1. Introduction

Multiwavelength-operating devices for wavelength-routed optical network have attracted considerable interest in recent years as the capacity of optical communication systems like wavelength-division multiplexing (WDM) systems extremely increases. To reduce the complexity of optical cross-connects (OXCs), multiwavelength fiber lasers have been significantly advanced because they have a variety of advantages such as multiwavelength operation, simple structure, low cost and insertion loss, and resulted in their versatile applications to dense wavelength division multiplexed (DWDM) systems, optical fiber sensors, optical instrument testing, spectroscopy and so on [1]–[10]. There are a range of methods to generate the multiwavelength output based on various gain media like erbium-doped fiber amplifiers (EDFAs) [1]–[6], semiconductor optical amplifiers (SOAs) [7], and Raman amplifiers [8]–[10]. The EDFA has been frequently utilized to fabricate the multiwavelength fiber laser. However, since the EDFA is a homogeneous gain medium, it is necessary to suppress the homogeneous line broadening and the unstable mode competition of the EDFA for stable multiwavelength operation at room temperature. The SOA-based multiwavelength laser can operate stably at room temperature without supplementary treatments to suppress the homogeneous line broadening. However, it is not easy to achieve the high extinction ratio and the low noise figure [7]. The Raman amplifier-based multiwavelength fiber laser is also stable at room temperature and it has a lot of advantages such as a high extinction ratio and lasing wavelength flexibility corresponding to a pump wavelength [8]–[10]. However, the high pump power and quite long length of fibers are required to obtain the sufficient Raman gain to induce the multiwavelength output.

Several methods to mitigate the mode competition of the EDFA were proposed [1]–[6]. A conventional method is to use the cooling EDF down to cryogenic temperature with liquid nitrogen [1] or the frequency-shifted feedback technique within a laser cavity [2]. Another method is to utilize the hybrid gain medium which consists of both the EDFA and the SOA [3]–[5]. Recently, the wavelength switchable EDF laser based on both fiber Bragg gratings and photonic crystal fibers (PCFs), was reported [6]. Since the homogeneous line broadening of the EDFA can be effectively suppressed by four wave mixing of PCFs, the stable operation of the multiwavelength EDF laser could be realized [6]. However, it is necessary to cascade a lot of fiber gratings to increase the number of lasing wavelengths, which can induce an additional insertion loss. The functionalities of multiwavelength EDF laser such as the tunability of the lasing wavelength, the wavelength spacing, and the number of channels, were not sufficiently proposed and demonstrated.

In this paper, we propose and experimentally demonstrate a flexibly tunable multiwavelength EDF laser based on degenerate four-wave mixing in a dispersion shifted fiber (DSF). The stable operation of the multiwavelength EDF laser at room temperature can be achieved by alleviating the mode competition of the EDF with the DSF. The number of channels and wavelength spacing of the proposed multiwavelength laser can be controlled flexibly by the combination of two polarization maintaining fiber (PMF) segments in the intracavity Lyot-Sagnac filter. We successfully achieve 1.0 nm-spacing eleven channel and 0.8 nm-spacing seventeen channel lasing wavelengths at room temperature. The extinction ratio of the multiwavelength output is as high as ~ 50 dB. The output power of multiwavelength EDF laser is stable so that the peak fluctuation is less than ~0.6 dB. The lasing wavelength of the proposed multiwavelength fiber laser can be also controlled by the polarization controller, which has the great potential for application to the interleaved optical switching device. Since the proposed scheme uses the long length of DSF, it is also very useful for application to long-distance remote sensors.

2. Tunable multiwavelength erbium-doped fiber laser based on four-wave mixing effect in dispersion shifted fibers

Figure 1 shows the experimental schematic for the proposed multiwavelength EDF laser that operates stably at room temperature. The multiwavelength EDF laser is based on the ring cavity which is composed of a 980 nm pump source with the output power of 500 mW, a 980/1550nm WDM coupler, the 20 m-EDF, an isolator, the 1km-DSF, the Lyot-Sagnac filter, a polarization controller, and a 10/90 coupler. The absorption coefficient of the EDF is 8.27 dB/m. The principle of the stably operating the multiwavelength EDF laser at room temperature is based on the effect of four-wave mixing on the EDFA. Since the energy transfer from the higher power waves to the lower power waves is induced by the several degenerate four-wave mixing processes, the mode competition of the EDF can be degraded. The difference of power variation (ΔP_i – ΔP _i+1) between two photons (P _i+1 and P_i at the photon frequency of ω _i+1 and ω_i, respectively) by the degenerated four-wave mixing processes, can be written as [6]

where δ is the efficiency of the four-wave mixing process. For P _i+1 > P_i, the energy of the wave at ω_i+1 is moved to that at ω_i since the power variation at ω_i (ΔP_i,) is higher than that at ω_i+i (ΔP _i+1). On the contrary, for P_i > P _i+1, the energy of the wave at ω_i is transferred to that of ω _i+1 since the power variation at ω _i+1 (ΔP _i+1) is higher than that at ω_i(ΔP_i). It seems to be a self-stability function. Therefore, the four-wave mixing process can effectively suppress the homogeneous line broadening of EDF and the stable multiwavelength output at room temperature can be achieved.

