Optical limiting, pulse reshaping, and stabilization with a nonlinear absorptive fiber system

Optical limiting, pulse reshaping, and stabilization effects have been demonstrated based on a two-photon absorption mechanism with a dye-solution-filled hollow fiber system. The nonlinear absorptive medium is the solution of a new dye, trans-4-[p-(N-hydroxyethyl-N-methylamino)styryl]-N-methylpyridinium iodide (ASPI) in dimethyl sulfoxide, with which we filled a 20-cm-long quartz hollow fiber of 100-mum internal diameter. The input optical signal was a laser pulse train that contained ~30 pulses of 130-ps pulse width. When the input peak intensity reached 400-1000-MW/cm(2) levels, obvious optical limiting could be observed and the envelope of the transmitted pulse train became flatter and broader. By using another new dye solution, 4-[N-(2-hydroxyethyl)-N-(methyl)amino phenyl]-4?-(6-hydroxyhexyl sulfonyl)-stilbene (APSS) in benzyl alcohol, which interacted with a series of ~800-nm laser pulses of ~8-ns pulse width, we obtained a much higher nonlinear absorption coefficient and a superior optical peak-power stabilization effect.     

Two-photon Absorption and Nonlinear Optical Properties of A New Organic Dye DEASPI

A new organic dye trans-4- [p-(N,N-diethylamino) styryl]-N-methylpyridinium iodide (abbreviated' as DEASPI thereafter) with large two-photon absorption (TPA) cross section and excellent upconverted lasing properties was synthesized. The melting point and decompound point were measured to be 230 degreesC and 264.7 degreesC respectively. The molecular TPA cross section was measured to be sigma (2)=6.9x10(-48) cm(4).s/photon at 1064 nm by using an open aperture Z-scan system. The linear and nonlinear optical properties of this dye were systematically studied. The highest net upconversion efficiency from the absorbed pump energy to the output upconverted lasing energy is as high as 18.6% at the pump energy of 2.17 mJ from a mode-locked Nd:YAG ps laser. The nonlinear transmittance at the wavelengths from 720 to 1100 nm was measured. The dye solution also shows a clear optical power limiting effect.     

Resolution improvement in two-photon fluorescence microscopy with a single-mode fiber

The dependence of spectral broadening of an ultrashort-pulsed laser beam on the fiber length and the illumination power is experimentally characterized in order to deliver the laser for two-photon fluorescence microscopy. It is found that not only the spectral width but also the spectral blue shift increases with the fiber length and illumination power, owing to the nonlinear response in the fiber. For an illumination power of 400 mW in a 3-m-long single-mode fiber, the spectral blue shift is as large as 15 nm. Such a spectral blue shift enhances the contribution from the short-wavelength components within the pulsed beam and leads to an improvement in resolution under two-photon excitation, whereas the efficiency of two-photon excitation is slightly reduced because of the temporal broadening of the pulsed beam. The experimental measurement of the axial response to a two-photon fluorescence polymer block confirms this feature.     

Nonlinear optical stabilization of 1064-nm laser pulses with a two-photon absorbing liquid-dye salt system

Highly effective optical power-limiting and optical-stabilization performances have been achieved in a new type of two-photon absorbing medium. That is a liquid-dye salt system, trans-4-[p-(N,N-dimethoxyethylamino)styryl]-N-3,6,9-trioxadecylpyridinium tetrafluoroborate (abbreviated as ASDPT), which features an unusually high molar concentration (-1 M) of two-photon absorbing chromophores and the capability of withstanding a higher pulsed laser power and energy. The nonlinear transmission property and output-input characteristics were studied based on a 1-cm-long liquid-dye salt sample by use of nanosecond 1064-nm laser pulses with a repetition rate variable from 1 to 10 Hz. A superior optical-stabilization behavior of the studied material was demonstrated; the relative fluctuation of the nanosecond laser pulses can be reduced by more than two times simply by passing them through this highly concentrated nonlinear absorbing medium.     

Multimodal coherent anti-Stokes Raman spectroscopic imaging with a fiber optical parametric oscillator

We report on multimodal coherent anti-Stokes Raman scattering (CARS) imaging with a source composed of a femtosecond fiber laser and a photonic crystal fiber (PCF)-based optical parametric oscillator (FOPO). By switching between two PCFs with different zero dispersion wavelengths, a tunable signal beam from the FOPO covering the range from 840 to 930 nm was produced. By combining the femtosecond fiber laser and the FOPO output, simultaneous CARS imaging of a myelin sheath and two-photon excitation fluorescence imaging of a labeled axons in rat spinal cord have been demonstrated at the speed of 20 μs per pixel.     

