Mode-locked argon laser

Fig. 5.21 Wavelength-resolved decay profile of DODCI (3,3 -diethyloxacarbocyanm-iodid) in ethanol. Excitation hy mode-locked argon laser at 514 nm. Scattered excitation light forms a peak at the excitation wavelength...

Fig. 6.11 Measured pulses of a mode-locked argon laser at A = 488 nm (a) monitored with a fast photodiode and a sampling oscilloscope (500 ps/div). The small oscillations after the pulse are cable reflections (b) the attenuated scattered laser light was detected by a photomultiplier (single-photon counting) and stored in a multichannel analyzer. The time resolution is limited by the pulse rise times of the photodiode and photomultiplier, respectively [656]...

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. 

Pulse widths below 100 fs can be reached with this CPM technique [675, 676]. If the CPM ring dye laser is synchronously pumped by a mode-locked argon laser stable operation over many hours can be realized [677]. Using a novel combination of saturable absorber dyes and a frequency-doubled mode-locked Nd YAG laser as a pump, pulse widths down to 39 ps at A = 815 nm have been reported [678]. 

When a cw laser is mode locked its average output power decreases by a factor 3 which depends on various parameters of the active medium, such as the lifetime of the upper laser level. For mode-locked argon lasers 3 is about 2-4. The peak power, however, is increased by a factor 6v/(3Av). 

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.)) ... 

A mode-locked Tksapphire laser, which can produce sub-ps pulses and is usually pumped by CW argon or Nd-YAG lasers. The shortest pulses coming from this laser type are exceptionally short, of the order of a few femtoseconds. 

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. 

This Synchroscan [68] streak camera system has been used to study the time resolved fluorescence of trans-stilbene in the picosecond time regime. The experimental arrangement [69] is shown in Fig. 20. An acousto-optically mode-locked argon ion laser (Spectra Physics 164), modulated at 69.55 MHz was used to pump a dye laser. The fundamental of this dye laser, formed by mirrors M, M2, M3 and M4, was tunable from 565 to 630 nm using Rhodamine 6G and second harmonic output was available by doubling in an ADP crystal placed intracavity at the focal point of mirrors M5 and M6. The peak output power of this laser in the ultraviolet was 0.35W for a 2ps pulse which, when focused into the quartz sample cell of lens L, produced a typical power density of 10 KW cm-2. Fluorescence was collected at 90° to the incident beam and focused onto the streak camera photocathode with lens L3. The fluorescence was also passed through a polarizer and a bandpass filter whose maximum transmission corresponded to the peak of the trans-stilbene fluorescence. 

Fig. 20. Schematic diagram of the Synchroscan streak camera system. A Spectra Physics model 164 acousto-optically mode-locked argon ion laser modulated at 69.44MHz pumps the Rhodamine 6G dye laser formed by mirrors Mi, M2, M3 and M4. This dye laser typically produces pulses of 2 ps duration with an energy content of 0.6 nJ. The second harmonic is generated intracavity in an ADP crystal. The UV radiation is then coupled out through mirror Ms and a filter F2 is used to eliminate any transmitted visible light before focusing into the sample cell with lens Lt. The fluorescence is detected at 90 to the incident beam. A lens L2 collects the fluorescence which passes through a polarizer and a bandpass filter and then onto the slit of the streak camera. (After ref. 69.)...

Synchronous pumping Method of mode-locking a laser (here the dye laser) by gain modulation or repetitive pulsed pmnping, where the roimd-trip transit time in the cavity of the pumped laser is adjusted to be precisely equal to (synchronous with) the period of a mode-locked pumping laser (here, usually an argon laser or doubled Nd YAG laser). 

For the one-color picosecond pump probe experiments (see Sect. 3.2.2), an astigmatically compensated dye laser (Spectra Physics model 375) with rhodamine 6G/rhodamine B is used [190]. The dye laser is synchronously pumped by an actively mode-locked argon ion laser (Spectra Physics model 171). In the synchronous pumping process, the output pulses of the mode-locked ion laser are used to excite the dye laser, whose cavity length has been extended to be equal to the ion laser s cavity 1.8 m) [187]. The transmission of the dye laser s output coupler is 22%. With this setup the emitted dye laser pulses have a pulse duration of 1.38 ps and a bandwidth of 40 cm (see Fig. 2.3). The pulse repetition rate is 82.5 MHz and an average power of 120 mW is obtained over the tuning from 600 nm to 630 nm. 

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