We have proposed and demonstrated a multi-wavelength-switchable and uniform erbium-doped fiber laser using unbalanced in-line Sagnac Interferometer. By employing this simple scheme and through the proper control of the polarization controller, we were able to achieve uniform multi-wavelength operation of up to 84 laser lines with the signal-to-noise ratio over 20dB and 0.8-nm wavelength switching at room temperature. Again, we generated more than 300 lines, 0.1-m wavelength switching and good power stability (≦0.2dB) over 1570nm–1610nm in the experiment.
© 2007 Optical Society of America
The multi-wavelength-switchable fiber laser sources have not only been seriously considered for application in optical remote sensing and Dense Wavelength-Division Multiplexing transmission (DWDM) systems, but have also been used as a test instrument for optical components. Multi-wavelength fiber lasers using EDFs have been widely investigated due to unique advantages such as high saturation power and large gain. However, there are different approaches to achieving multi-wavelength generation with various gain mechanisms including semiconductor optical amplification (SOA) [1–4], erbium-doped fiber (EDF) [5, 6], hybrid SOA and EDF , Brillouin-erbium fiber [8–10], as well as Raman and EDF .
Up to now, 160 Brillouin Stokes lines [9, 10] and uniform multi-wavelength fiber based on hybrid Raman and EDF gains  have been reported. Recently, we presented a multi-wavelength-switchable and uniform erbium-doped fiber laser using unbalanced in-line Sagnac Interferometer where an sliced L-band ASE light source as a seed and a loop cavity have been introduced to suppress the strong homogenous line broadening of the EDF. It was designed to provide uniform multi-wavelength operation of up to 84 laser lines with a signal-to-noise ratio over 20dB and 0.8-nm wavelength switching. Again we can achieve the generation of more than 300 lines, 0.1-m wavelength switching and good power stability over 1570nm–1610nm at ≦0.2dB.
2. Experiments and Discussion
The experimental schematic for the proposed multi-wavelength-switchable and uniform erbium-doped fiber laser based on unbalanced in-line Sagnac Interferometer with polarization-maintaining fiber (PMF) is shown in Fig. 1. Two pieces of erbium-doped fiber, EDF1 and EDF2 were the gain medium, and one 1×2 optical coupler was used to generate the L-band amplified spontaneous emission (ASE) light source. The EDF1 and EDF2 with 20m and 2m were forward pumped through WDM1 and WDM2 combining 980nm and 1580nm with 0.5dB insertion loss by two pumping laser diodes at 980nm pumping LD1 (250mW) and LD2 (250mW), and generating the L band wavelengths as shown in Fig. 1 (a). The erbium-doped fiber (EDF) in such configuration has peak absorption coefficients of 30dB/m at 1530nm, cutoff wavelength at 900–1100nm, a numerical aperture of 0.19±0.03, and maximum polarization mode dispersion of 0.002ps/m.
The unbalanced in-line Sagnac Interferometer consisted of: one optical circulator with an insertion loss of 0.5dB and isolation greater than 40dB for all circulating ports; one optical polarizer with single-mode-fiber input port and PMF output port (with an extinction ratio of 30dB); one piece of PMF (with a birefringence value of 0.0003); one polarization controller and one optical mirror with 97% reflectivity. The optical circulator’s port two is connected to the input port of a polarizer, whose output port is spliced to PMF with a different angle. The multi-wavelength was amplified by a loop cavity, which was mainly formed by a 2×2 optical coupler, WDM3, 980nm pumping LD3 (100mW), 2m EDF and optical isolator, and output from its port to improves its own power stability within ±0.05dB.
The operating principle of unbalanced in-line Sagnac Interferometer is described as follows. A polarizer is used to produce a linear polarization. By adjusting the principal axis of a polarizer by splicing different angle with respect to both principal axes (fast-axis and slow-axis) of PMF, we can obtain two orthogonal linear polarizations with equal intensities (at 45°). One polarization controller is used to fine-tune the relative phase difference Φ=4π(Δn)Leff/λ between two orthogonal polarization modes, where Δn is the effective birefringence between two orthogonal polarization modes, Leff is the effective length of PMF and λ is wavelength. As reflected by a fiber mirror, the two states of polarization (SOP) propagate the same optical path twice and interfere at the spliced point. Therefore, the PMF and PC can easily control the wavelength spacing and shift of the multi-wavelength fiber laser with great flexibility.
