Multiwavelength fiber lasers have attracted a lot of interest recently due to their outstanding advantages such as multiwavelength operation, compact structure, portable design, and their versatile applications to dense wavelength division multiplexed systems, optical component testing, and spectroscopy^[1,2]. There are many different methods for generating multiwavelength output such as a Fabry–Perot cavity filter^[3], multiple fiber Bragg grating (FBG)^[4], tunable comb filter^[5], and several nonlinear effect methods (nonlinear optical loop mirror^[6], four wave mixing^[7], and nonlinear polarization rotation (NPR)^[8]). In the last decades, most of the previous work in multiwavelength fiber lasers was focused on 1 and 1.5 μm^[9] wavelengths. Since 2 μm lasers were promising sources for many applications, including LIDAR, spectroscopy, remote sensing, and medicine^[10–12], they have also attracted broad attention recently^[13–16]. Up to now, there have been only a few reports on multiwavelength fiber lasers at 2 μm. In 2013, Wang et al. reported a stable six wavelength Tm-doped fiber laser (TDFL) around 1.9 μm based on four wave mixing^[7]. In the same year, Peng et al. reported a 1.94 μm switchable dual-wavelength TDFL employing two high-birefringence FBGs^[17]. In 2014, Xue et al. demonstrated an all-fiber tunable Tm/Ho-codoped laser operating in the 2 μm wavelength region^[18]. At the same time, Hu et al. reported a multiwavelength laser at 2 μm based on a hybrid gain scheme consisting of a Brillouin gain medium and a Tm-doped fiber^[15]. In the previous reports, several specific elements were inserted into the laser cavity to generate multiwavelength operation^[19]. The structure of the cavity would be complicated in this case.

NPR is an efficient way to achieve passive mode-locking. Due to the transmission curve in Ref. [3], when the intensity surpasses the specific threshold, the positive feedback used to achieve mode-locking would become negative. As a matter of fact, in the negative region, the transmission at the isolator would decrease while the intensity in the cavity increased. In this region, the polarization rotation effect would efficiently suppress the mode competition between the different wavelengths, and then stable multiwavelength operation could be observed^[20,21].

In this Letter, we demonstrated a multiwavelength fiber laser employing polarization-maintaining undoped fiber (PMF) based on NPR. PMF was used to enhance the relative linear phase difference, and then the space between the adjacent wavelengths would decrease. At the same time, there would be more wavelengths that were supported in the region of emission cross section of Tm3+. A polarization-sensitive isolator (PSI) and two polarization controllers (PCs) were used to induce the intensity-dependent loss (IDL) and frequency-dependent loss (FDL) to alleviate mode competition caused by homogeneous gain broadening in an active fiber. An up to four wavelengths laser operating in a cavity was observed without the PMF. After the PMF was inserted into the cavity, stable dual wavelength operation was achieved. The output had an average power distribution. Spectral fluctuation at each wavelength was smaller than 0.9 dB. Dual wavelengths could be switched by rotating the PCs.

Figure 1 shows the experimental schematic of the proposed TDFL based on NPR. The ring cavity of the fiber laser consists of 1.5 m Tm-doped silica fiber (TDF), a 1550 nm homemade fiber laser, a wavelength division multiplexer (WDM), two PCs, a PSI, and an 80:20 coupler. The TDF has a 9 μm core covered by a 125 μm clad. Its absorption coefficient at 1550 nm is 7 dB/m. The 1550 nm fiber laser has a maximum output power of 900 mW. The WDM could transfer the 1550 nm pump laser into the TDF to provide the gain to maintain the balance in the cavity. The 80:20 coupler was used as an output coupler while 80% energy remained in the cavity. The PMF was inserted between the WDM and the coupler. It has a 15 μm core and a 130 μm clad. Its birefringence coefficient is 1.5×10−4 with a panda type. The PSI performed as the polarization-selected device to block the specific wavelength and ensure unidirectional operation. The two PCs were inserted into the fiber loop to adjust the polarization state of the light inside the ring cavity. An optical spectrum analyzer (OSA) with a wavelength resolution of 0.05 nm and an optical power meter (OPM) with the power resolution of 0.1 mW were, respectively, used to measure the spectrum and power of the output laser.

