Ultrafast Amplifier Research Articles

A novel nanoplasmonic-based diagnosis platform: Advances and emerging technologies

The development of a rapid, simple, inexpensive, and sensitive nucleic acid assay is critical in ultrafast diagnostic systems to prevent the rapid spread of novel infectious agents. However, current representative diagnostic methods, such as the polymerase chain reaction (PCR), are bulky, expensive, complex, and time-consuming. The plasmonic photothermal effect of plasmonic materials is widely used in biological applications because of excellent properties, such as strong optical properties and enhanced local electromagnetic field. PCR methods using plasmonic nanostructures are used to overcome the limitations of conventional PCR. Therefore, plasmonic photothermal-based PCR exhibit a rapid ramping rate, ultrafast amplification, high sensitivity, cost-effectiveness, and compactness, which can be used as a point-of-care device in situ. This study reviewed the mechanisms of plasmonic PCR and various materials, such as organic, inorganic, and hybrid materials, used in PCR. Furthermore, the properties, light source, amplification efficiency, and monitoring method of classified plasmonic materials were compared. This review introduced the disease diagnosis application using plasmonic PCR and evaluated the prospect of potential future applications as next-generation PCR.

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.

Nonlinear pulse compression of a 200 mJ and 1 kW ultrafast thin-disk amplifier.

We present a high-energy laser source consisting of an ultrafast thin-disk amplifier followed by a nonlinear compression stage. At a repetition rate of 5 kHz, the drive laser provides a pulse energy of up to 200 mJ with a pulse duration below 500 fs. Nonlinear broadening is implemented inside a Herriott-type multipass cell purged with noble gas, allowing us to operate under different seeding conditions. Firstly, the nonlinear broadening of 64 mJ pulses is demonstrated in an argon-filled cell, showing a compressibility down to 32 fs. Finally, we employ helium as a nonlinear medium to increase the energy up to 200 mJ while maintaining compressibility below 50 fs. Such high-energy pulses with sub-50 fs duration hold great promise as drivers of secondary sources.

Sophisticated and precise: design and implementation of a real-time optical detection system for ultra-fast PCR.

Quantitative Real-Time Polymerase Chain Reaction (qRT-PCR) has become indispensable in the realm of disease nucleic acid screening and diagnostics, owing to its remarkable precision and sensitivity, in which the real-time fluorescence detection system plays an extremely critical role. To solve the problems of long time and slow speed of traditional nucleic acid detection, PCR systems are evolving towards ultra-rapid configurations. Nonetheless, most extant ultra-rapid PCR systems either depend on endpoint detection for qualitative assessments due to inherent structural or heating constraints or circumvent the challenge of adapting optical systems to expeditious amplification systems, resulting in potential shortcomings in assay efficacy, volume, or expense. Consequently, this study proposed a design of a real-time fluorescence detection system for ultra-fast PCR, capable of executing six channels of real-time fluorescence detection. Through the meticulous calculation of the optical pathway within the optical detection module, effective regulation of system dimensions and the cost was accomplished. By devising an optical adaptation module, the signal-to-noise ratio was enhanced by approximately 307% without compromising the PCR temperature alteration rate. Ultimately, by employing a fluorescence model that accounted for the spatial attenuation effect of excitation light, as proposed herein, fluorescent dyes were arranged to evaluate the repeatability, channel interference, gradient linearity, and limit of detection of the system, which proved that the system had good optical detection performance. Finally, the real-time fluorescence detection of human cytomegalovirus (CMV) under 9 min ultra-fast amplification was achieved by a complete ultra-fast amplification experiment, which further validated the potential of the system to be applied to rapid clinical nucleic acid detection.

Modeling of mode-locked fiber lasers and amplifiers with accurate saturable gain evaluation

We present a numerical model for mode-locked lasers and ultrafast nonlinear amplifiers in which the saturated gain profiles along active fibers are judiciously computed using experimental pump powers as input. This eliminates the need for approximating the gain profile by using a small-signal gain coefficient and saturation energy to simulate pulse propagation in active fibers. Our model shows good agreement with experiments involving mode-locked cavities at 1 µm with a silica glass host doped with ytterbium ions. Accurate results are also obtained for continuous-wave and mode-locked laser cavities around 2.8 µm, which uses ZBLAN fiber doped with erbium ions. In the case of Er:ZBLAN fiber, we use the model to show regions of stable mode-locking delivering a single pulse as output and how the spectral width changes with variation in doping concentration and fiber lengths. Our model enables accurate numerical modeling of mode-locked fiber lasers and ultrafast amplifiers, and can be useful in guiding the design of new architectures, understanding complex intracavity laser dynamics, and optimizing device output.

