Mode-Locking of Lasers

Without frequency-selective elements inside the laser resonator the laser generally oscillates simultaneously on many resonator modes within the spectral gain profile of the active medium (Sect. 5.3). In this multi-mode-operation no definite phase relations exist between the different oscillating modes and the laser output equals the sum of the intensities of all oscillating modes which are more or less randomly fluctuating in time (Sect. 6.2). 

If the modulator is placed inside the laser resonator with the mirror separation d and the mode frequencies = vo m cjld (m = 0,1,2.), the sidebands coincide with resonator mode frequencies if the modulation frequency / equals the mode separation Av = cjld. The sidebands can then reach the oscillation threshold and participate in the laser oscillation. Since they pass the intracavity modulator they are also modulated and new sidebands y = vq i 2/ are generated. This continues until all modes inside the gain profile participate in the laser oscillation. There is, however, an important difference from normal multimode operation the modes do not oscillate independently, but are phase-coupled by the modulator. At a certain time the amplitudes of all modes have their maximum at the location of the modulator and this situation is repeated after each cavity round-trip time T = 2d c (Fig. 6.8c). We will discuss this in more detail The modulator has the time-dependent transmission 

For equal mode amplitudes A = Ao, (6.8) gives the total time-dependent intensity 

If the amplitude Ao is time independent (cw laser), this represents a sequence of equidistant pulses with the separation 

The peak power of the pulses, which can be derived from the intensity maxima in (6.9) at times t = 2nq 2 =q 2djc) q = Q,, 2.), is proportional to A. The pulse energy is therefore proportional to A AT a A. In between the main pulses (A - 2) small maxima appear, which decrease in intensity as A increases. 

For 2 = 7ic/d, the sideband cokA corresponds to the next resonator mode and generates the amplitude 

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The dithiobenzil complex [Ni 4-Me2NC6H4C(S)C(S)C6H4 2] has been used in laser technology.79 Mode locking of lasers with this complex makes possible the production of light pulses of very short duration and high energy.80... 

P.W. Smith, M.A. Duguay, E.P. Ippen, Mode-locking of lasers, in Progr. Quantum Electron, vol. 3 (Pergamon, Oxford, 1974)... 

Waves of different frequencies, or colours, can interfere to form a pulse if they have a fixed relative phase. This effect is known as chromatic coherence and is, for example, encountered in the mode locking of lasers. It can be measured by optical autocorrelation. 

RW. Smith Mode-locking of lasers. Proc. IEEE 58, 1342 (1970)... 

Basics on the subject of mode-locking of lasers are found in, for example, the well-known articles of Smith [179], Harris and McDuff [180], and Kuizenga and Siegman [181], as well as in the overview paper of New [182]. 

E. Schreiber, Femtosecond Real-Time Spectroscopy of Small Molecules and Clusters Habilitation thesis, Freie Universitat Berlin, Berlin-Dahlem, 1996. P.W. Smith, Mode-Locking of Lasers , Proc. IEEE 58, 1342 (1970). 

Asaki M T, Huang C-P, Garvey D, Zhou J, Kapteyn H C and Murnane M M 1993 Generation of 11 fs pulses from a self-mode-locked Tfsapphire laser Opt. Lett. 977 977-9... 

Another variation is the mode-locked dye laser, often referred to as an ultrafast laser. Such lasers offer pulses having durations as short as a few hundred femtoseconds (10 s). These have been used to study the dynamics of chemical reactions with very high temporal resolution (see Kinetic LffiASURELffiNTS). 

Gemini North Observatory/CTI Mode-locked SFG Laser. CTT is developing the first commercial solid-state Na LGS system. It will be installed on the center section of the 8-m Gemini North telescope, with the output beam relayed to a projector behind the secondary mirror. The projected beam is required to be 10-20 W power, with M2 < 1.5. The architecture is based on sum-frequency mixing two mode-locked solid-state Nd YAG lasers. The mode-locked format provides significantly higher peak intensity than CW, enabling more efficient SFG conversion. The laser is also free of the thermal and intensity transients that are inherent in the macro pulse format. The chosen... 

