The performance of mid-infrared fiber lasers operating on the 3.5 μm transition in erbium has improved significantly since the first demonstration that dual wavelength pumping allowed efficient operation. In this contribution, we will discuss the progress of fiber lasers that operate on this transition with an emphasis on advances towards short pulse generation and wavelength agility. Mode-locked operation using saturable absorption is a robust means of achieving ultra-short pulse operation in the near infrared but achieving this in the mid-infrared has been elusive. We will also describe our characterization of the mid-infrared performance of graphene, a material which has been very successfully applied to mode-locked pulse generation in the near infrared.
We report the first-of-its-kind compact and robust coherent source operating in mid-IR based on Fe:ZnSe chalcogenide gain medium optically pumped by Er:ZBLAN fiber laser. In the research, we study the CW operation of cryogenically cooled laser based on Fe:ZnSe single crystals with different doping level grown from the vapor phase on a single-crystal seed by using the concurrent-doping technology. The maximal output power achieved is 2.1 W with 59% slope efficiency with respect to absorbed pump power, which is close to the Stokes shift limit. Measured Fe:ZnSe output spectra indicate a significant influence of re-absorption on generation wavelength. For high doping levels and output powers, spectrum shifts to the red wing, which makes possible continuous tuning from 4012 to 4198 nm. As well, tunability of the laser in a wide range of temperature is investigated.
A 11.5 J at 40 ns output has been obtained from a diode-pumped cryo-cooled Yb:YAG ceramics active-mirror laser amplifier system. The system consists of two amplifier heads which has four Yb:YAG ceramic disks and two pump LD modules. The Yb:YAG ceramics are cooled by conventional cryostat from rear side and are pumped by LD modules from front-side. A pump pulse is delivered to Yb:YAG ceramics coaxially with a seed pulse to reduce damage risk at a dielectric coating of Yb:YAG ceramics due to simplified coating design. To realize this system design, a LD module has been developed to keep a rectangle pattern with side length of around 37 mm among imaging depth of about 10cm at working distance of about 410 mm. As an experimental result of two pass amplification, a 11.5 J pulse energy was obtained with input energy of 1.0 J and total pump energy of 90.2 J. Then, an optical-to-optical conversion efficiency was 11.6% and an extraction efficiency was estimated to be 42%. In our knowledge, this is the highest output energy with nano second pulse duration in cryo-cooled Yb:YAG active-mirror laser amplification scheme. A repetition rate of 0.05 Hz depends on a limitation of a repetition rate of the seed pulse. A dependence of small-signal-gain on pumping repetition rate of the active-mirror laser head was experimentally evaluated. From the experimental result, we have estimated a feasible repetition rate of over 5 Hz. A 10 Hz operation will be demonstrated to reduce a thermal resistance between Yb:YAG ceramics and cryostat. Finally, this laser amplifier system is installed to a 100-J class laser system as preamplifier.
We report the femtosecond laser inscription of fiber Bragg gratings (FBGs) in an Er-doped fluoride glass fiber used for lasing at a mid-infrared wavelength of 2.8 μm. FBG reflectivity and laser output power are observed with varying the index change of grating plane. We have tried to create high-index-contrast grating planes worked as Bragg reflector. The index change was estimated by fitting experimentally obtained reflectivity to its calculation. When using laser fluences of 25 and 40 J/cm<sup>2</sup>, the index change was found to be 0.7×10<sup>-3</sup> and 1.1×10<sup>-3</sup>, respectively. When using laser fluence of 25 J/cm<sup>2</sup>, FBG reflectivity increases up to 95% at the grating length of 4.0 mm. The case of using 40 J/cm<sup>2</sup> shows 97% at the grating length of 2.5 mm. These results are in agreement with the reflectivity calculation. The investigation of lasing evolution will contribute to more efficient fabrications of FBG and fiber laser system.
Short pulse operation of fiber lasers operating at wavelengths up 3 micron have been reported in recent years. At longer wavelengths, fiber lasers have only been demonstrated with a continuous operation mode. Short pulse operation in the mid-IR is necessary for utilizing such lasers in laser radars and for medical applications. Our previous numerical work suggested that Q-switching is possible on the 3.5 μm transition in erbium-doped ZBLAN in a similar manner to work demonstrated on the 2.8 μm transition in erbium. In this work we report on initial experimental results of a Q-switched, dualwavelength pumped fiber laser operating on the 3.5 μm transition in erbium-doped ZBLAN glass fibers. Using a hybrid fiber and open resonator configuration utilizing an acousto-optic modulator we demonstrated stable single pulse Q-switching while operating at repetition rates of 20 kHz and up to 120 kHz. The laser achieved a peak power of 8 W with pulse energy of 7 μJ while operating at 25 kHz. Long pulse widths on the order of 1 μs were obtained. The low peak power and long pulses are likely the result of both low gain of the transition and additional losses in the resonator which are currently being investigated. Our latest results will be presented.
LFEX is the world’s largest high-energy petawatt laser. So far it delivers 3 kJ/1 ps and is planed to finally deliver 10 kJ/10-20 ps. It has been constructed and became partially operational since 2008, and with full beams since 2014. LFEX is synchronized to nsec Gekko-XII laser for variety of experiments with nsec and psec simultaneous laser beams irradiating the targets for fast ignition and other high-energy density physics.
Quantum-defect-limited operation in a diode-pumped Yb:YAG oscillator have been demonstrated at low temperature. The highest slope efficiency of 90% was obtained with M<sup>2</sup>=20 at the crystal temperature below 70 K, which was close to the theoretical stokes efficiency of 91.2% (λ<sub>pump</sub>/λ<sub>laser</sub>=941nm/1030nm). An optical-to-optical efficiency and a laser gain were 74% and 8 cm<sup>-1</sup>, respectively, at a low pump intensity of 1.3 kW/cm<sup>2</sup>. After optimizing a spatial mode coupling between a diode pump laser and a TEM00 cavity mode on the crystal, 80% slope efficiency and 70% optical efficiency were still high at M<sup>2</sup>=1~1.5.