Advances in engineering of high contrast CARS imaging endoscopes

Abstract

The translation of CARS imaging towards real time, high resolution, chemically selective endoscopic tissue imaging applications is limited by a lack of sensitivity in CARS scanning probes sufficiently small for incorporation into endoscopes. We have developed here a custom double clad fiber (DCF)-based CARS probe which is designed to suppress the contaminant Four-Wave-Mixing (FWM) background generated within the fiber and integrated it into a fiber based scanning probe head of a few millimeters in diameter. The DCF includes a large mode area (LMA) core as a first means of reducing FWM generation by ~3 dB compared to commercially available, step-index single mode fibers. A micro-fabricated miniature optical filter (MOF) was grown on the distal end of the DCF to block the remaining FWM background from reaching the sample. The resulting probe was used to demonstrate high contrast images of polystyrene beads in the forward-CARS configuration with > 10 dB suppression of the FWM background. In epi-CARS geometry, images exhibited lower contrast due to the leakage of MOF-reflected FWM from the fiber core. Improvements concepts for the fiber probe are proposed for high contrast epi-CARS imaging to enable endoscopic implementation in clinical tissue assessment contexts, particularly in the early detection of endoluminal cancers and in tumor margin assessment.

Fig. 1

Numerical evaluation of the influence of W, the trench width, and δn, the trench depth, on (a) bending losses per ½ turn at Stokes wavelength of 1064 nm for a 2-cm bending radius; (b) LP₁₁ confinement losses at the pump wavelength of 816 nm. (c) Target index profile, designed to keep the bending losses at ~0.1 dB per ½ turn (2 cm bending radius) with mode field areas of 50 µm² at 816 nm and 80 µm² at 1064nm.

Fig. 2

(a) Picture of the fiber facet, (b) Relative radial refractive index profile of the INO-1006A11 DCF core, (c) 3D plot of the refractive index profile.

Fig. 3

(a) Block diagram of the CARS setup employed to characterize the contaminant FWM signal, A Spectra Physics Tsunami laser tuned to 816 nm was employed as the pump source and a Time-Bandwidth Lynx laser delivering 1064 nm acted as the Stokes source. The synchronization delay between the picosecond laser sources was modulated using a low frequency (~20 Hz) triangular wave. The emission of the pump and Stokes sources was coupled using a dichroic mirror and, after passing through a Glan–Thompson polarizer, was focused into the fiber core of the fiber under test, with the polarization aligned parallel to either the slow of the fast axis of the fiber. The fiber emission was filtered with a short pass filter to remove pump and Stokes wavelengths, allowing anti-Stokes/FWM wavelengths to be detected by a photomultiplier tube (PMT). The PMT signal was then amplified and converted from current to voltage for display on the oscilloscope along with the function generator output. (b) Typical oscilloscope traces showing FWM peaks detected at delays corresponding to air-glass interfaces, (c) graph showing typical actual FWM power vs actual delay.

Fig. 4

(a) Test setup for measurement of beam propagation factor. (b) Typical near field output beam profile obtained while purposely detuning the alignment of a single transverse mode beam launched in the optical fiber so as to induce a 50% reduction of the coupling efficiency from optimal alignment. M² = 1.05 ± 0.01.

Fig. 5

Images of the developed SFE probe head. (a) Side view. (b) Front view (DCF diameter is 245 µm).

Fig. 6

(a) Epi-CARS setup employing a Spectra-Physics Tsunami laser tuned to 816 nm at 44 mW and a Time-Bandwidth Lynx laser tuned to 1064 nm at 22 mW, for excitation of Raman transitions around 2850 cm⁻¹. A collimating lens and focusing lens (two Mitutoyo NIR 5X objectives) were used to deliver the pump and Stokes beams to the sample. Anti-Stokes light from the sample (λ_Anti-Stokes = 662 nm) was collected and directed back along the same path to the fiber, where it was collected by the core and surrounding inner cladding. The light was then directed to a PMT using a long-pass dichroic mirror and short-pass filtered to remove the pump and Stokes radiation. (b) Epi-CARS image of (6µm) polystyrene beads deposited on a mirror at ~2850 cm⁻¹ using the DCF-based SFE. (c) Signal detected at 662 while on and aside beads.

Fig. 7

(a) Forward CARS setup. The pump and Stokes beams were delivered to the sample as for the epi-CARS setup except that a long-pass filter was employed between the focusing and collimating lens for some images. Anti-stokes light was collected in the forward direction by an objective lens and, after removal of pump and Stokes radiation with a short-pass filter, directed onto a PMT. Images of micro-algae obtained by optical transmission microscopy (b) and F-CARS without (c) and with (d) filtering of the FWM contaminating signal generated in the probe fiber. Different regions of the same sample were used for CARS vs optical microscopy.

Fig. 8

(a) Schematic of the DCF with MOF. (b) SEM images of the DCF tip with MOF. (c) Measured MOF spectral transmittance. (d) Calculated DCF CARS collection efficiency as a function of MOF size for a variety of modeled samples (identified by #4,5,7,9,10,11,12,13, the corresponding tissue parameters can be found in Table 4 of Veilleux et al., Proceedings of the SPIE, Volume 7558, article id. 75580D, 2010).

Fig. 9

CARS images of 15µm polystyrene beads taken using the forward-CARS setup with (a) DCF without MOF and no free-space LPF, (b) DCF without MOF with free-space LPF, (c) DCF with MOF without free-space LPF and (d) DCF with MOF and free space LPF.

Fig. 10

CCD images and linear intensity profiles of FWM emission at proximal end of INO DCF in epi-CARS setup. (a) Image obtained without spatial filter in place, (b) Image with spatial filter in conjugate plane, (c) Intensity profile along a horizontal line coinciding with the center of the fiber tip image.

Fig. 11

(a) SEM image taken at a shallow angle showing curvatures of the fiber distal facet close to the MOF; (b) Estimated power recoupling into LP₀₁ mode at 663 nm after back-reflection from a misaligned mirror.