Optical path difference velocity

To overcome the problems discussed previously it is necessary to use an FT-IR instrument with very good S/N performance and good stability at lower optical path difference velocities. Perkin Elmer 1800 series or 1700 series (particularly models 1720/1760) FT-IR spectrometers coupled to an MTEC model 100 PA photoacoustic detector adequately meet these requirements. 

In step scan mode the optical path difference is changed incrementally, the mirror does not scan continuously. It steps to a position, collects data, and moves to the next position. Data are collected at each step with the path difference held constant. At each step the mirror is jittered at a constant frequency, a procedure that modulates the incident beam. The ability to select a constant modulation frequency, independent of the mirror velocity and wavenumber, results in a constant probed depth over the entire spectral range. This is advantageous over the varying sampling depth observed when using rapid scanning,... 

X is the optical path difference, intensity measured with the detector, and B is the radiation density of the source. The frequency of the signal at the detector for a radiation with a frequency v is a function of the velocity v by which the optical path difference is changed ... 

There are many other types of two-beam interferometers besides the one originally described by Michelson (see Chapter 5). Many of these interferometers do not vary the path difference between two beams by a single mirror moving at constant velocity. Except for stationary interferometers used for Fourier transform spectroscopy (Section 5.6), an optical element or combination of optical elements is moved so that the optical path difference is changed at a certain rate, known as the optical velocity or OPD velocity, V. For the Michelson interferometer, V = 2V. In general, the Fourier frequency for radiation of wavenumber v is given by... 

With optical detection, the overall time resolution is limited by the different velocities of fast electrons and photons in condensed media this results in loss of synchronization as the two beams pass through the sample cell. This desynchronization is approximately 10 ps cm in water [145], so the optical path length has to be reduced proportionally to achieve the improved time resolution provided by subpicosecond pulses. There is thus a compromise between having short time resolution (short optical path) and high absorbance signals (long optical path). 

Most of the methods used to measure picosecond phenomena are based on optical delay lines where the picosecond laser pulse is divided by a beam splitter and the two replica pulses travel different path lengths before they are recombined. The measurement of a time interval At is thus transferred to that of a path difference ax = cAt where c is the velocity of light. 

---