We propose and numerically demonstrate a cascade pulsing mechanism in a CW-pumped Er3+:ZBLAN all-fiber laser system. In the design, the laser was pumped at 980 nm and passively Q-switched at 1.6 μm. The Q-switched resonance reduced the population on 4I13/2 of the erbium gain fiber, thereby creating a population inversion between the levels of 4I11/2 and 4I13/2, and instantly inducing an intense gain-switched pulse at 2.7 μm. Sequential 2.7-μm single-mode pulsing with a pulse energy of 170 μJ and a peak power of 6 kW was achieved with an absorbed pump power of 0.65 W.
© 2012 OSA
Intense mid-inferred lasers near the water absorption peak of 3 μm are useful for medical applications. This fact has spurred the development of erbium ZBLAN fiber lasers. The lasing of Er3+ at 2.7-2.8 μm is attributed to the transition from 4I11/2 to 4I13/2. The intrinsic lifetimes of τ2, τ1 and τ21 in an Er3+ doped ZBLAN fiber are 6.9, 9 and 16.9 ms, respectively [1,2]. Due to the small branching ratio from 4I11/2 to 4I13/2, i.e., ϕ21 = τ2/τ21, a positive population inversion and continuous-wave (CW) lasing can be obtained [2,3]. However, the laser efficiency and power scaling are limited by N1, the easily accumulated population of 4I13/2. Several approaches have been developed to reduce N1 and improve laser efficiency, including the use of 792-nm excited-state-absorption (ESA) pump on the 4I13/2 level , lifetime quenching by energy transfer (ET) between Er3+ and a Pr3+ co-dopant [5,6] and energy-transfer-upconversion (ETU) in highly erbium-doped fibers . In addition, the de-excitation of 4I13/2 by 1.6-μm co-lasing in cascade transitions is an efficient solution for reducing the heat caused by multi-phonon relaxation . Opposite to these efforts directed towards de-populating the level 4I13/2 of Er3+, herein we propose an idea that the population of 4I13/2, N1, accumulated in the gain fiber can serve as an inherent absorber for 2.7-μm pulsing operation.
Similar to CW power scaling, the Q-switched pulse power can be increased using the fibers that are core-pumped at 792 nm , co-doped with praseodymium , or highly erbium-doped . The Q-switches that have been employed were traditional rotating mirrors , shutters , germanium acousto-optic modulators [9,11] and passive saturable absorbers as semiconductor InAs epilayers  and a liquefying gallium mirror . Pulsed power scaling has recently been achieved by S. Tokita et al. , who used a laser consisting of a double-cladding highly erbium-doped ZBLAN multimode fiber with a core diameter of 35 μm. In the aforementioned study, the laser was 75-W pumped and actively Q-switched to achieve an average output power of 12 W at 2.8 μm, a pulse energy of 100 μJ and a peak power of 0.9 kW. In comparison, although passive techniques are simple and cost-effective, progress on their performance enhancement has been relatively slow. Thus far, a fiber-type saturable absorber Q-switch (SAQS) has not been developed for erbium ZBLAN fiber lasers at this mid-IR range. Unlike the traditional Q-switching and pulse-pumped gain-switching [14,15], in the present study, we propose and numerically demonstrate a cascade pulsing mechanism, where a large N1 served as an inherent absorber for populating a comparable N2, and an intense 2.7-μm pulse was induced by the sudden removal of N1 by intra-cavity 1.6-μm Q-switching. With a 980-nm CW pump, sequential 2.7-μm pulsing with a pulse energy of 170 μJ and a peak power of 6 kW was passively achieved with an absorbed pump power of 0.65W.
