We demonstrate a self-Q-switched, all-fiber, tunable, erbium laser at 1530 nm with high pulse repetition rates of 0.9-10 kHz. Through the use of an auxiliary 10-mW, 1570 nm laser that shortened the relaxation time of erbium, sequentially Q-switched pulses with pulse energies between 4 and 6 μJ and pulse widths of 40 ns were steadily achieved. A peak pulse power of 165 W was obtained.
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Fiber lasers have been Q-switched using traditional bulk Q-switches and fiber-pigtailed ones. Sophisticated alignment techniques were necessary for reducing the coupling losses between the bulk Q-switches and the fiber cores. High-power, multimode pump diodes, double-cladding fibers and pump-signal combiners were often employed to increase pump efficiency and build up large laser gains that compensated for the coupling losses. All-fiber Q-switched lasers with fiber-pigtailed Q-switches are of great interest because of their alignment-free characteristics and low cavity loss that is essential for efficient Q-switching performance. The fiber-pigtail Q-switches can be active devices as piezoelectric actuators , acousto-optical modulators [2–4] and magnetostrictive transducers , or passive ones such as semiconductor saturable-absorber mirrors (SESAM) [6,7] and saturable-absorber fibers [8–12]. Passive Q-switching with a saturable absorber fiber is the simplest and most economical approach for producing high-power laser pulses. In addition, a solid-state saturable absorber Q-switch (SAQS) fiber has a high damage threshold and can hold an enormous laser gain for energy release into one Q-switched laser pulse. In spite of these advantages, the SAQS fibers are seldom considered in the literature. One of the criteria for a SAQS laser is that the absorption cross section, σa, of the SAQS should be larger that the emission cross section, σe, of the gain medium. Furthermore, to achieve sequentially Q-switched pulses using a continuous-wave (CW) pump source, the relaxation lifetime of the SAQS, τa2, should be shorter than that of the gain medium, τg2. There has been a breakthrough in the last four years in finding SAQS fibers for ytterbium fiber lasers. Fibers doped with bismuth  and rare earth ions such as samarium , thulium  and holmium  have been demonstrated as efficient SAQS materials for ytterbium fiber lasers within the wavelength range from 1050 nm to 1125 nm where the σe of Yb is relatively small. Moreover, a Ho-doped fiber was reported as a SAQS fiber for use in a Tm fiber laser operating at 2-2.1 μm .
Erbium fiber lasers emitting at the so-called eye-safe wavelength of 1.5-1.6 μm are the preferred laser devices for light detection and ranging (LIDAR) and other applications that require Q-switched operation. Few bulk crystals such as U:CaF2  Co:ZnS and Co:ZnSe  have been reported to be efficient SAQS materials for Er:glass lasers. Nevertheless, there has been no fiber-type SAQS material demonstrated for erbium fiber lasers except for erbium fiber itself. Recently, we demonstrated self-Q-switched, all-fiber erbium lasers in a ring scheme  and a standing-wave resonator , where erbium fibers served as both 3-level lasers and 2-level saturable absorbers. Since the cross sections, σa and σe, of erbium are comparable, techniques of photon density enhancement in the SAQS fibers, such as double-passing routes and mismatch of mode field areas (MFA), were developed to activate SAQSing functions. Due to lifetime matching, τa2 = τg2, in a self-Q-switched laser, a saturable-amplifier pump switch (SAPS) was introduced for acquiring sequentially Q-switched pulses . The function of the SAPS was to passively switch the pump intensity and delay the recovery of the gain population after Q-switched pulses. Although a passively Q-switched, all-fiber laser was demonstrated at low repetition rates, the peak pulse power decreased quickly (i.e. less pulse energy and larger pulse width) with an increasing pump power. The peak pulse power was 100 W at the lowest repetition rate of 250 Hz and decreased to 20 W at 1 kHz (i.e. ~10/τ2). The low-rate Q-switching was primarily due to the long relaxation lifetime of erbium (10 ms). Techniques shortening the relaxation lifetime of the erbium SAQS fiber could solve this inherent drawback and broaden the applications of the device.
