We report an erbium-doped fiber laser passively Q-switched by a few-layer molybdenum disulfide () saturable absorber (SA). The few-layer is grown by the chemical vapor deposition method and transferred onto the end-face of a fiber connector to form a fiber-compatible SA. The laser cavity is constructed by using a three-port optical circulator and a fiber Bragg grating (FBG) as the two end-mirrors. Stable Q-switched pulses are obtained with a pulse duration of 1.92 μs at 1560.5 nm. By increasing the pump power from 42 to 204 mW, the pulse repetition rate can be widely changed from 28.6 to 114.8 kHz. Passive Q-switching operations with discrete lasing wavelengths ranging from 1529.8 to 1570.1 nm are also investigated by using FBGs with different central wavelengths. This work demonstrates that few-layer can serve as a promising SA for wideband-tunable Q-switching laser operation.
© 2015 Chinese Laser Press
Passively Q-switched fiber lasers have received considerable attentions due to their versatile applications in optical imaging, micromachining, medicine, and telecommunications [1,2]. In recent years, different kinds of saturable absorbers (SAs) have been used in fiber lasers to achieve passively Q-switched pulses, such as transition-metal-doped bulk crystals , semiconductor saturable absorber mirrors (SESAMs) , and single-wall carbon nanotubes (SWCNTs) . Due to the compatibility with optical fibers, SESAMs and SWCNTs are widely used in passively Q-switched fiber lasers. However, SESAM is regarded as an expensive SA for passively Q-switched fiber lasers because of its complex fabrication and packaging design. Although the fabrication of SWCNT is simple and cost-effective, bandgap engineering or diameter/chirality control of the SWCNT is often necessary for realizing a broadband saturable absorption. Thus, there are always strong motivations to explore new high-performance SAs for pulsed laser systems. Graphene, a well-known two-dimensional (2D) nanomaterial, has been successfully used as an excellent SA for pulsed laser applications [6–8]. The success of graphene greatly motivates scientific researchers to seek other graphene-like nanomaterials for photonics applications.
Atomic-layered molybdenum disulfide () is a representative 2D material of transition metal dichalcogenides (TMDs) , which is now attracting continuous attention of the scientific community due to its exceptional optical properties, including saturable absorption behavior  and ultrafast carrier dynamics . Recently, taking its advantage of the excellent saturable absorption, few-layer has been successfully exploited as a promising SA used in pulsed lasers [12–18]. Woodward et al. demonstrated a passively Q-switched ytterbium-doped fiber laser (YDFL) with –polymer composite and achieved stable Q-switched pulses at 1.06 μm wavelength [13,14]. Wang et al. reported broadband few-layer SAs by introducing suitable atomic defects, which were successfully used in passively Q-switched solid-state lasers operating at 1.06, 1.42, and 2.1 μm . By using a few-layer SA, we firstly demonstrated a passively mode-locked erbium-doped fiber laser (EDFL) emitting 1.28 ps pulses at 1568.9 nm . Then, a SA was used to Q-switch an EDFL that had a short linear cavity with two fiber Bragg gratings (FBGs) as the end-mirrors . These experimental results paved the way for few-layer used as a promising broadband SA for pulsed laser applications. Most recently, Luo et al. demonstrated 1, 1.5, and 2 μm fiber lasers Q-switched by a few-layer –polymer composite as broadband SA . It should be noted that the few-layer SAs used in Refs. [12–14,18] were prepared by liquid-phase exfoliation (LPE), which contain nano-flakes with different layers, and the prepared films are not always uniform. The chemical vapor deposition (CVD) technique allows growing high-quality films with precisely controlled number of layers. However, to date, the CVD-grown few-layer as a broadband Q-switcher for pulsed fiber lasers has not yet been fully explored.
In this paper, we demonstrate a tunable Q-switched EDFL using few-layer as a SA. The CVD method was employed to grow few-layer film, which was sandwiched between two fiber connectors (FCs) with a fiber adapter to form a fiber-compatible SA. The laser cavity was constructed by using a three-port optical circulator (OC) and a FBG as the two end-mirrors. Stable Q-switched pulses were obtained with a pulse duration of 1.92 μs at 1560.5 nm. The pulse repetition rate of the laser could be widely changed from 28.6 to 114.8 kHz by increasing the pump power from 42 to 204 mW. By using FBGs with different central wavelengths, passive Q-switching operations with discrete lasing wavelengths ranging from 1529.8 to 1570.1 nm were also achieved.
