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Tm–Ho codoped fiber based all fiber amplification of a gain-switched 2 μm fiber laser
Optik 125 (2014) 6198–6200
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Tm–Ho codoped fiber based all fiber amplification of a gain-switched 2 m fiber laser Mengmeng Tao a,∗ , Yan Yan a , Xisheng Ye a,b , Yong Wu a , Pengling Yang a , Guobin Feng a a
State Key Laboratory of Laser Interaction with Matter, Northwest Institute of Nuclear Technology, Xi’an 710024, PR China Research Center of Space Laser Information Technology, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, PR China b
a r t i c l e
i n f o
Article history: Received 6 November 2013 Accepted 5 June 2014 Keywords: Tm–Ho codoped fiber Gain-switching Gain-switched mode-locking All fiber amplification
a b s t r a c t A Tm–Ho codoped fiber amplifier system is built. And, amplification of a gain-switched Tm–Ho codoped fiber laser is investigated. Average output of 300 mW is obtained at repetition rate of tens of kHz with an amplification gain bigger than 11 dB. And, pulse amplification efficiency of resonantly pumped Tm–Ho codoped single clad fiber is comparable with 793 nm pumped Tm-doped double clad fiber. The maximal pulse energy generated is about 13.1 J, corresponding to a peak power of 282 W at 20 kHz. During the amplification process, gain-switching, partially modulated gain-switched mode-locking and 100% modulated gain-switched mode-locking are observed sequentially. At gain-switching mode, the laser output enjoys a narrow linewidth of 0.31 nm, while at gain-switched mode-locking mode, the spectral linewidth broadens to 0.6 nm. © 2014 Elsevier GmbH. All rights reserved.
1. Introduction Tm–Ho codoped fiber (THDF) has been studied for decades [1,2] as it offers an important approach to obtain 2 m laser sources. Up to now, high power CW operation [3], active/passive Q-switching operation [4,5], gain-switching operation [6–8] and mode-locking operation [9–11] have been realized with THDF. Besides, THDF has also been explored for supercontinuum generation [6,11,12] and saturable absorption [13,14]. In our previous works, we investigated the absorption characteristics of THDFs around 1550 nm. Exploiting THDF as the saturable absorber, Q-switching, Q-switched mode-locking and mode-locking of Er-doped fiber lasers are achieved [13]. And, absorption cross-section of THDF around 1550 nm is recalibrated [14]. Based on these researches, resonantly pumped, gain-switched 2 m Tm–Ho codoped single clad fiber laser are built and optimized [7,8]. At stable gain-switching operation mode, directly controlled by the pulsed pump, repetition rate of the 2 m signal is widely tunable from several kHz to more than 100 kHz. With different pump pulse energies, pulse duration ranges from more than 1 s to shorter than 100 ns. Peak power up to tens of watts is reached. At stable gain-switched mode-locking operation mode, sub-pulse
∗ Corresponding author. E-mail address: [email protected] (M. Tao). http://dx.doi.org/10.1016/j.ijleo.2014.06.136 0030-4026/© 2014 Elsevier GmbH. All rights reserved.
repetition rate of tens of MHz with duration in the ns scale is generated. In this paper, we investigate the amplification characteristics of THDF amplifier in the shortwave range. Average output power, pulse energy and peak power of the THDF amplifier at different repetition rates are presented. Experiments show that this amplification system provides a gain bigger than 11 dB for the gainswitched input while still maintaining its narrow linewidth feature.
