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Pulsed wave (PW) Doppler ultrasound is routinely used in the clinic to assess blood flow. Our annual Doppler quality assurance tests revealed unexpectedly large errors in measurement of maximum velocity, exceeding our tolerance (error >20%), when using certain scanners with small Doppler sample volume dimensions. The aim of this study was to assess the dependence of maximum velocity estimates on PW Doppler sample volume size. A flow phantom with known steady flow was used to acquire maximum velocity estimates (maximum velocities of 24, 39 and 85 cm/s and sample volume range of 0.3–20 mm) with a variety of transducers and scanners in clinical use (51 probes from 4 manufacturers). Selected acoustic outputs were characterized using free-field hydrophone measurements. All maximum velocity estimates were within our tolerance for sample volume sizes ≥1.5 mm, although maximum velocity estimates typically increased with decreasing sample volume size. Errors exceeding our tolerance were commonly found for one manufacturer when using smaller sample volumes, resulting in up to 75% overestimation. Although intrinsic spectral broadening based on transit time considerations may help explain our findings, the sample volume dependence raises potential clinical concerns that users should be aware of and which manufacturers should consider addressing.

In this work, the experimental study of spectral broadening due to stimulated Raman scattering with respect to variation of input power of pulsed laser, as well as a 4 m length high-numerical aperture fiber with 100 and 500 m of non-zero dispersion-shifted fiber is reported. The results showed extreme broadening of spectrum which is known as supercontinuum generation, originated by self-phase modulation, four- wave mixing and dominated by cascaded stimulated Raman scattering. Supercontinuum spectrum achieved by total width of 850 nm and a remarkable flatness in the range of 1500 to 1600 nm. Furthermore, output spectrum showed a total of five Stokes waves. Numerical results are also presented.

Low-power continuous-wave (cw) pumped supercontinuum sources exhibit poor spectral flatness and high sensitivity to the position of pump-wavelength with respect to the zero-dispersion of the nonlinear fiber. We propose a technique to overcome these limitations by the simple addition of a section of transmission fiber (such as Corning LEAF) before the nonlinear broadening stage. The pump incoherence introduced by this fiber in the C-band allows for a smooth, equalized, input wavelength flexible supercontinuum at low powers. With the proposed two-stage spectral broadening, we have successfully demonstrated for the first time, a cw pump wavelength agile all-fiber supercontinuum module with spectra spanning from 1300 nm to 2000 nm using relatively short lengths of nonlinear fiber. Similar supercontinuum is obtained for output powers from 80 mW up to a maximum in this work of 550 mW making it an ideal source for broadband low-power applications.

We demonstrate flat supercontinuum (SC) generation in a piece of step-index tellurite fiber pumped by a noise-like mode-locked fiber laser in the deep normal dispersion region. The noise-like pulse emitted from a simple all-fiber mode-locked laser is used as the pump directly and the cascaded stimulated Raman scattering leads to the spectral broadening to the long wavelength region. Flat SC spanning from $\sim 1570$ nm to $\sim 1900$ nm is obtained and no obvious spectral defects are observed. These results show the great advantage of noise-like pulse pumping in the deep normal dispersion region and the potential of noise-like pulse pumped mid-infrared SC generation.

For successful targeted alpha radiotherapy (TAT), verifying the accurate position and distribution of a targeted radiotherapeutic agent in a patient or phantom is important. This paper, describes our investigation of depth-of-interaction (DOI) Compton imaging for the two γ-rays emitted during TAT with the 225Ac radioactive isotope. We optimized the design parameters of the DOI Compton camera, for example, the inter-detector distance, based on the figure of merit (FOM). The performance of DOI Compton imaging for TAT was improved because Doppler broadening and the energy uncertainty are inversely proportional to the radiation energy and the position uncertainty of the depth information is decreased. After the contrast phantom and the resolution phantom had been designed, two reconstruction algorithms, the filtered back-projection (FBP) algorithm and the maximum-likelihood expectation maximization (MLEM) algorithm, were applied to each reconstructed phantom image, and the qualities of the reconstructed images for the two γ-rays (218 keV and 440 keV) were compared. In the quantitative evaluation of the reconstructed images, the MLEM reconstruction algorithm performed better than the FBP algorithm. Based on Monte Carlo simulation studies, the DOI Compton images of the 225Ac radioactive isotope emitting two γ-rays demonstrated the capability of imaging a targeted radiotherapeutic agent in TAT.

