We demonstrate all-fiber passively Q-switched erbium lasers at 1570 nm using Tm3+-doped saturable-absorber fibers. The absorption cross section of a Tm3+-doped fiber at 1570 nm was measured in a bleaching experiment to be about 1.44 × 10−20 cm2. With a thulium-doped fiber, sequential pulses with a pulse energy of 9 μJ and a pulse duration of about 420 ns were stably produced at repetition rates in the range 0.1 to 2 kHz. The maximum pulse repetition rate was 6 kHz, limited by the maximum pump power of a 980-nm laser diode, about 230 mW.
©2010 Optical Society of America
Q-switched all-fiber lasers are of great interest because of their flexibility, large accumulated one-trip gain, high beam quality and intense power confined in mode field diameters of only a few micrometers. In addition, all-fiber lasers are alignment-free and have low cavity losses that are essential for efficient Q-switching performance. Q-switching in all-fiber lasers has been achieved by applying a mechanical force on highly dispersive fiber Bragg gratings (FBG). The mechanical force or vibration could be provided by piezoelectric actuators , acousto-optics modulators [2,3], or magnetostrictive transducers . Modulation of the Q factor can also be realized in passive approaches using semiconductor saturable-absorber mirrors [5,6] and solid-state saturable-absorber fibers [7–14].
The advantages of the solid-state saturable-absorber Q-switch (SAQS) fibers are their ability to hold enormous gain excited in the gain fiber from lasing and their high damage threshold for high-power Q-switched pulses. The saturable absorber Q-switching criterion, considering level degeneracy and mode-field-area (MFA) mismatch  is:
Only a few SAQS fibers have been demonstrated in the literature, and most are for ytterbium-doped fiber lasers [7–11]. Recently, we reported a few techniques to enable the self-Q-switching mechanism in all-fiber erbium lasers [12–14]. In spite of the success of the self-Q-switching methods, its efficiency and structural simplicity could be overshadowed by the use of a natural SAQS that has an inherently high σa. Here we propose and demonstrate a passively Q-switched erbium all-fiber laser using a thulium-doped SAQS fiber. Thulium-doped fiber is the first SAQS fiber material (except for erbium itself) demonstrated for an erbium all-fiber laser.
The energy transition 3H6-3F4 of Tm3+ has a very broad emission wavelength range, from 1.6 to 2.1 μm, and an absorption band from 1.5 to 1.9 μm. Tm3+ silicate fibers co-doped with aluminum, germanium, and Zinc modifiers have exhibited varying emission and absorption cross sections. The reported absorption cross section at 1.6 μm was about 3.5-4.5 × 10−21 cm2 in Al/Ge co-doped fibers  and about 8 × 10−21 cm2 in Zn-boro-silicate fibers . These values were larger than the emission cross sections of Er3+ doped fiber at 1.6 μm, σe~1 × 10−21 cm2, suggesting a possible realization of a passively Q-switched erbium fiber laser using a thulium saturable-absorber fiber.
A thulium fiber (model Tm134) employed in the experiment was obtained from the manufacturer CorActive. The core glass host was alumino-phospho silicate. The fiber had a small core diameter of 3 μm and a core aperture number (NA) of 0.18. A large mode-field diameter (MFD) of about 10 μm and a weak confinement factor of 0.17 at 1570 nm were estimated. The thulium fiber was originally designed for single-mode guiding of wavelengths as short as 1210 nm and 790 nm. Figure 1 shows the absorption spectrum of the Tm134 fiber and the normalized σa(λ), which is the absorption spectrum divided by the numerically calculated confinement factor, Γ(λ), and further normalized by the peak value at 1210 nm. The spectrum of the thulium fiber shows an absorption band in the 3H6-3F4 transition that is relatively strong compared to the peak absorption at 1210 nm. This absorption strength (3H6-3F4) was found stronger than the values reported in the literature.
As the wavelength decreases from 1.6 to 1.57 μm, the σa of the SAQS Tm fiber decreases by a factor of 1.7 and the σg of the Er gain fiber increases by a factor of 2.6, leading to an increasingly unfavorable Q-switching condition. According to the σa values in the literature and the Q-switching criteria, an erbium laser should not be Q-switched with a thulium fiber at wavelengths at or shorter than 1570 nm. Nevertheless, as demonstrated in the later experiment, erbium lasers were efficiently Q-switched with thulium fibers at 1570 nm, indicating higher-than-expected σa values for the employed thulium fibers. The σa of a bulk crystal could be measured in a bleaching experiment with a best-fit curve of the modified Avizonis-Grotbeck’s equation . Since light power is partially confined in the fiber core, we further modified Avizonis-Grotbeck’s equation to consider the confinement factor of fiber samples in the bleaching experiment. The σa of a Tm SAQS fiber was determined to be 1.44 × 10−20 cm2 at 1570 nm.
