Dual-Wavelength Q-switched Fiber Laser Generation by Incorporating 2D Materials in D-SMS Fiber Structure

We demonstrate a dual-wavelength Q-switched fiber laser generation by inserting a combination of SM-MM-SM fiber and a d-shape structure. This combination is known as D-SMS fiber. By side-polishing an SM fiber with 1-dB loss, the d-shape fiber structure is obtained. Then the d-shape SM fiber is spliced to 25 cm long MM and SM fiber to form a D-SMS fiber structure. After inserting D-SMS fiber, a stable dual-wavelength presence at 1562.8 nm and 1563.8 nm (1 nm wavelength spacing). The pulse train generates once we deposited a graphene oxide solution onto the surface of the d-shape structure. This Q-switching operation mode obtained from 45 mW to 111 mW pump power with a maximum pulse energy of 56 nJ. The repetition rate and pulse width have been recorded from 34 kHz to 55 kHz and from 10.22 µs to 6.47 µs, respectively.


Introduction
A passively Q-switched fiber lasers can be used for outstanding intentions like remote sensing and laser processing.Passive Q-switched fiber laser is frequently used due to cost-effective and more robust compared to active Q-switched fiber laser [1].In other hand, Dual-wavelength fiber laser sources generation has gained tremendous interest in optical sensing and microwave generation [2].Integration between dual-wavelength laser and Q-switching operation laser creating a compact laser source which be able to open wider applications in new laser technology development.Robustness, compact in size and alignment-free are a great choice for all-fiber based laser technology.
The saturable absorber is the main element in the fiber laser cavity.Graphene is under twodimensional material has excellent photoelectric properties and due to its unique structures, it can attract too much attention [3] in radiation environments including in outer space [4].The advantages of graphene are cheap in cost, stable, and excellent in performance of optical properties [5].Due to this factor, the usage of graphene is widely used as a saturable absorber because of low saturable absorption and high modulation depth.Graphene has some disadvantages in terms of weak light-matter interaction and oxidation issues when the material is under high-energy laser irradiation .
In this paper, we demonstrate the integration of SM-MM-SM fiber and d-shape structure in generating dual-wavelength Q-switched erbium-doped fiber laser (EDFL).This fiber integration is called as D-SMS fiber.The graphene oxide solution is coated onto d-shape surface area to induce saturable absorption characteristics in the D-SMS fiber structure.The SM-MM-SM fiber structure induces the Mach-Zehnder interferometer (MZI) effect.Thus, generates stable dual-wavelength in Qswitching operation.

Fiber Laser Configuration
Figure 1(a) illustrates the graphene PVA as a passive saturable absorber and the 2.4 m long erbiumdoped fiber (EDF) as a gain medium integrated into the fiber laser ring cavity.The EDF (FIBERCORE, I-25) has a core diameter of 9 µm and a numerical aperture (NA) of 0.16, with an absorption coefficient of 27 dB/m at 980 nm.With 980/1550 wavelength division multiplexing (WDM), a single-mode laser diode pump source at 980 nm wavelength is pumped to the EDF.The one-way direction of laser operation in the ring cavity is established using an isolator.To observe the laser performances, about 10% of the laser intensity is tapped out from the laser cavity via a 10-dB optical coupler.The proposed D-SMS fiber structure is integrated into the laser cavity, between the isolator and optical coupler as in the insert image of Figure 1(a).Figure 1(b) shows the length of the d-shape fiber that has been polished and the length of the d-shape region is 1.75 mm with 0.08 mm remaining core and cladding.The total laser cavity length is approximately 7 m long.The continuous-wave spectrum of EDFL with and without D-SMS fiber structure is shown in Figure 1(c).Without D-SMS, the peak laser generated at 1564.9 nm with a cavity efficiency of 7%.Once the D-SMS fiber structure is added, the generated laser experienced a blue shift and peak lasing presence at 1562.6 nm.The total shift difference is approximately 2.3 nm, determining additional insertion loss has been induced from the proposed D-SMS fiber structure.

Laser Performance
The laser performance in this paper is reported based on the EDFL with D-SMS fiber structure, where the d-shape surface area is coated with a graphene oxide solution as a passive saturable absorber.This commercially available graphene oxide solution has a 0.05 wt% concentration of graphene oxide and approximately 56% is a carbon element.Figure 2 shows the measured output spectrum of the dualwavelength Q-switched EDFL ring cavity.At 52 mW pump power, the lasing was generated at 1562.8 nm and 1563.8 nm.The obtained 1 nm peak wavelength separation is matched with 25 cm long MMF in the D-SMS fiber structure.Thus, meets the MZI condition where the dual-wavelength was observed.To further characterize the Q-switching operation, the laser output is connected to a 300 MHz Oscilloscope via 1.2 GHz InGaAs Photodetector. Figure 3 presents the temporal performance analysis of dual-wavelength Q-switched EDFL from 45 mW to 111 mW pump power.The repetition rate and pulsewidth plotting trend in Figure 3(a) show the typical trend for Q-switching operation laser (repetition rate: 34 kHz to 55 kHz; pulsewidth: 10.22 µs to 6.47 µs).The shortest pulsewidth is 6.47 µs, obtained at 111 mW pump power.Figure 4 shows the obtained dual-wavelength Q-switched EDFL has a laser efficiency of 3.4 %.The obtained output power increases from 0.87 mW to 3.11 mW, while the pulse energy relatively is from 25 nJ to 56 nJ.Stable pulsing operation starts at 52 mW pump power.The obtained dual-wavelength Qswitched EDFL has a peak power from 2.45 mW to 8.66 mW.At the maximum pump power, the maximum output power and pulse energy are 3.11 mW and 56 nJ, respectively.

Conclusion
A combination between SM-MM-SM fiber structure and d-shape structure (D-SMS) demonstrates a dual-wavelength EDFL.Once the d-shape surface area of the D-SMS fiber structure is coated with graphene oxide solution, the EDFL laser induces the Q-switching operation from 45 mW to 111 mW pump power.The stable pulsing starts at 52 mW pump power.The D-SMS fiber structure coated with graphene oxide has generated the dual-wavelength Q-switched EDFL with maximum output power, pulse energy, and peak power of 3.11 mW, 56 nJ, and 8.66 mW, respectively.

Figure 1 :
Figure 1: (a) EDFL ring configuration.Insert image is D-SMS fiber structure.(b) Microscopic image of d-shape fiber.(c) Output spectrum with and without D-SMS fiber structure at threshold pump power.

Figure 2 :
Figure 2: Output spectrum of EDFL with D-SMS fiber structure at 52 mW pump power.

Figure 4 :
Figure 4: Output power and pulse energy of EDFL.