A multi-wavelength Erbium-doped fiber (EDF) laser based on four-wave-mixing is proposed and experimentally demonstrated. The 5km single mode fiber in the cavity enhances the four-wave-mixing to suppress the homogenous broadening of the erbium-doped fiber and get the stable multi-wavelength comb. The lasing stability is investigated. When the pump power is 300mW, the fiber laser has 5-lasing lines and the maximum fluctuation of the output power is about 3.18dB. At the same time, a laser with 110m high nonlinear fiber (HNFL) is demonstrated. When the pump power is 300mW, it has 7-lasing lines (above −30dBm) and the maximum fluctuation is 0.18dB.
© 2013 OSA
Multi-wavelength Erbium-doped fiber lasers (EDFL) have attracted a lot of interests because of its potential wide application in many fields, such as optical fiber sensors system and optical communication system. However, at room temperature it is difficult for EDFL to attain stable multi-wavelength comb because of the homogenous broadening effect of the erbium-doped fiber. A lot of solutions have been proposed to solve the problem, such as liquid nitrogen cooling , using a hybrid gain medium , stimulated scattering effect in fiber [3–5], frequency shifting , four-wave-mixing [7–10], piezoelectric- transducer-based phase modulations , a nonlinear optical loop mirror (NOLM) , and nonlinear polarization rotation (NPR) [13–15]. As a cheap high-performance nonlinear medium, Single mode fiber (SMF) is widely used as nonlinear gain medium [3–5] and nonlinear phase accumulated device [13–15].
Four-wave-mixing effect is a good way to suppress the homogenous broadening of the erbium-doped fiber at room temperature. Since the mode competition can be effectively suppressed by four-wave mixing effect or an inhomogeneous loss mechanism of the high nonlinear fiber, stable multi-wavelength operation could be achieved. In order to enhance four-wave mixing effect, several kinds of fiber are inserted into the cavity, such as dispersion-shifted fiber , photonic crystal fiber [8, 10] and highly nonlinear fiber . In this paper, we investigate the performance of four-wave-mixing effect of the SMF in the cavity. The multi-wavelength comb with five lines is achieved when the pump power is 300mW. The stability of the multi-wavelength comb is studied. At the same time, a multi-wavelength fiber laser with 110m HNFL working as the nonlinear medium in the cavity is presented. The results show that the fiber laser with 110m HNFL has more line and better stability.
2. Experimental setup
A schematic diagram of the experimental setup is shown in Fig. 1. The laser’s ring cavity consists of 5km SMF, a 980/1550 wavelength division multiplexing (WDM) coupler, a section of EDF, a 10:90 fiber coupler, and a polarization independence isolator (ISO), a Sagnac loop filter. The SMF is produced by Yangtze Optical Fibre and Cable Company. Its mode field diameter at 1550nm is 9.9~10.9 µm and the dispersion at 1550nm is 18 ps/(ns.km). The EDF pumped by a laser diode is the gain medium, the type of the EDF is Nufern EDFC-980-HP and its absorption coefficient at 1530 nm is 6.0 ± 1.0 dB/m. The maximum output power of the pump is 300 mW. The length of EDF used in our experiment is 12 m. The Sagnac loop filter, which consists of a 50:50coupler a 10m long PMF and a polarization controller, serves as the periodic filter in the cavity. Figure 2 shows the transmission spectrum of the Sagnac loop. The wavelength spacing between adjacent reflection bands is 0.95 nm. The 10% port of the fiber coupler acted as the output port of the laser. An AQ6370 optical spectrum analyzer (OSA) with resolution of 0.02 nm was used to measure the output spectra of the laser.
3. Results and discussion
The threshold pump power is 10mW. When the pump power is 10mW, there are lasing wavelength at 1559.00 nm and 1560.90 nm, but the power is not stable, because the power is too low to stimulate four wave mixing. Figure 3 shows the output spectra at different time. Due to homogenous broadening effect in EDF, the mode competition induces the fluctuation of the output lines.
When the pump power increases to 100mW, the output of the laser has stable 4 lines. To prove the lasing wavelength stability of the laser, I measured the spectra every 5mins for 4 times of room-temperature operation. The measurement result indicated by the 3D diagram is shown in Fig. 4. The result shows that there are four lines oscillating stably, which locate at 1558.18nm, 1559.13nm, 1560.09nm, 1561.05nm. The peak at 1561.99nm is not stable enough. When the pump power increases to 300mW, the output has 5 stable peaks locating at 1556.72nm, 1557.68nm, 1558.63nm, 1559.58nm and 1560.53nm and an unstable peak locating at 1561.48nm. Figure 5 shows the spectra every 5mins for 5 times.
Figure 6 shows the power variation of the lines in Fig. 5. The peak locating at 1559.58nm has the best stability, the power fluctuation is 0.08dB. The power of the peaks locating at 1556.72nm has the biggest fluctuation, which is 3.18dB.
In order to improve the stability of the laser, a 110m long HNLF takes the place of the 5km SMF. The HNLF with flat near-zero dispersion is also produced by Yangtze Optical Fibre and Cable Company. Its mode field diameter at 1550nm is 4.07µm.The zero-dispersion wavelength of the HNFL is 1550nm. The remaining dispersion is −2.0~2.0 ps/(nm﹒km) over the wavelength range 1480-1580 nm. The threshold pump power of the laser is also 10mW. When the pump power increases to 100mW, the laser has a stable multi-wavelength comb output. In order to verify the stability of the proposed multi-wavelength fiber laser, we repeated 6 times scan outputs in 30 minutes with 5 minutes interval, as shown in Fig. 7. The output spectra of the laser have very good stability. As the pump power increases more, the multi-wavelength comb has more lines and keeps stable. Figure 8 shows the output spectra at different pump power, 100mW, 200mW, 250mW, 300mW.
Finally, we investigate the stability of the laser in detail when the pump power is 300mW. Figure 9 shows the output spectra in half hour at 5 minutes interval. In Fig. 9, the output spectra have no obvious fluctuation in power and wavelength.
Figure 10 shows the power variation of the lines in Fig. 9 whose power are more than −30dBm. There are 7 lines locating at 155.68nm, 1556.64nm, 1557.59nm, 1558.54nm, 1559.50nm, 1560.45nm and 1561.41nm. As the statistical results shown in Fig. 10, the power fluctuation of the peak locating at 1555.68nm is the biggest, which is 0.18dB. The laser has very good stability and performance. According to the theory in reference 10, the four wave mixing effect in the cavity is related to three parameters of the fiber, the nonlinearity coefficient, the length and the dispersion. Although the length is much short, the HNFL has much bigger nonlinearity coefficient and flat zero-close dispersion, which enhances the four wave mixing in the cavity and improves the performance of the multi-wavelength laser.
We have investigated multi-wavelength erbium-doped fiber lasers based on four-wave-mixing effect in single mode fiber and high nonlinear fiber. When the pump power is 300mW, the fiber laser with 5km SMF output 5-lasing lines and the maximum power fluctuation is about 3.18dB; the laser with 110m HNFL output 7-lasing lines (above −30dBm) and the maximum power fluctuation is 0.18dB. The multi-wavelength fiber laser is practical for a lot of applications, such as the optical communications, optical testing and measurement and optical sensors systems.
The authors gratefully acknowledge support from National Nature Science Foundation of China (No. 60925019, 61090393, 61177036), Science and Technology Innovation Team of Sichuan Province (No. 2011JTD0001), State Key Laboratory of Advanced Optical Communication Systems and Networks (Project No. 2008SH08) and the Fundamental Research Funds for the Central Universities (Project No. ZYGX2012J064, ZYGX2010J064).
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