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Abstract
In this paper, we report on high-power stable nanosecond pulse generation at ~2.1 μm from an integrated Tm–Ho all-fiber master oscillator power amplifier (MOPA) system. A total output power of 128.5 W is generated from the Tm–Ho hybrid MOPA, with an average power of 99.1 W from Ho emission at 2116 nm; the corresponding pulse repetition frequency and pulse width are 161 kHz and 322 ns, respectively, leading to a peak power of 1.91 kW. The Tm–Ho integrated master oscillator is designed to operate at 1980 and 2116 nm, where the former wavelength serves as the pump of the Ho-doped fiber. Stable laser pulses are generated from both the Tm and Ho oscillators owing to mutual modulation of emission from the two lasers. The prospects for further scaling in output power at ~2.1 μm using Tm-Ho integrated MOPA system are discussed.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Pulsed fiber lasers around 2 μm are attractive for material processing [1], medical applications [2], gas detection [3], and infrared countermeasures. They are also ideal pump sources to generate 3–5 μm mid-infrared lasers by nonlinear frequency conversion [4]. Different approaches to generate laser pulses at ~2 μm using Tm [5, 6], Ho [7], or Tm/Ho co-doped [8] fiber lasers have been investigated. Although Tm-doped fiber lasers can deliver a powerful output, the efficiency will decrease significantly when they are operated above 2 μm owing to the limited gain bandwidth [7]. In comparison, the emission wavelength of Ho-doped fiber covers a wide band from 2000 to 2200 nm, enabling generation of highly efficient pulsed lasers above 2 μm.
For pulsed laser operation with Ho-doped fiber, gain-switching [9] and Q-switching [10] have been demonstrated to generate nanosecond laser pulses. To date, saturable-absorber-based passively Q-switched Ho-doped all-fiber lasers have been successfully demonstrated and show the advantages of robust operation, compactness, and simplicity [10, 11]. Specially designed fiber configurations, such as, a double-clad fiber with a Ho-doped fiber core surrounded by a Tm-doped cladding can also exhibit unstable pulse modulation behavior [12]; the Tm transition is modulated, and the Ho operates in the gain-switched condition. Further, an extra laser located in the laser’s emission sideband can suppress the laser emission through population consumption [13]. These results indicate that an extra laser with spectral overlap can control the lasing, providing the potential to generate laser pulses. In addition, the output power of typical Q-switched Ho-fiber lasers is relatively low, and further power scaling is limited owing to the lack of suitable pump sources and fiber devices. For a pump source at 1150 nm, direct emission from rare-earth-doped lasers is difficult. Moreover, the pump laser at 1950 nm from the Tm fiber laser will damage the outer cladding of the commercial double-clad Ho-doped fiber owing to strong absorption of common low-index polymers in this spectral range. To date, the most powerful pulsed Ho-doped fiber laser has a power of 45 W at 2.09 μm using a specially designed triple-clad all-glass Ho fiber and a free-space configuration [7]. A cascaded single-mode Tm–Raman fiber amplifier was employed recently to amplify the laser at 2147 nm [14]; the output power and efficiency were limited by the pump power and nonlinear Raman gain coefficient at this wavelength. Further, Raman fiber with a length of dozens of meters is unfavorable for high power generation at 2.1 μm owing to the high intrinsic fiber loss.
In this paper, we report a high-power nanosecond fiber laser source above 2.1 μm using a hybrid Tm–Ho all-fiber master oscillator power amplifier (MOPA) system. An average output power of approximately 99.1 W at around 2116 nm was generated from a Tm–Ho cascaded fiber amplifier with a pulse repetition frequency of 161 kHz and pulse width of 322 ns, the seed was provided by a Tm–Ho integrated master oscillator. This configuration relies on multistage large-mode-area Tm and Ho fiber amplifiers to improve the laser efficiency and power level; the short device length of the Ho fiber can also minimize the fiber loss and nonlinear effects.
