A high energy all-fiber format nanosecond thulium-doped fiber laser at 2050 nm with a master oscillator power amplifier (MOPA) configuration is presented in this paper. The seed oscillator is a linearly polarized gain-switched fiber laser pumped by a 1550 nm fiber laser. The output pulse of the seed has a polarization extinction ratio (PER) better than 16 dB with a maximal output power of 470 mW. After two-stage double- cladding fiber amplifiers, the average power at 40 kHz was boosted up to 40.5 W. The output pulse has a maximum pulse energy of 1 mJ with a pulse width of 100 ns, which corresponds to a peak power of 10 kW. To the best of our knowledge, it is the highest single pulse energy ever reported for a nanosecond thulium-doped all-fiber MOPA system at 2050 nm.
© 2015 Optical Society of America
Infrared lasers with wavelengths longer than 1.4 μm have attracted great attentions due to their plentiful eye-safe atmospheric applications in recent years. High energy thulium-doped fiber lasers (TDFLs) at 1.9-2.1 μm are useful in many scientific and technical applications [1, 2 ], such as remote sensing [3, 4 ], bio-medical treatment , communication [6, 7 ] and nonlinear frequency conversion [8–10 ]. There are many reports of TDFLs with high output pulse energy. The highest pulse energy ever presented in fiber laser was achieved by Gaida et al. with a working wavelength at 1964 nm . To date, almost all of these works focus on the 1900-2000 nm region which locates in the high gain band of thulium-doped fibers (TDFs) , while little investigation focus on the longer wavelengths. In fact, there are also many important applications which require the TDFLs operating at 2050 nm. It is worth mentioning that compact fiber lasers with high beam quality at 2050 nm are highly desirable in remote sensing benefiting from the low transmission loss in the atmospheric window . Also, high power fiber lasers around 2050 nm are ideal pump lasers for ZnGeP2 (ZGP) crystal based optical parameter oscillators (OPO) with emitting wavelength locating at 3-5 μm middle infrared region as the ZGP crystal defects will absorb light with wavelength shorter than 2 μm [9, 14–17 ]. The linear absorption coefficient of the ZGP could decrease from 0.15 cm−1 at 2000 nm to nearly 0.02 cm−1 at 2050 nm .
Recently, Lucas et al. reported a linearly polarized MOPA which could emit single frequency 110 ns, 1 kW peak power pulses at 2050 nm, where the average power and pulse energy were up to 2 W and 0.11 mJ, respectively. Due to the single frequency characteristic of the seed laser, the slope efficiency in the main TDF amplifier was only 20% . In 2014, Gebhardt et al. demonstrated a ZGP based OPO , where the pump source was a 3.36 W TDFL at 1.98 μm with a pulse duration of 7 ns and a repetition rate of 4 kHz. In order to obtain pulse trains with high pulse energy and high average power at the same time, gain-switched (GS) fiber laser was also a good choice which could eliminate the need for the expensive optical modulator at 2 μm. In 2008, Creeden et al. reported a TDF amplifier system with an average output power of 21 W at 100 kHz, which was seeded by a GS fiber laser operating at 1995 nm. Considering the pulse width of 30 ns, the peak power reached 7 kW. Nonetheless, the whole system was not in a strictly all-fiber format. In 2012, Simakov et al. presented an all-fiber monolithic pulsed source based on a TDF-MOPA configuration. This source produced 200 µJ pulses with 20 ns duration at a repetition rate of 75 kHz resulting in up to 12 W of linearly polarized light at 2.044 µm . In fact, it is more difficult to achieve high power output at 2050 nm that at 2000 nm. The emission cross section of the TDF at 2050 nm is only 1.95 × 10−25 m2 while the corresponding number at 2000 nm is 3.15 × 10−25 m2 , which results in a much lower lasing efficiency and hinders high power pulse operation of TDFL consequently. Currently, only a few experiments on GS-TDFL with wavelength longer than 2000 nm have been reported, and it is still worth further investigation.
