Cascade-Gain-Switching for Generating 3.5-$\mu$m Nanosecond Pulses From Monolithic Fiber Lasers

Abstract

We propose a novel laser configuration that can output 3.5-µm nanosecond laser pulses based on a simple and monolithic fiber structure. Cascade-gain-switching, which converts the wavelength of nanosecond pulses from 1.55 µm to 3.5 µm by two successive gain-switching processes. Instead of using expensive pump sources at special wavelengths and bulky active or passive modulation elements for Q-switching or mode-locking, the cascade gain-switching only requires the pumping of an electric-modulated 1.55-µm pulsed laser and two continuous-wave (CW) 975-nm laser diodes. They are all standard products for fiber optic communication applications, which can greatly lower the cost of mid-infrared laser pulse generation. To investigate the feasibility of this configuration, we numerically simulated the cascade-gain-switching processes by comprehensive rate-equation models. In single-shot regime, for stable 3.5-µm pulsed lasing, the CW 975-nm pump should be turned on at least ∼ 500 µs ahead of the 1.55-µm pulsed pump. It shows that the pulse width of the 1.55-µm pump has major impact on the temporal shape of the intermediate 1.97-µm pulse while has neglected influence on the generated 3.5-µm pulse. On the other hand, increasing the CW pump power can significantly improve the output peak power and shorten the pulse when the pump power is less than ∼ 4 W. In the repetitive-pulse regime, we found the 3.5-µm pulse train can be stably outputted when the repetition rate is <= 100 kHz. As the repetition rate increases, the duration of reaching the stable operation increases. When the repetition rate is large than 100 kHz, the stable operation cannot be established because the rate of consuming the population on the 4 I 11/2 level of Er 3+ ions is faster than the rate of building the population.

Two-photon absorption and saturable absorption of mid-IR in graphene

“…21,22 We use a 100 fs tunable laser source at a range of wavelengths, ranging from 1.55 µm (0.80 eV), where measured saturation intensity is compared with values in literature, 2,5,8 up to 3.5 µm (0.35 eV), where recent advances have been made in fiber lasers. [23][24][25][26][27][28][29] We show that 2PA limits the effective modulation depth, as well as the slope of the nonlinear transmission curve, at mid-IR wavelengths. We also show that saturation intensities of femtosecond pulses follow the empirical relation I s ∝ E 3 ph where E ph is the photon energy, while for longer pulses, I s ∝ E ph .…”

From Symmetry to Asymmetry: The Use of Additional Pulses to Improve Protection against Ultrashort Pulses Based on Modal Filtration

“…Particular attention should be given for the possibility of pulse trains to influence REE. For instance, the results of a novel laser configuration that can output 3.5 µm nanosecond laser pulses based on a simple and monolithic fiber structure are known [11].…”

Microjoule-level widely tunable gain-switched thuliumdoped fiber laser

“…Gain-switched (GS) thulium-doped fiber lasers (TDFL) combine multiple features, such as simple cavity geometries, high oscillator output energies (from sub-μJ to mJ), narrow linewidth and wide spectral tunability in 1700-2200 nm range. These properties make GS TDFLs promising tools for middle-infrared (MIR) light generation through nonlinear conversion and supercontinuum broadening [1][2], for LIDAR and sensing applications [3], as well as pumping sources for other laser materials [4]. Recently, a number of GS TDFLs, distinguished by their design and functionality, has been presented.…”