Chih-Hsuan Lin

Supercontinuum generation by noise-like pulses transmitted through normally dispersive standard single-mode fibers

We report generation of broadband supercontinuum (SC) by noise-like pulses (NLPs) with a central wavelength of 1070 nm propagating through a long piece of standard single-mode fibers (~100 meters) in normal dispersion region far from the zero-dispersion point. Theoretical simulations indicate that the physical mechanism of SC generation is due to nonlinear effects in fibers. The cascaded Raman scattering is responsible for significant spectral broadening in the longer wavelength regions whereas the Kerr effect results in smoothing of SC generated spectrum. The SC exhibits low threshold (43 nJ) and a flat spectrum over 1050-1250 nm.

A controllable noise-like operation regime in a Yb-doped dispersion-mapped fiber ring laser

We report the generation of tunable high-energy noise-like pulses with a super-broadband spectrum from a Yb-doped dispersion-mapped fiber ring laser. Self-starting noise-like operation can be maintained over a relatively large range of pumping powers (4–13 W). The corresponding output power varies from 0.1 to 1.45 W. The maximum 3 dB spectral bandwidth of the noise-like pulses is about 48.2 nm while the output energy is as high as 47 nJ, limited by optical damage of the components. The central wavelength of the noise-like pulses can be tuned easily over ~12 nm. The bandwidth and duration of the generated wave packets can also be controlled. The use of a negative dispersion-delay line and spectral filter are found to be important for generating such high-power noise-like operation. Experimental results are in good agreement with theoretical simulations.

Robust diode-end-pumped Nd:GdVO4 laser passively mode-locked with saturable output coupler

Passively mode-locked diode-pumped solid-state (DPSS) Nd:GdVO4 laser by using a saturable output coupler (SOC) in stable two-mirror linear cavity design is demonstrated. The laser was pumped by low-power single laser diode without any active cooling and external optics. Robust CW mode-locked operation was obtained with a repetition of 150–250 MHz, the pulsewidth was 12 ps, the lowest threshold for CW mode-locked was 185 mW. The optimal condition is found when the laser provides the set of characteristics: low CW mode-locking threshold (∼260 mW), compact size (total cavity length 60 cm), good efficiency (∼20%), and very stable output. At the output power of 270 mW an average power fluctuations within ±2% for long-term operation was obtained.

Femtosecond laser ablation of polymethylmethacrylate via dual-color synthesized waveform

We have demonstrated the laser ablation of PMMA using dual-color waveform synthesis of the fundamental (ω) and its second-harmonic (2ω) of a femtosecond Ti:Sapphire laser. A modest and yet clear modulation (∼22%) in ablated area versus relative phase between the 2ω and ω beams with a power-ratio of 15% (28/183 mW) is revealed. This is explained qualitatively by the dependence of ablation on multiphoton ionization of which the rate is related to the relative phase of the synthesized waveform. At higher peak power ratios, the modulation decreases rapidly, as the two-photon-ionization rate of the 2ω dominates over that of the three- to four- photon ionization of the ω beam. This technique demonstrates the feasibility of phase-controlled laser processing of materials.

Optimal design of a high-power picosecond laser system using a dual-stage ytterbium-doped fibre amplifier

An average power as high as 60 W with 73 W of pumping was achieved for an ytterbium-doped fibre-based dual-stage amplifier (MOFA) system seeded by a diode-pumped solid-state (DPSS) laser. The corresponding optical conversion efficiency is 80%. The laser system generates a steady pulse train with a pulse width of 11 ps at a repetition rate of 250 MHz or a peak power of 21.8 kW. Moreover, the output beam quality M2 ≈ 1.6. The length and pumping power for the Yb-doped fibres were optimized to suppress stimulated Raman scattering (SRS) and amplified spontaneous emission (ASE) while maintaining desirable output characteristics.

Effective pulse recompression after nonlinear spectral broadening in picosecond Yb-doped fiber amplifier

We report the performance of a picosecond master-oscillator power amplifier (MOPA) system based on a diode-pumped solid-state (DPSS) seed laser and Yb-doped fiber amplifier. An average power of 28 W at ∼200 MHz repetition rate is achieved by using only one amplification stage. We found that positive nonlinear phase shift induced by nonlinear effect in the active fiber can be effectively compensated by a grating pair. A pulse duration of ∼1.6 ps is shown after recompression.

Laser Ablation of Polymethylmethacrylate (PMMA) by Phase- Controlled Femtosecond Two-Color Synthesized Waveforms

Single-shot laser ablation of polymethylmethacrylate (PMMA) was studied using dualcolor waveform synthesis of the fundamental (ω) and its second harmonic (2ω) of a femtosecond Ti: Sapphire laser. Changing the relative phase of the fundamental (ω) and second-harmonic (2ω) outputs of the exciting laser resulted in clear modulation of the ablated area. The modulation as well as the dependence of the ablation threshold on the relative phase between the ω and 2ω beams correlated closely with the theoretical model of laser breakdown (ablation) of transparent materials through photoionization in the intermediate regime (Keldysh parameter γ ≈ 1.5). Our study illustrates the potential applications of using phase-controlled synthesized waveform for laser processing of materials.

Fiber-laser-generated Noise-like Pulses and Their Applications

We describe generation and amplification of mediumand high-energy noise-like pulses (NLPs) using Yb-doped optical fibers. We also demonstrate supercontinuum (SC) genera‐ tion techniques in which NLPs serve as the pump. SC pumped by NLPs has been em‐ ployed successfully in optical coherence tomography systems.

Progress in Short-Pulse Yb-Doped Fiber Oscillators and Amplifiers

In this chapter, we first considered practical aspects in design and construction of relatively high-average-power picosecond Yb-doped fiber laser systems. Employing a highly stable diode-pumped solid-state laser as the seed source together with proper design of the fiber amplifiers, we were able to achieve an average output power of ~60 W with 73 W pumping using just 2 amplifier stages based on regular non-PM Yb-doped fiber. Applying modulation technique to generate pulse bursts at 700 kHz allowed us to optimize dynamically saturated amplifier and extract higher energies from the MOFA. This was used to improve the nonlinear conversion efficiency in the cases of second (16 % vs. 4 %) and fourth (8 % vs. 2 %) harmonic generation compared to regular pulse trains at ~250 MHz. We will also describe mode-locking techniques of fiber-based oscillators based on ring type cavities with NPE port. Our simulation results based on coupled nonlinear Schrodinger equations showed the possibility to generate either regular mode-locked pulse trains or noise-like pulses in such oscillators. Novel scheme of supercontinuum generation by noise-like pulses in normally dispersive single-mode fibers was demonstrated. The SC exhibits low threshold (43 nJ) and flat spectrum over the wavelength range of 1,050–1,250 nm.

Experimental and Theoretical Study of the Generation and Non-Linear Conversion of Picosecond Bursts from an Amplified Ytterbium-Doped Fiber Laser System

Abstract This article presents an experimental and theoretical study of the generation of picosecond bursts by a non-polarization-maintaining ytterbium-doped fiber master oscillator fiber amplifier system. The peak power and pulse energy of the burst are higher than 45 kW and 350 nJ at 700 kHz, respectively. The master oscillator fiber amplifier was used to generate 3 W of green and 200 mW of UV light with conversion efficiencies of 16% and 8%, respectively. The enhancement of conversion efficiency by the pulse burst compared with regular pulses was analyzed and attributed to dynamically saturated gain of the pump-power-limited fiber amplifier.