In this paper, an all fiber nanosecond thulium-doped MOPA system at 2.05 μm with pulse energy up to 1 mJ is demonstrated. The repetition rate and the pulse duration of the GS seed oscillator could be tuned by adjusting the corresponding parameters of the pump source. With a two-stage amplifier, the average power of the output pulse could be scaled up to 40.5 W, which is limited by the available pump power at present. The repetition rate and pulse width are 40 kHz and 100 ns, respectively, corresponding to a peak power of 10 kW. To the authors’ best knowledge, this result gives the maximum pulse energy of nanosecond pulse at 2050 nm in an all fiber format ever reported so far.
2. Experimental setup
The high pulse energy nanosecond all-fiber thulium-doped MOPA system consists of a fast GS seed oscillator, two stages thulium doped fiber amplifiers (TDFAs). In the seed oscillator, there are a high-reflectivity fiber Bragg grating (HR-FBG, reflectivity of 99%, bandwidth of 0.8 nm), an output coupler (OC-FBG, reflectivity of 10%, bandwidth of 0.2 nm), and a piece of 0.2 m long double clad thulium-doped fiber (DC-TDF) with the thulium dopant concentration of 2 wt.%. The core/inner cladding diameter of the TDF is 10/130 µm and corresponding numerical aperture (NA) of it is 0.15/0.46, respectively. The pump source is a MOPA system seeded by an electric modulated DFB laser at 1550 nm. The repetition rate could be tuned within a wide range similar to Ref . The pigtails of this pair of FBGs and the TDF are all polarization maintaining (PM) fibers with a panda style. As the operating wavelength of the FBG is refraction index related, and the index of the PM fiber has a little distinction at different polarization states, the center wavelength of the FBG varies at different polarization states. The competition between the operating wavelengths of different polarization states would be detrimental to the stability of the system. To eliminate the competition, the pigtail of the HR-FBG and the TDF were fusion spliced parallel, while the pigtail of OC-FBG and the TDF were fusion spliced perpendicularly. Parallel splicing means the fast axes of the two fibers are parallel when splicing, while perpendicularly splicing means the fast axes of the two fibers are perpendicular. In this way, the polarization state supported by the slow axis of the TDF was suppressed, so that the output pulses were all linearly polarized and the output spectrum has only one peak . Besides, the HR-FBG and the OC-FBG are all designed to operate around 2050 nm.
Then, the pulsed seed laser was amplified via a two-stage TDF amplifier chain, as depicted in Fig. 1 . A 793 nm LD, a (2 + 1) × 1 combiner, an isolator and a 4 m long DC-TDF (core diameter = 10 μm/NA = 0.15 and inner cladding diameter = 130 μm/NA = 0.46) were employed in the first stage amplifier. The absorption efficiency of the TDF-10/130 at 793 nm was about 4 dB/m. A circulator was used to protect the first stage amplifier from being impaired by the amplified spontaneous emission (ASE) generated in the second stage amplifier and the backward light. The second stage amplifier is the main stage amplifier, which was comprised of a (6 + 1) × 1 combiner, three 793 nm LDs, a piece of 4.5 m long large mode area (LMA) DC-TDF and a segment of 1 m passive fiber. The core diameter and inner clad diameter of the passive fiber and the DC-TDF are the same, being 25 μm and 250 μm (NA = 0.11/0.46), respectively. The absorption efficiency of the DC-TDF at 793 nm was about 5 dB/m. The TDF was coiled into a ring with diameter of 140 mm, which could suppress the high-order transverse modes and absorb the pump power more sufficiently. The 1 m long passively fiber and the TDF were fusion spliced and the unabsorbed pump laser was dumped just after the fusion splicing joint. The fiber was cleaved flatly, with the flatness less than 0.4 degree, after which, the end face was washed in the ultrasonic cleaning machine. As the transverse section of the inner cladding in the TDF is octagonal, manual adjustment is necessary in the splicing machine, so that the core of the fiber can be seen clearly, making it easy to precisely align the fiber. Also, the temperature of the splice filament, the filament time and the position of the fiber all need to be optimized. In such a situation, the splicing loss would be comparatively small, which would facilitate thermal management of the splicing joints as well. The output end face of the passive fiber was cleaved with an 8-degree angle to reduce the backward light induced by the Fresnel reflection at the end face. The backward light power was monitored at port 3 of the circulator. The DC-TDF and the adjacent splicing joints were all placed on a water-cooled heat sink to release the spare heat and protect the laser system. An optical spectrum analyzer (OSA) with 0.05 nm resolution was employed to measure the spectrum of output beam. The temporal characteristic of the pulse train was detected using a 200 MHz photo-detector and recorded through a 1.5 GHz digital oscilloscope.
