Burst-mode thulium all-fiber laser delivering femtosecond pulses at a 1 GHz intra-burst repetition rate

High-sensitivity and high-speed measurements of ultrashort pulses as short as 74 fs at 1.9 µm using a GRENOUILLE device

Ultrashort laser pulse sources in the wavelength range of 1.8 to 2 µm have many potential applications including medicine, materials processing, and sensing. In the use of such lasers, a crucial task is to measure their pulse's temporal intensity and phase. Such measurement devices are most useful when they are simple to build and operate and also have high speed and high sensitivity. The GRENOUILLE measurement device with few components, no moving parts, sensitivity of hundreds of picojoules, and measurement speed of hundreds of milliseconds, is commonly used to solve this problem at other wavelengths. In this paper, the measurement of ultrashort pulses by a GRENOUILLE device, developed using a silicon matrix sensor, for pulses in the wavelength range of 1.8 to 2 µm has been demonstrated. It is shown that ultrashort pulses with durations of 74 to 900 fs and a maximum spectral FWHM of 85 nm can be measured with this device. The recently developed ultra-reliable RANA approach was used for pulse retrieval from the measured traces. The device's performance was validated by comparing its measurements with those obtained by the robust FROG technique.

Ultra-short pulse burst laser around 1.98 µm obtained through an all-fiber nonlinear wavelength converter and Tm-doped fiber amplifier

We report an all-fiber ultra-short pulse burst laser operating at around 1.98 µm that is obtained through a nonlinear wavelength converter and Tm-doped fiber amplifier. A mode-locked Er-doped fiber laser was first built and then amplified in subsequent amplifiers to an average power of 1.3 W. Ultra-short pulse burst output was achieved through a pulse multiplier and a fiber-pigtailed acousto-optic modulator. It was then injected into an all-fiber nonlinear wavelength converter constructed from P-doped fiber and Tm-doped fiber, obtaining an ultra-short pulse burst laser of 540 mW around 1.98 µm. Its average output power was then amplified to 4.33 W in a Tm-doped fiber amplifier with an intra-burst pulse repetition frequency of 0.9 GHz, a burst repetition frequency of 200 kHz, and a duty cycle of 2%, corresponding to about 200 pulses within each burst. This 1.98 µm pulse burst laser has enormous potential to be applied in bio-medical areas.

Efficient surface polishing using burst and biburst mode ultrafast laser irradiation

The use of laser irradiation for micromachining is widely applicable and has many benefits. One of the main uses is that it is possible to mill and polish the sample using the same laser system. State-of-the-art laser systems with high average optical power and burst regimes are widely used in technology. The main advantages of burst regimes are the closer fluence values to optimal fluences and residual heat reusage for subsequent pulses. In this study, the influence of MHz burst, GHz burst, and bibursts was investigated for significant surface polishing of copper and stainless-steel samples. Z-scan experiments were performed to determine the optimal number of sub-pulses inside the burst for the lowest surface roughness.

Worst-Case X-ray Photon Energies in Ultrashort Pulse Laser Processing

Ultrashort pulse laser processing can result in the secondary generation of unwanted X-rays if a critical laser irradiance of about 10¹³ W cm^-2 is exceeded. Spectral X-ray emissions were investigated during the processing of tungsten and steel using three complementary spectrometers (based on CdTe and silicon drift detectors) simultaneously for the identification of a worst-case spectral scenario. Therefore, maximum X-ray photon energies were determined, and corresponding dose equivalent rates were calculated. An ultrashort pulse laser workstation with a pulse duration of 274 fs, a center wavelength of 1030 nm, pulse repetition rates between 50 kHz and 200 kHz, and a Gaussian laser beam focused to a spot diameter of 33 μm was employed in a single pulse and burst laser operation mode. Different combinations of laser pulse energy and repetition rate were utilized, keeping the average laser power constant close to the maximum power of 20 W. Peak irradiances I₀ ranging from 7.3 × 10¹³ W cm^-2 up to 3.0 × 10¹⁴ W cm^-2 were used. The X-ray dose equivalent rate increases for lower repetition rates and higher pulse energy if a constant average power is used. Laser processing with burst mode significantly increases the dose rates and the X-ray photon energies. A maximum X-ray photon energy of about 40 keV was observed for burst mode processing of tungsten with a repetition rate of 50 kHz and a peak irradiance of 3 × 10¹⁴ W cm^-2.

