1. Introduction

Erbium-doped fiber active sources have attracted much research interests [1, 2] due to the emission at the low loss optical communication window and the so-called eye-safe wavelength range [3], despite its relatively lower power and efficiency as compared with those of the ytterbium-doped fiber sources [4, 5]. During the last approximate twenty years, much attention has been paid to the erbium-doped superfluorescent fiber source (ED-SFS) [1, see the review in sec. 3.3] with low temporal coherence and high spatial coherence, and until now it is still an active research topic. The output power and the spectral bandwidth are the two among the most important parameters of the ED-SFS. By inserting an inline cascaded long-period fiber grating filter [6], or by utilizing the inherently flat gain profile of the erbium-doped fiber (EDF) in the L-band [7], ED-SFS with ultra-flat broad bandwidth of tens of nanometers has been achieved, however, with a limited output power of less than 200 mW [8]. By utilizing a multi-stage configuration to release the danger of instantaneous resonant lasing [9], ytterbium sensitized ED-SFS with 1 W output power has been constructed [10]. The ytterbium sensitized erbium-doped gain fiber (also called as the erbium-ytterbium co-doped fiber) in these high power ED-SFSs has many advantages over conventional EDF such as the partially released erbium-doping concentration limitation and the high pump absorption coefficient. However, due to the un-flattened gain profile of the ytterbium-sensitized EDF, these sources exhibit either a sharp structure [10] or a large hump [11, 12] in the spectrum. To date, no report demonstrates ED-SFS that simultaneously exhibits watt-level high output power and broadband flat spectrum.

In this paper, we demonstrate for the first time an ED-SFS that simultaneously exhibits watt-level high output power and flat broad bandwidth. To obtain a broadband flat spectrum, we chose the conventional single-mode EDF as the gain medium and utilize its inherently flat gain profile in the L-band. However, a single-mode EDF should be pumped by a single-mode fiber pump source, while commercial single-mode fiber pigtailed laser diodes (LD) are limited to be several hundreds of milliwatt, which is not competent for a watt-level fiber source. This is the primary reason why conventional single-mode EDF has not been utilized in watt-level SFSs [10, 11, 12]. In order to resolve this problem, we introduce a resonant pumping technique (refer to direct excitation of the upper laser manifold [3]) into the ED-SFS. We firstly construct a high power C-band ytterbium-sensitized ED-SFS with a relatively narrow bandwidth and up to 1.8 W output power from a standard single-mode fiber (the highest power of ED-SFS reported up to now). Then, by utilizing this C-band SFS as the pump and by utilizing a dual-stage configuration to suppress lasing behavior, an L-band SFS with 1 W output power and 34.3 nm bandwidth (FWHM) is demonstrated. As compared with laser pump sources, the C-band SFS pump source itself is intrinsically free of self-pulsing and spiking behavior, and hence benefits the stability of the L-band output.

2. Experimental setup

The experimental setup of the proposed L-band SFS is schematically shown in Fig. 1. Left of the broken line is an L-band ASE seed source which is actually an ED-SFS constructed in double-pass forward pumping configuration, yet with an extra section of un-pumped EDF between the WDM and the reflector to improve its performance. Right of the broken line is a double-pass backward pumped EDFA that acts as an L-band power amplifier, which is resonantly pumped by a C-band SFS as indicated in the dashed frame. The EDF used in the double-pass EDFA exhibits a peak absorption of about 390 dB at 1530 nm. An optical circulator is utilized to realize the double-pass amplification. The pump source itself is a tri-stage C-band SFS that consists of a low power ASE seed source, an EDF preamplifier, and an erbium-ytterbium co-doped fiber power amplifier, with an optical tunable filter (OTF) between the seed source and the preamplifier to control the wavelength and the spectrum shape. The fiber at the output port of the C-band SFS is standard single-mode fiber that makes it possible to pump conventional single-mode EDF.

