We demonstrate an all-fiber thulium-doped femtosecond laser by using microfiber coated with gold nanorods (MF-GNRs) as a saturable absorber (SA). The MF-GNR SA exhibits a broadband absorption from 1000 nm to 3000 nm. By placing the MF-GNR SA into a thulium-doped fiber laser (TDFL) cavity, a stable passively mode-locked laser with a central wavelength of ∼1943.5 nm is achieved for a threshold pump power of ∼870 mW. Furthermore, a femtosecond laser with a pulse width of ∼404 fs, a maximum average power of ∼45.5 mW, and a repetition rate of ∼25.66 MHz is obtained for a pump power of 2.1 W. The maximum average power of the laser is increased by ten times and the pulse width is reduced from several picoseconds to 404 fs compared to previously reported 2 μm mode-locked lasers based on GNR SA because of the weak photothermal effect and high laser damage threshold of MF-GNRs SA. To the best of our knowledge, this is the first time all-fiber thulium-doped femtosecond lasers based on MF-GNR SAs have been reported. These results show that MF-GNRs are promising SAs for constructing 2 μm femtosecond fiber lasers.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
All-fiber thulium (Tm)-doped fiber lasers that operate around an eye-safe wavelength range of 2 μm have been rapidly developed over the last decade due to a variety of applications including biomedical diagnostics, molecular spectroscopy, surgery, remote sensing, free-space communication and mid-infrared supercontinuum generation [1–5]. Tm3+ based active fibers have wide and smooth gain bandwidth from 1.8 to 2.1 μm, implying that ultrashort pulse laser can be generated from Tm-doped fiber lasers. Sub-picosecond pulses can be passively produced from a laser cavity through the use of a saturable absorber (SA). Until now, semiconductor SA mirrors (SESAMs) are the most common used SA for constructing mode-locked fiber lasers. However, their fabrication procedures are very complex. Hence, developing new SAs for ultrashort pulse laser generation plays an important role in the ultrafast photonics and related applications. In recent years, carbon nanotubes (CNTs) and graphene as SAs have been intensively investigated for ultrashort pulse generation at 2 μm [6–10]. At present, femtosecond mode-locked Tm-doped fiber lasers by using CNTs or graphene as SAs are also demonstrated experimentally. Furthermore, a series of two-dimensional materials, such as black phosphorus, topological insulators, and transition metal dichalcogenides (TMDs) are also used as SAs for femtosecond pulses lasers generation [11–13]. This has further led to the investigations of other forms of novel materials.
Metal nanoparticles have been successfully used in bioimaging, photothermal therapy, sensing, catalysis, surface-enhanced Raman scattering (SERS), and photonic devices due to the presence of collective oscillations of electrons called surface plasmon resonance (SPR) effect [14–17]. The SPR is particularly dependent on the metal choice and its shape as well as the size and the morphology of their assemblies. Gold nanomaterials is the most commonly used plasmonic metal because of its inert nature and its plasmonic resonances lying in the visible to infrared range, which can be tuned by altering the morphologies and size of gold nanostructures. In comparison with gold nanosphere, gold nanorod (GNR) has two SPR peaks resulting from the non-spherical symmetric structure [18–20]. One is transverse SPR (TSPR) originating from electron resonance that is perpendicular to the axial of rod. The other one is longitudinal SPR (LSPR) originating from the electron resonance that is along the axial of rod and the LSPR peaks can be tuned from visible to mid-infrared wavelength region by varying the aspect ratio of GNRs. Compared to the above SAs, GNRs also have large third-order nonlinearity, fast response time, simple manufacturing process, and good compatibility with optical fibers, indicating that it can be an ideal candidate saturable absorption material for wideband ultrashort pulse laser generation [21,22]. Up to now, mode-locked and Q-switched fiber lasers at different wavebands (1 μm, 1.56 μm, and 2 μm) by using GNRs as SAs have been demonstrated. In the majority of these works, the GNRs are fabricated into filmy composites and attached between two fiber ferrules as SAs for achieving passively mode-locked or Q-switched lasers [23–29]. In this case, the fabricating process is relatively simple and easy, but the mechanical damage by direct physical contact and the interaction between GNRs and the propagating light in the fiber core might also cause optically induced thermal damage of the GNRs SA. Moreover, short nonlinear interactions with a GNRs film in a direct interaction scheme might limit the pulsating ability of the SA, which can degenerate the laser performance. Hence, the obtained pulse durations are also at the level of few tens picoseconds. At present, 4.02 ps mode-locked pulse with a maximum average power of 6 mW at 2 μm was obtained by using GNRs as SA in our previous work. However, femtosecond laser generation at 2 μm based on GNRs SA was not realized up to now. In order to avoid the drawbacks described above, an alternative approach is to employ microfiber coated with GNRs (MF-GNRs) as SA. This scheme guarantees a long nonlinear interaction length and reduced optical power in the interaction region, potentially providing a large modulation depth and a high optical damage threshold of the GNRs SA [30–32].
