Passively Harmonic Mode-Locked Fiber Laser With a High Signal-to-Noise Ratio via Evanescent-Light Deposition of Bismuth Telluride $(\hbox{Bi}_{2}\hbox{Te}_{3} )$ Topological Insulator Based Saturable Absorber

Passively harmonic mode locking (HML) operation had been demonstrated in an erbium-doped fiber laser with a microfiber-based topological insulator (TI) $\hbox{Bi}_{2}\hbox{Te}_{3} $ saturable absorber (SA) . It was found that the pulse train possessed different orders of HML (with a tunable repetition rate from 232 to 390 MHz) due to different incident pump powers. The spectra exhibited typical features of conventional solitons (perfect Gaussian profile with Kelly sidebands) with no continuous wave component. The measured signal-to-noise ratio (SNR) reached 60 dB, whereas the pulse duration was kept around 1.32 ps without significant change. The experimental observation revealed that the microfiber-based TI device could indeed be employed as a high-performance SA for further applications in ultrafast photonics.

SECTION 1. Introduction

Due to the low reliance on polarization states of saturable absorber (SA), passively mode-locked fiber laser based on SA have recently attract great attention [1]–​ [3]. In order to search for high-performance SA, different kinds of nonlinear optical material including semiconductor saturable absorber mirrors (SESAMs), single-wall carbon nanotubes (SWCNTs), graphene, and graphene oxide (GO) have been widely exploited [4]–​ [6]. However, SESAMs are usually considered as expensive and complicated-fabrication devices; moreover, its operation bandwidth is limited at tens of nanometers [7]. Concerning SWCNTs, it needs additional band-gap engineering technologies by controlling the diameter and chirality in order to fit for special wavebands [8]. On the other hand, graphene SA, owing to its Dirac electron property, shows some advantages, such as wavelength-independent saturable absorbing characters, low saturable absorbing threshold, and large modulation depth [9]. Recently, topological insulators (TIs), as a rising material, have attracted extensive interest in the field of photonics. It was experimentally found that $\hbox{Bi}_{2}\hbox{Te}_{3}$, $\hbox{Bi}_{2}\hbox{Se}_{3}$, and $\hbox{Sb}_{2}\hbox{Te}_{3}$ are also characterized by graphene-like electronic-band structure and exhibits Dirac-like linear band dispersion [10], [11]. Bernard et al. found that TIs exhibited saturable absorption behavior around at the wavelength of 1.55 $\mu\hbox{m}$ [12]. On this basis, several different research teams had achieved ultrafast fiber laser mode-locked by inserting the TIs based SA into the laser cavity [13], [14]. However, in these reports, the TI based SA was fabricated with a quartz plate or onto the fiber end facet, which rendering the interaction length between the light and the SA very short. So the impact of the nonlinear effect of the SA on the pulse shaping was not evident. Whereas, in combination with the evanescent wave operation mechanism, Luo et al. had experimentally demonstrated the generation of 2 GHz harmonic mode-locked fiber laser by a microfiber-based $\hbox{Bi}_{2}\hbox{Te}_{3}$ SA [15]. This structure could effectively increase the interaction length between the light and the TIs. In this case, the nonlinear effect of the TI-based SA in fiber laser could be enhanced, making it very suitable for generating HML pulse. For the wide applications as astronomical frequency combs of high repetition rate pulse fiber lasers [16], such a result was of great value. However, the modulation depth of the fabricated SA device in that report was very low ( $\sim$1.7%) , and the authors did not measure laser signal-noise-ratio (SNR) , which was an important characteristic of the mode-locked fiber laser.

