High Peak Power Laser Pulses Research Articles

Experimental study of injection seeded single-longitudinal-mode Q-switched fiber laser

--The role of injection seeding power on the operation and output characteristics of a single-longitudinal-mode (SLM) injection seeded Q-switched fiber laser is investigated. The mode coupling between seeding laser and the potential longitudinal-modes of the oscillator changes as the injection seeding power increase. So the Q-switched laser may operate at different regimes which can be distinguished by the pulse features. Only with a proper range of injection seeding power that is not too larger or too small, the operation of injection seeded Q-switched fiber laser with narrow-linewidth, high peak power and narrow pulsewidth laser pulses is possible. The Q-switched fiber laser linewidth is narrowed through inserting a dual-OCs oscillator into the laser cavity. 200-ns, 1-W peak power SLM Q-switched laser pulse with nearly transform-limited linewidth is obtained when the pump power is 300 mW.

Generation of megawatt soliton at 1680 nm in very large mode area antiresonant fiber and application to three-photon microscopy

The spectral window around 1700 nm is interesting for in-depth multiphoton microscopy of intact tissues due to reduced scattering and absorption in this wavelength range. However, wide adoption of this excitation range will rely on the availability of robust and cost-effective high peak power pulsed lasers operating at these wavelengths. Here, we report on a fiber-based femtosecond laser providing up to 95 nJ, 85 fs pulses at 1800 nm. The laser system makes use of a fiber-based chirped pulse amplifier emitting at 1560 nm followed by an in-house fabricated very large mode area antiresonant fiber for soliton self-frequency shift. Megawatt-peak power pulses at the repetition rate of 1 MHz are available directly at the output of the flexible fiber. We illustrate the potential of the source for biological microscopy by recording three-photon-excited fluorescence images of mouse nervous tissue. The flexible fiber tailored to propagate megawatt solitons in the biologically relevant window around 1700 nm opens the way to deep brain imaging of freely moving animals via miniaturized endomicroscopes.

Time-correlated single photon Raman spectroscopy at megahertz repetition rates

Significant improvements in time-correlated single photon counting (TCSPC) Raman spectroscopy acquisition times can be achieved through exploitation of megahertz (MHz) laser repetition rates. We have developed a TCSPC Raman spectroscopy system based on a high peak power (>40W) pulsed laser, a high pulse repetition rate (40 MHz), a custom f/1.5 spectrometer, and a 512 spectral channel × 16 time bin single photon avalanche diode line sensor. We report millisecond Raman spectrum acquisition times, a peak Raman count rate of 104 kcps, and a linewidth aggregated count rate of 440 kcps with a diamond sample. This represents a three-order-of-magnitude increase in measured Raman count rate in comparison with a 104 kHz pulsed laser operating at 300 W and a four-order-of-magnitude increase over a 0.1 W pulsed laser operating at 40 MHz. A Raman-to-fluorescence ratio of 4.76 is achieved with a sesame oil sample at a 20 MHz repetition rate. Achieving high count rates and Raman-to-fluorescence ratios unlocks the potential of combined Raman/fluorescence lifetime spectroscopy for imaging and other short acquisition time applications.

Increasing the brightness of harmonic XUV radiation with spatially-tailored driver beams

Bright high harmonic sources can be produced by loosely focussing high peak power laser pulses to exploit the quadratic scaling of flux with driver spot size at the expense of a larger experimental footprint. Here, we present a method for increasing the brightness of a harmonic source (while maintaining a compact experimental geometry) by spatially shaping the transverse focal intensity distribution of a driving laser from a Gaussian to supergaussian. Using a phase-only spatial light modulator we increase the size and order of the supergaussian focal profiles, thereby increasing the number of harmonic emitters more efficiently than possible with Gaussian beams. This provides the benefits of a loose focussing geometry, yielding a five-fold increase in harmonic brightness, whilst maintaining a constant experimental footprint. This technique can readily be applied to existing high harmonic systems, opening new opportunities for applications requiring bright, compact sources of coherent short wavelength radiation.

Characterization and application of nonlinear plastic materials for post-CPA pulse compression.

We demonstrate the three-fold post-chirped-pulse-amplification (post-CPA) pulse compression of a high peak power laser pulse using allyl diglycol carbonate (CR39), which was selected as the optimal material for near-field self-phase modulation out of a set of various nonlinear plastic materials, each characterized with respect to its nonlinear refractive index and optical transmission. The investigated materials could be applied for further pulse compression at high peak powers, as well as for gain narrowing compensation within millijoule-class amplifiers. The post-CPA pulse compression technique was tested directly after the first CPA stage within the POLARIS laser system, with the compact setup containing a single 1 mm thick plastic sample and a chirped mirror pair, which enabled a substantial shortening of the compressed pulse duration and, hence, a significant increase in the laser peak power without any additional modifications to the existing CPA chain.

