Cascaded gain-switching in the mid-infrared region

In this report, we demonstrate mid-infrared dual-waveband (i.e., ~3 μm and ~2 μm) pulses from a cascaded gain-switched Ho3+-doped ZBLAN fiber laser by the use of hybrid pumping of 1150 nm CW and pulse LDs for the first time. Stable ~3 μm gain-switched pulses with the maximum output power 262.14 mW and shortest pulse duration of 0.824 μs were first gained at the repetition rate of 80 kHz and wavelength of 2928.5 nm. Then stable ~2 μm gain-switched pulses at 2068 nm were achieved at a switchable repetition rate between 40 kHz and 80 kHz. The maximum output power and shortest pulse duration were 75.23 mW and 0.787 μs, respectively (not simultaneously). Between them, there is a power-dependent μs-order time delay. This dual-waveband laser source has great potential in laser surgery, material processing.

ZBLAN fiber laser at the range of 2710~2830 nm in which gain-switched mode-locking was observed for the first time 24 . However, until now, all the mid-infrared gain-switched fiber lasers emitted at a single waveband (~3 μm or ~2 μm).
In this report, we demonstrate a dual-waveband gain-switched Ho 3+ -doped ZBLAN fiber laser at ~3 μm and ~2 μm based on the mechanism of cascaded gain-switching for the first time. Stable ~3 μm gain-switched pulses at a repetition rate of 80 kHz and ~2 μm gain-switched pulses at a switchable repetition rate between 40 kHz and 80 kHz were produced simultaneously with a time delay of several μs. The proposed pulse-switching method could be also applied in multi-waveband pulses generation.

