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We experimentally demonstrate femtosecond harmonic mode-locking of a fiber laser using a bulk-structured Bi2Te3 topological insulator (TI)-deposited on a side-polished fiber as a mode-locker. A bulk-structured Bi2Te3 TI film was prepared at a thickness of ~20 μm using a mechanical exfoliation method. Using the mode-locker in an erbium-doped fiber ring cavity, it was experimentally shown that harmonically mode-locked pulses with temporal widths of 630 ~700 fs could readily be generated upto the 55th harmonics. The pulse repetition rate was shown to be tunable from the fundamental resonance frequency of 14.07 MHz to the harmonic frequency of 773.85 MHz with increasing pump power. The pumping efficiency was measured at ~3.36 MHz/mW. The side mode suppression ratio (SMSR) was observed to be more than 27.3 dB over all harmonic orders, while the corresponding signal-to-noise ratio (SNR) ranged from 46.3 to 63 dB.
© 2015 Optical Society of America
1. Introduction
Passively mode-locked lasers have great potential in many applications such as fundamental scientific research, material processing, medicine, and sensing [1]. In particular, mode-locked lasers based on optical fiber have distinctive merits, such as alignment-free operation, excellent stability, and high beam quality [2, 3] compared to their solid state counterparts. For the generation of passively mode-locked optical pulses from a cavity, a nonlinear transfer function that allows only high intensity beams to be transmitted without being absorbed needs to be incorporated into the cavity. It is well known that this type of nonlinear transfer function can be realized by using the nonlinear polarization rotation (NPR) effect within a fiberized cavity incorporating a polarizer [4] or using a nonlinear optical loop mirror (NOLM) [5].
Even if mode-locked pulses can be readily produced using the aforementioned methods the requirement of precise polarization adjustment, which renders the mode-locking operation vulnerable to environmental changes, makes those methods less preferable in practical and commercial applications. On the other hand, a single passive device called a “saturable absorber” is very attractive since it can provide the required nonlinear transfer function irrespective of beam polarization status. The commonly used saturable absorbers are based on semiconductors and they have been widely used in the field of ultra-fast pulsed lasers [1, 6, 7]. Recently, carbon nanomaterials such as carbon nanotubes (CNTs) and graphene have been found to have saturable absorption properties comparable or superior to semiconductors. Over the past decade scientific and practical investigations on the use of saturable absorbers, based on CNTs and graphene for stable ultra-fast pulsed lasers, have been intensively conducted due to their outstanding properties such as ultrafast recovery time, low saturable absorbing threshold, and large modulation depth [8–29]. Between the two, graphene has been regarded as the better material platform for high performance saturable absorbers due to its very low loss and boundless operating bandwidth [30, 31].
Recently, another Dirac material, called a “topological insulator (TI)” has been receiving a great amount of attention in the field of ultrafast photonics because TIs were found to exhibit substantial nonlinear saturable absorption [32–35]. As a matter of fact, TIs have been a hot topic in the field of condense-matter physics due to their unique electronic properties [36, 37]. TIs are known to have an ordinary insulating band gap in the bulk, whereas their gapless edge or surface has protected conducting states. So far, a variety of materials such as HgTe, Bi1-xSbx, Bi2Se3, Bi2Te3, and Sb2Te3 have been experimentally identified as topological insulators [38–40]. The use of such unique electronic materials for ultrafast photonics was proposed by Bernard et al. in 2012 [32]. Since then, quite a few experimental studies have been conducted to figure out the applicability TIs for the implementation of high performance, wideband saturable absorbers suitable for passive mode-locking or Q-switching operation in lasers [33–35, 41–48]. Furthermore, another 2-dimensional material of MoS2 has also emerging as a new saturable absorption material [49].
One interesting topic in the field of ultrafast lasers is the realization of harmonic mode-locking in a passive way since harmonic mode-locking enables us to overcome the repetition rate limit of conventional mode-locked lasers operating at a fundamental resonance frequency [50, 51]. High repetition-rate-pulsed fiber lasers can be useful in some applications such as optical fiber communications and astronomical spectroscopy [52, 53]. It is known that harmonically mode-locked pulses are produced from a cavity since pulse splitting occurs when large photon energies exists within the cavity [50]. It is believed that the energy quantization effect is the main reason for the pulse splitting [51]. Initially, multiple soliton pulses move randomly; however, they are evenly distributed in the cavity (with the same pulse characteristics) under a certain cavity conditions and beam polarization states. Many investigations into passive harmonically mode-locked fiber lasers have been conducted both experimentally and theoretically [54, 55]. It was demonstrated that saturable absorbers based on CNTs and graphene could be used for generating harmonic mode-locked pulses [56–60]. There have also been three experimental reports on the use of TI-based saturable absorbers for harmonic mode-locking [61–63]. Even if it is agreed that those three works, based on high quality nano-structured TIs, successfully demonstrated the great potential of TI-based saturable absorbers as harmonic mode-lockers, it should be noted that those three lasers produced limited output temporal width performance of ~2.49 [61], ~2.2 ps [62], and ~1.3 ps [63]. The limited temporal performance is attributable to unoptimized cavity dispersion. Note that femtosecond pulse output performance has been readily demonstrated in the case of harmonically mode-locked lasers using CNTs and graphene [57–60].
