We demonstrated a dispersion-managed, high-power, Tm-doped ultrashort pulse fiber laser using a single-wall-carbon-nanotube (SWNT) polyimide film. SWNTs with a diameter of 1.6 nm were synthesized with the enhanced direct injection pyrolytic synthesis (e-DIPs) method, and thin polyimide films in which SWNTs were dispersed were developed as saturable absorbers in the wavelength range λ = 1.8–2.0 µm. An all-fiber type, passively mode-locked, ultrashort-pulse Tm-doped fiber laser was demonstrated using the developed SWNT films. Wavelength tuning operation with gain fiber control and dispersion management of the developed fiber laser were investigated. Stable soliton and dissipative soliton mode locking operations were observed. High-power (102.6 mW) single-pulse mode-locking operation was achieved in a large positive dispersion regime. The repetition rate was 21.6 MHz, and the corresponding pulse energy was 4.75 nJ. To the best of our knowledge, this is the highest power operation of a Tm-doped fiber laser using carbon nanotubes and film-type devices with nano-carbon materials. The developed laser showed self-staring, stable performance and is useful for practical applications.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Wavelengths in the 2 µm range are important for spectroscopy, materials processing, laser imaging detection and ranging (LIDAR), biomedical imaging, medicine, etc. A Tm-doped fiber laser operating in the wavelength range around λ = 2 µm is a useful and practical ultrashort-pulse laser source. The first passively mode-locked ultrashort-pulse Tm-doped fiber laser was demonstrated using a nonlinear polarization rotation (NPR) scheme in the soliton mode-locking regime . Then, stretched pulse and dispersion-managed, high-power ultrashort-pulse operations were demonstrated using the NPR scheme [2–4]. Dispersion-managed dissipative soliton mode-locking in a Tm/Ho fiber laser was demonstrated with a semiconductor saturable absorption mirror (SESAM) .
One of the issues in Tm-doped fiber lasers is the inherent instability owing to the effect of water vapor absorption. An all-fiber configuration is effective for eliminating the effect of water vapor absorption. Using nano-carbon materials, such as single wall carbon nanotubes (SWNTs) and graphene, we can fabricate a transmission-type saturable absorber, which will enable us to construct an all-fiber configuration.
The first demonstration of a Tm-doped fiber laser using an SWNT film was reported in 2008 . Subsequently, dispersion management and wavelength-tuning operation were reported by some groups [7–11]. Passively mode-locked Tm-doped fiber lasers using graphene devices were demonstrated with film-type and evanescent-type devices [12–14].
In this work, we demonstrated an ultrashort-pulse Tm-doped fiber laser using an SWNT polyimide film. SWNTs working at wavelengths around the 2 µm range were synthesized, and a free-standing polyimide film device dispersed with SWNTs was developed for the first time. A stable, high power, passively mode-locked Tm-doped fiber laser was demonstrated using the developed SWNT polyimide film. Wideband operation was confirmed by varying the characteristics of the Tm-doped fiber. Dispersion management in the cavity was employed, and the characteristics were investigated experimentally.
2. Development of SWNT film
In order to realize a Tm-doped fiber laser using SWNTs, first, we made a film-type device in which SWNTs were dispersed. As the material of the film-type device, we used polyimide, which is one of the toughest polymer materials. In our previous work, we investigated an Er-doped ultrashort-pulse fiber laser using a polyimide film in which SWNTs were dispersed . In that case, SWNTs with a diameter of 1.2 nm, which showed saturable absorption around the 1.55 µm wavelength region, were used as the saturable absorber. The HiPco and laser ablation methods were used to synthesize the SWNTs .
A Tm-doped fiber (TDF) has a wide gain band in the wavelength range from 1.7 to 2.1 µm. SWNTs with a diameter of 1.6 nm show saturable absorption around the 1.8-2.0 µm region. Here, the enhanced direct injection pyrolytic synthesis (e-DIPS) method was utilized to prepare the SWNTs with a narrow diameter distribution around 1.6 nm . First, 5 mg of SWNTs were ultrasonically dispersed in 30 ml of polyimide solution. The dispersion was further purified by centrifugation. Then, carbon nanotube (CNT)-polyimide films were prepared by using a film applicator. After evaporation of the solvent, we finally obtained thin CNT-polyimide self-standing films with thicknesses of 30-60 µm.
Figure 1(a) shows the observed microscope image of a polyimide film in which SWNTs were dispersed. Here we made three films (A–C) with different absorbances. The parameters of the films were summarized in Table 1. The thicknesses of the films were 35–45 µm. The densities of SWNT were 0.05, ∼0.08, and 0.1 wt% in films A, B, and C, respectively. The polyimide film was almost homogeneous, and only a few particles were observed in the field of view for three films. The observed small particles were considered to be aggregations of SWNTs.