An original technique is developed for spectral broadening of femtosecond pulses with compensation for the nonlinear spatial phase during the propagation through nonlinear media with effective cubic nonlinearity of different signs. It was shown that in the region of 1.5 μm, the proposed scheme with BBO crystals at the first stage and NaCl at the second stage allows, using chirped mirrors, the formation of few-cycle pulses of about 7 fs duration with a small B-integral. The possibility of focusing of pulses compressed in the proposed scheme with a large Strehl ratio is demonstrated.

[EN]We use self-calibrating dispersion scan to experimentally detect and quantify the presence of pulse train instabilities in ultrashort laser pulse trains. We numerically test our approach against two different types of pulse instability, namely second-order phase fluctuations and random phase instability, where the introduction of an adequate metric enables univocally quantifying the amount of instability. The approach is experimentally demonstrated with a supercontinuum fibre laser, where we observe and identify pulse train instabilities due to nonlinear propagation effects under anomalous dispersion conditions in the photonic crystal fibre used for spectral broadening. By replacing the latter with an all-normal dispersion fibre, we effectively correct the pulse train instability and increase the bandwidth of the generated coherent spectrum. This is further confirmed by temporal compression and measurement of the output pulses down to 15 fs using dispersion scan., The authors acknowledge funding from the Junta de Castilla y León (SA287P18) and FEDER funds; Spanish Ministerio de Economía y Competitividad (MINECO) (FIS2017-87970-R, EQC2018-004117-P, DI-15-07461, PTQ-15-07708); European EUREKA program CDTI-INNO-20171026; CCDR-N via the project Nanotechnology-based functional solutions (NORTE-01-0145-FEDER-000019); Fundação para a Ciência e a Tecnologia (FCT), Portugal (grants ‘UltraGraf’ M-ERA-NET2/0002/2016, M-ERA-NET2/0004/2016, UID/NAN/50024/2013, PTDC/FIS-OTI/32213/2017); Network of Extreme Conditions Laboratories - NECL and CCDR-N (NORTE-07-0124-FEDER-000070, NORTE-01-0145-FEDER-022096); PT2020 (program 05/SI/2017 – SI I&DT Empresarial - DI, grant no. 33573; program 04/SI/2019 Projetos de I&D Industrial à Escala Europeia, grant no. 045932). BA acknowledges funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No. 798264.

The results of investigation of the possibility of reducing the duration of the second-harmonic radiation pulse generated by the starting complex of an THL-100 laser system by conversion of a pre-chirped fundamental frequency pulse are presented. A front-end compressor of the complex is used to obtain a fundamental harmonic pulse chirp at a central wavelength of 950 nm with duration of 70 fs. It is shown that a small excess of the group velocity dispersion (±3200–4000 fs2) of fundamental frequency radiation increases the width of the second-harmonic radiation spectrum by a factor of 1.5–2. A compensation for the excess dispersion of radiation pulses with broadened spectrum makes it possible to reduce the second-harmonic radiation pulse duration approximately proportionally to its spectral broadening and to obtain a minimal pulse duration of 35 fs.

Through numerical simulations, we study the effects of weak continuous wave (CW) or weak pulse triggering on the picosecond (ps) pulse pumped supercontinuum (SC) generation. To actively improve the spectral broadening and control both phase and intensity stability of the SC generation, we fix the frequency shift of the trigger pulse (with respect to the pump signal) with different durations and then tune the peak power. Detailed simulation results show that either a weak CW or weak pulse trigger can greatly improve the spectral broadening of SC spectrum generated by a ps pulse pump. Compared to weak CW trigger, a weak femtosecond (fs) pulse or ps pulse trigger can assist the generation of SC with much better coherence and SNR, but precise control of the time delay between the pump and trigger is required.

The field of nonlinear optics is a well-established discipline that relies on macroscopic media and employs propagation distances longer than a wavelength of light. Recent progress with electromagnetic metamaterials has allowed for the expansion of this field into new directions of new phenomena and novel functionalities. In particular, nonlinear effects in thin, artificially structured materials such as metasurfaces do not rely on phase-matching conditions and symmetry-related selection rules of natural materials; they may be substantially enhanced by strong local and collective resonances of fields inside the metasurface nanostructures. Consequently, nonlinear processes may extend beyond simple harmonic generation and spectral broadening due to electronic nonlinearities. This article provides a brief review of basic concepts and recent results in the field of nonlinear optical metasurfaces.