Thus, in this work we have conducted an experimental investigation to precisely figure out the ultimate potential of a TI-based saturable absorber as an ultrafast harmonic mode-locker suitable for the generation of femtosecond pulses with a high repetition rate. This paper demonstrates that our harmonically mode-locked fiber laser can produce output pulses with temporal widths of 630-700 fs at various harmonic resonance frequencies. In particular, output pulses of ~630 fs are shown to be obtained at a repetition rate of 773.85 MHz (the 55th harmonic). The saturable absorber used in this investigation was prepared on a side-polished fiber platform with a bulk-structured, 20 μm-thick TI deposited film. It is experimentally shown that the bulk-structured TI-deposited side-polished fiber can readily provide two key functions of saturable absorption and high nonlinearity for harmonic pulse formation. Note that the evanescent field interaction scheme allows for higher nonlinearity due to its long lightwave interaction length [57, 61]. The tuning capability of the repetition rate depending on the pump power was extensively investigated.
2. Fabrication and characterization of a saturable absorber
Figure 1 shows the schematic and a photo of the bulk-structured Bi2Te3 TI-deposited side-polished fiber used in this study. A side-polished fiber platform was prepared as follows, First, SMF28 fiber was affixed to a V-grooved quartz block. Second, the cladding of the SMF28 fiber was polished until the distance between the polished flat surface of the fiber and the fiber core was ~10 μm. Third, a small amount of index-matching oil was spread on the flat surface to induce a better evanescent field interaction between the bulk-structured Bi2Te3 TI film and oscillating beam, this also reduces the scattering loss caused by the roughness of the polished surface. Finally, a TI film was transferred onto the side-polished fiber. The bulk-structured Bi2Te3 TI film was obtained using a mechanical exfoliation method [64]. We used scotch tape to detach the thick film from the bulk single crystal Bi2Te3 (99.999%, Alfa Aesar). The thickness of the detached TI film was measured at ~20 μm by using an alpha-step surface profiler (Surface Profiler P-10, KLA-Tencor). This indicates that the small variations of the Bi2Te3 film thickness are negligible for evanescent field interaction because the thickness is already ~12 times larger than the wavelength of the oscillating beam within the cavity [65].
We conducted experimental characterization measurements on the TI film, such as scanning electron microscopy (SEM), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS) to analyze the material characteristics of the prepared bulk-structured Bi2Te3 TI film. Figure 2(a) shows the SEM images of the Bi2Te3 film surface with two different scales. As shown in this Fig., distinctive layered structures, which are generally shown in nano-structured Bi2Te3, are not present. Figure 2(b) shows the measured Raman spectrum of the Bi2Te3 film with four peaks: Eg1 at 41 cm−1, A1g1 at 64 cm−1, Eg2 at 104 cm−1 and A1g2 at 137 cm−1. These four peaks are typical features of bulk-structured Bi2Te3 [66]. The A1u1 peak, which is commonly shown in nano-structured Bi2Te3 films due to symmetry breaking in the layered structure, was not observed. Figure 2(c) shows the measured XPS spectrum of the Bi2Te3 film for Bi 4f, whereas Fig. 2(d) shows the Te 3d spectrum. The two peaks at 162.8 and 157.5 eV shown in Fig. 2(c) are consistent with the binding energies of Bi 4f5/2 and Bi 4f7/2, whereas the two peaks at 582.8 and 572.4 eV shown in Fig. 2(d) are consistent with the binding energies of Te 3d3/2 and Te 3d5/2 [67]. Additional small peaks were also observed both in the Bi 4f and Te 3d regions due to the oxidation of Bi and Te atoms on the film surface [67].
Fig. 2 (a) Measured SEM images of the Bi2Te3 layer surface with two different scales. (b) Measured Raman spectrum. Measured XPS for (c) Bi 4f and (d) Te 3d.