2. Experimental setup and results Experimental setup of the THDF amplification system is illustrated in Fig. 1. The seed laser is a gain-switched THDF laser resonantly pumped by a pulsed 1550 nm fiber laser. With two high reflective fiber Bragg gratings (HR-FBG) as the cavity mirrors, the total length of the linear laser cavity is ∼3 m, including 0.8 m THDF (CorActive) as the active fiber. Reflection wavelength of the HRFBGs lies around 1.92 m. The estimated Tm ion concentration is about 5 × 1025 m−3 , and, the Ho ion concentration is much smaller. Absorption coefficient of the THDF is 13.3 dB/m at 1550 nm. 50% of seed is lead out through an output coupler (OC) for amplification. In the amplification stage, 1.55 m THDF is exploited. And, the CW 1550 nm pump is launched through a 1550/2000 nm WDM with a backward pump scheme. The output 2 m signal is monitored with a fast HgCdTe detector (response time < 3 ns) and a 4 GHz oscilloscope (Tektronix TDS
M. Tao et al. / Optik 125 (2014) 6198–6200
6199
Table 1 Parameters of the THDF laser at 1 W pulsed pump. Repetition rate (kHz)
Average output power (mW)
Pulse energy (J)
Peak power (W)
20 40 80
15.3 22.6 24.1
0.77 0.57 0.30
10.6 5.52 1.61
7404). The spectrum is measured with an Agilent 86140B spectrum analyzer in the extended working range. Output parameters of the THDF laser are listed in Table 1. And, linewidth of the laser output is no bigger than 0.3 nm [7]. Average output of the THDF amplifier is shown in Fig. 2(a). Slope efficiencies for 20 kHz, 40 kHz and 80 kHz pulse amplification are 26.1%, 29.3% and 30.3%, respectively, which are comparable with Tm-doped double clad fiber amplifiers (21% at 20 kHz in Ref. [6]; 32% at 14.7 MHz in Ref. [15]; 26.5% at 26 kHz and 31% at 40 kHz in Ref. [16]). And, as can be found, lasers with higher repetition rate can be amplified more efficiently [16]. The maximum output power of this amplification system is about 301 mW at 80 kHz, corresponding to a gain of 11 dB. Fig. 2(b) gives the corresponding pulse energy at different repetition rates. The pulse energy limitation Em for gain fiber is expressed as [17] Em =
hvA s (a + e )
(1)
where h is the laser photon energy, A is the doping area, s is the overlapping factor of the laser, and a , e are the absorption and emission cross-sections at the signal wavelength. The estimated maximal pulse energy for Tm-doped fiber with the same parameter as the THDF used is about 13 J ( a and e are taken from Ref. [18]). And, the measured maximal pulse energy generated in THDF is 13.1 J at 20 kHz, limited by the pump power. Besides, both of the average output power and the pulse energy increase almost linearly with the pump power. Peak power of the output pulses is depicted in Fig. 3. For 40 kHz and 80 kHz pulses, the peak power increases linearly with the pump as the output laser maintains stable gain-switching operation during the amplification process. However, for 20 kHz pulses, the laser experiences different operation modes, including stable gain-switching, stable gain-switched
mode-locking and unstable gain-switched mode-locking, as illustrated in Fig. 3. Waveforms of stable gain-switched pulses are shown in Fig. 4. Fig. 4(a) presents a stable gain-switched pulse train at 20 kHz. And, in Fig. 4(b), is a Gauss-shaped single gain-switched pulse with a pulse duration of 113.5 ns. The highest peak power at gainswitching operation mode is about 108 W. Further increasing the pump power, the gain-switched pulse gets mode-locked with partial modulation as shown in Fig. 5(a). With higher pump power, the gain-switched mode-locked pulse becomes fully modulated as presented in Fig. 5(b). However, beyond 900 mW pump power, the 100% modulated mode-locked pulse becomes unstable, featuring transition between different pulse states, as can be seen in Fig. 5(c) and (d). And, at this operation mode, the peak power scales to 282 W with a narrow pulse duration of about 46 ns. At gain-switched mode-locking operation, the gain-switched repetition rate maintains 20 kHz, while the mode-locked repetition rate is 35 MHz, related to the cavity length of the THDF laser [8]. And, the gain-switched envelop duration is around 120 ns, comparable to the gain-switched pulse, while the duration of the subpulse in the gain-switched envelop is about 8.8 ns. Fig. 6(a) illustrates the laser spectra at different repetition rates. The spectra show a same central wavelength of 1920.3 nm with different linewidths. At 40 kHz, 291 mW output and 80 kHz, 301 mW output, where the amplifier is operating in the gain-switching
HR-FBG Pulsed Pump 0.8 m THDF HR-FBG CW Pump 2 μm Output
WDM
1.55 m THDF
OC
Fig. 1. Experimental setup of the all fiber amplification system.
