Tungsten disulfide saturable absorbers for 67 fs mode-locked erbium-doped fiber lasers

In this paper, we demonstrate 67 fs pulse emitting with tungsten disulfide (WS2) in mode-locked erbium-doped fiber (EDF) lasers. Using the pulsed laser deposition method, WS2 is deposited on the surface of the tapered fiber to form the evanescent field. The fibertaper WS2 saturable absorber (SA) with the large modulation depth is fabricated to support the ultrashort pulse generation. The influences of the WS2 SA are analyzed through contrastive experiments on fiber lasers with or without the WS2 SA. The pulse duration is measured to be 67 fs, which is the shortest pulse duration obtained in the mode-locked fiber lasers with two dimensional (2D) material SAs. Compared to graphene, topological insulator, and other transition metal dichalcogenides (TMDs) SAs, results in this paper indicate that the fiber-taper WS2 SA with large modulation depth is a more promising photonic device in mode-locked fiber lasers with the wide spectrum and ultrashort pulse duration. © 2017 Optical Society of America OCIS codes: (160.4330) Nonlinear optical materials; (140.4050) Mode-locked lasers; (140.3510) Lasers, fiber. References and links 1. M. Aliofkhazraei and N. Ali, Two-Dimensional Nanostructures (CRC, 2012). 2. Z. W. Pan, Z. R. Dai, and Z. L. Wang, “Nanobelts of semiconducting oxides,” Science 291(5510), 1947–1949 (2001). 3. S. Z. Butler, S. M. Hollen, L. Cao, Y. Cui, J. A. Gupta, H. R. Gutiérrez, T. F. Heinz, S. S. Hong, J. Huang, A. F. Ismach, E. Johnston-Halperin, M. Kuno, V. V. Plashnitsa, R. D. Robinson, R. S. Ruoff, S. Salahuddin, J. Shan, L. Shi, M. G. Spencer, M. Terrones, W. Windl, and J. E. 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Introduction
Two dimensional (2D) material is a type of layered compounds, which are composed of the single layer or multiple layers of atoms [1].They have attracted much attention in basic and applied researches to the construction of photonic devices [2,3].In the meanwhile, fiber lasers have been applied in such fields as optical communications, nonlinear optics and laser processing [4].In all kinds of fiber lasers, passively mode-locked fiber lasers can be used to generate ultrashort pulses efficiently.There are two main techniques for passively modelocked fiber lasers: nonlinear polarization evolution (NPE) technique and saturable absorbers (SAs) technique [5,6].
Fiber lasers with the NPE technique have the advantages of ultrashort pulse duration and simple structure [4,5].However, compared to other mode locking mechanism, the relatively high pump power has been needed for the mode-locked operation.Besides, they have been influenced by environment conditions easily, and usually have the relatively high modelocking threshold [7][8][9][10][11][12][13][14][15][16][17][18][19].SAs, such as semiconductor saturable absorber mirrors (SESAMs) and 2D material SAs, can be used to overcome those disadvantages.SESAMs are relatively flexible for the design of the laser cavity, but the fabrication process of SESAMs is complicated, the price is expensive, the damage threshold is low, and the bandwidth is narrow [7,8].With the development of the material technology, 2D materials as broadband saturable absorption materials have been applied to the SAs preparation successfully, and 2D material SAs in fiber lasers have shown excellent performance [9][10][11][12][13][14][15][16][17][18][19].
In this paper, tungsten disulfide (WS 2 ), as a type of TMDs, will be used as the saturable absorption material to prepare the SAs.Compared to other work on the WS 2 SA [45], we have improved the preparation method for the WS 2 SA, which has smaller waist diameter, longer fused zone, higher nonlinearity and larger modulation depth.In order to enhance the reliability of the WS 2 SA, the WS 2 material will be deposited on the tapered fiber.Besides, to avoid being oxidized of the WS 2 , the WS 2 layer on the tapered fiber surface will be protected by the gold film.Based on the NPE and WS 2 SA technique, the mode-locked erbium-doped fiber (EDF) laser will be demonstrated, and optical properties of the WS 2 SA and fiber lasers will be measured.Although the hybrid mode-locked scheme has been reported in our previous work [12], the performance of WS 2 materials has never been verified in the hybrid mode-locked EDF lasers.Besides, the pulse duration is related to many factors, but the modulation depth plays a great role.Thus, it is necessary to enhance the modulation depth of SAs.The modulation depth of the WS 2 SA in this paper is larger than that of the Sb 2 Te 3 SA in the previous work [12].Moreover, in order to examine the influences of the WS 2 SA, the contrastive experiments with or without the WS 2 SA will be carried out.

