CW mode

A further advantage, compared with the alexandrite laser, apart from a wider tuning range, is that it can operate in the CW as well as in the pulsed mode. In the CW mode the Ti -sapphire laser may be pumped by a CW argon ion laser (see Section 9.2.6) and is capable of producing an output power of 5 W. In the pulsed mode pumping is usually achieved by a pulsed Nd YAG laser (see Section 9.2.3) and a pulse energy of 100 mJ may be achieved. 

The energy input into a CO2 laser is in the form of an electrical discharge through the mixture of gases. The cavity may be sealed, in which case a little water vapour must be added in order to convert back to CO2 any CO which is formed. More commonly, longitudinal or, preferably, transverse gas flow through the cavity is used. The CO2 laser can operate in a CW or pulsed mode, with power up to 1 kW possible in the CW mode. 

New impetus was given to photomedicine by development of lasers that are compatible with the clinical environment. These include HeNe, Ar ion, mby, and tunable dye lasers operating in the continuous wave (cw) mode. Prior to the advent of lasers in medicine, only the treatment of newborn jaundice, and the appHcation of long wavelength uv irradiation in conjunction with adininistration (or topical appHcation) of psoralen class sensitizers to treatment of skin diseases (86), principally psoriasis, were clinically important phototherapies. 

Sum-frequency mixing of two solid-state YAG lasers in a nonlinear crystal (see Ch. 20) to generate 589 nm in CW, CW mode-locked and macromicro pulse formats. The Nd YAG lasers can be pumped by flashlamps, but higher efficiency is obtained using diode lasers. 

For EPy-doped PMMA film, a 308 nm excimer laser (Lumonics TE 430T-2, 6ns) was used as as exposure source. We used a tine-correlated single photon counting systen (18) for measuring fluorescence spectra and rise as well as decay curves of a snail ablated area. The excitation was a frequency-doubled laser pulse (295 nm, lOps) generated from a synchronously punped cavity-dumped dye laser (Spectra Physics 375B) and a CW mode-locked YAG laser (Spectra Physics 3000). Decay curves under a fluorescence microscope were measured by the same systen as used before (19). 

Tksapphire lasers can operate both in CW mode and in pulsed mode. There are two types of pulsed Tksapphire laser ... 

The most convenient means of making time-resolved SH measurements on metallic surfaces is to use a cw laser as a continuous monitor of the surface during a transient event. Unfortunately, in the absence of optical enhancements, the signal levels are so low for most electrochemical systems that this route is unattractive. A more viable alternative is to use a cw mode-locked laser which offers the necessary high peak powers and the high repetition rate. The experimental time resolution is typically 12 nsec, which is the time between pulses. A Q-switched Nd YAG provides 30 to 100 msec resolution unless the repetition rate is externally controlled. The electrochemical experiments done to date have involved the application of a fast potential step with the surface response to this perturbation followed by SHG [54, 55,116, 117]. Since the optical technique is instantaneous in nature, one has the potential to obtain a clearer picture than that obtained by the current transient. The experiments have also been applied to multistep processes which are difficult to understand by simple current analysis [54, 117]. 

Laser II A femtosecond mode-locked dye laser (Coherent, Satori) synchronously pumped using a cw mode-locked and frequency-doubled Nd YAB laser (Coherent, Antares), generating pulses in 76 MHz repetition rate and 250-fs fwhm. 

Laser III A picosecond mode-locked and cavity-dumped dye laser (Spectra-Physics, 375B and 344S) synchronously pumped using a cw mode-locked argon ion laser (Spectra-Physics, 2030-18), generating tunable (530-830 nm) pulses in 4-MHz repetition rate and 10-ps fwhm. 

Figure 25. Excitation and photoluminescence (solid and dashed lines) of x% Mn2+ CdS nanocrystals, where. v = 0 (a), 0.8 (fe), 2.5 (c), and 4.8 (d). The solid luminescence spectra were collected in CW mode, and the dashed luminescence spectra were collected with a pulsed excitation source and a 2-ms delay between excitation and emission detection. Note that the intensities of (b)-(d) are referenced to that of (a). [Adapted from (82).]...

The forward and reflected electric power is measured in continuous wave (CW) mode before the exposures using a digital power meter (HP Model) and a dual directional coupler (Werlatone Model C1373). 