Experimental Investigation of Ultrafast Yb:fiber Oscillators Based on Several Dominant Mode Locking Methods

To find out which kind of mode-locked fiber oscillator is superior, six kinds of typical mode-locked Yb:fiber oscillators, including nonlinear polarization evolution (NPE)-based oscillators (i.e. conventional NPE-based oscillator, polarization maintaining (PM) NPE-based oscillator), nonlinear amplifying loop mirror (NALM)-based oscillators (i.e. figure-8 oscillator, all-fiber figure-9 oscillator, free-space figure-9 oscillator), and semiconductor saturable absorption mirror (SESAM)-based oscillator, were presented and crosswise compared. Except for the conventional NPE-based one, all the others were constructed with all-PM fiber components. Experimentally, detailed crosswise comparisons were carried out quantitatively and qualitatively for different oscillators in terms of spectral bandwidth, spectral shape, pulse repetition rate, signal-to-noise ratio (SNR), mode-locking threshold, self-starting mode-locking pump power, robustness, component life, construction simplicity, and so on. In addition, subjective sensations on each oscillator were also provided, which could be helpful for decision-making when one wants to make or purchase an oscillator. Finally, the all-fiber figure-9 oscillator was suggested to be the best choice even it needs higher pumping power, which could be an ideal seed for high power ultrafast laser amplifiers.

Advantages of YLF host over YAG in power scaling at cryogenic temperatures: direct comparison of Yb-doped systems

We have investigated the cryogenic performance of Yb:YAG and Yb:YLF crystals in rod-geometry to understand the pros and cons of each material for the development of ultrafast lasers and amplifier systems. We have performed detailed spectroscopic (absorption, emission, lifetime), temperature, lasing, and thermal-lens measurements with Yb:YLF and Yb:YAG crystals under almost identical conditions. Our analysis has shown that despite the higher thermal conductivity of Yb:YAG, due to its smaller quantum defect, the peak/average temperatures reached under similar pumping conditions is lower in Yb:YLF crystals. Moreover, since the YLF host has a negative thermo-optic coefficient, that balances other positive contributions to thermal lensing, overall Yb:YLF rods possess a much weaker thermal lens than Yb:YAG under similar conditions. As a result of these benefits, we have shown that Yb:YLF rods perform better than Yb:YAG in cryogenic lasing experiments in terms of attainable power performance and laser output beam quality. In terms of gain per pass, the Yb:YAG medium is superior, however, the gain bandwidth is much broader in Yb:YLF systems that make it more suitable for ultrafast pulse laser/amplifier development. We have further shown that, the asymmetric thermal lens behavior of Yb:YLF favors laser operation in E//c axis over E//a axis. The comparison in this study has been performed in rod geometry and for Yb-doping, however, we believe that, to first order, the discussion could be extended to YAG/YLF laser systems doped with other ions (Pr, Nd, Er, Tm, Ho) and to other lasing geometries such as slab and thin-disk.

High-average-power ultrafast laser amplifier with along-the-side end pumping

An approach to develop laser amplifiers is proposed based on the use of standard rod active elements with end diode pumping, the beam size of which is significantly smaller than the element cross section, and the beam center is shifted maximally close to the element cooled side surface. This approach allows effective gain and good thermal management, and offers technological simplicity. On the basis of this approach, an ultrafast Yb:YAG amplifier with > 100 W average power, reasonable depolarization, and good beam quality is developed.

