Fiber-based Femtosecond Laser Research Articles

Performance Enhancement of Photoconductive Antenna Using Saw-Toothed Plasmonic Contact Electrodes

A photoconductive logarithmic spiral antenna with saw-toothed plasmonic contact electrodes is proposed to provide a higher terahertz radiation compared with the conventional photoconductive antenna (PCA). The use of saw-toothed plasmonic contact electrodes creates a strong electric field between the anode and cathode, which generates a larger photocurrent and thereby effectively increases the terahertz radiation. The proposed PCA was fabricated and measured in response to an 80 fs optical pump from a fiber-based femtosecond laser with a wavelength of 780 nm. When the proposed antenna is loaded with an optical pump power of 20 mW and a bias voltage of 40 V, a broadband pulsed terahertz radiation in the frequency range of 0.1–2 THz was observed. Compared to the conventional PCA, the THz power measured by terahertz time domain spectroscopy (THz-TDS) increased by an average of 10.45 times.

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.

Measurement of the apex angle of a small prism by an oblique-incidence mode-locked femtosecond laser autocollimator

A major modification has been made to the conventional mode-locked femtosecond laser autocollimator so that the apex angle of a small prism, which is a transparent object difficult to be measured by the conventional optical methods, can be measured. In the oblique-incidence optical setup, a collimated mode-locked femtosecond laser is at first made incident to a target prism, and the laser beam reflected from the prism surface is then projected onto a reflective-type grating for angle measurement based on the principle of the femtosecond laser autocollimator. With the robust signal processing technique in the femtosecond laser autocollimator where the spectrum of the first-order diffracted beams emanated from the grating is captured by the fiber detector, high-precision angle measurement can be realized even on a surface having a small area for measurement and/or on a surface having low reflectivity. An oblique-incidence femtosecond laser autocollimator with a fiber-based femtosecond laser source is developed, and its basic characteristics are then evaluated in experiments. An experimental setup composed of the oblique-incidence femtosecond laser autocollimator and a high-precision rotary table is also constructed for prism angle measurement. To avoid the influence of the internally reflected beam in a prism, a method employing the characteristic of Brewster's angle is newly proposed, and its feasibility is verified through experiments. The uncertainty analysis on the measurement of the prism apex angle by the oblique-incidence mode-locked femtosecond laser is also carried out based on GUM.

Femtosecond supercontinuum generation around 1560 nm in hollow-core photonic crystal fibers filled with carbon tetrachloride.

We investigated experimentally supercontinuum generation in hollow-core photonic crystal fibers with cores infiltrated with carbon tetrachloride. As a pump source, we used a standard fiber-based femtosecond laser with a central wavelength at 1560 nm and a pulse duration of 90 fs. The first investigated fiber has a zero-dispersion wavelength at 1740 nm and generates a supercontinuum in the wavelength range from 1350 to 1900 nm. The second fiber has a zero-dispersion wavelength at 1440 nm, and the observed supercontinuum spectrum ranges from 1000 to 1900 nm. We numerically analyzed coherence of simulated supercontinuum pulses and noted that the observed supercontinuum spectra had a potential for high coherence. While the dynamics of supercontinuum generation in each of the investigated cases was revealed to be in agreement with the established state of the art in nonlinear fiber optics, our results are the first demonstration of such dynamics, to the best of our knowledge, leading up to octave spanning supercontinuum spectra in liquid-filled hollow-core silica fibers under pumping with a small-footprint femtosecond laser.

Shot-Noise-Limited Two-Color Stimulated Raman Scattering Microscopy with a Balanced Detection Scheme

Stimulated Raman scattering (SRS) microscopy has been considered a useful technique for investigating chemical components by selectively targeting the vibration mode of chemical structures. Its practical application to the observation of molecular structures and dynamics in complicated biological environments requires broad spectral coverage with both high resolution and a high signal-to-noise ratio. Here, we demonstrate a two-color SRS microscopy employing a balanced detection scheme and a spectral focusing method. Two different SRS signals are generated with pump and Stokes laser pulse pairs in perpendicular polarization, where each of them acts as an intensity reference for the other, significantly reducing the background noise level close to the shot-noise limit even with a fiber-based femtosecond laser system. The high spectral resolution comparable to that of spontaneous Raman scattering spectroscopy is achieved with the spectral focusing method. The two-color SRS images are obtained for a mixture of polymer beads and for the distributions of lipids and proteins in U2OS cells.

