Laser Amplifier Research Articles

Femtosecond laser activation of the photochemistry of bismuth and associated three-dimensional sub-micron fluorescence patterning

Localized sub-micrometer-scale visible and near-IR fluorescence structures have been achieved in three-dimension (3D) thanks to femtosecond laser-activated photochemistry of Bismuth in a Bismuth-doped phosphate glass. These structures exhibit high fluorescence contrast with good spatial resolution with spectral emission in the red and near-IR ranges. These fluorescence properties arise from femtosecond laser-induced multi-photon absorption and the creation of free electrons, which activates local redox reactions of Bi3+ that allow for the dose-dependent formation of low valence Bismuth ions such as Bi2+ and Bi+. These new species promote not only new selective fluorescence emission properties in the VIS and near-IR spectral range but also co-localized positive refractive index changes. The thermal treatment of 3D patterns evidences the possibility of Bismuth cluster formation with good thermal stability up to the glass temperature transition. These observations open the way for laser-inscribed photonic integrated circuits for potential laser amplification devices in the second telecommunication windows around 1.3 μm.

Cpa-free Yb:YAG thin-disk regenerative amplifier laser with hundreds of watts, hundreds of kHz, and hundreds of picoseconds

This paper introduces a CPA-free Yb:YAG thin-disk regenerative amplifier laser system. The seed pulse is generated by a passive mode-locked picosecond oscillator based on a semiconductor saturable absorber mirror, with a center wavelength of 1030.7 nm, a pulse width of 7.7 ps, and a repetition frequency of 26.3 MHz. After stretched and pre-amplified by an all-fiber front stage, the seed with a pulse width of 201.3 ps and a pulse energy of 15.2 nJ was obtained, then injected into the thin-disk regenerative amplifier. Yb:YAG crystal was chosen as the material for the thin-disk, with 100 μm thickness, 9 mm diameter and 7 at.% doping rate. In order to increase the one-way round-trip gain of the thin-disk regenerative amplifier, a symmetrical dual-pass resonant cavity was designed to double the number of passing through the crystal on a round trip in the regenerative resonant cavity to reduce the times of cycles, intra-cavity loss and improve optical efficiency. We used a 969 nm fiber-coupled semiconductor laser as the pump source to pump the Yb:YAG thin-disk crystal. With zero-phonon line pumping technology, the quantum loss and thermal effect were reduced. In the end, we obtained a regenerative amplification output with an average power of 104.5 W, a repetition frequency of 200 kHz, a pulse width of 143.9 ps, and a spectrum width of 0.39 nm. The amplified pulse had quite good beam quality with Mx2 = 1.09 and My2 = 1.14.

2 × 4 kW near-single-mode laser output assisted by an optimized bidirectional oscillating-amplifying integrated fiber laser configuration

Bidirectional output oscillating-amplifying integrated fiber laser (B-OAIFL) can achieve the two-ports laser amplification based on a single cavity, showcasing a promising prospect. In order to improve both the laser power and beam quality, we first simulate and optimize the stimulated Raman scattering (SRS) effect in the B-OAIFL. The simulation results show the SRS effect can be suppressed by optimizing the diameter as well as the length of the active fiber at different locations. With the guidance of theoretical and experimental analysis for the combined suppression of SRS and transverse mode instability (TMI), a near-single-mode B-OAIFL with 2 × 4 kW was demonstrated. Based on this foundation, we further devoted ourselves to the pursuit of the optimization of the structure and performance. The necessity of the configuration of side pump, which was initially introduced for its exceptional performance in stabilizing temporal chaos, was reevaluated in detail. With its negative impacts on efficiency improvement and SRS suppression were analyzed and verified, we removed this configuration and finally demonstrated a more simplified design with superior performance. A total bidirectional output of 8105 W was achieved, with an O-O efficiency of 79.6% and a near-single-mode beam quality of M A 2∼1.36,M B 2∼1.63. No signs of TMI were observed, and the signal-to-SRS suppression ratio was over 38 dB. The results still demonstrate a promising potential for power scaling based on this configuration and parameters.

