Pump Pulse Repetition Rate Research Articles

Characteristics and suppression of beam distortion in a high repetition rate nanosecond stimulated Brillouin scattering phase conjugation mirror

Abstract The stimulated Brillouin scattering phase conjugation mirror (SBS-PCM) based on liquid media is widely used in high-power laser systems due to its robust thermal load capacity, high energy conversion efficiency and improved beam quality. Nevertheless, with an increase in the pump repetition rate, thermally-induced blooming and optical breakdown can emerge, leading to distortions in the Stokes beam. In this study, we delved into the thermal effects in liquid SBS-PCMs employing hydrodynamic analysis, establishing a relationship between beam profile distortion and the thermal convection field. We calculated the temperature and convection velocity distribution based on the pump light parameters and recorded the corresponding beam profiles. The intensities of the beam profiles were modulated in alignment with the convection directions, reaching a velocity peak of 2.85 mm/s at a pump pulse repetition rate of 250 Hz. The residual sum of squares (RSS) was employed to quantify the extent of beam profile distortion relative to a Gaussian distribution. The RSS escalated to 7.8, in contrast to 0.7 of the pump light at a pump pulse repetition rate of 500 Hz. By suppressing thermal convection using a high-viscosity medium, we effectively mitigated beam distortion. The RSS was reduced to 0.7 at a pump pulse repetition rate of 500 Hz, coinciding with a twentyfold increase in viscosity, thereby enhancing the beam quality. By integrating hydrodynamic analysis, we elucidated and mitigated distortion with targeted solutions. Our research offers an interdisciplinary perspective on studying thermal effects and contributes to the application of SBS-PCMs in high-repetition-rate laser systems by unveiling the mechanism of photothermal effects.

Feedback Methods for Vector Measurements Using an All-Optical Atomic Magnetometer.

In this work, we look to compare three methods of feedback for the ultimate purpose of measuring the transverse vector components of a magnetic field using a synchronous light-pulse atomic scalar magnetometer with a few tens of fT/Hz sensitivity in Earth-field-scale magnetic environments. By applying modulation in the magnetic field to orthogonal axes, the respective vector components may, in principle, be separated from the scalar measurement. Success of this technique depends in significant part on the ability to measure and respond to these perturbations with low measurement uncertainty. Using high-speed least-squares fitting, the phase response of the atomic spins relative to the first harmonic of the optical pump pulse repetition rate is monitored and correspondingly adjusted into resonance with the natural Larmor precession frequency. This paper seeks to motivate and compare three distinct methods of feedback for this purpose. As a first step toward the full development of this technique, the present work uses a simplified version with modulation applied only along the bias field. All three methods investigated herein are shown to provide results that match well with the scalar magnetometer measurements and to depend on both the applied modulation amplitude and optimal feedback response to achieve low relative uncertainty.

Electro-optic comb pumped optical parametric oscillator with flexible repetition rate at GHz level.

We present a gigahertz (GHz)-repetition-rate optical parametric oscillator (OPO) pumped by an electro-optic comb at 1.03 µm, delivering sub-picosecond signal pulses across 1.5-1.7 µm from a MgO-doped periodically poled LiNbO3 crystal. Using a pump power of 5 W at 14.2 GHz repetition rate, 378 mW of signal power is obtained at 1.52 µm from a subharmonic cavity, corresponding to a signal extraction efficiency of 7.6%. By cascading a Mach-Zehnder modulator, the pump pulse repetition rate can be divided by any integer number from one to 14, allowing the OPO to operate with a flexible repetition rate from 1 to 14.2 GHz. A strategy leading to quasi-continuous repetition rate tunability of the OPO is also discussed.

Effects of pump pulse energy and repetition rate on beam quality in a high-power mid-infrared ZnGeP2 OPO.

We report on changes of beam quality with rising pump pulse energy and repetition rate in a high-power mid-infrared ZnGeP2 (ZGP) OPO in a non-planar cavity. The ZGP OPO generates 25 W in 3 - 5 μm pumped by a 10 kHz Q-switched Ho:LLF MOPA system at 2 μm. The beam quality factor increases from nearly diffraction-limited at OPO threshold to a M2 = 1.8 at maximum pump power. The degradation can be attributed to thermal lensing and gain guiding effects in the nonlinear crystal. To discriminate between those effects, we monitored the beam quality as a function of the repetition rate at the highest pump pulse energy using a Pockels cell as a pulse picker. Therefore, the heat load inducing thermal lensing increases linearly, whereas gain guiding remains nearly unaffected. M2 shows a minimum of 1.2 at 1 kHz, starting from 1.3 at 100 Hz and rising to 1.8 at 10 kHz. Those findings have been reproduced by numerical simulations. Further investigations show the transient evolution of beam quality, revealing the thermal lens build-up process to the steady-state.