Terahertz hyperspectral imaging with dual chip-scale combs

Hyperspectral imaging is a spectroscopic imaging technique that allows for the creation of images with pixels containing information from multiple spectral bands. At terahertz wavelengths, it has emerged as a prominent tool for a number of applications, ranging from nonionizing cancer diagnosis and pharmaceutical characterization to nondestructive artifact testing. Contemporary terahertz imaging systems typically rely on nonlinear optical downconversion of a fiber-based near-infrared femtosecond laser, requiring complex optical systems. Here, we demonstrate hyperspectral imaging with chip-scale frequency combs based on terahertz quantum cascade lasers. The dual combs are free-running and emit coherent terahertz radiation that covers a bandwidth of 220 GHz at 3.4 THz with ∼10 μW per line. The combination of the fast acquisition rate of dual-comb spectroscopy with the monolithic design, scalability, and chip-scale size of the combs is highly appealing for future imaging applications in biomedicine and the pharmaceutical industry.

Coherent supercontinuum generation in tellurite glass regular lattice photonic crystal fibers

We report on designing, fabrication and experimental characterization of highly nonlinear, tellurite glass photonic crystal fibers with engineered normal dispersion characteristics for coherent supercontinuum generation. Effectively single mode, air-hole lattice fibers, with measured, all-normal dispersion profiles as flat as -10 to -50 ps/mn/km over 1500-2400 nm wavelengths are developed and investigated. Supercontinuum spectra are measured for these fibers, with a spectral width covering 1100-2600 nm wavelengths under pumping with a robust, fixed-wavelength erbium fiber-based femtosecond laser, delivering 90 fs pulses, centered at 1560 nm with peak power below 40 kW. To the best of our knowledge, this is the first engineered microstructured fiber, which due to its high nonlinearity, enables a self-phase modulation and optical wave breaking-based supercontinuum pumped with a turn-key, 40 kW femtosecond laser at spectral widths obtainable with previous all-normal dispersion fiber designs only under pumping with systems delivering peak power in the MW range.

Fiber-Based High-Energy Femtosecond Pulses Tunable From 920 to 1030 nm for Two-Photon Microscopy

We demonstrate a fiber-based femtosecond laser source, which can deliver 37 MHz, ~100 fs laser pulses tunable from 920 nm to 1030 nm with up to 10 nJ pulse energy. Pumped by an ultrafast Yb-doped fiber laser, a short piece of tapered photonic crystal fiber with lower dispersion was used to generate self-phase-modulation filtered for the tunable pulses. We believe that this new type of high-energy ultrafast fiber laser source can be an ideal solution for the two-photon fluorescence microscopy applications.

Robust Phase Retrieval Using Group-Delay- Dispersion-Scanned Second-Harmonic Generation Demonstrated in a Femtosecond Fiber Chirped-Pulse Amplification System

We propose and demonstrate a phase retrieval method using a novel variant of the dispersion scan (d-scan) technique via both simulations and experimental measurements on a femtosecond fiber laser. The method combines a map of group-delay d-scan second-harmonic generation (SHG) spectra together with the fundamental spectrum as inputs. In order to ensure that the technique is robust when pulse-to-pulse fluctuations are present, we use only the wavelengths of the resulting SHG peaks, and avoid the areas with low signal in the borders of the data trace. Simulations confirmed that phase-retrieval is accurate even with high levels of laser fluctuation. The tradeoff is that pulses that have abruptly changing or highly modulated phase profiles, are not retrievable. The d-scan uses the compressor gratings intrinsic to fiber chirped-pulse amplification (CPA) systems, so the method is low cost. For experimental verification, we used a 470-fs ytterbium-fiber CPA system. However, the method is applicable to measurements on fiber-based femtosecond laser systems, which generally have an order of magnitude less bandwidth compared with the Ti:sapphire lasers used to test earlier variants of the d-scan approach.

Saturable Absorption in 2D Ti3 C2 MXene Thin Films for Passive Photonic Diodes.

