Laser Amplifier Research Articles

Measurement of gain-saturation properties of a Q-switched Nd:YAG laser oscillator with a three-pass amplifier and corresponding beam diagnosis

Here, a homemade Q-switched Nd: YAG laser oscillator alongside a three-pass amplifier has been designed and fabricated. The oscillator operates nearly in the fundamental mode and the spot size/phase front curvature is determined by making use of ABCD law regarding the thermal lensing effect. To scale-up the pulse energy in a compact volume, a three-pass amplifier arrangement is coupled to the oscillator. The systematical measurements to obtain the small-signal gain and saturation properties are carried out in terms of various pumping energies. Simultaneously, the output beam from the oscillator and triple-pass amplifier are well diagnosed to determine output energy, pulse duration, spectral width, M2 factor and the radius of curvature. Despite the oscillator pulse giving out 60 mJ, 13 ns however, the output pulse of 800 mJ, 1 Hz, 9 ns at 1064 nm with FWHM 0.024 nm and beam quality factor M2= 2.21 are determined in favor of multi-pass amplifier. Scaling up the pulse energy up to 800 mJ it is shown that the output beam alters its phase front from the initial planar wave to the spherical curvature during the propagation through a multi-pass amplifier due to the elevation of a radial temperature gradient. Eventually, the novelty of this manuscript arises from the effect of energy scale-up on the gain and saturation in laser amplifier and their correlation with the output beam properties.

Dynamic modal characteristics of transverse mode instabilities in ytterbium-doped fiber laser oscillator

In recent years, transverse mode instability (TMI) has been widely observed in fiber laser amplifier systems. The transverse mode instability phenomenon in fiber laser oscillators is less studied. Here, we focus on the dynamical output properties, i.e., its temporal signal and modal characteristics in a 30-μm-core-diameter ytterbium (Yb)-doped fiber laser oscillator. The TMI occurs at a pumping power around 310 W. Different from amplifiers, the basic oscillation frequency is quite low, at around 100 Hz, changing with time and pump power. When the fiber laser oscillator operates beyond TMI threshold at 357 W or 377 W for a while, the temporal fluctuation slowly disappears together with a decreased oscillation frequency, and appears again later. Based on the mode decomposition technique, we find that during the period of fluctuation disappearance at 357 W, the power output stays low and the output beam is still a mixture of fundamental mode and higher-order modes. The fundamental mode content is calculated to be averagely higher when temporal fluctuation disappears, increasing from ∼57% to ∼63%. Our results indicate complex interaction between the fiber laser oscillation and the TMI effect, and calls for more attention into understanding TMI in fiber laser oscillators.

Spectrally flat mid-infrared supercontinuum pumped by a high power 2 µm noise-like pulse.

We demonstrate spectrally flat high-power mid-infrared supercontinuum (MIR SC) generation with record-breaking power of 33.1 W and power conversion efficiency of 75.06%. It is pumped by a 2 µm master oscillator power amplifier system consisting of a figure-8 mode-locked noise-like pulse seed laser and dual-stage Tm-doped fiber amplifiers with repetition rate of 4.08 MHz. Through cascading a piece of ZBLAN fiber with 13.5 µm large core diameter by direct-low-loss fusion splicing, SCs with spectral ranges of 1.9-3.68 µm, 1.9-3.84 µm, 1.9-4.02 µm and average powers of 33.1 W, 29.8 W, 25.9 W are generated. To the best of our knowledge, all of them have achieved the highest output power under the same condition of MIR spectrum range. This high-power all-fiber MIR SC laser system has relatively simple architecture, high efficiency and flat spectrum, demonstrating the advantages of 2 µm noise-like pulse pump in high-power MIR SC generation.

