We report on a commercial laser system based on a Yb fiber oscillator with cross-filter mode lock (CFML) mechanism that is integrated with a programmable pulse shaper. The laser is self-starting and stable in a wide temperature range, 15- 50°C, resilient to vibrations and shock. It can serve as a seed for high-power femto- and pico- second systems or be implemented as a standalone unit, as illustrated in this paper. The master oscillator is outputting strongly chirped pulses, with the spectrum centered at 1030 nm and having the full bandwidth of up to 90 nm. It operates at 11 MHz repetition rate, with the pulse energy of at least 10 nJ at the output. When equipped with an additional power amplification module, the oscillator yields the same spectral output and repetition rate, but the pulse energy can be increased up to 400 nJ. The laser output is fully coherent, and pulses are compressible down to the transform limit (TL). For demanding femtosecond applications, the laser system is being configured with a static grating compressor and a compact spectral phase shaper. The pulse shaper utilizes a liquid-crystal spatial light modulator for active phase control which enables high-finesse pulse compression as well as arbitrary manipulation of the pulse waveform. With the use of the pulse shaper, the oscillator output is compressed down to 57 fs, which is within 7% from the TL pulse duration, 53 fs, calculated from the experimental laser spectrum.
We report on industrial-grade femtosecond Yb fiber lasers with >100μJ pulse energy and <300fs pulse duration using a tunable all-fiber pulse shaper. The rugged, compact phase modulator is a lossless addition to the standard chirped-pulseamplification scheme. The automated multichannel phase control across the optical bandwidth enables generation of near transform-limited pulses at the laser output, improves unit-to-unit reproducibility of laser pulse characteristics, and reduces laser build time.
The transition of femtosecond lasers from the laboratory to commercial applications requires real-time automated pulse compression, ensuring optimum performance without assistance. Single-shot phase measurements together with closed-loop optimization based on real-time multiphoton intrapulse interference phase scan are demonstrated. On-the-fly correction of amplitude, as well as second- and third-order phase distortions based on the real-time measurements, is accomplished by a pulse shaper.
We demonstrate a modular and versatile experimental setup that enables straightforward compression (and then shaping)
of ultrashort laser pulses at the imaging plane of a two-photon microscope. A commercially available pulse shaper is
used in conjunction with a commercially available broadband Ti:Sapphire oscillator to produce sub-8fs pulses at the
focus of a high-numerical-aperture objective. Automated adaptive pulse compression, based on multiphoton intrapulse
interference phase scan (MIIPS), is verified in situ by shaper assisted interferometric autocorrelation. Two-photon
excited fluorescence image of a mouse kidney slide is obtained to confirm microscopic sectioning capabilities.
We formulate a simple strategy for mitigation of laser-induced damage through pulse shaping and demonstrate
experimentally the effect of laser pulse duration on the degree of optically induced damage for two-photon microscopy
imaging. We use a broadband Ti:Sapphire laser source, aided with a shaper, and adjust both the laser pulse duration and
energy to maintain constant two-photon excitation efficiency. The damage is assessed by the dynamics of two-photon
excited autofluorescence intensity and sample morphology during prolonged laser exposure. We observe that for a 5-μm
layer of skin tissue the damage rate is independent of the pulse shape, which suggests that the primary damage
(bleaching) mechanism stems from the two-photon excitation itself. For optically thick dried blood samples, taken as
another example, the data suggests that the damage is driven by
one-photon absorption. In the later case, it is favorable
to use shorter laser pulses to mitigate photodamage while maintaining adequate intensity of two-photon excited
autofluorescence.
High-order dispersion of ultrashort laser pulses (with ~100 nm bandwidth) is shown to account for significant reduction
of two-photon excitation fluorescence and second harmonic generation signal produced at the focal plane of a laser-scanning
two-photon microscope. The second- and third-order corrections recover 20-40% of the signal intensity
expected for a transform-limited laser pulse, while the rest depends on the proper compensation of higher-order terms. It
can be accomplished through the use of a pulse shaper by measuring and correcting all nonlinear spectral phase
distortions.
An adaptive pulse shaper controlled by multiphoton intrapulse interference phase scanning (MIIPS) was used, together with a prism-pair, to measure and cancel high-order phase distortions introduced by a high-numerical-aperture objective and other dispersive elements of a two-photon laser-scanning microscope. The delivery of broad-bandwidth (~100 nm), sub-12-fs pulses was confirmed by interferometric autocorrelation measurements at the focal plane. A comparison of two-photon imaging with transform-limited and second-order-dispersion compensated laser pulses of the same energy showed a 6-to-11-fold improvement in the two-photon excitation fluorescence signal when applied to cells and tissue, and up to a 19-fold improvement in the second harmonic generation signal from a rat tendon specimen.