High-efficiency 940- and 969-nm brightness-maintaining wavelength-multiplexed LD-pumped 240-W thin-rod Yb:YAG amplifier

A high-efficiency ultrafast laser amplifier based on thin-rod Yb:YAG was demonstrated, featuring a 940- and 969-nm brightness-maintaining wavelength-multiplexed laser diode (LD)-pumping method. Two high-brightness LDs (940 nm and 969 nm) were spectrally combined into one beam spatially with a dichroic mirror, thus enabling twice pump power while maintaining high brightness. A maximum signal power of 240 W was obtained at a repetition rate of 1 MHz, with a power extraction efficiency (PEE) of ∼51%. To the best of the authors' knowledge, this is the first report of >50% efficiency as well as the highest average power operating at the fundamental mode for thin-rod Yb:YAG amplifiers. The beam quality factors (M2) of the amplified signal were measured to be 1.72 and 1.12 for the horizontal and vertical directions, respectively. A preliminary pulse compression was conducted at a signal power of 80 W with a chirped volume Bragg grating (CVBG) compressor. The compressed pulse duration was 744 fs with an average power of 66.5 W, corresponding to a compression efficiency of 83.1%.

All-fiber ultrafast amplifier at 1.9\xa0\u03bcm based on thulium-doped normal dispersion fiber and LMA fiber compressor

The duration reduction and the peak power increase of ultrashort pulses generated by all-fiber sources at a wavelength of 1.9,upmu hbox {m} are urgent tasks. Finding an effective and easy way to improve these characteristics of ultrafast lasers can allow a broad implementation of wideband coherent supercontinuum sources in the mid-IR range required for various applications. As an alternative approach to sub-100 fs pulse generation, we present an ultrafast all-fiber amplifier based on a normal-dispersion germanosilicate thulium-doped active fiber and a large-mode-area silica-fiber compressor. The output pulses have the following characteristics: the central wavelength of 1.9,upmu hbox {m}, the repetition rate of 23.8 MHz, the energy per pulse period of 25 nJ, the average power of 600 mW, and a random output polarization. The pulse intensity and phase profiles were measured via the second-harmonic-generation frequency-resolved optical gating technique for a linearly polarized pulse. The linearly polarized pulse has a duration of 71 fs and a peak power of 128.7 kW. The maximum estimated peak power for all polarizations is 220 kW. The dynamics of ultrashort-pulse propagation in the amplifier were analyzed using numerical simulations.

Reconstructing algorithm for frequency-resolved optical gating based on intelligent seeker optimization

Frequency-resolved optical gating (FROG) is a common technique of ultrashort pulse measurement. It reconstructs the test pulse by an iterative two-dimensional phase retrieval algorithm from a FROG trace. Now the most widely used FROG algorithm is principal component generalized projection (PCGP), yet its accuracy of pulse retrieval drops obviously under noise condition, and its iterative speed slows down significantly as the size of FROG trace increases. Actually, most of ultrashort pulses delivered from ultrafast oscillators and amplifiers as well as created in most scientific experiments are of smooth spectral phases, so that they can be determined by a few of dispersion coefficients. In this paper, we propose a FROG algorithm based on seeker optimization algorithm (SOA). After recording the spectrum of the test pulse, several main dispersion coefficients of the spectral phase of the pulse are searched directly by the SOA algorithm to fit the corresponding FROG trace. Then the shape of the test pulse can be uniquely reconstructed. Since this algorithm mainly operates in a spectral domain and its routine of iteration is much simpler than PCGP’s, faster speed and higher accuracy of pulse retrieval can be expected. In order to prove the advantages of SOA-FROG algorithm, numeral simulations are performed for test pulses with varying dispersion, in the cases without noise and with 1%, 5%, 10%, 20% noise levels, respectively. The simulation results show that the new algorithm performs much better than PCGP in accuracy and iteration speed. In the case without noise, 97% test pulses reach the condition of rigid convergence (FROG error <i>G</i> ≤ 10<sup>–4</sup>) after 1500 iteration circles by using the SOA, with an average FROG error <i>G</i> < 10<sup>–5</sup>. So the accuracy of pulse reconstruction by SOA is at least three orders of magnitude higher than by PCGP. In cases with different noise levels, the accuracy of pulse reconstruction by SOA is also much higher than by PCGP. By means of background-subtraction and filtering on the FROG traces, the retrieved pulse profiles almost accord with reality. Typically for a 256 × 256 FROG trace, SOA-FROG iterates 100.8 circles per second, about 5 times faster than PCGP. After 300 iteration circles by SOA in about 3 s, most of test pulses can finish their routines of reconstruction and reach high accuracy. Besides SHG-FROG, the SOA-FROG algorithm can also be utilized in other FROG techniques based on the 3<sup>rd</sup> order nonlinear optical effects. In summary, the SOA-FROG is expected to be suitable to the real-time pulse measurement with high accuracy in most of application cases. Yet some measures of improvement should be taken to reconstruct complex pulses with rough spectral phases or/and broken spectra.

