1. Introduction

Wide-band erbium amplified spontaneous emission (ASE) source has been not only intensively studied as an essential source for passive optical network and wavelength-division multiplexing (WDM) communication system [1, 2], but also used as a test instrument for optical components [3]. A wide range of wavelength, high power stability and moderate linewidth of effectiveness are essential performances of an erbium-doped fiber light source for fiber-optic gyroscopes and spectral slicing WDM system [4, 5]. Recently, most of the investigations have been carried out to study ASE light source with the fiber grating-based and fiber-mirror configurations under specific designs [6–10], which also take the advantage of erbium-doped fiber as the gain medium to amplify the feedback wavelength in both of the conventional wavelength band (C-band) and the long wavelength band (L-band).

In this paper, we proposed a high-power C+L-band ASE light source using an optical circulator with double-pass and bi-directional configuration, which was designed to provide a range covering both C-band and L-band with high rate output power. Without the needs of additional flattening filters, this configuration is expected to have good flattening spectra output by means of its own waveform reshaping abilities. The pumping efficiency and power stability for 8-hour interval measurements at the environmental temperature variations ranging from 20 °C to 30 °C also have experimental confirmations for the predictions of resonant lasing relaxing in the proposed configuration.

2. Experiments and discussions

Figure 1 shows the proposed double-pass and bi-directional pumping configuration for high-power C+L-band ASE source. Two pieces of erbium-doped fiber, EDF1 and EDF2 used as the gain medium, were inserted in both sides of an optical circulator (OC1). The EDF1 with 16 m was forward pumped through a fused wavelength division multiplexer (WDM1) combining 1480 nm and 155 0nm with 0.5 dB insertion loss by a 1480 nm pumping laser diode (1480-LD) to generate the C-band and L-band wavelengths. The EDF2 with 3 m, separated from the EDF1 by the optical circulator, was double pumped through a WDM2 and WDM3 combining 980 nm and 1550 nm with 0.5 dB insertion loss by two pieces of 980 nm-pumping laser diodes (980-LD1, 980-LD2), which not only generate the C-band wavelengths but also amplify the C-band and L-band wavelengths. The erbium-doped fiber in such configuration has peak absorption coefficients of 30 dB/m at 1530nm, cutoff wavelength at 900–1100 nm, a numerical aperture of 0.19±0.03 and maximum polarization mode dispersion of 0.002 ps/m. The lengths of the two pieces of erbium doped fiber are optimized according to the EDF1 for the L-band 5 to 6 times greater than the EDF2 for the C-band.

 

Fig. 1. Proposed double-pass and bi-directional configuration for high-power C+L-band ASE source

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Two pieces of 3 dB optical couplers (C1 and C2) were inserted at the1550 nm-port of WDM1 and WDM3 to feedback the ASE source and reshape its waveform without adding any spectra-flattening components. The optical circulator with an insertion loss of 0.5 dB and isolation greater than 40 dB for all circulating ports was placed to control one-way mechanism of ASE source and used as an output port O1 at its port 3 to measure optical power and spectra, where the isolation of port 1 to port 2 can avoid the resonant lasing and suppress the backward ASE power into EDF1.

 

Fig. 2. Optimum C+L-band ASE spectra for different 980 nm and 1480 nm pumping powers.

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Figure 2 illustrates the optimum output spectra of C+L-band ASE source from 1520 nm to 1620 nm measured by an optical spectrum analyzer (OSA) with a resolution of 0.1 nm. The different cases of EDF2 pumped by 980-LD1 and 980-LD2 with 250/250, 200/200, 150/150, 100/100 and 50/50 mW each, incorporated with those for EDF1 pumped by 1480-LD with 42, 41, 40, 39 and 36 mW to adjust the optimum output spectra, represent the ASE spectra as (1), (2), (3), (4) and (5). A power ripple ΔP_r in Fig. 2 was measured from the valley power at 1537 nm to the peak power, where a linewidth was measured for the output spectra of ASE source, was defined as -3 dB power level at 1537 nm. As shown in Fig. 2, the -3 dB linewidth in (1), (2), (3), (4) and (5) is respectively 81.0, 81.8, 82.6, 83.7 and 84.5 nm with a power ripple of 5.0, 5.3, 5.7, 6.2 and 7.1 dB. Therefore, we found that a power ripple in (1) can be reduced to 5.0 dB under sacrifice for the -3 dB linewith of 81.0 nm, compared with the case (5).

 

Fig. 3. C+L-band ASE spectra generated by two 980 nm LDs with 250 mW each incorporating different 1480 nm pumping powers.