In this paper, we demonstrate a self-Q-switched, all-fiber, erbium laser that emits at 1530 nm using an auxiliary 1570 nm erbium laser applied to the SAQS erbium fiber. Both wavelengths, 1530 and 1570 nm, are in the same transition band of erbium. The dopant Er3+ has a broad band of energy transition between 4I13/2 and 4I15/2, which corresponds to an emission and absorption wavelength range from 1.48 to 1.6 μm. According to the specifications of the SAQS erbium fiber provided by the manufacturer nLight, the emission and absorption cross sections at 1530 nm, σe,1530 and σa,1530 are about the same: 6 × 10−21 cm2. The cross sections, σe,1570 and σa,1570, are 2.6 × 10−21 cm2 and 1.3 × 10−21 cm2. These values might differ slightly for different products and manufacturers. The effective absorption population, defined as Na,λ = [ Na1-(σe,λ/σa,λ)Na2], is wavelength-dependent, and the cross-section ratio, σa,λ /σe,λ, is the population ratio of Na2 to Na1 in the state of full absorption saturation, that is to say, Na,λ = 0. When the SAQS is fully saturated by a large 1570-nm laser, Ia,1570, we have Na,1570 = 0, Na2 = NaT/3 and Na1 = 2NaT/3 that indicates Na,1530 = NaT/3, where NaT is the total erbium dopants of the SAQS fiber. Therefore, the initial value of Na,1530, called Nai, for Q-switching can be tuned between NaT and NaT/3 with an Ia,1570 applied on the SAQS fiber. More importantly, an Ia,1570 can shorten the relaxation lifetime of Na,1530. When the SAQS fiber is bleached by a Q-switched pulse at 1530nm, Na,1530 is zero and Na,1570 is -NaT/2 (i.e. gain in the SAQS for Ia,1570). This instant status will soon be modified by the Ia,1570 back to the initial condition, Na,1530 = Nai, for next Q-switching. To simplify the discussion on the effect of Ia,1570 on the erbium lifetime, we assumed uniform distribution of Na2, Na1 and Ia,1570 in the SAQS and derived the effective lifetime of Na,1530 as:
Figure 1 shows the schematic design of a self-Q-switched, erbium, all-fiber laser, pumped with a continuous-wave (CW) 980-nm laser diode (LD). The Q-switched laser was stabilized, and the pulse repetition rate was tunable using a 1570-nm laser that was also an erbium fiber laser pumped with a 980-nm LD (not shown). Therefore, the laser system could be simplified using only one high-power pump LD and a pump splitter with a properly designed ratio. The gain medium was a 210-cm erbium-doped fiber with a core diameter of 14 μm and an absorption loss of 19 dB⋅m−1 at 1530 nm, manufactured by the company Coractive. The SAQS was a 20-cm Er fiber, made by the manufacturer nLight, with a relatively smaller core diameter of 4 μm and an absorption loss of 110 dB⋅m−1 at 1530 nm. The mismatch of the mode field areas (MFA) between the gain and the SAQS resulted in high photon density and fast absorption saturation in the SAQS fiber, thereby giving rise to Q-switching action. The numerical aperture (NA) number and the mode field diameter of the SAQS were 0.2 and 6.5 μm, indicating a confinement factor Γ of about 0.53. Thus, the saturation power, Is,1570, was determined by Eq. (1) to be 1.2 mW. The 980/1530 nm WDM inside the resonator was used to protect the SAQS from the pump power. Similarly, a 1530/1570 nm WDM was employed to prevent the gain fiber from being stimulated by the 1570-nm laser. All components were core-fusion spliced. The length of the resonator was about 4 meters.
The Q-switching performances related to the pulse repetition rate, Rpr, with Ia,1570 = 0 and Ia,1570 = 10 mW are compared and shown in Fig. 2(a) and (b) . Both the cases with and without Ia,1570 were stable when Rpr was within 0.9-10 kHz and unsteady when Rpr was below 0.9 kHz. Instead of stabilizing the laser at low Rpr (<1 kHz) as the function of a saturable-amplifier pump switch (SAPS) , the tuning source, Ia,1570, primarily improved the Q-switching efficiency at high Rpr by affecting the relaxation lifetime. Without the Ia,1570, a pulse had a full-width-at-half-magnitude (FWHM) of about 0.9 μs and an energy of 1.1 μJ at a repetition rate, Rpr, of 0.9 kHz near the laser threshold. The 0.9 kHz value of Rpr indicated a 1.1-ms recovery time (i.e. τa2/9) for Na,1530 after being bleached by a Q-switched pulse. Assuming the SAQS was fully bleached by each pulse, it can be calculated that Na,1530 switched between 0 to about 0.1NaT. A higher pump power would give less time for Na,1530 recovery, leading to a higher repetition rate and smaller pulse energy. The pulsing output was still observed with a pump power larger than 100 mW where the pulse had a pulse width of microseconds and a very low pulse energy. Such low-efficiency Q-switching is referred to as “Q-fluctuation” and can only hold a small amount of energy in the gain medium and will have low extraction efficiency of the gain population.
