We present a compact thulium-doped chirped pulse amplifier producing 241 fs pulses with 1 μJ energy. The system is seeded with the Raman shifted soliton generated by the combination of an erbium-doped femtosecond laser and a nonlinear fiber. The Tm-doped large mode area fiber yields output power of 71 W, corresponding to pulse energy of 2.04 μJ, with a slope efficiency of 52.2%. The amplified pulses have been compressed to a duration time of 241 fs, using a folded Treacy grating setup. The pulse energy is measured to be 1.02 μJ, corresponding to a peak power of ~3 MW. To the best of our knowledge, this is the highest average power and pulse energy generated from an all-fiber, Raman shifted soliton seeded thulium-doped chirped pulse amplifier system.
© 2016 Optical Society of America
Pulsed 2 μm fiber laser sources have attracted intense interests in recent years  for its wide applications in remote sensing , medicine , plastic engineering , mid-IR frequency comb , nonlinear mid-IR supercontinuum generation [6,7], optical parametric generation  and so on. To produce picosecond or femtosecond pulses at 2 μm wavelength regime, mode-locking and nonlinear frequency conversion are two major approaches. Thulium fiber based mode-locked oscillators are usually operating at the wavelengths below 2 μm, which may be affected by water absorption around 1.9 μm [9,10]. Because the material dispersion values of silica glass around 2 μm is positive and relatively high, to generate pulses with pulse durations from picoseconds to hundreds of femtoseconds and repetition rates of MHz, normally dispersion-managed mode-locked thulium-doped fiber lasers are employed [11–13]. Using erbium-doped femtosecond fiber lasers to generate Raman shifted solitons have been proved to be a powerful and effective way to obtain ultrafast pulses at 2 μm regime, in comparison with the alternative 2 μm mode-locked oscillators. This is because the well-developed 1.5 μm fiber-based components are mature, stable and costless. The center wavelength of the generated Raman solitons beyond 1.5μm can be shifted by controlling the pumping power level [5,14–16,18–21]. This can be achieved through either the soliton self-frequency shift (SSFS) [14,15] or spectral broadening in highly nonlinear fibers [5,16,18–21]. So far, the shortest pulse width of 27 fs has been achieved using this approach, with three additional compression stages at 2 μm .
However, there are still some drawbacks for this kind of sources. For example, extra noises can be introduced from the nonlinear frequency generation processes , and the configuration of the laser source can be complex compared with thulium doped mode-locking oscillators. Moreover, the Raman-shifted source usually delivers pulse energy of pJ- to nJ-level, which is an obvious barrier to extend this sort of sources for some applications requiring higher pulse energy.
For further power scaling, nonlinear amplification and chirped pulse amplification (CPA) are the two common ways to generate 2 μm high-energy pulses with sub-ps pulse duration.
To date, the highest reported pulse energy is 31 nJ using the nonlinear amplification approach, seeded by the Raman shifted soliton from an ultrafast erbium-doped fiber laser . In that work, the shifted Raman soliton pulses were positively pre-chirped by a segment of normal dispersion fiber (NDF), before being amplified in a thulium-doped large mode area (LMA) fiber. Pulses with a width of 108 fs were obtained during the nonlinear amplification with center wavelength of 1980 nm. However, there is a trade-off between the fiber nonlinearity and the dispersion during the nonlinear amplification. This limits the achievable pulse energy since excessively high nonlinearity will cause pulse breakup.
On the other hand, pulse energy of 470 μJ with peak power up to 2 GW has been realized in thulium-doped chirped pulse amplification system seeded by 2 μm thulium-doped fiber oscillator. The generation of the pulses with such high energy is benefited from the usage of a large-pitch thulium-doped fiber, as well as complex free-space stretchers and high efficiency grating compressors .
