Spectral domain second harmonic optical coherence tomography

Optical coherence tomography (OCT) provides micrometer scale structural imaging by coherent detection of light backscattered from a sample. The significance of OCT would be greatly enhanced by the capability to measure molecular specific signals. Observation of fluorescent markers is unfortunately not possible using OCT because fluorescence is not a coherent process. Instead, the methods being researched to extend OCT to include molecular contrast are, amongst others: transient absorption (pump-probe imaging), Coherent Anti-Stokes Raman Scattering (CARS), and second harmonic generation. With any of these techniques, the quality of the images obtained is limited by the conversion efficiency of the nonlinear process, which is inevitably much less than 1 and thus has resulted in low SNR and long acquisition times in previously reported work. Recent publications have demonstrated a sensitivity advantage of 20-30dB for spectral domain (SD) techniques in OCT over conventional time domain acquisition. The increased sensitivity of SD OCT systems stands to benefit the small signal powers observed in molecular contrast OCT. We have constructed a prototype spectral domain second harmonic OCT system utilizing a 130 femtosecond Nd:Glass laser and a pair of custom spectrometers for simultaneous acquisition of the fundamental and second harmonic signals. We report a 10dB increase in sensitivity while imaging 100 times faster than in initial reports of second harmonic OCT using time domain systems.