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Abstract
We demonstrated a mode-locked fiber laser based on a novel photonic device that combined optical microfiber coupler (OMC) and saturable absorption materials. The stable ultrafast laser was formed based on the interaction between the deposited Indium Antimonide (InSb) and the evanescent field on OMC. Different from optical microfiber (OM), OMC can directly output the mode-locked laser without additional beam splitting devices, which further improves the integrated characteristics of the fiber laser. The pulse duration of the output pulse is 405 fs at the central wavelength of 1560 nm. To the best of our knowledge, this is the first time that optical microfiber coupler based saturable absorber (OMC-SA) for mode-locked fiber laser is demonstrated.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Ultrafast fiber lasers have been widely used in optical communication [1–4], biomedicine [5], material processing [6,7] and sensing [8] because of their compact structure and cost-effective advantages. Nonlinear saturable absorber (SA) is a widely used in ultrafast laser. Currently, semiconductor saturable absorption mirrors (SESAM) [9–11] are the commonly used SAs. However, the SESAM manufacturing process is complicated and the cost is relatively high. In recent years, single wall carbon nanotubes (SWNTs) [12–19] and graphene [20–23] also show their advantages as promising candidates for SA. Materials such as transition metal sulfides (TMDs) [24,25], black phosphorus (BP) [26], transition metal oxides (TMOs) [27–30] demonstrated excellent light intensity modulation ability because of their high electron mobility.
SAs are usually integrated into fiber structures to promote the interaction between light and materials, such as D-shaped fibers [12,31], hollow-core fibers [32–34], sandwiched between two fiber connectors [35,36], and OM [13,37]. Take OM as an example, its evanescent field transmission characteristics can realize the interaction between light and material, constructing the SA. OMC also has evanescent field transmission characteristics. It has been verified to be highly sensitive to changes in environmental parameters and are therefore widely used in the field of sensing [38,39]. In addition, it has also been made into optical fiber devices such as all-optical modulators and filters [40–42]. Because these devices have the merits of simple manufactured, low cost, and easy integration with fiber systems, there are recent reports on the application of OMC in fiber lasers. In 2015, Chen et al. used OMC to assist the wavelength tuning of Q-switched lasers [43]. In 2017, Ahmad et al. proposed an OMC-based laser which can generate stable single-wavelength, dual-wavelength and three-wavelength lasers from its two output ports [44]. The mode selective coupler that uses two different optical fibers to fuse is also more and more widely used in optical fibers laser to generate orbital angular momentum beam [45,46]. OMC shows great application potential in the development of new fiber lasers because of its powerful function expansion. Unlike in sensing, OMC used in lasers are required to produce stable lasers without interference from the external environment, such as temperature. In 2020, Yu et al. fabricated graphene-OMC integrated device and applied it to multi-wavelength fiber lasers [42]. The research result shows that benefiting from the excellent heat transfer performance of graphene, the multi-wavelength laser constructed with graphene-OMC combined devices has good stability. It also shows prospects to get new devices, such as SAs, filters, all-optical modulator and tunable mode selective coupler. In this paper, we demonstrate a novel photonic device that integrates SA into OMC, the light will interact with the material in the evanescent field of the OMC.
Group III-V compounds (such as InSb, GaSb, InN, GaN) is a widely used semiconductor material. For example, GaN and GaAs were widely used in LED solid-state lighting equipment [47]. Because of their excellent optical properties, Group III-V compounds are gaining more and more attention in optical intensity modulation [48,49]. Among them, InSb has the narrowest bandgap (∼0.18 eV), which means it is suitable for infrared applications. Therefore, in this work, a combination of InSb and OMC is used to form a novel photonic device and applied to a mode-locked laser. InSb was decorated to the prepared OMC by magnetron sputtering method. Then, the microstructure of the synthesized sample was observed. We achieved stable mode-locked operation in an erbium-doped fiber laser (EDFL) based on OMC-SA at the center wavelengths of 1560 nm. The results show that, as a novel fiber structure, OMC-SA achieves a good interaction between light and material and provides a platform and optional proposal for novel fiber lasers.
2. Fabrication and characterization of OMC-SA
The OMC in this paper is made by using an improved flame brushing technique [50]. For manufacturing it, the two single mode fibers (SMF) are twisted to ensure that the two optical fibers are fixed, then fuzed and lengthened at high temperatures. Figure 1(a) shows the sketch diagram of OMC. Its waist length is 3 mm and the waist diameter of the single tapered fiber is about 11µm. Figure 1(b) shows approximate shape of the cross-section of OMC, and the a is taken as the diameter of the single tapered fiber.