Figure 1(b) shows the measured absorption spectra of the developed SWNT polyimide films. Strong peaks were observed around the 1.9 µm range, which corresponds to the S1 band absorption of SWNTs with a diameter of 1.6 nm. As the density of SWNTs was increased, the magnitude of the absorption increased.
Next, we examined the saturable absorption properties of the developed SWNT polyimide films. For accurate evaluation, we used the z-scan technique. A Raman-shifted soliton pulse with a temporal width of 130 fs was generated in the 1920 nm wavelength range with an Er-doped ultrashort-pulse fiber laser and polarization maintaining fiber , and was used as the probe pulse for the z-scan measurement . The soliton pulse was focused with an optical lens, and the position of the polyimide film being examined was continuously scanned around the focal spot area, and the variation of the transmitted beam power was observed. The spot size of the beam was observed with a beam profiler, and the peak power density was estimated.
Figure 2 shows the observed saturable absorption properties of the developed polyimide films. As the peak power density was increased, the magnitude of the absorption gradually decreased, and saturable absorption properties were observed clearly. The measured data points of the absorption ratio α were fitted by the following equation:
From Fig. 2, we can see that the larger the magnitude of the absorption was, the larger the modulation depth was. The observed modulation depths were 12.3, 20.5, and 29.8%, the non-saturable absorption ratios were 27.4, 34.4, and 35.6%, and the saturation power fluences were 710, 620, and 1007 MW/cm2 for films A–C, respectively. It was considered that these numbers are adequate for stable passive mode-locking of ultrashort-pulse fiber lasers. The maximum average power irradiated into the film was 10 mW, and no damage to the film was observed during this measurement.
3. Development of the Tm-doped ultrashort-pulse fiber laser with SWNT film
3.1 Soliton mode-locked Tm-doped fiber laser
Next, we developed a Tm-doped ultrashort-pulse fiber laser using the developed polyimide film in which SWNTs were dispersed. The cavity consisted of single-mode fiber devices as can be seen in Fig. 3. As the TDF, we used 1.6 m of anomalous dispersive TDF (SCF-TM-8/125). The core diameter was 8.1 µm, and the magnitude of the absorption was 13 dB/m at a wavelength of 1567 nm. For the pump beam, the output of a single-mode DFB-LD at 1550 nm (Thorlabs SFL1550S) was amplified with an Er/Yb co-doped double clad fiber amplifier (Pritel FA-33). The maximum output power from the fiber amplifier was 2 W. The pump beam was introduced into the fiber laser cavity with a 1550/2000 WDM fiber coupler. A 75:25 fiber coupler was used as the output coupler in order to increase the output power and to reduce the irradiation power into the film.
The SWNT film was inserted between angled PCFs and was used as the mode-locker of this laser. An in-line type polarization controller was used to control the condition of the oscillating beam. The two polarization-insensitive optical isolators were used in order to achieve the stable operation and protect the SWNT film. An all-fiber configuration was realized to suppress the effect of the absorption of H2O in the air. Since the highest power was obtained for film A, the results obtained with film A were summarized in this section.
Figure 4 shows the characteristics of the output pulses from the developed fiber laser when the net cavity dispersion D = -0.26 ps2. Since the single-mode fiber devices showed anomalous dispersion properties in this wavelength range, stable single-pulse passive mode-locking was achieved with this configuration. In the optical spectra shown in Fig. 4(a), sech2 shaped pulse spectra with Kelly sidebands, which are characteristic of soliton mode-locking, were observed. The sharp dips in the main pulse spectrum were due to the absorption of water in the air. Figure 4(b) shows the autocorrelation trace observed with an autocorrelator (Femtochrome FR-103PD). The temporal width of the autocorrelation trace was 805 fs full width at half-maximum (FWHM), and the corresponding pulse width was 522 fs FWHM under the assumption of a sech2 pulse.
Figure 4(c) shows the pulse train observed with a fast pin photodiode (EOT ET-5000) and a digital oscilloscope (Yokogawa DL9040L). A clean pulse train was observed stably. The temporal separation between each pulse was 22.5 ns. Figure 4(d) shows the RF spectra observed using a fast pin photodiode and an RF spectrum analyzer (Anritsu MS2830A). Equal-frequency-spanning RF spectra with almost constant amplitude were observed up to a few GHz range, and stable passive mode-locking was confirmed. Figure 4(d) shows the enlarged RF spectra of the fundamental frequency. The repetition frequency was 44.5 MHz, and the SNR was ∼68 dB. This means that the noise level of the output pulse train was low enough for conventional applications.