 

Fig. 1. Schematic diagram of the high power flat L-band SFS. The configuration of the C-band pump source is illustrated in the dashed frame. ASE: amplified spontaneous emission; EDF: erbium-doped fiber; EDFA: erbium-doped fiber amplifier; EYDF: erbium-ytterbium co-doped fiber; LD: laser diode; LDA: laser diode array; WDM: wavelength division multiplexer; OTF: optical tunable filter; R: reflector; SFS: superfluorescent fiber source.

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3. Properties of the C-band pump source

The C-band SFS pump source is constructed in a tri-stage configuration as illustrated in the dashed frame in Fig. 1. The design intends to obtain a high power wavelength controllable C-band SFS with a relatively narrow bandwidth. The properties of the source are shown in Fig. 2 and Fig. 3.

 

Fig. 2. Spectra of the C-band SFS pump source. (a) Spectra at different points as indicated in Fig. 1; (b) Spectra at various emission wavelengths tuned by the OTF.

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The spectra at different points indicated in Fig. 1 are presented in Fig. 2(a). The first stage is actually a double-pass backward ED-SFS that exhibits a typical spectrum as shown by line A in Fig. 2(a). The output power from the fist stage is about 30 mW, of which only 0.14 mW left when filtered by the OTF with a spectrum shown as line B in Fig. 2(a). Then, the power is amplified by the preamplifier stage to about 38 mWwith spectrum shown as line C in Fig. 2(a). This seed source is then injected into the power amplifier and scaled to be larger than 1.8 W with a typical spectrum shown as line D in Fig. 2(a).

The peak wavelength of the SFS can be tuned continuously by adjusting the OTF from 1540 to 1555 nm free of lasing behavior till the maximum 980 nm LDA pump power (5.9 W) with spectra shown in Fig. 2(b), while when the wavelength approaches 1562 nm, resonant lasing occurs at high power levels. This phenomenon is caused by the fact that the gain of the power amplifier reaches itsmaximum and the seed source power begin to descend at these wavelengths which results in an increased net gain in the power amplifier. The lasing behavior can probably be eliminated by further increasing the output power of the preamplifier so as to further decrease the effective average gain of the power amplifier. However this effort has not been carried out because a source near 1560 nm is not desirable to pump EDF due to the decreased absorption coefficient of EDF at this wavelength.

As can be seen from Fig. 2(b), there is a large hump near 1560 nm in the spectrum of the C-band SFS. This is caused by the maximum gain of the power amplifier at this wavelength range. However, the main peak still occupies most of the energy. The integrated power ±2 nm around the main peak occupies 83%, 96%, 98% and 99% of the total power respectively for the four spectra peaked at 1540, 1545, 1550 and 1555 nm in Fig. 2(b). It should be noted that, the spectra in Fig. 2(b) were measured under ~1.4 W output power. At larger power levels, the above values should be slightly larger.

Figure 3 illustrates the power property of this C-band SFS when peaked at 1545 nm. As can be seen from the figure that, the output power increases straightly with the 980 nm diode pump power, with a slight saturation at high power levels (>5 W pump power). This saturation phenomenon indicates that further power scaling should not only increase the 980 nm diode pump power but also increase the seed power of the power amplifier. A maximum output power of 1.86Wwas obtained under 5.9W980 nm LDA pump power.We note that this is the highest reported superfluorescent power in the 1.55 µm band up to now. Taking into account the total ~0.2W980 nmdiode pump power of the seed and the preamplifier stage, the overall conversion efficiency of this C-band SFS is about 31%.

 

Fig. 3. Output power versus 980 nm LDA pump power of the C-band SFS pump source.

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4. Properties of the L-band SFS

As illustrated in Fig. 1, the L-band SFS consists of a low power ASE seed source and a high power EDF amplifier. The seed source exhibits a maximum output power of 16 mW in the L-band with spectrum shown in the inset of Fig. 4(a). The amplifier is in double-pass configuration backward pumped by the C-band SFS peaked at 1545 nm. The original intent of using the seed source includes shaping the spectrum and saturating the amplifier, so as to extract as much power as possible from the amplifier and to suppress the possible instantaneous resonant lasing.