The WS 2 SA preparation and characterization
The WS 2 SA is composed of three parts: the tapered fiber, WS 2 nanomaterial, and gold film.The SMF-28e fiber is made into the tapered fiber by the fused biconical taper devices.The waist diameter of the tapered fiber is 12 μm, and the effective length of the fused zone is 3 mm.Using the pulsed laser deposition (PLD) method, the WS 2 is deposited on the fused zone of the tapered fiber.When the lights are propagated in the tapered fiber, they will leak out in the fused zone, and interact between the evanescent fields of the propagating lights.Through controlling the length of the fused zone, we can change the interaction length between the lights and WS 2 .Besides, the different waist diameters of the tapered fibers can lead to different nonlinear effects.When the waist diameter is smaller, the nonlinearity of SAs will be stronger.In this paper, the waist diameter of the tapered fiber is 12 μm, which is quit frangible.In order to avoid being broken off and oxidized for WS 2 SAs, the WS 2 layer deposited on the tapered fiber is protected by the gold film.The microstructure and morphologies of the WS 2 SA are confirmed via scanning electron microscope (SEM) in Fig. 1.We can see that the fiber is the tapered fiber in Fig. 1(a), and there are some materials on the surface of the tapered fiber.Figure 1(b) is the enlarged part of the film surface of the tapered fiber.Wherein, it is found that the film surface of the WS 2 SA is uniformly covered with ~40 nm monodisperse WS 2 nanoparticles, indicating the superior dispersibility of the WS 2 .In order to understand the atomic arrangement and measure the phonon spectrum of the WS 2 for the SA, we measure the Raman spectra as shown in Fig. 2

Experimental setup
Fig. 4. Configuration of the passively mode-locked EDF laser with the fiber-taper WS 2 SA.LD is the laser diode, WDM is the wavelength-division multiplexer, SMF is the signal mode fiber, Col. is the collimator, QWP is the quarter wave plate, HWP is the half wave plate, PBS is the polarization beam splitter, ISO is the polarization-dependent isolator, and EDF is the erbiumdoped fiber.).The free-space length of the ring cavity is about 22 cm.The net cavity dispersion is difficult to be determined because of the hybrid structure of the fiber-taper WS 2 SA, which is composed by the inner tapered fiber and the outer coated WS 2 film.Without the fiber-taper WS 2 SA, the fiber laser is a typical setup of fiber lasers with the NPE technique.In this paper, the fiber-taper WS 2 SA is added between the EDF and WDM.In this case, the mode-locked scheme of the fiber laser is a hybrid mode-locked scheme with the NPE and WS 2 SA technique.Compared to the typical NPE fiber laser, the WS 2 SA in our fiber laser can be acted as the component to narrow pulse duration and reduce the mode-locked threshold.

Experimental results
Using the WS 2 SA, an all-fiber passively mode-locked EDF laser has been demonstrated without the NPE technique [45].In this section, we mainly discuss the characteristics of EDF lasers with or without the WS 2 SA technique in Fig. 4, and highlight the importance of the WS 2 SA with the large modulation depth in the hybrid mode-locked EDF laser when the preparation method for the WS 2 SA is improved.

1) EDF laser with NPE and WS 2 SA technique:
With NPE and WS 2 SA technique, the EDF laser can be mode-locked with the 87 mW pump power, which is the modelocked threshold for our fiber laser.With 680 mW pump power and using an optical spectrum analyzer (Yokogawa AQ6315A), the central wavelength of the optical spectrum is measured to be 1540 nm, and the 3 dB spectral width is 114 nm in Fig. 5(a).With an optical intensity autocorrelator (Femtochrome, FR-103XL), the pulse duration is measured to be 67 fs as shown in Fig. 5(b).Due to the high pump power, there is a small pedestal on the autocorrelation trace of