Spectral broadening in pulsed-wave Doppler caused by the transit-time effect deteriorates the frequency resolution and may cause overestimation of maximum velocities in high-velocity blood flow regions and for large beam-to-flow angles. Data-adaptive spectral estimators have been shown to provide improved frequency resolution, especially for small ensemble lengths, but offer little or no improvement when the transit-time effect dominates. In this work, a method is presented that combines a data-adaptive spectral estimation method, the power spectral Capon, and 2-D tracking Doppler to enable improved frequency resolution for both high and low velocities. For each velocity, a time signal is extracted by tracking scatterers over time and space to decrease the transit-time effect, and power spectral Capon is used for spectral estimation. The method is evaluated using simulations, flow phantom recordings, and recordings from healthy and stenotic carotid arteries. Simulation results showed that the spectral width was decreased by 60% compared to 2-D tracking Doppler for velocities around 2.3 m/s using 12 time samples. The reduction was estimated to be 66% using the flow phantom results for 0.85-m/s mean velocity. A 5-dB SNR gain was observed from the in vivo results compared with Welch’s method. Computer simulations confirm that in the presence of velocity gradients or out-of-plane motion, the proposed method can be used to reduce spectral broadening by requiring shorter observation windows.

A silicon nitride horizontal slot waveguide with silicon dioxide cladding is designed to achieve flat negative dispersion ( $\text {nm}\cdot \text {km}$ )) over an octave-spanning bandwidth (782 to 2100 nm). Simulations show that, when a 10-fs pulse with 1-W peak power is input into a 5-cm long waveguide, the pulse width becomes 448 times wider and its peak power is reduced to $2.23\times 10^{-3}$ W. After passing through another ideal positive dispersion element, we find that the output pulse peak power can be recompressed close to 1 W in the case of different input pulse widths. Moreover, for the input pulse with a much higher peak power, the pulse width of the output pulse can be compressed to a much narrower level because of the spectral broadening introduced by optical nonlinearity. A 100-fs input pulse with 10-kW peak power can be compressed to 12.5 fs with a peak power of 63 kW. Considering the time-domain broadening and the peak power reduction, such a slot waveguide with a flat negative dispersion could be used as a time stretcher for 10-fs level pulse in on-chip chirped pulse amplification (CPA) system.

A multipass cell for nonlinear compression to few-cycle pulse duration is introduced composing dielectrically enhanced silver mirrors on silicon substrates. Spectral broadening with 388 W output average power and 776 µJ pulse energy is obtained at 82% cell transmission. A high output beam quality ( M 2 < 1.2 ) and a high spatio-spectral homogeneity (97.5%), as well as the compressibility of the output pulses to 6.9 fs duration, are demonstrated. A finite element analysis reveals scalability of this cell to 2 kW average output power.

An optical input pulse with a duration of nanosecond to femtosecond propagates through a dispersive nonlinear medium, it is subject to an extreme spectral broadening termed as white light supercontinuum (SC). In PCF, dispersion characteristics can be modified by the waveguide geometric parameters and thereby the nonlinear dynamics can be altered. In this article, the fiber parameters like core diameter, doping percentage, and pitch are varied and analyzed to achieve a conclusive zero-dispersion wavelength (ZDW) and a high nonlinearity. The numerical study presents different designs of solid-core photonic quasi-crystal fiber (PQF) and selection of highly nonlinear fiber with an effective area of 1.14 mm2 and nonlinearity of 196 W-1-km-1 at 780 nm exhibiting two-zero dispersion wavelength for supercontinuum generation in different regimes. The results exhibit a significant pulse broadening effect when the pump pulse (FWHM) was varied from 25 fs to 100 fs. Similarly, the influences of the pulse energy and peak average power on the bandwidth of the generated SC are studied