In this contribution, we would like to fill this gap. We have demonstrated an Erbium-doped fiber (EDF) laser passively mode-locked by microfiber-based $\hbox{Bi}_{2}\hbox{Te}_{3}$ SA. Different HML states with a repetition rate tunable from 232 MHz to 390 MHz were achieved. The spectra exhibited perfect Gaussian profile and Kelly sidebands without any CW component. Under different input pump powers, the 3-dB bandwidth of the spectra was kept at 2.1 nm without change and the pulse duration fixed at 1.32 ps unvaried. The measured SNR was 60 dB. The obtained SNR in mode locking fiber (based on nonlinear polarization rotation, SESAM film, or $\hbox{MoS}_{2}$ film) was usually between 50 dBm to 60 dBm [2], [5], [17]. The reported SNR obtained in microfiber-based mode-locked laser was 50 dBm [18]. Therefore, the obtained SBR in our experiment was on the average level in fiber lasers, but a high value in microfiber-based lasers. In addition, we had observed the mode-locking state for 4 hours, and there was no significant degradation of the laser spectra, pulse train on oscilloscope, as well as the output power. Compared with the conventional SESAM absorbers or film-based absorbers of other material, in the Evanescent-Light-based absorber, only part of the transmitting light power acted with the absorber material, and it was uneasy to damage the absorber device. So the potential lifetime of such absorber device could be extended dramatically. Our result proved that the TI base SA possessed a high performance as a nonlinear photonic device in fiber laser system.

SECTION 2. Fabrication and Characteristic Measurement of the Microfiber-Based $\hbox{Bi}_{2}\hbox{Te}_{3}$ SA

The first step was to fabricate the $\hbox{Bi}_{2}\hbox{Te}_{3}$-polyvinyl alcohol (PVA) solution. The $\hbox{Bi}_{2}\hbox{Te}_{3}$ nano-platelets were synthesized by hydrothermal intercalation. The $\hbox{Bi}_{2}\hbox{Te}_{3}$ nano-platelets were dispersed in deionized water. Sodium dodecyl sulfate power was put into the $\hbox{Bi}_{2}\hbox{Te}_{3}$ dispersion using as a surfactant. The dispersion was ultrasonically agitated for 6 hours. Then some PVA power was dissolved in deionized water with ultrasonic agitation at 90 °C for 3 hours. The $\hbox{Bi}_{2}\hbox{Te}_{3}$ dispersion and PVA solution uniformly mixed and were prepared for using. The second act was the fabrication of tapered fiber similar to those in [18]. A bare single-mode-fiber (SMF, Corning SMF-28) was heated by an alcohol flame and stretched at the same time. And the waist diameter of the fiber could be tapered down to 23.8 $\mu\hbox{m}$ as shown in Fig. 1(a). Then the microfiber was fixed on a U-shaped frame. After the preparation of the microfiber, we injected a CW laser (home-made EDF laser: operating wavelength of 1563 nm as shown in Inset-I of Fig. 1(b); output power of 18 mW) into the microfiber as shown in Fig. 1(b). We observed it with an infrared viewer, and at the same time, we tested the output power out of the microfiber with a pump power continuously. However, no evanescent wave and output power change had been observed. Then, we placed a glassplate under the microfiber; the distance between the microfiber and the surface of the glassplate was about 2 mm. After that, the $\hbox{Bi}_{2}\hbox{Te}_{3}$-PVA solution was dripped onto the glassplate until it covered the microfiber. Just at this time, we still couldn't observe evanescent wave and the output power had no distinct decreasing. However, just after several seconds, very weak evanescent wave light from the microfiber could be seen through infrared viewer, and it became brighter and brighter. Meanwhile, the monitored output power began to decrease. When the output power decreased to 16 mw, the brightness of the evanescent light no longer changed. The Inset-II of Fig. 1(b) showed the observed evanescent light-spot (the upper bright one was the evanescent light from the microfiber and the lower dim one was the corresponding image formed by the optical platform), this picture was taken through the infrared viewer. The process took only about one minute. Afterwards, we separated the microfiber from the $\hbox{Bi}_{2}\hbox{Te}_{3}$-PVA solution and cut off the CW light source. Compared with other similar reports [19]–​ [21], the deposition time was much shorter. After that, we transferred it to a microscope, and observed the situation of the $\hbox{Bi}_{2}\hbox{Te}_{3}$ deposition with a magnification of 50-fold objective lens (combination with a 10-fold eyepiece). As shown in Fig. 1(c), the deposition was a little matte and tightly surrounded the tapped fiber. The diameter of the deposition was measured to be $\sim\! 86\ \mu\hbox{m}$, the length of that was $\sim\!\! 162\ \mu\hbox{m}$. Finally, the as-fabricated microfiber-based $\hbox{Bi}_{2}\hbox{Te}_{3}$ SA was evaporated at room temperature.