In this experiment, the observed maximum output power was 39.4 mW, and the corresponding pulse energy was 0.88 nJ, which were limited by the single pulse operation limit. Generally speaking, it is difficult to achieve high power in the soliton mode-locking regime owing to the single pulse operation limit and damage to the film. It was considered that the achieved high average power in the soliton mode-locking regime was due to the large mode-field diameter (MFD) and low nonlinear coefficient of the fibers in the 2 µm range, and the high damage threshold of the developed film.
The oscillation wavelength depends on the length and properties of the TDF. Figure 5 shows the variation of the optical spectra at the fiber laser output for several TDFs. Here we used two kinds of TDF: a highly doped (H) TDF (SM-TSF-5/125) and a conventional (C) TDF (SCF-TM-8/125). The magnitude of the absorption was 340 dB/m for the H-TDF and 13 dB/m for the C-TDF at λ=1560 nm. The lengths of the TDFs were from 10 to 50 cm for the H-TDF, and from 1.4 to 3.0 m for the C-TDF. As shown in Fig. 5, stable soliton mode-locking was achieved for each length. By reducing the length of the H-TDF from 50 to 10 cm, the center wavelength of the output pulse was shifted from 1947 nm to 1895 nm. For the C-TDF, the center wavelength of output pulse was shifted from 1874 nm to 1854 nm as the length of the C-TDF was reduced from 3.0 m to 1.4 m. It is considered that this wavelength shift is caused by the balance between absorption and amplification in the TDF. We can tune the oscillation wavelength by changing the length and doping ratio of the TDF. Since there is no absorption spectra above 1930 nm, the clean pulse spectra were observed in Figs. 5(a) and (b).
As the film dependence, when film B was used, the stable single pulse mode-locking was obtained when the 3.0 m of C-TDF was used. The output power for film B was lower than that of film A. When 1.4 m of C-TDF and HTDF were used, the stable single pulse mode-locking was not obtained for film B. For film C, although the multiple-pulse mode-locking or unstable single pulse mode-locking were observed with C-TDF, the stable single-pulse mode-locking was not obtained. It was considered that since the optical loss in film A was the lowest, the best performance was obtained among the three films.
3.2 Dispersion managed Tm-doped fiber laser
Next, we examined the dispersion management of the Tm-doped fiber laser [9,10,20]. Figure 6 shows the configuration of developed Tm-doped fiber laser. For the dispersion management, we used ultra-high numerical aperture fiber (UHNA) 4, which has a strong normal dispersion property, β2 = + 85∼89 ps2/km, and MDF = 2.2 µm. For the TDF, we used SCF-TM-8/125, which was used in the previous section. The other parts consisted of conventional SMF devices. The additional WDM in front of the SWNT film was added to remove the non-absorbed pump laser. We controlled the magnitude of the net cavity dispersion from negative to large positive by varying the length of UHNA4 between 0 and 4.23 m. The corresponding repetition rate was 21.6–42.3 MHz.
Figure 7 shows the measured β2 for the fibers used in the fiber laser as a function of wavelength in the range of interest. Here, we used a supercontinuum (SC) interferometer for chromatic dispersion measurement . A low-noise, coherent SC was generated with a normal-dispersion highly nonlinear fiber, and was used for the measurement . The SMF and TDF had anomalous dispersion properties and positive β3 in this range. The UHNA4 had strong normal-dispersion properties and negative β3 in this range. As shown in Fig. 7, the magnitude of measured β2 in UHNA4 at λ = 1900 nm was 0.089 ps2/m, which was almost in agreement with those reported in the previous works [23,24]. By changing the length of UHNA4, we controlled the net cavity dispersion of the Tm-doped fiber laser from negative to positive, and we examined the properties of mode-locking and the output pulses.
Figures 8 and 9 show the variation of the optical spectra and the characteristics of output pulses when the length of UHNA4 was varied. The film A was used as the saturable absorber for this experiment. In the anomalous dispersion range, sech2 like shaped pulses with Kelly sidebands were observed, and soliton mode-locking operation was obtained. When D = -0.23 ps2, the temporal width of the autocorrelation trace was ∼600 fs, and the corresponding pulse width was ∼363 fs. The average power was 20–39 mW, which was limited by the single-pulse mode-locking operation limit. Figure 10 shows the long-term stability of the optical spectra for soliton mode-locking condition when D = -0.0169 ps2. Although the spectra were shown in linear scale, the spectral traces were well overlapped for 1 hour, and the high long-term stable operation was confirmed.
For the net dispersion region around zero, although unstable or noise-like pulse mode-locking was observed, stable mode-locking was not obtained at the initial conditions. Similar behavior was observed in Ref. . However, when we adjusted the polarization controller, stable mode-locking was achieved for those cases. In Fig. 8, we can see that the center wavelength was slightly shifted toward longer wavelengths, and sech2-like shaped pulses with Kelly sidebands were observed. It was considered that the net dispersion became anomalous due to the red-shift of the center wavelength, and stable mode-locking was obtained for the same cavity configurations.