Results
In our experiment, the CW pump scheme was first switched on. Its launched pump power was adjusted to and clamped at 1.94 W, the threshold of ~2 μm laser. At this pump power, 92.6 mW ~3 μm CW laser was gotten. If further increasing the CW pump power, obvious ~3 μm self-pulsing was observed, its cause was also under investigation. Then the pulsed pump scheme was switched on, working at the available maximum modulation frequency of 80 kHz (limited by the large capacitance of the LDs) and a moderate duty cycle of 40% (corresponding to the pulse duration of 5 μs). The ~3 μm CW laser was immediately converted into unstable multi-pulsing which may be resulted from the combined effects of self-pulsing and gain-switching under low pulse pump power 25 . While the induced ~2 μm pulsing was also seen with sharply fluctuated amplitudes and varied pulse-to-pulse temporal intervals. Continue to increase the launched pulse pump power (LPPP) until 0.45 W, ~3 μm and ~2 μm pulsing became stronger and stronger but still unstable. When the LPPP was increased to 0.46 W at the fixed launched CW pump power (LCWPP) of 1.94 W. Stable ~3 μm gain-switched pulses at the same repetition rate of 80 kHz as pump pulses were obtained with a μs-order build-up time (calculated from front edge of pump pulse to that of ~3 μm pulse in a period). At this pump power, self-pulsing had been strongly suppressed by the gain-switching and almost negligible influence on the final output. With populations on 5 I 7 level being periodically modulated by ~3 μm gain-switched pulses, stable ~2 μm gain-switched pulses were then induced with a μs-order time delay (calculated from ~2 μm pulse peak to its adjacent ~3 μm pulse peak) but at a halved repetition rate of 40 kHz as shown in Fig. 1(a). This process was termed as cascaded gain-switching. In this case, to induce one ~2 μm gain-switched pulse, two ~3 μm gain-switched pulses were needed due to its low pulse energy. Thus this temporal state was called as "two for one" ("2-1") state for short by us. The left inset of Fig. 1(a) shows the zoomed pump, ~3 μm and ~2 μm gain-switched pulses. Their pulse durations were 5 μs, 1.59 μs and 1.39 μs, respectively. The corresponding build-up time and time delay were 5.5 μs and 3.45 μs, respectively. This stable "2-1" state could be kept until the LPPP of 1.46 W at the fixed LCWPP of 1.94 W. Across this pump range, the build-up time and time delay decreased from 5.5 μs to 4.29 μs and 3.45 μs to 1.65 μs, respectively, with the increase of LPPP, as shown in the right inset of Fig. 1(a), as a result of faster populations accumulation on 5 I 6 and 5 I 7 levels. Figure 1(b) shows the optical and RF spectra of ~3 μm and ~2 μm gain-switched pulses measured at the LPPP of 1.46 W while LCWPP of 1.94 W. It is seen that the ~2 μm optical spectrum centered at 2067.8 nm is smooth but many peaks are involved in the ~3 μm optical spectrum centered at 2928.4 nm. To make this clear, the optical spectrum of stable gain-switched pulses only under pulse pumping (LPPP: 1.8 W) was also measured as shown in Fig. 1(b). The similar optical spectrum shape indicated the multi-peaks resulted from intra-cavity mode distribution instead of CW pumping. High SNRs of >58 dB (~3 μm) and >53 dB (~2 μm) also confirmed quite pure dual-waveband gain-switching.
Further increasing the LPPP to 1.58 W at the fixed LCWPP of 1.94 W, stable ~2 μm gain-switched pulses at the same repetition rate of 80 kHz as ~3 μm gain-switched pulses were gained instead as shown in Fig. 2(a). In this case, only one ~3 μm gain-switched pulse could induce one ~2 μm gain-switched pulse suggesting more populations accumulated on 5 I 7 level in one pump period as a result of the boosted ~3 μm pulse energy. This temporal state was thus called as "one for one" ("1-1") state for short by us. The left inset of Fig. 2(a) shows the zoomed pump, ~3 μm and ~2 μm gain-switched pulses in one period, yielding the pulse durations of 5 μs, 1.048 μs and 1.054 μs, respectively. The corresponding build-up time and time delay were 4.17 μs and 2.5 μs, respectively. Note that at the LPPP range from 1.46 W to 1.58 W while the LCWPP of 1.94 W, it is the transition state jumping between "2-1" and "1-1" states. Until the allowable maximum LPPP of 1.8 W at the fixed LCWPP of 1.94 W, this stable "1-1" state could be always observed. At this moment, if further increasing the LCWPP at the LPPP of 1.8 W, this state was still maintained until the LCWPP of 2.03 W. The right inset of Fig. 2(a) displays the build-up time and time delay of stable "1-1" state with the increased launched total pump power (LTPP) from 3.52 W (LCWPP: 1.94 W, LPPP: 1.58 W) to 3.83 W (LCWPP: 2.03 W, LPPP: 1.8 W) as the process above. It is observed that both decreased almost linearly from 4.17 μs to 3.68 μs and 2.5 μs to 2.25 μs, respectively. At the LTPP of 3.83 W (LCWPP: 2.03 W, LPPP: 1.8 W), the optical and RF spectra were also measured as shown in Fig. 2(b). The center wavelengths were almost unchanged at 2068 nm and 2928.5 nm. And the SNRs were further improved to > 60 dB (~3 μm) and > 55 dB (~2 μm). Once the LCWPP was increased beyond 2.03 W at the maximum LPPP of 1.8 W, however, unstable ~3 μm multi-pulsing was observed due to excessive populations accumulated on 5 I 6 level which cannot be fully depleted by one ~3 μm gain-switched pulse and the influence of self-pulsing mechanism, then inducing unstable ~2 μm gain-switching. Note that the way to adjust both CW and pulsed pump schemes wouldn't influence the laser characteristics. It meant that both temporal state and output performance of dual-waveband gain-switched pulses were certain at the specific pair of LCWPP and LPPP. Figure 3(a) shows the repetition rates and pulse durations of both ~3 μm and ~2 μm gain-switched pulses as a function of the LTPP. It is seen that ~3 μm gain-switched pulses always operate at the same repetition rate of 80 kHz as pump pulses. But ~2 μm gain-switched pulses presented a suddenly doubled repetition rate from 40 kHz to 80 kHz with increasing the LTPP from 3 μm gain-switched pulse duration continued to decrease from 1.054 μs to 0.84 μs. Their output powers and pulse energies as a function of the LTPP were also recorded and calculated as show in Fig. 3(b). Both their output powers increased almost linearly with the increased LTPP. At the LTPP of 3.83 W (LCWPP: 2.03 W, LPPP: 1.8 W), the maximum output power of 262.14 mW of ~3 μm gain-switched pulses at a slope efficiency of 8.8% was achieved. The corresponding maximum pulse energy was 3.28 μJ. While the maximum output power of 75.23 mW of ~2 μm gain-switched pulses was also obtained at a slope efficiency of 3.9%. But their maximum pulse energy of 1.4 μJ was gained at the LTPP of 3.4 W (LCWPP: 1.94 W, LPPP: 1.46 W) as a result of comparatively low repetition rate of 40 kHz. At this pump power, the long-term stabilities of output powers were also measured. The low power fluctuations of ±4% (~3 μm) and ±3% (~2 μm) within 5 hours indicated the high stability of this dual-waveband pulsed laser. As addressed before, once the LCWPP exceeded 2.03 W at the allowable maximum LPPP of 1.8 W, the unstable ~3 μm multi-pulsing appeared leading to unstable ~2 μm gain-switching, thus impeded further output scaling of stable dual-waveband gain-switching. At this time, we adjusted the LPPP slightly beyond 1.8 W but at a very short time, and observed the reappearance of stable dual-waveband gain-switching. Although no more data were recorded in order to keep LDs damage from overloaded operation beyond the maximum allowable LPPP of 1.8 W, the phenomenon indicated further output scaling was indeed limited by the LPPP in our case. Actually, the similar phenomenon had been observed in a previously established hybrid pumped ~2 μm gain-switched Tm 3+ -doped fiber laser 25 which manifested there was always an allowable maximum LPPP for each specific LCWPP to realize stable gain-switching. Besides, fast gain-switching using narrow pulse pumping was verified to be a feasible way to postpone the arrival of multi-pulsing 26 . The μs-order pump pulse duration in our case was thus another limitation factor for output scaling.
Moreover, the effects of pump modulation frequency and pulse duration on the temporal evolution of dual-waveband laser were also experimentally studied. First, the pump pulse duration was fixed at 5 μs by adjusting the cycle duty when the modulation frequency was varied. We found that at a little lower modulation frequency of 70 kHz, the similar temporal evolution was observed just with lower maximum power and energy. However, when the modulation frequency was decreased to 60 kHz and even lower, it became hard to observe stable "1-1" or even "2-1" state as the way above to increase LTPP due to the only multi-pulsing of ~3 μm laser as a result of limited PLLL. Second, the pump modulation frequency was fixed at 80 kHz. At the available lowest cycle duty of 20% which depended on the used function generator of our modulator, although stable "1-1" state was not observed mainly due to the limited PLLL, it was reasonably predicted. However, when the cycle duty was increased to 50%, multi-pulsing of ~3 μm laser instead of stable "1-1" state followed stable "2-1" state due to the reduced pump pulse energy threshold of ~3 μm multi-pulsing with the increased cycle duty. This point has been verified in our recent another investigation on ~3 μm gain-switched Ho 3+ -doped ZBLAN fiber laser. If continuing to increase the cycle duty to 60% and even larger, both "2-1" and "1-1" states could not be observed any more. The results indicated that high modulation frequency and small pump pulse duration facilitated stable dual-waveband gain-switched pulses generation.

