Mode-locked Titanium Sapphire Laser Research Articles

Phase locking of a mode-locked titanium–sapphire laser-based optical frequency comb to a reference laser using a fast piezoelectric actuator

We phase lock an octave-spanning optical frequency comb, generated using a mode-locked titanium-sapphire laser and a photonic-crystal fiber, to a continuous-wave laser line-narrowed to a reference cavity. To phase lock the pulse-repetition frequency, the cavity length of the mode-locked laser is controlled by using a fast piezoelectric-actuated mirror of a servo bandwidth up to 80 kHz. The residual phase noise is 0.35 rad, and 89% of the power is concentrated to the carrier. To apply the system to optical frequency-ratio measurements and to evaluate the phase locking, a simultaneous frequency measurement of the beat between the other mode of the comb and another laser line-narrowed to a different resonance of the common reference cavity is demonstrated.

Ultrafast Laser Energy Density and Retinal Absorption Cross-Section Determination by Saturable Absorption Measurements

Laser pulse nonlinear transmission measurements through saturable absorbers of known absorption parameters allow the measurement of their energy density. On the other hand, nonlinear transmission measurements of laser pulses of known energy density through absorbing media allow their absorption parameter determination. The peak energy density w0P of second harmonic pulses of a mode-locked titanium sapphire laser at wavelength λP = 400 nm is determined by nonlinear energy transmission measurement TE through the dye ADS084BE (1,4-bis(9-ethyl-3-car-bazovinylene)-2-methoxy-5-(2’-ethyl-hexyloxy)-benzene) in tetrahydrofuran. TE(w0P) calibration curves are calculated for laser pulse peak energy density reading w0P from measured pulse energy transmissions TE. The ground-state absorption cross-section σP and the excited-state absorption cross-section σex at λP, and the number density N0 of the retinal Schiff base isoform RetA in pH 7.4 buffer of the blue-light adapted recombinant rhodopsin fragment of the histidine kinase rhodopsin HKR1 from Chlamydomonas reinhardtii were determined by picosecond titanium sapphire second harmonic laser pulse energy transmission measurement TE through RetA as a function of laser input peak energy density w0P. The complete absorption cross-section spectrum σ

Line-by-line pulse shaping with spectral resolution below 890 MHz

Line-by-line pulse shaping is demonstrated on a 890 MHz repetition rate mode-locked titanium sapphire laser. The high resolution pulse shaper is based on a virtual imaged phased array (VIPA) with a free spectral range of 25 GHz. For our implementation, the mask repeats every VIPA free spectral range, which corresponds to every 28 comb lines. Individual frequency modes from the laser are also resolved using the same VIPA paired with a diffraction grating to achieve a resolution of 357 MHz. Several output waveforms are compared with simulation to understand differences with the ideal case.

Nonlinear optical properties of lanthanum-modified lead zirconate titanate investigated by the femtosecond Z-scan technique

We used a closed-aperture Z-scan technique to study the nonlinear optical refractive index of lanthanum-modified lead zirconate titanate (PLZT) (9/65/35) ceramics. Our laser light source was a mode-locked titanium-sapphire laser, the output pulse width was approximately 100 femtoseconds at a wavelength of 790 nm, and the repetition rate was 10 kHz. We found that PLZT had a large, positive nonlinear optical refractive index, n2 = +(3.1 ± 0.6) × 10−8 esu, and its third-order susceptibility was calculated to be χ(3) = (9 ± 1.8) × 10−9 esu. When an electric field (E) was applied to the PLZT, n2 increased along in direct proportion to E2. We speculate that the increase in n2 was caused by the more ordered structural arrangement of the PLZT imposed by the applied electric field. The results show that PLZT has potential applications in fast, electric-field-controlled nonlinear optical elements.

Amplification of kW peak power femtosecond pulses in single quantum well InGaAs tapered amplifiers

The amplification of ps and fs pulses with peak powers of up to 4.5kW has been investigated in a single quantum well InGaAs tapered amplifier. The pulses with durations of 100fs or 2ps were generated by a modelocked titanium–sapphire laser. The amplified pulses indicate strong gain saturation and carrier generation due to photon absorption in the laser active region which causes a temporal broadening of the amplified pulses as well as modifications of the optical spectrum. The gain recovery time was measured by a pump–probe experiment. The experimental results are analyzed with respect to the sub-ps gain dynamics which is described by a relaxation time approximation.