3. Experimental results and discussion
3.1 Gain-switched TDFL
In the GS-TDFL experiment, first of all, the repetition rate of the pump pulse was set at 40 kHz. The GS pulse could be observed when the incident pump power reached ~1 W. The rather large threshold of the seed oscillator may attribute to the low reflectivity of the OC-FBG. At the beginning, the pulses were unstable. However, with the pump power increasing, the fluctuation of the pulse train got smaller . Steady GS operation was obtained when the pump power was 1.12 W, and the measured pulse train is shown in Fig. 2(a) . The center wavelength of the output pulse was 2049.73 nm with a 3 dB bandwidth of 0.07 nm, which is approaching the resolution limitation of the OSA. The repetition rate of the output pulse train is 40 kHz, which is consistent with the pump source. Fig. 3 , shows the evolution of the output pulse with the rise of the pump power. When the oscillator operated near the threshold, the pump residue and the signal pulse revealed the similar intensity. Further increasing the pump power, the intensity of the signal pulse at 2050 nm rose dramatically, whereas the pump residue at 1550 nm kept unchanged. With the growth of the pump power, the energy density rose rapidly. The built-time of the signal pulses would decrease consequently, which is the typical characteristic of GS fiber lasers . In addition, the gap between pump residue and the signal pulses decreased continually in the temporal domain. Finally, stable operation with pulse duration of 70 ns was achieved at the maximum pump power as depicted in Fig. 2(b).
The relationship between the output pulse duration and the pump pulse energy at different repetition rates were also investigated. The average power threshold of the oscillator operating at 100 kHz was a little higher than that at 40 kHz. However, as shown in Fig. 4 , the threshold of the pulse energy was much lower. It is manifest that the solid line, which represents the 100 kHz pulse train, locates at the left of the dash line of the 40 kHz pulse train. In Fig. 4(a), the output pulse duration decreased with the growth of pump pulse energy at both repetition rates. The narrowest pulses of 50 ns were achieved at the repetition rates of 100 kHz. On top of that, in Fig. 4(b), the ascending lines represent the rising of the output pulse energy with the increase of pump power. The efficiency of the pulse energy transition at 40 kHz was slightly lower than that at 100 kHz, which was induced by the relatively low duty ratio at 40 kHz. For both repetition rates, the pulse energy increased linearly without the sign of saturation, which indicates that they are mainly constrained by the pump power. The PER of the output pulse train at 2050 nm was also measured with a fiber-packaged polarization beam splitter (PBS). The PERs at both repetition rates were above 16 dB.
3.2 Amplifier chain
Limited by the available pump power at the main stage amplifier, the repetition rate of the pump source was set at 40 kHz for the following pulse energy scaling. The average power of seed pulses was 470 mW. Then, the seed pulse was amplified in the first-stage power amplifier. The average power reached 2.05 W with 12 W incident pump light at 793 nm. The corresponding output spectrum of the pulses at 2.05 W is shown in Fig. 5 . Obviously, there is no ASE in the measured spectrum. Only two weak side-spikes located at both sides of the signal wavelength symmetrically. The side-spikes were induced by the nonlinearity of modulation instability (MI), which is a typical effect occurring when pulses with high peak power propagated in abnormal-dispersed fiber . The calculated maximal peak power after the first amplifier is 700 W. The circulator could protect the first stage amplifier from being impaired by the backward laser and the ASE generated in the main amplifier and monitor the power of backward light simultaneously. The insertion loss measured from port 1 to port 2 of the circulator at 2050 nm is ~1.1 dB.
