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Abstract
We proposed and experimentally demonstrated combining a nonlinear optic saturable absorber and a wideband-tunable spectral filter in a single graphene oxide (GO) film deposited fiber optic device. The GO film was prepared on the cleaved facet of an optical fiber applying two sequential processes: the electrical deposition to make a thick GO film using an arc fusion splicer, followed by the laser pulse drilling to form a multi-layered GO film. The GO deposited fiber facet and a pristine fiber facet formed an asymmetric Fabry-Perot interferometer (FPI), whose spectral response was flexibly controlled by adjusting the air gap between them. An all-fiber ring laser cavity was built using the proposed device as a tunable saturable absorber along with erbium-doped fiber as a gain medium in the L-band. Stable Q-switching laser pulse trains were successfully generated, whose pulse duration was in the order of a few microseconds and its peak wavelength was tunable over 40nm from 1564 to 1604nm covering both C-and L-bands. At a certain condition, we also obtained Q-switching pulses simultaneously lasing at the double wavelengths, 1573.3 and 1586.7nm. Detailed device fabrication processes and laser characteristics are described to elucidate the high potential of 2-dimensional material films in nonlinear optics.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Ultrafast fiber lasers have realized novel functionalities playing important roles in telecommunications, military, medical and industrial applications, and therefore there have been continuously increasing research efforts investigating new nonlinear optical materials that can enhance laser pulse characteristics [1–4]. One of the key-devices in laser pulse generation has been the saturable absorber (SA) that selectively increases the transmission for the high peak intensity pulses enabling passive Q-switching and mode-locking [5,6]. Recently, two-dimensional (2-D) materials such as black phosphorus, transition metal dichalcogenide (TMDs), and topological insulators (TI) as well as some of the biomaterials have shown high potentials as an efficient all-fiber SA device for fiber lasers [7–10]. Among them, graphene-based SAs have been intensively investigated due to their wide spectral range of operation, high nonlinear optic coefficient, cost-effective deposition process [11–18]. Graphene oxide (GO), an oxidized form of graphene, has shown even stronger chemical and environmental stability than graphene enabling efficient mode-locking and Q-switching in various fiber lasers [19–23].
Erbium-doped fibers (EDFs) have been used as an optical gain medium in prior fiber lasers and most of the pulsed lasers have been within C-band (1530∼1565 nm) taking advantage of the inherently high gain in the band. Extension of the optical gain toward L-band (1565∼1620 nm) has enabled the wavelength division multiplexing capacity increase in optical communications, yet reports on pulsed lasers in L-band have been scarce despite various potential applications such as in-situ real-time monitoring of optical transmission links without interrupting C-band channels. It would be highly desirable to integrate both a high optical nonlinearity and a wide spectral tuning capability in a single SA device to generate laser pulse trains at a desired wavelength within the whole gain band of EDF. Dual-wavelength pulsed fiber lasers have received recent attention due to their high potentials in terahertz generation, light detection and ranging systems (lidars), and multi-modal sensor applications [24–26]. Yet, compact dual-wavelength Q-switching fiber laser in L-band has not been fully investigated.
In this study, we report a novel compact all-fiber GO thin-film device that can function both as a SA and tunable filter that enabled a wideband tunable single-wavelength Q-switching within C- and L-band. Using this device, we obtained Q-switching at the longest wavelength at λ=1604.9 nm and a single wavelength tunable over 40 nm from 1564 to 1604 nm. We further achieved a unique double wavelength Q-switching in L-band, for the first time to the best knowledge of the authors. Detailed fabrication procedures for the proposed device and its optical characteristics are explained and the Q-switching performances are discussed in the following sections.