Impaired Spine Stability Underlies Plaque-Related Spine Loss in an Alzheimer's Disease Mouse Model

Dendritic spines, the site of most excitatory synapses in the brain, are lost in Alzheimer's disease and in related mouse models, undoubtedly contributing to cognitive dysfunction. We hypothesized that spine loss results from plaque-associated alterations of spine stability, causing an imbalance in spine formation and elimination. To investigate effects of plaques on spine stability in vivo, we observed cortical neurons using multiphoton microscopy in a mouse model of amyloid pathology before and after extensive plaque deposition. We also observed age-matched nontransgenic mice to study normal effects of aging on spine plasticity. We found that spine density and structural plasticity are maintained during normal aging. Tg2576 mice had normal spine density and plasticity before plaques appeared, but after amyloid pathology is established, severe disruptions were observed. In control animals, spine formation and elimination were equivalent over 1 hour of observation ( approximately 5% of observed spines), resulting in stable spine density. However, in aged Tg2576 mice spine elimination increased, specifically in the immediate vicinity of plaques. Spine formation was unchanged, resulting in spine loss. These data show a small population of rapidly changing spines in adult and even elderly mouse cortex; further, in the vicinity of amyloid plaques, spine stability is markedly impaired leading to loss of synaptic structural integrity.

Femtosecond-laser-encoded distributed-feedback color center laser in lithium fluoride single crystals

Laser-active F2 centers were produced in lithium fluoride (LiF) at a concentration of 2×1018 cm−3 by irradiating focused femtosecond (fs) laser pulses from a mode-locked titanium sapphire laser (wavelength ∼800 nm, emission pulse duration ∼100 fs). This technique was used to write waveguides embedded in LiF crystals. A refractive index change estimated from a guide propagation method was approximately +1% at a wavelength of 633 nm. Refractive index-modulated volume-type gratings were also encoded inside LiF crystals by a single interfered fs laser pulse. The distributed feedback laser structure was fabricated using the gratings thus encoded, which exhibited a room-temperature F2-color center laser oscillation at 707 nm. This demonstrates a DFB color center laser operating at room temperature utilizing photon written, permanent Bragg gratings.

Up-Conversion and Two-Photon Excitation Fluorescence Properties of Phloxine B.

A dye, Phloxine B, a common food coloring and one of the active components of a photoreactive insecticide was recently approved by Food and Drug Administration as D&C Red #28 for use in drugs and cosmetics. Phloxine B is also one of the most widely use stain in fluorescence microscopy. However, in spite of the widespread interest in multi-photon spectroscopy and imaging, no information is available on the electronic transitions properties of Phloxine B with red edge fluorescence excitation and with multi-photon excitation.In the present report we described the steady state and time-resolved fluorescent properties of Phloxine B with up-convert photon excitation and with two-photon excitation. We examined the electronic transitions properties of Phloxine B when was excited by femtosecond pulses from a mode-locked titanium sapphire laser or a mode-locked optical parametric oscillator laser. Phloxine B under excitation wavelengths above 775 nm was found to display two-photon excitation fluorescence with a spectrum maximum at 580 nm, which emission is consistent with one-photon excitation fluorescence spectrum. At the red edge excitation of Phloxine B, from 590 nm to 650 nm, we observed one-photon excitation fluorescence indicating that Phloxine B behaves like upconverting dye with one-photon excitation process.

The use of a mode-locked laser for ultraviolet radiometry

A mode-locked titanium sapphire laser, tuneable over the region from 680 nm to 1000 nm, has been used to generate monochromatic laser radiation over the region from 250 nm to 450 nm by second and third harmonic generation, with power outputs of between 50 mW and 200 mW above 350 nm and around 30 mW between 250 nm and 300 nm. The (pulsed) radiation was assessed as follows for its suitability as a source for the calibration of detectors in continuous-wave (cw) applications. (i) Radiation at 356.41 nm was used to calibrate a trap detector for spectral responsivity against the National Physical Laboratory (NPL) cryogenic radiometer, and this value was compared with the one obtained using a cw laser at the same wavelength: the values differed by 0.06%. (ii) The linearity of response of the trap detector was determined at 362.0 nm. It was found to be linear to within 0.01% for powers up to 1.5 mW. The laser was then used to calibrate a trap detector for spectral responsivity against the cryogenic radiometer over the region from 350 nm to 360 nm, with an estimated standard uncertainty of less than 0.1%.

Multifocal multiphoton microscopy

We present a real-time, direct-view multiphoton excitation fluorescence microscope that provides three-dimensional imaging at high resolution. Using a rotating microlens disk, we split the near-infrared light of a mode-locked titanium-sapphire laser into an array of beams that are transformed into an array of high-aperture foci at the object. We typically scan at 225 frames per second and image the fluorescence with a camera that reads out the images at video rate. For 1.4 aperture oil and 1.2 water immersion lenses at 780-nm excitation we obtained axial resolutions of 0.84 and 1.4mum , respectively, which are similar to that of a single-beam two-photon microscope. Compared with the latter setup, our system represents a 40-100-fold increase in efficiency, or imaging speed. Moreover, it permits the observation with the eye of high-resolution two-photon images of (live) samples.

