Cavity-dumping

Often an acousto-optic switch is used, for example, for argon lasers and cw dye lasers [648]. Its basic principle is explained in Fig. 6.6. A short ultrasonic pulse with acoustic frequency / and pulse duration T 1 //s is sent nit = to through a fused quartz plate inside the laser resonator. The acoustic wave produces a time-dependent spatially periodic modulation of the refractive index n(t,z), which acts as a Bragg grating with the grating constant A = Cs//, equal to the acoustic wavelength A where Cg is the sound velocity. When an optical wave Eocos((ot — k r) with the 

The fraction rj depends on the modulation amplitude of the refractive index and thus on the power of the ultrasonic wave. 

When the optical wave is reflected at an acoustic wave front moving with the velocity Vs, its frequency co suffers a Doppler shift, which is, according to (6.1) with 

The average output power Pc the light pulse is then with co Q and 

The repetition rate / of the extracted pulses can be varied within wide limits by choosing the appropriate repetition rate of the ultrasonic pulses. Above a critical frequency /c, which varies for the different laser types, the peak power of the extracted 

The technique of cavity dumping is mainly applied to gas lasers and cw dye lasers. One achieves pulse durations AT = 10—100ns, pulse-repetition rates of 0—4 MHz, and peak powers that may be 10—100 times higher than for normal cw operation with optimized transmission of the output coupler. The average power depends on the repetition rate /. Typical values for f = 10 -4 X 10 Hz are 0.1—40% of the cw output power. The disadvantage of the acoustic cavity dumper compared to the Pockels cell of Fig. 11.5 is the intensity modulation of the pulse at the frequency 2f . 

Often an acousto-optic switch is used, e.g., for argon lasers and CW dye lasers [11.24]. Its basic principle is explained in Fig. 11.6. A short ultra- 

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Pshenichnikov M S, de Boeij W P and Wiersma D A 1994 Generation of 13 fs, 5 MW pulses from a cavity-dumped Ti sapphire laser Opt. Lett. 19 572-4... 

From these vievqjoints, we have developed a femtosecond NIR laser microscope with a home-built cavity dumped chromium forsterite (Cr F) laser as an excitation light source whose output wavelength is centered at 1260 run. In the following the set-up of the NIR laser microscope and its application to multiphoton imaging are presented. 

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). 

The commercially available laser source is a mode-locked argon-ion laser synchronously pumping a cavity-dumped dye laser. This laser system produces tunable light pulses, each pulse with a time duration of about 10 picoseconds, and with pulse repetition rates up to 80 million laser pulses/second. The laser pulses are used to excite the sample under study and the resulting sample fluorescence is spectrally dispersed through a monochromator and detected by a fast photomultiplier tube (or in some cases a streak camera (h.)) ... 

The excitation pulse tram is used to trigger of the excitation. Mode-locked lasers and cavity-dumped lasers are versatile excitation sources however, they are not ideal to implement portable instruments. Externally modulated solid state microlasers are a... 

The femtosecond fluorescence up-conversion setup has been described elsewhere [13,14]. Briefly, a second harmonic (SH) of a home-made chromium-forsterite femtosecond laser tunable from 610 to 660 nm was used to excite the sample (Fig.2) [14]. The pulse duration of the SH pulses was about 50 fs at the full width at half maximum (FWHM). We were successful in the cavity-dumping operation of this laser [14] and kept the repetition rate as low as 4 MHz. Reduction of the repetition rate was necessary to avoid multiple hits of the same location of the sample as small as possible. The excitation intensity, controlled by a neutral density filter before the sample cell, was (0.5-l)xl012 photons/cm2/pulse. Special care was taken to work at the lowest excitation light intensity so that the effect of the exciton-exciton annihilation process was negligible. 

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. 