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We also have measured the output ASE spectra pumped by 980-LD1 and 980-LD2 with 250/250 mW, incorporating 1480-LD with 10, 20, 30, 40 and 50 mW, as shown in Fig. 3. Such a lasing effect occurs in the cases of 1480-LD with 10, 20 and 30 mW owing to the lower C+L-band spectra and strong feedback of the C-band generated by 980-LD1 and 980-LD2 with 250 mW each. On the contrary, the cases of 1480-LD with 40 and 50 mW incorporating 980-LD1 and 980-LD2 with 250 mW each, restricted the resonant lasing effect as a result of the stronger C+L-band spectra from EDF1 pumped by 980-LD1 and 980-LD2. However, the output ASE spectrum shifted to the L-band when the pumping power at 1480 nm was increased to 50 mW. In comparison with Fig. 3, Fig. 4 illustrates the output spectra of the C+L-band ASE pumped by 980-LD1 and 980-LD2 with 100/100 mW, especially while being pumped by 1480-LD with 10, 20, 30, 40 and 50 mW. No resonant lasing effect existed even though the pumping power for 980-LD1 and 980-LD2 with 100 mW was applied. Note that the larger the pumping power is at 980 nm, the easier the lasing effect will be, and thus the larger pumping power at 1480 nm is adopted.

 

Fig. 4. C+L-band ASE spectra generated by two 980 nm LDs with 100 mW each incorporating different 1480 nm pumping powers.

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Fig. 5. Measured characteristics of output power and power ripple against 980 nm and 1480 nm pumping power.

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Based on the above-mentioned results, Fig. 5 shows the effects of pumping power on the measured C+L-band performances of output power P_ASE and power ripple ΔP_r for the optimum adjustment at 1480-power. With the increase of pumping power at 980 nm above 200/200 mW and 1480 nm above 40 mW, the output power of the C+L-band ASE source was increased to 177.8 mW without any resonant lasing. However, the cases of pumping power above 100/100 mW at 980 nm and above 20 mW at 1480 nm were not indicated here due to resonant lasing or a shift to the L-band. ΔP_r is the output power variation between the peak power and valley power at 1537 nm in the C+L-band spectra, which a power ripple of 5.0 dB can be achieved in the cases of pumping powers with 250/250 mW at 980 nm and 40 mW at 1480 nm. Several data points corresponding to pumping power with 10, 20, 30 and 50 mW at 1480 nm were not indicated in Fig. 5 for different power levels at 980 nm since either the C-band or L-band ASE spectra appeared only.

 

Fig. 6. Measured characteristics of pumping efficiency and linewidth against 980 nm and 1480 nm pumping power.

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Figure 6 illustrates the dependence of the performances of pumping efficiency and linewidth against the pumping power. The cases of pumping efficiency for 40 mW and 50 mW at 1480 nm incorporating the different pumping power at 980 nm are shown in this figure. However, the data in other cases are not indicated here due to resonant lasing at the C-band and the spectral shift to the L-band wavelength region for the other cases. Pumping efficiency for 200/200 mW and 250/250 mW at 980 nm and 40 mW and 50 mW at 1480 nm can be increased to about 32.8%. All the cases of the pumping power about 40 mW at 1480 nm incorporating all the pumping powers at 980 nm can achieve the minimum power ripple to measure -3 dB linewidth, which were decreased to 81.0 nm (1525.1–1606.1 nm) with an increase of the pumping powers at 980 nm.

Moreover, we also measured power stability ΔP_S for the cases of the pumping power with 250/250 mW at 980 nm and 40 mW at 1480 nm as shown in Fig. 7, which the 8-hour measurements with 1400 counts were taken under the temperature environment of 20 °C to 30 °C. The beginning output power of the C+L-band ASE source was 22.49 dBm at 25 °C. Power stability ΔP_S during 8-hour measurement was approach to an average of ±0.013 dB. From the results summarized in Table 1, it is obvious that this design can provide the high-power and stable C+L-band ASE source without any flattening filter. Therefore, two pumping LDs with 250/250 mW at 980 nm and one pumping LD with 42 mW at 1480 nm can be used to achieve the results of high output power and stability with the larger linewidth and smaller power ripple.

 

Fig. 7. Measured stability of output power in the temperature cycle of 20–30 °C.

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Table.1. Summarized table of optimum pumping powers for proposed configuration

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3. Conclusions

We have proposed and experimentally demonstrated a high-power C+L-band erbium amplified spontaneous emission source using an optical circulator with double-pass and bi-directional configuration. The performances of high output power of 177.8 mW with a ripple of 5 dB and a wide line width of 81.0 nm (1525.1–1606.1 nm) without adding any external spectra-flattening filters can be easily obtained under this configuration. In addition, the power stability with ±0.013 dB in 8-hour measurements of the temperature cycle has been also discussed to achieve high pumping efficiency of 32.8%. This kind of high-power C+L-band ASE source is very useful for the applications of DWDM filter test and spectra-slicing WDM transmission systems.