Molecularly imprinted polymers (MIPs) have the potential to provide a unique combination of high chemical selectivity and environmental stability and are, therefore, being widely studied in chemical sensor applications. Optical interrogation of the MIP-chemical interaction is very convenient for the detection of fluorescent compounds, but is problematic for the detection of non-fluorescent species. Doping MIPs with Eu3+ is one approach that can facilitate the optical detection of non-fluorescent species. Eu3+ has absorption in the near UV and the doped MIP can, therefore, be excited with a commercially available laser diode at 375nm. In the present paper MIPs doped with Eu3+ and imprinted to methyl salicylate (MES), a chemical warfare agent simulant, were prepared in the form of a thin film on a quartz substrate. Non-imprinted (Blank) polymer films were also prepared using the same imprinting procedure, but without introducing the MES template. Both polymers were tested to MES and the structurally similar compound methyl 3,5-dimethylbenzoate (DMB) in hexane. For MES, the fluorescence intensity of the MIP was significantly stronger than for the Blank, while for the methyl 3,5-dimethylbenzoate, the Blank polymer exhibited the stronger fluorescence signal. A portable chemical sensor employing differential fluorescence from MIP/Blank polymer pairs is under development and allows target discrimination without the need for spectroscopic analysis of the emission spectra.
Fiber optic-based chemical sensors are created by coating fiber Bragg gratings (FBG) with the glassy polymer cellulose
acetate (CA). CA is a polymeric matrix capable of localizing or concentrating chemical constituents within its structure.
Some typical properties of CA include good rigidity (high modulus) and high transparency. With CA acting as a sensor
element, immersion of the gratings in various chemical solutions causes the polymer to expand and mechanically strain
the glass fiber. This elongation of the fiber sections containing the grating causes a corresponding change in the
periodicity of the grating that subsequently results in a change in the Bragg-reflected wavelengths. A high-resolution
tunable fiber ring laser interrogator is used to obtain room-temperature reflectance spectrograms from two fiber gratings
at two different wavelengths - 1540nm and 1550nm. The graphical representation from this device enables the display
of spectral shape, and not merely shifts in FBG central wavelength, thereby allowing for more comprehensive analysis of
how different physical conditions cause the reflectance profile to move and alter overall form. Wavelength shifts on the
order of 1 to 80 pm in the FBG transition edges and changes in spectral shape are observed in both sensors upon
immersion in a diverse selection of chemical analytes.
A promising new fiber optic sensor is under development that combines fiber Bragg gratings coated with polymer materials for sensitive and rapid detection and identification of chemical and/or biological agents. Volumetric expansion of the polymer coating transfers characteristic strain to the Bragg grating, modifying directly its grating period rather than sensing through a change of effective guide index of refraction. The optical interrogation of the sensor element utilizes a sensitive transmission spectroscopy technique with a balanced receiver that minimizes polarization and laser intensity noise problems. A compact, rugged, all-solid-state laser at 1550 nm is being adapted for rapid tuning between discrete preset locked wavelengths. To accompany use of this laser at these few discrete wavelengths, a sampled (superstructure) fiber Bragg grating is being designed using coupled mode theory. Hence, the need for a continuously tunable laser, often with moving mechanical optical elements, and its attendant reference etalon will be avoided entirely. This process exploits a novel vernier effect between the discrete laser wavelengths and the sampled grating responses to create 'signatures' for an artificial neural network. Therefore, the total spectral response pattern of strain can constitute a unique fingerprint used to identify and quantify chemical agents or biomarkers. The sensor is intended for applications requiring multi-functionality, sensitivity, speed, mobility and remote operability in vibrational, electromagnetic, and explosive environments.
KEYWORDS: Luminescence, Polymers, Digital signal processing, Molecules, Chemical analysis, Sensors, Polymerization, Fluorescence spectroscopy, Glasses, Biological and chemical sensing
Fluorescence represents one of the most attractive approaches for chemical sensing due to the abundant light produced by most fluorophores, resulting in excellent detection sensitivity. However, the broad and overlapping emission spectra of target and background species have made it difficult to perform species identification in a field instrument because of the need to perform spectral decomposition and analysis. This paper describes a new chemical sensing strategy based on differential fluorescence measurements from molecularly imprinted polymers, which eliminates the need to perform any spectral analysis. Species identification is accomplished by measuring the differential light output from a pair of polymers-one imprinted to a target species and the other identical, but not imprinted. The imprinted polymer selectively concentrates the target molecule and controls the energy (wavelength) of the emitted fluorescence signal and the differential output eliminates common mode signals associated with non-specific background interference. Because no spectral analysis is required, the sensors can be made extremely small and require very little power. Preliminary performance parameters from a prototype sensor are presented and discussed.
