V. A. Kuz'menko
The classical pump-probe scheme for experiments with femtosecond laser is proposed to use for study the inequality of forward and reversed transitions in optics.
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The experiments with broadband down-converted light clearly show inequality of forward and reversed processes in optics.
Connection between the concept of entanglement and origin of nonlinear phenomena in optics is discussed.
The experiments with weak collinear probe pulses for time reversal noninvariance study in optics are proposed and discussed.
We demonstrate that thousands of periodic nano-craters are fabricated on a subwavelength-diameter tapered optical fiber, an optical nanofiber, by irradiating with just a single femtosecond laser pulse. A key aspect of the fabrication is that the nanofiber itself acts as a cylindrical lens and focuses the femtosecond laser beam on its shadow surface. We also demonstrate that such periodic structures on the nanofiber, act as a 1-D photonic crystal (PhC). Such PhC structures on the nanofiber will strongly enhance the field confinement in such a tapered fiber-based system and may open new avenues …
We report on fabrication of tubular optical waveguides buried in ZBLAN glass based on transverse femtosecond laser direct writing. Irradiation in ZBLAN with focused femtosecond laser pulses leads to decrease of refractive index in the modified region. Tubular optical waveguides of variable mode areas are fabricated by forming the four sides of the cladding with slit-shaped femtosecond laser pulses, ensuring single mode waveguiding with a mode field dimension as small as ~ 4 μm.
Time-reversal symmetry is important to optics. In linear optics, a time-reversed process to laser emission enables total absorption of coherent light fields into an optical cavity of loss by time-reversing the original gain medium. In nonlinear optics, time symmetry exists for some well-known processes such as parametric up/down conversion, sum/difference frequency generation, however, combining exact time-reversal symmetry with nonlinear wave mixings is yet explored till now. Here, we demonstrate time reversed wave mixings for second harmonic generation (SHG) and optical parametric amplificat…
We combine a tunable continuous-wave optical parametric oscillator and a femtosecond Ti:Sapphire laser frequency comb to provide a phase-coherent bridge between the visible and mid-infrared spectral ranges. As a first demonstration of this new technique we perform a direct frequency comparison between an iodine stabilized Nd:YAG laser at 1064 nm and an infrared methane optical frequency standard at $3.39 μ$m.
We propose to use femtosecond direct laser writing technique to realize dielectric optical elements from photo-resist materials for the generation of structured light from purely geometrical phase transformations. This is illustrated by the fabrication and characterization of spin-to-orbital optical angular momentum couplers generating optical vortices of topological charge from 1 to 20. In addition, the technique is scalable and allows obtaining microscopic to macroscopic flat optics. These results thus demonstrate that direct 3D photopolymerization technology qualifies for the realization of…
Lossless linear wave propagation is symmetric in time, a principle which can be used to create time reversed waves. Such waves are special 'pre-scattered' spatiotemporal fields, which propagate through a complex medium as if observing a scattering process in reverse, entering the medium as a complicated spatiotemporal field and arriving after propagation as a desired target field, such as a spatiotemporal focus. Time reversed waves have previously been demonstrated for relatively low frequency phenomena such as acoustics, water waves and microwaves. Many attempts have been made to extend these…
Optical frequency combs are laser sources which are capable of generating discrete, equal-spaced and highly coherent comb modes. Optical frequency comb technique provides a significant bridge to transfer the stability between optical frequency and radio frequency. The advances of this technology greatly promote the development of precision spectroscopy, optical time/frequency transfer, optical frequency division, long-distance transfer of time/frequency references and high-precision distance measurement. Benefiting from the wide spectral outputs, femtosecond lasers have become the best choice …
Understanding the physical mechanisms of the refractive index modulation induced by femtosecond laser writing is crucial for tailoring the properties of the resulting optical waveguides. In this work we apply polarized Raman spectroscopy to study the origin of stress-induced waveguides in diamond, produced by femtosecond laser writing. The change in the refractive index induced by the femtosecond laser in the crystal is derived from the measured stress in the waveguides. The results help to explain the waveguide polarization sensitive guiding mechanism, as well as providing a technique for the…
An influence of pass-through optics on femtosecond laser pulse filamentation in ambient air is analyzed for the first time both experimentally and numerically. Propagation of high-power femtosecond laser pulse through solid optical elements introduces spatiotemporal phase modulation due to the Kerr effect. This modulation may have a strong ef-fect on the pulse filamentation in air. We demonstrated that the phase modulation obtained in the thin pass-through dielectric plate reduces the distance to the filament onset and increases the plasma channel length
We investigate beam pointing dynamics in filamentation in gases driven by high repetition rate femtosecond laser pulses. Upon suddenly exposing a gas to a kilohertz train of filamenting pulses, the filament is steered from its original direction to a new stable direction whose equilibrium is determined by a balance among buoyant, viscous, and diffusive processes in the gas. Results are shown for Xe and air, but are broadly applicable to all configurations employing high repetition rate femtosecond laser propagation in gases.
