Synchroneous Pumping with Mode-Locked Lasers

For the synchroneous pumping the mode-locked pump laser LI which delivers short pulses with the time separation T = 2di/c is employed to pump 

The optimum gain for the dye-laser pulses is achieved if they arrive in the active medium (dye jet) at the time of maximum inversion AN(t) (Fig. 11.14). If the optical-cavity length 62 of the dye laser is properly matched to the length d of the pump laser, the round-trip times of the pulses in both lasers become equal and the arrival times of the two pulses in the amplifying dye jet are synchronized. Due to saturation effects the dye-laser pulses become much shorter than the pump pulses and pulse widths below 1 ps have been achieved [11.42-44]. For the experimental realization of accurate synchronization one end mirror of the dye-laser cavity is placed 

For many applications the pulse repetition rate f = c/2d (which is f = 150MHz for d = Im) is too high. In such cases the combination of syn-chroneous pumping and cavity dumping (Sect. 11.1.2) is helpful, where only every k-th pulse (k 10) is extracted due to Bragg reflexion by an ultrasonic pulsed wave in the cavity dumper. The ultrasonic pulse now has to be synchronized with the mode-locked optical pulses in order to assure that the ultrasonic pulse is applied just at the time when the mode-locked pulse passes the cavity dumper (Fig. 11.15b). 

There are several versions for the experimental realizations of mode-locked or synchroneously pumped lasers. Table 11.1 gives a short summary on typical operation parameters, advantages and disadvantages of the different techniques. More detailed representations of this subject can be found in [11.40-47]. 

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Ultrafast fluorescence quenching dynamics were studied by the fluorescence-up-conversion method with femtosecond mode-locked laser systems. For the studies of oxazine dyes, a synchronously pumped hybrid mode-locked dye laser with group velocity... 

For synchronous pumping the mode-locked pump laser Li, which delivers short pulses with the time separation T = 2d /c, is employed to pump another laser L2 (for example, a cw dye laser or a color-center laser). This laser L2 then operates in a pulsed mode with the repetition frequency / = l/T. An example, illustrated by Fig. 6.14, is a cw dye laser pumped by an acousto-optically mode-locked argon laser. 

Fig.11.15. Cavity-dumped synchroneously-pumped dye laser system with synchronization electronics [Courtesy Spectra Physics] (a) Experimental setup (p.d. pulse delay, f.d. frequency divider, ph.o. phase-locked oscillator. A amplifier), (b) Correct synchronization of a mode-locked laser pulse with the time of maximum output coupling of the cavity dumping pulse...

Time resolved hole burning spectra were measured by means of a femtosecond transient absorption spectrometer system. A second harmonics of a mode locked cw Nd + YAG laser (Quantronix, 82MHz) was used for a pumping source. A synchronously pumped rhodamine 6G dye laser with a saturable absorber dye jet (DODCl/DQOCI) and dispersion compensating prisms in the cavity was used. The output of the dye laser (lOOfs fwhm, 600pJ/pulse) was... 

Transient spectroscopy experiments were performed with a pump-probe spectrometer [7] based on a home-made original femtosecond Ti saphire pulsed oscillator and a regenerative amplifier system operated at 10 Hz repetition rate. The Tirsaphire master oscillator was synchronously pumped with doubled output of feedback controlled mode-locked picosecond pulsed Nd YAG laser. The pulse width and energy of Ti saphire system after the amplifier were ca. 150 fs and 0.5 mJ, respectively, tunable over the spectral range of 760-820 nm. The fundamental output of the Ti saphire system (790 nm output wavelength was set for present study) splitted into two beams in the ratio 1 4. The more intense beam passed through a controlled delay line and was utilized for sample... 

Nanosecond absorbance transients were measured with a single beam instrument (11). Excitation was provided by a 6-8 ns, 1 mJ 610 nm pulse from a NdrYAG pumped rhodamine dye laser. Absorption transients were either detected with a Hamamatsu R928 (A<900 nm) or R406 (A>900 nm) photomultiplier operating with a 2.5 ns response time. Picosecond absorbance transients were measured with a double beam apparatus (11). 1.5 ps, 1 mJ, 610 nm excitation pulses were generated by using the output of a mode-locked Ar" laser to synchronously pump a rhodamine dye laser. 

The ultrafast photodissociation dynamics of the Na3 C state was analyzed with time-resolved two-color TPI spectroscopy in the picosecond regime. The two excitation wavelengths required, independently tunable for the pump and the probe pulse, were generated by a home-built synchronization of two mode-locked titanium sapphire lasers. The deconvoluted real-time spectra can be well described by a single exponential decay with a time constant strongly... 

A common way to mode-lock a dye laser that is pumped by an ion laser is to use synchronous pumping. An actively locked ion laser with the same cavity length as the dye laser is then used (the length of the dye-laser... 

Fig.9.20. (a) Active mode locking by acousto-optic modulation, (b) Synchronous pumping of a dye laser equipped with a cavity dumper, (c) Passive mode-locking using a saturable absorber... 

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 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 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 time-resolved Raman spectra were measured with a picosecond time-resolved Raman spectrometer which employs a standard pump-probe technique. The details of the spectrometer have been publish elsewhere. The followings are concise description of the apparatus The output from a synchronously pumped mode-locked dye laser is amplified with the output from a cw Nd YAG regenerative amplifier. The second harmonic (294 nm, 2 kHz, 1-2 mW) of the amplified light (588 nm, 3.2 ps, 2 kHz, 15 mW) was used as a... 

The earliest subpicosecond systems incorporated dye laser technology. Shank, Ippen, and their colleagues at the Bell Laboratories [34] were the first to develop mode-locked subpicosecond lasers and to show how to compress pulses to very short values. With the colliding-pulse mode-locked (CPM) laser they achieved reduced pulse widths well into a subpicosecond range. Two approaches based on synchronously pumped dye lasers and colliding pulse dye lasers are commonly employed to produce subpicosecond pulses. These are briefly discussed below. 

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