Insteadly, R. A. Sims et al reported chirped pulse amplification seeded by Raman soliton pulses from a 1.9 μm mode-locked thulium doped fiber laser . The shifted Raman solitons were positively stretched by a chirped Bragg grating (CBG). The pulse repetition rate is then reduced down to 100 kHz by an electro-optic modulator. After the amplification of a thulium-doped LMA fiber and the compression of grating pairs, pulses with energy of 1 μJ and pulse width below 500 fs were finally obtained. The CBG in the experiment was also used as spectral filter to remove the unwanted spectral portion lying in the thulium gain bandwidth. However, the system is not an all-fiber device, due to the insertion of the free-space CBG stretcher. This makes the system complex and inconvenient.
In this work, we demonstrate a compact all-fiber thulium-doped chirped pulse amplification system seeded by a Raman-shifted erbium-doped fiber laser. The average output power of the amplified pulses was measured to be 71 W with a slope efficiency of 52.2%. The repetition rate has been measured to be 34.8 MHz. The pulses are then compressed to a duration time of 241 fs with 35.4 W average output power corresponding to a peak power of ~3 MW.
2. Experimental setup
The schematic setup of Tm-doped fiber CPA system is shown in Fig. 1. It consists of an all fiber Raman soliton generator, a fiber stretcher, two stages of fiber amplifiers and a free-space grating compressor. The Raman soliton generator is based on an amplified mode-locked erbium-doped fiber (EDF) laser and a short piece of highly nonlinear fiber (HNLF). The mode-locked EDF laser consists of a wavelength division multiplexer (WDM), a 20% output coupler, a fiber optical circulator coupled with a commercial available SESAM (semiconductor saturable absorber mirror) mode locker and a piece of EDF with the dispersion of −12 ps/nm/km at 1550 nm. The total cavity length is about 5.75 m, which corresponds to 34.8 MHz fundamental repetition rate. A fiber polarization controller (PC) is placed before the amplification stage to optimize the compression pulse quality. The amplifier consists of ~4 m EDF with the same parameters as the oscillator gain fiber. After amplification, the pulses are compressed with 110 cm single mode fiber (SMF-28). A short segment of HNLF was spliced directly to the SMF-28 fiber to generate the supercontinuum. The HNLF has a mode field diameter of 2.23 μm, a dispersion of 2.187 ps/nm/km at 1550 nm, and a nonlinearity γ of ~9 W−1km−1 respectively. An additional 16 cm-long Tm:Ho-doped fiber (TH512, Coractive Inc.) is used to further enhance the signal intensity at 2010 nm. Then the enhanced Raman soliton pulses are temporally stretched using ~110 m ultrahigh numerical aperture fiber (UHNA-4, Nufern Inc.) with a dispersion of ~-48.5 ps/nm/km at 1.9 μm and amplified in two-stage thulium-doped fiber amplifier. The pre-amplifier consists of a 3.5 m-long double clad single mode thulium-doped fiber (TDF). The TDF is with a core diameter of 10 μm, an outer diameter of 130 μm (Nufern Inc.), and an NA of 0.46 for the second cladding with cladding absorption of ~3 dB/m at 793 nm. A commercial pump combiner is used to deliver pump light to the gain fiber from the multimode pump diode, which has a center wavelength of 793 nm and output power of 12 W with 0.22 NA, 105/125 μm fiber pigtail. In the main amplifier stage, 1.9 m LMA thulium doped fiber (Nufern Inc.) is used to boost the signal power to tens of watts. The active fiber has a core diameter of 25 μm, an NA of 0.09, an inner cladding diameter of 250 μm and a cladding NA of 0.46 with cladding absorption of ~9.5 dB/ m at 793 nm. The active fiber is water-cooled down to 12 °C to dissipate the heat accumulation as well as promoting efficient two-for-one cross-relaxation during high power operation. The pump source consists of 5 diodes operating at 793 nm with 105 μm (NA = 0.22) pigtail fibers. The total output power of these pump diodes is ~148 W. A (6 + 1) × 1 high power pump combiner (ITF Technologies Inc.) is used to deliver pump light to the gain fiber with a coupling efficiency of ~95%. The output end of the LMA thulium-doped fiber was angle cleaved to frustrate parasitic lasing. After amplification a dichroic mirror (DM) is used to filter the unabsorbed pump light and the pulses are compressed with a folded Treacy grating compressor consisting of two 560 grooves per mm, fused silica transmission gratings. A fused silica wedge is placed after the compression output to measure the optical and radio-frequency (RF) spectra as well as pulse characteristics.