Fig. 2. Average output (a) and pulse energy (b) of the THDF amplifier at different repetition rates. The square, circle and triangle represent the experimental data for 20 kHz, 40 kHz and 80 kHz repetition rate, respectively.
Fig. 3. Peak power of the THDF amplifier at different repetition rates. The square, circle and triangle represent the experimental data for 20 kHz, 40 kHz and 80 kHz, respectively. GS: gain-switching mode; GS-ML: gain-switched mode-locking mode.
Fig. 4. Stable gain-switched 2 m laser signal at 20 kHz: (a) Stable gain-switched pulse train at 34 mW output (40 s/div); (b) A single gain-switched pulse with a pulse duration of 113.6 ns (200 ns/div).
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M. Tao et al. / Optik 125 (2014) 6198–6200
long THDF which is comparable with 793 nm pumped Tm-doped double clad fibers. At high repetition rate, temporal and spectral characteristics of the seed laser are well maintained. At low repetition rate, gain-switched laser pulses are led into gain-switched mode-locking operation with spectral linewidth broadened. Supercontinuum generation is feasible with further amplification. Acknowledgment The authors would like to thank Junwei Zhao, Ping Wang and Zhenbao Wang from the State Key Laboratory of Laser Interaction with Matter for their generous help during the experiment. And, we are also grateful to Yongsheng Zhang for his instructions on manuscript preparation. This work is partially supported by the State Key Laboratory of Laser Interaction with Matter under Grant No. SKLLIM1409. References Fig. 5. Pulse evolution of 2 m laser signal at 20 kHz: (a) A single partially modulated gain-switched mode-locking pulse with a pulse duration of 121.6 ns at 98 mW output (80 ns/div); (b) A single 100% modulated gain-switched mode-locking pulse with a pulse duration of 120 ns at 169 mW output (80 ns/div); (c) Snapshot of an unstable pulse with a pulse duration of 46 ns at 262 mW output (40 ns/div). (d) Fast acquisition waveform featuring transition between different pulse states at 262 mW output (80 ns/div).
Fig. 6. Laser spectra of 2 m laser signal: (a) Spectra at different repetition rates. The dotted line, the dashed and the solid line are laser spectra at 20 kHz (193 mW), 40 kHz (291 mW) and 80 kHz (301 mW), respectively; (b) Wide range spectrum from 1400 nm to 2000 nm.
mode, the typical linewidth is about 0.3 nm, within the 0.5 nm reflection bandwidth of the HR-FBGs. While at 20 kHz, 193 mW output, where the system works in the gain-switched mode-locking mode, the linewidth is broadened to 0.6 nm, indicating that more longitudinal modes are oscillating. Fig. 6(b) gives a wide range view of the laser spectrum from 1400 nm to 2000 nm. As can be seen, the laser spectrum features a single line around 1920 nm with no unabsorbed 1550 nm pump or ASE around 1950 nm. 3. Conclusions Pulse amplification characteristics of a THDF amplifier are experimentally studied in the shortwave range. An average output gain bigger than 11 dB is obtained through a piece of 1.55 m