For the net normal dispersion region, the Kelly sidebands disappeared, and dissipative soliton mode-locking was obtained. Steep spectral edges were observed, which are characteristic of dissipative soliton mode-locking [20,25]. The average power, spectral width, and output power were increased as the magnitude of the net dispersion was increased. The maximum average power was 102.6 mW when D = + 0.103 ps2. It is interesting to note that widely broadened spectra were observed in the net normal dispersion region of 0.0928–0.095 ps2 for the first time. It was considered that the high power inside the cavity induced strong self-phase modulation, and a wideband pulse spectrum was generated. The estimated irradiation power on the SWNT film was 33 mW. In this case, the stable operation was sustained for hours but we sometimes saw the damage of the film. Thus it was considered that the damage threshold was slightly above 33 mW.
Figure 11 shows the characteristics of the output pulses when D = +0.103 ps2. In this case, stable dissipative soliton mode-locking operation was achieved, and short pulses with a wide spectral width were obtained. The repetition frequency was 21.4 MHz, and the average power was 102.6 mW. The corresponding pulse energy was as high as 4.75 nJ. The spectral width was 32 nm, and the temporal width of the autocorrelation trace was 15 ps. The corresponding pulse width was 10.6 ps under the assumption of a Gaussian-shaped pulse. Since this pulse has up-chirping properties, we applied dispersion compensation using conventional SMF. The length of the SMF was optimized by the cut back method. The shortest pulse was observed when the length of the SMF was 4.6 m. The temporal width of the autocorrelation trace was 240 fs, and the estimated pulse width was 156 fs under the assumption of a sech2-shaped pulse. The pulse width in RMS was estimated to be 151 fs. The corresponding pulse compression ratio was as high as 62.4, and the time–bandwidth product was 0.686. For the pulse train shown in Fig. 11(c), an equally separated, stable pulse train with almost constant intensity was observed. For the RF spectra shown in Fig. 11(d), (a) clean fundamental RF frequency with SNR > 69 dB was observed. High-harmonics spectra with almost constant intensity were observed up to a few GHz, and stable mode-locking was confirmed.
Table 2 shows the comparison of the output performance for three films at different cavity dispersion condition. When film B was used, stable mode-locking was achieved between D = -0.01 ps2 and +0.1 ps2. The maximum power was 85 mW, which was lower than that for film A. In the anomalous dispersion range, stable mode-locking was not achieved. For film C, stable mode-locking was achieved between D = -0.01 ps2 and +0.055 ps2. The maximum power was 20 mW, which was much lower than that for film A. The best performance was achieved when film A was used. It was considered that the lowest absorption loss of film A contributed to the best performance of the fiber laser.
We investigated a passively mode-locked, ultrashort-pulse, Tm-doped fiber laser using single wall carbon nanotubes (SWNTs) dispersed in a polyimide film. SWNTs with a diameter of 1.6 nm, which showed saturable absorption properties in the λ = 1.8–2.0 µm range, were selectively synthesized with the enhanced direct injection pyrolytic synthesis (e-DIPs) method, and a free-standing polyimide film in which the SWNTs were dispersed was fabricated for the first time. This SWNT polyimide film device realized both the large modulation depth and the high damage threshold simultaneously. The saturable absorption properties of the developed film were investigated using the z-scan method with a wavelength tunable soliton pulse at 1.92 µm. Clear saturable absorption properties with a modulation depth of 12.3–29.8% were observed. An all-fiber type, passively mode-locked ultrashort-pulse Tm-doped fiber laser with an SWNT film was developed. Self-starting, stable operation of single-pulse, soliton mode-locking was achieved. The observed maximum average power was up to 39.4 mW, which was limited by the single pulse operation limit. Continuous wavelength tuning operation from 1854 to 1947 nm was observed by changing the length and doping ratio of the Tm-doped fiber.
Finally, a dispersion-managed, high-power, ultrashort-pulse Tm-doped fiber laser with an SWNT film was investigated. Stable operation was obtained for soliton and dissipative soliton mode-locking regimes. Thanks to the high performance of the SWNT polyimide film and high output coupling ratio of the fiber laser cavity, high-average-power (102.6 mW) operation was achieved in the large normal dispersion regime. The corresponding pulse energy was 4.75 nJ. As far as we know, this is the highest average power for an ultrashort-pulse Tm-doped fiber laser with SWNTs and a film-type device fabricated with nano-carbon materials. A 156 fs ultrashort pulse was obtained after dispersion compensation with a single-mode fiber. A widely broadened spectrum with a spectral width of ∼100 nm was observed in the large normal dispersion region. The developed fiber laser showed self-staring, stable performance and is promising for practical applications, such as multi-photon microscopy and laser processing.
The authors declare no conflicts of interest.
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