1. Introduction

Multi-wavelength fiber lasers are cost-effective sources in optical wavelength division multiplexed (WDM) systems, fiber sensors and optical instrument testing [1–3]. Several methods of multi-wavelength fiber lasers have been reported in previous works [4–6]. Erbium-doped fiber (EDF) is a gain medium with primarily homogeneous broadening at room temperature, which leads to strong mode competition and unstable lasing. Therefore, it is difficult to obtain simultaneous multi-wavelength lasing in erbium-doped fiber lasers (EDFLs). Even though this problem can be remedied by cooling EDF at cryogenic temperature, e.g., in liquid nitrogen (77K), to reduce the homogeneous broadening of EDF, such a technique is not well suited to practical applications. Therefore, the stable multi-wavelength oscillations of EDFLs at room temperature are important. Many approaches have been investigated to overcome this problem [7–11].

The switching and tuning of resonance wavelength in multi-wavelength fiber lasers is very important in many practical applications. Fiber Bragg gratings (FBGs) are excellent all-fiber wavelength-selective devices and have been used in multi-wavelength fiber lasers [12–15]. In general, FBGs written in multimode or few-mode optical fibers have multiple resonance peaks in the transmission and reflection spectra. The characteristics and applications of few-mode fiber gratings (FMFGs) and multimode fiber gratings (MMFGs) have been described in Refs. 16 and 17.

Recently, we investigated the characteristics of multi-wavelength oscillation in erbium-doped fiber using a few-mode fiber grating (FMFG) [15]. In this work, besides multi-wavelength oscillation reported previously, we demonstrate a novel and simple multi-wavelength erbium-doped fiber (EDF) laser configuration using an FBG written in few-mode side-hole fiber with an elliptical core and enhanced birefringence. It will be shown that the number of lasing wavelengths can be also controlled by adjusting the state of the polarization controller (PC).

2. System configuration and operation principle

The modified chemical vapor deposition (MCVD) method was used to fabricate the photosensitivity few-mode fiber with Ge-B co-doped in the core region [18–19]. The side-hole fiber with an elliptical core was fabricated by over-jacketing (19 X 25 tube) and collapsing after cutting both sides of the few-mode fiber preform. Figure 1 shows the refractive index profile of the fabricated few-mode fiber. The amount of germanium and boron was 300 SCCM and 20 SCCM, respectively. The heating temperature of boron was about 40 °C. The fiber was drawn at the temperature of 1930 °C and the capstan speed was 30 m/min. The relative index difference Δ of the few-mode fiber was 1.4 %.

 

Fig. 1. The refractive index profile of the fabricated few-mode fiber

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In general, a side-hole fiber has two air channels running along the fiber, which can be either cylindrical or elliptical [20]. Figure 2 shows the cross-section of the fabricated side-hole fiber with an elliptical core. The major/minor axes of the core and the side-hole diameter is 12 µm/6.6 µm and 20~25 µm, respectively.

 

Fig. 2. The cross-section of few-mode side-hole fiber with an elliptical core.

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Prior to fabrication of the FMFG, the few-mode fiber was loaded with hydrogen under 100 bars and 100 °C for 5 days. The FMFG was fabricated by the interferometer method with frequency-doubled argon-ion laser. The period of the phase mask was 1060 nm, the length of FMFG was 1.5 cm, the output power of the laser was 80 mW, and the UV exposure time was about 2 minutes. In contrast to FBG with one resonance wavelength written in single-mode fiber, FBG written in FMFG can have several resonance wavelengths due to the presence of multiple core modes that satisfy the phase matching condition.

Figure 3 shows the configuration of the proposed multi-wavelength erbium-doped fiber laser. The cavity of laser consists of a Sagnac fiber loop mirror, a WDM coupler, 10 m-long EDF, a polarization controller (PC) and an FMFG written in side-hole fiber with an elliptical core. The Sagnac fiber loop acts as a broad-band reflector. The EDF was pumped with a 980 nm laser diode with the output power of 110 mW through a WDM coupler. The threshold power for multi-wavelength oscillation was 33 mW.

 

Fig. 3. Configuration of the multi-wavelength erbium-doped fiber laser.

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The fibers in the cavity were all single-mode fibers (SMF) except the section of the FMFG. We used a mechanical splicer instead of the fusion splicer to facilitate adjustment of the transverse offset between the SMF and the few-mode fiber for better control of the mode excitation condition [21].