Discussion
For gain-switching, pulse duration was a key parameter associated with practical applications. In this case, the shortest pulse durations of ~3 µm and ~2 µm gain-switched pulses were 824 ns and 787 ns, respectively which were much larger than some obtained in the wavelength region of 1~2 µm (as small as several hundreds and even tens of ns) 20 . According to our recent investigation on ~3 µm single-waveband gain-switching (unpublished) combined with the previously reported results 20,27 , it was found that the gain-switched pulse duration was related to a set of parameters mainly including pump pulse energy, pump modulation frequency, pump pulse duration, cavity length, output coupler ratio and operation wavelength. For the former three which were relatively feasible to control for our current scheme, higher pump pulse energy, lower pump modulation frequency and smaller pump pulse duration were beneficial for obtaining narrower pulses. In our case, however, the modulation frequency could not be tuned to too low in order to get stable dual-waveband gain-switching. Thus, the allowable maximum pump pulse energy and minimum pump pulse duration limited the achievement of shorter pulse duration. For the latter two, although there were optimal output coupler ratio and operation wavelength to minimize pulse duration, additional components (e.g., partially reflected mirror, wavelength selector) were needed which significantly complicated the system, despite the compact but not cost-effective mid-infrared ZBLAN FBG 28 . For the cavity length (=gain fiber length + free-space length) directly related to the pulse duration, the intra-cavity free-space length in our scheme was about 15 cm which was hard to be further shortened considering the self-volume of the mount used for the mirror/lens. On the other hand, it was impractical to sharply shorten the gain fiber as well in order to maintain enough gain since it was lowly doped.

