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We propose and experimentally demonstrate a tunable multi-wavelength SOA fiber laser based on a Sagnac loop mirror using elliptical core side-hole fiber. We fabricated different types of elliptical core side-hole fiber with elliptical or circular cladding shape. The measured modal birefringence of the fabricated fiber was 1.7×10-4 (elliptical shape) and 1.16×10-4 (circular shape), respectively. By carefully adjusting the polarization controller inside the cavity, we could obtain 18 discrete channels with SNR over 30 dB and channel spacing of 0.8 nm and 10 discrete channels with SNR over 30 dB and channel spacing of ∼1.4 nm. The proposed fiber laser was rather stable and the temporal power fluctuation was less than 0.8 dB. In addition, by thermally heating the elliptical core side-hole fiber in the Sagnac loop, we could obtain a tunable multi-wavelength fiber laser.
©2007 Optical Society of America
1. Introduction
Multi-wavelength fiber lasers have attracted great interest because of their potential application in optical fiber sensing, optical communications, instrument testing and so on thanks to their various advantages, such as multi-wavelength operation, low cost and low insertion loss. Semiconductor optical amplifier (SOA)-based multi-wavelength fiber lasers exhibit stable operation because the SOA has the property of primarily inhomogeneous broadening and thus can support simultaneous oscillation of many lasing wavelengths [1–4].
Several techniques have been proposed to achieve multi-wavelength fiber lasers [5–11]. Among them, the use of a fiber Fabry-Perot filter increases the insertion loss of the cavity, and a Mach-Zehnder filter is sensitive to environmental changes due to the difference in the optical path lengths of the two arms. The Sagnac loop filter incorporating high birefringence fiber has the advantages of simple configuration and better stability compared with the filters based on Fabry-Perot and Mach-Zehnder interferometers.
In this paper, we propose and experimentally demonstrate a multi-wavelength fiber laser based on a Sagnac loop mirror using elliptical core side-hole fiber with elliptical or circular shape cladding. In addition, we demonstrate a tunable multi-wavelength fiber laser by thermally heating the elliptical core side-hole fiber in the Sagnac loop.
2. Experiments and results
To fabricate the elliptical core side-hole fiber, we used the MCVD process. Figure 1 shows the cross section of the fabricated side-hole fiber with an elliptical core. To fabricate the elliptical-shape fiber with elliptical core [Fig. 1(a)], it was fabricated by over-jacketing (19 X 25 tube) and collapsing after cutting both sides of preform [12]. The fiber was drawn at the temperature of 1930 °C. The relative index difference Δn (peak) was 0.02. The major/minor axes of the core and the side-hole diameter were 12 μm/6.6 μm and 20 ∼ 25 μm, respectively. To fabricate the circular-shape fiber with elliptical core [Fig. 1(b)], two holes of 5-mm diameter were drilled on both sides of the core in the preform [13]. The core with elliptical shape was made by partially collapsing the holes during fiber drawing at 2000 °C. The major/minor axes of the core and the side-hole diameter were 6.25 μm/3.8 μm and 18 μm, respectively. The relative index difference Δn (peak) was 0.018.
Fig. 1. The cross-section of fabricated elliptical core side-hole fiber with (a) an elliptical shape, (b) a circular shape.
Generally, the wavelength separation between two transmission peaks of the Sagnac loop mirror’s output is given by Δλ=λ 2/BL (where B is the modal birefringence, L is the length of the elliptical core side-hole fiber, and λ is the operation wavelength) [14]. From the transmission spectrum of a Sagnac loop mirror as shown in Fig. 2, the modal birefringence (B) of the fabricated elliptical core side-hole fiber was 1.7×10-4 (elliptical shape, Δn=0.02) and 1.16×10-4 (circular shape, Δn=0.018), respectively.
Fig. 2. (a). The configuration of a Sagnac loop mirror, (b) the transmission spectra of elliptical core side-hole fiber with an elliptical shape (L=10 m, Δλ=1.4 nm), and a circular shape (L=26 m, Δλ=0.8 nm), respectively.
Figure 3 shows the experimental setup of the proposed fiber laser. The input beam from two SOAs (or one SOA) is split into two counter-propagating beams by the 3 dB coupler, which recombine at the coupler after travelling through the fiber loop. Due to the large birefringence of the elliptical core side-hole fiber, the beams polarized along the fast axis and slow axis experience different optical path lengths. The channel spacing of the fiber laser is determined by the birefringent component only, which is the elliptical core side-hole fiber within the Sagnac loop mirror. The SOA1 and SOA2 (Alcatel, part no. 3CN00199CE) provided the peak small signal gain of 28.7 dB at 1550 nm with 0.5 dB polarization dependence when driven with 200 mA dc current. The laser output was taken from a 95:5 coupler, which provides 5 % for the output and 95 % for feedback to the cavity.