MXenes comprise a new class of 2D transition metal carbides, nitrides, and carbonitrides that exhibit unique light-matter interactions. Recently, 2D Ti3 CNTx (Tx represents functional groups such as OH and F) was found to exhibit nonlinear saturable absorption (SA) or increased transmittance at higher light fluences, which is useful for mode locking in fiber-based femtosecond lasers. However, the fundamental origin and thickness dependence of SA behavior in MXenes remain to be understood. 2D Ti3 C2 Tx thin films of different thicknesses are fabricated using an interfacial film formation technique to systematically study their nonlinear optical properties. Using the open aperture Z-scan method, it is found that the SA behavior in Ti3 C2 Tx MXene arises from plasmon-induced increase in the ground state absorption at photon energies above the threshold for free carrier oscillations. The saturation fluence and modulation depth of Ti3 C2 Tx MXene is observed to be dependent on the film thickness. Unlike other 2D materials, Ti3 C2 Tx is found to show higher threshold for light-induced damage with up to 50% increase in nonlinear transmittance. Lastly, building on the SA behavior of Ti3 C2 Tx MXenes, a Ti3 C2 Tx MXene-based photonic diode that breaks time-reversal symmetry to achieve nonreciprocal transmission of nanosecond laser pulses is demonstrated.

Robust optical-frequency-comb based on the hybrid mode-locked Er:fiber femtosecond laser.

We demonstrate an optical frequency comb in which an Er:fiber-based femtosecond laser employs nonlinear amplifier loop mirror (NALM) and nonlinear polarization evolution (NPE) mode-locking mechanisms. The laser combines advantages of good robustness of NALM and low noise feature of NPE. Our experimental results show that the hybrid mode-locked laser has high power, low relative intensity noise and self-started property, enabling the construction of a robust optical frequency comb system. In-loop relative instabilities of both stabilized repetition rate and carrier-envelope-offset frequency are well below 1 × 10-17 at 1 second integration time.

Fiber-based 1150-nm femtosecond laser source for the minimally invasive harmonic generation microscopy.

Harmonic generation microscopy (HGM) has become one unique tool of optical virtual biopsy for the diagnosis of cancer and the in vivo cytometry of leukocytes. Without labeling, HGM can reveal the submicron features of tissues and cells in vivo. For deep imaging depth and minimal invasiveness, people commonly adopt 1100- to 1300-nm femtosecond laser sources. However, those lasers are typically based on bulky oscillators whose performances are sensitive to environmental conditions. We demonstrate a fiber-based 1150-nm femtosecond laser source, with 6.5-nJ pulse energy, 86-fs pulse width, and 11.25-MHz pulse repetition rate. It was obtained by a bismuth borate or magnesium-doped periodically poled lithium niobate (MgO:PPLN) mediated frequency doubling of the 2300-nm solitons, generated from an excitation of 1550-nm femtosecond pulses on a large mode area photonic crystal fiber. Combined with a home-built laser scanned microscope and a tailor-made frame grabber, we achieve a pulse-per-pixel HGM imaging in vivo at a 30-Hz frame rate. This integrated solution has the potential to be developed as a stable HGM system for routine clinical use.

Watt-level fiber-based femtosecond laser source tunable from 2.8 to 3.6 μm.

The development of compact and reliable ultrafast sources operating in the mid-infrared region could lead to major advances in both fundamental and applied sciences. In this Letter, we report on a simple and efficient laser system based entirely on erbium-doped fluoride glass fibers that generates high-energy Raman soliton pulses tunable from 2.8 to 3.6 μm at a high average output power. Stable 160 fs pulses at 3.4 μm with a maximum energy of 37 nJ, a corresponding average output power above 2 W, and an estimated peak power above 200 kW are demonstrated. This tunable source promises direct applications in laser processing of polymers and biological materials.

Focus on microscopy and photonics

Focus on microscopy and photonics

Direct analysis of intact biological macromolecules by low-energy, fiber-based femtosecond laser vaporization at 1042 nm wavelength with nanospray postionization mass spectrometry.

A fiber-based laser with a pulse duration of 435 fs and a wavelength of 1042 nm was used to vaporize biological macromolecules intact from the condensed phase into the gas phase for nanospray postionization and mass analysis. Laser vaporization of dried standard protein samples from a glass substrate by 10 Hz bursts of 20 pulses having 10 μs pulse separation and <50 μJ pulse energy resulted in signal comparable to a metal substrate. The protein signal observed from an aqueous droplet on a glass substrate was negligible compared to either a droplet on metal or a thin film on glass. The mass spectra generated from dried and aqueous protein samples by the low-energy, fiber laser were similar to the results from high-energy (500 μJ), 45-fs, 800-nm Ti:sapphire-based femtosecond laser electrospray mass spectrometry (LEMS) experiments, suggesting that the fiber-based femtosecond laser desorption mechanism involves a nonresonant, multiphoton process, rather than thermal- or photoacoustic-induced desorption. Direct analysis of whole blood performed without any pretreatment resulted in features corresponding to hemoglobin subunit-heme complex ions. The observation of intact molecular ions with low charge states from protein, and the tentatively assigned hemoglobin α subunit-heme complex from blood suggests that fiber-based femtosecond laser vaporization is a "soft" desorption source at a laser intensity of 2.39 × 10(12) W/cm(2). The low-energy, turnkey fiber laser demonstrates the potential of a more robust and affordable laser for femtosecond laser vaporization to deliver biological macromolecules into the gas phase for mass analysis.

