Dual-wavelength lasers have been developed rapidly over recent years, due to their numerous potential applications in wide areas such as laser ranging, remote sensing, medicine, and optical communications. To date, dual-wavelength generation based on neodymium-doped materials in the spectral range from 1.04 to 1.12 μm has been reported in substantial research, as neodymium ions can emit radiation in two or more spectrally spaced wavelengths between the 4F3/2 and 4I11/2 transitions. Due to 12 lines at this transition corresponding to the luminescence spectrum of neodymium ions in crystals, most studies have focused on bulk neodymium-doped solid-state lasers including isotropic crystal lasers and anisotropic crystal lasers. However, neodymium-doped fiber laser (NDFL) attracts less attention because of the immature optical fiber manufacturing technology. In addition, there is a rare report on dual-wavelength neodymium-doped all-fiber lasers so far. Compared with solid-state lasers, all-fiber lasers have the natural advantages of compact structure, convenient portability, and cost-effectiveness. Therefore, exploring an effective method to achieve dual-wavelength operation in all-fiber NDFL would be a promising subject.
Recently, BDF has been reported as a promising proposal to operate as a long-pass filter. The spectrum of the supercontinuum source coupled with BDF is suppressed before 1050 nm, the cut-off wavelength of BDF. It is explicitly stated that BDF is a potential medium for filtering applications. In this paper, a dual-wavelength NDFL is successfully demonstrated by incorporating a piece of novel BDF as a spectral filter and loss modulator in the ring cavity. Dual-wavelength lasing is achieved successfully at 1080.8 nm and 1090.2 nm simultaneously. Due to the nonlinear effect aroused by the BDF, a pure sinusoidal continuous wave with a repetition of 9.1 MHz is generated. To the best of our knowledge, it is the first time that BDF is operating as a spectral filter to achieve a dual-wavelength all-fiber laser experimentally.
The experimental setup of the dual-wavelength all-fiber NDFL with BDF as a spectral filter. The all-fiber ring cavity consists of a wavelength division multiplexer (WDM), a 17 m long neodymium-doped fiber (NDF), a newly developed BDF, and an 80:20 output coupler (OC). The NDF is model DF1000 from FIBER CORE with a numerical aperture of about 0.2 and a core/cladding diameter of 3.5/125 μm. The absorption at 1.08 μm is 8.5 dB/m. The BDF has a numerical aperture of 0.3 and a core diameter of 12.7 μm. An 808 nm commercial laser diode with a maximum output power of 228 mW is connected to the WDM for pumping the active fiber. The OC extracts 20% of power from the ring cavity for the purpose of monitoring. The optical spectrum is monitored by an optical spectrum analyzer (Anritsu MS9710C) with a resolution of 0.05 nm. The waveform is measured by an oscilloscope (GWINSTEK GDS-3352) with a minimum power division of 2 mV. The frequency spectrum is detected by a radio frequency (RF) spectrum analyzer (Anritsu Ms2683A) with a span of 7.8 GHz.
A piece of BDF with a core diameter of 12.7 μm and NA of 0.31, is inserted into the cavity between the NDF and the OC. The OC is made from a single-mode optical fiber with a core diameter of 3.7 μm (refractive index of the core glass is ncore = 1.470, cut-off wavelength λc = 820 nm), and outer diameter of 125 μm (refractive index of the cladding glass, nclad = 1.461). Since the core diameter of BDF is significantly larger than the core of OC and NDF, mode interference occurs inside the BDF to act as a spectral filter for a dual-wavelength generation. In addition, the BDF also consists of a high content of GeO2 and thus it has a high non-linear coefficient due to the formation of bismuth-related active centers associated with germanium.
Then the spectrum evolution of NDFL against different pump powers is investigated after inserting the 30 cm BDF in the cavity. The output spectra of the NDFL with and without the BDF are both included for the purpose of comparison. The threshold pump power of the proposed laser is 37 mW for continuous wave operation. As can be seen, the emission spectrum of the laser without BDF has a single-exponent shape with a center wavelength of 1088.8 nm. After the BDF is inserted into the cavity, the spectrum width of the laser becomes narrower, and the center wavelength shifts to 1090.2 nm. This is attributed to the polarization-dependent loss of the BDF, which increases the cavity loss and forces the laser to operate at a longer wavelength with a higher net gain. As the pumping power is further increased to 52 mV, the spectrum of the laser without BDF has a broader gain spectral width than before. It is noted that the center wavelength of the laser with BDF maintains the same at 1090.2 nm. As the pump power is increasing to 59 mW, the spectral width of the laser without BDF keeps broadening. Meanwhile, a dual-wavelength operation is observed in the laser with BDF due to the emission spectrum extended by the enhanced pump power. The short wavelength is stimulated simultaneously at 1080.8 nm, and the long wavelength remains the same as it was at 1090.2 nm.
In summary, using the novel BDF with a 30 cm length as a spectral filter, we have experimentally investigated the formation of dual-wavelength and sinusoidal CW generation in an all-fiber NDFL. It is found that the nonlinear characteristic of BDF is able to suppress the mode competition to generate dual-wavelength laser output and balance the power intensity between different wavelengths. The filtering function of BDF can be applied to many applications such as wavelength selection in WDM spectroscopy, fiber sensing, and optical communication. Furthermore, the proposed all-fiber laser provides a simple configuration, low cost, compact structure, and stable operation under room temperature.
Author: Prof. Dr. Moh. Yasin, M.Si. (Corresponding & First Author)
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https://www.sciencedirect.com/science/article/pii/S0022231322006603