The Q-switching-induced gain-switching (QSIGS) mechanism in a CW-pumped Er:ZBLAN all-fiber laser is depicted in Fig. 1 . In the design, a 980-nm pump laser and a lightly erbium-doped double-cladding ZBLAN fiber were employed. As a result, the previously discussed ESA pump, ET with co-doped Pr3+ and ETU between highly-doped Er3+ pairs were negligible. Basically, the system was initially subjected to self-terminating lasing caused by the long lifetime of τ1 and the populated N1. Because the resonance of 1.6 μm was initially suppressed by a Tm3+-doped silica fiber, large amounts of N1 and N2 in the gain fiber were created by the 980-nm CW pump. Tm3+-doped fiber is an efficient SAQS material for erbium fiber lasers at emission wavelengths greater than 1570 nm [16,17]. When N1 reached the threshold, the erbium laser was saturable-absorber Q-switched at 1.6 μm by the thulium fiber. Due to the large ratio of the emission cross section (σ10) to the absorption cross section (σ01) of Er3+ (σ10 and σ01 of Er3+ ZBLAN fibers are 1.28 × 10−21 and 0.32 × 10−21 cm2 at 1.6 μm, respectively ), the Q-switched pulse in the cavity efficiently reduced N1 to the ground state (4I15/2), and a positive population inversion was created between the 4I11/2 and 4I13/2, which yielded a gain-switched pulse at 2.7 μm. According to the thermal management , the low Er3+ concentration and 1.6-μm co-pulsing used in the present system can reduce the heat generated by multi-phonon relaxations and improve the pulsed power scaling.
In the cascade transitions of 4I13/2 → 4I15/2 and 4I11/2 → 4I13/2, the pulses of 1.6 and 2.7 μm increase each other’s pulse energies when they are temporally overlapped. Since a gain-switched pulse follows a Q-switched pulse, to obtain effective pulse overlap, the gain of 2.7 μm must be present at the lasing threshold when Q-switching occurs, and the instantly switched gain of 2.7 μm must be larger than the Q-switched gain of 1.6 μm. These two criteria are satisfied due to the nature of self-terminating lasing and the relatively large σ12 at 2.7 μm compared with the σ10 at 1.6 μm (see Eq. (1) for explanation). The spectrum of σ12 cannot be easily measured and has not been thoroughly studied. Nevertheless, the fluorescence spectrum of Er-doped ZBLAN glass is known to range from 2.6 to 2.9 μm and peak at 2.72 μm, and the peak σ21 at room temperature is 5.7 × 10−21 cm2 [1,19]. Red shifts from 2.7 to 2.8 μm are often observed in the resonators with broadband mirrors [14,20,21], which implies that the ratio of σ21 to σ12 increases with an increase in the wavelength. Due to Stokes shift, σ12 is comparable to σ21 at or below 2.72 μm. In the simulation, σ12 and σ21 were assumed to be 5 × 10−21 cm2 at 2.7 μm. Hence, fiber Bragg gratings (FBGs) of 2.7 μm on the ZBLAN fiber must be employed to operate under a preferred large σ12 and avoid red shifts in the wavelength. In 2007, highly reflective FBGs were written on ZBLAN fibers using 800-nm femtosecond pulses  and have been applied in CW high-power all-fiber Er:ZBLAN lasers [7,23].
A large modulation depth of 2.7-μm gain-switching provided by the inherent N1 is easily arranged and only limited by the concern of fiber damage threshold. The modulation depth ratio of gain-switching to Q-switching, rmd, can be derived as:Eq. (1), assuming a negligible non-saturable loss in the 1.6-μm resonator, the modulation depth of Q-switching provided by the absorption strength of the thulium fiber was approximated as MdQ = 10/ln(10)⋅(σ10NgQ/Ag1), where NgQ was the threshold gain population of Q-switching. Due to the negligible non-saturable loss, the extraction efficiency of NgQ was presumably 100%. Furthermore, because the resonator of 2.7 μm was already at the threshold when Q-switching occurred, the modulation depth of gain-switching, MdG, was defined to be MdG = 10/ln(10)⋅(σ12/Ag2)⋅ΔN1, where ΔN1 was the change in N1 corresponding to 100% extraction of NgQ by Q-switching, i.e., ΔN1 = NgQ/(1 + g10). In the later simulation, rmd was calculated to be 1.87 and MdG was approximately 18.7 dB.