[1] K. Oh, T.F. Morse, A. Kilian, L. Reinhart, P.M. Weber, Continuous-wave oscillation of thulium-sensitized holmium doped silica fiber laser, Opt. Lett. 19 (1994) 278–280. [2] C. Ghisler, W. Luthy, H.P. Weber, J. Morel, A. Woodtli, R. Dandliker, V. Neumann, H. Berthou, G. Kotrotsios, A Tm sensitized Ho silica fiber laser at 2.04 m pumped at 809 nm, Opt. Commun. 109 (1994) 279–281. [3] S.D. Jackson, S. Mossman, High-power diode-cladding-pumped Tm, Ho-doped silica fiber laser, Appl. Phys. B 77 (2003) 489–491. [4] M. Eichhorn, S.D. Jackson, High-pulse-energy, actively Q-switched Tm-Hocodoped silica 2 m fiber laser, Opt. Lett. 33 (10) (2008) 1044–1046. [5] S. Kivisto, R. Koskinen, J. Paajaste, S.D. Jackson, M. Guina, O.G. Okhotnikov, Passively Q-switched Tm, Ho-doped silica fiber laser using a highly nonlinear saturable absorber and dynamic gain pulse compression, Opt. Express 16 (26) (2008) 22058–22063. [6] W.Q. Yang, B. Zhang, J. Hou, R. Xiao, R. Song, Z.J. Liu, Gain-switched and modelocked Tm/Ho-codoped 2 m fiber laser for mid-IR supercontinuum generation in a Tm-doped fiber amplifier, Laser Phys. Lett. 10 (2013) 045106. [7] M. Tao, L. Zhang, Y. Wu, Z. Wang, P. Yang, G. Feng, X. Ye, Gain-switching and gain-switched mode-locking operation of a Tm/Ho co-doped fiber laser, Laser Phys. 23 (2013) 095109. [8] M. Tao, J. Zhao, Y. Yan, Z. Wang, P. Yang, G. Feng, X. Ye, Experimental investigation of gain-switched Tm-Ho co-doped single clad fiber lasers, Laser Phys. 23 (2013) 105101. [9] Q. Wang, J. Geng, Z. Jiang, T. Luo, S. Jiang, Mode-locked Tm-Ho-codoped fiber laser at 2.06 m, IEEE Photon. Technol. Lett. 23 (11) (2011) 1041–1135. [10] B. Fu, L. Gui, X. Li, X. Xiao, H. Zhu, C. Yang, Generation of 35-nJ nanosecond pulse from a passively mode-locked Tm, Ho-codoped fiber laser with graphene saturable absorber, IEEE Photon. Technol. Lett. 25 (15) (2013) 1447–1449. [11] S. Kivisto, T. Hakulinen, M. Guina, O.G. Okhotnikov, Tunable Raman soliton source using mode-locked Tm-Ho fiber laser, IEEE Photon. Technol. Lett. 19 (12) (2007) 934–936. [12] W.Q. Yang, B. Zhang, J. Hou, R. Xiao, Z.F. Jiang, Z.J. Liu, Mid-IR supercontinuum generation in Tm/Ho codoped fiber amplifier, Laser Phys. Lett. 10 (2013) 055107. [13] M. Tao, J. Wu, J. Peng, Y. Wu, P. Yang, X. Ye, Experimental demonstration of an Er-doped fiber ring laser mode-locked with a Tm-Ho co-doped fiber saturable absorber, Laser Phys. 23 (2013) 085102. [14] M. Tao, X. Ye, P. Wang, T. Yu, Z. Wang, P. Yang, G. Feng, A Tm-Ho codoped fiber based 38 nm wideband wavelength tunable passively Q-switched Er-doped fiber laser, Laser Phys. 23 (2013) 105104. [15] J. Liu, P. Wang, High-energy near transform-limited pulses from an ultrafast thulium-doped all-fiber MOPA, IEEE Photon. Technol. Lett. 24 (16) (2012) 1384–1386. [16] J. Swiderski, M. Michalska, G. Maze, Mid-IR supercontinuum generation in a ZBLAN fiber pumped by a gain-switched mode-locked Tm-doped fiber laser and amplifier system, Opt. Express 21 (7) (2013) 7851–7857. [17] J. Min, P. Tayebati, Stable 10 ns, kilowatt peak-power pulse generation from a gain-switched Tm-doped fiber laser, Opt. Lett. 32 (13) (2007) 1797–1799. [18] F. Wang, D. Shen, H. Chen, D. Fan, Q. Lu, Modeling and optimization of stable gain-switched Tm-doped fiber lasers, Opt. Rev. 18 (4) (2011) 360–364.
