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1.The Beginning (1990-1994)

The first DOSI instrument was constructed by Bruce Tromberg at the Beckman Laser Institute (BLI).  The instrument was a multi-wavelength, multi-modulation frequency FDPM system confined to an optical table.  In reality, it was a phase and modulation fluorescence instrument.  The general electronic hardware for the detection of frequency domain signals is virtually the same (an idea championed by the Gratton Laboratory).  The original paper was published in 1993.
Instrument schematic of the original BLI DOSI instrument by Bruce Tromberg, excerpted from the original paper.

Two laser sources (L) were available: an Argon-Ion laser, using primarily the laser line at 514nm, and a dye laser with DCM dye which lases from about 600-677 nm.  These lasers are continuous wave (i.e., no modulation) so the beam was modulated by sending the beam through a Pockel's cell (PC).  Modulation frequencies up to about 250 MHz were available in steps of a few MHz by using a series of harmonic comb generators (HCG) driven by a frequency generator (FG).  A small portion of the modulated beam was stripped off as a reference using a beam splitter (BS).  The laser light was focused into an optical fiber (F1) and delivered to a sample.  Another optical fiber (F2) collected the diffuse reflectance from the sample. 

The modulated light was sent to a Photomultiplier tube (PMT), with a modification that allowed for the PMT to be modulated a a different frequency than the laser source.  This idea behind modulating the source and detector at different frequencies is sometimes called cross-correlation, heterodyne or amplitude modulation. The basic principle is to beat towether two waves at different frequencies (the frequency difference is called the cross-correlation freqeuncy) to pooduce a beating effect.  The end result, after some straightforward mathematical passages reveals that the high-frequency signal is downconverted to a low-frequency signal yet maintains the amplitude and phase information of the original signal. This is the basic principle behind an AM radio.

FDPM instrument deigned by Bruce Tromberg in 1993 that was fashioned from a commercial phase and modulation fluorescence instrument.

This system was one of the first multi-frequency photon migration instruments described in the literature.  The authors demonstrated that multi-frequency information content could recover the absorption and scattering properties of tissue-like phantoms.  While single modulation frequency information is sufficient for measuring tissue optical properties, the multi-frequency approach better approximates the information-rich time domain pulse (which contains all modulation frequencies).