We report on fabrication of a microtoroid resonator of a high-quality factor (i. e., Q-factor of ~3.24x10^6 measured under the critical coupling condition) using femtosecond laser three-dimensional (3D) micromachining. Coupling of light into and out of the microresonator has been realized with a fiber taper that is reliably assembled with the microtoroid. The assembly of the fiber taper to the microtoroid is achieved by welding the fiber taper onto the sidewall of the microtoroid using CO2 laser irradiation. The integrated microresonator maintains a high Q-factor of 3.21x10^5 as measured in ai…
We report a study on the optical dipole force on a beam of neutral three-level like atomic system induced by a femtosecond super-Gaussian pulse. We show that maximum coherence between the ground state |1> and the excited state |2> could be achieved using a train of femtosecond pulses. In addition, it is possible to control the trajectory of the atoms in an atomic beam by using the same scheme. The robustness of the scheme against the variation of the pulse parameters is also investigated.
This work represents a particular application of waveguide fabricated by femtosecond laser micromachining technology. More specifically, we report the development of an optical modulator based on the fabrication of single-mode optical waveguide structures buried in X-cut lithium niobate crystal with the femtosecond laser direct writing method. Here, change in refractive index profile is measured using near field intensity profile measurement method at optimized writing conditions. It has been observed that the change refractive index (Δn) is in the range of 10-4. Finally, the behavior of femto…
Fiber-optic nonlinear wavelength converter for adaptive femtosecond biophotonics
针对超快激光缺乏可调谐光纤传输脉冲的问题，提出光纤非线性波长转换器，实现宽范围可调脉冲输出，并应用于实时活体成像。
We experimentally demonstrated a high-order optical vortex pulsed laser based on a mode selective all-fiber fused coupler composed of a single-mode fiber (SMF) and a few-mode fiber (FMF). The fused SMF-FMF coupler inserted in the cavity not only acts as mode converter from LP01 mode to LP11 or LP21 modes with a broadband width over 100 nm, but also directly delivers femtosecond vortex pulses out of the mode locked cavity. To the best of our knowledge, this is the first report on the generation of high-order pulse vortex beams in mode-locked fiber laser. The generated 140 femtosecond vortex bea…
Recent works [Y.D. Chong {\it et al.}, Phys. Rev. Lett. {\bf 105}, 053901 (2010); W. Wan {\it et al.}, Science {\bf 331}, 889 (2011)] have shown that the time-reversed process of lasing at threshold realizes a coherent perfect absorber (CPA). In a CPA, a lossy medium in an optical cavity with a specific degree of dissipation, equal in modulus to the gain of the lasing medium, can perfectly absorb coherent optical waves at discrete frequencies that are the time-reversed counterpart of the lasing modes. Here the concepts of time-reversal of lasing and CPA are extended for optical radiation emitt…
We present a rapid-scan, time-domain terahertz spectrometer employing femtosecond Er:fiber technology and an acousto-optic delay with attosecond precision, enabling scanning of terahertz transients over a 12.4 ps time window at a waveform refresh rate of 36 kHz, and a signal-to-noise ratio of $1.7 \times 10^5/\sqrt{\rm Hz}$. Our approach enables real-time monitoring of dynamic THz processes at unprecedented speeds, which we demonstrate through rapid 2D thickness mapping of a spinning teflon disc at a precision of $10\,\rm nm/\sqrt{\rm Hz}$. The compact, all-optical design ensures alignment-fre…
We present experimental evidence of plasmonic-enhanced optical tweezers, of polystyrene beads in deionized water in the vicinity of metal-coated nanostructures. The optical tweezers operate with a continuous wave (CW) near-infrared laser. We employ a Cu/Au bilayer that significantly improves dissipation of heat generated by the trapping laser beam and avoid de-trapping from heat convection currents. We investigate the improvement of the optical trapping force, the effective trapping quality factor, and observe an exponential distance dependence of the trapping force from the nanostructures, ex…
