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In the paper, a 3 µm mid-infrared (MIR) high energy ultrafast Er:ZBLAN fiber laser and amplification system is presented. The 3µm seed pulses were generated through an Yb-doped ultrafast fiber laser pumped optical parametric amplification (OPA). A pulse energy of 12.4 µJ is generated at a repetition rate of 100 kHz with a pulse duration of 103 fs. A Er:ZBLAN fiber amplifier was used to further boost the chirped pulse energy to 84 µJ.
© 2015 Optical Society of America
1. Introduction
Er-doped ZBLAN fiber lasers, which can emit mid-IR (MIR) light at 2.65-2.9 μm through the transition from the upper energy level 4I11 ∕2 to the lower laser level 4I13 ∕2, have attracted much attention recently [1–8 ]. Some significant progresses on high power, high energy Er-doped ZBLAN fiber lasers have been achieved. In [5], an average power of 20.6 W continuous-wave (CW) output at 2.8 μm has been demonstrated with passive cooling and a slope efficiency of up to 35.4% with respect to the absorbed pump power. In [6], a CW power of 24W was obtained from Er-doped ZBLAN fiber laser by applying an efficient cooling with a combination of fluid cooling over the entire length of the fiber and conductive cooling at both end-faces of the fiber. In [7], pulse energy up to 100 μJ and pulse duration down to 90 ns has been realized in an actively Q-switched 2.8 μm fiber laser oscillator with an average output power of more than 12W. In [8], a passively mode-locked Er-doped ZBLAN fiber laser has been demonstrated in which a Fe2+:ZnSe crystal served as the saturable absorber. However, the spectrum bandwidth for the mode-locked pulses was only 0.6 nm with a pulse energy of 0.93 nJ. To obtain mode-locked seed pulses with broadband spectrum and to amplify broadband pulses at 3 µm regime are still big challenges to researchers.
In this paper, we present a laser system for generating 3 µm ultrafast pulse train and amplifying it to energy of 84 µJ. Optical parametric chirped pulse amplification (OPCPA), first demonstrated by Dubietis et al. [9], has become a well-known and rapidly developing technique to obtain high energy ultrafast pulses [10]. Periodically poled lithium (PPLN) based OPCPA has been successfully demonstrated to amplify Mid-IR ultrafast pulses [11–14 ]. In our work, we were able to combine all the state-of-the-art fiber laser solutions. The MIR ultrafast pulse train was generated and amplified through Yb doped ultrafast fiber laser pumped optical parametric generation (OPG) and OPA using MgO-doped PPLN crystals. Pulse energy of up to 12.4 µJ was obtained with compressed pulse duration of 103 fs. This is the highest pulse energy obtained from a fiber laser pumped OPA. In the 20 ps pulse pumped OPCPA system, pulse energy of up to 37.3 µJ was obtained. The pulse train was further amplified in an Er:ZBLAN fiber amplifier to boost the pulse energy to 84 µJ with pulse duration of around 24 ps. In a separate experiment, pulse energy of 201 µJ was obtained from a Q-switched Er:ZBLAN fiber laser with pulse duration of 33.5 ns. Both are the highest pulse energies obtained from ps and ns Er:ZBLAN fiber laser systems respectively, to the best of our knowledge.
2. High energy 3 µm pulsed laser system
The schematic diagram of 3 µm high energy pulsed laser system is shown in Fig. 1 . It consisted of a 1035 nm high energy fs pump laser, a MgO:PPLN OPG to generate 3 µm pulse train, a MgO:PPLN OPA to boost pulsed 3µm idler wavelength and a final stage of Er-doped ZBLAN fiber amplifier. The pump laser is a 1 µm high energy mode-locked femtosecond fiber laser, which is commercially available (Uranus-mJ series, PolarOnyx Laser, Inc., San Jose, California). The laser generates pulses with energy up to 0.5 mJ and pulse duration around 1 ps at repetition rate of 100 kHz. The output spectrum has a central wavelength of 1035 nm with a full width at half maximum (FWHM) bandwidth of 5.6 nm. In this experiment, a chirped pump pulses was used in order to obtain high pulse energy at 3 µm from OPCPA and fiber amplifier system. The signal beam for OPCPA was generated by SC radiation from a piece of polarization maintaining (PM) single mode fiber. An uncoated wedged fused silica window was used to reflect a small portion of the pump laser beam for SC generation. The main portion of the 1 µm pump laser beam was going through a delay stage and then injected into the MgO:PPLN crystal and overlapped with the signal beam for an OPCPA process. Idler wavelength of 3 µm was generated and delivered into the final stage of high energy Er:ZBLAN fiber amplifier.
