1. Introduction

High-power ultrafast lasers are important sources for a number of applications including material processing [1, 2], pump source for optical parametric oscillator (OPO) [3], and mid-infrared broadband supercontinuum generation [4–7]. Ultrafast thulium-doped fiber lasers, which extend the wavelength range of fiber lasers from 1.8 to 2.1 µm, have been rapid developed for the last several years [8–19] and the average output power of the ultrafast thulium-doped fiber amplifiers have reached hundred watt [20, 21]. Recently, the highest achieved peak powers from thulium-doped fiber chirped pulse amplification (CPA) systems are on the order of hundreds tens of MW [22, 23]. The broad and smooth gain spectrum of thulium-doped fiber makes it a well-suited gain medium for generation of ultrashort pulses and widely-tunable wavelength. Most notably, Haxsen et al. achieved high pulse energy and pulse peak power femtosecond pulses using CPA technique in a large-mode-area (LMA) thulium-doped fiber amplifier [18], which produces 5.4 W average output power and 151 nJ pulse energy. Wan et al. demonstrated a high-power CPA system based on a picosecond oscillator and multistage fiber amplifiers [19]. The final thulium-doped fiber power amplifier boosts the average output power to 36 W, corresponding to pulse energy of 1.1 μJ. In our previous work [20], we demonstrated a picosecond thulium-doped all-fiber amplifier with average output power of 120 W, corresponding to the pulse duration of 16 ps. However, the fiber oscillator and amplifiers were made from non-polarization-maintaining (PM) fibers. The system may suffer from the environmentally instability that means the laser source is sensitive to externally-induced changes, like significant temperature variations and mechanical perturbations which will influence the fiber birefringence property. One effective method to eliminate this environmentally instability is to build a PM thulium-doped all-fiber MOPA configuration where the light polarizes only along the slow or fast axis in the PM fiber and PM-fiber components. More recently, Stutzki et al. demonstrated a high-power thulium-doped fiber CPA emitting a record compressed average output power of 152 W and 4 MW pulse peak power by utilizing free-space pump PM LMA thulium-doped photonic crystal fibers (PCF) with 50 μm core diameter [21]. Using free-space Kagome-type PCF compressor in combination with a large-pitch rod-type PM thulium-doped PCF, pulses less than 70 fs at a pulse peak power of 200 MW and average power of 2 W were recently demonstrated [23].

In this contribution, for the first time, to our knowledge, we demonstrated a high-power linearly-polarized picosecond thulium-doped all-fiber-integrated MOPA system by using PM thulium-doped fiber and PM-fiber components. The final PM thulium-doped all-fiber power amplifier yielded 240 W of average output power with a polarization extinction ratio (PER) of >15 dB. The pulse duration of 45 ps at 127 MHz repetition rate results in a pulse peak power of 42 kW in the final fiber power amplifier. This eliminates the need for novel rod-type PCF fiber or free-space coupling to fiber amplifiers, improving reliability and robustness of the fiber laser source.

2. Experimental setup and results

The high-power linearly-polarized picosecond thulium-doped all-fiber-integrated MOPA system consists of a passively mode-locked PM thulium-doped all-fiber oscillator and a three stage PM thulium-doped all-fiber amplifier. The schematic setup of the MOPA system is shown in Fig. 1. The seed source was a semiconductor saturable absorber mirror (SESAM) mode-locked linearly-polarized picosecond thulium-doped all-fiber oscillator working at 1984 nm. The total cavity length of the fiber oscillator is 0.79 m, which included 20 cm PM double-clad thulium-doped fiber with 10 µm core pumped with a home-made continuous-wave 1550 nm fiber laser. A PM wavelength division multiplexer (WDM) was used to deliver pump light and the efficient pump coupling to the core of PM double-clad thulium-doped fiber was over 95%.

 

Fig. 1 Schematic setup of the high-power linearly-polarized picosecond thulium-doped all-fiber MOPA. SESAM, semiconductor saturable absorber mirror; PM MFA, polarization-maintaining mode field adaptor.

Download Full Size | PDF

The cavity output coupler is a narrow-bandwidth PM fiber Bragg grating (FBG) with 70% reflectivity at a center wavelength of 1984 nm (full-width at half-maximum of 2 nm). The narrow-bandwidth PM FBG also has the function of a spectral filter to balance the nonlinearity induced spectrum broadening effect. Therefore, we can obtain easily stable passively mode-locked laser pulses. One end of the PM double-clad thulium-doped fiber is fusion spliced to the PM FBG; the other end is perpendicularly cleaved and butted to a SESAM. The SESAM has a modulation depth of 8%, non-saturable loss of 5%, relaxation time of 500 fs, and saturation fluence of 20 µJ/cm². In our experiment, the power was measured with a wavelength insensitive thermal power meter (Gentec-EO, UP55G-500F-H12). The optical spectrum was measured by an optical spectral analyzer (YOKOGAWA AQ6375). The ultrafast pulse was characterized by an autocorrelator (Femtochrome, FR-103XL). A 25 GHz real-time oscilloscope (Agilent, DSO-X92504A) and a 12.5 GHz InGaAs photodetector (EOT, ET-5000F) were used to measure time characteristics. The beam quality factor was measured by a scanning slit beam profilers system (BeamScope-P8).