The principle of our proposed tunable Q-switching laser scheme is illustrated in Fig. 1(a). We used a ring fiber laser cavity where EDF served as an optical gain medium and the proposed graphene oxide saturable absorber (GO-SA) provided both the optical nonlinearity and the spectral tunability. The tunable GO-SA consisted of two single mode fibers (Corning SMF-28) cleaved at 90°, GO layers are deposited one of them and the other is a pristine fiber to form an asymmetric Fabry-Perot interferometer (FPI). By varying the air gap distance (L) between the two fiber facets, the tunable GO-SA device provided both spectral tunability and optical nonlinearity at the same time. In Fig. 1(b), some of the optical characteristics of the tunable GO-SA are shown, which is the transmission of the device measured using an EDF amplified spontaneous emission (ASE) light source. It is noted that the device can provide transmission peaks centered at L-band, dual wavelengths in L-band, and C-band by varying the air gap distance. The ring laser cavity was successfully Q-switching passively and the corresponding laser spectra are schematically shown in Fig. 1(c). By combining the optical nonlinearity of GO film and a Fabry-Perot interferometer, we were able to achieve not only the tunable Q-switching in C- and L-bands but also the double wavelength Q-switching in L-band, which has never been reported.
Fig. 1. Schematic diagram of the proposed all-fiber integrated SA-tunable wavelength filter for Q-switching laser using graphene oxide saturable absorber by controlling the distance (L) between fiber end face.
In this study, we experimentally demonstrated a spectrally tunable SA that enabled Q-switching of EDF laser at the longest wavelength and tuning laser in both C-and L-band, for the first time. The fabrication method is based on laser-driven self-exfoliation which was reported by the authors [27]. After forming a thick laser of GO by applying an electric arc on the GO colloid droplet on the fiber facet, we obtained a multi-layered GO thin film by laser drilling at the fiber core and a detailed process has been described in the previous report [27]. In this work, we further optimized the liquid precursor concentration and the arc condition to minimize the insertion loss (IL) of the GO-SA, which enabled the longest wavelength Q-switching and the wide spectral tuning covering both C-and L-band. Figure 1 illustrates our proposed scheme for spectrally tunable Q-switching of an EDF laser using a GO-SA-Fabry Perot. A pristine fiber end and GO deposited fiber end form an all-fiber Fabry-Perot to tune the lasing wavelength, while the GO thin film provided the SA functionality. In the following, we will discuss in detail nonlinear transmission characteristics, and Q-switching using the GO-SA.
2. Wideband tunable GO-SA in a Fabry-Perot interferometer and its lasing characteristics in the spectral domain
Thin films of 2-D materials have been deposited on the surfaces of optical fibers by various methods such as optical deposition, drop-casting, spraying colloidal solutions, spin coating, polymer composite film, and transfer of exfoliated materials to name a few [28–31]. Recently the authors’ group developed a new technique to obtain a multi-layer GO film selectively only at the fiber core [27]. The process consisted of three processes: 1) dip coating of the precursor colloidal solution to form a single droplet on the fiber facet, 2) evaporation of the solvent and thick-film formation by applying an electric arc, and 3) laser-driven self-exfoliation to form a multi-layered GO film at the core. Our laser-driven self-exfoliation (LDSE) deposition method can provide not only the highly nonlinear optic 2-D material deposition on fiber core but also the real-time monitoring SA fabrication process to significantly increase the process reproducibility. Detailed deposition conditions and physical characterization results of GO thin film for the process using a fusion splicer are given in Ref. [27].
The proposed device consists of two fiber facets facing each other where one of them is deposited with multi-layered GO film on the core and the other is a pristine facet. The two fiber facets and the air-gap between them form an asymmetric Fabry-Perot interferometer, which can inherently provide spectral tuning capability as well as optical nonlinearity. The intensity of the FPI is given by Eq. (1)
Fig. 2. (a) Experimental set-up for measuring the spectral tuning characteristics of a cleaved fiber pair which consisted of GO deposited fiber facet and pristine facet. (b) The two fiber facets separated by a variable air-gap formed a micro Fabry-Perrot interferometer and its transmission peaks are plotted as a function of the air-gap distance, L. (c) The fiber ring laser cavity schematic diagram where the GO-SA FPI was inserted. L was varied from 8 to 80µm. (d) The laser output spectra for various Ls in the GO-SA FPI. (e) Nonlinear transmission for the GO-SA. (LD: pump laser diode at λ=980 nm OSA: optical spectrum analyzer, EDF-ASE: erbium-doped fiber amplified spontaneous emission source LD: laser diode, WDM: 980 nm/1550 nm wavelength division multiplexer, ISO: optical isolator for C-and L- band, PC: polarization controller, EDF: Erbium-doped fiber, PD: Photodetector).