2. Nanosecond pulse generation
The experimental setup of the seed laser in the integrated Tm–Ho fiber MOPA system is shown in Fig. 1. In the oscillator, a Tm-doped laser cavity composed of a fiber loop mirror (FLM1) and a fiber Bragg grating (FBG1) (low reflectivity of 10% at 1980 nm, 3 dB reflective bandwidth of 1 nm) was located inside the Ho-doped laser cavity, which was composed of fiber loop mirrors FLM1 and FLM2. FLM1 and FLM2 were made of two tapered couplers (Labbang Inc.). The true power ratios of the two couplers were measured to be ~51:49 and ~10:90 at 1980 nm, ~38:62 and ~14:86 at 2116 nm, respectively. Therefore, the initial effective reflectivities of FLM1 and FLM2 at 1980 nm are ~100% and 36% at 1980 nm and ~94% and ~48% at 2116 nm. The total fiber length of the fiber loop in each FLM was shortened to 1.5 m to increase the threshold of nonlinear effects. A 2.7 m single-mode double-clad Tm-doped fiber [SM-TDF, 10 μm (0.15 NA) core diameter and 130 μm (0.46 NA) cladding diameter] with a cladding absorption coefficient of 3 dB/m at 793 nm was employed. Two 12 W pigtailed laser diodes (LDs) at 793 nm were used to pump the SM-TDF through a (2 + 1) × 1 combiner, and the available pump power coupled into the cladding of the TDF was measured to be 21.2 W. A home-made cladding pump stripper (CPS) was spliced to the end of the TDF to eliminate the unabsorbed pump light. Subsequently, the laser at 1980 nm was coupled into the core of a 3 m double-clad Ho-doped fiber [SM-HDF, 10 μm (0.15 NA) core diameter and 130 μm (0.46 NA) cladding diameter]. The small-signal core absorption of the SM-HDF is estimated to be ~40 dB/m at 1980 nm. FLM2 acted as the output coupler for the Ho-doped laser cavity with a transmissivity of ~52%. The laser pulses were monitored using two fast InGaAs PIN photodiodes responding at 1200–2600 nm (DET10D/M, Thorlabs Inc.) and recorded by a 350 MHz real-time digital oscilloscope (DSO-X 3034 A, Agilent Inc.). Laser spectra were recorded by an optical spectrum analyzer (AQ6375, YOKOGAWA Inc.) with a resolution of 0.2 nm.
Fig. 1 Experimental setup of the pulsed seed laser. LD: laser diode; SM-TDF: single-mode Tm-doped fiber; SM-HDF: single-mode Ho-doped fiber; FLM: fiber loop mirror; CPS: cladding pump stripper.
Fiber loop mirror FLM1 acted as a common feedback mirror for the Tm and Ho fiber lasers; both the Tm and Ho laser signals can leak out of the free port of FLM1. A dichroic mirror with high transmissivity at 1980 nm and high reflectivity at 2100–2150 nm was used to filter out the Tm laser signal for measurement of the temporal profile. Pulse profiles of the Ho laser were recorded directly after FLM2. Laser spectra were measured directly from the free port of FLM1. Figure 2 shows the laser characteristics of the hybrid Tm–Ho fiber integrated resonator under different pump powers. In the experiment, the threshold pump power of the Tm fiber laser at 1980 nm was 1.4 W. The temporal profile shows a typical disordered self-pulsing train of continuous-wave (CW) fiber lasers, as shown in Fig. 2(a). However, when the pump power was increased to 2 W, stable pulse emission occurred at ~2116 nm, as shown in Fig. 2(b). The Ho fiber laser exhibited broadband operation owing to the broadband feedback from the FLMs. As the pump power was increased further, the average output powers of both lasers continued to increase with pulses kept stable. Figure 2(c) shows a typical pulse train and single-pulse profile under a 15.8 W pump power. The pulse-to-pulse amplitude fluctuation was estimated to be less than 10%. The pulsed oscillation at two wavelengths showed obvious correlation with a time delay varying from 18.7 μs to 621 ns with the pump power increasing from 2 W to 21.2 W, as shown in Fig. 2(d). This behavior is consistent with that in gain-switched lasers where the time needed for photon density buildup in a gain-switched laser is reduced as the pump rate increases [15]. The pulse width and pulse repetition rate with respect to the pump power are illustrated in Fig. 2(e). The pulse width of the Tm laser decreased from 573 to 196 ns, and the Ho laser pulse width decreased from 1.61 μs to 390 ns. The pulse repetition frequency increased from 26 to 161 kHz. Laser pulses at both wavelengths remained stable at the highest pump power; therefore, the performance could be further improved by simply increasing the LD pump power.
