1. Introduction

Multiwavelength fiber lasers are of great interest for their potential applications in optical communications, fiber-optic sensors, optical instrumentation, and microwave photonic systems. Both semiconductor optical amplifiers (SOA) [1] and erbium-doped fiber amplifiers (EDFA) [2–6] have been used for generation of multiwavelength fiber laser. Compared to SOA-based multiwavelength fiber lasers, multiwavelength lasers with EDFA have advantages in their higher saturated power, lower polarization-dependent gain (PDG) and flatter gain spectrum. The main challenges for erbium-doped fiber (EDF) ring lasers to achieve stable multiwavelength lasing at room temperature are the strong homogeneous line broadening and the cross-gain saturation. Previously, several approaches have been proposed. Cooling the EDF to 77K by liquid nitrogen can suppress the homogenous line broadening and the cross-gain saturation [2], but this technique is impractical in many applications. To obtain room temperature multiwavelength lasing, a specially designed erbium-doped two core fiber has been used to provide inhomogeneous gain through macroscopic spatial hole-burning [3]. Recently, multiwavelength lasing has been demonstrated by use of a frequency shifter or a phase modulator [4–6]. Other methods, such as optical feedback and nonlinear gain in optical fiber have also been exploited [7–8]. However, some of these designs require many optical components and some are unstable. More recently, multiwavelength lasing was reported by incorporating a highly nonlinear fiber (HNLF) in the ring cavity [9]. Based on the self-phase modulation and the four-wave mixing (FWM) in the HNLF, 488 channels with a wavelength spacing of 10GHz were obtained. However, the signal-to-spontaneous-noise ratio of the laser was below 20dB, which was not suitable for some applications. Meanwhile, X. Liu et al. [10] and A. Zhang et al. [11] proposed multiwavelength fiber ring lasers by adding a length of highly nonlinear photonic crystal fiber (HN-PCF) into the ring cavity. The HN-PCF has a flat and low dispersion profile over a wide bandwidth, and therefore generates an FWM-induced dynamic gain flattening mechanism in the ring cavity, which further enables the multiwavelength operation.

In this letter, we investigated the stable room temperature multiwavelength lasing in an erbium-doped fiber laser by incorporating a section of commercial highly nonlinear fiber and a narrowband Fabry-Perot filter (FPF). Different from the previous works, we found our operation was mainly based on an inhomogeneous loss mechanism. With the co-operation of the HNLF and the FPF, the cavity loss is not only wavelength dependent, but also intensity dependent. Then, the mechanism automatically balances the power at different wavelengths and enables the multiwavelength operation. A similar effect has been successfully applied in a multiwavelength Raman fiber laser by Q. Wang et al. [12] Based on the mechanism, we observed more than 70 wavelengths simultaneous lasing with the same frequency space of 25GHz. The laser had a total output power of ∼3.2dBm, a bandwidth of 0.012nm (∼1.5GHz) and a signal-to-spontaneous-noise ratio of ∼44dB.

2. Experiment setup

A schematic of the experimental setup is shown in Fig. 1(a). The gain of the fiber laser was provided by an erbium-doped fiber amplifier (EDFA), whose maximum output power was ∼33dBm. A polarization independent isolator was used to ensure the unidirectional cavity. A FPF with a wavelength space of 0.2nm (25GHz) and a linewidth of 0.02nm served as a wavelength selector. Fig. 1(b) depicts the transmission spectrum of the FPF, which was obtained by an ASE source and an optical spectrum analyzer (ANDO AQ6317, resolution 0.01nm). A section of 1km commercial HNLF was inserted in the laser cavity. The zero-dispersion wavelength of the HNLF was ∼1555.5nm, and the dispersion slope, nonlinear coefficient were 0.018ps/(nm²∙km) and 10/(W∙km), respectively. The laser output was extracted from the cavity by a 10/90 fiber coupler, with which 90% power was fed back into the EDFA.

 

Fig. 1. (a) Setup for the multiwavelength fiber laser. C, 10/90 coupler; I, optical isolator; OSA, optical spectrum analyzer; BPF, optical bandpass filter. (b) Transmission spectrum of the FPF

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3. Results and discussion

At the first step, we inserted a tunable bandpass filter (3dB bandwidth: 0.6nm) in the cavity. When the pump current was low, only a single wavelength at 1561.850nm was lasing. Then, we increased the pump current and measured the linewidth (FWHM) before and after the FPF. The result is shown in Fig. 2. The linewidth before the FPF (blue triangle) increases with the optical power that injected into the HNLF while that after FPF (orange square) almost keep constant. As we all know, a fiber laser readily oscillates on multiple longitudinal modes as a result of its long cavity [13]. Although the FPF in the laser cavity can limit the number of modes over which the laser may oscillate, it can not prevent laser oscillation on more than one longitudinal mode in a single wavelength channel [14]. Therefore, with a section of HNLF in the cavity, the nonlinear wave mixing between the closely-spaced longitudinal modes can cause broadening of the emission spectrum [15]. Moreover, a channel with higher power will experience more serious spectrum broadening in HNLF, corresponding to an increased linewidth before the FPF. Then, the broadened channels are filtered by the FPF, where the broadened parts are discarded as a loss. For a clear show of the above process, we replaced the 25GHz FPF with a 40GHz one, whose linewidth is 0.03nm. The result is also depicted in Fig. 3 (before FPF, red triangle; after FPF, green square). Although the accuracy of the measured data is limited by the resolution of our OSA, the trend agrees well with that of previous work [15]. From the result, we can conclude that if multiple wavelengths are lasing, channels with higher power will suffer higher losses after the FPF, and therefore obtain relative lower gain after traveling a cycle in the loop. This process automatically balances the power between the lasing wavelengths and in some ways reduces the cross-gain saturation in EDF. Based on the above mechanism, multiwavelength operation of the erbium-doped fiber laser is possible. In our experiment, we also observed a sudden power fall of the firstly lasing wavelength when a second wavelength began lasing. As expected, the linewidth was decreased with the power fall as well.