A sub-2 Kelvin cryogenic magneto-terahertz scattering-type scanning near-field optical microscope (cm-THz-sSNOM)

We have developed a versatile near-field microscopy platform that can operate at high magnetic fields and below liquid-helium temperatures. We use this platform to demonstrate an extreme terahertz (THz) nanoscope operation and to obtain the first cryogenic magneto-THz time-domain nano-spectroscopy/imaging at temperatures as low as 1.8 K, magnetic fields of up to 5 T, and with operation of 0–2 THz. Our Cryogenic Magneto-Terahertz Scattering-type Scanning Near-field Optical Microscope (or cm-THz-sSNOM) instrument is comprised of three main equipment: (i) a 5 T split pair magnetic cryostat with a custom made insert, (ii) a custom sSNOM instrument capable of accepting ultrafast THz excitation, and (iii) a MHz repetition rate, femtosecond laser amplifier for broadband THz pulse generation and sensitive detection. We apply the cm-THz-sSNOM to obtain proof of principle measurements of superconductors and topological semimetals. The new capabilities demonstrated break grounds for studying quantum materials that require an extreme environment of cryogenic operation and/or applied magnetic fields in nanometer space, femtosecond time, and THz energy scales.

Ultrafast 3D nanofabrication via digital holography

There has been a compelling demand of fabricating high-resolution complex three-dimensional (3D) structures in nanotechnology. While two-photon lithography (TPL) largely satisfies the need since its introduction, its low writing speed and high cost make it impractical for many large-scale applications. We report a digital holography-based TPL platform that realizes parallel printing with up to 2000 individually programmable laser foci to fabricate complex 3D structures with 90 nm resolution. This effectively improves the fabrication rate to 2,000,000 voxels/sec. The promising result is enabled by the polymerization kinetics under a low-repetition-rate regenerative laser amplifier, where the smallest features are defined via a single laser pulse at 1 kHz. We have fabricated large-scale metastructures and optical devices of up to centimeter-scale to validate the predicted writing speed, resolution, and cost. The results confirm our method provides an effective solution for scaling up TPL for applications beyond laboratory prototyping.

Down-conversion and up-conversion photoluminescence studies of Er3+ doped calcium zinc borosilicate glass for optical fiber amplifiers

A series of Er3+ ion doped calcium zinc borosilicate (CZBS) glass with composition 15ZnO-(10-x)CaO-10SiO2-65B2O3-xEr2O3 (x = 0.1, 0.5, 1.0, 1.5, 2.0 mol%) has been synthesized using conventional melt quenching technique. The non-crystalline nature of the as-synthesized glass is established through X-Ray diffraction (XRD). Fourier Transform Infrared (FT-IR) spectral studies present a full picture of the functional groups present in the glass system. The absorption, up-conversion (UC) and down-conversion (DC) photoluminescence (PL) and corresponding PL decay dynamics, and laser power dependent PL are investigated to better understand the optical properties and energy transfer mechanism in the glass matrices. The radiative properties such as transition probability (AR), branching ratios (βR), radiative lifetimes (τR) and near infrared (NIR) emission quantum efficiency (η) are estimated. The absorption, emission cross-sections and optical gain coefficients reveal the potential applications as laser amplifiers. The results favorably suggest the potential utility of these Er3+ doped CZBS glasses in solid-state lighting and optical communication devices.

Theoretical optimization of cavity configurations for fiber lasers: enhancement of mode oscillation and slow-light effect

This article theoretically investigates the optimization of various fiber Bragg cavities for sustaining laser oscillation and amplification in fiber lasers designed as distributed feedback reflectors and/or distributed Bragg reflectors. Coupled mode theory with the transfer matrix method have been used to obtain reflection and transmission spectra as well as group delay and dispersion for characterizing the proposed cavities. The optimum configuration gives symmetrical spectra with single-mode oscillation in the stop-band manifesting an enhanced slow-light process. Various cavity parameters such as cavity configuration, length, pitch, and structure of the grating are found to have a significant effect on the field confinement, oscillated modes, and dispersion of the spectrum. Cavities formed by phase-shifted FBGs showed better photon confinement, and better reinforcement of single-mode oscillations. By incorporating an extra etalon-like cavity (etalon), uniform spectra with wider stop-band can be manifested and single-mode oscillations with remarkable delay for the slow-light effect are maintained.

