Imaging of biological tissue, as opposed to analysis of solutions or matrices of various sorts, imposes particular difficulties, primarily because biological tissue is highly scattering at optical wavelengths. Other challenges arise due to unique length and time scales of interest and vulnerability of the tissue to damage in light fields of excessive intensity.
Thus, light signals arising in scattering biological tissue are not only highly attenuated due to propagation though the tissue of both any probe field and the light signal arising in the medium, but, additionally, those weak light signals are characterized by low spatial and temporal coherence, since mean free paths through the medium are extremely short.
Moreover, low light input is always preferable in biological environments so as to limit any possible damage or interference to the biological environment. Currently, detection of these low level light signals has primarily relied on high-sensitivity photo-detectors, e.g. photo multiplier tubes or avalanche photodiodes. However, the sensitivity of these state-of-the-art detectors is reaching their theoretical limit, and fundamentally new technologies are called for in order to further improve detection efficiency.
Various modalities of optical amplification are known for boosting weak light signals, and these may be taken to include such technologies as heterodyne detection, stimulated emission or stimulated Raman scattering amplification, and optical parametric amplification (OPA). And while OPA has been demonstrated to be advantageous in amplification of ultraweak light, due to amplification ratios as high as 108, broad gain bandwidths of up to hundreds of nanometers, and no threshold requirement for the light to be amplified, there are many challenges that suggest that OPA schemes may not be suitable for amplifying signals used for optical imaging of highly scattering biological tissue. First, the phase-matching criteria of OPA require high degree of coherence of light to be amplified, but light signals generated in scattering biological samples usually have low spatial and temporal coherence. In addition, to achieve the high pump intensity required in OPA, pulsed lasers with high peak power but low repetition rate have typically been used. Unfortunately, such conditions are not suitable for practical imaging of biological samples because the low pulse repetition rate results in a long image formation time. These critical challenges have precluded the application of OPA to optical imaging of scattering biological tissue.