THz generation by optical rectification of intense near-infrared pulses in organic crystal BNA.

Generation of terahertz radiation by optical rectification of intense near-infrared laser pulses in N-benzyl-2-methyl-4-nitroaniline (BNA) is investigated in detail by carrying out a complete characterization of the terahertz radiation. We studied the scaling of THz yield with pump pulse repetition rate and fluence which enabled us to predict the optimal operating conditions for BNA crystals at room temperature for 800 nm pump wavelength. Furthermore, recording the transmitted laser spectrum allowed us to calculate the nonlinear refractive index of BNA at 800 nm.

Temporal dynamics of diode-pumped circulation-free liquid dye lasers

A highly stable diode-pumped circulation-free liquid dye laser in a plano-concave cavity is reported. The design is simple (no fabrication process step required and no fluid circuitry), compact (∼ cm sized), and cost-effective. An optical efficiency of 18% with an M2 of 1 is reported with an excellent photostability—no efficiency drop was seen after 1.4 × 106 pulses at 50 Hz, a value comparable to flowing systems. We show that thermal effects are central in the stability and also on the dynamics of this laser. The laser build-up and shutdown dynamics are studied in detail for different pump pulse durations/repetition rates; they reveal pulse shortening with an increasing pump pulse duration and repetition rate that are shown to be due to thermal lensing diffraction losses. This laser structure offers a very convenient and simple platform for testing or harvesting solution-processable gain materials.

20.6 W Mid-Infrared Supercontinuum Generation in ZBLAN Fiber With Spectrum of 1.9–4.3 μm

A 20.6-W mid-infrared supercontinuum (MIR-SC) generation is demonstrated in a piece of step-index ZBLAN (ZrF4−BaF2−LaF3−AlF3−NaF) fiber. A silica-fiber-based SC laser operating at 1.9–2.6 μm is used as a pump source. Owing to the specially designed ZBLAN fiber which perfectly matches the pigtail of the pump source in mode-field-diameter, direct fusion splicing was achieved between the ZBLAN fiber and the pigtail of the pump source instead of being connected via a piece of transition fiber. The coupling loss of the fusion splicing joint between the silica fiber and the ZBLAN fiber was measured as 0.26 dB@2000 nm. Robust fiber endcap made of multimode AlF3 fiber also benefited high-power MIR-SC generation. Spectral and power characteristics of the generated SC under different pump pulse duration and pulse repetition rate were carefully studied. A 20.6-W SC was obtained with an overall spectral coverage and a 10-dB long wavelength edge of 1.92–4.29 μm and 4.08 μm, respectively. This is, to the best of the authors’ knowledge, the first demonstration of 20-W level SC laser based on ZBLAN fiber with long wavelength edge extending beyond 4 μm.

High-power mid-infrared supercontinuum generation in a fluoroindate fiber with over 2 W power beyond 3.8 µm.

An all-fiber mid-infrared supercontinuum (MIR-SC) laser source with a power of over 2 W beyond 3.8 µm is demonstrated. The SC laser involves a silica-fiber-based SC laser as a pump source and a piece of fluoroindate (InF3) fiber as a nonlinear medium. The influence of pump pulse repetition rate on the SC characteristics is carefully studied. In the InF3 fiber, the pump pulse with spectral coverage of 1.9-2.6 µm is converted into MIR-SC with a broadest spectral coverage of 1.9-4.9 µm and a maximal average power of 11.8 W. Up to 2.18 W is measured in the spectral region beyond 3.8 µm, which, to the best of the authors' knowledge, demonstrates the record SC power in this waveband to date.

Optical characteristics and parameters of gas discharge plasma on a mixture of cadmium diodide vapor with helium and small addition of nitrogen

Наведено результати дослідження спектральних, інтегральних і ресурсних характеристик випромінювання газорозрядної плазми атмосферного тиску на багатокомпонентних сумішах дийодиду кадмію з гелієм і малої добавки молекулярного азоту. Створення газорозрядної плазми і збудження компонент робочої суміші здійснювалося імпульсно-періодичним (частота проходження імпульсів 18–20 кГц, тривалість імпульсів 150 нс) бар’єрним розрядом. Виявлено випромінювання у видимій області спектру ексиплексних молекул монойодиду кадмію, атомів кадмію. Встановлено закономірності в змінах оптичних характеристик плазми в залежності від частоти проходження імпульсів накачки, компонентного і кількісного складу сумішей. Представлено обговорення результатів досліджень. Дані досліджень представляють інтерес для створення ексиплексних газорозрядних джерел видимого випромінювання з максимумом випромінювання на довжині хвилі λ=650 нм.

