Photostability enhancement of Pyrromethene 567 and Perylene Orange in oxygen-free liquid and solid dye lasers

We investigated the effect of oxygen on the photostability of the laser dyes Pyrromethene 567, Perylene Orange, and Rhodamine 590 by determining their longevity of laser operation when pumped by the second harmonic of aQ -switched Nd:YAG laser. In solution, dissolved oxygen accelerated the photodegradation of Pyrromethene 567 and Perylene Orange but not Rhodamine 590. The photostability of Pyrromethene 567 was also found to be dependent on the solvent and on the lifetime of singlet oxygen. Deoxygenated Pyrromethene 567 doped polycom glass and modified poly(methyl methacrylate) (MPMMA) samples were tested for longevity of laser operation. A factor of 6 improvement in photostability was found for Pyrromethene 567 in MPMMA upon deoxygenation, and the total absorbed energy per mole of dye molecules to one-half output pulse energy was 36 GJ mol-1 .     

Role of the stimulated-emission rate in the photostability of solid-state dye lasers

The lasing and photostability characteristics of Rhodamine 6G and Pyrromethene 567 dyes dispersed in polymeric host materials have been investigated as a function of the intensities of incident pump and signal beams in a longitudinally pumped dye laser in an oscillator-amplifier configuration. A substantial reduction in the rate of photodegradation was observed under lasing conditions and with increasing signal intensity in a dye amplifier, establishing that the service lives of these materials improve with an increase in the rate of stimulated emission. We observed approximately 62% amplifier efficiency at 2 Hz operation and 10% reduction in amplifier efficiency at 10 Hz operation after exposure of 72,000 pulses by use of a Pyrromethene disk.     

Near-resonance enhanced O2 detection for dual-broadband pure rotational coherent anti-Stokes Raman scattering with an ultraviolet-visible setup at 266 nm

Broadband and dual-broadband coherent anti-Stokes Raman scattering (CARS) are widely established tools for nonintrusive gas diagnostics. Up to now the investigations have been mainly performed for electronic nonresonant conditions of the gas species of interest. We report on the enhancement of the O2-N2 detection limit of dual-broadband pure rotational CARS by shifting the wavelength of the narrowband pump laser from the commonly used 532-266 nm. This enhancement is caused when the Schumann-Runge absorption band is approached near 176 nm. The principal concept of this experiment, i.e., covering the Raman resonance with a single- or dual-broadband combination of lasers in the visible range and moving only the narrowband probe laser near or directly into electronic resonant conditions in the UV range, should also be applicable to broadband CARS experiments to directly exploit electronic resonance effects for the purpose of single-shot concentration measurements of minority species. To quantify the enhancement in O2 sensitivity, comparative measurements at both a 266 and a 532 nm narrowband pump laser wavelength are presented, employing a 4-dicyanomethylene-2-methyl-6-(p-dimethylaminostyryl)-4H-pyram (DCM) dye laser as a broadband laser source at 635 nm. An increase of approximately equal to 13% in the ratio of the rotational CARS cross sections of O2 and N2 was obtained. The broad spectral width of the CARS excitation profile was approximately equal for both setups. Further enhancement should be achievable by shifting the narrowband pump laser closer toward 176 nm, for example, with a frequency-doubled optical parametric oscillator or an excimer laser. The principal concept of this experiment should also be applicable to broadband CARS experiments to directly exploit electronic resonance effects of the narrowband pump laser with electronic transitions of minority species for the purpose of single-shot concentration measurements of those species.     

Preparation and properties of laser dye-ORMOSIL composites

Various organic laser dyes have been incorporated into organically modified silicates (ORMOSILs), derived from polydimethylsiloxane and tetraethoxysilane. These sol-gel derived PT-ORMOSILs proved to be good hosts for the laser dyes in terms of stability and optical gain. Some of the dyes, for example, Rhodamine 610, Rhodamine 620, and Rhodamine 640, have shown higher stability in PT-ORMOSILs than in methyl alcohol. The stability of Rh6G in a PT-ORMOSIL is 76% higher in dry nitrogen than in air, because of the contribution of oxygen and/or moisture to the dye degradation processes. The fluorescence intensity for Rh610 was enhanced by a factor of 3.9 as the temperature was cooled from room temperature to 77 K.     