It is also conceivable that one laser pulse could be used as both the photolysis and the probe laser. By studying the RR spectrum as a function of power, the Raman spectrum of the species formed during the pulse duration and enhanced at the laser frequency used for photolysis will be observed. It is conceivable that the amplified picosecond pulses produced from the mode locked-cavity dumped techniques (16) could be used to determine the RR spectra of species formed in the pico- and hopefully in the sub-picosecond time scale. 

Figure 14. Experimental apparatus for picosecond, time-resolved CD measurements using a mode-locked, Q-switched, cavity dumped pump laser. P, polarizer PC, Pockels cell Q, quarter-wave plate RHP, rotating half-wave plate S, sample cell PMT, photomultiplier tube. From ref. [42].

Fig. 4. Diagram of a cavity dumped, passively mode-locked dye laser. In this version, the saturable absorber is in a free flowing dye stream. In more recent experiments, the saturable absorber flows in contact with a 100% reflectivity resonator mirror (see text).

The apparatus used to perform vibrational relaxation experiments in supercritical fluids consists of a picosecond mid-infrared laser system and a variable-temperature, high-pressure optical cell (68,73). Because the vibrational absorption lines under study are quite narrow (<10 cm-1), a source of IR pulses is required that produces narrow bandwidths. To this end, an output-coupled, acousto-optically Q-switched and mode-locked Nd YAG laser is used to synchronously pump a Rhodamine 610 dye laser. The Nd YAG laser is also cavity-dumped, and the resulting 1.06 pm pulse is doubled to give an 600 u.l pulse at 532 nm with a pulse duration of "-75 ps. The output pulse from the amplified dye laser ("-35 uJ at 595 nm, 40 ps FWHM) and the cavity-dumped, frequency-doubled pulse at 532 nm... 

The eccitation source is based on a mode-locked, cavity dumped argon-ion laser (Spectra Hiysics Model 166/366) and provides a train of narrow, highly stable intense li t pulses with repetition rates selectable from single shot to l(X)MHz. The wave-... 

Using mode-locked cavity-dumped pulses from ion laser, second harmonic (267.25 nm) FWHM-300 ps... 

Using cavity-dumped dye laser Hilses (580 nm) FWHM —7 ns Corresponding to monomer emission only Cbnesponding to excimer emission only... 

Fluorescence lifetimes were measured by time-correlated single photon counting using a mode-locked, synchronously pumped, cavity-dumped pyridine I dye laser (343 nm) or Rhodamine 6G dye laser (290 nm). Emissive photons were collected at 90° with respect to the excitation beam and passed through a monochromator to a Hamamatsu Model R2809U microchannel plate. Data analysis was made after deconvolution (18) of the instrument response function (FWHM 80 ps). 

Figure 1. Q-switched, mode-locked Nd YAG laser with two synchronously pumped dye lasers PC = Pockels cell POL = polarizer with escape window DLl, DL2 = cavity dumped dye lasers PMT = photomultiplier tube. (Reproduced from Ref. 7. Copyright 1986 American Chemical Society.)...

All solutions were relatively dilute such that the OD at 290nm was << 0.1. The fluorescence Intensity was sufficiently low that all decay curves were obtained at the Center for Fast Kinetics Research at the University of Texas. This system uses Standard photon-counting electronics but the excitation source is a synch-pumped cavity-dumped dye laser pumped by a NdiYAG laser... 

In order to overcome this obstacle, we used a synchronously pumped, mode-locked dye laser, cavity-dumped at 4 MHz and time-correlated single-photon counting detection (18). Because of the higher sensitivity of this experimental system we were able to work at low e, using aqueous rhodamine B solutions with concentrations down to 10" M. To examine the dependence of the fluorescence decays on we chose to work with surface-solution interfaces, so as to minimize the problems associated with inhomogeneous surface coverage, which arise with dry surfaces... 

Lytle, F.E., and Kelsey, M.S. (1974) Cavity dumped argon ion laser as an excitation source in time resolved fluorimetry. Analytical Chemistry, 46, 855 860. 

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