2. EXPERIMENTAL SETUP OF Q-SWITCHED FIBER LASER
A schematic diagram of our compact passively Q-switched EDFL is shown in Fig. 1(a). A 0.6 m long erbium-doped fiber (EDF; Liekki Er80-8/125) was used as the gain medium. The laser was pumped by a 980 nm laser diode (LD) through a wavelength division multiplexer (WDM). Other fiber used in the cavity is the standard single-mode fiber (SMF). The total cavity length is about 2.6 m. The SA was spliced into the laser cavity for passive Q-switching. A three-port OC and a FBG were used as the two end-mirrors of the laser. The FBG with 95% reflectivity has a 3 dB spectral bandwidth of 0.1 nm and a central wavelength of 1560.5 nm. The 5% intracavity laser was output from the FBG. With the proposed laser cavity configuration, the self-mode-locking effect can be effectively suppressed by the narrow-band filtering of the FBG. Moreover, the intracavity OC was not only used as a high-reflection mirror, but also as a router for partly recycling the residual pump light and increasing the laser pumping efficiency . The laser output performance was measured using an optical power meter, an optical spectrum analyzer (Yokogawa; AQ 6370C), and a 2 GHz photodetector together with a 500 MHz oscilloscope (Tektronix; TDS3052C). The radio frequency (RF) spectrum was measured by a RF spectrum analyzer (Advantest; R3267).
Few-layer film used in this work was synthesized in a tube furnace by the CVD method as previously presented in Ref. . The Raman spectrum of the as-grown film was measured using a Renishaw 100 Raman spectrometer with 514 nm laser. As shown in Fig. 1, the film exhibited two Raman characteristic bands at 404.0 and , corresponding to the and modes, respectively . The peak frequency difference () between the and modes can be used to identify the number of layers . In our case, the value of the as-grown sample was , corresponding to a layer number of 4–5. The film was then transferred and attached onto the end-face of a FC [Fig. 1(c)]. The FC coated with few-layer was connected to another clean FC with a fiber adapter to form a SA. The SA parameters were measured by a power-dependent transmission measurement system with 250 fs pulses at a wavelength of 1550 nm, which has been elaborated in Ref. . The optical transmittance of the SA at different input powers was recorded, as shown in Fig. 2, exhibiting saturable absorption property that the transmittance increased with incident light intensity. The experimental data for power-dependent transmittance were fitted by , where is the modulation depth, is the incident intensity, is the saturation intensity, and is the nonsaturable absorbance. The parameters and were determined to be 28.5% and , respectively. Our measured saturation intensity for few-layer is much lower than those of SWCNTs  and graphene  at 1550 nm, which could be beneficial for Q-switched pulse generation in fiber lasers.
We note that the direct band gap of monolayer is about 1.8 eV (0.66 μm) while the indirect band gap of few-layer is about 1.2 eV (0.99 μm) . When irradiated by photons with an energy greater than the band gap, few-layer can be excited by absorbing one photon and exhibits saturable absorption at high excitation intensities due to the Pauli blocking effect . It seems that few-layer are beyond the application as a SA for pulsed lasers at around 1.55 μm. However, several groups have experimentally demonstrated that exhibits saturable absorption property at around 1.55 μm [10,12–18], and some mechanisms have been proposed to explain the intriguing saturable absorption observed at such waveband, such as bandgap reduction induced by introducing suitable S atomic defects  and coexistence of semiconducting and metallic states . Recently, Woodward et al. proposed a mechanism based on edge states in bandgap , which is responsible for the wideband saturable absorption exhibited by few-layer , despite operating at photon energies lower than the material bandgap. Our experimental results show that the CVD-grown few-layer has significant saturable absorption at 1550 nm. Further investigation is needed to identify the detailed mechanisms for the saturable absorption we observed.
3. RESULTS AND DISCUSSION
Continuous wave (CW) operation of the proposed laser started at a pump power of about 30 mW, and self-starting Q-switching operation was achieved when the pump power was increased to 41.8 mW. As shown in Fig. 3, typical pulse trains were achieved under different pump powers. The pulse repetition rate monotonously increases from 28.6 to 110.1 kHz by varying the pump power from 42 to 194 mW. Unlike mode-locking operation where the pulse repetition rate is determined by the cavity length, the proposed Q-switched fiber laser has a tunable repetition rate, which is dependent on the pumping strength .
Figure 4 shows typical characteristics of Q-switched pulses generated at a pump power of 102 mW. The optical spectrum [Fig. 4(a)] of Q-switched pulses is centered at 1560.5 nm and the 3 dB spectral width is about 0.06 nm. A recorded oscilloscope trace of the Q-switched pulse train is shown in Fig. 4(b). The time interval between adjacent pulses is about 14.9 μs. Figure 4(c) presents a zoom-in single pulse profile. The pulse has a symmetric intensity profile with a pulse duration (FWHM) of 2.18 μs. Figure 4(d) shows the corresponding RF spectrum with a 40 kHz span and 30 Hz resolution bandwidth. The pulse repetition rate is 66.9 kHz, matching with the pulse period of 14.9 μs. The signal-to-noise ratio (SNR) is , indicating good Q-switching stability. Moreover, as shown in the inset of Fig. 4(d), the RF spectrum in a wider span has no other frequency component except the fundamental and harmonic frequency, further confirming the stability of the Q-switching operation. We have also tested the long-term stability of the Q-switching operation at the pump power of 102 mW. In this case, the laser was turned on over 4 h in the conventional laboratory condition. Relative fluctuation of average output power was about . This indicated that the Q-switched fiber laser exhibited the good stability in the room environment.
