Optical delay

The spectral resolution of the Fourier transfonned data is given by the wavelength corresponding to the maximum optical delay. We have and therefore AD = 1/5,. Therefore, if 0.1 cm spectral... 

The practical difficulties are considerable These include the recording of realtime events and minimization of the duration of the laser pulse. Special mode-locked lasers are used to generate the abrupt pulses needed, and optical delays can use the velocity of light to provide the timing. 

The optical delay results form the augmentation of the length of a dispersive material and the deformation of the optical waveguide. Con-... 

Figure 13. Left Optical delay as a function of the stretching parameter. Right Transmission close to too % upper curve and contrasts as function of the generated delay. Low contrast corresponds to a fringes packet spreading.

Figure 14. One of the three contrasts (left) and phase closure (right)evolution as functions of the optical delay to be generated between the telescopes of a three aperture stellar interferome-ter(OPD between T1 - T2 and T2 - T3 reported in the horizontal plane).

The characterization of the laser pulse widths can be done with commercial autocorrelators or by a variety of other methods that can be found in the ultrafast laser literature. " For example, we have found it convenient to find time zero delay between the pump and probe laser beams in picosecond TR experiments by using fluorescence depletion of trans-stilbene. In this method, the time zero was ascertained by varying the optical delay between the pump and probe beams to a position where the depletion of the stilbene fluorescence was halfway to the maximum fluorescence depletion by the probe laser. The accuracy of the time zero measurement was estimated to be +0.5ps for 1.5ps laser pulses. A typical cross correlation time between the pump and probe pulses can also be measured by the fluorescence depletion method. 

Fig. 3.9. Principle of a wide field FLIM system with simultaneous detection of two time gates. The fluorescence image is split into two images and one of the images is optically delayed with respect to the other. Both images are detected simultaneously with the same time-gated detector.

With the photographic flash lamp the light pulse has a duration of several microseconds at best. The Q-switched pulsed laser provides pulses some thousand times faster, and the kinetic detection technique remains similar since photomultiplier tubes and oscilloscopes operate adequately on this time-scale. The situation is different with the spectrographic technique electronic delay units must be replaced by optical delay lines, a technique used mostly in picosecond spectroscopy. This is discussed in Chapter 8. 

We have seen that the limitations of the time characteristics of electronic devices requires the use of optical delays between the pump and probe pulses in ps flash photolysis. There are also indirect ways of using optical properties to measure the kinetics of laser pulses and of fluorescence, known as autocorrelation and up-conversion . These rely on the non-linear properties of certain materials or chemical systems, i.e. they are based on fast biphotonic processes. 

Up-conversion. This is similar to autocorrelation, but the laser pulse is mixed with the luminescence light of a sample in an SHG crystal to generate light of frequency vL + vE. The time between the laser pulse (vL) and the luminescence emission (vE) is varied by means of an optical delay path, and the light of added frequency is detected by a usual PM tube via optical filters. 

Grimes. G. Microwave Fiber-Optic Delay Lines Coming of Age in 1992, Microwave J. 61 (August 1992). 

Fig. 5.6. A block diagram of an optical coherence tomography/Raman spectroscopy system C, circulator RSOD, rapid scanning optical delay BP, 785 bandpass BSO, beam shaping optics DM1, dichroic mirror at 990 nm DM2, dichroic mirror at 800-950 nm LP, long pass at 808 nm GP, galvanometer pair BD, balanced detector BPF, electronic band-pass filter AI-AO DAQ, analog input-output data acquisition (reprinted with permission from [34]. Copyright 2008 Optical Society of America)...

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