4.3 GHz fundamental repetition rate passively mode-locked fiber laser using a silicate-clad heavily Tm3+-doped germanate core multimaterial fiber

We report a silicate-clad heavily Tm³⁺-doped germanate core multimaterial fiber that is successfully drawn by using a rod-in-tube method. This new fiber has a high gain per unit length of 6.11 dB/cm at 1.95 µm, which is, to the best of the authors' knowledge, the highest gain per unit length reported so far for Tm³⁺-doped glass fibers. By virtue of this high-gain glass fiber, an all-fiber-integrated passively mode-locked fiber laser with a fundamental repetition rate up to 4.3 GHz is demonstrated. Remarkably, the generated pulse operating at 1968 nm exhibits a signal-to-noise ratio of >76 dB in the radio-frequency domain. These results suggest that the silicate-clad heavily Tm³⁺-doped germanate core multimaterial fiber can act as a key building block for high repetition rate mode-locked fiber lasers at 2 µm.

Hectowatt-level GHz burst-mode all-fiber laser based on dissipative soliton resonance

We demonstrate a high power Yb-doped burst-mode all-fiber laser system operating at GHz intra-burst repetition rate. To our knowledge, it is the first report utilizing dissipative soliton resonance (DSR) to generate tunable burst-mode rectangular pulses. Due to the tunable duration and the rapid rise/fall time for DSR pulses, a 1-10 ns adjustable burst pulse duration is achieved. The intra-burst with sinusoidal waveform can be tuned from 0.8 GHz to 1.5 GHz and actively modulated by an electro-optic modulator (EOM). Amplified by a three-stage Yb-doped fiber amplifier (YDFA), the output power achieves 304 W at 10 ns of burst duration, and the maximum peak power reaches over 50 kW at 2 ns of burst duration. This laser system is anticipated to be applied to generate high power arbitrary microwave signal.

Compact chirped-pulse amplification systems based on highly Tm3+-doped germanate fiber

We report the fabrication of a dual cladding large mode area thulium-doped germanate fiber (TDGF). The fiber has a core diameter of 20 µm, a high Tm³⁺ ion concentration of 3cm³×10²⁰/cm³, and a hexagonal inner cladding to enhance pump absorption when cladding-pumped. Using a short fiber length, we demonstrate a compact 300 fs chirped-pulse amplification system operating at 1925 nm, investigating both core- and cladding-pumped implementations. By cladding pumping a 65 cm long fiber we produced an average power of 14.1 W (limited by thermally induced damage) and a peak power of 2.17 MW at a pulse repetition rate of 15.7 MHz. Core pumping a 19 cm length of TDGF produced 2.3 W of average-power and 16 MW peak-power pulses at 0.39 MHz. The performance is already comparable to the state-of-the-art success achieved with flexible silica fibers. Considering the rapid improvements in glass quality and the scope for further increasing the doping concentration, this fiber type holds great potential for pulsed fiber lasers in the 1.5-3 µm wavelength region.

Mode-locked pulses from a Thulium-doped fiber Mamyshev oscillator

We present experimental results of the generation of ultrashort pulses in the 2 µm wavelength region by a fiber Mamyshev oscillator, along with the simulation of the pulse propagation in the cavity. The Mamyshev oscillator emitted pulses with energies of 3.55 nJ at a repetition rate of 15 MHz and optical spectra with bandwidths of 48 nm. The pulses propagated in anomalous dispersive Thulium-doped fiber sections with dispersion compensation sections of normal dispersive fibers.

Femtosecond CPA hybrid laser system with pulse-on-demand operation

In this manuscript we present a true pulse-on-demand concept of a hybrid CPA laser system, consisting of a chirped-pulse fiber amplifier and an additional solid-state amplifier, capable of generating femtosecond pulses on demand without an external optical modulator/shutter. Pulse-on-demand operation is achieved by introducing idler pulses with a few nanoseconds duration and selectively switching between the femtosecond and idler pulses. The idler pulses are used to maintain a constant population inversion in the fiber amplifier as well as in the solid-state amplifier. Second harmonic generation (SHG) unit then effectively filters out the idler pulses due to their low peak power, leaving only a stable femtosecond pulse train. This concept is demonstrated on a CPA hybrid system that can generate pulses with up to 200 µJ at 515 nm with a pulse duration under 450 fs. As there is no optical modulator at the laser output, the presented concept also enables further power scaling.

Compact, high repetition rate, 4.2 MW peak power, 1925 nm, thulium-doped fiber chirped-pulse amplification system with dissipative soliton seed laser

We report the demonstration of a high average- and peak-power, 1925 nm, thulium-fiber based chirped pulse amplification (CPA) system. A compact, dissipative soliton thulium-fiber, mode-locked seed produced pre-chirped pulses with 25 ps duration, 45 mW output power and repetition rate of 15.7 MHz. After stretching to 105 ps in 83 m of normal dispersion fiber, the pulses were amplified in a core-pumped pre-amplifier and a cladding pumped power amplifier to average output powers of 28 W and 30 W with forward and backward pumping, respectively, with the output power limited only by the available pump power. After a pair of fused silica transmission gratings with an efficiency of 71%, the amplified pulses were re-compressed to 297 fs yielding pulses with a peak power of 4.2 MW (backward pumped) and a pulse energy of 1.27 µJ.