Due to the unique ultra-short pulse duration and high peak power, femtosecond (fs) laser has emerged as a powerful tool for many applications but has rarely been studied for 3D printing. In this paper, welding of both bulk and powder materials is demonstrated for the first time by using high energy and high repetition rate fs fiber lasers. It opens up new scenarios and opportunities for 3D printing with the following advantages - greater range of materials especially with high melting temperature, greater-than-ever level of precision (sub-micron) and less heat-affected-zone (HAZ). Mechanical properties (strength and hardness) and micro-structures (grain size) of the fabricated parts are investigated. For dissimilar materials bulk welding, good welding quality with over 210 MPa tensile strength is obtained. Also full melting of the micron-sized refractory powders with high melting temperature (above 3000 degree C) is achieved for the first time. 3D parts with shapes like ring and cube are fabricated. Not only does this study explore the feasibility of melting dissimilar and high melting temperature materials using fs lasers, but it also lays out a solid foundation for 3D printing of complex structure with designed compositions, microstructures and properties. This can greatly benefit the applications in automobile, aerospace and biomedical industries, by producing parts like nozzles, engines and miniaturized biomedical devices.
This paper presents the newest development of high energy, high power ultrafast fiber lasers in 1 μm, 2 μm and 3 μm regimes at PolarOnyx Inc. For the 1 μm mJ fiber laser, a Yb-doped PCF optical amplifier was built to boost pulse energy to 1.1 mJ at 100 kHz, with pulse duration of 710 fs. In the kW experiment, up to 1050 W average power was obtained with a repetition rate of 80 MHz and pulse duration of 850 fs. In the Tm-doped fiber laser experiment, an average power of up to 76 W was achieved at wavelength of 2012 nm with a repetition rate of 31 MHz and pulse duration of 870 fs. In the Er:ZBLAN fiber laser experiment, an average power of up to 142 mW was achieved at wavelength of 2784 nm with a repetition rate of 16.4 MHz and pulse duration of 5 ps. This work lays out a foundation for further energy and power scaling of ultrafast fiber lasers.
In the paper, a Yb:YAG single crystal fiber is used for the first time to amplify week image signal. It was longitudinally pumped by a fiber-coupled laser diode with a maximum power of 150W at 940 nm. The image amplifier provided low noise and high gain amplification. A spatially uniform amplification gain of up to 10.2 dB at wavelength of 1030 nm was obtained.
We describe in detail a high energy, high power ultrafast thulium-doped fiber laser system. The pulse energy of 156 μJ was realized. The laser system is comprised of a mode-locked 2020-nm seed oscillator and multiple-stage power/energy amplifiers. The seed oscillator output pulses at a repetition rate of 2.5 MHz. The seed pulses were stretched with the anomalous dispersion fiber to the duration of 320 ps. An acousto-optic modulator was used as a pulse picker to lower the repetition rate. A two-stage preamplifier was used to boost the pulse energy to 3 μJ. The pulse energies of up to 156 μJ and the average power of 15.6 W were obtained from the final stage of power amplifier at a repetition rate of 100 kHz with a slope efficiency of 26%. The pulse durations of 780 fs were obtained after pulse compression. High optical signal-to-noise ratio (OSNR) and low background noise were also achieved at this low repetition rate.
A high energy, high power ultrafast Tm doped fiber laser system was successfully developed. Pulse energy of 156 μJ and average power of 15.6 W were achieved. The laser system consisted of a mode-locked 2020 nm seed oscillator and multiple-stage power amplifiers. The seed included 30 m-long dispersion compensating fiber and emitted slightly chirped pulses with spectrum bandwidth of 8 nm. The mode-locking was stable and self-started. Repetition rate of seed oscillator was 2.5 MHz. The seed pulses were stretched with anomalous dispersion fiber to the duration of 320 ps. An AOM was used as a pulse picker to lower the repetition rate. A two-stage pre-amplifier was used to boost the pulse energy to 3 μJ. Pulse energies of up to 156 μJ were obtained from the final stage of power amplifier at a repetition rate of 100 kHz. Pulse durations of 780 fs were obtained after pulse compression. High OSNR and low background noise were also achieved at this low repetition rate.
