Oscillator-amplifier System Research Articles

13.5 μJ, 20 kHz Repetition Rate, Single Frequency Pr3+:YLF Master Oscillator Power Amplifier System

This article describes a master oscillator and power amplifier (MOPA) system with a single longitudinal mode (SLM) and high-repetition-frequency Pr3+:YLF active medium that was end-pumped by two 444 nm laser diodes. The Pr3+:YLF MOPA laser system produced a maximum pulse energy of 13.5 μJ with a pulse width of 130.2 ns at a pulse repetition frequency of 20 kHz, translating to a peak power of around 103.7 W. The Pr3+:YLF MOPA laser system’s output wavelength was 639.7 nm, and the line-width of its laser spectra was roughly 168 MHz. Additionally, at the highest output level, the laser beam quality did not decrease much due to amplification.

Multi-megawatt pulses at 50 MHz from a single-pump Mamyshev oscillator gain-managed amplifier laser.

We have developed a compact all-PM-fiber ytterbium-doped Mamyshev oscillator-amplifier laser system generating compressed pulses of 102 nJ and 37 fs, thus having over 2 MW of peak power, at a repetition rate of 52 MHz. The pump power from a single diode is shared between a linear cavity oscillator and a gain-managed nonlinear amplifier. The oscillator is self-started by pump-modulation and a linearly polarized single-pulse operation is achieved without filter tuning. The cavity filters are near-zero dispersion fiber Bragg gratings with a Gaussian spectral response. To our knowledge, this simple and efficient source has the highest repetition rate and average power among all-fiber multi-megawatt femtosecond pulsed laser sources and its architecture holds potential for generating higher pulse energies.

High peak power and energy scaling in the mid-IR chirped-pulse oscillator-amplifier laser systems.

The paper introduces a new route towards the ultrafast high laser peak power and energy scaling in a hybrid mid-IR chirped pulse oscillator-amplifier (CPO-CPA) system, without sacrificing neither the pulse duration nor energy. The method is based on using a CPO as a seed source allowing the beneficial implementation of a dissipative soliton (DS) energy scaling approach, coupled with a universal CPA technique. The key is avoiding a destructive nonlinearity in the final stages of an amplifier and compressor elements by using a chirped high-fidelity pulse from CPO. Our main intention is to realize this approach in a Cr2+:ZnS-based CPO as a source of energy-scalable DSs with well-controllable phase characteristics for a single-pass Cr2+:ZnS amplifier. A qualitative comparison of experimental and theoretical results provides a road map for the development and energy scaling of the hybrid CPO-CPA laser systems, without compromising pulse duration. The suggested technique opens up a route towards extremely intense ultra-short pulses and frequency combs from the multi-pass CPO-CPA laser systems that are particularly interesting for real-life applications in the mid-IR spectral range from 1 to 20 μm.

Investigation of Amplification Parameters in Q-switched Fiber Oscillator-Amplifier Systems

Investigation of Amplification Parameters in Q-switched Fiber Oscillator-Amplifier Systems

High-power DUV picosecond pulse laser with a gain-switched-LD-seeded MOPA and large CLBO crystal.

We report the generation of a 266 nm deep ultraviolet (DUV) picosecond pulse with an average output power of 14 W by the fourth-harmonic generation (FHG) from two consecutive frequency-doubling stages of a 1064 nm pulse based on a gain-switched-laser-diode (LD)-seeded hybrid fiber/solid-state master oscillator and power amplifier (MOPA) system. Through the gain-switched operation of a narrow-spectral-linewidth distributed-feedback laser diode and by using a Yb-doped fiber and a two-stage ${\rm Nd}:{{\rm YVO}_4}$Nd:YVO4 solid-state amplifier, we achieved an average power of 46.5 W near the Fourier transform limit for a 13 ps pulse with a repetition rate of 200 kHz. The narrow linewidth pulse characteristics enabled highly efficient frequency conversion, and the efficiency of conversion from 532 to 266 nm was 54%, and from 1064 to 266 nm was 31%. The beam quality factor ${M^2}$M2 of the generated DUV pulse was below 1.2. The highly efficient FHG process resulted in appeasing thermal stress caused by nonlinear absorption in the crystal, and more than 5000 h of continuous operation were achieved without any power down or beam profile degradation.

