Additive-pulse mode locking

K. Tamura, H.A. Haus, E.P. Ippen, Self-starting additive pulse mode-locked erbium fiber ring laser. Electron. Lett. 28, 2226 (1992)... 

FIGURE 16 Additive pulse mode-locked laser. The output of the color center laser is coupled to an optical fiber. Self-phase modulation in the fiber adds bandwidth to the pulse, which is coupled back to the laser. The combination produces femtosecond-duration pulses. (OC, output coupler BS, beam splitter BTP, birefringent tuner plate HR, high reflector PZT, piezotransducer). 

Soliton ring fiber lasers can be also realized with active mode locking by polarization modulation [11.77], or by additive-pulse mode locking (APM). In the latter technique the pulse is split into the two arms of an interferometer and the coherent superposition of the self-phase modulated pulses results in pulse shortening [11.55]. 

Several laser systems have been used in our time-resolved PM measurements. For the ultrafast measurements, a colliding pulse mode-locked (CPM) dye laser was employed [11]. Its characteristic pulsewidth is about 70 fs, however, its wavelength is fixed at 625 nin (or 2.0 cV). For ps measurements at various wavelengths two synchronously pumped dye lasers were used (12], Although their time resolution was not belter than 5 ps, they allowed us to probe in the probe photon energy range from 1.25 cV to 2.2 cV. In addition, a color center laser... 

In passive mode-locking, an additional element in the cavity can be a saturable absorber (e.g., an organic dye), which absorbs and thus attenuates low-intensity modes but transmits strong pulses. Kerr lens mode-locking exploits the optical Kerr63 or DC quadratic electro-optic effect here the refractive index is changed by An = (c/v) K E2, where E is the electric field and K is the Kerr constant. 

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). 

TCSPC is Ulnstrated in Fig. 3a. In addition to a mode-locked laser for pnlsed excitation and a detector with high time resolntion (nsnally a micro-channel plate photomultiplier tube capable of time-resolution of 20-30 ps), the required instrumentation inclndes constant-fraction discriminators to generate electrical pnlses triggered by fluorescence photons and by the reference (the excitation pulse), a time-to-amphtude converter or other device to measnre the time lag between reference and flnorescence connts, and a multichannel scaler to accumnlate... 

Fig. 5.8 Hyper-Raman scattering spectta of SWNTs excited by ps-pulsed radiation of 790 nm from the mode-locked Ti sapphrre laser. SWNTs exhibited the G-band and D-band peaks in addition to the hyper-Rayleigh scattered peak [27]...

Experimental Setup. The first example of a one-color pump-probe experiment on a picosecond time scale was given by Shank et al., where hemoglobin was studied, and dates back to the first mode-locked Ar" synchronously pumped dye laser systems developed by Shank and Ippen. Since then many different groups have made use of the one-color pump-probe technique. " A simplified scheme of such a setup is given in Fig. 3. A high repetition rate (76 MHz) frequency-doubled (532 nm) mode-locked Nd-YAG laser (or Ar ion laser) is used to pump a dye laser with 50-100 psec pulses. In the dye laser one dye jet functions as a lasing medium leading to pulses which can be shorter than 10 psec. An additional... 

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