In this letter, we report a high-power all-fiber mid-infrared (MIR) supercontinuum (SC) laser source based on a short piece of centimeter level germania-core fiber (GCF). The pump source at the wavelength of 2 μm was directly generated from a single-mode fiber laser. The all-fiber seed oscillator was passively mode-locked by a SESAM to generate pulse duration of 62 ps at a fundamental repetition rate of 44.31 MHz in a linear cavity. After all-fiber thulium-doped fiber amplifier (TDFA), we obtained a high-power pump source with a spectral coverage of 1.9~2.2 μm. We only use 12 cm long 94 mol. % GCF for spectral broadening pumped by TDFA. In addition, we measured the spectral broadening after 12 cm GCF in different length of dispersion compensation fibers (DCF) corresponding to different pulse width and peak power. As a result, we obtained a broadband SC source based on 12 cm 94 mol. % GCF extending from 1630 to 3205 nm and a maximum output average power of 15.07 W with a slope efficiency of 34.2%. The 20 dB spectral bandwidth was over 1000 nm spanning from 1.89 to 2.92 μm. To the best of our knowledge, this is the first demonstration of 15-W level SC laser based on germania-core fiber with long wavelength edge extending beyond 3.2μm. Further broadening of spectrum is limited to pump power.

A simple and cheap technique for post-compression of mid-IR laser pulses relies on nonlinear spectral broadening via SPM in mm-thick optical windows (plates) followed by recompression. Efficient broadening requires peak intensities of ~hundred GW/cm2, which in case of high average power (>10W) drivers puts serious requirements on the used optical materials’ thermal properties, purity, homogeneity, surface quality and LIDT of coatings. Several materials (YAG, fluorides, KBr, Si, ZnSe, ZnS) were tested with 3.2 µm, 13W (130 µJ, 100 kHz), 48 fs pumping. Measured nonlinear absorption, wavefront distortion, spectral broadening and long-term survival as a function of intensity will be shown.

Compression of 42 fs, 0.29 mJ pulses from a Ti:Sapphire amplifier down to 8 fs (approximately 3 optical cycles) is demonstrated by means of spectral broadening in a compact multi-pass cell filled with argon. The efficiency of the nonlinear pulse compression is limited to 45 % mostly by losses in the mirrors of the cell. The experimental results are supported by 3-dimensional numerical simulations of the nonlinear pulse propagation in the cell that allow us to study spatio-spectral properties of the pulses after spectral broadening.

In fundamental papers from 1962, Heffener and Haus showed that it is not possible to construct a linear noiseless amplifier [Proc. IRE 50, 1604 (1962)PIREAE0096-839010.1109/JRPROC.1962.288130; Phys. Rev. Lett. 128, 2407 (1962)PHRVAO0031-899X10.1103/PhysRev.128.2407]. This implies that amplifier intrinsic noise sources induce random perturbations on the phase of the incoming optical signal, which translates into spectral broadening. Achieving the minimum induced phase fluctuation requires a phase measurement method that introduces minimum uncertainty, i.e., optimum phase measurement. We demonstrate that a measurement method based on heterodyne detection and extended Kalman filtering approaches optimum phase measurement in the presence of amplifier noise. A penalty of 5 dB (numerical) and 15 dB (experimental) compared to quantum limited spectral broadening is achieved. Spectral broadening reduction of 44 dB is achieved, compared to when using the widely employed phase measurement method, based purely on the argument of the signal field. Our results reveal new scientific insights by demonstrating a phase measurement method that enables to approach minimum phase fluctuation, induced by amplifier noise. An impact is envisioned for phase-based optical sensing systems, as optical amplification could increase sensing distance with minimum impact on the phase.

A proposal for additional temporal compression and peak power enhancement of intense ( > T W / c m 2 ) femtosecond laser pulses using two thin plane-parallel plates is presented. The first ultrathin plate (order of mm) induces spectral broadening due to self-phase modulation, and the second ultrathin plate (order of micron) corrects the spectral phase. The elimination of the negative dispersive multilayer coating from the scheme offers an improved laser-induced damage threshold for the post-compression process.

Three-dimensional particle-in-cell simulations are conducted to investigate the self-compression of a sub-terawatt (TW) laser pulse when propagating in a thin, dense gas target. In this way, the interplay of ionization of gas atoms, defocusing and diffraction of laser fields cause the phase modulation and spectral broadening required to realize a short duration for the output pulse 1 , 2 .

