Saturable absorbers

CCL) can be easily incorporated into this configuration. It gives us the additional spectral range of 0.77 to 0.86 eV. The typical pulsewidth of the mode-locked CCL is about 20 ps, but can be improved to 200 fs by using a saturable absorber. A germanium detector has been used for the detection of the CCL probe beam. 

NIR region (see Chapter 9.13), complexes of 1,2-dithiolene (DT) and related ligands have attracted considerable attention for their (largely cubic) NLO properties. The complex (156) (a.k.a. BDN) is a highly photochemically stable, saturable absorber and has hence found extensive applications in laser Q-switching. The cubic NLO properties of (156) have been studied by DFWM148,403-407 and more recently, Z-scan.408 Time-resolved DFWM has been applied to square planar Co, Ni, Cu, or Pt complexes of 1,2-benzenedithiolate (BDT) or 1,2-aminobenzenethiolate ligands by Lindle and co-workers.409,410... 

Perry, J. W. Organic and Metal-containing Reverse Saturable Absorbers for Optical Limiters. In Nonlinear Optics of Organic Molecules and Polymers, Nalwa, H. S. Miyata, S., Eds CRC Boca Raton FL 1997, pp 813-840. 

Cyanines have been widely used as laser dyes, and as saturable absorbers in modelocked and Q-switched laser systems. 8, 50) The propensity of most cyanines to photooxidize which makes them useful in photographic film and as saturable absorbers makes them less than desirable as fluorophores in other applications. The use of... 

The incorporation of Cr" + ions in crystals is presently an active research subject, due to the possibility of realizing new broadly tunable solid state lasers in the infrared, which will operate at room temperature. Moreover, the spectroscopic properties of this ion are particularly useful in the development of saturable absorbers for Q-switching passive devices. At the present time, Cr + YAG is the most common material employed as a passive Q-switch in Nd YAG lasers. This is because the ions provide an adequate absorption cross section at the Nd + laser wavelength (1.06 /um), together with the good chemical, thermal, and mechanical properties of YAG crystals, which are required for stable operation. 

CnLiSAF laser crystal does not require any cooling. As a result, relatively inexpensive femtosecond CnLiSAF lasers with optical output powers up to 45 mW, robustly modelocked with a semiconductor saturable absorber mirror (SESAM), are achievable from entbely self-contained and portable units with dimensions of 22 cm x 28 cm. [A version of this laser design incorporating one pah of pump laser diodes is shown in figure 10] A second pah of pump laser diodes can easily be incorporated onto the baseplate to maximize operational performance (figure 12). 

The second category comprises the flash photolysis experiments using the short high power light pulses from Q-switched lasers, furthermore all investigations of time-dependent behavior of excited dye molecules, which play an important role as active material in dye lasers or as saturable absorbers in passive Q-switched giant pulse lasers. 

The idea of using the same medium as absorber and active material has been proposed and realized by several authors 340-343) Leg and Skolnick 40) used a neon gas discharge at low current and low pressure as saturable absorber inside the cavity of a He-Ne laser oscillating at X = 6328 A. The Lamb-dip halfwidth obtained was 30 Mc/sec compared to 1500 Mc/sec for the doppler line. The disadvantage of this arrangement is that the frequency of the neon transitions depends upon pressure and current 341) in the absorption cell, and this limits the stability and reproducibility of the Lamb dip center frequency. 

This effect can be used to produce shorter pulses than are obtainable by modelocking alone. Using five transits through a saturable absorber cell (and 4 ampli-... 

Much simpler than the use of active isolators, such as rotators making use of the Faraday effect, is again a saturable absorber cell which can easily be adjusted in length, dye composition and concentration to give the desired low light level attenuation and high light level transmission 46>. 

One drawback of dye lasers as compared to solid-state lasers is the short fluorescence lifetime rp or energy storage time, which implies a quick inversion decay when pumping stops. For this reason one cannot Q-switch a dye laser. On the other hand, dye lasers can be mode-locked by saturable absorbers 52> in much the same way as solid-state lasers, and many investigations have shown that one can obtain psec pulse in this way over a wide spectral region 53,54)... 

The observed ultraviolet absorption spectrum of l212 (for a display of the recorded spectrum see Section I.G of Wiberg s review article on the cyclopropyl group14) consists of three broad bands, of which the first (60,000 to 66,000 cm-1 7.44-8.18 eV) possesses a maximum at 63,000 cm 1 (7.8 eV oscillator strength/ 0.12), far to the red of most saturated absorbers and in the region of the n->n transitions of unsaturated molecules. This observation has... 

Reverse saturable absorption is another important nonlinear optical effect. Reverse saturable absorbers (RSAs) act as optical limiters by absorbing laser radiation. Therefore, they are particularly useful for laser protection, both civilian and military. Colorless infrared-absorbing RSAs which can absorb green laser radiation are especially important as they safeguard the eyes of pilots, tank commanders, etc. from enemy lasers [61],... 

L. Roso-Franco, Self-Reflected Wave inside a Very Dense Saturable Absorber, Physical Review Letters 55, 2149 (1995)... 

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 c.w. dye laser can also be passively mode-locked and two different arrangements have been used. The first employed two free flowing dye streams, one for the laser dye and the other for the absorber (see Fig. 4) [18, 19]. In the alternative arrangement, the saturable absorber dye flows in a narrow channel of variable thickness (0.2—0.5mm) and in contact with a 100% broadband reflectivity mirror. With an absorber thickness of 0.5 mm, output pulses of 1 ps duration have been obtained [20]. Pulses as short as 0.3ps were produced when the DODCI cell length was shortened to 0.2 mm. The subpicosecond pulses produced in this arrangement were transform-limited in bandwidth. 

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. 

Mode-locked laser A laser in which many resonant modes are coupled in phase, to yield a train of very short pulses (e.g. ps pulses). The coupling of the modes is obtained by modulation of the gain in the resonator, and can be active (electro-optic modulation of the losses or of the pump intensity), or passive (with a saturable absorber). 

Q-switched laser A laser in which the state of the device introducing important losses in the resonant cavity and preventing lasing operation is suddenly switched to a state where the device introduces very low losses. This increases rapidly the Quality factor of the cavity, allowing the build-up of a short and very intense laser pulse. Typical pulse durations are in the ns range. The Q-switching may be active (a rotating mirror or electro-optic device) or passive (a saturable absorber). 

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).

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... 

The simplest gas-solid containment systems conceptually are the direct adsorption ones. These accomplish adsorption on solids such as activated carbon, or alkalized alumina at relatively low temperatures and ordinary pressures [46]. In a separate unit a more concentrated sulfur dioxide stream is produced when the saturated absorbent is regenerated by heating. This is a more economically attractive feed to an acid plant or for liquefaction or sulfur generation. 

---