2.1.

Microfiber-Based WS₂-SA and its Application in Passively Mode-Locked EDF Laser

We employed the PLD method to fabricate the SA device. In this process, the ${\mathrm{WS}}_2$ target was placed into a vacuum chamber where the vacuum degree was set at $5\times {10}^4\mathrm{Pa}$. A high energy Nd:YAG laser (SL II-10, Surelite) could emit $2\mathrm{mJ}/\text{pulse}$ laser beam, which was delivered into the chamber and focused on the target to inspire out of the plasma plume. When arrived to the microfiber, the W and S elements would grow on the side surface of microfiber. In the experiment, the deposition time was 2 h, and the deposition temperature was fixed at room temperature. To verify that the film was really deposited on microfiber, we executed a scanning electron microscope (SEM) on the ${\mathrm{WS}}_2$ film morphology, as shown in Fig. 2. The waist region of microfiber had a diameter of $\sim 16\mu \mathrm{m}$. The guided light in taper region would effectively penetrate into the film and would be modulated along the microfiber. Figure 2(a) shows that a layer of ${\mathrm{WS}}_2$ film clearly stuck tightly on the side of microfiber. The thickness of ${\mathrm{WS}}_2$ film was measured to be $\sim 1\mu \mathrm{m}$. Figure 2(b) shows the film morphology. It illustrates that the film was quite different from the previously reported SA with relatively uniform size and thickness by LPE method.

Fig. 2

SEM characteristic of the microfiber-based ${\mathrm{WS}}_2$-SA. (a) ${\mathrm{WS}}_2$ layer on fiber and (b) film morphology.

For checking the Raman shift property of the as-prepared materials, the Raman spectra were measured by using a Raman spectrometer (LabRAM HR Evolution) with a laser at 514 nm. Figure 3(a) shows the measured Raman spectra of ${\mathrm{WS}}_2$ film on quartz glass together with that of bare quartz glass. Notably, these peaks were also observed for the film with locations of LA(M) at $174{\mathrm{cm}}^{-1}$, 2LA(M) at $350{\mathrm{cm}}^{-1}$, ${\mathrm{E}}_{2\mathrm{g}}^1$ at $356{\mathrm{cm}}^{-1}$ and ${\mathrm{A}}_{1\mathrm{g}}$ at $420.7{\mathrm{cm}}^{-1}$, where the LA(M) and 2LA(M) are the longitudinal acoustic modes, ${\mathrm{E}}_{2\mathrm{g}}^1$ is an in-plane optical mode, and ${\mathrm{A}}_{1\mathrm{g}}$ corresponds to the out-of-plane vibrations along the $c$-axis direction of the S atoms.

Fig. 3

(a) Raman spectra of bare quartz glass and ${\mathrm{WS}}_2$ film on quartz glass. (b) Measured linear transmission (inset) and nonlinear transmission.

We also measured the linear and nonlinear transmission of the SA device, as shown in Fig. 3(b). The linear transmission was at the level of 65.6% at 1560 nm by using an ASE source (Glight, 1250 to 1650 nm) and optical spectrum analyzer (OSA). The power-dependent nonlinear transmission is the key parameter to evaluate the mode-locking ability of SA. The SA of our fiber-integrated ${\mathrm{WS}}_2$-SA was investigated by standard two-arm experiment. A homemade fs laser (central wavelength: 1562 nm, repetition rate: 22.5 MHz, pulse duration: 650 fs, average power: 12 mW) was utilized as test source, a variable optical attenuator was applied to continuously change the input optical intensity into the sample. A 50:50 optical coupler (OC) was used to split the laser into two arms with the 50% arm for power-dependent transmission measurement of SA device and the 50% arm for reference. A two-channel power meter with measuring range from $10\mu \mathrm{W}$ to 10 mW was used to measure the power. The modulation depth, saturation intensity, and nonsaturable loss were 7.8%, $189\mathrm{MW}/{\mathrm{cm}}^2$, and 25.7%, respectively. Furthermore, the SA might suffer from two-photon absorption as the transmittance declined when the intensity of input light exceeds around $450\mathrm{MW}/{\mathrm{cm}}^2$.