First of all, the PMF was removed from the cavity. The laser could operate in a stable regime when the pump power was about 115 mW. The relatively high threshold of this TDFL was mainly attributed to the polarization-dependent loss at the PSI. When the pump power was just above the threshold, the laser operated in a single-wavelength state, as shown in Fig. 2(a). At this time, the output power was about 1.8 mW. We continually increased the pump power and adjusted the polarization state in the cavity, until four wavelengths were observed at the output. As we see in Fig. 2(b), the space between adjacent wavelengths was identical, which was 2.2 nm. But the peak power at each wavelength had 2.56 dB jitter. The output power of the four wavelengths state was 56 mW with about 500 mW pump power. The four wavelengths operation was unstable because of the mode-competition and was easily transferred into the mode-locking state.

After splicing the PMF into the cavity, the threshold of the cavity was 150 mW, which was a little higher than before. This was mainly attributed to the mode mismatch at the splicing point between the PMF and the single-mode fiber (SMF)-28. Dual-wavelength operation was observed by carefully increasing the pump power and finely adjusting the orientation of the PCs. Once the dual wavelength appeared, nearly uniform power distribution could be obtained by slowly rotating the PCs and adjusting the pressure on the fiber. Switchable single wavelength at each wavelength could be achieved by slightly changing the phase difference induced by the PCs, as shown in Fig. 3.

According to the theory of the NPR mechanism^[20], the model of the cavity is illustrated in Fig. 4(a), in which θ is the angle between the polarization direction of the input light and the fast axis of the SMF, and α is the angle between the polarization direction of the polarizer and the fast axis of the SMF. The transmittivity of the model can be expressed as $$\begin{gathered} T=sin2(θ)sin2(α)+cos2(θ)cos2(α) \\ +0.5sin(2θ)sin(2α)cos(Δφ1+Δφ2), \end{gathered}$$(1)where Δφ1 and Δφ2 are the linear and nonlinear phase differences between the two orthogonal polarization components, respectively. Δφ1 includes the phase difference induced by PCs and instinct birefringence of the fiber. The phase difference induced by instinct birefringence of the fiber could be expressed as Δφb=2π(nx−ny)L/λ, and the phase difference induced by PCs was constant when the PCs were fixed.

By adjusting the phase difference bias induced by the PCs, the cavity could operate either in regime I or II. When the PMF was not in the cavity, the phase difference induced by birefringence of the fiber was too faint, so the nonlinear phase difference could affect the transmission efficiency. The four wavelengths operation shown in Fig. 2 was working in regime II, while regime I was the mode-locking region, which we were not concerned with in this paper. After inserting the PMF into the cavity, Δφb was much larger than the nonlinear phase difference, which had a slight influence to the transmission curve. Rotating PCs could only change the phase bias of the cavity. The wavelength at which the transmittivity was higher was much more easily emitted than others. The space between the dual wavelengths shown in Fig. 3 was 9.8 nm, which was determined by the PMF length and its birefringence. According to this theory, we could obtain dual-wavelength operation by changing the PMF length in the cavity.

In conclusion, a multiwavelength fiber laser employing PMF based on NPR is presented. PMF and PSI between two PCs can act as a “saturable absorber”, which will induce the IDL and FDL to alleviate mode competition caused by homogeneous gain broadening in an active fiber. Without the PMF, an up to four wavelengths laser operating with space of 2.2 nm in a cavity can be obtained by finely adjusting the PCs. Stable dual-wavelength operation is achieved after the PMF is inserted into the cavity. The peak difference at each wavelength is smaller than 0.9 dB. A dual or more wavelengths laser with varied spaces can be obtained by changing the PMF length in the cavity. The multiwavelength fiber laser can be employed in more practical applications.