Numerical modeling of pulse-pumped ultrafast regenerative amplifier with unstable pump laser

A numerical model of a regenerative amplifier based on the Frantz-Nodvik equation with spatially dependent stored energy density in the active medium has been developed. The model was used to investigate the stability of the output energy of the amplifier associated with fluctuations of the pump energy and pump beam position. It was found that, same as in the known case with the fluctuations of the pump energy, the influence of the fluctuations of the pump beam position is much weaker at oversaturated operation of the regenerative amplifier. Opposite to fluctuations of the pump energy, fluctuations of the pump beam position result in highly-asymmetric histogram of the output energy, which is highly skewed to the lower energies.

Low noise all-fiber amplification of a coherent supercontinuum at 2 \xb5m and its limits imposed by polarization noise

We report a low noise, broadband, ultrafast Thulium/Holmium co-doped all-fiber chirped pulse amplifier, seeded by an Erbium-fiber system spectrally broadened via coherent supercontinuum generation in an all-normal dispersion photonic crystal fiber. The amplifier supports a − 20 dB bandwidth of more than 300 nm and delivers high quality 66 fs pulses with more than 70 kW peak power directly from the output fiber. The total relative intensity noise (RIN) integrated from 10 Hz to 20 MHz is 0.07%, which to our knowledge is the lowest reported RIN for wideband ultrafast amplifiers operating at 2 µm to date. This is achieved by eliminating noise-sensitive anomalous dispersion nonlinear dynamics from the spectral broadening stage. In addition, we identify the origin of the remaining excess RIN as polarization modulational instability (PMI), and propose a route towards complete elimination of this excess noise. Hence, our work paves the way for a next generation of ultra-low noise frequency combs and ultrashort pulse sources in the 2 µm spectral region that rival or even outperform the excellent noise characteristics of Erbium-fiber technology.

Pure-rotational 1D-CARS spatiotemporal thermometry with a single regenerative amplifier system.

We report spatiotemporal pure-rotational coherent anti-Stokes Raman spectroscopy (CARS) in a one-dimensional imaging arrangement obtained with a single ultrafast regenerative amplifier system. The femtosecond pump/Stokes photon pairs, used for impulsive excitation, are delivered by an external compressor operating on a ∼35% beam split of the uncompressed amplifier output (2.5 mJ/pulse). The picosecond 1.2 mJ probe pulse is produced via the second-harmonic bandwidth compression (SHBC) of the ∼65% remainder of the amplifier output (4.5 mJ/pulse), which originates from the internal compressor. The two pump/Stokes and probe pulses are spatially, temporally, and repetition-wise correlated at the measurement, and the signal generation plane is relayed by a wide-field coherent imaging spectrometer onto the detector plane, which is refreshed at the same repetition rate as the ultrafast regenerative amplifier system. We demonstrate 1 kHz cinematographic 1D-CARS gas-phase thermometry across an unstable premixed methane/air flame-front, achieved with a single-shot precision <1% and accuracy <3%, 1.4 mm field of view, and an excellent <20µm line-spread function.

Optimal Control of SOAs With Artificial Intelligence for Sub-Nanosecond Optical Switching

Novel approaches to switching ultra-fast semiconductor optical amplifiers using artificial intelligence algorithms (particle swarm optimisation, ant colony optimisation, and a genetic algorithm) are developed and applied both in simulation and experiment. Effective off-on switching (settling) times of 542 ps are demonstrated with just 4.8% overshoot, achieving an order of magnitude improvement over previous attempts described in the literature and standard dampening techniques from control theory.

All-fiber short-wavelength tunable mode-locked fiber laser using normal dispersion thulium-doped fiber.