Figure 4 shows the schematic of mode-locked fiber laser with our ${\mathrm{WS}}_2$-SA device. The pump source was a laser diode (LD) with emission centered at 974.5 nm. A piece of 2.4 m EDF was used as the laser gain medium with absorption coefficient of $25\mathrm{dB}/\mathrm{m}$ at 980 nm (IsoGainTM I-25, Fibercore). The pump was delivered into EDF via a 980/1550 fused wavelength-division multiplexer (WDM) coupler. A polarization independent isolator (PI-ISO), placed after the EDF, was used to ensure unidirectional operation and eliminate undesired feedback from the output end facet. A fused fiber OC was used to extract 30% energy from the cavity. A polarization controller (PC), consisting of three spools of SMF-28 fiber, was placed in the ring cavity after the ISO. The ${\mathrm{WS}}_2$-SA was inserted between the PC and the WDM coupler. Apart from the gain fiber, all the fiber devices in cavity were made by SMF-28 fiber. The laser performance was observed using an OSA (Yokogawa, AQ6370B), 1 GHz digital oscilloscope (Tektronix, DPO7104C), 3 GHz RF spectrum analyzer (Agilent, N9320A) coupled with a 15 GHz photodetector (EOT, ET-3500FEXT), and an optical autocorrelator (APE, PulseCheck).

Fig. 4

Schematic of mode-locked fiber laser with ${\mathrm{WS}}_2$-SA.

Figure 5(a) shows the typical spectrum of mode-locked pulses at the pump power of 65 mW. The generated optical soliton was centered at 1560 nm with a 3 dB spectral width of 6.75 nm. The appearance of pronounced Kelly sidebands indicated that the laser was in soliton operation. The radio frequency (RF) spectrum of the laser was shown in Fig. 5(b). The fundamental repetition frequency was 19.57 MHz with an SNR of 64 dB measured with a 1 kHz resolution bandwidth (RBW). Figure 5(c) shows the autocorrelation trace. It had a full width at half maximum ($T_{\mathrm{FWHM}}$) of 609 fs, corresponding to pulse duration ($\mathrm{\Delta }t$) of $\sim 395\mathrm{fs}$ if a ${\mathrm{sech}}^2$ pulse profile was assumed. The time-band product (TBP) was calculated to be 0.328 and deviated from the transform-limited value of 0.315, indicating that some chirp was included in the output pulse that had a broader bottom. The average output power was 1.5 mW, thus the output pulse energy was about 76.6 pJ.

Fig. 5

Performance of mode-locked fiber laser. (a) Spectrum, (b) RF spectrum measured with a 1 kHz RBW, and (c) measured pulse duration.

2.2.

Fiber-Tip-Integrated WS₂-SAM and its Application in Passively Mode-Locked and Q-Switched EDF Lasers

The MST was employed to fabricate the integrated ${\mathrm{WS}}_2$-SAM. The ${\mathrm{WS}}_2$ target was with the diameter of 49 mm, thickness of 3 mm, and purity of more than 99.5%. All the coating processes occurred in vacuum with the sputtering insert argon gas at pressure of ${10}^3\mathrm{Pa}$. During the deposition process, the AC voltage was applied to be 100 W in the chamber. The sputtered tungsten and sulfur atoms were ejected out from the target and then condensed on the fiber end. The synthesis lasted around 1 h at room temperature, forming a thin ${\mathrm{WS}}_2$ film as the SA layer. Subsequently, the Au target was excited under the DC voltage for 3 min until the Au atoms formed a tightly thin film with thickness of $\sim 150\mathrm{nm}$ on the SA layer. The gold film (GF) here not only functioned as a high reflective mirror, but also as a protective barrier that isolated the inner SA layer from the contamination, corrosion, and oxidation. Figure 6(a) shows the three-dimensional (3-D) view of the fabricated sample by confocal scanning microscopy. It indicated that the device was compact. To testify whether the SA was grown on the fiber end, we took out several samples from the chamber before the Au-film fabrication, which was direct deposited ${\mathrm{WS}}_2$ film on pollution-free quartz glasses in the same conditions as production ${\mathrm{WS}}_2$-SAM. Figure 6(b) shows the SEMs of the as-prepared sample at different scales. It illustrates that a layer of ${\mathrm{WS}}_2$ film was successively deposited on the quartz glass. The film was composited by thin layer of nanoparticles with diameters from 10 nm to around 40 nm, thus we can deduce that the ${\mathrm{WS}}_2$ film were also deposited on the fiber tip.