57-fs, Er-doped all-fiber all-polarization-maintaining amplifier based on a hybrid DCF/FBG scheme for dispersion compensation

In this paper we experimentally demonstrate a femtosecond all-polarization- maintaining laser amplifier in an all-fiber scheme, which consists of a mode-locked oscillator and a two-stage amplifier. A hybrid method of dispersion compensation is proposed based on a dispersion compensation fiber and two fiber Bragg gratings with different group velocity dispersion. The output pulse from the amplifier is 57 fs with average power of 150 mW and repetition rate of 60 MHz, corresponding to peak power of 38.6 kW. By evaluation of pulse shaping, one third of group velocity dispersion in the amplifier was compensated by the pair of gratings, and the remained amount compensated by the dispersion compensation fiber.

Frequency offset modeling in presence of ASE noise and corresponding low-complexity solution for discrete spectrum modulated nonlinear frequency division multiplexing system

For discrete spectrum modulated nonlinear frequency division multiplexing (DS-NFDM) transmission, the combined effect of laser frequency offset (FO) and amplifier spontaneous emission (ASE) noise would cause a perturbative shift of eigenvalues and corresponding spectral distortion. Moreover, the classical 4th-power feedforward frequency offset estimation (FOE) scheme is not suitable for DS-NFDM system with low baud rate. In this paper, we firstly deduce an FO impairment model with ASE noise in nonlinear Fourier domain (NFD). Meanwhile, we have proposed a training sequence-assisted FOE (TS-FOE) scheme, which effectively reduces the impact of ASE noise. The 2 GBaud DS-NFDM 16QAM simulation results illustrate that this scheme achieves an FOE error within 1 MHz using a training sequence (TS) with length of 64. Besides, its FOE range achieves [−Rs/2, +Rs/2], where Rs denotes the baud rate of signal. Furthermore, the effectiveness of TS-FOE scheme has been verified by 2 GBaud NFDM QPSK experimental system. The results demonstrate that when optical signal-to-noise ratio (OSNR) is set 13 dB, the TS-FOE scheme presents stable FO compensation performance for different FOs within 1 GHz, and the corresponding bit error rates (BERs) are all below the 7% hard decision forward error correction (HD-FEC) threshold. Meanwhile, its complexity is on the order of OL and related with the length L of TS, which is as one thousandth as that of angle search FOE scheme.

Watt-level deep-UV subnanosecond laser system based on Nd-doped fiber at 229 nm.

We report an efficient deep-UV master-oscillator power amplifier (MOPA) laser system at 229 nm that generates 350 ps pulses at 2 MHz repetition rate with an average power of 1.2 W. The use of a polarization-maintaining large mode area neodymium-doped fiber operating on the 4F3/2→4I9/2 transition allows high-power laser emission of up to 28 W near 915 nm in the sub-nanosecond regime with low spectral broadening. Two nonlinear frequency conversion stages (LBO + BBO crystals) in a single-pass configuration directly convert the IR laser emission to deep UV. This laser demonstrates the great potential of Nd3+-doped fiber lasers to produce high-power deep-UV emission.

Influence of defects on the femtosecond laser damage resistance of multilayer dielectric gratings.

Multilayer dielectric (MLD) gratings with high diffraction efficiency and a high laser-induced damage (LID) threshold for pulse compressors are key to scaling the peak and average power of chirped pulse amplification lasers. However, surface defects introduced by manufacturing, storage, and handling processes can reduce the LID resistance of MLD gratings and impact the laser output. The underlying mechanisms of such defect-initiated LID remain unclear, especially in the femtosecond regime. In this Letter, we model dynamic processes in interactions of a 20-fs near-infrared (NIR) laser pulse and a MLD grating design in the presence of cylindrically symmetrical nodules and particle contaminants and cracks at the surface. Utilizing a dynamic model based on a 2D finite difference in time domain (FDTD) field solver coupled with photoionization, electron collision, and refractive index modification, we study the simulation results for the damage site distribution initiated by defects of various types and sizes and its impact on the LID threshold of the grating design.