With proper adjustment of SESAM reflective coupling efficiency with the PM double-clad thulium-doped fiber end, stable self-started CW mode-locked pulses of the fiber oscillator occurred at 200 mW incident pump power. The PM fiber oscillator generated stable nearly transform-limited picosecond pulses with fundamental repetition rate of 127 MHz is presented in Fig. 2(a). With the incident pump power of 300 mW, the average output power of the passively mode-locked PM fiber oscillator was 6 mW. The pulse duration can’t characterized by our autocorrelator (FR-103XL) because of low average and peak power of the mode-locked fiber oscillator. Owing to the all-PM all-fiber-integrated configuration, the linearly-polarized laser pulses are obtained with the PER of >20 dB, measured by a mid-IR linear polarizer (Thorlabs, LPMIR100). Figure 2(b) shows the optical spectrum of the passively mode-locked PM thulium-doped fiber oscillator, which was measured by an optical spectral analyzer with resolution of 0.05 nm. The central lasing wavelength of the fiber oscillator was around 1984 nm, which is same to the resonant peak of the PM FBG; the 3 dB bandwidth was 0.9 nm. Figure 3(a) shows the RF spectrum of the passively mode-locked PM thulium-doped fiber oscillator, the fundamental peak located at the cavity repetition rate of 127 MHz has a signal-to-background ratio of 65 dB, indicating that the mode-locked state was stable.

 

Fig. 2 (a) Pulse train of the passively mode-locked PM thulium-doped fiber oscillator. (b) Output spectrum of the passively mode-locked PM thulium-doped fiber oscillator.

Download Full Size | PDF

 

Fig. 3 (a) RF spectrum of the passively mode-locked PM thulium-doped fiber oscillator. (b) Autocorrelation trace of the pulse of the first PM thulium-doped fiber preamplifier.

Download Full Size | PDF

The mode-locked pulses from the PM thulium-doped fiber oscillator were amplified by one stage PM thulium-doped fiber preamplifier. A high-power broadband isolator is followed after the fiber oscillator to ensure the laser would not be affected by the backward light from the PM thulium-doped fiber preamplifier. In the first PM fiber preamplifier, the gain medium is a 3 m PM double-clad thulium-doped fiber (cladding-absorption of 4.7 dB/m at 793 nm), characterized by the same parameters as passively mode-locked fiber oscillator mentioned above. A high-power fiber-pigtailed multimode diode at 793 nm is employed as the pump source, and the maximum output power of 6 W. The first PM fiber preamplifier produced 1 W average output power at incident pump power of 5 W. Figure 3(b) shows the autocorrelation trace of the first PM thulium-doped fiber preamplifier. The FWHM of the autocorrelation trace was 15 ps. Assuming a sech² pulse shape typical for solitons with 1.54 deconvolution factor, the pulse duration is about 10 ps. The autocorrelation trace (Fig. 3(b)) measured in the 100 ps span confirms the single-pulse operation without any signs of pre- or post-pulses. The center wavelength and the spectral bandwidth of the first PM fiber preamplifier were 1984 nm and 0.9 nm respectively.

In order to decrease the detrimental effects of fiber nonlinearity in the later thulium-doped fiber power amplifier, a piece of 100 m silica-based ultra-high numerical aperture (UHNA) fiber having a positive group velocity dispersion (GVD) of about 90 ps²/km at 2 µm wavelength is used as a pulse stretcher, which is directly spliced to the PM passive fiber by an arc-fusion splicer, and it is spliced with PM passive fiber with a splice loss of less than 0.5 dB. The UHNA fiber had a mode field diameter of 2.2 µm, and a NA of 0.35 to reduce bend-induced losses at 2 µm wavelength region. The UHNA fiber stretches the laser pulse from the original nearly transform limited duration of 10 ps up to ∼125 ps, as shown in Fig. 4(a). Due to the strong self-phase modulation (SPM) effect, the spectrum is centered at around 1980 nm and has a 10 dB bandwidth of 49 nm, as shown in Fig. 4(b). In the second PM thulium-doped fiber preamplifier, the active fiber was a 3 m PM double-clad thulium-doped fiber, characterized by the same parameters as the PM fiber oscillator mentioned above. Two fiber-pigtailed diodes at 793 nm are employed as the pump source, and the total output power of 24 W. The average output power of the second PM thulium-doped fiber preamplifier was 3 W for 14 W 793 nm incident pump power. The pulse duration after the second PM thulium-doped fiber preamplifier was measured by our 25 GHz real-time oscilloscope and a 12.5 GHz InGaAs photodetector, and the FWHM was about 115 ps.