Fast fabrication of micro-optical elements for generation of optical vortex beams based on the q-plate design is demonstrated by femtosecond (fs) laser ablation of gold film on glass. Q-plates with diameter of ~0.5 mm were made in ~1 min using galvanometric scanners with writing speed of 5 mm/s. Period of gratings of 0.8 micrometers and groove width of 250 nm were achieved using fs-laser ablation at 343 nm wavelength. Phase and intensity analysis of optical vortex generators was carried out at 633 nm wavelength and confirmed the designed performance. Efficiency of spin-orbital conversion of th…
We show the existence of nonlinear resonant states in a higher-order nonlinear Schrödinger model that appertains to the wave propagation in femtosecond fiber optics, under certain parametric regime. These nonlinear resonant states are analytically illustrated in terms of Gaussian beams, Airy beams, and periodic beams that resulted due to the presence of quadratic, linear, and constant type of `smart' potentials, respectively, of the ensuing model. Interestingly, the nonlinear chirp associated with each of these novel resonant states can be efficiently controlled, by varying the self-steepening…
We report a mode-locked Ti:sapphire femtosecond laser with 5GHz repetition rate. Spectral broadening of the 24 fs pulses in a microstructured fiber yields an octave-spanning spectrum and permits self-referencing and active stabilization of the emitted femtosecond laser frequency comb (FLFC). The individual modes of the 5GHz FLFC are resolved with a high-resolution spectrometer based on a virtually imaged phased array (VIPA) spectral disperser. Isolation of single comb elements at a microwatt average power level is demonstrated. The combination of the high-power, frequency-stabilized 5GHz laser…
Pump-probe experiments combining pulses from a X-ray FEL and an optical femtosecond laser are very attractive for sub-picosecond time-resolved studies. Since the synchronization between the two independent light sources to an accuracy of 100 fs is not yet solved, it is proposed to derive both femtosecond radiation pulses from the same electron bunch but from two insertion devices. This eliminates the need for synchronization and developing a tunable high power femtosecond quantum laser. In the proposed scheme a GW-level soft X-ray pulse is naturally synchronized with a GW-level optical pulse, …
The interaction of chirped femtosecond laser pulses with hybrid materials - materials comprised of plasmon sustaining structures and resonant molecules - is scrutinized using a self-consistent model of coupled Maxwell-Bloch equations. The optical properties of such systems are examined with the example of periodic sinusoidal gratings. It is shown that under strong coupling conditions one can control light transmission using chirped pulses in a spatiotemporal manner. The temporal origin of control relies on chirps non-symmetric in time while the space control is achieved via spatial localizatio…
We show that an arbitrary body or aggregate can be made perfectly absorbing at discrete frequencies if a precise amount of dissipation is added under specific conditions of coherent monochromatic illumination. This effect arises from the interaction of optical absorption and wave interference, and corresponds to moving a zero of the elastic S-matrix onto the real wavevector axis. It is thus the time-reversed process of lasing at threshold. The effect is demonstrated in a simple Si slab geometry illuminated in the 500-900 nm range. Coherent perfect absorbers are novel linear optical elements, a…
Low loss, optical channel waveguides have been successfully produced by high repetition rate, femtosecond laser inscription in a Li-Zn fluoroborate glass (64.9B2O3 + 25Li2O + 10ZnF2 + 0.1Er2O3). High quality waveguides were produced at 500 kHz, 1 MHz and 2 MHz laser repetition rates, showing a refractive index contrast in the range of 3-6 x 10-3 depending on various fluences. Dependence of experimental parameters such as average laser power, pulse repetition rate and writing speed on the properties of fabricated waveguides has been discussed. The comparison of optical and compositional charact…
In magnetoplasmonics, it is possible to tailor the magneto-optical properties of nanostructures by exciting surface plasmon polaritons (SPPs). Thus far, magnetoplasmonic effects have been considered static. Here, we describe ultrafast manifestations of magnetoplasmonics by observing the non-trivial evolution of the transverse magneto-optic Kerr effect within 45-fs pulses reflected from an iron-based magnetoplasmonic crystal. The effect occurs for resonant SPP excitations, displays opposite time derivative signs for different slopes of the resonance, and is explained with the magnetization-depe…
