Long-wavelength optical coherence tomography at 1.7 microm for enhanced imaging depth

Multiple scattering in a sample presents a significant limitation to achieve meaningful structural information at deeper penetration depths in optical coherence tomography (OCT). Previous studies suggest that the spectral region around 1.7 microm may exhibit reduced scattering coefficients in biological tissues compared to the widely used wavelengths around 1.3 mum. To investigate this long-wavelength region, we developed a wavelength-swept laser at 1.7 microm wavelength and conducted OCT or optical frequency domain imaging (OFDI) for the first time in this spectral range. The constructed laser is capable of providing a wide tuning range from 1.59 to 1.75 microm over 160 nm. When the laser was operated with a reduced tuning range over 95 nm at a repetition rate of 10.9 kHz and an average output power of 12.3 mW, the OFDI imaging system exhibited a sensitivity of about 100 dB and axial and lateral resolution of 24 mum and 14 mum, respectively. We imaged several phantom and biological samples using 1.3 mum and 1.7 microm OFDI systems and found that the depth-dependent signal decay rate is substantially lower at 1.7 microm wavelength in most, if not all samples. Our results suggest that this imaging window may offer an advantage over shorter wavelengths by increasing the penetration depths as well as enhancing image contrast at deeper penetration depths where otherwise multiple scattered photons dominate over ballistic photons.