Tunable solid-state laser based on modified polymethyl methacrylate with methanol doped with Pyrromethene 580

Solid-state dye material was fabricated by doping a laser dye Pyrromethene 580 (PM580) into the polymer host polymethyl methacrylate. Methanol was also injected into the host to improve the optical properties. The broadband and narrowband laser performances of the sample were studied in this paper. When the selected solid-state dye PM580 was placed in a Shoshan-type oscillator, narrow linewidth operation with a tuning range of 52 nm and good photostability was demonstrated. The narrowband laser output slope efficiency of 42.7% was obtained, and the corresponding broadband laser slope efficiency was 66.0%. To the best of our knowledge, the narrowband slope efficiency and tunable range are the best under similar conditions so far. For broadband and narrowband lasers, the beam quality factors were estimated to be M(y)(2)=10.7 and 3.9 in the vertical direction.     

Fabrication and characterization of dye mixture doped polymer optical fiber as a broad wavelength optical amplifier

Rhodamine 6G and Rhodamine B dye mixture doped polymer optical fiber amplifier (POFA), which can operate in a broad wavelength region (60 nm), has been successfully fabricated and tested. Tunable operation of the amplifier over a broad wavelength region is achieved by mixing different ratios of the dyes. The dye doped POFA is pumped axially using 532 nm, 10 ns laser pulses from a frequency doubled Q-switched Nd: YAG laser and the signals are taken from an optical parametric oscillator. A maximum gain of 22.3 dB at 617 nm wavelength has been obtained for a 7 cm long dye mixture doped POFA. The effects of pump energy and length of the fiber on the performance of the fiber amplifier are also studied. There exists an optimum length for which the amplifier gain is at a maximum value.     

Comparison of laser performance of dye molecules in sol-gel, polycom, ormosil, and poly(methyl methacrylate) host media

Laser performance is described for Rhodamine 590, Pyrromethene 567, Perylene red, and Perylene orange in inorganic porous sol-gel glass, poly(methyl methacrylate)(PMMA), a composite of porous sol-gel glass with PMMA and organically modified silicate ormosil glass. Lasers were excited with a flash-lamp-pumped dye laser in the long-pulse-length regime (3 μs, 506 nm, 300 mJ) and a second-harmonic Nd:YAG laser in the short-pulse-length regime (6 or 15 ns, 532 nm, 60 mJ). The feasibility of long-pulse-length operation is demonstrated, detailed characteristics of short-pulse operation are described, and laser damage measurements are given. The nonpolar perylene dyes had better performance in partially organic hosts, and the ionic rhodamine and pyrromethene dyes performed best in the inorganic sol-gel glass host.     

Highly efficient wideband continuum generation in a single-mode optical fiber by powerful broadband laser pumping

By pumping a single-mode optical fiber with a powerful broadband nonselective dye laser, we obtain a high-efficiency wideband continuum (530-930 nm) with nonlinear conversion efficiency exceeding 90%. Experimental conditions for a coherent regime of broadband stimulated Raman scattering are created, which in combination with the broadband self-phase modulation and the four-photon parametric processes leads to a spectral broadening and to the continuum formation. The influence of the pump laser spectral linewidth on the nonlinear conversion efficiency is analyzed and investigated by comparative experiments at narrow-band and broadband laser excitations.     

Precisely tunable, narrow-band pulsed dye laser

Anarrow-band, precisely tunable dye laser pumped by an injection-seeded YAG laser is described. The laser achieves an output of 100 mJ/pulse and 40% efficiency when one uses Rhodamine 6G dyes. The output pulse is Gaussian both in time and spatial profile. The laser oscillator employs an intracavity étalon that is repetitively pressure scanned over one free spectral range while the grating successively steps to consecutive étalon modes. We pressure scanned the étalon under computer control using a bellows. Methods are described for calibrating the tuning elements for absolute precision. We demonstrated that the laser has an absolute precision of ±0.4 pm over a 1.0-nm scan. This accuracy is achievable over the wavelength range of a dye.     