Figure 3.6-10 Schematic diagram of a femtosecond time-resolved CARS apparatus. YAG, cw mode-locked Nd YAG laser ML, mode locker PL, polarizer A s, apertures LP, laser pot DM, dichroic mirror DLl, femtosecond dye laser SA, saturable absorber CLFB, cavity-length feedback system DL2, picosecond dye laser W, tuning wedge E, etalon FD, fixed delay VD, variable delay BS, beam splitter P s, half-wave plates (when necessary) F s, filters S, sample MC, monochromator PMT, cooled photomultiplier. (Okamoto and Yoshihara, 1990).

The basis of the experimental femtosecond CARS apparatus developed by Okamoto and Yoshihara (1990) which is reproduced in Fig. 3.6-10 is essentially the same as that of Leonhardt et al. (1987) and Zinth et al. (1988) with the addition of the possibility to change the polarization of the laser radiation. The main parts of the system are two dye lasers with short pulses and high repetition rates, pumped by a cw mode-locked Nd YAG laser (1064 nm, repetition rate 81 MHz). The beam of the first dye-laser which produces light pulses with 75-100 fsec duration is divided into two parts of equal intensities and used as the pump and the probe beam. After fixed (for the pump beam) and variable (for the probe beam) optical delay lines, the radiation is focused onto the sample together with the Stokes radiation produced by the second laser (DL2), which is a standard synchronously pumped dye laser. The anti-Stokes signal generated in the sample is separated from the three input laser beams by an aperture, an interference filter, and a monochromator, and detected by a photomultiplier. For further details we refer to Okamoto and Yoshihara (1990). 

The present limitatkins in time resolution for the time-correlated photon counting technique are due to the time jitter in the detection electronics and the transit time spread in the photomultqjlier tube ( 500 ps). Mth future improvements in these components and using cw mode-locked lasers as an excitation source, deconvolution of fluorescence lifetimes to a few tens of picoseconds oidd be achieved. Alter-... 

Prichard BN, Owens CW. Mode of action of beta-adrenergic blocking drugs in hypertension. Clin Physiol Biochem 1990 8(Suppl 2) 1-10. 

Lasers for solid sampling should always be operated in the pulsed mode. The continuous, CW mode, as often used for drilling and welding in engineering, would have serious drawbacks. The application of a CW laser provides much lower intensities than the peak intensities of pulsed laser. There would be continuous removal from the sample, which is closer to ordinary evaporation than the explosion-like ablation process of pulsed lasers. 

The parameter gv depends on the shape of the laser pulse and the duty cycle. Pulses with a Gaussian temporal profile result in gv = 0.664 while gv = 0.588 is found for hyperbolic-secant square (sech2) pulses. Combination of Eqs. (40) and (41) results in Eq. (42), showing that pulsed lasers are more efficient excitation sources compared to lasers operating in cw mode. 

Lasers in cw mode have gc= 1 and require some orders of magnitude more average power in comparison with a mode locked Ti sapphire laser. This may result in photodestruction of the chromophore and is not desired. 

NMR Measurements - 1H- and 13C- NMR spectra of the polymers in CDCI3 solution at ambient temperature were recorded at 300 and 20 MHz, respectively, using Varian HR-300 (CW mode) and CFT-20 (FT mode) spectrometers. The 13C-NMR spectra were the result of over 100 K accumulations using a pulse width of lOy sec. Tetramethylsilane was used as an internal standard. 

Excimer lamps were selected to study the low fluence irradiation region, where linear (no ablation) photochemistry is taking place. This is the fluence range (e.g., insert in Fig. 47 of the previous chapter), where a linear relation between reaction products and laser fluence is observed. This may correspond to the range of linear photochemistry, i.e., below the threshold of ablation (see, e.g., Figs. 25 and 26), or the so-called Arrhenius tail. The excimer lamps emit at the same wavelengths as the excimer lasers, but with incoherent radiation, and in quasi-CW mode. The peak photon fluxes of the lamps are low compared to the excimer laser, suggesting that multiphoton processes are not important. Thin films of the triazene polymer on quartz substrates were irradiated with the excimer lamps under different conditions, i.e., in Ar, air, and 02. 

In fluorescence spectrometry, the intensity of fluorescence is proportional to the intensity of the radiation source (see Section 16.15). Various continuum UV sources are used to excite fluorescence (see below). But the use of lasers has gained in importance because these monochromatic radiation sources can have high relative intensities. Table 16.5 lists the wavelength and power characteristics of some common laser sources. Only those that lase in the ultraviolet region are generally useful for exciting fluorescence. The nitrogen laser (337.1 nm), which can only be operated in a pulsed mode (rather than continuous wave, or CW, mode), is useful... 

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