We demonstrate a gain-switched thulium-doped fiber laser (TDFL) built in an all-fiber format producing nanosecond pulses with variable wavelength in the 2 μm waveband. The laser features tunable operation in an ultra-wide spectral region of 1765 – 2055 nm (24 THz). The nearly 300 nm tunability doubles the record tuning range of existing gain-switched fiber lasers, and to the best of our knowledge, presents the broadest tuning range that has been reported for a monolithic pulsed rare earth doped fiber laser to date. The TDFL can operate at a repetition rate of 2.5 – 100 kHz with a pulse width as short as ~200 ns. Influences of various system parameters on the laser performance are investigated in detail.
© 2016 Optical Society of America
Lasers working in the nominally eye-safe 2 μm region (1.65 – 2.2 μm) have attracted significant attention due to their wide range of applications in, for example, medicine, optical communications, remote sensing, material processing, as well as nonlinear frequency conversion [1–3]. For instance, important gas species like CO2 and H2O present characteristic absorption lines in the 2 μm waveband , which makes 2 μm lasers attractive sources for environmental monitoring. On the other hand, the 2 μm spectral region overlaps with the atmospheric transmission window and is thus useful for applications such as free space communications . In the context of fiber optics communications, underpinned by the recent development in hollow-core photonic band-gap fibers [6,7], the 2 μm waveband is stimulating much research interest as a potential transmission window away from the conventional 1.5 μm band for future optical communications systems [8,9]. Two microns also serves as a good pumping band for accessing the mid-infrared through nonlinear approaches such as supercontinuum generation [10–14] and optical parametric oscillation [15,16]. For many of the aforementioned applications, a widely tunable source working in the pulsed regime is required or highly desired due to the flexibility in operation wavelength and temporal characteristics it can provide.
The thulium-doped fiber laser (TDFL) is a promising candidate for achieving wideband tunable laser operation at 2 μm. This is made possible by the 3F4 -> 3H6 transition of thulium (Tm3+) doped in silica glass that features a distinctively broad emission spectrum spanning over 30 THz (1600 – 2200 nm) [17,18]. Rapid progress has been made in the development of widely tunable (silica-based) TDFLs with many demonstrations of tuning ranges exceeding 100 nm [19–31]. Using a compression tuned FBG and core-pumping (pump source being a 1565 nm fiber laser), Daniel et al. demonstrated an all-fiber tunable TDFL working at a wavelength as short as 1660 nm , which is extremely short for Tm given the strong three-level behavior it exhibits at such wavelengths. The longest operation wavelength of a TDFL reported to date is ~2200 nm, which could be achieved either from an all-fiber wavelength-selectable setup based on fiber Bragg gratings  or from a tunable setup based on a diffraction grating , both using cladding-pumping (pump source being 790 nm diode lasers). For all-fiber diode-pumped TDFLs, which features better compactness, reliability, and efficiency compared to lasers involving free-space parts and/or fiber laser pumping, the broadest tuning range currently is 255 nm (1820 – 2075 nm) achieved through using a fiberized grating and 1550 nm core-pumping . A similar configuration with a 1550 nm fiber laser as the pump and a thulium/holmium co-doped fiber as the gain medium achieved a tuning range of 303 nm (1727 – 2030 nm) , which is the widest wavelength coverage of any monolithic rare earth doped fiber laser to date (note that its tuning range mostly falls in the gain spectrum of thulium thus holmium is expected to be unnecessary for achieving this result). Even broader tunability can be realized through combining different thulium gain stages [20,21,30] and up to 400 nm (1710 – 2110 nm) total tunability has been achieved so far by combining cladding-pumped and core-pumped gain stages with a common (free-space) acousto-optic tunable filter (AOTF) as the wavelength selective element . In terms of high power performance, 100 W output power has been reported for >100 nm tuning range [34,35]. It is worth mentioning that the gain spectrum of thulium makes it difficult for TDFLs to access >2070 nm [17,18], in which case holmium (Ho3+) is a better choice especially when the required pump sources for Ho (1150 nm diode laser or 1950 nm fiber laser) are becoming commercially available . A number of tunable holmium-doped fiber lasers (HDFLs) have been demonstrated, typically covering 50 – 150 nm in the spectral range of 2000 – 2170 nm [19,37–40]. Nonlinear approaches such as parametric conversion can also offer wide wavelength tunability at 2 μm and are currently under investigation [41,42].
