Abstract
We describe a novel non-linear detection method for optical tomography that does not rely on detection of interference fringes and is free of optical background. The method exploits temporally coherent broadband illumination such as ultrashort pulses, and a non-linear two-photon detection process such as sum-frequency generation (SFG). At the detection stage, the reference beam and the sample beam are mixed in a thick non-linear crystal, and only the mixing term, which is free of optical background, is detected. Consequently, the noise limitations posed by the background in standard OCT (excess and shot noise), do not exist here. Due to the non-linearity, the signal to noise ratio scales more favorably with the optical power compared to standard OCT, yielding an inherent improvement for high speed tomographic scans. Careful design of phase matching in the crystal enables non-linear mixing which is both highly efficient and broadband, yielding both high sensitivity and high depth resolution.
Highlights
The need for imagery with very high depth resolution of scattering media, such as biological samples, has led to the development of non-linear optical microscopy and optical coherence tomography [1,2,3,4,5,6,7]
As opposed to conventional Optical coherence tomography (OCT), where the two beams are directly interfered on a photodetector, here the two beams are first mixed in a nonlinear medium, where sum-frequency generation (SFG) is induced, and only the result of this mixing is detected in a process very similar to cross correlation of ultrashort pulses
Coherent two-photon OCT is related to quantum OCT [23,24], a method that relies on the quantum mechanical properties of time-energy entangled photon pairs emitted via broadband down-conversion
Summary
The need for imagery with very high depth resolution of scattering media, such as biological samples, has led to the development of non-linear optical microscopy and optical coherence tomography [1,2,3,4,5,6,7]. Optical coherence tomography (OCT), on the other hand, utilizes only linear elastic scattering in the sample, so high intensities are not necessary [6,7]. While excess noise can be reduced (with some cost in dynamic range) by optimization of the reference beam intensity and by balanced detection [8], quantum shot noise of the reference beam sets an inherent limit on the signal to noise ratio (SNR) in OCT. Integration times in OCT must remain short compared to the typical uncontrolled movement time of the (sometimes living) imaged sample
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