Mode-Locked YDFL Using Topological Insulator Bismuth Selenide Nanosheets as the Saturable Absorber

Abstract

Fiber lasers have long remained relevant for various applications worldwide in many industries. This paper presents a mode-locked ytterbium-doped fiber laser (YDFL) using our home-made topological insulator Bi₂Se₃ nanosheets (TI Bi₂Se₃) as the saturable absorber. The fabricated TI Bi₂Se₃ is transported to the end of the fiber ferrule using an optical deposition process, which is a key ingredient for initiating a pulsed fiber laser. With a pump power of 211.1 mW, the captured repetition rate and pulse width are 8.3 MHz and 6.2 ns, respectively. The length of the setup configuration is approximately 20 m, which corresponds to an output power measurement of 12.4 mW with a calculated pulse energy of 1.5 nJ. There are no significant Kelly sidebands, but the strong stability of the pulsed laser is defined by a high signal-to-noise ratio (SNR) of around 60.35 dB.

1. Introduction

Pulsed-fiber laser technology has evolved dramatically over the years due to its efficiency, accuracy, reliability, and compatibility with many applications across many industries. Q-switched or mode-locked are both types of these remarkable fiber lasers, with individual interest depending on the pulses’ repetition rates and pulse widths. Q-switched pulsed-fiber lasers typically have a kilohertz repetition rate and a nanoseconds pulse width, which is somewhat slow and large compared to mode-locked pulses. Still, they have their advantages, such as large power output and pulsed energy, suitable for use in dentistry for pain relief, environmental monitoring, and medical diagnostics [1,2]. Mode-locked pulsed lasers are now used extensively in various applications, including communications, medical treatment, medical surgery, and industrial micromachining. Mode-locked pulses have a high repetition rate in the megahertz range and narrow pulse width in the picosecond range.

Nonlinear polarization rotation (NPR) and saturable absorber (SA) are two common methods for generating mode-locked pulses [3,4]. However, the SA method attracts more attention due to its simple experimental configuration and high probability of generating the fiber laser pulses. For more than three decades, these pulses have been demonstrated with carbon-based materials such as graphene [5,6,7], carbon nanotubes (CNTs) [8,9], and silicon–germanium (SiGe) [10]. Aside from being used as the SA, they are also used to enhance the operation of field effect transistors (FETs) [11]. The discovery of graphene led to the development of other novel 2D materials, such as topological insulators (TIs) [12,13], transition metal dichalcogenides (TMDs) [14,15,16,17], black phosphorus (BP) [18,19], MXenes [20,21], and Max phase [22,23]. These two-dimensional materials possess exceptional electrical and optical properties that make them suitable for use as sensors [24], modulators [25], photodiodes [26], and transistors [27]. For example, graphene is applied to a tapered fiber to enable temperature measurement [28], and MXene is used for an all-optical wavelength converter, useful for future development of the direct optical signal-processing applications [29]. The different sample thicknesses of rare earth oxide also have been investigated as a potential saturable absorber candidate for output laser performance [30]. The latest work by Zhang et al. also shows the success of implementing the organic porous crystal structure class materials (NiO–MOF) as high-performance ultrafast lasers, achieving the harmonic mode-locking at femtoseconds level [31].

TI is an intriguing two-dimensional material with a solo Dirac cone and a big band-gap comparable to graphene. Furthermore, TI is a material that has a bulk band gap like an insulator but conducts at its surface, making it a promising candidate for basic research and applied physics. For instance, Bi₂Se₃ with ∆E~0.3 eV exhibits low saturation intensity, high modulation depth, and broadband saturable absorption properties. TI has proven to be an effective and efficient SA for initiating Q-switching and mode-locking [32,33]. The pervasive way to integrate SA into a fiber laser cavity is to place a thin film of SA between two fiber connectors. Other integration methods are used to coat 2D materials on tapered and D-shaped fibers [34]. In this work, a TI Bi₂Se₃ was deposited onto the fiber ferrule and integrated into ytterbium-doped fiber (YDF) to create a laser pulse. Mode-locking is observed at a repetition rate of 8.3 MHz and a pulse width in the picosecond range.

2. Preparation of TI Bi₂Se₃

The Bi₂Se₃ nanosheets (Sigma-Aldrich, Subang Jaya, Malaysia) were used as our saturable absorber optical component. First, 7 mg of Bi₂Se₃ nanosheets were mixed with 60 mL of isopropyl alcohol using the hot plate stirrer. This blend was stirred continuously for one day using a magnetic stirrer and further subjected to ultrasonic treatment for six hours, resulting in a Bi₂Se₃ suspension. Subsequently, this Bi₂Se₃ suspension was placed in an ultrasonic bath with a frequency of 40 kHz for about 30 min to yield a heterogenous Bi₂Se₃ mixture.