Metal Vapor Active Element Design

A novel metal vapor active element is constructed. The functions of metal vaporization and metal vapor excitation are divided between two different sources, which is a feature of the new element. Metal vapors are formed by induction heating of a dispenser located outside the active zone. The excitation is carried out by the commutation of a high-voltage pump pulse. The efficiency of the design has been tested in the excitation of copper atoms. Under conditions of independent heating of the dispenser, stable lasing was obtained at 510.6 and 578.2 nm lines at a pumping pulse repetition rate of 15 kHz.

Cascaded Raman scattering based high power octave-spanning supercontinuum generation in graded-index multimode fibers

A new method to generate multi-watt-level, octave-spanning, spectrally flat supercontinua stemmed from cascaded Raman scattering in graded-index multimode fibers is reported. Formation dynamics of supercontinua are investigated by studying the effect of fiber length and core size. High power handling capacity of the graded-index multimode fibers is demonstrated by power scaling experiments. Pump pulse repetition rate is scaled from kHz to MHz while pump pulse peak power remains same and ~4 W supercontinuum is achieved with 2 MHz pump repetition rate. To the best of our knowledge, this is the highest average power and repetition supercontinuum source ever reported based on a graded-index multimode silica fiber. Spatial properties of the generated supercontinua are measured and Gaussian-like beam profiles obtained for different wavelength ranges. Numerical simulations are performed to investigate underlying nonlinear dynamics in details and well-aligned with experimental observations.

High repetition rate gain-switched 194 μm fiber laser pumped by 156 μm dissipative soliton resonance fiber laser

A dissipative soliton resonance (DSR) mode-locked Er:Yb fiber laser has been used to pump a thulium fiber laser to generate gain-switched pulses at high repetition rates. Here 412ns long DSR pulses with a center wavelength of around 1.56μm at a repetition rate of 410kHz have been fed to a thulium fiber laser, resulting in generation of gain-switched pulses at 1.94μm. The minimum pulse width achieved was 256ns with an average power of 4.6W at 66% slope efficiency. Gain-switched pulses at 520kHz and 750kHz were generated through changing the pump pulse repetition rate by modifying the DSR cavity. To the best of our knowledge, this is the first demonstration of a high repetition rate gain-switched thulium fiber laser pumped by a DSR mode-locked fiber laser. As DSR pulses can be generated with high seed average power and energy independent of the operating wavelength regime as well as mode-locking technique, the proposed method can be applied to generate gain-switched pulses at high repetition rates and various wavelengths without the need of any optical or electrical modulators.

All-Fiber-Integrated High-Power Supercontinuum Sources Based on Multi-Core Photonic Crystal Fibers

The obstacles of power scaling the supercontinuum (SC) source based on single-core photonic crystal fiber (PCF) are analyzed. The combination of high-power fiber lasers and multi-core PCFs would be a feasible method to obtain an all-fiber-integrated high-power broadband SC source (covering visible range). In this paper, we present a comprehensive study of high-power SC generation in multi-core PCFs. Comparative experiments are performed by using a high-power pulse-repetition-rate-tunable picosecond fiber laser to pump two kinds of home-made seven-core PCFs. The influences of PCF structure (fiber dispersion property) and pulse repetition rate (pulse peak power) on the SC generation in multi-core PCFs are investigated in detail. When the picosecond fiber laser at a pulse repetition rate of 1.9 GHz is adopted as the pump, 116 W SC spanning from 800 to 1700 nm is generated in 1# seven-core PCF. Also 64 W visible SC spanning at least 500-1700 nm is demonstrated in 2# seven-core PCF at a pump pulse repetition rate of 480 MHz. The potential of extending the spectral range and scaling the output power for the SC source based on multi-core PCFs are analyzed and discussed.

Laser amplifier based on a neodymium glass rod in diameter

A unique large-aperture neodymium glass rod amplifier is experimentally studied. The small-signal gain distribution is measured at different pump energies. The aperture-averaged gain is found to be 2.3. The stored energy , the maximum possible pump pulse repetition rate, and the depolarisation in a single pulse and in a series of pulses with a repetition rate of one pulse per five minutes are calculated based on the investigations performed. It is shown that the use of this amplifier at the exit of the existing laser can increase the output pulse energy from 300 to .

