Mode-locking in Er-doped fiber laser based on mechanically exfoliated Sb₂Te₃ saturable absorber

1. Introduction

Mode-locked fiber lasers are essential sources for various applications, like basic science, material processing and precise optical metrology. Ultrafast fiber lasers utilize saturable absorption effect in order to start the pulsed operation. Various types of saturable absorbers (SA), the semiconductor saturable absorber mirrors (SESAMs) [1,2], single wall carbon nanotubes (SWNTs) [3,4], graphene SAs [5–20] and graphene oxide [21–23] were extensively investigated in last years. Well-developed and optimized technology of SESAMs makes them useful in industrial and commercial lasers. However, SESAMs have also some limitations like complicated and expensive molecular beam epitaxy (MBE) manufacturing technology and relatively narrowband operation range. Those limitations can be overcome by using so called Dirac-materials as saturable absorbers. They are characterized by very broadband and flat absorption spectrum and less complicated manufacturing processes, like chemical vapor deposition (CVD) [5–8, 18–20, 24], chemical exfoliation [10–14] and mechanical exfoliation [15–17]. Graphene has been the first 2D material widely used by the laser community to obtain mode-locked operation in fiber [5–17] and bulk lasers [18–20]. The broadband operation of graphene saturable absorber has been confirmed by solid state lasers operating in spectral range from 0.8 µm to 2.5 µm [18–20], widely tunable erbium fiber laser [12] and simultaneous mode-locked operation at 1.56 µm and 1.94 µm using common SA [8]. Recently, a new class of Dirac-materials called three-dimensional topological insulators (TI) has been strongly investigated [25–28]. It was found that Bi₂Te₃, Bi₂Se₃ and Sb₂Te₃ are characterized by graphene-like electronic-band structure [28]. The very-first results on saturable absorption in Bi₂Te₃ were presented by Bernard et al. [29]. The absorption parameters of Bi₂Se₃ and Bi₂Te₃ were further investigated by C. Zhao et al. [30,31] and Lu et al. [32] The Z-scan measurements performed at 1550 nm have shown that three-dimensional TI are characterized by high modulation depth, reaching 27% (Bi₂Se₃) [30] and 28% (Bi₂Te₃) [31], saturation intensities of 0.49 GWcm^-2 and 0.48 GWcm^-2_, respectively and low non-saturable losses of 1%. Both, bismuth selenide and bismuth telluride SA manufactured using chemical methods were used to obtain mode-locked operation of erbium-doped fiber lasers [30,31]. The very first results on an erbium mode-lock fiber laser using the third topological insulator Sb₂Te₃ up till now have been presented only by the authors of the paper [33]. Comprising to graphene, where in order to obtain high modulation depth the multilayer graphene need to be used - the modulation depth but also the non-saturable losses (around 1% per layer [20]) are scaled with graphene layers number [5,20], the first results indicates that topological insulator SAs are characterized by relatively high modulation depth with low non-saturable losses. Combination of the graphene-like band structure and high modulation depth with low losses cause that topological insulators can be considered as a perspective material for mode-locked lasers.

In this paper we present an erbium-doped fiber laser mode-locked by mechanically exfoliated antimony telluride saturable absorber. The Sb₂Te₃ absorption layers were directly deposited onto fiber ferule making the resonator fully fiberized. The laser was capable of generating soliton pulses centered at 1558.6 nm with 1.8 ps duration and repetition rate of 4.75 MHz.

2. Sb₂Te₃ layers preparation and characterization

The SA was prepared by mechanical exfoliation method extensively developed for graphene [15–17]. As a base material for SA preparation a commercially available high purity Sb₂Te₃ (Goodfellow) was used (inset in Fig. 1(a)). In the first step, antimony telluride flakes were obtained by precise scraping of the Sb₂Te₃ lumps with a sharp blade. Then, the flakes were mechanically exfoliated using scotch tape to adjust the thickness. Afterwards, the antimony telluride flakes were pressed with a fiber ferule, previously ultrasonically cleaned in the presence of isopropyl alcohol. The optical microscope (Fig. 1(a)) and scanning electron microscope (Fig. 1(b)) images indicate that the transferred layer very well cover the fiber core.

 

Fig. 1 Fiber facet with transferred Sb₂Te₃ layer: a) optical microscope image (inset: high purity Sb₂Te₃ bulk material), scanning electron microscope image

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In order to characterize the transferred layer, the morphology of Sb₂Te₃ layers was investigated with atomic force microscope (AFM) in tapping mode (Veeco Nanoman V AFM with Bruker MPP-11100-10 silicon probe). The visual insight of the antimony telluride surface is presented in the 2D plot (Fig. 2(a)).It is clearly seen that the core surface is almost entirely covered by Sb₂Te₃ layer. The cross section profiles show that the multilayer Sb₂Te₃ thickness changing in the range from 40 nm to 100 nm (40-100 quintuples bonded with Van der Waals forces [28,34]). The deposited layer composition was confirmed with EDS spectrum taken using Hitachi SU 6600 scanning electron microscope equipped with a NORAN System 7 energy dispersive spectrometer (EDS) and depicted in Fig. 2(b). It consist of typical lines characteristics for antimony telluride [35,36].

