We demonstrated an all-normal-dispersion nonlinear polarization rotation mode-locked Tm:ZBLAN fiber laser in the 2 μm wavelength band. All fibers in the experiment were ZBLAN fibers with normal dispersion in the wavelength band. An average power of 63 mW with a characteristic cat-ear shaped optical spectrum of an ANDi laser was obtained. The center wavelength and the spectral bandwidth were 1880 nm and ~80 nm, respectively. The repetition rate was 70.6 MHz and the corresponding pulse energy was 0.9 nJ. The pulse duration directly from the oscillator was 860 fs and it was compressed to 107 fs.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
In the last two decades, mode-locked fiber lasers with normal dispersion active fibers have attracted huge attention. Active fibers with normal dispersion can strongly reduce nonlinearity and avoid the soliton fission effect during the amplification [1–4]. The local contribution of self-phase modulation and normal dispersion in the active fibers also enables parabolic amplification, leading a much better handling of accumulated nonlinearity, higher power scalability and broader spectral bandwidth than amplification with negative dispersion active fibers [5–9]. In the 1 μm wavelength band, all-normal-dispersion (ANDi) mode-locked Yb fiber lasers have been strongly investigated in the last decade due to their much higher potential in power scalability than general soliton and stretched pulse mode-locked fiber lasers [10–12]. Very recently, an ANDi large mode-area fiber Mamyshev oscillator above 1 μJ pulse energy with a compressed pulse duration of 41 fs and peak intensity of 13 MW was reported . In the 1.5 μm wavelength band, a Mamyshev oscillator with a normal dispersion active Er fiber above 30 nJ pulse energy with ~100 fs compressed pulse duration was also reported . Although mode-locked Tm or Ho fiber lasers in the 2 μm wavelength band have attracted huge attention due to their potential advantages and applications [15,16], mode-locked fiber lasers with normal dispersion active fibers have very limited numbers of reports in this wavelength band so far [17–19]. As silica glass naturally shows large negative material dispersion in this wavelength band, large normal waveguide dispersion with a very small core diameter of less than 3 μm is required to achieve normal dispersion silica fibers. Instead of ANDi lasers, dissipative soliton (DS) lasers constructed with a negative dispersion gain fiber and positive dispersion passive fiber were investigated in the wavelength band [20,21]. DS lasers are very attractive, but nonlinear effects in negative dispersion gain fibers and small core passive normal dispersion fibers limits their power scalability. Fluoride glass (ZBLAN) fibers have a much smaller negative material dispersion than silica glass fiber  and can have a normal dispersion with a standard core size(e. g. ~6 μm). Pulses as short as 45 fs were obtained from a stretched pulse mode-locked Tm:ZBLAN fiber laser using a grating based dispersion control element .
Here, we have demonstrated the first all-normal-dispersion mode-locked Tm:ZBLAN fiber laser in the 2 μm wavelength band to our knowledge. An average power of 63 mW with a characteristic cat-ear shaped optical spectrum of an ANDi laser was obtained. The center wavelength and the spectral bandwidth were 1880 nm and ~80 nm, respectively. The repetition rate was 70.6 MHz and the corresponding pulse energy was 0.9 nJ. The pulse duration directly from the ANDi oscillator was ~860 fs and it was compressed to ~107 fs by a reflection type grating pair.
2. Experimental setup
The schematic picture of our ANDi mode-locked Tm:ZBLAN fiber laser is depicted in Fig. 1. The gain fiber was a 1.3 m Tm:ZBLAN fiber with a Tm doping level of 10,000ppm, core diameter of 6.2 ± 0.2μm and NA of 0.2 (FiberLabs Inc. Japan). The effective mode-field diameter was ~7.4 μm. To each end of the active fiber, passive ZBLAN fibers of the same diameter and NA were mechanically jointed instead of spliced . Their length were 1 m and 0.5 m, respectively. The jointing was done by FiberLabs Inc. and the estimated loss for the each mechanical joint was about 0.3 dB. The calculated GVD of the fiber is ~6800 ± 2200 fs2/m and the total GDD of the ZBLAN fibers was 19040 ± 6160 fs2. The pump beam from a home-built 1555 nm Er:Yb fiber MOPA was coupled into the core of the Tm:ZBLAN fiber via a dichroic mirror. The coupling efficiency was about 70% (measured with the passive ZBLAN fiber). The pump wavelength is far from the absorption peak of Tm:ZBLAN fiber , but the core pumping and the high Tm doping level leads to ~70% absorption efficiency. The effective absorption coefficient for core pumping is strongly affected by saturation, so the ratio of the emission and absorption cross sections is important. To achieve ANDi laser operation, we used two different Gaussian shaped bandpass filters. The first one has a transmission bandwidth of 10 nm (FWHM) with a peak transmittance of 73% at a center wavelength of 1887 nm. The second one has a transmission bandwidth of 35 nm (FWHM) with a peak transmittance of 84% at a center wavelength of 1870 nm. With a combination of nonlinear polarization rotation (NPR) effect in the fibers, a half waveplate, two quarter waveplates, and an isolator (Faraday rotator and two PBSs), the ANDi NPR mode-locking was obtained with either BPF. We have no dispersion data of the BPFs, but the total dispersion of these free space components (lenses, mirrors, isolator, BPF.) should be much smaller than the total positive dispersion of the ZBLAN fibers, because the total length of them is below 10 mm. So the cavity can be considered all-normal-dispersion state. The laser output was extracted from the input PBS of the isolator.