Figure 2(c) schematically depicts the ring laser cavity with the proposed device implemented for both Q-switching and spectral tuning. A 1.5 m long erbium-doped fiber (EDF, Er80-8/125, Liekki) was used as the gain medium which was pumped by a 980 nm laser diode (LD) using a wavelength division multiplexer (WDM). An isolator (ISO) was inserted at the end of EDF to maintain unidirectional laser operation and a polarization controller (PC) was used to optimize the polarization conditions within the ring cavity. The laser was then extracted from the cavity using a fiber coupler with a 10% output transmission ratio. A photodetector (PD) is connected to the output coupler to convert the laser to the electrical signal. The laser operation was monitored using an optical spectrum analyzer (Agilent 86140B), a 1 GHz digital oscilloscope (Tektronix TDS 744a), and a 7 GHz radio frequency (RF) spectrum analyzer (Keysight N9320B). Figure 2(d) summarizes the spectra of the Q-switching laser for various Ls from 8 to 80µm. The corresponding laser wavelength blue-shifted from 1604.6 to 1565.6 nm with a tuning range of 40 nm.
We measured the nonlinear transmission characteristics of the prepared GO-SA sample using a mode-locked Er-doped fiber laser (EDFL) with a repetition rate of 80 MHz and a pulse width of 80fs. Figure 2(e) shows the normalized transmissions as a function of the input optical power. The modulation depth was measured to be 4.3% within the available input laser power range.
And, we observed the unique double wavelength Q-switching consistent with Fig. 2(b). Note that using a similar fiber optic GO-SA the authors’ group has achieved an efficient Q-switching at λ=1600.5 nm the longest wavelength in the L-band at the time of report [27]. By further optimizing the multi-layer GO film thickness for lower insertion loss, we were able to set a new record of Q-switching in L-band further extending the lasing wavelength at λ=1604.6 nm, In the following sections, the lasing characteristics of Q-switching pulse train in the optical spectral domain, the frequency domain, and the temporal domain are discussed in detail.
3. Tunable Q-switching fiber laser characteristics
3.1 Q-switching in the longest wavelength in the L-band
Fig. 3. Laser characteristics Q-switching fiber laser at the longest wavelength in L-band. Here the air gap distance in the GO-SA FPI was 8µm and the incident pump power was 237.4 mW for (a) the optical spectrum, (b) the RF spectrum, and (c) the oscilloscope trace. We varied the incident pump power to find out its impact on (d) the pulse repetition rate and the pulse width, (e) the average output power and the pulse energy, and (f) the peak power and duty cycle
To achieve Q-switching pulse trains at the longest wavelength in L-band, we set the air gap distance L=8µm and the insertion loss was ∼0.6 dB. The measured optical spectrum is shown in Fig. 3(a) for the incident pump power of 237.4 mW. The peak was located at λ=1604.9 nm, the longest wavelength Q-switching in L-band, and its spectral width was Δλ ∼0.55 nm. The optical spectrum did not vary significantly as the pump power decreased, due to the inherent FPI in the GO-SA to lock the lasing wavelength (see Fig. 2(b)). The corresponding RF spectrum and temporal pulse traces are shown in Figs. 3(b) and 3(c), respectively. The RF peaks corresponding to the pulse repetition rate of 20.5kHz and its harmonics were observed. The signal to noise ratio (SNR) exceeded 31.0 dB to confirm stable pulse train generation in our laser system. The Q-switching pulse trains in the time domain are shown in Fig. 3(c), where the full width at half maximum (FWHM) was measured to be 6.8µs. In contrast to the optical spectrum, the responses in the frequency and the temporal domain were significantly dependent on the pump power as summarized in Figs. 3(d)–3(f). As the pump power increased from 105.2 mW to 237.4 mW, the repetition rate increased from 6.9 to 20.5kHz, while the FWHM pulse width decreased from 14.5 to 6.8µs (see Fig. 3(d)). The average output power increased nearly linearly from 0.5 to 2.6 mW and similarly the pulse energy increased from 72.4 to 126.8nJ as shown in Fig. 3(e). The corresponding peak power increased from 4.7 to 17.5 mW, while the duty cycle increased in the range from ∼8.8 to ∼13.2% as in Fig. 3(f).