In the main amplifier, the (6 + 1) × 1 combiner delivered pump light to the TDF from three fiber-pigtailed multimode LDs, which could combine a total power of ~100 W at 793 nm. The evolution of the output spectrum versus the increase of output power is shown in Fig. 6(a) . It is found that no obvious ASE noise exists in the final output. The broadening of the spectrum on both sides was induced by the nonlinearity of MI and the distinction between the signal peak and the MI spikes in the spectrum is above 30 dB. Fig. 6(b), displays the output spectrum at 40.5 W with a linear scale. In this case, it can be found that most of the energy was focused at ~2049.7 nm. The output pulse shape is depicted in Fig. 6(c). The pulse width broadens to 100 ns at the maximum pump power, while the Gaussian shape was kept. Fig. 6(d), gives the evolution of average output power versus the pump power. The average output power increased almost linearly with the rise of the pump power. When the pump power reaches 93.6 W, the average output power achieves the maximum, which is 40.5 W. The linear-fitted slope efficiency of the main amplifier is about 42%, and the output power saturation was not observed at the maximum pump power. It is expected that higher average output power would be obtained by adding more pump source. Since the repetition rate is 40 kHz, the corresponding pulse energy is estimated to be 1 mJ, which is the maximum pulse energy ever reported in an all-fiber thulium-doped MOPA system at the long wavelength of 2050 nm.
It is a little surprise that the outline of the spectrum kept almost the same when the average power surpassed 30 W as shown in Fig. 6(a). The peak power at 30 W and 40 W were 8.3 kW and 10 kW, respectively. As we all know, MI would induce more broadening in the spectrum with the peak power increasing in a piece of fiber . Then we re-measured the pulse profile with another faster InGaAs photo-detector (bandwidth of 8 GHz). Fig. 7(a) , shows the single pulse envelop of the seed pulses from the GS-TDFL. It was found that fine structured mode-locked sub-pulses were contained in one GS pulse envelop . The sub-pulses play an important role in increasing the peak power. The high peak power may be useful for supercontinuum generation or OPO. The interval between the adjacent sub-pulses is about 6 ns, which corresponds to the cavity round-trip time. These sub-pulses may be caused by the inherent nonlinearities in the laser cavity, which was strong enough to influence mode-locked type pulses formation . There is also a basis under the sub-pulses. The output pulse shape after the main amplifier is depicted in Fig. 7(b). Explicitly, Frantz-Nodvik relation governs the pulse shape evolution, and an apparent spike at the front edge of the pulses appeared . It can also be seen that the basis of the pulses was enlarged remarkably, which may be the explanation of the stable spectrum. When the pulses propagate in the gain-fiber, the basis of the pulse may be enlarged rapidly compared to the sub-pulses, as the duty ratio of the basis was much larger than that of the sub-pulses. In the situation of high average power, most energy was focused in the basis while the peak power of the sub-pulses kept constant. In this case, the spectrum broadening induced by the MI effect would keep at the similar level because of the almost unchanged peak powers . Continuous growth of the basis could be expected when further increasing the pump power and it would lead to a nearly “saturated” peak power while higher pulse energy was accumulated. As a result, the nonlinear effect would have less influence on the spectrum at higher average output power. If the spike at the front edge was included in the calculation, the peak power could be a little higher than 10 kW.
In summary, the first experimental realization of pulse energy exceeding 1 mJ in a thulium-doped MOPA system at 2050 nm was demonstrated in this paper. A linearly polarized GS-TDFL was adopted as the seed laser and pulse scaling of the seed laser was realized with a two-stage amplifier. In the experiment, nearly 20 dB increment of the pulse energy was achieved, with maximal pulse energy of 1 mJ and average power of 40.5 W, when the seed pulses have a repetition rate of 40 kHz. The amplified pulse shape detected by a low-speed detector was Gaussian like, and shows a temporal broadening from 70 ns to 100 ns during the amplification. Furthermore, finely structured gain-switched mode-locked characteristic was observed using a high-speed detector, and the pulse shape after amplification was measured. The pulse energy may be increased while keeping the peak power almost unchanged. The slope efficiency of the main amplifier is 42%. We could also expect to obtain high pulse energy and average output power by increasing the incident pump power. Considering the compactness and the robustness of the fiber laser system, we believe that the high pulse energy TDF-MOPA system at 2050 nm will be widely used in many practical applications.
This work is supported by the State Key Program of National Natural Science of China (Grant No. 61235008) and the Natural Science Foundation for Distinguished Young Scholars of China (Grant No.61405254).
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