1. Introduction

Multi-wavelength fiber lasers have attracted considerable interest in recent years because of their potential applications in optical wavelength division multiplexed (WDM) systems, fiber sensors and fiber optic instrumentation [1–5]. Since the large homogeneous line broadening of erbium-doped fiber (EDF) at room temperature leads to strong mode competition and loss of stability in the output power level, it is difficult to achieve simultaneous multi-wavelength operation in erbium-doped fiber lasers (EDFLs). Though cooling of EDF at cryogenic temperature is often used to reduce the homogeneous broadening of EDF, such a technique is not well suited to practical applications. In order to achieve stable multi-wavelength lasing at room temperature, numerous techniques have been proposed [5–9]. Most of them try to enhance inhomogeneous broadening in EDFs by using the techniques like multiple quantum-well waveguide [5], erbium-doped twin-core fiber [6], multimode fiber [7], acousto-optic frequency shifter [8], and elliptical core EDF [9]. Fiber Bragg gratings (FBGs) are ideal wavelength-selective components for fiber lasers. Various methods have been demonstrated to realize switchable multi-wavelength fiber lasers by using cascaded FBG cavities [10], polarization-dependent loss element [11], a tree, inline topology FBG [12], an FBG written in Hi-Bi fiber [13–14] and a sampled FBG [15]. Most of these methods have disadvantages such as complex configuration, high cost and low flexibility. In general, FBGs written in multimode or few-mode optical fibers have multiple resonance peaks in the transmission or reflection spectra. The characteristics and applications of few-mode fiber gratings (FMFGs) and multimode fiber gratings (MMFGs) have been discussed in Refs. 16 and 17.

In this work, we demonstrate a simple erbium-doped fiber (EDF) laser configuration using an FBG written in a few-mode fiber to obtain stable multi-wavelength oscillation at room temperature. This method has the advantage of stable operation and small variation in the output power and a rather simple configuration compared with techniques mentioned above. Moreover, the laser can be switched between the dual- and triple-wavelength lasing oscillations by adjusting the polarization controller inside the cavity.

2. System configuration and operation principle

The Ge-B co-doping method in the core region using modified chemical vapor deposition (MCVD) was used to fabricate few-mode fiber [18–20]. 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 1.3% and the core and cladding diameters were 9.64 µm and 125 µm, respectively. Since the V-number of the fabricated fiber is about 4.6, the fiber can guide four modes.

 

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

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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, and 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. The reflection spectrum of FMFG was obtained using an optical spectral analyzer (OSA) with 0.1 nm resolution. Figure 2 shows the measured reflection spectrum of the fabricated FMFG. The resonance wavelengths are 1547.14 nm (7 dB), 1550.21 nm (10dB), and 1553.09 nm (15 dB), respectively. The 3 dB bandwidth is approximately 0.25 nm.

 

Fig.2. The reflection spectrum of fabricated FMFG.

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The polarization dependence of the FMFG was measured by varying the state of the PC. The output power variation for the three resonance wavelengths was 0.5~1.5 dB and the shift of the resonance wavelength was negligible small below the measurement resolution. The polarization-dependent loss (PDL) was 2.54 dB, which was larger than that of SMF (0.95 dB). In comparison to the fiber Bragg grating written in single-mode fiber, the FMFG has multiple reflection wavelengths corresponding to the polarization modes that satisfy the phase matching condition independently. 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, EDF, a polarization controller (PC) and an FMFG. The length of the active medium EDF was 15 m. The Sagnac fiber loop acts as a broad-band reflector. The EDF was pumped by 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. The fibers in the cavity were all single-mode fibers (SMF) except the section of the FMFG. We used a mechanical splicer instead of a fusion splicer for better control of the splice between the fibers. Two fibers in the mechanical splicer were carefully connected while monitoring the output spectrum to minimize the effect of the mode field mismatch on the laser operation. The measured overall splice loss when the few-mode fiber and the single-mode fiber were fusion-spliced was less than 0.1 dB and virtually no increase in the splice loss was noticed in comparison to the case of splicing two identical single-mode fibers.

 

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

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3. Experimental results

With the output power of the pump laser diode set at 55 mW and with the PC properly adjusted, simultaneous three-wavelength lasing oscillations could be obtained as shown in Fig. 4. The resolution bandwidth of the measured spectra was 0.08 nm. The lasing wavelengths of the channels are 1547.16 nm, 1550.10 nm, and 1553.07 nm, respectively. Therefore, the wavelength separations among the adjacent lasing wavelengths are 2.94 nm and 2.97 nm, respectively. The 3-dB bandwidth is 0.1 nm, and the sidemode suppressive ratio (SMSR) is over 28 dB. The overall output power from the multiwavelength laser was approximately 1 µW, which amounts to about 0.002% efficiency.

 

Fig. 4. Simultaneous three-wavelength oscillations of the laser using a few-mode fiber grating (FMFG).

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Figure 5 shows the results of repeated output power measurements of the three-wavelength fiber laser. A total of ten measurements were made at two minutes’ interval. This result indicates that the peak power variation in each resonance wavelength was 1.6 dB. Although it was larger than the case of two-wavelength oscillations (<0.4 dB) [21] because of the homogeneous gain broadening in EDF, we believe that they are sufficiently stable at room temperature for practical applications.

 

Fig. 5. The stability of simultaneous three-wavelength laser (10 times repeated scans).

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Fig. 6. Output spectra of dual-wavelength laser operation with different states of PC.

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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. The lasing modes were switched by varying the state of the PC and the change was reversible. Figure 6(a) shows dual-wavelength operation at 1547.16 nm and 1550.11 nm with 3-dB bandwidths of 0.1 nm. Figure 6(b) shows dual-wavelength laser operation at 1550.11 nm and 1553.07 nm with 3-dB bandwidths of 0.1 nm, and the sidemode suppressive ratio (SMSR) is about 30 dB. Even though change of the output amplitude is quite sensitive to the state of the PC, the shift of the resonance wavelength was estimated to be less than the measurement resolution of 0.08 nm.

4. Summary

In this work, simple simultaneous dual- and triple-wavelength erbium-doped fiber lasers based on a few-mode fiber grating (FMFG), which has multiple resonance peaks in the transmission or reflection spectra, have been proposed and experimentally demonstrated. The lasing wavelengths for the triple-wavelength operation were 1547.16 nm, 1550.10 nm, and 1553.07 nm, respectively. The 3-dB bandwidths were 0.1 nm. The wavelength separations among the adjacent lasing wavelengths were 2.94 nm and 2.97 nm, and the sidemode suppressive ratio (SMSR) is about 28 dB. Furthermore, the laser has fairly stable room-temperature operation. The output power variation for each wavelength was 1.6 dB over 120 second period at room temperature. By properly adjusting the polarization controller, the laser operation could be switched between the dual- and triple-wavelength lasing operations. Even though change of the output amplitude is quite sensitive to the state of the PC, the shift of the resonance wavelength was estimated to be less than the measurement resolution of 0.08 nm. This method has the advantages of stable operation and simple configuration compared with other previously reported techniques.