3. Results and discussion
In our first experiment, we used the BPF of 10 nm bandwidth. Under the proper alignment of the waveplates, the NPR ANDi mode-locked operation automatically started above the pump power level of 280 mw [Fig. 2 (a)]. The maximum average power of 67 mW was obtained at the pump power of 430 mW. The laser slope efficiency in the mode-locked operation was lower than that in the CW operation, probably due to the lower output coupling efficiency from the PBS in the NPR mode-locked operation. The optical spectra at the average power levels of 49 mW and 67 mW are shown in Fig. 2(b). Both spectra show the characteristic cat-ear like shape of an ANDi laser. The spectral bandwidth increased with the average power and the FWHM of the spectrum at 67 mW average power was ~50 nm. Both spectra have thesame center wavelength of ~1883 nm and fine dips in the same positions, which should be the H2O vapor absorption lines . The pulse energy was 0.95 nJ. We measured intensity autocorrelation (AC) with a 0.4 mm thick type2 KTP crystal. The measured pulse duration directly from the oscillator was 800 fs assuming a Gaussian shaped pulses [Fig. 2(c)] which was much broader than the transform limited pulse duration of the spectrum. The measured pulse train and the RF spectrum indicate stable mode-locked operation at the repetition rate of 70.6 MHz with above 70 dB signal to noise ratio (Fig. 3), which is in good agreement with the total cavity length.
Then we replaced the BPF of 10 nm bandwidth with the 35 nm one. With the 35 nm bandwidth filter, the NPR ANDi mode-locked operation automatically started above the pump power level of 270 mW too. The maximum average power of 63 mW was obtained at the pump power level of 405 mW [Fig. 4(a)]. The optical spectra at the average power levelsof 48 mW and 63 mW are shown in Fig. 4(b), respectively. The spectrum of 63 mW average power showed clear cat-ear shape with the bandwidth of ~80 nm at the center wavelength of 1880 nm. When we increased the pump power above 405 mW, the spectral component at the central part started to grow and the mode-locked laser operation stopped. The measured AC trace of the pulses directly from the oscillator is shown in Fig. 5(a). The AC trace was fitted assuming a Gaussian shaped pulses and the pulse duration was estimated to be 860 fs. The calculated transform limited pulse duration from its optical spectrum was 99 fs [Fig. 5(b)] so that the pulses directly from the oscillator were chirped ~9 times. We compressed the pulses by a single pass reflection type grating pair (300/mm). The compressed pulse duration as a function of the dispersion value of the compressor is shown in Fig. 5(c). With the grating separation of ~1.9 cm, corresponding to the GDD of ~-21,000 fs2, the shortest pulse duration of 107 fs was obtained [Fig. 5(d)]. Although we can see a pedestal in the AC trace, which should be result of the uncompensated higher order dispersion, the compressed pulse duration of 107 fs is very close to its transform limited pulse duration of 99 fs indicating a high quality of the chirped pulses.
To deepen the understating, we simulated our ANDi laser numerically with a commercial software (Fiberdesk)  under fast saturable absorber assumption. The parameters used for the simulation are show in Table 1. The gain parameter was modified in order to obtain similar average power level as in the experiment. The simulation result showed a chirped pulse duration of ~860 fs [Fig. 6(a)] with a cat-ear shaped spectrum at ~80 nm bandwidth [Fig. 6(b)]. When we increased the gain about 2%, mode-locked operation stopped. This is similar to our experimental result. In the experiment, we used the commercially available Tm:ZBLAN fiber with a core diameter of 6.2 ± 0.2μm with GVD of 6800 ± 2200 fs2/m. The total GDD of our current setup was 19040 ± 6160 fs2, which is a much smaller value than in a typical ANDi laser. In ANDi type lasers, higher pulse energy with a higher breathing ratio of a chirped pulse duration can be expected with a larger total normal dispersion of the cavity. To obtain an estimate of possible future of pulse energy from our ANDi laser, we changed the GDD of the fibers to 24400 fs2/m in the simulation, which is available by a custom ZBLAN fiber with a core size of 5 μm and NA of 0.2. The 5 μm core fiber also has a similar mode-field diameter of 7.4 μm, which could have a higher propagation loss, but we ignored it in the simulation. According to the simulation, the fiber would allow above 10 nJ pulse energy with over 110 nm spectral bandwidth at ~4 ps chirped pulse duration. We also varied the output coupling efficiency and the length of the fibers in our simulation and found that this would allow further energy scaling. (We did not show the detail here as it is outside the scope of this paper). The main limitation of the energy scaling should come from the core size as we cannot increase the core size of the ZBLAN fiber while still maintaining normal dispersion with the waveguide dispersion.
In conclusion, we have demonstrated the first time all-normal-dispersion mode-locked Tm:ZBLAN fiber laser in the 2 μm wavelength band to our knowledge. An average power of 63 mW with a characteristic cat-ear shaped optical spectrum of an ANDI laser was obtained. The center wavelength and the spectral bandwidth were 1880 nm and ~80 nm, respectively. The repetition rate was 70.6 MHz and the corresponding pulse energy was 0.9 nJ. The pulse duration directly from the ANDi oscillator was ~860 fs and it was compressed to ~107 fs by a reflection type grating pair. The pulse energy is still below 1 nJ as the total normal dispersion of our current set-up was only 19040 ± 6160 fs2 that is much lower than typical ANDi laser and probably limiting the available pulse energy of our current ANDi laser. We believe much higher pulse energy can be available from the 2 μm ANDi laser by simply increasing the normal dispersion and output coupling efficiency of the cavity.
The Amada Foundation, and Photon Frontier Network Program of the Ministry of Education, Culture, Sports, Science and Technology, Japan.
We thank the FiberLabs Inc. Ryuki Nakatani for preparing the Tm:ZBLAN fibers and Institute for Molecular Science, Dr. Yutaka Nomura for private discussion about Tm:ZBLAN fiber lasers.
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