2. Modeling and simulation
The passive cascade pulsing mechanism in a lightly erbium-doped ZBLAN fiber laser is relatively simple to model compared with the simulations of highly doped ZBLAN CW lasers, where the processes of ET and ETU must be considered. Additionally, due to the low brightness of the pump applied in the double-cladding erbium fiber, the ESA pump was negligible. Therefore, most of the Er3+ atoms were presumably distributed among 4I11/2, 4I13/2 and 4I15/2, and the summation of N2, N1 and N0 was a constant value. The rate Eqs. of the cascading pulsing mechanism were modified based on Siegman’s Eqs . and are shown below:
Figure 2 shows the correlations between the populations, N2, N1, N0, Na, and the output 1.6- and 2.7-μm pulses during the cascade pulsing operation. In the beginning, the absorption population of Tm3+, Na, was quickly saturated at the onset of a 1.6-μm Q-switched pulse. Next, the Q-switched pulse reduced N1 to N0 and induced a 2.7-μm gain-switched pulse. Due to the self-terminating lasing at 2.7 μm (shown later in Fig. 3 ), a constant difference between N2 and N1 was maintained until Q-switching occurred, as shown in the inset of Fig. 2. Because the gain of 2.7 μm was already at the threshold and σ12 was larger than σ10, the gain-switched pulse appeared instantly, reached a peak and ended quickly, presenting a pulse width of 21 ns which was much shorter than the Q-switched pulse width of 192 ns. The pump power density (Ip) in Fig. 2 was approximately 0.7 times the saturation pump density (Ips), which was defined as hνp/(σ02τ1ϕ21). The average absorbed pump power was calculated using the second term in Eq. (2), and was equal to 647 mW. As a result, the output pulses of 1.6 and 2.7 μm possessed pulse energies of 257 and 170 μJ and peak powers of 1.2 and 6 kW, respectively. In the pulse overlap, the 1.6- and 2.7-μm pulses increased each other’s gains, and higher energy extractions were obtained. The extra amounts of energy contributed to the 1.6 and 2.7-μm pulses by pulse overlap were calculated to be 115 and 57 μJ, respectively.
The stable sequential pulses of 2.7 and 1.6 μm are shown in Fig. 3, along with the 2.7-μm spikes and their correlations on smaller time scales plotted in the insets. Because Er:ZBLAN is initially a 4-level laser medium, the relaxation oscillation at 2.7 μm occurred immediately after 980-nm pumping, as observed in inset (1). A spike appeared with an energy of 1 to 2 μJ and peak powers less than 2 W at an oscillation frequency of 43 kHz, as shown in inset (2). After an extended operating time, approximately 3000 gain-switched pulses were produced, and the average output powers of the spikes, the Q-switched pulses and the gain-switched pulses were calculated to be 76.8, 118.7 and 78.7 mW, respectively. At some pump levels, the spikes interfered with the gain-switched pulses, broadened the pulse durations and lowered the peak powers. However, the relaxation oscillation did not have a significant effect on the pulse energy and repetition rate due to the small spike energy. Figure 4 shows the variations of the pulse characteristics versus the pump normalised by Ips. Due to the long relaxation lifetime of Tm3+ (i.e., τa2 ~0.4 ms), the pulsing performance decreased with an increase in the repetition rate. When Ip/Ips was greater than 3, the system transitioned to CW lasing at 1.6 and 2.7 μm because a proper modulation depth of Q-switching could no longer be provided by the thulium fiber. A longer thulium fiber can provide larger modulation depths of Q- and gain-switching and can increase the range of pulse repetition rates as well as the pulse energies and peak powers.
We have modeled and numerically demonstrated a novel cascade-pulsed mechanism in a CW-pumped erbium ZBALN all-fiber laser system. The population on 4I13/2 of the erbium ZBLAN fiber was used as an inherent absorber at 2.7 μm and a gain medium at 1.6 μm, and was modulated by a coupled passively Q-switched resonator to induce intense 2.7-μm pulses. A large modulation depth of gain-switching can be easily arranged and was 18.7 dB in the design. The overlap of 1.6- and 2.7-μm pulses in the cascade transitions increased each other’s gain and contributed higher energy extractions. As a result, by use of a 980-nm CW pump, sequential 2.7-μm gain-switched pulses with a pulse energy of 170 μm and a peak power of 6 kW was passively achieved. The proposed technique of Q-switching-induced gain-switching is a potential mechanism for producing most intense 2.7-μm pulses in an all-fiber laser scheme.
The authors acknowledge support from the National Science Council of Taiwan (Project No. NSC 100-2628-E-006-030-MY3) and the Industrial Technology Research Institute, Tainan, Taiwan (FY101 Prospection Cooperation Project).
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