2) EDF laser with NPE technique:
Only with the NPE technique, the EDF laser can also be mode locked.The problem "what role does the WS 2 SA play?" will be generated.
In order to answer this question, we will use the SMF-28e fiber to replace the WS 2 SA under the same repetition rate.The corresponding results are presented in Fig. 6.The central wavelength of optical spectrum is a slight red shift to 1549 nm, and the 3 dB spectral width is reduced to 82 nm in Fig. 6(a).Correspondingly, the pulse duration increases to 95 fs, the SNR decreases to 85 dB, and the timing jitter increases to 950 fs.According to those results, we find that the WS 2 SA can be used to compress the pulse duration, increase the SNR, and decrease the timing jitter to enhance the stability of the fiber laser.

3) EDF laser with NPE and tapered fiber technique:
In order to further emphasize WS 2 's influence on the fiber-taper WS 2 SA, we replace the fiber-taper WS 2 SA with the same tapered fiber in Fig. 5, that is, the waist diameter is 12 μm, and the effective length of the fused zone is 3 mm.Figure 7 presents the corresponding measurement results.Compared to Fig. 6, the performances of the fiber laser continue to fall.The 3 dB spectral width is only 44 nm, the pulse duration becomes wider from 95 fs to 108 fs.Besides, the SNR continue to change as low as 80 dB.Due to the addition of the tapered fiber, the timing jitter increases to 15 ps.Those results indicate that the WS 2 has great influence on the WS 2 SA during the interaction between evanescent fields of propagating beams.According to above three experiments, we carry out the contrastive investigations on fiber lasers with or without the WS 2 SA, and the corresponding results are listed in Table 1.The enhancement of SNR indicates that the addition of the WS 2 SA can improve the stability of fiber laser.In addition, due to the large modulation depth, it is easier to get the shorter pulse than ever before.Furthermore, compared to other 2D material SAs in fiber lasers in Table 2, the 3 dB spectral width of this fiber laser is the widest, the SNR is the highest, the modulation depth is the largest, and the pulse duration is the shortest.Although 37.4 fs pulse generation from the EDF laser has been reported, the direct output pulse has been measured to be sub-100 fs [64], which is wider than 67 fs reported in this paper.Thus, the WS 2 SA with the large modulation depth is the excellent photonic device to shorten the pulse duration and optimize the related indicators of mode-locked fiber lasers.

Conclusion
In this paper, the mode-locked EDF laser with the large modulation depth WS 2 SA has been demonstrated.With the PLD method, the fiber-taper WS 2 SA has been prepared.The modulation depth has been measured to be 35.1% with the balanced twin-detector method.
The contrastive investigations on fiber lasers with or without the WS 2 SA have been conducted.With the WS 2 SA, the mode-locked pulse with 114 nm spectral width, 67 fs pulse duration, 93 dB SNR and 290 fs timing jitter has been obtained.To our knowledge, for modelocked fiber lasers with 2D material SAs (such as graphene SAs, topological insulator SAs, and other 2D TMDs SAs), the 3 dB spectral width is the widest, the pulse duration is the shortest, the SNR is the highest, and the modulation depth is the largest in this paper.Results are helpful to exploit new photonic devices for fiber lasers with the wide spectrum, ultrashort pulse duration and high stability.

Funding
We

Fig. 1 .
Fig. 1.(a) The SEM image for the taper waist of the WS 2 SA.The waist diameter is 12 μm, and the effective length of the fused zone is 3 mm.(b) Film surface of the WS 2 SA.

Fig. 2 .
Fig. 2. (a) Raman spectrum of the WS 2 on the fiber-taper SA.Two optical phonon modes are at 355.8 cm −1 and 418 cm −1 , respectively.The typical longitudinal acoustic mode is at 350 cm −1 .(b) Nonlinear saturable absorption of the WS 2 SA.The modulation depth is 35.1%, the saturation intensity is 19.8 MW/cm 2 , and the non-saturable loss is 59.5%.
(a).The lines observed at 355.8 cm −1 and 418 cm −1 correspond to 1 2g E and 1gA mode.The typical longitudinal acoustic mode is at 350 cm −1 .Those peaks correspond to the double-mode longitudinal acoustic feature due to the splitting in nanoparticles, in-plane inverse oscillation of W and S, and inplane inverse oscillation between two S atoms of the WS 2[61,62].All Raman characteristic peaks of the WS 2 correspond well to the previous researches[63], indicating the high purity of WS 2 nanoparticles on the fiber-taper SA.With the balanced twin-detector method in Fig.3, we measure the nonlinear saturable absorption of the WS 2 SA.The pulse source is a homemade fiber laser with 1550 nm center wavelength, 80 MHz repetition rate and 200 fs pulse duration.Variable optical attenuator (VOA) is used to control the power level of an optical pulses through rotating the button above.As shown in Fig.2(b), the modulation depth, saturation intensity and nonsaturable loss are measured to be 35.1%,22.8 MW/cm 2 and 57.9%, respectively.