We present full phase stabilization of an amplified Yb:fiber femtosecond frequency comb using an intra-cavity electro-optic modulator and an acousto-optic modulator. These transducers provide high servo bandwidths of 580 kHz and 250 kHz for frep and fceo, producing a robust and low phase noise fiber frequency comb. The comb was self-referenced with an f - 2f interferometer and phase locked to an ultra-stable optical reference used for the JILA Sr optical clock at 698 nm, exhibiting 0.21 rad and 0.47 rad of integrated phase errors (over 1 mHz - 1 MHz) respectively. Alternatively, the comb was l…
Diamond's nitrogen vacancy (NV) center is an optically active defect with long spin coherence times, showing great potential for both efficient nanoscale magnetometry and quantum information processing schemes. Recently, both the formation of buried 3D optical waveguides and high quality single NVs in diamond were demonstrated using the versatile femtosecond laser-writing technique. However, until now, combining these technologies has been an outstanding challenge. In this work, we fabricate laser written photonic waveguides in quantum grade diamond which are aligned to within micron resolutio…
We report the synthesis and measurement of an ultra-precise and extremely stable optical frequency in the telecommunications window around 1543 nm. Using a fibre-based femtosecond frequency comb we have phase-stabilised a fibre laser at 194 THz to an optical frequency standard at 344 THz, thus transferring the properties of the optical frequency standard to another spectral region. Relative to the optical frequency standard, the synthesised frequency at 194 THz is determined to within 1 mHz and its fractional frequency instability is measured to be less than 2*10^{-15} at 1 s, reaching 5*10^{-…
We propose a novel technique of microscopy to overcome the effects of both scattering and limitation of the accessible depth due to the objective working distance. By combining Laser Optical Feedback Imaging (LOFI) with Acoustic Photon Taging (APT) and Synthetic Aperture (SA) refocusing we demonstrate an ultimate shot noise sensitivity at low power (required to preserve the tissues) and a high resolution beyond the microscope working distance. More precisely, with a laser power of 10mW, we obtain images with a micrometric resolution over ~8 transport mean free paths, corresponding to 1.3 times…
Intrinsic to the process of high-order harmonic generation is the creation of plasma and the resulting spatiotemporal distortions of the driving laser pulse. Inside a high finesse cavity where the driver pulse and gas medium are reused, this can lead to optical bistability of the cavity-plasma system, accumulated self-phase modulation of the intracavity pulse, and coupling to higher order cavity modes. We present an experimental and theoretical study of these effects and discuss their implications for power scaling of intracavity high-order harmonic generation and extreme ultraviolet frequency…
Femtosecond laser written waveguides in sapphire for visible light delivery
本研究通过飞秒激光在Sapphire中刻写弯曲波导，实现728nm光的单模传输，损耗为1.9(3)dB/cm，展示了在Sapphire中构建集成光学波导的潜力。
We introduce a wide-field magneto-optical microscope to probe magnetization dynamics with femtosecond temporal and sub-micrometer spatial resolution. We carefully calibrate the non-linear dependency between the magnetization of the sample and the detected light intensity by determining the absolute values of the magneto-optical polarization rotation. With that, an analytical transfer function is defined to directly map the recorded intensity to the corresponding magnetization, which results in significantly reduced acquisition times and relaxed computational requirements. The performance of th…
We have measured the frequency of the $6s^2S_{1/2} - 5d^2D_{3/2}$ electric-quadrupole transition of $^{171}$Yb$^+$ with a relative uncertainty of $1\times 10^{-14}$, $ν_{Yb}$ = 688 358 979 309 312 Hz $\pm$ 6 Hz. A femtosecond frequency comb generator was used to phase-coherently link the optical frequency derived from a single trapped ion to a cesium fountain controlled hydrogen maser. This measurement is one of the most accurate measurements of optical frequencies ever reported, and it represents a contribution to the development of optical clocks based on an $^{171}$Yb$^+$ ion standard.