The multiwavelength operation of the proposed scheme is based on the multiple PMF-based Lyot-Sagnac filter. In general, it is difficult to control the wavelength spacing of multichannel filter based on an optical loop mirror. However, once multiple PMF segments are cascaded within the loop mirror, the wavelength spacing and the number of channels can be flexibly controlled by the combination of multiple PMF segments [5]. In Fig.1, the dashed square shows the tunable multichannel filter based on the Lyot-Sagnac filter. The tunable Lyot-Sagnac filter is composed of a 50/50 coupler, multiple PMF segments, and an all-fiber polarization controller (PC) with the half-wave and quarter-wave plates. Since the half-wave plates control the relative phase difference between two orthogonal polarization modes within the loop, the combination of multiple PMF segments can be changed. In our experiments, two combinations of two PMF segments like L ₁+L ₂ and L ₁-L ₂ could be obtained since two pieces of PMF segments were located in the loop. The wavelength spacing (Δλ) of the Lyot-Sagnac filter is inversely proportional to Δn∙L products of PMF segments. The wavelength spacing in the Lyot-Sagnac filter with two PMF segments can be written as [5]

where Δn is the effective birefringence between two orthogonal polarization modes and L is the length of the PMF. Therefore, two distinct transmission filters with two different wavelength spacing can be created by two different values of Δn∙L product depending on the relative angle of input polarization. In the experiments, we used two PMF segments with the length of 6 m (L1) and 1 m (L2), respectively. Their birefringence (Δn) is about 0.00045. According to Eq. (1), the wavelength spacing can be changed by two combinations of multiple PMF segments like 0.8 nm and 1.0 nm. If multiple PMF segments with N are serially connected within the Lyot-Sagnac filter, the number of combinations of PMF segments can be determined by 2^N-1 corresponding to the relative phase difference between two orthogonal polarization modes [5]. The number of channels can be also controlled depending on the wavelength spacing. Therefore, the multiwavelength fiber laser with the multiple functionalities like the tunability of the wavelength spacing, and the number of channels, can be achieved.

Figure 2 shows output spectra of the tunable multiwavelength EDF laser with different wavelength spacing and the number of channels corresponding to the combination of two PMF segments in the PMF Lyot-Sagnac filter. Based on degenerate four-wave mixing of the DSF, the stable multiwavelength EDF laser at room temperature could be obtained. Its extinction ratio was as high as ~ 50 dB. Based on the properties of Lyot-Sagnac filter, we successfully achieved 1.0 nm-spacing eleven channel and 0.8 nm-spacing seventeen channel lasing wavelengths as seen in Fig.2.

Figure 3(a) shows output spectra of the tunable multiwavelength fiber laser for two different rotation angle of the quarter-wave plate. The peak wavelength position could be effectively controlled by the rotation angle of the quarter-wave plate. All of channels were shifted into 0.5 nm when the rotation angle of the quarter-wave plate was 90°. It is also applicable to an interleaved optical switching device once the quarter-wave plate is replaced with a fast electro-optic PC. Figure 3(b) shows the continuous tuning of lasing wavelength shift with the rotation angle of the quarter-wave plate. The output of proposed multiwavelength EDF laser was so stable that the power fluctuation was less than ~ 0.6 dB. Figure 4 shows the variation of power variation with scanned time.

 

Fig.1. Schematic of the proposed multiwavelength EDF laser based on degenerate four-wave mixing in the DSF. The multiwavelength operation can be achieved by the tunable PMF Lyot-Sagnac filter (dashed square). PC: Polarization controller. LD: Laser diode. EDF: Erbium-doped fiber. DSF: Dispersion-shifted fiber. PMF: Polarization-maintaining fiber. OSA: Optical spectrum analyzer.

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Fig. 2. Output spectra of the tunable multiwavelength EDF laser depending on the combination of PMF segments in the Lyot-Sagnac filter. The extinction ratio was as high as ~ 50 dB. (a) Δλ = 1.0 nm, number of channels = 11. (b) Δλ = 0.8 nm, number of channels = 17.

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Fig.3. (a) Output spectra of the proposed tunable multiwavelength EDF laser when the polarization angle of the quarter-wave plate was 0° (solid line) and 90° (solid line), respectively, and (b) the continuous tuning of lasing wavelengths with the variation of polarization angle of the quarter-wave plate.

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Fig. 4. Power fluctuation of the multiwavelength EDF laser with scanned time. The power fluctuation was less than ~0.6 dB.

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Figure 4 shows the output spectra of the Raman fiber laser as the bending curvature along the few-mode FBG changes. For the positive bending of the few-mode FBG, the three lasing wavelengths shifted into the longer wavelength because of the tension strain. Contrarily, the compression strain, which was caused by the negative bending, induced the shift of three lasing wavelengths into the shorter wavelength. Figure 5 shows the lasing wavelength shift as a function of the bending curvature change. The similar tunability of 2.45 nm/m^-1 was observed in both tension and compression strain due to the symmetric mechanical characteristics of silica under stress. The dynamic range was more than 15 nm.

3. Discussion and conclusion

In summery, we proposed and experimentally demonstrated a flexibly tunable multiwavelength EDF laser based on degenerate four-wave mixing in the DSF. As the homogeneous line broadening phenomenon of the EDF was readily suppressed by various four-wave mixing processes of the DSF with the length of 1 km, the stable operation of the multiwavelength EDF laser at room temperature could be achieved successfully. We achieved 1.0 nm-spacing eleven channel and 0.8 nm-spacing seventeen channel lasing wavelengths corresponding to the combination of two PMF segments in the intracavity Lyot-Sagnac filter. The extinction ratio of the multiwavelength output was as high as ~ 50 dB. The multiwavelength EDF laser output was very stable and the peak fluctuation was less than ~0.6 dB. We also achieved the interleaved optical switching performance based on the proposed tunable multiwavelength EDF laser corresponding to the rotation angle of the quarter-wave plate. The proposed multiwavelength EDF laser has the great potential for applications to WDM systems, optical interleavers, long-distance remote sensors, and so on.