3.2 Dual-wavelength Q-switching laser in the L-band
Fig. 4. Laser characteristics Q-switching fiber laser at dual-wavelength in L-band. Here the air gap distance in the GO-SA FPI was 56µm and the incident pump power was 259.0 mW for (a) the optical spectrum, (b) the RF spectrum, and (c) the oscilloscope trace. We varied the incident pump power to find out its impact on (d) the pulse repetition rate and the pulse width, (e) the average output power and the pulse energy, and (f) the peak power and duty cycle
To achieve Q-switching pulse trains at dual-wavelength in L-band, we set the air gap distance L=56µm consistent with Fig. 2(b). IL increased to ∼1.6 dB and the optical gain shifted toward a shorter wavelength. The measured optical spectrum is shown in Fig. 4(a) for the incident pump power of 259.0 mW. The peaks were located at 1573.3 and 1586.7 nm and their spectral widths were Δλ ∼0.95 and ∼0.70 nm, respectively. The peaks in the optical spectrum maintained their locations against the pump power variation, due to the inherent FPI nature. The corresponding RF spectrum and temporal pulse traces are shown in Figs. 4(b) and 4(c), respectively. The RF peaks corresponding to the pulse repetition rate of 31.3kHz and its harmonics were observed. The SNR was 21.7 dB, which is lower than that of Fig. 3(b). Q-switching at dual-wavelength might be less stable than the single wavelength Q-switching, and we observed that the polarization controller played a critical role to maintain the Q-switching at the dual-wavelength. The polarization-dependent loss (PDL) of GO-SA is estimated to be ∼0.3 dB. Further enhancement of the signal to noise ratio could be achieved by using a polarization-maintaining fiber laser cavity. The Q-switching pulse trains are shown in the time domain in Fig. 4(c), where the full width at half maximum (FWHM) was measured to be 3.6 µs, which is the shortest among three cases discussed in this section. The laser characteristics were measured in the frequency domain and the temporal domain as the incident pump power increased from 127.2 to 259.0 mW. The results are summarized in Figs. 4(d)–4(f). The pulse repetition rate increased from 14.8 to 31.3kHz, while the pulse width decreased from 8.5 to 3.6µs as in Fig. 4(d). The average output power linearly increased from 0.6 to 1.8 mW, while the pulse energy increased from 40.5 to 57.5nJ (see Fig. 4(e)). The peak power increased from 4.5 to 15.0 mW while the duty cycle was in the range from ∼8.9 to ∼11.3% as shown in Fig. 4(f).