Internal phase control of fiber laser array based on photodetector array

Abstract Coherent beam combining (CBC) of fiber laser array is a promising technique to realize high output power while maintaining near diffraction-limited beam quality. To implement CBC, an appropriate phase control feedback structure should be established to realize phase-locking. In this paper, an innovative internal active phase control CBC fiber laser array based on photodetector array is proposed. The dynamic phase noises of the laser amplifiers are compensated before being emitted into free space. And the static phase difference compensation of emitting laser array is realized by interference measurement based on photodetector array. The principle of the technique is illustrated and corresponding simulations are carried out, and a CBC system with four laser channels is built to verify the technique. When the phase controllers are turned on, the phase deviation of the laser array is less than λ/20, and ∼ 95% fringe contrast of the irradiation distribution is obtained. The technique proposed in this paper could provide a reference for the system design of a massive high-power CBC system.

A new longitudinal pumping scheme that uses elliptical laser beams to improve the compactness and efficiency of high-power dye lasers

Liquid dye lasers are intrinsically suitable for high-average-power operation because their flowing laser gain media can effectively remove the heat produced by laser amplification; thus, liquid dye lasers have a significant place in various laser applications. However, to sufficiently remove the dye solution heated by the laser, a very cumbersome dye flow system is required to achieve a high flow velocity in the dye cell of a high-power dye laser, which limits its applications. In this paper, we found that the pressure and power required to drive the high-speed dye flow are both proportional to the square of the laser beam size along the direction of the flow. Based on this result, we proposed a novel longitudinal pumping scheme that uses elliptical laser beams. This new scheme can significantly reduce the size and energy consumption of dye flow loops in high-power dye lasers. We also developed a dye laser system with a main amplifier using a 1 mm × 4 mm elliptical input laser beam. The amplifier produced an average power of 135 W with a repetition rate of 10 kHz and a conversion efficiency of 39.5%. The M2 factor of the output beam was better than 1.7. The flow parameters to drive the dye flow loops were 2.1×105 Pa, and 0.43 L/s, which gave a clearing ratio of 2 for the amplifier. This new pumping scheme enables high-power dye lasers to be more compact and efficient, making them more flexible for various applications.

Intra-oscillator high harmonic source reaching 100-eV photon energy

Resonant enhancement inside an optical cavity has been a wide-spread approach to increase efficiency of nonlinear optical conversion processes while reducing the demands on the driving laser power. This concept has been particularly important for high harmonic generation XUV sources, where passive femtosecond enhancement cavities allowed significant increase in repetition rates required for applications in photoelectron spectroscopy, XUV frequency comb spectroscopy, including the recent endeavor of thorium nuclear clock development. In addition to passive cavities, it has been shown that comparable driving conditions can be achieved inside mode-locked thin-disk laser oscillators, offering a simplified single-stage alternative. This approach is less sensitive to losses thanks to the presence of gain inside the cavity and should thus allow higher conversion efficiencies through tolerating higher intensity in the gas target. Here, we show that the intra-oscillator approach can indeed surpass the much more mature technology of passive enhancement cavities in terms of XUV flux, even reaching comparable values to single-pass sources based on chirped-pulse fiber amplifier lasers. Our system operates at 17 MHz repetition rate generating photon energies between 60 eV and 100 eV. Importantly, this covers the highly attractive wavelength for the silicon industry of 13.5 nm at which our source delivers 60 nW of outcoupled average power per harmonic order.

Measurement of Group Delay Ripples of Chirped Fiber Bragg Gratings for CPA Lasers, and Their Effect on Performance

The deleterious effect of group delay ripples (GDR) on the performance of a chirped fiber Bragg grating used as a stretcher in a chirped pulse amplification (CPA) laser is analyzed through simulations of CFBGs with various amounts of noise. We show that GDR with a standard deviation of less than one-half the transform-limited pulse duration are required for consistent good performance. We furthermore describe a simple method to measure the group delay response of such CFBGs written in polarization-maintaining fiber, using the beat spectrum of the reflections from the two polarization axes after passing through a polarizer. The method can be used to extract GDR, as well as the phase response of the CFBG, which is used to predict the pulse recompression performance of a CPA laser. The method is theoretically described, and we show that despite limitations on its spatial resolution, it can capture the most deleterious GDR. Experimental measurements of GDR as low as 161 fs in an actual CFBG are demonstrated using our method, indicating a resolution better than 50 fs and very good reproducibility, with pulse recompression performance in agreement with the measurement prediction.