We report an all-fiber high pulse energy ultrafast laser and amplifier operating at the short wavelength side of the thulium (Tm) emission band. An in-house W-type normal dispersion Tm-doped fiber (NDTDF) exhibits a bending-induced distributed short-pass filtering effect that efficiently suppresses the otherwise dominant long wavelength emission. By changing the bending diameter of the fiber, we demonstrated a tunable mode-locked Tm-doped fiber laser with a very wide tunable range of 152 nm spanning from 1740 nm to 1892 nm. Pulses at a central wavelength of 1755 nm were able to be amplified in an all-fiber configuration using the W-type NDTDF, without the use of any artificial short-pass filter or pulse stretcher. The all-fiber amplifier delivers 2.76 ps pulses with an energy of ∼32.7 nJ without pulse break-up, due to the normal dispersion nature of the gain fiber, which marks so far, the highest energy amongst fiber lasers in the 1700 nm-1800 nm region.

Ultrafast thin-disk multi-pass amplifier system providing 1.9 kW of average output power and pulse energies in the 10 mJ range at 1 ps of pulse duration for glass-cleaving applications.

An ultrafast Yb-doped thin-disk multi-pass laser amplifier system with flexible parameters for material processing is reported. We can generate bursts consisting of four pulses at a distance of 20 ns and a total energy of 46.7 mJ at a repetition rate of 25 kHz. In single-pulse operation, 1.5 kW of average output is achieved at 400 kHz when optimizing for a beam quality of M2 = 1.5. Alignment for maximum output power provides 1.9 kW at the same repetition rate. All results are obtained without chirped-pulse amplification in the multi-pass set-up. The application potential of the system is demonstrated exploring its performance in materials processing of dielectrics. Cleaving of 3.8-mm-thick SCHOTT borofloat glass with a velocity of 1200 mm/s is demonstrated with 300 W of input power. Single-pass modification of 30 mm borosilicate glass is enabled with a Bessel beam at 1 kW of average power delivered by four-pulse bursts of an energy of 30 mJ.

Direct amplification of sub-300 fs pulses in a versatile thin-disk multipass amplifier

We report on a kW-class ultrafast laser amplifier emitting sub-300 fs pulses with a pulse peak power exceeding 1 GW. It is based on the direct amplification of 260 fs pulses in a versatile thin-disk multipass amplifier which provides fast modulation of the amplified beam on the workpiece by the implementation of a novel and robust polarization multiplexing scheme for the seed beam that is launched into the amplifier. Furthermore, the laser can be operated in a single-pulse or burst mode with a variable number of pulses per burst. Second harmonic conversion of the amplified beam by means of a lithium triborate crystal additionally led to an output power of up to 282 W in the green spectral range.

Ultra low-noise coherent supercontinuum amplification and compression below 100 fs in an all-fiber polarization-maintaining thulium fiber amplifier.

We report an ultra-low noise, polarization-maintaining, ultrafast Thulium-doped all-fiber chirped pulse amplifier, seeded by a polarized all-normal dispersion (ANDi) supercontinuum (SC) driven by an ultrafast Erbium-fiber laser. The system comprises only polarization-maintaining fibers and delivers 96 fs pulses with 350 mW output power at 100 MHz, centered at 1900 nm. The integrated relative intensity noise (RIN) in the range of 10 Hz - 10 MHz is only 0.047% at the amplifier output, which is virtually identical to the RIN of the Erbium-fiber laser driving the SC. Therefore, neither the SC generation nor the amplification process introduce significant excess noise. The RIN of our system is an order of magnitude lower than similar systems previously seeded with Raman solitons. This highlights the superior noise properties of ANDi SC and their potential as ultra-low noise seed sources for broadband, high power ultrafast fiber amplifiers and frequency combs.

Ultrafast thin-disk multipass laser amplifier scheme avoiding misalignment induced by natural convection of the ambient air

We report on an approach for a compact ultrafast thin-disk multipass laser amplifier making use of a highly compact geometric folding scheme. The setup is also suitable to minimize the effects caused by natural convection of hot air in front of the thin-disk on the amplified laser beam as it facilitates to orient the laser disk with its axis in the vertical direction. The efficacy of this approach is analyzed with finite-element method simulations of the heated laser crystal in ambient air with different orientations of the thin laser disk. The experiments confirm a significant improvement of the amplifier performance in terms of stability and an increase of the output power with nearly diffraction-limited beam quality (<italic>M</italic>² ≤ 1.4) by a factor of 3 with respect to the conventional orientation of the laser disk.