To obtain the gain spectra, we used a broadband source, which had about -27 dBm output power at 1550 nm. As the injection current of SOA increased, the gain also increased. Figure 4 shows the gain spectra of one SOA and two SOAs, when injection current of SOA was 200 mA. The maximum gain was ∼23 dB (one SOA) and ∼35 dB (two SOAs), respectively. In case of two SOAs, the gain was higher than that of one SOA.
Due to the large birefringence of the fiber, the number of lasing wavelengths and the channel wavelengths were sensitive to the polarization controller setting. By carefully adjusting the polarization inside the cavity, we could obtain the maximum number of lasing wavelengths with one SOA or two SOAs driven with the injection current of 200 mA. In case of one SOA as shown in Figs. 5(a) and 5(c), the number of lasing wavelengths with less than 3 dB power variation among them was 8 for the elliptical shape and 12 for the circular shape, respectively. In case of two SOAs, which increased the number of lasing wavelengths and produced uniform and broad output power spectrum [15], the number of lasing wavelengths was 10 for the elliptical shape and 18 for the circular shape as shown in Figs. 5(b) and 5(d), respectively. The channel spacing was ∼1.4 nm and ∼0.8 nm (100 GHz at 1550 nm, i.e. WDM ITU-grid spacing) with SNR > 30 dB, respectively.
Fig. 5. The output spectra of the fiber laser. (i) elliptical shape (L=10 m, Δλ=1.4 nm): (a) one SOA, (b) two SOAs, (ii) circular shape (L=26 m, Δλ=0.8 nm): (c) one SOA, (d) two SOAs
Fig. 6. Repeatedly scanned output spectra of the multi-wavelength fiber laser. (a) elliptical core side-hole fiber with an elliptical shape (L=10 m, Δλ=1.4 nm), and (b) a circular shape (L=26 m, Δλ=0.8 nm).
Figure 6 shows the repeatedly scanned spectra of the multi-wavelength fiber laser output. A total of eight measurements were made at two minutes’ interval. The output power was rather stable and the temporal power fluctuation was less than 0.8 dB.
In the Sagnac loop mirror, the phase-shift sensitivity is given by the simple formula [16]:
where is the phase difference, B is birefringence, T is temperature, λ is the X wavelength, and L is the length of PM side-hole fiber. In general, when the temperature is increased in the PM fiber region of the Sagnac loop, the resonance wavelengths of the Sagnac loop, which determine the resonance wavelength of multi-wavelength fiber laser, are shifted toward the shorter wavelength due to the decrease of birefringence of PM fiber [17]. In the Sagnac loop mirror, the temperature dependence of the resonance wavelengths is mainly determined by the characteristics and length of the PM fiber [18]. Figure 7 shows output spectra of the tunable multi-wavelength fiber. We could control almost up to π-phase wavelength shift at ΔT=5°C (elliptical shape, L=10 m, Δλ=1.4 nm) and ΔT=2°C (circular shape, L=26 m, Δλ=0.8 nm), respectively.
Fig. 7. Output spectra of the tunable multi-wavelength fiber laser: (a) an elliptical shape (L=10 m, Δλ=1.4 nm), and (b) a circular shape (L=26 m, Δλ=0.8 nm).
3. Conclusion
In this work, we proposed and experimentally demonstrated a tunable multi-wavelength fiber laser based on a Sagnac loop mirror using elliptical core side-hole fiber. For experimental measurements, we fabricated different types of elliptical core side-hole fiber with elliptical or circular cladding shape. The modal birefringence (B) of the fabricated elliptical core side-hole fiber was 1.7×10-4 (elliptical shape, Δn=0.02) and 1.16×10-4 (circular shape, Δn=0.018), respectively. By carefully adjusting the polarization controller inside the cavity, we could obtain 18 discrete channels with SNR over 30 dB and channel spacing of 0.8 nm and 10 discrete channels with SNR over 30 dB and channel spacing of ∼1.4 nm, respectively. The proposed fiber laser was rather stable and the temporal power fluctuation was less than 0.8 dB. Moreover, by thermally heating the fiber in the Sagnac loop, we could tune the resonance wavelengths up to ∼π phase. The fiber laser proposed in this work will be very useful for applications in WDM systems, sensing, and instrument testing.
Acknowledgments
This work was performed under the partial support from the Second-Phase of the Brain Korea-21 Project, the Basic Program Project of KOSEF (Grant No. R01-2006-000-11088-0), and the Next-Generation Growth Engine Project of MOCIE.
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