Optical delay line

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. 

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

Each pulse was split with a beam splitter (9) into pump and probe parts with energies of 2 nJ and 0.04 nJ, respectively, that were directed onto the sample at angles of incidence of 7° and 27°, respectively. A variable optical delay line (10) in the pump path allowed the time delay between the two pulses to be varied with a minimum step size of 1.67 fs. The position of a hollow retro-reflecting prism (11) in the delay line was varied through a few wavelengths at high frequency in order to remove any coherence oscillations around the zero delay position. The maximum time delay was 2 ns. 

Figure 3-24 Simplified diagram of the experimental apparatus for the optically sensitized triplet generation/TR3 studies. Requisite time delays were obtained either by optical delay or by the two-laser experimental configuration. The optical delay line is not to scale its actual length was approximately 120 ft. (Reproduced with permission from Ref. 82. Copyright 1981 American Chemical Society.)...

Figure 13 Schematic of the setup of the pump-probe experiment with polarization resolution for the probing of the induced change in sample transmission. X/2 half-wave plate P1-P3 polarizers L1-L4 lenses D1-D5 detectors Ch chopper VD optical delay line. The sample is permanently moved in a plane perpendicular to the beams in order to avoid accumulative thermal effects.

The basis of the experimental femtosecond CARS apparatus developed by Okamoto and Yoshihara (1990) which is reproduced in Fig. 3.6-10 is essentially the same as that of Leonhardt et al. (1987) and Zinth et al. (1988) with the addition of the possibility to change the polarization of the laser radiation. The main parts of the system are two dye lasers with short pulses and high repetition rates, pumped by a cw mode-locked Nd YAG laser (1064 nm, repetition rate 81 MHz). The beam of the first dye-laser which produces light pulses with 75-100 fsec duration is divided into two parts of equal intensities and used as the pump and the probe beam. After fixed (for the pump beam) and variable (for the probe beam) optical delay lines, the radiation is focused onto the sample together with the Stokes radiation produced by the second laser (DL2), which is a standard synchronously pumped dye laser. The anti-Stokes signal generated in the sample is separated from the three input laser beams by an aperture, an interference filter, and a monochromator, and detected by a photomultiplier. For further details we refer to Okamoto and Yoshihara (1990). 

The various probe beams can be coupled into the same singlewavelength, dual-channel pulse-probe transient optical absorption set-up. A one-meter-long optical delay line is used to control the variable time delay between the electron and the probe pulses. Approximately half of the probe beam is deflected onto a reference photodiode while the other half of the beam is slightly focused into the sample, which is placed in front of the output window of the accelerator. Subsequently, the probe beam is then transported to the sample photodiode. (Alternatively, in some laboratories the probe and reference beams are transported into the detection room by long, low-OH silica optical fibers in order to reduce electronic noise pickup on the detector signal cables.)... 

The theory of sum-frequency conversion of incoherent light can be found in the literature [133-134]. The intensity of the sum-frequency light at a given delay time is proportional to the correlation function of the fluorescence intensity with the intensity of the oj beam. A time profile of the fluorescence intensity is obtained by change of the arrival time of one of the pulses with the aid of an optical delay line. 

The infrared and green pulses are subsequently separated by a dichroic beam splitter. The infrared pulses travel directly to the ultrafast optical kerr effect shutter (24) while the green pulses are diverted through an optical delay line which consists of one movable and two fixed prisms. The shutter consists of a cell of carbon disulfide placed between two crossed polarizers,... 

Lasers with ultrashort pulse lengths down to a few femtoseconds (1 fs = 10 15 s) are now available commercially. However, photomultipliers and the associated electronic digitizers are not sufficiently fast to follow waveforms much below 1 ns. Therefore, devices with pico- and femtosecond time resolution use optical delay lines to define the time delay between the excitation pulse and the probe pulse (Figure 3.16). By focusing part of the... 

The temporal resolution of the two different crystal setups is determined by the pulse length of the pump pulse the remaining fundamental of the regenerative amplifier. The pulse length of the 1064-nm fundamental is about 100 ps for the harmonics the pulse lengths are about 70 and 60 ps for 532 and 355/266 nm, respectively. Thus, our temporal resolution is about 20 ps by applying standard deconvolution methods. The optical delay line (computer controlled) of the pump beam determines the maximum time scale of the experiment, about 8 ns, but it can be doubled to 16 ns by implementing a double-pass setup. 

One of the most important yet simple ideas that ignited excitement about fem-tochemistry is wavepacket interferometry (Salour and Cohen-Tannoudji, 1977 Scherer, et al., 1990, 1991, 1992 Jonas and Fleming, 1995 Weinacht, et al., 1999), an optical form of Ramsey-fringe spectroscopy (Ramsey, 1990). A molecular system is subjected to two identical optical pulses created by splitting one pulse at a beam splitter. The two pulses are called the pump and the probe . The time delay between pump and probe pulses is scanned systematically using an optical delay line. The optical arrangement is very similar to that of a Fourier Transform Spectrometer (Heller, 1990). The difference in the paths traveled by the pump and probe pulses, Ad, before the two pulses are recombined at a second beam splitter corresponds to a time delay, At = Ad/c, where c is the speed of light. 

The measurement of the time-dependent depolarization of the fluorescence from molecules rotating on a time-scale comparable to the fluorescence decay time, enables information to be derived concerning the molecular reorientation motion. A review of these techniques has been published. A method involving an optical delay line has been used to record time-resolved fluorescence depolarization methods using only 1 photodetector, and thus some of the possible instrumental distortions are removed. ... 

---