Two-photon excitation of ethidium bromide labeled DNA

We examined the steady state and time-resolved emission of DNA stained with ethidium bromide (EB) when excited with 90 fs pulses from a mode-locked titanium sapphire laser. Over the wavelength range from 840 to 880 nm EB-DNA was found to display two-photon excitation, with a cross-section near 7 × 10 −50 cm 4s/photon. Frequency-domain intensity decay measurements revealed similar multi-exponential intensity decays for one- and two-photon excitation. Time-resolved anisotropy decay measurements revealed similar correlation times, but different amplitudes as has been observed previously for two- versus one-photon excitation. These results indicate that two-photon excitation of EB-DNA can be accomplished with the fundamental output of a Ti:sapphire laser without obvious heating or perturbation of the DNA.

Highly sensitive detection using laser two-photon excited fluorescence in capillary electrophoresis

The laser two-photon excited fluorescence (LTPF) has been applied to a capillary electrophoresis (CE) system using a self mode-locked titanium-sapphire laser. The analytes were coumarine 440 (7-amino-4-methyl coumarine) and DCCS (7-diethyl aminocoumarine-3-carboxylic acid succinimidyl ester). The two-photon excited fluorescence emission spectrum of DCCS was identical to the one-photon excited fluorescence emission spectrum. The LTPF quantum efficiency increased through the addition of methanol to the buffer solution, and, as a result, the detection limit decreased. The detection limit of coumarine 440 was 8.0 amol, this value was as low as that obtained by one-photon excited fluorescence.

Two-photon spectroscopy of laser-cooled Rb using a mode-locked laser

Using laser cooled samples of 85Rb and 87Rb in a magneto-optical trap, high resolution spectroscopy of the two-photon transitions 5 S 1 2 −5 D 3 2 , 5 S 1 2 −5 D 5 2 and 5 S 1 2 −7 S 1 2 have been investigated using the output from a mode-locked titanium sapphire laser. The hyperfine constants for the 7S terms of both isotopes have been measured with improved accuracy.

Three-photon excitation in fluorescence microscopy

We show experiments proving the feasibility of scanning fluorescence microscopy by three-photon excitation. Three-photon excitation fluorescence axial images are shown of polystyrene beads stained with the fluorophore 2,5-bis(4-biphenyl)oxazole (BBO). Three-photon excitation is performed at an excitation wavelength of 900 nm and with pulses of 130 fs duration provided by a mode-locked titanium sapphire laser. Fluorescence is collected between 350 and 450 nm. The fluorescence image signal features a third-order dependence on the excitation power, also providing intrinsic 3-D imaging. The resolution of a three-photon excitation microscope is increased over that of a comparable two-photon excitation microscope.

Time-resolved fluorescence microscopy using two-photon excitation

We have developed a high sensitivity time-resolved two-photon scanning microscope. At an excitation wavelength of 960 nm, a spatial point spread function of 0.3 μm (FWHM) radially and 0.9 μm (FWHM) axially is measured for an 1.25 N.A. objective. The light source is a mode-locked titanium-sapphire laser. The time resolution is 400 ps with common chromophores used in microscopy. Time resolution is obtained using the frequency-domain heterodyning technique in which the laser is synchronized at a very high cross-correlation frequency to the rest of the electronics. We demonstrate spatial and time resolution using well-characterized fluorescent microspheres. We show two applications of two-photon time-resolved fluorescence microscopy: time-resolved imaging of multiple dye labeled cells and quantitative cellular calcium concentration using a lifetime indicator.

Time‐resolved fluorescence microscopy using two‐photon excitation

We have developed a high sensitivity time-resolved two-photon scanning microscope. At an excitation wavelength of 960 nm, a spatial point spread function of 0.3 μm (FWHM) radially and 0.9 μm (FWHM) axially is measured for an 1.25 N.A. objective. The light source is a mode-locked titanium-sapphire laser. The time resolution is 400 ps with common chromophores used in microscopy. Time resolution is obtained using the frequency-domain heterodyning technique in which the laser is synchronized at a very high cross-correlation frequency to the rest of the electronics. We demonstrate spatial and time resolution using well-characterized fluorescent microspheres. We show two applications of two-photon time-resolved fluorescence microscopy: time-resolved imaging of multiple dye labeled cells and quantitative cellular calcium concentration using a lifetime indicator.

A dye laser-based amplifier for a CW mode-locked titanium sapphire laser and its use in the production of tunable ultraviolet light pulses

A multi-staged dye amplifier pumped by the second harmonic of a Q-switched Nd:YAG laser is used to amplify the output pulses from a commercial cw mode-locked titanium sapphire laser. The amplified light is frequency-doubled and the resulting blue pulses pass through a second dye amplifier pumped by the third harmonic of the Q-switched laser. The blue and red light beams are then frequency summed to produce tunable ultraviolet light. Their cross-correlation indicates that the temporal width of the uv pulses is about five picoseconds.