2. Experiments of saturable absorber Q-switching and bleaching
The schematic diagram of a passively Q-switched all-fiber erbium laser using a thulium SAQS fiber is shown in Fig. 2 .
The laser cavity was simple, consisting of two FBGs, one erbium fiber and one thulium fiber. One 980/1570 nm WDM was located outside the cavity to couple the pump input and pulse output. The FBGs had a high reflectivity (HR) near 100% and a low reflectivity (LR) about 20% at 1570nm. The erbium gain fiber (model Er80-8/125) was manufactured by nLight to the following specifications: a core diameter of 8 μm, a MFD of 9.5 μm, and an absorption loss of 34.6 dB⋅m−1 at 1570 nm. The gain fiber was 40 cm long. The thulium fiber (Tm134 by Coractive) was a 1.8 meter long with an initial absorption loss of 6 dB at 1570 nm. In spite of the disadvantage of a large MFA in the Tm134 fiber, sequentially Q-switched pulses were successfully produced. The pulse had a stable pulse energy of 9 μJ and pulse width of 420 ns at repetition rates in the range 0.1-2 kHz, as shown in Fig. 3 . The pulse repetition rate was proportional to the pump power, and limited to 6 kHz by the maximum power of the LD, about 230 mW.
When the repetition rate, Rp, was larger than about 2 kHz, the pulse energy started to decrease because the SAQS could not fully recover in time after a pulse and less gain population was excited before the next pulsing. It indicates that the relaxation lifetime (3F4) of the thulium fiber should be near and less than 0.5 ms that was the inversion of the largest Rp before the degrading of pulse energy. The estimated lifetime is consistent with the reported lifetime range of 0.3-0.6 ms [15,18]. The stability of thulium Q-switch was evaluated by the standard deviation of the repetition rate with a constant CW pump. By sampling of a thousand times on an oscilloscope, the deviation of the repetition rate were about ± 3.4% at 1 kHz, and slowly increased to ± 4.6% at 6 kHz. Furthermore, a long-term (110 hours) operation with a constant pump power of 100 mW (Rp~1 kHz) was tested for observing the photodarkening in Tm fiber reported in the literature . The average output power started initially at 9.1 mW, decreased with time in the early 40 hours and became stabilized at 8 mW afterward as shown in Fig. 4 . The Rp remained stably at 1 kHz, thereby indicating a near 11% degradation of pulse energy by photodarkening.
Observing the Q-switching performances with various ratios of pgAa/(paAg) could help reveal the σa value of the thulium fiber. An erbium fiber (model Er110-4/125 by nLight) and a thulium-holmium co-doped fiber (model TH512 by CorActive) were tested in the experiment. The characteristics of the fibers and the performances at a pulse repetition rate of 1 kHz are listed in Table 1 . Because of the relatively small pgAa/(paAg), the case using Er80 and TH512 fibers had the best Q-switching performance. It should be noted that multi-peak and distorted pulse shapes were observed in the cases using the TH512 fiber. The physics of these complex shapes was not clear and was attributed to the interaction between Tm3+ and Ho3+ dopants because they were not found in the cases using the Tm134 fiber. On the other hand, the case using the Er110 and Tm134 fibers had the lowest Q-switching efficiency, indicating operation near the critical condition. An educated guess, by σa ~[pgAa/(paAg)] × (1.5σg) near the Q-switching criterion, suggested that the σa of thulium should be larger than and close to 11.8 × 10−20 cm2 at 1570 nm. This estimated σa value was larger by a factor of 4.5 and 2.5 than the previously reported data [15,16].
To determine the precise σa value, a Tm134 fiber with an initial absorption loss of about 2 dB was tested in a bleaching experiment, as illustrated in Fig. 5(a) . The input pulse source was the Q-switched laser using the Er80 and TH512 fibers as indicated in Table 1. The pulse repetition rate was set to be 0.5 kHz, providing an average power of 8 mW. Because of the low repetition rate, full recovery of the absorption population of the thulium fiber from every pulse bleaching was expected. A manual attenuator and a power splitter were employed to control and monitor the incident pulse energy upon the thulium fiber. The fusion loss between the bare fiber and the thulium fiber was measured by a 1570-nm CW signal to be 0.9 dB. Therefore, the true pulse energy, Ei, that entered the tested sample could be estimated and modulated in a range of 0.6-10 μJ. The Ei and the corresponding internal transmission were obtained as shown in Fig. 5(b). The modified Avizonis-Grotbeck’s equation has been used to determine the σa and the excited-state absorption cross section, σESA, of bulk saturable absorbers . To include the effect of power confinement in the fiber core, we redefine the parameters and further modify the equation to be:
With a lack of excited-state absorption of 1.57 μm from the 3F4 level of Tm3+, σESA is negligible. The σa of Tm3+ was determined to be 1.44 × 10−20 cm2 with a best-fit numerical solution of Eq. (2) to the experimental data. The parameters for the best-fit bleaching curve were consistent with the sample characteristics.