In this paper, we demonstrated an all-fiber thulium-doped femtosecond laser generation by using MF-GNRs as SA. Stable passively mode-locked laser with a central wavelength of ∼1943.5 nm was achieved for a threshold pump power of ∼870 mW. Furthermore, femtosecond laser with a pulse width of ∼404 fs, a maximum average power of ∼45.5 mW, and a repetition rate of ∼25.66 MHz was obtained for a pump power of 2.1 W. The maximum average power of the laser was increased by ten times and the pulse width was reduced from several picoseconds to 404 fs compared to our previous 2 μm mode-locked lasers based on GNRs SA because of weak photothermal effect and high laser damage threshold of the MF-GNRs SA . These results show that the MF-GNRs is an effective SA for femtosecond pulse generation at 2 μm.
2. Preparation and characterization of GNRs and MF-GNRs SA
We used a seed-mediated growth method to prepare GNRs . First, gold seed solution was formed by mixing 5 mL of 0.034 mol/L HAuCl4 aqueous solution and 20 mL of 2 mol/L hexadecyltrimethyl ammonium bromide (CTAB) solution in a beaker. The mixture solution was stirred at 60°C for about 15 min. Then 1 mL of 0.0005 mol/L freshly NaBH4 solution was introduced into the above solution. The color of the solution was changed from yellow to dark brown immediately. The seed solution was kept for 1 hour at 60°C and then used for the synthesis of GNRs. Next, the growth solution was prepared by adding 0.03 mol CTAB and 0.0007 mol ortho-hydroxybenzoic acid into 31.25 ml deionized water in a baker, and the solution was vigorous stirring at 60°C. Then 1 mL of 0.004 mol/L AgNO3 solution, 0.156 mL of 0.00065 mol/L ascorbic acid aqueous solution and 31.25 mL of 0.03375 mol/L HAuCl4 aqueous solution were added into the above mentioned solution. The color of the solution was changed from orange to colorless at once. 0.226 mL of 12 mol/L HCl was used to adjust the PH value of growth solution. Finally, GNRs solution was obtained by added 0.1 mL seed solution into the growth solution and kept at room temperature for 8 hours. Figure 1(a) shows the transmission electron microscopy (TEM) image of the as-synthesized GNRs. The rod-shape nanoparticles were obtained and only a small amount of the spherical nanoparticles existed in the GNRs sample. By counting more than 100 nanoparticles, the aspect ratio varies from 5 to 9. Almost 35% of GNRs have an aspect ratio of 6.5, as can be seen from Fig. 1(b).