3. Experimental results

In general, FMFG has strong polarization dependence due to the damage cracks formed on one side of the core during the process of grating fabrication, and the multiple resonance wavelengths have different polarization states from one another [16]. Also, in case of FBGs written in the side-hole fiber with an elliptical core, the two resonance wavelengths are separated due to the polarization dependence [22]. It has been reported that the high birefringence of EDF with elliptical core can lead to simultaneous oscillation of multiple wavelengths if the polarization direction of the pump light is properly adjusted [11]. In contrast, the EDF is not birefringent in our case, and the birefringence of the FMFG in combination with the state of the PC determines the state of polarization inside the cavity and the number of the lasing lines. Therefore, by adjusting the state of the PC, we could change the state of polarization inside the cavity and the number of the lasing lines.

Figure 4 shows the output spectra of the proposed multi-wavelength laser with different settings of the PC. The resolution of the optical spectrum analyzer (OSA) was 0.08 nm. Figure 4(a) shows dual-wavelength laser operation at 1533.12 nm and 1535.19 nm with 3-dB bandwidths of 0.1 nm, and the side-mode suppression ratio (SMSR) is over 30 dB. Figure 4(b) shows triple-wavelength laser operation at 1532.47 nm, 1533.83 nm, and 1534.80 nm, respectively. Therefore, the wavelength separations among the adjacent lasing wavelengths are 1.36 nm and 0.97 nm, respectively. The 3-dB bandwidth is 0.1 nm, and SMSR is over 25 dB. Figure 4(c) shows quadruple-wavelength laser operation at 1532.47 nm, 1533.80 nm, 1534.80 nm, and 1535.98 nm, respectively. The wavelength separations are 1.33 nm, 1.0 nm, and 1.18 nm. The SMSR is over 20 dB. Moreover, by changing the state of PC, a bifurcation structure due to the simultaneous oscillation of the split resonance peaks was observed as shown in Figs. 4(d)–(f), which is due to the birefringence of the elliptical core. For example, Fig. 4(d) shows the simultaneous oscillation of the split resonance peaks near 1533.6 nm, while Fig. 4(b) shows oscillation of only one of them.

The stability test was performed by repeated 10-times scan at intervals of 2 minutes. In case of dual-wavelength oscillation, the peak power variations for each resonance wavelength were measured to be less than 0.4 dB [23]. In case of triple-wavelength oscillation, the peak power variations in each resonance wavelength were measured to be less than 1.5 dB [15]. Moreover, in case of more than triple-wavelength oscillation, as the number of lasing wavelengths was increased, this multi-wavelength laser was not stable because of the competition among different wavelengths in the gain medium. Of course, if the EDF is cooled in liquid nitrogen, stable multi-wavelength oscillations can be expected.

 

Fig. 4. Output spectra of the multi-wavelength fiber laser using FMFG written in side-hole fiber with an elliptical core at different settings of the PC. (a)–(c) wavelength switching, (d)–(f) simultaneous oscillation of the split resonance peaks.

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4. Summary

In this work, a simple and novel erbium-doped fiber laser based on a few-mode fiber grating (FMFG) written in a side-hole fiber with an elliptical core has been proposed and experimentally demonstrated. The lasing wavelengths for the dual-wavelength operation were 1533.12 nm and 1535.19 nm, respectively. The 3-dB bandwidths were 0.1 nm and the side-mode suppression ratio (SMSR) was over 30 dB. Furthermore, the laser had fairly stable room-temperature operation. The output power variation for each wavelength was less than 0.4 dB over 120-second period at room temperature.

The lasing wavelengths for the triple-wavelength operation were 1532.47 nm, 1533.83 nm, and 1534.80 nm, respectively. The wavelength separations among the adjacent lasing wavelengths were 1.36 nm and 0.97 nm, and the SMSR was over 25 dB. The output power variation for each wavelength was less than 1.5 dB over 120-second period at room temperature. The lasing wavelengths for the quadruple-wavelength laser operation were 1532.47 nm, 1533.80 nm, 1534.80 nm, and 1535.98 nm, respectively. The wavelength separations were 1.33 nm, 1.0 nm, and 1.18 nm. The SMSR was over 20 dB. In this case, multi-wavelength laser was not stable because of the competition among different wavelengths in the EDF.

By properly adjusting the polarization controller, the laser operation could be switched among the dual-, triple- and quadruple-wavelength lasing operations. Using the high birefringence due to the elliptical core, the number of lasing wavelengths also could be controlled by adjusting the state of the polarization controller. This method has the advantages of novel and simple configuration compared with other previously reported techniques.