The optical deposition process was performed by injecting 60 mW of 980 nm laser at one end of the SMF-28 fiber ferrule and immersing another end onto the TI suspension for 30 min. The fiber ferrule was then removed from the TI suspension and allowed to evaporate for another 20 min. This procedure was repeated three times to improve the Bi₂Se₃ adhesion to the fiber core.

Next, the following process was carried out to simplify our SA’s characterization. The Bi₂Se₃ thin film was formed using the spin coater machine to produce the homogenous distribution of Bi₂Se₃, assisted with polyvinyl alcohol film, and later placed in an oven for further drying. Field emission scanning electron microscopy (FESEM), Raman spectroscopy, and energy-dispersive x-ray (EDX) analysis were used to characterize the SA. Figure 1 shows the results of the characterization. The FESEM image of Bi₂Se₃ synthesized from Bi₂Se₃ nanosheets revealed layers similar to the complex flakes shown in Figure 1a. The compositions of Bi₂Se₃ were analyzed by EDX and are shown in Figure 1b. According to the atomic force microscopy (AFM) topography measurement in Figure 1c, a few-layer of Bi₂Se₃ was found to have a thickness of about 3.8 nm. The Raman spectroscopy of Bi₂Se₃ in Figure 1d shows that multiple vibration modes have been assigned in three apparent peaks as indicated in Raman’s rule choices [35]: $A_{1g}^1$ mode at 72 cm⁻¹, $E_g^2$ mode at 132 cm⁻¹, and $A_{1g}^2$ mode at 173 cm⁻¹. The frequency range scanned was examined in three prominent peaks and was consistent with previously published work [36].

The nonlinear optical response property for Bi₂Se₃ was investigated using the twin balance detection method [37]. Figure 1e shows the obtained nonlinear transmission curve. The curve is fitted with the following equation T(I) = 1 − (α_s × exp(−I/I_sat)−α_ns). Where T(I) is the transmission, α_s is the modulation depth, I is the input intensity, I_sat is the saturation intensity, and α_ns is the non-saturable absorption. Upon fitting the measured experimental data to the above equation, it can be concluded that the I_sat, α_s, and α_ns for Bi₂Se₃ are 18 MW/cm², 39.8%, and 9.8%, respectively.

3. Experimental Setup

Figure 2 illustrates the experimental configuration of the proposed TI-based SA mode-locked YDFL. The gain medium used was 3-m-long YDF. The remaining optical fiber components consisted of a wavelength division multiplexer (WDM), a polarization controller (PC), an isolator, and a 95:5 coupler. A 980-nm laser diode (LD) was injected into the YDF via the 980/1064-nm WDM. At 1020 nm, the absorption of ytterbium ions was 23 dB/m, while the core and cladding diameters of the YDF used were 4 μm and 125 μm. A deposited TI Bi₂Se₃ SA is positioned between the additional SMF and PC. An additional 15-m-long SMF-28 was introduced to balance the cavity’s dispersion and nonlinearity, leading to stable mode-locking pulses in the current cavity.

The role of the isolator was to restrict the direction of light propagation, while the polarization of the light was controlled by PC. We extracted 5% of the light out from the configuration via the 95:5 coupler for monitoring and characterization. The other 95% of the light was circulated back into the laser cavity. An optical spectrum analyzer (OSA-Yokogawa AQ6370B, Tokyo, Japan, 0.02 nm resolution) was used to characterize the pulse spectrum. A photodetector 1.2 GHz bandwidth (Thorlabs DET01CFC, Bergkirchen, Germany) with a standard oscilloscope (Tektronix TDS3052C, Boston, MA, USA) was used to examine the pulse train measurement in an electrical signal form plugged into a 50:50 output coupler. Pulse stability was determined using a radio frequency (RF) spectrum analyzer. The cavity consisted of 20-m length of all-fiber components and operated in anomalous dispersion with group delay dispersion (GDD) of −0.3042 ps².

4. Results and Discussion

To examine the pulse threshold, the pump power was increased steadily. When the pump power reached 110.5 mW, the self-started mode-locking operation was observed as continuous-wave (CW) lasing broadened in the OSA. The mode-locked operation was still steadily maintained while the pump was progressively increased until a maximum level of 211 mW was reached. Figure 3 shows the mode-locked optical spectrum captured at 211 mW. The central operating wavelength of the pulse spectrum was found to be 1051.7 nm with a 3 dB bandwidth measurement of 1.04 nm. No significant Kelly sidebands were observed in the optical spectrum. Figure 4 shows the pulse train profile at a pump power of 211 mW. The full resolution of the pulse train is shown in Figure 4a, whereas the close-up train is shown in Figure 4b. The peak-to-peak spacing on the oscilloscope trace was measured at 120.5 ns, corresponding to a pulse repetition rate of 8.3 MHz.