Taking advantage of flame-brushing technique, a microfiber was drawn from a single mode fiber. We fabricated the microfiber by stretching the fiber with heat produced by a stable flame. During this process, we monitored the waist diameter of the microfiber and the insertion loss of the SMF by using an optical microscope and a power meter, respectively. When the waist diameter was ~20 μm and the insertion loss of the SMF was about 4 dB, we stopped heating the fiber and then obtained the microfiber. The experimental setup for the fabrication of MF-GNRs SA was shown in Fig. 2(a). The microfiber was immersed into a GNRs solution droplet on a slide glass. A 1980 nm continuous wave (CW) laser was injected into the microfiber and the output power was monitored by a power meter to detect the start of GNRs deposition and consequently to control the deposition time. We stopped the light injection when the GNRs deposition loss increased by 2 dB, and the total loss was about 6 dB. Finally, the fabricated MF-GNRs SA was evaporated at room temperature for more than 24 hours. We also gave a scanning electron microscope (SEM) characterization (JEOL, 7500F) of the MF-GNRs SA at a scale of 10 μm as shown in Fig. 2(b). It can be seen that the waist diameter of the microfiber is ~20 μm. We choose some regions of the GNRs deposited on microfibers randomly. One of them was shown in the inset of Fig. 2(b). It can be seen that the GNRs are randomly arranged along the fiber direction. These results manifest the existence and the arrangement of GNRs on the microfiber. In addition, we injected the visible light into the MF-GNRs by using a red light laser diode at 635 nm, in order to intuitively observe the evanescent field which passed through the MF-GNRs SA. The evanescent field scattered by GNRs was clearly seen here, as shown in Fig. 2(c).
The absorption spectra of the as-synthesized GNRs solution and film were measured by using an ultraviolet-visible-near infrared spectrophotometer (UV-3600 Shimadzu), as shown in Fig. 3(a). The GNRs solution (the red solid line) had two absorption peaks at 532 nm and 950 nm, respectively. The absorption peak at 532 nm was caused by the TSPR of GNRs and the other one at 950 nm was caused by the LSPR of GNRs. In order to investigate the absorption characteristics at 2 μm, we dropped the GNRs solution of the same concentration on the cleaned CaF2 glass sheet. After drying in the vacuum oven for 8 hours to avoid the strong absorption between 1.4 μm and 2.1 μm of liquid water. We measured the absorption spectrum of the CaF2 glass sheet coated with GNRs (the black solid line), as shown in Fig. 3(a). In comparison with GNRs solution, the GNRs film also has two absorption peaks, but the absorption peaks have a little bit red shift and the absorption spectrum caused by LSPR shows broadening due to the aggregation and self-assembly of GNRs . The inset in Fig. 3(a) shows the photographs of GNRs solution and film. The color of the solution is red and the film is red-purple. To further examine the effects of the coated GNRs on the microfiber, we measured the absorption spectrum of the MF-GNRs SA with a supercontinuum light source (NKT Photonics Superk COMPACT) as shown in Fig. 3(b). We also used a microfiber that not coated with GNRs as a reference sample, in order to eliminate the effect of insert loss of microfiber. Interestingly, the absorption band of the MF-GNRs SA covered from 1200 to 2400 nm and had an absorption peak at 2200 nm due to the intensive interaction between GNRs and microfiber. This result shows that the MF-GNRs might be used as a SA for constructing femtosecond pulse lasers at 2 μm.