3. Fiber laser pumped OPG and OPA at 3µm
The nonlinear crystal used to generate 3 µm pulse train is a 3 mm-long MgO doped PPLN crystal. Because in the final stage amplifier, Er:ZBLAN fiber has a gain bandwidth from 2.65 µm to 2.9 µm, centered at 2.78 µm, a PPLN crystal grating period of 29.98 µm and operation temperature of 120 °C were selected to obtain phase-matching for the required idler wavelength. PPLN has a cross section of 1 mm by 1 mm. Up to 2.5 W 1030 nm pump power at 100 kHz repetition rate was delivered into the PPLN crystal. Pump beam was focused with a beam diameter of 150 µm. Output power of of 55 mW and 25 mW were obtained respectively from signal and idler wavelengths. The idler beam was passing through a dichroic mirror and injected into a 20 mm-long PPLN crystal for OPA. In the OPA stage, both pump beam and idler beam were focused with a beam diameter of 375 µm. The pump beam delay was carefully adjusted to make it overlapped with signal beam thus the output power was optimized. The amplified idler power as a function of pump power is shown in Fig. 2(a) . Up to 1.24 W idler power (12.4 µJ pulse energy) was obtained with 12.4 W pump power. The spectra for the output idler beam at maximum output power level from OPA are shown in Fig. 2(b). The idler beam had a central wavelength of 2786 nm and a FWHM spectrum bandwidth of 297 nm. The 3 µm pulse was compressed and pulse duration was measured by a home-made interferometric autocorrelator. A FWHM pulse duration as short as 103 fs was obtained (Fig. 3 ).
4. Optical parametric chirped pulse amplification (OPCPA)
In the above OPA experiment, short pump pulse was used for the simply configuration of amplifier. However, the output power was limited by the damaged threshold of high peak intensity and the poor signal-pump overlapping ratio due to different refractive indices, especially from a long crystal. In order to achieve higher pulse energy at 3 µm regime, an OPCPA system was tested. The OPCPA system was based on a 20 mm long MgO-doped PPLN crystal with a grating period of 29.98 µm. The pump power was continuously tuned by a combination of a half waveplate and a polarizer. Up to 19.6 W pump power at repetition rate of 100 kHz was delivered into the PPLN crystal. Both pump beam from high energy fiber laser and idler beam from OPG were stretched by a grating stretcher to ~20 ps and then focused into the PPLN crystal with beam diameters of 330 µm. The pump beam delay was carefully adjusted to make it overlapped with 3 µm beam thus the output power was optimized. The amplified signal power and idler power as a function of pump power are shown in Fig. 4 . Up to 3.77 W idler power (37.3 µJ pulse energy) and 6.3W signal power were obtained with 19.6 W pump power. The spectra for idler and signal beams at maximum output power level from OPCPA are shown in Fig. 5 . The amplified signal beam had a central wavelength of 1655 nm and a FWHM spectrum bandwidth of 38 nm [Fig. 5(a)]. The idler beam had a central wavelength of 2766 nm and a FWHM spectrum bandwidth of 91 nm [Fig. 5(b)]. The output pulse train of signal was measured with a 10 GHz InGaAs photodetector [Fig. 6(a) ]. The 100 kHz pulse train of idler output was measured by a fast InAs detector [Fig. 5(b)].
5. Er:ZBLAN fiber amplifier
Finally, the 3 µm pulse train from OPCPA was injected into the last stage of high energy Er:ZBLAN fiber amplifier to further boost the pulse energy. The setup of high energy amplifier is shown in Fig. 7(a) . The active medium is a 5 m-long Er-doped double clad ZBLAN fiber with a core diameter of 33 µm and a pump cladding diameter of 330 µm (OgMentum). The Er-ion concentration is 6 mol%. Both ends of the fiber were polished with an angle of around 10 degrees to avoid parasitic lasing. The 3 µm pulse train from OPA was focused by an antireflection (AR) coated CaF2 lens onto one end of the gain fiber. In order to achieve higher pulse energy, a germanium acousto-optic modulator (AOM) was used as a pulse picker to lower the pulse repetition rate. A diffraction efficiency of up to 90% at 2.8 µm was measured, including the transmission losses from AOM input and output surfaces. The optical rising and falling time of the AOM was 100 ns/mm beam size and the amplifier was adjusted to be operated when the RF power to the AOM was on. The reverse pumping scheme was used in this experiment. Up to 11W pump beam from a fiber coupled laser diode with central wavelength of 975 nm was delivered into the gain fiber. The pump beam was collimated first and then focused onto the end of the Er:ZBLAN fiber with a beam diameter of 280 µm. The pump beam was reflected by a dichroic mirror which had a high reflectivity at pump wavelength and high transmittance at signal wavelength near 3 µm regime. The amplified 3 µm wavelength passed through the dichroic mirror and then was collimated by another AR coated lens to output.