The output spectrum of the L-band SFS is shown in Fig. 4. The spectrum shape changes a lot with seed power. When no seed is injected, the source should produce the backward ASE of the pumped EDF, however two sharp humps appear near 1560 nm as indicated by the dotted line in Fig. 4(a). These humps adjacent to 1560 nm are caused by the sub-peak of the C-band SFS pump source in combination with the filtering effect of the WDM. From Fig. 2 we can see that the C-band SFS exhibits a sub-peak near 1560 nm, which acts as a signal source when the seed power is very low. This signal is firstly amplified in the pumped EDF and then filtered by the WDM before being guided out together with the ASE component of the EDF, resulting in an output spectrum shown as the dotted line in Fig. 4(a). However, as the seed power increases, the sub-peak of the C-band SFS pump source begin to act as a pump source because the inverted population is greatly consumed by the seed source amplification process. So, the two humps are suppressed with a relatively higher seed power. Thus, a broadband flat spectrum is finally obtained as shown by the real line in Fig. 4(a). The two humps can probably be thoroughly eliminated by using a substituted pump source without sub-peak. Figure 4(b) shows the output spectrum variation with 1545 nm pump power. As compared with the seed power, the 1545 nm pump power has less influence on the output spectrum shape.

 

Fig. 4. Spectrum of the high power flat L-band SFS variation with (a) seed source power and (b) 1545 nm pump power. Inset in (a) is the spectrum of the seed source.

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Fig. 5. Output power versus 1545 nm pump power under various seed source powers of the high power flat L-band SFS. Inset shows the output power variation with seed power.

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Unlike the previously reported SFS [12], no resonant lasing occurs during the whole process of changing the seed power and the 1545 nm pump power from zero to the highest values, due to its relatively flat spectrum which results in a lower net gain over a broader wavelength range. Although the spectrum of the seed source is a bit higher at shorter wavelength range (inset in Fig. 4(a)), flattened spectrum is still obtained from the L-band SFS by properly lengthening the EDF length of the amplifier. As demonstrated in the experiment, an ultimate flat spectrum needs a proper EDF length pair (the EDF length of the seed source and that of the amplifier). If we increase the EDF length of the seed source to obtain a much flattened seed spectrum, the EDF length of the corresponding amplifier should be properly reduced in order to obtain an ultimate flat spectrum, and vice versa.

Figure 5 illustrates the power properties of the L-band SFS. The output power increases rapidly with an increasing seed power and saturated soon as indicated by the inset. The output power increases monotonically with the 1545 nm pump power and becomes saturated gradually with a low seed source power (1.5 mW), while no obvious premonition of saturation is observed with a high seed source power (16 mW) (Fig. 5). The latter indicates the possibility of further scaling the L-band SFS to even higher power levels. A maximum output power of 1.01 W has been achieved under 16 mW seed power and 1.84 W 1545 nm pump power, giving an optical conversion efficiency of about 54%. The -3 dB and -20 dB bandwidth of the L-band SFS at the maximum power is 34.3 nm (1568.0 – 1602.3 nm) and 51.9 nm (1561.8 – 1613.7 nm), respectively.

5. Conclusion

In conclusion, an L-band SFS that simultaneously exhibits watt-level high output power and broadband flat spectrum is demonstrated in a dual-stage configuration with a resonant pumping scheme. The source is free of lasing behavior due to its relatively flat spectrum which results in a lower net gain over a broader wavelength range. The optical conversion efficiency from the 1545 nm pump source to the L-band output is about 54% and that from the original 980 nm pump to the L-band output is about 17% (31%×54%). The results provide guidelines to a more promising scheme of L-band ED-SFS with 1.55 µm LD pump, which should probably be preferred in future accompanying the continuous progress in LD manufacture. The high conversion efficiency of this resonant pumping scheme should partially relieve the source from thermal effects for high power operating through reduction of the non-radiation relax.