In the paper, a 2 μm high energy fs fiber laser is presented based on Tm doped fiber at a low repetition
rate. The seed laser was designed to generate pulse train at 2 μm at a pulse repetition rate of 2.5 MHz.
The low repetition rate seed oscillator eliminated extra devices such as AO pulse picker. Two-stage
fiber amplifier was used to boost pulse energy to 0.65 μJ with chirped pulse amplification.
Bulk Erbium-doped lasers are widely used for long-distance telemetry and ranging. In some applications such as
coherent Doppler radars, laser outputs with a relatively long pulse width, good beam profile and pulse shape are
required. High energy Q-switched Er:glass lasers were demonstrated by use of electro-optic (E/O) Q-switching or
frustrated total internal reflection (FTIR) Q-switching. However, the output pulse durations in these lasers were fixed to
relatively small values and extremely hard to tune. We report here on developing a novel and compact Q-switched Er:Yb
co-doped phosphate glass laser at an eye-safe wavelength of 1.5 μm. A rotating mirror was used as a Q-switch. Co-linear
pump scheme was used to maintain a good output beam profile. Near-perfect Gaussian temporal shape was obtained in
our experiment. By changing motor rotation speed, pulse duration was tunable and up to 240 ns was achieved. In our
preliminary experiment, output pulse energies of 44 mJ and 4.5 mJ were obtained in free-running and Q-switched
operation modes respectively.
An ultra-low repetition rate high energy 200 ns Er:Yb co-doped fiber laser has been developed by using a master
oscillator power amplifier configuration at an eye-safe 1.53 μm wavelength . A modulated pump scheme was used to
suppress ASE accumulation between pulse intervals. Combined with pulse shaping technology to mitigate pulse
narrowing effect and SBS effect, a maximum of 480 μJ pulse energy was obtained. In the stable, long-term running
mode, pulse energy up to 204 μJ, were obtained with pulse durations of 200 ns at Hz level.
A novel and compact hybrid resonant amplifier has been demonstrated for further scaling energy/power level from 1.55
μm fiber lasers by using Er3+/Yb3+ co-doped phosphate glass. The seed laser is a pulse shaping fiber laser at an eye safe
wavelength of 1.55 μm. The wavelength was temperature controllable and was stabilized at one of amplifier's resonance
wavelengths. Pulse shaping technology provides a vital solution in generating different input wave formats, for both CW
and pulsed outputs. For Pulsed amplification, the pulse duration can be varied from microsecond to nanosecond and
repetition rate from a few Hz to 250 kHz. Gain as high as 20 dB was obtained for nanosecond pulses at 10 Hz repetition
rate, comparing with a single pass gain of only 0.64 dB. High OSNR, high extinction ratio and low background noise
were also achieved at this low repetition rate by our new amplification method. In our CW input experiment, an optical
conversion efficiency of up to 20% was obtained.
This new optical amplifier is very compact. The size of the amplifier is less than 5 mm. It has a great potential for broad
Fiber laser is becoming an enabling technology for coherent Lidar applications and free space communications due to its
high efficiency, compact size and reliable operation. In these applications, high extinction rate and high OSNR are
musts for a fiber laser. However, current Q-switched fiber lasers and solid state lasers cannot meet these criteria.
Innovative approach has to be conceived to meet the requirements.
In this paper, we will discuss our researches on high energy/power ns pulsed fiber lasers. Modulation schemes for seed
laser in getting various optical waveforms to accommodate pulse narrowing in high power amplification and reduce SBS,
high power operation of fiber amplifiers, nonlinearity mitigation in high power fiber lasers, and trade-offs among the
parameters such as OSNR, power/energy scaling, extinction ratio, interpulses background noise (contrast ratio), and
efficiency will be discussed.