Steady state and lifetime fluorescence measurements were acquired to measure the binding activity associated with molecularly imprinted polymer (MIP) microparticles imprinted to dipicolinic acid. Dipicolinic acid is a unique compound associated with the sporulation phase of spore-forming bacteria (e.g., genus Bacillus and Clostridium). Vinylic monomers were polymerized in a dimethylformamide solution containing the dipicolinic acid as a template. The resulting MIP was then pulverized and size selected into small microscale particles. Samplers were adapted incorporating the MIP particles within a dialyzer (500 MW). Tests were run on replicate samples of biologically active cultures representing both stationary phase and sporulation post fermentation products in standard media. The permeability of the membrane permitted diffusion of lighter molecular weight constituents from media effluents to enter the dialyzer chamber and contact the MIP. Extractions of the media were measured using steady state and lifetime fluorescence. Results showed dramatic steady state fluorescence changes as a function of excitation, emission and intensity and an estimated lifetime of 5.8 ns.
Nanoscale polymeric coatings are used in a variety of sensor systems. The influence of polymer coating morphology on sensor response was investigated and it was determined that coating morphology plays a particularly important role in transducers based on optical or acoustic resonance such as surface acoustic wave (SAW) or surface plasmon resonance (SPR) devices. Nanoscale polymeric coatings were deposited onto a number of miniature devices using a "solvent-free" deposition technique known as Rapid Expansion of Supercritical Solutions (RESS). In RESS, the supercritical solvent goes into the vapor phase upon fast depressurization and separates from the polymer. Therefore, dry polymer particles are deposited from the gas phase. The average diameter of RESS precipitates is about two orders of magnitude smaller than the minimum droplet size achievable by the air-brush method. For rubbery polymers, such as PIB and PDMS, the nanoscale solute droplets produced by RESS agglomerate on the surface forming a highly-uniform continuous nanoscale film. For glassy and crstalline polymers, the RESS droplets produce uniform particulate coatings exhibiting high surface-to-volume ratio. The coating morphology can be changed by controlling the RESS processing conditions.
A new scheme for generation of coherent radiation on the intersubband transition without population inversion between subbands is presented. The scheme is based on the resonant nonlinear mixing of the optical laser fields on the two interband transitions that are generated in the same active region and serve as a coherent drive for the infrared field. The two-wavelength lasing on the interband transitions can be achieved at substantially lower threshold current than population inversion and gain on the intersubband transition. This may ensure stable high-power room-temperature operation. Due to parametric, inversionless nature of generation, the proposed lasers are especially promising for the long- wavelength operation above 20 micron.
We investigate possible regimes of one -- or two -- color mode superradiance of femtosecond optical pulses and accompanying parametric generation of infrared pulses, which contain a few periods of field oscillations, in quantum-well heterolasers of micrometer length under cw pumping. The effects of strong magnetic field and inhomogeneous broadening of electronic levels of dimensional quantization in quantum wells are analyzed.
We analyze super-radiant lasing, i.e. superfluorescence (SF) under continuous pumping, due to collective interband recombination of electron-hole (eh) pairs in quantum wells (QWs) placed in a strong magnetic field oriented perpendicular to the well plane. In such semiconductor systems, cooperative radiation processes lead to generation of coherent femtosecond pulses and can be observed even at room temperature due to complete quantization of particle motion, high spectral density of carrier states, high spatial density of effective 'cyclotron quantum dots,' and partial suppression of intraband scattering. Our simulations based on the Maxwell-Bloch equations show that, in the case of two neighboring transitions between electron and hole Landau levels connected by scattering, two-color super-radiant lasing of the corresponding pair of resonance modes is possible under cascade cw pumping. We investigate analytically and numerically threshold conditions for this two-color lasing as well as for super-radiant lasing modified by 'discrete' inhomogeneous broadening due to fluctuations of the number of atomic layers in actual QW heterostructures. Also, we present an example of real design and discuss necessary conditions for a vertical cavity surface-emitting laser (VCSEL) to operate in the super-radiant regime.
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