Fig. 6

(a) 3-D image of ${\mathrm{WS}}_2$-SAM and (b) SEM of deposited ${\mathrm{WS}}_2$ film.

The Raman spectrum of the as-grown ${\mathrm{WS}}_2$ was measured by a Raman spectrometer (LabRAM HR Evolution) with laser at 488 nm. Figure 7(a) shows the characteristic Raman bands, e.g., two optical phonon modes (${\mathrm{E}}_{2\mathrm{g}}^1$ at $356{\mathrm{cm}}^{-1}$ and ${\mathrm{A}}_{1\mathrm{g}}$ at $417.5{\mathrm{cm}}^{-1}$) and typical longitudinal acoustic modes 2LA(M) at $349.8{\mathrm{cm}}^{-1}$, where the ${\mathrm{E}}_{2\mathrm{g}}^1$ is an in-plane optical mode and ${\mathrm{A}}_{1\mathrm{g}}$ corresponds to the out-of-plane vibrations along the $c$-axis direction of the S atoms. The main Raman bands agreed with the earlier reports. The linear transmission of the sample was measured in the range from 1000 to 2000 nm by using an ASE source and OSA. It was at the level of $92.8\pm 0.6\%$, and the transmittance at 1560 nm was 93.1%, as shown in Fig. 7(b). The nonlinear absorption curve gave a modulation depth of $\sim 4.48\%$, saturation intensity of $\sim 138\mathrm{MW}/{\mathrm{cm}}^2$, and nonsaturable loss of 2%, as shown in Fig. 7(b). The nonsaturable loss here might be the smallest value when compared with other ${\mathrm{WS}}_2$-SA reported in Refs. 103 and 105. We tend to believed that this remarkable improvement comes from the compactness of our device—no extra insertion loss is imported in this SAM. This design can allow the reflectivity up to 98% after the SA layer is saturable and only Au film is remained as a high reflective mirror.

Fig. 7

(a) Raman spectra of ${\mathrm{WS}}_2$ film. (b) Measured linear transmission (inset) and nonlinear transmission.

A simplest linear cavity was implemented to obtain self-starting mode-locking operation, as shown in Fig. 8. The pump source was a 976 nm LD with maximum output power of 410 mW. A 980/1550 nm WDM was utilized to deliver the 976 nm pump light into the laser cavity and extract the laser output. The cavity was constructed by fiber Bragg grating (FBG), EDF, and ${\mathrm{WS}}_2$-SAM. An FBG centered at 1560 nm was utilized as output coupler with 3-dB bandwidth of 0.2 nm and reflectivity of 88.52%. A 13 cm EDF (Liekki 110-4/125) was used as active media with absorption coefficient of $250\mathrm{dB}/\mathrm{m}$ at 980 nm. The ${\mathrm{WS}}_2$-SAM, serving as a light modulator and high reflective mirror, was spliced directly with the EDF. The other fibers in cavity were single mode fiber (Corning, SMF-28) with 16.2-cm length. At the output end, an isolator was added to eliminate undesired feedback from the output end facet.

Fig. 8

Schematic of mode-locked fiber laser with ${\mathrm{WS}}_2$-SAM.

The CW lasing state started to occur around the pump power of 6 mW, and the mode-locking operation would self-start and quickly stabilize when the pump power was beyond 30 mW. Figure 9 shows the measured spectrum, RF spectra, and pulse traces of the laser at the pump power of 410 mW. Figure 9(a) shows the spectrum of mode-locked pulses. The generated pulses are centered at 1560 nm with a 3-dB bandwidth of 0.031 nm. The mode-locking operated at a fundamental frequency of 352 MHz, corresponding well to the cavity round trip time. The RF spectrum in a 2 GHz span was also measured with RBW of 10 kHz and SNR of $\sim 61\mathrm{dB}$, as displayed in Fig. 9(b), certifying the stability of this mode-locking EDFL. Restricted by the bandwidth of our FBG, the output pulse had a width to be 1.04 ns, as shown in Fig. 9(c). Its long range stability had been characterized in a span of $1\mu \mathrm{s}$ by the inset. In the full pump range, the fundamental mode-locking state remained stable and no pulse-breaking and harmonic waves were observed on the oscilloscope. The power characteristic curve was shown in Fig. 9(d). It can be seen that the maximum output was 5.28 mW at 1560 nm.