Supercontinuum generation directly from a random fiber laser based on photonic crystal fiber.

Supercontinuum (SC) can be generated directly from a random fiber laser (RFL). However, its spectral bandwidth and flatness need to be further optimized for many practical applications. To solve this issue, a RFL based on random distributed Rayleigh scattering in photonic crystal fiber is demonstrated for the first time in this paper. The experimental results revealed that compared with the traditional single or double clad fiber, photonic crystal fiber not only can provide random distributed feedback effectively, but is also a superior nonlinear medium for SC generation which can realize better spectral width and flatness. A flat SC covering 400 nm to 2300 nm is obtained directly from a RFL based on photonic crystal fiber and the corresponding 20 dB bandwidth is more than 1600 nm, which is the widest ever reported to the best of our knowledge. The optical rogue waves caused by solitonic collisions can explain the instability of the output pulses in the time domain. This work proves that photonic crystal fiber can be used in RFL to provide random distributed feedback as well as nonlinear medium for spectrum broadening, and the spectral width and flatness of the generated SC is as good as the conventional method of using a high peak power pulsed laser to pump a piece of photonic crystal fiber, which can greatly reduce the cost of the SC and enrich the research scope of SC as well as RFL.

Investigating silicon wafer based substrates for dried-droplet analysis by Laser-Induced Breakdown Spectroscopy

This work communicates a critical assessment on the analytical capability of the three silicon wafer-based substrates; crystalline silicon (c-Si), oxide-coated silicon (SiO2-Si), and nitride-coated silicon (Si3N4-Si), for dried-droplet analysis by laser-induced breakdown spectroscopy. The methodology consists of loading, drying and analyzing steps. First, nanoliter volume of droplets are manually loaded onto the substrate and dried at room temperature. Then, the dry residue is subjected to high peak power (1.15 GW/cm2) laser pulses focused outside the minimum focal point condition and luminescent plasma is spectroscopically analyzed. Results revealed that nitride-coated substrate exhibits strong enhancements in signal intensity for most emission lines of the analyte species investigated: Cd, Cr, Cu, Mn, and Pb. Surface reflectivity and surface morphology were comparatively investigated to explore enhanced analytical performance of nitride-coated substrates. Experimental conditions were optimized and growth curves for all the elements are found linear with minimum regression constant of 0.96. LOD's of 62 pg Cd, 1.5 pg Cr, 0.5 pg Cu, 2 pg Mn and 11 pg Pb, in absolute amounts, were obtained. The accuracy and precision of the methodology were tested on certified reference water sample (CRM-TMDW), and ICP-multi-element standard sample (ICP-MES). The surface enhancement effect observed on Si3N4 coated substrates has improved the analytical capability of laser-induced breakdown spectroscopy for liquid analysis.

Intact primate brain tissue identification using a completely fibered coherent Raman spectroscopy system.

.Coherent Raman fiber probes have not yet found their way into the clinic despite their immense potential for label-free sensing and imaging. This is mainly due to the traditional bulky laser systems required to create the high peak power laser pulses needed for coherent Raman, as well as the complications that arise from the propagation of this type of energy through silica. Specifically, a coherent anti-Stokes Raman scattering (CARS) probe that could select its integration volume at high resolution, away from the tip of the fiber, is particularly interesting in the case of electrode implantation neurosurgeries, wherein it is possible to place optical fibers on-board the chronic electrode and provide optical guidance during its implantation, through the semi-transparent tip. To this clinical end, we have created an all fiber CARS system, consisting of small, rapidly tunable, turn-key fiber-lasers, capable of creating high wavenumber CARS spectra on the order of tens-of-milliseconds. The use of traditional silica fibers is made possible by the use of the laser’s long pulse-widths (25 ps). The probe itself has an outer diameter of allowing it to fit within commercially available metal tubes that can replace deep brain stimulation (DBS) stylets. Using this system, we identified brain tissue types in intact nonhuman primates’ brains and showed the ability to delineate white and gray matters with high resolution. Its advantages over spontaneous Raman stem from the orders of magnitude improvement in spatial resolution, its inherent translatability to three-dimensional (3-D) imaging, as well as the theoretical ability to remove parasitic Raman signal from probe encasements, such as a DBS electrode. The system is planned to have clinical implications in neurosurgical guidance as well as diseased tissue detection.