Space radiation test of saturable absorber for femtosecond laser.

We report a space radiation test performed on a semiconductor-type saturable absorber (SA) for its use in outer space as a key mode-locking component of fiber-based femtosecond pulse lasers. Gamma-ray effects on the nonlinear transmission behavior of the SA were evaluated by configuring a pump-probe experiment with femtosecond light pulses. Test results revealed that when the total ionizing dose of gamma-ray exposure reaches 120 krad, the SA encounters a sudden failure with its modulation depth and saturation fluence deteriorating from 11% to 6% and 40 to 110 μJ/cm2, respectively. On the other hand, no notable degradation was observed in its temporal response of absorption recovery.

Amplitude to phase conversion of InGaAs pin photo-diodes for femtosecond lasers microwave signal generation

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 typical detectors, commonly used in optical frequency metrology with femtosecond laser based optical frequency combs. When scanning the optical pulse energy, the coefficient which relates phase variations to energy variations is found to alternate between positive and negative values, with many (for high harmonics of the repetition rate) vanishing points. By operating the system near one of these vanishing points, the typical amplitude noise level of commercial-core fiber-based femtosecond lasers is sufficiently low to generate state-of-the-art ultra-low phase noise microwave signals, virtually immune to amplitude to phase conversion related noise.

Temporal cleaning of a high-energy fiber-based ultrafast laser using cross-polarized wave generation

We report the use of cross-polarized wave generation to perform both pulse shortening and temporal cleaning of a high-energy ytterbium-doped fiber-based femtosecond laser system. The nonlinear processes allow both a highly efficient nonlinear conversion of 20% and a large compression ratio of 3.5, with inherently improved coherent and incoherent contrasts. This results in the generation of 37 μJ, 115 fs pulses at a repetition rate of 100 kHz with high temporal quality.

Fiber-based combined optical coherence and multiphoton endomicroscopy

A fiber-based multimodal imaging system that combines multiphoton microscopy (MPM) with Fourier domain optical coherence microscopy (OCM) is reported. The system uses a fiber-based femtosecond laser, a fiber coupler, and a double-clad fiber (DCF) device. The fiber laser has a central wavelength of 1.04 μm and bandwidth of 29 nm. Longer excitation wavelength is used to increase penetration depth and increase the excitation efficiency for dyes, such as red fluorescent dyes. A single mode fiber coupler is used to replace the free-space beam splitter and one arm of the coupler is fused with a double-clad fiber device. The MPM and OCM share the same excitation light path in the core of a double-clad fiber, while the OCM and MPM signals were collected by the core and clad of the double-clad fiber, respectively. The performance of the introduced double-clad device is analyzed. The device can confine all the excitation light in the core and has a collection efficiency of 20% for the MPM signal. The efficiency can be further increased by fusing more multimode fibers with the DCF. Simultaneous optical coherence microscopic imaging, second harmonic generation imaging, and two-photon excitation fluorescence imaging are demonstrated in biological samples.

Nonlinear microscopy with fiber laser continuum excitation

A compact high-power fiber-based femtosecond laser system is presented for coherent anti-Stokes Raman scattering/second-harmonic generation (CARS/SHG) microscopy, and quantitatively compared with a conventional picosecond optical parametric oscillator (OPO)-based system. While the broad spectral width of the femtosecond pulses results in 2.5 times lower image contrast and limited spectral selectivity, lipid stores, myosin, and collagen filaments in living cells can clearly be identified at 60 times lower excitation powers compared to the picosecond system. Visually the images contain the same information. Together with simple operation, small footprint, and low cost, the capabilities of this high-power all-fiber-based laser system promise a more general use of nonlinear microscopy within the biosciences.