Figure 4
Figure4is the schematic diagram of the fiber laser.The ring cavity includes the 976 nm laser diode (LD, 680 mW maximum pump power), wavelength division multiplexer (WDM, 980 nm/1550 nm), fiber-taper WS 2 SA, erbium-doped fiber (EDF, Liekki 110-4/125, 12 fs 2 /mm), single mode fiber (SMF, SMF-28e, −22 fs 2 /mm), two fiber collimators, two half wave plates (HWP), two quarter wave plates (QWP), polarization-dependent isolator (ISO) and polarization beam splitting (PBS).In[12], Liu et al. have introduced how to use those optical devices.The total cavity length of the fiber laser is 1.54 m, and the repetition rate is about 135 MHz.The leading fibers of the WDM, collimators, and SA are all SMFs.The lengths of the EDF and SMFs are, respectively, 32 cm and 98 cm (25 cm for the WDM, 50 cm for the collimator, and 23 cm for the WS 2 SA).The free-space length of the ring cavity is about 22 cm.The net cavity dispersion is difficult to be determined because of the hybrid structure of the fiber-taper WS 2 SA, which is composed by the inner tapered fiber and the outer coated WS 2 film.Without the fiber-taper WS 2 SA, the fiber laser is a typical setup of fiber lasers with the NPE technique.In this paper, the fiber-taper WS 2 SA is added between the EDF and WDM.In this case, the mode-locked scheme of the fiber laser is a hybrid mode-locked scheme with the NPE and WS 2 SA technique.Compared to the typical NPE fiber laser, the

Fig. 5 .
Fig. 5. Experimental results of the passively mode-locked EDF laser with the fiber-taper WS 2 SA.(a) Optical spectrum of the generated pulses.The 3 dB spectral width is 114 nm at 1540 nm.(b) Intensity autocorrelation trace with 67 fs pulse duration.(c) Radio frequency (RF) spectrum with 93 dB SNR measured with 10 kHz RBW.(d) Phase noise measurement at 135 MHz, and the timing jitter is about 290 fs.

Fig. 5 (
b).The repetition rate of the fiber laser is about 135 MHz in Fig.5(c), which is measured by a radio frequency analyzer (ROHDE & SCHWARZ FSW26).With 10 kHz resolution bandwidth (RBW), the electrical signal to noise ratio (SNR) is measured to be 93 dB.Integrated from 1 MHz down to 100 Hz, we measure the timing jitter of the fiber laser is about 290 fs in Fig.5(d), which indicates that the mode-locked state of the fiber laser is stable.

Fig. 6 .Fig. 7 .
Fig. 6.Experimental results of the passively mode-locked EDF laser with the SMF in the same length.The difference with Fig. 4 is that the fiber-taper WS 2 SA is replaced with the SMF.(a) The 3 dB spectral width is 82 nm at 1549 nm.(b) Intensity autocorrelation trace with 95 fs pulse duration.(c) RF spectrum with 85 dB SNR measured with 10 kHz RBW.(d) Phase noise measurement at 135 MHz, and the timing jitter is about 0.95 ps.

Table 2 . Comparison of mode-locked EDF lasers with SAs based on different materials. GO is the graphene oxide.
thank the funding supports by the National Basic Research Program of China (973 Program Grant No. 2012CB821304), the National Natural Sciences Foundation of China (Grant Nos.11674036, 11078022 and 61378040), and by the Fund of State Key Laboratory of Information Photonics and Optical Communications (Beijing University of Posts and Telecommunications, Grant No. IPOC2016ZT04).