Fig. 1.

Microfiber (a), the process of $\hbox{Bi}_{2}\hbox{Te}_{3}$ deposition (b), and the microfiber after $\hbox{Bi}_{2}\hbox{Te}_{3}$ deposition (c).

In order to further investigate characteristics of the fabricated microfiber-based $\hbox{Bi}_{2}\hbox{Te}_{3}$ SA, we measured its linear and nonlinear transmission curve as shown in Fig. 2. First, we injected a broadband light source into the microfiber and measured its transmittance. Since the other part of the SA device was SMF-28 fiber, it was single mode only around 1.55 $\mu\hbox{m}$, so we gave the results of the wavelengths ranging from 1300 to 1700 nm here. As shown in Fig. 2(a), the linear transmittance of the SA device was about 23.1% at the wavelength of 1564 nm. Due to the instability of the light source, the measured curve was rough and performed strong spikes at the longer wavelength. Secondly, we tested the nonlinear transmittance of the SA device with a home-made dispersion management mode-locked fiber laser whose central wavelength was $\sim$1578 nm. As shown in Fig. 2(b), the saturable absorption data of the microfiber-based SA and the corresponding fitting curve were given as a function of the injected laser power. As can be seen in Fig. 2, the modulation depth was $\sim$4.8% and the nonsaturable loss was $\sim$73.4%. Correspondingly, the inserting loss of the microfiber based SA was about 7.34 dB. Compared with the results in Ref. [15], the inserting loss was a little high, but the modulation depth had been improved distinctly.

Fig. 2.

Linear (a) and nonlinear (b) transmittance curve of the microfiber based $\hbox{Bi}_{2}\hbox{Te}_{3}$ SA.

SECTION 3. Experimental Setup

After having prepared the microfiber-based $\hbox{Bi}_{2}\hbox{Te}_{3}$ SA, we inserted the mode-locking device into the fiber laser cavity. The configuration of proposed fiber laser system was schematically shown in Fig. 3.

Fig. 3.

Schematic of the fiber laser setup.

One 976-nm single-mode laser diodes (LD) with maximum power of 650 mW was used to provide pump through a 980/1053 nm wavelength-division-multiplexer (WDM). A 10-m EDF (Nufern with core diameter of around 8 $\mu\hbox{m}$, cladding diameter of 125 $\mu\hbox{m}$, Numerical Aperture of 0.11, and Core Absorption of about 4.5 dB/m at 976 nm) acted as the gain media in the oscillator. The other fibers in the laser system together with the pigtail of the passive components were Corning SMF-28 with dispersion parameter D of 17 ps/nm/km. The total length of the oscillator was $\sim$37 m, which corresponded to a fundamental repetition of $\sim$5.5 MHz. One polarization insensitive isolators (ISO) was used to ensure unidirectional operation in the oscillator. One set of polarization controller (PC) was used in the oscillator for adjusting the linear cavity birefringence and selecting laser wavelength. A fused optical coupler (OC) with 10% output was placed after it as the output port. The fabricated microfiber device worked as the absorber. In some reports, a spectral filer was incorporated in the oscillator to reach stable mode locking [5]. What's more, the filter could be used to controlling the pulse duration, spectral width and wavelength of the output [22], [23]. However, there were also some reports that had reaching stable mode locking without a filter [2], [17]. On the other hand, in our experiment, the bandwidth of the WDM, ISO, and output coupler were 40 nm, which could also act as a filter. Therefore, we did not embed a filter in our experiment. The output prosperities of the laser oscillator and amplifier were monitored by a power meter, an optical spectrum analyzer (OSA), an autocorrelator (AC), a radio-frequency analyzer (RFA), and a 6-GHz digital oscilloscope together with a home-made 2.5-GHz photodiode detector.