Acousto-optic holography for pseudo-two-dimensional dynamic light patterning.

Optical systems use acousto-optic deflectors (AODs) mostly for fast angular scanning and spectral filtering of laser beams. However, AODs may transform laser light in much broader ways. When time-locked to the pulsing of low repetition rate laser amplifiers, AODs permit the holographic reconstruction of 1D and pseudo-two-dimensional (ps2D) intensity objects of rectangular shape by controlling the amplitude and phase of the light field at high (20-200kHz) rates for microscopic light patterning. Using iterative Fourier transformations (IFTs), we searched for AOD-compatible holograms to reconstruct the given ps2D target patterns through either phase-only or complex light field modulation. We previously showed that phase-only holograms can adequately render grid-like patterns of diffraction-limited points with non-overlapping diffraction orders, while side lobes to the target pattern can be cured with an apodization mask. Dense target patterns, in contrast, are typically encumbered by apodization-resistant speckle noise. Here, we show the denoised rendering of dense ps2D objects by complex acousto-optic holograms deriving from simultaneous optimization of the amplitude and phase of the light field. Target patterns lacking ps2D symmetry, although not translatable into single holograms, were accessed by serial holography based on a segregation into ps2D-compatible components. The holograms retrieved under different regularizations were experimentally validated in an AOD random-access microscope. IFT regularizations characterized in this work extend the versatility of acousto-optic holography for fast dynamic light patterning.

1.2 kW, 20 kHz Nanosecond Nd:YAG Slab Laser System

In this paper, we develop a kW-level high-repetition-rate nanosecond master oscillator power amplifier (MOPA) laser system, employing a structure of fiber, Nd:YVO4, and Nd:YAG hybrid amplification. A tunable fiber seed source is used for adjustable pulse repetition frequency and pulse width. The Nd:YVO4 pre-amplifier, which is dual-end-pumped, achieves high gain while maintaining good beam quality, and the high-power side-pumped Nd:YAG slab main-amplifier enables efficient power amplification. The repetition rate of the output laser can be adjusted within the range of 1~20 kHz, and the pulse width can be tuned within the range of 10~300 ns. The seed output is 6 mW at a repetition frequency of 20 kHz; we achieve an average output power of 1240 W with a total power extraction efficiency of 39.1% and single-pulse energy of 62 mJ at a pulse width of 301 ns. This parameter-controllable high-power laser holds promise for applications in the laser cleaning of complex surface contaminants.

Self-injection-locked thin-film regenerative laser amplifier

Organic lasers based on distributed feedback (DFB) microcavities have been extensively investigated. However, the application of these lasers is limited by their low output power and large beam divergence. Therefore, laser amplifiers are needed to achieve practically applicable laser intensity and controllable lasing modes for far-field applications. In this work, we report self-injection-locked laser amplifiers using the combination of a DFB microcavity and a Bragg reflector, where a high-reflection mirror acts as the Bragg reflector and its feedback supplies the external-cavity injection. The coherent coupling between the DFB microcavity and the Bragg amplifier is crucial for achieving high conversion efficiency and high-contrast transverse modes. An amplification factor larger than 20 and a single output laser spot with high contrast that has been achieved. Such an integration design of the self-injected DFB microcavity amplifier can be directly utilized in the realization of high-performance thin-film laser sources for practical applications.

Transient pump-probe analysis of pump-induced lensing in laser amplifiers.

Understanding of pump-induced lensing in laser amplifiers is essential for the optimized design of high-power lasers with high spatial quality, but there is usually incomplete knowledge of the interplay between thermal and population induced lensing mechanisms, lensing under lasing and non-lasing conditions, and transient lensing effects under pulsed operation. This paper provides quantitative insight of pump-induced lensing effects by using experimental transient pump-probe measurements in an alexandrite laser amplifier end-pumped by a short pulse pump beam with Gaussian spatial intensity distribution. Lensing results are presented showing a large difference in lensing under both non-lasing and lasing conditions and distinction of the population lens and thermal lens contributions from their different response time. Different pump beam sizes are used to show the variation of the relative strength of the lensing mechanisms. Comparison of experimental results with the analytical transient theory developed in this paper for the Gaussian pump beam gives excellent agreement and quantitative information on the thermal and population contributions to the amplifier lens. This paper provides a methodology for quantitative investigation of pump-induced lensing in general laser amplifier systems, and potentially other classes of optical materials with residual optical absorption.