Dynamics of a harmonic oscillator coupled with a Glauber amplifier

A system of a quantum harmonic oscillator bi-linearly coupled with a Glauber amplifier is analysed considering a time-dependent Hamiltonian model. The Hilbert space of this system may be exactly subdivided into invariant finite dimensional subspaces. Resorting to the Jordan–Schwinger map, the dynamical problem within each invariant subspace may be traced back to an effective SU(2) Hamiltonian model expressed in terms of spin variables only. This circumstance allows to analytically solve the dynamical problem and thus to study the exact dynamics of the oscillator-amplifier system under specific time-dependent scenarios. Peculiar physical effects are brought to light by comparing the dynamics of such a system with that of two interacting standard oscillators.

Sub-50 fs, µJ-level pulses from a Mamyshev oscillator-amplifier system.

In this Letter, we present a Mamyshev oscillator with an output power of 332 mW and a pulse energy of 31 nJ, which is directly amplified in an additional fiber section. By balancing the gain-narrowing effect with self-phase modulation during the amplification, we achieved an average output power of 11 W, corresponding to a pulse energy of more than 1 µJ. The pulse duration can be compressed to sub-50 fs behind the amplification stage.

High-repetition-rate laser ultrasonic generation in carbon-fiber-reinforced plastics excited by a 3.2-3.4 μm ZGP master oscillator power amplifier system.

In this paper, we demonstrate 1-kHz-repetition-rate laser ultrasound in carbon-fiber-reinforced plastics (CFRPs). A ZGP master oscillator and power amplifier (MOPA) system was used to generate high-repetition-rate laser radiation in the spectral range 3.2-3.4μm. At the output wavelength of 3.4μm, the maximum output energy of the ZGP MOPA system is 5.62mJ with a pulse width of 24ns, corresponding to a peak power of approximately 233.9kW. The ultrasound was generated by the laser converted from 3.2 to 3.4μm in the graphite/epoxy composite. The maximum ultrasonic signal amplitude in the CFRP sample was 36.2mV in the condition of thermoelastic excitation at 3.4μm. Ablation occurred in the CFRP sample when the energy fluence was over 90 mJ/cm2. Compared in different samples, laser-ultrasound generation was influenced by the wavelength of the laser.

100 Hz operation in 10 PW/sr·cm2 class Nd:YAG Micro-MOPA.

An Nd:YAG Micro-MOPA, based on a microchip master oscillator and power amplifier system with gain aperture beam cleaning, could generate sub-ns 200 mJ pulses with extremely high brightness of 18 PW/sr·cm2 [Opt. Express268609 (2018)]. However, the system repetition rate was limited to 10 Hz, due to thermal problems occurring in the main amplifier rod under high-power operation. In this work, we achieved 100 Hz operation with pulse brightness of 11 PW/sr·cm2 by optical compensation of thermal lensing, which was evaluated through calculations and an experiment.

High repetition rate optical parametric oscillator at 1.73 μm with two KTP crystals

A novel method for high-repetition-rate optical parametric oscillator (OPO) at the wavelength of 1.73 μm was demonstrated with potassium titanyl phosphate (KTP) as the nonlinear crystal. The OPO based on KTP was pumped by a 1.064 μm pulsed laser beam from a two-stage diode-end-pumped electro-optically Q-switched Nd:YAG and Nd:YVO4 oscillator-amplifier system. For a Q-switched operation, in the LD pulse pumping regime, the pulse repetition frequency (PRF) was changed from 1 to 3 kHz. At PRFs as high as 2 kHz, the maximum average output power of 2.3 W was obtained at the signal wavelength of 1.725 μm. With a line width of 0.08 nm, the pluses delivered a near-diffraction-limited beam with M2 values of 1.37 and 1.41 in the horizontal and vertical directions, respectively. In this case, a pulse energy of 1.1 mJ and pulse width of 9.6 ns were recorded, and the corresponding optical-to-optical conversion efficiency and slope efficiency were 22.8% and 61.1%, respectively. This work provides an excellent candidate laser source for bond-selective imaging of deep tissue, and also provides a reliable theoretical as well as experimental basis for the industrialization of new solid-state laser.