 

Fig. 4 (a) Pulse duration from a piece of 100 m pre-chirp UHNA fiber having a positive GVD. (b) Output spectrum from a piece of 100 m pre-chirp UHNA fiber having a positive GVD.

Download Full Size | PDF

In the final PM thulium-doped all-fiber power amplifier, a segment of 4 m PM thulium-doped double-clad fiber were used as the gain medium. The fibers have a core diameter of 25 µm, a core NA of 0.09, inner cladding diameter of 400 µm and a NA of 0.46. To improve the conversion efficiency in the fiber power amplifier, the PM double-clad thulium-doped fiber was cooled to 8°C in a water-cooled heatsink and the coil bending radius of the active fiber in the power amplification process is maintained less than 10 cm [20]. Due to the quasi-three-level nature of the thulium-doped laser, cooling the active fiber to the lower temperature was very critical for achieving high conversion efficiency in the final PM fiber power amplifier [24–27]. The pump source of the fiber power amplifier from six temperature stabilized multimode laser diode modules emitting at 793 nm was delivered by a multimode fiber with a fiber diameter of 200 µm, which match to the pump fiber of the pump combiner. The total output power of these diode modules was 560 W. A PM (6 + 1) × 1 pump combiner was used to deliver pump light to the PM double-clad thulium-doped fiber with a coupling efficiency of 95%. The output end of the PM thulium-doped fiber was spliced to a piece of PM passive fiber with matched core, and the output facet was cleaved at 8° to frustrate parasitic lasing. A dichroic mirror was used to separate residual pump light from the signal.

The final PM thulium-doped fiber power amplifier average output power versus the 793 nm pump power is plotted in Fig. 5. The maximum average output power was 240 W, which corresponds to slope efficiency of 46% and the output power increased almost linearly with the increase of pump power. The linearly-polarized picosecond thulium-doped fiber MOPA system is extremely robust against the thermal variations and mechanical stress, and it can operate without fluctuations obviously for a long period of time. The average output power fluctuation of ± 1% was measured within 20 minutes, which indicates a good stability for the MOPA system. The PER at the highest output power was measured to be >15 dB. Owing to the large positive chirp of the output pulses from the second PM thulium-doped fiber preamplifier and the negative GVD of the PM thulium-doped fiber of the fiber power amplifer, the pulses get shorter during high power amplification in the final PM thulium-doped all-fiber power amplifier [28]. Figure 6(a) shows the pulse duration at maximum average output power. The pulse duration is measured of 45 ps using our 25 GHz real-time oscilloscope and 12.5 GHz InGaAs photodetector, corresponding to the pulse peak power of 42 kW. Figure 6(b) shows the 10 dB spectral bandwidth as a function of signal output power. The SPM effect results in the broaden of the output spectrum. The center wavelength is around 2000 nm, and the 10 dB spectral bandwidth was >140 nm. The amplified spontaneous emission in the final PM fiber power amplifier is 30 dB down compared with the amplified signal. The beam quality factor was measured to be M² of 1.4 at 200 W average output power using our scanning slit beam profilers system.

 

Fig. 5 Average output power of the final PM thulium-doped all-fiber power amplifier with the increase of 793 nm incident pump power.

Download Full Size | PDF

 

Fig. 6 (a) Pulse duration of the final PM fiber power amplifier. (b) Measured 10 dB spectral bandwidth of the final PM fiber power amplifier as a function of signal output power.

Download Full Size | PDF

3. Conclusion

In summary, we have demonstrated a 240 W average output power linearly-polarized thulium-doped all-fiber-integrated MOPA system delivering picosecond pulses by using nonlinearity amplification technique, which is, to the best of our knowledge, the highest average output power ever reported for a picosecond all-fiber-integrated laser at around 2 µm wavelength. The PER at the highest output power was measured to be >15 dB, further power scaling is limited by available pump power. The pulse duration of 45 ps at 127 MHz repetition rate results in a pulse energy of 1.89 µJ in the final PM thulium-doped fiber power amplifier. The MOPA system was constructed entirely of PM fibers and PM-fiber components which makes it more resistant to thermal and mechanical perturbations. This kind of high-power linearly-polarized picosecond thulium-doped all-fiber laser source with compact and simple design is in great demand for a variety of applications, such as coherent polarization beam combination [29], and frequency conversion in nonlinear crystals [30].