Interplay between dispersion and nonlinearity in optical fibers is a fundamental research topic of nonlinear fiber optics. Here we numerically and experimentally investigate an incoherent continuous-wave (CW) optical field propagating in the fiber with normal dispersion, and introduce a distinctive spectral evolution that differs from the previous reports with coherent mode-locked fiber lasers and partially coherent Raman fiber lasers [Nat. Photonics 9, 608 (2015).]. We further reveal that the underlying physical mechanism is attributed to a novel interplay between group-velocity dispersion (GVD), self-phase modulation (SPM) and inverse four-wave mixing (IFWM), in which SPM and GVD are responsible for the first spectral broadening, while the following spectral recompression is due to the GVD-assisted IFWM, and the eventual stationary spectrum is owing to the dominant contribution of GVD effect. We believe this work can not only expand the light propagation in the fiber to a more general case and help advance the physical understanding of light propagation with different statistical properties, but also benefit the applications in sensing, telecommunications and fiber lasers.

In this paper, the pixelated phase mask (PPM) method of interferometry is applied to coherence imaging (CI)—a passive, narrowband spectral imaging technique for diagnosing the edge and divertor regions of fusion plasma experiments. Compared to previous CI designs that use a linear phase mask, the PPM method allows for a higher possible spatial resolution. The PPM method is also observed to give a higher instrument contrast (analogous to a more narrow spectrometer instrument function). A single-delay PPM instrument is introduced as well as a multi-delay system that uses a combination of both pixelated and linear phase masks to encode the coherence of the observed radiation at four different interferometer delays simultaneously. The new methods are demonstrated with measurements of electron density ne, via Stark broadening of the Hγ emission line at 434.0 nm, made on the Magnum-PSI linear plasma experiment. A comparison of the Abel-inverted multi-delay CI measurements with Thomson scattering shows agreement across the 3 × 1019 < ne < 1 × 1021 m−3 range. For the single-delay CI results, agreement is found for ne > 1 × 1020 m−3 only. Accurate and independent interpretation of single-delay CI data at lower ne was not possible due to Doppler broadening and continuum emission.

We compare supercontinuum generation in $$\hbox {CaF}_2$$ CaF 2 crystal under tight and loose focusing of 150 fs, 515 nm second harmonic pulses from an amplified Yb:KGW laser at a repetition rate of 10 kHz. It is demonstrated that supercontinuum generation geometry applying loose focusing ($$\hbox {NA}=0.004$$ NA = 0.004 ) of the pump beam into a long (25 mm) $$\hbox {CaF}_2$$ CaF 2 sample is advantageous in terms of supercontinuum spectral extent and durability of damage-free operation of the nonlinear material as compared to a commonly used supercontinuum generation setup which employs tight focusing ($$\hbox {NA}=0.012$$ NA = 0.012 ) into a short (5 mm) sample and to setup which uses tight focusing into a long (25 mm) sample. More specifically, loose focusing into a long sample showed remarkably longer (20 min) damage-free operation of the nonlinear material, which was not translated with respect of the pump beam, while in tight focusing condition the sample is damaged just within 2 min of operation, leading to a complete extinction of the supercontinuum spectrum. The evolution of optical degradation of the nonlinear material in time and its impact to supercontinuum spectrum is studied in terms of filament-induced luminescence due to self-trapped exciton emission and light scattering at the pump wavelength indicating the onset of optical damage. Our findings are supported by the numerical simulations which compare relevant parameters related to filament propagation in tight and loose focusing conditions.

High-energy few-ps pulses from a Ho:YLF chirped pulse amplifier operating at a 1 kHz repetition rate are compressed in a two-stage arrangement to sub-90-fs duration. The energy of the compressed pulses is more than 20 mJ at an average power of 20 W. In the first stage, the duration of the 2.8 ps, 40 mJ pulses at 2.05 µm wavelength was reduced to 1.4 ps by using nonlinear propagation in air. Subsequently, the pulses were further compressed to 86 fs after spectral broadening in a 3-m-long Kr-filled stretched flexible hollow-core fiber. The high photon flux, peak power, and excellent beam quality and stability make this light source highly attractive for fs pulse generation in the extreme ultraviolet (XUV) to x-ray spectral range for time-resolved XUV spectroscopy or measurements of structural dynamics in solids.

The ultrabroadband supercontinuum (SC) source possesses a number of unique characteristics which have had a major impact in many research fields, ranging from precision metrology to high resolution imaging and ultrafast spectroscopy [1] . The generation of a supercontinuum is a highly complex process resulting from the propagation of intense light pulses in a nonlinear medium such as an optical fibre or waveguide. As a result of the very rich landscape of nonlinear dynamics, the input pulses injected into the fibre experience massive spectral broadening that can expand an initially narrowband input field to an output spanning from the visible to infrared [1] .

