In this article the ability of ultrahigh resolution ophthalmic optical coherence tomography (OCT) to image small choroidal blood vessels below the highly reflective and absorbing retinal pigment epithelium is demonstrated for the first time. A new light source (lambdac= 1050 nm, Deltalambda = 165 nm, Pout= 10 mW), based on a photonic crystal fiber pumped by a compact, self-starting Ti:Al2O3 laser has therefore been developed. Ex-vivo ultrahigh resolution OCT images of freshly excised pig retinas acquired with this light source demonstrate enhanced penetration into the choroid and better visualization of choroidal vessels as compared to tomograms acquired with a state-of-the art Ti:Al2O3 laser (Femtolasers Compact Pro, lc= 780 nm, Deltalambda= 160 nm, Pout= 400 mW), normally used in clinical studies for in vivo ultrahigh resolution ophthalmic OCT imaging. These results were also compared with retinal tomograms acquired with a novel, spectrally broadened fiber laser (MenloSystems, lambdac= 1350 nm, Deltalambda= 470 nm, Pout = 4 mW) permitting even greater penetration in the choroid. Due to high water absorption at longer wavelengths retinal OCT imaging at ~1300 nm may find applications in animal ophthalmic studies. Detection and follow-up of choroidal neovascularization improves early diagnosis of many retinal pathologies, e.g. age-related macular degeneration or diabetic retinopathy and can aid development of novel therapy approaches.

We have scattered photons from an interfering path of a multiple beam Ramsey interference experiment realized with a cesium atomic beam. It is demonstrated that in multiple beam interference the decoherence from photon scattering cannot only lead to a decrease but, under certain conditions, also to an increase of the Michelson fringe contrast. In all cases, the atomic quantum state loses information with photon scattering, as "which-path" information is carried away by the photon field. We outline an approach to quantify this which-path information from observed fringe signals, which allows for an appropriate measure of decoherence in multiple path interference.

Most time-resolved optical experiments rely either on external mechanical delay lines or on two synchronized femtosecond lasers to achieve a defined temporal delay between two optical pulses. Here, we present a new method which does not require any external delay lines and uses only a single femtosecond laser. It is based on the cross-correlation of an optical pulse with a subsequent pulse from the same laser. Temporal delay between these two pulses is achieved by varying the repetition rate of the laser. We validate the new scheme by a comparison with a cross-correlation measurement carried out with a conventional mechanical delay line.

A novel, compact, user friendly fiber laser with a broad emission bandwidth (MenloSystems, lambdac = 1375 nm, deltalambda = 470 nm, Pout = 4 mW) was used to achieve unprecedented sub-2-microm axial resolution optical coherence tomography (OCT) in nontransparent biological tissue in the 1300-nm wavelength region. Fresh human skin and arterial biopsies were imaged ex vivo with approximately 1.4-microm axial and approximately 3-microm lateral resolution and 95-dB sensitivity, demonstrating the great potential for clinical OCT applications of this stable, low-cost, and turn-on-key fiber laser.

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