When the 1570-nm laser was turned on, the Q-switching performance was much improved. By comparing the two cases (i.e. with and without Ia,1570) at the same Rpr in Fig. 2(a), the improved pulse energy with the Ia,1570 demonstrates faster and more extensive recovery of Na,1530. Due to this increased Na,1530 recovery, a higher pump power was required with the Ia,1570. Furthermore, the improved recovery of Na,1530 denoted a higher hold-off ratio of the gain population in the gain fiber, in turn leading to a better Q-switching performance with a shorter pulse width and higher peak power as clearly demonstrated in Fig. 2(b).
The relation between the pulsing characteristics and the Ia,1570 with a constant pump power of 60 mW is shown in Fig. 3 . As indicated in Eq. (1), a more intense Ia,1570 will contribute a shorter τa2’. Due to the increased Na,1530 recovery that occurs with a shorter τa2’, more time for pumping was needed to compensate for the increased Na,1530, which resulted in a decreasing Rpr. Consequently, better pulsing characteristics with an increasing Ia,1570, as higher pulse energy, shorter pulse width and stronger peak power, were achieved. It is interesting to note that the decrease in Rpr, and the increasing of pulse energy with the Ia,1570 nearly stopped when the Ia,1570 approached 10 mW. Such a steady state implied that the Ia,1570 was sufficient to provide for a nearly full recovery of Na,1530 back to the initial value of Nai for each subsequent Q-switched pulse. When the Ia,1570 was sufficient, the Q-switching efficiency depended upon the designed Nai and the fundamental physics of optimization.
We further improved the Q-switching performance by doubling the length of a SAQS fiber. The SAQS had an absorption loss of 44 dB at 1530 nm that was even larger than that of the gain fiber. Thus, the laser could not reach the threshold by pumping alone without the assistant of the Ia,1570. Stable, sequential, Q-switched pulses were achieved using a 10-mW Ia,1570, as shown in Fig. 4 . The pulse had a stable shape, a FWHM of about 40 ns, and a peak power of larger than 100 W along the pump range from 75 to 200 mW. The maximum pulse energy of 6 μJ and peak power of 165 W was achieved at the lowest Rpr of 0.1 kHz. The Rpr was proportional to the pump power and limited by the maximum 980-nm LD output. The high Q-switching efficiency was attributed to the high hold-off ratio of the gain population by the large Nai, which, in turn, lead to a high extraction efficiency of the pumped gain. By recording the readings on the oscilloscope, statistics showed that the deviation of the repetition rate, ΔRrp/Rrp was about 7% - 8% and the deviation of the peak power was about 8%. The stability showed no strong relation with Rrp, although it seemed that the stability was a little better (smaller ΔRrp/Rrp) when Rrp was higher. Since more pumping time was needed for the large gain population, the Q-switching was stabilized into a low-Rpr range from 0.1 to 1.5 kHz. Efficient Q-switching at higher Rpr should be achievable using a more intense pump LD. The results demonstrated here were better and more stable than what could be achieved using a saturable-amplifier pump switch .
We have demonstrated a self-Q-switched, all-fiber erbium laser emitting at 1530 nm through the use of an auxiliary laser at 1570 nm, Ia,1570, that allowed tunable and optimizable Q-switching performance. The Ia,1570 was applied to a SAQS erbium fiber to shorten the relaxation lifetime. The wavelengths of 1530 and 1570 nm are in the same band of energy transition between 4I13/2 and 4I15/2. A SAQS fiber with a 22-dB absorption loss was employed for demonstrating the effect of lifetime shortening, and the improvement on Q-switching at the repetition rate from 0.9 to 10 kHz. By doubling the length of the SAQS fiber and applying a 10-mW Ia,1570, sequential pulses with pulse energy of 6-4 μJ, steady pulse width of 38-40 ns and peak power of 165-105 W were achieved at repetition rate of 0.1-1.5 kHz. Efficient Q-switching at higher repetition rates is expected when a more intense pump source is employed.
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