Potassium titanyl arsenate based cascaded optical parametric oscillator emit at 2.5 µm derived by neodymium-doped yttrium lithium fluoride laser

We reported a potassium titanyl arsenate (KTA) based cascaded optical parametric oscillator (OPO). The secondary OPO signal light at 2.5 µm was obtained with two OPO processes in one non-critical phase matching cut KTA crystal. This cascaded OPO was driven by a Q-switched neodymium-doped yttrium lithium fluoride (Nd:YLF) laser at 1047 nm. Making full use of the negative thermal lens effect and long upper level fluorescence lifetime of Nd:YLF, signal power of 605 mW at 2503 nm was achieved with a pulse repetition frequency of 15 kHz and an incident diode pump power of 9.7 W. Therefore, the cascaded OPO derived by Q-switched Nd:YLF laser could provide high peak power pulsed laser emission in mid-infrared band.

Progress on electro-optic crystals for Q-switches

Electro-optic Q-switched laser systems with high peak power have been widely applied in medical, cosmetology, measurement, processing, military industry, and other fields, while the crystal for electro-optic Q-switch is the core optoelectronic material for outputting high peak power pulse laser. With the technological progress and the development of short pulse, high repeat frequency and high power laser system application, the performance of crystals for electro-optic Q-switch is constantly improved. The paper reviews the research progress on basic characteristics, crystal preparation technology and electro-optic Q-switch application technology of several important electro-optic crystals for Q-switches including lithium niobate (LN) crystal, deuterated potassium dihydrogen phosphate (DKDP) crystal, low temperature phase barium metaborate (BBO) crystal, rubidium oxygen titanium phosphate (RTP) crystal, langasite (LGS) crystal and potassium titanium oxide phosphate (KTP) crystal, and propose that the research on practical electro-optic crystals for Q-switches is urgent to make breakthroughs. For a long time to come, exploring the electro-optic crystal materials, with high electro-optic coefficient, high laser damage threshold, stable performance, high temperature applicability and low cost, will be still an important issue in the field of crystal materials research.

All-fiber femtosecond laser providing 9 nJ, 50 MHz pulses at 1650 nm for three-photon microscopy

The spectral window lying between 1.6 and 1.7 μm is interesting for in-depth multiphoton microscopy of intact tissues due to reduced scattering and absorption in this wavelength range. However, wide adoption of this excitation range will rely on the availability of robust and cost-effective high peak power pulsed lasers operating at these wavelengths. In this communication, we report on a monolithically integrated high repetition rate (50 MHz) all-fiber femtosecond laser based on a soliton self-frequency shift providing 9 nJ, 75 fs pulses at 1650 nm. We illustrate its potential for biological microscopy by recording three-photon-excited fluorescence and third-harmonic generation images of mouse nervous tissue and developing Drosophila embryos labeled with a red fluorescent protein.

The modified SRRS threshold criteria for high peak power laser pulses in long air-path transmission considering the near-field beam quality

This paper demonstrates that the stimulated rotational Raman scattering (SRRS) threshold for high peak power laser pulses propagating through a long air path can be influenced strongly by the near-field quality of the laser beams, and the relationship between the SRRS threshold and the near-field beam quality (i.e., spatial intensity modulation index and contrast ratio) can be evaluated quantitatively. By using our three-dimensional numerical model, which can describe the spatial–temporal evolution behaviors of SRRS and is verified by previously published SRRS experimental data, the criteria of the safe transmission distance for high peak power nanosecond laser pulses are obtained, and the modified SRRS threshold criterion formulas considering the near-field beam conditions are presented.

Mid-infrared nonlinear absorption in As2S3 chalcogenide glass.

We report mid-infrared (MIR) nonlinear absorption in As2S3 glasses which results from two-photon excitation of valence electron to the Urbach extension followed by strong linear absorption of excited states. The measured MIR nonlinear absorption can be 3 to 4 orders of magnitude stronger than the two-photon absorption in the near-infrared for similar laser intensities and does not result from contaminants, but it is intrinsic to As2S3 glasses. As2S3 fibers are widely used to generate supercontinuum by pumping them with high peak power laser pulses. For a 100 kilowatt peak power MIR soliton propagating in single mode As2S3 fiber, the nonlinear absorption can be of similar magnitude than the fiber background loss. Finally, for laser peak power around 1 MW, the MIR nonlinear absorption can be ~2 orders of magnitude larger than the fiber background loss in single mode As2S3 fiber.

Sapphire-fiber-based distributed high-temperature sensing system.

We present, for the first time to our knowledge, a sapphire-fiber-based distributed high-temperature sensing system based on a Raman distributed sensing technique. High peak power laser pulses at 532 nm were coupled into the sapphire fiber to generate the Raman signal. The returned Raman Stokes and anti-Stokes signals were measured in the time domain to determine the temperature distribution along the fiber. The sensor was demonstrated from room temperature up to 1200°C in which the average standard deviation is about 3.7°C and a spatial resolution of about 14 cm was achieved.

Control of broadband optically generated ultrasound pulses using binary amplitude holograms.