High-repetition-rate single-frequency Ho:YAG MOPA system.

A stable, 2.09μm single-frequency Ho:YAG master oscillator and power amplifier (MOPA) system at a pulse repetition frequency of 1.25kHz is demonstrated. The maximum output energy of the single-frequency injection-seeded laser is 13.76mJ, and the corresponding pulse width is 178.9ns. The seed laser is a continuous-wave Ho:YAG non-planar ring oscillator laser. The half-width of the pulse spectrum is measured to be 2.65MHz by using the heterodyne technique, which is about 1.06 times Fourier transform limited.

1645-nm single-frequency, injection-seeded Q-switched Er:YAG master oscillator and power amplifier

A 1645-nm injection-seeded Q-switched Er:YAG master oscillator and power amplifier system is reported. The master oscillator generates single-frequency pulse energy of 11.10 mJ with a pulse width of 188.8 ns at 200 Hz. An Er:YAG monolithic nonplanar ring oscillator is employed as a seed laser. The output pulse from the master oscillator is amplified to 14.33-mJ pulse energy through an Er:YAG amplifier, with a pulse width of 183.3 ns. The M2-factors behind the amplifier are 1.14 and 1.23 in x- and y-directions, respectively. The full width at half maximum of the fast Fourier transformation spectrum of the heterodyne beating signal is 2.84 MHz.

High-energy, stable single-frequency Ho:YAG ceramic amplifier system.

We demonstrate a 2090nm single-frequency, high-energy, Q-switched Ho:YAG ceramic master oscillator and power amplifier system that contains two-stage amplifiers. The maximum single-frequency pulse energy is 55.64mJ at a pulse repetition frequency of 200Hz. The half-width of the pulse spectrum is measured to be 3.96MHz by a heterodyne technique. To the best of our knowledge, 55.64mJ is the highest single-frequency output energy obtained from the Ho:YAG laser.

Generation of 47 fs Pulses from an Er:Fiber Amplifier**Supported by the National Key Research and Development Program of China under Grant No 2017YFB1104500, the Science and Technology Project of Guangdong Province under Grant Nos 2014B090903014, 2015B090920003, 2016B090917002 and 2016B090926004.

We demonstrate a self-starting erbium fiber oscillator-amplifier system based on the nonlinear polarization rotation mode-locked mechanism. The direct output pulse from the amplifier is 47 fs with an average power of 1.22 W and a repetition rate of 50 MHz, corresponding to a pulse energy of 24 nJ. The full width at half-maximum of the spectrum of the output pulses is approximately 93 nm at a central wavelength of 1572 nm so that the transform-limited pulse duration is as short as 39 fs. Due to the imperfect dispersion compensation, we compress the pulses to 47 fs in this experiment.

Chair-like pulses in an all-normal dispersion Ytterbium-doped mode-locked fiber laser.

We report, for what we believe is the first time, generation of stable chair-like pulses (a pulse shape with an initial long flat portion followed by a short high peak power portion resembling the shape of a chair) by mode locking of a Ytterbium (Yb)-doped fiber laser. Chair-like pulse shapes are achieved by implementing dual saturable absorbers, one based on a nonlinear optical loop mirror (NOLM) and the other based on nonlinear polarization rotation (NPR) inside the cavity. The transmission characteristics of the NOLM-NPR pair leading to the formation of chair-like pulses are numerically investigated. We also report the amplification characteristics of chair-like pulses in an external multistage Yb-doped fiber amplifier setup at different repetition rates of the pulse train. It was found that the chair-like pulses are suitable for amplification, and more than 10 W of average power at 460 kHz repetition rate have been obtained at total pump power of ∼20 W coupled to the power amplifier. At a lower repetition rate (115 kHz), ∼8 W of average power were obtained corresponding to ∼70 μJ of pulse energy with negligible contribution from amplified spontaneous emission or stimulated Raman scattering. We believe that such an oscillator-amplifier system could serve as an attractive tool for micromachining applications.