Fig. 1. (a) Sketch diagram of the OMC; (b) Sketch diagram of the approximate cross section of the OMC; (c) The SEM image for OMC-SA; (d) Film surface of the InSb on the OMC-SA; (e) Raman spectrum.
Using the magnetron sputtering method, the InSb is deposited on the OMC. Magnetron sputtering has the advantages of simple and fast operation, little damage to materials, good bonding effect, great repeatability, and the film thickness can be controlled by the coating time. The microstructure and film surface of OMC-SA were observed by scanning electron microscope (SEM), which are shown in Fig. 1(c) and Fig. 1(d), respectively. The Raman spectrum acquired with the 532 nm emission of InSb on the OMC-SA is shown in Fig. 1(e). The peak at 104 cm−1 is scattering by the A mode [51]. The peak at 142 cm−1 scattering by the transverse optical and transverse acoustic (TO-TA) phonon mode [52].
The nonlinear saturable absorption of the OMC-SA was measured by the balanced twin-detector method in Fig. 2(a). In this experiment, we use port 1 and port 3 as the parts circulating in the cavity. So when the nonlinear saturable absorption is measured, port 1 and port 3 were connected into the setup. As shown in Fig. 2(b), the modulation depth (MD), saturation intensity (αs) and non-saturable loss (αns) are 1.58%, 1.749 MW/cm2 and 90.08%, respectively. We can see that SA has an enormous non-saturable loss, which is because of the excessively long interaction length. This problem can be solved by sheltering the part except for the waist zone during decoration InSb on the OMC.
Fig. 2. (a) Experiment configuration of standard two-arm transmission. (b) The nonlinear saturable absorption of the InSb SA. (c) transmission spectrum of OMC-SA before and after covering InSb.
In order to further characterize the constructed OMC-SA, the amplified spontaneous emission (ASE) source and the optical spectrum analyzer (OSA, Yokogawa AQ6370D) were connected with the OMC-SA to show the transmission spectrum. As shown in Fig. 2(c), the coupler has a flat transmission spectrum, and the splitting ratio also changes because of the change of the external refractive index after coating.
3. Experimental results and discussion
The prepared OMC-SA is applied to the fiber mode-locked laser shown in Fig. 3. The ring cavity contains a 976 nm pump source, wavelength division multiplexer (980/1550 WDM), 3 m Erbium-doped fiber (Nufern, EDFC-980-HP) as the gain fiber, polarization-independent isolator (PI-ISO) ensures unidirectional transmission, polarization controller (PC), and the InSb SA based on OMC acts as both SA and an output device. The signal light is transmitted from port1 to port3, separated to port4 and output.
The whole cavity length should be 13 m, including 10 m single-mode fiber and 3 m Erbium-doped fiber. When the pump power is above the mode-locking threshold, above 32 mW, the erbium-doped fiber laser realizes mode-locking. Figure 4(a) shows the spectrum measured by OSA (Yokogawa AQ6370D) at a pump power of 60 mW, and the optical center wavelength is about 1560 nm. It can be seen that there is a continuous-wave component in the spectrum, and the 3 dB spectrum width is about 6.46 nm when it is ignored. The bump near 1530 nm is ASE noise because of the transmission loss of the OMC-SA. The spectrum was repeatedly recorded five times in one hour, showing the spectral stability of the fiber laser. Figure 4(b) shows the pulse duration measured by the optical intensity auto-correlator (APE Pulsecheck) is 405 fs. There will be noise at the bottom because our output power is low, so the pulse has an upward trend at the edge of the window. An autocorrelation trace with wider window is shown in Fig. 4(c). The trace of the pulse train depicted on the oscilloscope (Rohde and Schwarzr RTO2024) is shown in Fig. 4(d), which shows that the pulse interval is 64.1 ns. In Fig. 4(e), the repetition frequency of the fiber laser is 15.597 MHz, and the signal-to-noise ratio (SNR) is 80 dB, measured by radio frequency spectrum analyzer (Rohde and Schwarzr FSV13). Furthermore, the illustration also shows a uniform downward broadband radio frequency spectrum. Both of them show the good stability of the output pulse. Figure 4(e) shows the trend of the output power and estimated pulse energy with the pump power. The maximum output power is 2.59 mW under 377 mW pump power. The reason the output slope of the laser is so small is that InSb is evenly decorated in all parts of the OMC, including the waist zone and the taper zone. When the mode-locked pulsed laser is output, the energy will be absorbed by InSb again lead to power reduction. This problem can also be solved by sheltering the part except for the waist zone during fabrication.