 

Fig. 2. Linewidth (FWHM) as a function of the optical power that injected into the HNLF.

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Fig. 3. Three-dimensional plot of measured spectral evolution of the optical output versus the power that injected into the HNLF. OSA Resolution: 0.2nm.

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Next, we moved the tunable bandpass filter from the cavity. The output spectra measured as a function of the power that injected into the HNLF are depicted in the three-dimensional plot in Fig. 3. Here, we set the resolution of the OSA at 0.2nm to neglect the fine structure of the spectra. At a low injection power of 0.22W, only a few wavelengths lase around 1562 nm. The spectrum was observed to be unstable due to the insufficiently reduction of the cross-gain saturation in erbium-doped fiber. However, as the injection power increases, the spectrum extends and flats. The lasing state of wavelengths around 1562nm kept even when shaking the fiber, which denoted the successful cross-gain saturation reduction. Besides, the spectra were extended to the longer wavelength as the pump power increased. As known, parametric interchannel FWM would cause the spectra shifted to the shorter wavelength (normal dispersion area) for better phase matching. Therefore, we can surely conclude that the spectra broaden here is not dominated by the interchannel FWM as depicted in some previous works^9-11. This phenomenon can be easily understood in the following way: firstly, the dispersion of the HNLF at the lasing wavelengths is not equal to, although close to zero, therefore, the nonlinear wave mixing between the closely-spaced longitudinal modes occurs more easily than the interchannel FWM; secondly, the passband of the filter is very narrow, slight spectrum broaden of a channel would lead to a significant loop loss. As a result, the inhomogeneous loss mechanism dominates our operation.

Setting the pump power of the EDFA at its maximum value, the optical power that injected into the HNLF was estimated to be 1.9 W. The flat area of the spectrum is over 14nm, as shown in Fig. 4. Different from Fig. 3, Fig. 4 is captured by setting the resolution of the OSA at 0.01nm. From Fig. 4(a), over 70 wavelengths simultaneous lasing was observed. They had the same frequency separation space as that of the FPF (25GHz). The laser had a linewidth of ∼0.012nm (1.5GHz) and a signal-to-spontaneous-noise ratio of ∼44dB. The total output power was 3.2dBm. It should be emphasized that the output power can be increased to more than 190mW by moving the 90/10 coupler right after the EDFA. For a clear understanding of the fine structure of the laser output spectrum, Fig. 4(b) gives the expanded laser spectrum of Fig. 4(a). As shown, almost no power variation appears between these lasing wavelengths.

To figure out the role of the HNLF and FPF, we performed the same measurement for the laser configuration without incorporating the HNLF or FPF. For the configuration without the HNLF, multiple wavelengths (less than 10) lasing could also be obtained by careful gain equalization. However, when we slightly dither the fiber, some lasing wavelengths would fade due to their failure in the gain competition. For the configuration without the FPF, the result was the same with that in a recent work by J. H. Lee et al. [16]. We confirmed the continuous-wave supercontinuum generation in an erbium-doped fiber laser that incorporating a HNLF in the ring cavity. The modulation instability and stimulated Raman scattering mechanism for the generation of the continuous-wave supercontinuum may also help to the generation of the multiwavelength laser in our experimental setup.

 

Fig. 4. (a) Laser spectrum with a span of 20nm. (b) Expanded laser spectrum. OSA Resolution: 0.01nm.

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Finally, we quantitatively investigated the long–term stability of the output spectrum. We observed the output spectrum every 10 min for 2 hour while the pump power was set to a maximum value of the EDFA. The repeated scans are shown in Fig. 5. No significant spectral fluctuations were observed. The wavelengths shifted within 0.003 nm and the relative change of amplitudes was smaller than 0.05dB, which were at the resolution limit of our OSA.

 

Fig. 5. Repeated scans of the output optical spectrum. The time interval of each scan was 10 min.

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4. Conclusion

In conclusion, we have experimentally demonstrated a multiwavelength erbium-doped fiber laser based on inhomogeneous loss mechanism by incorporating a section of commercial HNLF and a narrowband FPF in the ring cavity. The proposed scheme can successfully reduce the cross-gain saturation in erbium-doped fiber laser and enable the multiwavelength operation. Using the method, over 70 wavelengths simultaneous lasing with a frequency space of 25GHz was achieved. The laser had a total output power of ∼3.2dBm, a bandwidth of 0.012nm (∼1.5GHz) and a signal-to-spontaneous-noise ratio of ∼44dB. The lasing states were observed to be very stable. The total output power can be further increased to more than 190mW. We believe this simple multiwavelength fiber laser is practical for a lot of applications, such as the optical communications, optical testing and measurement and microwave photonic systems.