Figure 5(a) shows the pulse repetition rate and pulse duration of the Q-switched fiber laser as functions of incident pump power. By increasing the pump power from 42 to 204 mW, the pulse repetition rate increased monotonously from 28.6 to 114.8 kHz, but the pulse duration decreased from 3.7 to 1.92 μs, which are typical features of passive Q-switching . At every specific pump power and repetition rate, the Q-switched pulse output was stable and no significant pulse-intensity fluctuation was observed on the oscilloscope. Figure 5(b) presents the average output power and correspondingly calculated pulse energy of the proposed laser versus pump power. The average output power almost increased linearly with the pump power and single-pulse energy changed from 5.8 to 8.2 nJ. Experimentally, we found that the pulse duration, and pulse energy remained almost unchanged at higher pump power (), suggesting that the SA could be obviously saturated. At the pump power of 204 mW, a minimum pulse duration of 1.92 μs was achieved, comparable to the Q-switched fiber lasers using SWCNTs , graphene , and topological insulators (TIs) . The minimum pulse duration obtain in our work could be further narrowed through shortening the cavity length and increasing the modulation depth of the -based SA [25,26].
Further increasing the pump power over 204 mW, the Q-switched pulse trains became unstable with strong amplitude fluctuation, and ultimately the Q-switching operation disappeared. This phenomenon was also observed in a Q-switched fiber laser with a TI-based SA, and explained by the thermal damage to the SAs . Subsequently, as we decreased the incident pump power below 204 mW, stable Q-switched pulses were observed again on the oscilloscope screen, which indicated that the -based SA had not been thermally damaged. In our experiment, the unstable Q-switching operation could be caused by the over-bleaching of SA under a strong pumping intensity . In addition, to check whether the passive Q-switching was attributed to the as-fabricated SA, the FC coated with few-layer was replaced with another common clean FC in the laser cavity. In this case, no Q-switched pulses were observed on the oscilloscope no matter how we adjusted the pump power in a wide range. This finding verified that the few-layer SA played a key role in passive Q-switching. We believe that the performance of Q-switched pulses emitted from our laser could be further improved by optimizing the SA parameters of few-layer and the cavity design.
To verify whether the few-layer SA could operate as a broadband passive Q-switcher, we replaced the FBG in the cavity with other ones that have the central wavelengths of 1529.8, 1539.6, 1550.1, and 1570.1 nm. At the same pump power of 102 mW, stable passively Q-switched pulses were achieved at different wavelengths ranging from 1529.8 to 1570.1 nm, as shown in Fig. 6(a). We also recorded the pulse repetition rate and pulse duration at different lasing wavelengths shown in Fig. 6(b). As the lasing wavelength varied from 1529.8 to 1570.1 nm, the repetition rate deceased from 72.4 to 62.8 kHz, and the pulse duration increased from 1.84 to 2.21 μs. This finding implied that the characteristics of Q-switching operation were wavelength-dependent, which could be attributed to the gain difference of EDF at different wavelengths . On the other hand, the central wavelength of Q-switching could be continuously tuned by applying an axial strain on the FBG. As an example, a wavelength range tunable from 1561.4 to 1564.8 nm was achieved when the FBG with a central wavelength of 1560.5 nm was used as the cavity mirror, as shown in the inset of Fig. 6(a). The tunable wavelength range can be further extended by using strain-induced tunable cascaded FBGs or highly stretchable FBGs .
We experimentally demonstrated a passively Q-switched EDFL based on a few-layer SA. The few-layer was prepared by the CVD method and sandwiched between two FCs with a fiber adapter to form a fiber-compatible -based SA. Stable Q-switched pulses at 1560.5 nm have been successfully obtained with a pulse duration of 1.92 μs and a pulse repetition rate tunable from 28.6 to 114.8 kHz. Passive Q-switched pulses at discrete lasing wavelengths ranging from 1529.8 to 1570.1 nm have been achieved. Our experimental results suggest that few-layer can serve as a promising SA for wideband-tunable pulsed laser applications.
This work was supported by the National Natural Science Foundation of China (Grant Nos. 61378028, 61475030, 61421002, and 61377037), the National Basic Research Program of China (2012CB315701), and the NCET Program (Grant No. NCET-13-0092).
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