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Nonlinear optical contrast enhancement for optical coherence tomography

Claudio Vinegoni, Jeremy S. Bredfeldt, Daniel L. Marks, and Stephen A. Boppart

Open Access Open Access

Abstract

We present a new interferometric technique for measuring Coherent Anti-Stokes Raman Scattering (CARS) and Second Harmonic Generation (SHG) signals. Heterodyne detection is employed to increase the sensitivity in both CARS and SHG signal detection, which can also be extended to different coherent processes. The exploitation of the mentioned optical nonlinearities for molecular contrast enhancement in Optical Coherence Tomography (OCT) is presented.

©2004 Optical Society of America

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Figures (7)
Fig. 1.
Fig. 1. Setup used to generate the Stokes and the Pump excitation fields. DPSS, diode-pumped solid-state laser; Regen, regenerative amplifier; OPA, optical parametric amplifier.

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Fig. 2.
Fig. 2. Setup of the interferometric CARS measurement system. DM, dichroic mirror; BS, beamsplitter; M, mirror; HPF, high-pass-filter; PH, pin-hole; PMT photomultiplier tube; PC, personal computer.

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Fig. 3.
Fig. 3. Interferogram of the pump beam detected at the beamsplitter BS2 of the setup shown in Fig. 2. The envelope of the interferogram was fitted using the modulus of the degree of the coherence function. In the inset is shown a detail of the interference pattern and its fit by the real part of the degree of coherence function (open circles: experimental data; solid line: fit). L c is the coherence length of the pulse. λ PUMP is the wavelength of the Pump signal.

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Fig. 4.
Fig. 4. Log-log plots of the intensity of the CARS signal as a function of (a) the intensity of the Pump field and (b) the intensity of the Stokes field (solid lines, curve fitting). The dotted line in (a) has a slope of 2. “m” is the angular coefficient of the solid lines [22].

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Fig. 5.
Fig. 5. CARS interferogram detected at the beamsplitter BS2 of the setup shown in Fig.2. The envelope of the interferogram was fitted using the modulus of the degree of the coherence function. In the inset is shown a detail of the interference pattern and its fit by the real part of the degree of coherence function (open circles: experimental data; solid line: fit). L c is the coherence length of the pulse. λ AS is the wavelength of the CARS signal.

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Fig. 6.
Fig. 6. Setup of the interferometric SHG measurement system. Two different SHG crystals (Type I) were inserted in the two arms of the interferometers. BS, beamsplitter; M, mirror; IF, interference filter; PH, pin-hole; PMT, photomultiplier tube; PC, personal computer.

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Fig. 7.
Fig. 7. SHG interferogram detected at the beamsplitter BS2 of the setup shown in Fig.6. The interferogram was recorded as the pathlength of the reference arm was scanned. The modulus of the degree of the coherence function was used to fit the envelope of the interferogram. The inset shows a detail of the interference pattern and its fit by the real part of the degree of coherence function (open circles: experimental data; solid line: fit). L c is the coherence length of the pulse. λ SHG is the wavelength of the SHG signal.

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Equations (3)

Equations on this page are rendered with MathJax. Learn more.

P ¯ = ε 0 ( χ ( 1 ) · E ¯ + χ ( 2 ) : E ¯ E ¯ + χ ( 3 ) ⋮ E ¯ E ¯ E ¯ + … )
γ ( τ ) = exp ( − i τ ω 0 − i δ ω 2 τ 2 4 )
τ c = ∫ − ∞ + ∞ ∣ γ ( τ ) ∣ 2 d τ
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