The widely tunable TDFLs discussed above all worked in the continuous wave (CW) regime. Pulsed operation of tunable TDFLs has also been reported, however, the technology is less well developed compared with its CW counterpart with only a limited number of demonstrations. Ultra-short (fs, ps) pulse generation typically employs the mode locking technique [43–48]. The first report of mode-locked tunable TDFLs dated back to the mid-1990s . In this work, mode-locking was achieved through nonlinear polarization rotation (NPR), which produced 350 – 500 fs pulses. Tunable operation was realized by adjusting the wave plates and birefringent tuning plate in the cavity. An aggregate tunability of 104 nm (1798 – 1902 nm) was demonstrated using different wave plate settings with a particular setting typically covering 50 nm. The topic was revisited by Fang et al. who demonstrated the first all-fiber tunable mode-locked laser at 2 μm . The laser employed a carbon nanotube saturation absorber as the mode-locking element and a fiber taper based tunable filter to select the wavelength. As a result, 1 ps pulse duration and 50 nm tuning range at 2 μm have been achieved. The widest tuning range of mode-locked tunable TDFL is currently 136 nm (1842 – 1978 nm) which was obtained through NPR in a diode-pumped monolithic cavity . It is worth noting that besides mode locking, nonlinear approaches (e.g., Raman wavelength shifting [49–56], parametric process , and time-lens ) is another promising way of generating ultra-short wavelength-tunable 2 μm pulses. This is currently an active area where many encouraging results have been produced. Further research effort in this field is therefore expected and certainly worthwhile.
In order to obtain ns and longer pulses for tunable TDFLs, different techniques have to be adopted. Q-switching is presently under study for achieving this purpose [59–63]. For example, Gutty et al. reported a Q-switched tunable TDFL producing 25 – 55 ns pulses with ~100 nm tunability at around 1.9 μm , whereby an acousto-optic modulator (AOM) and an AOTF were used as the Q-switch and wavelength selector, respectively. Up to 7 kW peak power was obtained by amplifying the output of this laser using a MOPA setup . A high power Q-switched tunable TDFL based on an AOM and diffraction grating was demonstrated by Li et al., which could cover 137 nm (1970 – 2107 nm) tuning range and deliver ~180 ns pulses with up to 5.6 kW peak power . The development of Q-switched tunable TDFLs is a new topic and we expect to see fast advances in this field. Apart from Q-switching, another common way of generating ns pulses is gain switching. It relies on directly modulating the pump source therefore the system possesses the distinct advantage of having a simpler configuration compared with Q-switched systems without the need of any additional in-cavity pulse generator  (e.g., an AOM  or a saturation absorber ). Although there have been a number of reports on gain-switched TDFLs operating at fixed wavelengths , this technique has not been demonstrated in the case of tunable TDFLs until very recently. Wang et al. reported a gain-switched TDFL with 140 nm (1860 – 2000 nm) tuning range . At 25 kHz repetition frequency, the laser produced pulses with 350 – 850 ns pulse width and 0.8 – 3.2 mW average power depending on the operation wavelength; the repetition frequency can be tuned from 5 – 25 kHz. In this work, laser tuning was achieved by adjusting a diffraction grating that forms one end of the laser cavity. The grating resulted in a free-space structure, which compromises the compactness and robustness of the laser and requires careful alignment and protection from environmental disturbance. Clearly, a monolithic fiber laser with improved performance is much preferred especially for applications with maintenance-free demands.
In this contribution, we present a gain-switched TDFL built in a monolithic structure, which generates wavelength-tunable nanosecond pulses at 2 μm. The laser wavelength can be tuned in an ultra-wide spectral region of 290 nm (1765 – 2055 nm, 24 THz). This doubles the record tuning range of existing gain-switched fiber lasers , and to the best of our knowledge, presents the broadest wavelength coverage that has been reported for monolithic pulsed rare earth doped fiber lasers to date.