3.3 Single wavelength Q-switching in the C-band
As we further increased the air-gap distance of the GO-SA FPI device to L=80µm the insertion loss further increased to IL=∼2.2 dB and therefore, the lasing wavelength further shifted to the C-band edge. Here we used the incident pump power of 280.8 mW and the optical spectrum is shown in Fig. 5(a), where the peak was observed at λ=1564.9 nm along with the spectral width Δλ=0.61 nm. The corresponding RF spectrum had a peak frequency at 47.6kHz with an SNR of 31.1 dB, in Fig. 5(b). Note that SNR in the figure was slightly higher than 31.0 dB in Fig. 3(b). The Q-switching pulse trains in the temporal domain are shown in Fig. 5(c), where the full width at half maximum (FWHM) was measured to be 4.1µs. The incident pump power was varied from 149.3 to 280.8 mW and its impacts on the laser characteristics are summarized in Figs. 5(d)–5(f). The repetition rate increased from 22.5 to 47.6kHz, while the pulse width decreased from 10.4 to 4.2µs (see Fig. 5(d)). The average output power increased from 0.2 to 1.6 mW linearly, and the corresponding pulse energy increased from 9.3 to 28.4nJ, as shown in Fig. 5(e). The peak power of the laser pulse increased from 0.8 to 6.4 mW. The duty cycle varied from ∼16.5 to ∼20.6% as shown in Fig. 5(f). Our experimental results confirmed that the graphene oxide layer on the fiber core fabricated in our method can act as an efficient saturable absorber for Q-switching in the fiber laser cavity. Furthermore, by changing the air-gap distance between two optical fibers, we achieved a stable Q-switching tunable fiber laser covering both C-and L-band.
Fig. 5. Laser characteristics Q-switching fiber laser in C-band. Here the air gap distance in the GO-SA FPI was 80µm and the incident pump power was 280.8 mW for (a) the optical spectrum, (b) the RF spectrum, and (c) the oscilloscope trace. We varied the incident pump power to find out its impact on (d) the pulse repetition rate and the pulse width, (e) the average output power and the pulse energy, and (f) the peak power and duty cycle
The laser performances of previously reported tunable Q-switching EDFL using graphene-based SAs are listed in Table 1. These prior attempts are a simple combination of two separate devices in a row and they still suffer from the large formfactor and some of them induced a high insertion loss that limit EDF laser wavelength confined to C-band, where the inherently high optical gain can compensate the insertion loss. Our proposed device provided the smallest formfactor and wide spectral tunability simultaneously in Q-switching EDFL.
Table 1. Comparison of tunable Q-switching fiber laser characteristics using graphene-based SA.a
In summary, this all-fiber integrated SA-tunable filter provided both high optical nonlinearity and wide spectral tunability in an all-fiber platform, which has never been realized in experiments thus far in 2D material applications. The proposed technique can provide an all-fiber nonlinear-optic platform opening new potentials of novel materials including other 2D materials in laser physics and engineering without additional tuning components.
4 Conclusion
In conclusion, we experimentally demonstrated a Q-switching Er-doped fiber laser in the C- and L-band using a novel all-fiber tunable saturable absorber. The saturable absorber (SA) was prepared on the core of a cleaved facet of a single mode fiber using laser-induced self-exfoliation of graphene oxide (GO) multi-layer. By introducing a variable air-gap between the GO deposited fiber and pristine fiber facets, an asymmetric all-fiber Fabry-Perot interferometer (FPI) was formed, which endowed the spectral tunability. Using the shortest air gap of L=8µm in the GO-SA FPI, we achieved the longest wavelength Q-switching in L-band with the peak wavelength at λ=1604.9nm and the pulse duration of 6.8µs. By adjusting the air-gap of GO-SA FPI to L=56µm and the polarization controller in the laser cavity, we successfully obtained stable dual-wavelength Q-switching in L-band at λ=1573.3nm and 1586.7nm with a pulse duration of 3.6µs. As the air-gap further increased to 80µm, the lasing wavelength reached C-band at λ=1564.9nm with a pulse duration of 4.2µs The proposed compact all-fiber SA-FPI can adopt other nonlinear optic materials and can be used as an all-fiber platform to generate short-pulse trains.
Funding
National Research Foundation of Korea (2019R1A2C2011293); Ministry of Science and ICT, South Korea (2019R1A2C2011293).
Disclosures
The authors declare no conflicts of interest.
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