Utilization of the Yellow Component of a Copper-Vapor Laser for Extending the Tuning Range of a Rhodamine 6G Dye Laser by Use of an Additional Dye in a Novel Coupled Resonator Scheme

We demonstrate the use of both 510.6- and 578.2-nm components and the extension of the tuning range of a Rhodamine 6G dye laser in a novel coupled resonator scheme. Rhodamine 6G is pumped by 510.6-nm light in one resonator and Sulforhodamine B is pumped by 578.2 nm in the other. The spectral tuning range of 564-609 nm of the Rhodamine 6G laser is extended up to 640 nm. A two-mirror arrangement ensures continuous tuning across the spectral ranges of the two dyes by rotation of a single plane mirror.     

Broadband coherent anti-Stokes Raman spectroscopy with a modeless dye laser

We develop a modeless dye laser for broadband coherent anti-Stokes Raman spectroscopy (CARS) and investigate the operational characteristics of the modeless laser. The energy efficiency of the modeless laser is 6%, and the beam divergence is 0.65 mrad. We construct a compact movable CARS system with the modeless laser and a graphite tube furnace to assess the accuracy of the CARS temperature. It is found that the difference between the averaged CARS temperature and the radiation temperature measured with an optical pyrometer is <2% at a temperature range from 1000 to 2400 K. We also measure the averaged CARS temperature drift owing to the variation of the spectral distribution of the modeless laser, which is <1.5% during 5 h of operation.     

Epoxy matrix for solid-state dye laser applications

The preparation and performance of an epoxy-based matrix impregnated with Pyrromethene 580 for solid-state dye laser applications are discussed. The matrix proved to be stable and efficient as a laser medium when pumped by a frequency-doubled, Q-switched Nd:YAG laser with a 10-ns pulse width. Stability measurements were performed on a 1-mm-thick epoxy sample, doped with Pyrromethene 580 at a concentration of 4 x 10(-3) M. When the sample was pumped at millijoule energy levels, the stability was measured to be ~55,000 pulses from a single spot on the sample before the power dropped by a factor of half.     

Efficient lasing of a mixture of organic dyes at 610 and 670 nm

The efficiency of the lasing regime of a pulsed tunable laser at the “exotic” wavelengths of 610 and 670 nm, which are needed in nuclear physics, is investigated. A method for performing model calculations of the active media for such a laser, i.e., working mixtures of high-molecular-weight dyes, is developed. The conditions specifying a quasiequilibrium window, in which the achievement of high output characteristics is possible for such a mixture at assigned wavelengths, are derived. It is interpreted on the basis of the law of alternating nonequilibria in the theory of nonequilibrium states. The results obtained are verified experimentally: a high-power miniature pulsed laser with an active medium consisting of a mixture of dye solutions with high output characteristics at the assigned wavelengths of 610 and 670 nm is created. Pis’ma Zh. Tekh. Fiz. 25, 1–7 (April 26, 1999)     

Holographic DFB Dye Lasers

A new simple type of DFB dye laser is proposed. The laser consists of a dye cell and a transparent phase holographic grating mounted on the front window of the cell. In the experiment, high-quality stable transparent holographic gratings were recorded in bichromated gelatine with a single mode He-Cd laser. When pumped perpendicular to the surface of the grating the DFB laser emitted a narrow spectral line independently of the width and the position of the pump spectrum. A linewidth of 0·01 nm was measured under pumping with broadband radiation of 20 nm spectral width. Either a set of holographic gratings of a different period or a fan-shaped grating may be used in the laser for stepped or continuous spectral tuning.     

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.     

Noise in single-shot broadband coherent anti-Stokes Raman spectroscopy that employs a modeless dye laser

The noise in single-shot coherent anti-Stokes Raman (CARS) spectroscopy that employs a broadband modeless dye laser (MDL) is examined and the results are compared with those of a conventional dye laser. The noise of the dye-laser, the nonresonant CARS, and the resonant N(2) CARS signals are determined. The use of a MDL is shown to result in substantially reduced CARS noise when the CARS signal is generated with a single-mode pump laser, but only a marginal reduction of noise is observed with a multimode pump source The noise measurements are compared with theoretical predictions that are based on models that assume modes of random amplitudes and phases in the multimode laser sources. The combination of a MDL and a single-mode pump laser is shown to increase the precision of single-shot N(2) CARS temperature measurements.     