2. Experimental setup
Figure 1 illustrates the setup of the broadly-tunable gain-switched TDFL. The TDFL had an all-fiber format and was built in a ring configuration with a total cavity length of 6.2 m (corresponding to a round trip time of 30 ns). The gain medium was a piece of thulium-doped fiber (TDF) with a core/cladding diameter of 9/125 μm. The TDF had a numerical aperture (NA) of 0.15 and core absorption of ~10 dB/m at 1550 nm. 5 m of TDF was chosen as the optimum length to achieve the broadest wavelength tuning range. The TDF was core-pumped by two in-house built 1550 nm fiber lasers coupled through two 1550/2000 nm fused wavelength division multiplexers (WDMs). One pump, which was an erbium ytterbium co-doped fiber laser (EYDFL), operated in the CW regime to overcome the loss of the cavity and its power was kept below the threshold of the tunable laser. The other pump, which acted as the gain switch of the system has a master oscillator power amplifier (MOPA) configuration consisting of a seed and a power amplifier. The seed was a 1550 nm diode laser with its repetition rate tunable from 1 kHz – 2 MHz and pulse width tunable from 10 – 200 ns, both limited by the pulsed current driver. The power amplifier was an erbium ytterbium co-doped fiber amplifier (EYDFA). The maximum average power after the EYDFA was 300 mW at 10 kHz. A fiberized diffraction-grating-based tunable filter was employed to select the wavelength of the laser. The filter had a 3 dB bandwidth of ~2.8 nm and an insertion loss of ~7 dB. The insertion loss is mainly decided by the coupling loss of the input/output fiber and the grating within the filter. The high insertion loss of the tunable filter presents a major contribution to the cavity loss and justifies the utilization of the CW pump. An isolator was spliced in-between the tunable filter and a WDM to ensure unidirectional propagation of the signal light in the cavity. A 90/10 fiber tap coupler was introduced after the tunable filter to extract 10% of the signal launching into it as the useful output while recycling the rest 90% back into the cavity. The high feedback ratio of the tap coupler was chosen to balance the high insertion loss of the cavity. A power meter and an optical spectrum analyzer (OSA) were used to measure the power and spectrum of the laser, respectively. The pulse shape of the laser output was detected by a photo detector (DET05D) and an oscilloscope with 1 GHz bandwidth.
3. Performance of the laser – main results
Figure 2 summarizes the performance of the TDFL when it was gain-switched at 10 kHz repetition frequency. The CW pump power was kept below the threshold for all measurements to ensure that no lasing was observed without the presence of the gain-switching pulsed pump (the CW pump power was regulated between 260 – 730 mW for different wavelengths). The average power of the pulsed pump was 300 mW, corresponding to a pump pulse energy of 30 µJ. Figure 2(a) plots the output average power and pulse width of the TDFL as a function of operation wavelength. The tuning range of the TDFL covered 1765 – 2055 nm where stable (which refers to single-pulse operation  as opposed to the situation where the repetition rate of the 2 μm pulses differs from that of the 1550 nm pump pulses ) pulsed operation could be achieved. The output average power was between −5.3 – 7.5 dBm (0.3 – 5.6 mW) corresponding to 0.03 – 0.56 µJ pulse energy and peak power in the range of 0.1 – 1 W. The shape of the output power tuning curve follows that of the Tm3+ gain curve . Further wavelength tuning towards <1750 nm is mainly limited by the severe reabsorption of the short wavelength component due to the strong three-level behavior of Tm3+ at such wavelengths. The increasing insertion loss of the tunable filter in this waveband is another limiting factor. On the long wavelength side, tuning towards >2050 nm is limited by the decreasing emission cross section of Tm3+ [17,18]. The maximum output power was obtained at 1965 nm, which is consistent with the relatively long length (5 m) of the gain fiber. The output pulse width was in the range of 215 – 850 ns (7 – 28 times the round trip time) for most of the tuning range; however, a sharp increase was observed at the long wavelength edge of the working window with the pulse width for 2055 nm reaching 6.04 µs. This rapid degradation is caused by the notable decrease in the emission cross section of Tm3+ at above 2050 nm [17,18]. The inset of Fig. 2(a) shows an exemplary output pulse shape. The output spectrum was of high quality as shown in Fig. 2(b). Over 50 dB optical signal-to-noise ratio (OSNR) was achieved for 1800 – 2050 nm whereas at the short wavelength edge the signal had over 35 dB OSNR.
4. Discussion – influence of various parameters on the laser performance
4.1 Pump pulse energy
To study the influence of the pump pulse energy on the TDFL performance, the TDFL was tested at a fixed wavelength and repetition frequency of 1930 nm and 10 kHz, respectively, while its output pulse energy and pulse duration were recorded at varied pump pulse energies (the pump pulse energy was changed by varying the gain of the EYDFA). Figure 3 plots the corresponding results. It can be clearly seen that there is a threshold effect, i.e., when the pump pulse energy exceeds a certain value, the laser performance changes dramatically. For the setup shown in Fig. 1 whereby the launched CW pump was set at 285 mW, the “threshold” pump energy was approximately 1.6 μJ. When the pump pulse energy was 0.67 μJ (40% the “threshold”), the output pulse energy and pulse duration were 0.07 μJ and 1330 ns, respectively. A marked change in the laser performance was observed when the pump pulse energy reached 1.6 μJ, with the output pulse energy increasing by a factor of 1.7 reaching 0.12 μJ and pulse duration dropping by a factor of 3 to 440 ns. From this point onwards, the output pulse energy grows linearly with the pump pulse energy and a steady decrease in the pulse duration was observed. At 30 μJ pump pulse energy (19 times the “threshold”), the output pulse energy increased to 0.24 μJ with no sign of roll-off whereas the pulse duration converged at ~300 ns. Note that the TDFL showed stable performance over the entire pump pulse energy range tested even for under “threshold” operation. This is due to the presence of the CW pump which was set close to the CW operation threshold of the TDFL. Without the CW pump, a large fraction of the pump pulse energy has to compensate for the cavity loss and thus the remaining useful pump pulse energy is compromised.
