Spectral phase modulation

Here we introduced the spectral phase-modulation function (p co), which is used to parameterize the laser pulse shape. Since M m) = 1 in this case, the modulus... 

In the following, we describe two prominent types of spectral phase modulation, each of which plays an important role in coherent control. Both types, namely sinusoidal (Section 6.2.1) and quadratic (Section 6.2.2) spectral phase modulation, are relevant for the experiments and simulations presented in this contribution. We provide analytic expressions for the modulated laser fields in the time domain and briefly discuss the main characteristics of both classes of pulse shapes. 

Periodic spectral phase-modulation functions have been used in numerous experiments and theoretical studies on coherent control of atoms [75-79] and molecules [24, 25, 42, 68, 73, 80-85]. Applying a sinusoidal phase-modulation function of the form... 

Figure 6.3 Shaped femtosecond laser pulses from sinusoidal spectral phase modulation of an 800 nm, 20 fs FWHM input pulse. The left column shows the modulated pulses in the frequency domain, decomposed into spectral amplitude (gray line and background) and modulation...

Quadratic phase modulation using the spectral phase modulation function... 

Fig. 1. Spectral phase modulation in the femtosecond pulses is characterized (a) and compensated (b) using the MIIPS method. SHG-FROG traces for the pulses in panels (a) and (b) are shown in panels (c) and (d).

Its basic principle [779] is illustrated in Fig. 6.85. The signal beam 1 interferes in a nonlinear medium with the signal beam 2, specially prepared by a conditional filter which consists of a spectral and a temporal phase modulator. The spectral phase modulator can be realized by an unbalanced Mach-Zehnder interferometer (see Vol. 1, Sect. 4.2.4) with a dispersive element placed in one of the arms. The spectral phase modulator breaks the symmetry of the measured signal and therefore avoids ambiguities caused by symmetric VAMPIRE spectrograms. 

Theoretical level populations. Sinee there are population variations on time seale shorter than some level lifetimes, a complete description of the excitation has been modeled solving optical Bloch equations Beacon model, Bellenger, 2002) at CEA. The model has been compared with a laboratory experiment set up at CEA/Saclay (Eig. 21). The reasonable discrepancy when both beams at 589 and 569 nm are phase modulated is very likely to spectral jitter, which is not modeled velocity classes of Na atoms excited at the intermediate level cannot be excited to the uppermost level because the spectral profile of the 569 nm beam does not match the peaks of that of the 589 nm beam. 

Photomultipliers are generally used to convert the spectral radiation to an electrical current and often phase-sensitive lock-in amplifiers are used to amplify the resulting current. AES and AFS require similar read-out systems because both methods are measuring small signals. The difficulty associated with both these methods is the separation of the signal for the atomic transition of interest from the background radiation emitted by excited molecular species produced in the atom reservoir. AFS phase locks the amplifier detection circuit to the modulation frequency of the spectral source. Modulation of the source is also used in AAS. 

The helium-neon (HeNe) laser immediately comes to mind, having a very useful spectral line at 633 nm for steady-state red/near-IR fluorescence studies. Kessler and Wolfbeis have demonstrated the fluorescence assay of the protein human serum albumin using the probe albumin blue excited with a red HeNe laser.(71) Another useful wavelength available from the green HeNe laser is at 543.5 nm and this has been used with phase-modulation fluorometry by Lakowicz etal. to study probes such as carboxy seminaphtorhodafluor-6 (SNARF-6) as a means of measuring pH.(72)... 

The multipulse excitation is generated by a modulation of the spectral phases,... 

Fig. 2. Optimisation of internal conversion with multipulse excitation, a) Experimental setup Coherent multipulse sequences are generated by periodic modulation of the spectral phases. The multipulse spacing b is optimised for maximal ratio IC/EET by an evolutionary algorithm, b) Histogram of optimal modulation 2re/viightfe (i.e. multipulse separation) by successive optimisations.

The pulse shaper is a device made of a pair of gratings and lenses, arranged in a zero dispersion compressor, and a liquid crystal modulator placed in the Fourier plane. Liquid crystal arrays allow one to influence independently spectral phase and amplitude with discretization of 128 pixels each. 

When an intense pulse of monochromatic laser light is focussed on a transparent liquid or solid, there is an emission of white light over a wide continuous spectral range. This process is known as self-phase modulation . We will not consider its physics. For our purpose it is important to note its photochemical implications. On the one hand, this pulse of white light can be used to provide a probe light in ps and fs flash photolysis (sections 8.1 and 8.2). On the other hand, it can be a source of stray light in some luminescence measurements. This comes as a surprise to many users of lasers for luminescence kinetics measurements, but it is an unavoidable problem. 

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