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Tm–Ho codoped fiber based all fiber amplification of a gain-switched 2 m fiber laser Mengmeng Tao a,∗ , Yan Yan a , Xisheng Ye a,b , Yong Wu a , Pengling Yang a , Guobin Feng a a
State Key Laboratory of Laser Interaction with Matter, Northwest Institute of Nuclear Technology, Xi’an 710024, PR China Research Center of Space Laser Information Technology, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, PR China b
a r t i c l e
i n f o
Article history: Received 6 November 2013 Accepted 5 June 2014 Keywords: Tm–Ho codoped fiber Gain-switching Gain-switched mode-locking All fiber amplification
a b s t r a c t A Tm–Ho codoped fiber amplifier system is built. And, amplification of a gain-switched Tm–Ho codoped fiber laser is investigated. Average output of 300 mW is obtained at repetition rate of tens of kHz with an amplification gain bigger than 11 dB. And, pulse amplification efficiency of resonantly pumped Tm–Ho codoped single clad fiber is comparable with 793 nm pumped Tm-doped double clad fiber. The maximal pulse energy generated is about 13.1 J, corresponding to a peak power of 282 W at 20 kHz. During the amplification process, gain-switching, partially modulated gain-switched mode-locking and 100% modulated gain-switched mode-locking are observed sequentially. At gain-switching mode, the laser output enjoys a narrow linewidth of 0.31 nm, while at gain-switched mode-locking mode, the spectral linewidth broadens to 0.6 nm. © 2014 Elsevier GmbH. All rights reserved.
1. Introduction Tm–Ho codoped fiber (THDF) has been studied for decades [1,2] as it offers an important approach to obtain 2 m laser sources. Up to now, high power CW operation [3], active/passive Q-switching operation [4,5], gain-switching operation [6–8] and mode-locking operation [9–11] have been realized with THDF. Besides, THDF has also been explored for supercontinuum generation [6,11,12] and saturable absorption [13,14]. In our previous works, we investigated the absorption characteristics of THDFs around 1550 nm. Exploiting THDF as the saturable absorber, Q-switching, Q-switched mode-locking and mode-locking of Er-doped fiber lasers are achieved [13]. And, absorption cross-section of THDF around 1550 nm is recalibrated [14]. Based on these researches, resonantly pumped, gain-switched 2 m Tm–Ho codoped single clad fiber laser are built and optimized [7,8]. At stable gain-switching operation mode, directly controlled by the pulsed pump, repetition rate of the 2 m signal is widely tunable from several kHz to more than 100 kHz. With different pump pulse energies, pulse duration ranges from more than 1 s to shorter than 100 ns. Peak power up to tens of watts is reached. At stable gain-switched mode-locking operation mode, sub-pulse
∗ Corresponding author. E-mail address: [email protected] (M. Tao). http://dx.doi.org/10.1016/j.ijleo.2014.06.136 0030-4026/© 2014 Elsevier GmbH. All rights reserved.
repetition rate of tens of MHz with duration in the ns scale is generated. In this paper, we investigate the amplification characteristics of THDF amplifier in the shortwave range. Average output power, pulse energy and peak power of the THDF amplifier at different repetition rates are presented. Experiments show that this amplification system provides a gain bigger than 11 dB for the gainswitched input while still maintaining its narrow linewidth feature.