Within a synchronously pumped optical parametric oscillator (SPOPO), the inherent synchronism between the pump and the resonating signal is the magic to partly transfer the coherence property of the pump to the signal. In our demonstration, Vernier effect is observed within a femtosecond SPOPO by simply detuning the FSR of the cavity, generating signal pulses at tunable repetition rate from several GHz to 1 THz with a maximum 22.58 nm full width half maximum (FWHM) bandwidth supporting 160 fs pulses covering the C- and L-bands of the telecom wavelength region. This technique offers a simple me…
In this work, for the first time of our knowledge, we present a well-supported procedure to fabricate ridge optical waveguides onto thin films of PLZT by femtosecond laser ablation. In order to achieve smooth guiding structures to guarantee good optical performance, we have studied the ablation threshold characteristics for different kinematic conditions of fs laser machining and also we have explored the different ablation regimens for several fluencies reached. Besides, we characterize the morphology and roughness of ridge waveguides through a modal and scattering loss analysis, for waveguid…
When a photo-diode is illuminated by a pulse train from a femtosecond laser, it generates microwaves components at the harmonics of the repetition rate within its bandwidth. The phase of these components (relative to the optical pulse train) is known to be dependent on the optical energy per pulse. We present an experimental study of this dependence in InGaAs pin photo-diodes illuminated with ultra-short pulses generated by an Erbium-doped fiber based femtosecond laser. The energy to phase dependence is measured over a large range of impinging pulse energies near and above saturation for two t…
We propose a feedback-assisted direct laser writing method to perform laser ablation of fiber optics devices in which their light-collection signal is used to optimize their properties. A femtosecond-pulsed laser beam is used to ablate a metal coating deposited around a tapered optical fiber, employed to show the suitability of the approach to pattern devices with small radius of curvature. During processing, the same pulses generate two-photon fluorescence in the surrounding environment and the signal is monitored to identify different patterning regimes over time through spectral analysis. T…
For fused silica irradiated by near-100-fs, 795-nm laser pulses with fluence approaching the damage threshold, the transient transmission spectroscopy based on a wavelength-degenerate pump-probe technique clearly presents two dynamic processes corresponding to the instantaneous effects of laser optical field and the delayed effects of free electron dynamics, respectively. The reversible, instantaneous process originates in third-order nonlinear optical responses (in particular the optical Kerr effect) ascribed to virtual optical-field ionization (VOFI) that significantly contributes to the non…
High-power femtosecond laser radiation propagates nonlinearly in air exhibiting pulse self-focusing and strong multiphoton medium ionization, which leads to the spatial fragmentation of laser pulse into highly-localized light channels usually called the filaments. The filaments are characterized by high optical intensity, reduced (even zero) angular spreading and can contain laser plasma or be plasmaless (postfilaments). The presence of optical turbulence on the propagation path dramatically changes pulse filamentation dynamics and in some cases causes pulse fragmentation enhancement and colla…
The four-wave mixing produced with two ultrashort phase-locked w-3w laser pulses propagating coherently in a two-level system in the infrared spectral region is shown to depend on the pulses relative phase. The Maxwell-Bloch equations are solved beyond the rotating-wave approximation to account for field frequencies which are largely detuned from the atomic resonance. The relative phase dominating the efficiency of the coupling to the 5w anti-Stokes Raman component is determined by sign of the total ac Stark shift induced in the system, in such a way that the phase influence disappears precise…
Electro-optic time transfer with femtosecond stability
本研究针对光纤频率梳不满足低SWaP-C需求的问题，采用可全芯片集成的电光梳实现双向时间频率传递，在1秒平均时间下不稳定度低至15 fs，为高稳定、灵活的低SWaP-C时间传递网络提供了可行路径。
Origin of recently discovered reversed (opposite sign) dark resonances was explained theoretically and verified experimentally. It is shown that the reason for these resonances is a specific optical pumping of ground state level in a transition when ground state angular momentum is smaller than the excited state momentum.
