commercial tunable band pass filter used in the previous works, the TCF coupler has much broader transmission bandwidth and will not restrain the number of Brillouin Stokes. In addition, it has the advantage of good compatibility in fiber laser, cost-effective structure. It is also found that tuning range of this MW-BEFL is related to the FSR of TCF coupler. The FSR chosen close to the bandwidth of EDF has benefit to achieve wide tuning range and more wavelengths stable oscillation. Besides, high BP power and low 980-nm pump power can further improve the tuning range. On the other hand, gain suppression induced by high-BP-power injection reduces the number of Stokes.