4.2 Pulse repetition frequency
The TDFL can work at different repetition frequencies in the range of 2.5 – 100 kHz. The corresponding laser performance, namely the output pulse energy and pulse width were recorded and shown in Fig. 4(a). The pulse energy decreased monotonically with increasing repetition frequency. This is because a higher repetition frequency corresponds to a lower gain-switching pump pulse energy (note that the pump power for the EYDFA was fixed for this test). In contrast, the output pulse width did not show strict monotonic dependence on the repetition frequency. When the laser operated at 2.5 – 10 kHz repetition rate, the output pulse width was relatively stable and fell in the range of 290 – 375 ns (it is interesting that the pulse width at 2.5 kHz is slightly larger compared with 5 kHz and 10 kHz cases; the reason is currently under investigation). However, for laser operation at 10 – 100 kHz repetition rate, the pulse width increased with increasing operation frequency and reached >1200 ns at 100 kHz. This can be explained by Fig. 3 which shows that the pulse duration saturates after the pump pulse energy exceeds a certain value (~5 times the “threshold”). Figure 4(b) plots exemplary oscilloscope traces of output waveforms. It is worth noting that operation at higher repetition frequencies led to slightly narrower tuning ranges, for instance, when the TDFL was gain-switched at 100 kHz repetition frequency, the corresponding tuning range was 1765 – 2025 nm, which was 30 nm narrower on the long wavelength side compared with the 10 kHz case.
4.3 Cavity length
The cavity length is mainly decided by the lengths of the passive and active fibers employed and affects the round trip time hence the output pulse width of the TDFL. This point was demonstrated by splicing an extra length of passive fiber to the setup shown in Fig. 1 and performing a cut-back measurement. Figure 5(a) shows the output pulse width as a function of cavity length. It can be clearly seen that the pulse width increased almost linearly with the cavity length. The range of cavity length tested (6.2 – 14.2 m) corresponds to a round trip time of 30 – 68 ns; therefore the output pulse width was 9 – 10 times the round trip time.
While the passive fiber length affects the system in a relatively straightforward way, the length of the TDF has a more significant influence on the TDFL performance, in particular, the pulse width, threshold, and tuning range. Figure 5(b) shows the dependence of the output pulse width and laser threshold (CW threshold at 1930 nm) on the TDF length. For the TDF lengths tested (3 – 6 m), the output pulse width was measured to be 246 – 365 ns, which was 10 – 12 times the corresponding round trip time. Compared with the case of varying passive fiber length, the output pulse width in this case has a less linear dependence on the round trip time. This arises from the complexity that the active fiber affects not only the round trip time but also the gain in the cavity, which is another important factor that influences the pulse width. On the other hand, a longer piece of TDF also led to a higher laser threshold as shown in Fig. 5(b). The cavity with 3 m of TDF had a threshold of 230 mW for CW operation at 1930 nm whereas at an increased TDF length of 6 m the threshold increased to 298 mW.
Another important effect of altering the TDF length is the change in the wavelength tuning range of the TDFL. It was observed that for the four TDF lengths under test the short wavelength edge of the working window stayed at ~1765 nm whereas the long wavelength edge kept red-shifting from 2030 nm to 2059 nm with increasing TDF length. The 5 m and 6 m TDFs had almost identical tuning range, however, since the 5 m piece has a shorter pulse width and lower threshold (as shown in Fig. 5(b)), it was chosen as the optimal length for the TDFL to achieve the widest tuning range.
4.4 Output coupling ratio
A set of output coupling ratios were tested by using different tap couplers to study its influence on the performance of the laser. Figure 6 plots the measured output pulse energy and pulse width at different output coupling ratios. In terms of output pulse energy, 70% output coupling ratio achieved the best performance (3.5 μJ output pulse energy, 35 mW average power), whereas a coupling ratio of 50% led to the shortest pulse width of 255 ns. Note that the output pulse energy shows a stronger dependence on the output coupling ratio than the pulse width does – the maximum output pulse energy is ~15 times the minimum value, whereas the maximum pulse width is only ~1.2 times the minimum value. Considering that the output pulse energy can be amplified afterwards and the pulse width is not affected much by the output coupling ratio, the tuning range of the TDFL is hence what we try to optimize in this work. It is interesting to note that although the 10% output coupling ratio corresponds to the lowest output pulse energy and longest pulse width, it is optimal in terms of achieving the broadest tuning range. 290 nm (1765 – 2055 nm) tunability was obtained when the output coupling ratio was set at 10%, in contrast to, for example, 270 nm (1765 – 2035 nm) and 200 nm (1805 – 2005 nm) for 30% and 50% coupling ratio, respectively.