Comparing Femtosecond Optical Tweezers with Conventional CW Optical Tweezers
本研究通过可切换模式的Ti:Sapphire激光器，对比不同功率下连续波与飞秒脉冲光镊对不同尺寸聚苯乙烯微球的捕获性能，发现脉冲光镊在小颗粒及高功率场景下稳定捕获效果更优。
The surface morphology of metal influences its optical absorptivity. Recent experiments have demonstrated that the femtosecond laser induced surface structures on metals could be dynamically controlled by the fluence of laser and the number of pulses. In this paper, we formulate an analytical model to calculate the optical absorption of a rough metallic surface by modeling the roughness as a fractal slab. For a given experimental image of the surface roughness, we characterize the roughness with a fractal parameter by using box-counting method. With this parameter as input, we calculate the ab…
The frequency comb created by a femtosecond mode-locked laser and a microstructured fiber is used to phase coherently measure the frequencies of both the Hg^+ and Ca optical standards with respect to the SI second as realized at NIST. We find the transition frequencies to be f_Hg=1 064 721 609 899 143(10) Hz and f_Ca=455 986 240 494 158(26) Hz, respectively. In addition to the unprecedented precision demonstrated here, this work is the precursor to all-optical atomic clocks based on the Hg^+ and Ca standards. Furthermore, when combined with previous measurements, we find no time variations of …
We demonstrate an efficient procedure for terahertz-wave radiation generation by applying a nonlinear wavelength conversion approach using a mid-infrared pump source. We used a 3.3 μm dual-chirped optical parametric amplification source and a 1.5 μm fiber laser for a comparison of energy conversion efficiencies. Nonlinear inorganic and organic crystals were used, and for the periodically-poled lithium niobate crystal, the efficiencies were 1.3*10^-4 and 5.6*10^-12 for the 3.3 μm source and the 1.5 μm source, respectively. We confirmed that nonlinear crystals could be pumped with tera-watt / cm…
We report about the dephasing of the spins states in a Garnet film excited by femtosecond laser pulses. It is shown that magneto-optical four-wave-mixing signals are efficiently generated and controlled with an external static magnetic field. The self-diffraction of either two or three femtosecond pulses allows us determining independently the coherent response time ($T_2$ < 50 fs) and the population relaxation, which occurs in several steps. After the laser induced demagnetization, the spin-lattice relaxation is the dominant mechanism ($T_{1,spin-lat}$ < 1.35 ps) and the precession of the mag…
Integrated femtosecond pulse and frequency comb sources are critical components for a wide range of applications. The leading approaches for on-chip pulse generation rely on mode locking inside microresonator with either third-order nonlinearity or with semiconductor gain. These approaches, however, are limited in noise performance, wavelength tunability and repetition rates. Alternatively, sub-picosecond pulses can be synthesized without mode-locking, by modulating a continuous-wave (CW) single-frequency laser using a cascade of electro-optic (EO) modulators. This method is particularly attra…
Fabrication of large area (sub 1 cm cross-section) micro-optical components in a short period of time (approximately 10 min) and with lesser number of processing steps is highly desirable and cost-effective. In the recent years, femtosecond laser fabrication technology has revolutionized the field of manufacturing by offering the above capabilities. In this study, a fundamental diffractive optical element, binary axicon i.e axicon with two phase or amplitude levels, has been designed in three configurations namely conventional axicon, photon sieve axicon (PSA) and sparse PSA and directly mille…