Tailoring the electromagnetic field transients has been a prominent research focus over the last decade. Advances in ultrashort pulse generation and stabilizing the carrier phase of the electromagnetic field relative to its envelope allowed for extension of coherent synthesis to optical frequencies and ultrashort pulse domain at tens of microjoules of energy. In parallel, ytterbium-doped lasers become a mature technology. They are able to deliver down to 1-picosecond scale pulses at hundreds of millijoule energy and kilowatt-scale average power, making them suitable frontends for scaling the energy and power of light transients. In this paper, we discuss two conceptual schemes, our experimental results, and technological challenges for generation of sub-cycle light transients based on Yb:YAG thin-disk lasers by direct and efficient spectral broadening of ytterbium-doped lasers, and by coherent combination of pulses from multiple broadband optical parametric amplifiers. Moreover, a conceptual design study for a novel synthesis scheme based on polarization splitting of a broadband spectrum and amplification of each polarization in a separate stage is presented. The novel sources hold promise for studying and controlling the nonlinear interactions of matter with custom-tailored light transients at a sub-cycle period of their electric field, opening up unprecedented opportunities in attoscience and strong-field physics.

Spectral broadening of a radially polarized (RP) pulse in a gas-filled hollow-core fiber (HCF) is experimentally studied. Broad spectrum covering from 540 nm to 1070 nm is obtained. It is found that the output spectrum preserves the RP state after nonlinear propagation in HCF, both for the components in the input pulse and for the newly generated spectral portion. This method shows great potential in generating RP sources with unprecedented spectral width, and may also be extended to other kinds of cylindrical vector beams.

We present investigations on the extent of spectral broadening occurring during the propagation of laser pulses in a Thin-Disk Multipass Amplifier (TDMPA) with the aim to exploit this nonlinear effect for pulse compression. The demonstrated compression of 2.05 mJ-pulses to a duration of about 136 fs FWHM at an average power of 409 W and a peak power of 9 GW is in good agreement with numerical pulse propagation simulations. Our spectral broadening and compression scheme introduced additional losses below 2%, is scalable to average powers of >1 kW with multi-mJ pulses, and strongly enhances the capabilities of ultrafast TDMPAs towards shorter pulses and higher peak powers.

The authors show the role of space-time focusing in the spectral broadening process of multipass cells.

Laser-induced breakdown spectroscopy is a promising method for rapidly measuring hydrogen and its isotopes, critical to a wide range of disciplines (e.g. nuclear energy, hydrogen storage). However, line broadening can hinder the ability to detect finely spaced isotopic shifts. Here, the effects of varying plasma generation conditions (nanosecond versus femtosecond laser ablation) and ambient environments (argon versus helium gas) on spectral features generated from Zircaloy-4 targets with varying hydrogen isotopic compositions were studied. Time-resolved 2D spectral imaging was employed to detail the spatial distribution of species throughout plasma evolution. Results highlight that hydrogen and deuterium isotopic shifts can be measured with minimal spectral broadening in a ∼ 10 Torr helium gas environment using ultrafast laser-produced plasmas.

The average power scaling of counter pumped Yb-doped monolithic fiber amplifiers emitting a diffraction-limited beam has been investigated. The characteristics of seed laser including linewidth and power are optimized, which are beneficial for the suppression of stimulated Brillouin scattering (SBS), nonlinear-induced spectral broadening and transverse mode instabilities (TMI). The fiber with a mode field area of 700 μm2 is able to amplify a 360 W narrow linewidth seed up to 4.05 kW average output power before the onset of TMI. Up to this power level, a narrow linewidth with (0.372 nm), high optical-to-optical efficiency of 67.6% with diffraction-limited beam quality and without any sign of TMI or stimulated Raman scattering for a spectral dynamic range of higher than -52 dB is obtained.

Recent work has shown that temporally stable optical sources are required in a narrow linewidth Yb-Raman fiber amplifier to suppress the spectral broadening phenomenon. Superfluorescent fiber sources (SFSs) with different spectral widths are used as the Raman-pumped lasers in a 200-watt level narrow linewidth Yb-Raman fiber amplifier for the first time to the best of our knowledge. The experimental results reveal that the spectral broadening phenomenon could be well controlled by using the broadband SFS. Therefore, the narrow linewidth operation could be well maintained during the power scaling process. Moreover, the suppression of the spectral broadening phenomenon would deteriorate when the spectral width of the SFS decreases. This work could provide a compact, low-cost choice for the Raman-pumped laser in narrow linewidth Yb-Raman fiber amplifiers.