3. Passive harmonic mode-locking of a 1.55 μm fiber laser
The measured nonlinear saturable absorption curve is shown in Fig. 3(a). This measurement was carried out using a passively mode-locked fiber laser with a temporal width of ~1 ps at 1550 nm. In this Fig. the incident pulse peak power was used in the x-axis since the beam intensity could not be estimated due to a lack of information on the mode field area at the very point where the evanescent field interaction occurred. The modulation depth and saturation power were measured to be ~3.75% and 61 W, respectively. The following formula, which is commonly used to fit the saturable absorber, was used for fitting [41].
where is the transmission, is the modulation depth, is the input pulse energy, is the saturation energy, and is the nonsaturable loss. The optical insertion loss and polarization dependent loss (PDL) of the Bi2Te3 TI-deposited side-polished fiber were measured to be ~3 dB and ~10 dB, respectively. It is well known that the side-polished fiber based saturable absorber is suitable for high power laser applications because only a small portion of light is interacting with deposited materials on the side-polished fiber without damage to the deposited material [12]. In order to check the high-power beam tolerance of the bulk-structured Bi2Te3 TI-deposited side-polished fiber we launched a 1550-nm continuous-wave (CW) amplified laser beam of a 500 mW power into it. No optical damage was observed. We believe that the optical damage threshold is much higher than the value.Fig. 3 (a) Measured nonlinear absorption curve of the bulk-structured Bi2Te3 TI-deposited side-polished fiber. (b) The laser schematic.
Figure 3(b) shows our laser schematic. A 2.3-m length of erbium-doped fiber (LIEKKITM Er20-4/125, nLIGHT Corporation) was used for the gain medium and it was pumped via a 980/1550 nm wavelength division multiplexing (WDM) by a 980-nm semiconductor laser diode with a maximum pump power of 234 mW. A polarization independent optical isolator was inserted in the laser cavity for unidirectional beam propagation. A polarization controller (PC) was also used to adjust the polarization state of the oscillating beam in the laser cavity. After the PC, the prepared bulk-structured Bi2Te3 TI-deposited side-polished fiber was located. Finally, a 10:90 output coupler was used to extract the output pulse. The total length of the laser cavity was ~14.7 m and its corresponding fundamental frequency was ~14.07 MHz. The estimated net cavity dispersion was approximately −0.189 ps2. It is critical to properly adjust both group velocity dispersion and Kerr nonlinearity of the fiberized cavity for the generation of harmonically mode-locked femtosecond pulses [57]. We thus reduced the cavity length bit by bit while characterizing the temporal properties of the output pulses, until the output pulse width became a femtosecond level. This cavity optimization was conducted in terms of only temporal width of the output pulses without considering the pumping efficiency.
The fundamental mode-locking operation of the laser was obtained when the pump power reached 18 mW while the PC was properly adjusted. Figure 4 shows the measured optical spectrum, oscilloscope trace and electrical spectrum of the output pulses at the fundamental repetition rate. In the optical spectrum of Fig. 4(a) we observed non-negligible CW components, these could be attributed to an imperfect mode-locking status [61].
Fig. 4 Measured (a) optical spectrum, (b) oscilloscope trace, (c) electrical spectrum of the output pulses at the fundamental repetition rate. (d) Measured output electrical spectrum over a 200-MHz span.
As the pump power was increased, while the PC within the cavity was finely adjusted, temporal splitting of optical pulses was observed due to the soliton energy quantization effect [51]. Then, stable, harmonically mode-locked optical pulses started to be produced at a pump power of 22 mW. Figure 5(a) shows the measured optical spectrum of the 55th harmonic output pulses, which was obtained with the maximum pump power of 234 mW. The center wavelength and 3-dB bandwidth were measured to be ~1555.9 nm and ~4.5 nm, respectively. Figure 5(b) shows the measured oscilloscope trace of the output pulses using a combination of a 16 GHz real time oscilloscope (DSA71604C, Tektronix) and a photodetector with a combined electrical bandwidth of 15 GHz. The period of the optical pulses was 1.29 ns, which corresponds to 773.85 MHz.
Fig. 5 (a) Measured optical spectrum and (b) oscilloscope trace of the 55th harmonic output optical pulses.
Figure 6(a) shows the measured autocorrelation trace of the 55th harmonic optical pulses where there temporal width was estimated to be ~630 fs. Considering the 3-dB bandwidth of ~4.5 nm, the estimated time-bandwidth product was ~0.35, which is slightly larger than that of transform-limited sech2 pulses. This indicates that the output pulses were slightly chirped. The measured electrical spectrum of the output pulses is shown in Fig. 6(b). A strong signal peak at 773.85 MHz was clearly observed and the supermode suppression ratio (SMSR) was measured to be larger than 39 dB over a 1.5 GHz frequency span, while the signal-to-noise ratio (SNR) was ~63 dB.
Fig. 6 (a) Measured autocorrelation trace and (b) electrical spectrum of the 55th harmonic output optical pulses. Inset: Measured electrical spectrum over a wide frequency span of 10 GHz..
