As reported previously, OCT sources emit short infrared pulses that may be seen due to two-photon vision. In this work, we quantified visibility of OCT sources, which may be beneficial for eye imaging system designers.
Pulsed near-infrared (NIR) light sources can be successfully applied for both imaging and functional testing of the human eye, as published recently. These two groups of applications have different requirements. For imaging applications, the most preferable is invisible scanning beam while efficiently visible stimulating beam is preferable for functional testing applications. The functional testing of human eye using NIR laser beams is possible due to two-photon vision (2PV) phenomenon. 2PV enables perception of pulsed near-infrared laser light as color corresponding to approximately half of the laser wavelength. This study aims to characterize two-photon vision thresholds for various pulse lengths from a solidstate sub-picosecond laser (λc = 1043.3 nm, Frep = 62.65 MHz), either of 253 fs duration or elongated by Martinez- type stretcher to 2 ps, and fiber-optic picosecond laser (λc = 1028.4 nm, Frep = 19.19 MHz, τp = 12.2 ps).
Development of new microperimetric tools dedicated for imaging of early functional changes in the retina may help in the monitoring of various ocular diseases progression e.g. Age-Related Macular Degeneration. Recently described two-photon vision may be applied to microperimetric devices. Many subjects with well-known disease history could be investigated with newly developed instrumentation that tests ability of human eye to perceive near infrared radiation. The main limitation of this new method is a very high cost of the femtosecond laser. Facing this problem, we try to replace the femtosecond laser with lower cost fiber-optic picosecond light source. To compare these two lasers, we constructed dedicated measurement system. We performed measurements of two-photon vision threshold on healthy subjects for two different light sources - sub picosecond Kerr mode-locking solid-state laser and fiber-based picosecond laser. Experiments were conducted for an open circle flickering stimulus with 0.5 deg. diameter, for retinal locations varying from 0 deg. to 5.8 deg., using 4-2-1 threshold strategy that is well-known from classical microperimetry. Values of obtained thresholds are only 5 times higher for the fiber laser than that obtained by using the femtosecond laser, while it was expected to be about over 16 times higher. This fact requires further investigations. Nevertheless, the idea of replacement of the latter laser by relatively cheap fiber-optic one in ophthalmic devices for two-photon vision studies seems to be potentially promising.
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