In this work, the linewidth performance of triangle-shaped pulsed fiber laser (TPL) with 6.5-ns pulse width was investigated numerically and experimentally. The spectral linewidth of TPL changed with different rising times and fixed pulse width. The minimum spectral linewidth could be obtained when the rising time or falling time is equal to 0. Besides, the self-phase modulation (SPM) could be suppressed when the rising time of TPL is less than 3.25 ns, and the tunable linewidth from 77.73 MHz to 86.69 MHz was obtained at 2.1-kW peak power of pulse. It was found that experimental results are consistent with the theoretical analysis.

We propose a gas sensor using quantum cascade lasers which achieves high sensitivity and gas selectivity using a compact configuration and simple signal processing. This sensor features a small-volume Herriott cell and a simple concentration quantification algorithm. Our invented Herriott cell offers 5 m optical path length with only 40 mL internal volume. To preform concentration determination, feature quantities are extracted from an absorption signal by correlating the measured absorption signal with predetermined reference signals. The extracted feature quantities are used for the determination of target and interfering gas concentrations with simple simultaneous linear equations. The method is also capable of correcting for various disturbances, such as spectral shift due to laser wavelength drift and spectral broadening due to partial pressure changes of coexisting gases, by extracting additional, appropriate, feature quantities. This concentration quantification algorithm dramatically reduces calculation time compared to conventional spectral curve fitting methods because hundreds of spectral data points are replaced with a small number of feature quantities used during calculation. This approach allows the construction of a compact, fast-responding and robust gas sensor. We demonstrate a multicomponent and real-time measurement with a prototype of the proposed gas sensor. The prototype showed sub-ppm detection limits for NO, NO2 and N2O with 0.1 s integration time, and the interferences from high concentrations of H2O and CO2 could be removed completely. Furthermore, it was also shown that the prototype demonstrates excellent robustness against ambient temperature changes and spectral broadening effects caused by coexisting gases.

Optical coherence tomography (OCT) is a well-established modality providing cross-sectional images of the human retina. OCT, however, does not provide high-resolution en face images of the outer retinal layers due to eye aberrations and the tradeoff between imaging depth and transverse resolution. Recently, we demonstrated the spatiotemporal optical coherence (STOC) that provides high-resolution, aberration-free volumetric images of the retina and cornea with the voxel rate of 8 GHz. Here, we show that STOC can be used for angiography, and allows us to estimate Doppler broadening, enabling visualization of the choroidal blood vessels in the human retina in vivo.

The principle of a Mamyshev oscillator depends on alternating spectral filtering between sections of spectral broadening by self-phase modulation. In the 2 µm wavelength range, this concept faces the difficulty that standard fibers are anomalous dispersive which limits the possible pulse energy to the pJ-regime without proper dispersion management. We applied ultra-high numerical aperture fibers with normal dispersion in order to achieve up-chirped pulses in an anomalous dispersive Thulium-doped gain fiber. With that design, we achieved mode-locked pulses with energies of 6.4 nJ and a compressed autocorrelation duration of 195 fs at a repetition rate of 16 MHz.

The rapid development of the optical-cycle-level ultra-fast laser technologies may break through the bottleneck of the traditional ultra-intense laser [i.e., Petawatt (PW, 1015 W) laser currently] and enable the generation of even higher peak-power/intensity lasers. Herein, we simulate an ultra-broadband concept for the realization of an Exawatt-class (EW, 1018 W) high peak-power laser, where the wide-angle non-collinear optical parametric chirped-pulse amplification (WNOPCPA) is combined with the thin-plate post-compression. A frequency-chirped carrier-envelope-phase stable super-continuum laser is amplified to high-energy in WNOPCPA by pumping with two pump-beamlets and injected into the thin-plate post-compression to generate a sub-optical-cycle high-energy laser pulse. The numerical simulation shows this hybrid concept significantly enhances the gain bandwidth in the high-energy amplifier and the spectral broadening in the post-compression. By using this concept, a study of a prototype design of a 0.5 EW system is presented, and several key challenges are also examined.