2. Experimental setup and results Experimental setup of the THDF amplification system is illustrated in Fig. 1. The seed laser is a gain-switched THDF laser resonantly pumped by a pulsed 1550 nm fiber laser. With two high reflective fiber Bragg gratings (HR-FBG) as the cavity mirrors, the total length of the linear laser cavity is ∼3 m, including 0.8 m THDF (CorActive) as the active fiber. Reflection wavelength of the HRFBGs lies around 1.92 m. The estimated Tm ion concentration is about 5 × 1025 m−3 , and, the Ho ion concentration is much smaller. Absorption coefficient of the THDF is 13.3 dB/m at 1550 nm. 50% of seed is lead out through an output coupler (OC) for amplification. In the amplification stage, 1.55 m THDF is exploited. And, the CW 1550 nm pump is launched through a 1550/2000 nm WDM with a backward pump scheme. The output 2 m signal is monitored with a fast HgCdTe detector (response time < 3 ns) and a 4 GHz oscilloscope (Tektronix TDS
M. Tao et al. / Optik 125 (2014) 6198–6200
6199
Table 1 Parameters of the THDF laser at 1 W pulsed pump. Repetition rate (kHz)
Average output power (mW)
Pulse energy (J)
Peak power (W)
20 40 80
15.3 22.6 24.1
0.77 0.57 0.30
10.6 5.52 1.61
7404). The spectrum is measured with an Agilent 86140B spectrum analyzer in the extended working range. Output parameters of the THDF laser are listed in Table 1. And, linewidth of the laser output is no bigger than 0.3 nm [7]. Average output of the THDF amplifier is shown in Fig. 2(a). Slope efficiencies for 20 kHz, 40 kHz and 80 kHz pulse amplification are 26.1%, 29.3% and 30.3%, respectively, which are comparable with Tm-doped double clad fiber amplifiers (21% at 20 kHz in Ref. [6]; 32% at 14.7 MHz in Ref. [15]; 26.5% at 26 kHz and 31% at 40 kHz in Ref. [16]). And, as can be found, lasers with higher repetition rate can be amplified more efficiently [16]. The maximum output power of this amplification system is about 301 mW at 80 kHz, corresponding to a gain of 11 dB. Fig. 2(b) gives the corresponding pulse energy at different repetition rates. The pulse energy limitation Em for gain fiber is expressed as [17] Em =
hvA s (a + e )
(1)
where h is the laser photon energy, A is the doping area, s is the overlapping factor of the laser, and a , e are the absorption and emission cross-sections at the signal wavelength. The estimated maximal pulse energy for Tm-doped fiber with the same parameter as the THDF used is about 13 J ( a and e are taken from Ref. [18]). And, the measured maximal pulse energy generated in THDF is 13.1 J at 20 kHz, limited by the pump power. Besides, both of the average output power and the pulse energy increase almost linearly with the pump power. Peak power of the output pulses is depicted in Fig. 3. For 40 kHz and 80 kHz pulses, the peak power increases linearly with the pump as the output laser maintains stable gain-switching operation during the amplification process. However, for 20 kHz pulses, the laser experiences different operation modes, including stable gain-switching, stable gain-switched
mode-locking and unstable gain-switched mode-locking, as illustrated in Fig. 3. Waveforms of stable gain-switched pulses are shown in Fig. 4. Fig. 4(a) presents a stable gain-switched pulse train at 20 kHz. And, in Fig. 4(b), is a Gauss-shaped single gain-switched pulse with a pulse duration of 113.5 ns. The highest peak power at gainswitching operation mode is about 108 W. Further increasing the pump power, the gain-switched pulse gets mode-locked with partial modulation as shown in Fig. 5(a). With higher pump power, the gain-switched mode-locked pulse becomes fully modulated as presented in Fig. 5(b). However, beyond 900 mW pump power, the 100% modulated mode-locked pulse becomes unstable, featuring transition between different pulse states, as can be seen in Fig. 5(c) and (d). And, at this operation mode, the peak power scales to 282 W with a narrow pulse duration of about 46 ns. At gain-switched mode-locking operation, the gain-switched repetition rate maintains 20 kHz, while the mode-locked repetition rate is 35 MHz, related to the cavity length of the THDF laser [8]. And, the gain-switched envelop duration is around 120 ns, comparable to the gain-switched pulse, while the duration of the subpulse in the gain-switched envelop is about 8.8 ns. Fig. 6(a) illustrates the laser spectra at different repetition rates. The spectra show a same central wavelength of 1920.3 nm with different linewidths. At 40 kHz, 291 mW output and 80 kHz, 301 mW output, where the amplifier is operating in the gain-switching
HR-FBG Pulsed Pump 0.8 m THDF HR-FBG CW Pump 2 μm Output
WDM
1.55 m THDF
OC
Fig. 1. Experimental setup of the all fiber amplification system.