We have demonstrated passively Q-switched all-fiber erbium lasers at 1570 nm using Tm-doped and Tm-Ho co-doped SAQS fibers. With a Tm-doped fiber, sequentially Q-switched pulses at repetition rates of 0.1-6 kHz were stably achieved using CW 980-nm pump powers of 60-230 mW. By use of a Tm-Ho co-doped fiber with a mode-field area of 8.6 μm, Q-switched pulses with pulse energy of 15.7 μJ and pulse width of 120 ns were obtained. Because of relatively larger cross-section ratios, σa/σg at 1.57-1.6 μm, better Q-switching performances were expected at higher wavelengths. We also modified the Avizonis-Grotbeck’s equation to consider the power confinement factor of the fiber core in the bleaching experiment. The absorption cross section of Tm was identified to be about 1.44 × 10−20 cm2 at 1570 nm by the best fit of a theoretical curve to the experimental data.
The authors would like to thank the fiber manufacturers, CorActive Inc. and INO, Canada, for providing the absorption spectrum of the Tm134 fiber and other fiber specifications. The authors are also grateful for the financial supports from NCKU Project of Promoting Academic Excellence & Developing World Class Research Centers (D98-3360).
References and links
1. N. A. Russo, R. Duchowicz, J. Mora, J. L. Cruz, and M. V. Andrés, “High-efficiency Q-switched erbium fiber laser using a Bragg grating-based modulator,” Opt. Commun. 210(3-6), 361–366 (2002). [CrossRef]
2. D. W. Huang, W. F. Liu, and C. C. Yang, “Q-switched all-fiber laser with an acoustically modulated fiber attenuator,” IEEE Photon. Technol. Lett. 12(9), 1153–1155 (2000). [CrossRef]
3. M. Delgado-Pinar, D. Zalvidea, A. Díez, P. Pérez-Millan, and M. V. Andrés, “Q-switching of an all-fiber laser by acousto-optic modulation of a fiber Bragg grating,” Opt. Express 14(3), 1106–1112 (2006). [CrossRef] [PubMed]
4. P. Pérez-Millán, A. Díez, M. V. Andrés, D. Zalvidea, and R. Duchowicz, “Q-switched all-fiber laser based on magnetostriction modulation of a Bragg grating,” Opt. Express 13(13), 5046–5051 (2005). [CrossRef] [PubMed]
5. R. Paschotta, R. Häring, E. Gini, H. Melchior, U. Keller, H. L. Offerhaus, and D. J. Richardson, “Passively Q-switched 0.1-mJ fiber laser system at 1.53 mum,” Opt. Lett. 24(6), 388–390 (1999). [CrossRef]
6. S. Kivistö, R. Koskinen, J. Paajaste, S. D. Jackson, M. Guina, and O. G. Okhotnikov, “Passively Q-switched Tm3+, Ho3+-doped silica fiber laser using a highly nonlinear saturable absorber and dynamic gain pulse compression,” Opt. Express 16(26), 22058–22063 (2008). [CrossRef] [PubMed]
8. A. A. Fotiadi, A. S. Kurkov, and I. M. Razdobreev, “All-fiber passively Q-switched Ytterbium laser,” 2005 Conference on Lasers and Electro-Optics Europe, p. 515.
10. A. S. Kurkov, E. M. Sholokhov, and O. I. Medvedkov, “All fiber Yb-Ho pulsed laser,” Laser Phys. Lett. 6(2), 135–138 (2009). [CrossRef]
13. T.-Y. Tsai, Y.-C. Fang, Z.-C. Lee, and H.-X. Tsao, “All-fiber passively Q-switched erbium laser using mismatch of mode field areas and a saturable-amplifier pump switch,” Opt. Lett. 34(19), 2891–2893 (2009). [CrossRef] [PubMed]
16. K. Annapurna, M. Das, and S. Buddhudu, “Spectral analysis of thulium doped zinc-boro-silicate glass,” Physica B 388(1-2), 174–179 (2007). [CrossRef]
17. V. G. Shcherbitsky, S. Girard, M. Fromager, R. Moncorg’e, N. V. Kuleshov, V. I. Levchenko, V. N. Yakimovich, and B. Ferrand, “Accurate method for the measurement of absorption cross sections of solid-state saturable absorbers,” Appl. Phys. B 74(4-5), 367–374 (2002). [CrossRef]
18. D. A. Simpson, “Spectroscopy of Thulium Doped Silica Glass,” (Victoria University, Melbourne, 2008).
19. M. M. Broer, D. M. Krol, and D. J. Digiovanni, “Highly nonlinear near-resonant photodarkening in a thulium-doped aluminosilicate glass fiber,” Opt. Lett. 18(10), 799–801 (1993). [CrossRef] [PubMed]