Gain recovery dynamics in active type-II semiconductor heterostructures

Type-II heterostructures as active layers for semiconductor laser devices combine the advantages of a spectrally broad, temperature stable, and efficient gain with the potential for electrical injection pumping. Their intrinsic charge carrier relaxation dynamics limit the maximum achievable repetition rates beyond any constraints of cavity design or heat dissipation. Of particular interest are the initial build up of gain after high-energy injection and the gain recovery dynamics following depletion through a stimulated emission process. The latter simulates the operation condition of a pulsed laser or semiconductor optical amplifier. An optical pump pulse injects hot charge carriers that eventually build up broad spectral gain in a model (Ga,In)As/GaAs/Ga(As,Sb) heterostructure. The surplus energies of the optical pump mimic the electron energies typical for electrical injection. Subsequently, a second laser pulse tuned to the broad spectral gain region depletes the population inversion through stimulated emission. The spectrally resolved nonlinear transmission dynamics reveal gain recovery times as fast as 5 ps. These data define the intrinsic limit for the highest laser repetition rate possible with this material system in the range of 100 GHz. The experimental results are analyzed using a microscopic many-body theory identifying the origins of the broad gain spectrum.

High Power Ytterbium-Doped Fiber Lasers Employing Longitudinal Vary Core Diameter Active Fibers

Thanks to the advantage of balancing nonlinear effects and transverse mode instability, vary core diameter active fiber (VCAF) has been widely used in high power ytterbium-doped fiber lasers in recent years. Up to now, VCAF has developed from the basic form of the original tapered fiber to the spindle-shaped and saddle-shaped fiber with different characteristics and has been applied in conventional fiber lasers, oscillating–amplifying integrated fiber lasers, and quasi-continuous wave fiber lasers and successfully improved the performance of these lasers. In the present study, a 6110 W fiber laser amplifier is realized based on a tapered fiber. The maximum output power of a fiber laser amplifier based on spindle-shaped fibers is 6020 W with a beam quality of M2~1.86. In this paper, we first introduce the basic concept of VCAF and summarize its main fabrication methods and advantages in high-power fiber laser applications. Then, we will present the recent research results of high-power fiber laser employing VCAF in our group and clarify the outstanding advantages of VCAF compared with the constant core diameter active fiber (CCAF).

Spectrum-tailored random fiber laser towards ICF laser facility

Broadband low-coherence light is considered to be an effective way to suppress laser plasma instability. Recent studies have demonstrated the ability of low-coherence laser facilities to reduce back-scattering during beam–target coupling. However, to ensure simultaneous low coherence and high energy, complex spectral modulation methods and amplification routes have to be adopted. In this work, we propose the use of a random fiber laser (RFL) as the seed source. The spectral features of this RFL can be carefully tailored to provide a good match with the gain characteristics of the laser amplification medium, thus enabling efficient amplification while maintaining low coherence. First, a theoretical model is constructed to give a comprehensive description of the output characteristics of the spectrum-tailored RFL, after which the designed RFL is experimentally realized as a seed source. Through precise pulse shaping and efficient regenerative amplification, a shaped random laser pulse output of 28 mJ is obtained, which is the first random laser system with megawatt-class peak power that is able to achieve low coherence and efficient spectrum-conformal regenerative amplification.

The Effect of Cooling Layer Thickness and Coolant Velocity on Crystal Thermodynamic Properties in a Laser Amplifier.

Laser diode pumped solid-state lasers (DPSSLs) have been widely used in many fields, and their thermal effects have attracted more and more attention. The laser diode (LD) side-pumped amplifier, as a key component of DPSSLs, is necessary for effective heat dissipation. In this paper, instead of the common thermal analysis based only on a crystal rod model, a fluid-structure interaction model including a glass tube, cooling channel, coolant and crystal rod is established in numerical simulation using ANSYS FLUENT for the configuration of an LD array side-pumped laser amplifier. The relationships between cooling layer thickness, coolant velocity and maximum temperature, maximum equivalent stress, inlet pressure and the convective heat transfer coefficient are analyzed. The results show that the maximum temperature (or maximum equivalent stress) decreases with the increase in the coolant velocity; at low velocity, a larger cooling layer thickness with more coolant is not conductive enough for improved heat dissipation of the crystal rod; at high velocity, when the cooling layer thickness is above or below 1.5 mm, the influence of the cooling layer thickness on the maximum temperature can be ignored; and the effect of the cooling layer thickness on the maximum equivalent stress at high velocity is not very significant. The comprehensive influence of various factors should be fully considered in the design process, and this study provides an important reference for the design and optimization of a laser amplifier and DPSSL system.

Compact laser modulation system for a transportable atomic gravimeter.