Studies on the two-photon pumped upconverted fluorescence and superradiance of a new organic dye material in solutions

The linear and nonlinear optical properties of a new organic dye, trans-4-[p-(N-ethyl-N-ethylamino)-styryl]-N-methyl-pyridinium tris(thiocyanato) cadmates (II), are reported in this paper. When pumped with a picosecond laser at the wavelength range of 850-1200 nm, intense upconversion fluorescence can be obtained. The upconversion efficiencies at different pump energies were measured when pumped with a 1064-nm laser beam from a mode-locked Nd:YAG laser. The highest upconversion efficiencies were measured to be 5.8% and 7.6% in dimethyl formamide (DMF) and methanol. The lifetime of the dye in DMF was measured to be 75 ps. The strongest nonlinear absorption was at the wavelength of 940 nm, and the highest upconversion efficiency was at the wavelength of 1030 nm. The difference of the two wavelengths was caused by excited state absorption in the dye at wavelengths shorter than 1000 nm. The dye solution in DMF and methanol show a clear optical power limiting effect.     

卤化银光纤式CO2激光手术刀

卤化银多晶光纤「ＡｇＣｌｘＢｒ（１－ｘ），０≤ｘ≤１」是一种性能优良的中红外传能光纤。光纤原料采用高真空熔炼、氯气气氛下区域融熔和高真厂长发瓿内单昌生长等特殊工艺方法提纯，光纤用热挤压法成型。已制成的直径Φ１．０（ｍｍ）光纤Ｌ＝１．６４（ｍ）输出ＣＯ２激光功率〉２０（Ｗ），损耗０．３￣０．５ｄＢ／ｍ，Ｆｏｒｕｒｉｅｒ红外光谱（ＦＴＩＲ）测量结果显示，光纤在４￣１６μｍ波段内有良好的透过率，用光纤制     

Experimental study of generalized self-filtering unstable resonators in an ablative-wall flash-lamp-pumped dye laser

The performance of a generalized self-filtering unstable resonator (GSFUR) that consists of two curved mirrors in a nonconfocal scheme with a low magnification of M = -1.62 in an ablative-wall flash-lamp dye laser is reported. The objective was to study the near- and far-field intensity distribution and the divergence of the laser beam. It was found that the output beam has a nearly Gaussian distribution with a pulse duration of ~400 ns FWHM, almost independent of the diameter of the field-limiting aperture, but increases slightly with the pumping rate. A diffraction-limited laser beam of 1.1 mrad was obtained from this laser cavity. The output energy was ~1 mJ when an intracavity glass plate was used as an output coupler. The required relations needed for the GSFUR design were also derived.     

Dependence of the laser two-photon ionization process in solution on the laser pulse width

The laser two-photon ionization process has been investigated using simultaneous irradiation of an UV beam and an IR beam. When the laser pulse width was 300 ps, it proceeded through a geminate pair state (a stepwise process), as indicated by the signal enhancement with simultaneous irradiation of the two laser beams. Although no signal enhancement was observed when the laser pulse width was 100 fs, and because molecules with no absorption at the laser wavelength showed an intense signal, the two-photon ionization excited by a femtosecond laser should proceed through a simultaneous two-photon process. The detection limit was quite susceptible to the laser fluctuations.     

Effect of scattering on nonlinear optical scanning microscopy imaging of highly scattering media

The intensity of two-photon excited fluorescence (TPF) generated by ultrashort laser pulses was measured as a function of the depth of a focal point inside highly scattering media. The purpose was to investigate the spatial location of TPF in a scattering medium. Owing to the scattering, the intensity of the incident beam as well as the generated TPF signal was attenuated exponentially as the focal point was scanned into the medium. As the scattering strength of the medium was increased, the TPF was not confined to the focal region and had a wider distribution. These observations show that the scattering will result in the degradation of the ability of optical depth sectioning of nonlinear optical scanning microscopy.     