3. Experimental results
The oscillator is a passively SESAM mode-locked erbium-doped fiber laser operating at stretched pulse mode-locking regime. The net dispersion of the cavity is estimated to be −0.048 ps2. The oscillator delivers an average output power of 1.5 mW with a pulse width of 1.1 ps and a repetition rate of 34.8 MHz. The output spectrum is shown in Fig. 2 (as the black line) with a center wavelength of 1559 nm and a spectrum bandwidth of 12 nm, indicating that the pulses are negatively chirped. The output pulses are amplified to about 68 mW by the erbium-doped fiber amplifier (EDFA). The combination of the distributed gain, the normal dispersion of the EDFA as well as self-phase modulation (SPM) effect leads to self-similar amplification with appropriate input pulse energy, in which the pulses have been evolved to parabolic shape with linear chirp . After amplification, the width of the spectrum expands to over 80 nm with nearly parabolic shape (see the red line in Fig. 2) and the amplified pulse width is measured to be 1.15 ps. In the experiment, the SMF-28 with anomalous dispersions is used to compress the chirped pulses. By carefully tailoring the SMF-28 fiber length with cutback method, the pulse compression is maximized when the SMF-28 length is 110 cm. The pulse duration after the compression fiber is measured to be ~70 fs and the bandwidth of the compressed pulse spectrum (as the blue line shown in Fig. 2) is ~84 nm.
An optimized short (13 cm) segment of HNLF is spliced with the SMF-28 to generate Raman shifted solitons through nonlinear spectral broadening process. The generated supercontinuum is recorded by an optical spectral analyzer (YOKOGAWA AQ6375). A broad and smooth supercontinuum spectrum ranging from 1.2 to 2.1 μm is generated after the HNLF under suitable pump power, (see the pink line in Fig. 3). The spectral broadening is dominated by soliton fission and soliton self-frequency shifting, as we can see the separated solitons at longer wavelength. The spectrum peak near 1950 nm is caused by the unabsorbed 976 nm pump signal. The supercontinuum can be further extended to beyond 2.3 μm with the highest pump power in the experiment (see the orange line in Fig. 3). The pump power has been optimized to generate Raman shifted soliton at the center wavelength of 2 μm, which is away from the water vapor absorption in the air near the 1.9 μm regime. This is important because the final stage of pulse compression is in the free space. The generated Raman soliton is centered at ~2010 nm with a full width half maximum (FWHM) bandwidth of ~93 nm. As we can see from the output spectrum, most of the spectral power remains near the wavelength between 1.5 and 1.7 μm. In order to fully utilize such residual spectral power and enhance the signal intensity at 2010 nm, the supercontinuum output is then coupled into a 16 cm Tm:Ho-doped fiber (TH512, Coractive Inc.) to achieve self-amplification process . The enhanced Raman soliton supercontinuum is shown in Fig. 3 (see the blue line).
Before the supercontinuum entered in the thulium doped fiber pre-amplifier, the Raman shifted pulses are temporally stretched to ~320 ps to avoid the nonlinearities in the amplifier introduced by high peak intensities inside the gain fiber. The output spectral power from 1900 nm to 2100 nm is estimated to be ~5 mW after the fiber stretcher. The stretched pulses are amplified in the first stage thulium-doped fiber amplifier. An output power of ~850 mW has been achieved after the isolator, with a pump power of ~7.6 W corresponding to a gain of 22 dB. Compared with the Raman soliton seed source, the spectrum bandwidth of the amplified pulses was decreased to 42 nm due to the gain narrowing after the amplification process. The center wavelength of the amplified pulses shows blue-shifted to 1984 nm because of the thulium gain location. The pulse width of the amplified Raman solitons decreases to ~180 ps with the narrowing of the amplified spectrum width. Due to the long pulse duration in the gain fiber, no significant spectral distortion was observed at this stage.