Fig. 2 Output characteristics of the hybrid Tm–Ho fiber oscillator. Output spectrum and temporal behavior of (a) 1.4 W pump power; (b) 2 W pump power; (c) Typical pulses train and single pulse profile at 15.8 W of pump power; (d) Delay time of the two pulsed laser signals at different pump powers; (e) Pulse width and pulse repetition frequency (PRF) at different pump powers.
Owing to the high absorption of the SM-HDF and the function of FBG1, Tm-doped laser oscillation can occur only between FLM1 and FBG1. Therefore, the hybrid oscillator is different from the passively Q-switched fiber laser using a saturable absorber [16], in which the Ho fiber should be inside the Tm laser cavity. The reason for this temporal behavior can be understood as follows. The emission spectra of the Tm and Ho ions exhibit a broadband overlap from 1900 to 2200 nm [17]; therefore, the 2116 nm laser will be amplified by the Tm fiber as well once the 2116 nm laser starts to emit. Then much of the populations on the upper level of Tm will be consumed, and the lasing at 1980 nm will be suppressed. This mutual modulation between 1980 and 2116 nm signal on the population of Tm and Ho ions results in the generation of stable and periodic pulsation of the output.
Figure 3(a) shows output power of the seed as a function of the 793 nm pump power from the output port of FLM2. The output power increased linearly with a slope efficiency of 14.8%. The low efficiency was mainly caused by the low transmissivity of FLM2. An output power of 2.9 W was generated at a pump power of 21.2 W, corresponding to a single pulse energy of 18 μJ and peak power of 46 W. Figure 3(b) shows the laser spectrum at the maximum output power. The center wavelength of the seed laser was 2116 nm with a 3 dB bandwidth of 0.18 nm. The laser at 1980 nm can be ignored, because the difference in spectral intensity compared with the 2116 nm laser was more than 38 dB.
Fig. 3 (a) Output power of the Ho laser with increasing 793 nm pump power; (b) Laser spectrum of the Ho laser at maximum output power.
3. Power scaling using integrated Tm–Ho fiber amplifier
The output from the seed laser was amplified by two-stage amplifiers to obtain a higher output power, as shown in Fig. 4. Another FBG (FBG2, high reflectivity of >99.6% at 1980 nm, 3 dB reflective bandwidth of 2 nm) was spliced after FLM2 to block the unabsorbed 1980 nm signal. The seed laser from the oscillator passed through an isolator to avoid any feedback from the amplifier, and it was then combined with a home-made CW 1980 nm Tm fiber laser using a 10/90 coupler. The output power available at 2116 and 1980 nm after the coupler were 1.8 W and 300 mW, respectively. They propagated together in one fiber and were coupled to the pre-amplifier through a home-made mode field adaptor. A 2.5 m piece of large-mode-area double-clad Tm-doped fiber [LMA-TDF, core diameter of 25 μm (0.09 NA) and inner cladding diameter of 400 μm (0.46 NA)] with a cladding absorption coefficient of 4 dB/m at 793 nm was used in the pre-amplifier. The Tm fiber was pumped by a 793 nm LD through a (2 + 1) × 1 combiner; the total pump power after the combiner was measured to be 46 W. The residual pump light in the inner cladding was stripped by a CPS. In the main amplifier, a piece of 3.8 m LMA-TDF with the same parameters as the pre-amplifier was pumped by six 793 nm LDs with a core diameter of 200 μm (0.22 NA). The total pump power launched into the fiber using a (6 + 1) × 1 combiner was measured to be 348 W. Similarly, the residual pump light in the inner cladding of the LMA-TDF was eliminated by a CPS. Then, all the signal power from the Tm fiber was coupled into the core of a 2.5 m large-mode-area Ho-doped fiber (LMA-HDF). The core diameter of the LMA-HDF was 25 μm (0.09 NA), and the inner cladding diameter was 250 μm (0.46 NA). The small-signal core absorption of the LMA-HDF was estimated to be ~36 dB/m at 1980 nm. In this setup, all the active fibers were water-cooled to ~10 °C to avoid any possible damage to the fibers. Finally, a home-made endcap was spliced to the end of the amplifier.
Fig. 4 Integrated Tm–Ho fiber amplifier. ISO: isolator; MFA: mode-field adapter; LMA-TDF: large-mode-area Tm-doped fiber; LMA-HDF: large-mode-area Ho-doped fiber.