Sensitivity of a three-mirror cavity to thermal and nonlinear lensing: Gaussian-beam analysis

We consider a compact three-mirror cavity consisting of a flat output coupler, a curved folding mirror, and an active medium with one facet cut at the Brewster angle and the other facet coated for unit reflectivity. We examine the sensitivity to thermal lensing and to self-focusing in the active medium of the Gaussian beam that is circulating in that cavity. We use a simple thin-lens model; the astigmatism of the beam that is circulating in the cavity and the nonlinear coupling between the field distributions along the two orthogonal axes are taken into account. We find configurations in which beam ellipticity is compensated for at either end of the cavity in the presence of thermal lensing. We have derived an analytical criterion that predicts the sensitivity of the beam size to nonlinear lensing. The ability of the cavity to favor self-mode locking is found to be sensitive to the strength of thermal lensing. In the absence of thermal lensing, cavities operated as telescopic systems (C = 0) or self-imaging systems (B = 0) are most appropriate for achieving self-mode locking, with nonlinear mode selection accomplished through saturation of the spatially varying laser gain. We identify conditions for which self-mode locking can be produced by variable-reflectivity output couplers with either maximum or minimum reflectivity at the center of the coupler. We use our model to estimate the nonlinear gain produced in laser cavities equipped with such output couplers. We identify a cavity configuration for which nonlinear lensing can simultaneously produce mode locking and correction of beam ellipticity at the output coupler.     

Investigation of the effects of SOA locations in the linear cavity of an O-band Brillouin SOA fiber laser

An investigation into the effect of semiconductor optical amplifier (SOA) location in an O-band Brillouin SOA fiber laser (BSFL) was performed. Better output peak power flatness was generated by placing the SOA after the nonlinear medium, which is a 20?km true wave fiber (TWF) than placing it before the TWF. A maximum power of six flat output peaks with average power of ?22.0?dBm for a BP (Brillouin pump) wavelength of 1320?nm was obtained, generated from a BSFL with a SOA located after the TWF, compared with three flat Stokes signals with the SOA before the TWF at a BP wavelength of 1310?nm. The flat peak power output for the O-band Brillouin fiber laser is important, especially in producing a good O-band source.     

Bessel-beam-pumped tunable distributed-feedback laser

A distributed-feedback (DFB) dye laser that is pumped by a standing Bessel-beam wave is constructed. Because of the long line focus of the Bessel beam, the laser medium is pumped in only a very thin filament (a few micrometers) along the optical axis. At the same time, longitudinal-mode selection is achieved because of the DFB effect. It is demonstrated that when the effective wavelength of the Bessel pump beam is varied, the Bragg wavelength for DFB is altered, and as a result the output wavelength can be tuned.     

Multimode laser emission from dye doped polymer optical fiber

Multimode laser emission is observed in a polymer optical fiber doped with a mixture of Rhodamine 6G (Rh 6G) and Rhodamine B (Rh B) dyes. Tuning of laser emission is achieved by using the mixture of dyes due to the energy transfer occurring from donor molecule (Rh 6G) to acceptor molecule (Rh B). The dye doped poly(methyl methacrylate)-based polymer optical fiber is pumped axially at one end of the fiber using a 532 nm pulsed laser beam from a Nd:YAG laser and the fluorescence emission is collected from the other end. At low pump energy levels, fluorescence emission is observed. When the energy is increased beyond a threshold value, laser emission occurs with a multimode structure. The optical feedback for the gain medium is provided by the cylindrical surface of the optical fiber, which acts as a cavity. This fact is confirmed by the mode spacing dependence on the diameter of the fiber.     

Superradiance and lasing properties of new two-photon absorption dye DEASPS

Pumped by picosecond pulses from a Nd:YAG laser, a new lasing dye, trans-4-[4′-(N,N-diethylamino)styryl]-N-methyl pyridinium methyl sulfate (abbreviated to DEASPS), shows both intense superradiance and strong lasing properties in benzyl alcohol solution. By using streak camera systems, the superradiance and lasing can be distinguished both spectrally and temporally. It has been found that the peak wavelength of lasing is at 620 nm with a red-shift of about 12 nm to the superradiance wavelength. The lasing pulse shows an oscillatory effect that it is not found in the superradiance pulse. The fluorescence lifetime is 529 ± 40 ps and the effective molecular two-photon absorption is (1.25 ± 0.1) × 10^?48 cm⁴ ·s·photon ^?1, measured using a nonlinear transmittance method. This dye shows effective optical limiting of the pumping wavelength.     

Nonlinear characteristic and optical limiting effect of oil red O azo dye in liquid and solid media

In view of a possible application in optical limiting devices for protection against laser radiation, the nonlinear optical absorption, refraction and optical limiting behavior of an organic dye, oil red O, under excitation with CW, Nd: YAG laser at 532 nm was studied. The nonlinear optical responses of the dye were studied both in solution (acetonitrile) and solid film, (methylmethacrylate [MMA]) respectively, using the single-beam Z-scan technique. The open aperture Z-scan of the solution and solid samples displayed reversible saturable absorption. The closed aperture Z-scan of the samples exhibited negative nonlinearity, which was larger in magnitude in the solid film compared to that in solution. The nonlinear refractive index was found to vary with concentration. Optical limiting characteristics of the dye at various concentrations were studied. The third-order nonlinearity of this dye is dominated by nonlinear absorption, which leads to strong optical limiting of the laser.     