We studied the evolution of the two-color laser filament emitted THz pulse energy with the influences of the infrared pump energy and air dispersion. The THz modulation induced by additional relative phase is verified by the two-color spectral broadening due to the intensity dependent nonlinearity.

This talk provides a review of laser pulse post-compression leveraged by the advent of the multi-pass spectral broadening scheme, including perspectives on expanding the limits of pulse duration and energy.

Publisher Summary This chapter deals with the properties of highly non-linear fibers. It reviews the techniques used to measure the non-linear parameter (γ). The four kinds of fibers that are developed to enhance the non-linear effects are also reviewed. The three major non-linear effects occurring inside the optical fibers—the self-phase modulation (SPM), the cross-phase modulation (XPM), and the four-wave mixing (FWM), are governed by a single non-linear parameter γ. This parameter is fixed for each glass material. The chapter elaborates that the use of the non-silica glasses provides an alternative approach to designing non-linear fibers. The SPM technique uses the broadening of the pulse spectrum. It measures the maximum value of the non-linear phase shift. In place of deducing the SPM-induced phase shift from spectral broadening, a related technique deduces it from the spectral changes occurring when light from two lasers operating at slightly different wavelengths is transmitted through the fiber. Measurements in such an approach can be performed with CW lasers. The chapter summarizes that the combination of unusual dispersive properties and a high value of γ leads to a variety of novel non-linear effects.

Spatial-spectral characteristics of Yb:KGW laser pulses passing through a multi-pass cell and a multiple-plate continuum are analyzed. The former exhibits slightly better spectral homogeneity under the same degree of spectral broadening due to less B-integral per pass.

We present an isothermal chamber designed and realized at the Italian Metrological Institute (INRiM) to stabilize the temperature of a quartz cell for Doppler-broadened precision spectroscopy of mercury atoms in the UV region for the practical realization of the new definition of kelvin. The challenge of this apparatus is the capability to control, with great accuracy, the temperature of a non-cylindrical quartz cavity containing the mercury vapors. The temperature was actively controlled and maintained stable within 0.05 mK at the triple point of water for 15 h, but longer periods are possible. The spatial uniformity was also characterized. The traceability of temperature measurements and the complete uncertainty budget are discussed.

Photon accelerators can spectrally broaden laser pulses with high efficiency in moving electron density gradients driven in a rapidly ionizing plasma. When driven by a conventional laser pulse, the group velocity walk-off experienced by the accelerated photons and deterioration of the gradient from diffraction and plasma refraction limit the extent of spectral broadening. Here we show that a laser pulse with a shaped space-time and transverse intensity profile overcomes these limitations by creating a guiding density profile at a tunable velocity. Self-photon acceleration in this profile leads to dramatic spectral broadening and intensity steepening, forming an optical shock that further enhances the rate of spectral broadening. In this new regime, multi-octave spectra extending from 400 to 60 nm wavelengths, which support near-transform-limited $l400$ as pulses, are generated over $l100 \ensuremath{\mu}\mathrm{m}$ of interaction length.

Lasing without inversion (LWI) in specific models of closed three-level systems is analyzed in terms of nonlinear dynamics. From a linear stability analysis of the trivial nonlasing solution of the homogeneously broadened systems with on-resonance driving and laser fields, we find that, near lasing threshold, resonant closed $\ensuremath{\Lambda}$ and $V$ schemes yield continuous-wave LWI while resonant cascade schemes can give rise to self-pulsing LWI. The origin of this different behavior is discussed. For parameters of a real cascade system in atomic ${}^{138}\mathrm{Ba}$ we check numerically that the self-pulsing solution is stable in a broad range of nonzero detunings. It is shown that the self-pulsing emission can still be observed when the typical residual Doppler broadening of an atomic beam is taken into account.

We propose a new architecture, double-pass multiple-plate compression (DPMPC), for nonlinear pulse compression. The design of double optical passes avoids pushing the nonlinear phase shift to the limit (as opposed to traditional single-stage MPC). The moderately broadened spectrum in the first optical pass permits quality dispersion compensation by highly dispersive mirrors (as opposed to multi-pass cell), which is beneficial for further spectral broadening in the second optical pass. As a result, DPMPC realizes the advantages of MPC and multi-pass cell simultaneously with a simple and compact geometry.