This repetition rate was consistent with the overall configuration length and showed that the mode-locked YDFL was operating at the fundamental repetition rate. At 211 mW pump power, mode-locked operation remained stable. The magnified view of the oscillation trace showed that the pulse width was 6.2 ns. The pulse width measurement on the oscilloscope was wider compared to the real pulse width value due to the optical measurement devices’ limitations. The minimum estimated pulse width was predicted using a theoretical calculation of time–bandwidth product (TBP), assuming that the pulse shape is the sech² pulse profile. The analysis showed that the pulse width from the laser configuration should be approximately 1.11 ps.

The stability of the pulse was verified with an RF spectrum analyzer as shown in Figure 5. The pulse’s fundamental frequency at 8.3 MHz, with a signal-to-noise ratio (SNR) of about 60.35 dB was recorded. It is worth noting that the laser operation was switched to CW when SA was removed from the configuration. This verified that the fabricated SA was responsible for the mode-locking operation, and it cannot be initiated without the presence of SA.

Figure 6 represents the relationship between average laser output power and pulse energy as a function of pump powers. Both the output power and the pulse energy increase linearly with increasing pump power. At maximum available pump power of 211 mW, recorded output power is 12.4 mW, corresponding to the 1.5-nJ calculated pulse energy.

Table 1. Ultrafast mode-locked laser performances at 1 µm using various SAs.

SA Material	Integration Method	Operating Wavelength	3 dB Bandwidth	Threshold (mW)	Pulse Width	Repetition Rate	Ref.
Graphene	Thin film	1964.9 nm	0.19 nm	567 mW	0.9–6.8 ns	927 kHz	[38]
GO	Thin film	1064.1 nm	0.477 nm	137 mW	2.3 ns	1.072 MHz	[39]
Graphene	Thin film	1069.5 nm	1.29 nm	45 mW	580 ps	0.9 MHz	[40]
Graphene	Tapered fiber	1061.8 nm, 1068.8 nm	4.5 nm, 2.16 nm	263 mW	2.64 ns	1.78 MHz	[41]
GO	Thin film	1077.68 nm	1.22 nm	106.5 mW	2.0 ns	1.583 MHz	[42]
GO	Thin film	1059.7 nm	1.93 nm	45 mW	189 ps	10.05 MHz	[43]
TMD MoS2	Thin film	1029.3 nm	2.3 nm	55 mW	336.5 ps	2.025 MHz	[44]
TMD PDS2	Side-polished fiber	1033 nm	3.7 nm	160 mW	375 ps	24.4 MHz	[45]
TMD PdSe2	Thin film	1067.37 nm	5.22 nm	135 mW	767.7 ps	3.77 MHz	[46]
BP	Thin film	1085.58 nm	0.23 nm	1322 mW	26 ns	13.5 MHz	[47]
BP	Thin film	1030.6 nm	0.11 nm	200 mW	400 ps	46.3 MHz	[47]
BP	Thin film	1067.1 nm	0.11 nm	258.6 mW	77.2 ns	0.39 MHz	[48]
TI Bi2Se3	Thin film	1031.7 nm	2.5 nm	153 mW	46 ps	44.6 MHz	[49]
TI Bi2Te3	SPF	1057.82 nm	3.69 nm	200 mW	230 ps	1.44 Mhz	[50]
Ti Bi2Te3	Tapered fiber	1052.5 nm	1.245 nm	230 mW	317 ps	19.8 MHz	[51]
Ti Sb2Te3	SPF	1039.4 nm	4.25 nm	320 mW	380 ps	17.07 MHz	[52]
Ti Bi2Te3	Tapered fiber	1063.4 nm	2.24 nm	220 mW	5.47 ns	6.2 MHz	[53]
TI Bi2Se3	Thin film	1065.08 nm	0.025 nm	105 mW	398 ns	527 kHz	[54]
TI Bi2Se3	Optical deposition	1051.7	1.04 nm	110.5 mW	6.2 ns	8.3 MHz	This work

below shows the compilation of work reported on mode-locked YDFL at the 1 µm region using various SAs. SAs have proven their capability to induce ultrafast mode-locking at 1 µm, specifically graphene and TI, where they exhibit broadband saturable absorption properties. Both graphene and TI SA demonstrated the ability to generate ultrafast mode-locking at 1 µm. The threshold mode-locking in our work was relatively low at 110.5 mW compared to other TI-based SA.

5. Conclusions

A homemade fabricated TI Bi₂Se₃ SA succeeded in initiating a mode-locked YDFL. The constructed mode-locked YDFL lased stably in a laboratory environment with a basic repetition rate of 8.3 MHz and a pulse width of 6.2 ns. The maximum output power and pulse energy are 12.4 mW and 1.5 nJ, with a pump power of 211 mW, respectively. In addition, the SNR from the RF spectrum is 60.35 dB, which indicates the strong stability of the mode-locking pulse to the experimental configuration.