Figure 2 illustrates the optimum and uniform output spectra of the proposed multiwavelength-switchable fiber laser for 1m PMF at spliced angles of 0, 15, and 30 degree as well as 45, 60, and 90 degree spliced angles. These measurements are taken from the output port as measured by an optical spectrum analyzer (OSA) with a resolution of 0.06 nm. The 45-degree spliced angle showed the best spectral performance in terms of generated laser lines and signal-to-noise ratio. On the other hand, the 0- and 90-degree spliced angle demonstrated the worst spectral performance. Therefore, we carried out the following experiments based on the 45-degree spliced angle.
Next, we measured the output spectra for 1m, 3m, 6m and 12m PMF as shown in Fig.3. By carefully adjusting PC, the red solid spectra in Fig. 3(a) can be shifted to the blue dotted line as well as the other cases. We also found that 19 generated laser lines in Fig. 3(a) can increase to 84, 166 and 318 lines as shown in Fig. 3(b), (c) and (d) by extending the PMF from 3m to 12m.
Furthermore, the power uniformity of less than 2dB can be provided from 1570–1600nm with the total measured output power being 8.61dBm.
Figure 4 illustrates the zoomed spectra for four cases over 1582–1588nm except the 1m PMF where the laser line generation is low. Fig. 4 (a) shows that the wavelength spacing Δλ, wavelength shift Δλ’, and signal-to-noise ratio of two interleaved wavebands are 3.3nm, 0.64nm and 27dB respectively for the 1m PMF. The wavelength spacing, shift, and signal-to-noise-ratio for 3m PMF is 0.8nm, 0.28nm and 20dB, for 6m PMF is 0.4nm, 0.15nm and 12dB, and for 12m PMF is 0.2nm, 0.08nm and 6dB as shown in Fig. 4 (b), (c) and (d) respectively.
The increase in PMF length resulted in the proportional decrease in the wavelength spacing, wavelength shift, signal-to-noise ratio, full width at half maximum (FWHM), and an increase in the generated laser line.
In order to demonstrate good stability of the multi-wavelength generation at room temperature, we display the repeated scanning spectra for 1m, 3m, 6m and 12m PMF at three minute intervals in a 45 minute period as shown in Fig. 5.
Moreover, we also measured power stability for four cases shown in Fig. 6(a–d), where the data were taken 4 times in a 3-minute period. Power stability during 3-minute measurement changes less than 0.2dB over the whole waveband.
From the results as summarized in Table 1, it is obvious that such design can provide the multi-wavelength generation so that the channel spacing in each waveband is 0.8nm/0.4nm/0.2nm (100GHz/50GHz/25GHz at 1550nm, i.e. WDM ITU-grid). Compared with the previous works [9–11], we not only achieved more generated laser lines but also provided more power stability and uniformity. Therefore, unbalanced in-line Sagnac Interferometer incorporating PMF can be used to achieve multi-wavelength generation and uniform power spectra with good power stability.
We have proposed and demonstrated a multi-wavelength-switchable and uniform erbium-doped fiber laser using unbalanced in-line Sagnac Interferometer. By employing this simple scheme and through the proper control of the polarization controller, we were able to achieve uniform multi-wavelength operation of up to 84 laser lines with the signal-to-noise ratio over 20dB and 0.8-nm wavelength switching at room temperature. Again, we experimentally demonstrated how to generate more than 300 lines, 0.1-m wavelength switching and good power stability ≦0.2dB over 1570nm–1610nm in 45-minute measurements at room temperature have been experimentally demonstrated. Such multi-wavelength-switchable laser source is very useful for the applications of DWDM devices and systems.
References and links
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