4.5 Performance improvement
The current power performance of the TDFL is largely limited by the high insertion loss of the passive components employed in the cavity. The total cavity loss is estimated to be ~10 dB (excluding the extraction ratio of the tap coupler), the major contributing factor being the tunable filter which alone presents ~7 dB insertion loss across the entire operation wavelength range. The maximum power of the signal circulating inside the cavity is therefore estimated to be two orders of magnitude higher than the extracted output power. With better passive components, an order of magnitude increase in output power should be easily achievable for the same configuration.
The temporal performance of the laser can be further improved in terms of achieving shorter pulse width. The output pulse width mainly depends on two factors, namely the round trip time of the cavity as well as the contrast between gain and loss (given in 2g0l/δ where 2g0l is the small signal gain and δ is the round trip loss) . In order to reduce the round trip time, a more heavily doped active fiber can be used [70–73] and pigtails of devices in the cavity should be kept as short as possible. We expect that a total cavity length of 0.5 – 1 m is possible, which corresponds to a round trip time of 2.5 – 5 ns and can potentially lead to a factor of 6 – 12 reduction in the output pulse width compared with current results (i.e., a minimum pulse width of ~18 – 35 ns is achievable). The gain-loss contrast is another factor to optimize in order to produce shorter output pulses. However, this is not straightforward as it does not affect the pulse width in a linear way . Besides, there are a number of parameters (e.g., output coupling ratio, pump power, component insertion loss) which can affect gain and/or loss hence a certain gain-loss contrast may be realized using different configurations. Here we show an example of pulse width reduction to sub-100 ns. Figure 7 plots the output average power and pulse width of a TDFL whose configuration is similar to the one shown in Fig. 1 except for a shorter length of TDF (3 m), a higher output coupling ratio (50%), and a lower loss tunable filter (5 dB, note that this filter was later damaged in another experiment thus cannot be used for this work). The power levels of the CW and pulsed pump were similar to those used to achieve the results above. As can be seen from Fig. 7, the shortest pulse width achieved in this case was 98 ns whereas the output power levels were also higher than those shown in Fig. 2(a) (up to 16 dBm / 40 mW has been achieved in this case). It is worth noticing that the tuning range of this particular TDFL, which covers 200 nm (1800 – 2000 nm), is narrower than the results shown in Fig. 2(a). Nonetheless, this demonstrates the possibility of achieving shorter pulses by modifying the experimental setup, in particular the gain-loss contrast (at the cost of a tunability compromise).
The discussion above indicates that reducing the insertion loss of passive components is crucial to improving the laser performance. In particular, it allows more power to be extracted as the useful output. Besides, a smaller loss value makes a broader range of gain-loss contrast for the same gain range, which offers more flexibility in laser design.
In conclusion, we have demonstrated a monolithic gain-switched widely-tunable TDFL at 2 μm. The tuning range of the laser covers 290 nm (1765 – 2055 nm), which is twice the record tuning range of existing gain-switched fiber lasers. Indeed, to the best of our knowledge, the TDFL developed in this work presents the broadest tuning range that has been reported for monolithic pulsed rare earth doped fiber lasers to date. Improved performance of the laser can be achieved with better 2 μm passive components (for instance, new version tunable filters can achieve insertion losses as low as 3 dB). The laser can find useful applications in, for example, sensing, optical communications, and characterizing devices developed for the emerging 2 μm spectral region. The laser also serves as a good seed source for MOPA configurations whereby multi-kW peak power  and 100 W level average power  can be achieved, making it suitable for higher power applications (e.g., LIDAR and nonlinear frequency conversion).
China Postdoctoral Science Foundation (2016M593022), National High Technology Research and Development Program of China (2015AA021101), National Natural Science Foundation of China (61235008).
The authors thank K. Yin, S.-P. Chen, B. Zhang, X. Qi, L. Yang, C. Lei, Z. Dou, X. Xi, P. Zhou for helpful discussions, X. Wang and X. Jin for sharing components, as well as K. Yin, S. Guo, and X. Yang for assistance with the experiment.
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