Adjustable coherence length sources for low coherence interferometry

We investigate the effect of light source coherence length on longitudinal and en-face OCT images of the retina. The sources used in this comparative study are a superluminescent diode (SLD), a superluminescent diode fitted with an interference filter at its output and a tunable coherence length three-electrode laser device (3EL). We show that the use of sources of shortest coherence length is ideal for longitudinal OCT imaging. However, there are reasons for using adjustable coherence length sources for en-face OCT imaging. The effect of adjustable coherence length (and implicitly spectrum FWHM) on the achievable signal to noise ratio in the OCT is also presented. An increase in the coherence length enhances the excess photon noise but, at the same time, increases the signal collected from scattering tissue due to a larger thickness of the coherence gated backscattering layer in the target tissue. This suggests that the signal to noise ratio should not change with the light source coherence length. Nevertheless, the effect of light source coherence length change on the signal to noise ratio is more complex due to other noise sources in the system.