Nowadays, atom-based quantum sensors are leaving the laboratory towards field applications requiring compact and robust laser systems. Here we describe the realization of a compact laser system for atomic gravimetry. Starting with a single diode laser operating at 780 nm and adding only one fiber electro-optical modulator, one acousto-optical modulator and one laser amplifier we produce laser beams at all the frequencies required for a Rb-87 atomic gravimeter. Furthermore, we demonstrate that an atomic fountain configuration can also be implemented with our laser system. The modulated system reported here represents a substantial advance in the simplification of the laser source for transportable atom-based quantum sensors that can be adapted to other sensors such as atomic clocks, accelerometers, gyroscopes or magnetometers with minor modifications.

40 W high beam quality Tm:YLF InnoSlab amplifier

We demonstrated a compact and excellent beam quality Tm:YLF InnoSlab amplifier at 1908 nm for the first time. The power of the seed laser was 10 W with beam quality factors of M 2 ⩽ 1.04. With the high-efficiency coupling of the pump light and the seed laser, the amplified power of 40.1 W was achieved under the pump power of 318.7 W without ASE. The beam quality factors M 2 were 1.07 and 1.37 in the vertical and horizontal directions, respectively. The RMS instability was measured to be less than 0.15% in 30 min. Meantime, we had established a general and simplified CW Tm:YLF laser amplifier model.

Up-sampling of electron beam simulation particles with addition of shot-noise

An algorithm and numerical code for the up-sampling of a system of particles, from a smaller to a larger number, is described. The method introduces a Poissonian ‘shot-noise’ to the up-sampled distribution, typical of the noise statistics arising in a bunch of particles generated by a particle accelerator. The algorithm is applied to a 6-Dimensional phase-space distribution of relatively few simulation particles, representing an electron beam generated by particle accelerator modelling software, for subsequent injection into an Free Electron Laser (FEL) amplifier which is used here to describe the model. A much larger number of particles is usually required to model the FEL lasing process than is required to model the electron beam accelerators that drive it. A numerical code developed from the algorithm was then used to generate electron bunches for injection into to an unaveraged 3D FEL simulation code, Puffin. Results show good qualitative and quantitative agreement with analytical theory. The program and user manual is available for download.

Evolution of orbital angular momentum spectrum of broadband Laguerre–Gaussian beam in OPCPA process

In this study, we numerically simulate the evolution of the orbital angular momentum (OAM) spectrum of a vortex laser beam in the optical parametric chirped pulse amplification (OPCPA) process, which is an effective technical method to realize ultra-intense and ultra-short vortex laser amplification. The results show that the proportion of the vortex laser beam with 100% topological charge (TC) of 1 decreases to 97.44% with the enhancement of the saturation amplification after amplification by a 15 mm length LBO pumped by a 526.5 nm laser with a pump intensity of 1.74 GW/cm2. Conversely, the beams with other topological charges generate and increase with the amplification. The simulation results are consistent with our previous experimental results. Meanwhile, compared with non-collinear OPCPA, collinear OPCPA can maintain well the proportion of TC l = 1.

Rapid-Scan Nonlinear Time-Resolved Spectroscopy over Arbitrary Delay Intervals

Femtosecond dual-comb lasers have revolutionized linear Fourier-domain spectroscopy by offering a rapid motion-free, precise, and accurate measurement mode with easy registration of the combs beat note in the radio frequency domain. Extensions of this technique already found application for nonlinear time-resolved spectroscopy within the energy limit available from sources operating at the full oscillator repetition rate. Here, we present a technique based on time filtering of femtosecond frequency combs by pulse gating in a laser amplifier. This gives the required boost to the pulse energy and provides the flexibility to engineer pairs of arbitrarily delayed wavelength-tunable pulses for pump–probe techniques. Using a dual-channel millijoule amplifier, we demonstrate programmable generation of both extremely short, fs, and extremely long (>ns) interpulse delays. A predetermined arbitrarily chosen interpulse delay can be directly realized in each successive amplifier shot, eliminating the massive waiting time required to alter the delay setting by means of an optomechanical line or an asynchronous scan of 2 free-running oscillators. We confirm the versatility of this delay generation method by measuring χ (2) cross-correlation and χ (3) multicomponent population recovery kinetics.