Fig. 7 Er:ZBLAN fiber experimental setup for (a) high energy amplifier and (b) Q-switched 3 µm fiber lasers.
To study the amplification capability of Er-doped ZBLAN fiber pulse amplifier, especially at low repetition rates, the Q-switched operation of 3 µm fiber laser was tested. As shown in Fig. 7(b), the amplifier was modified to a Q-switched laser cavity by adding two cavity mirrors. One end of the cavity was a gold mirror reflector. The other end was an output coupler mirror with a transmittance of 40%. When the AOM was running at 1 kHz repetition rate, the laser output nanosecond pulse with energy of up to 201µJ at pump power of 5W. The output pulse energy as a function of pump power is shown in Fig. 8(a) . A typical output pulse train captured by a fast InAs photodetector with the highest pulse energy of 201 µJ is shown in Fig. 8(b). The measured FWHM pulse duration was 33.5 ns. We believed that the pulse duration was mainly limited by the AOM rising time. Figure 8(c) shows a typical output spectrum. The central wavelength was 2784.5 nm and bandwidth was around 0.4 nm. The amplified spontaneous emission (ASE) background was also measured. When Q-switch was off all the time, the measured ASE power level was less than 10 mW, which was smaller than 5% of the averaged output power at 1 kHz Q-switched operation. In the experiment of Q-switched operation, the pump power was limited to 5W to protect the Er:ZBLAN fiber from possible thermal damage.
Fig. 8 Experiment result from Q-switched 3 µm Er:ZBLAN fiber laser. (a) Pulse energy as a function of pump power at 1 kHz; (b) Output pulse train with a pulse energy of 201 µJ; (c) A sample laser output spectrum.
In the Er:ZBLAN fiber amplifier experiment, the AOM was used as a pulse picker to reduce the pulse repetition rate to 10 kHz. The measured average power of 3 µm seeding pulses was 280 mJ right before the fiber end, corresponding to a pulse energy of 28 µJ. Up to 10.6 W 976 nm pump power was delivered into the gain fiber. The amplifier boosted the pulse energy to 84 µJ and boosted the average power to 840 mW. The output pulse energy as a function of pump power is shown in Fig. 9(a) . The output spectrum with maximum pulse energy obtained in this experiment is shown in Fig. 9(b). After amplification, the spectrum bandwidth was narrowed to 55 nm which was mainly due to the gain narrowing effect from Er:ZBLAN fiber. Er:ZBLAN fiber has a gain peak near 2.8 µm. Background noise was also estimated in this experiment. When there was no seeded pulses, ASE background of up to 35 mW was observed at the maximum pump power of 10.6W. Considering the ASE power can be further suppressed when the amplifier is seeded, the ASE was remained at a low level comparing with the amplified output power of 840 mW with the same pump power. The seed pulse duration was around 20 ps. As ZBLAN fiber has a GVD of 62 fs2/mm near 3 µm regime [15], after passing through a 5m-long ZBLAN fiber the pulse duration can be stretched for an extra 4 ps in the worst case. Thus, the amplified pulse duration was estimated to be less than 24 ps. To the best of our knowledge, the pulse energy of 84 µJ measured in this experiment is the highest pulse energy obtained from a picosecond chirped ultrafast Er:ZBLAN fiber laser/amplifier.
Fig. 9 Experiment results from 10 kHz Er:ZBLAN fiber amplifier. (a) Output pulse energy as a function of pump power; (b) A sample output spectrum.
5. Conclusions
In conclusion, we demonstrated the highest pulse energy (84 µJ) ultrafast 3 µm Er:ZBLAN fiber laser/amplifier system. The 103 fs seed laser pulse train was generated and amplified through MgO-doped PPLN OPG and OPA systems with pulse energy of 12.4 µJ and at a repetition rate of 100 kHz. To the best of our knowledge, this is the highest pulse energy obtained from a fiber laser pumped OPA system in the MIR spectrum range (around 3µm). An Er:ZBLAN fiber amplifier was used to further boost the pulse energy to 84 µJ with a broadband spectrum (55 nm). The output pulse duration was about 24 ps. A pulse energy of up to 201 µJ obtained in the experiment of Q-switch operation indicates the potential for further scaling of the energy at 3 μm. Further scaling of the pulse energy and reduction of the pulse width is ongoing in PolarOnyx.
Acknowledgment
This project is supported in part by DoE SBIR program DE-SC0009674.
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