2015 International Conference on Automation, Cognitive Science, Optics, Micro Electro-Mechanical System, and Information Technology (ICACOMIT). Bandung, Indonesia, October 29–30, 2015
978-1-4673-7408-8/15/$31.00©2015 IEEE
Multiwavelength Erbium Doped Fiber Laser based on
Nonlinear Polarization Rotation
Nurul Shahrizan Shahabuddin and Ruzita Abu Bakar Department of Electrical Engineering, Nilai University
Negeri Sembilan, Malaysia Email: shahrizan@nilai.edu.my
Abstract—A multiwavelength fiber ring laser is demonstrated
using a nonlinear polarization rotation (NPR) effect. An isolator and a photonic crystal fiber are incorporated in an Erbium doped fiber laser (EDFL) to generate intensity-dependent transmission induced by the NPR effect which allows the multiwavelength generation at room temperature. A stable 17 lasing lines in 1560 nm region are generated. The multiwavelength Erbium doped fiber laser (EDFL) exhibits spacing of 0.27 nm. EDF pump power was varied from 30 mW to 120 mW. Higher Erbium doped fiber (EDF) pump power generally leads to more multiwavelength generation at the expense of lower SNR due to higher noise level.
Keywords—multiwavelength laser; nonlinear polarization rotation; photonic crystal fiber;
I. INTRODUCTION
Wavelength division multiplexing (WDM) technology is now well established, and is being deployed in transmission and network systems worldwide. As the channel count continues to increase across the spectrum from S- to L-band, interest in laser sources that can provide simultaneous, multiwavelength operation grows. Applications for such sources include those in WDM transmitters, passive and active component characterization. Moreover, these sources may also be used as replacements of an equivalent number of discrete laser sources or tunable laser sources to reduce cost or the complexity in measurement procedures (Harun et al., 2009; Luo et al., 2009; Luo et al., 2009b; Luo et al., 2009c and Chen et al., 2009). Many techniques have been reported for multiwavelength generation in room temperature which are mainly based on four-wave mixing (FWM), stimulated Brillouin scattering (SBS) and spectral slicing (ahmad). In addition, the multiwavelength generation can be also achieved by adding intracavity components such as Fabry–Perot etalon (Yeh et al., 2009), Mach–Zhender fiber interferometer (Moghaddam et al., 2010), fiber Bragg gratings (Zhao et al., 2004) and intracavity polarizer (Chartier et al., 2005).
Lately, nonlinear polarization rotation (NPR) effect in optical fibers has also been used to demonstrate multiwavelength fiber lasers (Zhang et al., 2011). The NPR produces intensity dependent loss in the cavity, which then contributes to phase shifting and multiwavelength signal generation. This effect could overcome the unstable mode competition and generate multiwavelength output in Erbium-doped fiber laser (EDFL). NPR based multiwavelength fiber
laser can be realized using a simple structure as it does not mechanism in PCF using a standard EDF (with a low Erbium concentration) as the gain medium. The scanning electron micrograph (SEM) image of the PCF used is shown as insert in the figure. The PCF was made from pure silica with 17.4 wt% of Ge-doped core region. The cladding diameter of the PCF is 128 ± 5 µm with a single layer of acrylate at 250 ± 10 µm in diameter coating the PCF. The PCF exhibits a triangular core with an average diameter d of 2.1 ± 0.3 µm and an effective core area Aeff of 6.1 µm2. The average air hole diameter of the PCF is 0.8 µm with pitch of 1.5 µm. The attenuation of the PCF between 1510 – 1620 nm is < 9 dB/km with effective nonlinear coefficient of ~11 (Wkm)-1 at 1550 about 11 W-1 km-1 and a zero-dispersion wavelength of around 1550 nm. The gain medium, an 8 m long EDF (I-6, manufactured by Fibercore), is pumped by a 980 nm laser diode through a 980/1550 wavelength selective coupler (WSC). A 3-dB coupler is used to extract a portion of the laser as an output to be fed into an optical spectrum analyzer (OSA) and another one is employed to couple the Brillouin pump (BP) light to a tunable laser source (TLS). An isolator is used in the resonator to provide a unidirectional operation for the NPR based multiwavelength laser.
the multiple wavelength generation with constant spacing through the partially degenerated FWM process. When dual wavelengths (ωi and ωj) started to lase, the partially
degenerated FWM took place and generated both red shifted and blue shifted lines at frequency of = 2ωi– ωjand = 2ωj–
ωi. FWM annihilates photons from these waves to create new
photons at different frequencies, thus generate consecutive higher order lines. In addition, the FWM process could redistributes energy of the oscillating lines which allow the energy of a stronger wave to be transferred to weaker wave. This can also balance the gain competition in the EDF and thus, improves the stability and uniformity of the multiwavelength lines (Liu et al., 2005). The induced peak power variation ΔPifor an arbitrary oscillation peak at λican
be described as (Shum et al., 2006).
i m im i
i m i m i
i
m m
i
P
P
P
P
2
1
(1)where αm denotes the efficiency of the FWM process taking place among λi and λi ± m∙ ∆λ (corresponding frequency
are ωi and ωi ± m∙∆ω; ∆λ and ∆ω are the laser wavelength and
frequency spacing, respectively), Pi-m and Pi+m are power at
frequencies ωi-m and ωi+m, respectively. Fig. 2 shows the laser
oscillation with and without the isolator in the cavity. Without isolator in the cavity, unstable oscillation modes are observed around 1559 nm region. This is due to strong competition between self lasing cavity modes within the highest gain region. The isolator is added to enable NPR to take place and thus generates equally spaced multiwavelength oscillations. As shown in the figure, multiwavelength lines with spacing of 0.27 nm is generated at pump power of 80 mW.
Fig. 1. Configuration of the NPR based multiwavelength EDFL. Insert is the SEM image of the PCF.
Fig. 2. Laser oscillations at 80 mW pump power with and without isolator.
Fig. 3. Multiwavelength generation at multiple pump power.
III. CONCLUSION
The generation of multiwavelength laser is demonstrated using a NPR effect in EDF gain media with assistance of PCF. The intensity-dependent transmission induced by the NPR effect allows the multiwavelength generation at room temperature. The birefringent property PCF allowed the generation of equal spacing lines. With incorporation of PCF,
an NPR-based multiwavelength EDFL can be demonstrated using a standard EDF as the gain medium. Higher 980 nm pump power generally leads to more multiwavelength generation at the expense of lower SNR due to higher noise level. At 120 mW pump power, about 17 lines in 1560 nm region are generated. The multiwavelength EDFL exhibits spacing of 0.27 nm.
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