The nonlinear saturable absorption properties of the MF-GNRs SA at 2 μm were investigated by using the balanced twin-detector technique. The pump laser used for saturable absorption measurement was a home-made mode-locked fiber laser (center wavelength: ~1980 nm, repetition rate: ~37 MHz, pulse duration: ~500 fs). The dependence of the transmittance on the incident pump peak power densities of the MF-GNRs SA was shown in Fig. 4. The measurement was performed first by increasing the input power and then by decreasing the input power. The two measurement results are nearly unchanged, which confirmed the existence of saturable absorption in MF-GNRs SA. By fitting the data shown in Fig. 4 with the equation α (I) = αs /(1 + I/Is) + αns (where α (I) is the absorption coefficient, αs and αns are the saturable and nonsaturable absorption components, and I and Is are input and saturation intensities, respectively), the modulation depth, non-bleachable loss and saturation intensity were determined to be ~8.5%, ~39.2%, ~4 MW/cm2, respectively. In comparison with the GNRs film SA that reported in our previous work , the saturation intensity is decreased from 35.5 MW/cm2 to 4 MW/cm2 and the modulation depth is increased from 4.1% to 8.5%. The above results showed that the MF-GNRs SA could be used to construct ultrashort pulse mode-locked fiber lasers at 2 μm. In addition, we also synthesized GNRs with different aspect ratios, their absorption peaks were at 699 nm, 850 nm, and 1050 nm, respectively. We measured the saturable absorption properties of microfibers coated with these GNRs by using the same method, the results (saturation intensity: 3.5 MW/cm2, 4.3 MW/cm2, and 3.8 MW/cm2, Modulation depth: 8.4%, 8.7% and 8.6%) showed that the aspect ratios of GNRs only have a little effects on the saturable absorption properties. It is because that, as the aggregation of those GNRs with different aspect ratios occurs, those aggregated GNRs had similar absorption spectra caused by LSPR. In addition, the modulation depth of the MF-GNRs SA also depend on the concentration of the GNRs solution. In the future, we will adjust the modulation depth of the MF-GNRs by varying the concentration of the GNRs solution and also use the magnetron sputtering deposition method to ensure the uniform properties of the MF-GNRs SA [35,36].
3. Results and discussion
In order to investigate the performance of the as-synthesized MF-GNRs SA, a ring-cavity fiber laser incorporating the MF-GNRs SA was constructed, as shown in Fig. 5. A piece of 20 cm long single mode Tm-doped silica fiber (Nufern, SM-TSF-5/125) with a dispersion parameter of −12 ps2km−1 at 1.9 μm was employed as the laser gain medium. A 1570 nm fiber laser with a maximum output power of ~2.1 W was used as the pump source. The pump light was delivered into the laser cavity via a 1570/2000 nm wavelength division multiplexing (WDM) coupler. An isolator (ISO) and a polarization controller (PC) were installed inside the laser cavity to make sure of unidirectional operation and adjustment of the polarization states of the laser in the cavity, respectively. The propagating light inside the cavity was extracted via a 10 dB output coupler (OC). All fiber pigtails of the components we used in the experiment were made of single mode fibers (Coring SMF-28e) with a dispersion of −67 ps2km−1 at 1.9 μm. The total length of the ring cavity was ~7.8 m, corresponding to a repetition rate of 25.7 MHz. The total dispersion in the cavity was calculated to be −0.51 ps2. The output lasers were analyzed by using an optical spectrum analyzer (YOKOGAWA, AQ6375), a digital oscilloscope (Tektronix, DPO70604C), an autocorrelator (Aveata, AA-10DD) and a radio frequency (RF) spectrograph (Agilent, E4411B).
The mode-locked laser operation was achieved at the pump power of 870 mW with the help of adjustment the PC. The mode-locked laser spectrum was shown in Fig. 6(a). The spectral width at half-maximum was about 5.3 nm. Obviously Kelly bands due to the interference of dispersion waves were also observed, which was the typical feature of the solitary laser operation [37,38]. Output pulse trains of the above mode-locked laser was measured by using a digital oscilloscope, as shown in Fig. 6(b). The time interval between two adjacent pulses was about 38.97 ns, corresponding to a repetition rate of 25.66 MHz, which coincides with the above calculated value by using the length of the laser cavity (~7.8 m). The pulse laser was amplified by using a first stage Tm-doped fiber amplifier (TDFA) for obtaining high peak power to meet the criteria of the commercial autocorrelator. The TDFA was consists of a 1 m long TDF (Nufern, SM-TSF-5/125) forward pumped by a 2.1 W 1570 nm CW fiber laser. The pulse autocorrelation trace was measured after the first