Fig. 9

Performance of mode-locked fiber laser. (a) Spectrum, (b) RF spectrum measured with a 10 kHz RBW, (c) measured pulse duration and pulse trace, and (d) output power versus input power.

It was noted that no PC was inserted into the cavity for stabilizing the pulse, unlike that in Ref. 100. Also, the pump threshold was much lower than the mode-locking EDFL based on ${\mathrm{MoS}}_2$-PVA SA.¹⁰⁰ This is because that the high reflection of narrowband FBG and ${\mathrm{WS}}_2$-SAM was beneficial to the rapid growth of cavity energy. As a fiber-integrated photonic device, this SAM also demonstrated well-thermal stability as the EDFLs worked at the maximum pump power. It was interesting that the generated pulse was restricted in the ns regime in our experiment, which might come from the inherent property of ${\mathrm{WS}}_2$ nanoparticles. It was expected that the shorter pulse could be generated when the quality of ${\mathrm{WS}}_2$ film was improved by optimizing the deposition condition and applying the postprocessing.

The deposition condition and integrated fiber type would have great impact on the laser performance. Apart from the above ${\mathrm{WS}}_2$-SAM sample, we also integrated the ${\mathrm{WS}}_2$-SAM on a polarization-maintaining fiber (PMF, PM980). Before the deposition, the vacuum pressure specification in chamber was settled to ${10}^{-3}\mathrm{Pa}$ to remove various impurity gases. During the deposition, the RF power was fixed at 100 W. The SMF tips were coated with a thin ${\mathrm{WS}}_2$ nanomaterial functioned as SA layer during 1.5-h deposition process. Then the gold film was deposited under direct-current magnetron sputtering at the power of 80 W with $\sim 300\text{-}\mathrm{nm}$ thickness.

In this case, the same linear-cavity structure was implemented to obtain the pulse operation by the ${\mathrm{WS}}_2$-SAM on PMF, whereas the EDF was replaced by 23-cm length of Liekki 110-4/125 and the total cavity was 44-cm long. It was very interesting that the fiber laser could easily achieve the stable Q-switched pulse output at such a simple structure. The laser performance was measured and shown in Fig. 10. The Q-switched pulse started to occur around the pump power of 50 mW, and then the Q-switched operation would maintain stably even the pump power up to 600 mW. Figures 10(a) and 10(b) show the measured optical spectrum and RF spectrum at pump power of 450 mW. It can be seen that the central wavelength was at 1560 nm with a 3-dB bandwidth of 0.025 nm, while the fundamental frequency was at 296.7 kHz with an RF SNR of 40 dB under RBW of 1 kHz. Figure 10(c) plotted the typical Q-switched pulse trains. At pump of 50 mW, the pulses had an FWHM of $\sim 750\mathrm{ns}$. The duration of Q-switched pulses decreased when the pump power increased, which was agreed with Q-switching properties. At the maximum pump of 600 mW, the minimum pulse width was measured to be $\sim 160\mathrm{ns}$. Figure 10(d) shows the repetition rate and output power versus pump power. The repetition rate varied with a wide range from 91 to 318 kHz as a function of pump power. It was interesting to see that the output power almost linearly increased even when the pump power was up to 600 mW, indicating that the SAM functioned well without any extra protection at this pump level. The maximum output power was 17.3 mW, corresponding to the pulse energy of 54.4 nJ.

Fig. 10

Performance of Q-switched fiber laser. (a) Spectrum, (b) RF spectrum measured with a 1 kHz RBW, (c) measured pulse traces, and (d) output power versus input power.

For the direct comparison with the previous works using CNT/graphene/TI as Q-switcher in EDF laser, we summarized their Q-switched pulse parameters, as listed in Table 1. It is notable that our work has the largest output power or pulse energy; meanwhile, the pulse width of 160 ns is the shortest for the Q-switched EDFL with linear-cavity structure. It is expected that the pulse duration can be further shortened down to 100 ns by decreasing the length of the laser cavity and employing the high absorption gain fiber pumped by high power LD at 980 nm.

Table 1

Parameters of Q-switched EDFLs using CNT/graphene/TI and WS2-SAM.