The pre-amplified pulse train is further injected into the second amplifier for power scaling. Spectral modulation has been observed after the pre-amplified pulses passing through the LMA Tm-doped fiber, which is shown in Fig. 3 (see the olive line). This is probably due to the multimode interference (MMI) in the LMA gain fiber . Benefiting from the high absorption of the gain fiber, short fiber length can be expected to obtain high optical-to-optical power conversion efficiency and minimize nonlinear effects such as stimulated Brillouin scattering and stimulated Raman scattering. Figure 4 shows the average output power of the thulium-doped fiber power amplifier versus incident pump power. The maximum average power and the corresponding pulse energy of the output are measured to be 71 W (i.e., a gain of 18.9 dB) and 2.04 μJ respectively, when the incident pump power is 141 W. The average power increases linearly with the incident pump power with a slope efficiency of ~52.2%. No amplified spontaneous emission has been observed during the power amplification.
At the final stage, the amplified pulses are compressed by a folded Treacy compressor. The compressor has a slope efficiency of 49.8% (see Fig. 4), which is mainly limited by the diffraction efficiency of the gratings. The output power of the compressed pulses is measured to be 35.4 W, corresponding to pulse energy of 1.02 μJ, at the maximum pump power of 141 W. The optical spectrum of compressed pulses is shown in Fig. 5(a), with a FWHM of 28.4 nm at the center wavelength of 1970 nm. The output spectrum is not smooth with some modulation, we attribute this to the MMI in the LMA Tm-doped fiber and nonlinearities generated at high pulse energies.
Figure 5(b) illustrates the autocorrelation trace of the compressed pulses measured by a commercial second harmonic generation (SHG) autocorrelator (Femtochrome, FR-103XL) at the highest output power. The pulse duration after compression is 241 fs given a sech2 pulse intensity profile. The time bandwidth product (TBP) is calculated to be 0.53. The pedestal is seen in the autocorrelation trace. The formation of the pedestal can be attributed to the nonlinear phase shifts generated in the Raman shift processing, the uncompensated high-ordered dispersion of the stretcher fiber and the nonlinearities accumulated in the LMA fiber amplifier under high power operation. According to the area integral in Fig. 5(b), the pulse contains ~70% of the energy and the peak power is calculated to be ~3 MW. The average output power has been continuously measured for 5 minutes and the variation is less than 1%, indicating excellent power stability of the compressed pulses.
The output beam quality was measured using a scanning slit beam profilers system (BeamScope-P8). A near-field intensity profile of the laser beam is shown in Fig. 6. The beam quality factor M2 was measured to be 1.214 at average output power of 70 W.
Figure 7 shows the RF spectrum (after compression) measured by a spectrum analyzer (Agilent N9030A) and a 12.5 GHz InGaAs photodetector (Newport Inc.) at a span range of 1 MHz with the resolution bandwidth of 300 Hz. High signal-to-noise ratio of ~67 dB is observed without visible sidebands, indicating low amplitude fluctuations between pulses.
In conclusion, we have reported a compact thulium-doped chirped pulse fiber amplifier system, seeded by Raman shifted erbium-doped fiber laser. Output power of 71 W has been achieved from the LMA fiber amplifier with a slope efficiency of 52.2%. The chirped pulses are finally compressed to the duration time of 241 fs with the average power of 35.4 W, corresponding to ~3 MW peak power. The generated smooth and ultra-broad band spectrum of Raman shifted solitons indicates that it is a suitable seed source for multi-stage amplifiers.
National Natural Science Foundation of China (NSFC) (61527822, 61235010).
The authors acknowledge funding support from the National Natural Science Foundation of China.
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