Thermal Birefringence-Based beam shaping in high energy lasers

Repetition-rate high-energy laser systems with high beam quality have been attracting growing attentions in various applications. However, the degradation of beam quality caused by thermal effects is nearly unavoidable in high-energy laser amplifiers operating at high repetition rates. One primary thermal effect is thermal-induced birefringence, which typically induces polarization changes at various spatial positions within the beam. This study explores the potential of leveraging the thermal-induced birefringence effect to manipulate the polarization distribution of the beam and, consequently, control the spatial intensity distribution of the beam by using an additional polarizer. This method demonstrates promise for beam pre-compensation, mitigating thermal-induced depolarization, and other laser applications in high-energy lasers with high repetition rates.

Demonstration of a 1 TW peak power, joule-level ultrashort Tm:YLF laser

We report on the demonstration of a diode-pumped, Tm:YLF-based, chirped pulse amplification laser system operating at λ ≈ 1.9 µm that produces amplified pulse energies exceeding 1.5 J using a single 8-pass power amplifier. The amplified pulses are subsequently compressed to sub-300 fs durations by a diffraction grating pair, producing record >1 TW peak power pulses. To the best of our knowledge, this is the highest peak power demonstrated for any solid-state, near-2 µm laser architecture and illustrates the potential of Tm:YLF for the next generation of high-power, diode-pumped ultrashort lasers.

Versatile design for temporal shape control of high-power nanosecond pulsed fiber laser amplifier.

This research proposed a novel pulse-shaping design for directly shaping distorted pulses after the amplification. Based on the principle of the design we made a pulse shaper. With this pulse shaper, we successfully manipulate the pulse's leading edge and width to achieve an 'M'-shaped waveform in an amplification system. Comparative experiments were conducted within this system to compare the output with and without the integration of the pulse shaper. The results show a significant suppression of the nonlinear effect upon adding the pulse shaper. This flexible and effective pulse shaper can be easily integrated into a high-power all-fiber system, supplying the capability to realize the desired output waveform and enhance the spectral quality.

High fidelity amplification of radially-polarized laser in few-mode polarization-maintaining fiber amplifier

A high-power 20/400-μm few-mode polarization-maintaining (PM) fiber amplifier with a radially polarized output beam is firstly demonstrated and investigated. The reconstructed beam profiles and mode contents of the output laser were obtained using a numerical method. Experimentally, a home-made fiberized mode convertor was employed to convert the LP01 mode to be TM01 mode (radially polarized beam) with an average power of 2 W. In addition, the mode purity and polarization purity were 98.76%, and 98.5%, respectively. Based on a few-mode PM fiber amplifier, instead of the commonly used non-PM fiber amplifier, the radially polarized beam was scaled up to 192.1 W, while preserving the donut-shaped intensity distribution. The corresponding mode purity and polarization purity were measured to be approximately 97% and 98.2%, respectively, which indicates the polarization characteristic of the TM01 mode was preserved well. The beam quality factor (M2) was measured to be ∼2.2. Additionally, long-term stability in terms of both output power and polarization state were also verified experimentally. To the best our knowledge, this is the highest output power from a few-mode PM fiber amplifier laser system, while maintaining a high mode purity and a high polarization purity for radially polarized beam.

Applications of Microstructured Optical Fibers in Ultrafast Optics: A Review

With the development of laser technology, microstructured optical fibers (MOFs) have become an important part of ultrafast optics, providing excellent platforms for ultrafast laser pulse generation, amplification, and compression, promoting the development of fiber laser systems to generate high power, high pulse energy, and few-cycle duration pulses. MOFs extend the ultrafast laser spectrum to the vacuum ultraviolet (VUV) and even extreme ultraviolet (EUV) regions based on dispersive wave emission and high harmonic generation, as well as to the mid-infrared region based on soliton self-frequency shift (SSFS), contributing compact and low-cost light sources for precision microscopy and spectroscopy. In this paper, first several common types of MOFs are introduced, then the various applications of MOFs in ultrafast optics are discussed, mainly focusing on the aspects of ultrafast laser pulse scaling in pulse energy and spectral bandwidth, and finally the possible prospects of MOFs are given.