Fixed-wavelength operation of a copper-laser-pumped dye laser injection seeded by low-power He-Ne lasers

The design and operating characteristics of a dye laser pumped by a 3-W copper-vapor laser (CVL) and injection seeded by low-power (1-5 mW) He-Ne lasers at 633 nm are reported. An extremely simple optical arrangement is used wherein the output mirror of the He-Ne laser and a third mirror form the dye laser cavity. Laser efficiency in fixed-wavelength operation has been investigated for variable CVL pump power, He-Ne injection power and polarization, and cavity output coupling for a standard Rhodamine 590/Rhodamine 640 dye solution. Over 90% of free-running (unseeded) laser power is obtained in fixed-wavelength (seeded) operation at low CVL pump powers (≤1 W), dropping to approximately 60% at 3-W pump power. Maximum CVL pump to dye laser optical conversion efficiency in narrow-band, fixed-wavelength operation at 633 nm was 12%.     

Effective supercontinuum generation by using highly nonlinear dispersion-shifted fiber incorporated with Si nanocrystals

The dispersion-shifted fiber (DSF) incorporated with Si nanocrystals (Si-NCs) having highly nonlinear optical property was fabricated to investigate the effective supercontinuum generation characteristics by using the MCVD process and the drawing process. Optical nonlinearity was enhanced by incorporating Si nanocrystals in the core of the fiber and the refractive index profile of a dispersion-shifted fiber was employed to match its zero-dispersion wavelength to that of the commercially available pumping source for generating effective supercontinuum. The non-resonant nonlinear refractive index, n2, of the Si-NCs doped DSF measured by the cw-SPM method was measured to be 7.03 x 10(-20) [m2/W] and the coefficient of non-resonant nonlinearity, gamma, was 7.14 [W(-1) km(-1)]. To examine supercontinuum generation of the Si-NCs doped DSF, the femtosecond fiber laser with the pulse width of 150 fs (at 1560 nm) was launched into the fiber core. The output spectrum of the Si-NCs doped DSF was found to broaden from 1300 nm to wavelength well beyond 1700 nm, which can be attributed to the enhanced optical nonlinearity by Si-NCs embedded in the fiber core. The short wavelength of the supercontinuum spectrum in the Si-NCs doped DSF showed shift from 1352 nm to 1220 nm for the fiber length of 2.5 m and 200 m, respectively.     

Two-photon absorption and degenerate four-wave mixing studies of sulfide semiconductor nanoparticles in polymeric solutions

The two-photon absorption coefficients (beta) and the third-order nonlinear susceptibilities (chi(3)) of several semiconductor nanoparticles (CdS, Cd(x)Ag(1-x)S, and core-shell CdS/Ag2S) that are confined and stabilized by random and block ionomers have been measured by nonlinear transmission and degenerate four-wave mixing techniques using 21 picosecond laser pulses at near-infrared spectral region. The imaginary part of the third-order nonlinear susceptibility that is related to the two-photon absorption coefficient was then calculated. The absorptive nonlinearity of the nanoparticles (2 approximately 9 nm) was found to be dependent on the particle size, composition and wavelength, i.e., larger CdS particles exhibit higher two-photon absorption coefficients and the presence of Ag improves two-photon absorption of CdS nanoparticles. The obtained two-photon absorption coefficients of nanoparticles corrected for their volume fraction in solution are significantly greater that those of corresponding bulk semiconductors.     

Spatial distribution of two-photon-excited fluorescence in scattering media

Spatial distribution of two-photon-excited fluorescence (TPF) for dye beneath the surface of a highly scattering medium was investigated with picosecond laser pulses at 1064 nm. The active scattering media consisted of a suspension of polystyrene particles in a solution of Rhodamine 590 Tetrafluoroborate dye. With the increase of scattering strength of the medium, the location of the maximum TPF intensity was found to move closer to the surface away from the focal region; the intensity of TPF was not confined to the focal region as in the case of nonscattering medium but was more evenly distributed. The spatial resolution of nonlinear optical microscopy for probing a scattering medium is degraded. Taking into account the scattering of the medium qualitatively explains the observed TPF spatial distribution.