 

Fig. 2 Characterization of prepared Sb₂Te₃ absorber: a) AFM scan, b) EDS spectrum, c) Raman spectrum, d) linear absorption.

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To confirm crystalline structure of the exfoliated Sb₂Te₃ layers the Raman spectrum, using Renishaw inVia Raman Microscope equipped with microscope objective with magnification x100 and laser operating at 532 nm, was measured. The spectrum presented in Fig. 2(c) contains three main peaks at 69 cm⁻¹, 112.5cm⁻¹ and 165.5cm⁻¹. Comparing our results with the theoretical calculations done by Soso et al. [37] the measured peaks correspond to A¹_1g (62 cm⁻¹), Eg² (113cm⁻¹) and A²_1g (166 cm⁻¹). The obtained results fit very well the theoretical data and are consistent with previously reported Raman spectra of mechanically exfoliated few-quintuple layers thick Sb₂Te₃ [38]. To measure the linear absorption the fiber connector with deposited material was connected with the clean one using a fiber adapter. The measurement was done in the spectral range 1200nm to 1600nm using optical spectrum analyzer and white light source. The linear absorption has been changing from 80% to 60% (Fig. 2(d)). The obtained values are higher than that reported for Bi₂Se₃ and Bi₂Te₃ [30,31] because our sample was thicker. Hence, contribution in optical absorption process from the topologically protected surface states were dominated by bulk states. It was also the main reason of the non-flat absorption characteristic.

3. Experimental setup and results

Such prepared and characterized fiber connector containing Sb₂Te₃ layers was spliced to the fiber resonator presented in Fig. 3.The all-fiber resonator consist of 70 cm long erbium doped fiber (Liekki Er80-4/125, EDF), fiber isolator, polarization controller (PC) and output coupler (OC). The resonator was counter-directionally pumped by a 980 nm laser diode via wavelength division multiplexer (WDM).

 

Fig. 3 Sb₂Te₃ SA based mode-locked laser setup.

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The laser performance was observed using optical spectrum analyzer (Yokogawa AQ6370B), 13 GHz digital oscilloscope (Agilent Infiniium DSO91304A), 7 GHz RF spectrum analyzer (Agilent EXA N9010A) and an autocorrelator.

The laser starts to operate in the mode-locked regime at the pump power of 45 mW after careful adjustment of the PC. The stable operation in fundamental mode locking was observed up to the 90 mW of pump power. For higher pump powers pulse braking was observed. In order to eliminate the Q-switching instabilities the intracavity energy was increased by adding 35 m long fiber loop. Hence, the total fiber resonator length was around 41 m. The output coupling ratio of the OC was also investigated. The fundamental, stable mode-locked operation was obtained for output coupling ranging from 80% to 1%. The highest average output power of 0.5 mW was obtained for the output coupling of 70%. Figure 4(a) shows the generated optical soliton centered at 1558.6 nm with the full width at half maximum (FWHM) bandwidth of 1.8 nm. The pulse duration measured using an optical autocorrelator was 1.8 ps, assuming a sech² pulse shape (Fig. 4(b)). With 1.8 nm FWHM bandwidth (0.225 THz), the Time-Bandwidth Product (TBP) is equal to 0.405 in comparison to the 0.315 for sech² transform limited pulse.

 

Fig. 4 Generated optical solitony centered at 1558.6 nm: a) optical spectrum with indicated 3dB bandwidth. Inset: optical spectrum recorded with 30 nm span, b) 1.8 ps pulse autocorrelation.

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The RF spectrum of the laser is depicted in Fig. 5(a).The repetition rate was 4.75 MHz. The electrical signal to noise ratio (SNR) measured with 1 kHz resolution bandwidth (RBW) at 4 MHz span was higher than 60 dB. The corresponding oscilloscope trace is depicted in Fig. 5(b). The pulses are equally spaced by approx. 210 ns, corresponding to 4.75 MHz repetition frequency and 41 m cavity length. The pulse energy and peak power calculated for the 0.5 mW average power is 105 pJ and 58 W, respectively.

 

Fig. 5 a) RF spectrum of the mode-locked laser output measured with 4 MHz frequency span and 1 kHz RBW. Inset: spectrum in 500 MHz span, b) corresponding pulse train recorded with oscilloscope.

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4. Conclusion

Concluding, we demonstrate for the first time, to our knowledge an erbium-doped fiber mode-locked laser based on antimony telluride saturable absorber. The SA was prepared by mechanically exfoliation. The 40-100nm thick Sb₂Te₃ layer deposited onto the fiber core were fully characterized. The crystalline structure were confirm by Raman spectroscopy. Such prepared SA with linear absorption of 60% @ 1550 nm were used to picosecond pulse generation. The laser was capable to generate optical soliton centered at 1558.6 nm with the FWHM of 1.8 nm. The pulse duration and energy was 1.8 ps and 105 ps, respectively.