Simultaneous Two- and Three-Photon Deep Imaging of Autofluorescence in Bacterial Communities.

The intrinsic fluorescence of bacterial samples has a proven potential for label-free bacterial characterization, monitoring bacterial metabolic functions, and as a mechanism for tracking the transport of relevant components through vesicles. The reduced scattering and axial confinement of the excitation offered by multiphoton imaging can be used to overcome some of the limitations of single-photon excitation (e.g., scattering and out-of-plane photobleaching) to the imaging of bacterial communities. In this work, we demonstrate in vivo multi-photon microscopy imaging of Streptomyces bacterial communities, based on the excitation of blue endogenous fluorophores, using an ultrafast Yb-fiber laser amplifier. Its parameters, such as the pulse energy, duration, wavelength, and repetition rate, enable in vivo multicolor imaging with a single source through the simultaneous two- and three-photon excitation of different fluorophores. Three-photon excitation at 1040 nm allows fluorophores with blue and green emission spectra to be addressed (and their corresponding ultraviolet and blue single-photon excitation wavelengths, respectively), and two-photon excitation at the same wavelength allows fluorophores with yellow, orange, or red emission spectra to be addressed (and their corresponding green, yellow, and orange single-photon excitation wavelengths). We demonstrate that three-photon excitation allows imaging over a depth range of more than 6 effective attenuation lengths to take place, corresponding to an 800 micrometer depth of imaging, in samples with a high density of fluorescent structures.

Diode-pumped high-peak-power nanosecond Nd,Y: SrF2 regenerative laser amplifier

We demonstrate a regenerative laser amplifier based on laser-diode-pumped Nd, Y:SrF2 crystals with an output of approximately 9.78 mJ/10 Hz. A specially designed birefringent filter was inserted into the cavity to compensate the gain narrowing and amplify the seed pulse with a center wavelength of 1053.3 nm. To optimize the design of the resonator, the thermal wavefront of the laser gain module was simulated and experimentally measured. When a laser pulse with a duration of 9 ns and energy of 1 nJ was injected into the regenerative amplifier, the output energy was 9.78 mJ with peak power of 1.1 MW. The energy stability was 0.72 % RMS. The horizontal and vertical beam quality M2 was 1.18 and 1.23 respectively. The results show the potential of the developed amplifier for application in high-power laser facilities.

Bright continuously tunable vacuum ultraviolet source for ultrafast spectroscopy

Ultrafast electron dynamics drive phenomena such as photochemical reactions, catalysis, and light harvesting. To capture such dynamics in real-time, femtosecond to attosecond light sources are extensively used. However, an exact match between the excitation photon energy and a characteristic resonance is crucial. High-harmonic generation sources are advantageous in terms of pulse duration but limited in spectral tunability in the vacuum ultraviolet range. Here, we present a monochromatic femtosecond source continuously tunable around 21 eV photon energy utilizing the second harmonic of an optical parametric chirped pulse amplification laser system to drive high-harmonic generation. The unique tunability of the source is verified in an experiment probing the interatomic Coulombic decay in doped He nanodroplets across the He absorption bands. Moreover, we achieved intensities sufficient for driving collective processes in multiply excited helium nanodroplets, which have been previously observed only at free electron lasers.

Self-channeling of a multi-Joule 10 µm picosecond pulse train through long distances in air.

In the long-wave infrared (LWIR) range, where, due to wavelength scaling, the critical power of Kerr self-focusing Pcr in air increases to 300-400 GW, we demonstrate that without external focusing a train of picosecond CO2 laser pulses can propagate in the form of a single several-centimeter diameter channel over hundreds of meters. The train of 10 µm pulses, for which the total energy ≥20 J is distributed over several near-terawatt picosecond pulses with a maximum power ≤2Pcr, is generated naturally during short pulse amplification in a CO2 laser. It is observed that the high-power 10 µm beam forms a large diameter "hot gas" channel in the ambient air with a ≥ 50 ms lifetime. Simulations of the experiment show that such filamentation-free self-channeling regime has low propagation losses and can deliver multi-Joule/TW-power LWIR pulses over km-scale distances.