Conclusions
In summary, we demonstrated ~3 μm and ~2 μm dual-waveband pulses from a cascaded gain-switched Ho 3+ -doped ZBLAN fiber laser in a compact linear cavity. CW and pulse lasers at 1150 nm were used to pump the scheme simultaneously where the CW laser aimed at providing populations for 5I6 level while the pulse laser acted as a switch to trigger the ~3 μm pulses. At the pump modulation frequency of 80 kHz and cycle duty of 40%. Stable ~3 μm gain-switched pulses at the same repetition rate of 80 kHz were first obtained. With a μs-order time delay, stable ~2 μm gain-switched pulses were also achieved at a switchable repetition rate between 40 kHz and 80 kHz. The shortest pulse durations of ~3 μm and ~2 μm gain-switched pulses were 0.824 μs and 0.787 μs, respectively. The maximum pulse output power of 262.14 mW (~3 μm) and 75.23 mW (~2 μm) were mainly limited by 1150 nm pulse pump. In the next step, further output scaling of dual-waveband gain-switched pulses would be carried out by introducing more powerful and narrower 1150 nm pulse pump sources (e.g., pulsed version of 1150 nm Yb 3+ -doped silica fiber laser 29 ) combined with cascaded Ho 3+ -doped ZBLAN fiber based MOPA scheme considering their high SNRs. This demonstration puts forward a universal and compact approach to yield dual-and even multi-waveband pulses, and is beneficial for some applications in the mid-infrared region.

Methods
Design of gain-switched fiber laser. The schematic of the laser is shown in Fig. 4. The gain fiber was a custom-designed double-clad Ho 3+ -doped ZBLAN fiber (FiberLabs, Japan). It has a D-shaped inner clad with a diameter of 125 μm across the circular cross section and a NA of 0.5. Its core diameter and NA are 10 μm and 0.16, respectively. To obtain ~3 μm and ~2 μm components easier simultaneously (the simplified energy level structure is shown in the right inset of Fig. 4), low Ho 3+ ions dopant concentration of 1.2 mol.% was selected to weaken energy transfer between Ho 3+ ions thus reducing ~2 μm laser threshold. Its clad absorption coefficient at 1150 nm was measured to be 0.28 cm −1 using the cutback method and length was 4.5 m. Based on our offthe-shelf commercial 1150 nm LDs (Eagleyard Photonics Berlin) in lab, double-side pumped arrangement was designed to provide enough pumping for dual-waveband emissions. However, it would bring us the trouble that the pump pulses from two ends would temporally asynchronously change local gain in the cavity hence leading to chaotic gain-switching. To circumvent this problem, CW pumping was meanwhile introduced just to provide populations for 5 I 6 laser upper level when pulse pumping acted as a switch to trigger the ~3 μm and then ~2 μm pulses. The CW pump scheme includes two polarized combined 1150 nm LDs using a polarizing beam splitter (PBS). The output CW laser was launched into the gain fiber after focusing using a plano-convex CaF 2 lens (Thorlabs, LA5315) at a launching efficiency of 82%. A dichroic mirror with a transmission of 94% at 1150 nm and a high reflection of >95% at ~3 μm and ~2 μm was positioned between the PBS and CaF 2 lens at an angle of ~30% with respect to the pump beam. It was used to steer the ~3 μm and ~2 μm laser from the fiber core onto a gold-protected mirror as one side of the cavity. To prevent intra-cavity parasitic lasing, the fiber end close to the CW pump scheme was cleaved at an angle of 8°. The pulsed pump scheme is similar to the CW one just with additionally inserting a home-made modulator and 1 Ω resistance (monitoring instantaneous current) into the loop. The output pulsed laser was launched into the gain fiber using a same scheme as before. Here the fiber end was perpendicularly cleaved as the other side of the cavity and output coupler with the help of 4% Fresnel reflection. Another dichroic mirror same as before was also positioned between the PBS and CaF 2 lens to steer the ~3 μm and ~2 μm pulses out from the system.

Measurement of laser output characteristics.
The dual-waveband pulses were distinguished using a dichroic mirror (high transmission/reflection at ~3/2 μm) and then detected using an InAs detector (Judson, J12-18C-R01M) and an InGaAs (EOT ET-5000F), respectively, both connected with a 500 MHz digital oscilloscope. The radio frequency (RF) spectrum was captured by a RF spectrum analyzer (YIAI, China, AV4033A, 30Hz-18GHz). A monochromator based on nitrogen cooled photodiode (Princeton instrument Acton SP2300) was used to measure optical spectrum.