Figure 5 shows the pre-amplified output power versus the pump power and the laser spectrum at maximum output power. Because of the broad gain range of Tm ions, both 1980 and 2116 nm lasers can be amplified in the Tm-doped fiber amplifier. However, the 1980 nm laser can be amplified more efficiently owing to the higher laser gain. To measure the output power, a dichroic mirror with high transmissivity at 1980 nm and high reflectivity at 2100–2150 nm was used to separate the two wavelengths. The CW power at 1980 nm was amplified to 10.9 W, and the average power at 2116 nm was amplified to 6.5 W. The average slope efficiencies at 1980 and 2116 nm with respect to the 793 nm pump power were 26.5% and 11.3%, respectively. After the pre-amplifier, the pulse width was narrowed to 328 ns, because the stored energy in the TDF was insufficient [18]; the pulse energy and peak power were amplified to 40.4 μJ and 123.2 W, respectively.
Fig. 5 Output characteristics of the pre-amplifier. (a) Output power versus LD pump power; (b) Output spectrum at maximum output power.
The output from the pre-amplifier was further scaled using the Tm–Ho cascaded main amplifier. Figure 6(a) shows the output power measured after the Tm fiber amplifier. The total output power rose from 10.5 to 205.4 W with a slope efficiency of 55% with respect to the LD pump power. The laser power at 1980 nm was amplified from 3.6 to 166 W with an average slope efficiency of 44.4%. The laser power at 2116 nm first increased slowly to 49 W and then decreased to 39.5 W. The reason is that the majority of the pump power was transferred to 1980 nm quickly under the high pump power, leading to insufficient amplification at the longer wavelength of 2116 nm.
Fig. 6 Output performance of the main amplifier. (a) Output power after the TDF versus LD pump power; (b) Final output power from the endcap; (c) Typical seed pulse and amplified pulse profile at the maximum output power; (d) Final spectra at different total output powers.
The Ho-doped fiber (LMA-HDF) following the LMA-TDF in the main amplifier was used to transfer the power at 1980 nm to 2116 nm. When the output from the Tm fiber amplifier was coupled into the LMA-HDF, the laser at 2116 nm acted as the seed laser and was amplified by the LMA-HDF, where the 1980 nm laser served as the pump. Figure 6(b) shows the final output from the Ho fiber amplifier. The total output power and slope efficiency were 128.5 W and 34.6%, respectively. The laser power at 2116 nm was amplified linearly from 6.4 to 99.1 W, which corresponds to a single pulse energy of 615.5 μJ. The slope efficiency was 27% with respect to the 793 nm pump power. Only 29.4 W of laser power at 1980 nm was left. Figure 6(c) shows a typical seed pulse and an amplified single pulse profile at the maximum output power. After the main amplifier, the pulse width was measured to be 322 ns, corresponding to a peak power of 1.91 kW. The sub-pulses were also amplified in the amplifier, leading to obvious multiple spikes in a single pulse. However, the output pulse still maintained a shape similar to the input pulse. The final output spectra of the amplifier were recorded at different total output powers, as shown in Fig. 6(d). Under high peak power, self-phase modulation will lead to the spectral broadening, and the longer wavelength will be further amplified by the stimulated Raman scattering effect, increasing the power at longer wavelengths (>~2140 nm) [19]. We infer that the Raman peak at ~2340 nm will appear if we further increasing the pump power. Further, the laser power at long wavelength will be absorbed by the silica fiber owing to the high intrinsic loss of the silica fiber at this wavelength, slowing the power-enhancement rate at high pump powers. Based on our current experimental results, a better laser performance, including the power level and spectral quality should be anticipated, if further optimizing in the length of the Ho fiber and passive fiber in the main-amplifier.
4. Conclusion
In conclusion, we have demonstrated a hybrid Tm–Ho all-fiber MOPA system to generate 99.1 W of stable nanosecond laser pulses at approximately 2.1 μm with a pulse repetition frequency of 161 kHz. The laser performances in the oscillator and amplifier were discussed in detail. To the best of our knowledge, this is the first report on a nanosecond all-fiber laser above 2.1 μm with an average output power near 100 W.
Funding
National Key R & D Program of China (2017YFB1104400).
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