We report on fabrication of on-chip calcium fluoride (CaF2) microdisk resonators using water-assisted femtosecond laser micromachining. Focused ion beam (FIB) milling is used to create ultra-smooth sidewalls. The quality (Q)-factors of the fabricated microresonators are measured to be 4.2x10^4 at wavelengths near 1550 nm. The Q factor is mainly limited by the scattering from the bottom surface of the disk whose roughness remains high due to the femtosecond laser micromachining process. This technique facilitates formation of on-chip microresonators on various kinds of bulk crystalline material…
Reconfigurability of integrated photonic chips plays a key role in current experiments in the area of linear-optical quantum computing. We demonstrate a reconfigurable multiport interferometer implemented as a femtosecond laser-written integrated photonic device. The device includes a femtosecond laser-written $4\times 4$ multiport interferometer equipped with 12 thermooptical phase shifters, making it a universal programmable linear-optical circuit. We achieve a record fast switching time for a single nested Mach-Zender interferometer of $\sim10$ ms and quantitatively analyse the reconfigurab…
We put forward a co-axial pump(optical)-probe(X-rays) experimental concept and show performance of the optical component. A Bessel beam generator with a central 100 micrometers-diameter hole (on the optical axis) was fabricated using femtosecond (fs) laser structuring inside a silica plate. This flat-axicon optical element produces a needle-like axial intensity distribution which can be used for the optical pump pulse. The fs-X-ray free electron laser (X-FEL) beam of sub-1 micrometer diameter can be introduced through the central hole along the optical axis onto a target as a probe. Different …
We report on femtosecond laser writing of single mode optical waveguides in chalcogenide Gallium Lanthanum Sulphide (GLS) glass. A multiscan fabrication process was employed to create waveguides with symmetric single mode guidance and low insertion losses at 800 nm wavelength, compatible with Ti:Sapphire ultrafast lasers. μRaman and X-Ray microanalysis were used to elucidate the origin of the laser-induced refractive index change in GLS. Nonlinear refractive index measurements of the waveguides were performed by finding the optical switching parameters of a directional coupler, demonstrating t…
Direct compression of femtosecond optical pulses from a Ti:sapphire laser oscillator was realized with a cholesteric liquid crystal acting as a nonlinear 1D periodic Bragg grating. With a 6-um thick sample, the pulse duration could be compressed from 100 to 48 femtoseconds. Coupled-mode equations for forward and backward waves were employed to simulate the dynamics therein and good agreement between theory and experiment was obtained.
We report on the observation of backward lasing at 391.4 nm of nitrogen ions pumped by linearly polarized intense femtosecond pulses at 800 nm. The strongly enhanced spectral intensity at 391.4 nm, as well as the amplification of an externally injected backward seeding pulse, confirm that the backward 391.4 nm signal is due to optical amplification in the gas plasma. Compared to the forward emission at 391.4 nm, the optimal backward emission is achieved at a lower gas pressure around 10 mbar, which is due to asymmetry of the backward and forward directions rooted in the traveling excitation ge…
We show that a 1.13-GHz repetition rate optical pulse train with 0.70 fs high-frequency timing jitter (integration bandwidth of 17.5 kHz - 10 MHz, where the measurement instrument-limited noise floor contributes 0.41 fs in 10 MHz bandwidth) can be directly generated from a free-running, single-mode diode-pumped Yb:KYW laser mode-locked by single-walled carbon nanotube (SWCNT)-coated mirrors. To our knowledge, this is the lowest timing jitter optical pulse train with the GHz repetition rate ever measured. If this pulse train is used for direct sampling of 565-MHz signals (Nyquist frequency of t…
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