Fig. 2. Average output (a) and pulse energy (b) of the THDF amplifier at different repetition rates. The square, circle and triangle represent the experimental data for 20 kHz, 40 kHz and 80 kHz repetition rate, respectively.
Fig. 3. Peak power of the THDF amplifier at different repetition rates. The square, circle and triangle represent the experimental data for 20 kHz, 40 kHz and 80 kHz, respectively. GS: gain-switching mode; GS-ML: gain-switched mode-locking mode.
Fig. 4. Stable gain-switched 2 m laser signal at 20 kHz: (a) Stable gain-switched pulse train at 34 mW output (40 s/div); (b) A single gain-switched pulse with a pulse duration of 113.6 ns (200 ns/div).
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M. Tao et al. / Optik 125 (2014) 6198–6200
long THDF which is comparable with 793 nm pumped Tm-doped double clad fibers. At high repetition rate, temporal and spectral characteristics of the seed laser are well maintained. At low repetition rate, gain-switched laser pulses are led into gain-switched mode-locking operation with spectral linewidth broadened. Supercontinuum generation is feasible with further amplification. Acknowledgment The authors would like to thank Junwei Zhao, Ping Wang and Zhenbao Wang from the State Key Laboratory of Laser Interaction with Matter for their generous help during the experiment. And, we are also grateful to Yongsheng Zhang for his instructions on manuscript preparation. This work is partially supported by the State Key Laboratory of Laser Interaction with Matter under Grant No. SKLLIM1409. References Fig. 5. Pulse evolution of 2 m laser signal at 20 kHz: (a) A single partially modulated gain-switched mode-locking pulse with a pulse duration of 121.6 ns at 98 mW output (80 ns/div); (b) A single 100% modulated gain-switched mode-locking pulse with a pulse duration of 120 ns at 169 mW output (80 ns/div); (c) Snapshot of an unstable pulse with a pulse duration of 46 ns at 262 mW output (40 ns/div). (d) Fast acquisition waveform featuring transition between different pulse states at 262 mW output (80 ns/div).
Fig. 6. Laser spectra of 2 m laser signal: (a) Spectra at different repetition rates. The dotted line, the dashed and the solid line are laser spectra at 20 kHz (193 mW), 40 kHz (291 mW) and 80 kHz (301 mW), respectively; (b) Wide range spectrum from 1400 nm to 2000 nm.
mode, the typical linewidth is about 0.3 nm, within the 0.5 nm reflection bandwidth of the HR-FBGs. While at 20 kHz, 193 mW output, where the system works in the gain-switched mode-locking mode, the linewidth is broadened to 0.6 nm, indicating that more longitudinal modes are oscillating. Fig. 6(b) gives a wide range view of the laser spectrum from 1400 nm to 2000 nm. As can be seen, the laser spectrum features a single line around 1920 nm with no unabsorbed 1550 nm pump or ASE around 1950 nm. 3. Conclusions Pulse amplification characteristics of a THDF amplifier are experimentally studied in the shortwave range. An average output gain bigger than 11 dB is obtained through a piece of 1.55 m
