Figure 1
From: Dispersion-cancelled biological imaging with quantum-inspired interferometry
An optical-coherence-tomography system based on chirped-pulse interferometry.
(a) A simplified schematic of CPI. A pair of classical beams in the same spatial mode and with anticorrelated frequencies impinges on a beamsplitter and the two resulting paths overlap in a β-barium borate (BBO) crystal for sum-frequency generation (SFG) after one path experiences a variable delay, τ and the other passes through a dispersive material with a frequency-dependent wave-vector k(ω). The frequency offset, Δ, is swept over the bandwidth of the input pulses. The CPI signal then is the intensity of the SFG light near 2ω0 as a function of τ. This signal is inherently robust against unbalanced dispersion. (b) The experimental implementation. Broadband pulses from a titanium: sapphire laser pass through a 4-F pulse-shaper20,21, the light is then split into a beam that reflects from the sample in the focus of a lens and a beam that travels a variable-length delay. The delay and the x- and y-positions of the sample are motorized. A stack of BK7 glass in the reference arm introduces dispersion equal to that of the static optical elements in the sample arm, including the BK7 window and water layer in the sample holder, but excluding the samples themselves. The pulse shaper compensates for this static, balanced dispersion throughout the experiment such that the laser pulse is transform limited at the nonlinear crystal. The pulse shaper can add an additional phase shift, ϕ(ω), to produce the BARC pulse shape at the crystal. Inserting an extra 3-mm-thick BK7 window in the sample arm introduces a controlled amount of unbalanced dispersion. Light from the two interferometer arms is focused onto a nonlinear crystal and undergoes non-collinear SFG. The SFG light passes through a spatial filter and a monochromator and the signal is measured as a function of time delay using a photomultiplier. Illustration of the cross-sections of the two samples, (c) microscope coverglass slides and (d) a piece of onion. Each sample was held in a lens tube and placed behind a layer of distilled water and a 1-mm-thick BK7 window; the 2.7 mm water layer prevented drying of the onion sample.