stage TDFA with an output power of 200 mW. Figure 6(c) shows a single pulse profile of the above mode-locked fiber laser with a sech2 fitting curve. The pulse laser duration after amplification was measured to be 404 fs. The inset in Fig. 6(c) shows the mode-locked spectrum after amplification. The spectral width at half-maximum was about 10.8 nm. Considering the pulse duration of 404 fs, the calculated time-bandwidth product was about 0.357, which was higher than that (~0.315) of the transform-limited sech2 pulses, indicating that the output pulses are slightly chirped. As we know, the output pulse width in an ultrafast laser is strongly influenced by the cavity dispersion and reaches to a minimum when the net cavity dispersion approaches zero [39,40]. In the future, we will try to obtain ultrashort pulse (<200 fs) based on MF-GNRs SA by optimizing the dispersion parameters of the laser cavity. The output power increased linearly from 2.3 mW to 45.53 mW with the increase of the pump power from 870 mW to 2.1 W, and the corresponding slope efficiency was about 3.5%, the maximum pulse energy was about 1.78 nJ. The low slope efficiency of 3.5% was mainly caused by the low output ratio of the OC. Only 10% of the total power was extracted from the laser cavity. In the future, we will increase the output ratio through changing the OC, in order to improve the slope efficiency. In addition, we measured the optical damage threshold of MF-GNRs SA by using a 1943 nm femtosecond fiber laser and the measured value was about 3.15 GW/cm2. In the future, we will optimize the evanescent effects through varying the length of microfiber, further enhance the optical damage threshold of MF-GNRs SA. Compared to previously reported 2 μm mode-locked lasers based on GNRs SA, the maximum average power of the laser was increased by ten times and the pulse width was reduced from several picoseconds to 404 fs because of weak photothermal effect and high laser damage threshold of MF-GNRs SA. This result was mainly caused by the increase of the interaction length between GNRs SA and microfiber. Thus, it might weaken the photothermal effect in GNRs efficiently, potentially providing a large modulation depth and a high optical damage threshold of the GNRs SA.
In order to confirm the stability of the mode-locked lasers, we measured the radio frequency (RF) spectrum of the MF-GNRs SA based mode-locked pulse laser by using a high resolution RF spectrum analyzer (Agilent, E4411B) as shown in Fig. 7. A strong signal peak with an electrical signal to noise ratio (SNR) of 67 dB was obtained at a fundamental pulse repetition rate of 25.66 MHz (RBW:1kHz), the inset in Fig. 7 shows the wide span RF spectrum of 500 MHz, which indicated good mode-locking stability.
In addition, long term stability is a key issue for mode-locked fiber lasers based on the SA. In order to confirm the long term operation stability of MF-GNRs SA, we measured the emission spectrum of the mode-locked fiber laser based on MF-GNRs SA every 10 min for 1.5 hours. The measured emission spectra are shown in Fig. 8. It can be seen that neither central wavelength nor output power varies. These results confirm that the MF-GNRs SA is suitable for constructing femtosecond lasers at 2 μm.
In conclusion, we experimentally demonstrated femtosecond pulse laser generation at 2 μm based on MF-GNRs SA. Stable passively mode-locked laser with a central wavelength of ∼1943.5 nm was achieved for a threshold pump power of ∼870 mW. Furthermore, femtosecond laser with a pulse width of ∼404 fs, a maximum average power of ∼45.5 mW, and a repetition rate of ∼25.66 MHz was obtained for a pump power of 2.1 W. The maximum average power of the laser was increased by ten times and the pulse width was reduced from several picoseconds to 404 fs compared to our previous 2 μm mode-locked lasers based on GNRs SA because of weak photothermal effect and high laser damage threshold of the MF-GNRs SA. Our results showed that MF-GNRs could be used as SAs for constructing ultrashort pulse laser at 2 μm.
National Natural Science Foundation of China (NSFC) (61605219, 61527823, 61378004, 61605058, 11774132 and 11474132); Science and Technology Project of Jilin Province (20160520085JH); The Opened Fund of the State Key Laboratory on Integrated Optoelectronics and Tsinghua National Laboratory for Information Science and Technology Cross-discipline Foundation; The Key Technology Research and Development Project of Jilin Province (20180201120GX); The Major Science and Technology Tendering Project of Jilin Province (20170203012GX); The Joint Foundation from Equipment Pre-research and Ministry of Education (6141A02022413); The Outstanding Young Talent Fund Project of Jilin Province (20180520188JH).
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