In this work, the use of binary amplitude holography is investigated as a mechanism to focus broadband acoustic pulses generated by high peak-power pulsed lasers. Two algorithms are described for the calculation of the binary holograms; one using ray-tracing, and one using an optimization based on direct binary search. It is shown using numerical simulations that when a binary amplitude hologram is excited by a train of laser pulses at its design frequency, the acoustic field can be focused at a pre-determined distribution of points, including single and multiple focal points, and line and square foci. The numerical results are validated by acoustic field measurements from binary amplitude holograms, excited by a high peak-power laser.

Temporal Shaping of High Peak Power Pulse Trains from a Burst-Mode Laser System

It has been shown in the past that pulsed laser systems operating in the so-called “burst mode” are a beneficial approach to generate high peak power laser pulses at high repetition rates suitable for various applications. So far, most high-energy burst-mode laser systems put great effort into generating a homogeneous energy distribution across the burst duration, e.g., by shaping the pump pulse. In this work, we present a new shaping technique, which is able to produce arbitrary energy distributions within the burst by pre-shaping the seed pulse burst with a Pockels cell. Furthermore, this technique allows for the precompensation of any static modulations across the burst, which may be introduced during the subsequent amplification process. Therefore, a pulse burst with a uniform energy distribution can also be generated. The method is tested with an ultra-short pulse burst mode laser amplifier system producing bursts of a 1 ms duration with a pulse repetition rate of 1 MHz and a maximum output power of 800 W during the burst. Furthermore, a method to predict the influence of the amplifier on a non-uniformly shaped burst is presented and successfully tested to produce a pre-defined pulse shape after amplification.

Beam scanning for rapid coherent Raman hyperspectral imaging.

Coherent Raman imaging requires high-peak power laser pulses to maximize the nonlinear multiphoton signal generation, but accompanying photo-induced sample damage often poses a challenge to microscopic imaging studies. We demonstrate that beam scanning by a 3.5-kHz resonant mirror in a broadband coherent anti-Stokes Raman scattering (BCARS) imaging system can reduce photo-induced damage without compromising signal intensity. Additionally, beam scanning enables slit acquisition, in which spectra from a thin line of sample illumination are acquired in parallel during a single charge-coupled device exposure. Reflective mirrors are employed in the beam-scanning assembly to minimize chromatic aberration and temporal dispersion. The combined approach of beam scanning and slit acquisition is compared with the sample-scanning mode in terms of spatial resolution, photo-induced damage, and imaging speed at the maximum laser power below the sample-damage threshold. We show that the beam-scanning BCARS imaging method can reduce photodamage probability in biological cells and tissues, enabling faster imaging speed by using a higher excitation laser power than could be achieved without beam scanning.

700-kW-Peak-Power Monolithic Nanosecond Pulsed Fiber Laser

We report a high-pulse energy, high-peak power, and monolithic nanosecond pulsed fiber laser source at ~1064 nm in master oscillator-power amplifier configuration. The seed source is a directly modulated laser diode, producing nanosecond pulses as short as ~3 ns with ~1-W peak power and a tunable repetition rate. Three core-pumped fiber amplifier stages and two double-cladding fiber amplifier stages were built to boost the peak power and the pulse energy of the ~3-ns seed pulses to ~697 kW and ~2.3 mJ, respectively. The all-fiber construction of the whole laser system enables compact size, maintenance-free, and robust operation.

Eyesafe high peak power pulsed fiber lasers limited by fiber nonlinearity

We present design and performances of several eye-safe high peak power fiber lasers operating either around 1550nm or 2000nm. They share the limitations by nonlinear effects either Stimulated Brillouin Scattering for single frequency lasers or Kerr related effects for short pulse amplifiers. Performances above 1kW peak power for single frequency lasers and 26nJ for short pulse fiber lasers are reported. The influence of the saturation power of the fibers on the non-linearity is discussed and applied to a comparison between Erbium and Ytterbium co-doped fibers and Thulium doped fibers. The Brillouin gain properties in these fibers are also compared.

Peak-Power Instabilities of Laser Pulses Retroreflected by a Corner-Cube Retroreflector at Different Distances

In order to obtain the change in the return laser-pulse peak-power instability with the distance between two communication terminals in a free-space optical communication (FSO) asymmetric link, which uses a corner-cube retroreflector (CCR) and a high peak-power pulse laser, we present the peak power instability of laser pulses retroreflected by the CCR at different distances. The CCR bottom surface was a circle with a diameter of 1 in., and three mirrors of the CCR were coated with silver. The weather was cloudy with winds of Category 4. The distances from the transmitter to the CCR were 120, 1,550, 3,900, and 5,240 m. The laser emitted 100 laser pulses with a peak power of 5 MW at each distance. The results of the experiments show that the increase in both absolute and relative peakpower instabilities with the distance is approximately linear. The absolute peak-power instability is more than 58.7 mV and the relative peak-power instability is more than 11.8 % if the distance increases 1 km.