Mode-locking passive

Passive mode-locking is a technique which demands less experimental efforts than active mode-locking it can be applied to pulsed as well as to CW lasers. Pulse widths below 1 ps have been realized. Its basic principles can be understood as follows  

Instead of the active modulator a saturable absorber is put inside the laser resonator, close to one of the end mirrors (Fig. 11.12). The absorbing transition k) - i) takes place between the levels i) and k) with short relaxation times Tj, In order to reach oscillation threshold in spite of the absorption losses the gain of the active medium must be correspondingly high. In case of a pulsed pump source the emission of the active laser medium at a time shortly before threshold is reached, consists of fluorescence photons which are amplified by induced emission. The peak power of 

This nonlinear interaction of photons with the absorbing and the amplifying media leads under favorable conditions to mode-locked laser operation starting from a statistically fluctuating, unstable threshold situation. After this short unstable transient state the laser emission consists of a stable, regular train of short pulses with the time separation T = 2d/c, as long as the pump power remains above threshold (which is now lower than at the beginning because the absorption is saturated). 

Passive mode-locking can also be realized in CW lasers. However, the smaller amplification restricts stable operation to a smaller range of values for the ratio of absorption to amplification, and the optimum conditions are more critical than in pulsed operation [11.37,38]. With passively mode-locked CW dye lasers pulse widths down to 0.5 ps have been achieved [11.39]. 

More detailed representations of active and passive mode-locking can be found in [11.16,40,41]. 

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The requited characteristics of dyes used as passive mode-locking agents and as active laser media differ in essential ways. For passive mode-locking dyes, short excited-state relaxation times ate needed dyes of this kind ate characterized by low fluorescence quantum efficiencies caused by the highly probable nonradiant processes. On the other hand, the polymethines to be appHed as active laser media ate supposed to have much higher quantum efficiencies, approximating a value of one (91). 

Also, using dyes as laser media or passive mode-locked compounds requires numerous special parameters, the most important of which ate the band position and bandwidth of absorption and fluorescence, the luminiscence quantum efficiency, the Stokes shift, the possibiHty of photoisomerization, chemical stabiHty, and photostabiHty. AppHcations of PMDs in other technical or scientific areas have additional special requirements. 

The experimental setup is shown in Figure 5.1. Six picosecond (ps)-long pulses at 532 nm and 80 MHz repetition rate were delivered by a frequency-doubled, passively mode-locked NdYV04 laser (Hi-Q Laser Production, Austria). The maximum available average power of the laser was reduced by an external variable attenuator to about a few hundred milliwatts. The OPO gain material is a flux-grown KT10P04 crystal,... 

Eigure 5.8 shows a schematic of the proof-of-principle CARS endoscope system. The laser source consisted of a passively mode-locked 10 W Nd Y VO4 laser (High-Q Laser GmbH) operating at 1064 mn, which delivered transform-limited 7 ps pnlses... 

The third order optical susceptibility was measured by degenerate four-wave mixing (DFWM). A single pulse at 1.064 pm with a full width at half maximum of 35 ps was selected from the output of a passively mode locked Nd/YAG laser and split into three... 

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. 

The apparatus employs a passively mode locked Nd/YAG laser oscillator, a Pockel cell pulse extractor, and Nd/YAG laser amplifier to produce laser pulses at 1064 nm. Non-linear crystals convert 30% of this light to 355 nm, which is used for excitation of the sample. The optical path length of the 355 nm light is varied by a computer-controlled time delay stage. 

In 1997, the old dye laser record with respect to the shortest pulses was improved by a Ti sapphire laser. Pulses with a duration of 5 fs could be generated [9]. Despite the many advantages of solid-state femtosecond laser systems compared to dye lasers (power, compactness, no toxic dyes and solvents etc.), one drawback should be noted. The passive mode-locking process is not self-starting in many cases. 

Picosecond Transient Absorption Measurements.— Several picosecond excitation and probe experiments have been recently described. A 0.5 ps pulse from a passively mode-locked dye laser was utilized to excite and subsequently monitor the singlet absorption in tetraphenylethylene. A 5 ps relaxation time, ascribed to twisting around the ethylene double bond, and a long-lived twisted intermediate were observed. The involvement of an intermediate exciplex in the photoinduced hydrogen-atom transfer reaction of pyrene with diphenylamine was demonstrated... 

Differential absorption spectra (DAS) under excitation of high-energy transitions are studied. The pump is done by 15-ps pulses from passively mode-locked Nd YA103 laser (/l=1.08 pm). White-light continuum generated from D2O by a part of 15-ps laser pulse is used as a probe. Fig. 4 demonstrates DAS registered for sample p8 (see Fig. 1). The dots present values of differential absorption obtained from intensity-dependent transmission measurements for this glass at 1.08 pm and 1.54 pm. 

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