Fig. 4. Experimental results of the mode-locked EDFL with InSb OMC-SA. (a) Optical spectrum. (b) Intensity autocorrelation trace. (c) Intensity autocorrelation trace with a wider window. (d) Pulse sequence. (e) Radio frequency spectrum. (f) The variation trend of output power and estimated pulse energy with pump power.
Reference [49] used chemical vapor transport to prepare InSb and coated it on a 9µm OM with optical deposition method. In comparison, our OMC-SA has a larger loss, and we believe partially cover during coating will solve this problem in future work. Thanks to the shorter cavity length of our laser, our output pulse width is better than the 1.12ps in Ref. [49], and the signal-to-noise ratio is better than 55.7 dB in Ref. [49]. It shows that our high integration leads to fewer single-mode fibers and fewer devices.
In this experiment, a novel photonic device was constructed by decorating the SAs on the OMC. It can act as an SA and output device meanwhile in a mode-locked fiber laser. The mode-locked laser is output with no additional beam splitting device, and the fiber mode-locked laser is further integrated. We believe that all materials that can be applied in OM-SA will be suitable for OMC. We also used GaSb as a SA material in our experiments and got the mode-locked output. GaSb is also an antimonide, which is used in mode-locked lasers of various wavelengths [53,54]. We sputtered GaSb onto OMC by magnetron sputtering method, obtained OMC-SA and applied it in the fiber laser. Figure 5(a) shows the optical spectrum, which has a 3 dB bandwidth of 6.49 nm at the center wavelength of 1558.62 nm. Figure 5(b) shows the intensity autocorrelation trace with a pulse width of 396 fs. It is foreseeable that as an experiment platform, the OMC can be combined with many materials to get SAs. The experimental cases of mode-locked fiber laser based on OM will be very helpful and instructive to integrate OMC with various materials. Besides the diversity of materials, there are also many methods for materials preparation and transfer, such as pulsed laser deposition (PLD) [55] and chemical vapor deposition (CVD) [56]. The type of material and thickness determine the absorption characteristics. Choosing an OMC with appropriate microstructures such as diameter and waist length by precise control is the key to preparing OMC-SA.
Fig. 5. Experimental results of the mode-locked EDFL with GaSb OMC-SA. (a) Optical spectrum. (b) Intensity autocorrelation trace.
The OMC-SA prepared in this experiment is a novel photonic device that combines the mode-locking function and the beam splitting function. Through the improved flame brush technique, the microstructure parameters of OMC during the tapering process are precisely controlled, and OMC suitable for integration with antimonide is prepared. It has a single fiber waist diameter of 11um and a flat transmission spectrum. When the diameter of the OMC is smaller, it will have filtering characteristics. In order to integrate the filtering function into this device, materials with smaller absorption and a more optimized process level are required. Such a device will contribute to the development of multi-wavelength mode-locked lasers and wavelength-tunable mode-locked lasers. High-damage-resistant SA can be obtained by better thermal management, such as protect film. Compared with existing solutions such as sandwich-SA or OM-SA, our device will be more integrated and cost-effective. Its multi-port transmission characteristics, potential filtering characteristics [42] and all-optical modulation characteristics [40,41] provide functional scalability. It will provide an optional proposal and experimental platform for the novel fiber lasers.
4. Conclusion
In this paper, two kinds of antimonides were respectively decorated on the OMC by magnetron sputtering method, and a novel photonic device with both mode-locking and beam splitting functions was prepared. With the InSb OMC-SA, the mode-locked pulse with 6.46 nm spectral width, 405 fs pulse duration, and 80 dB SNR has been obtained. All the results show that the proposed OMC-SA has excellent performance as an ultrafast photonic device. The results are helpful to exploit novel photonic devices for fiber lasers with multi-dimensional light-field control.
Funding
Open Research Fund of State Key Laboratory of Pulsed Power Laser Technology (SKL2018KF04); State Key Laboratory of Information Photonics and Optical Communications (Beijing University of Posts and Telecommunications, IPOC2019ZZ01); Project of State Key Laboratory of Transducer Technology of China (SKT2001); National Natural Science Foundation of China (61805278); Fundamental Research Funds for the Central Universities (2019XD-A09-3)
Acknowledgments
Y. Yu and W. J. Liu are both corresponding authors, and contribute equally in this paper. We thank the College of Physics and Optoelectronic Engineering in Shenzhen University for preparing InSb and GaSb, and thank the Center of Material Science in National University of Defense Technology for characterization of SA.
Disclosures
The authors declare no conflicts of interest.
Data availability
No data were generated or analyzed in the presented research.
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