To carry out a spectroscopy, that is the structural and dynamical determination, of elementary processes in real time at a molecular level necessitates the application of laser pulses with durations of tens, or at most hundreds, of femtoseconds to resolve in time the molecular motions. Sub-100 fs laser pulses were realised for the first time from a colliding-pulse mode-locked dye laser in the early 1980s at AT T Bell Laboratories by Shank and coworkers by 1987 these researchers had succeeded in producing record-breaking pulses as short as 6fs by optical pulse compression of the output of mode-locked dye laser. In the decade since 1987 there has only been a slight improvement in the minimum possible pulse width, but there have been truly major developments in the ease of generating and characterising ultrashort laser pulses. 

In our tip-enhanced near-field CARS microscopy, two mode-locked pulsed lasers (pulse duration 5ps, spectral width 4cm ) were used for excitation of CARS polarization [21]. The sample was a DNA network nanostructure of poly(dA-dT)-poly(dA-dT) [24]. The frequency difference of the two excitation lasers (cOi — CO2) was set at 1337 cm, corresponding to the ring stretching mode of diazole. After the on-resonant imaging, CO2 was changed such that the frequency difference corresponded to none of the Raman-active vibration of the sample ( off-resonant ). The CARS images at the on- and off- resonant frequencies are illustrated in Figure 2.8a and b, respectively. 

Figure 4. A schematic of the colliding pulse mode-locked femtosecond laser system. Taken with permission from ref. 65.

Historically, this has been the most constrained parameter, particularly for confocal laser scanning microscopes that require spatially coherent sources and so have been typically limited to a few discrete excitation wavelengths, traditionally obtained from gas lasers. Convenient tunable continuous wave (c.w.) excitation for wide-held microscopy was widely available from filtered lamp sources but, for time domain FLIM, the only ultrafast light sources covering the visible spectrum were c.w. mode-locked dye lasers before the advent of ultrafast Ti Sapphire lasers. 

For EPy-doped PMMA film, a 308 nm excimer laser (Lumonics TE 430T-2, 6ns) was used as as exposure source. We used a tine-correlated single photon counting systen (18) for measuring fluorescence spectra and rise as well as decay curves of a snail ablated area. The excitation was a frequency-doubled laser pulse (295 nm, lOps) generated from a synchronously punped cavity-dumped dye laser (Spectra Physics 375B) and a CW mode-locked YAG laser (Spectra Physics 3000). Decay curves under a fluorescence microscope were measured by the same systen as used before (19). 

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. 

The light source can be a xenon lamp associated with a monochromator. The optical configuration should be carefully optimized because the electro-optic modulator (usually a Pockel s cell) must work with a parallel light beam. The advantages are the low cost of the system and the wide availability of excitation wavelengths. In terms of light intensity and modulation, it is preferable to use a cw laser, which costs less than mode-locked pulsed lasers. 

The autput of a mode-locked ruby laser 729) producing a train of pulses of 5 psec duration with a maximum peak power of 5 GW was focused into a cell pressurized with the sample gas. Pulse-energy conversion efficiencies into the Raman lines of up to 70 % have been obtained. The induced rotational lines are broadened this could be due to a strong optical Stark effect 730)... 

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,... 

With single-photon exposure, excitations may decay either through a variety of processes including chain scission and fluorescence (47). We would therefore expect to observe fluorescence from two-photon excitation as well. To observe the fluorescence, we used a Spectra Physics mode locked dye laser system, operating with Rhodamine 560 dye. This was focused onto the polymer film, and the emitted light collected into a spectrometer with a Princeton Instruments Optical Multichannel Analyzer (OMA) attachment. 

The method proposed allows direct absolute measurement of local concentration at the instant of the laser pulse in a low pressure flame. We believe that this method could be applied to higher pressure flames by the use of ultrashort duration laser pulses with the new mode locked dye laser technique. But until the detector technology allows such short time resolutions we think that collisional lifetimes studies must be pursued to obtain more precise evaluation of the fluorescence efficiency, and to have a better understanding of the redistribution phenomena involved in optical pumping. For this purpose we are now studying the decay of resolved fluorescence lines. 

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