34 mJ Ho:YAG ceramic master oscillator and power amplifier laser at 2097 nm.

We demonstrated a 2097 nm Ho:YAG ceramic master oscillator and power amplifier system pumped by Tm:YLF lasers. Laser characteristics of 0.6 at.% and 0.4 at.% Ho:YAG ceramics were studied in CW and Q-switched modes. An output energy of 20.6 mJ at a pulse repetition frequency of 200 Hz was achieved from the master oscillator. The pulse energy was amplified to 34 mJ by the Ho:YAG ceramic amplifier with a pulse duration of 54 ns.

A Preventive Measure against Fiber Damage of a Femtosecond 1.55-µm Er-doped Fiber Laser System using Multimode Optical Fiber

A system of passively-mode-locked figure-of-eight erbium-doped fiber laser oscillator-amplifier system was studied. Cooling the erbium-doped fiber down to 0 oC increased the output power of the oscillator to 1.8 times that of the room temperature. To avoid the optical damage of the fiber end, a graded-index multimode fiber of 8 mm in length was connected by fusion splice. The output power increased from 29 to 41 kW for direct amplification and from 72 to 93 kW for pulse extraction and amplification without any damage of the fiber end. The mechanisms of the temperature effect and the optical damage are discussed. Damage prevention by a factor of up to 25 is expected.

Narrow Linewidth 177.3-nm Nanosecond Laser With High Efficiency and High Power

We demonstrate a high-efficiency high average power vacuum ultraviolet (VUV) 177.3-nm laser with a narrow linewidth generated by second harmonic (SH) generation from a nanosecond (ns) 355-nm laser in the KBe2BO3F2 crystal. The 355-nm pumping source was a frequency-tripled Q-switched 1064-nm Nd:YAG oscillator-amplifier laser system, in which a quartz etalon was used to compress the linewidth of 1064-nm laser. Under a pump power of 5.2 W at 355 nm with a linewidth of 2 pm, a high average power of 146.5 mW at 177.3 nm with a linewidth of ~0.71 pm was obtained, corresponding to an SH conversion efficiency as high as 2.8%. This is, to the best of our knowledge, the highest output power and highest SH conversion efficiency for the ns 177.3-nm lasers. This VUV coherent light can be used as light source for fine precision spectroscopy and semiconductor photolithography owing to its narrow linewidth, high energy resolution, and high average power.

Efficient terahertz wave generation from GaP crystals pumped by chirp-controlled pulses from femtosecond photonic crystal fiber amplifier

A chirp-tunable femtosecond 10 W, 42 MHz photonic-crystal-fiber oscillator-amplifier system that is capable of delivering sub-60 fs light pulses at 1040 nm is used to demonstrate high-efficiency terahertz radiation generation via optical rectification in GaP crystals only a few millimeters in length. The optimization of the chirp of the fiber-laser pulses is shown to radically enhance the terahertz output, indicating one possible way to more efficiently use these extended nonlinear crystals in compact fiber-pumped terahertz radiation sources.

An all-photonic-crystal-fiber wavelength-tunable source of high-energy sub-100 fs pulses

We demonstrate a wavelength-tunable source of high-energy sub-100fs light pulses where all the key components, including a master oscillator, an amplifier, and a tunable frequency shifter, are based on photonic-crystal fibers (PCFs). Soliton self-frequency shift in a specifically designed all-solid photonic band-gap fiber with an effective mode area of 110μm2 is used in this system to transform a 1040nm, 50MHz, 100fs amplified output of a large-mode-area ytterbium-doped PCF oscillator–amplifier system into sub-100fs, 6.4nJ light pulses smoothly tunable within the range of wavelengths from 1160 to 1260nm.