[1] K. Oh, T.F. Morse, A. Kilian, L. Reinhart, P.M. Weber, Continuous-wave oscillation of thulium-sensitized holmium doped silica fiber laser, Opt. Lett. 19 (1994) 278–280. [2] C. Ghisler, W. Luthy, H.P. Weber, J. Morel, A. Woodtli, R. Dandliker, V. Neumann, H. Berthou, G. Kotrotsios, A Tm sensitized Ho silica fiber laser at 2.04 m pumped at 809 nm, Opt. Commun. 109 (1994) 279–281. [3] S.D. Jackson, S. Mossman, High-power diode-cladding-pumped Tm, Ho-doped silica fiber laser, Appl. Phys. B 77 (2003) 489–491. [4] M. Eichhorn, S.D. Jackson, High-pulse-energy, actively Q-switched Tm-Hocodoped silica 2 m fiber laser, Opt. Lett. 33 (10) (2008) 1044–1046. [5] S. Kivisto, R. Koskinen, J. Paajaste, S.D. Jackson, M. Guina, O.G. Okhotnikov, Passively Q-switched Tm, Ho-doped silica fiber laser using a highly nonlinear saturable absorber and dynamic gain pulse compression, Opt. Express 16 (26) (2008) 22058–22063. [6] W.Q. Yang, B. Zhang, J. Hou, R. Xiao, R. Song, Z.J. Liu, Gain-switched and modelocked Tm/Ho-codoped 2 m fiber laser for mid-IR supercontinuum generation in a Tm-doped fiber amplifier, Laser Phys. Lett. 10 (2013) 045106. [7] M. Tao, L. Zhang, Y. Wu, Z. Wang, P. Yang, G. Feng, X. Ye, Gain-switching and gain-switched mode-locking operation of a Tm/Ho co-doped fiber laser, Laser Phys. 23 (2013) 095109. [8] M. Tao, J. Zhao, Y. Yan, Z. Wang, P. Yang, G. Feng, X. Ye, Experimental investigation of gain-switched Tm-Ho co-doped single clad fiber lasers, Laser Phys. 23 (2013) 105101. [9] Q. Wang, J. Geng, Z. Jiang, T. Luo, S. Jiang, Mode-locked Tm-Ho-codoped fiber laser at 2.06 m, IEEE Photon. Technol. Lett. 23 (11) (2011) 1041–1135. [10] B. Fu, L. Gui, X. Li, X. Xiao, H. Zhu, C. Yang, Generation of 35-nJ nanosecond pulse from a passively mode-locked Tm, Ho-codoped fiber laser with graphene saturable absorber, IEEE Photon. Technol. Lett. 25 (15) (2013) 1447–1449. [11] S. Kivisto, T. Hakulinen, M. Guina, O.G. Okhotnikov, Tunable Raman soliton source using mode-locked Tm-Ho fiber laser, IEEE Photon. Technol. Lett. 19 (12) (2007) 934–936. [12] W.Q. Yang, B. Zhang, J. Hou, R. Xiao, Z.F. Jiang, Z.J. Liu, Mid-IR supercontinuum generation in Tm/Ho codoped fiber amplifier, Laser Phys. Lett. 10 (2013) 055107. [13] M. Tao, J. Wu, J. Peng, Y. Wu, P. Yang, X. Ye, Experimental demonstration of an Er-doped fiber ring laser mode-locked with a Tm-Ho co-doped fiber saturable absorber, Laser Phys. 23 (2013) 085102. [14] M. Tao, X. Ye, P. Wang, T. Yu, Z. Wang, P. Yang, G. Feng, A Tm-Ho codoped fiber based 38 nm wideband wavelength tunable passively Q-switched Er-doped fiber laser, Laser Phys. 23 (2013) 105104. [15] J. Liu, P. Wang, High-energy near transform-limited pulses from an ultrafast thulium-doped all-fiber MOPA, IEEE Photon. Technol. Lett. 24 (16) (2012) 1384–1386. [16] J. Swiderski, M. Michalska, G. Maze, Mid-IR supercontinuum generation in a ZBLAN fiber pumped by a gain-switched mode-locked Tm-doped fiber laser and amplifier system, Opt. Express 21 (7) (2013) 7851–7857. [17] J. Min, P. Tayebati, Stable 10 ns, kilowatt peak-power pulse generation from a gain-switched Tm-doped fiber laser, Opt. Lett. 32 (13) (2007) 1797–1799. [18] F. Wang, D. Shen, H. Chen, D. Fan, Q. Lu, Modeling and optimization of stable gain-switched Tm-doped fiber lasers, Opt. Rev. 18 (4) (2011) 360–364.