SECTION 4. Experimental Results and Analysis

Under different pump power, Q-switched mode locking, soliton rains, and bunched solitons could all be obtained in this experiment. However, we only paid attention to the HML state in this report. With appropriate PC orientation, stable self-started mode-locking could be achieved when the pump power reached 80 mW. The average output power was $\sim$5.3 mW. As the blue line showed in Fig. 4(a), the mode-locked spectrum had a 3-dB bandwidth of $\sim$2.1 nm with the center wavelength of 1564 nm. The spectrum displayed smooth Gaussian profile and symmetric Kelly sidebands, which were typical characteristics of conventional solitons. Moreover, there was no CW component on the spectrum so that we could speculate that the noise in our experiment was very low. The pulse train detected with an oscilloscope was shown in Fig. 4(b), in which pulses had relatively uniform intensity and temporal interval. The corresponding autocorrelation trace exhibited smooth profile as shown in Fig. 4(c), the blue curve was the experimental result, and the red one was the Sech2-fit result. Since it was conventional soliton, the pulse should be Sech2 shape in temporal domain. The full width at half maximum (FWHM) of the autocorrelation trace was determined to be $\sim$2.06 ps, as shown. If a Sech $^{2}$ temporal profile was assumed, the pulse width was estimated as $\sim$1.34 ps, the corresponding time-bandwidth product was $\sim$0.345, which was close to the transmission limit. The RF spectra in Fig. 4(d) showed that SNR was $\sim$60 dB and the fundamental peak was located at $\sim$232.14 MHz as determined by the pulse train. It corresponded to 42 ^th HML state. Above all we could confirm that the fiber laser worked at harmonic mode locking state not at Q-switched or bunched soliton.

Fig. 4.

Optical spectra (a), oscilloscope trace (b), autocorrelation traces (c), and RF spectra (d) of the mode-locked conventional soliton.

When increasing the pump power to 120 mW, the laser oscillator maintained mode-locking. Fig. 4(a) showed the output spectra at different pump power. It was obvious that the spectral intensity broadened monotonously with the enhancement of pump power, while the 3-dB bandwidth kept almost unchanged. The measured pulse duration was still $\sim$1.34 ps without distinct change. The pulse intensity became more uniform. The pulse interval narrowed and the pulse repetition rate, respectively, increased to $\sim$314.47 MHz ( $\hbox{Pump}=100\ \hbox{mW}$) and $\sim$390.02 MHz ( $\hbox{Pump}=120\ \hbox{mW}$), as shown in Fig. 5. They corresponded to 57th and 71st HML states.

Fig. 5.

Pulse trains when pump power was 100 mW (a) and 120 mW (b).

Further increasing or decreasing the pump power out of the stable mode-locking region, CW component would arise on the spectra as shown in Fig. 6, and the pulse train became unstable.

Fig. 6.

Spectra when pump power was 150 mW (red line) and 60 mW (blue line).

SECTION 5. Conclusion

In summary, we had demonstrated a passively HML fiber laser by inserting a microfiber-based $\hbox{Bi}_{2}\hbox{Te}_{3}$ SA. Taking advantage of the high nonlinearity and SA effect introduced by the SA device, HML operation could be easily initiated. With different input power, the repetition rates could vary from 232 MHz to 390 MHz, which respectively corresponded to the 42nd and 57th HML states. The SNR was measured as 60 dB, which proved the high performance of the SA device in our fiber laser.