Next, we measured the harmonic order variation of the output pulses while changing the pump power. Figure 7(a) shows the measured pulse repetition rate of the output pulses as a function of pump power. The pulse repetition rate decreased linearly from 773.85 to 14.07 MHz when the pump power was reduced from the maximum pump power of 234 to 18 mW. When the pump power was increased again from 18 mW to 234 mW, the same harmonic frequency change was observed. The pumping efficiency was ~3.36 MHz/mW. Figure 7(b) shows the measured average output optical power and pulse energy of the output pulses as a function of harmonic order (n). The average output power was observed to monotonously increase with the harmonic order increase, whereas the pulse energy was shown to remain between 1.32 and 2.52 pJ. The average output power increase is simply due to the increase of the pump power. The harmonic pulses were observed to possess pulse energies between 1.83 and 1.32 pJ, showing a slightly increasing trend in proportion to the harmonic order. We think that the optimization of our Bi2Te3 TI-deposited side-polished fiber in terms of insertion loss can help increase the pulse energy.
Fig. 7 (a) Harmonic mode-locking frequency of the output pulses as a function of pump power. (b) Measured average optical power and pulse energy of the output pulses as a function of harmonic order (n).
Next, the SMSR and SNR of the output optical pulses at each harmonic order (n) were measured as shown in Fig. 8(a). The SMSR was observed to be more than 27.3 dB over all harmonic orders, while the corresponding SNR ranged from 46.3 to 63 dB. Both the SMSR and SNR were, interestingly, observed to be improved with increasing harmonic order (n). Figure 8(b) shows the temporal width of the output pulses as a function of harmonic order (n), which was measured with an autocorrelator. The output pulse width was observed to be less than 700 fs for all harmonic orders. Note that the autocorrelation measurement of the output pulses at harmonic orders less than 12, was impossible due to limited output pulse peak power.
Fig. 8 (a) Measured SMSR, SNR, and (b) temporal width of the output pulses as a function of harmonic order (n).
Finally, we conducted additional experiments to check whether the NPR effect is a dominating harmonic mode-locking mechanism rather than saturable absorption of the Bi2Te3 film. We did this because our prepared Bi2Te3-deposited side-polished fiber exhibited a significant PDL of ~10 dB. For this final check we replaced the Bi2Te3-deposited side-polished fiber within the laser cavity of Fig. 3(b) with a commercially available micro-optic polarizer (OLCS-12-155, Opto-Link Corporation) that has a polarization extinction ratio larger than 30 dB. Figure 9 shows the optical spectrum and oscilloscope trace of the output pulses at the maximum pump power of 234 mW. Neither stable harmonic nor fundamental mode-locking could be obtained. As shown in Fig. 9(b) only unstable pulse formations were observed even though we tried to adjust the pump power and PC within the cavity. A polarizer with such a large polarization extinction ratio, which was inserted into the cavity, could induce the unstable soliton pulse generation under a high pump power through both nonlinear polarization rotation (NPR) and soliton-CW beam interaction due to anomalous dispersion-induced modulation instability [68]. However, in this case, the output pulses were very unstable and sensitive to the environmental condition changes. We also conducted the same experiment with a gold-deposited side-polished fiber with a lower PDL level of ~6.5 dB. Using this ~6.5 dB PDL device in the cavity in Fig. 3(b), it was impossible to obtain stable mode-locked pulses even at the fundamental repetition resonance frequency. These results are believed to confirm that the stable harmonic mode-locking of our laser in Fig. 3(b) was produced mainly due to saturable absorption of the Bi2Te3 film.
Fig. 9 The optical spectrum and oscilloscope trace of the output pulses from the laser cavity of Fig. 3(b), in which the Bi2Te3-deposited side-polished fiber was replaced with a commercially available micro-optic polarizer. The graphs were obtained at the maximum pump power of 234 mW.
4. Conclusion
We have experimentally demonstrated a harmonically mode-locked femtosecond pulse laser at 1555.9 nm that incorporates a bulk-structured Bi2Te3 TI-based saturable absorber. Using a 20-μm thick, bulk-structured Bi2Te3 TI-deposited side polished fiber within an erbium-doped fiber ring cavity as a harmonic mode-locker, it was shown that femtosecond optical pulses with temporal widths of 630-700 fs could readily be produced at various harmonic orders, the maximum order of which was 55, corresponding to a harmonic frequency of 773.85 MHz.
We believe that this experimental demonstration is further evidence to show the effectiveness of bulk-structured TIs, which do not require high precision and expensive nanomaterial processing facilities, as base saturable absorption materials for ultrafast laser applications.
Acknowledgments
This work was supported by the Korean Ministry of Trade, Industry and Energy within the project, “Development of Process & Equipment Technology to Engrave Roll Molds with 10-micron Scale Line Width using a Pulse-width Tunable Ultrafast Laser (10048726)”. This work was also supported by the Industrial Strategic Technology Development Program (10039226, Development of actinic EUV mask inspection tool and multiple electron beam wafer inspection technology) funded by the Ministry of Trade, Industry & Energy, Republic of Korea.
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