Design of pump pulses for InGaAs quantum well semiconductor disk lasers

It has been demonstrated experimentally that pulsed pumping can significantly improve thermal management, thus upgrade the output power of an optically pumped semiconductor disk laser (SDL). The transient heat conduction equation is solved by the use of the finite element method, and the maximum temperature rise of the active region in an InGaAs quantum well SDL under pulsed pumping is focused. Based on the numerical results, the influences of width, repetition rate, and shape of pump pulses on the maximum temperature rise are discussed, the optimized design of width, repetition rate, and shape of pump pulses are concluded, and the theoretical results are in good agreement with the reported experiments.

Gain-switched all-fiber laser with narrow bandwidth

Gain-switching of a CW fiber laser is a simple and cost-effective approach to generate pulses using an all-fiber system. We report on the construction of a narrow bandwidth (below 0.1 nm) gain-switched fiber laser and optimize the pulse energy and pulse duration under this constraint. The extracted pulse energy is 20 μJ in a duration of 135 ns at 7 kHz. The bandwidth increases for a higher pump pulse energy and repetition rate, and this sets the limit of the output pulse energy. A single power amplifier is added to raise the peak power to the kW-level and the pulse energy to 230 μJ while keeping the bandwidth below 0.1 nm. This allows frequency doubling in a periodically poled lithium tantalate crystal with a reasonable conversion efficiency.

Raman laser with intracavity conversion of laser radiation in a crystal

Intracavity Raman conversion of radiation of a Nd : YAG laser by a crystal into the first-Stokes component is studied in different operation regimes. In the case of passive -switching of the Nd : YAG laser, the SRS radiation has an average power of with a pulse train energy up to , an individual pulse duration of , and a pump pulse repetition rate of . In the case of active -switching, the average SRS power is at a pulse energy up to , a pulse duration of , and a pump pulse repetition rate of .

Optical characteristics of a HgBr excilamp

Optical characteristics of a coaxial HgBr excilamp on multicomponent mercury dibromide vapour mixtures with helium, nitrogen and sulfur hexafluoride are investigated under pumping by a pulse-periodic barrier discharge. Stable excilamp operation was demonstrated at a pump pulse repetition rate of . The component composition of the working system was determined, which provides a maximal average and pulsed specific radiation power of and , respectively, at the efficiency of 7.3% in the blue-green spectral range with the maximal radiation intensity at the wavelength of . The reduction in the radiation power after shots is 5%. Interpretation is given for the results of optimisation of excilamp characteristics.

Investigation of hundred-watt-level supercontinuum generation in photonic crystal fiber

A 101 W all-fiber supercontinuum soure is demonstrated by seeding a piece of photonic crystal fiber with a high-power pulse-repetition-rate-tuning picosecond fiber laser. By performing a series of comparative experiments, influences of the pump pulse repetition rate and the photonic crystal fiber length on the supercontinuum generation process are investigated in detail. How to improve the power level of the supercontinuum soure is analyzed and discussed. The research results will make a contribution to the further development of high-power supercontinuum soure.

Optical characteristics and plasma parameters of a blue-green exciplex lamp

The optical characteristics and plasma parameters of an exciplex lamp radiating in the blue-green spectral range are studied. A plasma is generated by an atmospheric-pressure barrier discharge initiated in a quaternary mixture including mercury dibromide, sulfur hexafluoride, nitrogen, and helium. It is shown that the exciplex lamp can radiate at an elevated repetition rate of pump pulses (1–12 kHz) under the conditions of mixture self-heating. A tradeoff between the radiation power and nitrogen partial pressure is found. The mean specific radiation power in the blue-green range at a level of 480 mW/cm3 is achieved at partial vapor pressures of mercury dibromide, sulfur hexafluoride, nitrogen, and helium of 0.70, 0.07, 4.00, and 117.20 kPa, respectively. The plasma parameters, namely, the electron energy distribution function; concentration, temperature, and mean energy of electrons; transport properties; and rate constants of elastic and inelastic electron scattering by the working mixture components are determined as functions of ratio E/N (where E is the electric field strength and N is the total concentration of mercury dibromide, sulfur hexafluoride, and nitrogen molecules and helium atoms). It is found that mercury monobromide molecules and also excited and higher energy states take part in the population of exciplex molecules HgBr* (B2Σ12/+ states) in the course of quenching these exciplexes by sulfur hexafluoride and nitrogen molecules.