Single pulse CARS noise: a comparison between single-mode and multimode pump lasers

An investigation of single pulse coherent anti-Stokes Raman spectroscopy (CARS) noise, determined by the analysis of broadband nonresonant spectra, is described. It is shown that the use of a single-mode rather than a multimode pump laser leads to a significant reduction of CARS noise (40%), down to the level exhibited by the Stokes spectral profile itself. This reduction in noise is attributed to the minimization of the effects due to random variations in the laser temporal profiles by using temporally smooth single-mode laser pumps. A measurement of detector shot noise is presented and its effect on CARS noise is described. The advantages of using a single-mode pump laser in CARS spectroscopy are discussed.     

Is dye mixture more suitable rather than single dye to fabricate dye sensitized solar cell?

The steady state and time resolved spectroscopic studies reveal that two xanthene dyes Rhodamine 6G (R6G) and Rhodamine B (RB), used in the present investigations, form ground state hydrogen -bonded complexes with meso-tetrakis(4-carboxyphenyl) porphyrin (TCPP). However, it is apparent that upon photoexcitation the H-bonding complexes formed in the ground state decompose into the individual reacting components. This presumption was confirmed from the observation of the presence of only static quenching mode in the steady state fluorescence of the dyes in presence of porphyrin. The photoelectrochemical properties of the free dyes and the mixtures of each dye with porphyrin are investigated by measuring incident photon-to-current conversion efficiency (IPCE) using ZnO electrode and also with TiO2 electrode. It is seen that Rhodamine B-porphyrin mixture has attained maximum IPCE among the four samples studied at approximately 550 nm using ZnO electrode. Using TiO2 electrode, slight improvement in the value of IPCE was found for the same mixture. Therefore Rhodamine B-porphyrin mixture may act as a good sensitizer for converting solar energy to electrical energy.     

Thermometry for turbulent flames by coherent anti-Stokes Raman spectroscopy with simultaneous referencing to the modeless excitation profile

An optimal system for temperature measurements by coherent anti-Stokes Raman spectroscopy (CARS) in turbulent flames and flows is presented. In addition to a single-mode pump laser and a modeless dye laser, an echelle spectrometer with a cross disperser is used. This system permits simultaneous measurement of the N2 CARS spectrum and the broadband dye laser profile. A procedure is developed to use software to transform this profile into the excitation profile by which the spectrum is referenced. Simultaneous shot-to-shot referencing is compared to sequential averaged referencing for data obtained in flat flames and in room air. At flame temperatures, the resultant 1.5% imprecision is limited by flame fluctuations, indicating that the system may have a single-shot imprecision below 1%. At room temperature, the 3.8% single-shot imprecision is of the same order as the best values reported for dual-broadband pure-rotational CARS. Using the unique shot-to-shot excitation profiles, simultaneous referencing eliminates systematic errors. At 2000 and 300 K, the 95% confidence intervals are estimated to be +/- 20 and +/- 10 K, respectively.     

Stray Light Rejection in Rotational Coherent Anti-Stokes Raman Spectroscopy by use of a Sodium-Seeded Flame

A common experimental problem with rotational coherent anti-Stokes Raman spectroscopy (CARS) is undesired spectral interference that is due to stray light from the primary laser beams. Also, for the most developed approach, dual-broadband rotational CARS, practical measurements often suffer from stray light interference from the narrow-band laser, inasmuch as the CARS signal is produced inherently in the spectral vicinity of the narrow-band laser beam. An optical filter does not provide a sufficiently sharp transmission profile, thus leading to signal loss and spectral distortion of the rotational CARS signal. An atomic filter consisting of a sodium-seeded flame is presented here as a solution to the problem, and its usefulness was demonstrated in dual-broadband rotational CARS experiments.